Thermolabile Organotitanium Monoalkyl Phosphates: Synthesis, Structures, and Utility as Epoxidation Catalysts and Single-Source Precursors for TiP2O7

A rigid calcium 2-ethylhexylphosphate one-dimensional polymer: synthesis, structure, thermal behaviour and decomposition chemistry

The reactivity of two different phosphate esters has been explored with calcium ions. The reaction of calcium oxide with a mixture of mono/bis 2-ethylhexyl phosphate and 1,10-phenanthroline leads to the formation of the 1-D polymer [(phen)Ca(ehpH)₂]_n (1). On the other hand, the reaction of Ca(OAc)₂ with trimethyl phosphate in the presence of 1,10-phenanthroline however yields needle-shaped colourless crystals of [(phen)Ca(OAc)₂]_n (2). Compounds 1 and 2 have been characterized by various spectroscopic and analytical techniques. The solid-state structures have been determined by single crystal X-ray diffraction studies. The arrangement of rigid and flexible parts in 1 makes it an interesting compound to be studied for its thermal behaviour. DSC experiments performed above and below room temperature reveal thermal transitions in both the ranges, where below room temperature transition is prominent. A combination of variable temperature powder and single crystal X-ray diffraction experiments provides further insights into these transitions. Solid-state bulk thermolysis of 1 at 500 °C leads to an amorphous material contaminated with graphitic carbon which upon further heating to 600 °C produces crystalline α-Ca(PO₃)₂.

Efficient Production of Epoxy-Norbornane from Norbornene by an Engineered P450 Peroxygenase

Epoxy-norbornane (EPO-NBE) is a crucial building block for the synthesis of various biologically active heterocyclic systems. To develop an efficient protocol for producing EPO-NBE using norbornene (NBE) as a substrate, cytochrome P450 enzyme from Pseudomonas putida (CYP238A1) was examined and its crystal structure (PDB code: 7X53) was resolved. Molecular mechanism analysis showed a high energy barrier related to iron-alkoxy radical complex formation. Therefore, a protein engineering strategy was developed and an optimal CYP238A1^NPV variant containing a local hydrophobic "fence" at the active site was obtained, which increased the H₂ O₂ -dependent epoxidation activity by 7.5-fold compared with that of CYP238A1^WT . Among the "fence", Glu255 participates in an efficient proton transfer system. Whole-cell transformation using CYP238A1^NPV achieved an EPO-NBE yield of 77.6 g ⋅ L^-1 in a 30-L reactor with 66.3 % conversion. These results demonstrate the potential of this system for industrial production of EPO-NBE and provides a new biocatalytic platform for epoxidation chemistry.

Alkali Metal Di-tert-butyl Phosphates: Single-Source Precursors for Homo- and Heterometallic Inorganic Phosphate Materials

The reaction of alkali metal acetates, M(OAc)·nH₂O (M = Li, Na, K), with thermally and hydrolytically unstable di-tert-butylphosphate ((^tBuO)₂PO₂H, dtbp-H) in a 1:1 molar ratio in MeOH at room temperature leads to clean formation of group 1 metal phosphates [Li(μ-dtbp)]_n (1), [Na(μ-dtbp)]_n (2), and [K₄(μ-dtbp)₄(μ-H₂O)₃]_n (3). All three compounds are essentially M/L 1:1 complexes. Owing to the presence of larger potassium ions, additional coordinated water molecules are found in 3, which has been further employed as a precursor for the synthesis of a mixed-metal phosphate polymer [CaK(μ-H₂O)₃(μ-dtbp)₃]_n (4) by reacting 3 with Ca(OAc)₂. Compounds 1-4 have been characterized by various analytical and spectroscopic techniques. Molecular structures of 1-4 have been established in the solid state by single-crystal X-ray diffraction studies, which reveal them to be one-dimensional polymers, where the adjacent metal centers are connected through -O-P-O- bridges formed by the dtbp ligand. These complexes are rare examples of analytically pure alkali metal alkyl phosphates bearing no additional N-donor ligands (other than dtbp ligands, only water molecules are coordinated to the metal centers). Therefore, these compounds can be employed as single-source precursors (SSPs) for nano-sized ceramic phosphates. The thermogravimetric analysis of 1-4 reveals the loss of thermally labile tert-butyl substituents of the organophosphate ligands to form organic-free phosphate materials in the temperature range 300-500 °C. Solvothermal decomposition of 1-3 in boiling toluene leads to the formation of corresponding dihydrogen phosphates M(H₂PO₄) (M = Li, Na, and K). The thermal decomposition of heterometallic 4 in the temperature range 400-800 °C leads to the formation of phase-pure mixed-metal calcium potassium metaphosphate CaK(PO₃)₃.

Nanocrystals perovskites photocatalyzed singlet oxygen generation for light-driven organic reactions

Lead halide perovskite nanocrystals were prepared and used as photocatalysts for the in situ ¹O₂ generation to perform hetero Diels-Alder, ene and oxidation reactions with suitable dienes and alkenes. The methodology has been reasonably standardized and made applicable to a variety of olefinic substrates. The scope of the method is finely illustrated by the results in all the tested reactions, which allowed to obtain desymmetrized hydroxy-ketone derivatives, unsaturated ketones and epoxides. Some limitations were also observed especially in the case of the alkene oxidations as well as poor chemoselectivity was somewhere observed. ¹O₂ generated by lead halide perovskite nanocrystals as photocatalyst in organic reactions.

Di-tert-butylphosphate Derived Thermolabile Calcium Organophosphates: Precursors for Ca(H2PO4)2, Ca(HPO4), α-/β-Ca(PO3)2, and Nanocrystalline Ca10(PO4)6(OH)2

Thermally and hydrolytically unstable di-tert-butyl phosphate (dtbp-H) has been used as synthon to prepare discrete and polymeric calcium phosphates that are convenient single-source precursors for a range of calcium phosphate ceramic biomaterials. The reactivity of dtbp-H toward two different calcium sources has been found to vary significantly, e.g., the reaction of Ca(OMe)₂ with dtbp-H in a 1:6 molar ratio in petroleum ether forms a mononuclear calcium hexa-phosphate complex [Ca(dtbp)₂(dtbp-H)₄] (1), whereas the change of calcium source to CaH₂, in a 1:2 molar ratio under otherwise similar reaction conditions, yields the calcium phosphate polymer, [Ca(μ-dtbp)₂(H₂O)₂·H₂O]_n(2). Compounds 1 and 2 have been extensively characterized by various spectroscopic and analytical techniques. The solid-state structures of both 1 and 2 have been determined by single-crystal X-ray diffraction studies. In discrete molecule 1, the central calcium ion is surrounded by two anionic dtbp and four neutral dtbp-H ligands in an octahedral coordination environment. Compound 2 is a one-dimensional polymer in which adjacent calcium ions are connected through double dtbp bridges. Solid-state thermolysis of bulk 1 in air leads to the exclusive formation of calcium metaphosphate β-Ca(PO₃)₂ in the entire temperature range of 400-800 °C. Thermal decomposition of polymer 2, however, can be fine-tuned to produce either α-Ca(PO₃)₂ or β-Ca(PO₃)₂ depending on the thermolysis conditions employed. Although the sample sintered at 600 °C produces exclusively α-form of Ca(PO₃)₂, the sample annealed at 800 °C or above produces β-form. Both α- and β-forms can also be successively formed one after other by a slow heating of a freshly prepared 2 on the powder diffractometer sample holder. Additional forms of ceramic phosphates have been prepared by solvothermal conditions because of the highly labile nature of the tert-butoxy groups of dtbp in 1 and 2. Solution decomposition of either 1 or 2 in boiling toluene at 140 °C in a sealed tube produces calcium dihydrogen phosphate [Ca(H₂PO₄)₂·H₂O] as the only product in the form of single crystals. Solution thermolysis of 2 in protic solvents such as water and methanol can be biased to produce other calcium phosphate biomaterials such as hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂]and calcium monohydrogen phosphate [Ca(HPO₄)] in the presence of additional calcium precursors such as CaO and Ca(OMe)₂, respectively.