Facile Access to Mono- and Dinuclear Heteroleptic N‑Heterocyclic Silylene Copper Complexes

Reaction of the heteroleptic N-heterocyclic chlorosilylene L­(Cl)­Si: (<b>1</b>; L = PhC­(N<i>t</i>Bu)₂) with [Cu­(tmeda)­(CH₃CN)]­[OTf] (<b>2</b>; tmeda = <i>N,N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine, OTf = OSO₂CF₃ (triflate)) affords the Cu­(I) complex [L­(Cl)­Si:→Cu­(tmeda)]­[OTf] (<b>3</b>) in high yield as the first example of a heteroleptic N-heterocyclic silylene copper complex. Similarly, the reaction of L­(O<i>t</i>Bu)­Si: (<b>4</b>; L = PhC­(N<i>t</i>Bu)₂) with <b>2</b> affords [L­(O<i>t</i>Bu)­Si: → Cu­(tmeda)]­[OTf] (<b>5</b>) and that of L­(NMe₂)­Si: (<b>6</b>) with <b>2</b> leads to [L­(NMe₂)­Si:→Cu­(tmeda)]­[OTf] (<b>7</b>). Complex <b>3</b> shows a rather strong interaction in the solid state between the O atom of the triflate anion and the three-coordinate Cu­(I) center with a Cu···O distance of 2.312 Å. In contrast, complex <b>7</b> features only a weak interaction (ca. 3.28 Å), while in complex <b>5</b> the cation and anion are fully separated. Strikingly, the reaction of the chelating oxo-bridged silylene :Si­(L)­(μ₂-O)­(L)­Si: (<b>8</b>) with the copper source [Cu­(CH₃CN)₄]­[OTf] (<b>9</b>) affords the dinuclear complex salt [Cu₂{η¹:η¹-LSi­(μ₂-O)­SiL}₂]­[OTf]₂ (<b>10</b>), featuring a novel metallacyclooctane dication, selectively in a good yield. Complex <b>10</b> also exhibits a very strong interaction between the copper centers in the dication and the oxygen atoms of triflate anions in the solid state, evidenced by a Cu···O separation of only 2.141 Å. All complexes were fully characterized

An Aliphatic Solvent-Soluble Lithium Salt of the Perhalogenated Weakly Coordinating Anion [Al(OC(CCl₃)(CF₃)₂)₄]⁻

The facile synthesis of a new highly aliphatic solvent-soluble Li⁺ salt of the perhalogenated weakly coordinating anion [Al­(OC­(CCl₃)­(CF₃)₂)₄]⁻ and its application in stabilizing the Ph₃C⁺ cation were investigated. The lithium salt Li­[Al­(OC­(CCl₃)­(CF₃)₂)₄] (<b>4</b>) was prepared by the treatment of 4 mol equiv of HOC­(CCl₃)­(CF₃)₂ with purified LiAlH₄ in <i>n</i>-hexane from −20 °C to room temperature. Compound <b>4</b> is highly soluble in both polar and nonpolar solvents, and it bears both CCl₃ and CF₃ groups, resulting in a lower symmetry around the Al center compared to that of Li­[Al­(OC­(CF₃)₃)₄] (<b>1</b>). Treatment of <b>4</b> with Ph₃CCl afforded the ionic compound [Ph₃C]­[Al­(OC­(CCl₃)­(CF₃)₂)₄] (<b>5</b>) bearing the Ph₃C⁺ cation with concomitant elimination of LiCl, suggesting the potential application of [Al­(OC­(CCl₃)­(CF₃)₂)₄]⁻ in stabilizing reactive cationic species. Compounds <b>4</b> and <b>5</b> were fully characterized by spectroscopic and structural methods

Synthesis and Unexpected Reactivity of Germyliumylidene Hydride [:GeH]⁺ Stabilized by a Bis(<i>N</i>‑heterocyclic carbene)borate Ligand

Employing the potassium salt of the monoanionic bis­(<i>NHC</i>)­borate <b>1</b> (<i>NHC</i> = <i>N</i>-<i>H</i>eterocyclic <i>C</i>arbene) enables the synthesis and isolation of the bis­(<i>NHC</i>)­borate-stabilized chlorogermyliumylidene precursor <b>2</b> in 61% yield. A Cl/H exchange reaction of <b>2</b> using potassium tri<i>sec</i>.-butylborhydride as a hydride source leads to the isolation of the first germyliumylidene hydride [HGe:⁺] complex <b>3</b> in 91% yield. The Ge­(II)–H bond in the latter compound has an unexpected reactivity as shown by the reaction with the potential hydride scavenger [Ph₃C]⁺[B­(C₆F₅)₄]⁻, furnishing the corresponding HGe: → CPh₃ cation in the ion pair <b>4</b> as initial product. Compound <b>4</b> liberates HCPh₃ in the presence of <b>3</b> to give the unusual dinuclear HGe: → Ge: cation in <b>5</b>. The latter represents the first three-coordinate dicationic Ge­(II) species stabilized by an anionic bis­(<i>NHC</i>) chelate ligand and a Ge­(II) donor. All novel compounds were fully characterized, including X-ray diffraction analyses

An Elusive Hydridoaluminum(I) Complex for Facile C–H and C–O Bond Activation of Ethers and Access to Its Isolable Hydridogallium(I) Analogue: Syntheses, Structures, and Theoretical Studies

The reaction of AlBr₃ with 1 molar equiv of the chelating bis­(N-heterocyclic carbene) ligand bis­(<i>N</i>-Dipp-imidazole-2-ylidene)­methylene (bisNHC, <b>1</b>) affords [(bisNHC)­AlBr₂]⁺Br^– (<b>2</b>) as an ion pair in high yield, representing the first example of a bisNHC–Al­(III) complex. Debromination of the latter with 1 molar equiv of K₂Fe­(CO)₄ in tetrahydrofuran (THF) furnishes smoothly, in a redox reaction, the (bisNHC)­(Br)­Al­[Fe­(CO)₄] complex <b>3</b>, in which the Al­(I) center is stabilized by the Fe­(CO)₄ moiety through Al­(I):→Fe(0) coordination. Strikingly, the Br/H ligand exchange reactions of <b>3</b> using potassium hydride as a hydride source in THF or tetrahydropyran (THP) do not yield the anticipated hydridoaluminum­(I) complex (bisNHC)­Al­(H)­[Fe­(CO)₄] (<b>4a</b>) but instead lead to (bisNHC)­Al­(2-<i>cyclo</i>-OC₄H₇)­[Fe­(CO)₄] (<b>4</b>) and (bisNHC)­Al­(2-<i>cyclo</i>-OC₅H₉)­[Fe­(CO)₄] (<b>5</b>), respectively. The latter are generated via C–H bond activation at the α-carbon positions of THF and THP, respectively, in good yields with concomitant elimination of dihydrogen. This is the first example whereby a low-valent main-group hydrido complex facilitates metalation of sp³ C–H bonds. Interestingly, when K­[BHR₃] (R = Et, <i>s</i>Bu) is employed as a hydride source to react with <b>3</b> in THF, the reaction affords (bisNHC)­Al­(O<i>n</i>Bu)­[Fe­(CO)₄] (<b>6</b>) as the sole product through C–O bond activation and ring opening of THF. The mechanisms for these novel C–H and C–O bond activations mediated by the elusive hydridoaluminum­(I) complex <b>4a</b> were elucidated by density functional theory (DFT) calculations. In contrast, the analogous hydridogallium­(I) complex (bisNHC)­Ga­(H)­[Fe­(CO)₄] (<b>9</b>) can be obtained directly in high yield by the reaction of the (bisNHC)­Ga­(Cl)­[Fe­(CO)₄] precursor <b>8</b> with 1 molar equiv of K­[BHR₃] (R = Et, <i>s</i>Bu) in THF at room temperature. The isolation of <b>9</b> and its inertness toward cyclic ethers might be attributed to the higher electronegativity of gallium versus aluminum. The stronger Ga­(I)–H bond, in turn, hampers α-C–H metalation or C–O bond cleavage in cyclic ethers, the latter of which is supported by DFT calculations

A Cyclic Germadicarbene (“Germylone”) from Germyliumylidene

By employing the chelate dicarbene <b>1</b>, the new chloro­germylium­ylidene complex <b>2</b> could be synthesized and isolated in 95% yield. Dechlorination of <b>2</b> with sodium naphthalenide furnishes the unique cyclic germadicarbene <b>3</b> which could be isolated in 45% yield. Compound <b>3</b> is the first isolable Ge(0) complex with a single germanium atom stabilized by a dicarbene. Its molecular structure is in accordance with DFT calculations which underline the peculiar electronic structure of <b>3</b> with two lone pairs of electrons at the Ge atom

From Unsymmetrically Substituted Benzamidinato and Guanidinato Dichlorohydridosilanes to Novel Hydrido N‑Heterocyclic Silylene Iron Complexes

Starting from the unsymmetric N,N′-substituted thiourea compounds (R)­N­(H)­C­(S)­N­(H)­(^<i>t</i>Bu) (<b>1</b>, R = Dipp: 2,6-^<i>i</i>Pr₂-C₆H₃; <b>2</b>, R = 1-adamantyl), the corresponding asymmetric carbodiimines (R)­NCN­(^<i>t</i>Bu) (<b>3</b>, R = Dipp; <b>4</b>, R = 1-adamantyl) are readily accessible in high yields upon reduction with LiHMDS (Li­[N­(SiMe₃)₂]). The reaction of compound <b>3</b> with PhLi followed by SiCl₄ afforded, in a one-pot reaction, the asymmetric benzamidinato-stabilized trichlorosilane [PhC­{(N^<i>t</i>Bu)­(NDipp)}]­SiCl₃ (<b>5</b>). Similarly, silanes [PhC­{(N^<i>t</i>Bu)­(NDipp)}]­SiHCl₂ (<b>6</b>), [(NMe₂)­C­{(N^<i>t</i>Bu)­(NDipp)}]­SiHCl₂ (<b>7</b>), and [PhC­{(N^<i>t</i>Bu)­(NAd)}]­SiHCl₂ (<b>8</b>) could also be isolated. All novel trichloro- or dichlorohydridosilanes were fully spectroscopically characterized and studied by single-crystal X-ray diffraction analyses, the latter revealing in all cases a distorted-trigonal bipyramidal five-coordinate silicon center. The reactions of silanes <b>5</b>–<b>8</b> with K₂[Fe­(CO)₄] were also explored: In the case of the reaction of silane <b>5</b> with K₂[Fe­(CO)₄], no reaction was observed even after prolonged heating. However, in the case of the silanes <b>6</b>–<b>8</b>, the selective formation of the corresponding hydrido Si^II:→Fe⁰ complexes [[R¹C­{(N^<i>t</i>Bu)­(NR²)}]­(H)­Si:→Fe­(CO)₄] (<b>9</b>, R¹ = Ph, R² = Dipp; <b>10</b>, R¹ = NMe₂, R² = Dipp; <b>11</b>, R¹ = Ph, R² = 1-adamantyl) could be achieved. Complexes <b>9</b>–<b>11</b> represent unprecedented hydrido-N-heterocyclic silylene complexes, bearing asymmetric ligand backbones. Complexes <b>9</b>–<b>11</b> were fully spectroscopically characterized, and in addition the single-crystal X-ray structure analysis of compound <b>10</b> is reported

A Cyclic Germadicarbene (“Germylone”) from Germyliumylidene

By employing the chelate dicarbene <b>1</b>, the new chloro­germylium­ylidene complex <b>2</b> could be synthesized and isolated in 95% yield. Dechlorination of <b>2</b> with sodium naphthalenide furnishes the unique cyclic germadicarbene <b>3</b> which could be isolated in 45% yield. Compound <b>3</b> is the first isolable Ge(0) complex with a single germanium atom stabilized by a dicarbene. Its molecular structure is in accordance with DFT calculations which underline the peculiar electronic structure of <b>3</b> with two lone pairs of electrons at the Ge atom

Isolable Diphosphorus-Centered Radical Anion and Diradical Dianion

Two salts containing diphosphorus-centered radical anion <b>1</b>^•– and diradical dianion <b>1</b>^2–•• were obtained by one- and two-electron reductions of an indenofluorene-bridging diphosphaalkene (<b>1</b>) with K and KC₈, respectively. The salts have been characterized by electron paramagnetic resonance (EPR) spectroscopy, UV–vis absorption spectroscopy, and single-crystal X-ray diffraction analysis. EPR spectroscopy and theoretical calculations reveal that the spin density of the radicals mainly resides on the phosphorus atoms, and <b>1</b>^2–•• has an open-shell singlet ground state. <b>1</b>^•– and <b>1</b>^2–•• represent the first isolable and structurally characterized diphosphorus-centered radical anion and dianion

Bis‑<i>N</i>‑Heterocyclic Carbene (NHC) Stabilized η⁶‑Arene Iron(0) Complexes: Synthesis, Structure, Reactivity, and Catalytic Activity

Reaction of FeCl₂ with the chelating bis-<i>N</i>-heterocyclic carbene (NHC) bis-(<i>N</i>-Dipp-imidazole-2-ylidene)­methylene (abbreviated {(^DippC:)₂CH₂}) (Dipp = 2,6-di-isopropylphenyl) affords the complex [FeCl₂{(^DippC:)₂CH₂}] (<b>1</b>) in high yield. Reduction of complex <b>1</b> with excess KC₈ with a 10-fold molar excess of PMe₃ affords the Fe­(II) complex [FeH­{(^DippC:)₂CH₂}­(PMe₃)­(η²-PMe₂CH₂)] (<b>2</b>) as a mixture of three stereoisomers. Complex <b>2</b>, the first example of any iron­(II) complex bearing mutually an NHC and PMe₃ ligand, is likely obtained from the <i>in situ</i>, reductively generated 16 VE Fe(0) complex, [Fe­{(^DippC:)₂CH₂}­(PMe₃)₂] (<b>2</b>′), following intramolecular C–H activation of one of the phosphorus-bound CH₃ groups. Complex <b>2</b> is unstable in aromatic solvents and forms, <i>via</i> a novel synthetic transformation involving intramolecular reductive elimination and concomitant PMe₃ elimination, the Fe (0) arene complex [Fe­{(^DippC:)₂CH₂}­(η⁶-C₆D₆)] (<b>4-<i>d</i></b>_<b>6</b>) in C₆D_6. Complex <b>4-<i>d</i></b>_<b>6</b> represents the first example of an NHC stabilized iron (0) arene complex. The transformation from <b>2</b> to <b>4-<i>d</i></b>_<b>6</b> can be accelerated at higher temperature and at 60 °C forms immediately. Alternatively, the reduction of <b>1</b> in the presence of toluene or benzene affords the complexes [Fe­{(^DippC:)₂CH₂}­(η⁶-C₇H₈)] (<b>3</b>) and [Fe­{(^DippC:)₂CH₂}­(η⁶-C₆H₆)] (<b>4</b>), selectively and in good yields. DFT calculations characterizing the bonding situation in <b>3</b> and <b>4</b> reveal similar energies of the HOMO and LUMO orbitals, with the LUMO orbital of both complexes located on the Dipp rings of the bis-NHC. The HOMO orbital reflects a π-back-bonding interaction between the Fe(0) center and the chelating NHC ligand, while the HOMO-1 is associated with the arene interaction with the Fe(0) site. The calculations do not suggest any noninnocence of the coordinated arene in either complex. Moreover, the ⁵⁷Fe Mössbauer spectrum of <b>4</b> at 80K exhibits parameters (δ = 0.43 mm·s^–1; Δ<i>E</i>_Q = 1.37 mm·s^–1) which are consistent with a five-coordinate Fe(0) system, rendering <b>3</b> and <b>4</b> the first examples of well-defined authentic Fe(0)-η⁶-arene complexes of the type [Fe­(η⁶-arene)­L₂] (L = η^1 or 2 neutral ligand, mono or bidentate). Some reactivitiy studies of <b>3</b> are also reported: The reaction of <b>3</b> with excess CO selectively yields the five-coordinate piano-stool complex [Fe­{(^DippC:)₂CH₂}­(CO)₃] (<b>6</b>) in near quantitative yields, while the reaction of complex <b>3</b> with C₆D₆ under heating affords by toluene elimination <b>4-d</b>_<b>6.</b> The catalytic ability of <b>4</b> was also investigated with respect to amide reduction to amines, for a variety of substrates using Ph₂SiH₂ as a hydride source. In all cases good to excellent yields to the corresponding amines were obtained. The use of <b>4</b> as a precatalyst represents the first example of a well-defined Fe(0) complex to effect this catalytic process

Bis(boryl anion)-Substituted Pyrenes: Syntheses, Characterizations, and Crystal Structures

The two new diboranes <b>1</b> and <b>2</b> connected by a pyrene moiety at the 1,6- and 1,3-positions, respectively, were synthesized, and their two-electron-reduction reactions were investigated. The doubly reduced species <b>1</b>^••2– is silent in electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic measurements, suggesting a quasi-quinoidal structure with a diradical character of <b>1</b>^••2–, which has a singlet–triplet gap of 6.6 kcal mol^–1 as determined by theoretical calculations. In contrast, the reduction product <b>2</b>^••2– is EPR active and theoretical calculations indicate that <b>2</b>^••2– has an open-shell singlet ground state with a singlet–triplet energy gap of 4.9 kcal mol^–1