Acidic dicationic iron(II) dihydrogen complexes and compounds related by H2 substitution

Article abstract of DOI:10.1021/ic990876a

[trans-Fe(H₂)(CO)(dppe)₂]²⁺ (3) (dppe = 1,2-bis(diphenylphosphino)ethane) was generated by protonation of [trans-FeH(CO)(dppe)₂]⁺ in CD₂Cl₂. [trans-Fe(H₂)(CO)(depe)

Full text of DOI:10.1021/ic990876a

6060
Inorg. Chem. 1999, 38, 6060-6068
Acidic Dicationic Iron(II) Dihydrogen Complexes and Compounds Related by H2
Substitution
Shaun E. Landau, Robert H. Morris,* and Alan J. Lough
Department of Chemistry, University of Toronto, 80 St. George Street,
Toronto, Ontario, M5S 3H6, Canada
ReceiVed July 23, 1999
[trans-Fe(H2)(CO)(dppe)2]2+ (3) (dppe ) 1,2-bis(diphenylphosphino)ethane) was generated by protonation of
[trans-FeH(CO)(dppe)2]+ in CD2Cl2. [trans-Fe(H2)(CO)(depe)2]2+ (6) (depe ) 1,2-bis(diethylphosphino)ethane)
was generated by the treatment of [trans-FeCl(CO)(depe)2]+ in CD2Cl2 with AgSbF6 under 1 atm of H2. Complex
3 is more acidic than trifluoromethanesulfonic acid (HOTf) in CD2Cl2, while 6 is suspected to be less acidic than
[Et2OH]+. 3[OTf]2 is stable to H2 loss under reduced pressure for several hours, an indication of strong three-
center (Fe-H2), two-electron σ-bonding. Both complexes 3 and 6 undergo H2 substitution reactions. There is
evidence of the formation of [trans-Fe(H2O)(CO)(dppe)2]2+ and [trans-Fe(OTf)(CO)(dppe)2]+, although these
complexes could not be isolated. [trans-FeY(CO)(depe)2]Y complexes (Y- ) [BF4]-, 7[BF4]; Y- ) [OTf]-,
8[OTf]) were isolated from the corresponding reactions of [trans-FeH(CO)(depe)2]Y with [Et2OH][BF4] or HOTf.
7[BF4] was structurally characterized by single-crystal X-ray diffraction. Attempts to grow crystals of 8[OTf]
yielded salts containing the complex [trans-Fe(H2O)(CO)(depe)2]2+ (9), which were structurally characterized by
single-crystal X-ray diffraction. Coordination of [BF4]- in 7[BF4] was demonstrated, by variable-temperature
31P{1H} NMR spectroscopy, to be dynamic. Dissolving 7[BF4] in methanol results in nucleophilic substitution at
B to yield the new complex [trans-FeF(CO)(depe)2]+ (10). An attempt to grow crystals of 10[BF4] from the
reaction mixture resulted in crystals of [H2(depe)][BF4], which were structurally characterized by single-crystal
X-ray diffraction.
Introduction
(NH3)5]2+, although dicationic, is not appreciably acidic com-
pared to most other dicationic dihydrogen complexes.6 The
comparatively high pKa of [Os(H2)(NH3)5]2+ is relevant to this
study as it relates to the favorable formation of the new aqua
In η2-H2 complexes metal-H2 bonding has both σ and π
components. The relative strength and importance of the two
bonding interactions has been a topic of considerable discussion
in the past, with emphasis being put on the importance of the
π component.1,2 However, this work3 and the recent work of
others4 on the synthesis of electron-deficient dicationic dihy-
drogen complexes, thought to possess little potential for metal-
H2 π-bonding, indicate that the stability of dihydrogen com-
plexes with respect to elimination of H2 is highly dependent on
the strength of the metal-H2 σ interaction.
complexes [Fe(H2O)(CO)(dppe)]2+ and [Fe(H2O)(CO)(depe)]2+
.
The strong σ-donor ligands H2O and NH3 are highly polarizable
and thus stabilize electron-deficient metal centers.
Dicationic dihydrogen complexes tend to be highly acidic
and are often susceptible to H2 ligand substitution reactions.
For this reason the counteranions chosen for these dihydrogen
complexes are critical since many of these complexes cannot
exist in the presence of even very weakly nucleophilic anions
such as [BF4]- and [OTf]-.
Also relevant to this study was the nature of the ancillary
ligand set and the effects of its variation on the stability and
reactivity of the complexes. Both of these new dicationic iron
dihydrogen complexes have a {(R2PCH2CH2PR2)2(CO)} (R )
Ph, Et) ancillary ligand set. In terms of electronics dppe (R )
Ph) is much less donating than depe (R ) Et), where the EL
parameters for these bidentate ligands are 0.72 V (2 × 0.36 V)
and 0.58 V (2 × 0.29 V), respectively.7 However, depe is much
less sterically bulky than dppe, where the cone angles about P
for these ligands are approximately 122° (PEt3) and 136°
(PMePh2), respectively.8 The effects of these electronic and
steric disparities will be discussed.
One of us previously proposed that for any d6 octahedral metal
complex with dinitrogen trans to a π acid ligand and having
E°(d5/d6) greater than or equal to 1 V, the corresponding
dihydrogen complex would be thermally unstable with respect
to H2 elimination.2 This report deals with the synthesis,
characterization, and reactivity of two dicationic dihydrogen
complexes of iron: [Fe(H2)(CO)(dppe)2]2+ and [Fe(H2)(CO)-
(depe)2]2+. These complexes, on the basis of ligand additivity
principles, are expected to have dinitrogen analogues with E°
values much greater than 1 V, and yet these complexes are
remarkably stable with respect to elimination of H2.
A number of dicationic dihydrogen complexes have been
reported recently including the highly acidic complexes
[Os(H2)(CO)(dppp)2]2+ 5 and [Fe(H2)(CNH)(dppe)2]2+.3 [Os(H2)-
Many of the known cationic dihydrogen complexes are highly
acidic. The generation of these highly acidic dihydrogen
* To whom correspondence should be addressed. E-mail: rmorris@
chem.utoronto.ca.
(5) Rocchini, E.; Mezzetti, A.; Ruegger, H.; Burckhardt, U.; Gramlich,
V.; Delzotto, A.; Martinuzzi, P.; Rigo, P. Inorg. Chem. 1997, 36, 711-
720.
(6) Harman, W. D.; Taube, H. J. Am. Chem. Soc. 1990, 112, 2261-2263.
(7) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271-1285.
(8) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(1) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120-128.
(2) Morris, R. H. Inorg. Chem. 1992, 31, 1471-1478.
(3) The communication of part of the present work: Forde, C. E.; Landau,
S. E.; Morris, R. H. J. Chem. Soc., Dalton Trans. 1997, 1663-1664.
(4) Luther, T. A.; Heinekey, D. M. Inorg. Chem. 1998, 37, 127-132.
10.1021/ic990876a CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/04/1999

Dicationic Iron(II) Dihydrogen Complexes
Inorganic Chemistry, Vol. 38, No. 26, 1999 6061
Table 1. Crystal Structure Data Acquisition and Solution Details
9[OTF]2‚H2O
{9[OTf]}2[{(OTf)2Ag}2(µ-depe)]
7[BF4]
empirical formula
fw
C23H52F6FeO9P4S2
830.50
C58H124Ag2F18Fe2O22P10S6
2345.32
C21H48B2F8FeOP4
669.95
cryst syst
space group
a/Å
b/Å
c/Å
â/deg
V/Å3
Z
monoclinic
Cc
monoclinic
P2(1)/c
monoclinic
P21/m
21.5801(5)
10.3554(2)
20.1249(5)
118.318(1)
3959.1(2)
4
1.393
0.717
200.0(1)
0.0566
0.1682
16.6339(6)
14.7060(5)
19.0563(6)
94.328(2)
4648.2(3)
2
1.676
1.126
100.0(1)
0.0361
0.1013
10.270(2)
15.014(3)
39.908(8)
96.23(3)
6117(2)
8
1.455
0.766
173.0(1)
0.0608
0.1560
d
calcd/(Mg m-3
)
µ(Mo ΚR)/mm-1
T/K
R1a [I > 2σ(I)]
wR2a (all data)
a R1 ) ∑(Fo - Fc)/∑(Fo). b wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
2
2
Table 2. Selected Bond Lengths
complexes by the treatment of some precursor complex with
molecular hydrogen therefore has potential application where
strong acids are required. The in situ generation of these acids
from molecular H2 has obvious advantages including handling
and storage.
bond length/Å
{9[OTf]}2-
9[OTF]2‚H2O {[(OTf)2Ag]2(µ-depe)}
7[BF4]
Fe(1)-P(1)
Fe(1)-P(2)
Fe(1)-P(3)
Fe(1)-P(4)
Fe(1)-C(1)
Fe(1)-O(2)
Fe(1)-F(4)
2.291(2)
2.285(2)
2.303(2)
2.301(2)
1.719(7)
2.051(5)
2.2940(9)
2.2917(9)
2.2770(9)
2.2730(9)
1.722(4)
2.037(2)
2.285(3)
2.299(3)
2.287(3)
2.302(3)
1.71(1)
Experimental Section
General Procedures. Unless otherwise stated, all manipulations and
reactions were carried out under an atmosphere of prepurified argon
or nitrogen (both gases were suitable) using standard Schlenk and
glovebox techniques. Unless otherwise stated, all solvents were
thoroughly dried over the appropriate drying agents and made free of
nitrogen and oxygen by distillation under argon. THF, Et2O, toluene,
and hexanes were refluxed over sodium wire with benzophenone
2.081(6)
Table 3. Selected Bond Anglesa
angle/deg
i
{9[OTf]}2-
indicator. CH2Cl2 was refluxed over calcium hydride. PrOH, EtOH,
9[OTF]2‚H2O {[(OTf)2Ag]2(µ-depe)} 7[BF4]
and MeOH were refluxed over iodine-activated magnesium turnings.
Acetone was refluxed over anhydrous CaSO4. CD2Cl2, acetone-d6, and
CDCl3 were obtained from Cambridge Isotopes Laboratories. These
deuterated solvents were degassed by freeze-pump-thaw (three cycles)
and dried by storing over molecular sieves (3A, beads, 8-12 mesh,
Aldrich Chemical Co.).
C(1)-Fe(1)-O(2)
C(1)-Fe(1)-P(2)
O(2)-Fe(1)-P(2)
C(1)-Fe(1)-P(1)
O(2)-Fe(1)-P(1)
P(2)-Fe(1)-P(1)
C(1)-Fe(1)-P(4)
O(2)-Fe(1)-P(4)
P(2)-Fe(1)-P(4)
P(1)-Fe(1)-P(4)
C(1)-Fe(1)-P(3)
O(2)-Fe(1)-P(3)
P(2)-Fe(1)-P(3)
P(1)-Fe(1)-P(3)
P(4)-Fe(1)-P(3)
178.3(2)
88.8(2)
90.3(2)
89.0(2)
89.5(1)
86.30(6)
90.2(2)
90.7(1)
178.65(7)
94.57(7)
91.4(2)
177.8(1)
89.4(1)
90.38(7)
90.9(1)
86.94(7)
86.02(3)
89.9(1)
90.36(7)
179.00(4)
94.70(3)
91.2(1)
175.8(4)
93.1(4)
90.8(2)
88.2(4)
90.5(2)
83.8(1)
87.7(3)
88.5(2)
178.4(1)
97.6(1)
93.8(4)
87.7(2)
94.0(1)
177.1(1)
84.6(1)
1H, 31P, and 19F NMR spectra were acquired on a Varian Gemini
1
300 MHz spectrometer operating at 300 MHz for H, 121 MHz for
31P, and 282 MHz for 19F. 2H NMR spectra were acquired on a Varian
Unity 400 MHz instrument operating at 61 MHz for 2H. These
instruments were used to acquire spectra in deuterated and nondeuter-
1
ated solvents. For the acquisition of H NMR spectra of samples in
90.2(2)
90.99(7)
94.02(3)
177.93(4)
85.29(3)
nondeuterated solvents, the following criteria were essential: deactiva-
tion of deuterium locking (set IN ) N on Gemini instruments),
minimization of pulse width (set PW ) 0.3 on Gemini instruments),
minimization of gain (set GAIN ) 0 on Gemini instruments). For good-
quality spectra a double-precision Fourier transform (set MATH ) D
on Gemini instruments) and high sample concentration were desirable.
The acquisition of 31P and 19F NMR spectra for samples in nondeu-
terated solvents required only the deactivation of deuterium locking.
1H NMR spectra were indirectly referenced to TMS via solvent peaks.
31P and 19F NMR spectra were referenced to external 85% H3PO4 and
external CFCl3, respectively. T1 measurements were made using the
inversion recovery method.
94.23(7)
179.39(7)
84.90(6)
a For 7[BF4], substitute F(4) for O(2).
Refinement was by full-matrix least-squares on F2 using all data
(negative intensities included). Hydrogen atoms were included in
calculated positions, except for the hydride atoms, which were refined
with isotropic thermal parameters. Structure solution and refinement
details are listed in Table 1. Selected bond lengths are given in Table
2, and selected bond angles are listed in Table 3. The bond lengths
and angles listed for [Fe(BF4)(CO)(depe)2][BF4] are for one of four
similar formula units.
Infrared spectra were acquired on a Nicolet Magna-IR spectrometer
550 or a Perkin-Elmer Paragon 500 FT-IR spectrometer.
Single-crystal X-ray diffraction data were collected using a Nonius
Kappa CCD diffractometer with graphite-monochromated Mo KR
radiation (λ ) 0.710 73 Å). A combination of 1° φ and ω (with κ
offsets) scans were used to collect sufficient data. The data frames were
integrated and scaled using the Denzo-SMN package.9 The structures
were solved and refined using the SHELXTL\PC V5.1 package.10
The following compounds were prepared by literature methods:
FeHCl(dppe)2,11 FeHCl(depe)2,12 [FeCl(CO)(depe)2]Cl.13 AgBF4,
AgOTf, AgSbF6, HBF4‚Et2O, HOTf, and DOTf were obtained from
Aldrich Chemical Co. The silver salts were heated at 100 °C under
(11) Aresta, M.; Giannoccaro, P.; Rossi, M.; Sacco, A. Inorg. Chim. Acta
1971, 5, 115-118.
(12) Chatt, J.; Hayter, R. G. J. Chem. Soc. 1961, 5507.
(13) Bellerby, J. M.; Mays, M. J.; Sears, P. L. J. Chem. Soc., Dalton Trans.
1976, 1232-1236.
(9) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326.
(10) Sheldrick, G. M. SHELXTL\PC V5.1; Bruker Analytical X-ray
Systems: Madison, WI, 1997.

6062 Inorganic Chemistry, Vol. 38, No. 26, 1999
Landau et al.
reduced pressure for several hours before use. dppe was obtained from
Digital Specialty Chemicals. depe was obtained from Strem. N2 gas
(grade 4.8), Ar gas (grade 5.0), CO gas (C.P. grade) and H2 gas (grade
4.0) were obtained from BOC Gases, Canada. D2 gas (C.P. grade) was
obtained from Matheson Gas Products, Canada.
31P{1H} NMR spectrum exhibited a major resonance for [FeH(CO)-
(dppe)2]+ and much less intense resonances for [Fe(H2O)(CO)(dppe)2]2+
and [Fe(OTf)(CO)(dppe)2]+ (approximately 5% each).
Observation of [Fe(HD)(CO)(dppe)2][OTf]2. A solution of [FeH-
(CO)(dppe)2][OTf] (0.032 g, 0.031 mmol) in CD2Cl2 (0.65 mL) was
treated with DOTf (0.056 g, 0.37 mmol). Some gas evolution was
observed. Isolation of the product was not attempted. 1H NMR
(CD2Cl2): δ -6.82 (t of qi, 1J(HD) ) 33.11 ( 0.05 Hz, 2J(HP) ) 3.3
Hz, 1H, FeH). 31P{1H} NMR (CD2Cl2): δ 67.4 (s).
Preparation of [FeCl(CO)(depe)2]Cl. This compound was prepared
by the literature method of Bellerby, Mays, and Sears.13 No NMR
spectroscopic data have been published for this compound, so it is
1
presented here. H NMR (CDCl3): δ 2.40 (m, 4H, backbone CH2):
Reaction of [Fe(H2)(CO)(dppe)2][OTf]2 with D2. HOTf (0.042 g,
0.280 mmol) was added to a solution of [FeH(CO)(dppe)2][OTf] (0.026
g, 0.025 mmol) in CD2Cl2 (0.65 mL). The solution was cooled to
approximately -130 °C (ethanol cooled to freezing point with liquid
N2), evacuated, and filled with 1 atm of D2 gas. The tube and contents
were shaken on reaching room temperature and immediately before
NMR examination. The H NMR spectrum exhibited a resonance at
11.33 ppm, which is attributed to DOTf, in addition to the intense
CD2Cl2 resonance.
2.15 (m, 8H, ethyl CH2), 2.01 (m, 8H, ethyl CH2), 1.82 (m, 4H,
backbone CH2), 1.33 (m, 12H, CH3), 1.27 (m, 12H, CH3). 31P{1H}
NMR (CDCl3): δ 66.7 (s). IR (CH2Cl2, cm-1): 1932 (CO).
Preparation of [FeH(CO)(depe)2][BF4] (1[BF4]). This method is
similar to that reported by Bancroft for the preparation of [FeH(CO)-
(depe)2][BPh4].14 A mixture of acetone (20 mL), FeHCl(depe)2 (1.273
g, 2.521 mmol), and NaBF4 (0.920 g, 8.38 mmol) was stirred for 12 h
under 1 atm of CO. The solvent was removed under vacuum, and the
residue was extracted with CH2Cl2 (20 mL). The solids were filtered
off, washed with CH2Cl2 (2 × 1.5 mL), and discarded. The combined
filtrate and washings were reduced in volume under reduced pressure
to 5 mL, and Et2O (25 mL) was slowly added with stirring to precipitate
the yellow product. The product was collected by filtration, washed
with Et2O (3 × 3 mL), and dried under vacuum. Yield: 63%. 1H NMR
(CDCl3): δ 2.01 (m, 8H, ethyl CH2), 1.84 (m, 4H, backbone CH2),
1.78 (m, 8H, ethyl CH2), 1.46 (m, 4H, backbone CH2), 1.23 (m, 12H,
2
Preparation of [Fe(BF4)(CO)(depe)2][BF4] (7[BF4]). A solution of
HBF4‚Et2O in Et2O (1.732 g, 85% by mass, 9.09 mmol) was added to
a solution of [FeH(CO)(depe)2][BF4] (0.410 g, 0.702 mmol) in CH2Cl2
(10 mL). The resulting solution was stirred for 3 h, after which time
50 mL of Et2O was added with stirring to precipitate an orange, pasty
solid. The pale yellow liquor was decanted off and discarded. The
residue was dissolved in CH2Cl2 (5 mL) and carefully layered with
Et2O (20 mL) in a test tube with an inner diameter of 2 cm. Slow
diffusion afforded large orange-brown crystals, which were collected
by filtration, washed with Et2O (3 × 1.5 mL), and dried under vacuum.
2
CH3), 1.10 (m, 12H, CH3), -10.99 (qi, J(HP) ) 47 Hz, 1H, FeH).
31P{1H} NMR (CDCl3): δ 86.1 (s). IR (Nujol, cm-1): 1918 (CO).
Preparation of [FeH(CO)(depe)2][OTf] (1[OTf]). The procedure
for the preparation of [FeH(CO)(depe)2][BF4] was followed, substituting
NaOTf (1.435 g, 8.34 mmol) for NaBF4. Quantities of other reagents
used: acetone (20 mL), FeHCl(depe)2 (1.552 g, 2.825 mmol). Yield:
1
Yield: 64%. H NMR (CDCl3): δ 2.4-1.9 (m, 24H, CH2), 1.34 (m,
24H, CH3). 31P{1H} NMR (CDCl3): δ 67.8 (qi, 2J(FP) ) 5.2 Hz).
31P{1H} NMR (CH2Cl2, 293 K): δ 67.7 (qi, 2J(FP) ) 5.2 Hz). 31P{1H}
NMR (CH2Cl2, 223 K): δ 68.3 (d, 2J(FP) ) 20.2 Hz). 19F NMR
(CDCl3): δ -147.6 (br s, 4F, coordinated [BF4]-), -152.3 (br s, 4F,
free [BF4]-). IR (Nujol, cm-1): 1944 (CO). IR (CH2Cl2, cm-1): 1944.4
(CO). Anal. Calcd for C21H48B2F8FeOP4: C, 37.64; H, 7.24. Found:
C, 37.26; H, 7.61.
1
64%. H NMR (CDCl3): δ: 2.01 (m, 8H, ethyl CH2), 1.84 (m, 4H,
backbone CH2), 1.78 (m, 8H, ethyl CH2), 1.46 (m, 4H, backbone CH2),
2
1.23 (m, 12H, CH3), 1.10 (m, 12H, CH3), -10.99 (qi, J(HP) ) 47
Hz, 1H, FeH). 31P{1H} NMR (CDCl3): δ 86.1 (s). IR (Nujol, cm-1):
1918 (CO).
A crystal of [Fe(BF4)(CO)(depe)2][BF4] was grown by slow vapor
diffusion of Et2O into a CH2Cl2 solution of this salt. The crystal was
analyzed by single-crystal X-ray diffraction.
Preparation of [FeH(CO)(dppe)2][OTf] (2[OTf]). The procedure
outlined for the preparation of [FeH(CO)(depe)2][BF4] was followed,
substituting NaOTf (1.435 g, 8.34 mmol) for NaBF4 and FeHCl(dppe)2
(1.055 g, 1.130 mmol) for FeHCl(depe)2. Quantities of other reagents
used: acetone (20 mL). The product was purified by reprecipitation
by slow addition of Et2O to a CH2Cl2 solution of the salt. Yield 70%.
1H NMR (CDCl3): δ 7.41-7.06 (m, 40H, phenyl H), 2.41 (m, 4H,
Observation of [FeF(CO)(depe)2][BF4] (10[BF4]). [Fe(BF4)(CO)-
(depe)2][BF4] (0.015 g, 0.022 mmol) was dissolved in CD3OD (0.65
mL) and analyzed by 1H and 31P{1H} NMR spectroscopy. Attempts to
isolate this salt by precipitation were unsuccessful as the product was
always contaminated with [Fe(BF4)(CO)(depe)2][BF4]. Slow diffusion
(several weeks) of Et2O into a MeOH solution of [Fe(BF4)(CO)(depe)2]-
[BF4] resulted in the crystallization of [H2(depe)][BF4]2 as evidenced
by a single-crystal X-ray diffraction structural determination. 1H NMR
(CD3OD): δ 2.4-1.7 (m, 24H, CH2), 1.31 (m, 12H, CH3), 1.23 (m,
2
backbone CH2), 2.16 (m, 4H, backbone CH2), -7.74 (qi, J(HP) )
47.1 Hz, 1H, FeH). 31P{1H} NMR (CDCl3): δ 84.8 (s). IR (Nujol,
cm-1): 1947 (CO).
Observation of [Fe(H2)(CO)(dppe)2][OTf]2 (3[OTf]2). HOTf (0.045
g, 0.30 mmol) was added to a solution of [FeH(CO)(dppe)2][OTf] (0.030
g, 0.029 mmol) in CD2Cl2 (0.65 mL) or CDCl3 (0.65 mL). Some gas
evolution was observed. Isolation of the product was not attempted.
1H NMR (CDCl3): δ 7.56, 7.40, 7.26, 6.62 (multiplets, 40H, phenyl
H), 2.99 (m, 4H, CH2), 2.58 (m, 4H, CH2), -6.67 (br s, 2H, FeH). 1H
NMR (CD2Cl2): δ -6.8 (br s, 2H, FeH. T1 data (300 MHz): 0.013 s
at 273 K, 0.011 s at 248 K, 0.011 s at 223 K, 0.023 s at 198 K. η2-H2
T1(min) ) 0.011 ( 0.001 s (235 K, 300 MHz, CD2Cl2). T1(min) was
found by fitting observed data to a general equation.15 31P{1H} NMR
(CD2Cl2): δ 67.4 (s). 31P{1H} NMR (CDCl3): δ 68.1 (s). IR (CH2Cl2),
cm-1: 2006 (CO).
Testing the Stability of [Fe(H2)(CO)(dppe)2][OTf]2. A solution
of [Fe(H2)(CO)(dppe)2][OTf]2 (prepared as previously outlined) was
exposed to partial vacuum for a period of 2 h. The solvent had
evaporated off within 15 min, and the excess HOTf had evaporated
off within the first hour. The orange solid residue remaining was
dissolved in CH2Cl2 and examined by 1H and 31P{1H} NMR spectros-
copy. The 1H NMR spectrum exhibited no η2-H2 resonance; however,
an intense upfield resonance was observed for [FeH(CO)(dppe)2]+. The
2
12H, CH3). 31P{1H} NMR (CD3OD): δ 72.8 (d, J(FP) ) 28.8 Hz).
19F NMR (CD3OD): δ -152.7 (s, 4F, free [BF4]-), -156.2 (s, 1F,
FeF).
Observation of [Fe(H2)(CO)(depe)2][SbF6]2 (6[SbF6]2). CD2Cl2
(0.65 mL) was distilled under partial static vacuum into an NMR tube
containing a mixture of [FeCl(CO)(depe)2]Cl (0.021 g, 0.037 mmol)
and AgSbF6 (0.032 g, 0.093 mmol). The tube was filled with 1 atm of
H2, and the contents were vigorously shaken several times over 1 h.
1
No attempt was made to isolate the product. H NMR (CD2Cl2): δ
1.8-2.5 (m, 24H, CH2), 1.26 (m, 24H, CH3), -9.82 (br s, 2H, FeH).
T1 data (300 MHz): 0.014 s at 260 K, 0.012 s at 248 K, 0.010 s at 236
K, 0.011 s at 229 K. η2-H2 T1(min) ) 0.010 ( 0.001 s (225 K, 300
MHz, CD2Cl2). T1(min) was found by fitting observed data to a general
equation.15 31P{1H} NMR (CD2Cl2): δ 69.5 (s). IR (CH2Cl2, cm-1):
1996 (CO).
Preparation of [Fe(OTf)(CO)(depe)2][OTf] (8[OTf]). [FeH(CO)-
(depe)2][OTf] (0.895 g, 1.38 mmol) was dissolved in 2 mL of CH2Cl2,
and HOTf (0.425 g, 2.83 mmol) was slowly added with stirring.
Addition of the acid resulted in vigorous gas evolution and darkening
of the solution (turned from yellow to intense orange). This solution
was stirred for 16 h to ensure complete reaction, and then it was filtered
through Celite. Et2O (15 mL) was slowly added to the filtrate with
stirring and the yellow product eventually oiled out. This mixture was
(14) Bancroft, G. M.; Mays, M. J.; Prater, B. E.; Stefanini, F. P. J. Chem.
Soc. A 1970, 2146-2149.
(15) Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H.; Schweitzer,
C. T.; Sella, A. J. Am. Chem. Soc. 1988, 110, 7031-7036.

Dicationic Iron(II) Dihydrogen Complexes
Inorganic Chemistry, Vol. 38, No. 26, 1999 6063
(depe)2]+. 2[OTf] may also be contaminated if the FeHCl(dppe)2
used in its preparation is contaminated with paramagnetic FeCl2-
(dppe), which reacts with CO to produce diamagnetic isomers
of FeCl2(CO)2(dppe).16 The cations 1 and 2 were each assigned
a trans configuration on the basis of the upfield quintet in their
1H NMR spectra and singlet in their 31P{1H} NMR spectra.
These yellow salts are air-stable in the solid state and when
dissolved in acetone, CH2Cl2, and CHCl3.
Observation of [Fe(H2)(CO)(dppe)2][OTf]2. The synthesis
of dihydrogen complexes is usually achieved via two general
routes: coordination of H2 or protonation of a classical hydride
ligand. In the case of [Fe(H2)(CO)(dppe)2]2+ only the second
of these two routes was employed since a suitable precursor
for the H2-coordination method, such as [FeX(CO)(dppe)2]+ (X
) halide), was unavailable. [FeCl(CO)(dppe)2][FeCl4] has been
reported in the literature.17 However, due to the poor yields cited
and paramagnetic nature of the anion, the generation of
[Fe(H2)(CO)(dppe)2]2+ using this precursor was not attempted.
The complete protonation of 2 (eq 1) required at least 10
equiv of HOTf to generate the new dihydrogen complex [trans-
Fe(H2)(CO)(dppe)2]2+ (3). The aqueous pKa of HOTf has been
stirred for another 12 h, during which time the oil had changed to a
yellow powder. The solid was collected by filtration, washed with 5 ×
3 mL of Et2O, and dried under vacuum. To ensure that the product
was free of water and HOTf, it was recrystallized by vapor diffusion
of Et2O into an EtOH solution of the salt, and the crystals obtained
were heated for 2 days at 100 °C while being exposed to vacuum.
1
Yield: 93%. H NMR (CDCl3): δ 2.29 (m, 8H, CH2), 2.13 (m, 8H,
CH2), 2.02 (m, 8H, CH2), 1.31 (m, 24H, CH3). 31P{1H} NMR
(CDCl3): δ 65.0 (s). 19F{1H} NMR (CDCl3): δ -77.7 (s, 3F,
coordinated [OTf]-), -78.4 (s, 3F, free [OTf]-). IR (Nujol, cm-1):
1949, 1940 (CO). IR (CH2Cl2, cm-1): 1943 (CO). Anal. Calcd for
C23H48F6FeO7P4S2: C, 34.77; H, 6.10. Found: C, 34.61; H, 6.21.
Repeated attempts to grow crystals of [Fe(OTf)(CO)(depe)2][OTf]
for X-ray structural determination were unsuccessful.
Observation of [Fe(H2O)(CO)(depe)2][OTf]2 (9[OTf]2). [Fe(OTf)-
(CO)(depe)2][OTf] (0.015 g, 0.019 mmol) was dissolved in acetone-d6
(0.65 mL), and distilled, degassed H2O (0.117 g, 6.49 mmol) was added
to the resulting solution. 1H NMR (acetone-d6): δ 4.58 (br s, 2H, H2O),
2.46-2.24 (m, 24H, CH2), 1.41 (m, 24H, CH3). 31P{1H} NMR (acetone-
d6): δ 67.8 (s). 31P{1H} NMR (CDCl3): δ 66.5 (s). 19F{1H} NMR
(CDCl3): δ -78.6 (s).
Crystals of 9[OTf]2 could not be grown from the reaction mixtures.
However, a crystal of [Fe(H2O)(CO)(depe)2][OTf]2‚H2O was grown
by slow vapor diffusion of Et2O into an EtOH (apparently wet) solution
of [Fe(OTf)(CO)(depe)2][OTf]. The crystal was analyzed by single-
crystal X-ray diffraction.
Observation of {[Fe(H2O)(CO)(depe)2][OTf]}2[{(OTf)2Ag}2(µ-
depe)]. A mixture of CH2Cl2 (10 mL), AgOTf (0.465 g, 1.46 mmol),
and [FeCl(CO)(depe)2]Cl (0.224 g, 0.395 mmol) was stirred for 12 h,
after which time the excess AgOTf and precipitated AgCl were removed
by filtration. The yellow filtrate was concentrated under reduced
pressure to 5 mL, and Et2O (25 mL) was added to precipitate the yellow
product. The product was collected by filtration, washed with Et2O (3
× 3 mL), and dried under vacuum. The isolated product was expected
to be [Fe(OTf)(CO)(depe)2][OTf], and its CDCl3 solution 31P{1H} NMR
spectrum exhibited only a resonance at 65.0 ppm. However, a single
crystal of {[Fe(H2O)(CO)(depe)2][OTf]}2[{(OTf)2Ag}2(µ-depe)] was
obtained by diffusion of Et2O into a CH2Cl2 solution of the product
isolated from this reaction. The crystal was analyzed by X-ray
diffraction.
(1)
estimated5 at approximately -5, indicating a pKa of less than
-5 for 3. The related and very acidic complex [trans-Os(H2)-
(CO)(dppp)2]2+ has been reported to have an estimated pKa of
-6.5
The 31P{1H} NMR spectrum for a yellow-orange CH2Cl2
solution of 3[OTf]2 consists of a singlet at 67.4 ppm, indicating
a trans configuration about iron in 3. Preparations of 3 must be
done in the absence of nucleophiles and bases including H2O.
For the preparation of 3[OTf]2 the reaction conditions are
extremely acidic and result in the gradual decomposition of the
complex as evidenced by the appearance of a 31P{1H} NMR
resonance at 10.9 ppm, which is attributed to protonated dppe.
Two additional species have been observed as singlets at 60.6
and 59.7 ppm in 31P{1H} NMR spectra for preparations of
3[OTf]2 in CH2Cl2. These complexes could not be isolated but
have been identified as [trans-Fe(H2O)(CO)(dppe)2]2+ (4) and
[trans-Fe(OTf)(CO)(dppe)2]+ (5), respectively, on the basis of
the observation that the addition of H2O to these reaction
mixtures results in, among other things, a reduction of the
intensity of the signal at 59.7 ppm, while the signal at 60.6
ppm increases in intensity. The replacement of H2 in 3 by H2O
is consistent with the findings of Kubas, where in a related study
of tungsten complexes W(CO)3(PR3)2 (R ) alkyl), H2O was
found to displace H2 at room temperature.18 A 1H NMR signal
for coordinated H2O in 4 could not be assigned definitively,
although there were several resonances near 3 ppm which were
Reaction of [Fe(H2)(CO)(depe)2][SbF6]2 with D2. A CD2Cl2
solution of [Fe(H2)(CO)(depe)2][SbF6]2 was prepared as described above
except that 1 equiv of NaOTf was added to facilitate H+/D+ exchange.
The mixture was cooled to approximately -130 °C (ethanol cooled to
freezing with liquid N2), evacuated, and filled with 1 atm of D2 gas.
The tube was shaken prior to NMR spectroscopic examination. 1J(HD)
for [Fe(HD)(CO)(depe)2][SbF6]2 was estimated from the 1H NMR
spectrum to be 33 ( 1 Hz. [trans-FeD(CO)(depe)2]+ was observed by
2
31P{1H} NMR spectroscopy as a triplet at 86.0 ppm with a J(DP) of
9.8 Hz, in addition to [FeH(CO)(depe)2]+ (85.9 ppm) and [Fe(η2-L)-
(CO)(depe)2]2+ (L ) H2, HD, D2; broad resonance at 69.5 ppm). A
multiplet pattern was observed at -10.46 ppm in the 1H NMR spectrum.
This pattern appeared to be a doublet of doublet of doublets with
2
approximate J(HP) values of 33, 26, and 20 Hz. The splittings may
all be assumed to be 31P couplings since no other hydride resonance
was observed for this complex. This complex is believed to be
[FeH(CO)(OTf)(depe)(η1-Et2PCH2CH2PEt2H)]+
Results and Discussion
possible candidates (in [Os(H2)(H2O)(dppe)2]2+
a
1H NMR
Synthesis of the Monohydride Complexes. [FeH(CO)-
(depe)2][BF4] (1[BF4]), [FeH(CO)(depe)2][OTf] (1[OTf]), and
[FeH(CO)(dppe)2][OTf] (2[OTf]) were prepared in yields of
63%, 64%, and 70%, respectively, by exposing acetone solutions
of the corresponding FeHCl(pp)2 (pp ) bidentate diphosphine
ligand) complexes to CO in the presence of NaBF4 or NaOTf.
Caution must be exercised when FeHCl(depe)2 is prepared for
use in the preparation of salts of 1 since any unreacted FeCl2-
(depe)2 will react with CO to produce salts of [FeCl(CO)-
resonance for coordinated H2O appears at 3.2 ppm19). These
(16) Manuel, T. A. Inorg. Chem. 1963, 2, 854-858.
(17) Gao, Y.; Holah, D. G.; Hughes, A. N.; Spivak, G. J.; Havighurst, M.
D.; Magnuson, V. R.; Polyakov, V. Polyhedron 1997, 16, 2797-2807.
(18) Kubas, G. J.; Burns, C. J.; Khalsa, G. R. K.; Van der Sluys, L. S.;
Kiss, G.; Hoff, C. D. Organometallics 1992, 11, 3390-3404.
(19) Bartucz, T. Y.; Golombek, A.; Lough, A. J.; Maltby, P. A.; Morris,
R. H.; Ramachandran, R.; Schlaf, M. Inorg. Chem. 1998, 37, 1552-
1562.

6064 Inorganic Chemistry, Vol. 38, No. 26, 1999
Scheme 1. Reactions of [Fe(H2)(CO)(dppe)2][OTf]2
Landau et al.
Scheme 2. Displacement of H2 by [BF4]- in
[Fe(H2)(CO)(depe)2][BF4]2
Scheme 3. Reactions of [FeCl(CO)(depe)2]+
Figure 1. Structure of [Fe(OH2)(CO)(depe)2][OTf]2‚H2O. Carbon atom
labels and hydrogen atoms have been omitted for clarity.
of nucleophiles and bases. No reaction was observed when
Na[B(C6H3-3,5-(CF3)2)4] was used instead of AgSbF6. The use
of AgBF4 and AgOTf salts instead of AgSbF6 resulted in the
generation (Scheme 3) of 7 and 8, respectively, which again
demonstrates the necessity to minimize the donor properties of
the anions. Complexes 6, 7, and 8 are trans according to their
singlet 31P{1H} NMR spectra. 7 and 8 generated from [FeCl-
(CO)(depe)2]Cl are contaminated with the corresponding CH2Cl2-
soluble silver salts, which makes the halide abstraction method
undesirable and unsuitable for the preparation of pure 7[BF4]-
and 8[OTf]. All future references to 7[BF4] and 8[OTf], unless
otherwise stated, will be to those prepared via the protonation
of [FeH(CO)(depe)2]+.
reactions are summarized in Scheme 1 and are assumed, without
direct evidence, to proceed via the unobserved intermediate
[Fe(CO)(dppe)2]+. The pathway to complex 4 involving the
unprecedented intermediate complex [Fe(OH3)(CO)(dppe)2]3+
may or may not operate and is suggested as a possibility only
due to the fact that the concentration of H2O in these highly
acidic reaction mixtures is expected to be negligible.
31P{1H} NMR spectra for CH2Cl2 solutions of 7[BF4] and
8[OTf] often reveal the presence of an impurity in the form of
a singlet at 66.4 ppm (CH2Cl2) or 66.5 ppm (CHCl3). This
impurity has been identified as [trans-Fe(H2O)(CO)(depe)2]2+
(9) on the basis of the fact that the addition of a large excess of
water to solutions of 8[OTf] results in the quantitative conver-
sion of 8 to the impurity.
Attempts to grow crystals of 8[OTf] have afforded only
crystals of 9[OTf]2‚H2O (Figure 1). This is peculiar given that
the solutions from which the crystals were grown were prepared
using dry solvents and appeared to be free of 9[OTf]2 according
to 31P{1H} NMR spectroscopy. These observations indicate that
9[OTf]2 is much less soluble than 8[OTf] in the recrystallization
solvents used (EtOH, acetone, and CH2Cl2). Complex 9 is
octahedral, with CO trans to H2O. The observed Fe-P bond
lengths are between 2.28 and 2.30 Å, which is typical for
octahedral bis(depe) complexes of iron.20-23 According to a
1989 review of the Cambridge Crystallographic Database (to
which all subsequent bond length comparisons will be made,
unless otherwise stated), the observed Fe-CO bond length of
1.719(7) Å and Fe-OH2 bond length of 2.051(5) Å are both
Synthesis of [Fe(H2)(CO)(depe)2][SbF6]2 and Related
Complexes. Monohydride complex 1 is not a practical precursor
for the synthesis of the new dihydrogen complex [trans-
Fe(H2)(CO)(depe)2]2+ (6) since a sufficiently strong acid with
a weakly coordinating conjugate base is not commercially
available. This predicament is exemplified by failed attempts
to prepare 6 by the protonation of 1 using excess [HOEt2][BF4]
or HOTf. These experiments resulted in the quantitative
conversion to [trans-Fe(BF4)(CO)(depe)2]+ (7) or [trans-Fe(OTf)-
(CO)(depe)2]+ (8), respectively. 6 has been observed as a low-
concentration transient species by 31P{1H} NMR spectroscopy
in the reaction of 1 with [HOEt2][BF4] (Scheme 2). The
generation of 7 via the protonation of 1 required the addition
of only 1 equiv of [Et2OH][BF4], and on the basis of the
vigorous gas evolution in this reaction, complex 6 is likely
significantly less acidic than [HOEt2]+. 2 is unchanged in the
presence of excess [HOEt2][BF4], which demonstrates that 3 is
much more acidic than 6.
The new dihydrogen complex 6 may be generated by the H2-
coordination method since [FeCl(CO)(depe)2]Cl, a convenient
precursor, is easily prepared in high yield by the treatment of
FeCl2(depe)2 with CO in methanol.13 This precursor, when
treated with AgSbF6 under 1 atm of H2 in CD2Cl2, affords
yellow-orange solutions of 6[SbF6]2. As in the preparation of
3[OTf]2, the preparation of 6[SbF6]2 must be done in the absence
(20) Baker, M. V.; Field, L. D.; Hambley, T. W. Inorg. Chem. 1988, 27,
2872-2876.
(21) Barclay, J. E.; Hills, A.; Hughes, D. L.; Leigh, G. J. J. Chem. Soc.,
Dalton Trans. 1988, 2871-2877.
(22) Evans, D. J.; Henderson, R. A.; Hills, A.; Hughes, D. L.; Oglieve, K.
E. J. Chem. Soc., Dalton Trans. 1992, 1259-1265.
(23) Hirano, M.; Akita, M.; Tani, K.; Kumagai, K.; Kasuga, N. C.; Fukuoka,
A.; Komiya, S. Oranometallics 1997, 16, 4206-4213.

Dicationic Iron(II) Dihydrogen Complexes
Inorganic Chemistry, Vol. 38, No. 26, 1999 6065
correspond to d(HH) values of 0.87 Å. A rather peculiar and
consistent observation is that, in general, 1J(HD) is between 32
and 34 Hz for complexes with HD trans to CO, regardless of
the other ligands involved.25,26a It appears that the magnetic and
structural features of a dihydrogen ligand which is trans to CO
are insensitive to changes in the remaining ligands. This is
consistent with the fact that the observed T1(min) values for
complexes 3 and 6 are 0.011 ( 0.001 s (235 K, 300 MHz,
CD2Cl2) and 0.010 ( 0.001 s (225 K, 300 MHz, CD2Cl2), which
are equal within experimental error. Similarly in the related
complexes [trans-Mn(H2)(CO)L2][BArf4], L ) dppe, depe, the
H-H distances are equal within experimental error.26
The fact that changes in the P4 ligand set confer changes in
the electronic properties of the metal is apparent by the
differences in the pKa and IR ν(CO) values for complexes 3
and 6. On the basis of reaction conditions necessary for the
generation of complexes 3 and 6, it appears that 3 (dppe
complex) is more acidic than 6 (depe complex). This is as would
be expected given that depe is more basic than dppe. The
observed IR ν(CO) values (CH2Cl2) for complexes 3 and 6 are
2006 and 1996 cm-1, respectively, which indicates greater
π-back-bonding in 6 than in 3.
2+
Figure 2. (a, left) Structure of the {[Fe(H2O)(CO)(depe)2][OTf]}2
cation (depe hydrogen atoms and carbon atom labels have been omitted
for clarity). (b, right) Structure of the [{Ag(OTf)2}2(µ-depe)]2- anion.
typical.24 The carbonyl C-O bond length is also typical at
1.165(9) Å. There is a hydrogen-bonding interaction between
the protons of the aqua ligand and a counterion. The hydrogen
atoms of the water ligand were not located or refined. The
shortest observed FeO‚‚‚OSO2CF3 distance is 2.721(7) Å. There
is also 1 equiv of noncoordinated water, O(1S), in the lattice
which may be hydrogen-bonded to the aqua ligand and/or a
triflate counterion.
For fast rotation of H2 (relative to the spectrometer frequency),
the observed T1(min) values correspond to d(HH) values of 0.84
and 0.83 Å for complexes 3 and 6, respectively; for restricted
motions these distances become 1.06 and 1.04 Å. The reasonable
1
agreement between d(HH) from J(HD) and T1(min) for fast-
As mentioned earlier, salts of 7 and 8 prepared using AgBF4
and AgOTf, respectively, are contaminated with the CH2Cl2-
soluble silver reagents. This became apparent when a crystal
was grown from the product obtained by the reaction of [FeCl-
(CO)(depe)2]Cl with AgOTf. The cation of this salt was found
to be an unusual hydrogen-bonded dimer of two [trans-Fe(H2O)-
(CO)(depe)2][OTf]+ (9[OTf]+) units (Figure 2a), while the anion
is [{Ag(OTf)2}2(µ-depe)]2- (Figure 2b). All of the bond lengths
for the cation, except for the Fe-P bonds, are equal within
experimental error to the corresponding bond lengths previously
stated for 9[OTf]2‚H2O. The hydrogen atoms of the aqua ligand
were located roughly in the O(2S)-O(2)-O(3SA) plane and
then refined isotropically. The aqua O-H bond lengths are
0.81(3) and 0.82(3) Å, while the OH‚‚‚OS hydrogen-bond
distances are 1.90(3) and 1.87(3) Å. The CH2Cl2 solution of
the isolated amorphous product exhibited a 31P{1H} NMR
spectrum identical to that of 8[OTf] isolated from the reaction
of HOTf and 1[OTf]. Thus, the product isolated from the
reaction of [FeCl(CO)(depe)2]Cl and AgOTf, may be 8[OTf]
contaminated with AgOTf or it may be 82[{(OTf)2Ag}2(µ-
depe)]. No 31P{1H} resonance was observed for the anion, but
this could be attributed to labile Ag-P bonds. The use of an
excess of the silver salt reagent is necessary since water
impurities in this reagent may be only partially removed by
heating under vacuum.
spinning H2 ligands is evidence that in these complexes the H2
ligands have a high rotational frequency relative to the
spectrometer frequency. Furthermore, the fast spinning of H2
in 3 and 6 is additional evidence of the poor π-base character
of the [Fe(CO)(pp)2]2+ moieties.
Importance of Metal-H2 σ-Bonding for Stability of η2-
H2 Complexes. The relative importance of the metal-H2 σ and
π bonds, in terms of the stability of η2-H2 complexes with
respect to the elimination of H2, is not well understood. The
synthesis of 3, 6, and other electron-deficient dihydrogen
complexes has shed light on the matter. [Fe(CO)(pp)2]2+
moieties are poor π-bases, and as such, Fe-H2 bonding in the
corresponding H2 complexes is primarily σ in character. In the
series of isoelectronic complexes [trans-M(H2)(CO)(dppe)2]n+
(M ) Mo,27,28 n ) 0; M ) Mn,26 n ) 1; M ) Fe, n ) 2, this
work) only the iron complex (3) is stable with respect to H2
loss under dinitrogen or argon atmospheres.3 In fact 3 eliminates
H2 only very slowly under vacuum, which may simply be a
consequence of the displacement of H2 by the counterion. It is
possible that the small degree of metal-to-H2 π-back-bonding
in [Fe(H2)(CO)(dppe)2]2+ is accompanied by a very strong three-
center, two-electron metal-H2 σ-bond, the net result of which
is surprising thermal stability. These findings are consistent with
previous findings where cationic complexes [Re(CO)3(PR3)2]+
were at least as effective at binding H2 as the isoelectronic
Interpretation of 1J(HD) of the HD Ligands and T1(min)
of the H2 Ligands. The generation of [Fe(HD)(CO)(dppe)2]2+
(3-d) was effected by the substitution of DOTf for HOTf in the
procedure for the preparation of 3[OTf]2. [Fe(HD)(CO)-
(depe)2]2+ (6-d) was not prepared directly but was generated in
situ by the treatment of 6[SbF6]2 with D2 gas. A linear
i
neutral complexes W(CO)3(PR3)2 (R ) Cy, Pr).29
(25) Morris, R. H. Transition Metal Sulphides. Chemistry and Catalysis,
Weber, T., Prins, R., van Santen, R. A., Eds.; Kluwer Academic
Publishers: London, 1998; pp 57-89.
(26) (a) King, W. A.; Scott, B. L.; Eckert, J.; Kubas, G. J. Inorg. Chem.
1999, 38, 1069-1084. (b) King, W. A.; Luo, X. L.; Scott, B. L.;
Kubas, G. J.; Zilm, K. W. J. Am. Chem. Soc. 1996, 118, 6782-6783.
(27) Kubas, G. J.; Burns, C. J.; Eckert, J.; Johnson, S. W.; Larson, A. C.;
Vergamini, P. J.; Unkefer, C. J.; Khalsa, G. R. K.; Jackson, S. A.;
Eisenstein, O. J. Am. Chem. Soc. 1993, 115, 569-581.
(28) Zilm, K. W.; Millar, J. M. AdV. Magn. Opt. Reson. 1990, 15, 163-
200.
1
correlation between d(HH) for an η2-H2 complex and J(HD)
for the corresponding HD isotopomer allows one to calculate
1
1
d(HH) from J(HD). The observed J(HD) values for 3-d and
6-d are 33.11 ( 0.05 and 33 ( 1 Hz, respectively, both of which
(24) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
(29) Heinekey, D. M.; Radzewich, C. E.; Voges, M. H.; Schomber, B. M.
J. Am. Chem. Soc. 1997, 119, 4172-4181.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.

6066 Inorganic Chemistry, Vol. 38, No. 26, 1999
Landau et al.
Scheme 5. Intramolecular Proton-Transfer Mechanism of
H/D Scrambling
Figure 3. Selected regions of 1H NMR spectra for the reactions of D2
with (a) [Fe(H2)(CO)(dppe)2]2+ and (b) [Fe(H2)(CO)(depe)2]2+
.
Scheme 4. H2/D2 Dissociation Mechanism of H/D
Scrambling
mechanism proposed. A mechanism similar to that in Scheme
5 has been established for H2 replacement in [FeH(H2)-
(dppe)2]+.30
Kinetics versus Thermodynamics in H2 Coordination. It
is clear that the binding of [BF4]- and [OTf]- is thermodynami-
cally favored over H2 in the case of [Fe(CO)(depe)2]2+
.
However, the situation with the dppe analogue is more
complicated. One would expect, given the greater electron-
deficient nature of the dppe moiety, that [BF4]- and [OTf]-
coordination would be favored over H2 coordination for 3 as is
the case for 6. However, 3 is reasonably stable in the presence
of [OTf]-, which cannot be said of 6. The most likely
explanation for the persistence of 3[OTf]2 in solution is kinetic,
where replacement of H2 by [OTf]- is possible and favorable
but rather slow due to the steric bulk of dppe. Exposure of
3[OTf]2 to vacuum for several hours does not result in loss of
H2. However, heating a toluene solution of 3[OTf]2 with excess
HOTf does result in replacement of H2 by [OTf]- as evidenced
by 31P{1H} NMR spectroscopy.
H2/D2 Exchange and H/D Scrambling. Exposure of CH2Cl2
solutions of 3 and 6 to D2 results in scrambling of H and D. In
these experiments the H NMR resonances for the HD com-
1
plexes have been observed in addition to resonances for free
H2 and HD (Figure 3). In the dppe case 2H NMR spectroscopy
revealed the presence of a substantial amount of DOTf, which
is a testament to the low pKa of complex 3. Although 3[OTf]2
and 6[SbF6]2 are thermally stable with respect to H2 elimination,
these experiments demonstrate that H2 coordination in 3 and 6
may be reversible. One possible mechanism for the observed
H/D scrambling is outlined in Scheme 4. This pathway is likely
a dissociative one in which the elimination of H2 yields a low-
concentration transient species, possibly stabilized by a C-H
bond or solvent Cl, which binds D2. The D2 isotopomers thus
produced are, like 3 and 6, very acidic and may to some extent
deuterate anions, glass, etc. The conjugate base deuteride
complexes are protonated in the reverse processes to yield HD
complexes. Finally, replacement of HD by H2 or D2 yields free
HD.
-
Unlike [BF4]- and [OTf]-, SbF6 exhibits no propensity to
displace H2 in 6. This is most probably due to electronic effects
since SbF6- is much less nucleophilic than [BF4]- and [OTf]-.
-
Sterics are probably not a factor in the coordination of SbF6
to the [Fe(CO)(depe)2]2+ moiety since Sb-F bond distances of
1.8 Å in SbF6 would put this ligand far removed from the
-
ethyl substituents of the depe ligands, assuming an Fe-F-Sb
angle greater than 160° like that observed for the Fe-F-B angle
in 7.
Solution NMR and X-ray Structure of [Fe(BF4)(CO)-
(depe)2][BF4]. In solution at room temperature the coordinated
[BF4]- of [Fe(BF4)(CO)(depe)2]+ is highly fluxional. Evidence
of the nonstatic nature of the [BF4]- coordination is the
spectroscopic equivalence of the fluorides for this ligand, which
gives rise to a quintet in the room-temperature 31P{1H} NMR
spectrum of 7[BF4]. This contrasts with the spectrum at -50
°C, which consists of a doublet with 31P coupling to only the
coordinated F of [BF4]- (Figure 4). Related variable-temperature
31P NMR spectra were observed for the complex cis-W(CO)3-
(NO)(PMe3)(FPF5), and an intramolecular anion exchange
mechanism was deduced.31
A second possible mechanism is illustrated in Scheme 5. In
this process the highly acidic H2 ligand protonates an Fe-P
bond and chelate ring-opening results. The resulting 16-electron
species (not observed) may be stabilized by some agostic
interaction with the newly formed P-H bond or by a C-H
bond. The vacant coordination site allows for coordination of
D2 and scrambling of H and D among the classical hydride and
dihydrogen ligands. Protonation of the classical deuteride ligand
by the dangling protonated phosphine and coordination of the
phosphine follow dissociation of HD. Evidence for this mech-
anism was the appearance of an eight-line multiplet (ap-
proximate J values of 33, 26, and 20 Hz) centered at -10.46
ppm in the 1H NMR spectrum for the reaction of 6 with D2, in
which NaOTf was added as a proton/deuteron shuttle. This
pattern can be assigned to the complex [FeH(CO)(OTf)(η2-
depe)(Et2PCH2CH2PEt2H)]+, which can be accounted for by the
Figure 5 depicts the structure of the cation of one of four
distinct formula units in the unit cell of 7[BF4]. The bonding in
the other three cations is similar. A particularly noteworthy
(30) Basallote, M. G.; Duran, J.; Fernandez-Trujillo, M. J.; Gonzalez, G.;
Manez, M. A.; Martinez, M. Inorg. Chem. 1998, 37, 1623-1628.
(31) Honeychuck, R. V.; Hersh, W. H. J. Am. Chem. Soc. 1989, 111, 6056-
6070.

Dicationic Iron(II) Dihydrogen Complexes
Inorganic Chemistry, Vol. 38, No. 26, 1999 6067
Figure 4. 31P{1H} NMR spectra of [Fe(BF4)(CO)(depe)2][BF4] in
CH2Cl2.
Figure 5. Structure of the [Fe(BF4)(CO)(depe)2][BF4] subunit A cation.
structural feature of 7 is the disparity between terminal and
bridging B-F distances (Table 4). In 7 the average bridging
B-F distance is 1.47 Å, while the average terminal B-F bond
length is 1.35 Å. The longer average bridging B-F distance
illustrates the pronounced electron-withdrawing effect of the
metal on the ligand. The average bridging B-F bond length
for 7 is greater than the typical value of 1.37(4) Å, while the
average terminal B-F bond length for 7 is comparable to the
corresponding typical value of 1.33(6) Å. This complex contains
has a rare FeII-(µ-F) bond. The Fe-F distance of 2.081(6) Å
is comparable to the ones previously observed.32 The Fe-P bond
lengths all lie between 2.28 and 2.32 Å, which is typical for
octahedral bis(depe) complexes of iron. The Fe-CO bond
lengths range from 1.71 to 1.76 Å (typical). The observed
carbonyl C-O bond lengths range from 1.12(1) to 1.18(1) Å,
which is not unusual.
Table 4. Coordinated [BF4]- B-F Bond Lengths for
[Fe(BF4)(CO)(depe)2][BF4]
bridging B-F
bond length/Å
terminal B-F
bond length/Å
subunit
A
B
C
D
1.47(1)
1.46(1)
1.46(1)
1.48(2)
1.38(1), 1.30(2), 1.33(2)
1.38(1), 1.32(1), 1.40(1)
1.36(2), 1.33(2), 1.39(1)
1.37(1), 1.31(2), 1.36(1)
The most noteworthy feature of this structure is the fairly strong
H(1P)-F(1) hydrogen bond, which is characterized by a bond
length of 2.46 Å. This compound evidently arises through the
slow decomposition of some complex in solution since no
31P{1H} NMR resonance for this species has been observed for
freshly prepared samples of 7[BF4] in MeOH. MeOH is not
sufficiently acidic to protonate depe. However, MeOH‚BF3, a
byproduct of this reaction, should be quite acidic. It is likely
that over time MeOH‚BF3 or [MeOH2]+ (if MeOH‚BF3 is more
acidic than [MeOH2]+) protonates the Fe-P bonds, eventually
resulting in the formation of the species observed. This may be
facilitated by a cis labilizing effect of the F- ligand.
Electrophilic Activation of [BF4]- in [Fe(BF4)(CO)-
(depe)2]+. The structural distortion of [BF4]- upon coordination
to the electron-deficient [Fe(CO)(depe)2]2+ moiety is consistent
with the observed reactivity of 7. Treatment of 7 with methanol
quickly and quantitatively affords a species which has been
identified as [trans-FeF(CO)(depe)2]+ (10) on the basis of its
doublet 31P{1H} NMR spectrum. This reaction likely proceeds
via an SN2 mechanism (eq 2). Although there are previous
Conclusions
[trans-Fe(H2)(CO)(dppe)2][OTf]2 (3[OTf]2) was generated by
the reaction of [trans-FeH(CO)(dppe)2][OTf] with HOTf. [trans-
Fe(H2)(CO)(depe)2][SbF6]2 (6[SbF6]2) was generated by the
reaction of AgSbF6 with [FeCl(CO)(depe)2]Cl in the presence
of molecular H2. Neither of these salts has been isolated. 3 is
more acidic than HOTf, and 6 is less acidic than [HOEt2]+ and
much less acidic than 3. ν(CO) of 3 is greater than that of 6.
The relative acidities and ν(CO) values of 3 and 6 are consistent
with the greater basicity of depe as compared to dppe. 3 is stable
with respect to H2 loss even when exposed to reduced pressure
for several hours, but under the extremely acidic conditions
required to produce 3, this complex slowly decomposes. 3 is
much less reactive to nucleophiles than 6. This relative reactivity
is contrary to the relative electronic deficiencies of the iron
centers in these complexes but may be attributed to the greater
steric hindrance of reactions for 3 compared to 6.
Changing the phosphine ligands from depe to dppe has a great
effect on the reactivity, pKa, and π-back-bonding but has little
effect on the structural (H-H distance) and magnetic properties
(2J(HD), T1) of the η2-H2 ligands. This is generally the case for
complexes with H2 trans to CO. The H2 ligands in 3 and 6 rotate
about the Fe-H2 bond axis much faster than 300 MHz, which
is consistent with the poor π-base character of iron in these
complexes.
(2)
examples33 of fluoride abstraction from [BF4]-, this is the first
reported example in which a tetrafluoroborato complex is an
observable and isolable intermediate.
The reaction illustrated in eq 2 above is apparently reversible
since the precipitation of 10[BF4] by the addition of Et2O to a
sample of 7[BF4] dissolved in CH2Cl2 or MeOH yields a product
which is highly contaminated with 7[BF4]. The 31P{1H} NMR
spectra of these reaction mixtures exhibit no resonance for
7[BF4]. Attempts to grow a single crystal of 10[BF4] were
unsuccessful. Over many weeks slow vapor diffusion of Et2O
into a sample of 7[BF4] dissolved in MeOH resulted in the
crystallization of the colorless salt [Et2HPCH2CH2PHEt2][BF4]2.
(32) Herold, S.; Lippard, S. J. Inorg. Chem. 1997, 36, 50-58.
(33) (a) Jia, G.; Morris, R. H.; Schweitzer, C. T. Inorg. Chem. 1991, 30,
593-594. (b) Ellis, R.; Henderson, R. A.; Hills, A.; Hughes, D. L. J.
Organomet. Chem. 1987, 333, C6-C10.

6068 Inorganic Chemistry, Vol. 38, No. 26, 1999
Landau et al.
The great stability of the dicationic complex 3 with respect
to H2 loss indicates that the increase in metal-H2 σ-bond
strength more than compensates for the reduced π-back-bonding
when compared to the analogous neutral Mo0 and monocationic
MnI complexes.
Complexes 3 and 6 facilitate H/D scrambling between
molecular H2 and D2. There is evidence for a chelate ring-
opening mechanism for 6. The rapid formation of [trans-
Fe(OTf)(CO)(depe)2]+ (8) and [trans-Fe(BF4)(CO)(depe)2]+ (7)
via the protonation of [FeH(CO)(depe)2]+ with HOTf and HBF4,
respectively, strongly indicates an H2-dissociation mechanism,
at least for 6.
There is NMR evidence for the formation of [trans-Fe(H2O)-
(CO)(dppe)2]+ (4) and [trans-Fe(OTf)(CO)(dppe)2]+ (5) in
reaction mixtures of 3; however, these complexes could not be
isolated. Complexes 7, 8, and [trans-Fe(H2O)(CO)(depe)2]2+ (9)
have been observed and characterized by NMR (7, 8, 9) and
IR (7, 8) spectroscopy and single-crystal X-ray diffraction (7,
9). The [BF4]- ligand of 7 is highly fluxional at room
temperature. This dynamic behavior contrasts with the observed
activation and reactivity of the [BF4]- ligand, which undergoes
nucleophilic substitution reactions with water and some alcohols
to afford the complex [trans-FeF(CO)(depe)2]+ (10). In the
presence of the acidic byproduct MeOH‚BF3, 10 slowly
decomposes in methanol solution to yield [HEt2PCH2CH2PEt2H]-
[BF4]2, which has been structurally characterized by single-
crystal X-ray diffraction. While we have observed the coordi-
nation of the anions [OTf]- and [BF4]- in this work, we have
not observed solvent complexes of the type [Fe(ClCH2Cl)(L)2-
(CO)]2+ (cf. the electrophilic cations [Re(CO)4(PR3)(ClCH2Cl)]-
[BArf]34).
Acknowledgment. This research was funded by an NSERC
research grant to R.H.M. and NSERC and OGS scholarships
to S.E.L.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determinations of 7[BF4], 9[OTF]2‚
H2O, {9[OTf]}2[{(OTf)2Ag}2(µ-depe)], and [HEt2PCH2CH2PEt2H]-
[BF4]2. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC990876A
(34) Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J. Inorg. Chem. 1999,
38, 115-124.