A Copper(I) Oxygenation Precursor in the Entatic State: Two Isomers of a Copper(I) Compound of a Rigid Tetradentate Ligand

• Published in: Chemistry : a European journal 2000-03, Vol.6 (5), p.914-919
• Main Authors: Börzel, Heidi, Comba, Peter, Hagen, Karl S., Katsichtis, Charis, Pritzkow, Hans
• Format: Article
• Language: English
• Subjects: alterdentate ligand; copper; entatic state; oxygenations; preorganization; rigid ligands
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/(SICI)1521-3765(20000303)6:5<914::AID-CHEM914>3.0.CO;2-K

Oxygenation of [CuI(L1)(NCCH3)]+ (L1=dimethyl 2,4‐bis(2‐pyridinyl)‐3,7‐diazabicyclo‐[3.3.1]‐nonane‐9‐on‐1,5‐dicarboxylate) leads to a relatively stable μ‐peroxo‐dicopper(II) product. The stability of this type of oxygenation product has been shown before to be the result of the square pyramidal geometry of L1; preorganization by a dinucleating ligand has been shown to increase the stability of the μ‐peroxo‐dicopper(II) compound. The structural data presented here indicate that destabilization of the copper(I) precursor is another important factor. There are two isomers of [CuI(L1)(NCCH3)]+; one is yellow, and the other is red. X‐ray crystallography indicates that one pyridinyl donor is not coordinated in the yellow compound and that the red compound is 5‐coordinate. In the light of the X‐ray structure of the metal‐free ligand and that of the corresponding copper(II) compound, it emerges that the ligand cavity is well suited for copper(II), whereas the copper(I) compounds are highly strained. This is supported by 1H NMR spectra of the copper(I) species where a fast dynamic process leads to line broadening and by electrochemical data, which indicate that the copper(II) products are exceptionally stable. Also presented are structural (copper(II)), electrochemical, and spectroscopic data (1H NMR, copper(I)) of the derivative [Cu(L2)(X)]n+ with a methyl substituent at the α‐carbon atom of the two coordinated pyridinyl groups (L2=dimethyl 2,4‐bis(2‐pyridinyl‐6‐methyl)‐3,7‐diazabicyclo‐[3.3.1] ‐ nonane ‐ 9 ‐ on ‐ 1,5 ‐ dicarboxylate). There are two structural forms of [CuII(L2)(X)]n+ (X=NCCH3, Cl), which depend on the steric demand of the fifth donor X. For both, van der Waals repulsion leads to a destabilization of the copper(II) products, and this is also evident from an increase in the reduction potential (−110 mV vs. −477 mV, Ag/Ag3). Die Oxygenierung von [CuI(L1)(NCCH3)]+ (L1=Dimethyl 2,4‐bis(2‐pyridinyl)‐3,7‐diazabicyclo‐[3.3.1]‐nonane‐9‐on‐1,5‐dicarboxylat) liefert ungewöhnlich stabile μ‐peroxo‐dikupfer(II) Produkte. Diese Stabilität wurde in einer früheren Arbeit darauf zurückgeführt, dass L1 eine quadratisch pyramidale Koordinationsgeometrie erzwingt; durch Ligandenpräorganisation für eine zweikernige Einheit kann die Stabilität noch weiter erhöht werden. In der vorliegenden Arbeit wird gezeigt, dass die Destabilisierung des Kupfer(I) Edukts ein weiterer entscheidender Faktor ist. Von [CuI(L1)(NCCH3)]+ existieren zwei Isomere, ein gelbes, bei dem nur ein