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

Abstract

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 RuIII(acac)2(HL-) (1), strongly π-accepting pap (= 2-phenylazopyridine) in RuII(pap) 2(L2-) (2)/[(pap)2RuII(μ-L 2-)RuII(pap)2](ClO4)2 ([4](ClO4)2), and reported moderately π-accepting bpy (= 2,2′-bypiridine) in [RuII(bpy)2(HL -)]PF6 ([5]PF6)/[(bpy)2Ru(μ- L2-)Ru(bpy)2](PF6)2 ([7](PF 6)2). 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 L2-. The nonplanarity of the coordinated L2- 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 L2-. 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 RuII(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)22+} derived dimeric [4](ClO4)2 was structurally characterized. It establishes the symmetric N,O-/N,O- bridging mode of nonplanar L 2- as in reported [7](PF6)2. Besides structural and spectroscopic characterization of the newly developed complexes, the ligand (HL-, L2-, L′2-, pap)-, metal-, or mixed metal-ligand-based accessible redox processes in 1n (n = +2, +1, 0, -1), 2n/3n (n = +2, +1, 0, -1, -2), and 4n (n = +4, +3, +2, +1, 0, -1) were analyzed in conjunction with density functional theory calculations. © 2014 American Chemical Society.