A series of twelve second coordination sphere (SCS) functionalized manganese tricarbonyl bipyridyl complexes are investigated for their electrocatalytic CO2 reduction properties in acetonitrile. A qualitative and quantitative assessment of the SCS functional groups is discussed with respect to the catalysts' thermodynamic and kinetic efficiencies, and their product selectivities. In probing a broad scope of functional groups, it is clear that only the aprotic ortho-arylester SCS is capable of promoting the highly desired low-overpotential proton-transfer electron-transfer (PT-ET) pathway for selective CO production. The ortho-phenolic analogues cause an increase in overpotential with a product selectivity favoring H2 evolution, consistent with a high-overpotential pathway via the anionic [Mn-H]- intermediate. Alternative aprotic Lewis base functional groups such as trifluoromethyl, morpholine and acetamide are shown to also be capable of intermediate manganese hydride generation. The tertiary amine substituent, 2-morpholinophenyl, exhibits a desirable product distribution characteristic of syn-gas (CO : H-2=30 : 48) with an impressive turnover frequency, while the secondary amine group, 2-acetamidophenyl, induces a notable shift in selectivity with a faradaic yield of 55 % for the formate (HCO2-) product. In addition to their catalytic properties, cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC) studies are presented to probe pre-catalyst electronic properties and the two-electron reduction activation pathway.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)2bpy vs mes2bpy ligands) and Lewis basic ((Ph)2bpy vs (MeOPh)2bpy vs (MeSPh)2bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO2 reduction mechanisms with [MnI(R2bpy)(CO)3(CH3CN)]+ precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)2bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO2 binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes2bpy-based precatalysts. Cyclic voltammetry conducted in CO2-saturated electrolyte for the (Ph)2bpy-based precatalyst [2-CH3CN]+ confirms that CO2 binding occurs at the two-electron-reduced activated catalyst [2]- in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO2 binding. This observation is supported by computed free energies of the parent-child reaction for [Mn-Mn]0 dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO2 binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH3CN]+ maintains a lower overpotential than [2-CH3CN]+ at the protonation-first pathway with comparable kinetic performance, whereas an ∼2-fold greater TOFmax is observed at its reduction-first pathway with an almost identical overpotential as [2-CH3CN]+. Notably, excessive steric bulk in the mes2bpy-based precatalyst [3-CH3CN]+ results in increased activation free energies of the C-OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH3CN]+ and [2-CH3CN]+. In fact, [3-CH3CN]+ requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Voltammetry recorded under 1 atm of CO2 with 2.8 M (5%) H2O establishes [2-CH3CN]+ to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie "on the fence" when providing sufficient steric bulk to hinder (but not prevent) [Mn-Mn]0 dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C-OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH3CN]+ possesses an increased steric presence relative to [2-CH3CN]+, this is offset by its capacity to stabilize the C-OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.

A series of nickel complexes with potentially redox active bis(aldimino)pyridine ligands [NNN] ([NNN] = 1,1'-(pyridine-2,6-diyl)bis(N-arylmethanimine), where aryl = 2,6-diisopropylphenyl, mesityl, 4-methoxyphenyl, 4-trifluoromethylphenyl, and 3,5-bis-(trifluoromethyl)phenyl) were synthesized, and their properties and reactivities were investigated as a function of the overall oxidation state of the system. (Ni[NNN])(2+) complexes of ligands featuring bulky electron-rich substituents (1a-Br-2 and 1b-Br-2, [NNN] = 1,1' (pyridine -2,6-diyl) bis (N-(2,6-diisopropylphenyl)methanimine) and 1,1'-(pyridine-2,6-diyl)bis(N-mesitylmethanimine), respectively) demonstrated five electrochemical reduction events, the first three of which were quasi-reversible. In contrast, only two quasi-reversible reductions were observed for the less bulky and electron-deficient N-aryl substituents 4-(trifluoromethyDphenyl and 3,5-bis(trifluoromethyl)phenyl. Chemical reduction of la-Br, and lb-Br, with 1 equiv of KC8 or CoCp*(2) forms (Ni[NNN])(0) complexes of the general formula Ni[NNN]Br (2a-Br and 2b-Br). Structural, spectroscopic, and theoretical studies reveal that these complexes feature significant unpaired spin density on the metal, consistent with "nickel(I)" character. This behavior is in contrast with previously reported bis(ketimino)pyridine systems, in which at the (Ni[NNN])(+) state the unpaired electron resided exclusively in the ligand. Further reduction forms a series of (Ni[NNN]) complexes, in which all of the potentially tridentate [NNN] ligands bind via only one iminopyridine unit; the second arm is left unbound in most complexes. Variable temperature NMR spectroscopy demonstrates that bound and unbound arms exchange via a postulated tridentate intermediate. Electrochemical reduction, via three sequential one-electron reductions, of la-Br, and lb-Br, in the presence of CO2/H+ forms an active catalyst for H-2 evolution at a glassy-carbon electrode surface, again emphasizing the unique redox chemistry of the bulky bis(aldimino)pyridine nickel complexes.

The electrochemical properties of two Ni(NNN)X2 pincer complexes are reported where X = Cl or Br and NNN is N,N'-(2,6-diisopropylphenyl)bis-aldiminopyridine. Cyclic voltammetry under 1 atm of CO2 suggests electrocatalytic CO2 reduction activity, however, bulk electrolysis shows a poor Faradaic efficiency for CO evolution with a high Faradaic yield for H2 evolution.

One-electron reduction of bis(aldimino)pyridine nickel(ii) forms a non-planar bis(aldimino)pyridine nickel halide featuring a significant spin density at the Ni center.