Electrophilic Activation of Silicon-Hydrogen Bonds in Catalytic Hydrosilations

Hydrosilation reactions represent an important class of chemical transformations and there has been considerable recent interest in expanding the scope of these reactions by developing new catalysts. A major theme to emerge from these investigations is the development of catalysts with electrophilic character that transfer electrophilicity to silicon via Si—H activation. This type of mechanism has been proposed for catalysts ranging from Group 4 transition metals to Group 15 main group species. Additionally, other electrophilic silicon species, such as silylene complexes and η3-H2SiRR' complexes, have been identified as intermediates in hydrosilation reactions. In this Review, different types of catalysts are compared to highlight the range of hydrosilation mechanisms that feature electrophilic silicon centers, and the importance of these catalysts to the development of new hydrosilation reactions is discussed

Size-Matched Radical Multivalency

Persistent π-radicals such as MV^+• (MV refers to methyl viologen, i.e., N,Nꞌ-dimethyl-4,4ꞌ-bipyridinum) engage in weak radical-radical interactions. This phenomenon has been utilized recently in supramolecular chemistry with the discovery that MV+• and [cyclobis(paraquat-p-phenylene)]2(+•) (CBPQT2(+•)) form a strong 1:1 host-guest complex [CBPQT⊂MV]3(+•). In this full paper, we describe the extension of radical-pairing-based molecular recognition to a larger, square-shaped diradical host, [cyclobis(paraquat-4,4ꞌ-biphenylene)]2(+•) (MS2(+•)). This molecular square was assessed for its ability to bind an isomeric series of possible diradical cyclophane guests, which consist of two radical viologen units that are linked by two ortho-, meta-, or para-xylylene bridges to provide different spacings between the planar radicals. UV-Vis-NIR measurements reveal that only the m-xylylene-linked isomer (m-CBPQT2(+•)) binds strongly inside of MS2(+•), resulting in the formation of a tetra-radical complex [MS⊂m-CBPQT]4(+•). Titration experiments and variable temperature UV-Vis-NIR and EPR spectroscopic data indicate that, relative to the smaller tris-radical complex [CBPQT⊂MV]3(+•), the new host-guest complex forms with a more favorable enthalpy change that is offset by a greater entropic penalty. As a result, the association constant (Ka = (1.12+/- 0.08) x 10^5 M^(-1)) for [MS⊂m-CBPQT]4(+•) is similar to that previously determined for [CBPQT⊂MV]3(+•). The (super)structures of MS2(+•), m-CBPQT2(+•), and [MS⊂m-CBPQT]4(+•) were examined by single-crystal X-ray diffraction measurements and DFT calculations. The solid-state and computational structural analyses reveal that m-CBPQT2(+•) is ideally sized to bind inside of MS2(+•). The solid-state superstructures also indicate that localized radical-radical interactions in m-CBPQT2(+•) and [MS⊂m-CBPQT]4(+•) disrupt the extended radical-pairing interactions that are common in crystals of other viologen radical cations. Lastly, the formation of [MS⊂m-CBPQT]4(+•) was probed by cyclic voltammetry, demonstrating that the radical states of the cyclophanes are stabilized by the radical-pairing interactions

Molecular Russian dolls

The host-guest recognition between two macrocycles to form hierarchical non-intertwined ring-in-ring assemblies remains an interesting and challenging target in noncovalent synthesis. Herein, we report the design and characterization of a box-in-box assembly on the basis of host-guest radical-pairing interactions between two rigid diradical dicationic cyclophanes. One striking feature of the box-in-box complex is its ability to host various 1,4-disubstituted benzene derivatives inside as a third component in the cavity of the smaller of the two diradical dicationic cyclophanes to produce hierarchical Russian doll like assemblies. These results highlight the utility of matching the dimensions of two different cyclophanes as an efficient approach for developing new hybrid supramolecular assemblies with radical-paired ring-in-ring complexes and smaller neutral guest molecules

Molecular Russian dolls

The host-guest recognition between two macrocycles to form hierarchical non-intertwined ring-in-ring assemblies remains an interesting and challenging target in noncovalent synthesis. Herein, we report the design and characterization of a box-in-box assembly on the basis of host-guest radical-pairing interactions between two rigid diradical dicationic cyclophanes. One striking feature of the box-in-box complex is its ability to host various 1,4-disubstituted benzene derivatives inside as a third component in the cavity of the smaller of the two diradical dicationic cyclophanes to produce hierarchical Russian doll like assemblies. These results highlight the utility of matching the dimensions of two different cyclophanes as an efficient approach for developing new hybrid supramolecular assemblies with radical-paired ring-in-ring complexes and smaller neutral guest molecules

Correcting Frost Diagram Misconceptions Using Interactive Frost Diagrams

Frost diagrams provide convenient illustrations of the aqueous reduction potentials and thermodynamic tendencies of different oxidation states of an element. Undergraduate textbooks often describe the lowest point on a Frost diagram as the most stable oxidation state of the element, but this interpretation is incorrect because the thermodynamic stability of each oxidation state depends on the specific redox conditions in solution (i.e., the potential applied by the environment or an electrode). Further confusion is caused by the widespread use of different, contradictory conventions for labeling the y-axis of these diagrams as either nE° or −nE°, among other possibilities. To aid in discussing and correcting these common mistakes, we introduce a series of interactive Frost diagrams that illustrate the conditional dependence of the relative stabilities of each oxidation state of an element. We include instructor’s notes for using these interactive diagrams and a written activity for students to complete using these diagrams.</p

Mingling Light, Oxygen, and Organometallics to Form Cobalt-Carbon Bonds in the Confines of a Metal-Organic Nanocage

First-row transition-metal complexes often show a propensity for forming reactive radical species, such as superoxide complexes (M–O2•) generated by the binding of O2 to the metal, or free alkyl-radicals formed via M–C homolysis. Such radicals are important intermediates in reactions catalyzed by synthetic metal complexes and metalloenzymes, but their high reactivity can lead to undesired side reactions such as quenching by solvent, oxygen, or other radicals. In this work, we show that confinement of a CoII porphyrin complex in a large porphyrin-walled M8L6 nanocage allows for the taming of radical reactivity to enable clean oxidative alkylation of the cobalt center with tetraalkyltin reagents via an unexpected process mediated by O2 and light, which usually promote homolytic decomposition of porphyrin-supported CoIII–alkyl bonds. Indeed, analogous CoIII–alkyl complexes in free solution degrade too quickly under the alkylating conditions to enable their clean formation. The nanocage also acts as a size-selective barrier for alkylating agents, allowing CoIII–alkyl formation using SnMe4 and SnEt4 but not SnBu4. Likewise, Co–C homolysis is facilitated by the persist radical reagent TEMPO but not by a bulky derivative of TEMPO. These results show that nanoconfinement is a promising strategy for guiding radical-based organometallic reactivity under otherwise prohibitive conditions

Silane–Isocyanide Coupling Involving 1,1-Insertion of XylNC into the Si–H Bond of a σ‑Silane Ligand

Complexes [PhBP^Ph₃]­RuH­(η³-H₂SiRR′) (R,R′ = Me,Ph, <b>1a</b>; RR′ = Ph₂, <b>1b</b>) react with XylNC (Xyl = 2,6-dimethylphenyl) to form Fischer carbene complexes [PhBP^Ph₃]­Ru­(H)[C­(H)­(N­(Xyl)­(η²-H–SiRR′))] (<b>2a</b>,<b>b</b>) that feature a γ-agostic Si–H bond. The ruthenium isocyanide complexes [PhBP^Ph₃]­Ru­(H)­(CNXyl)­(η²-HSiHRR′) (<b>6a</b>,<b>b</b>) are not intermediates as they do not convert to <b>2a</b>,<b>b</b>. Experimental and theoretical investigations indicate that XylNC is activated by initial coordination to the silicon center in <b>1a</b>,<b>b</b>, followed by 1,1-insertion into an Si–H bond of the coordinated silane and then rearrangement to <b>2a</b>,<b>b</b>

Silane–Isocyanide Coupling Involving 1,1-Insertion of XylNC into the Si–H Bond of a σ‑Silane Ligand

Complexes [PhBP^Ph₃]­RuH­(η³-H₂SiRR′) (R,R′ = Me,Ph, <b>1a</b>; RR′ = Ph₂, <b>1b</b>) react with XylNC (Xyl = 2,6-dimethylphenyl) to form Fischer carbene complexes [PhBP^Ph₃]­Ru­(H)[C­(H)­(N­(Xyl)­(η²-H–SiRR′))] (<b>2a</b>,<b>b</b>) that feature a γ-agostic Si–H bond. The ruthenium isocyanide complexes [PhBP^Ph₃]­Ru­(H)­(CNXyl)­(η²-HSiHRR′) (<b>6a</b>,<b>b</b>) are not intermediates as they do not convert to <b>2a</b>,<b>b</b>. Experimental and theoretical investigations indicate that XylNC is activated by initial coordination to the silicon center in <b>1a</b>,<b>b</b>, followed by 1,1-insertion into an Si–H bond of the coordinated silane and then rearrangement to <b>2a</b>,<b>b</b>