Syntheses, Characterization, and Multifaceted Coordination Chemistry of Hydrazonido Titanium Complexes.

The reaction of hydrazones with bis(π-η5:σ-η1-pentafulvene)titanium complexes leads to both hydrazonido and hydrazido complexes depending on the interaction of the hydrazone with the fulvene ligand of the metal complex. The molecular structures mostly reveal κ2N,N side-on coordination of the hydrazonido ligand due to the deprotonation of the N-H bond by one of the fulvene moieties. Instead of deprotonation, the reaction of the bis(adamantylidene fulvene)titanium complexes with cinnamon aldehyde phenylhydrazone leads to κ1N coordination. By using donating groups in the backbone of the hydrazone ligands, there are exceptions to this coordination mode due to the insertion of the C═N double bond into the Ti-Cexo bond of the pentafulvene moiety. Using 2-pyridinecarboxaldehyde phenylhydrazone, a formal κ3N,N,N ligand system is formed by the coordination of the pyridine nitrogen atom to the metal center via consecutive N-H deprotonation and insertion. Finally, the use of salicylaldehyde phenylhydrazone ultimately produces a complex with the κ3N,N,O coordination mode by double deprotonation of the hydrazone N-H and O-H functions. Because of its slow conversion to the final product, the intermediate was isolated as an insertion product with consecutive O-H deprotonation, showing a κ2N,O coordination mode of the hydrazido ligand.

From Coordination to Noncoordination: Syntheses and Substitution Lability Studies of Titanium Triflato Complexes.

A new concept for obtaining cationic complexes with triflate counteranions from coordinating triflato ligands was developed. Various routes are leading to titanium(IV) and titanium(III) triflato complexes efficiently. The reactions of pentafulvene titanium complexes with either triflic acid or silver triflate give the corresponding titanium(IV) triflato complexes in excellent yields. Hydrolysis of the titanium(IV) bistriflato complexes leads to cationic aqua complexes via displacement of the triflato ligand, which consequently acts as a noncoordinating anion. A functionalized titanium(IV) monotriflato complex was synthesized by insertion of a nitrile into the Ti-C bond and the triflato ligand was displaced by an NHC. While the titanium(IV) complexes are mostly inert toward substrates, the donor-free titanium(III) triflato complex is a strong Lewis acid and forms various adducts with monodentate Lewis bases. The titanium(III) complex was oxidized by reaction with TEMPO, resulting in a diamagnetic titanium(IV) complex. The reaction with bidentate ligands results in cationic titanium(III) complexes due to displacement of the triflato ligand by the bidentate ligands. Treatment with acetone leads to an aldol reaction of two acetone molecules and the formation of a cationic diacetone alcohol complex.

Elemental Magnesium as an Early Transition Metal Substitute: From Pentafulvene Coordination to Deprotonation of Amines.

The reaction of magnesium turnings and 6,6-di-para-tolylpentafulvene was investigated. Under mild conditions, the magnesium dissolves, forming the MgII complex 1 with a π-η5 : σ-κ1 coordinating ligand of the dimerized pentafulvene, analyzed by NMR and XRD investigations. As a magnesium pentafulvene complex was a possible intermediate, amines were employed as intercepting agents. Thereby, the amines were formally deprotonated by elemental magnesium, yielding the first examples of Cp'Mg(THF)2 NR2 complexes. This reaction competes with the formation of 1 and a consecutive formal [1,5]-H-shift forming an ansa-magnesocene. Employing amines with low basicity gave quantitative conversion to the amide complexes.

Titanium-Catalyzed Polymerization of a Lewis Base-Stabilized Phosphinoborane.

The reaction of the Lewis base-stabilized phosphinoborane monomer tBuHPBH2 NMe3 (2 a) with catalytic amounts of bis(η5 :η1 -adamantylidenepentafulvene)titanium (1) provides a convenient new route to the polyphosphinoborane [tBuPH-BH2 ]n (3 a). This method offers access to high molar mass materials under mild conditions and with short reaction times (20 °C, 1 h in toluene). It represents an unprecedented example of a transition metal-mediated polymerization of a Lewis base-stabilized Group 13/15 compound. Preliminary studies of the substrate scope and a potential mechanism are reported.

Vanadium Pentafulvene Complexes: Synthon for Unprecedented VanadoceneIII Derivatives.

The benzene ligand at CpV(η6 -C6 H6 ) (1) is exchanged for pentafulvenes. Using sterically demanding pentafulvenes gives a clean exchange reaction, yielding vanadium pentafulvene (2 a and 2 b) and benzofulvene complexes (3 a and 3 b). The molecular structures of the target compounds suggest a π-η5 :σ-η1 coordination mode with a vanadiumIII center. With the sterically low demanding 6,6-dimethylpentafulvene, a C-H activation at the leaving ligand is observed, yielding the ring-substituted vanadoceneII 4. The reactivity of the pentafulvene complexes was investigated. A series of unprecedented vanadoceneIII compounds were synthesized: Under mild conditions, E-H splitting of 4-tert-butylphenol, diphenylamine, and 2,6-diisopropylaniline yield well characterized examples of rare vanadoceneIII phenolate and amide complexes. Insertion reactions into the V-Cexo bond of the pentafulvene complexes by multiple bond containing substrates were found for acetone, 4-chlorobenzonitril and N,N'-dicyclohexylcarbodiimide.

Density Functional Theory Calculations for Multiple Conformers Explaining the Regio- and Stereoselectivity of Ti-Catalyzed Hydroaminoalkylation Reactions.

Hybrid Density Functional Theory (DFT) calculations for multiple conformers of the insertion reactions of a methylenecyclopropane into the Ti-C bond of two differently α-substituted titanaaziridines explain the experimentally observed differences in regioselectivity between catalytic hydroaminoalkylation reactions of methylenecyclopropanes with α-phenyl-substituted secondary amines and corresponding stoichiometric reactions of a methylenecyclopropane with titanaaziridines, which can only be achieved with α-unsubstituted titanaaziridines. In addition, the lack of reactivity of α-phenyl-substituted titanaaziridines as well as the diastereoselectivity of the catalytic and stoichiometric reactions can be understood.

Titanium-Catalyzed Hydroaminoalkylation of Ethylene.

The first examples of titanium-catalyzed hydroaminoalkylation reactions of ethylene with secondary amines are presented. The reactions can be achieved with various titanium catalysts and they do not require the use of high pressure equipment. In addition, the first solid-state structure of a titanapyrrolidine that is formed by insertion of an alkene into the Ti-C bond of a titanaaziridine is reported.

Electronic Transitions in Different Redox States of Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene-Bridged Titanium Complexes: Spectroelectrochemistry and Quantum Chemistry.

Multinuclear transition metal complexes bridged by ligands with extended π-electronic systems show a variety of complex electronic transitions and electron transfer reactions. While a systematic understanding of the photochemistry and electrochemistry has been attained for binuclear complexes, much less is known about trinuclear complexes such as hexaphenyl-5,6,11,12,17,18-hexaazatrinaphthylene-tristitanocene [(Cp2 Ti)3 HATN(Ph)6 ]. The voltammogram of [(Cp2 Ti)3 HATN(Ph)6 ] shows six oxidation and three reduction waves. Solution spectra of [(Cp2 Ti)3 HATN(Ph)6 ] and of the electrochemically formed oxidation products show electronic transitions in the UV, visible and the NIR ranges. Density functional theory (DFT) and linear response time-dependent DFT show that the three formally titanium(II) centers transfer an electron to the HATN ligand in the ground state. The optically excited transitions occur exclusively between ligand-centered orbitals. The charged titanium centers only provide an electrostatic frame to the extended π-electronic system. Complete active self-consistent field (CASSCF) calculation on a structurally simplified model compound, which considers the multi-reference character imposed by the three titanium centers, can provide an interpretation of the experimentally observed temperature-dependent magnetic behavior of the different redox states of the title compound in full consistency with the interpretation of the electronic spectra.

Spin Transition of an Iron(II) Organoborate Complex in Different Polymorphs and in Vacuum-Deposited Thin Films: Influence of Cooperativity.

Two polymorphic modifications (1-I and 1-II) of the new spin crossover (SCO) complex [Fe{H2B(pz)(pypz)}2] (pz = pyrazole, pypz = pyridylpyrazole; 1) were prepared and investigated by differential scanning calorimetry (DSC), magnetic measurements, Mößbauer, vibrational, and absorption spectroscopy as well as single-crystal and X-ray powder diffraction. DSC measurements reveal that upon heating the thermodynamically metastable form 1-II to ∼178 °C it transforms into 1-I in an exothermic reaction, which proves that these modifications are related by monotropism. Both forms show thermal SCO with T1/2 values of 390 K (1-II) and 270 K (1-I). An analysis of the crystal structures of 1-II and the corresponding Zn(II) (2) and Co(II) (3) complexes that are isotypic with 1-I reveals that form II consists of dimers coupled by strong intramolecular π···π interactions, which is not the case for 1-I. In agreement with these findings, investigations of thin films of 1, where significant π···π interactions should be absent, reveal SCO behavior similar to that of 1-I. These results underscore the importance of cooperativity for the spin-transition behavior of this class of complexes.

Synthesis, Crystal Structures, and Magnetic and Electrochemical Properties of Highly Phenyl Substituted Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene (HATNPh6)-Bridged Titanium Complexes.

Trinuclear titanium complexes coordinated by one ligand with three coordination sites have shown properties of mixed valency and a high number of reversible redox steps. Herein we report on the hexaphenyl-substituted derivative (Cp2Ti)3(µ3-HATNPh6) (2). On reaction of 2 with the ferrocenium salt [Cp2Fe]BF4, the cationic complexes [(Cp2Ti)3(µ3-HATNPh6)] n+ ( n = 1-3; 3-5) become available in a selective way. Cyclic voltammograms show 10 reversible redox states of the trinuclear species 2 without decomposition. In order to classify the degree of electronic communication between the titanium centers, comproportionation constants Kc, IVCT bands in NIR spectra, and magnetic measurements were analyzed. These parameters show strong coupling effects between the titanium centers but no full delocalization. In addition, single-crystal X-ray analysis of the neutral complex 2 and its oxidation products (1+ (3), 2+ (4), and 3+ (5)) revealed the geometric structure of the molecule in the solid state. For the cationic species anion-π interactions between the electron-deficient central ring of the HATNPh6 ligand and BF4- counterions were found.

Bis(η5 :η1 -pentafulvene)niobium(V) Complexes: Efficient Synthons for Niobium Carbene and Imido Derivatives.

Transition-metal carbene complexes and their reactivities are a key topic of chemistry. They are an integral part of researches in catalysis, organic synthesis, coordination chemistry, and numerous other areas. In this context, we report the synthesis of a low-valent bis(η5 :η1 -(di-p-tolyl)-pentafulvene)niobium chloride. Owing to the π-η5 :σ-η1 coordination mode of the pentafulvenes and the resulting high nucleophilic character of the exocyclic carbon atom of the ligand, the bis(η5 :η1 -pentafulvene)niobium complex is able to achieve the umpolung of a coordinated vinyl unit and the resulting formation of the first η5 :η1 cyclic niobium Schrock carbene complex. This new synthetic route is, in comparison to classical α-hydrogen elimination reactions or thermolysis of diazo compounds, completely unprecedented. The reactivity of the cyclic carbene function and the remaining fulvene ligand is demonstrated by double N-H bond activation of primary amines to niobium imido complexes.

From Five to Seven: Ring Expansion of Monoazadiene Titanium Complexes by Insertion of Aldehydes, Ketones and Nitriles.

The ring enlargement reactions at ambient temperatures of non C-terminus substituted monoazabutadiene (η4 -RN=CHCH=CH2 ) titanium complexes 2 are investigated. The insertion of aldehydes/ketones (five examples) and nitriles (four examples) into the Ti-C bonds result in expansion of the five-membered rings to uncommon seven-membered titanacycles 3 and 4 in good yields. These new compounds are fully characterized by NMR spectroscopy and single-crystal X-ray diffraction. In subsequent reactions, the seven-membered ring systems are protolyzed and the released organic fragments are isolated. Whereas the aldehyde/ketone insertion products 3 form substituted δ-amino alcohols 5 after reduction with NaBH3 CN, the nitrile insertion products 4 form substituted pyrroles 6 via cyclization.

Formation of Binuclear Zigzag Hexapentaene Titanium Complexes via a Titanacumulene [Ti=C=C=CH2 ] Intermediate.

The reaction of bis(η5 :η1 -pentafulvene)titanium complexes with an allylidenephosphorylide Ph3 P=C(H)- C(H)=CH2 leads to binuclear zigzag hexapentaene titanium complexes (Ti2a, Ti2b). The formation of the central C6 H4 unit can be described as a spontaneous double C-H bond activation process, leading to an R3 P=C=C=CH2 intermediate, as a synthon for a titanabutatriene fragment [(CpR )2 Ti=C=C=CH2 ] (R: 2-adamantyl, CH(p-tol)2 ). In a subsequent dimerization Ti2a and Ti2b are formed, proofed by single-crystal X-ray diffraction and NMR measurements. The reaction sequence is confirmed by DFT calculations.

Flexible Structural Features of Pentafulvene Titanium Derivatives: Isolation and Characterization of NHC Complexes.

The reaction of η(5),η(1)-pentafulvene titanium complexes with the strong N-heterocyclic carbene (NHC) donor 1,3,4,5-tetramethylimidazole-2-ylidene, leads to the formation of isolable NHC titanium adducts, featuring a haptotropic shift of the pentafulvene ligand, proved by single crystal X-ray diffraction as well as NMR spectroscopy studies.

Molecular and Electronic Structures of Mononuclear and Dinuclear Titanium Complexes Containing π-Radical Anions of 2,2'-Bipyridine and 1,10-Phenanthroline: An Experimental and DFT Computational Study.

Whereas reaction of [(η(5)-Cp*)Ti(IV)Cl3](0) (1) with 2 equiv of neutral 2,2'-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(η(5)-Cp*)Ti(III)(bpy(•))2](0) (2), performing the same reaction with only 1 equiv each of magnesium and bpy provides the dinuclear complex [{(η(5)-Cp*)Ti(µ-Cl)(bpy(•))}2](0) (3). Conducting the latter reaction using 1,10-phenanthroline (phen) in place of bpy resulted in formation of dinuclear [{(η(5)-Cp*)Ti(µ-Cl)(phen(•))}2](0) (4). The structures of 2, 3, and 4 have all been determined by high-resolution X-ray crystallography at 153 K; the Cpy-Cpy distances of 1.420(3) and 1.431(4) Å in the N,N'-coordinated bpy ligands of 2 and 3, respectively, are indicative of the presence of (bpy(•))(1-) ligands, rather than neutral (bpy(0)). The electronic spectra (300-1600 nm) of these two complexes are similar in form, and contain intense π → π* transitions associated with the (bpy(•))(1-) radical anion. Temperature dependent magnetic susceptibility measurements (4-300 K) show that mononuclear 2 possesses a temperature independent magnetic moment of 1.73 µB, which is indicative of an S = (1)/2 ground state. Broken symmetry density functional theory (BS-DFT) calculations yield a picture consistent with the experimental findings, in which the central Ti atom possesses a +3 oxidation state and is coordinated by a η(5)-Cp* ligand and two (bpy(•))(1-). Strong intramolecular antiferromagnetic coupling of these three unpaired spins, one each on the Ti(III) center and on the two (bpy(•))(1-) ligands, affords the experimentally observed doublet ground state. The magnetic susceptibility measurements for dinuclear 3 and 4 display weak but significant ferromagnetic coupling, and indicate that these complexes possess S = 1 ground states. The mechanism of the spin coupling phenomenon that yields the observed behavior was analyzed using BS-DFT calculations, and it was discovered that the tight π-stacking of the N,N'-coordinated (bpy(•))(1-)/(phen(•))(1-) ligands in these two complexes results from direct overlap of their SOMOs and formation of a two-electron multicentered bond. This yields a diamagnetic {(bpy)2}(2-)/{(phen)2}(2-) bridging unit whose doubly occupied HOMO is spread equally over both ligands. The two remaining unpaired electrons, one at each Ti(III) center, couple weakly in a ferromagnetic fashion to yield the experimentally observed S = 1 ground states.

Efficient access to titanaaziridines by C-H activation of N-methylanilines at ambient temperature.

Titanaaziridines or η(2)-imine titanium complexes are considered key intermediates of the titanium-catalyzed hydroaminoalkylation of alkenes. Herein, we present an efficient synthetic route to this class of compounds, starting from N-methylanilines and a bis(η(5):η(1)-pentafulvene)titanium complex. Consecutive reactions on the η(2)-methyleneaniline complexes, characterized for the first time, prove a high chemical versatility. In particular, hydroaminoalkylation products were found in reactions of the three-membered titanacycles with alkenes. For the first time, all the intermediates of the hydroaminoalkylation of alkenes were isolated and characterized.

Syntheses, Characterization, and Redox Activity of Ferrocene-Containing Titanium Complexes.

The chemistry of bis(π-η5:σ-η1-pentafulvene)titanium complexes is characterized by a broad range of E-H activation and Ti-C functionalization reactions, whereas ferrocene derivatives are easily accessible and redox-active compounds. The reaction of ferrocenealdehyde and -ketones with bis(π-η5:σ-η1-pentafulvene)titanium complexes result in the formation of bimetallic complexes via insertion of the C=O double bond of the aldehyde/ketone into the Ti-Cexo bond of the pentafulvene moiety. The reaction of bis(π-η5:σ-η1-pentafulvene)titanium complexes with ferrocenyl alcohols leads to alcoholate complexes via deprotonation of the OH group by the pentafulvene ligand. Because of the one remaining pentafulvene unit, further functionalization of the complexes is possible. In this work, we proceeded with 1,1'-bifunctionalized ferrocene derivatives for intramolecular follow-up reactions. 1,1'-Ferrocenedimethanol reacts with bis(π-η5:σ-η1-pentafulvene)titanium complexes in a double O-H deprotonation reaction to yield the dialcoholate complex. 1,1'-bis(phenylphosphine)ferrocene reacts differently as the double P-H deprotonation reaction results in the formation of a P-P linked phosphine. Therefore, we studied the reactivity of 1,1'-bis(phenylphosphine)ferrocene toward Rosenthal's reagent. As Rosenthal's reagent is regarded as a masked titanocene(II) species, it undergoes redox reactions toward H-acidic substrates, forming a paramagnetic Ti(III) complex.

(Carbazol-9-ido-κN)di-chlorido-(η5:η1-2,3,4,5-tetra-methyl-penta-fulvene)tantalum(V).

The reaction of (η5:η1-2,3,4,5-tetra-methyl-penta-fulvene)tantalum(V) dicarbazolide chloride (1) with etheric HCl results in the formation of the title compound (2), [Ta(C10H14)(C12H8N)Cl2]. The TaV atom has a distorted tetra-hedral coordination environment in a three-legged piano-stool fashion. The conformation of the penta-fulvene exocyclic C atom to the three other ligands is staggered and not eclipsed, as found in the crystal structure of 1. Inter-molecular inter-actions include π-π stacking, H⋯π inter-actions and weak C-H⋯Cl hydrogen bonds.

Crystal structures of the Lewis acid-base adducts {[(C6H11)2N]3Ti-LB}+[MeB(C6F5)3]-·1.5C7H8; LB = (C6H5)3PO (1), p-F-C6H4CN (2).

The reaction of [(C6H11)2N]3Ti-CH3 with the Lewis acid B(C6F5)3, followed by addition of the Lewis bases (C6H5)3PO and p-F-C6H4CN led to the complex salts tris-(di-cyclo-hexyl-amido)(tri-phenyl-phosphine oxide)titanium methyl-tris(penta-fluoro-phen-yl)borate toluene sesquisolvate, [Ti(C12H22N)3(C18H15OP)](C19H3F15B)·1.5C7H8, (1), and tris-(di-cyclo-hexyl-amido)(4-fluoro-benzo-nitrile)-titanium methyl-tris-(penta-fluoro-phen-yl)borate toluene sesquisolvate, [Ti(C12H22N)3(C7H4FN)](C19H3F15B)·1.5C7H8, (2), both crystallizing with 1.5 equivalents of toluene solvent mol-ecules. The Lewis acid-base adducts (1) and (2) can be described by dative donor bonds. The packing of the complex cations, anions and solvent mol-ecules in the crystal structure is consolidated by an intricate three-dimensional network of non-classical C-H⋯F inter-actions. Disorder of some of the cyclo-hexyl groups and the toluene solvent mol-ecules is observed.

Reaction of a bis(pentafulvene)titanium complex with an N-heterocyclic olefin: C-H-activation leads to resonance between a titanium vinyl and titanium alkylidene complex.

The N-heterocyclic olefin (NHO) ImMe4CH2 (2) (ImMe4CH2 = (MeCNMe)2CCH2) was employed for the synthesis of the titanium complex 3 derived from an NHO ligand precursor. By reacting 2 with the bis(π-η5:σ-η1-pentafulvene)titanium complex 1a, the terminal ylidic methylene unit of 2 is deprotonated by the quaternary exocyclic carbon atom of one pentafulvene ligand of 1a yielding the titanium complex 3 which bears an anionic NHO ligand (ImMe4CH-). 3 was characterized by NMR spectroscopy, single crystal X-ray diffraction and quantum chemical calculations. The latter highlight that 3 is best described as a titanium vinyl complex with significant contribution of the titanium alkylidene resonance structure.