Peter W. Wade

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.

Structurally reinforced macrocyclic ligands

The coordinating properties of structurally reinforced macrocyclic ligands are discussed. The ligand 6-1 2-aneN4 (1,4,7,1 O-tetraazabicyclo(8.2.2)tetradecane) has a very small cavity, and is able to compress the low-spin Ni(ll) Ion. In spite of the resulting high steric strain in (Ni(B-12-aneN,))2t, the complex is of high stability, because the free ligand itself is also of very high steric strain. In complexes of Cu(ll) with unreinforced macrocycles with cavities that are too large for the Cu(ll), the CU-N bonds are not stretched, but rather the coordination geometry of the Cu(ll) is distorted toward tetrahedral, and normal CU-N bonds are found. In contrast, a large cavity reinforced macrocycle such as NE-3,3-HP (1 1-methyl-1 l-nitro-1,5,9,13-tetraazabicyclo-(l1.3.2)tetradecane) is able to stretch the Cu-N bonds out to 2.09 A, since the homopiperazine reinforcing bridge prevents buckling of the ligand. However, with further increase in macrocyclic ring size, buckling of the ligand and tetrahedral distortion of the Cu(ll) occurs, showing the need for even more rigid reinforcing groups. The synthesis and properties of ligands containing the bispidine group, which has an adamantane-like structure, as a reinforcing group, is discussed.

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.

Metal ion size selectivity in ligands with groups containing the neutral oxygen donor atom. A crystallographic and thermodynamic study

Abstract The coordinating properties of open-chain ligands containing alcoholic or ethereal oxygen donors are examined. Addition of oxygen donors usually leads to complex stabilisation for large metal ions (Pb2+, Cd2+) and to less favourable effects on complex stability for small metal ions (Cu2+, Ni2+). The formation constants of these metal ions with the set of ligands RN(CH2CHOH·CH3)2 where R is H, CH2CHOH·CH3, CH2CH2OCH3, CH2CH2OCH2CH2OH, and CH2CHOCH2CH2CH2 are reported. The largest stabilisation for each case where R is an O-donor group relative to R = H occurs for Pb2+, the largest metal ion, while Cu2+, the smallest metal ion, shows the smallest stabilisation. The crystal structure of [Ni(HOCH2CH2NHCH2CH2NH2)2] (NO3)2 is reported. The space group is P 1 , with cell constants a = 13.098(3), b = 8.737(4), and c = 7.746(3) A, β = 112.66(3), β = 90.65(3), and γ = 85.03(2), and Z = 2. Disorder of the nitrate anions hindered refinement, with the result that a final conventional R factor of 0.0903 was achieved. The NiN bond lengths average 2.06(1) (secondary nitrogen) and 2.10(2) (primary nitrogen). The NiO bond lengths are rather long, averaging 2.15(1) A, which is used to support the idea that the steric effects are responsible for destabilising the complexes of small metal ions such as Ni(11) when neutral oxygen donors are present.

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.

Crystallographic and molecular mechanics study of the copper perchlorate complex of a larger reinforced macrocycle

The crystal structure of the five-co-ordinate copper(II) complex of 7-methyl-7-nitro-1,5,9,13-tetraazabicyclo[11.2.2]heptadecane, [CuL7(H2O)][ClO4]2·H2O, has been determined. The blue crystals crystallised in the orthorhombic space group Pbca with a= 14.625(4), b= 19.083(2), and c= 17.476(4)A, Z= 8, and R= 0.059. The copper ion is co-ordinated to four macrocyclic nitrogen atoms with an average Cu–N distance of 2.05 A, and a water molecule occupies an axial co-ordination site with Cu–O 2.22 A. The conformation and chemical properties of the complex are discussed with the aid of molecular mechanics calculations.

Structurally reinforced macrocyclic ligands that show greatly enhanced selectivity for metal ions on the basis of the match in size between the metal ion and the macrocyclic cavity

Addition of an extra bridging group between nitrogen donors on macrocyclic ligands to give a piperazine-like structure leads to much more rigid ligands which display selectivity for metal ions which is more strongly based on the match between the size of the metal ion and of the macrocyclic cavity than is the case for conventional macrocyclic ligands.

Crystallographic and molecular mechanics study of a cobalt(III) complex of a structurally reinforced macrocycle

The complex [CoL6(Cl)][CoCl4]·H2O, where L6 is the reinforced macrocycle 7-amino-7-methyl-1,5,9,13-tetraazabicyclo[11.2.2]heptadecane, has been prepared and its crystal structure determined. The green-black crystals are monoclinic, space group P21/c, with a= 8.778(2), b= 13.344(4), c= 19.380(8)A, β= 98.48(2)°, Z= 4, and a final conventional R= 0.0736. The cobalt is co-ordinated in a planar fashion in the cavity of the macrocycle, with one axial co-ordination site occupied by the pendant primary amine group, and the other by a chloride. The Co–N bond lengths to the tertiary nitrogens of the piperazine bridge of the macrocycle are 2.01(1) and 2.02(1)A, distinctly longer than those [1.97(1)A] to the secondary amines in the cavity of the macrocycle. The Co–N distance to the pendant primary amine is 2.01(1)A, and Co–Cl to the axially co-ordinated chloride trans to the pendant amine group is 2.26(1)A. Molecular mechanics calculations are used to analyse the highly distorted co-ordination sphere around the CoIII, where the N–Co–N angle involving the nitrogens of the piperazine bridge of the macrocycle is only 73.5(4)°. The calculations reproduce the co-ordination geometry around the CoIII well, including the variation in Co–N bond length and the highly distorted N–Co–N angles.