Probing the motion of the η²-dideuterium ligand by solution and solid-state ²H NMR spectroscopy

This is the authors' abstract. We don't add key points for this paper.

Variable temperature 2 H NMR is used to measure the T 1 min values of the η 2 -D 2 and D ligands in trans-[M(η 2 -D 2 )(D)(dppe) 2 ] + , M = Ru (1) and Os (2) in solution. The rapid spinning motion of the η 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 calculated to be 79 kHz for 1 and 81 kHz for 2 while motion-reduced coupling constant (C Q eff ) for the D in the η 2 -D 2 ligands in 1 and 2 are in the range 19-22 and 27-31 kHz, respectively. The actual C Q for these ligands with short D-D distances (<1 Å) should be at least 2C Q eff and probably 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 field gradient component of between 90° and 60° or between 50° and 40° with respect to the axis of D 2 rotation is an explanation for the small C Q eff . Therefore neither D-D nor M-D bonding dominates the electric field gradient direction in these M(η 2 -D 2 ) bonds. The complex [RuD 2 (C 5 Me 5 )(dppm)] + 3, which exists in solution as a 3.3:1 mixture of Ru(D···D) (3a) and Ru(D) 2 (3b) tautomers, has T 1 min (D) values that provide C Q eff values of 66 kHz for 3a and 71 kHz for 3b. The elongated D···D ligand in 3a with d(D···D) ~ 1.10 Å may be "static" compared to the 61.4 MHz 2 H spectrometer frequency and therefore have "compressed dihydride" character and display similar Ru-D bonds as in 3b. However, 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° flip of the D 2 as observed in 7. The complexes trans-[M(D···D)(Cl)(dppe) 2 ]PF 6 , M = Ru (4), Os (5), and Ru(η 2 -D 2 )(dppb)(µ-Cl) 3 RuCl(dppb) (6) also have motion-reduced C Q values. Some 2 H NMR quadrupole echo wide-line spectra of 1, 4, and Os(D···D)(Cl) 2 (CO)(P i Pr 3 ) 2 (7) (contaminated with 20% Os(H···D)) were recorded in the solid phase from 293 to 123 K. These also indicate that the C Q eff of the D in the D 2 ligands are motion-reduced. Simulation of the spectra of 7 are suggestive of a twofold reorientation of a D 2 ligand with a static C Q value in the range of 120-167 kHz, a tilt angle of the electric field gradient of about 50°, and an asymmetry parameter near to zero. Significantly, the MAS 2 H spectrum of 7 has a broad doublet, possibly due to a non-averaged D-D dipolar/quadrupolar interference phenomenon; this is first time this has been observed. Variable temperature 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 only other way of quantifying such an energy barrier is by use of an inelastic neutron scattering method.