Lanthanoid formamidinates and halogenoaluminate complexes of the rare earths and alkaline earths: synthesis and reactivity

Hossain, Md Elius (2017) Lanthanoid formamidinates and halogenoaluminate complexes of the rare earths and alkaline earths: synthesis and reactivity. PhD thesis, James Cook University.

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This thesis focuses on the synthesis and characterisation of halogenoaluminate π-arene complexes of rare earths and alkaline earths and compares the similarities between the two groups of metals. This thesis also discusses the reactivity of divalent rare earth N,N′- bis(aryl)formamidinate (ArForm) complexes.

Study of the synthesis of halogenoaluminate π-arene complexes of rare earths has yielded several new complexes containing η⁶-arene, [Ln(arene)(AlX₄)ₙ] (Ln = La, Ce, Pr, Nd, Gd, Sm, Eu, Yb; arene = toluene, mesitylene; X = Br, I; n = 2, 3). Divalent compounds of Sm, Eu and Yb have lattice solvated toluene molecules; however, the trivalent complexes are unsolvated. [Eu(η⁶-MeC₆H₅)(AlI₄)₂]ₙ.PhMe, [Yb(η⁶-MeC₆H₅)(AlI₄)₂]ₙ.1/2PhMe and [Eu(η⁶-MeC₆H₅)(AlBr₄)₂]ₙ.PhMe are the first examples of polymeric structures among these complexes. All of the complexes have nine coordinated metal centres; however, the ytterbium complex [Yb(η⁶-MeC₆H₅)(AlI₄)₂]ₙ.1/2PhMe has an eight coordinate Yb²⁺ centre, due to the lanthanoid contraction effect.

The iodoaluminate and bromoaluminate complexes of lanthanoids were found to be isostructural and comparable with the chloroaluminate complexes in the literature. The Ln- X (X = Br, I), Ln-centroid and the average Ln-C bond distances in the divalent complexes are longer than that of the trivalent complexes. This was expected, as the divalent ions are larger than trivalent ions. Furthermore, the Ln-I distances in the iodoaluminate complexes are longer than the Ln-Br distances in the bromoaluminate complexes. These differences are associated with the larger ionic radii of Ln²⁺ and I⁻ ions than the Ln³⁺ and Br⁻ ions, respectively.

The geometry of the complexes could be best described as distorted pentagonal bipyramidal, with the arene molecule at an axial position (the centroid-Ln-I/Br angles are close to the straight angle, 180o). The lanthanoid contraction was also evidenced by the gradual decrease of the Ln-X (X = Br, I), Ln-centroid and the average Ln-C bond distances in the trivalent complexes of lanthanoids (from lanthanum to gadolinium). This trend was also accessible among the divalent complexes from samarium to ytterbium, and there was an intense change in ytterbium as it is the smallest metal among the metals used for the synthesis here. The catalytic activity of [Nd(η⁶-C₆H₅Me)(AlI₄)₃] for isoprene polymerisation was performed at ambient temperature, and was less effective than the literature results of analogous chloroaluminate complexes.

Investigation of the iodoaluminate π-arene complexes of alkaline earths (Ae) has given several new complexes involving η⁶-arene, [Ae(arene)m(AlI₄)₂] (Ae = Ca, Sr, Ba; arene = toluene, mesitylene; m = 1, 2). The Ca-centroid and the average Ca-C distances in the mesitylene complex were somewhat reduced than that of the toluene complex. This fact evidenced a stronger Ca-arene interaction in mesitylene complexes than in the toluene analogue, probably due to more electron donating effect of three methyl groups in mesitylene molecule than by one methyl group in toluene. Both complexes have a zigzag polymer structure. The strontium complex is isostructural with the samarium and europium complexes reported in chapter 2 due to their similar atomic radii.

The barium complex [(Ba(η⁴-C₆H₅Me)₂(AlI₄)₂] has the barium centre sandwiched between two toluene molecules both in an η⁴-fashion giving the barium centre an eight coordination. Nevertheless, the other complexes have only one arene molecule bonded to the metal centre. The Ba-I, Ba-C contacts are comparable with other analogous complexes considering the ionic radii of metals and iodide.

Reactivity of divalent the formamidinate complexes [Yb(Form)₂(thf)₂] (Form = [RNCHNR]; R = 2,6-Me₂ (XylForm); 2,4,6-Me₃ (MesForm); 2,6-Et₂ (EtForm); 2,6-ⁱPr₂ (DippForm)) has been studied by using different oxidants such as Cl₃CCCl₃, BrCH₂CH₂Br and ICH₂CH₂I. Benzophenone has also been used to study the reactivity of the less sterically demanding ytterbium formamidinate complexes. Reactions of [Yb(DippForm)₂(thf)₂].2THF with BrCH₂CH₂Br and ICH₂CH₂I yielded the halide complexes [Yb(DippForm)₂X(thf)] (X = Br, I). However, all other divalent formamidinate complexes gave the homoleptic trisformamidnate complexes [Yb(Form)₃] (Form = XylForm, MesForm and EtForm). Reactions with benzophenone also resulted in the tris-formamidnate complexes. This is probably due to the redistribution of the less bulky formamidinates. The nmr spectrum of these complexes could not be integrated due to the paramagnetic nature of trivalent ytterbium.

With the intention of the isolation of cationic complexes [L₂Ln]⁺[AlX₄/BPh₄/SbCl₆]⁻, a range of halide abstraction reactions has been performed using different halide abstracting reagents such as AlX₃ (X = Cl, Br, I), SbCl₅ and AgBPh₄. Unexpectedly, all attempts to prepare cationic complexes from the halide abstraction reactions consistently gave ligand; however, on one occasion we isolated the reported complex [YbI₂(thf)₅]⁺ [YbI₄(thf)₂]⁻. The reaction between [Yb(DippForm)₂(thf)₂] and cobalt carbonyl gave the cobalt complex [Co(DippFormCO)(CO)₃].THF in place of the expected cationic complex [Yb(DippForm)₂]⁺[Co(CO)₄]⁻.

A deliberate RTP reaction using two formamidines of dissimilar steric bulk such as DippForH and XylFormH afforded the heteroleptic tris-formamidinate complex [Yb(DippForm)(XylForm)₂].PhMe for the first time. In addition, [Eu(XylForm)₂(μ- OH)(thf)]₂ and [Eu(EtForm)₂(thf)₂] have been synthesised by the RTP reactions involving XylFormH and EtFormH, respectively. [Yb(MesForm)₂(μ-OH)]₂ was also isolated by the similar procedure using ytterbium. The formation of [Yb(MesForm)₂(μ-OH)]₂ and [Eu(XylForm)₂(μ-OH)(thf)]₂ probably involves a trace amount of water and the presence of the hydroxyl group in the complexes was further confirmed from IR spectra.

Item ID: 57687
Item Type: Thesis (PhD)
ISSN: 1432-0975
Keywords: lanthanoids, Lanthanoid(II), Lanthanoid(III), Lanthanoid(IV), formamidinates
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Copyright Information: Copyright © 2017 Md Elius Hossain
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Appendix 3: Deacon, Glen B., Hossain, Md Elius, Junk, Peter C., and Salehisaki, Mehdi (2017) Rare-earth N,N'-diaryformamidinate complexes. Coordination Chemistry Reviews, 340. pp. 247-265.

Date Deposited: 03 May 2019 05:22
FoR Codes: 03 CHEMICAL SCIENCES > 0302 Inorganic Chemistry > 030202 f-Block Chemistry @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970103 Expanding Knowledge in the Chemical Sciences @ 100%
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