Crystallographic and copper-63,65 nuclear quadrupole resonance study of two-co-ordinated copper(I) cations with hindered substituted pyridine ligands

Abstract

The crystal structures and s3.s5Cu NOR spectra are reported of the two-co-ordinated complex cations formed between Cu' and hindered pyridine homologues, [CuLJX [L = 2,6-dimethylpyridine, X = BF,-1, PF,-2 or CI0,-( a phase) 3, (p phase) 4, (y phase) 5; L = 2,4,6-trimethylpyridine, X = BF,-6 or CuCI,-71. All complexes have essentially linear N-CU-N fragments, but whereas for 1, 2, 4 and 6 the copper atom is located on a two-fold axis and the two pyridine rings have dihedral angles of 60.4, 56.0, 58.1 and 51.3" respectively, 3, 5 and 7 have essentially planar structures with dihedral angles of 0, 3.0 and 0" respectively. In agreement with a previous study of the planar complex 3, the CU-N bond is significantly longer than in the non-planar conformation, and the corresponding 63,65Cu NOR resonance frequencies are less.

63Cu NQR Studies of Two-and Three-coordinated Complexes of Cu(I) with Siloxyl Ligands

“…The only previous measurement for an oxygen-coordinated compound of Cu(I) is that of Cu 2 0 -probably the first Cu(I) compound studied by NQR -where each copper atom is also linearly bonded to two oxygen atoms, and the resonance frequency (26.697 MHz at 87 K, [3]) is noticeably lower than those reported here. In accord with the trend that was found for the two-or three-coordinated cations with pyridine ligands [4,5] or the polyhalocuprate anions [6,7], the resonance frequencies of the three-coordinated species are significantly higher than those of the two-coordinated complexes. The observation of two resonance frequencies for Ia and a single resonance frequency for lib is in accord with their published crystal structures [2].…”

“…In particular we were unable to observe resonances from the two-coordinated complexes, (C 6 H 5 COOCu) 4 and ('BuOCu) 4 , and the three-coordinated complexes, (Ph 3 P) 2 CuOOCR and (RC0CH = C(0)R)CuMe 3 P, where the oxygen atom is bonded to carbon.…”

63Cu NQR spectra of dicoordinated Cu(I) cations with imidazole and pyrazole ligands

“…For the pyridine series, where a necessary condition for the formation of a dicoordinated species is that both ortho positions carry substituents, the Cu-N bond length can be varied to some extent by the relative orientation of the pyridine rings that modifies the steric interactions between these substituents, we have shown that there is a good correlation between the 63Cu NQR frequency and the Cu-N bond length [7]. If this correlation were to persist in the imidazole and pyrazole series, where there are no substituents adjacent to the coordinating nitrogen atom, then it would indicate that the shorter bond length can be ascribed to a decreased steric hindrance and that the marked difference in reactivity between the two series of complexes may not be due to any fundamental difference between the metal-ligand bonding in these two series of complexes but be only a consequence of the closer interaction between the ligand and the metal atom.…”

“…Only 1, 3, 6 and 9 are of known structure [1][2][3], all with an almost linear N-Cu-N geometry, with two almost coplanar five-membered rings and with two slightly different Cu-N bond lengths falling in the range 1.879-1.855. The frequencies of the analogous pyridine complexes fall in the range 39.1-41.9 MHz and the correlation equation [7] between the NQR frequency at room temperature, uRT, and the bond length Rcu N is given by ugx = 157.66-61.136Rcu N This equation applied to 1, 3, 6 and 9 predicts resonance frequencies of 44.0, 43.6, 43.0 and 43.5MHz respectively, differing by 4.0, 4.1, 4.5 and 2.6MHz from the observed frequencies. On the other hand the seven frequencies of the pyridine complexes fit the above equation with a maximum deviation of 0.33 MHz.…”

“…complexes is that the N-Cu bond lengths (1.85-1.88A) in the pyrazole and imidazole complexes [1][2][3] are significantly less than those (1.90-1.94A) of the analogous pyridine complexes [4][5][6][7]. For the pyridine series, where a necessary condition for the formation of a dicoordinated species is that both ortho positions carry substituents, the Cu-N bond length can be varied to some extent by the relative orientation of the pyridine rings that modifies the steric interactions between these substituents, we have shown that there is a good correlation between the 63Cu NQR frequency and the Cu-N bond length [7].…”