Many similarities exist between metallabenzenes and conventional arenes. Among these similarities are structural features such as ring planarity and the absence of bond length alternation, spectroscopic features such as downfield chemical shifts for ring protons, and chemical reactions such as electrophilic aromatic substitution and arene displacement from (arene)Mo(CO)3. All of these features, taken together, strongly support the thesis that metallabenzenes represent a new class of aromatic compounds, one in which metal d orbitals participate fully with carbon p orbitals in the formation of ring pi-bonds. However, it is also apparent that metallabenzenes are much more prone to isomerization reactions than are conventional arenes. This appears to be particularly true of first-row and second-row metallabenzenes, where the metal-carbon bond strengths are weaker. In these systems, carbene migratory insertion often leads to cyclopentadienyl-metal products. pi-Coordination of metallabenzenes to other metal centers generally stabilizes the metallabenzene moieties while maintaining their aromatic character. Among metallabenzenes coordinated in this way, there are representatives from all three transition-metal rows (Fe, Ni, Mo, Ru, and Ir). The metal atom in pi-coordinated metallabenzenes is displaced out of the ring and away from the complexing metal center. The reason for this displacement in most cases appears to be steric repulsion between ligands on the two metal centers. However, other subtle effects may contribute. For example, metal displacement leads to more favorable internal angles at the alpha-carbons and a better orientation of C alpha p orbitals toward the complexing metal center. While no dominant synthetic strategy for constructing metallabenzenes has emerged, cyclization reactions involving metal-thiocarbonyl, metal-alkylidyne, and metal-alkylidene precursors have proved useful. In addition, approaches involving pentadienyl reagents as the source of ring carbons have yielded notable successes. Vinylcyclopropene reagents have recently led to isolation of the first example of a metallabenzene valence isomer--a metallabenzvalene--and its subsequent conversion to a planar metallabenzene. Finally, interligand attacks of butadienyls on carbonyls have produced a variety of transient oxy- or alkoxy-substituted metallabenzene species. The development of new synthetic approaches, particularly systematic approaches that can be used with a variety of transition metals, is the key issue facing metallabenzene chemists. One hundred and thirty-five years after Kekulé's celebrated dream, aromatic chemistry continues to be a fascinating and provocative research topic. Metallabenzenes represent one of the "new frontiers" that promise to keep aromatic chemistry vibrant well into the 21st century.

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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...

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The first example of a stable metallapyrylium complex, [CHdC(Me)CHdC(Me)OdIr(PEt 3 ) 3 ] + -BF 4 -(2), has been prepared, and its reaction chemistry has been explored. Compound 2 is obtained in ∼50% yield upon treatment of mer-CHdC(Me)CHdC(Me)OIr(H)(PEt 3 ) 3 (3) with silver tetrafluoroborate in tetrahydrofuran. The other major product of this reaction, [mer-CHdC(Me)CH 2 C(Me)dOIr(H)(PEt 3 ) 3 ] + BF 4 -(4), is readily converted back to 3 (by treating with base) and can be reused. Compound 2 exhibits downfield 1 H NMR chemical shifts for its ring protons, consistent with its characterization as an aromatic metallacycle. The iridium center in 2 is reactive toward a variety of 2edonor reagents, including hydride reagents, methyllithium, chloride reagents, and trimethylphosphine. The products of these reactions are octahedral Ir(III) compounds containing the iridaoxacyclohexa-1,3-diene ring skeleton.The chloride reaction product, mer-CHdC(Me)CHdC(Me)OIr(Cl)(PEt 3 ) 3 ( 6), has been characterized by X-ray diffraction. Compound 2 also undergoes [4 + 2] cycloaddition reactions with various unsaturated substrates, including acetone, alkynes, alkenes, and sulfur dioxide. In each of these reactions, the substrate adds across iridium and the central carbon atom of the ring (C3) to produce octahedral Ir(III) compounds containing the iridaoxacyclohexa-1,4diene ring skeleton. The X-ray crystal structure of the sulfur dioxide cycloadduct, {fac-[CHd C(Me)CHC(Me)dOIrS(O) 2 ](PEt 3 ) 3 } + BF 4 -(13), has been obtained. Finally, treatment of 2 with nitrosobenzene generates a novel adduct containing two fused five-membered rings, {fac-[C(Me)CHdC(Me)OIrON(Ph)dCH](PEt 3 ) 3 } + BF 4 -(14). This reaction probably involves initial [2 + 2] cycloaddition, followed by rearrangement. The structure of 14 has been confirmed by X-ray diffraction.