5-Aminoorotic acid, a versatile ligand with the ability to exhibit differing co-ordination and hydrogen-bonding modes: synthesis and crystal structures of platinum(II) complexes

Abstract

The complex [Pt(cod)Cl₂](cod = cycloocta-1,5-diene, C₈H₁₂) reacted with 2 equivalents of PPh₃ and an excess of 5-aminoorotic acid (5-amino-2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid, H₄L) in the presence of silver(I) oxide to give two isomers of [Pt(PPh₃)₂(H₂L)]1 and 2. Complexes 1 and 2 can be separated by fractional crystallisation but, in CDCl₃ solution, each slowly converts into an equilibrium mixture of the two. Crystal structure determinations have shown that in both 1 and 2 the 5-aminoorotate ligand co-ordinates to the platinum atom as a dianion. In 1 this is achieved via deprotonation of the carboxylic acid and the loss of an amino NH₂ proton, leading to a six-membered chelate ring, whereas in 2 it is by deprotonation of the carboxylic acid and loss of the amido proton, leading to a five-membered chelate ring. This difference in co-ordination mode leads to a difference in the orientation of the hydrogen-bond donors and acceptors remaining on the ligand which, in turn, leads to different supramolecular structures, dimers for 1 and tetramers for 2, with the latter structurally similar to guanine tetrads. These naturally occurring units are stabilised by alkali-metal ions, but reaction of 2 with a compound such as NaBF₄ in a two-phase dichloromethane–water system led to [Pt₂(PPh₃)₄(HL)][BF₄] as the only platinum-containing product. A crystal structure determination showed that in this complex the ligand is trianionic, deprotonated at the carboxylic acid and both the amido and amino nitrogen atoms, co-ordinating to two platinum atoms via five- and six-membered chelate rings. When dppe [1,2-bis(diphenylphosphino)ethane] was used instead of PPh₃ only one isomer of [M(dppe)(H₂L)](M = Pt 3 or Pd 4) was observed for both platinum and palladium, containing the five-membered chelate ring.