This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017 New J. Chem., 2017, 41, 4157--4164 | 4157 Cite this: New J. Chem., 2017, 41, 4157 Luminescent closed shell nickel(II) pyridyl-azo- oximates and the open shell anion radical congener: molecular and electronic structure, ligand redox behaviour and biological activity† Shuvam Pramanik, a Suhana Dutta, b Sima Roy, a Soumitra Dinda, ab Tapas Ghorui, a Arup Kumar Mitra, b Kausikisankar Pramanik * a and Sanjib Ganguly * b Luminescent nickel(II) complexes of type [Ni II (L I ) 2 ], 2, have been synthesized using photosensitizer redox-active oxime ligands HL, 1, incorporating the p-acidic azo and pyridyl functions. The redox non- innocent behaviour of the ligands has been exploited to isolate the open shell Ni(II)-bound azo-oxime anion radical complexes of type Et 4 N[Ni II {(L I ) 2 }  ], 3, via reduction with NaBH 4 . The superior stabilization of the unpaired spin over the ligand framework has been established by EPR and DFT studies. The anti-bacterial activity of 2 has been scrutinized and it has been found to exhibit potential radical scavenging activity. Introduction Luminescent d 8 transition metal complexes are currently under intense investigation from both a theoretical perspective 1 as well as the point of view of applications. 2 From these perspectives, a lot of studies have already been performed on the emissive behaviour of palladium(II) and platinum(II), 3,4 nonetheless there is an astounding paucity of analogous scrutiny for the first member of the triad. Notably, the presence of accessible excited states has enriched the complexes of the higher congeners with fascinating luminescence properties. Recently, it has been found that nickel(II) compounds incorporating a photosensitizer (PS) oxime chromo- sphere can behave as catalysts for solar-driven water oxidation and proton reduction. 5 In this regard, the study of the molecular and electronic structures as well as the redox behaviour of novel nickel–oxime complexes is imperative for the understanding of the optoelectronic properties that are associated with electron transfer within metal–organic frameworks. In this context, and in line with our interest in the photo- physical properties of nickel(II) oximates, we have chosen to study photosensitizer redox-active oxime ligands HL, 1, containing the p-acidic azo and pyridyl functions. They behave as potentially redox non-innocent upon ligation and can also act as electron sinks owing to the presence in each of a low-lying empty p* orbital, delocalized primarily over the azo group with substantial contribution from the pyridyl and oxime moieties. 6–9 This property of the ligands has been explored theoretically and the results are summarized in Fig. 1. A comparison of the frontier Fig. 1 (a) LUMO of HL, 1. (b) Lewis structures of the neutral and radical anionic forms of 1. (c) Variations in interatomic bond distances for 1 (bond numbering shown in (b)). a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata, 700032, India. E-mail: kpramanik@hotmail.com b Department of Chemistry, St. Xavier’s College (Autonomous), Kolkata, 700016, India. E-mail: icsgxav@gmail.com † Electronic supplementary information (ESI) available: Experimental details, analytical and spectroscopic data of new compounds. CCDC 1521795. For ESI and crystallo- graphic data in CIF or other electronic format see DOI: 10.1039/c7nj00402h Received 3rd February 2017, Accepted 19th April 2017 DOI: 10.1039/c7nj00402h rsc.li/njc NJC PAPER