Pentacarbonyl[2‐(2′‐pyridyl)quinoxaline‐κ N 4 ]tungsten: A Combined Study of Its Conformational and Electronic Structure Based on Experimental and DFT‐TDDFT Data

Photolysis of W(CO) 6 in the presence of 2‐(2′‐pyridyl)quinoxaline (pq) results in the formation of W(CO) 4 (pq) via an intermediate product W(CO) 5 (N 4 ‐pq) ( 1 ). In 1 the pq ligand that has three nitrogen atoms – two pyridine and one quinoxaline type – is coordinated in a monodentate fashion. This is the first example, where the pq ligand is coordinated in a monodentate fashion and especially from a quinoxaline nitrogen atom (N 4 ). Complex 1 has been fully characterized and its crystal structure has unambiguously demonstrated that pq is preferably coordinated through N 4 rather than the N 1 and N 1′ atoms, while its packing is governed by π–π interactions and hydrogen bonds. The almost perfect “face‐to‐face” alignment of the condensed quinoxaline rings is attributed to the existence of the electron‐withdrawing groups, namely W(CO) 5 and the pyridine ring. The aforementioned preference for N 4 atoms has been elucidated by thermodynamic means and explained in terms of the frontier orbitals. Time‐dependent density functional theory has been used for the investigation of the excited states of 1 . The calculated vertical excitation energies are consistent with the experimental data showing that the low‐lying solvatochromic band of 1 in the UV/Vis spectra is best described by the term MLCT/LLCT.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)