Characterization of the structural and electronic properties of spin-coupled dinuclear copper(II) centers by proton NMR spectroscopy

The H-1 NMR spectra of a series of well-characterized mu-phenoxo and mu-alkoxo spin-coupled dicopper(II) complexes have been investigated. The complexes studied were [Cu-2(BPMP)(OH)](2+) (1) (BPMP = 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol), [Cu-2(CH(3)HXTA)(OH)](2-) (2) (CH(3)HXTA = N,N'-(2-hydroxy-5-methyl-1,3-xylylene)bis(N-carboxymethylglycine), [Cu-2(m-XYL)(OH)](2+) (3) (m-XYL = 2,6-bis[[bis(2-pyridylethyl)amino]methyl]phenol), and [Cu-2(TBHP)(OAc)](2+) (4) (TBHP = N,N,N',N'-tetrakis[(2-benzimidazolyl)methyl]-2-hydroxy-1,3-diaminopropane). The magnetic interactions of these complexes range from strongly antiferromagnetically to weakly ferromagnetically coupled. Both one- and two-dimensional (COSY) H-1 NMR methods were used to facilitate the assignment of the hyperfine shifted H-1 NMR signals of each complex. COSY experiments provide clear cross signals for resonances < 200 Hz wide. These data have facilitated the assignment of the hyperfine shifted H-1 NMR signals and have verified that the solid state structures exist in solution for each system studied. Assuming a paramagnetic dipolar relaxation mechanism, the crystallographically determined Cu-H distance (r) is proportional to T-1(1/6). All calculated Cu-H distances for 1-4 are within ca. 20% of the Cu-H distances derived from X-ray crystallography. These data indicate that a paramagnetic dipolar relaxation mechanism is the dominant proton relaxation pathway for spin-coupled dicopper(II) centers. Our results indicate that H-1 NMR spectroscopy is an excellent tool with which to probe the solution structures of spin-coupled dicopper(II) centers in model complexes as well as biological systems.