Coordination and ligand exchange dynamics of solvated metal ions

Recent developments in computer speed and capacity have opened the access to highly accurate molecular dynamics simulations based on quantum mechanically calculated forces for the chemically relevant region around ions in solution (QM/MM formalism). This accuracy, although still extremely consuming (30-300 days of CP time per simulation), is needed for reliable structural details and ligand exchange rates. A large number of main group and transition metal ions have been investigated by this approach, giving very detailed insight into the properties of these ions in solution and allowing to classify the ions by various characteristics. Most first-row transition metal ions have a very stable first hexa-coordinated solvation shell, whose vibrational distortions, however, strongly influence the dynamics of the second shell. The dynamical Jahn-Teller effect - shown to be a femto- and picosecond phenomenon - can strongly influence ligand coordination and exchange dynamics. A large number of ions with very labile solvation shell such as most main group ions, but also transition metal ions, e.g. Ag(I) and Hg(II), can change their coordination within the picosecond scale, leading to an almost simultaneous presence of several species hardly accessible by present experimental techniques. Among these ions, the structure breakers are of particular interest, and it could be shown that there are two types of them, one with a large and very labile first coordination shell such as Cs(I), the other characterised by a small first but an unusually large second solvation shell such as Au(I). Investigations of metal ions coordinated to ammonia ligands have shown that coordination to hetero-atoms can accelerate the ligand exchange reaction rates by several orders of magnitude, e.g. for Cu(II) and Ni(II). Simulations of ions in aqueous ammonia gave a very detailed picture of the complexity of species almost simultaneously present and illustrate the enormous difficulties encountered when trying to fit X-ray or neutron diffraction data for such systems. In general, ligand exchange rates situated in the picosecond range are far below the NMR scale, and as femtosecond laser pulse spectroscopy could not be applied so far to ionic solutions, accurate simulations have become a very important tool to access structure and dynamics of solvated ions. A number of VIDEO clips supplied on the Web as supporting material illustrates the processes occurring in solutions of the metal ions.     

Heterobimetallische phosphanido-verbrückte Zweikern-Komplexe - Synthese von cis-rac-[(η-C5H4R)2Zr{μ-PH(2,4,6−iPr3C6H2)}2M(CO)4] (RMe,MCr,Mo; RH,MMo)

Heterobimetallic Phosphanido-bridged Dinuclear Complexes - Syntheses of cis-rac-[(η-C₅H₄R)₂Zr{μ-PH(2,4,6-ⁱPr₃C₆H₂)}₂M(CO)₄] (R?Me, M?Cr, Mo; R?H, M?Mo) The zirconocene bisphosphanido complexes [(η-C₅H₄R)₂Zr{PH(2,4,6-ⁱPr₃C₆H₂)}₂] (R?Me, H) react with [(NBD)M(CO)₄] (NBD?norbornadiene, M?Cr, Mo) to give only one diastereomer of the phosphanido-bridged heterobimetallic dinuclear complexes cis-rac-[(η-C₅H₄R)₂Zr{μ-PH(2,4,6-ⁱPr₃C₆H₂)}₂M(CO)₄] [R?Me, M?Cr ( 1 ), Mo ( 2 ); R?H, M?Mo ( 3 )]. However, no reaction was observed between [(η-C₅H₅)₂Zr{PH(2,4,6-^tBu₃ C₆H₂)}₂] and [Pt(PPh₃)₄]. 1—3 were characterised spectroscopically. For 1—3 , the presence of the racemic isomer was shown by NMR spectroscopy. No reaction was observed at room temperature for 3 and CS₂, (NO)BF₄, Me₃NO or PH(2,4,6-Me₃C₆H₂)₂. With Et₂AlH or PhC?CH decomposition of 3 was observed.     

A study of peralkylated derivatives of bis(η6-benzene)chromium by gas-phase electronic absorption spectroscopy

The gas-phase electronic absorption spectra of (⁶-C₆R₆)₂Cr (R = Me (1) and Et (2)) reveal Rydberg structures, which disappear on going to the condensed phase. Each spectrum shows a Rydberg series converging to the ionization threshold. The first ionization potential determined as the series convergence limit is 4.662±0.008 eV for 1 and 4.667±0.019 eV for 2. The Rydberg bands are due to the transitions from the non-bonding MO 3d_z2 to the R4s and Rnp (n = 4—10) levels. The influence of methyl and ethyl substituents on the term values of the Rydberg transitions depends on the principal quantum number of the Rydberg MO.     

Electrochemical studies of Tl(I) crown-ether complexes in nonaqueous media

Summary Complexation equilibria of the Tl(I) ion with 18-crown-6 and dibenzo-18-crown-6 were studied polarographically in 10 nonaqueous solvents. The stability of the complexes is strongly influenced by the nature of solvents and varies with their Lewis basicities. It has been found that the logK _s value (K _s is the stability constant of the complex) can be well described by empirical relation logK _s=a DN+b, whereDN stands for the Gutmann donor number anda andb mark the regression coefficient. Addition of the second explanatory parameter, the acceptor number, is not statistically significant. This result is in agreement with the predominant role of Tl(I) ion solvation.

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Elektrochemische Untersuchungen von Tl(I)-Kronenetherkomplexen in nichtwäßrigen Medien

Zusammenfassung Es wurden die Komplexierungsgleichgewichte des Tl(I)-Ions mit 18-Krone-6 und Dibenzo-18-krone-6 polarographisch in 10 nichtwäßrigen Lösungsmitteln untersucht. Die Stabilität der Komplexe wird sehr stark vom Solvens beeinflußt, wobei eine starke Abhängigkeit von der Lewis-Basizität beobachtet wird. Es wurde festgestellt, daß die logK _s-Werte (K _s ist die Komplexstabilitätskonstante) gut mit der empirischen Beziehung logK _s=a DN+b beschrieben werden können, wobeiDN die Gutmann'sche Donorzahl unda undb die Regressionskonstanten bedeuten. Hinzunahme der Akzeptorzahl als zweiten Parameter bleibt statistisch insignifikant. Dieses Ergebnis stimmt mit dem dominierenden Einfluß der Tl(I)-Ionensolvatation überein.

     

Nitrate ion association with Nd3+

The overall stability constantsK ₁ andK ₂ of NdNO ₃ ²⁺ and Nd(NO₃) ₂ ⁺ complexes were determined (K ₁=1.77;K ₂=1.28) using an extraction method with tri-n-butyl phosphate as the extractant. The ratio ₁/₂ of the stepwise stability constants is discussed. It was established that the Nd(NO₃) ₂ ⁺ complex was an outer-sphere ion pair.

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Die Assoziation von Nitrat-Ion mit Nd³⁺

Zusammenfassung Mittels einer Extraktionsmethode wurden die StabilitätskonstantenK ₁ undK ₂ von NdNO ₃ ²⁺ - und Nd(NO₃)⁺-Komplexen bestimmt (K ₁=1.77;K ₂=1.28; Tri-n-butylphosphat als Extraktionsmittel). Das Verhältnis ₁/₂ der stufenweisen Stabilitätskonstanten wird diskutiert. Es stellte sich heraus, daß der Nd(NO₃)⁺-Komplex als ein Outer-Shere-Ionenpaar vorliegt.

     

Boron chelate complexes with 4-[N-(R-pyrid-2-yl)carbamoyl]-3-amino-2-cyanobuten-2-ic acid esters (codimers of N-(R-pyrid-2-yl)amides and the ethyl ester of cyanoacetic acid)

Mono- and binuclear boron chelates have been synthesized by the action of butylthiodibutylborane on the ethyl esters of 4-[N-(R-pyrid-2-yl)carbamoyl]-3-amino-2-cyanobuten-2-ic acids (codimers of N-(R-pyrid-2-yl) amides and the ethyl ester of cyanoacetic acid). Complexes of this type can exist in solution in the form of two tautomers: acetamide acid derivatives or the corresponding ketene N,O-acetals.N. D. Zelinski Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 9, pp. 2162–2170, September, 1992.     

Synthesis, characterization, and reactivity of half-sandwich Ru(II) complexes containing phosphine, arsine, stibine, and bismutine ligands

The synthesis and some reactions of the Ru(II) and Ru(IV) half-sandwich complexes [RuCp(EPh₃)(CH₃CN)₂]⁺ (E=P, As, Sb, Bi) and [RuCp(EPh₃)(η³-C₃H₅)Br]⁺ have been investigated. The chemistry of this class of compounds is characterized by a competitive coordination of EPh₃ either via a RuE or a η⁶-arene bond, where the latter is favored when the former is weaker, that is in going down the series. Thus in the case of Bi, the starting material [RuCp(CH₃CN)₃]⁺ does not react with BiPh₃ to give [RuCp(BiPh₃)(CH₃CN)₂]⁺ but instead gives only the η⁶-arene species [RuCp(η⁶-PhBiPh₂)]⁺ and [(RuCp)₂(μ-η⁶,η⁶-Ph₂BiPh)]²⁺. Similarly, the EPh₃ ligand can be replaced by an aromatic solvent or an arene substrate. Thus, the catalytic performance of [RuCp(EPh₃)(CH₃CN)₂]⁺ for the isomerization of allyl-phenyl ethers to the corresponding 1-propenyl ethers is best with E=P, while the conversion drops significantly using the As and Sb derivatives. By the same token, only [RuCp(PPh₃)(CH₃CN)₂]⁺ is stable in a non-aromatic solvent, whereas both [RuCp(AsPh₃)(CH₃CN)₂]⁺ and [RuCp(SbPh₃)(CH₃CN)₂]⁺ rearrange upon warming to [RuCp(η⁶-PhEPh₂)]⁺ and related compounds. In addition, the potential of [RuCp(EPh₃)(CH₃CN)₂]⁺ as precatalysts for the transfer hydrogenation of acetophenone and cyclohexanone has been investigated. Again aromatic substrates are clearly less suited than non-aromatic ones due to facile η⁶-arene coordination leading to catalyst's deactivation.     

γ-Zirconium and γ-Titanium Phosphates Platinum Intercalation Compound Preparation, characterization and thermal behaviour

Inorganic ion-exchangers with a layered structure such as γ-zirconium and γ-titanium phosphates, intercalated with organic diamines, are able to exchange Pt²⁺ ions to give new intercalation compounds that can be utilized in heterogeneous catalysis. The experiments performed at different temperatures (25 and 45°C), show different ion uptakes, greater at 45°C and for the materials derived from γ-zirconium phosphate. After platinum exchange, all the materials show an amorphization in the XRD if compared with their precursors. The thermal behaviour of the platinum materials is specific, depending on the exchanger used and the ligand inside the exchanger. Pt²⁺ ion has a catalytic effect on ligand elimination in the γ-zirconium phosphate platinum compounds, but not in those derived from γ-titanium. All the obtained yellow materials show a small step in the TG curves and simultaneously we have the Pt²⁺→Pt⁰ reduction: this is confirmed by XRD registered at the temperatures of the thermal effect, showing peaks at d_hkl=2.27 and 1.95 Å. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aromatic hydrocarbons, carbocyclic ligands spanning several oxidation states in both Main Group and transition elements. Recent advances with early transition d elements

In this paper some synthetic procedures to obtain (η⁶-arene)metal derivatives are reviewed. The metal-atom-arene-vapor co-condensation technique is the most appropriate to generate complexes of polycyclic aromatic hydrocarbons or heterocycles. As far as the aluminium halide-mediated synthesis is concerned, two classes of reaction are observed. When AlX₃ is used with a metal halide in the presence of an aromatic hydrocarbon in the absence of any reducing agent, AlX₃ can function as a dehalogenating agent, to give ionic compounds of general formula [M(η⁶-arene)n](AlX₄)_m, or it can add across the M---X bond with formation of M(μ-X)_nAlX_4−n systems. In both cases the metal displays its typical oxidation state. However, the use of AlX₃ in combination with aluminium (the Fischer-Hafner reducing system) affords ionic or covalent low-oxidation-state metal(η⁶-arene) complexes. Attention is focused on our most recent results concerning the synthesis, properties and reactivity of η⁶-arene derivatives of Group 4 and 5 elements, showing, inter alia, the first example of a tetraarylborate anion behaving as a 12-electron donor to one metal atom and low-valent η⁶-arene compounds as useful reagents in the inorganic and coordination chemistry of the corresponding metal in nonaqueous systems.     

Anionic copper complex fragmentations from enkephalins under low-energy collision-induced dissociation in an ion trap mass spectrometer

Peptide metallation with Cu2+ was explored in the negative ESI mode using an ion trap mass spectrometer. Under these conditions, the [(M-3H) + CuII]- species formed were investigated under low-energy collision-induced dissociation conditions. MS2 experiments indicate a very different behavior of CuII metallated complexes compared with [M-H]- species. CuII induces an easy loss of CO2 and specific side-chain cleavages (by radical losses) at the C-terminal residue, as observed previously by prompt 'in source' dissociation experiments. The loss of CO2 yields an unstable carbylide that leads to further dissociations involving the migration of a proton or a hydrogen radical (through the reduction of CuII). Multistage MS3 experiments were carried out to rationalize this behavior. Fragmentation pathways are proposed in order to explain the product ions observed. The side-chain radical loss at the C-terminus was demonstrated to be a consecutive process. Finally, evidence is provided that the specific side-chain cleavages can be used for the differentiation of Leu/Ile and Gln/Lys residues when they are located at the C-terminus. The existence of a zwitterionic form in the case of the anionic YGGFK-CuII complex is proposed.