Can Metallapyrimidines Be Aromatic? A Computational Study into a New Class of Metallacycles

The aromaticity of a series of metallapyrimidines involving second row transition metals was examined using density functional theory. Nucleus independent chemical shifts (NICS) placed above the ring (NICS(1)zz) were used to gauge the amount of aromaticity. Natural chemical shielding analysis (NCS) was employed to decompose the chemical shifts in terms of diamagnetic and paramagnetic contributions from individual molecular orbitals. While NICS(1)zz for niobapyrimidine, [(pz)2(Nb-pyr)](0), suggested slightly aromatic character, the NCS analysis shows this is due to the diamagnetic (field-free) contribution. Instead, the positive paramagnetic (field-induced) contribution suggests that niobapyrimidine may be slightly antiaromatic. A series of d(0) metallapyrimidines, [(pz)2(M-pyr)] with M = Y(III), Zr(IV), Nb(V), Mo(VI), Tc(VII), demonstrated similar behavior. Variation of the number of metal d electrons in a series of M(V) metallapyrimidines, [(pz)2(M-pyr)] where M = Mo, Tc, Ru, and Rh, showed strong evidence for aromaticity, with NICS(1)zz values of -15.4, -36.0, -31.6, and -22.4, respectively, that are comparable to benzene (-28.7). NCS analysis of the Tc(V), Ru(V), and Rh(V) complexes shows that aromaticity is favored by an unoccupied d-π orbital that serves as an acceptor to facilitate conjugation in the metallapyrimidine ring. This unoccupied orbital is not sufficient as the d(0) series of complexes demonstrated, and we propose that the occupied d-δ orbital prevents bond localization and enables aromaticity in these metallacycles.