Preparative and structural studies on iron(II)-thiolate cyanocarbonyls: Relevance to the [NiFe]/[Fe]-hydrogenases

Article abstract of DOI:10.1039/b311059a

A number of thermally stable iron(II)-thiolate cyanocarbonyl complexes, cis,cis-[Fe(CN)₂(CO)₂(CS₃-S,S)]^2- (1), mer-[Fe(CO)₂(CN)₃ (NCCH₃)]^- (2) mer-[Fe(CO)₃(CN)(CS₃ -S,S)]^- (3), cis-[Fe(CO)₂(CN)(S(CH₂)₂S (CH₂)₂S(CH₂)₂S-S,S,S)]^- (4), [Fe(CO)₂(CN)₃Br]^2- (5), mer-[Fe(CO)₂(CN)₃(m-SC₆H₄Br)]^2- (6) and mer-[Fe(CO)₂(CN)₃(SPh)]^2- (7) were isolated and characterized by IR and X-ray diffraction analysis. The extrusion of one strong σ-donor CN^- ligand instead of CO from the iron(II) center of the thermally stable complexes [Fe^II(CO)₂(CN)₃Br]^2- (5) containing less electron-donating bromide reflects the electron-rich character of the mononuclear [Fe^II(CN)₂(CO)₂(CS₃- S,S)]^2- (1) when ligated by the bidentate thiolate, and the combination of one cyanide, two carbonyls and a tridentate thiolate provides the stable complex 4 as a result of the reaction of complex 5 and chelating ligand [S(CH₂)₂S(CH₂)₂ S]^2-. The preference of the sixth ligand coordinated to the unsaturated [Fe^II(CO)(CN)₂(CS₃-S,S)]^2- Fe(II) center, the iron-site architecture of the bimetallic Ni-Fe active-site of [NiFe] hydrogenases, is a strong π-acceptor CO group. Scrutiny of the coordination chemistry of iron(II)-thiolate cyanocarbonyl species [Fe^II(CO)_x(CN)_y(SR)_z]^n- reveals that certain combinations of thiolate, cyanide and carbonyl ligands(3 ≤ y + z ≤ 4) bound to Fe(II) are stable and this could point the way to understand the reasons for Nature's choice of combinations of these ligands in hydrogenases.

Full text of DOI:10.1039/b311059a

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Preparative and structural studies on iron(II)–thiolate
cyanocarbonyls: relevance to the [NiFe]/[Fe]-hydrogenases
Chien-Hong Chen,a Yung-Su Chang,b Chien-Yeh Yang,b Tai-Nan Chen,a Chien-Ming Leea and
Wen-Feng Liaw*a
a Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan
b Department of Chemistry, National Changhua University of Education, Changhua, Taiwan.
E-mail: wfliaw@mx.nthu.edu.tw
Received 10th September 2003, Accepted 5th November 2003
First published as an Advance Article on the web 24th November 2003
A number of thermally stable iron()–thiolate cyanocarbonyl complexes, cis,cis-[Fe(CN)2(CO)2(CS3-S,S)]2Ϫ (1),
mer-[Fe(CO)2(CN)3(NCCH3)]Ϫ (2) mer-[Fe(CO)3(CN)(CS3-S,S)]Ϫ (3), cis-[Fe(CO)2(CN)(S(CH2)2S(CH2)2S-S,S,S)]Ϫ
(4), [Fe(CO)2(CN)3Br]2Ϫ (5), mer-[Fe(CO)2(CN)3(m-SC6H4Br)]2Ϫ (6) and mer-[Fe(CO)2(CN)3(SPh)]2Ϫ (7) were isolated
and characterized by IR and X-ray diffraction analysis. The extrusion of one strong σ-donor CNϪ ligand instead
of CO from the iron() center of the thermally stable complexes [FeII(CO)2(CN)3Br]2Ϫ (5) containing less electron-
donating bromide reflects the electron-rich character of the mononuclear [FeII(CN)2(CO)2(CS3-S,S)]2Ϫ (1) when
ligated by by the bidentate thiolate, and the combination of one cyanide, two carbonyls and a tridentate thiolate
provides the stable complex 4 as a result of the reaction of complex 5 and chelating ligand [S(CH2)2S(CH2)2S]2Ϫ. The
preference of the sixth ligand coordinated to the unsaturated [FeII(CO)(CN)2(CS3-S,S)]2Ϫ Fe() center, the iron-site
architecture of the bimetallic Ni–Fe active-site of [NiFe] hydrogenases, is a strong π-acceptor CO group. Scrutiny of
the coordination chemistry of iron()–thiolate cyanocarbonyl species [FeII(CO)x(CN)y(SR)z]nϪ reveals that certain
combinations of thiolate, cyanide and carbonyl ligands (3 ≤ y ϩ z ≤ 4) bound to Fe() are stable and this could point
the way to understand the reasons for Nature’s choice of combinations of these ligands in hydrogenases.
and Sellmann et al.,4 respectively. We have also shown
Introduction
that complexes [Fe(CO)4(CN)]Ϫ and [Fe(CO)3(CN)2Br]Ϫ serve
as the useful precursors to yield Fe–CN–CO–SR species,
[Fe(CO)2(CN)(SNHC6H4-N,S)]Ϫ, [Fe(CO)(CN)(SC5H3N)2]Ϫ,
[Fe(CN)2(CO)2(S2CR-S,S)]Ϫ, and [Fe(CN)2(CO)(S2CR-S,S)]Ϫ
(R = OEt, NEt2), respectively.5 We undertook the present
exploratory project aimed at expanding the range of Fe–CO–
CN–SR derivatives which may provide valuable insight
related to the structure, spectroscopic properties and reactivity
of iron-containing hydrogenases. Herein, we report the syn-
thesis and characterization of a number of the thermally stable
iron()–thiolate cyanocarbonyl complexes, cis,cis-[Fe(CO)2-
(CN)2(CS3-S,S)]2Ϫ (1), mer-[Fe(CO)2(CN)3(NCCH3)]Ϫ (2),
mer-[Fe(CO)3(CN)(CS3-S,S)]Ϫ (3), cis-[Fe(CO)2(CN)(S(CH2)2-
S(CH2)2S-S,S,S)]Ϫ (4), [Fe(CO)2(CN)3Br]2Ϫ (5), mer-[Fe(CO)2-
(CN)3(m-SC6H4Br)]2Ϫ (6), and mer-[Fe(CO)2(CN)3(SPh)]2Ϫ (7).
This study demonstrates that a certain total number of thiolate
and cyanide ligands ligated to the Fe() center provides signifi-
cant stabilization to the iron()–thiolate cyanocarbonyl species
During the past few years, iron–thiolate complexes with mixed
CNϪ and CO ligands have drawn the attention of bioinorganic/
bioorganometallic chemists in their pursuit of synthesizing
penta-/hexa-coordinate iron()–thiolate cyanocarbonyl com-
plexes,1–5 since the active-site [(Scys)2Ni(µ-Scys)2(µ-X)Fe(CO)-
(CN)2] (X = O, OH) of [NiFe]-hydrogenases isolated from
Desulfovibrio gigas has been established as a pyramidal [Fe-
(CN)2(CO)] unit with the opposite face coordinated to two
cysteines bridged to a nickel.6,7 The X bridge is assumed to be
absent in the reduced active state of the enzyme.6,7 EPR studies
indicated the nickel site serves as a redox center and the iron site
remains as Fe() in all spectrally defined redox states (Ni–A,
Ni–B, Ni–SI, Ni–C, Ni–R, Ni–L states) of the enzyme.8 In [Fe]-
hydrogenases, the X-ray crystal structure of [Fe]-hydrogenases
from Clostridium pasteurianum (CpHase)/Desulfovibrio desulfu-
ricans (DdHase) indicated that the active-site cluster consists of
a [4Fe–4S] cubane subcluster covalently bridged by a cysteine to
a dinuclear iron–thiolate–cyanocarbonyl subcluster with iron
atoms coordinated by CO and CNϪ groups.9,10
[FeII(CO)x(CN)y(SR)z]nϪ
.
The iron sites of both families of hydrogenases feature CNϪ
ligands in addition to cysteines and CO. A deep understanding
of the relative affinities of Fe() for cyanides vs. thiolates is
relevant to the entire theme of metallo-hydrogenases.2–5 There-
fore, a varying number of these donor groups can be assembled
in a designed complex to tune the redox and catalytic properties
of the iron center. In addition, structural models as [FeII-
(CO)x(CN)y(SR)z]nϪ may be appreciated as possibilities for inter-
mediates in the biosynthesis of [NiFe]-hydrogenase active-sites,
although the assembly of Fe–CN–CO building blocks and
nickel tetracysteines have been addressed recently by Casalot
and Rousset in the “Maturation of the [NiFe] Hydrogenases”.11
Recently, a few mononuclear iron–thiolate cyanocarbonyl
compounds ([CpFe(CN)2(CO)]Ϫ (Cp = η5-cyclopentadienyl),
[Fe(CN)2(CO)(S2C6H4-S,S)]2Ϫ, [(PS3)Fe(CN)(CO)]2Ϫ (PS3H3 =
Results and discussion
Reaction of K2CS3 and [Fe(CO)3(CN)2Br]Ϫ in 1 : 1 stoichio-
metry proceeds in CH3CN under N2 at ambient temperature
to form cis,cis-[Fe(CN)2(CO)2(CS3-S,S)]2Ϫ (1) identified by IR
and X-ray diffraction analysis (Scheme 1(a), (b)). The dianionic
complex 1 exhibits a two-band pattern in the νCO region (2016s,
1960s cmϪ1 (CH3CN) supporting a cis position of two CO
groups, and the two weak absorption bands 2111w, 2102w cmϪ1
assigned to the cyanide stretching frequencies support a cis
position of two CNϪ ligands. Exposure of complex 1 to 13CO
gives rise to a two-band pattern (1970s, 1915s cmϪ1) of IR νCO
bands at 25 ЊC overnight to afford 13C-enriched derivative
cis,cis-[Fe(CN)2(13CO)2(CS3-S,S)]2Ϫ
.
Reappearance of the
2016s, 1960s cmϪ1 bands on removal of the 13CO and replace-
ment with 12CO atmosphere demonstrated reversibility of CO
ligand lability of complex 1. In contrast, under similar reaction
condition, complex mer-[Fe(CO)2(CN)3(NCCH3)]Ϫ (2) was
tris(2-phenylthiol)phosphine), and [(S3)Fe(CN)2(CO)]2Ϫ (S3
=
bis(2-mercaptophenyl) sulfide(2Ϫ))) have been reported by
Darensbourg et al.,1 Rauchfuss et al.,2 Koch and co-workers3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 1 3 7 – 1 4 3
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Table 1 The known iron()–thiolate complexes [FeII(CO)x(CN)y-
complexes, a straightforward synthetic reaction of [Fe(CO)3-
(CN)I2]Ϫ (obtained from reaction of [Fe(CO)4(CN)]Ϫ with 1
equiv of I2) and K2CS3 in thf was conducted. The reaction
resulted in the formation of the thermally unstable [(CO)3-
(CN)Fe(CS3-S,S)]Ϫ (3) under N2 at ambient temperature
(Scheme 1(d)). Following extended periods of stirring at room
temperature, a thf solution of complex 3 converted into an
insoluble solid. Presumably, the role of increments of CNϪ lig-
and coordinated to the Fe() center in complex 1, compared to
complex 3, is to increase the electron density around the Fe()
center as well as to stabilize the FeII oxidation level. Interest-
ingly, the reaction of the chelating Na2[S(CH2)2S(CH2)2S]
ligand and [Fe(CO)3(CN)2Br]Ϫ yielded cis-[Fe(CO)2(CN)-
(S(CH2)2S(CH2)2S-S,S,S)]Ϫ (4) in thf at ambient temperature
(Scheme 1(e)). Formation of complex 4 can be interpreted as
displacement of one cyanide ligand resulting from chelation of
the tridentate [S(CH2)2S(CH2)2S]2Ϫ ligand. The extrusion of one
σ-donor CNϪ ligand instead of CO from iron() center as a
result of the reaction of [Fe(CO)3(CN)2Br]Ϫ and Na2[S(CH2)2-
S(CH2)2S] implicates the optimum electronic environment of
the [FeII(CO)2]2ϩ fragment ligated by two thiolates, one thio-
ether, and one cyanide (Table 1), as observed in the transform-
(thiolate)]nϪ
Compound
y ϩ thiolate
Reference
[Fe(CO)2(CN)(S2C6H4-S,S)]Ϫ
3
1 ϩ bidentate
1 ϩ bidentate
1 ϩ tridentate
2 ϩ bidentate
2 ϩ bidentate
2 ϩ bidentate
2 ϩ 2(monodentate)
2 ϩ bidentate
3 ϩ monodentate
3 ϩ monodentate
2 ϩ tridentate
5a
This work
This work
2
5b
5b
2
This work
This work
This work
4
4
[Fe(CO)(CN)2(S2C6H4-S,S)]2Ϫ
[Fe(CO)2(CN)2(S2COR-S,S)]Ϫ
[Fe(CO)2(CN)2(S2CNR2-S,S)]Ϫ
[Fe(CO)2(CN)2(SPh)2]2Ϫ
1
6
7
[Fe(CO)(CN)2(S3)]2Ϫ
2Ϫ
ation of [Fe(CN)2(CO)2(µ-SEt)2]2 into the complex [(CN)-
(CO)2Fe(µ-SEt)3Fe(CO)2(CN)]Ϫ accompanied by extrusion of
one σ-donor CNϪ ligand from each iron() center of the com-
plex [Fe(CN)2(CO)2(µ-SEt)2]22Ϫ when ligated by the third bridg-
ing ethylthiolate.12 Notably, both the electronic and steric
reasons (chelation) are responsible for the formation of com-
plexes 1–4. Upon alkylation of complex 4 by CF3SO3Me in thf
at ambient temperature, the infrared spectrum shows the carb-
onyl/cyanide stretching bands 2046s, 1999s (νCO), 2115w (νCN
)
cmϪ1 (thf ), attributed to the thiolate-alkylated reaction to form
the thermally unstable, neutral [Fe(CO)2(CN)(S(CH2)2S(CH2)2-
SMe-S,S,S)] (Scheme 1(f )).
As shown in Scheme 2(a), when [Me3Si][CN] was reacted
directly with [Fe(CO)3(CN)2Br]Ϫ in thf at room temperature, an
air-stable hexacoordinate FeII complex [Fe(CO)2(CN)3Br]2Ϫ (5)
was isolated as a light yellow solid after recrystallization from
CH3CN–diethyl ether. The reaction solution led to the isolation
of a mixture of fac-[Fe(CO)2(CN)3Br]2Ϫ and mer-[Fe(CO)2-
(CN)3Br]2Ϫ on addition of diethyl ether. The IR spectra (2039s,
1984br (νCO), 2123vw, 2115w, 2105m (νCN) cmϪ1 (CH3CN)) are
consistent with a mixture of stereochemically fluxional isomers.
Scheme 1
obtained from reaction of [Fe(CO)3(CN)2Br]Ϫ and Na2[o-
(SCH2)2C6H2-4,5-(CH3)2] (4,5-dimethyl-o-xylene-α,αЈ-dithiol)
in CH3CN at room temperature after the mixture solution was
precipitated by diethyl ether and extracted by thf (Scheme 1(c)).
We proposed that the formation of complex 2 results from
[o-(SCH2)2C6H2-4,5-(CH3)2]2Ϫ ligand dissociation of inter-
mediate
[Fe(CO)2(CN)2(o-(SCH2)2C6H2-4,5-(CH3)2-S,S)]2Ϫ
,
followed by CH3CN coordination and cyanide [CN]Ϫ binding.
The presumed intermediate [Fe(CO)2(CN)2(o-(SCH2)2C6H2-
4,5-(CH3)2-S,S)]2Ϫ was not detected spectrally. The loss of the
ligand [o-(SCH2)2C6H2-4,5-(CH3)2]2Ϫ from the FeII center
of the intermediate [Fe(CO)2(CN)2(o-(SCH2)2C6H2-4,5-(CH3)2-
S,S)]2Ϫ may be due to the electron-rich character around
the FeII center (the lack of electron-delocalization around the
FeS2C4 plane) and the more stable complex 2. Obviously, the
bidentate thiolates [CS3]2Ϫ and [o-(SCH2)2C6H2-4,5-(CH3)2]2Ϫ
,
render the [Fe(CO)2(CN)2] motif in different electronic
environments, induce differing stability to the [Fe(CO)2(CN)2]
unit (Table 1).
In order to evaluate the influence of CNϪ ligands on the
stability and reactivity of the FeII–thiolate cyanocarbonyl
Scheme 2
D a l t o n T r a n s . , 2 0 0 4 , 1 3 7 – 1 4 3
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Subsequent reactions of complex 5 with K2CS3 in CH2Cl2 pro-
duced the air and thermally stable complex 1 (Scheme 2(b)).
Apparently, the dianionic [CS3]2Ϫ ligand bound to the FeII metal
in a bidentate manner labilizes one cyanide ([CN]Ϫ) ligand to
yield complex 1. The thermodynamic stability of complex 1 vs.
3, 5 would then imply that the bidentate donor ligand [CS3]2Ϫ
substantially stabilizes the [FeII(CO)2(CN)2]0, rather than the
[FeII(CO)3(CN)]ϩ and [FeII(CO)(CN)3]Ϫ motifs (Table 1).
We further examined the preparation of iron()–thiolate
cyanocarbonyl species with the goal of investigating the influ-
ence of thiolate ligand(s) on the stability of the [FeII(CO)x-
(CN)y(SR)z]nϪ species. The reaction of 2 equiv. of Na[SC6H4Br]
with a CH3CN solution of [Fe(CO)2(CN)3Br]2Ϫ afforded
mer-[Fe(CO)2(CN)3(m-SC6H4Br)]2Ϫ (6), as determined by IR
and X-ray diffraction (Scheme 2(c)). In a similar fashion, the
ligand-displacement reaction was also displayed by reaction of
complexes [Fe(CO)2(CN)3Br]2Ϫ and Na[SPh] (1 : 3 molar ratio)
in CH3CN at 5 ЊC. IR spectroscopy of the CH3CN solution
indicated the formation of a mixture of mer-[Fe(CO)2(CN)3-
(SPh)]2Ϫ (7) and traces of trans-[Fe(CO)2(CN)4]2Ϫ (Scheme 2(d)
and (e)). The dominant product 7 was further purified by
extraction into thf. Following extended periods of stirring at
ambient temperature, a CH3CN solution of complex 7 con-
verted into complex 2 and trans-[Fe(CO)2(CN)4]2Ϫ identified by
IR and X-ray diffraction analysis (Scheme 2(e)). The thermo-
dynamic instability of complex 7 may be attributed to the
stronger σ-donor/π-donor properties of [SPh]Ϫ vs. [m-SC6H4-
Br]Ϫ ligands, although kinetic effects cannot be ruled out in the
absence of rate studies. With this assignment, we may conclude
that the less electron-donating monodentate thiolate ligand
stabilizes the [FeII(CO)2(CN)3]Ϫ motif.
shows carbonyl/cyanide stretching bands (1997s, 2047s (νCO),
2113w, 2122w (νCN) cmϪ1 (CH3CN)), shifting from 1960s, 2016s
(νCO) 2102w, 2111w (νCN) cmϪ1 (CH3CN), attributable to the
presence of the chelating [CS3Me]Ϫ ligand bound to the [Fe-
(CO)2(CN)2] motif. 1H NMR spectra (δ 2.499 (s) ppm (CDCl3)
([CS3Me]Ϫ) become even more straightforward to confirm
methylation of sulfur. There was no indication of nitrogen-site
alkylation. This result implicates the reaction of complex 1 and
electrophile occur at the more accessible, electron-rich sulfur
site, although the thermodynamic product 9 does not neces-
sarily reflect the initial, presumably, charge-controlled collision
complex. Further addition of CF3SO3Me to complex 9 in
equimolar proportions in thf produces the charge-controlled
collision product [Fe(CO)2(CN)(CNMe)(CS3Me-S,S)] (10),
characterized by the IR νCNR stretching frequency 2225 cmϪ1
1
(2024s, 2069s (νCO), 2128w, 2225m (νCN) cmϪ1 and H NMR
(δ 2.61 (s) (CNMe), 2.50 (s) (CS3Me-S,S) ppm (CDCl3))
(Scheme 4(b)).
Scheme 4
Structures
The molecular structures of complexes 1–7 are depicted in
Figs. 1–7, respectively. Selected bond distances and angles are
summarized in the captions of Figs. 1–7. The Fe–S bonds of
average length 2.311(1) Å, Fe–C(O) bonds of average length
1.786(4) Å and Fe–C(N) bonds of average length 1.953(4) Å in
complex 1 are within the range (2.2, 1.7 and 1.9 Å, respectively)
observed in [NiFe] hydrogenases from D. gigas.6 The two carb-
onyl groups of complex 2 are disposed in a cis arrangement and
are trans to the CNϪ and CH3CN groups, respectively. The
shorter Fe–C(5) bond (1.776(3) Å), as compared to 1.822(3) Å
for the Fe–C(6) bond, is attributed to the π-back bonding char-
acter in complex 2. The molecular structure of complex 3 anion
is that of a distorted octahedron with S(1)–Fe(1)–S(2) bond
angle of 74.04(2)Њ. The Fe–C(N) distance of 1.926(2) Å is com-
parable to that of the five-coordinate [Fe(CO)2(CN)(SNHC6H4-
N,S)]Ϫ (Fe–C(N) 1.926(6) Å).5a
In order to corroborate this interpretation we treated a
CH3CN solution of [Fe(CO)2(CN)3Br]2Ϫ with one equiv. of
Na[SEt] at ambient temperature. The IR νCO/νCN stretching
frequencies are consistent with the formation of trans-[Fe-
(CO)2(CN)4]2Ϫ
, complex 2, and [(CN)(CO)2Fe(µ-SEt)3Fe-
(CO)2(CN)]Ϫ, presumably, resulting from transformation of
the thermally unstable intermediate [Fe(CO)2(CN)3(SEt)]2Ϫ (8)
(Scheme 3(a) and (b)). Thus, we can project an electron donor
ability of the [SEt]Ϫ to [Fe(CO)2(CN)3]Ϫ unit to be more than
that of the [m-SC6H4Br]Ϫ ligand, and this explains the relative
instability of the intermediate 8.
Scheme 3
Fig. 1 ORTEP drawing and labeling scheme of the [Fe(CN)2(CO)2-
(CS3-S,S)]2Ϫ anion. Selected bond lengths (Å) and angles (Њ): Fe–C(2)
1.776(4), Fe–C(1) 1.796(4), Fe–C(3) 1.947(4), Fe–C(4) 1.960(3), Fe–S(2)
2.3109(10), Fe–S(1) 2.3119(11), S(1)–C(5) 1.725(4), S(2)–C(5) 1.719(4),
S(3)–C(5) 1.669(4) Å; S(2)–Fe–S(1) 74.41(4)Њ.
Complex
1 contains particularly interesting functional
groups, [CS3]2Ϫ and [CN]Ϫ, in which both the sulfur and nitro-
gen centers are potential sites of reactivity with electrophiles.
Nucleophilic reaction of complex 1 with the hard alkylating
reagent CF3SO3Me in equimolar proportions in CH3CN at
ambient temperature for 5 min yielded [N(PPh3)2][Fe(CO)2-
(CN)2(CS3Me-S,S)] (9) (Scheme 4(a)). The infrared spectra
The interesting feature of complex 4 is the asymmetry in the
FeII–S bond lengths (Fe–S(2) 2.286(2) Å vs. Fe–S(1) 2.334(2)
and Fe–S(3) 2.324(2) Å), which shows a difference of 0.043 Å
D a l t o n T r a n s . , 2 0 0 4 , 1 3 7 – 1 4 3
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Fig. 4 ORTEP drawing and labeling scheme of the [Fe(CO)2(CN)-
(S(CH2)2S(CH2)2S-S,S,S)]Ϫ anion. Selected bond lengths (Å) and
angles (Њ): Fe–C(2) 1.741(8), Fe–C(1) 1.778(8), Fe–C(3) 1.904(7), Fe–
S(2) 2.286(2), Fe–S(3) 2.3236(19), Fe–S(1) 2.3338(17) Å; S(2)–Fe–S(3)
87.96(8), S(2)–Fe–S(1) 88.47(8), S(3)–Fe–S(1) 94.53(17)Њ.
Fig. 2 ORTEP drawing and labeling scheme of the [Fe(CO)2(CN)3-
(NCCH3)]Ϫ anion. Selected bond lengths (Å): Fe–C(5) 1.776(3), Fe–
C(6) 1.822(3), Fe–C(7) 1.927(3), Fe–C(3) 1.950(2), Fe–C(4) 1.943(3),
Fe–N(1) 1.974(2) Å.
Fig. 3 ORTEP drawing and labeling scheme of the [Fe(CO)3(CN)-
(CS3-S,S)]Ϫ anion. Selected bond lengths (Å) and angles (Њ): Fe(1)–C(2)
1.803(2), Fe(1)–C(5) 1.833(3), Fe(1)–C(3) 1.870(3), Fe(1)–C(1) 1.926(2),
Fe(1)–S(1) 2.3315(6), Fe(1)–S(2) 2.3315(6), S(1)–C(4) 1.728(2), S(2)–
C(4) 1.743(2), S(3)–C(4) 1.644(2) Å; S(1)–Fe(1)–S(2) 74.04(2)Њ.
Fig.
5
ORTEP drawing and labeling scheme of the [Fe(CO)2-
(CN)3Br]2Ϫ anion. Selected bond lengths (Å): Fe–C(2) 1.8114(19), Fe–
C(1) 1.8162(19), Fe–C(5) 1.9659(19), Fe–C(3) 1.9874(18), Fe–C(4)
1.9883(19) Å.
between FeII–thioether and FeII–thiolate bonds. The two sulfur
atoms of thiolates are disposed in a cis position and the
positions trans to the sulfur atoms of thiolates are occupied
by two CO ligands. This feature, as observed in complex 4, can
be attributed to the electronic influence of the [S(CH2)2-
S(CH2)2S]2Ϫ ligand. Because of disorder of BrϪ and CNϪ
ligands in complex 5, the exact Fe–C(N) bond lengths are
poorly determined and the two carbonyl groups are disposed in
a cis position. Owing to the disorder of CNϪ and [SC6H4Br]Ϫ in
complex 6, the exact Fe–S bond length is calculated as 2.424(5)
Å, significantly longer than the Fe–S bond length of 2.375(1) Å
in complex 7.
Fig. 6 ORTEP drawing and labeling scheme of the [Fe(CO)2(CN)3-
(o-SC6H4Br)]2Ϫ anion. Selected bond lengths (Å): Fe–C(3) 1.850(9),
Fe–C(2) 1.856(6), Fe–C(1) 1.900(5), Fe–S(1) 2.424(5) Å.
Conclusion
Studies on the mononuclear iron()–thiolate cyanocarbonyl
complexes have led to the following results related to the struc-
ture, stability, and spectroscopic properties of the iron-site of
bimetallic Ni–Fe active-site of [NiFe]-hydrogenases. As shown
in Table 1, certain combinations of CO, CNϪ and thiolate
ligands bound to the Fe() center play an important role
in stabilizing the [FeII(CO)x(CN)y(SR)z]nϪ species. The stabiliz-
ation of various iron()–thiolate cyanocarbonyl complexes by
the particular combinations of thiolate, cyanide and carbonyl
ligation, (a) [FeII(CO)2(CN)3]Ϫ motif with monodentate thiolate
ligand, (b) [FeII(CO)2(CN)2]0 motif with bidentate thiolate
ligands ([S2COR]Ϫ and [CS3]2Ϫ 5b and (c) [FeII(CO)2(CN)]ϩ motif
)
D a l t o n T r a n s . , 2 0 0 4 , 1 3 7 – 1 4 3
140

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S,S)]). Diffusion of diethyl ether into a CH3CN solution of
complex 1 at Ϫ15 ЊC for 3 weeks yielded crystals suitable for
X-ray crystallography. IR (CH3CN): 1960s, 2016s (νCO), 2102w,
2111w (νCN) cmϪ1 13C NMR (CD3CN): δ 212.95 (d), 216.94 (d)
.
ppm (CO). Absorption spectrum (thf ) [λmax/nm (ε/MϪ1 cmϪ1)]:
362 (6536) (Found: C, 68.77; H, 4.82; N, 4.20. Calc. for C77H60-
N4O2P4S3Fe: C, 68.54; H, 4.48; N, 4.15%).
Reaction of [N(PPh3)2][Fe(CO)3(CN)2Br] and
Na2[o-(SCH2)2C6H2-4,5-(CH3)2-S,S]
A CH3CN solution (10 mL) containing [N(PPh3)2][Fe(CO)3-
(CN)2Br] (0.4051 g, 0.5 mmol) and Na2[o-(SCH2)2C6H2-
4,5-(CH3)2] (0.099 g, 0.5 mmol, obtained from reaction of
4,5-dimethyl-o-xylene-α,αЈ-dithiol and Na[OMe])5b was stirred
for 3 days at ambient temperature. The resulting mixture was
filtered to remove the insoluble solid, and the yellow solution
was then dried under vacuum. 5 ml of thf was added to the gray
solid, the yellow solution was separated from the light yellow
solid by filtration, and diethyl ether was slowly added to
precipitate the yellow solid mer-[N(PPh3)2][Fe(CO)2(CN)3-
(NCCH3)] (2) (yield 0.190 g, 48%). Recrystallization by
vapor diffusion of diethyl ether into a concentrated thf solution
of complex 2 at Ϫ15 ЊC afforded yellow crystals. IR: 2009s,
2056s (νCO), 2105w, 2117w (νCN) cmϪ1 (thf ); 2068s, 2023s
Fig.
7
ORTEP drawing and labeling scheme of the [Fe(CO)2-
(CN)3(SPh)]2Ϫ anion. Selected bond lengths (Å): Fe(1)–C(5) 1.784(4),
Fe(1)–C(1) 1.787(5), Fe(1)–C(2) 1.918(4), Fe(1)–C(3) 1.921(5), Fe(1)–
C(4) 1.927(4), Fe(1)–S(1) 2.3751(12) Å.
with tridentate thiolate ligands, can be attributed to the
optimum electronic environment of the Fe() center resulting
from strong σ-donor CNϪ, thiolate ligands as well as strong
π-acceptor CO ligands. Notably, a certain total number of
thiolate and cyanide ligands bound to the Fe() center stabilizes
the binding of carbon monoxide to the Fe() metal, and this
may point the way to understand the reasons for Nature’s
choice of combinations of these ligands in hydrogenases.
1
(νCO), 2133w, 2127w, 2118w (νCN) cmϪ1 (CH3CN). H NMR
(CDCl3): δ 2.15 (s) (NCCH3). Absorption spectrum (thf )
[λmax/nm (ε/MϪ1 cmϪ1)]: 340 (1109), 372 (757) (sh) (Found: C,
67.58; H, 4.15; N, 8.89. Calc. for C43H33N5O2P2Fe: C, 67.11; H,
4.32; N, 9.10%).
Preparation of mer-[N(PPh3)2][Fe(CO)3(CN)(CS3-S,S )] (3)
Experimental
A thf solution (10 mL) containing 0.366 g (0.5 mmol) of
[N(PPh3)2][Fe(CO)4(CN)] and 0.127 g (0.5 mmol) of I2 was
stirred at room temperature for 2 h. The reaction was moni-
tored immediately by IR. The spectrum (IR (thf ): 2122w (νCN),
2089m, 2047sh, 2039s (νCO) cmϪ1) was assigned to the formation
of [N(PPh3)2][Fe(CO)3(CN)I2]. In the same flask, 0.5 mmol of
K2CS3 (0.1022 g) was added, followed by stirring at room tem-
perature for 2 h. The reaction mixture was filtered to remove
KI, and hexane was added to precipitate the thermally unstable,
red–brown solid mer-[N(PPh3)2][Fe(CO)3(CN)(CS3-S,S)] (3)
(0.334 g, 74%). To grow X-ray diffraction quality crystals,
crystallization vials containing a concentrated thf solution of
complex 3 were placed in a septum-sealed container surrounded
by hexane. Upon standing at 0 ЊC for 3 weeks, red–brown crys-
tals of 3 form. IR (thf ): 2009s, 2033vs, 2076w, 2090vw (νCO),
2123w (νCN) cmϪ1. Absorption spectrum (thf ) [λmax/nm (ε/MϪ1
cmϪ1)]: 412 (182).
Manipulations, reactions, and transfers of samples were con-
ducted under nitrogen according to standard Schlenk tech-
niques or in a glove-box (argon gas). Solvents were distilled
under nitrogen from appropriate drying agents (diethyl ether
from CaH2, acetonitrile from CaH2–P2O5, methylene chloride
from P2O5, hexane and tetrahydrofuran (thf ) from sodium/
benzophenone) and stored in dried, N2-filled flasks over 4 Å
molecular sieves. Nitrogen was purged through these solvents
before use. Solvent was transferred to a reaction vessel via a
stainless steel cannula under positive pressure of N2. The
reagents bis(triphenylphosphoranylidene)ammonium chloride
([N(PPh3)2]Cl), iron pentacarbonyl, 4,5-dimethyl-o-xylene-α,αЈ-
dithiol, 2-bromothiophenol, 2-mercaptoethyl sulfide, sodium
bis(trimethylsilyl) amide, ethanethiol, potassium trithio-
carbomate monohydrate, and benzenethiol (Lancaster/Aldrich/
Acros/Fluka) were used as received. Infrared spectra of the νCO
/
νCN stretching frequencies were recorded on a Bio-Rad model
FTS-185 and PerkinElmer model spectrum one B spectro-
photometer with sealed solution cells (0.1 mm) and KBr
windows. UV–vis spectra were recorded on a GBC Cintra 10e
and a Hewlett Packard 71 spectrophotometers. Analyses of
carbon, hydrogen and nitrogen were obtained with a CHN
analyzer (Heraeus).
Preparation of cis-[N(PPh3)2][Fe(CO)2(CN)(S(CH2)2S(CH2)2-
S-S,S,S )] (4)
Compound [N(PPh3)2][Fe(CO)3(CN)2Br] (0.4051 g, 0.5
mmol)5b and Na2[S(CH2)2S(CH2)2S] (0.5 mmol, obtained from
reaction of NaOMe and 2-mercaptoethyl sulfide (HS(CH2)2-
S(CH2)2SH) in a 2 : 1 ratio in MeOH) were dissolved in 10 mL
of thf and stirred for 5 h under nitrogen at 40 ЊC. The resulting
mixture was filtered and diethyl ether–hexane was added to pre-
cipitate the orange solid cis-[N(PPh3)2][Fe(CO)2(CN)(S(CH2)2-
S(CH2)2S-S,S,S)] (yield 0.206 g, 47%). Diffusion of diethyl
ether into thf solution of complex 4 at Ϫ15 ЊC for 3 weeks
yielded yellow crystals suitable for X-ray crystallography. IR
(thf ): 1954s, 2009s (νCO), 2101w (νCN) cmϪ1. 1H NMR (CDCl3):
δ 3.08 (m) (S(CH2)2S). Absorption spectrum (thf ) [λmax/nm
(ε/MϪ1 cmϪ1)]: 338 (1900) (Found: C, 61.97; H, 5.47; N, 3.80.
Calc. for C43H38N2O2P2S3Fe: C, 62.31; H, 4.63; N, 3.38%).
Upon alkylation of complex 4 by CF3SO3Me in thf at ambient
temperature, the infrared spectrum shows the carbonyl/cyanide
stretching bands 2046s, 1999s (νCO), 2115w (νCN) cmϪ1 (thf ),
Preparation of cis,cis-[N(PPh3)2]2[Fe(CO)2(CN)2(CS3-S,S )]
(cis,cis-[N(PPh3)2][Et4N][Fe(CO)2(CN)2(CS3-S,S )]) (1)
Into a 50 mL Schlenk flask were loaded the starting materials
[N(PPh3)2][Fe(CO)3(CN)2Br] (0.5 mmol, 0.4051 g),5b K2CS3
(0.5 mmol, 0.102 g), and [N(PPh3)2]Cl (0.5 mmol, 0.287 g) (or
Et4NCl, 0.5 mmol, 0.084 g). A 10 mL portion of CH3CN was
added to give a yellow–brown solution. The solution mixture
was stirred at room temperature overnight. The resulting mix-
ture was filtered and diethyl ether was added to precipitate the
yellow–brown solid. 5 ml of thf was added to wash the yellow-
brown solid. Recrystallization from CH3CN–diethyl ether gave
the yellow–brown solid cis,cis-[N(PPh3)2]2[Fe(CO)2(CN)2(CS3-
S,S)] (0.465 g, 68%) (or [Et4N][N(PPh3)2][Fe(CO)2(CN)2(CS3-
D a l t o n T r a n s . , 2 0 0 4 , 1 3 7 – 1 4 3
141

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consistent with the formation of the thiolate-alkylated neutral
[Fe(CO)2(CN)(S(CH2)2S(CH2)2SMeS,S,S)]. 1H NMR (CDCl3):
δ 1.23 (s) (SCH3), 2.74 (m) (S(CH2)2S) ppm.
Preparation of [N(PPh3)2]2[Fe(CO)2(CN)3Br] (5)
A solution containing 0.487 g (0.6 mmol) of [N(PPh3)2]-
[Fe(CO)3(CN)2Br] and 80 µL (0.6 mmol) of Me3SiCN in thf
(8 mL) was added to 0.1722 g (0.3 mmol) of [N(PPh3)2]Cl and
the reaction mixture was stirred at ambient temperature for
2 days. A yellow solution accompanied by a light yellow solid
was formed. The mother-liquor was removed under a positive
pressure of N2 and the yellow solid was redissolved in CH3CN.
The resulting mixture was filtered and diethyl ether was then
added to precipitate a yellow solid, [N(PPh3)2]2[Fe(CO)2-
(CN)3Br] (yield 0.5388 g, 66%). Diffusion of diethyl ether into a
CH3CN solution of complex 4 at Ϫ15 ЊC for 3 weeks yielded
yellow crystals suitable for X-ray crystallography. IR (CH3CN):
2039s, 1984br (νCO), 2123vw, 2115w, 2105m (νCN) cmϪ1. Absorp-
tion spectrum (CH3CN) [λmax/nm (ε/MϪ1 cmϪ1)]: 324 (507), 383
(278) (Found: C, 68.35; H, 4.51; N, 4.87. Calc. for C77H60-
BrN5O2P4S5Fe: C, 68.66; H, 4.48; N, 5.20%).
Preparation of mer-[N(PPh3)2]2[Fe(CO)2(CN)3(SC6H4Br)] (6)
A CH3CN solution (10 mL) containing complex 5 (0.539 g,
0.4 mmol) and Na[SC6H4Br] (0.8 mmol, obtained from reaction
of NaOMe and 2-bromothiophenol (HSC6H4Br) in a 1 : 1 ratio
in MeOH) was stirred for 4 days at ambient temperature. The
solution mixture was filtered to remove the insoluble solid and
then diethyl ether was added to precipitate the yellow solid
[N(PPh3)2]2[Fe(CO)2(CN)3(SC6H4Br)] (6) (yield 0.380 g, 65%).
Recrystallization from a saturated CH3CN solution of complex
6 with diethyl ether diffusion at Ϫ10 ЊC gave orange–yellow
crystals suitable for X-ray diffraction. IR (CH3CN): 1978s,
2028s (νCO), 2101w, 2112w, 2119w (νCN) cmϪ1 1H NMR
.
(CD3CN): δ 6.59–6.71 (m), 6.92–6.99 (m), 7.78–7.81 (m), 8.02–
8.05 (m) ppm (SC6H4Br). Absorption spectrum (CH3CN)
[λmax/nm (ε/MϪ1 cmϪ1)]: 418 (468) (Found: C, 69.19; H, 5.66; N,
5.58. Calc. for C83H64N5O2P4SBrFe: C, 68.51; H, 4.43; N,
4.82%). The elemental analysis did not show good agreement
with the calculated values because of the extreme thermal
instability.
Preparation of mer-[N(PPh3)2]2[Fe(CO)2(CN)3(SPh)] (7)
Into a 50-mL Schlenk flask were loaded the starting materials
[N(PPh3)2]2[Fe(CO)2(CN)3Br] (0.539 g, 0.4 mmol), Na[SPh]
(0.176 g, 1.2 mmol), and [N(PPh3)2]Cl (0.459 g, 0.8 mmol). A
10 mL portion of CH3CN was added to give a yellow solution.
The solution mixture was stirred at 5 ЊC for 4 days. The IR
spectrum of the CH3CN solution shows peaks at 2118w, 2113w,
2100m, 2023s, 2000s, 1973s cmϪ1, consistent with the formation
of [N(PPh3)2][Fe(CO)2(CN)3(SPh)] (7) and [N(PPh3)2]2[Fe-
(CN)4(CO)2]. A 2 mL portion of diethyl ether was added and
the resulting mixture was filtered to remove the insoluble solid
trans-[N(PPh3)2]2[Fe(CN)4(CO)2]. Diethyl ether (8 mL) was
then added to precipitate the yellow solid. 5 ml of thf was
added to extract the yellow product and the yellow solid
mer-[N(PPh3)2]2[Fe(CO)2(CN)3(SPh)] (7) was isolated by wash-
ing with hexane (yield 0.076 g, 13.8%). Recrystallization from
CH3CN–diethyl ether gave yellow crystals suitable for X-ray
diffraction. IR (CH3CN): 1973s, 2023s (νCO), 2100w, 2111vw,
2118vw (νCN) cmϪ1 1H NMR (CD3CN): δ 7.51–7.69 (m),
.
7.31 (m) ppm (SC6H5). Absorption spectrum (thf ) [λmax/nm
(ε/MϪ1 cmϪ1)]: 424 (229) (Found: C, 72.75; H, 4.84; N, 5.42.
Calc. for C83H65N5O2P4SFe: C, 72.44; H, 4.76; N, 5.09%). Fol-
lowing extended periods of stirring at ambient temperature, a
CH3CN solution of complex 7 converted into complex 2 and
trans-[Fe(CO)2(CN)4]2Ϫ identified by IR and X-ray diffraction
analysis.
D a l t o n T r a n s . , 2 0 0 4 , 1 3 7 – 1 4 3
142

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2 T. B. Rauchfuss, S. M. Contakes, S. C. N. Hsu, M. A. Reynolds and
Reaction of [N(PPh3)2]2[Fe(CO)2(CN)3Br] and Na[SEt]
S. R. Wilson, J. Am. Chem. Soc., 2001, 123, 6933.
3 H.-F. Hsu, S. A. Koch, C. V. Popescu and E. Münck, J. Am. Chem.
Soc., 1997, 119, 8371.
4 D. Sellmann, F. Geipel and F. W. Heinemann, Chem. Eur. J., 2002, 8,
958.
Into a 50-mL Schlenk flask were loaded the starting materials
[N(PPh3)2]2[Fe(CO)2(CN)3Br] (0.539 g, 0.4 mmol) and Na[SEt]
(0.038 g, 0.4 mmol). A 10 mL portion of CH3CN was added to
give a brown solution. The solution mixture was stirred at room
temperature for 4 days. The IR νCO/νCN spectra shows the form-
ation of complex 2 (2068s, 2023s, 2133w, 2127w, 2118w cmϪ1),
trans-[Fe(CO)2(CN)4]2Ϫ (2104m, 2000s cmϪ1) and [N(PPh3)2]-
[(CO)2(CN)Fe(µ-SEt)3Fe(CN)(CO)2] (2106w, 2033w, 2020s,
1976s cmϪ1).
5 (a) W.-F. Liaw, N.-H. Lee, C.-H. Chen, C.-M. Lee, G.-H. Lee and
S.-M. Peng, J. Am. Chem. Soc., 2000, 122, 488; (b) W.-F. Liaw,
J.-H. Lee, H.-B. Gau, C.-H. Chen, S.-J. Jung, C.-H. Hung, W.-Y.
Chen, C.-H. Hu and G.-H. Lee, J. Am. Chem. Soc., 2002, 124,
1680.
6 (a) A. Volbeda, M.-H. Charon, C. Piras, E. C. Hatchikian, M. Frey
and J. C. Fontecilla-Camps, Nature, 1995, 373, 580; (b) A. Volbeda,
E. Garcin, C. Piras, A. L. de Lacey, V. M. Fernandez, E. C.
Hatchikian, M. Frey and J. C. Fontecilla-Camps, J. Am. Chem. Soc.,
1996, 118, 12989; (c) E. Garcin, X. Vernede, E. C. Hatchikian,
A. Volbeda, M. Frey and J. C. Fontecilla-Camps, Structure, 1999, 7,
557; (d ) R. P. Happe, W. Roseboom, A. J. Pierik, S. P. Albracht and
K. A. Bagley, Nature, 1997, 385, 126.
Crystallography
Crystallographic data of complexes 1–7 are summarized in
Table 2. The crystals of 1–7 are chunky. Each crystal was
mounted on a glass fiber. Diffraction measurements for com-
plexes 1, 2, 4, 6 were carried out at 150(2) K (293(2) K for
complexes 3, 7 and 120(1) K for complex 5) on a Siemens
SMART CCD diffractometer with graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å) and θ between 2.35 and
27.50Њ for complex 1, between 1.40 and 27.53Њ for complex 2,
between 1.44 and 27.51Њ for complex 3, between 1.26 and 27.55Њ
for complex 4, between 1.71 and 27.50Њ for complex 5, between
1.79 and 27.50Њ for complex 6 and between 1.66 and 27.53Њ for
complex 7. Least-squares refinement of the positional and
anisotropic thermal parameters for the contribution of all
non-hydrogen atoms and fixed hydrogen atoms was based on
F 2. Six atoms were refined isotropically for complex 5. Due to
disorder, the hydrogen atoms of the packing H2O and C4H8O in
complexes 2 and 4, respectively, have not been refined. A
SADABS13 absorption correction was made. The SHELXTL14
structure refinement program was employed.
7 (a) Y. Higuchi, H. Ogata, K. Miki, N. Yasuoka and T. Yagi,
Structure, 1999, 7, 549; (b) Y. Higuchi, T. Yagi and N. Yasuoka,
Structure, 1997, 5, 1671.
8 (a) A. L. de Lacey, E. C. Hatchikian, A. Volbeda, M. Frey, J. C.
Fontecilla-Camps and V. M. Fernandez, J. Am. Chem. Soc., 1997,
119, 7181; (b) R. P. Happe, W. Roseboom and S. P. J. Albracht,
Eur. J. Biochem., 1999, 259, 602; (c) H. Ogata, Y. Mizoguchi,
N. Mizuno, K. Miki, S.-I. Adachi, N. Yasuoka, T. Yagi,
O. Yamauchi, S. Hirota and Y. Higuchi, J. Am. Chem. Soc.,
2002, 124, 11628–11635; (d ) S. Foerster, M. Stein, M. Brecht,
H. Ogata, Y. Higuchi and W. Lubitz, J. Am. Chem. Soc., 2003, 125,
83–93.
9 (a) Y. Nicolet, C. Piras, P. Legrand, E. C. Hatchikian and J. C.
Fontecilla-Camps, Structure, 1999, 7, 13; (b) Y. Nicolet, A. L. De
Lacey, X. Vernede, V. M. Fernandez, E. C. Hatchikian and J. C.
Fontecilla-Camps, J. Am. Chem. Soc., 2001, 123, 1596; (c) J. W.
Peters, W. N. Lanzilotta, B. J. Lemon and L. C. Seefeldt, Science,
1998, 282, 1853; (d ) A. L. De Lacey, C. Stadler, C. Cavazza, E. C.
Hatchikian and V. M. Fernandez, J. Am. Chem. Soc., 2000, 122,
11232; (e) C. V. Popescu and E. Münck, J. Am. Chem. Soc., 1999,
121, 7877.
CCDC reference numbers 217010–217016.
See http://www.rsc.org/suppdata/dt/b3/b311059a/ for crystal-
lographic data in CIF or other electronic format.
10 (a) B. J. Lemon and J. W. Peters, J. Am. Chem. Soc., 2000, 122, 3793;
(b) B. J. Lemon and J. W. Peters, Biochemistry, 1999, 38, 12969;
(c) B. Bennett, B. J. Lemon and J. W. Peters, Biochemistry, 2000, 39,
7455.
Acknowledgements
11 L. Casalot and M. Rousset, Trends Microbiol., 2001, 9,
We gratefully acknowledge financial support from the National
Science Council (Taiwan).
228.
12 W.-F. Liaw, W.-T. Tsai, H.-B. Gau, C.-M. Lee, S.-Y. Chou, W.-Y.
Chen and G.-H. Lee, Inorg. Chem., 2003, 42, 2783.
13 G. M. Sheldrick, SADABS, Siemens Area Detector Absorption
Correction Program, University of Göttingen, Germany, 1996.
14 G. M. Sheldrick, SHELXTL, Program for Crystal Structure
Determination, Siemens Analytical X-ray Instruments Inc.,
Madison, WI, 1994.
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D a l t o n T r a n s . , 2 0 0 4 , 1 3 7 – 1 4 3
143