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

4-(α-Diazobenzyl)-2,2′-bipyridine 2, diazodi{4-(2,2′-bipyridyl)}methane 3, and 1,3-benzenediyl{4-(4′-methyl-2.2°prime;-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.