DETERMINATION OF THE FE-N-O ANGLE IN (FENO)(7) COMPLEXES USING MULTIPLE-SCATTERING EXAFS ANALYSIS BY GNXAS

The Fe-N-O bond angle in a series of {FeNO}(7) complexes has been probed by EXAFS, utilizing a new theoretical data analysis package, GNXAS. This package provides an integrated approach to the analysis of EXAFS data based on a full curved-wave, multiple-scattering theoretical treatment incorporating least-squares refinement. Since GNXAS is able to calculate all the signals relating to two-, three-, and four-atom correlation functions with the proper treatment of correlated distances and Debye-Waller factors, it is particularly well-suited for analysis of multiple-scattering effects and bond angle determination. EXAFS data were obtained on a series of crystallographically characterized {FeNO}(7) inorganic complexes with varying Fe-N-O angles to examine the sensitivity of the GNXAS fit to this angle. The compounds studied were Fe(TMC)NO (where TMC = 1,4,8,1 l-tetramethyl-l,4,8,11-tetraazacyclotetradecane) which has an Fe-N-O bond angle of 177.5(5)degrees, Fe(TACN)(N-3)(2)NO (where TACN = N,N',N''-trimethyl-1,4,7-triazacyclononane) which has an angle of 156(1)degrees, and Fe(salen)NO (where salen = N,N'-ethylenebis(salicylideneiminato)) which has a bond angle of 127(6)degrees at 175 degrees C and 147(5)degrees at 23 degrees C. EXAFS data for FeEDTA-NO (whose crystal structure has not been determined and thus the angle is unknown) were also obtained and analyzed using GNXAS to determine the Fe-N-O bond angle. Results are presented which indicate that it is possible to determine whether the Fe-N-O unit is bent or linear, with the GNXAS analysis being extremely sensitive when the angle is between 150 degrees and 180 degrees. Using this method the Fe-N-O angle in FeEDTA-NO is found to be 156(5)degrees. The results of this study establish that EXAFS analysis using GNXAS can provide reliable angular information for small molecules coordinated to transition metals with rather complex coordination environments. This study thus provides the basis for the determination of the coordination geometry of molecules like NO and O-2 to metalloprotein active sites.