Oxo-, Hydroxo-, and Peroxo-Bridged Fe(III) Phosphonate Cages

Alkali-regulated Fe6 and Fe18 molecular clusters and their structural transformation

A cyclic Fe6 like a “six-vane windmill” and a bricky Fe18 clusters as a “Chinese knot” were obtained. Moreover, the alkali-induced conversion from Fe6 to Fe18 has been achieved, realizing the nuclearity enlargement and structural regulation.

Iron(III)-Oxo Cluster Chemistry with Dimethylarsinate Ligands: Structures, Magnetic Properties, and Computational Studies

A program has been initiated to develop Fe^III/oxo cluster chemistry with the "pseudocarboxylate" ligand dimethylarsinate (Me₂AsO₂^-) for comparison with the well investigated Fe^III/oxo/carboxylate cluster area. The synthesis and characterization of three polynuclear Fe^III complexes are reported, [Fe₁₂O₄(O₂C^tBu)₈(O₂AsMe₂)₁₇(H₂O)₃]Cl₃ (1), Na₂[Fe₁₂Na₂O₄(O₂AsMe₂)₂₀(NO₃)₆(Me₂AsO₂H)₂(H₂O)₄](NO₃)₆ (2), and [Fe₃(O₂AsMe₂)₆(Me₂AsO₂H)₂(hqn)₂](NO₃) (3), where hqnH is 8-hydroxyquinoline. The Fe₁₂ core of 1 is a type never previously encountered in Fe^III carboxylate chemistry, consisting of two Fe₆ units each of which comprises two {Fe₃(μ₃-O^2-)} units bridged by three Me₂AsO₂^- groups and linked into an Fe₁₂ loop structure by two anti-anti η¹:η¹:μ Me₂AsO₂^- groups, a bridging mode extremely rare with carboxylates. 2 also consists of two Fe₆ units, differing in their ligation from those in 1, and this time linked together into a linear structure by a central {Na₂(NO₃)₂} bridging unit. 3 is a linear Fe₃ complex with no monatomic bridges between Fe^III ions, a very rare situation in Fe^III chemistry with any ligands and unprecedented in Fe carboxylate chemistry. The distinct differences observed in arsinate vs carboxylate ligation modes are rationalized largely based on the greater basicity of the former vs the latter. Variable-temperature dc and ac magnetic susceptibility data reveal all Fe₂ pairwise interactions to be antiferromagnetic. For 1 and 2, the different J_ij couplings were estimated by use of a magnetostructural correlation for high nuclearity Fe^III-oxo clusters and by density functional theory calculations using broken symmetry methods, allowing identification of their relative spin vector alignments and thus rationalization of their S = 0 ground states. The J_ij values were then used as input values to give excellent fits of the experimental χ_MT vs T data. For 3, the fits of the experimental χ_MT vs T data to the Van Vleck equation or with PHI gave a very weak J₁₂ = -0.8(1) cm^-1 (H = -2JŜ_i·Ŝ_j convention) between adjacent Fe^III ions and an S = ⁵/₂ ground state. These initial Fe^III arsinate complexes also provide structural parameters that help validate literature assignments of arsinate binding modes to iron oxide/hydroxide minerals as part of environmental concerns of using arsenic-containing herbicides in agriculture.

Assembling Homometallic Sb6 and Heterometallic Ti4Sb2 Oxo Clusters

Isolation and structural characterization of novel organoantimony(V)-based oxo clusters are reported. (RSb)₄(OH)₄(t-BuPO₃)₆ and (RSb)₂(O)(t-BuPO₃H)₆ independently in the presence of pyridine under solvothermal conditions afford the hexanuclear organoantimonate clusters [(RSb)₆(μ₃-O)₂(μ₂-O)₆(t-BuPO₃)₄], where R = p-i-PrC₆H₄ (1), p-ClC₆H₄ (2). Further, reaction of organostibonate phosphonate with Ti(OⁱPr)₄ in the presence of pyridine under solvothermal conditions afforded the mixed-metal titanium stibonate hexanuclear clusters [(RSb)₂Ti₄(μ₃-O)₂(μ₂-O)₂(t-BuPO₃)₄(μ-OCH₃)₄(OCH₃)₄], where R = p-i-PrC₆H₄ (3), p-ClC₆H₄ (4). Band gap measurements were performed on 1-4. They reveal a remarkable reduction in the band gap on moving from the heavier main-group-based oxo cages (1 and 2) to the titanium-incorporated oxo cages (3 and 4).

Nanoaggregates of iron poly-oxo-clusters obtained by laser ablation in aqueous solution of phosphonates

Laser ablation in liquid (LAL) emerged as a versatile technique for the synthesis of nanoparticles with various structures and compositions, although the control over products remains challenging in most cases. For instance, it is still difficult to drive the size of metal oxide crystalline domains down to the level of few atom clusters with LAL. Here we demonstrate that laser ablation of a bulk iron target in aqueous solution of phosphonates gives phosphonate-grafted iron oxo-clusters polymerized into nanoaggregates with Fe:ligand ratio of 2:1, instead of the usual nanocrystalline iron oxides. We attribute this result to the strong ability of phosphonate groups to bind iron oxide clusters and prevent their further growth into crystalline iron oxide. These laser generated poly-oxo-clusters are biocompatible and trackable by magnetic resonance imaging, providing interesting features for use in biological environments, such as nano-vehicles for iron administration. Besides, this method is promising for the generation of atom-scale metal-oxide clusters, which are ubiquitary in chemistry and of interest in biochemistry, catalysis, molecular magnetism and materials science.

Hexanuclear iron(iii) α-aminophosphonate: synthesis, structure, and magnetic properties of a molecular wheel

A new hexanuclear molecular iron phosphonate complex, [Fe₆(HAIPA)₁₂(OH)₆]·nH₂O (1·nH₂O) (H₂AIPA = NH₂(CH₃)₂CP(O)(OH)₂, (2-aminopropan-2-yl)phosphonic acid), was synthesized from Fe²⁺ and Fe³⁺ salts in water by interaction with the ligand sodium salt.

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.

Discrete and polymeric cobalt organophosphates: isolation of a 3-D cobalt phosphate framework exhibiting selective CO2 capture

Auxiliary ligand assisted control over the structural diversity has been achieved in the case of cobalt(ii) organophosphates.

2,6-Dimethylphenol derived H-phosphonate and α-hydroxyphosphonate: facile synthesis, crystal chemistry, supramolecular association and metal complexation

A H-phosphonate and an α-hydroxyarylphosphonate with active P–H and P–OH groups have been synthesized from 2,6-dimethylphenol and their aggregation behavior has been investigated.

Switching nuclearity and Co(ii) content through stoichiometry adjustment: {CoII6CoIII3} and {CoIICo4III} mixed valent complexes and a study of their magnetic properties

Controlling the Co^II content in polynuclear systems: {Co^II₆Co^III₃} and {Co^IICo₄^III} mixed valent complexes with distinct magnetic behaviours is reported.

Synthesis and characterization of nickel(II) phosphonate complexes utilizing pyridonates and carboxylates as co-ligands

The synthesis and structures of five new nickel complexes containing phosphonate ligands are reported. The compounds utilize pivalic acid (HPiv) and 6-chloro-2-pyridonate (Hchp) as co-ligands with the resulting complexes being of formulas [Ni10(chp)4(Hchp)4.5(O3P(t)Bu)3(Piv)5(HPiv)2(OH)6(H2O)4.5](HNEt3)·0.5MeCN·2.5H2O 1, [Ni12(chp)12(Hchp)2(PhPO3)2(Piv)5(HPiv)2(OH)2(H2O)6](F)·4.5MeCN·2H2O 2, [Ni10(chp)6(O3PCH2Ph)2(Piv)8(F)2(MeCN)4] 3, [Ni10(chp)6(O3PMe)2(Piv)8(F)2 (MeCN)4]·5MeCN·2H2O 4, and [Ni10(chp)6(O3PCH2Nap)2(Piv)8(F)2(MeCN)2(H2O)2] 5. The metallic core of compounds 1 and 2 display tetra- and hexa-capped trigonal prismatic arrangements, while the metallic and phosphorus core of 3, 4, and 5 display three face-sharing octahedra. Variable temperature direct current (dc) magnetic susceptibility measurements reveal dominant antiferromagnetic exchange interactions within each cluster, with diamagnetic spin ground states found.

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.