Design of multinuclear transition metal and lanthanide complexes: magnetic and luminescent properties

Guo, Zhilin (2021). Design of multinuclear transition metal and lanthanide complexes: magnetic and luminescent properties. University of Birmingham. Ph.D.

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Abstract

The work presented in this thesis focuses on the syntheses and characterisation of a series of multi-nuclear transition metal and lanthanide-based coordination complexes. Their structures, magnetic and luminescent properties are investigated by various experimental techniques including single crystal X-ray diffraction, UV-vis, SQUID magnetometry and time-resolved luminescent spectroscopy.

Rational design of molecular clusters with intricate supramolecular architectures have received continuous attention in many interdisciplinary fields. In comparison with those closed-shell bridging ligands that suffer from weak magnetic coupling between the metal centres due to the large spanning distance, the use of free radical bridges can produce efficient spin exchange beyond distance limit and further enhances magnetic interaction with other spin centres arising from the direct orbital overlap.

Toward this end, two neutral bis-bidentate pyridazine ligands, pydz and pzdz, with the versatile azido bridge are used in Chapter 2 and 3 to synthesise tetranuclear Co and Fe grid-like complexes [M\(^{II}\)\(_4\)(pydz)\(_4\)(N\(_3\))\(_4\)](BPh\(_4\))\(_4\)·4MeCN and [M\(^{II}\)\(_4\)(pzdz)\(_4\)(N\(_3\))\(_4\)](BPh\(_4\))\(_4\)·solvents. The Co complexes behave as typical field-induced SMMs with energy barriers of 36 and 56 K, respectively. The Fe complex with pydz ligand shows remarkable SCO behaviour (T\(_{1/2}\) = 230 K) which is first observed in azide-bridged systems, while the pzdz analogue only shows overall antiferromagnetic interaction.

In Chapter 4, tetrazine radical bpztz\(^{•-}\) is used to prepare tetranuclear complexes [M\(^{II}\)\(_4\)(bpztz\(^{•-}\))\(_4\)(N\(_3\))\(_4\)] (M = Co and Zn). Strong antiferromagnetic metal-radical interactions with \(J\) = -65 cm\(^{-1}\) was obtained in Co complex. Meanwhile, an unexpected metamagnetic behaviour at low temperatures as well as significant enhancement of SMM behaviour compared to the neutral pyridazine bridge are observed.

Another tetrazine ligand bptz is introduced in Chapter 5 making tetranuclear Co grids [Co\(^{II}\)\(_4\)(bptz\(^{•-}\))\(_4\)(N\(_3\))\(_4\)·MeOH along with two mixed-valence components [Co\(^{2.5}\)\(_4\)(bptz\(^{•-}\))\(_4\)(N\(_3\))\(_4\)](ClO\(_4\))\(_2\)·5MeOH\) and [Co\(^{II}\)\(_2\)Co\(^{III}\)\(_2\)(Hbptz)\(_4\)(N\(_3\))\(_4\)](OTF)\(_2\)·solvents. SMM behaviours are observed for the tetrazine radical based complexes with higher effective energy barriers compared to the pyridazine counterparts. However, the slow magnetic relaxation is switched off for the hydrogenated tetrazine-bridged one. The redox properties are also studied.

Lanthanide complexes with well-designed organic chromophores are considered as ideal candidates for luminescent materials for their intense emission, long lifetimes and high quantum yields. Polynuclear complexes, specially heterometallic complexes, are of particular interest when incorporating luminescence with magnetic properties that are able to provide additional functions.

Chapter 6 describes the homo- and hetero-dinuclear assemblies of lanthanide complexes using a bis-bidentate bistpOp ligand for shielding around the lanthanide ions. Characteristic emission for Eu\(^{3+}\), Tb\(^{3+}\), Dy\(^{3+}\), Sm\(^{3+}\) and Yb\(^{3+}\) ions are obtained with long lifetimes for [Eu\(_2\)(bistpOp)\(_3\)] and [Tb\(_2\)(bistpOp)\(_3\)] complexes. The formation of heterodinuclear [DyEu(bistpOp)\(_3]\) and [EuYb(bistpOp)\(_3\)] complexes are investigated in solution via luminescence titration. Significant energy transfer is observed from the spectra with efficiency above 50%.

A novel DTPA-bisamide derivative with terpyridine arms is successfully synthesised and characterised in Chapter 7 making lanthanide complexes. Photophysical studies prove that the ligand is a good antenna for sensitising visible lanthanide emissions of Tb\(^{3+}\) and Eu\(^{3+}\). Moreover, the heterometallic 3d-4f assembly of a macrocyclic complex Eu\(_2\)Fe\(_2\)\(^{4+}\) is studied by UV-vis titration.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):