•Hydrogenolysis of five different [XCX]-Pd-OH (X = C,N,O,P,S) complexes examined.•Effects of cis-donor atoms in pincer-Pd-OH complexes studied via DFT.•IES pathway favored over OA/RE pathway.•Hemilabile ligands do not lead to H2 cleavage of the Pd-OH bond.The hydrogenolysis of pincer palladium hydroxides was examined using DFT methods. Two possible mechanistic pathways -- oxidative addition/reductive elimination and internal electrophilic substitution -- were considered for each of five different PdOH complexes that varied in the identity of the donor atoms of the tridentate pincer ligand cis to the hydroxide. For each of the complexes examined, the internal electrophilic substitution pathway was found to be significantly lower in energy than an oxidative addition/reductive elimination pathway. For a PdOH complex bearing a hemilabile pincer ligand, addition of hydrogen resulted in dissociation of the labile arm of the ligand and hydrogenation of the Pd-OH bond to yield a hydride and bound water. This result is consistent with experimental observations for a similar system.A series of pincer-Pd-OH complexes containing strongly-bound cis-donor ligands is shown by DFT to undergo hydrogenolysis via an internal electrophilic substitution pathway. Use of hemilabile cis-ligands does not lead to hydrogenolysis of the Pd-OH bond, but rather hydrogenation to produce a water-bound Pd-H, matching the experimental results.Download high-res image (87KB)Download full-size image

Two new Zn(II) complexes have been prepared and evaluated for their capacity to activate and reduce CO2. The electrochemical properties of dichloro[phenyldi(2-pyridyl)phosphine-κ2-N,N′]zinc(II) 1 and dichloro[diphenyl-(2-pyridyl)phosphine-κ1-N]zinc(II) 2 are compared using cyclic voltammetry. Electrochemical results indicate that 2 leads to a facilitated CO2 reduction to evolve CO at a glassy carbon electrode.

Correction for ‘Facilitated carbon dioxide reduction using a Zn(II) complex’ by Elizabeth S. Donovan et al., Chem. Commun., 2016, DOI: 10.1039/c5cc07318a.

•Preparation of Ni complexes containing neutral HN(PiPr)2 (HL) ligands.•Crystal structures of Ni-HL complexes.•Electrochemical reduction of CO2 using [(HL)2Ni]X2 (X = BF4, NO3, ClO4).In its neutral state, bis(diisopropylphosphino)amine HL reacts in equimolar amounts with the nickel halides NiCl2·6H2O, NiBr2, and NiI2 in ethanol solutions to give the air- and moisture-stable P,P-chelated complexes (HL)NiX2 (X = Cl, Br, I). Under similar conditions, complexes of the form [(HL)2Ni]X2 (X = BF4, NO3, ClO4) were prepared from 2:1 ligand-metal ratios of Ni(BF4)2·6H2O, Ni(NO3)2·6H2O, or Ni(ClO4)2·6H2O. Deprotonation of the ligand with NaNH2 followed by reaction with NiI2 gives L2Ni when performed in Et2O, but leads to the co-crystal L2Ni·2[NCCHC(Me)NH2] when the solvent is acetonitrile. In addition to these Ni2+ compounds, the Ni0 complex (HL)2Ni can be prepared from a toluene solution of Ni(cod)2. Each complex has been characterized by a combination of IR and multi-nuclear NMR spectroscopies, as well as single-crystal X-ray diffraction. Electrochemical studies of the complexes revealed irreversible decomposition of the (HL)NiX2 (X = Cl, Br, I) series, but electrocatalytic CO2 reduction by the [(HL)2Ni]X2 (X = BF4, NO3, ClO4) compounds.Bis(diisopropylphosphino)amine (HL) reacts with NiX2 (X = Br, I) and NiX2·6H2O (X = Cl, BF4, NO3, ClO4) to form P,P-chelated complexes (HL)NiX2 (X = Cl, Br, I) and [(HL)2Ni]X2 (X = BF4, NO3, ClO4), each of which was crystallographically characterized. Electrocatalytic reduction of CO2 was observed for each of the latter set of compounds.

The P,P-chelated heteroleptic complex bis[bis(diisopropylphosphino)amido]indium chloride [(i-Pr2P)2N]2InCl was prepared in high yield by treating InCl3 with 2 equiv of (i-Pr2P)2NLi in Et2O/tetrahydrofuran solution. Samples of [(i-Pr2P)2N]2InCl in a pentane slurry, a CH2Cl2 solution, or in the solid state were exposed to CO2, resulting in the insertion of CO2 into two of the four M–P bonds to produce [O2CP(i-Pr2)NP(i-Pr2)]2InCl in each case. Compounds were characterized by multinuclear NMR and IR spectroscopy, as well as single-crystal X-ray diffraction. ReactIR solution studies show that the reaction is complete in less than 1 min at room temperature in solution and in less than 2 h in the solid–gas reaction. The CO2 complex is stable up to at least 60 °C under vacuum, but the starting material is regenerated with concomitant loss of carbon dioxide upon heating above 75 °C. The compound [(i-Pr2P)2N]2InCl also reacts with CS2 to give a complicated mixture of products, one of which was identified as the CS2 cleavage product [S═P(i-Pr2)NP(i-Pr2)]2InCl]2(μ-Cl)[μ-(i-Pr2P)2N)].

Palladium complexes of the novel unsymmetrical phosphine pyrazole-containing pincer ligands PCNH (PCNH = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole) and PCNMe (PCNMe = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) have been prepared and characterized through single-crystal X-ray diffraction and multinuclear 1H, 13C{1H}, and 31P{1H} NMR spectroscopy. In preparations of the monomeric hydroxide species (PCNH)Pd(OH), an unexpected N detachment followed by C–H activation on the heterocycle 5-position took place resulting in conversion of the monoanionic {P,C–,N} framework into a dianionic {P,C–,C–} ligand set. The dinuclear hydroxide-bridged species (PCNH)Pd(μ-OH)Pd(PCC) was the final product obtained under ambient conditions. The “rollover” activation was followed via 31P{1H} NMR spectroscopy, and dinuclear cationic μ-OH and monomeric PdII hydroxide intermediates were identified. DFT computational analysis of the process (M06//6-31G*, THF) showed that the energy barriers for the pyrazolyl rollover and for C–H activation through a σ-bond metathesis reaction are low enough to be overcome under ambient-temperature conditions, in line with the experimental findings. In contrast to the PCNH system, no “rollover” reactivity was observed in the PCNMe system, and the terminal hydroxide complex (PCNMe)Pd(OH) could be readily isolated and fully characterized.

The bis(phosphino)amines (R2P)NH(PR′2) (R = R′ = isopropyl; R = R′ = phenyl; R = isopropyl, R′ = phenyl) react with ZnEt2 to form complexes with two different structural motifs, either the homoleptic monomeric P,P-chelates Zn[N(i-Pr2P)2]2 and Zn[N(i-Pr2P)(Ph2P)]2 or the heteroleptic dimeric Zn2N2P2 heterocycles {EtZn[N(PPh2)2]}2 and {EtZn[N(PPh2)(i-Pr2P)]}2. In two cases, CO2 reacts with these complexes to give adducts Zn[O2CP(i-Pr2)NP(i-Pr2)]2 and Zn[O2CP(i-Pr2)NPPh2][Ph2PN(i-Pr2P)]ZnEt2, similar to adducts formed from the reaction of CO2 with frustrated Lewis pairs (FLPs). In the other two cases, reaction with CO2 results in cleavage and rearrangement of the N–P bonds to give either N(PPh2)3 or Ph2P(iPr2P)NPPh2. The zinc complexes and their CO2 products were characterized with a combination of single crystal X-ray diffraction and multinuclear NMR spectroscopy.

The synthesis and characterization of anionic, neutral, and cationic hydride complexes of platinum and palladium are reported utilizing the PNP (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine) ligand. Comparisons by IR spectroscopy and X-ray crystallography are made across the series. Evaluation of the metal hydride stretching frequencies of the cationic through anionic complexes shows a trend of increasing M–H bond activation. The reactivity of these metal hydrides with oxygen is evaluated and compared to previously reported oxygen insertion reactions.

Bis(di-i-propylphosphino)amine 1 reacts with B(C6F5)3 to form an adduct with concomitant N/P H-isomerization. This species reacts smoothly with carbon dioxide. An attempt to prepare an anionic derivative resulted in the formation of a novel heterocycle derived from the PNP ligand and B(C6F5)3.

The divalent metal complexes MII{(SC6H4-2-PR2)-κ2S,P}2 (3–7, and 9–11) (M = Zn, Sn, or Pb; R = iPr, tBu, or Ph), the Sn(IV) complexes Sn{(SC6H4-2-PR2)-κ2-S,P}Ph2Cl (12 and 13) (R = iPr and tBu), and the ionic Sn(IV) complexes [Sn{(SC6H4-2-PR2)-κ2-S,P}Ph2][BPh4] (14 and 15) (R = iPr and tBu) have been prepared and characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction when suitable crystals were afforded. The Sn(II) and Pb(II) complexes with R = Ph, iPr, or tBu (5, 6, 9, and 10) demonstrated ligand “folding” hinging on the P,S vector—a behavior driven by the repulsions of the metal/phosphorus and metal/sulfur lone pairs and increased M-S sigma bonding strength. This phenomenon was examined by density functional theory (DFT) calculations for the compounds in both folded and unfolded states. The Sn(IV) compound 13 (R = tBu) crystallized with the phosphine in an axial position of the pseudotrigonal bipyramidal complex and also exhibited hemilability in the Sn–P dative bond, while compound 12 (R = iPr), interestingly, crystallized with phosphine in an equatorial position and did not show hemilability. Finally, the crystal structure of 15 (R = tBu) revealed the presence of an uncommon, 4-coordinate, stable Sn(IV) cation.

The reaction of (Me3Si)(i-Pr2P)NH with ZnEt2 gives the complex {[(Me3Si)(i-Pr2P)N]ZnEt}2, which was characterized by multinuclear NMR spectroscopy and X-ray crystallography. This Zn complex reacts with CO2 to give an adduct with the proposed formula of [(Me3Si)N(i-Pr2PCO2)]ZnEt that undergoes further transformation into the crystallographically characterized di-adduct [(Me3Si)N(i-Pr2PCO2)]2Zn, in addition to a Zn6 cluster containing both P–CO2 adducts and fragments of the isocyanate i-Pr2P–NCO. These reaction pathways have been separately observed in related group 2 and group 14 complexes, but not previously within the same molecule. Reactions with the related heteroallene CS2 were also examined. The free ligand (Me3Si)(i-Pr2P)NH reacts with CS2 to form a bright red, crystalline adduct (Me3Si)[P(i-Pr)2(CS2)]NH. This adduct was treated with ZnEt2 to provide the same product that is obtained from the reaction of {[(Me3Si)(i-Pr2P)N]ZnEt}2 with CS2, proposed to be a dithiocarbamate complex, with the CS2 bound to the N instead of the P atom.Graphical abstractThe reaction of (Me3Si)(i-Pr2P)NH with ZnEt2 gives {[(Me3Si)(i-Pr2P)N]ZnEt}2, which was characterized by NMR spectroscopy and X-ray crystallography. This Zn complex reacts with CO2 to eventualy provide both the di-adduct [(Me3Si)N(i-Pr2PCO2)]2Zn and a Zn6 cluster containing both P–CO2 adducts and fragments of the isocyanate i-Pr2P–NCO. The ligand (Me3Si)(i-Pr2P)NH reacts with CS2 to form an adduct (Me3Si)[P(i-Pr)2(CS2)]NH. This adduct was treated with ZnEt2 to provide the same product as obtained from the reaction of {[(Me3Si)(i-Pr2P)N]ZnEt}2 with CS2.Highlights► Syntheses of Zn(II) complexes with amido ligands. ► Examined reactions of CO2 and CS2 with Zn complexes. ► CO2 adducts and complexes derived from fragments of phosphino-substituted isocyanates. ► CS2 adduct with free amine ligand.

The group 2 complexes [(Me3Si)(i-Pr2P)N]2M(THF)x (M = Mg, x = 1; M = Ca/Sr, x = 2) as well as an unusual dimagnesium complex {[(Me3Si)(i-Pr2P)N]3Mg}Mg(n-C4H9) have been prepared and characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction. Each complex was shown to react with CO2 under extremely mild conditions (15 min, 1 atm, room temperature) to give the isocyanate (i-Pr)2P–N═C═O. The independent syntheses of (i-Pr)2P–N═C═O and the carbodiimide dimer [(i-Pr)2PNCNP(i-Pr)2]2 are also reported.

In this report, we investigate the interactions of MexMCl3–x (x = 0–3, M = Al, Ga) with various aromatic and alkyl-substituted 1,4-diaza-1,3-butadiene RDAB ligands (or α-diimine ligands) to give a variety of structures in solution and in the solid state. In combination with other previously reported structures, certain general trends of reactivity of these species can be deduced, although there are still some unexplained modes of reactivity. The methylated Al species react with aromatic-substituted RDAB ligands to provide final products that result from C═N insertion into the Al–CH3 group followed by rearrangement reactions. The addition of methyl groups onto the backbone of the RDAB ligand is insufficient to stop the insertion and rearrangement processes from occurring. In the case of MeAlCl2 with the bulky DiPPDAB ligand, the reaction could be followed spectroscopically from the monoadduct through the inserted/rearranged final product. Methylated Ga species, however, are much less predictable in their behavior with aromatic-substituted RDAB ligands. Depending on the exact species and ratios used, coordinated adducts can be formed and identified, or inserted/rearranged products similar to the aluminum reactions can be obtained. Quite interestingly, cation/anion pairs can also be formed in which GaCl3 or MeGaCl2 act as a chloride acceptors. This behavior was unique and substantially different from the analogous Al reactions which formed either a dicoordinated adduct or an inserted/rearranged complex. When the stronger-donating alkyl-substituted RDAB ligands were used with Me2GaCl, only cation/anion pairs were obtained. Surprisingly, when the same reactions were performed using Me2AlCl as a reagent, irreproducible results were obtained.

Reaction of tris(diphenylphosphino)methane, (Ph2P)3CH (tdppm) with 1 equivalent of M(CO)3(EtCN)3(M = Mo 1a, M = W 1b) affords [Mo{η2-(Ph2P)3CH}(CO)3(EtCN)] 2a and [W{η2-(Ph2P)3CH}(CO)3(EtCN)] 2b, respectively. Single crystal structure determinations were performed for complexes 2a and 2b. Both structures adopt a distorted octahedral geometry about Mo and W atoms, with one CO and EtCN group occupying an axial position, while the phosphine ligand and two CO ligands occupy the equatorial positions. To our knowledge this is the first structurally-characterized W complex containing the tdppm ligand.Reaction of tris(diphenylphosphino)methane, (Ph2P)3CH (tdppm) with 1 equivalent of M(CO)3(EtCN)3 (M = Mo, W) gives [Mo{η2-(Ph2P)3CH}(CO)3(EtCN)] and [W{η2-(Ph2P)3CH}(CO)3(EtCN)]. Single crystal structure determinations were performed for complexes 2a and 2b. To our knowledge this is the first structurally-characterized W complex containing the tdppm ligand.Highlights► The ligand tris(diphenylphosphino)methane (Ph2P)3CH reacts with Mo or W complexes. ► Both Mo and W complexes have been structurally characterized. ► First example of a (Ph2P)3CH complex of W. ► Labile EtCN ligand is present in place of one CO to aid in subsequent reactions.

A series of Ni(II) and Pd(II) hydrides supported by PNP and PCP ligands, including iPr2PNP(CH3)PdH (iPr2PNP(CH3) = N(2-PiPr2-4-MeC6H3)2), iPr2PNP(CH3)NiH, iPr2PNP(F)PdH (iPr2PNP(F) = N(2-PiPr2-4-C6H3F)2), CyPhPNPPdH (CyPhPNP = N(2-P(Cy)(Ph)-4-MeC6H3)2), tBu2PCPPdH (tBu2PCP = 2,6-C6H3(CH2PtBu2)2), tBu2PCPNiH, Cy2PCPPdH (Cy2PCP = 2,6-C6H3(CH2PCy2)2), and Cy2PCPNiH, were prepared using literature methods. In addition, the new Ni and Pd hydrides Cy2PSiPMH (M = Ni, Pd; Cy2PSiP = Si(Me)(2-PCy2-C6H4)2) supported by PSiP ligands were synthesized. The analogous metal hydride complexes supported by the Ph2PSiP ligand (Ph2PSiP = Si(Me)(2-PPh2-C6H4)2) could not be prepared. Instead, the Ni(0) and Pd(0) η2-silane complexes Ph2PSiHPM(PPh3) (M = Ni, Pd; Ph2PSiHP = (H)Si(Me)(2-PPh2-C6H4)2), which have been proposed to be in equilibrium with Ph2PSiPMH (M = Ni, Pd) and PPh3, were prepared. Facile carbon dioxide insertion into the metal–hydride bond to form the metal formate complexes tBu2PCPM-OC(O)H (M = Ni, Pd) or Cy2PCPM-OC(O)H (M = Ni, Pd) was observed for PCP-supported species, and a similar reaction was observed for Cy2PSiP-supported hydrides to form Cy2PSiPM-OC(O)H (M = Ni, Pd). No reaction with carbon dioxide was observed for any complexes supported by PNP ligands. The η2-silane complex Ph2PSiHPPd(PPh3) reacted rapidly with carbon dioxide to give Ph2PSiPPd-OC(O)H and PPh3, while the corresponding Ni complex Ph2PSiHPNi(PPh3) did not react with carbon dioxide. DFT calculations indicate that carbon dioxide insertion is thermodynamically favorable for PSiP- and PCP-supported hydrides because the strong trans influence of the anionic carbon donor destabilizes the metal–hydride bond. In contrast, carbon dioxide insertion is thermodynamically unfavorable for the PNP-supported species. In the case of the η2-silane complexes, carbon dioxide insertion is thermodynamically favorable for Pd and unfavorable for Ni. This is because the equilibrium between the metal hydride and PPh3 and the η2-silane complex more strongly favors the metal hydride for Pd than for Ni. In the cases of metal hydrides, the thermodynamic favorability of carbon dioxide insertion can be predicted from the natural bond orbital charge on the hydride. The pathway for carbon dioxide insertion into the metal hydride is concerted and features a four-centered transition state. The energy of the transition state for carbon dioxide insertion decreases as the trans influence of the anionic donor of the pincer ligand increases.

In this report we detail the synthesis and characterization of new Sn(II) complexes containing the –N[(SiMe3)(ArF)] ligand [ArF = 3,5-(CF3)2C6H3)], specifically designed to provide both an electron-withdrawing –ArF group as well as the –SiMe3 group required for migration to oxygen. As well, we have also examined the reaction of the LiN[(SiMe3)(ArF) precursor with CO2 and observed the formation of a bicyclic lithium carbamate consisting of two 8-membered rings characterized by single crystal X-ray crystallography. The desired starting complex Sn[N(SiMe3)(ArF)]2 could be prepared directly via a metathesis reaction, although X-ray quality crystals could not be grown. The reactions of this complex with CO2, OCS, and CS2 are described. Reaction with CO2 results in an unexpected insertion reaction quite dissimilar to those seen in the literature. A proposed route that explains the surprising CO2 reaction chemistry and products is given. Reactions of Sn[N(SiMe3)(ArF)]2 with OCS led to multiple products, while reaction with CS2 led to simple insertion to form a dithiocarbamate. Interestingly, an attempt to purify Sn[N(SiMe3)(ArF)]2 by distillation led to an unexpected cyclization reaction with activation of an aromatic H atom. This ortho-metallated product from this cyclization reaction was clearly identified via X-ray crystallography.Graphical abstractLi and Sn complexes containing the new electron-withdrawing –N[(SiMe3)(ArF)] ligand [ArF = 3,5-(CF3)2C6H3)] have been prepared and their reactivities towards CO2, OCS, and CS2 examined. Sn[N(SiMe3)(ArF)]2 reacts with CO2 to produce a new O-silylated oleate ligand rather than a carbamate. A proposed route to the unexpected product is given. Reaction with CS2 led to insertion to form a dithiocarbamate. Attempts to isolate Sn[N(SiMe3)(ArF)]2 by distillation led to a unexpected ortho-metallation reaction.Highlights► Syntheses of Sn(II) complexes with electron-withdrawing ligands. ► Examined reactions of CO2, OCS, and CS2 with Sn complexes. ► Products contain bridging or chelating ligands as shown by X-ray crystallography. ► ortho-Metallation of aromatic rings with Sn(II).

An investigation of the CO2(g) insertion products for a series of fully characterized monomeric lanthanide 2,6-di-t-butyl-phenoxide compounds ([Ln(DBP)3]; Ln = Ce (1), Sm (2), Dy (3), Y (4), Er (5), Yb (6), and Lu (7)) was undertaken at low pressure (<5 psi). From the products isolated, only one CO2(g) molecule per molecule [Ln(DBP)3] was found to insert, forming either the [Ce(μc-O2C-DBP)(DBP)2]2 (8) or [Ln(μ-O2C-DBP)(DBP)2]2 (Ln = Sm (9), Dy (10), Y (11), Er (12), Yb (13), and Lu (14)) structure. The purity of the bulk powders of 8–14 were verified by FT-IR and elemental analyses; however solution structures could not be studied due to the low solubility of the complexes. Higher pressures to increase the degree of CO2(g) insertions did not alter the degree of substitution. The selectivity of CO2(g) insertion was attributed to an interaction of the methyl moieties of the DBP ligand blocking coordination sites on the Ln metal center, as observed in the solid and solution state of 1–7.An investigation of the CO2(g) insertion products for a series of fully characterized monomeric lanthanide 2,6-di-t-butyl phenoxide compounds ([Ln(DBP)3]; Ln = Ce (1), Sm (2), Dy (3), Y (4), Er (5),Yb (6), and Lu (7)) was undertaken at low pressure (<5 psi). From the products isolated, only one CO2(g) molecule per molecule [Ln(DBP)3] was found to insert, forming either the [Ce(μc-O2C-DBP)(DBP)2]2 (8) or [Ln(μ-O2C-DBP)(DBP)2]2 (Ln = Sm (9), Dy (10), Y (11), Er (12), Yb (13), and Lu (14)) structure. Higher pressures to increase the degree of CO2(g) insertions did not alter the degree of substitution. The selectivity of CO2(g) insertion was attributed to an interaction of the methyl moieties of the DBP ligand blocking coordination sites on the Ln metal center.

A series of pincer (tBuPCP)Pd(II)–OR complexes (tBuPCP = 2,6-bis(CH2PtBu2)C6H3, R = H, CH3, C6H5, CH2C(CH3)3, CH2CH2F, CH2CHF2, CH2CF3) were synthesized to explore the generality of hydrogenolysis reactions of palladium–oxygen bonds. Hydrogenolysis of the Pd hydroxide complex to generate the Pd hydride complex and water was shown to be inhibited by formation of a water-bridged, hydrogen-bonded Pd(II) hydroxide dimer. The Pd alkoxide and aryloxide complexes exhibited more diverse reactivity. Depending on the characteristics of the −OR ligand (steric bulk, electron-donating ability, and/or the presence of β-hydrogen atoms), hydrogenolysis was complicated by hydrolysis by adventitious water, a lack of reactivity with hydrogen, or a competing dissociative β-hydride abstraction reaction pathway. Full selectivity for hydrogenolysis was observed with the partially fluorinated Pd(II) 2-fluoroethoxide complex. The wide range of Pd–OR substrates examined helps to clarify the variety of reaction pathways available to late-transition-metal alkoxides as well as the conditions necessary to tune the reactivity to hydrogenolysis, hydrolysis, or dissociative β-hydride abstraction.

The P,P-chelated stannylene [(i-Pr2P)2N]2Sn takes up 2 equiv of carbon dioxide (CO2) to form an unusual product in which CO2 binds to the Sn and P atoms, thus forming a six-membered ring complex. Gentle heating of the solid product releases CO2, indicating that CO2 is bound as an adduct to the main-group complex. The groups bound to the CO2 fragment are not particularly sterically crowded or highly acidic, thus indicating that “frustrated” Lewis acid–base pairs are not required in the binding of CO2 to main-group elements.

In an attempt to perform a simple anion-exchange reaction on a pincer-carbene-ligated nickel complex using AgNO3, we instead obtained an unexpected three-dimensional (3D) Ag7 cluster containing a [Ag6] core in a twisted-bowtie geometry. The reverse-transmetalation reaction by which the carbene is transferred from nickel to silver is virtually unprecedented. The CNC pincer-carbene ligands exhibit unusual bridging modes of ligand bonding for all three donor atoms. Another unique feature is that the final structure exhibits a 3D structure brought about by the connection of two-dimensional layers of the [Ag6] core via a seventh Ag ion.

Two new unsymmetrical RPNPR′-type pincer ligands based on a bis(tolyl)amine framework have been synthesized and characterized by a variety of techniques, including X-ray crystallography. These ligands have been coordinated to Ni, Pd, and Pt precursors to provide a number of well-characterized group 10 halides. Conversion of these metal halides to metal hydrides was accomplished using borohydride reagents, or by direct interaction of the ligand with the zerovalent metal precursor. The insertion of oxygen into these hydrides in an attempt to prepare metal hydroperoxides has been examined; however, we were unable to obtain stable and isolable hydroperoxide species.

The interactions of the heteroallenes CO2, OCS, and CS2 with (Me2N)2Sn have been investigated. These CX2 species insert into the Sn–N bonds under mild conditions to provide products bis-(N,N-dimethylcarbamato)tin(II), [(Me2NCO2)2Sn]2, bis-(N,N-dimethylthiocarbamato)tin(II), [Me2NC(O)S]2Sn and bis-(N,N-dimethyldithiocarbamato)tin(II), (Me2NCS2)2Sn. These molecules have been fully characterized by traditional spectroscopic methods as well as by X-ray crystallography. The [Me2NC(O)S]2Sn product is the first example of a structurally characterized Sn(II) thiocarbamate. The solid-state structures of the final products vary depending on the heteroallene inserted. The CO2-inserted product is dimeric in the solid-state, with both bridging and chelating carbamate ligands. These dimers form a chain-like network via intermolecular Sn⋯O interactions. The monomeric thiocarbamate also shows a chain-like extended structure, through both Sn⋯O and Sn⋯S interactions, while the dithiocarbamate product has no significant intermolecular contacts.Graphical abstractThe interactions of CO2, OCS, and CS2 with (Me2N)2Sn have been studied. While each heteroallene inserts into the Sn–N bonds, the final structures of the resulting products vary in terms of bonding modes and whether an extended network is formed in the solid-state.Highlights► Syntheses of Sn(II) carbamates, thiocarbamates, and dithiocarbamates. ► Direct insertion reactions of the heteroallenes CO2, OCS, and CS2. ► Products contain bridging or chelating ligands as shown by X-ray crystallography. ► Discussion of solid-state geometries and reasons for the various structural motifs.

The synthesis of a new pincer ligand (tBuPCO = 2-(CH2PtBu2)-6-(CH2OCH3)C6H3) is reported. This ligand has been observed to coordinate in three different modes to palladium. The tBuPCO ligand coordinates in a monodentate fashion through the phosphine moiety in the dimeric [(tBuPCO)Pd(Cl)(μ-Cl)]2. Bidentate coordination is observed through the phosphine and the aryl ring in the binuclear [(tBuPCO)Pd(μ-OH)]2. The traditional tridentate coordination mode of a pincer is observed in the monomeric complex (tBuPCO)PdCl, wherein the ether oxygen provides the third point of attachment. Each of these novel palladium(II) complexes was characterized by NMR spectroscopy, elemental analyses, and single-crystal X-ray crystallography. A variety of other palladium(II) complexes of tBuPCO have also been prepared and characterized, including the hydroxide complex (tBuPCO)PdOH. The reactivity of the hydroxide complex with CO2, CO, and H2 is reported.

The heterocumulenes carbon dioxide (CO2), carbonyl sulfide (OCS), and carbon disulfide (CS2) were treated with bis(2,2,5,5-tetramethyl-2,5-disila-1-azacyclopent-1-yl)tin {[(CH2)Me2Si]2N}2Sn, an analogue of the well-studied bis[bis(trimethylsilyl)amido]tin species [(Me3Si)2N]2Sn, to yield an unexpectedly diverse product slate. Reaction of {[(CH2)Me2Si]2N}2Sn with CO2 resulted in the formation of 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane, along with Sn4(μ4-O){μ2-O2CN[SiMe2(CH2)2]}4(μ2-N═C═O)2 as the primary organometallic Sn-containing product. The reaction of {[(CH2)Me2Si]2N}2Sn with CS2 led to formal reduction of CS2 to [CS2]2−, yielding [{[(CH2)Me2Si]2N}2Sn]2CS2{[(CH2)Me2Si]2N}2Sn, in which the [CS2]2− is coordinated through C and S to two tin centers. The product [{[(CH2)Me2Si]2N}2Sn]2CS2{[(CH2)Me2Si]2N}2Sn also contains a novel 4-membered Sn−Sn−C−S ring, and exhibits a further bonding interaction through sulfur to a third Sn atom. Reaction of OCS with {[(CH2)Me2Si]2N}2Sn resulted in an insoluble polymeric material. In a comparison reaction, [(Me3Si)2N]2Sn was treated with OCS to yield Sn4(μ4-O)(μ2-OSiMe3)5(η1-N═C═S). A combination of NMR and IR spectroscopy, mass spectrometry, and single crystal X-ray diffraction were used to characterize the products of each reaction. The oxygen atoms in the final products come from the facile cleavage of either CO2 or OCS, depending on the reacting carbon dichalogenide.

The reaction of the bis(carbene) pincer ligand MeCNC with nickel acetate and Bu4NBr produced [(MeCNC)3Ni2]4+[Br]4 (2), a complex that contains the MeCNC ligand in both traditional tridentate chelating modes as well as in a unique, bridging mode between two Ni2+ ions. While 2 could not be crystallized or isolated in a pure form, the anion exchange of bromide with triflate yielded [(MeCNC)3Ni2]4+[OTf]4 (4), which could be recrystallized from methanol and isolated pure. Single-crystal X-ray analysis of 4 confirmed the unusual dual-coordination modes of the MeCNC ligand towards Ni2+ in these species, and represents the initial example of a Group 10 complex of a bridging CNC ligand.Single-crystal X-ray analysis of [(MeCNC)3Ni2]4+[OTf]4 (4) confirmed the unusual dual-coordination modes of the bis(carbene) pincer MeCNC ligand towards Ni2+ in these species, and further represents the initial example of a structurally-characterized Group 10 complex of a bridging MeCNC ligand.

The syntheses and full characterization of nickel hydrides containing the PCP “pincer”-type ligand, where PCP = 2,6-C6H3(CH2PR2)2 (R = tBu, cHex, and iPr), are reported. These Ni−H complexes are prepared by the conversion of (RPCP)NiCl precursors into the corresponding nickel hydrides by use of appropriate hydride donors. Surprisingly, although the (RPCP)NiCl precursors are quite similar chemically, the conversions to the hydrides were not straightforward and required different hydride reagents to provide analytically pure products. While NaBH4 was effective in the preparation of pure (tBuPCP)NiH, Super-Hydride solution (LiEt3BH in THF) was required to prepare either (cHexPCP)NiH or (iPrPCP)NiH. Attempts to prepare a Ni−H from (PhPCP)NiCl with a variety of hydride reagents yielded only the free ligand as an identifiable product. Two of the derivatives, tBu and cHex, have also been subjected to single crystal X-ray analysis. The solid-state structures each showed a classic, near-square planar arrangement for Ni in which the PCP ligand occupied three meridional ligand points with the Ni−H trans to the Ni−C bond. The resulting Ni−H bond lengths were 1.42(3) and 1.55(2) Å for the tBu and cHex derivatives, respectively.

The reaction of (PCP)i-PrPdCl and the commonly-used reductant K-Selectride® solution [K(sec-Bu3BH) in THF] does not yield a simple (PCP)i-PrPdH species, but rather an adduct of the Pd–H that contains bound K(sec-Bu3BH). This adduct has been characterized by X-ray crystallography and shown to be a centrosymmetric dimer in the solid state. The most unique feature of the structure is that the tri-coordinate K+ ion bonds only to the terminal palladium hydride and the two bridging boron hydrides. Despite being prepared in THF, no other donor ligands are bound to K+. This [H3K]+ bonding mode for K+ ions has not been previously reported.Reaction of (PCP)i-PrPdCl and K-Selectride® solution does not yield a simple (PCP)i-PrPdH species, but rather an adduct containing K(sec-Bu3BH). This adduct has been characterized by X-ray crystallography, revealing that the tri-coordinate K+ ion is bound only to hydrides. This [H3K]+ bonding mode for K+ ions was previously unknown.

The search for new ligands with interesting properties is a quest that can occupy much of a synthetic chemist’s time. We have recently discovered two new ligands based on an adamantyl-substituted, 2,6-iPr2-substituted phenyl (Dipp) system. In an attempt to prepare the extremely bulky amine [(ad)(2,6-iPr2C6H3)NH] (ad = adamantyl) we found instead that the adamantyl group attacks the aromatic ring in the 4-position to form a new primary amine, (4-ad-2,6-iPr2C6H2)NH2 (1) (ad-Dipp-NH2), characterized by normal techniques. Compound 1 could be converted into a Li salt by a 1:1 reaction with nBuLi, or converted into a more bulky silylamine, [(ad-Dipp)NH(SiMe3)] (3), by treatment with Me3SiCl. We have characterized the lithium salt by X-ray crystallography as the dimeric complex, [(ad-Dipp)NHLi(Et2O)]2 (2). The lithium amide can be used as a reagent towards metal halides, and we have discovered that its reaction with SnCl2 yields a compound with a tetrameric, [Sn–N]4 cubane-like cage structure. We have also demonstrated the ligand behavior of 3 by its reaction with Bu2Mg in THF to form a monomeric Mg-amide with two THF solvent molecules attached. These new ligands can provide advantages over conventional ligands in terms of improved solubility and ease of crystallization.We have prepared and characterized two new bulky amido ligands unexpectedly prepared in an attempt to synthesize [(adamantyl)(2,6-iPr2C6H3)NH]. Rather than reacting at the primary –NH2 functionality, the adamantyl group attacked the 4-position of the 2,6-iPr2C6H3NH2 group to yield substituted ligands. These new ligands afford steric bulk, improved solubility in organic solvents, and an increased tendency to form crystalline compounds.

A series of solvated calcium bis(amides) with the general formula Ca[N(R)(SiMe3)]2(solv)x (R = –SiMe2t-Bu, –SiPh2t-Bu, and –SiPh3) was prepared from the reaction of CaI2 with 2 equiv of the corresponding potassium amide. Solvent ligands used in this study include THF, pyridine (py), hexamethylphosphoranide (HMPA), and 4-dimethylaminopyridine (DMAP). The coordinated THF in Ca[N(SiMe2t-Bu)(SiMe3)]2(THF)2 could be replaced by stronger donating ligands. When –N[(SiPh3)(SiMe3)] was used as the amide ligand again a bis(THF) adduct was formed. Quite interestingly, when the more sterically-demanding ligand –N[(SiPh2t-Bu)(SiMe3)] ligand was used only one THF molecule could coordinate to Ca, leading to a rare example of a tri-coordinate Ca bis(amide). One of the starting amides, [KN(SiMe2t-Bu)(SiMe3)]2, was isolated and found to adopt a dimeric structure in the solid state. All complexes were characterized with 1H NMR, 13C NMR, elemental analyses, and X-ray crystallography when appropriate.We have prepared and analytically characterized a series of Ca bis(amides) containing bulky ligands which include a –SiMe3 group on each amide. The bulky silylamides utilized include –N[(SiMe2t-Bu)(SiMe3)], –N[(SiPh2t-Bu)(SiMe3)], and –N[(SiPh3)(SiMe3)]. While most of the Ca complexes prepared are four-coordinate solvated complexes the –N[(SiPh2t-Bu)(SiMe3)] ligand produces an extremely rare example of a three-coordinate solvated Ca amide complex.

Two new Zn(II) complexes have been prepared and evaluated for their capacity to activate and reduce CO2. The electrochemical properties of dichloro[phenyldi(2-pyridyl)phosphine-κ2-N,N′]zinc(II) 1 and dichloro[diphenyl-(2-pyridyl)phosphine-κ1-N]zinc(II) 2 are compared using cyclic voltammetry. Electrochemical results indicate that 2 leads to a facilitated CO2 reduction to evolve CO at a glassy carbon electrode.

Correction for ‘Facilitated carbon dioxide reduction using a Zn(II) complex’ by Elizabeth S. Donovan et al., Chem. Commun., 2016, DOI: 10.1039/c5cc07318a.

Two new unsymmetrical RPNPR′-type pincer ligands based on a bis(tolyl)amine framework have been synthesized and characterized by a variety of techniques, including X-ray crystallography. These ligands have been coordinated to Ni, Pd, and Pt precursors to provide a number of well-characterized group 10 halides. Conversion of these metal halides to metal hydrides was accomplished using borohydride reagents, or by direct interaction of the ligand with the zerovalent metal precursor. The insertion of oxygen into these hydrides in an attempt to prepare metal hydroperoxides has been examined; however, we were unable to obtain stable and isolable hydroperoxide species.