Oxo-rhenium(V) complexes containing bis(diphenylphosphino)ethane and catecholate ligands

N-Centered Tripodal Phosphine Re(V) and Tc(V) Oxo Complexes: Revisiting a [3 + 2] Mixed-Ligand Approach

N-Triphos derivatives (NP₃^R, R = alkyl, aryl) and asymmetric variants (NP₂^RX^R′, R′ = alkyl, aryl, X = OH, NR₂, NRR′) are an underexplored class of tuneable, tripodal ligands in relation to the coordination chemistry of Re and Tc for biomedical applications. Mixed-ligand approaches are a flexible synthetic route to obtain Tc complexes of differing core structures and physicochemical properties. Reaction of the NP₃^Ph ligand with the Re(V) oxo precursor [ReOCl₃(PPh₃)₂] generated the bidentate complex [ReOCl₃(κ²-NP₂^PhOH^Ar)], which possesses an unusual AA’BB’XX’ spin system with a characteristic second-order NMR lineshape that is sensitive to the bi- or tridentate nature of the coordinating diphosphine unit. The use of the asymmetric NP₂^PhOH^Ar ligand resulted in the formation of both bidentate and tridentate products depending on the presence of base. The tridentate Re(V) complex [ReOCl₂(κ³-NP₂^PhO^Ar)] has provided the basis of a new reactive “metal-fragment” for further functionalization in [3 + 2] mixed-ligand complexes. The synthesis of [3 + 2] complexes with catechol-based π-donors could also be achieved under one-pot, single-step conditions from Re(V) oxo precursors. Analogous complexes can also be synthesized from suitable ⁹⁹Tc(V) precursors, and these complexes have been shown to exhibit highly similar structural properties through spectroscopic and chromatographic analysis. However, a tendency for the {M^VO}³⁺ core to undergo hydrolysis to the {M^VO₂}⁺ core has been observed both in the case of M = Re and markedly for M = ⁹⁹Tc complexes. It is likely that controlling this pathway will be critical to the generation of further stable Tc(V) derivatives.

An N-centered tripodal heterofunctionalized phosphine ligand was used to generate a reactive “metal-fragment” based on the {M^VO}³⁺ (M = Re, ⁹⁹Tc) core for the formation of mixed-ligand [3 + 2] complexes. Characteristic lineshapes arising from an AA’BB’XX’ spin system are diagnostic of bidentate vs tridentate coordination modes of the ligand.

Oxorhenium(V) complexes of quinoline and isoquinoline carboxylic acids--synthesis, structural characterization and catalytic application in epoxidation reactions

Six novel oxorhenium(V) complexes incorporating quinoline and isoquinoline carboxylic acid derivatives were prepared in good yields. Relying on the experimental conditions, compounds with two chelate ligands [ReOCl(iqc)2]·MeOH (1), [ReO(OMe)(iqc)2] (2), [ReO(OMe)(mqc)2] (3) and [ReO(OMe)(8-qc)2] (4) and compounds incorporating one bidentate ligand [ReOCl2(iqc)(PPh3)] (5) and [ReOCl2(mqc)(PPh3)] (6) were synthesized (iqcH = isoquinoline-1-carboxylic acid, mqcH = 4-methoxy-2-quinolinecarboxylic acid and 8-qcH = 8-quinolinecarboxylic acid). The reported compounds were characterized by spectroscopic methods and single crystal X-ray diffraction analysis. In compounds 1 and 2, one chelate ligand occupies an axial and an equatorial position, while the other one occupies two equatorial positions, forming a cis-(N,N) isomer. In turn, complexes 3 and 4 show a rare ligand arrangement with two trans-N, trans-O chelate ligands in the equatorial plane and a linear axial [O=Re-OMe] unit. The complexes with one chelate ligand 5 and 6 are cis-(Cl,Cl)-isomers. All compounds were tested as potential catalysts in the epoxidation of cyclooctene with 3 equiv. of tert-butylhydroperoxide. The yield of cyclooctane oxide varies between 16 and 68% (50 °C, 24 h), and the catalytic competency of compounds 1-6 was discussed with regard to the structure of each complex.