167642-50-4Relevant academic research and scientific papers
Factors affecting the strength of X-H?H-M hydrogen bonds
Peris, Eduardo,Lee Jr., Jesse C.,Rambo, Joe R.,Eisenstein, Odile,Crabtree, Robert H.
, p. 3485 - 3491 (2007/10/02)
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]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 1H 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 1H 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 H-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 Irδ+-Hδ- 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 Ir-H group alters the strength of the H-bond is that the δ- charge on the Ir-H is affected. The higher the trans effect of Y, the higher the δ- charge and the stronger the NH?HIr bonding interaction.
