The [(DABCO)₇·(LiCH₂SiMe₃)₈] Octamer: More Aggregated than the Parent Starting Material [LiCH₂SiMe₃]₆ but Also Higher in Reactivity

Herein, we report on the reaggregation of hexameric trimethylsilylmethyllithium [LiCH₂SiMe₃]₆ with the donor base DABCO (1,4-diazabicyclo[2.2.2]­octane) to give the unprecedented octamer [(DABCO)₇·(LiCH₂SiMe₃)₈] (<b>1</b>). The structure consists of four dimers, forming Li₂C₂ four-membered rings, connected to two [(DABCO)₃·{(LiCH₂SiMe₃)₂}₂] chain fractions, interconnected by a single DABCO molecule. Interestingly, two different conformers of (LiCH₂SiMe₃)₂ dimers are present, caused by different steric demand. Higher steric strain in the center of the molecule causes an ecliptic arrangement of the Me₃Si group along the Si–C_α bond, while at the periphery the more relaxed staggered conformation is enabled. The reactivity of trimethylsilylmethyllithium coordinated by DABCO was tested in the benchmark reaction with toluene. Although the aggregation of <b>1</b> is much higher than that of the parent [LiCH₂SiMe₃]₆, the reactivity of the first is higher than that of the starting material, provided the octameric aggregation found in the solid state is maintained in nondonating solvents. While the hexamer would not react with toluene, the octamer gives benzyllithium, coordinated by DABCO. The reaction was monitored by ¹H NMR spectroscopy. Revisiting that known structure with modern technology revealed that [(DABCO)·(LiCH₂Ph)]_∞ (<b>2</b>) crystallizes in the space group <i>P</i>2₁. <b>2</b> still is the only benzyllithium compound featuring the η³-coordination mode to the C_<i>ortho</i> atom of the phenyl ring, presumably triggered by the singly donating DABCO molecule. More donor centers supersede this extra coordination to the carbanion

Reactivity of Stable Heteroleptic Silylene PhC(N<i>t</i>Bu)₂SiNPh₂ toward Diazobenzene and <i>N</i>‑Benzylidineaniline

The reaction of heteroleptic silylene LSiNPh₂ [L = PhC­(N<i>t</i>Bu)₂] with diazobenzene afforded product <b>6</b>. This involves one <i>o</i>-C–H bond activation at one of the phenyl groups of diazobenzene and migration of this hydrogen atom from the phenyl ring to one of the nitrogen atoms, which leads to the formation of the new C–Si and N–Si bonds. The reaction of benzylidineaniline with LSiNPh₂ results in the oxidative addition of the three-membered silaaziridine derivative <b>7</b>. Compounds <b>6</b> and <b>7</b> were fully characterized by elemental analysis, multinuclear NMR spectroscopy, and EI-MS spectrometry. The molecular structures of compounds <b>6</b> and <b>7</b> were established unequivocally by single-crystal X-ray structural analysis

Influence of Donor–Acceptor Distance Variation on Photoinduced Electron and Proton Transfer in Rhenium(I)–Phenol Dyads

A homologous series of four molecules in which a phenol unit is linked covalently to a rhenium­(I) tricarbonyl diimine photooxidant via a variable number of <i>p</i>-xylene spacers (<i>n</i> = 0–3) was synthesized and investigated. The species with a single <i>p</i>-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (<i>n</i> = 0) triggers release of the phenolic proton to the acetonitrile/water solvent mixture; a H/D kinetic isotope effect (KIE) of 2.0 ± 0.4 is associated with this process. Thus, the shortest dyad basically acts like a photoacid. The next two longer dyads (<i>n</i> = 1, 2) exhibit intramolecular photoinduced phenol-to-rhenium electron transfer in the rate-determining excited-state deactivation step, and there is no significant KIE in this case. For the dyad with <i>n</i> = 1, transient absorption spectroscopy provided evidence for release of the phenolic proton to the solvent upon oxidation of the phenol by intramolecular photoinduced electron transfer. Subsequent thermal charge recombination is associated with a H/D KIE of 3.6 ± 0.4 and therefore is likely to involve proton motion in the rate-determining reaction step. Thus, some of the longer dyads (<i>n</i> = 1, 2) exhibit photoinduced proton-coupled electron transfer (PCET), albeit in a stepwise (electron transfer followed by proton transfer) rather than concerted manner. Our study demonstrates that electronically strongly coupled donor–acceptor systems may exhibit significantly different photoinduced PCET chemistry than electronically weakly coupled donor–bridge–acceptor molecules

Influence of Donor–Acceptor Distance Variation on Photoinduced Electron and Proton Transfer in Rhenium(I)–Phenol Dyads

A homologous series of four molecules in which a phenol unit is linked covalently to a rhenium­(I) tricarbonyl diimine photooxidant via a variable number of <i>p</i>-xylene spacers (<i>n</i> = 0–3) was synthesized and investigated. The species with a single <i>p</i>-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (<i>n</i> = 0) triggers release of the phenolic proton to the acetonitrile/water solvent mixture; a H/D kinetic isotope effect (KIE) of 2.0 ± 0.4 is associated with this process. Thus, the shortest dyad basically acts like a photoacid. The next two longer dyads (<i>n</i> = 1, 2) exhibit intramolecular photoinduced phenol-to-rhenium electron transfer in the rate-determining excited-state deactivation step, and there is no significant KIE in this case. For the dyad with <i>n</i> = 1, transient absorption spectroscopy provided evidence for release of the phenolic proton to the solvent upon oxidation of the phenol by intramolecular photoinduced electron transfer. Subsequent thermal charge recombination is associated with a H/D KIE of 3.6 ± 0.4 and therefore is likely to involve proton motion in the rate-determining reaction step. Thus, some of the longer dyads (<i>n</i> = 1, 2) exhibit photoinduced proton-coupled electron transfer (PCET), albeit in a stepwise (electron transfer followed by proton transfer) rather than concerted manner. Our study demonstrates that electronically strongly coupled donor–acceptor systems may exhibit significantly different photoinduced PCET chemistry than electronically weakly coupled donor–bridge–acceptor molecules

Introducing NacNac-Like Bis(4,6-isopropylbenzoxazol-2-yl)methanide in s‑Block Metal Coordination

Within this work, the field of bulky methanides in metal coordination is exceeded by the synthesis of the versatile and promising bis­(4,6-isopropylbenzoxazol-2-yl)­methane (<b>7</b>) ligand platform. As an enhancement in this class of ligands, isopropyl (<i>i</i>Pr) substituents as steric-demanding groups have been successfully introduced in proximity to the coordination pocket, mimicking the shielding abilities of the ubiquitous NacNac ligand scaffold to improve the steric protection of a coordinated s-block metal cation. A percent buried volume (% V_bur) calculation as well as an electronic structure analysis shades light onto the shielding and electronic abilities of the ligand in comparison to other selected methanides and diketiminates. Upon deprotonation with a variety of different group 1 and 2 metalation agents, a row of novel s-block metal complexes of the parent deprotonated monoanionic ligand <b>7</b> was obtained and structurally, as well as spectroscopically, characterized. In particular, in this context, the alkali-metal precursor complexes [Li­(THF)₂{(4,6-<i>i</i>Pr-NCOC₆H₂)₂CH}] (<b>8</b>) and [K­{μ-(4,6-<i>i</i>Pr-NCOC₆H₂)₂CH}]_∞ (<b>9</b>) as well as the alkaline-earth-metal compounds [MgCl­(THF)₂{(4,6-<i>i</i>Pr-NCOC₆H₂)₂CH}] (<b>10</b>) and [M­(THF)_n{(4,6-<i>i</i>Pr-NCOC₆H₂)₂CH}₂] [M = Mg, <i>n</i> = 0 (<b>11</b>); M = Ca, <i>n</i> = 1 (<b>12</b>); M = Sr, <i>n</i> = 1 (<b>13</b>); M = Ba, <i>n</i> = 1 (<b>14</b>)] were successfully synthesized. Especially, the latter four exhibit interesting trends in the solid state as well as in solution within the metal series

Reactivity Studies of a Stable N‑Heterocyclic Silylene with Triphenylsilanol and Pentafluorophenol

The reaction of the stable N-heterocyclic silylene [CH­{(CCH₂)­(CMe)­(2,6-<i>i</i>Pr₂C₆H₃N)₂}­Si] (<b>1</b>) with triphenylsilanol and pentafluorophenol in a 1:2 molar ratio resulted in quantitative yields of the pentacoordinate silicon-containing compounds [CH­{(CMe)₂(2,6-<i>i</i>Pr₂C₆H₃N)₂}­Si­(H)­{OSiPh₃}₂] (<b>2</b>) and [CH­{(CMe)₂(2,6-<i>i</i>Pr₂C₆H₃N)₂}­Si­(H)­{OC₆F₅}₂] (<b>3</b>), respectively. Compounds <b>2</b> and <b>3</b> were formed by O–H bond activation of triphenylsilanol and pentafluorophenol. They were characterized by elemental analysis, NMR spectroscopy, and EI-MS spectrometry. In their solid-state structures the silicon atom is tetracoordinate in <b>2</b>, whereas it is pentacoordinate in <b>3</b>

Facile Access to the Functionalized N-Donor Stabilized Silylenes PhC(N<i>t</i>Bu)₂SiX (X = PPh₂, NPh₂, NCy₂, N<i>i</i>Pr₂, NMe₂, N(SiMe₃)₂, O<i>t</i>Bu)

Reactions of silylenes with organic substrates generally lead to silicon­(IV) compounds. Ligand substitution at the silicon­(II) atom of silylene, without changing the formal +2 oxidation state, is very rare. We report herein a straightforward route to functionalized silylenes LSiX (L = PhC­(N<i>t</i>Bu)₂ and X = PPh₂ (<b>1</b>), NPh₂ (<b>2</b>), NCy₂(<b>3</b>), N<i>i</i>Pr₂ (<b>4</b>), NMe₂ (<b>5</b>), N­(SiMe₃)₂ (<b>6</b>), O<i>t</i>Bu (<b>7</b>)). Silylenes <b>1</b>–<b>7</b> have been prepared in quantitative yield by a modified ligand exchange reaction of PhC­(N<i>t</i>Bu)₂SiCl (LSiCl) with the corresponding lithium or potassium salts. Compounds <b>1</b>–<b>7</b> were characterized by spectroscopic and spectrometric techniques. Single-crystal X-ray structures of <b>1</b>, <b>3</b>, and <b>4</b> were determined

Stabilization of Low Valent Silicon Fluorides in the Coordination Sphere of Transition Metals

Silicon­(II) fluoride is unstable; therefore, isolation of the stable species is highly challenging and was not successful during the last 45 years. SiF₂ is generally generated in the gas phase at very high temperatures (∼1100–1200 °C) and low pressures and readily disproportionates or polymerizes. We accomplished the syntheses of stable silicon­(II) fluoride species by coordination of silicon­(II) to transition metal carbonyls. Silicon­(II) fluoride compounds L­(F)­Si·M­(CO)₅ {M = Cr (<b>4</b>), Mo (<b>5</b>), W­(<b>6</b>)} (L = PhC­(N<i>t</i>Bu)₂) were prepared by metathesis reaction from the corresponding chloride with Me₃SnF. However, the chloride derivatives L­(Cl)­Si·M­(CO)₅ {M = Cr (<b>1</b>), Mo (<b>2</b>), W­(<b>3</b>)} (L = PhC­(N<i>t</i>Bu)₂) were prepared by the treatment of transition metal carbonyls with L­(Cl)­Si. Direct fluorination of L­(Cl)Si with Me₃SnF resulted in oxidative addition products. Compounds <b>4</b>–<b>6</b> are stable at ambient temperature under an inert atmosphere of nitrogen. Compounds <b>4</b>–<b>6</b> were characterized by NMR spectroscopy, EI-MS spectrometry, and elemental analysis. The molecular structures of <b>4</b> and <b>6</b> were unambiguously established by single-crystal X-ray diffraction. Compounds <b>4</b> and <b>6</b> are the first structurally characterized fluorides, after the discovery of SiF₂ about four and a half decades ago

A Remarkable End-On Activation of Diazoalkane and Cleavage of Both C–Cl Bonds of Dichloromethane with a Silylene to a Single Product with Five-Coordinate Silicon Atoms

The 1:1 reaction of benzamidinato-stabilized chlorosilylene PhC­(N<i>t</i>Bu)₂SiCl (<b>1</b>) with CH­(SiMe₃)­N₂ resulted in the formation of colorless [PhC­(N<i>t</i>Bu)₂Si­(Cl)­{N₂CH­(SiMe₃)}]₂ (<b>2</b>), which consists of a four-membered Si₂N₂ ring. Surprisingly, N₂ elimination from the diazoalkane did not occur, but rather an end-on activation of the nitrogen was observed. For the mechanism, we propose the formation of a silaimine complex <b>A</b> as an intermediate, which is formed during the reaction and dimerized under [2 + 2] cycloaddition to <b>2</b>. In contrast, treatment of <b>1</b> with dichloromethane afforded a 2:1 product, [{PhC­(N<i>t</i>Bu)₂Si­(Cl₂)}₂CH₂] (<b>3</b>), which is obviously formed by oxidative addition under cleavage of both C–Cl bonds and formation of two Si–Cl and two Si–C bonds. Both silicon atoms in <b>3</b> are five-coordinate. Compounds <b>2</b> and <b>3</b> were characterized by single-crystal X-ray studies, multinuclear NMR spectroscopy, and EI-mass spectrometry