Fluxionality and Isomerism of the Bis(dihydrogen) Complex RuH₂(H₂)₂(PCy₃)₂:  INS, NMR, and Theoretical Studies

Abstract

To study the fluxionality of the bis(dihydrogen) complex RuH(2)(H(2))(2)(PCy(3))(2) (1), NMR spectra were recorded in Freons (mixture of CDCl(3), CDFCl(2), and CDF(2)Cl). 1 was found to remain fluxional at all temperatures, but the presence of CDCl(3) necessary for its solubilization induces its transformation into, first, RuHCl(H(2))(2)(PCy(3))(2) (3) and the new ruthenium(IV) dihydride RuH(2)Cl(2)(PCy(3))(2) (4). 4 is produced selectively in pure CDCl(3) but reacts further to give a mixture of chloro complexes. 4 was isolated from the reaction of 1 with aqueous HCl in Et(2)O and shows a fluxional process attributed to the interconversion between two symmetrical isomers. The activation parameters of this process were obtained by (1)H NMR line shape analysis, as well as those corresponding to the exchange between 3 and free dihydrogen. The fluxionality of the dihydrogen-hydride system is also evident at a much faster time scale than that of NMR studies in the inelastic neutron scattering observations of the rotation of the dihydrogen ligands. The geometries and relative energies of several isomers of complexes 1, 3, and 4 were studied using density functional theory (DFT) and MP2 methods, together with a few coupled-cluster (CCSD(T)) calculations. In contrast to what might have been expected, the two hydrides and the two H(2) units of 1 lie in the same plane, due to the attractive "cis effect" created by the hydrides. The two H(2) ligands adopt cis positions in the lowest-energy isomer. Rotation of the two dihydrogen ligands has been analyzed using DFT calculations. A slight preference for a C(2) conrotatory pathway has been found with a calculated barrier in good agreement with the experimental INS value. Two low-energy isomers of 4 have been characterized computationally, both of which have C(2)(v)() symmetry, consistent with the solution NMR spectra.

Synthesis and Molecular Structure of Six-Coordinate Dichlorodihydridoruthenium(IV) and Five-Coordinate Vinylideneruthenium(II) Complexes

“…The C1ϪC2 distance is in the expected buted to the interconversion between two stereoisomers having C 2v symmetry. [11] Our observations are in agreement range and this is true also for the RuϪCl and RuϪP bond lengths. with these results.…”

“…[10] In contrast, the tricyclohexylphosphane derivative ence of NEt 3 as a base. After removal of the solvent and 13 which was prepared independently by Chaudret and coextraction of the residue with pentane, a red oil was obworkers [11] and by us (see below), shows a fluxional process tained which by comparison of the NMR data with those attributed to the interconversion of two symmetrical isopublished by Chaudret is the dihydrogenruthenium(II) demers. rivative 5.…”

“…with these results. [23] Compound 14 does not only react with CO to give Finally, a special comment is appropriate concerning the interconversion of the ruthenium(II) and ruthenium(IV) [RuHCl(CO) 2 (PCy 3 ) 2 ] [11] but also with phenylacetylene in the molar ratio of 1:10 to yield the vinylidene complex 15 complexes 5 and 4 (see Scheme 5). In the context of the preparation of 4 from 3 it has already been mentioned that (Scheme 4).…”

Stereoisomerism of bis(σ‐Zincane) Complexes: Evidence for an Intramolecular Pathway

“…DFT calculations revealed three minima, trans , cis‐1 and cis‐2 (Figure ). Both cis‐1 and cis‐2 are lower in energy than trans due to the trans ‐influence of the CO ligands . The structure of trans contains perpendicular dihydrogen ligands, and allows both H 2 ligands to be stabilised by backdonation from orthogonal d xz and d yz orbitals .…”

“…Both cis-1 and cis-2 are lower in energy than trans due to the trans-influence of the CO ligands. [32,57] The structure of trans contains perpendicular dihydrogen ligands, and allows both H 2 ligands to be stabilised by backdonation from orthogonal d xz and d yz orbitals. [21,58] C 3 -rotations from trans interconvert the isomers through degenerate trigonal prismatic transition states and, depending on the direction of rotation (C 3 1 or C 3…”