Potassium fluorenyl

Addition of a second crown produces the loose ion pair A, Cr,K, Cr. However, the complexation constant for adding the second crown is 1800 M 1 for the fluorenyl carbanion and only 200 M 1 for the picrate salt. The lower value for picrate may in part be due to less charge delocalization, e.g., the free ion dissociation constant for potassium fluorenyl in TEF is 1.6 x 10 7M (18) as compared to 9.2 x 10 M for potassium picrate (17). The two N02 substituents close to the 0 bond in picrate may also hinder the enlargement of this ionic bond and the insertion of a crown ether molecule because of electronic or sterlc effects. 

Loose ion pairs of such charge-localized oxyanion salts as potassium t-butoxlde may be difficult to form. This alkoxide is a tetrameric aggregate in THF (20). and crown addition breaks it down to the more reactive monomeric form. It is unlikely that with benzo-15-crown-5 a 2 1 crown-K loose ion pair can be formed similar to that found with potassium picrate or potassium fluorenyl. However, external complexation itself will slightly stretch the . bond, and this can have a profound effect on the anion reactivity (21). 

The first example of group 4 complexes comprising an annulated derivative with a pendant NHC group was reported in 2006 by Downing and Danopoulos (Scheme 14.20) [14,68]. The titanium (III) complex 37 was obtained by reaction of the Ti(IV) bis-amido precursor TiQ2(NMc2)2 with the corresponding potassium fluorenyl NHC salt and proceeded with Ti(IV) to Ti(III) reduction. Dimethyl-functionalized N-heterocyclic carbene complexes of Ti(IV), Ti(III), and Zr(IV) were also prepared by salt metathesis reactions, and the bidentate coordination mode was confirmed by various X-ray diffraction studies (complexes 38) [69]. 

One case of n—5 —n delocalization was demonstrated by Stevenson et al. (2006). The potassinm anion-radical salt of l-(9-methyl-9H-fluoren-9-yl)-4-methyl benzyl is characterized by the delocalization of an nnpaired electron within the fluorenyl moiety only. Its ESR spectrnm completely coincides with the spectrnm of the potassium anion-radical salt of the 9,9-dimethyl fluorene anion-radical in THE However, the cesium anion-radical salt of the fluorenyl methylbenzyl derivative produces the ESR spectrum corresponding to the placement of this cation between the fluorenyl and methylbenzyl moiety. The conditions of n—s—n delocalization appear An unpaired electron spends its time within both fluorenyl and methylbenzyl fragments. The situation is explained in Scheme 3.54. 

From the kinetics of the oxirane polymerization initiated hy alcoholate (l6) and hy fluorenyl potassium (27) and as fluorenyl and dihydronaphthalene mono anion (28) have approximately the same basicity, the ka2 over kpr0p ratio may be estimated to 20. Therefore, the length of the two growing polyether chains must be largely independent on the nature of the initiating site. 

Such mechanisms might be questioned since in a recent work Goodman and Arnon (55) demonstrated the absence of initiator s fragments in the resulting polymer. In the course of their studies, polymerisation of y-benzyl-i.-glutamate was initiated in dioxane by 9-fluorenyl potassium or by radio-active sodium methoxide. The polymer produced by the former initiator was precipitated and examined spectrophotometrically. For Mjl ratio of 60 its degree of polymerisation was found from its intrinsic viscosity, to be about 180 and its solution did not absorb at A = 300 mfi where a strong absorption band (e = 104) of the fluorenyl moiety should appear. On the other hand, all the fluorenyl residues were found in the solution left after precipitation of the polymer. 

Danopoulos and co-workers reported on the preparation of NHC ligands with pendant indenyl and fluorenyl groups.19 Deprotonation of the alkyl -indene or -fluorene imidazolium salts with one equivalent of potassium hexamethyldisilazide leads to NHCs functionalised with neutral indene or fluorene moieties (IndH-NHC and F1H-NHC). Further deprotonation with... 

To a solution of (-BuOK in /-BuOH [prepared from potassium (0.12 g, 0.003 g-atom) in f-BuOH (50 mL)] was added 2,2 -biphenylene-l,l-dichloro-3-(9-fluorenyl)cyclopropane (0.43 g, 0.001 mol). The solution was heated to reflux for 30 min to afford dark red crystals, which were recrystallized from dioxane yield 0.32 g (91%) mp 306°C. 

The overlap of the sodium and potassium 3p and 4p orbitals with the carbon 2p orbitals should not be nearly as effective as that of the lithium 2p. Those metals should therefore form complexes that fit the electrostatic model described in Figure 29 more closely. The potassium ion in (TMED)K fluorenyl is disolvated but not chelated by bridging TMEDA groups. The result is a polymeric system in the solid state with each fluorenyl group coordinated to two potassium atoms (Figure 30). The projection of one of the potassium atoms onto the fluorenyl plane is at a point within the periphery of the five-membered ring and also within 0.2 A of the position predicted for the electrostatic model. 

We can carry this further and ask given a positive point charge in the position noted above for one potassium atom, where is the next minimum in the electrostatic potential—that is, where will the second potassium atom be most likely to coordinate The lowest potential energy minimum calculated for a fluorenyl group with one interacting K atom is on the opposite side of the fluorenyl ring and vertically above the plane just to the inside of the Cu—Ci2 bond of the five-membered... 

The relatively strong Bronsted acidity of cyclopentadienes, indenes, and fluorenes, permits the formation of alkali metal compounds of the conjugate bases of these organic molecules by direct reaction with the metal. The effect of substituents on the rate of formation of 9-R-fiuorenyllithium compounds in THE solution and on the degree of ion pairing in solutions of these species has been determined.Potassium derivatives are commonly prepared by the reaction of the cyclopentadiene with potassium bis(trimethylsilyl)amide. Fluorenyl and indenyl compounds with the heavier alkali metals, Rb and Cs, are also prepared by reaction with the metal bis(trimethylsilyl)amide. The cyclopentadienyl ring in alkali metal compounds can also be coordinated to other metals. ... 

Related metallocenes utilizing indenyl and fluorenyl ligands have been described. The bis-indenyl and bis-(diisopropyl) indenyl systems can be easily prepared by metathesis from the potassium indenide and the heavier alkaline earth metal iodides. The resulting compounds adopt the familiar bent geometries despite the large steric bulk of the ligands,... 

The reaction of benzene with cesium and cesium alloys to form cesium benzenide is remarkable. In contrast benzene in 0.01 M solution in 2 1 by volume of THF and 1,2-dimethoxyethane with Na-K alloy according to ESR analysis gave (59) concentrations of radical anion at equilibrium of 10 to 10" M as the temperature decreased from -20° to -83 . The superior reducing power of cesium and its alloys was perhaps to be anticipated in view of the superior reducing power of cesium over potassium in aqueous solution and the appreciably lower ionization potential of cesium compared to potassium in the gas phase. These properties will be influenced by differential solvation of potassium and cesium ions by tetrahydrofuran and by the nature of the ion pairs produced. For 9-fluorenyl salts the fraction of solvent-separated ion pairs has been shown (52) to decrease rapidly in the order Li > Na > K > Cs and is a sensitive function of the solvating power of the medium. The cesium salt of fluorene in THF at -70°C has been shown to exist essentially entirely as contact ion pairs whereas the sodium and lithium salts were completely solvent-separated. The reluctance of cesium cations to become solvent-separated from counteranions means that cesium ions are available for strong electrostatic interaction with anions. 

Arnold and Liddle [37] have prepared X-ray quality crystals of a potassium complex containing a stable amino-pendant NHC. Also prepared and characterized were two potassium salts of NHCs bearing pendant indenyl and fluorenyl groups. Interactions between the potassium atom and the NHC and pendant ligands are present, as evident in the crystal structures of the complexes [38]. 

There are fewer reports of linear, acyclic, ion-binding polymers. It has been reported that poly(oxyethylene) improves the solubility of alkali metals in ethers such as tetrahydrofuran, dime thoxy ethane, and diglyme, stabilizes fluorenyl alkali metal compounds, accelerates Williamson reactions and accelerates several other nucleophilic reactions.All of these effects were attributed to the ability of poly(oxyethylene) to complex with cations in solution. Yanagida and coworkers studied the alkali metal cation complexation of poly(oxyethylene), using a picrate salt extraction technique similar to the one used by Pedersen and Frensdorff. Polymers with more than 23 oxyethylene units were effective iono-phores for potassium, with degrees of extraction (percent extracted) comparable to crown ethers. The extractability per oxyethylene unit was nearly constant, and the complex stability increased linearly with increasing numbers of repeating oxyethylene units. Seven oxyethylenes were the minimum number of repeat units necessary to bind potassium ion effectively in the aqueous phase. The less efficient extraction of short-chain poly(oxyethylene) is apparently caused by its hydrophilic character. 

The fluorenyl-ferrocene based NLO-phores have were prepared by condensing 2-bromo-, 2,7-dibromofluorene, and 2-nitrofluorene with ferrocene carboxaldehyde to afford 2a, 3a, and 4, respectively, all in reasonable yidd. For 2-nitrofluorene the base used is potassium tert-butoxide and for the bromofluorenes, lithium diisopropylamide (LDA). Complexes 2a and 3a undergo clean halogen-metal exchange Ity treatment with n-BuLi in tetrahydrofiiran (THF) at -78 °C and are thm converted to their respective carboxaldehyde and tributylstannyl derivatives by... 

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