Doubly pyridazine-bridged macrocyclic complexes of copper in +1, +2 and mixed valent oxidation states

Abstract

Transmetallation of the dilead(II) complex [Pb 2(L1′)](ClO 4) 4 [L1′ is the (4+4) Schiff-base macrocycle derived from 3,6-diformylpyridazine and 1,3-diaminopropane] with copper(II) perchlorate results in the formation of a dicopper(II) complex of the (2+2) Schiff-base macrocycle L1, Cu II 2(L1)(ClO 4) 4 ( 1). Copper(II) is unable to template the formation of 1 from the organic precursors. A series of six dicopper(II) complexes of L1 has been prepared from 1, including: [Cu II 2(L1)X 2](ClO 4) 2 (where X=Cl − 2, Br − 3, I − 4, NCS − 5), Cu II 2(L1)(H 2O) 2(NO 3) 2(ClO 4) 2 ( 6) and Cu II 2(L1)(H 2O) 2(ClO 4) 4 ( 7). Three of these dicopper(II) complexes, 1 2MeCN, 2 H 2O and 7, have been characterised by X-ray crystallography. In all three cases the (2+2) macrocycle provides a double pyridazine bridge between the two copper(II) ions. Magnetic studies show that the double pyridazine bridge mediates strong antiferromagnetic exchange between the copper(II) ions in all of these complexes (−2 J=412 to 532 cm −1). Electrochemical and spectroelectrochemical studies reveal that reduction of the dicopper(II) complexes occurs in two one electron steps, via stable mixed valent intermediates ( K c=3.8×10 5 to 8.6×10 6 in acetonitrile, MeCN), in marked contrast to all previously studied pyridazine-bridged dicopper complexes. The thiocyanate salt of the mixed valent complex, [Cu IICu I(L1)(NCS) 4Cu I]MeCN ( 9), has been isolated, by transmetallation of [Pb 2(L1′)](ClO 4) 4 with copper(I) followed by the addition of NaNCS, and structurally characterised. Finally, the previously reported grid complex, [Cu I 4(L1) 2](PF 6) 4 ( 10) [formed by templating L1 formation on copper(I) ions], is shown by NMR spectroscopy to exist in equilibrium with another species, presumed to be a dicopper(I) complex, [Cu I 2(L1)(MeCN) 2](PF 6) 2 ( 11), in acetonitrile solution. In support of this assignment, the structure of the dicopper(I) complex, [Cu I 2(L1)(PPh 3) 2](PF 6) 2 ( 12), isolated from reaction of 10 with two equivalents of PPh 3, is reported.