Utilization of hydrogen bonds to stabilize M-O(H) units: synthesis and properties of monomeric iron and manganese complexes with terminal oxo and hydroxo ligands.
Non-heme iron and manganese species with terminal oxo ligands are proposed to be key intermediates in a variety of biological and synthetic systems; however, the stabilization of these types of complexes has proven difficult because of the tendency to form oxo-bridged complexes. Described herein are the design, isolation, and properties for a series of mononuclear Fe(III) and Mn(III) complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethyl]aminato ([H(3)1](3-)), which creates a protective hydrogen bond cavity around the M(III)-O(H) units (M(III) = Fe and Mn). The M(III)-O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the M(III)-O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [Fe(III)H(3)1(O)](2-) complex also can be made using sulfoxides. These findings support the proposal of a high valent M(IV)-oxo species as an intermediate during dioxygen cleavage. Isotopic labeling studies show that oxo ligands in the [M(III)H(3)1(O)](2-) complexes come directly from the cleavage of dioxygen: for [Fe(III)H(3)1(O)](2-) the nu(Fe-(16)O) = 671 cm(-1), which shifts 26 cm(-1) in [Fe(III)H(3)1((18)O)](2-) (nu(Fe-(18)O) = 645 cm(-1)); a nu(Mn-(16)O) = 700 cm(-1) was observed for [Mn(III)H(3)1((16)O)](2-), which shifts to 672 cm(-1) in the Mn-(18)O isotopomer. X-ray diffraction studies show that the Fe-O distance is 1.813(3) A in [Fe(III)H(3)1(O)](2-), while a longer bond is found in [Fe(III)H(3)1(OH)](-) (Fe-O at 1.926(2) A); a similar trend was found for the Mn(III)-O(H) complexes, where a Mn-O distance of 1.771(5) A is observed for [Mn(III)H(3)1(O)](2-) and 1.873(2) A for [Mn(III)H(3)1(OH)](-). Strong intramolecular hydrogen bonds between the urea NH groups of [H(3)1](3-) and the oxo and oxygen of the hydroxo ligand are observed in all the complexes. These findings, along with density functional theory calculations, indicate that a single sigma-bond exists between the M(III) centers and the oxo ligands, and additional interactions to the oxo ligands arise from intramolecular H-bonds, which illustrates that noncovalent interactions may replace pi-bonds in stabilizing oxometal complexes.
MacBeth
Cora E
CE
Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
Gupta
Rajeev
R
Mitchell-Koch
Katie R
KR
Young
Victor G
VG
Lushington
Gerald H
GH
Thompson
Ward H
WH
Hendrich
Michael P
MP
Borovik
A S
AS
D005021:Ethylamines / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry*; D006860:Hydrogen Bonding; D017895:Manganese Compounds / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D014961:X-Ray Diffraction
D005021:Ethylamines; D005296:Ferrous Compounds; D017895:Manganese Compounds; D009942:Organometallic Compounds; D010087:Oxides; C027424:manganese oxide; C034236:ferrous oxide; D010100:Oxygen