SMM Behaviour of the Butterfly {CrIII 2 DyIII 2 } Pivalate Complex and Magneto-structurally Correlated Relaxation Thermal Barrier.

We are reporting the synthesis, single-crystal X-ray structure characterization, and magnetic property investigations of the pivalate butterfly {CrIII 2 LnIII 2 } complexes with Ln= Gd and Dy and the isostructural Y(III) sample. We found an anti-ferromagnetic Cr(III)-Ln(III) exchange interaction, which, as previously observed in related Cr(III)/Ln(III) systems, plays a key role in suppressing quantum tunnelling of magnetization and enhances the SMM performance in the Dy(III) complex. In fact, a pure Orbach relaxation mechanism, with absence of QT regime, is observed with a thermal barrier of 50 cm-1 , leading to magnetization hysteresis opening, measured with a commercial magnetometer, up to 3.6 K with a coercive field of 2.9 T. Analysis of SMM behaviour in literature-known butterfly {CrIII 2 DyIII 2 } complexes, reveals the existence of a magneto-structural correlation between Ueff , the thermal barrier size, and the mean Cr-Dy bond distances. Moreover, a clear correlation is found for the thermal barrier magnitude and the maximum temperature hysteresis opening and coercive field.

Remarkable Changes of the Acidity of Bound Nitroxyl (HNO) in the [Ru(Me3[9]aneN3)(L2)(NO)] n+ Family ( n = 1-3). Systematic Structural and Chemical Exploration and Bioinorganic Chemistry Implications.

This work demonstrates that the acidity of nitroxyl (HNO) coordinated to a metal core is significantly influenced by its coordination environment. The possibility that NO- complexes may be the predominant species in physiological environments has implications in bioinorganic chemistry and biochemistry. This (apparently simple) result pushed us to delve into the basic aspects of HNO coordination chemistry. A series of three closely related {RuNO}6,7 complexes have been prepared and structurally characterized, namely [Ru(Me3[9]aneN3)(L2)(NO)]3+/2+, with L2 = 2,2'-bipyridine, 4,4'-dimethoxy-2,2'-bipyridine, and 2,2'-bipyrimidine. These species have also been thoroughly studied in solution, allowing for a systematic exploration of their electrochemical properties in a wide pH range, thus granting access and characterization of the elusive {RuNO}8 systems. Modulation of the electronic density in the {RuNO} fragment introduced by changing the bidentate coligand L2 produced only subtle structural modifications but affected dramatically other properties, most noticeably the redox potentials of the {RuNO}6,7 couples and the acidity of bound HNO, which spans over a range of almost three pH units. Controlling the acidity of coordinated HNO by the rational design of coordination compounds is of fundamental relevancy in the field of inorganic chemistry and also fuels the growing interest of the community in understanding the role that different HNO-derived species can play in biological systems.

Electronic Energy Transduction from {Ru(py)4} Chromophores to Cr(III) Luminophores.

Despite the large body of work on {Ru(bpy)2} sensitizer fragments, the same attention has not been devoted to their {Ru(py)4} analogues. In this context, we explored the donor-acceptor trans-[Ru(L)4{(µ-NC)Cr(CN)5}2]4-, where L = pyridine, 4-methoxypyridine, 4-dimethylaminopyridine. We report on the synthesis and the crystal structure as well as the electrochemical, spectroscopical, and photophysical properties of these trimetallic complexes, including transient absorption measurements. We observed emission from chromium-centered d-d states upon illuminating into either MLCT or MM'CT absorptions of {Ru(L)4} or {Ru-Cr}, respectively. The underlying energy transfer is as fast as 600 fs with quantum efficiencies ranging from 10% to 100%. These results document that {Ru(py)4} sensitizer fragments are as efficient as {Ru(bpy)2} in short-range energy transfer scenarios.

Exploring the Slow Relaxation of the Magnetization in Co(III) -Decorated {Dy(III) 2 } Units.

We have prepared and structurally characterized a new member of the butterfly-like {Co(III) 2 Dy(III) 2 } single-molecule magnets (SMMs) through further Co(III) decoration, with the formula [Co(III) 4 Dy(III) 2 (OH)2 (teaH)2 (tea)2 (Piv)6 ] (teaH3 =triethanolamine; Piv=trimethylacetate or pivalate). Direct current (DC) susceptibility and magnetization measurements were performed allowing the extraction of possible crystal-field parameters. A simple electrostatic modeling shows reasonable agreement with experimental data. Alternating current (AC) susceptibility measurements under a zero DC field and under small applied fields were performed at different frequencies (i.e., 10-1500 Hz) and at low temperatures (i.e., 2-10 K). Multiple magnetization relaxation pathways are observed. Comparison with previously reported {Co(III) 2 Dy(III) 2 } complex measurements allows an overall discussion about the origin of the dynamic behavior and its relationship with crystal-field split ground multiplet sublevels.

Class III delocalization in a cyanide-bridged trimetallic mixed-valence complex.

The NIR and IR spectroscopic properties of the cyanide-bridged complex, trans-[Ru(dmap)4 {(µ-CN)Ru(py)4 Cl}2 ](3+) (py=pyridine, dmap=4-dimethylaminopyridine) provide strong evidence that this trimetallic ion behaves as a Class III mixed-valence species, the first example reported of a cyanide-bridged system. This has been accomplished by tuning the energy of the fragments in the trimetallic complex to compensate for the intrinsic asymmetry of the cyanide bridge. Moreover, (TD)DFT calculations accurately predict the spectra of the trans-[Ru(dmap)4 {(µ-CN)Ru(py)4 Cl}2 ](3+) ion and confirms its delocalized nature.

SMM features of a large lanthanide family of butterfly CrIII2LnIII2 pivalate complexes (Ln = Gd, Tb, Dy, Ho, Er, Tm and Yb).

In this work we report the synthesis, structural characterization and magnetic properties of a family of butterfly complexes {Cr2Ln2} with Ln = Tb (4), Ho (5), Er (6), Tm (7) and Yb (8), extending the family of previously reported isostructural compounds with Gd (1), Dy (2) and Y (3). As in 1 and 2, an anti-ferromagnetic Cr(III)-Ln(III) exchange interaction is found. For oblate ions 4 and 5, SMM behavior with a purely Orbach relaxation mechanism is observed with thermal barriers of 38 cm-1 (4) and 32 cm-1 (5). Complex 4 displays hysteresis opening up to 2.4 K with loss of magnetization at zero field. The prolate complex ions 6 and 7 are field-induced SMM's with dominant Orbach, direct and QTM relaxation mechanisms. Compound 8 did not show SMM properties.

Alkyl chain length influence of the functionalized diethanolamine ligand on the slow relaxation of the magnetization in {CoIII3DyIII3} complexes.

We report the synthesis, structural characterization and magnetic properties of a series of Co(III)/Dy(III) complexes built up from an N-alkyl derivatized amino-alcohol ligand diethanolamine, functionalized with CnH2n+1 alkyl chains (n = 3, 4, 6, 8 and 10). While n = 3 afforded a butterfly {CoIII2DyIII2} core, the other alkyl chains (n = 4, 6, 8) afforded hexanuclear triangle-in-triangle {CoIII3DyIII3} complexes as shown by single-crystal X-ray determinations. Infrared spectroscopy allows us to characterize the C10 derivative complex, which did not crystallize. Magnetic ac susceptibility data collected below 10 K at driving frequencies up to 10 kHz under zero-dc field and up to 1 T provide insight into the SMM behaviour of the studied complexes. The characteristic time of the magnetization dynamics can be understood in terms of the competing Raman, Direct and Orbach-like mechanisms. A relationship between the magnetization dynamics within the family of complexes and the increasing alkyl chain length is discussed.

Communication between remote moieties in linear Ru-Ru-Ru trimetallic cyanide-bridged complexes.

In this article, we report the structural, spectroscopic, and electrochemical properties of the cyanide-bridged complex salts trans-[(NC)Ru(II)(L)4(µ-CN)Ru(II)(py)4Cl]PF6 and trans-[Ru(II)(L)4{(µ-CN)Ru(II)(py)4Cl}2](PF6)2 (L = pyridine or 4-methoxypyridine). The mixed-valence forms of these compounds show a variety of metal-to-metal charge-transfer bands, including one arising from charge transfer between the remote ruthenium units. The latter is more intense when L = 4-methoxypyridine and points to the role of the bridging ruthenium unit in promoting mixing between the dπ orbitals of the terminal fragments.

Relative influence of noncovalent interactions on the melting points of a homologous series of 1,2-dibromo-4,5-dialkoxybenzenes.

Crystal structures are presented for two members of the homologous series of 1,2-dibromo-4,5-dialkoxybenzenes, viz. those with decyloxy and hexadecyloxy substituents, namely 1,2-dibromo-4,5-bis(decyloxy)benzene, C(26)H(44)Br(2)O(2), (II), and 1,2-dibromo-4,5-bis(hexadecyloxy)benzene, C(38)H(68)Br(2)O(2), (III). The relative influences which halogen bonding, π-π stacking and van der Waals interactions have on these structures are analysed and the results compared with those already found for the lightest homologue, 1,2-dibromo-4,5-dimethoxybenzene, (I) [Cukiernik, Zelcer, Garland & Baggio (2008). Acta Cryst. C64, o604-o608]. The results confirm that the prevalent interactions stabilizing the structures of (II) and (III) are van der Waals contacts between the aliphatic chains. In the case of (II), weak halogen C-Br···(Br-C)' interactions are also present and contribute to the stability of the structure. In the case of (III), van der Waals interactions between the aliphatic chains are almost exclusive, weaker C-Br···π interactions being the only additional interactions detected. The results are in line with commonly accepted models concerning trends in crystal stability along a homologous series (as measured by their melting points), but the earlier report for n = 1, and the present report for n = 10 and 16, are among the few providing single-crystal information validating the hypothesis.

Tailoring the exchange interaction in covalently linked basic carboxylate clusters through bridging ligand selection.

We are reporting new dimeric units of basic carboxylates bearing the {Fe(III)(2)M(II)O} motif for M = Co and Ni, covalently bound through the tetradentate bridging (LL) 2,2'-azopyiridine (azpy) and 2,3-di(2-pyridyl)quinoxaline ligands (dpq). We structurally characterized the hexanuclear clusters, and their magnetic properties have been fully analyzed. DFT calculations have been performed as a supplementary tool. All results evidence a weak antiferromagnetic interaction through the bridging ligands between isolated spin ground states arising from intra-Fe(2)MO core exchange couplings. Together with the pioneer 2,2'-bipyrimidine bridged systems, the new complexes reported constitute a family of complexes where the exchange interaction can be tuned by the selection of the bridging LL type ligand.

Chlorido(4,4'-dimethoxy-2,2'-bipyridine)(1,4,7-trimethyl-1,4,7-triazacyclononane)ruthenium(II) perchlorate acetonitrile disolvate and aqua(4,4'-dimethoxy-2,2'-bipyridine)(1,4,7-trimethyl-1,4,7-triazacyclononane)ruthenium(II) bis(perchlorate) dihydrate.

The title complexes, [RuCl(C(9)H(21)N(3))(C(12)H(12)N(2)O(2))]ClO(4)·2C(2)H(3)N, (I), and [Ru(C(9)H(21)N(3))(C(12)H(12)N(2)O(2))(H(2)O)](ClO(4))(2)·2H(2)O, (II), display similar structures with the Ru atom in a distorted octahedral environment. In the crystal packing of the chloride complex, (I), the Ru complex molecules are held together in pairs through C-H···Cl interactions of the 4,4'-dimethoxy-2,2'-bipyridine and chloride ligands. In the case of the aqua complex, (II), hydrogen bonding affords a tetrameric hydrogen-bonded network. These two structures are the first examples of complexes with the {Ru(1,4,7-trimethyl-1,4,7-triazacyclononane)} motif and an electron-rich substituted 2,2'-bipyridine ligand.

A dinuclear Co(III)/Co(II) complex based on the H2pmide ligand showing field-induced SMM behaviour.

We report the synthesis, structural characterization and SMM behaviour of a new mixed valence Co(II)/Co(III) dinuclear complex bearing the H2pmide ligand. Well defined molecule pairs are observed in the crystal structure, bound through H-bond interactions directed by aqua ligands. From DC magnetometry data, a spin-only Hamiltonian approach including an axial zero-field splitting term seems to be enough for reasonable modelling, with a sizeable D parameter close to 40 cm-1. The first order orbital contribution is extensively quenched due to strong distortion from octahedral symmetry of the Co(II) site. Quantum computation at the CASSCF level supports this interpretation. To model low temperature magnetization data, the H-bond intermolecular exchange interaction is required, with a magnitude close to -1 cm-1, well supported by broken-symmetry DFT computation. This exchange is highly anisotropic due to the existence of a well isolated Kramers doublet at the Co(II) site. AC magnetic susceptibility shows field-induced SMM behaviour with competing Orbach and Raman relaxation pathways as well as a quantum tunnelling process at the lowest probed temperatures. The Orbach thermal barrier agrees with the expected one from combined experimental and quantum computed DC magnetometry analysis.

Synthesis, structural characterization, and magnetic property study of {Cr3Ln3}, Ln = Gd and Dy complexes.

We have successfully prepared and structurally characterized triangle-in-triangle {Cr3Ln3} complexes with Ln = Gd and Dy employing alcohol-amine, N-methyldiethanolamine (H2mdea) and pivalate ligands. These complexes and the Yttrium analogue proved to be isostructural and crystallized in a P1 triclinic cell. DC and AC magnetic measurements were carried out and supported by quantum computations at DFT and CASSCF levels. DC magnetic data are dominated by the Cr(III)-Ln(III) antiferromagnetic interaction and by single-ion anisotropy in the case of the Dy(III) complex. Ln(III)-Ln(III) magnetic interactions are negligible, as well as Cr(III)-Cr(III) ones. From AC data, slow relaxation of the magnetization is observed at 0 DC applied magnetic field in the case of the Dy(III) complex below 4 K. From temperature and field dependence data, possible Raman and Orbach relaxation mechanisms are established in the absence of quantum tunnelling pathways, suggesting a successful suppression of the latter due to the Cr(III)-Dy(III) exchange interaction.

Anisotropic exchange interaction and field-induced SMM behaviour in a mixed valence {CoCo} complex.

We are reporting the synthesis and structural characterization of a new hexanuclear Co(ii)/Co(iii) complex starting from a versatile pivalate cobalt precursor and the racemic mixture of a chelating Schiff base type ligand. The main [CoII4CoIII2(µ3-OH)2(µ-OR)2(µ-OR')2(µ-OR'')2]6+ core is unprecedented and exhibits an inversion center that affords only two unique Co(ii) sites. We performed DC and AC magnetic measurements and analysed them in terms of the anisotropic exchange of ground Kramers doublets at each Co(ii) site due to their unquenched angular orbital contribution to the magnetic moment. Quantum computations support the experimental data treatment. The interplay of dominant antiferromagnetic exchange, inversion symmetry and a non-collinear main quantization axis affords an exchange energy spectrum with mostly non-magnetic states. Nevertheless, field induced SMM behaviour is observed at 1500 Oe and below 3 K which might be explained by the relaxation of the first excited magnetic state (which is populated enough) through the next closest excited state. The Orbach and/or Raman mechanism could be operative from the experimental and quantum computed results.

cis-2,2'-Bipyrimidine-bridged polynuclear complex: a stairway-like mixed-valent {Fe(4)} cluster.

We report the first example of a polynuclear discrete coordination compound exhibiting only bpym bridges and containing a first-row d transition metal. A smooth self-assembly one-pot synthetic route, starting from simply FeCl(2) and FeCl(3) hydrates, allowed us to prepare a tetranuclear Fe(4) cluster with a stairway-like structure and the formula cis-{[(H(2)O)Cl(3)Fe(III)-µ(bpym)Fe(II)Cl(2)]}(2)-µ(bpym) (1) . All spectroscopic data suggest that complex 1 is a valence-localized mixed-valent Fe(II)-Fe(III) cluster with typical Mössbauer lines for both sites, which do not change with temperature. Reflectance spectroscopy did not allow one to distinguish an intervalence charge-transfer band. However, time-dependent density functional theory (DFT) calculations predict a weak high-energy Fe(II) → Fe(III) transition. Regarding the magnetic properties, the high-spin Fe(II) and Fe(III) ions interact in a weakly antiferromagnetic way with isotropic J constants of only a few wavenumbers as derived from direct-current susceptibility and magnetization data. Broken-symmetry DFT calculations support these observations.

Structural characterization and magnetic property studies of a mixed-valence {CoIIICo} complex with a µ4-oxo tetrahedral {Co} motif.

We have synthesized and structurally characterized a new mixed valence pentanuclear Co complex, bearing a rare µ4-O-tetrahedral CoII4 unit, by employing a pyridine-like Schiff base ligand. We have performed DC magnetic susceptibility and magnetization measurements over polycrystalline samples and chemical quantum computations in order to understand the exchange interaction pattern within Co(ii) sites and ground state magnetic anisotropy. This new complex shows an overall antiferromagnetic exchange interaction whose strength strongly depends on the local symmetry of Co(ii) sites. Also, local ion magnetic anisotropy reveals a strongly axial behaviour with the lowest Kramers doublet (KD) at each Co(ii) ion sufficiently isolated from excited states at low temperatures. Two Co(ii) sites show tetrahedral symmetry and the spin only formalism including axial zero-field splitting (ZFS) term properly described them; on the other hand, the other two Co(ii) sites have distorted octahedral and square base pyramidal coordination spheres, and a strong orbital contribution leads to a failure of the spin only formalism. A model of four Seff = 1/2 exchange interacting sites is necessary in order to account for low temperature magnetization behaviour. In view of the strongly anisotropic KD states, the exchange interactions are forced to be modelled as anisotropic ones. Overall, experimental data and quantum chemical computations are in good agreement, supporting the proposed model for magnetic behaviour.

Nickel(II) complexes based on L-amino-acid-derived ligands: synthesis, characterization and study of the role of the supramolecular structure in carbon dioxide capture.

The formation of the symmetrical µ3-carbonate-bridged self-assembled trinuclear NiII complex Na2{[Ni(LO)2(H2O)]3(µ3-CO3)} (LO is the carboxylate anion of a L-tyrosine derivative), involves atmospheric CO2 uptake. The asymmetric unit of the complex comprises an octahedral coordination for the NiII with two L-tyrosine-based ligands, a water molecule and one O atom of the carbonate bridge. The Ni3-µ3-CO3 core in this compound is the first reported of this kind according to the Cambridge Structural Database (CSD). The supramolecular structure is mainly sustained by hydrogen bonds developed by the phenolic functionality of the L-tyrosine moiety of one ligand and the carboxylate group of a neighbouring ligand. The crystal packing is then characterized by three interpenetrated supramolecular helices associated with a diastereoisomer of the type R-supP, which is essential for the assembly process. Magnetic susceptibility and magnetization data support weak ferromagnetic exchange interactions within the novel Ni3-µ3-CO3 core. The NiII complex obtained under the same synthetic conditions but using the analogous ligand derived from the amino acid L-phenylalanine instead of L-tyrosine gives rise to to a mononuclear octahedral system. The results obtained for the different complexes demonstrate the role of the supramolecular structure regarding the CO2 uptake property for these NiII-amino-acid-based systems.

From monomers to geometry-constrained molecules: one step further toward cyanide bridged wires.

We report on the synthesis and properties of a family of linear cyanide bridged mixed-valence heptanuclear complexes with the formula: trans-[L(4)Ru(II){(mu-NC)Fe(III)(NC)(4)(mu-CN)Ru(II)L'(4)(mu-NC)Fe(III)(CN)(5)}(2)](6-) (with L and L' a para substituted pyridine). We also report on the properties of a related pentanuclear complex. These oligomers were purified by size exclusion chromatography, characterized by electrospray ionization (ESI) mass spectrometry and elemental analysis, and their linear shape was confirmed by scanning tunneling microscopy (STM). These complexes present a rich electrochemistry associated with the seven redox active centers. The redox potential split of identical fragments indicates that there is considerable communication along the cyanide bridged backbone of the compounds, even for centers more than 3 nm apart. This small attenuation of the interaction at long distances make these cyanide bridged compounds good candidates for molecular wires. Interestingly, the extent of the communication depends on the relative energy of the fragments, as evaluated by their redox potentials, providing a guide for improvement of this interesting property.

Direct C-H metalation with chromium(II) and iron(II): transition-metal host/benzenediide guest magnetic inverse-crown complexes.

Check M(etal)ate: The chessboard and the figures represent a special reaction in which different low-polarity metals can metalate arenes directly when they are brought into the right position. In a combination of queen (sodium) and knight (chromium or iron), it is possible for the knight (usually the weaker piece) to make a direct deadly hit on the king (benzene) in this game of elemental chess.

Structural and magnetic characterization of a mu-1,5-dicyanamide-bridged iron basic carboxylate [Fe3O(O2C(CH3)3)6] 1D chain.

We are reporting an unprecedented example of a mu-1,5-dicyanamide (dca)-bridged iron basic carboxylate, [Fe3O(O2C(CH3)3)6], 1D chain. As revealed from X-ray determination, the Fe3O cores are arranged in a zigzag configuration along the chain and strictly aligned in the same plane. The chains are well-isolated by the bulky tert-butyl groups. Magnetic measurements showed that the Fe3O units are weakly antiferromagnetically coupled (J = -0.6 cm(-1)) through the dca ligand while possessing a well-isolated S = 1/2 spin ground state arising from competing antiferromagnetic interactions.