Complexed bridging ligands: oxorhenium(V) compounds with mono-coordinated pyrazine or pyrimidine as possible building blocks for the construction of polynuclear architectures

The reactivity of oxorhenium(V) precursors with simple diaza ligands such as pyrazine (pyz) and pyrimidine (pym) was investigated. Treatment of ReOCl3(Me2S)(OPPh3) or ReO(OEt)Cl-2(PPh3)(2) with an excess of pyz or pym under a range of different conditions yielded a series of oxo- and dioxo-rhenium(V) compounds with one or more terminally coordinated diaza ligands, namely the dinuclear species Re2O3Cl4(pyz)(4) 1 and Re2O3Cl4(pym)(4) 2, and the mononuclear complexes trans,trans,trans-ReO(OEt)Cl-2(pyz)(2) 3, trans,trans,trans-ReO(OEt)Cl-2(pym)(2) 4, trans,trans,trans-ReO(OEt)Cl-2(pyz)(PPh3) 5, and [trans-ReO2(pyz)(4)][PF6] 7. The new compounds were characterized in solution and in the solid state by common spectroscopic techniques; the crystal structures of 1, 3, 5, and 7 were also determined. The dinuclear species 1 presents a nearly linear O=Re-O-Re=O group and each Re atom brings two cis chlorides and two mono-coordinated pyrazine units. Crystals of the dioxo species 7 contain one NaPF6 and two H2O molecules of crystallization; the four pyrazine bases terminally bound to Re interact also with Na, forming slightly distorted square cavities (side length ca. 7.4 Angstrom), with Na and Re alternating in the corners. Density functional calculations of the excitation spectra of 3 and 4 were also performed with the aim of investigating in more detail the influence of the nature of the nitrogen ligand on the experimental visible spectra. A preliminary investigation of the reactivity of compound 3 towards coordination compounds and metalloporphyrins was also made. Treatment of 3 with a slight excess of the ruthenium(II) complex cis,fac-RuCl2(Me2SO)(3)(CO) yielded the linear trinuclear species [{trans,trans,trans-ReO(OEt)Cl-2}(mu -pyz)(2){cis,cis,cis-RuCl2(Me2SO-S)(2)(CO)}(2)], in which both ends of the rhenium complex are bound to a ruthenium coordination compound. Similarly, treatment of 3 with an excess of [Ru(TPP)(CO)(EtOH)] (TPP = tetraphenylporphyrinate) led to the symmetrical trinuclear species [{trans,trans,trans-ReO(OEt)Cl-2}(mu -pyz)(2){Ru(TPP)(CO)}(2)], consisting of two face-to-face ruthenium porphyrins axially connected through a coordination compound; the corresponding canted trinuclear compound [{trans,trans,trans-ReO(OEt)Cl-2}(mu -pym)(2){Ru(TPP)(CO)}(2)] was similarly obtained from 4. Thus the new oxorhenium(V) species can be considered as potentially useful basic building blocks for the self-assembly of polynuclear architectures.