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. 2021 Apr 21;143(17):6351–6356. doi: 10.1021/jacs.1c02463

A Monomeric Aluminum Imide (Iminoalane) with Al–N Triple-Bonding: Bonding Analysis and Dispersion Energy Stabilization

Joshua D Queen †, Sini Irvankoski ‡, James C Fettinger †, Heikki M Tuononen ‡,*, Philip P Power †,*
PMCID: PMC8154528  PMID: 33882237

Abstract

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The reaction of :AlAriPr8 (AriPr8 = C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) with ArMe6N3 (ArMe6 = C6H3-2,6-(C6H2-2,4,6-Me3)2) in hexanes at ambient temperature gave the aluminum imide AriPr8AlNArMe6 (1). Its crystal structure displayed short Al–N distances of 1.625(4) and 1.628(3) Å with linear (C–Al–N–C = 180°) or almost linear (C–Al–N = 172.4(2)°; Al–N–C = 172.5(3)°) geometries. DFT calculations confirm linear geometry with an Al–N distance of 1.635 Å. According to energy decomposition analysis, the Al–N bond has three orbital components totaling −1350 kJ mol–1 and instantaneous interaction energy of −551 kJ mol–1 with respect to :AlAriPr8 and ArMe6N̈:. Dispersion accounts for −89 kJ mol–1, which is similar in strength to one Al–N π-interaction. The electronic spectrum has an intense transition at 290 nm which tails into the visible region. In the IR spectrum, the Al–N stretching band is calculated to appear at ca. 1100 cm–1. In contrast, reaction of :AlAriPr8 with 1-AdN3 or Me3SiN3 gave transient imides that immediately reacted with a second equivalent of the azide to give AriPr8Al[(NAd)2N2] (2) or AriPr8Al(N3){N(SiMe3)2} (3).

The chemistry of compounds with group 13 element-nitrogen bonding has been extensively studied.1−9 Current interest is driven by their applications as precursors for group III–V materials,10 H2 storage media,11−13 and an interest in M–N (M = Al–Tl) multiple bonding. Early work on the group 13 amine complexes showed they could be condensed at elevated temperature with release of RR′ (R,R′ = organic group or hydrogen): a common route to amide, imide, and nitride compounds14 (Scheme 1).

Scheme 1. Stepwise Condensation of Group 13 Amine Complexes to Nitrides.

Scheme 1

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The group 13 metal imides (also called N-iminometallanes) of formula [RMNR′]n (R = alkyl, aryl, hydrogen, halide; R′ = alkyl, aryl, silyl, hydrogen; M = Al–In; n = 4–8) were first studied in detail by Cesari and co-workers in the 1960s and ’70s, and several examples featuring cage structures with alternating metal and nitrogen vertices were structurally characterized.15−19 Roesky and co-workers characterized the quasi-isomeric tetrameric amido-Al(I) compound [AlN(Dipp)(SiMe3)]4 (Dipp = 2,6-iPr2–C6H3) with a tetrahedral Al4 core and terminal amide groups.20 The lower imido aggregates (n = 1–3) remain scarce but are especially interesting since M–N multiple bonding becomes possible. Thus, the unique trimer [Al(Me)N(Dipp)]3,21 which is an Al analogue of borazine (i.e., an “alumazine”), features relatively short (ca. 1.78 Å) Al–N bonds. The planar Al3N3 ring has 6-π electrons but has little aromatic character as shown by its reaction chemistry.22,23 Several dimeric, [RAlNR′]2 compounds with Al2N2 cores and short Al–N distances in the range 1.796–1.842 Å, which is slightly longer than that seen in the alumazine derivative, have also been reported.24−29

Monomeric RAlNR′ compounds remain unknown, which is probably a result of high association energies (cf. dimerization of HAlNH is exothermic by ca. 580 kJ mol–1).30 Their synthesis via hydrocarbon or dihydrogen elimination usually proceeds at elevated temperatures that often results in C–H activation of the ligands.31 However, an alternative synthesis by the reaction of organoazides with M(I) species at low temperatures avoids C–H activation. For example, Roesky and co-workers reported that the reaction of :AlCp* (Cp* = η5-C5Me5), formed by dissociation of (AlCp*)4 at elevated temperature, with R3SiN3 (R = iPr, Ph, tBu), gave the imido dimers {Cp*Al(μ-NSiR3)}2.26 Using the larger, chelated Al(I) β-diketiminate :AlDippNacNac (DippNacNac = HC{(CMe)(NDipp)}2) gave the transient imides DippNacNacAl = NR which reacted with a second equivalent of the azide to give cyclic AlN4 products DippNacNacAl[(NR)2N2].32,33 Attempts to stabilize the imide using more sterically demanding m-terphenyl azides failed to give an isolable aluminum imide, although this route did yield a corresponding Ga imide.34 The Al imide underwent C–H activation of a methyl group on a flanking ring of the DippNacNac ligand or C–C activation of the aryl ring of the nitrogen terphenyl ligand.35

Nonetheless, monomeric aluminum imides were obtained by coordinative blocking of the Al atoms. Cui and co-workers showed that addition of an NHC (N-heterocyclic carbene) to :Al[HC{(CtBu)(NDipp)}2] resulted in insertion of the Al atom into the N–C bond of the β-diketiminate ligand.36 This gave the four-coordinate terminal Al imide I (Figure 1) with a short (1.705(2) Å) Al–N bond. Recently, the groups of Coles and Aldridge separately reported that the reaction of anionic Al(I) aluminyls with organoazides gave terminal aluminum imides II and III (Figure 1) with Al–N distances of 1.7251(11) and 1.723(2) Å, supported by multidentate NON ligands that exist as dimers with bridging K+ cations.37,38 It was shown that the Al=N bonds reacted readily with small molecules such as CO and CO2.36−38

Figure 1.

Figure 1

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Structurally characterized terminal aluminum imides. Mes = 2,4,6-Me3C6H2; Dipp = 2,6-iPr2C6H3.36−38

No compounds of formula RAlNR′ in which Al and N are two-coordinate have been isolated and characterized. The reaction of laser-ablated Al atoms with NH3 gas gave the planar trans-bent parent compound HAlNH as a minor product, identified by IR spectroscopy in a solid argon matrix.39Ab initio computations by Davy and Jaffrey found HAlNH to be “quasi-linear” with only a 0.2 kcal mol–1 barrier between the linear and bent geometries and a short Al–N bond distance of 1.63 Å, which may be interpreted on the basis of Al–N triple bonding. Computations for HAlNH and MeAlNMe showed linear geometries with short (ca. 1.63–1.65 Å) Al–N distances,11,30,40,41 and NBO analysis of MeAlNMe by Gilbert indicated that it had an Al–N triple bond composed of one σ- and 2 π-bonds.42

Previously, our group described the synthesis of gallium and indium imides with two-coordination at both the group 13 metal and N atoms by reaction of an m-terphenyl azide with the dimetallenes AriPr4MMAriPr4 (M = Ga, In; AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2), which exist in equilibrium with :MAriPr4 monomers in solution.43 This suggested that a similar Al species could be isolable, but the lack of an analogous Al(I) precursor (i.e., ArAlAlAr or :AlAr) precluded its synthesis. Recently, we reported the monomeric alanediyl :AlAriPr8 (AriPr8 = C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) with a one-coordinate Al atom.44 We show here that its reaction with ArMe6N3 (ArMe6 = C6H3-2,6-(C6H2-2,4,6-Me3)2 gives the aluminum imide AriPr8AlNArMe6 (1) having two-coordinate Al and N atoms with a notably short Al–N bond length of 1.625(4) or 1.628(3) Å consistent with Al–N triple bonding. Additionally, the reaction of :AlAriPr8 with the less sterically demanding azides 1-AdN3 (1-Ad = 1-adamantyl) or Me3SiN3 gives transient imides which react immediately with a second equivalent of azide to give products featuring ring closure or silyl migration.

Compound 1 was prepared by reaction of :AlAriPr8 with ArMe6N3 (Scheme 2a) in hexanes at ambient temperature, giving immediate vigorous evolution of N2 and formation of a red solution. After ca. 5 min, the solids had dissolved and gas evolution had ceased. Storage at ca. −30 °C for 3 days gave orange plates of 1 in ca. 91% yield. The crystal structure of 1 (Figure 2) contains two crystallographically independent molecules. One of these lies along the 2-fold proper rotation axis of the I2/a space group and contains a strictly linear C–Al–N–C core. The second molecule maintains a planar C–Al–N–C array but deviates slightly from linearity at the Al (C–Al–N = 172.5(3)°) and N (Al–N–C = 171.4(2)°) atoms. The Al–N bond lengths of 1.625(4) and 1.628(3) Å are the shortest reported to date and agree with those calculated for HAlNH and MeAlNMe.11,30,40,41 The linear structure of 1 is in marked contrast to the heavier congeners AriPr4M=NAr′ (M = Ga, In; Ar′ = C6H3-2,6-(C6H2-2,6-Me2-4-tBu)2) which are strongly bent at the M and N atoms (Ga–N = 1.701(2) Å; C–Ga–N = 148.2(2)°; Ga–N–C = 141.7(3)°; In–N = 1.928(2) Å; C–In–N = 142.2(1)°; In–N–C = 134.9(2)°).43

Scheme 2. Synthesis of Compounds 1, 2, and 3.

Scheme 2

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(a) Synthesis of compound 1; (b) synthesis of compounds 2 and 3; Trip = 2,4,6-iPr3C6H2; Mes = 2,4,6-Me3C6H2; 1-Ad = 1-adamantyl.

Figure 2.

Figure 2

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Thermal ellipsoid plot (50%) of one of the crystallographically independent molecules of AriPr8AlNArMe6 (1). H atoms and n-hexane solvent not shown. Selected bond lengths (Å) and angles (deg) {values in braces correspond to the other crystallographically independent molecule of 1}: Al(1)–N(1): 1.625(4) {1.628(3)}, Al(1)–C(1): 1.935(4) {1.931(3)}, N(1)–C(43): 1.378(5) {1.366(4)}, C(1)–Al(1)–N(1): 180 {172.4 (2)}, Al(1)–N(1)–C(43): 180 {172.5(3)}, C(1)–Al(1)–N(1)–C(43): 0 {167.0(2)}.

The UV–vis electronic spectrum of 1 has a single absorbance at 290 nm which tails into the visible region, producing an orange color. Time-dependent DFT calculations on 1 suggest that the low-intensity absorption tail is mostly due to the HOMO → LUMO+1 transition at 387 nm, whereas the main feature is due to two high-intensity transitions at 287 and 316 nm. The Al–N stretching band of 1 was calculated to be ca. 1100 cm–1; however, no distinct spectral features are apparent for assignment of the band.

Imide 1 slowly decomposes in solution over ca. 12 h at ambient temperature as indicated by fading of the orange color to colorless. 1H NMR spectroscopy (Figure S3, Supporting Information) is consistent with decomposition via C–H activation of one of the methyl groups on the flanking rings that is analogous to that in DippNacNacAl=NAriPr4.35 A singlet at δ 3.50 ppm is assigned to the resulting amine proton, while the Al-CH2 group gives a multiplet at δ 0.05–0.13 ppm. Solid 1 is thermally stable at ambient temperature for at least several days but rapidly decomposes to a white solid above 83 °C.

Dispersion corrected DFT calculations for 1 at the PBE1PBE- GD3BJ/def2-TZVP level yield an optimized structure with a linear C–Al–N–C core and an Al–N bond length of 1.635 Å in excellent agreement with the crystal structure. The Kohn–Sham orbitals (Figure S15, Supporting Information) and those of the model system Ph-NAl-Ph (Figure 3) show three major components to the Al–N bond, one of σ-type and two nondegenerate of π-type. NBO analysis yielded three two-center Al–N bonding orbitals with occupations close to 2 electrons and ca. 90% localization on the N atom.45 Consequently, the calculated Wiberg bond index for the Al–N bond is only 0.89. More detailed bonding analyses using the ETS-NOCV method and fragments :AlAriPr8 and ArMe6N̈: at the geometries they adopt in 1 also revealed three primary contributions to the Al–N bond. The major component (−1120 kJ mol–1, ca. 83% of the total orbital interaction of −1350 kJ mol–1) involves charge flow from Al to N, whereas the two minor components (−100 and −102 kJ mol–1, each ca. 8% of the total orbital interaction) describe backdonation from N to Al. Taken as a whole, the Al–N bond in 1 has the formal characteristics of a triple bond with donation from Al to N greatly exceeding backdonation from N to Al. The calculated instantaneous interaction energy between :AlAriPr8 and ArMe6N: is −551 kJ mol–1 (cf. Gibbs interaction energy of −429 kJ mol–1 taking into account fragment relaxation) with significant stabilization, –89 kJ mol–1, from dispersion interactions. The possibility of charge-shift character in the Al–N bond has not yet been supported by computational data.46−48

Figure 3.

Figure 3

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Occupied PBE1PBE-GD3BJ/def2-TZVP orbitals of Ph-NAl-Ph localized on the AlN bond (NPh moiety on the left, AlPh moiety on the right; isosurface value ±0.05 au).

Addition of AdN3 or Me3SiN3 to :AlAriPr8 gives the transient imides AriPr8AlNR (R = 1-Ad, SiMe3), which immediately react with a second equivalent of the azide (Scheme 2b). Roesky, Aldridge, and co-workers have shown that organic azides with small substituents such as -SiMe3, -SiPh3, and 1-Ad react in a 2:1 ratio with :Al[DippNacNac] or an aluminyl anion to give planar AlN4 heterocycles.32,33,38 Reaction with the first equivalent of azide results in N2 loss and a highly reactive species with a terminal Al=NR moiety, which undergoes ring closure with a second equivalent of the azide. The reaction of :AlAriPr8 with 2 equiv of 1-AdN3 gave 2 (Figure 4, left) as colorless crystals. The Al–N bonds are 1.8126(9) and 1.8220(11) Å which are in the typical range for these AlN4 compounds.32,33,38 However, steric congestion between the terphenyl flanking rings and the adamantyl groups result in a deformation of the central ring of the terphenyl ligand, illustrated by torsion angles of C(1)–C(2)–C(3)–C(4) = 20.43(14)° and C(1)–C(6)–C(5)–C(4) = 20.06(14)°. The 1H and 13C{1H} NMR spectra of 2 also display broad signals indicating restricted movement of the 1-adamantyl and terphenyl flanking groups. The reaction of :AlAriPr8 with 2 equiv of Me3SiN3 gives the amido- azido-alane 3 (Figure 4, right) as colorless crystals in which silyl migration from the second equivalent of azide to the nitrogen atom of the transient imide has occurred. Such migrations have been observed in a number of reactions of Me3SiN3 with low valent main group compounds.49−54

Figure 4.

Figure 4

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Thermal ellipsoid plots (50%) of AriPr8Al[(NAd)2N2] (2, left) and AriPr8Al(N3){N(SiMe3)2} (3, right). Hydrogen atoms and toluene are not shown. Selected bond lengths (Å) and angles (deg): 2: Al(1)–C(1): 1.9710(12), Al(1)–N(1): 1.8126(9), Al(1)–N(4): 1.8220(11), N(1)–N(2): 1.3899(15), N(2)–N(3): 1.2643(16), N(3)–N(4): 1.3853(12), C(1)–Al(1)–N(1): 135.49(4), C(1)–Al(1)–N(4): 138.11(4), N(1)–Al(1)–N(4): 86.39(4). 3: Al(1)–C(1): 1.9775(14), Al(1)–N(1): 1.8088(12), N(1)–Si(1): 1.7518(12), N(1)–Si(2): 1.7546(12), Al(1)–N(2): 1.8210(13), N(2)–N(3): 1.2132(18), N(3)–N(4): 1.1407(19), C(1)–Al(1)–N(1): 143.91(6), C(1)–Al(1)–N(2): 102.62(6), N(1)–Al(1)–N(2): 113.44(6), N(2)–N(3)–N(4): 175.71(16).

Computationally, the reaction of :AlAriPr8 with Me3SiN3 yielded AriPr8AlN(N2)SiMe3 which readily releases N2 with a free energy barrier of 46 kJ mol–1 to afford AriPr8AlNSiMe3 at −306 kJ mol–1 (Figure S17, Supporting Information). Addition of a second equivalent of Me3SiN3 gave two products, cis- and trans-AriPr8Al[N(SiMe3)N2]NSiMe3, depending on the relative orientation of AriPr8AlNSiMe3 and Me3SiN3. The cis-isomer has the two SiMe3 groups on the same side of the dative Al–N bond and readily forms 3 via silyl migration, whereas the trans-isomer can form the SiMe3 analogue of 2 via ring closure. Of the two possible products, 3 is kinetically preferred and thermodynamically favored by 113 kJ mol–1. The potential energy surface is expected to be largely similar for the :AlAriPr8 1-AdN3 pair with the exception that substituent migration is energetically unfeasible and 2 is formed rapidly via ring closure.

In summary, the alanediyl :AlAriPr8 reacts with the m-terphenyl azide ArMe6N3 to yield the monomer AriPr8AlNArMe6 in which the Al and N atoms have linear, or almost linear, coordination and short Al–N distances of 1.625(4) or 1.628(3) Å, consistent with Al–N triple bonding. Calculations show that the Al–N bond is composed of strong σ-donation from the :AlAriPr8 moiety to the :N̈ArMe6 nitrene and weak π-donation from the latter to :AlAriPr8. The calculations also indicate a key contribution from dispersion energies that, together with steric effects from the terphenyl substituents, provide sufficient stabilization for the room-temperature characterization of 1.

Acknowledgments

We thank the US National Science Foundation (CHE-156551) for supporting this work and for the purchase of a dual source X-ray diffractometer (CHE-0840444). This project received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement #772510 to H.M.T.).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.1c02463.

  • Experimental details for compounds 1–3, NMR, electronic, and IR spectral data, and computational details (PDF)

  • (XYZ)

Accession Codes

CCDC 2065246–2065248 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

The authors declare no competing financial interest.

Supplementary Material

ja1c02463_si_001.pdf (1.7MB, pdf)
ja1c02463_si_002.xyz (86.3KB, xyz)

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