Stability and Dynamic Processes in 16VE Iridium(III) Ethyl Hydride and Rhodium(I) σ-Ethane Complexes: Experimental and Computational Studies

Abstract

Iridium(I) and rhodium(I) ethyl complexes, (PONOP)M(C2H5) (M = Ir (1-Et), Rh (2-Et)) and the iridium(I) propyl complex (PONOP)Ir(C3H7) (1-Pr), where PONOP is 2,6-(tBu2PO)2C5H3N, have been prepared. Low-temperature protonation of the Ir complexes yields the alkyl hydrides, (PONOP)Ir(H)(R) (1-(H)(Et)(+) and 1-(H)(Pr)(+)), respectively. Dynamic (1)H NMR characterization of 1-(H)(Et)(+) establishes site exchange between the Ir-H and Ir-CH2 protons (ΔG(exH)(‡)(-110 °C) = 7.2(1) kcal/mol), pointing to a σ-ethane intermediate. By dynamic (13)C NMR spectroscopy, the exchange barrier between the α and β carbons ("chain-walking") was measured (ΔG(exC)(‡)(-110 °C) = 8.1(1) kcal/mol). The barrier for ethane loss is 17.4(1) kcal/mol (-40 °C), to be compared with the reported barrier to methane loss in 1-(H)(Me)(+) of 22.4 kcal/mol (22 °C). A rhodium σ-ethane complex, (PONOP)Rh(EtH) (2-(EtH)(+)), was prepared by protonation of 2-Et at -150 °C. The barrier for ethane loss (ΔG(dec)(‡)(-132 °C) = 10.9(2) kcal/mol) is lower than for the methane complex, 2-(MeH)(+), (ΔG(dec)(‡)(-87 °C) = 14.5(4) kcal/mol). Full spectroscopic characterization of 2-(EtH)(+) is reported, a key feature of which is the upfield signal at -31.2 ppm for the coordinated CH3 group in the (13)C NMR spectrum. The exchange barrier of the hydrogens of the coordinated methyl group is too low to be measured, but the chain-walking barrier of 7.2(1) kcal/mol (-132 °C) is observable by (13)C NMR. The coordination mode of the alkane ligand and the exchange pathways for the Rh and Ir complexes are evaluated by DFT studies. On the basis of the computational studies, it is proposed that chain-walking occurs by different mechanisms: for Rh, the lowest energy path involves a η(2)-ethane transition state, while for Ir, the lowest energy exchange pathway proceeds through the symmetrical ethylene dihydride complex.

A Rhodium–Pentane Sigma‐Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques

“…This proposed fluxional process is closely related to the chain‐walking events that have directly8b, 16c,16e and indirectly23 been shown to occur in transient, or only stable at low temperature, transition‐metal alkane σ‐complexes in solution (Scheme 4). With the pentane complex [ 3 ][BAr F 4 ], similar processes could well be occurring in the solid state, connecting terminal (M⋅⋅⋅H 3 C) and internal (M⋅⋅⋅H 2 C) σ‐binding motifs, and the estimated barriers to this (15–25 kJ mol −1 ) are similar to experimentally measured values derived from solution‐measurements at very low temperature 8b.…”

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…Ther eaction between the corresponding diols and PCl 5 proceeds with evolution of five equivalents of HCl and the last proton is then transferred to the Et 2 Os olvent to form the desired [H(OEt 2 )][WCA]s alt. [281] [H(OEt 2 ) 2 ][B(Ar CF3 ) 4 ]w as also used to protonate the monomeric ring-expanded carbene gold hydride complexes [283,284] The use of am ore weakly coordinating anion has two benefits: 1) it generates as tronger acid upon irradiation to initiate polymerization and 2) cation-anion interactions during polymerization are reduced to give amore active catalyst. [183] Oxonium acids have also featured highly in the literature for the protonolysis of metal/main group element-carbon bonds to yield the corresponding cation supported by the WCA.Amultitude of examples are presented in areview on p-block cations.…”

The Electronics of CH Activation by Energy Decomposition Analysis: From Transition Metals to Main-Group Metals

“…An alkane coordination complex (or σ-complex) involves the direct interaction of one or more alkane CH bonds with the inner sphere of the transition metal. There are now several examples of direct experimental detection of alkane complexes [40][41][42]. From a modeling perspective, an alkane coordination complex is an energy minimum structure on the potential energy landscape.…”