Chelate control of diiron(I) dithiolates relevant to the [Fe-Fe]-hydrogenase active site

Article abstract of DOI:10.1021/ic0618706

The reaction of Fe₂(S₂C₂H ₄)(CO)₆ with cis-Ph₂PCH=CHPPh₂ (dppv) yields Fe₂(S₂C₂H₄)(CO) ₄(dppv), 1(CO)₄, wherein the dppv ligand is chelated to a single iron center. NMR analysis indicates that in 1(CO)₄, the dppv ligand spans axial and basal coordination sites. In addition to the axial-basal isomer, the 1,3-propanedithiolate and azadithiolate derivatives exist as dibasal isomers. Density functional theory (DFT) calculations indicate that the axial-basal isomer is destabilized by nonbonding interactions between the dppv and the central NH or CH₂ of the larger dithiolates. The Fe(CO) ₃ subunit in 1(CO)₄ undergoes substitution with PMe ₃ and cyanide to afford 1(CO)₃(PMe₃) and (EttN)[1(CN)(CO)₃], respectively. Kinetic studies show that 1(CO)₄ reacts faster with donor ligands than does its parent Fe ₂(S₂C₂H₄)(CO)₆. The rate of reaction of 1(CO)₄ with PMe₃ was first order in each reactant, k = 3.1 × 10^-4 M^-1 s^-1. The activation parameters for this substitution reaction, ΔH? = 5.8(5) kcal/mol and ΔS? = -48(2) cal/deg·mol, indicate an associative pathway. DFT calculations suggest that, relative to Fe ₂(S₂C₂H₄)(CO)₆, the enhanced electrophilicity of 1(CO)₄ arises from the stabilization of a "rotated" transition state, which is favored by the unsymmetrically disposed donor ligands. Oxidation of MeCN solutions of 1(CO) ₃(PMe₃) with Cp₂FePF₆ yielded [Fe₂(S₂C₂H₄)(μ-CO)(CO) ₂(dppv)(PMe₃)(NCMe)](PF₆)₂. Reaction of this compound with PMe₃ yielded [Fe₂(S ₂C₂H₄)-(μ-CO)(CO)(dppv)(PMe ₃)₂(NCMe)](PF₆)₂.

Full text of DOI:10.1021/ic0618706

Inorg. Chem. 2007, 46, 1655−1664
Chelate Control of Diiron(I) Dithiolates Relevant to the [Fe
Hydrogenase Active Site
−Fe]-
Aaron K. Justice,† Giuseppe Zampella,‡ Luca De Gioia,*,‡ Thomas B. Rauchfuss,*,†
Jarl Ivar van der Vlugt,† and Scott R. Wilson†
Department of Chemistry, UniVersity of Illinois, Urbana, Illinois 61801, and Department of
Biotechnology and Biosciences, UniVersity of Milano-Bicocca, Piazza della Scienza 1, 20126
Milan, Italy
Received September 28, 2006
The reaction of Fe2(S2C2H4)(CO)6 with cis-Ph2PCH
the dppv ligand is chelated to a single iron center. NMR analysis indicates that in 1(CO)4, the dppv ligand spans
axial and basal coordination sites. In addition to the axial basal isomer, the 1,3-propanedithiolate and azadithiolate
derivatives exist as dibasal isomers. Density functional theory (DFT) calculations indicate that the axial basal
dCHPPh2 (dppv) yields Fe2(S2C2H4)(CO)4(dppv), 1(CO)4, wherein
−
−
isomer is destabilized by nonbonding interactions between the dppv and the central NH or CH2 of the larger
dithiolates. The Fe(CO)3 subunit in 1(CO)4 undergoes substitution with PMe3 and cyanide to afford 1(CO)3(PMe3)
and (Et4N)[1(CN)(CO)3], respectively. Kinetic studies show that 1(CO)4 reacts faster with donor ligands than does
its parent Fe2(S2C2H4)(CO)6. The rate of reaction of 1(CO)4 with PMe3 was first order in each reactant, k
)
3.1
Sq ) −48(2)
mol, indicate an associative pathway. DFT calculations suggest that, relative to Fe2(S2C2H4)(CO)6, the
×
4
1
10- M- s-1. The activation parameters for this substitution reaction,
∆ ) 5.8(5) kcal/mol and ∆
Hq
cal/deg
‚
enhanced electrophilicity of 1(CO)4 arises from the stabilization of a “rotated” transition state, which is favored by
the unsymmetrically disposed donor ligands. Oxidation of MeCN solutions of 1(CO)3(PMe3) with Cp2FePF6 yielded
[Fe2(S2C2H4)(
µ-CO)(CO)2(dppv)(PMe3)(NCMe)](PF6)2. Reaction of this compound with PMe3 yielded [Fe2(S2C2H4)-
(
µ
-CO)(CO)(dppv)(PMe3)2(NCMe)](PF6)2.
Introduction
clear to us that further advances will follow from greater
control of the ligands and the redox poise of the diiron center.
The active site of the [Fe-Fe]-hydrogenases feature a
diiron tricarbonyl center that can be described as [Fe2(SR)2-
(CO)3L3Hn]z (n ) 0, 1; z ) unknown) where two L’s are
CN- and the third L is a µ-thiolate of a [4Fe-4S] cluster
(Figure 1).3,4 Relevant to developing active site mimics are,
therefore, trisubstituted derivatives of diiron(I) dithiolato
complexes of the type Fe2(SR)2(CO)3L3.
A few trisubstituted and even tetrasubstituted derivatives
are known for ligands of small cone angles and modest donor
ability, such as MeNC and P(OMe)3.6,7 We have, however,
shown that cyanide will not substitute more than two CO
ligands, despite its modest steric demands. This finding
The design of first-row metal centers capable of catalyzing
redox interconversions of hydrogen is a major goal of
contemporary science.1 The structural analyses of the active
sites of the two major families of hydrogenases has provided
the information required for generating structural models.
Replicating the reactivity of the enzymes has, however,
proven far more challenging. More progress has been
achieved in modeling the [Fe-Fe]-hydrogenases than
modeling of the nickel-iron enzymes,2,3 and it is increasingly
* To whom correspondence should be addressed. E-mail: luca.degioia@
unimb.it (L.D.), rauchfuz@uiuc.edu (T.B.R.).
† University of Illinois.
‡ University of Milano-Bicocca.
(1) Basic Research Needs for the Hydrogen Economy. Report of the Basic
Energy Sciences Workshop on Hydrogen Production, Storage, and
Use.
(2) Tye, J. W.; Hall, M. B.; Darensbourg, M. Y. Proc. Natl. Acad. Sci.
2005, 102, 16911-16912.
(3) Linck, R. C.; Rauchfuss, T. B. In Bioorganometallics: Biomolecules,
Labeling, Medicine; Jaouen, G., Ed.; Wiley-VCH: Weinheim, Ger-
many, 2005.
(4) Cammack, R.; Frey, M.; Robson, R. Hydrogen as a Fuel: Learning
from Nature; Taylor & Francis: London, 2001.
(5) Frey, M. ChemBioChem 2002, 3, 153-160.
(6) Lawrence, J. D.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2002,
41, 6193-6195.
(7) de Beer, J. A.; Haines, R. J. J. Organomet. Chem. 1972, 36, 297-
312.
10.1021/ic0618706 CCC: $37.00
Published on Web 02/06/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 5, 2007 1655

Justice et al.
Figure 1. Active site of the [Fe-Fe]- hydrogenase in the reduced and
oxidized forms.5
shows that both ligand size and basicity affect the degree of
substitution.8 Cyanide also has a tendency to form linear
M-CN-M′ ensembles,9 and metal cyanides are susceptible
to N protonation. Phosphine complexes, on the other hand,
exhibit more versatile solubility without the complicating
tendencies of cyanide.10 Tertiary phosphine ligands are well-
known to form mono- and disubstituted products, Fe2(SR)2-
(CO)6-x(PR3)x (x ) 1, 2),11 but strategies leading to trisub-
stituted [Fe(I)]2 species are rare.
One approach to controlling the degree of substitution
entails the use of chelating ligands, which could stabilize
complexes of the type Fe2(SR)2(CO)3(chel)L. Species such
as Fe2(SR)2(CO)4(diphosphine) and [Fe2(SR)2(CO)5]2(di-
phosphine) have been previously investigated.12,13 Complexes
where the diphosphine chelates to a single iron center have
been only lightly studied.13,14
Figure 2. Structure of 1(CO)4, with thermal ellipsoids set at 35%. Phenyl
ellipsoids and phenyl hydrogen atoms have been omitted. Selected bond
lengths (Å) and angles (deg): Fe(1)-Fe(2), 2.5249(9); Fe(1)-S(1), 2.2451-
(13); Fe(1)-S(2), 2.2575(13); Fe(1)-P(1), 2.1743(13); Fe(1)-P(2), 2.2070-
(13); Fe(1)-C(4), 1.754(5); Fe(2)-C(1), 1.783(5); Fe(2)-C(2), 1.779(5);
Fe(2)-C(3), 1.780(5); Fe(2)-Fe(1)-P(1), 154.05(5); Fe(2)-Fe(1)-P(2),
109.23(4); Fe(2)-Fe(1)-C(4), 105.11(15); P(1)-Fe(1)-P(2), 87.83(5);
P(1)-Fe(1)-C(4), 94.32(15); P(2)-Fe(1)-C(4), 88.88(15).
Scheme 1
From the following studies of chelated diiron dithiolates,
we have been able to extract detailed information on several
points relevant to the behavior of the active site. Of particular
interest was the influence of the dithiolate on the reactivity
of the diiron center with respect to substitution and oxidation.
Results
Fe2(SR)2(CO)4(dppv). Fe2(S2C2H4)(CO)4(dppv), 1(CO)4,
was prepared in 80% yield by decarbonylation of Fe2-
(S2C2H4)(CO)6 in the presence of cis-1,2-bis(diphenylphos-
phino)ethylene (dppv). The dppv ligand was chosen because
of its preference for chelating to a single iron center rather
than bridging between the two iron centers.13 When the
reaction was monitored by IR spectroscopy, we observed
no intermediates. Similarly, we prepared the 1,3-propanedithi-
olate (pdt) Fe2(S2C3H6)(CO)4(dppv), 2(CO)4, and the aza-
dithiolate (adt) Fe2[(SCH2)2NH](CO)4(dppv), 3(CO)4. All
three compounds display similar IR spectroscopic features.
The IR spectrum of 1(CO)4 features three νCO bands,
wherein the difference between the highest and lowest energy
peaks, ∆νCO, is 101 cm-1. DFT calculations (vide infra)
suggest that the highest energy band (exptl, 2023 cm-1;
theory, 2022 cm-1) can be approximated as the A mode of
the Fe(CO)3 subunit and that the lowest energy band (exptl,
1915 cm-1; theory, 1930 cm-1) is associated with the Fe-
(dppv)(CO) subunit. For comparison, ∆νCO is smaller (72
cm-1) for the symmetrcially substituted species Fe2(S2C3H6)-
(CO)4(PMePh2)2, whereas ∆νCO is 119 cm-1 for the
monophosphine Fe2(S2C3H6)(CO)5(PMe2Ph).11 The large
value of ∆νCO for 1(CO)4 is consistent with a structure
wherein dppv is chelated to a single iron center, and the high
νCO value indicates that the second iron center should be
susceptible to substitution.15
(8) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. Angew. Chem., Int. Ed. 1999, 38, 3178-3180. Schmidt, M.;
Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999, 121, 9736-
9737.
(9) Vahrenkamp, H.; Geiâ, A.; Richardson, G. N. J. Chem. Soc., Dalton
Trans. 1997, 3643-3651.
(10) van der Vlugt, J. I.; Rauchfuss, T. B.; Wilson, S. R. Chem.sEur. J.
2005, 12, 90-98.
(11) Li, P.; Wang, M.; He, C.; Li, G.; Liu, X.; Chen, C.; Åkermark, B.;
Sun, L. Eur. J. Inorg. Chem. 2005, 2506-2513.
(12) Song, L.-C.; Yang, Z.-Y.; Bian, H.-Z.; Hu, Q.-M. Organometallics
2004, 23, 3082-3084. Song, L.-C.; Yang, Z.-Y.; Bian, H.-Z.; Liu,
Y.; Wang, H.-T.; Liu, X.-F.; Hu, Q.-M. Organometallics 2005, 24,
6126-6135. Hogarth, G.; O’Brien, M.; Tocher, D. A. J. Organomet.
Chem. 2003, 672, 29-33. de Beer, J. A.; Haines, R. J.; Greatrex, R.;
Greenwood, N. N. J. Chem. Soc. 1971, 3271-3282. Hossain, G. M.
G.; Hyder, M. I.; Kabir, S. E.; Abdul Malik, K. M.; Miah, M. A.;
Siddiquee, T. A. Polyhedron 2003, 22, 633-640.
The geometry adopted by the dppv ligand (axial-basal
versus dibasal) is influenced by the nature of the chelating
(13) de Beer, J. A.; Haines, R. J.; Greatex, R.; Greenwood, N. N. J.
Organomet. Chem. 1971, 27, C33-C35.
(14) Song, L.-C.; Yan, C.-G.; Mao, X.-A. Gaodeng Xuexiao Huaxue Xuebao
1997, 18, 1093-1097.
(15) Tye, J. W.; Lee, J.; Wang, H.-W.; Mejia-Rodriguez, R.; Reibenspies,
J. H.; Hall, M. B.; Darensbourg, M. Y. Inorg. Chem. 2005, 44, 5550-
5552. Ekstro¨m, J.; Ott, S.; Sun, L.; Åkermark, B.; Eriksson, L. Acta
Crystallogr., Sect. E 2005, E61, m852-m853.
1656 Inorganic Chemistry, Vol. 46, No. 5, 2007

Chelate Control of Diiron(I) Dithiolates
ligands on the second iron center (Figure 4). The calculated
values for νCO (2022.3, 1969.9, 1951.3, and 1930.3 cm-1)
compare favorably with the observed spectrum (2023, 1953
(br), and 1915 cm-1), where the middle band consists of two
overlapping vibrational frequencies.
Relative to the edt derivative, adt destabilizes the axial-
basal isomers (Figure 5). In one axial-basal isomer, the NH
group is oriented toward the Fe(dppv)(CO) center and
experiences steric crowding with one phenyl group. An
attractive interaction between the polar N-Hδ+ and the π
system of the phenyl ring partially counterbalances steric
problems. In the other axial-basal isomer, the NH group of
adt points toward the axial CO on the Fe(CO)3 center. Even
in this case, however, the CH2 groups of adt clash with the
phenyl group of dppv. In all four conformers of Fe2[(SCH2)2-
NH](CO)4(dppv), the N-H is axial because of the anomeric
effect, in agreement with previous DFT studies.16 For the
pdt derivative, 2(CO)4, the trimethylene group destabilizes
the axial-basal isomer relative to the edt derivative because
of steric clashes with one phenyl ring. In the most stable
axial-basal isomer, the central CH2 of the pdt strap points
away from the phenyl group (see the Supporting Informa-
tion), whereas the axial-basal isomer in which the central
CH2 points to the phenyl group (not shown) is less stable
by 0.75 kcal/mol.
Figure 3. 31P NMR spectra of Fe2[(SCH2)2NH](CO)4(dppv) in a CD2Cl2
solution at temperatures of +20 (top), -40, and -60 °C (bottom).
dithiolate. For 1(CO)4, the 31P NMR spectrum displays a
single resonance at δ 97.4 at 20 °C. Upon cooling of the
sample to -60 °C, this peak splits into doublets at δ 97.2
and 99.5 (JP-P ) 16 Hz), indicative of an axial-basal
isomer.10 This species was further characterized crystallo-
graphically (Figure 2).
For the pdt and adt complexes, the 31P NMR spectrum
again consists of a singlet at 20 °C, but upon cooling of the
sample to -60 °C, new signals appeared, indicating the
presence of four isomers. On the basis of coupling and shift
patterns, we propose that two of these isomers are axial-
basal and the other two isomers are dibasal. Singlets at δ
85.7 and 81.6 are attributed to a pair of dibasal isomers that
differ with respect to the position of the nitrogen atom of
the (SCH2)2NH ligand (Scheme 1 and Figure 3). A pair of
partially resolved AB quartets (δ 100.3 and 95.0) is assigned
to axial-basal isomers that again differ with respect to the
orientation of the amine in (SCH2)2NH. A related pattern is
observed for 2(CO)4, except that the dibasal isomers are more
favored. At -80 °C, the dibasal isomer represents 18%
abundance for the S2C3H6 derivative but only 8% abundance
for the adt derivative. Thus, the central CH2 group of the
pdt more strongly affects the orientation of the dppv ligand
Substituted Derivatives of 1(CO)4. We found that 1(CO)4
reacted efficiently with PMe3 to give Fe2(S2C2H4)(CO)3-
(dppv)(PMe3), 1(CO)3(PMe3). Further substitution was not
observed even with a 5-fold excess of PMe3. The compound
1(CO)4 also reacted efficiently with Et4NCN to give Et4N-
[1(CN)(CO)3] (eq 1). The structures of 1(CO)3(PMe3) and
Et4N[1(CN)(CO)3] were confirmed crystallographically
(Figure 6).
than does the NH of the adt. In general, we find that the 31
P
NMR shifts for the dppv ligand in [FeI]2 species correlate
with stereochemistry: dppv ligands bound in the axial-basal
fashion absorb at >δ 90, whereas dppv ligands bound in
the dibasal geometry absorb at <δ 90.
The 31P NMR spectrum of 1(CO)3(PMe3) consists of
singlets at δ 94.7 and 20.6 at room temperature, assigned to
dppv and PMe3, respectively. Upon cooling of the sample
to -90 °C, these peaks split to give two sets of three signals
each, indicative of two isomers in an approximately 2:1 ratio.
On the basis of peak positions, the dppv ligand is assigned
to the axial-basal orientation in both isomers. Thus, the
isomers differ with respect to the axial versus basal positions
of the PMe3.
Kinetics of the Formation of Fe2(S2C2H4)(CO)3(dppv)-
(PMe3). To our surprise, 1(CO)4 was found to react with
PMe3 faster than the parent compound Fe2(S2C2H4)(CO)6.
In a competition experiment, a solution containing a 1:1 mix-
ture of Fe2(S2C2H4)(CO)6 and 1(CO)4 was treated with 3
equiv of PMe3. Interestingly, 1(CO)4 was consumed exclu-
DFT Calculations on Fe2(SR)2(CO)4(dppv). Using DFT
calculations, we probed the stereochemistry of the dppv
ligand for the three dithiolato complexes. For the 1,2-
ethanedithiolate (edt) derivative 1(CO)4, computed structural
parameters agree with experimental data. In particular, the
discrepancy among computed and crystallographically de-
termined bond distances is <0.05 Å. Similarly, only small
deviations are observed for bond angles (not shown).
Calculations are in good agreement with the experimental
observation that the most stable isomer is axial-basal. In
the axial-basal isomer, the dppv ligand does not clash with
the edt or the carbonyl ligands on the second iron center. In
contrast, the dibasal isomer would induce unfavorable steric
crowding between the phenyl rings and the basal carbonyl
(16) Lawrence, J. D.; Li, H.; Rauchfuss, T. B.; Be´nard, M.; Rohmer, M.-
M. Angew. Chem., Int. Ed. 2001, 40, 1768-1771.
Inorganic Chemistry, Vol. 46, No. 5, 2007 1657

Justice et al.
Figure 4. Axial-basal (left) and dibasal (right) isomers of Fe2(edt)(CO)4(dppv) as obtained by DFT calculations (distances in angstroms and computed
energy differences in kilocalories per mole relative to the more stable isomer).
Figure 5. Axial-basal (top) and dibasal (bottom) isomers of Fe2(adt)(CO)4(dppv), as obtained by DFT calculations (distances in angstroms and computed
energy differences in kilocalories per mole relative to the most stable isomer).
siVely. The result is remarkable because not only is the
hexacarbonyl less encumbered sterically but its effective
concentration is twice that of 1(CO)4 because it has two
equivalent Fe(CO)3 centers. We verified that the reaction of
1(CO)4 and PMe3 indeed proceeded via the expected bimo-
lecular pathway.17,18 The rate of the 1(CO)4 + PMe3 reaction
was examined under pseudo-first-order conditions over the
temperature range 0 to 30 °C. Illustrative spectra for this
conversion are presented in Figure 7. The temperature
dependence of the rate indicated ∆Hq ) 5.8(5) kcal/mol and
∆Sq ) -48(2) cal/deg‚mol. These values lead to a ∆Gq that
is smaller than that of previously reported reactions of
Fe2(SR)2(CO)6 with tertiary phosphines.17,19 Upon a change
(17) Ellgen, P. C.; Gerlach, J. N. Inorg. Chem. 1973, 12, 2526-2532.
(18) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 2001, 123, 3268-3278.
1658 Inorganic Chemistry, Vol. 46, No. 5, 2007

Chelate Control of Diiron(I) Dithiolates
Figure 6. Left: structure of 1(CO)3(PMe3) with thermal ellipsoids set at 35%. Phenyl ellipsoids and phenyl hydrogen atoms were omitted for clarity.
Selected bond lengths (Å) and angles (deg): Fe(1)-Fe(2), 2.5344(7); Fe(1)-S(1), 2.2633(11); Fe(1)-S(2), 2.2690(10); Fe(1)-P(1), 2.2006(11); Fe(1)-
P(2), 2.1918(10); Fe(1)-C(1), 1.755(4); Fe(2)-P(3), 2.2312(11); Fe(2)-C(2), 1.767(4); Fe(2)-C(3), 1.758(4); Fe(2)-Fe(1)-P(1), 108.38(3); Fe(2)-Fe-
(1)-P(2), 157.03(4); Fe(2)-Fe(1)-C(1), 101.08(12); P(2)-Fe(1)-P(1), 88.19(4); P(2)-Fe(1)-C(1), 93.97(12); C(1)-Fe(1)-P(1), 91.82(13). Right: structure
of the anion in Et4N[1(CN)(CO)3], with thermal ellipsoids set at 35%. Phenyl ellipsoids, phenyl hydrogen atoms, and the Et4N+ groups are not shown.
Selected bond lengths (Å) and angles (deg): Fe(1)-Fe(2), 2.5416(16); Fe(1)-S(1), 2.2453(12); Fe(1)-S(2), 2.2512(12); Fe(1)-P(1), 2.1828(13); Fe(1)-
P(2), 2.1763(12); Fe(1)-C(4), 1.733(3); Fe(2)-C(1), 1.916(3); Fe(2)-C(2), 1.766(3); Fe(2)-C(3), 1.751(3); Fe(2)-Fe(1)-P(1), 112.81(5); Fe(2)-Fe(1)-
P(2), 154.49(3); Fe(2)-Fe(1)-C(4), 101.07(10); P(2)-Fe(1)-P(1), 87.54(6); P(2)-Fe(1)-C(4), 94.50(10); P(1)-Fe(1)-C(4), 88.25(9).
Figure 7. IR spectra for the reaction of Fe2(S2C2H4)(CO)4(dppv) (0.014 M) with 20 equiv of PMe3 in a CH2Cl2 solution at 20 °C.
of the dithiolate from edt to pdt, the rate of substitution
decreased approximately 5-fold.
The electron-rich character of the Fe(CO)(dppv) center
induces rotation of the Fe(CO)3 group, favoring the formation
of a µ-CO structure in the transition state. The (dppv)(OC)-
Fe-µ-CO distance in the transition state is calculated as 2.020
Å. In contrast, the (OC)3Fe-µ-CO distance in the Fe2-
(S2C2H4)(CO)6 transition state is as long as 2.190 Å. The
“rotated” structure is more susceptible to attack by nucleo-
philes than is the ground-state rotamer (Figure 8).20
DFT Analysis of the Substitution Process. As a comple-
ment to the experimental results, we employed DFT to
examine the reactivity of 1(CO)4 and Fe2(S2C2H4)(CO)6. As
a model nucleophile, we chose KCN to allow comparison
with data previously obtained for related complexes.20 Energy
barriers for the reactions KCN + 1(CO)4 and KCN + Fe2-
(S2C2H4)(CO)6 were calculated to be 8.4 and 13.3 kcal/mol,
respectively. A similar trend was observed with PMe3 as the
nucleophile, with energy barriers ) 10.3 and 14.8 kcal/mol
for 1(CO)4 and Fe2(S2C2H4)(CO)6, respectively.21
Diferrous Derivatives [Fe2(S2C2H4)(µ-CO)(CO)2(dppv)L-
(NCMe)]2+. Oxidation of MeCN solutions of 1(CO)3(PMe3)
with 2 equiv of Cp2FePF6 afforded the green diferrous
derivative [Fe2(S2C2H4)(µ-CO)(CO)2(dppv)(PMe3)(NCMe)]-
(PF6)2, [1(µ-CO)(CO)2(PMe3)(NCMe)](PF6)2. The IR spec-
trum exhibited a band at 1909 cm-1, assigned to νµ-CO. Other
diferrous carbonyls exhibit νµ-CO bands of comparable
(19) Zampella, G.; Bruschi, M.; Fantucci, P.; Razavet, M.; Pickett, C. J.;
De Gioia, L. Chem.sEur. J. 2005, 11, 509-520.
(20) Georgakaki, I. P.; Thomson, L. M.; Lyon, E. J.; Hall, M. B.;
Darensbourg, M. Y. Coord. Chem. ReV. 2003, 238-239, 255-266.
(21) Corrections to energy barriers led, for the reaction between 1(CO)4
and PMe3, to a computed ∆Gq at 298 K of 24.4 kcal/mol, which is in
reasonable agreement with experimental data, considering that the
BP86 functional is known to underestimate reaction barriers.
(22) Boyke, C. A.; Rauchfuss, T. B.; Wilson, S. R.; Rohmer, M.-M.;
Be´nard, M. J. Am. Chem. Soc. 2004, 126, 15151-15160.
Inorganic Chemistry, Vol. 46, No. 5, 2007 1659

Justice et al.
Figure 8. DFT-optimized reactant, transition state, and product for the reaction 1(CO)4 + KCN (distances in angstroms).
high frequency.22,23 The 31P NMR data (δ 74.6 for the dppv
and δ 36.4 for the PMe3) indicate a Cs-symmetric species,
which requires that the PMe3 is axial and the dppv is dibasal
(eq 2).
resulted in the formation of [Fe2(S2C2H4)(µ-CO)(CN)(CO)2-
(dppv)(NCMe)]PF6, [1(µ-CO)(CN)(CO)2(NCMe)]PF6. On
the basis of its 31P NMR spectrum, which indicates equivalent
phosphorus ligands, this species has Cs symmetry. Consistent
with this conclusion, DFT calculations on these diferrous
species indicate that donor ligands will preferentially occupy
sites trans to µ-CO (Figure 9).
Compound 1(CO)3(PMe3) also smoothly oxidized with 2
equiv of Cp2FePF6 in an acetone solution at -40 °C. The
initial product, assumed to be [Fe2(S2C2H4)(µ-CO)(CO)2-
(dppv)(PMe3)(OCMe2)]2+, was identified by FT-IR spec-
troscopy. The νCO portion of the spectrum (2059, 2015, and
1912 cm-1) is quite similar in pattern and energy to that seen
for the MeCN adduct. The stabilities of the MeCN and
acetone complexes, however differ sharply: the acetone
complex decomposes upon warming of the reaction solution
to room temperature, while the MeCN complex is stable both
as a solid and in solution at room temperature for extended
periods.
Complex [1(µ-CO)(CO)2(PMe3)(NCMe)]2+ is reactive
toward further substitution, in contrast to the parent 1(CO)3-
(PMe3). The addition of 1 equiv of PMe3 to a solution of
[1(µ-CO)(CO)2(PMe3)(NCMe)](PF6)2 caused a color change
from green to red. New νCO peaks appeared at 1983 and 1888
cm-1, with the former being the single terminal carbonyl
and the latter the bridging carbonyl peak. On the basis of
1H and 31P NMR and IR spectroscopies and electrospray
ionization mass spectrometry (ESI-MS), the new compound
was identified as [1(µ-CO)(CO)(PMe3)2(NCMe)](PF6)2 (eq
3). The 31P NMR spectrum indicates that the dppv is dibasal
and that the complex lacks symmetry. This species is similar
to the recently described [Fe2(S2C2H4)(µ-CO)(CO)(PMe3)4-
(NCMe)](PF6)2.10 Spectroscopic evidence suggests that oxi-
dation of Et4N[1(CN)(CO)3] with 2 equiv of Cp2FePF6
Oxidations of pdt Derivatives. To explore the 2e-
oxidation of the pdt derivatives, we prepared Fe2(S2C3H6)-
(CO)3(dppv)(PMe3), 2(CO)3(PMe3). The preparation pro-
ceeded analogously to that for the synthesis of 1(CO)3(PMe3).
Low-temperature 31P NMR measurements show that this
species exists as three isomers. Interestingly, the dibasal
isomer is far less populated in this species than in its
tetracarbonyl precursor, apparently as a result of the PMe3
ligand, probably axially coordinated, pushing the dibasal CO
ligands toward the base and inducing nonbonded interactions
with the phenyl rings on the dppv.
Treatment of 2(CO)3(PMe3) with 2 equiv of Cp2FePF6 at
-40 °C resulted in an immediate change in the νCO region
of the IR spectrum to a pattern resembling that for [1(µ-
CO)(CO)2(PMe3)(NCMe)]2+, i.e., one bridging (νµ-CO
)
1922 cm-1) and two terminal (νCO-t ) 2055 and 2009 cm-1)
bands. The shift of νCO-t to higher energies is typical for
the oxidation to the diferrous state, and the broad, low-
intensity band is typical for νµ-CO. The energies of these
bands were, however, quite different for the pdt versus edt
derivatives: the µ-CO band occurs at 1921 cm-1 for the pdt
(23) Boyke, C. A.; van der Vlugt, J. I.; Rauchfuss, T. B.; Wilson, S. R.;
Zampella, G.; De Gioia, L. J. Am. Chem. Soc. 2005, 127, 11010-
11018.
1660 Inorganic Chemistry, Vol. 46, No. 5, 2007

Chelate Control of Diiron(I) Dithiolates
Figure 9. DFT-optimized structures of axial-basal and dibasal isomers of [Fe2(S2C2H4)(µ-CO)(CO)2(dppv)(PMe3)(NCMe)]2+ (distances in angstroms and
computed energy differences in kilocalories per mole relative to the most stable isomer). For the sake of brevity, only the four most relevant (and most
stable) isomers are shown.
Scheme 2
derivative, 12 cm-1 higher than that for the edt derivative.
This shift is compensated for by opposite shifts in νCO-t
,
which are each ca. 7 cm-1 lower than those for the edt
derivative. Thus, the data indicate that the terminal CO
ligands are relatively better π acceptors for the pdt derivative,
whereas the µ-CO ligand is relatively more of a π acceptor
in the edt derivative. The effect of the dithiolate on the
stability of the oxidized derivatives is profound: whereas
[1(µ-CO)(CO)2(PMe3)(NCMe)]2+ is robust at room temper-
ature, [2(µ-CO)(CO)2(PMe3)(NCMe)]2+ decomposed within
ca. 30 min at -40 °C. The addition of PMe3 to a solution of
[2(µ-CO)(CO)2(PMe3)(NCMe)]2+ at -40 °C did not yield
an isolable derivative. Oxidation of the adt derivative
Fe2[(SCH2)2NH](CO)3(PMe3)(dppv) did not yield stable or
observable diferrous derivatives.
paradigm,20 substitution at Fe2(SR)2(CO)6-nLn proceeds via
attack of the nucleophile on a rotamer that features an
exposed coordination site on one Fe(CO)3 subunit, trans to
the Fe-Fe bond (Scheme 2). The binding of a ligand L to
this “rotated isomer” ruptures the Fe-Fe bond, forming the
36e- adduct Fe2(SR)2(µ-CO)(CO)3L3. Such 36e- adducts
readily decarbonylate concomitant with reformation of the
Fe-Fe bond and subsequent rotation of the Fe(CO)2L to its
ground-state orientation.
Discussion
Reactivity of Fe2(SR)2(CO)4(dppv). Ligands substitute
1(CO)4 exclusively at the Fe(CO)3 subunit under mild
conditions. This reactivity enabled the preparation of other-
wise rare trisubstituted diiron(I) dithiolato complexes. Kinetic
studies indicate that 1(CO)4 is more electrophilic than the
parent hexacarbonyl. According to the prevailing mechanistic
Inorganic Chemistry, Vol. 46, No. 5, 2007 1661

Justice et al.
Pickett et al. have spectroscopically detected an adduct
of the type Fe2(SR)2(µ-CO)(CO)6-xLx in the cyanation of
Fe2[(SCH2)2C(Me)CH2SMe](CO)6.19 In their case, a pendent
SMe ligand stabilizes the 36e- intermediate. Recent DFT
calculations suggest that the “rotated” geometry is stabilized
by the presence of donor ligands in unsymmetrically
substituted complexes.24,25 Darensbourg and Hall calculate
that unsymmetrical substitution of the diiron center stabilizes
the rotated structure, thereby accelerating substitution.25 Our
results support the role of the rotated structure. Our calcula-
tions agree with these previous predictions: the dppv ligand
in 1(CO)4 stabilizes the rotated transition state by about 5
kcal/mol. For the reaction Fe2(S2C2H4)(CO)6 + PBu3, ∆Hq
) 13.6 kcal/mol and ∆Sq ) -31 cal/deg‚mol, whereas the
presently discussed 1(CO)4 + PMe3 reaction proceeds with
∆Hq ) 5.8(5) kcal/mol and ∆Sq ) -48(2) cal/deg‚mol. The
enthalpic advantage for the dppv complexes is attributed to
the stabilization of the rotated structure, which precedes
attack by the nucleophile. We do not presently understand
the characteristics of the ligands that will lead to such rate
accelerations.
In general, larger ligands (PMe3) are not tolerated for pdt
derivatives; thus, nature’s use of small ligands (CO and CN-)
on the “distal” iron (location of the variable coordination
site) is understandable. Consistent with this view, the nature
of the dithiolate strap was found to strongly affect the
stability of the derivatives in the series [Fe2(xdt)(µ-CO)(CO)2-
(dppv)(PMe3)(NCMe)]2+ [xdt ) edt, adt, pdt]. The edt
species was robust at room temperature, the pdt derivative
was observable at low temperature, and the adt derivative
could not be generated. Because adt is less bulky than pdt,
the instability of this oxidized compound indicates that a
combination of steric and electronic effects are relevant.
We had previously shown that Fe2(S2C2H4)(CO)4(PMe3)2
could be oxidized to give derivatives [FeII (S2C2H4)(µ-CO)-
2
(CO)2(PMe3)3(NCMe)]2+ and [FeII (S2C2H4)(µ-CO)(CO)-
2
(PMe3)4(NCMe)]2+. These reactions require the addition of
trapping donor ligands such as PMe3, which displace one or
two terminal CO ligands, stabilizing the higher oxidation
state. Being already trisubstituted, Fe2(SR)2(CO)3(dppv)L
underwent efficient oxidation without the need for exogenous
donor ligands (aside from the MeCN provided by the
solvent). Oxidation proceeded in acetone, but the adduct was
thermally unstable. In the diferrous state of the enzyme, the
so-called Hoxair, an axial site is proposed to be occupied by
water, which we propose is simulated by MeCN in our case.
In this and related work, we have not observed coordination
of water.
Effect of the Dithiolate (edt vs pdt vs adt) on Subfer-
rous Species. A general question arising from studies on
diiron dithiolato carbonyls concerns the influence of the
dithiolate on the reactivity and structure of the underlying
Fe2(CO)6-xLx subunit.6,18,22 An understanding of how the
dithiolate influences the reactivity of the underlying diiron
centers is relevant to the design of new dimetallic reaction
centers based on dithiolato bridges.
Conclusions
The rate of substitution was found to depend upon the
dithiolate ligand. Differences in rates must reflect differences
in the steric accessibility of the axial position because the
electronic properties of the Fe2(SR)2(CO)4(dppv) cores are
virtually identical, as indicated by νCO values. The reaction
of 1(CO)4 with PMe3 was ca. 5 times faster than the
corresponding reaction for the pdt derivative. This difference
is attributed to increased steric protection provided by the
pdt ligand, which positions its central CH2 group over the
axial sites on the diiron framework.
Studies on diiron dithiolato complexes with unsymmetrical
coordination environments have led to a better understanding
of the influence of the dithiolate on the stereochemistry of
these complexes. The unsymmetrical coordination environ-
ment enhances the overall reactivity of the subferrous
complexes, allowing a third ligand to be easily installed.
Trisubstituted complexes undergo oxidation to the diferrous
derivatives under highly controlled conditions, providing the
foundation for future detailed studies on the redox properties
of diiron dithiolates.
The relative steric bulk of the dithiolates is indicated by
the ratio of the dibasal versus axial-basal isomers for Fe2-
(xdt)(CO)4(dppv) (xdt ) adt, edt, pdt). At comparable
temperatures, the ratio of axial-basal/dibasal was >20:1
(edt), 12:1 (adt), and 7:1 (pdt). Thus, pdt and adt are sterically
more imposing than edt. The steric bulk of pdt versus adt
does not appear to influence the barrier for the interconver-
sion of the dibasal and axial-basal isomers. The turnstile
rotation of related compounds has been well studied.18,26
Impact of Dithiolate on Diferrous Species. Previous
work had shown that oxidative decarbonylation of Fe2(SR)2-
(CO)6-xLx (L ) PR3, RNC) gives diferrous derivatives.22,23
Experimental Section
Unless otherwise indicated, reactions were conducted using
Schlenk techniques at room temperature. Chemicals were purchased
from Aldrich, and solvents were either HPLC-grade from an
alumina filtration system or distilled under nitrogen over an
appropriate drying agent. NMR spectra were recorded at room
temperature on a Varian Mercury 500 MHz spectrometer. NMR
chemical shifts are quoted in ppm; spectra are referenced to
1
tetramethylsilane for H and 13C{1H} NMR and 85% H3PO4 for
31P{1H} NMR spectra. FTIR spectra were recorded on a Mattson
Infinity Gold FTIR spectrometer. Fe2(S2C2H4)(CO)6, Fe2(S2C3H6)-
(CO)6, and Fe2[(SCH2)2NH](CO)6 were prepared according to
27
literature procedures. PMe3 was distilled prior to use on a high-
vacuum line. Et4NCN and dppv were obtained from Aldrich.
ONMe3 was obtained from Aldrich and dried by repeated vacuum
sublimation.
Fe2(S2C2H4)(CO)4(dppv), 1(CO)4. A solution of 2.70 g (7.24
mmol) of Fe2(S2C2H4)(CO)6 in 35 mL of toluene was treated with
a solution of 0.54 g (7.24 mmol) of Me3NO in 15 mL of MeCN.
(24) Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2004, 43, 3733-
3741.
(25) Tye, J. W.; Darensbourg, M. Y.; Hall, M. B. Inorg. Chem. 2006, 45,
1552-1559.
(26) Adams, R. D.; Cotton, F. A.; Cullen, W. R.; Hunter, D. L.; Mihichuk,
L. Inorg. Chem. 1975, 14, 1395-1399. Flood, T. C.; DiSanti, F. J.;
Campbell, K. D. Inorg. Chem. 1978, 17, 1643-1646.
(27) Li, H.; Rauchfuss, T. B. J. Am. Chem. Soc. 2002, 124, 726-727.
1662 Inorganic Chemistry, Vol. 46, No. 5, 2007

Chelate Control of Diiron(I) Dithiolates
After stirring for 10 min, the reaction mixture was treated with a
solution of 2.87 g (7.24 mmol) of dppv in 20 mL of toluene. After
5 h, the solvent was removed in vacuum. The residue, a brown
solid, was extracted into 10 mL of CH2Cl2, and the product
precipitated as a light-brown powder upon the addition of 100 mL
of hexanes. The product was rinsed with ∼60 mL of hexanes.
Crystals were grown via slow evaporation of a MeCN-toluene
solution. Yield: 4.57 g (82%). 1H NMR (CD2Cl2): δ 7.7-7.1 (m,
20H, C6H5), 4.3 (s, 2H, PCH), 1.6 (m, 2H, SCH2), 1.1 (m, 2H,
SCH2). 31P NMR (CD2Cl2): δ 97.4 at 20 °C; 99.5 (d, JP-P ) 16
Hz), 97.2 (d, JP-P ) 16 Hz) at -60 °C. IR (CH2Cl2): νCO 2023,
1953, 1915 cm-1. FD-MS: m/z 712.02 (1(CO)4). Anal. Calcd for
C32H26Fe2O4P2S2 (found): C, 53.94 (53.73); H, 3.68 (3.63).
Fe2(S2C3H6)(CO)4(dppv), 2(CO)4, was prepared similarly.
Yield: 57%. 1H NMR (CD2Cl2): δ 7.7 - 7.1 (m, 20H, C6H5), 4.3
(s, 2H, PCH), 1.59 (m, 4H, SCH2), 1.29 (m, 2H, SCH2), 0.97 (m,
2H, CH2CH2CH2). 31P NMR (CD2Cl2): δ 96.9, 82.9 at 20 °C; 95.9
at 90 °C; 103.2, 100.0, 96.0, 95.5, 85.7, 82.3 at -90 °C. IR (CH2-
Cl2): νCO 2021, 1950, 1912 cm-1. FD-MS: m/z 726.5 (2(CO)4).
Anal. Calcd for C33H28Fe2O4P2S2 (found): C, 54.55 (54.60); H,
3.89 (3.87).
Calcd for C41H48Fe2N2O3P2S2Cl2 (found): C, 53.20 (52.97); H,
5.06 (5.22); N, 3.06 (3.02).
Kinetics. Pseudo-first-order reaction conditions were employed
for all kinetics studies, using 10 equiv or more of PMe3. Reactions
were performed in a Schlenk vessel designed to contain the in situ
IR probe (React IR). Typically, the reactor, a 50 mL flask, was
charged with 0.050 g (0.070 mmol) of 1 and flushed with N2 before
the addition of 5.0 mL of CH2Cl2. The IR probe was then inserted
into the 0.014 M reaction mixture, and the reaction mixture was
placed into a thermostatted bath. Following thermal equilibration,
a 0.78 M solution PMe3 in CH2Cl2 was injected rapidly. While
being magnetically stirred, the solution was analyzed by IR
spectroscopy typically at intervals of 15 s until the reaction was
judged complete and then at 30 s intervals for an additional 30
min. The rates of reaction were analyzed using the program Concert
(Mettler-Toledo). Pseudo-first-order rate constants were the slopes
of plots of ln(1(CO)4) vs time (seconds). Rates were determined
for the following concentrations of PMe3: 10, 20, 30, and 40 equiv
at 20 °C. The slope of the graph of kobs vs [PMe3] gave the second-
order rate constant. Activation parameters were deduced from a
graph of ln(kT-1) vs T-1 for T ) 0, 10, 20, and 30 °C for reactions
employing a 20 times excess of PMe3. The slope was taken as
∆Hq/R and the y intercept as ∆Sq/R + 23.8 [)ln(kb/h)]. We estimate
5% uncertainty in the rate constants. With this assumption, using
only the rate constants for the 273 and 303 K reactions, we
determined uncertainties in ∆Hq and ∆Sq.
Fe2(S2(CH2)2NH)(CO)4(dppv), 3(CO)4, was prepared similarly.
1
Yield: 26%. H NMR (toluene-d8): δ 7.7-7.1 (m, 20H, C6H5),
4.3 (s, 2H, PCH), 3.04 (m, 4H, SCH2N). 31P NMR (CD2Cl2, 20
°C): δ 97.6. 31P NMR (CD2Cl2, -60 °C): δ 101.4, 100.7 (JP-P
)
20 Hz), 97.2, 95.3 (JP-P ) 20 Hz), 86.0, 82.9. IR (CH2Cl2): νCO
2021, 1953, 1915 cm-1. FD-MS: m/z 727.0 (3(CO)4).
[Fe2(S2C2H4)(µ-CO)(CO)2(dppv)(PMe3)(NCMe)](PF6)2, [1(CO)3-
(PMe3)(NCMe)](PF6)2. A solution of 0.200 g (0.263 mmol) of
1(CO)3(PMe3) in 10 mL of MeCN was treated with a solution 0.174
g (0.526 mmol) of Cp2FePF6 in 5 mL of MeCN. The reaction
solution changed from dark red to green immediately. Solvent was
removed in vacuum, and the green-colored residue was rinsed with
∼30 mL of hexane. The reaction mixture was extracted into 3 mL
of MeCN, and the product precipitated as a green powder upon
Fe2(S2C2H4)(CO)3(dppv)(PMe3), 1(CO)3(PMe3). A solution of
0.30 g (0.421 mmol) of 1(CO)4 in 20 mL of toluene was treated
with a solution of 0.031 g (0.421 mmol) of Me3NO in 5 mL of
MeCN. To this solution was added 0.90 mL (0.450 mmol) of a
0.50 M solution of PMe3 in toluene. After 12 h, the solvent was
removed in vacuum. The dark-red residue was extracted into 5 mL
of CH2Cl2, and the product was precipitated as a dark-red powder
upon the addition of 50 mL of hexanes. Crystals were grown via
slow diffusion of hexanes into a CH2Cl2 solution. Yield: 0.28 g
(88%). 1H NMR (CD2Cl2): δ 8.0-7.2 (m, 20H, C6H5), 4.3 (s, 2H,
PCH), 1.6 (m, 2H, SCH2), 1.2 (d, JP-H ) 8 Hz, 9H, PCH3), 0.8
(m, 2H, SCH2). 31P NMR (CD2Cl2, 20 °C): δ 94.7, 20.6. 31P NMR
(CD2Cl2, -90 °C): δ 101.1, 96.3, 94.0, 92.7, 28.4, 11.5. IR (CH2-
Cl2): νCO 1943, 1892 cm-1. FD-MS: m/z 760.00 ([Fe2(S2C2H4)-
(CO)3(dppv)(PMe3)]). Anal. Calcd for C34H35Fe2O3P3S2 (found):
C, 53.68 (53.82); H, 4.64 (4.51).
Reaction of Fe2(S2C2H4)(CO)6 and 1(CO)4 with PMe3. A
solution of 0.050 g (0.134 mmol) of Fe2(S2C2H4)(CO)6 and 0.096
g (0.134 mmol) of 1(CO)4 in 10 mL of toluene was treated with
0.41 mL (0.402 mmol) of a 0.98 M solution of PMe3 in toluene.
After 1 h, the following IR spectrum was obtained: νCO 2075, 2035,
2002, and 1991 cm-1 for Fe2(S2C2H4)(CO)6 and νCO 1960, 1949,
1911, and 1895 cm-1 for 1(CO)3(PMe3).
1
the addition of 35 mL of Et2O. Yield: 0.214 g (75%). H NMR
(MeCN-d3): δ 8.8-7.5 (m, 20H, C6H5), 4.40 (s, 2H, PCH), 3.40
(m, 2H, SCH2), 3.03 (m, 2H, SCH2) 1.85 (d, JP-H ) 12 Hz, 9H,
PCH3). 31P NMR (MeCN-d3): δ 74.6 (d, JP-P ) 5 Hz), 36.4 (t,
JP-P ) 5 Hz). IR (MeCN): νCO 2061, 2018, 1909 cm-1. ESI-MS:
m/z 946.2 ([Fe2(S2C2H4)(µ-CO)(CO)2(dppv)(PMe3)(NCMe)]PF6+).
Anal. Calcd for C36H38Fe2NF12O3P5S2 (found): C, 39.60 (39.31);
H, 3.51 (3.45); N, 1.28 (1.24).
When a solution of 0.020 g (0.0263 mmol) of 1(CO)3(PMe3) in
5 mL of acetone was treated with 0.017 g (0.0526 mmol) of Cp2-
FePF6 in 5 mL of acetone at -40 °C, the solution color changed
from red to green. The IR spectrum indicated [Fe2(S2C2H4)(µ-CO)-
(CO)2(dppv)(PMe3)(OCMe2)]2+. IR (OCMe2): νCO 2059, 2015,
1912 cm-1. Upon warming of the solution to 20 °C, the IR spectrum
revealed that the complex had decomposed.
[Fe2(S2C2H4)(µ-CO)(CO)(dppv)(PMe3)2(NCMe)](PF6)2, [1(CO)2-
(PMe3)2(NCMe)](PF6)2. A solution of 0.280 g (0.368 mmol) of
1(CO)3(PMe3) in 10 mL of MeCN was treated with a solution of
0.243 g (0.736 mmol) of Cp2FePF6 in 5 mL of MeCN. The reaction
mixture became green and was immediately treated with 1.2 mL
(0.736 mmol) of a 0.59 M solution of PMe3 in MeCN. The solvent
was removed in vacuum, and the dark-red residue was rinsed with
∼30 mL of hexane. The solid was extracted into 3 mL of MeCN;
the product precipitated as a brown powder upon the addition of
Et4N[Fe2(S2C2H4)(CN)(CO)3(dppv)], Et4N[1(CN)(CO)3]. A
solution of 0.750 g (1.03 mmol) of 1(CO)4 in 15 mL of MeCN
was treated with a solution of 0.161 g (1.03 mmol) of Et4NCN in
10 mL of MeCN. The solution was stirred for 3 h and the sol-
vent removed in vacuum. The residue, a dark-red solid, was
recrystallized by extraction into 5 mL of CH2Cl2, followed by the
addition of 50 mL of hexanes. Crystals were grown via slow
diffusion of hexanes into a CH2Cl2 solution. Yield: 0.65 g (75%).
1H NMR (MeCN-d3): δ 8.1-7.2 (m, 20H, C6H5), 5.4 (s, 2H, PCH),
3.5 (q, 16H, NCH2CH3), 1.6 (m, 2H, SCH2), 1.4 (t, 24H,
NCH2CH3), 1.2 (m, 2H, SCH2). 31P NMR (MeCN-d3, 20 °C):
1
35 mL of Et2O. Yield: 0.285 g (68%). H NMR (MeCN-d3): δ
8.7-7.3 (m, 20H, C6H5), 4.40 (s, 2H, PCH), 3.40 (m, 2H, SCH2),
2.91 (m, 2H, SCH2), 2.53 (m, 3H, Fe-NCCH3), 1.43 (d, JP-H ) 10
Hz, 9H, PCH3), 1.20 (d, JP-H ) 11 Hz, 9H, PCH3). 31P NMR
δ 94.7. IR (MeCN): νCN 2072 cm-1; νCO 1943, 1890 cm-1
ESI-MS: m/z 710.30 ([Fe2(S2C2H4)(CN)(CO)3(dppv)]-). Anal.
.
(MeCN-d3): δ 74.5, 68.7, 24.3 (t, JP-P ) 27 Hz), 17.2 (t, JP-P
)
Inorganic Chemistry, Vol. 46, No. 5, 2007 1663

Justice et al.
Table 1. Details of Data Collection and Structure Refinement for Crystallography
complex
chemical formula
T (K)
1(CO)4
C32H26O4P2S2Fe2
193 (2)
1(CO)3(PMe3)
C34H35O3P3S2Fe2
193 (2)
Et4N[1(CN)(CO)3]
C40H46Fe2N2O3P2S2
193 (2)
cryst size (mm3)
cryst syst
space group
a (Å)
0.46 × 0.08 × 0.06
0.16 × 0.12 × 0.01
0.48 × 0.36 × 0.14
triclinic
triclinic
triclinic
P1h
P1h
P1h
9.5119(15)
12.1329(19)
28.710(5)
79.242(3)
89.738(3)
83.877(3)
3236.1(9)
1
10.4617(10)
12.3125(10)
15.2256(13)
76.276(5)
88.122(5)
89.143(4)
1904.1(3)
2
11.219(7)
11.505(7)
18.994(12)
88.297(10)
74.230(10)
66.601(9)
2157(2)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
2
calcd (Mg m-3
)
1.509
0.71073
0.9386/0.4782
34970/12272
12272/0/770
0.916
1.474
0.71073
0.9767/0.8247
58398/9444
9444/0/427
1.048
1.425
0.71073
0.8773/0.5901
18473/7884
7884/66/519
1.026
µ(Mo KR) (mm-1
)
max/min transmn
reflns measd/indep
data/restraints/parameters
GOF on F 2
Rint
0.0931
0.0513
0.0335
R1 [I > 2σ] (all data)a
wR2 [I > 2σ] (all data)b
max peak/hole (e-/Å3)
0.0513 (0.1268)
0.0901 (0.1070)
0.565/-0.411
0.0518 (0.0824)
0.1357 (0.1532)
0.834/-1.189
0.0374 (0.0603)
0.0862 (0.0944)
0.569/-0.335
2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) {[w(|Fo| - |Fc|)2]/∑[wFo ]}1/2, where w ) 1/σ2(Fo).
27 Hz). IR (MeCN): νCO 1983, 1888 cm-1. ESI-MS: m/z 994.4
([Fe2(S2C2H4)(µ-CO)(CO)(dppv)(PMe3)(NCMe)](PF6)+).
(Table 1); atomic positions were deduced from an E map or by an
unweighted difference Fourier synthesis. Hydrogen atom U’s were
assigned as 1.2Ueq for adjacent carbon atoms. Non-hydrogen atoms
were refined anisotropically. Successful convergence of the full-
matrix least-squares refinement of F 2 was indicated by the
maximum shift/error for the final cycle.
DFT Calculations. DFT calculations used both the BP8628,29
functional and a valence triple-ú basis set with polarization on all
atoms (TZVP).30 Stationary points of the energy hypersurface were
located using energy gradient techniques, and full vibrational
analysis has been carried out to further characterize each stationary
point. Free energy values have been obtained from the electronic
self-consistent-field energy considering three contributions to the
Oxidation of Et4N[1(CN)(CO)3] with Cp2FePF6. A solution
of 0.030 g (0.0357 mmol) of Et4N[1(CN)(CO)3] in 10 mL of MeCN
was treated with a solution of 0.023 g (0.0714 mmol) of Cp2FePF6
in 5 mL of MeCN at -40 °C under a CO atmosphere. The solution
was warmed to room temperature and the product spectroscopically
1
identified as [Fe2(S2C2H4)(µ-CO)(CN)(CO)2(dppv)(NCMe)]+. H
NMR (MeCN-d3): δ 8.8-7.3 (m, 20H, PC6H5), 4.17 (s, 2H, PCH),
3.4 (m, 2H, SCH2), 3.18 (m, 2H, SCH2). 31P NMR (MeCN-d3): δ
75.3. IR (MeCN): νCN 2139, 2121 cm-1. νCO 2068, 2028, 1895
cm-1. ESI-MS: m/z 751 ([Fe2(S2C2H4)(µ-CO)(CN)(CO)2(dppv)-
(NCMe)]+).
Fe(S2C3H6)(CO)3(dppv)(PMe3), 2(CO)3(PMe3), and Its Oxida-
tion. The preparation followed that described above for 1(CO)3-
(PMe3). Yield: 0.29 g (90%). H NMR (CDCl2): δ 8.0-7.3 (m,
20H, PC6H5), 4.2 (s, 2H, PCH), 1.8 (m, 2H, SCH2), 1.5 (m, 2H,
SCH2), 1.36 (d, JP-H ) 8 Hz, 9H, PCH3), 1.2 (m, 2H, CH2CH2-
CH2). 31P NMR (CD2Cl2, 20 °C): δ 92.9, 26.9. 31P NMR (CD2-
Cl2, -90 °C): δ 101.6, 96.4, 91.7, 90.2, 84.8, 34.2, 28.3, 26.7. IR
(CH2Cl2): νCO 1943, 1893 cm-1. FD-MS: m/z 774.6 ([Fe(S2C3H6)-
(CO)3(dppv)(PMe3)]+).
total partition function (Q), namely, qtranslational, qrotational, qvibrational
,
under the assumption that Q may be written as the product of such
terms. Solvation effects (CH3CN; ꢀ ) 37.5) have been evaluated
according to the COSMO approach.
1
Acknowledgment. This work was supported by the
National Institutes of Health and the Petroleum Research
Fund. We thank Tere´sa Prussak-Wieckowska for assistance
with the crystallographic analyses.
When a solution of 0.075 g (0.097 mmol) of 2(CO)3(PMe3) in
10 mL of MeCN was treated with 0.064 g (0.194 mmol) of Cp2-
FePF6 in 5 mL of MeCN at -40 °C under a CO atmosphere, the
solution color changed from red to green. The IR spectrum indicated
[Fe2(S2C3H6)(µ-CO)(CO)2(PMe3)(dppv)(NCMe)]2+. IR (MeCN):
νCO 2055, 2009, 1922 cm-1. Upon warming of the solution to
20 °C, the IR spectrum revealed that the complex had decomposed.
X-ray Crystallography. Crystals were mounted to 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
Supporting Information Available: X-ray crystallographic data
for 1(CO)4, (Et4N)[1(CN)(CO)3], and 1(CO)3(PMe3), details for
DFT calculations, graphs for kinetics, and determination of the
activation parameters. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC0618706
(28) Becke, A. D. Phys. ReV. A: Gen. Phys. 1988, 38, 3098-3100. Perdew,
J. P. Phys. ReV. 1986, B33, 8822-8824.
(29) Schaefer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829-
5835.
(30) Klamt, A. J. Phys. Chem. 1996, 100, 3349; Klamt, A. J. Phys. Chem.
1995, 99, 2224.
1664 Inorganic Chemistry, Vol. 46, No. 5, 2007