Metallocenium ions

Although the cation-anion interaction of metallocenium ions is very weak, the counteranion is likely to remain in proximity with the metal cation to form a contact ion pair in low-permittivity solvents such as toluene (commonly used in polymerization reactions). If the metal cation allows the counteranion to penetrate into the first coordination sphere, it can form an inner-sphere ion pair (ISIP). When the counteranion is relegated to the second coordinating sphere, the ion pair becomes an outer-sphere ion pair (OSIP), which is more ionic in nature than ISIPs. A schematic representation of the relationship between ISIPs and OSIPs is depicted in Scheme 2. This simple scheme shows us the principal elements that affect the cation-anion interactions (e.g., counteranion (Y ), ancillary ligands (L ), transition metal (M), and alkyl ligand (R)), and the subtle balance between these elements in the dynamic equilibria. 

The syndiotactic polypropylene has a melting point up to 133°C (2 74). (Me2C(Flu)(Cp))ZrCl2 and similar metallocenes, in combination with MAO or other perfluorinated borates, can produce chiral metallocenium ions in which chirahty is centered at the transition metal. Because of the flipping of the polymer chain, the metallocene alternates between the two enantiomeric configurations and produces a syndiotactic polymer (272, 175). It had not previously been possible to produce such pure syndiotactic polymer. 

The Catalyst System Eleven years ago, Kaminsky invented a novel olefin polymerization catalyst derived from Cp2ZrCl2 (Cp = 7 -5-C5H5) and methylaluminoxane (1), a result that has stimulated intense interest in synthesis and reactions of metallocenium ions. Important questions still remain, however, regarding the nature of the Kaminsky catalyst. These include (1) what is methylaluminoxane and how does it interact with Cp2ZrMe2 to initiate polymerization and (2.) what are the mechanisms of chain initiation, propagation, transfer and termination A collateral question is how these steps may be controlled. 

Metallocenes are useful electron donors as judged by their low (vertical) ionization potentials in the gas phase and oxidation potentials in solution (see Table 2). In fact, the electron-rich (19 e ) cobaltocene with an oxidation potential of E°ox = -0.9 V relative to the SCE [45] is commonly employed as a very powerful reducing agent in solution. Unlike the alkylmetals (vide supra), the HOMOs of metallocenes reside at the metal center [46] which accounts for two effects (i) Removal of an electron from the HOMO requires minimal reorganization energy which explains the facile oxidative conversion from metallocene to metallocenium. (ii) The metal-carbon bonding orbitals are little affected by the redox process, and thus the resulting metallocenium ions are very stable and can be isolated as salts. 

Selective alkylation of aromatic molecules (benzene, toluene) with a-chloronorbornene at room temperature to afford the 1 1 addition products m -l-chloro-2-arylnorbornane (aryl = Ph, C6H4Me-/>) in good yields has been achieved with the metallocenium ion pair [Cp 2ThMe][B(C6F5)4].1 4 The solution structure and aggregation of the catalyst has been studied by NOE and PGSE NMR spectroscopy this compound exists in toluene solution as simple (inner-sphere-type) ion pair with close cation-anion interactions.195,196... 

Another commonly observed decomposition pathway involves C-H bond activation. For example, the reaction of (l,3-But2Gp)2ZrMe2 with B(C6F5)3 yields a C-H activated 77s,771- tuck-in cation 710 (Scheme 173), which is inert with respect to ethylene polymerization. There are also significant metal-alkyl group effects on the thermodynamic stability and stereochemical mobility of the B(C6F5)3-derived Zr and Hf metallocenium ion pairs.538... 

Conversely, one- and two-electron reduced Mn derivatives can be combined with functional cations, such as metallocenium ions [40,44] and the organic radical mpyNIT (S = 1/2) in Fig. 5 [43]. The former have been prepared directly from neutral Mn complexes and metallocenes according to Eq. 7, while the compoimd (mpyNIT)[Mni20i2(02CPh)i6(H20)4] was synthesized by a one-pot reaction between mpyHIT" " and Mn -benzoate (Eq. 14). 

Table 1. Kinetic Data for Metallocenium Ion-Pair Formation/Dissociation and Symmetrization Processes in Toluene-cfe ...

On the other hand, for a given counterion, the polymerization activity of electrophilic Zr catalysts does not necessarily correlate with the intrinsic electrophilicity of [L2ZrCH3]+ as L is varied.The ion-pairing tendency of L2ZrCH3+ cations in solution is a key contributor to the overall propagation rate, and the stability of the ion pair for a given counterion is expected to decrease as L becomes more electron donating and sterically encumbered and, thus, more reactive despite the reduced electrophilicity of the metallocenium ion. ... 

Experimental Results. The solution redox potentials (18) and gas-phase photoelectron spectroscopy of metallocenes (79) have received much attention in the literature. We have recently used charge-transfer equilibria in the FTICR-MS to determine the values of aG for several metallocenium ions (Figure 1). Reference compounds are not widely available for alP values < 6.8 V, so the absolute values for many metallocenes of lower IP have not yet been determined. However, alP values in the desired range are known (77), and we are continuing these studies. 

Figure 1. Free energy ladder for electron attachment to metallocenium ions, CP2M, as deduced by charge-transfer equilibria. Reference compounds (Ref. 17) are shown on the left with -aG values. Values of (Equation 8) are shown on the vertical lines for various equilibria, and derived values of -aG for the metallocenes are shown on the right. Values in parentheses are estimates. Temperature 350 K.

Activation parameters for /I-methyl elimination have been determined recently for the Zr and Hf metallocenium ion pairs depicted in Eq. 6.24. The activation free energies for the Zr and Hf complexes are almost the same, as the more favorable A// found for Hf (22.5 (0.9) kcal/mol for Zr and 17.3(0.9) kcal/mol for Hf) is partially offset by the negative entropy of activation (4.3 (3.3) cal/mol for Zr and — 11.9 (3.4) cal/mol for Hf) [144]. However, small changes in the ancillary ligands have a stronger influence on the rates of elimination as shown by the stability of the three metalloceiuum complexes in Eq. 6.24 [144,145]. Steric factors play an important role since the less hindered cyclopentadienyl ligands lead to the more... 

Another parallelized autoclave for the hydrothermal synthesis of solids was introduced by Bein and coworkers [28]. In this system, the products are recovered by a customized centrifugation device and product identification is done via XRD using an automated XY-translation stage. The system was used to study the infiu-ence of different template molecules, notably of metallocenium ions, on the formation of aluminophosphates. 

Heise H, Kohler FH, Herker M, HUIct W (2002) Inter-and intramolecular spin transfer in molecular magnetic matmials. solid-state NMR spectroscopy of paramagnetic metallocenium ions. J Am Chem Soc 124 10823-10832... 

---