The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species.

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(µ-Cl)(DPEPhos)]2 and [Rh(DPEPhos)(MeOH)2]+ with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.

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).

A Phosphorus-Centred, Zirconocene-Bridged Tetraradical: Synthesis, Structure and Application as Molecular Double Switch.

Biradicals are important intermediates in the formation and breaking of a chemical bond. Their use as molecular switches is of particular interest. Much less is known about tetraradicals, which can, for example, consist of two biradical(oid) units. Here we report the synthesis of the first persistent phosphorus-centred tetraradical bound to a transition metal fragment. Starting from a zirconocene complex, rac-(ebthi)ZrCl2 (rac-(ebthi)=1,2-ethylene-1,10-bis(η5-tetrahydroindenyl), two cyclo-1,3-diphospha-pentane-1,3-diyls were successfully introduced, which finally led to the isolation of a deep green zirconcene-bridged bis(biradicaloid) complex (5) that can act as a double molecular switch. Under the influence of light (570 nm), this tetraradical forms a transannular bond in each of the two five-membered biradical units, leading to the formation of housane 5 h. Upon irradiation at 415 nm, the reverse reaction is observed, fully recovering tetraradical 5. Through single-crystal-to-single-crystal transformation, both stable species of the molecular switch could be structurally characterised using SCXRD. The switching under the influence of light and the activation of molecular hydrogen were analysed in solution using NMR and UV spectroscopy. It was found that the addition of one or two equivalents of molecular hydrogen can be switched on and off by light.

A General Concept for the Electronic and Steric Modification of 1-Metallacyclobuta-2,3-dienes: A Case Study of Group 4 Metallocene Complexes.

The synthesis of group 4 metal 1-metallacyclobuta-2,3-dienes as organometallic analogues of elusive 1,2-cyclobutadiene has so far been limited to SiMe3 substituted examples. We present the synthesis of two Ph substituted dilithiated ligand precursors for the preparation of four new 1-metallacyclobuta-2,3-dienes [rac-(ebthi)M] (M=Ti, Zr; ebthi=1,2-ethylene-1,10-bis(η5-tetrahydroindenyl)). The organolithium compounds [Li2(RC3Ph)] (1 b: R=Ph, 1 c: R=SiMe3) as well as the metallacycles of the general formula [rac-(ebthi)M(R1C3R2)] (2 b: M=Ti, R1=R2=Ph, 2 c: M=Ti, R1=Ph, R2=SiMe3; 3 b: M=Zr, R1=R2=Ph; 3 c: M=Zr, R1=Ph, R2=SiMe3) were fully characterised. Single crystal X-ray diffraction and quantum chemical bond analysis of the Ti and Zr complexes reveal ligand influence on the biradicaloid character of the titanocene complexes. X-band EPR spectroscopy of structurally similar Ti complexes [rac-(ebthi)Ti(Me3SiC3SiMe3)] (2 a), 2 b, and 2 c was carried out to evaluate the accessibility of an EPR active triplet state. Cyclic voltammetry shows that introduction of Ph groups renders the complexes easier to reduce. 13C CPMAS NMR analysis provides insights into the cause of the low field shift of the resonances of metal-bonded carbon atoms and provides evidence of the absence of the ß-C-Ti interaction.

Low-Temperature Isolation of a Labile Silylated Hydrazinium-yl Radical Cation, [(Me3 Si)2 N-N(H)SiMe3 ].

The oxidation of silylated hydrazine, (Me3 Si)2 N-N(H)SiMe3 , with silver salts led to the formation of a highly labile hydrazinium-yl radical cation, [(Me3 Si)2 N-N(H)SiMe3 ].+ , at very low temperatures (decomposition > -40 °C). EPR, NMR, DFT and Raman studies revealed the formation of a nitrogen-centered radical cation along the N-N unit of the hydrazine. In the presence of the weakly coordinating anion [Al{OCH(CF3 )2 }4 ]- , crystallization and structural characterization in the solid state were achieved. The hydrazinium-yl radical cation has a significantly shortened N-N bond and a nearly planar N2 Si3 framework, in contrast to the starting material. According to DFT calculations, the shortened N-N bond has a total bond order of 1.5 with a π-bond order of 0.5. The π bond can be regarded as a three-π-electron, two-center bond.

Synthesis, Coordination Chemistry, and Mechanistic Studies of P,N-Type Phosphaalkene-Based Rh(I) Complexes.

The synthesis of P,N-phosphaalkene ligands, py-CH═PMes* (1, py = 2-pyridyl, Mes* = 2,4,6-tBu-C6H2) and the novel quin-CH═PMes* (2, quin = 2-quinolinyl) is described. The reaction with [Rh(µ-Cl)cod]2 produces Rh(I) bis(phosphaalkene) chlorido complexes 3 and 4 with distorted trigonal bipyramidal coordination environments. Complexes 3 and 4 show a pronounced metal-to-ligand charge transfer (MLCT) from Rh into the ligand P═C π* orbitals. Upon heating, quinoline-based complex 4 undergoes twofold C-H bond activation at the o-tBu groups of the Mes* substituents to yield the cationic bis(phosphaindane) Rh(I) complex 5, which could not be observed for the pyridine-based analogue 3. Using sub- or superstoichiometric amounts of AgOTf the C-H bond activation at an o-tBu group of one or at both Mes* was detected, respectively. Density functional theory (DFT) studies suggest an oxidative proton shift pathway as an alternative to a previously reported high-barrier oxidative addition at Rh(I). The Rh(I) mono- and bis(phosphaindane) triflate complexes 6 and 7, respectively, undergo deprotonation at the benzylic CH2 group of the phosphaindane unit in the presence of KOtBu to furnish neutral, distorted square-planar Rh(I) complexes 8 and 9, respectively, with one of the P,N ligands being dearomatized. All complexes were fully characterized, including multinuclear NMR, vibrational, and ultraviolet-visible (UV-vis) spectroscopy, as well as single-crystal X-ray and elemental analysis.

On Silylated Oxonium and Sulfonium Ions and Their Interaction with Weakly Coordinating Borate Anions.

Attempts have been made to prepare salts with the labile tris(trimethylsilyl)chalconium ions, [(Me3 Si)3 E]+ (E=O, S), by reacting [Me3 Si-H-SiMe3 ][B(C6 F5 )4 ] and Me3 Si[CB] (CB- =carborate=[CHB11 H5 Cl6 ]- , [CHB11 Cl11 ]- ) with Me3 Si-E-SiMe3 . In the reaction of Me3 Si-O-SiMe3 with [Me3 Si-H-SiMe3 ][B(C6 F5 )4 ], a ligand exchange was observed in the [Me3 Si-H-SiMe3 ]+ cation leading to the surprising formation of the persilylated [(Me3 Si)2 (Me2 (H)Si)O]+ oxonium ion in a formal [Me2 (H)Si]+ instead of the desired [Me3 Si]+ transfer reaction. In contrast, the expected homoleptic persilylated [(Me3 Si)3 S]+ ion was formed and isolated as [B(C6 F5 )4 ]- and [CB]- salt, when Me3 Si-S-SiMe3 was treated with either [Me3 Si-H-SiMe3 ][B(C6 F5 )4 ] or Me3 Si[CB]. However, the addition of Me3 Si[CB] to Me3 Si-O-SiMe3 unexpectedly led to the release of Me4 Si with simultaneous formation of a cyclic dioxonium dication of the type [Me3 Si-µO-SiMe2 ]2 [CB]2 in an anion-mediated reaction. DFT studies on structure, bonding and thermodynamics of the [(Me3 Si)3 E]+ and [(Me3 Si)2 (Me2 (H)Si)E]+ ion formation are presented as well as mechanistic investigations on the template-driven transformation of the [(Me3 Si)3 E]+ ion into a cyclic dichalconium dication [Me3 Si-µE-SiMe2 ]2 2+ .

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.

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.

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.

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.

Isolation of Labile Pseudohalogen NSO Species.

A new synthetic approach enabled the generation of highly labile thionylimide, H-NSO, which was trapped by adduct formation with the bulky Lewis acid B(C6 F5 )3 and fully characterized. For comparison, a series of different Me3 Si-NSO Lewis acid adducts were studied. Treatment of Me3 Si-NSO with the silylium ion [Me3 Si](+) led to the formation of the hitherto unknown iminosulfonium ion [Me3 Si-N=S-O-SiMe3 ](+) , which could be isolated and fully characterized as a salt in the presence of weakly coordinating carborate anions.

Synthesis, structure, and reactivity of diazene adducts: isolation of iso-diazene stabilized as a borane adduct.

This work describes the synthesis and full characterization of a series of GaCl3 and B(C6 F5 )3 adducts of diazenes R(1) NNR(2) (R(1) =R(2) =Me3 Si, Ph; R(1) =Me3 Si, R(2) =Ph). Trans-PhNNPh forms a stable adduct with GaCl3 , whereas no adduct, but instead a frustrated Lewis acid-base pair is formed with B(C6 F5 )3 . The cis-PhNNPh⋅B(C6 F5 )3 adduct could only be isolated when UV light was used, which triggers the isomerization from trans- to cis-PhNNPh, which provides more space for the bulky borane. Treatment of trans-PhNNSiMe3 with GaCl3 led to the expected trans-PhNNSiMe3 ⋅GaCl3 adduct but the reaction with B(C6 F5 )3 triggered a 1,2-Me3 Si shift, which resulted in the formation of a highly labile iso-diazene, Me3 Si(Ph)NN; stabilized as a B(C6 F5 )3 adduct. Trans-Me3 SiNNSiMe3 forms a labile cis-Me3 SiNNSiMe3 ⋅B(C6 F5 )3 adduct, which isomerizes to give the transient iso-diazene species (Me3 Si)2 NN⋅B(C6 F5 )3 upon heating. Both iso-diazene species insert easily into one BC bond of B(C6 F5 )3 to afford hydrazinoboranes. All new compounds were fully characterized by means of X-ray crystallography, vibrational spectroscopy, CHN analysis, and NMR spectroscopy. All compounds were further investigated by DFT and the bonding situation was assessed by natural bond orbital (NBO) analysis.

Isolation of a labile homoleptic diazenium cation.

Following our interest in nitrogen chemistry, we now describe the synthesis, structure, and bonding of labile disilylated diazene, its GaCl3 adduct, and the intriguing trisilylated diazenium ion [(Me3 Si)2 NN-SiMe3 ](+), a dark blue and highly labile (Tdecomp >-30 °C) homoleptic cation of the type [R3 N2 ](+). Although direct silylation of Me3 Si-N=N-SiMe3 failed, the [(Me3 Si)2 N=N-SiMe3](+) ion was generated in a straightforward two-electron oxidation reaction from mercury(II) dihydrazide and Ag[GaCl4]. Moreover, previous structure data of Me3 Si-NN-SiMe3 were revised on the basis of new data.

Catalytic trimerization of bis-silylated diazomethane.

(Me(3)Si)(2)CNN isomerizes upon addition of traces of [Me(3)Si](+) ions to give (Me(3)Si)(2)NNC, which then undergoes an unusual trimerization reaction to give exclusively 4-diazenyl-3-hydrazinylpyrazole. As catalyst the isonitrilium ion, [(Me(3)Si)(2)NNC(SiMe(3))](+), was identified and fully characterized. Experiments and computations indicate a three-step reaction including isomerization of diazomethane, a C-C or N-C coupling, and a formal cycloaddition reaction. The kinetics and thermodynamics are discussed on the basis of DFT calculations.

Mono- and dinuclear zirconocene(iv) amide complexes for the catalytic dehydropolymerisation of phenylsilane.

The dehydropolymerisation of phenylsilane is investigated using group 4 metallocene amide complexes as catalysts. The dinuclear zirconocene amide complex Cp2Zr(NMe2)(µ-Me3SiC3SiMe3)Zr(NMe2)Cp2 (2) (Cp = η5-cyclopentadienyl) shows high activity in dehydrocoupling reactions, producing polyphenylsilanes with molecular weights ranging from 200 to 3000 g mol-1 and linear-to-cyclic product ratios of up to 80 : 20. Likewise, different ratios of oligomers and polymers with different tacticities could be described. Ansa-zirconocene amide complexes possessing the ebthi (ebthi = 1,2-ethylene-1,1'-bis(η5-tetrahydroindenyl)) ligand systems were prepared and evaluated for catalytic dehydropolymerisation in comparison to the dinuclear catalyst system.

How solvents affect the stability of cationic Rh(I) diphosphine complexes: a case study of MeCN coordination.

Cationic rhodium(I) diphosphine complexes, referred to as Schrock-Osborn catalysts, are privileged homogeneous catalysts with a wide range of catalytic applications. The coordination of solvent molecules can have a significant influence on reaction mechanisms and kinetic scenarios. Although solvent binding is well documented for these rhodium species, comparative quantifications for structurally related systems are not available to date. We present a method for systematic investigation and quantification of this important parameter, using MeCN which replaces diolefins and forms stable Rh(I) MeCN complexes. Using UV-vis and 31P{1H} NMR spectroscopy we determine and compare stability constants of different [Rh(PP)(NBD)]BF4 and [Rh(PP)(COD)]BF4 complexes (PP = diphosphine; COD = 1,5-cyclooctadiene; NBD = 2,5-norbornadiene) and discuss the influence of PP ligands and reaction temperature. A DFT study reveals the dependence of the stability on the thermodynamics of the exchange reaction. Using variable temperature NMR spectroscopy, the first mixed solvate complex could be verified as an intermediate in the MeCN-MeOH exchange.

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.