Isolation, X-ray structure and properties of an unusual pentacarbonyl(2,2 0 -pyridyl-quinoxaline) tungsten complex Irene Veroni a , Christiana A. Mitsopoulou a, * , Fernando J. Lahoz b a Inorganic Chemistry Laboratory, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografou 15771, Greece b Departamento de Quimica Inorganicas ICMA, Facultad de Ciencias, Universidad de Zaragozas CSIC, Zaragoza 50009, Spain Received 9 August 2006; received in revised form 28 September 2006; accepted 28 September 2006 Available online 7 October 2006 Abstract The first example of a monodentate complexation of 2-(2 0 -pyridyl)quinoxaline (pq) to a metal centre through N 4 is reported. Photo- chemical exchange of the THF ligand in W(CO) 5 THF by pq yields W(CO) 5 (N 4 -pq) (1), where the potentially bidentate pq ligand coor- dinates in an unusual monodentate fashion. Complex 1 is isolated as orange crystals and fully characterized on the basis of NMR, IR, UV–Vis and emission spectroscopy. The structure of 1 was determined by X-ray analysis. W(CO) 5 (N 4 -pq) (1) crystallizes in space group P2 1/n , monoclinic crystal system with a = 7.0237(5) A ˚ , b = 10.4618(8) A ˚ , c = 23.7768(18) A ˚ , Z = 4 and V = 1731.9(2) A ˚ 3 . Complex 1 exhibits intramolecular CHN and intermolecular CHO hydrogen bonds between the CH groups and nitrogen atoms of quinoxaline and CH groups and oxygen atoms of carbonyls, respectively, resulting in a supramolecular architecture in solid state. The preference to N 4 as coordination site is discussed in terms of electronic interactions. Solutions of 1 emits dually at 77 K while they are moderately instable at room temperature, as 1 undergoes chelation via a first-order kinetic process to form W(CO) 4 pq (2). The determined reaction rate of 1 in toluene is 2.3 · 10 5 s 1 (at 298 K) and is compared with literature values for other W(CO) 5 L (L:diimine) complexes. Ó 2006 Elsevier B.V. All rights reserved. Keywords: 2-(2 0 -Pyridyl)quinoxaline; Pentacarbonyl; Tungsten; Crystal structure; Hydrogen bonds; Emission 1. Introduction The quinoxaline class of ligands can meet modern expec- tations of metallo-organic chemistry, as they own low-lying LUMOs, contain two nitrogen atoms in the suitable for bridges para-position and can be easily functionalized, allowing us to tailor their properties [1]. Besides, quinoxa- line structure is recognized in a great number of naturally occurring compounds such as riboflavin (vitamin B 2 ), fla- voenzymes, molybdopterines and antibiotics of streptomy- ces type that are implicated in considerable intra- and inter- electron transfer biochemical processes. Quinoxaline deriv- atives can be used, from the one hand, as antibacterial, antiviral, anticancer, antifungal, antihelmintic and insecti- cidalagents [2] and from the other, as electron-withdrawing groups in p-conjugated polymer chemistry [3], as fluores- cent dyes [4] or as optical-based sensors for anions in organic media [5]. 2-(2 0 -Pyridyl)quinoxaline (pq)(Scheme 1) is a synthetic product – produced by an unexpected condensation reac- tion [6] – that is a variation on the commonly used bridging ligand 2,3-bis (2-pyridyl)quinoxaline (dpq). Pq combines the bridging properties of quinoxalines along with the che- lating efficiency of 2-2-bpy. Up until now, a plethora of metal complexes have been reported, where pq is bi-coordi- nated through N 1 and N 0 1 atoms [7], but there has never been any undoubted proof for coordination of the metal at N 4 . In spite of that, semi-empirical calculations have shown that the HOMO of pq is mostly a ‘lone’ electron pair on N 1 and N 4 [7n], thus these two nitrogen atoms should share almost the same propensity for coordination. The thermodynamic stability that offers the chelation of pq after its coordination at N 1 should be the major reason for the above experimental observation. 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2006.09.060 * Corresponding author. Tel.: +30 210 7274 452; fax: +30 210 8322828. E-mail address: cmitsop@chem.uoa.gr (C.A. Mitsopoulou). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 691 (2006) 5955–5963