Regioselective C-H Activation of Cyclometalated Bis-Tridentate Ruthenium Complexes

A series of bis-tridentate Ru(II) complexes consisting of trimethyl-4,4',4-tricarboxylate-2,2':6',2-terpyridine (Me(3)tctpy) and derivatized 6-phenyl-2,2'-bipyridine (pbpy) ligands are reported. Each complex is attached to a terminal triphenylamine (TPA) substituent at the central ring of pbpy through a thiophene bridge to benefit light absorption, while the anionic ring of pbpy is fiinctionalized with substituents to modulate the metal-based redox potential. The cyclometalation step was found to favor the isomer where the electron-donating groups (EDGs; i.e., -OEt, -SEt) are situated ortho to the organometallic bond rather than the sterically favored para position, while the para isomer is formed in exclusivity when electron-withdrawing groups (e.g., -CF3,) are installed on the anionic ring. Moreover, the distribution of the isomeric products is affected by the identity of the chalcogen: ortho:para = 1:0 and 3:1 where EDG = -OEt and -SEt, respectively. Because our molecular scaffold rules out certain cyclometalation pathways (e.g., oxidative addition, agostic interactions, a-bond metathesis), we are able to experimentally establish that the observed regioselectivity is in accordance with an electrophilic metalation where the relative stabilities of the products and carbanionic intermediates govern the ratio of the isomers formed.