Rare earth hydroxycinnamates and hydroxycinnamate-derivative complexes as corrosion inhibitors
Creators: name: Lee, Woei Yen Winnie
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: This thesis describes the synthesis and characterisation of rare earth n-hydroxycinnamate (n- HOc), 3-(n`-hydroxyphenyl)propionate (n-hpp) and (E)-3-(n`-pyridyl)acrylate (n-pya) complexes (n = 2-4). The corrosion inhibition properties for mild steel in an aqueous environment are assessed for a selection of these complexes, representative of each of the ligand systems. In Chapter 2, a crystal structure for the complex [Ce(4HOc)4(H2O)(MeOH)2].(MeOH) was determined. The compounds La(4HOc)3(H2O)4, Pr(4HOc)3(H2O)3 and Pr(4HOc)3(H2O)4 were also obtained complementing the rare earth-4HOc complexes previously prepared. Powder XRD, analysis together with elemental analysis and TGA analysis of these compounds indicate that their structural profile are independent from the extents of hydration. In accordance with the aim of this study to investigate the influence of the hydroxy-substitution position on the cinnamates, two further series of rare earth complexes, namely with 3- hydroxycinnamate and 2-hydroxycinnamate ligands were also prepared. The [RE(3HOc)3(H2O)n].x(H2O) (RE = La and Ce, n = 2, x = 0.5; RE = Pr-Nd, Gd, Er, and Y, n = 1, x = 1) complexes were analogous (based on unit cell data and powder XRD analysis) across the RE elements except Lu2(3HOc)5(OH)(H2O)6 which showed slight differences in the powder XRD. A ferric compound was also obtained, Fe2(3HOc)3(OH)3(H2O)2, and is the only non-rare earth compound in the thesis. The RE-2HOc compounds obtained can be grouped as follows, (Group I) [Ln(2HOc)3(H2O)].2(MeOH) (RE = La and Pr), (Group II) Ce(2HOc)3(H2O)5.5, (Group III) [Pr2(2HOc)5(OH)(H2O)2].4(H2O), (Group IV) Ln(2HOc)3(H2O)3, (Group V) [Gd(2HOc)3(solv)2]2.x(solv) (solv = MeOH and/or H2O; x = 0 or 6) and [Er(2HOc)3(H2O)2]2.10(H2O), and (Group VI) [RE(2HOc)3(H2O)2]2.x(solv) (RE = Gd, x(solv) = 2H2O, 4MeOH; RE = Er, x(solv) = 10H2O; RE = Y, x(solv) = 2H2O). Three crystal structures were determined for this series of complexes, [RE(2HOc)3(MeOH)2]2.x(MeOH) (RE = Gd, x = 6; RE = Er, x = 5) (Group V), and [Er(2HOc)3(H2O)2]2.10(H2O) (Group VI). Chapter 3 contains a first report for the exciting new RE 3-(n`-hydroxyphenyl)propionate complexes (n = 2 and 4). This chapter investigates the influence on the structural chemistry of the RE complexes when the the –CH=CH– linker in the carboxylate ligand is reduced to a flexible –CH2–CH2– linker. The lanthanoid contraction is evident across the RE-4hpp and RE-2hpp complex series. There are two types of RE-4hpp (RE = La, Ce-Nd, Ho and Lu) complexes showing six different structural motifs across the series. Type I is monometallic with the complexes [La4(4hpp)12(H2O)6].4(H2O), [Ce(4hpp)3(H2O)][Ce(4hpp)3(H2O)2].(H2O).2.5(EtOH), [Ho(4hpp)3(H2O)2] and [Lu(4hpp)3].(H2O). The Type II complexes, [NaRE2(4hpp)7(H2O)2].3(H2O) (RE = Ce and Pr) and [NaNd2(4hpp)7(H2O)(MeOH)].(H2O).3(MeOH), are bimetallic. Five structural motifs are observed for the RE-2hpp (RE = La, Ce-Pr, Gd and Lu) complexes with the general composition [RE(2hpp)3(H2O)n].(H2O) (RE = Ce and Pr, n = 1; RE = La, Gd and Lu, n = 2). A second lutetium complex, [Lu2(2hpp)6(H2O)4].[Lu(2hpp)3(H2O)2].9(H2O).(MeOH) was also identified. The effect of the replacement of the phenyl-ring of the hydroxycinnamates by a pyridyl-ring is reported in Chapter 4. The [RE(4pya)3(H2O)2].x(solv) (RE = La, Pr, Sm and Y; solv = H2O/MeOH) complexes and [Lu(4pya)3(MeOH)2].3(H2O) complex are obtained. For the RE- 2pya series, the complexes [RE(2pya)3(H2O)2].x(H2O) (RE = La, Pr and Sm) and [RE(2pya)3].2(H2O) (RE = Ho and Lu) were obtained. Two PrIII crystal structures were determined for the complexes [Pr(4pya)3(H2O)2].(H2O).(MeOH) and [Pr(2pya)3(H2O)2].4(H2O). The latter complexes have never been reported before. Finally, Chapter 5 involves the screening of selected rare earth carboxylate complexes reported in Chapters 2, 3 and 4 for their corrosion inhibition properties for mild steel in 0.01 M NaCl solution. Immersion tests were carried out as initial screenings and electrochemical measurements were also made. The effect of the different derivatisation of the hydroxycinnamate ligand in the rare earth complexes was investigated. The La(3HOc)3(H2O)4 complex was determined to have the best performance. The combined data obtained from the immersion tests and electrochemical measurements found that the –CH=CH– linker and the hydroxyphenyl-ring are essential functionalities for effective corrosion mitigation by the rare earth hydroxycinnamate-type complexes.