Group 6 complexes as electrocatalysts of CO2 reduction: strong substituent control of the reduction path of [Mo(3allyl) η (CO)2(x,x dimethyl2,2 bipyridine) ′ ′ (NCS)] (x = 46

A series of molybdenum tetracarbonyl complexes with dimethyl‐substituted 2,2′‐bipyridine (dmbipy) ligands were investigated by cyclic voltammetry (CV) combined with infrared spectroelectrochemistry (IR‐SEC) in tetrahydrofuran (THF) and N‐methyl‐2‐pyrrolidone (NMP) to explore their potential in a reduced state to trigger electrocatalytic CO2 reduction to CO. Addressed is their ability to take advantage of a low‐energy, CO‐dissociation two‐electron ECE pathway available only at an Au cathode. A comparison is made with the reference complex bearing unsubstituted 2,2′‐bipyridine (bipy). The methyl substitution in the 6,6′ position has a large positive impact on the catalytic efficiency. This behavior is ascribed to the advantageous positioning of the steric bulk of the methyl groups, which further facilitate CO dissociation from the one‐electron‐reduced parent radical anion. On the contrary, the substitution in the 4,4′ position appears to have a negative impact on the catalytic performance, exerting a strong stabilizing effect on the π‐accepting CO ligands and, in THF, preventing exploitation of the low‐energy dissociative pathway.

Herein, we present the cathodic paths of the Group-7 metal complex [Re­(3,3′-DHBPY)­(CO)3Cl] (3,3′-DHBPY = 3,3′-dihydroxy-2,2′-bipyridine) producing a moderately active catalyst of electrochemical reduction of CO2 to CO. The combined techniques of cyclic voltammetry and IR/UV–vis spectroelectrochemistry have revealed significant differences in the chemistry of the electrochemically reduced parent complex compared to the previously published Re/4,4′-DHBPY congener. The initial irreversible cathodic step in weakly coordinating THF is shifted toward much less negative electrode potentials, reflecting facile reductive deprotonation of one hydroxyl group and strong intramolecular hydrogen bonding, O–H···O–. The latter process occurs spontaneously in basic dimethylformamide where Re/4,4′-DHBPY remains stable. The subsequent reduction of singly deprotonated [Re­(3,3′-DHBPY-H+)­(CO)3Cl]− under ambient conditions occurs at a cathodic potential close to that of the Re/4,4′-DHBPY-H+ derivative. However, for the stabilized 3,3′-DHBPY-H+ ligand, the latter process at the second cathodic wave is more complex and involves an overall transfer of three electrons. Rapid potential step electrolysis induces 1e–-reductive cleavage of the second O–H bond, triggering dissociation of the Cl– ligand from [Re­(3,3′-DHBPY-2H+)­(CO)3Cl]2–. The ultimate product of the second cathodic step in THF was identified as 5-coordinate [Re­(3,3′-DHBPY-2H+)­(CO)3]3–, the equivalent of classical 2e–-reduced [Re­(BPY)­(CO)3]−. Each reductive deprotonation of the DHBPY ligand results in a redshift of the IR ν­(CO) absorption of the tricarbonyl complexes by ca. 10 cm–1, facilitating the product assignment based on comparison with the literature data for corresponding Re/BPY complexes. The Cl– dissociation from [Re­(3,3′-DHBPY-2H+)­(CO)3Cl]2– was proven in strongly coordinating butyronitrile. The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate [Re­(3,3′-DHBPY-2H+)­(CO)3(PrCN)]3–. Useful reference data were obtained with substituted parent [Re­(3,3′-DHBPY)­(CO)3(PrCN)]+ that also smoothly deprotonates by the initial reduction to [Re­(3,3′-DHBPY-H+)­(CO)3(PrCN)]. The latter complex ultimately converts at the second cathodic wave to [Re­(3,3′-DHBPY-2H+)­(CO)3(PrCN)]3– via a counterintuitive ETC step generating the 1e– radical of the parent complex, viz., [Re­(3,3′-DHBPY)­(CO)3(PrCN)]. The same alternative reduction path is also followed by [Re­(3,3′-DHBPY-H+)­(CO)3Cl]− at the onset of the second cathodic wave, where the ETC step results in the intermediate [Re­(3,3′-DHBPY)­(CO)3Cl]•– further reducible to [Re­(3,3′-DHBPY-2H+)­(CO)3]3– as the CO2 catalyst.

Marked increases in sulfur and molybdenum concentration in stalagmites have been proposed as possible evidence of volcanic activity in the past. Thus, speleothems have great potential to deliver long and continuous records of volcanic activity. However, little is known about the chemical nature of these impurities in the calcium carbonate (CaCO3) phases forming stalagmites, which hinders the rationalization of the incorporation mechanisms. While sulfur is known to incorporate as a sulfate anion in CaCO3 polymorphs, the nature and stability of molybdenum incorporation in these minerals has not been investigated yet. Here, we present a computer simulation study, based on density functional theory, comparing the thermodynamics of incorporation of sulfur and molybdenum as tetrahedral oxyanions [XO4] 2-(X=S, Mo) in anion sites of CaCO3 polymorphs (calcite, aragonite, vaterite, monohydrocalcite and ikaite). Among the different polymorphs, vaterite incorporates [XO4] 2ions most favourably, which reflects the relatively low density of this carbonate phase. We show that molybdate anions are very unstable (more so than sulfate anions) in the bulk of all three anhydrous carbonate phases, with respect to the formation of naturally occurring competing phases. Most of the Mo impurities found in typical calcite/aragonite stalagmites is therefore likely to concentrate at surface/interface regions such as grain boundaries. Using the calcite (10.4) surface as a model, we show that the energies of substitution are indeed much lower at the surface than at the bulk. Our results suggest that factors affecting the crystallinity of CaCO3 in stalagmites, and therefore the specific surface area, will have a significant effect on the concentration of incorporated molybdenum, which should be a key consideration when interpreting data from Mo-based speleothem archives.

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy) x (bpy) 3−x ] 2+ (CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, and x = 1−3, abbreviated as 1 2+ , 2 2+ , and 3 2+ ) undergo four (1 2+ ) or five (2 2+ and 3 2+ ) successive one-electron reduction steps between −1.3 and −2.75 V versus ferrocenium/ferrocene (Fc + /Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 2 2+ and 3 2+ being separated by 150−210 mV; reduction of the unsubstituted bpy ligand in 1 2+ and 2 2+ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Mebpy ligand shifts the ν(CN) IR band by ca. −45 cm −1 , enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2 + , 3 + , and 3 0 show distinct ν(C N) features due to the presence of both CN-Me-bpy and CN-Me-bpy •− , confirming that each reduction is localized on a single ligand. NIR spectra of 1 0 , 1 − , and 2 − exhibit a prominent band attributable to the CN-Me-bpy •− moiety between 6000 and 7500 cm −1 , whereas bpy •− -based absorption occurs between 4500 and 6000 cm −1 ; complexes 2 + , 3 + , and 3 0 also exhibit a band at ca. 3300 cm −1 due to a CN-Me-bpy •− → CN-Me-bpy interligand charge-transfer transition. In the UV−vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 1 2+ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(CN) IR band is shifted by −38 cm −1 and 5−7-fold-enhanced relative to the ground state.