REMOTE SHIELDING OF THE C-N LIGAND FOR NUCLEAR-MAGNETIC-RESONANCE OF THE RU(BPY)(CN)(4)(2-) AND RU(BPY)(2)(CN)(2) COMPLEXES

Remote shielding effect of the CN ligand for H-1 NMR was investigated for the Ru(L) (CN)(4)(2-) and Ru(L)(2)(CN)(2) complexes where L is bpy (= 2,2'-bipyridine), 4,4'-dmbpy and 5,5'-dmbpy (dmbpy = dimethyl-2,2'-bipyridine). The 6,6' protons of bpy in Ru(bpy) (CN)(4)(2-), both of which are located over CN, show a downfield shift of 0.74 ppm (9.42 ppm for complex, 8.68 ppm for free ligand, in DMSO). For Ru(bpy)(2)(CN)(2), one of the 6,6' protons of bpy is located over CN and the other is over the pyridine ring of bpy. The proton which is located over CN shows a downfield shift of 0.94 ppm (9.62 ppm for the complex, in DMSO). This is in contrast with the upfield shift (of 1.19 ppm) of the other proton located over the pyridine ring caused by the aromatic ring current effect. The downfield shift of the proton located over the CN ligand is caused by the pi-electrons of the triple bond circulating around the symmetry axis, which induces the magnetic field that adds to the applied field over the CN bond.