Kenneth R. Adam

Comparative molecular mechanics study of the low-spin nickel(II) complexes of an extended series of tetraaza macrocycles

Molecular mechanics calculations of metal complex structures are often not straight forward because of uncertainties concerning the appropriate force-field parameters for structural elements involving the metal. A systematic study aimed at extending the Allinger MM2 force field for use with N 4 -macrocyclic complexes of low-spin Ni(II) is reported. The application of the extended force field to the re-examination of the configurational isomers of [Ni(cyclam)] 2+ and their N,N,N,N-tetramethylated derivatives is reported

Studies involving nitrogen–oxygen donor macrocyclic ligands. Interaction of copper(II) with new O2N3-donor macrocycles and the X-ray analysis of aqua(1,12,16-triaza-3,4:9,10-dibenzo-5,8-dioxacyclo-octadecane)copper(II) diperchlorate hydrate

The interaction of Cu2+ with three new O2N3-donor macrocycles has been investigated. Complexes of the type CuL(ClO4)2.nH2O and CuLX(ClO4).nH2O (where L = macrocycle, X = Cl or Br, and n= 0, 1, or 2) have been isolated. Conductance studies indicate that the halide ions in the latter complexes are co-ordinated in dimethyl sulphoxide. All complexes show the expected magnetic moments for Cu2+ with S=½. Potentiometric titration studies in 95% methanol (I= 0.1 mol dm–3, [NMe4]Cl) indicate that the following equilibria involving Cu2+ occur : Cu2++ L ⇌ CuL2+ and Cu2++ HL+⇌ Cu(HL)3+ and the respective formation constants have been determined. The X-ray structure of [CuL3(OH2)][ClO4]2·H2O indicates that this complex has a pseudo-square-planar co-ordination geometry with the co-ordination sphere consisting of the three nitrogen atoms of the macrocycle [Cu–N, 1.97(3)–2.07(2)A] together with an oxygen from water [Cu–O, 1.96(2)A]. The ether oxygen donors of the macrocycle do not co-ordinate. Crystals are monoclinic, with space group P21/c, a= 16.856(7), b= 10.717(4), c= 16.738(18)A, β= 110.59(5)°, and Z= 4, R being 0.12 for 1 044 ‘observed’ reflections.

On the nature of the host–guest interaction between cyclam and 4-tert-butylbenzoic acid—a system pre-assembled for metal complex formation

The host–guest interaction between cyclam and 4-tert-butylbenzoic acid has been investigated by NMR titration, X-ray diffraction, neutron diffraction and semi-empirical MO calculation; the product represents a system preassembled for metal-ion complexation.

Comparative molecular mechanics study of the high-spin nickel(II) complexes of an extended series of tetraaza macrocycles

Abstract Force field parameters for modeling tetraaza macrocyclic ligand complexes of high-spin Ni(II) are reported. The MM2 force field has been modified and extended to include systems containing a tetraaza macrocyclic ligand coordinated around four positions of an octahedral nickel ion (that contains halide or other monodentate ligands in the remaining coordinations sites). X-ray structural data for twenty-one such high-spin complexes have been used in the calibration of an extended force field which is capable of modeling the geometries of complexes within the above category. However, it was found that relatively minor changes in particular force field parameters may lead to considerable variation in the relative strain energies for different stereoisomers of a given complex type. Comparative strain energies for metal complexes obtained in this manner should be considere, at best, to be only semiquantitative.

Evaluation of the semiempirical PM3(tm) method for modeling high- and low-spin nickel(II) complexes of an extended series of tetraaza macrocycles

Abstract The semiempirical PM3(tm) program, employing the recently released parametrization for high- and low-spin nickel(II), has been evaluated for modeling the structures of both high- and low-spin nickel(II) complexes of an extended series of tetraaza macrocyclic ligands. The results obtained were compared with those available from our previous parallel studies using the molecular mechanics procedure based on an extended MM2 force field. In general terms, the semiempirical method was found to be clearly less effective, using the available parametrization, than the molecular mechanics studies for modeling the structures of the above categories of nickel complexes.

Metal-ion recognition. Modeling the stability constants of some mixed-donor macrocyclic metal ion complexes—a simple model

A procedure for modeling the stability constants of the Zn(II), Cd(II), Ag(I) and Pb(II) complexes of an extended series of 17-membered, mixed-donor macrocycles incorporating nitrogen, oxygen and/or sulfur donor atoms is presented. The ligands fall into two categories—those incorporating a symmetrical arrangement of their donor atom set (9 examples) and those in which the donor set has an unsymmetrical arrangement (5 examples). Metal stability constants for the former have been reported previously while corresponding data for the latter was determined as part of the present investigation. Initial computations were based on the 1:1 stability constants for the systems incorporating a symmetrical arrangement of their donor atom set. The approach employed was to assume that the overall free energy for metal complexation can be partitioned into the sum of contributions from individual metal–donor interactions. Thus for a given metal ion type, the log K data corresponding to each of these ligand systems were employed to derive parameters that are characteristic of the respective metal ion–donor bond types present. The parameterisation derived from the above (previously reported) stability data was then employed to compute log K values for the complexes of both the symmetrical and unsymmetrical ligand series. In general, quite good agreement with the experimentally determined log K values for both series of complexes was obtained. The procedure thus points the way for the prediction of metal complex stabilities for other systems that incorporate closely related ligand types.

The Interaction of Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) with Mixed Donor Macrocycles Containing Pendant Carboxylic Acid Functions

The syntheses of mixed oxygen-nitrogen donor macrocycles incorporating two or three pendant carboxylic acid groups are described. Potentiometric titrations in water (I = 0.1; KNO3) at 25°C have been used to determine the stability constants for the 1: 1 (metal:ligand) complexes of Co(II). Ni(II), Cu(II), Zn(II), and Cd(II). The constants obtained are compared with the previously determined values for the corresponding complexes of the unsubstituted macrocyclic precursors. The results of these studies indicate that each carboxylate function participates in binding to the central metal. For some metal-ion/ligand systems there is evidence that ring size effects influence the overall stability patterns and that, in such cases, both the ether oxygens as well as the tertiary amines of the macrocyclic rings appear to bind to the metal.

Interaction of transition and post-transition metal ions with oxygen–nitrogen donor macrocycles incorporating pendant hydroxyethyl and carbarnoylethyl groups

The interaction of new derivatives of O2N2-donor macrocycles incorporating pendant hydroxyethyl or carbamoylethyl arms with cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) has been investigated. The thermodynamic stabilities of a range of 1 : 1 (metal:ligand) complexes of these macrocycles have been determined potentiometrically in 95% methanol (I= 0.1 mol dm–3, NEt4ClO4 or NMe4Cl; at 25 °C). Comparison of selected complexes with those of the corresponding parent (non-pendant arm) macrocycles indicates that the former derivatives show more varied co-ordination behaviour than the latter. X-Ray diffraction studies of five nickel(II) complexes have been made. The complexes containing respectively 14-, 15- and 16-membered rings and two pendant alcohol arms have similar distorted-octahedral structures. In each case the nickel ion is situated outside the macrocyclic ring with only the ring nitrogen atoms and the pendant alcohol oxygen atoms co-ordinating; the remaining sites are occupied by the oxygen atoms of a bidentate nitrate ion. In contrast, the structure of the related 15-membered, mono-pendant arm (alcohol) derivative has the nickel in the macrocyclic cavity; the near-octahedral co-ordination sphere consists of two ether oxygens and two nitrogens from the macrocyclic ring in the equatorial plane together with oxygen atoms from the pendant alcohol arm and a water molecule in the axial sites. The structure of a pendant bis(amide) derivative also incorporates the nickel in the macrocyclic cavity with the pendant amide groups co-ordinated in axial sites to yield an overall distorted-octahedral geometry.

Ligand design and metal-ion recognition. The interaction of copper(II) with a range of 16- to 19-membered macrocycles incorporating oxygen, nitrogen and sulfur donor atoms

The interaction of copper(II) with a series of quinquedentate macrocycles incorporating nitrogen, oxygen, and/or sulfur donor atoms is reported. These ligands form part of a structural matrix in which a systematic variation in donor atom pattern and ring size is present (and against which structure–function relationships are more clearly seen). Stability constants for the 1 : 1 copper complexes have been determined in 95% methanol (I= 0.1 mol dm–3, NEt4ClO4) at 25 °C. The study demonstrates the extremely large stability differentials [up to 1010 for copper(II)] that may be achieved through structural variation within the ligand framework employed for this study. The contribution of the respective donor atom types to overall complex stability was found to follow the expected order of NH(aliphatic) > NH(anilino) > S > O. Based on earlier X-ray studies, the molecular mechanics modelling of two representative examples is also reported.

Macrocyclic ligand design. Effect of donor-set and ringsize variation on silver(I)/lead(II) discrimination within an extended series of dibenzo substituted rings

The effect of systematic variation of the donor-atom set within dibenzo-substituted, 17- to 20-membered macrocycles on their ability to discriminate between silver(I) and lead(II) has been investigated. The log K values for the respective complexes, determined potentiometrically in 95% methanol (l= 0.1 mol dm–3, NEt4ClO4), show that successive replacement of either nitrogen or oxygen heteroatoms by thioether donors leads to increasing affinity for silver(I) relative to lead(II), as does the incorporation of an ‘aliphatic’ NON or NSN donor sequence in the structure. For the 17-membered system incorporating a S2N2S-donor set, exceptional discrimination (of the order of 109)was observed. The characterisation of selected complexes has included the crystal structure of the silver perchlorate complex of the 17-membered ON2S2macrocycle (L17). The silver ion in [AgL17]ClO4 has a very distorted trigonal-bipyramidal geometry with all metal to donor bond lengths appearing normal. The complex crystallises in the monoclinic space group P21/n with lattice parameters a= 19.410(4), b= 9.602(2), c= 12.747(3)A, β= 103.67(2)° and Z= 4.