Fe-only hydrogenase active site mimics: Fe2(CO)6(μ-ADT) (ADT = azadithiolate) clusters bearing pendant 2,2′:6′,2″-terpyridine domains and containing alkynylthienylene or alkynylphenylene spacers

Article abstract of DOI:10.1016/j.inoche.2009.06.003

We report the syntheses and single crystal structures of two Fe₂(CO)₆(μ-ADT) (ADT = azadithiolate) clusters bearing pendant tpy (tpy = 2,2′:6′,2″-terpyridine) domains and containing alkynylthienylene or alkynylphenylene spacers betwe

Full text of DOI:10.1016/j.inoche.2009.06.003

Inorganic Chemistry Communications 13 (2010) 457–460
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Fe-only hydrogenase active site mimics: Fe2(CO)6( -ADT) (ADT = azadithiolate)
l
clusters bearing pendant 2,20:60,200-terpyridine domains and containing
alkynylthienylene or alkynylphenylene spacers
*
*
Edwin C. Constable , Catherine E. Housecroft , Swarna Latha Kokatam, Elaine A. Medlycott,
Jennifer A. Zampese
Department of Chemistry, University of Basel, Spitalstrasse 51, CH 4056 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the syntheses and single crystal structures of two Fe2(CO)6(l-ADT) (ADT = azadithiolate) clus-
ters bearing pendant tpy (tpy = 2,20:60,200-terpyridine) domains and containing alkynylthienylene or alky-
nylphenylene spacers between the Fe-only hydrogenase active site mimic and tpy metal-binding unit.
Ó 2009 Elsevier B.V. All rights reserved.
Received 28 April 2009
Accepted 4 June 2009
Available online 9 June 2009
Keywords:
Fe-only hydrogenase
X-ray
2,20:60,200-terpyridine
Iron sulfur cluster
Sources of clean and renewable energy are at the forefront of
international research efforts, with hydrogen production being of
critical importance. The crystallographic characterization of the ac-
tive site of Fe-only hydrogenase [1] and the observation that a
molecular mimic of the active site in Fe-only hydrogenase can
function as an electrocatalyst for proton reduction [2,3], have revi-
talized interest in the properties of small, sulfur-containing iron
carbonyl clusters. It is proposed that the presence of the amine
group in the active site facilitates its catalytic role in proton reduc-
tion and H2 evolution [4]. A number of papers have now appeared
detailing the syntheses and catalytic properties of Fe-only hydrog-
enase active site models. Most are based upon an Fe2(CO)6S2 core
with azadithiolate (ADT) bridge, and the effects of varying the ter-
minal N substituent, or introducing phosphine in place of carbonyl
ligands have been investigated [5–16]. Selenium-containing model
clusters have also been reported [17]. In order to facilitate light-
driven proton reduction, model clusters covalently linked
to [Ru(bpy)3]2+ (bpy = 2,20-bipyridine) or [Ru(tpy)2]2+ (tpy =
2,20:60,200-terpyridine) photosensitizers [14,18–20] or associated
with (but not covalently linked to) [Ru(bpy)3]2+ [21,22] have been
studied. The photophysical properties of {M(tpy)2}n+ units are often
poorer than those of {M(bpy)3}n+ (bpy = 2,20-bipyridine) [23]. This
problem can be addressed by introducing thienylene or alkynyl
spacers to tpy to enhance electronic communication between the
latter and the appended functionality [24–33]. We recently de-
scribed approaches to compounds in which a tpy domain was cova-
lently linked to alkynylthienyl or alkynylbithienyl substituents
[34,35]. We now report the synthesis and structural characteriza-
tion of two Fe2(CO)6(l-ADT) clusters bearing pendant tpy domains,
and containing alkynylthienylene or alkynylphenylene spacers.
The synthesis of 1 (Scheme 1) in a one-pot reaction by reaction of
4-ethynylbenzaldehyde, 2-acetylpyridine and NH4OH [36] is sim-
pler than the published route [37]. The 1H NMR spectrum of 1 was
assigned by 2D techniques, the signals for HC2 and HC3 being distin-
guished by the observation of a NOESY HC2-HB3 cross peak. Alkyne 1
or 2 [34] was coupled to 5 [20,22] under Sonogashira conditions
[38,39] to give 3 or 4, respectively [40]. Both compounds are light-
sensitive and must be stored in the dark. The electrospray mass
spectra of 3 and 4 each showed a parent ion, with the only other in-
tense peak corresponding to loss of six CO ligands. In the IR spectra
of the compounds, the pattern of strong absorptions around
2000 cmꢀ1 was consistent with the presence of the Fe2(CO)6 unit.
On going from 1 to 3, the disappearance of the signal for the al-
kyne proton (d 3.19 ppm) and the appearance of a singlet at d
4.35 ppm for the N(CH2)2 unit, and signals at d 7.53 and 6.74 ppm
(assigned by NOESY) for the N-attached arene ring, confirmed the
formation of 3. The signals for the phenyltpy unit in the 1H NMR
spectrum of 3 were essentially a superimposition of those in 1.
Similarly, the 1H NMR spectrum of 4 showed the spectroscopic sig-
nature of ligand 2 [34] with an additional singlet at d 4.34 ppm, and
signals at d 7.50 and 6.73 ppm for ring D protons.
Single crystals of 3 and 4 suitable for X-ray diffraction [41] were
grown by layering a THF solution of 3 with hexane, or a CH2Cl2
* Corresponding authors. Tel.: +41 61 267 1008; fax: +41 61 267 1018.
E-mail address: catherine.housecroft@unibas.ch (C.E. Housecroft).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.06.003

458
E.C. Constable et al. / Inorganic Chemistry Communications 13 (2010) 457–460
H
solution of 4 with Et2O. Figs. 1 and 2 depict the molecular struc-
tures. Bond parameters within the Fe2(CO)6( -ADT) units in 3
l
H
and 4 are similar and are unexceptional. In 3, the tpy unit is
approximately planar (angles between the least squares planes of
rings containing atoms N1 and N2, and N2 and N3 are 1.6(1) and
5.0(1)°). The plane of the arene ring containing atom C16 is twisted
through 40.0(1)° with respect to the central pyridine ring to which
it is attached, and lies 7.2(1)° out of the plane containing the clus-
ter-attached arene ring. In contrast, the tpy, thiophene and arene
rings in 4 are closer to being coplanar (angles between the least
squares planes of rings containing atoms N1 and N2, N2 and N3,
N2 and S1, and S1 and C22 are 3.6(1), 11.7(1), 9.2(1) and 4.4(1)°).
These observations indicate that, in the solid state at least, com-
4
3
2
C
C
3
3
S
3
B
B
N
N
N
4
N
N
4
6
5
6
5
N
A
A
3
3
1
2
pound 4 exhibits a
ADT unit to the tpy domain.
p-system which extends from the bridging
(i)
(i)
The tpy units of adjacent molecules of 4 are p-stacked (distance
A
N
N
between rings containing N3 and N1i = 3.21 Å, symmetry code i = x,
1 + y, z). The stacks are arranged so that the lattice contains dis-
crete organic and inorganic cluster domains (Fig. 3). This results
in relatively short intermolecular carbonyl Oꢁ ꢁ ꢁO contacts of
3.0 Å, similar to those observed in Fe2(CO)9 [42] and Fe3(CO)12
[43]. In contrast, molecules of 3 are packed so that there is partial
interdigitation of organic and iron sulfur cluster domains (Fig. 4a).
The dominant feature of the packing is the centrosymmetric, box-
like four-molecule embrace shown in Fig. 4b which arises from
2
3
Fe(CO)3
Fe(CO)3
S
B
N
X
N
D
S
3
4
X = 1,4-C6H4
X = 2,5-thienylene
Fe(CO)3
S
S
I
N
Fe(CO)3
5
Scheme 1. Scheme for the synthesis of 3 and 4; (i) THF, 5, NEt3, [Pd(PPh3)2Cl2], CuI,
in the dark. Ring labelling for NMR assignments.
Fig. 2. Molecular structure of
4 (H atoms omitted); ellipsoids plotted
Fig. 1. Molecular structure of
3 (H atoms omitted) with ellipsoids plotted
at 40% probability level. Selected bond parameters: Fe1–Fe2 = 2.5193(6),
Fe1–S3 = 2.2423(7), Fe1–S2 = 2.2665(7), Fe2–S2 = 2.2505(7), Fe2–S3 = 2.2532(8),
S2–C28 = 1.849(2), S3–C29 = 1.861(3), N4–C25 = 1.397(2), N4–C29 = 1.427(3),
N4–C28 = 1.429(3), C20–C21 1.190(3) Å; Fe2–S2–Fe1 = 67.80(2), Fe1–S3–Fe2 =
68.17(2), C25–N4–C29 = 121.8(2), C25–N4–C28 = 120.4(2), C29–N4–C28 = 113.7(2),
C16–S1–C19 = 91.7(1)°.
at 40% probability level. Selected bond parameters: Fe1–Fe2 = 2.5029(9),
Fe1–S1 = 2.2564(9), Fe1–S2 = 2.2571(8), Fe2–S1 = 2.259(1), Fe2–S2 = 2.2690(9),
S1–C31 = 1.853(3), S2–C30 = 1.855(3), N4–C27 = 1.412(3), N4–C30 = 1.423(3),
N4–C31 = 1.427(3), C22–C23 = 1.195(4) Å; Fe1–S1–Fe2 = 67.33(3), Fe1–S2–Fe2 =
67.15(3), C27–N4–C30 = 121.3(2), C27–N4–C31 = 121.2(2), C30–N4–C31 = 114.2(2)°.
Fig. 3. Packing of molecules of 4, showing layered organic and iron sulfur cluster domains.

E.C. Constable et al. / Inorganic Chemistry Communications 13 (2010) 457–460
459
Fig. 4. (a) Packing of molecules of 3; and (b) the four-molecule embrace in 3.
edge-to-face CHꢁ ꢁ ꢁ
p interactions between the arene and pyridine
References
rings. The C28–H28A bond points towards the centroid of the pyr-
idine ring containing N1i (C28–H28Aꢁ ꢁ ꢁcentroid = 2.83 Å, symme-
try code i = 1 ꢀ x, 1/2 + y, 3/2 ꢀ z), while C21–H21A is directed
towards the centroid of the ring containing N1ii (C21–H21Aꢁ ꢁ ꢁcen-
[1] J.W. Peters, W.N. Lanzilotta, B.J. Lemon, L.C. Seefeldt, Science 282 (1998) 1853.
[2] C. Tard, X. Liu, S.K. Ibrahim, M. Bruschi, L. De Gioia, S.C. Davies, X. Yang, L.-S.
Wang, G. Sawers, C.J. Pickett, Nature 433 (2005) 610.
[3] F. Gloaguen, T.B. Rauchfuss, Chem. Soc. Rev. 38 (2009) 100.
[4] X. Liu, S.K. Ibrahim, C. Tard, C.J. Pickett, Coord. Chem. Rev. 249 (2005) 1641. and
references therein.
[5] J.D. Lawrence, H. Li, T.B. Rauchfuss, Chem. Commun. (2001) 1482.
[6] J.D. Lawrence, H. Li, T.B. Rauchfuss, M. Bénard, M.-M. Rohmer, Angew. Chem.
Int. Ed. 40 (2001) 1768.
[7] V. Vijaikanth, J.-F. Capon, F. Gloaguen, F.Y. Pétillon, P. Schollhammer, J.
Talarmin, J. Organomet. Chem. 692 (2007) 4177.
[8] G. Si, L.-Z. Wu, W.-G. Wang, J. Ding, X.-F. Shan, Y.-P. Zhao, C.-H. Tung, M. Xu,
Tetrahedron Lett. 48 (2007) 4775.
[9] G. Si, C. Ma, M. Hu, H. Chen, C. Chen, Q. Liu, New J. Chem. 31 (2007) 1448.
[10] G. Si, W.-G. Wang, H.-Y. Wang, C.-H. Tung, L.-Z. Wu, Inorg. Chem. 47 (2008)
8101.
troid = 2.71 Å, symmetry code ii = x, 1/2 ꢀ y, ꢀ /2 + z).
1
In conclusion, we have prepared and characterized two
Fe2(CO)6(l-ADT) clusters bearing pendant tpy domains coupled
through alkynylthienylene or alkynylphenylene spacers. The aim
of their incorporation is to enhance electronic communication be-
tween the iron sulfur cluster and tpy metal-binding unit. Future
studies will focus on the synthesis of heteroleptic ruthenium(II)
complexes containing 3 or 4.
[11] G. Eilers, L. Schwartz, M. Stein, G. Zampella, L. de Gioia, S. Ott, R. Lomoth, Chem.
Eur. J. 13 (2007) 7075.
Acknowledgements
[12] Y.-F. Tang, J.-L. Zhu, Acta Crystallogr., Sect. E 64 (2008) m1423.
[13] S. Jiang, J. Liu, Y. Shi, Z. Wang, B. Åkermark, L. Sun, Dalton Trans. (2007) 896.
[14] J. Ekström, M. Abrahamsson, C. Olson, J. Bergquist, F.B. Kaynak, L. Eriksson, L.
Sun, H.-C. Becker, B. Åkermark, L. Hammarström, S. Ott, Dalton Trans. (2006)
4599.
We thank the Swiss National Science Foundation, the Swiss
National Center of Competence in Research in Nanoscale
Science, the KTI (Project ANWASP) and the University of Basel for
support.
[15] L. Schwartz, G. Eilers, L. Eriksson, A. Gogoll, R. Lomoth, S. Ott, Chem. Commun.
(2006) 520.
[16] E.P.L. van der Geer, G. van Koten, R.J.M. Klein Gebbink, B. Hessen, Inorg. Chem.
47 (2008) 2849.
[17] S. Gao, J. Fan, S. Sun, X. Peng, X. Zhao, J. Hun, Dalton Trans. (2008) 2128.
[18] S. Ott, M. Borgström, M. Kritikos, R. Lomoth, J. Bergquist, B. Åkermark, L.
Hammarström, L. Sun, Inorg. Chem. 43 (2004) 4683.
[19] L. Sun, B. Åkermark, S. Ott, Coord. Chem. Rev. 249 (2005) 1653.
[20] S. Ott, M. Kritikos, B. Åkermark, L. Sun, Angew. Chem. Int. Ed. 42 (2003) 3285.
[21] Y. Na, J. Pan, M. Wang, L. Sun, Inorg. Chem. 46 (2007) 3813.
[22] Y. Na, M. Wang, J. Pan, P. Zhang, B. Åkermark, L. Sun, Inorg. Chem. 47 (2008)
2805.
Appendix A. Supplementary material
CCDC 724617 and 724618 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.inoche.2009.06.003.
[23] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Zelewsky,
Coord. Chem. Rev. 84 (1988) 85.

460
E.C. Constable et al. / Inorganic Chemistry Communications 13 (2010) 457–460
[24] A. Harriman, R. Ziessel, Chem. Commun. (1996) 1707.
[25] V. Grosshenny, A. Harriman, R. Ziessel, Angew. Chem. Int. Ed. 34 (1995) 1100.
[26] A.C. Benniston, V. Grosshenny, A. Harriman, R. Ziessel, Dalton Trans. (2004)
1227.
[27] E.C. Constable, C.E. Housecroft, E.R. Schofield, S. Encinas, N. Armaroli, F.
Barigelletti, L. Flamigni, E. Figgemeier, J.G. Vos, Chem. Commun. (1999) 869.
[28] S. Encinas, L. Flamigni, F. Barigelletti, E.C. Constable, C.E. Housecroft, E.
Schofield, E. Figgemeier, D. Fenske, M. Neuburger, J.G. Vos, M. Zehnder, Chem.
Eur. J. 8 (2002) 137.
[29] J. Hjelm, E.C. Constable, E. Figgemeier, A. Hagfeldt, R. Handel, C.E. Housecroft, E.
Mukhtar, E. Schofield, Chem. Commun. (2002) 284.
[30] E.C. Constable, R.W. Handel, C.E. Housecroft, A.F. Morales, L. Flamigni, F.
Barigelletti, J. Chem. Soc., Dalton Trans. (2003) 1220.
[31] J. Hjelm, R.W. Handel, A. Hagfeldt, E.C. Constable, C.E. Housecroft, R.J. Forster,
Electrochem. Commun. 6 (2004) 193.
[32] J. Hjelm, R.W. Handel, A. Hagfeldt, E.C. Constable, C.E. Housecroft, R.J. Forster,
Inorg. Chem. 44 (2005) 1073.
[33] C. Houarner-Rassin, F. Chaignon, C. She, D. Stockwell, E. Blart, P. Buvat, T. Lian,
F. Odobel, J. Photochem. Photobiol. 192 (2007) 56.
[34] E.C. Constable, E. Figgemeier, C.E. Housecroft, S. Latha Kokatam, E.A. Medlycott,
M. Neuburger, S. Schaffner, J.A. Zampese, Dalton Trans. (2008) 6752.
[35] E.C. Constable, E. Figgemeier, C.E. Housecroft, E.A. Medlycott, M. Neuburger, S.
Schaffner, S. Reymann, Polyhedron 27 (2008) 3601.
[40] 3:1 (48 mg, 0.144 mmol), 5 (75 mg, 0.127 mmol), [Pd(PPh3)2Cl2] (18 mg,
0.025 mmol), NEt3 (4.3 cm3) and CuI (14 mg, 0.074 mmol) were added to
THF (15 cm3) under argon in the dark. The reaction mixture was stirred in the
dark at room temperature for 20 h. Solvent was then removed in vacuo. The
product was purified by column chromatography in the dark (Al2O3, CH2Cl2)
and
3
was collected as the second, red fraction (12 mg, 12%). 1H NMR
(400 MHz, CDCl3) d/ppm 8.76 (s, 2H, HB3), 8.74 (d, J = 4.8 Hz, 2H, HA6), 8.69 (dd,
J = 8.0, 0.6 Hz, 2H, HA3), 7.91 (d, J = 8.1 Hz overlapping m, 4H, HC2+A4), 7.66 (d,
J = 8.0 Hz, 2H, HC3), 7.53 (d, J = 8.7 Hz, 2H, HD2), 7.37 (dd, J = 7.4, 4.8 Hz, 2H,
H
A5), 6.74 (d, J = 8.7 Hz, 2H, HD3), 4.35 ðs;4H;HC2HÞ. ESI MS m/z 795.0 [M + H]+
(calc. 795.0), 627.0 [M ꢀ 6CO + H]+ (calc. 627.0); IR (solid,
m
CO/cmꢀ1) 2069,
2021, 1998. 4:1 (55 mg, 0.162 mmol), 5 (36 mg, 0.061 mmol), [Pd(PPh3)2Cl2]
(9.0 mg, 0.013 mmol), NEt3 (10 cm3) and CuI (14 mg, 0.074 mmol) were added
to THF (15 cm3) under argon in the dark. The mixture was stirred in the dark at
room temperature for 16 h. After solvent removal, column chromatography in
the dark (Al2O3, hexane:ethyl acetate 7:3) yielded 4 as the second, red fraction
(50 mg, 24%). 1H NMR (400 MHz, CDCl3) d/ppm: 8.74 (d, J = 4.7 Hz, 2H, HA6),
8.67 (s, 2H, HB3), 8.64 (d, J = 7.9 Hz, 2H, HA3), 7.88 (td, J = 7.7, 1.5 Hz, 2H, HA4),
7.67 (d, J = 3.8 Hz, 1H, HC3), 7.50 (d, J = 8.7 Hz, 2H, HD2), 7.36 (m, 2H, HA5), 7.30
(d, J = 3.8 Hz, 1H, HC4), 6.73 (d, J = 8.7 Hz, 2H, HD3), 4.34 (s, 4H, HCH2). ESI MS m/
z 800.9 [M + H]+ (calc. 800.9), 631.9 [M ꢀ 6CO]+ (calc. 631.9); IR (solid, mCO
/
cmꢀ1) 2073, 2028, 2001.
[41] 3: C37H22Fe2N4O6S2, M = 794.43, orange plate, monoclinic, space group P21/c,
[36] 1: 4-Ethynylbenzaldehyde (0.490 g, 3.77 mmol) was added to
a stirring
a = 23.298(5), b = 10.238(2), c = 15.598(3) Å, b = 100.82(3)°, U = 3654.5(13) Å3,
mixture of 2-acetylpyridine (0.88 cm3, 0.95 g, 7.85 mmol), NH4OH (20 cm3,
25% aq. solution) in EtOH (30 cm3). KOH pellets (0.48 g, 8.6 mmol) were added
slowly to the stirring mixture. A yellow precipitate formed as the solution was
stirred for 15 h at room temperature. 1 was collected by filtration, washed
with EtOH and isolated as a light yellow solid (0.50 g, 40%). The melting point
(188–191 °C) agreed with that published [37]. 1H NMR (500 MHz, CDCl3) d/
ppm 8.73 (s overlapping d, 4H, HB3+A6), 8.68 (d, J = 7.9 Hz, 2H, HA3), 7.90 (d,
J = 8.2 Hz overlapping m, 4H, HC2+A4), 7.66 (d, J = 8.3 Hz, 2H, HC3), 7.36 (m, 2H,
Z = 4, Dc = 1.444 Mg mꢀ3
,
l
(Mo-K ) = 0.958 mmꢀ1
,
T = 223(2) K, 88,925
a
reflections collected, merging r = 0.0944. Refinement of 460 parameters
using 8383 independent reflections against F2 converged at final R1 = 0.0493
(R1 all data = 0.0544), wR2 = 0.1078 (wR2 all data = 0.1102), gof = 1.237. 4:
C35H20Fe2N4O6S3, M = 800.46, red needle, monoclinic, space group P21/c,
a = 33.425(7), b = 6.468(1), c = 15.499(3) Å, b = 93.50(3)°, U = 3344.6(12) Å3,
Z = 4, Dc = 1.590 Mg mꢀ3
, l , T = 223(2) K, 56,648
(Mo-K ) = 1.108 mmꢀ1
a
reflections collected, merging r = 0.1693. Refinement of 452 parameters
using 7687 independent reflections against F2 converged at final R1 = 0.0461
(R1 all data = 0.0497), wR2 = 0.1097 (wR2 all data = 0.1134), gof = 1.054.
[42] F.A. Cotton, J.M. Troup, J. Chem. Soc., Dalton Trans. (1974) 800.
H
A5), 3.19 (s, 1H, HC„CH).
[37] V. Grosshenny, F.M. Romero, R. Ziessel, J. Org. Chem. 62 (1997) 1491.
[38] K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett. 16 (1975) 4467.
[39] K. Sonogashira, J. Organomet. Chem. 653 (2002) 46.
[43] L.J. Farrugia, A.L. Gillon, D. Braga, F. Grepioni, Organometallics 18 (1999) 5022.