Boronated Cyanometallates

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Abstract
Thirteen boronated cyanometallates [M(CN-BR3)6]3/4/5- [M = Cr, Mn, Fe, Ru, Os; BR3 = BPh3, B(2,4,6,-F3C6H2)3, B(C6F5)3] and one metalloboratonitrile [Cr(NC-BPh3)6]3- have been characterized by X-ray crystallography and spectroscopy [UV-vis-near-IR, 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 π + B-C borane σ orbitals. CASSCF+NEVPT2 calculations including t1u and t2u orbitals reproduced t1u/t2u → t2g excitation energies. Many [M(CN-BR3)6]3/4- complexes exhibited highly electrochemically reversible redox couples. Notably, the reduction formal potentials of all five [M(CN-B(C6F5)3)6]3- anions scale with the LMCT energies, and Mn(I) and Cr(II) compounds, [K(18-crown-6)]5[Mn(CN-B(C6F5)3)6] and [K(18-crown-6)]4[Cr(CN-B(C6F5)3)6], are surprisingly stable. Continuous-wave and pulsed electron paramagnetic resonance (EPR; hyperfine sublevel correlation) spectra were collected for all Cr(III) complexes; as expected, 14N hyperfine splittings are greater for (Ph4As)3[Cr(NC-BPh3)6] than for (Ph4As)3[Cr(CN-BPh3)6].
Funding Information
  • Division of Chemistry (CHE-1763429)
  • National Institute of Standards and Technology
  • Arnold and Mabel Beckman Foundation
  • California Institute of Technology

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