Ethene Complexes of Bulky Titanocenes, Their Thermolysis, and Their Reactivity toward 2-Butyne

Detailed Density Functional Theory Study of the Cationic Zirconocene Compound [Cp(C5H4CMe2C6H4F)ZrMe]. ACS OMEGA 2022;7:35136-35152. [PMID: 36211080 PMCID: PMC9535714 DOI: 10.1021/acsomega.2c04053]

Detailed density functional theory studies at the B3LYP and PBE-D3 levels of theory were performed on the cationic compound [Cp(C₅H₄CMe₂C₆H₄F)ZrMe]⁺, with the F atom occupying either the ortho, meta, or para positions of the arene ring. In all cases, the arene ring coordinates with the cationic zirconium metal. The nature of this coordination is such that for the meta- or para-substituted arene ring, it is predominantly the ortho carbon atom of the C-H bond which interacts with the metal, as evident from noncovalent interaction studies. This is further corroborated by the natural bond orbital and quantum theory of atoms in molecular studies. In the case of the F atom being in the ortho position, we obtained clear-cut evidence for a Zr-F coordination.

Redox chemistry of discrete low-valent titanium complexes and low-valent titanium synthons

Titanium is a versatile metal that has important applications in practical synthesis, though this is typically limited to stoichiometric reactions or Lewis acid catalysis. Recently, interest has grown in using titanium and other early-metals for redox catalysis; however, notable limitations exist due to the thermodynamic preference of these metals to adopt high oxidation states. Nonetheless, discrete low-valent titanium (LVT) complexes and their synthons (titanium complexes which chemically behave as LVT sources) are known. Here, we detail the various ligand platforms that are capable of stabilizing LVT compounds and present the redox chemistry of these systems. This includes a discussion of recent developments in the use of LVT synthons for accessing fully reversible oxidative-addition/reductive-elimination reactions.

A Mononuclear and High-Spin Tetrahedral TiII Complex

A high-spin, mononuclear TiII complex, [(TptBu,Me)TiCl] [TptBu,Me- = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(TptBu,Me)TiCl2] with KC8. Complex [(TptBu,Me)TiCl] has a 3A2 ground state (assuming C3v symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined TiII complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(TptBu,Me)TiCl(THF)], which is impervious to N2 binding. However, in the absence of THF, the TiII complex captures N2 to produce the diamagnetic complex [(TptBu,Me)TiCl]2(η1,η1;μ2-N2), with a linear Ti═N═N═Ti topology, established by single-crystal X-ray diffraction. The N2 complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two TiIII centers covalently bridged by an N22- unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(TptBu,Me)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate TiII complex, namely, [(TptBu,Me)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated TiII-containing [(ΤptBu,Me)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear TiIV complexes of the type [(TptBu,Me)Ti═E(Cl)] (with E2- = NSiMe3, N2CPh2, O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N3SiMe3, N2CPh2, N2O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.

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.

Propene complexes of molybdenum and tungsten stabilized by intramolecular coordination of the 1-(quinol-8-yl)indenyl ligand

Synthesis, reactivity and properties of unusually stable propene complexes of molybdenum and tungsten are reported.

Hydrogenation of titanocene and zirconocene bis(trimethylsilyl)acetylene complexes

Reactions following the addition of dihydrogen under maximum atmospheric pressure to bis(trimethylsilyl)acetylene (BTMSA) complexes of titanocenes, [(η5-C5H5-nMen)2Ti(η2-BTMSA)] (n = 0, 1, 3, and 4) (1A-1D), and zirconocenes, [(η5-C5H5-nMen)2Zr(η2-BTMSA)] (n = 2-5) (4A-4D), proceeded in diverse ways and, depending on the metal, afforded different products. The former complexes lost, in all cases, their BTMSA ligand via its hydrogenation to bis-1,2-(trimethylsilyl)ethane when reacted at 80 °C for a prolonged reaction time. For n = 0, 1, and 3, the titanocene species formed in situ dimerised via the formation of fulvalene ligands and two bridging hydride ligands, giving known green dimeric titanocenes (2A-2C). For n = 4, a titanocene hydride [(η5-C5HMe4)2TiH] (2D) was formed, similarly to the known [(η5-C5Me5)2TiH] (2E) for n = 5; however, in contrast to this example, 2D in the absence of dihydrogen spontaneously dehydrogenated to the known Ti(iii)-Ti(iii) dehydro-dimer [{Ti(η5-C5HMe4)(μ-η1:η5-C5Me4)}2] (3B). This complex has now been fully characterised via spectroscopic methods, and was shown through EPR spectroscopy to attain an intramolecular electronic triplet state. The zirconocene-BTMSA complexes 4A-4D reacted uniformly with one hydrogen molecule to give Zr(iv) zirconocene hydride alkenyls, [(η5-C5H5-nMen)2ZrH{C(SiMe3)[double bond, length as m-dash]CH(SiMe3)}] (n = 2-5) (5A-5D). These were identified through their 1H and 13C NMR spectra, which show features typical of an agostically bonded proton, [double bond, length as m-dash]CH(SiMe3). Compounds 5A-5D formed equilibria with the BTMSA complexes 4A-4D depending on hydrogen pressure and temperature.

Decamethyltitanocene hydride intermediates in the hydrogenation of the corresponding titanocene-(η2-ethene) or (η2-alkyne) complexes and the effects of bulkier auxiliary ligands

¹H NMR studies of reactions of titanocene [Cp*₂Ti] (Cp* = η⁵-C₅Me₅) and its derivatives [Cp*(η⁵:η¹-C₅Me₄CH₂)TiMe] and [Cp*₂Ti(η²-CH₂[double bond, length as m-dash]CH₂)] with excess dihydrogen at room temperature and pressures lower than 1 bar revealed the formation of dihydride [Cp*₂TiH₂] (1) and the concurrent liberation of either methane or ethane, depending on the organometallic reactant. The subsequent slow decay of 1 yielding [Cp*₂TiH] (2) was mediated by titanocene formed in situ and controlled by hydrogen pressure. The crystalline products obtained by evaporating a hexane solution of fresh [Cp*₂Ti] in the presence of hydrogen contained crystals having either two independent molecules of 1 in the asymmetric part of the unit cell or cocrystals consisting of 1 and [Cp*₂Ti] in a 2 : 1 ratio. Hydrogenation of alkyne complexes [Cp*₂Ti(η²-R¹C[triple bond, length as m-dash]CR²)] (R¹ = R² = Me or Et) performed at room temperature afforded alkanes R¹CH₂CH₂R², and after removing hydrogen, 2 was formed in quantitative yields. For alkyne complexes containing bulkier substituent(s) R¹ = Me or Ph, R² = SiMe₃, and R¹ = R² = Ph or SiMe₃, successful hydrogenation required the application of increased temperatures (70-80 °C) and prolonged reaction times, in particular for bis(trimethylsilyl)acetylene. Under these conditions, no transient 1 was detected during the formation of 2. The bulkier auxiliary ligands η⁵-C₅Me₄^tBu and η⁵-C₅Me₄SiMe₃ did not hinder the addition of dihydrogen to the corresponding titanocenes [(η⁵-C₅Me₄^tBu)₂Ti] and [(η⁵-C₅Me₄SiMe₃)₂Ti] yielding [(η⁵-C₅Me₄^tBu)₂TiH₂] (3) and [(η⁵-C₅Me₄SiMe₃)₂TiH₂] (4), respectively. In contrast to 1, the dihydride 4 did not decay with the formation of titanocene monohydride, but dissociated to titanocene upon dihydrogen removal. The monohydrides [(η⁵-C₅Me₄^tBu)₂TiH] (5) and [(η⁵-C₅Me₄SiMe₃)₂TiH] (6) were obtained by insertion of dihydrogen into the intramolecular titanium-methylene σ-bond in compounds [(η⁵-C₅Me₄^tBu)(η⁵:η¹-C₅Me₄CMe₂CH₂)Ti] and [(η⁵-C₅Me₄SiMe₃)(η⁵:η¹-C₅Me₄SiMe₂CH₂)Ti], respectively. The steric influence of the auxiliary ligands became clear from the nature of the products obtained by reacting 5 and 6 with butadiene. They appeared to be the exclusively σ-bonded η¹-but-2-enyl titanocenes (7) and (8), instead of the common π-bonded derivatives formed for the sterically less congested titanocenes, including [Cp*₂Ti(η³-(1-methylallyl))] (9). The molecular structure optimized by DFT for compound 1 acquired a distinctly lower total energy than the analogously optimized complex with a coordinated dihydrogen [Cp*₂Ti(η²-H₂)]. The stabilization energies of binding the hydride ligands to the bent titanocenes were estimated from counterpoise computations; they showed a decrease in the order 1 (-132.70 kJ mol^-1), 3 (-121.11 kJ mol^-1), and 4 (-112.35 kJ mol^-1), in accordance with the more facile dihydrogen dissociation.

Synthesis, molecular and electronic structure of a stacked half-sandwich dititanium complex incorporating a cyclic π-faced bridging ligand

The thermally robust ground singlet state complex [bis(η⁵-pentamethylcyclopentadienyltitanium)-μ-(η⁴:η⁴-1,2,4,5-tetrakis(trimethylsilyl)cyclohexa-1-4-diene-3,6-diyl)] (3) arises from thermolysis of Cp*TiMe₃.

Displacement of ethene from the decamethyltitanocene-ethene complex with internal alkynes, substituent-dependent alkyne-to-allene rearrangement, and the electronic transition relevant to the back-bonding interaction

The titanocene-ethene complex [Ti(II)(η(2)-C2H4)(η(5)-C5Me5)2] (1) with simple internal alkynes R(1)C≡CR(2) gives complexes [Ti(II)(η(2)-R(1)C≡CR(2))(η(5)-C5Me5)2] {R(1), R(2): Ph, Ph (3), Ph, Me (4), Me, SiMe3 (5), Ph, SiMe3 (6), t-Bu, SiMe3 (7), and SiMe3, SiMe3 (8). In contrast, alkynes with R(1) = Me and R(2) = t-Bu or i-Pr afford allene complexes [Ti(II)(η(2)-CH2=C=CHR(2))(η(5)-C5Me5)2] (11) and (12), whereas for R(2) = Et a mixture of alkyne complex (13A) and minor allene (13) is obtained. Crystal structures of 4, 6, 7 and 11 have been determined; the latter structure proved the back-bonding interaction of the allene terminal double bond. Only the synthesis of 8 from 1 was inefficient because the equilibrium constant for the reaction [1] + [Me3SiC≡CSiMe3] ⇌ [8] + [C2H4] approached 1. Compound 9 (R(1), R(2): Me), not obtainable from 1, together with compounds 3–6 and 10 (R(1), R(2): Et) were also prepared by alkyne exchange with 8, however this reaction did not take place in attempts to obtain 7. Compounds 1 and 3–9 display the longest-wavelength electronic absorption band in the range 670-940 nm due to the HOMO → LUMO transition. The assignment of the first excitation to be of predominantly a b2 → a1 transition was confirmed by DFT calculations. The calculated first excitation energies for 3–9 followed the order of hypsochromic shifts of the absorption band relative to 8 that were induced by acetylene substituents: Me > Ph ≫ SiMe3. Computational results have also affirmed the back-bonding nature in the alkyne-to-metal coordination.