text: Lavello and Grubbs discovered the compound unexpectedly when trying to prepare noncarbonyl, low coordinate, Fe(0) complexes of N-heterocyclic carbenes (NHCs). They found that reactions of Fe(COT) and the NHC, 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes), produced tetrametallic, Fe(I)-Fe(0) mixed valent NHC-COT complexes. In an attempt to characterize intermediates of the unusual transformation, they employed the more sterically hindered NHC, 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr)  (with Dipp substituents).In benzene, the Dipp substituted NHC reacts with Fe(COT) to produce large black rhomboidal crystals of tris(cyclooctatetraene)triiron over 24 h at room temperature. Notably, the reaction was found to occur with catalytic amounts of NHC (10 mol%) yielding 67% of Fe(COT) after 24 h (turn over number=9.5). The synthesis is optimized when the reaction is conducted at 45 °C, yielding 95% conversion to the tris(cyclooctatetraene)triiron cluster. They also highlighted that heating Fe(COT) in benzene without any NHC to 100 °C for 24 h forms trace amounts of Fe(COT), but also large amounts of iron metal. Unsurprisingly, elemental analysis of the cluster affirms a 1:1 Fe:COT ratio.
  The formation of Fe(COT) from Fe(COT) has been calculated to be slightly exothermic(15 kcal/mol).
  Other NHCs lead to other unique mixed NHC-COT low valent iron complexes. Lavallo and Grubbs rationalize the transformation by emphasizing the capacity of NHCs to catalytically induce the formation of metal-metal bonds, where the steric hinderance of the NHC is essential, in particular, for the lability of the NHC (in coordination and dissociation) in the cycle. The bulky NHC is proposed to prevent reduction of COT by a bimetallic [(L)Fe(COT)] intermediate, where steric constraints block the bonding hapticity required to ligate a reduced form of COT. Another possibility put forward is that reduction of COT occurs only following coordination by a second carbene in the case of SIMes during the catalytic cycle. The sterically hindered NHC prevents such a transformation from occurring.