Agostic versus Terminal Ethyl Rhodium Complexes: A Combined Experimental and Theoretical Study

Abstract

Ethylene insertion into Rh−H bonds in complexes 8 bearing an anionic fac-triphosphane ligand gives hydrido 9 complexes, β-agostic species, or noninteracting ethyl derivatives 10 depending on the reaction conditions. Several chemical equilibria 11 between these species have been analyzed by NMR and DFT 12 calculations, which revealed that they are mainly controlled by the 13 entropy. Moreover, β-agostic species were found to be lower in 14 enthalpy than the corresponding hydride−ethylene complexes, 15 probably due to the steric pressure exerted by the bulky fac-16 triphosphane ligand. 17 ■ INTRODUCTION 18 Weak interactions between nonpolar C−H bonds and 19 transition metals have been proved to be of paramount 20 importance in organometallic chemistry and catalysis, con-21 tributing to major developments in the field of C−H bond 22 activation and/or functionalization processes, 1 as well as in 23 dehydrogenation reactions. 2 Both versions, intermolecular (σ-24 complexes) and intramolecular (agostic 3 complexes) are 25 known, the latter being important intermediates connecting 26 two fundamental reactions in organometallic chemistry, namely, 27 the insertion of olefins into M−H bonds and the reverse one, 28 the β-hydrogen elimination. Nowadays, the significance of 29 agostic species becomes evident from the large body of 30 literature in which the multiple roles they can play is 31 enlightened. 4 As a way of example, α-agostic interactions 32 have been proved to have a strong impact on the stereo-33 specificity and rate of olefin insertion in polymerization; 5 β-34 agostic species often lead to C−H activation reactions, 6 while 35 the γ-agostic ones have been reported to be crucial in the 36 stabilization of the propagating species in vinyl norbornene 37 polymerization. 7 Moreover, a delicate balance between 38 electronic versus steric factors can tip the stability of αversus 39 β-agostic compounds, 8 or even between βand γ-agostomers. 9 40 Furthermore, longer range interactions such as the rare δand 41 ε-agostic ones are involved in uncommon intramolecular 1,4-, 42 metal migration or 1,5-σ bond metathesis, respectively. 10 In 43 other instances, they are valuable intermediates connecting the 44 transition states that lead to C−H versus C−C activation 45 reactions, 11 and also documented is their participation in the 46 stabilization of highly unsaturated intermediatessuch as T-47 shaped d 8 -ML 3 complexes 12 requiring two agostic interac-48 tions in some cases. 13 49A key feature of these particular M−alkyl moieties can be 50 related to their lability, providing (or not) a vacant site on the 51 coordination sphere of the metal, which, in turn, significantly 52 impacts the reactivity of the complex. Consequently, the study 53 of the dynamics of such species has been the focus of much 54 attention from both experimental 14 and theoretical ap-55 proaches, 15 most of them related to the estimation of the 56 migratory insertion barriers in the context of the polymerization 57 of olefins. 16 58 The s...

Rhodium Complexes in P−H Bond Activation Reactions

“…This ratio could represent the relative rates for OPHPh 2 coordination to B / C (assuming a fast equilibrium B ⇄ C ), or alternatively the ratio of B and C in the equilibrium if coordination of OPHPh 2 were faster. However, considering the low Δ G values for hydride insertions into Rh−ethylene bonds, the first possibility seems to be more plausible.…”

“…The proposed equilibrium between complex 15 and intermediates B and C has been verifiedb yt he reactiono f15 with OPHPh 2 (in 1:2m olar ratio), which systematically gives am ixture of the mononuclear complexes 16 and 17 in a2 :1 ratio. Since no change of this ratio was observed on heating this mixture for prolonged time, complexes 16 and 17 are not in equilibrium and, most probably, they arise from intermediates B and C,r espectively.T his ratio couldr epresent the relative rates for OPHPh 2 coordination to B/C (assuming afast equilibrium BQC), or alternatively the ratio of B and C in the equilibrium if coordination of OPHPh 2 were faster.H owever, considering the low DG values for hydride insertions into RhÀethylene bonds, [49] the first possibility seems to be more plausible.…”

Pseudo‐tetrahedral Rhodium and Iridium Complexes: Catalytic Synthesis of E‐Enynes

“…However,t he equivalence of the three phosphorus atoms in the 31 P{ 1 H} NMR spectra would require an easy phosphorus donor exchange throughe ither trigonal-bipyramidal structures or PÀ Mb ond dissociation, which in our experience involve energy barriers larger than that calculated (1 kcal mol À1 ). [34] The DFT-computed optimized geometries of the modelcomplexes [M{MeB(CH 2 PMe 2 ) 3 }(HCCPh)] (M = Ir, 3';R h, 6')a s closed-shell speciesr eproduce quite wellt he experimental data found for 3 and 6.I np articular,t he agreement of the MÀ Pb ond lengths and P-M-P bond angles with the experimental data is remarkable. This geometrici rregularity characteristic of the metallicf ragments d 8 -M{MeB(CH 2 PMe 2 ) 3 }i nt he singlet state is retained in the complexes, whereas the HOMO-LUMO gap increases to 3.48 and 3.81 eV in 6' and 3',r espectively.T he representations and compositions of the MOs of the model complexes show strong mixing, and therefore ad irect comparison with the MOs of the fragment is difficult.…”

“…The alternative pseudo‐square‐pyramidal M III ‐metallacyclopropene description could also be considered to account for the low‐field shifts of protons and carbons. However, the equivalence of the three phosphorus atoms in the 31 P{ 1 H} NMR spectra would require an easy phosphorus donor exchange through either trigonal‐bipyramidal structures or P−M bond dissociation, which in our experience involve energy barriers larger than that calculated (1 kcal mol −1 ) …”

Mechanistic insight into the pyridine enhanced α-selectivity in alkyne hydrothiolation catalysed by quinolinolate–rhodium(i)–N-heterocyclic carbene complexes

“…, as a result of S-H oxidative addition and subsequent insertion of ethylene into the newly formed Rh-H bond, 30 whereas, in contrast to 9, the formation of the octahedral pyridine complex was not observed. Traces of 9 and 10 were detected.…”