DOI: 10.1002/chem.201404833

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Univalent Gallium Complexes of Simple and ansa-Arene Ligands: Effects on the Polymerization of Isobutylene Martin R. Lichtenthaler,[a] Steffen Maurer,[a] Robert J. Mangan,[b] Florian Stahl,[a] Florian Mçnkemeyer,[a] Julian Hamann,[a] and Ingo Krossing*[a] Abstract: Using [Ga(C6H5F)2] + [Al(ORF)4](1) (RF = C(CF3)3) as starting material, we isolated bis- and tris-h6-coordinated gallium(I) arene complex salts of p-xylene (1,4-Me2C6H4), hexamethylbenzene (C6Me6), diphenylethane (PhC2H4Ph), and m-terphenyl (1,3-Ph2C6H4): [Ga(1,4-Me2C6H4)2.5] + (2 + ), [Ga(C6Me6)2] + (3 + ), [Ga(PhC2H4Ph)] + (4 + ) and [(C6H5F)Ga(m1,3-Ph2C6H4)2Ga(C6H5F)]2 + (52 + ). 4 + is the first structurally characterized ansa-like bent sandwich chelate of univalent gallium and 52 + the first binuclear gallium(I) complex without a GaGa bond. Beyond confirming the structural findings by multinuclear NMR spectroscopic investigations and density functional calculations (RI-BP86/SV(P) level), [Ga(PhC2H4Ph)] + [Al(ORF)4](4) and [(C6H5F)Ga(m-1,3Ph2C6H4)2Ga(C6H5F)]2 + {[Al(ORF)4] }2 (5), featuring ansa-arene

ligands, were tested as catalysts for the synthesis of highly reactive polyisobutylene (HR-PIB). In comparison to the recently published 1 and the [Ga(1,3,5-Me3C6H3)2] + [Al(ORF)4] salt (6) (1,3,5-Me3C6H3 = mesitylene), 4 and 5 gave slightly reduced reactivities. This allowed for favorably increased polymerization temperatures of up to + 15 8C, while yielding HRPIB with high contents of terminal olefinic double bonds (acontents = 84–93 %), low molecular weights (Mn = 1000– 3000 g mol1) and good monomer conversions (up to 83 % in two hours). While the chelate complexes delivered more favorable results than 1 and 6, the reaction kinetics resembled and thus concurred with the recently proposed coordinative polymerization mechanism.

Introduction

[Al(ORF)4] anion and the weakly h6-coordinating C6H5F ligands.[8a] Overall, 1 has not only proven to be an excellent starting material for further gallium(I) chemistry, for example, crown ether,[10] phosphine,[8a, 11] N-heterocyclic carbene,[12] and aromatic nitrogen base[13] complexes of univalent gallium, but also an efficient catalyst for the polymerization of isobutylene (IB).[14]

Gallium(I) Chemistry The low-valent gallium halides GaX2 = Ga + [GaX4][1] (X = Cl, Br and I), “GaI”,[2] and GaCl[3] are important milestones in terms of synthesizing gallium in its metastable oxidation state + 1 and have been used as starting material for gallium(I) chemistry: for example, arene complexes,[4] cyclopentadienyl complexes[5] and carbene analogues[6] of univalent gallium, as well as metalloidal gallium clusters.[7] However, a gallium(I) source that is both easily accessible and also suitable for coordination chemistry was only developed in 2010, in our group: [Ga(C6H5F)2] + [Al(ORF)4] (1) (C6H5F = fluorobenzene; RF = C(CF3)3).[8] 1 was prepared by oxidation of elemental gallium with the silver(I) salt of the weakly coordinating [Al(ORF)4] anion[9] in C6H5F and the stabilization of gallium(I) derives from synergistic effects between the voluminous and perfluorinated

Highly Reactive Polyisobutylene Low molecular weight (Mn = 1000–2300 g mol1) polyisobutylene (PIB), which features a high content of terminal olefinic double bonds (a-content  60 %), is classified as highly reactive (HR-PIB), making post-polymerization functionalization reactions easily feasible.[15] Hence, HR-PIB is an essential precursor for lubricant and fuel additives[16] and together with its conventional counterpart (a-content < 60 %) constitutes the most important class of IB polymers.[17] The industrial method of synthesizing HR-PIB still relies on Lewis acid–base adducts of BF3 and alcohols/ethers as initiating species,[15a, 16b, 18] thus using gaseous, toxic and corrosive BF3 at polymerization temperatures of 10 8C and below. A growing number of initiating/catalyzing alternative systems has therefore been introduced and two promising approaches have evolved: (i) modified conventional Lewis acids, directly leading to HR-PIB[19] or enabling living polymerizations[20] of IB in combination with well-defined termination methods[21] and (ii) transition[17b, c, 22] and maingroup metal[14, 23] complexes of weakly coordinating anions (WCAs), functioning as single-site catalysts for the synthesis of

[a] M. R. Lichtenthaler, S. Maurer, F. Stahl, F. Mçnkemeyer, J. Hamann, Dr. I. Krossing Albert-Ludwigs-Universitt Freiburg, Institut fr Anorganische und Analytische Chemie and Freiburger Materialforschungszentrum (FMF) Albertstr. 21, 79104 Freiburg (Germany) E-mail: [email protected] [b] R. J. Mangan University of Oxford, Magdalen College High Street, Oxford OX1 4AU, (United Kingdom) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404833. Chem. Eur. J. 2014, 20, 1 – 10

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Full Paper in C6H5F at + 25 8C: [Ga(1,4-Me2C6H4)2.5] + [Al(ORF)4] (2), [Ga(C6Me6)2] + [Al(ORF)4] (3), [Ga(PhC2H4Ph)] + [Al(ORF)4] (4) and [(C6H5F)Ga(m-1,3-Ph2C6H4)2Ga(C6H5F)]2 + {[Al(ORF)4]}2 (5). Although the weakly coordinating [Al(ORF)4] anions are crucial in terms of stabilizing the + 1 oxidation state of gallium,[8a] they hardly affect the molecular structure of the cationic gallium(I) complexes.[13] Structural discussions therefore focus on the latter and the obtained bis- and tris-h6-coordinated [Ga(1,4-Me2C6H4)2] + (2 a + ), [Ga(1,4-Me2C6H4)3] + (2 b + ) and [Ga(C6Me6)2] + (3 + ) complexes (Figure 1) are in good agreement

HR-PIB. Each approach has been intensively discussed[14, 17c, 19i] and the importance of WCAs during the polymerization of IB has been stressed.[24] Herein we report on our efforts to prepare novel gallium(I) arene complexes, and their effectiveness in terms of catalyzing the polymerization of IB. As the activity of the catalysts likely correlates to the nature of the bridges of the ligands, the ansaarenes were chosen in relation to known ansa-metallocene ligands.[25]

Results and Discussion Orienting quantum chemical calculations Exchange reactions of the [Ga(C6H5F)2] + complex in 1 predominantly involve two or three equivalents of the ligand.[8a, 11, 12, 14] Calculating the energetics for exchange with p-xylene (1,4Me2C6H4), hexamethylbenzene (C6Me6), diphenylethane (PhC2H4Ph), and m-terphenyl (1,3-Ph2C6H4) in a corresponding manner, all ligand exchange reactions were found to be exothermic and exergonic by at least 0.8 kJ mol1 (Table 1). The advantages of an ansa-arene ligand, such as PhC2H4Ph, are clearly visible by comparing the standard reaction enthalpies (DrH8(gas)) and Gibbs free energies (DrG8(gas)); only with the PhC2H4Ph ligand is DrG8(gas) more favorable than DrH8(gas) for the exchange, due to the increase in entropy. The other ansaarene 1,3-Ph2C6H4 did not retain its original chelating coordination mode during the structural relaxation, but switched to a h6-interaction via its central phenyl ring, thus resembling a simple arene ligand. This behavior already pointed to a somewhat different reactivity than the other investigated systems (see XRD below).

Figure 1. Molecular structures of 2 a + , 2 b + , and 3 + . The [Al(ORF)4] anions and all of the hydrogen atoms were omitted for clarity. Thermal ellipsoids are set at 50 % probability.

with the earlier reported [Ga(o-C6H4F2)2] + [8a] (o-C6H4F2 = ortho[Ga(C6H5Me)2] + [8a] difluorobenzene), [Ga(C6H5F)2.5], + [8a, 14] (C6H5Me = toluene), and [Ga(1,3,5-Me3C6H3)2] + [14] (1,3,5Me3C6H3 = mesitylene) complexes. The more electron rich the arene ligands are, the more they interact with the gallium(I) cations (shortening of Gacent; Table 2, entry 1; cent = centroid of arene ring) and the weaker is the interaction with the corresponding [Al(ORF)4] anions (decreasing number of GaF contacts; Table 2, entry 5). In addition, both, the number of ligands coordinating the gallium(I) cations as well as the enclosing angle, correlate to the steric demand of the employed Syntheses and crystal structures of the gallium(I) arene comligand. Hence, the less demanding 1,4-Me2C6H4 ligand coordiplexes nates gallium(I) in a bent sandwich, but also a pseudo-trigonalplanar fashion and the cent-Ga-cent angle in 2 a + is smaller by We isolated single crystals from highly concentrated solutions 18.18 than that in 3 + (Figure 1 and Table 2). of 1 in 1,4-Me2C6H4 or 1 and C6Me6, PhC2H4Ph, or 1,3-Ph2C6H4 A comparison of 3 + and the earlier reported monoarene Table 1. Energetics of possibly occurring ligand exchange reactions of a [Ga(C6H5F)2] + complex with 1,4complex [Ga(C6Me6)] + [GaX4] Me2C6H4, C6Me6, PhC2H4Ph and 1,3-Ph2C6H4. The given standard reaction enthalpies (DrH8(gas)) and Gibbs free [4c] (X = Cl, Br) demonstrates the energies (DrG8(gas)) are zero-point-energy corrected and correspond to 298.15 K and 1.013 bar (RI-BP86/SV(P) level). weakly coordinating nature of the [Al(ORF)4] anions, rather DrG8(gas) Reaction DrH8(gas) 1 1 than coordinating the less nu[kJ mol ] [kJ mol ] cleophilic counterion + + [Ga(C6H5F)2] + 2 1,4-Me2C6H4 ![Ga(1,4-Me2C6H4)2] + 2 C6H5F 52 46 [Al(ORF)4] , the gallium(I) cation 62 18 [Ga(C6H5F)2] + + 3 1,4-Me2C6H4 ![Ga(1,4-Me2C6H4)3] + + 2 C6H5F 88 66 [Ga(C6H5F)2] + + 2 C6Me6 ![Ga(C6Me6)2] + + 2 C6H5F interacts with a second C6Me6 –[a] –[a] [Ga(C6H5F)2] + + 3 C6Me6 ![Ga(C6Me6)3] + + 2 C6H5F ligand, giving 3 + . [Ga(C6H5F)2] + + PhC2H4Ph![Ga(PhC2H4Ph)] + + 2 C6H5F 31 57 Turning to the ansa-arene li0.8[b] 35[b] [Ga(C6H5F)2] + + 1,3-Ph2C6H4 ![Ga(1,3-Ph2C6H4)] + + 2 C6H5F gands, [Ga(PhC2H4Ph)] + (4 + ; [Ga(C6H5F)2] + + 2 1,3-Ph2C6H4 ![Ga(1,3-Ph2C6H4)2] + + 2 C6H5F 53 50 –[a] –[a] [Ga(C6H5F)2] + + 3 1,3-Ph2C6H4 ![Ga(1,3-Ph2C6H4)3] + + 2 C6H5F Figure 2) is the first structurally characterized bent sandwich [a] No minimum energy structure was found for this coordination mode, which is most likely due to steric reachelate of univalent gallium and sons; [b] the coordination mode of the initially chelated complex changed during the structural relaxation; the 1,3-Ph2C6H4 ligand h6-interacted via the central phenyl ring with the gallium(I) cation, thus resembling a simple in good agreement with a simiarene rather than a chelating ligand. The energetics were therefore additionally calculated applying two or lar structure postulated by three equivalents of 1,3-Ph2C6H4. Schmidbaur et al. from solution

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Full Paper potential coordination site of the gallium(I) cation in 4 + remained unoccupied, making the latter more accessible for potential reactants and thus a promising candidate as catalyst for the polymerization of IB. From the reaction of 1 with one equivalent of 1,3-Ph2C6H4 in C6H5F, we surprisingly obtained the dication [(C6H5F)Ga(m-1,3Ph2C6H4)2Ga(C6H5F)]2 + (52 + ; Figure 2), instead of isolating the postulated [Ga(1,3-Ph2C6H4)] + complex (see Table 1). In 52 + , each gallium(I) cation is coordinated in a pseudo-trigonalplanar fashion by one C6H5F (Gacent = 303.0 pm) and two bridging 1,3-Ph2C6H4 ligands (average Gacent = 274.0 pm), without featuring any notable interactions to the second gallium(I) cation (GaGa = 981.2 pm) or the corresponding [Al(ORF)4] anions (closest GaF = 339.2 pm) (Table 2). The dicationic structure of 52 + is, to our knowledge, the first of its kind and significantly differs from earlier reported neutral and anionic 1,3-Ph2C6H4 complexes of gallium(I) by Power et al.[4j, 27] The fact, that the subsequently modelled formation of 52 + is thermodynamically disfavored in the gas phase, is attributable to the distinct coulomb repulsion of the dication (Table 3). Although the repulsion is reduced by solvation effects of C6H5F or C6H5Me by 53 kJ mol1 or 38 kJ mol1, respectively (conductor-like screening model calculations (COSMO)[28] at the RIBP86/SV(P) level), 52 + is unlikely to exist in solution and possibly fragments into two [Ga(1,3-Ph2C6H4)(C6H5F)] + cations (Table 3). The formation of 5 is therefore only conceivable in the solid state and induced by the higher lattice enthalpies (DlattH8) of an AX2 species with respect to an AX salt. This effect is best shown by determining DrH8(solid) for the formation of 5 (45.6 kJ mol1) by using a Born–Haber–Fajans cycle (Figure 3).

Table 2. Structural key parameters of the cationic gallium(I) complexes 2 a + , 2 b + , 3 + , 4 + and 52 + . Entry 1 2 3 4 5

Gacent[a] (av.) [pm] cent-Ga-cent[b] (av.) [8] torsion angle[c] (av.) [8] closest GaF [pm] GaF contacts[d]

2 a+

2 b+

3+

4+

52 +

257.2 135.0 – 294.8 4

280.5 120.0 0.352 368.5 0

251.7 153.1 – 415.8 0

262.0 119.8 – 335.1 0

283.6 120.0 1.087 339.2 0

[a] Average distance from the gallium(I) cation to the centroids of the arene rings (cent). The corresponding distances in the [Ga(o-C6H4F2)2] + [8a,b] (268.4 pm), [Ga(C6H5F)2] + [8a,b] (266.9 pm), [Ga(C6H5Me)2] + [8a,b] (263.3 pm), and [Ga(1,3,5-Me3C6H3)2] + [14] (259.5 pm) complexes show that more electron-rich arene ligands interact better with the gallium(I) cations; [b] angle formed by the centroids of the arene rings and the gallium(I) cation. In the case of a tris-coordinated gallium(I) cation, the average angle was given; [c] average torsion angle of the tris-coordinated gallium(I) cation; [d] number of GaF contacts shorter than the sum of the van der Waals radii to the corresponding [Al(ORF)4] anions (334 pm).[26] While the numbers for the [Ga(o-C6H4F2)2] + [8a,b] (2), [Ga(C6H5F)2] + [8a,b] (2), [Ga(C6H5Me)2] + [8a,b] (1) and [Ga(1,3,5-Me3C6H3)2] + [14] (0) complexes match the trend of a weakened interaction with the corresponding [Al(ORF)4] anions, the high number for 2 a + derives from the interaction with two disordered [Al(ORF)4] anions and likely is attributable to solubility issues during the crystallization.

Figure 2. Molecular structures of 4 + and 52 + . The [Al(ORF)4] anions and all of the hydrogen atoms were omitted for clarity. Thermal ellipsoids are set at 50 % probability.

Multinuclear NMR spectroscopy of the Gallium(I) arene complexes The obtained gallium(I) salts were dissolved in o-C6H4F2 and characterized using multinuclear NMR spectroscopy. While the 1 H NMR spectra revealed the nature of the coordinating ligands, that is, a singlet at 2.39 ppm and at 7.26 ppm for 1,4Me2C6H4 in 2, a singlet at 2.26 ppm for C6Me6 in 3, a singlet at 3.25 ppm and a multiplet at 7.46 ppm for PhC2H4Ph in 4 and distinct multiplets from 7.53 to 7.97 ppm for 1,3-Ph2C6H4 in 5, the 19F and 27Al NMR spectra of all salts featured characteristic signals, corresponding to intact [Al(ORF)4] anions: a singlet at

NMR experiments.[4h] Though Gacent in 4 + is with 262.0 pm relatively long compared to its non-chelated congeners, no notable interaction with the corresponding [Al(ORF)4] anions was observed (Table 2). In addition, the cent-Ga-cent angle of 4 + of 119.88 resembled those of tris-arene complexes, such as 2 b + or [Ga(C6H5F)3], + [8a] and not bis-arene complexes, such as 2 a + or 3 + (Table 2). While the structural divergences are attributable to the chelating effect of the PhC2H4Ph ligand, the third

Table 3. Formation and possible fragmentation of 52 + . DrH8(gas) and DrG8(gas)/DrG8(solv) are zero-point-energy corrected and correspond to 298.15 K and 1.013 bar (RI-BP86/SV(P) level).[a] Reaction

DrH8(gas) [kJ mol1]

DrG8(gas)/DrG8(solv) [kJ mol1]

2 [Ga(C6H5F)2] + + 2 1,3-Ph2C6H4 ![(C6H5F)Ga(m-1,3-Ph2C6H4)2Ga(C6H5F)]2 + + 2 C6H5F

+ 65

+ 128/ + 75 (C6H5F)[b]/ + 90 (C6H5Me)[c]

[(C6H5F)Ga(m-1,3-Ph2C6H4)2Ga(C6H5F)]2 + !2 [Ga(1,3-Ph2C6H4)(C6H5F)] +

116

174/92 (C6H5F)[b]/120 (C6H5Me)[c]

[a] The high DrH8(gas) and DrG8(gas)/DrG8(solv) values are attributable to the distinct coulomb repulsion of the [(C6H5F)Ga(m-1,3-Ph2C6H4)2Ga(C6H5F)]2 + dication; [b] to consider the solvation effects of C6H5F (relative permittivity er = 5.47 F m1), the solvent from which 5 was crystallized, conductor-like screening model calculations (COSMO)[28] were performed; [c] to consider the solvation effects of C6H5Me (er = 2.38 F m1), the solvent in which 5 was applied to catalyze the polymerization of IB, COSMO[28] calculations were performed.

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Figure 3. Born–Haber–Fajans cycle to calculate DrH8(solid) for the formation of 5 (A = [Al(ORF)4]). DlattH8 values of the gallium(I) salts were calculated using the Jenkins generalized Kapustinskii equation[29] and the ion volumes derived from the single-crystal structures of 1 and 5. DrH8(gas) was calculated at the RIBP86/SV(P) level and DsubH8 for 1,3-Ph2C6H4[30] and DvapH8 for C6H5F[31] were extrapolated from the given references. All enthalpies are given in kJ mol1.

74.9 ppm in the 19F and at 33.8 ppm in the 27Al NMR spectra (see Supporting Information: Figure S1, S2, S3). In addition, the 71Ga NMR spectra featured only one singlet clearly different from the known[8a, 14] signal of 1 (758 ppm in o-C6H4F2): 746 ppm for 2, 735 ppm for 3, 746 ppm for 4 and 751 ppm for 5 (see Supporting Information: Figure S4). This is a clear indicator for ligand exchange and the successful stabilization of the + 1 oxidation state of gallium. To probe the chelate and bridging effect of the ansa-arenes on the reactivity of the gallium(I) complexes, 4 and 5 were tested in terms of catalyzing the synthesis of HR-PIB.

Table 4. Polymerizations of IB using 4 or 5 as a catalyst.[a]

Similar to the earlier reported 1 and [Ga(1,3,5-Me3C6H3)2] + [Al(ORF)4] salt (6), low concentrations of 4 and 5 enabled IB polymerizations at high temperatures (T) in the non-carcinogenic and non-water-hazardous solvent C6H5Me, yielding HRPIB within two hours (Table 4). Beyond improving the quantity and quality of the obtained HR-PIB, the most notable effect of the ansa-arene ligands was the mitigation of the reactivity of the gallium(I) salts, thus allowing for temperatures as high as + 15 8C, whereas the highest possible T for 1 and 6 were 5 8C and  0 8C, respectively.[14] Above these temperatures, sudden changes in T (DT jumps) were observed, making the polymerizations and the properties of the IB polymers hard to control. While such DT jumps were not observed for 4 and 5 (cf. the aforementioned mitigation), the correlations between c, T, the a-content, Mn and the conversion of IB (conv.) resembled those of 1, albeit shifted to higher temperatures. Hence, varying c had little effect on the properties of the obtained HR-PIB and the amount of 5 was therefore reduced to the lowest, yet still workable, c of 0.002 mol % (10 mg). Increasing T, the conversion increased, a-contents remained constant within the margin of error, Mn values decreased and Mw/Mn values increased (Table 4). To obtain HR-PIB in good yields, temperatures should therefore be as high as possible, provided Mn values do not become too low. The highest T in these terms is + 15 8C, at which 4 and 5 feature similar reactivities. On decreasing T to + 5 8C, 4 seemed to be less reactive than 5, as there was an overall decrease of conversion of 61 % for 4 and only 26 % for 5 (Table 4, column 3). These findings are likely attributable to the nature of the ligands in 4 and 5, which, as described earlier,[14] affect the reactivity of the gallium(I) complexes during the initiatory phase of the polymerization in two &

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conv. [%]

a-content [%][c]

Mn [g mol1][d]

Mw/ Mn[e]

4 4 4 5 5 5 1[f] 6[f]

21 40 82 56 72 82 71 60

93 88 86 84 86 87 93 69

2140 1690 1140 2920 2190 940 1980 630

2.6 2.6 2.9 2.4 3.2 4.1 2.7 2.0

0.007 0.007 0.007 0.002 0.002 0.002 0.007 0.007

+5 + 10 + 15 +5 + 10 + 15 10 0

[a] Condensed IB (20 mL, 12.6 g, 225 mmol) was added to a solution of 4 or 5 in C6H5Me (50 mL) at the given temperature (T). Keeping T constant, the reaction mixture was stirred for two hours and finally quenched by adding isopropanol (5 mL). In the absence of 4 and 5 no polymerization was observed; [b] referring to the amount of IB used (20 mL, 12.6 g, 225 mmol); [c] both, a- and b-content were calculated using 1H NMR spectroscopy. The terminal olefinic double bonds featured characteristic signals at d(1H) 4.64 and 4.85 ppm, whereas the internal olefinic double bonds featured one signal at d(1H) 5.15 ppm; [d] the Mn of the obtained PIB was calculated using 1H NMR spectroscopy and additionally validated by gel permeation chromatography (GPC) measurements calibrated to polystyrene standards. As the values calculated by 1H NMR spectroscopy are more precise, they are shown here; [e] the polydispersity (Mw/Mn) was determined by GPC measurements calibrated to polystyrene standards; [f] for comparison, 1 and 6 featured a higher reactivity than 4 and 5.[14] Most notably, 4 yielded HR-PIB at + 5 8C with almost identical properties as 1 at 10 8C. However, despite the elevated T, the conversion of IB (conv.) for 4 was only 21 %, whereas that for 1 was 71 %.

Polymerization of IB

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Catalyst c [mol %][b] T [8C]

ways: (i) by altering the interaction with the highest occupied molecular orbital (HOMO) of an IB unit (Figure 4) and (ii) by having a direct impact on the thermodynamics of the formation of the catalytically active species, the cyclogalla(III)pentanium cations ([(IB)GaIII(C8H16)] + ) (Table 6). Before addressing both aspects, it is important to understand which gallium(I) species are initially active when 4 and 5 are applied in C6H5Me. We therefore investigated potential ligand exchange and addition reactions of 4 + , the fragmented 52 + and C6H5Me (Table 5). Inspection of Table 5 shows that all exchange reactions would be feasible based on the DrH8(gas) values. However, with inclusion of solvation energies and entropic contributions, only two DrG8(solv) values are exergonic. It may therefore be concluded that 4 + exists as such in C6H5Me, whereas 52 + potentially degrades to the [Ga(1,3-Ph2C6H4)(C6H5Me)] + or the [Ga(C6H5Me)2] + complex. Beyond being 7 kJ mol1 more exergonic, the formation of the [Ga(1,3-Ph2C6H4)(C6H5Me)] + com4

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Figure 4. Kohn–Sham orbitals of an IB unit, 4 + , the [Ga(1,3-Ph2C6H4)(C6H5Me)] + , [Ga(C6H5Me)2] + and [Ga(1,3,5-Me3C6H3)2] + complexes. Due to smaller energy gaps, the ambiphilic gallium(I) cations act as Lewis acids rather than as Lewis bases and IB is therefore considered to be an electron donor, reacting through its HOMO. The solvation effects in C6H5Me (er = 2.38 F m1, COSMO[28]) have a pronounced effect on the orbital energies of the gallium(I) complexes, while hardly influencing the IB unit; the negligible lowering of the HOMO energy of the IB unit from 5.87 eV to 5.90 eV is not shown. Gas phase (except COSMO), RI-BP86/SV(P) level, electron density cut off at 0.06.

Table 5. Potential ligand exchange and addition reactions of 4 + , the fragmented 52 + , and C6H5Me. DrH8(gas) and DrG8(gas)/DrG8(solv) are zero-point-energy corrected and correspond to 298.15 K and 1.013 bar (RI-BP86/ SV(P) level).[a] Reaction

DrH8(gas) [kJ mol1]

DrG8(gas)/DrG8(solv) [kJ mol1]

[Ga(PhC2H4Ph)] + + C6H5Me![Ga(PhC2H4Ph)(C6H5Me)] + [Ga(PhC2H4Ph)] + + 2 C6H5Me![Ga(C6H5Me)2] + + PhC2H4Ph [Ga(PhC2H4Ph)] + + 3 C6H5Me![Ga(C6H5Me)3] + + PhC2H4Ph [Ga(1,3-Ph2C6H4)(C6H5F)] + + C6H5Me![Ga(1,3-Ph2C6H4)(C6H5Me)] + + C6H5F [Ga(1,3-Ph2C6H4)(C6H5F)] + + C6H5Me![Ga(1,3-Ph2C6H4)(C6H5F)(C6H5Me)] + [Ga(1,3-Ph2C6H4)(C6H5F)] + + 2 C6H5Me![Ga(C6H5Me)2] + + 1,3-Ph2C6H4 + C6H5F [Ga(1,3-Ph2C6H4)(C6H5F)] + + 2 C6H5Me![Ga(1,3-Ph2C6H4)(C6H5Me)2] + + C6H5F [Ga(1,3-Ph2C6H4)(C6H5F)] + + 3 C6H5Me![Ga(C6H5Me)3] + + 1,3-Ph2C6H4 + C6H5F

27 7.8 22 16 25 13 27 27

+ 8.7/ + 13[b] + 22/ + 19[b] + 44/ + 42[b] 19/16[b] + 18/ + 20[b] 12/10[b] + 14/ + 19[b] + 9.8/ + 4.0[b]

[a] The positive DrG8(gas)/DrG8(solv) values are attributable to a loss in entropy; [b] to consider the solvation effects of C6H5Me (er = 2.38 F m1), COSMO[28] calculations were performed.

Table 6. Formation of the catalytically active [(IB)GaIII(C8H16)] + cation. DrH8(gas) and DrG8(gas)/DrG8(solv) are zero-point-energy corrected and correspond to 298.15 K and 1.013 bar (RI-BP86/SV(P) level).[a]

Reaction

DrH8(gas) [kJ mol1] DrG8(gas)/DrG8(solv) [kJ mol1]

[(IB)GaIII(C8H16)] + =

[Ga(PhC2H4Ph)] + + 3 IB![(IB)GaIII(C8H16)] + + PhC2H4Ph [Ga(1,3-Ph2C6H4)(C6H5Me)] + + 3 IB![(IB)GaIII(C8H16)] + + 1,3Ph2C6H4 + C6H5Me [Ga(C6H5Me)2] + + 3 IB![(IB)GaIII(C8H16)] + + 2 C6H5Me [Ga(1,3,5-Me3C6H3)2] + + 3 IB![(IB)GaIII(C8H16)] + + 2 1,3,5Me3C6H3

11 + 0.1

+ 88/ + 71[b] + 74/ + 53[b]

3.1 + 23

+ 67/ + 52[b] + 83/ + 60[b]

[a] The positive DrG8(gas)/DrG8(solv) values are attributable to a loss in entropy; [b] To consider the solvation effects of C6H5Me (er = 2.38 F m1), COSMO[28] calculations were performed. Chem. Eur. J. 2014, 20, 1 – 10

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5

plex is more likely, as the [Ga(C6H5Me)2] + complex is also the active species directly deriving from 1 (cf. DrG8(solv) = 26 kJ mol1 for this process) and as 1 and 5 feature different reactivities in terms of catalyzing the polymerization of IB, they are unlikely to yield the same active species. With this knowledge, we turn back to the course of the polymerization. In terms of (i) and comparing the lowest unoccupied molecular orbitals (LUMOs) of the gallium(I) complexes to the HOMO of an IB unit, the investigations are in good agreement with the experimental results (Figure 4). In C6H5Me, the energy gaps of 4 + (1.77 eV) and the [Ga(1,3-Ph2C6H4)(C6H5Me)] + complex (1.82 eV) are very similar, thus corresponding to similar reactivities of 4 and 5, as observed at + 15 8C. A comparison of both species to the recently published 1 and 6 however, was not straightforward. Therefore and given that there is an electronic origin to this effect,

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smaller energy gaps would be expected for 1 and 6, since the [Ga(C6H5Me)2] + and [Ga(1,3,5-Me3C6H3)2] + complexes showed higher reactivities.[14] Solely considering the simple arene complexes however, the reactivities are again correctly described as 1 features a higher reactivity than 6.[14] We therefore attribute the observed discrepancy to the steric factors and chelating nature of the ansa-arenes during the initiatory phase of the polymerization. Regarding (ii) and as 4, 5, and the other complexes yield HRPIB with similar properties, we believe that the different initiatory phases of the polymerization merge and subsequently one coordinative reaction path dominates, during which the original aromatic ligands play a minor role. The point of merging likely corresponds to the oxidative addition and formation of the catalytically active cyclogalla(III)pentanium cation ([(IB)GaIII(C8H16)] + (a more favored isomer than the coordinative [GaI(IB)3] + complex).[14] Evaluating the simple and ansa-arene complexes separately, the thermodynamics of the postulated reactions are in good agreement with the experimental results (Table 6). Hence, DrG8(solv) for the formation of the active [(IB)GaIII(C8H16)] + cation is more accessible, if it derives from the [Ga(1,3-Ph2C6H4)(C6H5Me)] + complex rather than from the [Ga(PhC2H4Ph)] + complex, thus corresponding to a higher reactivity of 5 compared to 4 (cf. Table 4 at + 5 8C and + 10 8C). Similarly, the [Ga(C6H5Me)2] + complex formed from 1, features a higher reactivity than the [Ga(1,3,5-Me3C6H3)2] + complex of 6.

General procedures All manipulations were performed using Schlenk or glovebox techniques in an argon atmosphere (< 1 ppm H2O and O2). C6H5F and o-C6H4F2 were dried over CaH2, distilled and had H2O contents below 5 ppm (Karl-Fischer titrations). C6H5Me was dried using a conventional M. Braun Grubbs apparatus and also had H2O contents below 5 ppm. Gaseous IB was dried by passing it over a column loaded with pure CaSO4 pellets. Because the obtained compounds contain large amounts of fluorine in chemically very stable CF3 groups, standard combustion analyses have proven to be incomplete. Therefore, characterizations were done on the basis of single crystal X-ray analysis and multinuclear NMR spectroscopy. Obtained single crystals were coated with perfluoroether oil and mounted on 0.1 mm micromounts at the respective crystallization temperature. The crystal structure data were collected from the shockcooled crystals at 100 K, on a Bruker SMART APEX2 CCD area detector diffractometer while using MoKa radiation. Data reduction was done with SAINT[32] and scaling of the data and absorption correction was performed by SADABS-2008/1[33] and SADABS-2012/1,[34] respectively, and for 3, due to non-merohedral twinning, by TWINABS-2012/1[35] using the hklf5 file. The structures were solved by intrinsic phasing using SHELXT[36] and were refined by full-matrix least-squares minimization on F2 using all reflections with SHELXL[37] in the ShelXle[38] GUI. For 4, the twin law 1 0 0 0 1 0 0 0 1 2 was additionally applied, with twin fractions of 46 % and 54 %. To attribute idealized positions to all hydrogen atoms, a riding model was used. The disorder of the [Al(ORF)4] anions was treated using DSR.[39] The graphical representations were prepared using the software Diamond 3.2i.[40] CCDC 1011767 (2), 1012058 (3), 1011996 (4) and 1011566 (5) contain the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Solution NMR spectra were recorded on a BRUKER AVANCE 400 MHz WB spectrometer, at room temperature, using flame-sealed NMR tubes. For measuring and processing of the data, the software Bruker Topspin 2.1 and 3.2 was used. Resonances were given in ppm and referenced to SiMe4 for the 1H NMR spectra, to CFCl3 for the 19F NMR spectra, to a 1.1 m solution of Al(NO3)3 in D2O for the 27Al NMR spectra and to a 1.1 m solution Ga(NO3)3 in D2O for the 71Ga NMR spectra.[41] GPC measurements were conducted at room temperature using an Agilent 1260 Isocratic Pump G1310B coupled with a set of PSS SDV columns and a Knauer RI K-2301 differential refractive index detector. The DFT calculations were performed using TURBOMOLE 6.4.[42]

Conclusion Applying 1 in combination with 1,4-Me2C6H4, C6Me6, PhC2H4Ph and 1,3-Ph2C6H4, we isolated bis- and tris-h6-arene coordinated gallium(I) complexes of weakly coordinating [Al(ORF)4] anions: [Ga(1,4-Me2C6H4)2.5] + , [Ga(C6Me6)2] + , [Ga(PhC2H4Ph)] + and [(C6H5F)Ga(m-1,3-Ph2C6H4)2Ga(C6H5F)]2 + . Being the first structurally characterized bent sandwich chelate and dicationic complex with no GaGa interaction of univalent gallium, 4 + and 52 + were additionally tested in terms of catalyzing the polymerization of IB. Both complexes featured slightly reduced reactivities compared to 1, thus allowing for favorably increased polymerization temperatures of up to + 15 8C, while yielding HR-PIB with high a-contents (84–93 %), low molecular weights (Mn = 1000–3000 g mol1) and good monomer conversions (up to 83 % in two hours). Though the reaction kinetics and the similar properties of the obtained HR-PIB support the proposed coordinative polymerization mechanism, a competing polymerization pathway based on protic impurities was recently addressed. Hence, we applied 2,6-di-tert-butyl-4-methylpyridine (DTBMP) as a proton-quenching species and were surprised to learn that the gallium(I) salts directly reacted with the so called “non-nucleophilic” DTBMP.[13] A polymerization in the presence of DTBMP was therefore not possible. In addition, potential termination reactions, regeneration of the gallium(I) salts, and an expansion of the scope of monomers are currently under investigation.

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Preparation of the gallium(I) salts Crystallization attempts were successful, if the simple and ansaarene ligands were applied in overstoichiometric and stoichiometric ratios, respectively. While the synthesis of 4 is exemplarily presented here, the similar syntheses of 2, 3 and 5 can be found in the Supporting Information. A highly concentrated solution of 1 (200 mg, 163 mmol, 1.00 eq) and PhC2H4Ph (26.6 mg, 143 mmol, 0.88 eq) in C6H5F (200 mL) was stored at + 25 8C. After 70 days, colorless platelet-shaped crystals had formed. The crystals were suitable for single-crystal X-ray analysis and 4 was additionally characterized by multinuclear NMR spectroscopy. 1H NMR (400 MHz, oC6H4F2, calibrated to o-C6H4F2 = 7.12 ppm,[43] 298 K): d = 3.25 (s, 4 H, CH2, PhC2H4Ph), 7.46 ppm (m, 10 H, Ar-H, PhC2H4Ph); 19F NMR (377 MHz, o-C6H4F2, calibrated to o-C6H4F2 = 139 ppm,[44] 298 K): d = 74.9 ppm (s, CF3); 27Al NMR (104 MHz, o-C6H4F2, calibrated to

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71

Ga NMR

are included in the Supporting Information. Also included are multinuclear NMR resonances and spectra (1H, 19F, 27Al, 71Ga) for the gallium(I) salts and the IB polymers, crystal structure data for 2, 3, 4, 5 and quantum-chemical data (optimized atomic coordinates, calculated frequencies) for the gallium(I) complexes, the ligands and the IB adducts.

Polymerization of IB A solution of 4 (16.4 mmol) or 5 (3.67 mmol) in C6H5Me (50 mL) was cooled to the desired starting temperature. While monitoring T with a VOLTCRAFT K101 Digital Thermometer, condensed IB (20 mL, 225 mmol) was added to the reaction mixture. After two hours of stirring at constant T, the polymerization was terminated by adding isopropanol (5 mL). Water (50 mL) was subsequently added and the obtained emulsion was stirred overnight. A potential evolution of a colorless gas is attributable to non-converted IB. After addition of tert-butyl methyl ether (50 mL), the combined organic phases were washed with water (2  50 mL) and the organic solvents subsequently removed under reduced pressure to afford HR-PIB as a medium-viscosity colorless gel, which was dried under high vacuum (102 mbar) and characterized using 1H NMR spectroscopy and GPC. 1H NMR (400 MHz, CDCl3, calibrated to CHCl3 = 7.26 ppm,[45] 298 K): d = 0.99 (s, CH3, PIB), 1.03 (s, PIB), 1.09 (s, PIB), 1.11 (s, CH3, PIB, repeating unit), 1.33 (s, PIB), 1.38 (s, CH2, PIB), 1.42 (s, CH2, PIB, repeating unit), 1.54 (s, H2O), 1.78 (s, PIB), 2.00 (s, CH2, PIB), 2.36 (s, CH3, C6H5Me), 4.64 (m, CH2, a-content, PIB), 4.81 (s,  C(CH3)2C(=CH2)C(CH3)2, PIB), 4.85 (m, CH2, a-content, PIB), 5.12 (s, C(CH3)=CHC(CH3)2, PIB), 5.15 (m, CH, b-content, PIB), 7.17 (mc, Ar H (2,4,6), C6H5Me), 7.26 ppm (mc, Ar H (3,5), C6H5Me).

Acknowledgements: This work was supported by the Albert-Ludwigs-Universitt Freiburg, the DFG in the Normalverfahren, the DAAD RISE Program 2013, and the JSPS Summer Program 2014. We would like to thank Fadime Bitgl and Dr. Harald Scherer for measurement of the NMR spectra, Boumahdi Benkmil B.Sc., Dr. Daniel Kratzert, and Dr. Alexander Higelin for their support regarding single-crystal X-ray crystallography, Marina Hagios for GPC services, Dr. Daniel Himmel for valuable discussions of the DFT calculations and Prof. Dr. Kazuo Takimiya for his generous hospitality. Keywords: density functional calculations · gallium · ligand effects · NMR spectroscopy · polymerization [1] a) W. Klemm, W. Tilk, Z. Anorg. Allg. Chem. 1932, 207, 175 – 176; b) L. A. Woodward, G. Garton, H. L. Roberts, J. Chem. Soc. 1956, 3723 – 3725; c) G. Garton, H. M. Powell, J. Inorg. Nucl. Chem. 1957, 4, 84 – 89; d) R. C. Carlston, E. Griswold, J. Kleinberg, J. Am. Chem. Soc. 1958, 80, 1532 – 1534; e) J. D. Corbett, A. Hershaft, J. Am. Chem. Soc. 1958, 80, 1530 – 1532; f) J. D. Beck, R. H. Wood, N. N. Greenwood, Inorg. Chem. 1970, 9, 86 – 90; g) T. Staffel, G. Meyer, Z. Anorg. Allg. Chem. 1987, 552, 108 – 112; h) A. Ait-Hou, R. Hillel, C. Chatillon, J. Chem. Thermodyn. 1988, 20, 993 – 1008; i) D. G. Tuck, Polyhedron 1990, 9, 377 – 386; j) A. P. Wilkinson, A. K. Cheetham, D. E. Cox, Acta Cryst. 1991, 47, 155 – 161. [2] a) J. D. Corbett, R. K. McMullan, J. Am. Chem. Soc. 1955, 77, 4217 – 4219; b) M. Wilkinson, I. J. Worrall, J. Organomet. Chem. 1975, 93, 39 – 42; c) G. Gerlach, W. Hçnle, A. Simon, Z. Anorg. Allg. Chem. 1982, 486, 7 – 21; d) M. L. H. Green, P. Mountford, G. J. Smout, S. R. Speel, Polyhedron 1990, 9, 2763 – 2765; e) R. J. Baker, C. Jones, Dalton Trans. 2005, 1341 – 1348; f) C. M. Widdifield, T. Jurca, D. S. Richeson, D. L. Bryce, Polyhedron 2012, 35, 96 – 100. [3] a) E. Gastinger, Angew. Chem. 1955, 67, 108 – 108; b) E. Gastinger, Z. Anorg. Allg. Chem. 1962, 316, 161 – 167; c) M. Tacke, H. Kreienkamp, L. Plaggenborg, H. Schnçckel, Z. Anorg. Allg. Chem. 1991, 604, 35 – 38. [4] a) H. Schmidbaur, U. Thewalt, T. Zafiropoulos, Organometallics 1983, 2, 1550 – 1554; b) H. Schmidbaur, U. Thewalt, T. Zafiropoulos, Chem. Ber. 1984, 117, 3381 – 3387; c) H. Schmidbaur, U. Thewalt, T. Zafiropoulos, Angew. Chem. Int. Ed. Engl. 1984, 23, 76 – 77; Angew. Chem. 1984, 96, 60 – 61; d) H. Schmidbaur, Angew. Chem. Int. Ed. Engl. 1985, 24, 893 – 904; Angew. Chem. 1985, 97, 893 – 904; e) H. Schmidbaur, W. Bublak, B. Huber, G. Mueller, Organometallics 1986, 5, 1647 – 1651; f) H. Schmidbaur, W. Bublak, B. Huber, G. Mller, Helvetica Chimica Acta 1986, 69, 1742 – 1747; g) H. Schmidbaur, R. Hager, B. Huber, G. Mller, Angew. Chem. Int. Ed. Engl. 1987, 26, 338 – 340; Angew. Chem. 1987, 99, 354 – 356; h) H. Schmidbaur, W. Bublak, A. Schier, G. Reber, G. Mller, Chem. Ber. 1988, 121, 1373 – 1375; i) G. A. Bowmaker, H. Schmidbaur, Organometallics 1990, 9, 1813 – 1817; j) N. J. Hardman, R. J. Wright, A. D. Phillips, P. P. Power, J. Am. Chem. Soc. 2003, 125, 2667 – 2679; k) M. Gorlov, L. Kloo, Coord. Chem. Rev. 2008, 252, 1564 – 1576. [5] a) D. Loos, H. Schnçckel, J. Organomet. Chem. 1993, 463, 37 – 40; b) C. Dohmeier, D. Loos, H. Schnçckel, Angew. Chem. Int. Ed. Engl. 1996, 35, 129 – 149; Angew. Chem. 1996, 108, 141 – 161; c) D. Loos, E. Baum, A. Ecker, H. Schnçckel, A. J. Downs, Angew. Chem. Int. Ed. Engl. 1997, 36, 860 – 862; Angew. Chem. 1997, 109, 894 – 896; d) R. A. Fischer, J. Weiß, Angew. Chem. Int. Ed. 1999, 38, 2830 – 2850; Angew. Chem. 1999, 111, 3002 – 3022; e) P. Jutzi, L. O. Schebaum, J. Organomet. Chem. 2002, 654,

Crystal structure data Crystal structure data for 2 (CCDC 1011767): C72H50Al2F72Ga2O8 ; Mw = 2604.52 g mol1 monoclinic; space group C2/c; a = 43.8166(16), b = 21.1329(8), c = 20.6470(8) ; b = 105.4480(19); V = 18 427.8(12) 3 ; Z = 8; 1calc = 1.878 g cm3 ; F(000) = 10 256; l = 0.71073 ; T = 100(2) K; absorption coefficient = 0.802 mm1, absorption correction: multi-scan, Tmin = 0.6769, Tmax = 0.7454; GooF = 1.073; R1 = 6.63 and wR2 = 17.35 for reflections I > 2s(I); R1 = 8.56 and wR2 = 18.82 for all reflections. Crystal structure data for 3 (CCDC 1012058): C40H36AlF36GaO4 ; Mw = 1361.39 g mol1 monoclinic; space group P21; a = 14.6296(3), b = 17.0745(4), c = 20.4678(4) ; b = 91.7590(12)8; V = 5110.31(19) 3 ; Z = 4; 1calc = 1.769 g cm3 ; F(000) = 2704; l = 0.71073 ; T = 100(2) K; absorption coefficient = 0.728 mm1, absorption correction: multi-scan, Tmin = 0.676995, Tmax = 0.745618; GooF = 1.001; R1 = 3.63 and wR2 = 7.70 for reflections I > 2s(I); R1 = 4.38 and wR2 = 8.03 for all reflections. Crystal structure data for 4 (CCDC 1011996): C30H14AlF36GaO4 ; Mw = 1219.11 g mol1; orthorhombic; space group P212121; a = 9.7010(5), b = 19.4750(10), c = 21.0583(11) ; V = 3978.5(4) 3 ; Z = 4; 1calc = 2.035 g cm3 ; F(000) = 2376; l = 0.71073 ; T = 100(2) K; absorption coefficient = 0.922 mm1, absorption correction: multi-scan, Tmin = 0.6880, Tmax = 0.7461; GooF = 1.102; R1 = 4.15 and wR2 = 9.20 for reflections I > 2s(I); R1 = 8.04 and wR2 = 10.47 for all reflections. Crystal structure data for 5 (CCDC 1011566): C80H38Al2F74Ga2O8 ; Mw = 1363.25 g mol1; monoclinic; space group C2/c; a = 39.4401(12), b = 9.8194(3), c = 28.7435(10) ; b = 119.902(3)8; V = 9649.9(6) 3 ; Z = 4; 1calc = 1.877 g cm3 ; F(000) = 5344; l = 0.71073 ; T = 100(2) K; absorption coefficient = 0.774 mm1, absorption correction: multi-scan, Tmin = 0.6913, Tmax = 0.7456; GooF = 1.124; R1 = 4.87 and wR2 = 13.72 for reflections I > 2s(I); R1 = 6.96 and wR2 = 15.13 for all reflections.

Supporting Information Text, figures, tables, and CIF files giving a detailed description of the syntheses of 2, 3, 4, and 5 as well as the polymerization of IB, Chem. Eur. J. 2014, 20, 1 – 10

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Received: August 13, 2014 Published online on && &&, 0000

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FULL PAPER & Gallium(I) Complexes

Ansa me this: The ansa-arene complexes [Ga(PhC2H4Ph)] + and [(C6H5F)Ga(m-1,3-Ph2C6H4)2Ga(C6H5F)]2 + enable polymerizations of isobutylene (IB) at temperatures as high as + 15 8C and yield highly reactive polyisobutylene (HR-PIB) of excellent quality in good yields (see Figure: Dimetrodon jaw, National Museum of Nature and Science, Tokyo).

M. R. Lichtenthaler, S. Maurer, R. J. Mangan, F. Stahl, F. Mçnkemeyer, J. Hamann, I. Krossing* && – && Univalent Gallium Complexes of Simple and ansa-Arene Ligands: Effects on the Polymerization of Isobutylene

Gallium(I) Complexes Similar to a Japanese bento box, gallium(I) chemistry offers a wide range of “different tastes” regarding fundamental and applied science. In their full paper on page && ff., I. Krossing et al. describe the ansa-arene complexes [Ga(PhC2H4Ph)2] + and [(C6H5F)Ga(m-1,3Ph2C6H4)2Ga(C6H5F)]2 + , featuring the weakly coordinating [Al(OC(CF3)3)4]  anion, that not only deliver interesting insights into the bonding nature of univalent gallium, but also are highly efficient catalysts for the polymerization of isobutylene.

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Chem. Eur. J. 2014, 20, 1 – 10

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ÝÝ These are not the final page numbers!

Univalent gallium complexes of simple and ansa-arene ligands: effects on the polymerization of isobutylene.

Using [Ga(C6 H5 F)2 ](+) [Al(OR(F))4 ](-) (1) (R(F) =C(CF3)3) as starting material, we isolated bis- and tris-η(6) -coordinated gallium(I) arene compl...
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