posted on 2009-01-26, 00:00authored byYoucef Boutadla, Omar Al-Duaij, David L. Davies, Gerald A. Griffith, Kuldip Singh
Reactions of 2-substituted pyridines HL with [MCl2Cp*]2 (M = Ir, Rh) and [RuCl2(p-cymene)]2 have been carried out in the presence and absence of sodium
acetate. 2-Phenylpyridine (HL1) is cyclometalated easily to form [MCl(L1)(ring)] 1a−c (M = Rh, Ir, ring = Cp*; M = Ru,
ring = p-cymene). However, in the case of 2-acetylpyridine
(HL2) sp3 CH activation occurs cleanly with rhodium
to form N,C chelate complex [RhCl(L2)Cp*] 2b, but the
reactions with iridium and ruthenium give unseparable mixtures of
products. The N,C cyclometalated products [MCl(L2)(ring)] 2a−c (M = Ir, Rh, ring = Cp*; M = Ru, ring = p-cymene) have been independently prepared from the lithium
enolates of 2-acetylpyridine. Notably, in the absence of acetate,
[RhCl2Cp*]2 shows no reaction with 2-acetylpyridine,
whereas [IrCl2Cp*]2 and [RuCl2(p-cymene)]2 react to form equilibrium mixtures
of the starting materials and N,O chelate complexes 4a,c, respectively. In the presence of KPF6 the N,O chelate complexes [MCl(HL2)(ring)][PF6] 4a,c,d (M = Ir, ring = Cp*; M =
Ru, ring = p-cymene, mesitylene) can be isolated.
These are not intermediates en route to the N,C cyclometalated products.
These results suggest that for CH activation to occur under
these mild conditions acetate must coordinate to the metal prior to
coordination of the ligand.