Binding of Specialty Phosphines to Metals: Synthesis, Structure, and Solution Calorimetry of the Phosphirane Complex [PtMe2(iPrBABAR-Phos)2]
Résumé
The complex [PtMe2(iPrBABAR-Phos)2] (3) was prepared in a clean and quantitative ligand substitution reaction from [PtMe2(cod)] (1; cod ) eta(4)-1,5-cyclooctadiene) and the phosphirane iPrBABAR-Phos (2). The structure of 3 was determined by X-ray diffraction. The Pt-P bonds (~2.26 Å) lie in the shorter range of PtII-P bonds, although the 1J(195Pt31P) coupling (1840 Hz) is quite small. The enthalpy for this ligand substitution reaction was measured by solution calorimetry and found to be exothermic by 11.8 kcal/mol, a relatively low exothermicity for a reaction involving a tertiary phosphine in this Pt system. Calculations using density functional theory (DFT) on the B3LYP level were applied using the simplified model reaction [PtH2(cod)] + 2(H2N)PC2H4 -> [PtH2{(H2N)PC2H4}2)] + cod, and these also gave a rather low substitution enthalpy (-17 kcal/mol). A charge decomposition analysis (CDA) was performed for Pt(II) and Pt(0) complexes with the simple P-amino phosphirane (H2N)PC2H4 and PH3 as ligands. Contrary to expectations, it is found that the phosphirane acts as a relatively good electron donor, while its electron-acceptor properties are not significantly different from those of other phosphines. The particularly low reaction enthalpy may thus be due to a low directionality of the donor orbitals toward the metal center.