Tibor Szilvási

A Fragile Zwitterionic Phosphasilene as a Transfer Agent of the Elusive Parent Phosphinidene (:PH)

The simplest parent phosphinidene, :PH (1), has been observed only in the gas phase or low temperature matrices and has escaped rigorous characterization because of its high reactivity. Its liberation and transfer to an unsaturated organic molecule in solution has now been accomplished by taking advantage of the facile homolytic bond cleavage of the fragile Si═P bond of the first zwitterionic phosphasilene LSi=PH (8) (L = CH[(C═CH2)CMe(NAr)2]; Ar = 2,6-(i)Pr2C6H3). The latter bears two highly localized lone pairs on the phosphorus atom due to the LSi═PH ↔ LSi(+)-PH(-) resonance structures. Strikingly, the dissociation of 8 in hydrocarbon solutions occurs even at room temperature, affording the N-heterocyclic silylene LSi: (9) and 1, which leads to oligomeric [PH]n clusters in the absence of a trapping agent. However, in the presence of an N-heterocyclic carbene as an unsaturated organic substrate, the fragile phosphasilene 8 acts as a :PH transfer reagent, resulting in the formation of silylene 9 and phosphaalkene 11 bearing a terminal PH moiety.

From a zwitterionic phosphasilene to base stabilized silyliumylidene-phosphide and bis(silylene) complexes.

The reactivity of ylide-like phosphasilene 1 [LSi(TMS)═P(TMS), L = PhC(NtBu)2] with group 10 d(10) transition metals is reported. For the first time, a reaction of a phosphasilene with a transition metal that actually involves the silicon-phosphorus double bond was found. In the reaction of 1 with ethylene bis(triphenylphosphine) platinum(0), a complete silicon-phosphorus bond breakage occurs, yielding the unprecedented dinuclear platinum complex 3 [LSi{Pt(PPh3)}2P(TMS)2]. Spectroscopic, structural, and theoretical analysis of complex 3 revealed the cationic silylene (silyliumylidene) character of the silicon unit in complex 3. Similarly, formation of the analogous dinuclear palladium complex 4 [LSi{Pd(PPh3)}2P(TMS)2] from tetrakis(triphenylphosphine) palladium(0) was observed. On the other hand, in the case of bis(cyclooctadiene) nickel(0) as starting material, a distinctively different product, the bis(silylene) nickel complex 5 [{(LSi)2P(TMS)}Ni(COD)], was obtained. Complex 5 was fully characterized including X-ray diffraction analysis. Density functional theory calculations of the reaction mechanisms showed that the migration of the TMS group in the case of platinum and palladium was induced by the oxidative addition of the transition metal into the silicon-silicon bond. The respective platinum intermediate 2 [LSi{Pt(TMS)(PPh3)}P(TMS)] was also experimentally observed. This is contrasted by the reaction of nickel, in which the equilibrium of phosphasilene 1 and the phosphinosilylene 6 [LSiP(TMS)2] was utilized for a better coordination of the silicon(II) moiety in comparison with phosphorus to the transition metal center.

Direct time-domain observation of attosecond final-state lifetimes in photoemission from solids

Clocking electrons as they exit a metal Einstein earned his Nobel Prize for a quantum-mechanical explanation of electron ejection from metals by light. More than a century later, attosecond spectroscopy has let researchers explore that process in real time. Tao et al. used attosecond pulse trains to distinguish the dynamics of electrons excited from a nickel surface into discrete states versus free space (see the Perspective by Bovensiepen and Liggues). They succeeded in resolving a time delay of ∼200 as that was associated with excitation into unoccupied band states. Science, this issue p. 62; see also p. 28 Attosecond pulses resolve the quantum mechanical delays associated with electron photoejection from a nickel surface. Attosecond spectroscopic techniques have made it possible to measure differences in transport times for photoelectrons from localized core levels and delocalized valence bands in solids. We report the application of attosecond pulse trains to directly and unambiguously measure the difference in lifetimes between photoelectrons born into free electron–like states and those excited into unoccupied excited states in the band structure of nickel (111). An enormous increase in lifetime of 212 ± 30 attoseconds occurs when the final state coincides with a short-lived excited state. Moreover, a strong dependence of this lifetime on emission angle is directly related to the final-state band dispersion as a function of electron transverse momentum. This finding underscores the importance of the material band structure in determining photoelectron lifetimes and corresponding electron escape depths.

From a Phosphaketenyl‐Functionalized Germylene to 1,3‐Digerma‐2,4‐diphosphacyclobutadiene

The first 4π-electron resonance-stabilized 1,3-digerma-2,4-diphosphacyclobutadiene [L(H)2Ge2P2] 4 (L(H)=CH[CHNDipp]2 Dipp=2,6-(i)Pr2C6H3) with four-coordinate germanium supported by a β-diketiminate ligand and two-coordinate phosphorus atoms has been synthesized from the unprecedented phosphaketenyl-functionalized N-heterocyclic germylene [L(H)Ge-P=C=O] 2 a prepared by salt-metathesis reaction of sodium phosphaethynolate (P≡C-ONa) with the corresponding chlorogermylene [L(H)GeCl] 1 a. Under UV/Vis light irradiation at ambient temperature, release of CO from the P=C=O group of 2 a leads to the elusive germanium-phosphorus triply bonded species [L(H)Ge≡P] 3 a, which dimerizes spontaneously to yield black crystals of 4 as isolable product in 67% yield. Notably, release of CO from the bulkier substituted [L(tBu)Ge-P=C=O] 2 b (L(tBu)=CH[C((t)Bu)N-Dipp]2 ) furnishes, under concomitant extrusion of the diimine [Dipp-NC((t)Bu)]2, the bis-N,P-heterocyclic germylene [DippNC((t)Bu)C(H)PGe]2 5.

An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies.

The silicon analogues of an acylium ion, namely, sila-acylium ions 2a and 2b [RSi(O)(NHC)2]Cl stabilized by two N-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO2 using the donor stabilized silyliumylidene cations 1a and 1b [RSi(NHC)2]Cl (1a, 2a; R = m-Ter = 2,6-Mes2C6H3, Mes = 2,4,6-Me3C6H2 and 1b, 2b; R = Tipp = 2,4,6-iPr3C6H2). Structurally, compound 2a features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions 2a and 2b with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of 2a to H2O afforded a stable silicon analogue of carboxylate anion as a dimer form, [m-TerSi(O)O]2(2-)·2[NHC-H](+) (3), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion 2b afforded cyclotetrasiloxanediol dianion [{TippSi(O)}4{(O)OH}2](2-)·2[NHC-H](+) (4). Metric and DFT (Density Functional Theory) evidence support that 2a and 2b possess strong Si═O double bond character, while 3 and 4 contain more ionic terminal Si-O bonds. Mechanistic details of the formation of different (SiO)n (n = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry.

Can low-valent silicon compounds be better transition metal ligands than phosphines and NHCs?

We investigated the potential application of experimentally characterized low-valent silicon compounds as transition metal ligands by computing the most important ligand properties, σ-donor and π-acceptor ability, ligand-to-metal charge transfer, and steric parameters and compared them to the generally used carbene and phosphine ligands. We found that several recently synthesized donor-stabilized low-valent silicon compounds can compete or even exceed the favorable features of commonly used carbene and phosphine ligands regarding all investigated ligand properties. We derive the general principles behind the enhanced features and conclude how even better low-valent silicon ligands can be designed with a slight modification of known compounds. Using our results as a database, one can choose an appropriate silicon-based ligand for transition metal catalysis.

Synthesis and Unexpected Reactivity of Germyliumylidene Hydride [:GeH]+ Stabilized by a Bis(N-heterocyclic carbene)borate Ligand

Employing the potassium salt of the monoanionic bis(NHC)borate 1 (NHC = N-Heterocyclic Carbene) enables the synthesis and isolation of the bis(NHC)borate-stabilized chlorogermyliumylidene precursor 2 in 61% yield. A Cl/H exchange reaction of 2 using potassium trisec.-butylborhydride as a hydride source leads to the isolation of the first germyliumylidene hydride [HGe:(+)] complex 3 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 4 as initial product. Compound 4 liberates HCPh3 in the presence of 3 to give the unusual dinuclear HGe: → Ge: cation in 5. The latter represents the first three-coordinate dicationic Ge(II) species stabilized by an anionic bis(NHC) chelate ligand and a Ge(II) donor. All novel compounds were fully characterized, including X-ray diffraction analyses.

A Bis(silylene)-Substituted ortho-Carborane as a Superior Ligand in the Nickel-Catalyzed Amination of Arenes.

The synthesis and structure of the first 1,2-bis(NHSi)-substituted ortho-carborane [(LSi:)C]2 B10 H10 (termed SiCCSi) is reported (NHSi=N-heterocyclic silylene; L=PhC(NtBu)2 ). Its suitability to serve as a reliable bis(silylene) chelating ligand for transition metals is demonstrated by the formation of [SiCCSi]NiBr2 and [SiCCSi]Ni(CO)2 complexes. The CO stretching vibration modes of the latter indicate that the Si(II) atoms in the SiCCSi ligand are even stronger σ donors than the P(III) atoms in phosphines and C(II) atoms in N-heterocyclic carbene (NHC) ligands. Moreover, the strong donor character of the [SiCCSi] ligand enables [SiCCSi]NiBr2 to act as an outstanding precatalyst (0.5 mol % loading) in the catalytic aminations of arenes, surpassing the activity of previously known molecular Ni-based precatalysts (1-10 mol %).

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(N-Dipp-imidazole-2-ylidene)methylene (bisNHC, 1) affords [(bisNHC)AlBr2](+)Br(-) (2) 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 3, 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 3 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] (4a) but instead lead to (bisNHC)Al(2-cyclo-OC4H7)[Fe(CO)4] (4) and (bisNHC)Al(2-cyclo-OC5H9)[Fe(CO)4] (5), 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 sp(3) C-H bonds. Interestingly, when K[BHR3] (R = Et, sBu) is employed as a hydride source to react with 3 in THF, the reaction affords (bisNHC)Al(OnBu)[Fe(CO)4] (6) 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 4a were elucidated by density functional theory (DFT) calculations. In contrast, the analogous hydridogallium(I) complex (bisNHC)Ga(H)[Fe(CO)4] (9) can be obtained directly in high yield by the reaction of the (bisNHC)Ga(Cl)[Fe(CO)4] precursor 8 with 1 molar equiv of K[BHR3] (R = Et, sBu) in THF at room temperature. The isolation of 9 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.

New Route to Access an Acyl‐Functionalized Phosphasilene and a Four‐Membered Si‐P‐C‐O Heterocycle

An acyl-functionalized phosphasilene, LSi(COtBu)=P(SiMe3) (L = PhC(NtBu)2) was synthesized on a new route by the addition of tBuCOCl to the phosphinosilylene LSiP(SiMe3)2 and subsequent Me3SiCl elimination. DFT studies elucidated its molecular structure, the influence of the acyl group on UV/Vis transitions, and revealed the mechanism. The intermediate LSi(COtBu)ClP(SiMe3)2, with a five-coordinate silicon center, was characterized by NMR spectroscopy and X-ray analysis. On the other hand, phosphasilene LSi(SiMe3)=P(SiMe3) reacted with tBuCOCl by a [2+2] cycloaddition of the silicon-phosphorus double bond and the carbon-oxygen double bond in addition to Me3SiCl elimination, thereby affording the novel, fully characterized compound LSi(SiMe3)[P=C(tBu)O] bearing a Si-P-C-O heterocycle with a phosphorus-carbon double bond. DFT studies suggest that two mechanisms occur simultaneously.