Aryl Carbon Atoms Research Articles

M-C Bond Homolysis in Coinage-Metal [M(CF3 )4 ]- Derivatives.

A comparative study of the homoleptic [M(CF3 )4 ]- complexes of all three coinage metals (M=Cu, Ag, Au) reveals that homolytic M-C bond cleavage is favoured in every case upon excitation in the gas phase (CID-MS2 ). Homolysis also occurs in solution by photochemical excitation. Transfer of the photogenerated CF3 . radicals to both aryl and alkyl carbon atoms was also confirmed. The observed behaviour was rationalized by considering the electronic structure of the involved species, which all show ligand-field inversion. Moreover, the homolytic pathway constitutes experimental evidence for the marked covalent character of the M-C bond. The relative stability of these M-C bonds was evaluated by energy-resolved mass spectrometry (ERMS) and follows the order Cu<Ag≪Au. The qualitatively similar and rather uniform behaviour experimentally observed for all three coinage metals gives no ground to suggest variation in the metal oxidation state along the group.

Chiral palladium pincer complexes for asymmetric catalytic reactions.

Palladium pincer complexes, containing a monoanionic terdentate ligand composed of an anionic aryl carbon atom and two mutually compatible donor sites, have aroused considerable interest since their first reports in the late 1970s. The high stability of the Pd pincer complexes and particularly their high modularity make these species ideal candidates for catalysis. Furthermore, the nature of the meridional coordination of the pincer ligands, and along with this their ability to enforce a stereo-specific environment around the Pd center, provide a good opportunity for developing chiral Pd pincer catalysts. Thus, a broad variety of chiral Pd pincer complexes have been prepared by the introduction of various stereochemical centers in the pincer skeletons. These chiral Pd pincer complexes have been successfully applied to many asymmetric catalytic reactions such as hydrophosphination reactions, allylation of aldehydes and imines, Michael and aldol reactions, Suzuki-Miyaura reactions as well as reactions of nitrile compounds with imines. This review focuses on the synthetic methods and the applications of chiral Pd pincer complexes in asymmetric catalysis.

The mechanism of catalytic methylation of 2-phenylpyridine using di-tert-butyl peroxide.

The mechanism of palladium chloride-catalyzed direct methylation of arenes with peroxides is elucidated by using the energetics computed at the M06 density functional theory. The introduction of a methyl group by tert-butyl peroxides at the ortho-position of a prototypical 2-phenyl pyridine, a commonly used substrate in directed C-H functionalization reactions, is examined in detail by identifying the key intermediates and transition states involved in the reaction sequence. Different possibilities that differ in terms of the site of catalyst coordination with the substrate and the ensuing mechanism are presented. The important mechanistic events involved are (a) an oxidative or a homolytic cleavage of the peroxide O-O bond, (b) C-H bond activation, (c) C-C bond activation, and (d) reductive elimination involving methyl transfer to the aromatic ring. We have examined both radical and non-radical pathways. In the non-radical pathway, the lowest energy pathway involves C-H bond activation prior to the coordination of the peroxide to palladium, which is subsequently followed by the O-O bond cleavage of the peroxide and the C-C bond activation. Reductive elimination in the resulting intermediate leads to the vital C-C bond formation between methyl and aryl carbon atoms. In the non-radical pathway, the C-C bond activation is higher in energy and has been identified as the rate-limiting step of this reaction. In the radical pathway, however, the activation barrier for the C-C bond cleavage is lower than for the peroxide O-O bond cleavage. A combination of a radical pathway up to the formation of a palladium methyl intermediate and a subsequent non-radical pathway has been identified as the most favored pathway for the title reaction. The predicted mechanism is in good agreement with the experimental observations on PdCl2 catalyzed methylation of 2-phenyl pyridine using tert-butyl peroxide.

Zinc compounds with or without Zn–Zn bond: Alkali metal reduction of LZnCl2 (L = α-diimine ligands)

A series of mononuclear and dinuclear zinc compounds, [Na(Et(2)O)](2)[((H)L(iPr))Zn-Zn((H)L(iPr))] x Et(2)O (1), [Zn(2)((H)L(iPr))(3)K(2)(Et(2)O)(2)](n) (2, (H)L(iPr) = [(2,6-(i)Pr(2)C(6)H(3))NCH](2)), [Zn((H)L(Mes))(2)] (4, (H)L(Mes) = [(2,4,6-Me(3)C(6)H(2))NCH](2)), and [ZnL(Mes)(2)Na(2)(THF)(2)] (5, L(Mes) = [(2,4,6-Me(3)C(6)H(2))NC(Me)](2)), and a potassium complex, [((H)L(iPr))(2)K(4)(THF)(2)](n) (3), have been synthesized by alkali metal reduction of the corresponding LZnCl(2) precursors. The Zn-Zn bond in 1 (2.3673(6) A) is slightly shorter than that in the analogous compounds [M(THF)(2)](2)[L(iPr)Zn-ZnL(iPr)] (L(iPr) = [(2,6-(i)Pr(2)C(6)H(3))NC(Me)](2), M = Na, 2.399(1) A; M = K, 2.393(1) A). Complexes 2 and 3 feature a two- and one-dimensional (linear chain) polymeric structure, respectively, linked by K-C bonds between the potassium ion and aryl carbon atoms. Notably, complex 2 contains a ligand whose central C(2)N(2) core is in the trans conformation, acting as a non-chelating bridge between the two zinc atoms. While the ligands in 1, 2, 3 and 5 are doubly reduced to the dianion, complex 4 contains a pair of monoanionic ligands.

Synthesis and crystal structure of diorganotin(IV) complexes with pyruvic acid benzoylhydrazone and pyruvic acid salicylhydrazone

Six diorganotin esters of Schiff-base ligands formulated as [R2SnLY]2, where L1 is C6H5CON2C(CH3)CO2 with Y = CH3CH2OH, R = mClC6H4CH2 (1), oFC6H4CH2 (2), pFC6H4CH2 (3) and L2 is 2-HOC6H4CON2C(CH3)CO2 with Y = CH3OH, R = oFC6H4CH2 (4), pFC6H4CH2 (5), mClC6H4CH2 (6) have been prepared and characterized by elemental analysis, IR, 1H and 119Sn NMR spectra. The crystal structures of complexes 1 and 4 have been determined by X-ray single crystal diffraction. The structure analyses reveal that the Sn atom in both 1 and 4 is seven-coordinate in distorted pentagonal bipyramid geometries with a planar SnO4N unit and two apical aryl carbon atoms, thus forming a dimeric molecule, which sits on a crystallographic center of symmetry. Intramolecular or intradimeric hydrogen bonds contribute to the stability and compactness of the crystal structures.

Synthesis, characterisation and biological activity of cycloaurated organogold(III) complexes with imidate ligands

The reactions of the cycloaurated gold(III) complexes (2-bp)AuCl 2 (2-bp = 2-benzylpyridyl) or (damp)AuCl 2 (damp = Me 2NCH 2C 6H 4) with an excess of sodium saccharinate (Nasacc), potassium phthalimidate (Kphth), or with isatin and trimethylamine in refluxing methanol results in the successful isolation of a series of new gold(III) imidate complexes. These were characterised by NMR and IR spectroscopies, and by X-ray structure determinations on (2-bp)Au(sacc) 2 and (2-bp)Au(phth) 2. In both structures, the planes of the saccharinate and the phthalimidate ligands are orientated almost perpendicular to the gold coordination plane. As expected from trans-influence considerations, the Au–N (imidate) bond lengths trans to the aryl carbon atoms are longer than the Au–N (imidate) bond lengths trans to the pyridyl groups. The complexes have also been characterised by electrospray ionisation MS; in the presence of halide ligands, one imidate ligand is readily displaced. Anti-tumour (P388 murine leukemia) and selected anti-microbial data for the new complexes are reported. Surprisingly, all three damp complexes had low anti-tumour activity, which is likely to be a consequence of the poor solubility of these complexes. The synthesis and characterisation of a related gold(III) bis(amidate) complex derived from sulfathiazole is also described.

Synthesis and structural characterization of diorganotin(IV) esters with pyruvic acid isonicotinyl hydrazone and pyruvic acid salicylhydrazone Schiff bases

Eight diorganotin esters of Schiff base ligands formulated as [R 2SnLY] 2, where L 1 is 4-NC 5H 4CON 2C(CH 3) CO 2 with Y = H 2O, R = Ph ( 1), PhCH 2 ( 2), m-ClC 6H 4CH 2 ( 3), and L 2 is 2-HOC 6H 4CON 2C(CH 3) CO 2 with Y = CH 3OH, R = PhCH 2 ( 4), o-ClC 6H 4CH 2 ( 5), m-ClC 6H 4CH 2 ( 6), o-FC 6H 4CH 2 ( 7), p-FC 6H 4CH 2 ( 8) have been prepared and characterized by elemental analysis, IR, 1H and 119Sn NMR spectra. The crystal structures of compounds 1, 2 and 4 have been determined by X-ray single crystal diffraction. Their structures show that the tin atoms of three compounds are all rendered seven-coordinated in distorted pentagonal bipyramid geometries with a planar SnO 4N unit and two apical aryl carbon atoms. A comparison of the IR spectra of the ligands with those of the corresponding compounds, reveals that the disappearance of the bands assigned to carbonyl unambiguously conforms that the ligands coordinate with tin in the enol form.

In vitro characterization of IroB, a pathogen-associated C -glycosyltransferase

Pathogenic strains of Escherichia coli and Salmonella enterica modify the tricatecholic siderophore enterobactin (Ent) by glucosylation of three aryl carbon atoms, a process controlled by the iroA locus [Hantke, K., Nicholson, G., Rabsch, W. & Winkelmann, G. (2003) Proc. Natl. Acad. Sci. USA 100, 3677-3682]. Here, we report the purification of the IroB protein and its characterization as the Ent C-glucosyltransferase. IroB transfers glucosyl groups from uridine-5'-diphosphoglucose to C5 of one, two, or three of the 2,3-dihydroxybenzoyl units of Ent to yield monoglucosyl-C-Ent (MGE), diglucosyl-C-Ent (DGE), and triglucosyl-C-Ent (TGE). DGE, also known as salmochelin S4, and macrolactone-opened derivatives have been isolated from the culture broths of S. enterica and uropathogenic E. coli [Bister, B., Bischoff, D., Nicholson, G. J., Valdebenito, M., Schneider, K., Winkelmann, G., Hantke, K. & Sussmuth, R. D. (2004) Biometals 17, 471-481], but MGE and TGE have not been reported previously. IroB has a k(cat) of approximately 10 min(-1) for the first C-glucosylation and is distributive, with sequential conversion and buildup of MGE and then DGE. The C5 to C1' regio-selectivity of the 2,3-dihydroxybenzoyl-glucose linkage at all three rings of TGE suggests a C5 carbanion, para to the C2 phenolate oxygen, as the carbon nucleophile in this novel enzymatic C-glucosylation.

Structure of [2,6-bis(dimethylamino)methyl]phenyltin tribromide hydrate

The reaction of the 2,6-bis[(dimethylamino)methyl]phenyllithium with tin(IV) bromide affords, after recrystallisation from toluene on the air, the organotin(IV) compound [SnBr 3{2,6-(Me 2NCH 2) 2C 6H 3}(H 2O)], 3, in which the N,C,N-ligand is bounded to Sn atom through the aryl carbon atom and one of the NMe 2 groups only, while the second NMe 2 group is protonated by hydrogen from the water molecule. The crystal structure analysis has shown dimeric structure of this compound, with unusually Sn–O–H–Br hydrogen bridges. This compound was characterised by NMR spectroscopy in solution and in the solid state and X-ray structure analysis.

Pseudotetrahedral Organocobalt(III) Compounds Containing Specific Coordination Sites for Brønsted Acids

Transmetallation of the lithiated derivative 2,6-bis[dimethyl(amino)methyl]phenyllithium with the cobalt(III) centre of the dimer bis[(η5-cyclopentadienyl)cobalt(iodo)(μ-iodo)] has been found to afford the organocobalt compound [CoI{2-(Me2NCH2)-6-(Me2NH+CH2)C6H3}(η5-C5H5)]I, 3, in which the potentially terdentate (pincer) ligand is bound to Co through the formerly lithiated aryl carbon atom and one NMe2 unit only, whilst the second NMe2 group is protonated by HI. In the presence of aqueous KPF6, 3 has been found to undergo both substitution of the iodide bound to Co by a water molecule and deprotonation of the Me2NH group to afford [CoI{2,6-(Me2NCH2)2C6H3}(η5-C5H5)(H2O)]PF6, 4a. It was found that the water ligand in 4a could be substituted by neutral ligands such as PMe2Ph as well as by Bronsted acids such as HCl, HI, HF, or HOAc to afford the compounds [CoX{2-(Me2NCH2)-6-(Me2NH+CH2)C6H3}(η5-C5H5)]PF6, 5a−d, where X = Cl, I, F, or OAc, respectively. Crystal structure analyses of some representative examples of these compounds, including 4a, have shown unambiguously that an intramolecular hydrogen bond is present between X and the proton of the Me2NH+ unit. 1H-NMR evidence that this interaction persists in solution has been found for compounds 4a, 5a, 5c, and 5d.

Preferred orientations in the binding of 4'-hydroxyacetanilide (acetaminophen) to cytochrome P450 1A1 and 2B1 isoforms as determined by 13C- and 15N-NMR relaxation studies.

The widely used analgesic/antipyretic agent 4'-hydroxyacetanilide (acetaminophen, APAP) is oxidized by cytochromes P450 to a potent cytotoxin, N-acetyl-p-benzoquinone imine (NAPQI), and a nontoxic catechol, 3',4'-dihydroxyacetanilide (3-hydroxyacetaminophen, 3-OH-APAP). There are marked differences in the ratios of these two products formed from different isoforms of cytochrome P450. For example, the ratio of NAPQI to 3-OH-APAP formed by rat liver CYP1A1 was found to be approximately 3:1, whereas the ratio of the same two products formed by rat liver CYP2B1 was approximately 1:5. Investigations of the binding of APAP to CYP1A1 and CYP2B1 were carried out to assess the possibility that different preferred orientations of APAP in the active sites of these isoforms may, in part, by responsible for their different product selectivities. Although the spectral dissociation constants (Ks congruent to 0.85 mM) and UV-vis binding spectra (type I; absorption minimum congruent to 420 nm, absorption maximum congruent to 390 nm) were similar for interactions of APAP with the two P450 isoforms, NMR longitudinal relaxation times (T1) of APAP nuclei were significantly different. Two isotopically substituted analogs of APAP, [2,3',5'-13C3]-4'-hydroxyacetanilide and 4'-hydroxyacet-[15N]-anilide, were synthesized, and their binding to purified CYP1A1 and CYP2B1 was examined by NMR spectroscopy. Paramagnetic relaxation times (T1p) for each of the labeled nuclei were calculated from the T1 values obtained before (ferric) and after (ferrous-CO) treatment with Na2S2O4 and CO. The Solomon-Bloembergen equation was then used to calculate distances of the isotopically labeled nuclei from the heme iron of each P450 isoform. The results were that the amide nitrogen approaches relatively close to the heme iron in CYP1A1 (3.64 +/- 0.51 A) whereas it is significantly further away (> 4.5 A) in CYP2B1. In contrast, the aryl carbon atoms ortho to the phenolic group of APAP approach closer to the heme iron of CYP2B1 (3.19 +/- 0.12 A) than to the heme iron of CPY1A1 (3.66 +/- 0.30 A). The results are consistent with the hypothesis that CYP1A1 produces NAPQI preferentially because of closer proximity of the heme iron to the amide nitrogen, whereas CYP2B1 produces 3-OH-APAP preferentially because of closer proximity of the heme iron to the phenolic oxygen in this isoform.

Metalation reactions. XIV. Regiospecific preparation of polysubstituted benzenes [formula omitted] mono- or di-lithiation reactions of aromatic thioethers

The preparation of polyfunctionalized aromatic thioethers either by one-step dilithiation or by two successive one-flask monometalation reactions is described. By acting on 1 two equal or different electrophiles one on the thiomethyl group and one in the ortho -position with respect to it are introduced; by acting on 11 and on 35 the substitution involves the thiomethyl carbon atom and that in the ortho -position with respect to the alkoxy group. In the case of the homologous isopropylthio ( 23) the substitution involves the two aryl carbon atoms in the ortho -position to both functions. In the case of the p -disubstituted isomers ( 49, 59) analogous behaviour to ortho isomers in one-step metalation reaction is observed, while the two hydrogen atoms in the ortho -positions to the methoxy group are substituted when two successive monometalations are employed. The metalation of 40 results low selective. The behaviour of 79 and 93 is analogous to 1, while 72, 88 and 96 undergo only one-step monometalation reactions.

Reactions of cyclopalladated compounds. Part 21. Various examples of sulphur-assisted intramolecular palladation of aryl and alkyl groups

The cyclopalladation reactions of several sulphur-containing ligands have been investigated. The thioethers benzyl methyl sulphide, methyl naphthyl sulphide, 2,6-dimethylphenyl methyl sulphide, and neopentyl phenyl sulphide have been metallated by palladium acetate in acetic acid at either aryl or alkyl carbon atoms to afford, after reaction with lithium chloride in acetone, dimeric chloride bridged cyclopalladated complexes in moderate (22%) to good (81%)yields. In these products each metal is part of a five-membered ring. The same reaction with 2-biphenyl methyl sulphide afforded a dimeric complex containing a six-membered cyclopalladated ring through palladation of the ortho position of the biphenyl group. 4,4′- Dimethoxythiobenzophenone and N,N,N′,N′-tetramethyl-thiourea have been palladated by using a ligand-exchange reaction with [{[graphic omitted]Me2-2)Cl}2] in the presence of stoicheiometric amounts of trifluoroacetic acid. This afforded good yields of dimeric palladocyclic complexes through the metallation of a carbon ortho to the thioketone, or of a methyl group of the thiourea respectively. With 1,3-bis(methylthiomethyl)benzene this latter reaction, by metallation of the 2 position of the aryl ring, afforded excellent yields of a monomeric compound in which both sulphur atoms are co-ordinated to the palladium centre.

Tricarbonylchromium(0) complexes of 1,3,5-triphenylbenzene: an X-ray crystallographic and high field NMR spectroscopic study

The reaction of 1,3,5-triphenylbenzene (TPB) with Cr(CO) 6 yields products in which a Cr(CO) 3 moiety is bonded to the central ring or to one, two or all three peripheral phenyl rings. The structure of the TPB · 3Cr(CO) 3 complex has been determined by single crystal X-ray diffraction techniques. The complex crystallizes in the space group P2 1/ n with unit cell parameters a 8.316(2) Å, b 15.498(5) Å, c 22.254(4) Å, β 92.18(2)°, V 2865.8(11) Å 3 and Z = 4. The structure has been solved by direct and Fourier methods and refined by block diagonal matrix least squares to R 1 and R 2 values of 0.053 and 0.053, respectively, for 2830 observed reflections. The structure reveals that the chromium tricarbonyl groups are bonded to the three peripheral phenyls and these aryl rings adopt dihedral angles of 35° −26° and 14° with respect to the central ring; these dihedral angles are remarkably similar to those in the free ligand itself. All three Cr(CO) 3 tripods are oriented to within a few degrees of being perfectly staggered with respect to the aryl carbon atoms. The 500 MHz 1H and 125 MHz 13C NMR spectra of these complexes reveal that the peripheral rings do not exhibit restricted rotation even at −90°C. This contrasts with the situation for (hexaphenylbenzene)Cr(CO) 3 the 1H NMR spectrum of which may indicate slowed rotation of the phenyl rings even at room temperature.

The stabilities of Meisenheimer complexes. Part 42. Kinetic studies of the reactions of methyl 3,5-dinitrobenzoate and of methyl 4-chloro-3,5-dinitrobenzoate with hydroxide ions in dimethyl sulphoxide–water mixtures. Competitive nucleophilic attack at aryl and carbonyl carbon atoms

Kinetic and equilibrium data, obtained by stopped-flow spectrophotometry, are reported for the reactions of methyl 3,5-dinitrobenzoate (1) and methyl 4-chloro-3,5-dinitrobenzoate (5) with hydroxide ions in dimethyl sulphoxide–water mixtures. There is evidence for competition between attack at carbonyl and aryl carbon atoms, the latter becoming relatively favoured as the proportion of dimethyl sulphoxide in the solvent is increased. It is shown that the major pathway in the formation of 4-hydroxy-3,5-dinitrobenzoate from (5) involves hydroxide attack at the carbonyl function followed by slow attack at the ring carbon atom carrying chlorine.

Synthetic applications of N–N linked heterocycles. Part 18—substituent effects on the 13C NMR spectra in N‐(4‐Aryliminio‐2,6‐dimethylpyridin‐1‐yl)pyridinium bistetra‐fluoroborates, and some variable‐temperature 1H NMR measurements

AbstractThe 13C and 1H NMR spectra have been recorded and fully interpreted for a series of N‐(4‐Aryliminio‐2,6‐dimethylpyridin‐1‐yl)pyridinium bistetrafluoroborates having a variety of phenyl para substituents. Correlations between 13C chemical shifts and both Hammett and Taft dual‐substituent parameters for the aryl substituents were found for C‐2, C‐6, C‐3 and C‐5 of the iminiopyridinyl ring (but not for C‐4), and also for the aryl carbon atom attached to the imino group. Data from variable‐temperature 1H NMR spectra suggest that isomerization about the iminium CN bond is by the little known deprotonation‐imine isomerization‐reprotonation mechanism.

Structural effects on valence tautomerism. Part 5. A carbon-13 nuclear magnetic resonance. Study on the effects of the π-acceptor ability of aryl substituents on the valence tautomerism of 7-aryl-2,5-di-t-butyl-cyclohepta-1,3,5-trienes

The cycloheptatriene (CHT)–norcaradiene (NCD) equilibria of eight 7-aryl-2,5-di-t-butylcyclohepta-1,3,5-trienes in CS2–CD2Cl2 have been studied with the use of variable-temperature 13C n.m.r. The equilibrium constants (K=[NCD]/[CHT]) below the coalescence temperatures were calculated from signal intensities; those at higher temperatures were obtained by use of the equation K=(δCHT–δ)/(δ–δNCD). The enthalpy (ΔHo) for the conversion of the CHT into the NCD tautomer decreases in the substituent order p-MeO > p-Me > H > p-Br ∼p-Cl > m-Cl > p-CF3 > m,m′-Cl2, with the ΔHo values for p-MeO and m,m′-Cl2 2.64 ± 0.11 and –0.81 ± 0.13 kJ mol–1, respectively. The ΔHo values are linearly correlated not only with the Hammett σ-constants, but also with the π-acceptor ability (the D value) of the aryl carbon atom adjacent to C(7) as estimated by INDO calculations; this supports Hoffmann and Gunther's donor–acceptor theory. The 13C signals of C(3) and C(4) in both the NCD and the CHT tautomer undergo downfield shifts as the D value of the aryl group increases; the shift of the former is 4.3 times more sensitive than that of the latter as evaluated by the Hammett plot. In addition, the difference in the 13C shift of the para-carbon atom between the NCD and the CHT tautomer is larger in the 7-(m,m′-dichlorophenyl) than in the 7-phenyl system. These results suggest that conjugative interaction between the cyclopropane ring in the NCD structure and the aryl group increases with increase in the π-acceptor ability of the aryl group.

The solid state structure of arene-mercury(II) complexes from magic-angle spinning carbon-13 NMR and the solution carbon-13 NMR of some complexes of mercury(II) with arenes having bulky aliphatic substituents

The cross-polarization magic angle spinning 13C NMR spectra of Hg(SbF 6) 2 - 2 Arene (Arene = C 6HMe 5, 1,2,4,5-C 6H 2Me 4, 1,2,3,4-C 6H 2Me 4, or C 6H 6) have been measured. The spectra of the complexes of C 6HMe 5 and 1,2,4,5-C 6H 2Me 4 are consistent with static η 1-bonding of the mercury to the arene at an unsubstituted carbon atom, while the spectra of the 1,2,3,4-C 6H 2Me 4 and C 6H 6 complexes show the arene to have time-averaged C s or C 2, and C 6 symmetry respectively, at the temperature of measurement (300 K). The reduced temperature 13C NMR spectra of Hg(Arene) n 2+ ( n = 1 or 2; Arene = 1,3,5-C 6H 3R 3 (R = Me, i-Pr, or t-Bu)) in SO 2 solution are also reported and affirm that in these intramolecularly mobile species the mercury bonds in an η 1-manner, with unsubstituted aryl carbon atoms being the strongly preferred point of mercury attachment. This site preference is further demonstrated by the solution 13C NMR spectra of Hg(Arene) n 2+ (Arene = 1,2,3,4-C 6H 2-Me 4, n = 1 or 2; Arene = 1,4-C 6H 4R 2, R = Me or t-Bu, n = 1). The spectra of the 1,4-C 6H 4R 2 complexes and Hg( p-C 6H 4-t-BuMe) 2+ provide clear evidence for steric influence of the binding site. Like Hg(C 6Me 6) 2 2+, but unlike most of the complexes of substituted benzenes which have been studied, Hg(1,3,5-C 6H 3-i-Pr 3) 2 2+ exchanges only slowly with excess free ligand.

Synthesis and crystal structures of complexes between caesium thiocyanate and two isomeric macrocyclic crown polyethers, 6,7,9,10,17,18,20,21-octahydro-7R,9R,18S,20S-tetramethyldibenzo[b,k][1,4,7,10,13,16]hexaoxacyclo-octadecin (isomer F) and 6,7,9,10,17,18,20,21-octahydro-7R,9R,18R,20R-tetramethyldibenzo[b,k][1,4,7,10,13,16]-hexaoxacyclo-octadecin (isomer G)

Two isomers, F and G, of tetramethyldibenzo-18-crown-6 each with 4 chiral carbon atoms give complexes with caesium thiocyanate; CsNCS(F) consists of centrosymmetrical dimeric molecules with Cs–N–Cs bridges, the other co-ordinating atoms being the six oxygens of F at 3·07–3·34 Å and the aryl carbon atoms at 3·79 Å, while CsNCS(G)2 consists of Cs(G)2+ cations having symmetry with 12 Cs–O contacts at 3·17–3·43 Å and disordered thiocyanate ions.