Inorganic Chemistry
Article
(THF): m/z 838.5 ([Fe(DMeOPrPE)2(NH3)H]+). The complex
reacted with the N2 purge gas to form [Fe(DMeOPrPE)2(N2)H]+ in
the mass spectrometer. Calcd for [Fe(DMeOPrPE)2(N2)H]+: m/z
849.43. Found: m/z 849.5 [M+]. The five-coordinate [Fe-
(DMeOPrPE)2H]+ species from loss of the labile NH3 ligand was
also observed. Calcd for [Fe(DMeOPrPE)2H]+: m/z 821.42. Found:
m/z 821.5 [M+]. The complete MS spectrum and isotope pattern,
which matches the calculated pattern, are found in the Supporting
Information. IR (neat): νNH 3329 and 3211 cm−1; δNH 1615 and
between the top and bottom pathways in Scheme 1. Obviously,
the mechanisms for several of the transformations (e.g., 8 → 9)
are considerably more intricate than suggested, and detailed
studies are underway to probe these intimate mechanisms now
that we have access to the intermediates.
It is interesting to note that both the top and bottom
pathways in Scheme 1 involve the “symmetric” protonation of
the coordinated N2 unit. In this regard, these pathways are
different from the “asymmetric” protonation pathway proposed
for some Mo−N2 complexes that form ammonia.
Recent studies suggest a “symmetric” protonation pathway
for nitrogenase3 (which has iron in its active site), and it may
well be a general conclusion that Fe−N2 species produce
ammonia via a symmetric protonation mechanism.
3
3
1258 cm−1. Using a liquid IR cell, νFeH was observed at 1961 cm−1.
Synthesis of trans-[Fe(DMeOPrPE)2(N2H4)H][BPh4] (9). A
N2H4/THF solution (1.7 mL, 0.072 M) was added to a stirring
THF solution of trans-[Fe(DMeOPrPE)2(N2)H][BPh4] (0.14 g, 0.120
mmol) under argon. The reaction was stirred for 12 h, during which
time the solution color changed from brown to bright yellow. The
solvent was allowed to evaporate, yielding an orange-yellow oil. The
15N isotopologue was synthesized in the same manner using 15N2H4.
31P{1H} NMR (THF-d8): δ 81.3 (s). 31P NMR (THF-d8): δ 81.3 (d,
With regard to nitrogenase, the spectroscopic data for the
intermediates in Scheme 1 are also useful for a comparison with
trapped intermediates in nitrogenase turnover.1−3 A subsequent
paper will report on one such comparison and the implications
for the mechanism of nitrogen fixation with nitrogenase.34
1
2JPH = 49 Hz). H NMR (THF-d8) of the hydride region: δ −29.3
(quintet, 2JHP = 49 Hz). 15N{1H} NMR (THF-d8): δ −317 (d, 1JNN
=
1
5 Hz), −383 (d, JNN = 5 Hz). 15N NMR (THF-d8): δ −317 (td,
1
1
1
1JNH = 63 Hz, JNN = 5 Hz), −383 (td, JNH = 69 Hz, JNN = 5 Hz).
1H−15N HMQC (THF-d8): δ 3.7 (d, 1JHN = 68 Hz), 2.9 (d, 1JHN = 59
Hz). MS analysis showed that the complex reacted with the N2 purge
gas to form [Fe(DMeOPrPE)2(N2)H]+ in the mass spectrometer.
Calcd for [Fe(DMeOPrPE)2(N2)H]+: m/z 849.43. Found: m/z
849.33 [M+]. Using a liquid IR cell, νFeH was observed at 1962 cm−1.
Synthesis of trans-Fe(DMeOPrPE)2(Cl)H. An excess of tetrae-
thylammonium chloride was added to a stirring THF solution of trans-
[Fe(DMeOPrPE)2(N2)H][BPh4] under argon. The reaction was
stirred for 2 h and filtered through Celite to remove [NEt4][BPh4],
and then the solvent was allowed to evaporate, yielding a bright-orange
oil. The product contained uncoordinated DMeOPrPE as an impurity.
The free ligand was removed by running the pentane solution through
a column of basic alumina and then washing the column with pentane
several times. The product, still bound to the alumina, was then
isolated by washing the column with diethyl ether. As discussed in the
text, the product was readily soluble in all organic solvents and thus
EXPERIMENTAL SECTION
■
Materials and Reagents. All manipulations were carried out
either in a Vacuum Atmospheres Co. glovebox (argon- or N2-filled) or
on a Schlenk line using argon or N2 gas. HPLC-grade THF, hexane,
and diethyl ether (Burdick and Jackson) were dried and deoxygenated
by passing them through commercial columns of CuO, followed by
alumina under an argon atmosphere. Commercially available reagents
were used as received. Deuterated solvents were obtained from
Cambridge Isotope Laboratories and degassed via three freeze−
pump−thaw cycles. trans-[Fe(DMeOPrPE)2(N2)H][BPh4] was syn-
thesized as previously reported.7
Instrumentation. NMR samples were sealed under argon or N2
in 7 mm J. Young tubes. 31P{1H} and 1H NMR spectra were recorded
on either a Varian Unity/Inova 300 spectrometer at an operating
frequency of 299.94 (1H) and 121.42 (31P) MHz or a Varian Unity/
Inova 500 spectrometer at an operating frequency of 500.62 (1H) and
202.45 (31P) MHz. The 1H and 31P chemical shifts were referenced to
the solvent peak and to an external standard of 1% H3PO4 in D2O,
respectively. Note that the 1H NMR data for the methyl and
methylene regions in complexes containing the DMeOPrPE
ligand were generally broad and uninformative and therefore are not
reported in the synthetic descriptions below. 15N NMR spectra were
recorded on a Varian Unity/Inova 500 spectrometer at an operating
frequency of 50 MHz. The 15N chemical shifts were referenced to an
external standard of neat nitromethane (set to 0 ppm). IR spectra were
recorded on a Nicolet Magna 550 FT-IR with OMNIC software.
Samples were prepared either as neat oils using NaCl windows, as KBr
pellets, or in solution using a CaF2 cell. Mass spectra were obtained
using an Agilent LC/MS mass spectrometer. The samples were
dissolved in THF and introduced into the ionization head (electro-
spray ionization, ESI) using the infusion method.
could only be obtained as an oil. 31P{1H} NMR (C6D6): δ 83.0 (s). 31
P
2
1
NMR (C6D6): δ 83.0 (d, JPH = 49 Hz). H NMR (C6D6) of the
2
hydride region: δ −32.5 (quintet, JHP = 49 Hz).
Alternative Synthesis of trans-[Fe(DMeOPrPE)2(N2H4)H]-
[BPh4]. To a stirring solution of trans-Fe(DMeOPrPE)2(Cl)H in
THF (0.033 g, 0.038 mmol) was added a solution of hydrazine in THF
(65 μL, 0.603 M, 0.039 mmol). NaBPh4 (0.013 g, 0.039 mmol) was
immediately added, and the mixture was allowed to stir for 1 h. The
bright-yellow solution was filtered through Celite. The NMR spectra
of the product synthesized by this route were identical with those
described above.
Reaction of trans-Fe(DMeOPrPE)2(Cl)H with 15N2H4. TlPF6
was added to a THF-d8 solution of trans-Fe(DMeOPrPE)2(Cl)H
under argon, and the solution was filtered through Celite directly into
a J. Young tube. To this NMR tube was added a 10-fold excess of
15N2H4. The initial 31P{1H} NMR spectrum showed a mixture of 9
Synthesis of trans-[Fe(DMeOPrPE)2(NH3)H][BPh4] (10). THF
saturated with NH3 (2 mL) was added to a stirring THF solution of
trans-[Fe(DMeOPrPE)2(N2)H][BPh4] (0.049 g, 0.042 mmol) under
argon. The solution was stirred for 2 days, over which time the
solution color changed from pale brown to bright yellow. Alternatively,
excess ammonia can be bubbled directly through a THF solution of
trans-[Fe(DMeOPrPE)2(N2)H][BPh4], yielding trans-[Fe-
(DMeOPrPE)2(NH3)H][BPh4] after 4 h. The solvent was allowed
to evaporate, leaving a bright-orange-yellow oil. 31P{1H} NMR
1
and 10. Because of the addition of excess hydrazine, a clean H NMR
spectrum could not be obtained. The tube was allowed to stand
overnight. Hexane and pentane were then used to precipitate the
products, and the resulting oil was washed twice with pentane. The oil
was then redissolved in toluene-d8 and filtered through Celite into
another J. Young NMR tube. The 31P{1H} NMR spectrum showed
seven different iron phosphine species. By a comparison of the
1
integrations in the 31P{1H} and H NMR spectra, five of the species
(C6D6): δ 82.0 (s). 31P NMR (C6D6): δ 82.0 (d, JPH = 48 Hz). H
2
1
could be assigned because they had been previously synthesized. The
other two species were assigned as 8 and 8b (see the Results and
Discussion section). Only the NMR data for these two species are
listed here. 8. 31P{1H} NMR (toluene-d8): δ 79.6 (s). 31P NMR
NMR (C6D6) of the hydride region: δ −0.86 (s, br) and δ −29.6
2
(quintet, JHP = 48 Hz). 15N{1H} NMR (THF-d8): δ −441 (s).
15N NMR (THF-d8): δ −441 (quartet, JN−H = 64 Hz). The 15N
1
2
1
isotopologue was prepared by degredation of trans-[Fe-
(DMeOPrPE)2(15N2H4)H][BPh4] prepared by the reaction of
15N2H4 with trans-[Fe(DMeOPrPE)2(N2)H][BPh4]. ESI-MS(+)
(toluene-d8): δ 79.6 (d, JPH = 49 Hz). H NMR (toluene-d8): δ 15.0
(dd, 1JHN = 60 Hz, 3JHH = 24 Hz), 14.1 (ddt, JHN = 48 Hz, 3JHH = 24
1
Hz, JHP = 5 Hz), −17.8 (quintet, JHP = 49 Hz). 8b. 31P{1H} NMR
3
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dx.doi.org/10.1021/ic201873a|Inorg. Chem. 2012, 51, 439−445