Frustrated Lewis pair-like splitting of aromatic C–H bonds and abstraction of halogen atoms by a cationic [(FPNP)Pt]+species

Abstract

This manuscript explores new chemistry that can be related to the unobserved 14-electron [(FPNP)Pt]+ transient (FPNP = (4-F-2-(iPr2P)C6H3)2N). Its reactivity can be accessed via abstraction of triflate from (FPNP)PtOTf (1) with K[B(C6F5)4] in various solvents serving as substrates. With benzene, toluene and fluorobenzene, net heterolytic splitting of an aromatic C–H bond across the N–Pt bond is observed, without any detectable intermediates, leading to the [(FPN(H)P)Pt–Ar]+ products (Ar = Ph, 3a; Ar = C6H4Me, 3b (ortho), 3c (meta), 3d (para); Ar = o-C6H4F, 3e). The latter can be alternatively prepared by protonation of the neutral (FPNP)Pt–Ar compounds. Compounds 3a–3e do not release free arene under thermolysis at 80 °C, and compounds 3b/c/d do not interconvert under ambient temperature. With chlorobenzene and bromobenzene, the kinetic product is the κ1-Cl or κ1-Br adduct [(FPNP)Pt–Cl–Ph]+ (4) or [(FPNP)Pt–Br–Ph]+ (10). Compound 4 rearranges into a C–H splitting product [(FPN(H)P)Pt–C6H4Cl]+ (3f), while 10 slowly reacts by formal transfer of Br atom to Pt. An analogous Cl atom transfer to Pt is observed upon the reaction of 1 with K[B(C6F5)4] in dichloromethane, producing [(FPNP)PtCl][B(C6F5)4] (9a) which features an oxidized FPNP ligand framework. X-Ray diffractometry established structures of [(FPN(H)P)Pt–C6H4F-o][B(C6F5)4] (3e, disordered rotamers), [(FPN(H)P)Pt–C6H4Me][B(C6F5)4] (disordered meta- and para-isomers 3c/d), and [(FPNP)PtCl][HCB11Cl11] (9b). DFT calculations at the PBE0 and M06-L levels on the free [(FPNP)Pt]+ cation predict a relatively small (10–12 kcal mol−1) separation between the singlet and the triplet states. The relatively low triplet energy is probably related to the viability of the unexpected halogen atom abstraction reactions.