Group 13/15 multiple bonds
Heteroatomic multiple bonding between group 13 and group 15 elements are of great interest in synthetic chemistry due to their isoelectronicity with C-C multiple bonds. Nevertheless, the difference of electronegativity between group 13 and 15 leads to different character of bondings comparing to C-C multiple bonds. Because of the ineffective overlap between p? orbitals and the inherent lewis acidity/basicity of group 13/15 elements, the synthesis of compounds containing such multiple bonds is challenging and subject to oligomerization. The most common example of compounds with 13/15 group multiple bonds are those with B=N units. The boron-nitrogen-hydride compounds are candidates for hydrogen storage. In contrast, multiple bonding between aluminium and nitrogen Al=N, Gallium and nitrogen, boron and phosphorus, or boron and arsenic are less common.
Synthesis
Suitable precursors are crucial for the synthesis of group 13/15 multiple bond-containing species. In most successfully isolated structures, sterically demanding ligands are utilized to stabilize such bondings.Boraphosphenes (P=B)
Boraphosphenes, also known as phosphoboranes, was first reported by Cowley and co-workers in the 1980s. was characterized by mass spectroscopy, and the corresponding dimer, diphosphadiboretane, was characterized by X-ray crystallography. The Power and co-workers later reported the structure of, which is the first B=P double bond observed in solid state. The synthesis of starts from treating in-situ generated Mes2BPHR with 1 equivalent of t-BuLi in Et2O, followed by crystallization at low temperature.Cyclic system with P-B multiple bonds
Isomerization of four-member P-B cycles was investigated by Bourissou and Bertrand. It was reported that cycle- isomerize to form cycle- upon irradiation. An example of five-membered ring was reported by Crossley suggesting that a reaction of 1,2-diphosphinobenzene with n-BuLi and Cl2BPh yielded a benzodiphosphaborolediide. Several six-membered ring systems involving P=B double bonds have been reported. One of the example is an analogue of borazine synthesizing from MesBBr2 and CyPLi.Arsinideneborates (As=B)
A similar strategy to access litigated arsinideneborate was reported by Power and co-workers after the establishment of synthesizing litigated phosphinideneborates. Crystallizing with two equivalence of TMEDA yielded . Ring-systems containing As-B multiple bonds haven't been reported yet.Group 13 imides (Al=N, Ga=N, In=N)
Synthesis of group 13 imides usually starts with low valent group 13 species stabilized by bulky ligands. A cycloaddition of monomeric Al or Ga compound with sterically bulky azide, TipTerN3, gives the iminotrielenes . Additionally, dimers of Ga or In were reported to form the iminotrielens with Mes'TerN3.Al-N triple bonds
Transient Al≡N triple bond species were also investigated by reacting monomeric alanediyl precursor with organic azides. The unstable Al≡N triple bond species was not capture but further rearrange to tetrazole and amino-azide alone, respectively.Phosphaalumenes and Arsaalumenes (P=Al, As=Al)
The development of Al=P and Al=As species faced the difficulty due to the tendency of oligomerization of the lewis acidic Al and lewis basic P/As. In 2021, Hering-Junghans, Braunchweig, and co-workers reported the synthesis of phosphaalumens and arsaalumens with Al precursors, 4. Reacting 4 with DipTer-AsPMe3 or DipTer-AsPMe3 at 1:4 ratio yielded the corresponding phosphaalumens/arsaalumens, which are stable and isolable.Gallium-pnictogen double bonds (Ga=Pn)
Synthesis and characterization of Ga=Sb species was reported by Schulz and Cutsail III with the reaction of Ga with . The resulting Sb radical species, 2Sb, was then reduced by KC8 to give . Utilizing the similar reaction pathway, a Ga=As species,, was successfully synthesized and stabilized. Interestingly, no radical formation was observed comparing to the case of Ga=Sb species. With the rapid development of gallium pnictogen in the late 2010s, the first phosphagallene species was reported by Goicoechea and co-workers in 2020. The reaction of with gave the phosphagallene, .Reactivities
Reactivities of boraphosphenes
B=P double bond species has been studied for bond activation. For example, C-F activation of tris(pentafluorophenyl)borane by NHC-stabilized phosphaboranes,, was reported by Cowley and co-workers. The C-F bond activation takes place at the para position, leading to the formation of C-P bond. Reactions of phenyl acetylene with the dimer of give an analogue of cycle-butene,, where C-C triple bond undergoes a -cycloaddition to P=B double bond.Phospha-bora Wittig reaction
Transient boraphosphene reacts with aldehyde, ketone, and esters to form phosphaboraoxetanes, which converts to phosphaalkenes and x heterocycles. This method provides direct access of phosphaalkenes from carbonyl compounds.Reactivities of group 13 imides
Compounds with group 13-N multiple bonds are capable of small molecule activation. Reactions of PhCCH or PhNH2 with NHC-stabilized iminoalane result in the addition of proton to N and -CCPh or -NHPh fragment to Al. The reaction with CO leads to the insertion of CO between the Al=N bond.Reactivities of Ga=Pn species
Small molecule activation takes place across the P-P=Ga bonds in phosphanyl-phosphagallenes species, where the Ga=P species behave as frustrated Lewis pairs. For example, the reaction of CO2 with results in the formation of a P=P-C-O-Ga five-membered ring species. In contrast, H2 addition to the P-P=Ga fragment in a 1,3-activation manner. E-H bond activation of protic and hydridic reagents was investigated as well. Reactions of toward amines, phosphines, alkynes resulted in the formation of . Reversible ammonia activation was observed under 1 bar pressure in the presence of a Lewis acid.Bonding and structures
B=P double bond
Natural bond orbital analysis of a borophosphide anion, −, suggested that the B-P double bonds are polarized to the P atom. The B=P ?-bond is mostly non-polar while the ?-bond is polarized to the phosphorus. DFT calculation at B3LYP/6-31G level revealed that the HOMO of − has great B-P ?-bonding character. In most reported phosphinideneborates, the phosphorus chemical shifts are much more deshielded than the starting materials, phosphinoboranes. The down-field resonances of phosphorus in 31P NMR suggest the delocalization of lone pairs into the empty p-orbital of boron.| Compound | 11B NMR | 31P NMR | d |
| 65.6 | 70.1 | 183.2 | |
| 63.7 | 55.5 | 182.3 | |
| 85.7 | 90.4 | 182.3 | |
| 58.9 | 113.2 | 183.6 | |
| 71.7 | -49.2 | 183.3 |