Single-Electron Transfer in Palladium Complexes of 1,4-Naphthoquinone-Containing Bis(pyrazol-1-yl)methane Ligands

Abstract

A 1,4-naphthoquinone-substituted bis(pyrazol-1-yl)methane ligand (N--N) has been synthesized and transformed into its corresponding Pd(II) chelate complex [(N--N)PdCl(2)]. Both N--N and [(N--N)PdCl(2)] have been fully characterized by NMR spectroscopy, spectro-electrochemistry, and X-ray crystallography. After treatment of [(N--N)PdCl(2)] with NEt(3), the signature of a 1,4-naphthosemiquinonate radical is visible in the UV-vis- and electron paramagnetic resonance (EPR) spectrum of the reaction mixture; the free ligand N--N does not react with NEt(3) under the conditions applied. It is therefore concluded that NEt(3) first reduces the Pd(II)-ion of [(N--N)PdCl(2)] to the zero-valent state and that this reaction is followed by a single-electron transfer from the metal atom to the 1,4-naphthoquinone moiety. The complex has been specifically designed to disfavor any direct Pd-to-naphthoquinone coordination. Electron transfer thus proceeds through space or, less likely, via sigma-bonds of the ligand framework.

Chemical behavior of ortho-hydroquinone-based bis(pyrazol-1-yl)methane ligands in the presence of palladium(II) chloride

“…All PdeN bond lengths (Pd1eN2 .43 A to the centroid. In this context it should be noted that the same feature was also found in the related naphtobenzoquinonyl bis(pyrazol-1-yl)methane palladium dichloride complex [16].…”

(Acetonitrile-κN)chloridobis[2-(pyridin-2-yl)phenyl-κ2C1,N]iridium(III)

“…For our work on redox active ligands, see: Jä kle et al (1996); Guo et al (2001); Margraf et al (2006); Kretz et al (2006); Phan et al (2011);Scheuermann et al (2008Scheuermann et al ( , 2009); Blasberg et al (2010Blasberg et al ( , 2011. For the synthesis of the starting materials, see: Blasberg et al (2011);Lowry et al (2004).…”

“…Up to now we have applied electro-active ligands with poly(pyrazol-1-yl)borate (Jäkle et al, 1996;Guo et al, 2001), diimine (Margraf et al, 2006;Kretz et al, 2006;Phan et al, 2011) and bis(pyrazol-1-yl)methane (Scheuermann et al, 2008;Scheuermann et al, 2009;Blasberg et al, 2010) donor groups. So far, ferrocenyl and mainly para-quinonyl units (quinone is used if the oxidation state is not defined) were used as the redox-active element.…”

A Dicobalt Complex with an Unsymmetrical Quinonoid Bridge Isolated in Three Units of Charge: A Combined Structural, (Spectro)electrochemical, Magnetic and Spectroscopic Study

“…[1] Even thoughs imilar bridging ligands with a[ O,N,O,N] or a[ N,N,N,N] donors et ( Figure 1) have been knownf or decades, their systematic use in coordination and organometallic chemistry has only been exploredi nr ecenty ears. [2] Metal complexes of these ligands, that are related to their parent [O,O,O,O]a nalogue through the [NR] for [O] isoelectronic analogy,h ave been studied for their fascinating electronic struc-tures, [3] for their intriguing stoichiometric/catalytic reactivity [4] and for their potential as switchablem olecular materials. [5] In comparison to the metal complexes of the three classes of ligandsm entioned above,c omplexesw ith bridging quinones containing an unsymmetrically substituted [O,O,O,N] donor set ( Figure 1) are extremelyr are.…”