Weak coordination

N-Alkylpyrroles may be obtained by the Knorr synthesis or by the reaction of the pyrrolyl metallates, ie, Na, K, and Tl, with alkyl haUdes such as iodomethane, eg, 1-methylpyrrole [96-54-8]. Alkylation of pyrroles at the other ring positions can be carried out under mild conditions with allyhc or hensylic hahdes or under more stringent conditions (100—150°C) with CH I. However, unless most of the other ring positions are blocked, poly alkylation and polymerisation tend to occur. N-Alkylation of pyrroles is favored by polar solvents and weakly coordinating cations (Na", K" ). More strongly coordinating cations (Li", Mg " ) lead to more C-alkylation. 

Exciting developments have occurred in the coordination chemistry of the alkali metals during the last few years that have completely rejuvenated what appeared to be a largely predictable and worked-out area of chemistry. Conventional beliefs had reinforced the predominant impression of very weak coordinating ability, and had rationalized this in terms of the relatively large size and low charge of the cations M+. On this view, stability of coordination complexes should diminish in the sequence Li>Na>K>Rb> Cs, and this is frequently observed, though the reverse sequence is also known for the formation constants of, for example, the weak complexes with sulfate, peroxosulfate, thiosulfate and the hexacyanoferrates in aqueous solutions. 

By contrast, in the more weakly coordinating solvent Et20, Me bridges and /X3-OR bridges can... 

Because of its generally rather weak coordinating ability quite small changes can determine whether... 

Carboranes as anew class of weakly coordinating anions for strong electrophiles, oxidants, and superacids 98ACR133. 

The importance of the o-hydroxyl moiety of the 4-benzyl-shielding group of R,R-BOX/o-HOBn-Cu(OTf)2 complex was indicated when enantioselectivities were compared between the following two reactions. Thus, the enantioselectivity observed in the reaction of O-benzylhydroxylamine with l-crotonoyl-3-phenyl-2-imi-dazolidinone catalyzed by this catalyst was 85% ee, while that observed in a similar reaction catalyzed by J ,J -BOX/Bn.Cu(OTf)2 having no hydroxyl moiety was much lower (71% ee). In these reactions, the same mode of chirality was induced (Scheme 7.46). We believe the free hydroxyl groups can weakly coordinate to the copper(II) ion to hinder the free rotation of the benzyl-shielding substituent across the C(4)-CH2 bond. This conformational lock would either make the coordination of acceptor molecules to the metallic center of catalyst easy or increase the efficiency of chiral shielding of the coordinated acceptor molecules. 

The author anticipates that the further development of transition metal catalysis in ionic liquids will, to a significant extent, be driven by the availability of new ionic liquids with different anion systems. In particular, cheap, halogen-free systems combining weak coordination to electrophilic metal centers and low viscosity with high stability to hydrolysis are highly desirable. 

Ionic liquids with wealdy coordinating, inert anions (such as [(CF3S02)2N] , [BFJ , or [PFg] under anhydrous conditions) and inert cations (cations that do not coordinate to the catalyst themselves, nor form species that coordinate to the catalyst under the reaction conditions used) can be looked on as innocent solvents in transition metal catalysis. In these cases, the role of the ionic liquid is solely to provide a more or less polar, more or less weakly coordinating medium for the transition metal catalyst, but which additionally offers special solubility for feedstock and products. 

The first successful hydrogenation reactions in ionic liquids were studied by the groups of de Souza [45] and Chauvin [46] in 1995. De Souza et al. investigated the Rh-catalyzed hydrogenation of cyclohexene in l-n-butyl-3-methylimidazolium ([BMIM]) tetrafluoroborate. Chauvin et al. dissolved the cationic Osborn complex [Rh(nbd)(PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions (e.g., [PFg] , [BFJ , and [SbF ] ) and used the obtained ionic catalyst solutions for the biphasic hydrogenation of 1-pentene as seen in Scheme 5.2-7. 

Rh(nbd) (PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions. 

An example of a biphasic, Ni-catalyzed co-dimerization in ionic liquids with weakly coordinating anions has been described by the author s group in collaboration with Leitner et al. [12]. The hydrovinylation of styrene in the biphasic ionic liq-uid/compressed CO2 system with a chiral Ni-catalyst was investigated. Since it was found that this reaction benefits particularly from this unusual biphasic solvent system, more details about this specific application are given in Section 5.4. 

The fact that ionic liquids with weakly coordinating anions can combine, in a unique manner, relatively high polarity with low nucleophilicity allows biphasic catalysis with highly electrophilic, cationic Ni-complexes to be carried out for the first time [26]. 

Attempted synthesis of RhY(CO)(PPh3)2 in undried solvents (Y = a weakly coordinating anion, e.g. BF4, C104, S03CF3) leads to... 

With a tridentate ligand Au(terpy)Cl3.H20 has, in fact, AuCl(terpy)2"1" with weakly coordinated chloride and water while Au(terpy)Br(CN)2 has square pyramidal gold(III) the terpyridyl ligand is bidentate, occupying the axial and one basal position [124]. Macrocyclic complexes include the porphyrin complex Au(TPP)Cl (section 4.12.5) cyclam-type macrocyclic ligands have a very high affinity for gold(III) [125],... 

Finally, an example of an x-ray structure of a cationic complex shall be mentioned. From the data for 12, a surprisingly weak coordination (Si —N 1.932(8) A [146, 147]) of the acetonitrile donor to the silicon is inferred. The deviation from a pure tetrahedral geometry at the silicon is the largest yet observed (Table 4). 

N,N -Chelation is also exhibited by the dianionic P(III)/P(V) ligands (25) in the MejSn complex (31) [39] and in the magnesium complex (32) [40], which is prepared by oxidation of [Mg(thf)2[ BuNP(p-N Bu)2PN Bu] by elemental tellurium [40]. One of the endocychc N Bu groups in (32) is also weakly coordinated to magnesium, thus providing an intramolecular base-stabihzation similar to that observed for complexes of type (8). 

The two procedures give rise to different results. In both cases acrylic acid, present in the form of acrylate, readily reacts with ammonia at r.t. forming a species characterized by an intense band at 1535 cm i indicating the formation of an amide. With increasing reaction temperature (100°C), however, in the case of procedure A the band at 1535 cm shifts to 1495 cm-i and a weak band forms at 1720 cm h The latter band is characteristic of undissociated and weakly coordinated acrylic acid. This indicates that at 100°C amide dissociates with formation of the free acid. When ammonia is instead present in the gas phase (procedure B), the amide species undergoes transformation to acrylonitrile with a maximum in the intensity Fig. 6 IR spectra of 1 torr acrylic of the vcn band at 2220 cm- at an evacuation acid in contact (5 min) with Sb V=l temperature of about 300°C. and evacuation at r.t (a), and fol- Coordinated acrylic acid and ammonia thus lowing evacuations at 100 (b) and react faster at r.t. to form acrylamide, but in 200°C (c). absence of ammonia which inhibits the re-... 

When the reaction is carried out in heptanol [61], the particles are monodisperse in size (3 nm), well dispersed in the solvent, and adopt the hep structure of bulk rutheniiun. They can be isolated and re-dissolved in various solvents, including d -THF for NMR analysis. In this case, it is clear that coordinated heptanol is present at the surface of the particles and acts as a weakly coordinating ligand. In this case, the presence of surface hydrides was demonstrated by NMR techniques. 

Finally, the term steric stabihzation coifid be used to describe protective transition-metal colloids with traditional ligands or solvents [38]. This stabilization occurs by (i) the strong coordination of various metal nanoparticles with ligands such as phosphines [48-51], thiols [52-55], amines [54,56-58], oxazolines [59] or carbon monoxide [51] (ii) weak interactions with solvents such as tetrahydrofuran or various alcohols. Several examples are known with Ru, Ft and Rh nanoparticles [51,60-63]. In a few cases, it has been estab-hshed that a coordinated solvent such as heptanol is present at the surface and acts as a weakly coordinating ligand [61]. 

To compare NEt3 with NOct3, there was no obvious effect to regulate the size distribution of nanoparticles, because they could not function as a capping ligand of nanoparticles due to their weak coordination to silver in the growth process of silver nucleus [17]. 

Preparations of heterocyclic thiosemicarbazone complexes with cobalt(II) salts of weakly coordinating anions (e.g., perchlorate and tetrafluoroborate) often result in cobalt(III) complexes due to air oxidation. The first 2-acetylpyridine thiosemicarbazone cobalt(III) complex was the diamagnetic [Co(15b-H)2]C104... 

PtMe2(OR)(N-N)(OH2)] OH (47) (Eq. 6.16) [30, 31]. These complexes, likely resulting from an oxidative ROH addition, were characterized by elemental analysis, IR and NMR spectroscopy, conductivity measurements and conversion to derivatives containing weakly coordinating bulky anions. These reachons are of interest because they represent the first examples of oxidation of plahnum(ll) complexes with alcohols and provide the first stable alkoxoplahnum(lV) complexes. The alkoxo-platinum(lV) bond is inert against solvolysis by alcohols, water and even dilute perchloric acid. 

The intermediate-spin ground state of the ferric compounds published by Jager and coworkers is also stabilized by a N4-macrocyclic ligand, [N4] which exist in different varieties of substitutions. The apical ligands are weakly coordinating halides or pseudohalides, such as iodide in the case of [Fe [N4]l] (20) [68]. The electronic structure was elucidated by EPR, Mbssbauer and DFT studies. 

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