Triplet ground state (S= 1) pegylated bis(aminoxyl) diradical: synthesis and the effect of water on magnetic properties

The synthesis and magnetic characterization of pegylated bis(aminoxyl) diradical with an S= 1 ground state are presented, revealing water-induced changes in the molecular conformation and magnetic properties.     

Complete characterization of the (D2O)2 ground state: high Ka rotation-tunneling levels

We report the observation of extensive a- and c-type rotation-tunneling (RT) spectra of (D2O)2 for Ka = 0-4. These data allow quantification of molecular constants and tunneling splittings for a number of previously unobserved RT states of (D2O)2. The vibrational ground state has thus been characterized to energies as high as those of some of the intermolecular vibrations, and we present the first test of the VRT(ASP-W) potential at these high Ka states.     

Synthesis and characterization of the complexes of pentane-2,4-dione with nickel(II) and cobalt(III): [Ni(acac)2].0.5CH3OH and [Co(acac)2NO3].2H2O (acac = pentane-2,4-dione)

Two mononuclear complexes, [Ni(acac)2].0.5CH3OH (1) and[Co(acac)2NO3].2H2O (2) (acac = pentane-2,4-dione), have been synthesized and characterized by single crystal X-ray analysis. Complex 1 crystallizes in the monoclinic space group P2(1)/c with a = 9.295(4), b = 11.450(5), c = 12.974(6) A, V = 1379.1(11) A(3),beta = 92.854(7), and Z = 4. Complex 2 crystallizes in the triclinic space group P(-1) with a= 8.153(9), b = 9.925(11), c = 10.355(12), V = 746.3(15) A(3), alpha = 70.530(16), beta =71.154(15), gamma = 80.698(16) and Z = 2. Complex 1 has a one-dimensional chain-like structure, which is extended by weak hydrogen contacts, while complex 2 shows a three-dimensional network structure.     

Ultrafast dynamics of 2E state formation in Cr(acac)3

Femtosecond time-resolved absorption spectroscopy has been used to elucidate the excited-state dynamics associated with formation of the (2)E excited state in a Cr(III) transition metal complex. Cr(acac)(3) (where acac is the deprotonated monoanion of acetylacetone) exhibits monophasic decay kinetics with tau = 1.1 +/- 0.1 ps following excitation into the lowest-energy ligand-field absorption band; the time constant is found to be independent of both excitation and probe wavelength across the entire (4)A(2) --> (4)T(2) absorption envelope. The lack of a significant shift in the excited-state absorption spectrum combined with the observed spectral narrowing is consistent with an assignment of this process as vibrational cooling (k(vib)) in the (2)E state. The data on Cr(acac)(3) indicate that intersystem crossing associated with the (4)T(2) --> (2)E conversion occurs at a rate k(ISC) > 10(13) s(-)(1) and furthermore competes effectively with vibrational relaxation in the initially formed (4)T(2) state. Excitation into the higher energy (4)LMCT state (lambda(ex) = 336 nm) gives rise to biphasic kinetics with tau( 1) = 50 +/- 20 fs and tau( 2) = 1.2 +/- 0.2 ps. The slower component is again assigned to vibrational cooling in the (2)E state, whereas the subpicosecond process is attributed to conversion from the charge-transfer to the ligand-field manifold. In addition to detailing a process central to the photophysics of Cr(III), these results reinforce the notion that the conventional picture of excited-state dynamics in which k(vib) > k(IC) > k(ISC) does not generally apply when describing excited-state formation in transition metal complexes.     

Isovalent and mixed-valent diruthenium complexes [(acac)2RuII (-bpytz)RuII(acac)2] and [(acac)2RuII(-bpytz)RuIII(acac)2](ClO4) (acac = acetylacetonate and bpytz = 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine): synthesis, spectroelectrochemical, and epr investigation

The title compounds involving the structurally characterized bridging ligand bpytz were characterized, showing very strong electrochemical stabilization of the mixed-valent RuIIRuIII state (Kc = 10(13.9)) but no detectable (epsilon < 20 M(-1) cm(-1)) intervalence charge-transfer band in the infrared region. In situ reduction of the neutral precursor produces a diruthenium(II) complex of the bpytz radical anion according to EPR spectroscopy, whereas oxidation of the mixed-valent form leads to a diruthenium(III) species.     

Vapour pressure measurements on (acac)M(substituted olefin)2 and (acac)M(CO)2 (M = Rh(I), Ir(I))

Vapour pressures and enthalpies of sublimation of the complexes (acac)M (olefin)₂ and (acac)M(CO)₂ (M = Rh(I), Ir(I); olefin = ethylene, propylene, vinyl chloride, vinyl acetate and methyl acrylate) were determined by the Koudsen effusion method.     

Unusual monodentate binding mode of 2,2'-dipyridylamine (L) in isomeric trans-(acac)2RuII(L)2, trans-[(acac)2RuIII(L)2]ClO4, and cis-(acac)2RuII(L)2 (acac = acetylacetonate). synthesis, structures, and spectroscopic, electrochemical, and magnetic aspects

The reaction of cis-Ru(acac)2(CH3CN)2 (acac = acetylacetonate) with 2,2'-dipyridylamine (L) in ethanolic medium resulted in facile one-pot synthesis of stable [(acac)2RuIII(L)]ClO4 ([1]ClO4), trans-[(acac)2RuII(L)2] (2), trans-[(acac)2RuIII)L)2]ClO4 ([2]ClO4), and cis-[(acac)2RuII(L)2] (3). The bivalent congener 1 was generated via electrochemical reduction of [1]ClO4. Although in [1]+ the dipyridylamine ligand (L) is bonded to the metal ion in usual bidentate fashion, in 2/[2]+ and 3, the unusual monodentate binding mode of L has been preferentially stabilized. Moreover, in 2/[2]+ and 3, two such monodentate L's have been oriented in the trans- and cis-configurations, respectively. The binding mode of L and the isomeric geometries of the complexes were established by their single-crystal X-ray structures. The redox stability of the Ru(II) state follows the order 1 < 2 < 3. In contrast to the magnetic moment obtained for [1]ClO4, mu = 1.84 muB at 298 K, typical for low-spin Ru(III) species, the compound [2]ClO4 exhibited an anomalous magnetic moment of 2.71 muB at 300 K in the solid state. The variable-temperature magnetic measurements showed a pronounced decrease of the magnetic moment with the temperature, and that dropped to 1.59 muB at 3 K. The experimental data can be fitted satisfactorily using eq 2 that considered nonquenched spin-orbit coupling and Weiss constant in addition to the temperature-independent paramagnetism. [1]ClO4 and [2]ClO4 displayed rhombic and axial EPR spectra, respectively, in both the solid and the solution states at 77 K.     

Crystallographic report: A coordination polymer containing [Zn(NO3)(H2O)2(btp)2]+ units bridged by btp ligands (btp = 2,6‐bis(N′‐1,2,4‐triazolyl)pyridine)

A zinc(II) coordination polymer has been formed from Zn(NO₃)₂ and 2,6‐bis(N′‐1,2,4‐triazolyl)pyridine (btp) ligands in which each zinc(II) atom is coordinated by three nitrogen donor atoms from btp and three oxygen donor atoms from a nitrate and two water molecules. Copyright © 2003 John Wiley & Sons, Ltd.     

Polymerization of methyl methacrylate by Ziegler-Natta type catalyst systems: Fe(acac)3-AlEt2Br and Fe(acac)3-ZnEt2

The polymerization of methyl methacrylate was carried out with the following Ziegler-Natta type initiating systems: Fe(AcAc)₃-AlEt₂Br, Fe(AcAc)₃-ZnEt₂ (acac = acetyl acetonate). Both the catalyst systems are active under homogeneous conditions in benzene at 40°C for methyl methacrylate polymerization. The polymerization kinetics suggests that the average rate of polymerization was first order with respect to [monomer] for both the catalyst systems, and the overall activation energies were found to be 14.0 and 12.8 kcal mol ^?1.     

Triplet state properties of the benzo[α]pyrene-DNA complex as determined by optically detected zero-field magnetic resonance

The zero-field transitions in the photoexcited triplet state of benzo[α]pyrene bound to DNA have been observed by optical detection of magnetic resonance of 2 K. The transition frequencies individual spin sublevel intersystem crossing rates were measured by monitoring the microwave-induced intensity changes of the triplettriplet absorption at 465.8 nm. The triplet state zero-field splitting and dynamics for the benzo[α]pyrene DNA complex are compared with these properties measured for benzo(α)pyrene in other solvent systems.     

2D LnIII RuIII 2 compounds constructed from trans-[Ru(acac)2(CN)2]-. Syntheses, structures, and magnetic properties

A series of cyano-bridged Ln(III)Ru(III)2 coordination polymers, Ph4P{Ln(NO3)2[Ru(acac)2(CN)2]2} [Ln = Tb (1), Dy (2), Er (3), Gd (4); Hacac = acetylacetone] have been synthesized by the reaction of Ln(NO3)3 with trans-Ph4P[Ru(acac)2(CN)2] in methanol. X-ray crystallographic determination reveals that these compounds are isostructural and have a wavy (4,4) layer structure with the Ln3+ ions bridged by trans-[Ru(acac)2(CN)2]-. Magnetic studies shows that the magnetic coupling between the Ln(III) and Ru(III) ions through the cyano bridges in 1-4 is negligibly weak.     

Sub-picosecond time-resolved IR spectroscopy of the vibrational dynamics of Rh (CO)2 (acac)

Sub-picosecond pulses are used to measure the transient IR spectra of vibrationally excited Rh(CO)₂ (acac) in dilute solutions of n-hexane and CCl₃H. Ground-state bleach and excited-state absorption features are observed at t_D>0, while interference-like spectra characteristic of the perturbed free induction decay (FID) of the probe polarization are seen at t_D<). At t_D>), the bleach signal exhibits an initial decay (n-hexane, 3–6 ps; CCl₃H, 2.2–4 ps) attributed to rapid v-v coupling between the symmetric and asymmetric stretch modes of the dicarbonyl, followed by much slower (n-hexane, 61 ps; CCl₃H, 101 ps) population relaxation. Calculated transient spectra and bleach decay curves obtained from a 5-level density matrix model of the coupled CO oscillators account for the data, including the effects due to perturbed FID and rapid v-v coupling.     

Synthesis and Photoluminescence of a Novel Iridium Complex （BuPhOXD）2Ir（acac） with Unit of 1, 3, 4-Oxadiazole

A novel cyclometalated iridium complex with 1, 3, 4-oxadiazole moiety was synthesized and characterized. Its UV and photoluminescent properties were studied. The strong UV absorption intensity around 462 nm attributed to spin-forbidden triplet metal-ligand charge transfer band and photoluminescence at 518 nm were observed. This indicated that achieved iridium complex could be used as an efficient electrophosphorescent material.     

Phosphorescent [3 + 2 + 1] coordinated Ir(iii) cyano complexes for achieving efficient phosphors and their application in OLED devices

A series of neutral [3 + 2 + 1] coordinated iridium complexes bearing tridentate bis-NHC carbene chelates (2,6-bisimidazolylidene benzene), bidentate chelates (C^N ligands, e.g. derivatives of 2-phenylpridine), and monodentate ions (halides and pseudo-halides, such as Br, I, OCN and CN ions) have been systematically designed and synthesized. X-ray single crystal structure characterization revealed that the nitrogen atom in C^N ligands is located trans to the carbon atom in the benzene ring in tridentate chelates, while the coordinating carbon atom in C^N ligands is located trans to the monodentate ligands. Photophysical studies reveal that the C^N ligands play a vital role in tuning the UV absorption and emission properties, while the tridentate bis-NHC carbene chelates influence the lowest absorption band and emission energy when compared to heteroleptic Ir(ppy)2(acac) [i.e. molar absorptivities at ∼450 nm for ppy-OCN and Ir(ppy)2(acac) are 350 M⁻¹ cm⁻¹ and 1520 M⁻¹ cm⁻¹ and emission maximum peaks are at 465 nm and 515 nm respectively]. Among monodentate ligands that the complexes bear, the group containing the cyanide ligand displays higher emission energy, higher photophysical quantum yields, longer triplet lifetimes and better electrochemical and thermal stabilities than those of cyanate and bromide. Particularly, a blue organic light-emitting diode (OLED) based on dfppy-CN exhibited a maximum external quantum efficiency of 22.94% with CIE coordinates of (0.14, 0.24). Furthermore, a small efficiency roll-off of 5.7% was observed for this device at 1000 cd m⁻².

Construction of [3 + 2 + 1] coordinated iridium(iii) cyano complexes for achieving high-efficiency phosphors and their application in blue OLEDs with low efficiency roll-off.     

Structural characterization and DFT study of V(IV)O(acac)2 in imidazolium ionic liquids

We report the structural characterization of vanadyl acetylacetonate in imidazolium room temperature ionic liquids--bbimNTf(2), bmimNTf(2), C(3)OmimNTf(2), bm(2)imNTf(2), bmimPF(6), bmimOTf, bmimBF(4), bmimMeCO(2), bmimMeSO(4), bmimMe(2)PO(4) and bmimN(CN)(2)--and organic solvents. The complex was characterized by visible electronic (Vis) and EPR spectroscopies. VO(acac)(2) shows solvatochromism in the selected ionic liquids and behaves as in organic solvents, evidencing coordination of the ionic liquid anion in the solvents with higher coordinating ability. The Lewis basicity order obtained for the IL anions was: PF(6)(-) < NTf(2)(-) < OTf(-)≈ MeCO(2)(-) < MeSO(4)(-) < BF(4)(-)≈ N(CN)(2)(-) < Me(2)PO(4)(-). The solvent effect on the spectroscopic data was tentatively examined using linear solvation energy relationships based on the Kamlet-Taft solvent scale (α, β and π*), however no suitable correlation was found with all data. The EPR characterization showed the presence of two isomers in bmimOTf, bmimMeCO(2) and bmimMe(2)PO(4), suggesting coordination of the ionic liquid anions in both equatorial and axial positions. The full geometry optimization of cis-/trans-VO(acac)(2)(OTf)(-) and cis-/trans-VO(acac)(2)(OTf)(mmim) structures was done at the B3P86/6-31G* level of theory. The calculations confirm that the anion OTf(-) is able to coordinate to VO(acac)(2) with the trans isomer being more stable than the cis by 4.8 kcal mol(-1).