Pyridine and related ligands in transition metal homogeneous catalysis

This review provides a broad overview of the literature related to the importance of pyridine and related ligands in homogeneous catalysis. In particular, it describes the various ways by which this ligand can stabilised the metal within a complex for homogeneous catalysis. We surveyed the important transition metal homogenous catalysts containing pyridine and related ligand acting as backbone for other ligands in homogeneous catalytic reactions explicitly from 2011 up to early 2014 and summarized their comparative catalytic activities.     

Role of ligands in homogeneous catalysis based on transition metals

The role of ligands in homogeneous catalysis by transition metals and the promising lines of research related to simulation of biocatalytic processes are considered.     

Decarboxylation syntheses of transition metal organometallics. III polyfluorophenylplatinum(II) complexes with nitrogen donor ligands

Summary The platinum(II) carboxylates,trans-Pt(O₂CR)₂(py)₂ and Pt(O₂CR)₂bpy (R=C₆F₅,p-HC₆F₄,m-HC₆F₄, oro-HC₆F₄; bpy=2,2-bipyridyl), have been prepared by reactions oftrans-Pt(OH)₂(py)₂ or Pt(OH)₂bpy with the appropriate polyfluorobenzoic acids, whilst [Pt(py)₄](O₂CC₆F₅)₂ has been obtained from reaction oftrans-PtCl₂(py)₂ with thallous pentafluorobenzoate in pyridine at room temperature. In boiling pyridine, the platinum(II) polyfluorobenzoates undergo either decarboxylation givingtrans-PtR₂(py)₂ and PtR₂bpy (R= C₆F₅,p-HC₆F₄, orm-HC₆F₄) complexes or substitution, giving [Pt(py)₄](O₂CC₆F₄H-o)₂ and [Ptbpy(py)₂](O₂CC₆F₄H-o)₂. Reactions oftrans-PtX₂(py)₂ and PtX₂bpy (X=Cl or Br) with appropriate thallous polyfluorobenzoates in boiling pyridine have yielded the complexestrans-PtR₂(py)₂, PtR₂bpy, PtCl(R)bpy (R=C₆F₅,p-HC₆F₄, orm-HC₆F₄ in each case),trans-PtCl(R)(py)₂ (R = C₆F₅ orm-HC₆F₄),trans-PtBr(C₆F₅)(py)₂, and PtBr(C₆F₅)bpy. The complexestrans-PtR₂(py)₂ (R=C₆F₅ orp-HC₆F₄) have also been prepared from potassium tetrachloroplatinate(II) and the appropriate thallous polyfluorobenzoate in boiling py, andtrans-Pt(C₆F₅)₂(py)₂ has been similarly obtained fromcis-PtCl₂(py)₂ and C₆F₅CO₂Tl. Significant decarboxylation was not observed on reaction oftrans-PtCl₂(py)₂ or PtCl₂bpy with thallous 2,3,4,5-tetrafluorobenzoate.Part II, ref. 4;Preliminary communication, ref. 3;     

A modular approach to structurally diverse bidentate chelate ligands for transition metal catalysis

A modular approach to a new class of structurally diverse bidentate P/N, P/P, P/S, and P/Se chelate ligands has been developed. Starting from hydroquinone, various ligands were synthesized in a divergent manner via orthogonally bis-protected bromohydroquinones as the central building block. The first donor functionality (L1) is introduced to the aromatic (hydroquinone) ligand backbone either by Pd-catalyzed cross-coupling (Suzuki coupling) with hetero-aryl bromides, by Pd-catalyzed amination, or by lithiation and subsequent treatment with electrophiles (e.g., chlorophosphanes, disulfides, diselenides, or carbamoyl chlorides). After selective deprotection, the second ligand tooth (L2) is attached by reaction of the phenolic OH functionality with a chlorophosphane, a chlorophosphite, or a related reagent. Some of the resulting chelate ligands were converted into the respective PdX2 complexes (X = Cl, I), two of which were characterized by X-ray crystallography. The methodology developed opens an access to a broad variety of new chiral and achiral transition metal complexes and is generally suited for the solid-phase synthesis of combinatorial libraries, as will be reported separately.     

Molecular recognition in homogeneous transition metal catalysis: a biomimetic strategy for high selectivity

Traditional methods for selectivity control in homogeneous transition metal catalysis either employ steric effects in a binding pocket or chelate control. In a supramolecular strategy, encapsulation of the substrate can provide useful shape and size selectivity. A fully developed molecular recognition strategy involving hydrogen bonding or solvophobic forces has given almost completely regioselective functionalization of remote, unactivated C-H bonds.     

Synthesis and characterization of new metal(II) complexes with formates and some nitrogen donor ligands

New mixed-ligands complexes with empirical formulae: M(2,4′-bpy)₂L₂·H₂O (M(II)Zn, Cd), Zn(2-bpy)₃L₂·4H₂O, Cd(2-bpy)₂L₂·3H₂O, M(phen)L₂·2H₂O (where M(II)=Mn, Ni, Zn, Cd; 2,4′-bpy=2,4′-bipyridine, 2-bpy=2,2′-bipyridine, phen=1,10-phenanthroline, L=HCOO⁻) were prepared in pure solid state. They were characterized by chemical, thermal and X-ray powder diffraction analysis, IR spectroscopy, molar conductance in MeOH, DMF and DMSO. Examinations of OCO⁻ absorption bands suggest versatile coordination behaviour of obtained complexes. The 2,4′-bpy acts as monodentate ligand; 2-bpy and phen as chelating ligands. Thermal studies were performed in static air atmosphere. When the temperature raised the dehydration processes started. The final decomposition products, namely MO (Ni, Zn, Cd) and Mn₃O₄, were identified by X-ray diffraction.     

Axial donating ligands: a new strategy for late transition metal olefin polymerization catalysis

An alpha-diimine ligand (1) containing an axial donating pyridine group is developed for late metal polymerization catalysis. Despite having no substitution on the bottom face of the ligand, the nickel and palladium complexes of 1 are highly active for ethylene polymerization, producing linear high molecular weight polymers. For example, 1-NiBr2 (3) forms PE with a Mn of up to 109 224 g/mol with 1.4 branches/1000 C's. Similarly, 1-PdMeCl (5) forms PE with a Mn of up to 880 379 g/mol with 5.1 branches/1000 C's. In sharp contrast, catalysts containing the control ligand (2) consisting of a noncoordinating phenyl group gave only low molecular weight branched oligomers. It is observed that AlMe2Cl plays a specific role in generating the active species for the pyridine-based complexes. Presumably, the pyridine group may interact with AlMe2Cl to form a bimetallic species which suppresses the beta-hydride elimination process, hence resulting in reduced chain transfer and more linear structure.     

Synthesis,physico-chemical and antimicrobial screening studies of some transition metal complexes with O:O donor ligands

The solid complexes of Mn^II, Fe^III, Co^II, Ni^II and Cu^II with 3-(3-furan-2yl-acryloyl)-6-methyl-pyran-2,4-dione(L¹) and 3-(3-thiophene-2yl-acryloyl)-6-methyl-pyran-2,4-dione (L²) have been synthesized and characterized by elemental analysis, conductometry, thermal analysis, magnetic, i.r., P-n.m.r., u.v.–vis, X-ray diffraction and antimicrobial study. From the analytical and spectral data, the stoichiometry of the complexes has been found to be 1:2 (metal:ligand). I.r. spectral data suggest that the ligand behaves as a dibasic bidentate ligand with O:O donor sequence towards metal ions. The physico-chemical data suggests distorted octahedral geometry for Cu^II complexes and octahedral geometry for all other complexes. The X-ray diffraction suggests an Orthorhombic crystal system for the Cu^II complex and Monoclinic crystal system for Co^II and Ni^II complexes of ligand L¹. The ligands and their metal complexes were screened for antibacterial activity against Staphylococcus aureus and Escherichia coli, and the fungicidal activity against Aspergillus flavus, Curvularia lunata and Penicillium notatum.     

Well-defined transition metal complexes with phosphorus and nitrogen ligands for 1,3-dienes polymerization

This article provides an overview on recent progress in the polymerization of 1,3-dienes catalyzed by transition metal complexes with phosphorus and nitrogen ligands. Polymers having different microstructures (cis-1,4; 1,2; mixed cis-1,4/1,2) and tacticity (iso- or syndiotactic) were obtained from various 1,3-dienes (1,3-butadiene, isoprene, 1,3-pentadienes, 1,3-hexadienes) depending on the catalyst used, clearly suggesting that the catalyst structure (i.e. metal nature, type of ligand) strongly affects the polymerization chemo- and stereoselectivity. However, as indicated by the results obtained in the polymerization of substituted butadienes, a fundamental role in determining the selectivity is also played by the type of monomer: polymers with different structure, some of them completely new, were obtained from different monomers with the same catalyst. All these observations permitted to confirm, and in some cases to improve, the knowledge on the diene polymerization mechanism.     

Hydrogen bonding as a construction element for bidentate donor ligands in homogeneous catalysis: regioselective hydroformylation of terminal alkenes

A new concept for the construction of bidentate ligands for homogeneous metal complex catalysis is described. The concept relies on the self-assembly of monodentate ligands through hydrogen bonding. As a prototype of such systems, 6-diphenylphosphanyl-2-pyridone (6-DPPon) was shown to form a chelate in the coordination sphere of a transition metal center through unusual pyridone/hydroxypyridine hydrogen bonding (X-ray). This hydrogen bonding stays intact in a catalytic reaction as proven upon highly regioselective hydroformylation of terminal alkenes. Regioselectivities and reactivities observed rank the 6-DPPon/rhodium system among the most active and regioselective catalysts for n-selective hydroformylation of terminal alkenes.     

Supramolecular approaches to generate libraries of chelating bidentate ligands for homogeneous catalysis

The process of catalyst discovery and development relying on combinatorial methods has suffered so far from the difficult access to structurally diverse and large libraries of ligands, in particular the structurally more complex class of bidentate ligands. A completely new approach to streamline the difficult ligand synthesis process is to use structurally less complex monodentate ligands that self-assemble in the coordination sphere of a metal center through noncovalent attractive ligand-ligand interactions to generate bidentate, chelating ligands. When complementary attractive ligand-ligand interactions are employed, it is even possible to generate libraries of defined chelate-ligand catalysts by simply mixing two different monomeric ligands. This Minireview summarizes the first approaches and results in this new field of combinatorial homogeneous catalysis.     

Self-assembly of bidentate ligands for combinatorial homogeneous catalysis: asymmetric rhodium-catalyzed hydrogenation

The first chiral ligand library based on self-assembly through complementary hydrogen-bonding was realized. From a 10 x 4 ligand library, catalysts that show excellent activity and enantioselectivity for the asymmetric rhodium-catalyzed hydrogenation have been identified.     

Halide effects in transition metal catalysis

Among the most common ligands found on transition metal catalysts are halide ions. Of the commercially available catalysts or pre-catalysts, most are halo-metal complexes. In recent years, manipulation of this metal-halide functionality has revealed that this can be used as a highly valuable method of tuning the reactivity of the complex. Variation of the halide ligand will usually not alter the nature of the system to the extent that it becomes unreactive but will impart sufficiently large changes that differences in reactivity or selectivity occur. These differences are a product of the steric and electronic properties of the halide ligand which has the ability to donate electron density to the metal occurs in a predictable manner. Despite the common perception in asymmetric catalysis that halide ligands are of secondary importance compared to chiral ligands, halide ligands have been found to exert dramatic effects on the enantioselectivity of asymmetric transformations. While the mechanism of action is known for relatively few of the cases, many intriguing and potentially synthetically useful trends are apparent. This review discusses the physical properties of the halides and their effects on stoichiometric and catalytic transition metal processes. The metal-halide moiety thus emerges as a tunable functionality on the transition metal catalyst that can be used in the development of new catalytic systems.     

Paramagnetic metal complexes of diamido donor ligands

Recently, considerable attention has been given to the use of multi-dentate amido ligands in the coordination chemistry of a range of transition metals as a means of accessing novel structural motifs and unusual reactivity. Presented herein is a perspective on transition and f-block metal complexes containing diamido donor ligands of the general form [NDN](2-) (D = NR, O, PR). Particular focus is given to paramagnetic metals, which have in general been studied much less than their diamagnetic counterparts despite their potential to exhibit unique structures and diverse reactivity patterns, in addition to their magnetic properties.     

二维共价有机框架材料中引入含氮配体用于络合金属催化

共价有机骨架材料(COF)也被称为“有机分子筛”,具有孔道结构开放有序、易于进行化学修饰改性、化学/热稳定性好等优点,是一种新型的有机聚合物多孔材料.近年来,以COF材料为催化剂载体负载金属化合物用于制备多相反应催化剂已经成为材料领域研究的热点,表现出高活性和高选择性.但是到目前为止,仍未找到简单有效地控制骨架中金属负载量和分散性的方法,这已成为该领域一个具有挑战性的课题.
　　本文以2,2’-联吡啶-5,5’-二甲醛作为其中一个结构基元,成功把联吡啶配体引入到二维材料中.除此之外,由于COF是以亚胺键联接构筑形成的,因此框架中同时存在联吡啶和亚胺键两种含氮配体.我们通过红外光谱、结构模拟、元素分析、热失重分析、透射电镜(TEM)、X-射线光电子能谱、电感耦合等离子体色谱等手段详细表征了所制备的二维共价有机框架材料对醋酸钯(Pd(OAc)2)分子的络合负载行为.
　　研究发现,联吡啶和亚胺键均可参与配位Pd(OAc)2,与亚胺键配位的Pd(OAc)2分布于框架的层与层之间,而与联吡啶配位的则部分占据了框架的孔道,导致孔径减小.另外,由于框架中的联吡啶配体含量可通过加入2,2’-联吡啶-5,5’-二甲醛含量的变化实现线性调控,因此也可调节与其配位的Pd(OAc)2含量,其负载量可控制在14.3–18.7 wt%,是目前已报道的二维COF中的最高值;另外, COF材料中调控金属负载量尚未见报道. TEM结果显示,负载在框架中的催化剂分子没有发生团聚,框架的孔道仍处于开放状态,因而反应底物可以自由地出入一维孔道并与络合的催化剂充分接触.另外,由于催化剂在框架内部可以达到分子级别的分散,而且其负载量和负载位置都易于控制,因而对有机反应表现出了优异的催化性能.
　　我们尝试了以不同Pd负载量的COF为多相催化剂催化Heck反应.结果发现, Pd(II)@75%BPy COF(Pd负载量为最高值18.7 wt%)的催化活性最高,对不同底物均表现出优异的催化性能,产率达73–96%,反应速率遵循一级动力学曲线.且催化剂经多次循环利用仍能保持高活性,框架的有序结构也未被破坏,因此该材料有望用于各种类型优异的多相反应催化剂.     

Ligational behaviour of a tetradentate NONS donor ligand towards some transition metal ions

Summary The synthesis and characterization of Cr^II, Mn^II, Fe^II, Co^II, Ni^II, Pd^II, Cu^II, Zn^II, Cd^II and UO ₂ ²⁺ complexes of 1-meotinoyl-4-phenyl-3-thiosemicarbazide (H₂NTS) are reported. I.r. spectral data show that the ligand behaves in a bidentate and/or tetradentate manner. An octahedral structure is proposed for the Cr^II, Fe^II and Ni^II complexes; a tetrahedral structure for the Mn^II, Co^II and Cu(NTS)·2H₂O complexes; and a square planar structure for the Pd^II and Cu(HNTS)Cl·H₂O complexes. The i.r. data suggest that the Fe^II complex contains a hydroxo bridge.     

Carbonyl substitution chemistry of some trimetallic transition metal cluster complexes with polyfunctional ligands

The trimetallic clusters [Ru₃(CO)₁₀(dppm)], [Ru₃(CO)₁₂] and [RuCo₂(CO)₁₁] react with a number of multifunctional secondary phosphine and tertiary arsine ligands to give products consequent on carbonyl substitution and, in the case of the secondary phosphines, PH activation. The reaction with the unresolved mixed P/S donor, 1-phenylphosphino-2-thio(ethane), HSCH₂CH₂PHPh ( LH₂), gave two products under various conditions which have been characterised by spectroscopic and crystallographic means. These two complexes [Ru₃(μ-dppm)(H)(CO)₇(LH)] and [Ru₃(μ-dppm)(H)(CO)₈(LH)Ru₃(μ-dppm)(CO)₉], show the versatility of the ligand, with it chelating in the former and bridging two Ru₃ units in the latter. The stereogenic centres in the molecules gave rise to complicated spectroscopic data which are consistent with the presence of diastereoisomers. In the case of [Ru₃(CO)₁₂] the reaction with LH₂ gave a poor yield of a tetranuclear butterfly cluster, [Ru₄(CO)₁₀(L)₂], in which two of the ligands bridge opposite hinge wingtip bonds of the cluster. A related ligand, HSCH₂CH₂AsMe(C₆H₄CH₂OMe), reacted with [RuCo₂(CO)₁₁] to give a low yield of the heterobimetallic Ru-Co adduct, [RuCo(CO)₆(SCH₂CH₂AsMe(C₆H₄CH₂OMe))], which appears to be the only one of its type so far structurally characterised.The secondary phosphine, HPMe(C₆H₄(CH₂OMe)) and its oxide HP(O)Me(C₆H₄(CH₂OMe)) also react with the cluster [Ru₃(CO)₁₀(dppm)] to give carbonyl substitution products, [Ru₃(CO)₅(dppm)(μ₂-PMe(C₆H₄CH₂OMe))₄], and [Ru₃H(CO)₇(dppm)(μ₂,η¹-P(O)Me(C₆H₄CH₂OMe))]. The former consists of an open Ru₃ triangle with four phosphide ligands bridging the metal-metal bonds; the latter has the O atom symmetrically bridging one Ru-Ru bond, the P atom being attached to a non-bridged Ru atom.     

New directions in supramolecular transition metal catalysis

Supramolecular chemistry has grown into a major scientific field over the last thirty years and has fueled numerous developments at the interfaces with biology and physics, clearly demonstrating its potential at a multidisciplinary level. Simultaneously, organometallic chemistry and transition metal catalysis have matured in an incredible manner, broadening the pallet of tools available for chemical conversions. The interface between supramolecular chemistry and transition metal catalysis has received surprisingly little attention. It provides, however, novel and elegant strategies that could lead to new tools in the search for effective catalysts, as well as the possibility of novel conversions induced by metal centres that are in unusual environments. This perspective describes new approaches to transition metal catalyst development that evolve from a combination of supramolecular strategies and rational ligand design, which may offer transition metal catalysts for future applications.     

Organic peroxide assisted transition metal hydrosilylation catalysis

Summary The catalytic activity of rhodium complexes for the hydrosilylation of substrates such as alkenes, 1,3-dienes, 1-alkynes, or ketones, is enhanced by the addition of organic oxidizing agents, such ast-butyl hydroperoxide, hydrogen peroxide, orm-chloroperbenzoic acid. Similar enhancement is found for the Group VIA hexacarbonyls in the hydrosilylation of 1,3-dienes.