Slicing the π in Three Unequal Pieces: Iridium Complexes with Alkyne, Iminoxolene, and Dioxolene Ligands

Abstract

The formally isoelectronic alkyne, iminoquinone, and benzoquinone ligands can all exhibit significant metal−ligand π bonding. Condensation of 2,6-bis(triisopropylsilylethynyl)aniline with 3,5-di-tert-butyl-1,2-benzoquinone affords a ligand with two such moieties, the alkyne-containing iminoquinone Tipsi. All three groups are installed around a single iridium center by successive treatment of [(coe) 2 IrCl] 2 with Tipsi and 3,5-di-tert-butyl-1,2benzoquinone to give (κ 2 ,η 2 -Tipsi)(3,5-t Bu 2 Cat)IrCl in two isomeric forms with cis iminoxolene and dioxolene groups and a bound alkyne. Structural, spectroscopic, and computational data indicate that the dioxolene and especially the iminoxolene are strongly engaged in π bonding but that the alkyne is not an effective π donor or acceptor. Pyridine displaces the bound alkyne in (κ 2 ,η 2 -Tipsi)(3,5-t Bu 2 Cat)IrCl to give two isomers of (κ 2 -Tipsi)(3,5-t Bu 2 Cat)Ir(py)Cl, both with the iminoxolene and dioxolene trans to each other. The reaction rates and modest stereospecificity of the dissociative substitution reaction are consistent with a mechanism with competitive, stereochemically distinct, pathways for forming square pyramidal intermediates, which rapidly isomerize to the trans isomer before irreversible trapping by pyridine. The five-coordinate intermediates funnel down to the trans isomer because it is stabilized by an additional metal−ligand π interaction that is not possible in the cis isomers.

Modulation of Isomerization and Ligand Exchange Rates by π Bonding in Bis(iminoxolene)iridium Pyridine Complexes

“…DFT calculations strongly support the latter notion, with the more hindered cis -(diso) 2 IrCl computed to have an isomerization barrier of 11.4 kcal mol –1 , much higher than the barrier for cis -(ap) 2 IrCl of 4.6 kcal mol –1 (Figure ). The latter value is in line with the calculated barriers of 1.3 to 8.7 kcal mol –1 for stereoisomerization of the various isomers of cis -(Tipsi)­(cat)­IrCl, which are also relatively unhindered around the metal center.…”

“…(Note that while the calculations on (diso) 2 Ir­(py)Cl accurately reproduce the relative binding of pyridine to the cis and trans isomers, they underestimate the absolute binding strength of pyridine by about 18 kcal mol –1 . The qualitative underestimation of pyridine binding has been observed previously in calculations on mixed iminoxolene-dioxolene iridium systems …”

“…When combined with the thermodynamics of the binding equilibrium measured by NMR (eq ), one can determine the activation parameters for pyridine dissociation from trans -(Diso) 2 Ir­(py)Cl as Δ H ⧧ = 25.7(6) kcal mol –1 and Δ S ⧧ = 20.4(19) cal mol –1 K –1 . This corresponds to a dissociation rate constant k –1 of 0.025 s –1 ( t 1/2 = 27 s) at 298 K. The rather facile dissociation of pyridine from the bis­(iminoxolene) complex contrasts with the ∼5 × 10 4 times slower dissociation from the analogous mono-iminoxolene mono-dioxolene complexes trans -(Tipsi)­(3,5- t Bu 2 Cat)­Ir­(py)Cl (Tipsi = N -(2,6-bis­(triisopropylsilylethynyl)­phenyl)-4,6-di- tert -butyl-2-imino- o -benzoquinone), which exchange with deuterated pyridine with rate constants of about 6 × 10 –5 s –1 (depending on the orientation of the catecholate) at 60 °C . The arylimino group makes the iminoxolene a much stronger donor than the dioxolene, and the metal is correspondingly less Lewis acidic.…”

“…The greater stability of the five-coordinate species with apical chloride was also invoked in the ligand substitution chemistry of the mixed iminoxolene-dioxolene complex (η 2 ,κ 2 -Tipsi)­(3,5- t BuCat)­IrCl (Tipsi = N -(2,6-bis­(triisopropylsilylethynyl)­phenyl)-4,6-di- tert -butyl-2-imino- o -benzoquinone) . In the iminoxolene-dioxolene complex, however, the experimental observation is the exclusive formation of trans -(κ 2 -Tipsi)­(3,5- t BuCat)­Ir­(py)Cl on the addition of pyridine to the cis complex of the internal alkyne.…”

“…This secondary π interaction was observed to have noticeable effects on the optical spectrum of (Diso) 2 IrCl. The qualitative effects of this π interaction on ligand substitution were discussed in a recent study of a mixed iminoxolene-dioxolene complex (κ 2 ,η 2 -Tipsi)­(3,5- t Bu 2 Cat)­IrCl (Tipsi = N -(2,6-bis­(triisopropylsilylethynyl)­phenyl)-4,6-di- tert -butyl-2-imino- o -benzoquinone) . In this complex, displacement of the bound alkyne by pyridine results in stereoisomerization from cis to trans.…”