Controlling carbon monoxide binding at di-iron units related to the iron-only hydrogenase sub-site

Article abstract of DOI:10.1039/b712805c

Carbon monoxide binding by displacement of a pendant hemi-labile ligand at a di-iron site can be substantially 'switched-on' via a ligand protonation pathway which is competitive with metal-metal bond protonation. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712805c

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Controlling carbon monoxide binding at di-iron units related to the
iron-only hydrogenase sub-site{
Fenfen Xu,a Ce´dric Tard,b Xiufeng Wang,a Saad K. Ibrahim,b David L. Hughes,b Wei Zhong,a Xirui Zeng,c
Qiuyan Luo,c Xiaoming Liu*a and Christopher J. Pickett*b
Received (in Cambridge, UK) 20th August 2007, Accepted 2nd November 2007
First published as an Advance Article on the web 16th November 2007
DOI: 10.1039/b712805c
Carbon monoxide binding by displacement of a pendant hemi-
labile ligand at a di-iron site can be substantially ‘switched-on’
via a ligand protonation pathway which is competitive with
metal-metal bond protonation.
The binding of carbon monoxide to the resting and reduced states
of the H-cluster of hydrogenase inhibits proton reduction/
hydrogen oxidation by blocking the substrate binding site. This
poisoning is reversible with enzyme activity restored by flushing
out or pumping-off the CO.1,2 In contrast, platinum electro-
catalysts are far less robust to poisoning by trace CO and removal
can only be effected by anodic oxidation of the adsorbate to CO2
at high potentials.3 Studies of the factors which control the
formation of the substrate or inhibitor binding at di-iron units are
thus of some relevance to both enzyme mechanism and to the
design of new artificial electrocatalysts.
Fig. 1 Views of representative X-ray crystallographic structures illustrat-
ing ‘closed’ A, ‘open’ B and protonated C forms of the di-iron units.{
Here we report (i) the structures of new systems with pendant N
or O groups ligating or proximal to a di-iron site which can bind
CO, (ii) how the energetics of CO binding are controlled by the
nature of the pendant groups, (iii) how protonation of the pendant
group can trigger extensive CO binding and (iv) how protonation
of the metal–metal bond is involved in an equilibration involving
hydride-on/CO-off.
favours CO binding by about two orders of magnitude over that
of the benzyl thioether derivative (Table 1, entries 2 and 7).
Behaviour at the two equilibrium extremes is displayed by
complexes with pendant CH2NH2 and CH2OH groups: the
equilibrium concentration of the pentacarbonyl A (X = CH2OH) is
below the detection limit in the FTIR experiment (1%) and
we were not able to effect the conversion of B (X = CH2OH)
to A (X = CH2OH) by refluxing in toluene under dinitrogen;
conversely, the equilibrium concentration of the hexacarbonyl B
(X = CH2NH2) is below the detection limit under CO at one
atmosphere. Fig. 1 shows the X-ray crystal structures of the amine,
pyridine and alcohol complexes.
The N-ligated pentacarbonyl diiron complexes A (X =
CH2NH2; X = 2-pyridine) are the first examples of this type of
{NS2}-tripodal ligation at a dinuclear site, their structures are
shown in Fig. 1. The thioether complexes used in this study were
prepared as described earlier or by simple extension of the general
method.4 Illustrative syntheses, together with analytical and
spectroscopic data are provided as ESI.{
The complexes A possess more or less hemi-labile pendant N or
S ligands (X) which can be replaced by CO to give complexes B,
Scheme 1. We have measured the equilibrium constant Keq at
293 K for this reversible binding of CO and the data are given in
Table 1. Table 1 shows that for the thioether series the equilibrium
position is dominated by electronic effects of the pendant group.
As an example, the electron-withdrawing p-cyanobenzene group
aDepartment of Chemistry, Nanchang University, Nanchang, 330031,
China. E-mail: xiaoming.liu@ncu.edu.cn; Fax: +86 791 3969 254;
Tel: +86 791 3969 254
bSchool of Chemical Sciences and Pharmacy, University of East Anglia,
Norwich, NR4 7TJ, United Kingdom. E-mail: c.pickett@uea.ac.uk;
Fax: +44 1603 458 553; Tel: +44 1603 592 486
cSchool of Chemistry and Chemical Engineering, Jinggangshan
University, Jian, 343000, China
{ Electronic supplementary information (ESI) available: Synthesis and
characterisation of the complexes. See DOI: 10.1039/b712805c
Scheme 1 Interconversions involving hemi-labile ligands, protons and
carbon monoxide at di-iron dithiolate units.
606 | Chem. Commun., 2008, 606–608
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Table 1 Equilibrium constants and estimated standard free energies for binding of CO to A
Entry
Complex
Keq (L mol21
)
DGu293 (kJ mol21
)
0
1
2
3
4
5
6
7
8
9
a
[Fe2(CO)5{CH3C(CH2S)2X}]a (X = CH2OH)
.103
,217
[Fe2(CO)4(CN){CH3C(CH2S)2X}]12 (X = CH2SCH3)
[Fe2(CO)5{CH3C(CH2S)2X}] (X = CH2SC6H4-p-CN)
[Fe2(CO)5{CH3C(CH2S)2X }] (X = CH2SC6H4-p-NO2)
625 ¡ 5
453 ¡ 11
362 ¡ 28
125 ¡ 19
13.7 ¡ 0.1
3.6 ¡ 0.1
3.2 ¡ 0.3
,100
215.68 ¡ 0.02
214.9 ¡ 0.1
214.4 ¡ 0.2
211.8 ¡ 0.4
26.38 ¡ 0.02
23.1 ¡ 0.1
22.8 ¡ 0.2
.0
+
[Fe2(CO)5{CH3C(CH2S)2X}] (X = CH2SC6H4-p-NH3
[Fe2(CO)5{CH3C(CH2S)2X }] (X = CH2SC6H4-p-NH2)
[Fe2(CO)5{CH3C(CH2S)2X}] (X = 2-pyridine)
[Fe2(CO)5{CH3C(CH2S)2CH2X}] (X = SCH2C6H5)
[Fe2(CO)5{CH3C(CH2S)2X}] (X = CH2SCH3)
[Fe2(CO)5{CH3C(CH2S)2X}] (X = CH2NH2)
)
,100
.0
Hexacarbonyl present in .99% concentration.
‘Remote’ protonation can switch the donicity of the pendant
group, X. For example, the conversion of the p-aniline group to
the p-anilinium cation increases the equilibrium constant in favour
of CO binding by an order of magnitude. This is undoubtedly a
consequence of decreasing the Fe–Sthioether bond enthalpy, Table 1
(entries 5 and 4).
Addition of the acid to A (X = CH2SMe) under CO does not
lead to the formation of B (X = CH2SHMe+), rather it results in
protonation of the metal–metal bond to give the bridging hydride
D, Scheme 1. The evidence for this is as follows: Fig. 3 (a) shows
the FTIR of A (X = CH2SMe) before and after reaction with
excess HBF4?Et2O. First, the infra-red spectral pattern after the
reaction is essentially identical to that before protonation except
Addition of the acid HBF4?Et2O to A (X = CH2NH2;
2-pyridine) under CO (1 atm.; rt; CH2Cl2) results in the rapid
formation of the hexacarbonyl with concomitant de-coordination
of the N-ligands as the pendant ammonium and pyridinium salts
respectively, Eqn (1). Monitoring the reactions by FTIR shows
that under CO the addition of 1 equivalent of the acid results in an
infra-red pattern in the carbonyl region essentially identical to that
of [Fe2{S(CH2)3S}(CO)6]6 for A (X = CH2NH2; X = 2-pyridine),
Fig. 2. The overall reversibility of the process is illustrated by
addition of base (triethylamine) which quantitatively restores
the spectrum of the parent species, Fig. 2. The proton switched
CO coordination was fully confirmed by isolation of C (X =
2-pyridinium) and X-ray crystallographic determination of its
structure, Fig. 1.
ð1Þ
Fig. 3 Bridging hydride formation (a) reversible protonation of the
metal–metal bond of complex A (X = CH2SMe) in dichloromethane
giving D (X = CH2SMe) (b) FTIR of hydride component D (X =
1
Fig. 2 FTIR spectral changes for complex A (X = CH2NH2) upon
CH2NH2) and (c) H NMR spectrum of D (X = CH2SMe) showing the
protonation/de-protonation in MeCN under CO (1 atm.) at 293 K.
hydride resonance.
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H+ could of course operate via this associative pathway, but here
again some circumspection is needed. It is clear that with ligated
X = CH2NH2 or CH2SMe the Fe–Fe bond is sufficiently basic to
be extensively protonated. The positive charge brought in would
reasonably be expected to both strengthen the Fe–X bond and
lower the tendency for electrophilic attack by CO. Just as bridging
and terminal CO switching can provide a low activation energy for
substititution, so it may be that migration or tunneling of a proton
from the bridging mode to the Fe–N bond allows concerted
associative attack by CO and dissociation of XH+.
In summary, we have demonstrated how the nature of a
pendant ‘hemi-labile’ ligand can control the extent of carbonyla-
tion of a di-iron unit and how this can be further modified by
protonation reactions, including equilibration between hydride-on/
CO-off. The nature of X both as a base and as a ligand, and the
influence it has on the basicity of the metal–metal bond, provides
control of the ligand hemi-lability and activity of the di-iron unit
and this is relevant to catalyst design.
Fig. 4 Experimental n(CO) for the hydrides D (X = CH2SMe, CH2NH2)
versus n(CO) for the conjugate bases A (X = CH2SMe, CH2NH2).
that all peaks are shifted by 80 ¡ 17 cm21 to higher frequencies;
this is indicative of retention of the basic structure of the complex.
Secondly, de-protonation fully restores the spectrum of the parent
compound, thus the observed shift to higher frequencies on
We thank the NSF (Grant Number: 20571038, China), the RSC
for providing a JWT Jones Travelling Fellowship (to XL), the
BBSRC (UK) and the EPSRC (Supergen 5, UK) for funding
this work.
1
protonation cannot be a consequence of oxidation. Thirdly, H
NMR (CD2Cl2) unequivocally shows a single peak at 219.8 ppm,
consistent with the formation of a bridging hydride, Fig. 3 (c).7
Notably, D provides the first example of a hydride at an
enzymatically relevant {2Fe3S} core.
Notes and references
{ CCDC 658089–658093. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b712805c
A closer inspection of the infra-red spectrum of A (X =
CH2NH2) following protonation under CO shows additional
bands with low intensity, Fig. 2. These bands are enhanced when
the protonation is carried out under dinitrogen, but the species
formed slowly decays.§ The resolved infra-red pattern closely
matches that of the hydride D (X = SMe), Fig. 3 (b), indicative of
the formation of D (X = CH2NH2). Convincingly, we find that the
plot of n(CO) for the protonated D (X = CH2SMe, CH2NH2)
versus non-protonated A (X = CH2SMe, CH2NH2) pairs shows an
extraordinary linearity (correlation coefficient, r = 0.9996), Fig. 4.
This unambiguously supports structurally analogous protonations,
i.e bridging hydride formation. In a wider context this type of
correlation may serve in the identification of the retention of
overall structural geometry before and after protonation.
§ The complex A (X = CH2NH2) reacts with the acid in the absence of CO
to give initially strong bands of the hydride at 2111, 2056 and 2009 cm21
that decay with the formation of the hexacarbonyl that must be formed by
the scavenging of CO from (oxidised) material. The complex A (X =
2-pyridine) reacts with HBF4?Et2O in the absence of CO in a similar
fashion. Notably solvato-adducts are not formed.
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608 | Chem. Commun., 2008, 606–608
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