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)][O3SCF3], 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)][O3SCF3], 6a, which was easily converted into its
neutral precursor by treatment with a base.