Coordination complexes, ionic liquids

Cu (L)Cl][NTf2] (L = bis(2-dimethylaminoethyl)-((l-methylimidazol-2-yl)methyl)-amine), mpt. 108°C, extending the ligand palette for transition metal coordination complex ionic liquids to include quadridentate tripodal chelates. 

Ionic liquids offer a highly polar but noncoordinating environment for chemistry. It is difficult to dissolve catalysts in nonpolar, noncoordinating molecular solvents such as hexane. Polar solvents, such as acetonitrile, tend to coordinate metal complexes. Ionic liquids such as the tetrafluoroborates offer a straightforward replacement of a solvent with a polar solvent that is noncoordinating. 

Adjustable coordination properties Ionic liquids have the potential to be polar yet weaMy coordinating toward transition metal complexes they may enhance reaction rates involving cationic electrophilic intermediates When they contain coordinating anions (e.g., Cl ) they may stabilize anionic transition metal intermediates (e.g., Pd Heck couphng catalysis)... 

Coordination numbers of 2-6 are found for mononuclear metal complex ionic liquid anions, mostly with halide ions. 

Traces of bases such as methylimidazole in the final ionic liquid product can play an unfavorable role in some common applications of ionic liquids (such as bipha-sic catalysis). Many electrophilic catalyst complexes will coordinate the base in an irreversible manner and be deactivated. 

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

In pyridinium chloride ionic liquids and in l,2-dimethyl-3-hexylimida2olium chloride ([HMMIMjCl), where the C(2) position is protected by a methyl group, only [PdClJ was observed, whereas in [HMIMjCl, the EXAFS showed the formation of a bis-carbene complex. In the presence of triphenylphosphine, Pd-P coordination was observed in all ionic liquids except where the carbene complex was formed. During the Heck reaction, the formation of palladium was found to be quicker than in the absence of reagents. Overall, the EXAFS showed the presence of small palladium clusters of approximately 1 nm diameter formed in solution. 

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. 

Some halogenometalate species have been observed to have formed spontaneously during spectroelectrochemical studies in ionic liquids. For example, [MoCl ] (which is hydrolyzed in water, is coordinated by solvent in polar solvents, and has salts that are insoluble in non-polar solvents) can only be observed in basic (X(A1C13) < 0.5 chloroaluminate ionic liquids [1]. FFowever, this work has been directed at the measurement of electrochemical data, rather than exploitation of the ionic liquids as solvents for synthesis [2]. It has been shown that the tetrachloroa-luminate ion will act as a bidentate ligand in acidic X(A1C13) > 0.5 chloroaluminate ionic liquids, forming [M(AlCl4)3] ions [3]. This was also the result of the spontaneous formation of the complexes, rather than a deliberate attempt to synthesize them. 

The only reports of directed synthesis of coordination complexes in ionic liquids are from oxo-exchange chemistry. Exposure of chloroaluminate ionic liquids to water results in the formation of a variety of aluminium oxo- and hydroxo-contain-ing species [4]. Dissolution of metals more oxophilic than aluminium will generate metal oxohalide species. FFussey et al. have used phosgene (COCI2) to deoxochlori-nate [NbOa5] - (Scheme 6.1-1) [5]. 

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]. 

The possibility of adjusting acidity/coordination properties opens up a wide range of possible interactions between the ionic liquid solvent and the dissolved transition metal complex. Depending on the acidity/coordination properties of the anion and on the reactivity of the cation (the possibility of carbene ligand formation from 1,3-dialkylimidazolium salts is of particular importance here [37]), the ionic liquid can be regarded as an innocent solvent, as a ligand precursor, as a co-catalyst or as the catalyst itself. 

However, unlike most conventional solvents, many ionic liquids combine high solvating power for polar catalyst complexes (polarity) with weak coordination (nucleophilicity) [38], It is this combination that enables a biphasic reaction mode with these ionic liquids even for catalyst systems which are deactivated by water or polar organic solvents. 

Ionic liquids have also been applied in transfer hydrogenation. Ohta et al. [110] examined the transfer hydrogenation of acetophenone derivatives with a formic acid-triethylamine azeotropic mixture in the ionic liquids [BMIM][PF6] and [BMIM][BF4]. These authors compared the TsDPEN-coordinated Ru(II) complexes (9, Fig. 41.11) with the ionic catalyst synthesized with the task-specific ionic liquid (10, Fig. 41.11) as ligand in the presence of [RuCl2(benzene)]2. The enantioselectivities of the catalyst immobilized by the task-specific ionic liquid 10 in [BMIM][PF6] were comparable with those of the TsDPEN-coordinated Ru(II) catalyst 9, and the loss of activities occurred one cycle later than with catalyst 9. 

In catalytic reactions, ionic liquids with controllable coordinating strengths and/or reactivities toward the catalyst are particularly important. The coordinating strength of the ionic liquids depends on the nature of the anions and on the metal complex involved in the catalysis. With a large selection of available anions, it is possible to tailor the ionic liquid to suit specific chemical reactions, and variation of the structure and the composition of the cations and anions can alter the solubility of organic reactants in the ionic liquid. 

The presence of PPh3 reduced the TOF from about 1400 to about 210 h which is of the same order of magnitude as that obtained under homogeneous conditions (207). The weak nucleophilic nature of the ionic liquid was again found to be essential, as it does not compete with the butadiene reactant for the coordination site at the metal complex. 

Figure 3.3-1 Incorporation of groups with high affinities for ILs (such as cobaltacenium (i), guanadinium (ii), sulfonate (iv), and pyridinium (v)) or even groups that are themselves ionic liquid moieties (such as imidazolium (iii)) as peripheral functionalities on coordinating ligands increases the solubility of transition metal complexes in ILs.

In none of the above cases has a reaction been performed whilst taking the EXAFS data. Hamill et al. [50] have investigated catalysis of the Heck reaction by palladium salts and complexes in room-temperature ionic liquids. On dissolution of palladium ethanoate in [BMIMj and N-butylpyridinium ([BP] ) hexafluorophos-phate and tetrafluoroborate ionic Hquids, and triethyl-hexyl ammonium bis(trifluo-romethanesulfonyl)imide, a gradual change from ethanoate coordination to the formation of palladium metal was observed in the Pd K-edge EXAFS, as shown in Figure 4.1-13. 

Scheme 5.2-7 Biphasic hydrogenation of 1 -pentene with the cationic Osborn complex [RhfnbdjfPPhjjjjjPFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions.

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