Theoretical Study On Structure And Property Of Iron Carbonyl Derivatives Fe（CO）_x（PR₃）_5-x

Iron carbonyl compounds have drawn more and more attentiondue to their efficiently catalytic activity and low price in recent years.Iron pentacarbonyl Fe(CO)5is known as an important transition metalcomplex, which mainly used for the preparation of other useful lowvalence metal complexes and clusters, the measurement reagents ofmany organic transformation, and some important catalytic processas the catalyst precursor. Fe(CO)5is widely employed in the researchof organometallic chemistry. Fe(CO)5can catalyze many reactions,such as hydrogenation, isomerization, Fischer-Tropsch synthesis,water gas reaction and hydrosilylation, etc. However, manyexperiments indicate that the reaction selectivity of Fe(CO)5is nothigh, the high temperature and ultraviolet light is needed to obtainactive catalytic species, many side reactions occur.To modulated the electronic structure and property of Fe(CO)5,many researchers tried to introduce other ligands (considerelectronic effect and structural effect) around the Fe center.Nonetheless, there is still a big challenge of synthetic thesecompounds since some ligand molecules need special productionand processing technology. Also for some poor stable Fe(CO)xLy, it is difficult to obtain direct geometry and electronic propertiesdepending on advanced experiment methods. Therefore, the quantumchemical calculation combining with experimental methods toprovide comprehensive and thorough understanding of species’sgeometry and electronic properties becomes important andmeaningful. It not only can enrich iron carbonyl chemistry, but alsoguide the development of effective and practical iron catalysts.In the present paper, phosphine ligands have been used to replacethe carbonyl in Fe(CO)5, which possesses trigonal bipyramidstructure, to form a series of iron carbonyl derivatives Fe(CO)x(PR3)5-x(x=1-4, R=H, F, Me, Cy, OPh, Ph). The main contents and results areas follows.(1) The possible isomers of various compounds Fe(CO)x(PR3)5-xhave been optimized at B3LYP and BP86level of density functionaltheory. Frequency analyses have been performed to obtain the trueminima. The most stable configuration of Fe(CO)x(PR3)5-xhas beengiven by calculating and comparing the energy difference betweeneach isomer. All structures have slightly distorted trigonal bipyramid.The more stable skeleton structure is with C3vsymmetry incompounds Fe(CO)4(PR3), except for R=F, C2vsymmetry at B3LYPwhile C3vsymmetry at BP86is suggested. In compounds Fe(CO)3(PR3)2,the D3hskeleton structure shows better stability. There are twoexceptions, that is, C2vsymmetry for R=F, and for R=OPh, Cssymmetry at B3LYP while C2vsymmetry at BP86. In Fe(CO)2(PR3)3, themost stable structure varies with the different phosphine ligands. ForR=Me, OPh, C2vskeleton structure is more stable; R=F, Ph, D3hskeleton structure show better stability; R=H, C2vsymmetry at B3LYP while D3hsymmetry at BP86. All of our attempts to optimize R=Cy structure of Fe(CO)2(PR3)3failed. With respect to Fe(CO)(PR3)4, nostructures were found for R=Cy, OPh, Ph due to the big steric effect.For R=H and F, C3vskeleton structure is more stable; R=Me, C2vsymmetry at B3LYP while C3vsymmetry at BP86is verified. Here, theenthalpies difference between two isomers (R=Me) is within1kcal/mol, which indicates that two isomers may exist together andtransform each other.(2) The enthalpies for the replacement of CO by PR3to form aseries of stable Fe(CO)x(PR3)5-x(x=1-4): When the number of thephosphine ligand PR3are the same, the value of reaction enthalpiesincreases in the order R=Me <OPh <Ph <F <Cy <H. When the R ofPR3is same, the value of reaction enthalpies increases quickly withthe number of PR3increasing. It is indicated that the replacementreaction occurs more and more difficultly, even seems to beimpossible under the thermal condition. This testifies that manyexperiments to generate Fe(CO)x(PR3)5-xneed ultraviolet light.(3) On the basis of the calculated first carbonyl and phosphinebond dissociation energy, it is found that the formation of triplet statecomplex requires low energy than that of singlet state. For the samestable compounds Fe(CO)x(PR3)5-xto produce triplet state complex,the enthalpy of carbonyl dissociation is more endothermic than thatof phosphine dissociation, which demonstrated that the dissociationof phosphine ligand is prior to carbonyl under the same condition.With the number of phosphine ligand increasing, when R=H, OPh, Ph,the carbonyl dissociation energy decreases, for example, thecarbonyl dissociations from Fe(CO)3(PR3)2compounds are more easy to take place than that from Fe(CO)4(PR3); when R=Cy, Me, F, thecarbonyl bond dissociation energy increases, the carbonyldissociations from Fe(CO)3(PR3)2become more difficult. ForFe(CO)4(PR3), the phosphine bond dissociation enthalpies increase inthe order R=H <Cy <F <Ph <OPh <Me; for Fe(CO)3(PR3)2, the order isR=H <Cy <OPh <Ph <F <Me. The phosphine bond dissociationenergy for bisubstitued compounds Fe(CO)3(PR3)2are lower thanmonosubstitued compounds Fe(CO)4(PR3) except for R=F.