Mono-, Di-, and Oligonuclear Complexes of Cu^IIIons andp-Hydroquinone Ligands:  Syntheses, Electrochemical Properties, and Magnetic Behavior

Abstract

Four highly soluble square-planar Cu(II) and Ni(II) complexes of siloxy-salens (2SiCu, 2SiNi) and hydroxy-salens (2Cu, 2Ni) have been synthesized. An X-ray crystal structure analysis was performed on 2SiCu, 2SiNi, and 2Ni. The compounds have been investigated by cyclic voltammetry, UV-vis-NIR spectroelectrochemistry, and EPR spectroscopy. According to these results, the monooxidized species [2SiCu]+ and [2SiNi]+ are to be classified as Robin-Day class II and III systems, respectively. Magnetic measurements on the dinuclear (PMDTA)Cu(II) complex 1Cu2 x (PF6)2 with deprotonated 1,4-dihydroxy-2,5-bis(pyrazol-1-yl)-benzene (1) linker revealed antiferromagnetic coupling between the two Cu(II) ions thereby resulting in an isolated dimer compound. Coordination polymers [1Cu]n(H2O)(2n) of Cu(II) ions and bridging p-hydroquinone linkers were obtained from CuSO4 x 5 H2O and 1,4-dihydroxy-2,5-bis(pyrazol-1-yl)benzene. X-ray crystallography revealed linear chains running along the crystallographic a-direction and stacked along the b-axis. Within these chains, the Cu(II) ions are coordinated by two pyrazolyl nitrogen atoms and two p-hydroquinone oxygen atoms in a square-planar fashion.

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

“…This is in contrast, for example, to the related chelate ligand 2-(pz)C 6 H 3 (OH) 2 , which tends to give insoluble coordination polymers with various metal ions. [18] Electrochemical investigations and chemical oxidations on a preparative scale clearly reveal that free p-hydroquinone ligands 2 a and 2 b as well as the Pd II complex [Pd2 bCl 2 ] undergo reversible electron transfer to their p-benzoquinone state (2 a ox , 2 b ox , [Pd2 b ox Cl 2 ]). This is an important proof-ofprinciple of our concept given the fact that we want the pquinone substituent to aid the redox chemistry of the complexed metal ion by acting as a two-electron reservoir.…”

“…Most importantly, the synthesis of discrete dinuclear Cu II complexes of A turned out to be difficult mainly because of a pronounced tendency for the components to form insoluble microcrystalline precipitates. [18] Discrete complexes of p-hydroquinonate ligands not only serve as important soluble model compounds for notoriously insoluble coordination polymers, but they are also interesting in their own right. With the aim of having more versatile p-hydroquinone ligands at hand, we therefore prepared a second generation of derivatives B ( Figure 1) equipped with bidentate tethers to occupy three coordination sites of the complexed metal ion.…”

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].…”

Straightforward Synthesis of Substituted p‐Quinones: Isolation of a Key Intermediate and Use as a Bridging Ligand in a Diruthenium Complex

“…[2,3] Substituted p-quinones have been extensively used as ligands in coordination chemistry in recent years. [4][5][6] The remarkable properties that such ligands impart to their metal complexes make such compounds useful in a variety of fields, such as homogenous catalysis, [7] supramolecular chemistry, [5,8] coordination polymers, [9,10] and as bridging ligands in combination with redox active metal centers such as ruthenium. [11][12][13][14] The last field has gained tremendous attention in recent years because of ambiguities arising in oxidation-state formulations also with "organometallic-type" non-innocent ligands.…”