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

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Prabir Ghosh , Ritwika Ray , Ankita Das , andGoutam Kumar Lahiri *

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

Inorg. Chem., 2014, 53 (19), pp 10695–10707

DOI: 10.1021/ic501852a

Publication Date (Web): September 23, 2014

Copyright © 2014 American Chemical Society

*E-mail: lahiri@chem.iitb.ac.in. Phone: +91 22 25767159. Fax: +91 22 25723480.

Synopsis

Isomeric 1+ and 2+ represent unrecognized N,O− and usual N,N coordination modes involving O−H···N and O−H···O hydrogen bonding interactions at the back face of monodeprotonated 2,2′-bipyridine-3,3′-diol, respectively. 1+ and 2+ exhibit distinctive structural, electrochemical, spectral, as well as noninnocent features.

Section:

Inorganic Chemicals and Reactions

Abstract

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]ClO4have 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 {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.