Versatile Coordination of 2-Pyridinetetramethyldisilazane at Ruthenium: Ru(II) vs Ru(IV) As Evidenced by NMR, X-ray, Neutron, and DFT Studies
Résumé
The novel disilazane compound 2-pyridinetetramethyldisilazane (1) has been synthesized. The competition between N-pyridine coordination and Si−H bond activation was studied through its reactivity with two ruthenium complexes. The reaction between 1 and RuH2(H2)2(PCy3)2 led to the isolation of the new complex RuH2{(η2-HSiMe2)N(κN-C5H4N)(SiMe2H)}(PCy3)2 (2) resulting from the loss of two dihydrogen ligands and coordination of 1 to the ruthenium center via a κ2N,(η2-Si−H) mode. Complex 2 has been characterized by multinuclear NMR experiments (1H, 31P, 13C, 29Si), X-ray diffraction and DFT studies. In particular, the HMBC 29Si−1H spectrum supports the presence of two different silicon environments: one Si−H bond is dangling, whereas the other one is η2-coordinated to the ruthenium with a JSiH value of 50 Hz. DFT calculations (B3PW91) were also carried out to evaluate the stability of the agostic species versus a formulation corresponding to a bis(σ-Si−H) isomer and confirmed that N-coordination overcomes any stabilization that could be gained by the establishment of SISHA interactions. There is no exchange between the two Si−H bonds present in 2, as demonstrated by deuterium-labeling experiments. Heating 2 at 70 °C under vacuum for 24 h, leads to the formal loss of one equivalent of H2 from 2 and formation of the 16-electron complex RuH{(SiMe2)N(κN-C5H4N)(SiMe2H)}(PCy3)2 (3) formulated as a hydrido(silyl) species on the basis of multinuclear NMR experiments. The dehydrogenation reaction is fully reversible under dihydrogen atmosphere. Reaction of Ru(COD)(COT) with 3 equiv of 1 under a H2 pressure led to the isolation of the new complex RuH{(SiMe2)N(κN-C5H4N)(SiMe2H)}3 (4) characterized as a hydridotrisilyl complex by multinuclear NMR techniques, X-ray and neutron diffractions, as well as DFT calculations. The 29Si HMBC experiments confirm the presence of two different silicon atoms in 4, with a signal at −14.64 ppm for three dangling Si-Me2H fragments and a signal at 64.94 ppm (correlating with the hydride signal) assigned to three Si-Me2N groups bound to Ru. Comparison of DFT and neutron parameters involving the hydride clearly indicates an excellent correlation. The Si−H distance of 2.15 Å is much shorter than the sum of the van der Waals radii and typically in the range of a significant interaction between a silicon and a hydrogen atom (SISHA interactions). In 4, three dangling Si−H groups remain accessible for further functionalization.