Decorating the Second Coordination Sphere in Pyrazolate-Based Dinickel(II) Complexes with H-Bond Donors

Compartmental pyrazolate-based ligands with bis(2-pyridinylmethyl) aminomethyl or (2-pyridinylmethyl)[2-(1-methylimidazolyl)methyl]aminomethyl chelate arms have been further elaborated by attaching pivaloylamido groups to the outer pyridine rings, giving the new ligands H3L1 and H3L2. The peripheral pivaloylamido groups may serve as potential H-bond donor or acceptor sites towards substrates in the bimetallic cavity. Protonation constants of the new ligand H3L1 as well as its nickel(II) speciation in solution have been studied by pH potentiometry, revealing pK(a) = 8.9 for the metal-bound water. Based on the X-ray crystallographic structure of [Ni-2(H2L1)(H3O2)(MeCN)(2)](ClO4)(2) (3) this pKa is rationalized in terms of two balancing effects, namely involvement of the pivaloylamido O atom in metal coordination and H-bonding between the pivaloylamido NH group and the resulting nickel-bound hydroxide. Complex 3 features a pyrazolate-bridged dinickel(II) core and a bridging HO center dot center dot center dot H center dot center dot center dot OH unit within the bimetallic cavity, which is involved in multiple H-bonding interactions.