Erin M. Leitao

Catalysis in service of main group chemistry offers a versatile approach to p -block molecules and materials

Catalytic reactions that enable the formation of new bonds to carbon centres play a pervasive role in the state-of-the-art synthesis of organic molecules and macromolecules. In contrast, the development of analogous processes as routes to main group compounds and materials has been much slower. Nevertheless, recent advances have led to a broad expansion of this field and now allow access to a wide range of catenated structures based on elements across the p block. These breakthroughs have already impacted areas such as hydrogen storage and transfer, functional inorganic polymers and ceramic thin films. Dehydrogenation and dehydrocoupling processes are particularly well developed and may be mediated by either transition metal or main group catalysts. Such pathways represent an increasingly attractive and convenient alternative to traditional routes, such as salt metathesis and reductive coupling reactions. An overview of this emerging area is presented in this Review with a focus on recent developments and future challenges.

Mechanism of Metal-Free Hydrogen Transfer between Amine–Boranes and Aminoboranes

The kinetics of the metal-free hydrogen transfer from amine-borane Me(2)NH·BH(3) to aminoborane iPr(2)N═BH(2), yielding iPr(2)NH·BH(3) and cyclodiborazane [Me(2)N-BH(2)](2) via transient Me(2)N═BH(2), have been investigated in detail, with further information derived from isotopic labeling and DFT computations. The approach of the system toward equilibrium was monitored in both directions by (11)B{(1)H} NMR spectroscopy in a range of solvents and at variable temperatures in THF. Simulation of the resulting temporal-concentration data according to a simple two-stage hydrogen transfer/dimerization process yielded the rate constants and thermodynamic parameters attending both equilibria. At ambient temperature, the bimolecular hydrogen transfer is slightly endergonic in the forward direction (ΔG(1)°((295)) = 10 ± 7 kJ·mol(-1); ΔG(1)(‡)((295)) = 91 ± 5 kJ·mol(-1)), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me(2)N═BH(2) (ΔG(2)°((295)) = -28 ± 14 kJ·mol(-1)). Systematic deuterium labeling of the NH and BH moieties in Me(2)NH·BH(3) and iPr(2)N═BH(2) allowed the kinetic isotope effects (KIEs) attending the hydrogen transfer to be determined. A small inverse KIE at boron (k(H)/k(D) = 0.9 ± 0.2) and a large normal KIE at nitrogen (k(H)/k(D) = 6.7 ± 0.9) are consistent with either a pre-equilibrium involving a B-to-B hydrogen transfer or a concerted but asynchronous hydrogen transfer via a cyclic six-membered transition state in which the B-to-B hydrogen transfer is highly advanced. DFT calculations are fully consistent with a concerted but asynchronous process.

Thermal Decomposition Modes for Four-Coordinate Ruthenium Phosphonium Alkylidene Olefin Metathesis Catalysts

The four-coordinate ruthenium phosphonium alkylidenes 1-Cy and 1-iPr, differing in the substituent on the phosphorus center, were observed to decompose thermally in the presence of 1,1-dichloroethylene to produce [H(3)CPR(3)][Cl]. The major ruthenium-containing product was a trichloro-bridged ruthenium dimer that incorporates the elements of the 1,1-dichloroethylene as a dichlorocarbene ligand and a styrenic vinyl group on the supporting NHC ligand. Spectroscopic, kinetic, and deuterium-labeling experiments probed the mechanism of this process, which involves a rate-limiting C-H activation of an NHC mesityl ortho methyl group. These studies provide insight into intrinsic decomposition processes of active Grubbs type olefin metathesis catalysts, pointing the way to new catalyst design directions.

Kinetic and Thermodynamic Analysis of Processes Relevant to Initiation of Olefin Metathesis by Ruthenium Phosphonium Alkylidene Catalysts

Initiation processes in a family of ruthenium phosphonium alkylidene catalysts, some of which are commercially available, are presented. Seven 16-electron zwitterionic catalyst precursors of general formula (H(2)IMes)(Cl)(3)Ru=C(H)P(R(1))(2)R(2) (R(1) = R(2) = C(6)H(11), C(5)H(9), i-C(3)H(7), 1-Cy(3)-Cl, 1-Cyp(3)-Cl, 1-(i)Pr(3)-Cl; R(1) = C(6)H(11), R(2) = CH(2)CH(3), 1-EtCy(2)-Cl; R(1) = C(6)H(11), R(2) = CH(3), 1-MeCy(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(2)CH(3), 1-Et(i)Pr(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(3), 1-Me(i)Pr(2)-Cl) were prepared. These compounds can be converted to the metathesis active 14-electron phosphonium alkylidenes by chloride abstraction with B(C(6)F(5))(3). The examples with symmetrically substituted phosphonium groups exist as monomers in solution and are rapid initiators of olefin metathesis reactions. The unsymmetrically substituted phosphonium alkylidenes are observed to undergo reversible dimerization, the extent of which is dependent on the steric bulk of the phosphonium group. Kinetic and thermodynamic parameters of these equilibria are presented, as well as experiments that show that metathesis is only initiated through the monomers; thus dedimerization is required for initiation. In another detailed study, the series of catalysts 1-R(3) were reacted with o-isopropoxystyrene under pseudo-first-order conditions to quantify second-order olefin binding rates. A more complex initiation process was observed in that the rates were accelerated by catalytic amounts of ethylene produced in the reaction with o-isopropoxystyrene. The ability of the catalyst to generate ethylene is related to the nature of the phosphonium group, and initiation rates can be dramatically increased by the intentional addition of a catalytic amount of ethylene.

Mechanisms of the Thermal and Catalytic Redistributions, Oligomerizations, and Polymerizations of Linear Diborazanes

Linear diborazanes R3N-BH2-NR2-BH3 (R = alkyl or H) are often implicated as key intermediates in the dehydrocoupling/dehydrogenation of amine-boranes to form oligo- and polyaminoboranes. Here we report detailed studies of the reactivity of three related examples: Me3N-BH2-NMe2-BH3 (1), Me3N-BH2-NHMe-BH3 (2), and MeNH2-BH2-NHMe-BH3 (3). The mechanisms of the thermal and catalytic redistributions of 1 were investigated in depth using temporal-concentration studies, deuterium labeling, and DFT calculations. The results indicated that, although the products formed under both thermal and catalytic regimes are identical (Me3N·BH3 (8) and [Me2N-BH2]2 (9a)), the mechanisms of their formation differ significantly. The thermal pathway was found to involve the dissociation of the terminal amine to form [H2B(μ-H)(μ-NMe2)BH2] (5) and NMe3 as intermediates, with the former operating as a catalyst and accelerating the redistribution of 1. Intermediate 5 was then transformed to amine-borane 8 and the cyclic diborazane 9a by two different mechanisms. In contrast, under catalytic conditions (0.3-2 mol % IrH2POCOP (POCOP = κ(3)-1,3-(OPtBu2)2C6H3)), 8 was found to inhibit the redistribution of 1 by coordination to the Ir-center. Furthermore, the catalytic pathway involved direct formation of 8 and Me2N═BH2 (9b), which spontaneously dimerizes to give 9a, with the absence of 5 and BH3 as intermediates. The mechanisms elucidated for 1 are also likely to be applicable to other diborazanes, for example, 2 and 3, for which detailed mechanistic studies are impaired by complex post-redistribution chemistry. This includes both metal-free and metal-mediated oligomerization of MeNH═BH2 (10) to form oligoaminoborane [MeNH-BH2]x (11) or polyaminoborane [MeNH-BH2]n (16) following the initial redistribution reaction.

Polyaminoborane main chain scission using N-heterocyclic carbenes; formation of donor-stabilised monomeric aminoboranes

The reaction of N-heterocyclic carbenes with polyaminoboranes [MeNH-BH2]n or [NH2-BH2]n at 20 °C led to depolymerisation and the formation of labile, monomeric aminoborane-NHC adducts, RNH-BH2-NHC (R = Me or H); a similar NHC adduct of Ph2N=BCl2 was characterized by single crystal X-ray diffraction.

In Situ Visualization of Block Copolymer Self-Assembly in Organic Media by Super-Resolution Fluorescence Microscopy.

Abstract Analytical methods that enable visualization of nanomaterials derived from solution self‐assembly processes in organic solvents are highly desirable. Herein, we demonstrate the use of stimulated emission depletion microscopy (STED) and single molecule localization microscopy (SMLM) to map living crystallization‐driven block copolymer (BCP) self‐assembly in organic media at the sub‐diffraction scale. Four different dyes were successfully used for single‐colour super‐resolution imaging of the BCP nanostructures allowing micelle length distributions to be determined in situ. Dual‐colour SMLM imaging was used to measure and compare the rate of addition of red fluorescent BCP to the termini of green fluorescent seed micelles to generate block comicelles. Although well‐established for aqueous systems, the results highlight the potential of super‐resolution microscopy techniques for the interrogation of self‐assembly processes in organic media.

Distannoxane speciation during esterification catalysis: revealing insights provided by electrospray ionization mass spectrometry

Dimeric tetraalkyldistannoxanes are have been reported to catalyze esterification reactions, but are difficult to investigate in detail due to the lack of suitable spectroscopic handles. Electrospray ionization mass spectrometry (ESI-MS), in conjunction with a tethered charge on a tin atom, reveals that immediate decomposition to mono-tin carboxylate compounds occurs in the presence of carboxylic acid.

Probing the Growth Kinetics for the Formation of Uniform 1D Block Copolymer Nanoparticles by Living Crystallization-Driven Self-Assembly

Living crystallization-driven self-assembly (CDSA) is a seeded growth method for crystallizable block copolymers (BCPs) and related amphiphiles in solution and has recently emerged as a highly promising and versatile route to uniform core-shell nanoparticles (micelles) with control of dimensions and architecture. However, the factors that influence the rate of nanoparticle growth have not been systematically studied. Using transmission electron microscopy, small- and wide-angle X-ray scattering, and super-resolution fluorescence microscopy techniques, we have investigated the kinetics of the seeded growth of poly(ferrocenyldimethylsilane)- b-(polydimethylsiloxane) (PFS- b-PDMS), as a model living CDSA system for those employing, for example, crystallizable emissive and biocompatible polymers. By altering various self-assembly parameters including concentration, temperature, solvent, and BCP composition our results have established that the time taken to prepare fiber-like micelles via the living CDSA method can be reduced by decreasing temperature, by employing solvents that are poorer for the crystallizable PFS core-forming block, and by increasing the length of the PFS core-forming block. These results are of general importance for the future optimization of a wide variety of living CDSA systems. Our studies also demonstrate that the growth kinetics for living CDSA do not exhibit the first-order dependence of growth rate on unimer concentration anticipated by analogy with living covalent polymerizations of molecular monomers. This difference may be caused by the combined influence of chain conformational effects of the BCP on addition to the seed termini and chain length dispersity.