Degradation of Hole Transport Materials via Exciton-Driven Cyclization.

Organic light-emitting diode (OLED) displays have been an active and intense area of research for well over a decade and have now reached commercial success for displays from cell phones to large format televisions. A more thorough understanding of the many different potential degradation modes which cause OLED device failure will be necessary to develop the next generation of OLED materials, improve device lifetime, and to ultimately improve the cost vs performance ratio. Each of the different organic layers in an OLED device can be susceptible to unique decomposition pathways, however stability toward excitons is critical for emissive layer (EML) materials as well as any layer near the recombination zone. This study will specifically focus on degradation modes within the hole transport layer (HTL) with the goal being to identify the general decomposition paths occurring in an operating device and use this information to design new derivatives which can block these pathways. Through post-mortem analyses of several aged OLED devices, an apparently common intramolecular cyclization pathway has been identified that was not previously reported for arylamine-containing HTL materials and that operates parallel to but faster than the previously described fragmentation pathways.

Tetrahalide complexes of the [U(NR)2]2+ ion: synthesis, theory, and chlorine K-edge X-ray absorption spectroscopy.

Synthetic routes to salts containing uranium bis-imido tetrahalide anions [U(NR)(2)X(4)](2-) (X = Cl(-), Br(-)) and non-coordinating NEt(4)(+) and PPh(4)(+) countercations are reported. In general, these compounds can be prepared from U(NR)(2)I(2)(THF)(x) (x = 2 and R = (t)Bu, Ph; x = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl(-), the [U(NMe)(2)](2+) cation also reacts with Br(-) to form stable [NEt(4)](2)[U(NMe)(2)Br(4)] complexes. These materials were used as a platform to compare electronic structure and bonding in [U(NR)(2)](2+) with [UO(2)](2+). Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U-Cl bonding interactions in [PPh(4)](2)[U(N(t)Bu)(2)Cl(4)] and [PPh(4)](2)[UO(2)Cl(4)]. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7-10% per U-Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands.

Phototriggered DNA phosphoramidate ligation in a tandem 5'-amine deprotection/3'-imidazole activated phosphate coupling reaction.

We report the preparation and use of an N-methyl picolinium carbamate protecting group for applications in a phototriggered nonenzymatic DNA phosphoramidate ligation reaction. Selective 5'-amino protection of a modified 13-mer oligonucleotide is achieved in aqueous solution by reaction with an N-methyl-4-picolinium carbonyl imidazole triflate protecting group precursor. Deprotection is carried out by photoinduced electron transfer from Ru(bpy)(3)(2+) using visible light photolysis and ascorbic acid as a sacrificial electron donor. Phototriggered 5'- amino oligonucleotide deprotection is used to initiate a nonenzymatic ligation of the 13-mer to an imidazole activated 3'-phospho-hairpin template to generate a ligated product with a phosphoramidate linkage. We demonstrate that this methodology offers a simple way to exert control over reaction initiation and rates in nonenzymatic DNA ligation for potential applications in the study of model protocellular systems and prebiotic nucleic acid synthesis.

A direct route to bis(imido)uranium(V) halides via metathesis of uranium tetrachloride.

Metathesis reactions between uranium tetrachloride and lithium 2,6-diisopropylphenylamide in the presence of 4,4'-dialkyl-2,2'-bipyridyl (R(2)bpy; R = Me, (t)Bu) or triphenylphosphine oxide (tppo) appear to generate bis(imido)uranium(IV) in situ. These extremely reactive complexes abstract chloride from dichloromethane to generate U(NDipp)(2)Cl(R(2)bpy)(2) or U(NDipp)(2)Cl(tppo)(3) (Dipp = 2,6-(i)Pr(2)C(6)H(3)). The preparation of the bromide and iodide analogues U(NDipp)(2)X(R(2)bpy)(2) was achieved by addition of CH(2)X(2) (X = Br, I) to the uranium(IV) solutions. The uranium(V) halides were characterized by X-ray crystallography and found to exhibit linear N-U-N units and short U-N bonds. Electrochemical measurements were made on the chloride bipyridine species, which reacts readily with iodine or ferrocenium to generate bis(imido)uranium(VI) cations.

Photorelease of primary aliphatic and aromatic amines by visible-light-induced electron transfer.

Visible-light-absorbing tris(bipyridyl)ruthenium(II) has been used to mediate electron transfer to N-methylpicolinium carbamates that undergo C-O bond fragmentation followed by spontaneous carbon dioxide release to give free amines. Release of several aliphatic and aromatic primary amines has been demonstrated under mild conditions using visible light.

A general and modular synthesis of monoimidouranium(IV) dihalides.

The conproportionation reaction between the dimeric diimidouranium(V) species [U(N(t)Bu)(2)(I)((t)Bu(2)bpy)](2) ((t)Bu(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridyl) and UI(3)(THF)(4) in the presence of additional (t)Bu(2)bpy yields U(N(t)Bu)(I)(2)((t)Bu(2)bpy)(THF)(2) (2), an unprecedented example of a monoimidouranium(IV) dihalide complex. The general synthesis of this family of uranium(IV) derivatives can be achieved more readily by adding 2 equiv of MN(H)R (M = Li, K; R = (t)Bu, 2,6-(i)PrC(6)H(3), 2-(t)BuC(6)H(4)) to UX(4) in the presence of coordinating Lewis bases to give complexes with the general formula U(NR)(X)(2)(L)(n) (X = Cl, I; L = (t)Bu(2)bpy, n = 1; L = THF, n = 2). The complexes were characterized by (1)H NMR spectroscopy and single-crystal X-ray diffraction analysis of compounds 2 and {U[N(2,6-(i)PrC(6)H(3))](Cl)(2)(THF)(2)}(2) (4). (The X-ray structures of 5 and 6 are reported in the Supporting Information.)

Exploring the coordination modes of pyrrolyl ligands in bis(imido) uranium(VI) complexes.

The preparation of a family of bis(imido) uranium(VI) complexes stabilized by mono- and bidentate pyrrolyl ancillary ligands is described. X-ray crystallographic studies of dipyrrolylmethane (dpm) derivatives show that the pyrrolyl coordination mode in these uranium(VI) ions is unexpected in comparison to analogous transition metal and lanthanide chemistry. The ability of the coordinated pyrrolyl moieties to undergo pyrrolyl isomerization has also been explored and demonstrates reactivity that is unique from structurally similar uranium(VI)-bis(cyclopentadienyl) derivatives.

Oxidative addition to U(V)-U(V) dimers: facile routes to uranium(VI) bis(imido) complexes.

The ability of dimeric bis(imido) uranium(V) complexes with the general formula [U(N(t)Bu)(2)(Y)((t)Bu(2)bpy)](2) (Y = I (1), SPh (2); (t)Bu(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridyl) to behave as two-electron reducing agents was examined with I(2), AgX (X = Cl, Br), PhEEPh (E = S, Se, Te), and chalcogen (O, S, Se) atom transfer reagents. The addition of I(2) and AgX to 1 leads to the formation of uranium(VI) dihalide complexes with the general formula U(N(t)Bu)(2)(I)(X)((t)Bu(2)bpy) (X = I (3), Cl (4), Br (5)). Complexes 1 and 2 can also reduce PhEEPh to generate uranium(VI) complexes with the general formula U(N(t)Bu)(2)(X)(EPh)((t)Bu(2)bpy) (X = I, E = S (6), Se (8), Te (10); X = SPh, E = S (7), Se (12)). These unsymmetrical complexes appear to be in equilibrium with the uranium(VI) complexes U(N(t)Bu)(2)(X)(2)((t)Bu(2)bpy) and U(N(t)Bu)(2)(EPh)(2)((t)Bu(2)bpy) (E = Se (9), Te (11)) and suggest that both U-I and U-E bonds possess a labile nature in bis(imido) uranium(VI) complexes. Complex 1 also reacts as a two-electron reductant toward chalcogen atom transfer reagents such as 4-methylmorpholine N-oxide, S(8), and Se to produce dimeric bis(imido) uranium(VI) complexes with the general formula [U(N(t)Bu)(2)(I)((t)Bu(2)bpy)](2)(mu-E) (E = O (13), S (14), Se (15)) and [U(N(t)Bu)(2)(I)((t)Bu(2)bpy)](2)(mu-eta(2):eta(2)-E(4)) (E = S (16), Se (17)). Density functional theory studies performed on a model complex of 13 indicate the presence of multiple bonding in the bridging U-O bond.

Cation-cation interactions, magnetic communication, and reactivity of the pentavalent uranium ion [U(NtBu)2]+.

Communication is important: The dimeric bis(imido) uranium complex [{U(NtBu)(2)(I)(tBu(2)bpy)}(2)] (see picture; U green, N blue, I red) has cation-cation interactions between [U(NR)(2)](+) ions. This f(1)-f(1) system also displays f orbital communication between uranium(V) centers at low temperatures, and can be oxidized to generate uranium(VI) bis(imido) complexes.

Uranium(VI) bis(imido) chalcogenate complexes: synthesis and density functional theory analysis.

Bis(imido) uranium(VI) trans- and cis-dichalcogenate complexes with the general formula U(N(t)Bu)(2)(EAr)(2)(OPPh(3))(2) (EAr = O-2-(t)BuC(6)H(4), SPh, SePh, TePh) and U(N(t)Bu)(2)(EAr)(2)(R(2)bpy) (EAr = SPh, SePh, TePh) (R(2)bpy = 4,4'-disubstituted-2,2'-bipyridyl, R = Me, (t)Bu) have been prepared. This family of complexes includes the first reported monodentate selenolate and tellurolate complexes of uranium(VI). Density functional theory calculations show that covalent interactions in the U-E bond increase in the trans-dichalcogenate series U(N(t)Bu)(2)(EAr)(2)(OPPh(3))(2) as the size of the chalcogenate donor increases and that both 5f and 6d orbital participation is important in the M-E bonds of U-S, U-Se, and U-Te complexes.

Synthesis and reactivity of bis(imido) uranium(VI) cyclopentadienyl complexes.

The bis(imido) uranium(VI)-C(5)H(5) and -C(5)Me(5) complexes (C(5)H(5))(2)U(N(t)Bu)(2), (C(5)Me(5))(2)U(N(t)Bu)(2), (C(5)H(5))U(N(t)Bu)(2)(I)(dmpe), and (C(5)H(5))(2)U(N(t)Bu)(2)(dmpe) can be synthesized from reactions between U(N(t)Bu)(2)(I)(2)(L)(x) (L=THF, x=2; L=dmpe, x=1) and Na(C(5)R(5)) (R=H, Me); these complexes represent the first structurally characterized C(5)H(5)-compounds of uranium(VI) and they further highlight the differences between UO(2)(2+) and the bis(imido) fragment.

Inner-sphere two-electron reduction leads to cleavage and functionalization of coordinated dinitrogen.

Activation of molecular nitrogen by transition metal complexes is an area of current interest as investigations using the inert N2 molecule to produce higher-value organonitrogen compounds intensify. In an attempt to extend the addition of hydride reagents E-H (where E = BR2, AlR2, and SiR3) to the dinitrogen complex ([NPN]Ta)2(mu-H)2(mu-eta1:eta2-N2) [1; where NPN = (PhNSiMe2CH2)2PPh], the reaction with zirconocene chlorohydride, [Cp2Zr(Cl)H]x, was examined. The crystalline product formed in 35% yield was determined to be ([NP(N)N]Ta)(mu-H)2(mu-N)(Ta[NPN])(ZrCp2) (2) in which the coordinated N2 has been cleaved to form a phosphinimide bridging between Ta and Zr and a triply bridging nitride. The mechanism of this reaction was examined to determine the fate of the chloride and hydride ligands attached to Zr in the starting zirconocene reagent. Using the zirconocene dihydride dimer ([Cp2ZrH2]2), a higher yield of 2 was obtained (76%), and H2 was also observed by 1H NMR spectroscopy. To probe the origin of the eliminated H2, the dideuterated dinitrogen complex ([NPN]Ta)2(mu-D)2(mu-eta1:eta2-N2) (d2-1) was allowed to react with ([Cp2ZrH2]2), which resulted in the formation of ([NP(N)N]Ta)(mu-D)2(mu-N)(Ta[NPN])(ZrCp2), (d2-2), with no evidence of hydrogen for deuterium scrambling between the starting zirconocene dihydride and the ditantalum dinitrogen complex. Studies into the use of preformed Zr(II) and Ti(II) reagents were also performed. The proposed mechanism involves initial adduct formation that facilitates inner-sphere electron transfer to cleave the N-N bond to form a species with bridging nitrides, one of which is transformed by nucleophilic attack of a phosphine donor to generate the observed phosphinimide.

Synthesis, reactivity, and DFT studies of tantalum complexes incorporating diamido-N-heterocyclic carbene ligands. Facile endocyclic C-H bond activation.

The syntheses of tantalum derivatives with the potentially tridentate diamido-N-heterocyclic carbene (NHC) ligand are described. Aminolysis and alkane elimination reactions with the diamine-NHC ligands, (Ar)[NCN]H(2) (where (Ar)[NCN]H(2) = (ArNHCH(2)CH(2))(2)(C(3)N(2)); Ar = Mes, p-Tol), provided complexes with a bidentate amide-amine donor configuration. Attempts to promote coordination of the remaining pendent amine donor were unsuccessful. Metathesis reactions with the dilithiated diamido-NHC ligand ((Ar)[NCN]Li(2)) and various Cl(x)Ta(NR'(2))(5-)(x) precursors were successful and generated the desired octahedral (Ar)[NCN]TaCl(x)(NR'(2))(3-)(x) complexes. Attempts to prepare trialkyl tantalum complexes by this methodology resulted in the formation of an unusual metallaaziridine derivative. DFT calculations on model complexes show that the strained metallaaziridine ring forms because it allows the remaining substituents to adopt preferable bonding positions. The calculations predict that the lowest energy pathway involves a tantalum alkylidene intermediate, which undergoes C-H bond activation alpha to the amido to form the metallaaziridine moiety. This mechanism was confirmed by examining the distribution of deuterium atoms in an experiment between (Mes)[NCN]Li(2) and Cl(2)Ta(CD(2)Ph)(3). The single-crystal X-ray structures of (p)(-Tol)[NCNH]Ta(NMe(2))(4) (3), (Mes)[NCNH]Ta=CHPh(CH(2)Ph)(2) (4), (p)(-Tol)[NCN]Ta(NMe(2))(3) (7), (Mes)[NCCN]Ta(CH(2)(t)Bu)(2) (11), and (Mes)[NCCN]TaCl(CH(2)(t)Bu) (14) are included.