Stereochemical control of iron(II) complexes containing a diphosphine ligand with a pendant nitrogen base

Article abstract of DOI:10.1021/om050071c

A series of cis and trans Fe(II) complexes containing the diphosphine ligand PNP (where PNP is bis((diethylphosphino)methyl)methylamine) have been prepared and isolated. These include cis-[Fe(PNP)₂(CH ₃CN)₂](BF₄)₂ (3), cis-[Fe(PNP) ₂(CH₃CN)(CO)](BF₄)₂ (4), trans-[Fe-(PNP)(dmpm)(CH₃CN)₂](BF₄) ₂ (5), trans-Fe(PNP)(dmpm)Cl₂ (6), trans-[Fe(PNP)(dmpm) (CO)-Cl]₂[FeCl₄] (7), trans-HFe(PNP)(dmpm)Cl (8), and trans-[HFe(PNP)(dmpm)(CH₃CN)]BPh₄ (9) (where dmpm is bis(dimethylphosphino)methane). In addition, the cations trans-[HFe-(PNP)(dmpm) (CO)]⁺ (10) and trans-[(H₂)Fe(PNP)(dmpm)(H)]⁺ (11) have been observed in solution. These complexes all possess a pendant base that bridges the two phosphorus atoms of the PNP ligand. A cis geometry is observed for those complexes containing two PNP ligands, whereas a trans geometry is observed for the complexes containing one PNP ligand and one dmpm ligand. The molecular structures of cis-[Fe(PNP)₂(CH ₃CN)(CO)](BPh₄)₂ and trans-[HFe(PNP)(dmpm) (CH₃CN)]BPh₄ have been confirmed by X-ray diffraction studies. Protonation of 5, 7, and 10 occurs at the nitrogen atom of the PNP ligand, and pK_a values are reported for the corresponding protonated complexes. The formation of the dihydrogen complex [trans-[(H ₂)Fe(PNP)(dmpm)(H)]⁺ and the protonated PNHP complex trans-[HFe-(PNHP)(dmpm)(CO)]²⁺ demonstrate that intramolecular heterolytic cleavage of the dihydrogen ligand can be controlled by varying the nature of the ligand trans to the incipient dihydrogen ligand.

Full text of DOI:10.1021/om050071c

Organometallics 2005, 24, 2481-2491
2481
Stereochemical Control of Iron(II) Complexes
Containing a Diphosphine Ligand with a Pendant
Nitrogen Base
Renee M. Henry,† Richard K. Shoemaker,† Rachel H. Newell,†
George M. Jacobsen,† Daniel L. DuBois,*,‡ and M. Rakowski DuBois*,†
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309,
and National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401
Received February 1, 2005
A series of cis and trans Fe(II) complexes containing the diphosphine ligand PNP (where
PNP is bis((diethylphosphino)methyl)methylamine) have been prepared and isolated. These
include cis-[Fe(PNP)2(CH3CN)2](BF4)2 (3), cis-[Fe(PNP)2(CH3CN)(CO)](BF4)2 (4), trans-[Fe-
(PNP)(dmpm)(CH3CN)2](BF4)2 (5), trans-Fe(PNP)(dmpm)Cl2 (6), trans-[Fe(PNP)(dmpm)(CO)-
Cl]2[FeCl4] (7), trans-HFe(PNP)(dmpm)Cl (8), and trans-[HFe(PNP)(dmpm)(CH3CN)]BPh4
(9) (where dmpm is bis(dimethylphosphino)methane). In addition, the cations trans-[HFe-
(PNP)(dmpm)(CO)]+ (10) and trans-[(H2)Fe(PNP)(dmpm)(H)]+ (11) have been observed in
solution. These complexes all possess a pendant base that bridges the two phosphorus atoms
of the PNP ligand. A cis geometry is observed for those complexes containing two PNP
ligands, whereas a trans geometry is observed for the complexes containing one PNP ligand
and one dmpm ligand. The molecular structures of cis-[Fe(PNP)2(CH3CN)(CO)](BPh4)2 and
trans-[HFe(PNP)(dmpm)(CH3CN)]BPh4 have been confirmed by X-ray diffraction studies.
Protonation of 5, 7, and 10 occurs at the nitrogen atom of the PNP ligand, and pKa values
are reported for the corresponding protonated complexes. The formation of the dihydrogen
complex [trans-[(H2)Fe(PNP)(dmpm)(H)]+ and the protonated PNHP complex trans-[HFe-
(PNHP)(dmpm)(CO)]2+ demonstrate that intramolecular heterolytic cleavage of the dihy-
drogen ligand can be controlled by varying the nature of the ligand trans to the incipient
dihydrogen ligand.
Introduction
ous structural models of the active site of the Fe-only
hydrogenase enzymes have been reported, and the
electrochemical properties and reactions of these com-
pounds with protons and molecular hydrogen have been
investigated.9-18 However, simple bimetallic compounds
that catalyze hydrogen oxidation or production with
high rates and low overpotentials have proven elusive.
The active site of the Fe-only hydrogenase enzyme
from Desulfovibrio desulfuricans is depicted by structure
1. On the basis of recent interpretations of the structural
features of the enzyme and its active site, it is thought
that a bridging nitrogen atom in the active site assists
the heterolytic cleavage of hydrogen and acts as a
The ability to oxidize hydrogen to protons using iron
complexes is of interest for both practical and funda-
mental reasons. Iron catalysts for hydrogen oxidation/
production would have many advantages for hydrogen
fuel cells and electrolyzers for hydrogen production. Iron
is cheap, abundant, and not toxic to the environment.
In addition, two classes of hydrogenase enzymes, the
Fe-only and Ni-Fe hydrogenases, incorporate iron in
their active sites. The structures of several hydrogenase
enzymes have been determined, and both classes con-
tain bimetallic complexes at the active site.1-8 Numer-
* To whom correspondence should be addressed. E-mail: dan_dubois@
nrel.gov (D.L.D); mary.rakowski-dubois@colorado.edu (M.R.D.).
† University of Colorado.
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P.; Darensbourg, M. Y. J. Am. Chem. Soc. 2004, 126, 12004-12014.
(10) Justice, A. K.; Linck, R. C.; Rauchfuss, T. B.; Wilson, S. R. J.
Am. Chem. Soc. 2004, 126, 13214-13215.
‡ National Renewable Energy Laboratory.
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Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001,
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10.1021/om050071c CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/08/2005

2482 Organometallics, Vol. 24, No. 10, 2005
Henry et al.
shuttle to transfer protons between the active site and
the proton conduction channel of the protein.1,8,14 If this
interpretation is correct, then the pendant base plays
an important role in the catalytic oxidation/production
of hydrogen by iron-only hydrogenases. Studies of metal
hydrides with pendant bases suggest that such an
interaction is reasonable.19-22
configurations, whereas Fe(II) complexes containing
PNP and dmpm (where dmpm is Me2PCH2PMe2) form
trans isomers. These results demonstrate that the
geometry of these octahedral iron complexes containing
a proximal base can be controlled by the proper selection
of diphosphine ligands. The different stereochemistries
are expected to lead to interesting differences in chemi-
cal reactivity as well. The syntheses and reactions of
several derivatives of the trans complexes are reported
in this paper, and a more complete development of the
chemistry of the cis derivatives will be reported in a
subsequent paper.
Results
Syntheses and Characterization of Complexes.
Addition of [Fe(CH3CN)6](BF4)2 to 2 equiv of the PNP
ligand in acetonitrile results in formation of cis-[Fe-
(PNP)2(CH3CN)2](BF4)2 (3), as shown in step 1 of reac-
tion 1. This compound can be isolated as red crystals,
Our laboratories have recently reported a simple
monometallic nickel complex containing a diphosphine
ligand with a pendant nitrogen base, [Ni(PNP)2]2+ (2;
where PNP is Et2PCH2NMeCH2PEt2).23 This complex
is a catalyst for hydrogen oxidation at low overpoten-
tials. The hydride acceptor ability of the Ni(II) center
and the proton acceptor ability of the pendant base are
matched in energy to provide a 5 kcal/mol driving force
for heterolytic cleavage of hydrogen. In addition, it has
been shown that the pendant base provides a pathway
for the rapid exchange of the hydride ligand with
protons in solution for the catalytic intermediates [HNi-
(PNP)2]+ and [HNi(PNHP)(PNP)2]2+ (where PNHP rep-
resents the N-protonated form of the PNP ligand).
This paper describes our efforts to synthesize octa-
hedral Fe(II) complexes that incorporate the PNP
ligand. Experimental and theoretical studies have shown
that the acidity of the coordinated dihydrogen ligand
in complexes of the type trans-[(H2)M(diphosphine)2-
(X)]n+ (where M ) Fe, Ru, Os) is strongly dependent on
the nature of both X and the diphosphine ligand.24-27
Strong π acceptors, such as CO (n ) 2), trans to the
dihydrogen ligand result in very acidic complexes,
whereas anionic π donors (n ) 1) result in weakly acidic
dihydrogen complexes. Similarly, systematic variation
of substituents on the diphosphine ligand cis to the
dihydrogen ligand have been reported to result in
significant changes in the pKa values of the dihydrogen
ligand. These results suggest that it should be possible
to tune the acidity of dihydrogen complexes containing
pendant bases to favor either intramolecular heterolytic
cleavage of hydrogen or the formation of dihydrogen
complexes. In this work it is shown that octahedral
Fe(II) complexes containing two PNP ligands adopt cis
and it is air-sensitive in solution and in the solid state.
The 31P NMR spectrum of this complex in CD3CN
(Figure 1, upward trace) is consistent with an AA′BB′
spin system with resonances centered at 22.0 and 14.5
ppm. This spectrum can be simulated using the param-
eters listed in the Experimental Section, as shown by
the downward trace in Figure 1. The AA′BB′ spin
system implies a cis geometry. Resonances for the
1
coordinated PNP ligand are observed in the H NMR
spectra recorded in acetonitrile-d3 (see Experimental
1
Section for specific asignments). H NMR spectra re-
corded approximately 2 min after adding acetonitrile-
d3 to solid [Fe(PNP)2(CH3CN)2](BF4)2 showed only free
acetonitrile at 1.96 ppm, indicating a half-life of less
than 1 min for the exchange of free and coordinated
acetonitrile. Similarly, in acetone-d6, a 10:1 mixture of
acetonitrile-d3 and cis-[Fe(PNP)2(CH3CN)2](BF4)2 re-
sults in an 85% loss in the intensity of the bound
acetonitrile resonance at 2.73 ppm in 3 min at room
temperature (23 ( 2 °C). This corresponds to a t1/2 value
of approximately 1 min in acetone. Two IR bands (KBr
pellets) are observed at 2324 and 2290 cm-1. These
bands are assigned to the coordinated acetonitrile
ligands. An electrospray ionization mass spectrum (ESI)
recorded on acetonitrile solutions resulted in peaks
(19) Lee, D.-H.; Patel, B. P.; Clot, E.; Eisenstein, O.; Crabtree, R.
H. Chem. Commun. 1999, 297-298.
(20) Chu, H. S.; Lau, C. P.; Wong, K. Y.; Wong, W. T. Organo-
metallics 1998, 17, 2768-2777.
(21) Laugh, A. J.; Park, S.; Ramachandran, R.; Morris, R. H. J. Am.
Chem. Soc. 1994, 116, 8356-8357.
(22) Custelcean, R.; Jackson, J. E. Chem. Rev. 2001, 101, 1963-
1980.
(23) Curtis, C. J.; Miedaner, A.; Ciancanelli, R. F.; Ellis, W. W.; Noll,
B. C.; DuBois, M. R.; DuBois, D. L. Inorg. Chem. 2003, 42, 216-227.
(24) Landau, S. E.; Morris, R. H.; Lough, A. J. Inorg. Chem. 1999,
38, 6060-6068.
-
assignable to the parent complex minus a BF4 anion
and for the sequential loss of the weakly bound aceto-
nitrile ligands. All experimental data are consistent
with the formulation of this complex.
Carbon monoxide reacts with cis-[Fe(PNP)2(CH3CN)2]-
(BF4)2 to form cis-[Fe(PNP)2(CH3CN)(CO)](BF4)2 (4), as
shown in step 2 of reaction 1. The four doublet of doublet
(25) Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C. T.;
D’Agostino, C. Inorg. Chem. 1994, 33, 6278-6288.
(26) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris,
R. H.; Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375-3388.
(27) Xu, Z.; Bytheway, I.; Jia, G.; Lin, Z. Organometallics 1999, 18,
1761-1766.

Stereochemical Control of Fe(II) Complexes
Organometallics, Vol. 24, No. 10, 2005 2483
Figure 1. Proton-decoupled 31P NMR spectrum of cis-[Fe(PNP)2(CH3CN)2](BF4)2: (up) experimental spectrum; (down)
simulated spectrum. The horizontal scale is in ppm (162 MHz). See the Experimental Section for simulation parameters.
of doublet resonances observed by 31P NMR are consis-
tent with the substitution of just one acetonitrile ligand,
resulting in the loss of all symmetry. The simulated
spectrum again matches the observed spectrum (see
Experimental Section for parameters). In acetonitrile-
d3, the resonance at 2.37 ppm, assigned to coordinated
acetonitrile, disappeared with a half-life of 20 min (see
Figure S1(a) in the Supporting Information). In acetone-
d6, addition of a 10-fold excess of acetonitrile-d3 results
in the disappearance of the resonance at 2.70 ppm,
assigned to coordinated acetonitrile, and the appearance
of a resonance at 1.93 ppm, corresponding to free
acetonitrile, with a half-life of 34 min. The IR spectrum
in solution (CH2Cl2) exhibits a band at 1991 cm-1 that
is assigned to the C-O stretching vibration. The obser-
vation of two bands in this region for samples in KBr
pellets is attributed to different environments in the
solid state. An X-ray diffraction study of cis-[Fe(PNP)2-
mental Section), but the reaction was not sufficiently
clean to be an attractive synthetic approach. However,
reaction of [Fe(CH3CN)6](BF4)2, PNP, and dmpm (where
dmpm is bis(dimethylphosphino)methane) in a 1:1:1
ratio, as shown in reaction 2, resulted in the formation
of trans-[Fe(PNP)(dmpm)(CH3CN)2](BF4)2 (5), in 55%
isolated yield. The 31P NMR spectrum of this red
complex is shown in the upward trace of Figure 2, and
the simulated spectrum is shown by the downward
trace. The four phosphorus nuclei form an AA′XX′ spin
system, which is consistent with a trans geometry, but
not with a cis geometry, which would result in four
1
nonequivalent nuclei. A singlet at 2.40 ppm in the H
NMR spectrum recorded in CD3CN is assigned to
coordinated acetonitrile. This resonance disappears with
a half-life of approximately 7.2 h at 23 ( 2 °C. This
observation indicates that the acetonitrile ligands in this
complex are not as labile as those of cis-[Fe(PNP)2(CH3-
CN)2](BF4)2 described above (t1/2 < 1 min). The IR
spectrum, mass spectrum, and elemental analyses of
this complex (see Experimental Section) are also con-
sistent with the formulation of 5.
Several additional trans derivatives have been syn-
thesized and isolated, as detailed in the Experimental
Section. Reaction of FeCl2 with a 1:1 mixture of PNP
and dmpm results in the formation of the emerald green
(λmax 673 nm, ꢀ ) 17 M-1 cm-1) paramagnetic complex
trans-Fe(PNP)(dmpm)Cl2 (6; step 1 of reaction 3). This
-
(CH3CN)(CO)](BPh4)2, obtained by metathesis of BF4
with BPh4-, is described below under Structural Stud-
ies.
These results indicate that octahedral iron(II) com-
plexes containing two PNP ligands prefer to adopt a cis
geometry. In these complexes, unfavorable steric inter-
actions between the ethyl substituents on the two
different PNP ligands are expected to disfavor formation
of trans complexes. To reduce this interaction, we
attempted to prepare trans-[Fe(PNP)(dppm)(CH3CN)2]-
(BF4)2 (where dppm is bis(diphenylphosphino)methane)
by reacting [Fe(CH3CN)6](BF4)2, PNP, and dppm in a
1:1:1 ratio. It was encouraging that some of the desired
product appeared to form, as indicated by 31P NMR
spectroscopy of the crude reaction mixture (see Experi-
FeCl2 + PNP + dmpm f
CO8
trans-Fe(PNP)(dmpm)Cl2
6
trans-[Fe(PNP)(dmpm)(CO)Cl]+
(3)
7
complex is similar to the previously reported Fe-
(diphosphine)2Cl2 complexes containing the ligands 1,2-
bis(diphenylphosphino)ethane (dppe) and 1,2-bis(dieth-
ylphosphino)ethane (depe).28-30 Generation of this
complex in situ followed by reaction with CO (1 atm)
results in the formation of trans-[Fe(PNP)(dmpm)(CO)-

2484 Organometallics, Vol. 24, No. 10, 2005
Henry et al.
Figure 2. Proton-decoupled 31P NMR spectrum of trans-[Fe(PNP)(dmpm)(CH3CN)2](BF4)2: (up) experimental spectrum;
(down) simulated spectrum. The horizontal scale is in ppm (162 MHz).
Scheme 1
Cl]2[FeCl4] (7; step 2 of reaction 3).31 The 31P NMR
spectrum of this carbonyl complex is again consistent
with a trans geometry. The observation by IR spectros-
copy of a single CO stretching frequency at 1936 cm-1
in dichloromethane solution suggests the presence of
one carbonyl ligand, which is also consistent with the
observation of the [Fe(PNP)(dmpm)(CO)Cl]+ cation (m/z
490) by ESI mass spectrometry.
Reaction of Fe(PNP)(dmpm)Cl2 with bis(triphenylphos-
phine)iminium borohydride in tetrahydrofuran (THF)
produces orange trans-HFe(PNP)(dmpm)Cl (8), which
is isolated in low to moderate yields. The 31P NMR
spectrum indicates an AA′XX′ spin system and a trans
geometry. The presence of a hydride ligand is confirmed
2
by the observation of a pentet resonance with JPH
)
ligand is confirmed by the observation of a triplet of
triplet of doublets at -19.8 ppm in the 1H NMR
spectrum. The triplet patterns arise from coupling to
the two pairs of phosphorus atoms associated with the
PNP and dmpm ligands, as indicated by broadband and
selective phosphorus decoupling experiments described
in the Experimental Section. The origin of the doublet
splitting of the hydride resonance is attributed to
coupling between the hydride ligand and one of the
protons of the methylene group of the dmpm ligand. A
broadband 31P-decoupled 1H NMR spectrum exhibits an
AB portion of an ABX spin system at 2.98 and 3.15 ppm
(2JHH ) 14 Hz) assigned to the methylene protons of
the dppm ligand. The resonance at 3.15 ppm exhibits
an additional coupling of 4 Hz. This coupling matches
that of the doublet splitting observed for the hydride
resonance. A broadband 31P-decoupled 1H gCOSY spec-
trum (Figure S2 in the Supporting Information) con-
firmed the coupling between the resonance at 3.15 ppm
and the hydride resonance at -19.8 ppm. An IR band
at 1826 cm-1 is assigned to the Fe-H stretching mode.
As expected, the frequency of this band is about 30 cm-1
higher than that of trans-HFe(PNP)(dmpm)Cl. The
coordinated acetonitrile ligand in this complex ex-
changes slowly with free acetonitrile. A singlet at 2.22
ppm in the 1H NMR spectrum is assigned to the
coordinated acetonitrile in CD3CN. This resonance
1
47 Hz at -27.0 ppm in the H NMR spectrum and an
IR band at 1797 cm-1 assigned to the Fe-H stretching
mode. In comparison, the complex HFe(depe)2Cl has a
hydride resonance at -29.1 ppm and an IR stretch at
1849 cm-1 29,30
An ESI mass spectrum of an acetonitrile
.
solution of 8 exhibited fragments consistent with loss
of chloride, loss of hydride, and the formation of [HFe-
(PNP)(dmpm)(CH3CN)]+. The latter cation presumably
forms by loss of chloride and solvent coordination, as
discussed in the next paragraph. Elemental analysis of
8 indicates the presence of only one chloride ligand, and
UV-visible data indicate that contamination with trans-
Fe(PNP)(dmpm)Cl2 is less than 5%.
As shown on the left side of Scheme 1, reaction of
trans-HFe(PNP)(dmpm)Cl with acetonitrile in the pres-
ence of sodium tetraphenylborate results in the forma-
tion of trans-[HFe(PNP)(dmpm)(CH3CN)](BPh4) (9). The
31P NMR spectrum again indicates an AA′XX′ spin
system and a trans geometry. The presence of a hydride
(28) Chatt, J.; Hayter, R. G. J. Chem. Soc. 1961, 5507-5511.
(29) Aresta, M.; Giannoccaro, P.; Rossi, M.; Sacco, A. Inorg. Chim.
Acta 1971, 5, 115-118.
(30) Mayes, M. J.; Prayter, B. E. Inorg. Synth. 1974, 15, 22-25.
(31) A similar compound, [FeCl(CO)(dppe)2][FeCl4], has been previ-
ously reported: Gao, Y.; Holah, D. G.; Hughes, A. N.; Spivak, G. J.;
Havighurst, M. D.; Magnuson, V. R.; Polyakov, V. Polyhedron 1997,
16, 2797-2807.

Stereochemical Control of Fe(II) Complexes
Organometallics, Vol. 24, No. 10, 2005 2485
slowly disappears with a half-life of 9.6 h at 23 ( 2 °C
(Figure S1(b)). An X-ray diffraction study of trans-[HFe-
(PNP)(dmpm)(CH3CN)](BPh4) is described below under
Structural Studies.
at -90 °C for T1min, a H-H distance of 0.89 Å can be
calculated, assuming rapid rotation of the dihydrogen
ligand, and a H-H distance of 1.12 Å, assuming slow
rotation.36-38 Dihydrogen complexes of iron with a trans
hydride ligand generally exhibit rapid rotation.39 How-
ever, the T1 value obtained for the dihydrogen ligand
at the lowest temperature of -90 °C may not be a true
Reaction of trans-[HFe(PNP)(dmpm)Cl] with CO and
NaPF6 in acetone-d6 in an NMR tube (top reaction in
Scheme 1) results in a 60/40 mixture of trans-[HFe-
(PNP)(dmpm)(CO)]+ (10) and trans-[Fe(PNP)(dmpm)-
(Cl)(CO)]+. The latter is spectroscopically identical with
the material obtained from the reaction of CO with Fe-
(PNP)(dmpm)Cl2 discussed above. The expected product,
trans-[HFe(PNP)(dmpm)(CO)]+, has a hydride reso-
nance at -7.06 ppm in the 1H NMR spectrum that
appears as a triplet of triplet of doublets. As discussed
for the analogous acetonitrile complex, this coupling
pattern can be attributed to coupling to the two diphos-
phine ligands and a proton of the methylene group of
the dmpm ligand. The 31P NMR spectrum exhibits an
AA′XX′ spectrum centered at 2.34 ppm (dmpm) and
44.29 ppm (PNP), as expected for a trans geometry.
Attempts to isolate trans-[HFe(PNP)(dmpm)(CO)]+ as
a pure compound were unsuccessful. The origin of trans-
[Fe(PNP)(dmpm)(Cl)(CO)]+, produced as a significant
side product in this reaction, is not known. The UV-
visible spectra of the starting materials indicate that
less than 5% of the original material is Fe(PNP)(dmpm)-
Cl2, which would be expected to give rise to trans-[Fe-
(PNP)(dmpm)(Cl)(CO)]+ when treated with CO. The
observation that trans-[HFe(PNP)(dmpm)Cl reacts with
CO in acetone-d6 to form a 60/40 mixture of trans-[HFe-
(PNP)(dmpm)(CO)]+ and trans-[Fe(PNP)(dmpm)(Cl)-
(CO)]+ indicates that the major portion of the trans-
[Fe(PNP)(dmpm)(Cl)(CO)]+ complex is produced by a
hydride transfer reaction, but we have been unable to
identify the hydride acceptor. However, this reaction is
very reproducible.
1
minimum. To obtain a JHD value for calculating the
H-H bond distance, the reaction of trans-[HFe(PNP)-
(dmpm)Cl] with deuterated tetramethylguanidinium
under HD gas in protioacetone was followed by 2H NMR.
A doublet (1JHD ) 30.8 Hz) observed at -8.01 ppm for
a spectrum recorded at -60 °C is assigned to the HD
ligand. Upon proton decoupling, this resonance collapses
to a singlet. A H-H bond distance of 0.90-0.92 Å was
1
calculated using this JHD value and relationships
proposed by Heinekey and Morris.40,41 The data above
are consistent with the formation of the dihydrogen
complex [(H2)Fe(PNP)(dmpm)H]+ with an H-H bond
distance similar to that observed for [(H2)Fe(dppe)2H]+
1
(0.92-0.94 Å from JHD).33,34 The trans-[(H2)Fe(PNP)-
(dmpm)H]+ complex is always accompanied by a sig-
nificant fraction of the starting complex, trans-[HFe-
(PNP)(dmpm)Cl], or a decomposition product tentatively
identified as the cis-dihydride, [(H)2Fe(PNP)(dmpm)].
As a result, the dihydrogen complex was not isolated.
Protonation of Complexes Containing the PNP
Ligand. Protonation reactions of trans-[Fe(PNP)(dmpm)-
(CH3CN)2]2+, trans-[Fe(PNP)(dmpm)(Cl)(CO)]+, and
trans-[HFe(PNP)(dmpm)(CO)]+ were studied in aceto-
nitrile at 23 ( 2 °C (reaction 4). The use of strong acids
When trans-[HFe(PNP)(dmpm)Cl] is reacted with H2
in the presence of 1 equiv of the tetramethylguani-
dinium cation (HTMG+) in acetone-d6 at 0 °C in an
NMR tube for 1.5 h (diagonal reaction in Scheme 1),
broad resonances are observed in the 1H NMR spectrum
at -7.9 and -11.4 ppm that are assigned to the species
[(H2)Fe(PNP)(dmpm)H]+ (11). Upon cooling to -60 °C,
these resonances sharpen into a pentet centered at
-11.76 ppm (2JPH ) 46 Hz) and a singlet at -7.90 ppm
with an integration ratio of 1.0:2.0. The former is
assigned to the hydride ligand and the latter to the
dihydrogen ligand of [(H2)Fe(PNP)(dmpm)H]+. When
the sample is warmed back up to 0 °C, the original
spectrum is observed. The exchange of a hydride ligand
with an H2 ligand has been observed for a number of
other [(H2)Fe(diphosphine)2H]+ complexes.32-35 Tem-
perature-dependent T1 measurements of these reso-
nances at 400 MHz gives a T1 value of 346 ms at -90
°C for the hydride ligand and a value of 20.7 ms at -90
°C for the resonance assigned to the dihydrogen ligand.
A plot of ln T1 versus 1/K is shown in Figure S3 in the
Supporting Information. Using the T1 value obtained
such as HBF4 and cyanoanilinium (pKa ) 7.5 in aceto-
nitrile)42 resulted in complete protonation of the PNP
ligand, as indicated by a constant shift in the resonance
assigned to the PNHP+ ligand when excess acid was
used. The use of weaker acids such as anisidinum (pKa
) 11.3), anilinium (pKa ) 10.6), and bromoanilinium
(pKa ) 9.6)42 resulted in smaller shifts intermediate
(36) Kubas, G. J. In Metal Dihydrogen and σ-Bond Complexes;
Fackler, J. P., Jr., Ed.; Modern Inorganic Chemistry; Kluwer Academic/
Plenum: New York, 2001; p 159.
(37) Morris, R. H. Can. J. Chem. 1996, 74, 1907-1915.
(38) Morris, R. H.; Wittebort, R. J. Magn. Reson. Chem. 1997, 35,
243.
(32) Hills, A.; Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. J. J.
Chem. Soc., Dalton Trans. 1993, 3041-3049.
(33) Morris, R. H.; Sawyer, J. F.; Shiralian, M.; Zubkowski, J. D. J.
Am. Chem. Soc. 1985, 107, 5581-5582.
(34) Ricci, J. S.; Koetzle, T. F.; Bautista, M. T.; Hofstede, T. M.;
Morris, R. H.; Sawyer, J. F. J. Am. Chem. Soc. 1989, 111, 8823-8827.
(35) Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. Inorg. Chem.
2004, 43, 3341-3343.
(39) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1996, 35, 6775-6783.
(40) Luther, T. A.; Heinekey, M. Inorg. Chem. 1998, 37, 127-132.
(41) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris,
R. H.; Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem.
Soc. 1996, 118, 5396-5407.
(42) Edidin, R. T.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc.
1987, 109, 3945-3953.

2486 Organometallics, Vol. 24, No. 10, 2005
Henry et al.
Figure 3. Drawings of the trans-[HFe(PNP)(dmpm)(CH3CN)]+ (left) and cis-[Fe(PNP)2(CH3CN)(CO)]2+ (right) cations,
indicating the atom-numbering schemes. Thermal ellipsoids are drawn at the 50% probability level.
Table 1. Selected Bond Distances (Å) and Bond
between those of the fully protonated species and the
unprotonated species. These shifts could be used to
Angles (deg)
trans-[HFe(PNP)(dmpm)-
cis-[Fe(PNP)2(CH3CN)-
determine the equilibrium constants shown below reac-
tion 4 (see Experimental Section for details).43 These
equilibrium constants were used to calculate the pKa
values of the coordinated PNHP+ ligands in acetonitrile
by adding log Keq to the pKa value of the corresponding
protonated base. The pKa values of trans-[Fe(PNHP)-
(dmpm)(CH3CN)2]3+ (9.4), trans-[Fe(PNHP)(dmpm)(Cl)-
(CO)]2+ (9.6), and trans-[HFe(PNHP)(dmpm)(CO)]2+
(10.5) in acetonitrile are similar to previously deter-
mined values for [HNi(PNHP)(PNP)]2+ (10.6), [Ni-
(PNHP)(dmpm)]3+ (8.7), and [Fe2{(SCH2)2N(H)Me}-
(CO)6]+ (7.6-10.6).14,23 The uncertainties indicated for
the equilibrium constants of the iron complexes are for
2 standard deviations based on 3-5 independent mea-
surements. The uncertainties in the pKa values include
the uncertainties in these equilibrium measurements,
as well as the uncertainty in the pKa values of the
protonated bases that are used as references ((0.1).
In the case of trans-[HFe(PNP)(dmpm)(CH3CN)]+,
protonation of the PNP ligand is followed by a slow loss
of H2 and the formation of trans-[Fe(PNP)(dmpm)(CH3-
CN)2]2+. The observed hydrogen loss from trans-[HFe-
(PNHP)(dmpm)(CH3CN)]2+ when anisidine is used as
a base under 1 atm of H2 can be used to calculate a
hydride donor ability of less than 61 kcal/mol for [HFe-
(PNP)(dmpm)(CH3CN)]+ (see the Supporting Informa-
tion for details).44 The hydride donor ability is the free
energy associated with the reaction of [HFe(PNP)-
(dmpm)(CH3CN)]+ to form [Fe(PNP)(dmpm)(CH3CN)2]2+
and H- in acetonitrile. Because coordination of aceto-
nitrile occurs in the course of this reaction, the solvent
plays an important role in determining the energetics
of this hydride transfer reaction. The hydride donor
ability of less than 61 kcal/mol for [HFe(PNP)(dmpm)-
(CH3CN)]+ compared to a value of 67 kcal/mol for [HNi-
(PNP)2]+ (also measured in acetonitrile) indicates that
(CH3CN)]+
(CO)]2+
Bond Distances
1.9186(18) Fe(1)-C(3)
Fe(1)-N(1)
Fe(1)-P(4)
Fe(1)-P(2)
Fe(1)-P(3)
Fe(1)-P(1)
Fe(1)-H(1)
N(1)-C(1)
1.795(7)
1.990(5)
2.297(2)
2.302(2)
2.321(2)
2.379(2)
1.118(8)
1.142(7)
2.2044(6)
2.2159(6)
2.2173(6)
2.2217(6)
1.53(2)
Fe(1)-N(1)
Fe(1)-P(3)
Fe(1)-P(2)
Fe(1)-P(4)
Fe(1)-P(1)
N(1)-C(1)
C(3)-O(1)
1.151(3)
Bond Angles
N(1)-Fe(1)-P(4)
N(1)-Fe(1)-P(2)
P(4)-Fe(1)-P(2)
N(1)-Fe(1)-P(3)
P(4)-Fe(1)-P(3)
P(2)-Fe(1)-P(3)
N(1)-Fe(1)-P(1)
P(4)-Fe(1)-P(1)
P(2)-Fe(1)-P(1)
P(3)-Fe(1)-P(1)
H(1)-Fe(1)-P(1)
H(1)-Fe(1)-P(2)
H(1)-Fe(1)-P(3)
H(1)-Fe(1)-P(4)
94.68(5)
93.78(5)
168.28(2)
89.63(5)
89.58(2)
98.58(2)
97.75(6)
97.18(2)
73.63(2)
169.52(2)
81.1(10)
83.9(9)
C(3)-Fe(1)-P(3)
87.7(2)
N(1)-Fe(1)-P(2) 88.7(2)
C(3)-Fe(1)-P(2)
87.1(2)
N(1)-Fe(1)-P(3) 173.4(2)
P(3)-Fe(1)-P(4)
P(3)-Fe(1)-P(2)
90.89(7)
95.42(7)
N(1)-Fe(1)-P(1) 87.93(17)
P(4)-Fe(1)-P(1)
P(2)-Fe(1)-P(1)
P(3)-Fe(1)-P(1)
C(3)-Fe(1)-P(1)
C(3)-Fe(1)-P(4)
94.02(7)
87.62(7)
97.36(8)
173.0(2)
90.7(2)
91.2(10)
87.5(9)
N(1)-Fe(1)-P(4) 84.8(2)
P(2)-Fe(1)-P(4)
173.22(7)
H(1)-Fe(1)-N(1) 177.7(10)
Fe(1)-N(1)-C(1)
C(3)-Fe(1)-N(1) 87.4(3)
174.49(17) C(1)-N(1)-Fe(1) 166.5(7)
N(1)-C(1)-C(2)
174.8(8)
the iron complex is at least a 6 kcal/mol better hydride
donor than the nickel complex.23
Structural Studies. A structural determination of
trans-[HFe(PNP)(dmpm)(CH3CN)](BPh4) was carried
out on crystals grown from an acetonitrile/ether solu-
tion. A drawing of the cation is shown on the left side
of Figure 3, and selected bond angles and bond distances
are given in Table 1. The hydride ligand and the
acetonitrile ligand occupy trans positions. The six-
membered ring formed by iron and the PNP ligand is
in the chair conformation, as observed in previous nickel
structures.23 The four-membered ring formed by iron
and dmpm is folded toward the hydride ligand with a
distance of 3.13 Å between the hydride ligand and the
nearest hydrogen atom of the methylene bridge of the
(43) Drago, R. S. Physical Methods for Chemists, 2nd ed.; Surfside
Scientific: Gainesville, FL, 1992; pp 290-295.
(44) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J. Am.
Chem. Soc. 2002, 124, 1918-1925.

Stereochemical Control of Fe(II) Complexes
Organometallics, Vol. 24, No. 10, 2005 2487
dmpm ligand. The Fe-P, Fe-N, and Fe-H distances
are all within ranges observed for similar low-spin
Fe(II) complexes.32-34 The phosphorus atoms of both
diphosphine ligands are slightly bent toward the hy-
dride ligand and away from acetonitrile with an average
N(1)-Fe(1)-P angle of 94.0°. The average N(1)-
Fe(1)-P bond angle is slightly larger for the dmpm
ligand (95.8°) than for the PNP ligand (92.2°). This may
be a reflection of the small dihedral angles between the
methyl groups of the dmpm ligand and the coordinated
acetonitrile ligand (i.e., N(1)-Fe(1)-P(2)-C(7) ) 2.3°
and N(1)-Fe(1)-P(1)-C(4) ) 10.2°). The P(1)-Fe(1)-
P(2) bond angle for the dmpm ligand (73.6°) is much
less than 90°, as expected, and the P(3)-Fe(1)-P(4)
angle of the PNP ligand (89.6°) is very close to the ideal
90°. These observed bond angles are similar to those
observed for [Ni(PNP)(dmpm)]2+: 73.8 and 94.2°, re-
spectively.20
An X-ray diffraction study of cis-[Fe(PNP)2(CH3CN)-
(CO)](BPh4)2 confirms the geometry assigned on the
basis of spectrosopic data for cation 4. A drawing of this
cation is shown on the right side of Figure 3, and
selected bond distances and angles are given in Table
1. The acetonitrile and carbonyl ligands occupy cis
positions with a N(1)-Fe(1)-C(3) angle of 87.4°. The
three angles defined by cis diethylphosphino groups not
constrained to a chelate ring, i.e., P(1)-Fe(1)-P(3),
P(1)-Fe(1)-P(4), and P(2)-Fe(1)-P(3), average 95.6°.
The two bond angles between phosphorus atoms within
a chelate ring, P(1)-Fe(1)-P(2) and P(3)-Fe(1)-P(4),
average 89.2°. Taken together, these bond angles sug-
gest that steric interactions between cis diethylphos-
phino groups not constrained to a chelate ring produce
a distortion of these groups toward the smaller CO and
acetonitrile ligands. In addition, the N(1)-Fe(1)-P(2)-
C(11) torsional angle is 5.5°, indicating that the aceto-
nitrile ligand and an ethyl group on P(2) are essentially
eclipsed. As a result, the P(2)-Fe(1)-N(1) angle (88.7°)
is slightly larger than the P(4)-Fe(1)-N(1) angle (84.8°),
and the acetonitrile ligand bends away from the ethyl
group on P(2) toward P(4); i.e., the Fe(1)-N(1)-C(1)
angle is 166.5°. A similar interaction occurs between
acetonitrile and C(4) (N(1)-Fe(1)-P(1)-C(4) ) 3.9°). A
somewhat less eclipsed form is observed for the CO
ligand and the axial substituents on P(3) and P(4)
(C(3)-Fe(1)-P(4)-C(22) ) 12.0° and C(3)-Fe(1)-P(3)-
C(15) ) 12.4°). The average Fe-P bond distance for
cation 4 is 2.32 Å. This distance is 0.11 Å longer than
the average Fe-P distance observed for the trans-[HFe-
(PNP)(dmpm)(CH3CN)]+ cation discussed in the preced-
ing paragraph. The Fe(1)-P(1) distance in 4 is at least
0.06 Å longer than the three remaining Fe-P bond
lengths, because of the greater trans influence of the
CO ligand. The Fe(1)-N(1) distance of 1.99 Å for [Fe-
(PNP)2(CH3CN)(CO)]2+ is also significantly longer than
the Fe(1)-N(1) distance of 1.92 Å for trans-[HFe(PNP)-
(dmpm)(CH3CN)]+. This larger bond distance is consis-
tent with the greater lability of the acetonitrile ligand
of cis-[Fe(PNP)2(CH3CN)(CO)]2+ compared to trans-
[HFe(PNP)(dmpm)(CH3CN)]+.
complexes.25,45,46 Therefore, the formation of cis-[Fe-
(PNP)2(CH3CN)2](BF4)2 from [Fe(CH3CN)6](BF4)2 and 2
equiv of PNP, as shown in step 1 of reaction 1, is
expected. The substitution of one ligand, as shown in
step 2, to form cis-[Fe(PNP)2(CH3CN)(CO)](BF4)2 indi-
cates that stepwise substitution of acetonitrile in this
complex may be a useful route to forming a series of cis
complexes containing a diphosphine ligand with a
pendant base. Further studies toward this end are in
progress in our laboratories. The cis geometry of [Fe-
(PNP)2(CH3CN)(CO)](BPh4)2 has been confirmed by an
X-ray diffraction study.
A more extensively studied class of dihydrogen com-
plexes are the trans-[(H2)Fe(diphosphine)2X]n+ com-
plexes, which are favored by the use of diphosphine
ligands that form four- or five-membered rings upon
coordination to the metal.32-35 To overcome the prefer-
ence of the PNP ligand to form cis complexes, diphos-
phine ligands with small chelate bite angles were
combined with PNP. This approach was suggested by
previous structural studies of [Ni(PNP)2]2+ and the
mixed-ligand species [Ni(PNP)(dmpm)]2+ 23
The former
.
has a large tetrahedral distortion, whereas the latter
cation is planar. It therefore seemed reasonable that this
mixed-ligand approach could be used to prepare trans-
[Fe(PNP)(dmpm)(X)(Y)]n+ complexes, which require a
planar arrangement of diphosphine ligands. As the
results described above for [Fe(PNP)(dmpm)(CH3CN)2]2+
,
[Fe(PNP)(dmpm)(CO)(Cl)]+, [HFe(PNP)(dmpm)(Cl)],
[HFe(PNP)(dmpm)(CO)]+, and [(H2)Fe(PNP)(dmpm)-
(H)]+ indicate, complexes with trans geometries that
incorporate the PNP ligand can be prepared using this
strategy. The geometry of one of these complexes, trans-
[HFe(PNP)(dmpm)(CH3CN)](BF4), has been confirmed
by X-ray diffraction.
Of major interest is the role of the pendant base of
the PNP ligand in the chemistry of these complexes. To
establish a baseline for the proton acceptor ability of
this ligand on coordination to iron, the pKa values of
the protonated PNHP+ ligand for trans-[Fe(PNHP)-
(dmpm)(CH3CN)2]3+ (9.4), trans-[Fe(PNHP)(dmpm)(Cl)-
(CO)]2+ (9.6), and trans-[HFe(PNHP)(dmpm)(CO)]2+
(10.5) were determined in acetonitrile. To facilitate
comparison on an aqueous scale, it has been suggested
that aqueous pKa values can be estimated by subtract-
ing 7.5 from pKa values determined in acetonitrile.47 The
observation that protonation of trans-[HFe(PNP)-
(dmpm)(CO)]+ occurs at the N atom of the PNP ligand
(see structure 12), and not at the hydride ligand to form
a dihydrogen complex, indicates that the incipient
dihydrogen ligand in this complex would be more acidic
than the protonated PNHP+ ligand. Therefore, this
(45) Lenero, K. A.; Kranenburg, M.; Guari, Y.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M.; Sabo-Etienne, S.; Chaudret, B. Inorg. Chem.
2003, 42, 2859-2866.
(46) Karsch, H. H. Chem. Ber. 1984, 117, 3123-3133.
(47) Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hy-
drides; Dedieu, A., Ed.; VCH: New York, 1991; pp 309-359.
Discussion
In general, octahedral complexes containing two
diphosphine ligands with large chelate bites form cis

2488 Organometallics, Vol. 24, No. 10, 2005
Henry et al.
dihydrogen complex has a pKa value of less than 10.5
in acetonitrile. In contrast, when a hydride ligand is
trans to the dihydrogen ligand in [trans-[(H2)Fe(PNP)-
(dmpm)(H)]+ (see structure 13), the dihydrogen ligand
is less acidic than the protonated PNHP+ ligand. As a
result, the dihydrogen complex with a trans hydride is
stable with respect to intramolecular heterolytic cleav-
age. The formation of [trans-[(H2)Fe(PNP)(dmpm)(H)]+
and trans-[HFe(PNHP)(dmpm)(CO)]2+ illustrates the
control of intramolecular heterolytic cleavage of the
dihydrogen ligand by varying the nature of the trans
ligand. It is possible that a similar effect may be
achieved in the Fe-only hydrogenases by the variation
of the CO ligand trans to the H2 activation site from a
bridging to a terminal position during the catalytic
cycle.15
difference of 102-103 for these cis and trans complexes.
The faster rate of exchange of acetonitrile for cis-[Fe-
(PNP)2(CH3CN)(CO)]2+ (t1/2 ) 34 min) compared to
trans-[HFe(PNP)(dmpm)(CH3CN)]+ (t1/2 ) 9.6 h) is
consistent with the longer Fe-N bond in the former
complex compared to the latter. The observation of steric
interactions between ethyl substituents on one PNP
ligand and coordinated acetonitrile for cis-[Fe(PNP)2-
(CH3CN)(CO)]2+ suggests steric interactions as well as
electronic features could be useful in tuning the rate of
ligand loss in the cis complexes. The observation that
acetonitrile substitution for [Fe(PNP)2(CH3CN)2]2+ is
faster than for cis-[Fe(PNP)2(CH3CN)(CO)]2+ is consis-
tent with acetonitrile being a better π donor (or at least
a poorer π acceptor) than CO. For (H2)OsHCl(CO)(Pi-
Pr3)2, the rate of H2 loss is reported to be nearly 106
times faster than for (H2)OsH2(CO)(PiPr3)2.39 In the
former, the chloride ligand is cis to the dihydrogen
ligand and can stabilize the five-coordinate intermediate
formed by hydrogen loss through back-bonding, whereas
in the latter complex, the cis hydride ligand is unable
to provide a stabilizing back-bonding contribution. A
more complete understanding of the kinetics of ligand
exchange for the iron complexes described in this work,
as well as the thermodynamic characteristics of the
Fe-H, Fe-(H2) and N-H bonds in these complexes, will
be important for further efforts to develop iron-based
catalysts for H2 production/oxidation.
For trans-[HFe(PNP)(dmpm)(CH3CN)]+, protonation
in acetonitrile leads to loss of H2 and the formation of
trans-[Fe(PNP)(dmpm)(CH3CN)2]2+. This reaction pre-
sumably involves the sequence of steps shown in reac-
tion 5. The dihydrogen intermediate trans-[(H2)Fe-
Summary
A number of Fe(II) complexes have been prepared
containing the PNP ligand, which has a pendant
nitrogen base similar to that thought to be present in
Fe-only hydrogenases. The stereochemistry of these
complexes can be controlled by the proper choice of a
second diphosphine ligand. When this ligand is PNP,
cis complexes are formed. If the second ligand is dmpm,
a trans geometry results. This combination of cis and
trans complexes provides an opportunity to systemati-
cally probe the role of cis and trans ligands in conjunc-
tion with a pendant base in H2 cleavage and proton-
transfer reactions. The acidity of the protonated PNP
ligand has been measured for three complexes with pKa
values ranging from 9.4 to 10.5 in acetonitrile. The
observation of trans-[HFe(PNHP)(dmpm)(CO)]2+ indi-
cates that intramolecular heterolytic cleavage of hydro-
gen is favored when CO occupies the trans position.
Conversely, the observation of trans-[(H2)Fe(PNHP)-
(dmpm)(H)]+ indicates that a trans hydride ligand
disfavors intramolecular heterolytic cleavage of H2 in
this series of complexes. The exchange of free and
coordinated acetonitrile ligands is faster for the cis
complexes compared to trans complexes and suggests
possible approaches to controlling solvent and H2 ex-
change.
(PNP)(dmpm)(CH3CN)]2+ (16) results from proton trans-
fer from the PNHP+ ligand of 15, as shown in step 2.
This complex is unstable with respect to loss of H2 and
coordination of acetonitrile (step 3). For the catalytic
production of hydrogen, it is desirable that the elimina-
tion of hydrogen be somewhat favorable to avoid product
inhibition. On the other hand, a large driving force will
lead to increased overpotentials for H2 production.
As reported previously, [Ni(PNP)2]2+ adds hydrogen
to form [HNi(PNHP)(PNP)]2+, with Ni acting as a
hydride acceptor and the N atom of one diphosphine
ligand acting as a proton acceptor. In contrast, trans-
[HFe(PNHP)(dmpm)(CH3CN)]2+ eliminates hydrogen.
Because the pKa values for the protonated PNP ligands
are the same within experimental error for these two
compounds, this difference in behavior can be attributed
to the greater hydride donor ability of trans-[HFe(PNP)-
(dmpm)(CH3CN)]+ (less than 61 kcal/mol) compared to
[HNi(PNP)2]+ (67 kcal/mol).
Although the development of a more complete series
of cis complexes is still in progress, our preliminary
comparisons of the cis and trans Fe(II) octahedral
complexes have already revealed significant reactivity
differences. The observation of a half-life of less than 1
min for the loss of acetonitrile from cis-[Fe(PNP)2(CH3-
CN)2]2+ in acetonitrile-d3, compared to a half-life of 7.2
h for [Fe(PNP)(dmpm)(CH3CN)2]2+, indicates a rate
Experimental Section
Spectral and Electrochemical Measurements. 1H NMR,
31P NMR, and variable-temperature NMR (VT NMR) spectra
1
were recorded on a Varian Inova 400 MHz spectrometer. H
chemical shifts are reported relative to residual solvent
protons. 31P NMR spectra were proton-decoupled unless stated
otherwise, and all chemical shifts are referenced to external
phosphoric acid. VT NMR experiments were allowed to equili-

Stereochemical Control of Fe(II) Complexes
Organometallics, Vol. 24, No. 10, 2005 2489
brate for 5 min at each temperature before spectra were
recorded. T1 measurements were made using the inversion-
recovery method. Spectral simulations were performed using
Nuts Software by Acron NMR Inc. Electrospray ionization
(ESI) mass spectra were collected using an HP 59987A
Electrospray with an HP 5989B mass spectrometer. Infrared
spectra of potassium bromide pellets and solution infrared
spectra were recorded on either a ThermoNicolet Avatar 360
FT-IR ESP spectrometer or a Mattson Satellite FTIR spec-
trometer. UV-vis spectra were recorded on an Agilent 8453
UV-vis spectrometer. Elemental analyses were performed by
Desert Analytics Laboratory, Tucson, AZ.
General Methods and Materials. All reactions were
performed using standard Schlenk techniques under argon or
handled in a glovebox under nitrogen, and all solvents were
degassed with argon. Acetonitrile, dichloromethane, ethanol,
hexanes, and aniline were dried over calcium hydride and
distilled prior to use. Ether, toluene, and tetrahydrofuran
(THF) were dried over sodium/benzophenone and distilled.
Acetone, acetone-d6, acetonitrile-d3, and toluene-d8 were dried
over 4 Å molecular sieves and degassed using three freeze-
pump-thaw cycles. Bis(triphenylphosphine)nitrogen(1+) boro-
hydride, bis(dimethylphosphino)methane, iron dichloride, p-
bromoaniline, 4-cyanoaniline, anisidine, 1,1,3,3-tetrameth-
ylguanidine, fluoroboric acid (48% aqueous solution), carbon
monoxide, and hydrogen were obtained from commercial
suppliers and used without further purification. Deuterium
(gas, 99.8%) and deuterium hydride (gas, 98%) were acquired
from Isotec. [Fe(CH3CN)6](BF4)2 and bis((diethylphosphino)-
methyl)methylamine (PNP) were prepared according to lit-
erature methods.23,48,49
2.01 (m) (16 H total, PCH2CH3); 1.10-1.50 (br m, 24 H,
PCH2CH3). IR (KBr; cm-1): νCO 2032 and 1981. IR (CD2Cl2;
cm-1): νCO 1991 cm-1
.
Attempted Preparation of [Fe(PNP)(dppm)(CH3CN)2]-
(BF4)2. PNP (20.2 µL, 0.070 mmol) was added, by syringe, to
a solution of [Fe(CH3CN)6](BF4)2 (33.3 mg, 0.070 mmol) in
acetonitrile-d3 (0.7 mL). After 30 min, this solution was added
to dppm (0.0268 g, 0.070 mmol), and the resulting solution
was monitored by 31P NMR for 6 days. A mixture of products
was observed at all times. The major components were
assigned to trans-[Fe(PNP)(dppm)(CH3CN)2](BF4)2 (an AA′XX′
spin pattern at 5.51 ppm (dppm) and 25.32 ppm (PNP)) and
cis-[Fe(PNP)2(CH3CN)2](BF4)2 (two second-order multiplets at
22.02 and 14.52 ppm as discussed above). Three other uni-
dentified singlets were observed at 12.5, 39.6, and 32.4 ppm.
[Fe(PNP)(dmpm)(CH3CN)2](BF4) (5). A solution of [Fe-
(CH3CN)6](BF4)2 (0.697 g, 1.46 mmol) in acetonitrile was added
to a solution of PNP (0.344 g, 1.46 mmol) and dmpm (0.232
mL, 1.46 mmol) in acetonitrile (total 50 mL), and the resulting
red-orange solution was stirred at room temperature for 1 h.
Removal of the solvent with a vacuum resulted in a tar. Red-
orange crystals were obtained by crystallization from a solvent
mixture of dichloromethane and ethanol (0.55 g, 55%). Anal.
Calcd for C20H47N3P4B2F8Fe: C, 35.17; H, 6.93; N, 6.15.
Found: C, 34.89; H, 6.98; N, 5.88. 1H NMR (CD3CN; ppm):
2.40 (s, 5.5 H, CH3CN); 3.59 (t, 2.4 H, PCH2P, 2JPH ) 11.2 Hz);
1.62 (t, 11.5 H, 2JPH ) 10.0 Hz, PCH3); 2.87 (s, 3.8 H, PCH2N);
2.50 (s, 2.9 H, NCH3); 1.80 (m, 8.1 H, PCH2CH3); 1.11 (m, 12.0
H, PCH2CH3). 31P NMR (CD3CN; ppm): δA -5.06 (dmpm); δX
2
2
2
29.15 (PNP); JAA′ ) 78.6 Hz, JAX ) 89.3 Hz, JAX′ ) -57.2,
2JXX′ ) 55.5 Hz. ESI+ (CH3CN; m/z): 514 {[Fe(PNP)(dmpm)-
(CH3CN)2](BF4)}+; 446 {[Fe(PNP)(dmpm)(CH3CN)2](F)}+. IR
[Fe(PNP)2(CH3CN)2](BF4)2 (3). A solution of [Fe(CH3CN)6]-
(BF4)2 (0.41 g, 0.86 mmol) in acetonitrile was added to a
solution of bis((diethylphosphino)methyl)methylamine (PNP,
0.40 g, 1.70 mmol) in acetonitrile (total 30 mL). The red-orange
solution was stirred at room temperature for 2 h. The volume
of the solution was reduced to approximately 10 mL by
applying a vacuum. Red crystals (0.36 g, 55%) were obtained
by addition of ether to this solution and cooling to -15 °C.
Anal. Calcd for C26H60N4P4B2F8Fe: C, 39.93; H, 7.73; N, 7.16.
Found: C, 39.72; H, 7.64; N, 7.15. 31P NMR (CD3CN; ppm):
(KBr; cm-1): νCN 2261, νBF ) 1051.
4
[Fe(Cl)2(PNP)(dmpm)] (6). A mixture of PNP (0.240 g,
1.02 mmol) and dmpm (0.160 g, 1.17 mmol) in 5 mL of toluene
was added to a suspension of FeCl2 (0.129 g, 1.02 mmol) in 60
mL of toluene, and the resulting green solution was stirred
for 3 h. Removal of the solvent with a vacuum resulted in an
oil. Emerald green crystals (0.215 g, 43%) were obtained by
crystallization from hexanes at -15 °C. Anal. Calcd for C16H41-
NP4Cl2Fe: C, 38.58; H, 8.30; N, 2.81. Found: C, 38.89; H, 8.32;
N, 2.88. NMR spectra were not observed due to the paramag-
netic nature of this compound. ESI+ (CH3CN; m/z): 462 [FeCl-
(PNP)(dmpm)]+. UV-vis (THF): λmax 411 nm, λmax 673 nm, ꢀ
2
2
2
δA 22.01, δB 14.50, JAA′ ) -26.7 Hz, JBB′ ) 49.6 Hz, JAB
)
2
43.2 Hz, JAB′ ) 67.2 (see structure 3 of the text for atom
labeling scheme). 1H NMR (CD3CN; ppm): 2.36 (m), 2.81 (m),
3.16 (m), 3.34 (m) (2 H each, PCH2N); 2.45 (s, 5.7 H, NCH3);
1.48 (m), 1.64 (m), 1.79 (m), 1.99 (m), 2.12 (m), 2.24 (m) (16 H
total, PCH2CH3); 1.07 (m), 1.15 (m), 1.23 (m) (24 H total,
PCH2CH3); 1.96 (s, 6.8 H, CH3CN). ESI+ (CH3CN; m/z): 695
{[Fe(PNP)2(CH3CN)2](BF4)}+, 613 {[Fe(PNP)2](BF4)}+, 545
{[Fe(PNP)2](F)}+. IR (KBr pellet; cm-1): νCN 2324 and 2290.
) 17 M-1 cm-1
.
[Fe(Cl)(CO)(PNP)(dmpm)]2(FeCl4) (7). A mixture of PNP
(0.624 g, 2.65 mmol) and dmpm (0.362 g, 2.66 mmol) in toluene
(10 mL) was added to a suspension of FeCl2 (0.337 g, 2.66
mmol) in toluene (100 mL), and the resulting emerald green
solution was stirred for 3 h. Carbon monoxide was bubbled
into this solution for 40 min. A yellow precipitate formed when
this solution was stirred overnight under a CO atmosphere.
Removal of the solvent with a vacuum resulted in a yellow
powder. Yellow-orange crystals (0.209 g, 20%) were obtained
by crystallization from warm acetone under CO. Anal. Calcd
for C34H82N2P8Cl6O2Fe3: C, 34.64; H, 7.01; N, 2.38. Found: C,
[Fe(PNP)2(CO)(CH3CN)](BF4)2 (4). Carbon monoxide was
bubbled into a solution of [Fe(PNP)2(CH3CN)2](BF4)2 (0.10 g,
0.13 mmol) in acetone (40 mL) for 20 min. The reaction flask
was closed, and then the solution was stirred overnight. The
volume of the solution was reduced to ca. 15 mL by applying
a vacuum, and then ether was added with stirring until the
solution began to get cloudy. Crystals formed upon cooling to
-15 °C for 2 weeks (0.035 g, 36%). Anal. Calcd for C25H57-
ON3P4B2F8Fe: C, 39.04; H, 7.47; N, 5.46. Found: C, 39.10; H,
7.26; N, 5.26. 31P NMR (acetone-d6; ppm): δZ 2.12 (ddd), δX
1
34.55; H, 7.35; N, 2.36. H NMR (CD3CN; ppm): 2.74 (m, 2.1
H, PCH2P); 0.88 (m), 0.94 (m) (15.2 H total, PCH3); 2.49 (m,
2.1 H, PCH2N); 2.29 (s, 3.1 H, NCH3); 1.63 (br m, 6.6 H, PCH2-
CH3); 0.03 ppm, 0.29 (br s, 12.0 H, PCH2CH3). 31P NMR (CD2-
Cl2; ppm): AA′XX′ spin pattern; δA -9.64 (dmpm), δX 29.46
2
11.62 (ddd), δM 16.68 (ddd), and δA 18.93 (ddd). JZX ) 46.2
(PNP); 2JAA′ ) 91.0 Hz, 2JAX ) 123.1 Hz, 2JAX′ ) -60.6, 2JXX′
)
2
2
2
2
Hz, JMZ ) 69.2 Hz, JMX ) 54.0, JAZ ) 48.9 Hz, JAX ) 62.2
59.2 Hz. ESI+ (CH3CN; m/z): 490 {[Fe(Cl)(CO)(PNP)-
(dmpm)]}+. IR (KBr; cm-1): νCO 1918 and 1923. IR (CH2Cl2;
cm-1): νCO 1936.
Hz, 2JAM ) -38.4 Hz (see structure 4 in text for atom-labeling
scheme). H NMR (acetone-d6; ppm): 3.16 (m), 3.40 (m), 3.60
1
(m), 3.78 (m), 4.15 (s) (8 H total, PCH2N); 2.70 (s, 3 H, CH3-
CN); 2.61 (s), 2.54 (s) (3 H each, NCH3); 2.47 (m), 2.26 (m),
[HFe(Cl)(PNP)(dmpm)] (8). A solution of [FeCl2(PNP)-
(dmpm)] (0.700 g, 1.40 mmol) in tetrahydrofuran (50 mL) was
added to a suspension of PPNBH4 (1.55 g, 2.80 mmol) in
tetrahydrofuran (100 mL). The resulting orange solution was
stirred for 2 h. The white precipitate was removed by filtration,
and the solvent was removed from the filtrate with a vacuum
(48) Hathaway, B. J.; Underhill, A. E. J. Chem. Soc. 1960, 3705-
3711.
(49) Heintz, P. A.; Smith, J. A.; Szalay, P. S.; Weisgerber, K. R.
Inorg. Synth. 2002, 33, 77.

2490 Organometallics, Vol. 24, No. 10, 2005
Henry et al.
to yield a red-orange solid. Orange feathery crystals were
obtained by dissolving this solid in hexanes and cooling at -15
°C overnight (0.205 g, 32%). The product was isolated by
filtration. Anal. Calcd for C16H42NP4ClFe: C, 41.44; H, 9.12;
N, 3.02; Cl, 7.64. Found: C, 41.33; H, 9.03; N, 3.06; Cl, 7.57.
31P NMR (toluene-d8; ppm): δA 3.96 (dmpm), δX 48.26 (PNP).
Observation of [HFe(H2)(PNP)(dmpm)]Cl. In a typical
experiment, acetone-d6 (0.7 mL) was cooled to 0 °C, degassed
with H2, and syringed into an NMR tube containing [HFe(Cl)-
(PNP)(dmpm)] (20 mg, 40 µmol), tetramethylguanidinium
triflate ([HTMG][OTf], 11.4 mg, 43 µmol), and H2 (1 atm). The
sample was kept at 0 °C for 1.5 h before the VT NMR
experiments were performed. 31P NMR (acetone-d6, -60 °C;
ppm): -6.65 (m, [HFe(H2)(PNP)(dmpm)]); 31.29 (m, [HFe(H2)-
(PNP)(dmpm)]); 4.3 (br, [HFe(Cl)(PNP)(dmpm)]); 48.7 (br,
[HFe(Cl)(PNP)(dmpm)]); 50.64 (d, impurity); 6.73 (br m,
impurity). 1H NMR (acetone-d6, -60 °C; ppm): -11.76 (pentet,
2
1H NMR (acetone-d6; ppm): -27.00 (pentet, JPH ) 47 Hz, 1
H, HFe); 1.06 (br m, 12 H, PCH2CH3); 1.50 (br m, 14.3 H,
PCH3); 1.67 (m), 1.79 (m), 1.89 (m) (7.1 H total, PCH2CH3);
2.33 (s, 3.0 H, NCH3); 2.69 (m), 2.68 (m) (2 H each, PCH2N);
3.02 (m), 3.30 (m) (1 H each, PCH2P). ESI+ (CH3CN; m/z): 428
{[HFe(PNP)(dmpm)]}+, 462 {[Fe(Cl)(PNP)(dmpm)]}+, 469
{[HFe(CH3CN)(PNP)(dmpm)]}+. IR (KBr; cm-1): νFeH 1793.
2
1.0 H, JPH ) 46 Hz, [HFe(H2)(PNP)(dmpm)]+); -7.90 (br s,
2.0 H, [HFe(H2)(PNP)(dmpm)]+). T1 data for [HFe(H2)(PNP)-
(dmpm)]+ (400 MHz): 0.346 s at 183 K, 0.317 s at 193 K,
0.0957 s at 213 K, 0.0707 s at 233 K, 0.0872 s at 253 K. T1
data for [HFe(H2)(PNP)(dmpm)]+: 0.0207 s at 183 K, 0.0229
s at 193 K, 0.0377 s at 213 K, 0.0639 s at 233 K, 0.0841 s at
[HFe(CH3CN)(PNP)(dmpm)](BPh4) (9). [HFeCl(PNP)-
(dmpm)] (0.207 g, 0.446 mmol) and NaBPh4 (1.408 g, 4.11
mmol) were added to a flask containing acetonitrile (150 mL).
The resulting yellow solution was stirred for 2 h, and the
product was precipitated by addition of water to form a solid
that was washed with ether. Yellow crystals (0.077 g, 23%)
were obtained from acetonitrile/ether solutions at -15 °C.
Anal. Calcd for C42H65N2P4BFe: C, 63.97; H, 8.31; N, 3.55.
Found: C, 63.22; H, 8.05; N, 3.33. 31P NMR (CD3CN; ppm):
AA′XX′ spin pattern; δA 3.32 (dmpm), δX 44.86 (PNP). 1H NMR
(CD3CN; ppm): -19.85 (ttd, 2JPH ) 49 Hz, 2JPH ) 43 Hz, 4JHH
) 4 Hz, 1 H, HFe); 1.05 (br m, 12.5 H, PCH2CH3); 1.45 (t),
1.53 (t) (2JPH ) 4 Hz, 12.3 H total, PCH3); 1.49 (m), 1.60 (m),
1.69 (m) (8.3 H total, PCH2CH3); 2.33 (s, 3.0 H, NCH3); 2.35
(m), 2.81 (m) (2 H each, PCH2N); 2.99 (m), 3.14 (m) (1 H each,
PCH2P); 2.22 (s, 3.0 H, NCCH3); 6.85 (m), 6.99 (m), 7.27 (m)
(20.1 H total, B(C6H5)4). Broadband 31P-decoupled 1H NMR
253 K, 0.0986 s at 273 K. A plot of ln T1 versus 1/K is shown
min
in Figure S3 in the Supporting Information. Using T1
)
-
0.0207 s and the equations dHH(fast) ) 4.61[T1min/ν]1/6 and dHH
(slow) ) 5.81[T1min/ν]1/6 gives dHH values of 0.89 and 1.12 Å
for fast and slow rotation, respectively.36-38 In these equations,
d is in Å, T1min is in s, and the spectrometer frequency, ν, is in
MHz.
1JHD Experiments. Acetone saturated with HD was sy-
ringed into an NMR tube containing [HFe(Cl)(PNP)(dmpm)]
(20 mg, 40 µmol), (DTMG)(OTf) (11.4 mg, 43 µmol), and HD
gas (1 atm). An additional quantity of HD gas (approximately
3 mL) was added with a syringe. The sample was maintained
at 0 °C for 1.5 h before the VT NMR experiments were run.
2
(CD3CN; ppm): -19.85 (d, JHH ) 4 Hz, HFe); 2.89 and 3.15
1
2H NMR (acetone, -60 °C): -8.01 (d, JHD ) 30.8 Hz, [(HFe-
2
4
(AB portion of ABX spin pattern, JHH ) 13 Hz, JHH ) 4 Hz,
(HD)(PNP)(dmpm)]+). This doublet collapsed to a singlet when
PCH2P). Broadband 31P-decoupled H gCOSY (CD3CN; ppm;
1
the spectrum was proton decoupled (all the other peaks
see Figure S2): -19.85 (HFe) correlated to 3.15 (one H of
PCH2P). Selective 31P-decoupled (PNP) 1H NMR (CD3CN;
ppm): -19.85 (br t, 2JPH ) 49 Hz, HFe). Selective 31P-decoupled
1
remained the same). Using this JHD value and dHH ) 1.42-
0.0167(JHD), a value of 0.90 Å is calculated for the HH bond
distance.41 Using dHH ) 1.44 - 0.0168JHD, a value of 0.92 Å is
calculated for the HH bond distance.40
1
2
(dmpm) H NMR (CD3CN; ppm): -19.85 (br t, JPH ) 43 Hz,
HFe). ESI+ (CH3CN; m/z): 428 {[HFe(PNP)(dmpm)]}+. IR
(KBr; cm-1): νFeH 1826; νCN ) 2230.
pKa Measurements. In a typical experiment, [Fe(Cl)(CO)-
(PNP)(dmpm)]+ (25 mg, 0.021 mmol), p-bromoanilinium
tetrafluoroborate (55 mg, 0.21 mmol), and p-bromoaniline (36
mg, 0.21 mmol) were weighed into an NMR tube and dissolved
in CD3CN (0.7 mL). The solution was monitored by 31P NMR
for several hours at 23 ( 2 °C to ensure that the 31P NMR
shifts were constant; however, equilibrium occurred in less
than 0.5 h. This experiment was repeated using 5-, 10-, and
15-fold excesses of 1:1 mixtures of p-bromoanilinium tetra-
fluoroborate/p-bromoaniline (BH+/B) based on the metal com-
plex. The ratio of [Fe(CO)(Cl)(PNHP)(dmpm)]2+/[Fe(CO)(Cl)-
(PNP)(dmpm)]+, (1 - x)/x, was calculated using the observed
31P NMR shift of the PNP ligand, δ(PNP), and the relationship
x(29.08 ppm) + (1 - x)(37.39 ppm) ) δ(PNP). In this equation,
x is the mole fraction of [Fe(CO)Cl)(PNP)(dmpm)]+ and 1 - x
is the mole fraction of [Fe(CO)(Cl)(PNHP)(dmpm)]2+. With no
acid added, the 31P NMR chemical shift observed for the PNP
ligand of [Fe(CO)(Cl)(PNP)(dmpm)]+ is 29.08 ppm. The shift
of the completely protonated PNP ligand, observed when 3
equiv of HBF4 was added to form [Fe(CO)(Cl)(PNHP)(d-
mpm)]2+, is 37.39 ppm. The ratio of [B]/[BH+] was held
constant at 1.0 using large excesses of both B and BH+. The
equilibrium constant (Keq(4) ) {[B]/[BH+]} × {[Fe(CO)(Cl)-
(PNHP)(dmpm)]2+}/{[Fe(CO)(Cl)(PNP)(dmpm)]+}) calculated
for three experiments was 1.1 ( 0.2 (where the uncertainty
represents two standard deviations). Addition of log Keq(4) to
the pKa value of p-bromoanilinium (9.6) in acetonitrile40 gives
a pKa value of 9.6 ( 0.2 for [Fe(CO)(Cl)(PNHP)(dmpm)]2+. The
uncertainty in the pKa value reflects uncertainties in both the
measurement ((0.1 at 2σ) and the reference base ((0.1).
[HFe(CO)(PNP)(dmpm)](PF6)/[Fe(Cl)(CO)(PNP)-
(dmpm)](PF6), (10-PF6/7-PF6) Mixture. Acetone (60 mL)
was added to a mixture of [HFeCl(PNP)(dmpm)] (0.110 g, 0.237
mmol) and NaPF6 (0.130 g, 0.774 mmol) in a Schlenk flask.
The resulting solution was purged with CO for 20 min and
then stirred under a CO atmosphere overnight. The solvent
was removed under vacuum to give a yellow oil that was
extracted with CH2Cl2 (2 × 20 mL). Ether was added to the
combined extracts. Yellow crystals (0.050 g) formed upon
cooling overnight to -15 °C. The following data indicated a
mixture of products. Similar mixtures were also obtained in
NMR tube experiments. 31P NMR (acetone-d6; ppm): -143.15
(septet, PF6), -7.74 (AA′XX′ spin pattern, 17%, [Fe(Cl)(CO)-
(PNP)(dmpm)](PF6)); 28.91 (AA′XX′ spin pattern, 18%, [Fe-
(Cl)(CO)(PNP)(dmpm)](PF6)); 2.34 (AA′XX′ spin pattern, 30%,
[HFe(CO)(PNP)(dmpm)](PF6)); 44.29 (AA′XX′ spin pattern,
30%, [HFe(CO)(PNP)(dmpm)](PF6)). 31P gCOSY (acetone-d6;
ppm): -7.74 ([Fe(Cl)(CO)(PNP)(dmpm)](PF6)) correlated to
28.91 ([Fe(Cl)(CO)(PNP)(dmpm)](PF6)); 2.34 ([HFe(CO)(PNP)-
(dmpm)](PF6)) correlated to 44.29 ([HFe(CO)(PNP)(dmpm)]-
1
2
(PF6)). H NMR (acetone-d6; ppm): -7.06 (ttd, JPH ) 52 Hz,
4
2JPH ) 42 Hz, JHH ) 3.5 Hz, 1.3 H, HFe (10)); 1.09-1.31 (m,
24 H, PCH2CH3 (7 and 10)); 1.75 (m), 1.81 (m) (19.6 H total,
PCH3 (7 and 10)); 1.52 (m), 1.89 (m), 1.97 (m), 2.10 (m) (16.2
H, PCH2CH3 (7 and 10)); 2.39 (s), 2.48 (s) (3.56 and 2.28 H,
respectively, NCH3 (7 and 10)); 2.32 (m), 3.06 (m) (PCH2N (7
and 10)); 3.40 (m, PCH2P (7 and 10)). Broadband 31P-decoupled
1H NMR (acetone-d6; ppm): -7.05 (d, 4JHH ) 3.5 Hz, HFe (10));
3.40 and 3.46 (AB portion of ABX spin pattern, 2JHH ) 15 Hz,
4JHH ) 3.5 Hz, PCH2P (10)). Broadband 31P-decoupled 1H
gCOSY (acetone-d6): -7.05 (HFe) correlated to 3.41 (one H of
PCH2P).
A similar procedure was used to determine the pKa values
shown below reaction 4 for [Fe(PNHP)(dmpm)(CH3CN)2]3+ and
[HFe(PNHP)(dmpm)(CO)]2+ cations. In the latter case, a

Stereochemical Control of Fe(II) Complexes
Organometallics, Vol. 24, No. 10, 2005 2491
mixture of [HFe(PNP)(dmpm)(CO)]+ and [Fe(Cl)(CO)(PNP)-
(dmpm)]+ was used. This mixture was prepared as described
above.
Table 2. Crystal Data and Structure Refinement
for [(H)Fe(PNP)(dmpm)(CH3CN)]BPh4 and
[Fe(PNP)2(CH3CN)(CO)](BPh4)2
Determination of t1/2 for Acetonitrile Loss. [Fe(PNP)2-
(CH3CN)(CO)](BPh4)2 (approximately 20 mg) was dissolved in
0.7 mL of acetonitrile-d3 at 23 ( 2 °C, and the normalized
integral of the acetonitrile resonance at 2.37 ppm in the proton
NMR was followed as a function of time. A first-order plot of
these data is shown in Figure S1(a) in the Supporting
Information. The half-life obtained in this manner was 20 min.
A similar procedure was used to determine half-lives of less
than 1 min for [Fe(PNP)2(CH3CN)2](BF4)2, 7.2 h for [Fe(PNP)-
(dmpm)(CH3CN)2](BF4)2, and 9.6 h for [HFe(PNP)(dmpm)(CH3-
CN)](BPh4) (Figure S1(b) in the Supporting Information).
Similar experiments were also performed for [Fe(PNP)2(CH3-
CN)(CO)](BPh4)2 (t1/2 ) 34 min) and [Fe(PNP)2(CH3CN)2](BF4)2
(t1/2 ≈ 1 min) using acetone-d6 as the solvent by addition of a
10-fold excess of CD3CN.
[(H)Fe(PNP)(dmpm)- [Fe(PNP)2(CH3CN)2]-
(CH3CN)]BPh4
(BPh4)2
formula
formula wt
cryst syst
a (Å)
C44H68BFeN3P4
829.55
C73H97B2FeN3OP4
1233.89
monoclinic
21.873(10)
13.629(6)
24.754(11)
90
109.824(10)
90
6942(6)
P21/n (No. 14)
4
1.181
0.71073
triclinic
11.7581(5)
12.0235(6)
17.3035(8)
105.7990(10)
95.9700(10)
98.8340(10)
2298.40(18)
P1h (No. 2)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
space group
Z
calcd density (g/cm3) 1.199
λ(Mo KR) (Å)
temp (K)
0.71073
153
154
X-ray Diffraction Studies of [HFe(CH3CN)(PNP)-
(dmpm)]BPh4. Crystals suitable for X-ray diffraction studies
were grown from an acetonitrile/ether mixture. A crystal of
dimensions 0.97 × 0.49 × 0.25 mm was mounted on a glass
fiber using Paratone-N oil, transferred to a Siemens SMART
diffractometer/CCD area detector, centered in the beam (Mo
KR; λ ) 0.71073 Å; graphite monochromator), and cooled to
-127 °C by a nitrogen low-temperature apparatus that had
been previously calibrated by a thermocouple placed at the
same position as the crystal. Preliminary orientation matrix
and cell constants were determined by collection of 60 10-s
frames, followed by spot integration and least-squares refine-
ment. A minimum of a hemisphere of data was collected using
0.3° $ scans at 30 s per frame. The raw data were integrated
and the unit cell parameters refined using Saint. Data analysis
was performed using XPREP. Absorption correction was
applied using SADABS. The data were corrected for Lorentz
and polarization effects, but no correction for crystal decay was
applied. Structure solutions and refinements were performed
(SHELXTL-Plus V5.0) on F2.
Preliminary data indicated a triclinic cell. Systematic
absences indicated space group P1h (No. 2). The choice of the
centric space group was supported by the successful solution
and refinement of the structure. All non-H atoms were refined
anisotropically. H atoms were placed in idealized positions and
were included in structure factor calculations but were not
refined (with the exception of the hydride ligand). Crystal-
lographic data and structure refinement data for [(H)Fe(PNP)-
(dmpm)(CH3CN)]BPh4 are shown in Table 2.
scan type, deg
θ range (deg)
no. of indep rflns
ω scans, 0.3
1.79-27.48
17 440 (R(int) )
0.0468)
ω scans, 0.3
1.73-27.48
15 911 (R(int) )
0.3327)
no. of rflns obsd
10 414
53 474
abs coeff (mm-1
)
0.500
0.353
R1 (for Fo > 4σFo)a
R1, wR2 (all data)b
GOFc
0.0540 for 8537 data
0.0658, 0.1515
1.016
0.1074 for 5699 data
0.2638, 0.3062
0.896
largest peak,
0.864, -0.667
1.233, -1.421
hole (e/Å-3
)
a R ) R1 ) ∑||Fo| - |Fc||/∑|Fo|. b Rw )wR2 ) [∑[w(Fo2 - Fc )2]/
2
∑[w(Fo2)2]]1/2
.
c GOF ) S ) [∑[w(Fo2 - Fc )2]/(M - N)]1/2, where M
2
is the number of reflections and N is the number of parameters
refined.
data and structure refinement data for [Fe(PNP)2(CO)(CH3-
CN)](BPh4)2 are shown in Table 2.
Acknowledgment. This research was supported by
the National Science Foundation (Grant No. CHE-
0240106). NMR instrumentation used in this work was
supported in part by the National Science Foundation
CRIF program (Grant No. CHE-0131003). D.L.D. wishes
to acknowledge the support of the United States De-
partment of Energy, Office of Science, Chemical and
Biological Sciences Division under Contract No. DE-
AC36-99GO10337.
Supporting Information Available: Figures giving
X-ray Diffraction Studies of [Fe(PNP)2(CH3CN)(CO)]-
(BPh4)2. Anion exchange between [Fe(PNP)2(CO)(CH3CN)]-
(BF4)2 and Na(BPh4) was used to produce the [Fe(PNP)2(CO)-
(CH3CN)](BPh4)2 salt. Crystals suitable for X-ray diffraction
studies were grown from an acetone/ether mixture. A crystal
of dimensions 0.45 × 0.5 × 0.55 mm was mounted on a glass
fiber, and preliminary orientation matrix and cell constants
were determined as described above. These data indicated a
monoclinic cell, and the P21/n space group was assigned. The
structure was refined as described in the preceding paragraph
for [HFe(CH3CN)(PNP)(dmpm)]BPh4, and the crystallographic
kinetic plots for ligand loss, the broadband 31P-decoupled H
1
gCOSY NMR spectrum of [HFe(PNP)(dmpm)(CH3CN)](BPh4),
T1 plots for [(H2)Fe(PNP)(dmpm)(H)]+, and a thermodynamic
cycle and X-ray structural data, including tables of crystal and
refinement data, atomic positional and thermal parameters,
and interatomic distances and angles for [HFe(PNP)(dmpm)-
CH3CN)](BPh4) and [Fe(PNP)2(CH3CN)(CO)](BPh4)2; X-ray
data are also given as CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM050071C