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 H 3 L 1 and H 3 L 2 . 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 H 3 L 1 as well as its nickel(II) speciation in solution have been studied by pH potentiometry, revealing p K a = 8.9 for the metal‐bound water. Based on the X‐ray crystallographic structure of [Ni 2 (H 2 L 1 )(H 3 O 2 )(MeCN) 2 ](ClO 4 ) 2 ( 3 ) this p K a 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 ··· H ··· OH unit within the bimetallic cavity, which is involved in multiple H‐bonding interactions.