Loss Of Neutral Ligand Research Articles

Preparation, spectral properties and thermal decomposition of new ternary complexes of La(III), Ce(III), Pr(III) and Nd(III) ions with N-phenylanthranilic acid and 1,10-phenanthroline

The new ternary complexes of the formula Ln2(nfa)5(phen)(OH)·nH2O (where n = 1 or 1.5; nfa – N-phenylanthranilate anion (C13H10NO2), phen – 1,10-phenanthroline (C12H8N2), Ln = La(III), Ce(III), Pr(III) and Nd(III)) were synthesized and characterized by elemental analysis, complexometry, infrared spectroscopy, XRD technique, simultaneous thermogravimetry and differential scanning calorimetry (TG–DSC), evolved gas analysis by TG–DSC coupled with FTIR and MS spectroscopy. The detailed thermogravimetic analysis showed that thermal decomposition of the complexes in nitrogen and air atmosphere occurs in several stages including water elimination, neutral ligand loss and N-phenylanthranilate anion decomposition, respectively. It was found that the thermal stability and the decomposition path of the studied compounds depend on the metal ion and the applied atmosphere. Low values of the dehydration enthalpies indicate that dehydration is connected with the release of crystallization water. By the use of the coupled TG–FTIR–MS techniques the decomposition pathways of the synthesized compounds were examined and the gaseous products released during pyrolysis and thermal oxidation of the complexes were identified.

Synthesis and Hydrolysis of Uranyl, Neptunyl, and Plutonyl Gas-Phase Complexes Exhibiting Discrete Actinide–Carbon Bonds

Gas-phase organoactinyl complexes possessing discrete An–C bonds (An = U, Np, Pu) were synthesized in a quadrupole ion trap by endothermic decarboxylation of [AnO2(O2C–R)3]− anion complexes in which a formally AnO22+ actinyl core is coordinated by three carboxylate ligands, with R = CH3 (methyl), CH3CC (1-propynyl), C6H5 (phenyl), C6F5 (pentafluorophenyl). Decarboxylation and competing ligand loss were studied computationally by density functional theory complementing experiment. Although decarboxylation was computed to be the energetically most favorable process in all cases, reduction from An(VI) to An(V) via neutral ligand loss was often prevalent, particularly for An = Np, Pu, presumably resulting from barriers associated with decarboxylation. Comparative hydrolysis rates of the An–C bonds were experimentally determined, and the chemical properties of these bonds were analyzed by the quantum theory of atoms in molecules. The measured hydrolysis rates differed by up to 3 orders of magnitude: the fastes...

Electrospray production and collisional dissociation of lanthanide/methylsulfonyl anion complexes: Sulfur dioxide anion as a ligand

Gas-phase lanthanide-SO2 complexes, Ln(CH3SO2)3(SO2)−, were produced by collision induced dissociation (CID) of Ln(CH3SO2)4− precursors prepared by electrospray ionization. For all lanthanides except Eu, CID of Ln(CH3SO2)4− resulted in CH3 loss to form Ln(CH3SO2)3(SO2)−, which spontaneously react with O2 to form Ln(CH3SO2)3(O2)−. CID of Eu(CH3SO2)4− produced only Eu(CH3SO2)3−, with reduction from Eu(III) to Eu(II). For Ln=Yb and Sm, the Ln(CH3SO2)4− underwent neutral ligand loss to form Ln(CH3SO2)3−, which reacted with O2 to yield Ln(CH3SO2)3(O2)−, recovering the Ln(III) oxidation state. The CID results show parallels to condensed-phase Ln3+/Ln2+ redox chemistry. Density functional theory (DFT) calculations on Ln(CH3SO2)3(SO2)− for Ln=La, Yb and Lu reveal that SO2 acts as a bidentate oxygen bound ligand for doublet ground-state La(CH3SO2)3(SO2)− and Lu(CH3SO2)3(SO2)−, while the ground state for Yb(CH3SO2)3(SO2)− is an open-shell singlet with a monodentate SO2 ligand. Loss of CH3 is computed to be much more favorable than neutral ligand loss for La(CH3SO2)4− and Lu(CH3SO2)4−, whereas both channels are comparable in energy for Yb(CH3SO2)4−, in accord with the experiments. DFT results for fragmentation of Cu(CH3SO2)2− reveal that formation of the organometallic complex, Cu(CH3SO2)(CH3)−, is energetically most favorable, in agreement with contrasting fragmentation pathways of copper and lanthanide complexes.

Interactions of hemoglobin and myoglobin with their ligands CN(-), CO, and O2 monitored by electrospray ionization-mass spectrometry.

Electrospray ionization (ESI)-mass spectrometry (MS) can provide information on protein-ligand interactions via detection of the corresponding complexes as gaseous ions. Unfortunately, some systems are prone to dissociation upon transfer into the gas phase. The reversible oxygen binding to hemoglobin (Hb) has been extensively studied in solution using a wide range of biophysical techniques. In addition to O2, ferrous (Fe(II)) Hb can bind CO. High affinity interactions with CN(-) are limited to the ferric (Fe(III)) state. In analogous fashion, CN(-), CO, and O2 bind to myoglobin (Mb). It remains unclear whether any of these ligand-bound forms can be observed by ESI-MS. In this work we demonstrate the successful detection of MbCN, while MbCO and MbO2 do not survive under ESI-MS conditions. Control experiments suggest that an older report of "MbO2" detection by ESI-MS may involve the misassignment of oxidation artifacts formed under corona discharge conditions. The situation is more favorable for ESI-MS studies on Hb. The most intense signal in the HbCN mass distribution corresponds to the expected complex with four cyanide moieties bound. Ligand loss during ESI-MS is around 20%. HbCO is detectable as well, albeit with a more noticeable level of ligand dissociation (∼50%) which produces the 2CO-bound state as the highest intensity ion in the spectrum. In addition, our data suggest that low levels of HbO2 can survive the transition into the gas phase, evident from +64 Da and +128 Da signals that can be assigned to Hb carrying two and four oxygen molecules, respectively. The application of collisional activation induces neutral ligand loss for all three Hb derivatives. It appears that this is the first report on the detection of MbCN, HbCO, and HbO2 in the gas phase. We hope that this work will pave the way towards future spectroscopic investigations of desolvated Mb and Hb, complementing the extensive literature on CN(-), CO, and O2 bound globins in the condensed phase.

Gas phase uranyl activation: formation of a uranium nitrosyl complex from uranyl azide.

Activation of the oxo bond of uranyl, UO2(2+), was achieved by collision induced dissociation (CID) of UO2(N3)Cl2(-) in a quadrupole ion trap mass spectrometer. The gas phase complex UO2(N3)Cl2(-) was produced by electrospray ionization of solutions of UO2Cl2 and NaN3. CID of UO2(N3)Cl2(-) resulted in the loss of N2 to form UO(NO)Cl2(-), in which the "inert" uranyl oxo bond has been activated. Formation of UO2Cl2(-) via N3 loss was also observed. Density functional theory computations predict that the UO(NO)Cl2(-) complex has nonplanar Cs symmetry and a singlet ground state. Analysis of the bonding of the UO(NO)Cl2(-) complex shows that the side-on bonded NO moiety can be considered as NO(3-), suggesting a formal oxidation state of U(VI). Activation of the uranyl oxo bond in UO2(N3)Cl2(-) to form UO(NO)Cl2(-) and N2 was computed to be endothermic by 169 kJ/mol, which is energetically more favorable than formation of NUOCl2(-) and UO2Cl2(-). The observation of UO2Cl2(-) during CID is most likely due to the absence of an energy barrier for neutral ligand loss.

Bis(dimethylphosphino)methane-ligated silver chloride, cyanide and hydride cluster cations: Synthesis and gas-phase unimolecular reactivity

Electrospray ionization mass spectrometry (ESI/MS) of mixtures of AgNO3 or AgBF4 with the capping ligand Me2P(CH2)PMe2 (dmpm, L) in a solution of 1:1 methanol:chloroform or acetonitrile revealed the formation of: dinuclear clusters [Ag2L2]2+, [Ag2(LH)L]+, and [Ag2ClL2]+; trinuclear clusters primarily as [Ag3Cl2L3]+; and the tetranuclear cluster [Ag4Cl3L3]+. Addition of NaBH4 to these solutions of dmpm-protected clusters results in the formation of the silver hydride cluster cations [Ag2HL2]+, [Ag3HL3]2+, [Ag3HL4]2+, [Ag3H(CN)L3]+ and [Ag3HClL3]+ as determined by ESI/MS. Use of NaBD4 confirmed that the borohydride is the source of the hydride in these clusters.The gas-phase unimolecular chemistry of selected clusters was examined in a LTQ-FT hybrid linear ion trap (LIT) mass spectrometer. Low-energy collision-induced dissociation (CID) was performed on the clusters [Ag3Cl2L3]+, [Ag3HClL3]+, [Ag3H(CN)L3]+, [Ag3HL3]2+, [Ag3HL4]2+ and their nascent fragments. Neutral ligand loss, core fission and ligand activation were observed to depend on the cluster stoichiometry. Thus, [Ag3XYL3]+ (X, Y=Cl; X=H; Y=Cl or CN) mainly fragment via ligand loss, [Ag3XYL2]+ undergoes both ligand loss and core fission, [Ag3XYL]+ undergoes core fission and [Ag2YL]+ undergoes ligand activation. Small yields of reductive elimination of HY are observed for [Ag3HYL2]+ (Y=Cl and CN). Density functional theory (DFT) calculations were used to calculate the energetics of the optimised structures for all parent, fragment ions and neutrals and to estimate the reaction endothermicities. Generally there is a reasonable agreement between the most abundant product ion formed and the predicted endothermicity, except for the reductive elimination reactions. Finally, electron-induced dissociation (EID) of [Ag3HL3]2+ and [Ag3HL4]2+ gave similar fragmentation channels to CID.

Experimental and Theoretical Studies on the Fragmentation of Gas-Phase Uranyl–, Neptunyl–, and Plutonyl–Diglycolamide Complexes

Fragmentation of actinyl(VI) complexes U(VI)O2(L)2(2+), Np(VI)O2(L)2(2+), and Pu(VI)O2(L)2(2+) (L = tetramethyl-3-oxa-glutaramide, TMOGA) produced by electrospray ionization was examined in the gas phase by collision induced dissociation (CID) in a quadrupole ion trap mass spectrometer. Cleavage of the C-O(ether) bond was observed for all three complexes, with dominant products being U(VI)O2(L)(L-86)(+) with charge reduction, and Np(VI)O2(L)(L-101)(2+) and Pu(VI)O2(L)(L-101)(2+) with charge conservation. The neptunyl and plutonyl complexes also exhibited substantial L(+) loss to give pentavalent complexes Np(V)O2(L)(+) and Pu(V)O2(L)(+), whereas the uranyl complex did not, consistent with the comparative An 5f-orbital energies and the An(VI)O2(2+)/An(V)O2(+) (An = U, Np, Pu) reduction potentials. CID of Np(V)O2(L)2(+) and Pu(V)O2(L)2(+) was dominated by neutral ligand loss to form Np(V)O2(L)(+) and Pu(V)O2(L)(+), which hydrated by addition of residual water in the ion trap; U(V)O2(L)2(+) was not observed. Theoretical calculations of the structures and bonding of the An(VI)O2(L)2(2+) complexes using density functional theory reveal that the metal centers are coordinated by six oxygen atoms from two TMOGA ligands.

Deuterium Kinetic Isotope Effects on the Dissociation of a Protein–Fatty Acid Complex in the Gas Phase

Deuterium kinetic isotope effects (KIEs) are reported for the first time for the dissociation of a protein-ligand complex in the gas phase. Temperature-dependent rate constants were measured for the loss of neutral ligand from the deprotonated ions of the 1:1 complex of bovine β-lactoglobulin (Lg) and palmitic acid (PA), (Lg + PA)(n-) → Lg(n-) + PA, at the 6- and 7- charge states. At 25 °C, partial or complete deuteration of the acyl chain of PA results in a measurable inverse KIE for both charge states. The magnitude of the KIEs is temperature dependent, and Arrhenius analysis of the rate constants reveals that deuteration of PA results in a decrease in activation energy. In contrast, there is no measurable deuterium KIE for the dissociation of the (Lg + PA) complex in aqueous solution at pH 8. Deuterium KIEs were calculated using conventional transition-state theory with an assumption of a late dissociative transition state (TS), in which the ligand is free of the binding pocket. The vibrational frequencies of deuterated and non-deuterated PA in the gas phase and in various solvents (n-hexane, 1-chlorohexane, acetone, and water) were established computationally. The KIEs calculated from the corresponding differences in zero-point energies account qualitatively for the observation of an inverse KIE but do not account for the magnitude of the KIEs nor their temperature dependence. It is proposed that the dissociation of the (Lg + PA) complex in aqueous solution also proceeds through a late TS in which the acyl chain is extensively hydrated such that there is no significant differential change in the vibrational frequencies along the reaction coordinate and, consequently, no significant KIE.

Ligand Dissociation and Core Fission from Diphosphine-Protected Gold Clusters

Highly monodisperse samples of diphosphine ligand (1,3-bis(diphenylphosphino)propane- or 1,5-bis(diphenylphosphino)pentane)-protected gold nanoparticles form rapidly in a mixed methanol/chloroform solvent environment. Methanol soluble octagold, decagold, and undecagold monolayer protected clusters yield very stable ion currents when introduced into a mass spectrometer via electrospray ionization. In addition to neutral ligand loss pathways, collision-induced dissociation generates [AuL]+ and [Au3L]+ (L = diphosphine ligand) as particularly stable product ions from all clusters considered. Furthermore, deca- and undecagold clusters are found to be more resistant to collision induced dissociation, and more susceptible to partial ligand losses than octagold clusters. This suggests that for the deca- and undecagold species, Au−P and P−Ph (Ph = phenyl) bonds within the ligand-protected clusters are of comparable strength.

Ligand Binding Mode to Duplex and Triplex DNA Assessed by Combining Electrospray Tandem Mass Spectrometry and Molecular Modeling

In this paper, we report the analysis of seven benzopyridoindole and benzopyridoquinoxaline drugs binding to different duplex DNA and triple helical DNA, using an approach combining electrospray ionization mass spectrometry (ESI-MS), tandem mass spectrometry (MS/MS), and molecular modeling. The ligands were ranked according to the collision energy (CE(50)) necessary to dissociate 50% of the complex with the duplex or the triplex in tandem MS. To determine the probable ligand binding site and binding mode, molecular modeling was used to calculate relative ligand binding energies in different binding sites and binding modes. For duplex DNA binding, the ligand-DNA interaction energies are roughly correlated with the experimental CE(50), with the two benzopyridoindole ligands more tightly bound than the benzopyridoquinoxaline ligands. There is, however, no marked AT versus GC base preference in binding, as supported both by the ESI-MS and the calculated ligand binding energies. Product ion spectra of the complexes with triplex DNA show only loss of neutral ligand for the benzopyridoquinoxalines, and loss of the third strand for the benzopyridoindoles, the ligand remaining on the duplex part. This indicates a higher binding energy of the benzopyridoindoles, and also shows that the ligands interact with the triplex via the duplex. The ranking of the ligand interaction energies compared with the CE(50) values obtained by MS/MS on the complexes with the triplex clearly indicates that the ligands intercalate via the minor groove of the Watson-Crick duplex. Regarding triplex versus duplex selectivity, our experiments have demonstrated that the most selective drugs for triplex share the same heteroaromatic core.

Size matters! Fragmentation chemistry of [Cu(L)n]2+ complexes of diacylglycerophosphocholines as a function of coordination number (n = 2–7)

[Cu(L)(n)](2+) complexes of 1,2-dihexanoyl-sn-glycero-3-phosphocholine (L = D6PC) are formed upon electrospray ionization mass spectrometry (ESI-MS) of an 8 mM solution of D6PC with 4 mM CuCl(2) in 10 mM ammonium acetate buffer, pH 6.1. The collision-induced dissociation (CID) reactions of the [Cu(L)(n)](2+) complexes were examined in a linear ion trap mass spectrometer. A rich fragmentation chemistry was observed, including: loss of a neutral ligand; intermolecular ligand-ligand S(N)2 methylation; metal ion induced ligand fragmentation via carboxylate abstraction; and phosphate abstraction. The dominant reaction channel depends on the size (n) of the complex. Thus loss of neutral ligand(s) is the sole reaction channel for n = 5-7. At n = 4, S(N)2 methylation and carboxylate abstraction start to compete with neutral ligand loss. At n = 2 the carboxylate abstraction and phosphate abstraction reactions dominate the CID spectrum. The carboxylate abstraction and phosphate abstraction reactions are likely to be driven via neighboring group pathways. PM3 calculations, carried out to compare competing neighboring pathways based on the relative stabilities of the product ions, suggest a preference for five-membered ring formation for ligand fragmentation involving both carboxylate and phosphate abstraction.

Metal- and Ligation-Dependent Fragmentation of [M(1,10-Phenanthroline)1,2,3]2+ Cations with M = Mn, Fe, Co, Ni, Cu, and Zn: Comparison between the Gas Phase and Solution

The core ions [ML(n)]2+ with n = 1-3, where L = 1,10-phenanthroline and M is a first-row transition metal, have been successfully transferred from aqueous solution into the gas phase by electrospraying and then probed for their stabilities by collision-induced dissociation in a triple quadrupole mass spectrometer. The triply ligated metal dications [ML3]2+ were observed to dissociate by the extrusion of a neutral ligand, while ligand loss from both [ML2]2+ and [ML]2+ was accompanied by electron transfer. Comparisons are provided between gas-phase stabilities and stabilities for ligand loss measured in aqueous solution at 298 K. The measured onset for ligand loss from [ML3]2+ is quite insensitive to the metal, while a distinct stability order has been reported for aqueous solution. Low level density functional theory (DFT) calculations predict an intrinsic stability order for loss of ligand from [ML2]2+, but it differs from that in aqueous solution. Substantial agreement was obtained for the stability order for the loss of ligand from [ML]2+ deduced from onset energies measured for charge separation, computed with DFT, and reported for aqueous solution where hydration seems less decisive in influencing this stability order. A qualitative potential-energy diagram is presented that allows the energy for charge separation to be related to the energy for neutral ligand loss from [ML]2+ and shows that IE(M+) is decisive in determining the intrinsic stability order for loss of ligand from [ML]2+.

Synthesis, structure and properties of monomeric strontium and barium complexes containing terminal η 2-3,5-di- tert-butylpyrazolato ligands

Treatment of Ba(N(SiMe 3) 2) 2(THF) 2 with tetraglyme or triglyme (1 equiv) followed by 3,5-di- tert-butylpyrazole (2 equiv) in toluene afforded bis(3,5-di- tert-butylpyrazolato)(tetraglyme)barium (68%) and bis(3,5-di- tert-butylpyrazolato)(triglyme)barium (32%) as colorless solids after crystallization from hexane. Similar treatment of Sr(N(SiMe 3) 2) 2(THF) 2 with tetraglyme, triglyme or N, N, N′, N″, N″-pentamethyldiethylenetriamine (PMDETA) (1 equiv) followed by 3,5-di- tert-butylpyrazole (2 equiv) in toluene afforded bis(3,5-di- tert-butylpyrazolato)(tetraglyme)strontium (50%), bis(3,5-di- tert-butylpyrazolato)(triglyme)strontium (73%) and bis(3,5-di- tert-butylpyrazolato)(PMDETA)strontium(II) (64%), respectively, as colorless solids. The X-ray crystal structures of these complexes revealed monomeric molecules in which each metal coordination sphere is capped by one neutral polydentate ligand and two terminal η 2-pyrazolato ligands. Volatility studies showed that these complexes fail to sublime, and rather decompose under sublimation conditions with neutral ligand loss for the barium complexes and loss of the neutral ligands and 3,5-di- tert-butylpyrazole for the strontium complexes.

Complexation of nickel(II) by ethylenediamine investigated by means of electrospray ionization mass spectrometry

Electrospray ionization (ESI) mass spectrometry is used to investigate the complexation of Ni(II) by ethylenediamine (en) in water/methanol solution of NiCl 2, both in the presence and absence of N, N-dimethylformamide (DMF). The fragmentations of the resulting Ni(II) complexes are studied using collision-induced dissociation (CID) including labeling experiments. The solvated ions formed upon ESI, in the absence of DMF, were: [Ni(en) n ] 2+ with n=2–5, [Ni(en) n Cl] + with n=1–3, and the chloro-bridged binuclear ions [Ni 2(en) n Cl 3] + with n=2–4. The dications [Ni(en) n ] 2+ show neutral ligand loss, proton transfer between the ligands, and electron transfer from the ligand to Ni(II). CID experiments and comparative studies including related ligands suggest that the multiply solvated dications [Ni(en) n ] 2+ with n>3 bear a hexacoordinate [Ni(en) 3 ] 2+ core with the additional ligands bound via hydrogen bonds. The monocations [Ni(en) n Cl] + with n=2 and 3 undergo evaporation of ethylenediamine and elimination of HCl. The bisligated monocation [Ni(en)Cl] + undergoes competing eliminations of HCl and NH 3 at low collision energies, and NiCl bond homolysis at elevated collision energies. The binuclear ions [Ni 2(en) n Cl 3] + show a loss of neutral NiCl 2 leading to the formation of [Ni(en) 2Cl] +. The ions formed upon ESI from solutions which contain DMF as a co-ligand are: [Ni(en) 3(DMF) n ] 2+ with n=0–4, [Ni(en) 2(DMF) n ] 2+ with n=0–2, the monocation [Ni(en) 2(DMF)Cl] + and the binuclear clusters [Ni 2(en) n Cl 3] + with n=2–4. The mixed solvated Ni(II) ions preferentially eliminate neutral DMF, indicating a favorable coordination of Ni(II) by ethylenediamine.

Fragmentation and charge transfer in gas-phase complexes of divalent metal ions with acetonitrile

The development of electrospray has enabled generation of gas-phase multiply charged metal ion complexes with various solvent molecules. These species exhibit rich fragmentation chemistry, involving competition among neutral ligand loss, ligand cleavage, and dissociative electron and proton transfer. Acetonitrile is a common aprotic solvent. Here we present a comprehensive MS/MS study on acetonitrile complexes of divalent metal cations. We measured the critical sizes below which dissociation channels other than the trivial neutral evaporation become operative and minimum sizes at which dications remain stable against charge reduction. For all sizes between the two, low-energy fragmentation patterns have been elucidated in detail.