The influence of phenyl substituents on the redox potentials of sterically hindered tripodal ligand/copper complexes

Abstract

The structural characterization of Cu(1) and Cu(I1) complexes of tris(6-phenyl-2-pyridylmethyl)amine (tppa), a derivative of the known tris(2-pyridylmethy1)amine (tpa), was recently reported. The phenyl substituents in [Cu(tppa)AN](CIOJ, stabilize the Cu(1) state by 300-480 mV relative to [Cu(tpa)AN](CIO,),. Reported here is the synthesis of the compounds bis(2-pyridylmethyl)-6-phenyl-2-pyridylmethylamine (Phtpa) and bis(6-phenyl-2-pyridylmethyl)-2-pyridylmethylamine (Ph,tpa) and the synthesis and cyclic voltammetry studies of their Cu(I1) perchlorate complexes.The mechanism by whiCh the phenyl groups increase the redox potential was found to be solvent-dependent: in AN, the increase in potential is primarily due to reduced local dielectric: in IBN, DMF, and DMA, the steric interaction between coordinated solvent and the phenyl substituents becomes important. The redox behavior of these complexes is consistent with anticipated modes of binding of solvent molecules in the inner sphere of the Cu(I1) complexes.

Coordination and Bioinorganic Chemistry of Aryl-Appended Tris(2-Pyridylmethyl)amine Ligands

“…[28,38] For each complex, the spectral data is consistent Cyclic voltammetry studies of 30 revealed that in various solvents the redox potential of this complex is positive (up to several hundred mV) relative to that of [(TPA)Cu(NCCH 3 )](ClO 4 ) 2 . [21,27] The more positive potential of 30 may be related to electron withdrawing effects of the phenyl substituents, or a reduction in the local dielectric due to steric shielding of the copper center by the phenyl groups. This shielding may limit access of a polar solvent to the metal center.…”

“…The Cu(II) complexes [(6-Ph 2 TPA)Cu(NCCH 3 )](ClO 4 ) 2 and [(6-PhTPA)Cu(NCCH 3 )](ClO 4 ) 2 were prepared to further examine how the hydrophobic phenyl appendages influence the electrochemical properties of the copper center in acetonitrile solution. [27] These studies revealed that each phenyl substituent imparts approximately a 100 mV increase in the redox potential of the metal center.…”

“…Aryl-appended TPA ligands have been used in the preparation of coordination complexes of Mn(II), [22,28] Fe(II) and Fe(III), [20,[23][24][25]28,34,35] Co(II), [22,28,31] Ni(II), [22,28,31,36] Cu(I) and Cu(II), [21,27,28,[37][38][39] and Zn(II). [22,28,31] Mn(II).…”

“…[24][25][26] A procedure for the preparation of o-d 1 -6-phenyl-2-pyridinecarboxaldehyde has also been reported. [23] To construct an aryl-appended tris(2-pyridylmethyl)amine (TPA) ligand starting from the aryl-appended carboxaldehyde, a common synthetic pathway involves (1) reduction of the aldehyde group using NaCNBH 3 [27] or NaBH 4 [28] to generate a primary alcohol, (2) treatment of this alcohol with SOCl 2 [21,22,27] or PBr 3 [28] to generate a 2-halomethyl intermediate, and (3) heating of this intermediate with a 2-(aminomethyl)pyridine derivative in the presence of base to produce a mono-, di-or triaryl-appended chelate ligand. An alternative synthetic route involves reductive amination via treatment of the aryl-appended pyridine carboxaldehyde with a 2-(aminomethyl)pyridine derivative in the presence of NaCNBH 3 .…”

“…An alternative synthetic route involves reductive amination via treatment of the aryl-appended pyridine carboxaldehyde with a 2-(aminomethyl)pyridine derivative in the presence of NaCNBH 3 . [23,[27][28][29] The amide-appended ligand bppppa was prepared via an alkylation pathway involving N,N-bis((6-phenyl-2-pyridyl)methyl)amine and 2-bromomethyl-6-pivaloylamidopyridine. [30,31] All aryl-appended TPA ligands reported to date that have been used to generate structurally characterized metal complexes are shown in Figure 1.…”