Nickel(II) complexes stabilized by bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine: Synthesis and characterization of complexes stabilized by a hydrogen bonding network

Abstract

Hydrogen bonds in metalloproteins are key in directing reactivity yet to be achieved in synthetic systems. We have been developing a synthetic system that uses hydrogen-bonding interactions to modulate the secondary coordination around a transition metal ion. This was accomplished with the ligand bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine (H(2)pmb), which contains two carboxyamido units appended from pyridine rings. Several nickel complexes were prepared and structurally characterized. In particular, we found that the appended carboxyamido groups either provide intramolecular H-bond donors or can be converted to bind directly to a metal center. We established that the complex Ni(II)H(2)pmb(Cl)(2) can be sequentially deprotonated with potassium tert-butoxide, causing coordination of the carboxyamido oxygen atoms and concomitant loss of the chloro ligands. The chloro ligands were also removed with silver(I) salts-in the presence of acetate ions, the complex Ni(II)H(2)pmb(κ(2)-OAc)(κ(1)-OAc) was isolated, in which an intramolecular H-bonding network occurs between the H(2)pmb ligand and the coordinate acetato ligands.

Fig. 1

The carboxyamido unit used to form an intramolecular H-bond (A), to form a bidentate ligand (B), and the H₂pmb ligand used in this study (C).

Fig. 2

Thermal ellipsoid diagrams of Ni^IIH₂ pmb(Cl₂) (A) and Ni^IIHpmb(Cl) (B). The ellipsoids are drawn at the 50% probability level and all non-carboxyamido hydrogen atoms are removed for clarity.

Fig. 3

Thermal ellipsoid diagrams Ni^IIpmb(THF) (A) and Ni^IIH₂ pmb(κ²-OAc)(κ¹-OAc) (B). The ellipsoids are drawn at the 50% probability level and all non-carboxyamido hydrogen atoms are removed for clarity.

Scheme 1

Preparative route to Ni^IIH₂pmb(Cl)₂.

Scheme 2

Preparative routes to Ni^IIHpmb(Cl), Ni^IIpmb(THF), and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc).