Synthetic and structural studies on [Fe2(SR)2(CN)x (CO)6-x]x- as active site models for Fe-only hydrogenases

Article abstract of DOI:10.1021/ja016071v

A series of models for the active site (H-cluster) of the iron-only hydrogenase enzymes (Fe-only H₂-ases) were prepared. Treatment of MeCN solutions of Fe₂(SR)₂(CO)₆ with 2 equiv of Et₄NCN gave [Fe

Full text of DOI:10.1021/ja016071v

12518
J. Am. Chem. Soc. 2001, 123, 12518-12527
Synthetic and Structural Studies on [Fe2(SR)2(CN)x(CO)6-x]x- as
Active Site Models for Fe-Only Hydrogenases
Fre´de´ric Gloaguen, Joshua D. Lawrence, Michael Schmidt, Scott R. Wilson, and
Thomas B. Rauchfuss*
Contribution from the Department of Chemistry, UniVersity of Illinois at Urbana-Champaign,
Urbana, Illinois 61801
ReceiVed April 24, 2001
Abstract: A series of models for the active site (H-cluster) of the iron-only hydrogenase enzymes (Fe-only
H2-ases) were prepared. Treatment of MeCN solutions of Fe2(SR)2(CO)6 with 2 equiv of Et4NCN gave
[Fe2(SR)2(CN)2(CO)4]2- compounds. IR spectra of the dicyanides feature four νCO bands between 1965 and
1870 cm-1 and two νCN bands at 2077 and 2033 cm-1. For alkyl derivatives, both diequatorial and axial-
equatorial isomers were observed by NMR analysis. Also prepared were a series of dithiolate derivatives
(Et4N)2[Fe2(SR)2(CN)2(CO)4], where (SR)2 ) S(CH2)2S, S(CH2)3S. Reaction of Et4NCN with Fe2(S-t-Bu)2-
(CO)6 gave initially [Fe2(S-t-Bu)2(CN)2(CO)4]2-, which comproportionated to give [Fe2(S-t-Bu)2(CN)(CO)5]-.
The mechanism of the CN--for-CO substitution was probed as follows: (i) excess CN- with a 1:1 mixture of
Fe2(SMe)2(CO)6 and Fe2(SC6H4Me)2(CO)6 gave no mixed thiolates, (ii) treatment of Fe2(S2C3H6)(CO)6 with
Me3NO followed by Et4NCN gave (Et4N)[Fe2(S2C3H6)(CN)(CO)5], which is a well-behaved salt, (iii) treatment
of Fe2(S2C3H6)(CO)6 with Et4NCN in the presence of excess PMe3 gave (Et4N)[Fe2(S2C3H6)(CN)(CO)4(PMe3)]
much more rapidly than the reaction of PMe3 with (Et4N)[Fe2(S2C3H6)(CN)(CO)5], and (iv) a competition
experiment showed that Et4NCN reacts with Fe2(S2C3H6)(CO)6 more rapidly than with (Et4N)[Fe2(S2C3H6)-
(CN)(CO)5]. Salts of [Fe2(SR)2(CN)2(CO)4]2- (for (SR)2 ) (SMe)2 and S2C2H4) and the monocyanides
[Fe2(S2C3H6)(CN)(CO)5]- and [Fe2(S-t-Bu)2(CN)(CO)5]- were characterized crystallographically; in each case,
the Fe-CO distances were ∼10% shorter than the Fe-CN distances. The oxidation potentials for Fe2(S2C3H6)-
(CO)4L2 become milder for L ) CO, followed by MeNC, PMe3, and CN-; the range is ∼1.3 V. In water,
oxidation of [Fe2(S2C3H6)(CN)2(CO)4]2- occurs irreversibly at -0.12 V (Ag|AgCl) and is coupled to a second
oxidation.
Introduction
bears a structural similarity to the Reihlen hexacarbonyls, there
are seven ligands and the Fe-Fe bond length of 2.6 Å is ∼0.1
Å longer than it is in the iron thiolate tricarbonyl dimers.8 IR
studies also showed that the hydrogenases bind CO, which
explains the ability of CO to inhibit the enzyme. The inhibiting
CO binds in the position occupied by water in one of the
crystallized enzymes; this same site is assumed to be the position
for binding H2.9 The coordination chemistry of the Fe-only
hydrogenases is also reminiscent of that of the Fe-Ni hydro-
genases, which feature an Fe(CN)2(CO) center linked via a pair
of thiolato ligands to a nickel center.10 There is little or no
sequence homology between these two families of hydrogenase
enzymes; that is, they appear to have evolved independently.11
More recently still, Fontecilla-Camps has proposed that the Fe2
The dinuclear hexacarbonyls of the general formula Fe2(SR)2-
(CO)6 have been known for over 70 years. Reihlen first prepared
Fe2(SEt)2(CO)6 by the carbonylation of aqueous Fe(II) solutions
in the presence of NaSEt via a procedure that was modeled after
Pavel’s preparation of Fe2(SEt)2(NO)4.1 Hieber and Spacu
prepared Fe2(SR)2(CO)6 via the reaction of thiols and Fe(CO)5.2
In the 1960s, King separated Fe2(SMe)2(CO)6 into three isomers
demonstrating the stereochemical features associated with the
bridging mercaptide group.3 In the 1980s, Seyferth et al.
developed a synthesis of [Fe2(S)2(CO)6]2-, an analogue of
Roussin’s red anion [Fe2(S)2(NO)4]2-,4 and showed that this
carbonyl anion could be alkylated to give Fe2(SR)2(CO)6.5 In
part, because of their considerable stability, many examples of
Fe2(SR)2(CO)6 have been reported, and extensive research
describes the physical and chemical properties of such com-
pounds.
centers are linked by the azadithiolate SCH2NHxCH2S.7
A
synthetic analogue supporting this hypothesis has been reported
by us.12
Until recently, it was generally assumed that the essential
chemistry of Fe2(SR)2(CO)6 was mature. It was, therefore, a
great surprise when crystallographic investigations of the so-
called Fe-only hydrogenases revealed a six-iron “H-cluster”
containing an Fe2(µ-SR)2(CN)2(CO)3Ln core (L ) H2O and a
thiolate-linked Fe4S4(SR)4 cluster).6,7 Whereas the diiron core
(6) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science
1998, 282, 1853-1858. Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C.
E.; Fontecilla-Camps, J. C. Structure 1999, 7, 13-23.
(7) Nicolet, Y.; de Lacey, A. L.; Verne`de, X.; Fernandez, V. M.;
Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123,
1596-1601.
(8) Dahl, L. F.; Wei, C. H. Inorg. Chem. 1963, 2, 328-333.
(9) Lemon, B. J.; Peters, J. W. Biochemistry 1999, 38, 12969-12973.
De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, E. C.; Fernandez,
V. M. J. Am. Chem. Soc. 2000, 122, 11232-11233.
(10) Fontecilla-Camps, J. C.; Ragsdale, S. W. AdV. Inorg. Chem. 1999,
47, 283-333.
(1) Pavel, O. Ber. Dtsch. Chem. Ges. 1882, 15, 2600-2615.
(2) Hieber, W.; Spacu, P. Z. Anorg. Allgem. Chem. 1937, 233, 852-
864. Reihlen, H.; Gruhl, A.; Hessling, G. v. Liebigs Ann. Chem. 1929, 472,
268-287.
(3) King, R. B. J. Am. Chem. Soc. 1962, 84, 2460.
(4) Roussin, M. L. Ann. Chim. Phys. 1858, 52, 285-303.
(5) Seyferth, D.; Henderson, R. S.; Song, L. C. Organometallics 1982,
1, 125-133.
(11) Adams, M. W. W.; Stiefel, E. I. Curr. Opin. Chem. Biol. 2000, 4,
214-220.
(12) Lawrence, J. D.; Li, H.; Rauchfuss, T. B.; Rohmer, M.-M.; Be´nard,
M. Angew. Chem., Int. Ed. Engl. 2001, 40, 1768-1771.
10.1021/ja016071v CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/22/2001

x-
Studies on [Fe2(SR)2(CN)x(CO)6-x
]
J. Am. Chem. Soc., Vol. 123, No. 50, 2001 12519
Scheme 1
Chart 1. Geometric Isomers of [Fe2(SR)2(CN)2(CO)4]2-
three signals assigned to the diequatorial (idealized C2V) and
axial-equatorial (C1 symmetry) isomers, henceforth referred
to as e,e and a,e isomers. A similar situation applies to the
dicyanides derived from monothiolates. Ignoring the orientation
of the alkyl substituent, four geometric isomers of [Fe2(SR)2-
(CN)2(CO)4]2- are possible (Chart 1).
Although differences in the IR spectra of e,e and a,e isomers
could be observed in nonpolar solvents, no indication of
isomerism appeared in the νCO bands of the dicyano species in
MeCN. The observed bands are broader because of the increased
polarity of the solvent required to dissolve these species; this
solvent-induced broadening may mask minor differences in the
IR signals of the individual isomers. Both the a,e and e,e isomers
of (Et4N)2[Fe2(SMe)2(CN)2(CO)4] cocrystallize, and these con-
tributors were resolved in the refinement.15 The location of the
CN- ligand differs for the e,e and a,e isomers, indicative of
the small difference in energy between the various cyanide
positions. There is no obvious pattern for the variations in the
a,e/e,e ratio between the hexacarbonyl and the corresponding
dicyano compounds (see Experimental Section). It was previ-
ously proposed that axially oriented substituents are favored by
steric repulsions with the basal CO ligands and that equatorial
substituents are favored by bulky donor ligands on the Fe.16
Our previous communication described the synthesis and
structural characterization of (Et4N)2[Fe2(S2C3H6)(CN)2(CO)4].13
In the course of that work, a number of questions came to the
fore that we address in this report. These questions are, in turn,
(i) the effect of diverse dithiolates on the structural chemistry
of the dicyanides, (ii) the elusiveness of the monocyanide
derivatives, and (iii) the effect of CN--for-CO substitution on
the redox properties of these dimers. In this work, we examined
the products resulting from the reaction of cyanide with the
Reihlen dimers. Our intention was to generate a range of dicyano
derivatives that could be used in subsequent modeling experi-
ments. In recent work, we indeed show that members of this
class of compounds are effective catalysts for hydrogen evolu-
tion from protons.14
In addition to the SMe derivative, we also prepared the
corresponding thiocresolate derivative (Et4N)2[Fe2(SC6H4Me)2-
(CN)2(CO)4]. Again, the a,e/e,e ratio differs for the hexacarbonyl
and dicyano compounds. Compound 3 was employed in a test
for the involvement of monometallic intermediates in the
cyanide substitution reactions. Treatment of excess CN- with
a 1:1 mixture of Fe2(SMe)2(CO)6 and Fe2(SC6H4Me)2(CO)6 gave
only (Et4N)2[Fe2(SMe)2(CN)2(CO)4] and (Et4N)2[Fe2(SC6H4-
Me)2(CN)2(CO)4]. No 1H NMR signals were observed that could
be assigned to the mixed thiolato derivatives.
Results
Synthesis of [Fe2(SR)2(CN)2(CO)4]2-. The deep red [Fe2(SR)2-
(CN)2(CO)4]2- derivatives were prepared by treatment of MeCN
solutions of Fe2(SR)2(CO)6 with 2 equiv of Et4NCN. IR and
1H NMR spectra indicated total consumption of the starting
hexacarbonyl in 30-60 min at room temperature. The dicya-
nides are unreactive toward further equivalents of cyanide
(Scheme 1).
IR spectra of the dicyanides feature three bands and a shoulder
between 1965 and 1870 cm-1 assigned to νCO; we also observe
bands at 2077 and 2033 cm-1 for νCN. IR spectra of all dicyano
species are quite similar, with a variation of no more than a
few wavenumbers between derivatives. As originally demon-
strated by King many years ago for the SMe derivative,3 the
1H NMR spectrum of Fe2(SR)2(CO)6 (R ) alkyl) consists of
Because structural studies on the H-cluster indicate a chelating
dithiolate ligand, we examined the cyanide substitution of a
variety of chelating dithiolato derivatives Fe2(µ-SXS′)(CO)6 (X
) CR2, C2H4, C3H6). We were unable to prepare cyanide-
substituted derivatives of 1,1-dithiolates, Fe2(S2CR2)(CO)6,
including methanedithiolate17 and 2,2-adamantanedithiolate.18
These hexacarbonyl derivatives react with Et4NCN, but NMR
analysis suggests that side reactions occur at the organosulfur
ligands, possibly involving C-S bond cleavage. Furthermore,
(15) Contakes, S. M. Synthesis of NoVel Organometallic Cyanometalates
and Phthalocyanines. Ph.D. Dissertation, University of Illinois, Urbana,
IL, 2001.
(16) Maresca, L.; Greggio, F.; Sbrignadello, G.; Bor, G. Inorg. Chim.
Acta 1971, 5, 667-674.
(13) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc.
1999, 121, 9736-9737.
(14) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 9476-9477.

12520 J. Am. Chem. Soc., Vol. 123, No. 50, 2001
Gloaguen et al.
Figure 1. Structure of the dianion in (Et4N)2[Fe2(S2C2H4)(CN)2(CO)4]
(3) with the thermal ellipsoids set at the 50% probability level. Selected
distances (Å) and angles (deg): Fe(1)-Fe(2), 2.5114(12); Fe(1)-C(1),
1.924(6); Fe(1)-C(2), 1.759(7); Fe(1)-C(3), 1.762(6); Fe(2)-C(4),
1.944(7); Fe(2)-C(5), 1.716(7); Fe(2)-C(6), 1.753(6); Fe(1)-S(1),
2.2446(17); Fe(1)-S(2), 2.2451(17); Fe(2)-S(1), 2.2737(18); Fe(2)-
S(2), 2.2652(16); S(1)-Fe(1)-S(2), 80.68(6); S(1)-Fe(2)-S(2), 79.63-
(6).
Figure 2. Voltammograms (200 mV s-1) of Fe2(S2C3H6)(CO)6 (a),
Fe2(S2C3H6)(CNMe)2(CO)4 (b), (Et4N)[Fe2(S2C3H6)(CN)(CO)5] (c), and
(Et4N)2[Fe2(S2C3H6)(CN)2(CO)4] (d) in MeCN-Bu4NPF6 at a glassy
carbon electrode. Anodic currents are positive.
Table 1. Redox Potentials of Fe2(S2C3H6)(CO)6-xLx (L ) CO,
PMe3, CNMe, CN-). Potentials Are Quoted against the Ag|AgCl
Reference Electrode (∼-0.2 V vs NHE)
compound
Ered1/V
Eox1/V
the benzenedithiolate Fe2(S2C6H4)(CO)6 was found to react with
Et4NCN resulting in redox reactions; these results are reported
separately.19
a
Fe2(S2C3H6)(CO)6
Fe2(S2C3H6)(CO)6
-1.25c
-1.16c
-1.37d
-1.64d
-1.73d
-1.86d
-2.14d
-2.28d
1.2d
b
Fe2(S2C3H6)(CO)5(CNMe)b
Fe2(S2C3H6)(CO)4(CNMe)2
[Fe2(S2C3H6)(CN)(CO)5]-b
Fe2(S2C3H6)(CO)4(PMe3)b
1.01d
0.65d
The solid-state structure of the 1,2-ethanedithiolate (Et4N)2-
[Fe2(S2C2H4)(CN)2(CO)4] (CN ) 12CN, 3, CN ) 13CN, 3a) is
nearly identical to that of the previously reported propanedithi-
olate derivative,13 with the CN ligands positioned for the C1
isomer (Figure 1). The most significant difference between the
two chelating dithiolato derivatives is the contraction of the
S-Fe-S angles by approximately 5° in the 1,2-ethanedithiolate.
This S-Fe-S angle contracts by an additional 8° in 1,1-
dithiolate Fe2[S2CHCH2C(O)CH3](CO)6.20
The dynamic properties of the new complexes were checked
by variable temperature NMR spectroscopy. It was already
known that species of the type Fe2(SR)2(CO)6 exist as isomers
on the basis of the relative orientation of the R groups, which
can be axial or equatorial, and Huttner had demonstrated that
the backbone of chelating dithiolate derivatives, for example,
Fe2(S2CnH2n)(CO)6, undergo ring refolding on the NMR time
scale.21 Fe2(S2C3H6)(CO)6 (1) shows a single CO resonance at
25 °C that splits into two singlets with intensity 2:1 at -80 °C,
consistent with the solid-state structure. In the room temperature
13C NMR spectrum of the dicyano derivatives, the CO and CN-
ligands appear as singlets because of a low-energy turnstile
interchange of the CO and CN- sites. The 13C NMR spectrum
of 13CN-enriched (30%) [Fe2(S2C2H4)(CN)2(CO)4]2- in the CN
region consisted of four singlets at -80 °C with rough
integrations of 1:3:3:2. On the basis of the number of isomers
for the [Fe2(SR)2(CN)2(CO)4]2- core, a maximum of five CN
signals are expected. A similar pattern is seen in the 13C NMR
spectrum of 2a, that is, four CN signals at low temperatures. In
b
0.57d
0.24c
[Fe2(S2C3H6)(CN)(CO)4(PMe3)]-b
[Fe2(S2C3H6)(CN)2(CO)4]2-b
0.05 d
-0.08, 0.10d
a THF-Bu4NPF6. b MeCN-Bu4NPF6. c Reversible. d Irreversible.
the case of 2a, we conclude that the orientation of the
trimethylene group has no effect on the 13C NMR shifts of the
CN signals.
Redox Properties. Because the hydrogenase enzymes exist
in at least two oxidation states,22 we were interested in probing
the redox properties of the dicyano species as well as related
derivatives. To facilitate comparisons, we focused especially
on the 1,3-propanedithiolate derivatives (Figure 2, Table 1).
Cyclic voltammetry (CV) measurements indicate that the first
reduction of Fe2(S2C3H6)(CO)6-xLx (x ) 1, 2; L ) CNMe,
PMe3, CN-) is irreversible, whereas the first reduction of the
hexacarbonyl complex is reversible. Hexacarbonyl 1 undergoes
irreversible oxidation at Ep ) 1.21 V. This oxidation reaction
involves two electrons on the basis of a comparison of ip values
red1
versus the first reduction (E1/2
) -1.16 V). Oxidation of 1
generates a species that undergoes irreversible reduction at
-0.98 V, as detected on the reverse scan. In other words,
oxidation of 1 follows an ECi mechanism whereby the oxidation
product is reducible at -0.98 V. Upon controlled-potential
electrolysis, oxidation of 1 at 1.21 V requires ∼1.8 F mol-1
.
The voltammogram recorded after electrolysis does not show
the -0.98 V reduction peak, indicative of the instability of the
oxidized species. Indeed, the IR spectrum of the electrolyzed
solution shows no νCO bands. The species generated by
reductionof a solution of 1 were found to be also unstable on
the time scale of electrolysis.
(17) Seyferth, D.; Womack, G. B.; Gallagher, M. K.; Cowie, M.; Hames,
B. W.; Fackler, J. P., Jr.; Mazany, A. M. Organometallics 1987, 6, 283-
294.
(18) Alper, H.; Chan, A. S. K. Inorg. Chem. 1974, 13, 232-236. We
confirmed the structure of Fe2(S2-2,2-C10H14)(CO)6: Linck, R. C. Unpub-
lished results.
(19) Rauchfuss, T. B.; Contakes, S. M.; Hsu, S. C. N.; Reynolds, M.
A.; Wilson, S. R. J. Am. Chem. Soc. 2001, 123, 6933-6934.
(20) Seyferth, D.; Womack, G. B.; Song, L. C.; Cowie, M.; Hames, B.
W. Organometallics 1983, 2, 928-930.
(21) Winter, A.; Zsolnai, L.; Huttner, G. Z. Naturforsch. 1982, 37b,
1430-1436.
Study of the redox properties of (Et4N)2[Fe2(S2C3H6)(CN)2-
(CO)4] (2) at anodic potential was difficult because a passivating
(22) Pierik, A. J.; Hulstein, M.; Hagen, W. R.; Albracht, S. P. J. Eur. J.
Biochem. 1998, 258, 572-578.

x-
Studies on [Fe2(SR)2(CN)x(CO)6-x
]
J. Am. Chem. Soc., Vol. 123, No. 50, 2001 12521
blue film is deposited on the electrode surface at E > 1.0 V,
necessitating polishing of the electrode between experiments.
Electropolymerization of 1 upon oxidation was confirmed by
controlled-potential electrolysis. The primary oxidation occurs
at Epox1 ) -0.08 (the oxidation peak shows a shoulder at 0.10
V), and the primary reduction occurs at Epred1 ) -2.28 V. The
observed peak potential value agrees with that previously
reported by Le Cloirec et al.23 The first oxidation of 2 is assigned
to a one-electron process in comparison of the peak current with
that of {Ni[S2C2(CN)2]2}2-, an internal standard whose diffusion
coefficient is expected to be comparable to that of 2. As
previously reported,23 2 is both soluble and stable in aqueous
electrolyte. No deposition of insoluble species was observed
upon oxidation of 2 in aqueous media, suggesting that the
oxidation product is different from that generated in MeCN.
At pH ) 5 (phthalate buffer), the first (irreversible) oxidation
occurs at -0.12 V (∼+0.38 V vs RHE), indicating that 2 cannot
directly reduce protons in aqueous electrolyte at this pH. No
catalytic behavior of 2 toward proton reduction was observed.
Oxidation of THF solutions of bis(isocyanide) 4 appears to
produce insoluble species causing passivation of the electrode.
In MeCN solution, however, the CV of 4 shows an oxidation
peak (0.65 V), less anodic than for the parent complex 1,
consistent with the superior donor properties of CNMe versus
CO ligands.24 Similar redox behavior was reported by Poilblanc
et al.,25 for Fe2(SMe)2(CO)4(PMe3)2.
Scheme 2
These experiments were motivated by the instability reported26
for Na[Fe2(S2C3H6)(CN)(CO)5]. We first developed routes to
the desired monocyanide salts using the decarbonylation agent
Me3NO (Scheme 2). Initial experiments confirmed that solutions
of 1 and PPh3 react at room temperature only upon addition of
Me3NO. The product, Fe2(S2C3H6)(CO)5(PPh3), was identified
by its characteristic IR spectrum.27 We found that the PPh3 could
be added well after the decarbonylation agent; this experiment
implies that the intermediate, possibly Fe2(S2C3H6)(CO)5-
(MeCN) or Fe2(S2C3H6)(CO)5(NMe3), has significant lifetime
in solution (Scheme 2). In fact, the IR spectrum of such solutions
demonstrated the absence of 1 and the formation of a pattern
typical of Fe2(S2C3H6)(CO)5L. Attempts to isolate this complex
were unsuccessful.
Oxidation of (Et4N)[Fe2(S2C3H6)(CN)(CO)5] (5) did not
produce any insoluble species blocking the electrode surface,
in contrast to the behavior of 2. The first oxidation of 5 occurs
at Epox1 ) 0.57 V, and the first reduction at Epred1 ) -1.73 V,
indicating that, as expected, 5 is more difficult to oxidize and
Salts of [Fe2(S2C3H6)(CN)(CO)5]- had been previously
obtained via the reaction of NaN(tms)2 with Fe2(S2C3H6)-
(CO)6,26,28 a method that obviates the use of free CN-. The
Na+ salt is apparently unstable, even in the solid state under an
inert atmosphere.26 The corresponding Et4N+ salt was, however,
completely stable even in solution for days. Analytically pure
samples of (Et4N)[Fe2(S2C3H6)(CN)(CO)5] (5) were obtained
via the reaction of Fe2(S2C3H6)(CO)6 with Me3NO followed by
addition of Et4NCN (Scheme 2). The structure of 5 was
confirmed by single-crystal X-ray diffraction (Figure 3). The
1H NMR spectrum of this monocyanide in the (CH2)3 region
consists of two broad signals at room temperature that split into
four multiplets at -40 °C. The overall pattern is consistent with
four types of methylene protons. The room temperature NMR
data differ slightly from those described previously in terms of
the coupling pattern,26 but this difference may be due to ion
pairing effects because alkali metal cations are well-known to
bind to MCN (e.g., Na2Fe(CN)4(CO)2(dmf)4).29 In the IR
spectrum of 5, all νCO bands are shifted toward lower energies
(vs 1) indicating that the cyanide influences both Fe centers
strongly. Similar electronic effects have been previously noted
for the phosphine derivatives.16,30
ox1
easier to reduce than dicyanide 2 (see above). The ratio ip
ip
of 5 are one- and two-electron processes, respectively.
/
red1
is ∼0.5, suggesting that the oxidation and the reduction
Poilblanc et al. have shown that the complexes of general
formula Fe2(SR)2(CO)4L2 are more difficult to reduce and easier
to oxidize when CO is replaced by donor ligands such as tertiary
phosphines.25 In the present work, we found that the reduction
becomes more cathodic, and the oxidation less anodic in the
following sequence: 1 > Fe2(S2C3H6)(CO)5(CNMe.) > 4 > 5
> Fe2(S2C3H6)(CO)4(PMe3)2 > 2. With respect to hexacarbonyl
1, the oxidation and reduction of 2 occur at potentials that are
∼1.3 V less positive and 1.2 V more negative, respectively.
This suggests that, beyond a possible role in hydrogen-bonding,
the cyanide and phosphine ligands modify the redox properties
of 1 in a similar way, that is, by acting as a strong donor ligand.
According to Le Cloirec et al.,23 the hydrogenase active site
(“H-cluster”) would be easier to oxidize than 2 to drive proton
reduction. Direct redox is, however, not the only mechanism
to consider. Another possibility involves a chemical reaction
between the bimetallic subunit and the protons to form a metal
hydride complex, which upon subsequent reduction could
produce hydrogen. Indeed, other complexes of the type Fe2-
(SR)2(CO)4(PMe3)2, which exhibit similar redox properties to
those of 2, are well-known to protonate at the Fe-Fe bond.25b
Synthesis of the Monocyano Derivatives of Fe2(S2C3H6)-
(CO)6. In this part of the study, we explored the preparation of
monosubstituted derivatives of the type [Fe2(S2C3H6)(CN)(CO)5]-.
Having established the considerable stability of 5, we were
perplexed by our inability to directly synthesize this species
from the reaction of 1 equiv of CN- and Fe2(S2C3H6)(CO)6.
For example, the activation parameters reported26 for the reaction
of Et4NCN with hexacarbonyl 1 and with 5 suggested that it
should be possible to generate at least some of the monocyanide
(23) Le Cloirec, A.; Best, S. P.; Borg, S.; Davies, S. C.; Evans, D. J.;
Hughes, D. L.; Pickett, C. J. Chem. Commun. 1999, 2285-2286.
(24) Madec, P.; Muir, K. W.; Pe´tillon, F. Y.; Rumin, R.; Scaon, Y.;
Schollhammer, P.; Talarmin, J. J. Chem. Soc., Dalton Trans. 1999, 2371-
2383.
(25) (a) Mathieu, R.; Poilblanc, R.; Lemoine, P.; Gross, M. J. Organomet.
Chem. 1979, 165, 243-252. (b) Arabi, M. S.; Mathieu, R.; Poilblanc, R. J.
Organomet. Chem. 1979, 177, 199-209.
(26) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 2001, 123, 3268-3278.
(27) Ellgen, P. C.; Gerlach, J. N. Inorg. Chem. 1973, 12, 2526-2532.
(28) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. Angew. Chem., Int. Ed. Engl. 1999, 38, 3178-3180.
(29) Jiang, J.; Koch, S. A. Angew Chem., Int. Ed. Engl. 2001, 40, 2629-
2631.

12522 J. Am. Chem. Soc., Vol. 123, No. 50, 2001
Gloaguen et al.
Figure 3. Structure of the anion in (Et4N)[Fe2(S2C3H6)(CN)(CO)5] (5)
with the thermal ellipsoids set at the 50% probability level. Selected
distances (Å) and angles (deg): Fe(1)-Fe(2), 2.5291(6); Fe(1)-C(1),
1.923(3); Fe(1)-C(2), 1.771(3); Fe(1)-C(3), 1.773(3); Fe(2)-C(4),
1.768(3); Fe(2)-C(5), 1.791(3); Fe(2)-C(6), 1.797(3); Fe(1)-S(1),
2.2520(8); Fe(1)-S(2), 2.2518(8); Fe(2)-S(1), 2.2678(8); Fe(2)-S(2),
2.2698(8); S(1)-Fe(1)-S(2), 85.80(3); S(1)-Fe(2)-S(2), 85.01(3).
from the room-temperature reaction of 1 and Et4NCN. Careful
study revealed that ∼15% yields of 5 can be realized by direct
reaction with 1 equiv of Et4NCN in both concentrated and dilute
MeCN solutions of 1 (0.1-0.01 M).
Figure 4. 500 MHz NMR spectra of the reaction of Fe2(S-t-Bu)2(CO)6
with 1 equiv of Et4NCN in CD3CN. Open circles (O) correspond to
Fe2(S-t-Bu)2(CO)6, open squares (0) correspond to [Fe2(S-t-Bu)2(CN)-
(CO)5]- (6), and filled diamonds (() correspond to [Fe2(S-t-Bu)2(CN)2-
Synthesis of the Monocyano Derivatives of Fe2(S2-t-Bu)-
(CO)6. Treatment of Fe2(S-t-Bu)2(CO)6 with an excess of
cyanide led exclusively to the dicyano species, which was
identified by its characteristic IR spectrum in the νCO re-
gion.13,23,28 Furthermore, the 1H NMR spectrum of the dicyanide
consists of three signals for the t-Bu group in a ratio of 1:1:
0.28 indicative of both e,e and a,e isomers as seen for Fe2(S-
t-Bu)2(CO)6.31 Solutions of [Fe2(S-t-Bu)2(CN)2(CO)4]2- are,
however, unstable, decomposing over a period of days leaving
free t-BuSH as the only identifiable product. The reaction of
Fe2(S-t-Bu)2(CO)6 with 1 equiv of Ph4PCN affords two new
products, the monocyano (Ph4P)[Fe2(S-t-Bu)2(CN)(CO)5] (6b)
(see later) and the dicyanide (Figure 4). Over 2 days in solution,
the signals due to the dicyanide and unreacted hexacarbonyl
decrease in intensity concomitant with the increase in intensity
of the signals for the monocyanide. Some free t-BuSH is also
observed. These observations led us to speculate that the
monocyanide arises via decomposition of [Fe2(S-t-Bu)2(CN)2-
(CO)4]2- in the presence of Fe2(S-t-Bu)2(CO)6, which serves
as a trapping agent for cyanide (eqs 1, 2).
(CO)4]2-
.
NMR spectroscopy. The IR spectrum of 6b consists of four
bands, one in the CN region and three in the CO region at 2086
cm-1 and at 2023, 1969, 1939 cm-1, respectively. Crystals of
K(18-crown-6)[Fe2(S-t-Bu)2(CN)(CO)5] (6c) were also obtained,
and these were examined by single-crystal X-ray diffraction
(Figure 5). The solid-state IR spectra of salts 6b and 6c in the
νCX (X ) N, O) region are similar. In the crystal, the anion
exists as two enantiomeric units with 89/11% occupancy,
situated such that the N of the cyanides are at approximately
the same position in the crystal and the Fe-Fe bonds are almost
parallel. Otherwise, the structure features the expected five CO
ligands indicated by short Fe-CO distances (1.75 Å) and longer
Fe-C(N) distances (1.92 Å). The CN group is cis to the Fe-
Fe bond in both isomers. The CN terminus is coordinated to
K+ (rN-K ) 2.70 Å),32 which is otherwise bound to the six
oxygen atoms of the crown ether.
Synthesis of [Fe2(S2C3H6)(CN)(CO)4(PR3)]-: Further In-
sights into the Substitution Mechanism. The logical sugges-
tion26,28 that [Fe2(S2C3H6)(CN)(CO)5]- is an intermediate in the
formation of [Fe2(S2C3H6)(CN)2(CO)4]2- from the hexacarbonyl
accords with the finding that isolated samples of [Fe2(S2C3H6)-
(CN)(CO)5]- react with Et4NCN to give [Fe2(S2C3H6)(CN)2-
(CO)4]2-. This result is necessary but not sufficient evidence
for the intermediacy of the pentacarbonyl monocyanide. Further
evidence, presented below, points to an alternative pathway for
the substitution process; the new evidence was obtained through
trapping studies.
Fe2(S-t-Bu)2(CO)6 + CN- f
1/2[Fe2(S-t-Bu)2(CN)2(CO)4]2- (1)
[Fe2(S-t-Bu)2(CN)2(CO)4]2- + Fe2(S-t-Bu)2(CO)6 f
2[Fe2(S-t-Bu)2(CN)(CO)5]-
(2)
(6)
The PPh4+ salt of [Fe2(S-t-Bu)2(CN)(CO)5]- was isolated in
nearly analytical purity and was further characterized by IR and
(30) de Beer, J. A.; Haines, R. J.; Greatrex, R.; Greenwood, N. N. J.
Chem. Soc. A 1971, 21, 3271-3282. de Beer, J. A.; Haines, R. J. J.
Organomet. Chem. 1972, 37, 173-188. de Beer, J. A.; Haines, R. J. J.
Organomet. Chem. 1972, 36, 297-313. de Beer, J. A.; Haines, R. J.;
Greatrex, R.; Greenwood, N. N. J. Organomet. Chem. 1971, 27, C33-
C35.
When Fe2(S2C3H6)(CO)6 is treated with Et4NCN in the
presence of a 4-fold excess of PMe3, we obtained ∼90% yield
of analytically pure (Et4N)[Fe2(S2C3H6)(CN)(CO)4(PMe3)] (7).
(31) Natile, G.; Maresca, L.; Bor, G. Inorg. Chem. Acta 1977, 23, 37-
42.
(32) Neier, R.; Trojanowski, C.; Mattes, R. J. Chem. Soc., Dalton Trans.
1995, 2521-2528.

x-
Studies on [Fe2(SR)2(CN)x(CO)6-x
]
J. Am. Chem. Soc., Vol. 123, No. 50, 2001 12523
Figure 5. Structure of the major enantiomer of the anion in [K(18-
crown-6)][Fe2(S-t-Bu)2(CN)(CO)5] (6c) with the thermal ellipsoids set
at the 50% probability level. Selected distances (Å) and angles (deg):
Fe(1)-Fe(2), 2.506(2); Fe(1)-C(11), 1.912(6); Fe(1)-C(12), 1.736-
(7); Fe(1)-C(13), 1.735(7); Fe(2)-C(21), 1.730(6); Fe(2)-C(22),
1.777(6); Fe(2)-C(23), 1.728(7); Fe(1)-S(1), 2.290(2); Fe(1)-S(2),
2.251(2); Fe(2)-S(1), 2.285(2); Fe(2)-S(2), 2.264(2); S(1)-Fe(1)-
S(2), 79.18(7); S(1)-Fe(2)-S(2), 79.03(8).
This new compound was fully characterized spectroscopically
as well as by single crystal X-ray diffraction (Figure 6). Two
independent diiron units are found in the unit cell. One of the
anions is structurally similar to {Fe2[(S2(CH2)n)](CN)2(CO)4}2-
where the basal CN- has been replaced by PMe3.33 In the other
anion in the asymmetric unit, both cyanide and phosphine
ligands are basal, being cis to the Fe-Fe bond and trans to the
same sulfur atom.
Figure 6. Structure of the anions in (Et4N)[Fe2(S2C3H6)(CN)(CO)4-
(PMe3)] (7) with the thermal ellipsoids set at the 50% probability level.
Selected distances (Å) and angles (deg): (Top) Fe(1)-Fe(2), 2.5365-
(11); Fe(1)-C(1), 1.919(6); Fe(1)-C(2), 1.742(6); Fe(1)-C(3), 1.741-
(6); Fe(2)-P(1), 2.2061(16); Fe(2)-C(4), 1.734(6); Fe(2)-C(5),
1.742(6); Fe(1)-S(1), 2.2708(15); Fe(1)-S(2), 2.2629(15); Fe(2)-S(1),
2.2740(15); Fe(2)-S(2), 2.2561(15); S(1)-Fe(1)-S(2), 85.25(5); S(1)-
Fe(2)-S(2), 85.33(5). (Bottom) Fe(3)-Fe(4), 2.5593(11); Fe(3)-C(12),
1.920(6); Fe(3)-C(13), 1.766(6); Fe(3)-C(14), 1.747(6); Fe(4)-P(2),
2.2055(17); Fe(4)-C(15), 1.763(6); Fe(4)-C(16), 1.747(7); Fe(3)-
S(3), 2.2727(15); Fe(3)-S(4), 2.2560(15); Fe(4)-S(3), 2.2635(15); Fe-
(4)-S(4), 2.2538(16); S(3)-Fe(3)-S(4), 84.67(5); S(3)-Fe(4)-S(4),
84.93(5).
The mechanistic significance of the synthesis of [Fe2(S2C3-
H6)(CN)(CO)4(PMe3)]- is clarified by the associated control
experiments (Scheme 3):
(i) Purified samples of [Fe2(S2C3H6)(CN)(CO)5]- and PMe3
reactonlysluggishly;forexample,[Fe2(S2C3H6)(CN)(CO)4(PMe3)]-
is formed in low yield (<5%) after 4 h at room temperature,
whereas our synthesis of 5 was conducted at -40 °C.
(ii) Solutions of Fe2(S2C3H6)(CO)6 do not react with excess
PMe3 in 30 min at -10 °C. In contrast, the reaction of Fe2-
(S2C3H6)(CO)6 with Et4NCN/PMe3 is complete after 30 min at
-10 °C.
Collectively, our observations show that Fe2(S2C3H6)(CO)6
reacts with CN- to produce an intermediate that is more reactive
toward PMe3 than is [Fe2(S2C3H6)(CN)(CO)5]-. Qualitatively
similar results were obtained when PPh3 was employed in place
of PMe3.
Scheme 3
Further evidence that 5 is not an intermediate in the formation
of dicyano species 2 from 1 was obtained through a second
competition experiment. As a preliminary control, we first
showed that a mixture of hexacarbonyl 1 and monocyanide 5
is stable in MeCN solution (Figure 7, spectrum d). Upon
treatment of this solution with 2 equiv of Et4NCN, 1 completely
disappeared, but monocyanide 5 remained. The changes are most
readily indicated by the disappearance of the νCO band at ∼2000
cm-1 and the persistence of the νCN band at approximately
∼2100 cm-1 for the monocyanide 5. Obviously, the monocyano
(33) In contrast, Fe2(SMe)2(CO)4(PMe3)2 and Fe2(SCH2CH2OH)2(CO)4-
(PPh3)2 have both phosphines trans to the Fe-Fe bond in the solid state:
(a) Le Borgne, G.; Grandjean, D.; Mathieu, R.; Poilblanc, R. J. Organomet.
Chem. 1977, 131, 429-438. (b) Song, L.; Wang, R.; Hu, Q.; Wang, H.
Jiegou Huaxue 1990, 9, 217-220.
species 5 reacts much more slowly with free cyanide than does
the hexacarbonyl species 1.

12524 J. Am. Chem. Soc., Vol. 123, No. 50, 2001
Gloaguen et al.
Figure 8. IR spectra of model compounds (a) (NEt4)[Fe2(S2C3H6)-
(CN)(CO)5] (5), (b) (Et4N)[Fe2(S2C3H6)(CN)(CO)4(PMe3)] (7), (c)
(Et4N)2[Fe2(S2C2H4)(CN)2(CO)4], (3) and (d) the reduced active enzyme
from D. desulfuricans.9,22 Spectra a and b were obtained on THF
solutions, and spectrum c was obtained on a MeCN solution.
Figure 7. IR spectra (MeCN solution): (a) Fe2(S2C3H6)(CO)6, (b)
(Et4N)[Fe2(S2C3H6)(CN)(CO)5], (c) (Et4N)2[Fe2(S2C3H6)(CN)2(CO)4],
(d) Fe2(S2C3H6)(CO)6 and (Et4N)[Fe2(S2C3H6)(CN)(CO)5], (e) product
of the reaction of Fe2(S2C3H6)(CO)6, (Et4N)[Fe2(S2C3H6)(CN)(CO)5],
and 2 equiv of Et4NCN.
An associative substitution mechanism is particularly interest-
ing because it implies an Fe2(SR)2L7 intermediate, the coordina-
tion number for which matches that for the Fe-only H2-ase
enzyme (in both the CO-inhibited form and the resting form
which has a terminal water ligand9) in the Clostridium pasteur-
ianum as well as in the DesulfoVibrio desulfuricans Fe-only
H2-ase enzyme (where it is suggested that a terminal hydride
or H2 occupies one coordination site7). The observation of
associative kinetics by Darensbourg et al.26 is consistent with
earlier studies on the substitution of Fe2(SMe)2(CO)6 by
phosphine ligands.16,27 What is surprising is the elusiveness of
[Fe2(S2C3H6)(CN)(CO)5]- from the reaction of Fe2(SR)2(CO)6
with 1 equiv of cyanide, which obviously indicates that
substitution by the second cyanide is competitive with the
binding of the first cyanide. Normally in CO substitution
reactions, the second CO is replaced far more slowly than the
first. For example, the reaction of Cp2Mo2(SMe)2(CO)2 with
CN- affords only the monocyanide [Cp2Mo2(SMe)2(CN)(CO)]-.34
The anomalously high disubstitution rate is consistent with our
finding that mixtures of CN- and PMe3 effect rapid conversion
of the hexacarbonyl to [Fe2(S2C3H6)(CN)(CO)4(PMe3)]-. We
propose that the unusual reactivity of Fe2(S2C3H6)(CO)6 toward
cyanide and a second donor ligand (CN- or PR3) is due to the
stabilization of the electrophilic adduct [Fe2(S2C3H6)(CN)-
(µ-CO)(CO)5]- (Scheme 4). Not only does this mechanism
accord with the associative kinetics, it also accommodates the
detection of a µ-CO intermediate by Pickett et al.35 (Scheme
4). The stabilization of [Fe2(S2C3H6)(CN)(µ-CO)(CO)5]- may
be attributed to the small size and high basicity of cyanide,
Conclusions
A range of derivatives of the type [Fe2(SR)2(CN)2(CO)4]2-
can be prepared by CN--for-CO substitution reactions. These
syntheses occur for alkyl- and arylthiolato derivatives and for
alkyldithiolates. The products are stable and have been fully
characterized by X-ray diffraction for three derivatives (S2C2H4,
S2C3H6, and (SCH2)2NMe12). IR spectra of the dicyanides and
the reduced form of the enzymes9,22 are similar (Figure 8), which
is important because νCO bands are quite sensitive to the electron
density at the metal center. For example, Liaw et al. have shown
that ferrous thiolato carbonyls have significantly higher fre-
quency νCO bands. In other words, our data suggest that the
reduced enzyme features Fe(I) centers.
Crystallographic studies and IR data indicate that the dicya-
nide derivatives are electronically very similar for a variety of
dithiolates. The variation among the solid-state structures implies
that steric or crystal packing forces control the stereochemistry
of the CO, CN-, and PMe3 ligands, especially considering the
fluxionality of these ligands in solution. Independent of any
electronic role, the dithiolate could have a functional role. It
has recently been suggested7 that the Fe atoms are bridged by
SCH2NHCH2S (or N-protonated derivatives thereof) where the
amine base participates as a hydrogen relay agent in the
heterolytic activation and formation of H2 ligands.
From the synthetic perspective, the unusual aspect of the
dicyanides is their ready formation even with substoichiometric
amounts of cyanide, that is, addition of 1 equiv of Et4NCN to
a solution of Fe2(SR)2(CO)6 gives primarily [Fe2(SR)2(CN)2-
(CO)4]2-. Competition experiments demonstrate that hexacar-
bonyl Fe2(S2C3H6)(CO)6 reacts with cyanide more rapidly than
does monocyanide [Fe2(S2C3H6)(CN)(CO)5]-.
(34) Absasq, M.-L.; Pe´tillon, F. Y.; Talarmin, J. J. Chem. Soc., Chem.
Commun. 1994, 2191-2192.
(35) Razavet, M.; Davies, S. C.; Hughes, D. L.; Pickett, C. J. Chem.
Commun. 2001, 847-848.

x-
Studies on [Fe2(SR)2(CN)x(CO)6-x
]
J. Am. Chem. Soc., Vol. 123, No. 50, 2001 12525
(SCH3)2(CO)6,3 Fe2(S-4-C6H4CH3)2(CO)6,39 Fe2(S2CH2)(CO)6,17 Fe2(S2-
2,2-C10H14)(CO)6,18 and Fe2(S-t-Bu)2(CO)6.40 The procedure is as
follows: 1.5 g of Fe3CO12 was suspended in 100 mL of toluene followed
by the addition of 1 equiv of the disulfide (R ) Me, t-Bu) or dithiol
(R ) (CH2)2, (CH2)3) or 2 equiv of monothiol (R ) S-4-C6H4CH3).
The reaction mixture was stirred at 70 °C until its color changed from
deep green to dark red. The reaction mixture was allowed to cool to
room temperature and filtered. The red filtrate was evaporated to dryness
under vacuum, and the residue was extracted with 3 × 10 mL of
hexanes. The combined extracts were reduced under vacuum to ∼5
mL and cooled to -20 °C to give red crystals of Fe2(SR)2(CO)6.
Yield: 75-85%. Fe2(S2C3H6)(CO)4(PMe3)2 was prepared in the usual
way by the thermal reaction of Fe2(S2C3H6)(CO)6 and PMe3.33a
Electrochemistry. The cyclic voltammetry experiments were con-
ducted in a ∼10-mL one-compartment glass cell. The counter electrode
was a Pt wire, the reference electrode a Ag|AgCl, KCl saturated
electrode, and the working electrode either a glassy carbon or a Pt disk
(3 or 2 mm in diameter, respectively). The concentration in electroactive
compound was ∼4 mM. Controlled-potential electrolysis was performed
at a Pt working electrode in a two-compartment cell.
Scheme 4
Crystallography. Crystals were mounted to a thin glass fiber using
oil (Paratone-N, Exxon). Data 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). Data were
collected at 198 K on a Siemens CCD diffractometer. Crystal and
refinement details are given in Table 2. The structures were solved
using SHELXTL by direct methods; correct atomic positions were
deduced from an E map or by an unweighted difference Fourier
synthesis. H atom Us were assigned as 1.2 times the Ueqs of adjacent
C atoms. Non-H atoms were refined with anisotropic thermal coef-
ficients. Successful convergence of the full-matrix least-squares refine-
ment of F2 was indicated by the maximum shift/error for the last cycle.
(Et4N)2[Fe2(S2C3H6)(CN)2(CO)4] (2). A solution of 1.00 g (2.59
mmol) of Fe2(S2C3H6)(CO)6 in 20 mL of MeCN at 0 °C was treated
with a solution of 0.80 g (5.12 mmol) of Et4NCN in 10 mL of MeCN,
and then the mixture was allowed to warm to room temperature. After
1 h further, the resulting dark red solution was evaporated to dryness.
The solid residue was washed with 15 mL of hexanes. Yield: 1.56 g
(94%). Anal. Calcd for C25H46Fe2N4O4S2: C, 46.74; H, 7.22; N, 8.72;
S, 9.98; Fe, 17.38. Found: C, 46.75; H, 7.21; N, 9.07; S, 9.63; Fe,
17.20. 1H NMR (400 MHz, CD3CN): δ 3.18 (q, 16H, NCH2CH3), 1.85
(t, 4 H, SCH2), 1.67 (m, 2H, CH2CH2CH2), 1.21 (t, 24H, NCH2CH3).
which is well-known to stabilize high coordination numbers,36
while the electrophilicity of the Fe(µ-CO)(CO)3 center can be
attributed to its coordination to four CO ligands and the
localization of the negative charge on the Fe(CN)(CO)2 center
due to the absence of an Fe-Fe bond. Otherwise, compounds
of the type [Fe2(µ-L)(SR)2(CO)4L′2]z are best known for L )
H.14,25b,37 Finally, the redox properties of the dicyanides are
pertinent because the hydrogenase enzymes are redox-active.
For the series [Fe2(S2C3H6)(L)2(CO)4] (L ) CO, CNMe, PMe3,
and CN-), the dicyanide is the most reducing. Thus, while one
could describe cyanide as “Nature’s trimethyl phosphine”, it is
in fact a much more strongly donating ligand and, importantly,
a ligand whose donor properties can be modulated by the pH
of its environment.38
IR (KBr): νCN ) 2078, 2029; νCO ) 1961, 1917, 1880, 1867 cm-1
.
(Et4N)2[Fe2(S2C3H6)(13CN)2(CO)4] (50% Enriched) (2a). A solution
of 0.20 g (1.30 mmol) of Et4NCN and 0.08 g (1.30 mmol) of K13CN
in 10 mL of CH3OH was stirred for 10 h. The solvent was removed
under vacuum, and the resulting solid was slurried in 5 mL of MeCN.
The suspension was filtered into a solution of 0.25 g (0.65 mmol) of
Fe2(S2C3H6)(CO)6 in 20 mL of MeCN at 0 °C. The reaction mixture
was allowed to warm to room temperature. After an additional 1 h, the
resulting dark red solution was evaporated to dryness under vacuum.
The residue was washed with 15 mL of hexanes. Yield: 0.19 g (46%).
1H NMR (500 MHz, CD3OD): δ 3.22 (q, 16H, NCH2CH3), 1.84 (t, 4
H, SCH2), 1.66 (m, 2H, CH2CH2CH2), 1.22 (t, 24H, NCH2CH3). 13C
NMR (500 MHz, CD3OD): δ 219.5 (s, 4C, FeCO), 164.5 (s, 2C,
FeCN), 53.5 (s, 8C, NCH2CH3), 31.6 (s, 2C, SCH2), 23.9 (s, 1C,
CH2CH2CH2), 7.8 (s, 8C, NCH2CH3). IR (CH2Cl2): νCN ) 2071, 2031;
Experimental Section
General Procedures. Organosulfur and organophosphorus com-
pounds, Et4NCN, and Fe(CO)5 were obtained from Aldrich and used
without further purification. Solvents were purified by degassing with
a nitrogen purge and were dispensed through two 1-m long columns
of active alumina. Reactions were carried out under an atmosphere of
purified nitrogen using either standard Schlenk techniques or in an inert
atmosphere glovebox.
νCO ) 1964, 1924, 1884 cm-1
.
(Et4N)2[Fe2(SCH3)2(CN)2(CO)4] (3). A solution of 0.41 g (1.11
mmol) of Fe2(SCH3)2(CO)6 in 20 mL of MeCN at 0 °C was treated
with a solution of 0.35 g (2.22 mmol) of Et4NCN in 5 mL of MeCN.
The reaction mixture was allowed to come to room temperature. After
1 h further, the resulting dark red solution was evaporated to dryness
under vacuum. The residual solid was washed with 15 mL of hexanes.
Yield: 0.58 g (83%). Anal. Calcd for C24H46Fe2N4O4S2: C, 45.72; H,
7.35; N, 8.89; S, 10.17; Fe, 17.72. Found: C, 45.97; H, 7.33; N, 8.98;
The following compounds were prepared by minor variations of
literature methods: Fe2(S2C2H4)(CO)6, Fe2(S2C3H6)(CO)6 (1),21 Fe2-
(36) Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition
Metals; Academic Press: London, 1976.
(37) (a) Fauvel, K.; Mathieu, R.; Poilblanc, R. Inorg. Chem. 1976, 15,
976-978. (b) Savariault, J.-J.; Bonnet, R.; Mathieu, R.; Galy, J. C. R. Acad.
Sci., Ser. IIC: Chim. 1977, 663-667. (c) Zhao, X.; Georgakaki, I. P.; Miller,
M. L.; Yarbrough, J. C.; Darensbourg, M. Y. J. Am. Chem. Soc. 2001,
123, 9710-9711.
(38) For example, see: Datta, D.; Mascharak, P. K.; Chakravorty, A.
Inorg. Chem. 1981, 20, 1673-1679.
1
S, 10.34; Fe, 17.59. H NMR (400 MHz, CD3CN): δ 3.21 (q, 16H,
NCH2CH3), 1.83 (s, 3H, 73% a,e isomer, SCH3), 1.74 (s, 6H, 27% e,e
(39) Treichel, P. M.; Crane, R. A.; Matthews, R.; Bonnin, K. R.; Powell,
D. J. Organomet. Chem. 1991, 402, 233-248.
(40) King, R. B.; Bisnette, M. B. Inorg. Chem. 1965, 4, 482-485.

12526 J. Am. Chem. Soc., Vol. 123, No. 50, 2001
Gloaguen et al.
Table 2. Details of Data Collection and Structure Refinement for 3, 5, 6c, and 7
3
5
6c
7
chemical formula
temperature (K)
crystal size (mm)
space group
a (Å)
b (Å)
c (Å)
C24H44Fe2N4O4S2
193(2)
C17H26Fe2N2O5S2‚0.9THF
193(2)
C26H42Fe2KNO11S2
193.2
C38H70Fe4N4O8P2S4
193(2)
0.01 × 0.48 × 0.80
0.36 × 0.18 × 0.08
0.03 × 0.03 × 0.76
0.04 × 0.12 × 0.22
P1h
P1h
P21/c
P21/c
7.7580(14)
10.0251(18)
19.747(4)
90.774(3)
98.760(4)
96.712(4)
1506.7(5)
2
7.6421(11)
10.4061(14)
16.846(2)
20.563(2)
8.7120(9)
20.342(2)
90
103.588(2)
90
3542.2(6)
4
1.424
11.1882(16)
32.862(5)
14.280(2)
90
95.300(3)
90
5228.0(13)
4
1.356
R (deg)
102.859(3)
91.197(14)
99.024
1287.7(3)
2
1.492
0.710 73
1.0000/0.7937
11 956/6015
6015/8/350
1.044
â (deg)
γ (deg)
V (Å3)
Z
D calcd (Mg m-3
µ (Mo KR, mm-1
max/min transn
)
)
1.385
0.710 73
0.9942/0.6117
7992/5237
5237/40/361
0.942
0.710 73
0.710 73
0.9671/0.8392
15 375/4950
4943/739/605
0.787
0.9967/0.7932
31 219/9225
9225/0/555
0.878
reflns measured/independent
data/restraints/parameters
GOF
Rint
0.0680
0.0277
0.1760
0.1108
R1[I > 2σ] (all data)a
Rw [I > 2σ] (all data)b
max peak/hole (e-/Å3)
0.0616 (0.1250)
0.1361 (0.1608)
0.617/-0.669
0.0380 (0.0529)
0.1061 (0.1135)
0.810/-0.376
0.0499 (0.0609)
0.1887 (0.0808)
0.350/-0.381
0.0490 (0.0808)
0.1352 (0.0970)
0.603/-0.396
2
a R ) Σ||Fo| - |Fc|/Σ|Fo|. b Rw ) {[w(|Fo| - |Fc|)2]/Σ[wFo ]}1/2, where w ) 1/σ2(Fo).
Table 3. a,e vs e,e Isomer Ratios in Fe2(SR)2(CO)6 and
Derivatives in MeCN Solution Determined by H NMR
Spectroscopy
under vacuum, and the resulting solid was extracted into 5 mL of
MeCN. The suspension was filtered into a solution of 0.24 g (0.65
mmol) of Fe2(S2C2H4)(CO)6 in 10 mL of MeCN. The reaction mixture
was stirred for 1 h further. The resulting dark red solution was
evaporated to dryness under vacuum. The residue was washed with 15
mL of hexanes and recrystallized by extraction into ∼5 mL of MeCN
followed by the addition of Et2O. Yield: 0.35 g (78%). 1H NMR (500
MHz, CD3OD, 25 °C): δ 3.28 (q, 16H, NCH2CH3), 1.96 (2, 4 H,
SCH2), 1.21 (t, 24H, NCH2CH3). 13C NMR (500 MHz, CD3OD): δ
219.5 (s, 4C, FeCO), 164.5 (s, 2C, FeCN), 53.5 (s, 8C, NCH2CH3),
31.6 (s, 2C, SCH2), 23.9 (s, 1C, CH2CH2CH2), 7.8 (s, 8C, NCH2CH3).
1
compound
a,e/e,e ratio
Fe2(SMe)2(CO)6
67/33
73/27
81/19
78/22
70/30
20/80
93/7
[Fe2(SMe)2(CN)2(CO)4]2-
Fe2(S-t-Bu)2(CO)6
[Fe2(S-t-Bu)2(CN)(CO)5]-
[Fe2(S-t-Bu)2(CN)2(CO)4]2-
Fe2(SC6H6Me)2(CO)6
[Fe2(SC6H4Me)2(CN)2(CO)4]2-
13
IR (MeCN): νCN ) 2077, 2033 (ν CN ) 2033 (s)); νCO ) 1965, 1924,
1886, 1870 cm-1
.
isomer, SCH3), 1.21 (t, 24H, NCH2CH3), 1.20 (s, 3H, 73% a,e isomer,
SCH3) (Table 3). IR (KBr): νCN ) 2072, 2037; νCO ) 1961, 1913,
Fe2(S2C3H6)(CO)5(CNMe) and Fe2(S2C3H6)(CO)4(CNMe)2 (4). A
solution of 0.500 g (1.30 mmol) of 1 in 60 mL of MeCN was treated
with 250 µL (4.60 mmol) of CNMe, followed by the addition of 0.290
g (2.61 mmol) of Me3NO in 20 mL of MeCN. After 5 h, TLC showed
the reaction was complete. The dark red solution was filtered and
evaporated to dryness under vacuum. The residue was extracted into 5
mL of toluene and passed through a 3 cm × 20 cm column of silica
gel, eluting with hexane-toluene 3:1. Two bands were observed: the
first orange band (Fe2(S2C3H6)(CO)5(CNMe); IR (hexane): νCN ) 2185;
νCO ) 2044, 2003, 1971 cm-1) was collected with ∼500 mL of eluant,
and the second red band (4) with ∼300 mL of toluene. Yield: 0.160
g (30%). 1H NMR (500 MHz, C6D6): δ 1.94 (s, 6H, NCH3), 1.73 and
1.40 (4 and 2H, S(CH2)3S). IR (MeCN): νCN ) 2170; νCO ) 2020,
1879 cm-1
.
(Et4N)2[Fe2(S-4-C6H4Me)2(CN)2(CO)4]. A solution of 0.53 g (2.59
mmol) of Fe2(S-4-C6H4Me)2(CO)6 in 10 mL of MeCN at 0 °C was
treated with a solution of 0.31 g (2.00 mmol) of Et4NCN in 5 mL of
MeCN. The reaction mixture was allowed to come to room temperature.
After an additional 1 h, the resulting dark red solution was evaporated
to dryness under vacuum. The remaining red solid was washed with 3
× 5 mL of hexanes. Yield: 0.71 g (91%). Anal. Calcd for C36H54-
Fe2N4O4S2: C, 55.25; H, 6.95; N, 7.16; S, 8.19; Fe, 14.27. Found: C,
54.23; H, 6.52; N, 6.49; S, 8.45; Fe, 14.33. 1H NMR (400 MHz, CD3-
CN): δ 7.51-6.76 (m, 8H, C6H4), 3.16 (q, 16H, NCH2CH3), 2.20 (s,
3H, 93.4% a,e isomer, C6H4CH3), 2.18 (s, 6H, 6.6% e,e isomer,
C6H4CH3), 2.17 (s, 3H, 93.4% a,e isomer, C6H4CH3), 1.19 (t, 24H,
NCH2CH3) (Table 3). IR (KBr): νCN ) 2092, 2078, 2033; νCO ) 1976,
2000, 1967, 1936 cm-1
.
Generation of [Fe2(S2C3H6)(MeCN)(CO)5]. A solution of 0.15 g
(0.38 mmol) of Fe2(S2C3H6)(CO)6 in 10 mL of MeCN was treated with
a solution of 0.031 g (0.38 mmol) ONMe3‚2H2O in 10 mL of MeCN.
The solution immediately became dark purple. The IR spectrum of this
solution showed νCO ) 2051, 2040, 1991, 1964, 1933 cm-1. Evaporation
of the solution gave Fe2(S2C3H6)(CO)6 and unidentified brown solids.
If instead of evaporation the solution was treated with 0.50 g (1.9 mmol)
PPh3, the solution color lightened to red-orange, and the IR spectrum
identified the main product as Fe2(S2C3H6)(CO)5(PPh3); νCO ) 2045,
1931, 1900 cm-1
.
(Et4N)2[Fe2(S2C2H4)(CN)2(CO)4] (3). A solution of 1.00 g (2.68
mmol) of Fe2(S2C2H4)(CO)6 in 10 mL of MeCN was treated with a
solution of 0.84 g (5.36 mmol) of Et4NCN in 5 mL of MeCN. The
reaction proceeded as for the propanedithiolate. After 1 h, the resulting
dark red solution was evaporated to dryness under vacuum. The residue
was washed with 5 mL of Et2O and recrystallized by dissolution in
MeCN, followed by dilution with Et2O. Yield: 1.37 g (82%). Single
crystals were grown from a concentrated MeCN solution that was
layered with Et2O. Anal. Calcd for C24H44Fe2N4O4S2: C, 45.87; H,
7.06; N, 8.91; Fe, 17.77. Found: C, 45.78; H, 6.91; N, 8.90; Fe, 17.95.
1H NMR (500 MHz, CD3OD): δ 3.28 (q, 16H, NCH2CH3), 1.96 (s, 4
H, SCH2), 1.21 (t, 24H, NCH2CH3). IR (MeCN): νCN ) 2077, 2033
1985, 1932 cm-1 27
.
(Et4N)[Fe2(S2C3H6)(CN)(CO)5] (5). A solution of 0.35 g (0.87
mmol) of Fe2(S2C3H6)(CO)6 in 10 mL of MeCN was treated with a
solution of 0.097 g (0.87 mmol) ONMe3‚2H2O in 10 mL of MeCN.
The solution was then cooled to at -40 °C and treated with a solution
of 0.137 g (0.87 mmol) of Et4NCN in 10 mL of MeCN. The reaction
mixture was allowed to warm to room temperature over the course of
2 h. The dark red solution was evaporated to dryness under vacuum.
The resulting red oil was extracted into 10 mL of THF, and this extract
(w); νCO ) 1965, 1924, 1886, 1870 cm-1
.
(Et4N)2[Fe2(S2C2H4)(13CN)2(CO)4] (50%Enriched) (3a). A solution
of 0.20 g (1.30 mmol) of Et4NCN and 0.08 g (1.30 mmol) of K13CN
in 15 mL of CH3OH was stirred for 30 min. The solvent was removed

x-
Studies on [Fe2(SR)2(CN)x(CO)6-x
]
J. Am. Chem. Soc., Vol. 123, No. 50, 2001 12527
was filtered. The volume was reduced to approximately 1 mL, and the
product was precipitated upon addition of 20 mL of hexanes and washed
with 45 mL of hexanes, giving a red solid. Single crystals were grown
from THF solution by overlaying with hexanes. Yield: 0.46 g (79%).
Anal. Calcd for C17H26Fe2N2O5S2: C, 39.71; H, 5.10; N, 5.45, S, 12.47.
reaction mixture was stirred for 120 h. The resulting dark red solution
was evaporated to dryness under vacuum. The solid residue was washed
with 15 mL of hexanes and recrystallized from THF by the addition of
Et2O. Single crystals of 11c were obtained by layering a concentrated
THF solution with Et2O. H NMR (400 MHz, CD3CN): δ 3.56 (s,
24H, OCH2), 1.32 (s, 18H, 22% e,e isomer, SC(CH3)3), 1.28 (s, 9H,
1
1
Found: C, 39.43; H, 5.06; N, 5.43, S, 12.47. H NMR (500 MHz,
CD3CN): δ 3.16 (q, 8H, NCH2CH3), 2.18 and 1.77 (br, 6H, S(CH2)3S),
1.19 (t, 12H, NCH2CH3). IR (THF): νCN ) 2094; νCO ) 2029, 1974,
77% e,a isomer, SC(CH3)3), 1.23 (s, 9H, 77% a,e isomer, SC(CH3)3)
(Table 3). IR (KBr): νCN ) 2087; νCO ) 2020, 1972, 1929 cm-1
.
1955, 1941, 1917 cm-1
.
(Et4N)[Fe2(S2C3H6)(CN)(CO)4(PMe3)] (7). A solution of 0.40 g (1.0
mmol) of Fe2(S2C3H6)(CO)6 in 10 mL of MeCN at -40 °C was treated
with a solution of 0.4 mL (4.0 mmol) of PMe3 in 10 mL of MeCN
followed by a solution of 0.15 g (0.95 mmol) of Et4NCN in 5 mL of
MeCN. The reaction mixture was warmed to -10 °C. After 30 min,
the IR spectrum showed that all starting material had been consumed.
The resulting dark red-purple solution was evaporated to dryness under
vacuum. The red oil was extracted with 10 mL of THF and filtered.
The volume was reduced under vacuum to ∼5 mL, and the product
was precipitated with 30 mL of pentane as an oil. The red oil was
washed with an additional 3 × 30 mL of pentane, giving a solid. Single
crystals were grown from toluene/hexanes at -20 °C. Yield: 0.50 g
(92%). Anal. Calcd for C19H35Fe2N2O4PS2: C, 40.59; H, 6.29; N, 4.98.
(Et4N)[Fe2(S-t-Bu)2(CN)(CO)5] (6a). A solution of 0.20 g (0.44
mmol) of Fe2(S-t-Bu)2(CO)6 in 10 mL of MeCN at 0 °C was treated
with a solution of 0.07 g (0.44 mmol) of Et4NCN in 5 mL of MeCN.
The reaction mixture was allowed to warm to room temperature. After
24 h further, the resulting dark red solution was evaporated to dryness
under vacuum. The remaining solid was washed with 15 mL of hexanes.
Yield: 0.22 g (86%). Anal. Calcd for C22H38Fe2N2O5S2: C, 45.06; H,
6.53; N, 4.78; Fe, 19.05. Found: C, 44.44; H, 6.64; N, 5.14; Fe, 19.45.
1H NMR (400 MHz, CD3CN): δ 3.13 (q, 8H, NCH2CH3), 1.32 (s,
18H, 21.9% e,e isomer, SC(CH3)3), 1.28 (s, 9H, 79% a,e isomer, SC-
(CH3)3), 1.23 (s, 9H, % a,e isomer, SC(CH3)3), 1.18 (t, 12H, NCH2CH3)
(Table 3). IR (KBr): νCN ) 2084; νCO ) 2019, 1965, 1950, 1926, 1905
cm-1. ESI-MS: m/z 456.1 (M-), 428.1 (M- - CO), 400.1 (M- - 2CO),
372.2 (M - 3CO).
1
Found: C, 40.65; H, 6.34; N, 4.94. H NMR (500 MHz, C6D6): δ
2.79 (q, 8H, NCH2CH3), 2.17 and 2.08 (br, 4H, SCH2), 1.89 and 1.68
(br, 2H, CH2CH2CH6), 1.36 (d, 9H, CH3), 0.87 (t, 12H, NCH2CH3).
31P NMR (500 MHz, C6D6): δ 28.7 (s). IR (THF): νCN ) 2078, 2036;
νCO ) 1971, 1931, 1895, 1880 cm-1. FAB-MS: M- 431.8 m/z.
Ph4PCN‚H2O. A solution of 1.55 g (23.80 mmol) of KCN in 10
mL of CH3OH was treated dropwise with a solution of 6.44 g (15.36
mmol) of PPh4Br in 10 mL of CH3OH. The resulting suspension was
evaporated to dryness, and the residue was extracted with 20 mL of
MeCN. The extract was concentrated and stored at -32 °C to give
colorless crystals. Yield: 2.50 g (42%). The water of crystallization
can be removed by vacuum-drying for 24 h. Anal. Calcd for C25H22-
NOP: C, 78.31; H, 5.78; N, 3.65. Found: C, 78.19; H, 5.40; N, 3.17.
Reaction of (Et4N)[Fe2(S2C3H6)(CN)(CO)5] (5) and Fe2(S2C3H6)-
(CO)6 (1) with 2 equiv of Et4NCN. A solution of 0.75 g (0.15 mmol)
of (Et4N)[Fe2(S2C3H6)(CN)(CO)5] and 0.56 g (0.15 mmol) of Fe2-
(S2C3H6)(CO)6 in 20 mL of MeCN at -40 °C was treated with a
solution of 0.46 g (0.31 mmol) of Et4NCN in 10 mL of MeCN. The
reaction mixture was allowed to warm to room temperature over the
course of 30 min. IR spectra were obtained after the solution had been
warmed to room temperature, after 1 h, and after 3h. The IR spectrum
showed that all Fe2(S2C3H6)(CO)6 had been consumed after 3 h. IR
(MeCN): νCN ) 2092, 2075, 2030; νCO ) 1975, 1963, 1921, 1883
cm-1 (see Figure 7).
IR (KBr): νCN ) 2069 cm-1
.
(Ph4P)[Fe2(S-t-Bu)2(CN)(CO)5] (6b). A solution of 0.20 g (0.44
mmol) of Fe2(S-t-Bu)2(CO)6 in 10 mL of MeCN at 0 °C was treated
with a solution of 0.17 g (0.44 mmol) of Ph4PCN‚H2O in 5 mL of
MeCN. The reaction mixture was allowed to come to room temperature
and stir for 24 h. The resulting dark red solution was evaporated to
dryness under vacuum. The remaining solid was washed with 25 mL
of hexanes. Yield: 0.14 g (40%). Anal. Calcd for C38H38Fe2NO5PS2:
C, 57.37; H, 4.81; N, 1.76; Fe, 14.04. Found: C, 57.57; H, 4.74; N,
1.55; Fe, 12.83. 1H NMR (400 MHz, CD3CN): δ 7.94-7.64 (m, 20H,
P(C6H5)4), 1.31 (s, 18H, e,e isomer, SC(CH3)3), 1.28 (s, 9H, e,a isomer,
SC(CH3)3), 1.23 (s, 9H, a,e isomer, SC(CH3)3) (Table 3). IR (KBr):
Acknowledgment. This research was supported by NIH and
DOE. F.G. thanks the CNRS (France) for a leave of absence.
S. Contakes is thanked for assistance and advice.
νCN ) 2086; νCO ) 2023, 1969, 1939 cm-1
.
Supporting Information Available: Crystallographic in-
formation and IR data (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
[K(18-crown-6)][Fe2(S-t-Bu)2(CN)(CO)5] (6c). A solution of 0.10
g (0.22 mmol) of Fe2(S-t-Bu)2(CO)6 in 10 mL of THF was treated with
a solution of 0.01 g (0.22 mmol) of KCN and 0.06 g (0.23 mmol) of
18-crown-6 in 15 mL of a 2:1 mixture of MeCN and CH3OH. The
JA016071V