The synthesis, crystal structure and spectroscopic properties of the neodymium(III) complex [Nd(btpa) Cl-2] Cl center dot 5H(2)O with btpa being the octadentate polypyridine ligand 6,6'-bis[bis(2-pyridylmethyl) aminomethyl]- 2.2'-bipyridine are reported. The structure shows that the Nd3+ ion is coordinated to seven nitrogen atoms of btpa and two chloride ions, leaving one uncoordinated pyridine group. The coordination polyhedron is best described as a monocapped cube. The btpa ligand forms a stable 1: 1 complex with Nd(CF3SO3)(3) and other selected triflates of the lanthanides(III) ions (Ln) in anhydrous acetonitrile. The stability constants of the Ln(btpa)(3+) complexes are quite similar along the series, due to a compensation between ligand-metal interaction and lanthanide desolvation. High resolution absorption spectroscopy at 4 K shows one equivalent crystallographic position of the Nd3+ ion in the structure. The Nd-btpa complexes in the solid-state and in AN solution upon excitation wavelength from the range of ligands absorption bands display NIR luminescence with the characteristic F-4(3/2) -> I-4(J) (J = 9/2, 11/2 and 13/2) transitions of the Nd3+ ion. The decay times of the F-4(3/2) state of Nd3+ equal 1.4 (293 K) and 2.3 mu s (77 K) (lambda(exc) = 350 nm) for the complexes in the solid-state as well as 0.43 (293 K) and 0.46 mu s (77 K) (lambda(exc) = 266 nm) for AN solution. (C) 2017 Elsevier B.V. All rights reserved.

A review of the electronic structure of transition metal thionitrosyl complexes is given. The optical spectra and EPR spectra of the complexes are discussed along with the results of theoretical calculations of the electronic structure including geometry optimizations. The sigma. donor and pi donor/acceptor properties of the NS ligand are compared with the isovalent NO ligand. The photochemistry of thionitrosyl complexes is presented. The reactivity of the photoreleased NS radical in solution is discussed. (C) 2015 Elsevier B.V. All rights reserved.

The synthesis and X-ray structure of a new member of the series of oxo-bridged, dinuclear chromium(III) complexes, the methyl isocyanide complex [(CH3NC)5CrOCr(CNCH3)5](PF6)4·2CH3CN, is reported. This constitutes only the third oxo-bridged, dinuclear chromium(III) complex with a homoleptic auxillay ligand sphere. Experimentally, the system shows unshifted narrow nuclear magnetic resonance (NMR) spectra that are consistent with calculations using broken symmetry density functional theory (DFT), which suggests it to be the strongest coupled, dinuclear chromium(III) complex known. Furthermore, we report the crystal structure and computed magnetic properties for [(bpy)2(SCN)CrOCr(NCS)(bpy)2](ClO4)2·2H2O (bpy = 2,2'-bipyridine), which differs from other reported oxo-bridged species by featuring a bent CrOCr(4+) core. We also interpret the spectacular 10-orders-of-magnitude variation in acid dissociation constant of the bridging hydroxo ligand in mono hydroxo-bridged dinuclear chromium(III) complexes, in terms of a valence bond model parametrized by metal-to-metal charge transfer (MMCT) and ligand-to-metal charge transfer (LMCT) energies.

A review of the advances since 1988 of the chemistry of hydroxo-bridged chromium(III) complexes will be presented. The structures of novel hydroxo- and oxo-bridged complexes will be described along with their optical and magnetic properties. The thermodynamics and kinetics of hydroxo bridge formation and cleavage and of other reactions will be discussed. The results of theoretical calculations of the optical and magnetic properties of the complexes are included. (C) 2014 Elsevier B.V. All rights reserved.

Single crystals of 1,3,4‐thiadiazolidine‐2,5‐dione (1) have been grown and the X‐ray crystal structure showed the molecule to be planar with sp2‐hybridized nitrogen atoms. Density functional theory calculations of nucleus‐independent chemical shifts profiles show that 1 is non‐aromatic. From actinometry, it was found that continuous photolysis of 1 in methanol at λ = 254 nm resulted in a decomposition with the quantum yield Φ = 0.076(7) mol/Einstein at room temperature.

The magnetic susceptibility of the dinuclear chromium(III) complex[(CH3CN)5CrOCr(NCCH3)5](BF4)4·2 CH3CN has been measured and analyzed. With a fitted value of the triplet energy J=650cm−1, the antiferromagnetic coupling is the strongest hitherto determined for an unsupported linear oxide-bridged dinuclear Cr(III) system. The applicability of DFT for computational prediction of exchange in strongly coupled chromium(III) dimers was examined and an optimal and accurate modeling approach was devised. By such modeling it was shown possible to reproduce experimental exchange coupling constants with small relative errors typically of less than 10% ranging from the strongest coupled systems to systems with moderately strong couplings. A significant influence (>20%) of the chemical nature of the peripheral, non-bridging ligands on the exchange coupling was found and rationalized.

A review of the chemistry of nitrosyl complexes of chromium published since 2002 is given including coordination complexes as well as organometallic complexes. The syntheses, electronic structures, optical absorption spectra, electron paramagnetic resonance parameters, photochemistry, reactivity, and magnetic properties of the complexes are presented.

The reaction of LnCl3 (Ln=Sm, Eu, Tb, Yb) with M(acacCN)3 (with M=Al, Cr, acacCN=3-cyanoacetylacetonate) in ethanolic solution yields a series of novel 3d-4f heterodinuclear complexes [(acacCN)2M(μ-acacCN)LnCl3]. For M=Al the complexes display strong 4f–4f lanthanide-centered luminescence due to energy transfer from the triplet state of the ligand acacCN to the lanthanide. Photophysical measurements of the complexes with M=Cr show efficient sensitization of both Ln(III) and Cr(III) through ligand excitation at 77K. Luminescence measurements indicate strong interactions between Ln(III) and Cr(III) and the degree of energy transfer between the two metals depends on the energy gap of the donor and acceptor levels.

Lanthanide-modified CdSe quantum dots (CdSe(Ln) QDs) have been prepared by heating a solution of Cd(oleate)(2), SeO2, and Ln(bipy)(S2CNEt2)(3) (bipy = 2,2'-bipyridine) to 180-190 degrees C for 10-15 min. The elemental compositions of the resulting CdSe(Ln) cores and CdSe(Ln)/ZnS core/shell QDs show this route to be highly reproducible. The optical absorption spectra of these composite materials are similar to those of the unmodified nanocrystals, but the QD-centered band edge photoluminescence (PL) is partially quenched. The time-gated emission and excitation spectra of the CdSe(Ln) cores display sensitized lanthanide-centered PL upon higher energy excitation of the nanocrystal host but not upon excitation at the lowest energy QD absorption band. Growth of the ZnS shell led to the depletion of about 60% of the lanthanide ions present together with depletion of nearly all of the lanthanide-centered PL. On these bases, we conclude that the lanthanide-centered PL from the CdSe(Ln) cores originates with Ln(3+)-related trap states associated with the QD surface.

A simple synthetic route to the preparation of a thin ZnS shell on CdSe quantum dot cores from the air-stable, single-molecular precursor zinc diethyldithiocarbamate, Zn(S(2)CNEt(2))(2), in the three-component solvent system octadec-1-ene/oleylamine/tri-n-octylphosphine (ODE/OLA/TOP) is presented. The one-pot synthesis proceeds through heating of the solution of CdSe cores and the amount of crystalline Zn(S(2)CNEt(2))(2) corresponding to a shell thickness of two monolayers of ZnS to 110-120 °C for 1-2 h. The role of the surfactants OLA and TOP and the significance of the temperature and the amount of Zn(S(2)CNEt(2))(2) have been investigated with optical absorption and luminescence spectroscopy. We show that the presence of both OLA and TOP is crucial for the low-temperature growth and that the amount of precursor corresponding to two monolayers of ZnS results in the highest quality of core/shell CdSe/ZnS quantum dots.

From high field and frequency electron paramagnetic resonance (EPR) powder spectra of the high spin chromium(II) complex trans-[Cr(NCCH3)4(FBF3)2] (1) recorded at T=5–40K the EPR parameters were found to be D=−2.19cm−1, E≈0cm−1, and g‖=g⊥=1.9833. The reaction between a MeCN solution of 1 and dioxygen gave dark green crystals of [(CH3CN)5CrOCr(NCCH3)5](BF4)4·2CH3CN (2). From the X-ray crystal structure of 2 at T=122K a Cr–O–Cr angle of 180° and a Cr–O distance of 1.7556(5)Å was found. This is the shortest among the known Cr–O distances in dinuclear complexes containing a CrOCr4+ core, and this can partly explain the strong antiferromagnetic coupling between the two chromium(III) centers in 2.

AbstractReview: structure, reactivity, optical, photochemical, and magnetic properties of thioitrosyl complexes of transition metals; 75 refs.

The novel S = 1/2 thionitrosyl complexes Cr(NS)(CN)(5)(3-), Cr(NS)(dmso)(5)(2+), and Cr(NS)(nmf)(5)(2+) (dmso = dimethyl sulfoxide, nmf = N-methylformamide) have been prepared, and their optical and electron paramagnetic resonance (EPR) spectra were studied. The values of the isotropic and anisotropic hyperfine and superhyperfine coupling constants A((53)Cr), A((14)N), and A((13)C) and of g were determined from the EPR spectra at room temperature and at 66 K. The values of A(⊥) and A(iso) in the thionitrosyl complexes were slightly higher than in the analogous nitrosyl complexes. A common feature in the optical absorption spectra of the thionitrosyl complexes in solution at 298 K is an absorption band around 600 nm with a vibronic structure whereas such a band is located around 450 nm in the analogous nitrosyl complexes. Density functional theory (DFT) studies of the series of complexes Cr(N)(H(2)O)(5)(2+), Cr(NO)(H(2)O)(5)(2+), Cr(NS)(H(2)O)(5)(2+), and Cr(NSe)(H(2)O)(5)(2+) show that the unpaired electron resides in a metal-based d(xy) orbital and that the electronic structure in the equatorial plane is similar in all four complexes and similar to Cr(3+). The σ donating ability was found in the order N(3-) ≫ NO < NS ≈ NSe and the π accepting ability in the order NO > NS ≈ NSe. Time dependent DFT calculations gave in all four complexes a d(x(2)-y(2)) ← d(xy) transition energy around 17 500 cm(-1).

The nitrosyl complex [Cr(dmso)(5)(NO)](PF6)(2) (1) (dmso = dimethyl sulfoxide) has been prepared by the solvolysis of [Cr(NCCH3)(5)(NO)](PF6)(2) in neat dmso. The optical absorption spectrum of 1 in dmso shows maxima at 734, 567, 450, 413, and 337 nm. Continuous photolysis of 1 with lambda = 365-580 nm light in dmso solution results in a release of NO with quantum yield, Phi, in the range 0.034-0.108 mol Einstein (1). Irradiation of a deoxygenated CH3CN solution of [Cr(NCCH3)(5)(NO)](PF6)(2) in the presence of excess of [Fe(S2CNEt2)(2)] results in a transfer of NO to the iron centre as shown from the characteristic EPR spectrum of [Fe(S2CNEt2)(2)(NO)] with A(iso)(N-14) = 12.2 x 10 (4) cm (1). The EPR parameters of 1 were determined: g(iso), g(parallel to) and g(perpendicular to) : 1.96725, 1.91881(4) and 1.992763(2); A(iso)(Cr-53), A(parallel to) (Cr-53) and A(perpendicular to)(Cr-53): 22.8 x 10 (4), 39 x 10 (4) and 15.8 x 10 (4) cm (1); A(iso)(N-14), A(parallel to) (N-14) and A(perpendicular to)(N-14): 5.9 x 10 (4), 2 x 10(4) and 7.540(4) x 10 (4) cm (1). (C) 2008 Elsevier B.V. All rights reserved.

An investigation on the thermodynamics of complex formation between Ag(I) ion and two tripodal ligands tris[(2-pyridyl)methyl]amine (TPA) and 6,6′-bis-[bis-(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine (BTPA) has been carried out in the aprotic solvents dimethylsulfoxide (DMSO) and dimethylformamide (DMF) by means of potentiometry and titration calorimetry. The results for TPA are compared with those already obtained for other aliphatic tripodal polyamines. In general, the TPA ligand forms complexes less stable than 2,2′,2″-triaminotriethylamine (TREN) and tris(2-(methylamino)ethyl)amine (Me 3 TREN) as a result of the combination of higher structural rigidity of TPA and lower σ-donor ability of pyridinic moieties with respect to primary and secondary amines. The same trend is found if the stability of Ag(I) complex with TPA is compared with that of tris(2-(dimethylamino)ethyl)amine (ME 6 TREN), despite the pyridinic nitrogen is formally a tertiary one. Theoretical calculations run to explain the reasons of this weaker interaction indicate that this difference is due to solvation, rather than to steric or σ-donor effects. The ligand BTPA is able to form bimetallic species whose relative stability is largely influenced by the different solvation of Ag(I) ion in DMSO and DMF rather than by the difference in the dielectric constants of these two media.

Photolysis of the thionitrosyl complex Cr(CH3CN)5(NS)2+ (1) in acetonitrile solution leads to the dissociation of nitrogen monosulfide (NS). In deaerated solution, this reaction is reversible, and flash photolysis studies demonstrate that NS reacts with Cr(CH3CN)62+ according to the rate law d[1]/dt = kon[Cr(CH3CN)62+][NS] (kon = 2.3 x 108 M-1 s-1 at 298 K). The photolysis of 1 in deaerated acetonitrile with added Fe(S2CNEt2)2 leads to the transfer of NS and the formation of a species concluded to be Fe(S2CNEt2)2(NS) based on its electron paramagnetic resonance spectrum. Analogous photolysis of 1 in the presence of added NO leads to clean formation of the nitrosyl complex Cr(CH3CN)5(NO)2+ (2) presumably by NO capture of the photoproduct Cr(CH3CN)6(2+) (3). When 1 was photolyzed in aerated acetonitrile solution, the reactive species 3 was trapped, thus leading to net photochemical transformations with excitation-wavelength-dependent quantum yields of 0.3-1.0 mol/Einstein. Mass spectroscopic studies of the product solutions demonstrate the formation of S8, presumably from the decomposition of NS. The quantitative photochemical behaviors of 1 and the nitrosyl analog 2 are compared.