Hydrogen-bonding cavities about metal ions: synthesis, structure, and physical properties for a series of monomeric M-OH complexes derived from water

The tripodal ligand NCH2CH2NHC(O)NHC(CH3)3]3 (H61]) was used to synthesize a series of monomeric complexes with terminal hydroxo ligands. The complexes Co(II/III)H31(OH)](2-/1-), Fe(II/III)H31(OH)](2-/1-), and Zn(II)H31(OH)](2-) have been isolated and characterized. The source of the hydroxo ligand in these complexes is water, which was confirmed with an isotopic labeling study for Co(III)H31(OH)](1-). The synthesis of M(II)H31(OH)](2-) complexes was accomplished by two routes. Method A used 3 equiv of base prior to metalation and water binding, affording yields of < or = 40% for Co(II)H31(OH)](2-). When 4 equiv of base was used (method B), yields ranged from 50% to 70% for all of the M(II)H31(OH)](2-) complexes. This improvement is attributed to the presence of an intramolecular basic site within the cavity, which scavenges protons produced during formation of the M(II)-OH complexes. The molecular structures of Zn(II)H31(OH)](2-), Fe(II)H31(OH)](2-), Co(II)H31(OH)](2-), and Co(III)H31(OH)](1-) were examined by X-ray diffraction methods. The complexes have trigonal bipyramidal coordination geometry with the hydroxo oxygen trans to the apical nitrogen. The three M(II)-OH complexes crystallized with nearly identical lattice parameters, and each contains two independent anions in the asymmetric unit. The complexes have intramolecular H-bonds from the urea cavity of H31](3-) to the coordinated hydroxo oxygen. All the complexes have long M-O(H) bond lengths (>2.00 A) compared to those of the few previously characterized synthetic examples. The longer bond distances in M(II)H31(OH)](2-) reflect the intramolecular H-bonds in the complexes. The five-coordinate Zn(II)H31(OH)](2-) has an average Zn-O(H) distance of 2.024(2) A, which is similar to that found for the zinc site in carbonic anhydrase II (2.05(2) A). The enzyme active site also has an extensive network of intramolecular H-bonds to the hydroxo oxygen. Co(II)H31(OH)](2-) and Fe(II)H31(OH)](2-) have one-electron redox processes at -0.74 and -1.40 V vs SCE. Both complexes can be chemically oxidized to yield their corresponding M(III)-OH complexes. Co(III)H31(OH)](1-), with an S = 1 ground state, is a rare example of a paramagnetic Co(III) complex.