New nitrosyl derivatives of diiron dithiolates related to the active site of the [FeFe]-hydrogenases

Article abstract of DOI:10.1021/ic801542w

Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the precursor Fe₂(S₂C_nH _2n)(dppv)(CO)₄ (dppv = cis-1,2- bis(diphenylphosphinoethylene). These studies expand the range of substituted diiron(I) dithiolato carbonyl complexes. From [Fe₂(S ₂C₂H₄)(CO)₃(dppv)(NO)]BF ₄ ([1(CO)₃]BF₄), the following compounds were prepared: [1(CO)₂(PMe₃)]BF₄, [1(CO)(dppv)]BF₄, NEt₄[1(CO)(CN)₂], and 1(CO)(CN)(PMe₃). Some of these substitution reactions occur via the addition of 2 equiv of the nucleophile followed by the dissociation of one nucleophile and decarbonylation. Such a double adduct was characterized crystallographically in the case of [Fe₂(S₂C ₂H₄)(CO)₃(dppv)(NO)(PMe₃) ₂]BF₄. This result shows that the addition of two ligands causes scission of the Fe-Fe bond and one Fe-S bond. When cyanide is the nucleophile, nitrosyl migrates away from the Fe(dppv) site, yielding a Fe(CN)₂(NO) derivative. Compounds [1(CO)₃]BF₄, [1(CO)₂(PMe₃)]BF₄, and [1(CO)(dppv)]BF ₄ were also prepared by the addition of NO⁺ to the di-, tri-, and tetrasubstituted precursors. In these cases, the NO⁺ appears to form an initial 36e^- adduct containing terminal Fe-NO, followed by decarbonylation. Several complexes were prepared by the addition of NO to the mixed-valence Fe(I)Fe(II) derivatives. The diiron nitrosyl complexes reduce at mild potentials and in certain cases form weak adducts with CO. IR and EPR spectra of 1(CO)(dppv), generated by low-temperature reduction of [1(CO)(dppv)]BF₄ with Co(C₅Me₅)₂, indicates that the SOMO is located on the FeNO subunit.

Full text of DOI:10.1021/ic801542w

Inorg. Chem. 2008, 47, 11816-11824
New Nitrosyl Derivatives of Diiron Dithiolates Related to the Active Site
of the [FeFe]-Hydrogenases
Matthew T. Olsen, Aaron K. Justice, Fre´de´ric Gloaguen, Thomas B. Rauchfuss,* and Scott R. Wilson
Department of Chemistry, UniVersity of Illinois, Urbana, Illinois 61801
Received August 12, 2008
Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the precursor Fe2(S2CnH2n)(dppv)(CO)4 (dppv
) cis-1,2-bis(diphenylphosphinoethylene). These studies expand the range of substituted diiron(I) dithiolato carbonyl complexes.
From [Fe2(S2C2H4)(CO)3(dppv)(NO)]BF4 ([1(CO)3]BF4), the following compounds were prepared: [1(CO)2(PMe3)]BF4, [1(CO)(d-
ppv)]BF4, NEt4[1(CO)(CN)2], and 1(CO)(CN)(PMe3). Some of these substitution reactions occur via the addition of 2 equiv of the
nucleophile followed by the dissociation of one nucleophile and decarbonylation. Such a double adduct was characterized
crystallographically in the case of [Fe2(S2C2H4)(CO)3(dppv)(NO)(PMe3)2]BF4. This result shows that the addition of two ligands
causes scission of the Fe-Fe bond and one Fe-S bond. When cyanide is the nucleophile, nitrosyl migrates away from the
Fe(dppv) site, yielding a Fe(CN)2(NO) derivative. Compounds [1(CO)3]BF4, [1(CO)2(PMe3)]BF4, and [1(CO)(dppv)]BF4 were also
prepared by the addition of NO+ to the di-, tri-, and tetrasubstituted precursors. In these cases, the NO+ appears to form an
initial 36e- adduct containing terminal Fe-NO, followed by decarbonylation. Several complexes were prepared by the addition
of NO to the mixed-valence Fe(I)Fe(II) derivatives. The diiron nitrosyl complexes reduce at mild potentials and in certain cases
form weak adducts with CO. IR and EPR spectra of 1(CO)(dppv), generated by low-temperature reduction of [1(CO)(dppv)]BF4
with Co(C5Me5)2, indicates that the SOMO is located on the FeNO subunit.
Introduction
models.3 Associated with their distinctive structure, these
nitrosyl compounds are Lewis acidic, which is unknown for
other [FeI]2 species but characteristic of the mixed valence
Hox state and its models. Our results indicated the likely
stability of other nitrosyl complexes. We therefore have
investigated the nitrosyl derivatives of the diiron(I) dithiolates
supported by cis-1,2-bis(diphenylphosphinoethylene) (dppv).
For studying the fundamental reactivity of diiron(I) dithi-
olates, we have shown that Fe2(S2CnH2n)(CO)4(dppv) (n )
2, 3) are useful reagents because of their high reactivity and
the simplified structures of the products.4 We expected,
incorrectly that NO+ would yield highly polar derivatives
of Fe2(S2CnH2n)(CO)4(dppv).
The substitution chemistry for compounds of the type
[Fe2(SR)2(CO)6-xLx]z is well-developed for L ) tertiary
phosphines and cyanide.1 These synthetic transformations
were mainly developed en route to functional and structural
mimics of the active site of the [FeFe]-hydrogenase enzymes.
The enzymes catalyze the redox interconversion of protons
and H2 and do so at near-thermodynamic potentials.2 The
high efficiency of the Fe-based catalysts has motivated
numerous studies on the synthesis and reactivity of diiron
dithiolato carbonyls.
Recently, we reported a series of diiron dithiolate-PMe3
complexes containing nitrosyl ligands. These compounds are
of interest because they adopt “rotated” structures as seen
in related mixed-valence FeIFeII complexes, the so-called Hox
Nitrosyl derivatives of iron thiolates are of course well-
known. The first synthetic iron-sulfide cluster, Roussin’s Red
anion, [Fe2S2(NO)4]2-, led to the corresponding “esters”,
Fe2(SR)2(NO)4 (R ) alkyl, aryl).5 The Roussin esters have
come under renewed scrutiny because of their connection
* Author to whom correspondence should be addressed. E-mail:
rauchfuz@uiuc.edu.
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J. H.; Soriaga, M. P.; Darensbourg, M. Y. J. Am. Chem. Soc. 2004,
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11816 Inorganic Chemistry, Vol. 47, No. 24, 2008
10.1021/ic801542w CCC: $40.75  2008 American Chemical Society
Published on Web 11/14/2008

New Nitrosyl DeriWatiWes of Diiron Dithiolates
pv)(NO)+ and Fe(CO)3 subunits, reminiscent of the structure
seen for [Fe2(S2CnH2n)(CO)3(PMe3)2(NO)]+.13
Scheme 1. Diiron Dithiolato Nitrosyl Complexes Described in This
Work
In addition to the νNO band at 1796 cm-1, the IR spectrum
of [1(CO)3]BF4 features two νCO bands for the Fe(CO)3
subunit. Relative to Fe2(S2C2H4)(CO)4(dppv), these bands are
shifted to higher energy by about 50 cm-1. A similar trend
is observed for the propanedithiolato complexes.
The 31P NMR spectrum of [1(CO)3]BF4 shows two singlets
in a 1:2 ratio, consistent with two isomers that differ with
respect to the position of the dppv chelate.4 The smaller
signal is assigned to a highly fluxional apical-basal isomer;
this signal was found to broaden upon cooling the sample
to -80 °C. A singlet assigned to the major isomer proved
temperature-invariant, consistent with the dibasal stereo-
chemistry. A similar trend is seen for [2(CO)3]BF4, except
that the equilibrium concentration of the apical-basal isomer
is barely detectable (Scheme 2, Table 1). The related
Fe2(S2C2H4)(CO)4(dppv) exhibits similar dynamics, although
the apical-basal isomer is far more stabilized (20:1). In the
-80 °C 31P NMR spectrum of [2(CO)3]+, four isomers are
observed, indicative of slowed folding of the propanedithi-
olate.
Substitution of [Fe2(S2CnH2n)(CO)3(dppv)(NO)]BF4 by
PMe3. The salt [1(CO)3]BF4 proved highly reactive toward
nucleophiles. Thus, treatment of [1(CO)3]BF4 with 1 equiv
of PMe3 at room temperature gave [1(CO)2(PMe3)]BF4. The
31P NMR spectrum of [1(CO)2(PMe3)]BF4 displayed singlets
in both the dppv and PMe3 regions, which implicates axial
PMe3 and dibasal dppv. The dibasal disposition of the dppv
ligand is further evidenced by the 31P NMR chemical shift
(δ74.1), following a pattern seen for related complexes.4
In a test of its relative electrophilicity, equimolar amounts
of Fe2(S2C2H4)(CO)4(dppv) and [1(CO)3]BF4 were treated
with 1 equiv of PMe3 at 20 °C. IR and 31P NMR measure-
ments revealed exclusive consumption of [1(CO)3]BF4. In
an attempt to elucidate some details of the substitution
mechanism, a solution of [1(CO)3]BF4 was treated with 1
equiv of PMe3 at -20 °C; we observed the complete
disappearance of free PMe3, partial consumption of
[1(CO)3]+, and the appearance of an intermediate species
(νNO ) 1727 cm-1). This reaction appears to rapidly give
the double adduct [1(CO)3(PMe3)2]+, followed by a slower
dissociation of PMe3 that reacts with remaining starting
materials (eqs 1 and 2).
to the so-called dinitrosyl iron complexes.6 Nitric oxide
inhibits [NiFe] hydrogenases,7 although the reactivity of NO
with [FeFe] hydrogenases has not been reported.
Both NO and NO+ are electrophiles;8 hence, it is worth
mentioning the reactivity of other electrophiles with
Fe2(SR)2(CO)6-xLx. Substituted diiron(I) dithiolates protonate
12
readily.9 Sources of SMe+,10 Br+, I+,11 and HgCl+ have
long been known to add across the Fe-Fe bond.
Results
[Fe2(S2CnH2n)(CO)3(dppv)(NO)]+ (n ) 2, 3). The salts
[Fe2(S2CnH2n)(NO)(CO)3(dppv)]BF4 (n ) 2, [1(CO)3]BF4;
n ) 3, [2(CO)3]BF4) were found to form efficiently upon
treatment of CH2Cl2 solutions of Fe2(S2CnH2n)(CO)4(dppv)
with NOBF4 at 20 °C (Scheme 1). Indicative of their high
electrophilicity, both [1(CO)3]BF4 and [2(CO)3]BF4 exhibit
limited stability in MeCN solutions at room temperature,
although CH2Cl2 and tetrahydrofuran solutions are stable for
hours. These mononitrosyl complexes were unreactive toward
additional equivalents of NOBF4 at room temperature.
The crystallographically determined structure of
[2(CO)3]BF4 revealed that the nitrosyl ligand is apical, the
cation having idealized Cs symmetry (Figure 1). The complex
exhibits a slight twisting distortion in both the Fe(dp-
(6) Foster, M. W.; Cowan, J. A. J. Am. Chem. Soc. 1999, 121, 4093–
4100. Stamler, J. S. S. D. J.; Loscalzo, J. Science 1992, 258, 1898–
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128, 3528–9. Huang, H.-W.; Tsou, C.-C.; Kuo, T.-S.; Liaw, W.-F.
Inorg. Chem. 2008, 47, 2196–2204.
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Ahmed, A.; Lewis, R. S Biotechnol. Bioeng. 2007, 97, 1080–1086.
(8) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University
Press: New York, 1992.
(9) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pe´tillon, F. Y.; Schollhammer,
P.; Talarmin, J.; Pichon, R.; Kervarec, N. Inorg. Chem. 2007, 46,
3426–3428.
(10) Treichel, P. M. C. R. A.; Matthews, R.; Bonnin, K. R.; Powell, D. J.
Organomet. Chem. 1991, 402, 233–248. Georgakaki, I. P.; Miller,
M. L.; Darensbourg, M. Y. Inorg. Chem. 2003, 42, 2489–2494.
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1976, 1749–1757.
[Fe2(S2CnH2n)(CO)3(dppv)(NO)]+ + 2PMe3 f
[Fe2(S2CnH2n)(CO)3(dppv)(PMe3)2(NO)]+ (1)
[Fe2(S2CnH2n)(CO)3(dppv)(PMe3)2(NO)]+ f
[Fe2(S2CnH2n)(CO)2(dppv)(PMe3)2(NO)] BF4 +
PMe3 + CO (2)
Upon warming to room temperature, the mixture of
[1(CO)3]+ and the intermediate converted with good ef-
ficiency to [1(CO)2(PMe3)]+. When a solution of [1(CO)3]+
was treated with excess PMe3 at -20 °C, electrospray
(12) Arabi, M. S.; Mathieu, R.; Poilblanc, R. Inorg. Chim. Acta 1977, 23,
L17-L18.
(13) Olsen, M. T.; Bruschi, M.; De Gioia, L.; Rauchfuss, T. B.; Wilson, S.
R. J. Am. Chem. Soc. 2008, 130, 12021–12030.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11817

Olsen et al.
Figure 1. Side-on (left) and end-on (right) views of [2(CO)3]BF4. Thermal ellipsoids are shown at 35% probability and are omitted on the phenyl rings for
clarity, as are the anions (one phenyl group is perpendicular to the plane of the page). Selected bond distances (Å): Fe(1)-Fe(2), 2.5931(7); Fe(1)-P(1),
2.2791(10); Fe(1)-P(2), 2.3128(10); Fe(1)-N(1), 1.659(2); Fe(2)-C(4), 1.806(3); Fe(2)-C(5), 1.806(3); Fe(2)-C(6), 1.785(3).
Scheme 2. Isomerization Processes for the Fe(dppv)(NO)+ Subunit
Figure 2. Structure of the cation in [2(CO)3(PMe3)2]BF4. Thermal ellipsoids
are shown at 35% probability and are omitted on the phenyl rings for clarity,
as are the anions. Selected bond distances (Å): Fe(1)-N(1), 1.618(9);
Fe(1)-C(1), 1.860(11); Fe(1)-P(1), 2.231(3); Fe(1)-P(2), 2.270(3);
Fe(1)-S(1), 2.323(3); Fe(2)-S(1), 2.376(2); Fe(2)-S(2), 2.306(3);
Fe(2)-C(2), 1.749(11); Fe(2)-C(3), 1.741(11); Fe(2)-P(3), 2.312(3);
Fe(2)-P(4), 2.270(3); Fe(1)-Fe(2), 3.969.
Table 1. Ratios of Apical-Basal/Dibasal Isomers (dppv Location) for
[Fe2(S2CnH2n)(CO)4-x(dppv)(NO)x]x+
(S2C2H4)(CO)4 (S2C2H4)(CO)3(NO) (S2C3H6)(CO)4 (S2C3H6)(CO)3(NO)
20:1
1:2
7:1
1:>25(est.)
ionization mass spectrometry (ESI-MS) analysis of the
reaction mixture confirmed the presence of a species corre-
sponding to [1(CO)3(PMe3)2]+. The related reaction of
[2(CO)3]+ with PMe3 gave similar results. The 31P NMR
spectrum of the intermediate in the PMe3 region (∼δ25)
revealed one major and two minor isomers, and in each, the
PMe3 groups are nonequivalent, as virtually required since
only low-symmetry products could be produced.
Solutions of [2(CO)3(PMe3)2]BF4 readily produced crystals
suitable for analysis by X-ray diffraction. The structure of
[2(CO)3(PMe3)2]+ features both octahedral and trigonal-
bipyramidal iron centers, which are assigned as Fe(II) and
Fe(0), respectively. The octahedral site features cis thiolates,
cis phosphines, and cis carbonyl ligands. Viewing the
pentacoordinate site as a trigonal bipyramid, the axial sites
are occupied by the acceptor ligands, CO and NO+ (Figure
2).
The crystallographic result shows that two molecules of
PMe3 added to the Fe(CO)3 site, inducing the migration of
CO and scission of Fe-Fe and Fe-S bonds.
Cyano-Nitrosyl Complexes. The treatment of
[1(CO)3]BF4 with 2 equiv of Et4NCN gave the dicyanide,
Et4N[1(CN)2(CO)]. Shown in Figure 3, variable-temperature
31P NMR measurements indicate that Et4N[1(CN)2(CO)]
exists exclusively as a single diastereomer in solution wherein
Figure 3. The 202 MHz 31P NMR spectra of Et4N[1(CN)2(CO)] (CD2Cl2
soln) at 20 (top) and -60 °C (bottom).
the dppv is apical-basal. We propose that this reaction
proceeds via relocation of the nitrosyl to the Fe(CN)2 site.
The IR spectrum of Et4N[1(CN)2(CO)] resembles that for
[1(CO)(dppv)]BF4 in the νCO region, but νNO occurs at 41
cm-1 lower energy. The switch of the dppv to apical-basal
geometry is also consistent with a Fe(dppv)CO site; in
Fe(dppv)NO sites, the dppv is invaribly dibasal in the
absence of extreme steric effects.
One equivalent of [1(CO)3]BF4 was found to rapidly
consume 2 equiv of Et4NCN. This behavior is analogous to
the reaction of PMe3 and [1(CO)3]BF4 (see above). When
11818 Inorganic Chemistry, Vol. 47, No. 24, 2008

New Nitrosyl DeriWatiWes of Diiron Dithiolates
Figure 5. Structure of 1(CN)(CO)(PMe3). Thermal ellipsoids are shown
at 35% probability and are omitted on the phenyl rings for clarity. Distances
(Å): Fe(1)-Fe(2), 2.275(4); Fe(1)-N(2), 1.631(12); Fe(1)-P(3), 2.275(4);
Fe(1)-C(4), 1.926(15); Fe(2)-P(1), 2.183(4); Fe(2)-P(2), 2.208(4);
Fe(2)-C(3), 1.713(14).
Figure 4. Structure of [1(CO)(dppv)]. Thermal ellipsoids are shown at
35% probability and are omitted on the phenyl rings for clarity. Selected
bond distances (Å): Fe(1)-Fe(2), 2.5528(13); Fe(2)-P(1), 2.253(2);
Fe(2)-P(2), 2.237(2); Fe(2)-N(1), 1.695(6); Fe(1)-C(3), 1.719(7);
Fe(1)-P(3), 2.2026(19); Fe(1)-P(4), 2.247(2).
[1(dppv)(CO)]BF4 is the only diiron ditholate nitrosyl
observed in this series that features a basal nitrosyl ligand
present in the major observed isomer. 31P NMR studies at
low temperatures showed that [1(CO)(dppv)]BF4 binds CO
reversibly (see Supporting Information).
examined by IR spectroscopy at -45 °C, the dicyanation of
[1(CO)3]BF4 generated a complex mixture characterized by
several bands in the νCO and νCN regions, although the region
1800-1600 cm-1 was blank,13 implying the presence of a
bridging NO ligand.
Oxidation of Fe2(S2C3H6)(CO)4(dppv) with 1 equiv of
FcBF4 followed by the addition of about 1 equiv of NO gave
[2(CO)3]BF4. We also prepared [1(CO)2(PMe3)]BF4 via the
mixed valence species [Fe2(S2C2H4)(CO)3(dppv)(PMe3)]BF4.4
Nitrosyl Migations: Fe2(S2C2H4)(CN)(CO)(dppv)(PMe3)
(NO). Whereas Fe2(S2C2H4)(CO)3(dppv)(PMe3) is inert to-
ward substitution by cyanide, its nitrosylated derivative
[1(CO)2(PMe3)]BF4 was found to react with 1 equiv of
cyanide to produce 1(CN)(CO)(PMe3). The 31P NMR
spectrum of 1(CN)(CO)(PMe3) consisted of two doublets in
the dppv region, indicative of a single diastereomer. Appar-
ently, rapid turnstile rotation4,14 equilibrates the diastereo-
meric rotamers. The structure of this species was confirmed
crystallographically; the compound is chiral and crystallizes
as the racemate (Figure 5). Carbonyl, cyanide, and nitrosyl
ligands were distinguished by their bond lengths; the
crystallographic refinement also clearly favored one set of
assignments. The dppv spans the apical-basal sites, as is
typical for Fe(CO)(dppv) centers.4 Cyanide and PMe3 are
basal and NO is apical, as is typical in this series of
compounds. Distinctively, the nitrosyl is no longer bound
to the same Fe as the dppv, a finding that implicates the
intramolecular migration of NO (eq 3).
Despite their high electrophilicity, neither [1(CO)3]BF4 nor
[2(CO)3]BF4 were observed to form adducts with CO, even
at -80 °C. This behavior contrasts with the monodentate
bis(phosphine) nitrosyl complexes, [Fe2(S2CnH2n)(CO)3
(PMe3)2(NO)]BF4 (n ) 2, 3), which reversibly bind CO.
Thus, it is not surprising that [1(CO)3]BF4 and [2(CO)3]BF4
react with excess PMe3 only near -20 °C, whereas at
comparable reactant concentrations, [Fe2(S2CnH2n)-
(CO)3(PMe3)2(NO)]BF4 react immediately with PMe3 at -80
°C.
NO As
a Trapping Agent for Mixed-Valence
Species: [Fe2(S2C2H4)(CO)2(dppv)2(NO)]+ and Related
Species. Treatment of [1(CO)3]BF4 with 1 equiv of dppv
slowlyandinefficientlyaffordedthebis(diphosphine),[1(CO)(d-
ppv)]BF4. Alternatively, [1(CO)(dppv)]BF4 also arises in the
reaction of the dicarbonyl14 Fe2(S2C2H4)(CO)2(dppv)2 with
NOBF4. This route suffered from the tendency of the
mononitrosyl to further react with NOBF4 to irreversibly
yield, inter alia, Fe(dppv)(NO)2. A potentially versatile route
to diiron nitrosyl complexes entails a two-step process that
involves one-electron oxidation of diiron(I) precursors fol-
lowed by treatment with NO. Oxidation of Fe2(S2C2H4)
(CO)2(dppv)214 and trapping with NO gave the corresponding
mononitrosyl [Fe2(S2C2H4)(NO)(CO)(dppv)2]BF4 ([1(dppv-
)(CO)]BF4). The 31P NMR spectrum of [1(dppv)(CO)]BF4
showed four broad singlets at 25 °C, which sharpened upon
cooling the sample to -80 °C. We conclude that the two
dppv ligands span apical-basal coordination sites, as observed
crystallographically in the solid state (Figure 4), but that the
dynamic racemization process is subject to a low activation
barrier. The precursor complex also exhibits a related
dynamic process, but at a higher barrier.14 The complex
[(CO)2(PMe3)Fe(S2C2H4)Fe(dppv)(NO)]+ + CN- f
(NO)(CN)(PMe3)Fe(S2C2H4)Fe(CO)(dppv) + CO (3)
A similar NO migration reaction had been implicated for
the cyanation of [Fe2(S2C3H6)(CO)4(PMe3)(NO)]BF4, which
gives Fe2(S2C3H6)(CN)(CO)3(PMe3)(NO).13
IR data for the compounds discussed in this work are given
in Table 2.
Electrochemistry of Nitrosyl Derivatives. Cyclic volta-
mmetric studies showed that replacement of CO by NO+
shifts the first reduction and first oxidation events by ca. 1
V anodically, an effect that is akin to that of protonation
(14) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B. Chem.
Commun. 2007, 2019–21.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11819

Olsen et al.
Table 2. IR Data (cm-1, CH2Cl2 soln) for Selected Compounds
compound
νCO
νNO
[1(CO)3]+
2070, 2006
2002, 1958
1928
1910
1923
1796
1775
1760
1734
1719
[1(CO)2(PMe3)]+
[1(CO)(dppv)]+
1(CO)(PMe3)(CN)
[1(CN)2(CO)]-
Figure 7. Cyclic voltammograms of complex [2(CO)3]BF4 in a 1 mM
CH2Cl2 (∼50 mM Bu4NPF6) solution as a function of [CF3CO2H] (0, 1, 2,
3, 5, 7, and 10 expressed as molar equiv). Scan rate 100 mV s-1 at a glassy
carbon electrode 0.3 cm in diameter.
Figure 6. Cyclic voltammogram of 1 mM [1(CO)(dppv)]BF4 in CH2Cl2
solution (50 mM Bu4NPF6, 20 °C, 100 mV/s).
Table 3. Redox Properties for Selected Nitrosyl Complexesa
compound
oxidation (V)
reduction (V)
Fe2(S2C2H4)(CO)4(dppv)
[1(CO)3]+
-0.06
> 1
-2.07b
-1.15d, -1.61d
-1.21d, -1.73d
[2(CO)3]+
> 1
Fe2(S2C2H4)(CO)3(dppv)(PMe3)4
[1(CO)2(PMe3)]+
-0.300b
0.850
-0.695c
0.22c
-1.51c, -1.99b
-1.62c
Fe2(S2C2H4)(CO)2(dppv)2
[1(CO)(dppv)]+
a Potentials are referenced against Ag/AgCl. Voltammograms recorded
in CH2Cl2 solution with 50 mM Et4NPF6, at 100 mV/s. b Irreversible.
c Reversible. d Reversible at scan rates <100 mV/s.
(Figure 6). Furthermore, the observed redox events are
typically reversible: the species, [1(CO)3]+, reduces quasi-
reversibly at -630 and -1.090 mV versus Ag/AgCl. The
bulky complex [1(CO)(dppv)]+ can be reversibly oxidized
and reduced (see Table 3).
Figure 8. Plots of the catalytic current as a function of the acid
concentration for [1(CO)3]+ (squares) and [2(CO)3]+ (triangles): ik and ip
are the peak currents in the presence and in the absence of acid, respectively.
(see the Supporting Information).15 We estimated (see the
Experimental Section) an overall catalytic rate constant k′
∼ 130 and 100 M-1 s-1 for [1(CO)3]+ and [2(CO)3]+,
respectively. These values compare well with those calculated
by the same procedure for cobaloxime catalysts, which are
known to be highly efficient.16 Note however that the overall
rate constant k′ does not reflect precisely the rate-determining
step but is a composite of equilibrium and rate constants.
At high [H+], the catalytic current becomes almost indepen-
dent of the acid concentration, indicative of a change in the
rate-determining step, which could be the release of H2, as has
Given their mild reduction potentials, we assayed the redox
properties of the diiron nitrosyl complexes in the presence
of acids. The simple dppv complexes [1(CO)3]BF4 and
[2(CO)3]BF4 exhibited catalytic waves at their primary
reductions, both of which were mild (-1.15 and -1.21 V,
respectively, versus the Fc0/+ couple, see Figure 7).
At the concentration ratio of [CF3CO2H]/([2(CO)3]+) e
20, no catalysis was observed at the potential of the second
reduction step. This behavior is explicable if the acid in the
diffusion layer is consumed during the first reduction wave.
A rough estimate of the catalytic efficiency of the FeNO
derivatives can be obtained from the variation of the catalytic
peak current with the acid concentration. As shown in Figure
8, the peak current varies linearly with the square root of
the acid concentration, which indicates that the rate-
determining step in the catalytic reaction is first-order in acid
(15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals
and Applications; J. Wiley: New York, 2001.
(16) Hu, X. L.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc. 2007,
129, 8988–8998.
11820 Inorganic Chemistry, Vol. 47, No. 24, 2008

New Nitrosyl DeriWatiWes of Diiron Dithiolates
Scheme 3. Pathway Proposed for Substitution of [2(CO)3]+ by PMe3
been proposed previously.16 Catalysis by [1(CO)3]+ and
[2(CO)3]+ is unaffected by the presence of CO, although the
corresponding hexacarbonylcatalysts are poisoned by CO.17
Compounds [1(CO)3]+ and [2(CO)3]+ are not protonated
even in the presence of excess triflic acid, consistent with
proton reduction catalysis that is initiated by electron-transfer
(E step) followed by protonation (C step). The sequence of
the two subsequent steps, EC or CE, is unknown. Consistent
with the EC mechanism, [1(CO)(dppv)]BF4 was chemically
reduced with ∼1.1 equiv of decamethylcobaltocene at -78
°C to generate the radical 1(CO)(dppv). The IR spectrum
indicated that reduction is localized on FeNO: νCO shifted
from 1918 to 1886 cm-1, but νNO shifted by nearly 180 cm-1
from 1748 to 1571 cm-1. Upon exposure of these solutions
to air, the signals for [1(CO)(dppv)]+ were restored. X-band
EPR spectrum of the frozen reduced solution revealed a
slightly rhombic signal (g ) 1.95350, 1.98051, 2.01355),
where the low field signal exhibits a large 60 MHz splitting,
consistent with 14N hyperfine coupling.
the two phosphines are dppv or PMe3. The complex
[Fe2(S2C3H6)(CO)3(PMe3)2(NO)]+ adopts a strongly distorted
“rotated” structure,13 whereas [1(CO)3]+ adopts the normal
distorted-C2V motif. We suggest that the electronic asymmetry
of the (PMe3)2 derivatives, which feature Fe(CO)2(PMe3) and
Fe(NO)(CO)(PMe3)+ sites, is greater than for the dppv
derivatives described in this work.
The new nitrosyl diiron(I) complexes were prepared by
three methods:
(1) Direct Electrophilic Nitrosylation by Attack of NO+. This
method is effective but limited by the range of substituted
diiron dithiolates, since the diiron center must contain some
donor ligands; the hexacarbonyls do not form isolable
derivatives upon reaction with NOBF4. We propose that the
electrophilic nitrosylation proceeds similarly to the proto-
nation of diiron complexes, that is, by attack of the
electrophile at a terminal metal site.9 We have shown that
related 36e- diiron nitrosyl complexes are prone to decar-
bonylation.13
(2) Nitrosylation by Attack of NO on Mixed Valence Diiron
Complexes. This method is complementary to the previous
one. It has been shown that NO+ reacts with some metal
carbonyls via inner-sphere electron transfer followed by
dissociation of NO.20 Many of the oxidized diiron carbonyl
dithiolates studied by us previously react with NO.
(3) Substitution of Diiron Nitrosyl Complexes. The consider-
able electrophilicity of [1(CO)3]BF4 is indicated both by the
rate and the stoichiometry of its substitution reactions. The
presence of the nitrosyl allows the preparation of highly
substituted diiron(I) complexes, which would be difficult to
prepare without NO+. These include [Fe2(S2C2H4)
(CN)2(CO)(dppv)(NO)]- and the chiral-at-metal derivative
Fe2(S2C2H4)(CN)(PMe3)(CO)(dppv)(NO). The facility of the
intermetallic migration of NO+ limits the range of isolable
substituted products. Also impressive is the rate of substitu-
tions: the cyanations of Fe2(S2C3H6)(CO)6 at 25 °C and
[2(CO)3]BF4 at -78 °C were found to proceed at comparable
rates. This temperature difference roughly corresponds to a
30% decrease in activation energy.
Discussion
Nitrosylation provides an easy means to significantly
modify the electronic environment of the diiron dithiolato
carbonyls without altering its formal oxidation state. Re-
placement of CO by NO+ causes νCO to increase by 50 cm-1
(Table 1). Such shifts in νCO are about half the effect induced
by protonation of related diiron complexes.18 The nitrosyl
derivatives exhibit an enhanced tendency to undergo sub-
stitution reactions as well as to serve as proton reduction
catalysts at potentials approaching the thermodynamic limit.19
The EPR spectrum of Fe2(S2C2H4)(CO)(N)(dppv)2 indicates
that reduction is localized on the FeNO subunit.
As shown in this and the previous report, NO+ induces
novel stereochemistry on the diiron site. In contrast to all
other ligands evaluated on diiron(I) dithiolates, NO+ displays
a high preference for the apical site. The complexes
characterized in this work all exhibit apical NO+ ligands,
except for the (dppv)2 derivatives where steric factors
destabilize the isomer containing with both dibasal and
apical-basal diphosphines. Even this complex is less stere-
ochemically rigid than the analogous carbonyl derivative.
The structure and reactivity of the complexes
[Fe2(S2C3H6)(CO)3(PR3)2(NO)]+ strongly depends on whether
One striking finding is the tendency of [Fe2(S2CnH2n)
(CO)3(dppv)(NO)]+ to undergo substitution via the intermediacy
of a 2:1 adduct, as illustrated by [(PMe3)2(CO)2Fe(S2CnH2n)
Fe(CO)(dppv)(NO)]+ (Scheme 3). These adducts form via the
scission of the Fe-Fe bond and one Fe-S bond. The 2:1
adducts illustrate the possibility that other [Fe(I)]2 species
substitute via mixed-valence adducts. Such a mechanism may
be relevant to the finding that attempted monocyanation of
(17) Borg, S. J.; Behrsing, T.; Best, S. P.; Razavet, M.; Liu, X.; Pickett,
C. J. J. Am. Chem. Soc. 2004, 126, 16988–16999.
(18) Zhao, X.; Hsiao, Y.-M.; Lai, C.-H.; Reibenspies, J. H.; Darensbourg,
M. Y. Inorg. Chem. 2002, 699–708. (a) Gloaguen, F.; Lawrence, J. D.;
Rauchfuss, T. B.; Be´nard, M.; Rohmer, M.-M. Inorg. Chem. 2002,
41, 6573–6582.
(20) Connelly, N. G.; Demidowicz, Z.; Kelly, R. L. Dalton Trans 1975,
2335–2340. Ashford, P. K.; Baker, P. K.; Connelly, N. G.; Kelly, R. L.;
Woodley, V. A. J. Chem. Soc., Dalton Trans. 1982, 477–479.
(19) Felton, G. A. N.; Glass, R. S.; Lichtenberger, D. L.; Evans, D. H.
Inorg. Chem. 2006, 45, 9181–9184.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11821

Olsen et al.
Table 4. Details of Data Collection and Structure Refinement
[2(CO)3]BF4
[1(CO)2(PMe3)]BF4
chemical formula
fw
T (K)
C32H28BF4Fe2NO4P2S2
815.12
193(2)
C35H39BCl4F4Fe2NO3P3S2
1019.01
193(2)
space group
a (Å)
b (Å)
P21/c
P21/n
10.094(4)
17.701(7)
19.493(8)
90
97.814(6)
90
15.2650(11)
19.3588(13)
15.5567(11)
90
95.573(3)
90
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
Z
4
4
V (Å3)
λ (Å)
3451(2)
0.71073
0.001569
0.01114
0.0278
4575.5(6)
0.71073
0.001479
0.01114
0.0406
Dcalcd (g cm-3
)
µ (cm-1
)
2
R (Fo )
2
Rw (Fo )
0.0568
0.1086
[1(CO)(dppv)]BF4
[1(CN)(CO)(PMe3)]BF4
[1(CO)3(PMe3)2]BF4
chemical formula
fw
T
space group
a (Å)
b (Å)
C58H54BCl6F4Fe2NO2P4S2
C33H35Fe2N2O2P3S2
760.36
193(2)
C38H46BF4Fe2NO4P4S2
967.27
1396.23
193(2)
P21
10.8808(16)
17.757(3)
16.692(2)
90
104.511(3)
90
2
3122.3(8)
0.71073
0.001485
0.00945
0.0610
0.1256
2
193(2)
j
Pna21
P1
19.629(5)
11.016(3)
32.186(9)
90
90
90
13.4269(19)
13.637(2)
14.435(2)
74.158(9)
73.229(9)
79.917(9)
2
2421.2(6)
0.71073
0.001327
0.00868
0.0773
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
Z
8
V (Å3)
λ (Å)
6959(3)
0.71073
0.001451
0.01124
0.0886
0.1186
Dcalcd (g cm-3
)
µ (cm-1
)
R (I > 2σ)a
Rw (I > 2σ)b
0.1628
a R ) ∑|Fo| - |Fc||/∑|Fo|. b Rw ) {[w(|Fo| - |Fc|)2]/∑[wFo ]}1/2, where w ) 1/σ2(Fo).
Fe2(S2C3H6)(CO)6 mainly affords the dicyanide.21The presence
of the NO ligand, a very strong acceptor, favors such electron-
transfer processes. Related Fe-S scission reactions occur upon
the reduction of diiron dithiolates.17,22
red reaction mixture was concentrated in vacuo. The addition of
50 mL of hexanes to the concentrated solution precipitated the dark-
red-colored product. Yield: 0.912 g (94%). 500 MHz H NMR
1
(CD2Cl2): δ 8.6-7.2 (m, 20H, C6H5), 3.0 (dd, 2H, PCH), 2.05 (m,
1H, SCH), 1.6 (m, 1H, SCH), 1.3 (m, 1H, SCH), 0.8 (m, 1H, SCH).
202 MHz 31P NMR (CD2Cl2, 20 °C): δ 77.1 (s, dppv), 72.0 (s,
dppv). 31P NMR (CD2Cl2, -80 °C): δ 78.5 (s, dppv), 73.3 (s, dppv).
IR (CH2Cl2): νCO ) 2070, 2006; νNO ) 1796 cm-1. ESI-MS: m/z
714.1 ([Fe2(S2C2H4)(CO)3(dppv)(NO)]+). Anal. calcd (found) for
C31H26BF4Fe2NO4P2S2: C, 46.86 (46.86); H, 3.25 (3.27); N, 1.60
(1.75).
[Fe2(S2C3H6)(CO)3(dppv)(NO)]BF4, [2(CO)3]BF4. This com-
pound was prepared following the method described for
[1(CO)3]BF4. Yield: 2.0 g (88%). 500 MHz 1H NMR (CD2Cl2, 20
°C): δ 8.5-8.2 (m, 20H, C6H5), 2.9 (bs, 2H, PCH), 2.6 (m, 4H,
(SCH2)2CH2), 2.0 (m, 2H, (SCH2)2CH2). 202 MHz 31P NMR
(CD2Cl2, 20 °C): δ 69.7 (s, dppv). 31P NMR (CD2Cl2, -80 °C): δ
76.7 (s, dppv), 75.1 (s, dppv), 72.4 (s, dppv), 69.5 (s, dppv). IR
(CH2Cl2): νCO ) 2069, 2005; νNO ) 1788 cm-1. ESI-MS: m/z 728.1
([Fe2(S2C3H6)(CO)3(dppv)(NO)]+). Anal. calcd (found) for
C31H26BF4Fe2NO4P2S2: C, 47.15 (46.56); H, 3.46 (3.41); N, 1.72
(1.86).
Experimental Section
Methods have been recently described.23 NOBF4 was purified
by rapid sublimation at 220 °C at ∼ 0.01 mmHg and was stored in
a dry glovebox at -30 °C. Since NOBF4 is poorly soluble, the
solid was ground to a fine powder prior to use in sensitive reactions.
Electrochemistry. Cyclic voltammetry experiments were con-
ducted in a ∼10-mL one-compartment glass cell, with a Pt wire
(counter electrode) and a glassy carbon working electrode. All
voltammagrams were referenced versus a Ag/AgCl reference
electrode (50 mM KCl).
[Fe2(S2C2H4)(CO)3(dppv)(NO)]BF4, [1(CO)3]BF4. A slurry of
0.141 g (1.21 mmol) of pulverized NOBF4 in 20 mL of CH2Cl2
was treated with a solution of 0.865 g (1.21 mmol) of
Fe2(S2C2H4)(CO)4(dppv) in 100 mL of CH2Cl2. The reaction
mixture was immediately cooled to 0 °C, and after 10 h, the deep
(21) Gloaguen, F.; Lawrence, J. D.; Schmidt, M.; Wilson, S. R.; Rauchfuss,
T. B. J. Am. Chem. Soc. 2001, 123, 12518–12527.
(22) Felton, G. A. N.; Vannucci, A. K.; Chen, J.; Lockett, L. T.; Okumura,
N.; Petro, B. J.; Zakai, U. I.; Evans, D. H.; Glass, R. S.; Lichtenberger,
D. L. J. Am. Chem. Soc. 2007, 129, 12521–12530. (a) Aguirre de
Carcer, I.; DiPasquale, A.; Rheingold, A. L.; Heinekey, D. M. Inorg.
Chem. 2006, 45, 8000–8002.
[Fe2(S2C2H4)(CO)2(NO)(PMe3)(dppv)]BF4, [1(CO)2(PMe3)]BF4.
To a mixture of 0.200 g (0.263 mmol) of Fe2(S2C2H4)-
(CO)3(dppv)(PMe3)4 and 0.030 g (0.263 mmol) of finely pulverized
NOBF4 was added 15 mL of CH2Cl2. After stirring for 5 min, the
solution was concentrated to 5 mL, and the dark red product was
precipitated upon addition of 30 mL of hexanes. Crystals were
(23) Justice, A. K.; Nilges, M.; Rauchfuss, T. B.; Wilson, S. R.; De Gioia,
L.; Zampella, G. J. Am. Chem. Soc. 2008, 130, 5293–5301.
11822 Inorganic Chemistry, Vol. 47, No. 24, 2008

New Nitrosyl DeriWatiWes of Diiron Dithiolates
grown via slow diffusion of hexanes into a CH2Cl2 solution of the
complex. Yield: 0.19 g (86%). 31P NMR (CD2Cl2, 20 °C): δ 74.1
(s, dppv), 25.0 (s, PMe3). 31P NMR (CD2Cl2, -70 °C): δ 78.1 (s,
dppv), 75.7 (s, dppv), 26.8 (s, PMe3), 23.0 (s, PMe3). IR (CH2Cl2):
νCO ) 2002, 1958, νNO ) 1775. In situ spectra (ReactIR 4000,
Mettler-Toledo) indicated the presence of [Fe2(S2C2H4)-
(CO)3(dppv)(PMe3)(NO)]BF4 after 3 h of vigorous stirring at -78
°C: (CH2Cl2): νCO ) 2037, 1992, 1957; νNO ) 1775. ESI-MS: m/z
762.2 ([Fe2(S2C2H4)(CO)(dppv)(PMe3)(NO)]+). Anal. calcd (found)
for C33H35BF4Fe2NO3P3S2: C, 46.67 (46.66); H, 4.15 (4.37); N,
1.65 (1.62).
1 min, followed by cooling again to -78 °C. The IR spectrum of the
reaction mixture indicated [1(CO)3(PMe3)2]BF4. The addition of 50
mL of hexanes to the reaction mixture yielded a dark brown oil that
was dried in vacuo. IR spectra of the redissolved material contained
traces of [1(CO)2(PMe3)]BF4, indicative of the thermal instability of
[1(CO)3(PMe3)2]BF4. 31P NMR (CD2Cl2, 0 °C), A, B, and C cor-
respond to three isomers: δ 98.4 (d, JP-P ) 48.3, dppv A), 98.0 (d,
JP-P ) 44.3, dppv B), 97.3 (d, JP-P ) 39.7, dppv C), 81.5 (d, JP-P
)
48.9, dppv A), 81.3 (d, JP-P ) 45.8, dppv B), 78.8 (d, JP-P ) 38.4,
dppv C), 25.3, 17.8 (d, J ) 64.1, PMe3 C), 17.0 (d, J ) 62.5, PMe3
A), 10.0 (ABq, J ) 292, JAB ) 245, PMe3 B), 6.4 (d, J ) 64.2, PMe3
C), 5.4 (d, J ) 65.6, PMe3 A).
Synthesis via Oxidation and Trapping with NO. To a solution
of 0.196 g (0.258 mmol) of Fe2(S2C2H4)(CO)3(dppv)(PMe3)4 in 15
mL of CH2Cl2, cooled to -45 °C, was added 0.070 g (0.256 mmol)
of FcBF4. The IR spectrum of the purple solution displayed signals
matching [Fe2(S2C2H4)(CO)3(dppv)(PMe3)]BF4.4 The reaction ves-
sel was sealed and to the cooled solution was injected 6 mL (0.268
mmol) of NO gas. After 1 h, the IR spectrum of the resulting deep
red solution matched that for [1(CO)2(PMe3)]BF4. The product
precipitated upon the addition of 60 mL of hexanes. Yield: 0.116
g (53%). Analogous procedures were followed for the sequential
oxidation and trapping of Fe2(S2C3H6)(CO)4(dppv) to give [Fe2(S2C3-
H6)(CO)3(dppv)(NO)]BF4: To a solution of 0.045 g (0.062 mmol)
of Fe2S2C3H6 in 5 mL of CH2Cl2, cooled to -45 °C, was added a
solution of 0.017 g (0.062 mmol) of FcBF4 in 5 mL of CH2Cl2. To
the resultant reaction mixture was added 3.2 mL of NO (0.124
mmol), and the reaction vessel was sealed. After 20 min, the IR
spectrum matched that of [2(CO)3]BF4, and the product was
precipitated upon the addition of 50 mL of hexanes. Yield: 77%.
[Fe2(S2C2H4)(CO)(dppv)2(NO)]BF4, [1(CO)(dppv)]BF4. To a
solution of 0.150 g (0.142 mmol) of Fe2(S2C2H4)(CO)2(dppv)2 in
20 mL of CH2Cl2, cooled to -45 °C, was added 0.039 g (0.142
mmol) of FcBF4. An immediate IR spectrum of the dark brown
solution displayed signals attributed to [Fe2(S2C2H4)(CO)(µ-
Crystallization of [Fe2(S2C3H6)(CO)3(dppv)(PMe3)2(NO)]BF4,
[2(CO)3(PMe3)2]BF4. Approximately 0.2 mL of PMe3 was distilled
onto a frozen solution of 0.070 g of [1(CO)3]BF4 in 7 mL of CH2Cl2.
Aliquots briefly warmed (<2 min) to room temperature displayed
an IR spectrum that indicated the presence of
[2(CO)3(PMe3)2(NO)]BF4. IR (CH2Cl2): νCO ) 2021, 1971, 1951;
νNO ) 1721 cm-1. NMR (CD2Cl2, -20 °C), A, B, and C correspond
to three isomers: δ 97.4 (d, JP-P ) 34.2, dppv A), 95.0 (d, JP-P
)
28.6, dppv B), 78.2 (d, JP-P ) 38.2, dppv A), 76.9 (d, JP-P ) 29.0,
dppv B), 15.9 (d, JP-P ) 71.6, PMe3 A), 15.2 (d, JP-P ) 72.4,
PMe3 C), 9.0 (ABq, J ) 964.5, JAB ) 193, PMe3 B), 4.1 (d, JP-P
,
PMe3 A), 2.8 (d, JP-P ) 71.9, PMe3 C). The dppv signals for isomer
C are not reported because of overlap with isomer A’s signals).
The reaction mixture was thawed to -45 °C followed by transfer
via cannula to a Schlenk tube cooled to -78 °C. This solution was
layered with 50 mL of a 1/1 mixture of Et2O and hexane and stored
at -30 °C. After 1 week, red rhombs were visible.
Reaction of [1(CO)3] with 1 equiv of PMe3. To a J. Young NMR
tube was added a solution of 0.025 g (0.03 mmol) of [1(CO)3]BF4
in 1 mL of CD2Cl2. The solution was frozen in liquid nitrogen,
and to it was added 0.3 mL of a 0.13 M solution of PMe3 in CH2Cl2.
The tube was immediately capped, immersed in liquid nitrogen,
and evacuated. The contents were allowed to warm and were
monitored by NMR spectroscopy at various temperatures. At -38
°C, no reaction was observed. Upon warming to 0 °C, partial
consumption of [1(CO)3(dppv)(NO)]BF4 and nearly complete
consumption of PMe3 were accompanied by the growth of several
peaks. These peaks were assigned to one major (“B”) and two minor
(“A” and “C”) intermediates (see above spectra assignments). Upon
warming to room temperature overnight, conversion of the remain-
ing intermediate(s) to [1(CO)2(PMe3)]BF4 was observed.
CO)(dppv)2]BF4: νCO ) 1959 (s), 1887 (w, br) cm-1 23 The reaction
.
vessel was sealed, and to the cooled solution was injected 3.6 mL
(0.161 mmol) of NO gas. After 1 h, the resultant dark brown
solution displayed an IR spectrum corresponding to [1(CO)(dp-
pv)]BF4. The solution was warmed to room temperature and
concentrated in vacuo to ∼5 mL. The product precipitated upon
the addition of 30 mL of Et2O. Impurities observed in the 31P NMR
spectrum can be removed after several recrystallizations from
CH2Cl2-Et2O. Yield: 0.080 g (49%). 1H NMR (CD2Cl2): δ 8.2-6.8
(m, 40H, dppv), 1.7-0.8 (m, 4 H, SCH2CH2S). 31P NMR (CD2Cl2,
20 °C): δ 101.7 (br s, dppv), 87.8 (broad s, dppv), 81.2 (br s, dppv),
NEt4[Fe2(S2C2H4)(CN)2(CO)(dppv)(NO)]. A solution of 0.503
g (0.63mmol) of [1(CO)3]BF4 in 30 mL of MeCN was cooled to
-45 °C followed by treatment with a solution of 0.199 g (1.3 mmol)
of NEt4CN in 30 mL of MeCN, also cooled to -45 °C. The solution
immediately darkened, and after 3 h, the dark brown solution was
allowed to warm to room temperature followed by stirring
overnight. After the solvent was removed in vacuo, the residue was
extracted into 5 mL of CH2Cl2, and the product was precipitated
by the addition of 30 mL of Et2O. IR (CH2Cl2): νCN ) 2100, νCO
) 1923, νNO ) 1719. 31P NMR (CD2Cl2, 20 °C): δ 96.1 (s, dppv).
31P NMR (CD2Cl2, -60 °C): δ 99.3 (d, JP-P ) 22.4, dppv), 94.1
(d, JP-P ) 22.4, dppv). ESI-MS: m/z 710.0 ([Fe2(S2C2H4)-
74.9 (br s, dppv). IR (CH2Cl2): νCO ) 1928, νNO ) 1760 cm-1
.
ESI-MS: m/z 1054.2 ([Fe2(S2C2H4)(CO)(dppv)2(NO)]+). Acceptable
CHN analyses were not be obtainable. Anal. calcd (found) for
C55H48BF4Fe2NO2P4S2: C, 57.87 (56.00); H, 4.24 (4.00); N, 1.23
(1.31). An excess of NOBF4 gave Fe(dppv)(NO)2:24 νNO ) 1718
and 1666 cm-1; ESI-MS: m/z 512.
Alternative routes were examined: the addition of dppv to 1(CO)3
and the treatment of Fe2(S2C2H4)(CO)2(dppv)2 with NOBF4. The
raw product contained the targeted complex as well as unidentified
impurities, as indicated by the 31P NMR and IR spectra. The
treatment of [Fe2(S2C2H4)(CO)2(dppv)2(NO)]BF4 with NOBF4
produced Fe(NO)2(dppv).24
(CN)2(CO)(dppv)(NO)]+),
682.0
([Fe2(S2C2H4)(CN)2
(dppv)(NO)]+). Suitable CHN analyses were not obtained.
[Fe2(S2C2H4)(CO)3(dppv)(PMe3)2(NO)]BF4, [1(CO)3(PMe3)2]-
BF4. Onto a frozen solution of 0.206 g (0.26 mmol) of [1(CO)3]BF4
in 10 mL of CH2Cl2 was distilled 1 mL of PMe3. The mixture was
allowed to thaw at -78 °C and was warmed in an ice water bath for
[Fe2(S2C2H4)(CO)(CN)(dppv)(PMe3)(NO)]BF4, [1(CO)(CN)-
(PMe3)]BF4. A solution of 0.221 g (0.26 mmol) of [1(CO)2(PMe3)]BF4
in 30 mL of CH2Cl2 was treated with a solution of 0.041 g (0.26 mmol)
of NEt4CN in 10 mL of MeCN. After 60 min, the dark-brown-colored
reaction mixture was evaporated in vacuo, and the residue was
extracted into 30 mL of toluene. The dark-red-colored extract was
(24) Guillaume, P.; Wah, H. L. K.; Postel, M. Inorg. Chem. 1991, 30, 1828–
31.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11823

Olsen et al.
concentrated, followed by dilution with 30 mL of hexanes to precipitate
the product. Yield: 0.065 g (33%). 1H NMR (d8-toluene): δ 8.5-6.7
(m, C6H5 and P2C2H2), 2.1-1.6 (m, S2C2H4), 1.26 (d, JP-H ) 9.9,
9H, PCH3). 31P NMR (CD2Cl2, 20 °C): δ 95.3 (d, JP-P ) 20.9, dppv),
74.2 (d, JP-P ) 24.5, dppv), 7.9 (s, PMe3). IR (CH2Cl2): νCN ) 2096,
from the voltammetric peak current, ip, of the catalyst in the absence
of acid:
5
3/2
1/2
1⁄2
(
)
ip ) 2.69 × 10 n AD COν
where ν is the potential scan rate.
νCO
) 1910, νNO ) 1734. FD-MS: m/z 760 ([Fe2(S2C2-
H4)(CN)(CO)(dppv)(PMe3)(NO)]+). Anal. calcd (found) for
C33H35Fe2N2O2P3S2: C, 52.54 (52.13); H, 4.72 (4.64); N, 3.53 (3.68).
Relative Electrophilicity of [Fe2(S2C2H4)(CO)3(dppv)(NO)]+. To
a solution of 0.015 g (0.02 mmol) of [Fe2(S2C2H4)(CO)3(dppv)
(NO)]BF4 and 0.014 g (0.02 mmol) of Fe2(S2C2H4)(CO)4(dppv) in
5 mL of CH2Cl2 was added 0.5 mL of a 0.19 M solution of PMe3
in CH2Cl2. The reaction was monitored by IR spectroscopy over
the course of 7 h, during which time the absorption bands for
[1(CO)3(NO)]BF4 decayed and those for [1(CO)3(PMe3)(NO)]BF4
increased. The bands for Fe2(S2C2H4)(CO)4(dppv) remained un-
changed.
X-Ray Crystallography. Crystals were mounted on a thin glass
fiber using Paratone-N oil (Exxon). Data, collected at 198 K on a
Siemens CCD diffractometer, were filtered to remove statistical outliers.
The integration software (SAINT) was used to test for crystal decay
as a bilinear function of X-ray exposure time and sin(Θ). The data
were solved using SHELXTL by direct methods; atomic positions were
deduced from an E map or by an unweighted difference Fourier
synthesis. H-atom U values were assigned as 1.2Ueq for adjacent C
atoms. Non-H atoms were refined anisotropically. Successful conver-
gence of the full-matrix least-squares refinement of F2 was indicated
by the maximum shift/error for the final cycle. The assignment of NO
versus CO was tested by refining the cations with these ligands
interchanged. Optimal refinements were found only for cases with the
Fe(dppv)(NO) centers. Structure refinement details of the discussed
compounds are given in Table 4.
Semiquantitation of Catalytic Proton Reduction. An electron
transfer followed by a fast catalytic reaction can be formulated by
the following equations:
E1/2
O + ne- h R
Acknowledgment. This work was supported by the
National Institutes of Health. We thank Tere´sa Prussak-
Wieckowska for assistance with the crystallographic analyses.
M.T.O. thanks the NIH Chemistry-Biology Interface Train-
ing Program for a fellowship. We thank the CNRS-UIUC
cooperation program for support for F.G. during his time at
UIUC. Dr. M. Nilges recorded and simulated the EPR
spectrum of [Fe2(S2C2H4)(CO)(NO)(dppv)2.
k′
R + Z f P
where k′ is the rate constant for the reaction of reagent Z with the
reduced species R to give product P. When the concentration of Z
is large compared to the concentration of the catalyst and the
potential sufficiently negative with respect to E1/2, the catalytic
current ik is given by the following relation:15
ik ) nFACO k ′ DC
√
z
Supporting Information Available: X-ray crystallographic data
and additional spectroscopic and synthetic details. This material is
available free of charge via the Internet at http://pubs.acs.org.
where n is the number of electrons involved in the catalytic process,
F the Faraday constant, A the area of the electrode, and D the
diffusion coefficient for the catalyst. For graphs of ik against Cz1/2
the slope is nFACO(k′D)1/2. The value for ACOD1/2 can be obtained
,
IC801542W
11824 Inorganic Chemistry, Vol. 47, No. 24, 2008