The synthesis and behaviour of pyrazine mononuclear carbonyl complexes of Rh(I), Ir(I), Ru(II) and Os(II)

The synthesis in high yields and the dissociative behaviour in the solid state and in solution of the mononuclear complexes cis-M(CO)₂Cl(pyz)] (M=Rh, Ir; PYZ=pyrazine) and fac-M(CO)₃Cl₂(pyz)] (M=Ru, Os) are reported. The mononuclear complexes of Rh and Ir are relatively labile with respect to pyrazine release. Particularly in the case of rhodium they generate even in the solid state the corresponding dinuclear complexes cis-Cl(CO)₂M(pyz)cis-M(CO)₂Cl] (M=Rh, Ir). The ¹H NMR spectra of these mononuclear Rh and Ir complexes in CHCl₃ solution show, at 25 and 60 °C, respectively, a fast and reversible dissociation of metal coordinated pyrazine, which is hindered by lowering the temperature. Crystallographic aspects of cis-Ir(CO)₂Cl(pyz)] have been investigated via single crystal X-ray diffraction. The mononuclear complexes of Ru and Os are more stable. In the solid state they do not rearrange, with release of pyrazine, to generate the related dimeric complexes with pyrazine as bridge. In solution, at room temperature, they do not dissociate quickly, although a mixture of monomeric and dimeric pyrazine complexes (ratio monomer to dimer 9:1 and 15:1 for Ru and Os, respectively) is slowly formed by a process which is reverted by addition of excess pyrazine, as expected for a dissociative equilibrium.