Synthesis of ionisable [2 x 2] grid-type metallo-arrays and reversible protonic modulation of the optical properties of the [Co4(II)L4]8+ species

The new bis-hydrazone based ligands A and B form ionisable [2 x 2] grid-type transition metal complexes whose properties may be modulated by multiple protonation/deprotonation as shown by the reversible change in optical properties of the [Co2(II)L4]8+ complexes depending on their protonation state.     

Self-assembly of heteroleptic [2 x 2] and [2 x 3] nanogrids

Using the HETPHEN concept a general and quantitative approach to the formation of heteroleptic nanogrids is illustrated.     

Einlagerungsphasen vom TypMe x[Ta2S2C]

Zusammenfassung Ta₂S₂C bildet mit den Übergangsmetallen der ersten langen Periode Einlagerungsphasen. Die Elemente Ti, V, Cr, Mn, Fe, Co und Ni besetzen teilweise die oktaedrischen Lücken in den Schwefeldoppelschichten von 3s-Ta₂S₂C. Kupfer wird in die tetraedrischen Lücken in 1s-Ta₂S₂C eingelagert. Die Temperaturabhängigkeit der paramagnetischen Suszeptibilitäten zeigtCurie-Weiss-Verhalten, das auf ferromagnetische Ordnung in Mn_x[Ta₂S₂C] und Fe_x[Ta₂S₂C] und antiferromagnetische Ordnung in V_x[Ta₂S₂C], Cr_x[Ta₂S₂C], Co_x[Ta₂S₂C] und Cu_x[Ta₂S₂C] bei tiefen Temperaturen hinweist. Die strukturellen und magnetischen Eigenschaften dieser Phasen werden diskutiert.

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Intercalation phases of Me_x[Ta₂S₂C] typ (Me=Ti, V, Cr, Mn, Fe, Co, Ni, Cu)

Ta₂S₂C forms intercalation phases with the transition metals of the first long period. The elements Ti, V, Cr, Mn, Fe, Co and Ni occupy the octahedral voids in the sulphur double layers of 3s-Ta₂S₂C. Copper is intercalated into the tetrahedral voids of 1s-Ta₂S₂C. The temperature dependence of the paramagnetic susceptibilities exhibitsCurie-Weiss behaviour indicating ferromagnetic ordering for Mn_x[Ta₂S₂C] and Fe_x[Ta₂S₂C] and antiferromagnetic ordering for V_x[Ta₂S₂C], Cr_x[Ta₂S₂C], Co_x[Ta₂S₂C], and Cu_x[Ta₂S₂C] at low temperatures. The structural and magnetic properties of these phases are discussed.

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Mit 2 Abbildungen

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Herrn Prof. Dr.Hans Nowotny in Verehrung gewidmet.     

Photocatalytic [2 + 2] cycloadditions of enones with cleavable redox auxiliaries

α,β-Unsaturated 2-imidazolyl ketones undergo [2 + 2] cycloaddition with a variety of Michael acceptors upon irradiation with visible light in the presence of Ru(bpy)(3)(2+). Cleavage of the imidazolyl auxiliary from the cycloadducts affords cyclobutane carboxamides, esters, thioesters, and acids that would not be accessible from direct cycloaddition of the corresponding unsaturated carbonyl compounds.     

Electrochemical and theoretical study of the redox properties of transition metal complexes with [Pt2S2] cores

The oxidation processes undergone by the [Pt2(mu-S)2] core in [Pt2(P[intersection]P)2(mu-S)2](P[intersection]P = Ph2P(CH2)nPPh2, n= 2,3) complexes have been analysed on the basis of electrochemical measurements. The experimental results are indicative of two consecutive monoelectronic oxidations after which the [Pt2(mu-S)2] core evolves into [Pt2(mu-S2)]2+, containing a bridging disulfide ligand. However, the instability of the monoxidised [Pt2(P[intersection]P)2(mu-S)2]+ species formed initially, which converts into [Pt3(P[intersection]P)3(mu-S)2]2+, hampered the synthesis and characterisation of the mono and dioxidised species. These drawbacks have been surpassed by means of DFT calculations which have also allowed the elucidation of the structural features of the species obtained from the oxidation of [Pt2(P[intersection]P)2(mu-S)2] compounds. The calculated redox potentials corresponding to the oxidation processes are consistent with the experimental data obtained. In addition, calculations on the thermodynamics of possible processes following the degradation of [Pt2(P[intersection]P)2(mu-S)2]+ are fully consistent with the concomitant formation of monometallic [Pt(P[intersection]P)S2)] and trimetallic [Pt3(P[intersection]P)3(mu-S)2]2+ compounds. Extension of the theoretical study on the [Pt2Te2] core and comparisons with the results obtained for [Pt2S2] have given a more general picture of the behaviour of [Pt2X2](X = chalcogenide) cores subject to oxidation processes.     

Spectroscopic and redox properties of amine-functionalized K2[OsII(bpy)(CN)4] complexes

We report the first examples of amine-functionalized K(2)[Os(II)(bpy)(CN)(4)] (bpy = 2,2'-bipyridine) complexes. The tetracyanoosmate complexes were prepared by UV irradiation (λ = 254 nm) of K(4)[Os(II)(CN)(6)] and primary amine-functionalized bpy ligands in acidic aqueous media. The aqueous solution pH dependences of the spectroscopic and redox properties of 4,4'- and 5,5'-substituted complexes have been investigated. The pendant amine functional groups and coordinated cyanide ligands are basic sites that can be sequentially protonated, thereby allowing systematic tuning of electrochemical and optical spectroscopic properties.     

Calorimetric study of correlated disordering in [Hdamel]2[Cu(II)(tdpd)2] x 2 THF crystal

The heat capacity of [Hdamel]2[Cu(II)(tdpd)2] x 2 THF was measured from 6 to 250 K by adiabatic calorimetry. There are four heat anomalies around 150 K associated with disordering in the orientation of the uncoordinated THF molecules and in the conformation of the out-of-plane allyl groups of [Hdamel](+) units. The total entropy of transition was determined to be 19.8 J K(-1) mol(-1), less than the 4R ln 2 (R = gas constant) expected from the crystal structure at room temperature. The smallness of the total entropy change on phase transitions proves the presence of the strong motional correlation between the adjacent allyl groups. The calorimetric conclusion agreed with the crystal structure at 200 K re-examined in this study.     

Self-assembly,characterisation, and crystal structure of multinuclear metal complexes of the [2 x 3] and [3 x 3] grid-type

The self-assembly of new multimetallic complexes of grid-type architecture is described. The binding of a set of tris-terdentate ligands, 1 a-1 d, based on terpyridine-like subunits, with different octahedrally coordinated metal ions leads to the formation of species whose structure depends strongly on the ligand, the metal ion, the counterion, the solvent, and the reaction conditions. Under suitable conditions, the [3 x 3] grid was obtained from the reaction of ligand 1 a with zinc tetrafluoroborate and from ligand 1 b with mercury triflate. The other ligands led to the formation of mainly one compound of composition [M(6)L(5)](12+), which has the structure of an incomplete [2 x 3] grid. The crystal structure of such a [2 x 3] grid, [Co(6)(1 d)(5)](12+), has been determined. In this complex, the three central pyrimidine-pyridine-pyrimidine non-coordinating sites adopt transoid NCbond;CN conformations. The much less stable cisoid conformations, the "pinching" of the coordination sites in the complex, the weaker donor strength of the central binding site, and the steric demand of the substituents are all factors contributing to the reluctance to produce the [3 x 3] structure. A subtle interplay between the nature of the metal, the steric demand of the ligand, the reaction conditions, and the type of counterion determine the product of self-assembly. The results obtained show that by tuning the parameters, complexes containing six or nine octahedrally coordinated metal ions in a well-defined grid-type arrangement are accessible. Both types of arrays, [2 x 3] and [3 x 3 ], are of interest as self-assembled inorganic architectures of well-defined structure and nuclearity that may be suitable prototypes for selective information storage media.     

Electrocatalytic redox neutral [3 + 2] annulation of N-cyclopropylanilines and alkenes

Although synthetic organic electrochemistry (EC) has advanced significantly, net redox neutral electrosynthesis is quite rare. Two approaches have been employed to achieve this type of electrosynthesis. One relies on turnover of the product by the reactant in a chain mechanism. The other involves both oxidation on the anode and reduction on the cathode in which the radical cation or the radical anion of the product has to migrate between two electrodes. Herein, a home-built electrochemistry/mass spectrometry (EC/MS) platform was used to generate an N-cyclopropylaniline radical cation electrochemically and to monitor its reactivity toward alkenes by mass spectrometry (MS), which led to the discovery of a new redox neutral reaction of intermolecular [3 + 2] annulation of N-cyclopropylanilines and alkenes to provide an aniline-substituted 5-membered carbocycle via direct electrolysis (yield up to 81%). A chain mechanism, involving the regeneration of the substrate radical cation and the formation of the neutral product, is shown to be responsible for promoting such a redox neutral annulation reaction, as supported by experimental evidence of EC/MS.

We report the use of an online electrochemistry/mass spectrometry platform to develop a redox neutral electrosynthesis of 5-membered rings via [3 + 2] annulation of N-cyclopropylanilines and alkenes, without additional oxidant, reductant or catalyst.     

Crystal structures and magnetic properties of new cyano-bridged two-dimensional grid-like bimetallic assemblies [Ni(tn)2]2[Cr(CN)5((NO)]OH*H2O and [NI(tn)2]2[Co(CN)6]NO3*2H2O (tn=1,3-propanediamine)

Two bimetallic assemblies, [Ni(tn)(2)](2)[Cr(CN)(5)(NO)]OH.H(2)O (1) and [Ni(tn)(2)](2)[Co(CN)(6)]NO(3).2H(2)O (2) (tn = 1,3-diaminopropane), have been prepared and structurally and magnetically characterized. Crystal data for 1 (2): space group P1 (P1), a = 8.698(3) (8.937(2)) A, b = 10.001(2) (9.863(1)) A, c = 10.158(2) (10.064(1)) A, alpha = 87.40(2) (86.064(10)) degrees, beta = 65.10(2) (65.489(10)) degrees, gamma = 81.63(2) (81.572(12)) degrees and Z = 1 (1). Both structures consist of two-dimensional grid-like polycations containing Ni-N triple bond C-M linkages (M = Cr or Co) and counteranions (OH, NO(3)). Magnetic studies of 1 showed that the complex displays a metamagnetic behavior originating from intralayer ferromagnetic and interlayer antiferromagnetic interactions. Long-range antiferromagnetic ordering was observed at T(N) = 3.3 K. Complex 2 exhibits intramolecular ferromagnetic interactions through the diamagnetic N triple bond C-Co-N triple bond C bridges, owing to superexchange involving the empty d(sigma) orbital of the diamagnetic Co(III) ion.     

Mild redox complementation enables H2 activation by [FeFe]-hydrogenase models

Mild oxidants such as [Fe(C(5)Me(5))(2)](+) accelerate the activation of H(2) by [Fe(2)[(SCH(2))(2)NBn](CO)(3)(dppv)(PMe(3))](+) ([1](+)), despite the fact that the ferrocenium cation is incapable of oxidizing [1](+). The reaction is first-order in [1](+) and [H(2)] but independent of the E(1/2) and concentration of the oxidant. The analogous reaction occurs with D(2) and proceeds with an inverse kinetic isotope effect of 0.75(8). The activation of H(2) is further enhanced with the tetracarbonyl [Fe(2)[(SCH(2))(2)NBn](CO)(4)(dppn)](+) ([2](+)), the first crystallographically characterized model for the H(ox) state of the active site containing an amine cofactor. These studies point to rate-determining binding of H(2) followed by proton-coupled electron transfer. Relative to that by [1](+), the rate of H(2) activation by [2](+)/Fc(+) is enhanced by a factor of 10(4) at 25 °C.     

Solvent-modulated reversible conversion of a [2 x 2]-grid into a pincer-like complex

The triazine derived ligand reacts with one equivalent of Co(II) salts to give complexes whose architecture depends on the solvent employed: the [2 x 2]-grid like tetranuclear complex and the pincer-like mononuclear complex, obtained respectively by crystallization from nitromethane and from acetonitrile may be interconverted reversibly, the grid-pincer conversion being markedly accelerated by adding an amine.     

A rigid cavity containing tetra-cobalt(III) [2 x 2] grid complex

The synthesis and structure of a rigid, cavity containing tetra-cobalt(III) [2 x 2] grid complex using an unusual bis(bipyridine)dimethoxynaphthyridine ligand is described.     

Supramolecular spintronic devices: spin transitions and magnetostructural correlations in [Fe4IIL4]8+ [2x2]-grid-type complexes

The magnetism of a series of tetranuclear complexes of the [Fe4IIL4]8+ [2x2]-grid-type was investigated, revealing the occurrence of spin transition behavior within this class of compounds. The phenomenon depends directly on the nature of the substituent R(1) in the 2-position on the central pyrimidine group of the ligand L. All Fe(II) ions in compounds with R(1) substituents favoring strong ligand fields (R(1)=H; OH) remain completely in the diamagnetic low-spin state. Only complexes bearing R(1) substituents attenuating the ligand field by steric (and to a lesser extent electronic) effects (R(1)=Me; Ph) exhibit spin transition behavior triggered by temperature. In general, gradual and incomplete transitions without hysteresis were observed for magnetically active complexes. The systems described provide approaches to the development of (supra)molecular spintronics.     

Chiral Resolution of Spin-Crossover Active Iron(II) [2x2] Grid Complexes

Chiral magnetic materials are proposed for applications in second-order non-linear optics, magneto-chiral dichroism, among others. Recently, we have reported a set of tetra-nuclear Fe(II) grid complex conformers with general formula C/S-[Fe₄L₄]⁸⁺ (L: 2,6-bis(6-(pyrazol-1-yl)pyridin-2-yl)-1,5-dihydrobenzo[1,2-d : 4,5-d′]diimidazole). In the grid complexes, isomerism emerges from tautomerism and conformational isomerism of the ligand L, and the S-type grid complex is chiral, which originates from different non-centrosymmetric spatial organization of the trans type ligand around the Fe(II) center. However, the selective preparation of an enantiomerically pure grid complex in a controlled manner is difficult due to spontaneous self-assembly. To achieve the pre-synthesis programmable resolution of Fe(II) grid complexes, we designed and synthesized two novel intrinsically chiral ligands by appending chiral moieties to the parent ligand. The complexation of these chiral ligands with Fe(II) salt resulted in the formation of enantiomerically pure Fe(II) grid complexes, as unambiguously elucidated by CD and XRD studies. The enantiomeric complexes exhibited similar gradual and half-complete thermal and photo-induced SCO characteristics. The good agreement between the experimentally obtained and calculated CD spectra further supports the enantiomeric purity of the complexes and even the magnetic studies. The chiral resolution of Fe(II)- [2×2] grid complexes reported in this study, for the first time, might enable the fabrication of magneto-chiral molecular devices.     

Synthesis and characterization of 2H-pyrano[3,2-c]coumarin derivatives and their photochromic and redox properties

A series of 2H-pyrano[3,2-c]chromen-5-one derivatives were synthesized and characterized. Their photochromic and redox properties were investigated by the UV-vis absorption spectroscopy. While compounds with one or two phenyl groups incorporated at the 2-position were present in both ring-opened (5a and 10a) and ring-closed (6a and 11a) forms, the incorporation of an N,N-dimethylamino group on either side of the aromatic ring resulted in formation of the ring-opened (5b and 10b) forms only. The ring-closed forms 13 and 18 with a methyl substituent at the 3-position of the pyran moiety failed to exhibit photochromic behavior. Compound 23 with an N,N-dimethylamino group on the aromatic ring displayed increasing shoulder absorption in the visible region and a distinct change of color upon UV irradiation. The non-fluorescent 10b instantly changed from dark red to colorless, when treated with sodium borohydride. The reduced 28 was blue fluorescent with a quantum yield of 0.46 and could be returned to its original color via DDQ oxidation.