Solution 31P and 113Cd NMR Studies of Phosphine Adducts of Monomeric Cadmium (Bisphenoxide) Complexes and the Solid-State Structures of (2,6-Di-tert-butylphenoxide)2Cd(PCy3) and (2,6-Di-tert-butylphenoxide)2Cd(PMe3)2

Abstract

Monomeric phosphine derivatives of cadmium phenoxides, (phenoxide)2CdLn, where the phenoxide ligands contain sterically demanding substituents in the 2,6-positions (2,6-di-tert-butylphenoxide and 2,6-diphenylphenoxide) are described. These derivatives were synthesized from the reaction of Cd[N(SiMe3)2]2 and the corresponding phenol followed by the addition of the phosphine. For the large PCy3 ligand (Tolman's cone angle (θ) = 170°) n = 1, whereas for the smaller Me3P (θ = 118°) and n-Bu3P (θ = 132°) ligands both mono- and bis(phosphine) derivatives (n = 1 and 2) were prepared. The (2,6-di-tert-butylphenoxide)2Cd(PCy3) and (2,6-di-tert-butylphenoxide)2Cd(PMe3)2 were characterized in the solid-state by X-ray crystallography. The structure of the monophosphine adduct of PCy3 consists of a near trigonal planar geometry about the cadmium center, where the average P−Cd−O angle of 131.4° is larger than the O−Cd−O angle of 96.72° with a Cd−P bond length of 2.5247(12) Å. On the other hand, the bis(phosphine) adduct, Cd(O-2,6-tBu2C6H3)2(PMe3)2 is a distorted tetrahedral structure with O−Cd−O and P−Cd−P bond angles of 116.7(6)° and 104.3(2)°, respectively. The average Cd−P bond length in this derivative was determined to be 2.737[5] Å. The monotricyclohexylphosphine derivatives of these cadmium bisphenoxides were shown by 31P NMR spectroscopy not to be undergoing facile exchange with free phosphine in solution at ambient temperature. On the contrary, the corresponding Me3P and n-Bu3P analogues readily undergo self-exchange with free phosphine in solution via a rapid equilibrium between monophosphine adduct plus free phosphine and the bis(phosphine) adduct. The 113Cd chemical shifts in the CdO2P moieties shift downfield and the 113Cd−31P coupling constants decrease by about 900 Hz upon binding an additional phosphine ligand.