Ambident Behavior of the “New Vitamin” Methoxatin (Cofactor PQQ) towards Metals: Coordinative Stabilization of the Pyrrolide Form and the Semiquinone Form

The biochemical function of the o ‐quinonoid “new vitamin” methoxatin (Cofactor PQQ) is coupled with the formation of the semiquinone form and possibly also with metal coordination. In contrast to “hard” copper(II) fragments, the “soft” [Ru(bpy) 2 ] 2⊕ , which is capable of π‐back bonding, coordinates via the two o ‐quinone oxygen atoms of the methoxatin; both the semiquinoid intermediate as well as the form with deprotonated pyrrole ring are thereby stabilized and shifted into a physiologically relevant potential‐ and pH‐range.