Structural characterization of [Fe(NO)(mnt)2]1−/2− salts

The iron dithiolene compounds [Fe2(mnt)4]2− [1]2− and [Fe(NO)(mnt)2]n (n = 1−, [2]1−; n = 2−, [2]2−) ([mnt]2− = maleonitriledithiolate = [(NC)2C2S2]2−) have been characterized structurally by X-ray diffraction as their [Et4N]+ salts at 100 K. Dianion [2]2− is prepared from [2]1− by reduction with Na[Et3BH] and is observed to have a bent Fe–NO angle at 149.9(5)° in contrast to the linear configuration of Fe–NO in [2]1− (180.0°). The change from linear to bent binding mode for NO, an increase of more than 0.1 Å in the Fe–N bond length, and the relative invariance of the Fe–S distances for [2]2− versus [2]1− indicate that the NO ligand is the site of reduction. The [Et3NH]+ complex of [2]1− was also identified by crystallography and found to have hydrogen bonding contacts between [Et3NH]+ and the cyano nitrogen atom of an [mnt]2− ligand. Furthermore, relatively close S⋯S contacts (3.602–3.615 Å) occur between [2]1− anions, which pack together in an offset, head-to-head fashion. These S⋯S contacts are absent in the structure of [Et4N][2]. Infrared spectra show an energy decrease for, and a significant broadening of, the NO bond stretching absorption peak in [2]2−, which is consistent with a bent NO ligand sampling a range of conformations both by facile pivoting about the Fe–N axis and by a breathing of the Fe–NO angle.

Reduction of [Fe(NO)(mnt)2]1− by [Et3BH]1− affords [Fe(NO)(mnt)2]2−. Crystallographic characterization shows Fe–NO angles of 180.0° and 149.9(5)° for mono- and dianion, respectively, consistent with NO ligand being the site of reduction. Infrared spectroscopy shows both an energy lowering (1836 → 1648 cm−1) and broadening of νNO, which accords with its change in binding mode.Figure optionsDownload as PowerPoint slideHighlights
► An improved preparation of [Fe(NO)(S2C2(CN)2)2]2− is reported.
► Crystal structures of [Fe(NO)(S2C2(CN)2)2]n (n = 1−, 2−) are described.
► Reduction of monoanion to dianion is NO ligand-based.
► A linear-to-bent NO binding mode change occurs upon reduction to dianion.
► Bent NO binding in [Fe(NO)(S2C2(CN)2)2]2− is shown by a broad NO stretching mode.