Ge=B π‐Bonding: Synthesis and Reversible [2+2] Cycloaddition of Germaborenes

Abstract Phosphine‐stabilized germaborenes featuring an unprecedented Ge=B double bond with short B⋅⋅⋅Ge contacts of 1.886(2) (4) and 1.895(3) Å (5) were synthesized starting from an intramolecular germylene–phosphine Lewis pair (1). After oxidative addition of boron trihalides BX3 (X=Cl, Br), the addition products were reduced with magnesium and catalytic amounts of anthracene to give the borylene derivatives in yields of 78 % (4) and 57 % (5). These halide‐substituted germaborenes were characterized by single‐crystal structure analysis, and the electronic structures were studied by quantum‐chemical calculations. According to an NBO NRT analysis, the dominating Lewis structure contains a Ge=B double bond. The germaborenes undergo a reversible, photochemically initiated [2+2] cycloaddition with the phenyl moiety of a terphenyl substituent at room temperature, forming a complex heterocyclic structure with GeIV in a strongly distorted coordination environment.

The chemistry of boron-element compounds containing B À Em ultiple bonds is ac hallenging field of research. [1] Currently,t he chemistry of B À Bd ouble and triple bonds is in the focus of interest. [1k] In terms of B=Edouble bonds with elements of Group 14, compounds exhibiting ad ouble bond between boron and carbon were reported more than thirty years ago by Berndt, Nçth and Paetzold et al. (Scheme 1, A-C). [1p-r] Adouble bond between boron and silicon was reported in 2006 (Scheme 1, D). [1o] Sekiguchi and co-workers reacted a1 ,1-dilithiosilane [Li 2 Si(SitBu 2 Me) 2 ]w ith 2,2,6,6-tetramethylpiperidinedichloroborane and isolated the low-valent boron compound as yellow crystals. [1o] Theborasilene features aS i = Bd ouble bond of 1.8379 (17) .Achloride adduct of aborasilene (Scheme 1, E)was isolated after reduction of the addition product between acyclic dialkylsilylene and dichloromesitylborane.I nt he chloride adduct, the double bond between boron and silicon [1.859(2) ]isonly slightly longer than in example D. [1s] Ac arbene adduct of ab orasilene exhibiting alonger Si À Bdistance of 1.990 (2) was reported by Inoue and co-workers. [1x] We have recently reported the synthesis of ap hosphinestabilized digermavinylidene. [2] Its synthesis started from an intramolecular Lewis pair between ag ermylene and ap hosphine (1). [3] After addition of germanium dichloride,t he addition product was reduced with as trong reducing agent such as {( Mes Nacnac)Mg} 2 ,a nd the digermavinylidene was isolated in 52 %y ield [ Mes Nacnac = {[(Mes)NC(Me)] 2 CH} À , Mes = 2,4,6-Me 3 C 6 H 2 ]. [4] Herein, we report on the synthesis of phosphine-stabilized germaborenes. [1ag] Thei ntramolecular Lewis pair 1 was reacted with boron trihalides,a nd the oxidative addition products 2 and 3 were isolated in high yield and characterized (Scheme 2). In the 11 BNMR spectrum, the tetracoordinated boron atoms of adducts 2 and 3 lead to resonances at À0.3 (2) and À13.1 ppm (3). Theresonances of the phosphorus atoms were found at 3.9 (2)a nd 6.8 ppm (3)i nt he 31 PNMR spectrum. These 11 Ba nd 31 PNMR resonances can be compared with signals of phosphine adducts of boron trihalides or organoboron dihalides. [5] In Figure 1, the molecular structure of the addition product 2 in the solid state is depicted. Them olecular structure of the BBr 3 addition product 3,i ncluding crystallographic details,i sp resented in the Supporting Information. TheG e ÀBb ond lengths of 2 [2.095(5) ]a nd 3 [2.089 (2) ] lie in the range of interatomic distances found for BÀGe single bonds [2.033(6)-2.260(13) ]. [6] TheB À Pdistances in 2 [2.017(5) ]a nd 3 [1.979 (2) ]a re closely related to B À P distances found in phosphine adducts of boron trihalides [1.952(2)-1.970(2) ]. [5c,d] To reduce the addition products 2 and 3,r eactions with magnesium were studied. Thebest results were obtained after activation of the magnesium with dibromoethane and addition of catalytic amounts of anthracene. [8] Reductions were carried out in tetrahydrofuran at ambient temperature (Scheme 2). In the course of the reduction the colorless solution turned deep red, and the reduction products 4 and 5 were isolated as single crystals from ap entane solution. The lowest-energy absorption bands observed in the UV/Vis spectra were located at l max (4) = 528 nm and l max (5) = 520 nm. Them olecular structures in the solid state of 4 and 5 were determined by single-crystal X-ray diffraction, and in Figure 2, an ORTEP of 4 is shown.
TheGe À Bbond lengths of 1.886(2) (4)and 1.895(3) (5) are short distances between these elements and point towards ad ouble bond. Recently,A peloig,D riess,a nd co-workers have published ab orylsilylgermylene stabilized by as ilylene donor. [9] Fort his molecule,r esonance structures with partial Ge = Bcharacter were discussed, and crystal-structure analysis gave abond length of 1.971 (2) .Inthe molecular structures of germaborenes 4 and 5,t he observed short Ge À Bi nteratomic distances go along with trigonal-planar coordination at these atoms,w hich is manifested by the sum of the bond angles around the Ge and Batoms [4: S angles at Ge 359. 68 8,B 359.88 8;5:Ge359.68 8,B360.08 8]. This gives rise to acharacterization of this bond as aclassic double bond. In comparison to the starting materials 2 and 3,w ef ound slightly shorter B À P distances in the reduction products [4:1 .888(2), 5:1 .878-(3) ]. The 31 PNMR signals of 4 and 5 show only slight changes in chemical shift in comparison to the starting materials 2 and 3.I nt he 11 BNMR spectrum, the resonances for the reduction products were found as doublets at lower field at 17.3 ppm, 1 J PB = 132 Hz (4)a nd 10.3 ppm, 1 J PB = 134 Hz (5), corroborating the coordination number three; the coupling with the neighboring phosphorus atom was resolved as adoublet. To evaluate the electronic structures of the germaborenes 4 and 5,q uantum-chemical calculations were carried out (see the Supporting Information). [10] The density functional theory optimized geometries of both compounds 4 and 5 are in good agreement with the solidstate structures [Ge À Bd istance in 4:e xp.1 .886 (2)  In Figure 3, the HOMO,representing the p-bond between Band Ge of the bromo-substituted germaborene 5,isshown. Anatural bond orbital (NBO) analysis reveals an occupancy of 1.94 electrons for the Ge À B s-bond and of 1.67 electrons for the p-bond. [12] Both bonds are slightly polarized: s-bond Ge(44.2 %) À B(55.8 %) and p-bond Ge(42.3 %) À B(57.8 %). This leads to aW iberg bond index (WBI) of 1.51, indicating ac ovalent GeÀBb ond in line with GeÀBm ultiple bond character.N BO and NRT( natural resonance theory) analyses on model compounds 4* and 5*,w here Ar* and Ph have been replaced by Me,were also performed. Among all of the suggested structures,the dominating Lewis structure contains aG e = Bd ouble bond as depicted in Figure 3. Similar to the results for 5,t he BÀGe s-bonds reveal an occupancy of 1.97 (4*)and 1.96 (5*)electrons,and the occupancies of the BÀGe p-bonds are 1.77 (4*)and 1.78 (5*)electrons,togive anatural

Angewandte Chemie
Communications 3152 www.angewandte.org B À Ge bond order of 1.73 (4* and 5*). Aresonance structure of notable contribution considers p-bonding from ah alide lone pair that is donated to boron (see the Supporting Information). However,n or elevant contribution of > 1% was obtained for astructure that contains aboron-based lone pair as ad escription of base-stabilized borylenes would implement. This further corroborates the proposed efficient orbital overlap allowing for authentic Ge = B p-bonding.
Both germaborenes 4 and 5 were exposed to the light of amercury vapor lamp.Upon using this lamp with arelatively broad spectral range,only mixtures of the starting germaborene (4,5)and the product of cycloaddition (6, 7)were isolated (4/6:3 0/70 %; 5/7:1 0/90 %). Using al ight source with aw avelength of 530 nm, which is close to that of the lowestenergy absorption band, quantitative formation of the [2+ +2] cycloaddition products 6 and 7 was observed by NMR spectroscopy (Scheme 3). Ac omputationally predicted absorption spectrum by TDDFT of 4 is in good agreement with the experimental spectrum. [13] Thenature of the lowestenergy excitation around l = 520 nm is dominated by at ran-sition from the Ge=B p-orbital into the p-system of the phenylidene moiety (see the Supporting Information). Upon heating compounds 4 and 5 under the exclusion of daylight for 68 hat608 8C, no transformation of either compound into the corresponding cycloaddition product was observed by NMR spectroscopy.
In the 11 BNMR spectrum and in the 31 PNMR spectrum, new resonances for compounds 6 and 7 were seen. The 11 BNMR signals were observed as singlets at À4.5 (6)a nd À10.1 ppm (7), indicating tetracoordination at the boron atoms.H owever, coupling with the neighboring phosphorus atom was not resolved. Crystals suitable for X-ray diffraction were obtained for both cycloaddition products from hexane or benzene solutions.I nb oth cases,f our stereogenic tetracoordinate atoms Ge,B ,C 4, and C5 are formed upon ring formation. Compound 7 crystallizes in the space group P2 1 , and we observed only one enantiomer by single-crystal X-ray diffraction, whereas compound 6 crystallizes in the centrosymmetric space group P2 1 /n. [14] In Figure 4anORTEP of the molecular structure of 6 in the solid state is depicted. TheGe= Bd ouble bond of the germaborene has reacted with the double bond of aT rip moiety of aterphenyl substituent giving rise to ac omplex polyheterocyclic system.
Otherwise,w hen stored under the exclusion of daylight, compounds 6 and 7 are stable.E ven heating up to 60 8 8Ci n benzene solution does not result in any reconversion of 6 or 7. Cycloaddition reactions between alkenes and arenes are widely used in organic synthesis because of their high efficiency and selective stereochemistry. [15] In the case of main group element chemistry,s ome rare examples of [4+ +2] cycloaddition reactions between unsaturated main group element compounds and arenes have been published. Recently,S tephan and co-workers reported ar eversible intramolecular [4+ +2] cycloaddition of ap hosphaalkene with an arene ring. [16] Sakurai et al. published the [4+ +2] addition of ad isilene. [17] [4+ +2] Cycloaddition reactions with an unsaturated dialuminium species were presented by Power et al. and Tokitoh et al. [18] Furthermore,X ie and co-workers generated 1,3-dehydro-o-carborane and isolated the products of a[4+ +2] cycloaddition. [19] Thehigh reactivity of unsaturated elementboron compounds,B =Si, was documented by Iwamoto and co-workers with the CH activation of borasilene. [1s] Cycloaddition reactions of alkynes with homodinuclear multiple bonds of Group 13 and 14 element compounds have been reported for B, Al, Ga, Si, Ge,and Sn. [18b,20] Germaborenes 4 and 5 undergo areversible [2+ +2] cycloaddition with an arene moiety upon photoactivation. In main group element chemistry,t he reported reaction is ar are example of this type of cycloaddition.
To conclude,f ollowing ar eaction sequence of oxidative addition and reduction, halide-substituted germaborenes with the first authentic Ge=Bd ouble bond were synthesized in as traightforward fashion starting from an intramolecular, stabilized germylene-phosphine Lewis pair. Thee lectronic structure of the Ge = Bd ouble bond was studied by DFT calculations and analyzed by the NBO NRTmethod. Thehigh reactivity of the Ge = Bd ouble bond was manifested by the reversible intramolecular [2+ +2] cycloaddition with aC =C double bond of aphenyl substituent.