Heterolytic H2 activation mediated by low-coordinate L 3Fe-(μ-N)-FeL3 complexes to generate Fe(μ-NH)(μ-H)Fe species

Article abstract of DOI:10.1021/ja0544509

The diiron μ-nitride complexes, {L₃Fe^II(μ-N)Fe^IIL₃}^- and L₃Fe^III(μ-N)Fe^IIL₃, heterolytically activate hydrogen (1 atm) at ambient temperature in solution (L₃ = [PhB(CH₂PPh₂)₃]^-). These transformations lead to structurally unique {L₃Fe^II(μ-NH)(μ-H)Fe^IIL₃}^- and L₃Fe^III(μ-NH)(μ-H)Fe^IIL₃ products. X-ray data establish a marked reduction in the Fe-Fe distance upon H₂ uptake, and spectroscopic data establish both Fe^IIFe^II species to be diamagnetic, whereas the Fe^IIIFe^II species, L₃Fe^III(μ-N)Fe^IIL₃ and L₃Fe^III(μ-NH)(μ-H)Fe^IIL₃, populate doublet ground states with thermally accessible higher spin states. Copyright

Full text of DOI:10.1021/ja0544509

Published on Web 09/03/2005
Heterolytic H2 Activation Mediated by Low-Coordinate L3Fe-(µ-N)-FeL3
Complexes to Generate Fe(µ-NH)(µ-H)Fe Species
Steven D. Brown, Mark P. Mehn, 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 July 5, 2005; E-mail: jpeters@caltech.edu
Recently, we described the characterization of an unusual bi-
metallic bridged iron nitride, {([PhBP3]Fe)2(µ-N)}-, distinct by vir-
tue of the low coordination number (CN ) 4) and oxidation state
(2+) of its respective iron centers ([PhBP3] ) PhB(CH2PPh2)3-).1,2
Because molecular nitride species might be intermediates of N2
reduction schemes,3 an avenue of interest was to examine its
reactivity toward proton sources1 and molecular hydrogen.4 In the
latter context, molecular iron nitrides, whether bridging or terminal,
are not known to react with H2, despite the proposition that such
reactions are mechanistically important during the Haber-Bosch
ammonia synthesis process.5 Herein, we establish the facile
heterolysis of H2 under mild conditions (rt, 1 atm H2) upon exposure
to low-coordinate L3FeII-(µ-N)-FeIIL3 and L3FeIII-(µ-N)-FeIIL3
complexes. The resulting diiron imide-hydride products, L3Fe-
(µ-NH)(µ-H)-FeL3, are structurally unique.6,7
The iron µ-nitrides featured in this study are {([PhBP3]Fe)2(µ-
N)}{Na(THF)5}, {1}{Na(THF)5}, a brown complex that consists
of two antiferromagnetically coupled high-spin Fe(II) centers,1 and
its neutral one-electron oxidation product, greenish black {([PhBP3]-
Figure 1. (Top) Solid-state molecular structure of ([PhBP3]Fe)2N (2) with
phosphino phenyl groups and two THF solvent molecules removed for
Fe)2(µ-N)} (2). Access to the latter species was predicted by the
cyclic voltammogram of {1}-, which showed a reversible redox
couple at -1.3 V versus Fc+/Fc. Exposure of {1}- to numerous
oxidants generated varying amounts of the FeIII(µ-N)FeII species
2. PCl3 proved synthetically most reliable in this regard and
eliminated NaCl and Cl2P-PCl2 as byproducts to afford 2 in 66%
crystallized yield. Because the reported structure of {1}- exhibited
an unexpectedly bent Fe-N-Fe linkage and also experienced
modest disorder about the N-atom,1 we undertook the X-ray
characterization of single crystals of 2 for comparison. Its core
structure is shown in Figure 1 (top).8 The Fe-N bond distances of
1.668(2) and 1.683(2) Å for 2 are similar to those observed for
{1}- (Fe-N ) 1.675(5) and 1.705(5) Å). The Fe-N-Fe linkage
exhibits an even more pronounced bend (142.4(1)°) than that for
the case of {1}-. An interesting contrast to the structure of {1}- is
that there is substantial asymmetry about the Fe-P bonds of 2,
perhaps due to a Jahn-Teller distortion that results from placement
of one unpaired electron in a nearly degenerate π-orbital set.
Specifically, Fe1 exhibits two long and one short Fe-P bond, and
Fe2 exhibits two short and one long Fe-P bond. A rhombic signal
consistent with the S ) 1/2 ground state of 2 was observed in its 4
K X-band EPR spectrum.9 The low-energy charge transfer and d-d
transitions apparent in the optical spectrum of {1}- appear to be
blue-shifted in the spectrum of 2 owing to its higher oxidation state.
Also, its solid-state SQUID magnetization data are most consistent
with an antiferromagnetically coupled FeIIFeIII spin system. The
magnetic moment of 2 increased from 1.82 µB at 4 K to 3.40 µB at
300 K, most likely indicative of a ground state doublet with
thermally accessible higher spin states.
clarity. Selected bond lengths (Å) and angles (°): Fe1-N, 1.683(2); Fe2-
N, 1.668(2); Fe1-N-Fe2, 142.4(1). (Bottom) Solid-state molecular structure
of [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] {3}{Na(THF)5}. Selected bond
lengths (Å) and angles (°): Fe1-N1A, 1.826(5); Fe2-N1A, 1.790(5); Fe1-
Fe2, 2.6595(9); Fe1-N1A-Fe2, 94.7(3).
Scheme 1
10
characterized complexes [PhBP3]Fe(CO)2 and [PhBP3]Fe(CO)2-
(NCO):1
2 + 5 CO f [PhBP3]Fe(CO)2 + [PhBP3]Fe(NCO)(CO)2 (1)
Exposure of a brown THF solution of {1}{Na(THF)5} to an
atmosphere of H2 at room temperature resulted in a color change
to green within minutes. Isolation and characterization of the
resulting product after 2 h demonstrated a net 1,2 H-H addition
across the Fe-N-Fe linkage to produce the bridged species
[([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] {3}{Na(THF)5} (Scheme
1). The absence of low-energy d-d transitions in the optical
spectrum of {3}- intimated a low-spin diamagnetic systems
plausible given the presence of two five-coordinate Fe(II) centers
supported by strong-field ligands. Complex {3}- is indeed dia-
Chemical confirmation of the µ-nitride functionality of 2 was
afforded upon its exposure to an atmosphere of CO, which rapidly
and quantitatively produced equimolar amounts of the previously
1
magnetic and was conveniently characterized by H, 31P, and 15N
9
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10.1021/ja0544509 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Crystals of {3}- suitable for XRD analysis were obtained (Figure
1 (bottom)). The bridging ligands are disordered over two positions
as a result of the NH and H ligands alternating positions randomly
throughout the crystal. However, electron density attributable to
both the imide hydrogen and the hydride ligand could be located
in the difference map, and the data were satisfactorily refined when
the Fe-H distances were constrained to be equal. The solution was
of sufficient quality to unequivocally establish the connectivity of
the diiron core and to provide reliable structural data concerning
the statistically more prevalent Fe-(µ-NH)-Fe linkage (ca. 70%
occupancy). In comparison with nitride {1}-, the Fe-N bond
lengths for {3}- are expanded by an average distance of 0.118 Å
(Fe-N ) 1.790(5) and 1.826(5) Å for {3}-). Also, the Fe-N-Fe
bond angle exhibits a severe contraction from 135.9(3) to 94.7(3)°
upon installation of the bridging hydride ligand. The Fe-Fe distance
of 2.6588(9) Å observed for {3}- is approximately 0.45 Å shorter
than that of {1}- and is similar to the Fe-Fe distances observed
in hydrogenase model compounds for which direct iron-iron
interactions have been implicated.12 Our attempts to obtain reliable
structural parameters for 4 have been frustrated thus far by severe
disorder problems in the crystals we have examined.
In summary, low-valent and low-coordinate bridging iron nitrides
stabilized by [PhBP3] ligands mediate facile H2 activation under
mild conditions (room temperature, 1 atm H2). In contrast to
previous work from our lab concerning the hydrogenation of low-
spin Fe(III) imides,4a complete scission of the Fe-N linkage is not
observed. This is presumably due to the presence of an additional
metal center that traps the imide-hydride as a bimetallic species.
Acknowledgment. We thank the NIH for financial support
(GM-070757 to J.C.P., GM-072291 to M.P.M.), and Dr. Angel J.
Di Bilio, Lawrence Henling, and Dr. Michael W. Day for assistance.
Supporting Information Available: Experimental and character-
ization data; crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 2. (A) Proton-coupled 15N NMR spectrum, (B) µ-H resonance
1
observed in the H NMR spectrum, and (C) µ-NH resonance observed in
1
the H NMR spectrum of {3}- (50% 15N). Peaks marked with an asterisk
are due to coupling with the 15N nucleus. (D) Cyclic voltammetry of {3}-
{Na(THF)5} in THF (0.3 M [TBA][PF6], 50 mV/s).
NMR spectroscopies. Its 31P NMR spectrum exhibited a singlet
1
resonance at δ 66 ppm, and its H NMR spectrum featured diag-
nostic resonances at δ +18.5 and -22.4 ppm for the µ-NH and
µ-H ligands, respectively. These resonances were absent in the 1H
NMR spectrum when {1}- was hydrogenated under D2. 15N-labeled
{3}- (50% 15N) was prepared by the hydrogenation of {1}- (50%
15N) and featured a doublet at δ 406 ppm (JN-H ) 63 Hz; Figure
2), shifted significantly upfield from the 15N resonance of its pre-
cursor {1}- (δ ) 801 ppm (s)).1 Additionally, the NH resonance
1
at δ +18.5 ppm in the H NMR spectrum of {3}- (50% 15N) ex-
hibited a doublet superimposed on a singlet, resulting from coupling
to the 50% 15N label (Figure 2). We were surprised to find that
vibrations associated with the imide and hydride ligands could not
be observed in the IR spectrum of {3}-, regardless of how the
samples were prepared (THF/KBr solution, KBr pellet, Nujol).
The cyclic voltammetry of {3}- (Figure 2) demonstrated an
irreversible oxidative process at -0.15 V and two low-potential
redox events at -1.25 and -2.58 V versus Fc+/Fc. The reversible
event at -1.25 V is indicative of an FeIIIFeII/FeIIFeII redox couple
akin to that observed for {1}-. This suggests that the hydrogenation
of neutral 2 might lead to a stable neutral imide hydride. In fact,
exposure of green 2 to an atmosphere of hydrogen generated neutral
{([PhBP3]Fe)2(µ-NH)(µ-H)} (4). This product could be alternatively
generated by the oxidation of {3}- with [NO][PF6]. The lower-
potential, pseudo-reversible couple observed in the cyclic voltam-
mogram of {3}- at -2.6 V presumably constitutes an FeII(µ-NH)(µ-
H)FeII/FeII(µ-NH)(µ-H)FeI redox process. Electrochemical access
to the reduced FeIIFeI species is noteworthy given the recent interest
in FeIIFeI bimetallic diiron hydride cores as possible intermediates
of biological H2 production.11
The 4 K X-band EPR spectrum of 4 featured the anticipated
axial signal for an S ) 1/2 electronic configuration. The absence of
a low-field signal in the spectrum was suggestive of the presence
of two low-spin iron centers. Delocalization of its single unpaired
electron over both metals was implied by weak phosphorus coupling
in the g region arising from all six phosphorus nuclei (simulation
provided Ax ) 12 G). As for its nitride precursor 2, solid-state
SQUID magnetization data for 4 confirmed the presence of one
unpaired electron at low temperature (µeff ) 2.12 µB at 4 K). We
were surprised to find that the moment increased as the temperature
was raised to a value of 3.30 µB at 300 K, again likely indicating
low-lying excited states despite the presumed strong field nature
of the system. Distinct from {3}-, a readily discernible vibration
for the NH ligand (IR; Nujol mull) could be observed at 3319 cm-1
for 4.
References
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Hildebrandt, P.; Lengen, M.; Grodzicki, M.; Trautwein, A. X.; Nuber,
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(8) Complete structural details for 2 and {3}{Na(THF)5} can be found in the
Supporting Information.
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species, can be found in the Supporting Information.
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