Mononuclear Ruthenium and Osmium Complexes with a Bicyclic Guanidinate Ligand: Synthesis and Catalytic Behavior in Olefin Isomerization Processes

The preparation of the first mononuclear Ru(II), Ru(IV) and Os(II) complexes containing the anion of the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (Hhpp) as a chelating ligand, namely [RuX{2-(N,N´)-hpp}(6-arene)] [arene = p-cymene, X = Cl (2a), Br (2b), I (2c); arene = C6Me6, X = Cl (7)], [RuCl{2-(N,N´)-hpp}(3:3-C10H16)] (9; C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) and [OsCl{2-(N,N´)-hpp}(6-p-cymene)] (11), is described. Compounds 2a-c, 7, 9 and 11 were fully characterized by elemental analysis, HRMS, and IR and NMR spectroscopy. In addition, the structure of 2a was unequivocally confirmed by single-crystal X-ray diffraction methods. The catalytic behavior of these guanidinate-metal complexes in the base-free redox isomerization of allylic alcohols was explored, with the ruthenium(IV) derivative 9 showing the best performances (TOF up to 5940 h-1). All the synthesized complexes proved to be also active in the isomerization of the allyl-benzene estragole into the industrially relevant 1-propenyl-benzene anethole, with a trans selectivity of up to 95%.