Role of cyclometalation in controlling the rates of ligand substitution atplatinum(II) complexes

The rates of chloride for triphenylphosphine substitution have been measure d in dichloromethane for a series of cyclometalated [Pt(N-N-C)Cl] complexes containing a number of terdentate N-N-C. anionic ligands, derived from dep rotonated alkyl-, phenyl-, and benzyl-6-substituted 2,2'-bipyridines. These rates have been compared with those of the corresponding [Pt(N-N)(C)Cl] (N -N = 2,2'-bipyridine; C = CH3 or C6H5) complexes having the same set of don or atoms but less constrained arrangements of the ligands. The reactions of the cyclometalated compounds occur as a single-stage conversion from the s ubstrate to the ionic pair [Pt(N-N-C)(PPh3)]Cl products. There is no eviden ce by H-1 and P-31{H-1} NMR spectroscopy for the formation of other Pt(II) species or of concurrent ring-opening processes. In contrast, ih the monoal kyl- or monoaryl-2,2'-bipyridine complexes, chloride substitution is follow ed by subsequent slower processes which involve the detachment of one arm o f the chelated 2,2'-bipyridine, fast cis to hans isomerization of the cis-[ Pt(PPh3)(2)(eta (1)-bipy)(R)](+) transient intermediate, and, eventually, t he release of free bipy, yielding trans-[Pt(PPh3)(2)(R)Cl] (R = Me or Ph). All reactions are first-order with respect to complex and phosphine concent ration, obeying the simple rate law k(obsd) =: k(2)[PPh3]. The values of th e second-order rate constant k(2) do not seem particularly sensitive to the nature of the bonded organic moiety (alkyl or aryl), to its structure (cyc lometalated or not), to the size of the ring, or to the number of alkyl sub stituents on it. The effects are those foreseen on the basis of an associat ive mode of activation. The only exception to this pattern of behavior is c onstituted by the complex [Pt(bipy(phi)-H)Cl] (bipy(phi) = 6-phenyl-2,2(bip yridine), which features a significant rate enhancement with respect to the analogue [Pt(bipy)(Ph)Cl] complex. The results of this work, together with those of a previous paper, Suggest that there is not a specific role of cy clometalation in controlling the reactivity, unless an in-plane aryl ring b ecomes part of the pi -acceptor system of the chelated 2,2'-bipyridine, beh aving as a cyclometalated analogue of the nitrogen terdentate 2,2':6',2 " - terpyridine.