Olefin Dimerization and Isomerization Catalyzed by Pyridylidene Amide Palladium Complexes

Highly Efficient Low-loaded PdOx/AlSiOx Catalyst for Ethylene Dimerization

Ethylene dimerization is an industrial process that is currently carried out using homogeneous catalysts. Here we present a highly active heterogeneous catalyst containing minute amounts of atomically dispersed Pd. It requires no co-catalyst(s) or activator(s) and significantly outperforms previously reported catalysts tested under similar reaction conditions. The selectivity to C4- and C6-hydrocarbons was about 80 % and 10 % at 42 % ethylene conversion at 200 °C using an industrially relevant feed containing 50 vol % ethylene, respectively. Our kinetic and catalyst characterization experiments complemented by density functional theory calculations provide molecular insights into the local environment of isolated Pd(II)Ox species and their role in achieving high activity in the target reaction. When the developed catalyst was rationally integrated with a Mo-containing olefin metathesis catalyst in the same reactor, the formed butenes reacted with ethylene to propylene with a selectivity of 98 % at about 24 % ethylene conversion.

Stable CAAC-Triazenes: A New Nitrogen Ligand System With Donor and Conformational Flexibility, and With Application in Olefin Activation Catalysis

N-heterocyclic imines such as pyridylidene amines impart high catalytic activity when coordinated to a transition metal, largely imposed by their electronic flexibility. Here, this donor flexibility has been applied for the first time to CAAC-based systems through the synthesis of CAAC-triazenes. These new ligands offer a larger π-conjugation that extends from the N-heterocyclic carbene through three nitrogens rather than just one, as observed in N-heterocyclic imines. We demonstrate the straightforward synthesis of three new CAAC-triazenes containing different substituents on the terminal triazene nitrogen. These compounds are remarkably stable up to 120 °C without loss of N2 as typically observed with similar triazenes. E-to-Z isomerization within the triazene is instigated by UV light and is partially reversible dependent on the triazene substituent. The quinoline-substituted CAAC-triazene 1-Q has been applied as an L,L'-type ligand in the synthesis of [PdCl2(1-Q)], [PdCl(Me)(1-Q)] and [Pd(Me)(H2O(1-Q)]+. E-to-Z ligand isomerization also occurs when coordinated to PdCl2, providing access to on-metal manipulation. The cationic complex [PdMe(H2O)(1-Q)]+ is a precatalyst for oligomerization of ethylene to form initially 2-butene and subsequently linear and branched C8-C12 products from butene activation. Moreover, isomerization of 1-hexene takes place efficiently with exceptionally low catalyst loading (10 ppm) and up to 74,000 turnover numbers.

Electronically flexible PYA ligands for efficient palladium-catalyzed α-arylation of ketones

Palladium-catalyzed cross-coupling chemistry and in particular ketone α-arylation has been relying on a rather narrow range of supporting ligands with almost no alternatives to phosphines and N-heterocyclic carbenes. Here we introduce a class of well-defined palladium(II) complexes supported by N,N'-chelating and electronically flexible pyridylidene amide (PYA)-pyridyl ligands as catalysts for efficient α-arylation of ketones. Steric and electronic variations of the N,N'-bidentate ligand indicate that the introduction of an ortho-methyl group on the pyridinum heterocycle of the PYA ligand enhances the arylation rate and prevents catalyst deactivation, reaching turnover numbers up to 7300 and turnover frequencies of almost 10 000 h-1, which is similar to that of the best phosphine complexes known to date. Introducing a shielding xylyl substituent accelerates catalysis further, however at the expense of lower selectivity towards arylated ketones. Substrate scope investigations revealed that both electron-rich and -poor aryl bromides as well as a broad range of electronically and sterically modified ketones are efficiently converted, including aliphatic ketones. Mechanistic investigations using Hammett and Eyring analyses indicated that both, oxidative addition and reductive elimination are relatively fast, presumably as a consequence of the electronic flexibility of the PYA ligand, while enolate coordination was identified as the turnover-limiting step.

Pd(II) complexes with chelating N-(1-alkylpyridin-4(1H)-ylidene)amide (PYA) ligands: Synthesis, characterization and evaluation of anticancer activity

The bidentate N-(1-Alkylpyridin-4(1H)-ylidene)amide (PYA) pro-ligands [H₂L_Bn][Cl]₂ (2), and [H₂L_Me][TfO]₂ (3) were prepared by simple alkylation reactions of the known compound, N,N-di(pyridin-4-yl)oxalamide (H₂L, 1). The Pd(II) complexes, [Pd(L_Bn)₂][Cl]₂ (4), [Pd(L_Me)₂][Cl][TfO] (5), Pd(L_Bn)Cl₂ (6) and Pd(L_Me)Cl₂ (7) were synthesized through reactions between these pro-ligands and suitable Pd(II) substrates in the presence of base. The molecular structures of 3 and 6 were obtained by single crystal X-ray structure determinations. Studies of the experimental and computational DNA binding interactions of the compounds 1-7 revealed that overall 4 and 6 have the largest values for the binding parameters K_b and ΔG_b^o. The results showed a good correlation with the steric and electronic parameters obtained by quantitative structure activity relationship (QSAR) studies. In-vitro cytotoxicity studies against four different cell lines showed that the human breast cancer cell lines MCF-7, T47D and cervical cancer cell line HeLa had either higher or similar sensitivities towards 4, 6 and 2, respectively, compared to cisplatin. In general, the cytotoxicity of the compounds, represented by IC₅₀ values, decreased in the order 4 > 6 > 2 > 5 > 3 > 1 > 7 in cancer cell lines. Apoptosis contributed significantly to the cytotoxic effects of these anticancer agents as evaluated by apoptosis studies.

Modulation of N^N'-bidentate chelating pyridyl-pyridylidene amide ligands offers mechanistic insights into Pd-catalysed ethylene/methyl acrylate copolymerisation

The efficient copolymerisation of functionalised olefins with alkenes continues to offer considerable challenges to catalyst design. Based on recent work using palladium complexes containing a dissymmetric N^N'-bidentate pyridyl-PYA ligand (PYA = pyridylidene amide), which showed a high propensity to insert methyl acrylate, we have here modified this catalyst structure by inserting shielding groups either into the pyridyl fragment, or the PYA unit, or both to avoid fast β-hydrogen elimination. While a phenyl substituent at the pyridyl side impedes catalytic activity completely and leads to an off-cycle cyclometallation, the introduction of an ortho-methyl group on the PYA side of the N^N'-ligand was more prolific and doubled the catalytic productivity. Mechanistic investigations with this ligand system indicated the stabilisation of a 4-membered metallacycle intermediate at room temperature, which has previously been postulated and detected only at 173 K, but never observed at ambient temperature so far. This intermediate was characterised by solution NMR spectroscopy and rationalises, in part, the formation of α,β-unsaturated esters under catalytic conditions, thus providing useful principles for optimised catalyst design.

Modular O- vs. N-coordination of pyridylidene amide ligands to iron determines activity in alcohol oxidation catalysis

A family of polydentate pyridine-substituted pyridylidene amide (PYA) complexes bound to iron(ii) was developed. The variation of the coordination set from NN-bidentate PYA to tridentate pincer-type pyPYA2 systems (pyPYA2 = 2,6-bis(PYA)pyridine) had a large influence on the binding mode to iron(ii), including a change from the N- to rare O-coordination of the PYA site and a concomitant shift of the predominant ligand resonance structure. These binding mode variations invoke changes in the reactivity of the complexes, which were probed in the peroxide-mediated oxidation of 1-phenylethanol to acetophenone. A comparison with uncomplexed FeCl2 indicated that bidentate NN coordination is unstable and presumably leads to the dissociation of FeCl2. In contrast, the tridentate ligand binding is robust. Remarkably, the tridentate PYA pincer coordination inhibits catalytic activity in the NNN binding mode, while the ONO coordination greatly enhances catalytic performance. Under optimized conditions, the bis-ligated ONO pincer iron complex [Fe(pyPYA2)2][2PF6] reaches full conversion within one hour (0.5 mol% catalyst loading) and under dilute conditions turnover numbers over 20 000 (0.005 mol% catalyst loading).

Highly Efficient Transfer Hydrogenation Catalysis with Tailored Pyridylidene Amide Pincer Ruthenium Complexes

The rational optimization of homogeneous catalysts requires ligand platforms that are easily tailored to improve catalytic performance. Here, it is demonstrated that pyridylidene amides (PYAs) provide such a platform to custom-shape transfer hydrogenation catalysts with exceptional activity. Specifically, a series of meta-PYA pincer ligands with differently substituted PYA units has been synthezised and coordinated to ruthenium(II) centres to form bench-stable tris-acetonitrile complexes [Ru(R-PYA-pincer)(MeCN)₃ ](PF₆ )₂ (R=OMe, Me, H, Cl, CF₃ ). Analytic studies including ¹ H NMR spectroscopy, cyclic voltammetry, and X-ray crystallography reveal a direct influence of the substituents on the electronic properties of the ruthenium center. The complexes are active in the catalytic transfer hydrogenation of ketones, with activities directly encoded by the PYA substitution pattern. Their perfomance improves further upon exchange of an ancillary MeCN ligand with PPh₃ . While complexes [Ru(R-PYA-pincer)(PPh₃ )(MeCN)₂ ](PF₆ )₂ were only isolated for R=H, Me, an in situ protocol was developed to generate these complexes in situ for R=OMe, Cl, CF₃ by using a 1:2 ratio of the complexes and PPh₃ . This in situ protocol together with a short catalyst pre-activation provided highly active catalytic systems. The most active pre-catalyst featured the methoxy-substituted PYA ligand and reached turnover frenquencies of 210 000 h^-1 under an exceptionally low catalyst loading of 25 ppm for the benchmark substrate benzophenone, representing one of the most active transfer hydrogenation systems known to date.

Ambidentate bonding and electrochemical implications of pincer-type pyridylidene amide ligands in complexes of nickel, cobalt and zinc

Pincer-type tridentate pyridyl bis(pyridylidene amide) (pyPYA2) ligand systems were coordinated to the Earth-abundant first row transition metals nickel, cobalt and zinc. A one-pot synthesis in water/ethanol afforded octahedral homoleptic bis-PYA complexes, [M(pyPYA2)2](PF6)2, whereas five-coordinate mono-PYA dichloride complexes, [M(pyPYA2)Cl2], were obtained upon slow addition of the ligand to the metal chlorides in DMF. Electrochemical measurements further revealed a facile oxidation of the metal centers from Ni2+ to Ni4+ and Co2+ to Co3+, respectively, while the Zn2+ system was redox inactive. These experiments further allowed for quantification of the much stronger electron donor properties of neutral N,N,N-tridentate pyPYA2 pincer ligands as compared to terpy. Remarkably, ortho-PYA pincer ligands feature amide coordination to the metal center via oxygen or nitrogen. This ambidentate ligand binding constitutes another mode of donor flexibility of the PYA ligand system, complementing the resonance structure dynamics established previously. NMR spectroscopic and MS analysis reveal that the meta-PYA ligand undergoes selective deuteration when coordinated to cobalt. This reactivity suggests the potential of this ligand as a transient proton reservoir for HX bond activation and, moreover, indicates the relevance of several resonance structures and therefore supports the notion that meta-PYA ligands are mesoionic.

Donor Strength Determination of Pyridinylidene-amide Ligands using Their Palladium-NHC Complexes

Pyridinylidene-amides (PYAs) are a relatively new type of N-donor ligands that can exist in three isomeric forms and adopt various resonance structures. This makes them electronically flexible, and in order to evaluate their electronic profile using the Huynh electronic parameter (HEP), seven structurally diverse mixed N-heterocyclic carbenes (NHCs)/PYA palladium complexes of the type trans-[PdBr₂(ⁱPr₂-bimy)(PYA)] were prepared and fully characterized by various spectroscopic and spectrometric methods. This study shows that PYAs are among the strongest, formally neutral N-donors, but they are still weaker than phosphines and organometallic ligands such as NHCs. Notably, the donating abilities of isomeric PYAs are distinct and can be further fine-tuned by the choice of two substituents making them structurally and electronically versatile. These characteristics and the ease of their preparation hold promise for a wide applicability in coordination chemistry.

Pd-catalyzed cascade reactions involving skipped dienes: from double carbopalladation to remote C-C cleavage

We report two ligand-controlled cascade reactions relying on the intramolecular carbopalladation of skipped dienes. The use of a bulky monodentate phosphine ligand affords [4,5]-spirocycles via sequential double carbopalladation, however bidentate phosphines promote a remote β-C-elimination process which does not rely on the use of strained or sterically hindered substrates.