Thermostructural Behavior in a Series of Lanthanide-Containing Polyoxotungstate Hybrids with Copper(II) Complexes of the Tetraazamacrocycle Cyclam: A Single-Crystal-to-Single-Crystal Transformation Study

Abstract

The series of fourteen isostructural [Cu(cyclam)]2[{Cu(cyclam)}4{(α-GeW11O39)Ln(H2O)(OAc)}2]•18H2O (1-Ln, Ln = La to Lu; cyclam = 1,4,8,11-tetraazacyclotetradecane) polyoxometalate-based hybrids reported herein represent: (i) the first example of a 2-dimensional covalent hybrid lattice involving the [{(α-XW11O39)Ln(H2O)(OAc)}2] narchetype; and (ii) the first structural characterization of such dimeric polyoxotungste for Ln = La and Pr, as well as for the combination of X = Ge and Ln = Ce, Nd, Sm or Lu. Compounds 1-Ln have been characterized by elemental analyses, infrared spectroscopy and thermogravimetry, and their thermo-structural behavior has been monitored by powder and single-crystal X-ray diffraction. The title compounds undergo two single-crystal-to-single-crystal transformations triggered by thermal dehydration to lead to the [{Cu(cyclam)}6{(α-GeW11O39)Ln(H2O)(OAc)}2]•4H2O intermediate (2-Ln, Ln = Eu, Er) and [Cu(cyclam)]0.5[{Cu(cyclam)}5.5{(α-GeW11O39)Ln(OAc)}2] (3-Ln, Ln = Ce, Eu) final anhydrous phases, the latter evidencing a coordinatively unsaturated derivative of the dimeric archetype for the first time. These transitions involve formation and disruption of Cu-OPOM bonds that result in different {Cu(cyclam)} 2+ moieties grafting at and releasing from Keggin surfaces, which reduces the dimensionality of 1-Ln to 1-dimensional covalent assemblies for 2-Ln and 3-Ln. While all 3-Ln phases rehydrate fully upon exposure to air for 24 h, the kinetics governing the crystal transitions back toward 1-Ln through 2-Ln depend on the nature of Ln. Under ambient moisture, the anhydrous structures fully revert back to the parent framework for Ln = La-to-Sm, while the Eu-to-Lu-containing samples afford mixtures of 1-Ln and 2-Ln and require immersion in water for the structural reversion to be completed. Single-crystal X-ray diffraction analyses on the rehydrated 1R-Ln samples (Ln = Ce, Eu and Er) support these observations.

Single-Crystal-to-Single-Crystal Cluster Transformation in a Microporous Molybdoarsenate(V)-Metalorganic Framework

“…Single crystals of 1 were slowly heated in an oven at a rate of 1 °C min −1 up to 50, 70, and 100 °C and subsequently quenched to 100 K to determine whether high temperature phases found in variable temperature PXRD studies can be analyzed by this technique. In contrast to our latest reports on POM/{Cu(cyclam)} systems, 21,22 the single crystallinity is not preserved upon evacuation of water molecules, and hence, it prevented us from performing scXRD experiments on thermally activated samples. We tried to overcome these difficulties by dehydrating crystals of 1 under vacuum (P = 10 −5 Pa) at room temperature for 1 h, and luckily, we were able to carry out a full data acquisition for the anhydrous 2 at 100 K. In fact, the PXRD pattern of 1 exposed to high vacuum is virtually identical to both the PXRD pattern recorded at 100 °C and the simulated one from the single crystal data for 2 (Figure 1b).…”

“…In order to determine whether 1 is able to exhibit a comparable thermostructural behavior to that displayed by related {Cu­(cyclam)} 2+ /POM systems in which the crystallinity is retained all along the dehydration process, ,,, VT-PXRD analyses were performed from room temperature to 600 °C (Figure S5). These studies showed that our hybrid compound is able to maintain its crystallinity up to 250 °C, which is in good agreement with the thermal stability range observed in TGA measurements.…”

“…First, the TGA curve (Figure S3) revealed a dehydration process below 100 °C (found, % 10.9; calcd for 11 H 2 O, % 10.6), which is followed by a plateau that could be associated with the presence of a stable anhydrous phase that extends from 100 to 225 °C. Thermal decomposition of the anhydrous form should proceed through the combustion of In order to determine whether 1 is able to exhibit a comparable thermostructural behavior to that displayed by related {Cu(cyclam)} 2+ /POM systems in which the crystallinity is retained all along the dehydration process, 12,13,21,22 VT-PXRD analyses were performed from room temperature to 600 °C (Figure S5). These studies showed that our hybrid compound is able to maintain its crystallinity up to 250 °C, which is in good agreement with the thermal stability range observed in TGA measurements.…”

“…The thermal evacuation of guest solvent molecules usually triggers the occurrence of relevant structural changes such as the rearrangement of the Cu–O bonding scheme . However, no major skeletal modification of the metal-oxo anion was observed so far, besides the isolation of coordinatively unsaturated 3d- or 4f-metal substituted clusters such as {α-SiW 11 O 39 Cu} and {(α-GeW 11 O 39 )­Ln­(OAc)} 2 , respectively.…”

Roles of Organic Fragments in Redirecting Crystal/Molecular Structures of Inorganic–Organic Hybrids Based on Lacunary Keggin-Type Polyoxometalates

“…When [RE( α -PW 11 O 39 ) 2 ] (RE = Ce III , Pr III ) dimers were connected by [Cu(en)(2,2′-bipy)] 2+ and [Cu(en) 2 ] 2+ , a 1D zigzag chain was formed whilst strong hydrogen bonding constructed a 2D lattice [ 196 ]. Another interesting example was observed in a series of some isostructural [Cu(cyclam)] 2 [{Cu(cyclam)} 4 {( α -GeW 11 O 39 )RE(H 2 O)(OAc)} 2 ]·18H 2 O (RE = La−Lu) hybrids where acetate ions connected two {( α -GeW 11 O 39 )RE} polyanions, while three type {Cu(cyclam)} 2+ moieties had different structural roles: surface-appended antenna unit, linking moieties, and charge-balancing cation [ 197 ].…”