Determination of the Stabilization Energy of Planar-Tetracoordinate Carbon in Dynamic Dinuclear (.mu.-Hydrocarbyl)bis(zirconocene) Cation Complexes and Detection of an Organometallic Memory Effect in Their Formation

Abstract

Bis(propyny1)zirconocene (7a) reacts with [Cp2ZrCH3(THF)]+[BPh]-(8a) to yield the [@-propynyl)@-7' :v2-MeCCMe)(Cp2Zr)2] cation complex 5b that contains a planar-tetracoordinate carbon atom. Complex 5b was characterized by X-ray diffraction. The planar-tetracoordinate carbon center (C2) of 5b exhibits four strong bonds to its nearest in plane neighbors, Le. to two carbon and two zirconium centers. In solution, complex 5b is dynamic. The C, symmetric planar-tetracoordinate carbon compound 5b undergoes a thermally induced automerization reaction [AG* (250 K) = 11.8 f 0.5 kcal mol-'] that proceeds through a C2,-symmetric geometry (6b) that has the characteristics of a transition state according to a theoretical analysis of the 5 == 6 =+ 5' rearrangement process. This means that the extra stabilization energy of planar-tetracoordinate carbon relative to ordinary planar-tricoordinate carbon in the "semi-fenestrane" environment of the complexes 5 amounts to ca. 10-12 kcal mol-'. The 5 --L 5' equilibrium is strongly influenced by substituents at the p-v':v2-RCCCH3 ligand and by electronically active Cp substitution. In addition, a strong memory effect is operative in the formation of 5. It can be observed by using suitably substituted pairs of isomeric complexes 5. Thus, the reaction between [Cp2ZrCD3(THF)]+ (8d) and (MeCp)zZr( -CW-CH3)2 (7e) gives only complex 5h, which exhibits the CH3 group at the planar-tetracoordinate carbon center C2, whereas the reaction of the complementary reagent combination [(MeCp)2ZrCD3(THF)]+ (8e)/Cp2Zr-(-C=C--CH3)2 (7a)l in this double-labeling experiment exclusively leads to the same type of product (Si) that has the CD3 group bonded to C2. This and other examples of this organometallic memory effect indicate that dimetallic intermediates are involved in the formation of the unusually structured stable planar-tetracoordinate carbon compounds 5.

Recent advances in planar tetracoordinate carbon chemistry

“…[38][39][40][41][42][43][44][45][46][47][48] A few realized compounds of nonclassical structures containing ptCs are shown in Figure 2. In order to achieve electronic stabilization they share a common feature: there is at least one atom different from carbon linked to the ptC.…”

Formation of Hypercoordinated Carbon inside a (μ-Hydrocarbyl)bis(group 4 metallocene) Framework by a Simple Protonation Route

“…These spectroscopic data indicate that the overall structure of the parent compound 9a is analogous to the substituted systems 9b − d , but that there are two different dynamic processes taking place in 9a , both being characterized by rather different rate constants. First, there is an equilibration process that leads to an interchange of the C 1 H 2 and C 2 H 2 methylene groups inside the metallacyclic framework and, consequently, to an intramolecular positional exchange between the Cp 2 Zr 1 and Cp 2 Zr 2 bent metallocene moieties (see Scheme ). The equilibration process of the Cp 2 ZrCH 2 moieties can be frozen out on the NMR time scale.…”

Recent Advances in Planar Tetracoordinate Carbon Chemistry

“…11 Using Hoffmann's strategy (also called the electronic approach), several ptC molecules have been proposed in silico 14,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and/or experimentally detected. [38][39][40][41][42][43][44][45][46][47][48] A few realized compounds of nonclassical structures containing ptCs are shown in Figure 2. In order to achieve electronic stabilization they share a common feature: there is at least one atom different from carbon linked to the ptC.…”