Copper(I) and silver(I) complexes of a new tetrahedrally-enforcing ligand containing two bipyridyl binding sites linked by a diphenyl disulfide bridge

Abstract

Conversion of the amino group of 6-(2-aminophenyl)-2,2Ј-bipyridine to a thiol resulted not in the expected N,N,S-terdentate chelating ligand HL [6-(2-sulfhydryl)-2,2Ј-bipyridine] but in the oxidised disulfide 2,2Ј-bis-[6-(2,2Ј-bipyridyl)]diphenyldisulfide, L᎐L. This ligand contains two bipyridyl arms linked by a diphenyl disulfide bridge in such a way that they cannot be coplanar, but tend to co-ordinate to a single metal ion with a substantial dihedral angle between the two ligand planes giving a pseudo-tetrahedral geometry. The crystal structure of L᎐Lؒ0.5C 6 H 5 CH 3 shows the presence of near-linear N ؒ ؒ ؒ S᎐S interactions (non-bonded N ؒ ؒ ؒ S; 2.75 Å) in which the pyridyl lone pair interacts with the σ* orbital of the disulfide fragment. The complexes [Cu(L᎐L)][PF 6 ] and [Ag(L᎐L)][PF 6 ] were prepared and are isostructural, both having a distorted four-co-ordinate metal co-ordination environment in which the two chelating bipyridyl fragments have dihedral angles of between 70 and 80 ЊC between them (i.e. nearly mutually perpendicular). The electronic spectrum of the Cu I complex [Cu(L᎐L)][PF 6 ], and the EPR spectrum of its one-electron oxidised product [Cu(L᎐L)] 2ϩ , both confirm that the rigidity of the ligand donor set does not permit much variation in the co-ordination environment about the copper ion. The ligand therefore imposes a relatively rigid pseudo-tetrahedral geometry on the metal centres.

3,11,19‐Trithia[3.3.3]pyridinophane: Structural Diversity in Its Transition Metal Complexes

“…The Cu I ion is in a tetrahedral environment with N−Cu−N angles ranging from 119.87(2) to 120.81(2)° for the eight‐membered rings and S−Cu−N angles ranging from 85.64(1) to 87.09(1)° for the five‐membered rings. The Cu−N and Cu−S distances are in the expected range 40−43. In the solid‐state, the molecules are not discrete but form a network structure through some hydrogen bonding and weak interactions.…”

Spectroscopic and structural properties of complexes of 3,3′-bis(2-benzimidazolyl)-2,2′-bipyridine with copper(I) and silver(I)

“…Invariably, the ligand requires tetrahedral geometry not only with d 10 metal ions, such as Cu(I), Ag(I) and Zn(II) [18], but also with d 7 ion, such as Co(II) [12], and with d 9 ion, such as Cu(II) [17], with comparatively larger intra-ligand angle due to flattening of the MN 4 unit akin to those of structurally similar 1 : 2 complexes of 2,2′-bis(2-imidazolyl) biphenyl [12]. The Cu-N bim distances, Cu (1) (3) Å, and are larger than those in 1, consistent for pseudo-tetrahedral geometries [13]. The differences in M − N distances are attributable primarily to difference in ionic radii of the metals.…”

“…As shown in figure 7, the potential of the Cu I -Cu II couples are at rather anodic potentials compared to those of the parent unsubstituted ligands due to steric hindrance in the ligand, imposing a tetrahedral geometry on the metal, and from the electrochemistry results, L confers similar properties on the metal center to ligands, such as 2,9-dimethyl-1,10-phenanthroline [26], 6,6′-disubstituted-2,2′-bipyridine [27], and related compounds [3]. This type of ligands tend to stabilize pseudo-tetrahedral Cu I and to destabilize planar Cu II complexes [13], consistent with the intra-ligand angles for the complexes of L with Cu(I) and Cu(II) [17]. These results obviously show that sterically hindered ligands, such as L, stabilize the tetrahedral geometry in the solid state and also in solution.…”

“…Thus, upon oxidation of Cu(I) to Cu(II), a large structural change occurs; consequently, imposition of a particular geometry on the metal allows control of the copper(I)-copper(II) redox couple [15]. Systems for which this process does not occur [10,11,13,16] are very rare [14]. Among the tetrahedrally enforcing ligands, 3,3′-bis(2-benzimidazolyl)-2,2′-bipyridine (L) [17] incorporating two benzimidazole units linked together through 2,2′-bipyridine is an optimal choice for achieving a tetrahedral environment.…”

“…Sterically hindered ligands which can impose a pseudo-tetrahedral geometry on metal ions are of interest for selective metal-ion extraction [1], models for mononuclear copper proteins [2][3][4][5], and study of the relationship between structure and properties of transitionmetal complexes [6][7][8][9][10][11][12][13]. Copper(I), with the d 10 electronic configuration, prefers tetrahedral coordination, whereas Cu(II), with d 9 electronic configuration, adopts five-or six-coordination [14].…”