Probing regioselective intermolecular hydrogen bonding to [Re(CO)H-2(NO)(PR3)(2)] complexes by NMR titration and equilibrium NMR methodologies

The hydrogen-bonding interaction of rhenium hydrides [Re(CO)H-2(NO)(PR3)(2) ](R=Me 1a, Et 1b, iPr Ic) with two different proton donors (hexafluoro-2-pr opanol (PFTB) and perfluoro-2-me thyl-2-propanol (PFTB)) was studied in sol ution using variable-temperature (VT) NMR spectroscopy. As a novel feature, ReH ... HX intermolecular hydrogen bonding was observed to be dependent on R, the HX acid strength, the HX concentration, temperature and solvent typ e. The regioselectivity of these interactions could be verified. Hydrogen b onding occurs preferably with the hydride H-a trans to NO, but also with th e hydride H-b trans to CO, and with the O-NO atom. These interactions show similar dependencies on the steric requirements of the phosphanes and the c hemical nature of the acidic substrates. NMR equilibrium constants and ther modynamic data (- Delta H = 2.3 - 62 kcal mol(-1)) are reported for the hyd rogen-bonded complexes in solution. Difference NOE measurements for 1a and 1b allowed us to confirm the regioselectivity of the hydrogen bonding to H- a and H-b, with the major interaction to H-a. From the NMR relaxation time measurements of the hydrides, hydrogen bond lengths were obtained (1.78 - 1 .94 Angstrom). A solvent effect was established with considerably smaller K and -Delta H values in toluene than in methylcyclohexane, which could be r elated to 'aromatic' hydrogen bonding between the fluorinated alcohols and toluene.