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 RuIII(acac)2(HL–) (1), strongly π-accepting pap (= 2-phenylazopyridine) in RuII(pap)2(L2–) (2)/[(pap)2RuII(μ-L2–)­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]­(PF6)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 RuII(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 L2– 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.