Organometallic Group 4 Bis ( Borylamide ) Complexes as Templates for Ziegler - Natta Catalysis

Chapter 1 Motivated by the isolobal and isoelectronic relationship between the do CpW(CR) fragment and the do bent metallocene core, the synthesis and reactivity of alkyl complexes of the type CpW(CAd)R 2 (Ad = 1-adamantyl; R = alkyl, Cl, NMe 2 ) are presented. AdCN reacts with W2 (OCMe3 )6 to provide W(CAd)(OCMe 3)3, from which CpW(CAd)C12 may be prepared in three steps. Alkylation of CpW(CAd)Cl 2 with Grignard reagents at -30 'C provides CpW(CAd)R 2 (R = CH2 Ph, CH 3, CH2CMe 3) and CpW(CAd)(CH 2CMe 3)C1 in 70-80 % isolated yield. In contrast to the benzyl and methyl complexes, the monoand dineopentyl derivatives partially tautomerize in benzene-d6 by migration of the alkyl ax-H atoms to the alkylidyne a-C atom forming equilibrium mixtures of CpW(CCMe 3)(CH 2Ad)Cl and CpW(CCMe 3 )(CH 2Ad)(CH 2 CMe 3), respectively. Each neopentyl and 1-adamantylmethyl complex displays 1H and 13C NMR parameters indicative of a-agostic alkyl C-H bonds. An X-ray study of CpW(CAd)(CH 2 CMe 3)2 reveals a bent metallocene-like structure in which the orientation of the neopentyl groups places one set of C-H bonds in between the two neopentyl ligands and just out of the Calkyl-W-Calkyl plane. The amido lone pair in complexes CpW(CAd)(NMe 2)X (X = Cl, CH2 CMe 3) (prepared in one and two steps from CpW(CAd)C12) strongly interacts with a metal acceptor orbital as demonstrated by a high barrier to rotation about the W-NMe 2 bond. CpW(CAd)(NMe 2 )(CH 2CMe 3) shows no evidence of o-agostic interactions and does not tautomerize by alkyl to alkylidyne ax-H migration. The primary amide CpW(CAd)(NHCMe 3)Cl irreversibly undergoes ox-N-H migration to the alkylidyne a-C atom, forming a kinetic isomer of the alkylidene CpW(NCMe 3)(CHCAd)Cl which slowly converts to its opposite, thermodynamic rotamer. The factors promoting ac-H transfer in the CpW(CAd)R 2 (R = alkyl, chloride) system are discussed in relation to o-H abstraction reactions in dx chemistry. Chapter 2 The synthesis of do borylamido and borylimido complexes is presented along with an investigation into the electronic nature of the M-N-B linkage. MC14 (M = Zr, Hf) react with 2 eq. [Li(OEt 2)NHBMes 2]2 to provide the homoleptic M(NHBMes 2)4 whereas TiC14 reacts with 3/2 eq. [Li(OEt 2)NHBMes 2 ]2 to afford Ti(NHBMes 2)3C1. Variable temperature 1H NMR studies show that these do borylamides possess barriers to N-B bond rotation (AGtrot = 15 16 kcal/mol) significantly lower than in related borylamines. a-Abstraction reactions lead to borylimido complexes. Heating M(NHBMes 2 )4 (M = Zr, Hf) in pyridine at 85 'C produces M(NBMes2)(NHBMes 2 )2py 2 and 1 eq. H2 NBMes 2 . The addition of 5/2 eq. [(Et 20)LiNHBMes 2 ]2 to TaC15 in toluene similarly results in H2NBMes 2 formation, yielding Ta(NBMes 2 )(NHBMes 2)3. Addressing the origin of the reduced barriers to N-B rotation in the dý borylamides, Sn(NHBMes 2)3 C1 is prepared from SnC14 and 3/2 eq. [(Et 20)LiNHBMes 2]2. In contrast to the analogous titanium complex, the barrier to N-B bond rotation for the tin tris(borylamide) is greater than 22 kcal/mol, suggesting that the reduced values of AG rot in the do complexes result from competition between the do metal and boron for acceptance of the nitrogen lone pair. Chapter 3 Titanium and zirconium derivatives of the new chelating bis(borylamido) ligand, [Mes 2 BNCH2CH 2NBMes 2] ([Ben] 2-), are prepared by treating MC14 (THF)2 (M = Ti, Zr) with (Ben)Mg(THF) 2 . Nitrogen-boron x-interactions in (Ben)TiC12 and (Ben)ZrC12 (THF) result in one mesityl group in each BMes2 unit occupying space roughly above and below the MC12 plane. (Ben)TiC12 is smoothly alkylated by Grignard reagents in dichloromethane to give (Ben)Ti(R)C1 (R = CH 2Ph, CH 2CMe 3) and (Ben)TiR 2 (R = Me, CH 2Ph), while unstable (Ben)ZrMe2 can be prepared from (Ben)ZrCl 2 (THF) and methyllithium in toluene. An X-ray study of (Ben)Ti(CH2Ph)Cl confirms the proposed ligand conformation and features a highly distorted "r12" benzyl ligand with Ti-Cla-Cipso angle of only 87.0 (5)o. (Ben)MMe 2 complexes cleanly decompose by metallation of the ortho methyl groups from mesityl rings on different boron atoms at room temperature (for Zr) or upon heating (for Ti). An X-ray crystal structure of (TwistBen)Zr shows it to be a dimer in which the two zirconium centers are bridged by two mesityl o-methylene groups. B(C 6F5)3 binds to a methyl group in (Ben)MMe 2 complexes in dichloromethane, but such compounds show little polymerization activity towards ethylene at 250 C and 1-2 atm as a consequence of strong anion binding. Chapter 4 The effect of ligand sterics on ion pairing and reactivity with olefins in cationic bis(borylamide) alkyl complexes is investigated utilizing the sterically demanding [Trip 2 BNCH 2 CH 2 NBTrip2] 2([BigBen] 2 -) (Trip = 2,4,6 triisopropylphenyl) ligand. (BigBen)ZrC12 is obtained from the reaction of Li2(BigBen)o4THF with ZrC14 (THF2)2 in toluene. Reaction of (BigBen)ZrC12 with Grignard reagents provides (BigBen)ZrMe 2 as well as the 3-H containing (BigBen)ZrR2 (R = Et, Bu), all which possess reasonable thermal stability at room temperature. The X-ray structure of (BigBen)ZrMe2 reveals a sterically congested coordination wedge in which the methyl groups are laterally shielded by the ligand o-isopropyl groups. Reaction of (BigBen)ZrMe2 with B(C6F 5 ) 3 in pentane allows the isolation of [(BigBen)ZrMe][MeB(C 6 F5)3] whereas (BigBen)ZrR2 (R = Me, Et, Bu) react with [HNMe 2 Ph][B(C 6 F 5 )4] in toluene-d8 to give the spectroscopically characterized [(BigBen)ZrR][B(C 6 F5)4 ] which do not bind NMe 2Ph. The thermal stability of these cations allows upper limits on the barriers to anion dissociation/reassociation (15.5 17.0 kcal/mol) to be placed by variable temperature 1H NMR spectroscopy. Whereas [(BigBen)ZrMe][B(C 6F5)4] does not react with C2H4 , CO, or H2 at 1-3 atm, [(BigBen)Zr(Me)(NH 3)][B(C 6F5 )4 ] may be prepared with 1 eq. NH 3(g). The stability of these alkyl cations towards C2H 4 and CO is discussed in terms extensive shielding of the coordination wedge due to the steric demands of the ligand. Chapter 5 Zirconium and hafnium derivatives of the less sterically demanding bis(borylamido) ligand [Cy 2BNCH 2 CH 2NBCy 2] ([CyBen] 2-) are prepared by treating in situ prepared solutions of M(CH 2 SiMe 3 )2 C12 (OEt 2 )2 (M = Zr, Hf) in ether with Li2 (CyBen)*OEt 2 . Reaction of (CyBen)M(CH 2 SiMe 3 )2 with 2 eq. 12 in dichloromethane provides the synthetically versatile (CyBen)MI2. (CyBen)MI 2 undergoes smooth reactions with Grignard reagents in dichloromethane to afford the f-H containing primary dialkyls (CyBen)MR 2 (R = Et, 'Bu, nhexyl; M = Zr, Hf) and secondary dialkyl (CyBen)Zr(CHBu 2 )2 , all of which possess a high degree of thermal stability. The X-ray structure of (CyBen)Zr(CH 2CH 3)2 reveals a symmetric, laterally open structure in which the ethyl groups lie sandwiched between two cyclohexyl groups flanking the coordination wedge. Reaction of the dialkyls (CyBen)ZrR2 (R = Et, iBu) with [Ph3 C][B(C 6F5)4 ] in toluene-d8 produces the cations [(CyBen)M(R)][B(C 6F5)4] which display sharp AA'BB' backbone 1H NMR resonances, indicating the presence of a relatively non-labile ligand adjacent to the Zr-alkyl group. Toluene binds to the cations [(CyBen)M(R)][B(C 6F5 )4 ] (R = Et, iBu, n-hexyl; M = Zr, Hf) similarly prepared in chlorobenzene-d5 at -30 'C, which show broad backbone resonances at low temperature whose coalescence temperature (0 70 'C) strongly depends on the amount of added toluene. Chlorobenzene solutions of [(CyBen)M(R)][B(C 6F5)4 1 (R = Et with M = Zr, Hf; R = Bu' with M = Zr) polymerize 225 eq. 1-hexene at -30 'C to nearly complete conversion producing a regioregular polymer with a substantial degree of isotacticity, but which is of high molecular weight and polydispersity (Mn = 0.4 1.3 x 105 g/mol, PDI = 5 6). In dichloromethane 225 eq. 1-hexene is polymerized to 50 76% conversion at -30 0 'C by (CyBen)Zr(CH 2CHMe 2)2 activated with [Ph3C][B(C 6F 5)4] in the presence of 5 15 eq. toluene to give polymers of lower molecular weight (Mn = 1.5 2.4 x 104 g/mol) and much lower polydispersity (PDI = 1.08 1.53), but these polymers are highly regioirregular. The polymerization results are discussed in terms of a "secondary" activation of the CyBen based alkyl cations, producing the active catalyst(s) responsible for the 1-hexene polymerization activity observed. Thesis Supervisor: Dr. Richard R. Schrock Title: Frederick G. Keyes Professor of Chemistry