Decarbonylation Of Carbon Suboxide In Iron Carbonyl Complexes: A Theoretial Study

Carbon Suboxide,as heterocumulenes,is not only widely used in inorganic chemistry,but also organometallic chemistry.Comparing with the stable CO₂,the carbon suboxide molecules is more active,such as undergoing polymerization at 100 °C,and through thermal decomposition to afford carbon simple substance and carbon monoxide at 900-1025 K.Besides,carbon suboxide and its heavier heterocumulenes EC3E?E = S,Se?can be stabilized as a ligand in metal complexes.Recently,Qian and coworkers reported that C₃O₂ served as C1 source for preparing carbon nanotubes?CNTs?in presence of Fe₂?CO?₉ catalysis at lowest temperature of 180 °C.Understanding the formation mechanisms of CNTs is therefore extremely important to figure out the key intermediates as well as the controlling steps.Furthermore,electronic structures of key imtermediates and transition states are important for searching new catalytic process with more general conditions.However,no direct theoretical study has been reported,excepting few related experimental and theoretical studies focus on the metal-ligand bonding.In order to figure out the possible formation mechanism of CNTs,M06L/DZP level calculations were applied to this catalytic process for finding above mentioned structures.The following results were obtained:1.While one C₃O₂ coordinated to Fe₂?CO?₈,carbone bonding mode was most favorate structure because of the energetic aspect.However,C₃O₂ in Fe₂?CO?_n?C₃O₂??n = 6 4?molecules were preferred the C₂O + CO bonding mode,because of the rupture of C=C bond in C₃O₂ ligand.The dissociation energies of Fe₂?CO?_n?C₃O₂??n = 8 4?with respect to sequentially losing CO were found to be gruadually increased,indicating the increased stability of Fe₂?CO?_n?C₃O₂??n = 8 4?.Meanwhile,most stale structure of Fe₂?CO?₄?C₃O₂?has 15 electron configuration iron atoms on both sides,which would be important for accepting other C₃O₂ group.2.Fe₂?CO?_m?C₃O₂?₂?m =5²?molecules were obtained by the addition of the second C₃O₂ ligand.The most stable structure of all complexes contained the bridge C₃O ligand,except for Fe₂?CO?₄?C₃O₂?2,with a concreted heterocumulene C₄O₂ ligand.Thus,the presence of C₄O₂ ligands were originated from coupling of two C₂O ligands obtained by cleavage of the olefinic bond in C₃O₂ ligand.Then,further loss of CO group from C₄O₂ ligand leads to a carbene ligand?C₃O?,which implements the growth of carbon chain.With the progress of decarbonylation,the bridged carbene ligand was bent towards iron centers.The gradually increased CO dissociation Gibbs energies indicate facilitating the decarbonylation during formation of the C3 O ligand.3.The Fe₂?CO?_z?C₃O₂?₃?z =3⁰?complexes formed by the addition of the third C₃O₂ group.The structural feature of Fe₂?CO?_z?C₃O₂?₃?z =3⁰?were similar to the former complexes [Fe₂?CO?_m?C₃O₂?₂?m =5²?],newly added C₃O₂ ligand can offer a carbon atom to form new carbene C₄O ligand by lossing two CO groups.With the progress of decarbonylation,the curvature of bridge carbene ligand was gradually increase.The most stable structure of the last molecule Fe₂?C₃O₂?₃ has bridege carbene ligand functions as 6 electrons donor,instead of usual 2 electrons mode.Moreover,right side iron atom of the structure was a coordinately unsaturated 16 electrons configuration,which leaves the corresponding coordination holes for acception of another C₃O₂.4.The carbyne structure appearing in the Fe₂?CO?₆?C₃O₂?molecule,can undergoing dimerization to obtain the dimer complex containing the acetylenic bond.Then,three of the polymer can undergo a cyclization reaction to afford a more stable six-membered ring complex molecule.Since the binding of Fe?CO?₄ ligand to carbocyclic ring is weak,the intermediate can continue reaction after metal ligand separation in pairs to obtain more stable multicyclic ring structures.This provides a new idea for how the carbon source accumulates to form multilayered carbon nanotubes.Through the above research,we have obtained two kinds of the accumulation of carbon sources,which lays a solid foundation for the study of mechanism to carbon nanotubes growth.Its also provides valuable theoretical reference for the preparation of related carbon materials?graphene,graphite alkyne,etc.?.