Oxidative Addition of π-Bonds and σ-Bonds to an Al(I) Center: The

Article pubs.acs.org/IC

Oxidative Addition of π‑Bonds and σ‑Bonds to an Al(I) Center: The Second-Order Carbene Property of the AlNacNac Compound Wolfgang W. Schoeller* and Guido D. Frey Faculty of Chemistry, University of Bielefeld, 33615 Bielefeld, Germany S Supporting Information *

ABSTRACT: The oxidative addition of π- and σ-bonds is studied by means of quantum chemical investigations at a MCSCF and density functional level of sophistication. The title compound (AlNacNac) induces first-order strong donor−acceptor abilities in the triplet state, giving rise to biradicaloid adducts. At second-order, it reveals carbene character. The energy barriers for the 1,2-addition reactions are fairly small, resulting from an oxidative addition, which differs from the classical 1,2-addition reaction of a carbene to an olefin. For the splitting of σ-bonds (H−X) the energy barriers are largely driven by the strengths of the H−X bonds. The metal Al increases continuously its oxidation state from the educt over the transition state to the product. This implies that in the latter complexes the metal is positive and the olefin overall negative in charge. Ethylene itself does not form a stable adduct; it is still in equilibrium with AlNacNac plus ethylene. However, electron releasing substituents stabilize the addition product. The stabilities of various three-membered ring systems are evaluated. Hydrogen splitting possesses a relatively large barrier.

■

INTRODUCTION The Al(I) compound AlNacNac, 1, was reported some time ago,1 and its reactivity toward acetylene was extensively studied in the prolific work of Roesky et al.2−4 Compound 1 (Ar = Aryl, Alk = Me; Scheme 1) is generally described as a carbene analogue, which features a lone pair at

In the present publication, we analyze the carbene character of the AlNacNac compound on the basis of quantum chemical calculations at a MCSCF (multiconfigurational self-consistent field) and DFT (density functional theory) level. The pathways for a comprehensive view of oxidative 1,2-addition reaction of specific π-bonds (ethylene, acetylene) and of σ(H−X)-bonds (X = H, C, Si, Ge, Sn) are evaluated.

■

Scheme 1

RESULTS AND DISCUSSION a. Qualitative Considerations. It is informative to analyze first the frontier molecular orbital system of 1 and compare it with that of a classical carbene. For the latter the frontier orbitals HOMO and LUMO are composed of the σ-orbital and a p-orbital;18 this is schematically illustrated by Ia in Figure 1. For a carbene (Ia), the electronic configurations that are lowest in energy are 1σ2 (singlet) and 3(σ)1(p)1 (triplet), as well-documented for species with a triplet ground state (e.g., methylene19) and with a singlet ground state (e.g., CCl2, CF220,21). The matter is different for the AlNacNac system. In Ib, a frontier orbital system is formed, which differs in some aspects considerably from Ia, the carbene system. Within C 2v symmetry, the 3 relevant orbitals have a1, a2, and b1 symmetry. The a1 molecular orbital is predominantly a nonbonding σorbital at the Al-atom and thus reminiscent of the HOMO of the carbene. The a2 molecular orbital Ib has no coefficient at the Al-atom, and thus, it is located in the ligand π-space. The b1

the Al-center and simultaneously a vacant orbital (at Al).5 In metal−organic chemistry, the presence of a metal-centered lone pair and an accessible vacant orbital is reminiscent of transition metal centers amenable for activation of robust bonds.6 Lately, it has been reported by Chu et al.7 that oxidative addition of 1 is also feasible for robust σ(H−X)-bonds (X = H, B, C, Si, N, P, O). The matter is also known for the Ga(I) complex with the NacNac ligand8 and for its analogous In-compound.9 Activation of the H−H bond has been found also for Ga(I)-aryls, via a 2 + 2 addition to the digallene RGaGaR,10 at frustrated lone pairs,11−14 and finally by carbenes15 and group 4 carbenoids.16,17 © XXXX American Chemical Society

Received: June 22, 2016

A

DOI: 10.1021/acs.inorgchem.6b01488 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

two separated electrons. The view is supported by analyses of the populations in the various electronic states, in accord with our previous investigations.22 b. Donor Interactions. As a consequence of the lower Lewis acidity in the ground state 1A1 (as compared with its first excited triplet state 3A2), a donor forms a stronger bond within the triplet (oxidation state +II for Al) as compared with a donor interaction within the singlet 1A1 (Al (+I)) state. The matter was studied hitherto for σ-donors interacting with 1. The resulting donor−acceptor adducts possess singlet−triplet energy separations, which are sizably smaller than those in the separated compounds.22b In the parent compound 1, these are larger, in the range 30−35 kcal/per mol, depending on the substituents at the ligand system L (L = NacNac).24 Overall, the matter is illustrated qualitatively in Scheme 3. Scheme 3

Figure 1. Sequence of molecular orbitals in a carbene (Ia, left) and the parent carbene analogue 1 (Ib, right).

orbital possesses again a symmetry plane, such as to allow πconjugation with the Al-atom. Thus, the LUMO, which in Ia is responsible for electrophilic addition toward an olefin20 is in Ib the LUMO+1. On the basis of these considerations, one can define Ia in first-order as a carbene and Ib in second-order as a carbene (analogue); depending on the sets of orbitals, one has to consider for a frontier orbital the carbene reactivity. In the later discussion (see section c), it will be shown that the 1,2addition properties are dictated by the product stabilities rather than the frontier orbital properties as shown in Ib. The important electronic states confined by these molecular orbitals for Ib are constituted by excitation from the a1 into the a2 or b1 orbitals. This gives rise to the singlet 1A1 and the two triplets 3A2 and 3B1. According to energy optimized MCSCF calculations (CAS(8,7)/TZVPP), the relative energies for these three states result in 1A1 0.0, 3A2 31.4, and 3B1 45.0 kcal/mol. The electronic excitation from the 1A1 ground state to 3A2 leads to the transfer of one electron from aluminum to the π-system of the ligand. Bonding in the three important electronic states is sketched in Scheme 2 by structures with various formal oxidation states at the metal atom.22a

The situation in A refers to the singlet−triplet (S−T) energy separation of the two separated species, i.e., donor system LAl (L = NacNac ligand) plus acceptor. In the interacting donor− acceptor adducts B, the S−T energy separations are sizably reduced. These considerations hold likewise, for σ-donors and πdonors as well. For the former, the concept has been evaluated for a selected variety of σ-donors (amines, phosphines, differently substituted N-heterocyclic imidazolidines).22b A prominent example for the intermediate formation of biradicaloid structures is the reaction of 1 (R = Dipp) with 2 (R″ = Me, i-Prop) to 325 (Scheme 4); this was rationalized in more detail recently.22b Furthermore, the degradation of white phosphorus with 1 has been rationalized as a reaction process over a sequence of biradicaloid intermediates.22a c. π-Donors and 1,2-Addition Reactions. What is the relationship to π-donor interactions? This aspect will be

Scheme 2

Scheme 4 To a first-order in the ground state (1A1), the heavy atom is Al(+I), which implies 2 electrons at the aluminum while in the first excited state (3A2) it changes to Al(+II). This oxidation state is known to be fairly unstable and reveals a strong Lewis acidity at the aluminum center.23 It implies a different (lower) Lewis acidity in the singlet ground state 1A1 too, when compared with the first excited triplet state 3A2. The higher triplet state 3B1 has an electron distribution very similar to that of the ground state 1A1, besides the fact that it has in addition B

DOI: 10.1021/acs.inorgchem.6b01488 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry evaluated in the following. The 1.2-addition reaction of 1 with acetylene serves as the archetypical example for the analysis.26 It proceeds in two subsequent steps (Scheme 5). Scheme 5

In the first step, alumina-cyclopropene 4 is formed at low temperature. At higher temperature one acetylene adds further under formation of 5. The reaction does not occur for the sterically demanding bis-trimethylsilyl-acetylene; it has to be prepared by a different route.27 To our knowledge the corresponding reaction with ethylene has not been reported. First, the electronic hypersurface for the reaction cascade was evaluated by energy minima and transition state location (RIBP86/TZVP), and all stationary points were characterized by frequency analyses. The explorations of electronic hypersurfaces were performed for model geometries with R(N) = Me, and the alkyl groups in the NacNac ligand were replaced by hydrogens. Our investigations will mimic the main features of the reactions, but do not account for the bulkiness of the various substituents in the experimental situation (see vide infra). We record here first our findings for the acetylene addition to 1, as shown in Figure 2. The reference points on the two electronic hypersurfaces are for the former (A) the energy sum of the individual components LAl + C2H2 and for the latter (B) 4 + C2H2. In accordance with the previous considerations acetylene forms a weak encounter complex as indicated in 4a (AlC = 1.995, 2.589 Å). On the basis of the electronic energy it is slightly more stable (ΔE = −4.4 kcal/mol) than the individual components, but with the addition of entropy contributions the energy profit is canceled by the entropy contributions (ΔG = 0.0 kcal/mol, RT, 1 atm). Hence, one may consider the complex as an equilibrium of both, 1 plus acetylene, as shown in 4a (Figure 2A). The pathway A, right, proceeds over the transition state TS1 (ΔG = +7.3 kcal/mol) to the final product 4. The reaction continues by bending in the encounter complex one hydrogen of the acetylene fragment from the trans- to the cisconformation. The energy barrier for the activation of this reaction channel is fairly small and in accord with the experiment, which is carried out at low temperature (−78 up to −50 °C).26 In the transition state TS1, a three-center bonding between Al and the two carbon atoms of the aceylene fragment is adopted (AlC = 2.056 [1.085 SEN], 2.459 [0.641 SEN] Å); it is also substantiated by shared electron numbers [SEN] from a corresponding population analysis (in brackets). The calculations reveal also an alternative pathway, (LAl + C2H2) → TS2 → 6, which is higher in activation energy than the one over TS1. The acetylene fragment is here in transposition to 1. In the transition state TS2, a two-center bonding between Al and one C is involved (AlC = 2.436 [0.571 SEN] Å, AlH = 2.550 Å, ∠HCAl = 83.4°). Species 6 refers to the alternative possible splitting path of acetylene. Product 6 is predicted to be more stable than 4. Since the transition state TS2 is higher in energy than TS1 for the alumina-cyclopropene formation, a corresponding product formation to 6 should not

Figure 2. Stationary points for the hypersurfaces of acetylene addition to 1 (R(N) = Me). Relative free energies (ΔG in kcal/mol, room temperature (RT), 1 atm) are with respect to components LAl + C2H2, at the RI-BP86/TZVP level. (A) Reaction of equilibrium 4a to 6 (C−H insertion), as well to 4 (three-membered ring formation), and (B) a second C2H2 addition from 4 to 5.

occur. To our knowledge, it has not been observed in the experiment; 1 does not tend to insert easily into H−C (sp2) bonds of alkynes (see vide infra). The reaction anticipated in B involves the addition of a second acetylene to 4. Its energy barrier for the addition process is higher than that for the first step. In the transition state the acetylene fragment is in cis-position to 1. The hydrogen at the aluminum (HAl = 1.900 Å [0.490 SEN]) forms a three-center bond with Al and the cis-positioned carbon atom (1.691 Å [0.318 SEN]) of the alumina-cyclopropene. The neighboring carbon (of the acetylene fragment) adopts a weak bond with Al (AlC = 2.476 Å [0.249 SEN]). One AlC bond of the three-membered ring is already stretched (AlC = 2.014 Å [1.143 SEN]), such as to facilitate hydrogen addition. One expects that the alumina-cyclopropenes are fairly stable compounds. At least 4 gains an appreciable binding energy of −24.4 kcal/mol (ΔG), with respect to decomposition into its corresponding fragments. However, this must be not so, repulsive interaction between sterically strongly demanding substituents at the AlNacNac system, e.g., R(N) = Dipp, and bulky substituents at the acetylene fragment may disadvantage product formation. This is witnessed in 4b (R(N) = Dipp, R″ (at acetylene fragment) = TMS (trimethylsilyl)) (Scheme 6, hydrogens are omitted for clarity). According to the calculations (RI-BP86-D3/TZVP), its binding energy with respect to fragmentation into components is reduced to ΔG = −15.5 kcal/mol, even though the electron withdrawing trimethylsilyl group tends to stabilize a negative charge (see vide infra). The calculated transition state TS1 (Figure 2), which reveals sterical crowding in a bulky system, C

DOI: 10.1021/acs.inorgchem.6b01488 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

comparison, the reactions of 1 with olefins are rather controlled by the product stability. For understanding of the oxidative addition mechanism, a second aspect is important. Carbon profits from isovalent hybridization, that is based on an equal space extension of σand π-orbitals to sp2-hybrid orbitals.32,33 The aluminum is a higher main group element; it is less prone to adopt hybrid orbitals since the σ-orbital tends to be nonbonding (orbital nonhybridization).34 The reaction starts with oxidation of the Al and increases steadily with enhancing coordination to the product. This view is substantiated by a population analysis of the reaction of A, right (Figure 2). The NBO analysis (atomic charges) results for LAl and participating C2H2 follow in the order L/Al/C2H2: (a) educt (1 plus acetylene) −0.653/ +0.653/0.0, (b) TS1 −1.152/+1.014/0.128, (c) product 4 0.485/+1.513/−1.027. Therefore, in the transition state TS1, electron density is shifted into the ligand L and is finally moved in the product substantially to the C2H2 fragment. In the simple picture of oxidation numbers, it is in 1 Al(+I) and in TS1 Al(+II). Upon following to the product, it would end with Al(+III) as the central atom surrounded by ligands with an overall negative charge −3. In reality, the oxidative 1,2-addition reaction product is best described as intermediate between covalent and ionic in bonding between an Al (cation) surrounded by ligands. However, a full Al(+III) oxidation state is not achieved. d. Ring Formation with Olefin Analogues. Hitherto in the experiment, only one representative of an aluminacyclopropene has been found.2 It raises the question whether other three-membered rings could be synthesized. In order to analyze this aspect in more detail, we performed DFT calculations on a selected variety of olefins added to 1. Product stabilities in the corresponding alumina-cyclopropenes were probed on the basis of (a) the exothermicity of the corresponding addition reaction (with respect to the educts) and (b) the singlet−triplet energy separations in the ring systems. The results of the investigations are collected in Table 1. The table includes enthalpy and free energy contributions as well, in order to allow a prediction of product stabilities at low temperature (less importance of entropy) as well as at room temperature.

Scheme 6

makes the 1,2-addition route for the bulky dimethylsilylacetylene and corresponding derivatives unlikely for the approach path (as compared with TS1). Species 4b was also synthesized by a different experimental route.27 The relative weak binding energies of these ring systems become even more apparent in the 1,2-addition reaction of 1 with ethylenes. A corresponding electronic hypersurface for this reaction was explored as well and is recorded in Figure 3.

Figure 3. Various stationary points for the hypersurface (RI-BP86/ TZVP) of ethylene addition to 1 (R(N) = Me). Relative free energies (ΔG in kcal/mol, at RT, 1 atm) are with respect to components LAl + C2H4. Bonding parameters are in ångstroms.

In a comparison with the acetylene addition, the ethylene addition is only slightly exothermic by ΔG = −3.1 kcal/mol. Hence, 7 is unlikely to be traced at room temperature, but it may exist at lower temperature. There, the entropy contributions, which facilitate fragmentation, play a minor role. Electron rich olefins (e.g., tetramethyl-ethylene) destabilize while electron withdrawing substituents stabilize the product formation. We have probed this aspect at RI-BP86/ TZVP level of investigation: tetramethyl-ethylene addition is endothermic by ΔGreac = +19.1 kcal/mol, while tetracyanoethylene is exothermic by ΔGreac = −16.3 kcal/mol. Overall, one interesting aspect becomes apparent from these quantum chemical investigations. The 1,2-addition product from acetylene yields more stable compounds than the ethylene analogue. It is contradictory to the expectations within organic chemistry, which imply that ring strain in cyclopropene is considerably higher than in cyclopropane.28,29 The latter is controlled in its 1,2-addition reactivity by the two frontier orbitals HOMO (σ-orbital) and LUMO (π-orbital) (see Figure 1) and is classified as electrophilic, nucleophilic, and ambiphilic.30,31 This classification which relies on frontier orbital theory31 requires strong exothermicity in a reaction. In

Table 1. Various Olefins Added to 1: (a) Reaction Energies ΔGreac and ΔHreac (in kcal/mol, at RT, 1 atm) and ΔG(S−T) (in kcal/mol) of the Products, at RI-BP86/TZVP Level 8a 8b 8c 8d 8e 8f 8g 8h 8i 8k 8l 8m a

D

AB

type

ΔGreac

ΔHreac

−ΔG(S−T)

HCCH NCMe CNMe PCMe OCMe2 SCMe2 MeNNMe MePPMe OCO SCS O2 N2

IIa IIb IIb IIa IIa IIa IIa IIa IIb IIa IIa IIa

−24.4 1.1 11.3 −22.8 −11.0 −32.7 −19.7 −31.6 −6.4 −21.7 −65.8a 38.5

−31.8 −9.5 0.8 −33.4 −22.1 −43.8 −31.3 −40.4 −15.5 −26.6 −78.6a 29.6

33.5 18.6 10.1 25.8 24.8 35.6 11.7 29.9 34.8 36.7 42.1 0.2

Energies were derived with reference to triplet O2. DOI: 10.1021/acs.inorgchem.6b01488 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry The computational results (for further details see Table S1 in the Supporting Information) reveal two different types of bonding (Scheme 7). Either a symmetrical arrangement of ring

Scheme 8

Scheme 7

atoms is attempted, as is sketched in IIa, or the atom (fragment) A tends to approach the axial position of the metal center, as depicted in IIb. In this case, B is in a plane with the LAl: fragment. For convenience the already studied (Figure 2) HCCH adduct (8a) is included in the table. When A = B, this refers to the cases of acetylene (8a), diazene (8g), diphosphene (8h), dioxygen (8l), and dinitrogen (8m) addition to 1. The dioxygen adduct (8l) appears by far to be the most stable in this list. Alternatively, the diazene adduct results in the least stability regarding its S−T energy separation. In other words, this species tends to adopt a biradicaloid character. The nitrile (8b) and isonitrile (8c) are endothermic with respect to addition to 1. The phosphaalkyne (8d) has a strong preference for addition, and the ketone (8e) is less, while the thioketone (8f) is even more prone toward addition. Interestingly for CO2 (8i) and even more pronounced for CS2 (8k), an aluminacyclopropene is favored. For the cases in which there is a strong difference in electronegativity between A and B, the more electronegative atom (e.g., 8i) tends to adopt the axial position of the resulting adduct. A full compilation of all product geometries are collected in the Supporting Information. The dioxygen (8l) forms a strong adduct with a large S−T energy separation, while dinitrogen is strongly endothermic with respect for reaction with 1. Overall, olefin fragments with strong π-bonds (8b, 8c) tend not to add, while their second row congeners (8d, 8f, 8k) possess weaker π-bonds and are ready to add. Our findings allow insight into the further expansion reactions to five-membered ring systems: (a) Adducts with a small S−T energy separation like further rearrangement reactions. A pertinent example is the reaction of 1 (L = NacNac, R = aryl) with diphenyl-diazobenzene;35 it forms an intermediate 8g-Ph, which is expected to have a small S−T energy separation (Table 1). Hence, it will react as a biradicaloid, and it undergoes further rearrangement to 9 by a sequence of rearrangement reactions. (b) A vast number of expansion reactions of 1 are known,3 e.g., 4 (R(N) = Ar) reacts with CO2 to 10 (Scheme 8). As a representative, the reaction 4 → 10 was studied by the DFT calculations for the model reaction with 4 (R(N) = Me instead of Ar, Me at 3,5-positions were again replaced by hydrogens) with CO2. In analogy to the reaction of 4 with C2H2, an activation energy of ΔG = 16.5 kcal/mol is required (with respect to the educts 4 and CO2) for the insertion process to 10. Final product 10 (R(N) = Me) is more stable by ΔG = −44.4 kcal/mol, when compared with the stability of the educts. Most noticeably, the energy separation results in ΔG(S−T) = −50.1 kcal/mol. This implies an increase of

stability in relation to the alumina-cyclopropene 4 (see Table 2). Table 2. Stationary Points for the Insertion Reaction of 1 into H−X Bondsa X

type

ΔG⧧

ΔGreac

−ΔG(S− T)

r1

r2

r3

H CH3 SiH3 GeH3 SnH3

C D D D

38.0 51.5 25.2 28.5 19.0

−22.6 −17.2 −21.5 −26.1 −27.6

47.3 47.1 45.0 44.3 42.1

1.919 2.418 1.979 1.985 2.097

1.507 1.593 2.509 2.567 2.755

1.278 1.727 1.658 1.761 1.924

a

Free energies (at RT, 1 atm) are in kcal/mol; bond lengths are in ångstrom units, at RI-BP86/TZVP level.

e. σ-Bond Addition. For completeness of the investigations on the oxidative addition reaction of 1, we studied also particular reactions including σ-bond splitting. In the work of Chu et al.,7 the reaction of 1 with almost all robust H−X bonds (X = H, B, C, Si, N, P, O) under H−X splitting are reported. One may note here that H−X cleavage has been found first on the corresponding Ga-compound8 and also partially for the Incongener.9 From the previous investigation, one can conclude that such a process occurs by a related reaction mechanism. In order to reveal the most important facets of the reaction mechanism, we have analyzed the oxidative 1,2-addition reaction for H−X bonds, where X is a group IV element. The results of these investigations are collected in Table 2. A priori two different transition states are possible (Scheme 9), either H or (fragment) X prefers the axial position. In the transition state to 11, a three-center bond between H−X and the metal atom is adopted (for further details, see Table S2 in the Supporting Information). It is the consequence of the strong Lewis acidity of the LAl: species. For all cases,7 the reaction energy is sizably exothermic, which is in agreement with the aforementioned findings on πbond addition. The final products 11 also possess S−T energy separations, which imply considerable singlet stability of these species. Of particular interest are the activation energies. These reflect the bond energies of the H−X bonds; e.g., the H−C and H−H bonds are rather strong. It requires large energy E

DOI: 10.1021/acs.inorgchem.6b01488 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

bonds follows the same vein. The oxidative 1,2-addition of a selected variety of H−X bonds (X = group 14 fragments) reveals energy barriers; larger than those for the olefin addition, they depend on the energy needed to break the H−X bond. It is largest for dihydrogen splitting and smallest for Sn−H splitting. (4) With regard to the classical 1,2-addition reaction in organic chemistry, the congener in aluminum chemistry is best characterized as an oxidative addition, in which the metal atom increases continuously its oxidation state from the educt to the product site.

Scheme 9

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01488. Cartesian coordinates of the investigated species, singlet ground states, and transition states (PDF)

quantities to break these bonds in the insertion reaction. In the actual experiments, the energy barriers will deviate, depending on the steric hindrance for the approach path; e.g., it has been reported that H3SiPh adds to 1 (R = Ar) at room temperature stirring overnight while for the bulkier H2SiMePh heating overnight at 70 °C is needed.7 The relative large energy barrier for the insertion in the H−C bond of CH4 accounts nicely with the experimental observation to avoid reaction with terminal C−H bonds in alkenes and alkynes. For the alkynes, addition at the π-system takes place.

■

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

THEORETICAL SECTION

ACKNOWLEDGMENTS Allocation of computer time was provided by the University of Bielefeld. We thank Dr. Thorsten Tönsing for computational assistance. Early work on this subject was generously supported by the Deutsche Forschungsgemeinschaft. Dedicated to M. Luise.

The calculations were performed with the Gamess,36 Orca 3.1,37 and Turbomole 6.238 sets of programs. The density functional calculations were performed at the BP86 level39 with the resolution of the identity (RI).40 As basis sets throughout Ahlrichs’ triple-ζ bases, the def2TZVP set was used.41 For the MCSCF wave function calculations with a larger basis set, the TZVPP basis was taken.41 The frequency calculations to locate the stationary points for the smaller model geometries were performed analytically, and entropies and free energies were determined by standard equations from statistical thermodynamics.42 For the fairly large structures, we performed the frequency calculations numerically. In order to account for van der Waals interactions in the large structures, dispersion corrections were applied according to the latest Grimme approach with the Becke− Johnson damping.43 The population analysis was conducted with the NBO scheme44 and for the determination of bond orders according to the method of shared electron numbers (SENs).45 The geometries of all stationary points are provided in the Supporting Information to this publication.

■

REFERENCES

(1) 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. (2) Roesky, H. W.; Kumar, S. S. Chemistry of aluminium(I). Chem. Commun. 2005, 4027−4038. (3) Nagendran, S.; Roesky, H. W. The Chemistry of Aluminum(I), Silicon(II), and Germanium(II). Organometallics 2008, 27, 457−492. (4) Roesky, H. W. Aluminum(I) chemistry. In Inorganic Chemistry in Focus II; Meyer, G., Naumann, D., Wesemann, L., Eds. Wiley-VCH, Verlag HmbH & Co.: Weinheim, 2005; pp 89−103. (5) Asay, M.; Jones, C.; Driess, M. N-Heterocyclic Carbene Analogues with Low-Valent Group 13 and Group 14 Elements: Syntheses, Structures, and Reactivities of a New Generation of Multitalented Ligands. Chem. Rev. 2011, 111, 354−396. (6) (a) Power, P. P. Main-group elements as transition metals. Nature 2010, 463, 171−177. (b) Power, P. P. Interaction of Multiple Bonded and Unsaturated Heavier Main Group Compounds with Hydrogen, Ammonia, Olefins, and Related Molecules. Acc. Chem. Res. 2011, 44, 627−637. (7) Chu, T.; Korobkov, I.; Nikonov, G. I. Oxidative Addition of σBonds to an Al(I) Center. J. Am. Chem. Soc. 2014, 136, 9195−9202. (8) Seifert, A.; Scheid, D.; Linti, G.; Zessin, T. Oxidative Addition Reactions of Element−Hydrogen Bonds with Different Polarities to a Gallium(I) Compound. Chem. - Eur. J. 2009, 15, 12114−12120. (9) Hill, M. S.; Hitchcock, P. B.; Pongtavornpinyo, R. Oxidative Addition Reactions of Alkyl Halides with the Group 13 Carbene Analogue [In{N(Dipp)C(Me)}2CH] (Dipp = 2,6-iPr2C6H3). Inorg. Chem. 2007, 46, 3783−3788. (10) (a) Zhu, Z.; Wang, X.; Peng, Y.; Lei, H.; Fettinger, J. C.; Rivard, E.; Power, P. P. Addition of hydrogen or ammonia to a low-valent

■

CONCLUSIONS The investigations can be summarized as follows: (1) The oxidative 1,2-addition of π-systems, such as acetylene and ethylene proceed over fairly low energy barriers. The energy profit for product formation from the educts is also small. It depends on the electron demand of the olefins. The products can be viewed intermediate in bonding as π-complexes of a positively charged Al with a negatively charged olefin and covalent bonded alumina-cyclopropenes or -propanes. This sighting is strongly emphasized for the ethylene product, which is only slightly more stable than the educts. Electron withdrawing substituents attached to the olefin emphasize a stronger stability of the product. (2) Bulky substituents pronounce the educt site, and the products can then be considered being in equilibrium with the monomers. Consequently, other nucleophiles can easily replace the olefins for alternative ring systems. A second nucleophilic fragment can also add to the olefin adduct, giving rise to stable fivemembered ring systems. This more general statement has been proven for the CO2 addition to 4. (3) The splitting of robust σF

DOI: 10.1021/acs.inorgchem.6b01488 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

iPr2C6H3] 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. (26) Zhu, H.; Chai, J.; Fan, H.; Roesky, H. W.; He, C.; Jancik, V.; Schmidt, H. G.; Noltemeyer, M.; Merrill, W. A.; Power, P. P. A Stable Aluminacyclopropene LAl(η2-C2H2) and Its End-On Azide Insertion to an Aluminaazacyclobutene. Angew. Chem., Int. Ed. 2005, 44, 5090− 5093. (27) Cui, C.; Koepke, S.; Herbst-Irmer, R.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G.; Wrackmeyer, B. Facile Synthesis of Cyclopropene Analogues of Aluminum and an Aluminum Pinacolate, and the Reactivity of LAl[η2-C2(SiMe3)2] toward Unsaturated Molecules (L = HC[(CMe) (NAr)]2, Ar = 2,6-i-Pr2C6H3). J. Am. Chem. Soc. 2001, 123, 9091−9098. (28) Gordon, M. S. Ring strain in cyclopropane, cyclopropene, silacyclopropane, and silacyclopropene. J. Am. Chem. Soc. 1980, 102, 7419−7422. (29) Schleyer, P. v. R.; Williams, J. E., Jr.; Blanchard, K. R. Evaluation of strain in hydrocarbons. The strain in adamantane and its origin. J. Am. Chem. Soc. 1970, 92, 2377−2386. (30) Rondan, N. G.; Houk, K. N.; Moss, R. A. Transition states and selectivities of singlet carbene cycloadditions. J. Am. Chem. Soc. 1980, 102, 1770−1776. (31) Schoeller, W. W.; Brinker, U. H. Towards a classification of singlet carbenes. Z. Naturforsch., B: J. Chem. Sci. 1980, 35, 475−476. (32) (a) Pitzer, K. Relativistic effects on chemical properties. Acc. Chem. Res. 1979, 12, 271−276. (b) Pyykko, P.; Desclaux, J. P. Relativity and the periodic system of elements. Acc. Chem. Res. 1979, 12, 276−281. (33) Kutzelnigg, W. Chemical bonding in the higher major group elements. Angew. Chem., Int. Ed. Engl. 1984, 23, 272−295. (34) (a) Schoeller, W. W.; Dabisch, T. Bonding in cyclosilanes. Effect of orbital nonhybridization. Inorg. Chem. 1987, 26, 1081−1086. (b) Schoeller, W. W.; Dabisch, T.; Busch, T. Bonding in cyclosilanes. Effect of orbital nonhybridization. Inorg. Chem. 1987, 26, 4383−4389. (35) Zhu, H.; Chai, J.; Fan, H.; Roesky, H. W.; Nehete, U. N.; Schmidt, H.-G.; Noltemeyer, M. A rearrangement of azobenzene upon interaction with an aluminum(I) monomer LAl {L = HC[(CMe) (NAr)]2, Ar = 2,6-iPr2C6H3}. Eur. J. Inorg. Chem. 2005, 2005, 2147− 2150. (36) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General atomic and molecular electronic structure system. J. Comput. Chem. 1993, 14, 1347−1363. (37) Neese, F. The ORCA program system. Wiley Interdisciplinary Reviews: Computational Molecular Science 2012, 2, 73−78. (38) Turbomole V6.2, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, University of Karlsruhe, http:// www.turbomole.com. (39) (a) Vosko, S.; Wilk, L.; Nusair, M. Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis. Can. J. Phys. 1980, 58, 1200−1211. (b) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (40) Von Arnim, M.; Ahlrichs, R. Performance of parallel TURBOMOLE for density functional calculations. J. Comput. Chem. 1998, 19, 1746−1757. (41) (a) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (b) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (42) McQuarrie, D. A. Statistical Mechanics; Harper & Row: New York, 1976.

group 13 metal species at 25°C and 1 atm. Angew. Chem., Int. Ed. 2009, 48, 2031−2034. (b) Caputo, C. A.; Koivistoinen, J.; Moilanen, J.; Boynton, J. N.; Tuononen, H. M.; Power, P. P. Counterintuitive Mechanisms of the Addition of Hydrogen and Simple Olefins to Heavy Group 13 Alkene Analogues. J. Am. Chem. Soc. 2013, 135, 1952−1960. (11) Stephan, D. W. Frustrated Lewis pairs”: a concept for new reactivity and catalysis. Org. Biomol. Chem. 2008, 6, 1535−1539. (12) Stephan, D. W.; Erker, G. Frustrated Lewis pairs: metal-free hydrogen activation and more. Angew. Chem., Int. Ed. 2010, 49, 46−76. (13) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Reversible, Metal-Free Hydrogen Activation. Science 2006, 314, 1124− 1126. (14) For activation of a silylium ion see: Schäfer, A.; Reißmann, M.; Schäfer, A.; Saak, W.; Haase, D.; Müller, T. A New Synthesis of Triarylsilylium Ions and Their Application in Dihydrogen Activation. Angew. Chem., Int. Ed. 2011, 50, 12636−12638. (15) (a) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Facile splitting of hydrogen and ammonia by nucleophilic activation at a single carbon center. Science 2007, 316, 439−441. (b) Frey, G. D.; Masuda, J. D.; Donnadieu, B.; Bertrand, G. Activation of Si-H, B-H, and P-H Bonds at a Single Nonmetal Center. Angew. Chem., Int. Ed. 2010, 49, 9444−9447. (16) (a) Peng, Y.; Ellis, B. D.; Wang, X.; Power, P. P. Diarylstannylene Activation of Hydrogen or Ammonia with Arene Elimination. J. Am. Chem. Soc. 2008, 130, 12268−12269. (b) Peng, Y.; Guo, J.-D.; Ellis, B. D.; Zhu, Z.; Fettinger, J. C.; Nagase, S.; Power, P. P. Reaction of Hydrogen or Ammonia with Unsaturated Germanium or Tin Molecules under Ambient Conditions: Oxidative Addition versus Arene Elimination. J. Am. Chem. Soc. 2009, 131, 16272−16282. (c) Wang, Y.; Ma, J. Silylenes and germylenes: The activation of H−H bond in hydrogen molecule. J. Organomet. Chem. 2009, 694, 2567− 2575. (17) (a) Protchenko, A. V.; Birjkumar, K. H.; Dange, D.; Schwarz, A. D.; Vidovic, S.; Jones, C.; Kaltsoyannis, N.; Mountford, P.; Aldridge, S. A Stable Two-Coordinate Acyclic Silylene. J. Am. Chem. Soc. 2012, 134, 6500−6503. (b) Dube, J. W.; Brown, Z. D.; Caputo, C. A.; Power, P. P.; Ragogna, P. J. Activation of gaseous PH3 with low coordinate diaryltetrylene compounds. Chem. Commun. 2014, 50, 1944−1946. (18) Hoffmann, R. Trimethylene and the addition of methylene to ethylene. J. Am. Chem. Soc. 1968, 90, 1475−1485. (19) Sherrill, C. D.; Leininger, M. L.; Van Huis, T. J.; Schaefer, H. F., III Structures and vibrational frequencies in the full configuration interaction limit: Predictions for four electronic states of methylene using a triple-zeta plus double polarization (TZ2P) basis. J. Chem. Phys. 1998, 108, 1040−1049. (20) (a) Schoeller, W. W. Electrophilicity and nucleophilicity in singlet carbenes. I. Dependence on electronegativity. Tetrahedron Lett. 1980, 21, 1505−1508. (b) Schoeller, W. W. Electrophilicity and nucleophilicity in singlet carbenes. II. Electrophilic selectivity. Tetrahedron Lett. 1980, 21, 1509−1510. (21) Carter, E. A.; Goddard, W. A., III New predictions for singlettriplet gaps of substituted carbenes. J. Phys. Chem. 1987, 91, 4651− 4652. (22) (a) Schoeller, W. W.; Frey, G. D. White phosphorus degradation with a NacNac aluminum carbene analogue: the biradical reaction mechanism. Inorg. Chem. 2014, 53, 4840−4846. (b) Schoeller, W. W.; Frey, G. D. On the acceptor properties of a Al-Nacnac carbene analogue, a density functional investigation. J. Organomet. Chem. 2013, 744, 172−177. (23) Uhl, W. Organoelement compounds possessing Al-Al, Ga-Ga, In-In, and Tl-Tl single bonds. Adv. Organomet. Chem. 2004, 51, 53− 108. (24) Schoeller, W. W. Neutral Carbene Analogues of Group 13 Elements: The Dimerization Reaction to a Biradicaloid. Inorg. Chem. 2011, 50, 2629−2633. (25) 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 = 2,6G

DOI: 10.1021/acs.inorgchem.6b01488 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (43) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. J. Comput. Chem. 2011, 32, 1456−1465. (44) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural population analysis. J. Chem. Phys. 1985, 83, 735−746. (45) Ehrhardt, C.; Ahlrichs, R. Population analysis based on occupation numbers. II. Relationship between shared electron numbers and bond energies and characterization of hypervalent contributions. Theor. Chim. Acta 1985, 68, 231−245.

H

DOI: 10.1021/acs.inorgchem.6b01488 Inorg. Chem. XXXX, XXX, XXX−XXX