Calcium carbene complexes with boranophosphorano side-arms: CaC[P(Ph)2BH3]2

Abstract

The ligand H₂C(PPh₂BH₃)₂ (4-H₂) reacted with one or a half equivalent of (para-tBu-C₆H₄CH₂)₂Ca·(THF)₄ to form the calcium complexes Ca[HC(PPh₂BH₃)₂]₂ (4-H)₂Ca and CaC(PPh₂BH₃)₂ (4-Ca), respectively. The crystal structures of their THF adducts (4-H)₂Ca·THF and [4-Ca·(THF)]₂ follow the same trends as observed for the corresponding iminophosphorano substituted complexes Ca[HC(PPh₂NR)₂]₂ and [CaC(PPh₂NR)₂]₂. The P–C bonds shorten upon gradual deprotonation, whereas the P–B and P–N bonds elongate. The geometries of DFT-optimized model systems and complete molecular structures show similar trends. Also the charge distribution within the boranophosphorano complexes is similar to that in the iminophosphorano complexes. The high positive charges on Ca (1.74–1.75) indicate a predominantly ionic ligand–Ca bonding. High negative charges on the central carbon atom (−1.103 in Ca[HC(PH₂BH₃)₂]₂ and −1.775 in [CaC(PH₂BH₃)₂]₂) compare well to those calculated for analogous iminophosphorano complexes (−1.126 Ca[HC(PH₂NH)₂]₂ and −1.847 [CaC(PH₂NH)₂]₂, respectively). Thus, in both types of complexes delocalization of electron density over the boranophosphorano or iminophosphorano substituents does not play a major role. Complex [4-Ca·(THF)]₂ does not react with adamantyl cyanide. Instead the adduct (4-Ca)₂·(THF)·(adamantyl-CN) formed (a crystal structure has been determined).