Formation of Ruthenium(II) Complexes with Unsymmetrical Terdentate Ligands

Three unsymmetrical terdentate ligands, 2-(2'-quinolyl)-1,10-phenanthroline (4), 2-(1'-isoquinolyl)-1,10-phenanthroline (5), and 3,3'-dimethylene-2-(2'-quinolyl)-1,10-phenanthroline (6), have been prepared by application of the Friedländer condensation. Ligand 4 forms predominantly a pentaaza-coordinated (N5) complex with Ru(II), [Ru(4-N,N',N")(4-N,N')Cl](PF(6)), whose structure was determined by X-ray analysis (C(42)H(26)ClF(6)N(6)PRu: monoclinic, C2/c, a = 29.308(3) Å, b = 15.205(2) Å, c = 19.505(2) Å, beta = 107.312(8) degrees, V = 8298(1) Å(3), Z = 8). Ligand 5 forms predominantly a hexaaza-coordinated (N6) complex, [Ru(5-N,N',N")(2)](PF(6))(2). The bridged species 6 shows intermediate behavior, and the rate for interconversion of the N5 to N6 complex has been measured. The stereochemical features of these two binding modes are examined, and both pi-stacking and steric effects are invoked to explain the observed behavior, leading to the proposal of a backside displacement mechanism for their interconversion.