Impact of {Os(pap)2} in fine-tuning the binding modes and non-innocent potential of deprotonated 2,2′-bipyridine-3,3′-diol

Abstract

The reaction of ctc-Os^II(pap)₂Cl₂ (pap = 2-phenylazopyridine, ctc = cis–trans–cis with respect to chlorides and pyridine/azo nitrogens of pap, respectively) and ambidentate 2,2′-bipyridine-3,3′-diol (H₂L) leads to the simultaneous formation of isomeric [Os^II(pap)₂(HL⁻)]⁺ (2⁺/3⁺), seven-membered chelate containing Os^II(pap)₂(L²⁻) (4) and diastereomeric [{Os^II(pap)₂}₂(μ-L²⁻)]²⁺ (5a²⁺ (meso, ΔΛ)/5b²⁺ (rac, ΔΔ/ΛΛ)). The reaction of 2,2′-biphenol (H₂L′) and ctc-Os^II(pap)₂Cl₂ yields Os^II(pap)₂(L′²⁻) (6), an analogue of 4. The identities of the newly designed complexes have been established by different analytical, spectroscopic and X-ray diffraction techniques. ¹H-NMR spectra of the complexes and single crystal X-ray structures of selective derivatives [2]ClO₄, [3]ClO₄, [5a](ClO₄)₂, and 6 establish the retention of the tc-configuration of the precursor {Os(pap)₂}. In isomeric 2⁺ and 3⁺, monodeprotonated HL⁻ is linked to the {Os^II(pap)₂} fragment through N,N and N,O⁻ donors, resulting in nearly planar five- and six-membered chelates with O–H⋯O⁻ and O–H⋯N hydrogen bonds at its back face, respectively. The O⁻,O⁻ donating L′²⁻ extends a severely twisted seven-membered chelate with the {Os(pap)₂} unit in 6. The N,O⁻/O⁻,N donors of deprotonated L²⁻ bridge the two {Os^II(pap)₂} units in a symmetric fashion in 5a²⁺, forming two moderately twisted six-membered chelates. Though the deprotonation of the O–H⋯N hydrogen bond in 2⁺ by another unit of {Os^II(pap)₂} generates a diastereomeric mixture of 5a²⁺ and 5b²⁺, attempts to deprotonate the relatively stronger O–H⋯O⁻ hydrogen bond in 2⁺ have failed. The isomeric 2⁺/3⁺, seven-membered chelate containing 4/6 and diastereomeric 5a²⁺/5b²⁺ exhibit distinctive ¹H-NMR and absorption spectra as well as electrochemical responses. The pap (NN) based two successive reductions and the participation of HL⁻, L²⁻, L′²⁻ in the oxidation processes of the respective complexes have been revealed using EPR and DFT calculated MOs and Mulliken spin density plots at the intermediate paramagnetic states.