Role of London Dispersion Interactions in Ga-Substituted Dipnictenes

From Neutral Diarsenes to Diarsene Radical Ions and Diarsene Dications

Diarsene [L(MeO)GaAs]2 (L=HC[C(Me)N(Ar)]2, Ar=2,6-iPr2C6H3, 4) reacts with MeOTf and MeNHC (MeNHC=1,3,4,5-tetra-methylimidazol-2-ylidene) to the diarsene [L(TfO)GaAs]2 (5) and the carbene-coordinated diarsene [L(MeO)GaAsAs(MeNHC)Ga(OMe)L] (6). The NHC-coordination results in an inversion of the redox properties of the diarsene 4, which shows only a reversible reduction event at E1/2=-2.06 V vs Fc0/+1, whereas the carbene-coordinated diarsene 6 shows a reversible oxidation event at E1/2=-1.31 V vs Fc0/+1. Single electron transfer reactions of 4 and 6 yielded [K[2.2.2.]cryp][L(MeO)GaAs]2 (8) and [L(MeO)GaAsAs(MeNHC)-Ga(OMe)L][B(C6F5)4] (9) containing the radical anion [L(MeO)GaAs]2⋅- (8⋅-) and the NHC-coordinated radical cation [L(MeO)GaAsAs(MeNHC)Ga(OMe)L]⋅+ (9⋅+), respectively, while the salt-elimination reaction of the triflate-coordinated diarsene 5 with Na[B(C6F5)4] gave [LGaAs]2[B(C6F5)4]2 (11) containing the dication [LGaAs]2 2+ (112+). Compounds 1-11 were characterized by 1H and 13C NMR, EPR (8, 9), IR, and UV-Vis spectroscopy and by single crystal X-ray diffraction (sc-XRD). DFT calculations provided a detailed understanding of the electronic nature of the diarsenes (4, 6) and the radical ions (8⋅-, 9⋅+), respectively.

Molecular Wind-Up Meter for the Quantification of London Dispersion Interactions

ConspectusThe experimental quantification of interactions on the molecular level provides the necessary basis for the design of functional materials and chemical processes. The interplay of multiple parameters and the small quantity of individual interactions pose a special challenge for such endeavors. The common method is the use of molecular balances, which can exist in two different states. Thereby, a stabilizing interaction can occur in one of the states, favoring its formation and thus affecting the thermodynamic equilibrium of the system. One challenge is determining the change in this equilibrium since various analytical methods could not be applied to fast-changing equilibria. A new and promising method for quantifying molecular interactions is the use of Molecular Wind-up Meters (MWM) in which the change in kinetics, rather than the effect on thermodynamics, is investigated. An MWM is transformed with an energy input (e.g. irradiation) into a metastable state. Then, the rate of thermal transformation back to the ground state is measured. The strength of interactions present in the metastable state controls the kinetics of the back reactions, allowing direct correlation. The advantage of this approach lies in the high sensitivity (energy differences can be larger by 1 order of magnitude) and, in general, allows the use of a broader range of solvents and analytical methods. An Azobenzene-based MWM has been established as a powerful tool to quantify London dispersion interactions. London dispersion (LD) represents the attractive part of the van der Waals potential. Although neglected in the past due to its weak character, it has been shown that the influence of LD on the structure, stability, and reactivity of matter can be decisive. Especially in larger molecules, its energy contribution increases overproportionately with the number of atoms, which has sparked increasing interest in the use of so-called dispersion energy donors (DED) as a new structural element. Application of the azobenzene-based MWM not only allowed the differentiation of bulkiness, but also systematically addressed the influence of the length of n-alkyl chains. Additionally, the solvent influence on LD was studied. Based on the azobenzene MWM, an increment system has been proposed, allowing a rough estimate of the effect of a specific DED.

Increasing Complexity in a Conformer Space Step-by-Step: Weighing London Dispersion against Cation-π Interactions

We report an evaluation of the importance of London dispersion in moderately large (up to 36 heavy atoms) organic molecules by means of a molecular torsion balance whose conformations "weigh" one interaction against another in the absence of solvents. The experimental study, with gas-phase cryogenic ion vibrational predissociation (CIVP) spectroscopy, solid-state Fourier transfer infrared (FT-IR), and single-crystal X-ray crystallography, is accompanied by density functional theory calculations, including an extensive search and analysis of accessible conformations. We begin with the unsubstituted molecular torsion balance, and then step up the complexity systematically by adding alkyl groups incrementally as dispersion energy donors (DEDs) to achieve a degree of chemical complexity comparable to what is typically found in transition states for many regio- and stereoselective reactions in organic and organometallic chemistry. We find clear evidence for the small attractive contribution by DEDs, as had been reported in other studies, but we also find that small individual contributions by London dispersion, when they operate in opposition to other weak noncovalent interactions, produce composite effects on the structure that are difficult to predict intuitively, or by modern quantum chemical calculations. The experimentally observed structures, together with a reasonable value for a reference cation-π interaction, indicate that the pairwise interaction between two tert-butyl groups, in the best case, is modest. Moreover, the visualization of the conformational space, and comparison to spectroscopic indicators of the structure, as one steps up the complexity of the manifold of noncovalent interactions, makes clear that in silico predictive ability for the structure of moderately large, flexible, organic molecules falters sooner than one might have expected.

Bis‐Phosphaketenes LM(PCO) 2 (M=Ga, In): A New Class of Reactive Group 13 Metal‐Phosphorus Compounds

Phosphaketenes are versatile reagents in organophosphorus chemistry. We herein report on the synthesis of novel bis‐phosphaketenes, LM(PCO)₂ (M=Ga 2 a, In 2 b; L=HC[C(Me)N(Ar)]₂; Ar=2,6‐i‐Pr₂C₆H₃) by salt metathesis reactions and their reactions with LGa to metallaphosphenes LGa(OCP)PML (M=Ga 3 a, In 3 b). 3 b represents the first compound with significant In−P π‐bonding contribution as was confirmed by DFT calculations. Compounds 3 a and 3 b selectively activate the N−H and O−H bonds of aniline and phenol at the Ga−P bond and both reactions proceed with a rearrangement of the phosphaethynolate group from Ga−OCP to M−PCO bonding. Compounds 2–5 are fully characterized by heteronuclear (¹H, ¹³C{¹H}, ³¹P{¹H}) NMR and IR spectroscopy, elemental analysis, and single crystal X‐ray diffraction (sc‐XRD).

Trapping reactions with highly unstable hydrides of antimony and bismuth

(Dipp₂NacNac)Ga (Dipp₂NacNac = HC{C(Me)N(Dipp)}₂; Dipp = 2,6-iPr₂C₆H₃) was used as a trapping reagent for the unstable compounds tBuSbH₂ and MeBiH₂ to yield (Dipp₂NacNac)GaH(SbHtBu) (1) and {(Dipp₂NacNac)GaH(BiMe)}₂ (2). Moreover, its reactions with a row of alkylated or arylated dichloro-bismuthenes resulted in either bridged species or the formation of a dibismuthane.

Selective 1,2 addition of polar X-H bonds to the Ga-P double bond of gallaphosphene L(Cl)GaPGaL

Gallaphosphene L(Cl)GaPGaL 1 (L = HC[C(Me)N(2,6-i-Pr₂-C₆H₃)]₂) reacts at ambient temperature with a series of polar X-H bonds, i.e. ammonia, primary amines, water, phenol, thiophenol, and selenophenol, selectively with 1,2 addition at the polar Ga-P double bond. The gallium atom serves as electrophile and the phosphorous atom is protonated in all reactions. The resulting complexes L(Cl)GaP(H)Ga(X)L (X = NH₂2, NHi-Pr 3, NHPh 4, OH 5, OXyl 6, SPh 7, SePh 8) were characterized by IR and heteronuclear (¹H, ¹³C{¹H}, ³¹P{¹H}) NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction.

Synthesis of Distibiranes and Azadistibiranes by Cycloaddition Reactions of Distibenes with Diazomethanes and Azides

Cycloaddition reactions of distibene [L(Me2N)GaSb]2 (L = HC[C(Me)NDipp]2; Dipp= 2,6-i-Pr2C6H3)=) with a series of organoazides RN3 (R = Ph, p-CF3-Ph, 1-adamantyl (ada)) yielded azadistibiranes [L(Me2N)GaSb]2NR (R = Ph 1, p-CF3-Ph...

Synthesis, structure and bonding nature of heavy dipnictene radical anions

Cyclic voltammetry (CV) studies of two L(X)Ga-substituted dipnictenes [L(R2N)GaE]2 (E = Sb, R = Me 1; E = Bi; R = Et 2; L = HC[C(Me)NDipp]2; Dipp = 2,6-i-Pr2C6H3) showed reversible reduction events. Single electron reduction of 1 and 2 with KC8 in DME in the presence of benzo-18-crown-6 (B-18-C-6) gave the corresponding dipnictenyl radical anions (DME)[K(B-18-C-6)][L(R2N)GaE]2 (E = Sb, R = Me 3; E = Bi, R = Et 4). Radical anions 3 and 4 were characterized by EPR, UV-vis and single crystal X-ray diffraction, while quantum chemical calculations gave deeper insight into the nature of the chemical bonding.

Reversible and Irreversible [2+2] Cycloaddition Reactions of Heteroallenes to a Gallaphosphene

[2+2] Cycloaddition reactions of gallaphosphene L(Cl)GaPGaL 1 (L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ) with carbodiimides [C(NR)2 ; R=i-Pr, Cy] and isocyanates [RNCO; R=Et, i-Pr, Cy] yielded four-membered metallaheterocycles LGa(Cl)P[μ-C(X)NR]GaL (X=NR, R=i-Pr 2, Cy 3; X=O, R=Et 4, i-Pr 5, Cy 6). Compounds 4-6 reversibly react with CO2 via [2+2] cycloaddition at ambient temperature to the six-membered metallaheterocycles LGa(Cl)P[μ-C(O)O]-μ-C(O)N(R)GaL (R=Et 7, i-Pr 8, Cy 9). Compounds 2-9 were characterized by IR and heteronuclear (1 H, 13 C{1 H}, 31 P{1 H}) NMR spectroscopy and elemental analysis, while quantum chemical calculations provided a deeper understanding on the energetics of the reactions.

CHAL336 Benchmark Set: How Well Do Quantum-Chemical Methods Describe Chalcogen-Bonding Interactions?

We present the CHAL336 benchmark set-the most comprehensive database for the assessment of chalcogen-bonding (CB) interactions. After careful selection of suitable systems and identification of three high-level reference methods, the set comprises 336 dimers each consisting of up to 49 atoms and covers both σ- and π-hole interactions across four categories: chalcogen-chalcogen, chalcogen-π, chalcogen-halogen, and chalcogen-nitrogen interactions. In a subsequent study of DFT methods, we re-emphasize the need for using proper London dispersion corrections when treating noncovalent interactions. We also point out that the deterioration of results and systematic overestimation of interaction energies for some dispersion-corrected DFT methods does not hint at problems with the chosen dispersion correction but is a consequence of large density-driven errors. We conclude this work by performing the most detailed DFT benchmark study for CB interactions to date. We assess 109 variations of dispersion-corrected and dispersion-uncorrected DFT methods and carry out a detailed analysis of 80 of them. Double-hybrid functionals are the most reliable approaches for CB interactions, and they should be used whenever computationally feasible. The best three double hybrids are SOS0-PBE0-2-D3(BJ), revDSD-PBEP86-D3(BJ), and B2NCPLYP-D3(BJ). The best hybrids in this study are ωB97M-V, PW6B95-D3(0), and PW6B95-D3(BJ). We do not recommend using the popular B3LYP functional nor the MP2 approach, which have both been frequently used to describe CB interactions in the past. We hope to inspire a change in computational protocols surrounding CB interactions that leads away from the commonly used, popular methods to the more robust and accurate ones recommended herein. We would also like to encourage method developers to use our set for the investigation and reduction of density-driven errors in new density functional approximations.

Multi-Talented Gallaphosphene for Ga-P-Ga Heteroallyl Cation Generation, CO2 Storage, and C(sp3 )-H Bond Activation

Gallaphosphene L(Cl)GaPGaL (2; L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ), which is synthesized by reaction of LGa(Cl)PCO (1) with LGa, reacts with [Na(OCP)(dioxane)2.5 ] to LGa(OCP)PGaL (3), whereas chloride abstraction with LiBArF 4 yields [LGaPGaL][BArF 4 ] (4; BArF 4 =B(C6 F5 )4 ). 4 represents a heteronuclear analog of the allyl cation according to quantum chemical calculations. Remarkably, 2 reversibly reacts with CO2 to yield L(Cl)Ga-P[μ-C(O)O]2 GaL (5), while reactions with acetophenone and acetone selectively give compounds 6 and 7 by C(sp3 )-H bond activation.

Mechanistic insights into the insertion and addition reactions of group 13 analogues of the six-membered N-heterocyclic carbenes: interplay of electrophilicity, basicity, and aromaticity governing the reactivity

Three fundamental concepts (aromaticity/basicity/electrophilicity), being heavily used in modern chemistry, have been applied in this work to study the chemical reactivity of six-membered-ring group 13 N-heterocyclic carbenes (G13-6-Rea; G13 = group 13 elements) using density functional theory (BP86-D3(BJ)/def2-TZVP). G13-6-Rea is isolobal to benzene. Two model reactions have been used in the present study: the insertion reaction of G13-6-Rea with methane and the [1 + 2] cycloaddition reaction of G13-6-Rea with ethene. Our theoretical analysis reveals that the chemical reactivity of B-6-Rea, Al-6-Rea, and Ga-6-Rea is governed by their HOMO (the sp²-σ lone pair orbital on the G13 element), and thus they can be considered nucleophiles. Conversely, the chemical behavior of In-6-Rea and Tl-6-Rea is determined by their LUMO (the vacant p-π orbital on the G13 element), and thus they can be considered electrophiles. On the basis of the VBSCD (valence bond state correlation diagram) model and ASM (activation strain model), this theoretical evidence demonstrates that the origin of activation barriers for the above model reactions is due to the atomic radius of the pivotal group 13 element in the six-membered-ring of G13-6-Rea. Accordingly, our theoretical conclusions suggest that the lower the atomic number and the smaller the atomic radius of the G13 atom, the higher the aromaticity of the six-membered-ring of G13-6-Rea and the smaller the singlet–triplet energy splitting ΔE_st of this N-heterocyclic carbene analogue, which will facilitate its chemical reactions. The theoretical findings originating from this study allow many predictions in experiments to be made.

On the basis of sophisticated theoretical models, the theoretical study demonstrated that the atomic radius of a group 13 element in a six-membered-ring NHC analogue plays an important role in determining its chemical reactivity.

A Mechanistic Study on Reactions of Group 13 Diyls LM with Cp*SbX2 : From Stibanyl Radicals to Antimony Hydrides

Oxidative addition of Cp*SbX2 (X=Cl, Br, I; Cp*=C5 Me5 ) to group 13 diyls LM (M=Al, Ga, In; L=HC[C(Me)N (Dip)]2 , Dip=2,6-iPr2 C6 H3 ) yields elemental antimony (M=Al) or the corresponding stibanylgallanes [L(X)Ga]Sb(X)Cp* (X=Br 1, I 2) and -indanes [L(X)In]Sb(X)Cp* (X=Cl 5, Br 6, I 7). 1 and 2 react with a second equivalent of LGa to eliminate decamethyl-1,1'-dihydrofulvalene (Cp*2 ) and form stibanyl radicals [L(X)Ga]2 Sb. (X=Br 3, I 4), whereas analogous reactions of 5 and 6 with LIn selectively yield stibanes [L(X)In]2 SbH (X=Cl 8, Br 9) by elimination of 1,2,3,4-tetramethylfulvene. The reactions are proposed to proceed via formation of [L(X)M]2 SbCp* as reaction intermediate, which is supported by the isolation of [L(Cl)Ga]2 SbCp (11, Cp=C5 H5 ). The reaction mechanism was further studied by computational calculations using two different models. The energy values for the Ga- and the In-substituted model systems showing methyl groups instead of the very bulky Dip units are very similar, and in both cases the same products are expected. Homolytic Sb-C bond cleavage yields van der Waals complexes from the as-formed radicals ([L(Cl)M]2 Sb. and Cp*. ), which can be stabilized by hydrogen atom abstraction to give the corresponding hydrides, whereas the direct formation of Sb hydrides starting from [L(Cl)M]2 SbCp* via concerted β-H elimination is unlikely. The consideration of the bulky Dip units reveals that the amount of the steric overload in the intermediate I determines the product formation (radical vs. hydride).

Synthesis of heteroleptic gallium-substituted antimony hydrides by stepwise β-H elimination

Heteroleptic stibanes Cp*(R)SbCl (R = Dip 1, N(SiMe₃)₂2, OB(NDipCH)₂3; Cp* = C₅Me₅; Dip = 2,6-i-Pr₂-C₆H₃) react with monovalent gallanediyl LGa (L = HC[C(Me)N(Dip)]₂) with elimination of 1,2,3,4-tetramethylfulvene, yielding heteroleptic metal-stabilized Sb hydrides [L(Cl)Ga](R)SbH (R = Dip 4, N(SiMe₃)₂5, OB(NDipCH)₂6). Compounds 1-6 were characterized by heteronuclear NMR (¹H, ¹¹B, ¹³C) and IR spectroscopy, and the solid-state structures of 4-6 were determined by single-crystal X-ray diffraction. A close correlation between the ¹H NMR chemical shift of the hydride ligand and the electronegativity of the Sb-coordinating atoms was revealed.

The unique β-diketiminate ligand in aluminum(i) and gallium(i) chemistry

Herein we present an overview of the last 10 years for aluminum(i) and gallium(i) stabilized by β-diketiminate ligands that undergo a series of oxidative addition reactions with molecules containing single and multiple bonds.

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.

Facile oxidative addition of O2 and S8 by an indium bis(carbene) analogue

The oxidative addition of oxygen towards an indium bis(carbenoid) yields an almost planar In₄O₂ ring, an isovalence-electronic analogue of dioxane, in which the indium is in the formal oxidation state +ii. The weaker oxidant sulfur, however, yields an eight-membered ring in a deck-chair conformation containing four indium(iii) centres and four sulfides in an alternating fashion. According to DFT calculations, this reactivity difference originates from kinetic rather than thermodynamic reasons. When the reaction with S₈ is performed with the in situ generated bis(carbenoid), a β-diketimine (NacNac) incorporating an S₄-chain bridging the two γ-carbon atoms of each NacNac unit is derived.

Exploring London Dispersion and Solvent Interactions at Alkyl-Alkyl Interfaces Using Azobenzene Switches

Interactions on the molecular level control structure as well as function. Especially interfaces between innocent alkyl groups are hardly studied although they are of great importance in larger systems. Herein, London dispersion in conjunction with solvent interactions between linear alkyl chains was examined with an azobenzene-based experimental setup. Alkyl chains in all meta positions of the azobenzene core were systematically elongated, and the change in rate for the thermally induced Z→E isomerization in n-decane was determined. The stability of the Z-isomer increased with longer chains and reached a maximum for n-butyl groups. Further elongation led to faster isomerization. The origin of the intramolecular interactions was elaborated by various techniques, including 1 H NOESY NMR spectroscopy. The results indicate that there are additional long-range interactions between n-alkyl chains with the opposite phenyl core in the Z-state. These interactions are most likely dominated by attractive London dispersion. This work provides rare insight into the stabilizing contributions of highly flexible groups in an intra- as well as an intermolecular setting.