Two-Dimensional (1)H NMR Studies on Octahedral Nickel(II) Complexes

The dinucleating ligand ethylene glycol-bis(beta-aminoethyl ether) N,N,N',N'-tetrakis(2-(1-ethylbenzimidazoyl)] (EGTB-Et; 1) was used to synthesize the dinuclear Ni(II) tetraacetonitrile complex cation Ni(2)(EGTB-Et)(CH(3)CN)(4)](2+) (2): triclinic space group P&onemacr; (a = 12.273(5) ?, b = 12.358(7) ?, c = 12.561(6) ?, alpha = 90.43(4) degrees, beta = 110.26(3) degrees, gamma = 99.21 (4) degrees, and Z = 1). The structure shows two identical octahedral Ni(II) centers each bound to two benzimidazole ring nitrogen atoms, one amine nitrogen atom, an ether oxygen atom, and two acetonitrile nitrogen atoms. The Ni(II) ions are tethered together by a diethyl ether linkage with a crystallographic center of inversion between the methylene carbons of this bridge. The Ni--Ni separation in 2 is 7.072 ?. The mononuclear Ni(II) complex cation Ni(Bipy)(2)(OAc)](+) (3) (Bipy = bipyridine) was synthesized and crystallographically characterized: monoclinic space group P2(1)/c (a = 9.269(4) ?, b = 8.348(4) ?, c = 14.623(7) ?, and beta = 102.46(4) degrees, Z = 2). The Ni(II) ions in 3 adopts a distorted octahedral geometry and is bound to four bipyridine ring nitrogen atoms and two carboxylate oxygen atoms. The average Ni-N and Ni-O distances are 2.062 and 2.110 ?. The electronic absorption spectra of both 2 and 3 were recorded in acetonitrile solution and are consistent with octahedral coordination geometries about the Ni(II) ions with Racah parameters of 840 and 820 cm(-)(1), respectively. Both one- and two-dimensional (1)H NMR techniques were used to assign the observed hyperfine shifted (1)H NMR resonances of 2 and 3 in acetonitrile solution. Clear COSY cross signals are observed between the aromatic protons of both the benzimidazole and pyridine protons of 2 and 3, respectively. The use of 2D NMR methods to assign inequivalent aromatic protons rather than synthetic methods such as substitution or deuteration are discussed.