Coordination Chemistry of Phosphinocarbonyls with Platinum

Abstract

This thesis reports the coordination chemistry of phosphinocarbonyl ligands with platinum and describes the influence of phosphine substituents on the mechanism of chelation and the coordination mode of the carbonyl moiety. The ligands synthesised were 2-diphenylphosphinobenzaldehyde (1), 2-diphenylphosphinoacetophenone (2), 2-bis(pentafluorophenyl)phosphinobenzaldehyde (3), and 2-di-tert-butylphosphinobenzaldehyde (4). Compounds 1, 3, and 4 were selected on the basis of their steric bulk and extent to which they donate electron density to the metal. Compound 2 contained the same phosphine substituents to 1, but is the methyl ketone analogue and therefore does not contain the CHO moiety. The cone angle and electronic parameter of compounds 1–4 were compared to the reported values of PPh3, PPh(C6F5)2, and PPhtBu2. Compounds 3 and 4 were similarly bulky, and had larger cone angles than 1. The electron donating capacity of compound 4 was greater than that of 1, and compound 3 was the least electron donating. A new synthetic method for the preparation of 4 is also reported. The coordination chemistry of ligands 1–4 was investigated with platinum(II) and platinum(0) starting materials to assess the influence of the steric and electronic parameters of the phosphine on the chelation of the ligand through the carbonyl to platinum. Coordination of the ligand went through the initial coordination of the phosphine and, depending on the identity of that phosphine, may be followed by chelation of the carbonyl moiety to form a P,C chelate. However, the site of the platinum–carbon bond in the P,C metallacycle depends on the ligand employed. Coordination of the phosphinoaldehyde ligands 1, 3, and 4 produced Pt-C bonds via the C-H activation of the aldehyde CHO group whereas for ketophosphine 2, C-H activation occurred at the α-methyl group. The rate at which C-H activation occurred increased with increasing electron donation from the phosphorus to platinum. Compound 4 chelates to platinum more rapidly than compound 1, while 3 did not undergo chelation at room temperature. Although chelation was only observed to occur via C-H activation, the final products of the coordination reactions of 1–4 with platinum starting materials differed depending on the identity of the ligand. The C-H activation of two molecules of 1 with platinum(II) or platinum(0) produced a platina-β-diketone, cis-[Pt(P,C-2-PPh2C6H4CO)2] (21), which is capable of coordinating to H+, Li+, BF2 +, and [Rh(1,5-cyclooctadiene)]+ between the mutually cis carbonyl groups. One carbonyl moiety of 21 can also undergo condensation with primary amines and ammonia to produce platina-β-ketoimine complexes. The ketone moiety of ligand 2 reacted with platinum(II) starting materials through C-H activation of the terminal methyl group to form the six-membered bis-chelate complex analogous to complex 21. The reaction of 2 with platinum(0) starting materials resulted in the formation of a platinum hydride intermediate which mediated chelation through the partial reduction of the ketone group of one ligand, to form the product, [Pt(P,C-2-PPh2C6H4COCH2)(P,C-2-PPh2C6H4C(OH)CH3)] (48) . The reaction of 3 with [PtMe2(1,5-hexadiene)] at elevated temperatures resulted in the formation of [Pt(P,C-2-PPh2C6H4)(P,C-2-PPh2C6H4CO)] (54) – a decarbonylated and ortho-metallated complex containing a four-membered metallacycle. The platinum-phosphorus bond in the four-membered ring of 54 has a bond distance of 2.385(2) Å – the longest Pt–P bond reported to date. Ligand 4 reacted rapidly with platinum(II) starting materials and produced numerous chelation products. Complexes of ligand 4 were only observed to contain mutually trans phosphines, likely due to the steric bulk of the tert-butyl substituents. Comparison of the coordination chemistry of ligands 1–4 suggests that the propensity toward C-H activation of the ligands is predominantly determined by the electronic character of the phosphine (although steric effects cannot be disregarded), and the more electron-rich the phosphine, the more rapidly chelation occurs.