Showing papers by "James Alexis Platts published in 2013"
TL;DR: In this article, the emission spectral fingerprint of simple coordination compounds of Oh, C4v and C2v symmetry is used as a diagnostic tool to assign U(IV) species in solution and by inference, in the presence of [UO2]2+.
Abstract: Emission spectroscopy has been used for the first time in a spectroscopic study of a family of uranium(IV) halide complexes in non-aqueous media. The room temperature electronic absorption spectra of the simple coordination compounds [Li(THF)4][UX5(THF)] (X = Cl, Br, I), [Et4N]2[UCl6] and UCl4 in THF have been recorded and all transitions assigned with the aid of a comprehensive computational study using CASSCF and CASPT2 techniques. Excitation into a band of f-d and LMCT character followed by energy transfer into the 5f-orbital manifold accounts for the UV-visible radiative transitions observed in the emission spectra, which have been fully assigned as arising from transitions from the 5f16d1 electronic configuration to envelopes of states arising from the ground state 5f2 configuration. The bonding in [Li(THF)4][UCl5(THF)] has been further elucidated utilising NBO and AIM calculations which describe the nature of the U–Cl bond as predominantly ionic with some dative covalent character and substantial overlap between the Cl 3p orbitals and 5f and 6d orbitals on uranium. These studies indicate that the emission spectral fingerprint of simple U(IV) compounds of Oh, C4v and C2v symmetry are similar and characteristic and may be used as a diagnostic tool to assign U(IV) species in solution and by inference, in the environment, in the presence of [UO2]2+.
54 citations
TL;DR: Electrochemical studies confirmed subtle perturbation of the Ir(III/IV) redox couple as a function of ligand variation and TD-DFT calculations indicate that UV-vis absorption and subsequent emission has substantial MLCT character, mixed with LLCT.
Abstract: The syntheses of five new heteroleptic iridium complexes [Ir(L1–4)2(Diobpy)]PF6 (where Diobpy = 4,4′-dioctylamido-2,2′-bipyridine) and [Ir(L3)2(bpy)]PF6 (where L = para-substituted 2,3-diphenylquinoxaline cyclometalating ligands; bpy = 2,2′-bipyridine) are described. The structures of [Ir(L3)2(Diobpy)]PF6 and [Ir(L3)2(bpy)]PF6 show that the complexes each adopt a distorted octahedral geometry with the expected trans-N, cis-C arrangement of the cyclometalated ligands. Electrochemical studies confirmed subtle perturbation of the IrIII/IV redox couple as a function of ligand variation. Luminescence studies showed the significant contribution of 3MLCT to the phosphorescent character with predictable and modestly tunable emission wavelengths between 618 and 636 nm. DFT studies provided approximate qualitative descriptions of the HOMO {located over the Ir(5d) center (11–42%) and the phenylquinoxaline ligand (54–87%)} and LUMO {located over the ancillary bipyridine ligands (ca. 93%)} energy levels of the five co...
47 citations
TL;DR: Eight cationic heteroleptic iridium(III) complexes were prepared in high yield from a cyclometalated iridium bridged-chloride dimer bearing two ethyl-2-phenylquinoline-4-carboxylate (epqc) ligands, and are luminescent with moderately long lifetimes indicative of phosphorescence.
Abstract: Eight cationic heteroleptic iridium(III) complexes, [Ir(epqc)2(N^N)]+, were prepared in high yield from a cyclometalated iridium bridged-chloride dimer bearing two ethyl-2-phenylquinoline-4-carboxylate (epqc) ligands. Two X-ray crystallographic studies were undertaken on selected complexes (where the ancillary ligand N^N = 4,4′-dimethyl-2,2′-bipyridine and 4,7-diphenyl-1,10-phenanthroline) each confirming the proposed formulations, showing an octahedral coordination at Ir(III). In general, the complexes are luminescent (620–630 nm) with moderately long lifetimes indicative of phosphorescence. Hydrolysis of the ethyl ester moieties of the epqc ligands gave the analogous cinchophen-based complexes, which were water-soluble and visibly luminescent (568–631 nm). The spectroscopic and redox characterisation of the complexes was complemented by DFT and TD-DFT calculations, supporting the assignment of dominant 3MLCT to the emissive character.
43 citations
TL;DR: QSAR calculations using VolSurf descriptors to model the lipophilicity of 53 Pt(iv) complexes with a diverse range of axial and equatorial ligands results in an accurate and robust model, that moreover highlights the importance of size and hydrophobicity terms and the modest relevance of hydrogen bonding.
Abstract: We report QSAR calculations using VolSurf descriptors to model the lipophilicity of 53 Pt(IV) complexes with a diverse range of axial and equatorial ligands. Lipophilicity is measured using an efficient HPLC method. Previous models based on a subset of these data are shown to be inadequate, due to incompatibility of whole molecule descriptors between carboxylato and hydroxido ligands. Instead, the interaction surfaces of complexes with various probes are used as independent descriptors. Partial least squares modelling using three latent variables results in an accurate (R2 = 0.92) and robust model (Q2 = 0.87) of lipophilicity, that moreover highlights the importance of size and hydrophobicity terms and the modest relevance of hydrogen bonding.
39 citations
TL;DR: In this paper, the photophysical properties of a fac-Re(CO)3-bisthiazole complex are reported, which is shown to break the Kasha-Vavilov rule and has unusual emission wavelength variation due to the presence of photoinduced isomerisation from the stable, ground-state N,N-bound thiazole complexes to S,N and S,S-donation in the photo-induced excited state.
Abstract: We report highly unusual photophysical properties of a fac-Re(CO)3–bisthiazole complex, which is shown to break the Kasha–Vavilov rule. Herein, we show that such a complex has unusual emission wavelength variation due to the presence of photo-induced isomerisation from the stable, ground-state N,N-bound thiazole complex to S,N- and S,S-donation in the photo-induced excited state. A triple excitation, triple emission profile along with extensive DFT calculations support this theory to showcase an unusual circumstance of suulfur donation to the fac-Re(CO)3 core upon luminescence. fac-Rhenium tricarbonyl complexes are extensively studied due to their fluorescent properties, high stability, and ready incorporation into conjugates with broad applicability in imaging and targeting. The spectroscopic features of the complex under study have broad fundamental interest while also offering potential for design of novel agents for use in biological imaging studies.
32 citations
TL;DR: An approach to overcome the challenge of quadruplex conformational flexibility is presented here, which uses a combined multiple molecular dynamics and sampling approach and two test small molecules have been used, RHPS4 and pyridostatin, which themselves have contrasting degrees of conformational flexible.
Abstract: G-quadruplexes are higher-order four-stranded structures formed from repetitive guanine-containing tracts in nucleic acids. They comprise a core of stacked guanine-quartets linked by loops of length and sequence that vary with the context in which the quadruplex sequence occurs. Such sequences can be found in a number of genomic environments; at the telomeric ends of eukaryotic chromosomes, in promoter regions, in untranslated sequences and in open reading frames. Quadruplex formation can inhibit telomere maintenance, transcription and translation, especially when enhanced by quadruplex-binding small molecules, and quadruplex targeting is currently of considerable interest. The available experimental structural data shows that quadruplexes can have high conformational flexibility, especially in loop regions, which has hampered attempts to use high-throughput docking to find quadruplex-binding small-molecules with new scaffolds or to optimize existing ones with structure-based design methods. An approach to overcome the challenge of quadruplex conformational flexibility is presented here, which uses a combined multiple molecular dynamics and sampling approach. Two test small molecules have been used, RHPS4 and pyridostatin, which themselves have contrasting degrees of conformational flexibility.
30 citations
TL;DR: It is shown that all drugs induce broadly similar changes to the regular helical structure of DNA, but that variations in ligand lead to subtle differences in complex geometry, with cisplatin exhibiting notably different properties to other drugs.
Abstract: Hybrid QM/MM calculations on adducts of five platinum-based anti-cancer drugs, namely cisplatin, oxaliplatin, lobaplatin, and heptaplatin are reported. Starting from the NMR structure of a cisplatin–DNA octamer complex (PDB entry 1AU5), we compare DNA binding of drugs that differ in their carrier ligands, and hence in their potential interactions with DNA. It is shown that all drugs induce broadly similar changes to the regular helical structure of DNA, but that variations in ligand lead to subtle differences in complex geometry, with cisplatin exhibiting notably different properties to other drugs. Cisplatin is also the most weakly bound of drugs considered here, and heptaplatin the most strongly bound. Differences in binding appear to be due to changes in the pattern of non-covalent interactions between drug and DNA, especially hydrogen bonding to oxygen in guanine and phosphate groups. Despite adopting very similar geometries, two isomers of lobaplatin (RRS and SSS) are found to have quite different binding energies, the latter being bound by up to 30 kcal mol−1 more than the former.
27 citations
TL;DR: In this article, the authors report benchmark interaction energies, estimated from the extrapolated MP2 basis set limit with CCSD(T) correction, for several model complexes and assess faster, more approximate methods including MP2 and its local approximation, along with various forms of density functional theory.
Abstract: Anion-π interactions are attractive interactions between electron-poor aromatic rings and electron-rich negative ions or groups. Predicted by theoretical studies in the late 1990s, the first experimental evidence of anion-π interactions was provided by two independent studies in 2004. Since then, the role of these interactions in chemical and in biochemical systems has been investigated. In this work we report benchmark interaction energies, estimated from the extrapolated MP2 basis set limit with CCSD(T) correction, for several model complexes. These are then used to assess faster, more approximate methods including MP2 and its local approximation, along with various forms of Density Functional Theory (DFT). The most promising of these are then used to describe the geometries and interaction energies of a series of complexes between electron-poor substituted benzene rings and anions as well as examples of such interactions in models of proteins.
25 citations
TL;DR: In this article, the interaction of triethylaluminum (TEA) with a solution of the paramagnetic Cr(I) bisphosphine complex [Cr(CO)41][Al(OC(CF3)3)4] (1 = Ph2PN(iPr)PPh2) has been studied using EPR and DFT.
20 citations
TL;DR: A genetically encoded precursor to an aryl nitrene, para-azidophenylalanine, was introduced site specifically into proteins to deduce if distinct environments were capable of caging a reactive organic intermediate.
17 citations
TL;DR: Zirconium and hafnium metallocene dihalides based on simple polyphenyl cyclopentadienes are stable room-temperature lumophores with large Stokes shifts, emitting from states with substantial LMCT character as mentioned in this paper.
TL;DR: It is shown that scaling the perturbative triples via the (T*) approximation improves agreement with benchmark values, and the performance of composite post-MP2 ab initio methods with small basis sets for description of noncovalent interactions is reported.
Abstract: We report the performance of composite post-MP2 ab initio methods with small basis sets for description of noncovalent interactions, using the S66 data set as a benchmark. For three representative complexes, it is shown that explicitly correlated coupled cluster (CCSD-F12a) methods yield interaction energies ca. 0.1 kcal/mol from the complete basis set limit with aug-cc-pVDZ. Triple excitations are not explicitly correlated in this approach, but we show that scaling the perturbative triples via the (T*) approximation improves agreement with benchmark values. Across the entire S66 data set, this approach results in a root-mean-square error (RMSE) of 0.13 kcal/mol or 3%, with well-balanced description of all classes of complex. The basis set dependence of traditional CCSD(T) interaction energies is examined, and the small 6-31G*(0.25) basis set is found to give particularly accurate results (RMSE = 0.15 kcal/mol, or 4%). We also employ spin component scaling (SCS) of CCSD-F12a data, which gives slightly better accuracy than CCSD(T*)-F12a if contributions from same- and opposite-spin pairs are optimized for this data set (RMSE = 0.08 kcal/mol, or 2%). Interpolation of local MP2 and MP3 is also shown to accurately reproduce benchmark data with both aug-cc-pVDZ (RMSE = 0.18 kcal/mol or 5%) and 6-31G*(0.25) (RMSE = 0.13 kcal/mol or 4%).
TL;DR: The solid-state structure of the known complex [Et4N][U(NCS)5(bipy)2] has been re-determined and a detailed spectroscopic and magnetic study has been performed in order to confirm the oxidation states of both metal and bipy ligand.
Abstract: The solid-state structure of the known complex [Et4N][U(NCS)5(bipy)2] has been re-determined and a detailed spectroscopic and magnetic study has been performed in order to confirm the oxidation states of both metal and bipy ligand. Electronic absorption and infrared spectroscopy suggest that the uranium is in its +4 oxidation state and this has been corroborated by emission spectroscopy and variable temperature magnetic measurements, as well as theoretical calculations. Therefore the bipy ligands are neutral, innocent ligands and not, as would be inferred from just a solid state structure, radical anions.
TL;DR: Investigation of ligand hyperfine couplings for the bis(acetylacetonato)-copper(ii) complex [Cu(acac)2] in frozen solution found hindered rotation appears to be caused by weak outer-sphere solvent interactions with the complex.
Abstract: ENDOR spectroscopy and DFT calculations have been used to thoroughly investigate the ligand hyperfine couplings for the bis(acetylacetonato)–copper(II) complex [Cu(acac)2] in frozen solution. Solutions of [Cu(acac)2] were prepared under anhydrous conditions, and EPR revealed that the g and CuA values were affected by traces of water present in the solvent. The ligand HAi hyperfine couplings were subsequently investigated by CW and pulsed ENDOR spectroscopy. Anisotropic hyperfine couplings to the methine protons (HAi = 1.35, −1.62, −2.12 MHz; aiso = −0.80 MHz) and smaller couplings to the fully averaged methyl group protons (HAi = −0.65, 1.658, −0.9 MHz; aiso = 0.036 MHz) were identified by simulation of the angular selective ENDOR spectra and confirmed by DFT. Since the barrier to methyl group rotation was estimated to be ca. 5 kJ mol−1 by DFT, rapid rotation of these –CH3 groups, even at 10 K, leads to an averaged value of HAi. However, variable temperature X-band Mims ENDOR revealed an additional set of hyperfine couplings which showed a pronounced temperature dependency. Using CW Q-band ENDOR, these additional couplings were characterised by the hyperfine parameters HAi = 3.45, 2.9, 2.62 MHz, aiso = 2.99 MHz and assigned to a hindered methyl group rotation. This hindered rotation of a sub-set of methyl groups occurs in 120° jumps, such that a large Adip and aiso component is always observed. Whilst the majority of the methyl groups undergo free rotation, a sub-set of methyl groups experience hindered rotation in frozen solution, through proton tunnelling. This hindered rotation appears to be caused by weak outer-sphere solvent interactions with the complex.
TL;DR: In this article, a successful description of the electron density in a Ga-containing compound using the present multipole model requires separation and independent radial scaling of the three Ga core shells.
Abstract: The present study shows that a successful description of the electron density in a Ga-containing compound using the present multipole model requires separation and independent radial scaling of the three Ga core shells It is also shown that two similar compounds only differing in the oxidation state of the Ga atom provides similar residual density maps and thus the deficiencies in the standard multipole model are very similar in the two cases Nevertheless, we find two fundamentally different core modified multipole models It is found that introduction of the modified core parameters in the modelling of the experimental data has no significant impact An analysis of the ELI-D is introduced to study the localization of electrons in the Ga-guanidinate moiety There is a clear analogy between the Laplacian and the ELI-D, however the latter is able to reveal details from the total density which is not available in the Laplacian Only in an analysis of the valence density alone can the Laplacian reveal the finer details
TL;DR: The geometry around the hydrogen-bearing nitrogen atoms, whether planar or non-planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O-C-N-H torsion angles occurs to maintain almost-linear hydrogen-bonding interactions.
Abstract: A series of urea-derived heterocycles, 5N-substituted hexahydro-1,3,5-triazin-2-ones, has been prepared and their structures have been determined for the first time. This family of compounds only differ in their substituent at the 5-position (which is derived from the corresponding primary amine), that is, methyl (1), ethyl (2), isopropyl (3), tert-butyl (4), benzyl (5), N,N-(diethyl)ethylamine (6), and 2-hydroxyethyl (7). The common heterocyclic core of these molecules is a cyclic urea, which has the potential to form a hydrogen-bonding tape motif that consists of self-associative equation image(8) dimers. The results from X-ray crystallography and, where possible, Laue neutron crystallography show that the hydrogen-bonding motifs that are observed and the planarity of the hydrogen bonds appear to depend on the steric hindrance at the α-carbon atom of the N substituent. With the less-hindered substituents, methyl and ethyl, the anticipated tape motif is observed. When additional methyl groups are added onto the α-carbon atom, as in the isopropyl and tert-butyl derivatives, a different 2D hydrogen-bonding motif is observed. Despite the bulkiness of the substituents, the benzyl and N,N-(diethyl)ethylamine derivatives have methylene units at the α-carbon atom and, therefore, display the tape motif. The introduction of a competing hydrogen-bond donor/acceptor in the 2-hydroxyethyl derivative disrupts the tape motif, with a hydroxy group interrupting the N[BOND]H⋅⋅⋅O[DOUBLE BOND]C interactions. The geometry around the hydrogen-bearing nitrogen atoms, whether planar or non-planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O-C-N-H torsion angles occurs to maintain almost-linear hydrogen-bonding interactions.