The Effect of Reduction on Rhenium(I) Complexes with Binaphthyridine and Biquinoline Ligands:  A Spectroscopic and Computational Study

Abstract

A number of rhenium complexes with binaphthyridine and biquinoline ligands have been synthesized and studied. These are [Re(L)(CO)3Cl] where L = 3,3'-dimethylene-2,2'-bi-1,8-naphthyridine (dbn), 2,2'-bi-1,8-naphthyridine (bn), 3,3'-dimethylene-2,2'-biquinoline (dbq), and 3,3'-dimethyl-2,2'-biquinoline (diq). This series represents ligands in which the electronic properties and steric preferences are tuned. These complexes are modeled using density functional theory (DFT). An analysis of the resonance Raman spectra for these complexes, in concert with the vibrational assignments, reveals that the accepting molecular orbital (MO) in the metal-to-ligand charge transfer (MLCT) transition is the LUMO and causes bonding changes at the inter-ring section of the ligand. The electronic absorption spectroelectrochemistry for the reduced complexes of [Re(dbn)(CO)3Cl], [Re(dbq)(CO)3Cl], and [Re(diq)(CO)3Cl] suggest that the singly occupied MO is delocalized over the entire ligand structure despite the nonplanar nature of the diq ligand in [Re(diq)(CO)3Cl]. The IR spectroelectrochemistry for [Re(dbn)(CO)3Cl], [Re(dbq)(CO)3Cl], and [Re(bn)(CO)3Cl] reveal that reduction lowers the CO ligand vibrational frequencies to a similar extent in all three complexes. The substitution of naphthyridine for quinoline has little effect on the nature of the singly occupied MO. These data are supported by DFT calculations on the reduced complexes, which reveal that the ligands are flattened out by reduction: This may explain the similarity in the properties of the reduced complexes.

Structural, Electronic, and Computational Studies of Heteroleptic Cu(I) Complexes of 6,6′-Dimesityl-2,2′-bipyridine with Ferrocene-Appended Ethynyl-2,2′-bipyridine Ligands

“…The mean absolute deviations (MADs) of experimental versus calculated ṽ using Raman vibrational frequencies can be used to quantify how effectively the calculated optimized geometry has modeled the compound in question. The technique used to generate MADs has been described in the literature and is adopted here. − Typically a MAD of less than 15 cm –1 is regarded as satisfactory, especially in the case of large, dynamic molecules such as the studied compound series. For every compound, a MAD of less than 15 cm –1 was attained and has been included with an associated scaling factor in Table S2.…”

Understanding excited-state structure in metal polypyridyl complexes using resonance Raman excitation profiles, time-resolved resonance Raman spectroscopy and density functional theory

“…The reason for this enhancement is that this normal mode, in which the CO ligands stretch and compress in phase, is one of the normal coordinates that shows distinct structural distortion on going from the ground to the resonant excited state (Fig. 1) [20,21]. This relationship between resonance Raman enhancements and ground-to excited-state structural displacements was first enunciated by Hirakawa and Tsuboi [19] and subsequently developed by others [22].…”

“…This has been used in a study of the spectroscopy and DFT modelling of [Cu(phen)(PPh 3 ) 2 ] + [46]. The comparison between experimental and calculated vibration data was parameterised by the frequency mean absolute deviation (MAD) [21,[47][48][49] for the most intense bands (generally those of >20% the most intense feature). In general it is found that if the MAD is <10 cm −1 then other electronic parameters correlate well [21,46,47,50,51].…”

Excited-state spectroscopic investigations of multinuclear complexes based on [Ru(bpy)3]2+ moieties connected to 2,2′-bipyridine and 2,2′;6′,2′′-terpyridine ligands

“…The MADs were determined as 4-11 cm −1 , with an average of 6 cm −1 , which was deemed to be a good correlation. [34][35][36]…”