Counterion coordination

In compounds (333) and (336) the amines substitute one of the dithiophosphate ligands to form a NiN4S2 core with the second dithiophosphate ligand serving as a counterion. Coordination number 5 is rare, but is found for complex (345) where one of thiophosphates switched to a monodentate coordination,879 and in compound (346) where only one additional phosphine adduct coordinates to Ni.880... 

TPMA ligand typically coordinates to the copper center in a tetradentate fashion, similarly to Me6TREN [117]. However, the role of counterion coordination (in particular Br and Cl ) in these complexes still remains very unclear. Recently, we were able to isolate and structurally characterize neutral CuI(TPMA)Cl [118] and CuI(TPMA)Br [119] complexes. To our surprise, both complexes were pseudopenta-coordinated (Fig. 6). In CuI(TPMA)Cl, the copper ion was coordinated by four nitrogen atoms with bond lengths of 2.0704(11), 2.0833(11), 2.0888(11) A for the equatorial Cu-N, and 2.4366(11) A for the axial Cu-N bonds, and a chlorine atom with a bond length of 2.3976(4) A. The molecular structure of Cu (TPMA)Br was similar to the structure of Cu (TPMA)Cl and the complex was also found to be pseudopentacoordinated in the solid state due to the coordination of TPMA (Cu-N... 

Examples of these effects can be found. In the case of the systems shown below, the cis isomer is known to be the more stable isomer127. This is especially striking for the dianion since there exists a large coulombic repulsion between the sulfur atoms. Here, however, the possibility of counterion coordination should not be disregarded as a potential source of the greater stabilization of the cis isomer. 

Lambert and co-workers chose to work with tetrakis(pentafluorophenyl)borate (33, TPFPB"). [7-10] A crystal structure analysis of the triethylsilyl cation in toluene solution with TPFPB" as counterion revealed that the closest cation-anion contact distance is 4.18 A, which is well outside the range of van der Waals distances suggesting that counterion coordination to R3Si+ is no longer a problem with anions such as 33. [7]... 

When competing Claisen condensation of the ester is a problem, the use of the sterically hindered t-butyl esters is recommended. Unlike with ketone enolates, the 0-alkylation of ester enolates generally is not a problem. Consequently, HMPA may be added to ester enolate alkylations to improve yields. Many S 2 reactions proceed more readily in HMPA than in THF, DME, or DMSO. A solvent for replacing the carcinogenic HMPA in a variety of alkylation reactions is l,3-dimethyl-3,4,5,6-tetrahy-dro-2(lH)pyrimidinone (A,A -dimethylpropyleneurea, DMPU), which also has a strong dipole to facilitate metal counterion coordination. ... 

Complexes [Re(CO)4PR3]+ stabilized by the [BArF]" counterion coordinate CH2CI2, H2, and Si-H bonds activating H2 heterolysis and C-Cl bond breaking. ... 

The aromatic shifts that are induced by 5.1c, 5.If and S.lg on the H-NMR spectrum of SDS, CTAB and Zn(DS)2 have been determined. Zn(DS)2 is used as a model system for Cu(DS)2, which is paramagnetic. The cjkcs and counterion binding for Cu(DS)2 and Zn(DS)2 are similar and it has been demonstrated in Chapter 2 that Zn(II) ions are also capable of coordinating to 5.1, albeit somewhat less efficiently than copper ions. Figure 5.7 shows the results of the shift measurements. For comparison purposes also the data for chalcone (5.4) have been added. This compound has almost no tendency to coordinate to transition-metal ions in aqueous solutions. From Figure 5.7 a number of conclusions can be drawn. (1) The shifts induced by 5.1c on the NMR signals of SDS and CTAB... 

The reactivity of mercury salts is a fimction of both the solvent and the counterion in the mercury salt. Mercuric chloride, for example, is unreactive, and mercuric acetate is usually used. When higher reactivity is required, salts of electronegatively substituted carboxylic acids such as mercuric trifiuoroacetate can be used. Mercuric nitrate and mercuric perchlorate are also highly reactive. Soft anions reduce the reactivity of the Hg " son by coordination, which reduces the electrophilicity of the cation. The harder oxygen anions leave the mercuric ion in a more reactive state. Organomercury compounds have a number of valuable synthetic applications, and these will be discussed in Chapter 8 of Part B. 

The weakness of the ri -coordination follows from the general stmcture determination of tetrakis(2-thienyl)borate, 44 (96IC7095). Counterions of potassium are ri -coordinated, one cation per two thienyl substituents belonging to the neighboring anions. 

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

The discriminatory emission properties between two-coordinate d ° gold(I) complexes and their respective three-coordinate counterparts have been demonstrated in the literature [6, 10-13]. As discussed in the later sections, Che and coworkers have rationalized that the extraordinarily large Stokes shift of the visible emission of [Au2(diphosphine)2] from the [5da 6pa] transition is due to the exciplex formation ofthe excited state with solvent or counterions [6]. Fackler [14—16] reported the photophysical properties of monomeric [AUL3] complexes, which show visible luminescence with large Stokes shifts (typically lOOOOcm ), suggesting significant excited-state distortion. Gray et al. [10] examined the spectroscopic properties of... 

ESR spectra (Table 1). The JV-cyclohexylthiosemicarbazone, 13, complex formed the expected [Fe(13-H)2] with FeCl as the counterion [141]. However, [Fe(13) (13-H)H20]C104 was isolated from ethanol. Bulkiness of the cyclohexyl group, and the perchlorate ion s greater ability to hydrogen bond are probably both important to the stability of this cation. The iron(III) center is considered six-coordinate with a tridentate 13-H, bidentate 13, and a coordinated water molecule. 

While related to its carbon analogs, the existence of the RsSi species as a free ion in condensed phases had been doubted for a long time. However, NMR characterization using bulky aryl substituents has provided evidence for the triply coordinated silicon cation. " However, definitive evidence was recently reported by the groups of Reed and Lambert with a silyl cation species bound to three mesityl groups and a carborane [HCBnMesBrg] counterion (Eig. 7.5). It was suggested that... 

The reactivity of enolates is also affected by the metal counterion. For the most commonly used ions the order of reactivity is Mg2+ < Li+ < Na+ < K+. The factors that are responsible for this order are closely related to those described for solvents. The smaller, harder Mg2+ and Li+ cations are more tightly associated with the enolate than are the Na+ and K+ ions. The tighter coordination decreases the reactivity of the enolate and gives rise to more highly associated species. 

In the present case, the electron hopping chemistry in the polymeric porphyrins is an especially rich topic because we can manipulate the axial coordination of the porphyrin, to learn how electron self exchange rates respond to axial coordination, and because we can compare the self exchange rates of the different redox couples of a given metallotetraphenylporphyrin polymer. To measure these chemical effects, and avoid potentially competing kinetic phenomena associated with mobilities of the electroneutrality-required counterions in the polymers, we chose a steady state measurement technique based on the sandwich electrode microstructure (19). 

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