Tufan K. Mukhopadhyay

A Highly Active Manganese Precatalyst for the Hydrosilylation of Ketones and Esters

The reduction of ((Ph2PPr)PDI)MnCl2 allowed the preparation of the formally zerovalent complex, ((Ph2PPr)PDI)Mn, which features a pentadentate bis(imino)pyridine chelate. This complex is a highly active precatalyst for the hydrosilylation of ketones, exhibiting TOFs of up to 76,800 h(-1) in the absence of solvent. Loadings as low as 0.01 mol % were employed, and ((Ph2PPr)PDI)Mn was found to mediate the atom-efficient utilization of Si-H bonds to form quaternary silane products. ((Ph2PPr)PDI)Mn was also shown to catalyze the dihydrosilylation of esters following cleavage of the substrate acyl C-O bond. Electronic structure investigation of ((Ph2PPr)PDI)Mn revealed that this complex possesses an unpaired electron on the metal center, rendering it likely that catalysis takes place following electron transfer to the incoming carbonyl substituent.

A Pentacoordinate Mn(II) Precatalyst That Exhibits Notable Aldehyde and Ketone Hydrosilylation Turnover Frequencies

Heating (THF)2MnCl2 in the presence of the pyridine-substituted bis(imino)pyridine ligand, (PyEt)PDI, allowed preparation of the respective dihalide complex, ((PyEt)PDI)MnCl2. Reduction of this precursor using excess Na/Hg resulted in deprotonation of the chelate methyl groups to yield the bis(enamide)tris(pyridine)-supported product, (κ(5)-N,N,N,N,N-(PyEt)PDEA)Mn. This complex was characterized by single-crystal X-ray diffraction and found to possess an intermediate-spin (S = (3)/2) Mn(II) center by the Evans method and electron paramagnetic resonance spectroscopy. Furthermore, (κ(5)-N,N,N,N,N-(PyEt)PDEA)Mn was determined to be an effective precatalyst for the hydrosilylation of aldehydes and ketones, exhibiting turnover frequencies of up to 2475 min(-1) when employed under solvent-free conditions. This optimization allowed for isolation of the respective alcohols and, in two cases, the partially reacted silyl ethers, PhSiH(OR)2 [R = Cy and CH(Me)((n)Bu)]. The aldehyde hydrosilylation activity observed for (κ(5)-N,N,N,N,N-(PyEt)PDEA)Mn renders it one of the most efficient first-row transition metal catalysts for this transformation reported to date.

Mechanistic Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and Carboxylate Hydrosilylation

We recently reported a bis(imino)pyridine (or pyridine diimine, PDI) manganese precatalyst, (Ph2PPrPDI)Mn (1), that is active for the hydrosilylation of ketones and dihydrosilylation of esters. In this contribution, we reveal an expanded scope for 1-mediated hydrosilylation and propose two different mechanisms through which catalysis is achieved. Aldehyde hydrosilylation turnover frequencies (TOFs) of up to 4900 min-1 have been realized, the highest reported for first row metal-catalyzed carbonyl hydrosilylation. Additionally, 1 has been shown to mediate formate dihydrosilylation with leading TOFs of up to 330 min-1. Under stoichiometric and catalytic conditions, addition of PhSiH3 to (Ph2PPrPDI)Mn was found to result in partial conversion to a new diamagnetic hydride compound. Independent preparation of (Ph2PPrPDI)MnH (2) was achieved upon adding NaEt3BH to (Ph2PPrPDI)MnCl2 and single-crystal X-ray diffraction analysis revealed this complex to possess a capped trigonal bipyramidal solid-state geometry. When 2,2,2-trifluoroacetophenone was added to 1, radical transfer yielded (Ph2PPrPDI·)Mn(OC·(Ph)(CF3)) (3), which undergoes intermolecular C-C bond formation to produce the respective Mn(II) dimer, [(μ-O,Npy-4-OC(CF3)(Ph)-4-H-Ph2PPrPDI)Mn]2 (4). Upon finding 3 to be inefficient and 4 to be inactive, kinetic trials were conducted to elucidate the mechanisms of 1- and 2-mediated hydrosilylation. Varying the concentration of 1, substrate, and PhSiH3 revealed a first order dependence on each reagent. Furthermore, a kinetic isotope effect (KIE) of 2.2 ± 0.1 was observed for 1-catalyzed hydrosilylation of diisopropyl ketone, while a KIE of 4.2 ± 0.6 was determined using 2, suggesting 1 and 2 operate through different mechanisms. Although kinetic trials reveal 1 to be the more active precatalyst for carbonyl hydrosilylation, a concurrent 2-mediated pathway is more efficient for carboxylate hydrosilylation. Considering these observations, 1-catalyzed hydrosilylation is believed to proceed through a modified Ojima mechanism, while 2-mediated hydrosilylation occurs via insertion.

Carbon Dioxide Promoted H+ Reduction Using a Bis(imino)pyridine Manganese Electrocatalyst

Heating a 1:1 mixture of (CO)5MnBr and the phosphine-substituted pyridine diimine ligand, (Ph2PPr)PDI, in THF at 65 °C for 24 h afforded the diamagnetic complex [((Ph2PPr)PDI)Mn(CO)][Br] (1). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic ((Ph2PPr)PDI)MnBr (2). The molecular structure of 1 was determined by single crystal X-ray diffraction, and density functional theory (DFT) calculations indicate that this complex is best described as low-spin Mn(I) bound to a neutral (Ph2PPr)PDI chelating ligand. The redox properties of 1 and 2 were investigated by cyclic voltammetry (CV), and each complex was tested for electrocatalytic activity in the presence of both CO2 and Brønsted acids. Although electrocatalytic response was not observed when CO2, H2O, or MeOH was added to 1 individually, the addition of H2O or MeOH to CO2-saturated acetonitrile solutions of 1 afforded voltammetric responses featuring increased current density as a function of proton source concentration (icat/ip up to 2.4 for H2O or 4.2 for MeOH at scan rates of 0.1 V/s). Bulk electrolysis using 5 mM 1 and 1.05 M MeOH in acetonitrile at -2.2 V vs Fc(+/0) over the course of 47 min gave H2 as the only detectable product with a Faradaic efficiency of 96.7%. Electrochemical experiments indicate that CO2 promotes 1-mediated H2 production by lowering apparent pH. While evaluating 2 for electrocatalytic activity, this complex was found to decompose rapidly in the presence of acid. Although modest H(+) reduction activity was realized, the experiments described herein indicate that care must be taken when evaluating Mn complexes for electrocatalytic CO2 reduction.

Investigation of formally zerovalent Triphos iron complexes

The reduction of Triphos [PhP(CH(2)CH(2)PPh(2))(2)] iron halide complexes has been explored, yielding formally zerovalent (κ(3)-Triphos)Fe(κ(2)-Triphos) and (κ(3)-Triphos)Fe(κ(2)-Bpy). Electrochemical analysis, coupled with the metrical parameters of (κ(3)-Triphos)Fe(κ(2)-Bpy), reveal an electronic structure consistent with a π-radical monoanion bipyridine chelate that is antiferromagnetically coupled to a low spin, Fe(I) metal center.

Reactivity of (Triphos)FeBr2(CO) towards sodium borohydrides

Abstract The addition of CO to (Triphos)FeBr2 (Triphos = PhP(CH2CH2PPh2)2) resulted in formation of six-coordinate (Triphos)FeBr2(CO). This coordination compound was found to have cis-bromide ligands and a mer-Triphos ligand by single crystal X-ray diffraction. Once characterized, the reactivity of this compound toward NaEt3BH and NaBH4 was investigated. Adding 1 eq. of NaEt3BH to (Triphos)FeBr2(CO) resulted in formation of (Triphos)FeH(Br)(CO), while the addition of 2.2 eq. afforded previously described (Triphos)Fe(CO)2. In contrast, adding 2.2 eq. of NaBH4 to (Triphos)FeBr2(CO) resulted in carbonyl dissociation and formation of diamagnetic (Triphos)FeH(η2-BH4), which has been structurally characterized. Notably, efforts to prepare (Triphos)FeH(η2-BH4) following 2.2 eq. NaBH4 addition to (Triphos)FeBr2 were unsuccessful. The importance of these observations as they relate to previously reported (Triphos)Fe reactivity and recent developments in Fe catalysis are discussed.

Isolation of Mn(I) Compounds Featuring a Reduced Bis(imino)pyridine Chelate and Their Relevance to Electrocatalytic Hydrogen Production

We report the preparation and electronic structure determination of chelate-reduced Mn(I) compounds that are relevant to electrocatalytic proton reduction mediated by [(Ph2PPrPDI)Mn(CO)][Br]. Reducing [(Ph2PPrPDI)Mn(CO)][Br] with excess Na-Hg afforded a neutral paramagnetic complex, (Ph2PPrPDI)Mn(CO). This compound was found to feature a low spin Mn(I) center and a PDI radical anion as determined by magnetic susceptibility measurement (1.97 μB), EPR spectroscopy ( S = 1/2), and density functional theory calculations. When [(Ph2PPrPDI)Mn(CO)][Br] was reduced with K-Hg, Mn(I) complexes with highly activated CO ligands were obtained. Recrystallization of the reduced product from diethyl ether solution allowed for the isolation of dimeric [(κ4-Ph2PPrPDI)Mn(μ-η1,η1,η2-CO)K(Et2O)]2 (νCO = 1710 cm-1, 1656 cm-1), while methyl tert-butyl ether treatment afforded dimeric [(κ4-Ph2PPrPDI)Mn(μ-η1,η1-CO)K(MTBE)2]2 (νCO = 1695 cm-1, MTBE = methyl tert-butyl ether). Addition of 18-crown-6 to these products, or conducting the K-Hg reduction of [(Ph2PPrPDI)Mn(CO)][Br] in the presence of 18-crown-6, allowed for the isolation of a monomeric example, (κ4-Ph2PPrPDI)Mn(μ-η1,η2-CO)K(18-crown-6) (νCO = 1697 cm-1). All three complexes were found to be diamagnetic and were characterized thoroughly by multinuclear 1D and 2D NMR spectroscopy and single crystal X-ray diffraction. Detailed analysis of the metrical parameters and spectroscopic properties suggest that all three compounds possess a Mn(I) center that is supported by a PDI dianion. Importantly, (κ4-Ph2PPrPDI)Mn(μ-η1,η2-CO)K(18-crown-6) was found to react instantaneously with either HBF4·OEt2 or HOTf to evolve H2 and generate the corresponding Mn(I) complex, [(Ph2PPrPDI)Mn(CO)][BF4] or [(Ph2PPrPDI)Mn(CO)][OTf], respectively. These products are spectroscopically and electrochemically similar to previously reported [(Ph2PPrPDI)Mn(CO)][Br]. It is believed that the mechanism of [(Ph2PPrPDI)Mn(CO)][Br]-mediated proton reduction involves intermediates that are related to the compounds described herein and that their ambient temperature isolation is aided by the redox active nature of Ph2PPrPDI.