Facile room-temperature synthesis of layered transition metal phosphonates via hitherto unknown alkali metal tert-butyl phosphonates

A facile room-temperature synthetic method is presented to produce alkali metal salts of tert-butyl phosphonic acid. The reaction between equimolar amounts of alkali metal carbonates and tert-butyl phosphonic acid in methanol results in the formation of [(tBuPO3H)Li(H2O)3·(H2O)] (1), [(tBuPO3)Na2(H2O)4]n (2), and [(tBuPO3H)K(H2O)]n (3). Solid-state structures of these compounds have been confirmed by single-crystal X-ray diffraction studies and further validated using numerous spectroscopic and analytical techniques. Compounds 1-3 are polymeric solids that are predominantly made up of a 1-D polymeric metal phosphonate chain. This synthetic approach leads to the formation of network structures/polymers in the solid state that otherwise are absent in solution due to the ionic nature of the interaction between the alkali metal ions and phosphonate anions. Apart from the multidentate nature of the phosphonate ligands, additional hydrogen bonding interactions involving water molecules, free P-OH groups, and PO moieties allow these chains to be propagated into 2-D sheets. We have further utilized the completely metalated sodium phosphonate 2 to synthesize layered metal phosphonates [(tBuPO3)Ca(H2O)]n (4), [(tBuPO3)Mn(H2O)]n (5) and [(tBuPO3)Co(H2O)]n (6) via a simple metathesis reaction.

Variation of counter quaternary ammonium cations of anionic cage germanoxanes as building blocks of nanoporous materials

Double-four ring (D4R)-type cage germanoxanes, having a fluoride anion in the cage, contain organic ammonium cations as counter cations outside the cage, and they are attractive as unique nano-building blocks of anionic porous materials. Although the variety of counter cations directly included in the cage germanoxane synthesis is limited, this study demonstrates that other tetraalkylammonium cations can be introduced by cation exchange in both discrete and cross-linked states. Tetraethylammonium (TEA) of a discrete cage germanoxane was replaced with tetrabutylammonium (TBA) in an organic solvent, which provides another starting material. TEA and TBA cations in cross-linked networks formed by hydrosilylation reactions of dimethylvinylsilylated cage germanoxanes with various oligosiloxanes as linkers were exchanged with tetramethylammonium (TMA) cations. The variation in the pore volume, which depends on the type of introduced counter cations and oligosiloxane linkers, is verified. In terms of bottom-up synthesis of nanoporous materials from cage-type germanoxanes, the selection of both the counter cation and cross-linker is important to vary the porosity.

A tetranuclear cobalt(ii) phosphate possessing a D4R core: an efficient water oxidation catalyst

The reaction of Co(OAc)2·4H2O with a sterically hindered phosphate ester, LH2, afforded a tetranuclear complex, [CoII(L)(CH3CN)]4·5CH3CN (1) [LH2 = 2,6-(diphenylmethyl)-4-isopropyl-phenyl phosphate]. The molecular structure of 1 reveals that it is a tetranuclear assembly where the Co(ii) centers are present in the alternate corners of a cube. The four Co(ii) centers are held together by four di-anionic [L]2- ligands. The fourth coordination site on Co(ii) is taken by an acetonitrile ligand. Changing the Co(ii) precursor from Co(OAc)2·4H2O to Co(NO3)2·6H2O afforded a mononuclear complex [CoII(LH)2(CH3CN)2(MeOH)2](MeOH)2 (2). In 2, the Co(ii) centre is surrounded by two monoanionic [LH]- ligands and a pair of methanol and acetonitrile solvents in a six-coordinate arrangement. 1 has been found to be an efficient catalyst for electrochemical water oxidation under highly basic conditions while the mononuclear analogue, 2, does not respond to electrochemical water oxidation. The tetranuclear catalyst has excellent electrochemical stability and longevity, as established by chronoamperometry and >1000 cycle durability tests under highly alkaline conditions. Excellent current densities of 1 and 10 mA cm-2 were achieved with overpotentials of 354 and 452 mV respectively. The turnover frequency of this catalyst was calculated to be 5.23 s-1 with an excellent faradaic efficiency of 97%, indicating the selective oxygen evolution reaction (OER) occurring with the aid of this catalyst. A mechanistic insight into the higher activity of complex 1 towards the OER compared to that of complex 2 is also provided using density functional theory based calculations.

Molecular enneanuclear CuII phosphates containing planar hexanuclear and trinuclear sub-units: syntheses, structures, and magnetism

Highly symmetric enneanuclear copper(ii) phosphates [Cu9(Pz)6(μ-OH)3(μ3-OH)(ArOPO3)4(DMF)3] (PzH = pyrazole, Ar = 2,6-(CHPh2)2-4-R-C6H2; R = Me, 2MeAr; Et, 2EtAr; iPr, 2iPrAr; and Ar = 2,6-iPr2C6H3, 2Dip) comprising nine copper(ii) centers and pyrazole, hydroxide and DMF as ancillary ligands were synthesized by a reaction involving the arylphosphate monoester, 1, copper(i)chloride, pyrazole, and triethylamine in a 4 : 9 : 6 : 14 ratio. All four complexes were characterized by single crystal structural analysis. The complexes contain two distinct structural motifs within the multinuclear copper scaffold: a hexanuclear unit and a trinuclear unit. In the latter, the three Cu(ii) centres are bridged by a μ3-OH. Each pair of Cu(ii) centers in the trinuclear unit are bridged by a pyrazole ligand. The hexanuclear unit is made up of three dinuclear Cu(ii) motifs where the two Cu(ii) centres are bridged by an -OH and a pyrazole ligand. The three dinuclear units are connected to each other by phosphate ligands. The latter also aid the fusion of the trinuclear and the hexanuclear motifs. Magnetic studies reveal a strong antiferromagnetic exchange between the Cu(ii) centres of the dinuclear units in the hexanuclear part and a strong spin frustration in the trinuclear part leading to a degenerate ground state.

Synthesis of Organosilyl-Functionalized Cage-Type Germanoxanes Containing Fluoride Ions

Eight corners of a double-four ring cage-type germanoxane, containing a fluoride ion, were successfully silylated by the combination of chlorosilanes and silazanes. Three different silyl groups, trimethylsilyl, dimethylsilyl, and dimethylvinylsilyl, were attached on the corners of germanoxane cage. The solubility and reactivity of the cage modified with dimethylvinylsilyl groups were significantly increased, allowing for further reaction. Hydrosilylation reaction between dimethylvinylsilylated cage geramanoxanes and dimethylsilylated cage siloxanes afforded porous solids. Functionalization of the corners of germanoxanes with silyl groups should provide valuable building blocks in various functional materials.

Phosphonate Based High Nuclearity Magnetic Cages

Transition metal based high nuclearity molecular magnetic cages are a very important class of compounds owing to their potential applications in fabricating new generation molecular magnets such as single molecular magnets, magnetic refrigerants, etc. Most of the reported polynuclear cages contain carboxylates or alkoxides as ligands. However, the binding ability of phosphonates with transition metal ions is stronger than the carboxylates or alkoxides. The presence of three oxygen donor sites enables phosphonates to bridge up to nine metal centers simultaneously. But very few phosphonate based transition metal cages were reported in the literature until recently, mainly because of synthetic difficulties, propensity to result in layered compounds, and also their poor crystalline properties. Accordingly, various synthetic strategies have been followed by several groups in order to overcome such synthetic difficulties. These strategies mainly include use of small preformed metal precursors, proper choice of coligands along with the phosphonate ligands, and use of sterically hindered bulky phosphonate ligands. Currently, the phosphonate system offers a library of high nuclearity transition metal and mixed metal (3d-4f) cages with aesthetically pleasing structures and interesting magnetic properties. This Account is in the form of a research landscape on our efforts to synthesize and characterize new types of phosphonate based high nuclearity paramagnetic transition metal cages. We quite often experienced synthetic difficulties with such versatile systems in assembling high nuclearity metal cages. Few methods have been emphasized for the self-assembly of phosphonate systems with suitable transition metal ions in achieving high nuclearity. We highlighted our journey from 2005 until today for phosphonate based high nuclearity transition metal cages with V(IV/V), Mn(II/III), Fe(III), Co(II), Ni(II), and Cu(II) metal ions and their magnetic properties. We observed that slight changes in stoichiometry, reaction conditions, and presence or absence of coligand played crucial roles in determining the final structure of these complexes. Most of the complexes included are regular in geometry with a dense arrangement of the above-mentioned metal centers in a confined space, and a few of them also resemble regular polygonal solids (Archimedean and Platonic). Since there needs to be a historical approach for a comparative study, significant research output reported by other groups is also compared in brief to ensure the potential of phosphonate ligands in synthesizing high nuclearity magnetic cages.

A vanadium(IV) pyrazolate metal-organic polyhedron with permanent porosity and adsorption selectivity

A vanadium(IV) pyrazolate-based open metal-organic polyhedron of [V3(μ3-O)O(OH)2(μ4-BPD)1.5(μ-HCOO)3] (BDP = benzene-1,4-bipyrazolate) formulation gives rise to a porous crystal structure exhibiting micro and mesoporosity which is useful for selective adsorption of gases.

Molecular and polymeric zinc(II) phosphonates: isolation of an octanuclear ellipsoidal ensemble

The reaction of zinc(II) perchlorate with trichloromethyl phosphonic acid at room temperature afforded, upon crystallization, a two-dimensional layered coordination polymer possessing a dinuclear repeat unit, [{Zn2(Cl3CPO3)2(H2O)3}·1.5H2O]n (1). Modification of the above reaction by involving a co-ligand afforded the tetranuclear complex, [{Zn4(η(1)-DMPzH)6(Cl3C-PO3)2}(μ-OH)2(ClO4)2] (2). The molecular structure of 2 reveals that the tetranuclear core is non-planar and consists of three contiguous inorganic rings which include one 8-membered Zn2P2O4 ring and two six-membered Zn2PO3 rings. Replacement of Zn(ClO4)2·6H2O with ZnCl2 under the same reaction conditions that afforded 2 allowed the formation of the dinuclear complex [{(ZnCl)2(η(2)-Pz)2(Cl3CPO3)}(Et3NH)2] (3). 3 possesses a bicyclic core containing a seven-membered Zn2N2O2P ring. In 3, the phosphoryl oxygen atom (P=O) is involved in a bifurcated hydrogen bonding interaction with the triethylammonium cation. The reaction of ZnCl2 and 2,3,5,6-(Me)4C6HCH2PO3H2 afforded the octanuclear complex [Zn8(Cl)6{2,3,5,6-(Me)4C6HCH2PO3}6(Et3N)2](Et3NH)2]·2n-hexane·3H2O (4). The core of 4 is ellipsoid-shaped with the end-end polar distance (C-C) being ~20 Å.

Di-, tri- and tetranuclear molecular vanadium phosphonates: a chloride encapsulated tetranuclear bowl

Synthesis and characterization of three dinuclear, two dinuclear and one tetranuclear vanadium phosphonates is being reported. These include a chloride encapsulated tetranuclear bowl-shaped complex.