Combined Low Temperature Rapid Scan and1H NMR Mechanistic Study of the Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II) Diphenyl Complex Relevant to Arene C−H Activation

Abstract

A detailed kinetic study of the protonation and subsequent benzene elimination reactions of a (diimine)Pt(II) diphenyl complex (denoted as (N-N)PtPh(2)) has been undertaken in dichloromethane solution with and without acetonitrile as a cosolvent. Spectroscopic monitoring of the reactions by UV-vis stopped-flow and NMR techniques over the temperature range -80 to +27 degrees C allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. Protonation of (N-N)PtPh(2) with HBF(4) x Et(2)O in CH(2)Cl(2)/MeCN occurs with a kinetic preference for protonation at the metal, rather than at a phenyl ligand, and rapidly produces (N-N)PtPh(2)H(NCMe)(+) (DeltaH(double dagger) = 29 +/- 1 kJ mol(-1), DeltaS(double dagger) = -47 +/- 4 J K(-1) mol(-1)). At higher temperatures, (N-N)PtPh(2)H(NCMe)(+) eliminates benzene to furnish (N-N)PtPh(NCMe)(+). This reaction proceeds by rate-limiting MeCN dissociation (DeltaH(double dagger) = 88 +/- 2 kJ mol(-1), DeltaS(double dagger) = +62 +/- 6 J K(-1) mol(-1), DeltaV(double dagger) = +16 +/- 2 cm(3) mol(-1)). Protonation of (N-N)PtPh(2) in dichloromethane in the absence of MeCN cleanly produces the Pt(II) pi-benzene complex (N-N)PtPh(eta(2)-C(6)H(6))(+) at low temperatures. Addition of MeCN to a solution of the pi-benzene complex causes an associative substitution of benzene by acetonitrile, the kinetics of which were monitored by (1)H NMR (DeltaH(double dagger) = 39 +/- 2 kJ mol(-1), DeltaS(double dagger) = -126 +/- 11 J K(-1) mol(-1)). When the stronger triflic acid is employed in dichloromethane/acetonitrile, a second protonation-induced reaction also occurs. Thus, (N-N)PtPh(NCMe)(+) produces (N-N)Pt(NCMe)(2)(2+) and benzene with no detectable intermediates (DeltaH(double dagger) = 69 +/- 1 kJ mol(-1), DeltaS(double dagger) = -43 +/- 3 J K(-1) mol(-1)). The mechanisms for all steps are discussed in view of the accumulated data. Interestingly, the data allow a reinterpretation of a previous report on proton exchange between the phenyl and benzene ligands in (N-N)PtPh(eta(2)-C(6)H(6))(+). It appears that the exchange occurs by a direct sigma-bond metathesis pathway, rather than by the oxidative cleavage/reductive coupling sequence that was proposed.