Redox Active Hexanuclear Mixed Valence Dicationic Ce(III)/Ce(IV) Coordination Clusters

Article abstract of DOI:10.1002/ejic.202000659

A series of mixed valence hexanuclear dicationic coordination clusters containing two μ₄-O^2– bridges with general formula M²⁺(X^–)₂ [M = Ce^IV₂Ce^III₄(μ_4<

Full text of DOI:10.1002/ejic.202000659

Communication
doi.org/10.1002/ejic.202000659
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Coordination Chemistry
Redox Active Hexanuclear Mixed Valence Dicationic Ce(III)/
Ce(IV) Coordination Clusters
Selvakumar Arumugam,[a] Bhaskaran Shankar,[a] and Kartik Chandra Mondal*[a]
Abstract: A series of mixed valence hexanuclear dicationic co- fraction. The magnetic susceptibility measurements of M2+(X–)2
ordination clusters containing two μ4-O2– bridges with general showed that the Ce(III) ions are weakly antiferromagnetically
formula M2+(X–)2 [M = CeIV2CeIII4(μ4-O)2(L–R)4(val)6(H2O)2; 12+
3
,
coupled. Thermal stability and molar conductivity of M2+(X–)2
2+: R = H, 22+: Me; 12+, 22+: X = CeIII2(val)3(NO3)4, 32+: X = NO3] were also studied. Complex 32+(X–)2 was further studied
were synthesized, isolated and characterized. The reaction of by X-ray photoelectron spectroscopy to confirm the oxidation
cerium(III)nitrate hexahydrate with Schiff base ligand (H2L–R) states of Ce(III/IV) ions. The M2+ cation was shown to catalyze
and o-vanillin (val-H) under basic condition led to the formation the TEMPO-free oxidation of functionalized benzyl alcohols
of M2+(X–)2 and the reaction progress was monitored by mass quantitatively to the corresponding benzaldehyde derivatives
spectrometric studies. The molecular structures of all complexes at 100 °C in presence of aerial O2 using DMF as the solvent.
were unambiguously characterized by single-crystal X-ray dif-
cerium ions. The isolation and characterization of mixed valence
Ce complexes have been rarely documented so far.[9,10] In this
context, it is noteworthy to mention that only one mixed va-
Cerium is a unique f-block element with two possible redox
active oxidation states (III/IV),[1] structural diversities and various
applications in different fields of chemistry.[1–15] Some of the
cerium complexes are extremely interesting for studying multi-
ple bonds between Ce-atom and Ccarbene- [polarized covalent
Ce(IV)–C σ- and π-multiple bond interactions with significant
covalent component],[2] nitrogen-[3] and oxygen-atoms.[2,3]
Complexes containing such chemical bonds are highly reactive
towards certain reactants. Ce(III) ions with certain coordination
geometries in suitable ligand fields are known to display single-
ion magnet (SIM) property.[4] Ce(III)-containing[5] complexes are
also reported to exhibit luminesce. Recently, the quantum yield
for the UV-luminophores of tris(guanidinate)-Ce(III) complex has
been shown to be enhanced by controlling the steric crowding
induced cone angles around Ce(III) ions.[5] Considering the oxid-
ation states, Ce(III) ions are more stable than Ce(IV) ions in gen-
eral. However, Ce(III) ions can easily undergo aerial oxidation to
form Ce(IV) ions in solution when coordinated by hard donor
ligands (e.g., N-atom) with higher electrostatic interactions.[6,7]
Ce–N/O bonds are more electrostatic in nature rather than
covalent, electron sharing bonds due to the difference in their
electronegativity values. Additionally, the counter cations like
Li+, Na+ and K+ also might play a crucial role in the redox poten-
tial of the Ce(III)/Ce(IV) system.[8] As a result, the ratio of Ce(III)
and Ce(IV) ions in solution largely depends upon the nature of
the reactants present and the donor atoms coordinated to the
lence CeIVCeIII complex,[9] an anti-ferromagnetically coupled
2
mixed valence CeIV7CeIII [10a] unit encapsulated within polyoxo-
3
tungstanates and two other high nuclearity Ce-clusters[10b]
have been reported till date (Scheme 1).
Scheme 1. Two selected previously reported mixed valence CeIV2CeIII and
CeIV3CeIII cores.
7
Oxidation of Ce(III) ions to Ce(IV) ions in solution has not
been thoroughly studied so far. Very recently, Ce(IV)-oxo com-
–
plexes were obtained via inner sphere nitrate (NO3 ) reduc-
–
tion[11] and by photolysis[12] of nitrate anion (NO3 ) coordinated
to Ce(III) ions. These synthetic reports[11–12] are quite captivat-
ing and helpful to rationalize the fate of a reaction involving
cerium ions in the absence of molecular oxygen as oxidizing
agents. The oxidation of Ce(III) ions to Ce(IV) ions is very impor-
tant and often desired. The use of Ce(III) ion is fruitful as a
precursor for the preparations of Ce(IV) containing com-
plexes.[13] It has been found that Ce(III) ions which are coordi-
nated by hard donors (e.g., the nitrogen donor ligands along
with alkoxide ligands), spontaneously undergo oxidation to
[a] S. Arumugam, Dr. B. Shankar, Dr. K. C. Mondal
Department of Chemistry, Indian Institute of Technology Madras
Chennai 600036, India
E-mail: csdkartik@iitm.ac.in
http://chem.iitm.ac.in/faculty/kartik/#
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000659.
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produce Ce(IV) ions when the reactions are carried out under containing two μ4-O2– bridges [H2L–R = diprotic Schiff base li-
aerobic conditions.[14–15] A few Ce complexes[16–17] have been gand, R = H, Me]. In addition, the TEMPO-free catalytic oxid-
shown to act as efficient catalysts too. For example, a Ce(IV) ations of functionalized benzylic alcohols by 32+ were shown to
containing complex has been reported to be utilized as catalyst proceed smoothly with benzylic radical stabilizing functional
for the alcohol oxidation in the presence of TEMPO.[16] A visible groups. The Ce(III) ion of cerium(III) nitrate hexahydrate is not a
light mediated δ-selective C–H functionalization of alkanols has free Ce3+ ion in solution; rather, it is coordinated by three
been shown to be catalyzed by a Ce(IV)-chromophore via in- chelating nitrates and four water ligands to form
duced ligand-to-metal charge transfer process.[17] Both Ce(III) [Ce(NO3)3(H2O)4·2H2O] having ten coordinated Ce(III) ions
and Ce(IV) ions have been stabilized by nitroxide containing (Scheme 2). Two lattice water molecules are outside the primary
ligands.[19] The redox chemistry at cerium center [Ce(III)–Ce(IV)] coordination sphere and form H-bonds with the nitrates on
has been shown to be easily controlled when tripodal nitroxide Ce(III) ion.
A 2:3:3:8 molar mixture of H2L–R, o-val-H,
ligands are chelated to the cerium centers.[20] Furthermore, Ce(NO3)3(H2O)4·2H2O and Et3N was reacted in acetonitrile
thermolysis of cerium complexes leads to the formation of ceria (MeCN) for 2 h leading to the formation of dark brown solution
(CeO2) which is used for catalytic conversion of black carbon (see SI for details). Dark black rods of complexes M2+(X–)2
particle into carbon dioxide (CeO2-catalyzed soot combustion [for 12+ and 22+: M = CeIV2CeIII4(μ4-O)2(L–R)4(val)6(H2O)2, X = CeI-
in car by Ford Motor Company).[18] There are literature prece-
2(val)3(NO3)4, R = H for 12+, R = Me for 22+] were isolated in
II
dents for polymeric CeIII3-caboxylate network,[21] clusters con- 20–25 % yield after one week by slow evaporation of MeCN
taining Ce(IV)4(μ4-O),[22] CeIV4(μ4-O)(μ-O)2,[23] and their perxo- solution at room temperature (Scheme 2).
analogue.[24] CeIV3(μ-O)3 and CeIV6(μ3-O)4(μ3-OH)4 cores have
When the reaction was carried out in the molar ratio of
been stabilized by heteropolyoxometalates.[25] CeIV6(μ3-O)8, 2:6:5:11 in MeCN, the yield of M2+(X–)2 was increased to 45–
[6]
[26–27]
CeIV6(μ3-O)4(μ3-O)4 and CeIV6(μ6-O)(μ-OH)x
been stabilized by carboxylate ligands. Syntheses and charac- Ce(NO3)3(H2O)4·2H2O and Et3N was reacted in methanol
terization of polynuclear coordination clusters of mixed valence (MeOH) for 2.5 to afford dark brown solution of
units have 47 %. However, the 2:3:3:8 molar mixture of H2L–R, val-H,
h
a
Ce(III/IV) ions are rare.[9–10] Schiff base ligand (containing two M2+(X–)2. Dark black blocks/hexagons of M2+(X–)2 [for 32+: M =
phenolic OH groups) stabilized the mono nuclear Ce(III)/Ce(IV) CeIV2CeIII4(μ4-O)2(L–R)4(val)6(H2O)2, R = H, X = NO3; (Scheme 3)]
complexes[16,28] and the dinuclear Ce(IV) complexes.[16] Here, were isolated in much improved yield of 75 % (See SI). When a
we report the syntheses, isolation and characterization of similar reaction was carried out in the same molar ratio but in
dicationic complexes of general formula M2+(X–)2 [for 12+ and half of the volume of MeOH, complex 4+(X–) [Ce(IV)3Ce(III)3(L)4-
–
2
2+: M = CeIV2CeIII4(μ4-O)2(L–R)4(val)6(H2O)2, R = H for 12+
,
(val)6(O)2(OCH3)2]+(NO3 ) (Scheme 3) was isolated as crystalline
R = Me for 22+, X = CeIII2(val)3(NO3)4, for 32+: R = H, X = NO3], brown powder which was characterized by ESI mass spectrome-
Scheme 2. Syntheses of complexes 12+(X–)2 and 22+(X–)2 val-H = o-vanillin.
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Scheme 3. Syntheses of complexes 32+(X–)2, 4+(X–) and 52+(X–)2 in MeOH (X = NOH3).
try as [Ce(IV)3Ce(III)3(L)4(val)6(O)2(OCH3)2]+ (m/z = 2803.9919) ing at the bottom of the conical flask after five days. The mass
and by other characterization techniques as well (see SI).
The previously reported reaction of Dy(NO3)3(H2O)4·2H2O
with H2L ligand produced the doubly μ3-OH bridged tetranu-
clear Dy(III)4(μ3-OH)2(L)4(HL)2 cluster.[29] Only a very few exam-
ples of mononuclear Ce-containing complexes have been char-
acterized by MALDI-TOF mass-spectrometry till today.[30] Very
recently several Ln-ion containing coordination clusters (Eu21,
Dy1-Dy10) with O/N-donating ligands have been reported.[31]
The solution dynamics of the corresponding reactions were
studied by mass spectrometry.[31] There has been a report with
the same ligand on the mechanistic investigation of the forma-
tion of Dy4 complexes in solution by mass spectrometric analy-
sis.[31d] A similar[29] reaction with Ce(NO3)3(H2O)4·2H2O led to
the formation of a light brown solution after 5 min of stirring
of the reaction mixture. The progress of the reaction was moni-
tored by ESI mass spectrometry (Figures S39–58) which showed
spectrum of these solutions showed the presence of the
dicationic [Ce6(O)2(L)4(val)6]2+ ions in solution (Scheme 3). The
Ce(III) ion is a redox active cation with E1/2 = 1.61 V and can
form Ce(IV) ion under certain reaction conditions.[6–8,14–15,19–27]
Ce(III) ion is prone to undergo oxidation when a reaction is
carried out under aerobic condition in the presence of a nitro-
gen donating[22–27] base/ligand possibly via formation of
Ce(III)(NO3)3(N-donating ligands) species as an intermediate. Re-
cently, Ce(IV)-oxo complexes were obtained via inner sphere
–
nitrate (NO3 ) reduction.[11] When we carried out the similar
reaction under argon atmosphere, the color of the solution re-
mained yellow even after one day suggesting that the aerial
oxygen molecules are required to oxidise Ce(III) to Ce(IV).[22–27]
The latter ion is responsible for the brown color of the reaction
mixture. In our case, the mixed oxygen (O3) and nitrogen (N)
donor dianionic (L–R)2– ligand provide sizeable ligand field to
oxidise Ce(III) and Ce(IV) ion in the presence of aerial oxygen
molecule (Scheme 2). The O-atom donating o-vanillin ligated
Ce(III) is less prone to undergo oxidation to Ce(IV). The ESI mass
spectrum recorded five days after the completion of reaction
under inert atmosphere showed the existence of dicationic
[Ce6(L)4(val)6(O)2]2+ {m/z = 1370.9896} indicating that the yellow
colored complex, [Ce6(L)4(val)6(O)2] can form under basic condi-
tions and subsequently can undergo aerial oxidation. When
H2L, H-val, Ce(NO3)3(H2O)4·2H2O and Et3N were reacted in the
molar ratio of 2:3:3:8 in MeCN, after 0.5 h the color of the reac-
tion solution changed to brown. The ESI mass spectrum
showed the formation of the tetranuclear Ce(III)4 complex
[Ce4(O)(L)4(H-val)2]+ {m/z = 1844.00} is formed (see Table S10)
which is in equilibrium with three Ce(III)4 complexes
[Ce4(O)(L)4(val)2(NO3)-H]+ {m/z = 1902.9707; H2L-H = H2L},
the formation of
a
tetranuclear coordination cluster,
[Ce4(O)(L)4(val)2(NO3)]-2H as an intermediate species. This sug-
gested the partial hydrolysis of H2L ligand to provide the source
–
of val– anion [with a lower isolated yield of 32+(NO3 )2, 15 %].
Thorough analysis of the mass spectra revealed the presence of
additional coordinated nitrate ions. The same reaction was also
carried out in pure MeOH, MeCN and mixture of solvents
MeOH/MeCN (3:1) to study the effect of solvent on the reaction
progress and the fate of the intermediates/products. The mass
analysis showed the formation of hexanuclear, monocationic
cerium clusters, [Ce6(O)2(L)4(val)6]+ and [Ce6(O)2(L)4(val)6-
(OMe)2]+ after 3 h when the reaction was carried out in a mix-
ture of MeOH/MeCN (3:1) (See SI, Figures S39–57). The forma-
tion of hexanuclear cerium complex was not observed when
the reaction was carried out either in MeOH or in MeCN sepa-
rately (after 2.5 h). The mixed solvent system was found to be [Ce4(O)(L)4(val)2(NO3)2(NO)-H]+ {m/z
=
1931.9614} and
essential for the formation of the Ce6 cluster as concluded by [Ce4(O)(L)4(val)2(NO3)2(NO)-3H]+ {m/z
=
1992.9414} (Figures
the ESI mass spectrometry. Dark black rods/blocks started form- S39–58). Additionally, there were a few more Ce(III)4 complexes
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in solution which were also observed to be present in different S82 for details). The masses of higher values corresponding to
compositions (m/z
=
2021.96, 2085.94, 2239.97, 2329.18; Ce6 were only observed (m/z 2760–2895) in the recorded mass
Scheme 5, Table S10).
spectrum. The overall progress of the reaction was summarized
in Scheme 4 and details of the experiment was given in the
Figures S79–S83.
Two similar reactions were carried out in MeOH and in a
mixture of MeOH/MeCN (3:1). The MeOH solution showed that
three complexes, [Ce4(O)(L)4(val)2(NO3)-H]+ {m/z = 1902.9707;
H2L-H = H2L}, [Ce4(O)(L)4(val)2(NO3)(NO)-2H]+ {m/z = 1931.9614}
and [Ce4(O)(L)4(val)2(NO3)2(NO)-3H]+ {m/z = 1992.9414} are pre-
dominating till one hour of stirring of reaction solution (See
Table S11). A few red crystals of the corresponding Ce4(O)-ana-
logue were isolated as complex [CeIII4(μ4-O)(L–H)4(HLH-H)3
To study the growth of the nuclearity, a similar reaction was
carried out in H2L, H-val, [Ce(NO3)3(H2O)4·2H2O + Dy(NO3)3-
(H2O)4·2H2O] and Et3N in the molar ratio of 2:3:[2:1]:8 in MeOH/
MeCN (3:1). ESI spectrum showed the exchange of the lanthan-
ides; a Ce(III)4 intermediate transforms to Ce(III)3Dy(III) interme-
diates (See Figure S58). The formation of hexanuclear species
was not observed by mass spectrometry after 3 h. The ionic
radius of Dy(III) is smaller than Ce(III) due to the lanthanide
contraction and hence, Ce(III)3Dy(III) unit forms easily by replac-
ing a Ce(III) ion in the Ce(III)4 unit.
(H-val)]2+(NO3 )2 {52+(NO3 )2} [(L–H)2– = dianionic Schiff base
ligand, HLH-H = zwitterionic form of H2L-H Schiff base ligand
with two acidic protons] under slightly modified reaction condi-
tions (Scheme 3, Figure 2). Additionally, two Ce(IV)Ce(III)4
–
–
complexes [Ce5(L)6(val)2(NO3)]+ {m/z
=
2510.0374} and
–
The yield of the Ce6 cluster [32+(NO3 )2] was low when only
[Ce5(L)6(val)(H-val)(NO3)2]+ {m/z = 2573.16} were formed. The ESI
mass spectra were recorded after 1.5 and 2.0 h as well. All the
above mentioned Ce(III)4 complexes and the two pentanuclear
Ce(IV)Ce(III)4 complexes were observed in ESI mass spectra (See
Table S11). However, no peak was observed related to the mass
of Ce6 unit [monocation and dication 3+ and 32+].
Ce(NO3)3(H2O)4·2H2O was treated with H2L-H in the presence of
Et3N since it involves the partial hydrolysis of H2L-H ligand for
the generation of o-vanillate anion which are required to stabi-
lize the mixed valence Ce(IV)2Ce(III)4(O)2 core along with di-
anionic ligand (L–H)2–. The yield is much more improved when
o-vanillin (val-H) is externally added in the solution. Noteworthy
to mention that H2L ligand is significantly stable in methanolic
solution and does not undergo hydrolysis in the absence of
Ce(III) ion. While, the ligand undergoes hydrolysis in the pres-
ence of Ce(III) ion, facilitated by the coordination of H2L ligand
to Ce(III) ions and shifting of a phenolic proton to N-atom of
imine bond (by increasing the polarity of C=N bond making
the nucleophilic attack at C-atom by water molecule) [see struc-
The reaction, which was carried out in the mixture of sol-
vents MeOH/MeCN (3:1), showed the formation of the mixed
valence Ce(IV)3Ce(III)3 complex [Ce(IV)3Ce(III)3(L)4(val)6(O)2-
(OCH3)2]+ {m/z
=
2803.9919} and its close analogue
[Ce(IV)3Ce(III)3(L)4(val)6(O)2 + H]+ {m/z = 2742.9109}. The di-
cationic analogue was observed at [Ce6(O)2(L)4(val)6]2+ {m/z =
1370.9896} after 3 h (Figures S39–58). Hence, from the ESI mass
spectrometry we concluded that the hexanuclear Ce complex
forms only after 3 h. Additionally, the mixture of solvents
(MeOH/MeCN) found to be advantageous for the progress of
the reaction. The cation [CeIV3CeIII3(L)4(val)6(O)2(OCH3)2]+ was
isolated as dark brown crystalline powder with nitrate counter
anion and further characterized by elemental analysis, conduc-
tivity and magnetic susceptibility measurements (here H2L-H =
H2L for simplicity) (see SI). Two independent reactions were car-
ried out in similar molar ratios of 2:3:3:8 as mentioned previ-
ously; one in pre-distilled MeOH under argon atmosphere to
investigate the role of aerial oxygen (see Figures S79–S83 for
details) and the other in presence of calculated volume of aerial
oxygen. In the first case, the yellow colored {Ce4} complex was
formed exclusively with a negligible amount of {Ce6} complex
–
–
ture of 52+(NO3 )2; Figure 2]. The Ce6 cluster [32+(NO3 )2] was
isolated in 75 % yield as dark black shiny blocks/hexagons (See
SI). When the Ce(NO3)(H2O)4·2H2O was treated with H2L–R and
val-H in the presence of Et3N, the observations of change in the
colors of the reactions are same. However, the yields of the
reactions increase significantly. (NH4)2CeIV(NO3)6 was experi-
mentally found to be not effective for the synthesis of 32+
,
suggesting the substitution of chelating nitrate anions from
dianionic [Ce(NO3)6]2– seems to be unfavourable.
Complex M2+(X–)2 is soluble in DMF and DMSO. The crystals
of this series of complexes do not loose crystallinity when they
are separated out from their respective mother liquors. They are
sparingly soluble in MeCN, MeOH, CH2Cl2 and CHCl3. The UV/Vis
–
spectrum of M2+(NO3 )2 in CH3CN shows multiple absorption
[due to inner sphere nitrate (NO3 ) reduction[11]]. The color of
–
bands in the range of 270–500 nm (ε = 14030.6 L mol–1 cm–1)
which is possibly due to MTLCT (Figures S5–S9). A Schiff base
chelated mono-nuclear Ce(III) complex absorbs at 370 nm (π→π*
transition), while its oxidized Ce(IV) analogue shows an absorp-
tion band at 594 nm (ligand π orbital to the vacant Ce-4f or-
bital)[16] [273 nm (ε = 258000 L mol–1 cm–1) and another at
373 nm (ε = 60000 L mol–1 cm–1) for M = 1; 290 nm (ε = 17880
L mol–1 cm–1), 375 nm (ε = 39760 L mol–1 cm–1) for M = 2; 365
(ε = 98000 L mol–1 cm–1), 306 (ε = 90000 L mol–1 cm–1), 365 (ε =
98000 L mol–1 cm–1), 515 (ε = 16562 L mol–1 cm–1) for M = 2 in
MeCN].
the solution remained yellow even after refluxing the reaction
solution under argon atmosphere for 3 h and the intensity of
the mass corresponding to {Ce6}-fragment did not change even
after 5 days. In the second case, the color of the reaction solu-
tion changed to intense brown with the progress of reaction
time. The incorporation of the aerial oxygen was required to
increase the (bottom up) nuclearity from Ce4 to Ce6 (see Figure
The hexanuclear mixed valence cerium clusters 12+(X–)2
–
22+(X–)2 are iso-structural. Hence, the structural aspect of
12+(X–)2 is only described here (Figure 1).
Scheme 4. Plausible mechanistic pathway for the formation of the dication
M2+ concluded by ESI mass spectrometry (see SI, Figures S-39–58).
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tween two CeIV2CeIII2(μ4-O) tetrahedrons (Figure 1, bottom, left).
This oxo-cerium CeIV2CeIII4(μ4-O)2 core has been further shielded
by deprotonated organic ligands (deprotonated H2L–R and
H-val). Both dianionic (L–R)2– and mono-anionic o-vanillinate
(val–) ligands utilize their phenolic O-atoms to bridge between
the Ce(IV)–Ce(III)/Ce(III)–Ce(III) ions. Ce1 and Ce3 have adopted
square antiprismatic coordination geometry with O6N2 and O8
donor sets, respectively. In contrast, Ce2 have O9 coordination
environment with a distorted monocapped square antiprism
coordination geometry. Ce3 center has a coordinated water
molecule (see Figure 1). Two μ4-O groups are located in the
same plane of the four Ce(III) ions. The ligand (L–R)2– displays
η1:η2:η1:η2:μ2 bridging mode (Figure S18). On the other hand,
o-vanillinate displays two different types of bridging modes
(η1:η2:η1:μ2 and η0:η1:η1:μ in Figure S18). Both Ce1 and Ce2 cen-
ters are chelated by pocket-1 of the L2– ligand (Figure S18). The
oxygen atoms of cluster core exhibit both μ- and μ4-bridging
modes. The range of Ce–O distance in cluster 12+ is between
2.241(4)–2.697(4) Å, whereas Ce2–N(1) and Ce2–N(2) distances
are 2.531(5) and 2.547(5) Å which are very close to those previ-
ously reported Ce(IV)–O/N bond lengths.[35] These bond param-
–
eters are significantly longer than those of [52+(NO3 )2] (See
caption of Figure 1–2).
Figure 1. Molecular structure of dication of complex 12+(X–)2 (top). H atoms
are omitted for clarity except OH and NH. Selected experimental bond
lengths [Å] and angles [°]: Ce1–O16 2.244(4), Ce1–O16i 2.241(4), Ce1–O8
2.297(4), Ce1–O1 2.313(4), Ce1–O2 2.389(4), Ce1–O7 2.402(4), Ce1–N2
2.531(5), Ce1–N1 2.547(5), Ce2–O16 2.615(4), Ce2–O7 2.508(4), Ce2–O11
2.507(4), Ce2–O2i 2.501(4), Ce2–O5 2.496(4), Ce3–O16 2.366(4), Ce3–O11
2.518(4), Ce3–O5 2.490(4); Ce1–O2–Ce2i 109.46(14), Ce1–O1–Ce3i 104.15(13),
Ce1i–O16–Ce1 107.20(15), Ce1i–O16–Ce3 114.74(15), Ce1–O16–Ce3
115.53(15), Ce1i–O16–Ce2 110.40 (13), Ce1–O16–Ce2 110.30(14), Ce3–O16–
Ce2 98.42(13). Ce–O cores (bottom, left) and mono anionic counterion (bot-
tom, right) of complex 12+(X–)2. X = [CeIII2(o-val)3(NO3)4]–.
The only difference between 12+(X–)2 and 32+(X–)2 is their
–
counter anions [for former, X = CeIII2(o-val)3(NO3)4 , for latter,
X = NO3; see SI]. 12+(X–)2 crystallized in triclinic P1¯ space group.
Molecular structure of 1 is shown in Figure 1 (top). The asym-
metric unit of 12+(X–)2 contains a cationic part consisting of
three cerium ions, two dianionic (L–H)2– ligands and three
mono anionic o-vanillinate (val–) ligands. Further, in asymmetric
unit one cation is stabilized by one dinuclear cerium complex
as an anion which consists of two cerium(III) ions, three mono-
anionic o-vanillinate ligands and four bidentate nitrate anions.
The X-ray single crystal analysis showed that there are
two Ce-ions containing discrete molecular units. A dicationic
[CeIV2CeIII4(μ4-O)2(L–H)4(val)6(H2O)2]2+ (Figure 1, top) and two
mono anionic [CeIII2(val)3(NO3)4]– are co-crystallized for charge
balance (Figure 1, bottom, right).
Figure 2. Molecular structure of the intermediate dicationic complex [CeIII
-
4
(μ4-O)(L)4(HLH)3(val-H)]2+(NO3
)
2
[52+(NO3–)2]. Selected experimental bond
–
lengths [Å] and angles [°]: Ce(1)–O(1) 2.701(15), Ce(2)–O(1) 2.596(13), Ce(3)–
O(1) 2.649(13), Ce(4)–O(1) 2.564(15); Ce(1)–O1–Ce(2) 90.6(4), Ce(2)–O1–Ce(3)
91.9(4), Ce(3)–O1–Ce(4) 92.7(5), Ce(4)–O1–Ce(1) 91.7(4), Ce3–O1–Ce1 163.0(7),
Ce4–O1–Ce2 156.1(7). All Ce-ions are Ce(III) concluded from bond valence
and charge balance calculations.
–
All Ce-ions of 52+(NO3 )2 are Ce(III) which has been con-
Complex 32+(X–)2 possesses two nitrate anions for charge cluded from bond valence and charge balance calculations. The
balance which is suggested by the elemental analysis (see SI). bond valence sum (BVS) calculation and charge balance consid-
The electron densities due to these two disordered nitrates are eration of [CeIV2CeIII4(μ4-O)2(L–H)4(val)6(H2O)2]2+ led to conclu-
possibly smeared and hence could not be refined. The di- sion that Ce2, Ce2′, Ce3, Ce3′ are trivalent and Ce1, Ce1′ are
cationic [CeIV2CeIII4(μ4-O)2(L–H)4(val)6(H2O)2]2+ (Figure 1) con- tetravalent (see BVS calculation in the SI and Figure 2). The
tains a CeIV2CeIII4(μ4-O)2 core sharing one common edge be- selected bond lengths are given under Figure 1, caption. The
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mono-anionic [CeIII2(val)3(NO3)4]– (Figure 1, bottom, right) con-
tains two triply phenoxide bridged Ce(III) ions, each of which is
further chelated by two nitrate anions. Both the cerium atoms
have adopted ten coordination number with a O10 donor set.
The BVS calculation showed that both of these cerium ions are
in the +3 oxidation state (see SI).
–
The molar conductivity measurements of 32+(NO3 )2 showed
that it is a weak electrolyte, which can be rationalized by mass
spectrometry (see SI). The solution showed the association of
–
the cation with its counter anion (NO3 ). [Ce6(L)4(val)6(O)2(NO3)-
(NO)]+ {m/z = 2833.9492} and [Ce6(L)4(val)5(O2)(NO3)2(NO)-2H]+
{m/z = 2892.9453} (see SI). Cyclic voltammetry of 32+(NO3 )2
–
showed one quasi-reversible electron transfer processes
(Scheme 5).
–
Figure 3. Cyclic voltammogram of 32+(NO3
) in MeCN solvent in presence
2
of 0.1 N tetrabutlyammonium perchlorate as the electrolyte. Reference elec-
trode is Ag/AgNO3, working electrode is glassy carbon electrode and plati-
num wire is used as the counter electrode. {32+
=
[CeIV2CeIII4(μ4-O)2-
(L–H)4(val)6(H2O)2]2+}.
Scheme 5. Quasi-reversible one electron process at the electrodes in solution
of M2+ ion in MeCN. ESI mass spectrometry also indicated the stability of
dication M2+ and trication M3+ [here M = 3]. The oxidized species was ob-
served by ESI mass spectrometry as [CeIV3CeIII3(L)4(val)6(O)2(OCH3)2]+ {m/z =
2803.9919} (see SI) and isolated as crystalline precipitate from a similar reac-
tion with half of the volume of methanolic reaction solution.
–
The black hexagonal crystals of 32+(NO3 )2 were grinded and
pressed to form a circular disc which was utilized further to
record the X-ray photoelectron (XPS) spectrum which showed
two absorption bands (binding energy Ce3d5/2 at 882.9 eV and
Ce3d3/2 at 901.7 eV) which are unambiguously attributed to the
–
–
The acetonitrile solution of complex 32+(NO3 )2 was studied +3 oxidation state[10] of cerium in 32+(NO3 )2 (Figure 4, left).
by cyclic voltammetry (CV). One electron quasi-reversible oxid- The characteristic XPS (3d3/2) peak at 914.0 eV corresponds to
ation process was observed at E1/2 = –0.11 V, suggesting that a transition from the initial state 3d104f0 to the final state 3d94f0
the dication 32+ can undergo one electron oxidation to form of Ce4+ ions.[10] The binding energy 1s O-atom at 530.2–
the tri-cationic 33+ ion (Figure 3, Scheme 5). The redox property 530.7 eV [for μ4-O2– of CeIV2CeIII4(μ4-O2–), 531.7 eV (for C–O/
of mono-nuclear N2O2-donor atoms containing salen bonded N–O) and 534 eV (for coordinated H2O) (Figure 4, right)]. These
Ce(IV) complexes was studied by cyclic voltammetry (CV).[28] values [a sharp band at 530.2–530.7 eV (green/red lines) for
The reported[28] E1/2 (–0.12 to –0.36 V) values are found to be O2– and broad band (pink line) at 534 eV for H2O] are close to
–
very close to the value corresponding to 32+(NO3 )2 in MeCN the values reported for CeO2 thin films which has been depos-
(E1/2 = –0.11 V). This half electrode potentials were reported to ited on silicon and silicon nitride substrates.[34]
be dependent on the interacting counter anions.[28]
The magnetic susceptibilities of powdered samples of
–
The oxidation states of the cerium ions were calculated by 22+(X–)2 [X = CeIII2(val)3(NO3)3] and 32+(NO3 )2 have been meas-
BVS and charge balance calculations and further confirmed by ured from 300 K to 5 K under the application of dc magnetic
X-ray photoelectron spectroscopy of the representative com- field of 0.5 T (Figure 5–Figure 6). The Ce(III) ion is paramagnetic
–
plex 32+(NO3 )2 (Figure 4).
with the expected value of ꢀT product of 0.80 cm3 K mol–1 with
Figure 4. X-ray photoelectron spectra of O-atoms (left) Ce-ions (right) of complex 32+(NO3–)2.
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one unpaired electron and gj = 6/7[32] while to Ce(IV) ion has a mon in lanthanide complexes due to presence of ligand field.
diamagnetic spin ground state10 with temperature independent The ꢀT product slowly decreases from 3.25 cm3 K mol–1 at 300 K
paramagnetic (TIP) contribution from Ce(IV) ions.[33] The to 1.55 cm3 K mol–1 at 5 K upon cooling the sample (Figure 6).
ꢀT product of 22+(X–)2 {22+ = [CeIV2CeIII4(μ4-O)2(L–Me)4(val)6- This can be attributed to the depopulation of the Stark levels
(H2O)2]2+ and X = CeIII2(val)3(NO3)3} is 6.53 cm3 K mol–1 at 300 K of Ce(III) ions and weak antiferromagnetic interactions between
which is close to the expected ꢀT value of 6.40 cm3 K mol–1 for the Ce(III) ions.
magnetically non interacting eight Ce(III) ions (fn; n = 1, one
unpaired electron gj = 6/7).
–
–
Complex [CeIV2CeIII4(L–H)4(val)6(H2O)2]2+(NO3 )2 32+(NO3 )2
spontaneously undergoes oxidation in solution to produce
[CeIV3CeIII3(L)4(val)6(OMe)2]+(NO3 ) {4+(NO3 )}. Dark brown color
–
–
crystalline powder of 4+(NO3 ) was isolated in 85 % yield (see
–
Figure 5. The ꢀT vs. T plot of 22+(X–)2, X = CeIII2(val)3(NO3)3. See SI for the
similar plot of 12+(X–)2 analogue.
The ꢀT product slowly decreases to 5.32 cm3 K mol–1 upon
cooling down to 150 K (Figure 5) from room temperature. This
can be attributed to the depopulation of the Stark levels of
Ce(III) ions in the presence of ligand fields. Below this tempera-
ture the decrease of the ꢀT product of 22+(X–)2 is relatively
faster. It finally reaches 2.18 cm3 K mol–1 at 5 K which suggests
that the magnetic interactions between Ce(III) ions are weakly
antiferromagnetic. The ꢀT vs. T plot indicates that there might
be temperature independent paramagnetic (TIP) contribution
due to the presence of Ce(IV) ions. Similar TIP behavior has
been observed in cerium complexes reported by Anderson
et al.[33] The ꢀT vs. T plot of 12+(X–)2 is similar to that of of
22+(X–)2 (see SI).
The theoretically calculated ꢀT value for four non interacting
Ce(III) ions is 3.20 cm3 K mol–1 (4f1; gj = 6/7).[32] The signature
–
of the temperature dependence of ꢀT of 32+(NO3 )2 {32+
=
[CeIV2CeIII4(μ4-O)2(L–H)4(val)6(H2O)2]2+} suggests the presence of
weak antiferromagnetic interactions between Ce(III) ions medi-
ated via μ4-O2– and μ-Ophenoxide bridges (Figure 1) and further
accompanied by depopulation of Stark levels[4,10] which is com-
Figure 7. Experimental (bottom) and simulated (top) isotopic distribution pat-
terns corresponding to the mono- and di-cations: [32+ = [CeIV2CeIII4(μ4-O)2-
(L–H)4(val)6(H2O)2]2+
]
(a) [Ce2Ce4(O)2(L)4(val)6(OMe)2]+, (b) [Ce2Ce4(O)2-
Figure 6. The ꢀT vs. T plot of 32+(NO3
)
2
(H = 0.5 T).
(L)4(val)6]2+ and (c) [Ce2Ce4(O)2(L)4(val)6(NO3)2(NO)-2H]+.
–
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SI) when the H2L-H, o-val-H, Ce(NO3)3(H2O)4·2H2O and Et3N are
reacted in the same molar ratio of 2:3:3:8 in reduced volume of
MeOH as solvent [half of the volume of MeOH compared to the
reaction which produce [CeIV2CeIII4(L–H)4(val)6(H2O)2]2+(NO3 )2
–
–
32+(NO3 )2]. The magnetic susceptibility, elemental analysis, ESI
mass spectrometry (Figure 7a) and molar conductivity measure-
ments indicated the formation of partially soluble
–
–
[CeIV3CeIII3(L)4(val)6(OMe)2]+(NO3 ) {4+(NO3 )}. The CV measure-
ments also supports this possible oxidation (Figure 3,
–
Scheme 5). The ꢀT product of 4+(NO3 ) is 2.407 cm3 K mol–1 at
298 K which is close to the value for three non-interacting Ce(III)
ions is 2.40 cm3 K mol–1 (4f1, gj = 6/7) (Figure 8).[32] The signa-
ture of the temperature dependence of ꢀT of 4+(NO3 ) {4+
=
–
[CeIV3CeIII3(μ4-O)2(L–H)4(val)6(OMe)2]+} indicate the presence of
weak antiferromagnetic interactions mediated via μ4-oxide and
μ-phenoxide bridges (Figure 1) and further accompanied by de-
population of Stark level.[4,10]
Scheme 6. Oxidation of alcohol to aldehyde catalyzed by 32+ (NO3– )2 in DMF
at 100 °C. [32+ = [CeIV2CeIII4(μ4-O)2(L–H)4(val)6(H2O)2]2+ ].
While the presence of electron widthdrawing Cl and CF3 func-
tional groups at the para-position of the benzene ring leads to
the slowing down of the rate of the reaction. Our report is
the second example Ce-ion catalyzed TEMPO free oxidation of
benzyl alcohol[35] although there are several previously re-
ported synthetic methods of TEMPO free oxidation alcohols to
aldehydes.[36–37] To have an insight into the reaction mecha-
nism, the UV/Vis and ESI mass spectrometric techniques were
–
utilized. Complex 32+(NO3 )2 was treated with O2 using a bal-
loon at 100 °C in DMF for 5 h and its UV/Vis spectrum and ESI
mass spectrum were recorded and compared with those of the
–
freshly prepared DMF solution of 32+(NO3 )2. A sharp decrease
in intensities of the absorption of bands at 344 and 407 nm
was clearly observed suggesting the reaction of 32+(NO3 )2
–
with aerial O2 at 100 °C in DMF (Figure S60). The absorption
band at 344 nm has disappeared and a new broad absorption
Figure 8. The ꢀT vs. T plot of [CeIV3CeIII3(L)4(val)6(OMe)2]+(NO3–) {4+(X–)}.
(H = 0.1 T). 4+(X–) was further characterized by ESI mass spectrometry as
[Ce(IV)3Ce(III)3(L)4(val)6(O)2(OCH3)2]+ {m/z = 2803.9919}.
–
band at 700 nm was observed when 32+(NO3 )2 was treated
with 4-methoxybenaldehyde at 100 °C for 30 min (Figure S64).
The dicationic species is clearly observed in freshly prepared
The ꢀT product slowly decreases from 2.40 cm3 K mol–1 at
300 K to 1.67 cm3 K mol–1 at 5 K upon cooling the sample (Fig-
ure 8). This can be attributed to the depopulation of the Stark
levels of Ce(III) ions and weak antiferromagnetic interactions
between the Ce(III) ions which is similar to that of 32+. The
molar conductivity measurements in MeCN shows that mono
cationic 4+ is less conducting than its dicationic analogue 32+
(see SI) in solution as expected from their respective charges.
–
DMF solution of 32+(NO3 )2 [Ce(IV)2Ce(III)4(L)4(val)6(O)2]2+
{m/z = 1370.9896} (Figures S59,61), while the corresponding
peak completely disappears when the solution was heated in
presence of O2 at 100 °C in DMF with additional ESI mass frag-
ments observed near m/z = 1500 (See SI; Figures S62–63). The
distance between one Ce(III) ion (containing a coordinated H2O
molecule; 2.515 Å) to other Ce(III) ion from the other side
Ce(IV)2 unit is 5.95 Å (Figure S65). ESI mass spectrum clearly
shows that [CeIV2CeIII4(μ4-O)2(L–H)4(val)6]2+ is stable after loss of
two coordinated water molecules. After the loss of water ligand,
the vacant site at the Ce(III) ion can be occupied by molecular
O2. The UV/Vis measurement shows an additional broad band
around 700 nm observed when 32+ is reacted with O2 in DMF
at 100 °C (Figure S65). ESI mass spectrum shows the disappear-
ance of the signature mass due to dication [32+-2H2O] at m/z =
1370.97 {[Ce6(C14H11NO3)4(C8H7O3)6(O)2]2+} (Figure S63). The
possible reaction mechanism for this mixed valence hexa-
Complex [CeIV2CeIII4(L)4(val)6(H2O)2]2+(NO3 )2 {32+(NO3 )2} is
prone to undergo oxidation in solution which has been con-
cluded from cyclic voltammetry (CV) measurements and mass
spectrometry. This mixed valence hexanuclear cationic cerium
–
–
–
cluster {32+(NO3 )2}, which was isolated as black blocks/hexa-
gons in 75 % yield was employed as a catalyst towards the
oxidation of functionalized benzyl alcohol at 70 °C in MeCN and
at 100 °C in DMF in the presence of O2 and 3 Å molecular sieves
(Scheme 6).
The optimized condition is found to be 1.5 mol-% loading of
–
catalyst 32+(NO3 )2 at 100 °C in DMF (see SI; Catalytic oxidation
–
nuclear cationic cerium cluster {32+(NO3 )2} catalysed TEMPO
in DMF for details). The reaction is complete after 26 h.
free oxidation of benzyl alcohol follows the initial substitution
The conversion is quantitative for R = H, Me and NO2. These of a bidente o-vanillin (o-val) ligand from one Ce(III) ion and
functional groups can stabilize their benzyl radicals. It also ra- subsequent binding of O2 with other Ce(III) ion from opposite
tionalizes the fast progress of the corresponding reactions. side of Ce(IV)2 unit of 32+ converting Ce(III) into Ce(IV) ion fol-
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lowed by exchange of H-atom from Obenzyl alcohol to O-atom of tron withdrawing Cl and CF3 analogues. All aldehydes were pu-
the bonded O2 ligand at Ce(IV) ion (see SI). This can possibly rified by extraction of DMF solution in n-hexane with excellent
lead to the formation of Ce(IV)–O–OH unit which further ab- yields.
stracts one H-atom from benzylic C-atom atom leading to the
Deposition Numbers 1916232 {for 12+[X–]2;
X =
[CeIII2(val)3-
(NO3)3]–}, 1916233 {for 22+[X–]2; X = [CeIII2(val)3(NO3)3]–}, 1916231
{for 22+[X–]2; X = [CeIII2(val)3(NO3)3]–}, and 1852407 {for 32+[X–]2;
X = (NO3)3} contain the supplementary crystallographic data for this
paper. These data are provided free of charge by the joint Cam-
bridge Crystallographic Data Centre and Fachinformationszentrum
Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.
formation of benzyl aldehyde and hydrogen peroxide (H2O2).
The later decomposes to H2O and O2. After the removal of H2O2,
Ce(IV) ion converts to Ce(III) which enters into the next catalytic
cycle. There are several other examples of TEMPO free oxid-
ations of alcohols.[36–37] Hydrogen peroxide (H2O2) was utilized
as an oxidizing agent along with several catalysts.[37] Several
other metal-based catalysts were also utilized for O2 mediated
TEMPO free oxidation of alcohols nearly one and half decades
ago.[36] Very recently, both peroxide bridged dinuclear Ce(IV)
and monomeric Ce(III) complexes were shown to oxidise the
aromatic alcohols to their corresponding aldehydes in high
yields.[35] The Ce(III) ion was shown to react with molecular oxy-
gen to get oxidized to form per-oxo bridged dinuclear Ce(IV)
complex which then oxidized the aromatic alcohols to their cor-
responding aldehydes.[35]
Acknowledgments
K.C.M. thanks SERB for ECR grant (ECR/2016/000890) and
Prof. T.P for XPS measurements. S.A. thanks CSIR for SRF.
B.S. thanks IIT Madras for Postdoctoral Fellowship.
Keywords: Solution dynamics · Dicationic mixed valence
cerium complex · Magnetic studies · XPS · Catalysis
The thermal stabilities of all the complexes were studied by
thermo gravimetric analysis (TGA) under inert atmosphere and
in open air (see SI). The thermal stability of each complex is
much higher under inter atmosphere and only partial thermal
decomposition has been observed till 930 °C, while each com-
plex completely decomposes to colorless micro-crystals CeO2
above 300 °C. The formation of CeO2 was further confirmed by
powder X-ray diffraction pattern (see SI).
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In conclusion, the reaction of cerium(III)nitrate hexahydrate
with O3N donating Schiff base and o-vanillin ligands in the pres-
ence of a base led to the isolation of mixed valence hexa-
nucleardicationicceriumclustersM2+(X–)2 [M=CeIV2CeIII4(μ4-O)2-
(L–R)4(val)6(H2O)2; 12+ and 22+ for X = CeIII2(val)3(NO3)4; 32+ for
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–
with a pentanuclear cerium cluster. Finally, in presence of aerial
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Received: July 9, 2020
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Communication
doi.org/10.1002/ejic.202000659
EurJIC
European Journal of Inorganic Chemistry
Coordination Chemistry
S. Arumugam, B. Shankar,
K. C. Mondal* ................................... 1–11
Redox Active Hexanuclear Mixed Va-
lence Dicationic Ce(III)/Ce(IV) Coor-
dination Clusters
A series of mixed valence hexanuclear action dynamics have been studied by
dicationiccoordinationclustersM2+(X–)2 ESI mass spectrometry. The dication 32+
containing two μ4-O2– bridges have has been studied by X-ray photoelec-
been synthesized, isolated and charac- tron spectroscopy. Inaddition, all the
terized by reacting cerium(III)nitrate complexes have been studied by mag-
hexahydrate with Schiff base ligand netic susceptibility measurements and
(H2L–R) under basic condition. The re- other techniques.
doi.org/10.1002/ejic.202000659
Eur. J. Inorg. Chem. 0000, 0–0
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11
© 2020 Wiley-VCH GmbH