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Synthesis, Structure, Spectroscopy, and Reactivity of a Metallabenzene 1

1997; American Chemical Society; Volume: 16; Issue: 4 Linguagem: Inglês

10.1021/om961012p

1520-6041

John R. Bleeke, Robert Behm, Yun-Feng Xie, Michael Y. Chiang, Kerry D. Robinson, A.M. Beatty,

Coordination Chemistry and Organometallics

A rare example of a stable metallabenzene complex has been synthesized in three high-yield steps from (Cl)Ir(PEt3)3. In the first step, (Cl)Ir(PEt3)3 is treated with potassium 2,4-dimethylpentadienide to produce the metallacyclohexadiene complex mer-CHC(Me)CHC(Me)CH2Ir(PEt3)3(H) (1b) via metal-centered CH bond activation. Treatment of 1b with methyl trifluoromethanesulfonate removes the hydride ligand, producing [CHC(Me)CHC(Me)CH2Ir(PEt3)3]+O3SCF3- (2). Finally, deprotonation of 2 with base yields the metallabenzene complex CHC(Me)CHC(Me)CHIr(PEt3)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 1H 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, CHC(Me)CHC(Me)CHIr(PEt3)2L (4a, L = PMe3; 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 PEt3 ligand replacements are observed. For example, treatment of 3 with 2 equiv of PMe3 or P(OMe)3 in toluene under reflux produces CHC(Me)CHC(Me)CHIr(PEt3)L2 (5a, L = PMe3; 5b, L = P(OMe)3). Treatment of 3 with excess PMe3 in toluene under reflux produces the tris-PMe3 substitution product (6), while similar treatment with excess CO leads to carbonyl insertion and CC coupling, ultimately yielding (3,5-dimethylphenoxy)Ir(PEt3)2(CO) (7). Treatment of compound 3 with I2, Br2, 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 O2, nitrosobenzene, maleic anhydride, CS2, and SO2. 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 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-iridacyclohexa-2,4-diene ring in a half-boat conformation. Finally, treatment of 3 with N2O 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+O3SCF3- regenerates compound 2, while treatment with 2 equiv of H+O3SCF3- produces [(η5-2,4-dimethylpentadienyl)Ir(PEt3)3]2+(O3SCF3-)2 (19). Treatment of 3 with excess BF3 leads to the production of a novel (η6-borabenzene)iridium complex (20). This reaction apparently involves initial attack of BF3 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 (η6-metallabenzene)metal complex [η6-CHC(Me)CHC(Me)CHIr(PEt3)3]Mo(CO)3 (21).