Half-sandwich group 8 borylene complexes: synthetic and structural studies and oxygen atom abstraction chemistry

Abstract

Cationic terminal borylene complexes, synthesized by halide abstraction, offer a versatile platform on which to gauge the effects on the electronic structure of the metal-ligand bond brought about by variation in the borylene substituent and the metal/ligand framework. While the borylene substituent exerts a strong influence on boron-centered electrophilicity and hence on metal-ligand π character and bond length (e.g., from 1.792(8) Å for [Cp^∗Fe(CO)₂(BMes)]⁺ to 2.049(4) Å for [CpFe(CO)₂{B(NCy₂)(4-pic)}]⁺), much smaller changes are effected by changes in the metal/ligand set. Introduction of stronger π donor ruthenium- and/or phosphine-containing fragments is readily brought about by extension of the halide abstraction approach; phosphines are readily introduced by carbonyl ligand substitution at the boryl precursor stage. Thus, the novel systems [CpRu(CO)₂{B(NCy₂)}]⁺[BAr^f₄]^-, [CpM(CO)(PMe₃){B(NCy₂)}]⁺[BAr^f₄]^- (M = Fe, Ru), and [(CpFe(CO){B(NCy₂)})₂(μ-dmpe)]²⁺[BAr^f₄]^-₂ have been synthesized and structurally characterized. The effects of substitution at the metal on the M-B and B-N bonds are relatively minor {e.g., Fe-B and B-N bond lengths of 1.821(4), 1.347(5) Å and 1.859(6), 1.324(7) Å for [CpFe(CO)(PMe₃){B(NCy₂)}]⁺[BAr^f₄]^- and [CpFe(CO)₂{B(NCy₂)}]⁺[BAr^f₄]^-, respectively}, presumably reflecting, at least in part, the mutually cis disposition of the borylene and phosphine/carbonyl ligands. The utility of cationic complexes containing formally subvalent boron-based ligands in oxygen atom abstraction chemistry has been demonstrated by the conversion of a range of isocyanates, R'NCO, to the corresponding (metal-coordinated) isonitriles, [CpFe(CO)₂(CNR')]⁺. Moreover, with a view to modeling potential intermediates, the mechanism of related chemistry with carbodiimide substrates, R'NCNR', has been investigated by structural and in situ ESI-MS approaches. While reactivity toward carbodiimides leads to the formation of a novel spirocyclic boronium complex, [CpFe(CO)₂C(NCy)₂B(NCy)₂CNCy₂]⁺[BAr^f₄]^- (for R' = Cy), by a double-insertion process, DFT studies imply that the analogous product of isocyanate insertion is unlikely to be an intermediate on the pathway to isonitrile formation. The presence of a number of facile competing reaction pathways (including metathesis) and the thermodynamic stability of the spirocyclic product mean that the product distribution is better explained in terms of competing pathways, rather than differing extents of reaction along similar trajectories.