Structure and Energetics of the Samarium Diiodide−HMPA Complex in Tetrahydrofuran

Shotwell, J. Brad; Sealy, Jennifer M.; Flowers, Robert A.

doi:10.1021/jo990484l

Cited by 74 publications

(50 citation statements)

References 28 publications

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“…6. These overall features of the UV spectra have been described elsewhere [9] , although in that case a substantial decrease in the absorption was noted over the whole wavelength range as HMPA was added. Kinetics of electron transfer processes: Rate constants k obs were measured for the reactions of SmI 2 with 1-iodobutane (Table 1) and with benzyl chloride (Table 2), for various concentrations of HMPA.…”

Section: Resultssupporting

confidence: 71%

“…[8b] However, the solution structures of the complexes may differ; indeed, isothermal titration calorimetry experiments have indicated that a complex accommodating six HMPA ligands is not thermodynamically favored in THF, even in the presence of large amounts of HMPA. [9] Rather it was suggested that [SmI 2 (hmpa) 4 ] was the reductant responsible for the unique reactivity exhibited by Sm II in all THF/HMPA mixtures.…”

Section: Introductionmentioning

confidence: 99%

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Evidence for Ionic Samarium(II) Species in THF/HMPA Solution and Investigation of Their Electron-Donating Properties

et al. 2000

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The fundamental nature of samarium(II) complexes in THF/HMPA (HMPA = hexamethylphosphoramide) solutions containing SmI2 has been clarified by means of cyclic voltammetry, conductivity measurements, UV spectroscopy, and kinetic measurements. The principal species is not [SmI2(hmpa)4] as previously suggested, but either the ionic cluster [Sm(hmpa)4(thf)2+2I- if four equivalents of HMPA is present in the THF solution or [Sm(hmpa)6]2+ 2I- in the presence of at least 10 equivalents of HMPA. The formal potential of the [Sm(hmpa)4(thf)2]3+ 2I-/[Sm(hmpa)4(thf)2]2- 2I- redox couple determined by cyclic voltammetry was -1.79 +/- 0.08 V versus SCE. The order of reactivity of the samarium(II) complexes was found to be [Sm(hmpa)6]2+2I- > [Sm(hmpa)4(thf)2]2+2I- > SmI2 in their respective reactions with 1-iodobutane and with benzyl chloride. Very high rate enhancements, of the order of 1,000-15,000-fold, were observed upon addition of HMPA to the THF solution containing SmI2, Comparison of these rate constants with the corresponding rate constants for electron transfer (ET) reactions involving aromatic radical anions revealed that none of the reactions studied can be classified as outer-sphere ET processes and that the inner-sphere electron-donating abilities of the [Sm(hmpa)4(thf)2]2+ 2I- and SmI2 complexes are comparable. The inner-sphere ET character of the transition state increases on going from 1-iodobutane and benzyl bromide to benzyl chloride and acetophenone.

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Section: Resultssupporting

confidence: 71%

Section: Introductionmentioning

confidence: 99%

Evidence for Ionic Samarium(II) Species in THF/HMPA Solution and Investigation of Their Electron-Donating Properties

et al. 2000

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“…In fact, the higher end of the concentration range examined in this study was very close to the solubility limit of SmI 2 in THF, indicating that the reagent is always monomeric under normal reaction conditions. 36 As a consequence, deaggregation is unimportant. Follow-up studies in With the VPO study completed, ITC was used to delineate the stoichiometry of the SmI 2 -HMPA complex and to determine the affinity of HMPA for Sm(II) in THF.…”

Section: Biographical Sketchmentioning

confidence: 99%

Mechanistic Studies on the Roles of Cosolvents and Additives in Samarium(II)-Based Reductions

Flowers¹

2008

Synlett

171

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This account describes work, performed by my research group during the past decade, designed to understand the mechanistic roles of cosolvents and additives in reductions involving samarium(II) iodide (SmI 2 ). Emphasis is placed on studies designed to elucidate the roles of the cosolvent hexamethylphosphoramide (HMPA) and proton donors (water, alcohols, and glycols) in the mechanisms of important bond-forming reactions and reductions initiated by SmI 2 .

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“…[34] The concentration of the SmI 2 was determined by iodometric titration. [35] 2-Butanone, 1-chlorobutane, styrene and 4-tolunitrile were purchased from Aldrich and distilled before use. Benzyl-(1-phenylethylidene)amine was synthesized according to a reported procedure.…”

Section: Methodsmentioning

confidence: 99%

Photoinduced Electron Transfer Reactions by SmI₂ in THF: Luminescence Quenching Studies and Mechanistic Investigations

Prasad

Knettle

Flowers

2005

Chemistry A European J

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Photoluminescence quenching studies of SmI2 in dry THF were carried out in the presence of five different classes of compounds: ketone, alkyl chloride, nitrile, alkene and imine. The free energy change (DeltaG0) of the photoinduced electron transfer (PET) reactions was calculated from the redox potentials of the donor (SmI2) and acceptors. The bimolecular quenching constants (k(q)) derived from the Stern-Volmer experiments parallel the free energy changes of the PET processes. The observed quenching constants were compared with the theoretically derived electron transfer rate constants (k(et)) from Marcus theory and found to be in good agreement when a value of lambda = 167 kJ mol(-1) (40 kcal mol(-1)) was used for the reorganization energy of the system. A careful comparison of the excited state dynamics of SmII in the solid state to the results obtained in solution (THF) provides new insight in to the excited states of SmII in THF. The activation parameters determined for the PET reactions in SmI2/1-chlorobutane system are consistent with a less ordered transition state and high degree of bond reorganization in the activated complex compared to similar ground state reactions. Irradiation studies clearly show that SmI2 acts as a better reductant in the excited state and provides an alternative pathway for rate enhancement in known and novel functional group reductions.

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Structure and Energetics of the Samarium Diiodide−HMPA Complex in Tetrahydrofuran

Cited by 74 publications

References 28 publications

Evidence for Ionic Samarium(II) Species in THF/HMPA Solution and Investigation of Their Electron-Donating Properties

Evidence for Ionic Samarium(II) Species in THF/HMPA Solution and Investigation of Their Electron-Donating Properties

Mechanistic Studies on the Roles of Cosolvents and Additives in Samarium(II)-Based Reductions

Photoinduced Electron Transfer Reactions by SmI₂ in THF: Luminescence Quenching Studies and Mechanistic Investigations

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Structure and Energetics of the Samarium Diiodide−HMPA Complex in Tetrahydrofuran

Cited by 74 publications

References 28 publications

Evidence for Ionic Samarium(II) Species in THF/HMPA Solution and Investigation of Their Electron-Donating Properties

Evidence for Ionic Samarium(II) Species in THF/HMPA Solution and Investigation of Their Electron-Donating Properties

Mechanistic Studies on the Roles of Cosolvents and Additives in Samarium(II)-Based Reductions

Photoinduced Electron Transfer Reactions by SmI2 in THF: Luminescence Quenching Studies and Mechanistic Investigations

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Photoinduced Electron Transfer Reactions by SmI₂ in THF: Luminescence Quenching Studies and Mechanistic Investigations