Transition Metal Fragments Research Articles

Coordination and structural properties of encumbering 6-mesityl-2-picolinate complexes

In an effort to enforce a sterically hindered environment in transition-metal and main-group 2-picolinate complexes, the synthesis of the encumbering derivative 6-mesityl-2-picolinate ((Mes)pic) is presented. The coordination and structural properties of (Mes)pic are demonstrated with a range of transition-metal and main-group fragments. The 6-position mesityl group of (Mes)pic is shown to alter both the primary and secondary coordination spheres of metal centers relative to the ubiquitous and unencumbered parent 2-picolinate anion.

Substitution of Tetracyanoethene by Ethynyl–Metal Complexes Gives Tricyanovinylethynyl (Tricyanobutenynyl) Derivatives: Syntheses, Protonation, and Addition of Metal–Ligand Fragments

A series of complexes containing the new tricyanovinylethynyl (3,4,4-tricyanobut-3-en-1-ynyl) ligand have been obtained by substitution of a CN group in tetracyanoethene upon reaction with the ethynyl complexes M(C≡CH)(PP)Cp′ (M = Ru, Os, (PP)Cp′ = (PPh3)2Cp; M = Ru, PP = dppe, Cp′ = Cp, Cp*). The reactions proceed in higher yield as the metal environment becomes more sterically hindered, the normal [2 + 2]-cycloaddition/ring-opened product M{C[═C(CN)2]CH═C(CN)2}(PP)Cp′ also being formed in some cases. The diynyl complex Ru(C≡CC≡CH)(dppe)Cp* reacts with tcne to give only the ring-opened adduct Ru{C≡CC[═C(CN)2]CH═C(CN)2}(dppe)Cp*. Protonation (HBF4 or HPF6) of Ru{C≡CC(CN)═C(CN)2}(dppe)Cp* afforded the vinylidene cation [Ru{═C═CHC(CN)═C(CN)2}(dppe)Cp*]+. A second transition-metal fragment MLn (MLn = Ru(PPh3)2Cp, M′(dppe)Cp* (M′ = Ru, Os), RuCl(dppe)2) can be added to the CN group trans to the metal center; electrochemical, spectroscopic, and computational studies indicate that there is little ground-state d...

Coordination Properties of 2,5-Dimesitylpyridine: An Encumbering and Versatile Ligand for Transition-Metal Chemistry

To overcome the unfavorable steric pressures associated with 2,6-disubstitution in encumbering pyridine ligands, the coordination chemistry of a 2,5-disubstituted variant, namely, 2,5-dimesitylpyridine (2,5-Mes(2)py), is reported. This diaryl pyridine shows good binding ability to a range of transition-metal fragments with varying formal oxidation states and coligands. Treatment of 2.0 equiv of 2,5-Mes(2)py with monovalent Cu and Ag triflate sources generates complexes of the type [M(2,5-Mes(2)py)(2)]OTf (M = Cu, Ag; OTf = OSO(2)CF(3)), which feature long M-OTf distances and a substrate-accessible primary coordination sphere. Combination of 2,5-Mes(2)py with Cu(OTf)(2) and Pd(OAc)(2) produces four-coordinate complexes featuring cis- and trans-2,5-Mes(2)py orientations, respectively. The four-coordinate palladium complex Pd(OAc)(2)(2,5-Mes(2)py)(2) is found to resist py-ligand dissociation at room temperature in solution, but functions as a precatalyst for the aerobic C-H bond olefination of benzene at elevated temperatures. This C-H bond activation chemistry is compared with a similar Pd-based system featuring 2,6-disubstituted pyridines. 2,5-Mes(2)py also readily supports mono- and dinuclear divalent Co complexes, and the solution-phase equilibria between such species are detailed. The coordination studies presented highlight the potential of 2,5-Mes(2)py to function as an encumbering ancillary for the stabilization of low-coordinate complexes and as a supporting ligand for metal-mediated transformations.

Exchange coupling through diamagnetic [Fe(CO)4]2−bridging ligands in a xenophilic cluster

The electronic structure of so-called 'xenophilic' clusters, which contain both organometallic fragments and Werner-type paramagnetic transition metal centres, presents a challenge to simple theories of bonding. Density functional theory shows clearly that the cluster Mn(2)(thf)(4)(Fe(CO)(4))(2) is best described as an exchange-coupled Mn(II)(2) dimer, the closed-shell organometallic [Fe(CO)(4)](2-) fragments acting simply as bridging ligands. The high-spin configuration of the Mn(II) ions leads to single occupation of the Mn-Fe σ* orbitals and therefore substantially weaker metal-metal bonding than in conventional low-valent organometallic clusters. The transition metal fragments are effective mediators of superexchange (J(calc) = -44 cm(-1)), leading to the measured effective magnetic moment of ~5 μ(B) at 300 K, considerably lower than the limiting value of 8.37 μ(B) for two uncoupled S = 5/2 Mn(II) centres.

Borido complexes via intermetallic metalloborylene transfer

Intermetallic borylene transfer has been applied to synthesise new borido complexes. Starting from [{(η(5)-C(5)Me(5))Fe(CO)(2))}(μ-B){Cr(CO)(5)}] (1) the borylene moiety has been transferred successfully to different transition metal fragments. In the manner described, two new compounds have been fully characterised in solution and by X-ray crystallography.

Dynamic behaviour of Ru and Ru–Pt complexes containing tetrahedro-P4 ligand

Dynamic processes involving the P(4) cage coordinated to transition metal fragments were observed for the mononuclear complex trans-[Ru(dppm)(2)(H)(η(1)-P(4))]BF(4) and for the bimetallic derivative trans-[Ru(dppm)(2)(H)(μ ,η(1:2)-P(4)){Pt(PPh(3))(2)}]BF(4) as demonstrated by NMR experiments and DFT calculations.

Computational Insight into the Rh-Mediated Activation of White Phosphorus

Density functional calculations on the reaction of white phosphorus with the ligand bis(diphenylphosphino)methyl (dppm) at a rhodium center are presented. The cationic transition metal fragment can react as a nucleophilic as well as an electrophilic species, driven by a simple twisting of the four-membered rings. As a consequence of the conformational controlled philicity, the insertion reaction into white phosphorus occurs with a small energy barrier. The white phosphorus tetrahedron can be chelated by two cationic transition metal fragments into an opened bicyclobutane moiety, strongly stabilized by π-stacking interactions of the phenyl groups at the two transition metal fragments. It causes a 2:1 coordination; in the first stage of the reaction two molecules of the fragment add to one molecule of white phosphorus. The resulting dicationic complex easily undergoes dissociation into a cationic monoaddition product plus one cationic transition metal fragment. The ring expansion reaction of one ligand is explained by a j-step mechanism in one intermediary product. One ligand of the transition metal fragment dissociates and facilitates, by a cascade of low-energy processes, the rearrangement of the P(4)-moiety. Under bipyramid formation a PP-bond is broken, and the free ligand finally attaches to one phosphorus atom. Overall the reaction can be divided in low-energy processes, which pass through different unstable intermediates and more high-energy processes, requiring ligand dissociation.

Radical-Type Reactivity of Metalla[1.1.1]propellanes of Group 14: Syntheses, Structures, and Reduction Chemistry of Transition Metal-Terminated Bicyclo[1.1.1]pentastannanes

Two selected metalla[1.1.1]propellanes of group 14, Sn5Dep6 (1) and Ge5Mes6 (2) (Dep = 2,6-Et2C6H3, Mes = 2,4,6-Me3C6H2), were reacted with [FeCp(CO)2]2 and [RuCp(CO)2]2. While 2 did not show any reaction with [FeCp(CO)2]2 or did not lead to any isolable product in the case of [RuCp(CO)2]2, the tin propellane 1 quantitatively (monitored by 1H NMR) afforded the first transition metal-terminated bicyclo[1.1.1]pentastannanes, [{MCp(CO)2}2{μ-Sn5Dep6}] (M = Fe (3), Ru = (4)), in 68% (3) and 66% (4) isolated yield. This behavior confirms the radical-type reactivity of metalla[1.1.1]propellanes, in this case 1. The title compounds 3 and 4 have been characterized in detail using various methods. X-ray structure analyses corroborated that the two ligand-free bridgehead tin atoms within the Sn5 scaffold are bonded to the transition metal fragments. Electrochemical studies revealed the reduction chemistry of 3 and 4 in THF solutions to be very interesting. A detailed preparative and electrochemical study, including ...

Chemical Bonding in Transition Metal Complexes with Beryllium Ligands [(PMe3)2M−BeCl2], [(PMe3)2M−BeClMe], and [(PMe3)2M−BeMe2] (M = Ni, Pd, Pt)†

The equilibrium geometries and bond dissociation energies of the 14 valence electron (VE) complexes [(PMe(3))(2)M-BeCl(2)], [(PMe(3))(2)M-BeClMe], and [(PMe(3))(2)M-BeMe(2)] with M = Ni, Pd, and Pt have been calculated using density functional theory at the BP86/TZ2P level. The nature of the M-Be bond was analyzed with the NBO charge decomposition analysis and the EDA energy decomposition analysis. The theoretical results predict the equilibrium structures with a T-shaped geometry at the transition metal where the PMe(3) ligands are in the axial positions. The calculated bond dissociation energies show that the M-E bond strengths are in the range of donor-acceptor complexes of divalent beryllium compounds with ammonia. The bond strength decreases when the substituent at beryllium changes from Cl to CH(3). The NBO analysis shows a negative charge at the BeX(2) fragment, which indicates a net charge flow from the transition metal fragment to the beryllium fragment. The energy decomposition analysis of the M-Be bonds suggests two donor-acceptor bonds with sigma and pi symmetry where the transition metal fragment is a double donor with respect to the beryllium ligand. The pi component of the [Ni]-->BeXX' donation is much smaller than the sigma component.

(MesCu)2(η3‐Si4)]4−: A Mesitylcopper‐Stabilized Tetrasilicide Tetraanion

Between Zintl and Wiberg: The tetrahedral silicon cluster [(MesCu)2Si4]4− was obtained from the reaction of K6Rb6Si17 and MesCu (Mes=2,4,6-Me3C6H2) in liquid ammonia. Through stabilization with transition metal fragments, highly charged silicon clusters can thus be transferred and handled in solution.

Methylated cyclodextrins as preorganisation platforms for the synthesis of multidentate chelating ligands aimed at transition metal coordination and industrially relevant catalysis

Methylated cyclodextrins constitute versatile scaffolds for the synthesis of chelating di-, tri- and tetradentate podands, which can act both as first and second coordination spheres towards various transition metal fragments. This review focuses on the methods developed in the authors’ group for accessing multisubstituted methylated CDs and their subsequent transformation into oxygen, nitrogen, phosphorus and sulfur chelators, the coordination and supramolecular properties of which are then discussed within the context of industrially relevant homogeneous catalysis. Les cyclodextrines méthylées constituent des plateformes polyvalentes pour la synthèse de podands chélatants di-, tri- et tétradentates capables de jouer le rôle de première et deuxième sphère de coordination vis-à-vis de métaux de transition. Cette revue décrit les différentes méthodes de synthèse utilisées par notre groupe pour accéder aux cyclodextrines méthylées multisubstituées et leur transformation en chélateurs oxygénés, azotés, phosphorés et soufrés. Les propriétés supramoléculaires et de coordination de ces derniers sont ensuite discutées dans le contexte de la catalyse homogène.

ChemInform Abstract: The Activation of η5-Pyrrole Complexes Toward Nucleophilic Attack

Abstract Pyrrole is a five-membered aromatic heterocycle in which the nitrogen lone pair is delocalized over the π system of the ring. As a result of this π electron rich character, it reacts readily with electrophiles, but is not susceptible to nucleophilic attack. The η 5 -coordination of pyrrole or of the pyrrolyl anion to certain transition metal fragments activates the heterocycle toward nucleophilic substitution or addition reactions. In the pyrrolyl complexes (C 4 H 4 N)Re(PPh 3 ) 2 (H)(I) and (C 4 H 4 N)Ru(PEt 3 ) 2 Cl, nucleophilic substitution reactions at the 2-position of the ring follow a pathway that involves hydrogen transfer from the ring to the metal ion with displacement of a labile halide ligand from the metal center. Nucleophilic addition of hydride and methoxide anions to the neutral pentamethylpyrrole ligands in [(MeNC 4 Me 4 )M(cymene)](OTf) 2 , M=Ru and Os, has been found to occur at the α carbon atom of the heterocycle. However, reactions of other pyrrole sandwich complexes with nucleophiles may display competing reaction sites.

Functionalized Porous Silicas with Unsaturated Early Transition Metal Moieties as Hydrogen Storage Materials: Comparison of Metal and Oxidation State

Hexagonally packed mesoporous silica (HMS) grafted with titanium, vanadium, and chromium organometallic fragments possessing metal oxidation states between (II) and (IV) were synthesized and characterized by XRD, nitrogen adsorption, XPS, and elemental analysis. These materials were then tested for their excess hydrogen storage capacity, with the purpose in mind of subtracting out the physisorption component and establishing which transition metal fragments and oxidation states functioned as the most effective binding sites for hydrogen. By varying the metal type and the metal oxidation state, as well as the ligand environment by exchanging the alkyl group with hydride via hydrogenolysis, the effects of these variations on the H2 adsorption capacity of the metal center as well as on the binding enthalpies of these systems were investigated. This study demonstrated that titanium is more effective at hydrogen binding than vanadium and chromium. Hence, HMS silica grafted with benzyl titanium(III) fragments c...

ClosoversusHyperclosoMetallaboranes: A DFT Study

The structures and electronic relationship of 9-, 10-, 11-, and 12-vertex closo and hypercloso (isocloso) metallaboranes are explored using DFT calculations. The role of the transition metal in stabilizing the hypercloso borane structures is explained using the concept of orbital compatibility. The hypercloso structures, C(6)H(6)MB(n-1)H(n-1) (n = 9-12; M = Fe, Ru, and Os) are taken as model complexes. Calculations on metal free polyhedral borane B(n)H(n) suggest that n vertex hypercloso structures need only n skeleton electron pairs (SEPs), but the structure will have one or more six-degree vertices, whereas the corresponding closo structures with n + 1 SEPs have only four- and five-degree vertices. This high-degree vertex of hypercloso structures can be effectively occupied by transition metal fragments with their highly diffused orbitals. Calculations further show that a heavy transition metal with more diffused orbitals prefers over a light transition metal to form hypercloso geometry. This is in accordance with the fact that there are more experimentally characterized hypercloso structures with the heavy transition metals. The size of the exohedral ligands attached to the metal atom also plays a role in deciding the stability of the hypercloso structure. The interaction between the borane and the metal fragments in the hypercloso geometry is analyzed using the fragment molecular orbital approach. The interconversion of the closo and hypercloso structures by the addition and removal of the electrons is also discussed in terms of the correlation diagrams.

The Assembly of Macrocyclic Bis‐ and Tetra‐β‐lactams with Embedded Platinum or Palladium Square‐Planar Centers

The synthesis, isolation, and full characterization of different types of stable, metal-assembled macrocyclic beta-lactams are reported. By using adequately functionalized bis-beta-lactams with defined stereochemistry as building blocks, a series of mono- and bimetallic Pd and Pt macrocycles has been prepared in good to quantitative yields. These novel structures combine the beta-lactam moiety with transition-metal fragments with cis-square-planar geometry and constitute a new class of metal-assembled cavities involving molecules with biological relevance as building blocks. By combining the adequate ligands, metallic fragments, and tuning the reaction conditions, different mono- and bimetallic macrocyclic beta-lactam cavities can be selectively obtained. Macrocycles with Pt-ethynyl groups are suitable to form host-silver triflate guest complexes in a tweezer fashion.

Borylene Transfer from Transition Metal Borylene Complexes

Transition metal borylene complexes (LnM═BR) were predicted to be possible sources of the borylene fragment (:BR) in 1973, decades before the first complex had even been isolated. Over the past 15 years, routes to a small range of borylene complexes have been developed by a handful of research groups, including a family of active borylene donor complexes. Through these and other borylene donors, the borylene transfer to transition metal fragments has recently emerged as the most reliable and mild route to new transition metal borylene complexes, with the synthesis of a number of novel “second-generation” borylene complexes. Borylene transfer to alkynes was found to be a dependable route to the unusual “borirene” architecture, and more recently, the insertion of a borylene fragment into an unactivated alkenyl C−H bond has hinted at further borylene insertions into element−element bonds. This review covers borylene transfer reactions to both transition metal and main group substrates, from the first adventitious reports in the late 1990s up to the current state of intentional employment of the borylene transfer reaction in synthesis.

Calculations on the Kinetics, Thermodynamics, and Selectivity of Methyl Radical Addition to Olefins Coordinated to d8 and d0 Transition-Metal Fragments: Two Distinct and Opposite anti-Evans−Polanyi Effects with Potential Practical Implications

DFT calculations predict that methyl radical addition to the propene and isobutene adducts of the [Rh(PCP)] and [(Cp)2Zr(OtBu)]+ fragments affords different kinetic and thermodynamic products. The regioselectivity in the Zr adducts is predicted to be reversed compared to that in the uncoordinated or the Rh-coordinated systems.

μ- and μ 4–η 2 coordination of A 2H 2 (A = C, Si, Ge, Sn and Pb) ligands with transition metals

The heavier analogs of C 2H 2 have been studied at the B3LYP level for their μ and μ 4–η 2 coordination properties with the transition metals. Based on known alkyne compounds, transition metal fragments [W 2(μ-NH)(Cp) 2(Cl) 2] and [Fe 4(CO) 12] have been chosen. The SBKJC relativistic effective core potentials and their associated basis sets were used on W, Fe, Sn and Pb, and the 6-31G(d) basis set was used on all other elements. All the complexes of Si 2H 2, Ge 2H 2, Sn 2H 2 and Pb 2H 2 are found to be local minima. The trans-twist nature of the ligand A 2H 2 (A = Si–Pb) is large in μ-coordinated complexes of W, and it is very small in μ 4–η 2 coordinated complexes of Fe. The electronic structure of these complexes was investigated using fragment molecular orbital method (FMO).

Syntheses and crystal structures of two compounds based on saturated keggin-type polyoxotungstates and mixed ligands

Two new polyoxotungstates with dafo and 4,4′-bipy ligands, [Ag(dafo)2]2{[Ag(4,4′-bipy)]2SiW12O40} (1) and [Cd2(dafo)4(4, 4′-bipy)(H2O)2](dafo) SiW12O40 (2) (dafo = 4, 5-diazafluoren-9-one, 4,4′-bipy = 4,4′-bipyridine), have been hydrothermally synthesized and characterized by elemental analyses, IR, TG and single crystal X-ray diffraction. Compound 1 consists of a one-dimensional (1D) chain constructed from [Ag(4,4′-bipy)]+ cations. With the connection of [Ag(4,4′-bipy)]+ cations and α-[SiW12O40]4− anions, two-dimensional (2D) structure was built. Because of the π–π stacking interactions between pyridyl rings of 4,4′-bipy from adjacent chains, three-dimensional (3D) structure was generated. Furthermore, the C–H ··· O weak interaction between and polytungstate made the structure more stable. Like 1, compound 2 is a three-dimensional (3D) supramolecular compound constructed from a transition-metal complex fragment [Cd2(dafo)4(4,4′-bipy)(H2O)2]4+, and free dafo through C–H ··· O weak interactions. To our best knowledge, it is the first time that saturated Keggin-type polyoxotungstates interact with dafo and 4,4′-bipy at the same time.

Synthesis and Electrochemical Properties of a New 1D Organic–Inorganic Hybrid Compound Based on Keggin-Type Heteropolyanions and Isopolyanions Decorated by Transition Metal Fragments

A new chain-like organic–inorganic hybrid compound composed of the heteropolyanions and isopolyanions, [{CuII(2,2′-bpy)}6(MoVMoVI5O22)] [PMoVI12O40] · H2O 1 (bpy = 2,2′-bipyridine) has been hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy, TG analysis, XPS spectrum and single-crystal X-ray diffraction. Crystal data for compound 1: monoclinic, space group P2(1)/c, a = 13.9382(14) A, b = 20.0300(19) A, c = 17.1757(17) A, β = 94.054(2)°, V = 4783.2(8) A3, Z = 2. The Keggin-heteropolyanion [PMo12O40]3− and the Cu-supporting isopolyanion [{Cu(2,2′-bpy)}6(Mo6O22)]3+ building units are bridged through the O–Cu–O bridges into 1D infinite chain. Note that, the [{Cu(2,2′-bpy)}6(Mo6O22)]3+ unit, which contains a rarely reported isopolyanion [Mo6O22]9− decorated by six [Cu(2,2′-bpy)] fragments. Furthermore, the electrochemical behavior of a 1-modified carbon paste electrode (1-CPE) and its electrocatalytic reduction of nitrite were investigated.