Ligand variation in molybdenum imido alkylidene complexes : a synthetic, structural, and catalytic study
Author(s)Lichtscheidl, Alejandro G. (Alejandro Gaston)
Massachusetts Institute of Technology. Dept. of Chemistry.
Richard R. Schrock.
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Chapter 1. A general introduction is given. Chapter 2. The biscarboxylate species, Mo(NR)(CHCMe 2Ph)(O 2CPh3)2 (R = 2,6-i-Pr2C6H3, 2,6- Me2C6H3, 2-t-BuC 6H4, or 1 -adamantyl) are compared to newly synthesized bis(terphenylcarboxylate) species, Mo(NR)(CHCMe 2Ph)(O 2CTer)2 (Ter = 2,6-diphenyl-4- methylphenyl or 2,6-diphenyl-4-methoxyphenyl). Preparation of bis(terphenylcarboxylate) species was accomplished through protonolysis of Mo(NR)(CHCMe2R')(Me2Pyr)2 with two equivalents of TerCO2H and one of them was characterized through X-ray crystallography. Photolysis experiments of many of the biscarboxylate complexes led to rate constants for the converstion of anti to syn species, which are much slower than bisalkoxide species. Trimethylphosphine adducts of selected triphenylacetate complexes have been isolated and studied in solution. Protonolysis of Mo(NAr)(CHCMe 2R')(Me 2Pyr)2 (Ar = 2,6-i-Pr 2C6H3) with one equivalent of TerCO2H led to the isolation of a handful of monocarboxylate species, Mo(NAr)(CHCMe 2Ph)(O 2CAr')(Me2Pyr). An X-ray structure of one of them was also characterized. Several of the bis(triphenylacetate) complexes and all of the monocarboxylates are active initiators for the regioselective polymerization of diethyl dipropargylmalonate (DEPDM). In the case of the latter compounds, activity towards olefins is also observed and briefly mentioned.(cont.) Chapter 3. Monaryloxide pyrrolide (MAP) molybdenum imido alkylidene complexes of the type Mo(NArx)(CHCMe 2R)(Me2Pyr)(OR') (Me2Pyr = 2,5-dimethylpyrrolide) have been prepared in which NArx is an ortho-substituted phenylimido group (X = Cl (NArci), CF3 (NAr ), i-Pr (NAr"), t-Bu (NArtBu), mesityl (NArM), or TRIP (TRIP = triisopropylphenyl; NArT)) and OR' = O-2,3,5,6-(C 6H5)4C6H (OTPP), O-2,6-(2,4,6-Me 3C6H2)2C6H3 (OHMT), or O-2,6-(2,4,6-i- Pr 3C6H2 )2 C6 H3 (OHIPT). The object was to explore to what extent relatively "large" NArM or NArT ligands would alter the performance of MAP catalysts in reactions that have been proposed to depend upon the relative size of the imido and OR' groups. Preliminary studies employing the ring-opening metathesis polymerization of 5,6-dicarbomethoxynorbornadiene as a measure of selectivity suggest that a single ortho-substituent in the phenylimido group, even in an NArm or NArT group, does not produce any unique behavior and that the outcome of the ROMP reaction correlates with the overall relative size of the imido and OR' group. Single crystal X-ray structures of six species that contain the new NArM or NArT groups are reported. Chapter 4. Several bipyridine adducts of molybdenum imido alkylidene bispyrrolide complexes of the type Mo(NR)(CHCMe 2Ph)(Pyr)2(bipy) (9-15; R = 2,6-i-Pr2C6H3 (Ar), adamantyl (Ad), 2,6- Me2 C6H3 (Ar'), 2-ClC 6H4 (ArcI), 2-i-PrC6-14 (AriPr), 2-t-BuC 6H4 (ArtBu), 2-MesitylC 6H4 (Arm), respectively; have been prepared using three different methods. Up to three isomers of the adducts are observed that are proposed to be the trans and two possible cis pyrrolide isomers of syn alkylidenes. Sonication of a mixture containing 9-15, HMTOH (2,6-(2',4',6'- Me3C6H2 )2 C6H30H), and ZnCl2(dioxane) led to the formation of MAP species of the type Mo(NR)(CHCMe 2Ph)(Pyr)(OHMT) (16-22), which were isolated by filtration and purified by recrystallization. DCMNBD (2,3-dicarbomethoxynorbornadiene) is polymerized employing 16- 22 as initiators to yield >98% cis,syndiotactic poly(DCMNBD). Attempts to prepare bipy adducts of bisdimethylpyrrolide complexes led to the formation of imido alkylidyne complexes of the type Mo(NR)(CHCMe 2R')(Me2Pyr)(bipy) (Me2Pyr = 2,5-dimethylpyrrolide); 23-28) through a ligand-induced migration of an alkylidene a proton to a dimethylpyrrolide ligand. Xray structures of Mo(NAr)(CHCMe 2Ph)(Pyr) 2(bipy) (9), Mo(NAriP)(CHCMe 2Ph)(Pyr)(OHMT) (20), Mo(NAr)(CCMe 2Ph)(Me2Pyr)(bipy) (23), Mo(NAr')(CCMe 2Ph)(Me2Pyr)(bipy) (24), Mo(NArT)(CCMe 3)(Me 2Pyr)(bipy) (ArT = 2-(2',4',6'-i-Pr 3C6H2)C6H4, 28) showed structures with the expected characteristic bond lengths and angles.(cont.) Chapter 5. The formation of Mo(NAr)(CHCMe2Ph)(OC(CF 3)2(CH 3))(O2CTerme) (Ar = 2,6-i-Pr2C6H3, O2CTerMe= 4,4'-dimethylterphenylcarboxylate, 1) from Mo(NR)(CHCMe2Ph)(OC(CF3)2(CH 3))2 via protonolys with TerMeCO2H, as well as formation of complexes of the general formula Mo(NR)(CHCMe2Ph)(OC(CF3)2(CH 3))(X) (R = 2,6-iPr2C6H3 (Ar), 2; 2,6-Me2C6H3 (Ar'), 3; 2- iPrC6H4 (Ar' ), 4; 1-adamantyl (Ad), 5; X = 2,2',4,4',6,6'-hexamethylterphenoxide (HMTO)) (2,6-Me2C6H3 (Ar'), 6, 2-iPrC6H4 (Ar'P'), 7; X = 2,2',4,4',6,6'-hexamethylterphenylamine (HMT(H)N)) (R = 1-adamantyl, X = 2,2',4,4',6,6'-hexamethylterphenyl (HMT), 8) via saltmetathesis of Mo(NR)(CHCMe 2Ph)(OC(CF 3)2(CH 3))2 with one equivalent of LiX is presented as an efficient synthetic route for the preparation of monoalkoxide monoanionic (MAX) complexes. The X-ray studies of 5, 7, and 8 show that complexes 5 and 8 have similar structures; whereas 7 differs considerably due to the overall spacial arrangement of the N(H)HMT in the complex, which is orthogonal to the arrangement of OHMT in 5 or HMT in 8. The distance of the terphenyl backbone is shown to be one Angstrom shorter for 8, in comparison to 5 and 7, which results in failed attempts to prepare Mo(NR)(CHCMe 2Ph)(OC(CF 3)2(CH 3))(HMT) complexes with bigger R groups. The catalytic reactivity complexes 1-8 is probed with diallyl ether (DAE), 1 -hexene, 1 -octene, and dicarbomethoxynorbornadiene (DCMNBD). For comparison, the same olefin-metathesis reactions are carried-out with the analogous monoanionic monopyrrolide (MXP) complexes of general formula Mo(NR)(CHCMe 2Ph)(Pyr)(X) (Pyr = C4H4N, 2,5- Me2C4H2N). Overall, the reactivity of MAX and MXP complexes differs least when cis or trans bond formation is involved, and differs most when tacticity is considered, showing that during catalysis the transition states may differ significantly between the two kinds of complexes. Appendix The preparation of pure (R)-Mo(NAr)(CHCMe 2Ph)(Me2Pyr)(OR*) (Ar = 2,6-iPr2C6H3, R*O (1S,2S,4R)-2-(4-ClC 6H4)-1,7,7-Me 3-bicyclo[2.2.1]heptan-2-oxide), Me2Pyr = 2,5-Me2C4H2N, 1) and Mo(NAd)(CHCMe 2Ph)(OR*)2 (Ad = 1-adamantyl, R*O = 3,3'-Br2-2'-(OSi(Me2)2(t-Bu))- 5,5',6,6',7,7',8,8'-H-[1,1'-binaphthalen]-2-oxide, 21) are discussed. X-ray studies of both complexes are also presented. The polymerization of diethyl dipropargylmalonate (DEPDM) is probed with a variety of monoalkoxidemonopyrrolide (MAP) complexes, including 1, that either contain PMe3 bound to the metal center or not. In addition, a variety of bisalkoxide/bisphenoxide complexes, among them 21, are also used to polymerize DEPDM.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2012.Vita. Cataloged from PDF version of thesis.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Dept. of Chemistry.
Massachusetts Institute of Technology