Scope and Mechanistic Studies of Catalytic Hydrosilylation with a High-Valent Nitridoruthenium(VI)

Expanding Zirconocene Hydride Catalysis: In Situ Generation and Turnover of ZrH Catalysts Enabling Catalytic Carbonyl Reductions

Despite the wide use and popularity of metal hydride catalysis, methods utilizing zirconium hydride catalysts remain underexplored. Here, we report the development of a mild method for the in situ preparation and use of zirconium hydride catalysts. This robust method requires only 2.5-5 mol % of zirconocene dichloride in combination with a hydrosilane as the stoichiometric reductant and does not require careful air- or moisture-free techniques. A key finding of this study concerns an amine-mediated ligand exchange en route to the active zirconocene hydride catalyst. A mechanistic investigation supports the intermediacy of an oxo-bridged dimer precatalyst. The application of this method to the reduction of a wide variety of carbonyl-containing substrates, including ketones, aldehydes, enones, ynones, and lactones, is demonstrated with up to 92% yield and exhibits broad functional group tolerability. These findings open up alternative avenues for the catalytic application of chlorozirconocenes, potentially serving as the foundation for broader applications of zirconium hydride catalysis.

Electrochemical reduction of functionalized carbonyl compounds: enhanced reactivity over tailored nanoporous gold

The effect of the pore size of nanoporous gold on electrochemical reduction of functionalized carbonyl compounds was investigated. NPG with a pore size of ∼30 nm significantly enhanced the reactivity with high chemo-selectivity at a low-potential. Typically, p-nitrobenzaldehyde reduction demonstrates a high turnover frequency (TOF) up to 232 000 h-1.

Cationic rhenium(iii) complexes: synthesis, characterization, and reactivity for hydrosilylation of aldehydes

A series of novel cationic Re(iii) complexes [(DAAm)Re(CO)(NCCH₃)₂][X] [DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C₆F₅ (a), Mes (b)] [X = OTf (2), BAr^F₄ [BAr^F₄ = tetrakis[3,5-(trifluoromethyl)phenyl]borate] (3), BF₄ (4), PF₆ (5)], and their analogue [(DAmA)Re(CO)(Cl)₂] [DAmA = N,N-bis(2-arylamineethyl)methylamino; aryl = C₆F₅] (6) were synthesized. The catalytic efficiency for the hydrosilylation reaction of aldehydes using 4a (0.03 mol%) has been demonstrated to be significantly more active than rhenium catalysts previously reported in the literature. The data suggest that electron-withdrawing substituents at the diamido amine ligand increase the catalytic efficiency of the complexes. Excellent yields were achieved at ambient temperature under neat conditions using dimethylphenylsilane. The reaction affords TONs of up to 9200 and a TOF of up to 126 h^-1. Kinetic and mechanistic studies were performed, and the data suggest that the reaction is via a non-hydride ionic hydrosilylation mechanism.

Mechanistic insights into the catalytic carbonyl hydrosilylation by cationic [CpM(CO)2(IMes)]+ (M = Mo, W) complexes: the intermediacy of η1-H(Si) metal complexes

The ionic S_N2-type mechanistic pathway initiated by silane end-on coordination on the metal centers, forming η¹-H(Si) Mo/W complexes, is the preferred reaction pathway for the two cationic cyclopentadienyl molybdenum/tungsten complexes, [CpM(CO)₂(IMes)]⁺ (M = Mo, W) in catalyzing carbonyl hydrosilylation.

Versatile Manganese Catalysis for the Synthesis of Poly(silylether)s from Diols and Dicarbonyls with Hydrosilanes

Poly(silylether)s are interesting materials because of their degradation property under hydrolytic conditions and have been prepared via hydrosilylation polymerization from dicarbonyl and hydrosilanes, and via dehydrogenative cross-coupling of diols and hydrosilanes under catalytic conditions. Here, we present a manganese-salen compound based on an inexpensive and nontoxic metal that could effectively catalyze both polymerization reactions with hydrosilane. A series of poly(silylether)s containing various aliphatic and aromatic backbones have been synthesized from diol and dicarbonyl substrates. Moderate to high yields of polymers with number-average molecular weights up to 15 kg/mol are obtained. Because of the dual activity of the manganese catalyst, unsymmetrical substrates with mixed functional groups, such as p-hydroxybenzaldehyde, p-hydroxy benzylalcohol, and 3-(4-hydroxyphenyl)-1-propanol, have been employed to afford poly(silylether)s with multiple silicon connectivity in the main chain.

Reduction of carbon dioxide and organic carbonyls by hydrosilanes catalysed by the perrhenate anion

A simple quaternary ammonium perrhenate salt catalyses the hydrosilylation of aldehydes, ketones, and carbon dioxide, and the methylation of amines using carbon dioxide. DFT calculations show that a perrhenate hypervalent silicate interacts directly with CO₂.

Imidazolium-based ionic liquid-catalyzed hydrosilylation of imines and reductive amination of aldehydes using hydrosilane as the reductant

Efficient imidazolium-based ionic liquid-catalyzed hydrosilylation of imines and reductive amination using a catalytic amount of [BMIm][FeCl₄].

Redox-Active-Ligand-Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands

Coordination of a redox-active pyridine aminophenol ligand to Ru(II) followed by aerobic oxidation generates two diamagnetic Ru(III) species [1 a (cis) and 1 b (trans)] with ligand-centered radicals. The reaction of 1 a/1 b with excess NaN3 under inert atmosphere resulted in the formation of a rare bis(nitrido)-bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru-N-Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a/1 b supports the oxidation of a nitride (N(3-) ) to half an equivalent of N2 . The trinuclear omplex is reactive toward TEMPO-H, tin hydrides, thiols, and dihydrogen.

Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn(III)(TBP8Cz)] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn(III)(TBP8Cz) to give Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] with protonation constants of 9.0 × 10(6) and 4.7 × 10(3) M(-1), respectively. The protonated sites of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn(III)(OTf)(TBP8Cz(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O2 to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] in the presence of O2 revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn(III)(OTf)(TBP8Cz(H)) reacted with O2 with a diffusion-limited rate constant to produce the putative Mn(IV)(O2(•-))(OTf)(TBP8Cz(H)), whereas the tripseptet excited state of [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] exhibited no reactivity toward O2. In the presence of HOTf, Mn(V)(O)(TBP8Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn(III)(OTf)(TBP8Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d12, suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn(III)(OTf)(TBP8Cz(H)), acts as an efficient photocatalyst for the oxidation of HMB by O2 to the alcohol.

Dehydrogenative coupling of alcohols and carboxylic acids with hydrosilanes catalyzed by a salen–Mn(v) complex

A manganese–salen complex is shown to be effective in catalyzing the dehydrogenative coupling of hydroxyl groups with hydrosilane.

Amine synthesis via transition metal homogeneous catalysed hydrosilylation

This review summarizes the preparation of amines involving homogeneous transition metal catalysed hydrosilylation including reductions of imines, amides, nitro and nitriles, reductive aminations and N-methylation of amines with CO₂.

p-Tolylimido rhenium(v) complexes with phenolate-based ligands: synthesis, X-ray studies and catalytic activity in oxidation with tert-butylhydroperoxide

The reactions of mer-[Re(p-NTol)X₃(PPh₃)₂] with phenolate-based ligands gave 16 new rhenium(v) complexes. Only a few of them exhibited high catalytic activity.

Catalytic two-electron reduction of dioxygen by ferrocene derivatives with manganese(V) corroles

Electron transfer from octamethylferrocene (Me8Fc) to the manganese(V) imidocorrole complex (tpfc)Mn(V)(NAr) [tpfc = 5,10,15-tris(pentafluorophenyl)corrole; Ar = 2,6-Cl2C6H3] proceeds efficiently to give an octamethylferrocenium ion (Me8Fc(+)) and [(tpfc)Mn(IV)(NAr)](-) in acetonitrile (MeCN) at 298 K. Upon the addition of trifluoroacetic acid (TFA), further reduction of [(tpfc)Mn(IV)(NAr)](-) by Me8Fc gives (tpfc)Mn(III) and ArNH2 in deaerated MeCN. TFA also results in hydrolysis of (tpfc)Mn(V)(NAr) with residual water to produce a protonated manganese(V) oxocorrole complex ([(tpfc)Mn(V)(OH)](+)) in deaerated MeCN. [(tpfc)Mn(V)(OH)](+) is rapidly reduced by 2 equiv of Me8Fc in the presence of TFA to give (tpfc)Mn(III) in deaerated MeCN. In the presence of dioxygen (O2), (tpfc)Mn(III) catalyzes the two-electron reduction of O2 by Me8Fc with TFA in MeCN to produce H2O2 and Me8Fc(+). The rate of formation of Me8Fc(+) in the catalytic reduction of O2 follows zeroth-order kinetics with respect to the concentrations of Me8Fc and TFA, whereas the rate increases linearly with increasing concentrations of (tpfc)Mn(V)(NAr) and O2. These kinetic dependencies are consistent with the rate-determining step being electron transfer from (tpfc)Mn(III) to O2, followed by further proton-coupled electron transfer from Me8Fc to produce H2O2 and [(tpfc)Mn(IV)](+). Rapid electron transfer from Me8Fc to [(tpfc)Mn(IV)](+) regenerates (tpfc)Mn(III), completing the catalytic cycle. Thus, catalytic two-electron reduction of O2 by Me8Fc with (tpfc)Mn(V)(NAr) as a catalyst precursor proceeds via a Mn(III)/Mn(IV) redox cycle.

New insights into hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds by rhenium(V)-dioxo complexes

The hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds catalyzed by high-valent rhenium(V)-dioxo complex ReO2I(PPh3)2 (1) were studied computationally to determine the underlying mechanism. Our calculations revealed that the ionic outer-sphere pathway in which the organic substrate attacks the Si center in an η(1)-silane rhenium adduct to prompt the heterolytic cleavage of the Si-H bond is the most energetically favorable process for rhenium(V)-dioxo complex 1 catalyzed hydrosilylation of imines. The activation energy of the turnover-limiting step was calculated to be 22.8 kcal/mol with phenylmethanimine. This value is energetically more favorable than the [2 + 2] addition pathway by as much as 10.0 kcal/mol. Moreover, the ionic outer-sphere pathway competes with the [2 + 2] addition mechanism for rhenium(V)-dioxo complex 1 catalyzing the hydrosilylation of carbonyl compounds. Furthermore, the electron-donating group on the organic substrates would induce a better activity favoring the ionic outer-sphere mechanistic pathway. These findings highlight the unique features of high-valent transition-metal complexes as Lewis acids in activating the Si-H bond and catalyzing the reduction reactions.

The unexpected mechanism underlying the high-valent mono-oxo-rhenium(V) hydride catalyzed hydrosilylation of C=N functionalities: insights from a DFT study

In this study, we theoretically investigated the mechanism underlying the high-valent mono-oxo-rhenium(V) hydride Re(O)HCl2(PPh3)2 (1) catalyzed hydrosilylation of C=N functionalities. Our results suggest that an ionic S(N)2-Si outer-sphere pathway involving the heterolytic cleavage of the Si-H bond competes with the hydride pathway involving the C=N bond inserted into the Re-H bond for the rhenium hydride (1) catalyzed hydrosilylation of the less steric C=N functionalities (phenylmethanimine, PhCH=NH, and N-phenylbenzylideneimine, PhCH=NPh). The rate-determining free-energy barriers for the ionic outer-sphere pathway are calculated to be ∼28.1 and 27.6 kcal mol(-1), respectively. These values are slightly more favorable than those obtained for the hydride pathway (by ∼1-3 kcal mol(-1)), whereas for the large steric C=N functionality of N,1,1-tri(phenyl)methanimine (PhCPh=NPh), the ionic outer-sphere pathway (33.1 kcal mol(-1)) is more favorable than the hydride pathway by as much as 11.5 kcal mol(-1). Along the ionic outer-sphere pathway, neither the multiply bonded oxo ligand nor the inherent hydride moiety participate in the activation of the Si-H bond.

Recent progress towards ionic hydrogenation: Lewis acid catalyzed hydrogenation using organosilanes as donors of hydride ions

Recent advances in ionic hydrogenation as well as its fundamental mechanism are summarized and discussed.

Does a multiply bonded oxo ligand directly participate in B–H bond activation by a high-valent di-oxo-molybdenum(vi) complex? A density functional theory study

The multiply bonded oxo ligand does not participate in the activation of the B–H bond with organic substrates of amides, amines, and nitriles by the high-valent oxo-molybdenum complex MoO₂Cl₂.

Mechanistic insights into B–H bond activation with the high-valent oxo-molybdenum complex MoO2Cl2

The ionic mechanistic model involving the heterolytic cleavage of the B–H bond is slightly energetically favorable than the [2+2] addition mechanism for the high-valent oxo-molybdenum complex MoO₂Cl₂activating the B–H bond.