Three-Dimensional Porous Architectures Based on MnII/III Three-Blade Paddle Wheel Metallacryptates

Article abstract of DOI:10.1021/acs.cgd.8b01921

Three metallacryptate supramolecular assemblies have been obtained using salicylhydroxamic acid derivatives (H₃L). The three ligands differ in the residue at the para position with respect to the hydroxamic function (-H, -NH₂, and -(

Full text of DOI:10.1021/acs.cgd.8b01921

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Article
Three-dimensional porous architectures based
II/III
on Mn three-blades paddlewheel metallacryptates
Vittoria Marzaroli, Giulia Spigolon, Giuseppe Lococciolo, Martina Quaretti, Clarissa Salviati,
Jeff W. Kampf, Giulia Licini, Luciano Marchio', Vincent L. Pecoraro, and Matteo Tegoni
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01921 • Publication Date (Web): 31 Jan 2019
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Crystal Growth & Design
Three-dimensional porous architectures based on
MnII/III three-blades paddlewheel metallacryptates
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Vittoria Marzaroli,a Giulia Spigolon,b Giuseppe Lococciolo,a Martina Quaretti,a Clarissa
Salviati,a Jeff W. Kampf,c Giulia Licini,b Luciano Marchiò,a Vincent L. Pecoraro*c and
Matteo Tegoni*a.
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a. Department of Chemistry, Life Sciences, and Environmental Sustainability, Parco
Area delle Scienze 11A, 43124 Parma, Italy. Tel. +39 0521 905424; Email:
matteo.tegoni@unipr.it
b. Department of Chemical Sciences, University of Padova via Marzolo 1, 35131
Padova, Italy.
c. Department of Chemistry, University of Michigan, Ann Arbor, MI, 48109-1055, USA.
Tel.: +1 734 7631519; Email: vlpec@umich.edu
Abstract
Three metallacryptate supramolecular assemblies have been obtained using
salicylhydroxamic acid derivatives (H3L). The three ligands differ for the residue in para
position with respect of the hydroxamic function (-H, -NH2 and –(4-pyridyl)). The core of
these supramolecules correspond to the formula [(MnII)2(MnIII)9(μ3-O)4(OAc)3(Shi)6]2+
(abbreviated to Mn11L6) in all three compounds. Additional acetate or hydroxide ions,
and DMF molecules are coordinated to the MnIII ions. With L3- = Shi3- the Mn11L6 units
assemble into a non-porous structure, where the metallacryptates are linked together
through NaI ions bridging between the units. Conversely, with L3- = p-aShi3- (NH2 group)
and p-pyShi (4-pyridyl group) the units assemble in parallel chains and into a MOF-like
structure, respectively. In both solids, a honeycomb porous architecture was obtained,
where infinite channels run along the crystal structure. With p-aShi3-, NaI bridging ions
and hydrogen bonds, provided by the presence of the amino group, characterize the
interactions between the parallel chains of supramolecules. On the other hand with p-
pyShi3-, the peripheral pyridyl groups point away from the metallacryptate units and are
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coordinated to available positions on MnIII ions of neighbouring molecules, with
formation of a peculiar porous-coordination polymeric architecture.
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Figure Channel-like cavities in compound 13. The surface of the hexagonal channels
is represented in green.
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Three-dimensional porous architectures based on
MnII/III three-blades paddlewheel metallacryptates
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Vittoria Marzaroli,a Giulia Spigolon,b Giuseppe Lococciolo,a† Martina Quaretti,a Clarissa
Salviati,a Jeff W. Kampf,c Giulia Licini,b Luciano Marchiò,a Vincent L. Pecoraro*c and
Matteo Tegoni*a.
a.
Department of Chemistry, Life Sciences, and Environmental Sustainability, Parco
Area delle Scienze 11A, 43124 Parma, Italy. Email: matteo.tegoni@unipr.it
b.
Department of Chemical Sciences, University of Padova, via Marzolo 1, 35131
Padova, Italy.
c.
Department of Chemistry, University of Michigan, Ann Arbor, MI, 48109-1055,
USA. Email: vlpec@umich.edu
Abstract
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Three metallacryptate supramolecular assemblies have been obtained using
salicylhydroxamic acid derivatives (H3L). The three ligands differ for the residue in para
position with respect of the hydroxamic function (-H, -NH2 and –(4-pyridyl)). The core of
these supramolecules correspond to the formula [(MnII)2(MnIII)9(μ3-O)4(OAc)3(Shi)6]2+
(abbreviated to Mn11L6) in all three compounds. Additional acetate or hydroxide ions,
and DMF molecules are coordinated to the MnIII ions. With L3- = Shi3- the Mn11L6 units
assemble into a non-porous structure, where the metallacryptates are linked together
through NaI ions bridging between the units. Conversely, with L3- = p-aShi3- (NH2 group)
and p-pyShi3- (4-pyridyl group) the units assemble in parallel chains and into a MOF-like
structure, respectively. In both solids, a honeycomb porous architecture was obtained,
where infinite channels run along the crystal structure. With p-aShi3-, NaI bridging ions
and hydrogen bonds, provided by the presence of the amino group, characterize the
interactions between the parallel chains of supramolecules. On the other hand with p-
pyShi3-, the peripheral pyridyl groups point away from the metallacryptate units and are
coordinated to available positions on MnIII ions of neighbouring molecules, with
formation of a peculiar porous-coordination polymeric architecture.
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Introduction
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There is great interest today in developing porous coordination polymers and, in
general, large ordered assemblies of metal complexes.1–6 These substances are
promising materials that may have applications in gas storage, separations of small
molecules, catalysis, luminescence and magnetism.7 For the construction of such
materials, it is sometimes hard to predict the topology of the final architecture and to
control the properties of the material. One successful approach for that purpose is
constructing a 3D architecture, using pre-assembled polynuclear complexes, that exhibit
at the molecular level the desired properties for the final material.8,9
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Metallacrowns, the inorganic analogues of crown ethers, provide an opportunity to
exploit these strategies for the construction of 3D architectures.10–13 These
metallamacrocyclic complexes are assembled from metal ions and bridging organic
ligands such as hydroxamic acids, and are characterized by the repetition of a cyclic (M-
N-O)n connectivity (Scheme 1a). In metallacrowns, the oxygen atoms of the
metallacycle point toward the center of the cavity, into which metal ions (from transition
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(d-) to lanthanide (f-) elements) can be encapsulated.11,14,15 Closely related to
metallacrowns are inverse metallacrowns and metallacryptates.16–23 Inverse
metallacrowns share with metallacrowns the cyclic repetition of a (M-N-O)n connectivity,
but the conformation of the metallacyclic ring brings the metals rather than the oxygen
to the interior point of the cavity (Scheme 1b).16–19 In these constructs, the cavity is,
therefore, occupied by anions such as oxide, hydroxide or halogenides, which
coordinate to the metal ions. More complex 3D assemblies using the same, or closely
similar, (M-N-O)n repeat can lead to metallacryptates, the inorganic analogues of
cryptates. In these molecules, one can often discern interconnected metallacrown or
inverse metallacrown motifs and connectivities that form a cage into which cations or
anions are trapped (Scheme 1c).20–23
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Scheme 1. General schematic representations of the metallacyclic rings of
metallacrowns (a), inverse metallacrowns (b) and inverse metallacryptates (c).
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We have carried out in the past years an extensive investigation of the functional
properties of metallacrowns.11,14,15,24,25 The most promising in terms of their possible
applications are their luminescence brightness,26–30 their single-molecule magnetic
behavior,21,31,40–42,32–39 and their selectivity as hosts for both anionic or cationic
species.43–51 These are all desirable properties to be incorporated into porous
coordination polymers. Because metallacrowns are stable in solution over a wide range
of conditions (e.g. pH, presence of competing ligands or metals, different
solvents),15,45,52 they serve as ideal building blocks for the assembly of 3D- coordination
polymers based on polymetallic species.11,12,53 Metallacrowns can be preassembled in
solution and, therefore, converted into the desired solid state architectures by variation
of the conditions of crystallization. This capacity to give rise to coordination polymers
has been documented.54–63
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Several three-dimensional MC-based architectures have been presented in the
literature in the past few years.54–63 However, among these only a few were reported to
possess real pores and none of them exhibited permanent porosity.59,61 We have
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recently provided an important advancement in this field by reporting the preparation of
the first three-dimensional coordination polymer based on a MC framework that exhibits
permanent porosity.10 This material was formed by CuII[12-MCCu,L-4] units, where the
ligand L is a derivative of salicylhydroxamic acid, connected by CuII ions to form
chessboard-like 2D layers that stack one above the other. The channels present in this
framework have the capacity of being evacuated at high temperature, and filled with gas
molecules with no loss of crystallinity.10
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More recently, efforts have been presented in the literature on making extended solids
of materials that may have individual units with SMM and MOF behavior.64–68 In view of
obtaining these types of functional materials, we decided to move from copper to
manganese assemblies. The latter can actually be more promising for revealing
interesting magnetic properties as the result of the great single ion paramagnetism and
magnetic anisotropy.69 In fact, the first documented single molecule magnets (SMMs)
were indeed collections of MnIII and MnIV.70,71 Furthermore, several metallacrowns and
metallacryptates containing mixed-valence Mn assemblies have been shown to exhibit
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slow magnetic relaxation that is consistent with SMMs.21,22,31,33–36 In this paper we
present the synthesis of three salicylhydroximate derivatives (Shi3-, Scheme 2) used to
prepare self-assembled metallacryptate constructs which are three-fold axial
symmetrical. On one hand, with all three ligands (H3L) the obtained metallacryptate
assemblies have general stoichiometry Mn11L6 and they contain both MnII and MnIII
ions. These molecules are the first examples of [2.2.2] metallacryptates, and among the
very few metallacryptates isolated to date.20–23 Perhaps more importantly, by rational
design of the ligands derivatives the three-fold symmetry of these molecules was fully
exploited to obtain of three new 3D frameworks among which two are porous. By X ray
structural characterization we demonstrated how a modification of the ligand by one
atom on the aromatic ring of Shi3- converts this molecular species into a 3D array
allowing us to move from a non-porous molecular packing to honeycomb-like
architectures where infinite channels run along the crystal structure.
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Scheme 2. Representation of H3Shi (a) and its two derivatives derivatives H3p-aShi (b,
5) and H3p-pyShi (c, 10).
Experimental
Materials and methods
Chemicals were purchased from Sigma-Aldrich and TCI Chemicals, and used without
further purification. Reactions performed under an inert atmosphere were carried out
using Schlenk glassware using nitrogen as the inert gas. Flash column chromatography
was performed using silica gel (230‐400 mesh). Melting points were recorded by Stuart
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Melting Point SMP11. H and C{1H} NMR spectra were recorded on Bruker Avance-
DPX-200, Bruker Avance 400, Varian MR400 and Varian Vnmrs500 spectrometers
using standard pulse sequences. Chemical shifts were referenced to the residual
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signals of the solvents. The 13C{1H} NMR spectra (50 MHz) were referenced to the
CDCl3 peak. The following abbreviations are used in reporting the multiplicity for NMR
resonances: s=single, d=doublet, dd= doublet of doublet, t= triplet, and m=multiplet. FT-
IR were recorded with a Perkin-Elmer FTIR 1650 or with a Perkin-Elmer FTIR Nexus
spectrometer using a Smart Orbit HATR accessory equipped with a diamond crystal.
Electrospray ionization mass spectra (ESI-MS) of compounds 2-5 were collected on a
Micromass LCT TOF electrospray ionization mass spectrometer, using a capillary
voltage of 3500 and a desolvation temperature of 350 °C. Samples (40 μM) were
injected through direct infusion using a syringe pump at 11 μL/min, and the spectra
recorded in full scan analysis mode in the range 100‐2000 m/z. High resolution
electrospray ionization mass spectrometry HRMS (ESI-QTOF) analyses of compounds
6-10 were performed with a Waters Xevo G2-S QTof in positive and negative mode
(mixture of water/acetonitrile 50:50 with 0.1% of formic acid as mobile phase) with the
following settings: solvent flow= 50 μL/min , dry gas = 800 L/h, dry Temp = 350 °C,
detector potential = 2 KV. MS peak intensity for each analysis is reported as
monoisotopic mass and the data were processed with Mass-Lynx 4.1.
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X-ray crystallography. Single crystal X ray diffraction data of 11 were collected at 293
K on a Bruker Smart APEXII CCD diffractometer (Mo Ka; l = 0.71073 Å). Intensities
were integrated from several series of exposure frames that covered the sphere of
reciprocal space.72 Single crystal X ray diffraction data of metallacryptates 12 and 13
were collected with a Rigaku AFC10K Saturn 944+ CCD-based X-ray diffractometer
equipped with a low temperature device and Micromax-007HF Cu-target micro-focus
rotating anode (λ = 1.54187 A) operated at 1.2 kW power (40 kV, 30 mA). Rigaku d*trek
images were exported to CrysAlisPro for processing.73 Absorption correction was
applied using the program SADABS.74 The data collection for 12bis was performed at
the X-ray diffraction beamline (XRD1) of the Elettra Synchrotron, Trieste (Italy).75
Datasets were collected at 100 K through the rotating crystal method. Data were
acquired using a monochromatic wavelength of 0.700 Å on a Pilatus 2M hybrid-pixel
area detector. The collected images were processes with CrysAlisPro.73 The structures
of compounds 11-13 were solved by direct methods ShelXT,76 and refined with full-
matrix least squares,77 using the Olex2 software package. The structures were analysed
with the Mercury 3.10 software package.78 Crystallographic data for 11-13 and for 12bis
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reported in this paper have been deposited with the Cambridge Crystallographic Data
Centre (the deposition CCDC number: 1849184-1849186).
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Synthesis of N-t-butyloxycarbonyl-4-aminosalicylic acid (2). 4-Aminosalicylic acid (1,
4.0 g, 0.026 mol) was dissolved in 50 mL of ethanol. Sodium hydroxide (2.1 g, 0.052
mol), dissolved in 20 mL of water, was added. Di-t-butyl dicarbonate (11.4 g, 0.052
mol), dissolved in 40 mL of ethanol, was added dropwise within 1 hour, and the solution
stirred overnight. The solvent was removed in vacuo and hydrochloric acid (1 M) was
added up to pH = 3. The precipitate was filtered off, washed with water, and dried under
reduced pressure at room temperature to obtain a white powder (yield 3.7 g, 93 %). FT-
IR (ATR, cm-1) 3361, 2884, 1737, 1621, 1588, 1452, 1298, 1220, 1192, 1138, 1048,
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1023, 984, 914, 872, 826, 775, 699, 643, 596, 540, 504, 464; H NMR (400 MHz, δ,
DMSO-D6) 9.71 (1H, s, Ar-NH), 7.64 (1H , d, J = 8.7 Hz, Ar-6H), 7.12 (1H, d, J = 2.1 Hz,
Ar-3H), 6.98 (1H, dd, J = 8.8, J = 2.1 Hz, Ar-1H), 1.47 (9H, s, Boc-CH3); ESI-MS
(QTOF, m/z) 276.25 ([M+Na]+), 292.21 ([M+K]+); Found: C, 56.8; H, 6.2; N, 5.6%. Calcd
for C12H15NO5: C, 56.9; H, 6.0; N, 5.5%.
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Synthesis
of
N-t-butyloxycarbonyl-4-aminosalicyl-O-benzylhydroxamate
(3).
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Compound 2 (5.00 g, 0.0198 mol) was dissolved in 120 mL of DMF. Triethylamine (5.5
mL, 0.0395 mol), O-benzylhydroxylamine hydrochloride (6.3 g, 0.040 mol), EDC
hydrochloride (4.54 g, 0.0395 mol) and DMAP (0.30 g, 0.0024 mol) were added to the
reaction mixture in this order. The resulting mixture was stirred for 48 hours at room
temperature. DMF was removed under reduced pressure, and the resulting yellowish oil
diluted with ethyl acetate (50 mL). The organic phase was washed with a saturated
NaHCO3 solution (3 x 50 mL), brine and then treated with anhydrous Na2SO4. The
solvent was removed in vacuo and the product purified by flash column chromatography
(silica gel, ethyl acetate/hexane 1:1 as the eluent). Pure compound 3 was isolated as a
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brownish powder (yield 2.74 g, 40 %). H NMR (400 MHz, δ, DMSO-D6) 11.98 (1H, s,
OBz-OH), 11.66 (1H, s, Ar-OH), 9.60 (1H, s, Ar-NH), 7.55 (1H, d, J = 8.7 Hz, Ar-6H),
7.42 (5H, m, OBz-H), 7.13 (1H, s, Ar-3H), 6.92 (1H, d, J = 8.7 Hz, Ar-1H), 4.92 (2H, s,
OBz-CH2); ESI-MS (QTOF, m/z) 737.28 ([(M-H)2Na]-); Found: C, 63.4; H, 6.3; N, 7.7%.
Calcd for C19H22N2O5: C, 63.7; H, 6.2; N, 7.8%.
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Synthesis of 4-aminosalicyl-O-benzylhydroxamate (4). Compound 3 (2.09 g, 5.83
mmol) was dissolved in a dicholormethane/TFA mixture (1:1 v:v), and the solution
stirred for 2 hours at room temperature. The solvent was removed in vacuo. The
resulting oil was dissolved in 50 mL of ethyl acetate, washed with water (3 x 50 mL),
brine, and treated with anhydrous Na2SO4. The solvent was removed in vacuo, yielding
compound 4 as a brownish powder (yield 1.9 g, 91 %,). FT-IR (ATR, cm-1) 2859, 2603,
1611, 1543, 1497, 1472, 1430, 1389, 1321, 1255, 1196, 1142, 1109, 1017, 974, 840,
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906, 857, 823, 796, 734, 717, 695, 602, 570, 539, 517, 492; H NMR (400 MHz, δ,
DMSO-D6) 7.49 – 7.26 (7H, m, OBz-H + Ar-1H), 6.03 (1H, d, J = 9.7 Hz, Ar-6H), 5.96 (s,
1H, Ar-3H), 5.79 (2H, s, Ar-NH2), 4.88 (2H, s, Ar-NH2); ESI-MS (QTOF, m/z) 281.22
([M+Na]+), 297.25 ([M+K]+), 539.25 ([M2+Na]+); Elemental analysis found: C, 65.4; H,
5.4; N, 10.6%. Calc for C14H14N2O3: C, 65.1; H, 5.5; N, 10.8%.
Synthesis of 4-aminosalicylhydroxamic acid (H3p-aShi, 5). Compound 4 (0.120 g,
0.464 mmol) was dissolved in 60 mL of methanol and hydrogenated in the presence of
Pd/C (0.018 g, 15% w/w) under H2 (pH2 = 30 psi) for 24 hours. The catalyst was filtered
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off from the pale pink solution, the solvent removed in vacuo, and pure H3p-aShi (5)
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isolated as a dark red powder (yield 0.050 g, 92 %). H NMR (400 MHz, δ, DMSO-D6)
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7.32 (1H, d, J = 8.6 Hz, Ar-6H), 6.00 (1H, dd, J = 8.6, J1, 3 2.2 Hz, Ar-3H), 5.94 (1H, d, J
= 2.2 Hz, Ar-3H), 5.71 (2H, s, Ar-NH2); ESI-MS (QTOF, m/z) 169.06 ([M+H]+); Found: C,
50.3; H, 4.9; N, 16.4%. Calcd for C7H8N2O3: C, 50.0; H, 4.8; N, 16.7%.
Synthesis of methyl 4-iodosalicylate (7). Conc. H2SO4 (2.02 mL) were slowly added at
room temperature to a solution of 2-hydroxy-4-iodobenzoic acid 6 (4.00 g, 15.15 mmol)
in dry methanol (37.2 mL). The reaction mixture was warmed to reflux for 12 hours.
After cooling to room temperature the solvent was removed to dryness. The crude
product was dissolved in CH2Cl2 and washed with a saturated NaHCO3 aqueous
solution and then with brine. The organic phase was dried over anhydrous Na2SO4,
filtrated and concentrated under vacuum to gain product 7 as white solid that was not
further purified (yield 3.57 g, 85%). m.p. 64-65 °C. FT-IR (KBr, cm-1) 3202, 2949, 2844,
1681; 1H-NMR (200 MHz, δ, CDCl3) 3.98 (3H, s, CH3), 7.27 (1H, dd, JA,B = 8.4, JB,O=1.7
Hz, Ar-HB), 7.44 (1H, d, JC,O = 1.6 Hz, Ar-HC), 7.54 (1H, d, J = 8.4 Hz, Ar-HA), 10.78
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(1H, s, OH); 13C1H-NMR (50 MHz, δ, CDCl3) 52.69 (CH3), 102.91 (I-C), 112.14(Ar-C),
127.19 (Ar-CH), 128.84 (Ar-CH), 130.85 (Ar-CH), 161.69 (C-OH), 170.50 (C=O); HR
ESI-MS (QTOF, m/z) 276.9353 ([M-]).
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Synthesis of methyl 4-iodo-2-(methoxymethoxy)benzoate (8). Methyl-4-iodosalicylate
7 (3.60 g, 13.04 mmol) was dissolved in dry DMF (36 mL) under a N2 atmosphere.
K2CO3 (7.20 g, 52.17 mmol) and chloromethyl methyl ether (2.08 mL, 27.39 mmol) were
slowly added at room temperature and the mixture stirred for 15 hours. The reaction
mixture was then diluted with water and extracted with ethyl acetate. The organic phase
was washed with brine, dried over anhydrous Na2SO4, filtered and concentrated under
vacuum, to obtain product 8 as white solid (yield 4.00 g, 95%). m.p. 76-80 °C. FT-IR
(KBr, cm-1) 3104, 3080, 3013, 2969, 2914, 2858, 2833, 1721; 1H-NMR (200 MHz, δ,
CDCl3) 3.55 (3H, s, O-CH3), 3.91 (3H, s, COOCH3), 5.27 (2H, s, O-CH2-O), 7.56 – 7.39
(2H, m, Ar-HA, Ar-HB), 7.61 (1H, s, Ar-HC); 13C1H-NMR (50 MHz, δ, CDCl3) 52.36 (O-
CH3), 56.72 (COOCH3), 95.42 (O-CH2-O), 99.77 (I-C), 121.04 (C-CO), 125.85 (Ar-C),
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131.12 (Ar-C), 132.68 (Ar-C), 157.09 (C-O), 166.23 (C=O); HR ESI-MS (QTOF, m/z)
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344.9590 ([M+Na]+).
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Synthesis of methyl-2-hydroxy-4-(pyridin-4-yl)benzoate (9). A mixture of methyl-4-
iodo-2-(methoxymethoxy)benzoate 8 (4.2 g, 13.04 mmol) and 4-pyridinylboronic acid
(26.08 mmol, 3.21 g) in a Schlenk tube under nitrogen, was dissolved with degassed
solutions of toluene (110 mL), ethanol (22 mL) and aqueous Na2CO3 (2 M, 37 mL). After
addition of Pd(PPh3)4 (1.50 g, 0.13 mmol) the reaction mixture was heated to 80 °C and
stirred for 15 h. The reaction mixture was then cooled to room temperature, filtered over
celite and then concentrated under vacuum. The residue was diluted with ethyl acetate
(50 mL) and extracted with HCl 1 M (4 x 50 mL). The aqueous phase was stirred for 2
hours, basified with saturated NaHCO3 solution to pH=5 and extracted with ethyl
acetate (4 x 100 mL). The organic phase was dried over anhydrous Na2SO4,
concentrated under reduced pressure, and the residue purified by silica gel column
chromatography (dichloromethane/methanol 95:5), affording the product 9 as a light-
yellow solid. (yield 2.64g, 89%). m.p. 120-128 °C. FT-IR (KBr, cm-1) 3150, 3063, 3015,
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2961, 1677; H-NMR (200 MHz, δ, CDCl3) 4.01 (3H, s, CH3), 7.17 (1H, dd, JA,B = 8.3,
JB,O = 1.8 Hz, Ar-HB), 7.27 (1H, d, J = 1.7 Hz, Ar-HC), 7.53 (2H, d, JD,E = 5.6 Hz, Py-HD),
7.96 (1H, d, JA,B = 8.3 Hz, Ar-HA), 8.72 (2H, d, J = 5.3 Hz, Py-HE), 10.88 (1H, s, OH);
13C1H-NMR (50 MHz, δ, CDCl3) 52.69 (CH3), 112.88 (C-CO), 116.19 (Ar-CH), 118.02
(Ar-CH), 121.86 (Py-CH), 130.94 (Ar-CH), 145.41 (Py-C), 147.11 (Ar-C), 150.54 (Py-
CH), 162.08 (C-OH), 170.38 (C=O); HR ESI-MS (QTOF, m/z) 230.0813 ([M+H]+).
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Synthesis of 4-(pyridin-4-yl)-salicylhydroxamic acid (10, H3p-pyShi). Hydroxylamine
hydrochloride (1.81 g, 26 mmol) and NaOH (1.56 g, 39 mmol) were dissolved in water
(60 ml) and a solution of methyl-2-hydroxy-4-(pyridin-4-yl)benzoate 9 (2.98 g, 13 mmol)
in 1,4-dioxane (40 ml) was added dropwise. The mixture was stirred at room
temperature for 12 hours and then the solvent concentrated under vacuum. The product
10 was purified by crystallization from diethyl ether and obtained as a light-yellow solid.
(yield 2.83 g, 95%). m.p. 178-184 °C. FT-IR (KBr, cm-1) 3455, 3273, 1632, 1598; 1H-
NMR (200 MHz, δ, DMSO-D6) 7.00 (2H, m, ArHC, Ar-HB), 7.64 (2H, d, J = 5.3 Hz, Py-
HD), 7.74 (1H, d, J = 7.8 Hz, Ar-HA), 8.58 (2H, d, J = 5.3 Hz, Py-HE); C1H-NMR (50
13
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MHz, δ, CDCl3) 114.39 (Ar-CH), 115.19 (Ar-CH), 121.65 (Ar-C), 121.95 (Py-CH), 128.38
(Ar-CH), 138.08 (Py-C), 148.08 (Ar-C), 150.71 (Py-CH), 161.88 (C-OH), 165.19 (C=O);
HR ESI-MS (QTOF, m/z) 231.0777 ([M+H]+).
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Synthesis of Mn11Shi6 (11), Mn11p-aShi6 (12) and Mn11p-pyShi6 (13). The three Mn11L6
metallacryptates (H3L = H3Shi, H3p-aShi and H3p-pyShi) were prepared using the
following procedure. Manganese(II) acetate tetrahydrate (122 mg, 0.50 mmol), the
ligand (32 mg of H3Shi, 34 mg of H3p-aShi, or 46 mg of H3p-pyShi, 0.21 mmol) and
sodium acetate trihydrate (28 mg, 0.2 mmol) were singly dissolved in 2 mL DMF. The
three solution were mixed under stirring in the following order: manganese(II) acetate,
H3L, and sodium acetate. The resulting dark brown solution was kept under stirring for 4
hours and then centrifuged to remove insoluble impurities. Slow evaporation of the
supernatant at room temperature gave dark brown crystals suitable for XRD analysis
within four weeks. Yields of compounds 11 and 13 were 28% and 2%, respectively. The
synthesis of 12 was attempted several times, obtaining few crystals of the compound
only twice. From the other synthesis, a 12-MC-4 of formula {MnII(OAc)2(O-DMF)5[12-
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MCMn,p-aShi-4]}(DMF)2 was obtained (compound 12bis). The structural description of this
12-MC-4 is provided as Supporting Information. The three complexes correspond to the
following formulas: Na4[(MnII)2(MnIII)9(μ3-O)4(OAc)9(Shi)6(DMF)3](DMF)3(H2O)6.75 (11),
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Na3.5H0.5[(MnII)2(MnIII)9(μ3-O)4(OAc)9(p-aShi)6](DMF)7.5(H2O)6
(12),
and
H[(MnII)2(MnIII)9(μ3-O)4(OAc)3(OH)3(p-pyShi)6](H2O)6 (13).‡ Several solvent molecules
(not refined) are present in the channels of compounds 12 and 13, as deduced by the
presence of residual electron density. ESI-MS (QTOF, m/z) of a solution of 11 in
methanol:
m/z
1796
[Mn11(O)4(Shi3-)6(OAc)3(OH)3]+;
1810
[Mn11(O)4(Shi3-
)6(OAc)3(OH)2(OMe)]+; 1824 [Mn11(O)4(Shi3-)6(OAc)3(OH)(OMe)2]+; 1838 [Mn11(O)4(Shi3-
)6(OAc)3(OMe)3]+. ESI-MS (QTOF, m/z) of a solution of 12bis in methanol: m/z 994
[Mn5(p-aShi3-)4(OAc)]+; Compounds 12 and 13 resulted insoluble in all solvents suitable
for ESI-MS characterization. Elemental analysis found for 11: C, 34.5; H, 4.3; N, 6.5.
Calc for C78H106.5Mn11N12Na4O52.75: C, 34.0; H, 3.9; N, 6.1%. Elemental analysis found
for 12bis: C, 40.5; H, 4.9; N, 13.2. Calc for C53H75Mn5N15O23: C, 40.7; H, 4.8; N, 13.4%.
Results and Discussion
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Synthesis of the ligands.
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The experimental procedure for preparing the ligand 5 (H3p-aShi) was optimized by
modifying a previously reported route for the synthesis of hydroxamic acids (Scheme
3).79 The Boc- protection of the amino group of the commercially available 4-
aminosalicylic acid was followed by the condensation of the carboxylic function with O-
benzylhydroxylamine. The (N-Boc, O-Bz) protected intermediate 3 was deprotected in
two step to yield the final H3p-aShi (5) product.
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Ligand 10 (H3p-pyShi) could be obtained in very high yields via a four-step synthetic
procedure from commercially available 4-iodosalicylic acid 6 (Scheme 4). After
protection of the carboxylic acid as the methyl ester80 and the phenol group as
methoxymethylether (MOM) (80% overall yield), Suzuki coupling,81,82 followed by an
acidic work-up allowed the insertion of the pyridine group and MOM protecting group
removal affording 9 in high yields (89%). The protection of the phenol and carboxylic
acid functional groups was necessary for obtaining an effective coupling reaction. Direct
conversion in very high yields (95%) of the methyl ester 9 into the hydroxamic acid 10
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was achieved using hydroxylamine hydrochloride and NaOH in a mixture of dioxane
and water at room temperature.81,82
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Scheme 3. Synthetic route to H3p-aShi (5) from 4-aminosalicylic acid (1). a) Boc2O,
NaOH, H2O/EtOH, rt; b) Benzylhydroxylamine hydrochloride, triethylamine, EDC,
DMAP, DMF, rt; c) TFA, CH2Cl2; d) H2, Pd/C, MeOH.
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Scheme 4. Synthetic route to H3p-pyShi (10) from 4-iodosalicylic acid (6). a) H2SO4,
MeOH, reflux, N2, 18 h; b) MOM-Cl, K2CO3, DMF, rt, N2, 12 h; c) 4-Py-B(OH)2,
Pd(PPh3)4, Na2CO3 aq. 2 M, Toluene/EtOH, 100 °C, N2; d) HCl 1 M; e) NH2OH·.HCl,
NaOH, H2O/dioxane, rt.
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General description of metallacryptate core in 11, 12 and 13.
The metallacryptates 11, 12 and 13 were isolated after reaction of the ligands with
manganese(II) acetate, and sodium acetate in DMF. All three metallacryptates (MCr)
11, 12, and 13 present a [MnII]2[MnIII]9 polynuclear core, where nine MnIII and two MnII
are found (Figure 1). In all three compounds six hydroxamate ligands are present, which
bridge between the manganese ions and give rise to a supramolecular complex that
presents a crystallographic C3 symmetry axis. Therefore, these assemblies are referred
to as Mn11L6, where H3L = H3Shi, H3p-aShi or H3p-pyShi. The shape of the
metallacryptate architecture is that of a three-bladed paddlewheel, where the blades are
related by the crystallographic C3 axis in all three compounds (Figures. 1 and 2). For
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these reasons, we will first describe the structure of the blades, and how they are
connected together to give a metallacryptate motif.
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Figure 1 View of the Mn11L6 molecular unit of 11 along (a) and perpendicularly to (b)
the crystallographic C3 axis. Light blue spheres = MnIII, purple spheres = MnII. The
acetate ions bridging between blades are represented with thin lines.
The three molecules present a very similar MCr architecture and similar connectivities
of the blades, although with significant differences. Figure 2 shows how two L3- ligands,
three MnIII and one oxide ion are bound together. Two MnII are found at the apices of
the assembly, sitting on the C3 axis, and coincide with the points of connections of the
three MCr blades. The two L3- ligands bridge between one MnII and one MnIII ion
through the bridging (N-,O-)-(O,O-) bis-chelating mode known for this ligand. The MnII
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ions (Mn1 and Mn2) are coordinated to the hydroxamic (O,O-) site of Shi3- (purple
spheres in Figure 2) in trigonal prismatic (11 and 12) or octahedral (13) geometry.
Conversely, the (N-,O-) site is occupied by a MnIII ion (Mn4 and Mn5, light blue spheres
in Figure 2). A third MnIII/L3- interaction involves the coordination of the hydroximate O-
atom to a MnIII (Mn3) which is found in the center of the blade and which is part of the
Mn3O core of the MCr assembly (see below).
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Figure 2 Representations of 11 (a), 12 (b) and 13 (c). The L3- ligands are represented
as sticks. Light blue spheres = MnIII, purple spheres = MnII. The cyan spheres in c
represent MnIII atoms of neighbouring MCr molecules. The crystallographic C3 axis is
represented in green in a, and is present in all three compounds. Labelling of atoms in b
is the same as in a.
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While the blades in compounds 11 and 12 are nearly planar, that of compound 13 is
domed (see Figure S1, Supporting Information). Also, the blade in 13 bares two pyridyl
groups that point away from the MCr unit and are coordinated to two neighbouring
molecules. These two structural features of the blades in 13 (i.e. concavity and
presence of pyridyl groups) have profound consequences on both the molecular MCr
architecture and on its assembly in the solid state, as discussed below.
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In all three compounds, the three blades connect together ideally at the C3 axis to
form a MCr motif which has the connectivity Mn1-O-N-Mn4-Ooxide-Mn5-N-O-Mn2. This
connectivity in compound 11 is represented as spheres in Figure 3a. Associated to this
connectivity is the [1.1.1] orthoester cryptand topology reported in Figure 3b.83–85 This
organic cryptand bears a C-CH3 connection points rather than a nitrogen as in Lehn-
type cryptands, allowing the first atom of the arm to be an oxygen.86 Finally, the MCr
architecture sequesters a [MnIII O]7+ unit, which is represented in yellow in Figure 3a.
3
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Figure 3 a) Ball and stick representation of the metallacryptate connectivity in 11. b)
representation of a [o-Me2-1.1.1] orthoester cryptand. The connectivity in a is obtained
from that in b by conceptually replacing the C-C bonds with MnIII-N bonds, and the C-
CH3 fragment with MnII. Light blue spheres = MnIII, purple spheres = MnII, blue spheres
= N, red spheres = O. The [MnIII O]7+ unit is represented in yellow.
3
In this description of the MCr topology the oxide oxygen in the center of the blade is
part of the MCr connectivity. Since an oxide ion is commonly found as a guest in
supramolecular assemblies, the topology of our molecules is perhaps more
appropriately described in terms of inverse metallacryptate (invMCr) motifs, as
represented in the Supporting Information (Figs. S2 and S3). For compounds 11 and
12, the invMCr connectivity is Mn1-O-N-Mn4-O-C-O-Mn5-N-O-Mn2 (Figure S2a). This
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Crystal Growth & Design
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description differs from the previous analysis in two aspects. On one hand, the
connectivity does not pass through the oxide oxygen of the blade, but rather through the
O-C-O atoms of the carboxylates bridging between Mn4 and Mn5. From this
perspective, the inverse metallacryptate topology is in analogy with that of a Lehn-type
[2.2.2] cryptand (Figure S2b), where the oxygen atoms are ideally substituted by MnIII.
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In this inverse metallacryptate the sequestered host in the invMCr is a [MnIII O4]+ unit,
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which is reported in yellow in Figure S2a. Finally, all the oxide ions in the molecule are
part of the latter [MnIII O4]+ guest unit.
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In compound 13 the invMCr connectivity is slightly different, since the two peripheral
MnIII of the blades (i.e. Mn4 and Mn5) are bridged by a hydroxide ion, and not by
carboxylates. Therefore, the analog cryptand is still a [2.2.2], but with a methylene
spacer rather than a propylene (Figure S3, Supporting Information).
It must be underlined that for both the metallacryptate and inverse MCr topologies, the
connectivity does not correspond to (Mn-N-O)n repeats as found in normal
metallacrowns and metallacryptates.87 Actually, starting from Mn1 the O-N-MnIII repeat
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