Tungsten(0)–carbonyl complexes of naphthylazoimidazoles Papia Datta a , Ashis Kumar Patra b , Chittaranjan Sinha a, * a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata, West Bengal 700 032, India b Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore, India article info Article history: Received 19 September 2008 Accepted 14 November 2008 Available online 12 January 2009 Keywords: W(0)–carbonyl Naphthylazoimidazole X-ray structure Electrochemistry DFT–TD-DFT calculations abstract The reaction of W(CO) 6 with 1-alkyl-2-(naphthyl-a-azo)imidazole (a-NaiR) has synthesized [W(CO) 5 (a- NaiR-N)] (a-NaiR-N refers to the monodentate imidazole-N donor ligand) at room temperature. The structure of [W(CO) 5 (a-NaiMe-N)] shows a monodentate imidazole-N coordination of 1-methyl-2-(naph- thyl-a-azo)imidazole (a-NaiMe). The complexes are characterized by elemental, mass and other spectro- scopic data (IR, UV–Vis, NMR). On refluxing in THF at 323 K, [W(CO) 5 (a-NaiR-N)] undergoes decarbonylation to give [W(CO) 4 (a-NaiR-N,N 0 )] (a-NaiR-N,N 0 refers to the imidazole-N(N), azo-N(N 0 ) bidentate chelator). Cyclic voltammetry shows metal oxidation (W 0 /W I ) and ligand reductions (azo/ azo – , azo – /azo @ ). The redox and electronic properties are explained by theoretical calculations using an optimized geometry. DFT computation of [W(CO) 5 (a-NaiMe-N)] suggests that the major contribution to the HOMO/HOMO  1 come from W d-orbitals and the orbitals of CO. The LUMOs are occupied by a-NaiMe functions. The back bonding interaction thus originates from the W(CO) n moiety to the LUMO of a-NaiR. A TD-DFT calculation has ascribed that HOMO/HOMO  1 ? LUMO is a mixture of metal-to- ligand and ligand-to-ligand charge transfer underlying the CO ? azoimine contribution. The complexes show emission spectra at room temperature. [W(CO) 4 (a-NaiR-N,N 0 )] shows a higher fluorescence quan- tum yield (/ = 0.05–0.07) than [W(CO) 5 (a-NaiR-N)] (/ = 0.01–0.02). Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Transition metal–polypyridine complexes are attractive due to their crucial role in catalyses, photophysics and photochemistry, electrochemistry, supramolecular properties, medicinal activity etc. [1–3]. Recent years have witnessed a great deal of interest in the synthesis of new ligands with a diimine group (–N@C–C@N–) [4,5] (in the framework of pyridine derivatives) or the design of isolectronic groups to the diimine function, such as the azoimine function, (–N@N–C@N–) [6–29] (Scheme 1). The azoimine function is capable of stabilizing low valent metal oxidation states [22–25]. Zero valent Group 6 metal carbonyls are also stabilized by the azo- imine (–N@N–C@N–) function [30–40]. The N-heterocyclic azo molecules belong to the azoimine family and have been used to ex- plore the chemistry of transition [6–25] and non-transition metals [26–29]. We have designed 1-alkyl-2-(naphthylazo)imidazole (2)a potential member of the azoimine family [41–43]. The present work is concerned with W(0)–carbonyl complexes of 1-alkyl-2- (naphthyl-a-azo)imidazoles (a-NaiR). The reaction of a-NaiR, with W(CO) 6 under ambient conditions (298 K) in the presence of Me 3 NO has synthesized the penta-carbonyl complexes, [W(CO) 5 (a-NaiR-N)] (3) (imidazole-N coordinated a-NaiR is abbre- viated as a-NaiR-N). Penta-carbonyl Group 6 transition metal com- plexes are a source of Fischer carbene precursors in the studies of organic transformations [44–46], and are even used as diagnostic probes in the determination of nucleosides by measuring m CO of the coordinated CO [47]. [W(CO) 4 (a-NaiR-N,N 0 )] exhibit high intense fluorescence emis- sions while [W(CO) 5 (a-NaiR-N)] are weak emitters. The free ligand is non-fluorescent. The complexes were characterized by different spectroscopic studies (IR, UV–Vis, Mass, NMR, fluorescence), and the electrochemical properties have been studied with cyclic voltammetry. One of the penta-carbonyl complexes, [W(CO) 5 (a- NaiMe-N)], has been structurally confirmed by a single crystal X- ray diffraction study. Theoretical calculations (DFT and TD-DFT) using an optimized molecular system have been carried out to pre- dict the electronic structure, the spectral, redox and photophysical properties. 2. Experimental 2.1. Materials and measurements The 1-alkyl-2-(naphthyl-a-azo)imidazoles (a-NaiR, 2; R = CH 3 (a), CH 2 –CH 3 (b), CH 2 Ph (c)) were prepared by the reported proce- dure [42]. W(CO) 6 and Me 3 NO  2H 2 O were Sigma–Aldrich reagents. The reactions were carried out under an extremely dry oxygen free atmosphere under atmos bags (Sigma–Aldrich). 0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.11.041 * Corresponding author. Fax: +91 033 2413 7121. E-mail address: c_r_sinha@yahoo.com (C. Sinha). Polyhedron 28 (2009) 525–533 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly