Bis(4-carboxylpyrazol-1-yl)acetic acid: a scorpionate ligand for complexes with improved water solubility.

Bis(4-carboxylpyrazol-1-yl)acetic acid (H 3 bcpza) (2), obtained in a one-pot synthesis, contains carboxyl groups that differ in their p K a values. The ligand exhibits heteroscorpionate κ 3 -N,N,O-coordination whereupon the peripheral carboxyl groups are not involved in metal binding. The corresponding carbonyl complexes [Mn(H 2 bcpza)(CO) 3 ] (4), [Re(H 2 bcpza)(CO) 3 ] (5) and [Ru(H 2 bcpza)Cl(CO) 2 ] (6a/6b) are partially soluble in water but readily soluble in PBS buffer.