Activation of Aromatic C−H Bonds by (dmpe)Pt(Me)X (X = Me, O2CCF3, OTf) Systems

Abstract

A series of complexes (dmpe)Pt(X)2 (X = Me, Cl, O2CCF3, OTf) have been prepared and their thermal stabilities examined. In contrast to (dmpe)Pt(Me)(Cl), which is stable up to 150 °C in benzene solution, thermolysis of (dmpe)Pt(Me)(O2CCF3) in benzene at 125 °C results in methane loss and the formation of (dmpe)Pt(Ph)(O2CCF3) as the major product, along with small amounts of the disproportionation products (dmpe)Pt(O2CCF3)2 and (dmpe)Pt(Ph)2 (8−10%). Kinetic studies in C6H6 and C6D6 indicate that the disappearance of (dmpe)Pt(Me)(O2CCF3) is first-order, with an overall kinetic isotope effect of 3.3(2). Methyl/aryl metathesis rates are not inhibited by added TBA+O2CCF3-. Thermolysis of (dmpe)Pt(Me)2 in benzene at 180 °C for 18 h gave a mixture of unreacted (dmpe)Pt(Me)2 (15%), (dmpe)Pt(Me)(Ph) (51%), and (dmpe)Pt(Ph)2 (34%). Thermolysis of (dmpe)Pt(Me)(O2CCF3) in either 1,2- or 1,4-difluorobenzene at 125 °C afforded 2:1:1 statistical mixtures of (dmpe)Pt(Ar)(O2CCF3), (dmpe)Pt(Ar)2, and (dmpe)Pt(O2CCF3)2 (Ar = 2,3-C6H3F2 or 2,5-C6H3F2, respectively). Thermolysis of (dmpe)Pt(Me)(O2CCF3) in toluene affords a mixture of ortho, meta, and para tolyl products, (dmpe)Pt(C6H4Me)(O2CCF3). The metathesis rate for polar 1,2-difluorobenzene is not significantly different than that observed for the nonpolar aromatic substrates. Surprisingly, the aryl/aryl exchange rates for (dmpe)Pt(Ph)(O2CCF3) in C6D6 and C7D8 are 10−20 times faster than observed methyl/aryl exchange rates. These data may be accommodated by a mechanism involving a rate-limiting addition of aryl C−H bonds directly to a 4-coordinate (dmpe)Pt(II) or a 3-coordinate (η1-dmpe)Pt(II) center.