Reactivity of a Platinum Iminoboryl Complex toward Lewis and Brønsted Acids

Cyclic (Alkyl)(Amino)Carbene-Iminoboryl Compounds with Three Formal Oxidation States

BN/CC isosterism is an effective strategy to build hybrid functional molecules with unique properties. In contrast to the alkynyl iminium salts derived from cyclic (alkyl)(amino)carbenes (CAACs) that feature only one reversible reduction wave, the isoelectronic cationic CAAC-iminoboryl adducts could be singly and doubly reduced smoothly. Both the resultant neutral radical and anionic azaborataallenes bear NBC-mixed allenic structures. The former radical has a high spin-density of 0.55e at CCAAC carbon, yet exhibits formal boron-centered radical reactivity. The latter azaborataallenes feature the nucleophilic CCAAC center and polar N(δ-)═B(δ+)═C(δ-) unit, and readily undergo nucleophilic substitution, isocyanide insertion, dipolar addition and cycloaddition reactions etc. The N-substituents have been shown to have a significant influence on the solid-state structure, thermal stability, and reactivity of azaborataallenes. This work showcases the allenic BN-unsaturated species as versatile building blocks in organic synthesis.

Acyclic Boryl Complexes of Copper(I)

Reaction of (6-Dipp)CuOtBu (6-Dipp=C{NDippCH2 }2 CH2 , Dipp=2,6-iPr2 C6 H3 ) with B2 (OMe)4 provided access to (6-Dipp)CuB(OMe)2 via σ-bond metathesis. (6-Dipp)CuB(OMe)2 was characterised by NMR spectroscopy and X-ray crystallography and shown to be a monomeric acyclic boryl of copper. (6-Dipp)CuB(OMe)2 reacted with ethylene and diphenylacetylene to provide insertion compounds into the Cu-B bond which were characterised by NMR spectroscopy in both cases and X-ray crystallography in the latter. It was also competent in the rapid catalytic deoxygenation of CO2 in the presence of excess B2 (OMe)4 . Alongside π-insertion, (6-Dipp)CuB(OMe)2 reacted with LiNMe2 to provide a salt metathesis reaction at boron, giving (6-Dipp)CuB(OMe)NMe2 , a second monomeric acyclic boryl, which also cuproborated diphenylacetylene. Computational interrogation validated these acyclic boryl species to be electronically similar to (6-Dipp)CuBpin.

Intramolecular C-N bond activation by a transient boryl anion

Using a flexible diamido framework, a bulky boron bromide has been prepared as a precusor to a boryl anion with an extremely wide N-B-N angle. Reduction of the compound with lithium metal resulted in intramolecular C-N bond activation and migration of an aryl group onto the boron centre. Reaction of the boron bromide with K[FeCp(CO)2] resulted in nucleophilic reactivity of a carbonyl oxygen and the cooperative activation of CO.

Iminoboranes With Parent B=NH Entity: Imino Group Metathesis, Nucleophilic Reactivity and N-N Coupling

While R2 C=N-R double bonds in organic imines are well established, the related iminoboranes R-B=N-R are either prone to oligomerization or can only be stabilized at sufficient steric congestion. In particular, the examples of unsubstituted parent B=N-H entity are rare. We demonstrate that the amino imidazoline-2-imine ligand system (HAmIm) not only gives rise to the isolation of a parent (AmIm)B=N-H iminoborane, but also to species of type (AmIm)B=N-SiMe3 with concomitant stabilization by lithium bromide. The double bond character in these systems is unambiguously corroborated by DFT calculations. The steric accessibility of the (AmIm)B=NH unit allows facile reactivity including metathesis reactions with C=O and C=S bonds, nucleophilic addition toward organic and organometallic carbonyl compounds, but also oxidative N-N coupling within a dimeric unit. Thus, the chemical behavior of the (AmIm)B=N-H and (AmIm)B=N-SiMe3 is distinctly different from that of organic imines.

BN Analogue of Butadiyne: A Platform for Dinitrogen Release and Reduction

Exploration of the metallomimetic chemistry of main group elements is of the utmost importance from the perspective of both fundamental research and potential applications. Here, we report the synthesis, bonding analysis, and reactivities of an isolable diiminoborane, Mes*B≡N─N≡BMes* (Mes* = 2,4,6-tri-tert-butylphenyl) (1), a BN analogue of butadiyne. This species is characterized by a conjugated B≡N─N≡B moiety, a structural feature that enables the controlled release of N2 when it is exposed to organic nitriles. Furthermore, the N2 unit in 1 could be reduced to an ammonium salt via cleavage of the BN triple bond. Our work shows a rare example of an unsaturated BN system, serving as a platform for both the release and reduction of N2. This discovery opens new pathways and holds substantial influence on the future design of functional main group N2 species.

Synthesis of Iminoboryl o-Carboranes by Lewis Base Promoted Aminoborirane-to-Iminoborane Isomerization

The iminoboryl o-carboranes (Me₃Si)-Cb-B≡N-R (Cb = B₁₀C₂H₁₀, 3a, R = SiMe₃; 3b, R = ^tBu) have been successfully synthesized by tetrahydrofuran (THF)-promoted isomerization from the corresponding o-carborane-fused aminoboriranes Cb{BN(SiMe₃)R} (2). The synthetic protocol of the previously reported borirane 2a was optimized. The borirane Cb{BN(SiMe₃)^tBu} (2b) and the iminoboranes 3a and 3b were fully characterized by NMR, IR, and single-crystal X-ray diffraction analyses. The borirane 2a isomerizes more readily than 2b. The kinetics study revealed a bimolecular mechanism between borirane and THF, which is in good agreement with the computationally proposed reaction pathway. The title compounds are thermally robust, but compound 3a dimerized in the presence of a catalytic amount of ^tBuNC to give the cyclodimer 4. Quick equilibrium between 4 and the isonitrile adduct 4·^tBuNC was observed in solution.

Boraiminolithium: An Iminoborane-Transfer Reagent

BN/CC isosterism can give rise to attractive molecules with unique physical or chemical properties. We report here the synthesis, characterization, and reactivities of the boraiminolithium species 2, a room-temperature-stable crystalline solid accessible through a facile dehydrohalogenation/deprotonation reaction. This species, bearing a polarized B≡N triple bond and an anionic N center, is the first example of a BN analogue to the well-known alkynyllithium molecules (lithium acetylides). It has demonstrated a remarkable ability for iminoborane-transfer reactions, which allows for the isolation of a series of unprecedented N-functionalized iminoboranes as well as novel main-group heterocycles. Stable boraiminolithium reagents may become powerful tools in the exploration of new BN-containing building blocks for synthetic chemistry and materials science.

Reversible dehydrogenation of a primary aryl borane

The consecutive activation of B-H bonds in mesitylborane (H2BMes; Mes = 2,4,6-(CH3)3C6H2) by a 16-electron rhodium(i) monocarbonyl complex, (iPrNNN)Rh(CO) (1-CO; iPrNNN = 2,5-[iPr2P[double bond, length as m-dash]N(4-iPrC6H4)]2N(C4H2)-) is described. Dehydrogenative extrusion of the {BMes} fragment led to the isolation of (iPrNNN)(CO)RhBMes (1-BMes). Addition of H2 gas to 1-BMes regenerated 1-CO and H2BMes, highlighting the ability of 1-CO to facilitate interconversion of {BMes} with dihydrogen. Reactivity studies revealed that 1-BMes promotes formal group transfer and that {BAr} fragments accessed by dehydrogenation are reactive entities.

Boranediyl- and Diborane(4)-1,2-diyl-Bridged Platinum A-Frame Complexes

Diplatinum A-frame complexes with a bridging (di)boron unit in the apex position were synthesized in a single step by the double oxidative addition of dihalo(di)borane precursors at a bis(diphosphine)-bridged Pt0 2 complex. While structurally analogous to well-known μ-borylene complexes, in which delocalized dative three-center-two-electron M-B-M bonding prevails, theoretical investigations into the nature of Pt-B bonding in these A-frame complexes show them to be rare dimetalla(di)boranes displaying two electron-sharing Pt-B σ-bonds. This is experimentally reflected in the low kinetic stability of these compounds, which are prone to loss of the (di)boron bridgehead unit.

The siliconyl, boronyl, and iminoboryl ligands as analogues of the well-known carbonyl ligand: predicted reactivity towards dipolar cyclooligomerization in iron/cobalt carbonyl complexes

The Fe(CO)₄(SiO), Co(CO)₄(BO), and Co(CO)₄(BNSiMe₃), complexes akin to the well-known Fe(CO)₅ are predicted by density functional theory to undergo exothermic oligomerization to give the oligomers containing Si_nO_n/B_nO_n/B₂N₂ rings with single bonds.

1,2-Halosilane vs. 1,2-alkylborane elimination from (boryl)(silyl) complexes of iron: switching between borylenes and silylenes just by changing the alkyl group

The results present a new elimination reaction, the 1,2-alkylborane elimination, as a new route to silylene complexes.

B═B and B≡E (E = N and o) multiple bonds in the coordination sphere of late transition metals

Because of their unusual structural and bonding motifs, multiply bonded boron compounds are fundamentally important to chemists, leading to enormous research interest. To access these compounds, researchers have introduced sterically demanding ligands that provide kinetic as well as electronic stability. A conceptually different approach to the synthesis of such compounds involves the use of an electron-rich, coordinatively unsaturated transition metal fragment. To isolate the plethora of borane, boryl, and borylene complexes, chemists have also used the coordination sphere of transition metals to stabilize reactive motifs in these molecules. In this Account, we summarize our results showing that increasingly synthetically challenging targets such as iminoboryl (B≡N), oxoboryl (B≡O), and diborene (B═B) fragments can be stabilized in the coordination sphere of late transition metals. This journey began with the isolation of two new iminoboryl ligands trans-[(Cy3P)2(Br)M(B≡N(SiMe3))] (M = Pd, Pt) attached to palladium and platinum fragments. The synthesis involved oxidative addition of the B-Br bond in (Me3Si)2N═BBr2 to [M(PCy3)2] (M = Pt, Pd) and the subsequent elimination of Me3SiBr at room temperature. Variation of the metal, the metal-bound coligands, and the substituent at the nitrogen atom afforded a series of analogous iminoboryl complexes. Following the same synthetic strategy, we also synthesized the first oxoboryl complex trans-[(Cy3P)2BrPt(BO)]. The labile bromide ligand adjacent to platinum makes the complex a viable candidate for further substitution reactions, which led to a number of new oxoboryl complexes. In addition to allowing us to isolate these fundamental compounds, the synthetic strategy is very convenient and minimizes byproducts. We also discuss the reaction chemistry of these types of compounds. In addition to facilitating the isolation of compounds with B≡E (E = N, O) triple bonds, the platinum fragment can also stabilize a diborene (RB═BR) moiety, a bonding motif that thus far had only been accessible when stabilized by N-heterocyclic carbenes (NHCs). In the new π-diborene [(Et3P)2Pt(B2Dur2)] (Dur = 2,3,5,6-Me4-C6H) complex, the diborene ligand receives electron density from Pt, leading to a strong Pt-B bond and a B═B bond. We attribute this result to the very short B═B bond distance (1.51(2) Å) while coordinated to platinum. Overall, an increasing number of chemists are examining the chemistry of multiply bound boron compounds. The isolation of an oxoboryl complex is of special interest not only from a structural standpoint but also because of its orbital similarities to the ubiquitous CO ligand. Detailed computational studies of the π-diborene complex [(Et3P)2Pt(B2Dur2)] show that the bonding properties of this molecule violate the widely accepted Dewar-Chatt-Duncanson (DCD) bonding model.

Transition metal borylene complexes: boron analogues of classical organometallic systems

In the ten years since the first examples of terminal borylene complexes were reported, rapid advances in the chemistry of this ligand class have been made, and parallels emerged with classical organometallic ligand systems (such as carbenes, vinylidenes and even CO) as well as with other subvalent main group ligand systems. This article reviews key developments in synthetic, structural and reaction chemistry, with particular attention focusing on recent discoveries in the field.