Pentamethylcyclopentadienyl Ligand Research Articles

Divalent metallocenes of the lanthanides - a guideline to properties and reactivity.

Since the discovery in the early 1980s, the soluble divalent metallocenes of lanthanides have become a steadily growing field in organometallic chemistry. The predominant part of the investigation has been performed with samarium, europium, and ytterbium, whereas only a few reports dealing with other rare earth elements were disclosed. Reactions of these metallocenes can be divided into two major categories: (1) formation of Lewis acid-base complexes, in which the oxidation state remains +II; and (2) single electron transfer (SET) reductions with the ultimate formation of Ln(III) complexes. Due to the increasing reducing character from Eu(II) over Yb(II) to Sm(II), the plethora of literature concerning redox reactions revolves around the metallocenes of Sm and Yb. In addition, a few reactivity studies on Nd(II), Dy(II) and mainly Tm(II) metallocenes were published. These compounds are even stronger reducing agents but significantly more difficult to handle. In most cases, the metals are ligated by the versatile pentamethylcyclopentadienyl ligand: (C5Me5). Other cyclopentadienyl ligands are fully covered but only discussed in detail, if the ligand causes differences in synthesis or reactivity. Thus, the focus lays on three compounds: [(C5Me5)2Sm], [(C5Me5)2Eu] and [(C5Me5)2Yb] and their solvates. We discuss the synthesis and physical properties of divalent lanthanide metallocenes first, followed by an overview of the reactivity rendering the full potential of these versatile reactants.

Methanol-Mediated Formation of an Iridium(III) NHC/Azolato Chelate Complex: An Experimental and Theoretical Study

One equivalent of methanol-d4 reacts with the iridium(III)-carbonato-NHC complex [Ir(Cp*)(NHC-theo)(O2CO)] [1] containing an N-heterocyclic carbene ligand with an N-tethered C8-iodotheophylline group (NHC-theo) to form the iridium(III) chelate complex [Ir(Cp*)(theophyllinato^NHC)I] [2] that bears a theophyllinato^NHC chelate ligand, an iodo ligand, and the pentamethylcyclopentadienyl ligand (Cp*). The methanol-d4 solvent is oxidized to formaldehyde-d2 that was identified by a detailed LC–MS study using the 2,4-dinitrophenylhydrazine (DNPH) method. A plausible rearrangement involving the unusual oxidative addition of the C8–I bond of the theophylline group to an iridium(III) center to give a seven-coordinate iridium(V) intermediate is supported by density functional theory calculations.

The cycloaddition reaction of ethylene and methane mediated by Ir+ to generate a half-sandwich structure IrHCp+

The cycloaddition reactions of methane and ethylene mediated by Ir+ have been designed and studied by the techniques of mass spectrometry in conjunction with theoretical calculations. Studies have shown that Ir+ can mediate the cycloaddition reaction of CH4 and two C2H4 to generate a half-sandwich structure IrHCp+ (Cp = η5-C5H5) including pentamethylcyclopentadienyl ligand by continuous dehydrogenation reaction with the forming of three C-C bonds and seven C-H bonds. The orbital analysis indicates the mechanism of the cyclization reaction to generation of pentamethylcyclopentadienyl ligand with odd number carbon atom depends on the overlap of π orbitals in -C2H2 and carbene, which is more difficult than the forming of cyclobutadiene ligand and benzene. This study may help to understand the reaction mechanism in the cycloaddition reactions of organic compounds, which will be useful to guide the rational design of new catalysts with tailored selectivity and increased efficiency.

Trinuclear scandium methylidyne complexes stabilized by pentamethylcyclopentadienyl ligands.

The first examples of scandium methylidyne complexes [(Cp*)Sc(μ2-X)]3(μ3-CH) (Cp* = C5Me5; X = Br, Me, OMe), free of Lewis acids, can be achieved in high yields from [(Cp*)ScMe2]2 through a facile route. The chemical and geometrical flexibility to incorporate organic substrates indicates a rich chemistry of complex [(Cp*)Sc(μ2-OMe)]3(μ3-CH).

Cp*Co(III)-Catalyzed C-H Hydroarylation of Alkynes and Alkenes and Beyond: A Versatile Synthetic Tool.

The use of earth-abundant first-row transition metals, such as cobalt, in C–H activation reactions for the construction and functionalization of a wide variety of structures has become a central topic in synthetic chemistry over the last few years. In this context, the emergence of cobalt catalysts bearing pentamethylcyclopentadienyl ligands (Cp*) has had a major impact on the development of synthetic methodologies. Cp*Co(III) complexes have been proven to possess unique reactivity compared, for example, to their Rh(III) counterparts, obtaining improved chemo- or regioselectivities, as well as yielding new reactivities. This perspective is focused on recent advances on the alkylation and alkenylation reactions of (hetero)arenes with alkenes and alkynes under Cp*Co(III) catalysis.

Synthesis of a Half-Sandwich Hydroxidoiridium(III) Complex Bearing a Nonprotic N-Sulfonyldiamine Ligand and Its Transformations Triggered by the Brønsted Basicity

Synthesis and reactivities of a new mononuclear hydroxidoiridium(III) complex with a pentamethylcyclopentadienyl (Cp*) ligand are reported. The hydroxido ligand was introduced into an iridium complex having a nonprotic amine chelate derived from N-mesyl-N’,N’-dimethylethylenediamine by substitution of the chloride ligand using KOH. The resulting hydroxidoiridium complex was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The hydroxido complex was able to deprotonate benzamide and acetonitrile, and showed an ability to accept a hydride from 2-propanol to generate the corresponding hydrido complex quantitatively. In the reaction with mandelonitrile, a cyanide anion was transferred to the iridium center in preference to the hydride transfer. The cyanidoiridium complex was also identified in the reaction with acetone cyanohydrin, and could serve as catalyst species in the transfer hydrocyanation of benzaldehyde.

Crystal structure of the η4-ketimine titanium complex (di-phenyl-amido-κN){3-methyl-6-[(4-methyl-phen-yl)(phenyl-aza-nid-yl)methyl-idene]cyclo-hexa-2,4-dien-1-yl-κ2N,C1}(η5-penta-methyl-cyclo-penta-dien-yl)titanium(IV).

The mol-ecular structure of the title titanium(IV) half-sandwich complex, [Ti(η5-C10H15)(η4-C21H19N)(C12H10N)], shows a three-legged piano-stool geometry at the central TiIV atom, comprising of one penta-methyl-cyclo-penta-dienyl ligand, one bidentate ketimine ligand in an η4-coordination mode and one monodentate di-phenyl-amide ligand. Except for van der Waals forces, there are no significant inter-molecular inter-actions in the crystal.

Electronic and Steric Tuning of a Prototypical Piano Stool Complex: Rh(III) Catalysis for C-H Functionalization.

The history of transition metal catalysis is heavily steeped in ligand design, clearly demonstrating the importance of this approach. The intimate relationship between metal and ligand can profoundly affect the outcome of a reaction, often impacting selectivity, physical properties, and the lifetime of a catalyst. Importantly, this metal-ligand relationship can provide near limitless opportunities for reaction discovery. Over the past several years, transition-metal-catalyzed C-H bond functionalization reactions have been established as a critical foundation in organic chemistry that provides new bond forming strategies. Among the d-block elements, palladium is arguably one of the most popular metals to accomplish such transformations. One possible explanation for this achievement could be the broad set of phosphine and amine based ligands available in the chemist's toolbox compatible with palladium. In parallel, other metals have been investigated for C-H bond functionalization. Among them, pentamethylcyclopentadienyl (Cp*) Rh(III) complexes have emerged as a powerful mode of catalysis for such transformations providing a broad spectrum of reactivity. This approach possesses the advantage of often very low catalyst loading, and reactions are typically performed under mild conditions allowing broad functional group tolerance. Cp*Rh(III) is considered as a privileged catalyst and a plethora of reactions involving a C-H bond cleavage event have been developed. The search for alternative cyclopentadienyl based ligands has been eclipsed by the tremendous effort devoted to exploring the considerable scope of reactions catalyzed by Cp*Rh(III) complexes, despite the potential of this strategy for enabling reactivity. Thus, ligand modification efforts in Rh(III) catalysis have been an exception and research directed toward new rhodium catalysts has been sparse. Recently, chiral cyclopentadienyl ligands have appeared allowing enantioselective Rh(III)-catalyzed C-H functionalization reactions to be performed. Alongside chiral ligands, an equally important collection of achiral cyclopentadienyl-derived ligands have also emerged. The design of this new set of ligands for rhodium has already translated to significant success in solving inherent problems of reactivity and selectivity encountered throughout the development of new Rh(III)-catalyzed transformations. This Account describes the evolution of cyclopentadienyl ligand skeletons in Rh(III)-catalysis since the introduction of pentamethylcyclopentadienyl ligands to the present. Specific emphasis is placed on reactivity and synthetic applications achieved with the new ligands with the introduction of achiral mono-, di-, or pentasubstituted cyclopentadienyl ligands exhibiting a stunning effect on reactivity and selectivity. Furthermore, an underlying question when dealing with ligand modification strategies is to explain the reason one ligand outperforms another. Conjecture and speculation abound, but extensive characterization of their steric and electronic properties has been carried out and information about electronic and steric properties of the ligands all contribute to our understanding and give crucial pieces to solve the puzzle.

Structure and Reactivity of a Ru-based Peroxide Complex as a Reactive Intermediate of O2-Promoted Activation of a C–H Bond in a Cp* Ligand

We report the first example of the characterization of a Ru-based peroxide intermediate of O2-promoted activation of a C–H bond of the pentamethylcyclopentadienyl (Cp*) ligand. The peroxide complex activates the C–H bond to form a tetramethylfulvene complex. We propose a proton-coupled electron-transfer (PCET) mechanism of the C–H bond activation based on the structures and properties of the peroxide and tetramethylfulvene complexes.

Solid state structure of (pentamethylcyclopentadienyl)(2,4-dimethylpentadienyl)iron, Fe(C5Me5)(2,4-C7H11), and its incorporation into silica aerogels

The structure of Fe(C5Me5)(2,4-C7H11) has been determined (C7H11=dimethylpentadienyl), and found to be isomorphous with its ruthenium analogue. The Fe–C bond distances for the open pentadienyl ligand have been found to be about 0.02Å shorter than those for the aromatic pentamethylcyclopentadienyl ligand, whereas for the simple cyclopentadienyl analogue, the Fe–C bond distances for the two ligand types were nearly identical. While the structural data support proposals that the pentadienyl ligand is more strongly bound, the complex reacts readily with silica aerogel, during which it is the nonaromatic pentadienyl ligand which is removed via protonation. Solid state 13C NMR spectra are consistent with incorporation of the metal into one major environment, with lesser degrees of incorporation into alternative environments.

Expanding the Family of Uranium(III) Alkyls: Synthesis and Characterization of Mixed‐Ligand Derivatives

The generation of uranium(III) alkyls supported by hydrotris(pyrazolyl)borate (Tp) and pentamethylcyclopentadienyl (Cp*) ligands is reported. Mixed ancillary ligand frameworks were synthesized by treating TpUI2(THF)3 (1) and Cp*UI2(THF)3 with potassium hydrotris(pyrazolyl)borate salts. Addition of one equivalent of potassium hydrotris(3,5‐dimethylpyrazolyl)borate (Tp*) generated TpTp*UI (2), while treatment of Cp*UI2(THF)3 with either KTp or KTp* resulted in the respective formation of Cp*TpUI(THF) (3) or Cp*Tp*UI(THF) (4). Alkylation of 2 with KCH2Ph or NaCH2SiMe3 furnished TpTp*UCH2Ph (2‐CH2Ph) or TpTp*UCH2SiMe3 (2‐CH2SiMe3). Similarly, treatment of 3 with NaCH2SiMe3 formed Cp*TpUCH2SiMe3 (3‐CH2SiMe3), whereas treatment of 4 with KCH2Ph generated Cp*Tp*UCH2Ph (4‐CH2Ph). All compounds were characterized by multinuclear NMR, IR, and electronic absorption spectroscopy. Compounds 2‐CH2Ph, 3, and 3‐CH2SiMe3 were structurally characterized using X‐ray crystallography as well.

Crystal structure of bis-{μ2-[(2-imino-cyclo-pentyl-idene)methyl-idene]aza-nido-κ(2) N:N'}bis-[(η(5)-penta-methyl-cyclo-penta-dien-yl)zirconium(IV)] hexane monosolvate.

The title compound, [Zr2(C10H15)4(C6H6N2)2]·C6H14, was obtained by the stoichiometric reaction of adipo­nitrile with [Zr(C10H15)2(η2-Me3SiC2SiMe3)]. Intra­molecular nitrile–nitrile couplings and deprotonation of the substrate produced the (1-imino-2-enimino)­cyclo­pentane ligand, which functions as a five-membered bridge between the two metal atoms. The ZrIV atom exhibits a distorted tetra­hedral coordination sphere defined by two penta­methyl­cyclo­penta­dienyl ligands, by the imino unit of one (1-imino-2-enimino)­cyclo­pentane and by the enimino unit of the second (1-imino-2-enimino)­cyclo­pentane. The cyclo­pentane ring of the ligand shows an envelope conformation. The asymmetric unit contains one half of the complex and one half of the hexane solvent mol­ecule, both being completed by the application of inversion symmetry. One of the penta­methyl­cyclo­penta­dienyl ligands is disordered over two sets of sites with a refined occupancy ratio of 0.8111 (3):0.189 (3). In the crystal, the complex mol­ecules are packed into rods extending along [100], with the solvent mol­ecules located in between. The rods are arranged in a distorted hexa­gonal packing.

Facile C–H, C–F, C–Cl, and C–C Activation by Oxatitanacyclobutene Complexes

Aryl ketones react readily with oxatitanacyclobutenes bearing pentamethylcyclopentadienyl ligands to form unique titanocene complexes resulting from Cp* modification and C–H activation. An intermediate in this reaction is intercepted with various functional groups to form carbonyl insertion, C–F activation, and cyclopropane ring-opening products.

Crystal structure of μ-oxido-1,1'κ(2) O:O-bis{tetra-μ-oxido-1:2κ(2) O:O;1:3κ(2) O:O;2:3κ(4) O:O-tris[1,2,3(η(5))-penta-methyl-cyclo-penta-dien-yl]-trianglo-trititanium(IV)}.

The title polynuclear organometallic titanium(IV) oxide, [{Ti3(η5-C5Me5)3(μ-O)4}2(μ-O)], exhibits two Ti3O4 cores bridged by an O atom located on a twofold axis. All metal centres present the typical three-legged piano-stool coordination environment, where one site is occupied by a penta­methyl­cyclo­penta­dienyl ligand linked in an η5-coordination fashion, while three bridging O atoms fill the other three sites.

Can a pentamethylcyclopentadienyl ligand act as a proton-relay in f-element chemistry? Insights from a joint experimental/theoretical study.

Isomerisation of buta-1,2-diene to but-2-yne by (Me(5)C(5))(2)Yb is a thermodynamically favourable reaction, with the Δ(r)G° estimated from experimental data at 298 K to be -3.0 kcal mol(-1). It proceeds in hydrocarbon solvents with a pseudo first-order rate constant of 6.4 × 10(-6) s(-1) and 7.4 × 10(-5) s(-1) in C(6)D(12) and C(6)D(6), respectively, at 20 °C. This 1,3-hydrogen shift is formally forbidden by symmetry and has to occur by an alternative pathway. The proposed mechanism for buta-1,2-diene to but-2-yne isomerisation by (Me(5)C(5))(2)Yb involves coordination of methylallene (buta-1,2-diene) to (Me(5)C(5))(2)Yb, and deprotonation of methylallene by one of the Me(5)C(5) ligands followed by protonation of the terminal methylallenyl carbon to yield the known coordination compound (Me(5)C(5))(2)Yb(η(2)-MeC[triple bond, length as m-dash]CMe). Computationally, this mechanism is not initiated by a single electron transfer step, and the ytterbium retains its oxidation state (II) throughout the reactivity. Experimentally, the influence of the metal centre is discussed by comparison with the reaction of (Me(5)C(5))(2)Ca towards buta-1,2-diene, and (Me(5)C(5))(2)Yb with ethylene. The mechanism by which the Me(5)C(5) acts as a proton-relay within the coordination sphere of a metal also rationalises the reactivity of (i) (Me(5)C(5))(2)Eu(OEt(2)) with phenylacetylene, (ii) (Me(5)C(5))(2)Yb(OEt(2)) with phenylphosphine and (iii) (Me(5)C(5))(2)U(NPh)(2) with H(2) to yield (Me(5)C(5))(2)U(HNPh)(2). In the latter case, the computed mechanism is the heterolytic activation of H(2) by (Me(5)C(5))(2)U(NPh)(2) to yield (Me(5)C(5))(2)U(H)(HNPh)(NPh), followed by a hydrogen transfer from uranium back to the imido nitrogen atom using one Me(5)C(5) ligand as a proton-relay. The overall mechanism by which hydrogen shifts using a pentamethylcyclopentadienyl ligand as a proton-relay is named Carambole in reference to carom billiards.

Crystal structure of μ-1κC:2(η(2))-carbonyl-carbonyl-1κC-chlorido-2κCl-μ-chlorido-borylene-1:2κ(2) B:B-[1(η(5))-penta-methyl-cyclo-penta-dien-yl](tri-cyclo-hexyl-phosphane-2κP)iron(II)platinum(II) benzene monosolvate.

In the mol-ecular structure of the dinuclear title compound [η(5)-(C5(CH3)5)(CO)Fe{(μ-BCl)(μ-CO)}PtCl(P(C6H11)3)]·C6H6, the two metal atoms, iron(II) and platinum(II), are bridged by one carbonyl (μ-CO) and one chlorido-borylene ligand (μ-BCl). The Pt(II) atom is additionally bound to a chloride ligand situated trans to the bridging borylene, and a tri-cyclo-hexyl-phosphane ligand (PCy3) trans to the carbonyl ligand, forming a distorted square-planar structural motif at the Pt(II) atom. The Fe(II) atom is bound to a penta-methyl-cyclo-penta-dienyl ligand [η(5)-C5(CH3)5] and one carbonyl ligand (CO), forming a piano-stool structure. Additionally, one benzene solvent mol-ecule is incorporated into the crystal structure, positioned staggered relative to the penta-methyl-cyclo-penta-dienyl ligand at the Fe(II) atom, with a centroid-centroid separation of 3.630 (2) Å.

(Cyanido-κC)(2,2-di-phenyl-acetamido-κ(2) N,O)bis-(η(5)-penta-methyl-cyclo-penta-dien-yl)zirconium(IV).

In the title compound, [Zr(C10H15)2(C14H12NO)(CN)], the ZrIV atom is coordinated by two penta­methyl­cyclo­penta­dienyl ligands, the amidate ligand via the N and O atoms, and an additional C N ligand. The four-membered metallacycle is nearly planar (r.m.s. deviation = 0.008 Å). In the crystal, the mol­ecules are connected into centrosymmetric dimers via pairs of N—H⋯N hydrogen bonds.

Heterodinuclear complexes of 4,5-diazafluorene derivatives displaying η(5),κ(2)-[N,N] and η(5),κ(1)-N coordination modes.

The syntheses and structures for a series of heterodinuclear complexes of 4,5-diazafluorenyl (L(-)) and 3,6-dimesityl-4,5-diazafluorenyl (LMes(-)) ligands are reported herein. In all these heterodinuclear complexes, the Ru(II) centre is sandwiched between a pentamethylcyclopentadienyl (Cp*) ligand and the C5 ring of L(-) or LMes(-) in a double η(5) fashion, while the other metal (Fe(II), Co(II), Pt(II), or Cu(I)) is bound to the N-donors.

Chlorido(dimethyl 2,2′-bipyridine-4,4′-dicarboxylate-κ2N,N′)(η5-pentamethylcyclopentadienyl)rhodium(III) chloride 1-hydroxypyrrolidine-2,5-dione disolvate

The title complex, [Rh(C10H15)Cl(C14H12N2O4)]Cl·2C4H5NO3, has been synthesized by a substitution reaction of the precursor [bis(2,5-dioxopyrrolidin-1-yl) 2,2'-bipyridine-4,4'-dicarboxylate]chlorido(pentamethylcyclopentadienyl)rhodium(III) chloride with NaOCH3. The Rh(III) cation is located in an RhC5N2Cl eight-coordinated environment. In the crystal, 1-hydroxypyrrolidine-2,5-dione (NHS) solvent molecules form strong hydrogen bonds with the Cl(-) counter-anions in the lattice and weak hydrogen bonds with the pentamethylcyclopentadienyl (Cp*) ligands. Hydrogen bonding between the Cp* ligands, the NHS solvent molecules and the Cl(-) counter-anions form links in a V-shaped chain of Rh(III) complex cations along the c axis. Weak hydrogen bonds between the dimethyl 2,2'-bipyridine-4,4'-dicarboxylate ligands and the Cl(-) counter-anions connect the components into a supramolecular three-dimensional network. The synthetic route to the dimethyl 2,2'-bipyridine-4,4'-dicarboxylate-containing rhodium complex from the [bis(2,5-dioxopyrrolidin-1-yl) 2,2'-bipyridine-4,4'-dicarboxylate]rhodium(III) precursor may be applied to link Rh catalysts to the surface of electrodes.

Synthesis, Structure, and Reactivity of Group VI Metal Complexes Bearing Group IV Metallocenyldiphosphine Moieties and a Pentamethylcyclopentadienyl Ligand

A series of group VI metal complexes bearing a group IV metallocenyldiphosphine moiety and a pentamethylcyclopentadienyl ligand have been prepared and structurally characterized. Reactions of trinu...