Planar Möbius aromatic pentalenes incorporating 16 and 18 valence electron osmiums

Abstract

Aromaticity, a highly stabilizing feature of molecules with delocalized electrons in closed circuits, is generally restricted to 'Hückel' systems with 4n þ 2 mobile electrons. Although the Möbius concept extends the principle of aromaticity to 4n mobile electron species, the rare known examples have complex, twisted topologies whose extension is unlikely. Here we report the realization of osmapentalenes, the first planar Möbius aromatic complexes with 16 and 18 valence electron transition metals. The Möbius aromaticity of these osmapentalenes, documented by X-ray structural, magnetic and theoretical analyses, demonstrates the basis of the aromaticity of the parent osmapentalynes. All these osmapentalenes are formed by both electrophilic and nucleophilic reactions of the in-plane p component of the same carbyne carbon, illustrating ambiphilic carbyne reactivity, which is seldom observed in transition metal chemistry. Our results widen the scope of Möbius aromaticity dramatically and open prospects for the generalization of planar Möbius aromatic chemistry.

CCCCC pentadentate chelates with planar Möbius aromaticity and unique properties

“…The sum of the angles in the fused five-membered rings (5MRs) and the 6MR are 540.1°, 540.0°, and 719.7°, which are close to the ideal values of 540° and 720°. The bond distances of Os1–C1 (2.088 Å), Os1–C4 (2.107 Å), Os1–C7 (2.090 Å), and Os1–C11 (2.025 Å) are within the range of those of osmapentalene (1.926 to 2.139 Å) reported previously ( 7 ), indicating the delocalized metal-carbon bonds. The Os1–C12 (2.253 Å) bond distance is similar to that of the Os–C( sp 3 ) bond in complex 1 (2.272 Å) ( 9 ).…”

“…The sum of these values (−31.4 kcal mol −1 ) is close to the global ISE value (−29.0 kcal mol −1 ) of 2′ , indicating that such carbon chain chelates benefit from extra stability arising from quite strong 12c-12e d π -p π π-conjugation. Note that the global ISE value of 2′ is comparable to that computed for the osmapentalene (−31.4 kcal mol −1 ) ( 7 ), which has planar Craig-type Möbius aromaticity pertaining to its 4 ne (8c-8e) d π -p π π-conjugation.…”

“…A doublet signal at 13.22 parts per million (ppm) in the 1 H NMR spectrum is assigned to the H1 proton, which is shifted slightly downfield compared to those in the spectra of osmapentalene (11.94 to 12.46 ppm) ( 7 ). The signals of other protons on the fused 5MRs and 6MR are observed in the aromatic region (8.71 to 7.55 ppm).…”

Adaptive aromaticity in S0 and T1 states of pentalene incorporating 16 valence electron osmium

“…Note that Baird's rule can also be applied to the S 1 state [28][29][30] . Recently, Xia and co-workers synthesized several novel aromatic osmapentalynes and osmapentalenes 31,32 . Through density functional theory (DFT) calculations, we revealed that these organometallics exhibit Craig-type Möbius aromaticity resulting from eight-center eight-electron (8c-8e) d π -p π conjugation.…”

Probing the Origin of Challenge of Realizing Metallaphosphabenzenes: Unfavorable 1,2-Migration in Metallapyridines Becomes Feasible in Metallaphosphabenzenes

“…In comparison, the isomerization from metallaphosphabenzene II to nonaromatic analogue III has never been reported. Our ongoing interest in aromaticity2526272829303132 and reaction mechanisms3334353637 has led us to test the hypothetical isomerization of metallaphosphabenzene II to the nonaromatic analogue III as described in Fig 1A. Here we carry out thorough density function theory (DFT) calculations on this issue.…”