Activation Volume Measurement for C−H Activation. Evidence for Associative Benzene Substitution at a Platinum(II) Center

Abstract

The reaction of the platinum(II) methyl cation [(N-N)Pt(CH(3))(solv)](+) (N-N = ArN[double bond]C(Me)C(Me)[double bond]NAr, Ar = 2,6-(CH(3))(2)C(6)H(3), solv = H(2)O (1a) or TFE = CF(3)CH(2)OH (1b)) with benzene in TFE/H(2)O solutions cleanly affords the platinum(II) phenyl cation [(N-N)Pt(C(6)H(5))(solv)](+) (2). High-pressure kinetic studies were performed to resolve the mechanism for the entrance of benzene into the coordination sphere. The pressure dependence of the overall second-order rate constant for the reaction resulted in Delta V(++) = -(14.3 +/- 0.6) cm(3) mol(-1). Since the overall second order rate constant k = K(eq)k(2), Delta V(++) = Delta V degrees (K(eq)) + Delta V(++)(k(2)). The thermodynamic parameters for the equilibrium constant between 1a and 1b, K(eq) = [1b][H(2)O]/[1a][TFE] = 8.4 x 10(-4) at 25 degrees C, were found to be Delta H degrees = 13.6 +/- 0.5 kJ mol(-1), Delta S degrees = -10.4 +/- 1.4 J K(-1) mol(-1), and Delta V degrees = -4.8 +/- 0.7 cm(3) mol(-1). Thus DeltaV(++)(k(2)) for the activation of benzene by the TFE solvento complex equals -9.5 +/- 1.3 cm(3) mol(-1). This significantly negative activation volume, along with the negative activation entropy for the coordination of benzene, clearly supports the operation of an associative mechanism.

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

“…One important finding has been that low-temperature protonation of (diimine)Pt(II) dialkyl and Pt(II) diaryl complexes lead to observable, but thermally sensitive, Pt(IV) hydridoalkyl and hydridoaryl complexes that eliminate the respective hydrocarbons upon heating. 60,[76][77][78][79][80][81] Scheme 9 summarizes the mechanistic picture that has emerged for these reactions at (diimine)Pt(II) systems 60,63,74,[82][83][84][85] and at related Pt species with bidentate ligands. Substitution of π-benzene for an aqua (or TFE) ligand occurs as a solvent assisted process for which there is controversy over an associative mechanism.…”

A theoretical study of the original Shilov reaction involving methane activation by platinum tetrachloride in an acidic aqueous solution

“…Bercaw et al have found from activation volume measurements [24] that the substitution of a solvent molecule by benzene at the platinum (II) methyl cation [(N-N)Pt-(CH 3 )(solv)] + (N-N = ArN@C(Me)C(Me)@NAr, Ar = 2, 6-(CH 3 ) 2 C 6 H 3 , solv = CF 3 CH 2 OH) takes place via an associative mechanism. The substitution of ligands in square planar Pt(II) complexes is in general assumed to take place by an associative mechanism [25][26][27].…”

Platinum ω-Alkenyl Compounds as Chemical Vapor Deposition Precursors. Mechanistic Studies of the Thermolysis of Pt[CH2CMe2CH2CH═CH2]2 in Solution and the Origin of Rapid Nucleation

“…In contrast, for δ-C–H activation, we propose that benzene acts as a weakly coordinating solvent, so that C–H bond cleavage (or formation) is the RDS and a large primary KIE is seen, whereas C 6 F 6 is a strongly coordinating solvent, so that formation of the σ complex is the RDS and a small and possibly inverse KIE is seen (Scheme ). ,− It is known that C 6 F 6 binds more strongly than C 6 H 6 to Pt 0 because it is a better π acceptor and that C 6 F 6 forms stable η 2 complexes with Ir I or Rh I , which are isoelectronic with Pt II .…”