A dithiolate-bridged (CN)2(CO)Fe-Ni complex reproducing the IR bands of [NiFe] hydrogenase

Article abstract of DOI:10.1021/ic900017s

A dithiolate-bridged dinuclear Fe-Ni complex, which has the desired fac-(CN)₂(CO) ligand set at iron, has been synthesized. Its CN/CO bands in the IR spectrum reproduce those of the Ni-A, Ni-B, and Ni-SU states, which indicate that these octahe

Full text of DOI:10.1021/ic900017s

Inorg. Chem. 2009, 48, 2358-2360
A Dithiolate-Bridged (CN)2(CO)Fe-Ni Complex Reproducing the IR
Bands of [NiFe] Hydrogenase
Soichiro Tanino, Zilong Li, Yasuhiro Ohki, and Kazuyuki Tatsumi*
Department of Chemistry, Graduate School of Science, and Research Center for Materials
Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
Received January 6, 2009
Chart 1
A dithiolate-bridged dinuclear Fe-Ni complex, which has the
desired fac-(CN)2(CO) ligand set at iron, has been synthesized.
Its CN/CO bands in the IR spectrum reproduce those of the Ni-A,
Ni-B, and Ni-SU states, which indicate that these octahedral
FeII centers have similar electronic properties. This result verifies
the assignment of a (CN)2(CO)FeII moiety in the active site of [NiFe]
hydrogenase.
in the enzymatic function is not yet well understood.7 On
Hydrogenases are essential for hydrogen metabolisms of
many microorganisms,1 and the crystal structures of [NiFe],
the basis of electron paramagnetic resonance, IR, and
electrochemical measurements, various states of [NiFe]
hydrogenase have been identified as summarized in Chart
1, where the active site consists of a common (CN)2(CO)Fe
moiety linked to a Ni atom by two cysteinyl thiolate bridges.
[FeFe], and [Fe] hydrogenases have been determined.2-4
A
unique feature of the [NiFe] hydrogenase is that the Fe center
carries CO and CN ligands,5,6 while the role of these ligands
In the course of our studies of structural and functional
models of the active sites of [NiFe] hydrogenase,8 we have
reported the syntheses of (PPh4)[(CN)2(CO)2Fe(µ-pdt)Ni-
* To whom correspondence should be addressed. E-mail: i45100a@
nucc.cc.nagoya-u.ac.jp.
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(S2CNR2)] [1; pdt ) 1,3-propanedithiolate, S2CNR2
)
dithiocarbamates (R ) Et, R2 ) -(CH2)5-)]8a and a series
of bis- and tris(thiolate)-bridged Fe(CO)3Ni complexes.8c
Other dinuclear Fe-Ni model complexes previously reported,
which have ligand sets such as aminethiolates, phosphines,
η5-C5H5, or nitric oxide, match the enzyme active sites less
well compared with those in this report.9 Although the model
complexes 1 are unique in that they possess the crucial CO/
CN ligand set on iron, the number of CO ligands differs
from that found in the enzyme active sites. This paper
describes the synthesis of a thiolate-bridged (CN)2(CO)-
Fe-Ni complex, [(CN)2(CO)Fe(µ-tpdt)Ni(S2CNEt2)]- (2;
(5) (a) Happe, R. P.; Roseboom, W.; Pierik, A. J.; Albracht, S. P. J.;
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2358 Inorganic Chemistry, Vol. 48, No. 6, 2009
10.1021/ic900017s CCC: $40.75  2009 American Chemical Society
Published on Web 02/17/2009

COMMUNICATION
Scheme 1
Figure 1. Structure of the complex anion of 2 with CH3OH as a crystal
solvent. The thermal ellipsoids are given at the 50% probability level.
Selected bond distances (Å): Fe-Ni ) 3.2960(5), Fe-C1 ) 1.900(3),
Fe-C2 ) 1.904(2), Fe-C3 ) 1.789(3), Fe-S1 ) 2.2982(7), Fe-S2 )
2.2593(7), Fe-S3 ) 2.3129(9), Ni-S1 ) 2.2021(9), Ni-S3 ) 2.1952(7).
tpdt ) 3-thiapentanedithiolate10), from the reaction of
preformed [(CN)2(CO)Fe(tpdt)K]- (3) and (PPh3)NiBr-
(S2CNEt2). With an Fe(CN)2(CO) fragment mimicking the
enzyme active sites, complex 2 shows CN and CO bands
closely resembling those of the Ni-A, Ni-B, and Ni-SU
states of [NiFe] hydrogenase.
reaction mixture exhibits a set of CN/CO bands, which
indicates that 3 is the major product.15
We have successfully used 3, generated in situ, as a
synthon of the target dinuclear Fe-Ni complex 2. Treatment
of a methanol solution of 3 with an acetone solution of
(PPh3)NiBr(S2CNEt2) at -40 °C resulted in a dark-brown
solution, from which the dithiolate-bridged dinuclear com-
plex 2 was isolated as a dark-brown powder in 51% yield
based on (PPh4)[4]. Single crystals of 2 were grown at 0 °C
from an ether-layered methanol-acetonitrile solution, and
X-ray analysis revealed that the desired (CN)2(CO)Fe moiety
is retained during the formation of the Fe-Ni dinuclear
structure. Complex 2 crystallized with one methanol mol-
ecule and one acetonitrile molecule each in the asymmetric
unit as crystal solvents. Figure 1 shows the crystal structure
of 2·CH3OH ·CH3CN, and selected bond distances are given
in the caption. The Fe and Ni centers are bridged by the
two thiolato S atoms of tpdt, and the Fe(µ-S)2Ni rhombus is
slightly puckered along the S1-S3 vector with a dihedral
angle of 14.7°. The Fe-Ni distance of 3.2960(5) Å is long,
as are those of the oxidized forms of [NiFe] hydrogenase
(2.8-2.9 Å). The Ni atom is further coordinated by the
dithiocarbamate S atoms, conforming a square-planar ge-
ometry. With coordination of the thioether S atom of tpdt,
the Fe center adopts an octahedral geometry. In the X-ray
structure, the CN and CO ligands are distinguishable based
on significantly different Fe-C bond lengths. The two long
Fe-C bonds of 1.900(3) and 1.904(2) Å are assigned to CN
ligands, while the short Fe-C bond of 1.789(3) Å is
associated with CO, because CN is a weaker π acid. The
two CN ligands are chemically inequivalent, being trans to
a bridging thiolato sulfur or to a thioether S atom, although
their Fe-C bond lengths are nearly identical. The asymmetric
nature of 2 is corroborated by 1H NMR, which exhibits eight
sets of signals for the tpdt protons. The 1H-1H NMR COSY
spectrum further identified which sets of signals are coupled
in the 1H NMR spectrum. The coordination geometry at the
Fe atom resembles that of [(CN)2(CO)Fe(“S3”)]2- [“S3” )
bis(2-mercaptophenyl)sulfide].12 Noteworthy here is the
hydrogen bonding observed in the crystal between a CN
ligand and the methanol molecule with a N · · · O distance of
2.923(6) Å. This hydrogen bond may be relevant to that
Two structurally characterized thiolate complexes of iron,
[(CN)2(CO)Fe(bdt)]2- (bdt ) 1,2-benzenedithiolate)11 and
[(CN)2(CO)Fe(“S3”)]2- [“S3” ) bis(2-mercaptophenyl)sul-
fide],12 have been reported that have the essential
(CN)2(CO)Fe moiety. We have prepared (PPh4)[Fe(CN)2(CO)2-
(pdt)K] from (PPh4)[(CN)2(CO)3FeBr] ((PPh4)[4])13 and
K2(pdt), for use as a building block for the synthesis of 1.8a
Liaw et al. have reported the synthesis of the sulfur tridentate
(tpdt2-) complex, [N(PPh3)2][(CN)(CO)2Fe(tpdt)], from the
analogous reaction of [N(PPh3)2][4] with Na2(tpdt) in tet-
rahydrofuran at 40 °C.14 In contrast, when we carried out
the reaction between (PPh4)[4] and K2(tpdt) in methanol at
80 °C, the formation of 3 was detected by electrospray
ionization mass spectrometry. The IR spectrum of the
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Inorganic Chemistry, Vol. 48, No. 6, 2009 2359

COMMUNICATION
1900-2150 cm-1 region resembles closely that of the Ni-A
state of Allochromatium Vinosum hydrogenase.5c The νCN
and νCO values of the Ni-B and Ni-SU states are also
similar, while those for the other hydrogenase states of A.
Vinosum are systematically lower. Notably, a CH3CN solution
of half-sandwich iron complex K[(η5-C5H5)Fe(CN)2(CO)]
also gives CO/CN bands that match those of the active site.17
On the other hand, for the mononuclear dithiolate complex
anions containing the (CN)2(CO)Fe moiety, [(CN)2(CO)-
Fe(bdt)]2-,11 and [(CN)2(CO)Fe(“S3”)]2-,12 their IR spectra
in CH3CN match less well those of the Ni-A, Ni-B, and
Ni-SU states of the [NiFe] enzyme (Figure 2).
Figure 2. IR spectra of the Ni-A form of hydrogenase and
2·CH3OH·CH3CN in KBr (left). At right, the CN/CO stretching frequencies
(cm-1) of 2·CH3OH·CH3CN observed in KBr and CH3CN are compared
with those of various states of the [NiFe] hydrogenase from A. Vinosum.5c
In summary, we have synthesized a dithiolate-bridged
dinuclear Fe-Ni complex 2, which has the desired fac-
(CN)2(CO) ligand set at iron. The square-planar NiII center
of 2 differs from the Ni sites of Ni-A/Ni-B and Ni-SU
states of [NiFe] hydrogenase, which are thought to contain
pentacoordinated NiIII and NiII centers.2,5,6 Nevertheless, the
CN/CO bands in the IR spectrum of 2 reproduce those of
the Ni-A, Ni-B, and Ni-SU states, which indicate that
both of these octahedral FeII centers have similar electronic
properties. This result verifies the assignment of a
(CN)2(CO)FeII moiety in the active site of [NiFe] hydroge-
nase.
found in the protein crystal structures of hydrogenases. For
instance, the O(H) and N(H) atoms of Ser499 in the enzyme
of DesulfoVibrio fructosoVorans are both located 2.9 Å away
from a CN nitrogen of the active site, while the other CN
ligand may also form a hydrogen bond with one of the two
N(H) groups of Arg476 (NCN · · · NNH ) 2.8 Å).16
The IR spectrum of 2 · CH3OH · CH3CN in KBr is repro-
duced in the lower left part of Figure 2, where the two bands
at 2094 and 2083 cm-1 are assigned to C-N stretching
modes (νCN) and an intense νCO band appears at 1944 cm-1.
The CN and CO bands are both shifted to lower frequencies
compared with those of (PPh4)[(CN)2(CO)2Fe(µ-
Acknowledgment. This research was financially supported
by Grants-in-Aid for Scientific Research (Grants 18GS0207
and 18064009) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan. We thank Prof.
Roger E. Cramer at the University of Hawaii for fruitful
discussion and for careful reading of the manuscript.
pdt)Ni(S2CNEt2)] (νCN ) 2108 and 2092 cm-1, νCO
)
2031-2015 and 1975-1959 cm-1) because the smaller
number of CO ligands in 2 leads to greater π-back-donation
from the Fe center. The CN-CH3OH hydrogen bond found
in the X-ray structure of 2 · CH3OH · CH3CN may also
contribute to the νCN shift. Given the facial (CN)2(CO) ligand
arrangement on Fe in the dinuclear Fe-Ni structure, a
sensible comparison between the CN/CO bands of 2 and
[NiFe] hydrogenase, which are listed on the right side of
Figure 2, can be made. In fact, the IR spectrum of 2 in the
Supporting Information Available: Experimental details and
spectral data and information on X-ray analysis and a CIF file of
the X-ray crystallographic data for 2. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC900017S
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Hatchikian, E. C. J. Biol. Inorg. Chem. 2003, 8, 129–134.
D. J.; Darensbourg, M. Y. J. Am. Chem. Soc. 1998, 120, 10103–10114.
2360 Inorganic Chemistry, Vol. 48, No. 6, 2009