Terminal versus Bridging Boryl Coordination in N-Heterocyclic Carbene Copper(I) Boryl Complexes: Syntheses, Structures, and Dynamic Behavior

Synthesis, Structures, and Reactivity of Organostannanides and Organogermanides of Copper(I)

Organogermane and organostannane compounds are valuable reagents in cross coupling reactions, and copper(I) germanide and stannanide complexes can provide convenient access to these compounds. This review presents the chemistry of copper(I) germanide and stannanide complexes, with a particular focus on systems at the frontier of organic and inorganic chemistry where structural characterisation of coordination complexes facilitates rationalisation of organic mechanisms. These species show both similarities to, and significant divergences from their lighter silanide congeners. For example, they are all viable sources of the relevant organotetranide anion, but in the cases of both germanium and tin, the tetranides can be accessed via direct deprotonation of the corresponding tetranes, a reaction unknown for silicon. Further divergences between copper(I) germanides and stannanides are highlighted; whilst both can be used in productive organic transformations to access organotetranes, catalytic reactions are only reported for germanium. The rather striking ability of triphenylstannanides to acts as sources of the phenyl anion are discussed; the mechanism of this reaction is still subject to discussion, but its absence in the chemistry of germanium and silicon is now well-rationalised. We conclude this review by considering potential research directions in the synthesis and exploitation of copper(I) germanides and stannanides.

Copper Catalyzed Borylation of Alkynes: An Experimental Mechanistic Study

The copper catalyzed hydroboration of alkynes with B2pin2 was studied by in detail studies of individual relevant steps along the catalytic pathway. A number of reaction steps were retraced by in situ NMR spectroscopy as well as central intermediates and side-products were isolated and comprehensively characterized. A copper boryl complex is central to the catalytic process by inserting the terminal alkyne substrate into the B-Cu bond. The selectivity of this step - depending on the NHC auxiliary ligand - determines the α/β selectivity observed in the product. The latter complex is protonated by the auxiliary alcohol reagent resulting in hydroboration product formation and formation of a Cu alkoxido complex. Reaction of the latter with B2pin2 results in the regeneration of the central copper boryl complex. This alcoholysis step depends on the acidity of the alcohol, in particular on the relative acidity of the alcohol vs. the alkyne substrate. A number of side reactions leading to the hydrogenation product of the alkyne substrate and a bis hydroborated product were identified and studied in some detail. It is concluded that the performance of a particular catalytic system depends crucially on the relative acidities of the reagents and generalizations may be difficult.

Diborane Reductions of CO2 and CS2 Mediated by Dicopper μ-Boryl Complexes of a Robust Bis(phosphino)-1,8-naphthyridine Ligand

A dinucleating 1,8-naphthyridine ligand featuring fluorene-9,9-diyl-linked phosphino side arms (PNNPFlu) was synthesized and used to obtain the cationic dicopper complexes 2, [(PNNPFlu)Cu2(μ-Ph)][NTf2]; [NTf2] = bis(trifluoromethane)sulfonimide, 6, [(PNNPFlu)Cu2(μ-CCPh)][NTf2], and 3, [(PNNPFlu)Cu2(μ-OtBu)][NTf2]. Complex 3 reacted with diboranes to afford dicopper μ-boryl species (4, with μ-Bcat; cat = catecholate and 5, with μ-Bpin; pin = pinacolate) that are more reactive in C(sp)-H bond activations and toward activations of CO2 and CS2, compared to dicopper μ-boryl complexes supported by a 1,8-naphthyridine-based ligand with di(pyridyl) side arms. The solid-state structures and DFT analysis indicate that the higher reactivities of 4 and 5 relate to changes in the coordination sphere of copper, rather than to perturbations on the Cu-B bonding interactions. Addition of xylyl isocyanide (CNXyl) to 4 gave 7, [(PNNPFlu)Cu2(μ-Bcat)(CNXyl)][NTf2], demonstrating that the lower coordination number at copper is chemically significant. Reactions of 4 and 5 with CO2 yielded the corresponding dicopper borate complexes (8, [(PNNPFlu)Cu2(μ-OBcat)][NTf2]; 9, [(PNNPFlu)Cu2(μ-OBpin)][NTf2]), with 4 demonstrating catalytic reduction in the presence of excess diborane. Related reactions of 4 and 5 with CS2 provided insertion products 10, {[(PNNPFlu)Cu2]2[μ-S2C(Bcat)2]}[NTf2]2, and 11, [(PNNPFlu)Cu2(μ,κ2-S2CBpin)][NTf2], respectively. These products feature Cu-S-C-B linkages analogous to those of proposed CO2 insertion intermediate.

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.

The borderless world of chemical bonding across the van der Waals crust and the valence region

The definition of the van der Waals crust as the spherical section between the atomic radius and the van der Waals radius of an element is discussed and a survey of the application of the penetration index between two interacting atoms in a wide variety of covalent, polar, coordinative or noncovalent bonding situations is presented. It is shown that this newly defined parameter permits the comparison of bonding between pairs of atoms in structural and computational studies independently of the atom sizes.

Nickel boryl complexes and nickel-catalyzed alkyne borylation

The first nickel bis-boryl complexes cis-[Ni( ⁱ Pr₂Im^Me)₂(Bcat)₂], cis-[Ni( ⁱ Pr₂Im^Me)₂(Bpin)₂] and cis-[Ni( ⁱ Pr₂Im^Me)₂(Beg)₂] are reported, which were prepared via the reaction of a source of [Ni( ⁱ Pr₂Im^Me)₂] with the diboron(4) compounds B₂cat₂, B₂pin₂ and B₂eg₂ ( ⁱ Pr₂Im^Me = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B₂cat₂ = bis(catecholato)diboron; B₂pin₂ = bis(pinacolato)diboron; B₂eg₂ = bis(ethylene glycolato)diboron). X-ray diffraction and DFT calculations strongly suggest that a delocalized, multicenter bonding scheme dictates the bonding situation of the NiB₂ moiety in these square planar complexes, reminiscent of the bonding situation of "non-classical" H₂ complexes. [Ni( ⁱ Pr₂Im^Me)₂] also efficiently catalyzes the diboration of alkynes using B₂cat₂ as the boron source under mild conditions. In contrast to the known platinum-catalyzed diboration, the nickel system follows a different mechanistic pathway, which not only provides the 1,2-borylation product in excellent yields, but also provides an efficient approach to other products such as C-C coupled borylation products or rare tetra-borylated compounds. The mechanism of the nickel-catalyzed alkyne borylation was examined by means of stoichiometric reactions and DFT calculations. Oxidative addition of the diboron reagent to nickel is not dominant; the first steps of the catalytic cycle are coordination of the alkyne to [Ni( ⁱ Pr₂Im^Me)₂] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)₂(η²-cis-(Bcat)(R)C[double bond, length as m-dash]C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni( ⁱ Pr₂Im^Me)₂(η²-cis-(Bcat)(Me)C[double bond, length as m-dash]C(Me)(Bcat))] and [Ni( ⁱ Pr₂Im^Me)₂(η²-cis-(Bcat)(H₇C₃)C[double bond, length as m-dash]C(C₃H₇)(Bcat))].

B2pin2-Mediated Cascade Cyclization/Aromatization Reaction: Facial Access to Functionalized Indolizines

Herein, a B₂pin₂-mediated radical cascade cyclization/aromatization reaction of enaminone with pyridine is described. This strategy provides a practical way for the construction of valuable functionalized indolizines under metal-, external oxidant-, and base-free conditions, which could be compatible with various kinds of functional groups, such as halogen, π-system, heterocycle, ferrocenyl, etc. A preliminary mechanism investigation indicated that the pyridine-boryl radical formed in situ triggered the reaction to occur.

Electrospray Ionization Tandem Mass Spectrometry and DFT Survey of Copper(I) Ate Complexes Containing Coordinated Borohydride Anions

Copper(I) borohydride ate complexes of the type Cat⁺[XCu(BH₄)]^- have been previously postulated as intermediates in the reactions of copper salts with borohydride. Negative ion electrospray ionization of an acetonitrile solution of copper(I) phenylacetylide with a 10-fold excess of sodium borohydride (NaBH₄) revealed the formation of a diverse range of mononuclear, dinuclear and trinuclear cuprates with different numbers of BH₄^-, H^- and CN^- ligands, the latter likely being formed by abstraction of CN^- from the acetonitrile solvent. Collision-induced dissociation was used to examine the fragmentation reactions of the following borohydride containing cuprates: [Cu(H)(BH₄)]^-, [Cu(BH₄)₂]^-, [Cu(BH₄)(CN)]^-, [Cu₂(H)(BH₄)₂]^-, [Cu₂(H)₂(BH₄)]^-, [Cu₂(BH₄)₂(CN)]^-, [Cu₂(H)(BH₄)(CN)]^-, [Cu₃(H)(BH₄)₃]^-, [Cu₃(H)₂(BH₄)₂]^-, [Cu₃(H)₃(BH₄)]^-, [Cu₃(BH₄)₂(CN)₂]^-, and [Cu₃(H)(BH₄)₂(CN)]^-. In all cases, BH₃ loss is observed. For many of the dinuclear and trinuclear complexes cluster fragmentation by loss of CuH was also observed. In the case of [Cu₂(H)₂(BH₄)]^- and [Cu₃(H)₃(BH₄)]^-, loss of H₂ was also observed. DFT calculations were used to explore potential structures of the various borohydride-containing cuprates and to predict the overall reaction energetics for the various fragmentation channels.

Copper-Catalyzed Borylation of Acyl Chlorides with an Alkoxy Diboron Reagent: A Facile Route to Acylboron Compounds

Herein, the copper-catalyzed borylation of readily available acyl chlorides with bis(pinacolato)diboron, (B2 pin2 ) or bis(neopentane glycolato)diboron (B2 neop2 ) is reported, which provides stable potassium acyltrifluoroborates (KATs) in good yields from the acylboronate esters. A variety of functional groups are tolerated under the mild reaction conditions (room temperature) and substrates containing different carbon-skeletons, such as aryl, heteroaryl and primary, secondary, tertiary alkyl are applicable. Acyl N-methyliminodiacetic acid (MIDA) boronates can also been accessed by modification of the workup procedures. This process is scalable and also amenable to the late-stage conversion of carboxylic acid-containing drugs into their acylboron analogues, which have been challenging to prepare previously. A catalytic mechanism is proposed based on in situ monitoring of the reaction between p-toluoyl chloride and an NHC-copper(I) boryl complex as well as the isolation of an unusual lithium acylBpinOBpin compound as a key intermediate.

Robust dicopper(i) μ-boryl complexes supported by a dinucleating naphthyridine-based ligand

Copper boryl species have been widely invoked as reactive intermediates in Cu-catalysed C–H borylation reactions, but their isolation and study have been challenging. Use of the robust dinucleating ligand DPFN (2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine) allowed for the isolation of two very thermally stable dicopper(i) boryl complexes, [(DPFN)Cu₂(μ-Bpin)][NTf₂] (2) and [(DPFN)Cu₂(μ-Bcat)][NTf₂] (4) (pin = 2,3-dimethylbutane-2,3-diol; cat = benzene-1,2-diol). These complexes were prepared by cleavage of the corresponding diborane via reaction with the alkoxide [(DPFN)Cu₂(μ-O^tBu)][NTf₂] (3). Reactivity studies illustrated the exceptional stability of these boryl complexes (thermal stability in solution up to 100 °C) and their role in the activation of C(sp)–H bonds. X-ray diffraction and computational studies provide a detailed description of the bonding and electronic structures in these complexes, and suggest that the dinucleating character of the naphthyridine-based ligand is largely responsible for their remarkable stability.

Cu(i) boryl species have been widely invoked as reactive intermediates in Cu-catalysed C–H borylations, but their isolation has been challenging. In this work, thermally robust dicopper(I) boryl complexes have been synthesized and studied in detail.

Computational study of Cu-catalyzed 1,2-hydrocarboxylation of 1,3-dienes with CO2: Pauli repulsion-controlled regioselectivity of Cu–Bpin additions

The mechanism and origin of regioselectivity of Cu-catalyzed 1,2-hydrocarboxylation of 1,3-dienes with CO2 were computationally investigated. The results show that CO2 not only acts as a carboxylation reagent, but also...

Access to metastable [GeH2]n materials via a molecular "bottom-up" approach

We describe the application of a mild, molecular-based, hydride metathesis protocol for the preparation of metastable germanium(II) dihydrides with compositions approaching [GeH₂]_n. The common starting material for this work [Ge(O^tBu)₂] was prepared in a high yield and shown to undergo O^tBu/H exchange at Ge with the hydride sources pinacolborane (HBpin), catecholborane (HBcat), and diisobutylaluminum hydride (DIBAL-H) to give the [GeH₂]_n materials as yellow to orange solids. Heating one of these [GeH₂]_n materials to 200 °C affords a narrowing of the optical band gap (from 2.5 eV) and the generation of amorphous Ge. Reaction of [Ge(O^tBu)₂] with excess H₃B·SMe₂ in toluene at 70 °C provides a convenient route to thin films of amorphous Ge, including its deposition onto soft substrates, such as polyethyleneterephthalate (PET). Accompanying computations give insight into the energetics of O^tBu/H exchange at Ge, and reveal a general thermodynamic preference for branched structures of [GeH₂]_n oligomers over linear forms as the Ge chain becomes longer. We also show that [Ge(O^tBu)₂] is a suitable pre-catalyst for the borylation of aldehydes.

Copper-Catalyzed Ring-Opening Reactions of Alkyl Aziridines with B2pin2: Experimental and Computational Studies

The possibility to form new C-B bonds with aziridines using diboron derivatives continues to be a particularly challenging field in view of the direct preparation of functionalized β-aminoboronates, which are important compounds in drug discovery, being a bioisostere of β-aminoacids. We now report experimental and computational data that allows the individuation of the structural requisites and of reaction conditions necessary to open alkyl aziridines using bis(pinacolate)diboron (B₂pin₂) in a regioselective nucleophilic addition reaction under copper catalysis.

A stable ring-expanded NHC-supported copper boryl and its reactivity towards heterocumulenes

Reaction of bis(pinacolato)diboron with (6-Dipp)CuO^tBu generates a ring-expanded N-heterocyclic carbene supported copper(I) boryl, (6-Dipp)CuBpin. This compound showed remarkable stability and was characterised by NMR spectroscopy and X-ray crystallography. (6-Dipp)CuBpin readily dechalcogenated a range of heterocumulenes such as CO₂, isocyanates and isothiocyanates to yield (6-Dipp)CuXBpin (X = O, S). In the case of CO₂ catalytic reduction to CO is viable in the presence of excess bis(pinacolato)diboron. In contrast, in the case of iso(thio)cyanates, the isocyanide byproduct of dechalcogenation reacted with (6-Dipp)CuBpin to generate a copper(I) borylimidinate, (6-Dipp)CuC(NR)Bpin, which went on to react with heterocumulenes. This off-cycle reactivity gives selective access to a range of novel boron-containing heterocycles bonded to copper, but precludes catalytic reactivity.

N-H deprotonation of a diaminodialkoxido diborane(4) - a structural study on bifunctional Lewis acids/bases and their dimerisation to B(sp2)2B(sp3)2N2 six membered rings

The N-H deprotonation of the diaminodialkoxido diborane(4) pinB-Bdab (1) (pin: (OCMe₂)₂, dab: 1,2-(NH)₂C₆H₄), is crucial for the electrophilic N-functionalisation towards unsymmetrical diborane(4) reagents. An N-H deprotonated diborane(4) comprises Lewis basic nitrogen atoms and at the same time Lewis acidic boron atoms. This bifunctionality governs its reactivity and structural chemistry. Whilst bases such as Na(hmds), tBuLi or Li(tmp) readily effect a single deprotonation of 1, the second deprotonation is less straightforward and cleanly only achieved with Li(tmp) as a strong but little nucleophilic base. The N-H deprotonated diborane(4) derivatives readily dimerise to give B(sp²)₂B(sp³)₂N₂ six-membered ring Lewis base adducts. The structural chemistry of this class of compounds was studied in detail in the solid state by single crystal X-ray diffraction as well as in solution by NMR spectroscopy.

Ambiphilic Al-Cu Bonding

Copper-alumanyl complexes, [LCu-Al(SiN^Dipp )], where L=carbene=NHC^iPr (N,N'-diisopropyl-4,5-dimethyl-2-ylidene) and ^Me2 CAAC (1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) and featuring unsupported Al-Cu bonds, have been prepared. Divergent reactivity observed with carbodiimides and CO₂ implies an ambiphilicity in the Cu-Al interaction that is dependent on the identity of the carbene co-ligand.

Synthesis and characterization of N-heterocyclic carbene-MOEt2 complexes (M = Cu, Ag, Au). Analysis of solvated auxiliary-ligand free [(NHC)M]+ species

We report the synthesis, characterization and computational analysis of coinage metal-ether complexes supported by N-heterocyclic carbenes (NHC), SIPr and Et2CAAC. The related water adducts are also included. The [(NHC)M]+(M = Cu, Ag, Au) species show the noteworthy ability to bind Et2O and H2O. This interaction towards Et2O and H2O is partly ascribed to a σ-hole bonding with an almost linear disposition, taking advantage of the enhanced σ-hole potential evaluated for such [(NHC)M]+ species. This enhanced ability is larger than those found for non-covalent interactions involving main group species.

Stability and decomposition of copper(i) boryl complexes: [(IDipp)Cu–Bneop], [(IDipp*)Cu–Bneop] and copper clusters

Synthesis and characterisation of NHC copper boryl complexes [(NHC)Cu–Bneop] and their decomposition to low-valent copper clusters.

Copper-Catalyzed Triboration of Terminal Alkynes Using B2 pin2 : Efficient Synthesis of 1,1,2-Triborylalkenes

We report herein the catalytic triboration of terminal alkynes with B2 pin2 (bis(pinacolato)diboron) using readily available Cu(OAc)2 and Pn Bu3 . Various 1,1,2-triborylalkenes, a class of compounds that have been demonstrated to be potential matrix metalloproteinase (MMP-2) inhibitors, were obtained directly in moderate to good yields. The process features mild reaction conditions, a broad substrate scope, and good functional group tolerance. This copper-catalyzed reaction can be conducted on a gram scale to produce the corresponding 1,1,2-triborylalkenes in modest yields. The utility of these products was demonstrated by further transformations of the C-B bonds to prepare gem-dihaloborylalkenes (F, Cl, Br), monohaloborylalkenes (Cl, Br), and trans-diaryldiborylalkenes, which serve as important synthons and have previously been challenging to prepare.

Copper-Catalyzed Triboration: Straightforward, Atom-Economical Synthesis of 1,1,1-Triborylalkanes from Terminal Alkynes and HBpin

A convenient and efficient one-step synthesis of 1,1,1-triborylalkanes was achieved via sequential dehydrogenative borylation and double hydroborations of terminal alkynes with HBpin (HBpin=pinacolborane) catalyzed by inexpensive and readily available Cu(OAc)2 . This process proceeds under mild conditions, furnishing 1,1,1-tris(boronates) with wide substrate scope, excellent selectivity, and good functional-group tolerance, and is applicable to gram-scale synthesis without loss of yield. The 1,1,1-triborylalkanes can be used in the preparation of α-vinylboronates and borylated cyclic compounds, which are valuable but previously rare compounds. Different alkyl groups can be introduced stepwise via base-mediated deborylative alkylation to produce racemic tertiary alkyl boronates, which can be readily transformed into useful tertiary alcohols.