Theodor Agapie

A new series of tetranuclear Fe clusters displaying an interstitial μ4-F ligand was prepared for a comparison to previously reported μ4-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the μ4-F clusters were characterized in five redox states, from FeII3{FeNO}8 to FeIII3{FeNO}7, with NO stretching frequencies ranging from 1680 to 1855 cm–1, respectively. Despite accessing more reduced states with an F– bridge, a two-electron reduction of the distal Fe centers is necessary for the μ4-F clusters to activate NO to the same degree as the μ4-O system; consequently, NO reactivity is observed at more positive potentials with μ4-O than μ4-F. Moreover, the μ4-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher-charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small-molecule activation.

Stoichiometrically Activated Catalysts for Ethylene Tetramerization using Diphosphinoamine-Ligated Cr Tris(hydrocarbyl) Complexes

Co-reporter:Nathanael A. Hirscher and Theodor Agapie

Organometallics November 13, 2017 Volume 36(Issue 21) pp:4107-4107

Publication Date(Web):October 23, 2017

DOI:10.1021/acs.organomet.7b00706

A new, stoichiometric activation mode is presented for Cr-PNP (PNP = diphosphinoamine) complexes for ethylene tetramerization catalysis. To access suitable precatalysts, two robust Cr(III) multiaryl compounds were synthesized as THF adducts. These complexes are supported by a facially coordinated bis(aryl) ligand with an additional ether donor. From these precursors, Cr-PNP tris(hydrocarbyl) complexes were synthesized. Using 1 equiv of Brønsted acid as an activator, an active species for the catalytic tetramerization of ethylene was produced, without the need for excess alkylaluminum reagents.

Intramolecular C–H and C–F Bond Oxygenation by Site-Differentiated Tetranuclear Manganese Models of the OEC

Co-reporter:Kurtis M. Carsch, Graham de Ruiter, and Theodor Agapie

Inorganic Chemistry August 7, 2017 Volume 56(Issue 15) pp:9044-9044

Publication Date(Web):July 21, 2017

DOI:10.1021/acs.inorgchem.7b01022

The dangler manganese center in the oxygen-evolving complex (OEC) of photosystem II plays an important role in the oxidation of water to dioxygen. Inspired by the structure of the OEC, we synthesized a series of site-differentiated tetra-manganese clusters [LMn3(PhPz)3OMn][OTf]x (2: x = 2; 3: x = 1) that features an apical manganese ion—distinct from the others—that is appended to a trinuclear manganese core through an μ4-oxygen atom bridge. This cluster design was targeted to facilitate studies of high-valent Mn-oxo formation, which is a proposed step in the mechanism for water oxidation by the OEC. Terminal Mn-oxo species—supported by a multinuclear motif—were targeted by treating 2 and 3 with iodosobenzene. Akin to our previously reported iron complexes, intramolecular arene hydroxylation was observed to yield the C–H bond oxygenated complexes [LMn3(PhPz)2(OArPz)OMn][OTf]x (5: x = 2; 6: x = 1). The fluorinated series [LMn3(F2ArPz)3OMn][OTf]x (8: x = 2; 9: x = 1) was also synthesized to mitigate the observed intramolecular hydroxylation. Treatment of 8 and 9 with iodosobenzene results in intramolecular arene C–F bond oxygenation as judged by electrospray ionization mass spectrometry. The observed aromatic C–H and C–F hydroxylation is suggestive of a putative high-valent terminal metal-oxo species, and it is one of the very few examples capable of oxygenating C–F bonds.

Ethylene Tetramerization Catalysis: Effects of Aluminum-Induced Isomerization of PNP to PPN Ligands

Co-reporter:Alejo M. Lifschitz, Nathanael A. Hirscher, Heui Beom Lee, Joshua A. Buss, and Theodor Agapie

Organometallics April 24, 2017 Volume 36(Issue 8) pp:1640-1640

Publication Date(Web):April 13, 2017

DOI:10.1021/acs.organomet.7b00189

Diphosphinoamines (PNP) are commonly used to support Cr-catalyzed ethylene trimerization and tetramerization. Although isomerization of PNP to a PPN (iminobisphosphine) species has been established, such reactivity has not been studied in detail in the context of Cr-based selective ethylene oligomerization catalysis. Herein, we show that precursors that are stable as PNP frameworks can isomerize to PPN species in the presence of chlorinated aluminum activators relevant to ethylene oligomerization catalysis. Isomerization changes the pattern of reactivity of the ligands, making them more susceptible to nucleophilic attack by alkyl groups, resulting in a variety of degradation products. The isomerization-mediated degradation of PNP ligands leads to the formation of unwanted polymerization catalysts in ethylene tetramerization systems, thus providing insight into the formation of Cr species that affect the overall selectivity and activity values. For example, independently prepared [R2PNR]— leads to potent Cr polymerization catalysts. The susceptibility for isomerization is dependent on the nature of the N-substituent of the PNP precursor. Electron donating N-substituent i-Pr, which disfavors the PPN isomer compared to p-tolyl, and minimization of water contamination correlate with higher oligomerization activity and lower polymer byproducts. More broadly, the present study demonstrates the significant impact that Al-activators can have on the structure and behavior of the supporting ligand leading to detrimental reactivity.

Terminal Molybdenum Phosphides with d Electrons: Radical Character Promotes Coupling Chemistry

Co-reporter:Joshua A. Buss;Dr. Paul H. Oyala; Theodor Agapie

Angewandte Chemie International Edition 2017 Volume 56(Issue 46) pp:14502-14506

Publication Date(Web):2017/11/13

DOI:10.1002/anie.201707921

AbstractA terminal Mo phosphide was prepared through the group transfer of both P and Cl atoms from chloro-substituted dibenzo-7λ3-phosphanorbornadiene. This compound represents the first structurally characterized terminal transition-metal phosphide with valence d electrons. In the tetragonal ligand field, these electrons populate an orbital of dxy parentage, an electronic configuration that accommodates both metal d electrons and a formal M≡P triple bond. Single-electron oxidation affords a transient open-shell terminal phosphide cation with significant spin density on P, as corroborated by continuous wave (CW) and pulse electron paramagnetic resonance (EPR) characterization. Facile P−P bond formation occurs from this species through intermolecular phosphide coupling.

A CaMn4O2 model of the biological oxygen evolving complex: synthesis via cluster expansion on a low symmetry ligand

Co-reporter:Heui Beom Lee;Emily Y. Tsui

Chemical Communications 2017 vol. 53(Issue 51) pp:6832-6835

Publication Date(Web):2017/06/22

DOI:10.1039/C7CC02313H

Using a new multinucleating ligand featuring two dipyridyl alkoxide moieties and a carboxylate moiety, low symmetry tetranuclear complexes 1-M (M = Mn, Fe, and Co) have been synthesized. Complex 1-Mn was used as a precursor for the synthesis of a pentanuclear CaMn4O2 cluster (3) with the same metal stoichiometry as the biological OEC.

Dioxygen Reduction by a Pd(0)–Hydroquinone Diphosphine Complex

Co-reporter:Kyle T. Horak

Journal of the American Chemical Society 2016 Volume 138(Issue 10) pp:3443-3452

Publication Date(Web):March 7, 2016

DOI:10.1021/jacs.5b12928

A novel p-terphenyl diphosphine ligand was synthesized with a noninnocent hydroquinone moiety as the central arene (1-H). Pseudo-tetrahedral 4-coordinate Ni0 and Pd0–quinone (2 and 3, respectively) complexes proved accessible by metalating 1-H with the corresponding M(OAc)2 precursors. O2 does not react with the Pd0–quinone species (3) and protonation occurs at the quinone moiety indicating that the coordinated oxidized quinonoid moiety prevents reactivity at the metal. A 2-coordinate Pd0–hydroquinone complex (4-H) was prepared using a one-pot metalation with PdII followed by reduction. The reduced quinonoid moiety in 4-H shows metal-coupled reactivity with small molecules. 4-H was capable of reducing a variety of substrates including dioxygen, nitric oxide, nitrous oxide, 1-azido adamantane, trimethylamine n-oxide, and 1,4-benzoquinone quantitatively producing 3 as the Pd-containing reaction product. Mechanistic investigations of dioxygen reduction revealed that the reaction proceeds through a η2-peroxo intermediate (Int1) at low temperatures followed by subsequent ligand oxidation at higher temperatures in a reaction that consumed half an equivalent of O2 and produced water as a final oxygenic byproduct. Control compounds with methyl protected phenolic moieties (4-Me), displaying a AgI center incapable of O2 binding (7-H) or a cationic Pd–H motif (6-H) allowed for the independent examination of potential reaction pathways. The reaction of 4-Me with dioxygen at low temperature produces a species (8-Me) analogous to Int1 demonstrating that initial dioxygen activation is an inner sphere Pd-based process where the hydroquinone moiety only subsequently participates in the reduction of O2, at higher temperatures, by H+/e– transfers.

Mechanism of Molybdenum-Mediated Carbon Monoxide Deoxygenation and Coupling: Mono- and Dicarbyne Complexes Precede C–O Bond Cleavage and C–C Bond Formation

Co-reporter:Joshua A. Buss and Theodor Agapie

Journal of the American Chemical Society 2016 Volume 138(Issue 50) pp:16466-16477

Publication Date(Web):November 21, 2016

DOI:10.1021/jacs.6b10535

Deoxygenative coupling of CO to value-added C≥2 products is challenging and mechanistically poorly understood. Herein, we report a mechanistic investigation into the reductive coupling of CO, which provides new fundamental insights into a multielectron bond-breaking and bond-making transformation. In our studies, the formation of a bis(siloxycarbyne) complex precedes C–O bond cleavage. At −78 °C, over days, C–C coupling occurs without C–O cleavage. However, upon warming to 0 °C, C–O cleavage is observed from this bis(siloxycarbyne) complex. A siloxycarbyne/CO species undergoes C–O bond cleavage at lower temperatures, indicating that monosilylation, and a more electron-rich Mo center, favors deoxygenative pathways. From the bis(siloxycarbyne), isotopic labeling experiments and kinetics are consistent with a mechanism involving unimolecular silyl loss or C–O cleavage as rate-determining steps toward carbide formation. Reduction of Mo(IV) CO adducts of carbide and silylcarbyne species allowed for the spectroscopic detection of reduced silylcarbyne/CO and mixed silylcarbyne/siloxycarbyne complexes, respectively. Upon warming, both of these silylcarbynes undergo C–C bond formation, releasing silylated C2O1 fragments and demonstrating that the multiple bonded terminal Mo≡C moiety is an intermediate on the path to deoxygenated, C–C coupled products. The electronic structures of Mo carbide and carbyne species were investigated quantum mechanically. Overall, the present studies establish the elementary reactions steps by which CO is cleaved and coupled at a single metal site.

Intramolecular C–H and C–F Bond Oxygenation Mediated by a Putative Terminal Oxo Species in Tetranuclear Iron Complexes

Co-reporter:Graham de Ruiter; Niklas B. Thompson; Michael K. Takase

Journal of the American Chemical Society 2016 Volume 138(Issue 5) pp:1486-1489

Publication Date(Web):January 13, 2016

DOI:10.1021/jacs.5b12214

Herein we report the intramolecular arene C–H and C–F bond oxygenation by tetranuclear iron complexes. Treatment of [LFe3(PhPz)3OFe][OTf]2 (1) or its fluorinated analog [LFe3(F2ArPz)3OFe][OTf]2 (5) with iodosobenzene results in the regioselective hydroxylation of a bridging pyrazolate ligand, converting a C–H or C–F bond into a C–O bond. The observed reactivity suggests the formation of terminal and reactive Fe-oxo intermediates. With the possibility of intramolecular electron transfer within clusters in 1 and 5, different reaction pathways (FeIV-oxo vs FeIII-oxo) might be responsible for the observed arene hydroxylation.

A trans-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper–Lewis Acid Complex

Co-reporter:Davide Lionetti; Graham de Ruiter

Journal of the American Chemical Society 2016 Volume 138(Issue 15) pp:5008-5011

Publication Date(Web):March 30, 2016

DOI:10.1021/jacs.6b01083

The reduction of nitric oxide (NO) to nitrous oxide (N2O) is a process relevant to biological chemistry as well as to the abatement of certain environmental pollutants. One of the proposed key intermediates in NO reduction is hyponitrite (N2O22–), the product of reductive coupling of two NO molecules. We report the reductive coupling of NO by an yttrium–tricopper complex generating a trans-hyponitrite moiety supported by two μ-O-bimetallic (Y,Cu) cores, a previously unreported coordination mode. Reaction of the hyponitrite species with Brønsted acids leads to the generation of N2O, demonstrating the viability of the hyponitrite complex as an intermediate in NO reduction to N2O. The additional reducing equivalents stored in each tricopper unit are employed in a subsequent step for N2O reduction to N2, for an overall (partial) conversion of NO to N2. The combination of Lewis acid and multiple redox active metals facilitates this four electron conversion via an isolable hyponitrite intermediate.

Modulation of Proton-Coupled Electron Transfer through Molybdenum–Quinonoid Interactions

Co-reporter:Justin T. Henthorn and Theodor Agapie

Inorganic Chemistry 2016 Volume 55(Issue 11) pp:5337-5342

Publication Date(Web):May 26, 2016

DOI:10.1021/acs.inorgchem.6b00331

An expanded series of π-bound molybdenum–quinonoid complexes supported by pendant phosphines has been synthesized. These compounds formally span three protonation–oxidation states of the quinonoid fragment (catechol, semiquinone, quinone) and two different oxidation states of the metal (Mo0, MoII), notably demonstrating a total of two protons and four electrons accessible in the system. Previously, the reduced Mo0–catechol complex 1 and its reaction with dioxygen to yield the two-proton/two-electron oxidized Mo0–quinone compound 4 was explored, while, herein, the expansion of the series to include the two-electron oxidized MoII–catechol complex 2, the one-proton/two-electron oxidized Mo–semiquinone complex 3, and the two-proton/four-electron oxidized MoII–quinone complexes 5 and 6 is reported. Transfer of multiple equivalents of protons and electrons from the Mo0 and MoII catechol complexes, 1 and 2, to H atom acceptor TEMPO suggests the presence of weak O–H bonds. Although thermochemical analyses are hindered by the irreversibility of the electrochemistry of the present compounds, the reactivity observed suggests weaker O–H bonds compared to the free catechol, indicating that proton-coupled electron transfer can be facilitated significantly by the π-bound metal center.

Effect of the Mn Oxidation State on Single-Molecule-Magnet Properties: MnIII vs MnIV in Biologically Inspired DyMn3O4 Cubanes

Co-reporter:Po-Heng Lin, Emily Y. Tsui, Fatemah Habib, Muralee Murugesu, and Theodor Agapie

Inorganic Chemistry 2016 Volume 55(Issue 12) pp:6095-6099

Publication Date(Web):June 9, 2016

DOI:10.1021/acs.inorgchem.6b00630

Inspired by the ferromagnetic coupling in the cubane model CaMnIV3O4 of the oxygen-evolving complex of photosystem II, 3d–4f mixed-metal DyMn3O4 clusters were prepared for investigation of the magnetic properties. For comparison, YMnIV3O4 and YMnIV2MnIIIO4 clusters were investigated as well and showed ferromagnetic interactions, like the calcium analogue. DyMnIV3O4 displays single-molecule-magnet properties, while the one-electron-reduced species (DyMnIV2MnIIIO4) does not, despite the presence of a MnIII center with higher spin and single-ion anisotropy.

Combination of Redox-Active Ligand and Lewis Acid for Dioxygen Reduction with π-Bound Molybdenum−Quinonoid Complexes

Co-reporter:Justin T. Henthorn; Sibo Lin

Journal of the American Chemical Society 2015 Volume 137(Issue 4) pp:1458-1464

Publication Date(Web):January 10, 2015

DOI:10.1021/ja5100405

A series of π-bound Mo−quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal–arene interactions between Mo and the π system of the quinonoid fragment. The Mo–catechol complex (2a) was found to react within minutes with 0.5 equiv of O2 to yield a Mo–quinone complex (3), H2O, and CO. Si- and B-protected Mo–catecholate complexes also react with O2 to yield 3 along with (R2SiO)n and (ArBO)3 byproducts, respectively. Formally, the Mo–catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O2 oxygens. Unreactive by itself, the Mo–dimethyl catecholate analogue reduces O2 in the presence of added Lewis acid, B(C6F5)3, to generate a MoI species and a bis(borane)-supported peroxide dianion, [[(F5C6)3B]2O22–], demonstrating single-electron-transfer chemistry from Mo to the O2 moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O2 with pendant acids weaker than B(C6F5)3. Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O2 is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O2 species.

Nitric Oxide Activation by Distal Redox Modulation in Tetranuclear Iron Nitrosyl Complexes

Co-reporter:Graham de Ruiter; Niklas B. Thompson; Davide Lionetti

Journal of the American Chemical Society 2015 Volume 137(Issue 44) pp:14094-14106

Publication Date(Web):September 21, 2015

DOI:10.1021/jacs.5b07397

A series of tetranuclear iron complexes displaying a site-differentiated metal center was synthesized. Three of the metal centers are coordinated to our previously reported ligand, based on a 1,3,5-triarylbenzene motif with nitrogen and oxygen donors. The fourth (apical) iron center is coordinatively unsaturated and appended to the trinuclear core through three bridging pyrazolates and an interstitial μ4-oxide moiety. Electrochemical studies of complex [LFe3(PhPz)3OFe][OTf]2 revealed three reversible redox events assigned to the FeII4/FeII3FeIII (−1.733 V), FeII3FeIII/FeII2FeIII2 (−0.727 V), and FeII2FeIII2/FeIIFeIII3 (0.018 V) redox couples. Combined Mössbauer and crystallographic studies indicate that the change in oxidation state is exclusively localized at the triiron core, without changing the oxidation state of the apical metal center. This phenomenon is assigned to differences in the coordination environment of the two metal sites and provides a strategy for storing electron and hole equivalents without affecting the oxidation state of the coordinatively unsaturated metal. The presence of a ligand-binding site allowed the effect of redox modulation on nitric oxide activation by an FeII metal center to be studied. Treatment of the clusters with nitric oxide resulted in binding of NO to the apical iron center, generating a {FeNO}7 moiety. As with the NO-free precursors, the three reversible redox events are localized at the iron centers distal from the NO ligand. Altering the redox state of the triiron core resulted in significant change in the NO stretching frequency, by as much as 100 cm–1. The increased activation of NO is attributed to structural changes within the clusters, in particular, those related to the interaction of the metal centers with the interstitial atom. The differences in NO activation were further shown to lead to differential reactivity, with NO disproportionation and N2O formation performed by the more electron-rich cluster.

Investigations of the Effect of the Non-Manganese Metal in Heterometallic-Oxido Cluster Models of the Oxygen Evolving Complex of Photosystem II: Lanthanides as Substitutes for Calcium

Co-reporter:Po-Heng Lin; Michael K. Takase

Inorganic Chemistry 2015 Volume 54(Issue 1) pp:59-64

Publication Date(Web):December 18, 2014

DOI:10.1021/ic5015219

We report the syntheses and electrochemical properties of nine new clusters ([LLnMnIV3O4(OAc)3(DMF)n]+ (Ln = La3+, Ce3+, Nd3+, Eu3+, Gd3+, Tb3+, Dy3+, Yb3+, and Lu3+, n = 2 or 3)) supported by a ligand (L3–) based on a 1,3,5-triarylbenzene motif appended with alkoxide and pyridine donors. All complexes were obtained by metal substitution of Ca2+ with lanthanides upon treatment of previously reported LMn3CaO4(OAc)3(THF) with Ln(OTf)3. Structural characterization confirmed that the clusters contain the [LnMn3O4] cubane motif. The effect of the redox-inactive centers on the electronic properties of the Mn3O4 cores was investigated by cyclic voltammetry. A linear correlation between the redox potential of the cluster and the ionic radii or pKa of the lanthanide metal ion was observed. Chemical reduction of the LMnIV3GdO4(OAc)3(DMF)2 cluster with decamethylferrocene, resulted in the formation of LGdMnIV2MnIIIO4(OAc)3(DMF)2, a rare example of mixed-valence [MMn3O4] cubane. The lanthanide-coordinated ligands can be substituted with other donors, including water, the biological substrate.

Tuning of Metal Complex Electronics and Reactivity by Remote Lewis Acid Binding to π-Coordinated Pyridine Diphosphine Ligands

Co-reporter:Kyle T. Horak, David G. VanderVelde, and Theodor Agapie

Organometallics 2015 Volume 34(Issue 19) pp:4753-4765

Publication Date(Web):August 20, 2015

DOI:10.1021/acs.organomet.5b00562

Metal complexes supported by ligands with chemically modifiable pendant groups are of interest for controlling reactivity. We report on the coordination chemistry and reactivity of a multidentate phosphine ligand framework that contains a Lewis acid binding site. 3,5-Bis(2-phosphinophenyl)pyridine coordinates low-oxidation-state metal centers such as Ni0 and Pd0 via the phosphine donors and the π system of the heterocycle. Electrophilic reagents such as B(C6F5)3, Me+, and BCy2OTf bind the available pyridine nitrogen, generating the Ni complexes 2Ni-B(C6F5)3, 2Ni-Me, and 2Ni-BCy2OTf, respectively. Analogous compounds were prepared for Pd (2Pd, 2Pd-B(C6F5)3, and 2Pd-H). The effect of Lewis acid binding was evaluated by single-crystal X-ray diffraction studies and spectroscopy. Lewis acid binding to 2Pd leads to a stronger η1 interaction between the metal and the heterocycle π system. Ni binds in an η2 fashion, but the Lewis acid free species is not monomeric. Ni coordination results in disruption of pyridine aromaticity, as indicated by localization of double- and single-bond character in the solid state. CO adducts were prepared for Lewis acid free (4Ni) and Lewis acid bound species (H+-, Me+-, and B(C6F5)3-bound; 4Ni-H, 4Ni-Me, and 4Ni-B(C6F5)3) that show a significant shift of the CO stretching frequency from 1930 to 1966–1976 cm–1, respectively, indicating communication of ligand electronics to the metal center. An NO adduct (5Ni) with negligible metal–pyridine interactions was obtained upon sequential reaction of 2Ni with [OMe3][BF4] and then [NO][BF4]. Treatment of 2Ni with silanes and boranes results in pyridine dearomatization involving heteroatom–H bond activation, with the heteroatom binding to the pyridine nitrogen and the hydrides delivered to the ortho position of pyridine. This reactivity demonstrates that the pendant pyridine is drastically affected by metal binding, enabling unusual ligand-based substrate activation. The described chemistry highlights a strategy for tuning the properties of metal centers by ligand postsynthetic modifications.

Heterometallic Effects in Trinuclear Complexes Supported by p-Terphenyl Diphosphine Ligands

Co-reporter:Kyle T. Horak, Sibo Lin, Jonathan Rittle, and Theodor Agapie

Organometallics 2015 Volume 34(Issue 18) pp:4429-4432

Publication Date(Web):September 8, 2015

DOI:10.1021/acs.organomet.5b00579

A series of isostructural trinuclear complexes of metal composition Pd2Fe, Pd2Co, and Ni2Fe was synthesized in a stepwise fashion. Using dinuclear precursors (PdI2 and NiI2) supported by p-terphenyl diphosphine ligands allowed for the selective incorporation of a single additional metal center. The effects of both metal composition and ligand electronics on cluster properties were studied. These results highlight the importance of metal–metal interactions in mixed-metal sites of structurally analogous clusters.

Molecular Mixed-Metal Manganese Oxido Cubanes as Precursors to Heterogeneous Oxygen Evolution Catalysts

Co-reporter:Sy Suseno;Dr. Charles C. L. McCrory;Dr. Rosalie Tran;Dr. Sheraz Gul;Dr. Junko Yano; Theodor Agapie

Chemistry - A European Journal 2015 Volume 21( Issue 38) pp:13420-13430

Publication Date(Web):

DOI:10.1002/chem.201501104

Abstract

Well-defined mixed-metal [CoMn₃O₄] and [NiMn₃O₄] cubane complexes were synthesized and used as precursors for heterogeneous oxygen evolution reaction (OER) electrocatalysts. The discrete clusters were dropcasted onto glassy carbon (GC) and indium tin oxide (ITO) electrodes, and the OER activities of the resulting films were evaluated. The catalytic surfaces were analyzed by various techniques to gain insight into the structure-function relationships of the electrocatalysts’ heterometallic composition. Depending on preparation conditions, the Co-Mn oxide was found to change metal composition during catalysis, while the Ni–Mn oxides maintained the NiMn₃ ratio. XAS studies provided structural insights indicating that the electrocatalysts are different from the molecular precursors, but that the original NiMn₃O₄ cubane-like geometry was maintained in the absence of thermal treatment (2-Ni). In contrast, the thermally generated 3-Ni develops an oxide-like extended structure. Both 2-Ni and 3-Ni undergo structural changes upon electrolysis, but they do not convert into the same material. The observed structural motifs in these heterogeneous electrocatalysts are reminiscent of the biological oxygen-evolving complex in Photosystem II, including the MMn₃O₄ cubane moiety. The reported studies demonstrate the use of discrete heterometallic oxide clusters as precursors for heterogeneous water oxidation catalysts of novel composition and the distinct behavior of two sets of mixed metal oxides.

Aryl Ether Cleavage by Group 9 and 10 Transition Metals: Stoichiometric Studies of Selectivity and Mechanism

Co-reporter:Guy A. Edouard, Paul Kelley, David E. Herbert, and Theodor Agapie

Organometallics 2015 Volume 34(Issue 21) pp:5254-5277

Publication Date(Web):October 28, 2015

DOI:10.1021/acs.organomet.5b00710

The reactivity of terphenyl diphosphines bearing aryl–methyl ether or aryl–aryl ether moieties with M0 (M = Ni, Pd, Pt), M′I (M′ = Co, Rh, Ir), or MII centers was investigated to gain mechanistic insight into intramolecular aryl–ether bond cleavage in structurally related metal complexes. RhI converts the aryl–methyl ether moiety to an aryl C–H bond. This is similar to reactivity previously observed at Ni0 that involves C–O oxidative addition, β-H elimination liberating CH2O, reductive elimination of an aryl C–H bond, and decarbonylation of CH2O. IrI leads to unselective aryl and alkyl C–O bond activation. In the presence of excess CO, RhI and IrI display a shift in selectivity and reactivity and cleave the alkyl C–O bond. CoI does not perform C–O cleavage. Alkyl C–O bond activation was observed with MII–halide complexes with loss of MeCl via a Lewis acid–base mechanism. Pd0 and Pt0 cleave selectively the O–Me bond via oxidative addition. With a diaryl ether moiety, Pd0 and Pt0 are found to be capable of performing aryl C–O bond activation. Various levels of interactions between the central arene and the metal center were observed, and these were correlated with trends in bond activation. Overall, selective cleavage of the stronger aryl ether C–O bond was observed only with Ni0 and RhI. Pd0 and Pt0 can perform aryl ether C–O cleavage, but if available, they will cleave the weaker O–Me bond. This study provides insight into the relative reactivity of group 9 and 10 metal centers with aryl ether bonds and suggests future directions for designing systems for metal-catalyzed cleavage of ether C–O bonds in synthetic methodology as well as lignin deoxygenation.

Molybdenum Catalyzed Ammonia Borane Dehydrogenation: Oxidation State Specific Mechanisms

Co-reporter:Joshua A. Buss ; Guy A. Edouard ; Christine Cheng ; Jade Shi

Journal of the American Chemical Society 2014 Volume 136(Issue 32) pp:11272-11275

Publication Date(Web):July 17, 2014

DOI:10.1021/ja5059923

Though numerous catalysts for the dehydrogenation of ammonia borane (AB) are known, those that release >2 equiv of H2 are uncommon. Herein, we report the synthesis of Mo complexes supported by a para-terphenyl diphosphine ligand, 1, displaying metal–arene interactions. Both a Mo0 N2 complex, 5, and a MoII bis(acetonitrile) complex, 4, exhibit high levels of AB dehydrogenation, releasing over 2.0 equiv of H2. The reaction rate, extent of dehydrogenation, and reaction mechanism vary as a function of the precatalyst oxidation state. Several Mo hydrides (MoII(H)2, [MoII(H)]+, and [MoIV(H)3]+) relevant to AB chemistry were characterized.

Toward Models for the Full Oxygen-Evolving Complex of Photosystem II by Ligand Coordination To Lower the Symmetry of the Mn3CaO4 Cubane: Demonstration That Electronic Effects Facilitate Binding of a Fifth Metal

Co-reporter:Jacob S. Kanady ; Po-Heng Lin ; Kurtis M. Carsch ; Robert J. Nielsen ; Michael K. Takase ; William A. Goddard ; III

Journal of the American Chemical Society 2014 Volume 136(Issue 41) pp:14373-14376

Publication Date(Web):September 20, 2014

DOI:10.1021/ja508160x

Synthetic model compounds have been targeted to benchmark and better understand the electronic structure, geometry, spectroscopy, and reactivity of the oxygen-evolving complex (OEC) of photosystem II, a low-symmetry Mn4CaOn cluster. Herein, low-symmetry MnIV3GdO4 and MnIV3CaO4 cubanes are synthesized in a rational, stepwise fashion through desymmetrization by ligand substitution, causing significant cubane distortions. As a result of increased electron richness and desymmetrization, a specific μ3-oxo moiety of the Mn3CaO4 unit becomes more basic allowing for selective protonation. Coordination of a fifth metal ion, Ag+, to the same site gives a Mn3CaAgO4 cluster that models the topology of the OEC by displaying both a cubane motif and a “dangler” transition metal. The present synthetic strategy provides a rational roadmap for accessing more accurate models of the biological catalyst.

Arene non-innocence in dinuclear complexes of Fe, Co, and Ni supported by a para-terphenyl diphosphine

Co-reporter:Kyle T. Horak, Alexandra Velian, Michael W. Day and Theodor Agapie

Chemical Communications 2014 vol. 50(Issue 34) pp:4427-4429

Publication Date(Web):03 Mar 2014

DOI:10.1039/C4CC00838C

Cofacial Fe2, Co2, and Ni2 complexes supported by a para-terphenyl diphosphine ligand were prepared. Central arene deplanarization and a μ2:(η3,η3) coordination mode suggest partial bisallyl character in the Fe2 and Co2 complexes. An oxidation induced shift in Fe2–arene binding highlights the non-innocent nature of the arene ligand.

Bimetallic Zirconium Amine Bis(phenolate) Polymerization Catalysts: Enhanced Activity and Tacticity Control for Polyolefin Synthesis

Co-reporter:Madalyn R. Radlauer and Theodor Agapie

Organometallics 2014 Volume 33(Issue 13) pp:3247-3250

Publication Date(Web):June 18, 2014

DOI:10.1021/om500608j

Binucleating multidentate amine bis(phenolate) ligands with rigid terphenyl backbones were designed to support two zirconium centers locked in close proximity. Polymerizations of propylene or 1-hexene with the synthesized bimetallic precatalysts resulted in polymers with significantly higher isotacticity (up to 79% mmmm) in comparison to the stereoirregular polymers produced with previously reported Cs-symmetric monometallic analogues. The bimetallic precatalysts also display higher activity (up to 124 kg of poly(1-hexene) (mmol of Zr)−1 h–1), in comparison to the monometallic analogues, and among the highest activities reported for nonmetallocene catalysts. The stereocontrol is consistent with a bimetallic mechanism involving remote steric interactions with the ligand sphere of the second metal center.

Carbon dioxide cleavage by a Ni2 complex supported by a binucleating bis(N-heterocyclic carbene) framework

Co-reporter:Emily Y. Tsui, Theodor Agapie

Polyhedron 2014 84() pp: 103-110

Publication Date(Web):

DOI:10.1016/j.poly.2014.06.041

Dioxygen Reactivity with a FerroceneLewis Acid Pairing: Reduction to a Boron Peroxide in the Presence of Tris(pentafluorophenyl)borane

Co-reporter:Justin T. Henthorn ; Theodor Agapie

Angewandte Chemie 2014 Volume 126( Issue 47) pp:13107-13110

Publication Date(Web):

DOI:10.1002/ange.201408462

Abstract

Ferrocenes, which are typically air-stable outer-sphere single-electron transfer reagents, were found to react with dioxygen in the presence of B(C₆F₅)₃, a Lewis acid unreactive to O₂, to generate bis(borane) peroxide. Although several Group 13 peroxides have been reported, boron-supported peroxides are rare, with no structurally characterized examples of the BO₂B moiety. The synthesis of a bis(borane)-supported peroxide anion and its structural and electrochemical characterization are described.

Dioxygen Reactivity with a FerroceneLewis Acid Pairing: Reduction to a Boron Peroxide in the Presence of Tris(pentafluorophenyl)borane

Co-reporter:Justin T. Henthorn ; Theodor Agapie

Angewandte Chemie International Edition 2014 Volume 53( Issue 47) pp:12893-12896

Publication Date(Web):

DOI:10.1002/anie.201408462

Abstract

Bimetallic Coordination Insertion Polymerization of Unprotected Polar Monomers: Copolymerization of Amino Olefins and Ethylene by Dinickel Bisphenoxyiminato Catalysts

Co-reporter:Madalyn R. Radlauer ; Aya K. Buckley ; Lawrence M. Henling

Journal of the American Chemical Society 2013 Volume 135(Issue 10) pp:3784-3787

Publication Date(Web):February 20, 2013

DOI:10.1021/ja4004816

Dinickel bisphenoxyiminato complexes based on highly substituted p- and m-terphenyl backbones were synthesized, and the corresponding atropisomers were isolated. In the presence of a phosphine scavenger, Ni(COD)2, the phosphine-ligated syn-dinickel complexes copolymerized α-olefins and ethylene in the presence of amines to afford 0.2–1.3% α-olefin incorporation and copolymerized amino olefins and ethylene with a similar range of incorporation (0.1–0.8%). The present rigid catalysts provide a bimetallic strategy for insertion polymerization of polar monomers without masking of the heteroatom group. The effects of the catalyst structure on the reactivity were studied by comparisons of the syn and anti atropisomers and the p- and m-terphenyl systems.

Dipalladium(I) Terphenyl Diphosphine Complexes as Models for Two-Site Adsorption and Activation of Organic Molecules

Co-reporter:Sibo Lin ; David E. Herbert ; Alexandra Velian ; Michael W. Day

Journal of the American Chemical Society 2013 Volume 135(Issue 42) pp:15830-15840

Publication Date(Web):September 25, 2013

DOI:10.1021/ja406696k

A para-terphenyl diphosphine was employed to support a dipalladium(I) moiety. Unlike previously reported dipalladium(I) species, the present system provides a single molecular hemisphere for binding of ligands across two metal centers, enabling the characterization and comparison of the binding of a wide variety of saturated and unsaturated organic molecules. The dipalladium(I) terphenyl diphosphine toluene-capped complex was synthesized from a dipalladium(I) hexaacetonitrile precursor in the presence of toluene. The palladium centers display interactions with the π-systems of the central ring of the terphenyl unit and that of the toluene. Exchange of toluene for anisole, 1,3-butadiene, 1,3-cyclohexadiene, thiophenes, pyrroles, or furans resulted in well-defined π-bound complexes which were studied by crystallography, nuclear magnetic resonance (NMR) spectroscopy, and density functional theory. Structural characterization shows that the interactions of the dipalladium unit with the central arene of the diphosphine does not vary significantly in this series allowing for a systematic comparison of the binding of the incoming ligands to the dipalladium moiety. Several of the complexes exhibit rare μ–η2:η2 or μ–η2:η1(O or S) bridging motifs. Hydrogenation of the thiophene and benzothiophene adducts was demonstrated to proceed at room temperature. The relative binding strength of the neutral ligands was determined by competition experiments monitored by NMR spectroscopy. The relative equilibrium constants for ligand substitution span over 13 orders of magnitude. This represents the most comprehensive analysis to date of the relative binding of heterocycles and unsaturated ligands to bimetallic sites. Binding interactions were computationally studied with electrostatic potentials and molecular orbital analysis. Anionic ligands were also demonstrated to form π-bound complexes.

Heterometallic Triiron-Oxo/Hydroxo Clusters: Effect of Redox-Inactive Metals

Co-reporter:David E. Herbert ; Davide Lionetti ; Jonathan Rittle

Journal of the American Chemical Society 2013 Volume 135(Issue 51) pp:19075-19078

Publication Date(Web):December 4, 2013

DOI:10.1021/ja4104974

A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Fe3M(μ4-O)(μ2-OH) core that remains intact upon changing M or the oxidation state of iron. Electrochemical studies reveal that the reduction potentials (E1/2) span a window of 500 mV and depend upon the Lewis acidity of M. Using the pKa of the M-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between E1/2 and acidity, with a slope of ∼70 mV per pKa unit. The current study of [Fe3MO(OH)] and previous ones of [Mn3MOn] (n = 2,4) moieties support the generality of the above relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations, as applied to clusters of different redox-active metals.

Metal-templated ligand architectures for trinuclear chemistry: tricopper complexes and their O2 reactivity

Co-reporter:Davide Lionetti, Michael W. Day and Theodor Agapie

Chemical Science 2013 vol. 4(Issue 2) pp:785-790

Publication Date(Web):26 Nov 2012

DOI:10.1039/C2SC21758A

A trinucleating framework was assembled by templation of a heptadentate ligand around yttrium and lanthanides. The generated complexes orient three sets of two or three N-donors each for binding additional metal centers. Addition of three equivalents of copper(I) leads to the formation of tricopper(I) species. Reactions with dioxygen at low temperatures generate species whose spectroscopic features are consistent with a μ3,μ3-dioxo-tricopper complex. Reactivity studies were performed with a variety of substrates. The dioxo-tricopper species deprotonates weak acids, undergoes oxygen atom transfer with triphenylphosphine to yield triphenylphosphine oxide, and performs hydrogen atom abstraction from tetramethylpiperidine-N-hydroxide (TEMPO-H). Thiophenols reduce the oxygenated species to a Cu3I complex and liberate two equivalents of disulfide, consistent with a four-electron four-proton process.

Role of oxido incorporation and ligand lability in expanding redox accessibility of structurally related Mn4 clusters

Co-reporter:Jacob S. Kanady, Rosalie Tran, Jamie A. Stull, Luo Lu, Troy A. Stich, Michael W. Day, Junko Yano, R. David Britt and Theodor Agapie

Chemical Science 2013 vol. 4(Issue 10) pp:3986-3996

Publication Date(Web):19 Jul 2013

DOI:10.1039/C3SC51406D

Photosystem II supports four manganese centers through nine oxidation states from manganese(II) during assembly through to the most oxidized state before O2 formation and release. The protein-based carboxylate and imidazole ligands allow for significant changes of the coordination environment during the incorporation of hydroxido and oxido ligands upon oxidation of the metal centers. We report the synthesis and characterization of a series of tetramanganese complexes in four of the six oxidation states from MnII3MnIII to MnIII2MnIV2 with the same ligand framework (L) by incorporating four oxido ligands. A 1,3,5-triarylbenzene framework appended with six pyridyl and three alkoxy groups was utilized along with three acetate anions to access tetramanganese complexes, Mn4Ox, with x = 1, 2, 3, and 4. Alongside two previously reported complexes, four new clusters in various states were isolated and characterized by crystallography, and four were observed electrochemically, thus accessing the eight oxidation states from MnII4 to MnIIIMnIV3. This structurally related series of compounds was characterized by EXAFS, XANES, EPR, magnetism, and cyclic voltammetry. Similar to the ligands in the active site of the protein, the ancillary ligand (L) is preserved throughout the series and changes its binding mode between the low and high oxido-content clusters. Implications for the rational assembly and properties of high oxidation state metal–oxido clusters are presented.

Synthetic Cluster Models of Biological and Heterogeneous Manganese Catalysts for O2 Evolution

Co-reporter:Emily Y. Tsui ; Jacob S. Kanady

Inorganic Chemistry 2013 Volume 52(Issue 24) pp:13833-13848

Publication Date(Web):December 16, 2013

DOI:10.1021/ic402236f

Artificial photosynthesis has emerged as an important strategy toward clean and renewable fuels. Catalytic oxidation of water to O2 remains a significant challenge in this context. A mechanistic understanding of currently known heterogeneous and biological catalysts at a molecular level is highly desirable for fundamental reasons as well as for the rational design of practical catalysts. This Award Article discusses recent efforts in synthesizing structural models of the oxygen-evolving complex of photosystem II. These structural motifs are also related to heterogeneous mixed-metal oxide catalysts. A stepwise synthetic methodology was developed toward achieving the structural complexity of the targeted active sites. A geometrically restricted multinucleating ligand, but with labile coordination modes, was employed for the synthesis of low-oxidation-state trimetallic species. These precursors were elaborated to site-differentiated tetrametallic complexes in high oxidation states. This methodology has allowed for structure–reactivity studies that have offered insight into the effects of different components of the clusters. Mechanistic aspects of oxygen-atom transfer and incorporation from water have been interrogated. Significantly, a large and systematic effect of redox-inactive metals on the redox properties of these clusters was discovered. With the pKa value of the redox-inactive metal–aqua complex as a measure of the Lewis acidity, structurally analogous clusters display a linear dependence between the reduction potential and acidity; each pKa unit shifts the potential by ca. 90 mV. Implications for the function of the biological and heterogeneous catalysts are discussed.

Hydrogen Evolution Catalyzed by Aluminum-Bridged Cobalt Diglyoximate Complexes (Eur. J. Inorg. Chem. 22-23/2013)

Co-reporter:Paul Kelley;Michael W. Day

European Journal of Inorganic Chemistry 2013 Volume 2013( Issue 22-23) pp:

Publication Date(Web):

DOI:10.1002/ejic.201390098

Hydrogen Evolution Catalyzed by Aluminum-Bridged Cobalt Diglyoximate Complexes

Co-reporter:Paul Kelley;Michael W. Day

European Journal of Inorganic Chemistry 2013 Volume 2013( Issue 22-23) pp:

Publication Date(Web):

DOI:10.1002/ejic.201300806

Abstract

Invited for the front cover of the Cluster Issue on Small-Molecule Activation by Reactive Metal Complexes is the group of Theodor Agapie at Caltech. The cover image shows the core of the crystal structure of an aluminum-bridged cobalt diglyoximate catalyst for hydrogen evolution.

Hydrogen Evolution Catalyzed by Aluminum-Bridged Cobalt Diglyoximate Complexes

Co-reporter:Paul Kelley;Michael W. Day

European Journal of Inorganic Chemistry 2013 Volume 2013( Issue 22-23) pp:3840-3845

Publication Date(Web):

DOI:10.1002/ejic.201300309

Abstract

The syntheses of several cobalt diglyoximate complexes connected by one or two aluminum bridges are described. The aluminum centers are supported by tunable tetradentate diamine bisphenoxide ligands. Electrochemical investigations revealed that the number of aluminum bridges and the nature of the substituents on the phenoxide ligands significantly affect the cobalt reduction potentials. The present aluminum–cobalt compounds are electrocatalysts for proton reduction to hydrogen at potentials negative relative to those of the boron- and proton-bridged analogs. The reported synthetic strategies allow modulation of the reduction potentials and the secondary coordination sphere interactions by tuning the ancillary ligands bound to aluminum.

Trinuclear Nickel Complexes with Metal–Arene Interactions Supported by Tris- and Bis(phosphinoaryl)benzene Frameworks

Co-reporter:Sandy Suseno, Kyle T. Horak, Michael W. Day, and Theodor Agapie

Organometallics 2013 Volume 32(Issue 23) pp:6883-6886

Publication Date(Web):November 4, 2013

DOI:10.1021/om400976x

Triphosphine and diphosphine ligands with backbones designed to facilitate metal–arene interactions were employed to support multinuclear Ni complexes. Di- and trinuclear metal complexes supported by a triphosphine containing a triarylbenzene linker display diverse metal–arene binding modes. Multinuclear Ni halide complexes were isolated with strongly interacting metal centers bound to opposite faces of the coordinated arene. Upon reaction of the trinickel diiodide complex 2 with disodium tetracarbonylferrate, a cofacial triangulo nickel(0) complex, 4, was isolated. The Ni03 cluster motif can also be supported by a para-terphenyl diphosphine, where a terminal carbon monoxide ligand replaces the third phosphine donor. All multinuclear complexes feature strong metal–arene interactions, demonstrating the use of an arene as a versatile ligand design element for small clusters.

Intramolecular Arene C–H to C–P Functionalization Mediated by Nickel(II) and Palladium(II)

Co-reporter:Sandy Suseno and Theodor Agapie

Organometallics 2013 Volume 32(Issue 11) pp:3161-3164

Publication Date(Web):May 23, 2013

DOI:10.1021/om400424a

A tris(phosphine) ligand with a triarylbenzene backbone was employed to support mono-nickel(II) and -palladium(II) complexes. Two phosphine arms coordinated to the metal center, while the third phosphine was found to form a C–P bond with dearomatization of the central arene. Deprotonation effected the rearomatization of the central ring and metal reduction from M(II) to M(0). The overall conversion corresponds to a functionalization of an unactivated arene C–H bond to a C–P bond. This transformation represents a rare type of mechanism of C–H functionalization, facilitated by the interactions of the group 10 metal with the arene π system. This conversion is reminiscent of and expands the scope of recently reported intramolecular rearrangements of biaryl phosphine ligands common in group 10 catalysis.

Arene CH Amination at Nickel in TerphenylDiphosphine Complexes with Labile MetalArene Interactions

Co-reporter:Dr. David E. Herbert;Nadia C. Lara ; Theodor Agapie

Chemistry - A European Journal 2013 Volume 19( Issue 48) pp:16453-16460

Publication Date(Web):

DOI:10.1002/chem.201302539

Abstract

The meta-terphenyl diphosphine, m-P₂, 1, was utilized to support Ni centers in the oxidation states 0, I, and II. A series of complexes bearing different substituents or ligands at Ni was prepared to investigate the dependence of metal–arene interactions on oxidation state and substitution at the metal center. Complex (m-P₂)Ni (2) shows strong Ni⁰–arene interactions involving the central arene ring of the terphenyl ligand both in solution and the solid state. These interactions are significantly less pronounced in Ni⁰ complexes bearing L-type ligands (2-L: L=CH₃CN, CO, Ph₂CN₂), Ni^IX complexes (3-X: X=Cl, BF₄, N₃, N₃B(C₆F₅)₃), and [(m-P₂)Ni^IICl₂] (4). Complex 2 reacts with substrates, such as diphenyldiazoalkane, sulfur ylides (Ph₂SCH₂), organoazides (RN₃: R=para-C₆H₄OMe, para-C₆H₄CF₃, 1-adamantyl), and N₂O with the locus of observed reactivity dependent on the nature of the substrate. These reactions led to isolation of an η¹-diphenyldiazoalkane adduct (2-Ph₂CN₂), methylidene insertion into a NiP bond followed by rearrangement of a nickel-bound phosphorus ylide (5) to a benzylphosphine (6), Staudinger oxidation of the phosphine arms, and metal-mediated nitrene insertion into an arene CH bond of 1, all derived from the same compound (2). Hydrogen-atom abstraction from a Ni^I–amide (9) and the resulting nitrene transfer supports the viability of Ni–imide intermediates in the reaction of 1 with 1-azido-arenes.

Oxygen Atom Transfer and Oxidative Water Incorporation in Cuboidal Mn3MOn Complexes Based on Synthetic, Isotopic Labeling, and Computational Studies

Co-reporter:Jacob S. Kanady ; Jose L. Mendoza-Cortes ; Emily Y. Tsui ; Robert J. Nielsen ; William A. Goddard ; III

Journal of the American Chemical Society 2012 Volume 135(Issue 3) pp:1073-1082

Publication Date(Web):December 15, 2012

DOI:10.1021/ja310022p

The oxygen-evolving complex (OEC) of photosystem II contains a Mn4CaOn catalytic site, in which reactivity of bridging oxidos is fundamental to OEC function. We synthesized structurally relevant cuboidal Mn3MOn complexes (M = Mn, Ca, Sc; n = 3,4) to enable mechanistic studies of reactivity and incorporation of μ3-oxido moieties. We found that MnIV3CaO4 and MnIV3ScO4 were unreactive toward trimethylphosphine (PMe3). In contrast, our MnIII2MnIV2O4 cubane reacts with this phosphine within minutes to generate a novel MnIII4O3 partial cubane plus Me3PO. We used quantum mechanics to investigate the reaction paths for oxygen atom transfer to phosphine from MnIII2MnIV2O4 and MnIV3CaO4. We found that the most favorable reaction path leads to partial detachment of the CH3COO– ligand, which is energetically feasible only when Mn(III) is present. Experimentally, the lability of metal-bound acetates is greatest for MnIII2MnIV2O4. These results indicate that even with a strong oxygen atom acceptor, such as PMe3, the oxygen atom transfer chemistry from Mn3MO4 cubanes is controlled by ligand lability, with the MnIV3CaO4 OEC model being unreactive. The oxidative oxide incorporation into the partial cubane, MnIII4O3, was observed experimentally upon treatment with water, base, and oxidizing equivalents. 18O-labeling experiments provided mechanistic insight into the position of incorporation in the partial cubane structure, consistent with mechanisms involving migration of oxide moieties within the cluster but not consistent with selective incorporation at the site available in the starting species. These results support recent proposals for the mechanism of the OEC, involving oxido migration between distinct positions within the cluster.

Nickel-Mediated Hydrogenolysis of C–O Bonds of Aryl Ethers: What Is the Source of the Hydrogen?

Co-reporter:Paul Kelley ; Sibo Lin ; Guy Edouard ; Michael W. Day

Journal of the American Chemical Society 2012 Volume 134(Issue 12) pp:5480-5483

Publication Date(Web):March 6, 2012

DOI:10.1021/ja300326t

Mechanistic studies of the hydrogenolysis of aryl ethers by nickel were undertaken with (diphosphine)aryl methyl ethers. A Ni(0) complex containing Ni–arene interactions adjacent to the aryl–O bond was isolated. Heating led to aryl–O bond activation and generation of a nickel aryl methoxide complex. Formal β-H elimination from this species produced a nickel aryl hydride which can undergo reductive elimination in the presence of formaldehyde to generate a carbon monoxide adduct of Ni(0). The reported complexes map out a plausible mechanism of aryl ether hydrogenolysis catalyzed by nickel. Investigations of a previously reported catalytic system using isotopically labeled substrates are consistent with the mechanism proposed in the stoichiometric system, involving β-H elimination from a nickel alkoxide rather than cleavage of the Ni–O bond by H2.

Bimetallic Effects on Ethylene Polymerization in the Presence of Amines: Inhibition of the Deactivation by Lewis Bases

Co-reporter:Madalyn R. Radlauer ; Michael W. Day

Journal of the American Chemical Society 2012 Volume 134(Issue 3) pp:1478-1481

Publication Date(Web):January 6, 2012

DOI:10.1021/ja210990t

Dinickel complexes supported by terphenyl ligands appended with phenoxy and imine donors were synthesized. Full substitution of the central arene blocks rotation around the aryl–aryl bond and allows for the isolation of atropisomers. The reported complexes perform ethylene polymerization in the presence of amines. The inhibiting effect of polar additives is up to 250 times lower for the syn isomer than the anti isomer. Comparisons with mononuclear systems indicate that the proximity of the metal centers leads to the observed inhibitory effect on the deactivation of the catalysts.

Metallomacrocycles as ligands: synthesis and characterisation of aluminium-bridged bisglyoximato complexes of palladium and iron

Co-reporter:Paul Kelley, Madalyn R. Radlauer, Abraham J. Yanez, Michael W. Day and Theodor Agapie

Dalton Transactions 2012 vol. 41(Issue 26) pp:8086-8092

Publication Date(Web):14 May 2012

DOI:10.1039/C2DT30285C

Dialuminiummacrocycles based on bisglyoximato moieties were prepared and their coordination chemistry with FeII and PdII was investigated. The bridging aluminium centers were supported by several types of tetradentate diphenoxide diamine ligands. The nature of the ancillary ligands bound to aluminium was found to affect the overall geometry and symmetry of the metallomacrocycles. Enantiopure, chiral diphenoxide ligands based on the (R,R)-trans-1,2-diaminocyclohexane backbone afforded cleanly one metallomacrocycle isomer. The size and electronic properties of remote substituents on aluminium-bound ligands affected the binding mode and electronic properties of the central iron. A structurally characterized iron complex shows trigonal prismatic coordination mode, with phenoxide bridges between iron and aluminium. Increasing the size of the phenoxide substituents led to square bipyramidal coordination at iron. Employing p-NO2- instead of p-tBu-substituted phenoxide as supporting ligands for aluminium caused a 0.27 V positive shift of the FeIII/FeII reduction potential. These results indicate that the present synthetic approach can be applied to a variety of metallomacrocycles based on bisglyoximato motifs to affect the chemistry at the central metal.

Nickel Hydrides Supported by a Non-Innocent Diphosphine Arene Pincer: Mechanistic Studies of Nickel−Arene H-Migration and Partial Arene Hydrogenation

Co-reporter:Sibo Lin ; Michael W. Day

Journal of the American Chemical Society 2011 Volume 133(Issue 11) pp:3828-3831

Publication Date(Web):February 23, 2011

DOI:10.1021/ja200368y

Nickel hydrides supported by a terphenyl diphosphine were synthesized and found to undergo nickel-to-arene H-transfers. Some of the resulting complexes also undergo the reverse (C-to-Ni) H-migration, indicating the potential for storing H-equivalents in this type of pincer ligand. NMR spectroscopy, single crystal X-ray diffraction, and isotopic labeling studies investigating the mechanism of these processes are discussed.

Gating NO Release from Nitric Oxide Synthase

Co-reporter:Charlotte A. Whited ; Jeffrey J. Warren ; Katherine D. Lavoie ; Emily E. Weinert ; Theodor Agapie ; Jay R. Winkler ;Harry B. Gray

Journal of the American Chemical Society 2011 Volume 134(Issue 1) pp:27-30

Publication Date(Web):December 7, 2011

DOI:10.1021/ja2069533

We have investigated the kinetics of NO escape from Geobacillus stearothermophilus nitric oxide synthase (gsNOS). Previous work indicated that NO release was gated at position 223 in mammalian enzymes; our kinetics experiments include mutants at that position along with measurements on the wild type enzyme. Employing stopped-flow UV–vis methods, reactions were triggered by mixing a reduced enzyme/N-hydroxy-l-arginine complex with an aerated buffer solution. NO release kinetics were obtained for wt NOS and three mutants (H134S, I223V, H134S/I223V). We have confirmed that wt gsNOS has the lowest NO release rate of known NOS enzymes, whether bacterial or mammalian. We also have found that steric clashes at positions 223 and 134 hinder NO escape, as judged by enhanced rates in the single mutants. The empirical rate of NO release from the gsNOS double mutant (H134/I223V) is nearly as rapid as that of the fastest mammalian enzymes, demonstrating that both positions 223 and 134 function as gates for escape of the product diatomic molecule.

Trinuclear first row transition metal complexes of a hexapyridyl, trialkoxy 1,3,5-triarylbenzene ligand

Co-reporter:Emily Y. Tsui, Jacob S. Kanady, Michael W. Day and Theodor Agapie

Chemical Communications 2011 vol. 47(Issue 14) pp:4189-4191

Publication Date(Web):02 Mar 2011

DOI:10.1039/C0CC05608A

Trinuclear complexes of MnII, FeII, CoII, NiII, CuII, and ZnII were synthesized using a ligand architecture based upon a 1,3,5-triarylbenzene core decorated with six pyridines and three alkoxide moieties. Characterization viaX-ray diffraction, NMR, and magnetism studies is discussed.

Reversible Halide-Modulated NickelNickel Bond Cleavage: MetalMetal Bonds as Design Elements for Molecular Devices

Co-reporter:Steven T. Chao;Nadia C. Lara;Sibo Lin;Michael W. Day ; Theodor Agapie

Angewandte Chemie International Edition 2011 Volume 50( Issue 33) pp:7529-7532

Publication Date(Web):

DOI:10.1002/anie.201102797

Trinucleating Copper: Synthesis and Magnetostructural Characterization of Complexes Supported by a Hexapyridyl 1,3,5-Triarylbenzene Ligand

Co-reporter:Emily Y. Tsui;Dr. Michael W. Day ; Theodor Agapie

Angewandte Chemie International Edition 2011 Volume 50( Issue 7) pp:1668-1672

Publication Date(Web):

DOI:10.1002/anie.201005232

Reversible Halide-Modulated NickelNickel Bond Cleavage: MetalMetal Bonds as Design Elements for Molecular Devices

Co-reporter:Steven T. Chao;Nadia C. Lara;Sibo Lin;Michael W. Day ; Theodor Agapie

Angewandte Chemie 2011 Volume 123( Issue 33) pp:7671-7674

Publication Date(Web):

DOI:10.1002/ange.201102797

Trinucleating Copper: Synthesis and Magnetostructural Characterization of Complexes Supported by a Hexapyridyl 1,3,5-Triarylbenzene Ligand

Co-reporter:Emily Y. Tsui;Dr. Michael W. Day ; Theodor Agapie

Angewandte Chemie 2011 Volume 123( Issue 7) pp:1706-1710

Publication Date(Web):

DOI:10.1002/ange.201005232

A Synthetic Model of the Mn3Ca Subsite of the Oxygen-Evolving Complex in Photosystem II

Co-reporter:Jacob S. Kanady;Emily Y. Tsui;Michael W. Day

Science 2011 Volume 333(Issue 6043) pp:733-736

Publication Date(Web):05 Aug 2011

DOI:10.1126/science.1206036

A model compound sheds light on the puzzling role of calcium in the metal cluster that oxidizes water during photosynthesis.

Synthesis and C−C Coupling Reactivity of a Dinuclear NiI−NiI Complex Supported by a Terphenyl Diphosphine

Co-reporter:Alexandra Velian ; Sibo Lin ; Alexander J. M. Miller ; Michael W. Day

Journal of the American Chemical Society 2010 Volume 132(Issue 18) pp:6296-6297

Publication Date(Web):April 16, 2010

DOI:10.1021/ja101699a

Mono- and bimetallic complexes of nickel supported by a terphenyl diphosphine have been synthesized. The reported complexes show diverse metal−arene interactions in the solid state. Reactions of an o,o′-biphenyldiyl dinickel complex with CO and dichloroalkanes lead to fluorene derivatives, indicating the formation of carbon−carbon bonds at a bimetallic moiety.

A CaMn4O2 model of the biological oxygen evolving complex: synthesis via cluster expansion on a low symmetry ligand

Co-reporter:Heui Beom Lee, Emily Y. Tsui and Theodor Agapie

Chemical Communications 2017 - vol. 53(Issue 51) pp:NaN6835-6835

Publication Date(Web):2017/04/25

DOI:10.1039/C7CC02313H

Arene non-innocence in dinuclear complexes of Fe, Co, and Ni supported by a para-terphenyl diphosphine

Co-reporter:Kyle T. Horak, Alexandra Velian, Michael W. Day and Theodor Agapie

Chemical Communications 2014 - vol. 50(Issue 34) pp:NaN4429-4429

Publication Date(Web):2014/03/03

DOI:10.1039/C4CC00838C

Trinuclear first row transition metal complexes of a hexapyridyl, trialkoxy 1,3,5-triarylbenzene ligand

Co-reporter:Emily Y. Tsui, Jacob S. Kanady, Michael W. Day and Theodor Agapie