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Imination of carbonyl compounds is important in heterocycle synthesis and in the biosynthesis of amino acids 1,2 but side reactions can occur, such as aminal formation. Both base and acid catalysts ( e.g., ZnCl2) are known but these tend to be unselective. Eisch and Sanchez have reported a useful aluminum complex capable of selectively catalyzing imination. Our interest in metal complexes with pendant hydrogen bonding groups led us to test [(bq-NH 2)IrH(H2O)(PPh3)2](BF4) (1) (bq-NH2 = 2-aminobezoquinoliate) 5 for catalytic imination. Since we expected the aldehyde reagent but not the imine product to bind to the metal, selectivity for imine formation seemed likely. In this study p-tolualdehyde ( pArCHO) is indeed found to bind to 1 to give a crystalline derivative, [(bq-NH2)IrH(p-ArCHO)(PPh3)2](BF4) (2) (Scheme 1), and imine formation from p-ArCHO and MesNH2 (2,4,6trimethylphenylamine) is catalyzed by complexes 1 and 2. The complex 2 was obtained by the reaction of 1 with excess p-ArCHO in CH2Cl2 (Scheme 1). Crystallization of 2 from dichloromethane/hexane yields single crystals suitable for X-ray structure determination. A summary of crystal parameters and refinement results is given in Table 1 and selected bond lengths and angles are complied in Table 2. The ORTEP view of the cationic portion of the complex is depicted in Figure 1. The most important feature of 2 is that the pendant NH2 group is capable of hydrogen bonding to the carbonyl oxygen atom of the aldehyde. Although the NH2 hydrogens are not observed, the N-O distance of 2.86 Å is consistent with an N-H O hydrogen bond. 6 Two distinctive ν (NH) IR bands at 3359 and 3471 cm −1, shifted from the free ligand band at 3411 cm −1, also indicate hydrogen bonding. In the H NMR spectrum at low temperature (below -40 C), the H-bonded NH is observed at 7.70 ppm, whereas the non-hydrogen-bonded NH is observed at 5.85 ppm. We examined catalytic imination of p-ArCHO with MesNH2 in the presence of complexes 1 or 2. When equivalent amounts of p-ArCHO and MesNH2 were treated with 5 mol% 1 or 2 at 25 C in CD2Cl2 in an NMR tube (Scheme 2), we found an average rate of 18 turnovers/hr. Control reactions without catalyst or with an NH 3 ligand, 3, blocking the p-ArCHO binding site, showed that no detectable amount of imine was formed. Therefore, binding of an aldehyde to the iridium ion seems essential for the imine formation. We next tested the catalytic imination of p-tolualdehyde with an analogous iridium complex, 4 7 lacking a pendant NH2 group. The aqua ligand in 4 could be easily replaced by

Imination Catalysis via Two-point Binding of Substrate Aldehyde via a Metal and a Pendant Hydrogen-Bonding Group