Ancillary Ligand Modification Research Articles

Ancillary ligand modulation effect on crystallization, metal aquation, fluorescence, lipophilicity, electrochemistry and anticancer activities of Pt(II) complexes

The potential applications of Pt complexes depend on the coordinating auxiliary ligand(s) and ancillary/leaving ligand(s) attached to the metal. Pt(II) complexes bearing ONS-donor salicylaldimine auxiliary ligand with general formula LPtX, (L = ONS-donor salicylaldimine ligand) with functionalized ancillary ligand X (X = Cl, Br, I and OAc) (C1a-C4a and C1b-C4b) were synthesized using a well-established method. All these complexes were characterized in solution and in solid state using multiple analytical techniques. The effect of ancillary ligand modification was studied on multiple features including crystallization, metal aquation, fluorescence, lipophilicity, electrochemistry and anticancer activities of these complexes. Depending on the ancillary ligands, from single crystal X-ray analysis different structural features in solid state or arrangement in crystal packing were observed. The ancillary ligand (Cl, Br, I, and OAc) hydration in DMSO‑d6/D2O mixture to form cationic [L-Pt-H2O]+ was analyzed by time dependent 1H NMR spectroscopy. The ancillary ligands also affected fluorescence of these complexes, a trend of decreasing emission with increasing atomic mass of the attached atom as ancillary ligand was observed. Lipophilicity (log P) of “a” series was measured as reference series and we found the order depending on the ancillary ligand in each complex as I > Br > OAc > Cl, showing C3a lipophilicity was the highest and C1a was the lowest. Finally, these complexes were checked for their anticancer effect and the IC50 was calculated using MTT assay. These complexes showed better anticancer effect as compared to cisplatin. The anticancer effect was not linear with the previously discussed properties showing the anticancer activity may depend on the sum of multiple structural parameters and also depend on the nature of cancer cells, so it was not conclusive using a single set of parameters to comply with the other.

A Tailored Versatile and Efficient NHC-Based NNC-Pincer Manganese Catalyst for Hydrogenation of Polar Unsaturated Compounds.

The reactivity of metal-hydride complexes can be harnessed by the modification of ancillary ligands. With the aim of improving the hydride-donor ability of the key Mn-H intermediate and reducing steric hindrance, we herein report the rational design of a versatile and efficient NHC-based NNC-pincer Mn catalyst for hydrogenation reactions. This newly developed catalyst exhibited higher activity than the corresponding NNP-pincer Mn catalyst owing to its reduced steric hindrance and enhanced Mn-H σ-bonding orbital energy level through a π-antibonding interaction. Using this highly active NNC-pincer Mn catalyst, a rich array of polar unsaturated compounds (>80 examples) including esters, N-heteroarenes, amides, carbonates, and urea derivatives, were successfully hydrogenated under relatively mild conditions. This work represents a rare example of a general phosphine-free Mn-catalyzed hydrogenation system.

A Tailored Versatile and Efficient NHC‐Based NNC‐Pincer Manganese Catalyst for Hydrogenation of Polar Unsaturated Compounds

AbstractThe reactivity of metal‐hydride complexes can be harnessed by the modification of ancillary ligands. With the aim of improving the hydride‐donor ability of the key Mn−H intermediate and reducing steric hindrance, we herein report the rational design of a versatile and efficient NHC‐based NNC‐pincer Mn catalyst for hydrogenation reactions. This newly developed catalyst exhibited higher activity than the corresponding NNP‐pincer Mn catalyst owing to its reduced steric hindrance and enhanced Mn−H σ‐bonding orbital energy level through a π‐antibonding interaction. Using this highly active NNC‐pincer Mn catalyst, a rich array of polar unsaturated compounds (>80 examples) including esters, N‐heteroarenes, amides, carbonates, and urea derivatives, were successfully hydrogenated under relatively mild conditions. This work represents a rare example of a general phosphine‐free Mn‐catalyzed hydrogenation system.

Reinterpreting the Fate of Iridium(III) Photocatalysts─Screening a Combinatorial Library to Explore Light-Driven Side-Reactions.

Photoredox catalysts are primarily selected based on ground and excited state properties, but their activity is also intrinsically tied to the nature of their reduced (or oxidized) intermediates. Catalyst reactivity often necessitates an inherent instability, thus these intermediates represent a mechanistic turning point that affords either product formation or side-reactions. In this work, we explore the scope of a previously demonstrated side-reaction that partially saturates one pyridine ring of the ancillary ligand in heteroleptic iridium(III) complexes. Using high-throughput synthesis and screening under photochemical conditions, we identified different chemical pathways, ultimately governed by ligand composition. The ancillary ligand was the key factor that determined photochemical stability. Following photoinitiated electron transfer from a sacrificial tertiary amine, the reduced intermediate of complexes containing 1,10-phenanthroline derivatives exhibited long-term stability. In contrast, complexes containing 2,2'-bipyridines were highly susceptible to hydrogen atom transfer and ancillary ligand modification. Detailed characterization of selected complexes before and after transformation showed differing effects on the ground and excited state reduction potentials dependent on the nature of the cyclometalating ligands and excited states. The implications of catalyst stability and reactivity in chemical synthesis was demonstrated in a model photoredox reaction.

Improving the chemical stability of blue heteroleptic iridium emitter FIrpic in the lowest triplet state through ancillary ligand modification: a theoretical perspective.

With the aim of providing a deeper understanding of the underlying degradation mechanisms associated with the lifetime of blue emitters during the decay process of blue PhOLEDs, quantum chemistry studies were performed to examine the chemical degradation mechanism of common sky blue emitter iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C2')picolinate (FIrpic) and its derivatives with density functional theory (DFT) calculations. For these Ir(III) emitters, the Ir-N1 bond between the ancillary ligand (picolinate) and central iridium will be broken by external light stimuli, which is followed by conversion from the initial emissive metal-to-ligand charge transfer (3MLCT) state to the non-emissive metal centered (3MC) state. The potential energy change for the photo-induced degradation path is then dominated by the energy levels of the 3MT and 3MC states, which are related to the triplet transition energy and the Ir-N1 bond strength, respectively. Thereby, the Ir-N1 bond dissociation in the lowest triplet state will be much harder to proceed if the S0 → T1 transition energy gets more energetically stable or the bond strength gets larger. It is believed that strategic modification of the ancillary ligand, especially by substitution of electron-donating groups at the para position of the pyridyl N atom or extension of the p-electron delocalization, is an effective and easy way to enhance the photochemical stability of the typical blue emitter FIrpic.

Effect of β-Ketoiminato Ancillary Ligand Modification on Emissive Properties of New Iridium Complexes.

A series of new bis(benzo[h]quinolinato) Ir(III) complexes with modified β-ketoiminato ancillary ligands were synthesized, and their electrochemical, photophysical properties were determined with the support of theoretical calculations. Moreover, all the synthesized heteroleptic Ir(III) complexes were examined as dopants of the host-guest type emissive layers in solution-processed phosphorescent organic light emitting diodes (PhOLEDs) of a simple structure. As expected on the basis of voltammetry measurements as well as DFT calculations, all the compounds appeared to be green emitters. Their examination showed that alteration of β-ketoiminato ligand structure causes frontier orbitals' energy levels to be slightly changed, while significantly affecting photoluminescence and electroluminescence efficiencies of iridium phosphors containing these ligands. It was also found that modification of ancillary ligands might enhance charge trapping on the dopant, thus increasing its efficiency, especially in electroluminescence. From among the iridium complexes studied, the compound bearing 1-naphthyl group bonded to the nitrogen atom of the ancillary ligand proved to be the most efficient emitter. The PhOLED fabricated on the basis of this dopant has reached a luminance level of 16000 cd/m2, current efficiency close to 12 cd/A, and an external quantum efficiency around 3.2%.

Ancillary Ligand Modification via Reductive Elimination at Nickel(II)

The nickel(II) complex [κ3-BnPPC]NiBr (1), stabilized by an orthometalated-phosphine benzyl group and two bulky phosphine donors, results in facile reductive elimination with certain organometallic...

Bis-Cyclometalated Iridium Complexes with Chelating Dicarbene Ancillary Ligands

In this work, we report that covalent postsynthetic modification can be used for the preparation of a class of bis-cyclometalated iridium complexes featuring Chugaev-type chelating dicarbene ligands. Bis-cyclometalated iridium complexes with electron-deficient aryl isocyanide ancillary ligands react with hydrazine to form neutral dicarbene complexes. The neutral iridium carbene complexes have a basic site that can be protonated by strong acid, permitting access to complexes in two protonation states and allowing an additional layer of control over the key properties. These new Chugaev-type iridium complexes exhibit blue phosphorescence at both room temperature and 77 K. Compared to their bis-isocyanide precursors, the electrochemical and photophysical properties of these new complexes are substantially perturbed, demonstrating the concept that the electronic structure and excited state dynamics can be controlled by ancillary ligand modification. Furthermore, the emission spectra and excited-state dynamics...

Theoretical investigation on the effect of ancillary ligand modification for highly efficient phosphorescent platinum(ii) complex design

Reasonable modification of ancillary ligands for Pt(ii) complexes can effectively improve the quantum efficiency and strengthen the rigidity of luminescent materials in organic light-emitting diodes.

Tuning the electronic and photophysical properties of platinum(II) complexes through ancillary ligand modification: a theoretical study

In this work, six Pt(II) complexes have been studied via density functional theory (DFT)/time-dependent DFT caculations to explore the influence of different ancillary ligand on electron structures, photophysical properties and radiative decay processes. Moreover, the self-consistent spin–orbit coupling TDDFT was used to calculate zero-field splitting, radiative rate and radiative lifetime to unveil the radiative deactivation processes for these complexes. The results indicated that [Pt(ppy)(ppz)] (ppy = 2-phenylpyridine and ppz = 5-(2-pyridyl)-pyrazole) has a higher radiative decay rate constant and a smaller nonradiative decayrate constant than that of [Pt(ppy)(acac)] (acac = acetylacetonate). Furthermore, complex 5, with dimesityboron added on the 3′-position of the pyrazole ring in [Pt(ppy)(ppz)], shows great potential to serve as an efficient blue-green light emitter in OLED.

Low-voltage, high-brightness and deep-red light-emitting electrochemical cells (LECs) based on new ruthenium(ii) phenanthroimidazole complexes.

Light-Emitting Electrochemical Cells (LECs) with a simple device structure ITO/Ru complex/Ga:In were prepared by using heteroleptic ruthenium(ii) complexes containing 2-(2-hydroxyphenyl)-1-(4-bromophenyl)-1h-imidazo[4,5-f][1,10]phenanthroline (hpbpip) as the π-extended ligand. After ancillary ligand modification, the [Ru(hpbpip)(dmbpy)2](ClO4)2 complex shows a deep red electroluminescence emission (2250 cd m(-2) at 6 V) centered at 685 nm, 65 nm red-shifted compared to the [Ru(bpy)3](ClO4)2 benchmark red-emitter at a very low turn voltage (2.6 V), demonstrating its potential for low-cost deep-red light sources. Moreover, the PL quantum yield of the [Ru(hpbpip)(bpy)2](ClO4)2 complex was revealed to be higher (0.121) than the benchmark standard [Ru(bpy)3](2+) (0.095).

Introduction of new ancillary ligands to the iridium complexes having 2,3-diphenylquinolinato ligands for OLED

We investigated the effect of an ancillary ligand (AL) on the emission color and luminous efficiencies of its complex, Ir(4-Me-2,3-dpq) 2(AL), where 4-Me-2,3-dpq represents 4-methyl-2,3-diphenylquinolinato ligand. We expected that ancillary ligand modification by introduction of the bulky substituent to the complexes might allow luminous efficiency increase by reduction of T–T annihilation. Furthermore, some ancillary ligands may contribute to fine-tuning of their complex emission colors by influencing the energy level of Ir d-orbitals upon the orbital mixing. As new ancillary ligands substituting for acac which is a typical AL in the iridium complexes, pyrazolone-based ligands, 4-R-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one series (przl-R), were prepared, where R represents C 6H 5, C 6H 4CH 3 and C 6H 4Cl. These ligands were chelated to the iridium center to yield a new series of the iridium complexes, Ir(4-Me-2,3-dpq) 2(przl-R). The X-ray crystal structure of Ir(4-Me-2,3-dpq) 2(przl-C 6H 4Cl) was determined. The electrochemical and luminescence properties of the iridium complexes were investigated. The effect of the przl-substituents on the emission colors of the complexes was not significant. On the other hand, the luminous efficiencies of Ir(4-Me-2,3-dpq) 2(przl-C 6H 5) and Ir(4-Me-2,3-dpq) 2(przl-C 6H 4CH 3) were higher than that of Ir(4-Me-2,3-dpq) 2(acac).