Neutral carbene analogues of the heaviest Group 13 elements: Consideration of electronic and steric effects on structure and stability

The neutral monovalent Group 13 β-diketiminato complexes [CH-{(CF ₃)₂CN-2,6-ⁱPr₂C₆H ₃}₂ln] and [CH{(Me)₂CN-2,6-ⁱPr ₂C₆H₃}₂Tl] have been synthesised by analogous 'one-pot' procedures involving reaction of [K.N(SiMe₃) ₂] with the appropriate β-imino-enamine and Group 13 iodide. The structure of the indium complex reveals that replacement of the ligand backbone methyl groups of the previously reported complex [CH{(Me) ₂CN-2,6-ⁱPr₂C₆H₂} ₂ln] with trifluoromethyl substituants results in only minor modifications to the dimensions of the NCCCNIn metallacycle. The electronic structures of both indium species were interrogated by DFT calculations to reveal similar frontier molecular orbital schemes. In agreement with calculations performed previously on the aluminium and gallium complexes, [CH{(Me)₂CN-2,6-ⁱPr₂C₆H ₃}₂Al] and [CH{(Me)₂CN-2,6-ⁱPr ₂C₆H₃}₂Ga], the HOMO in both indium species comprises a metal-based sp-hybrid while the LUMO is a ligand-based orbital of π symmetry. The vacant indium p-orbital is represented by the LUMO + 1. Although incorporation of the fluorinated substituents results in a stabilisation of the system overall, the stabilities and observed structural features of the complexes are reasoned to be primarily a result of the steric profile of the very bulky ligands and not through any redistribution of the electron density within the cyclic species. The thallium complex is isostructural to the analogous and previously reported aluminium, gallium and indium species. The greater stability of the monovalent state however is reflected in a reordering of the orbital energies and a stabilisation of the metal-based orbitals in the frontier region of the MO scheme.