Revelation of Varying Coordination Modes and Noninnocence of Deprotonated 2,2′-Bipyridine-3,3′-diol in {Os(bpy)2} Frameworks

The reaction of 2,2′-bipyridine-3,3′-diol (H2L) and cis-OsII(bpy)2­Cl2 (bpy = 2,2′-bipyridine) results in isomeric forms of [OsII(bpy)2(HL–)]­ClO4, [1]­ClO4 and [2]­ClO4, because of the varying binding modes of partially deprotonated HL–. The identities of isomeric [1]­ClO4 and [2]­ClO4 have been authenticated by their single crystal X-ray structures. The ambidentate HL– in [2]­ClO4 develops the usual N,N bonded five-membered chelate with a strong O–H···O hydrogen bonded situation (O–H···O angle: 160.78°) at its back face. The isomer [1]­ClO4 however represents the monoanionic O–,N coordinating mode of HL–, leading to a six-membered chelate with the moderately strong O–H···N hydrogen bonding interaction (O–H···N angle: 148.87°) at its backbone. The isomeric [1]­ClO4 and [2]­ClO4 also exhibit distinctive spectral, electrochemical, electronic structural, and hydrogen bonding features. The pKa values for [1]­ClO4 and [2]­ClO4 have been estimated to be 0.73 and –. The O–H···O group of HL– in 2+ remains invariant in the basic region (pH 7–12), while deprotonation of O–H···N group of HL– in 1+ estimates the pKb value of 11.55. This indeed has facilitated the activation of the exposed O–H···N function in [1]­ClO4 by the second {OsII(bpy)2} unit to yield the L2– bridged [(bpy)2OsII(μ-L2–)­OsII(bpy)2]­(ClO4)2 ([3]­(ClO4)2). However, the O–H···O function in [2]­ClO4 fails to react with {OsII(bpy)2}. The crystal structure of [3]­(ClO4)2 establishes the symmetric N,O–/O–,N bridging mode of L2–. On the other hand, the doubly deprotonated L′2– (H2L′ = 2,2′-biphenol) generates structurally characterized twisted seven-membered O–,O– bonded chelate (torsion angle >50°) in paramagnetic [OsIII(bpy)2­(L′2–)]­ClO4 ([4]­ClO4). The electronic structural aspects of the complexes reveal the noninnocent potential of the coordinated HL–, L2–, and L′2–. The Kc value of 49 for 33+ reveals a class I mixed-valent OsIIOsIII state.