Adduct Formation of Dichloridodioxidomolybdenum(VI) and Methyltrioxidorhenium(VII) with a Series of Bidentate Nitrogen Donor Ligands
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Abstract The stability of a variety of bidentate N‐base adducts of MoO 2 Cl 2 and (CH 3 )ReO 3 (MTO) was investigated in thf and CH 2 Cl 2 as solvents. The formation constants were determined from the spectrophotometric data based on 1:1 adduct formation. The adduct formation constants for [MoO 2 Cl 2 L 2 ] (L 2 = bidentate nitrogen ligand) are 10 4 –10 6 times higher than those for [(CH 3 )ReO 3 L 2 ] with the same ligands under the same conditions. The adduct stability of both systems is very sensitive to the electronic nature of the ligands and increases with their donor ability. Hammett correlations of the formation constants against σ give relatively large negative values for the reaction constants ( ρ Re = –5.9, ρ Mo = –6.6). The stability is also governed by steric and strain factors. Thus, sterically hindered 6,6′‐disubstituted‐2,2′‐bipyridines do not form adducts with MTO, and only 6,6′‐dimethyl‐ and 6,6′‐diphenyl‐2,2′‐bipyridines form adducts with MoO 2 Cl 2 . However, these adducts are much less stable than other methyl derivatives of 2,2′‐bipridine adducts. The steric strain between the two methyl groups in 3,3‐dimethyl‐2,2′‐bipyridine influences the bipyridine planarity upon complexation and reduces the adduct stability. The thermodynamic parameters (enthalpy and entropy) were determined from temperature‐dependence studies. The adduct stability is mainly due to the strongly exothermic binding of the nitrogen‐bidentate ligand. The entropy change is small and has little effect on adduct stability.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)Steric properties of crystallographically and computationally determined structures of linear palladium(0) and square planar palladium(II) complexes of di(tert-butyl)neopentylphosphine (P(t-Bu)2Np), tert-butyldineopentylphosphine (P(t-Bu)Np2), and trineopentylphosphine (PNp3) have been determined. Structures of linear palladium(0) complexes show that steric demand increases as tert-butyl groups are replaced with neopentyl groups (P(t-Bu)2Np < P(t-Bu)Np2 < PNp3). In square planar palladium(II) complexes, PNp3 gives the smallest steric parameters, whereas P(t-Bu)Np2 has the largest steric demand. The change in the steric demand of PNp3 compared to P(t-Bu)2Np and P(t-Bu)Np2 results from a significant conformational change in PNp3 depending on the coordination number of the metal. The steric properties of these ligands were also probed by measuring the equilibrium constant for coordination of free phosphine to dimeric [(R3P)Pd(μ-Cl)Cl]2 complexes. Binding equilibria follow the same trend as the steric parameters for square planar complexes with PNp3 having the highest binding constant. In contrast to the normal trend, the neopentylphosphines show increased pyramidalization at phosphorus with increasing steric demand. We hypothesize that this unusual dependence reflects the low back side strain of the neopentyl group, which allows the ligand to be more pyramidalized while still exerting a significant front side steric demand.
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Steric properties of ligands are an important parameter for tuning the reactivity of the corresponding complexes. For various ligands used in mononuclear complexes, methods have been developed to quantify their steric bulk. In this work, we present an expansion of the buried volume and the G-parameter to quantify the steric properties of 1,8-napthyridine-based dinuclear complexes. Using this methodology, we explored the tunability of the steric properties associated with these ligands and complexes.
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Steric properties of ligands are an important parameter for tuning the reactivity of the corresponding complexes. For various ligands used in mononuclear complexes, methods have been developed to quantify their steric bulk. In this work we present an expansion of the buried volume and G-parameter to quantify the steric properties of 1,8-napthyridine based dinuclear complexes. Using this methodology, we explored the tuneability of the steric properties associated with these ligands and complexes.
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Steric energy and the semi-empirical method of calculating steric energy are discussed in the language of potential surfaces. Equations are given relating ΔH00 for several types of reactions commonly used to exhibit steric effects experimentally to the steric energy of the molecules involved in the reactions. The equations of Westheimer and Mayer for the calculation of the steric energy of a molecule are generalized in several respects. The difference in ΔH00 (formation) between cis and trans−2-butene is calculated as a steric effect. Because of various complications the application of the method in its present form to molecules such as H2O, NH3, PF3, etc., is rather unsatisfactory.
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