An NHC-Silyl-NHC Pincer Ligand for the Oxidative Addition of C−H, N−H, and O−H Bonds to Cobalt(I) Complexes

C(sp)-H, S-H, and Sn-H Bond Activation with a Cobalt(I) Pincer Complex

This study focuses on the stoichiometric reactions of {2,6-(iPr2PO)2C6H3}Co(PMe3)2 with terminal alkynes, thiols, and tin hydrides as part of an effort to develop catalytic, two-electron processes with cobalt. This specific Co(I) pincer complex proves to be effective for cleaving the C(sp)-H, S-H, and Sn-H bonds to give oxidative addition products with the general formula {2,6-(iPr2PO)2C6H3}CoHX(PMe3) (X = alkynyl, thiolate, and stannyl groups) along with the free PMe3. These reactions typically reach completion when the substituents on acetylene, sulfur, and tin are electron-withdrawing groups (e.g., phenyl, pyridyl, and alkenyl groups). In contrast, alkyl-substituted acetylenes, 1-pentanethiol, and tributyltin hydride are partially converted due to the equilibria with the corresponding oxidative addition products. The Co(I) pincer complex is not a hydrothiolation catalyst but capable of catalyzing the hydrostannation of terminal alkynes with Ph3SnH to produce β-(Z)-alkenylstannanes selectively.

Recent Advances on the Chemistry of Transition Metal Complexes with Monoanionic Bidentate Silyl Ligands

The chemistry of transition-metal (TM) complexes with monoanionic bidentate (κ2-L,Si) silyl ligands has considerably grown in recent years. This work summarizes the advances in the chemistry of TM-(κ2-L,Si) complexes (L=N-heterocycle, phosphine, N-heterocyclic carbene, thioether, ester, silylether or tetrylene). The most common synthetic method has been the oxidative addition of the Si-H bond to the metal center assisted by the coordination of L. The metal silicon bond distances in TM-(κ2-L,Si) complexes are in the range of metal-silyl bond distances. TM-(κ2-L,Si) complexes have proven to be effective catalysts for hydrosilylation and/or hydrogenation of unsaturated molecules among other processes.

X-type silyl ligands for transition-metal catalysis

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.

Iron complexes supported by silyl-NHC chelate ligands: synthesis and use for double hydroboration of nitriles

Iron complexes bearing new silyl-NHC bidentate ligands were synthesised by treating Fe3(CO)12 with a mixture of N-(hydrosilyl)methyl imidazolium salts and a base. These complexes showed high performance in the catalytic double hydroboration of nitrile with pinacolborane (HBpin) to produce N,N-bis(boryl)amine by a combination of UV irradiation and mild heating (60 °C). The product yields for the hydroboration of aromatic and aliphatic nitriles reached 85%-95% (NMR) using an iron complex (5 mol%). Reducing the loading amount of the iron complex to 0.5 mol% still afforded the products in high yields. An analogous ruthenium complex, which was similarly synthesised using Ru3(CO)12, showed lower activity. Stoichiometric reactions of the iron complex with nitriles afforded Fe(0)-N-silylimine complexes, which may be dormant states in nitrile hydroboration. A catalytic mechanism including Fe(0) N-silylimine species is proposed.

Experimental and Theoretical Study of NiII - and PdII -Promoted Double Geminal C(sp3 )-H Bond Activation Providing Facile Access to NHC Pincer Complexes: Isolated Intermediates and Mechanism

We report the first examples of metal-promoted double geminal activation of C(sp3 )-H bonds of the N-CH2 -N moiety in an imidazole-type heterocycle, leading to nickel and palladium N-heterocyclic carbene complexes under mild conditions. Reaction of the new electron-rich diphosphine 1,3-bis((di-tert-butylphosphaneyl)methyl)-2,3-dihydro-1H-benzo[d]imidazole (1) with [PdCl2 (cod)] occurred in a stepwise fashion, first by single C-H bond activation yielding the alkyl pincer complex [PdCl(PCsp 3 H P)] (3) with two trans phosphane donors and a covalent Pd-Csp 3 bond. Activation of the C-H bond of the resulting α-methine Csp 3 H-M group occurred subsequently when 3 was treated with HCl to yield the NHC pincer complex [PdCl(PCNHC P)]Cl (2). Treatment of 1 with [NiBr2 (dme)] also afforded a NHC pincer complex, [NiBr(PCNHC P)]Br (6), but the reactions leading to the double geminal C-H bond activation of the N-CH2 -N group were too fast to allow identification or isolation of an intermediate analogous to 3. The determination of six crystal structures, the isolation of reaction intermediates and DFT calculations provided the basis for suggesting the mechanism of the stepwise transformation of a N-CH2 -N moiety in the N-CNHC -N unit of NHC pincer complexes and explain the key differences observed between the Pd and Ni chemistries.

Reactivity of a T-shaped cobalt(I) pincer-complex

The reactivity of a paramagnetic T-shaped cobalt(i) complex, [(iPrboxmi)Co], stabilised by a monoanionic bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)], accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)], [(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-supported cobalt(i) complex to homolytically cleave bonds and thus its distinct one-electron reactivity. Furthermore, a square-planar cobalt(ii) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene. Putting such species in the context of the previously investigated hydroboration catalysis, its stoichiometric reaction with pinacolborane revealed its potential conversion into a cobalt(ii) hydride complex, thus confirming its original attribution as off-cycle species.

A motif for heteronuclear C[triple bond, length as m-dash]E (E = Si, Ge, Sn, Pb) bonding: Lewis acid-base pair strategy

The design and characterization of the heteronuclear group 14 C[triple bond, length as m-dash]E (E = Si, Ge, Sn, Pb) triple bonds have attracted intensive interest in the past few decades. In the current work, utilizing the advantages of N-heterocyclic carbenes (NHCs) and Lewis acid-base pair strategy, we theoretically designed a new class of compounds III-1, i.e., (NHCAR)C[triple bond, length as m-dash]E(Al(C6F5)3). Quantum chemical calculations showed that these singlet compounds possess very favourable isomerization, fragmentation and dimerization stabilities at the B3LYP/def2-TZVPP//B3LYP/def2-SVP level. The calculated bond lengths of CE in III-1 are 1.63 Å for Si, 1.70 Å for Ge, 1.91 Å for Sn and 2.01 Å for Pb, respectively, which are close to or even shorter than the known C[triple bond, length as m-dash]E bond lengths. In addition, the significant Mayer bond order values, two orthogonal π orbitals and one σ orbital between the C and E atoms also indicate the characteristics of triple bonds. Based on several bonding analyses, strong delocalization is found to exist between the C[triple bond, length as m-dash]E core and NHCAR forming a weak C[double bond, length as m-dash]C double bond. Hence, such obtained C[triple bond, length as m-dash]E species also can be described by their resonace structures as cunmulene analogs. In all, III-1 proposed here not only presents a universal C[triple bond, length as m-dash]E motif for all the heavier group 14 elements, but also provides a new strategy for the design and synthesis of heteronuclear group 14 triple bonds in the future.

Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron Dialkyl Complexes

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CH₂SiMe₃)₂, have been developed. The first route consists of addition of two equivalents of LiCH₂SiMe₃ to the iron dihalide complex, (CNC)FeBr₂, while the second relies on addition of the free CNC ligand to readily-prepared (py)₂Fe(CH₂SiMe₃)₂ (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ( ^Ar CNC)Fe(CH₂SiMe₃)₂(N₂) ( ^Ar CNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC-chelate. In contrast, the alkyl-substituted variant, (^tBuACNC)Fe(CH₂SiMe₃)₂ (^tBuACNC = 2,6-(^tBu-imidazol-2-ylidene)₂pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the ( ^Ar CNC)Fe(CH₂SiMe₃)₂(N₂) derivatives to an H₂ atmosphere resulted in formation of the corresponding iron hydride complexes ( ^Ar CNC)FeH₄. These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H₂, generating isotopologues and isotopomers of ( ^Ar CNC)Fe(H _n )(D_4-n ) (n = 0-4). When (3,5-Me₂ ^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (3,5-Me₂ ^MesCNC = 2,6-(2,4,6-Me₃-C6H2-imidazol-2-ylidene)₂-3,5-Me₂-pyridine) was treated successively with H₂ and then N₂, the corresponding reduced dinitrogen complex (3,5-Me₂ ^MesCNC)Fe(N₂)₂ was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me₂ ^MesCNC)Fe(CH₂SiMe₃)₂(N₂).

σ-Silane Platinum(II) Complexes as Intermediates in C-Si Bond-Coupling Processes

Platinum complexes [Pt(NHC')(NHC)][BAr^F ] (in which NHC' denotes a cyclometalated N-heterocyclic carbene ligand, NHC) react with primary silanes RSiH₃ to afford the cyclometalated platinum(II) silyl complexes [Pt(NHC-SiHR')(NHC)][BAr^F ] through a process that involves the formation of C-Si and Pt-Si bonds with concomitant extrusion of H₂ . Low-temperature NMR studies indicate that the process proceeds through initial formation of the σ-SiH complexes [Pt(NHC')(NHC)(HSiH₂ R)][BAr^F ], which are stable at temperatures below -10 °C. At higher temperatures, activation of one Si-H bond followed by a C-Si coupling reaction generates an agostic SiH platinum hydride derivative [Pt(H)(NHC'-SiH₂ R)(NHC)][BAr^F ], which undergoes a second Si-H bond activation to afford the final products. Computational modeling of the reaction mechanism indicates that the stereochemistry of the silyl/hydride ligands after the first Si-H bond cleavage dictates the nature of the products, favoring the formation of a C-Si bond over a C-H bond, in contrast to previous results obtained for tertiary silanes. Furthermore, the process involves a trans-to-cis isomerization of the NHC ligand before the second Si-H bond cleavage.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

High-Oxidation-State 3d Metal (Ti-Cu) Complexes with N-Heterocyclic Carbene Ligation

High-oxidation-state 3d metal species have found a wide range of applications in modern synthetic chemistry and materials science. They are also implicated as key reactive species in biological reactions. These applications have thus prompted explorations of their formation, structure, and properties. While the traditional wisdom regarding these species was gained mainly from complexes supported by nitrogen- and oxygen-donor ligands, recent studies with N-heterocyclic carbenes (NHCs), which are widely used for the preparation of low-oxidation-state transition metal complexes in organometallic chemistry, have led to the preparation of a large variety of isolable high-oxidation-state 3d metal complexes with NHC ligation. Since the first report in this area in the 1990s, isolable complexes of this type have been reported for titanium(IV), vanadium(IV,V), chromium(IV,V), manganese(IV,V), iron(III,IV,V), cobalt(III,IV,V), nickel(IV), and copper(II). With the aim of providing an overview of this intriguing field, this Review summarizes our current understanding of the synthetic methods, structure and spectroscopic features, reactivity, and catalytic applications of high-oxidation-state 3d metal NHC complexes of titanium to copper. In addition to this progress, factors affecting the stability and reactivity of high-oxidation-state 3d metal NHC species are also presented, as well as perspectives on future efforts.

[(NHC)CoR2]: pre-catalysts for homogeneous olefin and alkyne hydrogenation

[(ItBu)Co(CH₂SiMe₃)₂] serves as an efficient, homogeneous olefin hydrogenation catalyst.

Synthesis, structures, and reactivity studies of cyclometalated N-heterocyclic carbene complexes of ruthenium

Dehydrochlorination of Cp*Ru(IPr)Cl leads to an unusual C–C bond activation, yielding a cyclometalated Ru complex bearing an NHC-C(sp²) ligand. Reactivity studies of cyclometalated Ru complexes were explored.