Boronated Cyanometallates

Fourteen boronated cyanometallates ([M(CN-BR3)6]3/4/5– (M = Cr, Mn, Fe, Ru, Os, R = BPh3, B(2,4,6,-F3C6H2)3, B(C6F5)3) have been characterized by X-ray crystallography and spectroscopy [UV-vis-NIR, NMR, IR, spectroelectrochemistry, and magnetic circular dichroism (MCD)]; CASSCF+NEVPT2 methods were employed in calculations of electronic structures. For (t2g)5 electronic configurations, the lowest energy ligand-to-metal charge transfer (LMCT) absorptions and MCD C terms in the spectra of boronated species have been assigned to transitions from cyanide σ+π + B-C borane σ orbitals. CASSCF+NEVPT2 calculations including t1u and t2u orbitals reproduced t1u/t2u → t2g excitation energies. All ([M(CN-BR3)6]3/4− complexes exhibited highly electrochemically reversible redox couples. Notably, the formal potentials of all five [M(CN-B(C6F5)3)6]3− anions scale with LMCT energies; and Mn(I) and Cr(II) compounds, (K(18-crown-6))5[Mn(CN-B(C6F5)3)6] and (TBA)4[Cr(CN-B(C6F5)3)6], are surprisingly stable. Continuous wave and pulsed electron paramagnetic resonance (hyperfine sublevel correlation) spectra were collected for all Cr(III) complexes; as expected, 14N hyperfine splittings are greater for (TBA)3[Cr(NC-BPh3)6] than for (TBA)3[Cr(CN-BPh3)6]. Using (TBA)4[Fe(CN-B(C6F5)3)6] and (TBA)3[Fe(CN)6], a model flow battery was constructed and found to have an 80% energy effi-ciency.