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

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)]PF(6) (1-3), [Cu(RL)(SO(4))] (4-6), and [Cu(RL)(NO(3))(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-H(py) 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-H(py) atoms), N.N (between the N(py) atoms), and C.C (between the 6-Me carbon atoms), the Cu-N(py), Cu-N(MeCN), and Cu-O bond distances, and the tilt of the pyridine rings. The copper coordination geometries in 4-6, where a SO(4) 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 NO(3) ligands coordinate in a monodentate mode. The structures of 1-9 in solution are investigated by means of the electronic, (1)H NMR, and ESR spectroscopy. The (1)H 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.

Reversible O2-Binding and Activation with Dicopper and Diiron Complexes Stabilized by Various Hexapyridine Ligands. Stability, Modulation, and Flexibility of the Dinuclear Structure as Key Aspects for the Dimetal/O2 Chemistry

“…[64][65][66] We found that copper complexes with sterically hindered tripyridine ligands can be structurally modulated by introducing an alkyl group at the bridgehead position of the ligand. 67 We prepared three sterically hindered tripyridine ligands: tris-(6-methyl-2-pyridyl)methane (H6M3t), 68 1,1,1-tris(6-methyl-2-pyridyl)ethane (Me6M3t), 67 and 1,1,1-tris(6-methyl-2-pyridyl)propane (Et6M3t). 67 The chemical structures of H6M3t, Me6M3t, and Et6M3t are shown in Chart 4.…”

“…67 We prepared three sterically hindered tripyridine ligands: tris-(6-methyl-2-pyridyl)methane (H6M3t), 68 1,1,1-tris(6-methyl-2-pyridyl)ethane (Me6M3t), 67 and 1,1,1-tris(6-methyl-2-pyridyl)propane (Et6M3t). 67 The chemical structures of H6M3t, Me6M3t, and Et6M3t are shown in Chart 4. Crystal structures of Cu I complexes with these ligands are shown in Fig.…”

“…We call this ''pyridine shift.'' 67 The pyridine shift between the Cu I complexes of H6M3t and Me6M3t is schematically drawn on the basis of the crystal structures in Scheme 5. The pyridine shift leads to subtle but significant changes in the copper coordination structures.…”

“…Electronic absorption and 1 H NMR spectra of the Cu I complexes are consistently explained by the crystal structures. 67 By using M6M4h (see Chart 3), which is a sterically hindered hexapyridine ligand methylated at the bridgehead positions, we succeeded in greatly improving the reversibility for the O 2 -binding of the -…”

“…5). 35 Though the overall structure of 5 35 is similar to that of the -2 : 2 -Cu 2 O 2 complex of H6M4h, 34 in the former, steric repulsion between the bridgehead methyl groups and 3-py H atoms causes a pyridine shift, 67 leading to significant structural differences. The pyridine shift enhances the steric hindrance of the 6-Me groups and thus causes an elongation 2 -Cu 2 O 2 complexes reported so far.…”