Gallium "Shears" for C=N and C=O Bonds of Isocyanates

Reversible Coupling of Germylone with Isocyanates

The germylone dimNHCGe (5, dimNHC=diimino N-heterocyclic carbene) undergoes a [2+2] cycloaddition with isocyanates RNCO (R=4-tolyl or 3,5-xylyl) to furnish novel alkyl carboxamido germylenes 7 (R=4-tolyl) and 8 (R=3,5-xylyl), featuring a C-C bond between the former carbene carbon and the isocyanate moiety. Heating a mixture of 8 with 4-tolyl isocyanate to 100 °C results in isocyanate metathesis, demonstrating reversible C-C bond formation on the reduced germanium compound. DFT calculations suggest that this process occurs via the reductive dissociation of isocyanate from 8 that regenerates the parent Ge(0) compound 5.

Assembly of metallo-macrocycles through reductive C-C coupling of alkylnitriles by an Mg-Mg-bonded compound

Low-valent metal complexes have attracted much research interest owing to their novel reactivities toward small molecules. Herein the reactivity of the α-diimine-ligated, Mg-Mg-bonded compound [K(THF)₃]₂[LMg-MgL] (1, L = [(2,6-iPr₂C₆H₃)NC(Me)]₂^2-) with aliphatic nitriles has been studied. Complex 1 readily activates n-alkylnitriles (RCN; R = propyl, butyl, and pentyl) to afford the unique trinuclear magnesium metallo-macrocyclic complexes, [LMg(μ-{(NC-C(R)C(CH₂R)-NH})]₃[K₃(Solv)₆] (2-4: R = -(CH₂)_nCH₃, n = 2, 3, or 4; Solv = THF/DME), through a reductive deprotonation of the α-H of one nitrile molecule and C-C coupling between this α-carbon and the cyanide (CN) group of another nitrile, followed by a 1,3-H shift. The results demonstrate the possibility of assembling supramolecular architectures based on the α-diimine [LMg] fragment through small molecule activation.

Cycloaddition of isoselenocyanates to sodium and magnesium metallacycles

Heterocumulenes SeCNR (R = C₆H₄OMe-2, C₆H₄Me-2) undergo facile cycloaddition to [(H-dpp-bian)Na(Et₂O)₂] (1) (H-dpp-bian = N-protonated 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) resulting in cycloadducts [(H-dpp-bian)Na(SeCNR)(DME)] (2, 3), which are the first cycloadducts derived from a sodium metallacycle reported so far. A comparative reaction of [(dpp-bian)Mg(THF)₃] (10) with SeCNR gives magnesium cycloadducts [(dpp-bian)Mg(SeCNR)(Solv)₂] (11, 12), which undergo fast decomposition at room temperature. New compounds are characterized by NMR, EPR, and IR spectroscopy, and elemental and X-ray diffraction analysis. Their electronic structures and reaction pathways were probed using DFT calculations.

Calix[4]pyrrolato gallate: square planar-coordinated gallium(iii) and its metal–ligand cooperative reactivity with CO2 and alcohols

Forcing a priori tetracoordinate atoms into planar configuration represents a promising concept for enhanced reactivity of p-block element-based systems. Herein, the synthesis, characterization, and reactivity of calix[4]pyrrolato gallates, constituting square planar-coordinated gallium(iii) atoms, are reported. Unusual structural constraint-induced Lewis acidity against neutral and anionic donors is disclosed by experiment and rationalized by computations. An energetically balanced dearomatization/rearomatization of a pyrrole unit enables fully reversible metal–ligand cooperative capture of CO₂. While alcohols are found unreactive against the gallates, a rapid and selective OH-bond activation can be triggered upon protonation of the ligand. Secondary ligand–sphere modification adds a new avenue to structurally-constrained complexes that unites functional group tolerance with unconventional reactivity.

Ideally square-planar coordinated gallium(iii) species is isolated and fully characterized. Spontaneous metal–ligand cooperative reactivity towards CO₂ is observed, while OH-bond activation of alcohols can be triggered by protonation of the ligand.

Main-group metal complexes of α-diimine ligands: structure, bonding and reactivity

α-Diimine ligands, in particular 1,4-diazabutadiene (dad) and bis(iminoacenaphthene) (bian) derivatives, have been widely used for coordination with various metals, including main-group, transition, and lanthanide and actinide metals. In addition to their tunable steric and electronic properties, the dad and bian ligands are redox-active and can readily accept one or two electrons, converting into the radical-anionic (L˙^-) or dianionic (enediamido, L^2-) form, respectively. This non-innocence brings about rich electronic structures and properties of the ligands and complexes thereof. For example, the dad ligands in their three redox levels can effectively stabilize a series of metal centers in different oxidation states, including low-valent metals. Moreover, these ligands can serve as electron reservoirs and can participate in reactions toward other molecules with or without metals. Therefore, such ligands are extremely useful in the areas of low-valent complexes and small molecule activation. Herein, we will discuss the use of dad (and bian) ligands in the stabilization of metal-metal-bonded compounds, in particular those of main-group metals, as well as small molecule activation by these (low-valent) metal coordination species where the non-innocence of the ligands plays a key role.

Reactions of Iso(thio)cyanates with Dialanes: Cycloaddition, Reductive Coupling, or Cleavage of the C═S or C═O Bond

The dialanes [(dpp-Bian)Al-Al(dpp-Bian)] (1) and [(dpp-dad)Al(THF)-(THF)Al(dpp-dad)] (2) (dpp-Bian = 1,2-[(2,6-iPr₂C₆H₃)NC]₂C₁₂H₆, dpp-dad = [(2,6-iPr₂C₆H₃)NC(CH₃)]₂) react with some isothiocyanates, isocyanates, and diphenylketene via [2 + 4] cycloaddition of the C═O or C═S bond across the C═C-N-Al fragment to afford complexes [L(X═C-Y)Al-Al(X═C-Y)L] with an intact Al-Al single bond (3, L = dpp-Bian, X = PhN, Y = O; 4, L = dpp-Bian, X = Ph₂C, Y = O; 6, L = dpp-dad, X = BnN, Y = S; 7, L = dpp-dad, X = tBuN, Y = O; 8, L = dpp-dad, X = iPrN, Y = S; and 9, L = dpp-dad, X = CyN, Y = S). A mixed C═N and C═O mode cycloadduct, [(dpp-Bian)(TosN═C-O)Al-Al(TosN-C═O)(dpp-Bian)] 5, was obtained in the reaction of 1 with tosylisocyanate. Heating the solution of 3 resulted in a thermal transformation and a change of the cycloaddition mode from C═O to C═N to give the product [(dpp-Bian)(PhN-C═O)Al(O)Al(PhN-C═O)(dpp-Bian)] 10. The reduction of 7 and 8 with Na yielded the products [Na(THF)_n]₂[(dpp-dad^-H)(X═C-Y)Al]₂ (12, X = iPrN, Y = S, n = 2 and 13, X = tBuN, Y = O, n = 3) in which one of the methyl groups of the backbone of the initial dpp-dad ligand was dehydrogenated. When 2 was reacted with the bulky adamantyl isocyanate AdNCO, the C-C coupling of two substrates occurred to form 14 [(dpp-dad)Al(O═C-NAd)₂Al(dpp-dad)] in which the coupled dianionic oxamide ligand bridged two Al atoms in a μ,η⁴-N,O/N,O mode. Moreover, in the presence of 2.0 equiv of Na metal, precursor 2 reacts with tBuNCS, p-TolylNCS, or Me₃SiNCO, possibly through the reduced Al^I intermediate, to yield the sulfur- or oxygen-bridged dimer [Na(solv)_n]₂[(dpp-dad)Al(μ-E)]₂ (15, E = S, solv = THF, n = 3 and 16, E = O, solv = DME, n = 2) upon C═S or C═O bond cleavage. Dialane 1 reacts with dimethylsulfone to give a Lewis adduct [(dpp-Bian)(Me₂SO₂)Al]₂ (17), which releases dimethylsulfone upon heating. The diamagnetic compounds 3-10 and 12-17 were characterized by NMR and IR spectroscopy. The molecular structures of 3-17 were established by single-crystal X-ray diffraction analysis. Electronic structures of the compounds and possible isomers have been examined by DFT calculations.

Activation and modification of carbon dioxide by redox-active low-valent gallium species

The activation of carbon dioxide by metallylene [(dpp-bian)GaNa(DME)2] (dpp-bian = 1,2-bis[(2,6-di-isopropylphenyl)imino]acenaphthene) under mild conditions is described. Furthermore, the reaction of the activation complex [(dpp-bian)Ga(CO2)2Ga(dpp-bian)][Na(DME)2]2 (2) with diphenylketene, cyclohexyl isocyanate, and phenyl isocyanate leads to the elimination of carbon monoxide and the formation of derivatives of oxocarboxylic acid [(dpp-bian)GaOC(O)C(Ph)2C(CPh2)O][Na(DME)2] (6) and carbamate derivatives [(dpp-bian)GaN(Cy)C(O)N(Cy)C(O)O]2[Na(DME)2]2 (7) and [(dpp-bian)GaN(Ph)C(O)O]2[Na(DME)2]2 (8), respectively. Complexes have been characterized by NMR, IR spectroscopy, elemental analysis, and X-ray diffraction analysis. Their electronic structures have been examined by DFT calculations. The possible mechanism of the modification reaction is proposed and supported by the investigation of 13CO2-enriched samples and DFT calculations.

Reversible Addition of Carbon Dioxide to Main Group Metal Complexes at Temperatures about 0 °C

The reaction of dialane [LAl-AlL] (1; L=dianion of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene, dpp-bian) with carbon dioxide results in two different products depending on solvent. In toluene at temperatures of about 0 °C, the reaction gives cycloadduct [L(CO₂ )Al-Al(O₂ C)L] (2), whereas in diethyl ether, the reaction affords oxo-bridged carbamato derivative [L(CO₂ )(Et₂ O)Al(μ-O)AlL] (3). The DFT and QTAIM calculations provide reasonable explanations for the reversible formation of complex 2 in the course of two subsequent (2+4) cycloaddition reactions. Consecutive transition states with low activation barriers were revealed. Also, the DFT study demonstrated a crucial effect of diethyl ether coordination to aluminium on the reaction of dialane 1 with CO₂ . The optimized structures of key intermediates were obtained for the reactions in the presence of Et₂ O; calculated thermodynamic parameters unambiguously testify the irreversible formation of the product 3.

Transformation of carbodiimides to guanidine derivatives facilitated by gallylenes

The reductive coupling of carbodiimides RN[double bond, length as m-dash]C[double bond, length as m-dash]NR (R = 2,6-iPr2C6H3, Cy, iPr) by using [(dpp-bian)GaNa(dme)2] (1); [(dpp-dad)GaNa(thf)3] (2a) and [(dpp-dad)GaK(thf)4Ga(dpp-dad)][K(thf)6] (2b) led to the guanidinate derivatives [(dpp-bian)Ga(NCy)2C[double bond, length as m-dash]NCy][Na(thf)2] (3); [LGaN(R)C(RN)N(R)C(RN)][M] L = dpp-bian, M = Na(dme)2, R = iPr, (4a); L = dpp-dad, M = Na(thf)3, R = iPr, (4b); R = Cy, (4c); M = K(thf)4, (4d); L = dpp-bian, M = Na(dme)2, R = Cy, (4e) and [(dpp-dad)Ga(2,6-iPr2C6H3N)2C][Na(thf)2] (5).

Reduction of carbodiimides by a dialumane through insertion and cycloaddition

Dialumane reacts with carbodiimides, RNCNR (R = Cy, iPr, dipp, tBu), through insertion, [2+4] cycloaddition and a hydrogen transfer process.

Alkali metal reduction of 1,3,2-diazaborol and 1,3,2-diazagermol derivatives based on 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene

The reduction of borol (5) and germol (7) derivatives with sodium in thf proceeds with a transfer of two electrons and one proton to dpp-bian ligand and results in the formation of the products 6 and 8, which consist of the diamagnetic mono-anionic ligand.