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

Thorium(IV)-antimony complexes exhibiting single, double, and triple polar covalent metal-metal bonds

There is continued burgeoning interest in metal-metal multiple bonding to further our understanding of chemical bonding across the periodic table. However, although polar covalent metal-metal multiple bonding is well known for the d and p blocks, it is relatively underdeveloped for actinides. Homometallic examples are found in spectroscopic or fullerene-confined species, and heterometallic variants exhibiting a polar covalent σ bond supplemented by up to two dative π bonds are more prevalent. Hence, securing polar covalent actinide double and triple metal-metal bonds under normal experimental conditions has been a fundamental target. Here we exploit the protonolysis and dehydrocoupling chemistry of the parent dihydrogen-antimonide anion, to report one-, two- and three-fold thorium-antimony bonds, thus introducing polar covalent actinide-metal multiple bonding under normal experimental conditions between some of the heaviest ions in the periodic table with little or no bulky-substituent protection at the antimony centre. This provides fundamental insights into heavy element multiple bonding, in particular the tension between orbital-energy-driven and overlap-driven covalency for the actinides in a relativistic regime.

Synthesis and Application of Alkali Metal Antimonide-A New Approach to Antimony Chemistry

Alkali metal dihydrogen-antimonides [M(L)_x SbH₂ ], short: alkali metal antimonides (M=Li, Na, K, Rb, Cs; 1: L=pmdta; 2: L=crown-ether), were prepared from stibine and n-Butyllithium, M(hmds) (hmds=hexamethyldisilazane) or MOtBu, respectively. We developed a generally applicable synthesis route for these compounds and the obtained compounds were examined on their stability depending on the alkali metal and stabilizing additives used, whereby the use of appropriate crown-ethers allowed their isolation and characterization at room temperature. Moreover, the 1,4-dioxane adduct [Na(dioxane)_x SbH₂ ] was the appropriate starting compound for the synthesis of the first primary silylstibane (Me₃ Si)₃ SiSbH₂ (3) which was characterized by NMR and IR spectroscopy. Reaction of 3 with (Dipp₂ NacNac)Ga (Dipp₂ NacNac=HC{C(Me)N(Dipp)}₂ ; Dipp=2,6-iPr₂ C₆ H₃ ) resulted in the formation of (Dipp₂ NacNac)GaH(SbHSi(SiMe₃ )₃ ) (4) which was furthermore characterized by single crystal x-ray diffraction.

Recent advances in stable main group element radicals: preparation and characterization

Radical species are significant in modern chemistry. Their unique chemical bonding and novel physicochemical properties play significant roles not only in fundamental chemistry, but also in materials science. Main group element radicals are usually transient due to their high reactivity. Highly stable radicals are often stabilized by π-delocalization, sterically demanding ligands, carbenes and weakly coordinating anions in recent years. This review presents the recent advances in the synthesis, characterization, reactivity and physical properties of isolable main group element radicals.

X-Band Parallel-Mode and Multifrequency Electron Paramagnetic Resonance Spectroscopy of S = 1/2 Bismuth Centers

The recent successes in the isolation and characterization of several bismuth radicals inspire the development of new spectroscopic approaches for the in-depth analysis of their electronic structure. Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for the characterization of main group radicals. However, the large electron-nuclear hyperfine interactions of Bi (²⁰⁹Bi, I = 9/2) have presented difficult challenges to fully interpret the spectral properties for some of these radicals. Parallel-mode EPR (B₁∥B₀) is almost exclusively employed for the study of S > 1/2 systems but becomes feasible for S = 1/2 systems with large hyperfine couplings, offering a distinct EPR spectroscopic approach. Herein, we demonstrate the application of conventional X-band parallel-mode EPR for S = 1/2, I = 9/2 spin systems: Bi-doped crystalline silicon (Si:Bi) and the molecular Bi radicals [L(X)Ga]₂Bi^• (X = Cl or I) and [L(Cl)GaBi(^MecAAC)]^•+ (L = HC[MeCN(2,6-iPr₂C₆H₃)]₂). In combination with multifrequency perpendicular-mode EPR (X-, Q-, and W-band frequencies), we were able to fully refine both the anisotropic g- and A-tensors of these molecular radicals. The parallel-mode EPR experiments demonstrated and discussed here have the potential to enable the characterization of other S = 1/2 systems with large hyperfine couplings, which is often challenging by conventional perpendicular-mode EPR techniques. Considerations pertaining to the choice of microwave frequency are discussed for relevant spin-systems.

Bisstibane-distibane conversion via consecutive single-electron oxidation and reduction reaction

peri-Substituted naphthalene complexes (Trip₂Pn)₂Naph (Pn = Sb 1, Bi 2) were synthesised and their redox behaviour investigated. Oxidation of 1 with [Fc][BAr^F] (BAr^F = B(C₆F₅)₄) yielded [(Trip₂Sb)(TripSb)Naph][BAr^F] (3) containing the stibane-coordinated stibenium cation [(Trip₂Sb)(TripSb)Naph]⁺. Subsequent reduction of 3 with KC₈ yielded distibane (TripSb)₂Naph (4). 1-4 were characterised by NMR (¹H, ¹³C) and IR spectroscopy as well as single-crystal X-ray diffraction (sc-XRD), while their electronic structures were analysed by quantum chemical computations.

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).

Sequential Reduction of Borylstibane to an Electronically Nonsymmetric Diboryldistibene Radical Anion

Understanding the formation of metal-metal bonds and their electronic structures is still a scientific task. We herein report on the stepwise synthesis of boryl-substituted antimony compounds in which the antimony atoms adopt four different oxidation states (+III, +II, +I, +I/0). Sb-C bond homolysis of Cp*[(HCNDip)₂B]SbCl (1; Cp* = C₅Me₅; Dip = 2,6-iPr₂C₆H₃) gave diboryldichlorodistibane [(HCNDip)₂BSbCl]₂ (2), which reacted with KC₈ to form diboryldistibene [(HCNDip)₂BSb]₂ (3) and traces of cyclotetrastibane [(HCNDip)₂B]₃Sb₄Cl (5). One-electron reduction of 3 yielded the potassium salt of the diboryldistibene radical anion [(HCNDip)₂BSb]₂̇^-, [K(18-c-6)(OEt₂)][{(HCNDip)₂BSb}₂] (4), which exhibits an unprecedented inequivalent spin localization on the Sb-Sb bond and hence an unsymmetric electronic structure. Compounds 1-4 were characterized by heteronuclear nuclear magnetic resonance (NMR) (¹H, ¹³C, ¹¹B), infrared (IR), ultraviolet-visible (UV-vis) spectroscopy (3, 4), and single crystal X-ray diffraction (sc-XRD, 1-5), while the bonding nature of 3 and 4 was analyzed by quantum chemical calculations. EPR spectroscopy resolves the dissimilar Sb hyperfine tensors of 4, reflecting the inequivalent spin distribution, setting 4 uniquely apart from all previously characterized dipnictene radical anions.

Geminal C-Cl and Si-Cl bond activation of chloromethanes and chlorosilanes by gallanediyl LGa

The activation of relatively inert E-X σ-bonds by low-valent main group metal complexes is receiving increasing interest. We here confirm the promising potential of gallanediyl LGa (L = HC[C(Me)N(Dip)]₂, Dip = 2,6-i-Pr₂C₆H₃) to activate E-Cl (E = C, Si) σ-bonds of group 14 element compounds. Equimolar reactions of LGa with chloromethanes and chlorosilanes EH_xCl_4-x (E = C, x = 0-2; E = Si, x = 0, 1) occurred with E-Cl bond insertion and formation of gallylmethanes and -silanes L(Cl)GaEH_xCl_3-x (E = C, x = 2 (1), 1 (2), 0 (3); E = Si, x = 1 (4)). In contrast, consecutive insertion into a geminal E-Cl bond was observed with two equivalents of LGa, yielding digallyl complexes [L(Cl)Ga]₂EH_xCl_2-x (E = C, x = 2 (5); E = Si, x = 1 (6), 0 (7)). Compounds 1-7 were characterized by heteronuclear NMR (¹H, ¹³C, ²⁹Si (4, 6)), IR spectroscopy and elemental analysis, and their solid-state structures were determined by 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, 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.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

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.

Stibine as a reagent in molecular chemistry - targeted synthesis of primary and secondary stibanyl-gallanes and their lighter homologues

The reactions of SbH₃ with one or two equivalents of (Dipp₂NacNac)Ga (Dipp₂NacNac = HC{C(Me)N(Dipp)}₂; Dipp = 2,6-iPr₂C₆H₃) yield the primary and secondary stibanes (Dipp₂NacNac)GaH(SbH₂) (3) and {(Dipp₂NacNac)GaH}₂(SbH) (5). Their lighter homologs were obtained from the analogous reactions with phosphine and arsine. All compounds were characterized using heteronuclear NMR-spectroscopy, IR-spectroscopy and single-crystal X-ray diffraction.

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.

From π-Bonded Gallapnictenes to Nucleophilic, Redox-Active Metal-Coordinated Pnictanides

A comprehensive reactivity study of gallapnictenes LGaEGa(Cl)L (E=As, Sb; L=HC[C(Me)N(Ar)]2 , Ar=Dip=2,6-i-Pr2 C6 H3 ) proved the nucleophilic character of the pnictogen and the electrophilic nature of the Ga atom. Reactions of LGaEGa(Cl)L with imidazolium chloride [IPrH][Cl] yielded {[LGa(Cl)]2 E- }{IPrH+ } (E=As 1, Sb 2), and those with HCl and MeI gave pnictanes [LGa(Cl)]2 EH (E=As 5, Sb 6) and L(I)GaE(Me)Ga(Cl)L (E=As 7, Sb 8). Pnictanides 1 and 2 also react with [H(OEt2 )2 ][BArF 4 ] (BArF 4 =B(C6 F5 )4 ) to 5 and 6, while reactions with MeI yielded [LGa(Cl)]2 EMe (E=As 9, Sb 10). Single electron oxidation reactions of pnictanides 1 and 2 gave the corresponding radicals [LGa(Cl)]2 E. (E=As, Sb).

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.

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*=C₅Me₅) to group 13 diyls LM (M=Al, Ga, In; L=HC[C(Me)N (Dip)]₂, Dip=2,6‐iPr₂C₆H₃) 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]₂Sb^. (X=Br 3, I 4), whereas analogous reactions of 5 and 6 with LIn selectively yield stibanes [L(X)In]₂SbH (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]₂SbCp* as reaction intermediate, which is supported by the isolation of [L(Cl)Ga]₂SbCp (11, Cp=C₅H₅). 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]₂Sb^. 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]₂SbCp* 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).

Applications of electron paramagnetic resonance spectroscopy to heavy main-group radicals

The exploration of heavy main-group radicals is rapidly expanding, for which electron paramagnetic resonance (EPR) spectroscopic characterisation plays a key role. EPR spectroscopy has the capacity to deliver information of the radical's electronic, geometric and bonding structure. Herein, foundations of electron-nuclear hyperfine analysis are detailed before reviewing more recent applications of EPR spectroscopy to As, Sb, and Bi centred radicals. Additional diverse examples of the application of EPR spectroscopy to other heavy main group radicals are highlighted.