Molecular Main Group Metal Hydrides.

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Metal-Free Directed Site-Selective Csp3 -H Borylation of Saturated Cyclic Amines.

The borylation of Csp3 -H bonds is a challenging transformation that is typically restricted to transition metal catalysis. Herein, we report the site-selective metal-free Csp3 -H borylation of saturated cyclic amines. It is possible to selectively borylate piperidine derivatives at the α or ß positions according to the reaction conditions. The mechanism was supported by NMR spectroscopy, calorimetry experiments and density functional theory (DFT) computations. It suggests that the piperidine is dehydrogenated by complexation with BBr3 to produce an enamine intermediate, which is in turn borylated at either the α or ß position according to the reaction conditions.

Frustrated Lewis Pair Chelation and Reactivity of Complexed Parent Iminoborane and Aminoborane.

An intramolecular phosphine-borane frustrated Lewis pair (FLP) chelate, iPr2P(C6H4)BCy2 or PB (Cy = cyclohexyl), was used to coordinate aminoborane (H2BNH2) and iminoborane (HBNH) units via donor-acceptor stabilization. Attempts to induce dehydrogenation from these B-N adducts with known metal catalysts (or pre-catalysts) have been unsuccessful thus far, and related observations were noted with an H2BNH2 complex supported by a modified FLP chelate bearing a geometrically constrained bicyclic 9-borabicyclo(3.3.1)nonane (BBN) unit. Treatment of the iminoborane adduct [PB{HBNH}] with a chlorinating agent led to ligand activation via B-C bond cleavage instead of the expected H/Cl exchange at boron to give [PB{ClBNH}]. Nucleophilic attack at the boron center in [PB{HBNH}] was observed upon addition of BnK (Bn = benzyl), yielding the amidoborate complex [PB{H(Bn)BNH}{K(THF)2}].

Frustrated Lewis Pair Chelation as a Vehicle for Low-Temperature Semiconductor Element and Polymer Deposition.

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, i Pr2 P(C6 H4 )BCy2 (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH2 }] and [PB{GeH2 }] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine-borane ligand (PB) after element deposition, and 2) the single-source precursor [PB{SiH2 }] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe2 }] was also prepared, and shown to release poly(dimethylsilane) [SiMe2 ]n upon heating. Overall, this study introduces a "closed loop" deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.

Frustrated Lewis pair adducts of alkynyl-capped tetrelenes.

The frustrated Lewis pair (FLP) adducts PB{ECl2} (PB = iPr2P(C6H4)BCy2; Cy = cyclohexyl; E = Si, Ge) were used to access a bis(alkynyl)-functionalized silylene and a germylene; the goal behind preparing these species was to obtain new unsaturated main group polymers [E(CCSiMe3)2]n upon heating. While the silylene adduct PB{Si(CCSiMe3)2} was stable up to 150 °C, the heavier element congener PB{Ge(CCSiMe3)2} underwent a complicated rearrangement process accompanied by Cy-group migration and Ge(II)-alkyne coordination. Density functional theory computations were performed to understand the mechanistic pathway for the unusual rearrangement of PB{Ge(CCSiMe3)2}.

Frustrated Lewis pair-ligated tetrelenes.

The reactivity of [PB{SiX2}] (X = Cl, Br; PB = 1,2-iPr2(C6H4)BCy2; E = Si, Ge) adducts is described, with an initial focus on reduction attempts to access [PB{E}]x species; however, in all cases only free PB ligand was formed as the soluble product. Moreover, computations were performed to evaluate the energy penalty associated with EX2 dissociation from the PB chelates. Moving up the periodic table, the formal methylene adduct [PB{CH2}] was isolated and its reactivity was compared with its heavier element congeners of [PB{EH2}]. We also introduce new phosphine-borane frustrated Lewis pair (FLP) chelates and explore preliminary coordination chemistry with these ligands.

Access to adducts of parent iminoborane isomers, HBNH and NBH2, using frustrated Lewis pair chelation.

Adducts of the parent iminoborane isomers, HBNH and NBH2, have been prepared, each stabilized by the frustrated Lewis pair (FLP) chelate iPr2P(C6H4)BCy2 (PB). PB{HBNH} was accessed via dehydrohalogenation, while the corresponding isomer PB{NBH2} was obtained from the borylation of the formal nitrene-FLP complex PB{NH}.