Influence of Ligand Modifications on Structural and Spectroscopic Properties in Terphenyl Based Heavier Group 14 Carbene Homologues

The synthesis and characterization of a series of heavier group 14 element (Ge, Sn, and Pb) carbene homologues based on the electronically modified, 2,6-dimesityl substituted terphenyl ligands Ar-#-3,5-Pr-i(2), Ar-#-4-SiMe3, and Ar-#-4-Cl (Ar-#-3,5-Pr-i(2) = C6H2-2,6-Mes(2)-3,5-Pr-i(2); Ar-#-4-Cl = C6H2-2,6-Mes(2)-4-Cl; Ar-#-4-SiMe3 = C6H2-2,6-Mes(2)-4-SiMe3; Mes = C6H2-2,4,6-Me-3) are presented. The consequences of introducing electron withdrawing and -releasing substituents on the solid state structures of the newly synthesized germylenes, stannylenes, and plumbylenes as well as their Mossbauer, NMR and UV-vs spectroscopic properties are presented and discussed in the context of a second order Jahn-Teller type mixing of frontier orbitals with appropriate symmetry. Experimental findings were supported by DFT calculations. More electron withdrawing ligands lead to a bonding situation with higher contribution of p-orbitals from the central heavier group 14 element in a-bonding toward the ligands and thus increased s-electron character of the lone pair. Furthermore, this results in an increase in the energy separation between the frontier orbitals. Experimentally, these changes are manifested in narrower bending angles at the heavy tetrel atoms and hypsochromic in their UV-vis spectra. In contrast, derivatives of more electron rich m-terphenyl ligands are characterized by a smaller HOMO-LUMO gap and wider interligand angles.