Rhenium and Technetium Complexes with Pyridylphosphines

Technetium and rhenium co-ordination chemistry is of particular interest due to the favourable nuclear properties of 99mTc (Eg = 140 keV, half-life t1/2 = 6.02 h) which makes this isotope the workhorse for diagnostic nuclear medicine. The b-emitting rhenium nuclides 186Re and 188Re are under investigation for possible applications in radioimmunotherapy. Fundamental co-ordination chemical studies form the basis for the development of new Tc and Re radiopharmaceuticals.
Diphenyl(2-pyridyl)phosphine (PPh2py) and tris(2-pyridyl)phosphine (Ppy3) are a versatile ligands which can co-ordinate as a monodentate or chelating ligands depending on the requirements of the metal center.
The potentially bidentate ligand PPh2py reacted with (NEt4)2[MI(CO)3X3] complexes (M = Re, Tc) to give (NEt4)[MI(CO)3X2(PPh2py-P)] or [MI(CO)3X(PPh2py-P)2] depending on the amount of the ligand used. The reaction with (NBu4)[TcVINCl4] yielded [TcVNCl2(PPh2py-P)2] whereas from the reaction with (NBu4)[ReOCl4] the complexes [ReVOCl3(PPh2py-P,N)], [ReVOCl3(OPPh2py-O,N)], [ReIVCl4(OPPh2py-O,N) and [ReIVCl3(OH)(OPPh2py-O,N)] have been isolated. Reduction of the metal center and formation of diphenyl(2-pyridyl)phosphine oxide (OPPh2py) occurs using an excess of PPh2py and heating of the reaction mixtures under reflux.
All products have been characterised spectroscopically and by X-ray structure analysis. Monodentate co-ordination via phosphorus has been found for the rhenium(I) carbonyl complexes and [TcVNCl2(PPh2py-P)2]. In the latter compound a trigonal-bipyramidal coordination sphere is formed with the phosphines as axial ligands. The chelated complexes show small N-Re-P and N-Re-O bite angles due to the 4-membered or 5-membered chelate rings. The pyridine nitrogen occupies the axial position (trans to "O2-") in [ReOCl3-(PPh2py-P,N)] whereas equatorial co-ordination is found in [ReOCl3(OPPh2py-O,N)].
The reaction of (NEt4)2[ReI(CO)3Br3] with Ppy3 gave [Re(CO)3Br(Ppy3-N,N')] whereas with o ...