A Measureable Equilibrium between Iridium Hydride Alkylidene and Iridium Hydride Alkene Isomers

Abstract

Dedicated to Professor H. Werner on his 70th birthdayRearrangement between isomeric M-olefin and M-alkylidene units (in which M is a transition-metal atom) is a fundamental transformation key to many homogeneous and heterogeneous catalytic processes.[1] However, whereas conversion of a metal alkylidene into the corresponding metal alkene has been known for some time, particularly for cationic electrophilic metal alkylidenes, [2,3] the reverse transformation is a rarely observed process, known only for a few transition-metal complexes that have few d electrons.[4] Moreover, detection of a thermoneutral (or almost thermoneutral) redistribution of this kind, that is, a process whereby the alkene and the alkylidene isomers exist in equilibrium, finds little precedent and appears to be limited to some complexes of the early transition metals.[5] We report herein that a d 6 Tp Me 2 Ir III fragment allows attainment of an equilibrium mixture of the hydride alkylidene and hydride alkene structures 1 and 2 a (Scheme 1), through a reaction that implies the activation of sp 3 CÀH bonds of o-C 6 H 4 (OH)CH 2 CH 3 . The thermal reaction of TpMe 2 Ir(C 6 H 5 ) 2 (N 2 ) [6a] and 2-ethylphenol (ca. 1:2.5 molar ratio, cyclohexane solvent, 60 8C) produces the hydride alkylidene 1 (Scheme 1) as the main product (95 % conversion into products as determined by NMR spectroscopy; 90 % yield of isolated 1). Presumably, the reaction starts with alcoholysis of one of the IrÀC 6 H 5 bonds (structure A, Scheme 1), and this is then followed by a facile, double activation [6,7] of the methylenic CÀH bonds of the ortho ethyl substituent. The CÀH bond cleavage is highly regioselective. Monitoring by 1 H NMR spectroscopy reveals that under kinetic control, the alternative reaction product, namely the hydride alkene isomer 2 a that derives from b-as opposed to a-hydride elimination in the last CÀH activation step, amounts to only about 5 % of the total reaction product (IrÀH signals at d = À20.40 for 1 and À17.49 ppm for 2 a, with intensity ratio of ca. 100:5

Activation of Small Molecules by the Metal–Amido Bond of Rhodium(III) and Iridium(III) (η5-C5Me5)M-Aminopyridinate Complexes

“…Figure shows the solid-state structure of the molecules of this complex, corroborating the formulation anticipated in Scheme on the basis of spectroscopic data. The Ir1–C20 bond to the carbene carbon atom has a length of 1.911(12) Å, very similar to the distances found for other cationic and neutral carbene complexes of iridium­(III) reported by our group. − The Ir1–N1 bond to the pyridine ring has a distance of 2.113(8) Å, which is also comparable to the Ir–N pyridine distances found in related complexes described in this paper or previously. , …”

Tautomerisation of 2‐Substituted Pyridines to N‐Heterocyclic Carbene Ligands Induced by the 16 e− Unsaturated [TpMe2IrIII(C6H5)2] Moiety

“…The applicability of the latter in related systems has been previously tested. [11,46,58] The three functionals yield qualitatively similar results, although BHandH predicts somewhat higher energies for intermediates and transition states (see supporting information). Frequency calculations at the same levels of theory were used to characterise all minima and transition states.…”

Formation of a Delocalized Iridium Benzylidene with Azaquinone Methide Character via Alkoxycarbene Cleavage

“…This isomerization presumably proceeds through initial tautomerization of the alkylidene to the olefin, 31−34 a reversible process in related iridium systems. 35 The allyl complex 6b appears to be a thermodynamic product of isomerization that does not revert on treatment with pyridine. The parent compound 5a has not been observed to undergo the same rearrangement on standing at room temperature or during attempts at crystallization, presumably as a result of kinetic factors.…”

“…Attempts to crystallize 5b exclusively gave pure samples of 6b , which we have characterized as the η 3 -allyl resulting from isomerization of 5b (Figure ). This isomerization presumably proceeds through initial tautomerization of the alkylidene to the olefin, − a reversible process in related iridium systems …”