Seleniranium Ions Undergo π-Ligand Exchange via an Associative Mechanism in the Gas Phase

Abstract

Collision-induced dissociation mass spectrometry of the ammonium ions 4a and 4b results in the formation of the seleniranium ion 5, the structure and purity of which were verified using gas-phase infrared spectroscopy coupled to mass spectrometry and gas-phase ion-mobility measurements. Ion-molecule reactions between the ion 5 (m/z = 261) and cyclopentene, cyclohexene, cycloheptene, and cyclooctene resulted in the formation of the seleniranium ions 7 (m/z = 225), 6 (m/z = 239), 8 (m/z = 253), and 9 (m/z = 267), respectively. Further reaction of seleniranium 6 with cyclopentene resulted in further π-ligand exchange giving seleniranium ion 7, confirming that direct π-ligand exchange between seleniranium ion 5 and cycloalkenes occurs in the gas phase. Pseudo-first-order kinetics established relative reaction efficiencies for π-ligand exchange for cyclopentene, cyclohexene, cycloheptene. and cyclooctene as 0.20, 0.07, 0.43, and 4.32. respectively. DFT calculations at the M06/6-31+G(d) level of theory provide the following insights into the mechanism of the π-ligand exchange reactions; the cycloalkene forms a complex with the seleniranium ion 5 with binding energies of 57 and 62 kJ/mol for cyclopentene and cyclohexene, respectively, with transition states for π-ligand exchange having barriers of 17.8 and 19.3 kJ/mol for cyclopentene and cyclohexene, respectively.

Evidence that π‐ligand exchange reactions of chalcogen iranium ions proceed via Hückel pseudocoarctate transition states

“…The choice of systems to investigate associative π‐ligand exchange transition states with the above tools includes those studied both theoretically and experimentally in the gas phase previously. [ 18,24–26 ] These are outlined in Scheme 4 with an array of both identity and nonidentity exchange reactions bringing the total to 12 systems.…”

“…Reaction and racemisation pathways of chalcogen (ChS, Se or Te) iranium ion 1 with nucleophiles (adapted from Denmark et al [ 16 ] and Lim et al [ 24 ] )…”

“…To avoid potential dissociative transfer facilitated by solvents, counter ions and impurities, studies in the gas phase have investigated both the thermodynamic and kinetic factors controlling associative transfer (or lack thereof) of PhS + , PhSe + and PhTe + moieties between alkenes (Scheme 3). [ 24–26 ] Structures of the iranium ions 27–28 in particular, produced by electrospray ionisation of appropriate precursors 25–26 , were supported by high‐resolution mass spectrometry, infrared multiple photon dissociation and density functional theory (DFT) calculations with little if any isomerisation to the related onium ions occurring. [ 24,25 ] Thiiranium ions 10 , 19 and 27 were not observed to undergo transfer in the gas phase and instead produced an electrophilic addition product, contrasting with experiments by Denmark et al, which suggest that this transfer is feasible in the solution phase.…”

“…[ 24–26 ] Structures of the iranium ions 27–28 in particular, produced by electrospray ionisation of appropriate precursors 25–26 , were supported by high‐resolution mass spectrometry, infrared multiple photon dissociation and density functional theory (DFT) calculations with little if any isomerisation to the related onium ions occurring. [ 24,25 ] Thiiranium ions 10 , 19 and 27 were not observed to undergo transfer in the gas phase and instead produced an electrophilic addition product, contrasting with experiments by Denmark et al, which suggest that this transfer is feasible in the solution phase. [ 12 ] Where there was a thermodynamic driving force, the PhSe + moiety was transferred between alkenes in ion–molecule reactions (IMRs) ( 28 to 30 ), but identity exchange reactions suggested that the intrinsic kinetic barrier could also dominate reaction outcomes ( 11 to 14 compared with 20 to 23 ).…”

“…Experimental π‐ligand exchange reactions of chalcogen iranium ions 10–12, 19–21 and 27–28 in the gas phase performed in a linear ion trap quadrupole (LTQ) mass spectrometer (adapted from the references herein [ 24–26 ] ). LG, leaving group…”