Nickel-Catalyzed C-C Activation of Vinylcyclobutane with Visible Light: Scope, Mechanism, and Application to Chemically Recyclable Polyolefins.

N-heterocyclic carbene (NHC)-supported nickel complexes were investigated for the oxidative ring-opening of vinylcyclobutane (VCB) and photocatalytic activity. Addition of VCB to in situ generated [(NHC)Ni(0)] compounds furnished (NHC)Ni(VCB)2 that underwent oxidative addition and conversion to the corresponding Ni(II) alkyl, allyl-metallacycles. The (NHC)Ni(C6H10) metallacycles were isolated, characterized, and exhibited high thermal and chemical stability. Irradiation with visible light at ambient temperature produced a mixture of ethylene and 4-vinylcyclohexene and 1,5-cyclooctadiene, cycloaddition dimers of butadiene, arising from formal retro-[2 + 2] cycloaddition. A mixture of hexadiene products arising from ß-H elimination from the metallacycle was also observed. Free ethylene also underwent a secondary reaction to form cyclopropane products through formal [2 + 1] cycloaddition. A series of sterically and electronically modified NHC ligands was evaluated to establish the structure-activity relationship governing the rate of photocatalytic conversion of VCB and the resulting product distribution. Isotopic labeling experiments, resting state analysis, and independent synthesis of a range of nickel bis(olefin) complexes provided insight into the mechanism of the reaction and origins of the organic product mixture. (NHC)Ni-catalysis was also applied toward the retro-[2 + 2] depolymerization of (1,n'-divinyl)-oligocyclobutane to butadiene dimers.

Boronated Cyanometallates.

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].