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

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