A Study On Structures, Bonding Energies And Aromaticity For Mono-and Bi-Nuclear Sandwich Complexes Within Density Functional Theory

Since the ferrocene had been successfully synthesized, people have beenstudying it because of special structure, property and extensive application. Thesynthesis and theoretical study on sandwich complexes analogous to ferrocenehave become a focus of attention by experimental and theoretical chemists.Recently, 《 Science 》 peported the recentest results about this field: Timono-nuclear decaphosphametallocene [Ti(η5-P5)2]2-and bi-Zinc decamethyldi-zincocene [Zn2(η5–C5Me5)2] have been successfully synthesized one by one. Afterthat, the synthesis and theoretical study on novel mono-nulear and bi-nuclearmetallocene becomes a focus of attention by experimental and theoretical chemists.However, mono-nuclear and bi-nuclear decaphosphametallocenes having specialstabilities and activer catalysis reported less by now, especially, the structure,bonding energy and aromaticity of these complexes. In this thesis, based on the results about the structure, bonding energy andaromaticity of [Ti(η5-P5)2]2-and [Zn2(η5–C5Me5)2] within density founctionaltheory, we design two series of novel mono-nuclear decaphosphametallocenes[M(η5–P5)2] and bi-nuclear decaphosphametallocenes [M2(η5–P5)2] ( M = Be, Mg,Ca, Zn and Cd). Then, the quantum method-DFT–B3LYP-have been applied tooptimize the molecular equilibrium geometries and compute the Hessianfrequencies. The bond length, natural charge, Wiberg bond index and Laplacianelectronic density for these optimized equilibrium geometries have been caculatedby Natural Bond Orbitals (NBO) and Atom in Molecule (AIM) menthod.Meantime, in order to calculate the interaction strength between metals andbetween metal and ligands, and to make sure the stability orders of thees sandwichcomplexes, the interactional energies between molecular fragments in theseoptimized equilibrium geometries have been calculated with imaginary reactionpath and classical electrostatistic potential energy fomula. In addition, theelectronic structures and electronic currents for these equilibrium geometries havebeen analyzed using the Frontier Molecular Orbital and MO correlational diagramfor interaction between molecular fragments. Finally, because the most importantcharacteristic of sandwich complex is aromatic, Nucleus Independent ChemicalShifts(NICS) has been computed with GIAO-NMR menthod. The magnitude ofcenter aromaticity, exterior aromaticity and interior aromaticity are defined asNICS(0), NICS(1) and NICS(–1), respectively. In order to study the total NICSdistribution in outer cyclo-P5 plane and the various bonds NICS contributions, thetotal NICS has been dissected into various bonds NICS contributions using NBOprogram. The following is the main results:1. D5h, D5h and C5v configurations is a equilibrium geometry for P5 -,[Ti(η 5-P5)2]2– and [Ti(η 5-P5)]-, respectively. Three equilibrium geometries are allvery stable, and have large aromaticity. The order of stability and aromaticity isfollowing: P5-<[Ti(η 5-P5)2]2– <[Ti(η 5-P5)]-. Large aromaticity for two complexes[Ti(η 5-P5)2]2–and [Ti(η 5-P5)]-is contributed to the big electronic ring currents ofTi–P5 covalent bond, but electonic ring currents in latter is bigger than that informer.2. The D5d symmetric configuration for bi-Zinc metallocene [Zn2(η5–C5H5)2] islocal minimum on potential energy surface(PES), but the D5h symmetricconfiguration is a saddle point on PES. Four Zn-contaning half-sandwichcomplexes with C5v symmetric configuration are also local minima on PES. Znsandwich complexes, Zn half-sandwich complexex, Zn2 Sandwich complexes andZn2 half-sandwi ch complex all have aromaticity except that Zn+1 half-sandwichcomplex have ant iaromaticity. However, their aromatic degree is obvius different.Comparsion the sandwich complexes or the half-sandwich complexes with blockbuilding C5H5-, because of the effect of the Zn22+ unit (or Zn2+), a significantπ-electron density can be further transferred from the ring towards the outer ofcyclo-C5H5, especially towards the inner cyclo-C5H5, such that the center aromatic-ity becomes low, and the interior aromaticity becomes large. The [Zn(η5–C5H5)](2A1, C5v ) has larger binding energy and slightly lower aromaticity thantheη5–C5H5–, it features a simple bona file monovalent zinc compound, Thecomputed small dissociation energy indicates that [Zn(η5–C5H5)] (2A1, C5v) couldbe synthezed from the dissociation of [Zn2(η5–C5H5)2] (1A1, D5d). The Zn2+sandwich complex is not a minimum with much lower aromaticity than that of theη5–C5H5–, while the slip-sandwich complex, (η5–C5H5)Zn(η1–C5H5) (1A1, C1 ), is aminimum with center and exterior aromaticity close to that of the η5–C5H5– andwith large interior aromaticity.3. The sandwich complexes[M(η 5-P5)2] and [M2(η 5-P5)2] (M = Be, Mg andCa) with D5 symmetry are minima, but The D5h and D5d symmetry configurationsare saddle points on the flat potential energy surface. The M–M bond and M–P5bond are a weak σ covalent bond. The M–M bond plays a dominant role in thestability of both series of complexes. As M varies from Be to Ca, the M–M and theM –P5 bond length increase, while the bonding energy and the Wiberg bond Indexdecrease. The P?P bond length in these complexes is slightly elongated withrespect to in the P5 anion. The distance between M and the P5 ring in themononuclear sandwich complexes is shorter than that in the binuclear complexes.The P5 rings in both series of complexes are aromatic. However, because of theeffect of the M22+ unit (or M2+), a significant π-electron density can be furthertransferred from the ring towards the outer of cyclo-P5, especially towards the innercyclo-P5, such that the center aromaticity becomes low, and exterior aromaticity andinterior aromaticity, especially the interior aromaticity, become large. Themagnitude of NICS decreases as M varies from Be to Ca.4. The (η5–P5)MN(η5–P5) and (η5–C5H5)MN(η5–P5) ( M , N = Zn, Cd ) withan staggered (9 o ≤ D(E–M–M–E') ≤ 36o ) configuration are local minima except for theeclipsed CpCd2(P5) (C5v), and all the D5h and the D5d symmetric configurations aretransition states. Two seriers sandwich complexes all have larger aromaticity. TheM–M (or M–N ) bond in the bi-nuclear phosphametallocenes is a weak σ covalentbond. The strength of the M–M (or M–N) bond plays a decisive role on thestability of the bi-nuclear phosphametallocenes. However, the M?(η5–P5) (orM?Cp) bonding mainly is ionic. Among the different bi-nuclearphosphametallocenes with the same ligands, the bonding energies of the M–M (orM–N) bond and of the M?(η5–P5) (or M?Cp) bond decrease as M varying from Znto Cd. Among the different phosphametallocenes with the same metals, thebonding energies of the M–M (or M–N ) bond and the M?ligand bond increasewith ligand varying from the (η5–P5) to the Cp.