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Tuck-In Complex:
[edit]Overview:
[edit]A tuck-in complex is an organometallic molecule where a methyl group on a methyl substituted π-coordinated ring ligand bonds to the central metal. The term tuck-in is coined from the fact that this reaction cause the ligand to bend back towards the metal as if it were being tucked in. During the process of tucking in, the methyl group loses a C-H bond that is replaced with a C-M bond. In cases where molecules undergo a "tuck-in", hydrogen gas often is produced. In many other "tuck-in" reactions the ring methyl reacts displacing another methyl bound to the metal liberating methane. When first discovered a "tuck-in" was thought to be a form of organic reaction called ortho-metallation[1]. They came to be known as cyclometallations instead because of the new metal-containing ring that was formed in the process. This process readily happens at d0 centers because it involves no intermediate where the metal uses non-valence electrons. The most common cases of "tucked-in" complexes involve a pentamethylcyclopentadienyl ring (Cp*). The reaction is not limited to Cp*, as many other aromatic rings also share the same properties, such as benzene or cycloheptatriene, with methyl substitution. The first use of the term "tucked-in" in the scientific literature was by Cloke et al [2].
Synthesis:
[edit]Forming a tucked-in complex is a relatively simple reaction. Because it is an intramolecular rearrangement it requires no external chemicals or catalysts. With the addition of heat or light, an α-CH on a ring methyl is activated. The more electron rich the system is the more this kind of reaction can happen. One ring can have multiple methyls that "tuck-in" (see double "tuck-in" example below) [2].
Reactivity:
[edit]Tucked-in complexes are relatively common and often used as starting materials for other more complicated reactions. They are usually a fairly stable intermediate step between reactants and products. A common usage of tucked-in complexes is to adapt the cyclometalic ring. The ring can be bigger than three components allowing for additions of simple alkyl chain or larger multi-substituted aromatic systems[3]. A tucked-in complex can have the tucked bond broken leading to a cation metal. This leads to new pathways and reactions that the molecule can do that it was not able to before[4].
It is not entirely uncommon for tuck-in complex to form dimers with another complex. These molecules are more commonly call tuck-over complexes. A similar reaction occurs as in tuck in but the methyl bonds to a different metal atom. The hydrogens that would have been released from the reaction are instead trapped as bridging hydrides. Uranium complexes bound in this manner have unusual characteristics and can donate two, four or six electrons when reacted. [5]
Identification:
[edit]The current and most widely used method to detect a "tuck-in" complex is H1 NMR. When a Cp* is bound to a metal it has free rotation about the metal-Cp bond, making all of the hydrogen on the methyls equivalent. When a methyl tucks-in, the Cp* no longer has free rotation, which makes the hydrogens inequivalent[6]. In addition, the methlene(CH2) connecting to the metal and the ring is distinct.
References
[edit]- ^ Harder, S. “Intramolecular C-H activation in alkaline earth metal complexes” 42 Angew. Chem. Int. Ed. 2003 3430-3434. anie.200351055
- ^ a b Cloke, G. N.; Green, J. C.; Green, M. H.; and Morley, C. P. "Metal Atom Synthesis and Photochemistry of Bis(q-pentamethylcyclopentadienyl)-tungsten Dihydride” J. Chem. Soc, Chem. Communication 1985 3430-3434. doi:10.1039/C39850000945 Cite error: The named reference "Chem001Geoffrey" was defined multiple times with different content (see the help page).
- ^ Sun, Y.; Rupert, E.; Spence, H.; Piers, W. E.; Parvez, M.; and Yap, G. P. "Intramolecular Ion-Ion Interactions in Zwitterionic Metallocene Olefin Polymerization Catalysts Derived from “Tucked-In” Catalyst Precursors and the Highly Electrophilic Boranes” 119, J. Am. Chem. Soc. 1997 pg. 5132-5143. S0002-7863(97)00140-6
- ^ Erik B. Tjaden,’ Gary L. Casty,’ and Jeffrey M. Stryker’. "Metallacyclobutanes from Central Carbon Alkylation of Early Metal q3-Allyl Complexes. Nucleophilic Additions to Cationic, Lewis Base-Free Zirconium and Titanium Permethylmetallocene q3-Allyl Complexes” 115, J. Am. Chem. Soc. 1993 9814-9815.
- ^ Elizabeth Montalvo, Kevin A. Miller, Joseph W. Ziller, and William J. Evans*. "Reactivity of Tuck-in and Tuck-over Uranium Metallocene Complexes" 29, Organometallics 2010 4159-4170 DOI:10.1021.
- ^ Patrick N. Riley, Jennifer R. Parker, Phillip E. Fanwick, and Ian P. Rothwell*. “Formation of Tantalum “Tuck-in” Complexes by Activation of Methyl C-H Bonds in Pentamethylcyclopentadiene Groups by Carbazole Ligation” 18 Organometallics 1999 3579-3583.