Initial members of the family of molecular mid-valent high-nuclearity iron nitrides:: [Fe4N2X10]4- and [Fe10N8X12]5- (X = Cl-, Br-)

Current theoretical and experimental evidence points toward X = N as the identity of the interstitial atom in the [MoFe7S9X] core of the iron-molybdenum cofactor cluster of nitrogenase. This atom functions with mu(6) bridging multiplicity to six iron atoms and, if it is nitrogen as nitride, raises a question as to the existence of a family of molecular iron nitrides of higher nuclearity than known dinuclear Fe-III,Fe-IV species with linear [Fe-N-Fe](5+),(4+) bridges. This matter has been initially examined by variation of reactant stoichiometry in the self-assembly systems [FeX4](1-)/(Me3Sn)(3)N (X = Cl-, Br-) in acetonitrile. A 2:1 mol ratio affords [Fe4N2Cl10](4-) (1), isolated as the Et(4)N(+)salt (72%). This cluster has idealized C-2h symmetry with a planar antiferromagnetically coupled [Fe-4(III)(mu(3)-N)(2)](6+) core containing an Fe2N2 rhombus to which are attached two FeCl3 units. DFT calculations have been performed to determine the dominant magnetic exchange pathway. An 11:8 mol ratio leads to [Fe10N8Cl12](5-) (3) as the Et4N+ salt (37%). The cluster possesses idealized D-2h symmetry and is built of 15 edge- and vertex-shared rhomboids involving two,mu(3)-N and six mu(4)-N bridging atoms, and incorporates two of the core units of 1. Four FeN2Cl2 and four FeN3Cl sites are tetrahedral and two FeN5 sites are trigonal pyramidal. The cluster is mixed-valence (9Fe(III) + Fe-IV); a discrete Fe-IV site was not detected by crystallography or Mossbauer spectroscopy. The corresponding clusters [Fe4N2Br10](4-) and [Fe10N8Br12](5-) are isostructural with 1 and 3, respectively. Future research is directed toward defining the scope of the family of molecular iron nitrides.