Theoretical Studies of [MYR2] n Isomers (M = B, Al, Ga; Y = N, P, As; R = H, CH3): Structures and Energetics of Monomeric and Dimeric Compounds (n = 1, 2)

Interaction of Small Nitriles Occurring in the Atmosphere of Titan with Metal Ions of Meteoric Origin

Meteoric material injected into the atmosphere of Titan, Saturn's moon, can react with nitriles and other organic compounds that constitute Titan's atmosphere. However, specific chemical outcomes have not been fully explored. To understand the fates of meteoric metal ions in the Titan environment, reactions of Mg+ and Al+ with CH3CN (acetonitrile) and C2H5CN (propionitrile) were carried out using a drift cell ion reactor at room temperatures (300 K) and reduced temperatures (∼193 K) and modeled using density functional theory and coupled-cluster theory. Analysis of reactant ion electronic state distributions via electronic state chromatography revealed that Mg+ was produced in our instrument exclusively in its ground (2S) state, whereas Al+ was produced in both its 1S ground state and 3P first excited state. Mg+(2S) and Al+(1S) produce association products exclusively with both CH3CN and C2H5CN. Primary association reactions with C2H5CN occurred with higher reaction efficiencies than those with CH3CN. Mg+(2S) sequentially associates up to four nitrile ligands, and Al+(1S) associates up to three, each via the nitrile nitrogen. Computed binding energies are strongest for the first ligand and diminish with subsequent nitriles. Mg+(2S) exhibits a stronger preference for binding nitriles than Al+(1S) because its unpaired electron delocalizes to the nitrile ligands through back-bonding, whereas the lone pair on Al+(1S) remains localized on the metal center. Al+(3P) exhibited evidence of bimolecular product formation with both nitriles. Computational modeling of Al+(3P) with CH3CN suggests that the major product, AlCH3+, is kinetically favored over the more energetically stable product, Al+(HCN).

Synthesis and Reactivity of Germyl-Substituted Gallapnictenes

Decarbonylation of the phospha- and arsaketenyl germylenes (L)GeECO (E = P, As; L = CH[C(Me)NAr]2, Ar = 2,6-iPr2C6H3) in the presence of the Ga(I) precursor (L)Ga afforded the corresponding germyl-substituted gallaphosphene 1 and gallaarsene 3, respectively. Both 1 and 3 are examples of unsaturated chains with heavier group 13/15/14 elements. The germylene center and the polarized Pn = Ga (Pn = P or As) double bond provide multiple sites for small-molecule activation. For example, gallaarsene 3 reacted with adamantyl azide in a formal [3 + 2]-cyclization to give 4 containing a GaAsN3 heterocycle, clearly underlining the analogy between the As = Ga and C-C multiple bonds. By contrast, the reaction of 3 with Me-I afforded the 1,3-addition product 5, which indicates frustrated Lewis pair character in 3. DFT calculations indicate that the Ga-P/As bonds are highly polarized toward the pnictogen center. EDA-NOCV calculations further support this description and additionally shed light on ambiguous bonding scenarios in 4 and 5. These calculations prove that orbital interactions are outweighed by electrostatic interactions, resulting in polar bonds with significant ionic character.

Aluminium and Gallium Silylimides as Nitride Sources

Terminal aluminium and gallium imides of the type K[(NON)M(NR)], bearing heteroatom substituents at R, have been synthesised via reactions of anionic aluminium(I) and gallium(I) reagents with silyl and boryl azides (NON=4,5-bis(2,6-diisopropyl-anilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene). These systems vary significantly in their lability in solution: the N(Sii Pr3 ) and N(Boryl) complexes are very labile, on account of the high basicity at nitrogen. Phenylsilylimido derivatives provide greater stabilization through the π-acceptor capabilities of the SiR3 group. K[(NON)AlN(Sit BuPh2 )] offers a workable compromise between stability and solubility, and has been completely characterized by spectroscopic, analytical and crystallographic methods. The silylimide species examined feature minimal π-bonding between the imide ligand and aluminium/gallium, with the HOMO and HOMO-1 orbitals effectively comprising orthogonal lone pairs centred at N. Reactivity-wise, both aluminium and gallium silylimides can act as viable sources of nitride, [N]3- , with systems derived from either metal reacting with CO to afford cyanide complexes. By contrast, only the gallium system K[(NON)Ga{N(SiPh3 )}] is capable of effecting a similar transformation with N2 O to yield azide, N3 - , via formal oxide/nitride metathesis. The aluminium systems instead generate RN3 via transfer of the imide fragment [RN]2- .

Heavier group 13/15 multiple bond systems: synthesis, structure and chemical bond activation

Heavier group 13/15 multiple bonds have been under investigation since the late 80s and to date, several examples have been published, which shows the obsoleteness of the so-called double bond rule. Especially in the last few years, more and more group 13/15 multiple bonds became synthetically feasible and their application in terms of small molecule activation has been demonstrated. Our group has recently shown that the combination of the pnictinidene precursor ^DipTer-Pn(PMe₃) (Pn = P, As) in combination with Al(I) synthons afforded the first examples of phospha- and arsaalumenes as isolable and thermally robust compounds. This feature article is intended to show the recent developments in the field, to outline early synthetic approaches and to discuss strategies to unlock the synthetic potential of these elusive chemical bonds.

A Monomeric Aluminum Imide (Iminoalane) with Al-N Triple-Bonding: Bonding Analysis and Dispersion Energy Stabilization

The reaction of :AlAriPr8 (AriPr8 = C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) with ArMe6N3 (ArMe6 = C6H3-2,6-(C6H2-2,4,6-Me3)2) in hexanes at ambient temperature gave the aluminum imide AriPr8AlNArMe6 (1). Its crystal structure displayed short Al-N distances of 1.625(4) and 1.628(3) Å with linear (C-Al-N-C = 180°) or almost linear (C-Al-N = 172.4(2)°; Al-N-C = 172.5(3)°) geometries. DFT calculations confirm linear geometry with an Al-N distance of 1.635 Å. According to energy decomposition analysis, the Al-N bond has three orbital components totaling -1350 kJ mol-1 and instantaneous interaction energy of -551 kJ mol-1 with respect to :AlAriPr8 and ArMe6N̈:. Dispersion accounts for -89 kJ mol-1, which is similar in strength to one Al-N π-interaction. The electronic spectrum has an intense transition at 290 nm which tails into the visible region. In the IR spectrum, the Al-N stretching band is calculated to appear at ca. 1100 cm-1. In contrast, reaction of :AlAriPr8 with 1-AdN3 or Me3SiN3 gave transient imides that immediately reacted with a second equivalent of the azide to give AriPr8Al[(NAd)2N2] (2) or AriPr8Al(N3){N(SiMe3)2} (3).

Synthesis and structures of gallaarsenes LGaAsGa(X)L featuring a Ga-As double bond

Three equivalents of LGa {L = HC[C(Me)N(2,6-i-Pr₂C₆H₃)]₂} react with AsX₃ (X = Cl, Br) by insertion into two As-X bonds, followed by the elimination of LGaX₂ and formation of LGaAsGa(Cl)L (1) and LGaAsGa(Br)L (2). According to single crystal X-ray analysis, 1 and 2 each exhibit one Ga-As single bond and one Ga-As double bond. The π-bonding contribution (9.71 kcal mol^-11 and 9.44 kcal mol^-12) was proved by variable temperature (VT) ¹H NMR spectroscopy, while the electronic structure of 1' was studied by quantum chemical calculations.

Synthesis of a Gallaarsene {HC[C(Me)N-2,6- i-Pr2-C6H3]2}GaAsCp* Containing a Ga═As Double Bond

Cp*AsCl₂ (Cp* = C₅Me₅) reacts with one equivalent of LGa (L = HC[C(Me)N(2,6- i-Pr₂C₆H₃)]₂) with formation of L(Cl)GaAs(Cl)Cp* 1, whereas the reaction with two equivalents of LGa yielded gallaarsene LGaAsCp* 2 containing a Ga═As double bond and (η¹-Ga(Cp*)L(η²-GaL)(μ-As₃) 3. Compounds 2 and 3 were structurally characterized by single crystal X-ray diffraction, and the π-bonding contribution in 2 was analyzed by temperature-dependent ¹H NMR spectroscopy (9.65 kcal mol^-1) and by quantum mechanical computation.

Triply Bonded Gallium≡Phosphorus Molecules: Theoretical Designs and Characterization

The effect of substitution on the potential energy surfaces of triple-bonded RGa≡PR (R = F, OH, H, CH₃, SiH₃, SiMe(SitBu₃)₂, SiiPrDis₂, Tbt (C₆H₂-2,4,6-{CH(SiMe₃)₂}₃), and Ar* (C₆H₃-2,6-(C₆H₂-2,4,6-i-Pr₃)₂)) compounds was theoretically examined by using density functional theory (i.e., M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp). The theoretical evidence strongly suggests that all of the triple-bonded RGa≡PR species prefer to select a bent form with an angle (∠Ga-P-R) of about 90°. Moreover, the theoretical observations indicate that only the bulkier substituents, in particular, for the strong donating groups (e.g., SiMe(SitBu₃)₂ and SiiPrDis₂) can efficiently stabilize the Ga≡P triple bond. In addition, the bonding analyses (based on the natural bond orbital, the natural resonance theory, and the charge decomposition analysis) reveal that the bonding characters of such triple-bonded RGa≡PR molecules should be regarded as [Formula: see text]. In other words, the Ga≡P triple bond involves one traditional σ bond, one traditional π bond, and one donor-acceptor π bond. Accordingly, the theoretical conclusions strongly suggest that the Ga≡P triple bond in such acetylene analogues (RGa≡PR) should be very weak.

Advances in the development of complexes that contain a group 13 element chalcogen multiple bond

The advances in the synthesis and isolation of complexes that contain a group 13 element chalcogen multiple bond are accounted for.

A monotopic aluminum telluride with an Al=Te double bond stabilized by N-heterocyclic carbenes

Aluminum chalcogenides are mostly encountered in the form of bulk aluminum oxides that are structurally diverse but typically consist of networks with high lattice energy in which the chalcogen atoms bridge the metal centres. This makes their molecular congeners difficult to synthesize because of a pronounced tendency for oligomerization. Here we describe the isolation of the monotopic aluminum chalcogenide (L(Dip)N)AlTe(L(Et))2 (L(Dip)=1,3-(2,6-diisopropylphenyl)-imidazolin-2-imine, L(Et)=1,3-diethyl-4,5-dimethyl-imidazolin-2-ylidene). Unique features of (L(Dip)N)AlTe(L(Et))2 are the terminal position of the tellurium atom, the shortest aluminum-tellurium distance hitherto reported for a molecular complex and the highest bond order reported for an interaction between these elements, to the best of our knowledge. At elevated temperature (L(Dip)N)AlTe(L(Et))2 equilibrates with dimeric {(L(Dip)N)AlTe(L(Et))}2 in which the chalcogen atoms assume their common role as bridges between the metal centres. These findings demonstrate that (L(Dip)N)AlTe(L(Et))2 comprises the elusive Al=Te double bond in the form of an N-heterocyclic carbene-stabilized species.

THEORETICAL STUDY OF THE STRUCTURES AND PROPERTIES OF (Cl2AlN3)n(n = 2–4) CLUSTERS

Density functional theory has been employed to study the molecular geometries, energies, infrared vibrational spectra, and thermodynamic properties of the clusters ( Cl2AlN3 )n(n = 1–4) at the B3LYP/6-311+G* level. The optimized clusters ( Cl2AlN3 )n(n = 2–4) all possess cyclic structures formed by Al atoms bridged by the α-nitrogen of the azide groups, and azido in azides has linear structure. The relationships between geometrical parameters and oligomerization degree n are discussed. The gas-phase structures of the trimers prefer to exist in boat-twisting conformation. As for the tetramer, the S4 symmetry structure is the most stable. The infrared spectra are obtained and assigned by vibrational analysis. Trends in thermodynamic properties with temperature and oligomerization degree n are discussed, respectively. A study of their thermodynamic properties suggests that monomer 1A forms clusters (2A, 3A and 3B) can occur spontaneously in the gas phase at temperatures up to 800 K. However, monomer 1A forms tetramers can occur spontaneously at temperatures below 600 K.

Reactivity of borylenes toward ethyne, ethene, and methane

The electronic and geometric structure of various substituted borylenes BR (where R = H, F, Cl, Br, CH(3), Ph, NH(2), NHMe, and NMe(2)) in their lowest singlet and triplet electronic states was investigated by computational means using hybrid density functional (B3LYP) and second-order Møller-Plesset perturbation theories combined with 6-311+G** and cc-pVTZ basis sets. The reactivity of singlet borylenes towards prototypical saturated and unsaturated hydrocarbons was examined by the MP2 method in conjugation with the cc-pVTZ basis set and also by coupled cluster [CCSD(T)] computations in combination with the aug-cc-pVTZ basis set. To study the energetics and the mechanism of the addition reaction of borylenes to unsaturated CC bonds, ethyne and ethene are chosen as model compounds. The insertion reaction of borylene into a C-H bond of methane was also investigated. The addition reactions of borylenes to multiple C-C bonds are strongly exothermic. In case of the BH molecule the reactions proceed without barrier and are the most exothermic. For the insertion reaction of borylenes into methane, two approaches could be identified. Again, the smallest reaction barriers and highest reaction energies were computed for the BH insertion, while the highest barriers and the smallest exothermicities were obtained for the BF molecule. On the basis of frontier molecular orbital energies, barrier heights, reaction energies, and transition state geometries BH is the most electrophilic borylene, followed by BPh, while aminoborylenes and BF are the most nucleophilic ones among the investigated derivatives. Accordingly, reactions of BH have the smallest barriers (if there is one at all) and the largest reaction energies, while the reactions of BF have the highest barriers and the smallest reaction energies.

On the directed gas phase synthesis of the imidoborane molecule (HNBH)--an isoelectronic molecule of acetylene (HCCH)

The elementary reaction of ground state boron atoms, (B((2)P(j))), with ammonia (NH(3)(X(1)A(1))) was conducted under single collision conditions at a collision energy of 20.5 ± 0.4 kJ mol(-1) in a crossed molecular beams machine. Combined with electronic structure calculations, our experimental results suggested that the reaction was initiated by a barrier-less addition of the boron atom to the nonbonding electron pair of the nitrogen atom forming a weakly bound BNH(3) collision complex. This intermediate underwent a hydrogen shift to a doublet HBNH(2) radical that decomposed via atomic hydrogen loss to at least the imidoborane (HBNH(X(1)Σ(+)) molecule, an isoelectronic species of acetylene (HCCH(X(1)Σ(g)(+))). Our studies are also discussed in light of the isoelectronic C(2)H(3) potential energy surface accessed via the isoelectronic carbon-methyl system.

The road to 13-15 nano structures: structures and energetics of (MYH(2))(4) tetramers (M = B, Al, Ga; Y = N, P, As)

A(III)B(V) compounds are important for the development of advanced ceramic semiconductor and nanoelectronic materials, solar cell elements, and light-emitting diodes. A series of these group 13-15 compounds of the general formula M(4)Y(4)H(8) (M = B, Al, Ga; Y = N, P, As) has been theoretically studied at the B3LYP/TZVP level of theory. The stability of different isomer structures is discussed to reveal the competitiveness of group 13-13, group 13-15, and group 15-15 bonding. Preferential bonding patterns are established, and trends in the stability with respect to M and Y are also discussed. New structural types, featuring perfectly planar M(4) and Y(4) rings, have been established.