A rare bond between a soft metal (FeI) and a relatively hard base (RO-, R = phenolic moiety)

Article abstract of DOI:10.1016/j.inoche.2010.06.026

Reacting a bidentate ligand H₂L, 2-(2-methoxybenzyl)-2- methylpropane-1,3-dithiol, with Fe₃(CO)₁₂ formed a diiron hexacarbonyl complex (1Me) from which a diiron hexacarbonyl complex (1H) pendant with a phenolic group was derived via in-situ demethylation. Further deprotonation of complex 1H gave a diiron pentacarbonyl species (1) in which a rare bond between the soft metal Fe^I and the relatively hard base phenolate formed, Fe^I-OR (R = phenolic moiety). This bonding may be a suitable mimic of the bonding feature, {Fe^IFe^I}R-OH/ OH₂ found in the [FeFe]-hydrogenase.

Full text of DOI:10.1016/j.inoche.2010.06.026

Inorganic Chemistry Communications 13 (2010) 1089–1092
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A rare bond between a soft metal (FeI) and a relatively
hard base (RO−, R = phenolic moiety)
Wei Zhong a, Ying Tang a, Giuseppe Zampella b, Xiufeng Wang a, Xinlei Yang c, Bin Hu c, Jiang Wang c,
Zhiyin Xiao a, Zhenhong Wei a, Huanwen Chen c, Luca De Gioia b, Xiaoming Liu a,
⁎
a
Department of Chemistry/Institute for Advanced Study, Nanchang University, Nanchang 330031, China
Department of Biotechnology and Biosciences, University of Milano-Bicocca, 20106 Milano, Italy
b
c
College of Chemistry, Biology and Materials Sciences, East China Institute of Technology, Fuzhou 344000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Reacting a bidentate ligand H2L, 2-(2-methoxybenzyl)-2-methylpropane-1,3-dithiol, with Fe3(CO)12 formed
a diiron hexacarbonyl complex (1Me) from which a diiron hexacarbonyl complex (1H) pendant with a
phenolic group was derived via in-situ demethylation. Further deprotonation of complex 1H gave a diiron
pentacarbonyl species (1) in which a rare bond between the soft metal FeI and the relatively hard base
phenolate formed, FeI-OR (R = phenolic moiety). This bonding may be a suitable mimic of the bonding
feature, {FeIFeI}R-OH/OH2 found in the [FeFe]-hydrogenase.
Received 24 May 2010
Accepted 10 June 2010
Available online 17 June 2010
Keywords:
[FeFe]-hydrogenase
Diiron model complex
Phenolate
© 2010 Elsevier B.V. All rights reserved.
DFT calculations
Tandem mass spectrometry
In the past decade, great efforts have been devoted to the
bioinorganic chemistry of [FeFe]-hydrogenase in order to understand
the enzymatic chemistry of high efficiency in catalysis of both
hydrogen evolution and oxidation [1–12]. Modelling the unusual
features of the dimetallic centre such as, {FeIIFeI}–OH2/OH−, “semi-
bridging” CO, and proximal connection to a {Fe4S4}-cubane via a
cysteinate bridge, Fig. 1 (left) [13–15], have been the subject under
intensive investigations. Indeed, some synthetic systems mimicked
elegantly some of the peculiar enzymatic structural features, for
example, a synthetic H-cluster framework [1], a diiron complex with
“semi-bridging” CO and an unoccupied “vacant site” at {FeIIFeI}
oxidation states level [10,11]. However, to date model complexes
featuring {FeIFeI}-OR or {FeIIFeI}-OR (R = organic moiety) moiety
have not been reported.
demethylated product, [Fe2(μ-SCH2)2CMe(CH2-o-C6H4OH)(CO)6]
(1H), which bears a pendant phenolic group. Deprotonation of the
pendant phenol led to a species possessing an infrared spectral profile
similar to those of other diiron pentacarbonyl complexes reported
previously [3,5,8,20]. Infrared spectroscopic analysis, tandem mass
spectrometric, and theoretical investigations suggested that this species
with an FeI-OR bond, [Fe2(μ-SCH2)2CMe(CH2-o-C6H4O)(CO)5]− (1),
takes the geometry found for other neutral diiron pentacarbonyl
complexes reported in the literatures [5,6].
The dithiol ligand (H2L) was prepared via a 7-step synthesis,
Scheme 1. With the phenol in this ligand protected, complex 1Me was
formed by following a well-established procedure, with a yield over
85%. Demethylation of complex 1Me with BBr3 afforded complex 1H
in good yield (ca. 60%). In acetonitrile, both complexes show
characteristic infrared bands of diiron hexacarbonyl complexes and
have nearly superimposable spectral profile since demethylation
occurs far away from the Fe–Fe centre. The identities of complexes
1Me and 1H were further, unambiguously, confirmed by microanal-
ysis and 1H NMR spectroscopy. In addition, complex 1Me was also
crystallographically analysed, Fig. 1 (right), showing that the
geometry of this complex is essentially identical to other diiron
hexacarbonyl complexes reported previously [5,6].
Systems featuring the {FeIFeI} oxidation states have been the most
accessible in modelling the diiron sub-unit of the enzyme. Assembling
a system with {FeIFeI}-OR would be the first step leading to its
oxidised form, {FeIIFeI}-OR via one electron oxidation. Indeed,
previous attempts made by using ligands containing carboxylic acid,
ether oxygen, and alcoholyl group were unsuccessful [16–19].
Herein, we report a novel bidentate ligand (H2L), 2-(2-methox-
ybenzyl)-2-methylpropane-1,3-dithiol, its diiron hexacarbonyl com-
plex, [Fe2(μ-SCH2)2CMe(CH2-o-C6H4OMe)(CO)6] (1Me), and its
Treatment of a solution of complex 1H with excess of NaH in MeCN
at 0 °C caused distinct change in colour, from light to dark red, and
generated a group of new IR absorption bands at 2023.8, 1954.2,
1892.6 cm− 1, Fig. 2 (dashed line). On average, the stretching
frequency of the newly produced species shifted to low frequency
by nearly 60 cm−1 compared to its parent complex 1H. Presumably,
⁎
Corresponding author. Tel./fax: +86 791 3969254.
E-mail address: xiaoming.liu@ncu.edu.cn (X. Liu).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.06.026

1090
W. Zhong et al. / Inorganic Chemistry Communications 13 (2010) 1089–1092
Fig. 1. Schematic view of the H-cluster of the [FeFe]-hydrogenase (left) and crystal structure of complex 1Me (right).
the pendant phenol of 1H was first deprotonated by NaH to produce
an intermediate with a pendant phenolate which then subsequently
binds to the proximal iron to form an anionic diiron pentacarbonyl
complex by expelling one CO molecule, Scheme 2.
The final product showed characteristic spectral pattern of diiron
pentacarbonyl complexes [3,5,8,20]. Plotting the average absorption
bands of those diiron pentacarbonyl complexes reported in literatures
against those corresponding bands of the proposed complex 1 produces
an excellent linear correlation, Fig. 2. The linear correlation strongly
suggests that the final species formed is characterised by a geometry
similarto that of other diiron pentacarbonyl complexes[5]. Compared to
previously reported diiron pentacarbonyl species [3,5,8,20], these
absorption bands of the resultant species show a large “red” shift by
about 30 cm−1on average, Fig. 3, which strongly suggests the binding of
the pendant phenolate to the Fe–Fe centre to form a complex featuring a
{FeIFeI}-OR core, Scheme 2. Compared to themimic containinga {Fe4S4}-
Scheme 1. Synthetic route leading to ligand H2L, complexes 1Me, and 1H.

W. Zhong et al. / Inorganic Chemistry Communications 13 (2010) 1089–1092
1091
Fig. 2. Infrared spectra of complexes 1Me, 1H, and 1.
Fig. 3. Plot of average infrared absorption bands of the diiron pentacarbonyl complexes
reported in the literature, [Fe2(μ-SCH2)2CMe(X)(CO)5] (X=CH2SMe, CH2NH2, pyridin-2-yl)
against the corresponding bands of complex 1.
cubane [1], the absorption bands of the proposed complex 1 are even
further downshifted on average by 15 cm−1, which is indicative of a
strong electronic effect exerted by the phenolate on the Fe–Fe centre via
direct binding.
correlated to its structure, the observed linear correlation between the
computed and experimentally observed bands provides further support
for the structural assignment of complex 1.
In addition to the infrared spectroscopic evidence shown above,
the identity of the proposed species 1 was further unambiguously
established by tandem mass spectrometry and DFT calculations even
though characterising the proposed species by NMR spectroscopy and
crystallography turned out to be unsuccessful due to the relatively
poor stability of such derivative (refer also to Fig. S1). As shown in Fig.
S3a, a molecular peak at 477 corresponding to the proposed diiron
pentacarbonyl anion 1 is dominant and related fragments, which are
obtained by stripping CO off from the parent anion in the manner of
one-by-one by using tandem mass spectrometry, are also evident, Fig.
S3b–f. Possible dissociation pathways for those observed intermedi-
ates are depicted in Fig. S3g.
DFT calculations indicate that the reaction leading to species 1
isomer featuring five coordinated CO ligands in which the RO group is
directly bound to one iron atom as shown in Fig. 4 is exothermic
(−16.3 kcal mol−1). The DFT structure of species 1 is characterised by
overall geometry and bonding parameters typical of analogous diiron
complexes [3,5,8]. DFT calculations generated IR absorption bands at
2039.1 1988.6, 1979. 5, 1967.3, and 1900.7 cm−1 for species 1, of which
the three bands around 1980 cm−1 are corresponding to the relatively
broad absorption band centred at 1954.2 cm−1 observed experimen-
tally, Fig. 2. Plotting the computed against the bands observed
experimentally produced a very good linear correlation (R=0.99) as
shown in Fig. S3. Since the spectroscopic profile of a molecule is closely
In summary, we have described a route leading to a novel
phenolate-bound diiron pentacarbonyl anion (1), which possesses a
rare bond between a soft metal and a relatively hard base, phenolate.
Its identity and structural features were established by infrared
spectroscopic and tandem mass spectrometric analyses, as well as DFT
calculations. The reported synthetic approach may provide a starting
point to access mimics for the dimetallic core, {FeIIFeI}–OH2/OH−, of
the enzyme. In the light of the structural and electronic resemblances
to the dimetallic centre of the enzyme, the presented phenolate-based
complex may be another appropriate system to further explore its
bioinorganic chemistry relevant to the diiron sub-unit of the enzyme,
in addition to other categories of models investigated intensively in
the past decade, that is, complexes featuring {Fe2S2}-core [21–23],
aza-containing complexes [24,25], and {Fe2S2X}-based models (X=S
or N) [1,5,8,26].
Acknowledgments
We thank the Natural Science Foundation of China (Grant No.
20571038, 20871064), Ministry of Education of China, and Centre of
Analysis and Testing at Nanchang University for supporting this
work.
Scheme 2. Conversion of complexes 1H into 1.

1092
W. Zhong et al. / Inorganic Chemistry Communications 13 (2010) 1089–1092
relative to the definition of an entatic state in the active site, Coord. Chem. Rev.
238 (2003) 255–266.
[8] X.M. Liu, S.K. Ibrahim, C. Tard, C.J. Pickett, Iron-only hydrogenase: synthetic,
structural and reactivity studies of model compounds, Coord. Chem. Rev. 249
(2005) 1641–1652.
[9] L.C. Sun, B. Akermark, S. Ott, Iron hydrogenase active site mimics in supramo-
lecular systems aiming for light-driven hydrogen production, Coord. Chem. Rev.
249 (2005) 1653–1663.
[10] C.M. Thomas, T. Liu, M.B. Hall, M.Y. Darensbourg, Series of mixed valent Fe(II)Fe(I)
complexes that model the hox state of [FeFe]hydrogenase: redox properties,
density–functional theory investigation, and reactivities with extrinsic CO, Inorg.
Chem. 47 (2008) 7009–7024.
[11] T. Liu, M.Y. Darensbourg, A mixed-valent, Fe(II)Fe(I), diiron complex reproduces
the unique rotated state of the [FeFe]hydrogenase active site, J. Am. Chem. Soc.
129 (2007) 7008–7009.
[12] M. Bruschi, G. Zampella, P. Fantucci, L. De Gioia, DFT investigations of models
related to the active site of [NiFe] and [Fe] hydrogenases, Coord. Chem. Rev. 249
(2005) 1620–1640.
[13] A.S. Pandey, T.V. Harris, L.J. Giles, J.W. Peters, R.K. Szilagyi, Dithiomethylether as a
ligand in the hydrogenase H-cluster, J. Am. Chem. Soc. 130 (2008) 4533–4540.
[14] J.W. Peters, W.N. Lanzilotta, B.J. Lemon, L.C. Seefeldt, X-ray crystal structure of the
Fe-only hydrogenase (Cpl) from Clostridium pasteurianum to 1.8 angstrom
resolution, Science 282 (1998) 1853–1858.
[15] Y. Nicolet, C. Piras, P. Legrand, C.E. Hatchikian, J.C. Fontecilla-Camps, Desulfovibrio
desulfuricans iron hydrogenase: the structure shows unusual coordination to an
active site Fe binuclear center, Structure 7 (1999) 13–23.
[16] P.I. Volkers, T.B. Rauchfuss, S.R. Wilson, Coordination chemistry of 3-mercapto-2-
(mercaptomethyl)propanoic acid (dihydroasparagusic acid) with iron and nickel,
Eur. J. Inorg. Chem (2006) 4793–4799.
Fig. 4. Structural view of complex 1 with its positioned sodium cation including selected
bond lengths (Å) derived from DFT calculations. Colour legends for atoms: turquoise
(Fe), purple (Na), yellow (S), red (O), green (C), and grey (H).
[17] C.M. Thomas, O. Rudiger, T. Liu, C.E. Carson, M.B. Hall, M.Y. Darensbourg, Synthesis
of carboxylic acid-modified [FeFe]-hydrogenase model complexes amenable to
surface immobilization, Organometallics 26 (2007) 3976–3984.
[18] J.F. Capon, S. Ezzaher, F. Gloaguen, F.Y. Petillon, P. Schollhammer, J. Talarmin, T.J.
Davin, J.E. McGrady, K.W. Muir, Electrochemical and theoretical investigations of
the reduction of [Fe-2(CO)(5)L{mu-SCH2XCH2S}] complexes related to [FeFe]
hydrogenase, New J. Chem. 31 (2007) 2052–2064.
Appendix A. Supplementary data
[19] Y.W. Wang, Z.M. Li, X.H. Zeng, X.F. Wang, C.X. Zhan, Y.Q. Liu, X.R. Zeng, Q.Y. Luo, X.
M. Liu, Synthesis and characterisation of three diiron tetracarbonyl complexes
related to the diiron centre of [FeFe]-hydrogenase and their protonating,
electrochemical investigations, New J. Chem. 33 (2009) 1780–1789.
[20] S.J. George, Z. Cui, M. Razavet, C.J. Pickett, The di-iron subsite of all-iron
hydrogenase: mechanism of cyanation of a synthetic 2Fe3S — carbonyl assembly,
Chem. Eur. J. 8 (2002) 4037–4046.
Supplementary data associated with this article can be found in the
online version, at doi:10.1016/j.inoche.2010.06.026.
References
[1] C. Tard, X.M. Liu, S.K. Ibrahim, M. Bruschi, L. De Gioia, S.C. Davies, X. Yang, L.S.
Wang, G. Sawers, C.J. Pickett, Synthesis of the H-cluster framework of iron-only
hydrogenase, Nature 433 (2005) 610–613.
[21] M. Schmidt, S.M. Contakes, T.B. Rauchfuss, First generation analogues of the
binuclear site in the Fe-only hydrogenases: Fe-2(mu-SR)(2)(CO)(4)(CN)(2)(2-),
J. Am. Chem. Soc. 121 (1999) 9736–9737.
[2] A.K. Justice, T.B. Rauchfuss, S.R. Wilson, Unsaturated, mixed-valence diiron
dithiolate model for the H-ox state of the [FeFe] hydrogenase, Angew. Chem.
Int. Ed. 46 (2007) 6152–6154.
[3] M. Razavet, S.C. Davies, D.L. Hughes, C.J. Pickett, {2Fe3S} clusters related to the di-
iron sub-site of the H-centre of all-iron hydrogenases, Chem. Commun. (2001)
847–848.
[22] A. Le Cloirec, S.P. Best, S. Borg, S.C. Davies, D.J. Evans, D.L. Hughes, C.J. Pickett, A di-
iron dithiolate possessing structural elements of the carbonyl/cyanide sub-site of
the H-centre of Fe-only hydrogenase, Chem. Commun. (1999) 2285–2286.
[23] E.J. Lyon, I.P. Georgakaki, J.H. Reibenspies, M.Y. Darensbourg, Carbon monoxide
and cyanide ligands in a classical organometallic complex model for Fe-only
hydrogenase, Angew. Chem. Int. Ed. 38 (1999) 3178–3180.
[4] L. Schwartz, G. Eilers, L. Eriksson, A. Gogoll, R. Lomoth, S. Ott, Iron hydrogenase
active site mimic holding a proton and a hydride, Chem. Commun. (2006)
520–522.
[5] F.F. Xu, C. Tard, X.F. Wang, S.K. Ibrahim, D.L. Hughes, W. Zhong, X.R. Zeng, Q.Y. Luo,
X.M. Liu, C.J. Pickett, Controlling carbon monoxide binding at di-iron units related
to the iron-only hydrogenase sub-site, Chem. Commun. (2008) 606–608.
[6] C. Tard, C.J. Pickett, Structural and functional analogues of the active sites of the
[Fe]-, [NiFe]-, and [FeFe]-hydrogenases, Chem. Rev. 109 (2009) 2245–2274.
[7] I.P. Georgakaki, L.M. Thomson, E.J. Lyon, M.B. Hall, M.Y. Darensbourg, Fundamen-
tal properties of small molecule models of Fe-only hydrogenase: computations
[24] J.D. Lawrence, H.X. Li, T.B. Rauchfuss, M. Benard, M.M. Rohmer, Diiron
azadithiolates as models for the iron-only hydrogenase active site: synthesis,
structure, and stereoelectronics, Angew. Chem. Int. Ed. 40 (2001) 1768–1771.
[25] J.D. Lawrence, H.X. Li, T.B. Rauchfuss, Beyond Fe-only hydrogenases: N-
functionalized 2-aza-1,3-dithiolates Fe-2[(SCH2)(2)NR](CO)(x) (x=5,6), Chem.
Commun. (2001) 1482–1483.
[26] M. Razavet, S.C. Davies, D.L. Hughes, J.E. Barclay, D.J. Evans, S.A. Fairhurst, X.M. Liu,
C.J. Pickett, All-iron hydrogenase: synthesis, structure and properties of {2Fe3S}-
assemblies related to the di-iron sub-site of the H-cluster, Dalton Trans. (2003)
586–595.