DOI: 10.1002/chem.201402293

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Activation of Elemental Sulfur by Aluminum Dihydride: Isolation of Mono- and Bis(hydrogensulfide) Complexes of Aluminum Daniel Franz and Shigeyoshi Inoue*[a] Abstract: The conversions of the dimeric imidazolin-2-iminato aluminum dihydride {m-LDippAlH2}2 (1) with 1/4 or 1/2 equivalents of S8 gave the elusive hydrogensulfide {mLDippAl(H)SH}2 (2) and the bisthiol {m-LDippAl(SH)2}2 (3), respectively (LDipp = NC(N(Dipp)CH)2, Dipp = 2,6-diisopropylphenyl). Notably, these transformations proceeded in the absence of a promoter. The reaction of {m-LMesAlH2}2 (4), a less bulky congener of 1, with phenyl disulfide produces the phenyl sulfide {m-LMesAl(H)SPh}2 (5; LMes = NC(N(Mes)CH)2, Mes = 2,4,6-trimethylphenyl). The hitherto unknown compounds 2–5 were characterized by using spectroscopic methods and single-crystal X-ray diffraction analysis.

Scheme 1. Activation of elemental sulfur by the b-diketiminato aluminum dihydrides I[6] and III[7] (Dipp = 2,6-diisopropylphenyl, Mes = 2,4,6-trimethylphenyl).

hydrogensulfide L’Al(H)SH [L’ = N(Dipp)C(Me)CHC(Me)N(Dipp)], which was not characterized. In 2007, the scope of this chemistry was expanded by MoyaCabrera and co-workers, who converted the less sterically congested aluminum dihydride III with 3/8 equivalents of S8 and isolated the m-S-bridged dinuclear aluminum hydrogensulfide IV, most notably, refraining from the addition of P(NMe2)3 or any other kind of promoter (Scheme 1).[7] It has to be pointed out that the authors excluded the formation of a bis(hydrogensulfide) complex similar to II as an intermediate in this transformation. Interestingly, the implementation of b-diketiminato aluminum sulfides as ligands towards metal centers has been reported;[8] however, the full potential of this coordination chemistry remains somewhat unexploited. To date, we conclude that 1) the uncatalyzed S8 activation to an aluminum complex containing two terminal hydrogensulfide moieties of the type L’’Al(SH)2 (L’’ = ligand) has yet to be described and that 2) an aluminum monohydrogensulfide in the form of L’’Al(H)SH is an elusive intermediate species. The imidazolin-2-iminato ligand may act as either a 2s- or a 2p- or a 4p-electron donor.[9, 10] In the course of the last two decades, the properties of transition-metal complexes of this ancillary ligand have been studied thoroughly by Tamm and co-workers,[10] but it has been reported only scarcely on the chemistry of its main-group-element compounds.[11] It has not been until lately that Bertrand and co-workers linked the strongly related imidazolidin-2-imino group to a phosphorus center and enriched the field of main-group-element chemistry with intriguing complexes marked by prominent phosphorus mononitride,[12] phosphinonitrene,[13] or iminophosphonium[14] moieties. Based on that, our utter interest lies in exploring its coordination chemistry towards Group 13 and 14 metal atoms.

It is beyond doubt that the diverse field of element–element bond activation continues to be a topic of prominent interest to the scientific community. In comparison to the vastly explored realm of related transition-metal-mediated reactions,[1] the application of well-defined molecular main-group-element complexes[2, 3] as activators or even catalysts remains negligible. Whereas respective reports majorly focus on bond-activation methods by low-valent main-group-metal compounds[2] or frustrated Lewis pairs,[3] related examples for main-group metal–hydride complexes with high valencies are rare.[4] Apparently, reports about the reactivity of aluminum hydrides without the involvement of transition metals majorly focus on their use as hydride-transfer reagents towards Lewis acids.[5] In 2003, Roesky and co-workers described the transformation of the bulky b-diketiminato aluminum dihydride I to the aluminum bis(hydrogensulfide) complex II by reaction with 1/4 equivalents of S8 in the presence of a catalytic amount of P(NMe2)3 (Scheme 1).[6] In the absence of this promoter, a mixture of several products was obtained, and only a small amount of II was formed after prolonged reaction time. Remarkably, this transformation was reported to proceed via an unstable intermediate, that is, the aluminum hydride

[a] Dr. D. Franz, Prof. Dr. S. Inoue Institut fr Chemie, Anorganische Chemie Technische Universitt Berlin, Strasse des 17. Juni 135 Sekr. C2, 10623 Berlin (Germany) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402293. It contains full experimental details and crystallographic data. Chem. Eur. J. 2014, 20, 1 – 6

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Scheme 2. Syntheses of the imidazolin-2-iminato aluminum hydrogensulfides 2 and 3.

In 2012, we reported on the synthesis of a new iminosilylene.[15] Very recently, we described the synthesis and reactivity of the bulky imidazolin-2-iminato aluminum dihydride 1 (Scheme 2), which possesses dimeric structure.[9] In accordance with our quantum mechanical calculations, we concluded that this compound is a stronger hydride donor than the aluminum dihydride I. Thus, we set out to investigate on how the qualities of 1 could be exploited for the activation of nonpolar element–element bonds. After conversion of 1 with 1/4 equivalents of S8, the 1H NMR analysis (C6D6) of the reaction mixture strongly suggested a desired compound with one SH substituent attached to each aluminum center, as was concluded by the intensity ratio between a resonance at d = 2.05 ppm and the signals of the imino groups (see II: d(1H) = 0.88 ppm, SH, C6D6).[6] Examination of the product by high-resolution mass spectrometry established the formulation of the dimeric aluminum hydrogensulfide 2 (Scheme 2), and we isolated an analytically pure sample of the compound in moderate yield (43 %) that was characterized by 1H and 13C{1H} NMR spectroscopy, as well as by elemental analysis. In the infrared spectrum (ATR technique), bands produced by the AlH bonds in 2 were observed at v˜ = 1873 and 1832 cm 1 (cf. 1: v˜ = 1830 and 1798 cm 1 for AlH)[9] . The dimeric nature and the configuration of 2 are evidenced by single-crystal structure determination (Figure 1). As one essential structural motif, the X-ray study revealed a central Al2N2 moiety with the two SH groups assuming positions on the same side of the four-membered ring in a pseudo-cis arrangement. When comparing the parameters of the Al2N2 ring in 2 with the parent aluminum hydride 1,[9] we found that the monosulfurization at the aluminum center has only a negligible effect on the geometry of this moiety. The Al S distances in 2 are 2.2503(5) and 2.2518(5) , which is a little elongated with respect to II (Al S = 2.223(1) and 2.217(1) )[6] . In the context of considering the presence of cis, as well as trans forms of 2 in solution or the solid state, it has to be pointed out that the related bulky imino-substituted aluminum hydride triflate dimers {m-LDippAl(H)OTf}2[9] and {m[16] tBu3PAl(H)OTf}2 had also been structurally characterized as their cis isomers only. &

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Figure 1. Molecular structure of 2 in the solid state. Hydrogen atoms (except those on aluminum and sulfur) have been omitted for clarity; displacement ellipsoids are at the 30 % probability level. Selected bond lengths [] and angles [8]: S(1) Al(1) 2.2503(5), S(2) Al(2) 2.2518(5), Al(1) N(1) 1.895(1), Al(1) N(4) 1.883(1), N(1) C(1) 1.305(2); S(1)-Al(1)-N(1) 105.00(4), S(1)-Al(1)N(4) 119.79(4), N(1)-Al(1)-N(4) 85.91(5), Al(1)-N(1)-Al(2) 93.44(5), Al(1)-N(1)C(1) 132.40(9), Al(2)-N(1)-C(1) 134.13(9).

To investigate if 2 was indeed an intermediate species in the transformation of 1 into a bis(hydrogensulfide) species, we converted 2 with another 1/4 equivalents of S8 at elevated temperature and after prolonged reaction time {m-LDippAl(SH)2}2 (3) was isolated in 86 % yield (Scheme 2). Furthermore, the compound was prepared independently by the direct reaction of 1 with 1/2 equivalents of S8 and characterized by 1H and 13 1 C{ H} NMR spectroscopy, as well as by elemental analysis and high-resolution mass spectrometry. The latter suggests the dimeric formulation of 3. The introduction of two functional SH groups to each aluminum center is indicated by a signal in the 1 H NMR spectrum (C6D6) at d = 1.86 ppm that integrates to two protons per imino moiety. In the IR spectrum, 3 produces a weak peak at 2577 cm 1 that is assigned to the stretching band of the SH bonds and corresponds to the value of 2549 cm 1, which was reported for II.[6] Notably, compound 3 is marked by low solubility in toluene, C6D6, and ethereal solvents (e.g., THF and Et2O) at ambient temperature. We were fortunate to obtain single crystals of 3(toluene)1.5 suitable to X-ray diffraction analysis after slow cooling of a reaction mixture in toluene. The structural analysis confirmed that a dimer with four SH groups attached to the aluminum atoms of the Al2N2 ring was formed (Figure 2). One may take note that compared to {m-LDippAl(H)SH}2 (2), the geometry of this four-membered ring is essentially unaffected by the introduction of the second sulfur atom to each aluminum center, which accounts for the high rigidity of this moiety. The Al S bond lengths in 3(toluene)1.5 range from 2.231(1) to 2.240(1) , which is slightly shorter than observed for 2 and marginally longer than the corresponding values reported for L’Al(SH)2 (II).[6] With regard to the different reactivities of I and III towards yellow sulfur (Scheme 1), we were interested to compare the 2

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Scheme 3. Synthesis of the phenyl sulfide 5.

The infrared spectrum of 5 exhibited a prominent peak at 1878 cm 1 with a shoulder at 1853 cm 1, which are similar values to those of the AlH bonds in 2. Crystals of 5(toluene)2 suitable for X-ray diffraction analysis were obtained by slow cooling of a toluene solution from 100 8C to room temperature. The molecular structure in the solid state showed the formation of a centrosymmetric dimer (Figure 3). The Al S distance is 2.2867(7) , which is longer than the values observed for the hydrogensulfides II, 2, and 3. Remarkably, the phenyl sulfide groups are positioned at opposite faces of the Al2N2 ring plane, and it is reasonable to assume that steric interaction is minimized by this type of pseudo-trans configuration taking into account the imino ligands as well. The two imidazolin rings attached to the Al2N2

Figure 2. Molecular structure of 3(toluene)1.5 in the solid state. Hydrogen atoms (except those on sulfur) and toluene molecules have been omitted for clarity; displacement ellipsoids are at the 30 % probability level. Selected bond lengths [] and angles [8]: S(2) Al(1) 2.231(1), S(4) Al(2) 2.240(1), Al(1) N(1) 1.888(2), Al(1) N(4) 1.880(2), N(1) C(1) 1.311(3); S(1)-Al(1)-S(2) 115.11(4), N(1)-Al(1)-N(4) 87.42(8), Al(1)-N(1)-Al(2) 92.31(8), Al(1)-N(1)-C(1) 135.4(2), Al(2)-N(1)-C(1) 132.2(2).

outcome of the conversion of 1 with this element to its reaction with a less sterically hindered congener. Thus, we synthesized the less bulky compound 4 following an analogous procedure as was established for 1,[9] and converted it with 1/4 equivalents, as well as with 1/2 equivalents of S8 (see the Supporting Information). We were surprised to find out that 4 reacted with yellow sulfur in a far less controlled fashion. In both cases, complex product mixtures formed with several imidazolin-2-iminato species and three resonances produced by chemically different SH moieties, as was observed in the proton NMR spectrum (C6D6). This strongly suggests that steric protection of the hydrogensulfide group is a requirement for the isolation of aluminum complexes containing terminal SH ligands at the metal center. To rule out that electronic properties would render 4 generally unsuitable for the reaction with unpolarized S S bonds, we converted it with phenyl disulfide, which is a common target for testing the activation of nonpolar covalent bonds.[3c,e] Even though the reaction proceeded at room temperature, as was concluded from NMR spectroscopic control (C6D6), heating was required to gain synthetic access to 5, in which one SPh ligand is attached to each aluminum atom (Scheme 3). In contrast, the more sterically hindered 1 was found to be inert towards phenyl disulfide at room temperature. The synthesis of 5 illustrates the introduction of an organyl sulfide group to an aluminum center by using the disulfide as precursor in a reductive approach. We scarcely found similar examples for this type of conversion in the literature and, thus, want to point out a related report by Berben and co-workers.[17] Chem. Eur. J. 2014, 20, 1 – 6

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Figure 3. Molecular structure of 5(toluene)2 in the solid state. Hydrogen atoms (except those on aluminum) and toluene molecules have been omitted for clarity; displacement ellipsoids are at the 30 % probability level. Selected bond lengths [] and angles [8]: S(1) Al(1) 2.2867(7), S(1) C(22) 1.773(2), Al(1) N(1) 1.884(1), Al(1)’ N(1) 1.887(2), N(1) C(1) 1.306(2); Al(1)S(1)-C(22) 111.65(7), S(1)-Al(1)-N(1) 101.93(5), S(1)-Al(1)-N(1)’ 108.05(5), N(1)Al(1)-N(1)’ 85.69(6), Al(1)-N(1)-Al(1)’ 94.31(6), Al(1)-N(1)-C(1) 131.9(1), Al(1)’N(1)-C(1) 133.8(1).

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Communication core in 5 are co-planar and tilted with respect to the central four-membered ring, thus, a minimum of interference between the mesityl groups and the SPh moieties is realized. In contrast, the planes of the two five-membered imidazolin rings in the cis-configured 2 are tilted by 768 with respect to each other (Figure 1). As a consequence, the very bulky 2,6-diisopropylphenyl groups avoid contact, and only one six-membered ring per ligand faces an SH substituent as a direct neighbor, notably, with the Al S vector inclined away from its plane. Presumably, this arrangement in 2, that is, orthogonal imidazolin ringplanes combined with SH substituents positioned on the same side of the Al2N2 core is a compromise between the both unfavorable steric congestion of the Dipp groups and interaction of these bulky moieties with the SH substituents. Alternative orientations would result in higher proximity of the Dipp groups to each other (planes coplanar, substituents cis or trans) or closer contacts between the Dipp groups and the SH moieties (planes orthogonal, substituents trans). Overall, we consider the trans-arrangement in 5 to result from the reduced steric hindrance of the mesityl substituents compared to the Dipp moieties in cis-configured 2 rather than to account for the increased bulkiness of the SPh groups in comparison to the SH ligands. In conclusion, activation of S8 and phenyl disulfide has been achieved by using imino-stabilized aluminum dihydride complexes. We have isolated the dimeric imidazolin-2-iminato complex {m-LDippAl(H)SH}2 (2), which is stable at ambient temperature. To the best of our knowledge, 2 is the first example for a molecular compound, in which a protic SH group and a hydride functionality are unified at the same aluminum atom. The key to the synthesis of this hydrogensulfide lies in the strong electron-donating character of the imidazolin-2-iminato ligand, and we conclude that steric shielding of the reactive sites is a requirement for the isolation of 2. Moreover, we have illustrated the general concept of activating nonpolar S S bonds via implementation of the molecular aluminum hydrides {m-LDippAlH2}2 (1), {m-LDippAl(H)SH}2 (2), and {m-LMesAlH2}2 (4) in the synthesis of {m-LDippAl(SH)2}2 (3) and {m-LMesAl(H)SPh}2 (5). We currently investigate further applications of 1 and 4 in bond activations, as well as the coordination chemistry of the new aluminum sulfides with respect to transition-metal complexes.

[2]

[3]

[4]

[5]

[6] [7]

[8]

Acknowledgements [9] [10]

We are exceptionally grateful to the Alexander von Humboldt foundation (Sofja Kovalevskaja Program) for financial support. We thank Dr. Elisabeth Irran for structural refinement of 3 (toluene)1.5. We thank Paula Nixdorf for the measurement of single crystals.

[11]

[12]

Keywords: aluminum · bond activation · hydrides · reactive intermediates · sulfur

[13] [14]

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Communication [15] S. Inoue, K. Leszczyn´ska, Angew. Chem. 2012, 124, 8717; Angew. Chem. Int. Ed. 2012, 51, 8589. [16] S. Courtenay, D. Walsh, S. Hawkeswood, P. Wei, A. K. Das, D. W. Stephan, Inorg. Chem. 2007, 46, 3623.

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[17] T. W. Myers, A. L. Holmes, L. A. Berben, Inorg. Chem. 2012, 51, 8997. Received: February 21, 2014 Published online on && &&, 0000

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COMMUNICATION & Reactive Intermediates D. Franz, S. Inoue* && – && Activation of Elemental Sulfur by Aluminum Dihydride: Isolation of Mono- and Bis(hydrogensulfide) Complexes of Aluminum

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S S bond activation: Conversion of a dimeric aluminum dihydride containing a bulky strong electron-donating imidazolin-2-iminato ligand with S8 resulted in the formation of an elusive aluminum–hydride–hydrogensulfide

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complex. Transformation to the iminosubstituted aluminum bis(hydrogensulfide) complex was achieved by reaction with S8 at elevated temperature (see scheme).

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Activation of elemental sulfur by aluminum dihydride: isolation of mono- and bis(hydrogensulfide) complexes of aluminum.

The conversions of the dimeric imidazolin-2-iminato aluminum dihydride {μ-L(Dipp)AlH2}2 (1) with ¼ or ½ equivalents of S8 gave the elusive hydrogensul...
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