Comprehensive Computational Study of Decamethyldizincocene Formation. 1. Reaction of ZnR₂ Reagents with Decamethylzincocene

Abstract

Computational methods were used to study the surprising 2004 synthesis of decamethyldizincocene, Zn2(η(5)-C5Me5)2, which was the first molecule to have a direct, unbridged bond between two first-row transition metals. The computational results show that the methyl groups of decamethylzincocene, Zn(η(5)-C5Me5)(η(1)-C5Me5), affect the transition-state stability of its reaction with ZnEt2 (or ZnPh2) through steric hindrance, and this could possibly allow a counter-reaction, the homolytic dissociation of Zn(η(5)-C5Me5)(η(1)-C5Me5) into Zn(η(5)-C5Me5)(•) and (η(1)-C5Me5)(•), to occur, and because no such steric hindrance occurs when zincocene, Zn(η(5)-C5H5)(η(1)-C5H5), is used as a reactant, its dissociation never occurs regardless of what ZnR2 reagent is used.

Comprehensive Computational Study of Decamethyldizincocene Formation. 2. Reaction of KH/ZnCl2 with Decamethylzincocene

“…Our proposed formation route of 3 from 1 and KH is fairly straightforward to understand, but it still involves free radicals; radical processes are difficult to control, and yields can be inconsistent. − The addition of one equivalent of ZnCl 2 into the reaction mixture does not change the basic reaction scheme all that much since Steps 1a, 1b, 2b, and 2c remain, but there is one major difference: the radical dissociation and reassociation steps (Steps 2a and 3a in Scheme ) become highly unfavorable and are bypassed by the reaction of Me 5 CpZnK with ZnCl 2 to form Me 5 CpZn 2 Cl and KCl (Step 2e, Scheme ).…”

“…For steps involving KH, diffuse functions were added to the hydride hydrogens by using 6-31++G(d,p) . Transition state structures were confirmed using the imaginary mode displacement method (used in Part I of this study) . However, because of the redox nature of some of the reactions in this work, some transition states were able to relax to different minima with only minor differences in the scalar value, so these structures were more rigorously confirmed using the intrinsic reaction coordinate (IRC) method. , Ball and stick molecular images (Figures S1–S8) and Cartesian geometries for the reactants, products, intermediates, and transition states were rendered using Gaussview and are included in the Supporting Information.…”

“…As interesting as our recent theoretical studies on the formation of decamethyldizincocene Zn 2 (η 5 -C 5 Me 5 ) 2 via ZnEt 2 and ZnPh 2 may be, , that reaction is not at all efficient at forming the dizinc product. − The Carmona group found that any number of reducing agents can be reacted with decamethylzincocene Zn(η 5 -C 5 Me 5 )(η 1 -C 5 Me 5 ) to form Zn 2 (η 5 -C 5 Me 5 ) 2 : potassium hydride (KH) has been found to be the most convenient. − The most effective synthesis of 3 involves reacting 1 with KH and ZnCl 2 in a 1:2:1 stoichiometric ratio (Scheme ). At some point during the formation of 3 , the Zn 2+ ions are reduced by KH to Zn + ; but because 6 does not form when 5 is used (rather the zincate salt, 9 ), the mechanisms needed to be investigated in detail.…”

“…The reactions of 1 and 5 with KH were studied first, and then the addition of ZnCl 2 was taken into consideration. In order to maintain an unambiguous link between this article and Part 1 of this study, 2 the chemical numbering in Scheme 1 of this paper continues the ordering set of the previous study. 2…”

“…In order to maintain an unambiguous link between this article and Part 1 of this study, 2 the chemical numbering in Scheme 1 of this paper continues the ordering set of the previous study. 2…”