Crystal structures and properties of some 4-substituted pyridine N-oxide copper (II) complexes ; The crystal and molecular structure of (+)-Dinitrobis (L-Arginine) Cobalt (III) nitrate dihydrate

Abstract

Single crystal studies of three 4-substituted pyridine N-oxide complexes of copper(II) chloride have been undertaken. The intensity data for each of the crystals reported were collected by counter methods. Two of the complexes have been determined to be oxygen bridged dimers. The structural features of these two complexes are used in discussing electronic factors affecting the superexchange mechanism which is operative in reducing the magnetic moments of many dimeric copper(II) complexes. The 2:1 complex of 4-methyl pyridine N-oxide and copper(II) chloride crystallizes in two distinct forms--a green complex and a yellow complex. The green complex belongs to space group P2_1/c. The structure was refined by least-squares methods to a conventional R factor of 8:7% for 687 observed intensities. The complex has a trans square-planar geometry about the copper ions. There are no close intermolecular approaches along the axial sites about the copper. The yellow complex belongs to space group P2_1/n. The structure has been refined by least-squares methods to a conventional R factor of 6.1% for 1469 observed intensities. The complex is composed of oxygen bridged dimers. There is no copper-chlorine bridging network present. The geometry about the copper ions is described as a distorted square-based pyramid with a chloride ion occupying the apical site [Cu-Cl bond length of 2.354(3)Å]. A loose oxygen bridging system within the dimer is indicated by a Cu-0 bond length of 2.153(6)Å. The 1:1 complex of 4-phenyl pyridine N-oxide and copper(II) chloride belongs to space group P1. The structure was refined by least-squares methods to a conventional R factor of 5.8% for 3648 observed intensities. The unit cell contains two crystallographically independent molecules. Both molecules are composed of oxygen bridged dimers. One dimer unit has a cis planar geometry about the copper ions. Associated with that dimer is a tight oxygen bridging system [Cu1-O1 = 1.978(5)Å and Cu-O1 = 1.986(4)Å]. The second dimer has a square-based pyramidal geometry about the copper ions with a water oxygen occupying the apical site at a distance of 2.356(6)Å. The oxygen bridging system in that dimer is not as tightly bound as in the first dimer (Cu2-O2 = 1.980(5)Å and Cu2-O2' = 2.040(6)Å]. The complex of (+)-dinitrobis(L-arginine)cobalt(III) nitrate dihydrate belongs to space group P1. The structure was solved using the Ps Patterson function and regular Fourier techniques and has been refined by least-squares methods to a conventional R factor of 7.6% for 1889 observed intensities. The two arginine ligands are bidentate, bonding to the cobalt through an amino nitrogen and carboxylate oxygen. The guanidyl group of arginine is not found in the coordination sphere of the cobalt. Anomalous dispersion techniques have been used to confirm the absolute configuration of the amino acid as L-arginine. The absolute configuration about the cobalt ion has been determined as the R configuration.