Olefin Substitution in (silox)3M(olefin) (silox = tBu3SiO; M = Nb, Ta): The Role of Density of States in Second vs Third Row Transition Metal Reactiv
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- 作者：Kurt F. Hirsekorn ; Elliott B. Hulley ; Peter T. Wolczanski ; Thomas R. Cundari
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：January 30, 2008
- 年：2008
- 卷：130
- 期：4
- 页码：1183 - 1196
- 全文大小：528K
- 年卷期：v.130,no.4(January 30, 2008)
- ISSN：1520-5126
文摘
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 (
G
diss), and in combinationwith calculated olefin binding free energies (G
bind), activation free energies for olefin association (Gassoc)to (silox)3Nb (1) were estimated. For 2-ole, substitution was not observed prior to rearrangement toalkylidenes. Instead, activation free energies for olefin association to (silox)3Ta (2) were measured, andwhen combined with Gbind (calcd), estimates of olefin dissociation rates from 2-ole were obtained. Despitestronger binding energies for 1-ole vs 2-ole, the dissociation of olefins from 1-ole is much faster than thatfrom 2-ole. The association of olefins to 1 is also much faster than that to 2. Linear free energy relationships(with respect to Gbind) 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 dissociationfrom (HO)3M(C2H4) (M = Nb, 1'-C2H4; Ta, 2'-C2H4) are asymmetric and have orbitals consistent with eithersinglet or triplet states. The rearrangement of (silox)3Nb(trans-Vy,Ph-cPr) (1-VyPhcPr) to (silox)3Nb=CHCH=CHCH2CH2Ph (3) is consistent with a diradical intermediate akin to the transtion state for substitution. Thedisparity between Nb and Ta in olefin substitution chemistry is rationalized on the basis of a greater densityof states (DOS) for the products (i.e., (silox)3M + ole) where M = Nb, leading to intersystem crossingevents that facilitate dissociation. At the crux of the DOS difference is the greater 5dz2/6s mixing for Ta vsthe 4dz2/5s mixing of Nb. This rationalization is generalized to explain the nominally swifter reactivities of4d vs 5d elements.
Gdiss), and in combinationwith calculated olefin binding free energies (Gbind), activation free energies for olefin association (Gassoc)to (silox)3Nb (1) were estimated. For 2-ole, substitution was not observed prior to rearrangement toalkylidenes. Instead, activation free energies for olefin association to (silox)3Ta (2) were measured, andwhen combined with Gbind (calcd), estimates of olefin dissociation rates from 2-ole were obtained. Despitestronger binding energies for 1-ole vs 2-ole, the dissociation of olefins from 1-ole is much faster than thatfrom 2-ole. The association of olefins to 1 is also much faster than that to 2. Linear free energy relationships(with respect to Gbind) 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 dissociationfrom (HO)3M(C2H4) (M = Nb, 1'-C2H4; Ta, 2'-C2H4) are asymmetric and have orbitals consistent with eithersinglet or triplet states. The rearrangement of (silox)3Nb(trans-Vy,Ph-cPr) (1-VyPhcPr) to (silox)3Nb=CHCH=CHCH2CH2Ph (3) is consistent with a diradical intermediate akin to the transtion state for substitution. Thedisparity between Nb and Ta in olefin substitution chemistry is rationalized on the basis of a greater densityof states (DOS) for the products (i.e., (silox)3M + ole) where M = Nb, leading to intersystem crossingevents that facilitate dissociation. At the crux of the DOS difference is the greater 5dz2/6s mixing for Ta vsthe 4dz2/5s mixing of Nb. This rationalization is generalized to explain the nominally swifter reactivities of4d vs 5d elements.