Characterization of the Structural and Electronic Properties of Spin-Coupled Dinuclear Copper(II) Centers by Proton NMR Spectroscopy
The 1H 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 [Cu2(BPMP)(OH)]2+ (1) (BPMP = 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol), [Cu2(CH3HXTA)(OH)]2- (2) (CH3HXTA = N,N ‘-(2-hydroxy-5-methyl-1,3-xylylene)bis(N-carboxymethylglycine), [Cu2(m-XYL)(OH)]2+ (3) (m-XYL = 2,6-bis[[bis(2-pyridylethyl)amino]methyl]phenol), and [Cu2(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) 1H NMR methods were used to facilitate the assignment of the hyperfine shifted 1H 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 1H 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 T11/6. 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 1H 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.