Spectroelectrochemical study of complexes [Mo(CO)2(η 3-allyl)(α-diimine)(NCS)] (α-diimine = bis(2,6-dimethylphenyl)-acenaphthenequinonediimine and 2,2′-bipyridine) exhibiting different molecular structure and redox reactivity Dedicated to Professor Maria Jose? Calhorda on the occasion of her 65th birthday.

Article abstract of DOI:10.1016/j.jorganchem.2014.01.015

The redox properties and reactivity of [Mo(CO)₂(η ³-allyl)(α-diimine)(NCS)] (α-diimine = bis(2,6-dimethylphenyl)-acenaphthenequinonediimine (2,6-xylyl-BIAN) and 2,2′-bipyridine (bpy)) were studied using cyclic voltammetry and IR/UV-Vis spectroelectrochemistry. [Mo(CO)₂(η³-allyl)(2,6- xylyl-BIAN)(NCS)] was shown by X-ray crystallography to have an asymmetric (B-type) conformation. The extended aromatic system of the strong π-acceptor 2,6-xylyl-BIAN ligand stabilises the primary 1e^--reduced radical anion, [Mo(CO)₂(η³-allyl)(2,6-xylyl-BIAN ^?-)(NCS)]^-, that can be reduced further to give the solvento anion [Mo(CO)₂(η³-allyl)(2,6-xylyl-BIAN)(THF) ]^-. The initial reduction of [Mo(CO)₂(η³- allyl)(bpy)(NCS)] in THF at ambient temperature results in the formation of [Mo(CO)₂(η³-allyl)(bpy)]₂ by reaction of the remaining parent complex with [Mo(CO)₂(η³-allyl) (bpy)]^- produced by dissociation of NCS^- from [Mo(CO) ₂(η³-allyl)(bpy^?-)(NCS)]^-. Further reduction of the dimer [Mo(CO)₂(η³-allyl)(bpy) ]₂ restores [Mo(CO)₂(η³-allyl)(bpy)] ^-. In PrCN at 183 K, [Mo(CO)₂(η³-allyl)(2, 6-xylyl-BIAN^?-)(NCS)]^- converts slowly to 2e ^--reduced [Mo(CO)₂(η³-allyl)(2,6-xylyl- BIAN)(PrCN)]^- and free NCS^-. At room temperature, the reduction path in PrCN involves mainly the dimer [Mo(CO)₂(η ³-allyl)(bpy)]₂; however, the detailed course of the reduction within the spectroelectrochemical cell is complicated and involves a mixture of several unassigned products. Finally, it has been shown that the five-coordinate anion [Mo(CO)₂(η³-allyl)(bpy)] ^- promotes in THF reduction of CO₂ to CO and formate via the formation of the intermediate [Mo(CO)₂(η³-allyl) (bpy)(O₂CH)] and its subsequent reduction.

Full text of DOI:10.1016/j.jorganchem.2014.01.015

Journal of Organometallic Chemistry xxx (2014) 1e12
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Spectroelectrochemical study of complexes [Mo(CO)2(h a-
3-allyl)(
diimine)(NCS)] (
a
-diimine ¼ bis(2,6-dimethylphenyl)-
acenaphthenequinonediimine and 2,20-bipyridine) exhibiting
different molecular structure and redox reactivity
*
ꢀ
Joanne Tory, Gaël Gobaille-Shaw, Ann M. Chippindale, Frantisek Hartl
Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The redox properties and reactivity of [Mo(CO)2(
h
3-allyl)(
a
-diimine)(NCS)]
(
a
-diimine
¼
bis(2,6-
dimethylphenyl)-acenaphthenequinonediimine (2,6-xylyl-BIAN) and 2,20-bipyridine (bpy)) were studied
Received 16 November 2013
Received in revised form
12 January 2014
using cyclic voltammetry and IR/UVeVis spectroelectrochemistry. [Mo(CO)2(
was shown by X-ray crystallography to have an asymmetric (B-type) conformation. The extended aromatic
system of the strong
-acceptor 2,6-xylyl-BIAN ligand stabilises the primary 1eꢀ-reduced radical
anion, [Mo(CO)2(
3-allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ, that can be reduced further to give the solvento anion
[Mo(CO)2(
3-allyl)(2,6-xylyl-BIAN)(THF)]ꢀ. The initial reduction of [Mo(CO)2( 3-allyl)(bpy)(NCS)] in THF at
ambient temperature results in the formation of [Mo(CO)2(
3-allyl)(bpy)]2 by reaction of the remaining
parent complex with [Mo(CO)2(
3-allyl)(bpy)]ꢀ produced by dissociation of NCSꢀ from [Mo(CO)2(h3
allyl)(bpyꢁꢀ)(NCS)]ꢀ. Further reduction of the dimer [Mo(CO)2( 3-allyl)(bpy)]2 restores [Mo(CO)2(h3
allyl)(bpy)]ꢀ. In PrCN at 183 K, [Mo(CO)2(
[Mo(CO)2(
involves mainly the dimer [Mo(CO)2(
the spectroelectrochemical cell is complicated and involves a mixture of several unassigned products. Finally,
it has been shown that the five-coordinateanion [Mo(CO)2(
3-allyl)(bpy)]ꢀ promotes inTHFreduction of CO2
3-allyl)(bpy)(O2CH)] and its subsequent
h
3-allyl)(2,6-xylyl-BIAN)(NCS)]
Accepted 16 January 2014
p
h
Dedicated to Professor Maria José Calhorda
on the occasion of her 65th birthday.
h
h
h
h
-
-
Keywords:
Molybdenum
Allyl ligand
h
h
3-allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ converts slowly to 2eꢀ-reduced
h
3-allyl)(2,6-xylyl-BIAN)(PrCN)]ꢀ and free NCSꢀ. At room temperature, the reduction path in PrCN
3-allyl)(bpy)]2; however, the detailed course of the reduction within
a
-Diimine ligand
X-ray analysis
Spectroelectrochemistry
Carbon dioxide
h
h
to CO and formate via the formation of the intermediate [Mo(CO)2(
h
reduction.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
abundant first row transition-metal centre, has recently been
shown to act as an electrochemical catalyst for the conversion of
CO2 to CO in the presence of small concentrations of acid, with an
overall 2eꢀ reduction to [Mn(CO)3(bpy)]ꢀ [9,10]. Much less atten-
tion in this regard has been devoted so far to Group 6 metal
There is significant and sustained interest in the catalytic
reduction of CO2 to materials that may be used in fuel cells, as
chemical fuel sources, or as a feedstock for organic synthesis. A
number of different transition-metal complexes have been identi-
fied as potential redox catalysts for CO2 reduction, including
macrocyclic complexes, phosphine complexes, and complexes with
carbonyl a-diimine complexes. Only recently we have encountered
similar catalytic activity towards CO2 reduction exhibited by the
structurally related 2eꢀ reduced complex [Mo(CO)3(bpy)]2ꢀ [11].
Another family of Group 6 metal carbonyls, of the type [MoII(-
a
-diimine ligands [1e5]. The Group 7 complexes, [Re(CO)3(bpy)Cl]
and related species, are known to act as catalyst precursors for
efficient electrochemical and photochemical CO2 reduction, with
the active catalysts being five-coordinate, 1eꢀ reduced radical
or 2eꢀ reduced anionic species [6e8]. The manganese
analogue, [Mn(CO)3(bpy)Br], containing the cheaper and more
CO)2(
h
3-allyl)(LXL)X] (LXL ¼ chelating bidentate ligand, X ¼
anionic monodentate ligand), are known to have variable stereo-
chemistry. These complexes usually adopt either a symmetric
structure with both LXL donor atoms trans to equatorial carbonyl
ligands (type A), or an asymmetric structure where the
h
3-allyl
ligand and a carbonyl ligand are trans to LXL (type B) as seen in
Chart 1 [12e14]. Trans-dicarbonyl isomers, although rare, are also
observed [15]. For the cis-dicarbonyl complexes the less common
B-type structure occurs particularly when LXL is a diphosphine or a
* Corresponding author.
E-mail address: f.hartl@reading.ac.uk (F. Hartl).
0022-328X/$ e see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2014.01.015
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2
J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
2. Experimental
X
L
2.1. Materials
CO
CO
L
L
CO
CO
Mo
Mo
All solvents were freshly distilled under a nitrogen atmosphere.
Tetrahydrofuran (THF) and hexane were distilled from benzophe-
none/sodium, acetonitrile (MeCN) over P2O5, and butyronitrile
(PrCN) and dichloromethane (DCM) over CaH2. The supporting
electrolyte, Bu4NPF6 (TBAH, Aldrich), was recrystallised twice from
L
X
A
B
absolute ethanol and dried under vacuum. [Mo(CO)2(h
3-allyl)(-
Chart 1. The “A” and “B” type structures observed for [Mo(CO)2(h
3-allyl)(LXL)X]
MeCN)2(NCS)] [22] and 2,6-xylyl-BIAN [21] were prepared ac-
cording to literature procedures. The previously reported
complexes.
complex, [Mo(h
3-allyl)(CO)2(bpy)(NCS)] [23,24], was prepared for
non-rigid bidentate ligand; although, many exceptions are known
in the literature [13e15], and the nature of the X ligand also affects
the conformation. For both the A and B structural types, the axial
the purpose of this comparative study by the facile thermal sub-
stitution reaction of [Mo(h3-allyl)(CO)2(MeCN)2(NCS)] with 2,20-
bipyridine and identified by its IR spectrum in CH2Cl2:
n(CO) at
h
3-allyl ligand adopts the orientation where the open face is over
1950, 1867 cmꢀ1 (CN): 2079 cmꢀ1
,
n
.
Its purity was further
confirmed by 1H NMR spectroscopy. Elemental analysis was carried
out by MEDAC Ltd. 1H NMR spectra were recorded on a Bruker
NanoBay spectrometer. All electrochemical and spectroelec-
trochemical measurements were carried out under an inert atmo-
sphere of dry N2 or argon, using Schlenk techniques. Solutions were
saturated with CO2 at normal pressure by bubbling it through a frit.
the two carbonyl groups. Quantum mechanical (DFT, EHMO) cal-
culations have shown that this is the most stable arrangement for
both isomers [13,16].
There are few electrochemical studies of [Mo(CO)2(h3
-
allyl)(LXL)X] complexes reported in the literature, and these only
consider their anodic behaviour. A single one-electron reversible
oxidation is observed at 0.5e0.7
V
vs. SCE for
a range
3-allyl)(LXL)X] (LXL
¼
bpy, Ph2PCH2PPh2,
2.1.1. [Mo(CO)2(
A solution of 2,6-xylyl-BIAN (0.59 g, 1.5 mmol) in DCM (15 mL)
was added to a solution of [Mo(
3-allyl)(CO)2(NCS)(MeCN)2] (0.5 g,
h
3-allyl)(2,6-xylyl-BIAN)(NCS)]
of [Mo(CO)2(
h
Ph2PCH2CH2PPh2, Ph2AsCH2CH2AsPh2; X ¼ Cl, O2CCF3) complexes
in dichloromethane (DCM), resulting in the formation of the
h
[Mo(CO)2(h
3-allyl)(LXL)X]þ cation with retention of the stereo-
1.5 mmol) in DCM (15 mL) under an atmosphere of dry argon. The
mixture was heated under reflux for 3 h, and then reduced to half
its volume and the solid complex precipitated with hexane (10 mL).
The dark-green precipitate was filtered and washed with cold
hexane under inert conditions. Yield: 80e90%. The complex was
crystallised from DCM/hexane.
chemistry. Substitution of the chelating bidentate LXL ligand for a
pair of monodentate ligands results in an electrochemically and
chemically irreversible oxidation process [17]. Oxidation of com-
plexes with strongly
chelating ligands is less reversible at slower scan rates than for
complexes containing -diimine ligands such as bpy, 1,10-
phenanthroline (phen) and N,N0-di-tertbutyl-1,4-diazabuta-1,3-
diene (tBu-DAB). IR spectroelectrochemistry of [Mo(CO)2(h3
p-accepting diphosphine and diarsine
IR in CH2Cl2
n , n
(CO): 1946, 1871 cmꢀ1 (CN): 2075 cmꢀ1. UVevis
a
in CH2Cl2
(
lmax): 219, 241, 324, 338, 370 and 725 nm. 1H NMR
-
(400 MHz, CD2Cl2) dppm: 7.96 (2H, d, 2,6-xylyl-BIAN), 7.35 (8H, m,
2,6-xylyl-BIAN), 6.41 (2H, d, 2,6-xylyl-BIAN), 3.48 (1H, m, Hmeso),
3.12 (2H, br, Hsyn), 2.36 (6H, s, 2,6-xylyl-BIAN), 2.22 (6H, s, 2,6-xylyl-
BIAN), 1.27 (2H, d, Hanti). Anal. Calc. for C34H29MoN3O2S(CH2Cl2)0.5
(682.09): C, 57.19; H, 4.17; N, 5.80%. Found: C, 57.21; H, 4.40; N,
5.62%.
allyl)(LXL)X] usually shows the two n(CO) bands shifted to higher
frequencies by more than 100 cmꢀ1 upon oxidation, in line with the
dominant metal localization of the HOMO [18].
In this paper, the electrochemical behaviour of [Mo(CO)2(h3
-
allyl)(bpy)(NCS)], together with its ability to catalyse the reduction
of CO2, are investigated. To date, no studies of the reduction of
complexes of the type [Mo(CO)2(h a-diimine)X] have been
3-allyl)(
2.2. X-ray structure determination
reported, and predictions of the electrochemical mechanisms can
only be based on comparisons with similar complexes such
A
crystal of [MoII(CO)2( 3-allyl)(2,6-xylyl-BIAN)(NCS)] was
h
as [M(CO)3(
complex, [Mo(CO)2(
BIAN bis(2,6-dimethylphenyl)-acenaphthenequinonediimine,
Chart 2), is used for comparative electrochemical studies as the
extended -delocalised aromatic system of the 2,6-xylyl-BIAN
a
-diimine)X] (M ¼ Mn, Re) [7,9,19,20]. The related
mounted under Paratone-N oil and flash cooled to 150 K in a stream
of nitrogen in an Oxford Cryostream cooler. Single-crystal X-ray
h
3-allyl)(2,6-xylyl-BIAN)(NCS)] (2,6-xylyl-
¼
intensity data (Table 1) were collected using an Agilent Gemini S
ꢁ
Ultra diffractometer (Cu K
a
radiation (
l
¼ 1.54180 A)).
p
The data were reduced within the CrysAlisPro software [25]. The
structure was solved using the program Superflip [26] and all non-
hydrogen atoms located. Least-squares refinements on F were
carried out using the CRYSTALS suite of programs [27]. The non-
hydrogen atoms were refined anisotropically. Each hydrogen
atom of the 2,6-xylyl-BIAN and allyl ligands was placed geometri-
ligand [21] can stabilise the reduction products.
ꢁ
cally with a CeH distance of 0.95 A and a Uiso of 1.2 times the value
of Ueq of the parent C atom. The positions of the hydrogen atoms
were then refined with riding constraints. There are two molecules
of [Mo(CO)2(h
3-C3H5)(NCS)(C27N2H24)] in the asymmetric unit,
which are mirror images of each other, which give rise to four
molecules in the unit cell. The unit cell also contains two large
solvent-accessible voids, each with a volume of 129 A , located at
(0.480, 0.519, 0.785) and (0.520, 0.481, 0.215). Each of these voids
corresponds to a disordered molecule of dichloromethane, which
could not be modelled as discrete atomic sites. PLATON SQUEEZE
N
N
N
N
3
ꢁ
Chart 2. The ligands 2,20-bipyridine (bpy, left) and bis(2,6-dimethylphenyl)-ace-
naphthenequinonediimine (2,6-xylyl-BIAN, right).
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J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
3
Table 1
detector or connected to a separate Bio-Rad FTS 60 MCT detector
unit (for measurements at 183 K). UVeVis spectroelectrochemistry
Crystallographic data for [MoII(CO)2(
h
3-allyl)(2,6-xylyl-BIAN)(NCS)].
was
performed
using
a
Scinco
S-3100
diode-array
Formula
(C34H29MoN3O2S)(CH2Cl2)0.5
spectrophotometer.
Mr
682.088
Thin-layer UVeVis and IR spectroelectrochemical measure-
ments were carried out at 293 K (THF, PrCN) and 183 K (PrCN)
using OTTLE cells [29e31] equipped with Pt minigrid working
and auxiliary electrodes, an Ag microwire pseudoreference elec-
trode and CaF2 windows. The course of spectroelectrochemical
experiments was monitored by thin-layer cyclic voltammetry
conducted with a PA4 potentiostat (Laboratory Devices, Polná,
Czech Republic). The studied samples contained a 10ꢀ3 M (UVe
Vis spectroelectrochemistry) or 3 ꢂ 10ꢀ3 M (IR spectroelec-
trochemistry) Mo complex and 3 ꢂ 10ꢀ1 M Bu4NPF6 supporting
electrolyte.
Crystal system
Space group
Z
Triclinic
P-1
4
ꢁ
a/A
11.0821(4)
16.9712(6)
17.9778(4)
101.499(3)
103.627(3)
99.944(3)
3133.24(19)
1.446
ꢁ
b/A
ꢁ
c/A
ꢃ
ꢃ
a
/
b
/
ꢃ
/
g
3
ꢁ
V/A
Dcalc/g cmꢀ3
Crystal habit
Crystal dimensions/mm
Radiation
Green plate
0.1 ꢂ 0.05 ꢂ 0.04
ꢁ
Cu Ka (1.54080 A)
T/K
m
150
5.114
/mmꢀ1
3. Results and discussion
R(F), Rw(F)
3.55, 3.49
3.1. Crystal structure of [Mo(CO)2(
h
3-allyl)(2,6-xylyl-BIAN)(NCS)]
software enabled the contribution to diffraction of these disordered
solvent molecules to be calculated, and thus it was possible to
produce solvent-free diffraction intensities [28].
The crystal structure for the green complex [MoII(CO)2(h3
-
allyl)(2,6-xylyl-BIAN)(NCS)] shows that it has the asymmetric B-
type structure with the allyl ligand trans to one of the imino-N
coordination sites (Fig. 1). This is in contrast to the structure of
2.3. Cyclic voltammetry
the red [MoII(CO)2( 3-allyl)(bpy)(NCS)] complex [24], which is A-
h
Cyclic voltammograms were recorded using an EG&G PAR
Model 283 potentiostat operated with M770 v.4.23 software.
Airtight, single-compartment, three-electrode cells were used with
a 0.422 mm2 platinum microdisc working electrode polished with
type. The bond lengths in the 2,6-xylyl-BIAN complex are similar to
those reported for [MoII(CO)2( 3-allyl)(bpy)(NCS)], although the
h
a-
diimine and NCS MoeN and carbonyl MeC bonds are slightly
longer and the CO and CS bonds slightly shorter for the [MoII(-
CO)2(
h p-
3-allyl)(2,6-xylyl-BIAN)(NCS)] complex with the stronger
0.25 mm diamond paste, a platinum wire auxiliary electrode and an
accepting 2,6-xylyl-BIAN ligand compared to 2,20-bipyridine.
Ag wire pseudoreference electrode. The ferrocene/ferrocenium (Fc/
Fcþ) redox couple served as an internal reference for determination
of electrode potentials. The studied samples contained a 10ꢀ3 M Mo
complex and 10ꢀ1 M Bu4NPF6 supporting electrolyte. For some
measurements, the voltammetric cell was cooled to 200 K by using
an acetone/dry ice slurry.
It is remarkable that regardless of the much stronger p-acceptor
nature of the 2,6-xylyl-BIAN ligand compared to that of 2,20-
bipyridine (bpy), which is confirmed for the studied complexes
[Mo(CO)2(
redox potentials (Table 2), the
h
3-allyl)(LXL)(NCS)] by the large difference in their
n
(CO) wavenumbers of these com-
plexes are very similar (see Experimental and Table 3). A plausible
explanation for this surprising observation is the position of the
strong donor NCSꢀ ligand trans to one of the carbonyls in the
structure B, whereas both carbonyls bind trans to the nitrogen
atoms of the acceptor bpy ligand in the structure A (Chart 1).
2.4. Spectroelectrochemistry
IR spectroelectrochemical experiments were performed using a
Bruker Vertex 70v FT-IR spectrometer either equipped with a DTGS
Fig. 1. The two molecules of [MoII(CO)2(
h
3-allyl)(2,6-xylyl-BIAN)(NCS)] found in the asymmetric unit. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms are omitted
ꢁ
for clarity. Selected bond lengths (A): Mo(1)eN(1) 2.2863(17); Mo(10)eN(10) 2.2948(16); Mo(1)eN(2) 2.2298(16); Mo(10)eN(20) 2.2314(16); Mo(1)eN(3) 2.1828(18); Mo(10)e
N(30) 2.1819(19); Mo(1)eC(1) 1.975(2); Mo(10)eC(101) 1.954(2); Mo(1)eC(2) 1.959(2); Mo(10)eC(201) 1.968(3); Mo(1)eC(4) 2.325(2); Mo(10)eC(401) 2.336(2); Mo(1)eC(5)
2.191(2); Mo(10)eC(501) 2.208(2); Mo(1)eC(6) 2.309(2); Mo(10)eC(601) 2.325(2); N(3)eC(3) 1.169(3); N(30)eC(301) 1.162(3); S(1)eC(3) 1.622(2); S(10)eC(301) 1.627(2); O(1)e
C(1) 1.156(3); O(10)eC(101) 1.159(3); O(2)eC(2) 1.152(3); O(20)eC(201) 1.157(3). Selected bond angles (ꢃ): N(1)eMo(1)eN(2) 73.41(6); N(10)eMo(10)eN(20) 73.33(6); C(1)e
Mo(1)eC(2) 79.48(10); C(101)eMo(10)eC(201) 77.14(10).
Please cite this article in press as: J. Tory, et al., Journal of Organometallic Chemistry (2014), http://dx.doi.org/10.1016/j.jorganchem.2014.01.015

4
J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
Table 2
Electrochemical potentials for [Mo(CO)2(
BIAN) and their reduction products.
h
3-allyl)(LXL)(NCS)] (LXL ¼ bpy, 2,6-xylyl-
Complex
Solvent
Ep,a/V
Ep,c/V
ꢀ1.99
[Mo(CO2)(
h
3-allyl)(bpy)(NCS)]
THF
PrCN
MoII/III 0.20a R1
MoII/III 0.22a R1
ꢀ1.95
R1/O1 ꢀ1.89a,c
[Mo(CO2)(
(NCS)]
[Mo(CO2)(
h
h
h
3-allyl)(2,6-xylyl-BIAN)
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ
3-allyl)(2,6-xylyl-BIANꢁꢀ
3-allyl)(bpy)]2
THF
PrCN
THF
MoII/III 0.59 R1/O1 ꢀ1.16a
O1
ꢀ1.88b R2
ꢀ2.54b
ꢀ2.58c
ꢀ2.09
R2
R2
[Mo(CO2)(
)
(NCS)]ꢀ
[Mo(CO2)(
h
THF
PrCN
THF
PrCN
THF
R(D) ꢀ2.52
ꢀ2.82
O(D) ꢀ0.63c
[Mo(CO2)(
h
h
3-allyl)(bpy)]ꢀ
O10
O10
ꢀ1.74 R20
ꢀ1.74 R20
ꢀ2.77
ꢀ3.11
3-allyl)(2,6-xylyl-BIAN)
R20
Fig. 2. Cyclic voltammogram of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] in the THF/Bu4NPF6
[Mo(CO2)(
(THF)]ꢀ
electrolyte at room temperature recorded at a scan rate of 100 mV sꢀ1. The Fc/Fcþ
standard redox couple is marked with an asterisk.
a
E1/2 value.
b
c
Recorded at 2 V sꢀ1
Recorded at 200 K.
.
[Mo(CO)2(h
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ radical anion. From the combi-
nation of cyclic voltammetry and spectroelectrochemical results
(discussed below), and comparison with the well known
[Mn(CO)3(bpy)Cl] system [19,20], the anodic peak O10 has been
The formation of the B-type structure for [MoII(CO)2(h3
allyl)(2,6-xylyl-BIAN)(NCS)] is likely to arise from a combination of
steric and electronic factors, such as the bulkiness of the substituted
2,6-xylyl-BIAN ligand giving rise to possible interaction of the 2,6-
-
assigned to oxidation of the five-coordinate anion, [Mo(CO)2(h3
-
allyl)(bpy)]ꢀ.
The reduction of [Mo(CO)2(
[Mo(CO)2(
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ (Equation (1)) results in
concomitant dissociation of NCSꢀ to give the five-coordinate
h
3-allyl)(bpy)(NCS)] to unstable
xylyl substituent with the allyl ligand [32], and the strongly
p-
h
accepting nature of the 2,6-xylyl-BIAN ligand. Indeed, it has been
previously reported [12,13] that complexes with strongly
accepting diphosphine and diarsine ligands, rather than donor
p
a
-
-
radical [Mo(CO)2(
h
3-allyl)(bpyꢁꢀ)] (Equation (2)). [Mo(CO)2(h3
-
-
allyl)(bpyꢁꢀ)] undergoes 1eꢀ reduction to [Mo(CO)2(h3
diimine ligands, are more likely to form the asymmetric B-type
structure. The latter has also been reported for the related tri-
allyl)(bpy)]ꢀ (Equation (3)) directly at the potential of the parent
complex, as indicated by the O10 anodic counterwave. This behav-
iour is very similar to that observed for the related [Mn(CO)3(bpy)
Cl] complex, where the unstable [Mn(CO)3(bpyꢁꢀ)Cl]ꢀ radical anion
formed upon the 1eꢀ reduction instantaneously loses the Clꢀ ligand
and the resulting five-coordinate radical undergoes directly a sec-
ond 1eꢀ reduction (ECE) to give [Mn(CO)3(bpy)]ꢀ [19,20].
carbonyl cationic complexes [Mo(CO)3(h
3-C3H4Me)(Ar-BIAN)]þ
(Ar ¼ 2-Me-C6H4, 2,6-iPr2-C6H3) [33].
3.2. Cyclic voltammetry of [Mo(CO)2(
3.2.1. In THF at room temperature
h
3-allyl)(bpy)(NCS)]
[Mo(CO)2(
reduced parent complex to form
h
3-allyl)(bpy)]ꢀ is able to react with the yet non-
dimer, [Mo(CO)2(h3
-
The electrochemical behaviour of [Mo(CO)2(
h
3-allyl)(bpy)(NCS)]
a
was initially studied using cyclic voltammetry (Fig. 2). In
THF, this complex undergoes reversible Mo(II) / Mo(III) oxidation
at E1/2 ¼ 0.20 V vs. Fc/Fcþ, as reported in the literature [17,18].
allyl)(bpy)]2 (Equation (4)), as is also observed in the case of
[Mn(CO)3(bpy)]ꢀ [19,20]. The cathodic wave R(D) at Ep,c ¼ ꢀ2.52 V
corresponds to the reduction of [Mo(CO)2(h
3-allyl)(bpy)]2 to
The irreversible cathodic wave, R1, of [Mo(CO)2(
h
3-allyl)-
[Mo(CO)2(
a combination of [Mo(CO)2(
allyl)(bpyꢁꢀ)] (Equation (6)). The latter transient radical undergoes
h
3-allyl)(bpy)]ꢁ2ꢀ (Equation (5)), which dissociates to give
3-allyl)(bpy)]ꢀ and [Mo(CO)2(h3
-
(bpy)(NCS)] is observed at Ep,c ¼ ꢀ1.99 V (THF at ambient tem-
perature) with an anodic peak, O10, seen on the reverse scan at
Ep,a ¼ ꢀ1.75 V. At higher scan rates in THF (Fig. 3), or in PrCN at
lower temperatures recorded (see below), this is accompanied by a
second anodic peak, O1, due to the increased stability of the
h
immediate reduction to give a second molecule of [Mo(CO)2(h3
-
allyl)(bpy)]ꢀ. Reduction of [Mo(CO)2( 3-allyl)(bpy)]ꢀ occurs on the
h
R20 cathodic peak at Ep,c ¼ ꢀ2.82 V.
Table 3
IR and UVeVis absorption data for [Mo(CO)2(
h
3-allyl)(LXL)(NCS)] (LXL ¼ bpy, 2,6-xylyl-BIAN) and their reduction products.
Complex
Solventa
n
(CO)/cmꢀ1
n
(NC)/cmꢀ1
lmax/nm
[Mo(CO2)(
[Mo(CO2)(
[Mo(CO2)(
[Mo(CO2)(
h
h
h
h
3-allyl)(2,6-xylyl-BIAN)(NCS)]
THF
THF
THF
THF
MeCN
PrCN
PrCNb
PrCNb
PrCNb
THF
1946, 1872
1918, 1830
1885, 1798
1951, 1871
1950, 1866
1950, 1868
1946, 1864
1920, 1829
1893, 1796
1891, 1778, 1757
1892, 1775
1844, 1723
2075
2090
331, 373, 715
408, 508
3-allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ
3-allyl)(2,6-xylyl-BIAN)(THF)]ꢀ
3-allyl)(bpy)(NCS)]
2075
2083
2079
2085
2092
2147
340, 501
351, 478
[Mo(CO2)(
[Mo(CO2)(
[Mo(CO2)(
h
h
h
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ
3-allyl)(bpy)(PrCN)]ꢀ
3-allyl)(bpy)]2
369, 581
371, 538
388, 551, 585, 632, 735, 829
PrCN
THF
[Mo(CO2)(
h
3-allyl)(bpy)]ꢀ
a
The solvent contained the supporting electrolyte, 3 ꢂ 10ꢀ1 M Bu4NPF6.
b
Recorded in PrCN at 183 K. All other spectra recorded at room temperature.
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J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
5
h
iꢀ
h
i
ꢀ
ꢁ
ꢀ
ꢁꢀ
ꢁ
MoðCOÞ2 h3 ꢀallyl ðbpyÞ / MoðCOÞ2 h3 ꢀallyl bpyꢁꢀ
þ MoðCOÞ2 h3 ꢀallyl ðbpyÞ
2
h
iꢀ
ꢀ
ꢁ
(6)
3.2.2. In PrCN at room and low temperature
The cyclic voltammogram of [Mo(CO)2(
h
3-allyl)(bpy)(NCS)] was
recorded in PrCN to observe the difference in electrochemical
behaviour in a more strongly coordinating solvent. In PrCN at room
temperature (Fig. S1), the reversible Mo(II) / Mo(III) oxidation is
observed at E1/2 ¼ 0.22 V.
The irreversible bpy-based reduction closely resembles that
observed in THF, with the reduction (R1) of [Mo(CO)2(h
3-allyl)-
(bpy)(NCS)] to [Mo(CO)2(
h
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ at Ep,c ¼ ꢀ1.95 V,
and the resulting oxidation (O10) of [Mo(CO)2( 3-allyl)(bpy)]ꢀ at
h
Fig. 3. Cyclic voltammogram of [Mo(CO)2(
electrolyte at room temperature recorded at a scan rate of 2 V sꢀ1
h
3-allyl)(bpy)(NCS)] in the THF/Bu4NPF6
Ep,a ¼ ꢀ1.74 V. At 200 K (Fig. 4b) or increased scan rates (Fig. 4a), a
.
new cathodic peak (R2) is seen at Ep,c ¼ ꢀ2.54 V, corresponding to
the reduction of [Mo(CO)2(h -
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ to [Mo(CO)2(h3
allyl)(bpy)(NCS)]2ꢀ (Equation (7)). The unstable dianion immedi-
h
i
ately dissociates NCSꢀ and converts to [Mo(CO)2( 3-allyl)(bpy)]ꢀ
h
ꢀ
ꢁ
MoðCOÞ2 h3 ꢀ allyl ðbpyÞðNCSÞ
þ eꢀ4 MoðCOÞ2 h3 ꢀ allyl bpyꢁꢀ ðNCSÞ
(Equation (8)) observed on the reverse anodic scan (O10).
h
iꢀ
ꢀ
ꢁꢀ
ꢁ
Unlike in THF, the cathodic peak associated with reduction of
R1
(1)
(2)
the [Mo(CO)2(h
3-allyl)(bpy)]2 dimer in PrCN is barely observable at
room temperature on the CV timescale, as the strongly coordinating
PrCN solvent may partly inhibit its formation (Equation (9)). An
h
iꢀ
ꢀ
ꢁꢀ
ꢁ
MoðCOÞ2 h3 ꢀ allyl bpyꢁꢀ ðNCSÞ
MoðCOÞ2 h3 ꢀ allyl bpyꢁꢀ þ NCSꢀ
/
anodic peak, O(D), corresponding to the oxidation of [Mo(CO)2(h3
-
h
i
ꢀ
ꢁꢀ
ꢁ
allyl)(bpy)]2 is seen more clearly on the reverse anodic scan at
200 K; although, this only appears when first passing the wave R2
where [Mo(CO)2(h
3-allyl)(bpy)]ꢀ is formed, converting to the
h
i
ꢀ
ꢁꢀ
ꢁ
dimer at O10 (Fig. 4b).
MoðCOÞ2 h3 ꢀ allyl bpyꢁꢀ
þ eꢀ4 MoðCOÞ2 h3 ꢀ allyl ðbpyÞ
The reduction of the five-coordinate anion, [Mo(CO)2(h3
-
h
iꢀ
ꢀ
ꢁ
allyl)(bpy)]ꢀ, is observed at room temperature at Ep,a ¼ ꢀ2.77 V
R10
(3)
(4)
(cathodic wave R20). At 200 K (Fig. 4b), this peak is poorly defined,
probably due to coordination of PrCN to [Mo(CO)2(
(Equation (9)) revealed by IR spectroelectrochemistry (see below).
h
3-allyl)(bpy)]ꢀ
h
iꢀ
ꢀ
ꢁ
MoðCOÞ2 h3 ꢀ allyl ðbpyÞ
h
iꢀ
h
i
ꢀ
ꢁꢀ
ꢁ
ꢀ
ꢁ
MoðCOÞ2 h3 ꢀ allyl bpyꢁꢀ ðNCSÞ
þ MoðCOÞ2 h3 ꢀ allyl ðbpyÞðNCSÞ
h
i
ꢀ
ꢁ
h
i
2ꢀ
ꢀ
ꢁ
/ MoðCOÞ2 h3 ꢀ allyl ðbpyÞ þ NCSꢀ
þ eꢀ4 MoðCOÞ2 h3 ꢀ allyl ðbpyÞðNCSÞ
R2
(7)
(8)
2
h
i
h
i
ꢀ
ꢁ
2ꢀ
ꢀ
ꢁ
MoðCOÞ2 h3 ꢀ allyl ðbpyÞ
þ eꢀ/ MoðCOÞ2 h3 ꢀ allyl ðbpyÞ
MoðCOÞ2 h3 ꢀ allyl ðbpyÞðNCSÞ
2
h
iꢀ
h
iꢀ
ꢀ
ꢁ
ꢀ
ꢁ
h3
RðDÞ
(5)
/ MoðCOÞ2
ꢀ allyl bpy
ð
Þ
þ NCSꢀ
2
Fig. 4. Cyclic voltammograms of [Mo(CO)2(
h
3-allyl)(bpy)(NCS)] in the PrCN/Bu4NPF6 electrolyte a) at room temperature, recorded at v ¼ 2 V sꢀ1, and b) at 200 K, recorded at
v ¼ 100 mV sꢀ1
.
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6
J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
h
iꢀ
ꢀ
ꢁ
MoðCOÞ2 h3 ꢀ allyl ðbpyÞ
þ PrCN/ MoðCOÞ2 h3 ꢀ allyl ðbpyÞðPrCNÞ
h
iꢀ
ꢀ
ꢁ
(9)
3.3. Cyclic voltammetry of [Mo(CO)2(
BIAN)(NCS)]
h
3-allyl)(2,6-xylyl-
The electrochemical behaviour of the [Mo(CO)2(
h
3-allyl)(2,6-
xylyl-BIAN)(NCS)] complex was studied in THF (Fig. 5). The
anodic scan shows that, unlike for the analogous bpy complex (see
above), the Mo(II)/Mo(III) oxidation at Ep,a ¼ 0.59 V is irreversible.
Fig. 6. Cyclic voltammogram of [Mo(CO)2(h
3-allyl)(2,6-xylyl-BIAN)(NCS)] in the THF/
The 2,6-xylyl-BIAN ligand is a much stronger p-acceptor than bpy
Bu4NPF6 electrolyte at room temperature recorded at a scan rate of 100 mV sꢀ1
.
which enables it to stabilize the radical anion produced on the
initial reduction of [Mo(CO)2(
h
3-allyl)(2,6-xylyl-BIAN)(NCS)], but
h
iꢀ
ꢀ
ꢁꢀ
ꢁ
less able than bpy to stabilize the formally Mo(III) species produced
upon the oxidation.
On the cathodic scan, the initial 2,6-xylyl-BIAN-based redox
couple (R1/O1) is observed at E1/2 ¼ ꢀ1.16 V. This process is both
chemically and electrochemically reversible (Fig. 6), and is signifi-
cantly positively shifted compared with the analogous reduction of
MoðCOÞ2 h3 ꢀallyl 2;6ꢀxylylꢀBIANꢁꢀ ðNCSÞ
h
i
2ꢀ
ꢀ
ꢁ
þeꢀ4 MoðCOÞ2 h3 ꢀallyl ð2;6ꢀxylylꢀBIANÞðNCSÞ
R2
(11)
the bpycomplex as a resultof the extended
2,6-xylyl-BIAN ligand and a low-lying LUMO, which stabilizes the
1eꢀ reduced species, [Mo(CO)2( 3-allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ
p
-aromatic systemof the
h
i
2ꢀ
ꢀ
ꢁ
MoðCOÞ2 h3 ꢀ allyl ð2; 6 ꢀ xylyl ꢀ BIANÞðNCSÞ
h
iꢀ
h
ꢀ
ꢁ
/ MoðCOÞ2 h3 ꢀ allyl ð2; 6 ꢀ xylyl ꢀ BIANÞ þ NCSꢀ
(Equation (10)). The second cathodic wave R2 lies at Ep,c ¼ ꢀ2.09 V
and is irreversible due to dissociation of the NCSꢀ ligand from the
(12)
[Mo(CO)2(
h
3-allyl)(2,6-xylyl-BIAN)(NCS)]2ꢀ dianion to produce
3-allyl)(2,6-xylyl-BIAN)]ꢀ (Equations (11) and (12)), as
h
iꢀ
[Mo(CO)2(
h
ꢀ
ꢁ
MoðCOÞ2 h3 ꢀ allyl ð2; 6 ꢀ xylyl ꢀ BIANÞ
revealed by IR spectroelectrochemistry (see below).
h
iꢀ
A further cathodic peak, R20, is seen at Ep,c ¼ ꢀ3.11 V. The
negative potential shift of R20 from the R1 and R2 waves for the 2,6-
xylyl-BIAN complex is larger than was the case for the bpy complex.
This difference suggests that, unlike in the related bpy complex, the
ꢀ
ꢁ
þ THF/ MoðCOÞ2 h3 ꢀ allyl ð2; 6 ꢀ xylyl ꢀ BIANÞðTHFÞ
(13)
five coordinate anion [Mo(CO)2(
the strong -acceptor -diimine ligand is capable of coordinating
h
3-allyl)(2,6-xylyl-BIAN)]ꢀ with
3.4. Spectroelectrochemistry of [Mo(CO)2(
oxidation
h
3-allyl)(bpy)(NCS)]:
p
a
the donor solvent (even THF) on the CV timescale (Equation (13)).
The cathodic spectroelectrochemical data in THF support this
assumption (see below).
The oxidation of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] was shown to be
reversible at room temperature using cyclic voltammetry at
v ꢄ 100 mV sꢀ1 (Fig. 6). An IR monitored rapid potential step oxida-
tion was carried out in MeCN at room temperature (Fig. 7) to identify
h
i
ꢀ
ꢁ
MoðCOÞ2 h3 ꢀallyl ð2;6ꢀxylylꢀBIANÞðNCSÞ
þeꢀ4 MoðCOÞ2 h3 ꢀallyl 2;6ꢀxylylꢀBIANꢁꢀ ðNCSÞ
h
iꢀ
ꢀ
ꢁꢀ
ꢁ
and characterise the [MoIII(CO)2(
h
3-allyl)(bpy)(NCS)]þ cations.
(CO) bands at 1950 and
(CN) band at 2083 cmꢀ1 are replaced by a
(CO) band at 2069 cmꢀ1 and a broader, more intense band
at 2028 cmꢀ1 resulting from overlap of the
(CN) and second (CO)
bands. [MoIII(CO)2( 3-allyl)(bpy)(NCS)]þ is not completely stable
R1
On the timescale of seconds, the
1866 cmꢀ1 and the
narrow
n
(10)
n
n
n
n
h
under these conditions, with the decrease of the
n
(CO) band at
(CN) band at
(CO) bands
2069 cmꢀ1 accompanied by the increase of the
n
2028 cmꢀ1 indicating decarbonylation. Low intensity
n
are observed at 1959 and 1875 cmꢀ1, with the small shift in
wavenumbers pointing to the presence of a secondary (less elec-
tron rich) Mo(II) species.
Anodic IR spectroelectrochemistry was carried out on
[Mo(CO)2(
[MoIII(CO)2(
(CO) bands of [Mo(CO)2(
1864 cmꢀ1 shift to 2069 and 2019 cmꢀ1 upon oxidation to
[Mo(CO)2(
3-allyl)(bpy)(NCS)]þ, accompanied by the shift of the
(CN) band of the thiocyanate ligand from 2085 to 2034 cmꢀ1. The
shift of the (CO) bands is in good agreement with the values re-
ported in the literature for the related species [MoIII(CO)2(h3
allyl)(bpy)Cl]þ and [MoIII(CO)2( 3-allyl)(bpy)(O2CCF3)]þ [18]. The
h
3-allyl)(bpy)(NCS)] in PrCN at 183 K (Fig. 8) where
h
3-allyl)(bpy)(NCS)]þ becomes completely stable. The
3-allyl)(bpy)(NCS)] at 1947 and
n
h
h
n
n
Fig. 5. Cyclic voltammogram of [Mo(CO)2(h
3-allyl)(2,6-xylyl-BIAN)(NCS)] in the THF/
Bu4NPF6 electrolyte at room temperature recorded at a scan rate of 100 mV sꢀ1. The Fc/
Fcþ standard redox couple is marked with an asterisk.
-
h
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J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
7
3.6. Spectroelectrochemistry of [Mo(CO)2(
BIAN)(NCS)]: reduction
h
3-allyl)(2,6-xylyl-
It was shown with cyclic voltammetry that the initial reduction
of [Mo(CO)2(
3-allyl)(bpy)(NCS)] was irreversible (Fig. 2), whereas
a reversible reduction was seen for the analogous [Mo(CO)2(h3
h
-
allyl)(2,6-xylyl-BIAN)(NCS)] complex (Fig. 6). IR spectro-
electrochemistry was used to study the initial reversible
reduction of [Mo(CO)2(h
3-allyl)(2,6-xylyl-BIAN)(NCS)] in THF and
to identify the product formed (Fig. 9).
IR monitoring of the initial cathodic step of [Mo(CO)2(h3
-
allyl)(2,6-xylyl-BIAN)(NCS)] at E1/2 ¼ ꢀ1.16 V shows the shift of the
(CO) bands from 1946 and 1872 cmꢀ1 to 1918 and 1830 cmꢀ1
belonging to stable [Mo(CO)2(
3-allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ.
The
(CN) band shifts from 2075 to 2090 cmꢀ1 as the thiocyanate
ligand tends towards the less
-donating MoeN^CeSꢀ resonance
n
h
n
Fig. 7. Infrared spectroelectrochemistry of [Mo(CO)2(
MeCN/Bu4NPF6 electrolyte, showing the potential-step oxidation to [Mo(CO)2(h3
allyl)(bpy)(NCS)]þ within an OTTLE cell at room temperature. The
(CO) band labelled
with [Y indicates the slow decomposition of the primary cationic product (see the
main text).
h
3-allyl)(bpy)(NCS)] (Y) in the
p
-
form. The UVeVis spectroelectrochemistry (Fig. 10) of this largely
2,6-xylyl-BIAN-localised 1eꢀ reduction shows the intense absorp-
tion band of the parent complex at 715 nm replaced by a band at
n
522 nm due to [Mo(CO)2(h
3-allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ, in a
good agreement with the UVeVis spectrum of the free 2,6-xylyl-
n
(CN) band of the thiocyanate ligand shifts to lower wavenumbers
BIANꢁꢀ radical anion (Fig. 10 inset).
upon oxidation as the -donation from the anionic ligand to the
metal increases, and the Mo]N]C]S resonance form of the
ligand becomes more favourable.
p
The subsequent irreversible reduction of [Mo(CO)2(h
3-allyl)(2,6-
xylyl-BIANꢁꢀ)(NCS)]ꢀ at ꢀ2.09 V (R2) results in rapid dissociation of
the NCSꢀ ligand, as evidenced by the new (CN) band at 2056 cmꢀ1
n
[34](Fig.11). Thetwo new n
(CO) bandsarising at 1885and 1798 cmꢀ1
most likely belong to the 2eꢀ-reduced anion [Mo(CO)2( 3-allyl)(2,6-
h
3.5. Spectroelectrochemistry of [Mo(CO)2(
BIAN)(NCS)]: oxidation
h
3-allyl)(2,6-xylyl-
xylyl-BIAN)(THF)]ꢀ (Equations (11)e(13)), as deduced from the cy-
clic voltammetric data (see above). Another support for this
assignment comes from similar CO-stretching wavenumbers
of the closely related six-coordinate anion, [Mo(CO)2(h3
-
Cyclic voltammetry has shown that the predominantly metal-
based 1eꢀ oxidation of [Mo(CO)2( 3-allyl)(2,6-xylyl-BIAN)(NCS)]
is irreversible at moderate scan rates (Fig. 5). IR spectroelec-
trochemistry was used to monitor the
bands during this process (Fig. S2).
Upon oxidation in THF at ambient temperature, the parent
bands at 1946 and 1872 cmꢀ1 were lost, accompanied by the
appearance of low intensity
(CO) bands at 1960 and 1887 cmꢀ1
allyl)(bpy)(PrCN)]ꢀ, that was found stable only at 183 K (see Table 3).
h
In contrast, [Mo(CO)2(
h
3-allyl)(2,6-xylyl-BIAN)(THF)]ꢀ is stable
n(CO) and n(CN) absorption
already at room temperature, despite the weaker coordination of
THF compared to PrCN, which can be ascribed to the dominant
localization of the second cathodic step at the 2,6-xylyl-BIAN ligand.
n(CO)
In this respect, doubly reduced bpy is a much stronger
p-donor,
3-allyl)(bpy)]ꢀ
n
stabilizing the 5-coordinate geometry of [Mo(CO)2(
h
due to a secondary Mo(II) species. The n(CN) band of the thiocya-
inTHF (see below) and only PrCN can bind at low temperature. These
observations comply with the reduction paths previously reported
nate ligand shifted from 2075 to 2022 cmꢀ1, a value similar to the
wavenumbers of the decarbonylated product observed for
for [Re(CO)3(
coordinate anions [Re(CO)3(
a
-diimine)X] (X
¼
halide) involving both five-
-diimine li-
-diimine)
decomposition of [Mo(CO)2(h
3-allyl)(bpy)(NCS)]þ in THF. No band
a
-diimine)]ꢀ (for donor
a
was seen at 2055 cmꢀ1 corresponding to free NCSꢀ [34], indicating
that this ligand remains coordinated to the unknown ultimate
oxidation product.
gands) and their six-coordinate equivalents [Re(CO)3(
a
X]nꢀ (n ¼ 1, X ¼ donor solvent; n ¼ 2, X ¼ halide) [35].
Fig. 8. Infrared spectroelectrochemistry of [Mo(CO)2(
PrCN/Bu4NPF6 electrolyte, showing oxidation to inert [Mo(CO)2(
([) within an OTTLE cell at 183 K.
h
3-allyl)(bpy)(NCS)] (Y) in the
Fig. 9. Infrared spectroelectrochemistry of [Mo(CO)2(h
3-allyl)(2,6-xylyl-BIAN)(NCS)]
h
3-allyl)(bpy)(NCS)]þ
(Y) in the THF/Bu4NPF6 electrolyte, showing reduction to [Mo(CO)2(
h
3-allyl)(2,6-xylyl-
BIANꢁꢀ)(NCS)]ꢀ ([) within an OTTLE cell at room temperature.
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8
J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
Fig. 10. UVeVis spectroelectrochemistry of [Mo(CO)2(h
3-allyl)(2,6-xylyl-BIAN)(NCS)]
(Y) in the THF/Bu4NPF6 electrolyte, showing the reduction to [Mo(CO)2(h
3-allyl)(2,6-
Fig. 12. Infrared spectroelectrochemistry of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] (Y) in the
xylyl-BIANꢁꢀ)(NCS)]ꢀ ([) within an OTTLE cell at room temperature. Inset: the UVe
THF/Bu4NPF6 electrolyte, showing initial reduction and formation of [Mo(CO)2(h3
allyl)(bpy)]2 ([) within an OTTLE cell at room temperature.
-
Vis spectrum of free [2,6-xylyl-BIAN]ꢁꢀ in THF generated under the same conditions.
3.7. Spectroelectrochemistry of [Mo(CO)2(
reduction
h
3-allyl)(bpy)(NCS)]:
1922 cmꢀ1) in THF at room temperature [36]. Additional small
(CO) bands observed around 1811 and 1714 cmꢀ1 ([Y in Fig. 12)
n
remain unassigned. UVeVis spectroelectrochemical monitoring of
this reduction step (Fig. 13) shows the formation of two bands at
369 and 581 nm. The broad shape of this very intense (solvent
dependent, Table 3) MLCT band at 581 nm is similar to that
observed at 650 nm for the related metalemetal bonded dimer
[Mn(CO)3(bpy)]2 [19,20].
3.7.1. In THF at room temperature
The initial 1eꢀ reduction of [Mo(CO)2( 3-allyl)(bpy)(NSC)]
h
at ꢀ2.05 V is irreversible at room temperature, as seen by cyclic
voltammetry (Fig. 2). IR monitoring of this cathodic reaction in THF
(Fig. 12) shows a decay of the
complex at 2075 cmꢀ1, and 1951 and 1871 cmꢀ1, respectively. A
new
(CN) band at 2053 cmꢀ1 confirms that the NCSꢀ ligand has
n(CN) and n(CO) bands of the parent
The reduction of [Mo(CO)2(
results in the cleavage of the metalemetal bond (Equations (5) and
(6)) and the formation of [Mo(CO)2(
3-allyl)(bpy)]ꢀ with low-
energy
(CO) bands at 1844 and 1723 cmꢀ1. The large shift of the
(CO) bands of [Mo(CO)2(
3-allyl)(bpy)]ꢀ from those of the parent
complex [Mo(CO)2(
3-allyl)(bpy)(NSC)] (110e150 cmꢀ1) is similar
to the difference observed between the (CO) bands of related
h
3-allyl)(bpy)]2, as shown in Fig. 14,
n
dissociated [34].
h
As discussed above, the loss of the NCSꢀ ligand from
n
[Mo(CO)2(
coordinate radical, [Mo(CO)2(
readily reduced at ꢀ2.05
anion, [Mo(CO)2(
3-allyl)(bpy)]ꢀ (Equations (1)e(3)). The final
product observed is the dimer, [Mo(CO)2(
3-allyl)(bpy)]2, resulting
from reaction of [Mo(CO)2(
3-allyl)(bpy)]ꢀ with the yet non-
reduced parent complex (Equation (4)). The dimer has been iden-
tified by its three, typically fairly narrow, (CO) bands at 1891, 1778,
and 1757 cmꢀ1. The average
(CO) shift accompanying the reduc-
tion of [Mo(CO)2(
3-allyl)(bpy)(NSC)] to [Mo(CO)2( 3-allyl)(bpy)]2
h
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ results in the formation of a five-
n
h
h
3-allyl)(bpyꢁꢀ)], which is itself
to the five-coordinate
h
V
n
h
[Mn(CO)3(bpy)Cl] and [Mn(CO)3(bpy)]ꢀ [19,20]. The UVeVis spec-
trum (Fig. 15) shows the appearance of an intense band at 388 nm
accompanied by a structured visible absorption band with maxima
at 551, 585, 632, 735 and 829 nm. This complex pattern in the
visible region is characteristic for a family of 2eꢀ-reduced five-
coordinate carbonyl anions containing the bpy ligand, such as
[Mn(CO)3(bpy)]ꢀ absorbing at 547 nm with shoulders at 465, 607,
690 and 770 nm; the bands correspond to several close-lying MLCT/
LLCT transitions [37].
h
h
n
n
h
h
is comparable with that observed, e.g., for the cathodic conversion
of [RuII(CH3)(CO)2(iPr-DAB)(I)] (iPr-DAB ¼ N,N0-di-isopropyl-1,4-
diazabuta-1,3-diene) (n
(CO) at 2024 and 1958 cmꢀ1) to the
similar dimer [Ru(CH3)(CO)2(iPr-DAB)]2
(n(CO) at 1981, 1951 and
3.7.2. In PrCN at low temperature
The initial reduction of [Mo(CO)2(
was first monitored at 183 K using IR spectroelectrochemistry
h
3-allyl)(bpy)(NCS)] in PrCN
Fig. 11. Infrared spectroelectrochemistry showing the irreversible reduction of
[Mo(CO)2(
h
3-allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ (Y) to [Mo(CO)2(
h
3-allyl)(2,6-xylyl-
Fig. 13. UVeVis spectroelectrochemistry of [Mo(CO)2(
THF/Bu4NPF6 electrolyte, showing the initial reduction and formation of the dimer
[Mo(CO)2(
3-allyl)(bpy)]2 ([) within an OTTLE cell at room temperature.
h
3-allyl)(bpy)(NCS)] (Y) in the
BIAN)(THF)]ꢀ ([) in the THF/Bu4NPF6 electrolyte within an OTTLE cell at room
temperature.
h
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J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
9
Fig. 14. Infrared spectroelectrochemistry showing the reduction of [Mo(CO)2(h3
allyl)(bpy)]2 (Y) to [Mo(CO)2(
3-allyl)(bpy)]ꢀ ([) in the THF/Bu4NPF6 electrolyte,
within an OTTLE cell at room temperature. The bands labelled with asterisk belong to
remaining [Mo(CO)2(
3-allyl)(bpy)(NSC)] completely reduced during this step.
-
Fig. 16. Infrared spectroelectrochemistry of [Mo(CO)2(
h
3-allyl)(bpy)(NCS)] (Y) in the
to [Mo(CO)2(h3
-
h
PrCN/Bu4NPF6
electrolyte, showing
reduction
initially
allyl)(bpyꢁꢀ)(NCS)]ꢀ ([Y), then to [Mo(CO)2(
h
3-allyl)(bpy)(PrCN)]ꢀ ([) within an
h
OTTLE cell at 183 K.
(Fig. 16), as cyclic voltammetry has revealed that the reduced
for both solvento anions observed [Mo(CO)2(
h
3-allyl)(2,6-xylyl-
h
3-allyl)(bpy)(PrCN)]ꢀ are indeed very
temperature results in an increased stability of [Mo(CO)2(h3
allyl)(bpyꢁꢀ)(NCS)]ꢀ. Upon the initial reduction of [Mo(CO)2(h3
-
-
BIAN)(THF)]ꢀ and [Mo(CO)2(
similar.
allyl)(bpy)(NCS)] to [Mo(CO)2(
bands shift from 1946 and 1864 cmꢀ1 to 1920 and 1829 cmꢀ1. This
is accompanied by the positive shift of the
(CN) band at 2085 cmꢀ1
to 2092 cmꢀ1. The (CO) bands shift by 27e35 cmꢀ1 and the
(CN)
band by 7 cmꢀ1, i.e., in good agreement with the shifts of 30e
47 cmꢀ1 and cmꢀ1
respectively, observed for reduction
of [Mo(CO)2(
3-allyl)(2,6-xylyl-BIAN)(NCS)] to [Mo(CO)2(h3
allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ (Fig. 9).
The radical anion [Mo(CO)2(
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ is not
completely stable at 183 K, as shown by the simultaneous growth of
the
(CN) band of free NCSꢀ at 2056 cmꢀ1, and the appearance of a
secondary species with
(CO) bands at 1893 and 1796 cmꢀ1. The
h
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ, the
n
(CO)
3.7.3. In PrCN at room temperature
Reduction of [Mo(CO)2(
3-allyl)(bpy)(NSC)] in PrCN at room
n
h
n
n
temperature was monitored with spectroelectrochemistry to
investigate differences from the well documented cathodic path in
the less coordinating THF electrolyte (see above). Cyclic voltam-
metry showed that the R(D) cathodic peak corresponding to the
6
,
h
-
reduction of the dimer [Mo(CO)2(
tible in PrCN (Fig. S1), and evidence for the PrCN coordination and
absence of [Mo(CO)2(
3-allyl)(bpy)]2 was obtained by IR spec-
troelectrochemistry at 183 K (Fig. 16).
The IR spectrum of [Mo(CO)2(
(Fig. 17) features a
(CN) band at 2079 cmꢀ1 and
1950 and 1868 cmꢀ1. Upon the reduction, the new
(CN) band rising
at 2056 cmꢀ1 corresponds to free NCSꢀ, being accompanied by
h
3-allyl)(bpy)]2 was not percep-
h
h
n
n
h
3-allyl)(bpy)(NSC)] in PrCN
(CO) bands at
latter reduction product is formed in parallel with [Mo(CO)2(h3
-
allyl)(bpyꢁꢀ)(NCS)]ꢀ, so it must result from the dissociation of NCSꢀ
n
n
n
and subsequent reduction of the five-coordinate radical
[Mo(CO)2(
cussed previously (Equations (1)e(3)). The
ble secondary product lie at much larger wavenumbers than would
be expected for [Mo(CO)2(
3-allyl)(bpy)]ꢀ (1844 and 1723 cmꢀ1 in
h h
3-allyl)(bpyꢁꢀ)] to [Mo(CO)2( 3-allyl)(bpy)]ꢀ, as dis-
n
(CO) bands resulting from multiple species. Importantly, the sharp
(CO) band at 1892 cmꢀ1 and broad (CO) band at 1775 cmꢀ1
3-allyl)(bpy)]2 dimer is formed on
n
(CO) bands of the sta-
n
n
indicate that also the [Mo(CO)2(
h
h
the initial reduction at room temperature, as seen in THF (Equations
THF at room temperature, Fig.14). This difference, together with the
observation of a band rising at 2147 cmꢀ1 and likely belonging to
the CN-stretch of a coordinated PrCN molecule [38], point to the
(1)e(4); Table 3). A second sharp n(CO) band of an unassigned
product is observed at 1906 cmꢀ1. UVeVis spectroelectrochemistry
formation of the 6-coordinate (solvento) anion [Mo(CO)2(h3
-
(Fig. 18) shows the bands of the parent complex at 351 and 478 nm
allyl)(bpy)(PrCN)]ꢀ (Equation (9)). From Table 3, the
n(CO) values
Fig. 15. UVeVis spectroelectrochemistry showing the reduction of [Mo(CO)2(h3
allyl)(bpy)]2 (Y) to [Mo(CO)2(
3-allyl)(bpy)]ꢀ ([) in the THF/Bu4NPF6 electrolyte,
within an OTTLE cell at room temperature.
-
Fig. 17. Infrared spectroelectrochemistry of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] (Y) in the
h
PrCN/Bu4NPF6 electrolyte, showing initial reduction and formation of [Mo(CO)2(h3
allyl)(bpy)]2 ([ with asterisk) within an OTTLE cell at room temperature.
-
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10
J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
Fig. 18. UVeVis spectroelectrochemistry of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] (Y) in the
PrCN/Bu4NPF6 electrolyte, showing the initial reduction of the parent complex and
Fig. 19. Cyclic voltammogram of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] in the CO2 saturated
formation of [Mo(CO)2(
h
3-allyl)(bpy)]2 ([ with asterisk) within an OTTLE cell at room
THF/Bu4NPF6 electrolyte at room temperature recorded at a scan rate of 100 mV sꢀ1
.
temperature. The broad absorption band at 810 nm belongs to an unassigned reduction
product.
The Fc/Fcþ standard couple is marked with asterisk. Cf. Fig. 2 for differences when CO2
is absent.
are replaced by two bands at 371 and 538 nm, similar to those
reduction of the [Mo(CO)2(
h
3-allyl)(bpy)]2 dimer at Ep,c ¼ ꢀ2.52 V,
observed for [Mo(CO)2(h
3-allyl)(bpy)]2 in less polar THF (369,
resulting in the formation of [Mo(CO)2(h
3-allyl)(bpy)]ꢀ.
581 nm). A new broad absorption broad arises at 810 nm. The n(CO)
band at 1906 cmꢀ1 and the electronic absorption at 810 nm, both
not observed in THF, are tentatively assigned to a different dimeric
3.9. Spectroelectrochemistry of [Mo(CO)2(
reduction in CO2-saturated THF electrolyte
h
3-allyl)(bpy)(NCS)]:
product, with further
n(CO) bands within the broad band at
1775 cmꢀ1. In addition, the small band at 2179 cmꢀ1 points to PrCN
coordination in a secondary reduction product [38].
IR monitoring of the reduction of [Mo(CO)2(
h
3-allyl)(bpy)(NCS)]
(CO) bands at
n
(CN) band shifts to 2053 cmꢀ1 as
Continued controlled-potential reduction of [Mo(CO)2(h3
-
in CO2-saturated THF shows the initial appearance of
n
1916 and 1818 cmꢀ1 (Fig. 20). The
allyl)(bpy)]2 taking place in parallel with the other (dimeric?)
species leads to another set of unassigned (CO) bands at 1910,
1882, 1835 and 1762 cmꢀ1 not belonging to [Mo(CO)2(h3
allyl)(bpy)(PrCN)]ꢀ or [Mo(CO)2( 3-allyl)(bpy)]ꢀ. The correspond-
the NCSꢀ ligand dissociates. Small bands are also observed forming
at 1717 cmꢀ1 (the stretching mode of coordinated O2CHꢀ), and at
1890, 1634 and 1580 cmꢀ1. Notably, the intensity of the 13CO2 sat-
ellite peak at 2272 cmꢀ1 does not change significantly. The shift of
n
,
-
h
ing UVeVis spectra shows a typical bifurcated absorption band at
503 and 529 nm belonging to the [bpyꢁꢀ] ligand. In order to avoid
extensive speculative assignment, the reduction path in PrCN will
be investigated in the near future in more detail using other
methods such as EPR spectroscopy, applied to several derivatives of
the
be expected for the formation of an anionic species. Therefore, this
species is likely to be the neutral formate complex, [Mo(CO)2(h3
n
(CO) bands (35e53 cmꢀ1) is significantly smaller than would
-
allyl)(bpy)(O2CH)], reducible at a more negative cathodic potential
than the parent NCS-complex.
parent [Mo(CO)2(
range.
h
3-allyl)(bpy)(NCS)] over a wider temperature
Upon further reduction (Fig. 21), a new species was observed
Overall, the reduction path documented for [Mo(CO)2(h3
-
with
dimer [Mo(CO)2(
13CO2 band significantly decreased (Fig. 21, inset), and new
bands were seen rising rapidly at 1674 and 1641 cmꢀ1, belonging to
n
(CO) bands at 1889 and 1762 cmꢀ1, possibly relating to the
h
3-allyl)(bpy)]2 (Table 3). The intensity of the
(C]O)
allyl)(bpy)(NCS)] at room temperature closely resembles that of
[Mn(CO)3(bpy)X] [19,20], with the loss of the NCSꢀ ligand from
n
unstable [Mo(CO)2(
tual formation of the [Mo(CO)2(
reaction of [Mo(CO)2(
3-allyl)(bpy)]ꢀ with the parent complex in
both THF and PrCN. At low temperature in PrCN, the cathodic
behaviour of the [Mo(CO)2(
3-allyl)(bpy)(NCS)] complex more
closely resembles that of [Re(CO)3(bpy)X] [7] with formation of a
six-coordinate solvento anion, [Mo(CO)2(
3-allyl)(bpy)(PrCN)]ꢀ.
h
3-allyl)(bpyꢁꢀ)(NCS)]ꢀ resulting in the even-
h
3-allyl)(bpy)]2 dimer from the
h
h
h
3.8. Cyclic voltammetry of [Mo(CO)2(
saturated THF electrolyte
h
3-allyl)(bpy)(NCS)] in a CO2-
The cyclic voltammogram of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] was
also recorded in dry THF saturated with CO2 under normal atmo-
spheric pressure (Fig. 19), in order to probe the ability of the
complex to electrocatalyse CO2 reduction.
No change is seen to the irreversible cathodic wave, R1, at
Ep,c ¼ ꢀ1.99 V, but the intensity of the O10 anodic counter peak at
Ep,a ¼ ꢀ1.77 V has decreased upon addition of CO2. This behaviour
indicates a possible interaction of CO2 with the five-coordinate
Fig. 20. Infrared spectroelectrochemistry of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] (Y) in the
anion, [Mo(CO)2(
Further small anodic waves are observed at Ep,a ¼ ꢀ1.10 V
h
3-allyl)(bpy)]ꢀ, formed at Ep,c
¼ ꢀ1.99 V.
CO2-saturated THF/Bu4NPF6 electrolyte within an OTTLE cell. Inset: Decay of the 13CO2
satellite peak. The three product bands ([) labelled with asterisk belong likely to
and ꢀ0.83 V. A high (catalytic) current is seen following the
[Mo(CO)2(h
3-allyl)(bpy)(O2CH)].
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J. Tory et al. / Journal of Organometallic Chemistry xxx (2014) 1e12
11
Acknowledgements
The authors thank the University of Reading for provision of the
Chemical Analysis Facility (the CAF lab). FH is also grateful to the
University of Reading for a start-up grant covering this project.
Mr Henk Luyten (Purmerend, The Netherlands) is thanked for the
maintenance of the spectroelectrochemical cells in the Reading
Spectroelectrochemistry Laboratory (RSL) and Mr Nick Spencer for
assistance in collection of the single-crystal data in the CAF lab.
Dr. Qiang Zeng (University of Reading, RSL) kindly helped with the
preparation of the experimental setup for spectroelectrochemistry
at low temperatures.
Appendix A. Supplementary material
Fig. 21. Infrared spectroelectrochemistry of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] (Y) in the
CCDC 967234 (1) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
CO2 saturated THF/Bu4NPF6 electrolyte within an OTTLE cell. For the preceding non-
catalytic process see Fig. 20. Inset: decay of the 13CO2 satellite peak.
HCOꢀ3 that typically accompanies the CO production; the third
intense n
(C]O) band at 1605 cmꢀ1 corresponds to the catalytic
Appendix B. Supplementary data
production of free formate. In addition, small bubbles were devel-
oping at the working electrode minigrid, indicating the formation
of CO. The gas development limited the scope of the monitoring of
the electrocatalytic reaction to the initial 10 min.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.01.015.
Comparison of the cyclic voltammetry and IR spectroelec-
h
3-allyl)(bpy)(NCS)] in THF in the pres-
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4. Conclusions
The redox behaviour of [Mo(CO)2(h
3-allyl)(bpy)(NCS)] was
studied using both cyclic voltammetry and in situ UVeVis/IR
spectroelectrochemistry in THF and PrCN at 293 K and in PrCN at
200e183 K. The previously unreported [Mo(CO)2(h
3-allyl)(2,6-
xylyl-BIAN)(NCS)] complex was synthesised and characterised by
NMR, IR and UVeVis spectroscopy, and X-ray crystallography. The
extended
BIAN ligand stabilises the six-coordinate radical anion
[Mo(CO)2(
3-allyl)(2,6-xylyl-BIANꢁꢀ)(NCS)]ꢀ formed on the initial
reduction, and this assisted in the assignment of the products
formed by reduction of [Mo(CO)2(
3-allyl)(bpy)(NCS)] and in the
determination of the reduction path.
The reduction of [Mo(CO)2(
3-allyl)(bpy)(NCS)] in THF resulted
in dissociation of the spectator NCSꢀ ligand and concomitant
reduction of [Mo(CO)2(
3-allyl)(bpyꢁꢀ)] to give the five-coordinate
anion [Mo(CO)2(
3-allyl)(bpy)]ꢀ, that reacts with the yet unre-
duced parent complex to produce the dimer [Mo(CO)2(h3
allyl)(bpy)]2. [Mo(CO)2(
3-allyl)(bpy)]ꢀ is observed following the
reduction of [Mo(CO)2(
3-allyl)(bpy)]2 in THF, but in PrCN this
cathodic step is significantly more complex. Importantly, it has
been shown for the first time that [Mo(CO)2(
3-allyl)(bpy)]ꢀ is
capable of catalysing the reduction of CO2 to CO and formate via
formation and subsequent reduction of [Mo(CO)2(
3-allyl)(b-
p-aromatic system of the strongly p-accepting 2,6-xylyl-
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Please cite this article in press as: J. Tory, et al., Journal of Organometallic Chemistry (2014), http://dx.doi.org/10.1016/j.jorganchem.2014.01.015