Weakly coordinating counteranions

This method exploits the relative weakness of the E-H bond and involves the transfer of the hydride from the element to a strong Lewis acid, in most cases to trityl cation. The easy access of trityl salts with a wide variety of weakly coordinating counteranions is a clear advantage of this method. The reaction can be applied in polar solvents such as sulfolane, ethers and nitriles but also in chlorinated... 

In general, the active species in the polymerization of olefins with metallocenes comprise the cyclopentadienyl ancillary ligations (Ln), an electron-deficient metal center (M), and a weakly coordinative counteranion (cocatalyst) (X )... 

The catalytically active species formed by the treatment of 2,6-bis(imino) pyridine iron(II) chloride complexes with MAO is generally proposed to be a highly reactive monomethylated iron(ll) cation [LFe-Me] (L = 2,6-bis(imino)pyridine ligand) bearing a weakly coordinating counteranion [Me-MAO]. Both monochloride and monoalkyl cationic species are expected to be present in the solution, their relative concentration depending on the MAO/Fe ratio [24]. 

Pi, Z., Keimedy, J. P. Cationic Polymerizations at Elevated Temperatures by Novel Initiating Systems Having Weakly Coordinating Counteranions. 1. High Molecular Weight Polyisobutylenes, Nato Sci. Ser., Ser. E. ed.. In Ionic Polymerizations and Related Processes Puskas, J. E., Ed., Kluwer Dordrecht, Neth., 1999 Vol. 359, pp. 1-12. 

Propagation proceeds in a manner similar to that described for the traditional Ziegler-Natta initiators. The transition metal has two active sites—the polymer chain is held at one site (the one occupied by a methyl group in XXVII) and monomer at the other site (shown as the vacancy ). The reactivity of the active sites is high because the counteranion, which is either (ClMAO) or (OR MAO) or a mixture of the two, is a weakly coordinating anion. Reactivity is decreased when the counterion is strongly coordinating. 

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. 

Monodentate ketones, aldehydes, ethers and esters are weakly coordinating ligands towards nickel(II) and in general the complexes can only be prepared using anhydrous reagents and solvents and with large counteranions. 

The perchlorate ion CIO 3 is a recognized contaminant in groundwater as well as soil with more than a hundred contamination sites in 35 states.67 Perchlorate toxicity arises from its interference with thyroid function. CIO i is unique among the inorganic oxyanions in that it is weakly coordinating and when used as a counteranion with metal complexes, the salts yield high-quality single crystals. Nevertheless, the... 

Copper perchlorate and tetrafluoroborate salts have also been found to form coordination compounds with the neutral ligands L37 (n = 2 or 3), but the structures vary little due to nonparticipation of the counteranions C104 and BF4 for their weak coordinating ability 110, 111). In fact, they are 1 1 metal-ligand compounds in which... 

D.ii.c. Cationic Pd(II) Complexes. The most preferred coordination number for Pd(II) complexes is four. When more than two neutral ligands coordinate to a divalent palladium, the complex becomes cationic. Anions snch as hahde can be potential ligands, which may replace the coordinating neutral hgands. To prevent the counteranions from coordination, weakly coordinating anions (noncoordinating anions) such as Bp4 are frequently employed for these complexes. 

In 2008, Shibata and co-workers reported a cationic iridium bidentate phosphine complex catalyzed C—H bond alkylation reaction of 2-methy-lacetophenone with alkynes and alkenes (Scheme 5.61). Compared with alkynes, the use of styrenes as acceptors requires the weakly coordinated tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) as a counteranion of the iridium complex and a higher temperature to achieve good yields. When a norbornene was used as the acceptor, the sole example of an asymmetric ortho C—H bond alkylation product 175 was obtained in 58% yield and 70% ee. 

Neutral amidinate complexes [RuX /i (A,iV)-CMe(NPf)2 ( "ar i )] 256 have been synthesized by reacting the dimers [ RuX(/i-X)(7 -arene) 2] with the lithium salts of the appropriate ligands (Scheme 20)." The related cationic compounds 257 are obtained by exchange of the halogen atom with the weakly coordinating BAr 4 counteranion. 

A unique case of the chemistry of pincer-type complexes was the synthesis of the first carbonyl hemilabile pincer complex (PCN)Pt(CO) reported by Milstein and coworkers [15]. Hence, the carbonyl derivatives (32) and (33) were synthesized from the direct reaction of CH2CI2 solutions of the corresponding cationic complexes (30) and (31) with a slight excess of CO. Here, the weakly coordinated Bp4 counteranion is easily displaced by the better coordinating ligand CO (Scheme 2.17). 

As counteranions only very robust and extremely weakly coordinating anions [43] such as fluorinated tetraarylborates of the type [B(C6F5 R )4] (R = SiR a, H,... 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

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