Maria C. Milletti

Theoretical investigation of the π-bonding ability of P-, S-, and N-containing ligands in group 6 transition metal complexes

Abstract The π-bonding ability of several ligands bonding through N, P, and S to group 6 transition metals was studied via non-empirical molecular orbital calculations. The ability of the ligands to donate or accept π electrons to or from the metal was compared in 1,2-Mo2[P(t-Bu)2]2[NMe2]4 [I] and 1,2-W2[P(t-Bu)2]2[NMe2]4 [II]; (PSO)Cr(CO)4 [III] and (PSO)W(CO)4 [IV] (PSO = 2-diphenyl phosphino methyl-2-phenylthiomethyl-1-methoxy propane); and [W(2-Spy)(CO)4]− [V] and [Mo(2-Spy)(CO)4]− [VI] (2-Spy = 2-thiol pyridine). Mulliken population analyses were used to study how two different atoms compete for π-bonding with the metal. Both NMe2 and PMe2 are π donors to the metal in complexes [I] and [II]. Overall, the phosphido ligand is a better π donor than the amido ligand. The PSO ligand in complexes [III] and [IV] is a good π acceptor and phosphorus is a better π acceptor than sulfur in both complexes. The 2-Spy ligand in the two anionic complexes, [V] and [VI], is a good π acceptor. π back-donation from the metal d orbitals to the ligand is mostly through the nitrogen of the pyridine ring rather than the sulfur external to the pyridine ring. The electron density withdrawn from the metal through the nitrogen is delocalized throughout the π-conjugated ring. The π-accepting ability of the 2-Spy ligand in the two complexes is about the same. The observed differences in the π-bonding ability of the ligands are explained in terms of the accessibility of the ligand molecular ori two linking atoms in the same bidentate ligand is also made in terms of the type and energy of ligand orbitals (compound [III] vs [IV] and [V] vs [VI]). A direct comparison is made between electron density distribution and crystallographic bond lengths.

Theoretical investigation of substituent effects on the silicon–metal bond for a series of transition metal-substituted base-stabilized silylene complexes

Abstract Hartree–Fock ab initio calculations were used to investigate the effects of various silicon substituents on the silicon–ruthenium bond for four transition metal-substituted base-stabilized silylene complexes. To provide comparison information, one silyl complex, Cp*(PMe3)2Ru–Si[S(Tol-p)]3, and four base-free silylene complexes ([Cp*(PMe3)2RuSiMe2]BPh4, Cp*(PMe3)2RuSi[S(Tol-p)][Os(CO)4], a bis(silylene)nickel compound and [trans-(P(Cy3)2(H)PtSi(SEt)2]BPh4) were also studied. Results of the calculations indicate that the order of promotion of silylene character for base groups is NCMe>OTf≫S(Tol-p). Of those considered, the best substituent group in terms of transition metal–silicon double bond promotion is methyl followed by triflate. The worst group with respect to this property is S(Tol-p).

Theoretical study of the influence of the halide ligands on the metalmetal quadruple bond in the M2X4(PH3)4 (M = Mo, W; X = Br, Cl, I) series of complexes

Abstract The relationship between the metalmetal bond length and the nature of the halide ligands in complexes of the type M 2 X 4 (PH 3 ) 4 (M = Mo, W; X = Br, Cl, I) is investigated using the Fenske—Hall semi-empirical molecular orbital method. The relative positions of the molecular orbitals follow the pattern predicted by electronic spectral data, with the 1 (delta → delta * ), HOMO—LUMO transition and the higher-energy LMCT transition both red-shifting as the halide ligand charges from chlorine to bromine to iodine. This pattern is present for both the molybdenum and the tungsten complexes. Although halide ligand contributions to the metalmetal valence molecular orbitals are significant and are a strong function of ligand identity, this does not seem to influence the metalmetal bond length. This would suggest that changes in the bond order brought about by changes in the identity of the halide ligands are not large enough to cause a significant variation in the metalmetal bond length.

Hydrogen migration in Fe3(CO)9H4

Abstract The Fenske-Hall method of semi-empirical molecular orbital calculations was used to analyze the capped transition metal cluster Fe3(CO)9CH4. This type of cluster is of interest because it can be used as a model for the activation of hydrocarbons on metal surface catalysts. The process of hydrogen migration between the metal base and the cap was studied, with particular emphasis on the agostic MCH interaction, which occurs in some isomers of the compound. For the three isomers produced in the migration process, the primary distinction that leads to the variation in stabilities among these structures is the extent of metal-metal overlap between 3d orbitals of the iron atoms. It was found that, if there are hydrogen atoms bridging the metal centers at the base of the complex, the isomer is stabilized by the reduction of overlap between atomic orbitals on adjacent metal centers, thereby reducing the impact of filled metal-metal antibonding orbitals. Agostic interactions between cap hydrogens and the base also tend to stabilize the compound. This was found to be largely due to a reorganization of the bond between the carbon in the capping group and the agostic metals. In addition, Mulliken overlap populations indicate that the hydrogens involved in MCH agostic interactions also reduce metal-metal overlap.

A density functional study of the relative stability of intermediates in a McMurry coupling reaction

The intramolecular McMurry reaction is a relatively common method for assembling carbocycles in organic synthesis. Most typically, this reaction involves a reductive coupling mediated by Ti(II). However, there are few examples of intramolecular McMurry reactions in the presence of Lewis basic heteroatoms. In this work, we investigate the titanium-mediated McMurry coupling leading to a pyrrolidine methoxy keto ester. Specifically, we compare the relative energies of all possible reaction intermediates at the B3LYP/6-31G level of theory. The most stable intermediate is found to be the one resulting from deoxygenation of the α-methoxy ketone. The McMurry product is not predicted to form.

Theoretical analysis of transition states in the exchange reaction of hydrogen and methane involving σ-bond metathesis

Silica supported zirconium hydride species are used to model heterogeneous catalysts for industrially-relevant reactions such as hydrogenation of paraffins. This work explores the exchange reaction between methane and hydrogen in the presence of a silica-supported zirconium or titanium hydride catalyst in order to determine the preferred transition state. Calculations at the B3LYP/LanL2DZ level of theory are used to model two distinct pathways for the reaction. Orbital interactions are analyzed to elucidate the relative stability of the two transition states.

Ab initio investigation of the synthesis of (3-(2,3,4,5-tetramethylcyclopentadienyl)propoxy)titanium dichloride

Abstract The two-step synthesis of [η5:η1-C5Me4(CH2)3O]TiCl2 from [C5Me4(CH2)3OMe]TiCl3 is investigated through molecular orbital calculations. Results of ab initio, restricted Hartree–Fock calculations at the 6-311G(d) basis set level are reported for the reactants, products, and an intermediate, [C5Me4(CH2)3OMe]TiCl2(CHPPh3). These results provide insight into the mechanism of the second reaction, which is found to be a charge-controlled intermolecular nucleophilic attack. The nature of the titanium–ylid bond in the intermediate complex is also reported.

MOLECULAR ORBITAL ANALYSIS OF NUCLEOPHILIC ATTACK ON A PLATINUM ALLYL COMPLEX

Abstract The platinum allyl complex, [(η3‒CH2C(CH3)C˭CH2)Pt(PPh3)2]+, behaves differently to-ward nucleophiles depending on their hardness. In the reaction with a “hard” nucleophile, nucleophilic attack occurs at the metal center. A “soft” nucleophile bonds to the middle carbon of the allyl ligand. The results of molecular orbital calculations suggest that both reactions are orbital controlled, which points to the metal as the preferred site of attack. However, the soft nucleophile attacks the allyl ligand due to steric constraints. A Mulliken population analysis reveals that the platinum center is directly bonded to only the two end carbons of the allyl ligand. The effect of basis set size and substitution of hydrogens for phenyl groups on the results of the calculations was also investigated. The choice of basis set had the largest effect on the charge distribution of the molecule. On the other hand, basis set size and inclusion of phenyl substituents on the phosphine ligands had minimal effect on the optimized structure of the complex.

Mechanism of electrophilic attack in (η5-pentadienyl)Mn[(Me2PCH2)3CMe] and (η5-2,4-dimethylpentadienyl)Re(PMe2Ph)3

Abstract Electrophilic attacks onboth (η 5 -pentadienyl)Mn[(Me 2 PCH 2 ) 3 CMe] and (η 5 -2,4-dimethylpentadienyl)Re(PMe 2 Ph) 3 are predicted to occur at the metal center, based on the results of ab initio molecular orbital calculations. Attack occurs at the open side of the pentadienyl ligand, although for both complexes the thermodynamic product exhibits a different configuration from that of the kinetic product. In the thermodynamic product of the manganese complex the proton resides in a semibridging position between the side of the pentadienyl ligand and the metal center. In the rhenium complex, the proton migrates to the back side of the pentadienyl ligand and bonds strongly with the metal center.