Large-scale CO hydrogenation could offer a renewable stream of industrially important C chemicals while reducing CO emissions. Critical to this opportunity is the requirement for inexpensive catalysts based on earth-abundant metals instead of precious metals. We report a nickel-gallium complex featuring a Ni(0)→Ga(III) bond that shows remarkable catalytic activity for hydrogenating CO to formate at ambient temperature (3150 turnovers, turnover frequency = 9700 h), compared with prior homogeneous Ni-centered catalysts. The Lewis acidic Ga(III) ion plays a pivotal role in stabilizing catalytic intermediates, including a rare anionic d Ni hydride. Structural and in situ characterization of this reactive intermediate support a terminal Ni-H moiety, for which the thermodynamic hydride donor strength rivals those of precious metal hydrides. Collectively, our experimental and computational results demonstrate that modulating a transition metal center via a direct interaction with a Lewis acidic support can be a powerful strategy for promoting new reactivity paradigms in base-metal catalysis.

The Co(−I) dihydrogen complexes, [(η 2 -H 2 )CoML] − , where ML is the group 13 metalloligand, N(o-(NCH 2 P i Pr 2 )C 6 H 4 ) 3 M, and M is Al, Ga, or In, were previously reported (J. Am. Chem. Soc. 2017, 139, 6570−6573). In this work, the related Co(−I) end-on dinitrogen adducts, [(N 2 )CoML] − , were isolated and investigated as precatalysts for CO 2 hydrogenation. The Co−Ga catalyst was highly active, achieving 19,200 formate turnovers with an initial turnover frequency of 27,000 h −1 under 34 atm of 1:1 CO 2 /H 2 and using Verkade's proazaphosphatrane as a base at ambient temperature. The Co−Al catalyst was moderately active, while the Co−In complex was inactive. Hence, tuning the group 13 ion greatly influences the catalytic activity at the Co site. To elucidate the role of the group 13 support, experimental and theoretical mechanistic studies of the Co−Ga and Co−Al catalysts were conducted. The Co(−I) H 2 species are potent hydride donors with estimated thermodynamic hydricities (ΔG°H − ) of 32.0(1) and 37.4(1) kcal/mol in CH 3 CN for M = Al and Ga, respectively. By acting as masked Co(I) dihydrides, the Co(−I) H 2 species operate via an unusual Co(−I)/Co(I) redox cycle. After hydride transfer to CO 2 , the resulting intermediate is the Co(I) hydride complex, HCoML, which was independently synthesized and structurally characterized for M = Al and Ga. The Gibbs free energy for H 2 binding, ΔG°b ind (1 atm), to generate (η 2 -H 2 )HCoML was slightly more favorable for HCoGaL (−4.2(1) kcal/mol) than for HCoAlL (−2.7(1) kcal/mol). In the subsequent step, the deprotonation reaction to regenerate the initial catalyst was much more favorable for (η 2 -H 2 )HCoGaL (pK a of 31.4, CH 3 CN) than for (η 2 -H 2 )HCoAlL (pK a of 34.3). The straightforward substitution of Al with Ga perturbs the energy profile of the catalytic reaction (|ΔΔG°H − | = 5.4 kcal/mol, |ΔΔG°b ind | = 1.5 kcal/mol, and |ΔΔG°K a | = 4.0 kcal/mol) and thus provides a thermodynamic rationale for the higher catalytic efficiency of Co−Ga over Co−Al.

The synthesis and characterization of new Mn(I)- and Re(I)-centered organometallic complexes fashioned with 1,4-diazabutadiene (DAB) ligands is reported. Ten compounds of the type fac-(α-diimine)M(CO)3Br (M = Mn, Re) were obtained in moderate to excellent yield (35–80%) and high purity from the coordination of the five ligands with M(CO)5Br in refluxing ethanol. Despite the electronic similarity of DAB to 2,2′-bipyridyl, the complexes described herein were poor mediators of electrochemical CO2 conversion to CO, but provide insight into the role of redox-active ligands in catalysis. Additional characterization of the one-electron reduced rhenium compounds, relevant intermediates in CO2 reduction, by EPR and single-crystal X-ray analysis is described.

A triad of d cobalt dihydrogen complexes was synthesized by utilizing Lewis acidic group 13 metalloligands, M[N((o-CH)NCHPPr)], where M = Al, Ga, and In. These complexes have formal Co(-I) oxidation states, representing the only coordination complexes in which dihydrogen is bound to a subvalent transition metal center. Single-crystal X-ray diffraction and NMR studies support the assignment of these complexes as nonclassical dihydrogen adducts of Co(-I).

Dinuclear Cu(I) and Ag(I) complexes, Cu2[(2,6-Me2C6H3N)2C(H)]2, 1, Ag2[(2,6-Me2C6H3N)2C(H)]2, 2, Cu2[2,6-(i)Pr2C6H3N)2C(H)]2, 3, and Ag2[(2,6-(i)Pr2C6H3N)2C(H)]2, 4, were synthesized from reactions of [Cu(NCCH3)4][PF6] with Na[(2,6-R2C6H3N)2C(H)] and AgO2CCH3 with [Et3NH][(2,6-R2C6H3N2C(H)], R = Me, (i)Pr. Carbon disulfide was observed to insert into the metal-nitrogen bonds of 1 to produce Cu4[CS2(2,6-Me2C6H3NC(H)═NC6H3Me2)]4, 5, with a Cu4S8 core, which represents a rare transformation of dinuclear to tetranuclear species. Insertion is also observed with 2 and CS2, with the product likely being polymeric, 6. With the (i)Pr-derivatives, CS2 insertion was also observed, albeit at much slower rate, with 3 and 4 producing hexanuclear clusters, M6[CS2(2,6-Me2C6H3NC(H)═NC6H3Me2)]6, M = Cu, 7; Ag, 8. Complexes 1 and 5 display green luminescence, a feature not shared by their Ag(I) analogs nor with 3. Notably, oxygen acts as a collisional quencher of the luminescence from 1 and 5 at a rate faster than most metal-based quenchometric O2 sensors. For example, we find that complex 1 can be rapidly and reversibly quenched by oxygen, presenting a nearly 6-fold drop in intensity upon switching from nitrogen to an aerated atmosphere. The results here provide a platform from which further group 11 amidinate reactivity can be explored.