Update for Reactions of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes: A Never-Ending Story?

On Haptotropic Rearrangements of Diphosphene and Diarsene Ligands in Titanium Complexes

Dipnictenes of the type RE=ER (E=P, As, Sb, Bi) are the isovalence electronic heavier analogs of alkenes. Although diphosphenes and dipnictenes in general show a variety of binding modes in metal complexes, little is known about haptotropic shift reactions involving these ligands. Herein, we report an unprecedented η2 to η1 rearrangement of the dipnictene ligands in titanocene complexes of the type Cp2Ti(Pn2Ar2) (Pn=P, As; Ar=2,4,6-Me3-C6H2, Mes; 2,6-iPr2-C6H3, Dip; 2,4,6-iPr3-C6H2, Tip), initiated by Lewis basic ligands (L=MeCN, PMe3, AdNC, CO). In the presence of L the dipnictene ligand changes its hapticity from η2 to η1 and complexes of the general form Cp2Ti(L)(Pn2Ar2) with a succinctly different electronic structure are obtained. Electronically, the new complexes are best described as biradicaloids with antiferromagnetically coupled (via a π-bond) [Cp2TiIII]⋅+ and [Pn2Ar2]⋅- fragments. However, the biradical character of these systems is affected by the electronic features of the co-ligand and significantly decreases moving from PMe3/MeCN (σ-donors) to CNAd/CO (σ-donors/π-acceptors).

Cp2Ti(II) Mediated Rearrangement of Cyclopropyl Imines

Ti-catalyzed oxidative alkyne carboamination with alkenes and azo compounds can yield either α,β-unsaturated imines or cyclopropyl imines through a common azatitanacyclohexene intermediate. Herein, we report the synthesis of a model azatitanacyclohexene complex (3) through the ring-opening of a cyclopropyl imine with Cp2Ti(BTMSA) (BTMSA = bis(trimethylsilyl)acetylene). 3 readily undergoes thermal or reductant-catalyzed ring contraction to an azatitanacyclopentene (4), analogous to the proposed mechanism for forming α,β-unsaturated imines in the catalytic reaction. A cyclopropyl imine or an α,β-unsaturated imine could be liberated via the oxidation of 3 or 4 with azobenzene, respectively, further implicating the role of these metallacycles in the Ti-catalyzed carboamination reaction.

Intrinsic reactivity of [η5-1,3-(Me3Si)2C5H3]2U(η4-C4Ph2) in small molecule activation

The uranium metallacyclocumulene, [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(η⁴-C₄Ph₂) (3) was isolated from the reaction mixture containing [η⁵-1,3-(Me₃Si)₂C₅H₃]₂UCl₂ (1), potassium graphite (KC₈) and 1,4-diphenylbutadiyne (PhCC-CCPh) in good yield. The reactivity of 3 towards various small organic molecules was evaluated. For example, while complex 3 shows no reactivity towards alkynes and 2,2'-bipyridine, it may deliver the [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(II) fragment in the presence of Ph₂E₂ (E = S, Se) and Ph₃CN₃, or react as a nucleophile in the presence of carbodiimides, isothiocyanates, aldehydes, ketones, and pyridine derivatives, forming five-, seven- or nine-membered heterometallacycles. On the contrary, addition of Ph₂CS to 3 induces CS bond cleavage yielding the dithiolate complex [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[S₂(C₁₂H₅Ph₅)] (14). In contrast, the closely related, but sterically more encumbered uranium metallacyclocumulene [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(η⁴-C₄Ph₂) (4) features a more limited reactivity which is restricted to mono- and double insertions with small unsaturated organic molecules such as isothiocyanates, ketones and nitriles.

Bimetallic complexes that merge metallocene and pincer-metal building blocks: synthesis, stereochemistry and catalytic reactivity

This perspective is to illustrate the synthesis and applications of bimetallic complexes by merging a metallocene and a (cyclopentadienyl/aryl) pincer metal complex. Four possible ways to merge metallocene and pincer-metal motifs are reported and representative examples are discussed in more detail. These bimetallic complexes have been employed in some important catalytic reactions such as cross-coupling, transfer hydrogenation or synthesis of ammonia. The metallocene fragment may tune the electronic properties of the pincer ligand, due to its redox reversible properties. Also, the presence of two metals in a single complex allows their electronic communication, which proved beneficial for, e.g., the catalytic activity of some species. The presence of the metallocene fragment provides an excellent opportunity to develop chiral catalysts, because the metallocene merger generally renders the two faces of the pincer-metal catalytic site diastereotopic. Besides, an extra chiral functionality may be added to the bimetallic species by using pincer motifs that are planar chiral, e.g. by using the different substituents of pincer ligand "arms" or non-symmetrical arene groupings. Post-functionalization of pre-formed pincer-metal complexes, via η⁶-coordination with an areneophile such as [CpRu]⁺ and [Cp*Ru]⁺ presents a striking strategy to obtain diastereomeric metallocene-pincer type derivatives, that actually involve half-sandwich metallocenes. This approach offers the possibility to create diastereomerically pure derivatives by using the chiral TRISPHAT anion. The authors hope that this report of the synthetic, physico-chemical properties and remarkable catalytic activities of metallocene-based pincer-metal complexes will inspire other researchers to continue exploring this realm.

Take a "Snapshot" of New Syntheses, Reactions, and Characterizations from Unusual Unsaturated Ring Strained Group 4 Metallacycles

Recently published syntheses, reactions and characterizations of unusual unsaturated ring strained Group 4 metallocene metallacycles like metalla‐cyclocumulenes, ‐cycloallenes and ‐cycloalkynes with different ring size are updated for the last three years. There exist for some of these metallacycles, depending on the ring size, 7‐, 5‐ and 4‐membered compounds. The new results for these metallacycles are summarized here and considered in addition to the former published results. Additionally, several compounds of this type were now characterized by new reactions. For a better understanding of these compounds, some spectroscopical methods as well as theoretical calculations were published. Despite of these all‐C‐metallacycles, only in some cases the syntheses and reactions for the corresponding hetero‐metallacycles were published too. Examples for these metallaheterocyclic compounds will not be considered in this article. All these unusual ring strained compounds have a great potential for a lot of interesting synthetic applications in the future. Additionally, they are very interesting from the theoretical point of view.

1-Zirconacyclobuta-2,3-dienes: synthesis of organometallic analogs of elusive 1,2-cyclobutadiene, unprecedented intramolecular C-H activation, and reactivity studies

The structure, bonding, and reactivity of small, highly unsaturated ring systems is of fundamental interest for inorganic and organic chemistry. Four-membered metallacyclobuta-2,3-dienes, also referred to as metallacycloallenes, are among the most exotic examples for ring systems as these represent organometallic analogs of 1,2-cyclobutadiene, the smallest cyclic allene. Herein, the synthesis of the first examples of 1-zirconacyclobuta-2,3-dienes of the type [Cp'₂Zr(Me₃SiC₃SiMe₃)] (Cp'₂ = rac-(ebthi), (ebthi = 1,2-ethylene-1,1'-bis(η⁵-tetrahydroindenyl)) (2a); rac-Me₂Si(thi)₂, thi = (η⁵-tetrahydroindenyl), (2b)) is presented. Both complexes undergo selective thermal C-H activation at the 7-position of the ansa-cyclopentadienyl ligand to produce a new type of "tucked-in" zirconocene system, 3a and 3b, that possesses a η³-propargyl/allenyl ligand. Both types of complexes react with carbonyl compounds, producing enynes in the case of 2a and 2b, as well as η¹-allenyl complexes for 3a and 3b. Computational analysis of the structure and bonding of 2a and 3a reveals significant differences to a previously described related Ti complex. All complexes were fully characterised, including X-ray crystallography and experimental results were supported by DFT analysis.

Latest News: Reactions of Group 4 Bis(trimethylsilyl)acetylene Metallocene Complexes and Applications of the Obtained Products

Recently published reactions of group 4 metallocene bis(trimethylsilyl)acetylene (btmsa) complexes from the last two years are reviewed. Complexes like Cp'2 Ti(η2 -Me3 SiC2 SiMe3 ) and Cp2 Zr(py)(η2 -Me3 SiC2 SiMe3 ) with Cp' as Cp (cyclopentadienyl) and Cp* (pentamethylcyclopentadienyl) have been considered (py=pyridine). These complexes can liberate a reactive low-valent titanium or zirconium center by dissociation of the ligands and act as ''masked'' MII complexes (M=Ti, Zr). They represent excellent sources for the clean generation of the reactive coordinatively and electronically unsaturated complex fragments [Cp'2 M]. This is the reason why they were used for many synthetic and catalytic reactions during the last years. As an update to several review articles on this topic, this contribution provides an update with recent examples of preparative organometallic and organic chemistry of these complexes, acting as reagents for a wide range of coordinating and coupling reactions. In addition, applications and investigations concerning reaction products derived from this chemistry are mentioned, too.

Insertion Chemistry of Lutetacyclopropene toward Unsaturated C-O/C-N Bonds

Although the reaction chemistry of transition metallacyclopropenes has been well-established in the last decades, the reactivity of rare-earth metallacyclopropenes remains elusive. Herein, we report the reaction of lutetacyclopropene 1 toward a series of unsaturated molecules. The reaction of 1 with one equiv. of PhCOMe, Ar¹ CHO (Ar¹ =2,6-Me₂ C₆ H₃ ), W(CO)₆ , and PhCH=NPh provided oxalutetacyclopentenes, metallacyclic lutetoxycarbene, and azalutetacyclopentene via 1,2-insertion of C=O, C≡O, or C=N bonds into Lu-C_sp2 bond, respectively. However, the reaction between 1 and Ar² N=C=NAr² (Ar² =4-MeC₆ H₄ ) gave an acyclic lutetium complex with a diamidinate ligand by the coupling of one molecule of 1 with two carbodiimides, irrespective of the amount of carbodiimide employed. More interestingly, when 1 was treated with two equiv. of Ar¹ CHO, the reductive coupling of two C=O bonds was discovered to give a lutetium pinacolate complex along with the release of tolan. Remarkably, the reactivity of 1 is significantly different from that of scandacyclopropenes; these metallacycles derived from 1 all represent the first cases in rare-earth organometallic chemistry.

Ti-Catalyzed and -Mediated Oxidative Amination Reactions

Titanium is an attractive metal for catalytic reaction development: it is earth-abundant, inexpensive, and generally nontoxic. However-like most early transition metals-catalytic redox reactions with Ti are difficult because of the stability of the high-valent Ti^IV state. Understanding the fundamental mechanisms behind Ti redox processes is key for making progress toward potential catalytic applications. This Account details recent progress in Ti-catalyzed (and -mediated) oxidative amination reactions that proceed through formally Ti^II/Ti^IV catalytic cycles.This class of reactions is built on our initial discovery of Ti-catalyzed [2 + 2 + 1] pyrrole synthesis from alkynes and azobenzene, where detailed mechanistic studies have revealed important factors that allow for catalytic turnover despite the inherent difficulty of Ti redox. Two important conclusions from mechanistic studies are that (1) low-valent Ti intermediates in catalysis can be stabilized through coordination of π-acceptor substrates or products, where they can act as "redox-noninnocent" ligands through metal-to-ligand π back-donation, and (2) reductive elimination processes with Ti proceed through π-type electrocyclic (or pericyclic) reaction mechanisms rather than direct σ-bond coupling.The key reactive species in Ti-catalyzed oxidative amination reactions are Ti imidos (Ti≡NR), which can be generated from either aryl diazenes (RN═NR) or organic azides (RN₃). These Ti imidos can then undergo [2 + 2] cycloadditions with alkynes, resulting in intermediates that can be coupled to an array of other unsaturated functional groups, including alkynes, alkenes, nitriles, and nitrosos. This basic reactivity pattern has been extended into a broad range of catalytic and stoichiometric oxidative multicomponent coupling reactions of alkynes and other reactive small molecules, leading to multicomponent syntheses of various heterocycles and aminated building blocks.For example, catalytic oxidative coupling of Ti imidos with two different alkynes leads to pyrroles, while stoichiometric oxidative coupling with alkynes and nitriles leads to pyrazoles. These heterocycle syntheses often yield substitution patterns that are complementary to those of classical condensation routes and provide access to new electron-rich, highly substituted heteroaromatic scaffolds. Furthermore, catalytic oxidative alkyne carboamination reactions can be accomplished via reaction of Ti imidos with alkynes and alkenes, yielding α,β-unsaturated imine or cyclopropylimine building blocks. New catalytic and stoichiometric oxidative amination methods such as alkyne α-diimination, isocyanide imination, and ring-opening oxidative amination of strained alkenes are continuously emerging as a result of better mechanistic understanding of Ti redox catalysis.Ultimately, these Ti-catalyzed and -mediated oxidative amination methods demonstrate the importance of examining often-overlooked elements like the early transition metals through the lens of modern catalysis: rather than a lack of utility, these elements frequently have undiscovered potential for new transformations with orthogonal or complementary selectivity to their late transition metal counterparts.

Selective Coupling of Lanthanide Metallacyclopropenes and Nitriles via Azametallacyclopentadiene and η2-Pyrimidine Metallacycle

Exploring new lanthanide metallacycles and finding their unique chemistry different from the analogues of transition metals are of great interest and importance. In this work, we reported the synthesis, characterization, and reactivity toward nitriles of two lanthanide metallacyclopropenes: lutetacyclopropene 2a and dysprosacyclopropene 2b. The selective coupling of 2a and three molecules of PhCN was found for the first time to provide the unexpected fused lutetacycle 3a with one 1,6-dihydropyrimidine ring. Mechanistic studies by DFT calculations reveal that the triple insertion of PhCN into 2a proceeds through four key steps: the insertion of the first PhCN into 2a giving azalutetacyclopentadiene IM1, the insertion of the second PhCN into the Lu-N bond of IM1, the intramolecular electrocyclization providing a highly strained η²-pyrimidine metallacycle, and the insertion of the third PhCN into the Lu-C_sp3 bond. Isolation and characterization of two active intermediates, azalutetacyclopentadiene IM1 and η²-pyrimidine dysprosacycle, provide critical evidence for the formation of 3a. Furthermore, IM1 was also reported to react with TMSCN, isocyanides, or W(CO)₆ to furnish the fused [4,5] lutetacycles. The chemistry of two lanthanide metallacyclopropenes with nitriles is significantly different from these metallacyclopropenes of scandium and other metals. Most notably, the azalutetacyclopentadienes, η²-pyrimidine complex, and other metallacycles all represent the first examples in rare-earth organometallic chemistry; the formation of these new lutetacycles provides concrete evidence for understanding the mechanism of transition metal promoted or catalyzed [2+2+2] cycloaddition between alkynes and nitriles.

f-Block Phospholyl and Arsolyl Chemistry

The f‐block chemistry of phospholyl and arsolyl ligands, heavier p‐block analogues of substituted cyclopentadienyls (Cp^R, C₅R₅) where one or more CR groups are replaced by P or As atoms, is less developed than for lighter isoelectronic C₅R₅ rings. Heterocyclopentadienyl complexes can exhibit properties that complement and contrast with Cp^R chemistry. Given that there has been renewed interest in phospholyl and arsolyl f‐block chemistry in the last two decades, coinciding with a renaissance in f‐block solution chemistry, a review of this field is timely. Here, the syntheses of all structurally characterised examples of lanthanide and actinide phospholyl and arsolyl complexes to date are covered, including benzannulated derivatives, and together with group 3 complexes for completeness. The physicochemical properties of these complexes are reviewed, with the intention of motivating further research in this field.

Uranium versus Thorium: Synthesis and Reactivity of [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U[η2 -C2 Ph2 ]

The synthesis, electronic structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Addition of diphenylacetylene (PhC≡CPh) to the uranium phosphinidene metallocene [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂U=P‐2,4,6‐tBu₃C₆H₂ (1) yields the stable uranium metallacyclopropene, [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂U[η²‐C₂Ph₂] (2). Based on density functional theory (DFT) results the 5f orbital contributions to the bonding within the metallacyclopropene U‐(η²‐C=C) moiety increases significantly compared to the related Th^IV compound [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂Th[η²‐C₂Ph₂], which also results in more covalent bonds between the [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂U²⁺ and [η²‐C₂Ph₂]²⁻ fragments. Although the thorium and uranium complexes are structurally closely related, different reaction patterns are therefore observed. For example, 2 reacts as a masked synthon for the low‐valent uranium(II) metallocene [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂U^II when reacted with Ph₂E₂ (E=S, Se), alkynes and a variety of hetero‐unsaturated molecules such as imines, ketazine, bipy, nitriles, organic azides, and azo derivatives. In contrast, five‐membered metallaheterocycles are accessible when 2 is treated with isothiocyanate, aldehydes, and ketones.

Titanocene pnictinidene complexes

The phospha-Wittig reagent MesTerPPMe3 (MesTer = 2,6-{2,4,6-Me3-C6H2}-C6H3) and arsa-Wittig reagent DipTerAsPMe3 (DipTer = 2,6-{2,6-iPr2-C6H3}-C6H3) have been employed to synthesize the titanocene complexes Cp2Ti(PMe3)PnAr (Pn = P, As) with terminal phosphinidene or arsinidene ligands, respectively. Ab initio studies show that the description as singlet biradicaloids in their ground state is warranted.