Nickel binding enables isolation and reactivity of previously inaccessible 7-aza-2,3-indolynes.

N-Heteroaromatics are key elements of pharmaceuticals, agrochemicals, and materials. N-Heteroarynes provide a scaffold to build these essential molecules but are underused because five-membered N-heteroarynes have been largely inaccessible on account of the strain of a triple bond in that small of a ring. On the basis of principles of metal-ligand interactions that are foundational to organometallic chemistry, in this work we report the stabilization of five-membered N-heteroarynes in the nickel coordination sphere. A series of 1,2-bis(dicyclohexylphosphino)ethane nickel 7-azaindol-2,3-yne complexes were synthesized and characterized crystallographically and spectroscopically. Ambiphilic reactivity of the nickel 7-azaindol-2,3-yne complexes was observed with multiple nucleophilic, electrophilic, and enophilic coupling partners.

Exploring Electrophilic Hydrophosphination via Metal Phosphenium Intermediates.

Two Mo(0) phosphenium complexes containing ancillary secondary phosphine ligands have been investigated with respect to their ability to participate in electrophilic addition at unsaturated substrates and subsequent P-H hydride transfer to "quench" the resulting carbocations. These studies provide stoichiometric "proof of concept" for a proposed new metal-catalyzed electrophilic hydrophosphination mechanism. The more strongly Lewis acidic phosphenium complex, [Mo(CO)4(PR2H)(PR2)]+ (R=Ph, Tolp), cleanly hydrophosphinates 1,1-diphenylethylene, benzophenone, and ethylene, while other substrates react rapidly to give products resulting from competing electrophilic processes. A less Lewis acidic complex, [Mo(CO)3(PR2H)2(PR2)]+, generally reacts more slowly but participates in clean hydrophosphination of a wider range of unsaturated substrates, including styrene, indene, 1-hexene, and cyclohexanone, in addition to 1,1-diphenylethylene, benzophenone, and ethylene. Mechanistic studies are described, including stoichiometric control reactions and computational and kinetic analyses, which probe whether the observed P-H addition actually does occur by the proposed electrophilic mechanism, and whether hydridic P-H transfer in this system is intra- or intermolecular. Preliminary reactivity studies indicate challenges that must be addressed to exploit these promising results in catalysis.

Radical-Polar Crossover Catalysis with a d0 Metal Enabled by a Redox-Active Ligand.

Radical-polar crossover mechanisms are invoked in numerous late transition metal and photocatalyzed reactions. To the best of our knowledge, reductive radical-polar crossover mechanisms are not invoked for group 3 early transition metals due to their propensity to exist in high oxidation states. Through use of a redox-active (tris)amido ligand we have accessed this mechanism for use with early transition metals. This mechanism is showcased through enabling product formation for a wide variety of elimination products from α-halo substituted benzylic bromides. The mechanism of this new type of reactivity with Sc is explored, and Hammett analysis reveals an anionic intermediate. The wide functional group tolerance of this reaction is also demonstrated.

Reversible Silylium Transfer between P-H and Si-H Donors.

The Mo=PR2 π* orbital in a Mo phosphenium complex acts as acceptor in a new PIII -based Lewis superacid. This Lewis acid (LA) participates in electrophilic Si-H abstraction from E3 SiH to give a Mo-bound secondary phosphine ligand, Mo-PR2 H. The resulting Et3 Si+ ion remains associated with the Mo complex, stabilized by η1 -P-H donation, yet undergoes rapid exchange with an η1 -Si-H adduct of free silane in solution. The equilibrium between these two adducts presents an opportunity to assess the role of this new LA in catalytic reactions of silanes: is the LA acting as a catalyst or as an initiator? Preliminary results suggest that a cycle including the Mo-bound phosphine-silylium adduct dominates in the catalytic hydrosilylation of acetophenone, relative to a putative cycle involving the silane-silylium adduct or "free" silylium.

A thermolytic route to a polysilyne.

We report a safe and convenient method to prepare a new class of network polysilane, or polysilyne ([RSi]n). Simple thermolysis of a readily accessible linear poly(phenylsilane), [PhSiH]n, affords polysilyne [PhSi]n with concomitant evolution of monosilanes. This new polymer shows a hyperbranched structure with unique features not observed in known polysilynes prepared via hazardous Wurtz coupling routes. Despite these differences, our soluble, yellow polysilyne exhibits some important properties associated with the traditional random network structure: it absorbs up to 400 nm in the UV spectrum, yet is stable to photolysis under inert atmosphere. This efficient new synthetic route opens the door to exciting applications for these hyperbranched polymers in materials and device technologies.

Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes.

Kinetic profiles obtained from monitoring the solution-phase substitution chemistry of [Ru(η5-indenyl)(NCPh)(PPh3)2]+ (1) by both electrospray ionization mass spectrometry and 31P{1H} NMR are essentially identical, despite an enormous difference in sample concentrations for these complementary techniques. These studies demonstrate dissociative substitution of the NCPh ligand in 1. Competition experiments using different secondary phosphine reagents provide a ranking of phosphine donor abilities at this relatively crowded half-sandwich complex: PEt2H > PPh2H ≫ PCy2H. The impact of steric congestion at Ru is evident also in reactions of 1 with tertiary phosphines; initial substitution products [Ru(η5-indenyl)(PR3)(PPh3)2]+ rapidly lose PPh3, enabling competitive re-coordination of NCPh. Further solution experiments, relevant to the use of 1 in catalytic hydrophosphination, show that PPh2H out-competes PPh2CH2CH2CO2Bu t (the product of hydrophosphination of tert-butyl acrylate by PPh2H) for coordination to Ru, even in the presence of a 10-fold excess of the tertiary phosphine. Additional information on relative phosphine binding strengths was obtained from gas-phase MS/MS experiments, including collision-induced dissociation experiments on the mixed phosphine complexes [Ru(η5-indenyl)PP'P″]+, which ultimately appear in solution during the secondary phosphine competition experiments. Unexpectedly, unsaturated complexes [Ru(η5-indenyl)(PR2H)(PPh3)]+, generated in the gas-phase, undergo preferential loss of PR2H. We propose that competing orthometallation of PPh3 is responsible for the surprising stability of the [Ru(η5-indenyl)(PPh3)]+ fragment under these conditions.