Theoretical and experimental studies on intermetal communication in dinuclear complexes of ruthenium with conjugated bridging ligands
Date of Award
Doctor of Philosophy (PhD)
Dinuclear, Ruthenium, Conjugated bridging, Ligands
Ruthenium hydride addition to alkynes to give to alkenyl complexes, [RuH(CO)Cl(PR 3 ) 2 (CH=CHR)] was used to prepare conjugated diruthenium complexes. Addition of RuH(CO)Cl(PR 3 ) n = 2, 3 to 1,3-butadiyne and 1,4-diethenylbenzene gave the complexes [Ru(CO)Cl(PR 3 ) 2 ] 2 (µ-CH=CH-X-CH=CH) where R = P(iPr) 3 or PPh 3 and X = nothing or C 6 H 4 . These symmetrical, dinuclear metal complexes undergo one-electron oxidation to give rise to mixed-valence radical cations and then dications. These compounds were analyzed by various techniques such as Near-IR, UV-Visible, and EPR spectroscopies, cyclic voltammetry and theoretical methods.
Compounds with P(iPr) 3 as the phosphine ligand behave as completely delocalized class III mixed-valence compounds, as confirmed by spectroscopy, where the electron is spread throughout the molecule. The coupling parameter H ab , however, decreases from 0.87 eV (X = nothing) to 0.5 eV (X = C 6 H 4 ) when the length of the bridge is increased. For the PPh 3 compounds, conclusive results could not be obtained. The results for the compound with the longer bridge were affected by coordination of excess PPh 3 at low temperatures. This compound did not give good spectra when excess phosphine was removed.
In order to evaluate spectroscopic results for [Ru(CO)Cl(PPh 3 ) 2 ] 2 (µ-CH=CH-C 6 H 4 -CH=CH), the coordination behavior of PPh 3 at low temperatures had to be understood. To study this coordination behavior, an analogous monoruthenium complex [Ru(CO)Cl(PPh 3 ) 2 ](CH=CHC 6 H 5 ) was studied. Variable temperature 31 P and 1 H NMR were performed at seven different temperatures in the presence of PPh 3 and showed that the PPh 3 bound species at low temperature was highly favored. From the NMR data, thermodynamic and kinetic data were determined for the coordination process. The [Delta]H and [Delta]S values for the process of coordination were found to be -17.6 kcal/mol and -58.2 e.u and the [Delta]H [double dagger] and [Delta]S [double dagger] values for decoordination were 20.5 kcal/mol and 38.9 e.u.
A theoretical study was also undertaken on simplified but similar molecules in neutral, cationic and dicationic states. The DFT calculated vibrational frequencies showed good agreement with the experimental ones and the calculated electronic spectra showed reasonable correlation with experimental spectra. DFT computations were also consistent with strongly delocalized structures.
Synthesis of a jumper cable molecule was attempted. The bridging ligand, a tetra substituted benzdiimidazole molecule, was synthesized. This molecule was synthesized from relatively inexpensive starting materials in six steps and in two steps from a known compound. The intermediate products were easy to isolate and required minimal or no purification. The introduction of substituents was done only in the second to last step, making the synthesis a general one. The attachment of the bridging ligand to the ruthenium metal was unsuccessful so far.
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Seetharaman, Sripriya K., "Theoretical and experimental studies on intermetal communication in dinuclear complexes of ruthenium with conjugated bridging ligands" (2006). Chemistry - Dissertations. Paper 56.