Synthesis and group 9 complexes of macrocyclic PCP and POCOP pincer ligands

Abstract

A new dimension in pincer chemistry: introducing phosphine-based macrocyclic pincer ligands PCP-14 and POCOP-14.

Oxidative Addition of a Mechanically Entrapped C(sp)–C(sp) Bond to a Rhodium(I) Pincer Complex

“…The synthesis of 1 was achieved by reaction of trans ‐[Rh(POCOP‐14)Cl 2 (CO)] [9] with Ar′C 2 MgCl⋅LiCl and Me 3 NO promoted decarbonylation of the resulting bis(alkynyl)carbonyl complex 3 (Scheme 2, insert) [11] . The five coordinate intermediate 2 invoked in the latter step was not observed when the reaction was monitored in situ by NMR spectroscopy and in the absence of a decarbonylation agent 3 is thermally stable under an Ar atmosphere (363 K, 16 h, [D 8 ]toluene) [12] .…”

Oxidative Addition of a Mechanically Entrapped C(sp)–C(sp) Bond to a Rhodium(I) Pincer Complex

“…Further supporting the equilibrium formation of 2, heating a solution of 1 in [D 8 ]toluene under H 2 (1 atm) at 358 K resulted in formation of dihydrogen complex 7 (d 31P 198.3, 1 J RhC = 165 Hz), [9] with concomitant generation of two equivalents of Ar'CH 2 CH 3 (24 h, Scheme 3). This outcome is fully consistent with a reaction sequence involving rate-determining insertion of the metal into the C(sp)-C(sp) bond followed by hydrogenolysis, and notably occurs with a similar rate to the formation of 3 from 1 in the presence of excess CO at 358 K (t 1/2 % 4.5 h).…”

“…[5] In these isolated examples the substrate substituents play a decisive role and attempts to replicate this reactivity using diphenylbutadiyne proved unsuccessful. [6] Inspired by reports of unique metal-based reactivity using interlocked ligands [7,8] and as part of our research exploring the organometallic chemistry of macrocyclic pincer complexes, [9,10] we speculated that mechanical entrapment could be exploited to overcome the unfavorable kinetics associated with C(sp)-C(sp) bond oxidative addition relative to substitution of the diyne, and enable onward reactivity of the resulting bis(alkynyl) products to be explored. We herein describe work evaluating this hypothesis using rhodium complex 1, which features a macrocyclic phosphinite pincer ligand (POCOP-14) [9] and bulky aryl terminated 1,3-diyne (Ar'C 4 Ar', Ar' = 3,5-tBu 2 C 6 H 3 ; Scheme 1).…”

“…The synthesis of 1 was achieved by reaction of trans-[Rh(POCOP-14)Cl 2 (CO)] [9] with Ar'C 2 MgCl•LiCl and Me 3 NO promoted decarbonylation of the resulting bis-(alkynyl)carbonyl complex 3 (Scheme 2, insert). [11] The five coordinate intermediate 2 invoked in the latter step was not observed when the reaction was monitored in situ by NMR spectroscopy and in the absence of a decarbonylation agent 3 is thermally stable under an Ar atmosphere (363 K, 16 h, [D 8 ]toluene).…”

“…[20] The remarkable reactivity of 1 presented herein can be reconciled by a pronounced "catenand effect", the name given by Sauvage for the augmentation of a metal ions reactivity due to confinement within a threaded structure, [7] and this conclusion is substantiated by comparison to the electronically similar acyclic congener 5 (Scheme 4). [9,18] Insertion of rhodium into the C(sp)-C(sp) bond of Ar'C 4 Ar' in 1 is a reversible but thermodynamically uphill reaction that is associated with a formidable activation barrier of DG°2 98K = 120 AE 3 kJ mol À1 . At suitably high temperatures, however, the resulting bis(alkynyl) 2 is formed under equilibrium and can be trapped out with CO or intercepted through irreversible reaction with H 2 , conferring overall exergonic processes.…”