Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases

Abstract

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris(amido)amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt(II) and zinc(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes [Co(L)](-) with dioxygen reversibly generates a small amount of a cobalt(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn[N{CH2CH2NC(O)CH2N(CH2Ph)2}3)](-) ([Zn(TN(Bn))](-)) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes [M(L)](-) results in complete proteolysis and formation of the ligands H3L. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative and acidic conditions.

Synthesis and Structural Characterization of Tris(isopropylbenzimidazol-2-ylthio)methyl Zinc Complexes, [TitmPriBenz]ZnX: Modulation of Transannular Zn–C Interactions

“…In addition to the τ 4 and τ δ geometry indices, a τ′ geometry index has also been introduced to differentiate tetrahedral and trigonal-monopyramidal coordination geometries, and the τ′ values (1.01 and 1.05) are also in accord with the idealized value for a trigonal monopyramid (1.00). Although zinc compounds which feature trigonal-monopyramidal coordination are not common, several examples are known, as illustrated by {[Tism Pr i Benz ]-Zn}­[HB­(C 6 F 5 ) 3 ] . In this regard, a comparison of the structures of {[Titm Pr i Benz ]­Zn} + and {[Tism Pr i Benz ]­Zn} + (Table ) indicates that the former has a slightly more idealized trigonal-monopyramidal geometry, as indicated by (i) τ′ values that are closer to 1.0 and (ii) smaller displacements of the Zn from the [ N 3 ] plane towards the carbon.…”

Modulating the Primary and Secondary Coordination Spheres within a Series of CoII—OH Complexes

“…In an attempt to determine the influence of the secondary coordination sphere, ligands have been prepared that contain either H-bond donors or acceptors in order to understand the effects of intramolecular hydrogen bonding (H-bonding) networks on metal ion chemistry. 1–14 For example, Fout has reported systems that regulate the secondary coordination sphere by using ligands that can tautomerize to function as either H-bond acceptors or donors. 15–17 In addition, we have developed a family of symmetrical and hybrid tripodal ligands to probe how systematic changes in the H-bonding networks affect function.…”

Zinc and Magnesium Catalysts for the Hydrosilylation of Carbon Dioxide

“…Specifically, the magnesium and zinc hydride compounds, [κ 3 -Tism Pr i Benz ]ZnH and [Tism Pr i Benz ]­MgH, also undergo rapid hydride abstraction by B­(C 6 F 5 ) 3 to afford {[Tism Pr i Benz ]­Zn}-[HB(C 6 F 5 ) 3 ] ( 4 ) and {[Tism Pr i Benz ]­Mg}­[HB(C 6 F 5 ) 3 ] ( 5 ), respectively. The molecular structures of {[Tism Pr i Benz ]­M}[HB(C 6 F 5 ) 3 ] (M = Zn, Mg) have been determined by X-ray diffraction (Figures and ), which demonstrates that the compounds possess uncommon trigonal monopyramidal , geometries and exist as ion pairs. In this regard, the structure of {[Tism Pr i Benz ]­Mg}­[HB­(C 6 F 5 ) 3 ] is in marked contrast to that of the related compound, [To M ]­MgHB(C 6 F 5 ) 3 , which does not exist as an ion pair but rather possesses a distinct Mg–H–B interaction .…”