Английская Википедия:Group 13/15 multiple bonds

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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 p𝝅 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 (Ga=N), boron and phosphorus (B=P), or boron and arsenic (B=As) are less common.

Synthesis

Файл:(P(R)=BMes2Li(Et2O)2) 2.png

P(R)=BMes₂Li(Et₂O)₂ (R = phenyl, cyclohexane, mesitylene)

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. [(tmp)B=P(Ar)] (tmp= 2,2,6,6,-tetramethylpiperidina, Ar= 2,4,6-t-Bu₃C₆H₂) was characterized by mass spectroscopy (EI MS), and the corresponding dimer, diphosphadiboretane, was characterized by X-ray crystallography. The Power and co-workers later reported the structure of [P(R)=BMes₂Li(Et₂O)₂] (R = phenyl, cyclohexane, and mesitylene), which is the first B=P double bond observed in solid state. The synthesis of [P(R)=BMes₂Li(Et₂O)₂] 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

Файл:Cycle-R2PB(R')-B(R')-P(Ph)2.png

Photo-induced isomerization of cycle-[(iPr)₂PB(tBu)-B(tBu)-P(Ph)₂]

Isomerization of four-member P-B cycles was investigated by Bourissou and Bertrand. It was reported that cycle-[R₂PB(R')-B(R')-P(Ph)₂] (R = phenyl, isopropyl; R'= tert-butyl, 2,3,5,6-tetramethyl phenyl) isomerize to form cycle-[R₂P-B(R')=P(Ph)-B(R')(Ph)] 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 CyP(H)Li.

Файл:6-membered P=B.png

Synthesis of a borazine analogue containing P=B bonds

Файл:5-membered P=B 2.png

Synthesis of a benzodiphosphaborolediide

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 [As(Ph)=BMes₂Li(THF)₃] with two equivalence of TMEDA yielded [As(Ph)=BMes₂][Li(TMEDA)₂]. Ring-systems containing As-B multiple bonds haven't been reported yet.

Файл:Al=N, Ga=N, In=N 2.png

Synthesis of [{^DipNacnc}M=N-^TipTer] (M=Al, Ga) and [(^DipTer)M=N-^Mes'Ter] (M=Ga, In)

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 [2+3] cycloaddition of monomeric [^DipNacnc]Al or [^DipNacnc]Ga (^DipNacnc= HC{(CMe)(NDip)}₂) compound with sterically bulky azide, ^TipTerN3 (^TipTer = -C₆H₃-2,6-(C₆H₂-2,4,6-iPr₃)₂), gives the iminotrielenes [{^DipNacnc}M=N-^TipTer] (M=Al, Ga). Additionally, dimers of Ga(I) or In(I) were reported to form the iminotrielens [(^DipTer)M=N-^Mes'Ter] with ^Mes'TerN₃ (M = Ga, In; ^Mes'Ter =C₆H₃-2,6(Xyl-4-tBu)₂).

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 [iPr₂^TIPTerAl≡NR] (R = Ad, SiMe₃) was not capture but further rearrange to tetrazole and amino-azide alone, respectively.

Файл:Pn=Al 2.png

Synthesis of ^DipTerPnAlCp* (Pn = P, As)

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(I) precursors, [Al(I)Cp*]₄ (Cp* = pentamethylcyclopentadiene). Reacting [Al(I)Cp*]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)

Файл:Ga=As.png

^DipNacnc(Cl)Ga}}₂Sb

Synthesis and characterization of Ga=Sb species was reported by Schulz and Cutsail III with the reaction of [^DipNacnc]Ga (^DipNacnc= HC{(CMe)(NDip)}₂) with [Cp*SbCl₂]. The resulting Sb radical species, [^DipNacnc(Cl)Ga]₂Sb, was then reduced by KC₈ to give [^DipNacncGa=Sb-Ga(Cl)^DipNacnc]. Utilizing the similar reaction pathway, a Ga=As species, [^DipNacncGa=AsCp*], 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 [(HC)₂(NDip)₂PPCO] with [^DipNacncGa] gave the phosphagallene, [^DipNacncGa=P-P(NDip)₂(CH)₂].

Reactivities

Файл:C-F activation by B=P 2.png

C-F activation of tris(pentafluorophenyl)borane by [(tmp)(L)B=PMes*] (L = IMe₄)

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, [(tmp)(L)B=PMes*] (L = IMe₄), 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 [Mes*P=B(tmp)] give an analogue of cycle-butene, [Mes*P=C(Ph)-C(H)=B(tmp)], where C-C triple bond undergoes a [2+2]-cycloaddition to P=B double bond.

Phospha-bora Wittig reaction

Файл:Phospha-bora Wittig 2.png

Phospha-bora Wittig reaction

Transient boraphosphene [(tmp)B=PMes*)] (tmp = 2,2,6,6-tetramethylpiperidine, Mes* = 2,4,6-tri-tert-butylphenyl) reacts with aldehyde, ketone, and esters to form phosphaboraoxetanes, which converts to phosphaalkenes [Mes*P=CRR'] and [(tmp)NBO]_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

Файл:PPGA EH activation 6.png

Polar bonds activation by [^DipNacnc(RN)Ga-P-P(H)(NDip)₂(CH₂)₂]

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 [^DipNacncGa=P-P(NDip)₂(CH₂)₂] 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 [^DipNacncGa=P-P(NDip)₂(CH₂)₂] toward amines, phosphines, alkynes resulted in the formation of [^DipNacnc(E)Ga-P-P(H)(NDip)₂(CH₂)₂]. 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, [(Mes*)P=BClCp*]^-, suggested that the B-P double bonds are polarized to the P atom. The B=P 𝝈-bond is mostly non-polar while the 𝝅-bond is polarized to the phosphorus (71%). DFT calculation at B3LYP/6-31G level revealed that the HOMO of [(Mes*)P=BClCp*]^- has great B-P 𝝅-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.

Selected NMR chemical shifts (ppm) and bond length (pm) of anionic compounds with B=P bond

Compound	11B NMR	31P NMR	d(B-P)
[P(Cy)=BMes2Li(Et2O)2]	65.6	70.1	183.2(6)
[P(Mes)=BMes2Li(Et2O)2]	63.7	55.5	182.3(7)
[P(Ad)=BMes2Li(Et2O)x]	85.7	90.4	182.3(8)
[P(tBu)=BTip2Li(Et2O)2]	58.9	113.2	183.6(2)
[P(SiMe3)=BMes2Li(THF)3]	71.7	-49.2	183.3(6)

Selected NMR chemical shifts (ppm) and bond length (pm) of Lewis acid/base stabilized compounds with B=P bond

Compound	11B NMR	31P NMR	d(B-P)
[Cr(CO)5{(tmp)B=PC(Et)3}	62.9	-45.3	174.3(5)
[AlBr3{(tmp)B=P(tBu)}]	68.4	-59.8	178.7(4)
[(tmp)(DMAP)B=PTipTer]	41.2	57.3	180.92(17)
[Cp*(DMAP)B=PMes*]	52.3	96.7	179.5(3)
[Cp*(IMe4)B=PMes*]	48.5	192.9	180.67(15)
[Cp*B(Br)=PMes*][IiPrSiMe3]	54.9	75.2	180.39(16)
[(tmp)(DMAP)B=PMes*]	44.5	64.0	182.11(16)
[(tmp)(IMe4)B=PMes*]	43.9	151.5	183.09(16)

Ga-Pn double bond

Natural bond orbital analysis was reported for Ga=Sb and Ga=Bi containing species, where electron populates more on Sb and Bi (62% and 59%, respectively). The Lewis acidic Ga results in the delocalization of electrons in Sb and Bi.