Aluminum and Gallium Hydrazides as Active Lewis Pairs: Cooperative

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Aluminum and Gallium Hydrazides as Active Lewis Pairs: Cooperative C−H Bond Activation with H−CC−Ph and Pentafluorobenzene Werner Uhl,* Matthias Willeke, Frank Hengesbach, Alexander Hepp, and Marcus Layh Institut für Anorganische und Analytische Chemie der Universität Münster, Corrensstraße 30, D-48149 Münster, Germany S Supporting Information *

ABSTRACT: Hydroalumination or hydrogallation of a sterically encumbered hydrazone, H10C5N−NC(C9H14) (NC5H10 = piperidine, C(C9H14) = 2-adamantdiyl), afforded hydrazides that, depending on the steric shielding by the substituents at the metal atoms, had different molecular structures. While both di(tert-butyl)metal derivatives (1a, 1b) are monomeric in the solid state with highly strained MN2 heterocycles (M = Al, Ga), the dimethylmetal compounds (1c, 1d) are dimeric with M2N2 heterocycles and exocyclic N−N bonds. The latter compounds are highly dynamic in solution. 1d crystallized as a mixture of cis- and trans-isomers as detected by crystal structure determinations. These compounds react as active Lewis pairs by their specific donor−acceptor functionality and are able to activate C−H bonds of moderately acidic substrates. Reaction of 1a (M = Al) with H−CC−C6H5 afforded by C−H bond activation and release of H− CMe3 trialkynyl compound 4, in which three alkynyl groups and a neutral hydrazine ligand are bound to Al. 1b (M = Ga) gave only the known dimeric monoalkynyl derivative [(Me3C)2Ga−CC−C6H5]2 (5b). The sterically less shielded dimethyl compounds 1c and 1d similarly yielded trialkynylmetal compounds by methane and hydrogen elimination. In this case a hydrazone ligand is coordinated to the metal atoms. 1d reacted with pentafluorobenzene in an unprecedented reaction to yield a diaryl-methylgallium compound with the metal atom bound to two electron-withdrawing groups and a hydrazone ligand completing the coordination sphere of Ga.

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INTRODUCTION The activation of small molecules in stoichiometric or catalytic processes is the focus of current research activities. Only recently have frustrated Lewis pairs (FLPs),1 based on coordinatively unsaturated Lewis acidic and basic main group elements, found enormous interest in this field and proved to be powerful alternatives to the traditionally applied transition metal complexes and their dominant role in catalysis. Although hydrogen activation is considered as one of the most important reactions in FLP chemistry, many further highly interesting contributions confirm the wide applicability of these compounds in the coordination and activation of various substrates.1 In most cases boron-based FLPs have been applied, but recent reports have shown Al-based derivatives to form a similar efficient new class of FLPs. Their fascinating reactivity is now well documented in the literature.2 We have recently reported on the synthesis of two new classes of Al/N-based Lewis pairs and some preliminary results concerning their reactivity. One type of compounds is formed by hydroalumination of yneamines3 and features strained and unsaturated AlC2N heterocycles with relatively weak Al−N interactions. These compounds reacted with heterocumulenes such as carbodiimides3 or facilitated the unique cooperative oligomerization of cyanamides4 to yield linear oligomers instead of the thermodynamically strongly favored cyclic trimers. The second type of compounds comprises monomeric © XXXX American Chemical Society

Al hydrazides, which have strained AlN2 heterocycles and have been isolated only in very rare cases.5,6 One compound was obtained by hydroalumination of a sterically shielded hydrazone; it reacted with isocyanates, isothiocyanates, or carbon dioxide by insertion into an Al−N bond.5 The first reaction proceeded via an adduct, in which the CO group was coordinated by the Lewis pair via the Al atom and the Lewis basic N atom after cleavage of the Al−N donor−acceptor bond. This reaction confirms impressively the potential FLP activity of these Al−N compounds, although breakage of an Al−N bond is required as the initial reaction step. In continuation of this work we were interested in exploring the influence of Ga instead of Al as a Lewis acidic center and also the effect of less bulky substituents at the metal atoms on the structure and reactivity of these Lewis pairs. We were particularly interested in C−H bond activation as a highly interesting topic in current research7 and started with reactions of moderately acidic compounds (pKa ca. 25).

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RESULTS AND DISCUSSION The monomeric Al hydrazide (Me3C)2AlN[CH(C9H14)]− NC5H10 (1a, CH(C9H14) = 2-adamantyl, NC5H10 = piperReceived: August 16, 2016

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DOI: 10.1021/acs.organomet.6b00658 Organometallics XXXX, XXX, XXX−XXX

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Organometallics idinyl) was obtained in high yield by hydroalumination of the hydrazone H10C5N−NC(C9H14) [C(C9H14) = 2-adamantdiyl] with equimolar quantities of (Me3C)2AlH in refluxing nhexane (Scheme 1).5 At room temperature the hydrazone Scheme 1. Synthesis of Monomeric Group 13 Element Hydrazides Including a Numbering Scheme for the Adamantyl C Atoms Used in the Experimental Part

Figure 1. Molecular structure and atomic numbering scheme of compound 1b. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H31) have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−N(1) 2.153(1), Ga(1)− N(2) 1.892(1), Ga(1)−C(1) 1.992(8), N(1)−N(2) 1.476(2), N(2)− C(31) 1.405(3); C(1)−Ga(1)−C(1)′ 118.0(4), N(1)−Ga(1)−N(2) 42.2(1) N(1)−N(2)−Ga(1) 78.4(1), N(2)−N(1)−Ga(1) 59.4(1). Symmetry transformation to generate equivalent atoms: x, −y + 3/2, z.

with a Ga atom are extremely rare, and 1b is the first example of a structurally authenticated GaN2 heterocycle. If the steric demand of the substituents on the metal atoms was reduced, the reaction with the hydrazone proceeded for Al and Ga at much milder conditions as demonstrated for the synthesis of Me2M[N{CH(C9H14)}−NC5H10) (1c, M = Al; 1d M = Ga), which were both easily obtained by stirring stoichiometric quantities of the two starting materials in solution at room temperature (Scheme 1). Both compounds were highly dynamic in solution and showed overlapping signals in the 1H NMR spectra that precluded assignment of peaks to individual H or C atoms. The Ga derivative 1d was found to crystallize as a dimer in which two Me2Ga units are bridged by two N[CH(C9H14)]−NC5H10 fragments with exocyclic N−N bonds. The lower steric demand of the methyl groups allows dimerization that is not possible in the case of the more bulky CMe3 groups on the metal atoms (1a, 1b). Single crystals were obtained by recrystallization from 1,1,1-trifluorotoluene. They were found to be a conglomerate of two types of crystals consisting of a mixture of two isomers in which the adamantyl and piperidinyl substituents are trans or cis (Figures 2 and 3) relative to the slightly puckered Ga2N2 heterocycle [angles between the normals of the Ga2N planes 26.7° (cis) and 36.5° (trans)]. The shape of the crystals is indistinguishable, and we were not able to find a simple reproducible procedure that would allow the separation under a microscope. Fortuitously we picked a crystal of each isomer and were able to determine the structure of both forms. The Ga−N distances are, at about 2.08 Å, approximately in between the short and the long Ga−N distance found in 1b and are in the range of typical bridging Ga−N bonds. In solution the situation may be further complicated by the additional presence of monomers, which is consistent with the demonstrated reactivity of 1c/d as masked Lewis pairs (see below). The Al derivative 1c is likely to have a similar dimeric structure to 1d, but attempts to obtain single crystals from a variety of solvents (n-pentane, cyclopentane, toluene, trifluorotoluene, pentafluorobenzene) to confirm this hypothesis were

adduct (Me3C)2AlH·[H10C5N−NC(C9H14)] was isolated as an intermediate. It was converted to the hydrazide when heated under reflux conditions in n-hexane. Similarly, the reaction of (Me2HC-H2C)2AlH with H10C5N−NC(C9H14) required elevated temperatures (100 °C) and long reaction times (24 h) to achieve complete conversion.5 Surprisingly the corresponding Ga hydride (Me3C)2GaH reacted under similar conditions already at room temperature to yield the Ga hydrazide 1b in good yield. Since (Me 3 C) 2 AlH and (Me3C)2GaH are both trimeric in the solid state8,9 with almost identical bond parameters, the difference in reactivity may be a result of the higher stability of the hydrazone adduct in the case of the much harder Lewis acid Al as compared to Ga. Further, the higher polarity of the Al−H bond and the higher negative partial charge at its H atom may enhance the stability of oligomeric compounds and hinder the dissociation of the hydride in solution.10 The formation of 1a and 1b was evident in the 1H NMR spectrum from the disappearance of the characteristic broad signal of the metal-bound H atom and the appearance of a new signal for a H atom bonded to the α-C atom of the adamantyl group (2-Ad). Furthermore, the 13C NMR shift of the N-bound C atom of the adamantyl group changed from δ = 197 in the hydrazone to around δ = 60 in the metal hydrazides. 1b is isostructural to the Al analogue 1a5 and crystallizes as a monomer (Figure 1) with a strained threemembered GaN2 heterocycle [N(1)−Ga(1)−N(2) 42.2(1)°] and one short and one long Ga−N bond [Ga(1)−N(2) 1.892(1) Å; Ga(1)−N(1) 2.153(1) Å, donor−acceptor interaction]. The Ga−C bond lengths are around 2.00 Å and comparable to the Al−C distances found in the Al compound 1a, while the Al−N and Ga−N distances differ significantly [1.82 and 2.01 Å versus 1.89 and 2.15 Å]. The shorter distances observed for the Al compound reflect the higher ionic contribution to the Al−N bond. Three-membered heterocycles B


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Organometallics

Scheme 2. Formation of the Dismutation Product 2 and of the THF Adduct 3

Figure 2. Molecular structure and atomic numbering scheme of compound trans-1d. Only one of two independent molecules is shown. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H11 and H13) have been omitted. Selected bond lengths (Å) and angles (deg). Values for the second molecule are in square brackets: Ga(1)−N(11) 2.053(2) [2.076(2)], Ga(1)−N(13) 2.080(2) [2.097(2)], Ga(1)−C(151) 1.998(3) [1.975(3)], Ga(1)−C(152) 1.973(3) [1.982(3)], Ga(2)−N(11) 2.106(2) [2.081(2)], Ga(2)− N(13) 2.089(2) [2.099(2)], Ga(2)−C(161) 1.988(3) [1.980(3)], Ga(2)−C(162) 1.961(3) [1.984(3)], N(11)−N(12) 1.472(3) [1.477(3)], N(11)−C(11) 1.511(3) [1.499(3)], N(13)−N(14) 1.481(3) [1.476(3)], N(13)−C(13) 1.496(3) [1.502(3)]; N(11)− Ga(1)−N(13) 88.4(1) [87.4(1)], N(11)−Ga(2)−N(13) 86.7(1) [87.2(1)], Ga(1)−N(11)−Ga(2) 87.0(1) [86.8(1)], Ga(1)−N(13)− Ga(2) 86.7(1) [86.9(1)].

2 is poorly soluble in n-pentane and started to precipitate immediately. The precipitate was isolated after 16 h and washed with n-pentane to yield compound 2 as an analytically pure solid. The Al hydrazide 1c was best synthesized in toluene (high solubility for the hydrazone starting material) at room temperature and stored at this temperature as a standard solution for further reactions. Since the NMR spectroscopic characterization of 1c was unsatisfactory (see above), it was treated with an excess of THF (Scheme 2). Coordination of the Lewis base THF led to monomerization, and in high yield the THF adduct Me2Al[N{CH(C9H14)}−NC5H10](THF) (3) was isolated, which features a five-coordinate Al atom and a threemembered AlN2 heterocycle similar to 1a and R2Al[N{(CH(C9H14)}−NC5H10] (R = CMe3, CH2CHMe2).5 This adduct allowed the unambiguous identification of 1c and confirms that it is formed in high purity and that a complicated equilibrium in solution hinders the characterization by NMR spectroscopy. Attempts to obtain a similar simple adduct of the Ga analogue 1d by treatment with THF, pyridine, or PPh3 were not successful and led to a mixture of compounds and eventual decomposition of the starting material. The different behavior reflects the different Lewis acidity of these metal atoms with Ga atoms as the weaker Lewis acids. The chemical shifts of the piperidyl and 2-adamantyl substituents of 2 and 3 are very similar to those of the hydrazone. The chemical shifts of the metal-bound methyl groups are at about δ = −0.4 (1H) and −11 (13C), respectively. Both compounds are monomeric in the solid state (Figures 4, 5) and feature five-coordinate Al atoms with one (3) or two (2) three-membered AlN2 heterocylces. The coordination sphere of the metal atom in 3 may be described as a distorted tetrahedron in which one vertex is occupied by the hydrazide ligand. The coordination of 2 may be best described as a distorted quadratic pyramid with the methyl group in the axial position. The Al−N[CH(C9H14)] distances of about 1.82 Å are comparable to that of 1a, but the Al−N(pip) bond lengths are significantly longer [2.064(av) Å (2); 2.151(1) Å (3)] and consistent with the higher coordination number of the Al atom and a weaker Al−N donor−acceptor bond. The Al(1)−O(1) bond [1.999(1) Å] to the coordinatively bound THF molecule of compound 3 is nearly 0.1 Å longer than that of typical THF adducts with fourcoordinate Al atoms, which may be caused by the increased

Figure 3. Molecular structure and atomic numbering scheme of compound cis-1d. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H1) and solvent molecules (F3C−C6H5) have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−N(1) 2.093(4), Ga(1)−N(1)′ 2.102(4), Ga(1)−C(3) 1.991(5), Ga(1)−C(4) 1.992(5), N(1)−N(2) 1.477(5), N(1)−C(1) 1.496(6); N(1)−Ga(1)−N(1)′ 89.3(2), Ga(1)−N(1)−Ga(1)′ 87.5(2). Symmetry transformation used to generate equivalent atoms: y + 1/2, x−1/2, −z.

unsuccessful. The only product that crystallized reproducibly in low yield was the condensation product MeAl[N{CH(C9H14)}−NC5H10]2, 2 (Scheme 2), which is formed by elimination of AlMe3 from two formula units of the starting material. Compound 2 is a minor side product of the reaction, which was found to crystallize preferentially, thereby probably shifting the equilibrium to the condensation product. It was found that the best yields of 2 (28%) were obtained when the reaction between the hydrazone and Me2AlH was carried out in just enough n-pentane to solubilize the hydrazone. Compound C


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Organometallics

of about 25. Treatment of (Me3C)2Al[N{CH(C9H14)}− NC5H10] (1a) with Ph−CC−H in toluene and recrystallization of the product from cyclopentane afforded the hydrazine adduct 4. All tert-butyl groups of 1a were removed, and three alkynido groups were instead bound to the central Al atom. The coordination sphere of the metal atom is completed by a neutral hydrazine ligand, which is coordinated via the more basic piperidinyl N atom12 (Scheme 3). 4 was isolated in 73% Scheme 3. Treatment of 1a and 1b with Ethynes

Figure 4. Molecular structure and atomic numbering scheme of compound 2. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H1 and H3) have been omitted. Selected bond lengths (Å) and angles (deg): Al(1)−N(1) 1.809(1), Al(1)− N(2) 2.067(1), Al(1)−N(3) 1.808(1), Al(1)−N(4) 2.060(1), Al(1)− C(5) 1.964(2), N(1)−N(2) 1.486(2), N(3)−N(4) 1.493(2), N(1)− C(1) 1.454(3), N(3)−C(3) 1.473(3), N(1)−Al(1)−N(2) 44.5(1), N(3)−Al(1)−N(4) 44.8(1), N(1)−Al(1)−N(4) 116.0(1), N(2)− Al(1)−N(3) 116.6(1), N(1)−Al(1)−N(3) 122.4(1), N(2)−Al(1)− N(4) 147.7(1), C(5)−Al(1)−N(1) 120.1(1), C(5)−Al(1)−N(2) 106.6(1), C(5)−Al(1)−N(3) 117.6(1), C(5)−Al(1)−N(4) 105.7(1).

yield when the starting compounds 1a and the alkyne were mixed in toluene in the correct 1 to 3 stoichiometric ratio. The reaction was not completed after stirring of the mixture for 16 h at room temperature, as evident from an NMR spectrum. Under optimized conditions the solvent toluene was removed in vacuo, the raw product was treated with cyclopentane, and the product started to precipitate immediately. Combining the starting materials directly in cyclopentane in the absence of toluene led to a mixture of compound 4 and several unidentified compounds. Me3CH was identified as a byproduct (Scheme 3) by its characteristic 1H NMR spectrum [δ = 1.65 (decet, 3JHH = 6.6 Hz, 1 H); 0.86 (d, 3JHH = 6.6 Hz, 9 H)] in an experiment carried out in an NMR tube that was sealed immediately after the starting materials (1a and alkyne) were treated with benzene-d6 . Attempts to isolate possible intermediates such as (PhCC) 2 Al[N{CH(C 9 H 14 )}− NC5H10] by using only two equivalents of Ph−CC−H led to the isolation of compound 4 and unreacted starting material. The 1H NMR spectrum of 4 is characterized by the disappearance of the signal for the tert-butyl substituents and the appearance of signals of Ph groups in the aromatic region. There is an additional signal for the NH proton at δ = 3.87. The N−C−H hydrogen atom of the adamantyl group is shifted downfield by more than 0.5 ppm to δ = 4.29 compared to the starting compound 1a. This may be the result of the transformation of the anionic hydrazide group to a neutral hydrazine ligand. Both α-protons of the piperidinyl heterocycle show also a downfield shift compared to compounds 1 to 3. The characteristic ν(N−H) stretching vibration was observed in the IR spectrum at 3283 cm−1.

Figure 5. Molecular structure and atomic numbering scheme of compound 3. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H1) have been omitted. Selected bond lengths (Å) and angles (deg): Al(1)−N(1) 1.828(1), Al(1)−N(2) 2.151(1), Al(1)−C(3) 1.982(1), Al(1)−C(4) 1.971(1), Al(1)−O(1) 1.999(1), N(1)−N(2) 1.468(1), N(1)−C(1) 1.452(1), C(3)−Al(1)− C(4) 115.5(1), N(1)−Al(1)−N(2) 42.3(1).

coordination number of five.11 Other bond lengths and angles are unexceptional and similar to those of 1a and 1b. The monomeric metal hydrazides 1a and 1b have strained three-membered AlN2 heterocycles, and cleavage of an Al−N or Ga−N bond should allow their application as active Lewis pairs. Few reactions with heterocumulenes (isocyanate, CO2) have been reported, but C−H bond activation as a topic of high current research interest7 has never been considered previously. As suitable substrates we choose the moderately acidic organic compounds Ph−CC−H and H−C6F5, which have pKa values D


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Organometallics

Treatment of Al(CMe3)3 at room temperature or at 65 °C with an excess of Ph−CC−H led to the elimination of only one equivalent of HCMe3 and the formation of the dialkynylaluminum compound [(Me3C)2Al−CC−Ph]214b which was obtained previously from (Me3C)2Al−Cl and Li−CC−Ph. Excess tert-butylethyne, H−CC−CMe3, and Al(CMe3)3 gave an unselective reaction. After 14 days at room temperature the trialkylaluminum compound was consumed only partially, and the Al alkynide [(Me3C)2Al−CC−CMe3]215 was not detected as a product by NMR spectroscopy. The ammine adduct (Me3C)3Al←NEt3 has a strong Al−N donor−acceptor interaction. Its reaction with Ph−CC−H led even at 75 °C to the replacement of only one tert-butyl group by an alkynyl substituent and the release of H−CMe3, resulting again in the formation of the monoalkynide [(Me3C)2Al−CC−Ph]2.14b The Ga hydrazide (Me3C)2Ga[N{CH(C9H14)}−NC5H10] (1b) was much less reactive, and its reaction with an excess (four equivalents) of Me3C−CC−H yielded after 14 days at room temperature only the dimeric alkynide [(Me3C)2Ga− CC-CMe3]2 (5a) in 63% yield (Scheme 3). This compound has not been described previously. It was characterized by NMR [δ(13C): 144.9 (CC−CMe3), 79.1 (CC−CMe3)] and IR spectroscopy (νCC = 2135 and 2058 cm−1). The constitution with two Ga atoms bridged by two α-C atoms of alkynido groups (CC: 1.208(2) Å) was further confirmed by crystal structure determination. The structure is isotypic to the corresponding Al alkynide,15 and we abstain from a pictorial presentation. An NMR experiment with Ph−CC−H was consistent with the same behavior and gave the dimer [(Me3C)2Ga−CC−Ph]2 (5b)14b quantitatively. The different reactivity of 1a and 1b may tentatively be explained by the postulated mechanism shown in Scheme 4. The first step is likely the cleavage of an Al−N bond by ring opening and the approach of the α-C atom of the alkyne, which bears a relatively high negative partial charge, to the electropositive Al atom. Cooperative activation of the C−H bond and migration of the

Compound 4 is monomeric in the solid state (Figure 6) with a distorted tetrahedral coordination of the metal atom. The

Figure 6. Molecular structure and atomic numbering scheme of compound 4. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H1 and H01) have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Al(1)−N(2) 2.015(1), Al(1)−C(3) 1.939(2), Al(1)−C(4) 1.935(2), Al(1)−C(5) 1.939(1), N(1)−N(2) 1.469(2), N(1)−C(1) 1.482(2), C(3)−C(31) 1.211(2), C(4)−C(41) 1.209(2), C(5)−C(51) 1.211(2); N(2)−Al(1)−C(3) 108.5(1), N(2)−Al(1)−C(4) 106.1(1), N(2)−Al(1)−C(5) 100.8(1), C(3)−Al(1)−C(4) 111.3(1), C(3)−Al(1)−C(5) 114.0(1), C(4)− Al(1)−C(5) 115.2(1).

hydrazine ligand binds to the metal atom via the more basic piperidyl N atom, which is in an approximate anti position to the H atom of the NCH group (2-Ad, N(2)−N(1)−C(1)− H(1) 24°). The Al(1)−N(2) distance is, at 2.015(1) Å, in the typical range of coordinative Al−N bonds. The tetrahedral coordination sphere of Al is distorted toward a trigonal pyramid in which the alkyne substituents form the base, as evident from large C−Al−C angles (111° to 115°). The N−Al−C angles are significantly smaller (101° to 108°). The CC bond lengths (1.21 Å on average) correspond to the standard value, and the ethynyl groups deviate only slightly from linearity (Al−CC 175.0°; CC−C 178.7°). A relatively short intramolecular distance between an α-C atom of an ethynyl group (C(5)) and the H atom attached to nitrogen (H(01); 2.4 Å) as well as the small torsion angle H(01)−N(1)···Al(1)−C(5) of −10° may indicate a weak N−H···C hydrogen bonding.3,13 Accordingly, the angle Al(1)−C(5)−C(51) is, at 170.8(1)°, relatively small compared to the remaining two Al−CC angles (177.1° on average). Monoalkynylaluminum compounds are well established in the literature,14,15 but dialkynyl- or trialkynylaluminum species are rare.15−17 They were usually obtained by salt elimination or treatment of Al−H compounds with alkynes by hydrogen elimination. The reactions between 1a and Ph3Si−CC−H or Me3C− CC−H yielded, by an tert-butyl-alkynyl exchange, also the corresponding hydrazine adducts (R-CC)3Al[NC5H10−N(H){CH(C9H14)}] (R = SiPh3, CMe3) in high purity. Only the reaction of H−CC−SiPh3 afforded an amorphous solid, which, however, could not be purified by recrystallization. The NMR spectroscopic data given in the Experimental Section resulted from NMR experiments and allow an unambiguous identification. The generation of single crystals for crystal structure determinations failed. Therefore, we abstain from a more detailed discussion. A series of control experiments confirmed the unique reactivity of the active Lewis pair 1a.

Scheme 4. Postulated Mechanism for the Generation of 4

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Organometallics proton to an N atom of the hydrazide results in the formation of a zwitterionic intermediate. A comparable compound was isolated and fully characterized by the reaction of Ph−CC− H with the related Al/N-based active Lewis pair (Me3C)2Al− C(SiMe3)C(H)−(2,6-Me2NC5H8), which has a strained four-membered AlC2N heterocycle with a relatively weak Al− N donor−acceptor bond.3 An adduct resulted in this case that has the alkynido group bound to Al and the proton attached to N. Hydrogen bonding was observed between the α-C atom of the ethynyl group and the N−H proton. This compound dissociated partially in solution and did not show alkane elimination, which is characteristic of the reaction of the hydrazide 1a with phenylethyne. In Scheme 4 the high polarity of the Al−C bond and the acidity of the N−H group facilitates the elimination of HCMe3. This process is repeated for the second equivalent of Ph−CC−H. The approach of the third equivalent of Ph−CC−H and the activation of its terminal C−H bond results in the cleavage of the Al−N[(CH(C9H14)] bond, the subsequent transfer of the H atom to the piperidinyl N atom, and the formation of the hydrazine adduct 4. In the case of the Ga derivative 1b the weak polarizing capability of the Ga atom and the low polarity of the Ga−C bond (Ga has a higher electronegativity than Al) do not favor the elimination of HCMe3. The reaction with the alkyne leads directly to the elimination of the hydrazine and the formation of the thermodynamically stable [(Me3C)2Ga−CC−Ph]2 dimer (5b)14b with two bridging alkynide groups. The formation of dimers may be favored over a hydrazine adduct, [(Me3C)2Ga− CC−R][HN{CH(C9H14)}−NC5H10)], due to the relatively weak interaction of the comparably soft Ga atom with the hard N atom of the hydrazine. The free hydrazine was detected in the reaction mixture by its characteristic 1H NMR chemical shifts. The sterically less encumbered methyl derivatives showed again a different reaction behavior, and treatment of 1c and 1d with Ph−CC−H at room temperature led, via an immediate gas evolution, in good yields to the formation of the hydrazone adducts (Ph−CC)3M←NC(C9H14)−NC5H10 6a and 6b (Scheme 5), in which, probably for steric reasons, the less basic N atom NC(C9H14) is connected to the metal atom. These reactions clearly demonstrate the ability of the dimeric hydrazides to act as active Lewis pairs. The product formation is consistent with a reaction sequence that includes CH4 elimination by reaction of the methyl substituents with the activated hydrogen atom of the alkyne and retro-hydrometalation to yield a metal hydride18 and hydrazone (proposed mechanism in Scheme 6). This is followed by the reaction of the hydridic H atom of the metal hydride with the acidic proton of the alkyne by H2 elimination and adduct formation of the resulting metal alkynide with the hydrazone. Retrohydroalumination as the key step of these reactions has been observed several times for sterically less shielded Al and Ga hydrazides; these results have not been published yet. In the related reaction of uncoordinated Me2EH (E = Al, Ga) with Ph−C C−H the methyl groups were not reactive and H2 elimination led exclusively to the formation of dimeric alkynides (Me2E− CC−Ph)2,14b confirming that activation of the C−H bond by the active Lewis pair is a crucial step in the reactions of compounds 1c and 1d. 6a and 6b are highly sensitive and decompose in solution and in the solid state. Relevant NMR parameters for compounds 6 are the characteristic shifts of the CN group at about δ = 200 and of the alkynyl C atoms in a narrow range between δ = 103

Scheme 5. Reaction of 1c and 1d with Ph−CC−H and H− C6F5

and 109 in the 13C NMR spectra. Both groups are also easily identified in the IR spectrum [ν(CC) 2127 and 2139 cm−1; ν(CN) 1596 and 1621 cm−1]. In the 1H NMR spectrum the two magnetically inequivalent 1,3-Ad protons are shifted significantly to lower field (δ = 3.98 and 4.40) as compared to compounds 1 to 4 (δ = 1.5 to 2.5) or the free hydrazone H10C5N−NC(C9H14) (δ = 3.45 and 2.52).5 This may be caused by a syn orientation and a resulting short contact between the piperidyl N atom and one of the adamantyl H atoms (see crystal structure of 6b: N···H 2.3 Å; torsion angle C23−N2−C11−H11 −7.7°). This contact is also observed in the solid-state structure of the free hydrazone.5 In compound 6b the second H atom is approximately equidistant to two α-C atoms of the alkynyl groups (2.61 and 2.81 Å) and is located inside the deshielding cone of the alkyne (Figure 7). This effect may also be significant for the chemical shift of the NCH2 protons of the piperidyl substituents. The axial H atoms are in the solid state even closer to the remaining alkynyl substituent (2.58 and 2.51 Å), which is consistent with the unusually low field shift observed for these protons (δ = 4.40) and the large shift difference Δδ = 1.7 between axial and equatorial H atoms, which is typically less than 0.5 (cf. compounds 1 to 4). Single crystals suitable for crystal structure determination were only obtained for the Ga compound 6b. The molecular structure (Figure 7) is similar to that of 4. The Ga atom is coordinated in a distorted tetrahedral fashion by three alkynyl substituents14b,c,16f,19 and the imine N atom (N−Ga−C angles 103° to 107°; C−Ga−C angles 111° to 116°). The Ga(1)− N(1) distance is, at 2.064(1) Å, slightly longer than the Al−N distance in the hydrazine adduct 4 but in the typical range of dative Ga−N bonds. To further demonstrate the unique reactivity of these active Lewis pairs, 1d was treated with H−C6F5, whose pKa20 is comparable to that of alkynes. The reaction proceeded similarly to that of Ph−CC−H under mild conditions (room F


Article

Organometallics Scheme 6. Proposed Mechanism for the Formation of Compounds 6 by Retrohydroalumination and C−H Bond Activation

Figure 7. Molecular structure and atomic numbering scheme of compound 6b. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−N(1) 2.064(1), Ga(1)−C(3) 1.941(1), Ga(1)−C(4) 1.944(1), Ga(1)−C(5) 1.945(1), N(1)−N(2) 1.446(1), N(1)−C(1) 1.290(2), C(3)−C(31) 1.203(2), C(4)−C(41) 1.207(2), C(5)−C(51) 1.201(2); N(1)−Ga(1)−C(3) 102.6(1), N(1)−Ga(1)− C(4) 106.0(1), N(1)−Ga(1)−C(5) 107.2(1), C(3)−Ga(1)−C(4) 113.5(1), C(3)−Ga(1)−C(5) 116.0(1), C(4)−Ga(1)−C(5) 110.7(1).

temperature), and the hydrazone adduct 7a was obtained in 60% yield (Scheme 5). The molecular structure (Figure 8) has a methyl and two pentafluorophenyl groups bound to the central Ga atom. The distorted tetrahedral coordination sphere of the metal atom is completed by a hydrazone ligand, which is coordinated via its imine N atom. The Me group (H atom in 7b) is oriented syn relative to the piperidyl N atom (C(3)− Ga(1)−N(1)−N(2) 4.3°, H(1)−Ga(1)−N(1)−N(2) 2.7°). The Ga(1)−N(1) distance is, at 2.095(1) Å, slightly longer than in 6b despite the electronegative pentafluorobenzene substituents. The Ga−C distances differ significantly. A distance of 1.954(2) Å was detected for the Ga−Me bond [Ga(1)− C(3)], while larger distances of 2.018(av) Å resulted for the Ga−aryl bonds [Ga(1)−C(4) and Ga(1)−C(5)]. The bond length of 1.290(2) Å confirms the double-bond character of the CN bond to the adamantyl substituent. The formation of 7a from 1d can be rationalized by CH4 elimination, retrohydrogallation, H2 elimination, and adduct formation. One pentafluorobenzene molecule is deprotonated by a Me group; the second, by reaction with the intermediately formed Ga−H bond. In contrast to the trialkynyl products (6) only two aryl groups are coordinated to the Ga atom, and we have not been able to replace the second methyl group by treating 1d with an excess of H−C6F5. Beyond steric effects, the electronwithdrawing properties of both fluorinated substituents may

Figure 8. Molecular structure and atomic numbering scheme of compound 7a. The molecular structure of compound 7b is very similar but has a hydrogen atom instead of the methyl group attached to Ga. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) (values for 7b in brackets): Ga(1)−N(1) 2.095(1) [2.090(2)], Ga(1)−C(3) 1.954(2) [Ga(1)−H(1) 1.44(2)], Ga(1)−C(4) 2.021(2) [Ga(1)−C(3) 2.006(2)], Ga(1)−C(5) 2.014(2) [Ga(1)−C(4) 2.015(2)], N(1)−N(2) 1.452(2) [1.464(2)], N(1)−C(1) 1.290(2) [1.309(4)], N(1)−Ga(1)−C(3) 115.7(1) [N(1)−Ga(1)−H(1) 108(1)], N(1)−Ga(1)−C(4) 100.0(1) [N(1)−Ga(1)−C(3) 105.7(1)], N(1)−Ga(1)−C(5) 105.6(1) [N(1)−Ga(1)−C(4) 102.2(1)], C(3)−Ga(1)−C(4) 113.1(1) [H(1)−Ga(1)−C(3) 114.8(9)], C(3)−Ga(1)−C(5) 108.4(1) [H(1)−Ga(1)−C(4) 111(1)], C(4)−Ga(1)−C(5) 114.0(1) [C(3)−Ga(1)−C(4) 114.2(1)].

reduce the polarity and reactivity of the Ga−Me bond. For comparison the adduct Me3Ga←[NC(C9H14)−NC5H10] G


Article

Organometallics

M−N bonds. Cleavage of the long M−N bond should result in an active state with coordinatively unsaturated metal and N atoms similar to frustrated Lewis pairs. The dimeric compounds have M−N−M bridges with exocyclic N−N bonds. Although monomerization is required as the initiating first step, these dimers show a reactivity similar to that of the monomeric compounds. In previous investigations we reacted the sterically shielded Al compound 1a with heterocumulenes such as phenyl isocyanate or carbon dioxide and observed adduct formation with the CO group inserted into the Al−N donor−acceptor bond or insertion into the Al−N polar covalent bond.5 C−H bond activation is a fascinating possible application of these active Lewis pairs, which has not yet been investigated and may allow unprecedented transformations with the generation of new compounds. Therefore, we treated compounds 1 with H− CC−C6H5 and H−C6F5 as examples for moderately acidic organic materials. The alkyne resulted in the formation of trialkynylaluminum or -gallium compounds (4 and 6) by alkane release under mild conditions. A neutral hydrazine or hydrazone ligand is formed as a byproduct, which is coordinated to the metal atoms via one of the N atoms. Only with the sterically encumbered di(tert-butyl)gallium compound 1b did the reaction end with dimeric monoalkynyl derivatives, [(Me3C)2Ga−CC−R]2 (5), probably caused by the facile dissociation of an intermediate hydrazine adduct. Pentafluorobenzene reacted with the GaMe2 compound 1d to afford a similar hydrazone adduct (7a) in which two C6F5 groups were attached to the Ga atom. This compound may be formed by release of CH4, retro-hydrogallation, and H2 elimination. One methyl group did not react. Steric repulsion may hinder the attack of a third H−C6F5 molecule, and the electronwithdrawing groups may reduce the polarity of the Ga−C bond and the nucleophilicity of the CH3 group. Treatment of the acidic compounds with trialkylelement compounds, an amine adduct, or dimethylgallium hydride afforded monosubstitution products (with H−CC−R), gave unselective reactions, or gave no reaction at all (with H−C6F5). It is the specific cooperativity between N and metal atoms in these Al/ N-based active Lewis pairs that enables these unprecedented reactions. A proposed mechanism comprises the activation of the polar C−H bonds by an approach of the C atom to the Lewis acidic metal atom and of the H atom to a basic N atom. The C−H bond is cleaved, and the relatively high acidity of the resulting N−H functionality facilitates alkane elimination. A related activation of C−H bonds of terminal alkynes has previously been reported for Mg and Ga amides.23 Similar transformations may succeed with other organic acids having similar or smaller pKa values such as CHCl3 or cyclopentadiene and may result in the formation of unique compounds with unknown substitution patterns at the metal atoms. Particularly interesting are reactions in which the activated substrates are transferred to other compounds in stoichiometric or catalytic reactions. In further investigations it seems to be important and reasonable to extend these studies to Al amides, R2Al−NR′2, in order to gain a better insight into the mechanism of these transformations and in particular into the influence of the β-N atom of the hydrazide group on conversion and selectivity.

and the uncoordinated gallium hydride Me2GaH were both reacted with H−C6F5. The hydrazone adduct was stable in the presence of H−C6F5, and no change in the NMR spectra (1H, 19 F) was observed after 2 days at 65 °C in benzene. Me2GaH in contrast started to decompose in an unselective reaction to yield a number of unidentified products after 1 day at 65 °C in benzene, but there was no change in the 19F NMR spectrum, indicating no participation of the pentafluorophenyl group in the decomposition reaction. These observations underscore once more the unique propensity of the M−N-based Lewis pairs 1 to activate suitable substrates and to facilitate highly selective reactions in a cooperative fashion. C−H bond cleavage may be initiated in a way presented in Scheme 6. The di(tertbutyl)element compounds 1a and 1b do not react with H− C6F5, which may be caused by steric shielding. These compounds can be recrystallized or stored in this solvent over weeks without any sign of decomposition. The dimethylaluminum-based Lewis pair 1c reacted in an unselective way and afforded a mixture of unidentifiable products with a very complicated 19F NMR spectrum. We found only a few reports in the literature on C−H bond activation with pentafluorobenzene. In most cases highly reactive transition metal complexes were used, and one contribution reported on the insertion of Si(II) into the C− H bond.21 Transition metal based reactions show a competition between C−F and C−H bond activation. The 19F NMR spectra of the reaction mixtures confirm selective transformations with the active Lewis pair 1d without any hint of C−F bond cleavage. The 13C NMR and IR spectroscopic parameters of 7a [δ(CN) = 199.3, ν(CN) = 1636 cm−1) are similar to those of compounds 6. The signals of the H atoms in 1,3 positions of the adamantane skeleton are shifted by δ = 3.03 and 3.82 to a lower field of the 1H NMR spectrum, which also is in accordance with the data of compounds 6. This is again consistent with a relatively short N−H contact (N2···H11 2.3 Å; N2−N1−C11−H11 1.0°) in the solid state. The second signal may be shifted as a result of close proximity between the H atom and the aromatic ring of the pentafluorobenzene substituents (H···C distances to one ortho-C and two ipso-C atoms 2.68 to 2.79 Å). The methyl group bound to Ga has a chemical shift of δ(1H) = 0.58, which is at a relatively low field compared to other methylgallium compounds or related species with GaCH groups22 and may be influenced by the electronwithdrawing substituents. As a minor side-product we isolated a few single crystals of 7b, sufficient for an X-ray crystal structure determination, but not for a full characterization. 7b contains a H atom attached to Ga instead of the methyl group in 7a. It may result from the reaction of a Ga(H)Me species intermediately formed by dehydroalumination, which was not followed by H2 elimination as necessary for the formation of the main product (7a), but instead by the release of a second methane molecule in a minor competitive reaction. Some important structural parameters of 7b are added to the caption of Figure 8.

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CONCLUSION Hydroalumination or hydrogallation of a sterically shielded hydrazone afforded hydrazides (1), which, depending on the size of the substituents bound to the metal atoms, are monomeric (1a, 1b) or dimeric (1d) in the solid state. The monomeric forms have strained AlN2 or GaN2 heterocycles in their molecular cores with long (donor−acceptor) and short

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EXPERIMENTAL SECTION

General Considerations. All procedures were carried out under an atmosphere of purified argon in dried solvents (n-hexane, npentane, and cyclopentane with LiAlH4; toluene and THF with Na/ benzophenone; 1,1,1-trifluorotoluene, 1,2-difluorobenzene, and pentaH


Article

Organometallics

831 w, 797 vw ν(CC), ν(CN), ν(NN); 725 vs (paraffin); 671 m, 652 m, 619 m, 567 m, 536 s, 467 m, 453 m ν(GaC), ν(GaN), δ(CC). MS (20 eV, 413 K): m/z (%) 332 (14) [1/2M]+, 317 (5) [1/2 M − Me]+. Anal. Calcd for C34H62Ga2N4 (666.34): C 61.3, H 9.4, N 8.4. Found: C 60.7, H 9.1, N 8.2. Synthesis of MeAl[N{CH(C9H14)}−NC5H10]2 (2). A solution of H10C5N−NC(C9H14) (4.33 g, 18.6 mmol) in n-pentane (10 mL) was added to neat Me2AlH (1.08 g, 18.6 mmol). The suspension was stirred at room temperature overnight. The solvent was removed by filtration, and the colorless residue was washed twice with n-pentane (10 mL each) and dried in vacuo (1.33 g, 28% based on Me2AlH). Mp: 182 °C (dec). 1H NMR (400 MHz, C6D6, 300 K): δ 3.57 (s, 2 H, 2-Ad), 3.18 (d, 2JHH = 11.3 Hz, 4 H, NCH2), 2.64 (pseudo-t, JHH = 11.3 Hz, 4 H, NCH2), 2.48 (d, 2JHH = 11.8 Hz, 4 H, 4,8,9,10-Ad), 1.92 (s, overlap, 8 H, 4,8,9,10-Ad), 1.92 (s, overlap, 2 H, 5,7-Ad) 1.91 (s, overlap, 2 H, 5,7-Ad), 1.87 (s, 4 H, 1,3-Ad), 1.82 (s, 4 H, 6-Ad), 1.55 (d, 2JHH = 11.7 Hz, 4 H, 4,8,9,10-Ad), 1.47 (m overlapping, 8 H, NCH2CH2), 1.46 (m, overlap, 2 H, NCH2CH2CH2), 0.87 (s, 2 H, NCH2CH2CH2), −0.37 (s, 3 H, AlMe). 13C NMR (100 MHz, C6D6, 300 K): δ 58.6 (2-Ad), 55.7 (NCH2), 38.5 (6-Ad), 38.2 (4,8,9,10-Ad), 36.0 (1,3-Ad), 32.4 (4,8,9,10-Ad), 28.7 and 28.3 (5,7-Ad), 26.9 (NCH2CH2), 24.0 (NCH2CH2CH2), −13.6 (AlMe). IR spectrum (KBr plates, paraffin, cm−1): 1611 w; 1462 vs, 1375 vs (paraffin); 1346 m, 1306 m, 1287 m, 1263 s δ(CH3); 1217 m, 1190 s, 1167 s, 1152 s, 1110 m, 1096 s, 1067 s, 1030 s, 961 w, 907 w, 864 w, 847 vw, 800 s, 770 m ν(CC), ν(CN), ν(NN); 721 vs (paraffin); 700 s, 671 m, 633 s, 505 m, 486 w ν(AlC), ν(AlN), δ(CC). MS (EI, 20 eV, 413 K): 508 (58) [M]+, 492 (4) [M − CH4]+, 425 (16) [M − HNC5H10]+, 410 (12) [M − NC5H10 − CH4]+. Anal. Calcd for C31H53AlN4 (508.75): C 73.2, H 10.5, N 11.0. Found: C 72.9, H 10.3, N 11.3. Synthesis of Me2Al[N{CH(C9H14)}−NC5H10](THF) (3). A solution of 1c (0.5 M in toluene, 1.0 mL, 0.5 mmol) was treated with THF (2.0 mL, 1.8 g, 25 mmol). The mixture was stirred for 3 h. The solvent was removed in vacuo, and the residue recrystallized from 1,1,1trifluorotoluene (−28 °C) to yield compound 3 as a colorless solid (0.17 g, 94%). Mp: 116 °C (dec). 1H NMR (400 MHz, C6D6, 300 K): δ 3.58 (s, overlap, 1 H, 2-Ad), 3.58 (m, overlap, 4 H, OCH2), 2.96 (d, 2 JHH = 10.6 Hz, 2 H, NCH2), 2.75 (dt, 2JHH = 12.4 Hz, 3JHH = 2.6 Hz, 2 H, NCH2), 2.45 (d, 2JHH = 11.7 Hz, 2 H, 4,8,9,10-Ad), 1.94 (s, overlap, 4 H, 4,8,9,10-Ad), 1.92 (s, overlap, 2 H, 5,7-Ad), 1.84 (s, overlap, 2 H, 1,3-Ad), 1.83 (s, overlap, 2 H, 6-Ad), 1.61 (d, 2JHH = 12.0 Hz, 2 H, NCH2CH2), 1.52 (m, 4 H, NCH2CH2 and 4,8,9,10-Ad), 1.46 (s, overlap, 1 H, NCH2CH2CH2), 1.19 (m, 4 H, OCH2CH2), 0.95 (s, 1 H, NCH2CH2CH2), −0.44 (s, 6 H, AlMe). 13C NMR (100 MHz, C6D6, 300 K): δ 69.6 (OCH2), 59.6 (2-Ad), 55.9 (NCH2), 38.7 (6Ad), 38.4 (4,8,9,10-Ad), 35.8 (1,3-Ad), 32.4 (4,8,9,10-Ad), 28.4 (5,7Ad), 27.1 (NCH2CH2), 25.0 (OCH2CH2), 24.3 (NCH2CH2CH2), −9.3 (AlMe). IR spectrum (CsI plates, paraffin, cm−1): 1642 w; 1453 vs, 1372 vs (paraffin); 1366 vs, 1351 s, 1343 s, 1319 m, 1306 m, 1298 m, 1285 s, 1263 vs, 1249 m δ(CH3); 1230 s, 1220 vs, 1183 vs, 1160 s, 1152 s, 1112 s, 1097 vs, 1066 vs, 1024 vs, 986 w, 978 w, 963 s, 950 m, 939 m, 917 m, 913 m, 902 m, 873 s, 865 s, 847 m, 824 m, 797 s, 768 s, 756 vs, 738 vs ν(CC), ν(CN), ν(CO), ν(NN); 718 vs (paraffin); 682 vs, 672 vs, 653 vs, 626 s, 604 s, 578 w, 556 m, 493 s, 448 m, 440 m, 407 m, 368 w ν(AlC), ν(AlN), ν(AlO), δ(CC). Anal. Calcd for C21H39AlN2O (362.54): C 69.6, H 10.8, N 7.7. Found: C 69.4, H 10.4, N 7.7. Synthesis of (Ph−CC)3Al[N(H){CH(C9H14)}−NC5H10] (4). PhCCH (0.34 g, 3.33 mmol) was added at room temperature to a solution of (Me3C)2Al[N{CH(C9H14)}−NC5H10]5 1a (0.42 g, 1.11 mmol) in toluene (20 mL). The mixture was stirred for 16 h at room temperature. The solvent was removed in vacuo to yield a colorless oil, which was dissolved in cyclopentane (2 mL). After several minutes the product started to precipitate and was isolated as a colorless solid (0.46 g 73%). Mp: 163 °C (dec). 1H NMR (400 MHz, C6D6, 300 K): δ 7.55 (dd, 3JHH = 8.0 Hz, 4JHH = 1.6 Hz, 6 H, o-H), 7.00 (m, overlap, 6 H, mH,) 6.96 (m, overlap, 3 H, p-H), 4.29 (s, 1 H, 2-Ad), 3.87 (s, 1 H, NH), 3.69 (s, 2 H, NCH2), 3.12 (s, 2 H, NCH2), 2.39 (s, 2 H, 1,3-Ad), 2.06 (d, 2JHH = 12.3 Hz, 2 H, 4,8,9,10-Ad), 1.95 (d, 2JHH = 12.3 Hz, 2 H, 4,8,9,10-Ad), 1.75 (overlap, 2 H, 4,8,9,10-Ad), 1.71 (s, overlap, 1 H,

fluorobenzene with molecular sieves). NMR spectra were recorded in C6D6 or C4D8O at ambient probe temperature using the following Bruker instruments: Avance I (1H, 400.13; 13C, 100.6 MHz; 15N, 40.55 MHz); Avance III (1H, 400.03; 13C, 100.59; 29Si, 79.5; 15N, 40.54; 19F, 376.37 MHz). Signals were referenced internally to residual solvent resonances (chemical shift data in δ). 13C NMR spectra were all proton-decoupled. Elemental analyses were determined by the microanalytic laboratory of the Westfälische Wilhelms Universität Münster or by the company Pascher. IR spectra were recorded as paraffin mull between KBr or CsI plates on a Shimadzu Prestige 21 spectrometer; electron impact mass spectra on a Varian mass spectrometer. (H14C9)CN−NC5H10 was synthesized from 2adamantone and H2N−NC5H10,5 Me2GaH from Me2GaCl and LiH,24 (Me3C)2GaH from Ga(CMe3)3 and GaH3,9 and (Me3C)2Al[N{(CH(C9H14)}−NC5H10] 1a from (H14C9)CN−NC5H10 and (Me3C)2AlH.5 The assignment of NMR spectra is based on HMBC, H,H-ROESY, HSQC, and DEPT135 data. The numbering scheme for the assignment of the 2-adamantyl substituents follows IUPAC nomenclature (see Scheme 1). Synthesis of (Me3C)2Ga[N{CH(C9H14)}−NC5H10] (1b). A solution of H10C5N−NC(C9H14) (0.33 g, 1.42 mmol) in n-hexane (15 mL) was added at room temperature to a solution of (Me3C)2GaH (0.26 g, 1.41 mmol) in n-hexane (15 mL). The mixture was stirred for 16 h at room temperature. The solvent was removed in vacuo to yield a colorless oil, which was recrystallized from pentafluorobenzene (−28 °C) to give compound 1b as a colorless solid (0.48 g, 81%). Mp: 90 °C (dec). 1H NMR (400 MHz, C6D6, 300 K): δ 3.55 (s, 1 H, 2-Ad), 2.86 (s br, 4 H, NCH2), 2.56 (d, 2JHH = 12.5 Hz, 2 H, 4,8,9,10-Ad), 1.92 (m, 1 H, 5,7-Ad), 1.89 (m, 4 H, 4,8,9,10-Ad), 1.88 (m, 1 H, 5,7Ad), 1.79 (s, 2 H, 6-Ad), 1.74 (s, 2 H, 1,3-Ad), 1.53 (d, 2JHH = 12.5 Hz, 2 H, 4,8,9,10-Ad), 1.29 (s, 18 H, CMe3), 1.38 (m, 4 H, NCH2CH2), 1.13 (s, br, 2 H, NCH2CH2CH2). 13C NMR (100 MHz, C6D6, 300 K): δ 60.2 (2-Ad), 56.6 (NCH2), 38.6 (6-Ad), 37.8 (4,8,9,10-Ad), 36.3 (1,3-Ad), 32.0 (CMe3), 31.9 (4,8,9,10-Ad), 28.8 and 28.4 (5,7-Ad), 27.6 (NCH2CH2), 26.5 (CMe3), 24.3 (NCH2CH2CH2). MS (20 eV, 298 K): m/z (%) 416 (11) [M]+, 359 (17) [M − CMe3]+, 302 (25) [M − 2 CMe3]+. IR (KBr plates, paraffin, cm−1): 1638 w, 1533 w, 1510 w; 1460 vs, 1377 vs (paraffin); 1306 m, 1285 m, 1261 m δ(CH3); 1223 m, 1213 m, 1177 m, 1169 w, 1153 m, 1138 s, 1111 m, 1098 m, 1065 m, 1024 s, 1007 m, 978 m 961 m, 943 m, 932 m, 910 m, 901 w, 889 m, 872 w, 862 m, 841 w, 810 s, 773 w, 746 m ν(CC), ν(CN), ν(NN); 721 s (paraffin); 673 w, 623 w, 590 m, 532 s, 498 m, 471 m, 457 w ν(GaC), ν(GaN), δ(CC). Anal. Calcd for C23H43GaN2 (417.33): C 66.2, H 10.4, N 6.7. Found: C 65.7, H 10.1, N 6.5. Synthesis of Me2Al[N{CH(C9H14)}−NC5H10] (1c). A solution of H10C5N−NC(C9H14) (7.97 g, 34.2 mmol) in toluene (69 mL) was added at room temperature to neat Me2AlH (1.99 g, 34.3 mmol). The mixture was stirred overnight. Numerous attempts to crystallize 1c from a variety of solvents were unsuccessful and resulted always in the crystallization of small quantities of MeAl[N{CH(C9H14)}−NC5H10]2 (2, see below) as the only isolable product. An NMR spectrum of the crude product (1c) after removal of all volatiles showed a complicated equilibrium mixture of several compounds and could not be meaningfully interpreted. Addition of THF afforded a single product (3), which was isolated and characterized and confirmed the hypothesis of an equilibrium mixture. The solution (0.5 M; quantitative conversion assumed) of the crude product was used directly for further reactions. Synthesis of [Me2GaN{CH(C9H14)}−NC5H10]2 (1d). A solution of H10C5N−NC(C9H14) (4.25 g, 18.2 mmol) in n-pentane (50 mL) was added to neat Me2GaH (1.83 g, 18.2 mmol) at 0 °C. The mixture was allowed to warm to room temperature and stirred overnight. The solvent was removed in vacuo, and the residue was recrystallized from 1,1,1-trifluorotoluene (−30 °C) to yield compound 1d as colorless crystals (5.61 g, 92%). Solutions of 1d showed complicated NMR spectra similar to 1c, which could not be assigned (see Results and Discussion). Mp: 100 °C (dec). IR spectrum (KBr plates, paraffin, cm−1): 1877 vw, 1641 w, 1612 m; 1460 vs, 1377 vs (paraffin); 1346 sh, 1323 m, 1308 m, 1285 m, 1265 m δ(CH3); 1238 vw, 1202 s, 1148 w, 1099 m, 1059 m, 1034 s, 1003 m, 962 m, 935 m, 912 m, 889 m, 858 m, I


Article

Organometallics

removed in vacuo, and the residue dissolved in n-pentane (20 mL). When the mixture was treated with Ph−CC−H (0.31 g, 3.00 mmol) a solid started to precipitate. The mixture was stirred overnight and filtered. The solid residue was dried in vacuo (0.31 g, 55%). Compound 6a is highly sensitive and decomposes in solution and in the solid state. The originally colorless crystals became rapidly yellow at room temperature. Several attempts to obtain correct results from microanalysis with freshly crystallized samples failed due to this instability. Mp: 164 °C (dec). 1H NMR (400 MHz, C6D6, 300 K): δ 7.57 (m, 6 H, o-H), 7.01 (m, overlap, 6 H, m-H), 6.96 (m, overlap, 3 H, p-H), 4.49 (s, overlap, 2 H, NCH2), 4.47 (s, overlap, 1 H, 1,3-Ad), 3.97 (s, 1 H, 1,3-Ad), 2.76 (d, 2JHH = 10.0 Hz, 2 H, NCH2), 2.35 (d, 2 JHH = 13.1 Hz, 2 H, 4,8,9,10-Ad), 1.87 (m, 2 H, 4,8,9,10-Ad), 1.56 (s, overlap, 4 H, 4,8,9,10-Ad and 5,7-Ad), 1.50 (s, overlap, 4 H, 4,8,9,10Ad and NCH2CH2), 1.45 (s, overlap, 2 H, 6-Ad), 1.38 (s, overlap, 4 H, NCH2CH2 and NCH2CH2CH2). 13C NMR (100 MHz, C6D6, 300 K): δ 201.9 (CN), 132.3 (o-C), 128.4 (m-C), 127.6 (p-C), 126.1 (ipsoC), 108.8 (s br, PhCC), 108.1 (PhCC), 53.6 (NCH2), 39.6 (1,3Ad), 39.5 and 38.9 (4,8,9,10-Ad), 35.7 (6-Ad), 34.7 (1,3-Ad), 27.4 (5,7-Ad), 26.0 (NCH2CH2), 23.3 (NCH2CH2CH2). IR spectrum (KBr plates, paraffin, cm−1): 2127 m ν(CC); 1596 m, 1560 w ν(CN), phenyl; 1486 s, 1462 vs, 1377 vs (paraffin); 1289 vw, 1274 vw, 1211 m, 1176 vw, 1155 w, 1103 w, 1070 w, 1025 m, 976 w, 935 vw, 913 m, 847 vw, 800 s, 755 vs ν(CC), ν(CN), ν(NN); 722 s (paraffin); 691 s, 658 w (phenyl); 601 m, 592 m, 563 m, 535 s, 456 vw, 405 m, 398 m ν(AlC), ν(AlN), δ(CC). Synthesis of (Ph−CC)3Ga[(H14C9)CN−NC5H10] (6b). Ph− CC−H (0.40 g, 3.96 mmol) was added to a solution of 1d (0.44 g, 1.32 mmol, based on the monomer) in n-pentane (25 mL). There was a vigorous gas evolution, and a solid started to precipitate. The reaction was completed by stirring the mixture overnight. The solvent was separated by filtration, and the solid residue was dried in vacuo (0.67 g, 84%). Compound 6b is highly sensitive and decomposes in solution and in the solid state. The originally colorless crystals became rapidly yellow at room temperature. Several attempts to obtain correct results from microanalysis with freshly crystallized samples failed due to this instability. Mp: 164 °C (dec). 1H NMR (400 MHz, C6D6, 300 K): δ 7.56 (m, 6 H, o-H), 7.01 (m, overlap, 6 H, m-H), 6.98 (m, overlap, 3 H, p-H), 4.40 (s, overlap, 3 H, 1,3-Ad and NCH2), 3.98 (s, 1 H, 1,3-Ad), 2.74 (s, br, 2 H, NCH2), 2.32 (d, 2JHH = 12.4 Hz, 2 H, 4,8,9,10-Ad), 1.88 (d, 2JHH = 12.7 Hz, 2 H, 4,8,9,10-Ad), 1.58 (s, overlap, 2 H, 4,8,9,10-Ad), 1.57 (s, overlap, 2 H, 5,7-Ad), 1.51 (s, overlap, 4 H, 4,8,9,10-Ad and NCH2CH2), 1.47 (s, overlap, 2 H, 6Ad), 1.39 (s, overlap, 4 H, NCH2CH2 and NCH2CH2CH2). 13C NMR (100 MHz, C6D6, 300 K): δ 198.8 (CN), 132.2 (o-C), 128.4 (m-C), 127.6 (p-C), 125.8 (ipso-C), 106.8 (PhCC), 103.1 (br, PhCC), 53.5 (NCH2), 39.4 (4,8,9,10-Ad), 39.3 (1,3-Ad), 39.0 (4,8,9,10-Ad), 35.7 (6-Ad), 34.9 (1,3-Ad), 27.5 (5,7-Ad), 25.8 (NCH2CH2), 23.3 (NCH2CH2CH2). IR spectrum (KBr plates, paraffin, cm−1): 2139 w ν(CC); 1751 vw, 1724 vw, 1719 vw, 1701 vw, 1686 vw, 1674 vw, 1651 vw, 1603 m, 1560 w, 1543 w, 1522 w, 1506 w ν(CN), phenyl; 1456 vs, 1377 vs (paraffin); 1321 m, 1290 m, 1273 w, 1242 w, 1211 m, 1173 w, 1153 w, 1101 vw, 1084 w, 1067 w, 1024 w, 999 w, 974 w, 952 w, 935 vw, 914 w, 891 vw, 874 vw, 856 vw, 795 m, 754 s ν(CC), ν(CN), ν(NN); 723 s (paraffin); 693 s, 654 w (phenyl); 613 w, 565 m, 532 m, 515 w, 455 w, 420 vw ν(GaC), ν(GaN), δ(CC). Synthesis of [Me(C6F5)2Ga][(H14C9)CN−NC5H10] (7a). A solution of pentafluorobenzene (0.33 g, 1.96 mmol) in toluene (5 mL) was added at room temperature to a solution of compound 1d (0.33 g, 0.99 mmol) in toluene (10 mL). The mixture was stirred overnight. The solvent was removed in vacuo, and the residue recrystallized from cyclopentane at −30 °C to yield colorless crystals of compound 7a (0.39 g, 59%, the crystals included 0.4 equiv of cyclopentane). A few crystals of compound 7b were isolated as a byproduct. Characterization of 7a: Mp: 155 °C. 1H NMR (400 MHz, C6D6, 300 K): δ 3.82, (s, 1 H, 1,3-Ad), 3.20 (s, br, 2 H, NCH2), 3.03 (s, 1 H, 1,3-Ad), 2.72 (s, br, 2 H, NCH2), 1.48 (d, 3JHH = 13.8 Hz, 2 H, 4,8,9,10-Ad), 1.46 (s, 4 H, cyclopentane), 1.37 (s, 2 H, 5,7-Ad), 1.34 (s, 2 H, 4,8,9,10-Ad), 1.33 (s, 2 H, 6-Ad), 1.32 (s, 4 H, 4,8,9,10-Ad), 1.31 (s, 4 H, NCH2CH2), 1.29 (s, 1 H, NCH2CH2CH2), 0.81 (s, 1 H,

5,7-Ad), 1.70 (s, overlap, 1 H, 5,7-Ad), 1.64 (s, br, overlap, 2 H, NCH2CH2), 1.60 (s, 2 H, 6-Ad), 1.53 (d, 2JHH = 12.8 Hz, 2 H, 4,8,9,10-Ad), 1.34 (s, br, 2 H, NCH2CH2), 1.22 (s, br, 1 H, NCH2CH2CH2), 1.13 (s, br, 1 H, NCH2CH2CH2). 13C NMR (100 MHz, C6D6, 300 K): δ 132.3 (o-C), 128.4 (m-C), 127.9 (p-C), 125.5 (ipso-C), 108.8 (PhCC), 103.4 br, PhCC), 59.1 (2-Ad), 53.4 (NCH2), 37.6 (6-Ad, 4,8,9,10-Ad), 33.3 (1,3-Ad), 31.9 (4,8,9,10-Ad), 27.5 and 27.4 (5,7-Ad), 23.3 (NCH2CH2CH2), 21.0 (br, NCH2CH2). IR spectrum (KBr plates, paraffin, cm−1): 3283 m ν(NH); 2943 vs, 2841 vs (paraffin); 2743 w, 2669 w, 2656 w; 2124 s ν(CC); 2021 vw, 1964 vw, 1946 vw, 1890 vw, 1875 vw, 1803 vw, 1755 vw, 1674 vw (overtones, phenyl); 1591 m, 1570 w, 1520 w (phenyl); 1470 vs, 1454 vs, 1377 vs (paraffin); 1344 m, 1306 m, 1275 w δ(CH3); 1209 m, 1157 m, 1103 m, 1067 m, 1030 m, 1015 w, 989 m, 975 m, 956 m, 930 w, 910 m, 885 w, 843 w, 814 s, 802 s, 756 vs ν(CC), ν(CN), ν(NN); 721 s (paraffin); 691 s, 673 w (phenyl); 610 m, 600 m, 563 m, 538 s, 503 m, 471 m, 455 m ν(AlC), ν(AlN), δ(CC). Anal. Calcd for C39H41AlN2 (564.75): C 82.9, H 7.3, N 5.0. Found: C 82.4, H 7.3, N 4.8. Synthesis of (Ph3Si−CC)3Al[N(H){CH(C9H 14)}−NC5H 10]. Ph3Si−CC−H (3 equiv) was added at room temperature to a solution of (Me3C)2Al[N{CH(C9H14)}−NC5H10]5 1a (1 equiv) in toluene. The mixture was stirred for 16 h at room temperature (RT). The solvent was removed in vacuo, the residue was washed with nhexane, and (Ph3Si−CC)3Al[N(H){CH(C9H14)}−C5H10] was obtained as a colorless solid. The product was fully characterized by NMR spectroscopy, but the removal of some minor impurities by recrystallization was not successful. 1H NMR (400 MHz, C4D8O, 300 K): δ 7.68 (d, 3JHH = 6.3 Hz, 18 H, o-H), 7.34 (m, overlap, 9 H, p-H), 7.29 (m, overlap, 18 H, m-H), 2.97 (s, 1 H, 2-Ad), 2.54 (s, br, 4 H, NCH2), 2.21 (d, 2JHH = 11.3 Hz, 2 H, 4,8,9,10-Ad), 2.08 (s, br, 1JNH = 65 Hz, NH), 1.84 (overlap, 2 H, 4,8,9,10-Ad), 1.82 (overlap, 2 H, 5,7Ad), 1.81 (overlap, 2 H, 4,8,9,10-Ad), 1.72 (overlap, 5 H, 4,8,9,10-Ad, 6-Ad and 5,7-Ad), 1.55 (m, 4 H, NCH2CH2), 1.39 (overlap, 2 H, 4,8,9,10-Ad), 1.36 (overlap, br., 2 H, NCH2CH2CH2). 13C NMR (100 MHz, C4D8O, 300 K): δ 136.3 (o-C), 135.6 (ipso-C), 130.2 (p-C), 128.5 (m-C), 106.3 (Me3CCC), 100.2 (s, br, Me3CCC), 60.9 (2Ad), 58.7 (NCH2), 39.1 (6-Ad), 38.2 (4,8,9,10-Ad), 32.8 (1,3-Ad), 32.3 (4,8,9,10-Ad), 29.4 and 29.3 (5,7-Ad), 27.3 (NCH2CH2), 25.1 (s, br, NCH2CH2CH2). 29Si NMR (79.5 MHz, C4D8O, 300 K): δ −31.9 (1JSiC = 91.4 Hz). 15N NMR (40.5 MHz, C4D8O, 300 K): δ 121 (1JHN = 65 Hz, N(H)C−H(Ad)), 86 (NC5H10). Synthesis of (Me3C−CC) 3Al[N(H){CH(C9H14)}−NC5H10]. (Me3C−CC)3Al[N(H){CH(C9H14)}−NC5H10] was obtained in an NMR experiment from Me3C−CC−H and (Me3C)2Al[N{CH(C9H14)}−NC5H10]5 1a after 2 d at RT. Data of a preliminary NMR characterization: 1H NMR (400 MHz, C6D6, 300 K): δ 4.24 (s, br, 1 H, 2-Ad), 3.92 (s, 1 H, NH), 3.66 and 3.04 (s, br, 4 H, NCH2), 2.40− 1.20 (m, 20 H, 1,3,4,5,6,7,8,9,10-Ad, NCH2CH2, NCH2CH2CH2), 1.18 (s, 27 H, CMe3). 13C NMR (100 MHz, C6D6, 300 K): δ 117.2 (Me3C−CC), 90.7 (s, br, Me3C−CC), 57.5 (2-Ad), 52.5 (s, br, NCH2), 37.4 and 37.3 (CH2), 32.7 (s, br, CH), 31.6 (CH2), 28.0 (CMe3), 27.3 (br, CH), 23.4 and 20.7 (s, br, CH2), 31.2 and 28.0 (CMe3). Synthesis of [(Me3C)2Ga−CC−CMe3]2, 5a. A solution of 1b (0.31 g, 0.74 mmol) in toluene (25 mL) was treated at room temperature with an excess of Me3C−CC−H (0.30 g, 2.94 mmol). The mixture was stirred for 14 d. The solvent was removed in vacuo, and the residue recrystallized from n-pentane (−30 °C) to yield colorless crystals of [(Me3C)2Ga−CC−CMe3]2 (5a, 0.12 g, 61%). Mp: 227 °C (dec). 1H NMR (400 MHz, C6D6, 300 K): δ 1.41 (s, 18 H, GaCMe3), 1.09 (s, 9 H, CCMe3). 13C NMR (100 MHz, C6D6, 300 K): δ 144.9 (CC−CMe3), 79.1 (CC−CMe3), 32.1 (GaCMe 3 ), 30.8 (CC−CMe 3 ), 29.6 (CC−CMe 3 ), 26.1 (GaCMe3). IR spectrum (KBr plates, paraffin, cm−1): 2135 w, 2058 s ν(CC); 1465 vs, 1377 vs (paraffin); 1361 vs, 1310 w, 1260 w, 1242 s δ(CH3); 1200 s, 1165 s, 1094 w, 1009 s, 935 w, 889 vw, 812 vs ν(CC); 719 s (paraffin); 631 vw, 532 m, 424 s ν(GaC), δ(CC). Synthesis of (Ph−CC)3Al[(H14C9)CN−NC5H10] (6a). A solution of 1c (0.5 M in toluene, 2.0 mL, 1.0 mmol, based on the monomer) was transferred into a round-bottom flask. The solvent was J


Organometallics

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NCH2CH2CH2), 0.58 (s, 3 H, GaMe). 13C NMR (100 MHz, C6D6, 300 K): δ 199.3 (NC, 2-Ad), 149.0 (dm, 1JFC = 234 Hz, o-C), 141.0 (dm 1JFC = 250 Hz, p-C), 137.2 (dm, 1JFC = 254 Hz, m-C), 119.7 (t, 3 JFC = 45.5 Hz, ipso-C), 53.4 (NCH2), 39.1 (1,3-Ad), 39.0 and 38.2 (4,8,9,10-Ad), 35.3 (1,3-Ad and 6-Ad), 27.1 (5,7-Ad), 25.6 (NCH2CH2), 23.4 (NCH2CH2CH2), 2.2 (GaMe). 19F NMR (376 MHz, C6D6, 300 K): δ −122.2 (m, o-F), −154.9 (t, 3JFF = 19.9 Hz, pF), −161.5 (m, m-C). IR spectrum (KBr plates, paraffin, cm−1): 1917 m, 1636 s, 1599 s, 1553 w, 1506 vs ν(CN), phenyl; 1470 vs, 1377 vs (paraffin); 1319 s, 1288 m, 1265 s δ(CH3); 1234 sh w, 1207 w, 1150 w, 1103 w, 1072 vs, 1053 vs, 1031s, 1007 m, 959 vs, 912 s, 889 m, 870 w, 856 m, 839 w, 783 m ν(CC), ν(CN), ν(NN), ν(CF); 723 vs (paraffin); 654 w, 638 w, 623 w, 608 m, 590 w, 581 w, 565 m, 513 w, 488 w, 453 w ν(GaC), ν(GaN), δ(CC). MS (EI, 20 eV, 298 K): m/z (%): 635 (0.2) [M − CH3]+, 418 (4) [MeGa(C6F5)2]+, 403 (43) [Ga(C6F5)2]+. Anal. Calcd for C28H27F10GaN2(C5H10)0.4 (679.29; the sample was recrystallized from cyclopentane and contained after drying in vacuo 0.4 equiv of the solvent as determined by 1H NMR spectroscopy): C 53.0, H 4.6, N 4.1. Found: C 52.8, H 4.6, N 4.5. X-ray Crystallography. Crystals suitable for X-ray crystallography were obtained from pentafluorobenzene (−30 °C, 1b; −28 °C, 7b), 1,2-difluorobenzene (−28 °C, 2), benzene (4 °C, 3), 1,1,1trifluorotoluene (−30 °C, cis- and trans-1d), n-pentane (−28 °C, 6b), cyclopentane (−30 °C, 7a), or slow conconcentration of a cyclopentane/1,2-difluorobenzene solution at room temperature (4). Intensity data were collected on Bruker Quazar or D8-Venture diffractometers with monochromated Mo Kα radiation. The collection method involved ω scans. Data reduction was carried out using the program SAINT+.25 The crystal structures were solved by direct methods using SHELXTL.26 Non-hydrogen atoms were first refined isotropically followed by anisotropic refinement by full matrix leastsquares calculation based on F2 using SHELXTL.25 H atoms (except H(1) of 7b, which was refined isotropically) were positioned geometrically and allowed to ride on their respective parent atoms. The molecules of 1b reside on a mirror plane; a CMe3 and the adamantyl groups were disordered and refined on split positions (0.5:0.5). cis-1d crystallized with a strongly disordered 1,1,1trifluorotoluene molecule; only the central C6 ring could be refined. trans-1d crystallized with two independent molecules in the asymmetric unit. Both adamantyl groups of 2 were disorderd and refined on split positions (0.52:0.54; 0.54:0.46). 6b had two disordered phenyl groups (C(31), 0.47:0.53; C(51), 0.59:0.41). 7b crystallized with half a pentafluorobenzene molecule per formula unit, and the solvent molecule was disordered over a center of symmetry. Further details of the crystal structure determinations are available from the Cambridge Crystallographic Data Center on quoting the depository numbers CCDC-1495429 (1b), -1495430 (1d-cis), -1495431 (1d-trans), -1495432 (2), -1495433 (3), -1495434 (4), -1495435 (6b), -1495436 (7a), -1495437 (7b), and -1495438 (5a).

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ACKNOWLEDGMENTS We are grateful to the Deutsche Forschungsgemeinschaft (SFB 858) for generous financial support. K


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Aluminum and Gallium Hydrazides as Active Lewis Pairs: Cooperative

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