Jaap G. Haasnoot

Mononuclear, oligonuclear and polynuclear metal coordination compounds with 1,2,4-triazole derivatives as ligands

Abstract 1,2,4-Triazole and its derivatives have gained great attention as ligands to transition metals by the fact that they unite the coordination geometry of both pyrazoles and imidazoles, and in addition exhibit a strong and typical property of acting as bridging ligands between two metal centres. In this bridging capacity, the 1,2,4-triazole ligands show a great coordination diversity, especially when the triazole nucleus is substituted with additional donor groups. This property together with their strong σ donor properties and the relative ease of synthesis make them very appealing for the design of new polynuclear metal complexes with interesting properties. A number of X-ray structures have been evaluated in some detail in the present paper.

High-spin α low-spin transition in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O). X-ray crystal structure and magnetic, mössbauer and EPR properties

Abstract The FeII ion in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O) has distorted tetragonal symmetry with two trans-oriented NCS− ligands. It shows a very abrupt high-spin α low-spin transition at 123.5 K on cooling and 144.5 K on warming. The compound is rather stable in vacuo at room temperature; however, samples which have passed the spin transition once lose their water of hydration above 240 K in vacuo. The dehydrated substance does not show a spin transition; it is high spin in the whole temperature range. Mossbauer ligand-field spectra and magnetic behaviour of both the hydrated and non-hydrated compounds are discussed. The spin transition has been followed by EPR measurements with the aid of traces of Cu2+ ions which could be substituted for Fe2+ in the tetragonal structure. In the high-spin phase the EPR signal is very broad and featureless; in the diamagnetic low-spin phase it is very sharp and resolved in hyperfine and superhyperfine structures. This unusual method to follow the spin transition was shown to be quite generally applicable. In the crystal structure of Fe(NCS)2(C4H4N6)2(H2O) the distances FENCS are 2.125(3) A, and FeN(ligand) 2.180(3) and 2.188(2) A. The water molecule is connected to the non-coordinating ligand N-atom by hydrogen bonding.

Tetrazoles as ligands. Part IV. Iron(II) complexes of monofunctional tetrazole ligands, showing high-spin (p5T2g) ⇋ low-spin transitions

Abstract 1-Alkyl substituted tetrazoles and iron(III) tetrafluoroborate lead to the formation of hexacoordinated iron(II) complexes, viz. : [FeL 6 ](BF 4 ) 2 with L = 1-methyltetrazole (MTZ), 1-ethyltetrazole (ETZ), 1-propyltetrazole (PTZ) and 1-isopropyltetrazole (IPTZ). The complexes were identified and characterized by elemental analyses, IR and ligand field spectra and magnetic susceptibility measurements. The iron(II) ions are octahedrally surrounded by monodentate ligands and a mononuclear complex formation is obvious. At temperatures varying from 140–80 K the complexes show high-spin ( 5 T 2g ) ⇋ low-spin ( 1 A 1g ) transitions, recognizable by a colour change from white to purple. The transitions were followed by magnetic measurements.

Two Examples of Novel and Unusual Double-Layered, Two-Dimensional CuII Compounds with Bridging 1,3-Bis(1,2,4-triazol-1-yl)propane

The synthesis and characterisation of two new polymeric CuII complexes is described, i.e. {[Cu(btp)2(CH3CN)(H2O)](CF3SO3)2}n (1) and {[Cu(btp)2(CH3CN)2](ClO4)2}n (2), in which btp = [1,3-bis(1,2,4-triazol-1-yl)propane]. Compound 1 crystallizes in space group P21/c with a = 11.9337(15) A, b = 20.108(6) A, c = 12.748(6) A, β = 92.247(14)°, and Z = 4. Compound 2 crystallizes in space group Pna21 with a = 18.770(8) A, b = 12.648(8) A, and c = 12.019(8) A. The structures refined to R1 values of 0.0683 for 1 and 0.0846 for 2. In both structures the CuII ions are linked by the bridging ligands, resulting in two-dimensional networks. Two such curved layers are arranged on top of each other with center-to-center of layer distances of 2.12 A in 1 and 1.98 A in 2. Such double layers are separated from each other by 10.05 A in 1 and 9.385 A in 2. The space between the double layers is occupied with interstitial anions. No significant interaction between CuII ions is observed by EPR and magnetic susceptibility measurements. The compounds form a new class of a lattice engineered system held together by the CuII ions. – The coordination geometry of the copper ions is distorted octahedral, with the equatorial plane formed by the N4 nitrogens of the four triazole groups and the axial sites occupied by solvent molecules; acetonitrile and water in structure 1 and two acetonitrile molecules in structure 2. The two structures are related by a group–subgroup relationship, which appears to be the first such case in supramolecular chemistry. – The Cu–N vibrations in the FIR region are found at 274 cm–1 for 1, and at 276 cm–1 for 2. The ligand-field maxima are observed at about 16·103 cm–1, with a shoulder at about 12·103 cm–1. The νCN stretching vibrations of the acetonitrile molecules are found at 2303 and 2261 cm–1 for 1, and at 2313, 2294, 2278, and 2260 cm–1 for 2.

Spin-transition behaviour in chains of FeII bridged by 4-substituted 1,2,4-triazoles carrying alkyl tails

A family of polymeric 1-dimensional chains of iron(II) species showing the spin-crossover phenomenon has been synthesized using 4-n-alkyl-1,2,4-triazoles as bridging ligands. The influence of the length of the alkyl tails on the triazole ligands on characteristic features of the spin transition was studied, showing degrading of steepness with increasing length. A set of four counter ions has been used to access a wider range of transition temperatures. Large hysteresis loops are detected with small tails, mainly for the methyl and ethyl substituted products. In most cases longer tails weaken co-operativity and hysteresis gradually decreases to zero. However it is shown that with certain anions hysteresis remains, even with very long tails on the triazoles. Weakening of the co-operativity mainly arises from a diminution of the length of the polymeric chains with increasing alkyl tails on the triazole. This effect is anion dependent. A strong interaction along the polymeric chains is confirmed.

Structure-dependent in vitro cytotoxicity of the isomeric complexes [Ru(L)2Cl2] (L=o-tolylazopyridine and 4-methyl-2-phenylazopyridine) in comparison to [Ru(azpy)2Cl2]

The dichlorobis(2-phenylazopyridine)ruthenium(II) complexes, [Ru(azpy)2Cl2], are under renewed investigation due to their potential anticancer activity. The three most common isomers α-, β- and γ-[RuL2Cl2] with L=o-tolylazopyridine (tazpy) and 4-methyl-2-phenylazopyridine (mazpy) (α indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual cis, trans, cis positions, β indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual cis, cis, cis positions, and γ indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual trans, cis, cis positions) are synthesized and characterized by NMR spectroscopy. The molecular structures of γ-[Ru(tazpy)2Cl2] and α-[Ru(mazpy)2Cl2] are determined by X-ray diffraction analysis. The IC50 values of the geometrically isomeric [Ru(tazpy)2Cl2] and [Ru(mazpy)2Cl2] complexes compared with those of the parent [Ru(azpy)2Cl2] complexes are determined in a series of human tumour cell lines (MCF-7, EVSA-T, WIDR, IGROV, M19, A498 and H266). These data unambiguously show for all complexes the following trend: the α isomer shows a very high cytotoxicity, whereas the β isomer is a factor 10 less cytotoxic. The γ isomers of [Ru(tazpy)2Cl2] and [Ru(mazpy)2Cl2] display a very high cytotoxicity comparable to that of the γ isomer of the parent compound [Ru(azpy)2Cl2] and to that of the α isomer. These biological data are of the utmost importance for a better understanding of the structure–activity relationships for the isomeric [RuL2Cl2] complexes.

mer-[Ru(terpy)Cl3] (terpy = 2,2′:6′,2″-terpyridine) shows biological activity, forms interstrand cross-links in DNA and binds two guanine derivatives in a trans configuration

Abstract The compound mer-[Ru(terpy)Cl3] was found to be active as a cytostatic in L1210 leukemia cells, with an activity in between cisplatin and carboplatin. It was shown that the Ru complex covalently binds to DNA and this binding results in the formation of ∼2% interstrand cross-links. In addition, no Ru-promoted nicks are formed. The parent compound has also been reacted with the DNA model bases 9-methylhypoxanthine (9mhyp) and 9-ethylguanine (9egua). The solid complexes trans-[Ru(terpy)(B-κN7)2(H2O)](PF6)2, where B = 9mhyp or 9egua, were isolated and characterized in acetone solution by proton NMR. The bases are symmetrically arranged around the metal center, and involved in hydrogen bonding with the water ligand, most likely via their O6 atoms. The water ligand has been substituted by acetonitrile. The complexes trans-[Ru(terpy)(B-κN7)2(CH3CN)](PF6)2, where B = 9mhyp or 9egua, have been prepared in situ. Compared to the aqua complexes, the hydrogen bond donor molecule is lost and a rearrangement of the guanine bases is observed.

Increase in coordination number of lanthanide complexes with 2,2′-bipyridine and 1,10-phenanthroline by using β-diketonates with electron-withdrawing groups

Abstract The coordination chemistry of Ln(hfpd)3 (hfpd=1,1,1,5,5,5-hexafluoropentane-2,4-dionate) with phen and bpy depends on the size of the Ln3+ ion and on the used solvent. The complexes [Er(hfpd)3(phen)] (7) and [Er(hfpd)3(bpy)] (14) were obtained from the synthesis of Er(CF3SO3)3 with Hhfpd, CsOH and either 1,10-phenanthroline or 2,2′-bipyridine in acetonitrile. The structure of 7 was determined by X-ray crystallography. Similar reactions, but performed in methanol, with various other lanthanide elements resulted in isolation of five different types of complexes, according to stoichiometry and spectral properties. With elements later in the lanthanide series eight-coordinated complexes of the types [Ln(hfpd)3(bpy)] (for Ln=Dy, Ho and Yb) and [Ln(hfpd)3(phen)] (for Ln=Tb, Ho and Yb), like 7, were obtained, whereas with the early lanthanide elements ten-coordinated complexes of the types [Ln(hfpd)3(bpy)2] (for Ln=La and Sm) and [Ln(hfpd)3(phen)2] (for Ln=La, Ce, Pr and Nd) were isolated. The X-ray crystal structure of [La(hfpd)3(bpy)2] (9) was determined, which provided proof for ten-coordination around the La ion. In addition to [Sm(hfpd)3(bpy)2], the synthesis with Sm and bpy and a trace of water yielded a second compound: the nine-coordinated complex [Sm(hfpd)3(H2O)(bpy)]·(bpy) (11), which was structurally characterised by X-ray crystallography. The LnN distances vary largely, depending on the used N-donor and the Ln3+ ion, and do not run parallel with the Ln3+ ionic radius.

Spectral Properties and Crystal Structure of Bis(μ-thiocyanato-N,S)bis(thiocyanato-N)-tetrakis(5,7-dimethyl[1,2,4]triazolo[1,5-α]pyrimidine-N3)dicopper(II) and of Tetrakis(5,7-dimethyl[1,2,4]triazolo[1,5-α]pyrimidine-N3)platinum(II)hexa(thiocyanato-S)platinate(IV).

Abstract The preparation, spectroscopic studies and the crystal structure of Cu(dmtp) 2 (NCS) 2 ( I ) and Pt(dmtp) 4 Pt(SCN) 6 ( II ) (dmtp stands for the 5,7-dimethyl[1,2,4]triazolo[1,5- a ] pyrimidine ligand) are described. Crystals of I are monoclinic, space group C 2/ c , with a = 19.088(7), b = 11.516(8), c = 20.118(7) A, β = 104.51(4)°, Z = 28; crystals of II are monoclinic, space group P 2 1 / n , with a = 16.914(8), b = 11.474(9), c = 11.893(7) A, β = 91.52(4)°, Z = 2. The structures of I and II have been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least-squares to R = 0.044 for I and 0.051 for II . The structure of I consists of centrosymmetric dimers [Cu(dmtp) 2 (NCS) 2 ] 2 , in which the copper atoms, bridged by two thiocyanate groups, are in a square pyramidal arrangement involving two isothiocyanate nitrogen atoms and two triazole nitrogen atoms from two dmtp ligands in the basal plane and a thiocyanate sulphur atom from the centrosymmetric complex in the apical position. The structure of II consists of square planar [Pt(dmtp) 2 ] 2+ cations, in which the platinum atom is bound to four triazole nitrogen atoms from dmtp ligands and of octahedral [Pt(SCN) 6 ] 2− anions with the metal bound to sulphur atoms of thiocyanate anions.

A pyrazole ligand yielding both chloro-bridged dinuclear and tetranuclear copper(II) compounds. The crystal and molecular structure of bis[μ-chloro-chloro(3,4-dimethyl-5-phenylpyrazole)(4,5-dimethyl-3-phenylpyrazole)copper(II)] and of (μ4-oxo)hexakis(μ-chloro)tetrakis(3,4-dimethyl-5-phenylpyrazole)tetracopper(II)

Abstract The reaction of CuCl2·2H2O and 3(5),4-dimethyl-5(3)-phenylpyrazole (hereafter Hdmppz) gives green crystals that analyze as CuCl2(C11H12N2)2 (1). The green filtrate produces brown crystals of composition Cu4OCl6(C11H12N2)4 (2). The crystal structure of both compounds was determined from single-crystal X-ray data. Crystals of bis[μ-chloro-chloro-(3,4-dimethyl-5-phenylpyrazole)-(4,5-dimethyl-3-phenylpyrazole)copper(II)] (1) are triclinic, space group P 1 ; a = 9.071(1), b = 11.008(1), c = 11.358(1) A, α = 93.42(1), β = 97.50(1), γ = 96.16(1)°, Z = 1. The dinuclear unit is located on a crystallographic inversion center. The copper ion is five-coordinated by two pyrazole nitrogen atoms and one bridging and one non-bridging chloride anion. The coordination is best described as a distorted square pyramid with the long bridging chloride as the top. Surprisingly the two pyrazole ligands are coordinating through different nitrogen atoms, one through the nitrogen atom close to the phenyl substituent and the other through the nitrogen close to the methyl substituent. Crystals of (μ4-oxo)hexakis(μ-chloro)tetrakis(4,5-dimethyl-3-phenyl-pyrazole)tetra-copper(II) (2) are monoclinic, space group I2/a; a = 20.836(1), b = 11.161(1), c = 22.996(1) A, β = 104.54(1)°, Z = 4. The copper ions are coordinated by five ligands in a trigonal-bipyramidal arrangement. The pyrazole nitrogen and the oxo-anion form the two tops of the bipyramid and the chlorides form the basal plane. Magnetic susceptibility measurements of 1 and 2 show that there is considerable antiferromagnetic interaction in these compounds as can be expected. For 1 the experimental data could be fitted to a model for a dinuclear compound with 2J as the singlet triplet splitting and zJ′ the inter-dinuclear exchange. The best fit was found for g = 2.13, J/k = −5.8 K and zJ′/k = -0.22 K. For 2 a model was used with two different exchange parameters Ja and Jb. The best fit was obtained for g = 2.11 and Ja/k = −51 K and Jb/k= −33 K.