Phosphinite-Ni(0) mediated formation of a phosphide-Ni(II)-OCOOMe species via uncommon metal-ligand cooperation

Small Molecule Activation at the acriPNP Pincer-Supported Nickel Sites

ConspectusNickel pincer systems have recently attracted much attention for applications in various organometallic reactions and catalysis involving small molecule activation. Their exploration is in part motivated by the presence of nickel in natural systems for efficient catalysis. Among such systems, the nickel-containing metalloenzyme carbon monoxide dehydrogenase (CODH) efficiently and reversibly converts CO2 to CO at its active site. The generated CO moves through a channel from the CODH active site and is transported to a dinuclear nickel site of acetyl-coenzyme A synthase (ACS), which catalyzes organometallic C-S and C-C bond forming reactions. An analogous C-S bond activation process is also mediated by the nickel containing enzyme methyl-coenzyme M reductase (MCR). The nickel centers in these systems feature sulfur- and nitrogen-rich environments, and in the particular case of lactate racemase, an organometallic nickel pincer motif revealing a Ni-C bond is observed. These bioinorganic systems inspired the development of several nickel pincer scaffolds not only to mimic enzyme active sites and their reactivity but also to further extend low-valent organonickel chemistry. In this Account, we detail our continuing efforts in the chemistry of nickel complexes supported by acridane-based PNP pincer ligands focusing on our long-standing interest in biomimetic small molecule activation. We have employed a series of diphosphinoamide pincer ligands to prepare various nickel(II/I/0) complexes and to study the conversion of C1 chemicals such as CO and CO2 to value-added products. In the transformation of C1 chemicals, the key C-O bond cleavage and C-E bond (E = C, N, O, or S, etc.) formation steps typically require overcoming high activation barriers. Interestingly, enzymatic systems overcome such difficulties for C1 conversion and operate efficiently under ambient conditions with the use of nickel organometallic chemistry. Furthermore, we have extended our efforts to the conversion of NOx anions to NO via the sequential deoxygenation by nickel mediated carbonylation, which was applied to catalytic C-N coupling to produce industrially important organonitrogen compound oximes as a strategy for NOx conversion and utilization (NCU). Notably, the rigidified acriPNP pincer backbone that enforces a planar geometry at nickel was found to be an important factor for diversifying organometallic transformations including (a) homolysis of various σ-bonds mediated by T-shaped nickel(I) metalloradical species, (b) C-H bond activation mediated by a nickel(0) dinitrogen species, (c) selective CO2 reactivity of nickel(0)-CO species, (d) C-C bond formation at low-valent nickel(I or 0)-CO sites with iodoalkanes, and (e) catalytic deoxygenation of NOx anions and subsequent C-N coupling of a nickel-NO species with alkyl halides for oxime production. Broadly, our results highlight the importance of molecular design and the rich chemistry of organonickel species for diverse small molecule transformations.

Isolation of Cationic η3-Allenylnickel(II) Key Intermediate Complexes: Origins of Enantioselectivity and Regioselectivity in Nickel(0)-Catalyzed Asymmetric Propargylic Substitutions

Herein, we report the synthesis and isolation of cationic η3-allenylnickel(II) complexes that bear rac-BINAP as a bidentate ligand for the first time via Me3SiOTf-promoted C-O bond cleavage of propargylic tert-butyl carbonate. In contrast, in the presence of the monodentate phosphine ligand PEt3, treatment of propargylic tert-butyl carbonate with Ni(cod)2 resulted in a gradual C-O bond cleavage leading to η1-allenylnickel(II) complexes, i.e., trans-(PEt3)2Ni(η1-CPh═C═CHR)(OBoc). X-ray diffraction and NMR spectroscopy studies of [(η3-RCH-CCPh)Ni(rac-BINAP)](OTf) revealed that the complex adopts an η3-allenyl coordination mode both in the crystal lattice and in solution. A thorough structural comparison between [(η3-RCH-CCPh)Ni(rac-BINAP)](OTf) and palladium and platinum analogues revealed that the η3-allenyl moiety in the nickel complex is similar to that observed in palladium and platinum complexes, albeit that each Ni-C bond is shorter than the corresponding Pd-C and Pt-C bonds due to the smaller ionic radius of nickel to that of Pd or Pt. The reactions of either N-methylaniline or sodium N-methylanilide with [(η3-RCH-CCPh)Ni((R)-BINAP)](OTf) furnished (R)-PhC≡CCH(NMePh)Me as an asymmetric propargylic substitution (APS) product with excellent enantioselectivity. Furthermore, when the nickel-catalyzed APS reaction of propargylic tert-butyl carbonate with N-methylaniline was conducted in DMSO at 60 °C in the presence of 5 mol % of [(η3-RCH-CCPh)Ni((R)-BINAP)](OTf) and 7.5 mol % of sodium N-methylanilide as a catalytic precursor and an additive, respectively, (R)-PhC≡CCH(NMePh)Me was obtained in 79% yield with 90% ee. The experimental results and computational calculations strongly suggest that the nickel-catalyzed APS reaction might proceed via a cationic η3-allenylnickel(II) species as the key reaction intermediate.

Cleavage of Carbon Dioxide C=O Bond Promoted by Nickel-Boron Cooperativity in a PBP-Ni Complex

The synthesis and characterization of (tBu PBP)Ni(OAc) (5) by insertion of carbon dioxide into the Ni-C bond of (tBu PBP)NiMe (1) is presented. An unexpected CO2 cleavage process involving the formation of new B-O and Ni-CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster (tBu PBOP)2 Ni4 (μ-CO)2 (6). Mechanistic investigation of this reaction indicates a reductive scission of CO2 by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO2 activation reaction produces a three-coordinate (tBu P2 BO)Ni-acyl intermediate (A) that leads to a (tBu P2 BO)-NiI complex (B) via a likely radical pathway. The NiI species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give (tBu P2 BO)NiII (η2 -TEMPO) (7). Additionally, 13 C and 1 H NMR spectroscopy analysis using 13 C-enriched CO2 provides information about the species involved in the CO2 activation process.

Electronic Effect on Phenoxide Migration at a Nickel(II) Center Supported by a Tridentate Bis(phosphinophenyl)phosphido Ligand

A phosphide nickel(II) phenoxide pincer complex (2) reacts with CO(g) to give a pseudo-tetrahedral nickel(0) monocarbonyl complex (3) possessing a phosphinite moiety. This metal-ligand cooperative (MLC) transformation occurs with a (PPP)Ni scaffold (PPP^- = P[2-PⁱPr₂-C₆H₄]₂^-), which can accommodate both square planar and tetrahedral geometries. The 2-electron reduction of a nickel(II) species induced by CO coordination involves group transfer to generate a P-O bond. For better mechanistic understanding, a series of nickel(II) phenolate complexes (2a-2e, XC₆H₄O^- (X = OMe, Me, H, and CF₃) and pentafluorophenolate) were prepared. Kinetic experimental data reveal that a phenolate species with an electron-withdrawing group reacts faster than those with electron-donating groups. The reaction kinetic experiments were conducted in pseudo-first order conditions at room temperature monitored by UV-vis spectroscopy. A pentafluorophenolate nickel(II) complex (2e) reveals instantaneous reactions even at -40 °C to give a nickel(0) monocarbonyl species (3e) and the reverse reaction is also possible. According to kinetic experiments, the rate determining step (RDS) would be the formation of a 5-coordinate intermediate 4 with a negative entropy value (ΔS^‡ < 0), and a positive ρ value based on the Hammett plot indicates that the electron-deficient phenolate leads to a faster CO association. Furthermore, scramble experiments suggest that phenolate de-coordinates from the intermediate 4, which gives a (PPP)Ni-CO species 6. The cationic nickel monocarbonyl intermediate can possess a P^--Ni(II), P•-Ni(I), or even a P⁺-Ni(0) character. Such an inner-sphere electron transfer is suggested when a π-acidic ligand such as CO coordinates to a metal ion. Another possible reaction is homolysis of a Ni-O bond to give P^--Ni(I) or P•-Ni(0), when a phenoxyl radical is liberated. Considering the P-O bond formation, closed-shell nucleophilic and open-shell radical pathways are suggested. A phenolate pathway reveals a lower energy state for 2e relative to other complexes (2c and 2d), while its radical pathway undergoes via a higher energy state. Therefore, the formation of a P-O bond may occur with the binding of a closed-shell phenolate to the electron-deficient P center.

Oxidative Addition of a Phosphinite P-O Bond at Nickel

Oxidative addition is an essential elementary reaction in organometallic chemistry and catalysis. While a diverse array of oxidative addition reactions has been reported to date, examples of P-O bond activation are surprisingly rare. Herein, we report the ligand-templated oxidative addition of a phosphinite P-O bond in the diphosphinito aniline compound HN(2-OPⁱPr₂-3,5-^tBu-C₆H₂)₂ [H(P₂ONO)] at Ni⁰ to form (PONO)Ni(HPⁱPr₂) after proton rearrangement. Notably, the P-O cleavage occurs selectively over an amine N-H bond activation. Additionally, the ligand cannibalization is reversible, as addition of XPR₂ (X = Cl, Br; R = ⁱPr, Cy) to (PONO)Ni(HPⁱPr₂) readily produces either symmetric or unsymmetric (P₂ONO)NiX species and free HPⁱPr₂. Finally, the mechanisms of both the initial P-O bond cleavage and its subsequent reconstruction are investigated to provide further insight into how to target P-O bond activation.

Synthesis and reactivity of a μ-1,2-dinitrogen dinickel(II) complex with a C-H activated silaamidinate pincer ligand

Reaction of the silaamidinate nickel bromide LSi(NAr)₂NiBr₂Li(thf)(OEt₂) (L = PhC(NtBu)₂, Ar = 2,6-iPr₂C₆H₃, 1) with NaHBEt₃ led to intramolecular C-H activation with the formation of the μ-1,2-dinitrogen dinickel pincer complex [LSi(NAr)(NAr)Ni]₂(μ-1,2-N₂) (Ar = 2-C(CH₃)₂-6-iPrC₆H₃, 2). Single-crystal X-ray diffraction analysis of 2 disclosed a square planar Ni(II) atom bridged by N₂. Reaction of 2 with carbon monoxide and 2,6-dimethylphenyl isocyanide yielded square planar carbonyl and isocyanide complexes 3 and 4 with release of N₂. These results provide new approaches for the coordination of N₂ with nickel(II) species.

Two-Site O-H Addition to an Iridium Complex Featuring a Nonspectator Tricoordinate Phosphorus Ligand

The synthesis and reactivity of an ambiphilic iridium complex IrCl(PPh3)(L1) (1; L1 = P(N(o-N(2-pyridyl)C6H4)2)) featuring a chelating nontrigonal phosphorus triamide ligand is reported. The tandem Lewis basic Ir and Lewis acidic P of 1 achieve a two-site oxidative addition of phenol giving the iridaphenoxyphosphorane species IrHCl(PPh3)(L1OPh) (3'). In contrast, reactions of 1 with benzenethiol and benzeneselenol do not engage L1 and instead proceed via metal-centered oxidative addition of the chalcogen-hydrogen bond. These findings establish metal-ligand cooperation involving nonspectator reactivity of tricoordinate phosphorus ligands.

Ligand and solvent effects on CO2 insertion into group 10 metal alkyl bonds

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (^RPBP)M(CH₃) (^RPBP = B(NCH₂PR₂)₂C₆H₄⁻; R = Cy or ^tBu; M = Ni or Pd) to generate κ¹-acetate complexes of the form (^RPBP)M{OC(O)CH₃} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual ^RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (^RPBP)M(CH₃) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using (^tBuPBP)Pd(CH₃). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd) proceeds via an S_E2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.

The kinetics of carbon dioxide insertion into a pincer-supported palladium methyl complex are studied. The complex inserts carbon dioxide at room temperature, and we explore both solvent and Lewis acid effects on carbon dioxide insertion.

Ni(0)-promoted activation of Csp2 -H and Csp2 -O bonds

A dinickel(0)-N2 complex, stabilized with a rigid acridane-based PNP pincer ligand, was studied for its ability to activate C(sp2)-H and C(sp2)-O bonds. Stabilized by a Ni-μ-N2-Na+ interaction, it activates C-H bonds of unfunctionalized arenes, affording nickel-aryl and nickel-hydride products. Concomitantly, two sodium cations get reduced to Na(0), which was identified and quantified by several methods. Our experimental results, including product analysis and kinetic measurements, strongly suggest that this C(sp2)-H activation does not follow the typical oxidative addition mechanism occurring at a low-valent single metal centre. Instead, via a bimolecular pathway, two powerfully reducing nickel ions cooperatively activate an arene C-H bond and concomitantly reduce two Lewis acidic alkali metals under ambient conditions. As a novel synthetic protocol, nickel(ii)-aryl species were directly synthesized from nickel(ii) precursors in benzene or toluene with excess Na under ambient conditions. Furthermore, when the dinickel(0)-N2 complex is accessed via reduction of the nickel(ii)-phenyl species, the resulting phenyl anion deprotonates a C-H bond of glyme or 15-crown-5 leading to C-O bond cleavage, which produces vinyl ether. The dinickel(0)-N2 species then cleaves the C(sp2)-O bond of vinyl ether to produce a nickel(ii)-vinyl complex. These results may provide a new strategy for the activation of C-H and C-O bonds mediated by a low valent nickel ion supported by a structurally rigidified ligand scaffold.

Enthalpy-Controlled Insertion of a "Nonspectator" Tricoordinate Phosphorus Ligand into Group 10 Transition Metal-Carbon Bonds

Insertion of a tricoordinate phosphorus ligand into late metal-carbon bonds is reported. Metalation of a P^P-chelating ligand (L1), composed of a nontrigonal phosphorous (i.e., P(III)) triamide moiety, P(N(o-N(Ar)C6H4)2, tethered by a phenylene linker to a -PiPr2 anchor, with group 10 complexes L2M(Me)Cl (M = Ni, Pd) results in insertion of the nontrigonal phosphorus site into the metal-methyl bond. The stable methylmetallophosphorane compounds thus formed are characterized spectroscopically and crystallographically. Metalation of L1 with (cod)PtII(Me)(Cl) does not lead to a metallophosphorane but rather to the standard bisphosphine chelate (κ2-L1)Pt(Me)(Cl). These divergent reactivities within group 10 are rationalized by reference to periodic variation in M-C bond enthalpies.

Activation of Dinitrogen by Polynuclear Metal Complexes

Activation of dinitrogen plays an important role in daily anthropogenic life, and the processes by which this fixation occurs have been a longstanding and significant research focus within the community. One of the major fields of dinitrogen activation research is the use of multimetallic compounds to reduce and/or activate N2 into a more useful nitrogen-atom source, such as ammonia. Here we report a comprehensive review of multimetallic-dinitrogen complexes and their utility toward N2 activation, beginning with the d-block metals from Group 4 to Group 11, then extending to Group 13 (which is exclusively populated by B complexes), and finally the rare-earth and actinide species. The review considers all polynuclear metal aggregates containing two or more metal centers in which dinitrogen is coordinated or activated (i.e., partial or complete cleavage of the N2 triple bond in the observed product). Our survey includes complexes in which mononuclear N2 complexes are used as building blocks to generate homo- or heteromultimetallic dinitrogen species, which allow one to evaluate the potential of heterometallic species for dinitrogen activation. We highlight some of the common trends throughout the periodic table, such as the differences between coordination modes as it relates to N2 activation and potential functionalization and the effect of polarizing the bridging N2 ligand by employing different metal ions of differing Lewis acidities. By providing this comprehensive treatment of polynuclear metal dinitrogen species, this Review aims to outline the past and provide potential future directions for continued research in this area.

Metal–ligand cooperativity of a Co–P moiety

Metal–ligand cooperative redox reactions and intramolecular group transfer of a P–P containing dicobalt(i) species are shown.

A Nontrigonal Tricoordinate Phosphorus Ligand Exhibiting Reversible "Nonspectator" L/X-Switching

We report here a "nonspectator" behavior for an unsupported L-function σ³ -P ligand (i.e. P{N[o-NMe-C₆ H₄ ]₂ }, 1a) in complex with the cyclopentadienyliron dicarbonyl cation (Fp⁺ ). Treatment of 1a⋅Fp⁺ with [(Me₂ N)₃ S][Me₃ SiF₂ ] results in fluoride addition to the P-center, giving the isolable crystalline fluorometallophosphorane 1a^F ⋅Fp that allows a crystallographic assessment of the variance in the Fe-P bond as a function of P-coordination number. The nonspectator reactivity of 1a⋅Fp⁺ is rationalized on the basis of electronic structure arguments and by comparison to trigonal analogue (Me₂ N)₃ P⋅Fp⁺ (i.e. 1b⋅Fp⁺ ), which is inert to fluoride addition. These observations establish a nonspectator L/X-switching in (σ³ -P)-M complexes by reversible access to higher-coordinate phosphorus ligand fragments.

Monomeric alkoxide and alkylcarbonate complexes of nickel and palladium stabilized with the iPrPCP pincer ligand: a model for the catalytic carboxylation of alcohols to alkyl carbonates

Monomeric alkoxo complexes of the type [(iPrPCP)M-OR] (M = Ni or Pd; R = Me, Et, CH2CH2OH; iPrPCP = 2,6-bis(diisopropylphosphino)phenyl) react rapidly with CO2 to afford the corresponding alkylcarbonates [(iPrPCP)M-OCOOR]. We have investigated the reactions of these compounds as models for key steps of catalytic synthesis of organic carbonates from alcohols and CO2. The MOCO-OR linkage is kinetically labile, and readily exchanges the OR group with water or other alcohols (R'OH), to afford equilibrium mixtures containing ROH and [(iPrPCP)M-OCOOH] (bicarbonate) or [(iPrPCP)M-OCOOR'], respectively. However, [(iPrPCP)M-OCOOR] complexes are thermally stable and remain indefinitely stable in solution when these are kept in sealed vessels. The constants for the exchange equilibria have been interpreted, showing that CO2 insertion into M-O bonds is thermodynamically more favorable for M-OR than for M-OH. Alkylcarbonate complexes [(iPrPCP)M-OCOOR] fail to undergo nucleophilic attack by ROH to yield organic carbonates ROCOOR, either intermolecularly (using neat ROH solvent) or in intramolecular fashion (e.g., [(iPrPCP)M-OCOOCH2CH2OH]). In contrast, [(iPrPCP)M-OCOOMe] complexes react with a variety of electrophilic methylating reagents (MeX) to afford dimethylcarbonate and [(iPrPCP)M-X]. The reaction rates increase in the order X = OTs < IMe ≪ OTf and Ni < Pd. These findings suggest that a suitable catalyst design should combine basic and electrophilic alcohol activation sites in order to perform alkyl carbonate syntheses via direct alcohol carboxylation.

Cooperative activation of O–H and S–H bonds across the Co–P bond of an N-heterocyclic phosphido complex

A cobalt N-heterocyclic phosphido complex is shown to cleave element–hydrogen bonds via a metal–phosphorus ligand cooperative pathway.

Insertion of a Nontrigonal Phosphorus Ligand into a Transition Metal-Hydride: Direct Access to a Metallohydrophosphorane

The synthesis and reactivity of an NPN-chelating ligand containing a nontrigonal phosphorous triamide center (L1 = P(N( o-N(2-pyridyl)C₆H₄)₂) is reported. Metalation of L1 with RuCl₂(PPh₃)₃ gives RuCl₂(PPh₃)(L1) (2). By contrast, metalation of L1 with RuHCl(CO)(PPh₃)₃ yields RuCl(CO)(PPh₃)(L1^H) (3), a chelated 10-P-5 ruthenahydridophosphorane, via net insertion into the Ru-H bond. Hydride abstraction from 3 with Ph₃CPF₆ gives [RuCl(CO)(PPh₃)(L1)]PF₆ (4); reaction of 4 with NaBH₄ returns 3.

Selective Transformation of CO2 to CO at a Single Nickel Center

Carbon dioxide conversion mediated by transition metal complexes continues to attract much attention because of its future potential utilization as a nontoxic and inexpensive C1 source for the chemical industry. Given the presence of nickel in natural systems that allow for extremely efficient catalysis, albeit in an Fe cluster arrangement, studies that focus on selective CO₂ conversion with synthetic nickel species are currently of considerable interest in our group. In this Account, the selective conversion of CO₂ to carbon monoxide occurring at a single nickel center is discussed. The chemistry is based on a series of related nickel pincer complexes with attention to the uniqueness of the coordination geometry, which is crucial in allowing for particular reactivity toward CO₂. Our research is inspired by the efficient enzymatic CO₂ catalysis occurring at the active site of carbon monoxide dehydrogenase. Since the binding and reactivity toward CO₂ are controlled in part by the geometry of a L₃Ni scaffold, we have explored the chemistry of low-valent nickel supported by PP^MeP and PNP ligands, in which a pseudotetrahedral or square-planar geometry is accommodated. Two isolated nickel-CO₂ adducts, (PP^MeP)Ni(η²-CO₂-κ C) (2) and {Na(12-C-4)₂}{(PNP)Ni(η¹-CO₂-κ C)} (7), clearly demonstrate that the geometry of the nickel ion is crucial in the binding of CO₂ and its level of activation. In the case of a square-planar nickel center supported by a PNP ligand, a series of bimetallic metallacarboxylate Ni-μ-CO₂-κ C, O-M species (M = H, Na, Ni, Fe) were synthesized, and their structural features and reactivity were studied. Protonation cleaves the C-O bond, resulting in the formation of a nickel(II) monocarbonyl complex. By sequential reduction, the corresponding mono- and zero-valent Ni-CO species were produced. The reactivities of three nickel carbonyl species toward various iodoalkanes and CO₂ were explored to address whether their corresponding reactivities could be controlled by the number of valence d electrons. In particular, a (PNP)Ni(0)-CO species (13) shows immediate reactivity toward CO₂ but displays multiple product formation. By incorporation of a -CMe₂- bridging unit, a structurally rigidified ^acriPNP ligand was newly designed and produced. This ligand modification was successful in preparing the T-shaped nickel(I) metalloradical species 9 exhibiting open-shell reactivity due to the sterically exposed nickel center possessing a half-filled d _{x²- y²} orbital. More importantly, the selective addition of CO₂ to a nickel(0)-CO species was enabled to afford a nickel(II)-carboxylate species (22) with the expulsion of CO(g). Finally, the (^acriPNP)Ni system provides a synthetic cycle in the study of the selective conversion of CO₂ to CO that involves two-electron reduction of Ni-CO followed by the direct addition of CO₂ to release the coordinated CO ligand.

Synthesis and characterization of rhodium, iridium, and palladium complexes of a diarylamido-based PNSb pincer ligand

A new diarylmido-based pincer proto ligand (iPrPNHSbPh) with one –PPrⁱ₂ and one –SbPh₂ side donor has been synthesized.

[NiIII(OMe)]-mediated reductive activation of CO2 affording a Ni(κ1-OCO) complex

We report a novel pathway for the reductive activation of CO₂ by the [Ni^III(OMe)(P(C₆H₃-3-SiMe₃-2-S)₃)]^– complex, yielding the [Ni^III(κ¹-OCO˙^–)(P(C₆H₃-3-SiMe₃-2-S)₃)]^– complex.

Carbon dioxide is expected to be employed as an inexpensive and potential feedstock of C₁ sources for the mass production of valuable chemicals and fuel. Versatile chemical transformations of CO₂, i.e. insertion of CO₂ producing bicarbonate/acetate/formate, cleavage of CO₂ yielding μ-CO/μ-oxo transition-metal complexes, and electrocatalytic reduction of CO₂ affording CO/HCOOH/CH₃OH/CH₄/C₂H₄/oxalate were well documented. Herein, we report a novel pathway for the reductive activation of CO₂ by the [Ni^III(OMe)(P(C₆H₃-3-SiMe₃-2-S)₃)]^– complex, yielding the [Ni^III(κ¹-OCO˙^–)(P(C₆H₃-3-SiMe₃-2-S)₃)]^– complex. The formation of this unusual Ni^III(κ¹-OCO˙^–) complex was characterized by single-crystal X-ray diffraction, EPR, IR, SQUID, Ni/S K-edge X-ray absorption spectroscopy, and Ni valence-to-core X-ray emission spectroscopy. The inertness of the analogous complexes [Ni^III(SPh)], [Ni^II(CO)], and [Ni^II(N₂H₄)] toward CO₂, in contrast, demonstrates that the ionic [Ni^III(OMe)] core attracts the binding of weak σ-donor CO₂ and triggers the subsequent reduction of CO₂ by the nucleophilic [OMe]^– in the immediate vicinity. This metal–ligand cooperative activation of CO₂ may open a novel pathway promoting the subsequent incorporation of CO₂ in the buildup of functionalized products.

Facile Insertion of Rh and Ir into a Boron-Phenyl Bond, Leading to Boryl/Bis(phosphine) PBP Pincer Complexes

The unexpectedly facile insertion of Rh or Ir into a B-Ph bond (reversible for Rh) converts a borane/bis(phosphine) precursor into a boryl/bis(phosphine) PBP pincer ligand. Interconversions between the boryl/borane/borate central functionality are demonstrated in reactions with dihydrogen.