Jm Harrowfield

The Synthesis and Structure of Encapsulating Ligands: Properties of Bicyclic Hexamines

Template syntheses based on tris (ethane-1,2-diamine)cobalt(III) lead to cobalt(III) complexes of cage hexamines of the sarcophagine type ( sarcophagine = sar = 3,6,10,13,16,19- hexaazabicyclo [6.6.6] icosane ) rapidly and in high yield. Reduction of these species to their cobalt(II) forms enables the ligands to be removed in concentrated acids at elevated temperatures, and in hot aqueous solutions containing excess cyanide ion. The free sarcophagine and 1,8-diaminosarcophagine [(NH2)2sar or diamsar] ligands are strong bases, accepting up to four and five protons, respectively, in aqueous solution. In chloride medium, I = 1.0, at 298 K, pK1 = 11.95, pK2 = 10.33, pK3 = 7.17, pK4 ≈ 0 for sarcophagine , and pK1 = 11.44, pK2 = 9.64, pK3 = 6.49, pK4 = 5.48, pK5 ≈ 0 for diaminosarcophagine , with very similar values being found for triflate medium. Crystal structure determinations for both free bases, the chloride, sulfate, perchlorate and nitrate salts of diamsar , the complex of zinc chloride with sar, and the magnesium nitrate complex with diamsar show remarkably small variations in the cavity defined by the bicyclic ligands, though relatively subtle bond length and bond angle changes can be rationalized in terms of the effects of proton and metal ion binding. Exhaustive methylation of sarcophagine produces the highly lipophilic, hexatertiary base hexamethylsarcophagine , which, in the solid state, adopts quite different conformations and nitrogen-atom configurations to those of sar itself. All the ligands rapidly form metal ion complexes of generally exceptional kinetic and thermodynamic stability.

Synthesis with Chelated Imino Carboxylates: The Influence of Chelate Coligands on Reaction Pathways

Synthesis and X-ray crystallography have been used to explore the chemistry of chelates derived from the imines of pyruvic and phenylpyruvic acids held in a chiral environment on cobalt(III) as [Co(en)2(HNC(CH3)CO2)]2+ (en = ethane-1,2-diamine) and [Co(en)2(HNC(CH2C6H5)CO2)]2+. The latter complex was obtained by an especially facile dehydration of the corresponding β-phenylserine complex. Significant differences in both stereo- and regio-selectivity were detected in reactions of the two complexes, and, for the reaction of butenone with [Co(en)2(HNC(CH3)CO2)]2+, a pathway indetectable in previously studied reactions of the tetraammine analogue, and leading to a chelate of 4-methyl-5,6-dihydropyridine-2-carboxylate, was observed to be dominant. Kinetically determined stereospecificity was evident in the addition of ethyl cyanoacetate to [Co(en)2(HNC(CH3)CO2)]2+, whereas equilibrium control appears to exist in the stereoselective addition of nitromethane to both complexes. Crystal structure determinations on five new complexes have been used to establish these points and other important aspects of the chemical syntheses. A considerable number of new imino and amino acids can be derived through the reactions described, and the synthetic utility of both the tetraammine and bis ( ethanediamine ) complex ion systems in providing the free organic products has been subjected to preliminary evaluation through the isolation of t- leucine , N-methyl-2-phenylglycine, 2-aminomethylalanine, and some vinylglycine derivatives.

Calixarene Phosphorylation: Cation Control of Reaction Pathways

The hydroxide ion catalysed phosphorylation of p-t- butylcalix [4] arene by diethyl chlorophosphate under heterogeneous conditions shows a marked sensitivity to the nature of the phase transfer catalyst used. Thus, substitution of tetraethylammonium bromide for the tetrabutylammonium bromide prescribed for the synthesis of the calixarene tetraphosphate leads to a major reaction product becoming a bis (phosphate) in which the calixarene binds as a unidentate ligand to one phosphorus and as a bidentate ligand to the other. An X-ray crystal structure analysis on this new compound shows the calixarene to adopt a somewhat distorted cone conformation with an unusual orientation of one of the t-butyl substituents into the cone cavity. It also reveals that a significant diastereoselectivity must operate in the formation reaction. The only other reaction product detected in this system appears to be the calixarene tetraphosphate in a partial cone conformation.

Syntheses and Structures of Manganese(II) and Manganese (III) Nitrate Diaminosarcophagine Complexes

The syntheses of [ Mn ((NH3)2sar)](NO3)4.H2O and [ Mn ((NH3)2sar)](NO3)5.2H2O, manganese(II) and manganese(III) complexes of the cage amine ligand diaminosarcophagine ( di-aminosarcophagine = (NH2)2sar = 1,8-diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6] icosane ) in its diprotonated form are recorded, together with their single-crystal X-ray structure determinations at c. 295 K. The monoclinic P21 array of the manganese(II) complex (a 12.386(5), b 12.431(4), c 8.598(4) Ǻ, β 93.89(4)°, V 1321(1) Ǻ3,Z 2) is archetypical for similar complexes of a wide variety of transition metals; for the present determination, R was 0.027 for 2013 observed (I > 3σ(I)) reflections. The manganese(III) complex is monoclinic C 2/c, a 10.744(2), b 13.294(4), c 20.462(9) Ǻ, β 102.38(3)°, Z 4; R was 0.055 for 1629 observed reflections. Both structures show the six secondary nitrogen atoms of the ligand to be bound to the manganese ion in a configuration approximately halfway between a trigonal prism and an octahedron. The ligand is in the lel3 conformation. In the first complex, Mn -N distances, appropriate to high-spin manganese(II), range from 2.228(3) to 2.253(3) Ǻ, mean 2.238 Ǻ; in the second, surprisingly, the distances are even more closely ranged (unlike those of the sarcophagine analogue of the previous paper), 2.115(4)-2.127(4) Ǻ, the mean (2.122 Ǻ) being closely comparable to that recorded for the sar analogue, and show no appreciable variation attributable to the expected Jahn-Teller effect.

Synthesis, Structure and Redox Properties of Nickel Complexes of Cage Amine Ligands

The syntheses of complexes containing the [Ni( sar )]2+/3+ and [Ni((NH3)2sar)]4+ ions are described along with an X-ray crystal structure analysis of the salt [Ni((NH3)2sar)] (NO3)4.H2O ( sar is 3,6,10,13,16,19-hexaazabicyclo[6.6.6] icosane ; (NH3)2sar2+ is its 1,8-diammonio derivative). The NiIII ion is a powerful oxidant, with E′ = 0.90 V (v. n.h.e. at 298 K, I = 0.2, aqueous trifluoromethanesulfonate medium), but is relatively stable in dilute aqueous acid. Both the NiII and NiIII complexes of sar have been resolved into their enantiomeric forms, and their absorption, optical rotatory dispersion and circular dichroism spectra recorded and partly analysed. The electron self-exchange rate between enantiomeric forms in the different oxidation states has been measured by a stopped-flow circular dichroism method, and ket = (5.3±0.3)×103 dm3 mol-1 s-1 at 298 K, I = 0.2 (NaCF3SO3). The activation parameters, ΔH‡ 22±4 kJ mol-1 and ΔS‡ -100±12 J K-1 mol-1, and the rate constants are consistent with the comparatively small rearrangements required in the structures of the two ions relative to the optimal cavity radius for the cage of 2.05(1) Ǻ.