Authors:	East, Nathan R.
Advisor:	Heinze, Katja
Title:	Towards photoactive manganese(IV) and nickel(II) complexes
Online publication date:	1-Aug-2023
Year of first publication:	2023
Language:	english
Abstract:	With increased emphasis on sustainability, photochemistry with earth abundant 3d transition metal complexes has become increasingly important. Primary focus over the last decades has centered on optimising and developing charge transfer phosphorescence. However, in recent years octahedral 3d transition metal complexes containing low energy metal-centered states have received a great deal of attention, particularly with the [Cr(ddpd)2]3+ (ddpd = N,N’-dimethyl-N,N’-dipyridin-2-ylpyridine-2,6-diamine) as prime example. Such successes have guided very recent research with [V(ddpd)2]3+ as the first octahedral vanadium(III) complex to display room temperature phosphorescence. With the prospect of d2, d3, d4 and d8 electron configurations possessing low energy metal-centered states potentially capable of phosphorescence, research with other 3d metals in relevant oxidation states is of particular interest. To date only one d3 manganese(IV) and no d8 nickel(II) complexes show luminescence from metal-centered states. Part one of this work details the synthesis and characterization of an octahedral MnIV complex [Mn(dgpy)2]4+ (dgpy = 2,6-diguanidylpyridine) with a d3 electron configuration analogous to chromium(III). This begins with the synthesis of a manganese(II) precursor [Mn(dgpy)2]2+, and oxidation to the MnIV complex via a MnIII intermediate. Structural, magnetic, spectroscopic and computational studies for the complete series of oxidation states +II to +IV provides important information to aid in synthesis and design of potentially photoactive manganese(IV) complexes. It is shown that six-membered chelating ligands with mixture of strongly donating and accepting moieties are required to stabilise such a labile electron-rich d5 MnII, Jahn-Teller distorted d4 MnIII and electron-poor d3 MnIV complexes. The photophysics and photochemistry of [Mn(dgpy)2)4+ are subsequently described and documented in part two. The photophysics of such MnIV complexes are poorly understood due to extreme rarity. The present investigation reveals only the second example of MnIV phosphorescence, with luminescence in the low energy NIR-II region (1435 nm). The excited 2LMCT/2MC state (0.86 eV) of this complex was found to be long lived enough (1.6 ns) to participate in bimolecular chemistry. [Mn(dgpy)2)4+ was discovered to be strongly dual state photooxidative following NIR irradiation and is able to oxidise naphthalene dynamically via a 2LMCT/2MC state and also more difficult substrates like benzene statically via a 4LMCT state (1.46 eV). Bimolecular quenching of this photoreactive complex gives valuable insight into further photophysical dynamics and potential design of future photoactive MnIV transition metal complexes capable of bimolecular reactivity following low energy excitation (850 nm). The third part is an investigation into understanding the requirements necessary for a metal-centered emission from octahedral d8 nickel(II) complexes. To date metal-centered emission from octahedral nickel(II) is undocumented; and here an investigation is made to discover why. A strong ligand field is required to get the correct excited state ordering i.e. the intraconfigurational singlet states being lowest in energy. However, there is a limit to the ligand field strength that can be imposed on d8 systems and still maintain an octahedral coordination, with advantageous excited state ordering. To further examine the effect of ligand field strength on excited state ordering the series of complexes [Ni(dgpy)2)2+, [Ni(terpy)2)2+ (terpy = 2,2';6',2"-terpyridine), [Ni(phen)3)2+ (phen = 1,10-phenanthroline), [Ni(ddpd)2)2+ and [Ni(tpe)2]2+ (tpe = 1,1,1-tris(pyrid-2-yl)ethane) are synthesized and structural and electronic properties examined. Additionally, the influence of increased hydrostatic pressure on ligand field states of [Ni(ddpd)2)2+ is also investigated, to evaluate if the inter- and intraconfigurational states can be further separated. This investigation gives valuable information in the pursuit of metal-centered spin-flip emissive octahedral NiII complexes. Part four builds on part three by looking at the impact of increasing ligand field strength on NiII ligand field states. This is done by firstly increasing σ-donation to destabilize the eg* orbitals, with synthesis of the carbene complex [Ni(CNC)(NCN)]2+ (CNC = 1,1’-(pyridin-2,6-diyl)bis(3-methyl-1H-imidazol-3-ylidene)) and NCN = (1,3-bis(2-pyridyl)imidazolylidene). With increased σ-donation it is also conceivable that the complex will adopt square planar geometry, thus structural confirmation of an octahedral environment is the first step, and further characterization will follow. An alternative method of increasing ligand field strength is using π-acceptor ligands to stabilize the t2g orbitals. This method will also be shown with the synthesis of [Ni(dcpp)2]2+ (dcpp = (2,6-bis(2-carboxypyridyl)pyridine)). The dcpp ligand contains two π-accepting carbonyl groups which enhance the π-accepting ability of the ligand. A Lewis acid (Sc[OTf]3] will then be coordinated to the ligand carbonyl groups, and a second coordination sphere will give further insight into the influence of increasing ligand π-acceptance on ligand field states of [Ni(dcpp)2]2+. This investigation aims to further understand the ligand requirements for NiII in the pursuit of spin-flip emission.
DDC:	540 Chemie 540 Chemistry and allied sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-9302
URN:	urn:nbn:de:hebis:77-openscience-c98edf1c-d46f-4c31-9216-edfd64efd74f7
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	http://rightsstatements.org/vocab/InC/1.0/
Extent:	XI, 200 Seiten ; Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen