Ligand switching reactions

F. Insights into Competition Between Ion Core Transformations, Ligand Switching Reactions, and Association Reactions... 

Chemical reagents have been used in solution to footprint sites of interaction in noncovalent complexes. Only a limited set of studies have been performed in the gas phase using ion—molecule reactions between a charged noncovalent complex and various neutral reagents [72,73]. They can be classified into two areas H/D exchange reactions [72] and ligand switching reactions [73]. Recent results in these two areas are discussed below. 

Cluster ions can be formed by photon or electron interaction with a neutral cluster produced in a supersonic expansion [1321. Another process restricted to clusters is ligand-switching or the replacement of one ligand for another. Often exothermic ligand-switching reactions take place at rates near the gas kinetic limit, especially for small values of n [72, 133]. Chemical-reactivity studies as a function of cluster size show a variety of trends [93, 127. 133]. Proton-transfer reactions are often unaffected by solvation, while nucleophilic-displacement reactions are often shut down by as few as one or two solvent molecules. 

Thus, the ambient air can be directly introduced to achieve an on-line measurement in the PTR-MS operation. Due to the presence of electric field, in the reaction region, the ion energy is closely related to the reduced-field E/N, where E is the electric field and N is the number density of gas in the drift tube. In a typical PTR-MS measurement, E/N is required to set to an appropriate value normally in the range of 120-160 Td (1 Td = 10 Vcm /molecule), which may restrain the formation of the water cluster ions H30 (H20) n = 1-3) to avoid the ligand switch reaction with analyte M [6] ... 

The SOf anion, which is more accurately described as the electrostatically bonded complex S03 02, has been generated [12] by the three-body association of S03 with O2 as well as the ligand switching reaction SO3" (H2O) + O2 -> SO5--h H2O = 5 X 10"" cm s ). 

With P-donors a variety of different structures are formed. Two complexes (359a,b) with a five-coordinate square pyramidal geometry and one S-donor in the apical position are reported.934,933 In a related complex (360) the nickel center reveals a distorted square planar geometry, because one xanthate ligand switched to a monodentate coordination mode. 6 By reaction of [NiL2] complexes with Ph2P(CH2)2P(Ph)(CH2)2PPh2, compound (362) is formed, which consists of a five-coordinate cation and a hexacoordinate anion. 7... 

During the course of the work, we also conducted further studies of the reactions of proton hydrates with CH3COCH3 and CH3COOCH3. The reaction mechanisms were found to change from proton transfer to ligand switching and ultimately to an association process, which would be equivalent to adsorption in the case of bulk systems. 

Erb W, Neuville L, Zhu JP (2009) Ugi-post functionalization, from a single set of Ugi-adducts to two distinct heterocycles by microwave-assisted palladium-catalyzed cyclizations tuning the reaction pathways by ligand switch. J Org Chem 74 3109-3115... 

Mg+" reacts with alcohols via either condensation or via elimination. The ontcome is snbstrate dependent. Thns while n-BnOH reacts via condensation (eqnation 24), f-BnOH reacts via elimination (equation 25). No oxidative reactions (cf equations 21-23) are observed. Armentront and coworkers have examined the CID reactions of methanol addncts of Mg using Xe as the collision gas and fonnd competition between ligand loss (eqnation 26), ligand switching (equation 27) and C—O bond activation (eqnations 28 and 29). 

---