Metal-based carbon dioxide uptake and conversion

Abstract

A family of lanthanide-carbonate complexes formed from the fixation of CO2 were synthesised, of the form [LnIII4(H2L1)3(CO3)6(H2O)3]∙solvate where H2L1 = 2,6- bis(pyrazole-3-yl)pyridine; solvate = varying amounts of dimethyl sulfoxide (DMSO) and H2O; and Ln = Gd (1), Tb (2),Tb (3), Dy (4), Dy (5), Ho (6), Er (7), Tm (8), Yb (9), Yb (10), Lu (11) and Y (12). These used LnIII(NO3)3∙xH2O salts with the exception of 3, 5 and 10, which used LnIIICl3∙xH2O salts. The choice of lanthanide starting material had no effect on the final structures of 1 – 12, which all contain tetranuclear lanthanide species bridged by μ2/μ3 carbonate (CO3 2- ) anions which could only have come from atmospheric CO2. Coordinated nitrate anions were considered instead of the assigned carbonate anions, but were excluded on the basis that 3, 5 and 10 could be synthesised from non-nitrate starting materials. The crystal structure, which is analogous across 1 – 12, seems to rely on π-π interactions between H2L1 ligands from neighbouring complexes. An examination of these interactions was undertaken using Hirshfeld surface analysis/crystallographic data; periodic trends were analysed; and luminescence properties were reported for 2. Mechanistic studies of the uptake of CO2 by 1 – 12 were undertaken with the view to design future coordination compounds for CO2 capture. Computational (semi-empirical and DFT methods) studies and prediction of intermolecular interactions using Hirshfeld surfaces revealed the mechanism was akin to that of the enzyme carbonic anhydrase, and that DMSO aided the process. This was supported by experimental procedures such as NMR spectroscopy, which utilised a isotopic form of 12: [YIII4(H2L1)3( 13CO3)6(H2O)3]∙7DMSO∙17H2O (13). During the synthesis of 1 – 12, the by-products [YIII4(OH)4(H2L1)2(HL1)2(MeOH)3(OMe)(NO3)] (14), [YbIII(H2L1)(NO3)2(H2O)2]·NO3 (15) and [YbIII(H2L1)3]·3NO3 (16) were formed, which are novel complexes and were used in computational prediction of the mechanism. The previously dubbed ‘π-π interactions’ in 1 – 12 were discounted by the generated Hirshfeld surfaces, which suggest that these interactions are unlikely to be at play. As such, the novel ligand 2,6-bis(pyrazol-3-yl)-4-chloro-pyridine (H2L2), a chlorinated equivalent of H2L1, was synthesised to study the effect of the electronegative region on any potential π-π interactions. The novel coordination complex [DyIII6(HL2- )3(L22- )3(NO3 - )3(DMSO)(MeO- )(MeOH)2(O2- )2)(H2O)2(L22- )0.5]·2H2O·MeOH (17) was synthesised using the same synthetic procedure as that of 1 – 12, indicating that the presence of the chlorine on the ligand interrupts the capture of CO2. The mechanistic studies indicated that interaction with CO2/CO3 2- required a mononuclear inorganic complex with a coordinated H2O. Consequently, the pyridyl-based ligands (2,6-bis(5- methyl-1H-pyrazol-3-yl)pyridine (H2L3) and 2,6-bis(5-phenyl-1H-pyrazol-3-yl)pyridine (H2L4)) were synthesised with the aim of coordinating to cheaper metals, but only H2L3 was able to be coordinated to a metal: [ZnII(H2L3)Cl2]·DMSO (18).

Date of Award	Jun 2022
Original language	English
Awarding Institution	University of Brighton
Supervisor	Ian Gass (Supervisor)