Unusual Structural Features of Tetrakis(M-carboxylato)dirhodium(II), an Antitumor Agent, Bound to Azathioprine, a Biologically Active Mercaptopurine Derivative

We are investigating the chemistry of azathioprine (6- [1 -methyl-4-nitroimidazol- 5-yl)thio] purine, AZA), a derivative of the anticancer agent 6-mercaptopurine (6-MP). The considerable biological activity of AZA combined with its electronically and stereochemically versatile binding sites prompted us to investigate the X-ray crystal structure of the bis adduct [Rh₂(OAc)₄(AZA)₂]4DMAA (1). Crystals of 1 are monoclinic, space group P2/n, with unit cell parameters a = 18.053 (3) Å, b = 8.181 (4) Å, c = 19.523 (7) Å, α = 90°, β = 97.79 (5)°, Γ = 90°, V = 2856 (3) ų, and Z = 2. Residuals of R = 0.064 and R_w= 0.082 and a quality of fit of 2.18 were obtained. The molecule resides on a crystallographic center of inversion at the midpoint of the Rh-Rh bond with AZA coordinated to the two axial positions of the dirhodium tetraacetate cage through the purine nitrogen atom N(3). The metal-metal distance is Rh(l)-Rh(l') = 2.373 (3) Å and the axial ligand bond length is Rh(l)-N(3) = 2.23 (1) Å. Presumably, steric constraint imposed by the octahedral environment about the rhodium atom together with the bulk of the imidazole ring on S(l) inhibits metal binding at the N(7) site usually observed for purines. The structure is stabilized by hydrogen bonding between N(9)-H and the acetate O(2), detected in solution by the two inequivalent sets of acetates in the variable-temperature ¹H NMR spectrum of 1 at -70 °C. Compound 1 has been fully characterized by NMR, IR, and UV-visible spectroscopies as well as by FAB-MS and cyclic voltammetry.