A Mn6 Cluster inside a Mn10 Wheel: Characterization of a Mn16-Oximate Complex Resulting 1
from a Tetrazole-2-pyridylketoneoximate Ligand 2
3 4 5 6
Laura Alcazar,[a] Mercè Font-Bardia,[b] and Albert Escuer*[a] 7 8 9 10 11 12 13 14 15 16
[a] Departament de Química Inorgànica, Universitat de Barcelona, Av. Diagonal 645, 08028 Barcelona, Spain
17
E-mail: albert.escuer@ub.edu
18
http://www.ub.edu/inorgani/recerca/MagMol/magmol.htm
19
[b] Department de Cristallografia, Mineralogia i Dipòsits, Universitat de Barcelona, Martí Franquès s/n, 08028 Barcelona,
20 Spain 21 22 23 24
Keywords: Manganese / Mixed-valent compounds / Magnetic properties 25
26 27 28 29
ABSTRACT: 30
31
A new Mn16 topology consisting of one hexanuclear {MnII2MnIII4} unit coordinated inside a 32
decametallic {MnII6MnIII2MnIV2} wheel was synthesized from 2-pyridylcyanoxime and azido 33
ligands. A new tetrazole-2-pyridylketoneoximate ligand was obtained in situ by cyclization of the cyano 34
group and azide mediated by manganese cations. 35
36 37
Manganese clusters with a {Mn12O12(RCOO)16} core have been key compounds in SMM research 38
since its discovery.[1] This core can be envisaged as a MnIV cube inside a MnIII8 wheel, linked by oxo 39
and carboxylato bridges. In search of a SMM response, larger metallic wheels around in-plane defective 40
cubane fragments have been characterized,[2–5] which form a beautiful family of related structures with 41
the easy axis of the MnIII cations roughly aligned (Scheme 1). These larger complexes exhibit lower 42
spin ground states than MnIII8MnIV4 and, except for one of the MnIII10MnIV6 complexes,[3] small 43
magnetization reversal barriers[2,4] or even no SMM response[5] were reported. 44
Following our studies on the reactivity of 2-pyridyloximes with manganese cations,[6] we have explored 45
new synthetic routes employing the ligand 2-pyridylcyanoxime (pyC{CN}NOH) (Scheme 2). Air 46
oxidation of Mn(hfacac)2, pyC{CN}NOH, Cs(OH), and NaN3 in a 1:1:1:1 ratio in acetonitrile gives the 47
hexadecanuclear complex [MnII8MnIII6MnIV 2(N3)4(pyCOO)2 {(CH3COO)2pyC-48
{CN}NO}10(pyC{Tz}NO)2(O)10]·4MeCN (1·4MeCN), which shows a structural relationship with the 49
above-described MnIII10MnIV 6 complex (Scheme 1). In this work, we present the characterization of 50
complex 1, the study of its magnetic response, and the structural characterization of the new tetrazole-2-51
pyridylketoneoximate ligand (pyC{Tz}NO2–) (Scheme 2), which is the product of the cyclization 52
reaction between 2-pyridylcyanoxime and azide anions. 53
Mixed-valent complex 1 contains sixteen manganese atoms linked by means of ten pyC{CN}NO– 54
ligands in their η1:η1:η1:μ and η1:η1:η2:μ3 coordination modes, six μ4-O and four μ3-O oxo donors, 55
four end-on azido bridges, four syn-syn carboxylates, and two η1:η1:η1:η2:μ3 pyC{Tz}NO2– ligands 56
(Figure 1). The oxidation states have been assigned on the basis of bond lengths, distortion of the 57
coordination polyhedra, and BVS calculations (Table S1), which results in two tetravalent manganese 58
cations [Mn(5) in an octahedral MnO6 environment], six trivalent manganese cations [Mn(1,2,3) in 59
axially elongated MnO5, MnO5N, and MnO4N2 square-pyramidal or octahedral coordination 60
environments], and eight MnII cations {four of them [Mn(4,6)] in a distorted octahedral environment 61
and four MnO3N4 heptacoordinated cations Mn(7,8)}. 62
The oximato bridges involve all the manganese atoms with a wide range of Mn–N–O–Mn torsions, 63
Table S2. Two azido ligands are coordinated to the external wheel with a Mn(4)–N(19)–Mn(6) bond 64
angle of 93.6(3)° and another two to the inner hexanuclear unit with a Mn(2)–N(22)–Mn(8) bond angle 65
of 110.4(4)°. 66
Ten manganese cations Mn(3), Mn(4), Mn(5), Mn(6), Mn(7) (and symmetry equivalents) form a 67
{MnII6MnIII2MnIV2(O)8(N3)2} wheel around one {MnII2MnIII4(O)8(N3)2} defective dicubane 68
fragment. The hexanuclear fragment, composed of the Mn(1), Mn(2), Mn(8) cations (and their 69
symmetry equivalents), is linked to the wheel by means of six μ4-O and additional oximato bridges. The 70
structure of 1 shows closely related fragments with the above-described MnIII10MnIV6 complex,[3,4] 71
but in addition to the different manganese oxidation states, it shows a subunit of the inner defective 72
cubane placed perpendicularly to the mean wheel plane, which generates an unprecedented Mn16 core 73
(Figure 2). It is remarkable that complex 1, together with two Mn16 complexes containing methyl-2-74
pyridylketoneoximate[7] or 2,6-diacetylpyridineoximate,[8] exhibits the maximum nuclearity seen to 75
date in complexes that have 2-pyridyloxime ligands and is the largest Mn/2-pyridyloximate complex 76
containing azido bridges.[9] 77
Oximes and oximate–metal species exhibit a wide reactivity without preservation of the (–C–N=O) 78
function.[10] As an example, in pyridyloximate chemistry, the products of dehydration or rearrangement 79
into amides have often been observed, and, as a consequence, the presence of the pyCOO–/CH3COO– 80
ligands that result from oxidative deoximation promoted by high-valent cations is not rare.[10b] Much 81
more interesting is the unprecedented reactivity of 2-pyridylcyanoximate into the new tetrazole-2-82
pyridylketoneoxime ligand (Scheme 3). The new pyC{Tz}NO2–ligand shows evident structural 83
similarities with the di-2-pyridylketoximate ligand, but, in this case, the charge is 2–because of the 84
deprotonation of the oximate group and the loss of the tetrazole proton. In 1, the mean pyridine and 85
tetrazole planes are tilted 34°, and four donor atoms are involved in linking the manganese cations, 86
which, after coordination, forms two five and six-membered chelating rings (Scheme 2). 87
Although the reaction of nitriles with the azide anion to produce tetrazoles is well known, it requires 88
typically large reaction times (even days) and relatively high temperatures.[11] However, some time ago 89
Ellis and Purcell reported the synthesis of tetrazole that was assisted by CoIII;[12] more recently, 90
several authors have reported that the dipolar addition becomes easier when the reaction is performed on 91
a metallic coordination site: Sharpless et al. reported a synthetic method in the presence of ZnII as 92
catalysts,[13] with reduction of the reaction times to hours at 100 °C, and Henderson et al. reported the 93
room temperature syntheses, assisted by MnII, of several pyridyltetrazoles in 2005.[14] Thus, the 94
presence of the new tetrazole-2-pyridylketoneoximate ligand should be attributed to a similar reaction 95
mediated by the previous coordination of 2-pyridylcyanoximate ligands to one MnII cation and 96
subsequent cycloaddition of one azido anion. 97
Variable-temperature magnetic susceptibility measurements were performed on powdered samples of 1 98
under an external field of 0.03 T over the range 2–300 K. The χMT value decreases continuously from 99
44.1 cm3mol–1K at 300 K and continues to decrease on cooling to 10.0 cm3mol–1K at 2 K, which 100
indicates dominant antiferromagnetic interactions and a low S ground state (Figure 3). The room-101
temperature value is lower than the spin value of 56.75 cm3mol–1K expected for eight uncoupled MnII 102
(S = 5/2), six MnIII (S = 2) and two MnIV (S = 3/2) ions because of the moderately strong MnII–MnIII 103
and MnII–MnIII antiferromagnetic coupling mediated by the oximate bridges. 104
The magnetization measurements show a maximum value equivalent to twelve electrons under the 105
maximum field of 5 T. The magnetization still strongly increases under the maximum field and 106
saturation is not achieved because of continuous population of the low-lying larger spin levels under a 107
high external field. From the low temperature χMT value and the slope of the magnetization plot in the 108
lowfield region, an S = 4 ground state appears to be reasonable for 1. The easy axis of the six MnIII 109
cations are roughly parallel, but χ M out-of-phase signals were not found in the AC susceptibility 110
measurements above 2 K, as could be expected for a centrosymmetric cluster (Figure S2). 111
In conclusion, we have reported a mixed-valent MnII8MnIII6MnIV 2 complex from reaction of the 112
precursor Mn(hfacac)2 with the 2-pyridylcyanoxime and azido ligand in basic medium. The complex 113
exhibits an unprecedented topology consisting of a hexanuclear {MnII2MnIII4} defective cubane unit 114
coordinated inside a decanuclear {MnII6MnIII2MnIV2} wheel. Reaction of 2-pyridylcyanoximate with 115
azido anions, mediated by manganese cations, leads to the characterization of a new member of this 116
family of ligands containing a tetrazole group, which introduces changes in the total charge of the ligand 117
and offers new coordination possibilities. Further attempts to isolate pure tetrazole-2-118
pyridylketoneoxime are underway. 119
EXPERIMENTAL SECTION 121
122
Physical Measurements: Magnetic susceptibility measurements were carried out on polycrystalline 123
samples by using a Quantum Design MPMS-5 SQUID susceptometer working in the range 2–300 K 124
under magnetic fields of 0.3 T (300–30 K) and 0.03 T (30–2 K) to avoid saturation effects. Diamagnetic 125
corrections were estimated from Pascal tables. Infrared spectra (4000–400 cm–1) were recorded from 126
KBr pellets on a Bruker IFS-125 FT-IR spectrophotometer. 127
Syntheses: Reagents (Sigma–Aldrich Inc.) were used as received. The pyC{CN}NOH ligand was 128
prepared according to the improved[ 15] method reported in the literature.[16] 129
[{Mn16(N3)4(pyCOO)2(CH3COO)2pyC{CN}NO}10(pyC{Tz}NO)2-(O)10]·4MeCN (1·4MeCN): 130
(1·4MeCN) was obtained from the reaction of [Mn(hfacac)2]·3H2O (262 mg, 0.5 mmol), 131
pyC{CN}NOH (76 mg, 0.5 mmol) and NaN3 (33 mg, 0.5 mmol) in a basic medium of CsOH (83 mg, 132
0.5 mmol) in acetonitrile (20 mL) as solvent. After the mixture was stirred in open air for 30 min, the 133
solution became dark brown. Dark crystals (almost black, reddish by transparency) adequate for X-ray 134
diffraction were obtained after a week by crystallization in a closed vial or by layering with diethyl 135
ether; moderate yield (30 %). Dried C100H62Mn16N56O30 (1) (3406.95): calcd. C 35.25, H 1.83, N 136
23.02; found C 35.0, H 1.5, N 22.5. IR: ν˜ = 3424, 2919 (w), 2851 (w), 2221 (w, CN), 2069 (vs, end-on 137
N3), 1598 (s), 1465–1458 (s), 1426 (w), 1332 (w), 1212 (m), 1174 (m), 1152 (s), 1100 (s), 1059 (m), 138
1031 (vs), 783 (m), 713 (m9), 601 (m) cm–1. 139
Single-Crystal X-ray Structure Analyses: A black plate-like specimen of C108H74Mn16N60O30, 140
approximate dimensions 0.070 0.120 0.220 mm, was used for the X-ray crystallographic analysis. 141
The X-ray intensity data were measured on a Bruker APEX-II CCD system equipped with a graphite 142
monochromator and a Mo sealed tube (λ = 0.71073 Å). The integration of the data using a triclinic unit 143
cell yielded a total of 35261 reflections to a maximum θ angle of 22.06° (0.95 Å resolution), of which 144
9766 were independent (average redundancy 3.611, completeness = 99.2 %, Rint = 10.70%, Rsig = 145
13.67%) and 5077 (51.99%) were greater than 2σ(F2). The final cell constants of a = 14.732(3) Å, b = 146
17.427(3) Å, c = 18.052(3) Å, α = 63.703(7)°, β = 84.952(8)°, γ = 73.532(7)°, volume = 3980.7(13) Å3 147
are based upon the refinement of the XYZ-centroids of reflections above 20σ(I). The structure was 148
solved by using the Bruker SHELXTL Software Package and refined by using SHELXL.[17] The unit 149
cell contains one acetonitrile and one water molecule, which have been treated as a diffuse contribution 150
to the overall scattering without specific atom positions by SQUEEZE/PLATON. Details of the crystal 151
data and refinement are given in Table S3. CCDC-1028713 (1) contains the supplementary 152
crystallographic data for this paper. These data can be obtained free of charge from The Cambridge 153
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 154
ACKNOWLEDGEMENTS 156
157
Funds from CICYT Project CTQ2012-30662 are acknowledged. A. E. is thankful for financial support 158
from the Excellence in Research ICREA-Academia Award. 159
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200 . 201
Legends to figures 202
203
Scheme 1. Core of MnIII8MnIV4, MnIII9MnIV6, MnIII10MnIV6, and MnIII12MnIV9 complexes 204
consisting of MnIV cubane or defective cubane fragments inside MnIII wheels. Color key: MnIII, dark 205
green; MnIV, firebrick. 206
207
Scheme 2. 2-pyridylcyanoxime (top) and 2-pyridyltetrazoleketoxime (bottom) ligands and their 208
coordination modes found in compound 1. 209
210
Figure 1. Top: view of the molecular assembly of compound 1. The green bonds emphasize the inner 211
hexanuclear subunit. Bottom: labeled plots of the Mn10 wheel and the Mn6 subunits of 1. Color key: 212
MnII, yellow; MnIII, dark green; MnIV, firebrick; O, red; N, navy blue; C, black.. 213
214
Figure 2.. View of the relative position of the Mn10 wheel and the Mn6 fragment for complex 1. 215
216
Scheme 3. Cycloaddition of the azide anion to the nitrile function of the ligand pyC{CN}NO–. 217
218
Figure 3. χMT product vs. T plot for compound 1. Inset: magnetization plot measured at 2 K. 219
220 221
SCHEME 1. 222 223 224 225 226
SCHEME 2. 227 228 229 230 231
FIGURE 1. 232 233 234 235 236
FIGURE 2. 237 238 239 240 241 242
SCHEME 3. 243 244 245 246 247
FIGURE 3. 248 249 250 251 252
