A Straightforward Electrochemical Approach to Imine- and Amine-bisphenolate Metal Complexes with Facile Control Over Metal Oxidation State

Article abstract of DOI:10.1002/open.201600019

Synthetic methods to prepare organometallic and coordination compounds such as Schiff-base complexes are diverse, with the route chosen being dependent upon many factors such as metal–ligand combination and metal oxidation state. In this work we have shown that electrochemical methodology can be employed to synthesize a variety of metal–salen/salan complexes which comprise diverse metal–ligand combinations and oxidation states. Broad application has been demonstrated through the preparation of 34 complexes under mild and ambient conditions. Unprecedented control over metal oxidation state (M^II/III/IVwhere M=Fe, Mn) is presented by simple modification of reaction conditions. Along this route, a general protocol-switch is described which allows access to analytically pure Fe^II/III–salen complexes. Tuning electrochemical potential, selective metalation of a Mn/Ni alloy is also presented which exclusively delivers Mn^II/IV–salen complexes in high yield.

Full text of DOI:10.1002/open.201600019

DOI: 10.1002/open.201600019
A Straightforward Electrochemical Approach to Imine- and
Amine-bisphenolate Metal Complexes with Facile Control
Over Metal Oxidation State
[
a]
[a]
[b]
[a]
Michael R. Chapman, Susan E. Henkelis, Nikil Kapur, Bao N. Nguyen,* and
Charlotte E. Willans*
[
a]
Synthetic methods to prepare organometallic and coordination
compounds such as Schiff-base complexes are diverse, with
the route chosen being dependent upon many factors such as
metal–ligand combination and metal oxidation state. In this
work we have shown that electrochemical methodology can
be employed to synthesize a variety of metal–salen/salan com-
plexes which comprise diverse metal–ligand combinations and
oxidation states. Broad application has been demonstrated
through the preparation of 34 complexes under mild and am-
bient conditions. Unprecedented control over metal oxidation
II/III/IV
state (M
where M=Fe, Mn) is presented by simple modifica-
tion of reaction conditions. Along this route, a general proto-
col-switch is described which allows access to analytically pure
II/III
Fe –salen complexes. Tuning electrochemical potential, selec-
tive metalation of a Mn/Ni alloy is also presented which exclu-
II/IV
sively delivers Mn –salen complexes in high yield.
Introduction
[
13,14]
Schiff-base ligands are amongst the most widely studied chela-
tors in inorganic chemistry, with salen-type ligands (imine-
bisphenolates) forming a popular class of Schiff-bases. The ni-
trogen atoms on the backbone may also be saturated to form
salan-type (amine-bisphenolate) ligands. Salen-type ligands in
particular have been coordinated to a broad range of metal
centers and have been used widely in catalysis, in addition to
of a base.
The use of sensitive precursors can lead to com-
plicated product mixtures, particularly where metal–alkoxides
[3]
are used due to the process being an equilibrium reaction.
Metal-salts generated when metal halides are used may coordi-
nate to the Lewis-basic oxygen atoms of the salen, leading to
problems with purification.
We have previously developed an electrochemical method
[
1–8]
I
biomedical and materials applications.
Synthetic methods
for the preparation of Cu –N-heterocyclic carbene (NHC) com-
[
15–17]
to salen complexes are relatively diverse and are very much
dependent upon the desired metal and the oxidation state of
the metal center. A common method to prepare metal–salens
involves reaction of the ligand with metal acetate, with high
temperatures often being necessary (e.g., Fe and Mn) and
column chromatography being required for purification (e.g.,
plexes.
The route was found to be simple, efficient, and
versatile, and does not require the use of high temperatures or
basic conditions. During the electrochemical reaction, an imi-
dazolium (HL) ligand precursor is reduced at the cathode, re-
leasing H as the only by-product to form a free NHC (L). Con-
2
comitantly, oxidation of the sacrificial copper anode occurs, lib-
[
9]
+
Cu), often resulting in low yields. Other general methods in-
erating Cu ions into solution. These two species combine to
10
I
clude reaction of the ligand with metal alkoxide or amide
deliver the desired CuÀNHC complex. To complement these
[
11,12]
[18–25]
precursors,
or reaction with metal halide in the presence
findings and extend the current state-of-the-art,
it was hy-
pothesized that any HL ligand with an appropriate reduction
potential could be used with a range of metals to electro-
chemically prepare organometallic complexes in a simple and
economical manner. Herein, we report an important extension
to the electrochemical syntheses through the use of different
ligand types and transition metals (Scheme 1).
[
a] M. R. Chapman, S. E. Henkelis, Dr. B. N. Nguyen, Dr. C. E. Willans
School of Chemistry, University of Leeds
Woodhouse Lane, Leeds LS2 9JT (United Kingdom)
E-mail: c.e.willans@leeds.ac.uk
B.Nguyen@leeds.ac.uk
[b] Prof. N. Kapur
School of Mechanical Engineering, University of Leeds
Woodhouse Lane, Leeds LS2 9JT (United Kingdom)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
open.201600019. CCDC 1438308–1438320 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
ꢀ
2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.
Scheme 1. Electrochemical synthesis of metal–ligand complexes: a) reduc-
tion of HL ligand; b) oxidation of metal; c) combination of ligand and metal.
ChemistryOpen 2016, 00, 0 – 0
1
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Results and Discussion
At the outset, scope and generality of the method were evalu-
ated; therefore, ligand precursors 1a–1h were prepared using
[26–28]
literature procedures and fully characterized (Scheme 2).
Cyclic voltammetry of ligands 1a–1e indicated reduction po-
+
tentials in the range À2.04 to À2.55 V (vs. FeCp /FeCp ), simi-
2
2
lar to those found for imidazolium salts (see Supporting Infor-
mation). Ligand precursors 1a–1e were dissolved in hydrous
acetonitrile, and two metal electrodes of copper, nickel, or zinc
were inserted into the solution under an atmosphere of air. A
potential was applied across the electrodes, maintaining a con-
stant current of 50 mA for 90 min under aerobic conditions.
The complexes precipitated from solution and were collected
via vacuum filtration, to obtain analytically pure product in
good yield (Scheme 2 and Table 1).
Scheme 2. Electrochemical synthesis of metal-salen complexes. Reagents and
conditions: ligand precursor (1.0 mmol), Bu
4 4 3
NBF (0.03 mmol), CH CN
(
50 mL), applied potential (22–25 V, maintaining 50 mA current), rt, 90 min.
II
Yields available in Table 1. Zn complexes 4a–4e contain one molecule of
coordinating H O from solvent.
II
Figure 1. Top: molecular structure of Ni –salen 3b. Bottom: molecular struc-
2
II
ture of Ni –salen 3e. Ellipsoids are drawn at the 50% probability level;
hydrogen atoms are omitted for clarity.
Table 1. Isolated yields (%) of Cu–, Ni–, and Zn–salen complexes.
each metal through two phenoxide oxygen atoms and two
imine donors, with distorted square planar environments
around the metal centers (Figure 1).
II
II
II
Cu [Yield / %]
Ni [Yield / %]
Zn [Yield / %]
2
2
2
2
2
2
a (79)
b (54)
c (81)
d (86)
e (78)
f (58)
3a (91)
3b (84)
3c (80)
3d (77)
3e (88)
4a (88)
4b (79)
4c (83)
4d (77)
4e (94)
Interestingly, during the formation of Zn complexes, one
molecule of water coordinates to the metal center in each
case, which is consistent with microanalytical data. The solid-
state structure of 4e exhibits a coordinated dimethyl sulfoxide
molecule which has displaced the water during crystallization
(
Figure 2). In the absence of bulky ortho-substituents on the
II
In all cases, complexes were characterized by high-resolution
mass spectrometry (HRMS) and elemental analysis. Diamagnet-
phenoxide moiety, Zn -salens are known to dimerize through
axial coordination of one of the oxygen atoms of each ligand
II
II
II
[29]
ic Ni and Zn complexes were also subjected to NMR spectro-
to the Zn center of a neighboring unit. In the presence of
other donor ligands such as water or dimethyl sulfoxide, dime-
rization is suppressed.
I
scopic analysis. Whilst only Cu species were observed in the
[
15]
electrochemical synthesis of Cu–NHCs,
in the presence of
II
II
II
salen ligands, Cu (or Ni /Zn ) products were isolated solely.
This is likely due to the presence of two reducible HL sites per
Reduction (often hydrogenation) of the salen ligand produ-
ces a new tetradentate ligand, coined as salan. Such ligands
do not possess a C=N double bond, resulting in a more flexible
structure around the metal center which is more resistant to
hydrolysis. Several metal–salan complexes have been reported,
2
+
ligand, in addition to the stabilization of M complexes by
salen-type chelating ligands. Further, Ni-complexes 3b and 3e
were characterized in the solid-state using X-ray crystallogra-
phy, with both complexes illustrating one ligand coordinating
[30–33]
[31]
[33,34]
[35,36]
[37]
namely with Cu,
Co, Ni,
V,
and Zn. However,
&
ChemistryOpen 2016, 00, 0 – 0
www.chemistryopen.org
2
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

II
Figure 2. Molecular structure of Zn –salen 4e·(DMSO). Ellipsoids are drawn
at the 50% probability level; hydrogen atoms are omitted for clarity.
common to their syntheses are the use of refluxing tempera-
tures, acidic waste-streams, and often addition of strong base
to allow the products to precipitate from the reaction mixture
for isolation. Therefore, N-saturated salan precursors 1g–h un-
derwent electrochemical conversion in a similar manner to
their unsaturated analogues, using two copper plates with hy-
drous solvent under aerobic conditions. Following 90 min reac-
II
Figure 3. Top: molecular structure of Cu –salan 2g. Bottom: molecular struc-
II
ture of Cu –salan 2h. Ellipsoids are drawn at the 50% probability level;
hydrogen atoms are omitted for clarity.
II
tion time, analytically pure Cu –salan complexes 2g and 2h
were filtered from solution in 83 and 75% isolated yield, re-
spectively,
with no further purification required (Scheme 3).
II
Scheme 3. Electrochemical synthesis of Cu –salan complexes with isolated
yields. Reagents and conditions: ligand precursor (1.0 mmol), Bu
0.03 mmol), CH CN (50 mL), applied potential (22–25 V, maintaining 50 mA
current), rt, 90 min, yields shown in the scheme.
4 4
NBF
(
3
Formation of complexes 2g and 2h was monitored via
high-resolution mass spectrometry and confirmed by micro-
analysis. In addition, dark green needles of each complex were
isolated and analyzed by X-ray crystallography (Figure 3).
Applying the optimized (aerobic/non-anhydrous) procedure
III
Scheme 4. Selective electrochemical synthesis of Fe –salen (right route) and
II
Fe –salen (left route) complexes with isolated yields. Reagents and condition-
s: ligand precursor (1.0 mmol), Bu NBF (0.03 mmol), CH CN (50 mL), applied
4 4 3
potential (22–25 V, maintaining 50 mA current), rt, 90 min, yields shown in
the scheme.
to ligand precursors 1a–e using Fe electrodes, dinuclear
III
III
[
Fe (salen)] O complexes 5a–e were isolated and fully charac-
Fe . The solid-state structure of 5a displays a square-base pyr-
2
terized (Scheme 4, right). m-Oxo-bridges are a common feature
amidal geometry around the metal center, coordinating two
phenoxide oxygen atoms, two imine donors, and a bridging
III
III
[38]
of Fe -salens due to the high oxygen affinity of the Fe ion,
with such complexes being used in catalytic transformations,
[39]
III
oxo group (Figure 4). All Fe complexes were characterized via
or as precursors to add another ligand. The presence of oxo-
high-resolution mass spectrometry and elemental analysis.
II
III
bridges suggests that Fe -salens are initially formed electro-
Despite Fe –salens becoming very well established within
II
chemically, and subsequently oxidize in the presence of O to
the synthetic community, isolation of Fe –salen complexes is
2
ChemistryOpen 2016, 00, 0 – 0
www.chemistryopen.org
3
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Similar to our electrochemical methodology, Sousa and co-
workers developed an electrochemical procedure toward the
II/III
synthesis of Fe complexes of 1- and 2-substituted pyridines,
[42]
with control over oxidation state for one example. Despite
simple product isolation in most cases, the resulting FeL com-
2
plexes were not air- or moisture-sensitive, with each product
forming an unreactive hydrated complex. Through using anhy-
drous and anoxic conditions in the electrochemical synthesis
of Fe complexes, we have accessed highly air-sensitive, spec-
II
troscopically pure Fe -salens 6a, 6b, and 6e in a simple and
efficient manner in very high yields and relatively short reac-
tion times (Scheme 4, left). All complexes were characterized
via high-resolution mass spectrometry and microanalysis, with
single crystals of complex 6e also subject to X-ray crystallo-
graphic analysis (Figure 4). Magnetic susceptibility measure-
ments for each complex were performed in solution by means
of the Evans NMR method, with effective magnetic moments
II
calculated between 4.9–5.0 m , indicative of high-spin Fe (see
B
Supporting Information).
IV
Whilst Mn -oxo complexes of Schiff bases appeared in the
early 1990s, their syntheses involves reaction of the corre-
sponding Mn(L)(acac) with H O (L=Schiff base), or oxidation
2
2
III
+
of a basic solution containing [Mn (L)] via an over pressure of
[43,44]
air.
Pleasingly, use of a manganese/nickel alloy anode
(
88% Mn, 12% Ni) with ligand precursors 1a–e under aerobic/
non-anhydrous conditions led to electrochemical formation of
IV
their corresponding bis(m -oxo) [Mn (salen)] O complexes 7a–
2
2
2
e in high yield (Scheme 5, right). The use of this alloy enables
high concentration of a brittle (non-machinable) metal, such as
Mn, to be incorporated into the electrode surface. The poten-
2
+
tial required to liberate Mn ions into solution is 0.9 V more
III
Figure 4. Top: molecular structure of Fe –salen 5a, ellipsoids are drawn at
II
the 50% probability level. Bottom: molecular structure of Fe –salen 6e. Ellip-
soids are drawn at the 40% probability level; hydrogen atoms are omitted
for clarity.
much less common due to their air- and moisture-sensitivity,
which presents challenges regarding their syntheses and han-
II
dling. Such sensitivity largely originates from the Fe center
which is forced to occupy highly distorted square-planar ge-
ometry; the upshot of which is formation of a highly reactive
II
metal complex. One approach toward the preparation of Fe -
salens involves the organometallic reagent, Fe Mes , which is
2
4
prepared through reaction of ferrous halide with preformed
[
40]
mesityl Grignard.
However, Fe Mes4 is highly air-sensitive
2
and difficult to handle itself, as are other common reagents
II
such as Fe[N(SiMe ) ] which is prepared via reaction of Fe
3
2 2
halide with Li[N(SiMe ) ], followed by high-vacuum distillation
3
2
of the highly sensitive product. Another reported synthetic
[
9]
method involves reaction of ligand precursors with Fe(OAc) ,
2
III
though in our hands this route furnishes an Fe complex when
ligand precursor 1b is used (see Supporting Information), as
IV
[
41]
II
Scheme 5. Selective electrochemical synthesis of Mn –salen (right route)
do other literature examples. Fe –salens have been rendered
inactive in catalysis in some cases, which may be explained by
II
and Mn –salen (left route) complexes using a bimetallic Mn/Ni alloy, with iso-
lated yields. Reagents and conditions: ligand precursor (1.0 mmol), Bu
0.03 mmol), CH CN (50 mL), applied potential (22–25 V, maintaining 50 mA
current), rt, 90 min, yields shown in the scheme.
4 4
NBF
III [12]
the presence of Fe .
(
3
&
ChemistryOpen 2016, 00, 0 – 0
www.chemistryopen.org
4
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

2
+
negative than that of Ni , allowing selective metalation of
a–e at Mn with no corresponding Ni-salen detected by ele-
outlined within. All complexes were characterized by high-res-
olution mass spectrometry and microanalysis, and each exhib-
1
mental analysis, X-ray fluorescence, or atomic absorption spec-
its solution magnetic moments between 5.7–5.9 m , characteris-
B
IV
II
troscopy (see Supporting Information). All Mn complexes
tic of high-spin Mn (see Supporting Information). Single crys-
were characterized using high-resolution mass spectrometry
and elemental analysis, with complexes 7a and 7e further
probed by single crystal X-ray diffraction analysis (Figure 5 and
Supporting Information).
tals of complex 8e were further analyzed via X-ray crystallogra-
phy, illustrating a square-based pyramidal Mn metal center co-
II
ordinated to a single salen ligand, with one molecule of
solvating dimethyl sulfoxide (DMSO, crystallization solvent)
also occupying the coordination sphere (Figure 5, bottom).
Conclusion
Although metal–salen complexes may be accessed relatively
II
easily in some cases, in other circumstances (e.g., Fe - and
II
Mn –salens) high temperatures, sensitive precursors, and by-
product contamination can cause problems. Synthetic meth-
ods are diverse and are dependent upon the ligand, metal,
and metal oxidation state. This work describes a ‘one size fits
all’ approach to the synthesis of metal–salen and metal–salan
complexes using an electrochemical procedure that is simple,
efficient, clean, and high yielding. The electrochemical route
has been shown to be versatile with regard to ligand, metal,
and metal oxidation state, and could be applied to a broad
array of HL-type ligand precursors and metals. Furthermore,
a bimetallic alloy can be used in the system, with remarkable
control over metal selectivity through applied potential. It is
envisaged that this synthetic route may be used generally by
organometallic, synthetic, supramolecular, and materials scien-
tists to easily prepare organometallic and coordination species
which are otherwise difficult or impossible to access.
Experimental Section
II
II
II
III
IV
General procedure for Cu , Ni , Zn , Fe , and Mn com-
plexes
IV
A three-necked round bottomed flask equipped with stirrer bar
was charged with salen precursor (1.0 mmol), tetrabutylammonium
Figure 5. Top: molecular structure of Mn –salen 7a. Bottom: molecular
II
structure of Mn -salen 8e·(DMSO). Ellipsoids are drawn at the 50% probabili-
ty level; hydrogen atoms are omitted for clarity.
tetrafluoroborate (0.03 mmol), and CH CN (50 mL). Two metal elec-
3
trodes (30ꢁ10ꢁ1 mm) were introduced to the solution and a po-
tential applied via an external power supply [22.0–25.0 V] to main-
tain a constant current of 50.0 mA for 90 min. The resulting precip-
itate was collected via vacuum filtration, washed with CH Cl (2ꢁ
II
II
Akin to Fe –salens, analogous complexes of Mn are typically
2
2
prepared using Mn(OAc) , with subsequent exposure to air
10 mL), water (2ꢁ30 mL), and diethyl ether (3ꢁ30 mL) and dried in
vacuo to deliver the corresponding M–salen complex as a micro-
crystalline solid.
2
[9]
providing access to their higher-valent Mn congeners. How-
II
ever, the intermediate Mn –salen complexes are often not iso-
lated on account of their sensitivity, with our efforts regarding
reaction of precursor 1a with Mn(OAc) exclusively producing
2
II
II
III
General procedure for Fe and Mn complexes
an acetate-bridged [Mn (salen)(OAc)] polymer (see Supporting
n
Information). In terms of electrochemical synthesis, early work
by Sousa developed a protocol to salicylaldiminate complexes
A flame-dried three-necked round bottomed flask equipped with
stirrer bar was charged with salen precursor (1.0 mmol) and tetra-
butylammonium tetrafluoroborate (0.03 mmol) and further dried in
vacuo. Anhydrous (anoxic) CH CN (50 mL) was added via cannula,
3
and the solution further degassed via bubbling a stream of argon
for about 30 min through the solution. Two metal electrodes (30ꢁ
III
II
of Mn which are proposed to form via their neutral Mn coun-
[
45]
II
terparts.
However, isolation of Mn Schiff base complexes
which are not stabilized by additional pyridyl donors are much
less common due to their high reactivity.
1
0ꢁ1 mm) were introduced to the solution and a potential applied
Employing anhydrous/anaerobic conditions to our method,
via an external power supply (22.0–25.0 V, operating in CV mode)
to maintain a constant current of 50.0 mA for 90 min. The suspend-
ed precipitate was isolated via cannula filtration and washed with
II
analytically pure Mn -salen complexes 8a, 8b and 8e were ac-
cessed under equally mild and general conditions to those
ChemistryOpen 2016, 00, 0 – 0
www.chemistryopen.org
5
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

anhydrous (anoxic) CH CN (2ꢁ15 mL), followed by rinsing with an-
[15] B. R. M. Lake, E. K. Bullough, T. J. Williams, A. C. Whitwood, M. A. Little,
3
C. E. Willans, Chem. Commun. 2012, 48, 4887–4889.
hydrous (anoxic) diethyl ether (3ꢁ15 mL) before drying in vacuo to
deliver the corresponding M–salen complex as a (highly air- and
moisture-sensitive) powder.
[
16] E. K. Bullough, M. A. Little, C. E. Willans, Organometallics 2013, 32, 570–
77.
5
[
17] M. R. Chapman, Y. M. Shafi, N. Kapur, B. N. Nguyen, C. E. Willans, Chem.
Commun. 2015, 51, 1282–1284.
18] J. J. Habeeb, D. G. Tuck, F. H. Walters, J. Coord. Chem. 1978, 8, 27–33.
19] N. Kumar, D. G. Tuck, Can. J. Chem. 1982, 60, 2579–2582.
20] L. Bustos, J. H. Green, M. A. Khan, D. G. Tuck, Can. J. Chem. 1983, 61,
Full synthetic procedures to all ligand precursors and metal com-
plexes, analytical data, and crystallographic data can be found in
the Supporting Information. CV measurements, magnetic suscepti-
bility measurements, and atomic adsorption and X-ray fluorescence
data are also provided in the Supporting Information, in addition
to photographs to show the electrochemical reactor configuration.
[
[
[
2141–2146.
[
[
[
[
21] L. Matassa, N. Kumar, D. G. Tuck, Inorg. Chim. Acta 1985, 109, 19–21.
22] T. A. Annan, C. Peppe, D. G. Tuck, Can. J. Chem. 1990, 68, 423–430.
23] N. Kumar, D. G. Tuck, K. D. Watson, Can. J. Chem. 1987, 65, 740–743.
24] J. J. Habeeb, F. F. Said, D. G. Tuck, J. Chem. Soc. Dalton Trans. 1981, 118–
120.
Acknowledgements
[
25] T. A. Annan, J. E. Kickham, D. G. Tuck, Can. J. Chem. 1991, 69, 251–256.
26] D. Nartop, W. Clegg, R. W. Harrington, R. A. Henderson, C. Y. Wills,
Dalton Trans. 2014, 43, 3372–3382.
[
This work was supported by the University of Leeds and AstraZe-
neca. The XRF measurements were made at the Department of
Chemistry (York) by Prof. I. J. S. Fairlamb. The XRF instrument was
funded by a University of York Strategic Equipment grant, up-
graded by an Engineering and Physical Sciences Research Council
[27] T. A. Immel, M. Grꢂtzke, E. Batroff, U. Groth, T. Huhn, J. Inorg. Biochem.
012, 106, 68–75.
28] D.-N. Lee, H. Kim, L. Mui, S.-W. Myung, J. Chin, H.-J. Kim, J. Org. Chem.
009, 74, 3330–3334.
2
[
[
[
[
2
29] J. Sanmartꢃn Matalobos, A. M. Garcꢃa-Deibe, M. Fondo, D. Navarro, M. R.
Bermejo, Inorg. Chem. Commun. 2004, 7, 311–314.
30] R. Klement, F. Stock, H. Elias, H. Paulus, P. Pelikan, M. Valko, M. Mazur,
Polyhedron 1999, 18, 3617–3628.
31] M. Valko, R. Klement, P. Pelikan, R. Boca, L. Dlhan, A. Bottcher, H. Elias, L.
Muller, J. Phys. Chem. 1995, 99, 137–143.
[32] A. Boettcher, H. Elias, E. G. Jaeger, H. Langfelderova, M. Mazur, L. Muel-
(EPRSC) Core Capability grant (EP/K039660/1), involving the N8
Universities Equipment sharing initiative. The authors would also
like to thank Dr. A. N. Kulak (University of Leeds, School of
Chemistry) for assistance with atomic absorption spectroscopy
measurements.
ler, H. Paulus, P. Pelikan, M. Rudolph, M. Valko, Inorg. Chem. 1993, 32,
4131–4138.
Keywords: electrochemistry · organometallic compounds ·
salen · Schiff bases · synthesis
[33] I. Correia, A. Dornyei, T. Jakusch, F. Avacilla, T. Kiss, J. Costa Pessoa, Eur.
J. Inorg. Chem. 2006, 2819–2830.
[
34] A. Bçttcher, H. Elias, L. Mꢂller, H. Paulus, Angew. Chem. Int. Ed. Engl.
992, 31, 623–625; Angew. Chem. 1992, 104, 635–637.
1
[
[
[
[
1] T. Katsuki, Chem. Soc. Rev. 2004, 33, 437–444.
2] C. Baleiz¼o, H. Garcia, Chem. Rev. 2006, 106, 3987–4043.
3] P. G. Cozzi, Chem. Soc. Rev. 2004, 33, 410–421.
[
35] I. Correia, J. Costa Pessoa, M. T. Duarte, R. T. Henriques, M. F. M. Piedade,
L. F. Veiros, T. Jakusch, T. Kiss, A. Dornyei, M. M. C. A. Castro, C. F. G. C.
Geraldes, F. Avecilla, Chem. Eur. J. 2004, 10, 2301–2317.
4] J. K. H. Hui, Z. Yu, M. J. MacLachlan, Angew. Chem. Int. Ed. 2007, 46,
[
36] I. Correia, J. Costa Pessoa, M. T. Duarte, M. F. M. Piedade, T. Jakusch, T.
Kiss, M. M. C. A. Castro, C. F. G. C. Geraldes, F. Avecilla, Eur. J. Inorg.
Chem. 2005, 732–744.
7980–7983; Angew. Chem. 2007, 119, 8126–8129.
[
[
[
5] A. K. Crane, M. J. MacLachlan, Eur. J. Inorg. Chem. 2012, 17–30.
6] A. W. Kleij, Dalton Trans. 2009, 4635–4639.
7] S. Medina, A. S. Henderson, J. F. Bower, M. C. Galan, Chem. Commun.
[
37] I. Correia, A. Dornyei, F. Avecilla, T. Kiss, J. Costa Pessoa, Eur. J. Inorg.
Chem. 2006, 656–662.
2
015, 51, 8939–8941.
8] Y. Hitomi, Y. Iwamoto, A. Kashida, M. Kodera, Chem. Commun. 2015, 51,
702–8704.
9] Y. N. Belokon, J. Fuentes, M. North, J. W. Steed, Tetrahedron 2004, 60,
191–3204.
10] E. L. Whitelaw, M. G. Davidson, M. D. Jones, Chem. Commun. 2011, 47,
0004–10006.
11] F. M. Kerton, A. C. Whitwood, C. E. Willans, Dalton Trans. 2004, 2237–
244.
12] C. J. Whiteoak, R. T. M. de Rosales, A. J. P. White, G. J. P. Britovsek, Inorg.
Chem. 2010, 49, 11106–11117.
13] R. R. Chowdhury, A. K. Crane, C. Fowler, P. Kwong, C. M. Kozak, Chem.
Commun. 2008, 94–96.
[
[
[
38] M. Gerloch, E. D. McKenzie, A. D. C. Towl, Nature 1968, 220, 906–907.
39] S. K. Edulji, S. T. Nguyen, Organometallics 2003, 22, 3374–3381.
40] A. Klose, E. Solari, C. Floriani, A. Chiesi-Villa, C. Rizzoli, N. Re, J. Am.
Chem. Soc. 1994, 116, 9123–9135.
[
[
8
3
[
[
41] K. J. Gallagher, R. L. Webster, Chem. Commun. 2014, 50, 12109–12111.
42] J. Sanmartꢃn, M. R. Bermejo, J. A. Garcia-Vazquez, J. Romero, A. Sousa,
Transition Met. Chem. 1993, 18, 528–530.
[
[
[
[
[
1
[
[
43] J. W. Gohdes, W. H. Armstrong, Inorg. Chem. 1992, 31, 368–373.
44] E. Larson, M. S. Lah, X. Li, J. A. Bonadies, V. L. Pecoraro, Inorg. Chem.
2
1
992, 31, 373–378.
[
45] L. Luaces, M. R. Bermejo, J. A. Garcia-Vazquez, J. Romero, A. Sousa, Poly-
hedron 1996, 15, 3717–3724.
14] R. K. Dean, C. I. Fowler, K. Hasan, K. Kerman, P. Kwong, S. Trudel, D. B.
Leznoff, H.-B. Kraatz, L. N. Dawe, C. M. Kozak, Dalton Trans. 2012, 41,
Received: February 23, 2016
4806–4816.
Published online on && &&, 0000
&
ChemistryOpen 2016, 00, 0 – 0
www.chemistryopen.org
6
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
Smooth salen! Electrochemical synthe-
sis has provided easy access to 34 com-
plexes of diverse metal–ligand combina-
tions. Facile control over metal oxida-
tion state was possible through slight
modification of reaction conditions.
Remarkably, selective metallation using
a Mn/Ni alloy furnished analytically pure
Mn complexes in high yield.
M. R. Chapman, S. E. Henkelis, N. Kapur,
B. N. Nguyen,* C. E. Willans*
&& – &&
A Straightforward Electrochemical
Approach to Imine- and Amine-
bisphenolate Metal Complexes with
Facile Control Over Metal Oxidation
State
ChemistryOpen 2016, 00, 0 – 0
www.chemistryopen.org
7
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ