Enhancing Electron Accepting Ability of Triarylboron via π-Conjugation with 2,2‘-Bipy and Metal Chelation:  5,5‘-Bis(BMes₂)-2,2‘-bipy and Its Metal Complexes

The synthesis of a new diboron compound, 5,5‘-bis(BMes 2 )-2,2‘-bipy (B2bipy, 1 ), has been accomplished. The conjugation of the 2,2‘-bipy unit with the two BMes 2 groups in 1 has been found to greatly enhance the electron-accepting ability of the boron centers, compared to the biphenyl analogue. 1 is an effective chelate ligand for metal ions and three metal complexes; namely, [Cu I (B2bipy)(PPh 3 ) 2 ][BF 4 ], 2, Pt II (B2bipy)Ph 2, 3a, and Pt II (B2bipy)Me 2, 3b, have been synthesized. The binding of metal ions to B2bipy was found to further significantly increase the electron-accepting ability/Lewis acidity of the boron centers. Cyclic voltammetry diagrams revealed that the free ligand and all three complexes display two reversible reduction peaks that are separated by 0.31−0.40 V. The first reduction potential ( E 1/2 ) for 1, 2, 3a, and 3b was found to be −1.69, −1.36, −1.34, and −1.38 V, respectively, versus FeCp 2 +/0 in DMF, which supports that the electron-accepting ability of the metal complexes is much greater than that of BMes(C 6 F 5 ) 2 . The strong electron-accepting ability/Lewis acidity of the metal complexes and the free ligand was manifested by their ability to sequentially bind to 2 equiv of F - ions in non-alcoholic solvents with K 1 ≥ 10 8 and 10 9 M -1 for 1 and 3a, respectively, and to 1 equiv of F - ions in the presence of methanol/ethanol or water. The crystal structures of 1, its 2:1 fluoride adduct [NBu 4 ] 2 [ 1F 2 ], and 3a were established by X-ray diffraction analyses. The 1:1 fluoride adduct of 2, [Cu I (B2bipyF)(PPh 3 ) 2 ], 2F, isolated directly from the reaction of 2 with NBu 4 F in a 4:1 mixture of CH 3 OH and CH 2 Cl 2 was characterized. All metal complexes are intensely colored and display a characteristic metal-to-ligand (B2bipy) charge transfer absorption band in the visible region, which was found to be highly sensitive toward binding by anions such as fluorides.