Phosphine-Substituted (η⁵-Pentadienyl) Manganese Carbonyl Complexes: Geometric Structures, Electronic Structures, and Energetic Properties of the Associative Substitution Mechanism, Including Isolation of the Slipped η³-Pentadienyl Associative Intermediate

Abstract

The molecule (η5-Me2Pdl)Mn(CO)3 (η5-Me2Pdl = 2,4-dimethyl-η5-pentadienyl) has been prepared by a new method and used as a starting material to prepare the molecules (η5-Me2Pdl)Mn(CO) n (PMe3)3–n (n = 2, 1) by phosphine substitution for carbonyls. The first carbonyl substitution is achieved thermally in refluxing cyclohexane, and the second carbonyl substitution requires photolysis. At room temperature in benzene the associative intermediate (η3-Me2Pdl)Mn(CO)3(PMe3) that precedes the initial loss of carbonyl is observed. Single-crystal structures are reported for all complexes, including the associative intermediate of the first substitution in which the pentadienyl ligand has slipped to the η3 bonding mode. These molecules offer an opportunity to examine fundamental principles of the interactions between metals and pentadienyl ligands in comparison to the well-developed chemistry of metal cyclopentadienyl (Cp) complexes as a function of electron richness at the metal center. Photoelectron spectra of these molecules show that the Me2Pdl ligand has π ionizations at energy lower than that for the analogous Cp ligand and donates more strongly to the metal than the Cp ligand, making the metal more electron rich. Phosphine substitutions for carbonyls further increase the electron richness at the metal center. Density functional calculations provide further insight into the electronic structures and bonding of the molecules, revealing the energetics and role of the pentadienyl slip from η5 to η3 bonding in the early stages of the associative substitution mechanism. Computational comparison with dissociative ligand substitution mechanisms reveals the roles of dispersion interaction energies and the entropic free energies in the ligand substitution reactions. An alternative scheme for evaluating the computational translational and rotational entropy of a dissociative mechanism in solution is offered.

Electrochemical, Spectroscopic, and Computational Study of Bis(μ-methylthiolato)diironhexacarbonyl: Homoassociative Stabilization of the Dianion and a Chemically Reversible Reduction/Reoxidation Cycle

“…In the first reaction, translational entropy favors the product, and in the second reaction, translational entropy favors the reactants. The result that the same Gibbs energy change is obtained for both reactions indicates that solvation and the method we developed previously for the solution entropy are performing well for these complexes. There is little uncertainty that the dissociation energy is on the same scale as the axial/equatorial exchange energy, which as mentioned before is not observed up to 80 °C.…”

“…The following two dissociations were modeled in acetonitrile: [ ( μ ‐SMe ) 2 Fe 2 ( CO ) 6 ] 2 − ⇌ 2 [ false( SMe false) Fe ( CO ) 3 ] − [ ( μ ‐SMe ) 2 Fe 2 ( CO ) 6 ] 2 − + 2 NCMe ⇌ 2 [ false( SMe false) Fe ( CO ) 3 NCMe ] − In both cases, dissociation is disfavored computationally by 120 kJ/mol. The largest uncertainty in modeling dissociative or associative reactions in solution is the handling of the solution translational and rotational entropy contributions to the Gibbs energy . In the first reaction, translational entropy favors the product, and in the second reaction, translational entropy favors the reactants.…”

“…Photoelectron spectra were recorded using an instrument that features a 360 mm radius hemispherical analyzer with custom-designed photon source, sample cells, detection, and control electronics as described previously . Data collection and processing were carried out as described previously . The MeMe compound sublimed at room temperature.…”

“…Solvation Gibbs energies are estimated by the conductor-like screening model (COSMO) of solvation using default parameters for acetonitrile. Calculations of the standard oxidation and reduction potentials and Gibbs energies and equilibrium constants for reactions included the optimized electronic energies in solvent, the zero-point vibrational energies unscaled from harmonic frequency calculations, and thermal enthalpy and entropy contributions in solution at 298.15 K. The solution translational and rotational entropy was estimated as described before but regardless of the definition has little effect on the calculated potentials. Figures of the optimized geometries and molecular orbitals were created with program Visual Molecular Dynamics …”

Synthesis and Characterization of N-Donor-Functionalized Enantiomerically Pure Pentadienyl Ligands Derived from (1R)-(−)-Myrtenal

“…Pentadienyl (Pdl) and heteropentadienyl ligands are an integral part of the organometallic toolbox, and many pentadienyl complexes have been synthesized since 1962 . One of the most prominent protagonists in this field is Ernst, who showed that pentadienyls can adopt not only several conformations, namely the W , S , and U conformations (Chart ), but also multiple coordination modes such as η 1 , η 3 , and η 5 . − Nevertheless, most of these investigations have employed alkyl- and trimethylsilyl-substituted derivatives. ,,− …”