Abstract
A series of Pd(II) aryl complexes of a PNO-type pincer ligand bearing a weakly coordinated benzofuran side arm has been prepared. Crystallographic studies showed very long Pd–O distances of more than 2.3 Å, which is significantly longer than the Pd–O distances in structurally similar PNO-Pd complexes with an exocylic oxygen donor. Crystallographic and 19F NMR solution studies of complexes containing electron-donating (OMe) and electron-withdrawing (CF3) substituents in the position para to the benzofuran oxygen atom revealed the dependence of the Pd–O interactions on the nature of the aromatic group at the Pd center. The ability to influence these interactions by changing the electron density at the metal was demonstrated in the stoichiometric Sonogashira-type cross-coupling reactions between the Pd complexes and phenylacetylene, which proceed via a reversible aromatization/dearomatization of the pincer ligand. Complexes with the electron-poor CF3 group showed higher reactivity in comparison to their electron-neutral or -rich analogues. DFT studies of these systems provided further mechanistic insight into the origin of the observed reactivity patterns.
Highlights
Transition-metal pincer complexes have long become ubiquitous in organometallic chemistry.[1]
While the early studies of this family of compounds focused on the ligand coordination and bond activation reactions,[2] much of the more recent emphasis has been placed on their catalytic properties.[3]
We introduced the first palladium pincer system (1) that utilizes an aromatic fluorine atom as a side arm, which serves as a stabilizing “reluctant” donor (Scheme 1d).[14−16] The new system was highly reactive in activating E−H bonds (E= C, N, O, S) and, in several cases, Aryl−E (E = C, S) reductive elimination, which was successfully exploited in catalysis.[17]
Summary
Transition-metal pincer complexes have long become ubiquitous in organometallic chemistry.[1]. While the early studies of this family of compounds focused on the ligand coordination and bond activation reactions,[2] much of the more recent emphasis has been placed on their catalytic properties.[3] A interesting research area that rose meteorically in the past decade and a half involves systems with a cooperative mode of bond activation between the transition metal and the aromatic core of the pincer ligand In such systems, the metal center does not change its oxidation state as the ligand undergoes a reversible aromatization−dearomatization reaction (Scheme 1).[4] This bond activation approach, pioneered by the Milstein group,[5] has been highly efficient in the catalytic functionalization of a great variety of substrates bearing E−H bonds (E = N, O).[6,7] An important feature of many, albeit not all, of these catalytic transformations is the hemilabile behavior of the amine side arm in the PNN-type pincer systems (L = P, L′ = N; Scheme 1a). In addition to the scientific value of discovering a new reaction mechanism for extremely important and well-established catalytic transformations,[9] the cooperative cleavage of the E−
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