Sterically Hindered Carboxylate Ligands Support Water-Bridged Dimetallic Centers That Model Features of Metallohydrolase Active Sites

The synthesis and characterization of carboxylate-bridged dimetallic complexes are described. By using m-terphenyl-derived carboxylate ligands, a series of dicobalt(II), dicobalt(III), dinickel(II), and dizinc(II) complexes were synthesized.
The compounds are [Co2(μ-O2CArTol)2(O2CArTol)2L2] (1), [Co2(μ-OH2)2(μ-O2CArTol)2(O2CArTol)2L2] (2a−c), [Co2(μ-OH)2(μ-O2CArTol)2(O2CArTol)2L2] (3), [Ni2(μ-O2CArTol)4L2] (4), [Ni2(μ-HO···H)2(μ-O2CArTol)2(O2CArTol)2L2] (5), and
[Zn2(μ-O2CArTol)2(O2CArTol)2L2] (6), where ArTolCO2H = 2,6-di(p-tolyl)benzoic acid and L = pyridine, THF, or N,N-dibenzylethylenediamine. Structural analysis of these complexes revealed that additional bridging ligands can be
readily accommodated within the {M2(μ-O2CArTol)2}2+ core, allowing a wide distribution of M···M distances from
2.5745(6) to 4.0169(9) Å. Unprecedented bridging units {M2(μ-OH2)2(μ-O2CR)2}n+ and {M2(μ-HO···H)2(μ-O2CR)2}n+
were identified in 2a−c and 5, respectively, in which strong hydrogen bonding accommodates shifts of protons
from bridging water molecules toward the dangling oxygen atoms of terminal monodentate carboxylate groups.
Such a proton shift along the O···H···O coordinate attenuates the donor ability of the anionic carboxylate ligand,
which can translate into increased Lewis acidity at the metal centers. Such double activation of bridging water
molecules by a Lewis acidic metal center and a metal-bound general base may facilitate the reactivity of
metallohydrolases such as methionine aminopeptidase (MAP).