Synthesis and Structural Characterization of New Zirconium(IV) Bent Metallocenes comprising η8‐Cyclooctatetraenyl and Bulky Cyclopentadienyl Ligands

The new zirconium bent metallocenes (COT)Zr(Cp^tBu2)Cl ( 1 ) and (COT)Zr(Cp′′)Cl ( 2 ) were synthesized in a straightforward manner and in high yields ( 1 : 91 %, 2 : 86 %) by treatment of in situ‐prepared (COT)ZrCl₂(THF) with 1 equiv. of K(Cp^tBu2) or K(Cp′′), respectively (COT = η⁸‐cyclooctatetraenyl; Cp^tBu2 = η⁵‐1,3‐di‐tert‐butylcyclopentadienyl; Cp′′ = η⁵‐1,3‐bis(trimethylsilyl)cyclopentadienyl). Subsequent reaction of 1 with 1 equiv. of phenyllithium afforded the σ‐phenyl derivative (Cp^tBu2)Zr(COT)Ph ( 3 ) as orange crystals in 83 % isolated yield. All three new compounds were structurally characterized through single‐crystal X‐ray diffraction.     

Trifluoromethylation of [AuF3(SIMes)]: Preparation and Characterization of [Au(CF3)xF3−x(SIMes)] (x=1–3) Complexes

Trifluoromethylation of [AuF₃(SIMes)] with the Ruppert–Prakash reagent TMSCF₃ in the presence of CsF yields the product series [Au(CF₃)_xF_3−x(SIMes)] (x=1–3). The degree of trifluoromethylation is solvent dependent and the ratio of the species can be controlled by varying the stoichiometry of the reaction, as evidenced from the ¹⁹F NMR spectra of the corresponding reaction mixtures. The molecular structures in the solid state of trans-[Au(CF₃)F₂(SIMes)] and [Au(CF₃)₃(SIMes)] are presented, together with a selective route for the synthesis of the latter complex. Correlation of the calculated SIMes affinity with the carbene carbon chemical shift in the ¹³C NMR spectrum reveals that trans-[Au(CF₃)F₂(SIMes)] and [Au(CF₃)₃(SIMes)] nicely follow the trend in Lewis acidities of related organo gold(III) complexes. Furthermore, a new correlation between the Au−C_carbene bond length of the molecular structure in the solid state and the chemical shift of the carbene carbon in the ¹³C NMR spectrum is presented.     

Low-Valent Group 14 NHC-Stabilized Phosphinidenide ate Complexes and NHC-Stabilized K/P-Clusters

The N-heterocyclic carbene (NHC)-stabilized phosphinidenide, SIMesPK [SIMes=1,3-bis(2,4,6-trimethylphenyl)imidazolidine-2-ylidene], was used as an (NHC)P-transfer reagent for the synthesis of the low-valent Group 14 ate complexes K[(SIMesP)₃E] (E=Ge: 2 , Sn: 3 , Pb: 4 ), which were characterized by ¹H NMR, ³¹P NMR, IR spectroscopy as well as elemental and X-ray analysis. Furthermore, SIMesPK was used in reactions with potassium amides and alkoxides to form the molecular phosphorus–potassium clusters [K₄(SIMesP)₂(hmds)₂] [ 5 , hmds=N(SiMe₃)₂] and [K₆(SIMesP)₂(OtBu)₄] ( 6 ). Finally, the reaction of SIMesPK with Li[Al(OC₄F₉)₄] led to the potassium-rich ionic compound [(SIMesP)₄K₅][Al(OC₄F₉)₄] ( 7 ).     

Normal‐to‐Abnormal NHC Rearrangement of AlIII,GaIII, and InIII Trialkyl Complexes: Scope,Mechanism, Reactivity Studies,and H2 Activation

The present contribution reports experimental and theoretical mechanistic investigations on a normal‐to‐abnormal (C2‐to‐C4‐bonded) NHC rearrangement processes occurring with bulky group 13 metal NHC adducts, including the scope of such a reactivity for Al compounds. The sterically congested adducts (nItBu)MMe₃ (nItBu=1,3‐di‐tert‐butylimidazol‐2‐ylidene; M=Al, Ga, In; 1 a – c ) readily rearrange to quantitatively afford the corresponding C4‐bonded complexes (aItBu)MMe₃ ( 4 a – c ), a reaction that may be promoted by THF. Thorough experimental data and DFT calculations were performed on the nNHC‐to‐aNHC process converting the Al‐nNHC ( 1 a ) to its aNHC analogue 4 a . A nItBu/aItBu isomerization is proposed to account for the formation of the thermodynamic product 4 a through reaction of transient aItBu with THF–AlMe₃. The reaction of benzophenone with (nItBu)AlMe₃ afforded the zwitterionic species (aItBu)(CPh₂‐O‐AlMe₃) ( 6 ), reflecting the unusual reactivity that such bulky adducts may display. Interestingly, the nItBu/Al(iBu)₃ Lewis pair behaves like a frustrated Lewis pair (FLP) since it readily reacts with H₂ under mild conditions. This may open the way to future reactivity developments involving commonly used trialkylaluminum precursors.     

The σ‐Aromatic Clusters [Zn3]+ and [Zn2Cu]: Embryonic Brass

The triangular clusters [Zn₃Cp*₃]⁺ and [Zn₂CuCp*₃] were obtained by addition of the in situ generated, electrophilic, and isolobal species [ZnCp*]⁺ and [CuCp*] to Carmona’s compound, [Cp*Zn? ZnCp*], without splitting the Zn? Zn bond. The choice of non‐coordinating fluoroaromatic solvents was crucial. The bonding situations of the all‐hydrocarbon‐ligand‐protected clusters were investigated by quantum chemical calculations revealing a high degree of σ‐aromaticity similar to the triatomic hydrogen ion [H₃]⁺. The new species serve as molecular building units of Cu_nZn_m nanobrass clusters as indicated by LIFDI mass spectrometry.     

Pyridine‐Enhanced Head‐to‐Tail Dimerization of Terminal Alkynes by a Rhodium–N‐Heterocyclic‐Carbene Catalyst

A general regioselective rhodium‐catalyzed head‐to‐tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh–N‐heterocyclic‐carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π‐alkyne‐coordinated Rh^I species [RhCl(NHC)(η²‐HC?CCH₂Ph)(py)] ( 3 ) and [RhCl(NHC){η²‐C(tBu)?C(E)CH?CHtBu}(py)] ( 4 ) and the Rh^III–hydride–alkynyl species [RhClH{? C?CSi(Me)₃}(IPr)(py)₂] ( 5 ). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne C? H oxidative addition, alkyne insertion, and reductive elimination. A 2,1‐hydrometalation of the alkyne is the more favorable pathway in accordance with a head‐to‐tail selectivity.     

Synthesis of Unprecedented 4d/4f-Polypnictogens

A series of 4d/4f-polyarsenides, -polyarsines and -polystibines was obtained by reduction of the Mo-pnictide precursor complexes [{Cp^tMo(CO)₂}₂(μ,η^2:2-E₂)] (E=As, Sb; Cp^t=tBu substituted cyclopentadienyl) with two different divalent samarocenes [Cp*₂Sm] and [(Cp^Me4nPr)₂Sm]. For the reductive conversion of the Mo-stibide only one product was isolated, featuring a planar tetrastibacyclobutadiene moiety as an unprecedented ligand for organometallic compounds. For the corresponding Mo-arsenide a tetraarsacyclobutadiene and a second species with a side-on coordinated As₂²⁻ anion was isolated. The latter can be considered as reaction intermediate for the formation of the tetraarsacyclobutadiene.     

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 [Cp₂Ti], [Cp₂Zr], and [Cp*₂Zr] 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*₂Zr] compounds. Using [Cp*₂Ti] leads to the unexpected paramagnetic complexes [Cp*₂Ti^III(N,O‐acpy)] and [Cp*₂Ti^III(N,N′‐impy)]. This highlights the non‐innocent character of the pyridyl substrates.     

NHCs as Neutral Donors towards Polyphosphorus Complexes

The first adducts of NHCs (=N-heterocyclic carbenes) with aromatic polyphosphorus complexes are reported. The reactions of [Cp*Fe(η⁵-P₅)] ( 1 ) (Cp*=pentamethyl-cyclopentadienyl) with IMe (=1,3,4,5-tetramethylimidazolin-2-ylidene), IMes (=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene) and IDipp (=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene) led to the corresponding neutral adducts which can be isolated in the solid state. However, in solution, they quickly undergo a dissociative equilibrium between the adduct and 1 including the corresponding NHC. The equilibrium is influenced by the bulkiness of the NHC. [Cp′′Ta(CO)₂(η⁴-P₄)] (Cp′′=1,3-di-tert-butylcyclopentadienyl) reacts with IMe under P atom abstraction to give an unprecedented cyclo-P₃-containing anionic tantalum complex. DFT calculations shed light onto the energetics of the reaction pathways.     

Synthesis and Crystal Structures of t‐Butyl‐Pyridine Adducts of ZnR2 (R = Me,i‐Pr,t‐Bu,Cp*)

Lewis acid‐base adducts of the general type R₂Zn(4‐tBuPy)_x (R = Me 1 , iPr 2 , tBu 3 , Cp* 4 ; x = 1, 2) were obtained in high yields from reactions of ZnR₂ with the Lewis base 4‐tBu‐Pyridine. Compounds 1 – 4 were characterized by multinuclear NMR (¹H, ¹³C) and IR spectroscopy and elemental analyses, 1 and 4 also by X‐ray diffraction at single crystals.     

Synthesis,Bonding and Reactivity of a Terminal Titanium Alkylidene Hydrazido Compound

We report a detailed study of the reactions of the Ti?NNCPh₂ alkylidene hydrazide functional group in [Cp*Ti{MeC(NiPr)₂}(NNCPh₂)] ( 8 ) with a variety of unsaturated and saturated substrates. Compound 8 was prepared from [Cp*Ti{MeC(NiPr)₂}(NtBu)] and Ph₂CNNH₂. DFT calculations were used to determine the nature of the bonding for the Ti?NNCPh₂ moiety in 8 and in the previously reported [Cp₂Ti(NNCPh₂)(PMe₃)]. Reaction of 8 with CO₂ gave dimeric [(Cp*Ti{MeC(NiPr)₂}{μ‐OC(NNCPh₂)O})₂] and the “double‐insertion” dicarboxylate species [Cp*Ti‐{MeC(NiPr)₂}{OC(O)N(NCPh₂)C(O)O}] through an initial [2+2] cycloaddition product [Cp*Ti{MeC(NiPr)₂}{N(NCPh₂)C(O)O}], the congener of which could be isolated in the corresponding reaction with CS₂. The reaction with isocyanates or isothiocyanates tBuNCO or ArNCE (Ar=Tol or 2,6‐C₆H₃iPr₂; E=O, S) gave either complete NNCPh₂ transfer, [2+2] cycloaddition to Ti?N_α or single‐ or double‐substrate insertion into the Ti?N_α bond. The treatment of 8 with isonitriles RNC (R=tBu or Xyl) formed σ‐adducts [Cp*Ti{MeC(NiPr)₂}(NNCPh₂)(CNR)]. With Ar^F5CCH (Ar^F5=C₆F₅) the [2+2] cycloaddition product [Cp*Ti{MeC(NiPr)₂}{N(NCPh₂)C(Ar^F5)C(H)}] was formed, whereas with benzonitriles ArCN (Ar=Ph or Ar^F5) two equivalents of substrate were coupled in a head‐to‐tail manner across the Ti?N_α bond to form [Cp*Ti{MeC(NiPr)₂}{N(NCPh₂)C(Ar)NC(Ar)N}]. Treatment of 8 with RSiH₃ (R=aryl or Bu) or Ph₂SiH₂ gave [Cp*Ti{MeC(NiPr)₂}{N(SiHRR′)N(CHPh₂)}] (R′=H or Ph) through net 1,3‐addition of Si? H to the N? N?CPh₂ linkage of 8 , whereas reaction with PhSiH₂X (X=Cl, Br) led to the Ti?N_α 1,2‐addition products [Cp*Ti{MeC(NiPr)₂}(X){N(NCPh₂)SiH₂Ph}].     

Iodination of cyclo‐E5‐Complexes (E=P,As)

In a high‐yield one‐pot synthesis, the reactions of [Cp*M(η⁵‐P₅)] (M=Fe ( 1 ), Ru ( 2 )) with I₂ resulted in the selective formation of [Cp*MP₆I₆]⁺ salts ( 3 , 4 ). The products comprise unprecedented all‐cis tripodal triphosphino‐cyclotriphosphine ligands. The iodination of [Cp*Fe(η⁵‐As₅)] ( 6 ) gave, in addition to [Fe(CH₃CN)₆]²⁺ salts of the rare [As₆I₈]^2? (in 7 ) and [As₄I₁₄]^2? (in 8 ) anions, the first di‐cationic Fe‐As triple decker complex [(Cp*Fe)₂(μ,η^5:5‐As₅)][As₆I₈] ( 9 ). In contrast, the iodination of [Cp*Ru(η⁵‐As₅)] ( 10 ) did not result in the full cleavage of the M?As bonds. Instead, a number of dinuclear complexes were obtained: [(Cp*Ru)₂(μ,η^5:5‐As₅)][As₆I₈]_0.5 ( 11 ) represents the first Ru‐As₅ triple decker complex, thus completing the series of monocationic complexes [(Cp^RM)₂(μ,η^5:5‐E₅)]⁺ (M=Fe, Ru; E=P, As). [(Cp*Ru)₂As₈I₆] ( 12 ) crystallizes as a racemic mixture of both enantiomers, while [(Cp*Ru)₂As₄I₄] ( 13 ) crystallizes as a symmetric and an asymmetric isomer and features a unique tetramer of {AsI} arsinidene units as a middle deck.     

Binuclear CpV,Cp*V,and Cp*Ta Complexes containing Organochalcogenolato Bridges, μ‐ER (E = Sulfur,Selenium, Tellurium; R = Methyl,Phenyl, and Ferrocenyl)

Photolysis of the halfsandwich tetracarbonylmetal complexes CpV(CO)₄, Cp*V(CO)₄ and Cp*Ta(CO)₄ in solution in the presence of di(organyl)dichalcogenides E₂R₂ (E = S, Se, Te; R = Me, Ph, Fc) leads to diamagnetic doubly organochalcogenolato‐bridged compounds, [Cp⁽⁾M(CO)₂(μ‐ER)]₂. According to the X‐ray structure determinations carried out for [CpV(CO)₂(μ‐TeMe)]₂, [Cp*V(CO)₂(μ‐TePh)]₂ and [Cp*Ta(CO)₂(μ‐SPh)]₂, the molecular framework consists of a folded M₂(μ‐ER)₂ ring with the cyclopentadienyl ligands in cis‐configuration and the organyl substituents R in a syn‐equatorial arrangement, thus forming a bowl‐shaped molecule with the four terminal CO ligands protruding into the inner sphere. The M…M distances (in the range between 305 and 330 pm) are not considered to indicate direct bonding interactions. The vanadium complexes [Cp⁽⁾V(CO)₂(μ‐ER)]₂ are completely decarbonylated in the presence of an excess of E₂R₂ in boiling toluene, and in many cases the paramagnetic quadruply‐bridged products, [CpV(μ‐ER)₂]₂, can be isolated.     

Umsetzungen von Halbsandwich-Rhenium(V)-Oligochalkogenidkomplexen mit Acetylen-dicarbonsäuredimethylester. Molekülstrukturen der neuen 1,2-Dicarbomethoxy-ethen-1,2-dichalkogenat-Chelatverbindungen Cp*Re[S2C2(COOMe)2]2 und Cp*Re(NtBu)[Se2C2(COOMe)2]

Reactions of Halfsandwich Rhenium(V) Oligochalcogenide Complexes with Dimethyl Acetylene Dicarboxylate. Molecular Structures of the New 1,2-Dicarbomethoxy-ethene-1,2-dichalcogenate Chelate Compounds Cp*Re[S₂C₂(COOMe)₂]₂ and Cp*Re(N^tBu)[Se₂C₂(COOMe)₂] The reaction of Cp*Re(S₃)(S₄) ( 1a ) with dimethyl acetylene dicarboxylate (dmad) leads through the blue intermediate Cp*Re(S₄)[S₂C₂(COOMe)₂] ( 2a ) to the red bis(ethene-1,2-dithiolato) complex Cp*Re[S₂C₂(COOMe)₂]₂ ( 3a ). The product 3a is also formed in the reactions of dmad with the tetrasulfidorhenium complexes Cp*Re(L)(S₄) (L = O ( 4a ), N^tBu ( 5a )) while the analogous tetraselenidorhenium compounds Cp*Re(L)(Se₄) ( 4b and 5b ) are only transformed to Cp*Re(L)[Se₂C₂(COOMe)₂] (L = O ( 6b ), N^tBu ( 7b )). According to the X-ray crystal structure analyses, the (ethene-1,2-dithiolato)rhenium chelate rings in 3a are folded along the S …? S vector towards the Cp* ligand (angle between the planes ReS₂/S₂C₂ 159.2°), whereas the ReSe₂C₂ chelate ring in 7b is planar.     

Ringkontraktion durch NHC‐induzierte Pnictogen‐Abstraktion

Die Reaktion von [Cp′′′Co(η⁴‐P₄)] ( 1 ) (Cp′′′=1,2,4‐tBu₃C₅H₂) mit ^MeNHC (^MeNHC=1,3,4,5‐tetramethylimidazol‐2‐ylidene) führt über eine NHC‐induzierte Phosphorkationen‐Abstraktion zum Ringkontraktionsprodukt [(^MeNHC)₂P][Cp′′′Co(η³‐P₃)] ( 2 ), welches das erste Beispiel eines anionischen CoP₃‐Komplexes repräsentiert. Solche von NHCs induzierten Ringkontraktionsreaktionen lassen sich ebenfalls auf Tripeldecker‐Sandwich‐Komplexe anwenden. So werden die Komplexe [(Cp*Mo)₂(μ,η^6:6‐E₆)] ( 3 a , 3 b ) (Cp*=C₅Me₅; E=P, As) zu den Komplexen [(^MeNHC)₂E][(Cp*M)₂(μ,η^3:3‐E₃)(μ,η^2:2‐E₂)] ( 4 a , 4 b ) transformiert, wobei 4 b das erste strukturell charakterisierte Beispiel eines NHC‐substituierten As^I‐Kations darstellt. Darüber hinaus führt die Reaktion des Vanadium‐Komplexes [(Cp*V)₂(μ,η^6:6‐P₆)] ( 5 ) mit ^MeNHC zur Bildung der neuartigen Komplexe [(^MeNHC)₂P][(Cp*V)₂(μ,η^6:6‐P₆)] ( 6 ), [(^MeNHC)₂P][(Cp*V)₂(μ,η^5:5‐P₅)] ( 7 ) bzw. [(Cp*V)₂(μ,η^3:3‐P₃)(μ,η^1:1‐P{^MeNHC})] ( 8 ).     

Ring Contraction by NHC‐Induced Pnictogen Abstraction

The reaction of [Cp′′′Co(η⁴‐P₄)] ( 1 ) (Cp′′′=1,2,4‐tBu₃C₅H₂) with ^MeNHC (^MeNHC=1,3,4,5‐tetramethylimidazol‐2‐ylidene) leads through NHC‐induced phosphorus cation abstraction to the ring contraction product [(^MeNHC)₂P][Cp′′′Co(η³‐P₃)] ( 2 ), which represents the first example of an anionic CoP₃ complex. Such NHC‐induced ring contraction reactions are also applicable for triple‐decker sandwich complexes. The complexes [(Cp*Mo)₂(μ,η^6:6‐E₆)] ( 3 a , 3 b ) (Cp*=C₅Me₅; E=P, As) can be transformed to the complexes [(^MeNHC)₂E][(Cp*M)₂(μ,η^3:3‐E₃)(μ,η^2:2‐E₂)] ( 4 a , 4 b ), with 4 b representing the first structurally characterized example of an NHC‐substituted As^I cation. Further, the reaction of the vanadium complex [(Cp*V)₂(μ,η^6:6‐P₆)] ( 5 ) with ^MeNHC results in the formation of the unprecedented complexes [(^MeNHC)₂P][(Cp*V)₂(μ,η^6:6‐P₆)] ( 6 ), [(^MeNHC)₂P][(Cp*V)₂(μ,η^5:5‐P₅)] ( 7 ) and [(Cp*V)₂(μ,η^3:3‐P₃)(μ,η^1:1‐P{^MeNHC})] ( 8 ).     

Organometallic Cerium Complexes from Tetravalent Coordination Complexes

The use of tetravalent cerium alkoxides, nitrates, and triflates was studied as a direct route to [Ce^IV(carbene)] complexes. Protonolysis reactions between 1H‐imidazolium‐ or imidazoline (=4,5‐dihydro‐1H‐imidazole)‐containing alkoxide proligands HL (L=OCMe₂CH₂[1‐C(NCHCHNⁱPr)]) and HL^S (L^S=OCMe₂CH₂[1‐C(NCH₂CH₂NⁱPr)]) and Ce^IV tert‐butoxide, triflate, and nitrate compounds were studied to target [Ce^IV(N‐heterocyclic carbene)] complexes (of unsaturated and saturated carbenes, resp.). Instead, tetravalent cerium imidazolium [(O^tBu)₃Ce(μ‐O^tBu)₂(μ‐HL)Ce(O^tBu)₃], or imidazolinium adducts [(O^tBu)₃Ce(μ‐O^tBu)₂(μ‐HL^S)Ce(O^tBu)₃] were isolated. However, the salt metathesis of cerium triflate with KL provided a simple route to [CeL₄], which was significantly improved if an external oxidant, benzoquinone, was included in the mixture to maintain oxidation‐state integrity. Likewise, the salt metathesis of cerium triiodide with KL and added benzoquinone provided a straightforward route to [CeL₄].     

Hydrogen Generation by Water Reduction with [Cp*2Ti(OTf)]: Identifying Elemental Mechanistic Steps by Combined In Situ FTIR and In Situ EPR Spectroscopy Supported by DFT Calculations

A detailed mechanism of hydrogen production by reduction of water with decamethyltitanocene triflate [Cp*₂Ti^III(OTf)] has been derived for the first time, based on a comprehensive in situ spectroscopic study including EPR and ATR‐FTIR spectroscopy supported by DFT calculations. It is demonstrated that two H₂O molecules coordinate to [Cp*₂Ti^III(OTf)] subsequently forming [Cp₂*Ti^III(H₂O)(OTf)] and [Cp*Ti^III(H₂O)₂(OTf)]. Triflate stabilizes the water ligands by hydrogen bonding. Liberation of hydrogen proceeds only from the diaqua complex [Cp*Ti^III(H₂O)₂(OTf)] and involves, most probably, abstraction and recombination of two H atoms from two molecules of [Cp*Ti^III(H₂O)₂(OTf)] in close vicinity, which is driven by the formation of a strong covalent Ti? OH bond in the resulting final product [Cp*₂Ti^IV(OTf)(OH)].     

C?H Bond Activation/Borylation of Furans and Thiophenes Catalyzed by a Half‐Sandwich Iron N‐Heterocyclic Carbene Complex

A coordinatively unsaturated iron‐methyl complex having an N‐heterocyclic carbene ligand, [Cp*Fe(L^Me)Me] ( 1 ; Cp*=η⁵‐C₅Me₅, L^Me=1,3,4,5‐tetramethyl‐imidazol‐2‐ylidene), is synthesized from the reaction of [Cp*Fe(TMEDA)Cl] (TMEDA=N,N,N′,N′‐tetramethylethylenediamine) with methyllithium and L^Me. Complex 1 is found to activate the C? H bonds of furan, thiophene, and benzene, giving rise to aryl complexes, [Cp*Fe(L^Me)(aryl)] (aryl=2‐furyl ( 2 ), 2‐thienyl ( 3 ), phenyl ( 4 )). The C? H bond cleavage reactions are applied to the dehydrogenative coupling of furans or thiophenes with pinacolborane (HBpin) in the presence of tert‐butylethylene and a catalytic amount of 1 (10 mol % to HBpin). The borylation of the furan/thiophene or 2‐substituted furans/thiophenes occurs exclusively at the 2‐ or 5‐positions, respectively, whereas that of 3‐substituted furans/thiophenes takes place mainly at the 5‐position and gives a mixture of regioisomers. Treatment of 2 with 2 equiv of HBpin results in the quantitative formation of 2‐boryl‐furan and the borohydride complex [Cp*Fe(L^Me)(H₂Bpin)] ( 5 ). Heating a solution of 5 in the presence of tert‐butylethylene led to the formation of an alkyl complex [Cp*Fe(L^Me)CH₂CH₂tBu] ( 6 ), which was found to cleave the C? H bond of furan to produce 2 . On the basis of these results, a possible catalytic cycle is proposed.     

Synthesis of new iron–NHC complexes as catalysts for hydrosilylation reactions

A series of new piano‐stool iron(II) complexes comprising N‐heterocyclic carbene ligands [Fe(Cp)(CO)₂(NHC)]I (NHC = 1,3‐disubstituted imidazolidin‐2‐ylidene) have been synthesized and analyzed by ¹H NMR, ¹³C NMR, IR, elemental analysis and mass spectrometric techniques. These compounds were easily prepared from the reaction of disubstituted imidazolidin‐2‐ylidene with [FeI(Cp)(CO)₂] in toluene at room temperature. These complexes were tested in the catalytic hydrosilylation reaction of aldehydes and ketones with phenylsilane in solvent‐free conditions. After a basic hydrolysis step, the corresponding alcohols were obtained in good yields. Copyright © 2013 John Wiley & Sons, Ltd.