Bond Energies, Reaction Volumes, and Kinetics for σ- and π-Complexes of Mo(CO)₅L

Abstract

The photosubstitution reactions of molybdenum hexacarbonyl with σ and π donor ligands were investigated using photoacoustic calorimetry and computational methods in a series of linear alkane solvents (pentane, hexane, heptane, octane, decane, and dodecane). The results show that reaction volumes make a significant contribution to the photoacoustic signal and must be considered during thermodynamic calculations based on photoacoustic measurements. The enthalpies of CO substitution by an alkane solvent and subsequent substitution by each Lewis base were determined. Corresponding Mo-L bond energies (kcal mol(-1)) were calculated: L = linear alkanes (13), triethylsilane (26), 1-hexyne (27), 1-hexene (27), and benzene (17). The relative energies are in agreement with computational results. The experimental reaction volume for CO substitution by alkane was positive (15 mL mol(-1)) and negative or close to zero for alkane substitution by a Lewis base (for example, -11 mL mol(-1) for triethylsilane and 3.6 mL mol(-1) for benzene). The errors in the experimental and computational reaction volumes are large and often comparable to the reaction volumes. An improved calibration of the methods as well as a better understanding of the underlying physics involved is needed. For the Lewis bases reported in this study, the second-order rate constants for the displacement of a coordinated alkane are less than diffusion control (5 × 10(6)-4 × 10(7) M(-1) s(-1)) and decrease monotonically with the alkane chain length. The rate constants correlate better with steric effects than with bond energies. An interchange mechanism is consistent with the results.

Photochemistry of Chromium Arene Tricarbonyl Complexes with Tethered Pyridinyl and Propenyl Groups: Investigations of the Effect of Ring Size on Chelate Formation, Structure, and Linkage Isomerization

“…Previous results indicate that the metal−heptane bond should be about 12−15 kcal mol −1 . 26 For the moment, assume ΔV 1 = ΔV 2 = 0 for eqs 3 and 4; then the PAC determination leads to ΔH 1 = 25.3 kcal mol −1 and ΔH 2 = −13.2 kcal mol −1 for 10. Accounting for a 15 kcal mol −1 contribution of bonding to heptane results in bond dissociation energies in solution of 40 kcal mol −1 for Cr-CO and 28 kcal mol −1 for Cr−N.…”

“…The pressure transducer of a photoacoustic calorimeter responds to pressure changes caused by thermal expansion and reaction volumes that occur faster than or comparable to the response time of the transducer. 26 The amplitude of the PAC signal is expressed as a fraction of the energy of the absorbed photon. For processes much faster than the response time of the transducer, the relative amplitude (ϕ 1 ) depends on the energy of the photon (hν), the reaction enthalpy (ΔH 1 ), the volume change (ΔV 1 ) associated with processes much faster than the transducer response, the quantum yield (Φ), and the thermoelastic properties of the solvent (χ = (MW)β/C p ρ where MW = molecular weight, β = coefficient of thermal expansion, C p = molar heat capacity, and ρ = density) (eq 1).…”

“…Previous studies suggest that a bond dissociation results in a volume increase while bond formation results in a volume decrease; thus, a substitution tends to result in a cancellation in the volume changes. 26 The enthalpy of ligand substitution (CO dissociation followed by addition of tethered or untethered pyridine) can be estimated by adding eqs 3 and 4, yielding eq 5, where the volume terms cancel. If ΔV 1 + ΔV 2 = 0, then combining eqs 3 and 4 yields…”

Calibrating Reaction Enthalpies: Use of Density Functional Theory and the Correlation Consistent Composite Approach in the Design of Photochromic Materials

“…Also, the bistability of these organometallic photochromes requires an adequate free energy of activation (Δ G ⧧ > ∼30 kcal mol –1 ) for rearrangement such that the complex will not thermally isomerize between chelates at room temperature. Hence, the ground-state bond dissociation enthalpy (BDE) required to cleave a metal–ligand bond is of particular interest to researchers to aid in the rational design of new organometallic photochromes. ,,− Our lab and others have used photoacoustic calorimetry (PAC) to study the BDEs of various ligands in transition-metal (TM) complexes. − ,,,− …”

“…For example, near-UV irradiation tends to break the weakest bond in many of these complexes. 2 The thermodynamics of the linkage isomerization are dependent on several factors, including the relative metal−ligand bond strengths of tethered functional groups, steric effects, and the proximity of each functional group to the metal center preceding chelation. Also, the bistability of these organometallic photochromes requires an adequate free energy of activation (ΔG ⧧ > ∼30 kcal mol −1 ) for rearrangement such that the complex will not thermally isomerize between chelates at room temperature.…”