Models of the iron-only hydrogenase: Structural studies of chelating diphosphine complexes [Fe2(CO)4(-pdt) (κ2P,P′-diphosphine)]

Article abstract of DOI:10.1039/b706123b

Six chelating diphosphine complexes, [Fe₂(CO) ₄(-pdt)(κ²P,P′-diphosphine)], have been crystallographically characterised allowing differences between basal-apical and dibasal conformations to be analysed. The Royal Society

Full text of DOI:10.1039/b706123b

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Models of the iron-only hydrogenase: Structural studies of chelating
diphosphine complexes [Fe2(CO)4(l-pdt)(j2P,Pꢀ-diphosphine)]†
Fatima I. Adam, Graeme Hogarth,* Idris Richards and Benjamin E. Sanchez
Received 23rd April 2007, Accepted 1st May 2007
First published as an Advance Article on the web 15th May 2007
DOI: 10.1039/b706123b
Six chelating diphosphine complexes, [Fe2(CO)4(l-pdt)(j2P,
Pꢀ-diphosphine)], have been crystallographically charac-
terised allowing differences between basal–apical and dibasal
conformations to be analysed.
Dithiolate-bridged diiron complexes, [Fe2(CO)6(l-SCH2XCH2S)]
In 8 the relative arrangement of carbonyl ligands at the
(X = O, NH, CH2),1–20 have been widely studied over the past
7–8 years since they closely resemble the two-iron unit of the H-
cluster active site of iron-only hydrogenases.21–25 Despite significant
advances in the understanding of this system a number of
limitations still need to be addressed. A major limitation of all
current iron-only hydrogenase models is the unrotated nature of
the two square-planar iron centres. Thus, in current models the
two Fe(CO)2L units roughly eclipse one another (unrotated) while
in the enzyme they are rotated with one of the carbonyls residing in
the area between the two iron centres (rotated). A recent theoretical
study of the system by Tye, Darensbourg and Hall addressed this
issue concluding that “asymmetric substitution of strong donor
ligands is the most viable method of making better synthetic di-
iron complexes that will serve as both structural and functional
models” of the active site of iron-only hydrogenase.26
binuclear centre is strongly dictated by the triphosphine binding
mode. This would be less pronounced in tetracarbonyl chelate
complexes [Fe2(CO)4(l-pdt)(j2P,Pꢀ-diphosphine)] since the iron
tricabonyl unit would be free to adopt its preferred coordination
geometry. In order to probe this, and to assess if significant
structural variations result from preferential adoption of a dibasal
or apical–basal coordination geometry of the chelating ligand, we
have prepared a wide range of complexes of the type [Fe2(CO)4(l-
pdt)(diphosphine)]. Herein we report the crystal structures of
six tetracarbonyl chelate complexes, [Fe2(CO)4(l-pdt)(j2P,Pꢀ-
diphosphine)], and assess the role the diphosphine coordination
plays on the geometry at each metal centre.
Complexes 2–7 were prepared from the reaction of [Fe2(CO)6(l-
pdt)] (1) with the relevant phosphine under a range of different
conditions‡ and a crystal structure has been carried out on
each (Fig. 1 and 2).§ Key metric parametersare given in Table 1
together with data for 1 and 8. The chelate complexes fall into two
structural types. In 2–4, the diphosphine lies in the basal plane
which renders the two phosphorus atoms equivalent, while in 5–
7 apical–basal coordination geometry is seen. Dibasal complexes
2–4 are characterised by very small bite-angles of 71.32–74.55◦
and are adopted by ligands with a single atom linking unit. This
leads to quite short phosphorus–phosphorus distances of around
The only structurally characterised model complex to
date that fulfils this criteria is [Fe2(CO)3(l-pdt)(l,jP,j2Pꢀ,Pꢀꢀ-
triphos)] (8) (H2pdt = 1,3-propanedithiol, triphos = bis(2-
diphenylphosphinoethyl)phenylphosphine) prepared by ourselves
from the thermal reaction of triphos with [Fe2(CO)6(l-pdt)] (1).18
Here the chelating part of the tridentate phosphine adopts a
basal–apical coordination geometry in the solid state, while the
bridging groups are cis basal. In solution, a trigonal twist is
believed to interconvert the two basal sites of the dicarbonyl centre,
while the trigonal twist of the iron monocarbonyl centre is not
observed probably since it leads to an unfavourable twisting of the
triphosphine ligand.
˚
2.6 A. The thiolate ligands bridge the diiron centre somewhat
asymmetrically, bonds to the iron phosphine-substituted centre
˚
being around 0.04 A shorter than those to the iron tricarbonyl unit,
˚
which at an average of 2.26 A are more usual of those found in
all basal–apical and other substituted l-pdt complexes. Basal–
apical complexes 5–8 are characterised by significantly larger
ligand bite-angles of 86.80–94.96◦ and the dithiolate ligands bridge
approximately symmetrically.
Department of Chemistry, University College London, 20 Gordon Street,
London, UK WC1H 0AJ. E-mail: g.hogarth@ucl.ac.uk; Tel: +44
(0)2076794664
† CCDC reference numbers 614428, 614638, 639866–639870. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b706123b
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Fig. 1 Molecular structure of dibasal 4.
Fig. 2 Molecular structure of apical–basal 6.
Introduction of the chelating diphosphine leads to some devi-
ation away from the square-based pyramidal geometry found in
1 and derivatives with monodentate phosphine ligands. Addison,
Reedijk and co-workers have developed a simple way of assessing
the nature of trigonal bipyramidal and square pyramidal character
at a pentacoordinate metal centre.27 A structural index factor (s)
is expressed as s = (a − b)/60, where a and b are the largest
and second largest angles around the metal centre. Thus, s = 0
is expected for an ideal square pyramidal geometry, while s =
1 would represent an ideal trigonal bipyramidal structure. The
calculated s values for 1–8 are given in Table 1. The difference
between these values for the two iron centres is some measure of
the geometric asymmetry induced upon diphosphine chelation.
This is a maximum for the triphos complex 8 (Ds = 0.323) and
significant differences are also seen in dibasal 2 (Ds = 0.206) and
apical–basal 7 (Ds = 0.195) While in all the iron centres are best
described as square pyramidal, s values of 0.314 and 0.287 in 2 and
7 respectively are suggestive of significant trigonal bipyramidal
character. Interestingly, while the trigonal bipyramidal character
is seen at the iron tricarbonyl unit in dibasal 2, it is located at
the diphosphine-substituted centre in apical–basal 7. Further, the
2496 | Dalton Trans., 2007, 2495–2498
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greatest asymmetry is seen in 8 where the mono-substituted centre
shows significant deviation away from square pyramidal geometry
(s = 0.405).
structural and electronic asymmetry is believed to occur and thus
complexes such as those described above may act as more realistic
structural and functional models of this important enzymatic site.
The chelating phosphines can also be seen to play a role in
the eclipsed nature of the three non-sulfur substituents on each
metal centre. In 1 the two iron atoms are related by symmetry
and thus the three carbonyls on each iron atom are eclipsed.8
For apical–basal substitution this varies only slightly while for
dibasal complexes twisting away from the eclipsed state becomes
more pronounced. This is especially so in 2 where the average
torsional angle of 18.1◦ includes angles of 27.7(3) and 20.59◦
between C(1)–C(2) and P(1)–C(3) respectively. However, the most
pronounced distortion away from an eclipsed state is seen in the
triphos complex 8 where the torsional angle between P(1)–C(3)
is 49.4(2)◦, i.e. these substituents are virtually staggered (Fig. 3).
Despite this asymmetry, however, all these complexes adopt the
unrotated structure.
Note added in proof.
The X-ray structure of basal–apical [Fe2(CO)4(l-pdt)(j2-P,Pꢀ-
Ph2PCH2CH2PPh2)] has been recently reported by Schollhammer
and co-workers; S. Ezzaher, J. -F. Capon, F. Gloaguen, F. Y. Petil-
lon, P. Schollhammer, J. Talarmin, R. Pichon and N. Kervarec,
Inorg. Chem., 2007, 46, 3426.
Notes and references
‡ Selected spectroscopic data: 2 IR m(CO) (C6H14): 2023vs, 1952s,
1
1917m cm−1
;
31P{ H} NMR (CDCl3): d 12.5 (s); 3 IR m(CO) (CH2Cl2):
2017vs, 1948s, 1895m cm−1
;
31P{ H} NMR (CDCl3): d 98.2 (s, 80%),
1
91.6 (s, 20%); 4 IR m(CO) (CH2Cl2): 2019vs, 1949s, 1905m cm−1
;
31P{ H}
1
NMR (CDCl3): d 116.2 (brs, 25%), 100.9 (s, 75%); 5 IR m(CO) (CH2Cl2):
2019vs, 1949s, 1906w cm−1
;
31P{ H} NMR (CDCl3): d 88.1 (s, 90%), 80.5
1
(s, 10%); 6 IR m(CO) (CH2Cl2): 2019vs, 1946s, 1890m cm−1
;
31P{ H} NMR
1
(CDCl3): d 53.3 (s, 90%), 48.6 (s, 10%); 7 IR m(CO) (CH2Cl2): 2020vs, 1949s,
1
1891m cm−1
;
31P{ H} NMR (CDCl3): d 56.4 (brs, 75%), 48.4 (brs, 25%),
−26.0 (brs); 8 IR m(CO) (CH2Cl2): 1947s, 1889vs cm−1
;
31P{ H} NMR
1
(CD2Cl2): d 89.7 (d, J 17.2 Hz), 86.3 (dd, J 17.2, 9.2 Hz), 83.3 (br), 81.8
(brd, J 23.4 Hz), 63.9 (br), 61.6 (d, J 9.2 Hz).
§ Crystal data: [Fe2(CO)4(l-pdt)(j2P,Pꢀ-Ph2PCH2PPh2)] (2): red block,
¯
dimensions 0.14 × 0.13 × 0.11 mm, triclinic, space group P1, a =
˚
10.3556(13), b = 12.1406(15), c = 12.9298(16) A, a = 91.174(2), b =
◦
3
˚
98.599(2), c = 102.055(2) , V = 1569.6(3) A , Z = 2, F(000) 732, qcalc
=
1.511 g cm−3, l = 1.195 mm−1. 12622 reflections were collected, 7042
unique [R(int) = 0.0378] of which 5039 were observed [I > 2.0r(I)].
At convergence, R1 = 0.0543, wR2 = 0.1253 [I > 2.0r(I)] and R1
=
0. 0826, wR2 = 0.1387 (all data), for 379 parameters. CCDC 614428.
[Fe2(CO)4(l-pdt){j2P,Pꢀ-Ph2PN(Pri)PPh2}] (3): brown block, dimensions
¯
0.24 × 0.16 × 0.14 mm, triclinic, space group P1, a = 9.7227(10), b =
˚
10.3859(11), c = 18.5263(19) A, a = 84.913(2), b = 80.645(2), c =
◦
63.569(2) , V = 1652.7(3) A , Z = 2, F(000) 780, qcalc = 1.522 g cm−3
,
3
˚
l = 1.141 mm−1. 14333 reflections were collected, 7563 unique [R(int) =
0.0250] of which 6917 were observed [I > 2.0r(I)]. At convergence, R1 =
0.0421, wR2 = 0.1095 [I > 2.0r(I)] and R1 = 0. 0453, wR2 = 0.1122
(all data), for 406 parameters. CCDC 639866. [Fe2(CO)4(l-pdt){j2P,Pꢀ-
=
Ph2PN(CH2CH CH2)PPh2}] (4): red plate, dimensions 0.18 × 0.16 ×
0.04 mm, monoclinic, space group P21/n, a = 9.8036(6), b = 17.9672(12),
◦
3
˚
˚
c = 18.4965(12) A, b = 100.919(1) , V = 3199.0(4) A , Z = 4, F(000) 1552,
qcalc = 1.568 g cm−3, l = 1.178 mm−1. 27778 reflections were collected, 7641
unique [R(int) = 0.0281] of which 6556 were observed [I > 2.0r(I)]. At
convergence, R1 = 0.0330, wR2 = 0.0813 [I > 2.0r(I)] and R1 = 0. 0401,
wR2 = 0.0843 (all data), for 406 parameters. CCDC 639870. [Fe2(CO)4(l-
pdt)(j2P,Pꢀ-Ph2PC6H4PPh2)]·CH2Cl2 (5·CH2Cl2): brown block, dimen-
Fig. 3 Molecular structure of 8 looking down the iron–iron vector.
sions 0.24 × 0.08 × 0.07 mm, monoclinic, space grou◦p Pc, a = 10.1181(13),
3
˚
˚
b = 10.9537(14), c = 17.301(2) A, b = 106.262(2) , V = 1840.7(4) A ,
Z = 2, F(000) 880, qcalc = 1.554 g cm−3, l = 1.174 mm−1. 15525
reflections were collected, 8372 unique [R(int) = 0.0398] of which 7769
were observed [I > 2.0r(I)]. At convergence, R1 = 0.0381, wR2 = 0.0856
[I > 2.0r(I)] and R1 = 0. 0422, wR2 = 0.0874 (all data), for 451
parameters. CCDC 639867. [Fe2(CO)4(l-pdt){j2P,Pꢀ-Ph2P(CH2)3PPh2}]
(6): red block, dimensions 0.28 × 0.14 × 0.14 mm, triclinic, space group
In solution, interconversion of dibasal and apical–basal forms
can occur via a trigonal twist process. This has recently been
shown by De Gioia, Rauchfuss and co-workers to occur in
2
ꢀ
[Fe2(CO)4(l-pdt)(j P,P -cis-Ph2PCH CHPPh2)]19 which we have
independently prepared and characterised.28 Our structural stud-
ies show that this interconversion is associated with a ca. 12–25◦
change in the bite-angle of the diphosphine, which will in turn
effect the arrangement of carbonyl ligands.
=
¯
˚
P1, a = 10.4973(8), b = 11.1050(8), c = 16.5294(12) A, a = 73.689(1),
◦
3
˚
b = 84.670(1), c = 61.883(1) , V = 1628.5(2) A , Z = 2, F(000) 764,
qcalc = 1.514 g cm−3, l = 1.155 mm−1. 14343 reflections were collected,
7449 unique [R(int) = 0.0160] of which 6774 were observed [I > 2.0r(I)].
At convergence, R1 = 0.0273, wR2 = 0.0675 [I > 2.0r(I)] and R1
=
In conclusion we have reported for the first time the solid-
state structures of chelating diphosphine complexes [Fe2(CO)4(l-
pdt)(j2P,Pꢀ-diphosphine)], highlighting how dibasal and apical–
basal coordination modes lead to somewhat different structural
parameters. This structural asymmetry is matched by an electronic
asymmetry across the iron–iron bond, the relatively electron-
rich diphosphine-substituted centre being directly coupled to the
electron-deficient iron tricarbonyl unit. In the enzyme a similar
0. 0306, wR2 = 0.0694 (all data), for 525 parameters. CCDC 639868.
[Fe2(CO)4(l-pdt){j2P,Pꢀ-(Ph2PCH2)3CMe}] (7): red needle, dimensions
0.30 × 0.04 × 0.02 mm, orthorhombic, space group Pna21, a = 23.707(2),
3
˚
˚
b = 16.7698(14), c = 11.2147(10) A, V = 4458.5(7) A , Z = 4, F(000) 1976,
qcalc = 1.422 g cm−3, l = 0.896 mm−1. 36380 reflections were collected, 10394
unique [R(int) = 0.0904] of which 7859 were observed [I > 2.0r(I)]. At
convergence, R1 = 0.0447, wR2 = 0.0716 [I > 2.0r(I)] and R1 = 0. 0726,
wR2 = 0.0764 (all data), for 532 parameters. CCDC 639869. [Fe2(CO)3(l-
pdt){l,jP,j2Pꢀ,Pꢀꢀ-(Ph2PCH2CH2)2PPh}] (8):18 orange block, dimensions
This journal is
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¯
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0.16 × 0.15 × 0.04 mm, triclinic, space group P1, a = 9.4439(8), b =
◦
˚
9.6258(9), c = 22.437(2) A, a = 99.783(1), b = 95.331(2), c = 94.278(2) ,
V = 1992.8(3) A , Z = 2, F(000) 948, qcalc = 1.536 g cm−3, l = 1.127 mm−1
.
3
˚
17384 reflections were collected, 9087 unique [R(int) = 0.0228] of which
7520 were observed [I > 2.0r(I)]. At convergence, R1 = 0.0405, wR2
=
0.0950 [I > 2.0r(I)] and R1 = 0. 0510, wR2 = 0.0998 (all data), for 478
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2498 | Dalton Trans., 2007, 2495–2498
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©