Solvation Largely Accounts for the Effect of N-Alkylation on the Properties of Nickel(II/I) and Chromium(III/II) Cyclam Complexes

Abstract

The source of the effect of N-alkylation on the redox properties of Ni(II/I) and Cr(III/II) cyclam complexes has been investigated using DFT calculations. The structures of the anhydrous and hydrated complexes were optimized in the gas phase, and single point calculations were performed in a polarized continuum. The main results are the following: the decrease in outer sphere solvation upon N-alkylation is the major source of the relative stabilization of the lower oxidation state complexes by the tertiary amine ligands; tertiary amine nitrogen donors are stronger sigma-donors than the secondary amines, as predicted from the inductive effect of alkyls; steric strain elongates the metal-nitrogen bonds in the tertiary complexes and decreases the ligand strain energies; and the site of water binding to the complexes differs because of their different electronic structures (i.e., in the Ni complexes, the water molecules bind to the M[bond]N[bond]H sites, whereas in the Cr complexes they bind to the central metal cation). Outer sphere hydrogen bonding of water to the ligands in the coordination sphere lowers the ionization potentials by charge delocalization.

Thermodynamics of Complex Formation of Silver(I), Cadmium(II) and Cobalt(II) with Open‐Chain Polyamines in Dimethyl Sulfoxide and Molecular Dioxygen Binding to Cobalt(II) Complexes

“…The negative enthalpy and entropy values are typical of reactions involving complexation of metal ions by neutral ligands in aprotic solvents. [3,6,19] Data in water solutions are also available for all the trien systems, [7] whereas analogous data for the Co II /hmtrien system are reported only as estimated values. It should be remembered that Cd 2ϩ and Co 2ϩ are hexasolvated in both solvents, with the solvent molecules in a regular octahedral arrangement, whereas silver() is tetrahedrally solvated both in DMSO and in water.…”

“…In a very recent work [3] it was shown that N-alkylation causes several contradictory effects: on one hand, the nitrogen atoms become better σ-donors as a consequence of the introduction of electron-donating substituents, which shifts the redox potential of the M nϩ1 L m /M n L m couples cathodically, while on the other hand, solvation and steric effects shift the potential anodically. As the solvation effects are larger, N-alkylation stabilizes low-valent transition metal complexes and the redox potentials of the couples are shifted anodically for L ϭ tertiary amines.…”

“…As the solvation effects are larger, N-alkylation stabilizes low-valent transition metal complexes and the redox potentials of the couples are shifted anodically for L ϭ tertiary amines. [1,3,5,6] In addition, the stability constants of the complexes with the tertiary amines are smaller than those with primary and secondary amines, [3,5Ϫ9] this behaviour mainly being attri-…”

The Redox Chemistry of (N1‐[3‐(2‐aminoethylimino)‐1,1‐dimethylbutyl]ethane‐1,2‐diamine)nickel(II) Perchlorate, NiIIL1(ClO4)2, in Aqueous Solutions –A Pulse Radiolytic and an Electrochemical Study

“…The second reaction observed also obeys a first order rate law and its rate is proportional to [Br -], Figure 8; the large intercept suggests that this is an equilibrium process, see (9). The spectrum of the product of the transient formed in reaction (9) is very similar to that formed in reaction (8), Figure 7.…”

“…The redox potential of the couple in 0.1  NaClO 4 is cathodically shifted by 250 mV relative to its tetraaza macrocyclic analogue, [5] b) The macrocyclic complex has less hydrogen bonds with the solvent, i. e. is more hydrophobic. [6][7][8][9] c) The open ligand is less rigid and thus has a better ability to satisfy the electronic and stereochemical demands of the central metal ion.…”