Systematic Study of N‑Heterocyclic Carbene Coordinate Hydrosilylene Transition-Metal Complexes

An in-depth study of the synthesis and structures of N-heterocyclic carbene (NHC)-stabilized silylene transition-metal complexes is reported. An iron hydrosilylene complex, [<i>t</i>Bu₃Si­(NHC)­(H)­Si:→Fe­(CO)₄] (<b>2</b>), was synthesized starting from the corresponding hydrosilylene [<i>t</i>Bu₃Si­(NHC)­(H)­Si:] (<b>1</b>). Complex <b>2</b> was fully characterized, including X-ray diffraction analysis, which showed an unusual long Si–Fe bond length. A very long bond length was also observed in the novel hydrosilylene tungsten complex [<i>t</i>Bu₃Si­(NHC)­(H)­Si:→W­(CO)₅] (<b>3</b>). A series of NHC-stabilized silylene iron complexes ([R₂(NHC)­Si:→Fe­(CO)₄], where R = Cl (<b>4</b>), H (<b>5</b>), and Me (<b>6</b>)) were synthesized and fully characterized to investigate the influence of different substituents. The dihydrosilylene iron complex [H₂(NHC)­Si:→Fe­(CO)₄] (<b>5</b>) represents a new example of a donor–acceptor-stabilized parent silylene (H₂Si:). Density functional theory calculations were utilized to understand the influence of the electronic and steric effects of the silylene unit and its substituents on the Si–Fe bond in these iron complexes, in particular to rationalize the long Si–Fe bond in <b>2</b>

From a Zwitterionic Phosphasilene to Base Stabilized Silyliumylidene-Phosphide and Bis(silylene) Complexes

The reactivity of ylide-like phosphasilene <b>1</b> [LSi­(TMS)P­(TMS), L = PhC­(N<i>t</i>Bu)₂] with group 10 d¹⁰ 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 <b>1</b> with ethylene bis­(triphenylphosphine) platinum(0), a complete silicon–phosphorus bond breakage occurs, yielding the unprecedented dinuclear platinum complex <b>3</b> [LSi­{Pt­(PPh₃)}₂P­(TMS)₂]. Spectroscopic, structural, and theoretical analysis of complex <b>3</b> revealed the cationic silylene (silyliumylidene) character of the silicon unit in complex <b>3</b>. Similarly, formation of the analogous dinuclear palladium complex <b>4</b> [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 <b>5</b> [{(LSi)₂P­(TMS)}­Ni­(COD)], was obtained. Complex <b>5</b> 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 <b>2</b> [LSi­{Pt­(TMS)­(PPh₃)}­P­(TMS)] was also experimentally observed. This is contrasted by the reaction of nickel, in which the equilibrium of phosphasilene <b>1</b> and the phosphinosilylene <b>6</b> [LSiP­(TMS)₂] was utilized for a better coordination of the silicon­(II) moiety in comparison with phosphorus to the transition metal center

Synthesis of Intramolecularly Coordinated Aluminum and Gallium Compounds for the Preparation of [1]Ferrocenophanes

Two different ligands equipped with pyridine donor moieties, (2-H₄C₅N)­(Me₃Si)₂C (R′) and (2-H₄C₅N)­(Me₃Si)­(Me)C (R″), were applied in the preparation of aluminum and gallium dihalides that could be employed in salt metathesis reactions with 1,1′-dilithioferrocene. R′GaCl₂ (<b>1</b>) was accessible from LiR′ and GaCl₃ (21%), whereas the respective aluminum compound R′AlCl₂ (<b>3</b>^<b>Cl</b>) and its bromine analogue R′AlBr₂ (<b>3</b>^<b>Br</b>) could only be prepared through the intermediate species R′AlMe₂ (<b>2</b>) by the addition of Me₃SnCl and Br₂, respectively. An improved synthesis of the ligand precursor R″H, (2-H₄C₅N)­(Me₃Si)­(Me)­CH), is described. Attempted syntheses of R″AlX₂ starting from LiR″ and AlCl₃ or ClAlMe₂ gave the bis-ligand compounds R″₂AlCl (<b>4</b>) and R″₂AlMe (<b>6</b>), respectively. As deduced from proton NMR spectroscopy, the formation of <b>6</b> proceeded through the intermediate R″AlMe₂ (<b>5</b>) and was facilitated in the presence of tmeda. The formation of <b>4</b> and <b>6</b>, respectively, is diastereospecific, as only <i>rac</i> isomers were formed (<i>R</i>,<i>R</i>-Λ and <i>S</i>,<i>S</i>-Δ). Molecular structures of compounds <b>2</b>, <b>3</b>^<b>Br</b>, and <b>6</b> were determined by single-crystal X-ray analysis. Salt metathesis of the dihalides <b>1</b>, <b>3</b>^<b>Cl</b>, and <b>3</b>^<b>Br</b> with 1,1′-dilithioferrocene gave the respective galla- and alumina[1]­ferrocenes (<b>7</b> and <b>8</b>). Neither compound could be isolated and were only identified by ¹H NMR spectroscopy in reaction mixtures. Analytically pure polymers (<b>7</b>_{<b><i>n</i></b>}) of low molecular weight were found and investigated by DLS (<i>M</i>_w = 8.3 ± 2.5 kDa; DP_w = 17 ± 5)

From a Zwitterionic Phosphasilene to Base Stabilized Silyliumylidene-Phosphide and Bis(silylene) Complexes

The reactivity of ylide-like phosphasilene <b>1</b> [LSi­(TMS)P­(TMS), L = PhC­(N<i>t</i>Bu)₂] with group 10 d¹⁰ 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 <b>1</b> with ethylene bis­(triphenylphosphine) platinum(0), a complete silicon–phosphorus bond breakage occurs, yielding the unprecedented dinuclear platinum complex <b>3</b> [LSi­{Pt­(PPh₃)}₂P­(TMS)₂]. Spectroscopic, structural, and theoretical analysis of complex <b>3</b> revealed the cationic silylene (silyliumylidene) character of the silicon unit in complex <b>3</b>. Similarly, formation of the analogous dinuclear palladium complex <b>4</b> [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 <b>5</b> [{(LSi)₂P­(TMS)}­Ni­(COD)], was obtained. Complex <b>5</b> 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 <b>2</b> [LSi­{Pt­(TMS)­(PPh₃)}­P­(TMS)] was also experimentally observed. This is contrasted by the reaction of nickel, in which the equilibrium of phosphasilene <b>1</b> and the phosphinosilylene <b>6</b> [LSiP­(TMS)₂] was utilized for a better coordination of the silicon­(II) moiety in comparison with phosphorus to the transition metal center

[1.1]Ferrocenophanes and Bis(ferrocenyl) Species with Aluminum and Gallium as Bridging Elements: Synthesis, Characterization, and Electrochemical Studies

Salt-metathesis reactions between dilithioferrocene (Li₂fc·2/3tmeda) and intramolecularly coordinated aluminum and gallium species RECl₂ [R = 5-Me₃Si-2-(Me₂NCH₂)­C₆H₃; E = Al (<b>2a</b>), Ga (<b>2b</b>); and R = (2-C₅H₄N)­Me₂SiCH₂; E = Al (<b>3a</b>), Ga (<b>3b</b>)] gave respective [1.1]­ferrocenophanes ([1.1]­FCPs). Those obtained from <b>2a</b> and <b>2b</b>, respectively, were isolated as analytically pure compounds and fully characterized including single-crystal X-ray structure determinations [<b>4a</b> (Al): 43%; <b>4b</b> (Ga): 47%]. Bis­(ferrocenyl) compounds of the type REFc₂ [R = 5-Me₃Si-2-(Me₂NCH₂)­C₆H₃; E = Al (<b>5a</b>), Ga (<b>5b</b>); and R = (2-C₅H₄N)­Me₂SiCH₂; E = Al (<b>6a</b>), Ga (<b>6b</b>)] and R₂SiFc₂ [R = Me (<b>7</b>^<b>Me</b>); Et (<b>7</b>^<b>Et</b>)] were prepared, starting from respective element dichlorides and lithioferrocene (LiFc). Molecular structures of <b>6a</b>, <b>7</b>^<b>Me</b>, and <b>7</b>^<b>Et</b> were solved by single-crystal X-ray analyses. One of the two Fc moieties of <b>6a</b> was bent toward the open coordination site of the aluminum atom. The measured dip angles α* of the two independent molecules in the asymmetric unit were 11.9(5) and 13.3(5)°, respectively. The redox behavior of [1.1]­FCPs <b>4</b> and bis­(ferrocenyl) species <b>5</b>, <b>6</b>, <b>7</b>, and (Mamx)­EFc₂ [Mamx = 2,4-<i>t</i>Bu₂-6-(Me₂NCH₂)­C₆H₂; E = Al (<b>8a</b>), Ga (<b>8b</b>)] were investigated with cyclic voltammetry. While all gallium and silicon compounds gave meaningful and interpretable data, all aluminum compounds were problematic with the exception of <b>8a</b>. Aluminum species, compared to respective gallium species, are more sensitive and, presumably, fluoride ions or residual water from the electrolyte and solvent are causing degradation. The splitting between the formal potentials for bis­(ferrocenyl) species was significantly smaller (<b>5b</b>, <b>6b</b>, and <b>8b</b>: Δ<i>E</i>°′ = 0.138–0.159 V) than that of the [1.1]­FCP <b>4b</b> (Δ<i>E</i>°′ = 0.309 V). These results were explained by assuming an electrostatic interaction between the two iron centers; differences between bis­(ferrocenyl) species and [1.1]­FCPs are likely due to a more effective solvation of Fe-containing moieties in the more flexible bis­(ferrocenyl) species

[1.1]Ferrocenophanes and Bis(ferrocenyl) Species with Aluminum and Gallium as Bridging Elements: Synthesis, Characterization, and Electrochemical Studies

Salt-metathesis reactions between dilithioferrocene (Li₂fc·2/3tmeda) and intramolecularly coordinated aluminum and gallium species RECl₂ [R = 5-Me₃Si-2-(Me₂NCH₂)­C₆H₃; E = Al (<b>2a</b>), Ga (<b>2b</b>); and R = (2-C₅H₄N)­Me₂SiCH₂; E = Al (<b>3a</b>), Ga (<b>3b</b>)] gave respective [1.1]­ferrocenophanes ([1.1]­FCPs). Those obtained from <b>2a</b> and <b>2b</b>, respectively, were isolated as analytically pure compounds and fully characterized including single-crystal X-ray structure determinations [<b>4a</b> (Al): 43%; <b>4b</b> (Ga): 47%]. Bis­(ferrocenyl) compounds of the type REFc₂ [R = 5-Me₃Si-2-(Me₂NCH₂)­C₆H₃; E = Al (<b>5a</b>), Ga (<b>5b</b>); and R = (2-C₅H₄N)­Me₂SiCH₂; E = Al (<b>6a</b>), Ga (<b>6b</b>)] and R₂SiFc₂ [R = Me (<b>7</b>^<b>Me</b>); Et (<b>7</b>^<b>Et</b>)] were prepared, starting from respective element dichlorides and lithioferrocene (LiFc). Molecular structures of <b>6a</b>, <b>7</b>^<b>Me</b>, and <b>7</b>^<b>Et</b> were solved by single-crystal X-ray analyses. One of the two Fc moieties of <b>6a</b> was bent toward the open coordination site of the aluminum atom. The measured dip angles α* of the two independent molecules in the asymmetric unit were 11.9(5) and 13.3(5)°, respectively. The redox behavior of [1.1]­FCPs <b>4</b> and bis­(ferrocenyl) species <b>5</b>, <b>6</b>, <b>7</b>, and (Mamx)­EFc₂ [Mamx = 2,4-<i>t</i>Bu₂-6-(Me₂NCH₂)­C₆H₂; E = Al (<b>8a</b>), Ga (<b>8b</b>)] were investigated with cyclic voltammetry. While all gallium and silicon compounds gave meaningful and interpretable data, all aluminum compounds were problematic with the exception of <b>8a</b>. Aluminum species, compared to respective gallium species, are more sensitive and, presumably, fluoride ions or residual water from the electrolyte and solvent are causing degradation. The splitting between the formal potentials for bis­(ferrocenyl) species was significantly smaller (<b>5b</b>, <b>6b</b>, and <b>8b</b>: Δ<i>E</i>°′ = 0.138–0.159 V) than that of the [1.1]­FCP <b>4b</b> (Δ<i>E</i>°′ = 0.309 V). These results were explained by assuming an electrostatic interaction between the two iron centers; differences between bis­(ferrocenyl) species and [1.1]­FCPs are likely due to a more effective solvation of Fe-containing moieties in the more flexible bis­(ferrocenyl) species