title: Sterically hindered carboxylate ligands support water-bridged dimetallic centers that model features of metallohydrolase active sites.
abstract: 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 [Co(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (1), [Co(2)(mu-OH(2))(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (2a-c), [Co(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (3), [Ni(2)(mu-O(2)CAr(Tol))(4)L(2)] (4), [Ni(2)(mu-HO...H)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (5), and [Zn(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (6), where Ar(Tol)CO(2)H = 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 [M(2)(mu-O(2)CAr(Tol))(2)](2+) core, allowing a wide distribution of M...M distances from 2.5745(6) to 4.0169(9) A. Unprecedented bridging units [M(2)(mu-OH(2))(2)(mu-O(2)CR)(2)](n+) and [M(2)(mu-HO...H)(2)(mu-O(2)CR)(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).
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Dongwhan
authors.initials: D
authors.lastname: Lee
authors.forename: Pei-Lin
authors.initials: PL
authors.lastname: Hung
authors.forename: Bernhard
authors.initials: B
authors.lastname: Spingler
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Lippard
mesh_terms: D000626:Aminopeptidases / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D002264:Carboxylic Acids / Q000737:chemistry; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003035:Cobalt / Q000737:chemistry; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology; D006860:Hydrogen Bonding; D008024:Ligands; D063208:Methionyl Aminopeptidases; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D020027:Pyrococcus furiosus / Q000201:enzymology; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared; D014867:Water / Q000737:chemistry; D015032:Zinc / Q000737:chemistry
chemical_list: D002264:Carboxylic Acids; D008024:Ligands; D009942:Organometallic Compounds; D014867:Water; D003035:Cobalt; D009532:Nickel; D000626:Aminopeptidases; D063208:Methionyl Aminopeptidases; D015032:Zinc