No abstract

The cationic tropylium [(PCP′)Ir(CO)(H)(Cl)] + complex (4) (PCP′ ) [2,7-(CH 2 P t Bu 2 ) 2 C 7 H 4 ] + ) is generated from the neutral cycloheptatriene pincer complex (PCP)Ir(CO)(H)(Cl) (1) (PCP ) 2,7-(CH 2 P t Bu 2 ) 2 C 7 H 5 ) by a hydride abstraction from the CH-Ir fragment with 1 equiv of trimethylsilyl trifluoromethanesulfonate. Interestingly, the tropylium ligand backbone in complex 4 is deprotonated by a base to give the neutral Ir(III) compound (PCP′′)Ir(CO)(H)-(Cl) (5) (PCP′′ ) 2-(CHP t Bu 2 )-7-(CH 2 P t Bu 2 )C 7 H 4 ) with a π-system that extends into one of the phosphine bridges. Finally treatment of 5 with a further equivalent of a base removes HCl from the iridium center, forming the Ir(I) complex 6 with the same ligand backbone as in 5. The HCl elimination and the deprotonation reactions are reversible. Thus addition of 2 equiv of HCl to 6 gives at first 5 then the tropylium complex 7, which differs from 4 only by the counterion. The seven-membered aromatic tropylium system in 4 coordinates to the Mo(CO) 3 fragment, generating the bimetallic complex [(CO) 3 Mo(η 7 -PCP′)Ir(CO)(H)(Cl)] + (8). Quantum chemical calculations at various levels of theory illustrated the relative energetic stabilities of all iridium complexes.

When the cycloheptatriene iridium(iii) pincer complex (PCP)Ir(CO)(H)(Cl) (3) (PCP = 2,7-(CH(2)P(t)Bu(2))(2)C(7)H(5)) is treated with the bases NaH, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and lithium 2,2,6,6-tetramethylpiperidide (LiTMP) under various conditions different products are obtained. At elevated temperatures and with DBU or LiTMP as a base the trans dihydride (PCP')Ir(CO)(H)(2) (PCP' = 2-(CHP(t)Bu(2))-7-(CH(2)P(t)Bu(2))C(7)H(4)) (5) is formed where the pi-system extends into one of the phosphine bridges. This compound loses H(2) to give the square-planar iridium(I) carbonyl complex (PCP'IrCO). The dihydride 5 can also rearrange to the new isomeric iridium(I) carbonyl 6 (PCP''IrCO, PCP'' = 2,7-(CH(2)P(t)Bu(2))(2)C(7)H(5)). Thus the two hydrides have moved into the ligand backbone creating a methylene group in the 3-position of the cycloheptatriene ring. Alternatively, 6 is formed by a rearrangement from 6a which differs from 6 by having the methylene group in the 4-position of the cycloheptatriene ring. The iridium(I) carbonyl 6a in turn is made from 3 by treatment with DBU at room temperature. Interestingly, when compound is heated to reflux in THF the hydrogen bound at the metal carbon is shifted to a carbon atom in the cycloheptatriene ring generating a ring methylene group (3a). From this complex HCl is eliminated upon chromatography forming 6 as the final product. Quantum chemical calculations at various levels of theory illustrate the relative energetic stabilities of all iridium complexes.