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Reprin

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Coordination Chemistry

An Iron Nitride Complex

High on nitride : Discrete iron nitridecomplexes stabilized by N-anchored tris-(carbene) ligands have been synthesized(see picture). These high-valent FeIV�Ncomplexes are stable at room tempera-ture, which allows their full spectroscopicand—for the first time—crystallographiccharacterization.

C. Vogel, F. W. Heinemann, J. Sutter,C. Anthon, K. Meyer* 2681 – 2684

Keywords: azides · carbene ligands · iron ·M/ssbauer spectroscopy · nitrides

2008 – 47/14

Coordination ChemistryDOI: 10.1002/anie.200800600

An Iron Nitride Complex**Carola Vogel, Frank W. Heinemann, J�rg Sutter, Christian Anthon, and Karsten Meyer*

Coordination compounds of iron in high oxidation states havebeen invoked as reactive intermediates in biocatalyses.Iron(IV) ferryl species are examples of such highly reactivespecies that have long been known to be at the catalyticcenters of oxygenases.[1] Supported by X-ray diffractionstudies on nitrogenase, the iron nitride moiety has recentlybeen suggested to be present at the site of biological nitrogenreduction.[2] As a result, well-characterized high-valent ironcomplexes have been sought as biomimetic models fortransformations mediated by iron-containing enzymes.

To gain understanding of iron nitride reactivity and thepossible role of such species in biocatalysis, insight into themolecular structure of complexes stabilizing the [FeN]synthon is highly desirable. Whereas significant progress hasbeen made in the synthesis and spectroscopic elucidation ofFe=NR and Fe�N species,[3–6] X-ray crystallographic charac-terization of a complex with a terminal Fe�N functionality hasnot been accomplished.[7,8]

The first mononuclear FeIV=O entity crystallographicallycharacterized was stabilized in an octahedral environmentprovided by a macrocyclic tetra-N-methylated cyclamligand.[9] Similar cyclam derivatives also allow the stabiliza-tion and detailed spectroscopic characterization of octahedralFeV and FeVI nitride complexes in unusually high oxidationstates.[3,4, 10]

Recently, Peters and Betley developed a stunninglyredox-rich iron system employing the tripodal tris(phosphi-no)borate ligand system (PhBRP3

�), which stabilizes tetrahe-dral L3Fe=Nx species in oxidation states ranging from + I to+ IV.[7] Remarkably, this ligand system enabled the firstroom-temperature spectroscopic characterization of a termi-nal FeIV nitride species. Concentration-dependent coupling tothe FeI-N2-FeI dinuclear product, however, prevents crystal-lization of this nitride species.

We herein present the synthesis, spectroscopy, and mostsignificantly, the X-ray diffraction analysis of a discrete iron

nitride complex stabilized by the sterically encumbering N-anchored tris(carbene) ligand, tris[2-(3-aryl-imidazol-2-ylide-ne)ethyl]amine (TIMENR, R = xylyl (xyl), mesityl (mes)).[11]

Structurally and electronically related to the tetrahedralphosphinoborate ligand system by Peters and Betley,[7] thistripodal N-heterocyclic carbene (NHC) system coordinates ahigh-spin FeII center in a trigonal-planar fashion, thus formingfour-coordinate complexes with the metal ion in trigonal-pyramidal environments.

Under inert atmosphere, treatment of TIMENR withone equivalent of anhydrous ferrous chloride in pyridine atroom temperature yields the four-coordinate FeII complexes[(TIMENR)Fe(Cl)]Cl (1mes, 1xyl) as analytically pure, whitepowders in 80% yield (Scheme 1).

Reduction of the FeII complexes 1mes and 1xyl over sodiumamalgam in a solution of NaBPh4 in THF leads to theformation of deep red solutions of the reduced compounds.After 2 h, these solutions were filtered, concentrated, andcooled to �35 8C to yield dark red crystals of FeI complexes[(TIMENR)Fe]BPh4 (2mes, 2xyl). Complexes 1R and 2R werecharacterized by X-ray crystallography, spectroscopy, andelemental analysis. Detailed spectroscopy and further reac-tivity of these complexes will be reported in a full paper.

Addition of excess trimethylsilylazide to FeI complexes 2R

in THF results in elimination of Me6Si2 and formation of lightyellow solutions, containing pale yellow precipitates ofdivalent [(TIMENR)Fe(N3)]BPh4 (3R, Scheme 2).

Both azide complexes are easily identified by the intenseand very characteristic nas(N3) IR vibrational band at2094 cm�1. The crystal structures of compounds 3R (3mes seeFigure 1) show the unusual, close to linear azide coordination(a(Fe-Na-Nb) = 174.5(2)8 in 3mes, 166.9(2)8 in 3xyl) at the FeII

tris(carbene) complex.[12] Such linear azide coordination to ametal center is rare but has been observed before in stericallyencumbering ligand environments, in which similar cylindri-cal cavities are generated around the metal center.[13] The Fe�Na azide distances in 3mes and 3xyl were determined to be1.947(2) and 1.955(2) A, respectively. In both complexes, theamine anchor is not bound (3mes : d(Fe�N1) = 3.244(2); 3xyl :

Scheme 1. Synthesis of [(TIMENR)Fe(Cl)]Cl complexes 1R.

[*] C. Vogel, Dr. F. W. Heinemann, Dr. J. Sutter, Dr. C. Anthon,Prof. Dr. K. MeyerDepartment of Chemistry and PharmacyFriedrich Alexander University Erlangen-N7rnbergEgerlandstrasse 1, 91058 Erlangen (Germany)Fax: (+49)9131-852-7367E-mail: kmeyer@chemie.uni-erlangen.de

[**] We thank the Friedrich-Alexander University Erlangen-N7rnberg(FAU) and the DFG for financial support, as well as M. R. Bautistaand Y. Xie (UCSD) for help with the synthesis of ligands and ironstarting materials. Dr. E. Bill (MPI for Bioinorganic Chemistry,M7lheim/Ruhr) is gratefully acknowledged for helpful discussionsand Dr. C. Hauser (FAU) for assistance with preparation of themanuscript.

Supporting information for this article is available on the WWWunder http://www.angewandte.org or from the author.

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2681Angew. Chem. Int. Ed. 2008, 47, 2681 –2684 � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

d(Fe�N1) = 3.197(2) A), leaving the FeII ion coordinated in atrigonal-pyramidal coordination environment, in which themetal center is located 0.554(2) A (3mes) and 0.536(2) A (3xyl)above the trigonal plane of the three NHC ligands.

Azide compounds 3R are light sensitive and readily releasedinitrogen to form deeply colored products 4R (Scheme 2).Accordingly, exposure of pale yellow solutions of 3R in THFto light of a Xenon arc lamp at room temperature results ingradual formation of a purple solution and N2 gas evolution.Whereas the electronic absorption spectra of colorless azidecomplexes 3R are transparent in the visible region, the spectraof the photolysis products 4R (Scheme 2) show an intenseabsorption band centered at lmax 520 nm (e = 1980m�1 cm�1,see the Supporting Information) giving rise to the purplecolor. Photolysis of iron azides[10] was continued until no gasevolution was observed and the azide vibrational bands couldno longer be observed by IR spectroscopy. Instead, in thespectra of both purple photolysis products, a new bandappears at 1008 cm�1, which is assigned to the iron nitride

band n(Fe�14N). Samples of the 15N-labeled isotopomer, 50%15N labeled at the terminal nitride, show an additional band at982 cm�1 (see the Supporting Information). This 26 cm�1 shiftof the n(Fe�15N) stretch to lower frequency is predicted fromthe reduced mass calculation for a harmonic oscillator (calcd:28 cm�1). In the 1H NMR spectra (see the SupportingInformation), the purple, diamagnetic reaction productsshow the expected resonances for the functionalized tris-(carbene) chelator coordinated to the iron centers of 4R.Additionally, the 15N NMR spectrum of 15N-labeled 4mes

exhibits a resonance signal at d = 1121 ppm (referenced toNH3, see the Supporting Information). Slow diffusion ofdiethyl ether into THF solutions of 4R yields purple crystalsthat are air stable at room temperature and suitable for an X-ray structure analysis. The molecular structures of [(TIME-NR)Fe�N]BPh4 (Figure 1, 4mes ; for 4xyl see the SupportingInformation) confirm the light-induced transformation of theaxial azide to a terminal nitride ligand with short Fe�N bondsof 1.526(2) (4mes) and 1.527(3) A (4xyl). Such short M�Ndistances are typically observed for transition-metal com-plexes with triply bonded nitride ligands (M = Cr, Mn).[14–19]

The Fe�N distances in 4R are also very similar to the XASspectroscopically determined distances of the FeIV complex[(PhBRP3)Fe(N)] (d(Fe�N) = 1.51–1.55(2) A, R = iPr, CH2-Cy)[8] by Peters and co-workers, and the octahedral FeVI

nitrido species [(Me3cy-ac)Fe(N)] (d(Fe�N) = 1.57 A)reported by Wieghardt and co-workers.[4]

Like the FeII ions in azide precursors 3R, the iron centers innitrido complexes 4R are coordinated in trigonal-pyramidalfashion but with the FeIV ion located at only 0.427(3) (4mes)and 0.384(2) A (4xyl) above the trigonal tris(carbene) plane.Evidently, this smaller displacement from the trigonal coor-dination plane in 3R is a consequence of increased interactionof the anchoring amine nitrogen (4mes : d(Fe�N1) = 3.120(2);4xyl : d(Fe�N1) = 2.984(2) A) and the carbene ligands with thehigh-valent FeIV ions. This stronger coordination of thecarbenes in 4mes and 4xyl is reflected in significantly shorteraverage Fe�C distances of 1.941(5) and 1.956(4) A comparedto the weaker binding in the FeII azide precursors of d(Fe�C)av = 2.108(2) in 3mes and 2.101(2) A in 3xyl.

The zero-field MDssbauer spectra of 3mes and 4mes areshown in Figure 2. The spectra of crystalline samples of 3mes at77 K display a quadrupole doublet with an isomer shift d =

0.69(1) mms�1, a quadrupole splitting parameter DEQ =

2.27(1) mms�1, and a line width GFWHM = 0.48(1) mms�1.These parameters are characteristic of four-coordinate d6

high-spin (S= 2) FeII complexes.[20] Considering the trigonalcoordination of the tris(carbene) chelator we suggest a ligandfield, in which the paired electron is in an e orbital (e3a1

1e2). Incontrast, spectra of 4mes show a sharp quadrupole doublet(GFWHM = 0.26(1) mms�1) with very unusual MDssbauerparameters of d =�0.27(1) mms�1 and DEQ = 6.04(1) mms�1.

The negative isomer shift and remarkably large quadru-pole splitting are consistent with values expected for a d4, S=

0 FeIV�N species. In fact, the observed DEQ = 6.04(1) mms�1

for 4mes is very similar to that determined for the[(PhBiPrP3)Fe(N)] system (DEQ = 6.01(1) mms�1).[5] In bothcases, the valence and covalency contributions from thedominating Fe�N moiety affect the unusually large quadru-

Scheme 2. Synthesis of FeIV nitride complex 4mes.

Figure 1. Molecular structures of complex cations of [(TIMENmes)-Fe(N3)]BPh4 (3

mes, left) and [(TIMENmes)Fe(N)]BPh4 (4mes, right) in

crystals of 3mes·2.5THF and 4mes·Et2O. Hydrogen atoms and co-crystal-lized solvent molecules are omitted for clarity.

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2682 www.angewandte.org � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 2681 –2684

pole splitting parameter DEQ of the complexes in accordancewith the observed and calculated diamagnetic {(dxy)

2(dx2�y2)2}{(dz2)0(dxz)

0(dyz)0} electron ground state configuration

(Figure 3).

In contrast to this striking similarity in DEQ values, theisomer shift d =�0.27(1) mms�1 of 4mes differs significantlyfrom that reported for [(PhBiPrP3)Fe(N)] (d =�0.34 mms�1).The isomer shift determined by 57Fe MDssbauer spectroscopy

is directly related to the s electron density at the nucleus,which is shielded by electron density in the shells withnonzero angular momenta (p and d orbitals). Accordingly, theless negative d indicates that the iron ion in 4mes is lessoxidized than the tetrahedral iron center in[(PhBiPrP3)Fe(N)]. This is likely a consequence of a certaindegree of ligand-to-metal back bonding from the NHC ligandp system to empty iron d orbitals in trigonal-pyramidal 4mes.While it is intuitive that the electronic structure of four-coordinate iron nitride complexes is dominated by the strongFe�N interaction, the different isomer shifts clearly demon-strate that other factors, such as ligand-donor set andcoordination geometry, ultimately determine the detailedelectronic structure and complex stability of high-valent ironnitride species. Obviously, the neutral, potentially tetraden-tate tris(carbene) ligand and the anionic tridentate tris(phos-phino)borate, exhibit not only steric but also significantelectronic differences. This could also explain the differencesin color and stability of the deep purple aminocarbene FeIV

nitride system [(TIMENR)Fe�N]+ and the tan-colored, moretetrahedral, [(PhBiPrP3)Fe�N] complex prone to dimerization.To gain a better understanding of the intricate electronicstructure of 4R, in-depth computational and spectroscopicanalysis will be carried out.

In conclusion, photolysis of FeII azide complexes ofsterically encumbering nitrogen-anchored N-heterocyclic tris-(carbene) ligands generates discrete, air-stable FeIV nitridecomplexes. The X-ray crystallographically determined Fe�Nbonds are short at 1.53 A, and the high-valent FeIV centers inthese molecules possess trigonal-pyramidal coordinationenvironments. The molecular structures of iron nitride com-plexes 4R here reported are the first structural verification ofthe [FeN] synthon that has been recognized as the reactiveintermediate in the industrial Haber–Bosch process.

While iron nitride complexes 4R are remarkably air andmoisture stable at room temperature, they demonstratepromising reactivity with proton sources in preliminaryexperiments. Detailed reactivity investigations of the ironnitride complexes 4R are currently underway.

Experimental SectionDetailed descriptions of the syntheses as well as spectroscopic andcrystallographic details of all compounds are given in the SupportingInformation.

Received: February 5, 2008Published online: February 27, 2008

.Keywords: azides · carbene ligands · iron ·MGssbauer spectroscopy · nitrides

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Figure 2. MGssbauer spectra of complexes 3mes (top) and 4mes

(bottom). Spectra were recorded on solid samples at 77 K without anapplied magnetic field; solid lines are fits resulting in the parametersgiven in the text.

Figure 3. Electronic structure of 4R, S =0 (DFT, ORCA, B3LYP). Com-puted structural parameters (BP86): Fe�N1 3.278, Fe�N8 1.517, Fe�Cav 1.949 J; a(N8-Fe-C) 103.18.

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[11] X. L. Hu, K. Meyer, J. Organomet. Chem. 2005, 690, 5474.[12] Data were collected at 100 K on a Bruker Nonius KappaCCD

diffractometer using MoKa radiation (l = 0.71073 A, graphitemonochromator). Structures were solved by direct methods andrefined on F2 using full-matrix least-squares techniques. For3mes·2.5THF: triclinic, P1̄, a= 11.3035(5), b= 17.725(2), c=17.831(2) A, a = 88.056(10), b = 76.821(6), g = 78.047(7)8, V=

3402.7(6) A3, Z= 2, R1 = 0.0516 [I> 2s(I)], wR2 = 0.1292 (alldata). For 4mes·Et2O: triclinic, P1̄, a= 12.432(2), b= 12.632(2),

c= 22.614(3) A, a = 80.104(13), b = 84.734(9), g = 61.303(7)8,V= 3068.6(8) A3, Z= 2, R1 = 0.0450 [I> 2s(I)], wR2 = 0.0938(all data). CCDC-676549 (for 3mes·2.5THF), CCDC-676550 (for4mes·Et2O), CCDC-676551 (for 3xyl·MeCN), and CCDC-676552(for 4xyl·THF) contain the supplementary crystallographic datafor this paper. These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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Inorg. Chem. 1998, 37, 5180.[19] J. Bendix, K. Meyer, T. WeyhermLller, E. Bill, N. Metzler-Nolte,

K. Wieghardt, Inorg. Chem. 1998, 37, 1767.[20] N. N. Greenwood, T. C. Gibbs, M�ssbauer Spectroscopy, Chap-

man and Hall Ltd., London, 1971.

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2684 www.angewandte.org � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 2681 –2684