[ { "id": "authors:fc96x-jr256", "collection": "authors", "collection_id": "fc96x-jr256", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170414-073636496", "type": "article", "title": "The Electronic Influence of Ring Substituents and Ansa Bridges in Zirconocene Complexes as Probed by Infrared Spectroscopic, Electrochemical, and Computational Studies", "author": [ { "family_name": "Zachmanoglou", "given_name": "Cary E.", "clpid": "Zachmanoglou-C-E" }, { "family_name": "Docrat", "given_name": "Arefa", "clpid": "Docrat-A" }, { "family_name": "Bridgewater", "given_name": "Brian M.", "clpid": "Bridgewater-B-M" }, { "family_name": "Parkin", "given_name": "Gerard", "clpid": "Parkin-G" }, { "family_name": "Brandow", "given_name": "Christopher G.", "clpid": "Brandow-C-G" }, { "family_name": "Bercaw", "given_name": "John E.", "clpid": "Bercaw-J-E" }, { "family_name": "Jardine", "given_name": "Christian N.", "clpid": "Jardine-C-N" }, { "family_name": "Lyall", "given_name": "Mark", "clpid": "Lyall-M" }, { "family_name": "Green", "given_name": "Jennifer C.", "clpid": "Green-J-C" }, { "family_name": "Keister", "given_name": "Jerome B.", "clpid": "Keister-J-B" } ], "abstract": "The electronic influence of unbridged and ansa-bridged ring substituents on a zirconocene center has been studied by means of IR spectroscopic, electrochemical, and computational methods. With respect to IR spectroscopy, the average of the symmetric and asymmetric stretches (\u03bd_(CO(av))) of a large series of dicarbonyl complexes (Cp^R)_2Zr(CO)_2 has been used as a probe of the electronic influence of a cyclopentadienyl ring substituent. For unbridged substituents (Me, Et, Pri, But, SiMe_3), \u03bd_(CO(av)) on a per substituent basis correlates well with Hammett \u03c3_(meta) parameters, thereby indicating that the influence of these substituents is via a simple inductive effect. In contrast, the reduction potentials (E\u00b0) of the corresponding dichloride complexes (Cp^R)_2ZrCl_2 do not correlate well with Hammett \u03c3_(meta) parameters, thereby suggesting that factors other than the substituent inductive effect also influence E\u00b0. Ansa bridges with single-atom linkers, for example [Me_2C] and [Me_2Si], exert a net electron-withdrawing effect, but the effect is diminished upon increasing the length of the bridge. Indeed, with a linker comprising a three-carbon chain, the [CH_2CH_2CH_2] ansa bridge becomes electron-donating. In contrast to the electron-withdrawing effect observed for a single [Me_2Si] ansa bridge, a pair of vicinal [Me_2Si] ansa bridges exerts an electron-donating effect relative to that from the single bridge. DFT calculations demonstrate that the electron-withdrawing effect of the [Me_2C] and [Me_2Si] ansa-bridges is due to stabilization of the cyclopentadienyl ligand acceptor orbital, which subsequently enhances back-donation from the metal. The calculations also indicate that the electron-donating effect of two vicinal [Me_2Si] ansa bridges, relative to that of a single bridge, is a result of it enforcing a ligand conformation that reduces back-donation from the metal.", "doi": "10.1021/ja020236y", "issn": "0002-7863", "publisher": "American Chemical Society", "publication": "Journal of the American Chemical Society", "publication_date": "2002-08-14", "series_number": "32", "volume": "124", "issue": "32", "pages": "9525-9546" }, { "id": "authors:dazff-k8d64", "collection": "authors", "collection_id": "dazff-k8d64", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170808-082527490", "type": "article", "title": "Ancillary Ligand and Olefin Substituent Effects on Olefin Dissociation for Cationic Zirconocene Complexes Bearing a Coordinated Pendant Olefin", "author": [ { "family_name": "Brandow", "given_name": "Christopher G.", "clpid": "Brandow-C-G" }, { "family_name": "Mendiratta", "given_name": "Arjun", "clpid": "Mendiratta-A" }, { "family_name": "Bercaw", "given_name": "John E.", "clpid": "Bercaw-J-E" } ], "abstract": "A series of zirconocene complexes bearing 2,2-dimethyl-2-sila-4-pentenyl substituents (and methyl-substituted olefin variants) ((\u03b7^5-C_5H_5)_2Zr(CH_3)(CH_2SiMe_2CH_2CR^1\u2550CR^2R^3) (R^1, R^2, R^3 = H, CH_3, 1, 5\u22127), (\u03b7^5-C_5H_4CMe_3)_2Zr(CH_3)(CH_2SiMe_2CH_2CH\u2550CH_2) (2), {Me_2Si(\u03b7^5-C_5H_4)_2}Zr(CH_3)(CH_2SiMe_2CH_2CH\u2550CH_2) (3), and {1,2-(SiMe_2)_2(\u03b7^5-C_5H_3)_2Zr(CH_3)(CH_2SiMe_2CH_2CH\u2550CH_2) (4)) have been prepared. Methide abstraction with B(C_6F_5)_3 results in reversible coordination of the tethered olefin to the cationic zirconium center. The kinetics of olefin dissociation have been examined using NMR methods, and the effects of ligand variation for unlinked, singly [SiMe_2]-linked, and doubly [SiMe_2]-linked bis(cyclopentadienyl) arrangements have been compared (\u0394G^\u29e7 values for olefin dissociation vary from 11.4 to 15.6 kcal\u00b7mol^(-1) measured over the temperature range 223\u2212283 K). For the cation derived from 4 the kinetics for olefin dissociation and site epimerization (inversion at zirconium) can be distinguished. Additionally, with this ligand system competitive binding of the olefin and the [CH_3B(C_6F_5)_3] anion is observed. Methide abstraction from {1,2-(SiMe_2)_2(\u03b7^5-C_5H_3)_2}Zr(CH_3)(CH_2CMe_2CH_2CH\u2550CH_2) results in rapid \u03b2-allyl elimination with loss of isobutene to cleanly afford the allyl cation [{1,2-(SiMe_2)_2(\u03b7^5-C_5H_3)_2}Zr(\u03b7^3-CH_2CH\u2550CH_2)]^+.", "doi": "10.1021/om010363n", "issn": "0276-7333", "publisher": "American Chemical Society", "publication": "Organometallics", "publication_date": "2001-10-01", "series_number": "20", "volume": "20", "issue": "20", "pages": "4253-4261" } ]