SOLID-STATE H-2 AND C-13 NMR-STUDIES OF HYDROGEN-BOND DYNAMICS IN FERROCENE-1,1'-DIYLBIS(DIPHENYLMETHANOL)

Dynamic properties of the hydroxyl groups in a selectively deuterated crystalline sample of ferrocene-1,1'-diylbis(diphenylmethanol) (FBDPM) have been studied via variable-temperature wide-line H-2 NMR spectroscopy and high-resolution C-13 NMR spectroscopy. In crystalline FBDPM, the molecules form hydrogen-bonded dimers, with the O atoms of the four hydroxyl groups involved in this hydrogen bonding defining a folded trapezium. Each hydroxyl H atom is disordered between two equally populated positions, from which it is inferred that there are two plausible arrangements (clockwise and anticlockwise) of the eight-membered ring hydrogen bonded unit. The temperature dependences of the quadrupole echo H-2 NMR line shape in the temperature range 293-370 K, the H-2 NMR spin-lattice relaxation time in the temperature range 313-428 K, and the C-13 CP/MAS NMR spectrum in the temperature range 205-253 K demonstrate that the hydrogen-bonding arrangement is dynamic; this dynamic process is interpreted as interconversion between the clockwise and anticlockwise hydrogen-bonding arrangements. The observed temperature dependence of the NMR spectra is consistent with the following dynamic models: (i) transfer of each hydroxyl H atom between adjacent hydroxyl O atoms; (ii) a two-site pi jump motion of-each hydroxyl group about its C-O bond. In either case, it is inferred that the motions of the four hydroxyl groups in the hydrogen-bonded dimer are highly correlated. In general, these dynamic models could be distinguished on the basis of H-2 NMR spectroscopy, but for the specific geometry of the intermolecular hydrogen-bonding arrangement in FBDPM, both of these models fit the H-2 NMR data. On the assumption of Arrhenius behavior for the temperature dependence of the jump frequency, the activation energy for the dynamic process is estimated (from H-2 NMR spin-lattice relaxation time measurements and H-2 NMR line-shape analysis) to be in the range 53-65 W mol(-1).