Nuno A. G. Bandeira

A Metal-Free Synthesis of N-Aryl Carbamates under Ambient Conditions.

The first chemo- and site-selective process for the formation of N-aryl-carbamates from cyclic organic carbonates and aromatic amines is reported. The reactions proceed smoothly under extremely mild reaction conditions using TBD (triazabicyclodecene) as an effective and cheap organocatalyst, thus providing a sustainable and new methodology for the formation of a wide variety of useful N-aryl carbamate synthons in good to excellent yields. Computational investigations have been performed and show the underlying reason for the observed unique reactivity as related to an effective proton-relay mechanism mediated by the bicyclic guanidine base.

Coexistence of Intramolecular Ligand-Mediated and Through Hydrogen-Bond Magnetic Interactions in a Chain of Dicopper(II) Units

The reaction of 2,8-dimethyl-5,11-bis(pyridin-2-ylmethyl)-1,4,5,6,7,10,11,12-octahydroimidazo[4,5-h]imidazo[4,5-c][1,6]-diazecine (dimp) with copper(II) nitrate in water produces the compound [Cu(2)(dimp)(H(2)O)(2)(NO(3))(2)](NO(3))(2). The single-crystal X-ray structure shows the formation of hydrogen-bonded chains in the lattice that are formed by dicopper(II) units doubly connected by nitrate/water bridges. Within the one-dimensional chains, the Cu ions are separated by either intramolecular or intermolecular distances of 7.309(2) Å or 6.255(2) Å, respectively. The magnetic susceptibility data revealing weak antiferromagnetic exchange interactions between the copper(II) ions were interpreted by considering two possible models, namely, an isolated dinuclear and a 1-D chain picture. The latter leads to an alternation J(1) = -11.6 and J(2) = -3.0 cm(-1) along the chain. In order to clarify the relative strengths of the exchange couplings through hydrogen bonds and via the bridging dimp ligand, solution EPR studies and quantum chemical calculations were carried out. EPR studies unambiguously conclude on the existence of an exchange interaction J(a) mediated by the dinucleating dimp ligand, while the through-H coupling J(b) is physically absent in solution. On the basis of dinuclear units extracted from the X-ray data, J(a) was estimated around -5.0 cm(-1) from DFT-based calculations (M06 functional), whereas J(b) is negligible. In contrast, wave function configuration interaction calculations (DDCI) support a description where both inter- and intramolecular pathways coexist with a preeminent role of H bonds with J(a) = -2.8 and J(b) = -10.4 cm(-1). Not only are these values very consistent with the extracted set of parameters (J(1), J(2) = -11.6, -3.0 cm(-1)) but the possibility to generate leading exchange coupling through weak bonds is evidenced by means of wave function-based calculations.

Calculation of Magnetic Couplings in Hydrogen-Bonded Cu(II) Complexes Using Density Functional Theory

The performance of recent density functionals for computation of molecular magnetic coupling constants (J) in hydrogen-bonded systems is evaluated. A survey of six Cu(II) dinuclear complexes is considered. The global accuracy trend is GGAs <InvalidTag-GGAs < hybrid-GGAs ≈ hybrid meta-GGAs. Hybrid meta-GGAs do not generally provide any improvement over well-established hybrid functionals such as B3LYP. It is also seen that spin projection values agree best with experiment if one uses functionals that either have large quantities of exact exchange such as B2PLYP or functionals with long-range Coulomb screening such as CAM-B3LYP or LC-ωPBE, thus suggesting that these provide a description that is free from self-interaction errors.

Phosphinothiolates as Ligands for Polyhydrido Copper Nanoclusters

The reaction of [CuI(HSC6 H4 PPh2 )]2 with NaBH4 in CH2 Cl2 /EtOH led to air- and moisture-stable copper hydride nanoparticles (CuNPs) containing phosphinothiolates as new ligands, one of which was isolated by crystallization. The X-ray crystal structure of [Cu18 H7 L10 I] (L=(-) S(C6 H4 )PPh2 ) shows unprecedented features in its 28-atom framework (18 Cu and 10 S atoms). Seven hydrogen atoms, in hydride form, are needed for charge balance and were located by density functional theory methods. H2 was released from the copper hydride nanoparticles by thermolysis and visible light irradiation.

Cis–Trans Isomeric and Polymorphic Effects on the Magnetic Properties of Water-Bridged Copper Coordination Chains

The synthesis and magnetic characterization of vanillin-based Cu(II) mononuclear complexes of formula [Cu(van)(2)(H(2)O)(2)](H(2)O)(x) (van = vanillinate; x = 0, compound 1; x = 2, compounds 2 and 3) were performed. Despite the presence of very similar [Cu(van)(2)(H(2)O)(2)] moieties, the crystal structures exhibit distinct Cu···Cu contacts and display three different through-H-bond exchange-coupling pathways. As a result of the relative positions of the water molecules, the experimental (MAGSUS) exchange-coupling constants are dissimilar, i.e., J(1) = -3.0 cm(-1) (the data have been fitted to the Bleaney-Bowers equation considering a dimer; 2J = -6.0 cm(-1)), J(2) = -4.0 cm(-1) (the data have been fitted to the Bonner-Fischer equation for a chain of monomeric copper(II) units), whereas compound 3 is paramagnetic. Subsequently, the theoretical density functional theory (DFT) and wave function theory-based (DDCI) calculations were carried out to better understand the role of the water molecule as a mediator of the magnetic coupling. The use of localized orbitals allows one to elucidate the role of the H-bonds in generating exchange interactions. Since the exchange-coupling constants are strongly dependent on the mechanisms selectively introduced, the role of the H-bond is demonstrated.

Highly Efficient Chirality Transfer from Diamines Encapsulated within a Self-Assembled Calixarene-Salen Host.

A calix[4]arene host equipped with two bis-[Zn(salphen)] complexes self-assembles into a capsular complex in the presence of a chiral diamine guest with an unexpected 2:1 ratio between the host and the guest. Effective chirality transfer from the diamine to the calix-salen hybrid host is observed by circular dichroism (CD) spectroscopy, and a high stability constant K2,1 of 1.59×10(11)  M(-2) for the assembled host-guest ensemble has been determined with a substantial cooperativity factor α of 6.4. Density functional calculations are used to investigate the origin of the stability of the host-guest system and the experimental CD spectrum compared with those calculated for both possible diastereoisomers showing that the M,M isomer is the one that is preferentially formed. The current system holds promise for the chirality determination of diamines, as evidenced by the investigated substrate scope and the linear relationship between the ee of the diamine and the amplitude of the observed Cotton effects.

Influence of chemical and visual stimuli in food-search behaviour of Procambarus clarkii under clear conditions

Chemical communication may play a major role in aquatic environments because of visibility limitations, but when turbidity is reduced do chemical and visual stimuli interact to elicit food-search behaviour by Procambarus clarkii? Does P. clarkii use visual cues to detect its prey? We conducted behavioural experiments in a Y maze, under non-turbid conditions. In the experiments P. clarkii was exposed to chemical plus visual (CV) and visual only (V) stimuli transmitted by Chironomus riparius. The results revealed that P. clarkii detected CV and V cues associated with the presence of C. riparius. In controls, with no prey, crayfish displayed only searching behaviour, whereas in treatments they exhibited searching, detection and attack behaviours. Further, there were no significant differences between CV and V stimuli in the mean number of attacks performed by crayfish. Thus, our findings indicate that under low turbidity, visual cues are sufficient to mediate prey detection by P. clarkii.

Di- versus Trinuclear Copper(II) Cryptate for the Uptake of Dicarboxylate Anions

Searching for receptors selective for the binding of dicarboxylate anions, the copper(II) complexes of the known ditopic octaazacryptand (t2pN8), derived from bistren [tren = tris(2-aminoethyl)amine] linked by p-xylyl spacers, were re-examined, with the expectation of observing a selective binding of oxalate or malonate by bridging the two copper centers of the [Cu2(t2pN8)(H2O)2](4+) receptor. Solution studies involving the supramolecular species formed by the receptor and oxalate (oxa(2-)), malonate (mal(2-)), and succinate (suc(2-)) anions are reported. The determined association constants revealed the unexpected formation of a 3:1:1 Cu/t2pN8/anion stoichiometry for the cascade species with oxa(2-) and mal(2-), and the single crystal X-ray structural characterization confirmed the presence of tricopper(II) complexes, with an unusual binding mode for the dicarboxylate anions. Each of the two copper atoms binds four nitrogen donor atoms of the t2pN8 cryptand and one additional hydroxide group, which bridges to the third copper. The square planar environment of this one is complete with two oxygen atoms from the oxalate (or the malonate). The two copper centers bound to the tren heads are ∼6.5 Å apart, each one at about 3.5 Å from the third Cu center. These studies were complemented by SQUID magnetization measurements and DFT calculations. The magnetic susceptibility measurements of the oxalate cascade complex showed a strong magnetic coupling (J = - 210 cm(-1)) between the Cu centers at a short distance (3.5 Å), while the coupling between the two equivalent Cu atoms (∼6.5 Å) was only -70 cm(-1). This result was well reproduced by DFT calculations.

Accuracy of embedded fragment calculation for evaluating electron interactions in mixed valence magnetic systems: study of 2e-reduced lindqvist polyoxometalates.

Accurate quantum chemical calculations on real-world magnetic systems are challenging, the inclusion of electron correlation being the bottleneck of such task. One method proposed to overcome this difficulty is the embedded fragment approach. It tackles a chemical problem by dividing it into small fragments, which are treated in a highly accurate way, surrounded by an embedding included at an approximate level. For the vast family of medium-to-large sized polyoxometalates, two-electron-reduced systems are habitual and their magnetic properties are interesting. In this paper, we aim at assessing the quality of embedded fragment calculations by checking their ability to reproduce the electronic spectra of a complete system, here the mixed-metal series [MoxW6-xO19](4-) (x = 0-6). The microscopic parameters extracted from fragment calculations (electron hopping, intersite electrostatic repulsion, local orbital energy, etc.) have been used to reproduce the spectra through model Hamiltonian calculations. These energies are compared to the results of the highly accurate ab initio difference dedicated configuration interaction (DDCI) method on the complete system. In general, the model Hamiltonian calculations using parameters extracted from embedded fragments nearly exactly reproduce the DDCI spectra. This is quite an important result since it can be generalized to any inorganic magnetic system. Finally, the occurrence of singlet or triplet ground states in the series of molecules studied is rationalized upon the interplay of the parameters extracted.

Te-Te interactions in inorganic rings with sulfur donors

Te(II) forms binuclear complexes with bridging [{S=P(Ph)(2)}(2)N](-) ligands and axial aryl groups with very long (TeTe)-Te-... distances, while Te(I) analogues with [S2P(R)(2)](-) have no axial ligands and exhibit a Te-Te bond. No such selenium complexes have been found, although a square-planar mononuclear derivative of [{S=P(Ph)(2)}(2)N](-) is available for both Te and Se. In an attempt to study the nature of the (TeTe)-Te-... interaction, a variety of theoretical approaches and basis sets was used in order to find the best way to reproduce the (TeTe)-Te-... distance in the Te(II) derivatives. DFT calculations with the ADF program provided the best answer, since MP2 calculations are more computationally demanding. While the formal oxidation state of Te is a requirement for a Te-Te bond, the type of ligand strongly determines the geometry of the binuclear species. The smaller [S2P(R)(2)](-) leads to more asymmetric species, with Te forming a normal Te-S bond and a very weak one; with [{S=P(Ph)(2)}(2)N](-), both Te-S bonds have comparable lengths. Selenium analogues were found to behave similarly.