The Electronic and Solvatochromic Properties of [Co(L)(bipyridine)2]+ (L = o-catecholato, o-benzenedithiolato) Species: a Combined Experimental and Computational Study
Giacomo Cioncoloni, Hans M. Senn, Stephen Sproules, Claire Wilson and Mark D. Symes*  
WestCHEM, School of Chemistry, University of Glasgow, University Avenue, Glasgow, G12 8QQ, United Kingdom. *Email: mark.symes@glasgow.ac.uk

General Experimental Remarks: All solvents were obtained from Sigma Aldrich and used as supplied. 2,2′-bipyridine (≥99%), 5,5′-dimethyl-2,2′-bipyridine (98%), catechol (1,2-dihydroxybenzene, ≥99%), 1,2-benzenedithiol (96%), Co(NO3)2·6H2O (98%) and CoCl2·6H2O (98%) were supplied by Sigma Aldrich. Tetrabutylammonium hexafluorophosphate (TBA-PF6) (98%) was obtained from Alfa Aesar.
UV-Vis spectra were recorded on a JASCO V-670 spectrophotometer using 1 cm pathlength cuvettes. minute. Experiments performed at “room temperature” were carried out at 20 °C. Electrochemical experiments were performed as below. 
Electrochemical Methods: Electrochemical studies were performed in a single chamber cell in a three-electrode configuration using a CH Instruments CHI700 series potentiostat. The supporting electrolyte was 0.1 M TBA-PF6 in acetonitrile, unless otherwise noted. A large surface area strip of carbon felt (Alfa Aesar) was used as the counter electrode, along with an Ag/AgNO3 pseudo reference electrode. Potentials are reported relative to the ferrocene/ferrocenium couple, the position of which was judged by adding ferrocene to the samples analyzed. A boron-doped diamond disc electrode (area = 0.071 cm2, Windsor Scientific Ltd., UK) was used as the working electrode. Working electrodes were washed with acetone and deionized water prior to use. Cyclic voltammograms were collected at room temperature under an atmosphere of Ar at a scan rate of 100 mV/s. Measurements were conducted without stirring and with iR compensation enabled.
Computational Methods: All calculations were performed with the Gaussian 09 program. The structures of the complexes were optimized at the TPSS-D3/def2-TZVP, level in vacuum. We tested several other functionals (PBE-D3, M06-L, M06, ωB97-XD), both in vacuum and in water (using PCM), and found that TPSS-D3 gave the best agreement with experiment (metal–ligand bond lengths within 0.01 Å) and that solvation had a negligible effect on the structure (changes in metal–ligand bond lengths of ≤ 0.01 Å). We therefore used the TPSS-D3/def2-TZVP/vacuum structure in all subsequent calculations. TD-DFT was used to calculate vertical singlet excitation energies at the ωB97-XD/def2-TZVP/PCM level. The default IEF-PCM solvation model was used, which only includes electrostatic solvation effects; non-equilibrium solvation was used for excited states. NTO analyses were performed with Gaussian. Structures and orbitals were visualized with the program ChemCraft. The calculated S1 excitation energies in various solvents were found to be significantly overestimated (by ~0.6 eV) compared to the experimental band maxima. It is well-known that the positions of ligand-field excitations, particularly in first transition row complexes, are not predicted reliably by TD-DFT and are highly dependent on the chosen functional. Moreover, many such excitations have a certain amount of charge-transfer character, which is also problematic for TD-DFT. The latter problem is alleviated by using a long-range corrected functional like ωB97-XD. We did not attempt to identify the optimal functional for the systems under investigation as the deviations are systematic and constant for a given excitation and functional. Therefore, the shifts due to solvation and the nature of the excitations, which are the focus of the present work, can still be considered reliable.

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