Interannular conjugation in new iron(II) 5-aryl tetrazolate complexes

The synthesis of new Fe(II) 5-aryl tetrazolate complexes [CpFe(CO)(L)(N4C-C6H4-CN)] (4) [L = CO (4a); PPh3 (4b); P(OCH3)3 (4c); CN-2,6-Me2C6H3 (4d)] is described. The target compounds were obtained by addition of sodium azide to the parent 1,4-dicyanobenzene complexes [CpFe(CO)(L)(NC-C6H4-CN)][O3SCF3] (2a-d). X-ray molecular structure of 4c confirms the predictions based on NMR (1H, 13C) studies concerning the coplanarity between the tetrazole and phenyl rings, with consequent interannular conjugation effect. The multidentate nature of the tetrazole ring involves the presence of different sites that can undergo electrophilic attack; reaction of complexes 4a-c with methyl triflate afforded the methylated cationic complexes [CpFe(CO)(L)(CH3-N4C-C6H4-CN)][O 3SCF3], 5a-c, whose 1H and 13C NMR spectroscopy data suggested out-of-plane rotation of the phenyl ring and subsequent large reduction of interannular conjugation. Noteworthy, the same effect was obtained in a reversible way by addition of triflic acid to 4a, affording the protonated cationic complex [CpFe(CO)2(H-N4C-C6H4-CN)][O3 SCF3], 6a, which was easily converted into its neutral precursor by treatment with a base.