Absolute samarium-ligand bond disruption enthalpies in the series Cp′2Sm-R/X (Cp′ = η5-(CH3)5C5) have been measured by iodinolytic and alcoholytic isoperibol titration calorimetry of Cp′2Sm/(Cp′2Sm-I)n, (Cp′2Sm-OtBu)2/Cp′ 2Sm-R/X ensembles in toluene. Derived D(Cp′2Sm-R/X) values in toluene solution are as follows (kcal/mol, R/X): 47.0 (1.5), CH(SiMe3)2; 45.0 (1.5), η3-C3H5; 93.2, CCPh; 54.2 (3.0), H; 48.2 (1.8), NMe2; 82.4 (3.5), OtBu; 81.3 (1.0), OCH(tBu)2; 97.1 (3.0), Cl; 83.6 (1.5), Br; 69.4 (2.4), I; 73.4 (2.4), SnPr; 32.6 (2.0), PEt2. D(Cp′2Sm-THF) and D(Cp′2Sm(THF)-THF) values were also determined in toluene and are 7.3 (0.4) and 4.9 (1.0) kcal/mol, respectively, while D(Cp′2Sm(THF)-I) was found to be 72.7 (2.9) kcal/mol. The observed D(Sm-halogen) parameters are close to the D1 values of the corresponding samarium trihalides. Important trends in D(Cp′2Sm-R/X) include a relatively small value of D(Sm-H) - D(Sm-alkyl), a large value of D(Sm-I) - D(Sm-alkyl), and generally strong bonds to group 15 and group 16 ligands. A variety of Sm-centered ligand transposition and oxidative addition/reductive elimination processes are analyzed in light of the present data. The formation of strong Sm-heteroelement bonds makes an important contribution to the driving force. Hydrocarbon functionalization via dinuclear Sm(II) → Sm(III) oxidative addition processes is only expected to be exothermic in special cases.
|Number of pages||10|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1989|
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