Stereoisomerism as an Origin of Different Reactivities of Ir(III) PC(sp³)P Pincer Catalysts

Abstract

Two stereoisomers of pentacoordinate iridium(III) hydridochloride with triptycene-based PC(sp 3 )P pincer ligand (1,8bis(diisopropylphosphino)triptycene), 1 and 2, differ by the orientation of hydride ligand relative to the bridgehead ring of triptycene. According to DFT/B3PW91/def2-TZVP calculations performed, an equatorial Cl ligand can relatively easily change its position in 1, whereas that is not the case in 2. Both complexes 1 and 2 readily bind the sixth ligand to protect the empty coordination site. Variable temperature spectroscopic (NMR, IR, and UV−visible) studies show the existence of two isomers of hexacoordinate complexes 1•MeCN, 2•MeCN, and 2•Py with acetonitrile or pyridine coordinated trans to hydride or trans to metalated C(sp 3 ), whereas only the equatorial isomer is found for 1• Py. These complexes are stabilized by various intramolecular noncovalent C−H•••Cl interactions that are affected by the rotation of isopropyls or pyridine. The substitution of MeCN by pyridine is slow yielding axial Py complexes as kinetic products and the equatorial Py complexes as thermodynamic products with faster reactions of 1•L. Ultimately, that explains the higher activity of 1 in the catalytic alkenes' isomerization observed for allylbenzene, 1-octene, and pent-4-enenitrile, which proceeds as an insertion/elimination sequence rather than through the allylic mechanism.

Regioselective Isomerization of Terminal Alkenes Catalyzed by a PC(sp3)Pincer Complex with a Hemilabile Pendant Arm

“…Catalysis . Initial alkene isomerization studies were conducted using five complexes reported by us in the past as catalysts for the transfer dehydrogenation of alkanes [17a,b] . Compounds 1 – 4 possess hemilabile coordinating alkoxyl groups: 1 – 3 are characterized by mer‐ configuration of the PC( sp 3 )ligand and a rigid coordination of the sidearm having a different steric bulk complex, whereas 4 is a more flexible analogue with fac ‐coordination of the ligand.…”

Chiral Iodotriptycenes: Synthesis and Catalytic Applications

“…Triptycenes as structures for catalysts or reagents in organic transformations have not been explored intensively. Some work highlights the use of triptycenes as a ligand/catalyst incorporating metals, mostly pincer‐type ligands/complexes in organic transformations such as alkenes [3] and 2‐methyl‐3‐butenenitrile isomerization, [4] chemoselective transfer‐hydrogenation of α,β‐unsaturated ketones, [5] transfer dehydrogenations of alkanes, [6] bis‐hydroformylation of butadiene, [7] selective hydrocyanation of butadiene, [8] cyanation of aryl bromides, [9] and cross‐coupling reactions of aryl chlorides with phenylboronic acid [10] . Very recently, triptycenyl methyl sulfide has been explored for electrophilic aromatic halogenations with N ‐halosuccinimides through the formation of sulfonium salt 1 as the active species (Figure 1a) [11] .…”

Synthesis and Reactivity of titanium ‘POCOP’ pincer complexes

“…Since their initial discovery by Shaw in 1976 pincer complexes have held a privileged position in organometallic chemistry. [1][2][3] Pincer complexes are widely applied in catalysis for example in iron catalysed hydrogenations, [4][5][6][7][8][9][10][11][12][13] iridium and ruthenium catalysed alkane dehydrogenation, [14][15][16][17][18][19][20][21][22][23][24][25][26][27] and cross-coupling reactivity. [28][29][30][31][32][33][34][35][36][37][38] Pincer ligands also support interesting reactivity, for example acting as a two-electron sink to augment reactivity.…”