Adding a New Dimension to the Investigation of Platinum-Mediated Arene C−H Activation Reactions Using 2D NMR Exchange Spectroscopy. Dynamics of Pt(II) Phenyl/Benzene Site Exchange

Protonation of (N−N)PtPh2 (1; N−N = diimine ArNCMe−CMeNAr with Ar = 2,6-Me2C6H3
(a), 2,4,6-Me3C6H2 (b), 4-Br-2,6-Me2C6H2 (c), 3,5-Me2C6H3 (d), and 4-CF3C6H4 (e)) in the presence of MeCN
at ambient temperature generates (N−N)Pt(Ph)(NCMe)+ (2). At −78 °C, protonation of 1a yielded (N−N)PtPh2(H)(NCMe)+ (3a), which produced benzene and 2a at ca. −40 °C. Protonation of 1a−e in CD2Cl2/Et2O-d10 furnished (N−N)Pt(C6H5)(η2−C6H6)+ (4a−e). The π-benzene complexes 4a−c, sterically
protected at Pt, eliminate benzene at ca. 0 °C. The sterically less protected 4d−e lose benzene already at
−30 °C. SST and 2D EXSY NMR demonstrate that phenyl and π-benzene ligand protons undergo exchange
with concomitant symmetrization of the diimine ligand, most likely via oxidative insertion of Pt into a C−H
bond of coordinated benzene. The kinetics of the exchange processes for 4a−c were probed by quantitative
EXSY spectroscopy, resulting in ΔH⧧ of 70−72 kJ mol-1 and ΔS⧧ of 37−48 J K-1 mol-1. A large, strongly
temperature-dependent H/D kinetic isotope effect (9.7 at −34 °C; 6.9 at −19 °C) was measured for the
dynamic behavior of 4a versus 4a-d10, consistent with the proposed π-benzene C−H bond cleavage. The
fact that the π-benzene complex 4a is thermally more robust in the absence of MeCN than is the Pt(IV)
hydridodiphenyl complex 3a in the presence of MeCN agrees with the notion that arene elimination from
Pt(IV) hydridoaryl complexes occurs via Pt(II) π-arene intermediates that eliminate the hydrocarbon
associatively, in this case, promoted by MeCN. Compounds 1a, 1b, 1d, 2a, and 2b have been
crystallographically characterized.

(N−N)PtPh₂（1；其中N−N为二亚胺（diimine）ArN=CMe−CMe=NAr，Ar分别为2,6-二甲基苯基(a)、2,4,6-三甲基苯基(b)、4-溴-2,6-二甲基苯基(c)、3,5-二甲基苯基(d)与4-三氟甲基苯基(e)）在乙腈（MeCN）存在、室温条件下发生质子化反应，生成(N−N)Pt(Ph)(NCMe)+（2）。在−78 ℃时，1a发生质子化反应得到(N−N)PtPh₂(H)(NCMe)+（3a），该产物在约−40 ℃时可分解为苯与2a。在氘代二氯甲烷（CD₂Cl₂）/氘代乙醚（Et₂O-d₁₀）中对1a~e进行质子化，可得到(N−N)Pt(C₆H₅)(η²−C₆H₆)+（4a~e）。这类π-苯配合物4a~c在铂中心具有空间位阻保护，于约0 ℃时发生苯消除反应；而空间位阻更小的4d~e则在−30 ℃即可完成苯消除。选择性饱和转移谱（SST）与二维交换谱（2D EXSY）核磁共振研究表明，苯基与π-苯配体的质子会发生交换，同时伴随二亚胺配体的对称化，该过程最可能通过铂对配位苯的C−H键氧化插入路径实现。通过定量EXSY谱学对4a~c的质子交换过程动力学进行了探究，测得其活化焓ΔH⧧为70~72 kJ·mol⁻¹，活化熵ΔS⧧为37~48 J·K⁻¹·mol⁻¹。针对4a与其氘代衍生物4a-d₁₀的动态行为，测得其具有较大且随温度显著变化的氘-氢动力学同位素效应（KIE, kinetic isotope effect）：在−34 ℃时为9.7，在−19 ℃时为6.9，该结果与所提出的π-苯配体C−H键断裂机理相符。π-苯配合物4a在无乙腈存在时的热稳定性优于有乙腈存在时的四价铂氢合二苯基配合物3a，这一事实支持了如下观点：从Pt(IV)氢合芳基配合物中消除芳烃的过程，是通过二价铂（Pt(II)）的π-芳烃中间体进行的，该中间体以缔合方式消除芳烃，本案例中该过程由乙腈促进。化合物1a、1b、1d、2a与2b已通过单晶X射线衍射完成结构表征。