Catalytic reactions of bisphosphinite pincerligated iridium compounds p-X R (POCOP)IrHCl (POCOP) [2,6-(R 2 PO) 2 C 6 H 3 , R = i Pr, X = H (1); R = t Bu, X = COOMe (2); = H (3); = NMe 2 (4)] with primary and secondary silanes have been performed. Complex 1 is primarily a silane redistribution precatalyst, but dehydrocoupling catalysis is observed for sterically demanding silane substrates or with aggressive removal of H 2 . The bulkier compounds (2−4) are silane dehydrocoupling precatalysts that also undergo competitive redistribution with less hindered substrates. Products generated from reactions utilizing 2−4 include low molecular weight oligosilanes with varying degrees of redistribution present or disilanes when employing more sterically demanding silane substrates. Selectivity for redistribution versus dehydrocoupling depends on the steric and electronic environment of the metal but can also be affected by reaction conditions. ■ INTRODUCTIONPolysilanes are not primarily prepared by metal catalysis, despite how essential metal catalysis is for the preparation of specialty polyolefins. 1 This is a surprising dichotomy given the premium on monodisperse, linear polysilanes of high molecular weight to promote optimal σ-electron delocalization. 2,3 Reasons for the lack of industrial adoption of metal-catalyzed silane dehydrocoupling depend on the general category of catalyst employed. For example, well-studied and highly active d 0 metal catalysts tend to suffer from competitive backbiting and formation of cyclosilanes, which can limit molecular weight and increase polydispersity. 4 Despite the fact that the first report on silane dehydrocoupling used Wilkinson's catalyst, 5 initial observations of low dehydrocoupling activity with late metals stunted their development relative to d 0 metal catalysts. 6 Additionally, the use of late metal catalysts in dehydrocoupling is potentially problematic due to two metal-catalyzed side reactions, the oxidation of Si−Si bonds 7 and redistribution. 8 Thus, the potential advances that metal catalysis can offer to polysilane preparation cannot be realized until the factors that impact metal catalysis are better understood. Efforts to minimize these side reactions have included promoting hydrogen loss, 9 employing designer substrates, 10,11 inhibiting silyl coordination, 12 and rigorous exclusion of water and oxygen.While conditions for promoting linear polysilanes over cyclosilanes are known for d 0 metallocene complexes, general conditions to promote dehydrocoupling over redistribution are not known. 4 However, late metal catalysts using nickel, 13−16 rhodium, 17,18 and platinum 19 have shown dehydrocoupling activity and selectivity comparable to that of the group 4 metallocenes, although these methods of polymerization still generally suffer from lower than desired molecular weights and high polydispersity.Iridium pincer complexes bearing the POCOP ligand (POCOP = 2,6-( t Bu 2 PO) 2 C 6 H 3 − ) are thermally robust molecules that have shown high activity as dehydrogenation ca...

A B S T R A C T : κ 5 -( M e 3 S i N C H 2 C H 2 ) 2 N -(CH 2 CH 2 NSiMe 2 CH 2 )Zr (1) has been found to dehydrocouple amine borane substrates, RR′NHBH 3 (R = R′ = Me; R = t Bu, R′ = H; R = R′ = H), at low to moderate catalyst loadings (0.5−5 mol %) and good to excellent conversions, forming mainly borazine and borazane products. Other z i r c o n i u m c a t a l y s t s , ( N 3 N ) Z r X [ ( N 3 N ) = N -(CH 2 CH 2 NSiMe 2 CH 2 ) 3 , X = NMe 2 (2), Cl (3), and O t Bu (4)], were found to exhibit comparable activities to that of 1. Compound 1 reacts with Me 2 NHBH 3 to give (N 3 N)Zr-(NMe 2 BH 3 ) (5), which was structurally characterized and features an η 2 B−H σ-bond amido borane ligand. Because 5 is unstable with respect to borane loss to form 2, rather than β-hydrogen elimination, and 2−4 do not exhibit X ligand loss during catalysis, dehydrogenation is hypothesized to proceed via an outer-sphere-type mechanism. This proposal is supported by the catalytic hydrogenation of alkenes by 2 using amine boranes as the sacrificial source of hydrogen. ■ INTRODUCTIONApplication of amine boranes in materials science, for hydrogen storage, and in organic synthesis demonstrates the usefulness of these simple Lewis acid−base adducts as materials precursors and chemical reagents. 1−5 The most pointed driver for recent study of amine borane dehydrogenation has been the potential use of these molecules for hydrogen storage, owing to their high hydrogen content by weight and relative ease of hydrogen loss. However, thermal degradation of amine boranes is poorly controlled and time-consuming. 3,6 Thus, catalysts that operate under desirable conditions (i.e., mild temperatures and faster reaction times) to form products in a controlled manner have been sought. Some of the most active catalysts that have been reported are capable of operating at ambient temperatures using rare and expensive group 9 transition metals such as iridium and rhodium. 7−10 This is a highly active field of study, and catalysts from across the periodic table have been reported including transition-metal and main group compounds. 11−19 Group 4 catalysts were some of the earliest studied for amine borane dehydrocoupling and have elicited interest from several groups due to their high reactivity and mechanistic richness. Manners, 20−22 Chirik, 23 Rosenthal, 24 Wass, 25,26 Beweries, 24,27 and Baker 28 have all presented detailed studies of related group 4 metallocene complexes. These independent studies indicate that titanium compounds demonstrate greater activity than their heavier congeners and that increased electron donation and steric pressure from ancillary ligands decrease reactivity. Substrate selectivity was also observed for group 4 metallocenes, which demonstrated high activity for secondary amine borane dehydrocoupling, while these compounds were almost inert toward ammonia borane. The lower activity of these metallocene compounds with NH 3 BH 3 is perhaps due to competing formation of stable amido borane complexes that precipitate out of soluti...

Carboxylic acid-functionalized Pd and Pt PNP pincer complexes were used for the assembly of two porous Zr metal-organic frameworks (MOFs), 2-PdX and 2-PtX. Powder X-ray diffraction analysis shows that the new MOFs adopt cubic framework structures similar to the previously reported ZrO(OH)[(PCP)PdX], [PCP = 2,6-(OPAr)CH); Ar = p-CHCO, X = Cl, I] (1-PdX). Elemental analysis and spectroscopic characterization indicate the presence of missing linker defects, and 2-PdX and 2-PtX were formulated as ZrO(OH)(OAc)[M(PNP)X] [M = Pd, Pt; PNP = 2,6-(HNPAr)CHN; Ar = p-CHCO; X = Cl, I]. Postsynthetic halide ligand exchange reactions were carried out by treating 2-PdX with Ag(OSCF) or NaI followed by PhI(OCCF). The latter strategy proved to be more effective at activating the MOF for the catalytic intramolecular hydroamination of an o-substituted alkynyl aniline, underscoring the advantage of using halide exchange reagents that produce soluble byproducts.

Catalytic hydrophosphination of alkenes using a chiral, air-stable primary phosphine, (R)-[2'-methoxy(1,1'-binapthalen)-2-yl]phosphine, (R)-MeO-MOPH2, proceeds under mild conditions with a zirconium catalyst, [κ(5)-N,N,N,N,C-(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH]Zr (1), to selectively furnish anti-Markovnikov, air-stable secondary phosphines or tertiary phosphines with slight modification of the protocol. An intermediate in the catalysis, [(N3N)Zr(R)-MeO-MOPH] (4), was structurally characterized.