Phosphaethynolate Dimerization And Carbonyl Migration In Cyclopentadienyliron Carbonyl Systems:A Theoretical Study

The 2-phosphaethynolate anion PCO^–,which is the phosphorus analogue of the cyanate ion NCO^–,have received a lot of attention because it is the isoelectronic species of^–OCS,F^–CP,^–SCN.Especially the 2-phosphaethynolate anion have provided a novel path to synthesize other phosphorus containing heterocyclic compounds.Many experimental evidence had shown that the PCO ligand is an ideal phosphorus atom source through decarbonylation for synthesizing many interesting organic phosphorus derivatives as well as novel metal phosphide complexes.However,many aspects of these processes are currently unclear,especially the mechanism for M?P or M-P formation,dimerization mechanisms,and the generation of metal P₂ complexes using two PCO ligands as the phosphorus source.So we chose the cyclopentadienyliron carbonyl complexes FeCp（PCO）（CO）_n（n=2⁰）,and Fe₂Cp₂（PCO）₂（CO）_n（n=4⁰）discussed in this paper to explore such mechanisms.M06L/DZP level calculations were used to calculate the successive decarbonylation and cyclodimerization of the mononuclear and binuclear complexes.The electronic structures of key intermediates and transition states are important for exploring the mechanisms of decarbonylation and cyclodimerization processes.In addition,the non-metallic compounds with different substituents（R=Me,t-Bu,Mes,（ipc）₂B,t-BuPh₂Si）are used to explore the possibility of trimerization to give six memberd rings.The following results were obtained:1.One PCO ligand coordinated to FeCp（CO）₂ was found as mononuclear complexes with different resonance structures.The calculation results shows the P-bond structure R-P=C=O is energeticly more favorable than the O-bond structure R-O-C?P.With the decarbonylation processes happened in FeCp（PCO）（CO）_n（n=2⁰）,the electronic structures in iron atom tend to become unsturated,which could lead to the carbonyl migration to give the Fe?P triple bonds.2.The dimerization from two mononuclear complexes FeCp（CO）₂ was happened on the P=C double bonds to give a dimer containing P₂C₂ four membered ring,such[2+2]cycloaddition reaction is a thermodynamic controled process.With the decarbonylation processes happened in binuclear complexes,the carbonyl in PCO ligand migrate from the phosphorus atom to the iron atom to give diphosphide complexes with P₂ ligand.The calculation results shows the carbonyl migration processes are thermodynamically favorable.3.In the non-metallic compounds with hydrocarbon group,the R-P=C=O structure is thermodynamically more favorable than R-O-C?P structures.The energy gap between the two resonance structures decreasing with the increasing steric effect of the substituents.The trimerization in such compounds with hydrocarbon group seems unlikely without catalyst.In the boryl and silyl complexes,the R-O-C?P structure is thermodynamically more favorable than R-P=C=O structures.The polymerization tend to happen on the P?C triple bonds.The energy gap between the two resonance structures is related to the oxophilicity and the delocalization.Through the research,we have explained the mechanisms of PCO ligand for its cyclodimerization and carbonyl migration processes.It provides a valuable theoretical reference for PCO ligand to be used as a novel phosphorus atom source to synthesize other organic phosphorus-containing complexes.