Functionalization of Complexed N₂O in Bis(pentamethylcyclopentadienyl) Systems of Zirconium and Titanium

Abstract

Methyl triflate reacts with the metastable azoxymetallacyclopentene complex Cp*2Zr(N(O)NCPhCPh), generated in situ from nitrous oxide insertion into the Zr–C bond of Cp*2Zr(η2-PhCCPh) at −78 °C, to afford the salt [Cp*2Zr(N(O)N(Me)CPhCPh)][O3SCF3] (1) in 48% isolated yield. A single-crystal X-ray structure of 1 features a planar azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen position of the former Zr(N(O)NCPhCPh) scaffold. In addition to 1, the methoxy-triflato complex Cp*2Zr(OMe)(O3SCF3) (2) was also isolated from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of Cp*2Zr=O(NC5H5) with MeO3SCF3. Complex 2 could also be observed, spectroscopically, from the thermolysis of 1 (80 °C, 2 days). In contrast to Cp*2Zr(N(O)NPhCCPh), the more stable titanium N2O-inserted analogue, Cp*2Ti(N(O)NCPhCPh), reacts with MeO3SCF3 to afford a 1:1 mixture of regioisomeric salts, [Cp*2Ti(N(O)N(Me)CPhCPh)][O3SCF3] (3) and [Cp*2Ti(N(OMe)NCPhCPh)][O3SCF3] (4), in a combined 65% isolated yield. Single-crystal X-ray diffraction studies of a cocrystal of 3 and 4 show a 1:1 mixture of azoxymetallacyle salts resulting from methyl alkylation at both the β-nitrogen and the β-oxygen of the former Ti(N(O)NCPhCPh ring. As opposed to alkylation reactions, the one-electron reduction of Cp*2Ti(N(O)NCPhCPh) with KC8, followed by encapsulation with the cryptand 2,2,2-Kryptofix, resulted in the isolation of the discrete radical anion [K(2,2,2-Kryptofix)][Cp*2Ti(N(O)NCPhCPh)] (5) in 68% yield. Complex 5 was studied by single-crystal X-ray diffraction, and its solution X-band EPR spectrum suggested a nonbonding σ-type wedge hybrid orbital on titanium, d(z2)/d(x2–y2), houses the unpaired electron, without perturbing the azoxymetallacycle core in Cp*2Ti(N(O)NCPhCPh). Theoretical studies of Ti and the Zr analogue are also presented and discussed.

Synthetic chemistry with nitrous oxide

“…The diphenylacetylene complexes of permethyltitanocene and zirconocene react with N 2 O to give azoxymetallacyclopentene complexes (Scheme 14a). 93 Some organometallic samarium complexes are also able to insert N 2 O into metal-carbon bonds. The carbon-metal bond of the latter can be cleaved with a variety of substrates processing acidic protons.…”

“…Recently, it was shown that the labile azoxymetallacyclopentene complex can be trapped by reactions with MeO 3 SCF 3 (alkylation of the b-N-atom). 93 The reaction product can be isolated and is stable as a solid if stored at À35 1C. In case of Ti, the initial N 2 O adduct is more stable and can be used for further reactions.…”

“…Apart from an alkylation reaction with MeO 3 SCF 3 (O-and b-N-alkylation), it was shown that the complex can be reduced to give a stable radical anion. 93 Scheme 12 Reaction of organometallic compounds of the alkali and the alkaline-earth metals with N 2 O.…”

Synthesis and Characterisation of Bridged Titanocene Oxido Complexes and Their Reactions with Water

“…A very selective and general method of oxidising a titanocene centre was published by Andersen and co‐workers, who found that gaseous nitrous oxide is capable of introducing terminal oxido units into permethylated titanocenes [Cp* 2 Ti] 7. Earlier, this principle was also applied for other metallocenes of group 4–6 metals 8,9. Hence, we used this procedure for the synthesis of bridged titanocene oxido complexes that are potential starting materials for the generation of the corresponding dihydroxido species.…”

Steric and Electronic Influences of Internal Alkynes on the Formation of Thorium Metallacycles: A Combined Experimental and Computational Study

“…Metallacycles have rapidly emerged from laboratory curiosities to important intermediates in catalytic processes and in organic and material synthesis. − In this context metallacyclopropenes and metallacyclopentadienes of group 4 metallocenes are readily accessible from the reaction of Cp′ 2 M (Cp′ = (un)­substituted η 5 -cyclopentadienyl) with alkynes or from the reduction of Cp′ 2 MCl 2 in the presence of a suitable alkyne and the resulting metallacycles exhibit unusual intrinsic reactivity. , However, precise stoichiometric control of the added alkyne is usually a prerequisite to prevent the formation of metallacyclopentadienes and thereby to isolate the more strained metallacyclopropenes. However, in contrast to the plethora of group 4 metallacycles, , the chemistry of the corresponding actinide derivatives is underdeveloped . Therefore, thorium, which adopts an intermediate position between group 4 and actinide metals, represents an interesting synthetic target for further investigations, since its electronic ground state of 7s 2 6d 2 relates thorium to the group 4 elements.…”