Combinatorial transition-metal catalysis: mixing monodentate ligands to control enantio-, diastereo-, and regioselectivity

This review focuses on a new approach to combinatorial homogeneous transition-metal catalysis which goes beyond the traditional parallel preparation of modular ligands. It is based on the use of mixtures of monodentate ligands L(a) and L(b), which upon exposure to a transition metal (M) form not only the two homocombinations [ML(a)L(a)] and [ML(b)L(b)], but also the heterocombination [ML(a)L(b)]. If the latter is more reactive and selective than the homocombinations, an improved catalyst system is formed without the need to synthesize new ligands. Thus, the control of enantio,- diastereo-, and regioselectivity is possible.     

Metal‐Only Lewis Pairs by Reversible Insertion of Ruthenium and Osmium Fragments into Metal–Boron Double Bonds

New metal‐only Lewis pairs (MOLPs: Ru→Cr and Os→Cr) are prepared by the insertion of a zerovalent ruthenium or osmium complex into chromium–boron double bonds of borylene complexes. The reaction creates new borylene complexes (the first ever for osmium), and is crystallization‐controlled; re‐dissolving the complexes results in regeneration of the starting materials. A mechanism is proposed based on DFT calculations, along with a computational study of the unusual MOLPs.     

Fragmentation reactions of singly and doubly protonated thiourea- and sugar-substituted cyclams and their transition-metal complexes

Cyclam macrocycles tetrasubstituted with amino-, thiourea-, and sugar-terminated side chains are ionized by electrospray ionization mass spectrometry (ESI-MS) as singly or doubly protonated species or as transition-metal complexes. Their fragmentation behavior is examined in a Fourier-transform ion-cyclotron-resonance (FT-ICR) mass spectrometer by collision-induced dissociation (CID) experiments. Typically, fragmentation occurs within the side chains through a number of different 1,2-elimination reactions irrespective of the absence or presence of a transition metal ion such as Co(2+), Ni(2+), or Zn(2+). A remarkable exception is Cu(2+), which induces ring cleavage reactions. This is traced back to an electron transfer from the cyclam nitrogen atoms to the Cu(2+) ion. The electron transfer creates a cation-radical within the macrocycle, which induces typical fragmentation reactions such as alpha-cleavages that lead to fragmentation within the macrocycle. This interpretation is in line with fragmentation experiments on unsubstituted cyclam and its complexes.     

High-valent first-row transition-metal complexes of tetraamido (4N) and diamidodialkoxido or diamidophenolato (2N/2O) ligands: Synthesis, structure, and magnetochemistry

Introduced approximately two decades ago, macrocyclic deprotonated tetraamido (4N) and, nearly a decade earlier, acyclic diamidodialkoxido or diamidophenolato (2N/2O) ligand systems have been used, among other things, for the synthesis of a wide variety of high-valent complexes of iron, manganese, cobalt, vanadium, nickel, chromium, and copper. Structural, magnetic, and catalytic properties of these mononuclear, dinuclear, and polynuclear complexes created by the Collins group are reviewed. The present account continues an overview of complexes of this type published recently and devoted to iron species exclusively [Chanda et al., J. Inorg. Biochem., 100 (2006) 606], which provide the first highly effective small molecule mimics of peroxidase enzymes, called TAML activators. The story of the reviewed first-row complexes does not include the diverse and instructive chemistry discovered for osmium, but like the osmium chemistry, it derives its greatest significance from the fact that key members of the various species mark the steps along the design pathway that led to iron-TAML activators. Consideration is given to recent questioning in the literature of the innocence of a TAML system that was designed to be innocent. The reasons underlying the now 15-year old refocusing of our research program on oxidation catalysis and green chemistry with the associated termination of research into designed molecule-based magnetic materials are explained. Our closing contributions from the mid-1990s to the design of molecule-based magnetic materials are reviewed. Previously reported data are discussed in conjunction with newly obtained information on the complexes using density functional theory.     

Strained metallocenophanes and related organometallic rings containing pi-hydrocarbon ligands and transition-metal centers

The structures, bonding, and ring-opening reactions of strained cyclic carbon-based molecules form a key component of standard textbooks. In contrast, the study of strained organometallic molecules containing transition metals is a much more recent development. A wealth of recent research has revealed fascinating nuances in terms of structure, bonding, and reactivity. Building on initial work on strained ferrocenophanes, a broad range of strained organometallic rings composed of a variety of different metals, pi-hydrocarbon ligands, and bridging elements has now been developed. Such strained species can potentially undergo ring-opening reactions to functionalize surfaces and ring-opening polymerization to form easily processed metallopolymers with properties determined by the presence of the metal and spacer. This Review summarizes the current state of knowledge on the preparation, structural characterization, electronic structure, and reactivity of strained organometallic rings with pi-hydrocarbon ligands and d-block metals.     

Synthesis and use of dioxime ligands for treatment of leukemia and colon cancer cells

Two new vicinal dioxime ligands containing thiosemicarbazone units (L¹H₂ and L²H₂) were synthesized and characterized using ¹H NMR, ¹³C NMR, heteronuclear multiple quantum correlation, mass, infrared and UV–visible spectroscopies, elemental analysis and magnetic susceptibility measurements. In addition, homotrinuclear nickel(II), copper(II) and cobalt(II) complexes with a metal‐to‐ligand ratio of 3:2 for L¹H₂ and L²H₂ were prepared. Synthesis of nickel(II) complex containing a BF₂⁺ bridge was carried out using a precursor hydrogen‐bridged nickel(II) complex via the template effect. All metal–ligand complexes were tested against two human cancer cell lines (HL‐60 and HT‐29) for their antiproliferative and apoptotic activities. The results showed that [Co(L²H)₂(H₂O)₂] and [Ni(L²H)₂] exhibited the strongest antiproliferative activity with I_pC₅₀ values ranging between 5 and 10 μM, while [Co(L²H)₂(H₂O)₂] and [Ni(L²H)₂] both induced necrosis of HT‐29 cells and 60 and 65% apoptosis in HL‐60 cells, respectively.     

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.     

Nickel and cobalt complexes bearing β‐ketoamine ligands: syntheses,structures and catalytic behavior for norbornene polymerization

Late transition metal (nickel, cobalt) complexes (1, 2) with β‐ketoamine ligand (L) based on the pyrazolone derivative are synthesized by condensing 1‐phenyl‐3‐methyl‐4‐benzoyl‐5‐pyrazolone with p‐fluoroaniline, and then treating the β‐ketoamine (L) produced with the respective metal halide. The bis(β‐ketoamine)metal complexes can act as catalyst precursors for norbornene polymerization with activation by methylaluminoxane. The effects of the central metal variation in the complex on catalyst activities and polymer microstructure are described. Copyright © 2005 John Wiley & Sons, Ltd.     

Solid-phase synthesis of transition-metal complexes

This overview highlights recent progress in the field of selective construction of linear, oligonuclear transition-metal complexes by using solid-phase synthesis procedures. Two general protocols have been identified: formation of coordinative bonds between metal centres and bridging ligands and formation of covalent bonds between preformed kinetically inert transition-metal-containing building blocks in the chain growth step. Currently available suitable building blocks for the second approach are based on ferrocene units, bis(terpyridine)-ruthenium(II) moieties or metal porphyrins.     

Quantification of the trans influence in d8 square planar and d6 octahedral complexes: a database study

A systematic search of the Cambridge structural database was undertaken to quantify the trans influence in square planar and octahedral transition metal compounds. For square planar geometry, d⁸ metal centers were studied, while octahedral searches focused on low-spin d⁶ complexes. Two probe ligands (PL) were used to measure the effect of the trans ligand (TL), chloride, and triphenylphosphine (PPh₃). For the TLs O=CX₂, NR₃, pyridine, and Cl^? (X?=?any non-metal, R?=?H or hydrocarbon), the effects on the metal–probe ligand (M–PL) distance were statistically equal and were taken as essentially no trans influence. The other ligands studied showed significant decrease in the mean M–PL bond order, relative to the above ligands: SR₂?=?0.941; S=CX₂?=?0.887; PPh₃?=?0.825; phenyl?=?0.743; CR₃?=?0.719; hydride?=?0.685. Some variation in the trans influence is observed, based on the geometry of the metal center and the PL. In general, electron-donating, σ-bonding ligands lead to a larger trans influence, but π-bonding effects can also be important, particularly when the probe ligand also has π-bonding properties.     

Dendrizymes: Expanded ligands for enantioselective catalysis

Dendrizymes are a new class of expanded ligands, designed for enantioselective catalysis with transition metal complexes. These expanded ligands consist of a strongly binding chelate core, which is surrounded by space-filling dendrimer substituents, built-up by branching units and optically active groups. In a complex of such a dendrimer ligand a reaction is supposed to take place in the same way as in the pocket of an enzyme.     

Organometallic cluster complexes containing ynamine ligands

A comprehensive review of the chemistry of metal carbonyl cluster complexes containing ynamine ligands including syntheses, structures, bonding, and reactivity is presented.     

Diamination of olefins: synthesis, structures and reactivity of osmaimidazolidines

The diamination of certain olefins bearing electron-withdrawing substituents proceeds with well-defined bisimido and trisimido complexes of osmium. The products are obtained as osmaimidazolidines which are of unprecedented stability with regards to olefin functionalisation. Osmium complexes from related dihydroxylation or aminohydroxylation are significantly less stable and thereby promote catalytic reactions. This difference in reaction profile has been investigated and chiral osmium heterocycles obtained from olefin difunctionalisation were characterised by X-ray analysis for the first time. Kinetic studies on the reaction profile have also been carried out. An asymmetric version of this reaction is based on chiral non-racemic auxiliaries and leads to diastereomerically enriched osmaimidazolidines with up to 90 % de. This sequence represents the first asymmetric diamination of olefins. Attempts on the use of chiral ligands for direct asymmetric diamination as well as the consequences of osmaimidazolidine properties for a catalytic reaction are discussed.     

Shaping and enforcing coordination spheres: the implications of C3 and C1 chirality in the coordination chemistry of 1,1,1-tris(oxazolinyl)ethane ("trisox")

A key feature of tris(oxazolinyl)ethane ("trisox") ligands, which have shown broad scope in asymmetric catalysis, is the orientation and steric demand of their oxazoline substituents. This, along with the modularity of their synthesis determines their coordination chemistry. The possibility to combine oxazolines, in which the stereogenic centers adjacent to the N-donor atoms have different absolute configuration, whilst retaining their ability to coordinate as tripodal ligands, has been demonstrated by the synthesis of the enantiomerically pure C3-symmetric iPr-trisox(S,S,S) and C1-symmetric iPr-trisox(S,S,R) and their reaction with [Mo(CO)3(NCMe)3] yielding [Mo{iPr-trisox(S,S,S)}(CO)3] (1 a) and [Mo{iPr-trisox(S,S,R)}(CO)3] (1 b), respectively. The non-autocomplementarity of two homochiral trisox ligands at one metal center has been demonstrated by reaction of rac-C3 iPr-trisox with one equivalent of [Co(ClO4)2].6 H2O, giving the centrosymmetric heterochiral complex [Co(iPr-trisox)2](ClO4)2 (3), whereas an analogous reaction with the enantiopure ligand yielded a mixture of Co(II) complexes, which is characterized by the total absence of a [(trisox)2Co](+/2+) ion. The scope of the trisox ligand in terms of facial coordination to both early and late transition metals was demonstrated by the synthesis and structural characterization of the mononuclear complexes [ScCl3(iPr-trisox)] (4), [Fe(tBu-trisox)(NCMe)3](BF4)2 (5), and [Ru(eta6-p-cymene)(iPr-trisox)](PF6)2 (6). The facial coordination of their three ligating atoms to a metal center may be impeded if the transition-metal center stereoelectronically strongly favors a non-deltahedral coordination sphere, which is generally the case for the heavier d8-transition-metal atoms/ions. Reaction of iPr-trisox with [Rh(cod)2]BF4 led to the formation of the 16-electron d8-configured complex [Rh(iPr-trisox)(cod)](BF4) (7), which is oxidized by CsBr3 to give the Rh(III) complex [RhBr3(iPr-trisox)] (8) possessing a C3-symmetric structure with a kappa3-N-trisox ligand. The crystalline salts [M2(mu-Cl3)(iPr-trisox)2](PF6) (M=Fe(II): 9, Co(II): 10, Ni(II): 11), were prepared by addition of one molar equivalent of iPr-trisox and an excess of KPF6 to solutions of the anhydrous (FeCl2) or hydrated metal halides (CoCl2.6 H2O, NiCl2.6 H2O). All dinuclear complexes display weak magnetic coupling. For the mononuclear species [CuCl2(iPr-trisox)] (12) the removal of a chloride anion and thus the generation of a dinuclear chloro-bridged structure failed due to Jahn-Teller destabilization of a potential octahedral coordination sphere.