Design, synthesis and reactivity of novel carbazole-bis(azole) ligands for use in lanthanide and transition metal complexes
Date
2020-12-21
Authors
Gajecki, Leah
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Abstract
An array of carbazole-bis(azole) (CzTR and CzTrR, Cz = carbazole, T = tetrazole, Tr = triazole, R = Me, iPr, Bz and CH2Mes) pincer ligands were synthesized and fully characterized by NMR, X-ray crystallography and high resolution mass spectrometry. The physical properties of these ligands, as well as their applications in metal coordination compounds, were extensively studied. Mono, bis- and tris-ligand lanthanide (Y, Yb, Er, Sm, and Ce) complexes were synthesized and the solid state crystal structures of these complexes revealed the unusual binding modes of these ligands. The capability of these ligands to distort from planarity and bind in a fac type fashion, as well as bidentate, rather than tridentate modes was unexpected. The reactivity of the mono-ligand lanthanide complexes was explored and showed modest activity in ring-opening polymerization of lactones, as well as some unusual one electron redox chemistry.
Bis-ligand complexes of some first row transition metals (Fe, Co, Ni and Zn) were also explored. These air-stable complexes proved invaluable in studying the physical properties of the ligands themselves, as well as the metal complexes. The bis-ligand zinc complexes (CzTiPr)2Zn and (CzTriPr)2Zn showed reversible oxidation of the CzT and CzTr ligands at modest potentials, as well as ligand-based fluorescence. Two bis-ligand iron complexes were synthesised using sterically diverse ligands (CzTiPr)2Fe and (CzTCH2Mes)2Fe and while (CzTCH2Mes)2Fe showed temperature-independent paramagnetism, the similar (CzTiPr)2Fe complex showed a two-step thermally induced spin crossover phenomenon near room temperature in the solid state. The oxidation of (CzTiPr)2Fe yielded [(CzTiPr)2Fe]+ BF4-; we have speculated that this complex is initially oxidized at the ligand and then undergoes an electron transfer from the metal to the ligand to result in an Fe(III) complex.
Mono-ligand iron complexes using the CzTiPr ligand were also synthesized. While ligand redistribution hindered initial progress, the use of a bulky phenoxide ancillary ligand arrested the redistribution pathway and allowed the synthesis of (CzTiPr)Fe(OAr)(THF). This iron complex proved to be the most successful in terms of isolating stable and reactive complexes with our ligands, and several adducts (THF, tBuNC, TPPO) were synthesized. Synthetic and DFT studies of these complexes demonstrated the tendency for the Fe to bind σ-donor ligands, but not π-acceptor ligands. This led to the isolation of a thermally sensitive four-coordinate iron complex (CzTiPr)Fe(OAr). The catalytic activity of these Fe phenoxide complexes in the hydrosilylation of styrene with phenylsilane was explored. These catalysts were able to selectively produce the Markovnikov hydrosilylation product with good activity. The apparent preservation of the Fe(II) oxidation state throughout the catalytic transformation suggests a possible ligand-assisted mechanism where the CzT ligand acts as a proton reservoir.
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Keywords
Synthesis, lanthanide, metal, ligand, reactivity