RESUMEN
The tripodal ligand N[CH2CH2NHC(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.
Asunto(s)
Metaloproteínas/química , Metales/química , Fenómenos Químicos , Química Física , Cristalografía por Rayos X , Electroquímica , Espectroscopía de Resonancia por Spin del Electrón , Enlace de Hidrógeno , Ligandos , Modelos Moleculares , Estructura Molecular , Espectroscopía Infrarroja por Transformada de Fourier , Agua/químicaRESUMEN
Delivery of NO to specific targets is important in fundamental studies and therapeutic applications. Various methods have been reported for delivery of NO in vivo and in vitro; however, there are few examples of systems that reversibly bind NO. Reported herein is the development of a new polymer (P-1[Co(II)]) that reversibly binds NO. P-1[Co(II)] has a significantly higher affinity for NO compared to O(2), CO(2), and CO. The polymer is synthesized by template copolymerization methods and consists of a porous methacrylate network, containing immobilized four-coordinate Co(II) sites. Binding of NO causes an immediate color change, indicating coordination of NO to the site-isolated Co(II) centers. The formation of P-1[Co(NO)] has been confirmed by EPR, electronic absorbance, and X-ray absorption spectroscopies. Electronic and X-ray absorbance results for P-1[Co(II)] and P-1[Co(NO)] show that the coordination geometry of the immobilized cobalt complexes are similar to those of their monomeric analogues and that NO binds directly to the cobalt centers. EPR spectra show that the binding of NO to P-1[Co(II)] is reversible in the solid state; the axial EPR signal associated with the four-coordinate Co(II) sites in P-1[Co(II)] is quenched upon NO binding. At room temperature and atmospheric pressure, 40% conversion of P-1[Co(NO)] to P-1[Co(II)] is achieved in 14 days; under vacuum at 120 degrees C this conversion is complete in approximately 1 h. The binding of NO to P-1[Co(II)] is also observed when the polymer is suspended in liquids, including water.
Asunto(s)
Metales/metabolismo , Óxido Nítrico/metabolismo , Compuestos Orgánicos/química , Sitios de Unión , Metales/química , Análisis EspectralRESUMEN
Iron species with terminal oxo ligands are implicated as key intermediates in several synthetic and biochemical catalytic cycles. However, there is a dearth of structural information regarding these types of complexes because their instability has precluded isolation under ambient conditions. The isolation and structural characterization of an iron(III) complex with a terminal oxo ligand, derived directly from dioxygen (O2), is reported. A stable structure resulted from placing the oxoiron unit within a synthetic cavity lined with hydrogen-bonding groups. The cavity creates a microenvironment around the iron center that aids in regulating O2 activation and stabilizing the oxoiron unit. These cavities share properties with the active sites of metalloproteins, where function is correlated strongly with site structure.