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 mu6 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,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)3N (X = Cl-, Br-) in acetonitrile. A 2:1 mol ratio affords [Fe4N2Cl10]4- (1), isolated as the Et4N+ salt (72%). This cluster has idealized C2h symmetry with a planar antiferromagnetically coupled [Fe(III)4(mu3-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 D2h symmetry and is built of 15 edge- and vertex-shared rhomboids involving two mu3-N and six mu4-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 Mössbauer 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.
Bennett
Miriam V
MV
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
Stoian
Sebastian
S
Bominaar
Emile L
EL
Münck
Eckard
E
Holm
Richard H
RH
D001965:Bromides / Q000737:chemistry*; D002712:Chlorides / Q000737:chemistry*; D018360:Crystallography, X-Ray; D007501:Iron / Q000737:chemistry*; D015394:Molecular Structure; D008982:Molybdenum / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry*; D009591:Nitrogenase / Q000737:chemistry; D009942:Organometallic Compounds / Q000138:chemical synthesis*; D015204:Spectroscopy, Mossbauer
D001965:Bromides; D002712:Chlorides; D009942:Organometallic Compounds; D008982:Molybdenum; D007501:Iron; D009591:Nitrogenase; D009584:Nitrogen