Sensitivity of a Strained C–C Single Bond to
Charge Transfer: Redox Activity in Mononuclear and Dinuclear Ruthenium
Complexes of Bis(arylimino)acenaphthene (BIAN) Ligands
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Abstract
The
new compounds [Ru(acac)<sub>2</sub>(BIAN)], BIAN = bis(arylimino)acenaphthene
(aryl = Ph (<b>1a</b>), 4-MeC<sub>6</sub>H<sub>4</sub> (<b>2a</b>), 4-OMeC<sub>6</sub>H<sub>4</sub> (<b>3a</b>), 4-ClC<sub>6</sub>H<sub>4</sub> (<b>4a</b>), 4-NO<sub>2</sub>C<sub>6</sub>H<sub>4</sub> (<b>5a</b>)), were synthesized and structurally,
electrochemically, spectroscopically, and computationally characterized.
The α-diimine sections of the compounds exhibit intrachelate
ring bond lengths 1.304 Å < d(CN) < 1.334 and 1.425 Å
< d(CC) < 1.449 Å, which indicate considerable metal-to-ligand
charge transfer in the ground state, approaching a Ru<sup>III</sup>(BIAN<sup>•–</sup>) oxidation state formulation. The
particular structural sensitivity of the strained peri-connecting
C–C bond in the BIAN ligands toward metal-to-ligand charge
transfer is discussed. Oxidation of [Ru(acac)<sub>2</sub>(BIAN)] produces
electron paramagnetic resonance (EPR) and UV–vis–NIR
(NIR = near infrared) spectroelectrochemically detectable Ru<sup>III</sup> species, while the reduction yields predominantly BIAN-based spin,
in agreement with density functional theory (DFT) spin-density calculations.
Variation of the substituents from CH<sub>3</sub> to NO<sub>2</sub> has little effect on the spin distribution but affects the absorption
spectra. The dinuclear compounds {(μ-tppz)[Ru(Cl)(BIAN)]<sub>2</sub>}(ClO<sub>4</sub>)<sub>2</sub>, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine;
aryl (BIAN) = Ph ([<b>1b</b>](ClO<sub>4</sub>)<sub>2</sub>),
4-MeC<sub>6</sub>H<sub>4</sub> ([<b>2b</b>](ClO<sub>4</sub>)<sub>2</sub>), 4-OMeC<sub>6</sub>H<sub>4</sub> ([<b>3b</b>](ClO<sub>4</sub>)<sub>2</sub>), 4-ClC<sub>6</sub>H<sub>4</sub> ([<b>4b</b>](ClO<sub>4</sub>)<sub>2</sub>), were also obtained and investigated.
The structure determination of [<b>2b</b>](ClO<sub>4</sub>)<sub>2</sub> and [<b>3b</b>](ClO<sub>4</sub>)<sub>2</sub> reveals <i>trans</i> configuration of the chloride ligands and unreduced
BIAN ligands. The DFT and spectroelectrochemical results (UV–vis–NIR,
EPR) indicate oxidation to a weakly coupled Ru<sup>III</sup>Ru<sup>II</sup> mixed-valent species but reduction to a tppz-centered radical
state. The effect of the π electron-accepting BIAN ancillary
ligands is to diminish the metal–metal interaction due to competition
with the acceptor bridge tppz
