Title: Odd-Electron Bonds and Biradicals in Main Group Element Chemistry
Abstract: Make or break: Stable biradicaloid main group element compounds could provide deep insight into fundamental processes of bond making and bond breaking. Whereas for a long time such compounds were only known as highly reactive intermediates or postulated as transition states, recently through the isolation of the stable crystalline 1,3-diphospha-2,4-diborabicyclo[1.1.0]butane (1) a decisive step has been taken towards achieving a better understanding of the steps involved in making and breaking bonds. Radicals play a crucial role in bond-forming and bond-breaking processes.1 In hydrocarbon chemistry, however, carbon-centred radicals are usually observed as rather short-lived intermediates.2 Some selected recent examples serve as highlights to demonstrate that in main group element chemistry (apart from C) such reactive intermediates can be stabilized even to such an extent that they can be isolated in crystalline form. To study the generation of a new type of a genuine two-center, one-electron (2c1e) PP bond, Geoffroy, Mathey, Le Floch et al.3 developed the macrocyclic system 1. Compound 1 contains two phosphinine units, PC5R5, which are linked by two flexible siloxane bridges to form a twelve-membered heterocycle. In the ground state conformation, the two phosphinine ring planes are almost coplanar and the two phosphorus atoms approach each other to a distance of 3.26 Å (Scheme 1). Generation of a 2c1e PP bond (top) and 2c3e SS bond (bottom). This neutral precursor can be reversibly reduced at a potential of E=−1.85 V and E=−2.10 V (scan rate 5 V s−1, THF) to give the radical anion 2 and the dianion 3, respectively. Chemically, 1 was reduced with sodium naphthalenide. The combination of the results from X-ray analyses of the neutral precursor 1 and the fully reduced dianion 3 with the results from EPR spectroscopy and DFT calculations for the persistent phosphorus radical anion 2, demonstrated that in this case the unpaired electron resides in the single-occupied molecular orbital (SOMO; Scheme 1). This SOMO is partially delocalized over both phosphinine rings, but mainly localized in a PP σ bond. Notably, the PP distance in 2 has shortened considerably and lies between that of the neutral compound 1 and that of the fully reduced form 3, in which a 2c2e PP bond has formed and the phosphinine rings deviate strongly from planarity. The formation of the PP bond in 2 and 3 resembles the creation of SS bonds in the intensively studied radical cations of type II and dications of type III of polysulfur compounds (Scheme 1, bottom).4 The chemical process of bond making and the electronic configuration is different in these sulfur compounds. In the neutral precursor I, the nonbonding electron pairs of the two S entities interact to give a bonding and antibonding combination. One-electron oxidation leads to the radical cation [>S…︁S<].+ II with a 2c3e bond.5 Note that although the bond orders in 2 and II are the same (b.o.=0.5), the single electron occupies an SS antibonding orbital in II but a PP bonding orbital in the radical anion 2. Often, the radical cations II have lower oxidation potentials than their precursors which make their direct observation difficult. In the above-mentioned radicals, one unpaired electron is delocalized over two main group element centers forming either a 2c1e or 2c3e bond. Even closer to bond-forming and bond-breaking processes are biradicals,6 which in many cases are more appropriately designated as biradicaloids.7 These may be roughly divided into two classes: 1) delocalized biradicals (non-Kekulé molecules)8 for which trimethylenemethane (TMM)9a and tetramethyleneethane (TME)9b are the archetypal examples; 2) localized biradicals with two well-defined radical substructures that are not conjugated by way of a classical π system. Typical examples of this class are cyclopentane-1,3-diyl (CP(1,3))10 or cyclobutanediyl (CB).11 In all of these radicals the single electrons reside in π-type orbitals and the individual spins may either couple to give a triplet (TMM, TME) or a singlet state (CP(1,3)). The triplet states of CBs could be observed because the barrier for spin conversion impedes the ring closure to the corresponding bicyclo[1.1.0]butanes. On the other hand, singlet cyclobutanediyls are only predicted as transition states for the ring inversion of [1.1.0]cyclobutanes at about 50 kcal mol−1 (Scheme 2).12 Formulas of the delocalized biradicals trimethylenemethane (TMM), tetramethyleneethane (TME) (non-Kekulé molecules) and localized cylopentane-1,3-diyl (CP(1,3)) and cyclobutanediyl (CB). Strained Group 14 element compounds were identified as suitable candidates for the synthesis of stable biradicaloids (Scheme 3).13 For the silicon, germanium, tin, and lead analogues of bicyclo[1.1.0]butanes, quantum-chemical calculations13b, 14a–14c predict the phenomenon of bond stretch isomerism,15 that is two distinct minima on the potential energy surface (PES) which mainly differ by the length of one bond, as a result of the high ring strain and intrinsically low σ-bond energies. For the silicon and germanium compounds, the bond stretch isomers 4 and 5, which can be regarded as coupled π-type biradicaloids, are more stable by about 8 and 16 kcal mol−1, respectively, than the isomers 4′ and 5′ which possess usual EE bond lengths. For E=Sn, Pb only the isomers 6 and 7 with an elongated EbEb bond between the bridgehead atoms are found as minima. Calculated bridgehead EbEb bond lengths [Å] and interplane angles φ [°] for tetrametallabicyclo[1.1.0]butanes of Group 14. Synthetically accessible and structurally characterized tetrasilabicyclo[1.1.0]butanes are rare. The sterically encumbered derivative 8 has a normal SiSi bond length of 2.37 Å (Scheme 4).16 This is in accord with computations which predict that σ-electron-donating groups, for example tBu, at the bridgehead silicon atoms favor the closed form, while σ-electron-withdrawing substituents, for example F, at the bridging silicon atoms favor the open form.13b, 14a However, the physical and chemical properties of 8 indicate the energetically close relationship to the biradicaloid form. Crystals of 8 are thermochromic (intensely yellow at 170 °C, colorless at −196 °C) and the ring-inversion barrier for 8 is low (Ea≈15 kcal mol−1), as estimated by NMR methods from the coalescence of the inequivalent R groups. Also, the central SibSib bond is unusually reactive; even water can be quantitatively added to give the hydroxy derivative 9. A similar observation was made for the silyl-substituted tetrasila[1.1.0]bicyclobutane derivative 11, which was generated from the tetrasilacyclobutene derivative 10 under irradiation with light (λ>420 nm).17 In the photostationary state, 91 % conversion was reached, and 11 was quenched by hydrolysis at 0 °C to give 12. No further spectroscopic data were obtained for 11 because in the dark it rearranges quantitatively to the cyclobutene 10 (Scheme 4). Remarkably, this isomerization cycle can be repeated more than 10 times without notable side reactions. In contrast to these results, computations for the parent system predict that the bicyclobutane isomer should be thermodynamically more stable.14d On the other hand, such rearrangements, biradicaloid⇌cyclobutenoid, have some common character and predictions concerning the relative stabilities of the individual isomers may be highly substituent dependent (for another example see 20⇌23 in Scheme 7). Experimentally studied tetrasilabicyclo[1.1.0]butanes 8 and 11. The inequivalent R groups that are interconverted by ring inversion are indicated by white and gray circles. To our knowledge no other stable molecular metallabicyclobutanes of the heavier Group 14 elements have been reported to date; however, an interesting example of bond stretch isomerism and indications for biradicaloids in solid-state chemistry has been reported for the Zintl phase Ba3Ge4. This system is prepared from a stoichiometric reaction of the elements between 1120 and 1360 K.18 In the high-temperature β-modification only isolated [Ge4]6− ions with the butterfly bicyclo[1.1.0]butane structure are formed. The bridgehead GebGeb bond is long (2.71 Å), although shorter than the computationally predicted bond length of 3.03 Å. In the room-temperature stable α-modification, one half of the [Ge4]6− units exist as isolated ions with short GebGeb bonds (2.58 Å) that are aligned along the crystallographic b axis (intermolecular GeGe contacts: 3.65 Å). The other half forms a polymeric [Ge4] chain along the a axis. In this polymeric anion, the intermolecular GeGe bonds of 2.87 Å replace the intramolecular GebGeb bonds which are elongated to 3.27 Å. As indicated in Scheme 5, neither the number of GeGe bonds nor the charges change during the polymerization process nor the bivalent and trivalent germanium atoms are interchanged. Quantum-chemical considerations show that the intermolecular GebGeb bonds in the polymeric anion have considerable biradicaloid character. Surprisingly, although all calculations indicate a diamagnetic ground state a sizable temperature-dependent paramagnetism was observed which leaves the question open whether this is due to accessible triplet states or to impurities in the sample. Section of the anionic substructure of the α- and β-modification of Ba3Ge4. The long intermolecular contacts between the isolated [Ge4]6− ions represented by dotted lines. In the polymeric anion [Ge4] these distances are shortened and replace the elongated bridgehead GebGeb bonds. As one might expect, when the intrinsic σ-bond strength is weakened by taking an element from a higher period and the number of annulated three-membered rings is increased, the open biradicaloid form is stabilized even more. This was impressively demonstrated by Sita and Kinoshita who succeeded in isolating the pentastanna[1.1.1]propellane 14 (31 % yield) and the derivative 15 (1 % yield) from the reduction of the cyclotristannane 13 with lithium in THF (Scheme 6).19 Synthesis and reactivity of pentastanna[1.1.1]propellanes. The experimentally determined distances (X-ray analyses) between the bridgehead tin atoms Snb are 20 % longer than a regular SnSn single bond. That 14 and 15 have considerable singlet biradicaloid character is further corroborated by the ease by which both compounds can be reduced stepwise to give the radical anions 14.− and 15.− (E≈−1.4 V versus NHE) and the dianions 142− and 152− (E≈−1.9 V versus NHE), respectively. The formally electron-deficient bridgehead tin centers (seven valence electrons) are converted into eight-valence-electron-configured (SnR2)3Sn− entities. Also, 14 showed some unusual reactivity; for example, MeLi or MeI are cleanly added to the central SnbSnb bond to give 16 and 17, respectively (Scheme 6). Extensive ab initio calculations performed for E5[1.1.1]propellanes (E=C, Si, Ge, Sn),20 support the idea that although a bond-critical point indicative of a classical covalent bond can be found only for the carbon compound, there is a strong through-space interaction between the bridgehead centers in the σ-type biradicaloids for E=Si–Sn. The first biradicaloid species prepared in gram quantities (yields >60 %) were the 1,3-diphosphacyclobutane-2,4-diyls 20 and 21. These were isolated when two equivalents of the C-dichlorophosphaalkenes 18 or 19, respectively, were allowed to react with one equivalent of nBuLi at −100 °C (Scheme 7).21 The ring skeleton of these highly unusual P2C2 heterocycles is planar but both the carbon and the phosphorus centers are embedded in pyramidal coordination spheres. The comparatively high inversion barriers at phosphorus impede the formation of a planar 6π-conjugated heterocycle (like in the isoelectronic S2N2)22 and 20 and 21 have a high biradicaloid character. On the other hand, the coordination sphere at the phosphorus atoms is less pyramidal than in common phosphanes, which indicates some degree of π-donation from the phosphorus lone pairs to the carbon radical centers. Syntheses of 1,3-diphosphacyclobutan-2,4-diyls (Niecke biradicals). Simplistically, the electronic ground state may be approximated by the resonance structures A and B shown in Scheme 7 and diphosphacyclobutane-2,4-diyls may be described as weakly π-conjugated (delocalized) biradicaloids. Recent calculations by Schoeller et al. show that especially the substituents at the radicaloid carbon centers influence the electronic configuration of these Niecke biradicals; for example, C-bonded silyl groups (σ donor, π acceptor) stabilize the singlet state, while the combination, NR2 group at phosphorus and alkyl group at carbon, lowers the energy of the triplet state, thus providing the opportunity to easily control the biradical spin state.21e The pioneering preparative work to these biradicaloids was performed by Niecke et al. While the P-amino derivative 20 is unstable at room temperature and isomerizes rapidly and quantitatively in solution, and slowly even in the solid state, to the 1,2-dihydrodiphosphete 23, the Mes*-substituted compound 21 is stable up to 150 °C. The chloro substituents in 21 can be exchanged for a silyl group and a H atom to give 24. This compound can then be deprotonated by lithium di(isopropyl)amide (LDA) to give an unusual anionic carbene which subsequently reacts with AlMe3 to give the anionic Niecke biradical 25. The symmetry of the HOMO shown in Scheme 7 does not allow a thermal ring closure of the open P2C2 heterocycles to the thermodynamically more stable valence isomer 2,4-diphosphabicyclo[1.1.0]butane; however, photolysis leads almost quantitatively to 26. Biradical structures are also found among the more than 200 possible valence isomers of the formula C6H6. For the Cope rearrangement of the bicycloprop-2-enyl BCP one such structure is proposed as a transition state TSBCP (Scheme 8).23 Replacement of the four carbon atoms in the central four-membered ring by phosphorus atoms leads to compounds of type 29 which are no longer transition states but are stable.24 Bertrand et al. synthesized 29 from the reaction of the 1H-diphosphirene 27 with a catalytic amount of NEt3⋅BF3, and isolated it in the form of red crystals in 45 % yield. It seems likely that the diphosphirenyl radical 28 is generated as the intermediate in this reaction. According to calculations by Schoeller the unpaired electron in 28 is equally distributed over both phosphorus atoms and localized in the π* orbital of the PP bond. It is hence understandable that dimerization of 28 occurs through a π*–π* interaction25 and leads to the observed product 29. The central P4 framework can be best described as a 4c6π-electron system in which four π electrons are hosted in two PP bonds within each CP2 ring and the remaining two π electrons are localized in the HOMO (Scheme 8). Thus formally the two CP2 units are held together by two one-electron PP bonds, as indicated by the formula given in Scheme 8. This description is consistent with the observation that the intra-ring PP bonds are quite short, while the inter-ring PP bonds are very long (c.f. 2 in Scheme 1). A further inspection of the electronic properties of compounds of type 29 by quantum-mechanical calculations24, 26 predict that they can be transformed into genuine biradicals like 29′ by compression of the central P4 ring along the long PP vectors. This could be achieved either by changing the amino groups at the ring carbon atoms for alkyl or especially aryl groups, whereby the unpaired electrons will localize at the C-carbon atoms.26 It may also be possible to achieve this transition by applying pressure on the system.26 These exciting possibilities await, however, experimental verification. Cope rearrangement of bicycloprop-2-enyl BCP⇌BCP′ and synthesis of the stable diphosphirenyl dimer 29. As the icing on the cake, Bertrand, Schoeller et al. have recently successfully synthesized the crystalline, localized singlet, boron-centered biradicaloid 31 (Scheme 9).27 The (RP)2(CR)2 motif of the Niecke radicals was replaced by the isoelectronic (R2P)2(BR)2 unit (both contain 22 valence electrons for R=H). This had two important consequences for the stability: 1) the contribution of resonance structure B (Scheme 7) to the electronic ground state is largely diminished because the phosphorus lone pair has been transformed into a PC σ bond; 2) the heterocycle in 31 is expanded because of the intrinsically longer PB bonds (1.89 Å; the PC bonds in 20, 24, and 25 are 1.73–1.76 Å). Both factors favor an open singlet-biradical form. Synthesis of the Bertrand biradical 31. The synthesis of 31 proved to be simple: reaction of the 1,2-dichlorodiborane(4) 30 with two equivalents of the secondary lithium phosphide LiP(iPr)2 furnished 31 as yellow, air-sensitive but thermally highly stable (>200 °C) crystals in 63 % yield. The X-ray structure analysis revealed a perfectly planar P2B2 ring in which the transannular BB distance (2.57 Å) is 38 % longer than the longest BB bond reported so far.28 Interestingly, ab initio calculations show that for the parent molecule (H2P)2(BH)2 the planar form would be the transition state at 16.4 kcal mol−1 for the inversion of the 1,3-diphospha-2,4-diborabicyclo[1.1.0]butane 33. Note also that the symmetry of the HOMO shown in Scheme 9 allows a thermal disrotatorial electrocyclic ring closure. Although the mechanism is not known in detail, it can be assumed that the 1,2-diphosphanyldiborane(4) 32 should be one of the initial reaction intermediates. This structure, likely in a staggered conformation, should rapidly rearrange into the 1,3-diphospha-2,4-diborabicyclo[1.1.0]butane 33 which contains enough potential energy from the bulky groups to spring apart to give 31. In other words, the choice of the sterically demanding substituents that can be comfortably accommodated in the planar form of 31 allowed the crystallization of a strictly iso(valence)electronic and isostructural transition state analogue for the bicyclo[1.1.0]cyclobutane, C4H6, inversion. Using other substituents, it should be possible to adjust the BB distance such that any conformation on the internal reaction coordinate (IRC) for the bicyclo[1.1.0]butane inversion can be investigated. Futhermore, such studies on the transition of biradicaloids to biradicals (where 100 % unpaired spin density would be confined to one radical site) would allow fundamental deep insights into the coupling of electronic spins. Clearly, biradical(oid)s with main group elements are intrinsically more stable than their hydrocarbon counterparts. And many combinations are offered by the periodic system! Furthermore, the properties of biradicaloids can be fine-tuned by varing the substituents. Generally, the potential energy surfaces containing isomers of main group element compounds are much flatter than for comparable hydrocarbons. This facilitates the control over their distribution and mutual interconversions within a reasonable temperature range. From a purely synthetic point of view, the introduction of sterically demanding but electronically very different substituents is often easier to achieve in main group element chemistry than in classical hydrocarbon chemistry (simple salt metathesis reactions were involved in most of the above-cited examples, EHal + R−→ER + Hal−). Quantum-chemical calculations proved to be a highly valuable tool to guide the synthetic efforts for bi- and polyradicals, for the design of the electronic configuration, and finally for the development of new materials. As Berson wrote:6c “In this domain of chemistry one has to expect the unexpected and to be ready to find it.” Hopefully, this highlight will stimulate further interest and research efforts in this direction.
Publication Year: 2002
Publication Date: 2002-11-04
Language: en
Type: article
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 186
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