En Route to Chiral-at-Metal Ruthenium Complexes Containing Tripodal Tetradentate Ligands

Synthetic routes for ruthenium­(II) complexes bearing the {Ru­(κ4C,N,N′,P-L)} fragment (L = tripodal tetradentate ligand) are explored. Tripodal ligands LH1–LH3 diastereoselectively react with the dimer [{(Cy)­RuCl}2(μ-Cl)2] (Cy = η6-p-MeC6H4iPr), affording the p-cymene compounds [(Cy)­Ru­(κ3N,N′,P-LH)]­[SbF6] (1–3). Refluxing complexes 1–3 in MeCN gives [Ru­(κ3N,N′,P-LH)­(NCMe)3]­[SbF6]2 (4a–6a). In refluxing MeOH, isomers 4a–6a evolve to the corresponding mer complexes 4c–6c. Reaction of [RuCl2(PPh3)3] with LH1 affords [RuCl2(PPh3)­(κ3N,N′,P-LH1)] (7) as only one isomer. [RuClH­(CO)­(PPh3)3] reacts with ligands LH1–LH3 to give [RuH­(CO)­(PPh3)­(κ3N,N′,P-LH)]Cl (8–10). Complexes 8–10 were isolated as a mixture of only two diastereomers as pairs of enantiomers. Refluxing suspensions of complexes 8–10 in 2-ethoxyethanol afforded the corresponding metalated complexes [RuCl­(CO)­(κ4C,N,N′,P-L)] (11–13), in which the tripodal ligands L1–L3 act as tetradentate ligands. Only one stereoisomer was detected for complexes 11 and 13, but metalation at the C-2 or C-6 carbons gave a mixture of two isomers for complex 12. The new complexes have been characterized by analytical and spectroscopic means and by the determination of the crystal structures of the complexes 1, 2, 5c, 10a, 11a, 12a,b, and 13a by X-ray diffractometry