Search

4-(2,6-Di(2H-indazol-2-yl)pyridin-4-yl)benzoic acid (1) and 10-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)anthracene-9-carboxylic acid (2) were required for adsorption studies on Ag(111), with a view to subsequent iron(II) complexation and formation of well-ordered spin-responsive self-assembled monolayers. While the generation of these compounds has remained elusive, several intermediates and by-products were obtained, potentially useful as dipyrazolylpyridine-related derivatives and for metal ion coordination. 3,5-Dichloro-2,6-diindazolylpyridine-4-amine, which forms as a mixture of regioisomers, was synthesised, the mixture separated, and the components characterised (3,5-dichloro-2,6-di(2H-indazol-2-yl)pyridin-4-amine; 3,5-dichloro-2-(1H-indazol-1-yl)-6-(2H-indazol-2-yl)pyridin-4-amine; 3,5-dichloro-2,6-di(1H-indazol-1-yl)pyridin-4-amine). Their iron(II) complexes have been prepared and fully characterised, including single crystal X-ray structure determination. The complexes are instructive examples of the influence of ligand design (“steric jamming”) on the spin-crossover (SCO) activity of FeII centres. Bulky substitution, which entails twisted ligand conformation, increases intramolecular crowding. This prevents contraction of the metal coordination sphere, which would be a prerequisite for thermally inducible SCO. Mössbauer spectroscopy has revealed that the complexes remain predominantly high-spin (HS) between 20 and 200 K, and that a mixture of conformational HS isomers is present in the microcrystalline solid.

We report the chelation-assisted oxidative CH2 extrusion from a PtII-coordinated dipyridylmethane, resulting in the formation of a 2,2′-bipyridyl ligand. This unusual one-pot reaction (solvent: dimethylformamide, DMF), which is quantitative, proceeds in air at mildly elevated temperature, with CH2 → C=O conversion, decarbonylation, and ipso-ipso C–C coupling of the two pyridyl residues (“fragment coupling”). It constitutes, in effect, a chelate ring contraction by one member.

Formation of the O-O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High-valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O-O bond in solution, from non-heme, N5 -coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta-chloroperbenzoic acid is administered in excess. Oxygen-isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)-initiated free-radical pathways of the peroxides, which are typical of catalase-like reactivity, and iron-borne O-O coupling, which is unprecedented for non-heme/peroxide systems. Interpretation in terms of [FeIV (O)] and [FeV (O)] being the resting and active principles of the O-O coupling, respectively, concurs with fundamental mechanistic ideas of (electro-) chemical O-O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

The new titanium(IV) coordinative compound (dpp-bian)TiCl2 (2) was obtained by metathesis of the disodium salt (dpp-bian)Na2 (1) (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with TiCl4. It was shown that the chlorido ligands can be exchanged for chelating dpp-bian ligands, or tert-butoxy groups, with the formation of (dpp-bian)2Ti (3) and (dpp-bian)Ti(OtBu)2 (4), respectively. Complexes 2–4 were isolated and fully characterized. It was demonstrated that dpp-bian is in the reduced dianionic form and acts as a σ2π-donor ligand.

The coordination chemistry of the tetrapodal-pentadentate ligand L , which has a mixed N 3 O 2 donor set, has been studied in the presence of copper(I) sources. The metal-to-ligand stoichiometry of the mononuclear copper(I) complexes resulting from 1:1 metal/ligand mixtures is markedly dependent on the character of the counter ion, giving 1:1 ([Cu( L )Cl]) and 1:2 complexes ([Cu( L ) 2 ](PF 6 )), with coordinating chloride and non-coordinating hexafluorophosphate, respectively. 1 H NMR and UV–Vis spectroscopy and single crystal X-ray structure analysis reveal tetrahedral coordination in both cases, with the “soft” copper centre engaging only two pyridine donors out of the N 3 O 2 donor set. Upon chemical oxidation of [Cu( L ) 2 ](PF 6 ) with dioxygen or 3,5–di-tert-butyl-quinone, the same binuclear copper(II) complex [Cu 2 ( L –H) 2 ](PF 6 ) 2 is selectively formed, indicating metal-oxidation induced deprotonation of one ligand, extrusion of the second ligand and subsequent fusion of the coordination spheres via alkoxide bridges. Elongation along a Jahn-Teller active molecular axis in [Cu 2 ( L –H) 2 ] 2+ is affected by the nature of the counter ion, giving N 2 O 3 and N 3 O 2 coordination patterns in the hexafluorophosphate and in the chloride salt, respectively. In addition to the oxidised complex, isolated in close-to-quantitative yield, catechol is identified as the only reduction product, indicating a 2e − /2H + reduction of the quinone. Electron transfer from copper(I) to the chemical oxidant is suggested to steer the formation of base equivalents from O 2 and Q , which then drive ligand deprotonation and complex nucleation.

Deprotonation of ligand-appended alkoxyl groups in mononuclear copper(II) complexes of N,O ligands L-1 and L-2, gave dinuclear complexes sharing symmetrical Cu2O2 cores. Molecular structures of these mono-and binuclear complexes have been characterized by XRD, and their electronic structures by UV/Vis, H-1 NMR, EPR and DFT; moreover, catalytic performance as models of catechol oxidase was studied. The binuclear complexes with anti-ferromagnetically coupled copper(II) centers are moderately active in quinone formation from 3,5-di-tert-butyl-catechol under the estab-lished conditions of oxygen saturation, but are strongly activated when additional dioxygen is administered during catalytic turnover. This unforeseen and unprecedented effect is attributed to increased maximum reaction rates v(max), whereas the substrate affinity KM remains unaffected. Oxygen administration is capable of (partially) removing limitations to turnover caused by product inhibition. Because product inhibition is generally accepted to be a major limitation of catechol oxidase models, we think that our observations will be applicable more widely.

The title compound crystallized from the mock-up of a typical electroless copper bath (ECB) as used to deposit copper on printed circuit boards, consisting of a copper(II) salt, soda lye, l-(+)-tartrate as a complexing agent, and 2,2′-bypyridine derivative as a stabilizer. Its layer-like crystal structure is dominated by extensive π stacking and classical hydrogen bonding., Typical electroless copper baths (ECBs), which are used to chemically deposit copper on printed circuit boards, consist of an aqueous alkali hydroxide solution, a copper(II) salt, formaldehyde as reducing agent, an l-(+)-tartrate as complexing agent, and a 2,2′-bi­pyridine derivative as stabilizer. Actual speciation and reactivity are, however, largely unknown. Herein, we report on the synthesis and crystal structure of aqua-1κO-bis­(4,4′-dimeth­oxy-2,2′-bi­pyri­dine)-1κ2 N,N′;2κ2 N,N′-[μ-(2R,3R)-2,3-dioxidosuccinato-1κ2 O 1,O 2:2κ2 O 3,O 4]dicopper(II) octa­hydrate, [Cu2(C12H12N2O2)2(C4H2O6)(H2O)]·8H2O, from an ECB mock-up. The title compound crystallizes in the Sohncke group P21 with one chiral dinuclear complex and eight mol­ecules of hydrate water in the asymmetric unit. The expected retention of the tartrato ligand’s absolute configuration was confirmed via determination of the absolute structure. The complex mol­ecules exhibit an ansa-like structure with two planar, nearly parallel bi­pyridine ligands, each bound to a copper atom that is connected to the other by a bridging tartrato ‘handle’. The complex and water mol­ecules give rise to a layered supra­molecular structure dominated by alternating π stacks and hydrogen bonds. The understanding of structures ex situ is a first step on the way to prolonged stability and improved coating behavior of ECBs.

Formation of the O−O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O−O bond in solution, from non‐heme, N5‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O−O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [FeIV(O)] and [FeV(O)] being the resting and active principles of the O−O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O−O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

Monolayers of 4-(2,6-di(1H-pyrazol-1-yl)pyridine-4-yl)benzoic acid (DPP-BA) on Au substrates modified by an underpotential-deposited bilayer of Ag were studied by X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy. Highly crystalline layers are formed with molecules coordinatively bonding to the surface through the carboxylate moiety in a bidentate configuration. The molecules assemble to rows characterized by densely packed upright-orientated DPP units occupying an area of 41 A2. The DPP units adopt a trans–trans conformation as inferred from the N 1s XPS spectra taken at different photon energies. Their alignment alternates between adjacent rows, giving rise to a herringbone pattern. The pronouncedly different structure of the weakly commensurate self-assembled monolayer (SAM) of DPP-BA on Ag compared to the commensurate SAM of a DPP thiol on Au is a manifestation of the shift from substrate-directed assembly in the latter ca...

This work presents a detailed study of the photo-induced spin-state dynamics of the photochromic iron(II) complex 1, where the metal ion is in the field of a tripodal hexa-imine ligand with protolysable phenol groups. The nature of the complex’s ground state has been identified as a spin singlet by 1H NMR and steady-state UV/vis spectroscopies, and its distorted octahedral structure was analyzed via crystal structure determination. Sub-picosecond and nanosecond time-resolved laser flash photolysis experiments identify the long-lived quintet state of 1 as the selective product of photoexcitation in the UV/vis spectral region. Thermal barriers of spin-state interconversion as a function of solvent and added base are derived from temperature-dependent rates of transient decay. Ground-state recovery is found to be significantly affected by the solvent and is strongly enhanced, in particular, by base-driven solvolysis of the ligand’s phenol groups. Partial spontaneous deprotonation of the phenolic hydroxyl gro...

The ligand-field strength in metal complexes of polydentate ligands depends critically on how the ligand backbone places the donor atoms in three-dimensional space. Distortions from regular coordination geometries are often observed. In this work, we study the isolated effect of ligand-sphere distortion by means of two structurally related pentadentate ligands of identical donor set, in the solid state (X-ray diffraction, (57)Fe-Mössbauer spectroscopy), in solution (NMR spectroscopy, UV/Vis spectroscopy, conductometry), and with quantum-chemical methods. Crystal structures of hexacoordinate iron(II) and nickel(II) complexes derived from the cyclic ligand L(1) (6-methyl-6-(pyridin-2-yl)-1,4-bis(pyridin-2-ylmethyl)-1,4-diazepane) and its open-chain congener L(2) (N(1),N(3),2-trimethyl-2-(pyridine-2-yl)-N(1),N(3)-bis(pyridine-2-ylmethyl) propane-1,3-diamine) reveal distinctly different donor set distortions reflecting the differences in ligand topology. Distortion from regular octahedral geometry is minor for complexes of ligand L(2), but becomes significant in the complexes of the cyclic ligand L(1), where trans elongation of Fe-N bonds cannot be compensated by the rigid ligand backbone. This provokes trigonal twisting of the ligand field. This distortion causes the metal ion in complexes of L(1) to experience a significantly weaker ligand field than in the complexes of L(2), which are more regular. The reduced ligand-field strength in complexes of L(1) translates into a marked preference for the electronic high-spin state, the emergence of conformational isomers, and massively enhanced lability with respect to ligand exchange and oxidation of the central ion. Accordingly, oxoiron(IV) species derived from L(1) and L(2) differ in their spectroscopic properties and their chemical reactivity.

The formation of self-assembled monolayers (SAMs) on Au(111) from solution has been investigated for two ionic iron(II) complexes of the type [Fe(L)](BF4)2, where L is tripodal hexadentate and contains three thiocyanate anchor groups. The ligands (L1, L2; donor set: N6) are obtained by Schiff base condensation of a tripodal triamine (L1: tris-(2-aminoethyl)amine, ‘tren’; L2: 1,1′,1″- trimethyl(thiophosphoryl)trihydrazide) with 5-(4-thiocyanatobutoxy) pyridine-2-carbaldehyde. Layers of the complexes adsorbed on Au(111) from methanol solution have been characterised using scanning tunnelling microscopy (STM), infrared reflectance absorbance (IRRAS), X-ray photoelectron (XPS) and UV=Vis reflectance spectroscopies, as well as ellipsometry. Complex [Fe(L1)](BF4)2 deposits intact on a gold surface and retains its optical addressability. Elaboration of this result may provide access to a new class of self-assembled layers, employing salt-like tripodal coordination compounds with thiocyanate anchors. The second complex, [Fe(L2)](BF4)2, which contains a sulphur atom in the ligand backbone, is not sufficiently stable under the same conditions.

Tris-o-semiquinonato cobalt complexes react with a tetrapodal pyridine-derived ligand to form dinuclear cobalt compounds of general formula (OMP)[CoQ2]2, where OMP = 2,2′-(pyridine-2,6-diyl)bis(N1,N1,N3,N3-tetramethylpropane-1,3-diamine), Q = mono- or dianion of 3,6-di-tert-butyl-o-benzoquinone (complex 1) and it derivatives: 3,6-di-tert-butyl-4,5-N,N′-piperazino-o-benzoquinone (complex 2), and 3,6-di-tert-butyl-4-Cl-o-benzoquinone (complex 3). Single crystal X-ray crystallography of 1 and 3 indicates two bis-quinonato cobalt units bound by an OMP ligand, which acts as a bridge. Each central cobalt atom is chelated by one N1,N1,N3,N3-tetramethylpropane-1,3-diamine and two o-quinonato fragments. The nitrogen atom of the pyridine ring is uncoordinated. All complexes were characterized by NIR-IR and EPR spectroscopy, precise adiabatic vacuum calorimetry, and by variable-temperature magnetic susceptibility measurements. All data indicate a reversible thermally driven redox-isomeric (valence tautomeric) transformation in the solid state for all complexes.

A series of complexes of the new N4 chelate ligand L (L = 1-{6-[1, 1-bis(pyridin-2-yl)ethyl]pyridin-2-yl}-N,N-dimethylmethanamine) with intermediate to late divalent transition metal ions M was obtained by the reaction between L and the respective chloride salt or similar precursor in methanol: [MCl2L] (M = Mn, Fe, Co, Ni, Zn; Ru) and [CuClL]Cl. The stereochemical characteristics of the chelate ligand were studied by means of single-crystal X-ray diffraction, and quantified on the basis of several geometric parameters, including the tetragonal distortion Σ and the continuous symmetry measure S(Oh). The overall distortion of the coordination environment is predominantly determined by the steric demand of the central ion, while electronic or other subtler influences essentially contribute to the distortion of the ligand L. Unlike similar complexes, [MnIICl2L] cannot be oxidized to a manganese(III) complex by dioxygen, hydrogen peroxide, or iodosylbenzene. In [RuIICl2L], one chlorido ligand can be exchanged against small π-accepting molecules such as acetonitrile or dinitrogen. L offers an environment ideal for small metal ions (0.4–0.6 A), such as low-spin iron(II), which rationalizes the late onset of thermal spin crossover in the complex [FeIIL(NCS)2].

We have recently shown that the vacuum-deposited complex [Fe(II)L(NCS)2] (L: 1-{6-[1,1-di(pyridin-2-yl)ethyl]-pyridin-2-yl}-N,N-dimethylmethanamine) is capable of a thermally induced spin crossover (SCO) in direct contact with a graphite surface. The SCO significantly differs from the transition behaviour in the bulk phase (powder). In the present work, the assumption of virtually no intermolecular interaction in the powder is confirmed by comparison with the spin transition in acetone solution (T1/2 = 234[3] K, ΔT80 = 58[4] K), as monitored by temperature-dependent UV/Vis spectroscopy. The complex crystallises from chlorocarbons in the form of a number of pseudopolymorphs. Amongst these, the sufficiently stable solvate [Fe(II)L(NCS)2]·CHCl3 is investigated by variable-temperature single-crystal X-ray diffractometry. Its SCO behaviour (T1/2 = 240[3] K, ΔT80 = 35[4] K) correlates with features of molecular structure that are unambiguously identified by analysis of the tensor of thermal expansion. Following comprehensive comparison of spin-transition properties in different states of aggregation (also in relation to the newly synthesised high-spin iron(ii) and iron(iii) complexes [Fe(II)Cl2L] and [Fe(III)Cl2L]PF6), a mode of adsorption on graphite surfaces is proposed, that complies with all previous findings.

The N4 ligand 1-{6-[1,1-bis(pyridin-2-yl)ethyl]pyridin-2-yl}-N,N-dimethylmethanamine (L) is presented. It has been used to obtain the copper(I) complex [CuL(MeCN)]PF6 and the iron(II) complex [Fe(CN)2L]. Aerobic oxidation of the copper(I) complex is unspecific at room temperature. Anaerobic oxidation in solution, which is very sluggish, occurs with formation of the copper(II) complex [CuIIFL]PF6, suggesting concomitant hydrolysis of hexafluorophosphate initiated by traces of water. The iron(II) complex adsorbed at a TiO2 surface is an efficient photosensitiser of the support. Photoelectrochemical photocurrent switching (PEPS), which can be initiated by changing the electrochemical potential and photon energy, is very pronounced. The construction of a simple photoelectrochemical NOR logic gate has been demonstrated.

Copper(II), iron(II), and manganese(II) complexes of 2-methyl-2-(2-pyridinyl)-1,3-propanediamine (ppda) were synthesized and characterized. The reaction of copper(II), iron(II), or manganese(II) salts with ppda yields complexes [Cu(ppda)Cl2] (1), [Cu(ppda)2](OTf)2 (2), [Fe(ppda)2](OTf)2 (3), and [Mn(ppda)2](OTf)2 (4). Complex 1 is square pyramidal with one molecule of ppda coordinated in a tridentate fashion with the amine N atoms and with chlorido ligands in the basal plane, and the pyridyl N atom in the axial position. Six-coordinate, C2-symmetrical complex 2 contains two ppda ligands with the pyridyl rings from two separate ppda ligands cis to each other in a tetragonally compressed square bipyramidal structure. The axial amine Cu–N bond distances are about 0.2 A shorter than the equatorial bonds. Solution EPR spectra of 2 at 77 and 298 K display typical axial signals suggesting a tetragonally elongated structure in solution. Librational disorder of the solid state structure points to crystal packing disorder in the solid state that leads to the slight axial compression, while the molecular structure in solution is the more typical d x 2 - y 2 ground state of an axially elongated structure. Complexes 3 and 4 are isostructural with 2 in terms of the disposition of the two ppda ligands. However, the bond distances for 3 are all very similar to each other (2.00 ± 0.04 A), and for 4 they are even more alike (2.25 ± 0.01 A).

International audience; Synthetic diiron compounds of the general formula Fe2(μ-S2R)(CO)n(L)6−n (R = alkyl or aromatic groups; L = CN− or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and Kβ main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(I) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe–Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds.

This microreview highlights recent techniques used to determine the spin state at or around room temperature in nanoscale objects composed of iron(II) spin-crossover complexes. Complexes have been studied in the form of thin films, nanoparticles, single molecules or oligomolecular aggregates. Three approaches are presented: Surface plasmon polariton resonance in films on Ti/Au substrates, the determination of single-particle current–voltage characteristics between gold electrodes, and current-imaging tunnelling spectroscopy performed on highly oriented pyrolytic graphite-adsorbed strings of oligonuclear, bead-like complex aggregates.

Slow relaxation: Exergonic high-spin→low-spin relaxation after photoexcitation has been found to be exceedingly slow in a class of iron(II) complexes with hexadentate imine ligands. The thermal activation barriers that arise between the quintet- and singlet-spin manifolds are the highest ever recorded for spin crossover of isolated molecules in free solution (see figure).

The results of studies focussed on copper complexes of a variety of ligands with an NN4 donor set are reported. The permethylated tetrapodal ligand 2 forms a complex with copper(I) which, upon reaction with dioxygen at –90 °C, yields a product having a bis(μ-oxido)dicopper(III) core (“O-type” product, 10), as inferred from UV/Vis and resonance-Raman spectroscopic data. The UV/Vis spectrum of 10 has two bands at 300 and 404 nm, with extinction coefficients of 9400 and 10400 L mol–1 cm–1, respectively. Resonance-Raman spectra display two 16O/18O-sensitive bands which, based on the isotopic shifts and the absolute frequencies, are attributed to the Cu–O stretching modes of the O-type product. Complex 10 shows tyrosinase-like activity, as its reaction with sodium p-tert-butylphenolate at –90 °C in THF yields p-tert-butylcatechol, in an ortho-hydroxylation reaction (yield: 30 %). Two new rigid tetrapodal pentadentate ligands (the “superpods” 3 and 4) can be synthesized by condensation of the primary polyamine 1 with paraformaldehyde. Their copper(II) complexes (5 and 6) have been spectroscopically characterized. As ascertained by X-ray crystallography, 5 has the CuII ion in a tetragonal-pyramidal environment, with almost uniform Cu–N bond lengths (basal bonds: 2.052 A and 2.070 A; apical bond: 2.077 A). No significant Jahn–Teller distortion is observed here. In 6, the ligand acts as a multinucleating donor, which leads to the formation of a ladder-like cluster of [Cu(μ3-OH)] units containing a total of two ligands, six copper(II) ions, four hydroxido ligands and eight trifluoroacetate ions. Two of the trifluoroacetate ions are non-coordinating. Variable-temperature magnetic susceptibility data are reported for this hexanuclear copper(II) cluster. Copper(I) complexes of 1 and 3 have been characterized and allowed to react with molecular oxygen, which caused the decomposition of the complexes. The IR spectra of the oxygenation products have bands at 1652 and 1632 cm–1, respectively, which are absent in the spectra of 1 and 3, suggesting that amine functions have been oxidized to imines.

2-Methyl-2-(pyridin-2-yl)propane-1,3-diamine and formaldehyde are condensed to prepare the hexahydropyrimidine derivative, which is subsequently reacted with two equivalents of 2-vinylpyridine, to produce a novel, potentially pentadentate amine/imine ligand. Full NMR spectroscopic details are reported. The ligand, hexahydro-5-methyl-5-(pyridin-2-yl)-1,3-bis[2-(pyridin-2-yl)ethyl]pyrimidine, acts as a pentadentate in a series of first-row transition metal complexes (M = Ni 2+ , Fe 2+ , Zn 2+ , Cu 2+ ) but is tridentate towards Mn 2+ , in the corresponding dibromido complex. Single-crystal X-ray structure analyses reveal the metal ions to be hexacoordinate in the case of M = Ni 2+ , Fe 2+ , with and additional aqua or halido (Br − , Cl − ) ligand, or pentacoordinate (M = Zn 2+ , Cu 2+ , Mn 2+ ). Ferric complexes were not obtained, neither from complexation experiments employing iron(III), nor from oxidations using the iron(II) complex, and hydrogen peroxide or iodosylbenzene. In the case of the latter reactions, mass spectrometric data indicate oxidation of the hexahydropyrimidine core, with concomitant decomplexation of the ligand.

A mononuclear iron(II) complex is readily formed when combining equimolar amounts of iron(II) tetrafluoroborate hexahydrate, the pyridine-derived triphosphane C5H3N{2-[CMe(CH2PMe2)2]}{6-[CMe2(CH2PMe2)]} (2) and diethylphosphane (Et2PH) in methanol at room temperature. The chelate ligand is in fact pentadentate, as one of the methyl groups of the neopentyl-like sidearm engages in a C–H-bonded contact (agostic interaction) with the metal centre, in addition to the expected NP3 coordination. The remaining site of what is a distorted coordination octahedron is occupied by monodentate Et2PH. The autoclave reaction of this purple complex with CO (10 bar, ethanol solvent, 65 °C) yields a yellow microcrystalline precipitate, whose analysis reveals a mixture of two products, in an approximate ratio of 40:1. One of the products is the cis-dicarbonyl complex[Fe(2)(CO)2](BF4)2 (4), in which the chelate ligand acts as an NP3 donor, and the other is the monocarbonyl complex of the tetrapodal pentadentate NP4 ligand 1. The latter ligand is formed from 2 by incorporation of an additional PMe2 donor, in what, in effect, is a metal-mediated phosphenium group transfer. A mechanism is suggested for this reaction. Other species in the reaction mixture have been identified on the basis of mass spectra, and the full NMR spectroscopic assignment of complex 4 (1H, 31P, 13C) is reported.

A pyridine-derived tetraphosphane ligand (donor set: NP4) has been found to undergo remarkably specific C-P bond cleavage reactions, thereby producing a ligand with an NP3 donor set. The reaction may be reversed under suitable conditions, with regeneration of the original NP4 ligand. In order to investigate the mechanism of this reaction, the NP3 donor ligand C5H3N[CMe(CH2PMe2)2][CMe2(CH2PMe2)] (11) was prepared, and its iron(II) complex 4 generated from Fe(BF4)2 ・6H2O, with methyl diethylphosphinite (7) as an additional monodentate ligand. Ligand 11 has, in addition to the NP3 donor set, one methyl group in close contact with the iron center, reminiscent of an agostic M・ ・ ・H-C interaction. Depending on the stoichiometric amount of iron(II) salt, a side product 15 is formed, which has a diethylphosphane ligand instead of the phosphinite 7 coordinated to iron(II). While attempts to deprotonate the metal-coordinated methyl group in 4 were unsuccessful, the reaction was shown to occur in an alternative complex (18), which is similar to 4 but has a trimethylphosphane ligand instead of the phosphinite 7. The reaction of complex 15 with CO gave two different products, which were both characterized by single-crystal X-ray diffraction. One (19) is the dicarbonyl iron(II) complex of the triphosphane ligand 11, the other (3) is the carbonyl iron(II) complex of the tetraphosphane C5H3N[CMe(CH2PMe2)2]2 (1). This suggests an intermolecular mechanism for the C-P bond formation in question.

The temperature-dependent and photodynamic spin behaviour of three iron(II) complexes with different 2,6-dipyridyl-4-phenyltriazine ligands L1-L3 was investigated in the solid state and in solution (DMSO). The ligands differ in the substituent R in the 4-position of the phenyl ring (L1: R = H; L2: R = OCH 3 ; L3: R = SAc), which allows the electronic properties of the ligands to be finetuned. The magnetic data for the complex [Fe(L3) 2 ](BF 4 ) 2 in the solid state indicate an incomplete spin transition to the high-spin form upon warming from liquid helium temperature, reaching about 30 % at 400 K. There is circumstantial evidence for paramagnetic contributions compatible with spin transitions, namely, temperature-dependent NMR spectroscopic line shifts and line broadening in solution (DMSO). However, an efficient thermally induced spin crossover in solution is hindered by the substitution lability of the complexes, as has been detected and analyzed in an extended temperature-dependent UV/ Vis spectroscopic study. Essentially unaffected by thermally induced substitution lability, the transient dynamics of the iron(II) complexes after nanosecond laser flash excitation of their metal-to-ligand charge-transfer bands provide good evidence for efficient photoinduced spin transitions in solution in all cases. The range of measured lifetimes of the high-spin quintet states is in accord with previously published data. Importantly, in our series of iron(II) complexes, the lifetimes of the high-spin state reflect the electron-donating character of the ligands.

Graphical Abstract ,, ... und ehrt mir ihre Kunst!“ – Evaluierung historischer und neuer Synthesewege zu 1,5-Dihydroxy-6-oxo-1,6-dihydropyridin-2-carbonsäure und 1,3-Dihydroxy-2-oxo-3-pyrrolin-4-carbonsäure / “. . . and honour their art!” – Evaluation of Historical and New Routes to 1,5-Dihydroxy-6-oxo-1,6- dihydropyridine-2-carboxylic Acid and 1,3-Dihydroxy-2-oxo-3-pyrroline-4-carboxylic Acid

The ligand 2,6-bis(1H-pyrazol-1-yl)-4-(thiocyanatomethyl)pyridine (L) has been prepared from the hydroxymethyl precursor by OH/Br exchange and subsequent thiocyanate substitution. Two polymorphs of the unsolvated iron(II) complex [Fe(L)2](BF4)2 crystallize at room temperature from the same solution (MeOH), and side by side. One (yellow) is high spin between 4 and 350 K, whereas the other (red-brown) shows spin crossover, with a thermal spin-transition centred at 272 K (width ca. 50 K). Both forms have been fully characterized by variable-temperature structure determination and magnetic susceptibility measurements. At a given temperature, the solid-state structures differ strongly with respect to the coordination geometry at iron(II), the orientation of the thiocyanatomethyl substituents, and cation-anion contacts. The implications of these structural differences for the observed spin behaviour are discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Octahedral iron(II) complexes of a unique pyridine-derived tetrapodal pentadentate polyamine ligand, 2,6-C5H3N(CMe[CH2NH2]2)2, show temperature-dependent spin crossover (SCO) depending on the nature of a sixth monodentate ligand L (imidazol or pyridine derivative). For L = 1-methylimidazol, the redox behaviour of the complex, as determined by cyclic voltammetry, suggests an accompanying ligand exchange. Pyridine-4-thiol and the disulphides: 4-(2-methyldisulphanyl)pyridine, 4-(2-hexadecyldisulphanyl)pyridine and 1,2-bis([pyridine-4-yl]methyl)disulphane, were studied as mono-dentate ligands L, with a view to enable chemisorption of iron(II) complexes on a gold surface. In the case of pyridine-4-thiol, the participation of the thiolate functional group in iron coordination is difficult to suppress, whereas the disulphides enter into yet unrecorded redox chemistry with iron(II), yielding a di-iron(III) complex containing a persulphide bridge (S 2 2− ).

Self-assembled monolayers (SAMs) of a bis(pyrazol-1-yl)pyridine-substituted thiol (bpp-SH) on Au (111)/mica were studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Using substrates precoated with perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA), preparation at elevated temperatures yields highly ordered layers whose structure is described by a rectangular (5 x radical3) unit cell containing one molecule. The bis(pyrazol-1-yl)pyridine (bpp) units exhibit pi-stacking along the 112 direction, and they are tilted significantly. We conclude the three imine nitrogen atoms in the bpp headgroup adopt a trans,trans arrangement.

An unprecedented alkyne–nitrile metathesis takes place when the high-valent iron-nitrido dication [Fe(L)N]2+, with L=2,6-bis(2-methyl-1,3-diaminopropan-2-yl)pyridine, is reacted with alkynes in the gas phase under thermal conditions. While the detailed role of the alkyne substrate with respect to relative rates, regioselectivities, and branching ratios remains to be elucidated, the very existence of this novel metathesis reaction provides additional experimental evidence of a genuine, long-lived, formal iron(V)-nitrido dication.

Two novel spin crossover (SCO) compounds, namely [Fe(INMe)(pyN 4 )]Br 2 , and [Fe(IMMe)(pyN 4 )](OTf) 2 , where pyN 4 = 2,6-Bis(1′,3′-diamino-2′methyl-prop-2′yl)pyridine, INMe = isonicotinic acid methyl ester, IMMe = N -methyl-imidazole, and OTf = triflate, are characterized here both from experimental and theoretical viewpoints. In particular, we apply various density functionals and basis sets to obtain optimized geometries for low- (LS) and high-spin (HS) states, vibrational spectra, LS–HS splittings, and temperature-dependent UV/vis spectra. While geometries and spectra are in good agreement with experimental data, the well-known spin pairing problem makes it difficult to compute accurate LS–HS splitting energies and enthalpies. Based on TD–DFT calculations, the capacity of the compounds for use as reversibly photo-switchable molecules is discussed.

A tetrapodal pentadentate nitrogen ligand (2,6-bis(1,1-di(aminomethyl)ethyl)pyridine, 1) is used for the synthesis of the azido-iron(III) complex [(1)Fe(N3)]X2 where X is either Br or PF6. By means of electrospray ionization mass spectrometry, the dication [(1)Fe(N3)]2+ can be transferred into the gas phase as an intact entity. Upon collisional activation, [(1)Fe(N3)]2+ undergoes an expulsion of molecular nitrogen to afford the dicationic nitrido-iron species [(1)FeN]2+ as an intermediate, which upon further activation can intramolecularly activate C−H− and N−H bonds of the chelating ligand 1 or can transfer an 〈NH〉 unit in bimolecular reactions with activated olefins. The precursor dication [(1)Fe(N3)]2+, the resulting nitrido species [(1)FeN]2+, and its possible isomers are investigated by mass spectrometric experiments, isotopic labeling, and complementary computational studies using density functional theory.

Spin-state switching of transition-metal complexes (spin crossover) is sensitive to a variety of tiny perturbations. It is often found to be suppressed for molecules directly adsorbed on solid surfaces. We present X-ray absorption spectroscopy measurements of a submonolayer of [Fe(II)(NCS)2L] (L: 1-{6-[1,1-di(pyridin-2-yl)ethyl]-pyridin-2-yl}-N,N-dimethylmethanamine) deposited on a highly oriented pyrolytic graphite substrate in ultrahigh vacuum. These molecules undergo a thermally induced, fully reversible, gradual spin crossover with a transition temperature of T1/2 = 235(6) K and a transition width of ΔT80 = 115(8) K. Our results show that by using a carbon-based substrate the spin-crossover behavior can be preserved even for molecules that are in direct contact with a solid surface.

The reaction of the 2,6-diethylpyridine-derived tetraphosphane ligand C5H3N[CMe(CH2PMe2)2]2 (1Me) with nickel(II) or cobalt(II) perchlorate or tetrafluoroborate in methanol produces complexes of approximate square-pyramidal geometry (NP4 coordination). The nickel complexes are diamagnetic, with NMR spectral features (31P) that reflect a tetrahedral distortion of diametrically opposite phosphorus donors; this is also found in the solid state. In terms of bond lengthsand angles, all complexes differ significantly from theiranalogues containing the phenyl-substituted ligand C5H3N[CMe(CH2PPh2)2]2. The ligand 1Me is readily and completely oxidised to the corresponding tetrakis(phosphane oxide) upon reaction with NO. Slow oxidation of the cobalt(II) tetrafluoroborate complex of 1Me with aerobic oxygen in acetonitrile produces a trinuclear cobalt complex containing two equivalents of partially oxidised ligand (C2-symmetrical; donor set: [PMe2]2[P(O)Me2]2). Four phosphane oxide oxygen atoms coordinate the central cobalt(II) ion in tetrahedral fashion, whereas the lateral cobalt(II) ions are in a square-pyramidal environment provided by two PMe2 donors and three acetonitrile ligands in each case. The reaction of the cobalt(II) perchlorate complex of 1Me with carbonmonoxide gives an octahedral, 19-valence-electron dicarbonyl complex in which one of the dimethylphosphanyl groups is uncoordinated. A structural relative of 1Me, C5H3N[CMe(CH2PMe2)2][CMe2(CH2PMe2)] (2), which contains one fewer dimethylphosphanyl donor, can be prepared in a straightforward manner from 2-ethyl-6-isopropylpyridine in a four-stage process. In a series of mononuclear nickel(II) complexes, 2 employs only its three phosphane donors which, together with a monodentate ligand (acetonitrile, acetamide, or bromide), provide a tetragonal coordination environment for the metal ion which is intermediate between square planar and tetrahedral. The acetamide complex is generated from the acetonitrile complex by slow hydrolysis of the coordinated ligand. Full spectroscopic details for the complexes, as well as X-ray structure analyses, are reported. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Complex formation between FeX(2)6 H(2)O (X=BF(4) or ClO(4)) and the pyridine-derived tetrapodal tetraphosphane C(5)H(3)N[CMe(CH(2)PMe(2))(2)](2) (1) in methanol proceeds with solvent-induced cleavage of one PMe(2) group. Depending on the reaction temperature and the nature of the counterion, iron(II) is coordinated, in distorted square-pyramidal fashion, by the anionic remainder of the chelating ligand, C(5)H(3)N[CMe(CH(2)PMe(2))(2)][CMe(CH(2)PMe(2))(CH(2) (-))] (NP(3)C(-) donor set: X=BF(4), -50 degrees C: 2; X=ClO(4), RT: 4) or its protonated form C(5)H(3)N[CMe(CH(2)PMe(2))(2)][CMe(CH(2)PMe(2))(CH(3))], in which the methyl group is in agostic interaction with the metal centre (X=BF(4), RT: 3; X=ClO(4), +50 degrees C: 5). A monodentate phosphinite ligand Me(2)POMe, formed from the cleaved PMe(2) group and methanol, completes the coordination octahedron in both cases. Working in CD(3)OD (X=BF(4), RT) gives the deuterium-substituted analogue of 3, with ligands L(CH(2)D) (L=residual chelating ligand) and Me(2)POCD(3). A mechanism for the observed phosphorus-carbon bond cleavage is suggested. Complex 2, when isolated at -50 degrees C, is stable in the solid state even at room temperature. The reaction of 2 in methanol with carbon monoxide (10.5 bar) at elevated temperature forms, in addition to as yet unidentified side products, the carbonyl complex [(1)Fe(CO)](BF(4))(2) (7), in which the previous P--C bond cleavage has been reversed, reforming the original tetrapodal pentadentate NP(4) ligand 1. All compounds have been fully characterised, including X-ray structure analyses in most cases.

In the course of researching methods of sampling and monitoring toxic metals (as arsenic ions) in water and wastewaters, we selected the use of sorption for in-depth examination. Among other things, sorption modelling was investigated by the mechanism of surface complexation in order to describe the process or even predict the outcome for the expected rate, and in general, to develop this detection technique for pollutants. This investigation will include the application of goethite mineral as the suitable metal ion sorbent and the use of appropriate computer software. The relative thermodynamic aqueous speciation of the solution, in similar experimental conditions, was also studied.

The tetrapodal pentadentate NP4 ligand 2,6-C5H3N[CMe(CH2PPh2)2]2, which is readily prepared from the reaction of the corresponding tetrabromide with KPPh2, forms mononuclear complexes with nickel(II) and cobalt(II). In the case of nickel(II), the coordination mode adopted by the ligand is sensitive to the nature of the counterion: Use of NiCl2 gives a chloro complex as one of the products, which, as determined by X-ray crystallography, has a central ion coordinated by three phosphane and one chloro substituent in a distorted square planar fashion; by contrast, the reaction with Ni(BF4)2 gives a complex of approximate square-pyramidal geometry as the only product, in which all five atoms of the NP4 set are coordinated to the nickel ion, despite the steric bulk of the eight phenyl substituents. The same type of complex is obtained with Co(BF4)2 and Co(ClO4)2. Attempts to increase the coordination number of either complex beyond 5 have been unsuccessful. In particular, the cobalt(II) complex (17 valence electrons) is practically inert to reagents such as O2, NO, NO+, I2, Cl2 or H2. Full spectroscopic details for the complexes, as well as X-ray structure analyses, are reported. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

In the context of derivatisation studies of the pentaamine ligand 2,6-C5H3N[CMe(CH2NH2)2]2 (2-(6-(1,3-diamino-2-methylpropan-2-yl)pyridin-2-yl)-2-methylpropane-1,3-diamine, 1), we explored its zinc(II) coordination chemistry. With ZnBr2 in hot aqueous ethanol, in the absence of Lewis acid, the aqua complex [Zn(1)(H2O)]Br2 (2) is obtained, in which the pentaamine ligand acts as a square-pyramidal coordination cap. Single crystal structure data for the dihydrate of 2 are reported. In methanol solution, the complex is reactive towards carbon dioxide, and spectroscopic data (IR, 13C NMR) indicate the reversible formation of the dinuclear methyl carbonate complex [(Zn(1))2(μ2-(η1-O,η1-O)O2COCH3)]Br3.

International audience; The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1), which possesses four equivalent primary amino groups, has been used to prepare the diiron(III) oxo complex [(1)2Fe2(μ2-O)](ClO4)4(2), by aerobic oxidation of the mononuclear iron(II) aqua complex in methanol. This synthesis is unusual because iron(II) complexes of primary amines are normally unstable with respect to metal-induced ligand oxidation. X-ray as well as Mössbauer data indicate a high-spin electronic configuration for the metal centres in the dinuclear complex.

The use of the pyN4 coordination cap to synthesize high-nuclearity clusters supported by polycyanometallate units was explored. The heptanuclear bimetallic complexes 2 and 3 were obtained as their salts by reacting stoichiometric amounts of pyN4 and Ni(ac)2 with K3[Fe(CN)6] and K4[Mo(CN)8], respectively. The tetranuclear complex 4 formed as a by-product in the synthesis of 3. Temperature-dependent magnetic susceptibility measurements carried out for 2 revealed that this compound behaves as a paramagnet over the whole range of temperatures. Synthese und Struktur bimetallischer Cluster aus Polycyanometallat-Zentraleinheiten und [NiII(pyN4)]-Kappen Es wird uber die Synthese vielkerniger Cluster, in denen sich {(pyN4)Ni}-Koordinationsfragmente um Polycyanometallat-Einheiten gruppieren, berichtet. Die siebenkernigen bimetallischen Komplexe 2 und 3 wurden in Form der entsprechenden Salze aus der Umsetzung stochiometrischer Mengen pyN4 und Ni(ac)2 mit K3[Fe(CN)6] bzw. K4[Mo(CN)8] erhalten. Der vierkernige Komplex 4 entstand als Nebenprodukt bei der Synthese von 3. Temperaturabhangige Messungen der magnetischen Suszeptibilitat von 2 belegen, dass diese Verbindung sich uber den gesamten Temperaturbereich als Paramagnet verhalt.

With the intention of preparing tetrapodal pentadentate ligands having NS4 or NP4 donor sets, we investigated reactions of the previously reported tetratosylate 2,6-C5H3N[CMe(CH2OTs)2]2 (2) with thiourea or diphenylphosphide, but found them not to proceed cleanly, and to give mixtures of products. A derivative of 2 better suited to nucleophilic substitution is the corresponding tetrabromide 2,6-C5H3N[CMe(CH2Br)2]2(2,6-bis-(2-bromo-1-bromomethyl-1- methyl-ethyl)-pyridine, 3), which is obtained in excellent yield from 2 by treatment with LiBr in dimethylsulfoxide. The reaction of 3 with 4 eq of thiourea in refluxing ethanol gives a single product. Substitution is not quantitative, however, and the product likely is a bis(thiouronium) bis(bromide) salt. Similarly, the reaction of 3 with 4 eq of potassium O-ethyl xanthogenate displaces only two out of the four bromo substituents under the chosen conditions; workup then leads to what is formulated as a bis(thietane) derivative formed by intramolecular cyclisation. By contrast, nucleophilic substitution with NaSEt in ethanol is quantitative, and the thioether 2,6-C5H3N[CMe(CH2SEt)2]2(2,6-bis-(2-ethylsulfanyl-1-ethylsulfanylmethyl- 1-methyl-ethyl)-pyridine, 4) has been isolated in close to 60% yield. Likewise, and in spite of the considerable steric bulk amassed in the molecule, the reaction of 3 with an excess of KPPh2 in THF proceeds smoothly (even at -50 "C), to give the tetraphosphane 2,6-C5H3N[CMe(CH2PPh2)2]2(2,6-bis-{2-(diphenyl-phosphanyl)-1-[(diphenyl-phosphanyl)- methyl]-1-methyl-ethylg-pyridine, 5) in 65% yield. In order to assess possible pathways of oxidative degradation relevant to the coordination chemistry of this ligand, 5 was treated with NO in CH2Cl2 or ether at different temperatures. In two cases, reaction was observed to produce the oxide 2,6-C5H3N[CMe(CH2P(=O)Ph2)2]2 (6) as a colourless solid in near quantitative yield, with concomitant formation of N2O. All compounds have been characterised by 1H, 13C and 31P NMR spectroscopy (as applicable); IR spectroscopic and elemental analysis data are reported, and the crystal structure of 6 has been determined.

The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) forms mononuclear complexes with CuII, as shown for a series of compounds of the type [(1)Cu]X2 (X = Br, SCN, PF6, ClO4). The coordination environment of the copper ion is square-pyramidal, with the pyridine nitrogen atom of 1 in the apical position. The axial bond Cu−Npy is significantly longer than the four Cu−N bonds at the base of the pyramid, average values being 2.16 A and 2.03 A, respectively. When [(1)Cu]X2 (X = PF6, ClO4) is refluxed with an excess of acetone in methanol in the absence of base, the corresponding bis(isopropylideneimine) complexes [{(nac)2pyN4}Cu]X2 are obtained, in which two diametrically opposite primary amino groups of 1 have condensed with the ketone. The complex cations, which are now chiral, again have square-pyramidal coordinated CuII. In the presence of sodium methoxide, the condensation of [(1)Cu]X2 with acetone proceeds further, leading to the formation of one diacetone-amine-imine linkage in the product. A 12-membered 1,5,9-triazamacrocycle, incorporating the 2,6-disubstituted pyridine unit, is thus formed. Two podand nitrogen donors of the NN4 set (which retains its square pyramidal topology) remain: a primary amine and an isopropylidene imine group. The product forms as a mixture of both conceivable isomers (azomethine groups at the base of the pyramid cis or trans), which were separated and structurally characterised as the tribromocuprate(I) and hexafluorophosphate salts, respectively. The condensation of metal-free 1 with acetone in the presence of two equivalents of HBr yields the bis(aminal) salt (2,6-C5H3N{CMe[cyclo-CH2NH2C(CH3)2NHCH2-]}2)Br2, in which one of the secondary amino groups in each diazacyclohexane ring is protonated, as ascertained by X-ray crystallography. The aminal is a metal-free tautomer of the bis(isopropylideneimine) ligand (nac)2pyN4, to which it converts upon reaction with CuII under suitable conditions, providing an alternative synthetic route to complexes of the type [{(nac)2pyN4}Cu]X2.

The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) readily reacts with four equivalents of 2-hydroxyphenyl-substituted carbonyl compounds, such as (i) 2-hydroxybenzaldehyde, (ii) 2,4-dihydroxybenzaldehyde, (iii) 3,5-di-rm-butyl-2-hydroxybenzaldehyde, and (iv) 2-hydroxyacetophenone, to give the corresponding Schiff base derivatives with N N 4O .1 donor sets (i: 2; ii: 3, iii: 4; iv: 5). With ligand 5, two transition metal complexes (M = Ni11 (6), Cu11 (7)) have been isolated, which are dinuclear and contain square-planar M(N2O2) units, linked by a pyridine-derived spacer whose nitrogen atom is uncoordinated. The reaction of 1 with ferrocenecarbaldehyde affords, under neutral conditions and irrespective of an excess of the aldehyde present, the bis(aminal) 8 as the major product, in which the two 1,3-propylenediamine subunits of 1 have condensed with two carbaldehyde moieties to give two 1,3-diazacyclohexane rings. All compounds have been characterised by IR spectroscopy and elemental analysis, as well as 1H and 13C NMR spectroscopy (except paramagnetic 7). The crystal structures of compounds 2, 6, 7, and 8 have been determined.