Efficient Cobalt-Catalyzed Coupling of Amines and Siloxanes to Prepare Ceramics and Polymers

The phosphine-substituted aryl diimine cobalt catalyst, (Ph2PPrADI)Co, has been found to mediate the dehydrocoupling of diamines or polyamines to poly(methylhydrosiloxane) (PMHS) to generate hydrogen and crosslinked solids in an atom-efficient fashion. The resulting siloxane diamine and siloxane polyamine networks persist in the presence of air or water at room temperature and can tolerate temperatures of up to 1600 °C. Upon lowering the catalyst loading to 0.01 mol %, (Ph2PPrADI)Co was found to catalyze the dehydrocoupling of 1,3-propanediamine and PMHS (m = 35) to generate a siloxane diamine foam with a turnover frequency of 157 s–1 relative to diamine consumption, the highest activity ever reported for Si–N dehydrocoupling. Furthermore, upon systematically reducing the number of potential branch points, the (Ph2PPrADI)Co-catalyzed dehydrocoupling of diamines with hydride-terminated poly(dimethylsiloxane) (PDMS) was found to yield linear siloxane diamine polymers with molecular weights of up to 47,300 g/mol

Preparation and Hydrosilylation Activity of a Molybdenum Carbonyl Complex That Features a Pentadentate Bis(imino)pyridine Ligand

Attempts to prepare low-valent molybdenum complexes that feature a pentadentate 2,6-bis­(imino)­pyridine (or pyridine diimine, PDI) chelate allowed for the isolation of two different products. Refluxing Mo­(CO)₆ with the pyridine-substituted PDI ligand, ^PyEtPDI, resulted in carbonyl ligand substitution and formation of the respective bis­(ligand) compound (^PyEtPDI)₂Mo (<b>1</b>). This complex was investigated by single-crystal X-ray diffraction, and density functional theory calculations indicated that <b>1</b> possesses a Mo(0) center that back-bonds into the π*-orbitals of the unreduced PDI ligands. Heating an equimolar solution of Mo­(CO)₆ and the phosphine-substituted PDI ligand, ^Ph2PPrPDI, to 120 °C allowed for the preparation of (^Ph2PPrPDI)­Mo­(CO) (<b>2</b>), which is supported by a κ⁵-<i>N</i>,<i>N</i>,<i>N</i>,<i>P</i>,<i>P</i>-^Ph2PPrPDI chelate. Notably, <b>1</b> and <b>2</b> have been found to catalyze the hydrosilylation of benzaldehyde at 90 °C, and the optimization of <b>2</b>-catalyzed aldehyde hydrosilylation at this temperature afforded turnover frequencies of up to 330 h^–1. Considering additional experimental observations, the potential mechanism of <b>2</b>-mediated carbonyl hydrosilylation is discussed

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

The reduction of (^Ph₂PPrPDI)­MnCl₂ allowed the preparation of the formally zerovalent complex, (^Ph₂PPrPDI)­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 (^Ph₂PPrPDI)Mn was found to mediate the atom-efficient utilization of Si–H bonds to form quaternary silane products. (^Ph₂PPrPDI)Mn was also shown to catalyze the dihydrosilylation of esters following cleavage of the substrate acyl C–O bond. Electronic structure investigation of (^Ph₂PPrPDI)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

Hydrosilylation of Aldehydes and Formates Using a Dimeric Manganese Precatalyst

The formally zero-valent Mn dimer [(^Ph2PEtPDI)­Mn]₂ has been synthesized upon reducing (^Ph2PEtPDI)­MnCl₂ with excess Na/Hg. Single crystal X-ray diffraction analysis has revealed that [(^Ph2PEtPDI)­Mn]₂ possesses a κ⁴-PDI chelate about each Mn center, as well as η²-imine coordination across the dimer. The chelate metrical parameters suggest single electron PDI reduction and EPR spectroscopic analysis afforded a signal consistent with two weakly interacting <i>S</i> = ¹/₂ Mn centers. At ambient temperature in the absence of solvent, [(^Ph2PEtPDI)­Mn]₂ has been found to catalyze the hydrosilylation of aldehydes at loadings as low as 0.005 mol % (0.01 mol % relative to Mn) with a maximum turnover frequency of 9,900 min^–1 (4,950 min^–1 per Mn). Moreover, the [(^Ph2PEtPDI)­Mn]₂-catalyzed dihydrosilylation of formates has been found to proceed with turnover frequencies of up to 330 min^–1 (165 min^–1 relative to Mn). These metrics are comparable to those described for the leading Mn catalyst for this transformation, the propylene-bridged variant (^Ph2PPrPDI)­Mn; however, [(^Ph2PEtPDI)­Mn]₂ is more easily inhibited by donor functionalities. Carbonyl and carboxylate hydrosilylation is believed to proceed through a modified Ojima mechanism following dimer dissociation

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, ^PyEtPDI, allowed preparation of the respective dihalide complex, (^PyEtPDI)­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, (κ⁵-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-^PyEtPDEA)­Mn. This complex was characterized by single-crystal X-ray diffraction and found to possess an intermediate-spin (<i>S</i> = ³/₂) Mn­(II) center by the Evans method and electron paramagnetic resonance spectroscopy. Furthermore, (κ⁵-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-^PyEtPDEA)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)­(ⁿBu)]. The aldehyde hydrosilylation activity observed for (κ⁵-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-^PyEtPDEA)Mn renders it one of the most efficient first-row transition metal catalysts for this transformation reported to date

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

<p>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 <i>cis</i>-bromide ligands and a <i>mer</i>-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(<i>η</i>²-BH₄), which has been structurally characterized. Notably, efforts to prepare (Triphos)FeH(<i>η</i>²-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.</p

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 (<b>1</b>), that is active for the hydrosilylation of ketones and dihydrosilylation of esters. In this contribution, we reveal an expanded scope for <b>1</b>-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, <b>1</b> 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 (<b>2</b>) 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 <b>1</b>, radical transfer yielded (^Ph2PPrPDI<b>·</b>)­Mn­(OC<b>·</b>(Ph)­(CF₃)) (<b>3</b>), which undergoes intermolecular C–C bond formation to produce the respective Mn­(II) dimer, [(μ-<i>O</i>,<i>N</i>_py-4-OC­(CF₃)­(Ph)-4-H-^Ph2PPrPDI)­Mn]₂ (<b>4</b>). Upon finding <b>3</b> to be inefficient and <b>4</b> to be inactive, kinetic trials were conducted to elucidate the mechanisms of <b>1</b>- and <b>2</b>-mediated hydrosilylation. Varying the concentration of <b>1</b>, 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 <b>1</b>-catalyzed hydrosilylation of diisopropyl ketone, while a KIE of 4.2 ± 0.6 was determined using <b>2</b>, suggesting <b>1</b> and <b>2</b> operate through different mechanisms. Although kinetic trials reveal <b>1</b> to be the more active precatalyst for carbonyl hydrosilylation, a concurrent <b>2</b>-mediated pathway is more efficient for carboxylate hydrosilylation. Considering these observations, <b>1</b>-catalyzed hydrosilylation is believed to proceed through a modified Ojima mechanism, while <b>2-</b>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, ^Ph2PPrPDI, in THF at 65 °C for 24 h afforded the diamagnetic complex [(^Ph2PPrPDI)­Mn­(CO)]­[Br] (<b>1</b>). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic (^Ph2PPrPDI)­MnBr (<b>2</b>). The molecular structure of <b>1</b> 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 ^Ph2PPrPDI chelating ligand. The redox properties of <b>1</b> and <b>2</b> 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 <b>1</b> individually, the addition of H₂O or MeOH to CO₂-saturated acetonitrile solutions of <b>1</b> afforded voltammetric responses featuring increased current density as a function of proton source concentration (<i>i</i>_cat/<i>i</i>_p up to 2.4 for H₂O or 4.2 for MeOH at scan rates of 0.1 V/s). Bulk electrolysis using 5 mM <b>1</b> 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 <b>1</b>-mediated H₂ production by lowering apparent pH. While evaluating <b>2</b> 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

A New Spin on Cyclooctatetraene (COT) Redox Activity: Low-Spin Iron(I) Complexes That Exhibit Antiferromagnetic Coupling to a Singly Reduced η⁴‑COT Ligand

Formally zerovalent (κ³-phosphine)­Fe­(η⁴-COT) complexes supported by either Triphos (PhP­(CH₂CH₂PPh₂)₂) or Triphos* (H₃CC­(CH₂PPh₂)₃) have been prepared following chelate addition to (COT)₂Fe (COT = 1,3,5,7-cyclooctatetraene) and by reduction of the respective dibromide complexes in the presence of excess COT. The solid-state structure of each complex was determined by single-crystal X-ray diffraction, and close inspection of the metrical parameters revealed significant COT ligand reduction, independent of the coordination geometry about iron. While the neutral and dianionic forms of the redox-active COT ligand have historically received a great deal of attention, a dearth of information regarding the often-evoked radical monoanion form of this ligand prompted the full electronic structure investigation of these complexes using a range of techniques. Comparison of the Mössbauer spectroscopic data collected for both (Triphos)­Fe­(η⁴-COT) complexes with data obtained for two appropriate reference compounds indicated that they possess a low-spin Fe­(I) center that is antiferromagnetically coupled to a COT radical monoanion. Further evidence for this electronic structure determination by EPR spectroscopy and cyclic voltammetry is presented. A comparison of the solid-state metrical parameters determined in this study to those of related first-row transition-metal complexes has provided insight into the electronic structure analysis of related organometallic complexes