Harnessing Low-Valent Metal Centers through Non-Bonding Orbital Interactions

The synthesis, characterization, and computational analysis of a series of low-valent, In(I) complexes bearing the bis(imino)pyridine scaffold, {Ar′NCPh}2(NC5H3), is reported. A stepwise steric reduction of the aryl groups on the imine substituents around the coordination site, (Ar′ = 2,5-tBu2C6H3, 2,6-iPr2C6H3, 2,6-(CH3CH2)2C6H3) is explored through the spectroscopic and crystallographic examination of complexes [{Ar′NCPh}2(NC5H3)]In+(OTf)− (1−3). Compounds 1−3 displayed long In−N and In−OTf distances indicating only weak or no coordination. Application of the ligand with Ar′ = 2,6-(CH3)2C6H3 led to an In(III) bis(imino)pyridine complex, [{2,6-Me2C6H3NCPh}2(NC5H3)]In(OTf)2Cl 4 with coordinated ligand, chloride, and triflate groups. Computational analysis of the interactions between the In cation and the ligands (orbital populations, bond order, and energy decomposition analysis) point to only minimal covalent interactions of the In(I) cation with the ligands. Although it features three N donor centers, the bis(imino)pyridine ligand provides little ligand-to-metal donation. A thorough electronic structure analysis revealed a correlation of compound stability with the reduced contribution of the In(I) 5s lone electron pair to the highest occupied molecular orbital (HOMO) of the cation. This effect, originating from non-bonding orbital interactions between the metal and the ligand, is more prominent in sterically crowded environments. The discovery of this correlation may help in designing new low-valent complexes.