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New ligands : design and coordination chemistry with the transition metals and lanthanides

University of British Columbia

This thesis deals with three different topics: (i) The synthesis and characterization of cyclopentadienyldiphosphine complexes of zirconium(III), niobium(III), and molybdenum(III). (ii) The synthesis and characterization of the π-arene complexes of lanthanides. (iii) Coordination chemistry and catalytic activity of mixed donor ligands based on sp²-hybridized nitrogen donors. The first topic is discussed in Chapter 1 and illustrates the organometallic chemistry of the trivalent second row transition metals, namely Zr, Nb, and Mo with the cyclopentadienyldiphosphine ligand {η⁵-C₅H₃-1,3-(SiMe₂CH₂PR₂)₂}-abbreviated as [RP₂Cp] where R = i-Pr and Ph. The Zr(IV) complex [i-PrP₂Cp]ZrCl₃ undergoes reduction with Na/Hg to form the trivalent Zr derivative, [i-PrP₂Cp]ZrCl₂. This compound is characterized by EPR spectroscopy. [i-PrP₂Cp]ZrCl₂ takes part in a metathetical reaction with MeMgBr to yield the monomethyl derivative [i-PrP₂Cp]ZrMeCl which is also characterized by EPR spectroscopy. The reaction of the dichloro complex [i-prP₂Cp]ZrCl₂ with excess carbon monoxide results in the disproportionation of the Zr(III) complex to a Zr(IV) complex ([i-prP₂Cp]ZrCl₃) and a Zr(II) compound ([i-prP₂Cp]ZrCl(CO)₂). This reaction is reversible and upon removal of CO the starting material [i-PrP₂Cp]ZrCl₂ is formed. The niobium(III) starting material (NbCl₃(DME)) reacts with [RP₂Cp]Li to form [RP₂Cp]NbCl₂. These complexes are low spin and diamagnetic. [RP₂Cp]NbCl₂ complexes react with excess CO to form the CO adducts whose solid state structures have been determined. The Nb(IV) derivatives [RP₂Cp]NbCl₃ are formed via the reaction of [RP₂Cp]NbCl₂ with PbCl₂ or with PhNCO. These complexes can also be produced when Nb(O)Cl₃(THF)₂ is allowed to react with [RP₂Cp]Li. These Nb(IV) derivatives are EPR active and their solid state molecular structures show distorted octahedral geometries around the Nb center. When MoCl₃(THF)₃ reacts with [RP₂Cp]Li the corresponding Mo(III) complexes of the [RP₂Cp] ligand, [RP₂Cp]MoCl₂ are formed. These compounds are low-spin, paramagnetic complexes as evidenced by their EPR spectra. Their solid state molecular structures show that they are four-legged piano-stool complexes. The second topic is discussed in Chapter 3 and deals with the formation of the π-arene complexes of the lanthanides incorporating the diamidodiphosphine macrocycle [PhP(CH₂SiMe₃NSiMe₃CH₂)PPh]⁻² ([P₂N₂]) as the ancillary ligand. LnCl₃(THF)₃ reacts with Li₂[P₂N₂] to form dimeric complexes {[P₂N₂]Ln}₂(μ-Cl)₂ Ln = Y, Yb, and Lu. It was shown before that aryl lithiums such as phenyl lithium, m-tolyl lithium, and p-tolyl lithium react with {[P₂N₂]Y}₂((μ-Cl)₂ to form dinuclear complexes bridged by a biaryl unit (biphenyl, bi-m-tolyl and bi-p-tolyl, respectively) formed via C-C bond formation between two aryl moieties. The Y[P₂N₂] fragments are π-coordinated to the opposite sides of this bridging biaryl unit. In this work, it is shown that when {[P₂N₂]Y}₂(μ-Cl)₂ reacts with biphenyl lithium, a dinuclear complex is formed that is bridged by a bisbiphenyl unit. The solid state molecular structure of this compound reveals that the [P₂N₂]Y fragments are π-coordinated to the two middle rings of the bisbiphenyl moiety on the opposite faces. There is an inconsistency between the solid state and solution structures of this complex that shows a fluxional process exists in which the two [P₂N₂]Y units traverse along all four of the rings. {[P₂N₂]Ln}₂(μ-Cl)₂ complexes (Ln = Y, Lu) also react with various polycyclic aromatic hydrocarbons (naphthalene, substituted naphthalene, anthracene, tetracene, and pyrene) under reducing conditions to form dinuclear complexes in which two [P₂N₂]Ln moieties are η⁴ coordinated on the opposite sides of the different rings of the polycyclic aromatic units. An examination of the solid state molecular structure and the behavior of these compounds in solution shows that they are fluxional in solution. The fluxionality arises from the very fast migration of the [P₂N₂]Ln units along the rings. The third topic is discussed in Chapter 5. Mixed pyridine-oxazoline bidentate ligands 2-(2- oxazolin-2-ylmethyl)pyridine (pymox) are synthesized by the condensation of the appropriate aminoalcohol with 2-pyridylacetonitrile. When 1-methyl-2-amino-1-propanol (R = R' = Me) is used the product is an achiral ligand, Me-pymox. Chirality is introduced at the carbon center next to the nitrogen donor on the oxazoline ring by using chiral aminoalcohols such as valinol (R = H, R' = i-Pr), to yield i-Pr-pymox, and leucinol (R = H, R' = s-Bu), to form s-Bu-pymox. These ligands react with Rh(I) starting material [Rh(COD)(THF)₂]PF₆ to form [Rh(COD)(pymox)]PF₆ complexes. [Rh(COD)(Me-pymox)]PF₆ undergoes a fluxional process in the solution which is due to the conformational flipping or inversion of the six-membered chelate ring. The solid state structure of [Rh(COD)(i-Pr-pymox)]PF₆ is preserved in solution and the molecule does not show fluxionality. It can be seen from the X-ray solid state structure of [Rh(COD)(i-Pr-pymox)]PF₆ that the isopropyl substituent on the oxazoline ring is oriented perpendicular to the square plane of rhodium moiety. These chiral complexes have been used as catalyst precursors in asymmetric hydrosilylation of ketones. They show moderate enantioselectivity. The reason is explained on the basis of the solid state structure of chiral Rh(pymox) complexes in which the substituent is oriented away from the rhodium center and the site of catalytic reaction. The coordination chemistry of pymox ligands with Pd(II) are also examined. Pd(C₆H₅CN)₂Cl₂ reacts with Me-pymox to form Pd(Me-pymox)Cl₂. The coordination geometry around the palladium center is distorted square planar. In solution it displays a highly symmetric and simple ¹H NMR spectrum which suggests that a similar fluxional behavior to the one described for [Rh(COD)(Me-pymox)]PF₆ must be operational. [Pd(methallyl)Cl]₂ reacts with the pymox ligands to form [Pd(methallyl)(pymox)]PF₆. The solid state molecular structures of these complexes have been determined and show that the coordination geometry around the palladium center is slightly distorted square planar. As in [Rh(COD)(i-Pr-pymox)]PF₆, the isopropyl substituent on the oxazoline ring is oriented perpendicular to the square plane of palladium moiety. [Pd(methallyl)(Me-pymox)]PF₆ shows a fast selective synlanti ecxhange for the terminal methylene groups of the methallyl moiety In the ¹H NMR spectrum. The ¹H NMR spectrum of [Pd(methallyl)(i-Pr-pymox)]PF₆ shows that two diastereomers coexist in the solution but they don't undergo a fast syn/anti exchange on the NMR time scale. The catalytic activity of the chiral [Pd(methallyl)(pymox)]PF₆ complexes in asymmetric allylic alkylation reactions is studied and it is shown that they result in low enantioselectivities. This can be explained on the basis of the solid state structure; the chiral induction will be very low since the chiral group is far away from the reaction center.

Thesis/Dissertation

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2009-06-30

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http://hdl.handle.net/2429/9887

Chemistry

University of British Columbia

UBCV

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