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Unusual Reactions of NacNacAl with Urea and Phosphine Oxides Terry Chu,†,‡ Sergei F. Vyboishchikov,*,§ Bulat M. Gabidullin,⊥ and Georgii I. Nikonov*,† †

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario L2S 3A1, Canada Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Carrer Maria Aurèlia Capmany 69, Girona 17003, Spain ⊥ X-Ray Core Facility, Faculty of Science, University of Ottawa, 150 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada §

S Supporting Information *

ABSTRACT: The reaction of cyclic urea 1,3-dimethyl-2-imidazolidinone with the aluminum(I) compound NacNacAl (1) gives an unexpected adduct of urea with the isomerized aluminum(III) hydride NacNac′AlH(OSIMe) (3). A related reaction of 1 with phosphine oxides results in cleavage of the PO bond and formation of hydroxyl derivatives NacNac′Al(OH)(OPR3) [R = Ph (5) and Et (6)]. Density functional theory calculations revealed that these reactions proceed via a bimolecular mechanism in which either the basic aluminum(I) center or the transient AlO species deprotonate the methyl group of the NacNac ligand.

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INTRODUCTION Activation of strong σ bonds by oxidative addition is a key step in a wide array of catalytic reactions mediated by transitionmetal complexes. 1 Historically, this process has been considered to be the exclusive domain of d-block elements because of the accessibility and facile interconversion between several stable oxidation states. In contrast, bond activation by main-group elements is much less studied and is generally more challenging because of the larger energy separation of the usual oxidation states. Nevertheless, recently there has been spectacular progress in developing transition-metal-like reactivity with main-group compounds.2 The usual requirement for this reactivity is the availability of a high-energy highest occupied molecular orbital (HOMO) and a low-lying lowest unoccupied molecular orbital (LUMO) that have been realized in carbenoids and the heavier analogues of alkynes.2a,3 To date, the oxidative addition of robust, nonpolar single bonds, such as H−H,4 N−H,5 C−H,6 and Si−H,7 has been described for group 13−15 elements. We and others have recently reported that the electron-rich aluminum(I) compound NacNacAl8 (1; NacNac = [ArNC(Me)CHC(Me)NAr]−, where Ar = 2,6Pri2C6H3) can even oxidatively add very strong C−O (86 kcal mol−1)9 and C−F (116 kcal mol−1)9 bonds.10 This discovery led us to believe that this system may also be suited for the little-studied oxidative addition of multiple bonds. The validity of this hypothesis was confirmed with the successful addition of the CS bond (137 kcal mol−1)9 of thioureas and the PS bond (80 kcal mol−1)9 of triphenylphosphine sulfide, leading to © 2017 American Chemical Society

the terminal aluminum sulfides NacNacAlS(L) (L = NHC or SPPh3).11 Encouraged by these findings, we turned our attention to activation of the robust CO bond (179 kcal mol−1).9 To this end, we employed a cyclic urea as the substrate of choice because cleavage of the CO bond would net an Nheterocyclic carbene (NHC), enabling the whole process to be an allowed, two-electron oxidation of aluminum(I) into aluminum(III). The reaction yielded unexpected results, the details of which are reported below.

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RESULTS AND DISCUSSION Reaction of compound 1 with the cyclic urea 1,3-dimethyl-2imidazolidinone (2; OSIMe) at room temperature resulted in a complex mixture of products. However, repeating this reaction at −60 °C in toluene cleanly yielded an unexpected aluminum hydride NacNac′AlH(OSIMe) (3; NacNac′ = [ArNC(CH2)CHC(Me)NAr]2−), as shown in Scheme 1. Deprotonation of the weakly acidic methyl group in the backbone of the NacNac ligand was evinced from the observation in the 1H NMR spectrum of two singlets at 4.03 and 3.27 ppm (1H each) for the nonequivalent protons of the resulting CH2 moiety and a singlet at 1.77 ppm (3H) for the remaining CH3 group. The nonequivalent isopropyl methines gave rise to two multiplets at 4.18 and 4.01 ppm resulting from the overlap of four heptets. The methine signals were found to Received: March 23, 2017 Published: May 2, 2017 5993

DOI: 10.1021/acs.inorgchem.7b00716 Inorg. Chem. 2017, 56, 5993−5997

Article

Inorganic Chemistry

Monitoring the formation of 3 by 1H NMR spectroscopy at −70 °C did not reveal any intermediates. Therefore, we performed density functional theory (DFT) calculations14 to map a possible mechanism of formation of complex 3 starting from the hypothetical complex NacNacAl(OSIMe) (3′), which forms upon coordination of the cyclic urea to 1. The isomerization of 3′ to 3 is strongly exergonic with a ΔG298° value of about −24 kcal mol−1, but the manner in which hydrogen is transferred from a methyl group to the aluminum atom is far from obvious. Test calculations did not reveal a direct unimolecular process for such a hydrogen shift; therefore, ionic mechanisms were considered. In principle, the formation of ionic intermediates [NacNac′Al(OSIMe)] − and [NacNacAlH(OSIMe)]+ from two molecules of 3′ in toluene requires, according to our calculations, a ΔG298° value of 9.5 kcal mol−1. In reality, such an ionic (or quasi-ionic) mechanism was found to take place via successive bimolecular hydrogen transfers in which hydrogen from a backbone methyl of 3′ is transferred to the aluminum atom of a second molecule of 3′. In the first step of this process, a weakly bound intermediate 3-Int1 is formed, in which the geometries of both 3′ moieties are nearly intact (Scheme 2). 3-Int1 isomerizes exergonically (ΔG298° = −8.5 kcal mol−1) to give a second intermediate 3-Int2, in which the hydrogen has been transferred from the methyl group to the Al1 atom. The Δ⧧Go298 barrier of 15.3 kcal mol−1 (relative to 3-Int1) for this process is rate-determining. The transition state 3-TS1 exhibits C···H and Al···H distances of 1.64 and 1.82 Å, respectively, which are very long compared to typical C−H and Al−H bond lengths. For the reaction to proceed further, a reorientation of the two moieties (3-Int2 → 3-Int3) has to take place for the methyl group to be located in a position suitable for the second hydrogen shift. This hydrogen shift is formally analogous to the first hydrogen transfer but is extremely facile (Δ⧧Go298 of about 2.4 kcal mol−1) and can be described as a hydrogen uptake by the Al2 atom. The overall barrier for the second hydrogen shift calculated with respect to 3-Int2 is 10.2 kcal mol−1. In light of these results, why is a similar isomerization pathway not observed for the parent complex 1 (stable up to approximately 100 °C)? In fact, our DFT calculations yielded a slightly negative ΔG298° of −1.6 kcal mol−1 for the isomerization of NacNacAl (1) to NacNac′AlH (1′), showing that it may be a thermodynamically allowed process. Calculations of a bimolecular pathway found a weakly bound 1-Int1, analogous to 3-Int1, but hydrogen transfer from the methyl group to aluminum is very unfavorable kinetically, with the overall Δ⧧Go298 value reaching 34.3 kcal mol−1, thus making this pathway insurmountable even at elevated temperatures (Figure SI4 and Scheme SI1). The reason for such a high barrier is that, while the C−H bond in the transition state 1-TS1 (2.15 Å) is almost broken, this is not compensated for by forming a weaker Al−H bond (1.58 Å, the same as that in 1′). Moreover, it turns out that surpassing this barrier results in the formation of a new Al−C bond and therefore corresponds to the first step of a (nonexistent) polymerization rather than an isomerization reaction. While CO bond (179 kcal mol−1) activation by 1 was unsuccessful, we supposed that the addition of the weaker P O bond (110 kcal mol−1)9 across the aluminum(I) center should be more feasible. Indeed, a 1:1 reaction between triphenylphosphine oxide and 1 occurred readily but unexpectedly gave a mixture of NacNac′Al(OH)(OPPh3) (5), 1, and free PPh3. Cleavage of the PO bond was revealed

Scheme 1. Reaction of Cyclic Urea 2 with 1 To Give Aluminum Hydride 3

be coupled to eight doublets, ranging from 1.66 to 1.16 ppm, that correlate to the isopropyl methyl protons of the Ar group. A multiplet at 1.84 ppm and a singlet at 1.59 ppm were assigned to the methylene and methyl protons of the OSIMe ligand, respectively. These signals are shifted upfield with respect to the uncoordinated urea. The aluminum hydride cannot be observed directly by 1H NMR spectroscopy but its presence was confirmed by the IR stretch seen at 1792 cm−1. Previously, a base-induced rearrangement of compound 1 into an adduct of its Al(III) isomer, NacNac′AlH(L) (4; L = C(N(R)C(Me)−)2, R = Pri or Me), was reported by Roesky et al. for the reaction of 1 with an NHC.12 The reaction was performed in the solid state and required heating to 120 °C for 5 h. The structure of 3 was confirmed by X-ray diffraction analysis (Figure 1), which revealed a distorted tetrahedral

Figure 1. Molecular structure of 3 (thermal ellipsoids are shown at 30% for one of the two independent molecules; hydrogen atoms, except the aluminum hydride and methylene protons on C5, are omitted for clarity).

geometry at the aluminum atom sitting 0.483 Å below the plane defined by the NacNac′ ligand. The Al−N distances of 1.842(3) and 1.836(2) Å are shorter than the average Al−N distance in the parent compound 1 [1.957(2) Å]8 but are close to the respective bond lengths of 1.844(3) and 1.853(2) Å in the related complex 4,12a consistent with the higher oxidation state of aluminum in 3. The Al1−O1 distance of 1.810(2) Å is longer than the respective distance of 1.769(2) Å found in the adduct between AlCl3 and tetramethylurea reported by Nöth and co-workers.13 The greater distance is unsurprising because the bulky 2,6-Pri2C6H3 groups flanking the aluminum atom in 3 prevent a closer approach of the urea. In the backbone of the NacNac′ ligand, the internal C−C lengths, C2−C3 of 1.384(4) Å and C3−C4 of 1.428(4) Å, are indicative of double and single bonds, respectively, while a more pronounced difference is noted between the terminal C−C [1.464(4) Å] and CC [1.398(4) Å] bond lengths for the C1−C2 and C4−C5 bonds, respectively. 5994

DOI: 10.1021/acs.inorgchem.7b00716 Inorg. Chem. 2017, 56, 5993−5997

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Inorganic Chemistry Scheme 2. Bimolecular Mechanism for the Isomerization of 3′ to 3a

a

Free energies are in kilocalories per mole relative to 3′ + 3′.

by the production of free triphenylphosphine, as confirmed by 1 H and 31P NMR spectroscopy. With 2 equiv of Ph3PO, the quantitative formation of 5 was observed (Scheme 3). Complex 5 was fully characterized by NMR and IR spectroscopy but resisted all attempts at recrystallization.14 Scheme 3. Reaction of 1 with 2 equiv of Phosphine Oxide to Prepare NacNac′Al(OH)(OPR3)

Figure 2. Molecular structure of 6 (thermal ellipsoids are shown at 30%; hydrogen atoms, except the hydroxyl proton, are omitted for clarity). The molecule is bisected by a crystallographically imposed mirror plane running through Al1 and C3.

With the intention of producing a more crystalline analogue of 5, 2 equiv of Et3PO was reacted with 1 to prepare NacNac′Al(OH)(OPEt3) (6; Scheme 3). The 1H NMR spectrum of 6 is similar to that of 5, with two singlets at 4.00 and 3.17 ppm observed for the diastereotopic methylene protons and a singlet at 1.77 ppm for the methyl protons in the NacNac′ framework. The hydroxyl proton is displayed as a singlet at 0.43 ppm and is corroborated with an IR stretch at 3742 cm−1. A singlet is present at 77.0 ppm in the 31P NMR spectrum because of the coordinated triethylphosphine oxide moiety, shifted downfield relative to the free phosphine oxide (45.4 ppm). X-ray diffraction analysis of 6 revealed a distorted tetrahedral geometry for aluminum, with a mirror plane running through Al1 and C3 (Figure 2). The aluminum atom sits 0.396 Å below the plane defined by the N2C3 framework of the NacNac′ ligand, while the Al1−N1 distance was found to be 1.834(1) Å, comparable to the respective distance in 3. The Al1−O1 distance of 1.731(2) Å is close to the average Al−OH bond in related NacNac complexes (1.724 Å),15 while the Al1−O2 distance of 1.804(2) Å is similar to the respective bond length of 1.781(1) Å in the cationic complex [NacNacAlMe(O PEt3)][B(C6F5)4].16 Because of the crystallographically im-

posed mirror plane, the C1−C2 distance of 1.433(2) Å and the C2−C3 distance of 1.411(2) Å are intermediate between the distances for the single and double C−C bonds [cf. 1.464(4) Å for a C−C bond and 1.398(4) Å for a CC bond in the backbone of the NacNac′ ligand in 3]. The mechanism of oxidation of 1 by R3PO to give 5 or 6 was studied by DFT calculations, using Me3PO as a model phosphine oxide. In the first step, direct coordination of Me3PO to 1 affords a loose intermediate NacNacAl ← OPMe3 (4-Int1; ΔG298° = 2.8 kcal mol−1; Figure 3). It then undergoes a facile shift of PMe3 to the metal (Δ⧧Go298 = 7.5 kcal mol−1 relative to 1 + Me3PO) to give the oxophosphine complex NacNacAlO(PMe3) (4-Int2), which is 16.5 kcal mol−1 (ΔG298°) more stable than 4-Int1. Complex 4-Int2 has a short AlO bond (1.69 Å) and a very long Al−P bond (2.54 Å). Expectedly, 4-Int2 is very prone to phosphine dissociation (ΔG298° = 9.9 kcal mol−1) to yield the oxide NacNacAlO (7).17 The latter can subsequently isomerize to NacNac′Al(OH) (8), whose free energy is 15.9 kcal mol−1 lower than that of 7. Similarly to 3, the oxo complex 7 forms a loosely bound dimer 7-Int1, which undergoes a facile hydrogen transfer 5995

DOI: 10.1021/acs.inorgchem.7b00716 Inorg. Chem. 2017, 56, 5993−5997

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Inorganic Chemistry

favorable process. The final product NacNac′Al(OH)(OPMe3) (9; homologous to 6) is formed by a direct, highly exothermic coordination of Me3PO to 8 (ΔEe = −59.2 kcal mol−1; ΔG298° = −43.6 kcal mol−1). The rate of the overall transformation of 1 to 8 is determined by the free-energy difference of 20.7 kcal mol−1 between the transition state 7-TS and the intermediate 4-Int2.

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CONCLUSION In conclusion, the reaction of complex 1 with a cyclic urea resulted in facile isomerization into an aluminum(III) hydride stabilized by the coordination of urea, whereas the related reaction with phosphine oxides led to the first examples of P O bond cleavage on aluminum(I) and the formation of hydroxide complexes via deprotonation of the ligand backbone. The new products were characterized by NMR and IR spectroscopy along with X-ray crystal structures of 3 and 6. Formation of the final products can be accounted for by quasiionic bimolecular mechanisms involving two subsequent hydrogen transfers from a methyl group in the ligand backbone to either aluminum or oxygen (as an oxo ligand bound to aluminum).

Figure 3. Free-energy profile for the reaction 1 + OPMe3 → 7 + PMe3. ΔG298° values are given in kcal mol−1.

(Δ⧧Go298 = 7.3 kcal mol−1; Scheme 4 and Figure 4). The transition state 7-TS is a very late one and does not differ

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Scheme 4. Bimolecular Mechanism for the Formation of 8 from 7

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00716. Materials and methods, experimental procedures, 1H and 13 C{1H} NMR spectra of 3, 1H, 13C{1H}, and 31P{1H} NMR spectra of compounds 5 and 6, X-ray crystallography data, computational methods, ΔG° profile for the 1 → 1′ bimolecular isomerization mechanism, and molecular energies and structures (PDF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Georgii I. Nikonov: 0000-0001-6489-4160 Present Address ‡

Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545. Author Contributions

All authors contributed equally. Notes

The authors declare no competing financial interest.

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Figure 4. Free-energy profile for the bimolecular mechanism of isomerization of 7 to 8. ΔG298° values are given in kcal mol−1.

ACKNOWLEDGMENTS G.I.N. acknowledges financial support of the Petroleum Research Fund, administered by the American Chemical Society. T.C. is grateful to the Government of Ontario for an Ontario Graduate Scholarship. Financial support to S.F.V. from the Spanish Ministerio de Economı ́a y Competitividad (Grant CTQ2014-54306-P) is also greatly appreciated.

significantly, geometrically or energetically, from the intermediate 7-Int2. After reorientation of both aluminum moieties in 7-Int2, the second hydrogen atom relocates swiftly to the second oxygen atom. The isomerization of 7-Int2 to 7-Int3 is a highly exergonic (ΔG298° = −37.7 kcal mol−1) and kinetically 5996

DOI: 10.1021/acs.inorgchem.7b00716 Inorg. Chem. 2017, 56, 5993−5997

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Inorganic Chemistry

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(8) Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.; Hao, H.; Cimpoesu, F. Synthesis and Structure of a Monomeric Aluminum(I) Compound [{HC(CMeNAr) 2 }Al] (Ar = 2,6− iPr2C6H3): A Stable Aluminum Analogue of a Carbene. Angew. Chem., Int. Ed. 2000, 39, 4274−4276. (9) Standard bond energies. http://www.cem.msu.edu/~reusch/ OrgPage/bndenrgy.htm (accessed Nov 11, 2016). (10) (a) Chu, T.; Boyko, Y.; Korobkov, I.; Nikonov, G. I. Transition Metal-like Oxidative Addition of C−F and C−O Bonds to an Aluminum(I) Center. Organometallics 2015, 34, 5363−5365. (b) Crimmin, M. R.; Butler, M. J.; White, A. J. P. Oxidative addition of carbonfluorine and carbon-oxygen bonds to Al(I). Chem. Commun. 2015, 51, 15994−15996. (11) Chu, T.; Vyboishchikov, S. F.; Gabidullin, B.; Nikonov, G. I. Oxidative Cleavage of CS and PS Bonds at an AlI Center: Preparation of Terminally Bound Aluminum Sulfides. Angew. Chem., Int. Ed. 2016, 55, 13306−13311. (12) (a) Zhu, H.; Chai, J.; Stasch, A.; Roesky, H. W.; Blunck, T.; Vidovic, D.; Magull, J.; Schmidt, H.-G.; Noltemeyer, M. Reactions of the Aluminum(I) Monomer LAl [L = HC{(CMe) (NAr)}2; Ar = 2,6iPr2C6H3] with Imidazol-2-ylidene and Diphenyldiazomethane. A Hydrogen Transfer from the L Ligand to the Central Aluminum Atom and Formation of the Diiminylaluminum Compound LAl(NCPh2)2. Eur. J. Inorg. Chem. 2004, 2004, 4046−4051. For related silicon chemistry, see: (b) Driess, M.; Yao, S.; Brym, M.; van Wüllen, C.; Lentz, D. A New Type of N-Heterocyclic Silylene with Ambivalent Reactivity. J. Am. Chem. Soc. 2006, 128, 9628−9629. (c) Xiong, Y.; Yao, S.; Driess, M. An Isolable NHC-Supported Silanone. J. Am. Chem. Soc. 2009, 131, 7562−7563. (13) Bittner, A.; Männig, D.; Nöth, H. Solutions of Aluminium Trichloride in Tetramethylurea and the Molecular Structure of an Aluminium Trichloride Tetramethylurea Adduct. Z. Naturforsch., B: J. Chem. Sci. 1986, 41, 587−591. (14) See the Supporting Information for details. (15) Based on a search in the Cambridge Crystallographic Data Centre. (16) Stennett, T. E.; Pahl, J.; Zijlstra, H. S.; Seidel, F. W.; Harder, S. A Frustrated Lewis Pair Based on a Cationic Aluminum Complex and Triphenylphosphine. Organometallics 2016, 35, 207−217. (17) A dimer of 7 obtained by the reaction of 1 with oxygen has been reported, but it is currently not known whether it forms via the dimerization of 7 or through a different pathway: Zhu, H.; Chai, J.; Jancik, V.; Roesky, H. W.; Merrill, W. A.; Power, P. P. The Selective Preparation of an Aluminum Oxide and Its Isomeric C−H-Activated Hydroxide. J. Am. Chem. Soc. 2005, 127, 10170−10171.

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DOI: 10.1021/acs.inorgchem.7b00716 Inorg. Chem. 2017, 56, 5993−5997