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 = η(5)-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* = η(5)-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 η(4)-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.