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

Abstract

Variable temperature 2H NMR is used to measure the T1min values of the η2-D2 and D ligands in trans-[M(η2-D2)(D)(dppe)2]+, M = Ru (1) and Os (2) in solution. The rapid spinning motion of the η2-D2 ligand results in a much longer T1min than that of the terminal deuteride. The quadrupole coupling constant (CQ) for the terminal deuteride is calculated to be 79 kHz for 1 and 81 kHz for 2 while motion-reduced coupling constant (CQeff) for the D in the η2-D2 ligands in 1 and 2 are in the range 19-22 and 27-31 kHz, respectively. The actual CQ for these ligands with short D-D distances (<1 Å) should be at least 2CQeff and probably greater than that of the terminal deuteride but less than that of D2(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 D2 rotation is an explanation for the small CQeff. Therefore neither D-D nor M-D bonding dominates the electric field gradient direction in these M(η2-D2) bonds. The complex [RuD2(C5Me5)(dppm)]+ 3, which exists in solution as a 3.3:1 mixture of Ru(D···D) (3a) and Ru(D)2 (3b) tautomers, has T1min(D) values that provide CQeff 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 2H 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 CQ 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 D2 as observed in 7. The complexes trans-[M(D···D)(Cl)(dppe)2]PF6, M = Ru (4), Os (5), and Ru(η2-D2)(dppb)(µ-Cl)3RuCl(dppb) (6) also have motion-reduced CQ values. Some 2H NMR quadrupole echo wide-line spectra of 1, 4, and Os(D···D)(Cl)2(CO)(PiPr3)2 (7) (contaminated with 20% Os(H···D)) were recorded in the solid phase from 293 to 123 K. These also indicate that the CQeff of the D in the D2 ligands are motion-reduced. Simulation of the spectra of 7 are suggestive of a twofold reorientation of a D2 ligand with a static CQ 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 2H 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 T1 data for solid 7 are also reported, which allow the evaluation of the activation barrier to the twofold flipping motion of the D2; the only other way of quantifying such an energy barrier is by use of an inelastic neutron scattering method.Key words: dihydrogen, deuterium, NMR, ruthenium, osmium, complexes, dynamics, hydride, bonding.