New heteronuclear bridged borylene complexes that were derived from [{Cp*CoCl}₂] and mono-metal—carbonyl fragments

Abstract

The synthesis, structural characterization, and reactivity of new bridged borylene complexes are reported. The reaction of [{Cp*CoCl}₂] with LiBH₄⋅THF at −70 °C, followed by treatment with [M(CO)₃(MeCN)₃] (M=W, Mo, and Cr) under mild conditions, yielded heteronuclear triply bridged borylene complexes, [(μ₃-BH)(Cp*Co)₂ (μ-CO)M(CO)₅] (1–3; 1: M=W, 2: M=Mo, 3: M=Cr). During the syntheses of complexes 1–3, capped-octahedral cluster [(Cp*Co)₂ (μ-H)(BH)₄{Co(CO)₂}] (4) was also isolated in good yield. Complexes 1–3 are isoelectronic and isostructural to [(μ₃-BH)(Cp*RuCO)₂ (μ-CO){Fe(CO)₃}] (5) and [(μ₃-BH)(Cp*RuCO)₂(μ-H)(μ-CO){Mn(CO)₃}] (6), with a trigonal-pyramidal geometry in which the μ₃-BH ligand occupies the apical vertex. To test the reactivity of these borylene complexes towards bis-phosphine ligands, the room-temperature photolysis of complexes 1–3, 5, 6, and [{(μ₃-BH)(Cp*Ru)Fe(CO)₃}₂(μ-CO)] (7) was carried out. Most of these complexes led to decomposition, although photolysis of complex 7 with [Ph₂P(CH₂)nPPh₂] (n=1–3) yielded complexes 9–11, [3,4-(Ph₂P(CH₂)nPPh₂)-closo-1,2,3,4-Ru₂Fe₂ (BH)₂] (9: n=1, 10: n=2, 11: n=3). Quantum-chemical calculations by using DFT methods were carried out on compounds 1–3 and 9–11 and showed reasonable agreement with the experimentally obtained structural parameters, that is, large HOMO–LUMO gaps, in accordance with the high stabilities of these complexes, and NMR chemical shifts that accurately reflected the experimentally observed resonances. All of the new compounds were characterized in solution by using mass spectrometry, IR spectroscopy, and ¹H, ¹³C, and ¹¹B NMR spectroscopy and their structural types were unequivocally established by crystallographic analysis of complexes 1, 2, 4, 9, and 10.