Oxidative Addition of C-Cl Bonds to a Rh(PONOP) Pincer Complex

Straightforward procedures for the generation of rhodium(I) kappa Cl- chlorocarbon complexes of the form [Rh(PONOP-tBu)(kappa Cl-ClR)][BArF4] [R = CH2Cl, A; Ph, 1; Cy, 2; tBu, 3; PONOP-tBu = 2,6-bis(di-tert-butylphosphinito)-pyridine; ArF = 3,5-bis(trifluoromethyl)phenyl] in solution are described, enabling isolation of analytically pure A and crystallographic characterization of the new complexes 1 and 2. Complex 1 was found to be stable at ambient temperature, but prolonged heating in chlorobenzene at 125 degrees C resulted in formation of [Rh(PONOP-tBu)(Ph)Cl][BArF4] 4 with experimental and literature evidence pointing toward a concerted C(sp2)-Cl bond oxidative addition mechanism. C(sp3)-Cl bond activation of dichloromethane, chlorocyclohexane, and 2-chloro-2-methylpropane by the rhodium(I) pincer occurred under considerably milder conditions, and radical mechanisms that commence with chloride atom abstraction and involve generation of the rhodium(II) metalloradical [Rh(PONOP-tBu)Cl][BArF4] 6 are instead proposed. For dichloromethane, [Rh(PONOP-tBu)(CH2Cl)Cl][BArF4] 5 was formed in the dark, but facile photo-induced reductive elimination occurred when exposed to light. Net dehydrochlorination affording [Rh(PONOP-tBu)(H)Cl][BArF4] 7 and an alkene byproduct resulted for chlorocyclohexane and 2-chloro-2-methylpropane, consistent with hydrogen atom abstraction from the corresponding alkyl radicals by 6. This suggestion is supported by dynamic hydrogen atom transfer between 6 and 7 on the 1H NMR time scale at 298 K in the presence of TEMPO.

Automated summary

This study describes straightforward procedures for the synthesis of rhodium(I) kappa Cl- chlorocarbon complexes in solution, enabling the isolation of pure products and crystallographic characterization. The results suggest that oxidative addition of C(sp2)-Cl bonds may occur at higher temperatures, while activation of C(sp3)-Cl bonds may proceed through radical mechanisms.