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.

Equilibria and mesomerism/valence tautomerism of group 4 metallocene complexes.

In this account, examples of the influence of equilibrium and mesomerism/valence tautomerism on reactions of selected group 4 metallocene complexes will be discussed. Complexes of bis(trimethylsilyl)acetylene Me3SiC2SiMe3 (mono-functional alkynes: C[triple bond, length as m-dash]C) in Cp'2M(η2-Me3SiC2SiMe3) (Cp' = substituted η5-cyclopentadienyl), of α-di-Ph2P- (hetero-tri-functional: P, C[triple bond, length as m-dash]C, P) in Cp'2M(η2-Ph2C2PPh2), and of α-mono-SiH- (hetero-bi-functional: C[triple bond, length as m-dash]C, SiH) and α-di-SiH-substituted alkynes (hetero-tri-functional: SiH, C[triple bond, length as m-dash]C, SiH) in Cp'2M(η2-Me3SiC2SiMe2H) and Cp'2M(η2-HMe2SiC2SiMe2H), respectively, were considered. Additionally, unusual all-C-metallacycles like metallacyclopentynes Cp'2M[η2-C(R2)-C2-C(R2)-] (formed by coordination of tri-functional trienes: C[double bond, length as m-dash]C, C[double bond, length as m-dash]C, C[double bond, length as m-dash]C), five-membered metallacycloallenes Cp'2M[η2-C(R)[double bond, length as m-dash]C[double bond, length as m-dash]C(R)-C(R2)-] (formed by coordination of bi-functional enynes: C[double bond, length as m-dash]C, C[triple bond, length as m-dash]C), metallacyclocumulenes Cp'2M[η2-C(R)[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]C(R)-] (formed by coordination of bi-functional diynes: C[triple bond, length as m-dash]C, C[triple bond, length as m-dash]C), and seven-membered metallacyclocumulenes Cp'2M[η2-C(R)[double bond, length as m-dash]C(C2R)-C(R)[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]C(R)-] (formed by coordination of two bi-functional diynes: C[triple bond, length as m-dash]C, C[triple bond, length as m-dash]C), with M = Ti, Zr, and Hf were selected as examples for considerations of equilibrium and mesomerism/valence tautomerism. On the basis of spectroscopic investigations, their molecular structures and selected calculations, it is discussed how the different interpretations of the equilibrium and mesomerism/valence tautomerism are in line with observed reactivity patterns.

Carbon Monoxide Coupling Reactions: A New Concept for the Formation of Hexahydroxybenzene.

For linear and cyclic coupling reactions of CO, among other products, the formation of the hexapotassium salt of hexahydroxybenzene is of particular interesting. The interaction of metallic potassium and CO offers, via the assumed K[OC≡CO]K as the result of several carbon monoxide coupling reactions, the formation of C6 (OK)6 among other products. To date, only speculations exist about the reaction pathway for these products, which were first described by Liebig in 1834. A novel concept is suggested here, which consists of the single steps (i) reductive coupling of CO, (ii) formation of dihetero-metallacyclopentynes (cis-2,5-diheterobutatriene as formal ethylenedione O=C=C=O complexes), (iii) formation of its dinuclear 1-metalla-2,5-dioxo-cyclopentyne complexes by external coordination of the triple bond, (iv) insertion of CO into the M-C bond of the formed metallacyclopropene, and (v) the reductive elimination of C6 (OK)6 . The novel aspect of this concept is the formation of dihetero-metallacyclopentynes (in analogy to the well characterized all-C-metallacyclopentynes), which have not been considered in the mechanism of reductive CO coupling reactions. It is expected that the presence of transition-metal impurities would promote the reaction.

Vivid Wording for the Chemistry of Group 4 Metallocene Complexes.

Three selected examples for the use of unusual wording to describe the organometallic chemistry of Group 4 metallocenes are explained and discussed. The term "tuck(ed)-in" concerns the behavior of decamethyltitanocene [(C5 Me5 )2 Ti] and similar complexes in which one or two methyl groups form the titanium hydride complex [(C5 Me5 )(C5 Me4 CH2 )TiH] or other hydride complexes by C-H activation. In the so-called "merry-go-round reaction" the rearrangement of C atoms bound to titanium in organometallic molecules is described which corresponds to the rotation of two C atoms along with a rotation of the six-membered ring in a dihydroindenyl moiety at titanium. In the third example "migration" or "tobogganing" concerns the "sliding" of titanocene along the chain of a linear polyyne by coordination to one or more triple bonds. In all these reactions changes of the coordination mode of the metal at Cp or substrate ligands by intramolecular dynamics occur.

Visiting the Limits between a Highly Strained 1-Zirconacyclobuta-2,3-diene and Chemically Robust Dizirconacyclooctatetraene.

The reaction of the allene precursor Li2 (Me3 SiC3 SiMe3 ) with [Cp2 ZrCl2 ] (Cp=cyclopentadienyl) was examined. The selective formation of hitherto unknown linear, allene-bridged dizirconocene complexes [(Cp2 ZrCl)2 {-µ-(Me3 Si)C3 (SiMe3 )-}] and [(Cp2 Zr)2 {-µ-(Me3 Si)C3 (SiMe3 )-}2 ] was observed. Upon σ coordination of the allenediyl unit to {Cp2 Zr}, pyrophoric Li2 (Me3 SiC3 SiMe3 ) is tamed stepwise to yield a surprisingly robust 1,5-dizirconacyclooctatetra-2,3,6,7-ene with cumulated double bonds. This complex is unexpectedly inert against moisture, air, water and acetone. Surprisingly, it degrades under MS conditions to give the highly strained 1-zirconacyclobuta-2,3-diene. All compounds isolated have been fully characterised and the molecular structures are discussed. The stability and reactivity of these complexes are rationalised by DFT computations.

Reactions of Group†4 Metallocene Bis(trimethylsilyl)acetylene Complexes with Nitriles and Isonitriles.

This Review summarizes reactions of Group 4 metallocene bis(trimethylsilyl)acetylene complexes with nitriles and isonitriles. Selected examples of reactions of Group 4 metallocene alkyne complexes of the type Cp'2 M(η2 -Me3 SiC2 SiMe3 ) with different nitriles and isonitriles are described following the general L-M-S principle, in which stoichiometric and catalytic reactions are influenced by ligands L, metals M, and substrate substituents S. The influence of all these parameters is highlighted. The basic reactions with nitriles and in some cases with isonitriles were investigated in detail and are summarized. Selected further reactions of the products again depend on L, M, and S. Most interesting feature for this is the formation of mono- and polynuclear 1-metalla-2,5-diazacyclopenta-2,4-dienes, which show great potential for the synthesis of complex heterocycles.

Titanocene Silylpropyne Complexes: Promising Intermediates en route to a Four-Membered 1-Metallacyclobuta-2,3-diene?

Coordination of the alkyl-substituted alkynes Me3 SiC2 CH2 R (1: R=SiMe3 ; 2: R=N(SiMe3 )2 ) to titanocene centres [Cp'2 Ti] (Cp'=Cp, Cp*) yields stable alkyne complexes of the type Cp'2 Ti(η2 -Me3 SiC2 CH2 R) (3: Cp'=Cp, R=SiMe3 ; 5: Cp'=Cp, R=N(SiMe3 )2 ; 6: Cp'=Cp*, R=SiMe3 ) that are not prone to alkyne/allene isomerisation. When reacting alkyne 2 with Cp*2 TiCl2 and Mg formation of the complex Cp*2 Ti(III)(η3 -Me3 SiC2 CH2 ) (7) which displays a propargylic unit coordinated to the TiIII centre takes place. All complexes were fully characterised, the molecular structures for 5, 6, and 7 are discussed.

Redox-Disproportionation of a Decamethyltitanocene(III) Isonitrile Alkynyl Complex.

A mixed decamethyltitanocene(III) isonitrile alkynyl complex (7) was synthesized by the sequential introduction of the isonitrile and alkynyl ligands. Direct synthesis results in the formation of the diamagnetic decamethyltitanocene bis(isonitrile) (2) and bis(alkynyl) (3) complexes. Compound 7 undergoes disproportionation at room temperature to give 2 and 3. All complexes were fully characterized by IR, NMR, and EPR spectroscopy, and mass spectrometry. Molecular structures for complexes 2 and 7 are reported. The stability and reactivity of these complexes are rationalized by DFT computations.

Selective Reductions of N,N-Bis{chloro(aryl)-phosphino}-amines Yielding Three-, Five-, Six-, and Eight-Membered Cyclic Azaphosphanes.

To date, a plethora of λ3 -P nitrogen-containing compounds is known. A large number of them are used as ligands in coordination chemistry and homogeneous catalysis. PN-containing compounds tend to build up cyclic moieties, which have received less attention in regard to their application as ligands in transition metal chemistry. Hence, different dehalogenation reactions of N,N-bis{chloro(aryl)-phosphino}-amines have been developed to synthesize different P-N-P containing cyclic compounds. Their coordination behavior to group VI transition metal carbonyls was explored.

Reactivity Study of Pyridyl-Substituted 1-Metalla-2,5-diaza-cyclopenta-2,4-dienes of Group 4 Metallocenes.

In this work the reactivity of 1-metalla-2,5-diaza-cyclopenta-2,4-dienes of group 4 metallocenes, especially of the pyridyl-substituted examples, towards small molecules is investigated. The addition of H2 , CO2 , Ph-C≡N, 2-py-C≡N, 1,3-dicyanobenzene or 2,6-dicyanopyridine results in exchange reactions, which are accompanied by the elimination of a nitrile. For CO2 , a coordination to the five-membered cycle occurs in case of Cp*2 Zr(N=C(2-py)-C(2-py)=N). A 1,4-diaza-buta-1,3-diene complex is formed by H-transfer in the conversion of the analogous titanocene compound with CH3 -C≡N, PhCH2 -C≡N or acetone. For CH3 -C≡N a coupling product of three acetonitrile molecules is established additionally. In order to split off the metallocene from the coupled nitriles, we examined reactions with HCl, PhPCl2 , PhPSCl2 and SOCl2 . In the last case, the respective thiadiazole oxides and the metallocene dichlorides were obtained. A subsequent reaction produced thiadiazoles.

Multiple and Highly Selective Alkyne-Isonitrile C-C and C-N Couplings at Group†4 Metallocenes.

Reactions of the group 4 metallocene alkyne complexes [Cp'2 M(η(2) -Me3 SiC2 SiMe3 )] [Cp'2 =rac-(ebthi)=rac-1,2-ethylene-1,1'-bis(η(5) -tetrahydroindenyl): M=Ti, Zr, Hf; Cp'2 =Cp*2 (Cp*=η(5) -pentamethylcyclopentadienyl): M=Zr] with 2,6-dimethylphenyl isocyanide (2-xylyl isonitrile, XyNC) were investigated. Depending on the metal, the Cp' ligand, and/or the stoichiometry, as well as the reaction temperature and time, different products were obtained. The products included simple end-on coordination compounds of XyNC in addition to those of the coupling of Me3 SiC2 SiMe3 with two, three, or four isonitriles to form enimine complexes, aza-metallacycloallenes, and fused heterocyclic systems, respectively. One example of the latter tricyclic compounds was subject to demetalation by using HCl. Molecular structures of the heterometallacycles were determined by X-ray crystallography. The interconversion of the products was investigated by (1) H NMR spectroscopy.

Reactions of 2-Substituted Pyridines with Titanocenes and Zirconocenes: Coupling versus Dearomatisation.

Reactions of group 4 metallocene sources with 2-substituted pyridines were investigated to evaluate their coordination type between innocent and reductive dearomatisation as well as to probe the possibility for couplings. A dependence on the cyclopentadienyl ligands (Cp, Cp*), the metals (Ti, Zr), and the substrates (2-phenyl-, 2-acetyl-, and 2-iminopyridine) was observed. While 2-phenylpyridine is barely reactive, 2-acetylpyridine reacts vigorously with the Cp-substituted complexes and selectively with their Cp* analogues. With 2-iminopyridine, in all cases selective reactions were observed. In the isolated [Cp2 Ti], [Cp2 Zr], and [Cp*2 Zr] compounds the substrate coordinates by its pyridyl ring and the unsaturated side-chain. Subsequently, the pyridine was dearomatised, which is most pronounced in the [Cp*2 Zr] compounds. Using [Cp*2 Ti] leads to the unexpected paramagnetic complexes [Cp*2 TiIII (N,O-acpy)] and [Cp*2 TiIII (N,N'-impy)]. This highlights the non-innocent character of the pyridyl substrates.

Metallacyclocumulenes: a theoretical perspective on the structure, bonding, and reactivity.

Conspectus Transition metals help to stabilize highly strained organic fragments. Metallacycles, especially unsaturated ones, provide much variety in this area. We had a sustained interest in understanding new C-C bond formation reactions affected by binuclear transition metal fragments Cp2M. One such study led to the exploration of the bimetallic C-C cleavage and coupled complexes, where the acetylide ligands bridge two metal atoms. The underlying M-C interaction in these complexes inspired the synthesis of a five-membered cyclocumulene complex, which opened a new phase in organometallic chemistry. The metallacyclocumulene produces a variety of C-C cleavage and coupled products including a radialene complex. Group 4 metallocenes have thus unlocked a fascinating chemistry by stabilizing strained unsaturated C4 organic fragments in the form of five-membered metallacyclocumulenes, metallacyclopentynes, and metallacycloallenes. Over the years, we have carried out a comprehensive theoretical study to understand the unusual stability and reactivity of these metallacycles. The unique (M-Cß) interaction of the internal carbon atoms with the metal atom is the reason for unusual stability of the metallacycles. We have also shown that there is a definite dependence of the C-C coupling and cleavage reactions on the metal of metallacyclocumulenes. It demonstrates unexpected reaction pathways for these reactions. Based on this understanding, we have predicted and unraveled the stabilization factors of an unusual four-membered metallacycloallene complex. Indeed, our prediction about a four-membered heterometallacycle has led to an interesting bonding situation, which is experimentally realized. This type of M-C bonding is intriguing from a fundamental perspective and has great relevance in synthesizing unusual structures with interesting properties. In this Account, we first give a short prologue of what led to the present study and describe the salient features of the structure and bonding of the metallacyclocumulenes. The unusual reaction pathway of this metallacycle is explored next. Similar features of the metallacyclopentynes and metallacycloallenes are briefly mentioned. Then, we discuss the exploitation of the unique M-C bonding to design some exotic molecules such as a four-membered metallacycloallene complex. Our efforts to build a conceptual framework to understand these metallacycles and to exploit their chemistry continue.

Reactions of group 4 metallocenes with monosubstituted acetonitriles: keteniminate formation versus C-C coupling.

The reactions of the Group 4 metallocene dichlorides [Cp'2 MCl2 ] (1 a: M=Ti, Cp'=Cp*=η(5) -pentamethylcyclopentadienyl, 1 b: M=Zr, Cp'=Cp=η(5) -cyclopentadienyl) with lithiated MesCH2-C≡N gave [Cp*2 TiCl(N=C=C(HMes))] (3; Mes=mesityl) in the case of 1 a. For compound 1 b, a nitrile-nitrile coupling resulted in a five-membered bridge in 4. The reaction of the metallocene alkyne complex [Cp*2 Zr(η(2) -Me3 SiC2 SiMe3 )] (2) with PhCH2 C≡N led in the first step to the unstable product [Cp*2 Zr(η(2) -Me3 SiC2 SiMe3 )(NC=CH2 Ph)] (5). After the elimination of the alkyne, a mixture of products was formed. By variation of the solvent and the reaction temperature, three compounds were isolated: a diazadiene complex 6, a bis(keteniminate) complex 7, and 8 with a keteniminate ligand and a five-membered metallacycle. Subsequent variation of the Cp ligand and the metal center by using [Cp2 Zr] and [Cp*2 Ti] with Me3 SiC2 SiMe3 in the reactions with PhCH2-C≡N gave complex mixtures.

Formation of tri- and tetranuclear titanacycles through decamethyltitanocene-mediated intermolecular C-C coupling of dinitriles.

The reactions of [Cp*2 Ti(η(2) -Me3 SiC2 SiMe3 )] (Cp*=η(5) -pentamethylcyclopentadienyl) with various dicyano compounds were investigated. Nitrile-nitrile CC couplings result in multinuclear complexes owing to the bifunctionality of the substrates. Applying 1,3- or 1,4-dicyanobenzene led to tri- and tetranuclear complexes of the rare 1-metalla-2,5-diaza-cyclopenta-2,4-dienes. These are potential catalysts and were tested in the ring-opening polymerization of ε-caprolactone. The reaction with adiponitrile as alkyl dinitrile afforded a trinuclear 1-metalla-2,5-diaza-cyclopent-3-ene through additional protonation of the nitrogen atoms. The structure and bonding of the products were investigated by X-ray crystallography and DFT analysis to understand the molecular organization in the macrocycles.

Reactions of rac-(ebthi)M(η(2) -Me3 SiC2 SiMe3 ) (M=Ti, Zr) with aryl nitriles.

The reactions of the Group 4 metallocene alkyne complexes rac-(ebthi)M(η(2) -Me3 SiC2 SiMe3 ) (1 a: M=Ti, 1 b: M=Zr; rac-(ebthi)=rac-1,2-ethylene-1,1'-bis(η(5) -tetrahydroindenyl)) with PhCN were investigated. For 1 a, an unusual nitrile-nitrile coupling to 1-titana-2,5-diazacyclopenta-2,4-diene (2) at ambient temperature was observed. At higher temperature, the CC coupling of two nitriles resulted in the formation of a dinuclear complex with a four-membered diimine bridge (3). The reaction of 1 b with PhCN afforded dinuclear compound 4 and 2,4,6-triphenyltriazine. Additionally, the reactivity of 1 b towards other nitriles was investigated.

Activation of small molecules by phosphorus biradicaloids.

The reactivity of biradicaloid [P(µ-NTer)]2 was employed to activate small molecules bearing single, double, and triple bonds. Addition of chalcogens (O2 , S8 , Sex and Tex ) led to the formation of dichalcogen-bridged P2 N2 heterocycles, except from the reaction with molecular oxygen, which gave a P2 N2 ring featuring a dicoordinated P(III) and a four-coordinated P(V) center. In formal [2πe+2πe] addition reactions, small unsaturated compounds such as ethylene, acetylene, acetone, acetonitrile, tolane, diphenylcarbodiimide, and bis(trimethylsilyl)sulfurdiimide are readily added to the P2 N2 heterocycle of the biradicaloid [P(µ-NTer)]2 , yielding novel heteroatom cage compounds. The synthesis, reactivity, and bonding of the biradicaloid [P(µ-NTer)]2 were studied in detail as well as the synthesis, properties, and structural features of all addition products.

Unusual nitrile-nitrile and nitrile-alkyne coupling of Fc-C≡N and FC-C≡C-C≡N.

The reactions of the Group 4 metallocene alkyne complexes, [Cp*2M(η2-Me3SiC2SiMe3)] (1 a: M=Ti, 1 b: M=Zr, Cp*=η5-pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc-CN and Fc-C≡C-C≡N (Fc=Fe(η5-C5H5)(η5-C5H4)), is described. In case of Fc-C≡N an unusual nitrile­nitrile C-C homocoupling was observed and 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (3 a, b) were obtained. As the first step of the reaction with 1 b, the nitrile was coordinated to give [Cp*2Zr(η2-Me3SiC2SiMe3)(N≡C-Fc)] (2 b). The reactions with the 3-ferrocenyl-2-propyne-nitrile FcC≡C-C≡N lead to an alkyne­nitrile C-C coupling of two substrates and the formation of 1-metalla-2-aza-cyclopenta-2,4-dienes (4 a, b). For M=Zr, the compound is stabilized by dimerization as evidenced by single-crystal X-ray structure analysis. The electrochemical behavior of 3 a, b and 4 a, b was investigated, showing decomposition after oxidation, leading to different redox-active products.

Four-membered heterometallacyclic d° and d¹ complexes of Group†4 metallocenes with amidato ligands.

A study of the coordination chemistry of different amidato ligands [(R)N-C(Ph)O] (R=Ph, 2,6-diisopropylphenyl (Dipp)) at Group 4 metallocenes is presented. The heterometallacyclic complexes [Cp2M(Cl){κ(2)-N,O-(R)N-C(Ph)O}] M=Zr, R=Dipp (1 a), Ph (1 b); M=Hf, R=Ph (2)) were synthesized by reaction of [Cp2MCl2] with the corresponding deprotonated amides. Complex 1 a was also prepared by direct deprotonation of the amide with Schwartz reagent [Cp2Zr(H)Cl]. Salt metathesis reaction of [Cp2Zr(H)Cl] with deprotonated amide [(Dipp)N-C(Ph)O] gave the zirconocene hydrido complex [Cp2M(H){κ(2)-N,O-(Dipp)N-C(Ph)O}] (3). Reaction of 1 a with Mg did not result in the desired Zr(III) complex but in formation of Mg complex [(py)3Mg(Cl) {κ(2)-N,O-(Dipp)N-C(Ph)O}] (4; py=pyridine). The paramagnetic complexes [Cp'2Ti{κ(2)-N,O-(R)N-C(Ph)O}] (Cp'=Cp, R=Ph (7 a); Cp'=Cp, R=Dipp (7 b); Cp'=Cp*, R=Ph (8)) were prepared by the reaction of the known titanocene alkyne complexes [Cp2'Ti(η(2)-Me3SiC2SiMe3)] (Cp'=Cp (5), Cp'=Cp* (6)) with the corresponding amides. Complexes 1 a, 2, 3, 4, 7 a, 7 b, and 8 were characterized by X-ray crystallography. The structure and bonding of complexes 7 a and 8 were also characterized by EPR spectroscopy.

Bis(µ2-iso-propyl-imido-κ(2) N:N)bis-[(η(5)-cyclo-penta-dien-yl)(ethenolato-κO)titanium(IV)].

The title dinuclear half-sandwich complex, [CpTi(OCH=CH2)(µ2-N-iPr)]2 (Cp = cyclo-penta-dien-yl; iPr = isopropyl), was ob-tained from the reaction of Cp2TiCl2, n-butyl-lithium and iso-propyl-amine in tetra-hydro-furan. Each Ti(IV) atom is coordinated by one Cp ligand, one vin-yloxy unit and two bridging imido groups in a strongly distorted tetra-hedral geometry. There are two half mol-ecules in the asymmetric unit, such that whole mol-ecules being generated by inversion symmetry.