Synthetic, Structural, Spectroscopic and Theoretical Studies on Homo- and Heterometallic Paramagnetic Complexes

This thesis covers many areas within the field of molecular magnetism. With the exception of chapter 6, all novel complexes in this thesis have been synthesized using the bidentate ligand 2,2'-biphenol (LH2), usually in conjunction with various pyridyl co-ligands. Investigation of this ligand in the context of molecular magnetism has been immensely successful and has led to a plethora of beautiful novel complexes. In chapter 2 we report a new family of five mu-alkoxide bridged [Mn(III)2] dimers. These complexes (1-5) have been used towards a full magnetic and DFT analysis of all [Mn(III)-(OR)2-Mn(III)] dimers in the literature. We have categorized this type of dimeric Mn(III) complex into three distinct groups, based on the relative orientations of the Mn(III) ions' Jahn-Teller axes. This study is an extremely useful contribution to the field of molecular magnetism as we have concluded that the main structural feature that determines the magnetic exchange coupling in these systems is in fact the Jahn-Teller axes relative orientation. Since the magnetic skeletons of large polynuclear Mn(III) clusters are often made up of dimeric building blocks linked via alkoxide bridges, this study aids in the rationalization of magnetic interactions in these larger clusters. These findings have been published in the Wiley journal: Chemistry - A European Journal [1]. From here we proceeded to investigate the ability of 2,2'-biphenol to complex both manganese and copper ions, which led to the production of two novel Cu(II)¿Mn(III) dimers (6 and 7), each possessing a rare Jahn-Teller compressed Mn(III) ion, as detailed in chapter 3. To our knowledge, these are the first Cu(II)-Mn(III) dimers to possess a Mn(III) ion in a compressed octahedral environment. This chapter details another comparative study, in which we contrast the ferrimagnetic interaction between the Cu(II) and Mn(III) ions in one of our novel dimers (complex 6), with that of a ferromagnetic Cu(II)-Mn(III) Single-Molecule Magnet from the literature (complex 9). DFT calculations offer insight into the mechanism of magnetic coupling where the origin of the antiferromagnetic interaction is related to the nature of the Jahn-Teller distortion. The electronic structure and zero-field splitting of our Cu(II)-Mn(III) dimer (complex 6) was analysed using single crystal high-frequency high-field EPR. This interesting study has been submitted for publication to the RSC journal: Dalton Transactions [2]. Continuing on the topic of heterometallic chemistry, in chapter 4 we report a family of 1-D coordination polymers (complexes 10-15 and 17), whose nodes are comprised of alternating M(III) and Na(I) units (M = Mn / Fe). This M(III)-Na(I)-M(III)-Na(I) motif is extremely rare, and these complexes represent the first coordination polymers built using 2,2'-biphenol. These infinite chains have been obtained through the modification of the synthetic procedures used in the preparation of the [Mn(III)2] dimers described in chapter 2. By adding an excess of both 2,2'-biphenol and NaOH, 1-D Mn(III)/Na(I) coordination polymers are produced, rather than discrete [Mn(III)2] complexes. Additionally, in this study we show how modifying the functionality of the pyridyl co-ligand (from -CH3 to -CH2CH3) or changing the position of the ligand functional group (from the 3- to the 4-position), changes the nature of the polymer formed; from a M(III)/Na(I) chain whereby the metal ions are bridged via the coordinate bonds of intermediate L2- oxygen atoms, to an ionic 1-D array comprising [Na]+ and [M]- monomeric units linked via hydrogen bonding. Our initial findings have been published in the RSC journal CrystEngComm in 2010 [3], which was followed by a full paper in 2011 in the same journal [4]. In chapter 5 we move on to investigate the chemistry of 2,2¿-biphenol with the highly magnetically anisotropic Co(II) ion. Here we report the Co(II) analogues (complexes 18 and 19) to the homo- and heterometallic dimers of chapter 3, a ferromagnetic [Co(II)8] cluster (20), and 1- and 2-dimensional coordination polymers comprising magnetically dilute Co(II) nodes (21 and 22 respectively). This family of five novel complexes represent the first complexes of cobalt constructed using 2,2'-biphenol, while the [Co(II)8] cluster is the second largest discrete cluster ever synthesized using this ligand. This structural and magnetic study has been published in CrystEngComm [5]. Attempts to utilize 2-hydroxypyridine (2-hpH) as a co-ligand in conjunction with 2,2'-biphenol in the synthesis of Cu(II)-Ln(III) systems led to the production of eleven heterometallic complexes containing both Cu(II) and Ln(III) ions. This family comprises a family of [Cu(II)4Ln(III)4] clusters (23-25), alongside a series of 1-D coordination polymers comprising {Ln(III)2Cu(II)8} nodes (24-33). Although 2,2'-biphenol is not present in these complexes (2-hp being the only ligand present), these complexes significantly build on previous research. The gadolinium analogues in these families are currently being investigated for their ability to perform as molecular coolant materials, while the anisotropic terbium and dysprosium versions will be probed for SMM behaviour. Once these experiments have been completed, this study will be ready for publication. [1] N. Berg, T. Rajeshkumar, S. M. Taylor, E. K. Brechin, G. Rajaraman, and L. F. Jones, "What Controls the Magnetic Interaction in bis-mu-Alkoxo Mn(III) Dimers? A Combined Experimental and Theoretical Exploration.", Chemistry - A European Journal, vol. 18, no. 19, pp. 5906, 18, May 2012. [2] N. Berg, T. N. Hooper, J. Liu, C. C. Beedle, S. K. Singh, G. Rajaraman, S. Piligkos, S. Hill, E. K. Brechin, and L. F. Jones, "Synthetic, Structural, Spectroscopic and Theoretical Study of a Mn(III)-Cu(II) Dimer Containing a Jahn-Teller Compressed Mn ion," Dalton transactions, vol. 42, pp. 207-216, 2013. [3] N. Berg and L. F. Jones, "Alternating bimetallic Na/Mn covalent and ionic chains," CrystEngComm, vol. 12, no. 11, p. 3518, 2010. [4] N. Berg and L. F. Jones, "A series of alternating Na+/M3+ (M = Mn, Fe) covalent and ionic chains," CrystEngComm, vol. 13, no. 17, p. 5510, 2011. [5] N. Berg, S. M. Taylor, A. Prescimone, E. K. Brechin, and L. F. Jones, "Old dog, new tricks: 2,2'-biphenol as a bridging and book-end ligand in discrete and extended Co(II) architectures," CrystEngComm, vol. 14, no. 8, p. 2732, 2012