Precursors to water-soluble dinitrogen carriers. Synthesis of water-soluble complexes of iron(II) containing water-soluble chelating phosphine ligands of the type 1,2-bis(bis(hydroxyalkyl)phosphino)ethane

Article abstract of DOI:10.1021/ic025774q

The reactions of the water-soluble chelating phosphines 1,2-bis(bis(hydroxyalkyl)phosphino)ethane (alkyl = n-propyl, DHPrPE; n-butyl, DHBuPE; n-pentyl, DHPePE) with FeCl₂·4H₂O and FeSO₄·7H₂O were studied as rout

Full text of DOI:10.1021/ic025774q

Inorg. Chem. 2002, 41, 5453−5465
Precursors to Water-Soluble Dinitrogen Carriers. Synthesis of
Water-Soluble Complexes of Iron(II) Containing Water-Soluble
Chelating Phosphine Ligands of the Type
1,2-Bis(bis(hydroxyalkyl)phosphino)ethane
Warren K. Miller,† John D. Gilbertson,‡ Carmen Leiva-Paredes,‡ Paul R. Bernatis,†
,†
,‡
Timothy J. R. Weakley,‡ David K. Lyon,* and David R. Tyler*
Bend Research, Inc., 64550 Research Road, Bend, Oregon 97701, and Department of Chemistry,
UniVersity of Oregon, Eugene, Oregon 97403
Received June 5, 2002
The reactions of the water-soluble chelating phosphines 1,2-bis(bis(hydroxyalkyl)phosphino)ethane (alkyl ) n-propyl,
DHPrPE; n-butyl, DHBuPE; n-pentyl, DHPePE) with FeCl2‚4H O and FeSO ‚7H O were studied as routes to water-
2
4
2
soluble complexes that will bind small molecules, dinitrogen in particular. The products that form and their
stereochemistry depend on the solvent, the counteranion, and the alkyl chain length on the phosphine. In alcoholic
solvents, the reaction of FeCl2‚4H O with 2 equiv of DHBuPE or DHPePE gave trans-Fe(L2)2Cl2. The analogous
2
reactions in water with DHBuPE and DHPePE gave only cis products, and the reaction of FeSO ‚7H O with any
4
2
of the phosphines gave only cis-Fe(L2)2SO . These results are interpreted as follows. The trans stereochemistry of
4
the products from the reactions of FeCl2‚4H O in alcohols is suggested to be the consequence of the trans geometry
2
of the Fe(H O)4Cl2 complex, i.e., substitution of the water molecules by the phosphines retains the geometry of the
2
starting material. The formation of cis-Fe(DHPrPE)2Cl2 is an exception to this result because the coordination of
two −OH groups forms two six-membered rings, as shown in the X-ray structure of the molecule. DHBuPE and
DHPePE reacted with FeSO ‚7H O in water to initially yield cis-Fe(P ) SO compounds, but subsequent substitution
4
2
2 2
4
reactions occurred over several hours to give sequentially trans-Fe(DHBuPE)2(H O)(SO ) and then trans-[Fe-
2
4
(DHBuPE)2(H O)2]SO . The rate constants and activation reactions for these aquation reactions were determined
2
4
and are consistent with dissociatively activated mechanisms. The cis- and trans-Fe(L ) X(X) (Cl)2 or SO ) complexes
2 2
4
react with N , CO, and CH CN to yield trans complexes with bound N , CO, or CH CN. The crystal structures of
2
3
2
3
the cis-Fe(DHPrPE)2SO , trans-Fe(DHPrPE)2(CO)SO , trans-Fe(DHBuPE)2Cl2, trans-[Fe(DHBuPE)2(CO)(Cl)][B(C H ) ],
4
4
6 5 4
trans-Fe(DMeOPrPE)2Cl2, trans-Fe(DMeOPrPE)2Br2, and trans-[Fe(DHBuPE)2Cl2]Cl complexes are reported. As
expected from using water-soluble phosphines, the complexes reported herein are water soluble (generally greater
than 0.5 M at 23 °C).
Introduction
it must have a high selectivity for N2 over CH4. In a
homogeneous process, high selectivity for N2 over CH4
requires that the transition metal complex bind N2 strongly
and have high solubility in a solvent in which CH4 is poorly
The ability of iron-diphosphine complexes to coordinate
1
N2 suggests they may be useful compounds for separating
N2 from nitrogen-containing natural gas streams.2 For a
transition metal complex to be successfully deployed in a
nitrogen-removal process, it must exhibit reversible N2
absorption and desorption at process-related pressures and
(1) For overviews of this research area, see: (a) Henderson, R. A.; Leigh,
G. J.; Pickett, C. J. AdV. Inorg. Chem. Radiochem. 1987, 23, 197. (b)
Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115. (c) Leigh, G. J.
Sci. Prog. (Oxford) 1989, 291, 389-412. (d) Fryzuk, M. D.; Johnson,
S. A. Coord. Chem. ReV. 2000, 200-202, 379-409. (e) Bazhenova,
T. A.; Shilov, A. E. Coord. Chem. ReV. 1995, 144, 69-145. (f) Poveda,
A.; Perilla, I. C.; Perez, C. R. J. Coord. Chem. 200, 54, 427-440. (g)
Hidai, M.; Mizobe, Y. Top. Organomet. Chem. 1999, 3, 227-241.
(h) Shilov, A. E. New J. Chem. 1992, 16, 213-18.
* Author to whom correspondence should be addressed. D.R.T.: E-mail
dtyler@oregon.uoregon.edu.
† Bend Research, Inc..
‡ University of Oregon.
10.1021/ic025774q CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/26/2002
Inorganic Chemistry, Vol. 41, No. 21, 2002 5453

Miller et al.
Iron(II) chloride tetrahydrate (FeCl2‚4H2O) and iron(II) sulfate
heptahydrate (FeSO4‚7H2O) were obtained from Aldrich. Reagent
grade methanol, ethanol, and acetonitrile were deoxygenated with
an argon purge before being brought into the glovebox. The
anhydrous Na2SO4 (Aldrich) was used as received. CO was obtained
from Linde.
soluble. Water is an ideal solvent for such a process.
However, the known iron-diphosphine complexes are water
insoluble, and it is necessary to modify them accordingly.
The method we chose to impart water solubility was to use
phosphines that are water soluble. Sulfonated phosphines are
often used in this capacity,3,4 but we found that sulfonate
groups are often non-innocent and can affect the ability of
the iron compounds to react with N2 or other small
molecules. For example, when sulfonated phosphines were
used for the chemistry reported herein, N2 binding was
greatly decreased due to competitive binding of the sulfonate
group to the iron center.4d To avoid this problem, we decided
to focus on phosphine ligands that have hydroxyl groups as
the water-solubilizing entity. Examples of the phosphines
that have been developed are the 1,2-bis(bis(hydroxyalkyl)-
phosphino)ethane ligands and the related methoxy derivative,
shown below.4
Instrumentation and Procedures. 31P{1H} NMR were run on
either a VARIAN GEMINI 2000 NMR spectrometer (at BRI) at
121.47 MHz and referenced externally to 1% H3PO4 or on a Varian
Unity/Inova 300 spectrometer (at UO) at an operating frequency
of 299.95 and 121.42 MHz for 1H and 31P nuclei, respectively. The
1H and 31P{1H} NMR obtained on the latter instrument were
referenced to the solvent peak and to an external standard of 1%
H3PO4 in D2O, respectively. The samples were sealed under argon
in 5 mm tubes fitted with Teflon valves. Infrared spectra were
recorded on a Perkin-Elmer Paragon 1000 FT infrared spectropho-
tometer (at BRI) or on a Nicolet Magna 550 FT-IR spectrometer
(at UO) with OMNIC software. Samples were prepared as either
Nujol mulls with NaCl or AgCl windows or in solution with CaF2
or ZnSe cells. UV-vis spectra were recorded with a Perkin-Elmer
Lambda 6 spectrophotometer. Elemental analyses were performed
by E+R Microanalytical Laboratory, Inc., Corona, NY.
Kinetics Studies. All of the kinetics experiments were performed
in H2O in a thermostated cell. The water was purified to a resistivity
of 17-18 MΩ‚cm with a Barnstead Nanopure II system. The water
was bubbled with oxygen-free N2 for 1 h prior to use. The reactions
were monitored by recording the disappearance of cis-Fe(DHBuPE)2-
(SO4) at λmax ) 500 nm. The kinetics experiments in the presence
of Na2SO4 were carried out at 293 K. Values of the rate constants
reported are an average of at least three individual measurements.
In this paper, we report the synthesis and characterization
of the complexes formed in the reactions of these ligands
with FeCl2‚4H2O, and we compare these compounds to the
analogous complexes that have water-insoluble chelating
phosphine ligands such as depe (1,2-bis(diethylphos-
phino)ethane). In addition, we report the reactions of FeSO4‚
7H2O with the 1,2-bis(bis(hydroxyalkyl)phosphino)ethane
ligands. The reactions of these complexes with N2, CO, and
CH3CN are also discussed, and the crystal structures of the
cis-Fe(DHPrPE)2SO4, trans-Fe(DHPrPE)2(CO)SO4, trans-Fe-
(DHBuPE)2Cl2, trans-[Fe(DHBuPE)2(CO)(Cl)][B(C6H5)4],
trans-Fe(DMeOPrPE)2Cl2, trans-Fe(DMeOPrPE)2Br2, and
trans-[Fe(DHBuPE)2Cl2]Cl complexes are reported. As part
of this investigation, we also studied the effect of solvent
and counterion on the products obtained in the preparatory
reactions. These investigations led to the unexpected and
previously unreported results that the transition metal
compound stereochemistry is dependent on solvent, coun-
teranion, and alkyl chain length on the phosphine.
X-ray Structural Analyses. Crystals of 1, 5, 8, 9, and 10 for
X-ray work were manipulated under hydrocarbon grease and sealed
in special glass capillaries in the glovebox. The data crystals of 2
and 4, which remained stable during data collection, were coated
with epoxy resin and mounted on glass fibers. Cell dimensions and
orientation matrices were determined from the setting angles of an
Enraf-Nonius CAD-4 diffractometer for 25 centered reflections in
the following θ ranges: 1, 15-16°; 8, 14-15°; 2, 13-14°; 9, 12-
14°; 4, 14-15°; 5, 14-15°; and 10, 14-15°. Table 1 contains a
summary of crystal data and the final residuals; fuller tables with
particulars of data collection and structure refinement are in the
Supporting Information. Data were collected to θ 25° for 1, 2, 4,
5, and 10 and to θ 22.5° for 8. Because of crystal decay, data for
the weakly diffracting 9 were collected from two crystals, ranges
1.5-20° and 20-23° θ, and the data sets were corrected for decay
before they were combined. Structure solutions were obtained from
SIR92 E-maps.6 Small absorption corrections were applied to the
Experimental Section
Materials and Reagents. Unless otherwise noted, all manipula-
tions were carried out in an argon-filled Vacuum Atmospheres Co.
glovebox or on a Schlenk line with nitrogen. The 1,2-bis(bis-
(hydroxyalkyl)phosphino)ethane ligands were prepared as reported
previously.4b 1,2-Bis(diethylphosphino)ethane (depe) was obtained
from Strem Chemical Co. and used without further purification.
(5) The 1H NMR spectra of the molecules reported herein were relatively
broad and generally similar to the spectra of the uncoordinated
phosphine ligand. Therefore, they were nondiagnostic in terms of
checking for either purity of the sample or the identity of the molecule.
In contrast, the 31P{1H} NMR spectra of the molecules were simple,
had no overlapping peaks, and were spread over a wide range of
chemical shifts. Thus, they provided an excellent means for character-
izing and identifying the products and for checking their purity. For
(2) Lyon, D. K. Fe Phosphine Complexes for N2 Removal from Natural
Gas, U.S. Patent 5 225 174, 1993.
(3) See, for example: Cornils, B.; Wiebus, G. ChemTech 1995, January,
33.
(4) (a) Nieckarz, G. F.; Weakley, T. J. R.; Miller, W. K.; Miller, B. E.;
Lyon, D. K.; Tyler, D. R. Inorg. Chem. 1996, 35, 1721-1724. (b)
Baxley, G. T.; Miller, W. K.; Lyon, D. K.; Miller, B. E.; Nieckarz,
G. F.; Weakley, T. J. R.; Tyler, D. R. Inorg. Chem. 1996, 35, 6688-
6693. (c) Baxley, G. T.; Weakley, T. J. R.; Miller, W. K.; Lyon, D.
K.; Tyler, D. R. J. Mol. Catal. A 1997, 116, 191-198. (d) Miller, W.
K.; Lyon, D. K.; Tyler, D. R. Unpublished observations.
1
the sake of completeness, the H NMR spectra of the complexes are
1
reported in the Experimental Section. For comparison, the H NMR
spectra of the ligands are as follows: DHPrPE (MeOH), 1.52 (m, br,
8 H, 4 × P-CH2), 1.57 (m, br, 4 H, P-CH2CH2-P), 1.64 (m, br, 8
H, 4 × CH2), 3.61 (t, 8 H, 4 × CH2-OH), 4.8 (s, OH); DHBuPE
(D2O), 1.47 (m, br, 20 H), 1.59 (m, br, 8 H), 3.54 (t, 8 H, 4 × CH2-
OH), 4.80 (s, OH); DHPePE (MeOH), 1.38 (m, br, 28 H), 1.47 (m,
br, 8 H), 3.46 (t, 8 H, 4 × CH2OH), 4.8 (OH).
(6) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Cuagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, N. J. Appl. Crystallogr. 1994, 27, 435.
5454 Inorganic Chemistry, Vol. 41, No. 21, 2002

Synthesis of Water-Soluble Complexes of Iron(II)
Table 1. Crystallographic Data for trans-Fe(DHBuPE)2Cl2 (1), cis-Fe(DHPrPE)2SO4 (2), trans-Fe(DMeOPrPE)2Cl2 (4), trans-Fe(DMeOPrPE)2Br2 (5),
trans-[Fe(DHBuPE)2(CO)(Cl)][B(C6H5)4] (8), trans-Fe(DHPrPE)2(CO)SO4 (9), and trans-[Fe(DHBuPE)2Cl2]Cl (10)
1
2
4
5
8
9
10
compd
C36H80Cl2-
FeO8P4
891.67
P1h
10.487(3)
10.553(6)
12.432(3)
64.98(4)
81.00(2)
67.12(4)
1149(2)
1
C28H64FeO12P4S‚
0.5H2O
813.62
P21/n
10.023(2)
22.427(3)
17.195(3)
90
100.07(2)
90
3806(1)
4
C36H80Cl2-
FeO8P4
891.67
C2/c
19.747(2)
12.739(1)
19.783(2)
90
111.39(1)
90
4634(2)
4
C36H80Br2-
FeO8P4
980.57
C2/c
19.750(2)
12.780(3)
20.078(5)
90
111.76(2)
90
4707(2)
4
C61H100BCl-
FeO9P4
1203.5
P1h
10.495(2)
13.037(5)
24.822(7)
91.61(3)
99.18(2)
100.96(2)
3284(3)
2
C29H64-
FeO13P4S
C36H80Cl3-
FeO8P4
927.12
C2/c
14.830(2)
21.455(3)
15.306(4)
90
100.13(2)
90
4649(2)
4
fw
832.62
P21/n
11.6314(8)
15.943(2)
21.422(3)
90
100.152(8)
90
3910(2)
4
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
d
calc, g cm-3
1.289
1.420
1.278
1.384
1.217
1.414
1.325
T, °C
21
22
22
22
21
21
23
λ, Å
0.71073
6.24
0.93-1.00
3550
4031
0.033
0.076
0.71073
6.68
0.92-1.00
4361
6698
0.084
0.151
0.71073
6.19
0.94-1.00
2607
3079
0.041
0.096
0.71073
22.0
0.87-1.00
3299
4130
0.042
0.080
0.71073
4.14
0.96-1.00
6254
8558
0.063
0.114
0.71073
6.52
0.95-1.00
3368
5416
0.093
0.155
0.71073
6.78
0.94-1.00
2472
4091
0.077
0.155
µ, cm-1
rel trans coeff
no. of obsd reflnsa
total indep reflns
R(F)a,b
wR(F2)c
2
2
4
a I g σ(I). b R(F) ) ∑||Fo| - |Fc||/∑|Fo|.; c wR(F2) ) [∑w(|Fo| - |Fc| )2/∑w|Fo| ]1/2; all independent data.
data for 2 (based on the isotropically refined structure7) and for 5
(based on azimuthal scans). The precision of the analyses of 8, 2,
9, and 10 has been restricted by considerable disorder in the side
chains; some carbon atoms showed large vibrational anisotropy and
unusual apparent bond lengths, and alternative positions were
evident in difference maps for some oxygen atoms of the terminal
OH groups. The fractional oxygens were generally refined with
isotropic thermal parameters. In addition, each of the two inde-
pendent cations on crystal inversion centers in 8 clearly contained
two composite (CO)0.5Cl0.5 ligands in trans coordination sites.
Hydrogen atoms of CH2 groups were included at calculated,
updated, positions with B(H) ) 1.2Beq(C). Hydrogen atoms of OH
groups, where discernible, were included at the observed positions
without refinement. There was no evidence for solvent of crystal-
lization in any compound. The TEXSAN8 program suite, incorpo-
rating complex scattering factors,9 was used in all calculations.
Synthesis of cis-Fe(DHPrPE)2SO4 (2). FeSO4‚7H2O (1.70 g,
6.11 mmol) and DHPrPE (4.00 g, 12.3 mmol) were stirred overnight
in 20 mL of methanol to give a deep purple solution. 1-Propanol
(100 mL) was added with stirring and within a few moments a
microcrystalline purple solid formed. After 2 h, the solid was
collected by filtration, rinsed with 1-propanol, and dried in vacuo.
Yield: 4.1 g, 83%. Anal. Calcd for FeC28H64O12P4S: C, 41.80; H,
8.02; P, 15.40; Fe, 6.94. Found: C, 41.90; H, 8.20; P, 14.65, 14.40;
Fe, 6.49, 6.57. Dark purple X-ray quality crystals were isolated by
cooling a hot saturated 1-propanol solution containing a few drops
mixture of products. Separation of the mixture was not pursued
(see the Discussion section).
Synthesis of cis-Fe(DHBuPE)2(SO4) (3). A solution of DHBuPE
(1.374 g, 3.596 mmol) in 5 mL of ethanol was slowly added to a
stirred solution of FeSO4‚7H2O (0.500 g, 1.798 mmol) in ethanol
at room temperature. The solution was stirred for 24 h in a glovebox
and then filtered through a pipet containing glass wool. The solvent
was removed and a purple, oily residue was obtained. The residue
was washed with diethyl ether (3 × 25 mL) and then dried under
vacuum. The product was a purple solid (yield 87%). IR (Nujol):
ν(SO4) 1170, 1122, 947, 637, 607, and 532 cm-1. 1H NMR (CD3-
OD) at 23 °C: δ 1.7 (m, br), 2.2 (m, br), 3.6 (m, br), 4.9 (s, OH).
Anal. Calcd for C36H80FeO12P4S: C, 47.16; H, 8.80. Found: C,
47.41; H, 8.85.
Synthesis of trans-Fe(DHBuPE)2Cl2 (1). DHBuPE (5.00 g, 13.1
mmol) was dissolved in 40 mL of absolute ethanol. FeCl2‚4H2O
(1.30 g, 6.54 mmol) was added with stirring, giving a green-brown
homogeneous solution. The mixture was stirred for 48 h. Diethyl
ether (200 mL) was then added over 1-2 min to induce rapid
crystallization of a green microcrystalline solid. Yield: 4.95 g, 85%.
1H NMR (CD3OD) at 23 °C: δ 1.5 (m, br), 1.6 (m, br), 1.8 (m,
br), 1.9 (m, br), 2.0 (m, br), 2.3 (m, br), 3.6 (m, br), 3.7 (m, br),
4.9 (s, OH). Anal. Calcd for FeC36H80Cl2O8P4: C,48.49; H, 9.04;
P, 13.89; Fe, 6.26; Cl, 7.95. Found: C, 48.82; H, 9.46; P, 12.97;
Fe, 6.30; Cl, 8.04. Green X-ray quality crystals were grown from
a saturated ethanol solution by addition of a small amount of
toluene.
1
of methanol as a cosolvent. H NMR (CD3OD) at 23 °C: δ 1.3
(m, br), 1.7 (m, br), 2.0 (m, br), 2.3 (m, br), 3.2 (m, br), 3.5 (m,
br), 3.7 (m, br), 4.0 (m, br), 4.9 (s, OH).
Reaction of FeCl2‚4H2O with DHPrPE. DHPrPE (4.00 g, 12.3
mmol) and FeCl2‚4H2O (1.21 g, 6.09 mmol) were dissolved in 30
mL of methanol, giving a deep purple solution. The purple solid
was isolated by removing the methanol under vacuum. The 31P-
{1H} NMR (CD3OD) of the purple solid showed it was a complex
Synthesis of trans-[Fe(DHBuPE)2Cl2]Cl (10). DHBuPE (0.095
g, 0.25 mmol) was added to a stirred solution of FeCl3‚6H2O (0.032
g, 0.12 mmol) in 20 mL of absolute ethanol. The solution
instantaneously changed from a bright yellowish-green to a deep
purple. The mixture was stirred at room temperature for 24 h.
Diethyl ether (∼20 mL) was then added, causing a purple solid to
precipitate from the reaction solution. The solid was filtered and
washed with diethyl ether (∼20 mL). Crystals for X-ray analysis
(Figure S4) were grown by evaporation from ethanol and sealed in
a 0.7-mm glass capillary.
(7) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, 39, 158-166.
(8) TEXSAN: Texray Program for Structure Analysis, version 5.0;
Molecular Structures Corporation: 3200A Research Forest Drive, The
Woodlands, TX 77381, 1989.
(9) Cromer, D. T.; Waber, J. T. In International Tables for X-ray
Crystallography; Ibers, J. A., Hamilton, W. T., Eds.; Kynoch Press:
Birmingham, England, 1974; Vol. IV, pp 71 and 148.
Generation of trans-Fe(DHPePE)2Cl2. This complex was
generated in situ by dissolving DHPePE (0.345 g, 0.79 mmol) and
FeCl2‚4H2O (0.078 g, 0.39 mmol) in 5.0 mL of methanol. The
Inorganic Chemistry, Vol. 41, No. 21, 2002 5455

Miller et al.
Table 2. NMR Data
complex
31P NMR
temp (K)
solvent
a
trans-Fe(DEPE)2Cl2
62.5
55.0 (s, br)
53.2 (s, br)
223
193
296
296
296
193
toluene
CD3OD
CD3OD
H2O
H2O
CD3OD
trans-Fe(DHBuPE)2Cl2 (1)
trans-Fe(DHPePE)2Cl2
cis-Fe(DHBuPE)2Cl2
cis-Fe(DHPePE)2Cl2
cis-Fe(DEPE)2SO4
cannot be observed
cannot be observed
82.9 (t, J ) 34 Hz)
64.2 (t, J ) 34 Hz)
71.5 (t, J ) 33 Hz)
57.9 (t, J ) 33 Hz)
79.0 (t, J ) 34 Hz)
60.0 (t, J ) 34 Hz)
79.0 (t, J ) 34 Hz)
59.7 (t, J ) 34 Hz)
70.0 (t, J ) 34 Hz)
56.8 (t, J ) 34 Hz)
cannot be observed
cannot be observed
ν(CtO) 1906
63.4 (s)
cis-Fe(DHPrPE)2SO4 (2)
cis-Fe(DHBuPE)2SO4 (3)
cis-Fe(DHPePE)2SO4
193
193
193
296
CD3OD
CD3OD
CD3OD
H2O
cis-[Fe(DHPrPE)2]SO4 (2)
cis-[Fe(DHBuPE)2SO4] (3)
296
296
296
296
H2O
H2O
Nujol
CD3OD
1-propanol
CD3OD
CD3OD
cis-Fe(DHPePE)2SO4
trans-[Fe(DEPE)2(CO)Cl+]Clb
trans-[Fe(DHBuPE)2(CO)Cl+][BPh4] (8)
ν(CtO) 1928
62.9
69.1 (s)
trans-[Fe(DHPePE)2(CO)Cl+]Cl-
trans-Fe(DEPE)2(CO)(SO4)
296
296
ν(CtO) 1928
trans-Fe(DHPrPE)2(CO)SO4 (9)
trans-Fe(DHBuPE)2(CO)SO4
66.8 (s)
296
296
CD3OD
CD3OD
ν(CtO) 1927
64.3 (s)
ν(CtO) 1926
trans-Fe(DHPePE)2(CO)SO4
64.4
296
296
296
296
193
247
233
233
CD3OD
CH3CN
CH3CN
CH3CN
CD3OD
toluene
CD3OD
CD3OD
trans-Fe(DHPrPE)2(CH3CN)SO4
trans-Fe(DHBuPE)2(CH3CN)SO4
trans-Fe(DHPePE)2(CH3CN)SO4
trans-Fe(DMeOPrPE)2Cl2 (4)
trans-Fe(DMeOPrPE)2Br2 (5)
trans-Fe(DHBuPE)2(H2O)(SO4) (6)
trans-[Fe(DHBuPE)2(H2O)2](SO4)(7)
62.2 (s)
62.0 (s)
61.5 (s)
61.5
58.7
54.6 (s)
55.8 (s)
a Reference 11b. b Reference 11a.
resulting solution was yellow-brown. The complex was character-
ized by NMR (Table 2) but was not isolated.
the original vessel. Approximately 20 mL of the toluene was
removed under vacuum followed by addition of anhydrous n-hexane
(50 mL). Vacuum was applied to remove some of the hexane and
chill the mixture. A green crystalline product was obtained by
filtration followed by hexane rinse and drying in vacuo. Yield 8.0
g, 54%. H NMR (CD3OD) at 23 °C: δ 1.7 (m, br), 3.4 (s), 3.9
(m, br), 4.9 (s, OH). The bromide analogue 5 was prepared
similarly.
Synthesis of 1,2-Bis(dimethoxypropylphosphino)ethane
(DMeOPrPE). Methylallyl ether (34.0 g, 472 mmol) and 1,2-
diphosphinoethane (10.0 g, 106 mmol) were combined in a 500-
mL round-bottom flask containing 100 mL of methanol. VAZO
67 (DuPont, 2.5 g) was added as the free-radical initiator and the
flask was sealed with a rubber septum held in place with a steel
worm clamp. The flask was placed in an oil bath at 60 °C and
stirred for 2 days behind a blast shield. (Safety note: Because
round-bottom flasks are not designed to handle high pressures, it
is recommended that the temperature in the flask not exceed 60
°C. Use a blast shield for this procedure.) The mixture was cooled
to room temperature followed by workup under inert atmosphere
in an argon-filled glovebox. Solvent was removed under vacuum
with heating. The resulting yellowish syrup was then dissolved in
300 mL of water by stirring with dropwise addition of 50% aqueous
sulfuric acid until the pH was 4. The mixture was filtered to remove
the white flocculant impurity. The solution was then neutralized
with sodium carbonate and extracted three times with diethyl ether.
The ether extracts were combined and then dried over anhydrous
sodium sulfate. The ether solution was isolated by filtration and
the solvent removed with vacuum to give the pure ligand. Yield:
33.1 g, 82%. 31P{1H} NMR, -26.7 (s).
1
Synthesis of trans-Fe(DHBuPE)2(H2O)(SO4) (6). cis-Fe-
(DHBuPE)2(SO4) (0.50 g., 0.54 mmol) was dissolved in deionized
H2O under N2 and stirred for 30 min at 23 °C. The solution changed
color from purple to red and some yellow solid was observed. The
solution was filtered through glass wool in a pipet, the water was
removed under vacuum, and a red oily residue was obtained. The
red residue was washed 3 times with 10 mL of diethyl ether and
dried under high vacuum for 3 days. The compound was stored in
the glovebox. IR (Nujol): ν(H2O), 1653; ν(SO4), see Table 4. 31P-
{1H} NMR (CD3OD): δ 54.6 (s). Anal. Calcd for C36H82-
FeO13P4S: C, 46.25; H, 8.84. Found: C, 46.35; H, 8.84.
Synthesis of trans-[Fe(DHBuPE)2(H2O)2](SO4) (7). The syn-
thesis of trans-[Fe(DHBuPE)2(H2O)2](SO4) was carried out as above
except the solution was stirred for at least 2 h at 23 °C. A yellow-
orange solution resulted, which yielded a yellow oil on removal of
the water. A small amount of orange material precipitated when
the reaction was over and this was removed by filtration prior to
removing the solvent. However, if the starting material is compound
6, no precipitate is observed and the filtration step is not necessary.
31P{1H} NMR (CD3OD): δ 55.8 (s). 1H NMR (CD3OD) at 23 °C:
δ 0.5 (m, br), 1.0 (m, br), 1.3 (m, br), 1.9 (m, br), 2.5 (m, br), 2.7
Synthesis of trans-Fe(DMeOPrPE)2Cl2 (4) and trans-Fe-
(DMeOPrPE)2Br2 (5). DMeOPrPE (12.79 g, 33.4 mmol) and
FeCl2‚4H2O (3.32 g, 16.7 mmol) were dissolved in 30 mL of
anhydrous toluene with stirring under argon atmosphere at ambient
temperature. The resulting green solution was carefully decanted
into a clean flask, leaving a small amount of oily, red impurity in
5456 Inorganic Chemistry, Vol. 41, No. 21, 2002

Synthesis of Water-Soluble Complexes of Iron(II)
Table 3. Summary of the Reactions and Products
product from reaction with
Fe complex
phosphine (L2)
solventa
product
trans-Fe(L2)2Cl2
CO or CH3CN (L′)
FeCl2‚4H2O
DPPE, DPPP, and many
other water-insoluble
chelating phosphines
DEPE
DEPE
DHPrPE
DHBuPE
DHPePE
DMeOPrPE
DHPrPE
DHBuPE
DHPePE
DHPrPE
DHBuPE
DHPePE
DHPrPE
DHBuPE
DHPePE
THF or hexane
trans-Fe(L2)2(L′)Cl+
FeCl2‚4H2O
FeCl2‚4H2O
FeCl2‚4H2O
FeCl2‚4H2O
FeCl2‚4H2O
FeCl2‚4H2O
FeCl2‚4H2O
FeCl2‚4H2O
FeCl2‚4H2O
FeSO4‚7H2O
FeSO4‚7H2O
FeSO4‚7H2O
FeSO4‚7H2O
FeSO4‚7H2O
FeSO4‚7H2O
hexane
MeOH
alcohol
alcohol
alcohol
toluene
H2O
trans-Fe(L2)2Cl2
cis-Fe(L2)2Cl2
mixture of cis- and trans-Fe(L2)Cl2
trans-Fe(L2)2Cl2
trans-Fe(L2)2Cl2
trans-Fe(L2)2Cl2
mixture of cis- and trans-Fe(L2)Cl2
cis-Fe(L2)2Cl2
trans-Fe(L2)2(L′)Cl+
trans-Fe(L2)2(L′)Cl+
trans-Fe(L2)2(L′)Cl+
trans-Fe(L2)2(L′)Cl+
trans-Fe(L2)2(L′)Cl+
H2O
H2O
cis-Fe(L2)2Cl2
cis-Fe(L2)2SO4
cis-Fe(L2)2SO4
cis-Fe(L2)2SO4
+
alcohol
alcohol
alcohol
H2O
trans-Fe(L2)2(L′)SO4+
trans-Fe(L2)2(L′)SO4+
trans-Fe(L2)2(L′)SO4+
trans-Fe(L2)2(L′)SO4
cis-Fe(L2)2SO4
H2O
cis-Fe(L2)2SO4
H2O
cis-Fe(L2)2SO4
a Alcohol refers to methanol or ethanol.
Table 4. Selected Infrared Data
SO4 (cm-1
)
2-
compd
proposed coordination of the SO4
ν1
ν3
ν4
613 (s)
609
630 (s)
640, 605
630 (s)
637, 607, 584
ref
uncoordinated SO4
uncoordinated
uncoordinated
uncoordinated
uncoordinatede
uncoordinated
bidentate
bidentate
bidentate
bidentate
bidentate
bidentate
bridging
bridging
bridging
monodentate
monodentate
monodentate
monodentate
monodentate
983a (w)
1104 (s)
1088
1090 (s)
1150
25a
25b
2-
Cu(en)3SO4
FeSO4‚7H2O
980 (w)
c
trans-[Fe(DHBuPE)2(H2O)2]SO4(7)
cis-[Fe(DHPrPE)2]SO4 (2)
cis-Fe(DHBuPE)2SO4 (3)
Pd(phen)SO4
b
b
b
c
1088 (s)
947
955
930
993
856
860
971
995
960
c
1170, 1122c
1240, 1125,1040-1015
1235, 1125, 1020
1211, 1176, 1075
1296, 1172, 880
1300, 1170, 885
1166, 1096, 1053-1035
1170, 1105, 1055
1195, 1110, 1035
1130d
26a
26a
26c
26b
26b
26a
26a
26a
Pd(py)SO4‚H2O
Co(en)2SO4Br
647, 632, 515
662, 610
Ir(PPh3)2(CO)I(SO4)
Ru(PPh3)2(NO)Cl(SO4)
Cu(bipy)SO4‚2H2O
[(µ-NH2)(µ-SO4)Co2(NH3)4]-3
Pd(NH3)2SO4
trans-Fe(DHBuPE)2(H2O)(SO4) (6)
Pd(NH3)2SO4‚H2O
Co(en)2SO4Br‚2H2O
638, 604
617, 593
645, 652
613, 602
617, 593
970
978
965
939
1140-1110, 1050-1030
26a
26c
25b
b
1130, 1070
1114, 1070
1154
Cu(en)2SO4
Fe(DHBuPE)2(CO)SO4
a Raman Frequency. b This work. c The weak band in the 970-cm-1 region is obscured by the Nujol band at 973 cm-1
.
d The sulfate band in the 1050-
cm-1 region is obscured by strong DHBuPE bands. e Symmetry likely lowered by ion-pairing or other type of outersphere bonding to complex.
(m, br), 3.0 (m, br), 4.9 (s, OH). Anal. Calcd for C36H84FeO14P4S:
C, 45.38; H, 8.89. Found: C, 45.56; H, 9.08.
N2. The solution was subjected to 3 freeze-pump-thaw cycles
and the flask was then filled with CO at 1 atm and stirred for 1 h
at room temperature. The solution changed from purple to yellow.
The CO was released and the solution was filtered through a pipet
containing glass wool. The solvent was removed under vacuum
and a yellow oily residue was obtained. The residue was washed
three times (10 mL) with diethyl ether and dried under vacuum
overnight. A yellow solid was obtained in 80% yield. IR(CH3-
OH): ν(CO) 1928 cm-1. IR (Nujol): ν(CO) 1917 cm-1; ν(SO4)
Synthesis of trans-[Fe(DHBuPE)2(CO)Cl][B(C6H5)4] (8). trans-
Fe(DHBuPE)2Cl2 (1.50 g, 1.68 mmol) was dissolved in 30 mL of
1-propanol. NaBPh4 (0.59 g, 1.72 mmol) was added with stirring,
giving a green-brown homogeneous solution. The mixture was
stirred overnight at ambient temperature under 45 psig of carbon
monoxide in a sealed 120-mL Fisher-Porter bottle. The resulting
yellow mixture was filtered to remove NaCl, leaving a yellow
solution. Crystals were obtained by adding toluene to the solution
1154, 939, 617, and 593 cm-1
.
31P{1H} NMR (CD3OD): δ 66.4
1
1
(s). H NMR (CD3OD) at 23 °C: δ 1.7 (m, br), 1.8 (m, br), 2.0
(m, br), 2.1 (m, br), 2.5 (m, br), 2.8 (m, br), 3.6 (m, br), 4.9 (s,
OH). Anal. Calcd for C37H80FeO13P4S: C, 47.04; H, 8.53. Found:
C, 46.50; H, 9.37.
and letting it stand for several days. H NMR (CD3OD) at 23 °C:
δ 1.6 (m, br), 2.0 (m, br), 2.1 (m, br), 2.3 (m, br), 3.6 (m, br), 4.9
(s, OH), 6.8 (m, br), 6.9 (m, br), 7.3 (m, br). Anal. Calcd for
FeC61H100BClO9P4: C, 60.88; H, 8.38; P, 10.29; Fe, 4.64; Cl, 2.94.
Found: C, 62.84; H, 8.12; P, 11.97; Fe, 4.34; Cl, 3.46. In light of
the rather poor elemental analysis for this compound, the purity of
this compound was demonstrated by 31P{1H} NMR. See Figure
S1 in the Supporting Information.
Synthesis of trans-Fe(DHPrPE)2CO(SO4) (9). cis-Fe(DHPrPE)2-
SO4 (2.10 g, 2.61 mmol) was slurried in 30 mL of absolute ethanol
and placed in a Fischer-Porter tube. The tube was sealed and
removed from the glovebox, and the argon atmosphere was
exchanged for 40 psig of carbon monoxide. The sample was then
stirred for 12 h at 60 °C, after which time the sample was cooled
Synthesis of trans-Fe(DHBuPE)2(CO)(SO4). cis-Fe(DHBuPE)2-
(SO4) (0.50 g, 0.54 mmol) was dissolved in 5 mL of methanol under
Inorganic Chemistry, Vol. 41, No. 21, 2002 5457

Miller et al.
to room temperature, the CO atmosphere replaced with argon, and
the Fischer-Porter tube returned to the glovebox for workup. The
yellow solid was collected on a glass frit and dried under vacuum.
Yield: 1.66 g (1.99 mmol, 76%). 1H NMR (CD3OD) at 23 °C: δ
1.7 (m, br), 1.9 (m, br), 2.1 (m, br), 2.5 (m, br), 2.7 (m, br), 3.6
(m, br), 4.9 (s, OH). This synthesis was also carried out in methanol
with the same result except that the solvent must be removed in
vacuo to recover the product. X-ray quality crystals were obtained
by cooling a hot saturated 1-propanol solution of the product.
Synthesis of trans-Fe(DHBuPE)2(MeCN)2(SO4). cis-Fe(DHBu-
PE)2(SO4) (0.400 g; 0.436 mmol) was dissolved in 5 mL of
methanol in the glovebox and 2 drops of CH3CN was added to the
solution. The solution changed from purple to yellow immediately.
The reaction mixture was stirred for 30 min and the solution was
filtered through a short pipet containing glass wool. The solvent
was removed under vacuum and an oily residue was obtained. The
residue was washed with diethyl ether (3 × 10 mL) and the solid
thus obtained was dried in a vacuum. The product was a yellow-
orange solid (yield 87%). IR(MeOH): (νCN) 2257 cm-1. IR
Figure 1. Molecular structure of the trans-Fe(DHBuPE)2Cl2 (1) complex.
(Nujol): (νCN) 2266 cm-1 31P{1H} NMR (CD3OD): δ 61.7 (s).
.
Anal. Calcd for C40H86FeN2O12P4S‚2H2O: C, 46.42; H, 8.77; N,
2.71. Found: C, 46.43; H, 8.52; N, 2.67.
octahedral complexes with two equatorial bidentate phos-
phines and two trans chloride ligands (eq 1). In contrast,
Reactions of cis-Fe(DHPePE)2SO4 with CO and CH3CN. A
solution of FeSO4 (0.05 M) with 2 equiv of DHPePE in either water
or methanol was stirred under 45 psig of CO for several hours.
Although the solution changed color to yellow within a few minutes,
the extra reaction time was allowed to ensure completion of the
reaction. The reaction with acetonitrile was carried out in an NMR
tube by adding 1.1 equiv of acetonitrile to the cis-Fe(DHPePE)2SO4
solution, generated in situ as described above. The NMR spectra
of the products (presumably trans-[Fe(DHPePE)2(CO)SO4] and
trans-[Fe(DHBuPE)2(CH3CN)2]SO4), respectively, were recorded
(Table 2), but the products were not characterized further.
this study found that the reactions of Fe(II) salts with the
water-soluble DHPrPE, DHBuPE, and DHPePE ligands gave
cis and/or trans products, depending on the solvent, the anion,
and the alkyl chain length of the phosphine. The effect of
each of these parameters is discussed in the sections below.
Effect of Chain Length. The reaction of 2 equiv of
DHBuPE with FeCl2‚4H2O in ethanol formed a green
solution from which a green product was isolated by addition
of ether to the reaction solution. Elemental analysis suggested
the molecular formula Fe(DHBuPE)2Cl2, a result confirmed
by the X-ray crystal structure of the product (Figure 1). The
product has a trans geometry, analogous to the products that
are formed by reaction of FeCl2‚4H2O with the water-
insoluble DEPE, DPrPE, and DBPE chelating diphosphines.11
These latter complexes have broad 31P NMR resonances at
room temperature,11b and similarly, trans-Fe(DHBuPE)2Cl2
(1) also showed a very broad signal at room temperature.
This broad band sharpened considerably at -80 °C to give
a resonance centered at about 55 ppm.
Preparation of Dinitrogen Complexes. The Fe sulfate com-
plexes were stirred under dinitrogen in methanol but no dinitrogen
binding was observed, as indicated by NMR and IR spectroscopy.
The Fe chloride complexes were found to bind dinitrogen when
stirred in methanol in the presence of NaBPh4. The dinitrogen
complexes were prepared following the method previously reported
by Leigh.10 A typical preparation is the following for [Fe(Cl)(N2)-
(DMeOPrPe)2+]BPh4-. Fe(Cl)2(DMeOPrPe)2 (5.00 g, 5.60 mmol)
and NaBPh4 (1.92 g, 5.61 mmol) were dissolved in 40 mL of
anhydrous THF in a Fisher-Porter tube and stirred under 50 psig
of dinitrogen. The green solution immediately turned brown and
the pressure of the dinitrogen slowly decreased as the product
complex was formed. After 24 h, the solution was filtered to remove
the white precipitate of NaCl. The presence of metal-bound
dinitrogen was confirmed by FTIR spectroscopy of the solution,
which showed a stretch at 2094 cm-1. The 31P{1H} NMR of the
complex in acetone-d6 under 40 psig of dinitrogen (in a 5 mm NMR
tube with Teflon valve) showed a single peak at 60 ppm. Because
the dinitrogen binding is readily reversible, the product was not
isolated in pure form.
In contrast to the reaction above, the reaction of 2 equiv
of DHPrPE with FeCl2‚4H2O in ethanol gave a deep purple
solution. Removal of the solvent gave a solid whose 31P-
{1H} NMR spectrum in methanol-d4 showed the presence
of more than one product. (Peaks at ≈ 79, 71, and 53 ppm
suggested the presence of at least cis and trans isomers of
Results and Discussion
There are numerous reports on the reactions of iron(II)
chloride with water-insoluble 1,2-bis(bis(alkyl)phosphino)-
ethanes in nonaqueous solvents.11 These reactions yield green
(11) (a) Bellerby, J. M.; Mays, M. J.; Sears, P. L. J. Chem. Soc., Dalton
Trans. 1976, 1232-1236. (b) Baker, M. V.; Field, L. D.; Hambly, T.
W. Inorg. Chem. 1988, 27, 2872-2876. (c) Lewis, J.; Khan, M. S.;
Kakkar, A. K.; Raithby, P. R.; Fuhrmann, K.; Friend, R. H. J.
Organomet. Chem. 1992, 433, 135-139.
(10) Hughes; D. L.; Leigh; G. J.; Jiminez-Tenorio, M.; Rowley, A. T. J.
Chem. Soc., Dalton Trans. 1993, 75.
5458 Inorganic Chemistry, Vol. 41, No. 21, 2002

Synthesis of Water-Soluble Complexes of Iron(II)
Figure 3. Variable-temperature 31P{1H} NMR spectra of cis-Fe-
(DHBuPE)2SO4 in methanol-d4. The peaks labeled “#” and “*” are assigned
to the substitution products trans-Fe(DHBuPE)2(CD3OD)(SO4) and trans-
[Fe(DHBuPE)2(CD3OD)2]SO4 (see text).
a mixture of cis and trans isomers is formed. As discussed
further below, it is suggested that, with the DHPrPE ligand,
coordination of two hydroxyl groups stabilizes the cis isomer
by forming two six-membered rings; the extra stability this
chelation imparts to the molecule drives the conversion of
the trans to the cis isomer. Finally, it is noted that FeCl2‚
4H2O also reacts with water-insoluble chelating phosphine
ligands in THF to form trans products.11 Thus, with the
exception of the DHPrPE ligand, the hydroxy alkyl and
“regular” alkyl ligands react similarly with FeCl2‚4H2O.
Effect of Anion. In contrast to the results above with
FeCl2‚4H2O, the reaction of FeSO4‚7H2O with any of the
chelating water-soluble phosphines (DHPrPE, DHBuPE,
DHPePE) in methanol gave cis-Fe(L2)2SO4 (eq 2), as
Figure 2. Molecular structure of the cis-Fe(DHPrPE)2SO4 (2) complex.
Fe(DHPrPE)2Cl2.) Attempts to separate and isolate the
products of the reaction were unsuccessful, and therefore the
synthesis was repeated by using FeSO4‚7H2O in place of
FeCl2‚4H2O. (The strategy here was to replace the two
chloride ligands with a chelating ligand, which might
stabilize the cis isomer so it could be isolated. Note that in
2+
the solid state FeSO4‚7H2O contains Fe2+ as the Fe(H2O)6
complex,12 whereas FeCl2‚4H2O contains trans-Fe(H2O)4-
Cl2.13) As with FeCl2‚4H2O, a deep purple solution resulted
from the reaction, giving a product with the molecular
formula Fe(DHPrPE)2SO4. The NMR of the solution at -80
°C consisted of two triplet resonances at 57.9 and 71.5 ppm
(JP-P ) 34 Hz), suggestive of a cis geometry. Purple crystals
of X-ray diffraction quality were isolated and the crystal
structure is shown in Figure 2. As predicted from the NMR
spectrum, the complex indeed has a cis geometry. Note that
the hydroxyl groups on the propyl chains are non-innocent;
two of them coordinate to the Fe to form six-membered rings.
Similar coordination of the -OH groups in the DHBuPE
ligand in complex 1 (Figure 1) apparently does not occur,
probably because the resulting structure would have unfa-
vored seven-membered rings. (cis-[Fe(DHPrPE)2]SO4 is
designated as 2.)
indicated by the characteristic two resonances in the 31P-
{1H} NMR spectra at 193 K (Table 2). A sample spectrum,
that of cis-Fe(DHBuPE)2SO4 (3), is shown in Figure 3; note
that at room temperature the characteristic triplets are broad,
featureless resonances, a result usually attributed either to a
diamagnetic/paramagnetic crossover or to a rapid decoordi-
nation/re-coordination of one bonding atom in the bidentate
ligand.11b Likewise, reaction of FeSO4‚7H2O with DEPE in
methanol gave the cis product, again indicated by two triplet
resonances at -80 °C in the 31P NMR spectrum. Evidence
for a coordinated bidentate sulfate ligand in these complexes
is discussed next. (The two resonances at “*” and “#” ppm
in the spectra in Figure 3 are attributed to the methanolysis
products trans-[Fe(DHBuPE)2(CH3OH)(SO4)] and trans-[Fe-
(DHBuPE)2(CH3OH)2]SO4. These species are also formed
in other reactions and are discussed in a later section.)
Recall that the sulfate ligand is not coordinated to the
Fe in cis-[Fe(DHPrPE)2]SO4. To determine if cis-Fe-
(DHBuPE)2SO4 has a similar coordination sphere, repeated
attempts were made to grow crystals for X-ray analysis.
In the final experiment of this series, FeCl2‚4H2O was
reacted with 2 equiv of DHPePE in methanol. As was the
case with DHBuPE, a green solution formed containing the
trans isomer, as indicated by the single resonance at 53.2
ppm in the 31P NMR spectrum of the solution (Table 2).
The results above are summarized in Table 3 and as
follows: FeCl2‚4H2O reacts with DHBuPE and DHPePE in
methanol or ethanol to form trans-Fe(L2)2Cl2. With DHPrPE,
(12) Nicholls, D. In ComprehensiVe Inorganic Chemistry; Trotman, A. F.,
Ed.; Pergamon Press: New York, 1973; Vol. 3, p 1026.
(13) Penfold, B. R.; Grigor, J. A. Acta Crystallogr. 1959, 12, 850-854.
Inorganic Chemistry, Vol. 41, No. 21, 2002 5459

Miller et al.
the sections below on solvent effects and interpretations. 31
Unfortunately, the attempts were unsuccessful, and therefore
the infrared spectrum of the product was examined to
determine the coordination mode of the sulfate ligand.14
Sulfate can be present in a metal complex in several different
ways: as a monodentate ligand, as a bidentate ligand, as a
bridged bidentate complex, or as a noncoordinated counte-
rion.15 These coordination modes can be distinguished by
the fact that the sulfate ν3 and the ν4 modes each give rise
to two IR bands in the monodentate form, three in the
bidentate form, and only one when uncoordinated.15 The
infrared data for 2, 3, and related complexes are summarized
in Table 4. As expected from the X-ray structure, the IR
spectrum of 2 is consistent with the presence of a noncoor-
dinated sulfate ligand. (The bands at 1088 (s) and 630 (s)
cm-1 are assigned to uncoordinated sulfate. For comparison,
free sulfate in the form of Na2SO4 has bands around 1104
(s), 983 (w), and 613 (s) cm-1. See Table 4 for additional
comparisons to complexes having uncoordinated sulfate
counterions.15) Note the IR spectrum of 3 (Table 4; Figure
S2) is quite different from that of 2 and shows bands in the
sulfate region at 1170, 1122, 947, 637, 607, and 584 cm-1.
Suggested band assignments are shown in Table 4. (The
additional bands in the spectrum of 3 at 1025-1060, 981,
and 890 cm-1 (Figure S2) are assigned to the DHBuPE ligand
(Table S1 in the Supporting Information). For comparison,
Figure S2 also shows the spectrum of trans-Fe(DHBuPE)2Cl2
(1), which also has DHBuPE bands at 1025-1060, 985, and
890 cm-1. The 1025-1060 cm-1 absorption consists of
several overlapping bands, which likely obscure the sulfate
band in 3 that is also in this region. For additional reference,
the uncoordinated DHBuPE ligand exhibited strong IR bands
at 1025-1060 and 985 cm-1 (Table S1).) Comparing the
IR spectrum of 3 with the other complexes in Table 4
suggests that the sulfate group is chelating or bridging. The
latter possibility is unlikely because a dimer of formula
(DHBuPE)2Fe(µ-SO4)2Fe(DHBuPE)2 is severely sterically
congested due to the numerous hydroxy-butyl groups. For
that reason a monomeric complex with a chelating sulfate
ligand is proposed for 3 (eq 2).
P
NMR data for all the products are summarized in Table 2.
Solvent Effects. With three exceptions, the reactions of
FeSO4‚7H2O or FeCl2‚7H2O with 2 equiv of DHPrPE,
DHBuPE, or DHPePE in water or methanol formed cis-Fe-
(L2)2X (X ) (Cl)2 or SO4). The three exceptions to this
generalization, already discussed, are the reactions of FeCl2‚
4H2O with DHBuPE and DHPePE in methanol, both of
which give the trans product, and DHPrPE, which forms a
mixture of the cis and trans isomers. Note that the reaction
of FeCl2‚4H2O with DEPE in hexane gave the trans product,
but the reaction of FeSO4‚7H2O with DEPE in methanol gave
the cis product.
Interpretation of the Reactivity. The results in the
preceding sections are summarized and interpreted as follows.
In methanol or ethanol, the reactions of FeCl2‚4H2O with
2 equiv of any bidentate phosphine other than DHPrPE give
a trans product. This stereochemisty is suggested to be the
consequence of the trans geometry of the FeCl2‚4H2O starting
material,13 i.e., substitution of the water molecules by the
phosphines retains the geometry of the starting complex (eq
3). Fe(DHPrPE)2Cl2 is an exception to this result; some cis
product also forms because the -OH groups on two of the
ligands can coordinate to the Fe center, forming two chelating
six-membered rings. The cis isomer is preferred in this
molecule, probably for electronic reasons because the
π-donor O atoms are trans to the π-accepting phosphines.
Consistent with this interpretation, it is noted that when the
-OH groups in the DHPrPE ligand are replaced with -OMe
the resulting product has coordinated trans chloride ligands,
analogous to the trans-Fe(DHBuPE)2Cl2 (1) and trans-Fe-
(DHPePE)2Cl2 complexes. A crystal structure of the trans-
Fe(DMeOPrPE)2Cl2 complex (4) is shown in Figure 4. (The
crystal structure of the isomorphous trans-Fe(DMeOPrPE)2Br2
complex (5) is shown in the Supporting Information.)
All of the remaining reactions initially favor products with
cis stereochemistry. Thus, the bidentate ligands react with
FeCl2‚4H2O in water (where Fe(H2O)62+ is the predominant
species) to give predominantly cis products and with FeSO4‚
7H2O in water or alcohols to give predominantly cis products.
The cis isomers are the predominant product probably for
several reasons. In the case of FeSO4‚7H2O in methanol, a
coordinated bidentate sulfate counterion will force the two
chelating phosphines into a cis orientation (eq 4). The reasons
In water, DHPrPE reacted with FeSO4‚7H2O to yield
purple cis-[Fe(DHPrPE)2]SO4 (2), the same product formed
in methanol, as indicated by the 31P NMR spectrum of the
product in methanol (Table 2). DHBuPE and DHPePE also
reacted with FeSO4‚7H2O in water to initially yield purple
solutions. No 31P NMR signals were observed at ambient
temperature for either of these products. (Again, rapid
decoordination/re-coordination of one arm of the bidentate
phosphine ligand or a spin state change may prevent the
observation of signals for these compounds.11b) Each of these
latter two solutions underwent additional reactions over the
course of several hours to give orange solutions, the 31P NMR
spectra of which are characteristic of trans compounds. These
reactions were thoroughly investigated and are discussed after
(14) The infrared spectra were obtained in Nujol because the complex
degraded quickly when it was mixed with KBr.
(15) Nakamoto, N. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, Part B. Applications in Coordination, Organo-
metallic, and Bioinorganic Chemistry, 5th ed.; John Wiley & Sons:
New York, 1997; pp 79-82.
for a cis geometry in water are less clear. In water, the sulfate
ligand is likely not coordinated to the Fe center, and it may
5460 Inorganic Chemistry, Vol. 41, No. 21, 2002

Synthesis of Water-Soluble Complexes of Iron(II)
expected band in the 1050-cm-1 region is obscured by the
broad peaks attributed to the DHBuPE ligand, Table S1.)
Further support for a coordinated sulfate ligand in 6 comes
from the reaction of 3 in water in the presence of added
Na2SO4; only 6 formed and the formation of 7 was inhibited,
as would be expected by the common-ion effect (mass-law
retardation) for a mechanism involving the loss of a
monodentate sulfate ligand.
1
As was the case with 3, the H NMR spectra of 6 and 7
were not particularly diagnostic. Both complexes exhibited
a broad resonance for the alkoxybutyl groups of the phos-
phine ligands. Furthermore, the resonance for the H2O ligand
coordinated to the Fe atom in 6 was not observed, likely
because of rapid H/D exchange with the CD3OD solvent.
Finally, note that 6 and 7 were only soluble in H2O or CH3-
OH. They did not dissolve in other common deuterated
solvents such as acetone-d6, CD3Cl, or C6D6, and they
decomposed in ethanol.
Attempts to recrystallize 6 and 7 for X-ray analysis were
unsuccessful because they back-reacted to form 3 when
dissolved in methanol and set aside to crystallize. For
example, complex 6 reverted to 3 in about 7 days in
methanol. Thus, the 31P{1H} NMR spectrum of this crystal-
lization solution after 7 days showed a small residual
Figure 4. Molecular structure of the trans-Fe(DMeOPrPE)2Cl2 (4)
complex.
be that the formation of the cis isomer is statistically
determined. (Coordination of two bidentate ligands will result
in 5/6 cis and 1/6 trans isomer.) Alternatively, the cis geometry
may be directed by steric interactions between the hydroxy-
alkyl groups on the phosphines. Note that in aqueous
solution, the cis products slowly react to form trans com-
plexes (except in the case of the DHPrPE ligands for which,
as already discussed, the coordinated -OH groups stabilize
the cis isomer). These reactions were studied in detail and
are discussed in the next section.
Reaction of cis-Fe(DHBuPE)2SO4 with H2O. Synthesis
and Characterization of Fe(DHBuPE)2(L)nSO4 (L ) H2O;
n ) 1 or 2). DHBuPE and DHPePE reacted with FeSO4‚
7H2O in water to initially yield cis-Fe(P2)2SO4 compounds.
Subsequent reactions occurred over several hours to give new
products. These products were isolated for the case of P2 )
DHBuPE, and subsequent characterization identified the
products as trans-Fe(DHBuPE)2(H2O)(SO4) (6) and trans-
[Fe(DHBuPE)2(H2O)2]SO4 (7) (Scheme 1). The complexes
were synthesized and isolated pure (as indicated by elemental
analysis) by stirring 3 in water at room temperature for 0.5
(6) or 2 h (7). The trans stereochemistry of each molecule
was indicated by the 31P{1H} NMR spectra, which showed
a single resonance at δ 54.6 and 55.8 for 6 and 7,
respectively. Note that the chemical shifts of these resonances
are in agreement with other related trans iron chelating-
phosphine complexes.16
resonance for 6 and two new resonances at δ 79.0 (t, JP-P
)
34 Hz) and 60.0 (t, JP-P ) 34 Hz), assigned to 3. In addition,
two minor peaks at 57.7 (d, JP-P ) 40 Hz) and 56.8 (s) were
present. These peaks are conditionally assigned to the
methanol-substituted complexes trans-[Fe(DHBuPE)2(CH3-
OH)(SO4)] and trans-[Fe(DHBuPE)2(CH3OH)2]SO4 (Scheme
1). These complexes could not be isolated, but it is noted
that the same species grew in slowly when 3 was dissolved
in methanol-d4 (Scheme 1; Figure 3), which lends credence
to the suggestion that they are solvent-substituted species.
Complex 7 also backreacted: when 7 was dissolved in
methanol-d4 at room temperature it initially formed 6 and
then converted to 3, as monitored by 31P{1H} NMR. Again,
small amounts of the putative methanol-substituted com-
plexes trans-[Fe(DHBuPE)2(CH3OH)(SO4)] and trans-[Fe-
(DHBuPE)2(CH3OH)2]SO4 also formed.
The reaction sequence 3 f 6 f 7 is also reversible in the
solid state. When a solid sample of 6 (red) or 7 (orange)
was heated at 363 K under vacuum for 3 h, the solid turned
purple and the 31P{1H} NMR spectrum of the product in
methanol at 233 K showed the resonances for 3 with small
amounts of 6 or 7.
Kinetics Studies of the 3 f 6 f 7 Reactions in H2O.
The kinetics of the reaction of 3 in water to form 6 and 7
were monitored by following the disappearance of the
starting material at λmax ) 500 nm. The changes in absor-
bance with time were fit to a biexponential function. The
biphasic kinetics are consistent with two successive substitu-
tion reactions that occur according to eqs 5 and 6 with k5 )
(2.79 ( 0.29) × 10-3 s-1 and k6 ) (3.99 ( 0.07) × 10-4
s-1 at 25 °C.17,18
The IR spectrum of 6 is consistent with the presence of a
monodentate sulfate group: the sulfate bands at 1130, 638,
and 604 cm-1 are in good agreement with other monodentate
sulfate complexes reported in the literature (Table 4).15 (The
(16) (a) Hills, A.; Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. J. J.
Organomet. Chem. 1990, 391, C41. (b) Baker, M. V.; Field, L. D.;
Hambley, T. W. Inorg. Chem. 1988, 27, 2872. (c) Baker, M. V.; Field,
L. D. J. Organomet. Chem. 1988, 354, 351.
k5
98
Fe(DHBuPE)2SO4
Fe(DHBuPE) (H O)SO
2 2 4
+ H2O
(5)
3
6
Inorganic Chemistry, Vol. 41, No. 21, 2002 5461

Miller et al.
Scheme 1. Reaction of cis-Fe(DHBuPE)2SO4 (3) in H2O to Form Successively trans-Fe(DHBuPE)2(H2O)(SO4) (6) and
trans-[Fe(DHBuPE)2(H2O)2]SO4 (7), as Well as the Reactions of 3 and 6 in CH3OH to Form trans-[Fe(DHBuPE)2(CH3OH)(SO4)] and
trans-[Fe(DHBuPE)2(CH3OH)2]SO4
and ionic metal complex separate.19 There are few appropri-
ate aquation reactions of Fe(II) complexes in the literature
for comparison to reactions 5 and 6, but it is noted that the
k6
98
Fe(DHBuPE)2(H2O)SO4
+ H2O
6
2-
2+ + SO4 (6)
[Fe(DHBuPE)2(H2O)2]
2+
activation parameters for aquation of Fe(phen)(H2O)4 are
7
∆Hq ) 12.8 kcal/mol and ∆Sq ) -16 cal/mol, both of which
compare favorably to the values for reactions 5 and 6.20
Water Solubilities. The DHPrPE, DHBuPE, and DHPePE
complexes synthesized in this study are appreciably water
soluble. Selected solubilities in water at 23 °C are as
follows: cis-Fe(DHPrPE)2SO4 (2), >0.63 M; trans-Fe-
(DHPrPE)2(CO)SO4, 0.63 M; cis-Fe(DHBuPE)2SO4, 0.56 M;
trans-Fe(DHBuPE)2(CO)SO4, 0.16 M; and trans-FeCl2-
(DHBuPE)2, 0.43 M.
To probe the mechanisms of these aquation reactions, the
effect of added Na2SO4 and the temperature dependences of
the reaction rates were studied. The rate constants at various
temperatures over the range 282-307 K are summarized in
Table S2. The activation parameters, obtained from an Eyring
plot of the data, are ∆Hq5 ) 15.0 ( 1.8 kcal mol-1, ∆Sq
)
5
-21.0 ( 1.6 cal mol-1, ∆Hq6 ) 13.0 ( 0.5 kcal mol-1, and
∆Sq ) -31.3 ( 1.4 cal mol-1. In the experiments with
6
Reactions of cis- and trans-Fe(L2)2(X) with N2, CO, and
CH3CN in Alcohols. The complexes synthesized above can
be used to bind a variety of small molecules for possible
separation purposes. In the literature it is reported that the
trans-Fe(L2)Cl2 complexes (where L2 is one of the water-
insoluble chelating phosphine ligands) react with ligands such
as N2, CO, and CH3CN in methanol or acetone11a to form
complexes of the type trans-Fe(L2)(L′)Cl+ or trans-Fe(L2)-
added Na2SO4, the only product formed was 6; no 7 was
formed. In addition, the added SO42- retarded the formation
of 6 from 3. For example, when [SO42-] ) 1.50 × 10-4 M,
the rate of reaction 5 was 23 times smaller than in the absence
2-
of added SO4 (k5( with SO4 ) 1.22 × 10-4 s-1 vs
2-)
2-)
k5( without SO4
) 2.79 × 10-3 s-1 at 25 °C).18 The
inhibition by added SO42- suggests a dissociative mechanism
for both substitution reactions. Although positive values of
∆Sq are generally expected for dissociative processes, the
rather negative values for reactions 5 and 6 are likely
attributable to solvent ordering as the ionic sulfate ligand
2+
(L′)2 (L′ ) N2, CO, CH3CN, etc.). In this study, similar
reactivity and products were found for the reactions of the
Fe(L2)(X) complexes (X ) SO4 or (Cl)2; L2 ) one of the
water-soluble chlelating phosphines used in this study) with
N2, CO, and CH3CN in methanol. For example, trans-Fe-
(DHBuPE)2Cl2 (2) reacted with CO in methanol to give
[trans-Fe(DHBuPE)2(CO)Cl]Cl. The tetraphenylborate salt
was isolated as yellow crystals (designated as compound 8),
and the X-ray structure is shown in Figure 5. Note that the
trans products formed in these reactions regardless of reactant
stereochemistry, reaction solvent, counterion, or alkyl chain
length. The reaction of trans-Fe(DHBuPE)2Cl2 with N2 in
(17) A small amount of an orange-yellow solid precipitated from solution
after the kinetics runs were complete. A similar observation was made
when 3 was dissolved in H2O containing NaSO4 to suppress the
formation of 7, i.e., 6 formed along with a small amount of the orange-
yellow precipitate. However, the orange-yellow solid did not form
when 6 reacted in H2O to give 7. The orange-yellow solid was not
soluble in common organic solvents. The 31P{1H} NMR spectra of
the reaction solution in which the precipitate formed showed trace
amounts of free phosphine (δ -23.4) and phosphine oxide (δ 56.6).
The pH of the reaction solutions was 6.6. A decrease in pH increased
the rate, and above pH 7.5, a white precipitate (perhaps Fe(OH)2)
formed quickly.
(18) As expected for mass law retardation of a rate, the retardation was
dependent on the concentration of the SO42-, as shown in the following
([Na2SO4]/10-2 M, k1/10-4 s-1): 0, 27.9 ( 0.29; 0.15, 1.22 ( 0.03;
0.75, 1.00 ( 0.12; 1.50, 0.65 ( 0.08.
(19) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition
Metal Complexes, 2nd ed.; VCH Publishers: New York, 1991; pp
105-106.
(20) Bell R. S.; Sutin, N. Inorg. Chem. 1962, 1, 359.
5462 Inorganic Chemistry, Vol. 41, No. 21, 2002

Synthesis of Water-Soluble Complexes of Iron(II)
Scheme 2. Reactions of 2, 3, and 6 with CO
cis-Fe(DHBuPE)2SO4 (3) also reacted with 1 atm of CO
at room temperature in methanol (Scheme 2). The IR
spectrum of the reaction solution after 30 min showed a band
at 1928 cm-1, indicative of a carbonyl ligand.22 The 31P-
{1H} NMR exhibited only one resonance (at δ 66.4),
indicative of a trans geometry. The IR spectrum of the
product in Nujol showed bands corresponding to the sulfate
ligand coordinated in a monodentate fashion (ν(SO4) ) 1154,
939, 617, and 593 cm-1; the ν(CO) band was at 1917 cm-1
in Nujol (Figure S2)). The product is assigned the formula
trans-Fe(DHBuPE)2(CO)SO4 (Scheme 2).
Figure 5. Molecular structure of the trans-[Fe(DHBuPE)2(CO)(Cl)]-
[B(C6H5)4] (8) complex.
the presence of NaBPh4 gave [trans-Fe(DHBuPE)2(N2)Cl]-
BPh4. The NtN stretch was observed at 2095 cm-1.21 The
In a related reaction, cis-Fe(DHBuPE)2SO4 (3) was dis-
solved in methanol and two drops of CH3CN were added to
the solution. The starting material reacted immediately (as
indicated by an immediate color change from purple to
yellowish) and the IR spectrum in methanol showed a ν-
(CN) band at 2257 cm-1 (2266 cm-1 in Nujol), indicative
of a coordinated CH3CN ligand.23 The 31P{1H} NMR of the
product exhibited a single resonance at δ 61.7, indicative of
trans stereochemistry. cis-Fe(DHBuPE)2SO4 (3) is not soluble
in neat CH3CN, but a slurry of cis-Fe(DHBuPE)2SO4 in CH3-
CN reacted in the solid state to give the same complex as
was formed in MeOH. Elemental analysis of the product
showed the presence of two molecules of CH3CN. X-ray
structural analysis showed that the complex was trans-[Fe-
(DHBuPE)2(CH3CN)2]SO4 and not the alternative [Fe-
(DHBuPE)2(CH3CN)(SO4)]‚CH3CN (Scheme 3).24
reaction is reversible, as indicated by the decrease in the
intensity of the N2 peak in the IR spectrum when vacuum is
applied to the reaction solution. The band increased in
intensity on subsequent reexposure to N2. This reversible N2
binding was also observed by 31P{H} NMR, which showed
the reversible appearance and disappearance of a resonance
at 59 ppm. The trans-Fe(DMeOPrPE)2Cl2 complex reacted
analogously with N2 and CO, forming products with ν(Nt
N) at 2094 cm-1, 31P{H} 60 ppm and ν(CtO) at 1930 cm-1,
31P{H} 64 ppm, respectively.
trans-Fe(DHBuPE)2(H2O)(SO4) (6) also reacted with
In another example, cis-Fe(DHPrPE)2SO4 (2) reacted
readily with CO in methanol to give a yellow solution from
which trans-Fe(DHPrPE)2(CO)SO4 (9) was isolated (Scheme
2; for spectroscopic data see Table 2). The crystal structure
of this complex is shown in Figure 6; note the monodentate
coordination of the sulfate ligand. For comparison purposes,
it was found that cis-Fe(DEPE)2SO4 reacted with CO in
methanol to give the yellow trans-Fe(DEPE)2(CO)(OSO3)
complex. (This complex was not isolated as a solid, but was
characterized in solution by IR and NMR. The 31P NMR
spectrum showed a singlet at 69.1 ppm (23 °C) and the IR
showed a single, intense carbonyl stretch at 1928 cm-1.)
CO(g) and CH3CN (Schemes 2 and 3). The reaction with
(22) See ref 15, pp 126-148.
(23) See ref 15, pp 113-115.
(24) A capillary-mounted crystal of trans-[Fe(DHBuPE)2(CH3CN)2]SO4
diffracted weakly with broad peaks. Data collected on a CAD 4
diffractometer sufficed to show that the Fe atom lay on a cystallo-
graphic center of symmetry in space group C2/c and that the
composition was trans-[Fe(DHBuPE)2(CH3CN)2]SO4 and not the
alternative [Fe(DHBuPE)2(CH3CN)(SO4)]‚CH3CN. The refinement
was not pursued further because of the rather poor quality of the data.
(25) (a) Nakamoto, K.; Fujita, J.; Tanaka, S.; Kobayashi, M. J. Am. Chem.
Soc. 1957, 79, 4904. (b) Baldwin, M. E. Spectrochim. Acta 1963, 19,
315. (c) McWhinnie, J. Inorg. Nucl. Chem. 1964, 26, 21.
(26) (a) Eskenazi, R.; Raskovan, J.; Levitus, R. J. Inorg. Nucl. Chem. 1966,
28, 521 and references in therein. (b) Horn, R. W.; Weissberger, E.;
Collman, J. P. Inorg. Chem. 1970, 10, 2367. (c) Barraclough, C. G.;
Tobe, M. L. J. Chem. Soc. 1961, 1993.
(21) See ref 15, pp 173-176.
Inorganic Chemistry, Vol. 41, No. 21, 2002 5463

Miller et al.
Further Discussion of the X-ray Structures. The fol-
lowing additional points are noted concerning the X-ray
structures of the molecules.
trans-Fe(DHBuPE)2Cl2 (1). The structure is accurately
centrosymmetric. Note the hydroxybutyl side chains are
extended (Figure 1). The O(1), O(2), and O(4) atoms (located
in OH groups) are donors in O‚‚‚O intramolecular hydrogen
bonds (2.690-2.839(4) Å), and the O(3) atom is a donor in
an O‚‚‚Cl bond (3.087(2) Å). Thus, a full three-dimensional
network is present.
trans-[Fe(DHBuPE)2(CO)Cl][B(C6H5)4] (8). Two inde-
pendent trans-octahedral Fe(L2)(CO)Cl+ complexes are
present. They lie on crystallographic centers of symmetry
so that the trans ligands are necessarily disordered. In
addition, O(2) is disordered over two positions. The hy-
droxyalkyl side chains are extended and all oxygen atoms
have short intramolecular O‚‚‚O contacts, attributable to
hydrogen bonds.
cis-Fe(DHPrPE)2SO4 (2). Although the hydrogen atoms
of the -OH groups could not be clearly located, numerous
O‚‚‚O contacts under 2.9 Å suggest there is a network of
cation-cation and cation-anion hydrogen bonds. These
include two short contacts (2.530, 2.576(7) Å) each involving
an anion oxygen atom and a coordinated hydroxyl group.
Figure 6. Molecular structure of the trans-Fe(DHPrPE)2(CO)SO4 (9)
complex.
Scheme 3. Reactions of 3 and 6 with CH3CN
trans-Fe(DHPrPE)2(CO)(SO4) (9). Despite the partial
disorder, the main features of the structure are clear. The
iron atom exhibits octahedral coordination, with two DHPrPE
ligands in the equatorial plane and a carbonyl and a
monodentate sulfate ligand in axial psoitions. The Fe-C
bond length is 1.686(12) Å compared with 1.709, 1.717(16)
Å for the Fe-C bonds in the two disordered cations of Fe-
(DHBuPE)2(CO)Cl+. The Fe-C-O unit is nearly linear
(178.9(8)°). The hydroxyl groups avoid the vicinity of the
carbonyl oxygen, O(9). Most of the hydroxyl groups make
close contacts (2.58-2.74(2) Å) with hydroxyl or noncoor-
dinated sulfate oxygens in other molecules; in addition, an
intra-molecular contact O(4)‚‚‚O(11) (2.80(2) Å) may also
indicate a hydrogen bond.
CO(g) in methanol was immediate. The product was spec-
troscopically identical with the one formed by reaction of
cis-Fe(DHBuPE)2SO4 (3) in methanol in the presence of CO,
i.e., trans-Fe(DHBuPE)2(CO)SO4. Similarly, the reaction of
trans-Fe(DHBuPE)2(H2O)(SO4) (6) with CH3CN formed a
product spectroscopically identical with the one formed by
reaction of cis-Fe(DHBuPE)2SO4 (3) in methanol with CH3-
CN, namely trans-[Fe(DHBuPE)2(CH3CN)2]SO4. The reac-
tion of trans-Fe(DHBuPE)2(H2O)(SO4) (6) with CH3CN
suggests that CH3CN will substitute for coordinated water.
Finally, it is noted that reactions similar to those above
also occurred in aqueous solution. For example, when the
reactions of cis-Fe(DHPrPE)2Cl2 with CO and CH3CN were
carried out in water, the products were trans-Fe(L2)(L′)Cl+
(L′ ) CO, CH3CN; L′ ) CO, 31P{1H} NMR (CD3OD) δ
62.6 (s); ν(CtO) 1930 cm-1; L′ ) CH3CN, 31P{1H} NMR
(CD3OD) δ 61.8 (s)). Likewise, the reactions of cis-Fe-
(DHPrPE)2SO4 (2) with CO and CH3CN in water gave trans-
Fe(L2)(L′)(OSO3) (L′ ) CO, CH3CN; L′ ) CO, 31P{1H}
NMR (CD3OD) δ 66.8 (s); ν(CtO) 1927 cm-1; L′ ) CH3-
CN, 31P{1H} NMR (CD3OD) δ 62.3 (s)).
trans-Fe(DMeOPrPE)2Cl2 (4). The complexes lie on
crystallographic centers of symmetry. The Fe-Cl and Fe-P
bond lengths and chelate P-Fe-P bond angle are very
similar to those in complex 1. As was found for the latter,
the extended side chains are free from disorder, although in
this case there are no intermolecular contacts under 3.4 Å
and no possibility of intermolecular hydrogen bonds.
trans-Fe(DMeOPrPE)2Br2 (5). The compound is isomor-
phous and closely isostructural with the dichloro complex,
4.
trans-[Fe(DHBuPE)2Cl2]Cl (10). The trans-octahedral
[FeIII(DHBuPE)2Cl2]+ cation lies on a crystal center of
symmetry, and the ionic chlorine, Cl(2), on a crystal diad
axis. Compared with the FeII complex 2, the Fe-Cl bonds
are shortened [2.361(1) Å to 2.250(1) Å] and the Fe-P bonds
are lengthened [2.265, 2.284(1) Å to 2.334, 2.342(2) Å]. All
side chains are extended; one is disordered, and in addition
three of the four independent terminal oxygens have alterna-
tive sites. Several intermolecular O‚‚‚O contacts [2.48-2.91-
5464 Inorganic Chemistry, Vol. 41, No. 21, 2002

Synthesis of Water-Soluble Complexes of Iron(II)
(3) Å] and Cl(2)‚‚‚O contacts [2.88-2.33(1) Å] may indicate
hydrogen bonds.
With regard to the 1,2-bis(bis(hydroxyalkyl)phosphino)-
ethane ligands, it is noteworthy that these ligands were
originally designed to replace the more commonly used
sulfonated phosphines, which in our hands were not neces-
sarily innocent, i.e., the sulfonate group frequently bonded
to the metal and prevented the coordination of small
molecules. With one exception, this study found that the
-OH ligands are noninterfering. (The one exception was cis-
[Fe(DHPrPE)2]SO4 (2), in which two OH groups bonded to
the metal to form two six-membered rings.)
Conclusions. An important consideration in removing N2
from methane (e.g., natural gas purification) by using
reversible N2-binding to a metal complex in a homogeneous
process is to use a polar solvent to enhance the separation
by decreasing the solubility of methane in the carrier solvent.
Because water is the best solvent for a properly designed
absorbent (in that it provides the highest N2/CH4 selectivity),
potential dinitrogen-absorbing compounds ideally are water
soluble. This work showed that the water-soluble 1,2-bis-
(bis(hydroxyalkyl)phosphino)ethane ligands react with FeCl2
or FeSO4 to form water-soluble complexes of the general
type Fe(PP)2X (X ) (Cl)2 or SO4). Whether the products
have a cis or trans stereochemistry depends on the solvent,
the counterion, and the alkyl chain length of the phosphine.
Of importance to the long-range goal of finding a suitable
carrier molecule, the chloride or sulfate ligands in the Fe-
(PP)2X complexes are labile, and they are readily substituted
by N2, CO, or CH3CN. Quantitative measurements of the
N2-binding in water will be reported in a subsequent paper,
as will the synthesis of water-soluble complexes of the type
trans-Fe(PP)2(H)(X), which have superior N2-binding ability
compared to the complexes reported in this paper.
Acknowledgment. The authors acknowledge the National
Science Foundation for the support of this research. P.R.B.,
W.K.M., and D.K.L. acknowledge support from DOE-METC
and internal R&D funds from Bend Research, Inc.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determinations of 1, 2, 4, 5, 8, 9,
and 10; four figures showing the 31P{1H} NMR spectrum of trans-
[Fe(DHBuPE)2(CO)Cl][B(C6H5)4] (8), selected infrared spectra, and
the molecular structures of the trans-Fe(DMeOPrPE)2Br2 (5) and
trans-[Fe(DHBuPE)2Cl2]Cl (10) complexes; and two tables showing
phosphine ligand IR data and rate constants for reactions 5 and 6
as a function of temperature. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC025774Q
Inorganic Chemistry, Vol. 41, No. 21, 2002 5465