Structural Modulation of Cu(I) and Cu(II) Complexes of Sterically Hindered Tripyridine Ligands by the Bridgehead Alkyl Groups

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Abstract
Structures of Cu(I) and Cu(II) complexes of sterically hindered tripyridine ligands RL = tris(6-methyl-2-pyridyl)methane (HL), 1,1,1-tris(6-methyl-2-pyridyl)ethane (MeL), and 1,1,1-tris(6-methyl-2-pyridyl)propane (EtL), [Cu(RL)(MeCN)]PF6 (1−3), [Cu(RL)(SO4)] (4−6), and [Cu(RL)(NO3)2] (7−9), have been explored in the solid state and in solution to gain some insights into modulation of the copper coordination structures by bridgehead alkyl groups (CH, CMe, and CEt). The crystal structures of 1−9 show that RL binds a copper ion in a tridentate facial-capping mode, except for 3, where EtL chelates in a bidentate mode with two pyridyl nitrogen atoms. To avoid the steric repulsion between the bridgehead alkyl group and the 3-Hpy atoms, the pyridine rings in Cu(I) and Cu(II) complexes of MeL and EtL shift toward the Cu side as compared to those in Cu(I) and Cu(II) complexes of HL, leading to the significant differences in the nonbonding interatomic distances, H···H (between the 3-Hpy atoms), N···N (between the Npy atoms), and C···C (between the 6-Me carbon atoms), the Cu−Npy, Cu−NMeCN, and Cu−O bond distances, and the tilt of the pyridine rings. The copper coordination geometries in 4−6, where a SO4 ligand chelates in a bidentate mode, are varied from a square pyramid of 4 to distorted trigonal bipyramids of 5 and 6. Such structural differences are not observed for 7−9, where two NO3 ligands coordinate in a monodentate mode. The structures of 1−9 in solution are investigated by means of the electronic, 1H NMR, and ESR spectroscopy. The 1H NMR spectra show that the structures of 1−3 in the solid state are kept in solution with rapid coordination exchange of the pyridine rings. The electronic and the ESR spectra reveal the structural changes of 5 and 6 in solution. The bridgehead alkyl groups and 6-Me groups in the sterically hindered tripyridine ligand play important roles in modulating the copper coordination structures.