Ru-D2 Complex

In the case of the RU-D2 complex the data from the lineshape analysis and the Tj-relaxation data are combined in Fig. 21.22 which shows an Arrhenius plot of the temperature dependence ofXj2 and ki2- 

While the temperature dependence of Xj2 is very weak and nearly linear in the temperature window between 5 and 20 K, the incoherent exchange rate ki2 exhibits a strong non-Arrhenius behavior and varies from 5 x 10 s i at 5.4 K to ca. 2.5 x 10 s i at 103 K and ca. lO s i at 300 K. The rate data from both types of experiments overlap between 20 K and 100 K and there is an excellent agreement between the values. This indicates that both rates result from the same motional process. 

The simulation of the temperature dependence was performed assuming a thermally activated tunneling process, described by a Bell type of tunneling. The high temperature rate in the tunnel model was chosen as 4 x lOi which is expected from the Eyring equation. Since the observed increase in kj2 at low temperatures is not obtainable by a simple one-dimensional Bell model an effective power law potential was employed  

The solid line is the result of a fit of the temperature dependence ofthe incoherent rates using a modified Bell tunnel model (see text). 

The dashed line is a simple linear fit of the coherent tunnel rates. 

---

Fig. 4. Experimental and simulated H NMR spectra of the RU-D2 complex 1 at the indicated temperatures. The peaks marked with arrows originate from the QEC between the deuterons. (Reproduced with permission from Ref. 28. Copyright by The PCCP Owner Societies, 1999).

Figure 21.18 compares experimental H solid echo NMR spectra and the simulated H FID-NMR spectra of the RU-D2 complex trans-[Ru(D2)Cl(dppe)2[PFg (Ru-D2). At temperatures below 10 K the singularities of a satellite Pake pattern are visible as a splitting of the spectra at 60 kHz. This satellite Pake pattern is the... 

As a first experimental example. Fig. 21.19 displays the result of the Tj measurements on the same RU-D2 complex trans-[Ru(D2)Cl(dppe)2 PF6 (RU-D2) as above. Due to the low sensitivity of the sample the spin-lattice relaxation rates were measured only at some selected temperatures. The lowest Tj value (0.12 0.02 s) was found at 97 K. At low temperatures the Tj data show strong deviations from simple Arrhenius behavior. 

Sellmann s model is claimed to be consistent with the severe constraints imposed on this N2-dependent HD formation from D2 and protons. Other modeling studies have shown that protons can be transferred from acidic H2 ligands in cationic Ru-H2 complexes to N2 ligands in W(N2)2(P)4 complexes (P = phosphine donor), in some cases even forming ammonia [Eq. (22)] (88,89). 

Nickel-sulfur hydride complexes were a primary research target. However, the few which were found did not catalyze the H2 heterolysis according to Eqs. 45b or 46 (142). In the quest for other metal-sulfur complexes exhibiting the [M(H)(SH)] motif, the Rh and Ru hydride complexes [Rh(H)(CO)(S4)] (51, 143) and [Ru(H)(PCy3 HS4) (144) were found. They proved to catalyze a D2/ H+ exchange according to Eq. 47. In order to do so, the Rh complex required the addition of catalytic amounts of Brpnsted acids such as aqueous HC1 or HBF4. 

Figure 21.18 Experimental solid echo NMR spectra ofthe Ru-D complex tra s-[Ru(D2)CI(dppe)2]PF5 (Ru-Dj), measured in the temperature range 5.4 to 230 K. At temperatures below 8.8 K a splitting in the NMR li neshape is clearly visible (arrows). This splitting can be explained by a coherent...

Related studies have made use of 2H NMR spectroscopy in the solid state. The theoretical foundation for the study of dynamics in dihydrogen complexes was developed by Buntkowsky and coworkers.124 Solid-state 2H NMR spectroscopy has been used to study the dynamics of bound D2 in tntns- Ru( D2)CI(dppe)2 PI>, where evidence was found for coherent rotational tunneling, with a barrier of 6.2kcal/ mol.125 In more recent work, solid-state 2H NMR spectroscopy has been used to determine the structure and dynamics of several dihydrogen Ru complexes.126... 

Assignments unclear, in the case of the elongated Ru andOs complexes, these are highly mixed modes that could involve M-H modes (if present). "Estimated from observed D2 isotopomer bands. 

Rocek and coworkers [10] attempted to compare the activity of various transition-metal complexes in the hydrodehalogenation of 5-iodouracil with atmospheric H2 at 70-80 °C. The majority of the examined complexes decomposed to metal, but [(PPh3)2CoCl2], and [(PPh3)3RuCl2] in DMA (N, N-d i me lb yl -act I a-mide) remained homogeneous. The more active Ru complex catalyzed also the preparation of [5-2H]uracil using D2 instead of H2. 

There is an extensive series of dioxorutheniuni(VI) complexes with porphyrinato ligands, trans-[Ru (0)2(por)]. These include the nonsterically bulky porphyrins 4-X-TPP and sterically encumbered porphyrins such as TMP, DPP, 2,6-Cl-TPP, and 2,4,6-MeO-Tpp-322 porphyrins such as chiral picket-fence porphyrin (Pf-por), Z)2-porphyrins D2-... 

Rates for d2 -d6 complexes show no trend with ionic radius, however, on descending a column in the periodic table the rates always decrease Fe(III) > Ru(III) and Co(III) > Rh(IlI). 

For the Ru system the thiol hydride could not be detected, while for the Rh system and also [IrH2(HS(CH2)3SH)(PCy3)2]+(which similarly catalyzes D2/H + exchange (79)), the H2 complex could not be seen but is a transient. A related system, Ni(NHP Pr3)(S3) clearly shows that heterolysis of D2 can also occur at nickel sites, which may be relevant to H2 activation in [FeNi] hydrogenases (78). 

---