FACTORS AFFECTING THE STRENGTH OF X-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-H-M HYDROGEN-BONDS

The strengths of intramolecular H ... H and H ... Y hydrogen bonding between a ligating 2-aminopyridine NH group and a cis Ir-H bond or a cis halo group has been estimated (1.8-5.2 kcal/mol) in a series of compounds of general form [IrH2(Y)(2-C6H4NH2)(PPh(3))(2)1(n+) (Y = H-, F-, Cl-, Br-, I-, SCN-, and CN-, n = 0, and Y = CO and MeCN, n = 1) by a new method involving measuring the Ar-NH2 rotation barrier by H-1 NMR. The H-bonding interaction is surprisingly strong; in cases where both are possible, N-H ... H-Ir hydrogen bonding is preferred over N-H ... Cl-Ir H-bonding. The experimental barrier for C-N bond rotation in [IrH2(Y)(2-C6H4NH2)(PPh(3))(2)(n+) was in the range 7.6-11.0 kcal/mol, as determined by H-1 NMR. From a simple geometrical study it appears that the two H-bonded hydrogens can approach appropriately close to each other. In contrast, the geometry of the situation is not as favorable for N-H ... Y-Ir H-bonding for Y = F-, Cl-, Br-, and I-. From core potential ab initio studies, the II-bond strength was estimated to be in the range 5.7-7.1 kcal/mol, assuming that the intrinsic C-N rotation barrier is the same in free and coordinated 2-C5H4NH2. These unusual hydrogen bonds (A-H ... B) are proposed to be strong for an element B having the electronegativity of hydrogen because of (i) a favorable geometry which allows NH and IrH to approach very close to one another and (ii) the facility with which Ir-H may be polarized in the sense Ir-delta+-H-delta- on the approach of the N-H bond. The calculations also suggest that the reason changing the nature of the ligand Y trans to the H-bonded Lt-H group alters the strength of the H-bond is that the delta- charge on the Ir-H is affected. The higher the trans effect of Y, the higher the delta- charge and the stronger the NH ... HIr bonding interaction.