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

The reaction of ctc-Os-II(pap)(2)Cl-2 (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)(mu-L2-)](2+) (5a(2+) (meso, Delta Lambda)/5b(2+) (rac, Delta Delta/Lambda Lambda)). The reaction of 2,2'-biphenol (H2L') and ctc-Os-II(pap)(2)Cl-2 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. H-1-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 center dot center dot center dot O- and O-H center dot center dot center dot 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 5a(2+), forming two moderately twisted six-membered chelates. Though the deprotonation of the O-H center dot center dot center dot N hydrogen bond in 2+ by another unit of {Os-II(pap)(2)} generates a diastereomeric mixture of 5a(2+) and 5b(2+), attempts to deprotonate the relatively stronger O-H center dot center dot center dot O- hydrogen bond in 2(+) have failed. The isomeric 2(+)/3(+), seven-membered chelate containing 4/6 and diastereomeric 5a(2+)/5b(2+) exhibit distinctive H-1-NMR and absorption spectra as well as electrochemical responses. The pap (N=N) 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