Thermodynamic targeting of electrocatalytic CO2 reduction: advantages, limitations, and insights for catalyst design.

Herein is reported the electrocatalytic reduction of CO2 with the complex [Ni(bis-NHC)(dmpe)]2+ (1) (bis-NHC = 1,l':3,3'-bis(1,3-propanediyl)dibenzimidazolin-2,2'-diylidene; dmpe = 1,2-bis(dimethylphosphino)ethane). The hydricity of 1 was previously benchmarked to be , equating to a driving force of a minimum of ∼3.4 kcal mol-1 for hydride transfer to CO2. While hydride transfer to CO2 is thermodynamically favorable, electrocatalytic and infrared spectroelectrochemical (IR-SEC) experiments reveal that hydride transfer is blocked by direct reactivity with CO2 in the reduced, Ni(0) state of the catalyst, yielding CO via reductive disproportionation (2CO2 + 2e- = CO + CO32-) and concomitant catalyst degradation. Although thermodynamic scaling relationships provide guidance in catalyst targeting, the findings herein illustrate the fundamental kinetic challenges in balancing substrate reactivity and selectivity in the design of CO2 reduction electrocatalysts. Advantages and limitations of this scaling relationship as well as approaches by which divergence from it may be achieved are discussed, which provides insight on important parameters for future catalyst design.

Chelated [Zn(cyclam)]2+ Lewis acid improves the reactivity of the electrochemical reduction of CO2 by Mn catalysts with bulky bipyridine ligands.

This communication reports the use of a soluble Lewis acid complex, [Zn(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) as a co-catalyst coupled with Mn(Mesbpy)(CO)3Br (Mesbpy = 6,6'-dimesityl-2,2'-bipyridine) for the electrochemical reduction of CO2 to CO. Utilization of the soluble chelated Lewis acid avoids the use of sacrificial additives and prevents the formation of insoluble products such as MgCO3 or ZnCO3 that change the thermodynamics of CO2 reduction. The use of soluble Lewis acids greatly improves catalysis compared to previously reported systems that used sacrificial anodes.

Incorporation of Pendant Bases into Rh(diphosphine)2 Complexes: Synthesis, Thermodynamic Studies, And Catalytic CO2 Hydrogenation Activity of [Rh(P2N2)2](+) Complexes.

A series of five [Rh(P2N2)2](+) complexes (P2N2 = 1,5-diaza-3,7-diphosphacyclooctane) have been synthesized and characterized: [Rh(P(Ph)2N(Ph)2)2](+) (1), [Rh(P(Ph)2N(Bn)2)2](+) (2), [Rh(P(Ph)2N(PhOMe)2)2](+) (3), [Rh(P(Cy)2N(Ph)2)2](+) (4), and [Rh(P(Cy)2N(PhOMe)2)2](+) (5). Complexes 1-5 have been structurally characterized as square planar rhodium bis-diphosphine complexes with slight tetrahedral distortions. The corresponding hydride complexes 6-10 have also been synthesized and characterized, and X-ray diffraction studies of HRh(P(Ph)2N(Bn)2)2 (7), HRh(P(Ph)2N(PhOMe)2)2 (8) and HRh(P(Cy)2N(Ph)2)2 (9) show that the hydrides have distorted trigonal bipyramidal geometries. Equilibration of complexes 2-5 with H2 in the presence of 2,8,9-triisopropyl-2,5,8,9-tetraaza-1-phosphabicyclo[3,3,3]undecane (Verkade's base) enabled the determination of the hydricities and estimated pKa's of the Rh(I) hydride complexes using the appropriate thermodynamic cycles. Complexes 1-5 were active for CO2 hydrogenation under mild conditions, and their relative rates were compared to that of [Rh(depe)2](+), a nonpendant-amine-containing complex with a similar hydricity to the [Rh(P2N2)2](+) complexes. It was determined that the added steric bulk of the amine groups on the P2N2 ligands hinders catalysis and that [Rh(depe)2](+) was the most active catalyst for hydrogenation of CO2 to formate.

Synthesis and structural studies of nickel(0) Tetracarbene complexes with the introduction of a new four-coordinate geometric index, τδ.

The synthesis and characterization of two homoleptic chelating nickel(0) tetracarbene complexes are reported. These are the first group 10 M(0) (M = Ni, Pd, Pt) tetracarbene complexes. These species have geometries intermediate between C2v sawhorse and tetrahedral and show high UV-vis absorption in the 350-600 nm range, with extinction coefficients (ϵ) between 5600 and 9400 M(-1) cm(-1). Density functional theory analysis indicates that this high absorptivity is due to metal-to-ligand charge transfer. In order to better describe the unusual geometries encountered in these complexes, an adjustment to the popular τ4 index for four-coordinate geometries is introduced in order to better delineate between sawhorse and distorted tetrahedral geometries.