From spiropentane to butterfly and tetrahedral structures in tetranuclear iron carbonyl carbide chemistry

Abstract

Oxidative degradation of the octahedral dianion [Fe₆C(CO)₁₆]²⁻ with an interstitial carbon atom leads eventually to the neutral Fe₄C(CO)₁₃ cluster with a butterfly-shaped central Fe₄C unit. The complete series of related Fe₄C(CO)_n (n = 16, 15, 14, 13, 12, 11) derivatives have now been investigated using density functional theory. For the lowest energy Fe₄C(CO)_n (n = 16, 15, 14, 13) structures the geometries obey the n + f = 18 rule where f is the number of Fe–Fe bonds. This leads to a spiropentane geometry with two Fe–Fe bonds for Fe₄C(CO)₁₆, a central bent Fe–Fe–Fe–Fe chain for Fe₄C(CO)₁₅, a distorted trigonal pyramidal structure with four Fe–Fe bonds for Fe₄C(CO)₁₄, and the experimentally observed butterfly structure with five Fe–Fe bonds for Fe₄C(CO)₁₃. A symmetrical higher energy centered tetrahedral structure for Fe₄C(CO)₁₂ with six Fe–Fe bonds also follows the n + f = 18 rule. However, the lowest energy Fe₄C(CO)_n (n = 12, 11) structures are derived from the lowest energy Fe₄C(CO)₁₃ structure by removal of CO groups with retention of the central Fe₄C butterfly unit.