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Title: PCP pincer compounds: synthesis and application as methanol carbonylation catalysts
Author: Reynolds, Timothy James
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2011
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A range of 1,8-bis(PR2)anthracene - " Anthraphos " - compounds have been synthesised. Previously reported (where PR2 = -PPh2 (2.1) or -Pi-Pr2 (2.5)), and new (where PR2 = -PCYP2 (2.18), -PCyh2 (2.19) -Po-Tol2 (2.20) or -Po-Ans2 (2.21)) anthraphos structures have been prepared in 3 steps starting from 1,8-dichloroanthraquinone (2.7). Compound (2.7) was converted by chlorine- bromine substitution followed by reduction with Al(OCyhh into 1,8-dibromoanthracene (2.4), from which (2.1), (2.5) and (2.18)-(2.21) were prepared by double lithiation of (2.4) with n-BuLi and subsequent reaction with the corresponding ClPR2 reagent. 1,8-bis(di-tert-butylphosphino)anthracene (2.6) was prepared according to the literature procedure in 3 steps starting from 1,8-dichloroanthraquinone (2.7). On reaction of (2.5), (2.18) or (2.19) with [NiCl2(dme)l or [PtCl2(t-BuCNhl in the presence of i-Pr2NEt, cyclometallation of the C-H bond in the anthracene C(10)-position led to the new square-planar nickel and platinum pincer complexes (3.1)-(3.3) and (3.7)( 3.9), respectively. Reaction of (2.5), (2.18) or (2.19) with [PdCl2(CH3CNhl afforded the analogous new palladium pincer complexes (3.4)-(3.6) in the absence of i-Pr2NEt. On reaction of (2.5), (2.6), (2.18) or (2.19) with 0.5 eq. of [RhCl(COEhb cyclometallation of the C-H bond at the anthracene C(lO) position gave the corresponding new 5- coordinate Rh(llI) pincer complexes (3.10)-(3.13). Exposing solutions of (3.10)-(3.12) to CO gas led to coordination of CO, affording the 6-coordinate pseudooctahedral Rh (Ill) pincer complexes (3.14)-(3.16). Complexes (3.14)-(3.16) were also prepared by reaction of the diphosphines (2.5), (2.18) or (2.19) with 0.5 eq. of [RhCl(COhb at room temperature. Exposing a C6H6 solution of (2.6) to CO afforded the Rh(I) square-planar pincer complex (3.20) with elimination of HCl. Square-planar Rh (I) carbonyl pincer complexes (3.17)-(3.19) were prepared by dehydrochlorination of (3.14)-(3.16) with i-Pr2NEt at RT. The analogous complexes of diphosphines (2.20) and (2.21) were prepared by reaction of the diphosphines with 0.5 equivalents of [RhCl(CO)2b in the presence of i-Pr2NEt. 5-coordinate Ir(Ill) pincer complexes (3.25) and (3.26) were prepared under similar conditions to those in the literature, and the new complex (3.27) by reaction of (2.19) with 0.5 eq. of [IrCl(COEhb. Rh(I) complexes (3.17)-(3.19), (3.21) and (3.22) oxidatively add MeI at room temperature and subsequently undergo migratory insertion to give the corresponding 5-coordinate Rh(IH) acetyl species. Complex (3.20), with bulky tBu2P-substituents, was unreactive towards MeI under the same conditions. Kinetic studies with (3.17)-(3.19) and (3.21) and (3.22) showed that the second-order rate constants (kl ) for MeI oxidative addition varied by a factor of >850, depending on the structure of the diphosphine. Complex (3.22), with o-anisyl P-substituents, showed a second-order rate constant of 90.2 M-I S-1 (CH2Cl2, 23 CC) compared to 0.106-0.408 M-I S-1 for (3.17)-(3.19) and (3.21). The rate enhancement is . proposed to be due to neighbouring group participation of the methoxy groups. Rh(I) complexes (3.17)-(3.19) and (3.22) were used as pre-catalysts for HI-promoted methanol carbonylation under industrially relevant conditions, using CO and CO /H2 (95:5) as feed gasses. 31p{IH} NMR spectra of the exit solutions suggested the phosphines remained largely bound to the metal centres at the end of the reactions. Using pincer compounds as pre-catalysts, carbonylation rates over twice that of the Monsanto catalyst ([Rh(COhI2t) were observed using CO feed gas, and approaching 3 times using CO/H2. A mechanism for the formation of acetic acid and side-products from MeOH and CO catalysed by pincer compounds is proposed .
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available