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Title: Synthesis and characterization of chromium(II) complexes of macrocyclic and related nitrogen-donor ligands
Author: Jubb, Jayne
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 1990
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The stacking square planar complex [Cr(thiourea)2(NCS)2], previously determined by X-ray crystallography and discussed in an appended publication, was found to be non-conducting by measurements on both single crystals and a disc of the compound. A series of chromium(II) complexes was prepared using a variety of nitrogen-donor ligands, selected to provide sufficient rigidity to encourage a square planar geometry. These were characterized using diffuse reflectance and infrared spectroscopy, and magnetic susceptibility measurements over the range 293-87 K. Several of these may in fact be square planar, though suitable crystals have yet to be isolated. When attempts to synthesize the chromium(II) complex of 7,16-dihydro-5,7,12,14-tetramethyl-2,3:9,10-dibenzo-1,4,8,11 -tetraazacyclotetradecane-4,6,11,13-tetraene (tmtaaH2) by template methods failed, a chromium(II)-o-phenylenediamine (OPD) intermediate was sought around which a Schiff-base condensation might occur. This approach yielded instead a series of chromium(II)-OPD complexes which were subsequently characterized by diffuse reflectance and infrared spectroscopy, and magnetic susceptibility measurements over the range 293-87 K. The denticity of the OPD was investigated using the above methods, particularly infrared, and the results were confirmed in the cases of [Cr(OPD)2(NCS)2] and [Cr(OPD)4](CF3SO3)2 by X-ray crystallographic studies. [Cr(OPD)4](CF3SO3)2 is the first crystallographic demonstration of Jahn-Teller distortion in a chromium(II) complex with an N6 donor set. The findings that OPD can act as both a mono- and a bi-dentate ligand, even within the same complex, is consistent with earlier literature studies of other first row transition metal complexes of OPD. After metal insertion reactions into the free tmtaaH2 ligand did not produce Crtmtaa, efforts were switched to the method using butyllithium and anhydrous chromium(II) acetate. This yielded the desired Crtmtaa complex. Room temperature magnetic susceptibility measurements revealed the compound to be virtually diamagnetic and dimeric, confirming the ciystallographic results published by another group during the course of these studies. Proton NMR measurements indicated the compound to be truly diamagnetic. Following the success of this approach, a related chromium(II) complex of the ligand 6,13-Diphenyl-5,7,12,14-tetrahydro-2,3:9,10-dibenzo-1,4,8,11-teraazacyclotetradecane-4,6,11,13-tetraene, (dptaaH2) was synthesized and characterized using infrared and UV-visible solution spectroscopy, and magnetic susceptibility measurements over the range 293-87 K. The latter revealed the chromium(II) complex to be low-spin and monomeric. Single crystal X-ray crystallographic studies carried out on both the free ligand, dptaaH2, and the cobalt(II) complex, Codptaa, revealed them to be substantially less puckered than their tmtaa analogues. This has been ascribed to minimal steric interaction between the vinylic protons in dptaaH2 and Codptaa with the benzenoid and phenyl rings, contrasting the large interaction between the methyl groups and benzenoid rings in tmtaa complexes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available