Acid-Promoted sp3 C–H Bond Cleavage in a Series of (2-Allylphenoxo)ruthenium(II) Complexes. Mechanistic Insight into the Aryloxo–Acid Interaction and Bond Cleavage Reaction

A series of RuCp­[OC6H3(CH2CHCH2-2)­(R)]­(PPh3)n complexes (n = 2, R = H (1a); n = 1, R = 4-OMe (2b), 4-Me (2c), 4-Ph (2d), 4-Br (2e), 4-NO2 (2f), 6-OMe (2g), 6-Me (2h), 6-Ph (2i)) have been prepared in 27–76% yields. These 2-allylaryloxo complexes 1a and 2b–f are in equilibrium between RuCp­[OC6H3(CH2CHCH2-2)­(R)-κ1O]­(PPh3)2 (1) and RuCp­[OC6H3(CH2CHCH2-2)­(R)-κ1O,η2C,C′]­(PPh3) (2) in solution, and 2g–i do not react with PPh3. The equilibrium constant K1 (K1 = [2]­[PPh3]/[1]) is about the same for 1a and 2b–f (K1 = 0.07–0.31 M). In contrast to the conventional aryloxo complexes of the late transition metals, treatment of 1a and 2a–g with weak Brϕnsted acids (HOR) gives a rapid equilibrium with 2·HOR. The association constant K2 (K2 = [2·HOR]/([2]­[HOR])) increases on decreasing the pKa value of the acid employed and on increasing the induction effect of substituents at the 4-position in the aryloxo group. These features suggest present association being regarded as a simple acid–base interaction. Interestingly, further association of 2·HOR with the second acid leads to the cleavage of the benzylic C–H bond, giving RuCp­[C3H4{1-C6H3(OH-2)­(R)}-η3C,C′,C″]­(PPh3) (3). The thermodynamic and kinetic studies suggest formation of hydrogen bonds among two Brϕnsted acid molecules, lone-pair electrons in the aryloxo oxygen, and a benzylic methylene proton. Such association makes the Ru­(II) center more electrophilic to attack the benzylic carbon to give 3.