title: Manganese Tricarbonyl Complexes with Asymmetric 2-Iminopyridine Ligands: Toward Decoupling Steric and Electronic Factors in Electrocatalytic CO2Reduction.
abstract: Manganese tricarbonyl bromide complexes incorporating IP (2-(phenylimino)pyridine) derivatives, [MnBr(CO)3(IP)], are demonstrated as a new group of catalysts for CO2reduction, which represent the first example of utilization of (phenylimino)pyridine ligands on manganese centers for this purpose. The key feature is the asymmetric structure of the redox-noninnocent ligand that permits independent tuning of its steric and electronic properties. The α-diimine ligands and five new Mn(I) compounds have been synthesized, isolated in high yields, and fully characterized, including X-ray crystallography. Their electrochemical and electrocatalytic behavior was investigated using cyclic voltammetry and UV-vis-IR spectroelectrochemistry within an OTTLE cell. Mechanistic investigations under an inert atmosphere have revealed differences in the nature of the reduction products as a function of steric bulk of the ligand. The direct ECE (electrochemical-chemical-electrochemical) formation of a five-coordinate anion [Mn(CO)3(IP)]-, a product of two-electron reduction of the parent complex, is observed in the case of the bulky DIPIMP (2-[((2,6-diisopropylphenyl)imino)methyl]pyridine), TBIMP (2-[((2-tert-butylphenyl)imino)methyl]pyridine), and TBIEP (2-[((2-tert-butylphenyl)imino)ethyl]pyridine) derivatives. This process is replaced for the least sterically demanding IP ligand in [MnBr(CO)3(IMP)] (2-[(phenylimino)methyl]pyridine) by the stepwise formation of such a monoanion via an ECEC(E) mechanism involving also the intermediate Mn-Mn dimer [Mn(CO)3(IMP)]2. The complex [MnBr(CO)3(IPIMP)] (2-[((2-diisopropylphenyl)imino)methyl]pyridine), which carries a moderately electron donating, moderately bulky IP ligand, shows an intermediate behavior where both the five-coordinate anion and its dimeric precursor are jointly detected on the time scale of the spectroelectrochemical experiments. Under an atmosphere of CO2the studied complexes, except for the DIPIMP derivative, rapidly coordinate CO2, forming stable bicarbonate intermediates, with no dimer being observed. Such behavior indicates that the CO2binding is outcompeting another pathway: viz., the dimerization reaction between the five-coordinate anion and the neutral parent complex. The bicarbonate intermediate species undergo reduction at more negative potentials (ca. -2.2 V vs Fc/Fc+), recovering [Mn(CO)3(IP)]-and triggering the catalytic production of CO.
authors.affiliation: Department of Chemistry, University of Sheffield , Sheffield S3 7HF, U.K.
authors.forename: Steven J P
authors.initials: SJ
authors.lastname: Spall
authors.affiliation: Department of Chemistry, University of Sheffield , Sheffield S3 7HF, U.K.
authors.forename: Theo
authors.identifier: http://orcid.org/0000-0003-4975-0868
authors.initials: T
authors.lastname: Keane
authors.affiliation: Department of Chemistry, University of Reading , Whiteknights, Reading RG6 6AD, U.K.
authors.forename: Joanne
authors.initials: J
authors.lastname: Tory
authors.affiliation: Department of Chemistry, University of Sheffield , Sheffield S3 7HF, U.K.
authors.forename: Dean C
authors.initials: DC
authors.lastname: Cocker
authors.affiliation: Department of Chemistry, University of Sheffield , Sheffield S3 7HF, U.K.
authors.forename: Harry
authors.initials: H
authors.lastname: Adams
authors.affiliation: Department of Chemistry, University of Sheffield , Sheffield S3 7HF, U.K.
authors.forename: Hannah
authors.initials: H
authors.lastname: Fowler
authors.affiliation: Department of Chemistry, University of Sheffield , Sheffield S3 7HF, U.K.
authors.forename: Anthony J H M
authors.identifier: http://orcid.org/0000-0003-4803-3488
authors.initials: AJ
authors.lastname: Meijer
authors.affiliation: Department of Chemistry, University of Reading , Whiteknights, Reading RG6 6AD, U.K.
authors.forename: František
authors.initials: F
authors.lastname: Hartl
authors.affiliation: Department of Chemistry, University of Sheffield , Sheffield S3 7HF, U.K.
authors.forename: Julia A
authors.identifier: http://orcid.org/0000-0001-6883-072X
authors.initials: JA
authors.lastname: Weinstein