Abstract
This work describes an unexpected generation of a new 3D metal–organic framework (MOF), [Cu4(μ-Cl)6(μ4-O)Cu(OH)2(μ-PTA=O)4]n·2nCl-EtOH·2.5nH2O, from copper(II) chloride and 1,3,5-triaza-7-phosphaadamantane 7-oxide (PTA=O). The obtained product is composed of diamandoid tetracopper(II) [Cu4(μ-Cl)6(μ4-O)] cages and monocopper(II) [Cu(OH)2] units that are assembled, via the diamandoid μ-PTA=O linkers, into an intricate 3D net with an nbo topology. Magnetic susceptibility measurements on this MOF in the temperature range of 1.8–300 K reveal a ferromagnetic interaction (J = +20 cm–1) between the neighboring copper(II) ions. Single-point DFT calculations disclose a strong delocalization of the spin density over the tetranuclear unit. The magnitude of exchange coupling, predicted from the broken-symmetry DFT studies, is in good agreement with the experimental data. This copper(II) compound also acts as an active catalyst for the mild oxidation and carboxylation of alkanes. The present study provides a unique example of an MOF that is assembled from two different types of adamantoid Cu4 and PTA=O cages, thus contributing to widening a diversity of functional metal–organic frameworks.
Highlights
Over the last decades, the synthesis of metal−organic frameworks (MOFs) has seen a tremendous development with fascinating applications in catalysis,[1,2] magnetism,[3] biochemistry,[4] and materials science.[5]
Among a variety of organic linkers applied in MOF research, the cagelike aminophosphine 1,3,5-triaza-7-phosphaadamantane (PTA) and its P-oxide (1,3,5-triaza-7-phosphaadamantane 7-oxide, PTA O) are very interesting building blocks that feature a diamondoid geometry and several N,P- or N,O-sites for coordination.[15−17] despite their considerable use in aqueous organometallic chemistry, PTA and PTA O are underexplored as building blocks for the design of MOFs.[15−17] This might be explained by a difficulty in realizing multiple N,P- or N,O-coordination modes of PTA or PTA O cages, respectively.[18−20] the use of PTA O as a water-soluble and stable building block offers a prospective way toward the preparation of novel and structurally unique metal−organic architectures
In the course of the synthesis of 2, the formation of the ionic monocopper(II) intermediate [HPTA O]2[CuCl3(NO3)] (1) was observed ([H-PTA O]+ is a protonated form of PTA O). This hybrid inorganic−organic compound is considered as a precursor of 2, formed via a Cu-catalyzed dechlorination of 2chloroethanol.[29−33] Certainly, the source of chloride ions in compound 2 is 2-chloroethanol. The use of this chlorinated solvent is essential for the synthesis of 1 and 2, as these products are not generated in similar reactions starting from CuCl2 and PTA O in ethanol
Summary
The synthesis of metal−organic frameworks (MOFs) has seen a tremendous development with fascinating applications in catalysis,[1,2] magnetism,[3] biochemistry,[4] and materials science.[5]. No simple magnetostructural relationship was established relating the value of the magnetic exchange constant J to the Cu−Cl−Cu bonding angle or Cu−Cl or Cu···Cu distances in chloro-bridged copper(II) systems.[50] This may be due to the large variation in structural features observed, such as a number of distinct coordination geometries, which involve different orbitals in the exchange pathway. We first applied this approach to the tetranuclear cubanelike cation in [Cu4(NH3)4(HL8)4][CdBr4]Br2·3DMF· H2O (CSD refcode FEVYAH; H2L8 = diethanolamine) with the S = 2 ground spin state.[76] This compound does not exhibit spin delocalization, and its magnetic properties were precisely fitted with the support of a multifrequency, high-field EPR technique. The activity of 2 in the carboxylation of hydrocarbons[78,85] is comparable to those of some other systems reported in our earlier studies (Table S5 in the Supporting Information)
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