Synthesis, Structure, Spectroscopy, and Reactivity of a Metallabenzene¹

Abstract

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-CHC(Me)CHC(Me)CH2Ir(PEt3)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)CH2Ir(PEt3)3]+O3SCF3 - (2). Finally, deprotonation of 2 with base yields the metallabenzene complex CHC(Me)CHC(Me)CHIr(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, CHC(Me)CHC(Me)CHIr(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 CHC(Me)CHC(Me)CHIr(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 CC 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...

Recent Advances in Metallabenzene Chemistry

“…[22] In 1989, Bleeke et al reported the synthesis of the first stable iridabenzene 4 utilizing 2,4-dimethylpentadienide as a source for the carbon backbone. [17,29] Using the resultant iridabenzene as a starting point, the Bleeke group extensively examined the chemistry of such complexes. [17] When exposed to EAS conditions, it was found that the electrophiles react preferentially with the electron-rich metal center.…”

“…[17,29] Using the resultant iridabenzene as a starting point, the Bleeke group extensively examined the chemistry of such complexes. [17] When exposed to EAS conditions, it was found that the electrophiles react preferentially with the electron-rich metal center. Additionally, it was found that when exposed to dienophiles, these iridabenzenes behave more like cyclohexatrienes and undergo cycloaddition reactions.…”

“…[9,17] Recently, however, the first demonstration of electrophilic aromatic substitution was observed for osmabenzene 8 (Scheme 2). [16] The nitration of 8 was achieved with a solution of Cu(NO 3 ) 2 in Ac 2 O.…”

“…[17,29] A number of iridabenzene derivatives could be prepared from 4; for example, reactions with phosphines that are more electron-rich usually resulted in substitution of one or more PEt 3 ligand, depending on the sterics of the ligand. Carbon monoxide could substitute one ligand at room temperature, while at reflux a second equivalent would insert into the metallacycle to produce an iridium-phenoxide complex.…”

“…For example, cycloadditions with dienophiles and rearrangements to form cyclopentadienyl complexes suggest that these molecules are not as aromatic-that is, as stable-as their all-carbon analogues. [9,17,[19][20][21][22] While metallabenzenes have been known since 1982 [23] and were the subject of an excellent review in early 2001, [9] these metallacycles have been synthesized at an ever-increasing pace over the last few years. More importantly, new aromatic topologies (metallabenzynes, metallanaphthalenes) as well as new, versatile syntheses of such metallacycles have begun to emerge.…”