Ferracyclic carbamoyl complexes related to the active site of [Fe]-hydrogenase

Article abstract of DOI:10.1039/c3dt50642h

The active site of the [Fe]-hydrogenase features an iron(ii) centre bearing cis carbonyl groups and a chelating pyridine-acyl ligand. Reproducing these unusual features in synthetic models is an intriguing challenge, which will allow both better understanding of the enzymatic system and more fundamental insight into the coordination modes of iron. By using the carbamoyl group as a surrogate for acyl, we have been able to synthesize a range of ferracyclic complexes. Initial reaction of Fe(CO)₄Br₂ with 2-aminopyridine yields a complex bearing a labile solvent molecule, which can be replaced by stronger donors bearing phosphorus atoms to produce a number of derivatives. Introduction of a hydroxy group using this method is unsuccessful both with a free OH group and when this is silyl-protected. In contrast, the analogous reactions starting from 2,6-diaminopyridine does allow synthesis of complexes bearing a pendant basic group.

Full text of DOI:10.1039/c3dt50642h

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Ferracyclic carbamoyl complexes related to the active
site of [Fe]-hydrogenase†
Cite this: Dalton Trans., 2013, 42, 8140
Peter J. Turrell, Amanda D. Hill, Saad K. Ibrahim, Joseph. A. Wright and
Christopher J. Pickett*
The active site of the [Fe]-hydrogenase features an iron(II) centre bearing cis carbonyl groups and a che-
lating pyridine–acyl ligand. Reproducing these unusual features in synthetic models is an intriguing chal-
lenge, which will allow both better understanding of the enzymatic system and more fundamental
insight into the coordination modes of iron. By using the carbamoyl group as a surrogate for acyl, we
have been able to synthesize a range of ferracyclic complexes. Initial reaction of Fe(CO)4Br2 with 2-amino-
pyridine yields a complex bearing a labile solvent molecule, which can be replaced by stronger donors
bearing phosphorus atoms to produce a number of derivatives. Introduction of a hydroxy group using
this method is unsuccessful both with a free OH group and when this is silyl-protected. In contrast, the
analogous reactions starting from 2,6-diaminopyridine does allow synthesis of complexes bearing a
pendant basic group.
Received 7th March 2013,
Accepted 12th April 2013
DOI: 10.1039/c3dt50642h
www.rsc.org/dalton
Introduction
Biologically, activation of dihydrogen occurs by three phylogeneti-
cally-distinct types of hydrogenase enzyme: [NiFe], [FeFe]- and
[Fe].1,2 The redox-active [NiFe]- and [FeFe]-hydrogenases have
been the focus of studies both at the biological and synthetic
levels, with a very large number of models reported for the di-
iron system. In contrast, the redox-inactive [Fe]-hydrogenase has
to date received less attention. This enzyme catalyses the heteroly-
tic cleavage of dihydrogen into a proton and a hydride, delivering
the latter to the substrate methylenetetrahydromethanopterin.3,4
The crystal structure of the [Fe]-hydrogenase revealed that the
iron centre is ligated by an unusual acyl group (Fig. 1).5,6
Fig. 1 Active site of the [Fe]-hydrogenase (X = solvent).6
Prior to the confirmation of the structure of the [Fe]-hydro-
genase by crystallography, a small number of model com-
pounds had been synthesized based on the available
spectroscopic evidence.7 The design of the majority of these
focused on reproduction of a cis carbonyl arrangement at iron,
and supported the formulation of an iron(II) centre in the
enzyme,8–11 while others introduced pyridine and sulfur
ligands around the metal centre.12,13 Following the publication
of the revised crystal structure of the [Fe]-hydrogenase, Hu and
coworkers have described the synthesis of five- and six co-ordi-
nate acyl models (Fig. 2)14–17 and related dimeric sulfur-
Fig. 2 Monomeric thiolate-containing models of [Fe]-hydrogenase active site
reported by Hu and coworkers (left)15 and Pickett and coworkers (right).19
bridged structures.18 We have communicated a facile route to
ferracyclic analogues possessing an intramolecularly-generated
carbamoyl group,19 and described the formation of a thiolate-
containing structure which closely reproduces the geometry of
the enzyme active site (Fig. 2).
Energy Materials Laboratory, School of Chemistry, University of East Anglia, Norwich
Research Park, Norwich, NR4 7TJ, UK. E-mail: c.pickett@uea.ac.uk
†Electronic supplementary information (ESI) available. CCDC 922917–922924.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt50642h
Herein, we now report synthetic and structural details of
some new ferracyclic carbamoyl systems, including the
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identification of a key intermediate in the synthetic pathway,
and the first example of a structural model of [Fe]-hydrogenase
bearing a basic group at the 6-position on the pyridine ring.
Results and discussion
Formation of carbamoyl ligands has been previously reported
following reaction of rhenium and ruthenium carbonyl com-
plexes with amine-substituted nitrogen-containing hetero-
cycles.20–22 Evidently coordination of the heterocyclic nitrogen
atom to the metal centre allows intramolecular attack at an
electrophilic CO (Scheme 1). This approach was adapted to
synthesise the first ferracyclic carbamoyl structures.
Primary ring-forming reaction
Reaction of Fe(CO)4Br2 with 2-aminopyridine in CH2Cl2 results
in the formation of a yellow precipitate 1 with evolution of
carbon monoxide (Scheme 2). This material is insoluble in
non-coordinating solvents (see below) and was not previously
identified.19 In the coordinating solvent MeCN, 1 reacted to
give complex 2 which was fully characterised by X-ray crystallo-
graphy. Compound 2 provided a starting point for the syn-
Fig. 3 ORTEP representation of the anion of 1 showing 50% probability ellip-
soids; hydrogen atoms except for H(2) have been omitted for clarity. Selected
average bond lengths (Å) and angles (°) with estimated standard deviations:
Fe(1)–Br(1) 2.4578(9), Fe(1)–Br(2) 2.5185(8), Fe(1)–C(6) 1.921(5), Fe(1)–C(7)
1.769(5), Fe(1)–C(8) 1.844(6), Fe(1)–N(1) 2.004(4); Br(1)–Fe(1)–Br(2) 93.48(3), C(6)–
Fe(1)–N(1) 81.78(19), C(7)–Fe(1)–C(8) 89.1(2).
thesis of thiolate derivatives related to the active site of [Fe]- Fe–Br distance in Fe(CO)4Br2 [2.427(3) Å]. High-resolution mass
hydrogenase (Scheme 2). We have now found that by slow spectroscopy of bulk 1 (formed as the yellow insoluble powder)
diffusion of a solution of Fe2(CO)4Br2 in CH2Cl2 into 2-amino- confirmed the presence of an M− species at m/z 392.7999, with
pyridine in hexanes we can obtain well-formed crystals of 1 the appropriate isotope pattern for the presence of two bromide
suitable for X-ray analysis. This reveals that 1 is a salt with the groups. The attenuated total reflectance (ATR) infrared spectrum
anionic component bearing the anticipated carbamoyl func- of bulk 1 shows two CO stretches at 1982 cm−1 and 2040 cm−1
tionality and cis carbonyl groups: the cationic fragment is a along with a carbamoyl peak at 1660 cm−1
protonated 2-aminopyridine (Fig. 3). The Br–Fe–Br angle in 1 As reported earlier, dissolution of 1 in MeCN followed by
.
[93.48(3)°] is very similar to that in Fe(CO)4Br2, [average concentration and cooling yielded 2·MeCN as a crystalline
92.75(17)°, see ESI† for details], suggesting that the steric require- solid (Scheme 2), with X-ray diffraction confirming loss of
ment of the pyridinyl–carbamoyl ligand is small. The two Fe–- hydrogen bromide from 1 and retention of the carbamoyl and
Br distances in 1 are significantly different, with the bromide cis carbonyl groups.19 Here we note that the X-ray structure of
trans to the carbamoyl [2.5185(8) Å] longer than that trans to a 2 shows that the carbamoyl group retains significant double
carbonyl [2.4578(9) Å]; the latter is in line with the average bond character, with the CO bond distance [1.229(3) Å]
Scheme 1 Generalised mechanism for metallocyclic carbamoyl formation.
Scheme 2 Synthesis of 3.
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significantly longer than that for the two carbonyls [1.136(3) Å which the carbamoyl ligand has been lost entirely from the
and 1.086(3) Å] but shorter than would be anticipated for a metal centre (see ESI†).
C–O single bond. The C–N distance [1.383(3) Å] also confirms the
The reaction of 2 in THF with one equivalent of PPh3 gave a
partial double-bond character of this unit. Examination of the mixture of mono- and di-substituted compounds. Modifying
C–Fe and N–Fe bond lengths in 1 and 2 shows that they are the reaction procedure by carrying out and working up under
not significantly different, whilst the C–Fe–N angles in the two an atmosphere of CO allowed isolation of 3b, which exhibits
compounds are very similar [81.84(18)° and 82.68(9)°, respect- solution IR stretches at 2040 cm−1 and 1990 cm−1 in the carbo-
ively]. The carbamoyl ligand unit is therefore insensitive to the nyl region. Compound 3b gave satisfactory elemental analysis
wider metal environment: with the exception of compound 3a to support formulation as the monophosphine. The di-substi-
(vide infra), this pattern is seen for all of the monomeric carba- tuted complex Fe(CO)(PPh3)2(Br)(C6H5N2CO)·THF was isolated
moyl complexes we have examined.
as a minor product which was characterized by crystallography
(see ESI†).
Substitution chemistry of complex 2
Complex 2 possesses a potential labile solvato ligand, MeCN.
We have shown that this can be replaced by THF and reversibly
by CO:19 here we further explore the substitution of the solvato
ligands MeCN and THF with phosphines.
Basic functionality at the 6-position of the pyridine ring
With a secure route to models replicating the first coordi-
nation sphere of the active site of the [Fe]-hydrogenase, the
next target is replicating the wider environment around the
iron. The native enzyme contains a hydroxyl group pendant to
the putative hydrogen bind site,6 and the involvement of this
group in catalysis has been proposed.4 Theoretical studies
suggest that the lone pair of the oxygen sufficiently basic
enough to able to polarize the hydrogen–hydrogen bond, facili-
tating hydrogen activation.23,24
Slow addition of a dilute solution of PMe3 to 2 in THF
enabled the isolation of 3a in which the ligating solvent is
replaced by the phosphine ligand (Scheme 2, Fig. 4). Com-
pared to complexes 1 and 2, the carbamoyl ligand in 3a is
‘twisted’ such that the pyridine ring does not lie directly above
the Fe–P bond, reducing steric clash between the phosphine
and pyridine groups. In solution, the IR spectrum of 3a shows
maxima at 1934 cm−1, 1967 cm−1 and 2035 cm−1, absorptions
which are shifted to lower wavenumbers with respect to the
parent complex and indicative of better electron-donating
ability of the PMe3 ligand compared with MeCN at the Fe(II)
centre.
Based on the formation of 1 by reaction of 2-aminopyridine
with Fe(CO)4Br2, we sought to introduce a hydroxy group using
4a as the ligand precursor. Compound 4a (Fig. 5) has pre-
viously been synthesized from 2,6-diaminopyridine by reaction
with HCl25 or H2SO4,26 but in neither case is spectroscopic
data available. Modification of the procedure of Tisza and
Joos25 allowed us to reliably access 4a in good yield. We have
established by an X-ray diffraction study that 4a exists as the
keto form 4a′ in the solid state (see ESI†), but this does not
interfere with the reactivity of the hydroxy group in solution
(see below). Reaction of 4a with Fe(CO)4Br2 led to a very rapid
reaction with significant effervescence; IR indicated that all
bound CO had been lost in this process. Attempts to moderate
the reaction by cooling and dilution were unsuccessful. To
exclude the possibility that this failure was due to reaction at
two competing sites, we moved to protection of the hydroxyl
group in 4a. Silylation using (i-Pr)3SiCl under standard con-
ditions to yield 4b proceeded smoothly. However, reaction of
4b with Fe(CO)4Br2 gave similar results to that with 4a: loss of
carbonyl signals in the infrared. It is possible that HBr liber-
ated in the formation of the carbamoyl is able to attack the
silyl group, deprotecting in situ and thus leading to the same
reactivity seen with 4a.
Reaction of 2 with PMe3 in THF is evidently sensitive to
moisture and/or oxygen, as a by-product which co-crystallised
with 3a, [Fe(MeCN)4(PMe3)2][(Br3Fe)2O], has been identified, in
Fig. 4 ORETP representation of the structure of 3a showing 50% probability
ellipsoids; hydrogen atoms except for H(2) have been omitted for clarity.
Selected average bond lengths (Å) and angles (°) with estimated standard devi-
ations: Fe(1)–Br(1) 2.4914(6), Fe(1)–C(6) 1.981(3), Fe(1)–C(7) 1.760(4), Fe(1)–C(8)
1.779(4), Fe(1)–N(1) 2.006(3), Fe(1)–P(1) 2.3152(10); C(7)–Fe(1)–C(8) 90.19(16),
C(6)–Fe(1)–N(1) 82.10(13), Br(1)–Fe(1)–P(1) 87.86(3).
Fig. 5 Oxygen-functionalised aminopyridines.
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Scheme 3 Synthesis of 6.
orange blocks. A diffraction study confirmed the presence of a
free amino group in this material, along with retention of the
cis carbonyl arrangement as required. The geometry at iron is
similar to the other carbamoyl complexes reported here,
although it is notable that the bromide occupies the position
trans to the carbamoyl here, in contrast to the arrangement
in 3. This arrangement permits intramolecular hydrogen
bonding to take place, with an H(3a)⋯Br(1) distance of 2.36 Å.
The other protic hydrogen atoms in 6 are involved in inter-
molecular interactions as would be expected.
Electrochemistry
Cyclic voltammetry of 2 in MeCN-0.1 M [NBu4][BF4] at vitreous
carbon revealed irreversible waves for both oxidation and
reduction. Oxidation gave a single wave at Ep,ox = +1.36 V versus
Ag/AgCl, while reduction showed two waves at −1.17 V and
−1.43 V versus Ag/AgCl. Notably the natural centre in the
enzyme is redox inactive in the accessible physiological poten-
tial domain ca −1.0 to +1.0 V versus Ag/AgCl, and the observed
oxidation and reduction potentials of the synthetic complex is
in accord with this. The large HOMO–LUMO gap of some 2.5 V
is indicative of the stability of the Fe(II) structure cf. Pearson’s
principle of maximum hardness.27 Cyclic voltammetry of 3a
similarly revealed only irreversible oxidation and reduction
processes at Eopx = +1.26 V and Erped = −1.36 V versus Ag/AgCl,
respectively. Infrared spectroelectrochemistry of 3a in the
MeCN electrolyte showed depletion bands at Eapplied = Ep but
the absence of detectable growth bands in the CO region,
indicative of oxidative decarbonylation on the seconds time-
scale.
Fig. 6 ORTEP representation of the structure of 6·MeCN showing 50% prob-
ability ellipsoids; hydrogen atoms except for H(2), H(3a) and H(3b) have been
omitted for clarity. Selected average bond lengths (Å) and angles (°) with esti-
mated standard deviations: Fe(1)–Br(1) 2.5488(18), Fe(1)–C(6) 1.917(11), Fe(1)–
C(7) 1.789(13), Fe(1)–C(8) 1.826(14), Fe(1)–N(1) 2.077(8), Fe(1)–P(1) 2.270(3);
C(6)–Fe(1)–N(1) 82.5(4), C(7)–Fe(1)–C(8) 92.8(5).
We therefore turned to the introduction of an alternative
basic group in the form of the second amine group in 2,6-
aminopyridine. Carbamoyl formation on one side of this mole-
cule would be expected to leave the second amine group free
to participate in further chemistry. Following a similar pro-
cedure to that which produces compound 2, the diamine was
added to a solution of Fe(CO)4Br2 (Scheme 3). Effervescence
due to the liberation of carbon monoxide was observed and a
pale yellow precipitate was rapidly formed. Dissolution of the
yellow powder in MeCN gave a solution showing CO stretching
frequencies at 1999 cm−1 and 2055 cm−1; this was tentatively
assigned as 5. Even when stored in the dark at −20 °C, solu-
tions of 5 were unstable whilst evaporation of the solvent left
only intractable oils, precluding the acquisition of further
data. We therefore added PMe3 directly to a crude preparation
of 5 in THF, leading to the appearance of new IR bands at
1963 cm−1, 1985 cm−1 and 2042 cm−1. When crystallized from
concentrated solutions,‡ compound 6 (Fig. 6) was obtained as
Conclusions
Metallocyclic carbamoyl complexes are rare. We have now
identified and structurally characterised a key precursor on a
pathway which has provided access to a range substituted
ferracyclic structures, including new phosphine derivatives
(Scheme 2). In the [Fe]-hydrogenase structure nature deploys a
hydroxyl-ketonic group at the 6-position on the pyridine ring
in a ferracyclic structure and this is thought to be important in
heterolytic cleavage of dihydrogen acting as proton acceptor.
Although approaches to synthesise a synthetic analogue with a
similar framework were unsuccessful, we have shown that
the reaction of Fe(CO)4Br2 with 2,6-diaminopyridine affords a
ferracyclic structure which has a basic amino group juxtaposed
in the 6-position to the Fe centre, i.e. in the second
‡When crude 6 is crystallized slowly from dilute solution, partial decomposition
takes place leading to the isolation of Fe(CO)(PMe3)2{C6H6N3CO(FeBr3)}(MeCN).
This dinuclear material features a FeBr3− group bound to the carbamoyl oxygen.
See ESI† for further details.
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coordination sphere, and this complex has been structurally evaporate the residual THF. The solid was recrystallized from a
characterised.
minimum of MeCN at −20 °C, give of red crystals of X-ray
quality (8 mg, 3%). Found C 34.04, H 3.55, N 7.12%;
C11H14BrFeN2O3P requires C 33.95, H 3.63, N 7.20%. νmax
/
cm−1 (MeCN) 2035, 1967, 1934. m/z (CI−) 361.8, 359.9 (M −
H − CO). 31P NMR (121 MHz, CD2Cl2): δ −12.51.
Experimental
General
FeBr(C6H5N2O)(CO)2(PPh3) (3b)
Unless otherwise stated, reactions were carried out under
nitrogen using conventional air-sensitive techniques. Starting
materials were purchased from Aldrich or Alfa Aesar and were
used without further purification. Solvents were freshly dis-
tilled under an inert atmosphere of N2 from an appropriate
drying agent. NMR spectra were recorded at 25 °C in 5 mm
tubes using a Bruker Avance 300 spectrometer. FT-IR spectra
were recorded on a Perkin-Elmer SpectrumBX instrument,
fitted with a SensIR Technologies DuraSamplIR II ATR unit
where appropriate. Elemental analyses were performed by
London Metropolitan University. Cyclic voltammetric measure-
ments were carried out using an Autolab PGSTAT 30 potentio-
Solid PPh3 (211 mg, 0.805 mmol) was added to 2 (316 mg
0.805 mmol) in THF (20 cm3). The solvent volume was reduced
by sparging with CO, leading to the formation of a red-brown
precipitate. The final brown paste was washed with cold THF
(−20 °C, 5 cm3) to leave a red-brown powder (315 mg, 68%).
Found C 54.19, H 3.60, N 4.89%; C26H20BrFeN2O3P requires C
54.29, H 3.50, N 4.87%. νmax/cm−1 (THF) 2040, 1990, 1666,
1620. 31P NMR (121 MHz, CD2Cl2): δ 37.47.
6-Aminopyridin-2-ol (4a)
2,6-Diaminopyridine (12.5 g, 114 mmol) was refluxed in HCl
(10%, 300 cm3) for three hours. The solvent was distilled out at
atmospheric pressure to leave 50 cm3 of concentrated solution.
On cooling, a pale crystalline material precipitated and was
recovered by filtration. The crude material was redissolved in a
minimum of hot water and the pH adjusted to 11 by addition
of 40% NaOH solution. Cooling to 0 °C led to the formation of
needle-like crystals of the product. After washing with CH2Cl2,
this was recrystallized from hot water (4.0 g, 32%). Crystals
suitable for X-ray diffraction were obtained from this second
stat.
A
conventional three-electrode arrangement was
employed, consisting of a vitreous carbon working electrode, a
platinum wire as the auxiliary electrode, and Ag/AgCl as a
reference electrode. All measurements were done in dry
degassed MeCN in the presence of 0.1 M [NBu4][BF4] as the
supporting electrolyte at room temperature. Spectroelectro-
chemistry experiments were carried out using a Spectroelectro-
chemistry Partners SP-02 cell mounted on a Pike MIRacle ATR
unit fitted to a Bruker Vertex 80 spectrometer.
1
crystallization. H NMR (300 MHz, D2O): δ 5.54 (m, 1H), 5.57
Fe(CO)4Br2
(d, 1H, J = 3.9 Hz), 7.23 (t, 1H, J = 8.3 Hz). 13C NMR (75 MHz,
D2O): 92.23, 101.81, 145.75, 151.59, 163.81. m/z (EI+) 110.0.
This was synthesized as previously reported.19 Crystals suitable
for X-ray diffraction were grown by sublimation at reduced
pressure.
6-{[Tris(propan-2-yl)silyl]oxy}pyridin-2-amine (4b)
6-Aminopyridin-2-ol 4a (618 mg, 5.62 mmol) was dissolved in
THF (100 cm3) and cooled to −78 °C. Butyl lithium (1.6 M in
THF, 3.51 cm3) was added slowly, and the reaction stirred for
2 h before warming to room temperature. Tris(isopropyl)silyl
chloride (1.20 cm3, 5.62 mmol) was added, and reaction
stirred for 48 h. The solvent was removed at reduced pressure
and the product extracted from the residue with CH2Cl2. This
was then filtered and the solvent removed at reduced pressure
to leave a yellow/brown oil, which was used directly (910 mg,
[C5H7N2][Fe(C6H5N2O)(CO)2Br2)] (1)
A solution of 2-aminopyridine (12 mg, 0.13 mmol) in hexane
(5 cm3) was carefully layered onto a solution of Fe(CO)4Br2
(50 mg, 0.13 mmol) in CH2Cl2 (10 cm3). A small amount (ca
5 mg) of yellow crystalline [C5H7N2][Fe(C6H5N2O)(CO)2Br2)]
was formed upon the slow diffusion of the solutions through
each other over the course of five days and the isolated
material was characterized by X-ray diffraction. Found C 30.11,
H 2.31, N 11.01%; C13H12Br2FeN4O3·0.5(CH2Cl2) requires C
30.57, H 2.47, N 10.56%. m/z (orbitrap) calcd for anion
C8H5N2Br2FeO3 392.8001, found 392.7999.
1
61%). H NMR (300 MHz, CD2Cl2): δ 1.10 (d, 27H, J = 7.4 Hz),
1.35 (octet, 3H, J = 7.4 Hz), 6.04 (d, 1H, J = 1.7 Hz), 6.07 (d, 1H,
J = 1.7 Hz), 7.32 (t, 1H, J = 7.8 Hz). m/z (orbitrap) calcd for
C14H25N2OSi 267.1887, found 267.1892.
FeBr(C6H5N2O)(CO)2(PMe3) (3a)
FeBr(C6H6N3O)(CO)2(PMe3) (6)
A solution of PMe3 (0.065 cm3, 0.063 mmol) dissolved in THF
(10 cm3) was added drop wise to 2 (240 mg 0.607 mmol) in Solid 2,6-diaminopyridine (186 mg, 1.71 mmol) was added to
THF (10 cm3) over 30 minutes. During the addition, the solu- a solution of Fe(CO)4Br2 (560 mg, 1.71 mmol) in CH2Cl2
tion turned from orange-red to a darker red-purple. The (25 cm3); gas was evolved and a precipitate formed. After two
volume of the solution was reduced to 2 cm3 under vacuum hours, the precipitate was recovered by filtration and dried in
before cooling to −20 °C. A white solid precipitated; the super- vacuo; this gave 557 mg of yellow solid. The solid was taken up
natant was decanted and the solvent was removed under in THF (10 cm3), and
a
solution of PMe3 (0.14 cm3,
reduced pressure yielding a sticky oil. This was taken in MeCN 1.35 mmol) in THF (15 cm3) was added. After one hour, the
(ca 1 cm3) and evaporated in vacuum three times to co- volume was reduced to approximately 5 cm3 and the solution
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Table 1 Summary of crystallographic data for Fe(CO)4Br2, 1, 3a and 6·MeCN
Fe(CO)4Br2
1
3a
6·MeCN
Formula
M
Space group
C4Br2FeO4
327.71
C2
C8H5Br2FeN2O3, C5H7N2
487.94
P21/n
C11H14BrFeN2O3P
388.97
P21/a
C11H15BrFeN3O3P, C2H3N
445.04
P21/c
a/Å
b/Å
c/Å
6.7608(4)
10.2296(5)
12.2954(7)
103.801(3)
825.80(8)
120(2)
4
11.2197(6)
12.6286(7)
11.8538(6)
105.142(5)
1621.24(15)
140(2)
13.2378(3)
7.5324(3)
15.4174(5)
110.218(2)
1442.58(8)
120(2)
9.8203(17)
12.719(2)
15.139(2)
91.830(14)
1890.0(5)
140(2)
β/°
V/Å3
T/K
Z
4
4
4
Rint
R1 [I > 2σI]
0
0.074
0.042
0.056
0.040
0.165
0.091
0.068
wR2 (all data)
0.177
0.085
0.079
0.277
cooled to −20 °C overnight. The solution was filtered to (grant numbers BB/E023290/1 and EP/F047878/1). The authors
remove any white precipitate that had formed, then dried thank the EPSRC National Mass Spectrometry Service, Swansea
under vacuum to give a sticky orange oil. Recrystallization of for mass spectroscopy data, and the EPSRC National Crystallo-
this oil from a minimum of MeCN (∼3 cm3) at −20 °C gave a graphy Service for collection of diffraction data.32
mass of homogeneous crystals of 8 (51 mg, 7.5%). Compound
6 was isolated as orange blocks suitable for X-ray diffraction
References
study. Found C 32.46, H 3.89, N 10.21%; C11H16BrFeN3O3P
requires C 32.62, H 3.98, N 10.38%. νmax/cm−1 (THF) 1963,
1985, 2042. m/z (CI−) 376.9, 374.9 (M − H − CO).
1 C. Tard and C. J. Pickett, Chem. Rev., 2009, 109, 2245–2274.
2 J.-F. Capon, F. Gloaguen, F. Y. Pétillon, P. Schollhammer
and J. Talarmin, Coord. Chem. Rev., 2009, 253, 1476–1494.
3 S. Shima and U. Ermler, Eur. J. Inorg. Chem., 2011, 963–
972.
Crystallography
Crystals were suspended in oil, and one was mounted on a
glass fibre and fixed in the cold nitrogen stream of the diffracto-
meter. Data were collected an Oxford Diffraction Xcalibur-3
CCD diffractometer equipped with Mo-Kα (λ = 0.71073 Å) radi-
ation and graphite monochromator (1 and 6·MeCN), or a
Bruker–Nonius FR591 molybdenum rotating anode and con-
focal mirrors [Fe(CO)4Br2 and 3a]. Data were processed using
the CrysAlisPro program (1 and 6·MeCN)28 or the DENZO and
COLLECT programs [Fe(CO)4Br2 and 3a].29 Structures were
determined by the direct methods routines in the Superflip
program30 (1 and 3a) or SHELXS program31 [Fe(CO)4Br2 and
6·MeCN], and were refined by full-matrix least-squares
methods on F2 in SHELXL.31 Non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms
bound to nitrogen were located in the Fourier difference map
and freely refined where possible; in all other cases they were
included in idealized positions and their Uiso values were set
to ride on the Ueq values of the parent carbon atoms. Crystallo-
graphic data for previous-unpublished structures are summar-
ized in Table 1 and S1† (ESI†). CCDC 922917–922924 contain
the supplementary crystallographic data for Fe(CO)4Br2, 1, 3a
and 6·MeCN and the supporting structures.
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PJT and ADH thank the Engineering and Physical Sciences 14 D. Chen, R. Scopelliti and X. Hu, Angew. Chem., Int. Ed.,
Research Council (EPSRC) for Departmental Training Award 2010, 49, 7512–7515.
studentships. JAW thanks the Biotechnology and Biological 15 D. Chen, R. Scopellitti and X. Hu, Angew. Chem., Int. Ed.,
Sciences Research Council (BBSRC) and EPRSC for funding
2011, 50, 5671–5673.
This journal is © The Royal Society of Chemistry 2013
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