Aqueous electrocatalytic CO₂ reduction using metal complexes dispersed in polymer ion gels

We use fac-[Re(bpy)(CO)₃Cl] ([Re–Cl]) dispersed in polymer ion gel (PIG) ([Re]–PIG) to carry out electrocatalytic CO₂ reduction in water. Electrolysis at −0.68 V vs. RHE in a CO₂-saturated KOH and K₂CO₃ solution produces CO with over 90% Faradaic efficiency. The PIG electrode is readily combined with water oxidation catalysts to generate a full electrochemical cell. Additionally, we provide evidence that the PIG electrode can be implemented with other molecular catalysts

Aqueous electrocatalytic CO₂ reduction using metal complexes dispersed in polymer ion gels

We use fac-[Re(bpy)(CO)₃Cl] ([Re–Cl]) dispersed in polymer ion gel (PIG) ([Re]–PIG) to carry out electrocatalytic CO₂ reduction in water. Electrolysis at −0.68 V vs. RHE in a CO₂-saturated KOH and K₂CO₃ solution produces CO with over 90% Faradaic efficiency. The PIG electrode is readily combined with water oxidation catalysts to generate a full electrochemical cell. Additionally, we provide evidence that the PIG electrode can be implemented with other molecular catalysts

Electronic Structure of Tetracyanonickelate(II)

Tetracyanonickelate(II) has been a poster child of ligand field theory for several decades. We have revisited the literature assignments of the absorption spectrum of [Ni(CN) ₄]²⁻ and the calculated ordering of orbitals with metal d character. Using low-temperature single-crystal absorption spectroscopy and accurate ab initio and density functional quantum mechanical methods (NEVPT2-CASSCF, EOM-CCSD, TD-DFT), we find an ordering of the frontier d- and p-orbitals of xy < xz, yz < z² < z < x²–y² < x, y and assign the d-d bands in the absorption spectrum to ¹A_(1g) → ³B_(1g) < ³E_g < ³A_(2g) < ¹B_(1g) < ¹E_g < ¹A_(2g). While differing from all previous interpretations, our assignments accord with an MO model in which strong π-backbonding in the plane of the molecule stabilizes d_(xy) more than out-of-plane bonding stabilizes d_(xz) and d_(yz)

Tuning the formal potential of ferrocyanide over a 2.1 V range

We report the synthesis and characterization of homoleptic borane adducts of hexacyanoferrate(II). Borane coordination blueshifts d–d transitions and CN IR and Raman frequencies. Control over redox properties is established with respect to borane Lewis acidity, reflected in peak anodic potential shifts per borane of +250 mV for BPh_3 and +350 mV for B(C_6F_5)_3. Electron transfer from [Fe(CN-B(C_6F_5)_3)_6]^(4−) to photogenerated [Ru(2,2′-bipyridine)_3]^(3+) is very rapid, consistent with voltammetry data. Coordination by Lewis acids provides an avenue for selective modification of the electronic structures and electrochemical properties of cyanometalates

Structure, Spectroscopy, and Electrochemistry of Manganese(I) and Rhenium(I) Quinoline Oximes

Reactions of α- and β-diimine quinoline aldoximes with Mn(I) and Re(I) tricarbonyl halides afford quinoline oxime complexes. Both Mn(I) and Re(I) complexes experience severe geometric strain due to ligand steric interactions: 6-membered metallocycles exhibit more pronounced distortions than 5-membered ones, consistent with density functional theory structural analyses. Such distortions likely also affect reactivity patterns, as evidenced by Re(I)-induced deoximation of a quinoline variant containing a CF_3-ketoxime

Structure, Spectroscopy, and Electrochemistry of Manganese(I) and Rhenium(I) Quinoline Oximes

Reactions of α- and β-diimine quinoline aldoximes with Mn(I) and Re(I) tricarbonyl halides afford quinoline oxime complexes. Both Mn(I) and Re(I) complexes experience severe geometric strain due to ligand steric interactions: 6-membered metallocycles exhibit more pronounced distortions than 5-membered ones, consistent with density functional theory structural analyses. Such distortions likely also affect reactivity patterns, as evidenced by Re(I)-induced deoximation of a quinoline variant containing a CF_3-ketoxime

Electronic Structures, Spectroscopy, and Electrochemistry of [M(diimine)(CN-BR₃)₄]²⁻ (M = Fe, Ru; R = Ph, C₆F₅) Complexes

Complexes with the formula [M(diimine)(CN-BR₃)₄]²⁻, where diimine = bipyridine (bpy), phenanthroline (phen), 3,5-trifluoromethylbipyridine (flpy), R = Ph, C₆F₅, and M = Fe^(II), Ru^(II), were synthesized and characterized by X-ray crystal structure analysis, UV–visible spectroscopy, IR spectroscopy, and voltammetry. Three highly soluble complexes, [Fe^(II)(bpy)(CN-B(C₆F₅)₃)₄]²⁻, [Ru^(II)(bpy)(CN-B(C₆F₅)₃)₄]²⁻, and [Ru^(II)(flpy)(CN-B(C₆F₅)₃)₄]²⁻, exhibit electrochemically reversible redox reactions, with large potential differences between the bpy^(0/–) or flpy^(0/–) and MIII/II couples of 3.27, 3.52, and 3.19 V, respectively. CASSCF+NEVPT2 calculations accurately reproduce the effects of borane coordination on the electronic structures and spectra of cyanometallates

Tuning the formal potential of ferrocyanide over a 2.1 V range

We report the synthesis and characterization of homoleptic borane adducts of hexacyanoferrate(II). Borane coordination blueshifts d–d transitions and CN IR and Raman frequencies. Control over redox properties is established with respect to borane Lewis acidity, reflected in peak anodic potential shifts per borane of +250 mV for BPh_3 and +350 mV for B(C_6F_5)_3. Electron transfer from [Fe(CN-B(C_6F_5)_3)_6]^(4−) to photogenerated [Ru(2,2′-bipyridine)_3]^(3+) is very rapid, consistent with voltammetry data. Coordination by Lewis acids provides an avenue for selective modification of the electronic structures and electrochemical properties of cyanometalates

Electrocatalysis of CO_2 Reduction in Brush Polymer Ion Gels

The electrochemical characterization of brush polymer ion gels containing embedded small-molecule redox-active species is reported. Gels comprising PS–PEO–PS triblock brush polymer, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm-TFSI), and some combination of ferrocene (Fc), cobaltocenium (CoCp_2^+), and Re(bpy)(CO)_3Cl (1) exhibit diffusion-controlled redox processes with diffusion coefficients approximately one-fifth of those observed in neat BMIm-TFSI. Notably, 1 dissolves homogeneously in the interpenetrating matrix domain of the ion gel and displays electrocatalytic CO_2 reduction to CO in the gel. The catalytic wave exhibits a positive shift versus Fc^(+/0) compared with analogous nonaqueous solvents with a reduction potential 450 mV positive of onset and 90% Faradaic efficiency for CO production. These materials provide a promising and alternative approach to immobilized electrocatalysis, creating numerous opportunities for application in solid-state devices

A Super-Oxidized Radical Cationic Icosahedral Boron Cluster

While the icosahedral closo-[B₁₂H₁₂]²⁻ cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all 12 B–H vertices with alkoxy or benzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic, monoanionic, and neutral forms. Here, we evaluated the electrochemical behavior of the electron-rich B₁₂(O-3-methylbutyl)₁₂ (1) cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of 1 with [N(2,4-Br₂C₆H₃)₃]·⁺ afforded the isolable [1]·⁺ cluster, which is the first example of an open-shell cationic B₁₂ cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core. The oxidation of 1 is also chemically reversible, where treatment of [1]·⁺ with ferrocene resulted in its reduction back to 1. The identity of [1]·⁺ is supported by EPR, UV–vis, multinuclear NMR (¹H, ¹¹B), and X-ray photoelectron spectroscopic characterization