Revelation of varying coordination modes and noninnocence of deprotonated 2,2'-bipyridine-3,3'-diol in {Os(bpy)₂} frameworks.

Abstract

The reaction of 2,2'-bipyridine-3,3'-diol (H2L) and cis-Os(II)(bpy)2Cl2 (bpy = 2,2'-bipyridine) results in isomeric forms of [Os(II)(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 <0.2, respectively, thereby revealing the varying hydrogen bonding interaction profiles of O-H···N and O-H···O involving the coordinated HL(-). 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 {Os(II)(bpy)2} unit to yield the L(2-) bridged [(bpy)2Os(II)(μ-L(2-))Os(II)(bpy)2](ClO4)2 ([3](ClO4)2). However, the O-H···O function in [2]ClO4 fails to react with {Os(II)(bpy)2}. The crystal structure of [3](ClO4)2 establishes the symmetric N,O(-)/O(-),N bridging mode of L(2-). 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 [Os(III)(bpy)2(L'(2-))]ClO4 ([4]ClO4). The electronic structural aspects of the complexes reveal the noninnocent potential of the coordinated HL(-), L(2-), and L'(2-). The Kc value of 49 for 3(3+) reveals a class I mixed-valent Os(II)Os(III) state.