Substitution Enthalpy Research Articles

Binding of Specialty Phosphines to Metals: Synthesis, Structure, and Solution Calorimetry of the Phosphirane Complex [PtMe2(iPrBABAR-Phos)2

The complex [PtMe2((iPr)BABAR-Phos)(2)] (3) was prepared in a clean and quantitative ligand substitution reaction from [PtMe2(cod)] (1; cod = eta(4)-1,5-cyclooctadiene) and the phosphirane (iPr)BABAR-Phos (2). The structure of 3 was determined by X-ray diffraction. The Pt-P bonds (similar to2.26 Angstrom) lie in the shorter range of Pt-II-P bonds, although the (1)J((PtP)-Pt-195-P-31) coupling (1840 Hz) is quite small. The enthalpy for this ligand substitution reaction was measured by solution calorimetry and found to be exothermic by 11.8 kcal/mol, a relatively low exothermicity for a reaction involving a tertiary phosphine in this Pt system. Calculations using density functional theory (DFT) on the B3LYP level were applied using the simplified model reaction [PtH2(cod)] + 2(H2N)PC2H4 --> [PtH2{(H2N)PC2H4}(2))] + cod, and these also gave a rather low substitution enthalpy (-17 kcal/mol). A charge decomposition analysis (CDA) was performed for Pt(II) and NO complexes with the simple P-amino phosphirane (H2N)PC2H4 and PH3 as ligands. Contrary to expectations, it is found that the phosphirane acts as a relatively good electron donor, while its electron-acceptor properties are not significantly different from those of other phosphines. The particularly low reaction enthalpy may thus be due to a low directionality of the donor orbitals toward the metal center.

Volume of reaction, energetics, and kinetics of tetrahydrothiophene displacement of solvent from Mo(CO)5(alkane).

Laser-induced optoacoustic spectroscopy (LIOAS) was used to determine the enthalpies and volumes of reactions for each step in ligand substitution of molybdenum hexacarbonyl with tetrahydrothiophene (THT). The enthalpy for substitution of CO on Mo(CO)6 by an alkane and of coordinated alkane on Mo(CO)5(alkane) by THT are 114 and -99 kJ mol(-1), respectively. Likewise, the volumes of reaction are 13 and -16 ml mol(-1). These results allow the calculation of the bond energies for Mo-alkane (56 kJ mol(-1)) and Mo-THT (155 kJ mol(-1)). Furthers studies should reveal that, for certain cases, the volume of reaction can be neglected in the determination of enthalpies of ligand substitution by LIOAS.

Photoacoustic calorimetry and quantum yields of Mo(CO)(6) ligand exchange in linear alkanes: determination of volume of reaction, energetics, and kinetics of nucleophile displacement of alkane from Mo(CO)(5)(alkane).

Photoacoustic calorimetry (PAC) and actinometry studies were used to determine the enthalpies and volumes of reaction for the production of a transient intermediate, Mo(CO)(5)-alkane, and for its subsequent reaction with tetrahydrofuran (THF). Both the enthalpy and the volume of reaction contribute to the photoacoustic signal and have been resolved by changing the solvent thermal expansion properties with a series of linear alkanes. The enthalpies for substitution of CO on Mo(CO)(6) by an alkane and of coordinated alkane on Mo(CO)(5)(alkane) by THF are 30 and -14 kcal/mol, respectively. Likewise, the volumes of reaction are 18 and -1 mL/mol. From available data for the Mo--CO bond energy, these results allow the calculation of the Mo-alkane and Mo-THF bond energies (11 and 25 kcal/mol, respectively). The Mo-alkane result is 7 kcal/mol less than that from our previous PAC study, which ignored the volume of reaction, and is in better agreement with the results of kinetic studies. The large absolute difference in the reaction volumes for each step is partially attributed to a void volume created during the formation of the Mo--THF bond. In general, the volume of reaction cannot be neglected in the calculation of enthalpies of ligand substitution from PAC studies. The quantum yields for photosubstitution of Mo(CO)(6), in contrast to Cr(CO)(6), were found to be insensitive to the chain length of the alkane solvent.

Correlation between Structural and Solution Calorimetric Data for Cp*Ru(PR3)2Cl (Cp* = C5Me5) Complexes

Single-crystal X-ray diffraction studies were conducted bn the following compounds: Cp*Ru(PMe3)(2)Cl (1), Cp*Ru(PPhMe2)(2)Cl (2), Cp*Ru(PMePh2)(2)Cl (3), Cp*Ru(PPh3)(2)Cl (4), Cp*Ru(PEt3)(2)Cl (5), Cp*Ru(AsEt3)(2)Cl (6), Cp*Ru((PBu3)-Bu-n)(2)Cl (7), and Cp*Ru(dmpm)Cl (8). Structural information obtained from these X-ray studies can be correlated with enthalpies of ligand substitution previously determined from solution calorimetry. The cone angle of the phosphine ligand (monodentate) and the Ru-P bond distance were found to be proportional to the enthalpy of reaction.

A combined QM/MM study of ligand substitution enthalpies in the L2Fe(CO)3, RuCpL2Cl, and RuCp*L2Cl systems

A combined density functional and molecular mechanics approach (QM/MM) has been validated in a study of the substitution reactions: (i) (PH3)2Fe(CO)3 + 2ER3 ⇆ (ER3)2Fe(CO)3 + 2PH3(ER3 = PMe3, PEt3, PMePh2, PPh3, PCyPh2, PiPr3, PBz3, PCy3, AsEt3, AsPh3); and (ii) Cp'Ru(PH3)2Cl + 2ER3 ⇆ Cp'Ru(ER3)2Cl + 2PH3 (Cp' = C5H5, C5(CH3)5; ER3 = PMe3, PEt3, PnBu3, PMe2Ph, PMePh2, PPh3, AsEt3, P(OMe)3, P(OPh)3, P(OCH2)3CEt). The steric influence of the R substituents on the substitution enthalpies correlates well with experimental data. The combined QM/MM approach is also able to afford molecular structures in good accord with experimental estimates. Key words: combined QM/MM, ligand substitutions, organometallic complexes.

Reaction enthalpy of nucleophilic substitution of ethyl iodide in acetonitrile and its mechanistic significance

Enthalpies of reaction for nucleophilic substitution of ethyl iodide have systematically been determined in acetonitrile. Through the concurrent analysis of empirical correlations between the reaction enthalpies and the specific interaction enthalpies for relevant anions with those between the logarithmic rates and the specific interaction enthalpies, partial desolvation accompanying activation has been deduced to be the major contributor to activation thermodynamic parameters, while the propensity of the reacting central atom in the nucleophilic anion plays a crucial role in determining reaction thermodynamic parameters. Semi-empirical molecular orbital calculations have supported these ideas. The application of the Marcus equation to the analysis of reaction characteristics in these reactions is discussed.

Solution Thermochemical Study of Ligand Steric Influences on Substitution Enthalpies in the L2Fe(CO)3 System

The enthalpies of reaction of (BDA)Fe(CO)(3) (BDA = (C6H5)CH=CHO(CH3), benzylideneacetone) with a series of sterically demanding phosphine ligands, leading to the formation of(L)(2)Fe(CO)(3) complexes (L = phosphine) have been measured by solution calorimetry in THF at 50 degrees C. The range of reaction enthalpies spans some 7 kcal/mol. The overall relative order of stability established is as follows (PR(3); -Delta H, kcal/mol): PPh(3) < PCy(2)Ph < PCyPh(2) < PCy(3) < (PPr3)-Pr-i < PPh(2)Et < PBz(3). A quantitative analysis of ligand effect of the present and previously obtained data for L(2)Fe(CO)(3) complexes helps clarify the exact steric versus electronic ligand contributions to the enthalpy ofreaction in this system. Results of a single-crystal diffraction study for the complex diaxial-(PPh(2)Cy)(2)Fe(CO)(3) (2) show the molecule to be monoclinic, P2(1)/n, with a = 12.393(5) Angstrom, b = 15.811(6) Angstrom, c 18.029(7) Angstrom, alpha = gamma = 90 degrees, beta = 108.00(2)degrees, V = 43360(5) Angstrom(3), Z = 4, d(calcd) = 1.337 g cm(-1), n(obsd) = 3130, and R = 0.052. Electronic effects are overwhelmingly important in this system, yet a steric threshold of 135 degrees can be extracted from the QALE treatment which shows at which point steric factors begin to influence and contribute to the measured enthalpy of ligand substitution.

Enthalpies of reaction of ruthenium complex Cp*Ru(CH3CN)3+O3SCF3- (Cp* = .eta.5-C5Me5) with arenes. Solution thermochemical study of arene binding to the Cp*Ru+ fragment

The enthalpies of reaction of Cp*Ru(CH3CN)3OTf (Cp* = eta5-C5(CH3)5, OTf = O3SCF3) with a series of arenes have been measured by solution calorimetry. The results of this study are used to establish the following relative stability scale leading to the formation of Cp*Ru(arene)OTf complexes in THF (in kcal/mol): naphthalene, -1.7 +/- 0.1; benzene, -3.4 +/- 0.1; biphenyl, -3.6 +/- 0.1; toluene, -4.3 +/- 0.2; p-xylene, -4.6 +/- 0.2; (trimethylsilyl)benzene, -5.0 +/- 0.2; anisole, -5.4 +/- 0.2; mesitylene, -5.5 +/- 0.1; indole, -7.3 +/-0.2; NN-dimethylaniline, -7.5 +/- 0.1; p-bis(dimethylamino)benzene, -8.3 +/- 0.2. The solid-state structure of one of these arene complexes, Cp*Ru[C6H5(Si(CH3)3)]OTf, was determined by a low-temperature data collection X-ray crystallographic study. This investigation illustrates the presence of steric interactions between the phenyl TMS methyl groups and the Cp* methyl group and explains the small measured enthalpy of reaction. The enthalpies of arene substitution from Cp*Ru(CH3CN)3OTf span some 6.6 kcal/mol and vary as a function of the electron-donating ability of the arene. Factors affecting the Ru-arene bond energy are discussed, and thermodynamic comparisons with other organometallic systems are presented.

Physico-chemical applications of HPLC

Examples of high-performance liquid chromatography (HPLC) are given for the determination of adsorption equilibrium and distribution coefficients, calculation of excess adsorption isotherms from solutions ranging from 10−7 to 10−1 mole fraction, changes in activity coefficient of the component predominantly adsorbed from solution and substitution enthalpy. These examples stem from studies of HPLC and the static adsorption of benzene, anisole and benzaldehyde from solutions in n-heptane on macroporous silica and active carbon at various temperatures. A criterion is suggested for evaluating equilibrium in the HPLC process.