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-iPr2, Ar#-4-SiMe3, and Ar#-4-Cl (Ar#-3,5-iPr2 = C6H2-2,6-Mes2-3,5-iPr2; Ar#-4-Cl = C6H2-2,6-Mes2-4-Cl; Ar#-4-SiMe3 = C6H2-2,6-Mes2-4-SiMe3; Mes
= C6H2-2,4,6-Me3) 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 Mössbauer,
NMR and UV–vis 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 σ-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.