Susan M. Dobson

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.

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.

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-

Crystallographic study of the barium(II) complex of a lariat ether with long pendent arms

Abstract The crystal structure of the Ba(II) complex of the ligand L (L = 7,16-bis(2-o-hydroxyethyl-2- oxyethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) is reported. The complex [Ba(L)]I2·3H2O crystallizes in the monoclinic space group C2/c with a=11.378(1), b=15.682(2), c=18.222(2) A, β=96.28(8)°; Z=4, V=3231.87 A3, Dm=1.80 g cm−3, Dc=1.82 g cm−3. The final conventional R factor was 0.032. The barium is eleven coordinate in the complex, with a water molecule occupying an apical position. The BaO oxygen bond lengths average 2.874(3) A, and the BaN bond lengths 3.079(3) A. The structure is used to rationalize the higher stability of the BaII complex of L than is found for other metal ions in terms of the ability of barium to coordinate all the donor atoms of the ligand.

The conformation of pentaazamacrocyclics. I. Crystal structures of nickel (II) and copper (II) complexes of 1,4,7,11,14-pentaazacycloheptadecane

The crystal structures of [Ni(17-aneN5)H2O]Br2·3H2O (17-aneN5=1,4,7,11,14-pentaazacycloheptadecane) and of [Cu(17-aneN5)]CuBr4 are reported. Diffraction data using MoKα radiation were measured with a CAD-4 diffractometer and the structures refined by full-matrix least squares. The nickel compound has regular octahedral coordination with coordinated water completing the octahedron. The copper is approximately square-pyramidal with an apical Cu-N bond of 2.29(4) Å, compared to a mean bond length of 2.06(2) Å in the basal plane. As a result, the folding of the 17-membered macrocyclic rings is completely different in the two compounds.

Control of metal-ion size-based selectivity through the structure of the oxygen-donor pendant groups on lariat ethers. A crystallographic and thermodynamic study

Ligand protonation constants and formation constants of complexes of Cu2+, Cd2+, Ca2+, Sr2+, Pb2+ and Ba2+ with 7,16-disubstituted 1,4,10,13-tetraoxa-7,16-diazacyclooctadecanes where the substituents are MeOCH2CH2(L4), [graphic omitted]HCH2(tetrahydrofurfuryl, L5), and HOCH2CMe2(L7) have been determined. Steric and inductive effects alter the selectivities of the ligands such that the stability order for L4 is Ba2+ > Sr2+ > Ca2+, but the reverse for L7. The structure of the complex [KL7]I has been determined: colourless crystals, orthorhombic space group P12121, with a= 11.507(4), b= 13.222(6) and c= 17.911(4)A, Z= 4 and R= 0.049. The absolute structure was determined by statistical analysis. The K–L bond lengths of the potassium complexes of L2(substituent HOCH2CH2), L4 and L7 vary considerably. The origins of this variation have been analysed using molecular mechanics calculations, and different approaches to modelling the K–O and K–N bonds are discussed.

Structural reinforcement of a large macrocyclic ligand. A structural, molecular mechanics, and thermodynamic study

The complex of the macrocycles L5(1,10-dioxa-4,7,13,16-tetra-azacyclo-octadecane), L6(4,13-dioxa-1,7,10,16-tetra-azabicyclo[14.2.2] eicosane) and L7(4,13-dioxa-1,7,10,16-tetra-aza-tricyclo[14.2.2.27,10]docosane) have been synthesized and their complex formation constants determined with Cu2+, Ni2+, Zn2+, Cd2+, and Pb2+; the ions Sr2+ and Ba2+ were found not to complex with the ligands. The successive insertion of ethylene bridges into the 1,4-diaminoethane units in L5 enhances the selectivity of the ligand for large metal ions, with L7 binding only to Pb2+. The selectivity of these ligands and the depression of the formation constants with all metal ions upon successive insertion of the ethylene bridges are rationalised with the aid of molecular mechanics calculations. The crystal structure of the free ligand of L7 has been determined: space group P21/n, monoclinic, with a= 6.053(2), b= 14.190(8), c= 10.346(6)A and β= 106.82(4)°, and Z= 2. The final conventional R factor was 0.0627.

A crystallographic study of a red and a blue complex of copper(II) formed with different geometric isomers of a reinforced macrocycle

Two complexes of copper(II), one red and one blue, with the macrocyclic ligand L3, have been prepared (L3 occurs as both the syn and anti form of 6-methyl-6-nitro-1,4,8,11-tetraazabicyclo-[9,3,2]hexadecane). A structural study of the two complexes shows the blue one to be [CuL3(NO3)]ClO4 and the red one to be [CuL3(H2O)][ClO4]2. In the blue complex the nitro group on the ligand is syn to the ethylene part of the double bridge, while in the red complex it is anti. The structural study shows, further, that the co-ordination geometry around the copper is different in the two complexes: mean Cu–N distance (blue) 2.00 A; (red) 1.99 A; Cu–O to the axial oxygen 2.20 (blue), 2.34 A(red); the copper atom lies out of the plane of the four N-donors by 0.34 (blue), 0.25 A(red). The difference in colour is related to the difference in co-ordination geometry. The crystals of both complexes were orthorhombic, space group pnma, with Z= 4. Other details: (red)a= 18.683(2), b= 8.897(1), c= 13.558(2)A and final conventional R factor of 0.0531; (blue)a= 20.417(5), b= 9.033(4), c= 10.870(2)A and R 0.0941.

Structural and thermodynamic study of the effect of sterically hindering alkyl groups on complex stability

The balance between steric and inductive effects on the thermodynamic stability of complexes in aqueous solution has been examined as the bulk of N-alkyl substituents is increased along the series methyl, ethyl, isopropyl, tert-butyl. The ligands 7,16-diisopropyl-1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane (L3), and RN[CH2CH(CH3)OH]2(R = Me, L4; Pri, L5; or But, L6) have been prepared. The formation constants of these ligands, and of other ligands with bulky N-alkyl groups, have been determined with the metal ions CuII, ZnII, CdII, PbII and with CaII, SrII and BaII(L3 only). The N-isopropyl groups of ligand L3 cause a sharp drop in thermodynamic complex stability for all metal ions, whereas in the series L2–L4 lead(II) is unusual in showing a strong increase in complex stability in response to the inductive effects of the larger alkyl groups. In an attempt to understand the lowering of complex stability by the N-isopropyl groups of L3 the crystal structure of the complex [KL3]I was determined: monoclinic, space group P21/c, a= 12.603(2), b= 14.875(3), c= 13.877(3)A, β= 112.81(1)°, Z= 4, final R= 0.042. The structure indicates some stretching of the K–N bond by the presence of the N-isopropyl groups. The parallel between steric hindrance effects by bulky N-alkyl groups and hard and soft acid and base behaviour is discussed.