The article presents the synthesis and characterization of two mononuclear Re(V) complexes. DFT and TD-DFT methods were employed for exploring the electronic structure and absorption spectra. Furthermore, the complexes were also tested for catalytic activity in epoxidation reactions. Although the synthetic schemes and experimental section provides reliable procedures for the synthesis of these compounds, slight modifications are suggested to improve overall efficiency.
Synthesis of [ReVO(L1)Cl2], 1.
“A solution of HL1 (30 mg, 0.12 mmol) in acetone was added under stirring at room temperature to a solution of [ReOCl3(Me2S)(OPPh3)] (78 mg, 0.12 mmol) in acetone (25ml). The solution became immediately red and the resulting mixture was stirred for 10 h, with precipitation of a dark brown solid observed slowly over the course of the reaction.” “The dark brown colored solid mass was dissolved in hot acetonitrile. On slow evaporation of the resulting solution dark red colored crystals were obtained.”
On repeating the experiment in reduced pressure, it was observed that the actual time needed for precipitation of dark brown solid with yields around 70% was about 4 hours.
The reaction time mentioned can be greatly reduced by this technique. An alternative for crystallization can be slow evaporation of the original red colored solution in acetone. Using this solution also reduces the reaction time.
DFT and TD-DFT studies
The geometry optimization (Fig. 3) and electronic structure calculations (Table 4) were done using LANL2DZ basis set for Re, 6-31(g) basis set for H and the 6-31+G(d) basis set for C, N, O, and Cl atoms. However, while calculating NMR properties, 6-311+G(2d,p) was used for all other atoms except Re.
There seems to be an anomaly while calculation of electronic structures and NMR properties. It is well known that HOMO and LUMO energy gap plays a significant part in chemical shifts especially when bonded to metals. As is evident from the Table 4, in complex 1, the role of Re is more or less same in HOMO and LUMO, but the role of aromatic ring is significantly higher in HOMO while that of the electronegative O is almost insignificant compared to that in LUMO, the situation presses for uniformity in usage of basis sets for calculation of Geometry optimization and electronic structure along the lines of that used for NMR properties.
So, I would recommend using the 6-311+G(2d,p) basis set for all the atoms except Re. A similar modification is recommended for complex 2.