Gold(I), Gold(III), and Heterometallic Gold(I)–Silver(I) and Gold(I)–Copper(I) Complexes of a Pyridazine-Bridged NHC/Pyrazole Hybrid Ligand and Their Initial Application in Catalysis

The pyridazine-bridged NHC/pyrazole ligand L (HL = 3-[3-(2,4,6-trimethylphenyl)-3H-imidazolium-1-yl]-6-(3,5-dimethylpyrazol-1-yl)-pyridazine) that provides an organometallic and a classical N-donor compartment is shown to serve as a versatile scaffold for a variety of homo- and heterometallic gold­(I) carbene complexes. Complexes [LAuX] (1Cl, X = Cl; 1Br, X = Br), [L2Au]­(PF6) (2), [L2AuAg]­(BF4)­(PF6) (3), [L2AuAg3(MeCN)6]­(BF4)4 (5), and [L2AuCu]­(OTf)0.75(PF6)1.25 (6) have been characterized by X-ray crystallography. In all cases Au­(I) binds to the NHC site while the additional Ag­(I) in 3 or Cu­(I) in 6 is accommodated in the pyrazole-derived site. Both 3 and 6 form two-stranded helical structures; racemization of the P and M enantiomers is much more facile in the Ag­(I) case 3 but has a barrier of around 65 kJ/mol in the Cu­(I) case 6, which is rationalized on the basis of the different coordination chemistry preferences of these two metal ions. 3 may bind two further Ag­(I) ions to the central pyridazine N, giving 5. Treatment of 1Br with Br2 leads to bromination at the pyrazole C4 of the ligand backbone, yielding [LBrAuBr] (8). In contrast, 1Cl could be successfully oxidized to the Au­(III) complex [LAuCl3] (7) using PhICl2; both 7 and the gold­(I) complex 8 have been characterized crystallographically. Preliminary screening shows that 7, in combination with AgBF4, is a good catalyst for the etherification of 1-indanol with a variety of alcohol substrates and shows significantly higher activity than the gold­(I) catalyst 1Cl.