<i>Para</i>-Quinone-Containing Bis(pyrazol-1-yl)methane Ligands: Coordination Behavior Toward Co^II and a C−H Activation Reaction with Ce^IV

Two series of sterically demanding para-dimethoxyphenyl-substituted bis(pyrazol-1-yl)methane ligands have been synthesized, i.e., L2R,R′ = ((MeO)2C6H3)C(H)(pzR,R′)2 and L3R,R′ = ((MeO)2C6H3)C(Me)(pzR,R′)2 (R,R′ = 3-Me,5-Me; 3-Ph,5-H; 3-tBu,5-H). In the solid state, already the sterically least encumbered derivative L2Me2 is able to stabilize CoII complexes of the form [X2Co(L2Me2)] (X = Cl, NO3); in solution, these complexes are at an equilibrium with the 1:2 species [Co(L2Me2)2]2+. Oxidative demethylation of L3Ph and L3tBu with [Ce(NH4)2(NO3)6] leads to the corresponding para-benzoquinonyl-substituted bis(pyrazol-1-yl)methane ligands L4Ph and L4tBu. Contrary to that, the methyl derivative L3Me2 is transformed into the ortho-benzoquinone species L5Me2, which still contains one methoxy substituent while one oxygen atom has been newly introduced. The formation of L5Me2 requires (i) the admission of air, (ii) the presence of both methoxy substituents of L3Me2, and (iii) the presence of (methyl) substituents both at the exocyclic carbon atom and at the 5- positions of the pyrazolyl rings. The parent ortho-hydroquinonyl-substituted bis(pyrazol-1-yl)methane ligand ((HO)2C6H3)C(H)(pzH,H)2 is readily available from 3,4-dihydroxybenzaldehyde and (pzH,H)2SO/pyridine.