Improving Bispyridinylidene-based organic reducing agents by increasing solubility and reductant strength
University of New Brunswick
Recently, a tetrasubstituted bispyridinylidene (BPY) featuring methyl groups on the pyridyl nitrogens and four triphenyliminophosphorano groups at the para and ortho positions of the pyridine rings was reported. This compound is a powerful electron donor but isolated only in low yield because of low solubility. One goal of this work was to improve solubility and isolated yield by modification of the N-alkyl substituents. An N-propyl derivative was synthesized, as evidenced by [superscript 31]P NMR spectroscopy; however, this BPY derivative could not be isolated. The reduction potential, determined by cyclic voltammetry of the oxidized BPY, was -1.67 V vs SCE, which is a slight decrease in reduction strength compared to the N-methyl derivative (-1.70 V vs SCE). Because of challenges in isolating the compound, it was concluded that the N-propyl derivative gave no benefits over the N-methyl derivative. In order to develop new BPYs with increased reductive strength, the four triphenyliminophosphorano groups of the tetrasubstituted BPY were systematically changed to tricyclohexyliminophosphorano groups in pairs. Tricyclohexyliminophosphorano groups are stronger π-donors than triphenyliminophosphorano groups as shown by their Hammett constants (σ[subscript p][superscript +]) of -1.82 and -2.21, respectively. The primary target was a BPY featuring four tricyclohexyliminophosphorano groups, which was prepared in four steps, and characterized by [superscript 31]P, [superscript 1]H, [superscript 13]C and ROESY 2D NMR spectroscopy as well as cyclic voltammetry. Cyclic voltammetry showed a reduction potential of -1.87 V vs SCE, which surpasses the tetrasubstituted triphenyliminophosphorano BPY derivative in reductive strength by 170 mV vs SCE, and makes the new compound the most powerful neutral organic electron donor ever reported. In addition, a BPY featuring triphenyliminophosphorano groups at the two para positions and tricyclohexyliminophosphorano groups at the two ortho positions was prepared in situ as evidenced by [superscript 31]P NMR spectroscopy; however, was not isolated. Cyclic voltammetry analysis is pending, therefore reductive strength is unknown, but should fall between tetrakis(triphenyliminophosphorano) BPY and the tetrakis(tricyclohexyliminophosphorano) BPY. Lastly, the pyridinium salt precursor to the other targeted BPY, featuring two tricyclohexyliminophosphorano groups at the para positions and two triphenyliminophosphorano groups at the ortho positions, respectively, was prepared as evidenced by [superscript 31]P and [superscript 1]H NMR spectroscopy. Unfortunately, this compound could not be completely purified and the preparation of the new BPY remains to be done.