Rotational barrier of a molybdenum-molybdenum quadruple bond

The synthesis and characterization of molybdenum(II) porphyrin dimers containing unbridged metal-metal quadruple bonds are presented. Variable temperature 300 MHz ¹H NMR studies of meso-substituted derivatives provide novel solution evidence for both the existence of quadruple bonds and for a barrier to rotation about the metal-metal bonds in these complexes. The activation energy for this rotational process is 10.1 ± 0.5 kcal·mol^-1.