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Electrochemistry
Cyclic voltammograms (CV’s), square wave voltammograms (SW’s) and linear sweep voltammograms (LSV’s) were recorded on a Princeton Applied Research PARSTAT 2273 voltammograph running PowerSuite (Version 2.58). All experiments were performed in a dry three-electrode cell. A platinum wire was used as auxiliary electrode while a glassy carbon working electrode (surface area 3.14 mm2) was utilized after polishing on a Buhler polishing mat first with 1 micron and then with ¼ micron diamond paste. A silver wire was used as pseudo internal reference under an argon atmosphere inside an M Braun Lab Master SP glovebox filled with high purity argon (H2O and O2 < 5 ppm). All electrode potentials are reported using the potential of the ferrocene/ferrocenium redox couple [FcH/FcH+] (FcH = ( 5-C5H5)2Fe, Eo¢ = 0.00 V) as reference.23 However, decamethyl ferrocene, Fc*, was used as internal standard to prevent signal overlap with ferrocenyl of 1 and 2. Decamethylferrocene has a potential of -550 mV versus free ferrocene with E = 72 mV and ipc/ipa = 1 under the conditions employed.24 Analyte solutions (0.5 mmol dm-3) were prepared in dry CH2Cl2 in the presence of 0.1 mol dm-3 [(nBu4)N][PF6]. Analyses were performed at 20 oC. Data were exported to a spread sheet program for manipulation and diagram preparation (Fig. 2.3-2.5).
Computational details
Geometry optimizations without symmetry constraints were carried out using the Gaussian09 suite of programs.25 Electron correlation was partially taken into account using the hybrid functional denoted as B3LYP26 (and uB3LYP for radical cations) in combination with double- quality plus polarization def-SVP27 basis set for all atoms (this level is denoted B3LYP/def2- SVP). Calculation of the vibrational frequencies28 at the optimized geometries showed that the compounds are minima on the potential energy surface.
Spectroscopy
Electronic effects of the carbene substituents can be followed in solution by both NMR and IR spectroscopy. Since Ha (see atom numbering in Scheme 2.1) is the position closest to the site of coordination of the carbene carbon atom, the chemical shift of this proton is influenced most and is a sensitive probe for electronic ring substituent involvement. Significant downfield shifts of Ha were observed in the 1H NMR spectra for 1 – 6 (see Table 2.1), compared to free ferrocene (4.19 ppm) and thiophene (7.20 ppm). This is consistent with the electron-withdrawing effect of the metal carbonyl fragment bonded to the carbene ligand, comparable, for example, to an ester functionality,31 as well as the -delocalization of the (hetero)aryl rings towards stabilizing the electrophilic carbene carbon atom (Fig. 2.1). Less ring-involvement of the thienyl substituent is seen for both aminocarbene complexes 4 and 6, as reflected by the higher field Ha resonances.
For 4 and 6, a duplication of all the resonances is also observed in both the 1H and 13C spectra. This duplication is due to the formation of two different isomers of 4, rotamers A (anticonfiguration) and C (syn-configuration) (Scheme 2.2) with restricted rotation around the Ccarbene-N bond.32 For 6, up to three different isomers could be distinguished via NMR spectroscopy. These were ascribed to three different biscarbene complex isomers; in one case, a syn,syn-configuration for both carbene ligands, ( syn,syn-isomer, Experimental section), another where both carbene ligands have anti-configuration (anti,anti-isomer, Experimental section), and finally, the biscarbene complex featuring one carbene ligand with syn-, the other ligand with anti-configuration (syn,anti-isomer, Experimental section). The syn,anti-isomer displays two sets of signals for all observed signals, corresponding to the presence of two different carbene ligands within the molecule. Increased electron donation from the nitrogen lone pair towards the carbene carbon atom results in a Ccarbene-N bond order greater than one. The bonding situation is best described as an intermediate between the zwitterionic isomers A, C and the neutral carbene B. The carbene carbon resonances obtained from the 13C NMR spectra reflect this marked contribution from the carbene heteroatom substituent, -OEt vs –NHBu. In the case of 4 and 6, upfield shifts for the carbene carbon atom (271.9, 260.9ppm for 4, and 268.8, 261.3, 261.0 and 258.1, respectively, for the three isomers of 6), compared to 3 and 5 (316.4 and 321.9ppm, respectively).
Chapter 1: Introduction
Chapter 2: Electrochemical illumination of thienyl and ferrocenyl chromium(0) Fischer carbene complexes
Chapter 3: Substituent effects on the electrochemical, spectroscopic and structural properties of Fischer mono- and biscarbene complexes
Chapter 4: Synthesis and electrochemical investigation of ferrocenyl aminocarbene chromium(0) complexes
Chapter 5: An electrochemical an computational study of tungsten(0) ferrocenium species and intramolecular electronic interactions
Chapter 6: Metal-metal interaction in Fischer carbene complexes-A study on ferrocenyl and biferrocenyl tungsten alkylidene complexes
Chapter 7: Thiophenes modified by tungsten Fischer carbenes-Synthesis, solid state structure and electrochemical investigations
Chapter 8: Redox behavior of cymantrenyl Fischer carbene complexes in designing organometallic multi-tags
Chapter 9: Fischer-type gold(I) carbene complexes stabilized by aurophilic interactions
Chapter 10: Conclusion
