Reactions of Tin(II) Hydride Species with Unsaturated Molecules

Interrogating the anti-Insertion of Alkynes into Gold(III)

Alkyne hydrofunctionalizations are a powerful strategy to efficiently build up structural complexity. The selectivity of these reactions is typically governed by the interaction between the alkyne and a metal-hydride, which commonly proceeds via a well-understood syn-insertion mechanism. In contrast, anti-insertions are far less common, with proposed mechanisms often extrapolated from literature precedents rather than grounded in direct experimental evidence. While gold complexes rank among the most efficient catalysts for such transformations, the mechanistic understanding of the key alkyne insertion step remains incomplete. In this study, we demonstrate that stable gold(III)-hydrides, featuring a (P∧N∧C) ligand, undergo selective insertion of alkynes to yield the corresponding anti-Markovnikov Z-vinyl complexes. A combination of control experiments, kinetic studies, and computational analyses reveals a nonradical, bimolecular insertion process, in which water plays a pivotal role by accelerating the reaction and potentially stabilizing a highly reactive, T-shaped gold(I) intermediate. Notably, this is the first demonstration of the insertion of both activated and unactivated terminal and internal alkynes into a gold(III)-hydride complex.

Non-covalent interactions and charge transfer in the CO 2 activation by low-valent group 14 complexes

CONTEXT

The CO2 activation by low-valent group 14 catalysts encompasses the rupture of varied covalent bonds in a single transition state through a concerted pathway. The bond between the central main group atom and the hydride in the complex is elongated to trigger the formation of the C-H bond with CO2 accompanied by the concomitant formation of the E-O bond between the complex and CO2 to lead the corresponding formate product. Prior studies have established that besides the apolar nature of CO2 , its initial interaction with the complex is primarily governed by electrostatic interactions. Notably, other stabilizing interactions and the transfer of charge between catalysts and CO2 during the initial phases of the reaction have been ignored. In this study, we have quantified the non-covalent interactions and charge transfer that facilitate the activation of CO2 by group 14 main group complex. Our findings indicate that electrostatic interactions predominantly stabilize the complex and CO2 in the reactant region. However, induction energy becomes the main stabilizing force as the reaction progresses towards the transition state, surpassing electrostatics. Induction contributes about 50% to the stabilization at the transition state, followed by electrostatics (40%) and dispersion interactions (10%). Atomic charges calculated with the minimal basis iterative stockholder (MBIS) method reveal larger charge transfer for the back-side reaction path in which CO2 approaches the catalysts in contrast to the front-side approach. Notably, it was discovered that a minor initial bending of CO2 to approximately 176 ∘ initiates the charge transfer process for all systems. Furthermore, our investigation of group 14 elements demonstrates a systematic reduction in both activation energies and charge transfer to CO2 while descending in group 14.

METHODS

All studied reactions were characterized along the reaction coordinate obtained with the intrinsic reaction coordinate (IRC) methodology at the M06-2X/6-31 g(d,p) level of theory. Gibbs free energy in toluene was computed using electronic energies at the DLPNO-CCSD(T)/cc-pVTZ-SSD(E) level of theory. Vibrational and translational entropy corrections were applied to provide a more accurate description of the obtained Gibbs free energies. To better characterize the changes in the reaction coordinate for all reactions, the reaction force analysis (RFA) has been employed. It enables the partition of the reaction coordinate into the reactant, transition state, and product regions where different stages of the mechanism occur. A detailed characterization of the main non-covalent driving forces in the initial stages of the activation of CO2 by low-valent group 14 complexes was performed using symmetry-adapted perturbation theory (SAPT). The SAPT0-CT/def2-SVP method was employed for these computations. Charge transfer descriptors based on atomic population using the MBIS scheme were also obtained to complement the SAPT analyses.

Chromium Complexes with Benzanellated N-Heterocyclic Phosphenium Ligands-Synthesis, Reactivity and Application in Catalytic CO2 Reduction

A chromium complex carrying two benzanellated N-heterocyclic phosphenium (bzNHP) ligands was prepared by a salt metathesis approach. Spectroscopic studies suggest that the anellation enhances the π-acceptor ability of the NHP-units, which is confirmed by the facile electrochemical reduction of the complex to a spectroscopically characterized radical anion. Co-photolysis with H2 allowed extensive conversion into a σ-H2-complex, which shows a diverse reactivity towards donors and isomerizes under H-H bond fission and shift of a hydride to a P-ligand. The product carrying phosphenium, phosphine and hydride ligands was also synthesized independently and reacts reversibly with CO and MeCN to yield bis-phosphine complexes under concomitant Cr-to-P-shift of a hydride. In contrast, CO2 was not only bound but reduced to give an isolable formato complex, which reacted with ammonia borane under partial recovery of the metal hydride and production of formate. Further elaboration of the reactions of the chromium complexes with CO2 and NH3BH3 allowed to demonstrate the feasibility of a Cr-catalyzed transfer hydrogenation of CO2 to methanol. The various complexes described were characterized spectroscopically and in several cases by XRD studies. Further insights in reactivity patterns were provided through (spectro)electrochemical studies and DFT calculations.

Insights into the variations of kinetic and potential energies in a multi-bond reaction: the reaction electronic flux perspective

CONTEXT

The debate over whether kinetic energy (KE) or potential energy (PE) are the fundamental energy components that contribute to forming covalent bonds has been enduring and stimulating over time. However, the supremacy of these energy components in reactions where multiple bonds are simultaneously formed or broken has yet to be explored. In this study, we use the reaction electronic flux (REF), an effective tool for investigating changes in driving electronic activity when bond formation or dissociation occurs in a chemical reaction, to examine the fluctuations in the KE and PE in a multi-bond reaction. To that end, the activation of CO2 by low-valent group 14 catalysts through a concerted σ -bond metathesis mechanism is analyzed. The findings of this preliminary study suggest that the REF can be utilized as a tool to rationalize alterations in the KE and PE in a multi-bond reaction. Specifically, analyses across the reaction coordinate reveal that changes in the KE and PE precede activation in the REF, stimulating the electronic activity where bond formation or dissociation processes dominate.

METHODS

The activation of CO2 by the low-valent LEH catalysts (L = N,N'-bis(2,6-diisopropyl phenyl)- β -diketiminate; E = Si, Ge, Sn, and Pb) was studied along the reaction coordinate at the M06-2X/6-31 G(d,p)-LANL2DZ(E) level of theory. The respective minimum energy path calculations were obtained using the intrinsic reaction coordinate (IRC) procedure. The reaction electronic flux (REF) was calculated through the computation of the electronic chemical potential using the frontier molecular orbital approximation. Mayer bond orders along the reaction coordinate have been determined using the NBO 3.1 program in Gaussian16. Most of the reaction coordinate quantities reported in this study (REF, KE, PE, among others) have been determined using the Kudi program and custom Python scripts.

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Divergent Reactivity of a Cyclic Bis-Hydridostannylene: A Masked Sn(I) Diradicaloid

Herein, reactivity studies of a cyclic bis-hydridostannylene [(ADC)SnH]2 (1-H2) (ADC=PhC{(NDipp)C}2; Dipp=2,6-iPr2C6H3) with various unsaturated organic substrates are reported. Reactions of terminal alkynes (RC≡CH) with 1-H2 afford mixed acetylide-vinyl-functionalized bis-stannylenes via dehydrogenation and hydrostannylation. Treatment of 1-H2 with PhC≡CCH3 gives a unique distannabarrelene via dehydrogenative C(sp3)-H stannylation and hydrostannylation of the C≡CCH3 moiety. 1-H2 undergoes dehydrogenative [2+2]-cycloaddition reactions with diphenylacetylene, azobenzene, acetone, benzophenone, and benzaldehyde to form the 1,4-distannabarrelene derivatives. The elimination of H2 in these reactions suggests the masked-diradical property of 1-H2. In fact, these [2+2]-cycloaddition products are also accessible on treatments of the Sn(I) diradicaloid [(ADC)Sn]2 (1) with appropriate reagents. All compounds have been characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction. Moreover, the catalytic activity of 1-H2 has been shown for the hydroboration of unsaturated substrates.

Facile Access to Cationic Methylstannylenes and Silylenes Stabilized by E-Pt Bonding and their Methyl Group Transfer Reactivity

We describe a family of cationic methylstannylene and chloro- and azidosilylene organoplatinum(II) complexes supported by a neutral, binucleating ligand. Methylstannylenes MeSn:+ are stabilized by coordination to PtII and are formed by facile Me group transfer from dimethyl or monomethyl PtII complexes, in the latter case triggered by concomitant B-H, Si-H, and H2 bond activation that involves hydride transfer from Sn to Pt. A cationic chlorosilylene complex was obtained by formal HCl elimination and Cl- removal from HSiCl3 under ambient conditions. The computational studies show that stabilization of cationic methylstannylenes and cationic silylenes is achieved through weak coordination to a neutral N-donor ligand binding pocket. The analysis of the electronic potentials, as well as the Laplacian of electron density, also reveals the differences in the character of Pt-Si vs. Pt-Sn bonding. We demonstrate the importance of a ligand-supported binuclear Pt/tetrel core and weak coordination to facilitate access to tetrylium-ylidene Pt complexes, and a transmetalation approach to the synthesis of MeSnII :+ derivatives.

Reactivity of NHI-Stabilized Heavier Tetrylenes towards CO2 and N2 O

A heteroleptic amino(imino)stannylene (TMS2 N)(It BuN)Sn: (TMS=trimethylsilyl, It Bu=C[(N-t Bu)CH]2 ) as well as two homoleptic NHI-stabilized tetrylenes, (It BuN)2 E: (NHI=N-heterocyclic imine, E=Ge, Sn) are presented. VT-NMR investigations of (It BuN)2 Sn: (2) reveal an equilibrium between the monomeric stannylene at room temperature and the dimeric form at -80 °C as well as in the solid state. Upon reaction of the homoleptic tetrylenes with CO2 , both compounds insert two equivalents of CO2 , however differing bonding modes can be observed. (It BuN)2 Sn: (2) inserts one equivalent of CO2 into each Sn-N bond, giving carbamato groups coordinated κ2 O,O' to the metal center. With (It BuN)2 Ge: (3), the Ge-N bonds stay intact upon activation, being bridged by one molecule of CO2 respectively, forming 4-membered rings. Furthermore, the reactivity of 2 towards N2 O was investigated, resulting in partial oxidation to form stannylene dimer [((It BuN)3 SnO)(It BuN)Sn:]2 (6).

Indium(III)/2-benzoylpyridine chemistry: interesting indium(III) bromide-assisted transformations of the ligand

Reactions of 2-benzoylpyridine, (py)(ph)CO, with InX₃ (X = Cl, Br) in EtOH at room temperature have been studied. The InCl₃/(py)(ph)CO system has provided access to complex [InCl₃{(py)(ph)CO}(EtOH)]·{(py)(ph)CO} (1) and the byproduct {(pyH)(ph)CO}Cl (2). The reaction of InBr₃ with (py)(ph)CO has led to a mixture of (L)[InBr₄{(py)(ph)CO}] (3) and [In₂Br₄{(py)(ph)CH(O)}₂(EtOH)₂] (4), where L⁺ is the 9-oxo-indolo[1,2-a]pyridinium cation and (py)(ph)CH(O)^- is the anion of (pyridin-2-yl)methanol. Based on solubility and crystallisation time differences between the two components of the mixture, complex 4 was isolated in pure form, i.e. free from 3. The formations of the counterion L⁺ and the coordinated (py)(ph)CH(O)^- anion represent clearly InBr₃-promoted/assisted transformations. Reaction mechanisms have been proposed for the formation of 2, 3 and 4. Complex 4 could also be isolated by the reaction of InBr₃ and pre-formed (py)(ph)CH(OH) in EtOH. The solid-state structures of 1, 3 and 4 were determined by single-crystal X-ray crystallography, while the identity of the salt 2 was confirmed by microanalyses and a variety of spectroscopic techniques, including ESI-MS spectra. In the indium(III) complexes, the metal ions are 6-coordinate with a distorted octahedral geometry. The halogeno groups (Cl^-, Br^-) in the three complexes are terminal. The (py)(ph)CO molecule behaves as a N,O-bidentate (1.11) ligand in 1 and 3. A terminal EtOH ligand completes the coordination sphere of In^III in 1. The alkoxo oxygen atoms of the two 2.21 (py)(ph)CH(O)^- ligands doubly bridge the In^III centers in 4 creating a {InIII2(μ-OR)₂}⁴⁺ core; a nitrogen atom of one reduced organic ligand, two bromo ions and one terminal EtOH molecule complete the 6-coordination at each metal centre. Complexes 1, 3 and 4 were characterised by IR and Raman spectroscopies, and the data were discussed in terms of their known solid-state structures. Molar conductivity data and ¹H NMR spectra were used in an attempt to probe the behaviour of the complexes in DMSO. The to-date observed metal ion-assisted/promoted transformations of (py)(ph)CO are also discussed.

Molecular Ln(III)-H-E(II) Linkages (Ln=Y, Lu; E=Ge, Sn, Pb)

Following the alkane-elimination route, the reaction between tetravalent aryl tintrihydride Ar*SnH3 and trivalent rare-earth-metallocene alkyls [Cp*2 Ln(CH{SiMe3 }2 )] gave complexes [Cp*2 Ln(μ-H)2 SnAr*] implementing a low-valent tin hydride (Ln=Y, Lu; Ar*=2,6-Trip2 C6 H3 , Trip=2,4,6-triisopropylphenyl). The homologous complexes of germanium and lead, [Cp*2 Ln(μ-H)2 EAr*] (E = Ge, Pb), were accessed via addition of low-valent [(Ar*EH)2 ] to the rare-earth-metal hydrides [(Cp*2 LnH)2 ]. The lead compounds [Cp*2 Ln(μ-H)2 PbAr*] exhibit H/D exchange in reactions with deuterated solvents or dihydrogen.

Access to Monomeric Lead(II) Hydrides with Remarkable Thermostability and Their Use in Catalytic Hydroboration of Carbonyl Derivatives

We report on the remarkable stability of unprecedented, monomeric lead(II) hydrides M⁺[LPb(II)H]^- (M[1-H]), where L = 2,6-bis(3,5-diphenylpyrrolyl)pyridine and M = (18-crown-6)potassium or ([2.2.2]-cryptand)potassium. The half-life of [K18c6][1-H] of ∼2 days in tetrahydrofuran at 25 °C is significantly longer than those reported for dimeric lead(II) hydrides supported by bulky terphenyl ligands (few hours at low temperatures), which are the only examples known for lead(II) hydride compounds. The presence of a Pb-H bond in [1-H]^- was unambiguously identified by multinuclear NMR spectroscopy. Remarkably, a ¹H resonance of the hydride ligand was found at δ = 41.43 ppm (¹J_PbH = 1312 Hz). For reactivity study, [1-H]^- serves as an excellent hydroboration catalyst with high turnover numbers and turnover frequencies for several carbonyl compounds.

Reactivity of organogermanium and organotin trihydrides

The organogermanium and organotin trihydrides (TbbEH₃) [E = Ge (3), Sn (7)] with the Tbb substituent were synthesized by hydride substitution (Tbb = 2,6-[CH(SiMe₃)₂]₂-4-(t-Bu)C₆H₂). Deprotonation of the organoelement trihydrides 3 and 7 was studied in reaction with bases MeLi, BnK and LDA (Bn = benzyl, LDA = lithium diisopropylamide) to yield the deprotonation products (8-11) as lithium or potassium salts. Hydride abstraction from TbbSnH₃ using the trityl salt [Ph₃C][Al(OC{CF₃}₃)₄] gives the salt [TbbSnH₂][Al(OC{CF₃}₃)₄] (12) which was stabilized by thf donor ligands [TbbSnH₂(thf)₂][Al(OC{CF₃}₃)₄] (13). Tintrihydride 7 reacts with trialkylamine Et₂MeN to give as the product of a reductive elimination of hydrogen the distannane (TbbSnH₂)₂ (14). Transfer of hydrogen was observed in reaction of trihydrides TbbEH₃ (E = Ge, Sn) and Ar*GeH₃ with N-heterocyclic carbene (NHC). The NHC adduct TbbSnH(^iPrNHC) (15) was synthesized at rt and the germanium hydrides exhibit hydrogen transfer at higher temperatures to give Ar*GeH(^MeNHC) (16) and TbbGeH(^MeNHC) (17).

Synthesis, structure and reactivity of μ3-SnH capped trinuclear nickel cluster

Treatment of the trichlorotin-capped trinuclear nickel cluster, [Ni₃(dppm)₃(μ₃-Cl)(μ₃-SnCl₃)], 1, with 4 eq. NaHB(Et)₃ yields a μ₃-SnH capped trinuclear nickel cluster, [Ni₃(dppm)₃(μ₃-H)(μ₃-SnH)], 2 [dppm = bis(diphenylphosphino)methane]. Single-crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and computational studies together support that cluster 2 is a divalent tin hydride. Complex 2 displays a wide range of reactivity including oxidative addition of bromoethane across the Sn center. Addition of 1 eq. iodoethane to complex 2 releases H₂ (g) and generates an ethyltin-capped nickel cluster with a μ₃-iodide, [Ni₃(dppm)₃(μ₃-I)(μ₃-Sn(CH₂CH₃))], 4. Notably, insertion of alkynes into the Sn–H bond of 2 can be achieved via addition of 1 eq. 1-hexyne to generate the 1-hexen-2-yl-tin-capped nickel cluster, [Ni₃(dppm)₃(μ₃H)(μ₃-Sn(C₆H₁₁))], 5. Addition of H₂ (g) to 5 regenerates the starting material, 2, and hexane. The formally 44-electron cluster 2 also displays significant redox chemistry with two reversible one-electron oxidations (E = −1.3 V, −0.8 V vs. Fc^0/+) and one-electron reduction process (E = −2.7 V vs. Fc^0/+) observed by cyclic voltammetry.

The synthesis, structure, and reactivity of a μ₃-SnH capped trinuclear nickel cluster, [Ni₃(dppm)₃(μ₃-H)(μ₃-SnH)], is reported. This complex undergoes oxidative addition chemistry, alkyne insertion, and subsequent hydrogenation.

Pyridylpyrrolido ligand in Ge(II) and Sn(II) chemistry: synthesis, reactivity and catalytic application

In our previous communication, we have reported the synthesis of a new chlorogermylene (B) featuring a pyridylpyrrolido ligand. This study details the preparation of a series of new germylenes and stannylenes starting from B. A transmetallation reaction between B and SnCl₂ led to the analogous chlorostannylene (1) with the simultaneous elimination of GeCl₂. This is a very unusual example of transmetallation between two elements of the same group. The preparation of 1via lithiation led to the formation of 2 as a side product, where the ortho C-H bond of the pyridine ring was activated and functionalized with a ⁿBu moiety. Subsequently, B and 1 were used as precursors to generate germylene (4) and stannylene (5) featuring tris(trimethylsilyl)silyl (hypersilyl) moieties. We also prepared tetrafluoropyridyl germylene (6) by reacting 4 with C₅F₅N with the simultaneous elimination of (Me₃Si)₃SiF by utilizing the fluoride affinity of the silicon atom. As there is scarcity of Sn(II) compounds as single-site catalysts, we investigated 5 as a catalyst towards the hydroboration of aldehydes, ketones, alkenes and alkynes. All the compounds have been characterized by single-crystal X-ray diffraction and by state of the art spectroscopic studies.

Controlling Oxidative Addition and Reductive Elimination at Tin(I) via Hemi-Lability

We report on the synthesis of a distannyne supported by a pincer ligand bearing pendant amine donors that is capable of reversibly activating E-H bonds at one or both of the tin centres through dissociation of the hemi-labile N-Sn donor/acceptor interactions. This chemistry can be exploited to sequentially (and reversibly) assemble mixed-valence chains of tin atoms of the type ArSn{Sn(Ar)H}_n SnAr (n=1, 2). The experimentally observed (decreasing) propensity towards chain growth with increasing chain length can be rationalized both thermodynamically and kinetically by the electron- withdrawing properties of the -Sn(Ar)H- backbone units generated via oxidative addition.

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.

Hydroboration of carbonyls and imines by an iminophosphonamido tin(II) precatalyst

A novel three-coordinated tin(II) chloride [Ph₂P(N^tBu)₂]SnCl (1) supported by an N,N'-di-tert-butyliminophosphonamide having two phenyl groups on the phosphorus atom was synthesized by the reaction of the starting lithium iminophosphonamide [Ph₂P(N^tBu)₂]Li with SnCl₂·(dioxane) in toluene. The molecular structure of 1 was established by X-ray diffraction analysis. Tin(II) chloride 1 can act as an efficient precatalyst for the hydroboration of a wide variety of aldehydes, ketones, and imines at -10 °C. DFT calculations propose that hydroboration involves hydride transfer from the corresponding tin(II) hydride intermediate [Ph₂P(N^tBu)₂]SnH (10) to the carbonyl substrates via four-membered transition states (TS-12), affording three-coordinated tin(II) alkoxide intermediates [Ph₂P(N^tBu)₂]SnOR (13), followed by the stepwise reaction of 13 with HBpin (pin = pinacolate) to release the boronate esters and regenerate the tin(II) hydride 10. The stoichiometric reaction of the in site-generated 10 with benzophenone 2a at -10 °C led to the formation of 13. Moreover, 13 also stoichiometrically reacted with HBpin at -10 °C, forming the corresponding boronate ester 3a and 10 based on the ¹H NMR spectrum of the reaction mixture.

Hydrostannylation of carbon dioxide by a hydridostannylene molybdenum complex

Reaction of the aryltin(II) hydrides {Ar^iPr₄Sn(μ-H)}₂ or {Ar^iPr₆Sn(μ-H)}₂ (Ar^iPr₄ = -C₆H₃-2,6-(C₆H₃-2,6-ⁱPr₂)₂, Ar^iPr₆ = -C₆H₃-2,6-(C₆H₂-2,4,6-ⁱPr₃)₂) with two equivalents of the molybdenum carbonyl [Mo(CO)₅(THF)] afforded the divalent tin hydride transition metal complexes, Mo(CO)₅{Sn(Ar^iPr₆)H}, (1), or Mo(CO)₅{Sn(Ar^iPr₄)(THF)H} (2), respectively. Complex 1 effects the facile hydrostannylation of carbon dioxide, to yield Mo(CO)₅{Sn(Ar^iPr₆)(κ²-O,O'-O₂CH)}, (3), which features a bidentate formate ligand coordinating the tin atom. Reaction of 3 with the pinacolborane, HBpin (pin = pinacolato) in benzene regenerated 1 in quantitative yield. All complexes were characterized by X-ray crystallography, as well as UV-visible, IR, and multinuclear NMR spectroscopies. The isolation of 1 and 2 is consistent with the existence of monomeric forms of {Ar^iPr₄Sn(μ-H)}₂ and {Ar^iPr₆Sn(μ-H)}₂ in solution. Regeneration of 1 from 3via reaction with pinacolborane as the hydrogen source shows the catalytic potential of 1 in the hydrogenation of CO₂.

Reactions of a diborylstannylene with CO2 and N2O: diboration of carbon dioxide by a main group bis(boryl) complex

The reactions of the boryl-substituted stannylene Sn{B(NDippCH)2}2 (1) with carbon dioxide have been investigated and shown to proceed via pathways involving insertion into the Sn-B bond(s). In the first instance this leads to formation of the (boryl)tin(ii) borylcarboxylate complex Sn{B(NDippCH)2}{O2CB(NDippCH)2} (2), which has been structurally characterized and shown to feature a κ2 mode of coordination of the [(HCDippN)2BCO2]- ligand at the metal centre. 2 undergoes B-O reductive elimination in hexane solution (in the absence of further CO2) to give the boryl(borylcarboxylate)ester {(HCDippN)2B}O2C{B(NDippCH)2} (3) i.e. the product of formal diboration of carbon dioxide. Alternatively, 2 can assimilate a second equivalent of CO2 to give the homoleptic bis(borylcarboxylate) Sn{O2CB(NDippCH)2}2 (4), which can be prepared via an alternative route from SnBr2 and the potassium salt of [(HCDippN)2BCO2]-, and structurally characterized as its DMAP (N,N-dimethylaminopyridine) adduct. Structural and reactivity studies also point to the possibility for extrusion of CO from the [(HCDippN)2BCO2]- fragment to generate the boryloxy system [(HCDippN)2BO]-, a ligand which can be generated directly from 1via reaction with N2O. The initially formed unsymmetrical species Sn{B(NDippCH)2}{OB(NDippCH)2} has been shown to be amenable to crystallographic study in the solid state, but to undergo ligand redistribution in solution to generate a mixture of 1 and the bis(boryloxy) complex Sn{OB(NDippCH)2}2.

Low valent lead and tin hydrides in reactions with heteroallenes

Low valent organoelement hydrides of tin and lead, [(Ar*SnH)₂] and [(Ar*PbH)₂], were reacted with diorganocarbodiimide and adamantylisocyanate to give products of hydroelementation reactions. Carbon dioxide also reacts with both low valent hydrides, but a reaction product was only characterized in the tin hydride case. A hydride was transferred to the carbon atom and the formed formate anion [HCO₂]^- shows coordination at two tin atoms. Carbon disulfide reacts with the stannyl-stannylene isomer of the low valent organotin hydride. The stannyl part forms a Sn-C bond whereas the stannylene moiety coordinates at the two sulfur atoms. The dimeric organolead hydride exhibits transfer of both hydride ligands to the carbon atom of CS₂ to give a dithiol ligand [CH₂S₂]^2- bridging both organolead units.

Tetryl-Tetrylene Addition to Phenylacetylene

Phenylacetylene adds [Ar*GeH2 -SnAr'], [Ar*GeH2 -PbAr'] and [Ar'SnH2 -PbAr*] at rt in a regioselective and stereoselective reaction. The highest reactivity was found for the stannylene, which reacts immediately upon addition of one equivalent of alkyne. However, the plumbylenes exhibit addition to the alkyne only in reaction with an excess of phenylacetylene. The product of the germylplumbylene addition reacts with a second equivalent of alkyne and the product of a CH-activation, a dimeric lead acetylide, were isolated. In the case of the stannylplumbylene the trans-addition product was characterized as the kinetically controlled product which isomerizes at rt to yield the cis-addition product, which is stabilized by an intramolecular Sn-H-Pb interaction. NMR chemical shifts of the olefins were investigated using two- and four-component relativistic DFT calculations, as spin-orbit effects can be large. Hydride abstraction was carried out by treating [Ar'SnPhC=CHGeH2 Ar*] with the trityl salt [Ph3 C][Al(OC{CF3 })4 ] to yield a four membered ring cation.

CO2 Derivatives of Molecular Tin Compounds. Part 2: Carbamato, Formato, Phosphinoformato and Metallocarboxylato Complexes

Single-crystal X-ray diffraction structures of organotin compounds bearing hemicarbonate and carbonate ligands were recently reviewed by us—“CO2 Derivatives of Molecular Tin Compounds. Part 1: Hemicarbonato and Carbonato Complexes”, Inorganics 2020, 8, 31—based on crystallographic data available from the Cambridge Structural Database. Interestingly, this first collection revealed that most of the compounds listed were isolated in the context of studies devoted to the reactivity of tin precursors towards carbon dioxide, at atmospheric pressure or under pressure, thus highlighting the suitable disposition of Sn to fix CO2. In the frame of a second part, the present review carries on to explore CO2 derivatives of molecular tin compounds by describing successively the complexes with carbamato, formato, and phosphinoformato ligands, and obtained from insertion reactions of carbon dioxide into Sn–X bonds (X = N, H, P, respectively). The last chapter is devoted to X-ray structures of transition metal/tin CO2 complexes exhibiting metallocarboxylato ligands. As in Part 1, for each tin compound reported and when described in the original study, the structural descriptions are supplemented by synthetic conditions and spectroscopic data.

Reactivity of NHC/diphosphene-coordinated Au(I)-hydride

We report the reactivity of isolable Au(i)-hydride stabilized by an NHC-coordinated diphosphene towards substrates containing C-C and N-N multiple bonds (NHC = N-heterocyclcic carbene). Reactions with dimethyl acetylenedicarboxylate and azobenzene lead to a trans-addition of the Au(i)-H across the C-C triple bond and the N-N double bond, respectively. In contrast, the reaction with ethyl diazoacetate affords a gold(i)-hydrazonide as the 1,1-addition product to the terminal nitrogen atom. With phenyl acetylene, the corresponding Au(i)-alkynyl complex is obtained under the elimination of dihydrogen. Strikingly, diphosphene-containing Au(i)-hydride is more reactive - affording different products in some cases - than a related NHC-stabilized Au(i)-hydride without the mediating diphosphene moiety.

1,2-Insertion reactions of alkynes into Ge-C bonds of arylbromogermylene

1,2-Insertion reactions of alkynes into the Ge-C bonds in dibromodigermenes afford stable crystalline bromovinylgermylenes. In contrast to previously reported Lewis-base-supported vinylgermylenes, the bromovinylgermylene obtained from reaction of the bromogermylene with 3-hexyne via such an 1,2-insertion is a donor-free monomer. A feasible reaction mechanism, proposed on the basis of the observed experimental results in combination with theoretical calculations, suggests that the [1+2]-cycloadduct and the insertion product are the kinetic and thermodynamic product, respectively.

Organogermanium(II) Hydrides as a Source of Highly Soluble LiH

The reactions of monomeric C,N-chelated organogermanium(II) hydride L(H)Ge⋅BH₃ with organolithium salts RLi yielded lithium hydrogermanatoborates (Li(THF)₂ {BH₃ [L(H)GeR]})₂ . Compound (Li(THF)₂ {BH₃ [L(H)GePh]})₂ was used as a source of LiH for the reduction of organic C=O or C=N bonds in nonpolar solvents accompanied by the elimination of a neutral complex L(Ph)Ge⋅BH₃ . The interaction of (Li(THF)₂ {BH₃ [L(H)GePh]})₂ with the polar C=O bond was further investigated by computational studies revealing a plausible geometry of a pre-reactive intermediate. The experimental and theoretical studies suggest that, although the Li atom of (Li(THF)₂ {BH₃ [L(H)GePh]})₂ coordinates the C=O bond, the GeH fragment is the active species in the reduction reaction. Finally, benzaldehyde was reduced by a mixture of L(H)Ge⋅BH₃ with PhLi in nonpolar solvents.

Formation of Formic Acid Derivatives through Activation and Hydroboration of CO2 by Low-Valent Group 14 (Si, Ge, Sn, Pb) Catalysts

The chemistry of low-valent main group elements has attracted much attention in the past decade. These species are relevant because they have been able to mimic transition metal behavior in catalytic applications, with decreased material costs and diminished toxicity. In this contribution, we study the L¹EH catalysts (E = Si(II), Ge(II), Sn(II), and Pb(II); L¹ = [ArNC(Me)CHC(Me)NAr] with Ar = 2,6-iPr₂C₆H₃) for the formation of formic acid derivatives through hydroboration of CO₂. Detailed characterization of relevant structures on the potential energy surface enabled us to rationalize different paths for the hydroboration of CO₂. Interestingly, it was found that according to the activation energies for the whole catalytic cycle, the process of transformation of CO₂ becomes more favored going down group 14. However, an effective energetic decrease for the process (taking as the reference the uncatalyzed reaction between CO₂ and HBpin) is evidenced just from the germanium analogue. The trend in reactivity found in the present study is a direct consequence of the change in the central main group element, enabling enhanced polar character of the E-H (L¹EH in the CO₂ activation step) and E-O (metal formates in the hydroboration step) bonds as the atomic radius increases. The transient stabilization of reaction intermediates found in the hydroboration step was rationalized through the non-covalent interaction index (NCI) and symmetry-adapted perturbation theory (SAPT). This computational study highlights the reactivity trends in group-14-based hydride catalysts in hydrometalation and posterior hydroboration to form formic acid intermediates. We hope that this study will motivate further experimental work in low-valent lead chemistry.

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.

Deprotonation of Organogermanium and Organotin Trihydrides

Terphenyltin and terphenylgermanium trihydrides were deprotonated in reaction with strong bases, such as LiMe, LDA, or KBn. In the solid state, the Li salts of the germate anion 4 and 4a exhibit a Li-Ge contact. In the Li salt of the dihydridostannate anion 6a, the Li cation is not coordinated at the tin atom instead an interaction of the Li cation with the hydride substituents was found. Evidenced by ¹H-⁷Li-HOESY NMR spectroscopy the Li-salt of the deprotonated tin hydride 6a exhibits in toluene solution a contact between Li cation and hydride substituents, whereas in the ¹H-⁷Li-HOESY NMR spectrum of the homologous germate salt 4a, no crosspeak between hydride and Li signals was found. The organodihydridogermate and -stannate react as nucleophiles with low-valent Group 14 electrophiles. Thus, three compounds were synthesized: Ar-Ë'-EH₂-Ar (E', E = Sn, Ge; Pb, Ge; Pb, Sn; Ar = Ar', Ar*). Following an alternative synthesis Ar'SnH₂PbAr* was synthesized in reaction between [(Ar*PbH)₂] and [(Ar'SnH)₄] generated in situ. In reaction between low-valent organotin hydride [(Ar*SnH)₂] and organdihydridostannate [Ar*SnH₂]^- formation of distannate [Ar*₂Sn₂H₃]^- was found.

NHC-Coordinated Diphosphene-Stabilized Gold(I) Hydride and Its Reversible Conversion to Gold(I) Formate with CO2

An NHC-coordinated diphosphene is employed as ligand for the synthesis of a hydrocarbon-soluble monomeric AuI hydride, which readily adds CO2 at room temperature yielding the corresponding AuI formate. The reversible reaction can be expedited by the addition of NHC, which induces β-hydride shift and the removal of CO2 from equilibrium through the formation of an NHC-CO2 adduct. The AuI formate is alternatively formed by dehydrogenative coupling of the AuI hydride with formic acid (HCO2 H), thus in total establishing a reaction sequence for the AuI hydride mediated dehydrogenation of HCO2 H as chemical hydrogen storage material.

Reactivity patterns for the activation of CO2 and CS2 with alumoxane and aluminum hydrides

Carbon dioxide is readily fixed when reacting with either alumoxane dihydride [{MeLAl(H)}2(μ-O)] (1) or aluminum dihydride [MeLAlH2] (2) (MeL = HC[(CMe)N(2,4,6-Me3C6H2)]2-) to produce bimetallic aluminum formates [(MeLAl)2(μ-OCHO)2(μ-O)] (3) and [(MeLAl)2(μ-OCHO)2(μ-H)2] (5), respectively. Furthermore, [(MeLAl)2(μ-OCHO)2(μ-OH)2] (4) is easily obtained upon the reaction of 3 or 5 with H2O. The stability of the unusual dialuminum diformate dihydride core observed in 5 stems from the proximity of the Al centers allowing the formation of two Al-HAl bridges and precluding further hydride transfer to the HCO2 moieties. Contrary to this behavior, 1 and 2 react with CS2 giving cyclic alumoxane and aluminum sulfides [(MeLAl)2(μ-S)(μ-O)] (6) and [{MeLAl(μ-S)}2] (7), respectively. The molecular structures of 3-7 were characterized by IR, Raman, solution or solid-state (MAS) NMR spectroscopy and mass spectrometry and for 4-7 were characterized by X-ray diffraction studies. NMR kinetic studies and DFT calculations suggest that the mechanisms for the formation of 6 and 7 involve the transfer of a hydride group forming transient aluminum thioformate intermediates which proceed to form Al-S-Al moieties through the cleavage of C-S bonds and insertion of a sulfur atom, followed by the elimination of thioformaldehyde.

Calcium formamidinate derivatives by hydride insertion of carbodiimides

Insertion reactions of N,N′-dialkyl and N,N′-diaryl carbodiimides with a dimeric calcium hydride provide formamidinate derivatives whose structures on the steric demands of the carbodiimide reagent.

Low-valent group 14 element hydride chemistry: towards catalysis

The chemistry of group 14 element(ii) hydride complexes has rapidly expanded since the first stable example of such a compound was reported in 2000. Since that time it has become apparent that these systems display remarkable reactivity patterns, in some cases mimicking those of late transition-metal (TM) hydride compounds. This is especially so for the hydroelementation of unsaturated organic substrates. Recently, this aspect of their reactivity has been extended to the use of group 14 element(ii) hydrides as efficient, "TM-like" catalysts in organic synthesis. This review will detail how the chemistry of these hydride compounds has advanced since their early development. Throughout, there is a focus on the importance of ligand effects in these systems, and how ligand design can greatly modify a coordinated complex's electronic structure, reactivity, and catalytic efficiency.

Silicon-fluorine chemistry: from the preparation of SiF2to C–F bond activation using silylenes and its heavier congeners

The feisty nature of silicon(ii) fluorides has been harnessed by two cyclic alkyl amino carbene (cAAC) ligands and (cAAC)₂SiF₂has been isolated at room temperature and structurally characterized.

Reductive Elimination of Hydrogen from Bis(trimethylsilyl)methyltin Trihydride and Mesityltin Trihydride

Alkyltin trihydride [(Me₃ Si)₂ CHSnH₃ ] was synthesized and the reductive elimination of hydrogen from this species was investigated. A methyl-substituted N-heterocyclic carbene reacts with the organotrihydride in dependence on stoichiometry and solvent to give a series of products of the reductive elimination and dehydrogenative tin-tin bond formation. Besides characterization of the carbene adduct of the alkyltin(II) hydride, a Sn₄ chain was also isolated, encompassing two stannyl-stannylene sites, which are stabilized each as NHC-adducts. Complete dehydrogenation resulted to give either a carbene-stabilized distannyne or a metalloid Sn₉ -cluster salt. Reductive elimination of hydrogen was also achieved with an excess of diethylmethylamine to give the alkyltin(II) hydride as a Lewis base free tetramer [(RSnH)₄ ]. The method of cluster formation at low temperatures by hydrogen elimination was also transferred to the mesityl-substituted tin trihydride MesSnH₃ . In this case [(MesSn)₁₀ ], showing a [5]prismane structure, was isolated in good yield and characterized. NMR spectroscopic features of the propellane-type cluster [Trip₆ Sn₆ ] are reported.