9222
J. Am. Chem. Soc. 2001, 123, 9222-9223
Chart 1
Stepwise Reduction of Dinitrogen Bond Order by a
Low-Coordinate Iron Complex
Jeremy M. Smith,† Rene J. Lachicotte,† Karl A. Pittard,‡
Thomas R. Cundari,‡ Gudrun Lukat-Rodgers,§
Kenton R. Rodgers,§ and Patrick L. Holland*,†
Department of Chemistry, UniVersity of Rochester
Rochester, New York 14627
Department of Chemistry and the Computational
Research on Materials Institute, UniVersity of Memphis
Memphis, Tennessee 38152
in a stepwise fashion. A combination of synthetic, structural,
spectroscopic, and theoretical studies shows that a low coordina-
tion number at iron correlates with the ability to weaken N2.
In the following discussion, L represents the anion shown at
the right of Chart 1.9 Reduction of the three-coordinate iron(II)
complex LFeCl10 with naphthalenide under a purified N2 atmo-
sphere gives a highly air-sensitive, dark red, paramagnetic
compound, for which the structure LFeNNFeL was revealed by
X-ray crystallography (Figure 1a).11 This is a rare example of a
three-coordinate transition-metal dinitrogen complex.7,12 Consis-
tent with the low coordination number at iron and/or multiple
bonding (see below) between iron and N2, the Fe-N2 distances
are extremely short (1.77-1.78 Å). The most interesting feature
of this structure is that the bridging N2 ligand is stretched
substantially (N-N ) 1.182(5) Å; N-N in free N2 ) 1.098 Å).
The N-N elongation by almost 0.1 Å distinguishes this compound
from other crystallographically characterized iron-N2 complexes,
which have N-N distances within about 0.03 Å of that in free
N2.13,14 An intense band at 1778 cm-1 was observed in the
resonance Raman spectrum of LFeNNFeL with 514.5 nm
excitation. This band shifted to 1718 cm-1 in the spectrum of a
sample prepared from 15N2, consistent with a diatomic N-N
oscillator whose force constant is substantially smaller than that
of free N2 (2331 cm-1).4,15 Thus, structural and spectroscopic
evidence shows that iron binding has weakened the N-N bond
in LFeNNFeL relative to N2.
Department of Chemistry, North Dakota State UniVersity
Fargo, North Dakota 58105
ReceiVed April 26, 2001
ReVised Manuscript ReceiVed August 2, 2001
Conversion of atmospheric N2 into NH3 is one of the most
important chemical processes, because ammonia is the industrial
and biological precursor to many nitrogen-containing compounds.
Large-scale transformation of N2 and H2 into ammonia is
performed in industry by the Haber-Bosch process, using
“potassium-promoted” porous iron.1 A view of the N2-reducing
active site of iron-molybdenum nitrogenase, which contains
unusual iron atoms with only three sulfur donors, is shown in
Chart 1.2 The presence of iron in the active sites of this and other
nitrogenases3 suggests that iron is again important for activating
dinitrogen. Thus iron plays a major role in both natural and
industrial N2 reduction catalysis.
Paradoxically, synthetic iron/N2 complexes are viewed as
“unactivated” despite the importance of iron in the catalytic
processes described above.4 Examples of stepwise metal-promoted
N2 cleavage reactions use metals in groups 5 and 6 of the periodic
table.5 The driving force for these N-N cleavage reactions is
the formation of extremely strong metal-nitride bonds. The only
synthetic Fe/N2 complex in which the N-N bond is stretched is
the unusual complex Fe[NNMo(N3N)]3.6,7 Some iron/N2 com-
plexes produce ammonia on decomposition, but the intermediates
in this process are not known.6,8 In this report we describe three-
coordinate iron complexes that bind N2 and weaken its N-N bond
To evaluate the effects of coordination number on the geometry
of bound N2, we performed DFT calculations on five- and three-
coordinate iron complexes. Geometry optimization of {Fe(CO)2-
(PH3)2}2(µ-N2) gave bond lengths of Fe-N ) 1.893 Å and N-N
) 1.122 Å, in excellent agreement with the experimental
structures of {Fe(CO)2(PR3)2}2(µ-N2) (R ) C2H5, Fe-N )
1.87(1), 1.89(2) Å, N-N ) 1.13(2) Å; R ) OCH3, Fe-N )
1.876(9) Å, N-N ) 1.13(1) Å).14 Removal of all PH3 ligands to
give three-coordinate iron (Scheme 1), followed by geometry
† University of Rochester.
‡ University of Memphis.
§ North Dakota State University.
(1) Jennings, J. R., Ed. Catalytic Ammonia Synthesis; Plenum: New York,
1991. Ertl, G.; Kno¨zinger, H.; Weitkamp, J., Eds. Handbook of Heterogeneous
Catalysis; VCH: New York, 1997; Vol. 4, pp 1697-1748.
(2) Kim, J.; Rees, D. C. Nature 1992, 360, 553. Howard, J. B.; Rees, D.
C. Chem. ReV. 1996, 96, 2965. Burgess, B. K.; Lowe, D. J. Chem. ReV. 1996,
96, 2983.
Scheme 1. Five-Coordinate and Three-Coordinate Models
Evaluated by Density-Functional Theory
(3) Eady, R. R. Chem. ReV. 1996, 96, 3013. Krahn, E.; Weiss, B. J. R.;
Kro¨ckel, M.; Groppe, J.; Henkel, G.; Cramer, S. P.; Trautwein, A. X.;
Schneider, K.; Mu¨ller, A. J. Biol. Inorg. Chem. 2001, 6, in press.
(4) Leigh, G. J. Science 1995, 268, 827. Tuczek, F.; Lehnert, N. Angew.
Chem., Int. Ed. Engl. 1998, 37, 2636. Fryzuk, M. D.; Johnson, S. A. Coord.
Chem. ReV. 2000, 200-202, 379. Note that “activation” as measured by
geometry and by reactivity do not necessarily correlate. See: Leigh, G. J.
Acc. Chem. Res. 1992, 25, 177.
(9) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg. Chem.
1996, 3539.
(10) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2001,
in press.
(5) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.;
Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996,
118, 8623. Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.; Chiesi-
Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1998, 120, 437. Clentsmith, G. K.
B.; Bates, V. M. E.; Hitchcock, P. B.; Cloke, F. G. N. J. Am. Chem. Soc.
1999, 121, 10444. Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C.; Re, N.;
Rizzoli, C.; Chiesi-Villa, A. J. Am. Chem. Soc. 2000, 122, 3652.
(6) Shilov, A. E. New J. Chem. 1992, 16, 213. Bazhenova, T. A.; Shilov,
A. E. Coord. Chem. ReV. 1995, 144, 69.
(11) Selected data: LFeNNFeL, 69% yield, µeff(C6D6) ) 4.2 µB/Fe, UV-
vis (pentane): 519 (ꢀ ) 11.3 mM-1 cm-1), 940 (ꢀ ) 4.4 mM-1 cm-1) nm;
Na2[LFeNNFeL], 96% yield, µeff(C6D6) ) 1.7 µB/Fe, UV-vis (pentane): 699
(ꢀ ) 9.2 mM-1 cm-1) nm; K2[LFeNNFeL], 65% yield, µeff(C6D6) ) 2.3 µB/
Fe, UV-vis (pentane): 732 (ꢀ ) 12.0 mM-1 cm-1) nm.
(12) Jolly, P. W.; Jonas, K.; Kruger, C.; Tsay, Y.-H. J. Organomet. Chem.
1971, 33, 109.
(13) See Table S-3 for a list of literature Fe/N2 complexes.
(14) Berke, H.; Bankhardt, W.; Huttner, G.; von Seyerl, J.; Zsolnai, L.
Chem. Ber. 1981, 114, 2754. Kandler, H.; Gauss, C.; Bidell, W.; Rosenberger,
S.; Bu¨rgi, T.; Eremenko, I. L.; Veghini, D.; Orama, O.; Burger, P.; Berke, H.
Chem. Eur. J. 1995, 1, 541.
(15) Cohen, J. D.; Mylvaganam, M.; Fryzuk, M. D.; Loehr, T. M. J. Am.
Chem. Soc. 1994, 116, 9529.
(7) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R.; Reiff, W. M. Inorg.
Chem. 1999, 38, 243. A crystal structure shows that the N2 bonds are stretched
(N-N ) 1.20-1.27 Å), although it is not clear whether iron or molybdenum
is the main cause of the stretching.
(8) Hall, D. A.; Leigh, G. J. J. Chem. Soc., Dalton Trans. 1996, 3539.
10.1021/ja016094+ CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/24/2001