Six different well-defined rhodium and iridium N-heterocyclic carbene complexes, i.e., RhCl-(1,3-dimesityltetrahydropyrimidin-2-ylidene)(COD) þ Rh 2 (CF 3 COO) 3 (COD) À (8). Compounds 4 and 8 were characterized by X-ray analysis. The activity of the rhodium complexes increased in the order 5 > 3 > 1 > 2, indicating the necessity of strongly electron-withdrawing groups at the metal centers, thus increasing their nucleophilicity. In due consequence, the "softer" iridium complexes 4 and 6 exhibited significantly reduced catalytic activity albeit with enhanced selectivity. The syntheses of the metal complexes as well as a detailed study on their reactivity in the arylation of carbonyl compounds using equimolar amounts of arylboronic acid and carbonyl compound in the presence of 0.08 -1 mol % catalyst are presented.
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