3d Metal Complexes in T-shaped Geometry as a Gateway to Metalloradical Reactivity.

ConspectusLow-valent, low-coordinate 3d metal complexes represent a class of extraordinarily reactive compounds that can act as reagents and catalysts for challenging bond-activation reactions. The pursuit of these electron-deficient metal complexes in low oxidation states demands ancillary ligands capable of providing not only energetic stabilization but also sufficiently high steric bulk at the metal center. From this perspective, pincer ligands are particularly advantageous, as their prearranged, meridional coordination mode scaffolds the active center while the substituents of the peripheral donor atoms provide effective steric shielding for the coordination sphere. In a T-shaped geometry, the transition metal complexes possess a precisely defined vacant coordination site, which, combined with the often observed high-spin electron configuration, exhibits unusually high selectivity of these compounds with respect to one-electron redox chemistry. In light of the intractable reaction pathways typically observed with related electronically unsaturated 3d transition metal complexes, the pincer coordination mode enables the isolation of low-valent compounds with more controlled and unique reactivity. We have thus investigated a series of T-shaped metal(I) complexes using three different types of pincer ligands, which may be regarded as "metalloradicals" due to their selectively exposed unpaired electrons.These compounds display remarkably high thermal stability and represent rarely observed "naked" monovalent metal species featuring both monomeric and dimeric structures. Extensive reactivity studies using various organic substrates highlight a strong tendency of these paramagnetic compounds to undergo one-electron oxidation, leading to the isolation of a plethora of metal(II) species with reduced organic ligands as unusual structural elements. The exploration of C2 symmetric T-shaped Ni(I) complexes as asymmetric catalysts also shows success in enantioselective hydrodehalogenation of geminal dihalogenides. In addition, this specific class of low-valent, low-coordinate complexes can be further diversified by introducing redox-active pincer ligands or building homobimetallic systems with two T-shaped units.This Account focuses on the discussion of selected examples of iron, cobalt, and nickel pincer complexes bearing a [P,N,P] or [N,N,N] donor set; however, their electronic structure and radical-type reactivity can be broadly extended to other pincer systems. The availability of various types of pincer ligands should allow fine-tuning of the reactivity of the T-shaped complexes. Given the unprecedented reactivity observed with these compounds, we expect the studies of T-shaped 3d metal complexes to be a fertile field for advancing base metal catalysis.

Methyl Effects on the Stereochemistry and Reactivity of PPP-Ligated Iron Hydride Complexes.

Iron dihydride complexes are key intermediates in many iron-catalyzed reactions. Previous efforts to study molecules of this type have led to the discovery of a remarkably stable cis-FeH2 complex, which is supported by bis[2-(diisopropylphosphino)phenyl]phosphine (iPrPPHP) along with CO. In this work, the hydrogen on the central phosphorus has been replaced with a methyl group, and the corresponding iron carbonyl dichloride, hydrido chloride, and dihydride complexes have been synthesized. The addition of the methyl group favors the anti configuration for the Me-P-Fe-H moiety and the trans geometry for the H-Fe-CO motif, which is distinctively different from the iPrPPHP system. Furthermore, it increases the thermal stability of the dihydride complex, cis-(iPrPPMeP)Fe(CO)H2 (iPrPPMeP = bis[2-(diisopropylphosphino)phenyl]methylphosphine). The variations in stereochemistry and compound stability contribute greatly to the differences between the two PPP systems in reactions with PhCHO, CS2, and HCO2H.

On the Demise of PPP-Ligated Iron Catalysts in the Formic Acid Dehydrogenation Reaction.

The PPP-ligated iron complexes, cis-(iPrPPRP)FeH2(CO) [iPrPPRP = (o-iPr2PC6H4)2PR (R = H or Me)], catalyze the dehydrogenation of formic acid to carbon dioxide but lose their catalytic activity over time. This study focuses on the analysis of the species formed from the degradation of cis-(iPrPPMeP)FeH2(CO) over its course of catalyzing the dehydrogenation reaction. These degradation products include species both soluble and insoluble in the reaction medium. The soluble component of the decomposed catalyst is a mixture of cis-[(iPrPPMeP)FeH(CO)2][(HCO2)(HCO2H)x], protonated iPrPPMeP, and oxidation products resulting from adventitious O2. The precipitate is solvated Fe(OCHO)2. Further mechanistic investigation suggests that cis-[(iPrPPMeP)FeH(CO)2][(HCO2)(HCO2H)x] displays diminished but measurable catalytic activity, likely through the displacement of a CO ligand by the formate ion. The formation of Fe(OCHO)2 along with the dissociation of iPrPPMeP is responsible for the eventual loss of catalytic activity.

Iron Dihydride Complex Stabilized by an All-Phosphorus-Based Pincer Ligand and Carbon Monoxide.

PNP-pincer-stabilized iron carbonyl dihydride complexes are key intermediates in catalytic hydrogenation and dehydrogenation reactions; however, decomposition through these intermediates has been observed. This inspires the development of a PPP-pincer system that may show improved catalyst stability. In this work, bis[2-(diisopropylphosphino)phenyl]phosphine (or iPrPPHP) is used to react with FeCl2 under a carbon monoxide (CO) atmosphere to yield trans-(iPrPPHP)Fe(CO)Cl2. A subsequent reaction with NaBH4 produces syn/anti-(iPrPPHP)FeH(CO)Cl or cis,anti-(iPrPPHP)Fe(CO)H2, depending on the amount of NaBH4 employed. The cis-dihydride complex shows catalytic activity for the conversion of PhCHO to PhCH2OH (under H2) or PhCO2CH2Ph (under Ar). It also catalyzes the dehydrogenation of PhCH2OH to PhCHO and PhCO2CH2Ph, albeit with limited turnover numbers. A more efficient catalytic process is the dehydrogenation of formic acid to carbon dioxide (CO2), which can operate under additive-free conditions. Mechanistic investigation suggests that the cis-dihydride complex undergoes protonation with formic acid to release H2 while forming anti-(iPrPPHP)FeH(CO)(OCHO)·HCO2H, in which the CO ligand has shifted and the formate is hydrogen-bonded to formic acid. The hydrido formate complex loses CO2 under ambient conditions, completing the catalytic cycle by reforming the cis-dihydride complex.

Experimental Evidence of syn H-N-Fe-H Configurational Requirement for Iron-Based Bifunctional Hydrogenation Catalysts.

Iron hydrides supported by a pincer ligand of the type HN(CH2CH2PR2)2 (RPNHP) are versatile hydrogenation catalysts. Previous efforts have focused on using CO as an additional ligand to stabilize the hydride species. In this work, CO is replaced with isocyanide ligands, leading to the isolation of two different types of iron hydride complexes: (RPNHP)FeH(CNR')(BH4) (R = iPr, R' = 2,6-Me2C6H3, tBu; R = Cy, R' = 2,6-Me2C6H3) and [(iPrPNHP)FeH(CNtBu)2]X (X = BPh4, Br, or a mixture of Br and BH4). The neutral iron hydrides are capable of catalyzing the hydrogenation of PhCO2CH2Ph to PhCH2OH, although the activity is lower than for (iPrPNHP)FeH(CO)(BH4). The cationic iron hydrides are active hydrogenation catalysts only for more reactive carbonyl substrates such as PhCHO, and only when the NH and FeH hydrogens are syn to each other. The cationic species and their synthetic precursors [(iPrPNHP)FeBr(CNtBu)2]X (X = BPh4, Br) can have different configurations for the isocyanide ligands (cis or trans) and the H-N-Fe-H(Br) unit (syn or anti). Unlike tetraphenylborate, the bromide counterion participates in a hydrogen-bonding interaction with the NH group, which influences the relative stability of the cis,anti and cis,syn isomers. These structural differences have been elucidated by X-ray crystallography, and the geometric isomerization processes have been studied by NMR spectroscopy.

Steric Effects of HN(CH2CH2PR2)2 on the Nuclearity of Copper Hydrides.

Copper hydride clusters of the type (RPNHP)nCu2nH2n (RPNHP = HN(CH2CH2PR2)2; n = 2 and 3) have been synthesized from the reaction of (RPNHP)CuBr with KOtBu under H2 or in one pot from a 1:2:2 mixture of RPNHP, CuBr, and KOtBu under H2. With medium-sized phosphorus substituents (R = iPr and Cy), the phosphine ligands stabilize both hexanuclear and tetranuclear clusters; however, the smaller clusters are kinetic products and aggregate further over time. Use of a bulkier ligand tBuPNHP leads to the formation of only a tetranuclear cluster. Crystallographic studies reveal a distorted octahedral Cu6 unit in (iPrPNHP)3Cu6H6 (2a) and (CyPNHP)3Cu6H6 (2b), while a tetrahedral Cu4 unit exists in (CyPNHP)2Cu4H4 (2b') and (tBuPNHP)2Cu4H4 (2c'), all furnished with face-capping hydrides and bridging RPNHP ligands. The aggregations are maintained in solution, although hydrides are fluxional. These copper clusters are capable of reducing aldehydes and ketones to the corresponding copper alkoxide species. Ranking their reactivity toward N-methyl-2-pyrrolecarboxaldehyde gives 2b' > 2a, 2b ≫ 2c', which correlates inversely with the order of thermal stability (against decomposition and cluster expansion).

Nickel Hydride Complexes.

Nickel hydride complexes, defined herein as any molecules bearing a nickel hydrogen bond, are crucial intermediates in numerous nickel-catalyzed reactions. Some of them are also synthetic models of nickel-containing enzymes such as [NiFe]-hydrogenase. The overall objective of this review is to provide a comprehensive overview of this specific type of hydride complexes, which has been studied extensively in recent years. This review begins with the significance and a very brief history of nickel hydride complexes, followed by various methods and spectroscopic or crystallographic tools used to synthesize and characterize these complexes. Also discussed are stoichiometric reactions involving nickel hydride complexes and how some of these reactions are developed into catalytic processes.

Mechanistic Study of Nickel-Catalyzed Reductive Coupling of Ynoates and Aldehydes.

In this work, (1,5-hexadiene)Ni(SIPr) (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) is used in place of Ni(COD)2/SIPr·HBF4/KOtBu (COD = 1,5-cyclooctadiene) as a more robust catalyst for regioselective reductive coupling of ynoates and aldehydes with triethylsilane. The catalytic reaction of ethyl 3-(trimethylsilyl)propiolate and methyl 4-formylbenzoate shows first-order dependence on aldehyde and catalyst concentrations, inverse first-order dependence on [ynoate], and no dependence on [silane]. The kinetics data, coupled with deuterium-labeling experiments, support a mechanism involving dissociation of the ynoate from a catalytically dormant nickelacyclopentadiene intermediate prior to turnover-limiting formation of a catalytically active nickeladihydrofuran.

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.

Mechanistic Insights into Oxidative Oligomerization of p-Phenylenediamine and Resorcinol.

An efficient synthesis of a green dye from oxidative coupling of p-phenylenediamine (PPD) and resorcinol (in a 2:1 ratio) has been developed. Reactivity studies of this dye molecule with a variety of reagents (PPD, resorcinol, the oxidized form of the green dye itself, and a dinuclear indo dye) demonstrate that it cannot be the key reactive intermediate in reported oxidative oligomerization of PPD and resorcinol. However, the trinuclear species does form large aggregates. At least one viable pathway of oligomerization has been demonstrated with the dinuclear indo dye.

A diphenyl ether derived bidentate secondary phosphine oxide as a preligand for nickel-catalyzed C-S cross-coupling reactions.

A new bidentate secondary phosphine oxide (SPO) was synthesized from diphenyl ether via ortho-lithiation, phosphorylation with PhP(Cl)NEt2, and hydrolysis in an acidic medium. Nickel(0) species ligated with this new SPO was established as a more effective catalyst than Ni(0)-Ph2P(O)H for the cross-coupling of aryl iodides with aryl thiols.

Mechanistic studies of ammonia borane dehydrogenation catalyzed by iron pincer complexes.

A series of iron bis(phosphinite) pincer complexes with the formula of [2,6-((i)Pr2PO)2C6H3]Fe(PMe2R)2H (R = Me, 1; R = Ph, 2) or [2,6-((i)Pr2PO)2-4-(MeO)C6H2]Fe(PMe2Ph)2H (3) have been tested for catalytic dehydrogenation of ammonia borane (AB). At 60 °C, complexes 1-3 release 2.3-2.5 equiv of H2 per AB in 24 h. Among the three iron catalysts, 3 exhibits the highest activity in terms of both the rate and the extent of H2 release. The initial rate for the dehydrogenation of AB catalyzed by 3 is first order in 3 and zero order in AB. The kinetic isotope effect (KIE) observed for doubly labeled AB (k(NH3BH3)/k(ND3BD3) = 3.7) is the product of individual KIEs (k(NH3BH3)/k(ND3BH3) = 2.0 and k(NH3BH3)/k(NH3BD3) = 1.7), suggesting that B-H and N-H bonds are simultaneously broken during the rate-determining step. NMR studies support that the catalytically active species is an AB-bound iron complex formed by displacing trans PMe3 or PMe2Ph (relative to the hydride) by AB. Loss of NH3 from the AB-bound iron species as well as catalyst degradation contributes to the decreased rate of H2 release at the late stage of the dehydrogenation reaction.

Iron-based catalysts for the hydrogenation of esters to alcohols.

Hydrogenation of esters is vital to the chemical industry for the production of alcohols, especially fatty alcohols that find broad applications in consumer products. Current technologies for ester hydrogenation rely on either heterogeneous catalysts operating under extreme temperatures and pressures or homogeneous catalysts containing precious metals such as ruthenium and osmium. Here, we report the hydrogenation of esters under relatively mild conditions by employing an iron-based catalyst bearing a PNP-pincer ligand. This catalytic system is also effective for the conversion of coconut oil derived fatty acid methyl esters to detergent alcohols without adding any solvent.

Pincer-ligated nickel hydridoborate complexes: the dormant species in catalytic reduction of carbon dioxide with boranes.

Nickel pincer complexes of the type [2,6-(R(2)PO)(2)C(6)H(3)]NiH (R = (t)Bu, 1a; R = (i)Pr, 1b; R = (c)Pe, 1c) react with BH(3)·THF to produce borohydride complexes [2,6-(R(2)PO)(2)C(6)H(3)]Ni(η(2)-BH(4)) (2a-c), as confirmed by NMR and IR spectroscopy, X-ray crystallography, and elemental analysis. The reactions are irreversible at room temperature but reversible at 60 °C. Compound 1a exchanges its hydrogen on the nickel with the borane hydrogen of 9-BBN or HBcat, but does not form any observable adduct. The less bulky hydride complexes 1b and 1c, however, yield nickel dihydridoborate complexes reversibly at room temperature when mixed with 9-BBN and HBcat. The dihydridoborate ligand in these complexes adopts an η(2)-coordination mode, as suggested by IR spectroscopy and X-ray crystallography. Under the catalytic influence of 1a-c, reduction of CO(2) leads to the methoxide level when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, 1a, involves bulky substituents on the phosphorus donor atoms. Catalytic reactions involving 1b and 1c are less efficient because of the formation of dihydridoborate complexes as the dormant species as well as partial decomposition of the catalysts by the boranes.

Efficient and regioselective nickel-catalyzed [2 + 2 + 2] cyclotrimerization of ynoates and related alkynes.

A nickel-based catalytic system has been developed for [2 + 2 + 2] cyclotrimerization of various alkynes, especially ynoates. This catalytic system enables facile construction of substituted aromatic compounds in excellent yields with high regioselectivity.

A robust nickel catalyst for cyanomethylation of aldehydes: activation of acetonitrile under base-free conditions.

Nick of time: The nickel cyanomethyl complex 1 catalyzes the room temperature coupling of aldehydes with acetonitrile under base-free conditions. The catalytic system is long-lived and remarkably efficient with high turnover numbers (TONs) and turnover frequencies (TOFs) achieved. The mild reaction conditions allow a wide variety of aldehydes, including base-sensitive ones, to catalytically react with acetonitrile.

The curious cases of tetrahydrosalen-type ligands interacting with Ni(II): structures and ligand-based oxidation reactions.

This work centers around the nickel complexes derived from two tetrahydrosalen-type proligands: N,N'-bis(2-hydroxybenzyl)-o-phenylenediamine (H2salophan) and N,N'-bis(2-hydroxy-3-methylbenzyl)-o-phenylenediamine (H2salophan_Me). The reaction of H2salophan with Ni(OAc)2·4H2O generates a dinuclear complex Ni2(Hsalophan)2(OAc)2 or Na[Ni2(salophan)2(OAc)] when NaOH is added to assist ligand deprotonation. The reaction of H2salophan_Me with Ni(OAc)2·4H2O, however, yields a mononuclear complex Ni(Hsalophan_Me)2. Unlike the corresponding salen-type nickel complexes, these tetrahydrosalen-type complexes are paramagentic and air sensitive (in solution). Oxidation by O2 or peroxides results in dehydrogenation of the ligand backbone to form the salen-type complexes.

Quick installation of a 1,4-difunctionality via regioselective nickel-catalyzed reductive coupling of ynoates and aldehydes.

The development of efficient methods for the synthesis of molecules with 1,4-difunctionalities has been a dire need of the synthetic community. In this work, intermolecular reductive coupling of ynoates and aldehydes (in the presence of a silane) has been accomplished for the first time using catalytic amounts of Ni(COD)(2), an N-heterocyclic carbene ligand, and PPh(3). High regioselectivity has been demonstrated for the multicomponent coupling reactions, and more than a dozen invaluable silyl-protected γ-hydroxy-α,ß-enoates have been synthesized. This methodology provides a quick entry to many other 1,4-difunctional compounds and oxygen-containing five-membered rings. The intermediacy of metallacycles in the catalytic process has been established by deuterium-labeling experiments.

How does the nickel pincer complex catalyze the conversion of CO2 to a methanol derivative? A computational mechanistic study.

The mechanistic details of nickel-catalyzed reduction of CO(2) with catecholborane (HBcat) have been studied by DFT calculations. The nickel pincer hydride complex ({2,6-C(6)H(3)(OP(t)Bu(2))(2)}NiH = [Ni]H) has been shown to catalyze the sequential reduction from CO(2) to HCOOBcat, then to CH(2)O, and finally to CH(3)OBcat. Each process is accomplished by a two-step sequence at the nickel center: the insertion of a C═O bond into [Ni]H, followed by the reaction of the insertion product with HBcat. Calculations have predicted the difficulties of observing the possible intermediates such as [Ni]OCH(2)OBcat, [Ni]OBcat, and [Ni]OCH(3), based on the low kinetic barriers and favorable thermodynamics for the decomposition of [Ni]OCH(2)OBcat, as well as the reactions of [Ni]OBcat and [Ni]OCH(3) with HBcat. Compared to the uncatalyzed reactions of HBcat with CO(2), HCOOBcat, and CH(2)O, the nickel hydride catalyst accelerates the H(δ-) transfer by lowering the barriers by 30.1, 12.4, and 19.6 kcal/mol, respectively. In general, the catalytic role of the nickel hydride is similar to that of N-heterocyclic carbene (NHC) catalyst in the hydrosilylation of CO(2). However, the H(δ-) transfer mechanisms used by the two catalysts are completely different. The H(δ-) transfer catalyzed by [Ni]H can be described as hydrogen being shuttled from HBcat to nickel center and then to the C═O bond, and the catalyst changes its integrity during catalysis. In contrast, the NHC catalyst simply exerts an electronic influence to activate either the silane or CO(2), and the integrity of the catalyst remains intact throughout the catalytic cycle. The comparison between [Ni]H and Cp(2)Zr(H)Cl in the stoichiometric reduction of CO(2) has suggested that ligand sterics and metal electronic properties play critical roles in controlling the outcome of the reaction. A bridging methylene diolate complex has been previously observed in the zirconium system, whereas the analogous [Ni]OCH(2)O[Ni] is not a viable intermediate, both kinetically and thermodynamically. Replacing HBcat with PhSiH(3) in the nickel-catalyzed reduction of CO(2) results in a high kinetic barrier for the reaction of [Ni]OOCH with PhSiH(3). Switching silanes to HBcat in NHC-catalyzed reduction of CO(2) generates a very stable NHC adduct of HCOOBcat, which makes the release of NHC less favorable.

An efficient nickel catalyst for the reduction of carbon dioxide with a borane.

Nickel hydride with a diphosphinite-based ligand catalyzes the highly efficient reduction of CO(2) with catecholborane, and the hydrolysis of the resulting methoxyboryl species produces CH(3)OH in good yield. The mechanism involves a nickel formate, formaldehyde, and a nickel methoxide as different reduced stages for CO(2). The reaction may also be catalyzed by an air-stable nickel formate.