C → N coordination bonds in (CCC) → N + ← (L) complexes

Abstract

Quantum chemical calculations were performed on a series of novel divalent NI compounds, CCC → N+ ← CO (1), CCC → N+ ← N2 (2), CCC → N+ ← PPh3 (3), CCC → N+ ← C(NH2)2 (4), CCC → N+ ← NHCMe (5) CCC → N+ ← N-methyl-4-pyridylidene (6) and CCC → N+ ← Cyclopropenylidene (7), where CCC is a carbocyclic carbene (cyclohexa-2,5-diene-4-(diaminomethynyl)-1-ylidene). Complete optimization of 3D structures indicates that the chosen structures are the global minima on their respective potential energy surfaces (tautomeric alternatives are much less stable). The CCC → N+ coordination bond length is in the range of 1.353–1.399 Å, supporting the C → N coordination bond character. This is also supplemented by very low CCC → N bond rotational barriers (> 8 kcal/mol). The CCC → N ← L angles are in the range of 118°–131°, suggesting that there is no heteroallene-type character at the central nitrogen atom. Electron localization function, lone pair occupancy calculations and partial charge analysis indicate the presence of excess electron density at the N+ centre. The nucleophilicity of the designed compounds was further measured by calculating the proton affinity and complexation energies with various Lewis acids like BH3, AlCl3 and AuCl at the N+ centre. All these studies suggest the presence of divalent NI character in the designed compounds 1–7.