Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Guanidinates as Alternative Ligands for Organometallic Complexes

For decades, ligands such as phosphanes or cyclopentadienyl ring derivatives have dominated Coordination and Organometallic Chemistry. At the same time, alternative compounds have emerged that could compete either for a more practical and accessible synthesis or for greater control of steric and electronic properties. Guanidines, nitrogen-rich compounds, appear as one such potential alternatives as ligands or proligands. In addition to occurring in a plethora of natural compounds, and thus in compounds of pharmacological use, guanidines allow a wide variety of coordination modes to different metal centers along the periodic table, with their monoanionic chelate derivatives being the most common. In this review, we focused on the organometallic chemistry of guanidinato compounds, discussing selected examples of coordination modes, reactivity and uses in catalysis or materials science. We believe that these amazing ligands offer a new promise in Organometallic Chemistry.

Alkali Metal Complexes of a Bis(diphenylphosphino)methane Functionalized Amidinate Ligand: Synthesis and Luminescence

A novel bis(diphenylphosphino)methane (DPPM) functionalized amidine ligand (DPPM-C(N-Dipp)2 H) (Dipp=2,6-diisopropylphenyl) was synthesized. Subsequent deprotonation with suitable alkali metal bases resulted in the corresponding complexes [M{DPPM-C(N-Dipp)2 }(Ln )] (M=Li, Na, K, Rb, Cs; L=thf, Et2 O). The alkali metal complexes form monomeric species in the solid state, exhibiting intramolecular metal-π-interactions. In addition, a caesium derivative [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 was obtained by cleavage of a diphenylphosphino moiety, forming an unusual six-membered ring structure in the solid state. All complexes were fully characterized by single crystal X-ray diffraction, NMR spectroscopy, IR spectroscopy as well as elemental analysis. Furthermore, the photoluminescent properties of the complexes were thoroughly investigated, revealing differences in emission with regards to the respective alkali metal. Interestingly, the hexanuclear [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 metallocycle exhibits a blue emission in the solid state, which is significantly red-shifted at low temperatures. The bifunctional design of the ligand, featuring orthogonal donor atoms (N vs. P) and a high steric demand, is highly promising for the construction of advanced metal and main group complexes.

A bis(imino)carbazolate pincer ligand stabilized mononuclear gallium(I) compound: synthesis, characterization, and reactivity

The first bis(imino)carbazolate pincer ligand supported mononuclear Ga(I) compound LGa: (3) was synthesized and fully characterized. Oxidation of 3 with elemental selenium afforded the dinuclear Ga(III) compound [LGa(μ-Se)]₂ (4) bearing two bridging Se atoms. Ligand substitution of Cr(CO)₆ with 3 under UV light irradiation afforded the gallylene-chromium complex LGa: → Cr(CO)₅ (5). In addition, the attempted synthesis of the aluminium analogue LAl: through reduction of LAlI₂ (7) only led to isolation of the dinuclear Al(III) compound (LAlI)₂ (8), most probably formed through the C-C coupling of two imino radicals.

Insights into the Factors Controlling the Origin of Activation Barriers in Group 13 Analogues of the Four-Membered N-Heterocyclic Carbenes

The mechanisms of C-H bond insertion and alkene cycloaddition were investigated theoretically using five model systems: group 13 analogues of the four-membered nucleophilic N-heterocyclic carbenes (NHCs) (1E; E = group 13 element). The theoretical findings indicate that, except for 1B with H2C=CH2, these four-membered NHCs undergo insertion and [1 + 2] cycloaddition reactions with difficulty because their activation barriers are quite high (31 kcal/mol). The theoretically confirmed chemical inertness of the four-membered NHCs 1Ga and 1In might explain why they have been experimentally detected at room temperature. Additionally, our theoretical observations indicate that the reactivity of these four-membered NHCs featuring a central group 13 element follows the order 1B ≫ 1Al > 1Ga > 1In > 1Tl. The theoretical examination suggests that the smaller the atomic radius of the central group 13 element in the four-membered NHC analogue is, the larger the aromaticity of this carbenic molecule is, the higher the basicity of this carbenic molecule in nature is, the larger its nucleophilic attack on other oncoming molecules is, the smaller the barrier heights of its C-H bond insertion and [1 + 2] cycloaddition reactions will be, the higher its exothermicities for these products will be, and thus, the greater its reactivity will be. Moreover, the present theoretical findings reveal that the reactivity of 1B is governed by its highest occupied molecular orbital, a nonbonding sp2 lone pair orbital. In contrast, the reactivity of the four heavier 1E' (E' = Al, Ga, In, and Tl) molecules is mainly determined by their lowest unoccupied molecular orbital, a vacant p-π orbital. The conclusions gained from this study allow many predictions to be made.

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.

Group 13 Heavier Carbene Analogues Stabilized by the Bulky Bis(4-benzhydryl-benzoxazol-2-yl)methanide Ligand

On the basis of the bulky bis(4-benzhydryl-benzoxazyl-2-yl)methane ligand (^4-BzhH2Box₂CH₂), neutral monovalent group 13 complexes [M₁₃(^4-BzhH2Box₂CH)] [M₁₃ = Tl (1), In (2), or Ga (3)] have been synthesized by salt metathesis reaction of the corresponding potassium or sodium precursor and TlOTf, InOTf, or "GaI". The diiodido gallium species [GaI₂(^4-BzhH2Box₂CH)] (3a) was realized as a byproduct once the synthesis of 3 was carried out at higher temperatures. The synthesis of [AlI₂(^4-BzhH2Box₂CH)] (6) as a potential precursor for an aluminum(I) congener was accomplished by two alternative synthetic routes. During one of those procedures, [AlMe₂(^4-BzhH2Box₂CH)] (4) was synthesized in good yields by deprotonation with an AlMe₃ solution (method A). Subsequently, 4 was converted to the monoiodinated species [AlMeI(^4-BzhH2Box₂CH₂)] (5) using 1 equiv of I₂ or to 6 by iodination with 2 equiv of I₂ at 70 °C for 4 days. As an alternative, complex 6 could be prepared by iodination of 1 equiv of I₂ and [AlH₂(^4-BzhH2Box₂CH)] (7), which was previously obtained by facile reaction of ^4-BzhH2Box₂CH₂ and AlH₃NMe₂Et. All main products 1-7 were completely characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray structure determination. Alane 7 was additionally analyzed by solid-state fluorescence spectroscopy. Density functional theory calculations on [M₁₃(^4-BzhH2Box₂CH)] [M₁₃ = Tl (1), In (2), Ga (3), or Al] revealed that the complexes consist of monovalent group 13 cations coordinated by an anionic (^4-BzhH2Box₂CH) ligand similar to metallacycles incorporating a NacNac ligand.

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.

Reduction of organic azides by indyl-anions. Isolation and reactivity studies of indium-nitrogen multiple bonds

The synthesis of a new potassium-indyl complex, K[In(NONAr)] (NONAr = [O(SiMe2NAr)2]2-, Ar = 2,6-iPr2C6H3) and its reactivity with organic azides RN3 is reported. When R = 2,6-bis(diphenylmethyl)-4- t Bu-phenyl, a dianionic alkyl-amide ligand is formed via C-H activation across a transient In-Nimide bond. Reducing the size of the R-group to 2,4,6-trimethylphenyl (mesityl, Mes) enables oxidation of the indium and elimination of dinitrogen to afford the imide species, K[In(NONAr)(NMes)]. The anion contains a short In-Nimide bond, shown computationally to contain appreciable multiple bond character. Reaction of isolated imides with an additional equivalent of azide (R = Mes, SiMe3) generates tetrazenido-indium compounds K[In(NONAr){κ-N,N'-N4(Mes)(R)-1,4}], shown by X-ray crystallography to contain planar InN4 heterocycles in the anion.

Kinetic stabilization of low-oxidation state and terminal hydrido main group metal complexes by a sterically demanding N,N′-bis(2,6-terphenyl)triazenide

Thermally robust main group metal complexes featuring terminal hydride ligands are achieved by deploying a sterically demanding N,N′-bis(2,6-terphenyl)triazenide ligand.

Combined QTAIM and ETS-NOCV investigation of the interactions in ClnM[PhB(NtBu)2] complexes with M = Si & Ge (n = 0), As & Sb (n = 1), Te & Po (n = 2)

In this work, the nature of the chemical interactions between the metalloid atom (M = Si, Ge, As, Sb, Te, Po) and the nitrogen atoms in the bora-amidinate (bam) complexes (Cl_nM[PhB(N^tBu)₂]) are investigated, mainly via density-based indices. The descriptors used are derived using the quantum theory of atoms in molecules and natural orbitals for chemical valence approaches. It is shown that the strongest interaction is achieved with silicon. Indeed, it is generally the lightest metalloid in a particular group of the periodic table (i.e., Si, As, and Te for groups 14-16, respectively) that exhibits the strongest bond in the bam complex. This suggests that the atomic radius of the metalloid is a useful parameter for predicting the bonding strength. Extended transition state (ETS) decomposition results indicate that the interactions are more electrostatic than due to orbital interactions.

En Route to Bis-Carbene Analogues of the Heavier Group 13 Elements: Consideration of Bridging Group and Metal(I) Source

With the aim of exploring the effect of steric constraints imposed on the metal-metal interaction of bis-carbene analogues of thallium by the linking scaffold, seven dinuclear thallium diyls with a series of rigid, semirigid, and flexible bridging scaffolds were synthesized. The solid-state molecular structures were determined for four of these compounds by single-crystal XRD and compared with the results of DFT calculations, which were performed for all substances. These compounds serve as models to investigate the metal-metal distance in the absence of co-coordinated molecules (additional ligands, solvent molecules). In addition, the effect of the metal(I) precursor, and more specifically the counterion, on the synthetic access to bis-carbene analogues of indium and thallium was investigated. For indium, only InI yields the desired dinuclear indium diyl. With InBr no reaction was observed, and using InCl gave rise to a mononuclear indium(III) compound. For thallium, both TlI and TlBr allow access to the related bis-carbene analogue, although the yield with the latter is significantly lower. In contrast, no reactions were observed with TlCl and TlBF₄ .

Hydrophosphination-type reactivity promoted by bismuth phosphanides: scope and limitations

Phenyl isocyanate inserts into the Bi-P bond of the terminal phosphanide Bi(NON^Ar)(PCy₂) (NON^Ar = [O(SiMe₂NAr)₂]^2- Ar = 2,6-iPr₂C₆H₃) to afford the κ²N,O-phosphanylcarboxamidate complex. Liberation of the hydrophosphination product from the metal is achieved by reaction with HPPh₂. The diphenylphosphanide product, Bi(NON^Ar)(PPh₂), is however unstable and decomposes to generate reduced species, preventing catalytic turnover.

PNacPNacE: (E = Ga, In, Tl) – monomeric group 13 metal(i) heterocycles stabilized by a sterically demanding bis(iminophosphoranyl)methanide

Monomeric group 13 metal(i) complexes of gallium, indium and thallium stabilised by a sterically demanding bis(iminophosphoranyl)methanide ligand have been prepared and characterised.

Boryl substituted group 13 metallylenes: complexes with an iron carbonyl fragment

The first examples of boryl substituted aluminylene and gallylene complexes, [(DAB)B(THF)Al{Fe(CO)₃(μ-CO)}]₂ and [(DAB)BGa{μ-Fe(CO)₄}]₂ (DAB = {(C₆H₃Prⁱ₂-2,6)NCH}₂) have been prepared by reduction of MX₂(THF){B(DAB)} (M = Al or Ga, X = Cl or Br) with K₂[Fe(CO)₄]. Spectroscopic and crystallographic analyses of the compounds show them to be structurally distinct dimers, the latter of which possesses a close GaGa separation that computational analyses reveal has negligible bonding character.

Formation of an Unusual Bis(diguanidinate) Ligand via Nucleophilic Attack of a Guanidinate onto a Carbodiimide

During an investigation of the synthesis of the new mono- and bis(guanidinate) complexes [{iPr2NC(···NiPr)2}GaCl2] (2) and [{iPr2NC(···NiPr)2}2GaCl] (3) from the in situ reactions of the lithium guanidinate [{iPr2NC(···NiPr)2}Li]n (1) with GaCl3, an unexpected new guanidinate expansion reaction was uncovered in which it is found that the nucleophilic addition of 2 to the carbodiimide, iPrN=C=NiPr, gives [iPrN{C(NiPr)=NiPr}{C(=NiPr)NiPr2}GaCl2] (4), containing an unusual bis(diguanidinate) ligand.

The ligand-based quintuple bond-shortening concept and some of its limitations

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a Cr-Cr distance of 1.7056 (12) Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr(I) dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.

Oxidative addition reactions of element-hydrogen bonds with different polarities to a gallium(I) compound

The gallium(I) derivative [Ga({N(dipp)CMe}(2)CH)] (1; dipp = 2,6-diisopropylphenyl) undergoes facile oxidative addition reactions with various element-hydrogen bonds including N-H, P-H, O-H, Sn-H, and H-H bonds. This was demonstrated by its reaction with triphenyltin hydride, ethanol, water, diethylamine, diphenylphosphane, and dihydrogen. All products were characterized by means of single-crystal X-ray structure determination, NMR spectroscopy, IR spectroscopy, and mass spectrometry.