Hard/soft selectivity in ligand substitution reactions of beta-diketonate platinum(II) complexes

The reactivity of platinum(II) complexes of the type [PtCl(O,O’-acac)(L)] (1) and [Pt(O,O’-acac)(gamma-acac)(L)] (2) (L = DMSO, a; DMS, b), with a range of hard and soft nucleophiles such as dimethylsulfide (DMS, b), triphenylphosphine, (PPh3, c), ethylene (eta2-C2H4, d), carbon monoxide (CO, e), pyridine (py, f), and guanosine (Guo, g) has been investigated. Interestingly, the complexes 1a and 1b undergo selective substitution of the chloro or sulphur ligand depending on the hard/soft character of the incoming nucleophile. The soft incoming ligand replaces the softer one and the hard ligand replaces the harder one, giving [PtCl(O,O’-acac)(L)] complexes (1b, 1c, 1d and 1e in the reaction of 1a with L = DMS, PPh3, eta2-C2H4, CO, respectively), and [Pt(O,O’-acac)(DMSO)(L’)] (3f, 3g) and [Pt(O,O’-acac)(DMS)(L’)] (4f, 4g) species in the reaction of 1a and 1b with L’ = py and guo, respectively. In the cases of 2a and 2b complexes, where the sigma-bonded acac (gamma-acac) replaces the chloro ligand, only in the presence of an incoming soft nucleophile substituting the soft sulphur ligand the reaction occurs. Equilibrium constants for the substitution reactions were measured by 1H NMR spectroscopy. Variable temperature 1H NMR spectroscopy studies, performed for the reaction of 1a and 2a complexes with DMS, revealed that the selective substitution of DMSO with DMS takes place in both cases, according to a second-order kinetic law. The calculated values of DeltaH‡ and DeltaS‡ are consistent with an associative mechanism. NMR spectroscopic characterization (1H, 13C, 195Pt, 31P) for the complexes and crystal structures of isolated complexes ([PtCl(O,O’-acac)(L)] (1) and [Pt(O,O’-acac)(gamma-acac)(L)] (2), L = DMSO, 1a and 2a; L = DMS, 1b and 2b; L = PPh3, 1c and 2c) are herein reported and discussed.