SYNTHESIS, STRUCTURE, SPECTROSCOPY, AND REACTIVITY OF A METALLABENZENE

A rare example of a stable metallabenzene complex has been synthesized in three high-yield steps from (Cl)Ir(PEt(3))(3). In the first step, (Cl)Ir(PEt(3))(3) is treated with potassium 2,4-dimethylpentadienide t o produce the metallacyclohexadiene complex mer-CH=C(Me)CH=C(Me)CH2Ir( PEt(3))(3)(H) (1b) via metal-centered C-H bond activation. Treatment o f 1b with methyl trifluoromethanesulfonate removes the hydride ligand, producing [CH=C(Me)CH=C(Me)CH2Ir(PEt(3))(3)](O3SCF3-)-O-+ (2). Finall y, deprotonation of 2 with base yields the metallabenzene complex CH=C (Me)CH=C(Me)CH=Ir(PEt(3))(3) (3). The X-ray crystal structure of 3 sho ws the coordination geometry about iridium to be square pyramidal. The metallabenzene ring is nearly planar, and the ring pi-bonding is delo calized. In the H-1 NMR spectrum of 3, the ring protons (H1/H5 and H3) are shifted downfield, consistent with the presence of an aromatic ri ng current. Compound 3 reads with a variety of small 2e(-) ligands und er mild conditions to produce monosubstituted metallabenzenes, CH=C(Me ) CH=C(Me)CH=Ir(PEt(3))(2)L (4a, L = PMe(3); 4b, L = P(OMe)(3); 4c, L = CO), in which the unique Ligand L resides preferentially in a basal coordination site. Under more forcing conditions, additional PEt(3) li gand replacements are observed. For example, treatment of 3 with 2 equ iv of PMe(3) or P(OMe)(3) in toluene under reflux produces CH=C(Me)CH= C(Me)CH=Ir(PEt(3))L(2) (5a, L = PMe(3); 5b, L = P(OMe)(3)). Treatment of 3 with excess PMe(3) in toluene under reflux produces the tris-PMe( 3) substitution product (6), while similar treatment with excess CO le ads to carbonyl insertion and C-C coupling, ultimately yielding (3,5-d imethylphenoxy)-Ir(PEt(3))(2)(CO) (7). Treatment of compound 3 with I- 2, Br-2, or Ag+/NCMe results in oxidation, and the production of octah edral Ir(III) complexes (8a, 8b, and 9, respectively) in which the met allabenzene ring is retained. Compound 3 undergoes 4 + 2 cycloaddition reactions with electron-poor substrates, including O-2, nitrosobenzen e, maleic anhydride, CS2, and SO2. In each case, the cycloaddition sub strate adds across iridium and C3 of the metallabenzene ring, producin g octahedral products (10-14, respectively) with boat-shaped 1-iridacy clohexa-2,5-diene rings. In contrast, treatment of 3 with CO2 leads to a 2 + 2 cycloaddition reaction in which the substrate adds across the Ir-C5 bond. The resulting octahedral adduct (15) contains a 1-iridacy clohexa-2,4-diene ring in a half-boat conformation. Finally, treatment of 3 with N2O results in ring contraction and production of an iridac yclopentadiene species (16). Compound 3 reacts with electrophiles at t he electron-rich alpha ring carbons, C1/C5. Hence, treatment with 1 eq uiv of H+O3SCF3- regenerates compound 2, while treatment with 2 equiv of H+O3SCF3- produces dimethylpentadienyl)Ir(PEt(3))(3)](2+)(O3SCF3-)( 2) (19). Treatment of 3 with excess BF3 leads to the production of a n ovel (eta(6)-borabenzene)iridium complex (20). This reaction apparentl y involves initial attack of BF3 at ring carbon C5, followed by migrat ion of ring carbon C1 to boron. Compound 3 displaces p-xylene from (p- xylene)-Mo(CO)(3) in tetrahydrofuran, generating the (eta(6)-metallabe nzene)metal complex [eta(6)CH=C(Me)CH=C(Me)CH=Ir(PEt(3))(3)]Mo(CO)(3) (21).