Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes: Formation of the Corresponding Semiquinones, Quinhydrones and Quinones

Abstract

Pyrazolyl-substituted 1,4-dihydroxybenzene and 1,4-dihydroxynaphthene derivatives have been synthesized by reaction of 1,4-benzoquinone and 1,4-naphthoquinone, respectively, with pyrazole. Cyclovoltammetric measurements have shown that 1,4-benzoquinone possesses the potential to oxidize 2-(pyrazol-1-yl)-and 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene. The 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene reacts with air to give quantitatively black insoluble 2,5-bis(pyrazol-1-yl)-1,4-quinhydrone. Black crystals of 2,5-bis(pyrazol-1-yl)-1,4-quinhydrone suitable for X-ray diffraction were grown from methanol at ambient temperature (monoclinic C2/c). The poor yields of pyrazolylsubstituted 1,4-dihydroxybenzene and 1,4-dihydroxynaphthene derivatives can be explained by the formation of insoluble black quinhydrons in the reaction of benzoquinone and naphthoquinone with pyrazole. The dianions of 2-(pyrazol-1-yl)-and 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene react with oxygen to give the corresponding semiquinone anions. 2,5-Bis(pyrazol-1-yl)-1,4-benzoquinone shows two reversible one-electron reduction processes in cyclovoltammetric measurements, whereas pyrazolyl-substituted 1,4-dihdroxybenzene and -naphthene derivatives undergo irreversibile electrontransfer processes.

Radical ligand-containing single-molecule magnets

“…Efforts to generate benzosemiquinonoid-containing transition metal complexes with a nuclearity of greater than one have resulted in numerous dinuclear compounds, including those containing metal centers bridged by derivatives of 2,5-dihydroxy-1,4-benzoquinonoids [172][173][174][175][176][177][178][179][180][181][182][183][184][185][186][187][188][189], bis(pyrazolyl)benzoquinonoids [190][191][192][193], bis(phosphine)benzoquinonoids [194,195], bis(amino) benzoquinonoids [196][197][198][199][200][201][202], and bis(amino)diiminobenzoquinonoids [86,[203][204][205][206]. Indeed, the radical form of these ligands has shown enhanced magnetic coupling compared to their non-radical redox isomers [174,180,182].…”

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

“…[1] As a consequence, the assignment of a formal oxidation number to the metal centre in hydroquinonate complexes often becomes ambiguous such that the more meaningful physical oxidation number has to be established experimentally. [14] In our own studies, we initially employed the ditopic p-hydroquinone chelate ligand A [15] ( Figure 1). Treatment of A with Cu II ions leads to the purplecoloured coordination polymer D, which consists of magnetically isolated linear chains with Cu II ions in a square-planar coordination environment ( Figure 2).…”

2,5-Dihydroxybenzaldehyde

“…Data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in SHELXTL-Plus (Sheldrick, 1991); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2003). Recently, we have extended our studies to redox-active ligands which can be used to influence the electrochemical reactivity of transition metals since their redox activity is expanded upon complexation (Lerner et al, 2006;Kretz et al, 2007). The resulting complexes can undergo multi-electron transfer reactions which are the sum of the oxidation state changes of the metal center and the ligand (Margraf et al, 2006;Kretz et al, 2006).…”

“…The resulting complexes can undergo multi-electron transfer reactions which are the sum of the oxidation state changes of the metal center and the ligand (Margraf et al, 2006;Kretz et al, 2006). Due to their electrochemical reversibility, hydroquinone / quinone derivatives are candidates for redox-active ligands (Lerner et al, 2006;Kretz et al, 2007). In our studies we have used Schiff-Base derivatives which can conveniently be achieved by reaction of amines and aldehyde derivatives (Margraf et al, 2006;Kretz et al, 2007).…”