A Series of Manganese(III) Salen Complexes as a Result of Team-Based Inquiry in a Transnational Education Programme

Article abstract of DOI:10.1002/cplu.202000337

The development of a team-based approach to research-led transnational practical chemistry teaching is described in which a team of five Chinese students on an articulated transnational degree programme, supported by a team of academic and technical staff, carried out a study examining the structural chemistry of a series of manganese(III) salen complexes. A series of four crystallographically characterized manganese(III) salen complexes with ancillary carboxylate ligands are reported here. The carboxylate coordination modes range from the bridging syn-anti μ²-κO : κO′ mode observed in the predominant cyclohexanoate and isobutyrate species, to a capping terminal monodentate mode for the adamantanoate species, and an unusual mixture of bridging and terminal coordination modes observed in a second minor phase of the cyclohexanoate species. The variation on extended structures based on the weakly interacting aliphatic backbones may provide a useful basis for further structural studies.

Full text of DOI:10.1002/cplu.202000337

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A Series of Manganese(III) Salen Complexes as a Result of
Team-Based Inquiry in a Transnational Education
Programme
Danlei Yuan,[a, b] Ningqi Cai,[a, b] Jingxi Xu,[a, b] Danyang Miao,[a, b] Sheng Zhang,[a, b]
Sian E. Woodfine,[a] Daniela Plana,*[a] Chris S. Hawes,*[a] and Michael Watkinson*[a]
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The development of a team-based approach to research-led
transnational practical chemistry teaching is described in which
a team of five Chinese students on an articulated transnational
degree programme, supported by a team of academic and
technical staff, carried out a study examining the structural
chemistry of a series of manganese(III) salen complexes. A series
of four crystallographically characterized manganese(III) salen
complexes with ancillary carboxylate ligands are reported here.
The carboxylate coordination modes range from the bridging
syn-anti μ2-kO:kO’ mode observed in the predominant cyclo-
hexanoate and isobutyrate species, to a capping terminal
monodentate mode for the adamantanoate species, and an
unusual mixture of bridging and terminal coordination modes
observed in a second minor phase of the cyclohexanoate
species. The variation on extended structures based on the
weakly interacting aliphatic backbones may provide a useful
basis for further structural studies.
Introduction
oxidation processes continuously under development.[4] Our
interest in the field also began at this time when we were
employing manganese(III) complexes of tetradentate N2O2
donor Schiff base ligands as biomimetic models. During these
studies we found that the nature of ancillary carboxylate
ligands had a profound effect on the coordination chemistry
observed.[5] Subsequently, we utilised the diverse array of
carboxylate binding modes observed in these complexes to test
the widely held validity[6] of assigning them through the use of
vibrational spectroscopy.[7] The [Mn(salen)] cation has also been
widely used subsequently as a crystal engineering tecton, due
to the nature of its linear building unit with two vacant
coordination sites at the axial positions. These can be reliably
occupied by bridging ligands, including carboxylates, as well as
azides, cyanide and fluoride, to generate extended structures.[8]
Control and modulation of the bridging modes in these systems
is known to lead to interesting property relationships, influenc-
ing reactivity as well as the possibility of introducing electronic
coupling between adjacent metal ions.[9] With the continued
focus on the control of extended crystalline structures to access
these properties,[10] particularly fuelled by the interest in
coordination polymers and Metal-Organic Frameworks,[11] the
[Mn(salen)] cation is an attractive target for further study. Given
our combined interests in manganese-Schiff base chemistry and
crystal engineering[4a,12] we were attracted by the potential of
their application in the practical provision of our transnational
undergraduate programme.
Metallo-salen complexes and related species have found wide-
spread utility in recent years, particularly in the field of
asymmetric catalysis,[1] but also as biomimetic models, and in
medical and materials applications.[2] Within this very broad
class of systems manganese(III) salen complexes and their
analogues have long been known as important catalytic
species, epitomised by the seminal work of Jacobsen and
Katsuki in the catalytic enantioselective epoxidation of a
number of unfunctionalized alkenes.[3] In the 30 years since the
first reports of the Jacobsen-Katsuki asymmetric epoxidation, a
wide range of derivatives and analogues have been explored,
with improved methods of asymmetric epoxidation and other
[a] D. Yuan, N. Cai, J. Xu, D. Miao, S. Zhang, Dr. S. E. Woodfine, Dr. D. Plana,
Dr. C. S. Hawes, Prof. M. Watkinson
The Lennard Jones Laboratories
School of Chemical and Physical Sciences
Keele University
Keele ST5 5BG (United Kingdom)
E-mail: d.plana@keele.ac.uk
c.s.hawes@keele.ac.uk
m.watkinson@keele.ac.uk
[b] D. Yuan, N. Cai, J. Xu, D. Miao, S. Zhang
Nanjing Xiaozhuang University
Nanjing Shi
Jiangsu Sheng (P. R. China)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.202000337
Although not a new phenomenon, internationalisation has
increasingly become important for the UK Higher Education
(HE) sector, being one of the UK’s most important export
earners and contributing towards national wealth.[13] In the UK,
transnational education (TNE) has continuously increased over a
number of years, with an ever evolving landscape, including a
wide variety of modalities adopted across the sector, from
This article is part of a Special Collection on “Supramolecular Chemistry:
Young Talents and their Mentors”. More articles can be found under https://
onlinelibrary.wiley.com/doi/toc/10.1002/(ISSN)2192-6506.Supramolecular-
Chemistry.
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This
is an open access article under the terms of the Creative Commons Attri-
bution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
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branch campuses abroad to complete distance learning pro- Results and Discussion
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grammes, going through articulation agreements.[13,14] In partic-
ular, China has remained within the top 5 countries where UK
TNE is delivered, with a marked increase in the number of UK-
China joint HE initiatives in the last decade. This is due in part
to Chinese economic reform, as well as by their willingness to
adopt “Western” styles of teaching and learning within their
own educational culture. The most common TNE model
adopted between the two countries has been articulation
programmes, where Chinese students spend 1 (or 2) year(s) of
their full degree programme in a UK institution. In addition,
during the years spent studying in China, UK-based faculty
travel to deliver intensive teaching blocks, which are sometimes
also supplemented with distance learning.[14–15] A particularly
complex aspect of these programmes is the development of
practical and transferable skills; in the Chinese courses, the
focus tends to be on theory over practice, whereas in the UK,
direct utilization by the students of the knowledge gained and
links to real-world applications are heavily emphasised.[16]
In this study, we report the findings of a crystal engineering
pilot study investigating the effect of backbone steric bulk on a
series of manganese(III) tetradentate N2O2 Schiff base carbox-
ylate complexes, carried out with a group of students engaged
in the Keele University – Nanjing Xiaozhuang University BSc
Applied Chemistry programme. We explain how the design of
the practical course content was augmented with research
elements and how the experimental data were validated, and
report the findings of the study itself which include the
structures of four novel [Mn(salen)] complexes and detailed
analysis of their solid-state structures.
Teaching Approach
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The Keele-NXU BSc Applied Chemistry programme is an
articulated 3+1 joint degree, full details of which have been
previously published.[15,17,18] Briefly, the students are based in
Nanjing Xiaozhuang University for the first three years, moving
to the UK for their final year of study. Keele University staff
deliver eight “bridging modules” during the second and third
years of the programme, six in person, through “flying faculty”,
and two as distance learning on-line modules. The last year
consists of the NXU students essentially integrating into the
final year of the BSc Chemistry programme at Keele, with the
only exception being that the Chinese students do not under-
take the Chemistry Research Project module. This is replaced by
two specially designed lab-based modules, Applied Chemistry 1
and 2.
This approach was taken as Chinese students typically have
limited previous experience in practical techniques, particularly
in terms of instrumental analysis. Through minimal practical
provision delivered as part of the bridging modules, and
consultation with the UK-China Universities and Royal Society
of Chemistry (RSC) transnational degrees network, the need for
gradual development of the students’ practical skills was clearly
identified. The laboratory-based content within the bridging
modules is severely limited, in terms of time, but also
availability of instrumental techniques and specialised software,
familiarisation of the staff with the laboratory environment, and
differences in regulations and requirements between the two
countries.
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In most UK HE programmes, student-led inquiry-based
learning would be favoured over staff-led scripted experiments
for practical laboratory-based modules during the final year;
Daniela Plana obtained her Chemistry degree
from the Universidad Simón Bolívar, Venezue-
la, completed her PhD studies at the Univer-
Mike Watkinson was appointed as Head of
School of Chemical and Physical Sciences at
Keele University in 2018 after spending nearly
twenty years at Queen Mary, University of
London, where he started his independent
academic career. He is also the current Chair
of The Royal Society of Chemistry Interest
Group in Macrocyclic and Supramolecular
Chemistry. Throughout his career he has been
committed to the mentoring and support of
colleagues in both his own universities and
the wider community and derives huge
satisfaction from the successes they have
achieved, despite the advice he has given
them. This contribution to the Supramolecular
Chemistry: Young Talents and their Mentors
special collection came about from our on-
going efforts to support and enhance the
experience of students in research-focused
laboratory teaching. The work described here
combines the research interests and experi-
ence of Daniela and Chris under Mike’s
mentorship and project design.
sity of Manchester, UK, and took up
a
postdoctoral position at the University of
Bristol. As a Lecturer at Keele University, she
has become involved in chemical education
research, particularly regarding international
students, transnational education, team-based
learning, feedback and the use of technology
for teaching.
Chris Hawes completed his PhD at the Uni-
versity of Canterbury, New Zealand under the
supervision of Prof. Paul Kruger in 2012. He
subsequently went on to postdoctoral posi-
tions at Monash University, Australia and
Trinity College Dublin, Ireland. He was ap-
pointed as a Lecturer at Keele University, UK
in 2017, where his research involves ligand
design for the development of new MOFs for
carbon capture and chemical sensing applica-
tions, and using single crystal X-ray crystallog-
raphy to explore the structural chemistry of
coordination compounds.
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with open-ended research at this level a specific requirement of
professional accreditation bodies, such as the RSC.[19] However,
as previously discussed,[20] there may be different levels within
inquiry approaches and students should go through these
different stages. Not only is prior knowledge key to teaching
and learning, but prior learning experiences themselves shape
the way students learn.[21] Consequently, in order to avoid
‘problematising’ international students by simply expecting
them to seamlessly adapt to the new learning environment and
methods, the step between the teacher-led to the student-
centred approach was facilitated by the instructional scaffolding
embedded within the constructively aligned set of modules,
taking the students from scripted experiments to an open-
ended semester-long group research project, within the final
year of their degree in the UK.
The first of the two lab-based modules was designed as a
set of scripted laboratory practicals, spanning a wide range of
areas of chemistry, acting not only as a stepping stone between
two educational systems, but in reality as scaffolding of
practical knowledge, where the students learned to use
instrumental techniques and developed their experimental
skills. The subsequent Applied Chemistry 2 module, during
which the work presented here was developed, focused on
open-ended, collaborative project work, providing clear scaf-
folding or alignment between a more structured, supportive
learning environment in the first semester and more independ-
ent research work in the second.
As increased independent study was identified as a concern
by Chinese students on TNE chemistry programmes,[18] collabo-
rative projects were chosen as students would benefit from
working in groups, rather than individually; team-based
approaches have shown benefits, including the sharing of
responsibilities, interaction and integration, learning from and
supporting each other.[22] An added benefit of this approach is
preparing them for future work environments or postgraduate
studies, where research is typically a collaborative activity,
making group work an essential graduate attribute in higher
education.[23] Groups were supported throughout not only by
academic supervisors on their specific project, but by the same
module lead and technical member of staff throughout the full
academic year, providing continuity and familiarity, leading to
improved student support and gradual development of the
required practical skills.
For this project, we designed a study of manganese(III)
Schiff base complexes in which carboxylate co-ligands could be
varied to probe the effect on the structure and properties of
these compounds. This provided multiple avenues for explora-
tion by the students, opportunities for a more diverse range of
laboratory experiences, and the potential to generate multiple
datasets simultaneously from a parallel approach within the
group. Crystal engineering as a discipline works on the basis of
wide-ranging studies involving parallel experimentation to
discover and harness recurring synthons,[24] and useful data-
points can be achieved from a well-designed screen of
experimental conditions.[25] This lends itself well to the huge
opportunities that exist in undergraduate research,[26] and our
aim in this study was to establish a starting platform to build
upon and develop a larger experimental dataset of structural
information in further student-led studies. Variables can include
crystallization method, presence of co-formers, solvent/antisol-
vent mixtures, and compositional screens such as anion/cation
composition, isosteric replacement or steric modification of a
compound backbone. Here, a selection of Schiff base ligands
(salen, salpn, and their 3,5-di-tert-butyl derivatives and carbox-
ylate backbones with increasing levels of steric bulk (isopropyl,
cyclohexyl, adamantyl) were used (Figure 1). From these, three
new salen complexes were prepared and crystallised success-
fully, as well as the known manganese salpn isobutyrate
complex which we had previously reported in 1999.[5d] As such,
the salen complexes became the focus of the verification study.
Crystallisation can be a notoriously challenging technique
to achieve reproducibly at all levels due to the huge number of
experimental variables,[27] and this can be exacerbated in
undergraduate research laboratories by both time constraints
(in this case the complete research project module consisted of
seven 5-hour laboratory sessions) and the relative inexperience
of the researchers in crystallization methods. For these projects,
we mitigated this by subsequently pursuing independent
reproduction of the results originally obtained by the under-
graduate students by an experienced researcher. In this way,
the complete research team can streamline the discovery
process using the results of the conditions screen carried out by
the students to guide a subsequent, more detailed study, while
independently verifying experimental conditions, and filling in
the inevitable gaps in data collection from the time-constrained
laboratory course. We followed a similar approach a year
earlier,[28] when a group of 5 students from the 2017-18 NXU
cohort were assigned three new ligands to prepare and
crystallize as their zinc(II) complexes. In that study, the students
optimized reaction conditions for both the ligand and complex
synthesis, exploring their own hypotheses for both, while
subsequent reproducibility work revealed a second isomeric
form of one ligand and a further two crystalline phases of zinc
complexes.
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Figure 1. Structures of the ligands of interest to this study.
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Experimental Approach
by two deprotonated salen ligands and two cyclohexanoate
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ligands, with one disordered lattice ethanol molecule present.
As shown in Figure 2, each manganese ion is coordinated
equatorially by a tetradentate salen ligand, though the nature
of the axial interactions differs between the two sites.
Manganese atom Mn1 adopts a regular octahedral coordination
geometry with a monodentate carboxylate oxygen atom O5
occupying one site. The other position is occupied by oxygen
atom O7 of a bridging syn-anti bis-monodentate (μ2-kO:kO’)
carboxylate which links through oxygen atom O8 to the second
manganese site Mn2. While the two axial MnÀ O bonds to Mn1
are both relatively long (2.218(2) and 2.216(2) Å for O5 and O7,
respectively), Mn2 adopts a tighter 5-coordinate geometry, with
an axial carboxylate MnÀ O distance of 2.083(2) Å. A recipro-
cated close contact is also evident to the vacant axial
coordination site from the phenolate oxygen atom O3 from an
adjacent Mn-salen species, at a distance (Mn2-O3) of 2.550(2) Å.
As shown in Figure 3, when considering the dimerization of
two dinuclear units, the overall structure of 1-α can be
considered as a discrete tetranuclear chain with bridging
Preparation of the manganese precursors
The manganese carboxylate precursors were prepared following
a similar method to that reported by Zhou et al.[29] We found
that the isobutyrate and cyclohexanoate species could be
prepared in good yield and with elemental analysis generally
consistent with species of the type Mn(RCOO)2· xH2O, although
the formulations of similar compounds are known to vary and
include oxides, protonated carboxylic acids and solvents.[30] In
the absence of crystallographic evidence these formulations
were taken as working assumptions. However, we found that
adamantane-1-carboxylic acid suffered from poor solubility
under these conditions and exhibited a tendency to foam, and
the recovery of the subsequent manganese complex was in
poor yield. Furthermore, elemental analysis of this species
showed anomalously low carbon content and a C:H ratio
inconsistent with a solvate of the bis-carboxylate compound,
suggesting the presence of additional inorganics. The infrared
spectrum of this species showed, beyond the main absorbances
at 1491 and 1404 cmÀ 1, several additional shoulders at 1540,
1614 and 1678 cmÀ 1, as well as at least two overlapping
absorbances in the range 1435–1454 cmÀ 1, and is notably more
complex than the carboxylate regions of the other salts
(Figure S4 in the Supporting Information). Additional absorban-
ces in both regions are consistent with carbonate or bicarbon-
ate species.[31] Assuming an incomplete conversion of the
carbonate starting material, the elemental analysis data are
consistent with a mixed adamantanoate/carbonate formulation
in a 2:1 ratio, or, alternatively, an adamantanoate/bicarbonate
mixture of 1.25:0.75. Nonetheless, we were able to use all three
precursors as convenient manganese-carboxylate sources to
generate single crystals of the desired manganese(III) salen
complexes.
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Synthesis and Structure of [Mn(salen)(CyCOO)] 1
Reaction of the manganese cyclohexanoate starting material
with H2salen in ethanol immediately gave a brown solution
indicative of the expected oxidation to manganese(III). Several
methods were attempted to crystallise this material, by
diffusion of diethyl ether or toluene vapour, cooling the
solutions, or slow evaporation. We found diethyl ether diffusion
to be the most effective method, which yielded brown crystals
over the space of one week at room temperature. In the initial
screening performed by the students, we found an ethanol-
solvated phase 1-α; however, subsequent reproduction and
analysis of the bulk solid by X-ray powder diffraction indicated
that this phase was only a minor constituent of the bulk
(Figure S1 in the Supporting Information). When the reaction
was repeated, the second (unsolvated) phase 1-β was also
elucidated.
Figure 2. Structure of the dinuclear repeat unit of complex 1-α with
heteroatom labelling scheme. Hydrogen atoms and lattice ethanol molecules
are omitted for clarity. ADPs are rendered at 50% probability.
The diffraction data for 1-α were solved and refined in the
monoclinic space group P21/c, giving a structural model with an
asymmetric unit containing two manganese ions coordinated
Figure 3. The complete tetranuclear unit of complex 1-α showing the
phenolate bridging between the two central [Mn(salen)] units. Hydrogen
atoms and ethanol solvent molecules are omitted for clarity.
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through carboxylates and (Mn-salen)2 dimers. The chains are
capped by terminal monodentate carboxylates, which engage
in hydrogen bonding with the lattice ethanol molecule through
the non-coordinating oxygen atom O6. Interestingly, the two
unique salen groups adopt slightly different geometries from
one another. The salen group at the ends of the tetramer
adopts a near-planar geometry; considering the mean planes of
the two phenyl rings, the normal vectors of these planes are
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3
4
5
6
7
8
9
°
°
related by an angle of 12.7 , and a “fold” of 8.1 . However, the
central salen unit adopts a much more folded geometry, with
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°
°
interplanar angle of 24.9 and fold of 23.9 . Here, the folding of
the central salen is consistent with the steric clash between
aromatic groups and the close approach between the phenolic
oxygen atom and the adjacent manganese ion.
Each tetranuclear unit interacts with adjacent molecules
through a series of weak non-covalent interactions. Visualisation
through a normalized contact distance mapping on a Hirshfeld
surface of the tetranuclear complex (Figure S5 in the Support-
ing Information)[32] shows the shortest internuclear distances
involve the (disordered) ethanol solvent molecule, both in the
form of the classical OÀ H···O hydrogen bond from the ethanol
molecule, and also CÀ H···O contacts from several aromatic CÀ H
groups to the ethanol oxygen atom. Additionally, the contoured
surface of the tetramer allows for interdigitation between
adjacent complexes; although no significant π-π interactions
are observed between complexes (discounting the close
contact between the dimer at the centre of the complex),
several instances of CÀ H···O interactions are observed, such as
the contact between a salen methine CÀ H group and a
coordinating carboxylate oxygen atom (C26···O5 distance
Figure 4. (A) The structure of complex 1-β and bridging mode to adjacent
metal sites, with heteroatom labelling scheme. ADPs are rendered at 50%
probability. (B) The packing of adjacent [Mn(salen)] units via the syn-anti
bridging mode of the cyclohexanoate ligand. All hydrogen atoms are
omitted for clarity. Symmetry codes used to generate equivalent atoms: (i) 1-
x, 1= +y, 1= -z; (ii) 1-x, y-1= , 1= -z.
2
2
2
2
°
3.340(3) Å, CÀ H···O angle 159 ).
The structure of 1-β was determined after we discovered
that the material prepared in our attempts to isolate a bulk
sample of 1-α was mostly comprised of a second phase. The X-
ray diffraction data were solved and refined in the monoclinic
space group P21/c (as with 1-α). With a unit cell volume of less
than half that of 1-α (2025 Å3, cf. 4425 Å3 for 1-α), we expected
to find a single unique [Mn(salen)] environment within the
asymmetric unit. Indeed, the asymmetric unit of 1-β contains
one salen ligand bound to a single manganese ion, with one
cyclohexanoate ligand bound in the axial position and no
lattice solvent molecules, as shown in Figure 4A. Expansion
through crystallographic symmetry reveals a one-dimensional
polymeric chain, with both axial manganese sites occupied by
carboxylate oxygen atoms. The carboxylate ligand coordinates
in a syn-anti bis-monodentate μ2-kO:kO’ coordination mode,
similar to the bridging carboxylate in the structure of 1-α. The
two axial MnÀ O bonds are also comparable, at 2.193(2) and
2.214(2) Å for O3 and O4, respectively. However, each [Mn
(salen)] unit is subject to a twofold rotation about the b unit cell
axis, which coincides with the polymeric chain direction. This
contrasts with 1-α, where the carboxylate-bridged [Mn(salen)]
desymmetrization is observed along the chain as a result of
hydrogen bonding from lattice water molecules.
The more simplified extended structure of 1-β, as shown in
Figure 4B, provides some additional insights as to the crystal
packing effects at play in these systems. Like the central unit in
1-α, the salen group adopts a folded conformation, with a fold
°
angle of 26.9 and no twisting contribution. In this case, the
folding of the salen group may relate to the close approach of
adjacent salen groups along the polymeric chain, which
includes angled CÀ H···π contacts between aromatic groups, at a
minimum C···π(mean plane) distance of 3.33 Å for C12 to the
°
C1–C6 plane at an interplanar angle of 48.9 . Several weak
intramolecular CÀ H···π contacts are also evident from the
cyclohexyl group, which is flanked by two phenyl groups.
Compared to 1-α, in 1-β the lack of both solvent of
crystallisation and terminal carboxylate groups means consid-
erably fewer opportunities for directed intermolecular interac-
tions. While some CÀ H···O interactions are evident involving the
phenolate oxygen atoms, the carboxylate oxygen atoms are
largely shielded by the steric bulk of the salen groups. Likewise,
with no externally-oriented π surfaces, there are no notable
CÀ H···π or π···π interactions between adjacent chains. This is in
contrast to several similar carboxylate-bridged [Mn(salen)] and
[Mn(salpn)] complexes and related species with smaller carbox-
°
units are not symmetry-related and are twisted by 114 with
°
respect to each other about the Mn1-Mn2 vector. The 180
twist in 1-β is more reminiscent of the bridging mode in the
[Mn(salpn)] acetate derivative,[5a] although in that instance
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ylate backbones,[5a,d,7] such as in our previously reported acetate
and n-propanoate compounds, where interdigitation of adja-
cent chains was observed with CÀ H···π or face to face π–π
interactions evident.
1
2
3
4
We made several attempts to generate the solvated 1-α
phase in bulk, but were unsuccessful and continuously
generated only trace quantities compared to the β phase.
Interestingly, switching the reaction solvent from ethanol to
methanol provided a third crystalline phase; single crystal X-ray
diffraction showed this material to be [Mn(salen)(HCOO)], of the
same polymorph reported by Ghosh.[33] The generation of
formate under these reaction conditions, most likely from
oxidation of methanol, is not without precedent in similar
systems,[34] but may present an interesting opportunity for
crystal engineering in the future.
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Synthesis and Structure of [Mn(salen)(iPrCOO)] 2
The reaction of salen with “[MniPrCOO]” under analogous
conditions to those employed above gave a brown solution
which deposited yellow-brown crystals following diffusion of
diethyl ether vapour. Multiple repeats of the original synthesis
reproducibly gave a single phase, and X-ray powder diffraction
confirmed that the single crystal structure was indicative of the
bulk. The diffraction data were solved in the orthorhombic
space group P212121, as an inversion twin. The structural model
was refined to show the asymmetric unit contains one [Mn
(salen)] unit and a single isobutyrate ligand, with no lattice
solvent, as shown in Figure 5A.
The structure is closely comparable to 1-β above; the
isobutyrate ligand coordinates in a syn-anti bis-monodentate
μ2-kO:kO’ mode, giving a one-dimensional coordination poly-
mer oriented parallel to the b axis (Figure 5B). The two axial
MnÀ O bonds are statistically equivalent and similar to the
equivalent bonds in 1-β (2.206(2) and 2.195(2) Å for O4 and O3,
Figure 5. (A) The structure of complex 2 and bridging mode to adjacent
metal sites, with heteroatom labelling scheme. ADPs rendered at 50%
probability. (B) The packing of adjacent [Mn(salen)] units via the syn-anti
bridging mode of the isobutyrate ligand, which is closely comparable to that
observed in the cyclohexanoate case 1-β. All hydrogen atoms are omitted
for clarity. Symmetry codes used to generate equivalent atoms: (i) 1-x, 1= +y,
2
1= -z; (ii) 1-x, y-1= , 1= -z.
2
2
2
by this method, suggesting a mixed phase is not accessible
under these conditions.
°
respectively), while the phenyl-phenyl fold angle of 22.8 is also
comparable to that described above. The twofold rotation
relating adjacent [Mn(salen)] units along the propagation axis
similarly blocks the majority of possible CÀ H···O interactions and
other intra-chain contacts, and adjacent chains pack without
significant directional forces between each. The sterics of the
isobutyrate backbone are similar to the cyclohexyl, at least in
the immediate vicinity of the carboxylic acid group, and so
similar weak CÀ H···π interactions are evident as the isopropyl
group is sandwiched between two salen phenyl groups from
the two bridged [Mn(salen)] units.
Synthesis and Structure of [Mn(salen)(AdCOO)] 3
Reaction of the mixed adamantanoate/carbonate manganese
source with salen gave a brown solution of similar appearance
to the other complexes. Diffusion of diethyl ether vapour gave
a mixture of two crystalline phases; one, yellow/brown rods of
complex 3, was formed as a minor phase, while the dominant
phase, a brown microcrystalline deposit intergrown with the
minor phase, was formed unavoidably. Multiple reaction
conditions were explored to avoid the formation of the second
phase; the same outcome was observed with diffusion of
diethyl ether, toluene, dichloromethane or ethyl acetate vapour,
while slow evaporation or cooling of the alcohol solution failed
to produce any crystalline material.
In all cases, we were unable to generate the pure
adamantanoate complex 3 without the concurrent formation of
the microcrystalline byproduct. Elemental analysis of the bulk
(mixed) phase shows a low C:N ratio which is inconsistent with
any substantial amount of adamantanoate being present in this
material. The elemental analysis data are consistent with a
Given the close similarities between the unsolvated phase
1-β and complex 2, we attempted a mixed-ligand experiment in
which 0.5 equivalents of both manganese sources were
combined with 1 equivalent of salen. This mixture was
subjected to the same reaction and crystallisation conditions
which had given rise to the original phases. Following diffusion
of diethyl ether vapour, a small crop of yellow-brown crystals
was obtained; analysis by X-ray powder diffraction (Figure S3 in
the Supporting Information) showed that only 1-β was formed
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species of the general formula [{Mn(salen)(EtOH)}2(CO3)], sug-
gesting the presence of an inorganic anion in the major phase,
though as we were unable to generate single crystals of this
material its structure and connectivity is unclear. For the
purposes of studying the solid-state structure of this material, a
single crystal of 3 was manually separated from the mixture for
X-ray diffraction.
1
2
3
4
5
6
7
The diffraction data for 3 were solved in the triclinic space
group P-1, and the structure model contains a complete [Mn
(salen)(AdCOO)] molecule in the asymmetric unit as shown in
Figure 6. Unlike the structures of 1 and 2, in complex 3 the
manganese ion adopts a purely 5-coordinate geometry, with a
shorter axial MnÀ O bond to the monodentate carboxylate of
2.048(2) for Mn1À O3. The geometry of the manganese ion is
best described as square pyramidal (τ5 =0.1),[35] although with
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°
the folded conformation (fold angle 28.8 ) adopted by the salen
°
°
group both basal trans angles deviate from 180 (164.94(10)
°
and 159.01(10) for O2À Mn1À N1 and N2À Mn1À O1, respec-
tively).
Despite the larger size of the adamantyl group, the lower
coordination number in the structure of 3 allows for additional
intermolecular contacts beyond those seen in the previous
structures. Most notable are CÀ H···O interactions involving the
carboxylate oxygen atoms, as shown in Figure 7. Adjacent
complexes are linked by reciprocated CÀ H···O contacts from the
methine carbon atom C10 to the non-coordinating carboxylate
oxygen atom O4, at a C···O distance of 3.073(4) Å and CÀ H···O
°
angle of 158 . The opposite end of the salen group participates
in an unusual DDD···AAA motif of CÀ H···O contacts, where
methylene, methine and phenyl CÀ H groups donate non-
classical hydrogen bonds to the two carboxylate oxygen atoms
and the phenolic oxygen atom, respectively. These interactions
vary in their contact distances (3.616(4), 3.248(4) and 3.471(4) Å
for C8···O4, C7···O3 and C5···O1, respectively) and CÀ H···O angles
Figure 7. The main intermolecular interactions in the structure of complex 3.
(A) The reciprocated methine···carboxylate contacts between adjacent
molecules represented on a normalized contact distance mapping of the
Hirshfeld surface (rendered at surface isovalues À 0.5/+1.3). (B) The
DDD···AAA set of weak CÀ H···O interactions between adjacent molecules in
the structure of 3, where the methine···carboxylate interaction is the most
substantial.
°
(159, 176 and 146 , respectively), with the methine unsurpris-
ingly acting as the most effective donor. Some limited face-to-
face π···π interactions are also evident in the structure of 3,
though these are limited to partial overlaps of the outer edges
of the phenyl rings due to the bulk of the adamantyl group on
one face and the concave bend of the salen species blocking
close contacts to the other face.
Conclusion
Through the scaffolded increase of inquiry-based laboratory
work, the students were able to develop their practical skills
through the first semester and worked well during their group
research projects. The research presented here was particularly
suited to this programme, and the team-based approach, as it
provided the opportunity of parallel activities along multiple
avenues of exploration, using a diverse range of laboratory
techniques. The outcome of the course has been largely
positive for the students, many of whom have progressed onto
postgraduate degrees in the UK or at home in China.
Figure 6. The structure of complex 3 with heteroatom labelling scheme.
Hydrogen atoms are omitted for clarity. ADPs rendered at 50% probability.
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mationszentrum Karlsruhe Access Structures service www.ccdc.ca-
m.ac.uk/structures.
From the structures of complexes 1-α, 1-β, 2 and 3, it is
1
2
3
4
5
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7
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clear that large-scale structural changes in manganese salen
complexes can be effected by modification of the carboxylate
backbones. We observed similar syn-anti μ2-kO:kO’ coordina-
tion modes in both 1-β and 2, leading to very similar one-
dimensional coordination polymers, while the much larger
adamantyl backbone seemingly restricted the bridging ability
of the carboxylate, leaving a discrete 5-coordinate manganese
species 3. We also uncovered a minor solvated phase 1-α which
showed intermediate behavior of the axial carboxylate ligands,
with both syn-anti bridging and terminal capping behavior
observed. These results suggest a subtle energetic landscape
exists in the crystallization of these species where axial ligands
with intermediate steric bulk may give access to these bridging
geometries selectively, opening the scope for a wider crystal
engineering study, and potentially allowing new reactivity to be
explored.
Synthesis of the manganese carboxylate precursors
The manganese carboxylates were prepared following adapted
literature procedures.[29] A suspension of manganese(II) carbonate
in water (50 mg/mL) was heated at reflux in the presence of the
corresponding carboxylic acid (3 equivalents) for 24 hours. After
this period the solution was filtered hot, and the filtrate evaporated
to dryness to give a white to pale pink solid. This solid was slurried
in ice-cold acetonitrile, filtered and dried in air.
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“[Mn–CyCOO]”: Prepared following the general procedure from
MnCO3 (3.0 g, 26 mmol) and cyclohexanecarboxylic acid (10 g,
78 mmol). Yield 1.87 g (22% based on Mn). Found C, 51.91; H,
7.04%; Calculated for [Mn(CyCOO)2]·0.75H2O (C14H22O4Mn·0.75H2O)
C, 52.10; H, 7.34%; νmax(ATR, cmÀ 1) 3342 m br, 2924 s, 2851 m,
1682 w, 1538 s, 1413 s sh, 1359 m, 1343 m, 1325 m, 1280 m,
1225 m, 1181 w, 1137 w, 1074 w, 1037 w sh, 938 w, 923 w, 892 m,
875 w, 845 w, 801 w, 763 w.
“[Mn-iPrCOO]”: Prepared following the general procedure from
MnCO3 (3.0 g, 26 mmol) and isobutyric acid (7.2 mL, 78 mmol). Yield
5.4 g (84% based on Mn). Found C, 38.78; H, 5.80%; Calculated for
Experimental Section
[Mn(iPrCOO)2]·H2O (C8H16O5Mn) C, 38.88; H, 6.53%; νmax(ATR, cmÀ 1
)
Materials and Methods
3395 m br, 2966 m, 2929 w, 2871 w, 1541 s, 1471 m, 1406 s sh,
1376 w, 1359 m, 1305 s, 1281 s, 1169 w, 1097 m sh, 926 m, 843 w,
760 w sh, 637 m.
All reagents, starting materials and solvents were purchased from
Merck, Alfa Aesar or Fluorochem, were of reagent grade or higher,
and were used as received. Salen was prepared according to
published procedures.[36] Infrared spectra were measured using a
Thermo Scientific Nicolet iS10 instrument with an ATR sampling
“[Mn-AdCOO]”: Prepared following a modified general procedure in
which the total solvent volume was increased from 60 mL to
150 mL to allow for the very poor solubility of the starting material,
from MnCO3 (2.0 g, 17 mmol) and 1-adamantanecarboxylic acid
(9.3 g, 52 mmol). Analysis suggested that this material consists of a
mixture of manganese adamantanoate and carbonate or bicarbon-
ate. This material was used without purification for the subsequent
reaction. Yield 450 mg (7%–9% based on Mn). Found C, 45.84; H,
accessory. Elemental analysis was performed with
a Thermo
Scientific Flash 2000 CHNS elemental analyser, with vanadium
pentoxide as a combustion aid and calibrated against sulfanilamide.
X-ray powder diffraction patterns were measured on a Bruker D8
Advance powder diffractometer equipped with a CuÀ Kα source
(λ=1.54178 Å). Samples were mounted on a zero-background
silicon crystal sample holder. All samples were measured at room
6.52%;
(C14.5H25.5O7.75Mn)
Calculated
C,
for
45.98;
[Mn(0.75HCO3)(1.25AdCOO)]·3H2O
H, 6.78%; Calculated for
°
temperature in the range 2θ=5–55 . Data were compared against
[Mn2(AdCOO)2(CO3)]·2H2O (C23H38O11Mn2) C, 46.01; H, 6.38%; νmax
(ATR, cmÀ 1) 3355 m br, 2902 m sh, 2849 m, 1678 w, 1614 w, 1491 s
sh, 1438 m sh, 1404 s, 1365 w, 1342 m, 1314 m, 1289 w, 1260 w,
1180 w sh, 1105 w, 1091 w, 977 w sh, 912 m, 810 m, 759 m br,
680 m.
the simulated patterns from the single crystal data, measured at
150 K.
X-ray Crystallography
Crystal and refinement parameters are presented in Table S1. All
data were collected using a Bruker D8 Quest ECO diffractometer
equipped with a Mo Kα source (λ=0.71073 Å). Crystals were
mounted on Mitigen micromounts in NVH immersion oil, and all
collections were carried out using φ and ω scans, with data
collection and reduction controlled with the Bruker APEX-3 suite of
programs.[37] Multi-scan absorption corrections were applied using
SADABS,[38] and the corrected intensity data were solved using the
intrinsic phasing routine in SHELXT.[39] Structural models were
refined on F2 with full-matrix least squares procedures in SHELXL
operating within the OLEX-2 GUI.[40] All non-hydrogen atoms were
refined with anisotropic displacement parameters, while hydrogen
atoms were placed in riding positions and refined with isotropic
displacement parameters equivalent to 1.2 or 1.5 times the
isotropic equivalent of their carrier atom. Complex 2 was refined as
a two-component inversion twin. Specific collection and refinement
strategies are further outlined in the combined crystallographic
information file. Deposition Number(s) 1994437 (1-α), 1994438 (1-
β), 199439 (2), 1994440 (3) contain the supplementary crystallo-
graphic data for this paper. These data are provided free of charge
by the joint Cambridge Crystallographic Data Centre and Fachinfor-
Synthesis of [Mn(salen)(CyCOO)] 1
H2salen[36] (50 mg, 190 μmol) and “[MnCyCOO]” (60 mg, 180 μmol,
1 eq.) were combined in ethanol (10 mL), and the resulting brown
solution was heated at reflux for 2 hours. On cooling to room
temperature the solution was filtered, and the filtrate was subjected
to diffusion of diethyl ether vapour for 1 week, giving dark yellow-
brown crystals. Yield 21 mg (26%). X-ray powder diffraction (Fig-
ure S1 in the Supporting Information) confirmed that crystals
isolated by this method give predominantly the β-phase with only
trace amounts of the solvated α-phase present. Found C, 61.24; H,
5.57; N, 6.47%; Calculated for [Mn(salen)(CyCOO)] (C23H25N2O4Mn) C,
61.61; H, 5.62; N, 6.25%; νmax(ATR, cmÀ 1) 3020 m, 2964 s sh, 2910s,
2852 s, 1634 s, 1597 m, 1539 s, 1466 m, 1445 s sh, 1397 s sh,
1336 m, 1323 m, 1302 m, 1277 w, 1258 w, 1231 w, 1201 s sh,
1148 s, 1125 s, 1082 s, 1043 m, 1030 m, 972 m, 957 m, 900 s, 863 m,
855 m, 979 s, 751 s sh, 714 m, 646 m, 620 s.
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Synthesis of [Mn(salen)(iPrCOO)] 2
1
2
3
4
5
6
7
8
9
H2salen (50 mg, 190 μmol) and “[Mn-iPrCOO]” (40 mg, 162 μmol)
were combined in ethanol (8 mL) and heated at reflux for 2 hours.
On cooling to room temperature the brown solution was filtered,
and the filtrate was subjected to diffusion of diethyl ether vapour
for 1 week. After this period the resulting brown crystals were
isolated by filtration. Yield 25 mg (38%). Found C, 58.33; H, 5.07; N,
7.37%; Calculated for [Mn(salen)(iPrCOO)] (C20H21N2O4Mn) C, 58.83;
H, 5.18; N, 6.86%; νmax(ATR, cmÀ 1) 2964w sh, 2903 m sh, 2851 m,
1631 s, 1597 m, 1539 s sh, 1466 m, 1442 m sh, 1409 m sh, 1383 m,
1364 m, 1335 m, 1300 m, 1276 w, 1230 w, 1199 m, 1145 s, 1126 m,
1082 m, 1043 m, 1029 m, 973 m sh, 957 m, 900 s, 863 m sh, 839 w,
796 s, 750 s sh, 682 m, 620 s. Phase purity was confirmed by X-ray
powder diffraction (Figure S2 in the Supporting Information)
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Synthesis of [Mn(salen)(AdCOO)] 3
H2-Salen (50 mg, 190 μmol) and “[Mn-AdCOO]” (70 mg, 230 μmol)
were combined in ethanol (10 mL) and heated at reflux for 2 hours.
On cooling to room temperature, the brown solution was filtered
and the filtrate subjected to diffusion of diethyl ether. After one
week, a small quantity of brown rod crystals were formed as a
minor phase, dispersed with a second, microcrystalline phase. An
individual crystal for X-ray diffraction studies was isolated directly
from this mixture. Elemental analysis of the bulk material is broadly
consistent with the formulation [{Mn(salen)(EtOH)}2(CO3)], with the
adamantanoate product 3 present as a minor phase only. Found C,
55.43, H, 5.57, N, 7.57%; Calculated for [{Mn(salen)(EtOH)}2(CO3)]
(C37H40N4O9Mn2) C, 55.93; H, 5.07; N, 7.05%.
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The authors gratefully acknowledge the School of Chemical and
Physical Sciences, Keele University, for funding support. Parts of
this work are supported by a Royal Society research grant to
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: crystal engineering · education · manganese ·
salen · team-based inquiry
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Manuscript received: April 27, 2020
Revised manuscript received: May 12, 2020
Accepted manuscript online: May 13, 2020
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