Synthesis and Reactivity of Substituted Phosphanylalanes and -Gallanes Stabilized by N-Heterocyclic Carbenes

A series of N-heterocyclic carbene (NHC)-stabilized phosphanylgallanes and highly temperature sensitive phosphanylalanes bearing aliphatic and aromatic substituents at the phosphorus atom were synthesized via salt metathesis reactions of IDipp-coordinated (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) triel halide precursors with alkali metal phosphanides. The gallium derivatives IDipp∙GaH₂PH^tBu (1) and IDipp∙GaH₂P(^tBu)₂ (2) were isolated and structurally characterized, revealing monomeric species with distorted tetrahedral coordination at the group 13 element. The corresponding aluminum compounds IDipp·AlEt₂PHtBu (3), IDipp·AlEt₂PHPh (4), IDipp·AlEt₂PPh₂ (5), and IDipp·AlEt₂PH₂ (6) exhibit a strong dependence of their electronic and structural properties on the substitution pattern at the phosphorus atoms. X-ray diffraction studies show that Al─P bonds in the P atom substituted phosphanylalanes are elongated compared to IDipp·AlEt₂PH₂. Furthermore, computational analysis of the frontier molecular orbitals revealed the availability of the phosphorus lone pair. Comparison of donor abilities of the studied compounds with known substituted phosphanylboranes also indicates that these phosphanylalanes and -gallanes can serve as strong σ donating ligands. The reactivity of phosphanylgallanes toward SMe₂·BH₃ with formation of IDipp·GaH₂PPh₂·BH₃ (9) and IDipp·GaH₂PH₂·BH₃ (10), respectively, confirms the accessibility of the phosphorus lone pair and highlights their potential as Lewis base compounds.