Poly(2-thienyl)borates: An Investigation into the Coordination of Thiophene and Its Derivatives

The synthesis, characterization, and reactivity of a new sulfur-rich tridentate ligand, tetrakis(2-thienyl)borate (1(-)()), are reported along with a molecular orbital analysis of its coordination to a metal center. Unlike the analogous tetrakis((methylthio)methyl)borate (2(-)()), 1(-)() does not coordinate Mo(CO)(3) when reacted with (C(7)H(8))Mo(CO)(3). The sulfur atoms in both ligands are oriented to coordinate the metal in a pyramidal eta(1) sulfur-bound mode. Approximate molecular orbital calculations are used to compare the metal-ligand interactions in these related species, and the results indicate that the magnitude and polarizability of the electronic charge density of the lone pairs on the sulfur atoms dictate the coordination strength of the ligands. Simple Mulliken atomic charges and orbital occupation numbers are used to determine the extent of charge delocalization. While the conjugation of the sulfur lone pair electrons with adjacent pi bonds in the ligands decreases the corresponding Lewis basicity, the contribution from the aromaticity in the thienyl groups is negligible. During the course of these studies, the structure of K[1] was determined by X-ray diffraction. K[1]: monoclinic space group C2/c, with a = 16.00(2) Å, b = 7.680(7) Å, c = 16.22(2) Å, beta = 118.520(7) degrees, V = 1750(3) Å(3), Z = 4, R(F) = 0.0494, and R(w)(F(2)()) = 0.122. The crystal lattice contains one-dimensional chains of 1(-)() bridged by K ions.