The Geometric and Electronic Structure of [(cyclam‐acetato)Fe(N)]⁺: A Genuine Iron(V) Species with a Ground‐State Spin S=1/2

Abstract

Low‐spin, high‐valent iron centers: A high‐valent iron–nitrido species containing a FeV center is accessible by the photolysis of [(cyclam‐ac)FeIIIN3]+. The identity of the resulting [(cyclam‐ac)FeV(N)]+ species was verified by spectroscopic methods, magnetic susceptibility data, and DFT calculations (see picture). Unexpectedly, the analysis provides strong evidence for a low‐spin d3 (S=1/2) ground‐state electron configuration at the FeV center.

Fragmentation of the (Cyclam‐acetato)iron Azide Cation in the Gas Phase

“…However, it needs to be tested in every instance that the particular structures are also maintained on a unimolecular level and are not altered by subsequent rearrangements. In the present case, for example, we do not have any reasonable doubt that the nitrido species 2 + is formed as an intermediate Ϫ in fact it has been observed in frozen matrices [5] Ϫ but the experimental evidence obtained for the gaseous ion at least indicates that the long-lived ions bears a different connectivity.…”

“…By the same line of reasoning, the ∆m = -28 fragmentation is attributed to the expulsion of a nitrogen molecule from the azido group, see reaction (4), which is in perfect analogy to the occurrence of reaction (1) in the condensed phase. [5] If this mass loss were instead due to a fragmentation of the cyclic ligand, it would also be expected to occur for the methoxido complex, for which it is not observed experimentally.…”

“…[1] The field has gained significant momentum since the discovery of the first proven Fe IV intermediate in mononuclear nonheme iron enzymes [2] and the first crystal structure of an elusive Fe IV -oxo species by Rohde et al [3] More recently, it has proven possible to proceed beyond the oxidation state Fe IV and to obtain definitive spectroscopic evidence for genuine Fe V [4,5] and even Fe VI species. [6,7] The high-valent iron sites in these compounds are stabilized in a distorted octahedral environment by an innocent framework provided by ligands designed on the basis of the cyclam motif together with an extremely basic axial nitrido ligand, which is generated in situ by photolysis of the corresponding iron azide precursors.…”

Iron–dinitrogen coordination chemistry: Dinitrogen activation and reactivity

“…Several bridging and terminal nitride complexes of iron have been synthesized in a variety of oxidation states [53,77,[123][124][125][126][127][128]. Even though iron forms nitride complexes, one problem limiting the validity of the asymmetric reaction pathway is that no iron nitride complex has yet been generated directly from N 2 .…”

The Photochemical Route to Octahedral Iron(V). Primary Processes and Quantum Yields from Ultrafast Mid-Infrared Spectroscopy

“…29 It was exactly this band that was originally exploited to convert the complex in a frozen matrix into an iron(V) nitride. 23,29 Additional, yet to be assigned features appear in the UV spectrum as shoulders at 305, 280, and 245 nm and as a peak at 214 nm.…”