Oxidative addition of phosphine-tethered thiols to iron carbonyl: Binuclear phosphinothiolate complexes, (μ -SCH2CH2PPh2)2Fe2 (CO)4, and hydride derivatives

Article abstract of DOI:10.1021/ic010741g

The mononuclear complex Fe(CO)₄(PPh₂CH₂CH₂SH), 1, is isolated as an intermediate in the overall reaction of PPh₂CH₂CH₂SH with [Fe⁰(CO)₄] sources to produce binuclear bridging thiolate complexes. Photolysis is required for loss of CO and subsequent S-H activation to generate the metal-metal bonded Fe^I-Fe^I complex, (μ -SCH₂-CH₂PPh₂)₂Fe₂ (CO)₄, 2. Isomeric forms of 2 derive from the apical or basal position of the P-donor ligand in the pseudo square pyramidal S₂Fe(CO)₂P coordination spheres. This position in turn is dictated by the stereochemistry of the μ-S-CH₂ bond, designated as syn or anti with respect to the Fe₂S₂ butterfly core. Addition of strong acids engages the Fe^I-Fe^I bond density as a bridging hydride, [(μ-H)-anti-2]⁺[SO₃CF₃]^- or [(μ-H)-syn-2]⁺[SO₃CF₃]^-, with formal oxidation to Fe^II-H-Fe^II. Molecular structures of anti-2, syn-2, and [(μ-H)-anti-2]⁺[SO₃CF₃]^- were determined by X-ray crystallography and show insignificant differences in distance and angle metric parameters, including the Fe-Fe bond distances which average 2.6 A. The lack of coordination sphere rearrangements is consistent with the ease with which deprotonation occurs, even with the weak base, chloride. The Fe^I-Fe^I bond, supported by bridging thiolates, therefore presents a site where a proton might be taken up and stored as a hydride without impacting the overall structure of the binuclear complex.

Full text of DOI:10.1021/ic010741g

Inorg. Chem. 2002, 41, 699−708
Oxidative Addition of Phosphine-Tethered Thiols to Iron Carbonyl:
Binuclear Phosphinothiolate Complexes, (µ-SCH CH PPh2)2Fe2(CO)4, and
2
2
Hydride Derivatives
Xuan Zhao, Yui-May Hsiao, Chia-Huei Lai, Joseph H. Reibenspies, and Marcetta Y. Darensbourg*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
Received July 11, 2001
The mononuclear complex Fe(CO)4(PPh2CH CH SH), 1, is isolated as an intermediate in the overall reaction of
2
2
PPh2CH CH SH with [Fe0(CO)4] sources to produce binuclear bridging thiolate complexes. Photolysis is required
2
2
I
I
for loss of CO and subsequent S−H activation to generate the metal−metal bonded Fe−Fe complex, (µ-SCH -
2
CH PPh2)2Fe2(CO)4, 2. Isomeric forms of 2 derive from the apical or basal position of the P-donor ligand in the
2
pseudo square pyramidal S Fe(CO)2P coordination spheres. This position in turn is dictated by the stereochemistry
2
of the µ-S−CH bond, designated as syn or anti with respect to the Fe2S butterfly core. Addition of strong acids
2
2
I
I
engages the Fe−Fe bond density as a bridging hydride, [(µ-H)-anti-2]+[SO CF ]- or [(µ-H)-syn-2]+[SO CF ]-,
3
3
3
3
with formal oxidation to Fe −H−Fe . Molecular structures of anti-2, syn-2, and [(µ-H)-anti-2]+[SO CF ]- were
II
II
3
3
determined by X-ray crystallography and show insignificant differences in distance and angle metric parameters,
including the Fe−Fe bond distances which average 2.6 Å. The lack of coordination sphere rearrangements is
I
I
consistent with the ease with which deprotonation occurs, even with the weak base, chloride. The Fe −Fe bond,
supported by bridging thiolates, therefore presents a site where a proton might be taken up and stored as a
hydride without impacting the overall structure of the binuclear complex.
Introduction
phosphine ligands, Scheme 1.3,4 In fact, all species shown
in Scheme 1 resulting from the protonation of anionic trans-
[RSFe(CO)3(PR3)]- were indicated in IR and NMR spec-
troscopic studies. While spectroscopic evidence for the
interesting η2-RSH-Fe interaction was compelling, the
complex was nevertheless not isolable.4 Clearly electron
density at iron and the stability of intermediates are fine-
tuned by neutral ligand and thiolate donor ability.
Thiols are known to oxidatively add to low-valent transi-
tion metals, in many cases resulting in aggregation due to
the propensity of thiolates to serve as bridging ligands. Thus
the addition of PhSH, MeSH, and EtSH to iron(0) carbonyls
produces the dinuclear bridging thiolate iron(I) species with
release of H2, eq 1.1,2
An analogue of the thiolate-hydride monomeric inter-
mediate has been prepared by Stephan et al., from the
reaction of the phosphine-tethered thiol ligand PPh2CH2CH2-
SH, P-SH, with Vaska’s complex, trans-IrI(PPh3)2(CO)Cl.5
Displacement of a PPh3 ligand and oxidative addition
produced the octahedral Ir(III) hydride complex, IrIII(P-S)-
(H)(PPh3)(CO)Cl.5 For the Fe(0) carbonyl reactions such a
ligand would tether the bridging thiolate to an Fe(CO)2P
terminus and produce a binuclear complex. Thus, in order
to explore the sequence of metal-ligand interaction and S-H
Fe(CO)5, Fe2(CO)9, or Fe3(CO)12 + RSH f
[(µ-SR)Fe(CO)3]2 + H2 + nCO (1)
While this reaction is of obvious importance, and the
product is a foundation molecule in organometallic chemistry,
the mechanistic pathway of binuclear RS-H oxidative
addition has not been mapped. Earlier work demonstrated
that the all-CO Fe(0) compound provided no evidence of
intermediates in this conversion; however, a monomeric
thiolate-hydride (RS)FeII-H intermediate was stabilized by
(3) Liaw, W.-F.; Kim, C.; Darensbourg, M. Y.; Rheingold, A. L. J. Am.
Chem. Soc. 1989, 111, 3591.
(4) Darensbourg, M. Y.; Liaw, W.-F.; Riordan, C. G. J. Am. Chem. Soc.
1989, 111, 8051.
* Author to whom correspondence should be addressed. E-mail: marcetta@
mail.chem.tamu.edu.
(1) Hieber, W.; Spacu, P. Z. Anorg. Allg. Chem. 1937, 233, 353.
(2) King, R. B.; Bisnette, M. B. Inorg. Chem. 1965, 4, 1663.
(5) Stephan, D. W. Inorg. Chem. 1984, 23, 2207.
10.1021/ic010741g CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/22/2002
Inorganic Chemistry, Vol. 41, No. 4, 2002 699

Zhao et al.
Scheme 1
Experimental Section
Materials and Methods. All manipulations were performed
using standard Schlenk techniques under Ar or in an argon
atmosphere glovebox. Solvents were reagent grade and purified as
follows: Dichloromethane was distilled over phosphorus pentoxide
under N2, and acetonitrile was distilled once from CaH2 and once
from P2O5 and freshly distilled from CaH2 immediately before use.
Tetrahydrofuran, diethyl ether, and hexane were distilled from
sodium benzophenone ketyl under N2. The following materials were
of reagent grade and used as received: diiron nonacarbonyl, iron
pentacarbonyl, trimethylamine N-oxide dihydrate, methylphe-
nylphosphine, and trifluoromethanesulfonic acid (Aldrich Chemical
Co.), and deuterated solvents (Cambridge Isotope Laboratories).
Instrumentation. Infrared spectra were recorded on a Mattson
Galaxy 6021 or an IBM IR/32 using 0.1 mm NaCl cells or KBr
pellets. 1H and 31P NMR spectra were obtained on XL200E Varian
and a Unity+ 300 MHz superconducting NMR spectrometers.
Cyclic voltammograms were recorded on a BAS-100A electro-
chemical analyzer using AgNO3 as reference electrode, Pt as counter
electrode, and a glassy carbon working electrode with 0.1 M
[n-Bu4N][PF6] as electrolyte. All potentials were scaled to NHE
using ferrocene as an internal standard.15 Photochemical reactions
were performed in water-jacketed cells using a 450 W mercury
lamp. Elemental analyses were performed by Galbraith Laboratories,
Inc., Knoxville, TN.
Syntheses. The ligand PPh2CH2CH2SH was prepared as de-
scribed in the literature,16 and a similar approach yielded PPhMeCH2-
CH2SH. For the latter, 7.5 g (0.06 mol) PPhMe(H) was dissolved
in 70 mL of THF, to which 6 mL of 10 M n-BuLi was added
dropwise over 15 min with stirring, forming an orange solution.
After 30 min, 3.6 mL (0.06 mol) of ethylene sulfide in 125 mL of
THF was added dropwise over 1 h; the solution gradually changed
to light yellow. After stirring for another hour, 10 mL of a saturated
aqueous NH4Cl solution was added and stirred for 10 min. The
organic layer was separated and dried over Na2SO4. Distillation
under reduced pressure (130 °C/0.5 Torr) gave the product as a
colorless liquid (9.3 g, 84%).
oxidative addition, we have examined the reaction of the
P-SH thiol as well as PMePhCH2CH2SH and PMe2CH2CH2-
SH with [Fe0(CO)4] sources. In addition, we wished to
determine the structures possible for the chelate P-S ligand
in binuclear complexes of iron.
Described herein are such products and hydrides derived
from protonation of the iron-iron bond in isomeric forms
of (µ-SCH2CH2PPh2)2Fe2(CO)4. Isomerism may result from
phosphine position in the pyramidal Fe(CO)2PR3 units (trans
or cis to the Fe-Fe bond, which is respectively apical or
basal in regard to the square pyramidal S2FeL3 units),6 as
well as from the orientation of the R-C on the bridging
thiolate. Of the possibilities represented below, the syn and
The PMe2CH2CH2SH ligand preparation was modified slightly
from the literature report.17 Sodium metal (1.49 g or 65 mmol) was
placed in a 250 mL three-neck flask into which 100 mL of liquid
ammonia was condensed at dry ice/acetone bath temperature. To
the blue solution, 3.94 g (32 mmol) of tetramethyldiphosphane was
added dropwise. Over the course of 1 h, the stirred solution changed
to yellow, following which time a cooled (dry ice/acetone) portion
(3.8 mL, 64 mmol) of ethylene sulfide was transferred into it,
stirring being maintained for 1-2 h. The cooling bath was removed
to evaporate NH3; then 20 mL of saturated aqueous NH4Cl solution
was added slowly and stirred for 20 min. The organic layer was
separated and dried over Na2SO4. Distillation under reduced
pressure (70°/0.5 Torr) gave a colorless liquid (2.5 g, 32% yield).
Preparation of Fe(CO)4(PPh2CH2CH2SH) (1): Method A. Fe2-
(CO)9 (1.0 g, 2.74 mmol) was placed in a 500 mL Schlenk flask
with PPh2CH2CH2SH (0.7 g, 2.85 mmol). A total of 200 mL of
THF was added to give a brown solution. The reaction mixture
was stirred overnight, leading to a reddish brown solution, after
which time the solvent was removed under vacuum. This residue
was washed with 15 mL of hexane and dried under vacuum to
anti forms have been observed for monodentate RS- bridges7
and have been shown to exact important steric effects in PR3/
CO exchange reactions.8,9 The syn′ form has been observed
only in bidentate dithiolate bridges, as in ethanedithiolate-
diironhexacarbonyl, (µ-SCH2CH2S)Fe2(CO)6,10 or the pro-
panedithiolate analogue, (µ-pdt)Fe2(CO)6.11 The latter and
its dicyano derivative prepared from CN-/CO exchange have
found use as structural and spectroscopic models of the
dinuclear active site of Fe-only hydrogenase.12-14
(6) De Beer, J. A.; Haines, R. J.; Greatrex, R.; Greenwood, N. N. J. Chem.
Soc. A 1971, 3271.
(7) Borgne, G. L.; Grandjean, D. J. Organomet. Chem. 1977, 131, 429.
(8) Maresca, L.; Greggio, F.; Sbrignadello, G.; Bor, G. Inorg. Chim. Acta
1972, 5, 667.
(9) De Beer, J. A.; Haines, R. J.; Greatrex, R.; Greenwood, N. N. J.
Organomet. Chem. 1971, 27, C33.
(10) Messelhaeuser, J.; Lorenz, I. P.; Haug, K.; Hiller, W. Z. Naturforsch,
B: Anorg. Chem., Org. Chem. 1985, 40B, 1064.
(11) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. Angew. Chem., Int. Ed. 1999, 38, 3178.
(12) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc.
1999, 121, 9736.
(13) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 2001, 123, 3268.
(15) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19,
2854.
(16) Chatt, J.; Dilworth, J. R.; Schmutz, J. A. J. Chem. Soc., Dalton Trans.
1979, 1595.
(14) Nicolet, Y.; Lemon, B. J.; Fontecilla-Camps, J. C.; Peters, J. W. Trends
Biochem. Sci. 2000, 25, 138.
(17) Kita, M.; Yamamoto, T.; Kashiwabara, K.; Fujita, J. Bull. Chem. Soc.
Jpn. 1992, 65, 2272.
700 Inorganic Chemistry, Vol. 41, No. 4, 2002

Binuclear Phosphinothiolate Complexes
Table 1. Infrared Data in Carbonyl Stretching Frequency Region of the
Fe2S2 Derivatives
and drying under vacuum, anti-(µ-SCH2CH2PPh2)2Fe2(CO)4 (anti-
2) was obtained in 35% yield. See Table 1 for IR data. Elemental
anal. Calcd (found) for anti-2, C32H28P2S2O4Fe2: C, 53.8 (53.9);
H, 3.92 (4.10). X-ray quality crystals of both the syn and anti
isomers were obtained by hexane diffusion into THF and CH2Cl2
solutions, respectively.
compound
ν(CO), cm-1
Fe(CO)4(PPh2CH2CH2SH), 1
Fe(CO)4(PPhMeCH2CH2SH), 1a
Fe(CO)4(PMe2CH2CH2SH), 1b
syn-(µ-SCH2CH2PPh2)2Fe2(CO)4,
syn-2
2053 (m), 1981 (m), 1948 (s), 1941 (s)a
2047 (s), 2008 (m), 1985 (m), 1935 (s)a
2047 (ms), 1968 (ms), 1933 (s,b)b
1985 (m), 1960 (s), 1910 (s), 1903 (sh)b
Preparation of (µ-SCH2CH2PPhMe)2Fe2(CO)4. Photochemical
irradiation of 0.50 g (1.4 mmol) of [PPh(CH3)CH2CH2SH]Fe(CO)4
dissolved in 100 mL of THF required 2-3 h to reach complete
conversion to dimeric (µ-SCH2CH2PPhMe)2Fe2(CO)4. Following
vacuum removal of solvent the product was extracted into 50 mL
of diethyl ether. Removal of ether left a red-brown solid (0.20 g,
48% yield). The ν(CO) IR data is given in Table 1. Elemental anal.
Calcd (found) for C22H24P2S2O4Fe2: C, 44.8 (45.0); H, 4.10 (4.30).
Protonation of anti-(µ-SCH2CH2PPh2)2Fe2(CO)4, anti-2. At
room temperature, an excess of CF3SO3H (0.1 mL, 1 mmol) was
added to a solution of anti-2 (360 mg, 0.5 mmol) in 60 mL of
THF under argon. The color of the solution gradually changed from
dark red to orange-red. After stirring for 2 h, the solvent was
removed, giving a red oil. This residue was washed with 3 × 10
mL of diethyl ether or until the diethyl ether solution was colorless.
The product was then extracted into THF (∼50 mL) and filtered
through Celite. The solvent volume was reduced to 3 mL, and
addition of ∼40 mL of diethyl ether gave an orange-red solid (300
anti-(µ-SCH2CH2PPh2)2Fe2(CO)4, 1981 (w), 1960 (s), 1912 (s), 1904 (sh)b
anti-2
(µ-SCH2CH2PPh2)2Fe2(CO)4-, 3
(µ-SCH2CH2PPh2)2Fe2(CO)42-, 4
[(µ-H)-anti-2]+(SO3CF3)-
1959 (s), 1897 (s,br) 1732 (m)c
1959 (s), 1897 (s), 1732 (m)c
2053 (m), 2033 (s), 2000 (m, br)b
2049 (sh), 2039 (s), 1995 (m, br)b
[(µ-H)-syn-2]+(SO3CF3)-
a n-Hexane solution. b THF solutions. c Recorded as Na+ salts in CH3CN
solution.
give a red brown solid (0.60 g, 53% yield). Infrared data in the
ν(CO) region is given in Table 1. Elemental anal. Calcd (found)
for C18H15PSO4Fe: C, 52.2 (51.8); H, 3.65 (3.60).
Method B. The procedure is the same as in method A using
Fe3(CO)12 (1.0 g, 2 mmol) as iron source and PPh2CH2CH2SH (0.7
g, 2.85 mmol).
Method C. At -30 °C, Me3NO‚2H2O (113 mg, 1.02 mmol) in
methanol was slowly added to Fe(CO)5 (0.14 mL, 1.02 mmol) in
THF to give a pale pink solution. To this solution was added 20
mL of a THF solution of PPh2CH2CH2SH (250 mg, 1.02 mmol) to
give a reddish solution. The reaction mixture was allowed to warm
to room temperature and stirred for 2 h, after which time the solvent
was removed under vacuum. The residue was redissolved in THF
and filtered through Celite filter, and the filtrate was evaporated to
dryness under vacuum. Trituration of the solid in 40 mL of hexane
under N2 was followed by removal of the hexane by cannula. The
product was dried in a vacuum to give a red brown solid (295 mg,
70% yield).
Synthesis of Fe(CO)4[P(CH3)2CH2CH2SH] and Fe(CO)4[PPh-
(CH3)CH2CH2SH]. According to method A above, 0.54 g (4.4
mmol) of P(CH3)2CH2CH2SH and 1.62 g (4.4 mmol) of Fe2(CO)9
were placed in a 100 mL Schlenk flask, 20 mL of THF was added,
and the mixture was stirred for 4 h; a red brown solution was
formed. The solvent was removed under reduced pressure; the
residue was washed with hexane and dried under vacuum. Yield:
0.60 g (47%). A brown solid in similar yield was obtained from
reaction of the PPh(CH3)CH2CH2SH ligand with Fe2(CO)9 follow-
ing the same procedure. Infrared data are given in Table 1.
Preparation of (µ-SCH2CH2PPh2)2Fe2(CO)4, 2. Fe(CO)4(PPh2-
CH2CH2SH) (1.00 g, 2.42 mmol) in 150 mL of THF was irradiated
at 22 °C in a photochemical reaction vessel with a water-jacketed
450 W mercury lamp while the solution was gently purged with
argon in order to remove liberated carbon monoxide. Irradiation
was continued for about 2 h, resulting in complete Fe(CO)4(PPh2-
CH2CH2SH) disappearance as indicated by IR. Under Ar, the
solution was transferred to a Schlenk flask and the solvent removed
under vacuum. This red brown residue was washed with 3 × 10
mL portions of hexane and dried in vacuo. This residue was
extracted with 50 mL of ether, producing a red ether solution and
leaving a red solid. On vacuum removal of solvent from the ether
extract, 75 mg of a red-brown solid was recovered. The product
was identified as syn-(µ-SCH2CH2PPh2)2Fe2(CO)4 (syn-2), by X-ray
crystallography (vide infra) and infrared spectroscopy, Table 1.
Elemental anal. Calcd (found) for syn-(µ-SCH2CH2PPh2)2Fe2(CO)4,
C32H28P2S2O4Fe2: C, 53.8 (53.3); H, 3.92 (4.18).
1
mg, 65% yield). H NMR (d6-acetone): -16.9 (d of d, 1 H, Fe-
H), 1.25-3.42 (m, 8 H, PCH2CH2S), 6.90-8.10 (m, 20 H, C6H5).
Elemental anal. Calcd (found) for C33H29P2S3O7F3Fe2: C, 45.8
(45.7); H, 3.38 (3.28).
Protonation of syn-(µ-SCH2CH2PPh2)2Fe2(CO)4, syn-2. The
procedure is the same as described for anti-2. A red solid was
obtained in 65% yield. IR (THF, cm-1): ν(CO) 2052 (sh), 2039
1
(s), 1995 (m, br). H NMR (d6-acetone): -17.4 (t, 1 H, Fe-H),
1.58-3.41 (m, 8 H, PCH2CH2S), 7.20-8.20 (m, 20 H, C6H5).
Elemental anal. Calcd (found) for C33H29P2S3O7F3Fe2: C, 45.8
(45.3); H, 3.38 (3.44).
Deprotonation of {(µ-H)(µ-SCH2CH2PPh2)2Fe2(CO)4}+(SO3-
CF3)-. Typically, weighed samples of the hydride were placed into
a Schlenk flask inside an argon-filled glovebox. After the stoppered
flask was transported outside the box, THF or CH3CN solvents
were added, followed by the deprotonation agents Et3N, and NEt4-
CN or PPNCl, dissolved in CH3CN. The reactions were monitored
by IR or NMR techniques. For example, to a 15 mL THF solution
of the [(µ-H)-anti-2]+[SO3CF3]- (170 mg, 0.20 mmol) was added
Et3N (0.28 mL, 2 mmol). The orange-red solution turned dark red
immediately. New ν(CO) peaks were observed in the IR at 1989
(m, s), 1952 (s), 1921 (m), and 1904 (m) cm-1. This product showed
1
no upfield hydride resonance in the H NMR.
With identical quantities as above, 10 equiv of Et3N (0.28 mL,
2 mmol) and [(µ-H)-syn-2]+[SO3CF3]- (170 mg, 0.2 mmol) in 15
mL of THF were stirred at room temperature, requiring overnight
to produce the final product. The slow reaction produced a dark
red solution which exhibited ν(CO) bands at 1984 (w), 1960 (s),
1
and 1912 (s) cm-1. The triplet resonance at -17.3 ppm in the H
NMR spectrum of [(µ-H)-syn-2]+[SO3CF3]- disappeared.
Reduction of (µ-SCH2CH2PPh2)2Fe2(CO)4. Under argon, a
solution of (µ-SCH2CH2PPh2)2Fe2(CO)4 (200 mg, 0.28 mmol) as a
mixture of isomers in 60 mL of CH3CN was transferred to a flask
containing Na/Hg amalgam (13 mg Na, 0.57 mmol, in 0.6 mL of
Hg). The mixture was magnetically stirred for 2 h or until change
in the IR to ν(CO) ) 1959 (s), 1897 (s, br), 1732 (m) was complete.
This solution was anaerobically filtered through Celite into a flask
containing solid PPNCl (160 mg, 0.28 mmol). After being stirred
at ambient temperature for 1 h, the solution was concentrated to
The ether insoluble portion from the above reaction was dissolved
in 1 mL of THF, after which 15 mL of hexane was added to
precipitate a red brown solid. Upon removal of the mother liquor
Inorganic Chemistry, Vol. 41, No. 4, 2002 701

Zhao et al.
Scheme 2
10 mL. Diethyl ether was then slowly added to give a red brown
solid (210 mg, 60% yield). IR (CH3CN, cm-1): ν(CO) 1959 (s),
1897 (s), 1732 (m).
Reduction of [(µ-H)-2]+[SO3CF3]-. To a solution of 0.173 g
(0.2 mmol) of [(µ-H)-2]+[SO3CF3]- as a mixture of isomers
dissolved in 15 mL of CH3CN was added 0.113 g (0.6 mmol) of
Cp2Co in 10 mL of CH3CN. The orange red solution turned to red
brown gradually. After stirring overnight the resulting red brown
solution showed three ν(CO) bands at 1910, 1956, and 1986 cm-1
similar in position and pattern to those of 2.
,
X-ray Structure Determination. The X-ray crystal structures
were solved at the Crystal & Molecular Structure Laboratory Center
at Texas A&M University. X-ray crystallographic data were
obtained on a Nicolet R3m/V X-ray diffractometer with an oriented
graphite monochromator or a Siemens R3m/V single-crystal X-ray
diffractometer operating at 55 kV and 30 mA, Mï KR (λ ) 0.71073
Å) radiation equipped with a Siemens LT-2 cryostat. All crystal-
lographic calculations were performed with use of the Siemens
SHELXTL-PLUS program package.18 The structures were solved
by direct methods. Anisotropic refinement for all non-hydrogen
atoms was done by a full-matrix least-squares method. The hydride
was located in a difference Fourier map and was refined. Cell
parameter and collection data along with atomic coordinates and
equivalent isotropic displacement parameters are provided in the
Supporting Information.
Results
Syntheses of Fe(CO)4(PPh2CH2CH2SH), Anti and Syn
Forms of (µ-SCH2CH2PPh2)2Fe2(CO)4, and Conjugate
Acids. Reaction of PPh2CH2CH2SH with iron carbonyl
sources Fe2(CO)9 or Fe3(CO)12 in THF at ambient temper-
ature for several hours leads to a red brown solution
containing a carbonyl derivative which, on the basis of ν-
(CO) IR spectroscopy and subsequent derivative chemistry,
was identified as Fe(CO)4(PPh2CH2CH2SH) (1), Scheme 2.
This complex can also be prepared by decarbonylation of
Fe(CO)5 with Me3NO‚2H2O in methanol,19 followed by
reaction with 1 equiv of PPh2CH2CH2SH, as indicated in
Scheme 2. The IR spectrum of 1, Figure 1a, has CO
stretching frequencies similar to those of the monosubstituted
Fe(CO)4PR3.20 The two lowest energy bands in the hexane
solution ν(CO) IR spectrum, at 1948 (s) and 1941 (s) cm-1,
are assigned as an E band, split under pseudo C3V symmetry
as imposed by the asymmetrical PPh2R ligand at the axial
position of trigonal bipyramidal LFe(CO)4. The resolution
Figure 1. Solution infrared spectra of (a) Fe(CO)4(PPh2CH2CH2SH) in
hexane; (b) anti-(µ-SCH2CH2PPh2)2Fe2(CO)4 in THF; (c) syn-(µ-SCH2CH2-
PPh2)2Fe2(CO)4 in THF; (d) anti-[(µ-H)(µ-SCH2CH2PPh2)2Fe2(CO)4]+[CF3-
SO3]- in CH3CN; (e) syn-[(µ-H)(µ-SCH2CH2PPh2)2Fe2(CO)4]+[CF3SO3]-
in CH3CN; and (f) product from reduction of (µ-SCH2CH2PPh2)2Fe2(CO)4
by Na/Hg for 1 h at room temperature, in CH3CN.
of the E vibrational mode into two bands is not seen in THF.
Similar ν(CO) IR spectral patterns were observed for
products of the reaction of Fe2(CO)9 with the ligands
PPhMeCH2CH2SH, 1a, and PMe2CH2CH2SH, 1b. While the
trend noted in Table 1 is to lower ν(CO) position with
increasing number of methyl groups, the shift is minimal,
despite the concomitant increasing electron donor character
of the phosphines. This observation is consistent with simpler
Fe(CO)4(PR3) complexes and bespeaks the complicated
interplay of ν(CO) values, stretching force constants, and
electron density at iron.20
Complex 1 is stable in solution at room temperature as
well as elevated temperatures (refluxing THF) for extended
periods. Nevertheless, photochemical irradiation of a THF
solution of Fe(CO)4(PPh2CH2CH2SH) at room temperature
for 1-2 h (depending on the efficacy of the photochemical
lamp) resulted in compound 2, (µ-SCH2CH2PPh2)2Fe2(CO)4,
(18) (a) Sheldrick, G. SHELXTL-86 Program for Crystal Structure Solution;
Institut fu¨r Anorganische Chemie der Universita¨t: Tammanstrasse 4,
D-3400 Go¨ttingen, Germany, 1986. (b) Sheldrick, G. SHELXTL-93
Program for Crystal Structure Solution; Institut fu¨r Anorganische
Chemie der Universita¨t: Tammanstrasse 4, D-3400 Go¨ttingen, Ger-
many, 1993.
(19) Shvo, Y.; Hazum, E. J. Chem. Soc., Chem. Commun. 1975, 829.
(20) Conder, H. L.; Darensbourg, M. Y. J. Organomet. Chem. 1974, 67,
93.
702 Inorganic Chemistry, Vol. 41, No. 4, 2002

Binuclear Phosphinothiolate Complexes
Scheme 3
as a mixture of isomers, separable by differences in solubili-
ties and measured to be in anti:syn ratios ranging from 4:1
to 6:1. The gases released during this reaction were verified
by gas chromatography to be H2 and CO. The stick drawings
in Scheme 3 are based on the molecular structures as
determined by X-ray crystallography, vide infra.
The anti and syn isomers of (µ-SCH2CH2PPh2)2Fe2(CO)4,
so named as described above, showed almost identical
infrared spectra in the CO stretching frequency range,
differing only in relative intensities (Table 1 and Figure 1).
This observation was consistent with the ν(CO) IR spectra
of the anti and syn forms of [(µ-SR)Fe(CO)3]2 (R ) C6F5,
C6Cl5, CH3)8 and [(µ-SPh)Fe(CO)2P(OR)3]2 (R ) CH3, C2H5,
C3H7).21
As indicated in Scheme 3, anti-2 and syn-2 reacted with
a slight excess of CF3SO3H in THF solution, yielding orange
red solutions from which air-stable orange solids were
obtained on removal of solvent. Spectral properties are
presented below. The [(µ-H)-anti-2]+[SO3CF3]- crystallized
from CH3CN/Et2O in suitable crystallinity for X-ray crystal
structure analysis.
Figure 2. (a) Thermal ellipsoid (50%) and (b) ball-and-stick representations
of the molecular structure of anti-2.
trans to the µ-H, of 0.12 (Fe(1)) and 0.24 Å (Fe(2)). These
displacements are also affirmed by the Lapical-Fe-Lbasal
of >90°, ranging from 92.1° to 107.6°. An exception is the
P(1)-Fe(1)-S(1) angles in anti-2 and [(µ-H)-anti-2]+,
which at 88.4° and 87.2°, respectively, represent the restric-
tion of the P-S chelate bite angle and is maintained in all
structures.
Molecular Structures of anti-2, syn-2, and [(µ-H)-anti-
2]+[SO3CF3]-. X-ray crystallographic data are listed in the
Supporting Information. Molecular structures are presented
in Figures 2-4, and selected bond distances and angles for
anti-2 and [(µ-H)-anti-2]+ are found in Table 2. Analogous
data for syn-2 are given in Table 3.
The orientation of the R-C on the bridging thiolates dictates
the position of phosphorus atoms on each iron. For anti-2,
P(1) atom of the P-S ligand at the Fe(1) center is trans to
the metal-metal bond (in the apical position of the pseudo
square pyramid), while the P(2) atom attached on Fe(2) is
trans to the bridged thiolate sulfur S(1); i.e., P(2) is in the
basal position of the pseudo square pyramid. Both CO groups
at the Fe(1) centers are trans to bridged thiolates. The
structure of [(µ-H)-anti-2+] differs only from anti-2 in that
the bridging hydride takes the position of the Fe-Fe bond
density. The more symmetrical syn-2, Figure 3, positions
both P-donor sites cis to the Fe-Fe bond and trans to the
bridged thiolates. Two phenyl groups are below the Fe-Fe
bond vector.
The coordination spheres of iron in both binuclear neutral
complexes contain two CO, two S from bridging thiolates,
and one phosphine, in a distorted square pyramidal arrange-
ment. The pair of electrons in the metal-metal bond orbital
overlap resides trans to the apical position of the square
pyramid and fulfills an octahedral electronic distribution.
Nevertheless, the displacement of the iron atom from the
best basal planes toward the apical position of 0.25 and 0.42
Å (respectively for Fe(1) and Fe(2) in anti-2), and 0.33 and
0.28 Å (respectively for Fe(1) and Fe(2) in syn-2) is fully
consistent with known square pyramidal geometries and
indicates a modest structural requirement of the metal-metal
bond density. While the bridging hydride of [(µ-H)-anti-
2]+ renders the irons six-coordinate, there is still a displace-
ment of the iron atoms toward the “apical” ligands, i.e., those
Metric parameters for the butterfly Fe2S2 core structures
are substantially the same for all with acute Fe-S-Fe’s,
and as shown in Figure 5, the three structures closely overlay
in that core (rms deviation of 6.4%). The dihedral angle
defined by the intersections of S(1)Fe(1)Fe(2) and S(2)Fe-
Inorganic Chemistry, Vol. 41, No. 4, 2002 703

Zhao et al.
Figure 3. (a) Thermal ellipsoid (50%) and (b) ball-and-stick representations
of the molecular structure of syn-2.
(1)Fe(2) planes is 99.3° in syn-2 and is slightly smaller than
that of anti-2, 104.5°, and of [(µ-H)-anti-2]+, 105.6°. Con-
sistently, the distances between two thiolate sulfur “wing
tips” are 2.826 Å for syn-2 and 2.945 Å for anti-2. The ana-
logous distance in the [(µ-H)-anti-2]+ structure is 2.969 Å.
Figure 4. (a) Thermal ellipsoid (50%) and (b) ball-and-stick representations
of the molecular structure of [(µ-H)-anti-2]+.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
anti-(µ-SCH2CH2PPh2)2Fe2(CO)4, anti-2, and
The Fe-P and Fe-C bond distances displayed in Table
2 for anti-2 are typical in comparison to monodentate
analogues [Fe(CO)2L(µ-SR)]2 (L ) P(CH3)3, R ) CH3).7
There are no significant differences in lengths of the Fe-
CO bond trans to the bridged thiolate ligand and the Fe-
CO trans to the metal-metal bond. The average Fe-S and
Fe-P bond distances, 2.265(2) and 2.209(3) Å, respectively,
of the syn-2 isomer are not distinguishable from those of
the anti-2. This indicates that the disposition of the pendant
ethylene groups has no significant effect on the Fe2S2 core,
consistent with the similarity in ν(CO) IR pattern, Vide infra.
The Fe-Fe bond distance of 2.604(2) Å is slightly longer
for syn-2 than that found in anti-2, 2.562(1) Å, while the
Fe-Fe distance of the [µ-H-anti-2]+, 2.580(1) Å, shows little
effect of protonation. Other selected bond distances and
angles for syn-2 are shown in Table 3.
(µ-H)-anti-(µ-SCH2CH2PPh2)2Fe2(CO)4+, [(µ-H)-anti-2]+
anti-2
[(µ-H)-anti-2]+
Fe(1)-Fe(2)
Fe(1)-P(1)
Fe(2)-P(2)
Fe(1)-S(1)
Fe(2)-S(2)
Fe(2)-S(1)
Fe(1)-S(2)
Fe(2)-C(4)
Fe(2)-C(3)
Fe(1)-C(1)
Fe(1)-C(2)
Fe(1)-H
2.562(1)
2.216(1)
2.214(1)
2.264(1)
2.223(1)
2.271(1)
2.275(1)
1.777(2)
1.769(2)
1.766(3)
1.743(3)
2.580(10)
2.2360(14)
2.2235(13)
2.2671(13)
2.2470(13)
2.2665(13)
2.2851(13)
1.787(5)
1.797(5)
1.790(5)
1.789(5)
1.587(6)
Fe(2)-H
1.740(6)
S(1)Fe(1)S(2)
S(1)Fe(2)S(2)
Fe(1)S(1)Fe(2)
Fe(1)S(2)Fe(2)
P(1)Fe(1)C(1)
P(1)Fe(1)C(2)
P(1)Fe(1)S(1)
P(1)Fe(1)S(2)
C(4)Fe(2)P(2)
C(4)Fe(2)C(3)
P(2)Fe(2)S(2)
80.9(1)
81.9(1)
68.8(1)
69.4(1)
107.6(1)
98.8(1)
88.4(1)
92.1(1)
95.1(1)
99.8(1)
87.1(1)
81.42(5)
82.27(5)
69.37(4)
69.39(4)
95.10(15)
98.37(15)
87.15(5)
92.45(5)
98.09(15)
93.9(2)
The asymmetry in the P-donor positions of anti-2 produces
an asymmetric bridging hydride ligand; the bond distance
of that trans to the phosphine, Fe(1)-H(1), is 1.59(1) Å while
that trans to the CO, Fe(2)-H(1), is 1.74(1) Å. Although
the incorporation of hydride, with concomitant oxidation of
the FeIFeI to FeIIFeII, produces no structural changes in the
Fe2S2 core, electrochemical and spectral properties show
considerable differences.
87.84(5)
Electrochemistry. Electrochemical studies were per-
formed on mixtures of anti and syn compounds in acetonitrile
solutions; however, the separated, pure isomers showed very
similar electrochemical behavior. As shown in Figure 6, the
704 Inorganic Chemistry, Vol. 41, No. 4, 2002

Binuclear Phosphinothiolate Complexes
Table 3. Selected Distances (Å) and Angles (deg) for
syn-(µ-SCH2CH2PPh2)2Fe2(CO)4, syn-2a
bridged diiron carbonyl compound, [Fe(CO)3(PPh2)]2 (-1.2
V vs SCE, ∆Ep ) 45 mv).22 In addition, an irreversible
feature observed at +0.33 V is assumed to be S-based.
The cyclic voltammogram of (µ-H)(µ-SCH2CH2PPh2)2)-
Fe(1)-Fe(2)
Fe(2)-S(1)
Fe(1)-S(1)
Fe(2)-P(2)
Fe(2)-C(3)
Fe(1)-C(1)
C(1)-O(1)
C(3)-O(3)
2.603(2)
2.265(2)
2.250(2)
2.207(2)
1.785(9)
1.780(8)
1.147(9)
1.150(9)
Fe(2)-S(2)
Fe(1)-S(2)
Fe(1)-P(1)
Fe(2)-C(4)
Fe(1)-C(2)
C(2)-O(2)
C(4)-O(4)
2.269(2)
2.277(2)
2.211(2)
1.753(8)
1.753(8)
1.159(9)
1.179(9)
+
Fe2(CO)4 (SO3CF3)- has two irreversible redox processes
at -0.60 and -0.90 V, assigned to reductions to FeIIFeI and
FeIFeI species. Repetitive scanning resulted in the buildup
of waves at -1.71 and -2.28 V, consistent with the
formation of the neutral (µ-SCH2CH2PPh2)2Fe2(CO)4 parent
compound. This was confirmed by bulk chemical reduction
of the cationic hydride.
Chemical Reduction. On bulk chemical reduction of
neutral (µ-SCH2CH2PPh2)2Fe2(CO)4 with excess Na/Hg
amalgam in CH3CN under argon for about 1 h, the CO
stretching frequencies at 1989 (m), 1952 (s), and 1909 (m,
br) were observed to shift to 1956 (s), 1896, (s, br), and
1732 (m) cm-1, Figure 1f. A UV-vis spectral monitor of
the reaction also shows a decrease in the absorbance at 360
nm. The first reduction species, 3, was isolated as a dark
red-brown solid with PPN+ as counterion. This solid is highly
air sensitive even in the solid state and returns to the starting
material, neutral (µ-SCH2CH2PPh2)2Fe2(CO)4, on exposure
to O2. The IR spectrum, Figure 1f, is interpreted as an Fe0-
FeI dimer with a bridging CO group between the two iron
centers as proposed herein, complex 3. In support of this
S(1)-Fe(1)-S(2)
Fe(1)-S(1)-Fe(2)
S(1)-Fe(1)-P(1)
S(2)-Fe(1)-P(1)
C(1)-Fe(1)-P(1)
C(3)-Fe(2)-P(2)
C(1)-Fe(1)-C(2)
77.26(8)
70.43(7)
86.78(9)
159.54(9)
101.2(3)
95.6(3)
S(1)-Fe(2)-S(2)
Fe(1)-S(2)-Fe(2)
S(2)-Fe(2)-P(2)
S(1)-Fe(2)-P(2)
C(1)-Fe(1)-S(1)
C(3)-Fe(2)-S(2)
C(3)-Fe(2)-C(4)
77.10(8)
69.87(7)
86.72(8)
162.21(9)
103.4(3)
103.2(3)
97.9(4)
97.9(3)
a Estimated standard deviations are given in parentheses.
Figure 5. Overlay of the Fe2S2 cores of anti-2, syn-2, and [(µ-H)-anti-
2]+.
assignment, we note that the reported anionic species, [(µ-
SEt)(µ-CO)Fe2(CO)6]-,23 and [(µ-PPh2)(µ-CO)Fe2(CO)5-
(PPh2H)]-,24 displayed bridging CO frequencies at 1743 and
1710 cm-1, respectively. The spectral properties of an
intermediate in the CN-/CO substitution reaction of an FeI-
FeI dimer rendered unsymmetric due to a thioether on one
iron was also interpreted as having a bridging CO of ν(CO)
) 1780 cm-1.25
On continuation of the reduction of (µ-SCH2CH2PPh2)2-
Fe2(CO)4 by Na/Hg in CH3CN overnight under argon, two
new peaks in the CO stretching region grew in at 1941 (s)
and 1854 (s) cm-1, reaching a maximum intensity after
several days. This extremely air-sensitive compound 4 is
tentatively formulated as a dianionic Fe0Fe0 species as
described above in the electrochemistry results. Notably,
these reaction conditions do not reliably yield pure 4, but
rather often a mixture of 4 and the parent, neutral (µ-SCH2-
CH2PPh2)2Fe2(CO)4, 2. Since 2 readily forms on exposure
Figure 6. Cyclic voltammogram of 5 mM solution (µ-SCH2CH2PPh2)2-
Fe2(CO)4 in 0.1 M [(n-Bu)4N][PF6]/CH3CN with a glassy carbon working
electrode at a scan rate of 200 mV/s.
cyclic voltammogram of (µ-SCH2CH2PPh2)2Fe2(CO)4 in CH3-
CN (as a mixture of isomers) shows two features at potentials
between -1.5 and -3.0 V. One quasi-reversible wave is
centered at -1.67 V (∆Ep ) 0.60 V) and is assigned to a
one-electron reduction at an iron atom to give the [FeIFe0]-
-
or (µ-SCH2CH2PPh2)2Fe2(CO)4 (radical anion). A quasi-
reversible wave centered at -2.20 V is assigned to a second
electron reduction, presumed to have an [Fe0Fe0] center in
the dianion or (µ-SCH2CH2PPh2)2Fe2(CO)42-. The formation
of the radical anion and dianionic species is also confirmed
by chemical reduction (Vide infra). The finding of two one-
electron reductions for (µ-SCH2CH2PPh2)2Fe2(CO)4 contrasts
to the one two-electron step reported for the phosphido-
(21) De Beer, J. A.; Haines, R. J. J. Organomet. Chem. 1972, 36, 297.
(22) Collman, J. P.; Rothrock, R. K.; Finke, R. G.; Moore, E. J.; Rose-
Munch, F. Inorg. Chem. 1982, 21, 146.
(23) Seyferth, D.; Womack, G. B.; Dewan, J. C. Organometallics 1985, 4,
398.
(24) Yu, Y.-F.; Gallucci, J.; Wojcicki, A. J. Am. Chem. Soc. 1983, 105,
4826.
(25) Razavet, M.; Davies, S. C.; Hughes, D. L.; Pickett, C. J. Chem.
Commun. 2001, 847.
Inorganic Chemistry, Vol. 41, No. 4, 2002 705

Zhao et al.
reported to result from protonation of [Fe(CO)2(PMe2Ph)-
(µ-SMe)]2 with various acids.26,27 In this case only one
bridged hydride species, i.e., the syn isomer with both PMe2-
Ph ligands in apical (or trans to the µ-H) positions, was
reported. From this work and ours we can conclude that the
absolute value of the JP-H of P trans to the µ-H in [(µ-H)-
anti-2]+ is smaller than cis coupling.
The bridged hydride cations were readily (on time of
+
mixing) deprotonated by equimolar amounts of NEt4 CN-
in CH3CN, regaining the neutral parent complexes. Aceto-
nitrile solutions of bistriphenylphosphineimminium chloride,
PPN+Cl-, also rapidly reacted; however, a 5-fold excess of
chloride resulted in only partial deprotonation. Equilibrium
was established more rapidly for the [(µ-H)-anti-2]+ isomer.
In contrast, 5, 10, and 20 equiv of the strong but neutral
base, triethylamine, required hours to achieve equilibrium
with [(µ-H)-syn-2]+ as its triflate salt in THF. Qualitative
differences in rates suggest more rapid deprotonation reac-
tions of Et3N with [(µ-H)-anti-2]+[SO3CF3]-, consistent with
less steric hindrance in this isomer as compared to [(µ-H)-
syn-2]+ where the bridging hydride is sterically protected
by two flanking phenyl groups. Such a steric argument is
appealing in that for the [(µ-H)-anti-2]+ the bridged hydride
is trans to the better σ-donor ligand phosphine which should,
relative to [(µ-H)-syn-2]+, inhibit removal of hydrogen as a
proton should electronic factors predominate. Addition of
small (ca. 10%) amounts of PPN+Cl- facilitated rates of
deprotonation by NEt3.
Figure 7. 1H NMR spectrum in upfield region for (a) [(µ-H)-anti-2]+[SO3-
CF3]-; and (b) [(µ-H)-syn-2]+[SO3CF3]- in d6-acetone.
of compounds 3 and 4 to air or to protonic acids, we assume
that its formation after long reaction times results from
adventitious acid or O2.
Solution Spectral Properties of the Bridging Hydrides
and Deprotonation Reactions. Both anti-2 and syn-2 react
with a slight excess of CF3SO3H in THF solution, yielding
orange-red solutions whose ν(CO) infrared spectra displayed
three bands approximately 60 cm-1 higher in energy than
those of their neutral (µ-SCH2CH2PPh2)2Fe2(CO)4 precursors,
Table 1. As shown in Figure 1, traces d and e, the CN3CN
solution ν(CO) infrared spectra of the cationic hydrides show
greater distinctions between the isomeric forms than those
between the neutral parent precursors. The [(µ-H)-anti-2]+
spectrum is resolved into four peaks while that of [(µ-H)-
syn-2]+ shows only two, positioned at the average of the
higher and the two lower bands of the anti isomer.
The 1H NMR spectrum (Figure 7a) of [(µ-H)-anti-
2]+[SO3CF3]- showed a doublet of doublets in the upfield
region, centered at δ -16.85 with coupling constants of JP-H
) 23.8 and 4.8 Hz. In addition, the downfield resonance
region also shows two different sets of ethylene protons for
the ligand. The 31P NMR shows doublets at δ 88.1 and 79.1
ppm each with JP-P ) 11 Hz. This spectroscopic data is
consistent with the molecular structure of [(µ-H)-anti-2]+,
as described above and presented in Figure 4; i.e., the bridged
hydride is coupled to two nonequivalent phosphorus atoms
from nonequivalent P to S ethylene linkers.
It should be mentioned that deprotonation of [(µ-H)-anti-
2]+[SO3CF3]- by NEt3 is complicated by an isomerization
process, as yet not clarified. On addition of a stoichiometric
amount of NEt3 to solutions containing only [(µ-H)-anti-
2]+[SO3CF3]-, the triplet characteristic of the syn isomer [(µ-
H)-syn-2]+ grew in while the doublet of doublets for the
[(µ-H)-anti-2]+ diminished in intensity and broadened. It is
not clear whether the broadening is due to exchange of the
+
proton of [(µ-H)-anti-2]+ with HNEt3 or to quadrupolar
broadening from interaction of the metal-bound proton with
NEt3. In either case, we can assume that the observation
signifies similar basicities of NEt3 and the Fe-Fe bond
density of neutral binuclear anti-2. This effect was not
observed with the poorer base pyridine. The ability of NEt3
to partially remove a proton from metal carbonyl hydrides
such as HCo(CO)4, producing a Co-- - -H+- -NEt3 tight ion
pair (alternately represented as a metal-H-bonded species),
has been established.28,29 Likewise well-known are the effects
of steric blocks on the rates of deprotonation of metal
hydrides in [HMo(CO)2(diphos)2]+, as well as the proton-
carrying effect of hard bases such as chloride.30-32
1
The H NMR spectrum for the product resulting from
protonation of syn-2, Figure 7 b, has an upfield triplet
centered at δ -17.4 ppm which integrates as one proton
compared to 28 in the ligand resonance region. The triplet,
JP-H ) 21.0 Hz, results from equal coupling of the bridged
hydride and the two P atoms of the syn isomer. The
compound [(µ-H)-syn-2]+[SO3CF3]- is more thermally stable
than [(µ-H)-anti-2]+[SO3CF3]-. Notably, the 1H NMR
spectrum obtained on the product of protonation of the
mixture of syn-2 and anti-2 as isolated in the original
synthesis, eq 3, showed a mixture of [(µ-H)-anti-2]+ and
[(µ-H)-syn-2]+ isomers, stable in the dark, but undergoing
isomerization and increasing the [(µ-H)-syn-2]+ in the light.
Interestingly, the presence of NEt3 promotes isomerization
of [(µ-H)-anti-2]+ to [(µ-H)-syn-2]+, Vide infra.
(26) Fauvel, K.; Mathieu, R.; Poilblanc, R. Inorg. Chem. 1976, 15, 976.
(27) Savariault, J.-M.; Bonnet, J.-J.; Mathieu, R.; Galy, J. C. R. Acad. Sci.
Paris, t. 1977, 284.
(28) Calderazzo, F.; Fachinetti, G.; Marchetti, F.; Zanazzi, P. F. J. Chem.
Soc., Chem. Commun. 1981, 181.
(29) Mareque Rivas, J. C.; Brammer, L. Coord. Chem. ReV. 1999, 183,
43.
(30) Darensbourg, M. Y.; Ludvig, M. M. Inorg. Chem. 1986, 25, 2894.
(31) Liu, W.; Thorp, H. H. J. Am. Chem. Soc. 1995, 117, 9822.
(32) Kristja´nsdottir, S. S.; Norton, J. K. In Transition Metal Hydride;
Dedieu, A., Ed.; VCH: New York, 1992; pp 309-359.
+
An analogous (µ-H)[Fe(CO)2(PMe2Ph)(µ-SMe)]2 com-
plex, with δ ) -16.6 ppm (triplet, JP-H ) 4.4 Hz), was
706 Inorganic Chemistry, Vol. 41, No. 4, 2002

Binuclear Phosphinothiolate Complexes
Scheme 4
electron-donor ligand, the Fe(CO)4(PMe2CH2CH2SH) com-
plex, should be preferentially protonated. The trend in
reactivity is, however, opposite and is taken as evidence of
the predominating influence of Fe-CO bond lability from
the Fe0 parent complex on the reaction progress. Note that
the mechanism in Scheme 4 does not indicate possible
binucleating steps which might occur as follows: Increased
CO lability is expected for the FeII thiolate hydride which
would open a site for the first thiolate bridge formation. A
possibility is shown in the arrangement below whereby thiol
binding to one Fe0 results in increased acidity and protonation
of a second Fe0 generating a thiolate-bridged FeII hydride.
Subsequent protonation of the hydride by the dangling thiol
must be followed by several steps, including H2 loss, CO
loss, a second thiolate bridge formation and FeIIFe0 com-
proportionation to FeIFeI. Precedents for such intramolecular
H+/H- reactivity in simpler mononuclear complex chemistry
include the protonation of a Ru-H by an amine-H+,
tethered to η5-C5H4R.33-35
Effects of P-Donor Strengths in PPh(CH3)CH2CH2SH
and P(CH3)2CH2CH2SH Derivatives. Under photochemical
experimental conditions as similar as possible it was
established that, relative to complex 1, Fe(CO)4(PPh2CH2-
CH2SH), extended periods of irradiation of THF solutions
of Fe(CO)4(PPhMeCH2CH2SH) were required to produce the
dinuclear iron dicarbonylphosphino-thiolate analogue of
complex 2. Extended irradiation of solutions of Fe(CO)4-
(PMe2CH2CH2SH) under similar photolytic conditions caused
no reaction as indicated by the infrared spectral monitor.
Structures. Three features relating to the X-ray crystal
structures are considered most important:
(1) The orientation of the R-C on the bridging thiolates
dictates the position of phosphorus atoms on each iron. The
differences in syn, anti, and syn′ S-C arrangements in all
CO dinuclear iron complexes with monodentate µ-SR bridges
are of little consequence to electronic effects from the
bridging sulfur donor. However, in the case of dinuclear
complexes containing bidentate S-P complexes, the subse-
quent positioning of the P-donor ligand as a result of
directionality of the S-CR position produces isomers with
the metal-metal bond density or µ-H in different electronic
environments dependent on whether the P-donor is cis or
trans to that position.
(2) The molecular structures of three binuclear complexes
show insignificant differences in Fe-Fe distances despite
the fact that in the FeIFeI complexes there is a formal metal-
metal bond whereas in the FeII(µ-H)FeII complex there is
none. Other Fe-L distances and the Fe2S2 cores are also
the same. The Fe-Fe bond density therefore presents a site
in which a proton might be taken up and stored as a hydride
without impacting the overall structure of the binuclear
complex.
Discussion
Reaction Pathway. Although the overall reaction pathway
for the synthesis of binuclear phosphino thiolates from thiols
is still uncertain, Scheme 4 expresses the most obvious
features as highlighted in our work. A coordinately unsatur-
ated Fe(CO)4 fragment is preferentially captured by the
phosphine donor atom in the P-SH ligand (rather than the
thiol sulfur donor), and the 18-electron mononuclear Fe(0)
complex serves as precursor to the binuclear product.
Photolysis is required to eject CO and provide a second open
site which we propose is required for the binding and
activation of the tethered thiol. Yet another CO must be lost,
along with intermolecular generation of H2, in order to
complete the reaction.4 The different requirements of UV
irradiation times for the three phosphino thiols relate to the
electronic differences in the P-donor ligand consistent with
better donors inhibiting CO loss.
An alternative mechanism involving initial RSH binding
S-H oxidative addition, producing a binuclear intermediate
with bridging thiolates and dangling phosphines, is not
indicated as all three phosphinothiols should have produced
binuclear Fe2S2 products. In fact, the superior ligating ability
of phosphine over thiols was illustrated in the isolation of
the series, Fe(CO)4(PMe2CH2CH2SH), Fe(CO)4(PPhMeCH2-
CH2SH), and Fe(CO)4(PPh2CH2CH2SH). Should protonation
or oxidative addition of the thiol proton to the 18-electron
Fe(0) complex (without CO loss and binding of the thiolate)
govern the reaction pathway, the complex containing the best
(3) This study provides another example of the interesting
similarities of dinuclear FeIFeI complexes with the actiVe
site of Fe-only hydrogenase in terms of the Fe2S2 core. It
indicates that the Fe-Fe distances of 2.60 Å in the binuclear
(33) Ayllon, J. A.; Sayers, S. F.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret,
B. Organometallics 1999, 18, 3981.
(34) Caballero, A.; Jalo´n, F. A.; Manzano, B. R. Chem. Commun. 1998,
1879.
(35) Chu, H. S.; Lau, C. P.; Wong, K. Y. Organometallics 1998, 17, 2768.
Inorganic Chemistry, Vol. 41, No. 4, 2002 707

Zhao et al.
active site of Fe-only hydrogenases14 can be consistent with
an Fe-Fe bond and FeIFeI oxidation states as well as a face-
bridged bioctahedron in FeII(µ-H)FeII complexes.36
Scheme 5
The Bridging Hydride as a Proton. The basicity of
M-M bonds is sensitive both to the M and to substituent
ligands. Whereas (µ-pdt)Fe2(CO)6 cannot be protonated, the
(µ-pdt)Fe2(CO)4(PMe3)2 readily forms a stable bridging
hydride complex.37 Likewise Nataro and Angelici have
shown the PMe3 derivative of Cp2Mo2(CO)4(PR3)2 to be
“dramatically” more basic than its PMe2Ph analogue.38
Perhaps the most extraordinary result of our study is the
ease with which the bridging hydride can be extracted as a
proton from the Fe-Fe bond density by anionic bases, even
the very weak base, chloride. Thermodynamic driving forces
include the formation of beneficial ion pairs in the low-
polarity solvents which were used, as well as the reclamation
of the Fe-Fe bond density. Contributors to the kinetic barrier
include polarization of the electron density surrounding the
hydride which, a priori, might have been considered
significant in a binuclear bridging hydride, Scheme 5. On
the other hand, the electronic rearrangement is shared
between two metals and, as noted by the crystal structures,
exacts a minimal structural change. In this manner of electron
density delocalization, the enhanced acidity of bridging
hydrides in polynuclear complexes has previously been
described for H4Os4(CO)12.39,40 The efficacy of binuclear
active sites with M‚‚‚M distances within bonding range in
enzyme catalysis could lie in such a dispersal of electronic
changes as proton/electron coupled reactions take place.
Acknowledgment. We acknowledge financial support
from the National Science Foundation (CHE 98-12355 for
this work, CHE 85-13273 for the X-ray diffractometer and
crystallographic computing system) and contributions from
the R. A. Welch Foundation. Appreciation is expressed to
Matthew L. Miller for technical assistance with molecular
structure displays as well as to Susan Winters for manuscript
preparation.
Supporting Information Available: X-ray crystallographic
tables for anti-(µ-SCH2CH2PPh2)2Fe2(CO)4, syn-(µ-SCH2CH2PPh2)2-
Fe2(CO)4, and [(µ-H)-anti-2]+[SO3CF3]-. Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC010741G
(36) De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, E. C.;
Fernandez, V. M. J. Am. Chem. Soc. 2000, 122, 11232.
(37) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710.
(39) Kristija´nsdo´ttir, S. S.; Moody, A. E.; Weberg, R. T.; Norton, J. R.
Organometallics 1988, 7, 1983.
(40) Walker, H. W.; Pearson, R. G.; Ford, P. C. J. Am. Chem. Soc. 1983,
105, 1179.
(38) Nataro, C.; Angelici, R. J. Inorg. Chem. 1998, 37, 2975.
708 Inorganic Chemistry, Vol. 41, No. 4, 2002