Synthesis, structure and reactivity of Ni site models of [NiFeSe] hydrogenases

Article abstract of DOI:10.1039/c3dt52967c

A series of structural models of the Ni centre in [NiFeSe] hydrogenases has been developed which exhibits key structural features of the Ni site in the H₂ cycling enzyme. Specifically, two complexes with a hydrogenase-analogous four-coordinate 'NiS₃Se' primary coordination sphere and complexes with a 'NiS₂Se₂' and a 'NiS ₄' core are reported. The reactivity of the complexes towards oxygen and protons shows some relevance to the chemistry of [NiFeSe] hydrogenases. Exposure of a 'NiS₃Se' complex to atmospheric oxygen results in the oxidation of the selenolate group in the complex to a diselenide, which is released from the nickel site. Oxidation of the selenolate ligand on Ni occurs approximately four times faster than oxidation with the analogous sulfur complex. Reaction of the complexes with one equivalent of HBF₄ results in protonation of the monodentate chalcogenolate and the release of this ligand from the metal centre as a thiol or selenol. Unrelated to their biomimetic nature, the complexes serve also as molecular precursors to modify electrodes with Ni-S-Se containing particles by electrochemical deposition. The activated electrodes evolve H₂ in pH neutral water with an electrocatalytic onset potential of -0.6 V and a current density of 15 μA cm^-2 at -0.75 V vs. NHE.

Full text of DOI:10.1039/c3dt52967c

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Synthesis, structure and reactivity of Ni site models
of [NiFeSe] hydrogenases†
Cite this: Dalton Trans., 2014, 43,
4483
Claire Wombwell and Erwin Reisner*
A series of structural models of the Ni centre in [NiFeSe] hydrogenases has been developed which exhibits
key structural features of the Ni site in the H2 cycling enzyme. Specifically, two complexes with a hydro-
genase-analogous four-coordinate ‘NiS3Se’ primary coordination sphere and complexes with a ‘NiS2Se2’
and a ‘NiS4’ core are reported. The reactivity of the complexes towards oxygen and protons shows some
relevance to the chemistry of [NiFeSe] hydrogenases. Exposure of a ‘NiS3Se’ complex to atmospheric
oxygen results in the oxidation of the selenolate group in the complex to a diselenide, which is released
from the nickel site. Oxidation of the selenolate ligand on Ni occurs approximately four times faster than
oxidation with the analogous sulfur complex. Reaction of the complexes with one equivalent of HBF4
results in protonation of the monodentate chalcogenolate and the release of this ligand from the metal
centre as a thiol or selenol. Unrelated to their biomimetic nature, the complexes serve also as molecular
precursors to modify electrodes with Ni–S–Se containing particles by electrochemical deposition. The
activated electrodes evolve H2 in pH neutral water with an electrocatalytic onset potential of −0.6 V and a
current density of 15 μA cm−2 at −0.75 V vs. NHE.
Received 21st October 2013,
Accepted 5th December 2013
DOI: 10.1039/c3dt52967c
www.rsc.org/dalton
Introduction
The sustainable generation of the energy vector H2 would
provide a possible solution to a fossil-free economy,1 and the
improvement of H2 evolution catalysts is therefore attracting
much current attention.2 In nature, the ubiquitous metals
nickel and iron are employed in the active sites of [NiFe] and
[FeFe] hydrogenases to reversibly catalyse the interconversion
Fig. 1 (a) Molecular structure of the [NiFe(Se)] hydrogenase active
site. The highlighted atom (Se/S) is S in [NiFe] hydrogenases and Se in
of protons and electrons to H2 and operate at high rates at [NiFeSe] hydrogenases. A hydride or oxo-group can serve as the bridging
modest over-potentials.3 [NiFeSe] hydrogenases are a subclass
of the [NiFe] hydrogenases with a selenocysteine (Sec) in place
moiety X (the latter was only observed in oxidised [NiFe] hydrogenases)
and an active reduced state does not contain a bridging ligand X.
(b) Composition of a structural conformer in the oxidised [NiFeSe]
hydrogenase from Desulfovibrio vulgaris Hildenborough.9f,g,10
of a cysteine (Cys) residue terminally bound to the Ni centre
(Fig. 1).4 These selenium containing enzymes have been shown
to display a number of advantageous properties in comparison
with the conventional [NiFe] hydrogenases which make them
good candidates as H2 evolution biocatalysts: they show higher
H2 evolution activities than [NiFe] hydrogenases5 and have fast
reactivation times from O2 damage at a low redox potential.5b,6
They were also previously employed in photocatalytic H2
generation schemes5d,7 and even display catalytic H2 pro-
duction in the presence of O2.5b,7 Therefore [NiFeSe] hydro-
genases serve as an attractive blueprint and inspiration for the
development of synthetic H2 evolution catalysts.
The active site structure of the [NiFe(Se)] hydrogenases has
been well studied using spectroscopic5a,6a,8 and crystallo-
graphic4,9 techniques (Fig. 1). It consists of a dinuclear [NiFe]
centre bridged by two cysteine ligands. There is also a Cys and
a Sec or Cys residue terminally bound to Ni.
Christian Doppler Laboratory for Sustainable SynGas Chemistry, Department of
Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK.
E-mail: reisner@ch.cam.ac.uk
†Electronic supplementary information (ESI) available: Supporting table, figures
and X-ray structure refinement details. CCDC 975187 (n-Bu4N)[Ni(LSe)(MesS)],
939413 (n-Bu4N)[Ni(LS)(MesS)], 939414 (n-Bu4N)[Ni(LSe)(MesSe)] and 939415
(n-Bu4N)[Ni(LS)(MesSe)]. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3dt52967c
The Ni ion serves as the active redox centre: its oxidation
state varies between Ni(I) and Ni(III),8a,d–h whereas Fe remains
low spin Fe(II)8h in all active site states. A catalytically active
form of the enzyme has been shown to exist as Ni(III) with a
bridging hydride between the Ni and Fe centres (position X in
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Fig. 1a) and a reduced Ni(II) form does not contain a bridging
ligand X.8f,g,i The Cys or Sec residue highlighted in Fig. 1a is
proposed to act as a proton relay during H2 cycling.4,9a–c The
different properties of Sec compared with Cys, such as its
higher nucleophilicity and lower pKa,11 may cause the
increased H2 production activity of the [NiFeSe] subclass.5
We also note that there are other differences apart from the
active site Sec, which could affect the chemistry of the enzyme.
For example a medial [3Fe4S] cluster is present in D. baculatum
[NiFeSe] hydrogenase rather than a [4Fe4S] cluster in con-
ventional [NiFe] hydrogenases in the electron transfer chain
between the active site and the protein surface.4
Despite a large number of structural mimics of the [NiFe]
hydrogenase active site,12 no systematic series of Ni subsite
models with a stoichiometrically accurate ‘NiS3Se’ core has
been previously reported. Noteworthy approaches to mimic
selenium in hydrogenases are complexes with a ‘NiS3SeP’
centre such as penta-coordinate [Ni(SePh)(P(o-C6H4S)2-
(o-C6H4SH))]−,12j,k a number of Ni complexes with nitrogen
and selenium-donor ligands,12m,n and a series of diiron com-
plexes containing mixed sulfur/selenium bridging ligands
such as [Fe2(μ-SC3H6Se-μ)(CO)6].13
Herein we report on the synthesis, characterisation, reactiv-
ity towards O2 and protons, and electrochemical study of four-
coordinate Ni complexes with thiolate and selenolate ligands
as structural models of the Ni site of reduced [NiFeSe] hydro-
genases. The mononuclear model compounds display protona-
tion and O2 reactivity relevant to the hydrogenases.
Electrochemical studies reveal that the Ni complexes do not
operate as molecular H2 production catalysts. However, they
are useful precursors to catalytic Ni-containing particles for H2
production in pH neutral H2O.
Scheme 1 Synthetic pathway to models of the Ni-site in [NiFe(Se)]
hydrogenases: (a) Li, 4,4’-di-tert-butylbiphenyl, tetrahydrofuran, −90 °C.
(b) (i) Se, −90 °C to −50 °C; (ii) H3O+. (c) (i) NaBH4; (ii) H3O+. (d)
Ni(OCOCH3)2·4H2O, tetrahydrofuran–methanol, reflux. (e) NaMesE’ (E’ =
S or Se), n-Bu4NOH, tetrahydrofuran–methanol.
Results and discussion
Synthetic pathway to hydrogenase inspired Ni complexes
The synthetic route to structural models of the Ni site in
[NiFeSe] hydrogenase is outlined in Scheme 1. The novel tri-
dentate mixed donor ligand ‘LSe’–H2 was synthesised from
thianthrene. A catalytic amount of 4,4′-di-tert-butylbiphenyl
was used with two equivalents of lithium powder in tetrahydro-
furan at −90 °C to give a radical anion which is strong enough
to cleave one of the thianthrene carbon–sulfur bonds.14 Intro-
duction of elemental selenium into the lithiated product and
acidic workup gave the diselenide (‘LSe–H’)2, which can be sub-
sequently reduced with LiAlH4 to give the selenol ‘LSe’–H2 after
acidic workup. Refluxing a solution of ‘LSe’–H2 in tetrahydro-
furan with one equivalent of Ni(OCOCH3)2·4H2O in methanol
yields the complex [Ni(LSe)]n as an insoluble black precipitate
in 87% yield.
Fig. 2 Chemical structure of [Ni(LS)]n in the dimeric (n = 2)12l and tri-
meric (n = 3)12i forms.
reported procedure.12i The oligomeric structure in [Ni(LSe)]n
and [Ni(LS)]n can be broken upon reaction with one equivalent
of sodium mesityl selenolate or sodium mesityl thiolate
in tetrahydrofuran–methanol (4 : 1) at room temperature.
Addition of one equivalent of n-Bu4NOH yields the corres-
ponding four mononuclear Ni complexes (n-Bu4N)[Ni(LE)-
(MesE′)] (E = S or Se; E′ = S or Se, Scheme 1). Brown crystals of
the Ni complexes were grown from a saturated solution in
tetrahydrofuran–hexane giving the pure product in good yields
(68 to 79%). The compounds have been characterised by
1H NMR (Fig. S1–S8†), mass spectrometry (Fig. S9–S16†),
ATR-IR spectroscopy (Fig. S17†) and elemental analysis. The
tetraphenyl phosphonium salts of [Ni(LS)(MesS)]− and [Ni(LS)-
[Ni(LS)]n (Scheme 1) was previously characterised by Sellmann
and co-workers using single crystal X-ray analysis and it showed
dimeric and trimeric forms, depending on crystal growth con-
ditions (Fig. 2).12i,l This composition is therefore also likely for
[Ni(LSe)]n. [Ni(LS)]n has been prepared following a previously
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lengths and angles are summarised in Table 1). The complex
anions [Ni(LSe)(MesS)]− and [Ni(LS)(MesSe)]− contain a Ni2+
cation bound to three S and one Se donors and reflect the
primary ligand sphere of the Ni site in [NiFeSe] hydrogenases.4
The complexes display a distorted square planar arrange-
ment around the metal centre. The average trans S–Ni–SMes
and S–Ni–SeMes bond angle is 160.2
0.6°. The [NiFeSe]
hydrogenase displays angles of 168.0° and 106.4° between
Se–Ni–(μ-S) and Sterminal–Ni–(μ-S), respectively (PDB 1CC1).4
Crystallographic disorder of the thiolate and selenolate donors
in ‘LSe’ occurs in [Ni(LSe)(MesSe)]− (indicated as S/Se in Fig. 3)
and [Ni(LSe)(MesS)]− and we therefore only consider the bond
lengths from the Ni to the MesS and MesSe ligand in our dis-
cussion. The average Ni–Se(Mes) bond length at 2.306 0.003 Å
is longer than the Ni–S(Mes) distance at 2.189
0.003 Å.
A Ni–Se bond length of 2.46 Å and average Ni–S distance of
2.2 Å (at 2.15 Å resolution) was reported for the reduced
[NiFeSe] hydrogenase from Desulfomicrobium baculatum.4
Reactivity of the Ni complexes with atmospheric O2
A distinct difference between the [NiFe] and [NiFeSe] hydroge-
nases is the reactivity of their active sites with O2. EPR studies
showed that upon reactivity with atmospheric O2 the Ni in the
conventional [NiFe] hydrogenases is oxidised to Ni(III), in one
of two states known as the ‘ready’ (Ni–B) and ‘unready’ (Ni–A)
states.8a,d,e,h Crystal structures of the oxidised enzyme show an
oxygen containing ligand bridging the Ni and Fe centre (posi-
tion X, Fig. 1) in both oxidised states.9b,d,e The oxidised active
site in [NiFeSe] hydrogenase is quite different; the Ni centre is
not oxidised and it remains diamagnetic Ni(II).5a Single
crystal X-ray structures of the oxidised [NiFeSe] hydrogenase in
D. vulgaris and D. baculatum confirm that there is no bridging
oxo ligand between the metal centres. In fact, selenium
and/or sulfur are oxidised instead.9f–h This different active
site chemistry might explain the different rates for in-
activation and re-activation between [NiFe] and [NiFeSe]
hydrogenases.5b
Fig. 3 X-ray single crystal structures of complexes (n-Bu4N)[Ni(LE)-
(MesE’)] showing 50% probability ellipsoids (countercation not shown;
the asymmetric units of [Ni(LSe)(MesS)]−, [Ni(LS)(MesSe)]− and [Ni(LS)-
(MesS)]− contain two independent anions; images created using Ortep 3
for Windows15). S/Se indicates disorder of S and Se over two positions.
(MesSe)]− have also been prepared and characterised by 1H,
13C, 77Se NMR spectroscopy, ESI-MS and elemental analysis.
Structural analyses of Ni complexes
The single crystal X-ray structures of all four complexes were
We therefore investigated the reactivity of the Ni complexes
obtained as tetrabutylammonium salts (Fig. 3; selected bond with O2. As expected, all complexes are air sensitive and
Table 1 Selected bond lengths (Å) and angles (°) for complexes (n-Bu4N)[Ni(LE)(MesE’)] (see Scheme 1 for labeling)
(n-Bu4N)[Ni(LSe)(MesS)]
(E = Se, E′ = S)
(n-Bu4N)[Ni(LSe)(MesSe)]
(E = Se, E′ = Se)
(n-Bu4N)[Ni(LS)(MesSe)]
(E = S, E′ = Se)
(n-Bu4N)[Ni(LS)(MesS)]
(E = S, E′ = S)
Bond lengths
Ni–S
Ni–S′
Ni–E
Ni–E′
n.d.a
n.d.a
n.d.a
2.158(2)
2.124(2)
2.197(2)
2.3009(14)
2.155(2)
2.1630(12)
2.1254(13)
2.1987(12)
2.1947(14)
2.19642(14)
2.1257(17)
2.1263(18)
2.124(1)
2.131(3)
2.190(2)
2.3066(17)
2.1218(14)
2.1960(14)
2.1856(16)
n.d.a
n.d.a
n.d.a
2.1930(18)
2.183(2)
2.3102(9)
Bond angles
S–Ni–E
S–Ni–S′
S–Ni–E′
E–Ni–S′
E–Ni–E′
S′–Ni–E′
153.2a
89.6a
157.0a
90.5a
155.2a
90.0a
153.63(9)
92.12(9)
96.51(8)
89.47(9)
91.36(7)
158.64(7)
156.26(10)
91.39(10)
95.39(9)
89.78(10)
90.67(8)
152.78(5)
91.76(5)
97.07(5)
89.24(5)
91.64(5)
158.64(5)
156.14(5)
91.15(5)
96.05(6)
89.58(5)
90.71(6)
161.33(6)
90.9a
89.7a
90.6a
92.2a
91.3a
91.8a
96.9a
95.8a
95.7a
158.74(7)
161.21(8)
160.79(5)
161.97(8)
a n.d. = not accurately determined due to crystallographic disorder.
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disorder compared with the rest of the active site, suggesting
various protonation states.9a–c In the D. baculatum [NiFeSe]
hydrogenase the selenocysteine residue in the same position
exhibits this crystallographic disorder suggesting that it is the
proton relay in this case.4
Exposure of (n-Bu4N)[Ni(LS)(MesSe)] to one equivalent of
HBF4 in dichloromethane results in the protonation of the
mesityl selenolate group (Scheme 2), in analogy to protonation
in the enzyme active site. Mesityl selenol is released from the
metal centre and has been isolated from the solution and
characterised using 1H NMR spectroscopy (Fig. S23†).
The remaining ‘Ni(LS)’ centre precipitates as the black solid
[Ni(LS)]n (separated and characterised by ATR-IR and elemental
analysis). Diethyl ether was added to the filtrate to precipitate
n-Bu4NBF4, which was separated by filtration and characterised
using 1H NMR spectroscopy and mass spectrometry. The
solvent was removed from the remaining filtrate to give mesityl
selenol. A comparable reactivity pattern is observed with
(n-Bu4N)[Ni(LS)(MesS)]: the ‘MesS’ ligand is protonated and is
released from the metal centre as mesityl thiol (Scheme 2;
Fig. S24†). The products of protonation were separated and
characterised in the same way as with (n-Bu4N)[Ni(LS)(MesSe)].
The protonation of these complexes parallels the proposed
protonation of the selenocysteine and cysteine residue in the
hydrogenase.
Scheme 2 Reactivity of [Ni(LS)(MesSe)]− (E’ = Se) and [Ni(LS)(MesS)]−
(E’ = S) with O2 and HBF4.
convert completely to oxidised products over a maximum of
five days. (PPh4)[Ni(LS)(MesSe)] and (PPh4)[Ni(LS)(MesS)]
convert cleanly to [Ni(LS)]n and the dichalcogenide (MesSe)2 or
(MesS)2 (Scheme 2). The oxidation products were isolated and
characterised after exposing the complexes to atmospheric O2
in dichloromethane. [Ni(LS)]n was isolated as a black precipi-
tate and characterised by ATR-IR spectroscopy (Fig. S18†) and
elemental analysis. The dichalcogenide was isolated from the
supernatant and characterised by 1H NMR and ESI-MS
(Fig. S19–S21†).
The oxidation of the Se containing model emulates the
reactivity of the enzyme. Oxidation of Se from the −2 to −1 oxi-
dation state, and the formation of a Se–chalcogen bond conco-
mitant with the loss of Se from the metal centre are all
observed in the oxidised [NiFeSe] hydrogenase from Desulfovi-
brio vulgaris Hildenborough.9f,g Crystallographic analysis of
this oxidised enzyme shows that the selenium is bound to a
sulfur atom extrinsic to the active site. The source of the
S atom is proposed to be H2S in this sulfate reducing bacterium.
In one oxidised conformer the selenium is not bound to the
Ni centre and the extrinsic sulfur binds to Ni in its place
(Fig. 1b).9 f,g
Electrochemistry and electrodeposition of Ni complexes
The cyclic voltammograms (CVs) of all four complexes show an
irreversible reduction process at approximately −1.4 V vs. NHE
at a scan rate of 100 mV s−1 in acetonitrile, dichloromethane
and dimethylformamide containing n-Bu4NBF4 as a support-
ing electrolyte (Fig. 4a black trace, Fig. S25–26†). This wave is
assigned to the reduction of Ni(II) to Ni(I) based on previous
electrochemical studies with Ni–thiolate complexes.12g,16
A
catalytic wave grows at approximately −1.30 V vs. NHE with
increasing amounts of triethylammonium chloride (Et3NHCl)
in acetonitrile indicating electrocatalytic H2 production
(Fig. 4a, Fig. S27†). The overpotential required for the
reduction of protons is approximately 0.65 V under these con-
ditions, calculated from the half wave potential for catalytic
proton reduction in the presence of 10 equivalents of Et3NHCl
(−1.17 V vs. NHE, Fig. S28†) and the standard potential
for reduction of protons from Et3NHCl in acetonitrile (−0.51 V
vs. NHE).17
However, the proton reduction activity does not stem from
a molecular Ni species, but from a solid deposit formed on the
electrode surface at the applied negative potential. Here, the
Ni complexes act as molecular precursors to catalyst particles.
The same electrocatalytic response was observed after remov-
ing the working electrode following electrocatalytic proton
reduction in n-Bu4NBF4–acetonitrile (0.1 M) with Et3NHCl
(10 mM) and a Ni complex, rinsing it with acetonitrile and
The oxidation of (PPh4)[Ni(LS)(MesSe)] and (PPh4)[Ni(LS)-
1
(MesS)] was also followed by H NMR spectroscopy in deuter-
ated dichloromethane (Fig. S22†). Over the course of oxidation
the signals of the ‘Ls’ ligand at 6.8 to 7.5 ppm decrease as
[Ni(LS)]n precipitates from solution and the methyl signals for
the mesityl thiolate/selenolate ligand at 2.1 to 2.6 ppm
decrease as new methyl signals appear around 2.3 ppm for the
dichalcogenide oxidation product. The selenolate containing
complex (PPh4)[Ni(LS)(MesSe)] is oxidised more rapidly than
the all sulfur complex: it is completely converted within
24 hours, compared with 96 h for (PPh4)[Ni(LS)(MesS)]. This
observation supports the hypothesis that the fast reactivity of
Se with O2 prevents the nickel centre from being oxidised in
[NiFeSe] hydrogenases.9f–h
Reactivity of the Ni complexes with HBF4
Crystallographic evidence suggests that one of the terminal immersing it in a fresh, Ni complex free electrolyte solution
cysteine residues bound to the Ni centre in [NiFe] hydroge- (Fig. S29†). Electronic absorption spectroscopy of the nickel
nases (highlighted in Fig. 1a) acts as a proton relay during the complexes confirmed their stability in the presence of
catalytic cycle. This residue has increased crystallographic 10 equivalents of Et3NHCl in acetonitrile in the absence of an
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Fig. 5 SEM images and EDX spectra of a (a and b) Ni particle modified
glassy carbon electrode prepared from the electro-deposition of
(n-Bu4N)[Ni(LSe)(MesS)] (1 mM) and (c and d) Ni film on a glassy carbon
electrode prepared from the electrodeposition of Ni(NO3)2·6H2O
(1 mM). The deposits were formed with the compounds at an applied
potential of −1.33 V vs. NHE for 0.5 h in acetonitrile (0.1 M n-Bu4NBF4)
with Et3NHCl (10 mM).
Ni(NO3)2·6H2O as a precursor resulted in a Ni film (no sulfur
or selenium residues were detected) covering the electrode
surface (Fig. 5c and 5d) rather than Ni particle formation as
observed with the Ni complexes. Our observations demonstrate
that the molecular Ni precursors allow for the incorporation of
sulfur and selenium in the solid precipitate and the formation
of a distinct morphology compared to using Ni(NO3)2·6H2O.
Fig. 4 (a) CV of (n-Bu4N)[Ni(LSe)(MesS)] (1 mM) in acetonitrile containing
n-Bu4NBF4 (0.1 M) at a glassy carbon electrode with a scan rate of
100 mV s−1 with increasing concentrations of Et3NHCl: no acid (black),
1 (red), 2 (blue), 3 (pink), 5 (green) and 10 (purple) mM. (b) CV of a Ni
particle activated (solid trace) and unmodified (dashed trace) glassy
carbon electrode in a phosphate buffered aqueous solution (0.1 M) at
pH 7 at a scan rate of 100 mV s−1 at room temperature.
Electrocatalytic activity of Ni-activated electrodes in water
applied potential, demonstrating that the complexes were de-
posited electrochemically on the electrode at negative poten-
tials (Fig. S30†).
We subsequently investigated the activity of the Ni particle
modified electrode, which was formed by holding the potential
at −1.33 V vs. NHE for 0.5 h in a solution of the Ni complexes
(1 mM) and Et3NHCl (10 mM) in n-Bu4NBF4–acetonitrile
(0.1 M) and washing of the electrode. The Ni particle electrode
displayed a respectable activity in a pH neutral Ni free aqueous
phosphate solution (0.1 M; Fig. 4b). The onset of a catalytic
wave was observed at approximately −0.60 V vs. NHE, which
The oxidation wave at E = −0.25 V in Fig. 4a and S25–S27†
is attributed to the oxidation of a soluble product formed
during the irreversible reduction of the complexes. The redox
process cannot be assigned to the oxidation of deposited
material as it is not visible in the CV of a nickel modified
glassy carbon electrode in a fresh electrolyte solution.
Although the particle formation has no obvious relevance
to the biomimetic features of the complexes, there is currently
much interest in assembling heterogeneous electrocatalysts for
proton reduction from molecular precursors.18 We therefore
decided to investigate the Ni particle formation in more detail.
Scanning electron microscopy (SEM) and energy-dispersive
X-ray spectroscopy (EDX) analysis of a glassy carbon slide (total
surface area 1.6 cm2) modified with (n-Bu4N)[Ni(LSe)(MesS)]
(1 mM) at −1.33 V vs. NHE for 0.5 h showed the deposition of
nickel containing particles on the glassy carbon surface
(Fig. 5a).
corresponds to a small overpotential of 180 mV (E0, H+/H2
−0.42 V at pH 7).
=
The production of H2 was confirmed using controlled
potential electrolysis at −0.9 V vs. NHE with a glassy carbon
rod or fluoride doped tin oxide (FTO) coated glass electrode
(both approximately 1.6 cm2) modified with the Ni precursor
by the same procedure. SEM/EDX analysis also confirmed the
deposition of Ni on the FTO substrate (Fig. S31†). A Faradaic
yield of approximately 60 to 71% was observed for all four
deposits on glassy carbon and FTO glass modified with the Ni.
The total amount of H2 generated varied widely on glassy
carbon (10 to 2000 µmol after 20 h bulk electrolysis), but was
more reproducible on FTO coated glass (9 to 55 µmol after
10 h electrolysis, Fig. S32 and Table S1†).
EDX of the particles revealed that they contain Ni, S and Se
(Fig. 5b), but the exact composition and oxidation state of
the Ni–S–Se containing deposit remains unknown. Using
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Modifying the FTO electrode with Ni(NO3)2·6H2O by the the solvent peak. The 77Se NMR spectrum was recorded on a
same procedure resulted in the formation of comparable Bruker Avance 500 MHz BroadBand NMR spectrometer and
amounts of H2, but with a reduced Faradaic yield of only 32%, referenced against dimethyl selenide in d-benzene as an
suggesting that the molecular precursor does not only have an external reference at 0 ppm. The mass spectrum of (‘LSe–H’)2
effect on the composition and morphology of Ni deposit, but was carried out on a Waters ZQ HPLC-MS. The mass spectra of
also on the activity of the Ni-modified electrodes. We also com- ‘LSe’–H2 and dimesityl diselenide were recorded by the Univer-
pared the Ni particle electrodes with the benchmark proton sity of Cambridge Mass Spectrometry Service using a Bruker
reduction catalyst Pt.19 A metallic Pt foil with the same geo- Bio Apex 4.0 FTICR EI MS. The mass spectra of the metal com-
metrical surface area (approximately 1.6 cm2) was therefore plexes and inorganic salts were recorded on a Waters Quattro
tested as the working electrode under the same experimental LC electrospray ionisation mass spectrometer. Expected and
conditions. The activity of the Pt foil is approximately an order experimental isotope distributions of [Ni(LE)(MesE′)]− were
of magnitude higher than the Ni particle modified FTO elec- compared. Elemental analysis was carried out by the micro-
trodes in an aqueous phosphate solution at pH 7 (Fig. S32†).
analysis service of the Department of Chemistry, University
of Cambridge. IR spectra were recorded on a Perkin Elmer
SpectrumOne FTIR spectrometer with an ATR sampling acces-
sory. Electronic absorption spectra were recorded on an Agilent
Cary UV-Vis 50 Bio spectrometer. The SEM images were
obtained using a Philips XL30 132-10 electron microscope.
Energy-dispersive X-ray spectroscopy (edax PV7760/68 ME) was
used at a 15 kV acceleration voltage, spot size 4.0 and an
acquisition time of at least 100 s. The elements were assigned
and atomic ratios were identified using the built in software
(EDAX).
Conclusions
We have demonstrated the synthesis and characterisation of
the first series of complexes, which resemble the primary
coordination sphere of the Ni site in [NiFeSe] hydrogenases.
Single crystal X-ray structures are reported for all complexes.
Two complexes, [Ni(LS)(MesSe)]− and [Ni(LSe)(MesS)]−, display
the key structural features of the Se-containing enzyme such as
a distorted four-coordinate ‘NiS3Se’ environment.
Aerobic oxidation of the complex [Ni(LS)(MesSe)]− results in
the oxidation of the monodentate selenium ligand to form a
dichalcogenide, whereas Ni remained in the +2 oxidation
state. Comparable chemistry was observed in the active site of
D. vulgaris [NiFeSe] hydrogenase after exposure to O2.9f,g Reac-
tivity of [Ni(LS)(MesSe)]− with HBF4 leads to protonation of the
selenolate ligand, indicating that the selenium atom is indeed
a plausible protonation site during H2 cycling in [NiFeSe]
hydrogenases. Although unrelated to the biomimetic
composition of the Ni molecules, the complexes also act as
precursors to Ni-containing particles on an electrode surface,
which show high electroactivity in pH neutral aqueous
solution. Work is currently in progress to introduce iron in our
Ni site precursors and assemble a full structural [NiFeSe]
hydrogenase model complex.
X-ray crystallographic studies
All data were recorded with Mo Kα radiation (λ = 0.71073 Å) on
a Nonius Kappa CCD diffractometer fitted with an Oxford
Cryosystems Cryostream cooling apparatus. The single crystals
were mounted in Paratone N oil on the tip of a glass fibre and
kept under a stream of N2. Structure solution was carried out
using direct methods and refined by least squares (SHELXL-
2
97)21 using Chebyshev weights on Fo . The weighted R-factor,
wR and goodness of fit (GOF) are based on F2. The hydrogen
atoms were assigned to idealised positions and given thermal
parameters of 1.5 (methyl hydrogens) or 1.2 (non-methyl
hydrogens) times the thermal parameter of the carbon atom to
which they were attached. The tridentate ‘LSe’ in the complexes
(n-Bu4N)[Ni(LSe)(MesS)] and (n-Bu4N)[Ni(LSe)(MesSe)] exhibits
the same sort of disorder, in each case modelled as two poorly
resolved S/Se sites. Crystal data, data collection parameters,
and structure refinement details for the complexes are given in
Table 2. Selected bond lengths and angles are shown in
Table 1. The mean bond lengths and angles for the discussion
in the paper were calculated as follows: for a sample of
n observations xi, a weighted mean value (xu) with its standard
deviation (σ) was calculated using the following equations: xu =
Experimental section
Materials and methods
Unless otherwise stated all compounds were prepared using
an anhydrous and anaerobic MBraun glovebox or Schlenk
techniques. All starting materials were purchased from com-
mercial suppliers in the highest available purity for all ana-
lytical measurements and used without further purification.
Σi xi/n, σ = {Σi(xi − xu)2/[n(n − 1)]}1/2
.
Electrochemical measurements
Organic solvents were dried and deoxygenated prior to use.
CVs were recorded at room temperature under Ar using an
IviumStat or CompactStat potentiostat. A standard three elec-
trode cell was used for all measurements with a glassy carbon
disc working (3 mm diameter), a platinum foil counter and a
Ag/Ag+ (organic solutions) or a Ag/AgCl/KCl(sat) (aqueous solu-
Mesityl selenol20 and [Ni(LS)]n
according to literature procedures.
(Scheme 1) were prepared
12i
Physical measurements
1H and 13C NMR spectra were recorded on
a Bruker tions) reference electrode. For CVs recorded in acetonitrile
DPX-400 MHz spectrometer and the spectra referenced against containing n-Bu4NBF4 (0.1 M, electrochemistry grade, Sigma
4488 | Dalton Trans., 2014, 43, 4483–4493
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Table 2 Crystal data and structure refinement for complexes shown in Fig. 3 (for selected bond lengths and angles see Table 1)
Complex
(n-Bu4N)[Ni(LSe)(MesS)]
(n-Bu4N)[Ni(LSe)(MesSe)]
(n-Bu4N)[Ni(LS)(MesSe)]
(n-Bu4N)[Ni(LS)(MesS)]
Empirical formula
Formula weight
C
37H55NNiS3Se
C37H55NNiS2Se2
794.57
C37H55NNiS3Se
747.67
C37H55NNiS4
700.77
747.67
Temperature (K)
Space group
a (Å)
b (Å)
c (Å)
180(2)
P21/c
220(2)
P21/n
180(2)
P21/c
180(2)
P21/c
18.6006(4)
23.1712(5)
18.7401(5)
109.268(1)
7624.5(3)
8
1.303
1.653
0.46 × 0.10 × 0.02
3.51 to 25.34
43 959
10.6327(2)
22.5152(5)
16.1237(5)
94.708(1)
3846.95(16)
4
1.372
2.530
0.23 × 0.21 × 0.02
3.55 to 25.66
35 314
18.7224(2)
23.0398(2)
18.7818(2)
109.810(1)
7622.27(13)
8
1.303
1.654
0.28 × 0.23 × 0.10
3.51 to 22.49
52 166
18.5275(2)
23.1544(2)
18.7038(2)
109.270(1)
7574.26(13)
8
1.229
0.758
0.28 × 0.12 × 0.02
3.52 to 27.50
73 103
β (°)
V (Å3)
Z
ρcalc (g cm−3
)
μ (Mo Kα, mm−1
Crystal size (mm)
Θ range (°)
)
Total number of data
Number of unique data
Number of parameters
Completeness
13 739
393
0.984
0.0711
7112
304
0.974
0.0452
9870
602
0.992
0.0634
17 283
602
0.993
0.0757
a
R1
b
wR2
0.1765
0.1123
0.1672
0.1819
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2
.
Aldrich), the Fc/Fc+ couple was used as an external reference tetrahydrofuran (15 mL) was added at −90 °C and the resulting
and potentials were converted to the normal hydrogen elec- beige solution was stirred for eight hours during which time
trode (NHE) by adding 630 mV in acetonitrile.22 For CVs in a the temperature of the reaction mixture was allowed to slowly
pH 7 aqueous phosphate solution (0.1 M), potentials were con- reach −50 °C. Selenium powder (0.455 g, 5.76 mmol) was
verted by adding 0.2 V to the potential against Ag/AgCl/ added in one batch and the orange solution was allowed to
23
KCl(sat)
.
reach room temperature slowly overnight. Degassed water
(50 mL) was slowly added and the aqueous layer was extracted
with diethyl ether (4 × 50 mL) to remove DTBB and any
remaining unreacted starting material. The aqueous layer was
then acidified with aqueous HCl (2 M, 50 mL) and extracted
with dichloromethane (3 × 25 mL). The combined organic
layers were washed with aqueous HCl (2 M, 25 mL), and the
solvent was removed under vacuum to give a yellow gum of
crude (‘LSe–H’)2, which was used for the next step without
further purification.24 The crude (‘LSe–H’)2 was dissolved in
tetrahydrofuran (15 mL) and added dropwise to a solution of
LiAlH4 (68 mg, 1.79 mmol) in tetrahydrofuran (3 mL) with stir-
ring at room temperature. The resulting colourless solution
was stirred overnight. Aqueous HCl (2 M, 12 mL) was then
added dropwise and the product was extracted with diethyl
ether (3 × 20 mL). The combined extracts were washed with
aqueous HCl (2 M, 12 mL) and the solvent was removed under
high vacuum at room temperature to give the product as a
white solid, which was purified by recrystallisation from a satu-
rated solution in tetrahydrofuran at −35 °C. Yield: 494 mg,
29%. 1H NMR (400 MHz, CDCl3) δ/ppm = 7.47 (1H, dd), 7.33 (1H,
dd), 7.00–7.19 (6H, m), 4.05 (1H, s, SH), 1.91 (1H, s, SeH); EI-MS
(CHCl3) +ive: 297.94 (30%, LSe); elemental analysis calculated
(%) for C12H10S2Se C 48.48, H 3.39; found C 48.67, H 3.39.
Controlled potential electrolysis (CPE)
CPE in phosphate solution (0.1 M, pH 7) was carried out using
a fluoride doped tin oxide (FTO) coated glass electrode or
glassy carbon rod electrode (geometrical surface area in contact
with electrolyte solution approximately 1.6 cm2), a platinum
mesh counter electrode and a Ag/AgCl reference electrode.
CPE was carried out in an airtight electrochemical cell contain-
ing N2 with 2% methane as the internal standard for gas
chromatography, GC, analysis. The headspace gas was ana-
lysed using an Agilent 7890A GC equipped with a 5 Å mole-
cular sieve column, using N2 carrier gas with a flow rate of
approximately 3 mL min−1. The GC columns were kept at
40 °C and a thermal conductivity detector (TCD) was used.
Measurements were taken every 2 h and the cell was purged to
remove all hydrogen following each GC measurement. All
measurements were repeated at least three times. Faradaic
efficiency (%) = 100[H2 (mol) × 2F/Q (C)].
Synthesis of ‘LSe’–H2
The first step of this reaction must be carried out under argon.
Lithium (25% dispersion in oil, Sigma Aldrich) was washed
with hexane (3 × 2 mL). To the de-oiled Li metal (0.080 g,
11.53 mmol) was added 4,4′-di-tert-butylbiphenyl (DTBB,
0.115 g, 432 µmol) in tetrahydrofuran (3 mL) and the reaction
Synthesis of [NiLSe]n
mixture was cooled to −90 °C (acetone–liquid N2 bath), result- A solution of ‘LSe’–H2 (200 mg, 671 µmol) in tetrahydrofuran
ing in the formation of a bright blue solution of a radical (3 mL) was added dropwise to a solution of Ni(OCOCH3)2·
anion of DTBB.14 Thianthrene (1.25 g, 5.76 mmol) in 4H2O (167 mg, 671 µmol) in methanol (0.5 mL). The dark
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brown suspension was heated to reflux for four hours, where- tetrahydrofuran (3 mL), n-Bu4NOH·30H2O (261 mg, 326 µmol)
1
upon an insoluble black precipitate was separated by filtration, in methanol (1 mL). Yield 175 mg, 72%. H NMR (400 MHz,
washed with tetrahydrofuran (3 × 2 mL) and dried under THF-d8) δ/ppm = 7.60 (2H, d, LS), 7.09 (2H, d, LS), 6.83 (2H, t,
vacuum at room temperature. Yield 208 mg, 87%. Elemental LS), 6.72 (2H, t, LS), 6.66 (2H, s, MesSe), 3.41 (8H, m, n-Bu4N)
analysis calculated (%) for [C12H8NiS2Se]n C 40.72, H 2.28; 2.73 (6H, s, MesSe), 2.11 (3H, s, MesSe), 1.67 (8H, m, n-Bu4N),
found C 40.78, H 2.31. ATR-IR ˜ν/cm−1 = 3040, 1568, 1441, 1.36 (8H, m, n-Bu4N), 0.90 (12H, t, n-Bu4N); ESI-MS (CH2Cl2)
1424, 1239, 1154, 1085, 1038, 757, 746. [NiLSe]n is insoluble in −ive: 505 (100%, [Ni(LS)(MesSe)]−), +ive: 242 (100%, n-Bu4N+);
any common organic solvent.
elemental analysis calculated (%) for C37H55NiNS3Se C 59.44,
H 7.41, N 1.87; found C 59.59, H 7.35, N 1.93.
General procedure for synthesis of (n-Bu4N)[Ni(LE)(MesE′)]
(n-Bu4N)[Ni(LS)(MesS)]
To a solution of NaOMe in methanol was added mesityl thiol
or selenol. The solution was stirred for 20 min and then added
to a stirred suspension of [NiLE]n in tetrahydrofuran at room
temperature. The reaction mixture was stirred for one hour
until [NiLE]n fully dissolved giving a brown solution. A solution
of n-Bu4NOH·30H2O in methanol was added and the solution
was stirred for an additional 20 min. All solvents were removed
under high vacuum at room temperature and the brown
residue was taken up in tetrahydrofuran and filtered through a
Millex FG PTFE microfilter (pore size 20 µm). Hexane was
layered on top of the tetrahydrofuran solution and the undis-
turbed mixture gave brown crystalline needles of the product
after several days. The crystals were separated by filtration and
washed with hexane. X-ray diffraction quality single crystals
were selected directly from the reaction vessel.
NaOMe (65 mg, 1.21 mmol) in methanol (3 mL), mesityl thiol
(182 µL, 1.21 mmol), [Ni(LS)]n (400 mg, 1.30 mmol) in tetra-
hydrofuran (9 mL), n-Bu4NOH·30H2O (966 mg, 1.21 mmol) in
methanol (3 mL). Yield 672 mg, 79%. 1H NMR (400 MHz, THF-
d8) δ/ppm = 7.62 (2H, d, LS), 7.12 (2H, d, LS), 6.92 (2H, t, LS),
6.79 (2H, t, LS), 6.71 (2H, s, MesS), 3.49 (8H, m, n-Bu4N), 2.77
(6H, s, MesS), 2.17 (3H, s, MesS), 1.75 (8H, m, n-Bu4N), 1.43
(8H, m, n-Bu4N), 0.98 (12H, m, n-Bu4N); ESI-MS (CH2Cl2) −ive:
457 (100%, [Ni(LS)(MesS)]−), +ive: 242 (100%, n-Bu4N+);
elemental analysis calculated (%) for C37H55NiNS4 C 63.41, H
7.91, N 2.00; found C 63.46, H 7.92, N 2.08.
Synthesis of (PPh4)[Ni(LS)(MesSe)]
Elemental Se (24 mg, 300 μmol) was added to mesityl mag-
nesium bromide (300 μL of a 1 M solution in diethyl ether).
The solution was stirred overnight until the Se powder dis-
solved and the colour changed from orange to yellow. Tetra-
hydrofuran (2 mL) was added followed by [Ni(LS)]n (96 mg,
300 μmol) and this was stirred for one hour until [Ni(LS)]n dis-
solved giving a brown solution. A solution of PPh4Br (126 mg,
300 μmol) in methanol (0.5 mL) was added and the solution
was stirred for 20 min. The solution was concentrated to
dryness under reduced pressure. The brown residue was dis-
solved in tetrahydrofuran (3 mL) and filtered through a Millex
FG PTFE microfilter (pore size 20 µm). Diethyl ether was
added to precipitate the brown product which was separated
by filtration, washed with diethyl ether and dried under
vacuum. Yield 169 mg, 67%. 1H NMR (400 MHz, CDCl3) δ/ppm =
7.66 (4 H, m, PPh4), 7.52 (8H, m, PPh4), 7.49 (8H, m, PPh4),
7.42 (2H, d, LS), 7.04 (2H, d, LS), 6.79 (2H, t, LS), 6.70 (4H, m,
LS and MesSe), 2.52 (6H, s, MesSe), 2.08 (3H, s, MesSe);
13C NMR (500 MHz, CDCl3) δ/ppm = 156.6, 143.8, 134.4 (PPh4),
133.0, 130.8 (PPh4), 129.8, 128.3, 127.0, 126.9, 120.2, 117.8
(PPh4), 117.1 (PPh4), 27.2 (MesSe), 21.1 (MesSe); 77Se NMR
(500 MHz, CD2Cl2) δ/ppm = 605.4; ESI-MS (CD2Cl2): −ive: 505
(n-Bu4N)[Ni(LSe)(MesS)]
NaOMe (28 mg, 525 µmol) in methanol (1 mL), mesityl thiol
(79 µL, 525 µmol), [Ni(LSe)]n (186 mg, 525 µmol) in tetrahydro-
furan (4 mL), n-Bu4NOH·30H2O (420 mg, 525 µmol) in metha-
nol (0.75 mL). Yield 280 mg, 71%. 1H NMR (400 MHz, THF-d8)
δ/ppm = 7.52 (1H, d, LSe) 7.43 (1H, d, LSe), 7.38 (1H, d, LSe),
7.30 (1H, d, LSe), 6.79–6.97 (4H, m, LSe), 6.78 (2H, s, MesS), 3.34
(8H, m, n-Bu4N) 2.79 (6H, s, MesS), 2.46 (3H, s, MesS), 1.59 (8H,
m, n-Bu4N), 1.56 (8H, m, n-Bu4N), 1.37 (12H, t, n-Bu4N); ESI-MS
(CH2Cl2) −ive: 505 (100%, [Ni(LSe)(MesS)]−), +ive: 242 (100%,
n-Bu4N+); elemental analysis calculated (%) for C37H55NiNS3Se
C 59.44, H 7.41, N 1.87; found C 59.17, H 7.36, N 1.90.
(n-Bu4N)[Ni(LSe)(MesSe)]
NaOMe (7.6 mg, 141 µmol) in methanol (2 mL), mesityl
selenol (28.1 mg, 141 µmol), [Ni(LSe)]n (50 mg, 141 µmol) in
tetrahydrofuran (3 mL), n-Bu4NOH·30H2O (113 mg, 141 µmol)
in methanol (1 mL). Yield 77 mg, 68%. 1H NMR (400 MHz,
THF-d8) δ/ppm = 7.59 (1H, d, LSe), 7.52 (1H, d, LSe), 7.26 (1H,
d, LSe), 7.17 (1H, d, LSe), 6.78–6.88 (4H, m, LSe), 6.66 (2H, s,
MesSe), 3.41 (8H, m, n-Bu4N) 2.71 (6H, s, MesSe), 2.11 (3H, s,
MesSe), 1.66 (8H, m, n-Bu4N), 1.35 (8H, m, n-Bu4N), 0.90 (12H,
t, n-Bu4N); ESI-MS (CH2Cl2) −ive: 553 (100%, [Ni(LSe)
(MesSe)]−), +ive: 242 (100%, n-Bu4N+); elemental analysis
calculated (%) for C37H55NiNS2Se2 C 55.93, H 6.98, N 1.76;
found C 55.85, H 6.94, N 1.81.
(100%, [Ni(LS)(MesSe)]−), +ive: 339 (100%, PPh4 ); elemental
+
analysis calculated (%) for C45H39NiPS3Se C 63.99, H 4.65,
P 3.67; found C 63.60, H 4.50, P 3.60.
Synthesis of (PPh4)[Ni(LS)(MesS)]
To a solution of NaOMe (17 mg, 309 μmol) in methanol (1 mL)
was added mesityl thiol (50 μL, 309 μmol). The solution was
stirred for 20 min and then added to a stirred suspension of
(n-Bu4N)[Ni(LS)(MesSe)]
NaOMe (17.6 mg, 326 µmol) in methanol (2 mL), mesityl [NiLS]n (100 mg, 309 μmol) in tetrahydrofuran (3 mL) at room
selenol (64.9 mg, 326 µmol), [Ni(LS)]n (100 mg, 326 µmol) in temperature and the solution was stirred for one hour until
4490 | Dalton Trans., 2014, 43, 4483–4493
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[NiLS]n fully dissolved giving a brown solution. A solution of with diethyl ether and dried under vacuum (11 mg). The
PPh4Br in methanol was added and this was stirred for solvent was removed from the resulting filtrate to give crude
20 min. The solution was concentrated to dryness under dimesityl disulfide as a pale yellow solid, which was purified
1
reduced pressure. The brown residue was dissolved in tetra- by filtration through silica in diethyl ether (5.8 mg, 80%). H
hydrofuran (3 mL) and filtered through a Millex FG PTFE NMR (400 MHz, CDCl3) δ/ppm = 6.82 (2H, s), 2.18 (3H, s), 2.14
microfilter (pore size 20 µm). Diethyl ether was added to pre- (6H, s).
cipitate the brown product which was separated by filtration,
washed with diethyl ether and dried under vacuum. Yield
1
158 mg, 64%. H NMR (400 MHz, CD2Cl2) δ/ppm = 7.92 (4H, Reaction of (n-Bu4N)[Ni(LE)(MesE′)] with oxygen
m, PPh4), 7.78 (8H, m, PPh4), 7.62 (8H, m, PPh4), 7.49 (2H, d,
Reaction of (n-Bu4N)[Ni(LE)(MesE′)] with a n-Bu4N cation gives
LS), 7.14 (2H, d, LS), 6.89 (2H, t, LS), 6.80 (2H, m, LS and
the dichalcogenides (MesE′)2 along with a mixture of insoluble
MesS), 2.59 (6H, s, MesS), 2.17 (3H, s, MesS); 13C NMR
(500 MHz, CD2Cl2) δ/ppm = 155.4, 142.9, 140.3, 135.7, 134.4
products which could not be unequivocally isolated and
characterised.
(PPh4), 134.1, 130.7, 130.6 (PPh4), 129.2, 127.3, 127.3, 120.4,
117.8 (PPh4), 117.1 (PPh4), 24.2 (MesS), 20.8 (MesS); ESI-MS
(CH2Cl2); −ive: 457 (100%, [Ni(LS)(MesS)]−), +ive: 339 (100%,
Reaction of (n-Bu4N)[Ni(LS)(MesSe)] with HBF4
+
PPh4 ); elemental analysis calculated (%) for C45H39NiPS4
To a solution of (n-Bu4N)[Ni(LS)(MesSe)] (20 mg, 26.7 µmol)
in dichloromethane (3 mL) was added HBF4·Et2O (3.7 µL,
C 67.75, H 4.93, P 3.88; found C 67.40, H 4.95, P 3.91.
Reaction of (PPh4)[Ni(LS)(MesSe)] with oxygen
26.7 µmol) in dichloromethane (2 mL) and the reaction
mixture was stirred for 30 min, whereupon [Ni(LS)]n (5.5 mg,
64%) formed as a black precipitate. The solid was separated by
filtration and washed with dichloromethane, methanol and
diethyl ether, dried under vacuum and characterised by
ATR-IR and elemental analysis. Diethyl ether was added to the
remaining filtrate to precipitate crude n-Bu4NBF4, which was
separated by filtration, washed with diethyl ether and dried
(PPh4)[Ni(LS)(MesSe)] (116 mg, 137 μmol) in dichloromethane
(5 mL) was exposed to atmospheric O2 for one day with stirring
to give a brown solution containing dimesityl diselenide with a
black precipitate characterised as [Ni(LS)]n. Isolation and
characterisation of precipitate containing [Ni(LS)]n: The solid was
separated by filtration and washed with dichloromethane,
methanol and diethyl ether and dried under vacuum. ATR-IR
spectroscopy, elemental analysis and single crystal X-ray ana-
1
under vacuum. H NMR (400 MHz, CDCl3) δ/ppm = 3.20 (8H,
12l
m), 1.62 (8H, m), 1.44 (8H, m), 1.00 (12 H, t), ESI MS (CHCl3);
−ive: 87 (100%, BF4−) +ive: 242 (100%, n-Bu4N+). Diethyl ether
was removed from the remaining filtrate under vacuum to
yield mesityl selenol (4.3 mg, 79%). 1H NMR (400 MHz, CDCl3)
δ/ppm = 6.88 (2H, s), 2.34 (6H, s), 2.23 (3H, s), 1.24 (1H, s).
lysis identified the solid product as [Ni(LS)]n (22 mg, 53%).
ATR-IR ˜ν/cm−1 = 3036, 1442, 1425, 1251, 1092, 759, 727.
Isolation and characterisation of filtrate containing (MesSe)2:
Diethyl ether (15 mL) was added to the filtrate solution to
precipitate a tetraphenyl phosphonium salt as a pink solid
which was separated by filtration and washed with diethyl
1
ether (42 mg). H NMR (400 MHz, CD2Cl2) δ/ppm = 7.66 (4 H,
Reaction of (n-Bu4N)[Ni(LS)(MesS)] with HBF4
m, PPh4), 7.52 (8H, m, PPh4), 7.49 (8H, m, PPh4), ESI-MS
(CD2Cl2): +ive: 339 (100%); λmax/nm (acetonitrile) 226 nm. The To a solution of (n-Bu4N)[Ni(LS)(MesS)] (50 mg, 71.3 µmol)
solvent was removed from the resulting filtrate to give crude in dichloromethane (4 mL) was added HBF4·Et2O (9.7 µL,
dimesityl diselenide as a yellow solid, which was purified by 71.3 µmol) in dichloromethane (2 mL) and this was stirred for
filtration through silica in diethyl ether (10 mg, 36%). 1H NMR 30 min. [Ni(LS)]n (20.9 mg, 83%) and mesityl thiol (8 mg, 74%)
(400 MHz, CDCl3) δ/ppm = 6.77 (2H, s), 2.18 (3H, s), 2.14, (6H, formed in analogy to the reaction of (n-Bu4N)[Ni(LS)(MesSe)]
s), EI-MS 398.0; λmax/nm (acetonitrile) 289.
with HBF4 and were characterised by the same methods.
1H NMR of mesityl thiol (400 MHz, CDCl3) δ/ppm = 6.81 (2H,
s), 3.04 (1H, s), 2.27 (6H, s), 2.17 (3H, s).
Reaction of (PPh4)[Ni(LS)(MesS)] with oxygen
(PPh4)[Ni(LS)(MesSe)] (30.7 mg, 38.4 μmol) in dichloromethane
(5 mL) was exposed to atmospheric O2 for five days under stir-
ring to give a brown solution containing dimesityl disulfide
with a black precipitate, characterised as [Ni(LS)]n. Isolation
and characterisation of precipitate containing [Ni(LS)]n: The solid
Acknowledgements
was separated by filtration and washed with dichloromethane, This work was supported by the Engineering and Physical
methanol and diethyl ether and dried under vacuum. ATR-IR Sciences Research Council (EP/H00338X/2), the Christian
spectroscopy and elemental analysis identified the solid Doppler Research Association and the OMV group. We thank
product as [Ni(LS)]n (9.6 mg, 86%). Isolation and characteris- Mr Dirk Mersch for recording SEM images and Dr John Davies
12l
ation of filtrate containing (MesS)2: Diethyl ether (15 mL) was for collecting and refining the crystallographic data and
added to the filtrate to precipitate a tetraphenyl phosphonium providing a letter about refinement of the X-ray structures
salt as a pink solid which was separated by filtration, washed (see ESI†).
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F. M. Rusnak, M. Kolk, E. C. Duin, S. P. J. Albracht and
Notes and references
E.
Munck,
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296.9 (‘LSe–H’)−.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4483–4493 | 4493