Direct bromination and iodination of the trispyrazolyl borate ligand in TpRu(nbd)Cl and the effect of Tp4−X ligands on redox potentials and catalysis

Abstract

Traditional syntheses of Tp (trispyrazolylborate, HB(pz)3) ligands often require extreme conditions that are incompatible with some pyrazole derivatives. In contrast, an alternative direct functionalization of the Tp ligand could offer more compatible conditions. To prove this principle, direct bromination or iodination of TpRu(nbd)Cl is reported herein, and Sonogashira cross-coupling of the iodinated ligand with an alkyne could be performed at ambient temperature. This direct halogenation strategy allows rapid access to several halogenated Tp4−XRu(nbd)Cl derivatives from TpRu(nbd)Cl in a single step and to diversified ruthenium complexes having functionalized Tp ligands in minimal additional steps. The influence of the halogen substituents on the physical and chemical properties of the complexes was also examined. Ligand exchange in the presence of modified ligands was facile. Halogenation caused the standard reduction or oxidation potential to shift 200 mV in the positive direction and reduced efficacy in ruthenium catalyzed epoxyalkyne rearrangement during examination of a literature hypothesis relating catalysis to reduction and oxidation potentials.