S. Martin Nelson

Quadridentate versus quinquedentate co-ordination of some N5 and N3O2 macrocyclic ligands and an unusual thermally controlled quintet ⇌ singlet spin transition in an iron(II) complex

The complexes [FeIIL(CN)2]·xH2O of the macrocyclic Schiff-base ligands L1–L3 derived from the condensation of 2,6-diacetylpyridine with 3,6-diazaoctane- 1,8-diamine (L1), 3,7-diazanonane1,9-diamine (L2), and 3,6-dioxaoctane-1,8-diamine (L3) have been prepared and their magnetic susceptibilities and Mossbauer spectra studied over the temperature range 80–300 K. The complexes of the N5 macrocycles L1 and L2 have low-spin (S= 0) ground states and are assigned six-co-ordinate structures in which the macrocycle acts as a quadridentate ligand with one secondary amine group unco-ordinated. This contrasts with the situation in all previously characterised complexes of these macrocycles (containing other metal ions and/or other axial ligands) where all five potential donor atoms are co-ordinated in a pentagonal planar arrangement. The exceptional structure of the present complexes is attributed to the ligand-field stabilization energy associated with the low-spin, approximately octahedral configuration generated by the strong field cyanide ligands. The magnetic properties of [FeL3(CN)2]·H2O exhibit a complex variation with temperature. The thermodynamically stable forms are high-spin (S= 2) at ambient temperature and low-spin (S= 0) below 150 K while between 150 and 200 K the stable form appears to contain approximately equal numbers of high-spin and low-spin molecules. The structure of the low-spin form is believed to be six-co-ordinate, with one ether oxygen of the macrocycle unco-ordinated while the high-spin form may have either a six- or a seven-co-ordinate structure.

Bi-copper(I) and bi-copper(II) complexes of a 30-membered macrocyclic ligand: the inclusion of substrate molecules and the crystal and molecular structures of a µ-hydroxo- and a µ-imidazolato-complex

The preparation of the copper(II) complex [CuL][ClO4]2·H2O of a 30-membered ‘N6O4’ macrocyclic Schiff-base ligand, L, in which the metal ion is octahedrally co-ordinated to the six nitrogen atoms, is described. In contrast to analogous complexes of the other first-row transition-metal ions, the macrocycle in [CuL]2+ unfolds on treatment with excess of free metal ion to yield the binuclear complex [Cu2L][CIO4]4·2H2O. This complex is the starting point for the preparation of several derivatives containing small substrate ions [OH–, imidazolate anion (im), N3–, NCS–] intramolecularly bound between the metal centres. The structures of the µ-imidazolato- and µ-hydroxo-complexes have been determined. Crystals of [Cu2L(im)][ClO4]3, (I), are triclinic, with a= 12.89(1), b= 13.64(1), c= 16.51(1)A, α= 89.9(1), β= 114.1(1), γ= 121.8(1)°, Z= 2, and space group P. Crystals of [Cu2L(OH)][ClO4]3·H2O, (II), are triclinic with a= 12.634(9), b= 14.103(8), c= 14.689(8)A, α= 88.3(1), β= 118.4(1), γ= 115.6(1)°, Z= 2, and space group P. The two structures were solved by Patterson and Fourier methods from 1 567 and 2 786 above background reflections measured by a diffractometer and refined by full-matrix least squares to R 0.081 and 0.084 respectively. In both (I) and (II) the two CuII ions are each bonded to the three nitrogen atoms of the two planar trimethine groups (Cu–N 1.90–2.08 A), the fourth position of the equatorial square plane being occupied by one nitrogen of a bridging imidazolate anion [Cu–N 1.92(2), 1.98(2)A] or by the bridging hydroxo-oxygen atom [Cu–O 1.866(13), 1.912(10)A]. Cu ⋯ Cu separations are 5.99 and 3.57 A in (I) and (II) respectively. In (I), the plane of the imidazolat ring is inclined at 68.8, 79.1° to the two ‘N4’ co-ordination planes. In (II), the Cu–O(H)–Cu angle is 141.7(7)°. In both structures the metal ions are weakly bonded to perchlorate or water oxygen atoms in axial positions. Magnetic susceptibility measurements as a function of temperature establish antiferromagnetic super-exchange in (I) and (II) with J=–21.0 and –120 cm–1 respectively, where 2J is the singlet–triplet separation. Two heterobinuclear CuII/NiII complexes have been prepared by reaction of mononuclear [CuL]2+ with NiII salts. A bi-copper(I) complex [Cu2L][ClO4]2·H2O has also been prepared by reduction of the bi-copper(II) complex with BPh4– as well as by a metal-exchange method.

Dicopper(II) complexes of a macrocyclic ligand containing single hydroxo-, methoxo-, or 1,1-azido-bridges: synthesis, magnetic properties, electron spin resonance spectra, and the crystal and molecular structure of a µ-hydroxo-derivative

Reaction of 2,6-diacetylpyridine with 1,3-diaminopropane in methanol in the presence of M[ClO4]2(M = SrII or BaII) in 2:1:1 molar proportions yields the complexes [ML2(ClO4)2] of the new 20-membered macrocyclic tetraimine ligand L2 derived from the [2 + 2] condensation of 2 mol of diketone with 2 mol of diamine. The single Ba2+ in [BaL2(ClO4)2] may be replaced by two Cu2+ to afford, depending on the conditions, the complexes Cu2L2(OH)(ClO4)3·2H2O and Cu2L2(OMe)(ClO4)3˙2H2O formulated as containing two Cu2+ linked by, respectively, single hydroxo- and methoxo-bridges. Crystals of the µ-hydroxo-complex are tetragonal with a= 15.20(1), c= 14.50(1)A, Z= 4, and space group P42/n. 1 191 Reflections above background were measured by diffractometer and refined by Fourier methods to R 0.078. Each copper atom in the binuclear [Cu2L2(OH)(OH2)]3+ cation is bonded to the three nitrogen atoms of a trimethine group of the folded macrocycle [Cu–N 1.916(13)–2.066(12)A]. The approximate co-ordination square plane about each metal atom is completed by a shared oxygen atom, O(1)(presumed to be from OH–), at 1.916(9)A. The two Cu atoms are also linked in a shared axial position by a second oxygen atom, O(2)(presumed to be from H2O), at 2.519(12)A. The Cu–O(1)–Cu and Cu–O(2)–Cu angles are 110.3(7) and 77.3(4)°, respectively, and the Cu ⋯ Cu separation is 3.145(4)A. Magnetic susceptibility measurements in the temperature range 90–300 K establish that the two Cu2+ in each binuclear complex are antiferromagnetically coupled with J=–32 and –53 cm–1 for the µ-hydroxy- and µ-methoxo-derivatives, respectively, where 2J is the singlet–triplet separation. The appearance of a ‘ triplet ’ e.s.r. spectrum at g≈ 2 and a seven-line ΔM= 2 half-band spectrum at g≈ 4 for frozen methanol–dimethyl sulphoxide solutions of the µ-methoxo-complex indicate retention of the bridge in solution. Reaction of Cu2L2(OMe)(ClO4)3·2H2O with Na[N3] affords Cu2L2(N3)(ClO4)3 for which i.r. spectra suggest that the Cu2+ are intramolecularly linked by one nitrogen atom of the azide ion (µ-1,1-N3 bridging mode). E.s.r. spectra of frozen solutions of the monoazide show that the Cu2+ are weakly antiferromagnetically coupled. The structures and physical properties of the complexes are discussed in relation to the cavity size of the macrocycle L2.

A hydrogen-1 and carbon-13 nuclear magnetic resonance study of the bonding in some monosubstituted cyclopentadienyl complexes of rhodium(I) and the crystal and molecular structure of cyclo-octa-1,5-diene(η-methoxycarbonyl-cyclopentadienyl)rhodium(I)

The 1H and 13C n.m.r. spectra of a series of monosubstituted η-cyclopentadienylrhodium(I) complexes of the types [RhL2(C5H4X)] and [Rh(LL)(C5H4X)] are described (L = ethylene or CO; LL = a 1,3-, 1,4-. or 1,5-diene; X = Me, CMe3, CO2Me, CO2Et, CO2Pri, CHO, COCO2Et, or CN). For complexes where the substituent X is electron-withdrawing in nature and where the neutral ligand is ethylene or a non-conjugated diene the 1H spectra of the cyclopentadienyl ring protons show a novel temperature dependence (+40 to –60 °C) in CDCI3 involving a chemical-shift cross-over without line broadening of the H(2), H(5) and H(3), H(4) resonances. In contrast, the n.m.r. behaviour of the complexes of conjugated dienes is normal, The unusual spectra observed for the non-conjugated alkene complexes are attributed to unequal barriers for the rotation of the cyclopentadienyl ring leading, at the lower temperatures, to preferential population of a particular rotamer state. A single-crystal X-ray structure determination of [Rh(cyclo-octa-1,5-diene)(C5H4CO2Me)](1)suggests that the preferred rotamer in solution may be one in which the C5ring is bonded to the metal asan η-allyl via C(2), C(1), and C(5)[2.230(8), 2.256(6), 2.226(5)A] with a weaker interaction via an alkene function localized between C(3) and C(4)[2.303(8), 2.294(7)A]. A simple bonding model is proposed which relates the n.m.r. spectra of solutions to the structure of the solid and which accounts for the differences in behaviour between complexes of conjugated dienes on the one hand and those containing isolated double bonds on the other.Crystals of (1) are triclinic, space group P, Z= 2, with a= 7.051(8), b= 8.390(8), c= 12.660(10)A, α= 100.16(9). β= 101.33(8), γ= 105.95(11)° The structure was solved by Patterson and Fourier methods from 2 023 independent reflections collected by counter methods and refined to R 0.039.

Crystal and molecular structure and some properties of pyridinium µ-oxo-bis[trichioroferrate(III)]–pyridine

The crystal structure of the title complex, [Hpy]2[Cl3Fe–O–FeCl3]·py, prepared by hydrolysis of FeCl3 in aqueous ethanol-pyridine, has been determined. Crystals are monoclinic with a= 8.532(6), b= 13.914(11),c= 10.597(11)A, β= 110.8(1)°, Z= 2, and space group P21/m. The structure has been solved by Patterson and Fourier methods and refined by full-matrix least squares to R 0.056 for 1 236 independent above-background reflections measured by diffractometer. In the complex anion which has crystallographically imposed Cs symmetry, the two equivalent FeOCl3 units are bridged by the oxygen atom via short Fe–O bonds [1.755(3)A], the Fe–O–Fe angle being 155.6(7)°. The geometry about each metal is close to tetrahedral, the Cl–Fe–Cl and Cl–Fe–O angles being 108–111°. The Fe–Cl distances (2.208–2.219 A) are somewhat lengthened with respect to those in [FeCl4]–. Strong antiferromagnetic coupling between the S=5//2 spins in the binuclear anion is observed. The coupling constant, J=–92 cm–1, is not appreciably different from values reported for other binuclear µ-oxo-complexes in which the iron(III) ions are in a square-pyramidal or octahedral environment. Infrared and Mossbauer spectra are reported.

Co-ordination complexes of 2,2′-bi-2-thiazoline and 2,2′-bi-4,5-dihydrothiazine

A number of complexes of 2,2′-bi-2-thiazoline. its 4,4′-dimethyl and 5,5′-dimethyl derivatives, and of 2,2′-bi-4,5- dihydrothiazine with FeII, FeIII, CoII, NiII, CuII, ZnII, HgII, and Mo0 have been prepared. Structures ofthe complexes are deduced from infrared, electronic, 1H n.m.r., and Mossbauer spectra, magnetic properties, and electrical conductances of solutions. The combined evidence afforded by these properties is consistent only with a co-ordination mode involving the α-di-imine group. Cyclic voltammctry of acetonitrile solutions of the tris-chelated iron(II) perchlorates shows two redox waves, one of which is ascribed to oxidation of the metal and the other to reduction of the co-ordinated ligand. The free-energy differences associated with these two couples correspond closely in magnitude to the energies of the metal-to-ligand charge-transfer bands occurring in the visible spectra. The properties of the complexes are compared with those of corresponding complexes of 2,2′-bipyridine and related ligands.

Oxidative nitrogen ⋯ nitrogen coupling of nitriles at a dicopper site and the structure of a pentanuclear copper–triazolyl complex containing two-co-ordinate and three-co-ordinate copper(I)

Reaction of a di-copper(II) complex of a macrocyclic ligand with acetonitrile in the presence of O2 and H2O affords a pentanuclear complex containing two-co-ordinate and three-co-ordinate copper(I) atoms linked via triply bridging 3,5-dimethyl-1,2,4-triazolate groups.

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.

Binuclear macrocyclic copper(II) complexes as receptors for small bridging ligands: X-ray crystal and molecular structure of a µ-azido complex

In two binuclear copper(II) complexes of a 30-membered Schiffs base macrocyclic ligand the macrocycle adopts a folded conformation which allows intramolecular linkage of the metal atoms via small bridging ligand (N3– or OH–); the structure of the µ-azido complex has been solved by X-ray analysis.

The structure of a dinuclear copper(I) complex of a Schiff-base ligand containing a copper–copper bond

The copper(I) complex [Cu2L22][ClO4]2·H2O [L2= 2,6-di(2′-methoxyethyliminomethyl)pyridine] has been prepared by reaction of [Cu(MeCN)4][ClO4] with the open-chain Schiff base ‘N3O2’ ligand L2 derived from the condensation of one molecule of 2,6-diformylpyridine with two molecules of 1-amino-2-methoxyethane. Crystals of [Cu2L22][ClO4]2·H2O are monoclinic with a= 20.221 (8), b= 16.239(7), c= 11.345(8)A, β= 102.2(1)°, Z= 4, and space group P21/n. In the dimeric cation the two copper atoms are directly linked by a Cu–Cu bond of length 2.626(1)A. In contrast to other metal complexes of pyridyldi-imine ligands the inequivalent copper atoms in the present structure are not bonded to all three nitrogen atoms of the same trimethine group and, moreover, all four imine groups are severely twisted out-of-plane with respect to their respective pyridine rings. Each copper atom is strongly bonded to two imino-nitrogen atoms, one from each ligand molecule. Cu(1) is also weakly bound to two ether oxygens, one from each ligand. The remaining two ether oxygen atoms are not co-ordinated. An unusual feature of the structure is that the pyridine nitrogen atoms appear to act as (asymmetric) bridges between the metal centres [Cu(1)–N, 2.693(6), 2.676(6)A; Cu(2)–N, 2.237(6), 2.204(6)A]. I.r. and 1H n.m.r. spectra in CD3CN of the free ligand and complex are compared. Evidence from electronic spectra suggests that the integrity of the dimeric unit is largely retained in solution.