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...
Iridacyclohexadienone complexes, a new class of unsaturated six-membered metallacycles, have been synthesized, and their reactions with acids have been investigated. Treatment (“iridabenzene” 1) with nitrous oxide produces the iridacyclohexadienone which upon stirring in diethyl ether solution slowly isomerizes to (1,2,5-η-2,4-dimethylpenta-1,3-dien-5-oyl)Ir(PMe3)3 (3). Treatment of 2 with methyl trifluoromethanesulfonate leads to removal of the metal hydride and production In acetone solution, 4 forms an equilibrium mixture with a ring-contracted iridacyclopentadiene Treatment of 4 with trimethylphosphine generates the tetrakis(trimethylphosphine)iridacyclohexadienone Iridacyclohexadienone 2 reacts with trifluoromethanesulfonic acid to produce the iridacyclopentene In contrast, treatment of iridacyclohexadienones 4 and 6 with trifluoromethanesulfonic acid leads to protonation of oxygen and production of the corresponding “iridaphenols” 8 and 10. When 4 is treated with trifluoroacetic acid, an iridaphenol product is again generated (compound 9), but in this case the phenol proton forms an intramolecular hydrogen bond with the carbonyl oxygen of the trifluoroacetate ligand. The ring protons in compounds 8, 9, and 10 exhibit downfield 1H NMR chemical shifts consistent with aromatic character. Compounds 3, 4, 6, 7, 8, and 9 have been structurally characterized by single-crystal X-ray diffraction.
Treatment of (Cl)Ir(PEt3)3 with lithium 2,3-dimethyl-5-thiapentadienide leads to the production of (4) via C−H bond activation. Oxidation of 4 with silver tetrafluoroborate in tetrahydrofuran generates “iridathiabenzene”, (3). The structural and spectroscopic features of 3 are consistent with the presence of an aromatic ring in which the iridium center participates in ring π-bonding. Treatment of 3 with excess PMe3 or with PPN+Cl- leads to the production of (5) or (6), respectively. Each of these products features an iridathiacyclohexa-1,3-diene ring system. The reaction of 6 with 1/2 equiv of silver trifluoromethanesulfonate leads to the production of a novel iridium dimer, (7), in which the two iridium centers are bridged by the two sulfur atoms of the iridathiacyclohexa-1,3-diene rings, as well as a chloride ligand. Treatment of 3 with nitrosobenzene generates a [4 + 2] cycloadduct, (8), containing an iridathiacyclohexa-1,4-diene ring. Compound 3 cleanly displaces p-xylene from (η6-p-xylene)Mo(CO)3 in tetrahydrofuran, generating (9). When 9 is reacted with excess trimethylphosphine, PMe3 adds to the molybdenum center, causing the iridathiabenzene ring to slip from η6 to η4 coordination and forming (10). Finally, treatment of (η5-C5Me5)Ru(NCMe)3 +O3SCF3 - with 3 leads to clean displacement of the acetonitrile ligands by the iridathiabenzene ring and generation of the Ru sandwich compound (11). Compounds 3, 4, 6a, 7, 9, and 10 have been structurally characterized by X-ray diffraction.
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