Andrés E. Goeta

The observation of a large gauche preference when 2-fluoroethylamine and 2-fluoroethanol become protonated

The energies of the gauche and anti conformers of 2-fluoroethylamine, 2-fluoroethanol and their protonated analogues are calculated using density functional theory. Unlike the non protonated systems, the protonated systems show a strong gauche effect where the C-F and the C-(+)NH(3) or C-F and C-(+)OH(2) bonds are gauche rather than anti to each other. Single crystal X-ray diffraction studies of 2-fluoroethylammonium compounds identify the same conformational preference.

A symmetry-breaking spin-state transition in iron(III)

Stepping up: A two-step magnetic spin transition with accompanying structural phase transitions is reported for the first time for Fe III. The transitions are observed at 187 K and 90 K on cooling with a hysteretic transition recorded upon heating during the first crossover at 106 K. The intermediate phase persists over 97 K and contains an unprecedented [HS-HS-LS] motif with tripling of the unit cell. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Anion binding by Ag(I) complexes of urea-substituted pyridyl ligands

A series of Ag(I) complexes of ureidopyridyl ligands 1 and 2 have been prepared from oxo-anion salts. In all cases the new materials contain the AgL2+ cation interacting with oxo-anions via the urea moiety. The complexes containing the para ligand 2: [Ag(2)2]CF3SO3·2H2O (3), [Ag(2)2]CH3CO2·1.33H2O·MeOH (4) and [Ag(2)2]NO3·H2O (5), all exhibit remarkably similar chain-like structures based around a linear Ag(I) centre, despite the change in the counter-ion. A recurring R22(8) hydrogen-bonding ring motif between the urea group and the oxo-anion is observed in almost all cases. An exception to this trend is the anhydrous nitrate structure [Ag(2)]NO3 (6) in which the nitrate is coordinated in a bridging position between two silver centres, which adopt distorted trigonal pyramidal geometries. Structures containing the ligand 1, [Ag(1)2]CF3SO3·0.5H2O (7), [Ag(1)2]CF3SO3·H2O·MeCN (8), [Ag(1)2]2SO4 (9), [Ag(1)2]NO3·MeOH (10) and [Ag(1)2]NO3·0.5MeOH·0.5MeNO2 (11), display very different geometries, although the R22(8) is observed to persist throughout. The most notable of these structures are 10 and 11 in which the nitrate anion is chelated within a ‘pincer’ arrangement by the silver complex. The nitrate anion is situated asymmetrically within the cavity of the host complex. This discrete nitrate complex persists in solution with strong nitrate binding by the [Ag(1)2]+ host compared to other anions being observed.

Structural approach of the features of the spin crossover transition in iron(II) compounds

We have determined the crystal structures, both in high and low spin state, of four Fe(PM-L) 2 (NCS) 2 complexes, where PM is N-2′-pyridylmethylene and the aromatic subunit L is 4-aminoterphenyl (TeA), 4-(phenylazo)aniline (AzA), 4-aminobiphenyl (BiA) or 4-(phenylethynyl)aniline (PEA). As previously reported, these compounds undergo a spin crossover at low temperature with different features of transition: very smooth and incomplete for Fe(PM-TeA) 2 (NCS) 2 , smooth with almost no hysteresis for Fe(PM-AzA) 2 (NCS) 2 , unusually abrupt for Fe(PM-BiA) 2 (NCS) 2 and abrupt with a very large hysteresis (37 K) for Fe(PM-PEA) 2 (NCS) 2 . In Fe(PM-BiA) 2 (NCS) 2 , Fe(PM-TeA) 2 (NCS) 2 and Fe(PM-AzA) 2 (NCS) 2 the spin conversion is not associated with a large structural phase transition and the space group is the same above and below the temperature of transition: orthorhombic Pccn for the two first and monoclinic P2 1 /c for the third. On the other hand, Fe(PM-PEA) 2 (NCS) 2 undergoes a change in the crystal symmetry from P2 1 /c (high spin) to Pccn (low spin) which corresponds to a strong re-organisation of the iron atom network. The evolution as a function of temperature of the FeN 6 core as well as of the intramolecular characteristics are almost identical in all four compounds. To a first approximation, the crystal packing is similar in all of the structures except that the P2 1 /c structures develop an asymmetrical molecular environment. Nevertheless, a close examination of the intermolecular interactions, classified as intra- and inter-sheet, show some differences. The intrasheet and the intersheet interactions are stronger in Fe(PM-BiA) 2 (NCS) 2 and Fe(PM-PEA) 2 (NCS) 2 than either in Fe(PM-TeA) 2 (NCS) 2 where no ‘second’ neighbour intrasheet contacts are created, or in Fe(PM-AzA) 2 (NCS) 2 where the intersheet interactions are weak. Thus, the abruptness of the transition is attributed to the combination of close intrasheet and intersheet contacts. The hysteresis effect in Fe(PM-PEA) 2 (NCS) 2 is connected to the phase transition which could occur due to an irregular iron atom network associated with very short carbon-carbon intermolecular contacts at high temperature, not found in Fe(PM-AzA) 2 (NCS) 2 which shows the same irregular iron atom network.

The molecular structures and electrochemical response of “twisted” tetra(aryl)benzidenes

The compounds N,N,N′,N′-tetra(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (4), N,N,N′,N′-tetra(4-methylphenyl)-(2,2′-dimethyl)-(1,1′-biphenyl)-4,4′-diamine (5a) and N,N,N′,N′-tetra(4-methylphenyl)-(2,2′,6,6′-tetramethyl)-(1,1′-biphenyl)-4,4′-diamine (6a) undergo two reversible one electron oxidations. The first oxidation potential increases in the order 4 < 5a < 6a, while the separation between the first and second oxidation decreases in the reverse order 4 (0.30 V) > 5a (0.16 V) > 6a (0.00 V), reflecting the decreasing thermodynamic stability of the radical cations [4+] > [5+] > [6a+]. Electronic spectroscopy and spectroelectrochemistry (UV-Vis-NIR) confirm expectations, and the introduction of methyl groups at the 2,2′ and 6,6′ positions of the 1,1′-biphenyl moiety electronically decouple the arylamine moieties. In contrast, N,N,N′,N′-tetra(phenyl)-(2,2′-dimethyl)-(1,1′-biphenyl)-4,4′-diamine (5b) and N,N,N′,N′-tetra(phenyl)-(2,2′,6,6′-tetramethyl)-(1,1′-biphenyl)-4,4′-diamine (6b) give much less kinetically stable radical cations upon oxidation, which oligomerise/polymerise through the 4 positions of the N-phenyl groups via a step-growth process. The molecular and crystal structures of 4 and 6b are also reported.

Thermal and light induced polymorphism in iron(II) spin crossover compounds

The spin crossover complexes [Fe[H(2)B(pz)(2)](2)L]([H(2)B(pz)(2)](-)= dihydrobis(pyrazolyl)borate, L = 2,2[prime or minute]-bipyridine (1), bipy and 1,10-phenanthroline, phen (2)) undergo both thermal and light induced spin crossover, but the structure of the low spin and light induced high spin states for are different from that of the thermally induced high spin state and from those of.

The R21(6) hydrogen-bonded synthon in neutral urea and metal-bound halide systems

The recurrence of the R21(6) motif between urea and metal bound halides is explored through studies in the CSD, including examples of the closely related thiourea and guanidine-based structures. A series of compounds containing the isomeric pyridyl–urea ligands 1 and 2 attached to trans-dihalide metal units have been prepared and their X-ray crystal structures determined. A hydrogen-bonding motif between the metal-bound halide ligands and the urea groups on the pyridyl ligands is observed in most cases which takes the form of an R21(6) ring. Three copper(II) complexes, [Cu(2)2Cl2]·2MeOH 3, [Cu(2)2Br2(MeOH)] 4 and [Cu(2)2Cl2(EtOH)] 5 all retain this supramolecular synthon despite a changing coordination environment and alteration of the halide ligands. A series of solvates of the complex [Pd(1)2Cl2] (6–8) shows that the R21(6) synthon does not occur in the presence of strongly hydrogen-bond accepting solvents such as DMF (6) and MeCN (7) but is observed in the presence of the hydrogen bond donor solvent methanol (8). The motif also occurs within an octahedral cadmium system [Cd(1)4Cl2]·2H2O·2MeCN 9. In all cases in which the R21(6) synthon is observed chain-like structures form between the metal complexes. Two contrasting structures based around tetrahedral zinc(II) centres [Zn(1)2Cl2] (10) and [Zn(1)2I2]·0.5MeOH (11) do not display the bifurcated synthon, showing instead an R22(8) motif in the case of 10 or no ring-type halide interaction in 11.

A new precatalyst for the Suzuki reaction—a pyridyl-bridged dinuclear palladium complex as a source of mono-ligated palladium(0)

A dinuclear pyridyl-bridged palladium complex, trans-(P,N)-[PdBr(μ-C5H4N-C2,N)(PPh3)]21, was obtained from material isolated from the Suzuki cross-coupling reaction of 2-bromopyridine with 2,4-difluorophenylboronic acid in the presence of catalytic (PPh3)4Pd. Complex 1 is an effective precatalyst for the Suzuki cross-coupling reactions of a variety organoboronic acids and aryl bromides, and represents a useful source of mono-ligated palladium(0), “(Ph3P)Pd(0)”.

Studying the Origin of the Antiferromagnetic to Spin‐Canting Transition in the β‐p‐NCC6F4CNSSN. Molecular Magnet

The crystal structure of the spin-canted antiferromagnet beta-p-NCC(6)F(4)CNSSN* at 12 K (reported in this work) was found to adopt the same orthorhombic space group as that previously determined at 160 K. The change in the magnetic properties of these two crystal structures has been rigorously studied by applying a first-principles bottom-up procedure above and below the magnetic transition temperature (36 K). Calculations of the magnetic exchange pathways on the 160 K structure reveal only one significant exchange coupling (J(d1)=-33.8 cm(-1)), which generates a three-dimensional diamond-like magnetic topology within the crystal. The computed magnetic susceptibility, chi(T), which was determined by using this magnetic topology, quantitatively reproduces the experimental features observed above 36 K. Owing to the anisotropic contraction of the crystal lattice, both the geometry of the intermolecular contacts at 12 K and the microscopic J(AB) radical-radical magnetic interactions change: the J(d1) radical-radical interaction becomes even more antiferromagnetic (-43.2 cm(-1)) and two additional ferromagnetic interactions appear (+7.6 and +7.3 cm(-1)). Consequently, the magnetic topologies of the 12 and 160 K structures differ: the 12 K magnetic topology exhibits two ferromagnetic sublattices that are antiferromagnetically coupled. The chi(T) curve, computed below 36 K at the limit of zero magnetic field by using the 12 K magnetic topology, reproduces the shape of the residual magnetic susceptibility (having subtracted the contribution to the magnetization arising from spin canting). The evolution of these two ferromagnetic J(AB) contributions explains the change in the slope of the residual magnetic susceptibility in the low-temperature region.

Imaging proton migration from X-rays and neutrons

The short hydrogen bond in the urea–phosphoric acid system has been studied by multiple-temperature X-ray single crystal diffraction. The hydrogen atom is imaged using difference Fourier methods from these data and also from previous neutron diffraction data. The migration of the hydrogen atom is clearly observed using the X-ray difference Fourier maps. The hydrogen atom positions determined from these maps are more reliable than from X-ray refinements of this atom. A greater apparent shift of the proton is observed from X-ray than from neutron diffraction, as might be expected.