Metal chains versus metal triangles in trinuclear trimethylenemethane iron carbonyls related to Fe3(CO)12

The trimethylenemethane ligand is found in the experimentally known and structurally characterized mononuclear iron carbonyl derivative (CH2)3CFe(CO)3, which may be regarded as a substitution product of Fe(CO)5. The structures of trinuclear trimethylenemethane iron carbonyl derivatives similarly related to the well-known doubly bridged Fe3(CO)12 structure have now been investigated by density functional theory. The lowest energy [(CH2)3C]Fe3(CO)10 structures likewise have two CO bridges and can be derived from the known Fe3(CO)12 structure by replacement of two terminal CO groups with either an unsymmetrical edge-bridging or face-bridging trimethylene methane ligand while preserving the central Fe3 triangle. However, replacing four CO groups in Fe3(CO)12 with two trimethylenemethane ligands to give [(CH2)3C]2Fe3(CO)8 is predicted unexpectedly to give a very different type of structure. In this structure one edge of the Fe3 triangle in Fe3(CO)12 is broken to form a bent Fe–Fe–Fe chain with the trimethylenemethane ligands bonded to the end iron atoms of this triiron chain. This [(CH2)3C]2Fe3(CO)8 structure appears to be very favorable, since it lies ∼22 kcal/mol below alternative structures. We anticipate that both [(CH2)3C]Fe3(CO)10 and [(CH2)3C]2Fe3(CO)8 should be sufficiently stable to be synthesized from the known (CH2)3CFe(CO)3 by thermolysis or photolysis with suitable iron carbonyls.

Density functional theory predicts the lowest-lying [(CH2)3C]Fe3(CO)10 to have a central Fe3 triangle with two bridging CO groups. However, the closely related [(CH2)3C]2Fe3(CO)8 is predicted to have a central bent Fe–Fe–Fe chain with an angle of ∼114°.Figure optionsDownload as PowerPoint slide