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

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).