Gold(III) and palladium(II) complexes of glycylglycyl-L-histidine: crystal structures of [AuIII(Gly-Gly-L-His-H-2)]Cl ·H2O and [PdII(Gly-Gly-L-His-H-2)] ·1.5H2O and HisεNH deprotonation

Abstract

Proton NMR studies show that [AuCl ₄ ] ^- reacts slowly with glycylglycyl-L-histidine (Gly-Gly-L-His) (t _½ = 9.3 h at 310 K and pH* 2) in D ₂ O at pH* (meter reading) values as low as 1.5 to form the stable complex [Au ^III (Gly-Gly-L-His-H _-2 )]Cl ·H ₂ O 1 via one intermediate. Complex 1 is shown by X-ray crystallography to be square-planar with gold bound to the terminal NH ₂ [Au–N 2.049(10) Å], two deprotonated amide nitrogens [Au–N ^- 1.941(9), 2.006(10) Å] and HisδN [Au–N 2.038(9) Å] giving one six-membered and two five-membered chelate rings. At pH* 7 the reaction of [AuCl ₄ ] ^- with Gly-Gly-L-His follows a different course, apparently involving the formation of Au ^III cross-linked polymers. The anion [PdCl ₄ ] ^2- reacts rapidly with Gly-Gly-L-His also at pH* 2, and forms a similar square-planar complex [Pd ^II (Gly-Gly-L-His-H _-2 )] ·1.5H ₂ O 2 involving the terminal NH ₂ [Pd–N 2.058(7) Å], two deprotonated amide nitrogens [Pd–N ^- 1.943(7), 1.983(6) Å] and HisδN [Pd–N 2.016(6) Å]. By potentiometry, pK _a values of 2.58 (CO ₂ H), 8.63 (HisεNH, ‘pyrrole nitrogen’) and 11.50 (co-ordinated NH ₂ ) for 1 and 11.30 (HisεNH) for 2 were determined and confirmed by ¹ H NMR spectroscopy.