Relaxed but highly compact diansa metallacyclophanes

A series of monoansa [μ-1,1′-PR-3,3′-Co(1,2-C ₂B ₉H ₁₀) ₂] ^- and diansa [8,8′-μ-(1″,2″-benzene)-μ-1,1′-PR-3,3′-Co(1, 2-C ₂B ₉H ₉) ₂] ^- (R = Ph, ^tBu) cobaltabisdicarbollidephanes have been synthesized, characterized and studied by NMR, MALDI-TOF-MS, UV-visible spectroscopy, cyclic voltammetry, and DFT calculations. Single crystal X-ray diffraction revealed a highly relaxed structure characterized by the title angle α of 3.8° ([7] ^-), this being the smallest angle α for a metallacyclophane. In such compounds, the metal-to-phosphorus distance is less than the sum of their van der Waals radii. The availability of a phosphorus lone pair causes an electron delocalization through the metal, as shown by the abnormal ³¹P NMR chemical shift. Remarkably, the combination of a phosphine donor and a phenyl acceptor moieties causes a synergistic effect that is observed through the different techniques used in this study. The importance of having an available lone pair is demonstrated by the oxidation of phosphorus with hydrogen peroxide, sulfur, and elemental black selenium to produce the corresponding P ^V compounds. When the electron lone pair is used to form the bond with the corresponding chalcogen atom, the communication between the donor and acceptor moieties on the diansa metallacyclophane is shut down.