Synthesis and Reactivity of Nickel(II) Hydroxycarbonyl Species, NiCOOH-κC

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.

Opportunities for Insight into the Mechanism of Efficient CO2/CO Interconversion at a Nickel-Iron Cluster in CO Dehydrogenase

The reduction of CO2 with low overpotential and high selectivity is a crucial challenge in catalysis. Fortunately, natural systems have evolved enzymes that achieve this catalytic reaction very efficiently at a complex nickel-iron-sulfur cluster within carbon monoxide dehydrogenase (CODH). Extensive biochemical, crystallographic, and spectroscopic work has been done to understand the structures and mechanism involved in the catalytic cycle, which are summarized here from the perspective of mechanistic organometallic chemistry. We highlight the ambiguities in the data and suggest experiments that could lead to clearer understanding of the mechanism and structures of intermediates at the active-site cluster. These include parallel crystallography and spectroscopy, as well as the preparation of synthetic analogues that help to interpret structural and spectroscopic signatures.

Alkoxycarbonyl Groups in Metalloesters Showing Oxocarbenium-like Structure and Alkylating Reactivity

In contrast to the well-documented acylating reactivity, the alkylating reactivity of the alkoxycarbonyl group, as signified by its oxocarbenium-like resonance structure, remains almost unexplored. Herein, the first series of Co/Ni dinuclear metalloesters exhibiting the novel oxocarbenium-like alkoxycarbonyl groups were synthesized and characterized. In these deformed alkoxycarbonyl groups, the Ccarbonyl-Oalkoxyl bonds were contracted to 1.177(11)~1.191(9) Å with the elongations of the Ccarbonyl=Ocarbonyl bonds to 1.368(13)~1.441(9) Å. Meanwhile, the O-Calkyl bonds were also elongated to 1.522(11) ~1.607(15) Å, and were by far the longest O-Calkyl bonds reported for alkoxycarbonyl groups. As triggered by the long O-Calkyl distances, the alkylating reactivity of the oxocarbenium-like methoxycarbonyl group towards a series of C/N/O-nucleophiles via the rare BAL2 mechanism at ambient conditions was examined. Furthermore, the homo-etherifications of alcohols mediated by the Co/Ni dinuclear metalloesters were investigated. The yields followed the trend ethanol≫n-propanol≫n-butanol ≈n-pentanol, that closely related to the structure features of the alkoxycarbonyl groups in corresponding metalloesters: while the ethoxycarbonyl group showed the reactive oxocarbenium-like framework, the n-propoxycarbonyl group displayed the dioxocarbenium-like skeleton with a shorter O-Calkyl bond; In comparison, the classical frameworks with unactivated alkyl moieties were observed for n-butoxycarbonyl and n-pentoxycarbonyl groups.

Nitrous oxide activation by picoline-derived Ni-CNP hydrides

Oxygen atom transfer (OAT) from N2O to the Ni-H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni-CNP complexes efficiently catalyse the reduction of N2O with pinacolborane (HBpin) under mild conditions.

Direct synthesis of oxalic acid via oxidative CO coupling mediated by a dinuclear hydroxycarbonylcobalt(III) complex

Oxidative coupling of CO is a straightforward and economic benign synthetic route for value-added α-diketone moiety containing C₂ or higher carbon compounds in both laboratory and industry, but is still undeveloped to date. In this work, a rare coplanar dinuclear hydroxycarbonylcobalt(III) complex, bearing a Schiff-base macrocyclic equatorial ligand and a μ-κ¹(O):κ¹(O')-acetate bridging axial ligand, is synthesized and characterized. The Co(III)-COOH bonds in this complex can be feasibly photocleaved, leading to the formation of oxalic acid. Moreover, the light-promoted catalytic direct production of oxalic acid from CO and H₂O using O₂ as the oxidant with good selectivity (> 95%) and atom economy at ambient temperature and gas pressure based on this dicobalt(III) complex have been achieved, with a turnover number of 38.5. The ¹³C-labelling and ¹⁸O-labelling experiments confirm that CO and H₂O act as the sources of the -COOH groups in the dinuclear hydroxycarbonylcobalt(III) complex and the oxalic acid product.

Rapid Access to Carbon-Isotope-Labeled Alkyl and Aryl Carboxylates Applying Palladacarboxylates

Herein, we report a strategy for the formation of isotopically labeled carboxylic esters from boronic esters/acids using a readily accessible palladium carboxylate complex as an organometallic source of isotopically labeled functional groups. The reaction allows access to either unlabeled or full ¹³C- or ¹⁴C-isotopically labeled carboxylic esters, and the method is characterized by its operational simplicity, mild conditions, and general substrate scope. Our protocol is further extended to a carbon isotope replacement strategy, involving an initial decarbonylative borylation procedure. Such an approach allows access to isotopically labeled compounds directly from the unlabeled pharmaceutical, which can have implications for drug discovery programs.

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes

The redox non-innocent behavior of the diaryl-azo-oxime ligand L^NOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(L^NO-)Cl₂(PPh₃)₂] 4 and [Ir(L^NO-)₂Cl(PPh₃)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(L^NO-)Cl(CO₂H)(PPh₃)₂] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO₂H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 is primarily azo-oxime centric with very low contribution from the iridium center.

Residues surrounding the active centre of carbon monoxide dehydrogenase are key in converting [Formula: see text] to CO

The enzyme carbon monoxide dehydrogenase is capable of efficiently converting [Formula: see text] to CO and, therefore, can enable an affordable [Formula: see text] recycling strategy. The reduction of [Formula: see text] occurs at a peculiar nickel-iron-sulfur cluster, following a mechanism that remains little understood. In this study, we have used ab initio molecular dynamics simulations to explore the free energy landscape of the reaction. We predict the existence of a COOH ligand that strongly interacts with the surrounding protein residues and favours a mechanism where a [Formula: see text] molecule is eliminated before CO. We have taken advantages of the insights offered by our simulations to revisit the catalytic mechanism and the role of the residues surrounding the active centre in particular, thus assisting in the design of inorganic catalysts that mimic the enzyme.

Selective Transformation of Nickel-Bound Formate to CO or C-C Coupling Products Triggered by Deprotonation and Steered by Alkali-Metal Ions

The complexes [LtBu Ni(OCO-κ2 O,C)]M3 [N(SiMe3 )2 ]2 (M=Li, Na, K), synthesized by deprotonation of a nickel formate complex [LtBu NiOOCH] with the corresponding amides M[N(SiMe3 )2 ], feature a NiII -CO2 2- core surrounded by Lewis-acidic cations (M+ ) and the influence of the latter on the behavior and reactivity was studied. The results point to a decrease of CO2 activation within the series Li, Na, and K, which is also reflected in the reactivity with Me3 SiOTf leading to the liberation of CO and formation of a Ni-OSiMe3 complex. Furthermore, in case of K+ , the {[K3 [N(SiMe3 )2 ]2 }+ shell around the Ni-CO2 2- entity was shown to have a large impact on its stabilization and behavior. If the number of K[N(SiMe3 )2 ] equivalents used in the reaction with [LtBu NiOOCH] is decreased from 3 to 0.5, the deprotonated part of the precursor enters a complex reaction sequence with formation of [LtBu NiI (μ-OOCH)NiI LtBu ]K and [LtBu Ni(C2 O4 )NiLtBu ]. The same reaction at higher concentrations additionally led to the formation of a unique hexanuclear NiII complex containing both oxalate and mesoxalate ([O2 C-CO2 -CO2 ]4- ) ligands.

Catalytic hydrogenation of CO2 at a structurally rigidified cobalt center

Catalytic hydrogenation of CO₂ occurs at a cobalt center supported by a rigidified PNP ligand revealing higher catalytic performance.

κ3 -(N^C^C)Gold(III) Carboxylates: Evidence for Decarbonylation Processes

Gold(III) carboxylate species, stabilized by a κ³ -(N^C^C) ligand template, are presented herein. A η¹ -Au^III -C(O)-OH species has been characterized under cryogenic conditions as a result of the nucleophilic attack of an ammonium hydroxide onto a dinuclear μ-CO₂ -κ³ -(N^C^C)Au^III precursor. Thermal decomposition for these species proceeds by an unusual decarbonylation process, in contrast to typical decarboxylation pathways observed in related metallocarboxylic acids.

One metal is enough: a nickel complex reduces nitrate anions to nitrogen gas

A stepwise reduction sequence from nitrate to dinitrogen gas at a single nickel center was discovered. A PNP nickel scaffold (PNP- = N[2-P i Pr2-4-Me-C6H3]2) emerged as a universal platform for the deoxygenation of NO x substrates. In these reactions carbon monoxide acts as the oxygen acceptor and forms CO2 to provide the necessary chemical driving force. Whereas the first two oxygens are removed from the Ni-nitrate and Ni-nitrite complexes with CO, the deoxygenation of NO requires a disproportionation reaction with another NO molecule to form NO2 and N2O. The final deoxygenation of nitrous oxide is accomplished by the Ni-NO complex and generates N2 and Ni-NO2 in a relatively slow, but clean reaction. This sequence of reactions is the first example of the complete denitrification of nitrate at a single metal-site and suggests a new paradigm of connecting CO and NO x as an effective reaction pair for NO x removal.

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.

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.

The elusive abnormal CO2 insertion enabled by metal-ligand cooperative photochemical selectivity inversion

Direct hydrogenation of CO₂ to CO, the reverse water–gas shift reaction, is an attractive route to CO₂ utilization. However, the use of molecular catalysts is impeded by the general reactivity of metal hydrides with CO₂. Insertion into M–H bonds results in formates (MO(O)CH), whereas the abnormal insertion to the hydroxycarbonyl isomer (MC(O)OH), which is the key intermediate for CO-selective catalysis, has never been directly observed. We here report that the selectivity of CO₂ insertion into a Ni–H bond can be inverted from normal to abnormal insertion upon switching from thermal to photochemical conditions. Mechanistic examination for abnormal insertion indicates photochemical N–H reductive elimination as the pivotal step that leads to an umpolung of the hydride ligand. This study conceptually introduces metal-ligand cooperation for selectivity control in photochemical transformations.

The development of molecular catalysts for the reverse water–gas shift reaction is impeded by the general selectivity of CO₂ insertion into M–H bonds to formates. Here, the authors report that the selectivity of CO₂ insertion into a Ni–H bond can be inverted from normal to abnormal insertion upon switching from thermal to photochemical conditions.

Direct CO2 Addition to a Ni(0)-CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO

Addition of CO₂ to a low-valent nickel species has been explored with a newly designed ^acriPNP pincer ligand (^acriPNP^- = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO₂ conversion to CO found in carbon monoxide dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably accessed in the presence of a CO ligand, which can be prepared from a stepwise reduction of a cationic {(^acriPNP)Ni(II)-CO}⁺ species. All three Ni(II), Ni(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the d_x²-y² orbital. Accordingly, the CO ligand position changes from equatorial to axial, ∠N-Ni-C of 176.2(2)° to 129.1(4)°, allowing opening of a CO₂ binding site. Upon addition of CO₂ to a nickel(0)-CO species, a nickel(II) carboxylate species with a Ni(η¹-CO₂-κC) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO(g). This is a unique result markedly different from our previous report involving the flexible analogous PNP ligand, which revealed the formation of multiple products including a tetrameric cluster from the reaction with CO₂. Finally, the carbon dioxide conversion to CO at a single nickel center is modeled by the successful isolation of all relevant intermediates, such as Ni-CO₂, Ni-COOH, and Ni-CO.

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.

Formation of a tetranickel octacarbonyl cluster from the CO2 reaction of a zero-valent nickel monocarbonyl species

The CO₂ reaction of a nickel(0) complex involves multiple reaction pathways including tetranickel cluster formation via reductive disproportionation of CO₂.

Reaction of a polydentate cysteine-based ligand and its nickel(ii) complex with electrophilic and nucleophilic methyl-transfer reagents - from S-methylation to acetyl coenzyme A synthase reactivity

The L-cysteine derived N2S2 ligand precursor H2L and its nickel(ii) complex L2Ni2 were investigated with respect to their behaviour in contact with electrophilic and nucleophilic methylation reagents (H2L = (N,N'-dimethyl-(2R,5R)-bis-(sulfanylmethyl)-piperazine). Treatment of deprotonated L(2-) with MeI led to the selective methylation of the thiolate groups thus generating a novel potential ligand, Me2L, which is neutral and contains two thioether donors. The coordinating properties of Me2L were demonstrated by the synthesis of a first nickel(ii) complex: reaction with NiBr2 led to a mononuclear complex 2 where all donor atoms coordinate to the nickel ion, which completes its octahedral coordination sphere by the two bromide ligands. If, however, the complex [LNi]2 (1) is treated with MeI only one thiolate function per ligand moiety is methylated, while the other one remains a thiolate. This leads to [MeLNi](+) complex metal fragments, which trimerize including a μ3-bridging iodide ion to give the compound 3 that was tested with regards to ACS reactivity. While it behaved inert towards CO, attempts to replace the bridging iodide ligand by methyl units in reactions with nucleophilic methylation reagents led to a product, which could not be identified but reacted with CO. Work-up showed that this protocol had converted the thiolate function of MeL(-) into a thioester function, which corresponds to an ACS-like reactivity.

Nickel and iron pincer complexes as catalysts for the reduction of carbonyl compounds

The reductions of aldehydes, ketones, and esters to alcohols are important processes for the synthesis of chemicals that are vital to our daily life, and the reduction of CO2 to methanol is expected to provide key technology for carbon management and energy storage in our future. Catalysts that affect the reduction of carbonyl compounds often contain ruthenium, osmium, or other precious metals. The high and fluctuating price, and the limited availability of these metals, calls for efforts to develop catalysts based on more abundant and less expensive first-row transition metals, such as nickel and iron. The challenge, however, is to identify ligand systems that can increase the thermal stability of the catalysts, enhance their reactivity, and bypass the one-electron pathways that are commonly observed for first-row transition metal complexes. Although many other strategies exist, this Account describes how we have utilized pincer ligands along with other ancillary ligands to accomplish these goals. The bis(phosphinite)-based pincer ligands (also known as POCOP-pincer ligands) create well-defined nickel hydride complexes as efficient catalysts for the hydrosilylation of aldehydes and ketones and the hydroboration of CO2 to methanol derivatives. The hydride ligands in these complexes are substantially nucleophilic, largely due to the enhancement by the strongly trans-influencing aryl groups. Under the same principle, the pincer-ligated nickel cyanomethyl complexes exhibit remarkably high activity (turnover numbers up to 82,000) for catalytically activating acetonitrile and the addition of H-CH2CN across the C═O bonds of aldehydes without requiring a base additive. Cyclometalation of bis(phosphinite)-based pincer ligands with low-valent iron species "Fe(PR3)4" results in diamagnetic Fe(II) hydride complexes, which are active catalysts for the hydrosilylation of aldehydes and ketones. Mechanistic investigation suggests that the hydride ligand is not delivered to the carbonyl substrates but is important to facilitate ligand dissociation prior to substrate activation. In the presence of CO, the amine-bis(phosphine)-based pincer ligands are also able to stabilize low-spin Fe(II) species. Iron dihydride complexes supported by these ligands are bifunctional as both the FeH and NH moieties participate in the reduction of C═O bonds. These iron pincer complexes are among the first iron-based catalysts for the hydrogenation of esters, including fatty acid methyl esters, which find broad applications in industry. Our studies demonstrate that pincer ligands are promising candidates for promoting the first-row transition metal-catalyzed reduction of carbonyl compounds with high efficiency. Further efforts in this research area are likely to lead to more efficient and practical catalysts.

Formation of a nickel carbon dioxide adduct and its transformation mediated by a Lewis acid

An uncommon nickel dinitrogen adduct and its tendency toward CO2 binding are investigated using a (PP(Me)P)Ni scaffold. (PP(Me)P)Ni(N2) (1) and {(PP(Me)P)Ni}2(μ-N2) (2) were prepared and their treatment with CO2 revealed the formation of (PP(Me)P)Ni(η(2)-CO2) (3). This is a new type of CO2 binding for a zero-valent nickel center supported by three donor ligands, reminiscent of the CODH active site environment. Clear unique structural differences in 3 are evident when compared with previous 4-coordinate Ni-CO2 adducts. Compound 3 when treated with B(C6F5)3 gives the Lewis acid-base adduct (PP(Me)P)Ni{COOB(C6F5)3} (4) possessing a Ni-μ-CO2-κ(2)C,O-B moiety.