New insights in Mn–Ca chemistry from the use of oximate-based ligands: {MnII/III22Ca2} and {MnIV2Ca2} complexes with relevance to both low- and high-valent states of the oxygen-evolving complex

Article abstract of DOI:10.1016/j.poly.2018.04.020

The initial use of quinoline-2-aldoxime (qaoH) and 2,6-diacetylpyridine dioxime (dapdoH₂) in heterometallic Mn–Ca chemistry has afforded a mixed-valence {Mn^II/III₂₂Ca₂} cluster containing several {Mn₄CaO_x} subunits and a ‘butterfly’-like {Mn^IV₂Ca₂} complex, respectively. The one-pot reaction of Mn(O₂CPh)₂·2H₂O and Ca(O₂CPh)₂·H₂O with qaoH and NEt₃ in a 1:1:1:1 molar ratio in a solvent mixture comprising MeCN/MeOH gave complex [Mn₂₂Ca₂O₁₄(OH)₄(OMe)₆(O₂CPh)₂₂(qao)₂(MeCN)₂(H₂O)₄](OH)₂ (1; 6Mn^II/16Mn^III) in 20% yield. A similar reaction with that of 1, albeit with Mn(O₂CPh)₂·2H₂O and Ca(NO₃)₂·4H₂O in the presence of dapdoH₂ and NEt₃, in a molar ratio of 1:1:2:2 in MeCN/MeOH, led to the smaller nuclearity complex [Mn₂Ca₂(OMe)₂(NO₃)₂(dapdo)₄] (2; 2Mn^IV) in 40% yield. The reported compounds demonstrate structural and magnetic relevance to both low- and high-valent states of the OEC.

Full text of DOI:10.1016/j.poly.2018.04.020

Accepted Manuscript
New insights in Mn-Ca chemistry from the use of oximate-based ligands:
{MnII/III 22Ca2} and {MnIV 2Ca2} complexes with relevance to both low- and
high-valent states of the oxygen-evolving complex
Alysha A. Alaimo, Dimitris I. Alexandropoulos, Christos Lampropoulos,
Theocharis C. Stamatatos
PII:
S0277-5387(18)30197-9
https://doi.org/10.1016/j.poly.2018.04.020
POLY 13122
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
8 February 2018
17 April 2018
Please cite this article as: A.A. Alaimo, D.I. Alexandropoulos, C. Lampropoulos, T.C. Stamatatos, New insights in
Mn-Ca chemistry from the use of oximate-based ligands: {MnII/III 22Ca2} and {MnIV 2Ca2} complexes with relevance
to both low- and high-valent states of the oxygen-evolving complex, Polyhedron (2018), doi: https://doi.org/
10.1016/j.poly.2018.04.020
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New insights in Mn-Ca chemistry from the use of oximate-based ligands:
{MnII/III22Ca2} and {MnIV Ca2} complexes with relevance to both low- and
2
high-valent states of the oxygen-evolving complex
Alysha A. Alaimo,a Dimitris I. Alexandropoulos,a Christos Lampropoulosb and Theocharis
C. Stamatatos*a
a
Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, L2S 3A1 St.
Catharines, Ontario, Canada
b
Department of Chemistry, University of North Florida, 1 UNF Dr., Jacksonville, Florida
32224, USA
Dedicated to Professor Spyros P. Perlepes on the occasion of his 65th birthday and his
contributions to coordination chemistry and molecular magnetism communities
Keywords:
Mn-Ca complexes
Crystal structures
Oximato-based ligands
Oxygen-evolving complex
Magnetism
*Corresponding author: Dr. Theocharis C. Stamatatos (E-mail: tstamatatos@brocku.ca; Tel: +1-
905-688-5550 ext. 3400)
1

Abstract
The initial use of quinoline-2-aldoxime (qaoH) and 2,6-diacetylpyridine dioxime
(dapdoH2) in heterometallic Mn-Ca chemistry has afforded a mixed-valence {MnII/III22Ca2}
cluster containing several {Mn4CaOx} subunits and a ‘butterfly’-like {MnIV Ca2} complex,
2
respectively. The one-pot reaction of Mn(O2CPh)2∙2H2O and Ca(O2CPh)2∙H2O with qaOH and
NEt3 in a 1:1:1:1 molar ratio in a solvent mixture comprising MeCN/MeOH gave complex
[Mn22Ca2O14(OH)4(OMe)6(O2CPh)22(qao)2(MeCN)2(H2O)4](OH)2 (1; 6MnII/16MnIII) in 20%
yield. A similar reaction with that of 1, albeit with Mn(O2CPh)2∙2H2O and Ca(NO3)2∙4H2O in the
presence of dapdoH2 and NEt3, in a molar ratio of 1:1:2:2 in MeCN/MeOH, led to the smaller
nuclearity complex [Mn2Ca2(OMe)2(NO3)2(dapdo)4] (2; 2MnIV) in 40% yield. The reported
compounds demonstrate structural and magnetic relevance to both low- and high-valent states of
the OEC.
1. Introduction
The burgeoning development of polynuclear metal cluster chemistry in recent years was
spearheaded by the intense investigation of novel organic and/or inorganic bridging and
chelating ligands [1,2]. Ligands with N- and/or O-donor atoms have also contributed to the
emergence of many diverse physicochemical properties of cluster compounds. In the molecular
magnetism arena, for instance, the ligands were found to significantly affect the nature of the
magnetic exchange interactions between the metal ions [3], the orientation of magnetic
anisotropy axes [4], and subsequently the magnetization dynamics of the molecular compounds
[5]. Furthermore, different classes of organic bridging/chelating ligands have been employed as:
(i) antennas for energy transfer purposes in the field of optics [6], (ii) spectators and actors in
catalysis [7], and (iii) contrast agents in magnetic resonance imaging [8].
Bioinorganic chemistry is a thriving field of research and the impact of the ligands on the
biocatalytic and biomimetic properties of polynuclear metal complexes has been manifested by
the specific arrangement of the metal ions within the active site of Photosystem II (PSII) [9].
PSII is a multicomponent assembly of proteins and cofactors [10] that absorbs four photons to
2

sequentially oxidize a {Mn4CaO5} cluster compound (Scheme 1), known as the water-oxidizing
complex (WOC) or the oxygen-evolving complex (OEC), through a four-electron process,
namely the Kok cycle [11]. The latter involves a sequence of four Sn Kok states (n = 0 to 4). The
most oxidized state of the Kok cycle is S4, which participates in the O2 release, while the S1 is
dubbed as the dark-adapted or dark-stable state [11,12]. The OEC consists of an oxido-bridged
{Mn3CaO4} distorted cubane unit linked to a fourth, dangling Mn atom through one of the unit’s
oxido bridges and an additional bis-μ-O2- group (O5; Scheme 1) [13]. The inorganic core of the
native catalytic site is surrounded by bridging amino acid residues and terminally ligated H2O
molecules or water-derived ligands (W1-W4; Scheme S1). This ligand organization within the
OEC prompted scientists to attempt bench-top synthesis of structural analogues with similar -if
not the same- geometrical, electronic and eventually catalytic properties [14]. Such synthetic
endeavors pose several challenges, including (i) the formation of an oxido-bridged {Mn4Ca}
core with an extended cubane structure, (ii) the stabilization of Mn ions in high oxidation states,
and (iii) the choice of a ligand framework that would ideally support stability of the inorganic
core in solution and access to multiple oxidation states.
Scheme 1. Latest model for the {Mn4CaO5} inorganic core of the native OEC in PSII.
Thus, the quest for new organic chelating/bridging ligands for heterometallic Mn-Ca
chemistry is ongoing, with efforts focused on the incorporation of ligand moieties satisfying the
aforementioned requirements. Simple carboxylates, hydroxamic acids, and polyalcohol-based
ligands have been used in the synthesis of Mn-Ca complexes, leading to species with diverse
topologies, oxidation state descriptions, and spectroscopic/physicochemical properties [14].
Oximes and dioximes have been extensively used in homometallic Mn cluster chemistry geared
3

toward magnetism applications [15]; however, this class of ligands has no precedence in the Mn-
Ca literature of biomimetic species. These ligands exhibit some interesting structural and
electronic properties, among of which are their ability to (i) stabilize Mn ions in high oxidation
states [16], (ii) support the formation and crystallization of Mn-O2- inorganic cores [17], and (iii)
potentially coordinate to oxophilic metal ions, such as CaII, due to the strong α-nucleophilic
character of the oximate functionality. In addition, oximes are photosynthetically effective
ligands and, in conjunction with hydrazine and nitric oxide, can reduce the OEC to S-1, S-2 and S-3
states [18].
We herein report the initial employment of quinoline-2-aldoxime (qaoH) and 2,6-
diacetylpyridine dioxime (dapdoH2) in heterometallic Mn-Ca chemistry (Scheme 2), which has
afforded
the
complexes
and
[MnII MnIII16Ca2O14(OH)4(OMe)6(O2CPh)22(qao)2(MeCN)2(H2O)4](OH)2
(1)
6
[MnIV Ca2(OMe)2(NO3)2(dapdo)4] (2), respectively. Complex 1 is the highest in nuclearity Mn-
2
Ca cluster reported to date [19] and, although too high in nuclearity to model the exact structure
of the native OEC, the presence of cubane-containing subunits within its structure render it a
promising foundation for the low-valent species of OEC. Complex 2 is the first of its metal
stoichiometry (Mn:Ca = 2:2) to have the MnIV ions in such a high oxidation state and therefore it
can be related to one of the high-valent S3 or S4 states of the Kok cycle [11,12].
Scheme 2. Structural formulae and abbreviations of the oxime ligands used in this study.
2. Experimental
2.1. Materials and general methods
4

All manipulations were performed under aerobic conditions using chemicals and solvents
as received, unless otherwise stated. The organic ligands qaoH and dapdoH2 were prepared as
described elsewhere [20]. The starting materials Mn(O2CPh)2∙2H2O and Ca(O2CPh)2∙H2O were
synthesized in quantitative yields (>95%) from the reaction between MnCl2∙4H2O or
CaCl2∙2H2O and excess of PhCO2H in MeOH/H2O (1:1 v/v) under basic conditions.
2.2. Preparation of complexes
[Mn22Ca2O14(OH)4(OMe)6(O2CPh)22(qao)2(MeCN)2(H2O)4](OH)2 (1): To a stirred,
yellow solution of qaoH (0.03 g, 0.2 mmol) and NEt3 (28 μL, 0.2 mmol) in MeCN/MeOH (15:3
mL) the solids Mn(O2CPh)2∙2H2O (0.07 g, 0.2 mmol) and Ca(O2CPh)2∙H2O (0.06 g, 0.2 mmol)
were added together. The resulting dark orange suspension was stirred for 30 mins, during which
time all the solids dissolved, and the colour of the solution changed to brown. The solution was
filtered, and a mixture of Et2O/C6H14 (10 mL, 1/1 v/v) diffused into the filtrate. After a month,
dark red needle-like crystals of 1 appeared and were collected by filtration, washed with cold
MeCN (2 × 3 mL) and Et2O (2 × 3 mL), and dried in air; the yield was 20% (based on the total
available Ca). The crystalline solid was analyzed as 1∙4H2O. Anal. Calcd: C, 43.90; H, 3.40; N,
1.67%. Found: C, 43.75; H, 3.27; N, 1.81%. Selected IR data (cm‒1): 1594 (m), 1567 (m), 1534
(m), 1491 (w), 1447 (w), 1386 (vs), 1303 (w), 1173 (w), 1143 (w), 1067 (w), 1020 (m), 984 (w),
936 (w), 829 (w), 710 (vs), 686 (m), 675 (m), 634 (w), 600 (m), 568 (m), 476 (w), 450 (w).
[Mn2Ca2(OMe)2(NO3)2(dapdo)4] (2): To a stirred, colourless solution of dapdoH2 (0.08
g, 0.4 mmol) and NEt3 (56 μL, 0.4 mmol) in MeCN/MeOH (15:5 mL) the solids
Mn(O2CPh)2∙2H2O (0.07 g, 0.2 mmol) and Ca(NO3)2∙4H2O (0.05 g, 0.2 mmol) were added
together. The resulting ecru suspension was stirred for 2 h, during which time all the solids
dissolved, and the colour of the solution changed to dark brown. The solution was filtered and
left to evaporate slowly at room temperature. After one week, dark brown rod-like crystals of
2∙2MeCN appeared and were collected by filtration, washed with cold MeCN (2 × 3 mL) and
Et2O (2 × 3 mL), and dried in air; the yield was 40% (based on the total available Ca). The
crystalline solid was analyzed as lattice solvent-free 2. Anal. Calcd: C, 40.01; H, 3.71; N,
17.19%. Found: C, 40.22; H, 3.78; N, 17.12%. Selected IR data (cm‒1): 2808 (w), 1591 (m),
5

1533 (m), 1409 (m), 1365 (w), 1316 (m), 1278 (w), 1190 (w), 1167 (m), 1149 (m), 1118 (w),
1087 (m), 1065 (m), 1015 (s), 947 (m), 822 (w), 805 (s), 792 (m), 760 (m), 684 (m), 663 (s), 553
(vs), 478 (m), 440 (m).
2.3. Physical measurements
Infrared (IR) spectra were recorded in the solid state on a Bruker’s FT-IR spectrometer
(ALPHA’s Platinum ATR single reflection) in the 4000-400 cm-1 range. Elemental analyses (C,
H, and N) were performed on a Perkin-Elmer 2400 Series II Analyzer. Magnetic susceptibility
studies were performed in-house on a Quantum Design MPMS-XL SQUID susceptometer
equipped with a 7 T magnet and operating in the 1.8-400 K range. Samples were embedded in
solid eicosane to prevent torquing. Pascal’s constants were used to estimate the diamagnetic
correction, which was subtracted from the experimental susceptibility to give the molar
paramagnetic susceptibility (χΜ) [21].
2.4. Single-crystal X-ray crystallography
Crystals of the complexes 1 and 2 were selected and mounted on MiteGen dual thickness
micromounts TM using inert oil [22]. Diffraction data were collected at 100(2) K on a Bruker D8
VENTURE diffractometer equipped with a multilayer mirror monochromator and a Mo Kα
microfocus sealed tube (λ = 0.71073 Å). Images were processed with the software SAINT+ [23],
and absorption effects were corrected with the multi-scan method implemented in SADABS
[24]. The structures were solved using the Bruker SHELXTL inside the APEX-III software
package, and refined using the SHELXLE and PLATON programs [25]. The programs used for
molecular graphics were MERCURY [26] and DIAMOND [27].
Additonal information on the crystallographic data collection and structure refinement
details is summarized in Table 1.
Table 1. Crystallographic data for complexes 1 and 2
Parameter
1
2
6

Formula
Fw / g mol-1
Crystal type
Crystal size / mm3
Crystal system
Space group
a / Å
C184H162Mn22Ca2N6O76 C38H42Mn2Ca2N14O16
4962.13
1140.88
Brown needle
0.20 × 0.20 × 0.20
Triclinic
Dark brown rod
0.10 × 0.20 × 0.40
Monoclinic
P21/n
P-1
17.226(5)
18.581(4)
19.604(3)
68.823(4)
68.416(8)
68.981(8)
5253.1(2)
1
13.5003(12)
12.9259(12)
15.2726(13)
90
b / Å
c / Å
α / °
β / °
104.320(3)
90
γ / °
V / Å3
2582.3(4)
2
Z
Dc / g cm-3
μ / mm-1
 range / º
1.587
1.467
1.407
0.765
2.24 to 25.99
−21  h  21
−22  k  22
−24  l  24
221412
2.21 to 26.02
−16  h  16
−15  k  15
−18  l  18
39911
Index ranges
Reflections collected
Independent reflections
20527 (Rint = 0.0644)
R1 = 0.0531
wR2 = 0.1456
R1 = 0.0730
wR2 = 0.1597
5072 (Rint = 0.0773)
R1 = 0.0374
wR2 = 0.0871
R1 = 0.0596
wR2 = 0.0968
Final
R
indices
[I>2(I)]a,b
Final R indices (all data)
(Δρ)max,min / e Å-3
2.090 and -2.367
0.550 and -0.454
2
2 2
2
a R1 = (||Fo| – |Fc||)/|Fo|. b wR2 = [[w(Fo - Fc2)2]/ [w(Fo ) ]]1/2, w = 1/[σ2(Fo ) + [(ap)2 +bp],
2
where p = [max(Fo , 0) + 2Fc2]/3.
3. Results and discussion
7

3.1. Synthetic comments
The synthetic route followed for the synthesis of both 1 and 2 was based on the use of
simple metal salts (i.e., metal carboxylates and nitrates) in conjuction with the desired
chelating/bridging organic ligand (i.e., qaoH or dapdoH2) and an external base (i.e., NEt3) to
facilitate the deprotonation of the employed chelates. Therefore, the one-pot reaction of
Mn(O2CPh)2∙2H2O and Ca(O2CPh)2∙H2O with qaOH and NEt3 in a 1:1:1:1 molar ratio in a
solvent mixture comprising MeCN/MeOH gave a brown solution that, upon filtration and
layering with Et2O/hexanes, afforded dark red needle-like crystals of the mixed-valence complex
[Mn22Ca2O14(OH)4(OMe)6(O2CPh)22(qao)2(MeCN)2(H2O)4](OH)2 (1; 6MnII/16MnIII) in 20%
yield (based on the total available Ca). A similar reaction with that of 1, albeit with
Mn(O2CPh)2∙2H2O and Ca(NO3)2∙4H2O in the presence of dapdoH2 and NEt3, in a molar ratio of
1:1:2:2 in MeCN/MeOH, led to a dark brown solution that, upon filtration and slow evaporation
at room temperature, afforded dark-brown rod-like crystals of the new complex
[Mn2Ca2(OMe)2(NO3)2(dapdo)4] (2; 2MnIV) in 40% yield (based on the total available Ca).
The reactions that gave 1 and 2 are both oxidations, undoubtedly by O2 under the
prevailing basic conditions. Employment of different organic bases, such as NMe3, Bun N and
3
Me4NOH, did not afford crystalline materials but only oily products that we were not able to
further characterize. It becomes apparent that the reaction solvent has an important role in the
synthesis and crystallization of both 1 and 2; bridging MeO- groups and terminally-bound or
lattice MeCN molecules were found in the structures of 1 and 2 (vide infra). Reactions in
different alcohols did not yield any crystalline materials but only amorphous precipitates that we
were unable to re-dissolve and crystallize. Analogous reactions with different carboxylate-based
starting materials (i.e., acetates, propionates, pivalates, etc.), all failed to yield single-crystals
suitable for X-ray diffraction studies. Finally, by adjusting the experimental molar ratios of the
precursors to the stoichiometric equivalents, in an attempt to optimize the isolated yields, we
failed to reproduce the crystals of both complexes 1 and 2.
3.2. Description of structures
8

The formulae of 1 and 2 are based on metric parameters, charge-balance considerations,
and bond valence sum (BVS) calculations on the Mn and O atoms (ESI). The presence of OH-
counterions in 1 is not unusual in high-nuclearity metal cluster chemistry [28]. The molecular
structure of the centrosymmetric cation of 1 (Fig. 1, top) consists of 6 MnII, 16 MnIII and 2 CaII
ions held together by 12 µ4-O2‒, 2 µ3-O2‒, 4 µ3-OH‒, 4 µ3-OMe-, 2 µ-OMe‒ and 2 µ4-oximate
groups from two η1:η1:η3:µ4 qao- ligands (Scheme S1). Peripheral ligation about the
[MnII MnIII16Ca2(µ4-O)12(µ3-O)2(µ3-OH)4(µ3-OMe)4(µ-OMe)2(µ4-NO)2]22+ core (Fig. 1, bottom)
6
-
is provided by 22 PhCO2 groups that are either µ- or µ3-bridging and terminally bound (Scheme
S1), as well as two and four terminally coordinated MeCN and H2O molecules located on the
Ca1/Ca1′, Mn7/Mn7′ and Mn10/Mn10′ pairs, respectively. Four coordinated H2O molecules are
also present in the active site of the native OEC, two of which are bound to a Mn ion [13]. These
H2O molecules could serve as substrates for the overall catalytic reaction to proceed, including
subsequent deprotonations with metal-centered oxidations preceding O-O bond formation [11-
13].
9

Mn10
Mn4
O13
Mn2
Mn7′
O2
Mn1
O6′
O5
O1
O14
Mn5
O4′
O8
Mn11′
Mn6′
Mn8′
Mn3
N3
O10
Mn9
O7′
O10′
Ca1′
O11
O9′
O3′
O11′
N3′
Mn3′
O1′
O3
O9
O7
Ca1
Mn5′
Mn9′
Mn11
O5′
Mn8
Mn6
O8′
O4
O14′
Mn1′
O6
O2′
Mn7
Mn2′
Mn4′
Mn10′
O13′
Fig. 1. Molecular structure of the cation of 1 (top) and a labelled representation of its complete
core (bottom). Colour scheme: MnII purple, MnIII blue, CaII yellow, O red, N green, C gray. H
atoms are omitted for clarity. Symmetry operation for the primed atoms in 1: 1-x, 1-y, 1-z.
The unique topology of 1 can be described as four {MnIIMnIII O4} cubanes that are
3
connected to each other through a central {Mn2O4} unit comprising the atoms Mn8, Mn8′, O7
and O7′. Each of the external cubane subunits is further linked to a {MnIIMnIIICa(μ3-ΝΟ)} unit
through the bridging oxido, methoxido and oximate groups. As a result, within 1, there are four
symmetry-related {Mn4O4} distorted cubanes attached to two “dangling” CaII atoms. The
structures of these {Mn4CaO5} and {Mn4CaO6} subunits (Fig. 2) are intriguing, especially when
compared to the OEC. In particular, they possess a similar extended cubane topology to that of
the native site, with Mn∙∙∙Mn and Mn∙∙∙Ca closest separations of 2.876-3.686 Å and 3.376-3.756
Å, respectively, comparable to the corresponding values of 2.7-3.3 Å and ~3.4 Å for the OEC in
PSII. All Mn atoms in 1 are six-coordinate with distorted octahedral geometries. In the case of
the MnIII atoms [Mn(2-9) and their symmetry-related partners], the octahedra are axially
elongated due to the Jahn-Teller (JT) distortions, as expected for high-spin d4 ions in this
geometry. In addition, both CaII atoms in 1 are five-coordinate with an almost perfect square
10

pyramidal geometry (τ = 0.05) [29]. Finally, the space-filling representation (Fig. S2) shows that
1 adopts a nano-sized ellipsoidal conformation with dimensions of ~26 and ~20 Å, defined by
the longest C∙∙∙C distances, excluding the H-atoms.
Mn3
Mn10
O3
Mn11′
O14
O5
O13
O8
Ca1
O9
Mn4
O1
O11
Ca1
Mn9
Mn2
Mn5
Mn7′
O2
O11
O6′
Fig. 2. Two types of {Mn4Ca} extended cubane subunits found within 1. Colour scheme as in
Fig. 1.
The structure of 2 (Fig. 3) contains an unusual ‘butterfly’-like core where the high-
valence MnIV ions are located at the wing-tip positions and the two CaII ions occupying the
‘body’ sites. The metal ions are bridged by two μ-OMe- (O1, O1′) and the oximate arms of two
η1:η1:η1:η1:η1:µ3 and two η1:η1:η1:η2:µ3 dapdo2- ligands (Scheme S2). The dioximate ligands
appear to both chelate and bridge the MnIV and CaII ions by employing all four of their donor
atoms albeit under different configurations. This emphasizes the versatility, flexibility and ability
of dapdo2- to stabilize metal ions in various oxidation states, diverse sizes and different intrinsic
characteristics (hardness, acidity and oxophilicity). The six-coordinate MnIV ions possess
-
distorted octadedral geometries, whereas two bidentate chelating NO3 groups complete eight-
coordination around each CaII ion. The coordination geometry of both CaII ions is very distorted;
the program SHAPE [30] was used to determine the CShM values of 3.71 and 3.88 for square
antiprismatic and triangular dodecahedral geometries, respectively (Fig. S3, ESI). The Mn∙∙∙Mn
and Mn∙∙∙Ca distances are 6.717 Å and 3.561/4.067 Å, respectively. Complex 2 is the first non-
oxido bridged Mn-Ca complex that contains exclusively MnIV ions; this is clearly supported by
the oximate arms of dapdo2- ligands.
11

O8
N7
O5
N5′
O6
O7′
N6′
O3′
N4′
Ca1
O2′
O4
Mn1
N2
N2′
Mn1′
O4′
N3
O1
N1
N1′
O1′
O2
Ca1′
N3′
N4
O3
N6
O7
O6′
N5
O5′
N7′
O8′
Fig. 3. Labelled structure of 2. Colour scheme: MnIV olive green, CaII yellow, O red, N green, C
gray. H atoms are omitted for clarity. Symmetry operation for the primed atoms in 2: 1-x, 1-y, 1-
z.
Complexes 1 and 2 do not appear to retain their solid-state structures in solutions of
various solvent media (i.e., MeCN, CH2Cl2, and THF), as it was confirmed by electrospray
ionization mass spectrometry. We have instead focused on the solid-state characterization, which
includes IR spectroscopy (ESI) and magnetic studies.
3.3. Magnetic susceptibility studies
Variable-temperature direct-current (dc) magnetic susceptibility measurements were
performed on freshly-prepared and analytically-pure microcrystalline samples of 1 and 2 in the
temperature range 2-300 K in an applied field of 0.1 T. The data are shown as χΜT vs. T plots in
Fig. 4. The value of the χΜT product for 1 at 300 K is 41.74 cm3mol-1K, significantly lower than
the value of 74.25 cm3mol-1K (calculated with g = 2) expected for 6 MnII and 16 MnIII non-
interacting ions. The χΜT of 1 steadily decreases with decreasing T, reaching a value of 1.56
cm3mol-1K at 2 K. The overall magnetic response of 1 is indicative of predominant
12

antiferromagnetic exchange interactions between the metal centers, and the low χΜT value at 2 K
suggests a very small spin ground state value. The latter was confirmed to be S = 0 by
extrapolation of the χ′ΜT ac data down to 0 K (Fig. S4). The χ′ΜT is heading to a value of ~0
cm3mol-1K, in agreement with an S = 0 ground state. The structure of 1 is too complicated to
allow for any accurate and precise rationalization of the S = 0. Although discrete {Mn4O4}
cubanes at various oxidation state descriptions are generally known to be ferromagnetically
coupled for a wide range of Mn-O-Mn angles [31], such cubane subunits in 1 are linked to each
other via oxido groups, one of the strongest antiferromagnetic couplers in molecular magnetism.
The MnIV ions in complex 2 are well separated from each other and there is very weak to
negligible magnetic interaction between them, as indicated by the temperature independence of
the χΜT product from 300-5 K. The χΜT of 2 remains essentially constant to a value of ~3.7
cm3mol-1K, very close to the theoretical value of 3.75 cm3mol-1K (calculated with g = 2) for
two non-interacting MnIV (d3; S = 3/2) ions. The low-T decrease (< 5 K) of the χΜT could be
tentatively ascribed to the onset of some weak magnetic coupling between the metal centres, of
either intra- or intermolecular origin, zero-field splitting and/or Zeeman effects from the applied
dc field.
50
{Mn22Ca2} (1)
{Mn2Ca2} (2)
40
30
20
10
0
0
50
100
150
200
250
300
Temperature (K)
Fig. 4. χΜT vs. T plots for 1 and 2 in a 1 kG field.
13

4. Conclusions and perspectives
In conclusion, we have shown that oximate and dioximate chelating/bridging ligands can
support the formation and crystallization of structurally new Mn-Ca complexes with diverse
nuclearities, topologies, metal stoichiometries and oxidation state descriptions. Complexes 1 and
2 join a growing, but still small, list of structurally characterized Mn-Ca complexes [14, 19, 32],
thus emphasizing the continuous need for the development of new ligand systems and synthetic
routes for the isolation of new structural models of the OEC. Although the overall structures of
the reported compounds are strictly not similar to that of the native OEC, we nevertheless believe
that this work provides a foundation from which to tackle the synthesis of the discrete {Mn4Ca}
unit responsible for the O2 gas evolution. This could be accomplished by modifying the reaction
conditions that yielded 1 to foster lower nuclearity products, hopefully with the correct topology
and metal stroichiometry as those of the OEC. The incorporation of bridging oxido groups in 2
could also facilitate the formation of MnIV-Ca/O2- complexes with dioximate ligands. Both the
described strategies, and others, are currently under investigation.
Acknowledgements
This work was supported by the NSERC-DG, ERA and Brock University (to Th.C.S), the
Research Corp. (to C.L), and the National Science Foundation (grants DMR-1429428 / DMR-
1626332 to C.L).
Appendix A. Supplementary data
Crystallographic data (excluding structure factors) for the structures of complexes 1 and 2
have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as
supplementary publication numbers: CCDC-1588820 (1), -1588821 (2). Copies of these data can
be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 2EZ, UK;
FAX: (+44) 1223 336033, or online via www.ccdc.cam.ac.uk/data_request/cif or by emailing
data_request@ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in
the online version, at https://doi.org/...
14

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17

SYNOPSIS
The initial use of quinoline-2-aldoxime (qaoH) and 2,6-diacetylpyridine dioxime (dapdoH2) in
heterometallic Mn-Ca chemistry has afforded a mixed-valence {MnII/III22Ca2} cluster containing
several {Mn4CaOx} subunits and a ‘butterfly’-like {MnIV Ca2} complex, respectively; the
2
reported compounds demonstrate structural and magnetic relevance to both low- and high-valent
states of the OEC.
18

PICTOGRAM
19