Heterolytic cleavage of a Si-H bond by a metal-ligand cooperation of a cationic iridium amido complex and hydrosilylation of aldehydes

Heterolytic cleavage of a Si-H bond was achieved mediated by a metal-ligand cooperation of a cationic iridium amido complex. The reaction was applied to the catalytic hydrosilylation of benzaldehyde and its derivatives, affording the corresponding hydrosilylated products in moderate to good yields.

Cooperative B-H bond activation: Dual sites borane activation by redox active κ2-N,S-chelated complexes

Cooperative dual site activation of boranes by redox-active 1,3-N,S-chelated ruthenium species, mer-[PR₃{κ²-N,S-(L)}₂Ru{κ¹-S-(L)}], (mer-2a: R = Cy, mer-2b: R = Ph; L = NC₇H₄S₂), generated from the aerial oxidation of borate complexes, [PR₃{κ²-N,S-(L)}Ru{κ³-H,S,S′-BH₂(L)₂}] (trans–mer-1a: R = Cy, trans–mer-1b: R = Ph; L = NC₇H₄S₂), has been investigated. Utilizing the rich electronic behaviour of these 1,3-N,S-chelated ruthenium species, we have established that a combination of redox-active ligands and metal–ligand cooperativity has a big influence on the multisite borane activation. For example, treatment of mer-2a–b with BH₃·THF led to the isolation of fac-[PR₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(S₂C₇H₄)₂}] (fac-3a: R = Cy and fac-3b: R = Ph) that captured boranes at both sites of the κ²-N,S-chelated ruthenacycles. The core structure of fac-3a and fac-3b consists of two five-membered ruthenacycles [RuBNCS] which are fused by one butterfly moiety [RuB₂S]. Analogous fac-3c, [PPh₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(SC₅H₄)₂}], can also be synthesized from the reaction of BH₃·THF with [PPh₃{κ²-N,S-(SNC₅H₄)}{κ³-H,S,S′-BH₂(SNH₄C₅)₂}Ru], cis–fac-1c. In stark contrast, when mer-2b was treated with BH₂Mes (Mes = 2,4,6-trimethyl phenyl) it led to the formation of trans- and cis-bis(dihydroborate) complexes [{κ³-S,H,H-(NH₂BMes)Ru(S₂C₇H₄)}₂], (trans-4 and cis-4). Both the complexes have two five-membered [Ru–(H)₂–B–NCS] ruthenacycles with κ²-H–H coordination modes. Density functional theory (DFT) calculations suggest that the activation of boranes across the dual Ru–N site is more facile than the Ru–S one.

Redox-active ruthenium complexes supported by hemilabile κ²-N,S-chelated ruthenacycles undergo unusual dual site B–H bond activation through metal–ligand cooperation with free and bulky boranes.

Pincer-supported metal/main-group bonds as platforms for cooperative transformations

Electron-rich late metals and electropositive main-group elements (metals and metalloids) can be combined to provide an ambiphilic façade for exploring metal-ligand cooperation, yet the instability of the metal/main-group bond frequently limits the study and application of such units. Incorporating main-group donors into pincer frameworks, where they are stabilized and held in proximity to the transition-metal partner, can allow discovery of new modes of reactivity and incorporation into catalytic processes. This Perspective summarizes common modes of cooperativity that have been demonstrated for pincer frameworks featuring metal/main-group bonds, highlighting similarities among boron, aluminium, and silicon donors and identifying directions for further development.

Access to and Reactivity of Fe0 , Fe-I , FeI , and FeII PCcarbene P Pincer Complexes

Despite their promising metal-ligand cooperative reactivity, PC_carbene P pincer ligands are rarely reported for first-row transition-metal centres. Using a dehydration methodology, we report access to an Fe⁰ PC_carbene P pincer complex (1) that proceeds via an isolated α-hydroxylalkyl hydrido complex (3). Reversible carbonyl migration to the carbene position in 1 is found to allow coordination chemistry and E-H bond addition (E=H, B, Cl) across the iron-carbene linkage, representing a unique mechanism for metal-ligand cooperativity. The PC_carbene P pincer ligand is also found to stabilize formal Fe^II , Fe^I , and Fe^-I oxidation states, as demonstrated with synthesis and characterization of the complexes [11-X][BAr^F ₂₀ ] (X=Br, I), 12, and K[13]. Compound K[13] is found to be highly reactive, and abstracts hydrogen from a range of aliphatic C-H sources. Computational analysis by DFT suggests that the formal Fe^I and Fe^-I complexes contain significant carbene radical character. The ability of the PC_carbene P ligand scaffold to partake in metal-ligand cooperativity and to support a range of iron oxidation states renders it as potentially useful in many catalytic applications.

Halogen Bonding Involving I2 and d8 Transition-Metal Pincer Complexes

We systematically investigated iodine–metal and iodine–iodine bonding in van Koten’s pincer complex and 19 modifications changing substituents and/or the transition metal with a PBE0–D3(BJ)/aug–cc–pVTZ/PP(M,I) model chemistry. As a novel tool for the quantitative assessment of the iodine–metal and iodine–iodine bond strength in these complexes we used the local mode analysis, originally introduced by Konkoli and Cremer, complemented with NBO and Bader’s QTAIM analyses. Our study reveals the major electronic effects in the catalytic activity of the M–I–I non-classical three-center bond of the pincer complex, which is involved in the oxidative addition of molecular iodine I2 to the metal center. According to our investigations the charge transfer from the metal to the σ* antibonding orbital of the I–I bond changes the 3c–4e character of the M–I–I three-center bond, which leads to weakening of the iodine I–I bond and strengthening of the metal–iodine M–I bond, facilitating in this way the oxidative addition of I2 to the metal. The charge transfer can be systematically modified by substitution at different places of the pincer complex and by different transition metals, changing the strength of both the M–I and the I2 bonds. We also modeled for the original pincer complex how solvents with different polarity influence the 3c–4e character of the M–I–I bond. Our results provide new guidelines for the design of pincer complexes with specific iodine–metal bond strengths and introduce the local vibrational mode analysis as an efficient tool to assess the bond strength in complexes.

Cooperative B-H and Si-H Bond Activations by κ2-N,S-Chelated Ruthenium Borate Complexes

Cooperative E-H (E = B, Si) bond activations employing κ²-N,S-chelated ruthenium borate species, [PPh₃{κ²-N,S-(NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(NC₇H₄S₂)₂}], (1) are established. Treatment of 1 with BH₃·SMe₂ yielded the six-membered ruthenaheterocycle [PPh₃{κ²-S,H-(BH₃NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (2) formed by a hemilabile ring opening of a Ru-N bond and capturing of a BH₃ unit coordinated in an "end-on" fashion. On the other hand, the bulky borane H₂BMes shows different reactivity with 1 that led to the formation of the two dihydroborate complexes [{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (3) and [PPh₃{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru(κ²-N,S-C₇H₄NS₂)] (4), in which H₂BMes has been inserted into the Ru-N bond of the initial κ²-N,S-chelated ligand. In an attempt to directly activate hydrosilanes by 1, reactions were carried out with H₂SiPh₂ that yielded two isomeric five-membered ruthenium silyl complexes, namely [PPh₃{κ²-S,Si-(NSiPh₂)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a,b), and the hydridotrisilyl complex [Ru(H){κ²-S,Si-(SiPh₂NC₇H₄S₂}₃] (6). These complexes were generated by Si-H bond activation with the release of H₂ and the formation of N-Si and Ru-Si bonds. When the reaction of 1 was carried out in the presence of PhSiH_3, the reaction only produced the analogous complexes [PPh₃{κ²-S,Si-(NSiPhH)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a',b'). Density functional theory (DFT) calculations have been used to probe the bonding modes of boranes/silane with the ruthenium center.

H/D exchange under mild conditions in arenes and unactivated alkanes with C6D6 and D2O using rigid, electron-rich iridium PCP pincer complexes

The synthesis and characterization of an iridium polyhydride complex (Ir-H4) supported by an electron-rich PCP framework is described. This complex readily loses molecular hydrogen allowing for rapid room temperature hydrogen isotope exchange (HIE) at the hydridic positions and the α-C-H site of the ligand with deuterated solvents such as benzene-d6, toluene-d8 and THF-d8. The removal of 1-2 equivalents of molecular H2 forms unsaturated iridium carbene trihydride (Ir-H3) or monohydride (Ir-H) compounds that are able to create further unsaturation by reversibly transferring a hydride to the ligand carbene carbon. These species are highly active hydrogen isotope exchange (HIE) catalysts using C6D6 or D2O as deuterium sources for the deuteration of a variety of substrates. By modifying conditions to influence the Ir-Hn speciation, deuteration levels can range from near exhaustive to selective only for sterically accessible sites. Preparative level deuterations of select substrates were performed allowing for procurement of >95% deuterated compounds in excellent isolated yields; the catalyst can be regenerated by treatment of residues with H2 and is still active for further reactions.

Ln(ii) and Ca(ii) NCsp3N pincer type diarylmethanido complexes – promising catalysts for C–C and C–E (E = Si, P, N, S) bond formation

The first examples of Ln(ii) (Ln = Yb, Sm) and Ca [NC_sp3N] pincer type diarylmethanido complexes were synthesized and successfully used as efficient and selective precatalyst for intermolecular C–C and C–E bond formation.

Cleavage of Si-H, B-H, and C-H Bonds by Metal-Ligand Cooperation

Metal-ligand cooperation, in which metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. In contrast to the classical bond cleavage by active metal centers with spectator ligands, metal-ligand cooperation has enabled unprecedented reactivities. Especially, metal-ligand cooperative H-H bond cleavage has been extensively studied and applied to various catalysts. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H-H bond. This review summarizes the recent progress in the metal-ligand cooperative cleavages of Si-H, B-H, and C-H bonds and their catalytic applications.

Synthesis and Structures of Stable PtII and PtIV Alkylidenes: Evidence for π-Bonding and Relativistic Stabilization

Isolable cationic Pt^II and Pt^IV alkylidenes, proposed intermediates in catalytic organic transformations, are reported. The bonding in these species was probed by experimental, structural, spectroscopic, electrochemical and computational methods, providing direct evidence for π-bonding, the often-theorized relativistic stabilization of these species, and the influence of oxidation state.

Silylene induced cooperative B–H bond activation and unprecedented aldehyde C–H bond splitting with amidinate ring expansion

Silylene mediated B–H and aldehyde C–H bond splitting were realized under ambient conditions.

Direct oxide transfer from an η2-keto ligand to generate a cobalt PCcarbeneP(O) pincer complex

We report the direct carbonyl cleavage in a κ³-P′,(η²-C,O),P′′ ligand by a monomeric cobalt centre through metal–ligand cooperativity.

Isolation of two bis(silyl)nickel complexes with Si–Si bond formation in a single-crystal-to-single-crystal fashion

Two unique silyl nickel complexes that generate two new Si–Si single bonds have been prepared unprecedentedly, which exhibit reversible single-crystal-to-single-crystal transformations upon the removal and rebinding of the coordinating PEt₃ molecule.

Synthesis and reactivity of a PCcarbeneP cobalt(i) complex: the missing link in the cobalt PXP pincer series (X = B, C, N)

We report the first example of a cobalt PC_carbeneP pincer complex (1) featuring a central alkylidene carbon donor accessed through the dehydration of an alcoholic POP proligand.

We report the first example of a cobalt PC_carbeneP pincer complex (1) featuring a central alkylidene carbon donor accessed through the dehydration of an alcoholic POP proligand. Complex 1 shares bonding similarities with cobalt PBP and PNP pincer complexes where the donor atom engages in π-bonding with the cobalt centre, and thus completes the PXP (X = B, C, N) pincer ligand series for cobalt (for X donors that partake in M–L π-bonding). As compared to PBP and PNP pincer complexes, which are known to be good hydride and proton acceptors (respectively), complex 1 is found to be an effective hydrogen atom acceptor. Complex 1 partakes in cooperative ligand reactivity, engaging in several small molecule activations with styrene, bromine, carbon disulphide, phenyl acetylene, acetonitrile, hydrogen, benzaldehyde and water (through microreversibility). The mechanism for the formation of complex 1 is studied through the isolation and computational analysis of key intermediates. The formation of 1 is found to avoid C–H activation of the proligand, and instead proceeds through a combination of O–H activation, hydrogen atom transfer, β-hydride elimination and hydrogen activation processes.

Redox-state dependent activation of silanes and ammonia with reverse polarity (PCcarbeneP)Ni complexes: electrophilic vs. nucleophilic carbenes

A rigidified PC_alkylP ligand allowed for the synthesis and characterization of cationic and radical PC_CarbeneP nickel complexes in which the carbene anchor of the pincer framework is electrophilic rather than nucleophilic.

Cooperative bond activation reactions with carbene complexes

This review highlights the recent advances in the application of carbene complexes in bond activation reactions via metal–ligand cooperation.

A hybrid carbocyclic/N-heterocyclic carbene ligand

We report the synthesis and characterisation of the first complex bearing a bidentate carbocyclic/N-heterocyclic carbene ligand, which represents an unprecedented combination of these two carbene classes. Starting from an unsubstituted cycloheptatrienylidene-Pd complex, the ligand was built up in an on-site synthesis at the metal center. DFT calculations were employed to understand the crucial allyl-to-alkyl rearrangement of the carbocycle in this sequence and all its binding modes during the reaction cascade were crystallographically characterised.

Planar PtPd3 Complexes Stabilized by Three Bridging Silylene Ligands

A heterobimetallic PtPd₃ complex supported by three bridging diphenylsilylene ligands, [Pt{Pd(dmpe)}₃ (μ₃ -SiPh₂ )₃ ] (1, dmpe=1,2-bis(dimethylphosphino)ethane), has been synthesized from mononuclear Pd and Pt complexes. The hexagonal core composed of Pt, Pd, and Si atoms is slightly larger than that of the tetrapalladium complex, [Pd{Pd(dmpe)}₃ (μ₃ -SiPh₂ )₃ ] (2). Reaction of PhSiH₃ with complex 1 in the presence and absence of Ph₂ SiH₂ results in the formation of a tetranuclear complex with silyl and hydride ligands at the Pt center, [PtH(SiPh₂ H){Pd(dmpe)}₃ (μ₃ -SiHPh)₃ ] (3), and an octanuclear complex, [{Pt{Pd(dmpe)}₃ (μ₃ -SiHPh)₃ }₂ (κ² -dmpe)] (5), respectively. Both M-Si (M=Pt, Pd) bond lengths and the ²⁹ Si NMR chemical shifts of 1 and 2 are located between those of mononuclear late transition-metal complexes with a silylene ligand and complexes with donor-stabilized silylene ligands. CuI and AgI adducts of 1 and 2, formulated as [M(μ-M'I){Pd(dmpe)}₃ (μ₃ -SiPh₂ )₃ ] (M=Pt, Pd; M'=Cu, Ag), undergo elimination of CuI (AgI) and regenerate the tetrametallic complexes upon heating or addition of a chelating diphosphine. Elimination of AgI from 2-AgI occurs more rapidly than elimination of CuI from 2-CuI, as determined from the results of kinetics experiments.

Facile generation of iridium PCcarbeneP pincer complexes via water elimination from an alcohol proligand

We report the facile generation of Ir PC_carbeneP pincer systems.

Heterobimetallic Pd-K carbene complexes via one-electron reductions of palladium radical carbenes

Heterobimetallic Pd-K carbenes featuring Pd-C_carbene-K moieties were synthesized via an unprecedented sequential substitution/reduction reaction from a radical precursor, [{PC˙(sp²)P} ^tBuPdI] ([PC(sp²)P] ^tBu = bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methylene). Polymeric structures were observed in the solid state for the heterobimetallic compounds that can be interrupted in the presence of a donor solvent.