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Electrochemical Studies of Hexahapto-Dibenzo[A,E]Cyclooctatetraene Complexes of Chromiumtricarbonyl and Cationic ManganesetricarbonylWilliams, Jarquees 15 August 2014 (has links)
Electrochemical behavior of mono- and bimetallic chromiumtricarbonyl and cationic manganesetricarbonyl of fluxional dibenzo[a,e]cyclooctatetraene (DBCOT) complexes were studied via cyclic voltammetry over a range of scan rates (20 – 2000 mV/s) and temperatures (0 °C and 25 °C). The presented work displays electrochemical reduction mechanisms associated with eight-membered ring coordinated M(CO)3 systems that undergo rapid ring inversion in solution. The electrochemical studies of these complex systems exhibit comparitively similar behaviors, which suggest relatively undifferentiated mechanisms. Slight differences between the chromium and isoelectronic cationic manganese are seen in their chemical reactions in solution and the potential at which they reduce. The significance of the electrochemical studies of these complexes are justified by their potential contribution to nanotechnology considering the possibility of generating a cylindrical nanostructure containing the tub-shaped eight-membered ring ð-coordinated M(CO)3 system. The high probability of a haptotropic shift to the eight-membered ring upon reduction could prove to be beneficial to electrocatalysis.
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Studies on Hexahapto-Dibenzo[A,E]Cyclooctatetraene Complexes of Chromiumtricarbonyl and Cationic ManganesetricarbonylBandara, Nilantha 09 December 2011 (has links)
Mono- and dinuclear chromiumtricarbonyl and manganesetricarbonyl complexes of dibenzo[a,e]cyclooctatetraene (DBCOT) were synthesized and characterized. In the bis(chromiumtricarbonyl)DBCOT synthesis, the main product was the syn,anti isomer where the two Cr(CO)3 moieties coordinate to opposite faces of the DBCOT backbone. This complex exhibits three dynamic processes in solution. A ring inversion of the organic skeleton occurs while the two chromiumtricarbonyl moieties undergo tripod rotation. This is the first study where eight-membered ring inversion is studied for a pi-coordinated metallic system. The rate of inversion at various temperatures was determined by 1H NMR line shape analysis at two different field strengths (300 and 600 MHz). Compared with other cyclooctatetraene compounds reported, there is a large positive entropy of activation and a relatively high enthalpy of activation in this system. DFT calculations, using the B3LYP/6-31G** basis set, were performed to gain a better understanding of the experimental results. It is proposed that free rotation of both Cr(CO)3 groups in the planar transition state are responsible for the large entropy of activation for ring inversion in hexahapto,hexahapto-dibenzo[a,e]cyclooctatetraene-syn,anti-bis(tricarbonylchromium). The relatively large enthalpy of activation is due to a stabilizing interaction between the endo carbonyl groups on the syn-Cr(CO)3 and the remote arene. In the monometallic DBCOT complexes, the metal can either be inside or outside the tub conformation. Interestingly, the crystal structures show opposite orientations for the isoelectronic chromium and manganese systems. The Cr(CO)3 group is positioned anti relative to the DBCOT backbone while the Mn(CO)3+ is syn. It should be noted that the optimized gas phase geometries obtained through DFT calculations agree with the crystallographic results. Electrochemical studies were performed to investigate the change in redox behavior associated with coordination of mono and bis-chromium units to the DBCOT backbone. Four new organometallic crystal structures are reported in this dissertation. Different tripod orientations, DBCOT backbone angles, and metal orientation relative to the interior of the organic skeleton are found. C-H...O, C-H...pi, and charge assisted C-H...F hydrogen bonding interactions were observed in the solid state packing. The structural motifs found suggest these complexes could serve as organometallic synthons in supramolecular chemistry.
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