Synthesis, Structure, Spectroscopy, and Reactivity of a Metallabenzene1

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 to produce the metallacyclohexadiene complex mer -CH C(Me)CH C(Me)CH 2 Ir(PEt 3 ) 3 (H) ( 1b ) via metal-centered C H bond activation. Treatment of 1b with methyl trifluoromethanesulfonate removes the hydride ligand, producing [CH C(Me)CH C(Me)CH 2 Ir(PEt 3 ) 3 ] + O 3 SCF 3 - ( 2 ). Finally, 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 shows the coordination geometry about iridium to be square pyramidal. The metallabenzene ring is nearly planar, and the ring π-bonding is delocalized. In the 1 H NMR spectrum of 3, the ring protons (H1/H5 and H3) are shifted downfield, consistent with the presence of an aromatic ring current. Compound 3 reacts with a variety of small 2e - ligands under 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 ligand replacements are observed. For example, treatment of 3 with 2 equiv 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 leads to carbonyl insertion and C C coupling, ultimately yielding (3,5-dimethylphenoxy)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 octahedral Ir(III) complexes ( 8a, 8b, and 9, respectively) in which the metallabenzene ring is retained. Compound 3 undergoes 4 + 2 cycloaddition reactions with electron-poor substrates, including O 2, nitrosobenzene, maleic anhydride, CS 2, and SO 2 . In each case, the cycloaddition substrate adds across iridium and C3 of the metallabenzene ring, producing octahedral products ( 10 − 14, respectively) with boat- shaped 1-iridacyclohexa-2,5-diene rings. In contrast, treatment of 3 with CO 2 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-iridacyclohexa-2,4-diene ring in a half-boat conformation. Finally, treatment of 3 with N 2 O results in ring contraction and production of an iridacyclopentadiene species ( 16 ). Compound 3 reacts with electrophiles at the electron-rich α ring carbons, C1/C5. Hence, treatment with 1 equiv of H + O 3 SCF 3 - regenerates compound 2, while treatment with 2 equiv of H + O 3 SCF 3 - produces [(η 5 -2,4-dimethylpentadienyl)Ir(PEt 3 ) 3 ] 2+ (O 3 SCF 3 - ) 2 ( 19 ). Treatment of 3 with excess BF 3 leads to the production of a novel (η 6 -borabenzene)iridium complex ( 20 ). This reaction apparently involves initial attack of BF 3 at ring carbon C5, followed by migration of ring carbon C1 to boron. Compound 3 displaces p -xylene from ( p -xylene)Mo(CO) 3 in tetrahydrofuran, generating the (η 6 -metallabenzene)metal complex [η 6 -CH C(Me)CH C(Me)CH Ir(PEt 3 ) 3 ]Mo(CO) 3 ( 21 ).