Registro de acceso restringido Este recurso no está disponible en acceso abierto por política de la editorial. No obstante, se puede acceder al texto completo desde la Universitat Jaume I o si el usuario cuenta con suscripción. Registre d'accés restringit Aquest recurs no està disponible en accés obert per política de l'editorial. No obstant això, es pot accedir al text complet des de la Universitat Jaume I o si l'usuari compta amb subscripció. Restricted access item This item isn't open access because of publisher's policy. The full--text version is only available from Jaume I University or if the user has a running suscription to the publisher's contents.

The synthesis and X-ray crystal structure of a potassium adduct of a monoanionic CNC-pincer ligand featuring two mesoionic carbenes is reported. Owing to the peculiar electronic and steric properties of this ligand, the first neutral stable Ni(II)-hydride, and an unusual Cu(II) complex displaying a seesaw geometry, have been isolated.It is well known that tridentate pincer ligands not only give rise to robust catalysts, but also allow for isolating extremely reactive metal centers. 1 A large number of both neutral and monoanionic pincer ligands featuring N-heterocyclic carbenes (NHCs) have been prepared, and the corresponding complexes used as catalysts for various chemical transformations. 2 However, only four complexes, featuring a strongly donating amido-moiety as the central coordinating atom, flanked by two NHC wing-tip groups in a CNC-fashion, have been reported (A-C) (Chart 1). 3 Moreover, pincer ligands based on the novel generation of carbenes, namely mesoionic carbenes (MICs), 4 which are even stronger donors than NHCs, have been even less explored. For the CCC-tridentate binding mode, a handful of examples are known with imidazol-4-ylidenes, 4,5 whereas with 1,2,3-triazol-5-ylidenes, 6 binuclear bridged complexes 7 or mononuclear complexes with bidentate ligands where the central C-atom does not ligate, 8 are exclusively found. A neutral CNC-analogue of tridentate terpyridine, [2,6-bis(1,2,3-triazol-5-ylidene)-pyridine], is the only example of a bisMIC pincer acting as a tridentate ligand. 9 Here we report the synthesis of the first stable anionic CNC-tridentate ligand featuring terminal 1,2,3-triazol-5-ylidenes and a central amido functionality, its mononuclear tridentate Ni(II)-hydride and Cu(II) complexes.The planar carbazole backbone with its rigid geometry seemed an attractive choice for the design of a bis(mesoionic carbene)amido pincer-type ligand. The synthesis of the dicationic salt precursor, namely the bis(1,2,3-triazolium)carbazole 1, was achieved in 43% yield by an adapted version of the formal 1,3-dipolar cycloaddition between a 1,3-diaza-2-azoniaallene salt and a 1,8-diethynylcarbazole (Scheme 1). 6a Addition of 3 equivalents of potassium hexamethyldisilazide (KHMDS) to a THF solution of 1 at À78 1C resulted only in the deprotonation of the carbazole, keeping unchanged the two pendant 1,2,3-triazolium moieties. The cationic salt 2 was isolated in 93% yield as an air-and moisture-stable red solid. The monodeprotonation of 1 is indicated by the absence of the N-H resonance in the 1 H NMR spectrum, and by the presence of a triazolium C-H signal (2H) at 10.03 ppm (see ESI †). The structure of 1 was confirmed by an X-ray diffraction study (Fig. 1). When a large excess of KHMDS (5 equivalents) was added to a diethylether suspension of 1 at À78 1C, the potassium salt 3 could be isolated in good yield (72%) after extraction with hexanes. In the 1 H NMR spectrum, the disappearance of the acidic N-H and triazolium C-H signals confirmed the formation of the triply deprotonated compound 3. In th...

Registro de acceso restringido Este recurso no está disponible en acceso abierto por política de la editorial. No obstante, se puede acceder al texto completo desde la Universitat Jaume I o si el usuario cuenta con suscripción. Registre d'accés restringit Aquest recurs no està disponible en accés obert per política de l'editorial. No obstant això, es pot accedir al text complet des de la Universitat Jaume I o si l'usuari compta amb subscripció. Restricted access item This item isn't open access because of publisher's policy. The full--text version is only available from Jaume I University or if the user has a running suscription to the publisher's contents.

A series of ferrocenyl and thienyl mono-and biscarbene chromium(0) complexes 1-6 were synthesised.The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH 2 Cl 2 revealed that the carbene double bond of 1-6 is reduced to an anion radical, − Cr-C˙at formal reduction potentials <−1.7 V vs.FcH/FcH + . A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at E pa > 0.95 V. The reactivity towards follow-up chemical reactions of the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced radical anions as the latter was still observably reoxidised to the parent CrvC species at fast scan rates. The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1-6 are involved in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically shown to stabilise the CrvC centre almost as much as the NHBu, and much more than the ethoxy and thienyl groups.

The photophysical properties of as eries of Tshaped coinage d 10 metal complexes,s upportedb yabis-(mesoionic carbene)carbazolide( CNC) pincerl igand,a re explored. The series includes ar are new example of at ri-dentateT -shaped Ag I complex. Post-complexation modification of the Au I complex provides access to al inear cationic Au I complex following ligand alkylation,o rt he first example of ac ationic square planar Au III ÀFc omplex from electrophilic attack on the metal centre. Emissions ranging from blue (Cu I )t oo range (Ag I )a re obtained, with variable contributionso ft hermally-dependentf luorescencea nd phosphorescence to the observed photoluminescence. Green emissions are observed for all three gold complexes (neutralT -shaped Au I ,c ationic linear Au I and square planar cationic Au III ). The higher quantum yield and longer decay lifetimeo ft he linear gold(I) complex are indicative of increased phosphorescence contribution.In the development of efficient organic light emitting devices (OLEDs), basic requirements of phosphorescente mitters include high externalq uantum efficiencies (EQE) of the emission, coupled with appropriater adiative lifetimes in the ordero fm icroseconds to facilitate the intersystem crossing (ISC) from the triplet to the singlet state. [1] These specifications have been amply met using iridium(III) and platinum(II) emitters, [2] with an increasing number of reports detailing the utility of the lesser explored gold(III) complexes. Indeed, CNC- [3] and CCN- [4] cyclometallated gold(III) complexes excelling in emissioncolor tuning, solubility and thermals tability have recently yielded OLEDsw ith very high EQEs and similarly long device operationalh alf-lifetimes. [4b] Lowero xidation state gold(I), and the other d 10 coinage metals, copper(I) and silver(I), have also been the focus of concurrent investigationsi nto their use in OLEDs. One of the more successful design strategieso nt his front employs the linear bonding geometry of carbene-metal-amides (CMAs),i nw hich all three d 10 metals (Cu I ,A g I and Au I ), have similarly accomplished excellent EQE performance and/or high brightnessO LED operation, notably employing carbazolide derivativesa st he donor amidep artnersw ith the acceptor carbenes. [5] These CMAsc an display particularly short (ns) emission lifetimesi nt hermally assisted delayed fluorescence (TADF), basedo nt he rapid triplet-to-singlet ISC, unlike the heavya tom (metal) phosphorescente mitters relying on spinorbit coupling. [1b, 6] The dependency of photoluminescence on the coordination geometry of d 10 coinage metal complexes is well-known, [7] for example, 3-coordinate trigonal planar copper(I)c omplexes showed tunable behavior from pure phosphorescence to TADF dependingo nt he carbene-metal-amine dihedral angles. [7c] Extending 3-coordinate systemsb eyondt rigonal planar geometries to ag round state Jahn Teller-distorted T-shape has been an early theoretical target for photophysical tuning of the singlet-triplet gap. [8] However,t he availab...

Supporting Information PlaceholderA series of ten ferrocenyl, furyl and thienyl mono-and biscarbene chromium(0) complexes were synthesized and characterized spectroscopically and electrochemically. The single crystal structure of the biscarbene complex [(CO) 5 Cr=C(OEt)-Fu'-(OEt)C=Cr(CO) 5 ] (4a) was determined: C 20 H 12 Cr 2 O 13 ; triclinic; P1; a = 6.2838(5), b = 12.6526(9), c = 29.1888(19) Å, α = 89.575(2), β = 88.030(2), γ = 87.423(2)°; Z = 4. Results from an electrochemical study in CH 2 Cl 2 were mutually consistent with a computational study in showing that the carbene double bond of 1 -6 is reduced to an anion radical, -Cr-C• at formal reduction potentials < -1.7 V vs. FcH/FcH + . The Cr centers are oxidized in two successive one electron transfer steps to Cr(II) via the Cr(I) intermediate. Only Cr(I) oxidation is electrochemically irreversible. Dicationic Cr(II) species formed upon two consecutive oneelectron oxidation processes are characterized by a peculiar bonding situation as they are stabilized by genuine CH···Cr agostic interactions. With respect to aryl substituents, carbene redox processes occurred at the lowest potentials for ferrocene derivatives followed by furan complexes. Redox process in the thiophene derivatives occurred at the highest potentials. This result is mutually consistent with a 13 C NMR study that showed the Cr=C functionality of furyl complexes were more shielded than thienyl complexes. The NHBu carbene substituent resulted in carbene complexes showing redox processes at substantially lower redox potentials than carbenes having OEt substituents.

Vegetation changes around artificial watering points were monitored in the Kalahari Gemsbok National Park from 1978 to 1989. Both rainfall and grazing influenced the vegetation but rainfall appeared to be the more significant factor. The above-average rainfall during the years preceding 1978 contributed to a relatively high basal cover in 1978. Since 1978, however, below-average rainfall was recorded resulting in a decline in basal cover, presence, frequency and density of most of the plant species. It seems likely that a wet cycle commenced in 1988, which is reflected in higher rainfall and an increase in basal cover, presence, and density. Some species maintained a relatively constant presence, some increased, and some declined during the study period. The monitoring is being continued

4 ] to afforded the carbene-carbene coupled olefinic products and small amounts of the corresponding 2-ethyl(biheteroaryl)acetate. By contrast, the biscarbene complex of thienylfuran (15), afforded only the 2,9-diester of thienylfuran.