Rational Design of Rhodium Complexes Featuring κ4-N,N,N,N- and κ5-N,N,N,P,P-Bis(imino)pyridine Ligands

Hydrogen Production Using a Nickel Catalyst Combining Redox Activity and Pendent Base Effects

With the mounting need for clean and renewable energy, catalysts for hydrogen production based on earth abundant elements are of great interest. Herein, we describe the synthesis, characterization, and catalytic activity of two nickel complexes based on the pyridinediimine ligand that possess basic nitrogen moieties of pyridine and imidazole that could potentially serve as pendent bases to enhance catalysis. Although these ligands have previously been reported to be complexed to some metal ions, they have not been applied to nickel. The nickel complex with the pendent pyridines was found to be the most active of the two, catalyzing proton reduction electrochemically with an overpotential of 490 mV. The appearance of a wave that preceded the Ni(I/0) redox couple in the presence of protons suggests that protonation of a dissociated pyridine was likely. Further evidence of this was provided with density functional theory calculations, and a mechanism of hydrogen production is proposed. Furthermore, in a light-driven system containing Ru(bpy)32+ and ascorbic acid, TON of 1400 were obtained.

Acetate ion addition to and exchange in (1,5-cyclooctadiene)rhodium(I) acetate: relevance for the coagulation of carboxylic acid-functionalized shells of core-crosslinked micelle latexes

The solution behavior of complex [Rh(COD)(μ-OAc)]2 in the absence and presence of PPN+OAc- in dichloromethane has been investigated in detail by multinuclear NMR spectroscopy. Without additional acetate ions, the compound shows dynamic behavior at room temperature, consistent with an inversion of its C2v structure. Addition of PPN+OAc- reveals an equilibrated generation of [Rh(COD)(OAc)2]-. Rapid exchange is observed at room temperature between the neutral dimer and the anionic mononuclear complex, as well as between the anionic complex and free acetate. Lowering the temperature to 213 K freezes the exchange between the two Rh complexes, but fast exchange between the anionic Rh complex and free acetate maintains coalesced Me (1H and 13C) and COO (13C) NMR resonances. DFT calculations support the experimental data and lean in favour of a dissociative mechanism for the acetate exchange in [Rh(COD)(OAc)2]-. The acetate ligands in complex [Rh(COD)(μ-OAc)]2 are also exchanged in a biphasic (water/organic) system with the methacrylic acid (MAA) functions of hydrosoluble [MMA0.5-co-PEOMA0.5]30 copolymer chains (PEOMA = poly(ethylene oxide) methyl ether methacrylate), resulting in transfer of the Rh complex to the aqueous phase. Exchange with the MAA functions in the same polymer equally takes place for the chloride ligands of [Rh(COD)(μ-Cl)]2. The latter phenomenon rationalizes the coagulation of a core-crosslinked micelle (CCM) latex, where MMA functions are present on the hydrophilic CCM shell, when a dichloromethane solution of [Rh(COD)(μ-Cl)]2 is added.

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst

A cobalt complex bearing a κ-N3 P2 ligand is presented (1+ or CoI (L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1+ is stable under air at oxidation state CoI thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1+ reveals a two-electron CoI /CoIII oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset =-1.6 V vs Fc/Fc+ . In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1+ forms the cobalt hydride derivative CoIII (L)-H (22+ ), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII (L)-H (2+ ) species, which rapidly forms hydrogen and regenerates the initial CoI (L) (1+ ). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1+ .

Comparing the Electronic Structure of Iron, Cobalt, and Nickel Compounds That Feature a Phosphine-Substituted Bis(imino)pyridine Chelate

It was recently discovered that (^Ph2PPrPDI)Mn (PDI = pyridine diimine) exists as a superposition of low-spin Mn(II) that is supported by a PDI dianion and intermediate-spin Mn(II) that is antiferromagnetically coupled to a triplet PDI dianion, a finding that encouraged the synthesis and electronic structure evaluation of late first row metal variants that feature the same chelate. The addition of ^Ph2PPrPDI to FeBr₂ resulted in bromide dissociation and the formation of [(^Ph2PPrPDI)FeBr][Br]. Reduction of this precursor using excess sodium amalgam afforded (^Ph2PPrPDI)Fe, which possesses an Fe(II) center that is supported by a dianionic PDI ligand. Similarly, reduction of a premixed solution of ^Ph2PPrPDI and CoCl₂ yielded the cobalt analog, (^Ph2PPrPDI)Co. EPR spectroscopy and density functional theory calculations revealed that this compound features a high-spin Co(I) center that is antiferromagnetically coupled to a PDI radical anion. The addition of ^Ph2PPrPDI to Ni(COD)₂ resulted in ligand displacement and the formation of (^Ph2PPrPDI)Ni, which was found to possess a pendent phosphine group. Single-crystal X-ray diffraction, CASSCF calculations, and EPR spectroscopy indicate that (^Ph2PPrPDI)Ni is best described as having a Ni(II)-PDI^2- configuration. The electronic differences between these compounds are highlighted, and a computational analysis of ^Ph2PPrPDI denticity has revealed the thermodynamic penalties associated with phosphine dissociation from 5-coordinate (^Ph2PPrPDI)Mn, (^Ph2PPrPDI)Fe, and (^Ph2PPrPDI)Co.

An Aryl Diimine Cobalt(I) Catalyst for Carbonyl Hydrosilylation

Through the application of a redox-innocent aryl diimine chelate, the discovery and utilization of a cobalt catalyst, (Ph2PPrADI)Co, that exhibits carbonyl hydrosilylation turnover frequencies of up to 330 s–1 is...

Phosphorous-substituted redox-active ligands in base metal hydrosilylation catalysis

This article highlights the utilization of phosphine-containing redox-active ligands for efficient hydrosilylation catalysis. Manganese, iron, cobalt, and nickel precatalysts featuring these chelates have been described and leading activities for carbonyl, carboxylate, and ester C-O bond hydrosilylation have been achieved. Mechanistic studies have provided insight into the importance of phosphine hemilability.

Ligand dynamics and protonation preferences of Rh and Ir complexes bearing an almost, but not quite, pendent base

Pendent nucleophiles can assist transition metals mediate bond rearrangements (e.g. as proton acceptors), but can also act as inhibitory hemilabile ligands. This dual nature has been studied in a series of rhodium and iridium complexes that exhibit disparate nucleophile binding ability in the ground state and in protonation reactions.

The Emergence of Manganese-Based Carbonyl Hydrosilylation Catalysts

In recent years, interest in homogeneous manganese catalyst development has intensified because of the earth-abundant and nontoxic nature of this metal. Although compounds of Mn have largely been utilized for epoxidation reactions, recent efforts have revealed that Mn catalysts can mediate a broad range of reductive transformations. Low-valent Mn compounds have proven to be particularly effective for the hydrosilylation of carbonyl- and carboxylate-containing substrates, and this Account aims to highlight my research group's contributions to this field. In our initial 2014 communication, we reported that the bis(imino)pyridine-supported compound (^Ph2PPrPDI)Mn mediates ketone hydrosilylation with exceptional activity under solvent-free conditions. Silanes including Ph₂SiH₂, (EtO)₃SiH, (EtO)₂MeSiH, and (EtO)Me₂SiH were found to partially reduce cyclohexanone in the presence of (^Ph2PPrPDI)Mn, while turnover frequencies of up to 1280 min^-1 were observed using PhSiH₃. This led us to evaluate the hydrosilylation of 11 additional ketones and allowed for the atom-efficient preparation of tertiary and quaternary silanes. At that time, it was also discovered that (^Ph2PPrPDI)Mn catalyzes the dihydrosilylation of esters (by way of acyl C-O bond hydrosilylation) to yield a mixture of silyl ethers with modest activity. Earlier this year, the scope of these transformations was extended to aldehydes and formates, and the observed hydrosilylation activities are among the highest obtained for any transition-metal catalyst. The effectiveness of three related catalysts has also been evaluated: (^Ph2PPrPDI)MnH, (^PyEtPDEA)Mn, and [(^Ph2PEtPDI)Mn]₂. To our surprise, (^Ph2PPrPDI)MnH was found to exhibit higher carboxylate dihydrosilylation activity than (^Ph2PPrPDI)Mn, while (^PyEtPDEA)Mn demonstrated remarkable carbonyl hydrosilylation activity considering that it lacks a redox-active supporting ligand. The evaluation of [(^Ph2PEtPDI)Mn]₂ revealed competitive aldehyde hydrosilylation and formate dihydrosilylation turnover frequencies; however, this catalyst is significantly inhibited by pyridine and alkene donor groups. In our efforts to fully understand how (^Ph2PPrPDI)Mn operates, a thorough electronic structure evaluation was conducted, and the ground-state doublet calculated for this compound was found to exhibit nonclassical features consistent with a low-spin Mn(II) center supported by a singlet PDI dianion and an intermediate-spin Mn(II) configuration featuring antiferromagnetic coupling to PDI diradical dianion. A comprehensive mechanistic investigation of (^Ph2PPrPDI)Mn- and (^Ph2PPrPDI)MnH-mediated hydrosilylation has revealed two operable pathways, a modified Ojima pathway that is more active for carbonyl hydrosilylation and an insertion pathway that is more effective for carboxylate reduction. Although these efforts represent a small fraction of the recent advances made in Mn catalysis, this work has proven to be influential for the development of Mn-based reduction catalysts and is likely to inform future efforts to develop Mn catalysts that can be used to prepare silicones.

Isolation of a bis(imino)pyridine molybdenum(i) iodide complex through controlled reduction and interconversion of its reaction products

Analysis of previously reported [((Ph2PPr)PDI)MoI][I] by cyclic voltammetry revealed a reversible wave at -1.20 V vs. Fc(+/0), corresponding to the Mo(ii)/Mo(i) redox couple. Reduction of [((Ph2PPr)PDI)MoI][I] using stoichiometric K/naphthalene resulted in ligand deprotonation rather than reduction to yield a Mo(ii) monoiodide complex featuring a Mo-C bond to the α-position of one imine substituent, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI. Successful isolation of the inner-sphere Mo(i) monoiodide complex, ((Ph2PPr)PDI)MoI, was achieved via reduction of [((Ph2PPr)PDI)MoI][I] with equimolar Na/naphthalene. This complex was found to have a near octahedral coordination geometry by single crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy revealed an unpaired Mo-based electron which is highly delocalized onto the PDI chelate core. Attempts to prepare a Mo(i) monohydride complex upon adding NaEt3BH to ((Ph2PPr)PDI)MoI resulted in disproportionation to yield an equimolar quantity of (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH and newly identified ((Ph2PPr)PDI)MoH2. Independent preparation of ((Ph2PPr)PDI)MoH2 was achieved by adding 2 equiv. NaEt3BH to [((Ph2PPr)PDI)MoI][I] and a minimum hydride resonance T1 of 176 ms suggests that the Mo-bound H atoms are best described as classical hydrides. Interestingly, ((Ph2PPr)PDI)MoH2 can be converted to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI upon iodomethane addition, while ((Ph2PPr)PDI)MoH2 is prepared from (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI in the presence of excess NaEt3BH. Similarly, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI can be converted to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH with 1 equiv. of NaEt3BH, while the opposite transformation occurs following iodomethane addition to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH. Facile interconversion between [((Ph2PPr)PDI)MoI][I], (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH, and ((Ph2PPr)PDI)MoH2 is expected to guide future reactivity studies on this unique set of compounds.

Hydrogen production from water using a bis(imino)pyridine molybdenum electrocatalyst

Reduction of [(^Ph2PPrPDI)MoO][PF₆]₂ affords an unusual Mo(ii) oxo compound that mediates the electrocatalytic reduction of water.

Phosphine-iminopyridines as platforms for catalytic hydrofunctionalization of alkenes

A series of phosphine-diimine ligands were synthesized by the condensation of 2-(diphenylphosphino)aniline (PNH2) with a variety of formyl and ketopyridines. Condensation of PNH2 with acetyl- and benzoylpyridine yielded the Ph2P(C6H4)N═C(R)(C5H4N), respectively abbreviated PN(Me)py and PN(Ph)py. With ferrous halides, PN(Ph)py gave the complexes FeX2(PN(Ph)py) (X = Cl, Br). Condensation of pyridine carboxaldehyde and its 6-methyl derivatives with PNH2 was achieved using a ferrous template, affording low-spin complexes [Fe(PN(H)py(R))2](2+) (R = H, Me). Dicarbonyls Fe(PN(R)py)(CO)2 were produced by treating PN(Me)py with Fe(benzylideneacetone)(CO)3 and reduction of FeX2(PN(Ph)py) with NaBEt3H under a CO atmosphere. Cyclic voltammetric studies show that the [FeL3(CO)2](0/-) and [FeL3(CO)2](+/0) couples are similar for a range of tridentate ligands, but the PN(Ph)py system uniquely sustains two one-electron reductions. Treatment of Fe(PN(Ph)py)X2 with NaBEt3H gave active catalysts for the hydroboration of 1-octene with pinacolborane. Similarly, these catalysts proved active for the addition of diphenylsilane, but not HSiMe(OSiMe3)2, to 1-octene and vinylsilanes. Evidence is presented that catalysis occurs via iron hydride complexes of intact PN(Ph)py.