Jan C. A. Boeyens

The conformation of six-membered rings

Although an indefinite number of conformations exists for six-mem- bered rings, and although there is general consensus about the six classical forms (Bucourt and Hainaut, 1965; Cano et al., 1977; Romers et al., t974), there is no generally accepted nomenclature or symbolic formalism to distinguish among them. A quantitative basis for a classification exists in the literature (Cremer and Pople, 1975), and the purpose of this communication is to propose a logical nomenclature adapted to the classification. The analysis starts out from crystallographic fractional coordinates and involves transformation first to a set of Cartesian coordinates, and eventually to Cartesian coordinates of pucker for the ring system under consideration. From these coordinates, a set of three parameters of pucker in the form of polar coordinates (Q, O, ~) is obtained. These coordinates map out the conformation of the ring on the surface of a sphere, radius Q, and with poles at O = 0, 180% as suggested by Hendrickson (1967). Figure 1 depicts the surface of this sphere in two-dimensional polar projection. Each of the hexagons represents a canonical conformation in terms of the six classical forms. The symbols on the sides of the hexagons indicate the signs of the endocyclic torsion angles: the torsion angle definition of Klyne and Prelog (1960) is used. The established terminology for chair, boat, and half-chair forms is retained. Of the variant forms twist-boat/skew-boat and envelope/half- boat/sofa, the first form is preferred in each case. For the 1,3-diplanar form, the name screw-boat is proposed. This scheme provides a set of names with six different initials which can conveniently be used to represent the various types. For a unique description of ring conformation, a well-defined atomic numbering scheme is needed next. Whenever a single atom in a ring (e.g., 317

The stereochemical activity or non-activity of the inert pair of electrons on lead(II) in relation to its complex stability and structural properties: some considerations in ligand design

Abstract The role of the lone pair of electrons on Pb(II) in its coordination geometry and complex stability is examined. In a series of macrocyclic ligands where oxygen donors are successively replaced by nitrogen donors, it is found that when three or four nitrogens are present, there is a sudden marked increase in the rate of change of complex stability per nitrogen donor added. This is attributed to a change from a stereochemically inactive lone pair with approximately two or fewer nitrogen donors present, to an active lone pair. Below the transition point, the Pb(II) ion behaves as a large metal ion with rather ionic ML bonding. In this state it responds to added oxygen donor bearing groups as expected for such a metal ion. Thus, the size-related selectivity patterns of Pb(II) with the ligand DAK-22 (4,7,13,16-tetraoxa-1,10-diazacyclooctadecane-N,N′-diacetate) are as expected for its size. The protonation constants and formation constants of DAK-22 with several metal ions are reported. For the complexes formed by 12-aneN4 (1,4,7,10-tetraazacyclododecane) and 12-aneN3O (1-oxa-4,7,10-triazacyclododecane) the Pb(II) appears to have a stereochemically active lone pair. Thus, when N-(2-hydroxypropyl) groups are added to 12-aneN4 and 12-aneN3O to give the ligands THP-12-aneN4 and THP-12-aneN3O, the Pb(II) ion does not respond to the added hydroxyalkyl groups as might have been expected. It behaves as a smaller more covalent ion, and a study of the formation constants of THP-12-aneN4 and THP-12-aneN3 with Cu(II), Zn(II), Cd(II), Pb(II), Ca(II), Sr(II) and Ba(II) reveals lower than anticipated Pb/Zn selectivities. A crystallographic study of [Pb(C20Hn44N4O4)](NO3)2·C3H8O reveals that there is space between the O donors for a stereochemically active lone pair, but the lack of shortening of the PbN bonds suggests that the lone pair is not active. The complex crystallizes in the orthorhombic system, space group Pnma, with cell dimensions a=10.352(8), b=14.781(2), and c=21.850(4) A, Z=4. A final conventional R=0.056 was obtained. Although the ligand THP-12-aneN4 has four chiral carbon atoms, the crystal structure suggests that only the RRRR and SSSS enantiomers of the free ligand are obtained after recrystallisation from n-hexane. The structure indicates that the [Pb(THP-12-aneN4)]2+ cations are disordered, with 50% site occupancy by the RRRR and by the SSSS conformer.

Conformational analysis of ring pucker

The conformation of a general puckered ring is defined by a linear combination of normal atomic displacements, according to the irreducible representations of the DNh symmetry group. Each tw.odimensional representation contributes two uniquely defined primitive modes, superimposed on a onedimensional crown form that only exists for N even, adding up to N-3 primitive forms, for any N. The normalized linear coefficients are independent of the amplitude of pucker and of the ring numbering scheme. The formalism applies to any ring type and a quantitative characterization of conformations, intermediate between the conventional classical forms, is possible. It provides the basis for mapping conformations as a function of puckering parameters and a simple algorithm for the identification of the classical forms. The procedure relates general ring conformations to a few simple shapes, familiar to chemists, without losing the advantage of quantitative puckering analysis.

Molecular mechanics: theoretical basis, rules, scope and limits

Abstract Molecular mechanics is a simple model, and it is based on a classical parameterization of non-classical effects for the computation of molecular structure. The basic concept of force field calculations is discussed and, based on the theoretical frame, the emerging scope and limits of the approach, and rules for the application, interpretation of the results and their communication are presented.

The synthesis of complexes of novel structurally reinforced tetraaza-macrocyclic ligands of high ligand field strength: a structural and molecular mechanics study

Abstract The synthesis of the complexes of low-spin Ni(II) with the three novel ligands shown below is described. Molecular structures of these complexes have been determined by single-crystal analysis. Crystal data are as follows. Complex I, monoclinic, space group P21/n, with cell dimensions a = 8.692(1), b = 10.867(2) and c = 21.618(4) A and β = 93.00(1)°, Z = 4; final conventional R = 0.081. Complex II, orthorhombic, space group Pmcn, with cell constants a = 9.178(1), b = 14.936(2), c = 30.317(5) A and angles α = 90.00(1)°, β = 90.00(7)°, γ = 90.00(8)°, Z = 8; final R = 0.060. Complex III, monoclinic, space group C2/c, with cell dimensions a = 18.723(5), b = 10.710(2) and c = 21.162(7) A and β = 94.92°, Z = 8; final conventional R = 0.047. The complexes of low-spin Ni(II) have average NiN bond lengths of 1.91 (I), 1.89 (II), and 1.89 (III) A, which are very close to the strain-free NiN bond length of 1.91 A. The Ni(II) complex of III exhibits the highest ligand field (LF) strength reported to date for a complex of low-spin Ni(II) with saturated nitrogen donor groups. The crystallographic study on the complex shows that there is no significant shortening of the NiN bond, which supports the idea that it is the presence of the high donor-strength tertiary and secondary nitrogens in a system of low steric strain which leads to the high LF strength. Ways of designing tetraaza-macrocycles of even higher LF strengths are discussed. Molecular mechanics calculations are used to predict the steric strain in these target complexes and to explain why these macrocycles could not be synthesized.

Identification of the conformational type of seven-membered rings

A scheme for the complete characterization of seven-membered ring conformation is described. Parameters of puckering which are calculated directly from crystallographic coordinates are used to map the conformation onto a torus in relation to the various symmetrical forms.

Metal ion size selectivity of 1-Thia4, 7-diazacyclononane (9-aneN:S), and other tridentate macrocycles. A study by molecular mechanics calculation, structure determination, and formation constant determination of complexes of 9-aneN 2S

Abstract Several aspects of the coordination chemistry of the tridentate cyclononane type macrocycles are examined using molecular mechanics calculations, crystallography and formation constant determinations. The molecular mechanics calculations show that small metal ions coordinate best to these ligands, such that metal ions with a covalent radius of 1.25 A fit best into 9-aneS3 and 1.40 A fit best into 9-aneN3 (9-aneS3 = 1,4,7-trithiacyclononane, 9-aneN3 = 1,4,7-triazacyclononane). For mixed donor members of the series such as 9-aneN:S (1-thia4,7-diaza- cyclononane) the disparity in M-L bond length between the M-N and M-S bond lengths leads to a much higher strain situation than expected from the strain energies of the 9-aneN3 and 9-aneS3 complexes. This accounts for the order of ligand field strength in complexes of these ligands of 9-aneS3 > 9-aneN3 > 9-aneN:S. It is concluded that in the absence of the strain effects encountered in mixed donor ligands containing the thioether donor group, the latter group should always be higher in the spectrochemical series than ligands containing the secondary nitrogen donor. The formation constants of 9-aneN:S with Ni(II), Zn(II), Cd(II), Co(II), Fe(II), and Pb(II) are reported. Comparison of these with the formation constants for the 9-aneN:O and 9-aneN3 complexes shows that the macrocyclic effect (the difference in stability between the complex of the macrocycle and of its open chain analogue) is much higher for small metal ions, and small with large metal ions, in agreement with the molecular mechanics calculations which show that the cyclononane macrocycles coordinate best with small metal ions. The crystal structure of the complex [Cu(9-aneN:S)Br:] is reported: monoclinic, space group P21/n, with cell dimensions a = 7.603(1), b = 13.167(2), and c -- 10.873(2) A, and β = 91.94- (1)°, Z = 4. Final conventional R = 0.061. The un-

Sulphur-containing metal complexes

Abstract The sulphides SR1R2 (R1 = C2H5; R2 = C2H5, CH2Ph) react with the carbene complexes [(CO)5CrC(OC2H5)R] (R = Bu, Ph), to produce the neutral pentacarbonyl(thio)chromium(0) complexes [(CO)5CrSR1R2], which have been characterized by chemical analysis, IR, NMR and mass spectra. Confirmation of the structures of the new complexes comes from a single-crystal X-ray study of (CO)5CrS(C2H5)CH2Ph, the first such study of a pentacarbonyl(dialkylthio)metal complex.

Mapping the conformation of eight-membered rings

Etude permettant de decrire les conformations de cycle a 8 chainons a partir des coordonnees atomiques cartesiennes et cristallographiques

Crystal and molecular structure of tris (2,2,6,6-tetramethyl-3,5-heptanedionato) aquodysprosium(III), Dy(thd)3H2O

The crystal structure of Dy(thd)3H2O has been solved by three-dimensional X-ray methods at room temperature. The space group isP¯1 and the cell dimensions area = 14·21(1),b = 14·88(2),c = 11·60(2) Å, α = 99·76(3), β = 109·91(1), γ = 114·08(1) °.Z = 2,Dm = l·24 andDx = 1·238 gcm−3, respectively. Full-matrix least-squares refinement of atomic and individual isotropic thermal parameters, using 3015 intensities obtained by counter methods, terminated with a conventionalR of 0·058. The oxygen polyhedron around dysprosium has pure seven-coordination geometry, the seventh ligand being the water molecule. The water is hydrogen bonded to two oxygen atoms of a centre of symmetry related formula unit so that two formula units are in fact held together by hydrogen bonds in a dimer.