Reactivity of Rhenium(V) Oxo Schiff Base Complexes with Phosphine Ligands: Rearrangement and Reduction Reactions

Steric influence of salicylaldehyde-based Schiff base ligands on the formation of trans-[Re(PR3)2(Schiff base)]+ complexes

Complexes of the type trans-[Re(PR₃)₂(Schiff base)]⁺ (R = ethyl and/or phenyl) 2-7 were prepared by the reaction of (nBu₄N)[ReOCl₄] with H₂sal₂en or H₂sal₂ibn followed by addition of a tertiary phosphine. The trans-[Re(PR₃)₂(sal₂en)]⁺ complexes 2-4 were stable in solution, whereas the trans-[Re(PR₃)₂(sal₂ibn)]⁺ complexes 6-7 were observed to convert to their corresponding cis-[ReO(PR₃)(sal₂ibn)]⁺ products through a process involving ligand dissociation, metal oxidation, and Schiff base ligand rearrangement. The conversion of the trans-[Re(PR₃)₂(sal₂ibn)]⁺ complexes is likely driven by steric interactions between the bulky backbone gem-dimethyl groups of the sal₂ibn ligand and the phosphine ligands. These complexes were isolated and characterized by ¹H and ¹³C NMR, FT-IR spectroscopy, cyclic voltammetry, and single crystal X-ray diffraction. The results reported herein provide insight into the factors that drive trans-[Re(PR₃)₂(Schiff base)]⁺ complex formation. This will aid in the development of novel ^186/188Re therapeutic agents and the design of novel bifunctional N₂O₂ Schiff base ligands.

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

Technetium and Rhenium Schiff Base Compounds for Nuclear Medicine: Syntheses of Rhenium Analogues to 99mTc-Furifosmin

Rhenium, the third-row congener of technetium, is often used to develop the macroscopic chemistry of potential ^99mTc diagnostic radiopharmaceuticals. The rhenium analogues to ^99mTc-furifosmin are being developed for potential radiotherapy of multidrug-resistant tumors. Complexes of the form trans-[M^III(PR₃)₂(N₂O₂-Schiff base)]⁺ are of interest for the potential imaging and treatment of multidrug-resistant tumors. Reaction of the tetradentate Schiff ligand 4,4'-[(1 E,1' E)-[ethane-1,2-diylbis(azanylylidene)]bis(methanylylidene)]bis(2,2,5,5-tetramethyl-2,5-dihydrofuran-3-ol) (tmf₂enH₂) with the M(V) starting materials ( nBu₄N)[TcOCl₄] and ( nBu₄N)[ReOCl₄] gave the monomeric products trans-[TcOCl(tmf₂en)] and trans-[ReOCl(tmf₂en)], respectively. Reduction of in situ formed trans-[ReOCl(tmf₂en)] by various tertiary phosphines yielded disubstitued Re(III) products of the general type trans-[Re^III(PR₃)₂(tmf₂en)]⁺. The rhenium(III) compounds were found to be water-soluble and stable in aqueous solution. Reversible Re^III/Re^IV and Re^III/Re^II redox processes were observed at about 0.8-0.9 and -0.65 to -0.8 V, respectively, for each of the rhenium(III) species. Reaction of in situ formed trans-TcOCl(tmf₂en) with triethylphosphine yielded the reduced, disubstituted trans-[Tc(PEt₃)₂(tmf₂en)]PF₆. A reversible Tc^III/Tc^II redox couple was observed for the technetium(III) species, about 200 mV less negative than their rhenium(III) analogues, in addition to an irreversible Tc^III/Tc^IV process. All compounds were characterized using conventional spectroscopic techniques, single-crystal X-ray crystallography, and cyclic voltammetry.

Technetium-99m radiochemistry for pharmaceutical applications

Technetium-99m (^99m Tc) is a widely used radionuclide, and the development of ^99m Tc imaging agents continues to be in demand. This overview discusses basic principles of ^99m Tc radiopharmaceutical preparation and design and focuses on the ^99m Tc radiochemistry relevant to its pharmaceutical applications. The ^99m Tc complexes are described based on the most typical examples in each category, keeping up with the state-of-the-art in the field. In addition, the main current strategies to develop targeted ^99m Tc radiopharmaceuticals are summarized.

Novel rhenium(III, IV, and V) tetradentate N2O2 Schiff base mononuclear and dinuclear complexes

Reaction of (Bu4N)[ReOCl4] with the tetradentate Schiff base ligand α,α'-[(1,1-dimethylethylene)dinitrilo]di-o-cresol (sal2ibnH2) yields cis-[Re(V)OCl(sal2ibn)], which quickly forms trans-[μ-O(Re(V)O(sal2ibn))2] in solution. The dinuclear complex can also be isolated by the addition of base (Et3N) to the reaction mixture. Conversely, the mononuclear complex can be trapped as cis-[Re(V)O(NCS)(sal2ibn)] by addition of (Bu4N)SCN to the reaction mixture. Reduction of cis-[Re(V)O(NCS)sal2ibn] with triphenylphosphine gives the rare trans-[Re(III)(NCS)(PPh3)(sal2ibn)] and unique μ-oxo Re(IV) dimer trans-[μ-O(Re(IV)(NCS)(sal2ibn))2]. All of the complexes were characterized by (1)H and (13)C NMR, FT-IR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), cyclic voltammetry and single crystal X-ray diffraction.

Inorganic chemistry in nuclear imaging and radiotherapy: current and future directions

Radiometals play an important role in diagnostic and therapeutic radiopharmaceuticals. This field of radiochemistry is multidisciplinary, involving radiometal production, separation of the radiometal from its target, chelate design for complexing the radiometal in a biologically stable environment, specific targeting of the radiometal to its in vivo site, and nuclear imaging and/or radiotherapy applications of the resultant radiopharmaceutical. The critical importance of inorganic chemistry in the design and application of radiometal-containing imaging and therapy agents is described from a historical perspective to future directions.

Re(V) and Re(III) complexes with sal2phen and triphenylphosphine: rearrangement, oxidation and reduction

Reactions of Re(V), tetradentate Schiff base complexes with tertiary phosphines have previously yielded both rearranged Re(V) and reduced Re(III) complexes. To further understand this chemistry, the rigid diiminediphenol (N(2)O(2)) Schiff base ligand sal(2)phen (N,N'-o-phenylenebis(salicylaldimine)) was reacted with (n-Bu(4)N)[ReOCl(4)] to yield trans-[ReOCl(sal(2)phen)] (1). On reaction with triphenylphosphine (PPh(3)), a rearranged Re(V) product cis-[ReO(PPh(3))(sal(2)phen*)]PF(6) (2), in which one of the imines was reduced to an amine during the reaction, and the reduced Re(III) products trans-[ReCl(PPh(3))(sal(2)phen)] (4) and trans-[Re(PPh(3))(2)(sal(2)phen)](+) (5) were isolated. Reaction of sal(2)phen with [ReCl(3)(PPh(3))(2)(CH(3)CN)] resulted in the isolation of [ReCl(2)(PPh(3))(2)(salphen)] (3). The compounds were characterized using standard spectroscopic methods, elemental analyses and single crystal X-ray crystallography.

Metal complexes of Schiff bases derived from dicinnamoylmethane and aliphatic diamines

Two new Schiff bases containing olefinic linkages have been synthesized by condensing aliphatic diamines with dicinnamoylmethane under specified conditions. The existence of these compounds predominantly in the intramolecularly hydrogen bonded keto-enamine form was well demonstrated by their IR, 1H-NMR and mass spectral data. Dibasic tetradentate N2O2 coordination of the compounds in their [ML] complexes (M = Ni(II), Cu(II) and Zn(II)) was established on the basis of analytical and spectral data.

Rhenium(V) Oxo Complexes with Acetylacetone Derived Schiff Bases: Structure and Catalytic Epoxidation

Substitution reactions of rhenium(V) oxo precursors [ReOCl3(PPh3)2] or [NBu4][ReOCl4] with the bidentate acetylacetone-derived ketoamine ligands APOH = 4-anilino-3-penten-2-one, DPOH = 4-[2,6-dimethylanilino]-3-penten-2-one, and MTPOH = 4-[2-(methylthio)anilino]-3-penten-2-one gave the complexes [ReO(APO)Cl2(PPh3)] (1), [ReO(DPO)Cl2(PPh3)] (2), and [NBu4][ReOLCl3] (3, L = APO; 4, L = DPO; 5, L = MTPO), respectively. All complexes exhibit only one ketoamino chelate, independent of the amount of ligand added to the rhenium precursors. The complexes were characterized by 1H and 13C NMR spectroscopy. X-ray crystal structures of the complexes 1, 2, 4, and 5, including that of MTPOH, were determined, revealing the trans position of the two oxygen atoms and the trans-Cl,Cl conformation in 1 and 2, in contrast to most other rhenium complexes of this type where the cis-Cl,Cl conformation is observed. Coordination of the potentially tridentate ligand MTPOH in 5 is bidentate with a dangling thioether substituent. Compound 2 shows catalytic activity in the oxidation of cis-cyclooctene with tert-butylhydroperoxide.

Metal complexes of Schiff’s bases derived from 3-(arylazo)-2,4- -pentanediones with 2-aminophenol and 2-aminothiophenol

Phenylazo- and thiazolylazo-2,4-pentanediones on reaction with 2-aminophenol and 2-aminothiophenol yielded a new series of polydentate Schiff?s base ligands. The structure and tautomeric nature of these compounds and their metal complexes were established on the basis of their IR, 1H-NMR and mass spectral data. The spectral and analytical data revealed the condensation of both carbonyl groups of 3-(2-thiazolylazo)- 2,4-pentanedione with 2-aminophenol to form an N2O2 tetradentate ligand. Details on the formation of its [ML] complexes with Ni(II), Cu(II) and Zn(II) and the nature of their bonding are discussed based on analytical, IR, 1H-NMR and mass spectral data.

MTO Schiff-Base Complexes: Synthesis, Structures and Catalytic Applications in Olefin Epoxidation

Several Schiff-base ligands readily form complexes with methyltrioxorhenium(VII) (MTO) by undergoing a hydrogen transfer from a ligand-bound OH group to a ligand N atom. The resulting complexes are stable at room temperature and can be handled and stored in air without problems. Due to the steric demands of the ligands they display distorted trigonal-bipyramidal structures in the solid state, as shown by X-ray crystallography, with the O(-) moiety binding to the Lewis acidic Re atom and the Re-bound methyl group being located either in cis or trans position to the Schiff base. In solution, however, the steric differences seem not to be maintained, as can be deduced from (17)O NMR spectroscopy. Furthermore, the Schiff-base ligands exchange with donor ligands. Nevertheless, the catalytic behaviour is influenced significantly by the Schiff bases coordinated to the MTO moiety, which lead either to high selectivities and good activities or to catalyst decomposition. A large excess of ligand, in contrast to the observations with aromatic N-donor ligands, is detrimental to the catalytic performance as it leads to catalyst decomposition.