Darren S. Baldwin

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.

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.

Ligand design and metal-ion recognition. Comparison of the interaction of cobalt(II) and nickel(II) with 16- to 19-membered mixed-donor macrocycles

The interaction of cobalt(II) and nickel(II) with a range of 16- to 19-membered ring macrocycles incorporating nitrogen, oxygen and/or sulfur heteroatoms is reported. These ligands constitute an extensive array of related macrocyclic structures in which the positions of the donor atoms, their spacing, and the macrocyclic ring size all vary in a systematic manner. Emphasis has been given to the examination of structure–function relationships in the complexation behaviour. Physical measurements confirm the 1 : 1 metal to macrocyclic ligand stoichiometry of the respective complexes. Stability constants for the metal complexes have been determined potentiometrically in 95% methanol (I= 0.1 mol dm–3, NEt4ClO4). An X-ray crystallographic study of [NiL18(H2O)][NO3]2(L18= 5,6,7,8,10,11,12,13,19,20-decahydrodibenzo[f,I][1,8,11,4,15]oxadithiadiazacycloheptadecine) confirms that the nickel ion is six-co-ordinate with the complex cation exhibiting a distorted-octahedral geometry defined by all five donor atoms of the ON2S2 macrocycle and a water molecule; the macrocyclic backbone incorporating the N–O–N donor fragment is arranged meridionally. Molecular mechanics modelling of selected nickel(II) complexes has also been undertaken. As well as their considerable intrinsic interest, the results provide a potentially useful background upon which the design of new reagents for metal-ion discrimination may be based.

New macrocyclic ligands: XIV. synthesis and X-ray structures of potentially pentadentate ligands incorporating non-symmetrically arranged N4S-, N3OS-, N2O2S- and N2S2O-heteroatoms

The synthesis and characterization of new mixed-donor macrocyclic ligands incorporating nitrogen, sulfur and/or oxygen heteroatoms are described. The new 17- or 18-membered macrocyclic rings contain unsymmetrical arrangements of their heteroatoms in contrast to related, previously reported rings in which the donor sets are arranged symmetrically. The X-ray structures of the 17-membered rings incorporating N4O- and N4S-donor sets are presented.

A new series of quinquedentate macrocycles exhibiting systematic donor-atom variation: equilibrium and X-ray structural data for their interaction with copper(II)

A series of nine seventeen-membered macrocyclic ligands in which the donor atom set varies systematically is reported; each ligand reacts with CuII to yield a 1:1 complex, the stability of which is markedly dependent on the donor atom pattern; the X-ray structure of the CuII complex of the N4S donor confirms that all its donor atoms co-ordinate in the solid state; the structure of this metal-free macrocycle is also reported.

Dalton communications. Metal-ion recognition. Donor-set control of silver(I)/lead(II) discrimination using mixed-donor macrocyclic ligands

The systematic variation of the donor atom set in a dibenzo-substituted, 17-membered ring structure on the ability of the resultant systems to discriminate between silver(I) and lead(II) has been performed; the compound containing a S2N2S-donor set yielded discrimination of the order of 109 in favour of silver(I).