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. 2015 Apr 9:5:9584.
doi: 10.1038/srep09584.

Synthesis of aromatic aza-metallapentalenes from metallabenzene via sequential ring contraction/annulation

Tongdao Wang  1 , Feifei Han  1 , Haiping Huang  1 , Jinhua Li  1 , Hong Zhang  2 , Jun Zhu  1 , Zhenyang Lin  3 , Haiping Xia  1
Affiliations

Synthesis of aromatic aza-metallapentalenes from metallabenzene via sequential ring contraction/annulation

Tongdao Wang et al. Sci Rep. .

Abstract

The concept of aromaticity has long played an important role in chemistry and continues to fascinate both experimentalists and theoreticians. Among the archetypal aromatic compounds, heteroaromatics are particularly attractive. Recently, substitution of a transition-metal fragment for a carbon atom in the anti-aromatic hydrocarbon pentalene has led to the new heteroaromatic osmapentalenes. However, construction of the aza-homolog of osmapentalenes cannot be accomplished by a similar synthetic manipulation. Here, we report the synthesis of aza-osmapentalenes by sequential ring contraction/annulation reactions of osmabenzenes via osmapentafulvenes. Nuclear magnetic resonance spectra, X-ray crystallographic analysis, and DFT calculations all suggest that these aza-osmapentalenes exhibit aromatic character. Thus, the stepwise transformation of metallabenzenes to metallapentafulvenes and then aza-metallapentalenes provides an efficient and facile synthetic route to these bicyclic heteroaromatics.

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Figures

Figure 1
Figure 1. The proposed structure of aza-metallapentalene.
Figure 2
Figure 2. Previous work and initial attempt.
(a) Protonation of osmapentalyne produces aromatic osmapentalene. (b) Reactions of osmabenzene with amines, leading to the formation of osmacyclopentadiene with an exocyclic C = N bond.
Figure 3
Figure 3. Synthesis of osmacyclopentadiene 2-PF6.
(a) The reaction of osmabenzene 1-I with aniline produces osmacyclopentadiene 2-PF6. (b, c) X-ray structures of 1-I (b) and 2-PF6 (c) (50% probability level). Phenyl moieties in PPh3, the counter anion and the solvent molecules have been omitted for clarity. Selected bond lengths (Å) for 1-I: Os1-C1 2.065(7), Os1-C5 1.956(7), C1-C2 1.359(10), C2-C3 1.432(11), C3-C4 1.345(10), C4-C5 1.407(11), C4-I1 2.131(7), Os1-C01 1.925(8), C01-O01 1.145(9), Os1-I2 2.8161(8). Selected bond lengths (Å) for 2-PF6: Os1-C1 2.004(6), Os1-C4 2.183(7), Os1-I1 2.8347(5), C1-C2 1.372(9), C2-C3 1.430(9), C3-C4 1.386(9), C4-C5 1.418(9), C5-N1 1.309(8), Os1-C01 1.880(9), C01-O01 1.109(8).
Figure 4
Figure 4. Synthesis of aza-osmapentalene 4-(PF6)2.
(a) The reaction of osmacyclopentadiene 2-PF6 with substituted propynols followed by dehydroxylation produces aza-osmapentalene 4-(PF6)2. (b, c) X-ray structures of 3a-PF6 (b) and 4a-(PF6)2 (c) (50% probability level). Phenyl moieties in PPh3, the counter anion and the solvent molecules have been omitted for clarity. Selected bond lengths (Å) for 3a-PF6: Os1-C1 2.040(8), Os1-C4 2.100(7), Os1-C6 2.180(9), C1-C2 1.381(11), C2-C3 1.450(11), C3-C4 1.373(12), C4-C5 1.377(13), C5-N1 1.313(11), C6-N1 1.483(11), C6-C7 1.338(12), C7-C8 1.489(12), C8-O1 1.430(12). Selected bond lengths (Å) for 4a-(PF6)2: Os1-C1 2.041(4), Os1-C4 2.069(4), Os1-C6 2.082(4), C1-C2 1.407(5), C2-C3 1.410(5), C3-C4 1.394(5), C4-C5 1.381(5), C5-N1 1.374(5), C6-N1 1.396(5), C6-C7 1.465(5), C7-C8 1.337(5).
Figure 5
Figure 5. Aromaticity of aza-osmapentalenes: downfield 1H chemical shifts and resonance structures.
(a) The proton chemical shifts (ppm vs. tetramethylsilane) of the ring protons of osmabenzene 1, osmapentafulvene 2, osmabicycle 3a and aza-osmapentalyne 4a. (b) Six possible resonance structures for the aza-osmapentalenes 4.
Figure 6
Figure 6. Evaluation of aromaticity for osmabenzene 1, osmapentafulvene 2, osmacycle 3, and aza-osmapentalyne 4a by DFT calculations.
(a) The NICS values calculated for the rings in osmabenzene 1, osmapentafulvene 2, osmacycle 3, and aza-osmapentalyne 4a. (b) The ASE values calculated for the model complexes osmabenzene 1′, osmapentafulvene 2′, osmacycle 3′, and aza-osmapentalyne 4a′. The energies computed at the B3LYP level using the LanL2DZ basis set for osmium and the 6-311++G(d,p) basis sets for carbon and hydrogen include zero-point energy corrections.
Figure 7
Figure 7. Molecular orbitals calculated for the aza-osmapentalene model 4a″ and azapentalene.
The eigenvalues of the MO's are given in parentheses.
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