Probing the motion of the eta(2)-dideuterium ligand by solution and solid-state H-2 NMR spectroscopy

Variable temperature H-2 NMR is used to measure the T-1(min) values of the eta(2)-D-2 and D ligands in trans[M(eta(2)-D-2)(D)(dppe)(2)](+), M = Ru (1) and Os (2) in solution. The rapid spinning motion of the eta(2)-D-2 ligand results in a much longer T-1(min) than that of the terminal deuteride. The quadrupole coupling constant (C-Q) for the terminal deuteride is calculate d to be 79 kHz for 1 and 81 kHz for 2 while motion-reduced coupling constan t (C-Q(eff)) for the D in the eta(2)-D-2 ligands in 1 and 2 are in the rang e 19-22 and 27-31 kHz, respectively. The actual C-Q for these ligands with short D-D distances (<1 Angstrom) should be at least 2C(Q)(eff) and probabl y greater than that of the terminal deuteride but less than that of D-2(g), 227 kHz. A fast spinning (>>61 MHz) and tilting of the primary electric fi eld gradient component of between 90 degrees and 60 degrees or between 50 d egrees and 40 degrees with respect to the axis of D-2 rotation is an explan ation for the small C-Q(eff). Therefore neither D-D nor M-D bonding dominat es the electric field gradient direction in these M(eta(2)-D-2) bonds. The complex [RuD2(C5Me5)(dppm)](+) 3, which exists in solution as a 3.3:1 mixtu re of Ru(D...D) (3a) and Ru(D)(2) (3b) tautomers, has T-1(min)(D) values th at provide C-Q(eff) values of 66 kHz for 3a and 71 kHz for 3b. The elongate d D...D ligand in 3a with d(D...D) congruent to 1.10 Angstrom may be "stati c" compared to the 61.4 MHz H-2 spectrometer frequency and therefore have " compressed dihydride" character and display similar Ru-D bonds as in 3b. Ho wever, it is more likely that the static C-Q for the D of 3a is much larger than that of a terminal Ru-D but averaged to 66 kHz by a 180 degrees flip of the D-2 as observed in 7. The complexes trans-[M(DD)(Cl)(dppe)(2)]PF6, M = Ru (4), Os (5), and Ru(eta(2)-D-2)(dppb)(mu-Cl)(3)RuCl(dppb) (6) also ha ve motion-reduced C-Q values. Some H-2 NMR quadrupole echo wide-line spectr a of 1, 4, and Os(D...D)(Cl)(2)(CO)((PPr3)-Pr-i)(2) (7) (contaminated with 20% Os(H ... D)) were recorded in the solid phase from 293 to 123K. These a lso indicate that the C-Q(eff) of the D in the D-2 ligands are motion-reduc ed. Simulation of the spectra of 7 are suggestive of a twofold reorientatio n of a D-2 ligand with a static C-Q value in the range of 120-167 kHz, a ti lt angle of the electric field gradient of about 50 degrees, and an asymmet ry parameter near to zero. Significantly, the MAS H-2 spectrum of 7 has a b road doublet, possibly due to a non-averaged D-D dipolar/quadrupolar interf erence phenomenon; this is first time this has been observed. Variable temp erature T-1 data for solid 7 are also reported, which allow the evaluation of the activation barrier to the twofold flipping motion of the D-2; the on ly other way of quantifying such an energy barrier is by use of an inelasti c neutron scattering method.