Three-Coordinate Nickel and Metal-Metal Interactions in a Heterometallic Iron-Sulfur Cluster

Biological multielectron reactions often are performed by metalloenzymes with heterometallic sites, such as anaerobic carbon monoxide dehydrogenase (CODH), which has a nickel-iron-sulfide cubane with a possible three-coordinate nickel site. Here, we isolate the first synthetic iron-sulfur clusters having a nickel atom with only three donors, showing that this structural feature is feasible. These have a core with two tetrahedral irons, one octahedral tungsten, and a three-coordinate nickel connected by sulfide and thiolate bridges. Electron paramagnetic resonance (EPR), Mössbauer, and superconducting quantum interference device (SQUID) data are combined with density functional theory (DFT) computations to show how the electronic structure of the cluster arises from strong magnetic coupling between the Ni, Fe, and W sites. X-ray absorption spectroscopy, together with spectroscopically validated DFT analysis, suggests that the electronic structure can be described with a formal Ni1+ atom participating in a nonpolar Ni-W σ-bond. This metal-metal bond, which minimizes spin density at Ni1+, is conserved in two cluster oxidation states. Fe-W bonding is found in all clusters, in one case stabilizing a local non-Hund state at tungsten. Based on these results, we compare different M-M interactions and speculate that other heterometallic clusters, including metalloenzyme active sites, could likewise store redox equivalents and stabilize low-valent metal centers through metal-metal bonding.

Synthesis of Nickel(I)-Bromide Complexes via Oxidation and Ligand Displacement: Evaluation of Ligand Effects on Speciation and Reactivity

Nickel's +1 oxidation state has received much interest due to its varied and often enigmatic behavior in increasingly popular catalytic methods. In part, the lack of understanding about NiI results from common synthetic strategies limiting the breadth of complexes that are accessible for mechanistic study and catalyst design. We report an oxidative approach using tribromide salts that allows for the generation of a well-defined precursor, [NiI(COD)Br]2, as well as several new NiI complexes. Included among them are complexes bearing bulky monophosphines, for which structure-speciation relationships are established and catalytic reactivity in a Suzuki-Miyaura coupling (SMC) is investigated. Notably, these routes also allow for the synthesis of well-defined monomeric t-Bubpy-bound NiI complexes, which has not previously been achieved. These complexes, which react with aryl halides, can enable previously challenging mechanistic investigations and present new opportunities for catalysis and synthesis.

Machine Learning-Guided Development of Trialkylphosphine Ni(I) Dimers and Applications in Site-Selective Catalysis

Owing to the unknown correlation of a metal's ligand and its resulting preferred speciation in terms of oxidation state, geometry, and nuclearity, a rational design of multinuclear catalysts remains challenging. With the goal to accelerate the identification of suitable ligands that form trialkylphosphine-derived dihalogen-bridged Ni^(I) dimers, we herein employed an assumption-based machine learning approach. The workflow offers guidance in ligand space for a desired speciation without (or only minimal) prior experimental data points. We experimentally verified the predictions and synthesized numerous novel Ni^(I) dimers as well as explored their potential in catalysis. We demonstrate C-I selective arylations of polyhalogenated arenes bearing competing C-Br and C-Cl sites in under 5 min at room temperature using 0.2 mol % of the newly developed dimer, [Ni^(I)(μ-Br)PAd₂(n-Bu)]₂, which is so far unmet with alternative dinuclear or mononuclear Ni or Pd catalysts.

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.

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(NHC)2 ] as a Scaffold for Structurally Characterized trans [H-Ni-PR2 ] and trans [R2 P-Ni-PR2 ] Complexes

The addition of PPh2 H, PPhMeH, PPhH2 , P(para-Tol)H2 , PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2 ] (NHC=IiPr2 , IMe4 , IEt2 Me2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H-Ni-PR2 ] or novel trans [R2 P-Ni-PR2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4 >IEt2 Me2 >IiPr2 ) and phosphines are employed. P-P activation of the diphosphines R2 P-PR2 (R2 =Ph2 , PhMe) provides an alternative route to some of the [Ni(NHC)2 (PR2 )2 ] complexes. DFT calculations capture these trends with P-H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni-P bond. P-P bond activation from [Ni(NHC)2 (Ph2 P-PPh2 )] adducts proceeds with computed barriers below 10 kcal mol-1 . The ability of the [Ni(NHC)2 ] moiety to afford isolable terminal phosphido products reflects the stability of the Ni-NHC bond that prevents ligand dissociation and onward reaction.

One to Find Them All: A General Route to Ni(I)-Phenolate Species

The past 20 years have seen an extensive implementation of nickel in homogeneous catalysis through the development of unique reactivity not easily achievable by using noble transition metals. Many catalytic cycles propose Ni(I) complexes as potential reactive intermediates, yet the scarcity of nickel(I) precursors and the lack of a general, non-ligand-specific protocol for their synthesis have hampered progress in this field of research. This has in turn also limited the access to novel, well-defined Ni(I) species for the development of new catalytic reactions. Herein, we report a simple, general route to access a wide variety of Ni(I)-phenolate complexes via an unusual example of an olefinic Ni(I) complex, [Ni(COD)(OPh*)] (COD = 1,5-cyclooctadiene, OPh* = O(^tBu)₃C₆H₂). This route has proven to be highly efficient for several coordination numbers and ligand classes enabling access to the following complexes: [Ni(IPr)(OPh*)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), [Ni(dcype)(OPh*)] (dcype = 1,2-bis(dicyclohexylphosphino)ethane), [Ni(dppe)(OPh*)] (dppe = 1,2-bis(diphenylphosphino)ethane), and [Ni(terpy)(OPh*)] (terpy = 2,2':6',2″-terpyridine). Moreover, reacting [Ni(dcype)(OPh*)] with trimethylsilyl triflate has led to the isolation of a unique example of a cationic binuclear Ni(I)-arene complex. All these complexes have been characterized by single-crystal X-ray, DFT, and EPR analyses, thus providing crucial experimental and theoretical information about their coordination environment and confirming a d⁹ electronic structure for all complexes involved. Overall, this new synthetic approach offers exciting opportunities for the discovery of new stoichiometric and catalytic reactivity as well as the mechanistic elucidation of Ni-based catalytic cycles.

Boosting homogeneous chemoselective hydrogenation of olefins mediated by a bis(silylenyl)terphenyl-nickel(0) pre-catalyst

The isolable chelating bis(N-heterocyclic silylenyl)-substituted terphenyl ligand [SiII(Terp)SiII] as well as its bis(phosphine) analogue [PIII(Terp)PIII] have been synthesised and fully characterised. Their reaction with Ni(cod)2 (cod = cycloocta-1,5-diene) affords the corresponding 16 VE nickel(0) complexes with an intramolecular η 2-arene coordination of Ni, [E(Terp)E]Ni(η 2-arene) (E = PIII, SiII; arene = phenylene spacer). Due to a strong cooperativity of the Si and Ni sites in H2 activation and H atom transfer, [SiII(Terp)SiII]Ni(η 2-arene) mediates very effectively and chemoselectively the homogeneously catalysed hydrogenation of olefins bearing functional groups at 1 bar H2 pressure and room temperature; in contrast, the bis(phosphine) analogous complex shows only poor activity. Catalytic and stoichiometric experiments revealed the important role of the η2-coordination of the Ni(0) site by the intramolecular phenylene with respect to the hydrogenation activity of [SiII(Terp)SiII]Ni(η 2-arene). The mechanism has been established by kinetic measurements, including kinetic isotope effect (KIE) and Hammet-plot correlation. With this system, the currently highest performance of a homogeneous nickel-based hydrogenation catalyst of olefins (TON = 9800, TOF = 6800 h-1) could be realised.

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.

Simplified and versatile access to low valent Ni complexes by metal-free reduction of NiII precursors

Various well defined low valent Ni complexes were obtained by salt-free reduction of a commercially available Ni(ii) precursor.

Metallo-supramolecular assembly of protic pincer-type complexes: encapsulation of dinitrogen and carbon disulfide into a multiproton-responsive diruthenium cage

A protic pincer complex and rigid diphosphine linker formed a cage, which incorporated N₂ and CS₂ into the multiproton-responsive cavity.

Ni(i)–Ni(iii) cycle in Buchwald–Hartwig amination of aryl bromide mediated by NHC-ligated Ni(i) complexes

NHC-ligated Ni(i) intermediates in Buchwald–Hartwig amination of aryl halides were isolated and determined. The presence of a Ni(iii) intermediate was also indicated at low temperature.

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers whose high reactivity has been exploited in catalysis and the activation of small molecules featuring strong bonds (N₂, CO₂, and CO).

Synthesis and reactivity of a nickel(ii) thioperoxide complex: demonstration of sulfide-mediated N2O reduction

The “masked” terminal nickel sulfide [K(18-crown-6)][L^tBuNi^II(S)] mediates the reduction of N₂O by CO, via the thioperoxide complex [K(18-crown-6)][L^tBuNi^II(η²-SO)].

The thiohyponitrite ([SNNO]^2–) complex, [K(18-crown-6)][L^tBuNi^II(κ²-SNNO)] (L^tBu = {(2,6-ⁱPr₂C₆H₃)NC(^tBu)}₂CH), extrudes N₂ under mild heating to yield [K(18-crown-6)][L^tBuNi^II(η²-SO)] (1), along with minor products [K(18-crown-6)][L^tBuNi^II(η²-OSSO)] (2) and [K(18-crown-6)][L^tBuNi^II(η²-S₂)] (3). Subsequent reaction of 1 with carbon monoxide (CO) results in the formation of [K(18-crown-6)][L^tBuNi^II(η²-SCO)] (4), [K(18-crown-6)][L^tBuNi^II(S,O:κ²-SCO₂)] (5), [K(18-crown-6)][L^tBuNi^II(κ²-CO₃)] (6), carbonyl sulfide (COS) (7), and [K(18-crown-6)][L^tBuNi^II(S₂CO)] (8). To rationalize the formation of these products we propose that 1 first reacts with CO to form [K(18-crown-6)][L^tBuNi^II(S)] (I) and CO₂, via O-atom abstraction. Subsequently, complex I reacts with CO or CO₂ to form 4 and 5, respectively. Similarly, the formation of complex 6 and COS can be rationalized by the reaction of 1 with CO₂ to form a putative Ni(ii) monothiopercarbonate, [K(18-crown-6)][L^tBuNi^II(κ²-SOCO₂)] (11). The Ni(ii) monothiopercarbonate subsequently transfers a S-atom to CO to form COS and [K(18-crown-6)][L^tBuNi^II(κ²-CO₃)] (6). Finally, the formation of 8 can be rationalized by the reaction of COS with I. Critically, the observation of complexes 4 and 5 in the reaction mixture reveals the stepwise conversion of [K(18-crown-6)][L^tBuNi^II(κ²-SNNO)] to 1 and then I, which represents the formal reduction of N₂O by CO.

Mechanisms of catalytic reduction of CO2 with heme and nonheme metal complexes

The catalytic conversion of CO2 into valuable chemicals and fuels has attracted increasing attention, providing a promising route for mitigating the greenhouse effect of CO2 and also meeting the global energy demand. Among many homogeneous and heterogeneous catalysts for CO2 reduction, this mini-review is focused on heme and nonheme metal complexes that act as effective catalysts for the electrocatalytic and photocatalytic reduction of CO2. Because metalloporphyrinoids show strong absorption in the visible region, which is sensitive to the oxidation states of the metals and ligands, they are suited for the detection of reactive intermediates in the catalytic CO2 reduction cycle by electronic absorption spectroscopy. The first part of this review deals with the catalytic mechanism for the one-electron reduction of CO2 to oxalic acid with heme and nonheme metal complexes, with an emphasis on how the formation of highly energetic CO2˙ is avoided. Then, the catalytic mechanism of two-electron reduction of CO2 to produce CO and H2O is compared with that to produce HCOOH. The effect of metals and ligands of the heme and nonheme complexes on the CO or HCOOH product selectivity is also discussed. The catalytic mechanisms of multi-electron reduction of CO2 to methanol (six-electron reduced product) and methane (eight-electron reduced product) are also discussed for both electrocatalytic and photocatalytic systems.

Nickel(I) Aryl Species: Synthesis, Properties, and Catalytic Activity

In this work, Ni(I) aryl species that are directly relevant to cross-coupling have been synthesized. Transmetalation of (dppf)Ni^IX (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) with aryl Grignard reagents or aryl boronic acids in the presence of base produces Ni(I) aryl species of the form (dppf)Ni^I(Ar) (Ar = Ph, o-tolyl, 2,6-xylyl, 2,4,6-mesityl, 2,4,6-ⁱPr₃C₆H₂). The stability of the Ni(I) aryl species is inversely correlated to the steric bulk on the aryl ligand. The most unstable Ni(I) aryl species are the most active precatalysts for Suzuki-Miyaura reactions because they rapidly decompose to generate the active Ni(0) catalyst. This study shows that Ni(I) aryl species are initially formed in the activation of Ni(I) halide precatalysts for Suzuki-Miyaura reactions and establishes their stoichiometric and catalytic reactivity profile.

Nickel-catalyzed intermolecular carboiodination of alkynes with aryl iodides

Odd-valent nickel-catalyzed intermolecular carboiodination of alkynes with aryl iodides to form highly substituted and functionalized alkenyl iodides has been developed.

Synthesis, Characterisation and Reactions of Truly Cationic NiI -Phosphine Complexes

The recently published purely metallo-organic Ni^I salt [Ni(cod)₂ ][Al(OR^F )₄ ] (1, cod=1,5-cyclooctadiene, R^F =C(CF₃ )₃ ) provides a starting point for a new synthesis strategy leading to Ni^I phosphine complexes, replacing cod ligands by phosphines. Clearly visible colour changes indicate reactions within minutes, while quantum chemical calculations (PBE0-D3(BJ)/def2-TZVPP) approve exergonic reaction enthalpies in all performed ligand exchange reactions. Hence, [Ni(dppp)₂ ][Al(OR^F )₄ ] (2, dppp=1,3-bis(diphenylphosphino)propane), [Ni(dppe)₂ ][Al(OR^F )₄ ] (3, dppe=1,3-bis(diphenyl-phosphino)ethane), three-coordinate [Ni(PPh₃ )₃ ][Al(OR^F )₄ ] (4) and a remarkable two-coordinate Ni^I phosphine complex [Ni(PtBu₃ )₂ ][Al(OR^F )₄ ] (5) were characterised by single crystal X-ray structure analysis. EPR studies were performed, confirming a nickel d⁹ -configuration in complexes 2, 4 and 5. This result is supported by additional magnetization measurements of 4 and 5. Further investigations by cyclic voltammetry indicate relatively high oxidation potentials for these Ni^I compounds between 0.7 and 1.7 V versus Fc/Fc⁺ . Screening reactions with O₂ and CO gave first insights on the reaction behaviour of the Ni^I phosphine complexes towards small molecules with formation of mixed phosphine-CO-Ni^I complexes and oxidation processes yielding new Ni^I and/or Ni^II derivatives. Moreover, 4 reacted with CH₂ Cl₂ at RT to give a dimeric Ni^II ylide complex (4 c). As CH₂ Cl₂ is a rather stable alkyl halide with relatively high C-Cl bond energies, 4 appears to be a suitable reagent for more general C-Cl bond activation reactions.

A T-Shaped Nickel(I) Metalloradical Species

A T-shaped Ni^I complex was synthesized using a rigid acridane-based pincer ligand to prepare a metalloradical center. Structural data displays a nickel ion is embedded in the plane of a PNP ligand. Having a sterically exposed half-filled dx2-y2 orbital, this three-coordinate Ni^I species reveals unique open-shell reactivity including the homolytic cleavage of various σ-bonds, such as H-H, N-N, and C-C.

When Weaker Can Be Tougher: The Role of Oxidation State (I) in P- vs N-Ligand-Derived Ni-Catalyzed Trifluoromethylthiolation of Aryl Halides

The direct introduction of the valuable SCF3 moiety into organic molecules has received considerable attention. While it can be achieved successfully for aryl chlorides under catalysis with Ni0(cod)2 and dppf, this report investigates the Ni-catalyzed functionalization of the seemingly more reactive aryl halides ArI and ArBr. Counterintuitively, the observed conversion triggered by dppf/Ni0 is ArCl > ArBr > ArI, at odds with bond strength preferences. By a combined computational and experimental approach, the origin of this was identified to be due to the formation of (dppf)NiI, which favors β-F elimination as a competing pathway over the productive cross-coupling, ultimately generating the inactive complex (dppf)Ni(SCF2) as a catalysis dead end. The complexes (dppf)NiI-Br and (dppf)NiI-I were isolated and resolved by X-ray crystallography. Their formation was found to be consistent with a ligand-exchange-induced comproportionation mechanism. In stark contrast to these phosphine-derived Ni complexes, the corresponding nitrogen-ligand-derived species were found to be likely competent catalysts in oxidation state I. Our computational studies of N-ligand derived NiI complexes fully support productive NiI/NiIII catalysis, as the competing β-F elimination is disfavored. Moreover, N-derived NiI complexes are predicted to be more reactive than their Ni0 counterparts in catalysis. These data showcase fundamentally different roles of NiI in carbon-heteroatom bond formation depending on the ligand sphere.

Mechanistic Study of an Improved Ni Precatalyst for Suzuki-Miyaura Reactions of Aryl Sulfamates: Understanding the Role of Ni(I) Species

Nickel precatalysts are potentially a more sustainable alternative to traditional palladium precatalysts for the Suzuki-Miyaura coupling reaction. Currently, there is significant interest in Suzuki-Miyaura coupling reactions involving readily accessible phenolic derivatives such as aryl sulfamates, as the sulfamate moiety can act as a directing group for the prefunctionalization of the aromatic backbone of the electrophile prior to cross-coupling. By evaluating complexes in the Ni(0), (I), and (II) oxidation states we report a precatalyst, (dppf)Ni(o-tolyl)(Cl) (dppf = 1,1'-bis(diphenylphosphino)ferrocene), for Suzuki-Miyaura coupling reactions involving aryl sulfamates and boronic acids, which operates at a significantly lower catalyst loading and at milder reaction conditions than other reported systems. In some cases it can even function at room temperature. Mechanistic studies on precatalyst activation and the speciation of nickel during catalysis reveal that Ni(I) species are formed in the catalytic reaction via two different pathways: (i) the precatalyst (dppf)Ni(o-tolyl)(Cl) undergoes comproportionation with the active Ni(0) species; and (ii) the catalytic intermediate (dppf)Ni(Ar)(sulfamate) (Ar = aryl) undergoes comproportionation with the active Ni(0) species. In both cases the formation of Ni(I) is detrimental to catalysis, which is proposed to proceed via a Ni(0)/Ni(II) cycle. DFT calculations are used to support experimental observations and provide insight about the elementary steps involved in reactions directly on the catalytic cycle, as well as off-cycle processes. Our mechanistic investigation provides guidelines for designing even more active nickel catalysts.

Versatile (η6-arene)Ni(PCy3) nickel monophosphine precursors

Revisiting the solution behaviour of [(PCy₃)₂Ni]₂N₂ reveals a facile synthesis for the versatile Ni(PCy₃) transfer reagents, (η⁶-arene)Ni(PCy₃).

Complexes of Ni(i): a “rare” oxidation state of growing importance

The synthesis and diverse structures, reactivity (small molecule activation and catalysis) and magnetic properties of Ni(i) complexes are summarized.

Carbon dioxide binding at a Ni/Fe center: synthesis and characterization of Ni(η1-CO2-κC) and Ni-μ-CO2-κC:κ2O,O'-Fe

The degree of CO2 activation can be tuned by incorporating a distinct electronic coordination environment at the nickel center. A mononuclear nickel carboxylate species (Ni-CO2, 3) and a dinuclear nickel-iron carboxylate species (Ni-CO2-Fe, 5) were prepared. The structure of 3 reveals a rare η1-κC binding mode of CO2, while that of 5 shows bridging CO2 binding (μ2-κC:κ2O,O') between the nickel and iron, presented as the first example of a nickel-μ-CO2-iron species. The structural analyses of 3 and 5 based on XRD and DFT data reveal a higher degree of CO2 activation in 5, imparted by the additional interaction with an iron ion.

Influence of Ring-Expanded N-Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study

Potassium graphite reduction of the half-sandwich Ni(II) ring-expanded diamino/diamidocarbene complexes CpNi(RE-NHC)Br gave the Ni(I) derivatives CpNi(RE-NHC) (where RE-NHC = 6-Mes (1), 7-Mes (2), 6-MesDAC (3)) in yields of 40%-50%. The electronic structures of paramagnetic 1-3 were investigated by CW X-/Q-band electron paramagnetic resonance (EPR) and Q-band ¹H electron nuclear double resonance (ENDOR) spectroscopy. While small variations in the g-values were observed between the diaminocarbene complexes 1 and 2, pronounced changes in the g-values were detected between the almost isostructural species (1) and diamidocarbene species (3). These results highlight the sensitivity of the EPR g-tensor to changes in the electronic structure of the Ni(I) centers generated by incorporation of heteroatom substituents onto the backbone ring positions. Variable-temperature EPR analysis also revealed the presence of a second Ni(I) site in 3. The experimental g-values for these two Ni(I) sites detected by EPR in frozen solutions of 3 are consistent with resolution on the EPR time scale of the disordered components evident in the X-ray crystallographically determined structure and the corresponding density functional theory (DFT)-calculated g-tensor. Q-band ¹H ENDOR measurements revealed a small amount of unpaired electron spin density on the Cp rings, consistent with the calculated SOMO of complexes 1-3. The magnitude of the ¹H A values for 3 were also notably larger, compared to 1 and 2, again highlighting the influence of the diamidocarbene on the electronic properties of 3.