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 N[CH2CH2NHC(O)NHC(CH3)(3)](3) ([H(6)1]) was used to syn thesize a series of monomeric complexes with terminal hydroxo ligands. The complexes [Co(II/III)H(3)1(OH)](2-/1-), [Fe(II/III)H(3)1(OH)](2-/1-), and [ Zn(II)H(3)1(OH)](2-) have been isolated and characterized. The source of th e hydroxo ligand in these complexes is water, which was confirmed with an i sotopic labeling study for [Co(III)H(3)1(OH)](1-). The synthesis of [M(II)H (3)1(OH)](2-) complexes was accomplished by two routes. Method A used 3 equ iv of base prior to metalation and water binding, affording yields of less than or equal to 40% for [Co(II)H(3)1(OH)](2-). When 4 equiv of base was us ed (method B), yields ranged from 50% to 70% for all of the M(II)H(3)1(OH)] (2-) complexes. This improvement is attributed to the presence of an intram olecular basic site within the cavity, which. scavenges protons produced du ring formation of the M-II-OH complexes. The molecular structures of [Zn(II )H(3)1(OH)](2-), [Fe(II)H(3)1(OH)](2-), [Co(II)H(3)1(OH)](2-), and [Co(III) H(3)1(OH)](1-) were examined by X-ray diffraction methods. The complexes ha ve trigonal bipyramidal coordination geometry with the hydroxo oxygen trans to the apical nitrogen. The three M-II-OH complexes crystallized with near ly identical lattice parameters, and each contains two independent anions i n the asymmetric unit. The complexes have intramolecular H-bonds from the u rea cavity of [H(3)1](3-) to the coordinated hydroxo oxygen. All the comple xes have long M-O(H) bond lengths (> 2.00 Angstrom) compared to those of th e few previously characterized synthetic examples. The longer bond distance s in [M(II)H(3)1(OH)](2-) reflect the intramolecular H-bonds in the complex es. The five-coordinate [Zn(II)H(3)1(OH)](2-) has an average Zn-O(H) distan ce of 2.024(2) Angstrom, which is similar to that found for the zinc site i n carbonic anhydrase H (2.05(2) Angstrom). The enzyme active site also has an extensive network of intramolecular H-bonds to the hydroxo oxygen. [Co(I I)H(3)1(OH)]2- and [Fe(II)H(3)1(OH)]2- have one-electron redox processes at -0.74 and -1.40 V vs SCE. Both complexes can be chemically oxidized to yie ld their corresponding M-III-OH complexes. [Co(III)H(3)1(OH)](1-), with an S = 1 ground state, is a rare example of a paramagnetic Co-III complex.