Olefin Substitution in (silox)3M(olefin) (silox = tBu3SiO; M = Nb, Ta): The Role of Density of States in Second vs Third Row Transition Metal Reactivity

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This article discusses the role of density of states in second vs third row transition metal reactivity.

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Hirsekorn, Kurt F.; Hulley, Elliott B.; Wolczanski, Peter T. & Cundari, Thomas R., 1964- January 8, 2008.

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This article discusses the role of density of states in second vs third row transition metal reactivity.

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Reprinted with permission from the Journal of the American Chemical Society. Copyright 2008 American Chemical Society.

Abstract: The substitution chemistry of olefin complexes (silox)3M(ole) (silox = tBu3SiO; M = Nb (1-ole), Ta (2-ole); ole = C2H4 (as 13C2H4 or C2D4), C2H3Me, C2H3Et, cis-2-C4H8, iso-C4H8, C2H3Ph, cC5H8, cC6H10, cC7H10 (norbornene)) was investigated. For 1-ole, substitution was dissociative (∆Gǂdiss), and in combination with calculated olefin binding free energies (∆G˚bind), activation free energies for olefin association (∆Gǂassoc) to (silox)3Nb (1) were estimated. For 2-ole, substitution was not observed prior to rearrangement to alkylidenes. Instead, activation free energies for olefin association to (silox)3Ta (2) were measured, and when combined with ∆G˚bind(calcd), estimates of olefin dissociation rates from 2-ole were obtained. Despite stronger binding energies for 1-ole vs 2-ole, the dissociation of olefins from 1-ole is much faster than that from 2-ole. The association of olefins to 1 is also much faster than that to 2. Linear free energy relationships (with respect to ∆G˚bind) characterize olefin dissociation from 1-ole, but not olefin dissociation from 2-ole, and olefin association to 2, but not olefin association to 1. Calculated transition states for olefin dissocation from (HO)3M(C2H4) (M = Nb, 1´-C2H4; Ta, 2´-C2H4) are asymmetric and have orbitals consistent with either singlet or triplet states. The rearrangement of (silox)3Nb(trans-Vy,Ph-cPr) to (silox)3Nb=CHCH=CHCH2CH2Ph (3) is consistent with a diradical intermediate akin to the transtion state for substitution. The disparity between Nb and Ta in olefin substitution chemistry is rationalized on the basis of a greater density of states (DOS) for the products (i.e., (silox)3M + ole) where M = Nb, leading to intersystem crossing events that facilitate dissociation. At the crux of the DOS difference is the greater 5dz2/6s mixing for Ta vs the 4dz2/5s mixing of Nb. This rationalization is generalized to explain the nominally swifter reactivities of 4d vs 5d elements.

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  • Journal of the American Chemical Society, 2008, Washington DC: American Chemical Society, pp. 1183-1196

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  • Publication Title: Journal of the American Chemical Society
  • Volume: 130
  • Page Start: 1183
  • Page End: 1196
  • Pages: 14
  • Peer Reviewed: Yes

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  • January 8, 2008

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  • March 2, 2012, 11:47 a.m.

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  • July 22, 2013, 2:42 p.m.

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Hirsekorn, Kurt F.; Hulley, Elliott B.; Wolczanski, Peter T. & Cundari, Thomas R., 1964-. Olefin Substitution in (silox)3M(olefin) (silox = tBu3SiO; M = Nb, Ta): The Role of Density of States in Second vs Third Row Transition Metal Reactivity, article, January 8, 2008; [Washington, DC]. (digital.library.unt.edu/ark:/67531/metadc77142/: accessed August 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.