2,2′-Bipyridines carrying a diazo moiety and their complexes with bis(hexafluoroacetylacetonato)copper: synthesis, structures, electronic and magnetic properties of their photoproducts in frozen solutions

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Abstract
4-(α-Diazobenzyl)-2,2'-bipyridine 2, diazodi{4-(2,2'-bipyridyl)}methane 3, and 1,3-benzenediyl{4-(4'-methyl-2,2'-bipyridyl)diazomethane} 4 were prepared as new types of spin sources/magnetic couplers. The molecular structures of bipyridine ligands, 2, 3, 4, and [Cu(hfac)2·2] were revealed by X-ray analyses. Photolysis of 2, 3, 4, and their complexes with Cu(hfac)2 in MTHF frozen solutions at cryogenic temperature was followed by UV–Vis and EPR spectroscopy. UV–Vis and EPR spectra after photolysis showed strong two absorptions; at 508 and 471 nm for 2, and 511 and 476 nm for 3, and characteristic sets of EPR signals due to the triplet states; |D/hc| = 0.418, and 0.436 cm−1 and |E/hc| = 0.021 and 0.022 cm−1 for 2 and 3, respectively. Diazo compound 4 after photolysis also showed absorptions at 506 and 471 nm and EPR signals characteristic of a quintet. Their complexes with Cu(hfac)2 showed a red shift of 3–14 nm in the visible spectra and gave EPR spectra characteristic of the high-spin complexes under similar conditions. Magnetic susceptibility measurements on a SQUID magnetometer were carried out using a sample similar to the one employed in the UV–Vis and EPR studies. The field dependence of magnetization in frozen solution suggested that the ground-state spin multiplicities of the photoproducts of 2, 3, and 4 were triplet, triplet, and quintet, respectively. Similarly, those of their complexes with Cu(hfac)2 were quartet, quintet, and septet, respectively.

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