Colin Cairns

Binuclear copper(II) complexes of a 24-membered macrocyclic Schiff base ligand containing intramolecularly bound substrate molecules and ions: X-ray structure of a µ-imidazolate complex

In some bi-CuII complexes of a 24-membered macrocyclic Schiff base ligand, the metal centres are linked intramolecularly by the imidazolate anion, pyrazine, and other small bridging ligands; the X-ray structure of a µ-imidazolate complex is described.

Nickel(II) complexes of quinquedentate macrocyclic ligands and the crystal and molecular structure of the ring-opened hydrolysis product [11-(6-acetyl-2-pyridyl)-3,7,10-triazadodec-10-enylamine-NN′N″N‴N″″]-aquanickel(II) diperchlorate

In contrast to several other first-row transition-metal (and non-transition-metal) ions, NiII is ineffective as a template for the cyclic condensation of 2,6-diacetylpyridine with 4,7-diazadecane-1,10-diamine and related tetraamines to yield quinquedentate macrocyclic ligands. However, nickel(II) complexes of the 17-membered macrocycle L3 may be prepared by replacement of AgI from the complex [AgL3][ClO4]. On the basis of electronic spectra and other physical measurements, these complexes [NiL3(X)][ClO4](X = ClO4, NCS, or N3) are shown to have six-co-ordinate structures in which L3 adopts a new conformation, i.e. one in which it occupies five sites of a distorted octahedron. Replacement of AgI from the cavity of the less flexible 16-membered macrocycle L2 in dry ROH (R = Me or Et) yields distorted octahedral complexes of the new, more flexible, macrocycles L9 or L10 formed by addition of ROH across one azomethine bond of L2. The driving force for these reactions, which do not occur with (pentagonal) complexes of L2 with other metal ions, is believed to arise from a strong preference for the d8 ion for an orthogonal disposition of donor atoms. If the metal exchange is carried out in the presence of water, or if the L9 and L10 complexes are treated with water, a ready hydrolysis occurs to yield octahedral complexes of the ring-opened ligand L11. Crystals of the title complex [NiL11(OH2)][ClO4]2 are monoclinic, with a= 15.292(12), b= 19.807(15), c= 8.432(8)A, β= 106.54(8)°, Z= 4, space group P21/a. 1 514 Reflections above background have been measured by diffractometer and refined by full-matrix least squares to R 0.076. The ligand occupies five sites of a distorted octahedron around the metal ion [Ni–N 2.099(12), 1.999(14), 2.312(13), 2.084(11), and 2.085(12)A]. A water molecule at 2.126(8)A completes the co-ordination sphere.

The synthesis, properties, and the crystal and molecular structures of five-co-ordinate copper(I) and silver(I) complexes of a quinquedentate macrocyclic ligand having an ‘N3S2’ donor set

The 17-membered macrocyclic ligand L1 containing the ‘N3S2’ donor set has been synthesised [and isolated as the silver(I) complex] by the template action of silver(I) salts on the cyclic Schiff-base condensation of 2,6-diacetyl-pyridine with 1,10-diamine-4,7-dithiadecane. The copper(II) ion was ineffective as a template for the synthesis. However, copper(II) complexes of L1 could be obtained from [AgL1]+via metal exchange (transmetallation). Sodium tetraphenylborate is a reducing agent for [CuIIL1]2+, affording [CuIL1]+ in good yield. Possible mechanisms for the reducing action of [BPh4]– are discussed. Hydrogenation of [AgL1]+ salts using Na[BH4] give the free reduced macrocycle RL1(i.e. L1+ 4H) from which a new range of macrocyclic complexes may be obtained. Infrared and electronic spectra of the silver(I) and copper(I) complexes of L1 are reported. The copper(I) complexes are unreactive to both O2 and CO. Crystals of [CuL1][ClO4] are orthorhombic with a= 6.844(7), b= 12.361(11), c= 24.072(21)A, Z= 4, and space group P212121. Crystals of [AgL1][BPh4] are monoclinic with a= 13.132(11), b= 25.001(12), c= 12.479(12)A, β=105.7(1)°, Z= 4, and space group P21/c. The two structures were solved by Patterson and Fourier methods from 927 and 2 621 reflections above background measured by diffractometer and refined by full-matrix least squares to R 0.077 and 0.071 respectively. While the co-ordination geometry of both complexes is a distorted trigonal bipyramid there are marked differences in the macrocycle conformations in the two cases and, for the copper(I) complex, in the metal–nitrogen bond lengths. These are discussed in relation to the sizes of the two metal ions and of the macrocycle hole size.

Iron(II) complexes of a potentially quinquedentate macrocyclic ligand having an ‘N3S2’ donor set and the crystal and molecular structures of a high-spin (S= 2) complex and a low-spin (S= 0) complex

A series of iron(II) complexes of the 17-membered macrocyclic ligand L, having an ‘N3S2’ donor set, derived from the Schiff-base condensation of 2,6-diacetylpyridine with 4,7-dithiadecane-1, 10-diamine, are described. Physical measurements (magnetic, Mossbauer) characterise the complexes as high-spin (S= 2) or low-spin (S= 0) depending on the nature of the associated unidentate ligands (halide, NCS–, MeOH, MeCN, pyridine, or NH3). Single-crystal X-ray structure determinations of two of the complexes reveal a versatility in the conformations and co-ordination modes of the macrocycle. Crystals of the low-spin complex [FeL(NCS)][BPh4](1) are monoclinic with a= 31.640(8), b= 11.019(8), c= 11.155(7)A, β= 96.40(3)°, Z= 4, and space group P21/a. Crystals of the high-spin complex [FeLCl(HOMe)][ClO4](2) are monoclinic with a= 18.512(11), b= 18.279(12), c= 8.125(9)A, β= 113.1(2)°, Z= 4, and space group P21/a. The two structures were solved by Patterson and Fourier methods from 2 548 and 1 772 reflections above background measured by diffractometer and refined by full-matrix least squares to R 0.047 and 0.102 respectively. In both (1) and (2), the co-ordination geometry of the metal ion is distorted octahedral. In (1), the macrocycle adopts a ‘wrap-around’ conformation to occupy five of the octahedral sites [Fe–N 1.981(6), 1.870(6), 1.987(7); Fe–S 2.307(3), 2.258(3)A], the sixth being filled by the nitrogen atom of the –NCS group [1.965(9)A]. In (2), one sulphur atom of the macrocycle is not bonded to the metal while the other is only weakly co-ordinated [2.806(5)A]cis to the trimethine nitrogens [Fe–N 2.205(11), 2.092(10), 2.200(12)A]. The remaining two octahedral sites are occupied by the Cl– ion (trans to the pyridine nitrogen) and the oxygen atom of the MeOH molecule [Fe–Cl 2.304(4), Fe–O 2.205(16)A]. Possible structures for the remaining complexes of the series including three modifications of [FeL(NCS)2] are considered.

A realistic model for heme-containing catalases and peroxidases: the X-ray structural characterisation of a non-porphyrin iron(III) macrocyclic complex, and the mechanism of its peroxidation of aromatic substrates

In acidic buffered aqueous solution the complex dichloro{meso-2,12-dimethyl-3,7,11,17-tetra-azabicyclo[11.3.1] heptadeca-1 (17),13,15-triene}iron(III) tetrafluoroborate, [Fe(meso-L′)Cl2]BF4, exhibits both catalase- and peroxidase-like activity. The predominant cation in aqueous buffered solution at pH 4.65 is the mixed species [Fe(meso-L′)(OH)(H2O)]2+. The catalase- and peroxidase-like activity is proposed to occur via a high oxidation state intermediate rather than through involvement of free hydroxyl radicals. Quantitative compliance with the algebraic forms of theoretical rate laws fails to distinguish between these possibilities. However, the kinetics of dioxygen evolution in the presence of hydroxyl radical traps leads to the elimination of the hydroxyl radical model. In addition, the peroxidase-like reactivity of substituted benzenes toward the [Fe(meso-L′)(OH)(H2O)]2+–H2O2 model system parallels that expected for an electrophilic oxidant, and not that of free OH radicals. Parallel experiments with Fentons reagent support this view. An X-ray structural determination on the dichloro complex indicates that the macrocycle adopts a folded conformation allowing the two chloride ligands to occupy cis positions in the co-ordination sphere. This stereochemistry is proposed to be retained in aqueous solution, and may allow bidentate co-ordination of hydrogen peroxide, a structural feature that may be critical to the catalase- and peroxidase-like activity. The iron(III) complex crystallises in the orthorhombic system, a= 10.046(2), b= 13.322(2), c= 15.262(3)A, space group Pnma, with four molecules per unit cell. Final residuals had values of 0.042 and 0.043, for R and R′, respectively, upon convergence for 1 525 observed reflections. Both the cationic, macrocyclic complex and the BF4 anion display crystallographically imposed mirror symmetry. The iron(III) ion displays an approximately octahedral geometry, with co-ordination angles ranging from 77 to 95°. An analysis of associated torsion angles suggests that the folded conformation of the macrocycle is almost strain free.

Three-, four-, and five-co-ordination modes of a potentially quinquedentate ‘nitrogen–sulphur’ macrocyclic ligand: the crystal and molecular structures of two copper(II) complexes

The 17-membered Schiff-base ‘N3S2’ macrocycle (L) derived from the cyclic condensation of 2,6-diacetylpyridine with 1,10-diamino-4,7-dithiadecane forms complexes with CuII of formulae CuLXY and CuLY2 where X = Cl, Br, or NCS and Y = ClO4, BPh4, or NCS. The crystal and molecular structures of two of the complexes have been determined: [CuL][ClO4]2 is orthorhombic, space group P21cn(no. 33), with a= 11.934(5), b= 13.528(6), c= 14.581(7)A, and Z= 4; [CuL(NCS)][ClO4]·0.5H2O is monoclinic, space group C2/c, with a= 37.831(30), b= 8.187(5), c= 16.364(11)A, β= 102.9(1)°, and Z= 8. Diffraction data for both crystals have been refined by full-matrix least squares (1 364 reflections to R 0.068 and 1 196 reflections to R 0.082, respectively). Both structures have a distorted square-pyramidal structure in which the trimethine unit of the macrocycle occupies three positions of the square plane [Cu–N 1.93–2.08 A]. In [CuL][ClO4]2 the fourth position is occupied by one sulphur atom of the macrocycle [Cu–S 2.377(4)A], the other sulphur atom being axially sited at 2.470(4)A. In [CuL(NCS)][ClO4]·0.5H2O the square plane is completed by the thiocyanate group [Cu–N 1.854(19)A], the axial position being occupied by one ligand sulphur atom at 2.745(5)A, the other sulphur atom being unco-ordinated. In both structures there is a weak interaction with a perchlorate oxygen in the axial position trans to the sulphur atom. Infrared and e.s.r. spectra suggest that [CuL(NCS)][BPh4] has a dimeric (NCS-bridged) structure in the solid state whereas solid [CuL(NCS)2] appears to contain both bridging and terminally N-bonded NCS groups, with neither of the macrocycle sulphur atoms co-ordinated.