Nitrogen adduction by three coordinate group 10 organometallic cations: platinum is favoured over nickel and palladium

The Magnesium Dication and Water Synergistically Promote the Protonolysis of Two of the B-C Bonds in the Tetraphenylborate Anion

Analytes are sampled from both solution phase and gas-phase environments during the ESI process, and thus, the mass spectrum that is measured can reflect both solution and gas-phase conditions. In the gas-phase regime, ion-molecule reactions can influence the types of ions that are observed. Herein, the synergistic effects of a Lewis acid (Mg2+) and background water are shown to lead to protonolysis of two of the B-C bonds of the tetraphenylborate ion in the gas phase, giving rise to different ions at different reaction times in ESI-MS/MS experiments in a linear ion trap mass spectrometer. At short reaction times (1 ms), the expected adduct [Mg(BPh4)]+ is observed. At 10 ms, [(HO)Mg(BPh3)]+ and [(HO)2Mg(BPh2)]+ are observed. At 100 ms, the water adducts [(HO)2Mg(BPh2)(H2O)]+ and [(HO)2Mg(BPh2)(H2O)2]+ appear, and these become the dominant ions at longer reaction times. DFT calculations provide a plausible explanation as to why only [(HO)Mg(BPh3)]+ and [(HO)2Mg(BPh2)]+ but not [(HO)3Mg(BPh)]+ are observed.

Gas-Phase Synthesis and Reactivity of Ligated Group 10 Ions in the Formal +1 Oxidation State

Electrospray ionization of the group 10 complexes [(phen)M(O₂CCH₃)₂] (phen=1,10-phenanthroline, M = Ni, Pd, Pt) generates the cations [(phen)M(O₂CCH₃)]⁺, whose gas-phase chemistry was studied using multistage mass spectrometry experiments in an ion trap mass spectrometer with the combination of collision-induced dissociation (CID) and ion-molecule reactions (IMR). Decarboxylation of [(phen)M(O₂CCH₃)]⁺ under CID conditions generates the organometallic cations [(phen)M(CH₃)]⁺, which undergo bond homolysis upon a further stage of CID to generate the cations [(phen)M]^+· in which the metal center is formally in the +1 oxidation state. In the case of [(phen)Pt(CH₃)]⁺, the major product ion [(phen)H]⁺ was formed via loss of the metal carbene Pt=CH₂. DFT calculated energetics for the competition between bond homolysis and M=CH₂ loss are consistent with their experimentally observed branching ratios of 2% and 98% respectively. The IMR of [(phen)M]^+· with O₂, N₂, H₂O, acetone, and allyl iodide were examined. Adduct formation occurs for O₂, N₂, H₂O, and acetone. Upon CID, all adducts fragment to regenerate [(phen)M]^+·, except for [(phen)Pt(OC(CH₃)₂)]^+·, which loses a methyl radical to form [(phen)Pt(OCCH₃)]⁺ which upon a further stage of CID regenerates [(phen)Pt(CH₃)]⁺ via CO loss. This closes a formal catalytic cycle for the decomposition of acetone into CO and two methyl radicals with [(phen)Pt]^+· as catalyst. In the IMR of [(phen)M]^+· with allyl iodide, formation of [(phen)M(CH₂CHCH₂)]⁺ was observed for all three metals, whereas for M = Pt also [(phen)Pt(I)]⁺ and [(phen)Pt(I)₂(CH₂CHCH₂)]⁺ were observed. Finally, DFT calculated reaction energetics for all IMR reaction channels are consistent with the experimental observations.

Gas-Phase Reactions of the Group 10 Organometallic Cations, [(phen)M(CH3)]+ with Acetone: Only Platinum Promotes a Catalytic Cycle via the Enolate [(phen)Pt(OC(CH2)CH3)]+

Electrospray ionisation of the ligated group 10 metal complexes [(phen)M(O2CCH3)2] (M = Ni, Pd, Pt) generates the cations [(phen)M(O2CCH3)]+, whose gas-phase chemistry was studied using multistage mass spectrometry experiments in an ion trap mass spectrometer with the combination of collision-induced dissociation (CID) and ion-molecule reactions (IMR). A new catalytic cycle has been discovered. In step 1, decarboxylation of [(phen)M(O2CCH3)]+ under CID conditions generates the organometallic cations [(phen)M(CH3)]+, which react with acetone to generate the [(phen)M(CH3)(OC(CH3)2)]+ adducts in competition with formation of the coordinated enolate for M = Pt (step 2). For M = Ni and Pd, the adducts regenerate [(phen)M(CH3)]+ upon CID. In the case of M = Pt, loss of methane is favored over loss of acetone and results in the formation of the enolate complex, [(phen)Pt(OC(CH2)CH3)]+. Upon further CID, both methane and CO loss can be observed resulting in the formation of the ketenyl and ethyl complexes [(phen)Pt(OCCH)]+ and [(phen)Pt(CH2CH3)]+ (step 3), respectively. In step 4, CID of [(phen)Pt(CH2CH3)]+ results in a beta-hydride elimination reaction to yield the hydride complex, [(phen)Pt(H)]+, which reacts with acetic acid to regenerate the acetate complex [(phen)Pt(O2CCH3)]+ and H2 in step 5. Thus, the catalytic cycle is formally closed, which corresponds to the decomposition of acetone and acetic acid into methane, CO, CO2, ethene and H2. All except the last step of the catalytic cycle are modelled using DFT calculations with optimizations of structures at the M06/SDD 6-31G(d) level of theory.

Effect of adduct formation with molecular nitrogen on the measured collisional cross sections of transition metal-1,10-phenanthroline complexes in traveling wave ion-mobility spectrometry: N2 is not always an "inert" buffer gas

The number of separations and analyses of molecular species using traveling wave ion-mobility spectrometry-mass spectrometry (TWIMS-MS) is increasing, including those extending the technique to analytes containing metal atoms. A critical aspect of such applications of TWIMS-MS is the validity of the collisional cross sections (CCSs) measured and whether they can be accurately calibrated against other ion-mobility spectrometry (IMS) techniques. Many metal containing species have potential reactivity toward molecular nitrogen, which is present in high concentration in the typical Synapt-G2 TWIMS cell. Here, we analyze the effect of nitrogen on the drift time of a series of cationic 1,10-phenanthroline complexes of the late transition metals, [(phen)M](+), (M = Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, and Hg) in order to understand potential deviations from expected drift time behaviors. These metal complexes were chosen for their metal open-coordination site and lack of rotameric species. The target species were generated via electrospray ionization (ESI), analyzed using TWIMS in N2 drift gas, and the observed drift time trends compared. Theoretically derived CCSs for all species (via both the projection approximation and trajectory method) were also compared. The results show that, indeed, for metal containing species in this size regime, reaction with molecular nitrogen has a dramatic effect on measured drift times and must not be ignored when comparing and interpreting TWIMS arrival time distributions. Density-functional theory (DFT) calculations are employed to analyze the periodic differences due to the metal's interaction with nitrogen (and background water) in detail.

Gas-phase studies of metal catalyzed decarboxylative cross-coupling reactions of esters

Metal-catalyzed decarboxylative coupling reactions of esters offer new opportunities for formation of C–C bonds with CO2as the only coproduct. Here I provide an overview of: key solution phase literature; thermochemical considerations for decarboxylation of esters and thermolysis of esters in the absence of a metal catalyst. Results from my laboratory on the use of multistage ion trap mass spectrometry experiments and DFT calculations to probe the gas-phase metal catalyzed decarboxylative cross-coupling reactions of allyl acetate and related esters are then reviewed. These studies have explored the role of the metal carboxylate complex in the gas phase decarboxylative coupling of allyl acetate proceeding via a simple two-step catalytic cycle. In Step 1, an organometallic ion, [CH3ML]+/–(where M is a group 10 or 11 metal and L is an auxillary ligand), is allowed to undergo ion-molecule reactions with allyl acetate to generate 1-butene and the metal acetate ion, [CH3CO2ML]+/–. In Step 2, the metal acetate ion is subjected to collision-induced dissociation to reform the organometallic ion and thereby close the catalytic cycle. DFT calculations have been used to explore the mechanisms of these reactions. The organometallic ions [CH3CuCH3]–, [CH3Cu2]+, [CH3AgCu]+and [CH3M(phen)]+(where M = Ni, Pd and Pt) all undergo C–C bond coupling reactions with allyl acetate (Step 1), although the reaction efficiencies and product branching ratios are highly dependant on the nature of the metal complex. For example, [CH3Ag2]+does not undergo C–C bond coupling. Using DFT calculations, a diverse range of mechanisms have been explored for these C–C bond-coupling reactions including: oxidative-addition, followed by reductive elimination; insertion reactions and SN2-like reactions. Which of these mechanisms operate is dependant on the nature of the metal complex. A wide range of organometallic ions can be formed via decarboxylation (Step 2) although these reactions can be in competition with other fragmentation channels. DFT calculations have located different types of transition states for the formation of [CH3CuCH3]–, [CH3Cu2]+, [CH3AgCu]+and [CH3M(phen)]+(where M = Ni, Pd and Pt). Of the catalysts studied to date, [CH3Cu2]+and [CH3Pd(phen)]+are best at promoting C–C bond formation (Step 1) as well as being regenerated (Step 2). Preliminary results on the reactions of [C6H5M(phen)]+(M = Ni and Pd) with C6H5CO2CH2CH=CH2and C6H5CO2CH2C6H5are described.

Copper mediated decyano decarboxylative coupling of cyanoacetate ligands: Pesci versus Lewis acid mechanism

A combination of gas-phase ion trap multistage mass spectrometry (MSⁿ) experiments and density functional theory (DFT) calculations have been used to examine the mechanisms of the sequential decomposition reactions of copper cyanoacetate anions, [(NCCH₂CO₂)₂Cu]⁻.

Decarboxylative-coupling of allyl acetate catalyzed by group 10 organometallics, [(phen)M(CH3)]+

Gas-phase carbon-carbon bond forming reactions, catalyzed by group 10 metal acetate cations [(phen)M(O2CCH3)](+) (where M = Ni, Pd or Pt) formed via electrospray ionization of metal acetate complexes [(phen)M(O2CCH3)2], were examined using an ion trap mass spectrometer and density functional theory (DFT) calculations. In step 1 of the catalytic cycle, collision induced dissociation (CID) of [(phen)M(O2CCH3)](+) yields the organometallic complex, [(phen)M(CH3)](+), via decarboxylation. [(phen)M(CH3)](+) reacts with allyl acetate via three competing reactions, with reactivity orders (% reaction efficiencies) established via kinetic modeling. In step 2a, allylic alkylation occurs to give 1-butene and reform metal acetate, [(phen)M(O2CCH3)](+), with Ni (36%) > Pd (28%) > Pt (2%). Adduct formation, [(phen)M(C6H11O2)](+), occurs with Pt (24%) > Pd (21%) > Ni(11%). The major losses upon CID on the adduct, [(phen)M(C6H11O2)](+), are 1-butene for M = Ni and Pd and methane for Pt. Loss of methane only occurs for Pt (10%) to give [(phen)Pt(C5H7O2)](+). The sequences of steps 1 and 2a close a catalytic cycle for decarboxylative carbon-carbon bond coupling. DFT calculations suggest that carbon-carbon bond formation occurs via alkene insertion as the initial step for all three metals, without involving higher oxidation states for the metal centers.

Rapid differentiation of ortho-, meta-, and para-isomers of halogenated phenylmethylidene hydrazinecarbodithioates by metal complexation and electrospray ionization mass spectrometry

RATIONALE

Development of mass spectrometry (MS)-based methods for isomeric differentiation remains a challenging analytical task, and has attracted the interest of many research groups. It is relevant to develop a general method to differentiate the isomeric halogenated phenylmethylidene hydrazinecarbodithioates (MX, X  =  F, Cl, Br).

METHODS

Diluted CH3 CN solutions containing NiCl2 and a title isomer (MX) were analyzed by electrospray ionization tandem mass spectrometry (ESI-MS(n)) in a quadrupole ion trap instrument equipped with an ESI source. Theoretical calculations were performed using the density functional theory (DFT) method at the uB3LYP/6-31+G(2d,p) level.

RESULTS

In MS(3) experiments, the complex [MX + SCH3 + Ni](+) ion, resulting from dissociation of the ESI-generated complex [2MX - H + Ni](+) ion, undergoes ligand-exchange reactions with residual gas molecules, such as water, acetonitrile, and nitrogen in the ion trap, and the o-isomers [Mo-X + SCH3 + Ni](+) were found to undergo the characteristic HX elimination reactions to afford several unique ions. Each set of three isomers [MX + SCH3 + Ni](+) show significantly different reactivity, which has been corroborated by MS(4) experiments and theoretical calculations.

CONCLUSIONS

A rapid method based on metal complexation and tandem mass spectrometric (MS(n)) analysis has been developed to differentiate three sets of positional isomers of halogenated phenylmethylidene hydrazinecarbodithioates (MX, X =  F, Cl, Br).