Chun-Yi Lin
Published: 2017
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This dissertation primarily focuses on the synthesis, characterization and reactivity studies of a series of linear two-coordinate, late transition metal complexes. Except Chapter 1, all the chapters were published as peer-reviewed articles in academic journals. Chapter 1 consists of two parts: the first part provides a brief introduction and review on the current state of the field for two-coordinate, open-shell transition metal complexes. Chapter 2 details the synthesis, spectroscopic characterization of three strictly linearly coordinated Fe, Co and Ni complexes of formula M{N(SiMee3)Dipp}2. It was found from dispersion-corrected DFT calculations that attractive dispersion forces played an important role in stabilizing these species with unusual geometries. SQUID magnetic measurement showed that both Fe{N(SiMe3)Dipp}2 and Co{N(SiMe3)Dipp}2 have high magnetic moments compared to their spin-only value, which is due to the unquenched orbital angular momentum (OAM) contribution. Several collaborating efforts that resulted in a number of different publications are also included. Slow magnetic relaxation studies on Fe{N(SiMe3)Dipp}2 revealed a spin reversal barrier (U[subscript eff]) of 181 cm−1, a second highest value to date (June 2017) for a mononuclear transition metal complex. 57Fe Mößbauer spectroscopy showed a record high internal field of 162 T created by the unquenched OAM, which is the highest internal field reported so far for any Fe containing compound. Chapter 3 gives details of the reduction of M{N(SiMe3)Dipp}2 (M = Fe, Co, Ni). It was found that these species can be reduced to their +1 oxidation state and at the same time, remain two-coordinate at the metal center. These complexes, with formula of [K(18-crown-6)][M{N(SiMe3)Dipp}2], formed a rare series of the two-coordinate M(I) complexes. SQUID magnetic measurement showed that Fe and Co species had a high magnetic moment compared to their spin-only value, which is again due to the unquenched orbital angular momentum. In a similar fashion, chapter 4 reports the reduction of the first two-coordinate transition metal complexes, Mn{C(SiMe3)3}2, which was originally reported by Eaborn and coworkers in 1985. Mn{C(SiMe3)3}2 was reduced with KC8 in the presence of two different alkali metal ion complexing agents, 18-crown-6 and 15-crown-5. It was found that although the anion remained two-coordinate, the geometries differ due to the shallow potential well for the C−Mn−C coordination. In addition, although they have near-linear manganese coordination but the orbital magnetism is almost completely quenched as a result of 4s−3d[subscript z]2 orbital mixing which affords a non-degenerate ground state. Chapter 5 presents a serendipitous finding during our course of investigating the magnetic properties of three-coordinate transition metal complexes, M{N(SiMe3)Dipp}2(L) (M = Fe, Co, Ni; L = Lewis base), we found that the complexation of the Lewis bases are reversible. Using electronic and NMR spectroscopy, we were able to determine the binding constants of THF, THT (tetrahydrothiophene), pyridine, DMAP (4-dimethylaminopyridine), PMe3, PCy3 and IMes (1,3-bis(1,3,5-trimethylphenyl)imidazol-2-ylidene) to M{N(SiMe3)Dipp}2 and the binding strengths was found to be in the order of DMAP > pyridine > PMe3 > PCy3 ≈ IMes > THF > THT and those of the respective metals was in the order of Fe > Co > Ni. Finally, Chapter 6 is a Review that stems from our interest in two-coordinate transition metal complexes of +1 oxidation state. In particular, although Ni(I) complexes are far less numerous than complexes of its neighboring oxidation states (Ni(0) and Ni(II)), they have many applications that include small molecule activation, catalysis and magnetism.