Experimental and Computational Study of the Transformation of Terminal Alkynes to Vinylidene Ligands ontrans-(Chloro)bis(phosphine)Rh Fragments and Effects of Phosphine Substituents

Abstract

Experimental and computational evidence points to unimolecular transformation of terminal alkynes on the title Rh(I) metal fragments. Lack of isotopic scrambling in double-crossover experiments is inconsistent with a previously proposed bimolecular pathway. Focusing on a unimolecular manifold, alkyne binding to the metal forms the Rh(I) alkyne π-complex 2, which isomerizes to the Rh(III) hydrido-(alkynyl) species 4, ultimately leading to Rh(I) vinylidene product 5. In making alkyne-free precursors, use of heterocyclic ligand (i-Pr) 2 PIm′ (1b, Im′ ) 1-methyl-4-tert-butylimidazol-2-yl) led to species 8 with a labile P,N chelate, whereas a geometrically similar o-tolyl ligand suffered metalation at the methyl group and was unsuitable for alkyne transformation studies. Kinetic studies comparing 1b and (i-Pr) 2 PPh (1c) allowed determination of rate constants for the alkyne binding event and conversion of 2 to 5 (the latter, k 2-5 , being 9.6 times faster for 1b). Based on a scan of the two-dimensional reaction surface, combined density functional/molecular mechanics calculations predict that η 2 -(C,H) alkyne complex 3 is in a fast equilibrium with the lower energy hydrido(alkynyl) complex 4, and neither species is expected to be present at observable concentrations. Eyring model estimates of the rate constants from these computational data predict the available experimental values in this work to within a factor of 2 and the ratio of the rate constants k 2-5 for 1b and 1c to within 10%. The calculations also agree with the qualitative observation that reaction rates are faster for both ligands 1b and 1c than for (i-Pr) 3 P and predict that reactions using triphenylphosphine will be faster than those with (i-Pr) 3 P. NMR coupling constants, particularly 1 J CC values, were used to evaluate bonding and back-bonding in isotopomers of 2a-c and 5a-c derived from H 13 C 13 CH.

Structures, Mechanisms, and Results in Bifunctional Catalysis and Related Species Involving Proton Transfer

“…We have provided evidence for the profound effects of a substituent next to heterocyclic nitrogen on both chelate stability and catalytic ability [3,8,33,34,[39][40][41][42][43][44][45][46]. Ruthenium complexes 7a and 7b (Fig.…”

Heteroatoms moving protons: Synthetic and mechanistic studies of bifunctional organometallic catalysis

“…Ligand loss from 8 could be adjusted by selecting an appropriately labile co-ligand L, whereas chelate opening in 7 could be adjusted by varying the steric demand of R 1 next to the coordinating nitro gen, as well as by varying the sterics and electronics of the phosphine R groups. In a series of papers in the past 8 years we have provided experimental evidence for the profound role of R 1 in both chelate stability and also catalytic ability [5,[22][23][24][25][26][27][28][29][30][31][32], where a tert-butyl group often emerges as an excellent choice. For example (Fig.…”

“…Using doubly labeled alkyne H 13 C 13 CH allowed us to measure all possible C-H couplings in both the alkyne and vinylidene complexes. Previous work from our labs [27,31] showed that 1 J CC could be used much as CO infrared stretching frequencies are used to discern metal-ligand backbonding and ligand electronic effects. In the vinylidene products, within experimental uncertainty 1 J CC and 1 J CH are the same, from which we can conclude that the ligand electronics are the same.…”

Counteranion and Solvent Assistance in Ruthenium-Mediated Alkyne to Vinylidene Isomerizations

“…1 These observations have been interpreted in terms of the electron-donating abilities of the basic heterocyclic phosphine substituents as well as their steric properties. 14 In recent years, an increasing amount of evidence has revealed the influence that ion pairing can have in organometallic transformations, 25 particularly in those involving proton transfer steps. Ion-pairing formation can modify the energy barrier of a proton transfer reaction by stabilization or destabilization of reaction intermediates, as shown in the kinetic study of the deprotonation of the cationic dihydrogen complex trans-[FeH(η 2 -H 2 )(dppe) 2 ] + with NEt 3 .…”