Spectroscopy, electrochemistry, and reactivity studies of group 8 transition metal complexes containing tetradentate dianionic ONNO and ONCN ligands

Abstract

Dianionic chelating ligands stabilize transition-metal ions in high oxidation states and enhance chemical/thermal stabilities of their complexes. Development of Group 8 metal complexes supported by tetradentate dianionic (TDDA) ligands is valuable in different areas of coordination chemistry. This thesis mainly describes structural, photophysical, and reactivity studies of Group 8 metal complexes bearing [O^N^N^O]- and [O^N^C^N]-based TDDA ligands. A series of polypyridyl ruthenium(III) and osmium(IV) complexes with bis(alkoxide)-, alkoxide/carboxylate-, or bis(carboxylate)-functionalized [O^N^N^O] ligands, including 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol) (H2DL3.1) and 6'-(2-hydroxypropan-2-yl)-[2,2'-bipyridine]-6-carboxylic acid (H2DL3.2), were synthesized and structurally-characterized, and their oxidation chemistry was examined. The large bite angles of the [O^N^N^O] ligands in the complexes (up to 148.34(18)o) allow formation of seven-coordinate complexes. The pKa values for bis(alkoxide)ruthenium(III) and -osmium(IV) complexes are 4.35 and 4.27, respectively. In 0.1 M CF3SO3H aqueous solutions, the bis(alkoxide) complexes are protonated and oxidized via PCET as revealed by electrochemistry, SC-XRD, HR-ESI-MS, EPR, rR, and 1H NMR measurements. The strong π-donating and basic alkoxide groups induce a redox-leveling and protonated sequence MIV(H2O)->MV(OH)MVIO that switched off water oxidation reactivity in ruthenium(V) state via protonation at pH 1, unlike bis(carboxylate) analogues following a redox sequence MIII(H2O)MIV(OH)MVO at pH 1. The alkoxide groups also lower oxidative reactivity towards organic substrates. Compared with bis(carboxylate)-OsVO complex (DO-H: 86.5 kcal mol-1), the second-order rate constants of HAT reactions between alkyl aromatics and bis(alkoxide)-OsV(OH) complex (DO-H: 84.4 kcal mol-1) are decreased by an order of magnitude with a different thermodynamic control. Using [O^N^C^N] ligands including 2-(6-(3-(pyridin-2-yl)phenyl)pyridin-2-yl)phenol (H2DL4.2), a series of cyclometalated iron(II/III) and ruthenium(II/III) complexes were synthesized and structurally characterized. These strong-field [O^N^C^N] ligands enforce all complexes in low-spin state at 298 K as revealed by 1H NMR, magnetic susceptibility, and EPR measurements. VT-UV-Vis studies suggested no spin-crossover of iron(II/III) complexes at up to 323 K. A glassy solution of the bis(arylisocyanide)ruthenium(II) complex [RuII(DL4.2)(XylNC)2] displays a weak and broad emission band (2-Me-THF; λem: 680 nm; τ: 0.27 μs) at 77 K while the others are non-emissive. DFT/TDDFT calculations revealed that 1dd excited states of the d6 complexes are strongly destabilized, and 1MLCT/1ILCT transitions are dominated in the visible region. For bis(trimethylphosphine)iron(II) complex [FeII(DL4.2)(PMe3)2] with panchromatic absorption, computational, spectroelectrochemical, and fs-TA studies suggested deactivation of a possible 3MLCT state (τ: 14 ps) via a low-lying 3dd state as one of decay channels. Compared with the d6 complexes, there exist low-lying 2dd excited states of d5 bis(trimethylphosphine) complexes; the possible 2LMCT states are short-lived (τ: 11.7-12.6 ps). Apart from the [O^N^N^O] and [O^N^C^N] polypyridyl ligands, a highly bulky 6,6’-bis(2,4,6-triphenylphenyl)-2,2’-bipyridine (tppbpy) ligand was also synthesized, and a number of low-coordinate coinage metal copper(I/II) and silver(I) complexes stabilized by tppbpy or 2,4,6-triisopropylphenyl 2,2’-bipyridine (tripbpy) were prepared and structurally characterized. The tppbpy ligand provides a considerably larger shielding effect on the metal center than the tripbpy ligand (computated percent buried volume %Vbur: 62% vs. 53.1% for their copper(II) complexes). In solid state, the cationic d10 Cu(I)-tppbpy and M(I)-tripbpy (M = Cu, Ag) complexes are emissive (λem: 481-622 nm) with lifetimes of 1.48-7250 μs at 77 K suggesting triplet parentage of their emissions.