Isomeric pyridylcarbenes and their Si and Ge heavier analogues at DFT: stabilization through π-p or n-p interaction

To realize the effects of the pyridyl group on the stability, multiplicity, and geometry of isomeric pyridylcarbenes and their heavier Si and Ge analogues, a theoretical study was performed at B3LYP/6-311++G(d,p)//B3LYP/6-31G(d). The behavior of nitrogen atom was totally different in each multiplicity (singlet and triplet), structural isomer (ortho-, meta-, and para-), and divalent center (C, Si, and Ge). All pyridylcarbenes have triplet ground states, while the stable silylene and germylene analogues are singlet. The pyridyl group stabilizes both singlet and triplet states divalent centers with more pronounced effects on the singlet states in the order: carbene>germylene>silylene. While all planar species benefit from common π-p conjugative interaction of the pyridyl ring, in the ortho-isomers of 2-pyridylsilylene and 2-pyridylgermylene there is another interaction, n-p, that leads to two stable non-planar conformers. This finding is confirmed by NBO charges, calculated UV-vis spectra, philicity indices (N and ω), and isodesmic reactions.

All planar 2-pyridylcarbenes, silylenes, and germylenes benefit from common π-p conjugative interaction of the pyridyl ring, but in the ortho-isomers of 2-pyridylsilylene and germylene there is another interaction, n-p, leads to two stable perpendicular conformers.