Synthesis of bimetallic trifluoroacetates through a crystallochemical investigation of their monometallic counterparts: The case of (A, A′)(CF3COO)2·: N H2O (A, A′ = Mg, Ca, Sr, Ba, Mn)

Article abstract of DOI:10.1039/c6dt04152c

Owing to their potential as single-source precursors for compositionally complex materials, there is growing interest in the rational design of multimetallic compounds containing fluorinated ligands. In this work, we show that chemical and structural principles for a materials-by-design approach to bimetallic trifluoroacetates can be established through a systematic investigation of the crystal-chemistry of their monometallic counterparts. A(CF₃COO)₂·nH₂O (A = Mg, Ca, Sr, Ba, Mn) monometallic trifluoroacetates were employed to demonstrate the feasibility of this approach. The crystal-chemistry of monometallic trifluoroacetates was mapped using variable-temperature single-crystal X-ray diffraction, powder X-ray diffraction, and thermal analysis. The evolution with temperature of the previously unknown crystal structure of Mg(CF₃COO)₂·4H₂O was found to be identical to that of Mn(CF₃COO)₂·4H₂O. More important, the flexibility of Mn_x(CF₃COO)_2x·4H₂O (x = 1, 3) to adopt two structures, one isostructural to Mg(CF₃COO)₂·4H₂O, the other isostructural to Ca₃(CF₃COO)₆·4H₂O, enabled the synthesis of Mg-Mn and Ca-Mn bimetallic trifluoroacetates. Mg_0.45Mn_0.55(CF₃COO)₂·4H₂O was found to be isostructural to Mg(CF₃COO)₂·4H₂O and exhibited isolated metal-oxygen octahedra with Mg²⁺ and Mn²⁺ nearly equally distributed over the metal sites (Mg/Mn: 45/55). Ca_1.72Mn_1.28(CF₃COO)₆·4H₂O was isostructural to Ca₃(CF₃COO)₆·4H₂O and displayed trimers of metal-oxygen corner-sharing octahedra; Ca²⁺ and Mn²⁺ were unequally distributed over the central (Ca/Mn: 96/4) and terminal (Ca/Mn: 38/62) octahedral sites.

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Synthesis of bimetallic trifluoroacetates through a
crystallochemical investigation of their
Cite this: DOI: 10.1039/c6dt04152c
monometallic counterparts: the case of (A, A’)
(CF3COO)2·nH2O (A, A’ = Mg, Ca, Sr, Ba, Mn)†
B. Dulani Dhanapala,a Natalie A. Mannino,a Laura M. Mendoza,a
K. Tauni Dissanayake,a Philip D. Martin,a Leopoldo Suescunb and
Federico A. Rabuffetti*a
Owing to their potential as single-source precursors for compositionally complex materials, there is
growing interest in the rational design of multimetallic compounds containing fluorinated ligands. In this
work, we show that chemical and structural principles for a materials-by-design approach to bimetallic
trifluoroacetates can be established through a systematic investigation of the crystal-chemistry of their
monometallic counterparts. A(CF3COO)2·nH2O (A = Mg, Ca, Sr, Ba, Mn) monometallic trifluoroacetates
were employed to demonstrate the feasibility of this approach. The crystal-chemistry of monometallic
trifluoroacetates was mapped using variable-temperature single-crystal X-ray diffraction, powder X-ray
diffraction, and thermal analysis. The evolution with temperature of the previously unknown crystal struc-
ture of Mg(CF3COO)2·4H2O was found to be identical to that of Mn(CF3COO)2·4H2O. More important,
the flexibility of Mnx(CF3COO)2x·4H2O (x
= 1, 3) to adopt two structures, one isostructural to
Mg(CF3COO)2·4H2O, the other isostructural to Ca3(CF3COO)6·4H2O, enabled the synthesis of Mg–Mn
and Ca–Mn bimetallic trifluoroacetates. Mg0.45Mn0.55(CF3COO)2·4H2O was found to be isostructural to
Mg(CF3COO)2·4H2O and exhibited isolated metal–oxygen octahedra with Mg2+ and Mn2+ nearly equally
distributed over the metal sites (Mg/Mn: 45/55). Ca1.72Mn1.28(CF3COO)6·4H2O was isostructural to
Ca3(CF3COO)6·4H2O and displayed trimers of metal–oxygen corner-sharing octahedra; Ca2+ and Mn2+
were unequally distributed over the central (Ca/Mn: 96/4) and terminal (Ca/Mn: 38/62) octahedral sites.
Received 29th October 2016,
Accepted 23rd December 2016
DOI: 10.1039/c6dt04152c
www.rsc.org/dalton
view of their potential as single-source precursors for composi-
tionally complex functional materials. Mishra et al. reported
1. Introduction
Metal–organic compounds containing fluorinated ligands con- the synthesis of a series of bimetallic compounds featuring
stitute an important class of precursors employed in the pre- Na and Y as metals and tfaH, tetrahydrofuran, and di-, tri-,
paration of functional materials and nanomaterials.1 Since the and tetraglyme as ligands.3 These compounds were found to
work of Roberts,2 trifluoroacetic acid (CF3COOH, tfaH) has serve as single-source precursors for NaYF4 upconverting nano-
been extensively used as a ligand to synthesize monometallic crystals. Likewise, Tereshchenko et al. reported the synthesis
fluorinated precursors of alkali, alkaline-earth, transition, and of a series of bimetallic trifluoroacetates featuring an alkali
rare-earth metals. Recently, there has been growing interest in and divalent nickel or cobalt as metals.4,5 These compounds
the preparation of multimetallic fluorinated compounds in featured triangular trinuclear complex anions, which are
potentially interesting in view of their magnetic properties.6
Despite their potential as precursors for functional
aDepartment of Chemistry, Wayne State University, Detroit, MI 48202, USA.
materials, the rational assembly of multimetallic compounds
E-mail: far@chem.wayne.edu
bCryssmat-Lab/Cátedra de Física/DETEMA, Facultad de Química, Universidad de la
containing trifluoroacetate ligands (tfa) remains challenging.
With the aim of addressing this challenge, we hypothesized
that a systematic investigation of the crystal-chemistry of
monometallic trifluoroacetates yields chemical and structural
principles that enable a materials-by-design approach to multi-
metallic compounds. A(tfa)2·nH2O (A = Mg, Ca, Sr, Ba, MnII)
trifluoroacetates were employed as the materials platform to
República, Montevideo 11800, Uruguay
†Electronic supplementary information (ESI) available: (1) Additional crystallo-
graphic information of Mg(tfa)2·4H2O, Mn3(tfa)6·4H2O, Mg0.45Mn0.55(tfa)2·4H2O,
and Ca1.72Mn1.28(tfa)6·4H2O, including CIF files; (2) Rietveld analysis of
Mnx(tfa)2x·4H2O; (3) crystallochemical features of alkaline-earth trifluoroacetates;
and (4) thermograms and differential thermograms of Mg(tfa)2·4H2O and
Mnx(tfa)2x·4H2O. See DOI: 10.1039/c6dt04152c
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test this hypothesis. Alkaline-earth trifluoroacetates have been solution was obtained upon quantitative dissolution of the
used as precursors to functional materials such as super- metal carbonate. Crystallization of Mg(tfa)2·4H2O was accom-
conducting YBa2Cu3O7 thin films,7,8 Er:Yb:SrF2 upconversion plished by solvent evaporation at room temperature, which
nanocrystals,9 and MgF2 and CaF2 fluoride composites and was induced by flowing dry nitrogen gas over the precursor
ceramics.10–12 Yet, an investigation of the structures and solution.20 This approach to crystallization of metal trifluoro-
crystal-chemistry of alkaline-earth trifluoroacetates crystallized acetates differs from that first proposed by Roberts, which
directly from a mixture of tfaH and water has not been per- achieves solvent evaporation by heating under air.2 A needle-
formed. Boyle et al. reported a study of the crystal-chemistry valve rotameter was employed to regulate the nitrogen flow
and structures of alkaline-earth trifluoroacetates, but these rate. The flow rate was maintained constant at 35 mL min−1
were recrystallized from basic solvents such as pyridine and for 24 h. Next, it was increased to 200 mL min−1 and main-
tetrahydrofuran.13 A similar situation is observed in the family tained constant for 4 days. A colorless polycrystalline solid was
of trifluoroacetates containing first-row transition metals such obtained at the bottom of the reaction flask. Single crystals of
as divalent manganese. Several researchers have synthesized Mg(tfa)2·4H2O were recovered from the powder for structural
and structurally characterized manganese(II) complexes con- determination.
taining trifluoroacetate ligands. However, in all cases, these
2.2. Crystallization of Mn3(tfa)6·4H2O
contained ligands other than tfa and H2O (e.g., pyridine,14
benzonitrile14 tetrahydrofuran,15 bipyridine,16 acetic acid17). MnCO3 (99.9%, Sigma Aldrich) was used as received. A mixture
Manganese(II) trifluoroacetate was first crystallized directly of 3 mL of double-distilled water and 3 mL of tfaH was pre-
from a mixture of tfaH and H2O by Baillie et al. in 1968.18 pared in a 50 mL two-neck round-bottom flask. Then, 1 mmol
Interestingly, its crystal structure was not elucidated until of MnCO3 (115.20 mg) was added to this solution. A brown,
2000, when Olejnik and Lis reported the solution of the high- optically transparent solution was obtained. Crystallization of
temperature (>215 K) phase of Mn(tfa)2·4H2O;19 the structure Mn3(tfa)6·4H2O was accomplished by a solvent evaporation
of the low-temperature counterpart was not reported and, to approach identical to that employed for the magnesium com-
the best of our knowledge, its solution was not further pound. The only difference was that the nitrogen flow rate was
pursued. Finally, no attempts at performing a systematic com- maintained constant at 70 mL min−1 for 12 days. Single crys-
parison between the crystal-chemistry of different families of tals of Mn3(tfa)6·4H2O suitable for structural determination
monometallic trifluoroacetates (e.g., alkali and alkaline-earth were then recovered.
vs. transition metals, transition metals vs. rare-earths, etc.)
have been reported.
2.3. Crystallization of Mg–Mn and Ca–Mn bimetallic
trifluoroacetates
Herein, we report a systematic investigation of the crystal-
chemistry of A(tfa)2·nH2O (A = Mg, Ca, Sr, Ba, Mn) and the syn- A mixture consisting of 1 mmol of MgCO3 (CaCO3) and
thesis and structural solution of bimetallic trifluoroacetates 1 mmol of MnCO3 was used in the synthesis of Mg–Mn
containing Mg–Mn and Ca–Mn as metal centers. Variable- (Ca–Mn) bimetallic trifluoroacetate. Crystallization was accom-
temperature single-crystal X-ray diffraction, powder X-ray diffr- plished by a solvent evaporation approach identical to that
action, and thermal analysis were employed to map the described above. The only difference was that the nitrogen
chemical and structural relationships within the A(tfa)2·nH2O flow rate was maintained constant at 70 mL min−1 for 9 days.
(A = Mg, Ca, Sr, Ba, Mn) family of monometallic trifluoro- Gelatin-like products were obtained, from which a few single
acetates. Emphasis was placed on (i) identifying common crystals suitable for structural determination were carefully
crystallochemical patterns encountered in alkaline-earth and isolated.
manganese trifluoroacetates, and (ii) utilizing those patterns to
synthesize bimetallic trifluoroacetates containing alkaline-earth
2.4. Single-crystal X-ray diffraction
and divalent manganese cations as metal centers. Critical to
2.4.1. Mg(tfa)2·4H2O. Single-crystal X-ray structure determi-
accomplishing these goals were the solutions of the structures nations were carried out at 100, 143, 160, 200, and 300 K.
of Mg(tfa)2·4H2O and of Mn3(tfa)6·4H2O, as well as understand- Colourless crystals of Mg(tfa)2·4H2O appeared as parallelepipeds.
ing their evolution in the 100–300 K temperature range.
A crystal with approximate dimensions 0.10 × 0.20 × 0.20 mm,
that turned out to be a two-component twin, was selected for
structure determination at 100 K. The geometric relation
between the two domains suggested this crystal had a higher
symmetry at room temperature, leading us to perform a more
complete structural analysis. A crystal with approximate dimen-
2. Experimental
2.1. Crystallization of Mg(tfa)2·4H2O
MgCO3 (99.9%, Sigma Aldrich) and anhydrous tfaH (99%, sions 0.13 × 0.15 × 0.42 mm was then selected for structure deter-
Sigma Aldrich) were used as received. Double-distilled water mination at 300 K, as well as for study of the reversible phase
was employed as the solvent. A mixture of 5 mL of double- transition to the low-temperature polymorph. Crystals were
distilled water and 1 mL of tfaH was prepared in a 50 mL two- mounted in Paratone N oil or epoxy in a Bruker X8 Apex single
neck round-bottom flask. Then, 2 mmol of MgCO3 (168.63 mg) crystal diffractometer. X-ray intensities were measured at the
were added to this solution. A colorless, optically transparent desired temperature using Mo Kα radiation (λ = 0.71073 Å).
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Frames were integrated using the Bruker SAINT software. both triclinic and monoclinic symmetries; structure refinement
Experimental data were corrected for Lorentz, polarization, and details are given for both models and discussed below. Crystal
absorption effects; for the latter, the multiscan method was data of Mg(tfa)2·4H2O were deposited in the Cambridge
employed using Bruker TWINABS software. The structure was Crystallographic Data Centre with numbers 1443800 (100 K),
solved using
a dual space approach as implemented in 1505493 (300 K), 1505494 (200 K), 1505495 (160 K, monoclinic),
SHELXT21 and difference Fourier maps as embedded in the 1505496 (160 K, triclinic), and 1505497 (143 K). Additional struc-
SHELXL-2014/7 software22 running under ShelXle23 and visual- tural details are given in the ESI (Tables S1–S18 and Fig. S1–S6†).
ized using the VESTA software.24 Hydrogen atom positions were
2.4.2. Mn3(tfa)6·4H2O. Single-crystal X-ray structure deter-
determined for the 100 and 300 K structures employing differ- mination was carried out at 100 K. Colourless crystals of
ence Fourier maps; these were later used as a starting point for Mn3(tfa)6·4H2O appeared as parallelepipeds. A crystal with
structural solution at other temperatures (143, 160, and 200 K). approximate dimensions 0.13 × 0.29 × 0.49 mm, that turned
Hydrogen positions were refined with the following constraints: out to be a multicomponent twin, was selected and processed
(1) an O–H bond distance of 0.840(0.001) Å; (2) a H⋯H distance as a four-component twin; components below 5 wt% were dis-
of 1.360(0.002) Å, corresponding to a 104.5° angle; and (3) Uiso
=
regarded. The structure was solved by intrinsic phasing
1.5Ueq of the parent oxygen atom. The final refined positions of methods. Data collection and structure refinement procedures
hydrogen atoms defined a reasonable hydrogen-bond network. were similar to those employed for the magnesium com-
An unconstrained approach yielded unreasonable bond dis- pounds. Table 2 summarizes crystal data, data collection, and
tances and angles. Table 1 summarizes crystal data, data collec- structure refinement details. Crystal data of Mn3(tfa)6·4H2O
tion, and structure refinement details for the five datasets col- were deposited in the Cambridge Crystallographic Data Centre
lected at 100, 143, 160, 200, and 300 K. For the diffraction data with number 1505498. Additional structural details are given
collected at 160 K, structure refinement was attempted using in the ESI (Tables S19–S21 and Fig. S7†).
Table 1 Crystal and structural determination data of Mg(tfa)2·4H2O
Chemical formula
Formula weight (g)
Temperature (K)
Wavelength (Å)
Crystal system
Space-group
Mg(tfa)2·4H2O
322.41
100
0.71073
Triclinic
ˉ
P1
322.41
143
0.71073
Triclinic
ˉ
P1
322.41
160
0.71073
Triclinic
ˉ
P1
322.41
160
322.41
200
322.41
300
0.71073
Monoclinic
C2/c
13.243(2),
12.8685(19),
8.1543(13)
90, 120.701(7),
90
0.71073
Monoclinic
C2/c
13.2595(9),
12.7856(9),
8.1950(7)
90, 120.562(6),
90
0.71073
Monoclinic
C2/c
13.2064(6),
12.6063(6),
8.3132(4)
90, 118.849(2),
90
a, b, c (Å)
8.0985(9), 9.0458(11), 8.1395(4), 9.0727(5), 8.142(2), 9.099(3),
9.3604(11)
9.3369(5)
9.250(3)
α, β, γ (°)
88.324(6), 65.021(5),
71.131(6)
88.102(3),
65.910(2), 70.795(2) 67.112(14),
69.439(14)
590.32(5)
2, 1
0.27
87.850(16),
Volume (Å3)
583.55(12)
2, 1
0.27
587.2(3)
2, 1
0.27
1194.9(3)
4, 0.5
0.26
1196.30(17)
4, 0.5
0.26
1212.24(10)
4, 0.5
0.26
Z, Z′
μ (mm−1
)
Crystal size (mm)
0.20 × 0.20 ×
0.10
0.42 × 0.15 ×
0.13
0.42 × 0.15 ×
0.13
0.42 × 0.15 ×
0.13
0.42 × 0.15 ×
0.13
0.42 × 0.15 ×
0.13
Tmin, Tmax
θ range (°)
0.691, 0.745
2.4–25.3
0.600
0.885, 0.967
2.4–28.4
0.669
0.885, 0.967
2.4–28.4
0.669
0.885, 0.967
2.4–28.3
0.666
0.885, 0.967
2.4–28.3
0.668
0.885, 0.967
2.4–28.3
0.667
(sin θ/λ)max (Å−1
)
Index ranges
−9 ≤ h ≤ 9
−10 ≤ k ≤ 10
−11 ≤ l ≤ 11
−10 ≤ h ≤ 10
−12 ≤ k ≤ 12
−12 ≤ l ≤ 12
−10 ≤ h ≤ 10
−11 ≤ k ≤ 11
−12 ≤ l ≤ 12
5346, 5346, 3230
−17 ≤ h ≤ 17
−16 ≤ k ≤ 17
−10 ≤ l ≤ 18
16 302, 1402,
841
−17 ≤ h ≤ 17
−17 ≤ k ≤ 17
−10 ≤ l ≤ 10
31 440, 1480,
1159
−17 ≤ h ≤ 17
−16 ≤ k ≤ 16
−11 ≤ l ≤ 11
31 593, 1511,
1313
Measured,
34 199, 34 199, 29 303 9250, 9250, 7136
independent, and
observed [I > 2σ(I)]
reflections
Refinement method Full-matrix least squares on F2
Weighting scheme
w = 1/[σ2(Fo2) +
w = 1/[σ2(Fo2) +
w = 1/[σ2(Fo2) +
(0.2P)2] where P =
(Fo2 + 2Fc2)/3
w = 1/[σ2(Fo2) +
(0.0897P)2 +
w = 1/[σ2(Fo2) +
(0.1187P)2 +
w = 1/[σ2(Fo2) +
(0.0433P)2 +
0.3461P] where
(0.0299P)2 +
(0.1059P)2 +
0.1114P] where
P = (Fo2 + 2Fc2)/3
34 199, 197, 12
0.2653P] where
P = (Fo2 + 2Fc2)/3
9250, 173, 0
22.3022P] where 0.7162P] where
P = (Fo2 + 2Fc2)/3 P = (Fo2 + 2Fc2)/3 P = (Fo2 + 2Fc2)/3
Reflections,
parameters,
restraints
5346, 173, 6
1402, 120, 72
1480, 148, 107
1511, 149, 100
H atom treatment
Only H-atom
coordinates refined
0.26, −0.27
3.1%
7.8%
1.04
H-atom parameters constrained
Δmax, Δmin (e Å−3
R[F2 > 2σ(F2)]
wR(F2)
)
0.73, −0.41
6.0%
17.7%
1.05
1.51, −1.21
10.6%
31.0%
0.54, −0.89
11.0%
31.8%
0.67, −0.53
6.0%
17.6%
1.08
0.27, −0.18
2.7%
7.8%
S
1.07
1.07
1.05
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Table 2 Crystal and structural determination data of Mn3(tfa)6·4H2O
Mg0.45Mn0.55(tfa)2·4H2O and Ca1.72Mn1.28(tfa)6·4H2O, respect-
ively. Both structures were solved using a dual space approach.
Mg0.45Mn0.55(tfa)2·4H2O exhibited the same type of twinning
observed in Mg(tfa)2·4H2O. The observed electronic density of
the metal site was found to be larger (lower) than that expected
for magnesium (manganese), suggesting a partial substitution
of manganese with magnesium. Thus, metal site occupancy was
refined for manganese and magnesium atoms with constrained
coordinates and thermal parameters yielding a Mg/Mn occu-
pancy ratio of 45/55, in good agreement with the nominal com-
position of the starting mixture. Likewise, mixed-metal site
occupancies were observed in Ca1.72Mn1.28(tfa)6·4H2O. Metal
site occupancies for the two crystallographically distinct sites in
this structure were refined independently, using constrained
atomic positions and anisotropic displacement parameters.
Split atomic positions were also refined for crystallographically
distinct trifluoroacetate ligands. Table 3 summarizes crystal
data, data collection, and structure refinement details. Crystal
data of Mg0.45Mn0.55(tfa)2·4H2O and Ca1.72Mn1.28(tfa)6·4H2O
were deposited in the Cambridge Crystallographic Data Centre
with numbers 1507757 and 1510812, respectively. Additional
structural details are given in the ESI (Tables S22–S28 and
Fig. S8 and S9†).
Chemical formula
Formula weight (g)
Temperature (K)
Wavelength (Å)
Crystal system
Space-group
a, b, c (Å)
Mn3(tfa)6·4H2O
915.00
100
0.71073
Monoclinic
P21/n
9.0195(5), 9.0431(5), 15.9167(9)
90, 90.694(3), 90
1298.14(13)
2, 0.5
α, β, γ (°)
Volume (Å3)
Z, Z′
μ (mm−1
)
1.64
Crystal size (mm)
Tmin, Tmax
0.49 × 0.29 × 0.13
0.641, 0.794
2.6–28.8
θ range (°)
(sin θ/λ)max (Å−1
)
0.678
Index ranges
−12 ≤ h ≤ 12
0 ≤ k ≤ 12
0 ≤ l ≤ 21
Measured, independent and
observed [I > 2σ(I)] reflections
Refinement method
4315, 4315, 3639
Full-matrix least squares on F2
w = 1/[σ2(Fo2) + (0.038P)2 + 4.929P]
where P = (Fo2 + 2Fc2)/3
Weighting scheme
Reflections, parameters,
restraints
4315, 368, 586
H atom treatment
Only H-atom coordinates refined
1.20, −0.76
5.2%
12.2%
1.05
Δmax, Δmin (e Å−3
R[F2 > 2σ(F2)]
wR(F2)
)
S
2.5. Room-temperature powder X-ray diffraction and Rietveld
analysis of Mnx(tfa)2x·4H2O
2.4.3. Mg–Mn and Ca–Mn bimetallic trifluoroacetates.
Single-crystal X-ray structure determinations were carried out
at 100 K. Crystals with approximate dimensions 0.36 × 0.33 ×
Polycrystalline Mnx(tfa)2x·4H2O (x = 1, 3) was characterized
using a Bruker daVinci powder diffractometer operated at
40 kV and 40 mA. Cu Kα radiation (λ = 1.5418 Å) was employed.
Diffractograms were collected in 8–80° 2θ range using a step
0.14 and 0.44
× 0.36 × 0.21 mm were selected for
Table 3 Crystal and structural determination data of Mg–Mn and Ca–Mn trifluoroacetates
Chemical formula
Formula weight (g)
Temperature (K)
Wavelength (Å)
Crystal system
Space-group
a, b, c (Å)
Mg0.45Mn0.55(tfa)2·4H2O
339.19
100
0.71073
Triclinic
ˉ
P1
8.1926(6), 9.0995(7), 9.4154(7)
88.761(4), 63.982(3), 71.251(4)
591.40(8)
2
0.79
Ca1.72Mn1.28(tfa)6·4H2O
889.46
Monoclinic
P21/n
9.0523(5), 9.1619(5), 16.3326(10)
90, 92.621(3), 90
1353.15(13)
2
α, β, γ (°)
Volume (Å3)
Z
μ (mm−1
)
1.13
Crystal size (mm)
Tmin, Tmax
0.36 × 0.33 × 0.14
0.829, 0.889
2.4–30.7
0.44 × 0.36 × 0.21
0.661, 0.801
2.5–34.8
θ range (°)
(sin θ/λ)max (Å−1
)
0.717
0.804
Index ranges
−10 ≤ h ≤ 11
−12 ≤ k ≤ 13
0 ≤ l ≤ 13
−14 ≤ h ≤ 14
0 ≤ k ≤ 14
0 ≤ l ≤ 25
8732, 8732, 6192
Measured, independent and observed
[I > 2σ(I)] reflections
Refinement method
6410, 6410, 5166
Full-matrix least squares on F2
Weighting scheme
w = 1/[σ2(Fo2) + (0.0257P)2 + 0.1582P]
w = 1/[σ2(Fo2) + (0.0553P)2 + 0.3102P]
where P = (Fo2 + 2Fc2)/3
8732, 397, 288
where P = (Fo2 + 2Fc2)/3
Reflections, parameters, restraints
H atom treatment
6410, 198, 12
Only H-atom coordinates refined
0.44, −0.30
3.0%
6.6%
1.03
Δmax, Δmin (e Å−3
R[F2 > 2σ(F2)]
wR(F2)
)
0.82, −0.82
4.0%
11.3%
1.07
S
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size of 0.016° and a step time of 3 s. Details of the Rietveld other two (trans positioned) belong to the carboxylate groups
analysis25,26 of the powder diffraction pattern are given in the of the trifluoroacetate anions, which act as monodentate
ESI (Fig. S10†).
ligands (Fig. 1a). The Mg–O bond lengths are in the
2.0259(18)–2.0995(16) Å range. The O–Mg–O bond angles of
176.19(8)–177.43(7)° (trans) and 87.67(7)–94.15(7)° (cis) corres-
pond to a nearly regular octahedron. Both trifluoroacetate
ligands are oriented with the –CF3 groups in opposite corners
of the magnesium–oxygen octahedral monomer; C–C bonds
are almost parallel to the O1′–Mg–O1 line connecting the
oxygen atoms of the two carboxylate groups. The remaining
two oxygen atoms O2′ and O2 are both hydrogen-bonded to
the O1W water molecule. As a result, both carboxylates are
nearly coplanar, with a dihedral angle of 21.67(11)° between
C1′–O1′–O2′–Mg and C1–O1–O2–Mg planes. Magnesium–
3. Results and discussion
3.1. Low-temperature phase of Mg(tfa)2·4H2O (100 K)
The crystal structure of the low-temperature phase of
Mg(tfa)2·4H2O is shown in Fig. 1a–e. Mg(tfa)2·4H2O crystallizes
ˉ
in the triclinic P1 space-group with Z = 2. The compound has a
monomeric structure in which the Mg2+ cation is octahedrally
coordinated by oxygen atoms: four of these atoms belong to
water molecules in a square-planar configuration, while the
Fig. 1 Crystal structure of Mg(tfa)2·4H2O at 100 K. Mg: orange, C: dark grey, O: red, F: green, H: purple. (a) First coordination sphere of the Mg2+
cation. Intramonomer hydrogen-bonds are depicted with black dashed lines. (b) Hydrogen-bond network connecting a magnesium–oxygen
monomer with four neighboring monomers. Hydrogen bonds are depicted with black dashed lines. –CF3 groups are omitted for clarity. (c)
Polyhedral representation of the structure in the [010] direction, where zig-zag chains running parallel to the a axis are observed. (d) Hydrogen
bonds between zig-zag chains. Intrachain and interchain interactions are depicted with long- and short-dashed lines, respectively. –CF3 groups are
omitted for clarity. (e) Polyhedral representation of the structure in the [100] direction, where one-dimensional channels running along the a axis are
observed. The channels are lined with –CF3 groups.
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Table 4 Donor (D)–acceptor (A) hydrogen-bond geometry (Å) in
Mg(tfa)2·4H2O at 100 K
ing atoms O1 (acceptor), O2W′ (donor and acceptor), and O3W
(donor). Each chain is connected to two equivalent chains
along the [010] and [001] directions via the two strongest
hydrogen bonds in the structure involving O2 and O2W on one
side, and O2′ and O2W′ on the other side. Since the shortest
hydrogen bonds are those connecting chains, the true nature
of the hydrogen-bond network is three dimensional. As a
result, each magnesium–oxygen monomer is connected to two
monomers within the same chain and to two other monomers
located in neighboring chains. Four adjacent chains define
infinite channels running parallel to the [100] direction
(Fig. 1e); these channels are lined with the –CF3 groups of the
trifluoroacetate ligands. Finally, it should be noted that weak
O–H⋯F interactions involving O2W′, F1′, and F3 may play a
role in determining the conformation of –CF3 groups.
D–H⋯A
D–H
H⋯A
D⋯A
D–H⋯A
O1W–H1⋯O2
0.816
0.792
0.832
0.823
0.814
0.814
0.840
0.840
0.783
0.794
2.088
2.038
1.851
1.983
2.045
2.479
1.853
2.582
2.064
2.068
137.85
142.70
173.61
161.92
152.25
136.42
172.10
115.61
150.28
149.80
2.747
2.711
2.680
2.777
2.791
3.120
2.687
3.041
2.771
2.783
O1W–H1′⋯O2′
O2W–H2B⋯O2i
O2W–H2A⋯O1′ii
O2W′–H2A′⋯O1iii
O2W′–H2A′⋯F3iii
O2W′–H2B′⋯O2′iv
O2W′–H2B′⋯F1′iv
O3W–H3⋯O2W′iii
O3W–H3⋯O2Wii
(i) –x, 2 − y, 1 − z; (ii) –x, 1 − y, 1 − z; (iii) 1 − x, 1 − y, 1 − z; (iv) 1 − x,
1 − y, −z.
oxygen octahedral monomers are connected to each other via
a complex three dimensional hydrogen-bond network depicted
in Fig. 1b; the geometric parameters of the network are given
in Table 4. Each magnesium–oxygen monomer is connected to
four others via hydrogen-bond interactions involving all four
coordinating water molecules and both pairs of oxygen atoms
of the carboxylate groups. The carboxylate oxygen atoms O2′/
O2, which are not coordinated to the metal center, are simul-
taneously involved in both intra- and intermonomer hydrogen-
bond. Olejnik and Lis provided an hierarchy of the hydrogen-
bonding network for the high-temperature phase of
Mn(tfa)2·4H2O,19 which is isostructural to the high-tempera-
ture phase of Mg(tfa)2·4H2O (vide infra). Although
subjective, this hierarchy shows that interactions between
Mg(OH2)4(OCOCF3)2 octahedra lead to the formation of zig-zag
chains running along the [100] direction (Fig. 1c). Intra- and
interchain hydrogen-bonding interactions can be distin-
guished (Fig. 1d). Intrachain interactions consist of four hydro-
gen bonds involving atoms O1′ (acceptor), O3W (donor), and
O2W (donor and acceptor), plus four hydrogen bonds involv-
3.2. High-temperature phase of Mg(tfa)2·4H2O (300 K) and
reversible phase transition
The crystal structure of the high-temperature phase of
Mg(tfa)2·4H2O is shown in Fig. 2a using a stick representation.
The structure of the low-temperature phase (100 K) is included
in Fig. 2b using an identical representation to facilitate com-
parison. Inspection of these figures demonstrates the remark-
able similarity between the monoclinic and triclinic crystal
structures. Only two differences are observed: (i) at 300 K
Mg(tfa)2·4H2O crystallizes in the monoclinic C2/c space-group
with Z = 4 (Z′ = 0.5), and (ii) –CF3 groups are rotationally dis-
ordered at 300 K, with each fluorine atom distributed between
three different positions with fractional occupancies of 53, 30,
and 17%. No changes are observed in the hydrogen-bond
network; the fluorine atom with the highest fractional occu-
pation in the disordered, high-temperature phase has the
same contacts as the fluorine atoms in the ordered, low-temp-
erature phase (see Table 5).
Fig. 2 Stick representations of the crystal structures of Mg(tfa)2·4H2O at (a) 300 and (b) 100 K. Mg: orange, C: brown, O: red; fluorine and hydrogen
atoms have been omitted for clarity. Unit cells are depicted with solid lines.
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Table 5 Donor (D)–acceptor (A) hydrogen-bond geometry (Å) in
Mg(tfa)2·4H2O at 300 K
phase transition at ∼160 K. Diffraction data at 160 K was
modeled using both triclinic and monoclinic symmetries (see
Table 1). Both models yielded consistent results, demonstrat-
ing that at 160 K the crystal is on the verge of the phase tran-
sition; splitting of the –CF3 groups was reduced to two crystal-
lographically distinct positions in both cases. Judging from
the refinement residuals, the monoclinic model performed
slightly better; thus, unit cell constants extracted from the
monoclinic structure at 160 K were employed in the plots
shown in Fig. 3. Temperature-dependent changes in the unit
cell are accommodated through a monotonous expansion of
the c axis and a contraction of the β angle upon increasing the
temperature; overall, this results in an increase in the unit cell
volume. Splitting of the reflections was observed upon cooling
from 200 to 143 K and was partially reversed upon heating to
D–H⋯A
D–H
H⋯A
D⋯A
D–H⋯A
O2W–H2A⋯O1i
O2W–H2B⋯O2ii
O1W–H1⋯O2
0.84
0.84
0.83
0.82
0.83
0.84
0.84
0.82
2.02
1.87
2.10
2.10
2.10
1.87
2.72
2.10
2.7963(10)
2.6978(10)
2.7545(8)
2.8363(7)
2.7545(8)
2.6978(10)
3.220(5)
154
170
135
149
135
170
120
149
O3W–H3⋯O2Wiii
O1W–H1⋯O2
O2W–H2B⋯O2ii
O2W–H2B⋯F1ii
O3W–H3⋯O2Wiii
2.8363(7)
(i) x, −y + 1, z − 1/2; (ii) −x + 1/2, −y + 3/2, −z + 1; (iii) −x + 1, −y + 1,
−z + 1.
Following the structure determination at 300 K, the tri- 200 K (see ESI†). Upon cooling to 143 K the crystal was
clinic-to-monoclinic phase transition was investigated to eluci- observed to be a twin with two triclinic components, as
ˉ
date (i) the transition temperature, (ii) whether it is reversible, expected for a C2/c to P1 phase transition. Furthermore, the
and (iii) the driving force for the transition. To this end, the geometric relationship between these two components was
crystal was subjected to the following cooling/heating found to be identical to that observed at 100 K and consistent
27
ˉ
sequence: 300 K → 200 K → 143 K → 160 K → 143 K → 200 K. with a C2/c to P1 twin-domain splitting. Altogether, these
Diffraction data for structure determination were collected in observations demonstrate that Mg(tfa)2·4H2O undergoes a
the last three steps of this sequence. The evolution with temp- reversible monoclinic-to-triclinic phase transition upon
erature of the unit cell constants and volume is plotted in cooling below 160 K and that this transition leads to crystal
Fig. 3; data points at 100 and 143 K correspond to pseudo- twinning. An identical phase transition was observed by
monoclinic cell constants. Inspection of Fig. 3a–c demon- Olejnik and Lis in Mn(tfa)2·4H2O at 215 K;19 this confirms the
strates that Mg(tfa)2·4H2O undergoes a triclinic-to-monoclinic structural similarity between the magnesium and manganese
trifluoroacetates. As mentioned earlier, the only difference
between the low-temperature (triclinic) and high-temperature
(monoclinic) phases is the rotational disorder of the –CF3
ˉ
groups in the latter. Thus, the transition from a P1 to a C2/c
symmetry upon increasing the temperature appears to be
related to this phenomenon. Indeed, when the –CF3 groups
are disordered, the magnesium–oxygen monomer possesses a
twofold rotation axis, which combined with the inversion sym-
metry of the lattice leads to a 2/m symmetry.
3.3. Map of structural relationships in Mg(tfa)2·4H2O,
Mn(tfa)2·4H2O, Mn3(tfa)6·4H2O, and Ca3(tfa)6·4H2O
Our next step was to establish chemical and structural relation-
ships between monometallic trifluoroacetates. In the follow-
ing, we focus on relationships existing between alkaline-earth
and manganese trifluoroacetates; for completeness, a systema-
tic comparison of the crystal-chemistries of Mg, Ca, Sr, and Ba
trifluoroacetates is provided in the ESI (Table S29 and
Fig. S10†). As mentioned earlier, the high-temperature phase
of Mg(tfa)2·4H2O is isostructural to the high-temperature
phase of Mn(tfa)2·4H2O reported by Olejnik and Lis.19 This led
us to investigate whether this relationship holds at low temp-
eratures as well. Although Olejnik and Lis acknowledged a
phase transition leading to crystal twinning at 215 K,15 they
Fig. 3 Evolution with temperature of the unit cell edges’ length
(top panel), angles (middle panel), and volume (bottom panel) of
Mg(tfa)2·4H2O. For the triclinic unit cell (100 and 143 K), pseudo-mono-
clinic cell constants are plotted. The dotted line depicts the estimated
did not provide a structural model for the low-temperature
phase of Mn(tfa)2·4H2O. Our attempt to perform a structural
investigation of Mn(tfa)2·4H2O at 100 K resulted in two inter-
esting findings: (i)
Mn3(tfa)6·4H2O, which is stable in the 100–298 K temperature
a
new compound of formula
triclinic-to-monoclinic transition temperature (160 K). Dashed lines are
guides-to-the-eye.
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Scheme 1 Structural relationships in Mg, Mn, and Ca trifluoroacetates. The ability of Mnx(tfa)2x·4H2O compounds to pivot between their mag-
nesium and calcium counterparts is highlighted.
range; and (ii) the observation that this new compound is ESI (Table S19†). The observation of a pseudo-tetragonal unit
isostructural to Ca3(tfa)6·4H2O.20,28 These findings allowed cell (i.e., a and b differ by 0.03 Å and β = 90.69°) is likely to be
us to construct a map of structural relationships between at the origin of the observed structural disorder and crystal
Mg(tfa)2·4H2O, Mn(tfa)2·4H2O, Mn3(tfa)6·4H2O, and Ca3(tfa)6· twinning. No structural changes were observed via single-
4H2O (Scheme 1).
crystal X-ray diffraction in the 100–298 K temperature range.
The crystal structure of Mn3(tfa)6·4H2O at 100 K is shown in The structure is identical to that of Ca3(tfa)6·4H2O reported by
Fig. 4. The structure is monoclinic (space-group P21/n) and fea- Khristov et al.28 and Dissanayake et al.20 (see ESI, Fig. S11†).
tures (i) trimers of manganese–oxygen corner-sharing octahe- The observation of a trimeric structure of manganese trifluoro-
dra, and (ii) layers running parallel to the b axis and the tri- acetate in the 100–298 K temperature range was unexpected
fluoromethyl groups directed outwards. The central manga- considering the monomeric nature of the previously known
nese atom is coordinated to six oxygen atoms belonging to car- high-temperature phase of Mn(tfa)2·4H2O. These observations
boxylate groups, whereas the terminal manganese atoms are were reconciled through Rietveld analysis of powder X-ray diffr-
coordinated to four oxygen atoms belonging to carboxylate action data (see ESI, Fig. S11†). The room-temperature X-ray
groups and two oxygen atoms from two water molecules. The diffraction pattern of polycrystalline Mnx(tfa)2x·4H2O could be
two coordinating water molecules are cis positioned. The struc- adequately fit as a mixture of the trimeric (space-group P21/n)
ture shows severe disorder of: (i) one of the two crystallographi- and monomeric (space-group C2/c) phases in weight fractions
cally distinct manganese atoms (displaced ∼0.15 Å off the of ∼11 and 89%, respectively. Therefore, two compounds of
center of symmetry), (ii) two trifluoroacetate moieties, formula Mnx(tfa)2x·4H2O can be distinguished: the C2/c mono-
(iii) both water molecules, and (iv) –CF3 groups (rotationally meric tetrahydrate (x = 1) stable above 215 K and originally
disordered). Details on the fractional occupancies of the reported by Olejnik and Lis,19 and the previously unknown
manganese, carbon, oxygen and fluorine sites are given in the P21/n trimeric hydrate (x = 3) stable in the 100–298 K range.
Both compounds coexist at room temperature. More impor-
tant, Mnx(tfa)2x·4H2O compounds are isostructural to the high-
temperature phase of Mg(tfa)2·4H2O (monomeric) on the one
hand, and to Ca3(tfa)6·4H2O (trimeric) on the other hand. As
will be shown in the next section, the chemical and structural
flexibility of Mnx(tfa)2x·4H2O to behave as its magnesium or
calcium counterparts constitutes a synthetic principle to
direct the preparation of Mg–Mn and Ca–Mn bimetallic
trifluoroacetates.
3.4. Crystal structures of Mg–Mn and Ca–Mn bimetallic
trifluoroacetates
Fig. 4 Crystal structure of Mn3(tfa)6·4H2O (100 K). Mn: purple; C:
brown; O: red; F: light blue; H: pink. Manganese–oxygen octahedra are
arranged in trimers (left panel). Layers running parallel to the b axis are
observed (right panel). The structure remains unchanged in the
Crystallization of Mg–Mn and Ca–Mn bimetallic trifluoroace-
tates was attempted using mixtures containing Mg2+ : Mn2+
and Ca2+ : Mn2+ in a 1 : 1 molar ratio, respectively. The crystal
structures of the resulting Mg0.45Mn0.55(tfa)2·4H2O and
Ca1.72Mn1.28(tfa)6·4H2O are shown in Fig. 5a and b, respect-
100–298 K temperature range. Severe structure disorder was observed;
atom splitting is not shown for clarity (see text for details).
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Fig. 5 Crystal structures of (a) Mg0.45Mn0.55(tfa)2·4H2O and (b)
Ca1.72Mn1.28(tfa)6·4H2O (100 K). Mg: orange; Mn: purple; Ca: blue; C:
brown; O: red; F: light blue; H: pink. Mg2+ and Mn2+ are nearly equally
distributed over the octahedral sites (Mg/Mn: 45/55). Ca2+ and Mn2+ are
unequally distributed over the central (Ca/Mn: 96/4) and terminal (Ca/
Mn: 38/62) octahedral sites of the corner-sharing metal–oxygen
trimers. Ca1.72Mn1.28(tfa)6·4H2O exhibits structural disorder (see text for
details).
Fig. 6 Metal–oxygen bond distances in monometallic and bimetallic
trifluoroacetates. All six distances are given for each octahedral site.
Average distances are depicted with hollow symbols. For bimetallic
compounds, the composition-weighted average distance for each site is
depicted with red symbols.
ively. Mg0.45Mn0.55(tfa)2·4H2O is isostructural to the low-temp-
2+
erature phase of Mg(tfa)2·4H2O (triclinic, P1). Mg and Mn2+ position-weighted average of bond distances encountered in
ˉ
are nearly equally distributed over the metal–oxygen octahedra
(Mg/Mn: 45/55). Ca1.72Mn1.28(tfa)6·4H2O is isostructural to
Mg(tfa)2·4H2O and Mn(tfa)2·4H2O, which equals 2.1293 Å
(2.0683 0.45 2.1792 0.55). In the case of
×
+
×
Ca3(tfa)6·4H2O and Mn3(tfa)6·4H2O (monoclinic, P21/n). Metal– Ca1.72Mn1.28(tfa)6·4H2O, the average metal–oxygen bond dis-
oxygen octahedra form corner-sharing trimers, with Ca2+ and
tance for the central site of the trimer is 2.358 Å. This is in
Mn2+ unequally distributed over the central (Ca/Mn: 96/4) and
good agreement with the composition-weighted average of dis-
terminal (Ca/Mn: 38/62) sites. As observed in Mn3(tfa)6·4H2O, tances encountered in Mn3(tfa)6·4H2O and Ca3(tfa)6·4H2O,
the structure exhibits severe disorder. Metal–oxygen bond dis-
tances encountered in Mg–Mn and Ca–Mn bimetallic trifluor-
which equals 2.327 Å (2.195 × 0.04 + 2.332 × 0.96). The close
agreement of these two values is consistent with the central
oacetates and in their monometallic counterparts are given site being almost exclusively occupied by Ca2+. Bond valence
in Table
6 and plotted in Fig. 6. In the case of
analysis using parameters R0 = 1.967 Å and b = 0.37 gives
Mg0.45Mn0.55(tfa)2·4H2O, the average metal–oxygen bond dis-
further support to this finding: a bond valence sum of +2.13 is
tance is 2.1298 Å. This is in excellent agreement with the com- thus obtained for the central site of the trimer, consistent with
Table 6 Metal–oxygen bond distances (Å) in mono- and bimetallic trifluoroacetates
Mg(tfa)2·4H2Oa
Mg0.45Mn0.55(tfa)2·4H2Oa
Mn(tfa)2·4H2Ob
Mn3(tfa)6·4H2Oa
Ca1.72Mn1.28(tfa)6·4H2Oa
Ca3(tfa)6·4H2Oc
ˉ
P1, monomeric
C2/c, monomeric
P21/n, trimeric
4 H2O + 2 tfa (trans)
Central: 6 tfa
Ca/Mn: 96/4
2.264(7)
2.273(8)
2.372(7)
2.343(4)
2.447(3)
2.451(3)
〈2.358〉
2.0259(18)
2.0405(18)
2.0672(18)
2.0819(18)
2.0946(16)
2.0995(16)
〈2.0683〉
2.0949(11)
2.1053(10)
2.1314(11)
2.1450(10)
2.1510(10)
2.1514(11)
〈2.1298〉
2.136(4)
2.157(3)
2 × 2.193(2)
2 × 2.198(2)
2.101(13)
2.118(12)
2.125(14)
2.133(13)
2.328(14)
2.362(13)
〈2.195〉
2 × 2.282(4)
2 × 2.294(4)
2 × 2.421(2)
〈2.1792〉
〈2.332〉
Terminal: 4 tfa + 2 H2O (cis)
Ca/Mn: 38/62
2.147(9)
2.168(16)
2.182(11)
2.188(18)
2.226(3)
2.280(3)
〈2.199〉
2.1860(13)
2.1888(18)
2.236(8)
2.318(4)
2.332(4)
2.368(3)
2.379(4)
2.385(4)
2.464(3)
〈2.374〉
2.241(9)
2.2681(11)
2.3231(12)
〈2.241〉
a Data at 100 K. This work. b Data at 220 K. Ref. 19. c Data at 298 K. Ref. 20.
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it being near fully occupied by Ca2+ ions.29 Likewise, the (National Science Foundation MRI-1427926). Prof. Massimo
average metal–oxygen bond distance for the terminal site is Nespolo (CRM2, Université de Lorraine, France) is gratefully
2.241 Å, in good agreement with the composition-weighted acknowledged for helpful discussions.
average of distances encountered in the parent monometallic
compounds, which equals 2.266 Å (2.199 × 0.62 + 2.374 ×
0.38). The consistency of these results is quite remarkable,
especially in the case of the Ca–Mn bimetallic trifluoroacetate,
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4. Conclusions
In conclusion, the synthesis of Mg–Mn and Ca–Mn bimetallic
trifluoroacetates was accomplished following a systematic
investigation of the crystal-chemistry of A(tfa)2·nH2O (A = Mg,
Ca, Sr, Ba, Mn) monometallic trifluoroacetates. Crystallo-
chemical relationships were established between Mg(tfa)2·4H2O,
Mn(tfa)2·4H2O, Mn3(tfa)6·4H2O, and Ca3(tfa)6·4H2O. The evolu-
tion with temperature of the crystal structure of Mg(tfa)2·4H2O
was found to be identical to that of Mn(tfa)2·4H2O, with both
systems exhibiting a phase transition leading to crystal twinning
at 160 and 215 K, respectively. The ability of Mnx(tfa)2x·4H2O to
pivot between the structures of the high-temperature phase of
Mg(tfa)2·4H2O and that of Ca3(tfa)6·4H2O enabled the synthesis
of bimetallic trifluoroacetates containing Mg–Mn and Ca–Mn as
metal centers. Mg0.45Mn0.55(tfa)2·4H2O was found to be isostruc-
tural to Mg(tfa)2·4H2O, with Mg2+ and Mn2+ nearly equally
distributed over the octahedral sites (Mg/Mn: 45/55).
Ca1.72Mn1.28(tfa)6·4H2O was found to be isostructural to
Ca3(tfa)6·4H2O, with Ca2+ and Mn2+ unequally distributed over
the central (Ca/Mn: 96/4) and terminal (Ca/Mn: 38/62) sites of
the corner-sharing metal–oxygen trimers. Analysis of metal–
oxygen bond distances in the bimetallic trifluoroacetates gave
further support to site occupancies extracted from single-crystal
X-ray diffraction. This work demonstrates that chemical and
structural principles enabling the rational design of multimetal-
lic trifluoroacetates can be established through a systematic
investigation of the crystal-chemistry of their monometallic
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Acknowledgements
The authors would like to acknowledge the financial support
of the National Science Foundation (DMR-1606917) and the 11 S. Fujihara, M. Tada and T. Kimura, Controlling factors for
Department of Chemistry at Wayne State University. They also
thank the Lumigen Instrument Center at Wayne State
the conversion of trifluoroacetate sols into thin metal fluor-
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