Abstract
Careful optimization of the reaction conditions provided access to the particularly small tetraruthenium macrocycle 2Ru2Ph-Croc, which is composed out of two redox-active divinylphenylene-bridged diruthenium entities {Ru}-1,4-CH=CH-C6H4-CH=CH-{Ru} (Ru2Ph; {Ru} = Ru(CO)Cl(PiPr3)2) and two likewise redox-active and potentially non-innocent croconate linkers. According to single X-ray diffraction analysis, the central cavity of 2Ru2Ph-Croc is shielded by the bulky PiPr3 ligands, which come into close contact. Cyclic voltammetry revealed two pairs of split anodic waves in the weakly ion pairing CH2Cl2/NBu4BArF24 (BArF24 = [B{C6H3(CF3)2-3,5}4]− electrolyte, while the third and fourth waves fall together in CH2Cl2/NBu4PF6. The various oxidized forms were electrogenerated and scrutinized by IR and UV/Vis/NIR spectroscopy. This allowed us to assign the individual oxidations to the metal-organic Ru2Ph entities within 2Ru2Ph-Croc, while the croconate ligands remain largely uninvolved. The lack of specific NIR bands that could be assigned to intervalence charge transfer (IVCT) in the mono- and trications indicates that these mixed-valent species are strictly charge-localized. 2Ru2Ph-Croc is hence an exemplary case, where stepwise IR band shifts and quite sizable redox splittings between consecutive one-electron oxidations would, on first sight, point to electronic coupling, but are exclusively due to electrostatic and inductive effects. This makes 2Ru2Ph-Croc a true “pretender”.
Highlights
Their inherent symmetry and the relative ease of their fabrication by the self-assembly of mononuclear metal-coligand nodes or dinuclear clamps and ditopic organic linkers as well as their structural adaptability render metallamacrocycles a appealing class of compounds [1,2,3,4,5,6,7]
The lack of specific near infrared (NIR) bands that could be assigned to intervalence charge transfer (IVCT) in the mono- and trications indicates that these mixed-valent species are strictly charge-localized. 2Ru2Ph-Croc is an exemplary case, where stepwise IR band shifts and quite sizable redox splittings between consecutive one-electron oxidations would, on first sight, point to electronic coupling, but are exclusively due to electrostatic and inductive effects
Through-bond coupling in metallamacrocyclic architectures was first indicated by sizable redox splittings between individual oxi-dations in trinuclear macrocyclic 3-hydroxypyridinone-bridged half-sandwich ruthenium complexes [19], and later demonstrated for related adenine-bridged congeners with RuII([9]aneS3) metal nodes ([9]aneS3 = 1,4,7-trithiacyclononane) [20,21]
Summary
Their inherent symmetry and the relative ease of their fabrication by the self-assembly of mononuclear metal-coligand nodes or dinuclear clamps and ditopic organic linkers as well as their structural adaptability render metallamacrocycles a appealing class of compounds [1,2,3,4,5,6,7]. Viable candidates can be found amongst aromatic bis(β-ketoenolates) such as 5,8-dioxido-1,4-anthraquinones or 1,4-dioxido-9,10anthraquinones Representatives of this ligand class and their amino-/imino congeners have a demonstrated ability to foster electronic coupling between two interlinked, electronrich ruthenium complex entities and/or to actively contribute or even dominate redox processes, leading to variable and often intricate charge state distributions [36,37,38,39,40,41]. The croconate dianion undergoes two reversible one-electron oxidations at low potentials as divinylarylene-bridged diruthenium complexes [23,47,48,49,50,51,52] The issues of the loci of the anodic processes and of the presence or absence of electronic coupling in the mixed-valent states are addressed by a multimethod approach, including IR, UV/Vis/NIR, and EPR spectroscopy with support by quantum chemistry
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
