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QSARs and Inorganic Chemistry

QSARs and Inorganic Chemistry. What is QSAR? Quantitative Structure-Activity Relationship Way to quantitatively correlate structure to physical properties or biological activity Can you correlate systematic changes in structure and/or composition to a measurable trend in properties?

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QSARs and Inorganic Chemistry

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  1. QSARs and Inorganic Chemistry • What is QSAR? • Quantitative Structure-Activity Relationship • Way to quantitatively correlate structure to physical properties or biological activity • Can you correlate systematic changes in structure and/or composition to a measurable trend in properties? • Related to the physical-organic chemistry concept of Hammett parameters • Hammett asked “How do electronic effects influence reaction equilibria, Keq?” • Original studies used the dissociation of p-substituted benzoic acids • As early example of a linear-free energy relationship

  2. Hammett Equation • Substituent Constant • Quantitative description of electron donating or withdrawing ability of a substituent • These are defined values! • σz = log Kz – log KH • Plot log (K/K0) vss • Slope is r • IfEWG increases K, r = positive • IfEDG increases K, r = negative • The Hammett equation has been modified to understand correlations in rate (k), thermodynamic values (ΔG, ΔH, ΔS), coupling constants (J, aH), etc. in place of equilibrium constants

  3. Modification of σ • Substituent constants (σ) are not “one size fits all” • Formally, σ describes electronic effects seen in para-substituted benzoic acids • Includes both inductive and resonance effects • σ has been modified to separate out these two effects • These values are redefined as σR and σI , resonance and inductive respectively • Additional modifications to σ have been published • These include: • σ. = radical intermediates • σ-= negatively charged intermediates • σ+ = positively charged intermediates • σm = meta substituted compounds…

  4. How is this applied to inorganic chemistry? • Correlating structure and property relationships can give information regarding: • Mechanistic information • k/K changes with changing properties • Intermediates • Predictive power • Regular trends can be elucidated • Help guide future studies/synthetic efforts • Structural changes can be made to: • Ligands • Metal center

  5. Example 1: Ligand Substitution in Coordination Complexes • “Linear free‐energy relationships in semiquinone species and their Mn(II) and Cu(II) complexes” • Is there a correlation between substituent and physical properties for semiquinone complexes? • Correlation found for electronic transitions (see below) and redox potentials • Different strengths of correlations found for Cu(II) and Mn(II) complexes • Rationalized on the possible exchange pathways present in Cu(II) vsMn(II) Δυ σ Mn(II) complex Δν versus σm, or σ−. Blue squares are the MLCT transition. Green squares are n π* transition. Δν = νH – νZ. Sloop, J. C., Shultz, D. A., Marcus, M. B. and Shepler, B. J. Phys. Org. Chem., 2012, 25, 101–109. 

  6. Example 2: Property Evaluation • “MesoporousThin Films of “Molecular Squares” as Sensors for Volatile Organic Compounds” • Is there are correlation between electronic structure of guest and binding constant in rhenium-based molecular squares? • Rational design of materials for specific guest absorption • Binding stronger for groups with electron donating groups • The authors suggest the driving force for binding is, in part, a charge-transfer interaction between the electron-rich aromatic guests and the electron- deficient pyrazine ligands. • Guests with electron withdrawing groups have lower electron transfer rates. Toluene Binding constant p-fluorotoluene Benzene Fluorobenzene σof guest molecule Keefe, M.H.; Slone, R.V.; Hupp, J.T.; Czaplewski, K.F.; Snurr, R.Q.; Stern, C.L. Langmuir, 2000, 16, 3964–3970.

  7. Example 3: Properties of Metal Ions • “Estimating Bioconcentration Factors, Lethal Concentrations and Critical Body Residues of Metals in the Mollusks… Using Ion Characteristics” • Relating metal bioconcentration factors and LC50s to properties of metal • Regression plots of acute toxicity vs metal properties were generated: • Correlation of LC50 and covalent index is strong and significant! Van Kolck, M.; Huijbregts, M.A.J.; Veltman, K.; Hendriks, A.J. Environmental Toxicology and Chemistry, 27, 2008, 272–276.

  8. Further Reading • T.H. Lowry, K. S. Richardson. Mechanism and Theory in Organic Chemistry, 2nd ed. Harper Collins, 1987, pp 143 – 159. • Walker, J. Newman, M.C., EnacheM. Fundamental QSARs for Metal Ions. Taylor & Francis, Boca Raton, FL, 2012. • Journals that publish QSAR/SAR related research • http://www.qsarworld.com/literature-qsar-journals.php • Review with values of σ for many organic and inorganic substituents • C. Hansch, A. Leo and R. W. Taft (1991). "A survey of Hammett substituent constants and resonance and field parameters". Chem. Rev. 91 (2): 165–195. • http://pubs.acs.org/doi/abs/10.1021/cr00002a004

  9. Learning Outcomes • Define QSAR • Describe the Hammett equation including definitions of each variable • Give examples of how QSAR can be used to predict properties of inorganic systems

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