Mononuclear (O,O′ or N,N′) and Heterodinuclear (O,O′ and N,N′) Transition-Metal Complexes of ortho-Quinoid Bis(pyrazol-1-yl)methane Ligands

The ortho-hydroquinone-substituted bis­(3,5-dimethylpyrazol-1-yl)­methane ligand (HO)₂C₆H₃–C­(H)­(pz^Me,Me)₂ (7) has been synthesized and fully characterized. Together with its bis­(3-tert-butylpyrazol-1-yl)­methane congener (HO)₂C₆H₃–C­(H)­(pz^tBu)₂ (6), 7 was employed in oxidation and complexation studies. 6 has been oxidized with 2,3-dichloro-5,6-dicyano-para-benzoquinone to the corresponding ortho-benzoquinone form 9 on a preparative scale. Pure samples of 9 are stable for several hours in solution and for approximately one day in the solid state. Attempts at a N,N′ coordination of [PdCl₂] to 6 led to decomposition of the −C­(H)­(pz^tBu)₂ moiety, whereas an introduction of [PdCl₂] exclusively at the N,N′ site was possible in high yield for ligand 7. A selectively O,O′-chelated [(p-cym)­Ru]²⁺ complex was accessible for ligand 6, but not for 7. In contrast, [(ppy)₂Ir]⁺ and [(Cp*)­Ir]²⁺ gave O,O′ complexes with both donors 6 and 7 (Hppy = 2-phenylpyridine; HCp* = pentamethylcyclopentadiene). The heterodinuclear complex [(Cp*)­Ir­(O)₂C₆H₃–C­(H)­(pz^Me,Me)₂PdCl₂] (16) was obtained from 14 and [(MeCN)₂PdCl₂].