Addressing Challenges in Palladium‐Catalyzed Cross‐Coupling Reactions Through Ligand Design

The development of palladium‐catalyzed cross‐coupling reactions has revolutionized the synthesis of organic molecules on both bench‐top and industrial scales. While significant research effort has been directed toward evaluating how modifying various reaction parameters can influence the outcome of a given cross‐coupling reaction, the design and implementation of novel ancillary ligand frameworks has played a particularly important role in advancing the state‐of‐the‐art. This Review seeks to highlight notable examples from the recent chemical literature, in which newly developed ancillary ligands have enabled more challenging substrate transformations to be addressed with greater selectivity and/or under increasingly mild conditions. Throughout, the importance and subtlety of ligand effects in palladium‐catalyzed cross‐coupling reactions are described, in an effort to inspire further development and understanding within the field of ancillary ligand design.     

Direct Observation of Aryl Gold(I) Carbenes that Undergo Cyclopropanation,C−H Insertion,and Dimerization Reactions

Mesityl gold(I) carbenes lacking heteroatom stabilization or shielding ancillary ligands have been generated and spectroscopically characterized from chloro(mesityl)methylgold(I) carbenoids bearing JohnPhos‐type ligands by chloride abstraction with GaCl₃. The aryl carbenes react with PPh₃ and alkenes to give stable phosphonium ylides and cyclopropanes, respectively. Oxidation with pyridine N‐oxide and intermolecular C?H insertion to cyclohexane have also been observed. In the absence of nucleophiles, a bimolecular reaction, similar to that observed for other metal carbenes, leads to a symmetrical alkene.     

Direct Observation of Aryl Gold(I) Carbenes that Undergo Cyclopropanation,C−H Insertion,and Dimerization Reactions

Mesityl gold(I) carbenes lacking heteroatom stabilization or shielding ancillary ligands have been generated and spectroscopically characterized from chloro(mesityl)methylgold(I) carbenoids bearing JohnPhos‐type ligands by chloride abstraction with GaCl₃. The aryl carbenes react with PPh₃ and alkenes to give stable phosphonium ylides and cyclopropanes, respectively. Oxidation with pyridine N‐oxide and intermolecular C?H insertion to cyclohexane have also been observed. In the absence of nucleophiles, a bimolecular reaction, similar to that observed for other metal carbenes, leads to a symmetrical alkene.     

Necroptosis Induced by Ruthenium(II) Complexes as Dual Catalytic Inhibitors of Topoisomerase I/II

Inducing necroptosis in cancer cells is an effective approach to circumvent drug‐resistance. Metal‐based triggers have, however, rarely been reported. Ruthenium(II) complexes containing 1,1‐(pyrazin‐2‐yl)pyreno[4,5‐e][1,2,4]triazine were developed with a series of different ancillary ligands ( Ru1 ‐ 7 ). The combination of the main ligand with bipyridyl and phenylpyridyl ligands endows Ru7 with superior nucleus‐targeting properties. As a rare dual catalytic inhibitor, Ru7 effectively inhibits the endogenous activities of topoisomerase (topo) I and II and kills cancer cells by necroptosis. The cell signaling pathway from topo inhibition to necroptosis was elucidated. Furthermore, Ru7 displays significant antitumor activity against drug‐resistant cancer cells in vivo. To the best of our knowledge, Ru7 is the first Ru‐based necroptosis‐inducing chemotherapeutic agent.     

Mechanistic insights into regioselective polymerization of 1,3‐Dienes catalyzed by a bipyridine‐ligated iron complex: A DFT study

The regioselective polymerizations of isoprene and 3‐methyl‐pentadiene catalyzed by a cationic iron (II) complex bearing bipyridine ligand have been computationally studied. Having achieved an agreement between calculation and experiment, it is found that the open‐shell unpaired 3d‐electrons localize on Fe center rather than partially distribute on the redox‐active bipyridine ligand. The steric effect plays a more important role in controlling the regioselectivity in comparison with electronic factors. The deformation energy is mainly contributed by monomer and Fe‐alkyl moieties rather than the bipyridine ligands themselves, although noncyclopentadienyl ancillary ligands are often deformed in most insertion transition states for selective polymerization of olefin. © 2016 Wiley Periodicals, Inc.     

Novel, highly efficient blue-emitting heteroleptic iridium(III) complexes based on fluorinated 1,3,4-oxadiazole: tuning to blue by dithiolate ancillary ligands

Novel blue-emitting phosphorescent iridium(III) complexes with fluorinated 1,3,4-oxadiazole derivatives as cyclometalated ligands and dithiolates as ancillary ligands have been synthesized and fully characterized; highly efficient OLEDs have been achieved using these complexes in the light-blue to blue-emitting region.     

Strongly Luminescent Cyclometalated Gold(III) Complexes Supported by Bidentate Ligands Displaying Intermolecular Interactions and Tunable Emission Energy

A series of charge‐neutral Au^III complexes, which comprise a dicarbanionic C‐deprotonated biphenyl ligand and bidentate ancillary ligands ([Au(C^C)(L^X)]; L^X=β‐diketonate and relatives (O^O), quinolinolate and relatives (N^O), and diphosphino (P^P) ligands), were prepared. All the complexes are emissive in degassed CH₂Cl₂ solutions and in thin‐film samples with Φ _em up to 18 and 35 %, respectively, except for 5 and 6 , which bear (N^O)‐type ancillary ligands. Variation of the electronic characteristics of the β‐diketonate ancillary ligand was demonstrated to be a viable route for tuning the emission color from blue‐green (peak λ _em at ca. 466 nm for 1 and 2 ; 501 nm for 4 a and 4 b ) to orange (peak λ _em at 585 nm for 3 ), in contrast to the common observations that the ancillary ligand has a negligible effect on the excited‐state energy of the Au^III complexes reported in the literature. DFT/time‐dependent (TD) DFT calculations revealed that the energies of the ³ππ*(C^C) and the ³ILCT(O^O) excited states (ILCT=intraligand charge transfer) switch in order on going from O^O=acetylacetonate (acac) to aryl‐substituted β‐diketonate ligands. Solution‐processed and vacuum‐deposited organic light‐emitting diode (OLED) devices of selected complexes were prepared. The vacuum‐deposited OLED fabricated with 2 displays a sky‐blue emission with a maximum external quantum efficiency (EQE) of 6.71 % and CIE coordinates of (0.22, 0.40). The crystal structures of 7 and 9 reveal short intermolecular Au^III⋅⋅⋅Au^III contacts, with intermetal distances of 3.408 and 3.453 Å, respectively. DFT/TDDFT calculations were performed on 7 and 9 to account for the noncovalent interactions. Solid samples of 1 , 3 , and 9 exhibit excimeric emission at room temperature, which is rarely reported in Au^III complexes.     

含长链β-二酮环金属铂配合物的电子光谱和二阶非线性光学性质的 理论研究

在B3LYP/LanL2DZ(6-31＋＋G**)理论水平对标题化合物进行结构优化和电子光谱与二阶非线性光学性质计算. 结果显示, 重金属的配合导致Pt原子与苯环, 吡啶环, β-二酮羰基环构成较大的共轭体系, 使得分子由基态到第一激发态的p→p*和n→p*跃迁伴随MLCT电荷转移, 对应的最大吸收波长在406 nm左右, 属于近紫外区, β-二酮碳链的长度对结构和电子光谱影响很小, 与实验结果一致. 长链β-二酮环金属铂配合物分子具有较好的非线性光学性质.     

Towards deep‐blue phosphorescence: molecular design and property prediction of iridium complexes with pyridinylphosphinate ancillary ligand

A series of iridium complexes ( 1 – 5 ), which consist of two 2‐(2,4‐difluorophenyl)pyridine (dfppy)‐based primary ligands and one pyridinylphosphinate ancillary ligand, have been investigated theoretically for screening highly efficient deep‐blue light‐emitting materials. Compared with the reported dfppy‐based emitter 1 , the designed iridium complexes 3 – 5 with the introduction of a stronger electron‐withdrawing (–CN, –CF₃ , or o‐carborane) group and a bulky electron‐donating (tert‐butyl) group in dfppy ligands can be achieved to display the emission peaks at 443, 442, and 447 nm, respectively. The electronic structures, absorption and emission properties, radiative and nonradiative processes of their excited states, and charge injection and transport properties of the iridium complexes are analyzed in detail. The calculated results show that designed iridium complexes have comparable radiative and nonradiative rate constants with 1 , and are expected to have similar quantum efficiency with 1 . Meanwhile, these designed complexes keep the advantages of the charge transport properties of 1 , indicating that they are potential iridium complexes for efficient deep‐blue phosphorescence. This work provides more in‐depth understanding the structure–property relationship of dfppy‐based iridium complexes, and shed lights on molecular design for deep‐blue phosphorescent metal complexes.     

Theoretical studies on structural and spectroscopic properties of photoelectrochemical cell ruthenium sensitizers,derivatives of AR20

Special efforts were devoted to improve the absorption behavior of AR20 in visible region. Density functional theory (DFT)‐based approaches were applied to explore the electronic structure properties of AR20 and its derivatives. Time‐dependent DFT results indicate that the ancillary ligand controls the molecular orbital (MO) energy levels and masters the absorption transition nature. The deprotonation of anchoring ligand not only affects the frontier MO energy levels but also determines the energy gaps of highest occupied MO–lowest unoccupied MO (LUMO) and LUMO–LUMO+1. Introducing thiophene groups into ancillary ligands would enhance the efficiency of the final dye‐sensitized solar cell (DSSC). The absorption intensity of the thiophene substituted derivatives of AR20 is irrelevant with environment circumstance change, such as pH value. This special nature prognosticates the thiophene‐substituted derivatives of AR20 which would have a broad application in DSSC. © 2012 Wiley Periodicals, Inc.     

Dimeric Palladium Complexes with Bridging Aryl Groups: When are they Stable?

Stable dimeric palladium(II ) complexes of general formula [Pd₂(μ‐R)₂(η³‐allyl)₂] (R=haloaryl, mesityl) have been prepared. Their X‐ray crystal structures, determined for some of the complexes, show that the two coordination square planes are usually coplanar. The haloaryl complexes are fluxional in solution, showing exchange between cis and trans isomers (relative to the orientation of the two allyl groups in the dimer) through solvent‐assisted associative bridge splitting. A number of other ancillary ligands (O,O, S,S, or C,N donors) failed to stabilize the bridging situation. Also, bridging phenyls were unstable. The reasons for this behavior and the formation of alternative compounds in attempts at synthesizing them are fully analyzed and explained. Stable aryl bridges seem to be favored by a combination of factors: the use of ancillary ligands of small size and lacking electron lone pairs, and the use of aryl ligands reluctant to homo and hetero C? C coupling. These seem to be more important factors in the stabilization of bridging aryl complexes than the strength of the bridges themselves.     

A Fluxional Copper Acetylide Cluster in CuAAC Catalysis

A molecularly defined copper acetylide cluster with ancillary N‐heterocyclic carbene (NHC) ligands was prepared under acidic reaction conditions. This cluster is the first molecular copper acetylide complex that features high activity in copper‐catalyzed azide–alkyne cycloadditions (CuAAC) with added acetic acid even at ?5 °C. Ethyl propiolate protonates the acetate ligands of the dinuclear precursor complex to release acetic acid and replaces one out of four ancillary ligands. Two copper(I) ions are thereby liberated to form the core of a yellow dicationic C₂‐symmetric hexa‐NHC octacopper hexaacetylide cluster. Coalescence phenomena in low‐temperature NMR experiments reveal fluxionality that leads to the facile interconversion of all of the NHC and acetylide positions. Kinetic investigations provide insight into the influence of copper acetylide coordination modes and the acetic acid on catalytic activity. The interdependence of “click” activity and copper acetylide aggregation beyond dinuclear intermediates adds a new dimension of complexity to our mechanistic understanding of the CuAAC reaction.     

Tuning the Excited—state Properties of Cyclometalated Platinum（Ⅱ）Complexes of 6—Pheny1—2,2’bipyridine by Ancillary Acetylide Ligand

A series of luminescent cyclometalated platinum(Ⅱ）complexes,(C^N^N)Pt(C≡CR)[HC^N^N=4-(4-tolyl)-6-phenyl-2,2’-bipyridine;R=4-chlorophenyl(1),phenyl(2) and 4-tolyl(3)],were synthesized,and their spectroscopic properties have been examined.These complexes are brightly emissive both in fluid solution and in the solid state,attributed to triplet metal-to-ligand charge transfer(^3MLCT)state.The excited state energy can be tuned by ancillary acetylide ligands.The emission lifetimes in dichloromethand solution at room temperature were up to 1.64 μs and the emission quantum yields were in the range of 0.03-0.15.     

Weak Ligand‐Field Effect from Ancillary Ligands on Enhancing Single‐Ion Magnet Performance

A series of bis‐pentamethylcyclopentadienyl‐supported Dy complexes containing different ancillary ligands were synthesized and characterized. Magnetic studies showed that 1 Dy [Cp*₂DyCl(THF)], 1 Dy’ [Cp*₂DyCl₂K(THF)]_n, 2 Dy [Cp*₂DyBr(THF)], 3 Dy [Cp*₂DyI(THF)] and 4 Dy [Cp*₂DyTp] (Tp=hydrotris(1‐pyrazolyl)borate) were single‐ion magnets (SIMs). The 1D dysprosium chain 1 Dy’ exhibited a hysteresis at up to 5 K. Furthermore, 3 Dy featured the highest energy barrier (419 cm^?1) among the complexes. The effects of ancillary ligands on single‐ion magnetic properties were studied by experimental, ab initio calculations and electrostatic analysis methods in detail. These results demonstrated that the QTM rate was strongly dependent on the ancillary ligands and that a weak equatorial ligand field could be beneficial for constructing Dy‐SIMs.     

A Lantern-Shaped Pd(II) Cage Constructed from Four Different Low-Symmetry Ligands with Positional and Orientational Control: An Ancillary Pairings Approach

One of the key challenges of metallo-supramolecular chemistry is to maintain the ease of self-assembly but, at the same time, create structures of increasingly high levels of complexity. In palladium(II) quadruply stranded lantern-shaped cages, this has been achieved through either 1) the formation of heteroleptic (multi-ligand) assemblies, or 2) homoleptic assemblies from low-symmetry ligands. Heteroleptic cages formed from low-symmetry ligands, a hybid of these two approaches, would add an additional rich level of complexity but no examples of these have been reported. Here we use a system of ancillary complementary ligand pairings at the termini of cage ligands to target heteroleptic assemblies: these complementary pairs can only interact (through coordination to a single Pd(II) metal ion) between ligands in a cis position on the cage. Complementarity between each pair (and orthogonality to other pairs) is controlled by denticity (tridentate to monodentate or bidentate to bidentate) and/or hydrogen-bonding capability (AA to DD or AD to DA). This allows positional and orientational control over ligands with different ancillary sites. By using this approach, we have successfully used low-symmetry ligands to synthesise complex heteroleptic cages, including an example with four different low-symmetry ligands.     

Imidazolin‐2‐ylidenaminophosphines as Highly Electron‐Rich Ligands for Transition‐Metal Catalysts

A variety of chemical transformations benefit from the use of strong electron‐donating ancillary ligands, such as alkylphosphines or N‐heterocyclic carbenes when electron‐rich metal centers are required. Herein, we describe a facile and highly modular access to monodentate and bidentate imidazolin‐2‐ylidenamino‐substituted phosphines. Evaluation of the phosphine’s electronic properties substantiate that the formal replacement of alkyl or aryl groups by imidazolin‐2‐ylidenamino groups dramatically enhance their donor ability beyond that of alkylphosphines and even N‐heterocyclic carbenes. The new phosphines have been coordinated onto palladium(II) centers, and the beneficial effect of the novel substitution patterns has been explored by using the corresponding complexes in the palladium‐catalyzed Suzuki–Miyaura cross‐coupling reaction of non‐activated aryl chloride substrates.     

Effect of ancillary ligands on the electronic structures, DNA-binding and spectral properties of [Co(L)2(pip)] (L = phen, bpy, en, tap)

Studies on the electronic structures and related properties of a series of Co(Ш) complexes have been carried out, using the density functional theory (DFT) at the B3LYP/LanL2DZ level. The effect of the ancillary ligands on their electronic structures, DNA-binding affinities and spectra was revealed. The results show that an ancillary ligand has quite important effect on electronic structures and DNA-binding properties of these Co(Ш) complexes. The ancillary ligand possessing a great conjugated structure can effectively improve the DNA-binding affinity of the complex. Meanwhile, introducing a stronger electronegative N atom on the skeleton of ancillary ligand can obviously reduce the LUMO energy of the complex. Based on these findings, a designed complex 4 can be expected to have the greatest K_b value in complexes 1–4. So it may be able to control the interaction between the complex and DNA-base-pairs via varying ancillary ligands. In addition, the electronic absorption spectra of these complexes were calculated and simulated in aqueous solution using the time-dependent DFT (TDDFT) method and the effect of the ancillary ligands on the spectra was also explored. The calculated absorption spectra of these complexes in aqueous solution are in a satisfying agreement with the experimental results, and the properties of experimental absorption bands were theoretically explained in detail.     

Porphyrins Fused to N‐Heterocyclic Carbenes (NHCs): Modulation of the Electronic and Catalytic Properties of NHCs by the Central Metal of the Porphyrin

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N‐heterocyclic carbene ligands 1 M . Rhodium(I) complexes 6 M – 9 M were prepared by using 1 M ligands with different metal cations in the inner core of the porphyrin (M=Ni^II, Zn^II, Mn^III, Al^III, 2H). The electronic properties of the corresponding N‐heterocyclic carbene ligands were investigated by monitoring the spectroscopic changes occurring in the cod and CO ancillary ligands of [( 1 M )Rh(cod)Cl] and [( 1 M )Rh(CO)₂Cl] complexes (cod=1,5‐cyclooctadiene). Porphyrin–NHC ligands 1 M with a trivalent metal cation such as Mn^III and Al^III are overall poorer electron donors than porphyrin–NHC ligands with no metal cation or incorporating a divalent metal cation such as Ni^II and Zn^II. Imidazolium salts 3 M (M=Ni, Zn, Mn, 2H) have also been used as NHC precursors to catalyze the ring‐opening polymerization of L ‐lactide. The results clearly show that the inner metal of the porphyrin has an important effect on the reactivity of the outer carbene.     

A Self‐Assembled Luminescent Host That Selectively Senses ATP in Water

Metal‐ion‐directed self‐assembly has been used to construct kinetically inert, water‐soluble heterometallic Ru₂Re₂ hosts that are potential sensors for bioanions. A previously reported metallomacrocycle and a new derivative synthesised by this approach are found to be general sensors for bioanions in water, showing an “off–on” luminescent change that is selective for nucleotides over uncharged nucleobases. Through a change in the ancillary ligands coordinated to the ruthenium centres of the host, an “off–on” sensor has been produced. Whilst this host only shows a modest enhancement in binding affinities for nucleotides relative to the other two host systems, its sensing response is much more specific. Although a distinctive “off–on” luminescence response is observed for the addition of adenosine triphosphosphate (ATP), related structures such as adenine and guanosine triphosphate (GTP) do not induce any emission change in the host. Detailed and demanding DFT studies on the ATP‐ and GTP‐bound host–guest complexes reveal subtle differences in their geometries that modulate the stacking interactions between the nucleotide guests and the ancillary ligands of the host. It is suggested that this change in stacking geometries affects solvent accessibility to the binding pocket of the host and thus leads to observed difference in the host luminescence response to the guests.     

Detection of Palladium(I) in Aerobic Oxidation Catalysis

Palladium(II)‐catalyzed oxidation reactions exhibit broad utility in organic synthesis; however, they often feature high catalyst loading and low turnover numbers relative to non‐oxidative cross‐coupling reactions. Insights into the fate of the Pd catalyst during turnover could help to address this limitation. Herein, we report the identification and characterization of a dimeric Pd^I species in two prototypical Pd‐catalyzed aerobic oxidation reactions: allylic C−H acetoxylation of terminal alkenes and intramolecular aza‐Wacker cyclization. Both reactions employ 4,5‐diazafluoren‐9‐one (DAF) as an ancillary ligand. The dimeric Pd^I complex, [Pd^I(μ‐DAF)(OAc)]₂, which features two bridging DAF ligands and two terminal acetate ligands, has been characterized by several spectroscopic methods, as well as single‐crystal X‐ray crystallography. The origin of this Pd^I complex and its implications for catalytic reactivity are discussed.