Ambidentate Thiocyanate and Cyanate Ligands in Dinitrosyl Iron Complexes

To explore the effect of delocalization in the Fe­(NO)2 unit on possible linkage isomerism of ambidentate ECN– ligands, E = S and O, anionic DNICs, dinitrosyl iron
complexes, (SCN)2Fe­(NO)2– (1) and (OCN)2Fe­(NO)2– (2) were synthesized by the reaction of in
situ-generated [Fe­(CO)2(NO)2]+ and PPN+ECN–. Other {Fe­(NO)2}9 (Enemark–Feltham notation) complexes, (N3)2Fe­(NO)2– and (PhS)2Fe­(NO)2–, were prepared for comparison.
The X-ray diffraction analysis of 1 and 2 yielded the typical tetrahedral structures of DNICs with two slightly
bent Fe–N–O oriented toward each other, and linear FeNCE
units. The ν­(NO) IR values shift to lower values for 1 > 2 > (N3)2Fe­(NO)2– > (PhS)2Fe­(NO)2–, reflecting the increasing donor ability of the ancillary
ligands and consistent with the redox potentials of the complexes,
and the small trends in Mössbauer isomer shifts. Computational
studies corroborate that the {Fe­(NO)2}9 motif
prefers N-bound rather than E-bound isomers. The calculated energy
differences between the linkage isomers of 1 (Fe-NCS
preferred over Fe-SCN by about 6 kcal/mol) are smaller than those
of 2 (Fe-NCO preferred over Fe-OCN by about 16 kcal/mol),
a difference that is justified by the frontier molecular orbitals
of the ligands themselves.