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

Abstract

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 pKa = 8.9 for the metal‐bound water. Based on the X‐ray crystallographic structure of [Ni2(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···H···OH unit within the bimetallic cavity, which is involved in multiple H‐bonding interactions.

Modelling Binuclear Metallobiosites: Insights from Pyrazole‐Supported Biomimetic and Bioinspired Complexes

“…As opposed to the above findings, reduction of the flexidentate pivaloylamido groups in the supporting scaffold of 47 allowed for isolation of a binuclear complex possessing a bridging unit containing two hydroxido ligands ( 48 , Scheme ) 121. Although the two cobalt(II) ions in complex 48 are now spanned by a water‐derived array, the protonation state differs from that typically observed for similar metal separations, such as the O 2 H 3 tether in 46 119. A striking difference between complexes 46 and 48 is the number of N–H donors provided by the ligand periphery; one vs. two for each metal‐binding compartment in each complex, respectively.…”

“…A few novel binucleating pyrazole‐based ligand systems possessing features capable of emulating secondary sphere hydrogen‐bonding interactions have appeared in the literature 119–122. The potential of the introduced peripheral groups to alter coordination behavior is well illustrated by comparing the different motifs adopted by exogenous water‐derived ligands in some of the corresponding complexes.…”

“…The potential of the introduced peripheral groups to alter coordination behavior is well illustrated by comparing the different motifs adopted by exogenous water‐derived ligands in some of the corresponding complexes. In binuclear structure 46 (Scheme ) two nickel ions are spanned by a pyrazolato‐bridging ligand designed to enforce a large metal separation, which results in the O 2 H 3 array often encountered in these systems (See above) 119. However, in this case pivaloylamido moieties appended to the pyridine rings of the chelate arms provide two N–H donors, one for each binding compartment.…”

“…In spite of having a very similar intermetallic distance, the closely related cobalt(II) structure ( 47 , Scheme ) shows only a single terminally bound hydroxido ligand 120. This can be partially attributed to the flexidentate nature of the pivaloylamido residues which, in addition to serving as hydrogen‐bond donors, are able to coordinate directly to the metal ion through their oxygen atoms (as either neutral or anionic ligands, depending on the protonation state) 119,120. In 47 , such a situation is observed for one of four of these groups, whereby it acts as an oxy‐anionic ligand to cobalt.…”

Supramolecular Perturbation of Metal–Metal Bonding in Cyclodextrin-Encapsulated (NHC)Cu-FeCp(CO)2 Complexes

“…Fundamental understanding of the chemistry of such systems advances the ability to design catalysts for a sustainable energy economy. − In this regard, one important aspect to consider is the interplay between primary and secondary coordination spheres . Few controlled studies have analyzed correlations between secondary coordination sphere effects and behavior of binuclear metal complexes. − …”