Co-reporter:Charles C. Mokhtarzadeh, Alex E. Carpenter, Daniel P. Spence, Mohand Melaimi, Douglas W. Agnew, Nils Weidemann, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa
Organometallics June 12, 2017 Volume 36(Issue 11) pp:2126-2126
Publication Date(Web):May 25, 2017
DOI:10.1021/acs.organomet.7b00164
The dimeric bridging carbonyl complexes [(μ-CO)2[CpCo]2]n (n = 0, 1−) have occupied a central position in the understanding of metal–metal bonding interactions when bridging ligands are present. Based on simple electron-counting formalisms, these dimers have been proposed to possess formal Co–Co bond orders of 2 and 1.5, respectively. However, this simple bonding scheme has been contrasted by molecular orbital theory considerations, as well as spectroscopic data that probes M–M bonding interactions generally. While this system has received considerable attention, there has been a long-standing synthetic limitation in that the doubly reduced dianionic dimer, [(μ-CO)2[CpCo]2]2–, has not been amenable to isolation, thereby precluding an analysis of ostensible full-integer reduction in a homologous series. Accordingly, herein is presented the synthesis of a homologous, three-membered series of bridging-isocyanide [(μ-CNAr)2[CpCo]2]n dimers, including the dianionic member. Structural and spectroscopic analyses of these [(μ-CNAr)2[CpCo]2]n dimers, which feature the m-terphenyl isocyanide CNArMes2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3), reveal that this series possesses similar overall properties to the bridging carbonyl counterparts. However, high-resolution X-ray crystallographic studies have revealed important structural differences that were not discernible in older studies of the carbonyl complexes. Also presented is the synthesis of the bridging-isocyanide/η6-arene dimers [Co2((η6-Mes)(μ-CNArMes))2]n (n = 0, 1+), which are valence isoelectronic to the mono- and dianionic [(μ-CNAr)2[CpCo]2]n derivatives. Structural and spectroscopic studies of these η6-arene complexes, as well as the related neutral nickel dimer (μ-CNArMes)2[CpNi]2 provide evidence for an electronic structure environment dominated by M→(CN)π* back-bonding interactions, rather than direct M–M bonding. This conclusion is supported by DFT-derived molecular orbital analysis on the bridging-isocyanide [(μ-CNArMes2)2[CpCo]2]n dimers.
Co-reporter:Brandon R. Barnett;Liezel A. Labios;Julia M. Stauber;Curtis E. Moore;Arnold L. Rheingold
Organometallics February 27, 2017 Volume 36(Issue 4) pp:944-954
Publication Date(Web):February 13, 2017
DOI:10.1021/acs.organomet.7b00035
Despite the large number of judiciously designed ligands that have been exploited in palladium-catalyzed cross-coupling protocols, the incorporation of ligands bearing appreciable π-acidic properties has remained significantly underexplored. Herein, we demonstrate that well-defined and low-coordinate Pd0 complexes supported by m-terphenyl isocyanides function as competent catalysts for the Suzuki–Miyaura cross-coupling of aryl bromides and arylboronic acids. Two-coordinate Pd(CNArDipp2)2 was active for the coupling of unhindered aryl bromides at room temperature in 2-propanol, while increasing the temperature to 60 °C allowed for the use of mono- or di-ortho-substituted aryl bromides. Oxidative addition of the aryl bromide was shown to proceed via a dissociative mechanism, implicating monoligated Pd(CNArDipp2) as the catalytically active intermediate. Attempts to access this fleeting species via activation of the PdII monoisocyanide PdCl(η3-C3H5)(CNArDipp2) with alkoxide base yielded the dinuclear PdI species (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2. Although dinuclear PdI complexes are often produced as off-cycle species when using complexes of the type PdCl(η3-allyl)L as precatalysts, this represents the first time that the comproportionation product (μ-allyl)(μ-Cl)[PdL]2 has been observed to undergo nucleophilic substitution with alkoxide, despite the fact that activating conditions for these precatalysts typically employ alkoxide bases. Remarkably, this alkoxide complex can undergo β-hydride elimination with expulsion of acetone and propene to produce two equivalents of catalytically active Pd(CNArDipp2), which can self-aggregate to yield the isolable tripalladium cluster Pd3(η2-Dipp-μ-CNArDipp2)3. This cluster is catalytically competent for the Suzuki–Miyaura reaction and functions as a formal source of monoligated Pd(CNArDipp2) in solution.
Co-reporter:Dr. Bron R. Barnett;Michael L. Neville;Dr. Curtis E. Moore; Arnold L. Rheingold; Joshua S. Figueroa
Angewandte Chemie 2017 Volume 129(Issue 25) pp:7301-7305
Publication Date(Web):2017/06/12
DOI:10.1002/ange.201702151
AbstractWhile interest in cooperative reactivity of transition metals and Lewis acids is receiving significant attention, the scope of known reactions that directly exploit the polarized reverse-dative σ-bond of metal-borane complexes (i.e., MBR3) remains limited. Described herein is that the platinum (boryl)iminomethane (BIM) complex [Pt(κ2-N,B-Cy2BIM)(CNArDipp2)] can effect the oxidative insertion of a range of unsaturated organic substrates, including azides, isocyantes, and nitriles, as well as CO2 and elemental sulfur (S8). In addition, alkyl migration processes available to the BIM framework allow for post-insertion reaction sequences resulting in product release from the metal center.
Co-reporter:Dr. Charles C. Mokhtarzadeh;Dr. Curtis E. Moore; Arnold L. Rheingold; Joshua S. Figueroa
Angewandte Chemie International Edition 2017 Volume 56(Issue 36) pp:10894-10899
Publication Date(Web):2017/08/28
DOI:10.1002/anie.201705877
AbstractThe encumbered tetraisocyanide dianion Na2[Fe(CNAr )4] reacts with two molecules of CO2 to effect reductive disproportionation to CO and carbonate ([CO3]2−). When the reaction is performed in the presence of silyl triflates, reductive disproportionation is arrested by silylative esterification of a mono-CO2 adduct. This results in the formation of four-coordinate terminal iron carbynes possessing an aryl carbamate substituent owing to the direct attachment of an C(O)OSiR3 group to an isocyanide nitrogen atom. Crystallographic, spectroscopic, and computational analyses of these iron–carbon multiply bonded species reveal electronic structure properties indicative of a conformationally locked iron carbyne unit.
Co-reporter:Dr. Bron R. Barnett;Michael L. Neville;Dr. Curtis E. Moore; Arnold L. Rheingold; Joshua S. Figueroa
Angewandte Chemie International Edition 2017 Volume 56(Issue 25) pp:7195-7199
Publication Date(Web):2017/06/12
DOI:10.1002/anie.201702151
AbstractWhile interest in cooperative reactivity of transition metals and Lewis acids is receiving significant attention, the scope of known reactions that directly exploit the polarized reverse-dative σ-bond of metal-borane complexes (i.e., MBR3) remains limited. Described herein is that the platinum (boryl)iminomethane (BIM) complex [Pt(κ2-N,B-Cy2BIM)(CNArDipp2)] can effect the oxidative insertion of a range of unsaturated organic substrates, including azides, isocyantes, and nitriles, as well as CO2 and elemental sulfur (S8). In addition, alkyl migration processes available to the BIM framework allow for post-insertion reaction sequences resulting in product release from the metal center.
Co-reporter:Douglas W. Agnew;Curtis E. Moore;Arnold L. Rheingold
Dalton Transactions 2017 vol. 46(Issue 20) pp:6700-6707
Publication Date(Web):2017/05/23
DOI:10.1039/C7DT01102D
Using the stable metalloradical Mn(CO)3(CNArDipp2)2, we report the formation of manganese-main group complexes via the single-electron functionalization of main group halides. The reactions occur in a simple 1 : 1 stoichiometry, and demonstrate the utility of using stable open-shelled organometallics as precursors for metal-main group compounds. This has enabled the preparation of manganese complexes bearing terminal –EXn substituents, as shown through the isolation of Mn(SnCl)(CO)3(CNArDipp2)2 and Mn(BiCl2)(CO)3(CNArDipp2)2 from SnCl2 and BiCl3, respectively. Through this approach, we have also isolated Mn(SbF2)(CO)3(CNArDipp2)2 from SbF3, which serves as a unique example of a terminal –SbF2 complex. Although the metalloradical functionalization of binary main group halides provides the desired main group adduct in yields comparable to nucleophilic activation using the manganate Na[Mn(CO)3(CNArDipp2)2], the former approach is shown to be far more atom-economical with respect to Mn. Additionally, we have found that Mn(CO)3(CNArDipp2)2 also serves as a convenient precursor to MnF(CO)3(CNArDipp2). The latter is an analogue to the elusive monofluoride FMn(CO)5.
Co-reporter:Douglas W. Agnew, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa
Organometallics 2017 Volume 36(Issue 2) pp:
Publication Date(Web):December 30, 2016
DOI:10.1021/acs.organomet.6b00792
Presented herein is a general design strategy for the formation of coordinatively unsaturated Mn(I) fragments via the cis labilization of CO using metal carboxylates and subsequent deprotection by a range of abstraction agents. A consistent trend is observed in the ability of carboxylate ligands to facilitate CO dissociation as a function of the electron-releasing properties of the carboxylate R group. This is in agreement with previous theoretical work on cis-labilizing ligands. A variety of κ2-coordinated carboxylates function as site-protection ligands for the [Mn(CO)2(CNArDipp2)2]+ scaffold and can be liberated using a range of synthetic techniques. The coordinatively unsaturated species thus formed are shown to be strong metal-based Lewis acids and can engage in agostic-type interactions; in addition, they can be trapped as the corresponding solvento complexes by ethereal solvents.
Co-reporter:Douglas W. Agnew, Milan Gembicky, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa
Journal of the American Chemical Society 2016 Volume 138(Issue 46) pp:15138-15141
Publication Date(Web):November 1, 2016
DOI:10.1021/jacs.6b10460
The preparation of 3D and 2D Cu(I) coordination networks using ditopic m-terphenyl isocyanides is described. The incorporation of sterically encumbering substituents enables the controlled, solid-state preparation of Cu(I) tris-isocyanide nodes with a labile solvent ligand in a manner mirroring solution-phase chemistry of monomeric complexes. The protection afforded by the m-terphenyl groups is also shown to engender significant stability towards heat as well as acidic or basic conditions, resulting in robust single-metal-node networks that can transition from 3D to 2D extended structures.
Co-reporter:Douglas W. Agnew, Matthew D. Sampson, Curtis E. Moore, Arnold L. Rheingold, Clifford P. Kubiak, and Joshua S. Figueroa
Inorganic Chemistry 2016 Volume 55(Issue 23) pp:12400-12408
Publication Date(Web):November 18, 2016
DOI:10.1021/acs.inorgchem.6b02299
To circumvent complications with redox-active ligands commonly encountered in the study of manganese electrocatalysts for CO2 reduction, we have studied the electrochemistry of the manganese mixed carbonyl/isocyanide complexes XMn(CO)3(CNArDipp2)2 (X = counteranion), to evaluate the pairing effects of the counteranion and their influence over the potential necessary for metal-based reduction. The complexes described herein have been shown to act as functional analogues to the known homoleptic carbonyl manganese complexes [Mn(CO)5]n (n = 1–, 0, 1+). The m-terphenyl isocyanide ligand CNArDipp2 improves the kinetic stability of the resulting mixed carbonyl/isocyanide systems, such that conversion among all three oxidation states is easily effected by chemical reagents. Here, we have utilized an electrochemical study to fully understand the redox chemistry of this system and its ability to facilitate CO2 reduction and to provide comparison to known manganese-based CO2 electrocatalysts. Two complexes, BrMn(CO)3(CNArDipp2)2 and [Mn(THF)(CO)3(CNArDipp2)2]OTf, have been studied using infrared spectroelectrochemistry (IR-SEC) to spectroscopically characterize the redox states of these complexes during the course of electrochemical reactions. A striking difference in the necessary potential leading to the first one-electron reduction has been found for the halide and triflate species, respectively. Complete selectivity for the formation of CO and CO32– is observed in the reactivity of [Mn(CO)3(CNArDipp2)2]− with CO2, which is deduced via the trapping and incorporation of liberated CO into the zerovalent species Mn(CO)3(CNArDipp2)2 to form the dimers Mn2(CO)7(CNArDipp2)3 and Mn2(CO)8(CNArDipp2)2.
Co-reporter:Bron R. Barnett; Arnold L. Rheingold ; Joshua S. Figueroa
Angewandte Chemie International Edition 2016 Volume 55( Issue 32) pp:9253-9258
Publication Date(Web):
DOI:10.1002/anie.201604903
Abstract
Owing to their unique topologies and abilities to self-assemble into a variety of extended and aggregated structures, the binary platinum carbonyl clusters [Pt3(CO)6]n2− (“Chini clusters”) continue to draw significant interest. Herein, we report the isolation and structural characterization of the trinuclear electron-transfer series [Pt3(μ-CO)3(CNArDipp2)3]n− (n=0, 1, 2), which represents a unique set of monomeric Pt3 clusters supported by π-acidic ligands. Spectroscopic, computational, and synthetic investigations demonstrate that the highest-occupied molecular orbitals of the mono- and dianionic clusters consist of a combined π*-framework of the CO and CNArDipp2 ligands, with negligible Pt character. Accordingly, this study provides precedent for an ensemble of carbonyl and isocyanide ligands to function in a redox non-innocent manner.
Co-reporter:Alex E. Carpenter, Arnold L. Rheingold, and Joshua S. Figueroa
Organometallics 2016 Volume 35(Issue 14) pp:2309-2318
Publication Date(Web):June 2, 2016
DOI:10.1021/acs.organomet.6b00297
Reported here is the synthesis and characterization of the tetrakis(m-terphenyl isocyanide)cobalt hydride HCo(CNArMes2)4 (1; ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3). Monohydride 1 serves as a well-defined isocyano analogue of the tetracarbonyl hydride HCo(CO)4. While tetrakis-phosphine analogues of HCo(CO)4 have been reported previously, these compounds have failed to exhibit a reactivity profile that can be compared and contrasted with HCo(CO)4 in a systematic fashion. Herein, HCo(CNArMes2)4 (1) is shown to be a readily accessed and reactive complex that allows for this comparison. For example, HCo(CNArMes2)4 (1) is found to decompose smoothly to the κ1-C-iminoformyl complex Co(η6-(Mes)-κ1C-C(H)NArMes2)(CNArMes2) (2). Kinetic analysis of this decomposition and that of the d1-isotopomer DCo(CNArMes2)4 (1-d1) revealed a unimolecular process characterized by a large primary kH/kD isotope effect (3.2(6)) and no dependence on the presence of free CNArMes2. These data point to rate-limiting hydride α-migration and formation of the κ1-C-iminoformyl species [Co(κ1-C-C(H)═NArMes2)(CNArMes2)3] as a critical intermediate. Indeed, ligand substitution reactions of HCo(CNArMes2)4 (1), as well as 13C-labeling experiments of the decomposition product 2, demonstrate that hydride α-migration is the dominant mechanistic feature of this system. Most notably, this behavior is in contrast with that of HCo(CO)4, for which it has been established that CO ligand dissociation is the initial mechanistic feature. Additional support for the critical role of hydride α-migration in HCo(CNArMes2)4 (1) was obtained by the development of catalytic CNArMes2 1,1-hydrogenation to form a stable and isolable methylenimine.
Co-reporter:Alex E. Carpenter, Chinglin Chan, Arnold L. Rheingold, and Joshua S. Figueroa
Organometallics 2016 Volume 35(Issue 14) pp:2319-2326
Publication Date(Web):July 7, 2016
DOI:10.1021/acs.organomet.6b00375
The m-terphenyl isocyanide complex, HCo(CNArMes2)4 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3), serves as a unique example of a well-defined isocyano analogue to HCo(CO)4. Given the well documented Brønsted acidity of HCo(CO)4 in both protic and nonprotic media, the Brønsted acidity of HCo(CNArMes2)4 was assessed for a quantitative comparison. Acid bracketing experiments in THF solution revealed that HCo(CNArMes2)4 has a Morris relative pKαTHF value of 38.5–40.7, which is considerably higher than that of HCo(CO)4 (pKαTHF(calc) = 11.4) and thereby indicates insignificant Brønsted acidity. Furthermore, the relative acidity of HCo(CNArMes2)4 rivals that of tetra-phosphine cobalt hydrides (i.e., HCo(PR3)4; pKαTHF(calc) ≥ 48), despite the good π-acidity properties of the isocyano unit. To systematically determine the effect of substituting an isocyanide for a CO ligand on the acidity of the Co–H unit in HCoL4 complexes, the full series of HCo(CO)n(CNArMes2)4–n monohydrides and [Co(CO)n(CNArMes2)4–n]− (n = 1–4) metalates were prepared and characterized. Acid bracketing studies on the [Co(CO)n(CNArMes2)4–n]− metalates in THF solution revealed a regular progression of increasing pKαTHF values as isocyanides are added to the Co center. However, the monoisocyanide tricarbonyl hydride, HCo(CO)3(CNArMes2), possesses a pKαTHF value of 28.6–32.5, which is also significantly higher than that of HCo(CO)4 and the monophosphine complex HCo(CO)3(PPh3). Accordingly, the unconventional ability of isocyanide ligands to function as stronger σ-donors than organophosphines is discussed within the context of both the Brønsted acidity and spectroscopic features of the HCo(CO)n(CNArMes2)4–n monohydrides.
Co-reporter:Bron R. Barnett; Arnold L. Rheingold ; Joshua S. Figueroa
Angewandte Chemie 2016 Volume 128( Issue 32) pp:9399-9404
Publication Date(Web):
DOI:10.1002/ange.201604903
Abstract
Owing to their unique topologies and abilities to self-assemble into a variety of extended and aggregated structures, the binary platinum carbonyl clusters [Pt3(CO)6]n2− (“Chini clusters”) continue to draw significant interest. Herein, we report the isolation and structural characterization of the trinuclear electron-transfer series [Pt3(μ-CO)3(CNArDipp2)3]n− (n=0, 1, 2), which represents a unique set of monomeric Pt3 clusters supported by π-acidic ligands. Spectroscopic, computational, and synthetic investigations demonstrate that the highest-occupied molecular orbitals of the mono- and dianionic clusters consist of a combined π*-framework of the CO and CNArDipp2 ligands, with negligible Pt character. Accordingly, this study provides precedent for an ensemble of carbonyl and isocyanide ligands to function in a redox non-innocent manner.
Co-reporter:Brandon R. Barnett, Curtis E. Moore, Perumalreddy Chandrasekaran, Stephen Sproules, Arnold L. Rheingold, Serena DeBeer and Joshua S. Figueroa
Chemical Science 2015 vol. 6(Issue 12) pp:7169-7178
Publication Date(Web):17 Sep 2015
DOI:10.1039/C5SC03104D
Complexes bearing electron rich transition metal centers, especially those displaying coordinative unsaturation, are well-suited to form reverse-dative σ-interactions with Lewis acids. Herein we demonstrate the generality of zerovalent, group 10 m-terphenyl isocyanide complexes to form reverse-dative σ-interactions to Tl(I) and Ag(I) centers. Structural and spectroscopic investigations of these metal-only Lewis pairs (MOLPs) has allowed insight into the electronic consequences of Lewis-acid ligation within the primary coordination sphere of a transition metal center. Treatment of the bis-isocyanide complex, Pt(CNArDipp2)2 (ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) with TlOTf (OTf = [O3SCF3]−) yields the Pt/Tl MOLP [TlPt(CNArDipp2)2]OTf (1). 1H NMR and IR spectroscopic studies on 1, and its Pd congener [TlPd(CNArDipp2)2]OTf (2), demonstrate that the M → Tl interaction is labile in solution. However, treatment of complexes 1 and 2 with Na[BArF4] (ArF = 3,5-(CF3)2C6H3) produces [TlPt(CNArDipp2)2]BArF4 (3) and [TlPd(CNArDipp2)2]BArF4 (4), in which Tl(I) binding is shown to be static by IR spectroscopy and, in the case of 3, 195Pt NMR spectroscopy as well. This result provides strong evidence that the M → Tl linkages can be attributed primarily to σ-donation from the group 10 metal to Tl, as loss of ionic stabilization of Tl by the triflate anion is compensated for by increasing the degree of M → Tl σ-donation. In addition, X-ray Absorption Near-Edge Spectroscopy (XANES) on the Pd/Tl and Ni/Tl MOLPs, [TlPd(CNArDipp2)2]OTf (2) and [TlNi(CNArMes2)3]OTf, respectively, is used to illustrate that the formation of a reverse-dative σ-interaction with Tl(I) does not alter the spectroscopic oxidation state of the group 10 metal. Also reported is the ability of M(CNArDipp2)2 (M = Pt, Pd) to form MOLPs with Ag(I), yielding the complexes [AgM(CNArDipp2)2]OTf (5, M = Pt; 6, M = Pd). As was determined for the Tl-containing MOLPs 1–4, it is shown that the spectroscopic oxidation states of the group 10 metal in 5 and 6 are essentially unchanged compared to the zerovalent precursors M(CNArDipp2)2. However, in the case of 5 and 6, the formation of a dative M → Ag σ-bonding interaction facilitates the binding of Lewis bases to the group 10 metal trans to Ag, illustrating the potential of acceptor fragments to open up new coordination sites on transition metal complexes without formal, two-electron oxidation.
Co-reporter:Brandon R. Barnett, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa
Chemical Communications 2015 vol. 51(Issue 3) pp:541-544
Publication Date(Web):10 Nov 2014
DOI:10.1039/C4CC08037H
The activation of carbon dioxide, organonitriles, and terminal acetylenes by (boryl)iminomethanes derived from isocyanide 1,1-hydroboration is described. Also detailed is the generality of hydroboration of m-terphenylisocyanides with hydroboranes of differing Lewis acidities.
Co-reporter:Charles C. Mokhtarzadeh, Grant W. Margulieux, Alex E. Carpenter, Nils Weidemann, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa
Inorganic Chemistry 2015 Volume 54(Issue 11) pp:5579-5587
Publication Date(Web):May 12, 2015
DOI:10.1021/acs.inorgchem.5b00730
Reported here are synthetic studies probing highly reduced iron centers in an encumbering tetraisocyano ligand environment. Treatment of FeCl2 with sodium amalgam in the presence of 2 equiv of the m-terphenyl isocyanide CNArMes2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) produces the disodium tetraisocyanoferrate Na2[Fe(CNArMes2)4]. Structural characterization of Na2[Fe(CNArMes2)4] revealed a tight ion pair, with the Fe center adopting a tetrahedral coordination geometry consistent with a d10 metal center. Attempts to disrupt the cation–anion contacts in Na2[Fe(CNArMes2)4] with cation-sequestration reagents lead to decomposition, except for the case of 18-crown-6, where a mononuclear complex featuring a dianionic 1-azabenz[b]azulene ligand was isolated in low yield. Formation of this 1-azabenz[b]azulene is rationalized to proceed by an aza-Büchner ring expansion of a CNArMes2 ligand mediated by a coordinatively unsaturated Fe center. Disodium tetraisocyanoferrate Na2[Fe(CNArMes2)4] is readily protonated by trimethylsilanol (HOSiMe3) to produce the monohydride ferrate salt, Na[HFe(CNArMes2)4], the anionic portion of which serves as an isocyano analogue of the hydrido-tetracarbonyl metalate [HFe(CO)4]−. Treatment of Na[HFe(CNArMes2)4] with methyl triflate (MeOTf; OTf = [O3SCF3]−) at low temperature in the presence of dinitrogen yields the five-coordinate Fe(0) complex Fe(N2)(CNArMes2)4. The formation of Fe(N2)(CNArMes2)4 in this reaction is consistent with the intermediacy of the neutral tetraisocyanide Fe(CNArMes2)4. The decomposition of Fe(N2)(CNArMes2)4 to the dimeric complex [Fe(η6-(Mes)-μ2-C-CNArMes)]2 and a seven-membered cyclic imine derived from a CNArMes2 ligand is presented and provides insight into the ability of CNArMes2 and related m-terphenyl isocyanides to stabilize zerovalent four-coordinate iron complexes in a strongly π-acidic ligand field.
Co-reporter:Brandon R. Barnett; Liezel A. Labios; Curtis E. Moore; Jason England; Arnold L. Rheingold; Karl Wieghardt
Inorganic Chemistry 2015 Volume 54(Issue 14) pp:7110-7121
Publication Date(Web):July 1, 2015
DOI:10.1021/acs.inorgchem.5b01252
The redox-noninnocence of metal-coordinated C-organo nitrosoarenes has been established on the basis of solid-state characterization techniques, but the solution-phase properties of this class of metal-coordinated radicals have been relatively underexplored. In this report, the solution-phase properties and dynamics of the bis-nitrosobenzene diradical complex trans-Pd(κ1-N-PhNO)2(CNArDipp2)2 are presented. This complex, which is best described as containing singly reduced phenylnitroxide radical ligands, is shown to undergo facile nitrosobenzene dissociation in solution to form the metalloxaziridine Pd(η2-N,O-PhNO)(CNArDipp2)2 and thus is not a persistent species in solution. An equilibrium between trans-Pd(κ1-N-PhNO)2(CNArDipp2)2, Pd(η2-N,O-PhNO)(CNArDipp2)2, and free nitrosobenzene is established in solution, with the metalloxaziridine being predominantly favored. Efforts to perturb this equilibrium by the addition of excess nitrosobenzene reveal that the formation of trans-Pd(κ1-N-PhNO)2(CNArDipp2)2 is in competition with insertion-type chemistry of Pd(η2-N,O-PhNO)(CNArDipp2)2 and is therefore not a viable strategy for the production of a kinetically persistent bis-nitroxide radical complex. Electronic modification of the nitrosoarene framework was explored as a means to generate a persistent trans-Pd(κ1-N-ArNO)2(CNArDipp2)2 complex. While most substitution schemes failed to significantly perturb the kinetic lability of the nitrosoarene ligands in the corresponding trans-Pd(κ1-N-ArNO)2(CNArDipp2)2 complexes, utilization of para-formyl or para-cyano nitrosobenzene produced bis-nitroxide diradical complexes that display kinetic persistence in solution. The origin of this persistence is rationalized by the ability of para-formyl- and para-cyano-aryl groups to both attenuate the trans effect of the corresponding nitrosoarene and, more importantly, delocalize spin density away from the aryl-nitroxide NO unit. The results presented here highlight the inherent instability of metal-coordinated nitroxide radicals and suggest a general synthetic strategy for kinetically stabilizing these species in solution.
Co-reporter:Douglas W. Agnew;Dr. Curtis E. Moore; Arnold L. Rheingold ; Joshua S. Figueroa
Angewandte Chemie International Edition 2015 Volume 54( Issue 43) pp:12673-12677
Publication Date(Web):
DOI:10.1002/anie.201506498
Abstract
The 17e− monoradical [Mn(CO)5] is widely recognized as an unstable organometallic transient and is known to dimerize rapidly with the formation of a MnMn single bond. As a result of this instability, isolable analogues of [Mn(CO)5] have remained elusive. Herein, we show that two sterically encumbering isocyanide ligands can destabilize the MnMn bond leading to the formation of the isolable, manganese(0) monoradical [Mn(CO)3(CNArDipp2)2] (ArDipp2=2,6-(2,6-(iPr)2C6H3)2C6H3). The persistence of [Mn(CO)3(CNArDipp2)2] has allowed for new insights into nitrosoarene spin-trapping studies of [Mn(CO)5].
Co-reporter:Douglas W. Agnew;Dr. Curtis E. Moore; Arnold L. Rheingold ; Joshua S. Figueroa
Angewandte Chemie 2015 Volume 127( Issue 43) pp:12864-12868
Publication Date(Web):
DOI:10.1002/ange.201506498
Abstract
The 17e− monoradical [Mn(CO)5] is widely recognized as an unstable organometallic transient and is known to dimerize rapidly with the formation of a MnMn single bond. As a result of this instability, isolable analogues of [Mn(CO)5] have remained elusive. Herein, we show that two sterically encumbering isocyanide ligands can destabilize the MnMn bond leading to the formation of the isolable, manganese(0) monoradical [Mn(CO)3(CNArDipp2)2] (ArDipp2=2,6-(2,6-(iPr)2C6H3)2C6H3). The persistence of [Mn(CO)3(CNArDipp2)2] has allowed for new insights into nitrosoarene spin-trapping studies of [Mn(CO)5].
Co-reporter:Alex E. Carpenter ; Andrew J. McNeece ; Brandon R. Barnett ; Alexander L. Estrada ; Charles C. Mokhtarzadeh ; Curtis E. Moore ; Arnold L. Rheingold ; Charles L. Perrin
Journal of the American Chemical Society 2014 Volume 136(Issue 44) pp:15481-15484
Publication Date(Web):October 15, 2014
DOI:10.1021/ja508956q
Reported here are the isolation, structural characterization, and decomposition kinetics of the four-coordinate pentachloroethyl nickel complex, NiCl(CCl2CCl3)(CNArMes2)2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3). This complex is a unique example of a kinetically persistent β-chloroalkyl in a system relevant to coordination–insertion polymerization of polar olefins. Kinetic analysis of NiCl(CCl2CCl3)(CNArMes2)2 decomposition indicates that β-chloride (β-Cl) elimination proceeds by a unimolecular mechanism that does not require initial dissociation of a CNArMes2 ligand. The results suggest that a direct β-Cl elimination pathway is available to four-coordinate, Group 10 metal vinyl chloride polymerization systems.
Co-reporter:Brandon R. Barnett, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa
Journal of the American Chemical Society 2014 Volume 136(Issue 29) pp:10262-10265
Publication Date(Web):July 9, 2014
DOI:10.1021/ja505843g
The synthesis of a three-coordinate Pt–borane complex featuring a bidentate “LZ” (boryl)iminomethane (BIM) ligand is reported. Unlike other LZ-type borane ligands featuring a single-donor buttress, the small bite angle enforced by the BIM ligand is shown to promote a significant metal–borane reverse-dative σ-interaction akin to multiply strapped metalloboratranes. The steric accessibility of the reactive Pt → B bond fostered by the BIM ligand allows for a rich reactivity profile toward small molecules that exploit metal–borane cooperative effects. The unligated (boryl)iminomethane BIM is also synthetically accessible and functions as a Frustrated Lewis Pair (FLP). The ability of the free BIM to effect bond activation reactions is contrasted with the behavior seen in the corresponding platinum-bound complexes.
Co-reporter:Stephen P. George, Curtis E. Moore, Alex E. Carpenter, Arnold L. Rheingold, James M. Blackwell, Joshua S. Figueroa
Inorganica Chimica Acta 2014 Volume 422() pp:146-151
Publication Date(Web):1 October 2014
DOI:10.1016/j.ica.2014.06.033
•Metal coordinated benzylcarbamates have been surveyed as two-stage photobase generators.•A new silylative transformation of [PF6]− anion to triflate anion has been introduced.•Coordination of benzylcarbamates to the [CpRu]+ fragment does not inhibit base generation.•DFT calculations suggest that [CpRu(η6-benzylcarbamate)]X salts may serve as two-stage PBGs.•Thin-film studies show that base generation occurs under typical photolithographic conditions.In order to develop a new class of transition-metal-based, two-stage photobase generators (PBGs), the synthesis, structural characterization, spectroscopic properties and base-generation ability of a series of [CpRu(η6-benzylcarbamate)]X salts (X = [PF6]−, [O3SCF3]−) are reported. Treatment of [CpRu(NCMe)3]PF6 with the N,N-dialkylcarbamate PBGs 2-nitrophenylmethyl N,N-dicyclohexylcarbamate (2NC), 4-nitrophenylmethyl N,N-dicyclohexylcarbamate (4NC) and 3,5-dimethoxyphenylmethyl N,N-dicyclohexylcarbamate (DMC) results in η6-complexation of the arene fragment and formation of the corresponding [CpRu(η6-benzylcarbamate)]PF6 salt. The [PF6]− salt, [CpRu(η6-DMC)]PF6, is converted to the triflate salt (triflate = [O3SCF3]− = [OTf]−) by treatment with Me3SiOTf. The molecular structures of the [CpRu(η6-benzylcarbamate)]X salts were determined by X-ray diffraction and varied by the orientation of the carbmamate unit relative to the η5-Cp ligand. Experimental UV–Vis data and Time-Dependent DFT calculations suggest that the [CpRu(η6-benzylcarbamate)]+ cations possess high-energy excitations that potentially render these salts as useful two-stage PBGs. The photobase generation ability of the [CpRu(η6-benzylcarbamate)]X salts at 254 nm irradiation is assessed in both acetonitrile solution and polymethacrylate thin films. The irradiation studies demonstrate that η6-coordination of a benzylcarbamate PBG does not deactivate photolytic base generation.A series of [CpRu(η6-benzylcarbamate)]+ cations are prepared for use as a new class of transition-metal-based photobase generators (PBGs).
Co-reporter:Treffly B. Ditri ; Alex E. Carpenter ; Donald S. Ripatti ; Curtis E. Moore ; Arnold. L. Rheingold
Inorganic Chemistry 2013 Volume 52(Issue 22) pp:13216-13229
Publication Date(Web):October 30, 2013
DOI:10.1021/ic402130p
Presented herein are synthetic and structural studies exploring the propensity of m-terphenyl isocyanide ligands to provide flanking-ring η6-arene interactions to zerovalent molybdenum centers. The alkyl-substituted m-terphenyl isocyanides CNArMes2 and CNArDipp2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3; ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) react with Mo(η6-napthalene)2 in a 3:1 ratio to form tris-isocyanide η6-arene Mo complexes, in which a flanking mesityl or 2,6-diisopropylphenyl group, respectively, of one isocyanide ligand is bound to the zerovalent molybdenum center. Thermal stability and reactivity studies show that these flanking ring η6-arene interactions are particularly robust. To weaken or prevent formation of a flanking-ring η6-arene interaction, and to potentially provide access to the coordinatively unsaturated [Mo(CNArR)3] fragment, the new halo-substituted m-terphenyl isocyanides CNArClips2 and CNArDArF2 (ArClips = 2,6-(2,6-Cl2C6H3)2(4-t-Bu)C6H2; ArDArF2 = 2,6-(3,5-(CF3)2C6H3)2C6H3) have been prepared. Relative to their alkyl-substituted counterparts, synthetic and structural studies demonstrate that the flanking aryl rings of CNArClips2 and CNArDArF2 display a lower tendency toward η6-binding. In the case of CNArDArF2, it is shown that an η6-bound 3,5-bis(trifluoromethyl)phenyl group can be displaced from a zerovalent molybdenum center by three molecules of acetonitrile. This observation suggests that the CNArDArF2 ligand effectively masks low-valent metal centers in a fashion that provides access to low-coordinate isocyano targets such as [Mo(CNArR)3]. A series of Mo(CO)3(CNArR)3 complexes were also prepared to compare the relative π-acidities of CNArMes2, CNArClips2, and CNArDArF2. It is found that CNArDArF2 shows increased π-acidity relative to CNArMes2 and CNArClips2, despite the fact that its electron-withdrawing CF3 groups are fairly distal to the terminal isocyano unit.
Co-reporter:Alex E. Carpenter;Isabel Wen;Dr. Curtis E. Moore; Arnold L. Rheingold ; Joshua S. Figueroa
Chemistry - A European Journal 2013 Volume 19( Issue 32) pp:
Publication Date(Web):
DOI:10.1002/chem.201301997
Co-reporter:Patrick W. Smith, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa
Dalton Transactions 2012 vol. 41(Issue 26) pp:8031-8038
Publication Date(Web):17 Apr 2012
DOI:10.1039/C2DT30585B
In an effort to enforce a sterically hindered environment in transition-metal and main-group 2-picolinate complexes, the synthesis of the encumbering derivative 6-mesityl-2-picolinate (Mespic) is presented. The coordination and structural properties of Mespic are demonstrated with a range of transition-metal and main-group fragments. The 6-position mesityl group of Mespic is shown to alter both the primary and secondary coordination spheres of metal centers relative to the ubiquitous and unencumbered parent 2-picolinate anion.
Co-reporter:Alex E. Carpenter;Grant W. Margulieux;Matthew D. Millard;Dr. Curtis E. Moore;Dr. Nils Weidemann;Dr. Arnold L. Rheingold ;Dr. Joshua S. Figueroa
Angewandte Chemie 2012 Volume 124( Issue 37) pp:9546-9550
Publication Date(Web):
DOI:10.1002/ange.201205058
Co-reporter:Alex E. Carpenter;Grant W. Margulieux;Matthew D. Millard;Dr. Curtis E. Moore;Dr. Nils Weidemann;Dr. Arnold L. Rheingold ;Dr. Joshua S. Figueroa
Angewandte Chemie International Edition 2012 Volume 51( Issue 37) pp:9412-9416
Publication Date(Web):
DOI:10.1002/anie.201205058
Co-reporter:Mason A. Stewart, Curtis E. Moore, Treffly B. Ditri, Liezel A. Labios, Arnold L. Rheingold and Joshua S. Figueroa
Chemical Communications 2011 vol. 47(Issue 1) pp:406-408
Publication Date(Web):17 Sep 2010
DOI:10.1039/C0CC02742A
The m-terphenyl isocyanides CNArMes2 and CNArDipp2 support five-coordinate, isocyanide/carbonyl monoanions of manganese. For CNArDipp2, a bis-isocyanide anion is available that is remarkably well behaved upon reaction with electrophiles. Most notable is the formation of an unprecedented chloride-substituted metallostannylene.
Co-reporter:Treffly B. Ditri, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa
Inorganic Chemistry 2011 Volume 50(Issue 20) pp:10448-10459
Publication Date(Web):September 16, 2011
DOI:10.1021/ic2015868
Synthetic studies are presented addressing the oxidative decarbonylation of molybdenum and tungsten complexes supported by the encumbering m-terphenyl isocyanide ligand CNArDipp2 (ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3). These studies represent an effort to access halide or pseudohalide M/CNArDipp2 species (M = Mo, W) for use as precursors to low-coordinate, low-valent group 6 isocyanide complexes. The synthesis and structural chemistry of the tetra- and tricarbonyl tungsten complexes trans-W(CO)4(CNArDipp2)2 and trans-W(NCMe)(CO)3(CNArDipp2)2 are reported. The acetonitrile adducts trans-M(NCMe)(CO)3(CNArDipp2)2 (M = Mo, W) react with I2 to form divalent, diiodide complexes in which the extent of decarbonylation differs between Mo and W. In the molybdenum example, the diiodide, dicarbonyl complex MoI2(CO)2(CNArDipp2)2 is generated, which has an S = 1 ground state in solution. Paramagnetic group 6 MX2L4 complexes are rare, and the structure of MoI2(CO)2(CNArDipp2)2 is discussed in relation to other diamagnetic and C2v-distorted MX2L4 complexes. Diiodide MoI2(CO)2(CNArDipp2)2 reacts further with I2 to effect complete decarbonylation, producing the paramagnetic tetraiodide complex trans-MoI4(CNArDipp2)2. The reactivity of the trans-M(NCMe)(CO)3(CNArDipp2)2 (M = Mo, W) complexes toward benzoyl peroxide is also surveyed, and it is shown that dicarboxylate complexes can be obtained by oxidative or salt-elimination routes. The reduction behavior of the tetraiodide complex trans-MoI4(CNArDipp2)2 toward Mg metal and sodium amalgam is studied. In benzene solution under N2, trans-MoI4(CNArDipp2)2 is reduced by Na/Hg to the η6-arene-dinitrogen complex, (η6-C6H6)Mo(N2)(CNArDipp2)2. The diiodide-η6-benzene complex (η6-C6H6)MoI2(CNArDipp2)2 is an isolable intermediate in this reduction reaction, and its formation and structure are discussed in context of putative low-coordinate, low-valent molybdenum isocyanide complexes.
Co-reporter:Julia M. Stauber, Andrew L. Wadler, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa
Inorganic Chemistry 2011 Volume 50(Issue 15) pp:7309-7316
Publication Date(Web):June 28, 2011
DOI:10.1021/ic201205p
To overcome the unfavorable steric pressures associated with 2,6-disubstitution in encumbering pyridine ligands, the coordination chemistry of a 2,5-disubstituted variant, namely, 2,5-dimesitylpyridine (2,5-Mes2py), is reported. This diaryl pyridine shows good binding ability to a range of transition-metal fragments with varying formal oxidation states and coligands. Treatment of 2.0 equiv of 2,5-Mes2py with monovalent Cu and Ag triflate sources generates complexes of the type [M(2,5-Mes2py)2]OTf (M = Cu, Ag; OTf = OSO2CF3), which feature long M-OTf distances and a substrate-accessible primary coordination sphere. Combination of 2,5-Mes2py with Cu(OTf)2 and Pd(OAc)2 produces four-coordinate complexes featuring cis- and trans-2,5-Mes2py orientations, respectively. The four-coordinate palladium complex Pd(OAc)2(2,5-Mes2py)2 is found to resist py-ligand dissociation at room temperature in solution, but functions as a precatalyst for the aerobic C–H bond olefination of benzene at elevated temperatures. This C–H bond activation chemistry is compared with a similar Pd-based system featuring 2,6-disubstituted pyridines. 2,5-Mes2py also readily supports mono- and dinuclear divalent Co complexes, and the solution-phase equilibria between such species are detailed. The coordination studies presented highlight the potential of 2,5-Mes2py to function as an encumbering ancillary for the stabilization of low-coordinate complexes and as a supporting ligand for metal-mediated transformations.
Co-reporter:Neil C. Tomson ; Liezel A. Labios ; Thomas Weyhermüller ; Joshua S. Figueroa ;Karl Wieghardt
Inorganic Chemistry 2011 Volume 50(Issue 12) pp:5763-5776
Publication Date(Web):May 25, 2011
DOI:10.1021/ic2005979
Studies on the coordination of nitrosoarene (ArNO) ligands to late-transition metals are used to provide the first definition of the geometric, spectroscopic, and computational parameters associated with a PhNO electron-transfer series. Experimentally, the Pd complexes PdCl2(PhNO)2, PdL2(PhNO)2, and PdL2(TolNO) (L = CNArDipp2; ArDipp2 = 2,6-(2,6-iPr2C6H3)2-C6H3) are characterized as containing (PhNO)0, (PhNO)•1–, and (TolNO)2– ligands, respectively, and the structural and spectroscopic changes associated with this electron transfer series provide the basis for an extensive computational study of these and related ArNO-containing late-transition metal complexes. Most notable from the results is the unambiguous characterization of the ground state electronic structure of PdL2(PhNO)2, found to be the first isolable, transition metal ion complex containing an η1-N-bound π-nitrosoarene radical anion. In addition to the electron transfer series, the synthesis and characterization of the Fe complex [Fe(TIM)(NCCH3)(PhNO)][(PF6)2] (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) allows for comparison of the geometric and spectroscopic features associated with metal-to-ligand π-backbonding as opposed to (PhNO)•1– formation. Throughout these series of complexes, the N–O, M–N, and C–N bond distances as well as the N–O stretching frequencies and the planarity of the ArNO ligands provided distinct parameters for each ligand oxidation state. Together, these data provide a delineation of the factors needed for evaluating the oxidation state of nitrosoarene ligands bound to transition metals in varying coordination modes.
Co-reporter:Brian M. Emerich, Curtis E. Moore, Brian J. Fox, Arnold L. Rheingold, and Joshua S. Figueroa
Organometallics 2011 Volume 30(Issue 9) pp:2598-2608
Publication Date(Web):April 14, 2011
DOI:10.1021/om200209w
Details are presented regarding a convenient synthesis of the nickel tris-isocyanide complex Ni(CNArDipp2)3 (ArDipp2 = 2,6-(2,6-[i-Pr]2C6H3)2C6H3). A previous synthesis of a Ni tris-isocyanide complex relied on a Tl(I) coordination-site protection strategy to discourage the formation of a tetrakis-isocyano complex. However, protecting-group-free access to Ni(CNArDipp2)3 is enabled by the encumbering m-terphenyl isocyanide CNArDipp2. Treatment of Ni(COD)2 with CNArDipp2 affords Ni(COD)(CNArDipp2)2, which is readily oxidized to NiI2(CNArDipp2)2 upon addition of I2. Reduction of NiI2(CNArDipp2)2 with Mg metal generates Ni(CNArDipp2)3 and does not require the addition of a third equivalent of CNArDipp2. Tris-isocyanide Ni(CNArDipp2)3 is active toward Lewis acid binding and oxidative addition reactions. Treatment of Ni(CNArDipp2)3 with TlOTf (OTf = [O3SCF3]−) generates the salt [TlNi(CNArDipp2)3]OTf, in which the Ni center functions as a Lewis base toward Tl. The Ni center in Ni(CNArDipp2)3 also oxidatively adds across C–X bonds in a number of alkyl, aryl, and main-group halides and is accompanied by varying degrees of CNArDipp2 ligand loss. Formation of η2-iminoacyl complexes deactivates the Ni center toward additional reactivity.
Co-reporter:Matthew D. Millard ; Curtis E. Moore ; Arnold L. Rheingold
Journal of the American Chemical Society 2010 Volume 132(Issue 26) pp:8921-8923
Publication Date(Web):June 15, 2010
DOI:10.1021/ja1037808
Detailed herein are synthetic, spectroscopic and reactivity studies for two isolable four-coordinate iridium(I) monohydride complexes of the simple formulation HIrL3. Such complexes have been postulated as reactive species in several transformations, but definite evidence for their existence has remained elusive. To stabilize these complexes, the methyleneadamantyl substituted phosphine ligand P(CH21Ad)(i-Pr)2 (abbreviated LmAd) was employed because of the resistance of the adamantane cage toward cyclometalation reactions. Treatment of the dihydride−chloride complex, H2IrCl(LmAd)2 with PhMgBr under N2 afforded the square planar complex HIr(N2)(LmAd)2. Contrastingly, treatment of H2IrCl(LmAd)2 with Li[HBEt3] under N2 generates the trihydride complex H3Ir(LmAd)2, which possesses an agostic interaction between the LmAd ligand and the Ir center. Dissolution of HIr(N2)(LmAd)2 in Et2O or C6D12 rapidly establishes an equilibrium mixture with the cyclometalated complex H2Ir(κ2-P,C-LmAd)(LmAd). Despite the equilibrium between HIr(N2)(LmAd)2 and H2Ir(κ2-P,C-LmAd)(LmAd), addition of 2 equiv of H2 or 1 equiv of H2O to the mixture cleanly generates the pentahydride complex H5Ir(LmAd)2 or the dihydride hydroxide complex H2Ir(OH)(LmAd)2, respectively. Sequential addition nBuLi and 12-crown-4 (12-c-4) to a HIr(N2)(LmAd)2/H2Ir(κ2-P,C-LmAd)(LmAd), mixture provides the salt [Li(12-c-4)2][HIr(κ2-P,C-LmAd)(LmAd)], which contains another four-coordinate Ir(I) monohydride. 31P{1H} NMR studies provide evidence that four-coordinate HIr(N2)(LmAd)2 is deprotonated en route to [Li(12-c-4)2][HIr(κ2-P,C-LmAd)(LmAd)]. [Li(12-c-4)2][HIr(κ2-P,C-LmAd)(LmAd)] deprotonates both H2N(2,6-(i-Pr)2C6H3) and HOC6F5 under an N2 atmosphere to regenerate HIr(N2)(LmAd)2/H2Ir(κ2-P,C-LmAd)(LmAd) equilibrium mixtures.
Co-reporter:Grant W. Margulieux ; Nils Weidemann ; David C. Lacy ; Curtis E. Moore ; Arnold L. Rheingold
Journal of the American Chemical Society 2010 Volume 132(Issue 14) pp:5033-5035
Publication Date(Web):March 21, 2010
DOI:10.1021/ja1012382
The encumbering m-terphenyl isocyanide ligand, CNArMes2 (Mes = 2,4,6-Me3C6H2), is used to stabilize homoleptic tetraisocyanide complexes of cobalt in the 1−, 0, and 1+ charge state. Most importantly, these complexes serve as isolable analogues of the binary carbonyl complexes [Co(CO)4]−, Co(CO)4, and [Co(CO)4]+. Sodium amalgam reduction of CoCl2 in the presence of CNArMes2 provides the salt Na[Co(CNArMes2)4], which can be oxidized with 1 equiv of ferrocenium triflate (FcOTf) to the neutral complex, Co(CNArMes2)4. X-ray diffraction, FTIR spectroscopy, and low-temperature EPR spectroscopy reveal that Co(CNArMes2)4 modulates between D2d- and C2v-symmetric forms. DFT calculations are used to rationalize this structural modulation in terms of thermal access to low-energy b2-symmetric C−Co−C bending modes. Treatment of Na[Co(CNArMes2)4] with 2 equiv of FcOTf, followed by addition of Na[BArF4], provides the salt [Co(CNArMes2)4]BArF4, which contains a diamagnetic, square planar monovalent cobalt center. The molecular and electronic structures of [Co(CNArMes2)4]BArF4 are compared and contrasted to the reported properties of the carbonyl cation, [Co(CO)4]+.
Co-reporter:Nils Weidemann, Grant W. Margulieux, Curtis E. Moore, Arnold L. Rheingold, Joshua S. Figueroa
Inorganica Chimica Acta 2010 Volume 364(Issue 1) pp:238-245
Publication Date(Web):15 December 2010
DOI:10.1016/j.ica.2010.08.024
Detailed herein are synthetic, spectroscopic and structural studies on trisisocyanide cobalt halide complexes featuring the encumbering m-terphenyl isocyanide CNArMes2 (Mes = 2,4,6-Me3C6H3). Addition of CNArMes2 to CoI2 in a 3:1 molar ratio provides the mononuclear complex, CoI2(CNArMes2)3, which can be oxidized to six-coordinate CoI3(CNArMes2)3 upon treatment with 0.5 equivalents of I2. Contrastingly, addition of CNArMes2 to CoBr2 provided the dinuclear complex Br2Co(μ2-Br)2Co(CNArMes2)3 irrespective of the molar ratios employed. FTIR analysis on these Co(II) and Co(III) complexes is used to assess the relative π-basicities of the cobalt centers toward the CNArMes2 ligands. Treatment of CoX2 (X = Cl, Br and I) with three CNArMes2 ligands followed by the addition of granulated Zn provides the pseudo-tetrahedral complexes XCo(CNArMes2)3. FTIR, magnetic and X-ray crystallographic studies are used to determine both the ground state electronic structure and relative π-basicities of these complexes.Despite encumbering steric properties, the m-terphenyl isocyanide CNArMes2 stabilizes four-, five- and six-coordinate cobalt halide complexes. A range of formal oxidation states is also accommodated by a tris-CNArMes2 structural motif. Such variations in coordination number and oxidation state are used to rationalize the electronic structure and π-basicity properties of the central cobalt atom.
Co-reporter:Liezel A. Labios ; Matthew D. Millard ; Arnold L. Rheingold
Journal of the American Chemical Society 2009 Volume 131(Issue 32) pp:11318-11319
Publication Date(Web):July 24, 2009
DOI:10.1021/ja905338x
Mg metal reduction of the divalent precursor PdCl2(CNArDipp2)2 (Dipp = 2,6-diisopropylphenyl) provides the isolable, two-coordinate Pd(0) bis-isocyanide, Pd(CNArDipp2)2, which is the first stable monomeric isocyanide complex of zerovalent palladium. Variable temperature 1H NMR and FTIR studies on Pd(CNArDipp2)2 in the presence of added CNArDipp2 revealed that free and coordinated isocyanide undergo rapid exchange, but the components do not form a stable tris-isocyanide complex. Bis-isocyanide Pd(CNArDipp2)2 is active for oxidative addition reactions and readily reacts with benzyl chloride and mesityl bromide to form Pd(Cl)(Bz)(CNArDipp2)2 and Pd(Br)(Mes)(CNArDipp2)2, respectively. Room-temperature Suzuki−Miyaura cross-coupling reactions are mediated by Pd(CNArDipp2)2. Coordinatively and electronically unsaturated substrates also react with Pd(CNArDipp2)2. Addition of thallium(I) triflate (TlOTf) to Pd(CNArDipp2)2 results in the salt [TlPd(CNArDipp2)2]OTf, while addition of O2 results in the peroxo complex (η2-O)Pd(CNArDipp2)2. Most remarkably, 2 equiv of nitrosobenzene react with Pd(CNArDipp2)2 to form the square planar complex (κ1-N-PhNO)2 Pd(CNArDipp2)2, the geometry of which strongly suggests the formation of a divalent Pd center. With the aid of density functional theory calculations, this valence change is rationalized in terms of a formal reduction of the bond order in each NO unit to 1.5.
Co-reporter:Treffly B. Ditri ; Brian J. Fox ; Curtis E. Moore ; Arnold L. Rheingold
Inorganic Chemistry 2009 Volume 48(Issue 17) pp:8362-8375
Publication Date(Web):August 4, 2009
DOI:10.1021/ic9010828
A synthetic procedure for the sterically encumbered m-terphenyl isocyanide CNArDipp2 (Dipp = 2,6-diisopropylphenyl) is presented. In comparison to the less encumbering m-terphenyl isocyanide ligand CNArMes2, the steric attributes of the flanking Dipp groups effectively control the extent of CNArDipp2 ligation to monovalent Cu and Ag centers and zero-valent Mo centers. Direct structural comparisons of Cu(I) and Ag(I) complexes of both CNArDipp2 and CNArMes2 are made. It was found that only two CNArDipp2 ligands are accommodated by monovalent Cu and Ag centers, whereas three CNArMes2 units can readily bind. As demonstrated by both 1H NMR and FTIR spectroscopic studies, addition of a third equivalent of CNArDipp2 to [(THF)2Cu(CNArDipp2)2]OTf in C6D6 solution results in slow isocyanide exchange. However, rapid isocyanide exchange is observed when an additional equivalent of CNArDipp2 is added to (TfO)Ag(CNArDipp2)2. Three CNArMes2 ligands react smoothly with fac-Mo(CO)3(NCMe)3 to afford the octahedral complex fac-Mo(CO)3(CNArMes2)3, which can be converted irreversibly to the mer isomer upon heating in solution. Contrastingly, addition of CNArDipp2 to fac-Mo(CO)3(NCMe)3 results in a mixture of both the tetracarbonyl and the tricarbonyl complexes trans-Mo(CO)4(CNArDipp2)2 and trans-Mo(NCMe)(CO)3(CNArDipp2)2, respectively, in which the encumbering CNArDipp2 ligands are in a trans-disposition. Ultraviolet irradiation of the preceding mixture in NCMe/Et2O under an argon flow provides exclusively the tricarbonyl complex trans-Mo(NCMe)(CO)3(CNArDipp2)2. Addition of free CNArDipp2 to trans-Mo(NCMe)(CO)3(CNArDipp2)2 does not result in the binding of a third isocyanide unit by the Mo center as determined by 1H NMR spectroscopy. Treatment of trans-Mo(NCMe)(CO)3(CNArDipp2)2 with the Lewis base pyridine (py) affords the complex fac,cis-Mo(py)(CO)3(CNArDipp2)2 as determined by X-ray diffraction. Notably, the encumbering nature of the CNArDipp2 units forces a cis Ciso−Mo−Ciso angle of about 100°.
Co-reporter:BrianJ. Fox;MatthewD. Millard;AntonioG. DiPasquale Dr.;ArnoldL. Rheingold Dr. ;JoshuaS. Figueroa Dr.
Angewandte Chemie International Edition 2009 Volume 48( Issue 19) pp:
Publication Date(Web):
DOI:10.1002/anie.200990096
Co-reporter:BrianJ. Fox;MatthewD. Millard;AntonioG. DiPasquale Dr.;ArnoldL. Rheingold Dr. ;JoshuaS. Figueroa Dr.
Angewandte Chemie International Edition 2009 Volume 48( Issue 19) pp:3473-3477
Publication Date(Web):
DOI:10.1002/anie.200806007
Co-reporter:BrianJ. Fox;MatthewD. Millard;AntonioG. DiPasquale Dr.;ArnoldL. Rheingold Dr. ;JoshuaS. Figueroa Dr.
Angewandte Chemie 2009 Volume 121( Issue 19) pp:
Publication Date(Web):
DOI:10.1002/ange.200990098
Co-reporter:BrianJ. Fox;MatthewD. Millard;AntonioG. DiPasquale Dr.;ArnoldL. Rheingold Dr. ;JoshuaS. Figueroa Dr.
Angewandte Chemie 2009 Volume 121( Issue 19) pp:3525-3529
Publication Date(Web):
DOI:10.1002/ange.200806007
Co-reporter:Brian J. Fox ; Queena Y. Sun ; Antonio G. DiPasquale ; Alexander R. Fox ; Arnold L. Rheingold
Inorganic Chemistry 2008 Volume 47(Issue 19) pp:9010-9020
Publication Date(Web):August 30, 2008
DOI:10.1021/ic8010986
The synthesis of the m-terphenyl isocyanide ligand CNArMes2 (Mes = 2,4,6-Me3C6H2) is described. Isocyanide CNArMes2 readily functions as a sterically encumbering supporting unit for several Cu(I) halide and pseudo halide fragments, fostering in some cases rare structural motifs. Combination of equimolar quantities of CNArMes2 and CuX (X = Cl, Br and I) in tetrahydrofuran (THF) solution results in the formation of the bridging halide complexes (μ-X)2[Cu(THF)(CNArMes2)]2. Addition of CNArMes2 to cuprous chloride in a 2:1 molar ratio generates the complex ClCu(CNArMes2)2 in a straightforward manner. Single-crystal X-ray diffraction has revealed ClCu(CNArMes2)2 to exist as a three-coordinate monomer in the solid state. As determined by solution 1H NMR and FTIR spectroscopic studies, monomer ClCu(CNArMes2)2 resists tight binding of a third CNArMes2 unit, resulting in rapid isocyanide exchange. Contrastingly, addition of 3 equiv of CNArMes2 to cuprous iodide readily affords the tris-isocyanide species, ICu(CNArMes2)3, as determined by X-ray diffraction. Similar coordination behavior is observed in the tris-isocyanide salt [(THF)Cu(CNArMes2)3]OTf (OTf = O3SCF3), which is generated upon treatment of (C6H6)[Cu(OTf)]2 with 6 equiv of CNArMes2 in THF. The disparate coordination behavior of the [CuCl] fragment relative to both [CuI] and [CuOTf] is rationalized in terms of structure and Lewis acidity of the Cu-containing fragments. The putative triflate species [Cu(CNArMes2)3]OTf itself serves as a good Lewis acid and is found to weakly bind C6H6 in an η1-C manner in the solid-state. Density Functional Theory is used to describe the bonding and energetics of the η1-C Cu−C6H6 interaction.
Co-reporter:Alex E. Carpenter; Charles C. Mokhtarzadeh; Donald S. Ripatti; Irena Havrylyuk; Ryo Kamezawa; Curtis E. Moore; Arnold. L. Rheingold
Inorganic Chemistry () pp:
Publication Date(Web):February 20, 2015
DOI:10.1021/ic5030845
To assess the relative electronic influence of highly substituted aryl isocyanides on transition metal centers, a series of C4v-symmetric Cr(CNR)(CO)5 complexes featuring various alkyl, aryl, and m-terphenyl substituents have been prepared. A correlation between carbonyl-ligand 13C{1H} NMR chemical shift (δCO) and calculated Cotton–Kraihanzel (C–K) force constant (kCO) is presented for these complexes to determine the relative changes in isocyanide σ-donor/π-acid ratio as a function of substituent identity and pattern. For nonfluorinated aryl isocyanides possessing alkyl or aryl substitution, minimal variation in effective σ-donor/π-acid ratio is observed over the series. In addition, aryl isocyanides featuring strongly electron-releasing substituents display an electronic influence that nearly matches that of nonfluorinated alkyl isocyanides. Lower σ-donor/π-acid ratios are displayed by polyfluorinated aryl isocyanide ligands. However, the degree of this attenuation relative to nonfluorinated aryl isocyanides is not substantial and significantly higher σ-donor/π-acid ratios than CO are observed in all cases. Substituent patterns for polyfluorinated aryl isocyanides are identified that give rise to low relative σ-donor/π-acid ratios but offer synthetic convenience for coordination chemistry applications. In order to expand the range of available substitution patterns for comparison, the syntheses of the new m-terphenyl isocyanides CNArTripp2, CNp-MeArMes2, CNp-MeArDArF2, and CNp-FArDArF2 are also reported (ArTripp2 = 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3); p-MeArMes2 = 2,6-(2,4,6-Me3C6H2)2-4-Me-C6H2); p-MeArDArF2 = 2,6-(3,5-(CF3)2C6H3)2-4-Me-C6H2); p-FArDArF2 = 2,6-(3,5-(CF3)2C6H3)2-4-F-C6H2).
Co-reporter:Brandon R. Barnett and Joshua S. Figueroa
Chemical Communications 2016 - vol. 52(Issue 96) pp:NaN13839-13839
Publication Date(Web):2016/10/27
DOI:10.1039/C6CC07863J
Transition metal complexes that contain metal-to-ligand retrodative σ-bonds have become the subject of increasing studies over the last decade. Lewis acidic “Z-type ligands” can modulate the electronic structure of their resultant complexes in a manner distinct from 2e− donor ligands, and can also engage in cooperative reactivity with a Lewis basic transition metal. In this Feature article, we summarize our work with transition metal isocyanide complexes of group 10 metals that have exploited metal-based σ-type Lewis basicity. While the complexes Ni(CNArMes2)3, Pd(CNArDipp2)2 and Pt(CNArDipp2)2 were initially targeted as analogues to unstable, low-coordinate metal carbonyls, it soon became apparent that these zero-valent metal centers bore appreciable Lewis basic qualities due largely to the enhanced σ-donor/π-acid ratio of isocyanides compared to CO. Detailed spectroscopic and structural studies of metal-only Lewis pairs (MOLPs) formed from these complexes have furthered our understanding of the electronic structure perturbations effected by Z-type ligand binding. In addition, the platinum (boryl)iminomethane (BIM) complex Pt(κ2-N,B-Cy2BIM)(CNArDipp2) has illuminated a general ligand design strategy that can engender significant reverse-dative interactions with buttressed Lewis acids, and also has expanded the known scope of cooperative reactivity that can be realized at a transition metal–borane linkage.
Co-reporter:Douglas W. Agnew, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa
Dalton Transactions 2017 - vol. 46(Issue 20) pp:NaN6707-6707
Publication Date(Web):2017/04/18
DOI:10.1039/C7DT01102D
Using the stable metalloradical Mn(CO)3(CNArDipp2)2, we report the formation of manganese-main group complexes via the single-electron functionalization of main group halides. The reactions occur in a simple 1:1 stoichiometry, and demonstrate the utility of using stable open-shelled organometallics as precursors for metal-main group compounds. This has enabled the preparation of manganese complexes bearing terminal –EXn substituents, as shown through the isolation of Mn(SnCl)(CO)3(CNArDipp2)2 and Mn(BiCl2)(CO)3(CNArDipp2)2 from SnCl2 and BiCl3, respectively. Through this approach, we have also isolated Mn(SbF2)(CO)3(CNArDipp2)2 from SbF3, which serves as a unique example of a terminal –SbF2 complex. Although the metalloradical functionalization of binary main group halides provides the desired main group adduct in yields comparable to nucleophilic activation using the manganate Na[Mn(CO)3(CNArDipp2)2], the former approach is shown to be far more atom-economical with respect to Mn. Additionally, we have found that Mn(CO)3(CNArDipp2)2 also serves as a convenient precursor to MnF(CO)3(CNArDipp2). The latter is an analogue to the elusive monofluoride FMn(CO)5.
Co-reporter:Charles C. Mokhtarzadeh, Arnold L. Rheingold and Joshua S. Figueroa
Dalton Transactions 2016 - vol. 45(Issue 37) pp:NaN14569-14569
Publication Date(Web):2016/07/21
DOI:10.1039/C6DT02789J
Synthetic studies targeting an m-terphenyl isocyanide analogue of the unstable 16e−, S = 1 complex CpCo(CO) are reported (Cp = η5-C5H5). The m-terphenyl isocyanide CNArDipp2 (ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) is shown to readily bind to both CpCoI2 and Cp*CoI2 fragments (Cp* = η5-C5Me5) and provide mono-isocyanide starting materials that are suitable for chemical reduction. Treatment of CpCoI2(CNArDipp2) with KC8 produces the bridging isocyanide dimer, [CpCo(μ-CNArDipp2)]2, thereby indicating that the steric combination of Cp and CNArDipp2 ligands does not allow for the production of mononuclear complexes. However, Cp*CoI2(CNArDipp2) with KC8 under an N2 atmosphere results in the formation of the complex, Cp*Co(N2)(CNArDipp2), which is a unique two-legged piano stool complex featuring a coordinated dinitrogen ligand. The N2 ligand in Cp*Co(N2)(CNArDipp2) is shown to be labile and, upon removal by application of vacuum, leads to the production of an η4-coordinated 1-azabenz[b]azulene complex by aza-Büchner cyclization of the CNArDipp2 ligand. This cyclization reaction is rationalized via the intermediacy of the unobserved 16e− species [Cp*Co(CNArDipp2)]. While this intramolecular aza-Büchner cyclization prevents isolation of [Cp*Co(CNArDipp2)], the dinitrogen complex Cp*Co(N2)(CNArDipp2) is shown to serve as a reliable synthon for this 16e− species upon reaction with small molecule substrates. Both free CNArDipp2 and diphenylacetylene react with Cp*Co(N2)(CNArDipp2) to form two-legged piano stool complexes. In addition, Cp*Co(N2)(CNArDipp2) reacts readily with 0.5 and 1.0 equivalents of P4 to produce poly-phosphorus products resulting from P–P single bond cleavage.
Co-reporter:Brandon R. Barnett, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa
Chemical Communications 2015 - vol. 51(Issue 3) pp:NaN544-544
Publication Date(Web):2014/11/10
DOI:10.1039/C4CC08037H
The activation of carbon dioxide, organonitriles, and terminal acetylenes by (boryl)iminomethanes derived from isocyanide 1,1-hydroboration is described. Also detailed is the generality of hydroboration of m-terphenylisocyanides with hydroboranes of differing Lewis acidities.
Co-reporter:Mason A. Stewart, Curtis E. Moore, Treffly B. Ditri, Liezel A. Labios, Arnold L. Rheingold and Joshua S. Figueroa
Chemical Communications 2011 - vol. 47(Issue 1) pp:NaN408-408
Publication Date(Web):2010/09/17
DOI:10.1039/C0CC02742A
The m-terphenyl isocyanides CNArMes2 and CNArDipp2 support five-coordinate, isocyanide/carbonyl monoanions of manganese. For CNArDipp2, a bis-isocyanide anion is available that is remarkably well behaved upon reaction with electrophiles. Most notable is the formation of an unprecedented chloride-substituted metallostannylene.
Co-reporter:Brandon R. Barnett, Curtis E. Moore, Perumalreddy Chandrasekaran, Stephen Sproules, Arnold L. Rheingold, Serena DeBeer and Joshua S. Figueroa
Chemical Science (2010-Present) 2015 - vol. 6(Issue 12) pp:NaN7178-7178
Publication Date(Web):2015/09/17
DOI:10.1039/C5SC03104D
Complexes bearing electron rich transition metal centers, especially those displaying coordinative unsaturation, are well-suited to form reverse-dative σ-interactions with Lewis acids. Herein we demonstrate the generality of zerovalent, group 10 m-terphenyl isocyanide complexes to form reverse-dative σ-interactions to Tl(I) and Ag(I) centers. Structural and spectroscopic investigations of these metal-only Lewis pairs (MOLPs) has allowed insight into the electronic consequences of Lewis-acid ligation within the primary coordination sphere of a transition metal center. Treatment of the bis-isocyanide complex, Pt(CNArDipp2)2 (ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) with TlOTf (OTf = [O3SCF3]−) yields the Pt/Tl MOLP [TlPt(CNArDipp2)2]OTf (1). 1H NMR and IR spectroscopic studies on 1, and its Pd congener [TlPd(CNArDipp2)2]OTf (2), demonstrate that the M → Tl interaction is labile in solution. However, treatment of complexes 1 and 2 with Na[BArF4] (ArF = 3,5-(CF3)2C6H3) produces [TlPt(CNArDipp2)2]BArF4 (3) and [TlPd(CNArDipp2)2]BArF4 (4), in which Tl(I) binding is shown to be static by IR spectroscopy and, in the case of 3, 195Pt NMR spectroscopy as well. This result provides strong evidence that the M → Tl linkages can be attributed primarily to σ-donation from the group 10 metal to Tl, as loss of ionic stabilization of Tl by the triflate anion is compensated for by increasing the degree of M → Tl σ-donation. In addition, X-ray Absorption Near-Edge Spectroscopy (XANES) on the Pd/Tl and Ni/Tl MOLPs, [TlPd(CNArDipp2)2]OTf (2) and [TlNi(CNArMes2)3]OTf, respectively, is used to illustrate that the formation of a reverse-dative σ-interaction with Tl(I) does not alter the spectroscopic oxidation state of the group 10 metal. Also reported is the ability of M(CNArDipp2)2 (M = Pt, Pd) to form MOLPs with Ag(I), yielding the complexes [AgM(CNArDipp2)2]OTf (5, M = Pt; 6, M = Pd). As was determined for the Tl-containing MOLPs 1–4, it is shown that the spectroscopic oxidation states of the group 10 metal in 5 and 6 are essentially unchanged compared to the zerovalent precursors M(CNArDipp2)2. However, in the case of 5 and 6, the formation of a dative M → Ag σ-bonding interaction facilitates the binding of Lewis bases to the group 10 metal trans to Ag, illustrating the potential of acceptor fragments to open up new coordination sites on transition metal complexes without formal, two-electron oxidation.
Co-reporter:Patrick W. Smith, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa
Dalton Transactions 2012 - vol. 41(Issue 26) pp:NaN8038-8038
Publication Date(Web):2012/04/17
DOI:10.1039/C2DT30585B
In an effort to enforce a sterically hindered environment in transition-metal and main-group 2-picolinate complexes, the synthesis of the encumbering derivative 6-mesityl-2-picolinate (Mespic) is presented. The coordination and structural properties of Mespic are demonstrated with a range of transition-metal and main-group fragments. The 6-position mesityl group of Mespic is shown to alter both the primary and secondary coordination spheres of metal centers relative to the ubiquitous and unencumbered parent 2-picolinate anion.
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