Hard/soft selectivity in ligand substitution reactions of β-diketonateplatinum(II) complexes

Abstract

The reactivity of platinum(II) complexes of the type [PtCl(O,O′-acac)(L)] (1) and [Pt(O,O′-acac)(γ-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 (η²-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 sulfur 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, η²-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 σ-bonded acac (γ-acac) replaces the chloro ligand, only in the presence of an incoming soft nucleophile substituting the soft sulfur 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 ΔH^‡ and ΔS^‡ 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)(γ-acac)(L)] (2), L = DMSO, 1a and 2a; L = DMS, 1b and 2b; L = PPh₃, 1c and 2c) are herein reported and discussed.