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The 2‐aryl‐3,4,5,6‐tetraphenyl‐1,2‐azaborinines 1‐EMe\(_{3}\) and 2‐EMe\(_{3}\) (E=Si, Sn; aryl=Ph (1), Mes (=2,4,6‐trimethylphenyl, 2)) were synthesized by ring‐expansion of borole precursors with N\(_{3}\)EMe\(_{3}\)‐derived nitrenes. Desilylative hydrolysis of 1‐ and 2‐SiMe\(_{3}\) yielded the corresponding N‐protonated azaborinines, which were deprotonated with nBuLi or MN(SiMe\(_{3}\))\(_{2}\) (M=Na, K) to the corresponding group 1 salts, 1‐M and 2‐M. While the lithium salts crystallized as monomeric Lewis base adducts, the potassium salts formed coordination polymers or oligomers via intramolecular K⋅⋅⋅aryl π interactions. The reaction of 1‐M or 2‐M with CO\(_{2}\) yielded N‐carboxylate salts, which were derivatized by salt metathesis to methyl and silyl esters. Salt metathesis of 1‐M or 2‐M with methyl triflate, [Cp*BeCl] (Cp*=C\(_{5}\)Me\(_{5}\)), BBr\(_{2}\)Ar (Ar=Ph, Mes, 2‐thienyl), ECl\(_{3}\) (E=B, Al, Ga) and PX\(_{3}\) (X=Cl, Br) afforded the respective group 2, 13 and 15 1,2‐azaborinin‐2‐yl complexes. Salt metathesis of 1‐K with BBr\(_{3}\) resulted not only in N‐borylation but also Ph‐Br exchange between the endocyclic and exocyclic boron atoms. Solution \(^{11}\)B NMR data suggest that the 1,2‐azaborinin‐2‐yl ligand is similarly electron‐withdrawing to a bromide. In the solid state the endocyclic bond length alternation and the twisting of the C\(_{4}\)BN ring increase with the sterics of the substituents at the boron and nitrogen atoms, respectively. Regression analyses revealed that the downfield shift of the endocyclic \(^{11}\)B NMR resonances is linearly correlated to both the degree of twisting of the C\(_{4}\)BN ring and the tilt angle of the N‐substituent. Calculations indicate that the 1,2‐azaborinin‐1‐yl ligand has no sizeable π‐donor ability and that the aromaticity of the ring can be subtly tuned by the electronics of the N‐substituent.
A cyclic alkyl(amino)carbene (CAAC)‐stabilized dicoordinate aminoborylene is synthesized by the twofold reduction of a [(CAAC)BCl\(_{2}\)(TMP)] (TMP=2,6‐tetramethylpiperidyl) precursor. NMR‐spectroscopic, X‐ray crystallographic and computational analyses confirm the cumulenic nature of the central C=B=N moiety. Irradiation of [(CAAC)B(TMP)] (2) resulted in an intramolecular C−C bond activation, leading to a doubly‐fused C\(_{10}\)BN heterocycle, while the reaction with acetonitrile resulted in an aryl migration from the CAAC to the acetonitrile nitrogen atom, concomitant with tautomerization of the latter to a boron‐bound allylamino ligand. One‐electron oxidation of 2 with CuX (X=Cl, Br) afforded the corresponding amino(halo)boryl radicals, which were characterized by EPR spectroscopy and DFT calculations. Placing 2 under an atmosphere of CO afforded the tricoordinate (CAAC,CO)‐stabilized aminoborylene. Finally, the twofold oxidation of 2 with chalcogens led, in the case of N\(_{2}\)O and sulfur, to the splitting of the B−C\(_{CAAC}\) bond and formation of the 2,4‐diamino‐1,3,2,4‐dichalcogenadiboretanes and CAAC‐chalcogen adducts, whereas with selenium a monomeric boraselenone was isolated, which showed some degree of B−Se multiple bonding.
A cyclic alkyl(amino)carbene‐stabilized 1,4‐diborabenzene (DBB) ligand enables the isolation of 18‐electron two‐legged parent piano‐stool Fe\(^{0}\) and Ru\(^{0}\) complexes, [(η\(^{6}\)‐DBB)M(CO)\(_{2}\)], the ruthenium complex being the first of its kind to be structurally characterized. [(η\(^{6}\)‐DBB)Fe(CO)\(_{2}\)] reacts with E\(_{4}\) (E=P, As) to yield mixed DBB‐cyclo‐E\(_{4}\) sandwich complexes with planar E\(_{4}\)\(^{2-}\) ligands. Computational analyses confirm the strong electron‐donating capacity of the DBB ligand and show that the E\(_{4}\) ligand is bound by four equivalent Fe−P σ bonds.
Cyclic alkyl(amino)carbene-stabilized (cyano)hydroboryl anions were synthesized by deprotonation of (cyano)dihydroborane precursors. While they display boron-centered nucleophilic reactivity towards organohalides, generating fully unsymmetrically substituted cyano(hydro)organoboranes, they show cyano-nitrogen-centered nucleophilic reactivity towards haloboranes, resulting in the formation of hitherto unknown linear 2-aza-1,4-diborabutatrienes.
The reactions of carbodiimides with the iron arylborylene complex [Fe=BDur(CO)\(_{3}\)(PMe\(_{3}\))] (Dur=2,3,5,6-Me\(_{4}\)C\(_{6}\)H) and the iron bis(borylene) complex [Fe{=BDur}{=BN(SiMe\(_{3}\))\(_{2}\)}(CO)\(_{3}\)] yield a wide variety of temperature-dependent products, including known FeBNC and novel FeBNB metallacycles, complexes of N-heterocyclic boracarbene and spiro-boracarbene ligands and a unique 1,3,2,4-diazadiborolyl pianostool complex, characterized by NMR spectroscopy and X-ray crystallography. The product distributions can be rationalized by considering sequences of cycloaddition, metathesis, insertion, and C−H activation pathways mainly governed by sterics.
Reduction of (CAAC)BBr\(_2\)(NCS) (CAAC=cyclic alkyl(amino)carbene) in the presence of a Lewis base L yields tricoordinate (CAAC)LB(NCS) borylenes which undergo reversible E/Z-isomerization. The same reduction in the absence of L yields deep blue, bis(CAAC)-stabilized, boron-doped, aromatic thiazolothiazoles resulting from the dimerization of dicoordinate (CAAC)B(NCS) borylene intermediates.
The parent borylene (CAAC)(Me\(_{3}\)P)BH, 1 (CAAC=cyclic alkyl(amino)carbene), acts both as a Lewis base and one-electron reducing agent towards group 13 trichlorides (ECl\(_{3}\), E=B, Al, Ga, In), yielding the adducts 1-ECl\(_{3}\) and increasing proportions of the radical cation [1]\(^{•+}\) for the heavier group 13 analogues. With boron trihalides (BX\(_{3}\), X=F, Cl, Br, I) 1 undergoes sequential adduct formation and halide abstraction reactions to yield borylboronium cations and shows an increasing tendency towards redox processes for the heavier halides. Calculations confirm that 1 acts as a strong Lewis base towards EX3 and show a marked increase in the B−E bond dissociation energies down both group 13 and the halide group.
The reduction of a cyclic alkyl(amino)carbene (CAAC)-stabilized organoberyllium chloride yields the first neutral beryllium radical, which was characterized by EPR, IR, UV/Vis spectroscopy and X-ray crystallography. DFT calculations show significant spin density at beryllium and confirm donor–acceptor bonding between an alkylberyllium radical fragment and a neutral CAAC ligand.
The reductive coupling of an NHC-stabilized aryldibromoborane yields a mixture of trans- and cis-diborenes in which the aryl groups are coplanar with the diborene core. Under dilute reduction conditions two diastereomers of a borirane-borane intermediate are isolated, which upon further reduction give rise to the aforementioned diborene mixture. DFT calculations suggest a mechanism proceeding via nucleophilic attack of a dicoordinate borylene intermediate on the aryl ring and subsequent intramolecular B-B bond formation.
Whereas the reduction of N-heterocyclic carbene (NHC)-stabilised cymantrenyldibromoboranes, (NHC)BBr\(_2\)Cym, in benzene results in formation of the corresponding diborenes (NHC)\(_2\)B\(_2\)Cym\(_2\), a change of solvent to THF yields a borylene of the form (NHC)\(_2\)BCym, stabilised through its boratafulvene resonance form.