Complexation of Diphenyl(phenylacetenyl)phosphine to Rhodium(III) Tetraphenyl Porphyrins: Synthesis and Structural, Spectroscopic, and Thermodynamic Studies

The coordination of diphenyl(phenylacetenyl)phosphine (DPAP) to (X)RhIIITPP (X = I or Me (3); TPP = tetraphenyl porphyrin) was studied in solution and in the solid state. The iodide is readily displaced by the phosphine, leading to the bis-phosphine complex [(DPAP)2Rh(TPP)](I) (4). The methylide on rhodium in 3 is not displaced, leading selectively to the mono-phosphine complex (DPAP)(Me)Rh(TPP) (5). The first and second association constants, as determined by isothermal titration calorimetry and UV-vis titrations, are in the range 10^4-10^7 M-1 (in CH2Cl2). Using LDI-TOF mass spectrometry, the mono-phosphine complexes can be detected but not the bisphosphine complexes. The electronic spectrum of 4 is similar to those previously reported with other tertiary phosphine ligands, whereas (DPAP)(I)Rh(TPP) (6) displays a low energy B-band absorption and a high energy Q-band absorption. In contrast to earlier reports, displacement of the methylide on rhodium in 5 could not be observed at any concentration, and the electronic spectra of 4 and 5 are almost identical. Isothermal titration calorimetry experiments showed that all binding events are exothermic, and all are enthalpy driven. The largest values of ΔG° are found for 6. The thermodynamic and UV-vis data reveal that the methylide and the phosphine ligand have an almost identical electronic trans-influence on the sixth ligand.