Structural Diversity of Calcium, Strontium, and Barium Complexes with Reduced Forms of the 3,6‐Di‐tert‐butyl‐o‐benzoquinone Ligand

• Published in: European journal of inorganic chemistry Vol. 2019; no. 39-40; pp. 4373 - 4383
• Main Authors: Sinitsa, Dmitry K., Sukhikh, Taisiya S., Petrov, Pavel A., Nadolinny, Vladimir A., Konchenko, Sergey N., Pushkarevsky, Nikolay A.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 31-10-2019
• Subjects: Alkaline earth metals; Barium; Benzoquinone; Calcium; Catecholate; Coordination compounds; Inorganic chemistry; Ligands; Multinuclear complexes; NMR; Nuclear magnetic resonance; Oxidation; Quinones; Redox‐active ligands; Semiquinolate; Strontium; Tetrahedra; Tetrahydrofuran; Triangles
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201900678

Molecular complexes of Ca, Sr, and Ba with the reduced forms of 3,6‐di‐tert‐butyl‐o‐benzoquinone as ligands (catecholate, 3,6‐Cat2–, or semiquinone 3,6‐SQ–‧) were synthesized and structurally characterized. Trinuclear complexes [M3(3,6‐Cat)3(thf)6] (1, M = Ca; 2, M = Sr) or tetranuclear complex [Ba4(3,6‐Cat)4(thf)6] (3) crystallize from tetrahydrofuran (thf) solutions. In the structures of 1 and 2 metal atoms form a nearly right triangle with two µ3‐bridging 3,6‐Cat ligands; in 3, the metal atoms form a tetrahedron contracted along a twofold axis, and all the catecholates are µ3‐bridging. In nonpolar media the complexes 1–3 lose coordinated thf, upon which 1 is converted into complex [Ca4(3,6‐Cat)4(thf)4] (4), structurally similar to 3. Complex 1 is oxidized by one equivalent of the quinone to yield mixed‐ligand complex [Ca3(3,6‐SQ)2(3,6‐Cat)2(thf)4] (5), structurally similar to 1. Successive oxidation by the quinone leads to dissociation to mononuclear species, [Ca(3,6‐SQ)2(thf)3] (5), which possesses pentagonal‐bipyramidal coordination with two semiquinolate ligands lying in an equatorial plane. Solution behavior of the complexes is studied by NMR (for catecholates) and EPR (for semiquinolates) spectroscopy.
Reduction of 3,6‐di‐tert‐butyl‐o‐benzoquinone by alkaline earth metals provides a convenient way to catecholates, which crystallize as trinuclear (Ca, Sr) or tetranuclear (Ca, Ba) complexes, depending on the solvent. Partial oxidation to semiquinolates affords mixed‐ligand trinuclear species, while homoleptic semiquinolates are more stable as mononuclear complexes.