Dinitrogen chemistry from trigonally coordinated iron and cobalt platforms

Article abstract of DOI:10.1021/ja036687f

This report establishes that trigonally coordinated "[PhBP^iPr₃]M" platforms (M = Fe, Co) will support both π-acidic (N₂) and π-basic (NR) ligands at a fourth binding site. The N₂ complexes of iron that are descr

Full text of DOI:10.1021/ja036687f

Published on Web 08/15/2003
Dinitrogen Chemistry from Trigonally Coordinated Iron and Cobalt Platforms
Theodore A. Betley and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received June 14, 2003; E-mail: jpeters@caltech.edu
Scheme 1
The Chatt-type nitrogen reduction scheme is well-established for
certain molecular systems that feature molybdenum.1 For example,
Schrock has recently demonstrated that a single, amido-supported
molybdenum center can support various N2-derived intermediates
(e.g., M(N2H), M(N2H2), M(N), M(NH)), oxidation states, and redox
couples to favorably mediate the complete reduction of nitrogen
to ammonia in a catalytic fashion.1c,d One key feature of this and
other competent N2-reducing molybdenum systems is their ability
to accommodate a π-acidic N2 ligand, as well as π-basic function-
alities derived from N2 (e.g., N3-, NH2-), at a single site.1,2 To
date, very few first-row ion platforms allow N2 to be taken up and
derivatized in a similar fashion (e.g., Fe(N2) f Fe(N2H)).3,4 Iron
is particularly noteworthy in this regard, given its possible if not
likely role in biological (and industrial) nitrogen fixation.5,6
Recent work by our group has shown that trigonally coordinated
iron and cobalt subunits of the form “[PhBP3]M” ([PhBP3] )
[PhB(CH2PPh2)3]-) will support a strongly π-donating ligand at a
fourth site along their pseudo-three-fold axis (e.g., [PhBP3]Fet
NR).7 Simple electronic structure considerations suggest that
these same [PhBP3]M subunits should also accommodate
strongly π-acidic ligands at the fourth site,7-9 a feature that might
complement nitrogen reduction schemes at iron. Herein we
demonstrate that second-generation “[PhBPiPr3]Fe” ([PhBPiPr3] )
Scheme 2
[PhB(CH2PiPr2)3]-) and related cobalt systems coordinate, activate,
and promote the methylation of nitrogen at the fourth binding site.
Moreover, the N2 ligand can be replaced by nitrene (NR) through
an oxidative group-transfer reaction.7,8a The iron-N2 species
discussed are the first thoroughly characterized, 4-coordinate
complexes of their type.4
Entry to this reaction manifold begins with the recently reported
precursors [PhBPiPr3]MX (M ) Fe, X ) Cl (1); M ) Co, X ) I
(2)).9 A THF solution of yellow 1 or green 2, stirred under a blanket
of nitrogen in the presence of Mg0, produced the anionic,
paramagnetic dinitrogen complexes {[PhBPiPr3]Fe(N2)}{MgCl-
(THF)2} (3a, 84%) and {[PhBPiPr3]Co(N2)}2{Mg(THF)4} (4a, 78%)
as red-brown and red crystals, respectively (Scheme 1). An XRD
of 7 is shown in Figure 1. The structure of 8 is isomorphous and
isostructural.11 A crystallographically imposed inversion center
reflects one-half of each dinuclear anion into the other, suggesting
that these species are likely delocalized at low temperature. A
modest elongation of the N2 ligand is observed in each case (N-
N′ 1.171(4) for 7, and 1.147(4) Å for 8), and the local geometry of
each metal center is perhaps best described as trigonal monopyra-
midal, with N2 lying in an equatorial position.
study of single crystals of 4a suffered from modest disorder but
2+
confirmed a Mg(THF)4 dication sandwiched by two anionic
Co(N2)- units, in accord with related structure types.10,11 The IR
spectra of 3a and 4a show intense νNN bands that shift to higher
energy upon addition of 18-crown-6 (18-C-6) to encapsulate the
Mg2+ ion (νNN(
in cm-1 for 3a, 1830(1769); 3b, 1884(1822);
15
NN)
4a, 1863(1802); 4b, 1896(1842)). Gentle oxidation by ferrocenium
(Cp2Fe+) in THF produced the neutral, dinuclear N2-bridged
products {[PhBPiPr3]Fe}2(µ-N2) (5, 88%) and {[PhBPiPr3]Co}2(µ-
N2) (6, 92%) (Scheme 2). Both 5 and 6 were also obtained directly
from halides 1 and 2 when Na/Hg amalgam was used as the
reductant in place of Mg0. Furthermore, extended exposure of 5
and 6 to sodium amalgam produced the mixed-valence (M0/MI)
complexes [([PhBPiPr3]Fe)2(µ-N2)][Na(THF)6] (7, 93%) and
[([PhBPiPr3]Co)2(µ-N2)][Na(THF)6] (8, 89%). XRD data were
obtained on single crystals of 7 and 8, and the molecular structure
Complexes
5
and
6
provided clean access to the
“[PhBiPrP3]MI” fragment, as exemplified by their ability to undergo
rapid oxidation upon addition of either tolyl or adamantyl azide to
afford trivalent imide products. For example, burgundy [PhBPiPR3]-
FetNAd (9, 69%) and red [PhBPiPr3]CotN-p-tolyl (10, 93%) were
isolated and thoroughly characterized (Scheme 2), and the solid-
state structure of each was determined.11 The crystal structure of
complex 9 (Figure 2) features a very short Fe-N bond (1.638(2)
Å) and an almost linear Fe-N-C bond vector (176°), consistent
with the triple-bond formulation we have proposed previously.7
9
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10.1021/ja036687f CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
[PhBiPrP3]CosNdNMe (12, 68%), which exhibited a very similar
νNN( NN) stretch 1599(1542) cm-1 and a solution moment indicative
15
of low-spin cobalt(II) (1.92 µB). Complex 4a also reacted cleanly
with trimethylsilyl chloride to generate [PhBPiPr3]CosNdNSiMe3
(13, 79%; νNN( NN) ) 1654(1596) cm-1; 2.08 µB). The latter product
15
could be obtained in good yield by a more direct procedure that
involved stirring a solution of chloride 2 in the presence of Me3-
SiCl and Na/Hg amalgam under nitrogen.
These data allow us to summarize several salient features of the
chemistry described herein: The [PhBPiPr3] ligand has been used
to isolate a single iron center in a pseudo-tetrahedral environment
in which a single binding site is compatible with coordination of
N2, diazenido (N2Me), and imide (NR). Moreover, N2 uptake/
coordination by 4-coordinate iron has been established for the
formal oxidation states M(0), M(+0.5), and M(+1). Methylation
of Fe0(N2)- to produce FeII(N2Me) constitutes a two-electron redox
process at iron. The additional observation of a separate two-electron
redox process, that of oxidative nitrene transfer (FeLI f FeIIIt
NR), establishes that the “[PhBPiPr3]Fe” platform will support four
formal oxidation states (Fe0, FeI, FeII, FeIII), in addition to two
discrete two-electron redox couples (Fe0/II and FeI/III). An analogous
set of observations has been made with the related cobalt system.
These features collectively motivate further development of the
present first-row systems into possible models for functional
nitrogen fixation.
Figure 1. Displacement ellipsoid representation of {([PhBPiPr3]Fe)2(µ-N2)}-
{Na(THF)6} (7). Hydrogen atoms have been removed for clarity. Selected
bond distances (Å) and angles (deg), for 7: Fe-N 1.813(2), N-N′ 1.171-
(4), Fe-P1 2.292(1), Fe-P2 2.278(1), Fe-P3 2.290(1); N′-N-Fe 178.0-
(3), P1-Fe-P2 96.93(4), P1-Fe-P3 97.96(4), P2-Fe-P3 97.22(4),
N-Fe-P1 118.88(9), N-Fe-P2 109.66(9), N-Fe-P3 129.83(9).
Acknowledgment. This work was supported by a DOE PE-
CASE award. T.A.B. is grateful for a Department of Defense
graduate research fellowship.
Supporting Information Available: Complete experimental pro-
cedures and characterization data, and crystallographic data (PDF, CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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4-coordinate Fe(N2) species, [EtFe(N2)(PPh3)2]Mg(THF)4], but no struc-
tural data for this latter species. See: Yamamoto, A.; Miura, Y.; Ito, T.;
Chen, H., Iri, K.; Ozawa, F. Organometallics 1983, 2, 1429.
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(7) (a) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002,
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(11) Crystallographic data obtained for 4a, 8, and 10 have been placed in the
Supporting Information.
Figure 2. Displacement ellipsoid representation of [PhBPiPr3]FetNAd (9).
Hydrogen atoms have been removed for clarity. Selected bond distances
(Å) and angles (deg), for 9: Fe-N 1.638(2), Fe-P1 2.260(1), Fe-P2 2.297-
(1), Fe-P3 2.263(1); C28-N-Fe 176.0(2), P1-Fe-P2 91.05(2), P1-Fe-
P3 91.56(3), P2-Fe-P3 92.39(3), N-Fe-P1 121.62(7), N-Fe-P2
130.03(7), N-Fe-P3 120.27(7).
Structural data for related 10 have been placed in the Supporting
Information.
The anionic N2 adducts 3 and 4 appear well-poised for further
elaboration at the coordinated N2 functionality. This is significant
because the direct conversion of coordinated N2 to a coordinated
diazenido (N2R-) and/or hydrazido (N2R22-) species by simple
addition of an electrophile (e.g., R+) has, to our knowledge, not
been established for a first-row transition metal ion.3,4 We were
thus gratified to find that the simple addition of methyl tosylate
(MeOTs) to a THF solution of 3 effected a modest color change
15NN)
from brown to gold, along with a concomitant shift in its νNN(
vibration to 1597(1538) cm-1, characteristic of a coordinated,
monodentate diazenido functionality.1 A neutral, benzene-soluble
product was isolated from the reaction mixture that analyzed as
the diazenido complex [PhBPiPr3]FesNdNMe (11, 56%), consistent
with its magnetic susceptibility (S ) 2, 4.93 µB, Evans). A similar
protocol converted 4a to the amber-colored complex
JA036687F
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J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 10783