Review pubs.acs.org/Organometallics
Heavier Main Group Dimetallene Reactivity: Effects of Frontier Orbital Symmetry† Christine A. Caputo and Philip P. Power* Department of Chemistry, The University of California at Davis, 1 Shields Avenue, Davis, California 95616, United States ABSTRACT: The major theme of this review concerns the reaction pathways of heavier main group 13 dimetallenes with olefins, cyclic olefins, polyolefins, other unsaturated molecules, and hydrogen. For gallium derivatives, these reactions proceed readily in high yield under ambient conditions. Early investigations showed that the group 13 dimetallenes dissociated to one-coordinate metallanediyl monomers, which suggested that they reacted as monomeric species due to their more open, less hindered structures. However, recent DFT calculations by Tuononen and co-workers show that the reaction of the monomers with olefins or hydrogen involves very large energy barriers and that the dimetallenes react instead as undissociated dimers. In contrast to the corresponding all-carbon systems, where [2 + 2] cycloadditions are forbidden because of π−π* symmetry mismatch, the dimetallenes react with olefins via two stepwise, symmetry-allowed [2 + 2] cycloadditions to give 1,4-digallacyclohexanes. The addition of the H2 proceeds by a different mechanism, initially involving a 1,2-dihydride intermediate to ultimately yield two ArGaH2 molecules, which recombine to give Ar(H)Ga(μ-H)2Ga(H)Ar. The corresponding dialuminene is more highly reactive and reacts with solvent toluene to give an unusual [2 + 4] cycloaddition product. Calculations reveal that there is an enhanced diradical character in its bonding and that the reaction with propene affords an open-shell transition state involving a dangling CH3Ċ HCH2Al moiety with unpaired spin density also on aluminum.
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INTRODUCTION The synthesis of a variety of neutral low-valent heavy group 13 and 14 element species of formula RMMR (M = Al−Tl, Si−Pb; R = aryl or silyl group) in the past decade has led to a widespread interest in their peculiar structures and bonding, which arises from the fact that they possess trans-bent or strained geometries that are in sharp contrast to their lighter element counterparts, which have uniformly linear structures.1 The strained geometry arises from the diminished ability of these elements to hybridize owing to the different core and spatial characteristics of the valence s and p orbitals of the heavier elements in relation to boron or carbonthe lightest members of these groups.2 The bent geometries can also be rationalized in terms of second-order interactions between valence orbitals of like symmetry.3 Such interactions can occur when a hypothetical molecule with linear geometry undergoes a trans-bending vibration which changes the symmetry of both the in-plane π orbital and the σ* orbital to a bu representation in the C2h (local symmetry) space group. This second-order or pseudo Jahn−Teller distortion is common in the heavier main group, multiply bonded molecules because the separations of the energy levels are much less (ca. 50%) than they are in the corresponding light element derivatives due to the weaker
bonding. This results in stronger second-order interactions, which can result in large geometrical changes. Thus, the lightest element derivative, HBBH, was calculated to have a linear structure in which the degenerate π bonds are each occupied by one electron to give a triplet ground state,4 whereas in the heavier elements (Al−Tl) the trans-bent geometry becomes stabilized so that the HOMO is now an n− lone pair (or slipped π bonding) orbital and the LUMO is an out of plane empty π orbital,5 as illustrated in Figure 1. Thus, in this case both the HOMO and the LUMO have the same π ymmetry. Stable examples of group 13 dimetallenes are known for the elements Ga, In, and Tl.6 An aluminum congener can apparently be generated in solution, but this quickly undergoes cyclization with aromatic solvents, and no stable example has been isolated to date.7 An early investigation of the physical properties and chemistry of the digallene AriPr6GaGaAriPr66b concluded that the Ga−Ga bond was weak, which was confirmed by later computational work, in collaboration with Guo and Nagase.5a Digallenes such as AriPr6GaGaAriPr6 and 4(CH3)3SiAriPr4GaGaAriPr4Si(CH3)3-4 were shown to react with a variety of unsaturated molecules, which generally underlined the weakness of the Ga−Ga bonds.6b−d They were also shown to react readily with H2 or NH3 to give bridged hydride or amido products with no Ga−Ga bonds.8 These early results were consistent with the view (if they did
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This Award Article summarizes, including more recent data, one of the major themes of a lecture presented on March 26th, 2012, at the 243rd American Chemical Society National Meeting at San Diego, CA, in receipt of the 2012 ACS Award in Organometallic Chemistry, sponsored by the Dow Corporation. © 2013 American Chemical Society
Received: January 4, 2013 Published: April 11, 2013 2278
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Figure 1. Schematic drawing of the frontier molecular orbitals in heavier group 13 dimetallenes.
Scheme 1. Reaction of AriPr6GaGaAriPr6 with 2,3-Dimethyl1,3-butadiene6b
not prove) that the digallenes reacted as dissociated gallanediyl monomers. This article will highlight recent results obtained in our laboratory, in which a low-valent heavier main group dimetallene, the digallene AriPr4GaGaAriPr4, reacts with a variety of small molecules, including hydrogen, olefins, cyclic polyolefins, and related species. In contrast with the earlier view, the results show that the digallenes react as intact dimers, underlining the importance of the symmetry of the frontier molecular orbitals on the rate and course of the reactions. In addition, we compare briefly their reactivity with that of their heavier group 14 analogues as well as with that of alkenes. The results indicate that although the heavier group 13 element dimetallenes are extensively dissociated in solution, they react as dimeric species because of their lower energy reaction pathways as a result of the stabilization of HOMO frontier orbital through M−M bonding.
featuring a GaC4 ring derived from a simple incorporation of a gallanediyl monomer by cyclization with the diene was observed. This was rationalized on the basis that the GaC4 ring was not formed from the gallanediyl monomer for reasons of strain. As will be seen, more recent investigations, which are the subject of this review, readily account for the lack of the five-membered GaC4 ring product by showing that the reaction of monomeric gallanediyls with olefins is energetically disfavored, whereas the reactions of the digallenes which lead to the ten-membered-ring
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FRONTIER ORBITALS AND THE REACTION OF GROUP 13 DIMETALLENES WITH OLEFINS Unlike the alkene counterparts, which require activation by the addition of electron-withdrawing or -donating groups, addition of a transition-metal catalyst, or application of forcing conditions (i.e., heat, pressure), the cycloaddition reactions of the heavy group 13 metal alkene analogue, digallene, with simple alkenes as well as polyolefins proceed rapidly under ambient conditions. An early investigation of digallene reactivity with olefins had shown that the treatment of 2,3dimethyl-1,3-butadiene with AriPr6GaGaAriPr6 proceeded readily at 0 °C to afford the species {AriPr6GaCH2C(CH3)C(CH3)CH2}2 (Scheme 1), containing a 10-membered Ga2C8 ring with no Ga−Ga bonding.6b Although it was known at that time that AriPr6GaGaAriPr6 was extensively dissociated to :GaAriPr6 gallanediyl monomers in solution, no monomeric AriPr6GaCH2C(CH3)C(CH3)CH2 gallacyclopentene product
species {AriPr6GaCH2C(CH3)C(CH3)CH2}2 are symmetrically and thermodynamically favored. The increased reactivity of the dimetallenes lies in the symmetry and relative energies of their frontier molecular orbitals (FMO).9 The heavy group 13 element dimetallenes have a HOMO of π symmetry and a LUMO that is also of π symmetry (Figures 1 and 2). The heavy group 14 analogues have a similar π HOMO but instead possess a n+ type LUMO. In classic organic chemistry, unsaturated species such as alkenes conform to the Woodward−Hoffmann rules for cycloaddition reactions,10 but the FMOs of an alkene differ in that they consist of a HOMO of π symmetry and a LUMO of π* symmetry. The symmetry of their FMOs dictates their reactivity with other polyolefins. The Woodward−Hoffmann rules predict the outcome of ring-closing events for long-chain polyolefins in accordance with the symmetry of the frontier orbitals.10 If the HOMO and LUMO of the reacting species have the same symmetry, the reactions are “symmetry allowed” 2279
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Figure 2. Frontier molecular orbitals of group 13 dimetallenes, group 14 dimetallynes, and olefins.
Scheme 2. Formal [2 + 2 + 2] Cycloaddition of Digallene with Simple Olefins11
and those that do not are “symmetry forbidden” and require significantly more energy to occur. The Woodward−Hoffmann rules also explain why photochemical excitation of olefins causes inversion of the FMO symmetry of one of the two olefin components, resulting in the occurrence of a “symmetry allowed” pericyclic reaction. Cycloaddition reactions of heavy group 13 alkene analogues with simple alkenes afford 1,4-digallacyclohexane structures (Scheme 2).11 If we consider the digallene to be a 2π equivalent, the reaction is a formal [2 + 2 + 2] cycloaddition. The first step in the reaction with simple olefins involves a simple [2 + 2] cycloaddition between two π bonds to afford a 1,2-digallacyclobutane, which is not isolated but which reacts quickly with another olefin equivalent to give the 1,4digallacyclohexane. Organic substrates rarely undergo thermally induced, symmetry-forbidden [2 + 2] cycloadditions. However, reactivity can be induced by using UV light to promote a ground-state electron to the LUMO, thus allowing the reaction to proceed. This is seen with the symmetry-forbidden [2 + 2] cycloaddition of alkenes to form a cyclobutane ring, if the system is irradiated. These reactions are often carried out with enone substrates, where one alkene component is activated by an adjacent ketone. 12 The first report of a [2 + 2] photocycloaddition reaction was in 1908 by Ciamician, who showed the formation of carvone camphor was possible upon exposure of carvone to sunlight for 1 year.13 It can be seen in Figure 3 that the HOMO of ethylenea π orbital of b3u symmetrymatches the LUMO of the digallenea π orbital of au symmetry (in the C2h point group)so that their interaction, which leads to the formation of the 1,2-digallacyclobutane ring, is symmetry-allowed. Our analysis of the shapes of the frontier orbitals involved in the formation of the 1,2-digallacyclobutane intermediate, in collaboration with the group of Tuononen,14 shows that the reaction is facilitated to an even greater degree if the conformation of the digallene is changed from trans to cis, as shown in Figure 3. The energy required for this conformational change is compensated for by formation of the two Ga−C bonds. In this case, not only are interactions of the HOMOs of ethylene and the LUMO of the cis (or trans) digallene allowed by symmetry, the interaction of the LUMO of ethylene (b2g symmetry) and HOMO of the cis-digallene (b2 symmetry) is also favorable, whereas the interaction with the HOMO of the trans-digallene is less strongly favored for spatial reasons. This
Figure 3. Key frontier orbitals of ethylene and digallene along with a schematic description of the key orbital interactions involved in the formation of the proposed 1,2-digallacyclobutane intermediate. Orbital symmetries for each molecule are given according to their respective point groups.
can be seen by inspection of the orbital pictures in Figure 3, where the HOMO of the cis-digallene not only matches the symmetry of the ethylene LUMO but also resembles the orbital pattern of the occupied b orbital of the 1,2-digallacyclobutane. The second step is the cycloaddition involving a σ bond and an additional 1 equiv of the alkene. This transformation is rare in organic systems, as the energy penalty is high. There are transition-metal-catalyzed examples of [2 + 2 + 2] cyclization reactions in all-carbon systems; however, most often these occur when the 2π reacting partners are alkynes, forming aromatic systems.15 To the best of our knowledge, there are no examples of cyclotrimerization reactions that furnish a saturated cyclohexane ringanalogous to the 1,4-digallacyclohexane ring formed in the case of the group 13 dimetallene. This would require breaking three C−C double bonds and forming three C−C single bonds. Overall, cleavage of the GaGa double bond in the digallene costs only ∼9 kcal/mol−1, and breaking the π bonds in the two alkenes may be estimated to cost ca. 130 kcal/mol−1. However, four new Ga−C single bonds (Ga−C BDE = 68 kcal/mol−11a) are formed in the overall reaction with 2 equiv of ethylene, which easily compensates for breaking the 2280
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Ga−Ga and C−C π bonds. Thus, the reaction (Scheme 1) is a thermodynamically favorable process, even allowing for the unfavorable entropy change. Nonetheless, the reverse of these processesthe uncatalyzed thermal [2 + 2 + 2] cycloreversion of a cyclohexane to three olefinsis known.16 Inspection of Figure 3 shows that the π HOMO of ethylene matches the symmetry of the LUMO of the 1,2-digallacyclobutane, so that the addition of the second equivalent of ethylene is also symmetry allowed. An interaction between the ethylene LUMO and the HOMO-1 orbital of the 1,2digallacyclobutane is also symmetry allowed, but it is unclear at present if this interaction is important. A significant steric effect was observed in these reactions, likely due to the extreme steric bulk of the terphenyl ligands used to stabilize the reactive gallium centers. No internal olefins reacted with digallene (i.e. trans- or cis-2-butene). When digallene reacted with excess 1-substituted alkenes, the resultant 1,4-digallacyclohexane ring was found as a twist-boat conformer with ethylene and was exclusively in a chair conformation when R was larger than H (R = CH3, C6H5, C4H9). In the case where R was CH3, we observed a mixture of equatorial and axial isomers in the solid state in a 46% to 54% ratio. In solution the ring was flexible and a major and minor isomer were observed by 1H NMR spectroscopy, but the signals could not be explicitly assigned. As the size of the R group was increased to Ph, only the equatorial isomer was observed, both in solution by 1H NMR spectroscopy and in the solid state. Recent calculations14 indicate that the reaction mechanism proceeds by an initial cyclization reaction that occurs after isomerization from trans-digallene to cis-digallene (an energy penalty of only ca. 5 kcal mol−1) to furnish an intermediate 1,2digallacyclobutene species, which could not be isolated experimentally. This first step is essentially spontaneous under ambient conditions, as a small energy was calculated for the transformation of
