A copper(I) oxygenation precursor in the entatic state: Two isomers of a copper(I) compound of a rigid tetradentate ligand

Oxygenation of [CU1(L-1)(NC-CH3)](+) (L-1) = dimethyl 2,4-bis(2-pyridinyl) -3,7-diazabicyclo-[3.3.1]-nonane-9-on-1,5-dicarboxylate) leads to a relativ ely stable mu-peroxo-dicopper(II) product. The stability of this type of ox ygenation product has been shown before to be the result of the square pyra midal geometry of L-1; preorganization by a dinucleating ligand has been sh own to increase the stability of the mu-peroxo-dicopper(II) compound. The s tructural data presented here indicate that destabilization of the copper(I ) precursor is another important factor. There are two isomers of [Cu-1(L-1 )(NCCH3)](+); one is yellow, and the other is red. X-ray crystallography in dicates 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 struct ure of the metal-free ligand and that of the corresponding copper(II) compo und, it emerges that the ligand cavity is well suited for copper(II), where as the copper(I) compounds are highly strained. This is supported by (HNMR) -H-1 spectra of the copper(I) species where a fast dynamic process leads to line broadening and by electrochemical data, which indicate that the coppe r(II) products are exceptionally stable. Also presented are structural (cop per(II)), electrochemical, and spectroscopic data (H-1 NMR, copper(I)) of t he derivative [(Cu(L-2)(X)](n+) with a methyl substituent at the alpha-carb on atom of the two coordinated pyridinyl groups (L-2 = dimethyl 2,4-bis(2-p yridinyl-(6-methyl)-3,7-diazabicyclo-[3.3.1]-nonane-9-on-1,5-dicarboxylate) . There are two structural forms of [(CuL2)-L-II) (X)](n+) (X = NCCH3, Cl), which depend on the steric demand of the fifth donor X. For bath, 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 v s. -477 mV, Ag/AgNO3).