Redox-Active p-Quinone-Based Bis(pyrazol-1-yl)methane Ligands: Synthesis and Coordination Behaviour

• Published in: Chemistry : a European journal 2008-03, Vol.14 (8), p.2590-2601
• Main Authors: Scheuermann, Sebastian, Kretz, Tonia, Vitze, Hannes, Bats, Jan W., Bolte, Michael, Lerner, Hans-Wolfram, Wagner, Matthias
• Format: Article
• Language: English
• Subjects: ligand design; N,O ligands; O ligands; palladium; pyrazolylmethanes; quinones
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200701615

The synthesis, structural characterisation and coordination behaviour of mono‐ and ditopic p‐hydroquinone‐based bis(pyrazol‐1‐yl)methane ligands is described (i.e., 2‐(pz2CH)C6H3(OH)2 (2 a), 2‐(pz2CH)‐6‐(tBu)C6H2(OH)2 (2 b), 2‐(pz2CH)‐6‐(tBu)C6H2(OSiiPr3)(OH) (2 c), 2,5‐(pz2CH)2C6H2(OH)2 (4)). Ligands 2 a, 2 b and 4 can be oxidised to their p‐benzoquinone state on a preparative scale (2 aox, 2 box, 4ox). An octahedral NiII complex [trans‐Ni(2 c)2] and square‐planar PdII complexes [Pd2 bCl2] and [Pd2 boxCl2] have been prepared. In the two PdII species, the ligands are coordinated only through their pyrazolyl rings. The fact that [Pd2 bCl2] and [Pd2 boxCl2] are isolable compounds proves that redox transitions involving the p‐quinone substituent are fully reversible. In [Pd2 boxCl2], the methine proton is highly acidic and can be ed with bases as weak as NEt3. The resulting anion dimerises to give a dinuclear macrocyclic PdII complex, which has been structurally characterised. The methylated ligand 2‐(pz2CMe)C6H3O2 (11ox) and its PdII complex [Pd11oxCl2] are base‐stable. A new class of redox‐active ligands is now available with the potential for applications both in catalysis and in materials science. Reverse transitions: Mono‐ and ditopic, redox‐active p‐quinone‐based bis(pyrazol‐1‐yl)methane ligands (see figure) have been prepared. These ligands can be oxidised on a preparative scale and complexes with palladium can be prepared that are active in CC coupling reactions.