An electrospray-active N-heterocyclic carbene ligand for real-time analysis of organometallic reactions.

A charge-tagged N-heterocyclic carbene (NHC) has been synthesized and its utility in allowing the dynamic behaviour of metal complexes to be monitored in real time using electrospray ionization mass spectrometry demonstrated. This compound was used to prepare different metal-NHC complexes, and the kinetic behaviour of complex formation and ligand exchange was monitored in real time through the use of pressurized sample infusion electrospray mass spectrometry (PSI-ESI-MS).

Electrospray ionization tandem mass spectrometry of 4-aryl-3,4-dihydrocoumarins.

We have investigated the gas-phase fragmentation reactions of 11 synthetic 4-aryl-3,4-dihydrocoumarins by electrospray ionization tandem mass spectrometry (ESI-MS/MS) on a quadrupole-time-of flight (Q-TOF) hybrid mass spectrometer. We have also estimated thermochemical data for the protonated coumarins (precursor ion A) and product ion structures by computational chemistry at a B3LYP level of theory to establish the ion structures and to rationalize the fragmentation pathways. The most abundant ions in the product ion spectra of coumarins 1-11 resulted from CHO, CO, CHO, CHO, CHO, and CHOH eliminations through retro-Diels-Alder (RDA) reactions, remote hydrogen rearrangements (β-eliminations), and β-lactone ring contraction.

Mausolates: Large-Cavity Chelates with Potential as Delivery Vehicles in Nuclear Medicine.

A new type of diborate clathrochelate (cage) ligand featuring nine inwardly pointing nitrogen donors that form a large, rigid cavity, termed a mausolate, is presented. The cavity size and high denticity make this an attractive delivery vehicle for large radionuclides in nuclear medicine. Metal mausolate complexes are stable to air and water (neutral pH) and display extremely high thermal stability (>400 °C).

Mechanistic study of the atomic layer deposition of cobalt: a combined mass spectrometric and computational approach.

Cobaltcarbonyl--butylacetylene (CCTBA) is a conventional precursor for the selective atomic layer deposition of Co onto silicon surfaces. However, a limited understanding of the deposition mechanism of such cobalt precursors curbs rational improvements on their design for increased efficiency and tuneable selectivity. The impact of using a less reactive internal alkyne instead of a terminal alkyne was investigated using experimental and computational methods.

OptiMS: An Accessible Program for Automating Mass Spectrometry Parameter Optimization and Configuration.

Mass spectrometers have an enormous number of user-changeable parameters that drastically affect the observed mass spectrum. Using optimal parameters can significantly improve mass spectrometric data by increasing signal stability and signal-to-noise ratio, which decreases the limit of detection, thus revealing previously unobservable species. However, ascertaining optimal parameters is time-consuming, tedious, and made further challenging by the fact that parameters can act dependently on each other.

Continuous addition kinetic elucidation: catalyst and reactant order, rate constant, and poisoning from a single experiment.

Kinetic analysis of catalytic reactions is a powerful tool for mechanistic elucidation but is often challenging to perform, limiting understanding and therefore development of these reactions. Establishing order in a catalyst is usually achieved by running several reactions at different loadings, which is both time-consuming and complicated by the challenge of maintaining consistent run-to-run experimental conditions. Continuous addition kinetic elucidation (CAKE) was developed to circumvent these issues by continuously injecting a catalyst into a reaction, while monitoring reaction progress over time.

Microstructural Analysis of Benzoxazine Cationic Ring-Opening Polymerization Pathways.

Herein, an evaluation of the initial step of benzoxazine polymerization is presented by mass spectrometry, with a focus on differentiating the phenoxy and phenolic products formed by distinct pathways of the cationic ring opening polymerization (ROP) mechanism of polybenzoxazine formation. The use of infrared multiple photon dissociation (IRMPD) and ion mobility spectrometry (IMS) techniques allows for differentiation of the two pathways and provides valuable insights into the ROP mechanism. The results suggest that type I pathway is favored in the initial stages of the reaction yielding the phenoxy product, while type II product should be observed at later stages when the phenoxy product would interconvert to the most stable type II phenolic product.

Handling considerations for the mass spectrometry of reactive organometallic compounds.

Mass spectrometry is a powerful tool in disparate areas of chemistry, but its characteristic strength of sensitivity can be an Achilles heel when studying highly reactive organometallic compounds. A quantity of material suitable for mass spectrometric analysis often represents a tiny grain or a very dilute solution, and both are highly susceptible to decomposition due to ambient oxygen or moisture. This complexity can be frustrating to chemists and analysts alike: the former being unable to get spectra free of decomposition products and the latter often being poorly equipped to handle reactive samples.

A mechanistic investigation of the Pd-catalyzed cross-coupling between -tosylhydrazones and aryl halides.

The cross-coupling of -tosylhydrazones and aryl halides forms carbon-carbon bonds, producing 1,1-disubstituted alkenes. Though it has proven extremely useful in several fields of chemistry, its mechanism remains experimentally unexplored. Combining benchtop NMR and real-time mass spectrometry afforded the ability to monitor the catalytic intermediates as well as the rate of product formation.

Real-time analysis of methylalumoxane formation.

Methylalumoxane (MAO), a perennially useful activator for olefin polymerization precatalysts, is famously intractable to structural elucidation, consisting as it does of a complex mixture of oligomers generated from hydrolysis of pyrophoric trimethylaluminum (TMA). Electrospray ionization mass spectrometry (ESI-MS) is capable of studying those oligomers that become charged during the activation process. We have exploited that ability to probe the synthesis of MAO in real time, starting less than a minute after the mixing of HO and TMA and tracking the first half hour of reactivity.

Regio- and diastereoselective Pd-catalyzed aminochlorocyclization of allylic carbamates: scope, derivatization, and mechanism.

The regio- and diastereoselective synthesis of oxazolidinones via a Pd-catalyzed vicinal C-N/C-Cl bond-forming reaction from internal alkenes of allylic carbamates is reported. The oxazolidinones are obtained in yields of 44 to 95% with high to excellent diastereoselectivities (from 6 : 1 to >20 : 1 dr) from readily available precursors. This process is scalable, and the products are suitable for the synthesis of useful amino alcohols.

Are Methylaluminoxane Activators Sheets?

Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5-coordinate and 4-coordinate Al, are likely precursors to ion-pairs seen during the hydrolysis of trimethylaluminum (Me Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me] ) and [Me Al] abstraction, involving this donor, were studied by polarizable continuum model calculations in fluorobenzene (PhF) and o-difluorobenzene (DFB) media. These studies suggest that low MW, 5-coordinate sheets ionize by [Me Al] abstraction, while [Me] abstraction from Me Al-OMTS is the likely process for higher MW 4-coordinate sheets.

Spectroscopic Studies of Synthetic Methylaluminoxane: Structure of Methylaluminoxane Activators.

Hydrolysis of trimethylaluminum (Me Al) in polar solvents can be monitored by electrospray ionization mass spectrometry (ESI-MS) using the donor additive octamethyltrisiloxane [(Me SiO) SiMe , OMTS]. Using hydrated salts, hydrolytic methylaluminoxane (h-MAO) features different anion distributions, depending on the conditions of synthesis, and different activator contents as measured by NMR spectroscopy. Non-hydrolytic MAO was prepared using trimethylboroxine.

Dynamic Ion Speciation during the Hydrolysis of Aryltrifluoroborates*.

Organotrifluoroborates serve as coupling partners during transmetalation in the Suzuki-Miyaura reaction but require hydrolysis prior to the coupling reaction. Their anionic nature allows study of their hydrolysis by electrospray ionization mass spectrometry (ESI-MS) through real-time monitoring, complemented by pH analysis. The induction period varied according to the borates employed, and a dynamic series of equilibria for numerous ions was observed during hydrolysis.

Trichloro(Dinitrogen)Platinate(II).

Zeise's salt, [PtCl (H C=CH )] is the oldest known organometallic complex, featuring ethylene strongly bound to a platinum salt. Many derivatives are known, but none involving dinitrogen, and indeed dinitrogen complexes are unknown for both platinum and palladium. Electrospray ionization mass spectrometry of K [PtCl ] solutions generate strong ions corresponding to [PtCl (N )] , the identity of which was confirmed through ion-mobility spectrometry and MS/MS experiments that proved it to be distinct from its isobaric counterparts [PtCl (C H )] and [PtCl (CO)] .

Step-by-step real time monitoring of a catalytic amination reaction.

The multiple reaction monitoring mode of a triple quadrupole mass spectrometer is used to examine the Buchwald-Hartwig amination reaction at 0.1% catalyst loading in real-time using sequential addition of reagents to probe the individual steps in the cycle. This is a powerful new method for probing reactions under realistic conditions.

Structure, Anion, and Solvent Effects on Cation Response in ESI-MS.

The abundance of an ion in an electrospray ionization mass spectrum is dependent on many factors beyond just solution concentration. Even in cases where the analytes of interest are permanently charged (under study here are ammonium and phosphonium ions) and do not rely on protonation or other chemical processes to acquire the necessary charge, factors such as cation structure, molecular weight, solvent, and the identity of the anion can affect results. Screening of a variety of combinations of cations, anions, and solvents provided insight into some of the more important factors.

Assigning the ESI mass spectra of organometallic and coordination compounds.

Electrospray ionization mass spectrometry (ESI-MS) is a useful technique for solving organometallic and coordination chemistry characterization problems that are difficult to address using traditional methods. However, assigning the ESI mass spectra of such compounds can be challenging, and the considerations involved in doing so are quite different from assigning the mass spectra of purely organic samples. This is a tutorial article for organometallic/coordination chemists using ESI-MS to analyze pure compounds or reaction mixtures.

PythoMS: A Python Framework To Simplify and Assist in the Processing and Interpretation of Mass Spectrometric Data.

Mass spectrometric data are copious and generate a processing burden that is best dealt with programmatically. PythoMS is a collection of tools based on the Python programming language that assist researchers in creating figures and video output that is informative, clear, and visually compelling. The PythoMS framework introduces a library of classes and a variety of scripts that quickly perform time-consuming tasks: making proprietary output readable; binning intensity vs time data to simulate longer scan times (and hence reduce noise); calculating theoretical isotope patterns and overlaying them in histogram form on experimental data (an approach that works even for overlapping signals); rendering videos that enable zooming into the baseline of intensity vs time plots (useful to make sense of data collected over a large dynamic range) or that depict the evolution of different species in a time-lapse format; calculating aggregates; and providing a quick first-pass at identifying fragments in MS/MS spectra.

Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes.

Kinetic profiles obtained from monitoring the solution-phase substitution chemistry of [Ru(η-indenyl)(NCPh)(PPh)] (1) by both electrospray ionization mass spectrometry and P{H} 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: PEtH > PPhH ≫ PCyH.

Modifying methylalumoxane via alkyl exchange.

Methylalumoxane (MAO) ionizes highly selectively in the presence of octamethyltrisiloxane (OMTS) to generate [MeAl·OMTS] [(MeAlO)(MeAl)Me]. We can take advantage of this transformation to examine the reactivity of a key component of MAO using electrospray ionization mass spectrometry (ESI-MS), and here we describe the reactivity of this pair of ions with other trialkyl aluminum (RAl) components. Using continuous injection methods, we found EtAl to exchange much faster and extensively at room temperature in fluorobenzene (t∼2 s, up to 25 exchanges of Me for Et) than iBuAl (t∼40 s, up to 11 exchanges) or OctAl (t∼200 s, up to 7 exchanges).

Synthesis, characterization and mass-spectrometric analysis of [LSn(IV)F] salts [L = tris ((1-ethyl-benzoimidazol-2-yl)methyl)amine, x = 1-4].

A series of complexes with the formulae [(BIMEt3)SnF4-x][OTf]x with x = 1-4 has been synthesized by successive fluoride abstraction from SnF4 with TMSOTf in the presence of the tetradentate nitrogen donor BIMEt3 (tris ((1-ethyl-benzoimidazol-2-yl)methyl)amine). Single crystal X-ray diffraction and heteronuclear NMR spectroscopic analysis provided insight into these new main group cations. Electrospray ionization mass spectrometric analysis on solutions containing the different salts allowed for successful detection of the cations [(BIMEt3)SnF]3+, [(BIMEt3)SnF2]2+ and [(BIMEt3)SnF3]+.

Oxidation of Methylalumoxane Oligomers.

The anions formed from methylalumoxane (MAO) and suitable donors (e.g. octamethyltrisiloxane) are amenable to mass spectrometric (MS) analysis.

Oxidation of Titanocene(III): The Deceptive Simplicity of a Color Change.

Reduction of red CpTiCl (Cp = cyclopentadienyl) with zinc dust in acetonitrile produces a blue solution of [CpTi(NCMe)], which when exposed to air rapidly discolors to bright yellow. This behavior makes the blue solution a handy visual indicator for the presence of oxygen, but the chemistry is considerably more complicated than the primary colors suggest at first glance. Real-time mass spectrometric and colorimetric analysis reveals that oxidation from Ti(III) to Ti(IV) produces a host of oxygen-containing complexes, whose appearance parallels the observed color changes.

Phenol-selective mass spectrometric analysis of jet fuel.

Bromobenzyl compounds react selectively with phenols via the Williamson ether synthesis. An imidazolium charge-tagged bromobenzyl compound can be used to reveal phenol impurities in jet fuel by analysis via electrospray ionization mass spectrometry. The complex matrix as revealed by Cold EI GC/MS analysis is reduced to a few simple sets of compounds in the charge-tagged ESI mass spectrum, primarily substituted phenols and thiols.