Significant influence of coligands toward varying coordination modes of 2,2'-bipyridine-3,3'-diol in ruthenium complexes.

The varying coordination modes of the ambidentate ligand 2,2'-bipyridine-3,3'-diol (H2L) in a set of ruthenium complexes were demonstrated with special reference to the electronic features of the coligands, including σ-donating acac(-) (= acetylacetonate) in Ru(III)(acac)2(HL(-)) (1), strongly π-accepting pap (= 2-phenylazopyridine) in Ru(II)(pap)2(L(2-)) (2)/(pap)2Ru(II)(μ-L(2-))Ru(II)(pap)22 (42), and reported moderately π-accepting bpy (= 2,2'-bypiridine) in [Ru(II)(bpy)2(HL(-))]PF6 ([5]PF6)/(bpy)2Ru(μ-L(2-))Ru(bpy)22 (72). The single-crystal X-ray structures reveal that, in paramagnetic and electron paramagnetic resonance active 1 and reported diamagnetic [5]PF6, nearly planar monoanionic HL(-) coordinates to the metal ion via the N,N donors forming a five-membered chelate ring with hydrogen-bonded O-H···O function at the backbone of the ligand framework, as has also been reported in other metal complexes. However, structurally characterized diamagnetic 2 represents O(-),O(-) bonded seven-membered chelate of fully deprotonated but twisted L(2-). The nonplanarity of the coordinated L(2-) in 2 does not permit the second metal fragment {Ru(pap)2} or {Ru(bpy)2} or {Ru(acac)2} to bind with the available N,N donors at the back face of L(2-). Further, the deprotonated form of the model ligand 2,2'-biphenol (H2L') yields Ru(II)(pap)2(L'(2-)) (3); its crystal structure establishes the expected O(-),O(-) bonded seven-membered chelate of nonplanar L'(2-) as in reported Ru(II)(bpy)2(L'(2-)) (6), although {Ru(acac)2} metal precursor altogether fails to react with H2L'. All attempts to make diruthenium complex from {Ru(acac)2} and H2L failed; however, the corresponding {Ru(pap)2(2+)} derived dimeric 42 was structurally characterized. It establishes the symmetric N,O(-)/N,O(-) bridging mode of nonplanar L(2-) as in reported 72. Besides structural and spectroscopic characterization of the newly developed complexes, the ligand (HL(-), L(2-), L'(2-), pap)-, metal-, or mixed metal-ligand-based accessible redox processes in 1(n) (n = +2, +1, 0, -1), 2(n)/3(n) (n = +2, +1, 0, -1, -2), and 4(n) (n = +4, +3, +2, +1, 0, -1) were analyzed in conjunction with density functional theory calculations.