Characterization of the Structural and Electronic Properties of Spin-Coupled Dinuclear Copper(II) Centers by Proton NMR Spectroscopy

The H NMR spectra of a series of well-characterized μ-phenoxo and μ-alkoxo spin-coupled dicopper(II) complexes have been investigated. The complexes studied were [Cu(BPMP)(OH)] (1) (BPMP = 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol), [Cu(CHHXTA)(OH)] (2) (CHHXTA = N,N'-(2-hydroxy-5-methyl-1,3-xylylene)bis(N-carboxymethylglycine), [Cu(m-XYL)(OH)] (3) (m-XYL = 2,6-bis[[bis(2-pyridylethyl)amino]methyl]phenol), and [Cu(TBHP)(OAc)] (4) (TBHP = 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 NMR methods were used to facilitate the assignment of the hyperfine shifted H 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 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. 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 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.