Dinitrogen binding, P4-activation and aza-Büchner ring expansions mediated by an isocyano analogue of the CpCo(CO) fragment

Abstract

Synthetic studies targeting an m-terphenyl isocyanide analogue of the unstable 16e⁻, S = 1 complex CpCo(CO) are reported (Cp = η⁵-C₅H₅). The m-terphenyl isocyanide CNAr^Dipp2 (Ar^Dipp2 = 2,6-(2,6-(i-Pr)₂C₆H₃)₂C₆H₃) is shown to readily bind to both CpCoI₂ and Cp*CoI₂ fragments (Cp* = η⁵-C₅Me₅) and provide mono-isocyanide starting materials that are suitable for chemical reduction. Treatment of CpCoI₂(CNAr^Dipp2) with KC₈ produces the bridging isocyanide dimer, [CpCo(μ-CNAr^Dipp2)]₂, thereby indicating that the steric combination of Cp and CNAr^Dipp2 ligands does not allow for the production of mononuclear complexes. However, Cp*CoI₂(CNAr^Dipp2) with KC₈ under an N₂ atmosphere results in the formation of the complex, Cp*Co(N₂)(CNAr^Dipp2), which is a unique two-legged piano stool complex featuring a coordinated dinitrogen ligand. The N₂ ligand in Cp*Co(N₂)(CNAr^Dipp2) is shown to be labile and, upon removal by application of vacuum, leads to the production of an η⁴-coordinated 1-azabenz[b]azulene complex by aza-Büchner cyclization of the CNAr^Dipp2 ligand. This cyclization reaction is rationalized via the intermediacy of the unobserved 16e⁻ species [Cp*Co(CNAr^Dipp2)]. While this intramolecular aza-Büchner cyclization prevents isolation of [Cp*Co(CNAr^Dipp2)], the dinitrogen complex Cp*Co(N₂)(CNAr^Dipp2) is shown to serve as a reliable synthon for this 16e⁻ species upon reaction with small molecule substrates. Both free CNAr^Dipp2 and diphenylacetylene react with Cp*Co(N₂)(CNAr^Dipp2) to form two-legged piano stool complexes. In addition, Cp*Co(N₂)(CNAr^Dipp2) reacts readily with 0.5 and 1.0 equivalents of P₄ to produce poly-phosphorus products resulting from P–P single bond cleavage.