Ring contraction of metallacyclobutadiene to metallacyclopropene driven by π- and σ-aromaticity relay

π-Aromaticity is an important driving force in directing the synthesis of aromatic compounds; in contrast, reactions induced by σ-aromaticity are uncommon. Here we report a strategy based on π- and σ-aromaticity relays to realize the structurally defined ring contraction of metallacyclobutadiene to metallacyclopropene. This reaction involves the release of the π-antiaromaticity of metallacyclobutadiene to afford a π-aromatic intermediate, followed by ring reclosure to generate σ-aromatic metallacyclopropene. The ring opening–reclosing mechanism and versatile switching of the aromaticity of the metallacyclic species are supported by experimental results and theoretical calculations. This work demonstrates the importance of aromaticity relay with the successive decrease of energy in reactions and will stimulate efforts in exploiting the vital role of aromaticity in synthetic chemistry. Reactions induced by σ-aromaticity are uncommon compared with those induced by π-aromaticity. Now, a π- and σ-aromaticity relay strategy is developed to realize the ring contraction of metallacyclobutadiene to metallacyclopropene. This reaction involves the release of π-antiaromaticity to afford a π-aromatic intermediate, followed by ring reclosure to form σ-aromatic metallacyclopropene.

Aromaticity is one of the most fundamental and interesting topics in organic chemistry 1,2 . Traditional π-aromaticity is characterized by π-electron delocalization in closed circuits of unsaturated compounds 3 and σ-aromaticity is characterized by the delocalization contributed by σ-electrons, which was first proposed by Dewar to explain the abnormal magnetic behaviour of cyclopropane 4 . Subsequently, other systems, such as hydrogen clusters 5 , inorganic rings 6 , metal clusters 7-10 and metallacyclopropenes [11][12][13][14] , featuring delocalized σ-conjugation, were suggested to have σ-aromatic character. The terms π-and σ-aromaticity are used to describe the electron delocalization of many cyclic compounds. Both π-and σ-aromaticity endow molecules with aromatic stabilization, leading to products with lower energies. Therefore, aromaticity-driven reactions play a crucial role in synthetic chemistry 15,16 . Currently, π-aromaticity-driven strategies are well known to guide the synthesis of aromatic compounds, but reactions induced by σ-aromaticity have seldom been reported 17,18 .
The synthesis and transformation of small heterocycles are valuable in synthetic chemistry. The small metallacycles, metallacyclobutadienes and metallacyclopropenes, are intriguing species because of their rich reactivity and catalytic applications 19,20 . Metallacyclobutadienes are well known in alkyne metathesis 21,22 and polymer synthesis 23 , while metallacyclopropenes play important roles in organometallic chemistry, such as ring-expansion polymerization for macrocyclic polyenes 24 , selective coupling 25 and activation of C-H bonds 26 . Thus, the synthesis, reactivity and structural properties of these metallacycles have attracted continuous attention 27-29 . In general, small metallacycles tend to undergo ring expansion by insertion of unsaturated species into the M-C bond or to participate in rearrangement/addition processes, resulting in opening of the metallacycles. However, the ring-contraction reaction of small metallacycles is challenging due to ring-strain effects, especially in the smallest four-to-three ring-contraction reaction. Considerable effort has been devoted to studying such ring-contraction reactions, such as the migration process in metallacyclobutanes 30 and the conversion of metallacyclobutenes to several metal-allene complexes 31 . However, ring contraction of metallacyclobutadienes was proposed as a key step only for the formation of metallacyclopropenes via reactions of metal carbynes with alkynes/phosphaalkynes 32,33 . 1 H NMR spectrum of 3a displays a characteristic signal at δ = 12.81 ppm attributed to C 1 H. Both the C 9 H and C9 signals at δ = 6.45 and 113.6 ppm, respectively, suggest that C9 is a vinyl carbon atom. Similar structures of 3b and 3c were also determined by single-crystal X-ray diffraction (Supplementary Figs. 3 and 4).
Chemical reactions of metallacyclobutadienes have been investigated previously, including the [2 + 2] retrocycloaddition to afford carbyne complexes and alkynes 21,22 , alkyne/CO insertion and the formation of metallabenzenes/other cyclic carbene complexes or reductive elimination products, such as η 5 -cyclopentadienyl and η 3 -cyclopropenyl complexes 41,42 . Metallacyclobutadienes have also been proposed as intermediates in the formation of vinylcarbene complexes 43 or metallacyclopropenes from reactions of metal carbynes with alkynes/ phosphoalkynes 32,33 . In principle, metallacyclobutadienes tend to undergo ring expansion or ring opening. In fact the transformation of 2a to 3a may represent the first observation of a structurally welldefined ring contraction of metallacyclobutadiene to metallacyclopropene. Analogous complexes as η 2 -1-metalla(methylene)cyclopropenes (η 2 -allenyl complexes) have been reported, which have usually been synthesized by transformations from η 2 -alkyne complexes [44][45][46] , or reactions of metal sources with unsaturated substrates, such as allenes and alkynes 15 .

Theoretical and experimental investigation of the mechanism
Density functional theory (DFT) calculations were performed to investigate the mechanism of the formation of 3a. The computed Gibbs free energy profile of the key reaction steps is shown as a black line in Fig. 3. The initial attack of CF 3 COOH led to the cleavage of the Os-C9 single bond in 2a and the formation of C9-H, generating Int 4a (Int, intermediate) via a transition state (TS1). The protonation process has Experimental evidence for the ring contraction of metallacyclobutadienes remains sparse.
Here we report an aromaticity relay strategy to realize the structurally defined ring contraction of metallacyclobutadiene to metallacyclopropene ( Fig. 1). Combined experimental and computational studies indicate that the transformation occurs via two consecutive steps along a ring opening-reclosing pathway and involves versatile aromaticity switches in the metallacycles. Initially, the acid-promoted release of the π-antiaromaticity of the osmacyclobutadiene moiety afforded a vinylcarbene fragment. Subsequently, the unusual generation of a strained three-membered ring from vinylcarbene occurred and was found to be driven by the σ-aromaticity of the osmacyclopropene. This aromaticity relay provides driving forces that lead to transformations inaccessible by other methods.

Synthesis and characterization of 1, 2a-2c and 3a-3c
We previously developed a series of aromatic metal bridgehead polycyclic frameworks with a triphenylphosphonium substituent attached to the metallacycle 34,35 . The bulky triphenylphosphonium group stabilizes the metallacyclic skeletons but may reduce the reactivity due to its steric and electron-withdrawing properties. Accordingly, we designed and synthesized an osmapentalyne (1) with a chlorine substituent instead of triphenylphosphonium at C2 through the treatment of OsCl 2 (PPh 3 ) 3 with a multiyne (L1) in the presence of excess tetrabutylammonium chloride. A plausible mechanism for the formation of compound 1 is presented in Supplementary Scheme 1. The structure of 1 was identified by NMR spectroscopy and high-resolution mass spectrometry. When osmapentalyne 1 was treated with different terminal alkynes, including p-toluenesulfonylacetylene, methyl propiolate and propiolic acid, all bearing electron-deficient groups, the [2 + 2] cycloaddition reactions occurred within 5 min, affording osmacyclobutadienes 2a-2c in high yields (>90%) (Fig. 2a). The driving force for this reaction was attributed to the release of the large ring strain caused by extreme distortion of the M≡C-C carbyne carbon angle in the five-membered ring of osmapentalyne 35 . Similar reactions have been investigated previously 19,36 .
The structure of complex 2a was confirmed by single-crystal X-ray diffraction analysis. As shown in Fig. 2b, complex 2a has a planar metallatricyclic structure, as reflected by the mean deviation from the leastsquares plane (0.022 Å) through Os1 and C1-C9. In the four-membered ring (4MR) of 2a, the bond lengths of Os1-C7 (2.064 Å), C7-C8 (1.425 Å), C8-C9 (1.338 Å) and Os1-C9 (2.218 Å) show a notable bond alternation (Fig. 2c). The Wiberg bond index of Os1-C8 is calculated to be 0.02, indicating a negligible interaction ( Supplementary Fig. 20). The structure of complex 2a was further supported by NMR spectroscopy. In the 1 H NMR spectrum, the singlet signal attributed to C 1 H at δ = 13.29 ppm was assigned to OsCH. Notably, the signal attributed to C 8 H of metallacyclobutadiene 2a was observed at δ = 5.41 ppm, showing a remarkable upfield shift compared with those of aromatic metallacycles 36 . In the 13 C{ 1 H} NMR spectrum, the characteristic signal of C7 in 2a was observed at δ = 173.3 ppm, shifting 136.7 ppm compared with the signal of C7 in complex 1 (310.0 ppm), revealing the conversion from metal carbyne to metal carbene. The main NMR chemical shifts (δ) of 1 and 2a are shown in Fig. 2d. The [2 + 2] cycloaddition reaction of metal carbynes with alkynes has been used to synthesize metallacyclobutadienes containing, for example, tantalum, molybdenum, tungsten and rhenium 19 . In contrast, this method has rarely been used for the synthesis of metallacyclobutadienes containing late transition metals [36][37][38] , the reactivity of which has rarely been studied. With osmacyclobutadienes 2a-2c in hand, we investigated their chemical reactions.
Complexes 2a-2c were treated with 10 equiv. of CF 3 COOH in CH 2 Cl 2 at r.t. for 3 h, resulting in reddish-brown complexes 3a-3c in isolated yields of 40-60% (Fig. 2a). The structure of 3a was confirmed by single-crystal X-ray diffraction. As shown in Fig. 2b, the metal was an energy barrier of 14.2 kcal mol −1 and is exergonic by 9.9 kcal mol −1 ; therefore, this reaction is theoretically facile. Subsequently, elimination of C 8 H yields the final energetically favourable product (3a) from Int 4a with an energy barrier of 10.7 kcal mol −1 , and this process is exergonic by 16.9 kcal mol −1 . Control experiments were performed to further confirm the proposed mechanism. Different amounts of acids were tested in reactions monitored by 31 P{ 1 H} NMR spectroscopy. As shown in Fig. 4b, upon reaction of complex 2a with 10 equiv. of CF 3 COOH, product 3a was obtained exclusively. However, when the reaction was carried out in the presence of 20 equiv. of CF 3 COOH, a new singlet at 17.45 ppm in the 31 P{ 1 H} NMR spectrum, attributed to species 4a, was observed. A series of parallel experiments showed that the content of 4a increased gradually as the amount of acid increased. When the acid level exceeded 50 equiv., the relative ratio of 3a and 4a remained almost unchanged. It can be assumed that the excess acid stabilized intermediate 4a.
When dilution experiments were conducted by the addition of CH 2 Cl 2 to the reaction mixture of 3a and 4a, compound 4a was gradually converted into 3a with decreasing acid concentration. Based on these results, 4a was concluded to be an intermediate in the formation of 3a, and the transformation from intermediate 4a to final product 3a was inhibited by excess CF 3 COOH. Other acids were tested in the formation of 4a. Fortunately, upon treatment of 2a with excess HBF 4 ·Et 2 O, a 4a analogue named 4A was isolated as the main product. This analogue was completely converted into 3a by heating the in situ mixture to 55 °C (Fig. 4a).
Single-crystal X-ray diffraction showed that 4A is an osmapentalene bearing a vinyl group attached at C7. As shown in Fig. 4c, the metal is coordinated with six atoms, namely, three carbon atoms (C1, C4 and C7), two phosphorus atoms and a chlorine atom, leading to a coordinated unsaturated 16-electron osmium centre. The bond lengths of Os1-C1 (1.962 Å) and Os1-C7 (2.021 Å) are comparable to previous observations in osmapentalenes (1.926-2.084 Å) 47 . The C8-C9 bond length (1.323 Å) is consistent with that of a typical C=C double bond. The C7-C8 bond connects the metallabicycle with a vinyl group, and the distance (1.464 Å) indicates a C-C single bond. In the 1 H NMR spectrum, the two doublets at δ = 6.99 and 6.10 ppm were assigned to C8H and C9H, respectively. The coupling constant of the HC8=C9H group was 14.40 Hz, confirming the E-isomer configuration. The DFT computations for the 13 C NMR spectra of compounds have been performed , and were basically consistent with the experimental results. All the data confirm that the cation of structure 4A is identical to that of Int 4a in the proposed mechanism.
Additional DFT calculations were performed to examine the formation of 3a under acidic conditions in the presence of HBF 4 ·Et 2 O (blue line in Fig. 3). The formation of 3a through TS1′ was associated with a much higher exothermicity of 23.4 kcal mol −1 to form Int 4A than to form Int 4a (9.9 kcal mol −1 ). The species Int 4A is thermodynamically favoured and is more stable than Int 4a. In addition, the conversion of Int 4A to 3a via TS2′ proceeded with a substantially higher computed barrier of 20.8 kcal mol −1 (10.7 kcal mol −1 for TS2), which suggests that it should proceed slowly at room temperature. The kinetic process of the conversion of 4A to the final product 3a was investigated at 40 and 55 °C ( Supplementary Fig. 11 Article https://doi.org/10.1038/s44160-022-00194-2 The combined experiments and DFT calculations confirmed that ring contraction proceeds through a ring opening-reclosing pathway involving acid-mediated protonation and deprotonation via vinylcarbene species (4A). Notably, the Gibbs free energy profile suggested that 3a with a strained metallacyclopropene is more stable than 4A. To address this issue, the aromaticity of each of these species was investigated.

Theoretical studies of aromaticity
Nucleus-independent chemical shift (NICS) calculations 48 were performed based on simplified model compounds 2a′, 3a′ and 4A′, in which PH 3 groups were used to replace the PPh 3 ligands and the phenyl ring attached to C3 was omitted. As shown in Fig. 5a (Fig. 5a). Anisotropy of the induced current density (ACID) 49 analysis also supported the aromaticity changes in these complexes ( Fig. 5b and Supplementary Figs. 15-17). Distinct counterclockwise circulation of 2a′ (Fig. 5b, left) was observed in the fused osmacyclobutadiene, suggesting antiaromaticity, whereas the clockwise circulation observed in two fused 5MRs demonstrated the aromaticity of 4A′ (Fig. 5b, middle), and the clockwise circulation along the periphery of the whole metallatricycle of 3a′ (Fig. 5b, right) indicated expansion of the global aromaticity.
To further examine our hypothesis of the (anti)aromaticity of complexes 2a′ and 3a′ and the σ and π contributions therein, diverse theoretical criterions were employed. The main occupied π molecular orbitals for 2a′ and 3a′ have been defined ( Supplementary Fig. 12). Canonical molecular orbital (CMO) NICS calculations were performed to identify the σ-and π-orbital contributions separately. Sophisticated (CMO) NICS including components of the σ and π contributions to the a, b and c rings in complexes 2a′ and 3a′ at distances d (0.0-3.0 Å) were studied (Supplementary Fig. 13). The computed NICS(0) π,zz value is positive for 2a′ (42.3 ppm) and the computed NICS(0) σ,zz value is negative for 3a′ (−59.6 ppm), suggesting a π-antiaromatic character of the 4MR in 2a′ and a σ-aromatic character of the 3MR in 3a′. Notably, ring b has a more negative NICS(1) zz value (−3.7 ppm) than ring c (3.3 ppm) in 2a′. This could be attributed to an anticlockwise current in the 4MR, which could enhance the ring current of the central ring b because these two ring currents proceed in the same direction along the M-C7 bond (Supplementary Fig. 14a). Additionally, the bifurcation values 0.27 of the electron localization function basins contributed by π component (ELF π )basins for the C7-C8 bond in the metallacyclobutadiene framework of 2a′ also suggest antiaromaticity ( Supplementary  Fig. 14b) 50 . The dissected NICS(0) and NICS(1) zz in complex 3a′ were selected to evaluate the nature of the possible σ-aromaticity in the 3MR. The total diamagnetic contribution of the NICS(0) value for the 3MR from the six occupied π highest occupied molecular orbitals (HOMO, HOMO-1, HOMO-3, HOMO-19, HOMO-25 and HOMO-28) was −9.2 ppm, whereas the NICS(0) value from all the σ orbitals (−28.4 ppm) was much more negative, indicating that σ-aromaticity is dominant in the 3MR and is the major contribution to aromaticity in the 3MR in 3a′ (Fig. 5c), which is consistent with the σ-aromatic character of reported metallacyclopropenes [11][12][13][14] . The σ-aromaticity in the unsaturated 3MR of 3a′ is further supported by ACID analysis contributed by different orbitals. As shown in Fig. 5d, the current density vectors plotted on the ACID isosurface indicate a diatropic ring current in the 3MR which appears only in the σ system, whereas the diatropic ring current in the π system is displayed only along the periphery of the fused 5MR in 3a′, confirming σ-aromaticity in the three-membered metallacyclopropene and π-aromaticity in fused 5MR of 3a′, respectively ( Supplementary Fig. 18).
The stability of osmacyclopropene in 3a′ can be investigated by means of two isodesmic reactions (Fig. 5e). The endothermic (+36.8 and +37.7 kcal mol −1 ) nature of the cleavage of the Os-C or C-C bonds in the 3MR in these isodesmic reactions also confirms the aromatic stabilization of the osmacyclopropene unit 51 .
Theoretical calculations indicated the antiaromaticity of metallacyclobutadiene in complex 2, aromaticity in fused metallapentalene 4 and enhanced σ-aromaticity of metallacyclopropene in complex 3 which revealed that aromaticity plays an important role in lowering the energy associated with the transformation of 2 → 3. The ring opening of osmacyclobutadiene in 2a to form 4A involves the release of antiaromaticity accompanied by the reinforcement of the π-aromaticity in osmapentalene and can be viewed as a π-aromaticity-driven process. Next, the ring-reclosing process from 4 → 3 further expands the aromaticity system of the metallacycles with newly formed σ-aromaticity. Thus, the ring-contraction reaction is driven by successive enhancements of the aromaticity accompanied by successive decreases in energy during the transformation and various changes in (anti)aromatic properties.

Thermal and chemical stability and optical properties
Thermal stability tests indicated that aromatic species 3a-3c are notably more stable than complexes 2a-2c. In the solid state, complexes 2a-2c are stable at r.t. and start to decompose at 100 °C for 2a and 2c or 110 °C in the case of 2b. In comparison, complexes 3a-3c exhibit higher thermal stability even when heated to 180 °C in the case of 3a, 160 °C for 3b and 140 °C for 3c in air for 3 h (Supplementary Table 11).
In addition, the chemical reactivity of 3a was investigated. Treatment of 3a with N-chlorosuccinimide or N-bromosuccinimide led to the formation of complexes 5a and 5b, respectively, in high yields (both 95%) (Fig. 6a). The structure of 5b shows that C9 is now substituted by a bromine atom, indicating the electrophilic substitution of the vinyl group. Treatment of 3a with excess 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) at 40 °C led to the formation of complex 6a. As shown in Fig. 6b, the bond lengths of C10-O1 and C11-O2 (1.162 and 1.193 Å, respectively) indicate a C=O double bond character, confirming that the 2,5-dihydrofuran unit of complex 3a was oxidized to form aldehydes. The NMR spectrum showed peaks attributed to C 10 H and C 11 H at δ = 9.04 and 10.00 ppm, respectively, and together with the signals of C10 (δ = 192.7 ppm) and C11 (δ = 188.9 ppm), these can be attributed to the two aldehyde groups.
The Mulliken charge and natural population analysis charge calculations showed that the electron charge of the C9 site is more negative than that of the C1 atom, which is explained well by the observation of electrophilic substitution preferentially occurring at the vinyl group ( Supplementary Fig. 19). Electrophilic substitution and an oxidation reaction both occurred at the exocyclic positions of 3a, suggesting the high stability and resistance to oxidation of the metallatricyclic moiety, in accordance with its aromatic character.
The ultraviolet-visible absorption spectra of metallacycles 2a-2c display an absorption maximum λ max at approximately 470 nm, slightly redshifted compared with that of osmapentalyne (1) (Fig. 6c). They also have weak and broad absorption band tails at 700-800 nm, which are typical bathochromic shifts of near-infrared bands attributed to the decrease in optical band gaps, in accordance with the antiaromatic character [52][53][54] . The near-infrared absorption of 2a-2c exhibits photothermal properties; for example, the temperature of the solution containing 1.00 mg ml −1 for 2c increased from 25.5 °C to 46.6 °C within 7 min under irradiation at 808 nm by a laser at a laser power density of 1.0 W cm −2 (Fig. 6d). Metallacycles 3a-3c gave rise to two obvious absorption peaks at wavelengths of ~340 and ~510 nm. Compared with 3a, electrophilic substitution products 5a and 5b exhibited similar absorption properties, while the oxidation product 6a gave rise to two slightly redshifted broad absorption peaks.

Conclusion
We have described an unusual acid-induced ring contraction of metallacyclobutadiene to metallacyclopropene via a ring opening-reclosing process. The successful isolation of the key intermediate and the  field vector is orthogonal with respect to the ring plane and is directed upward (the corresponding paratropic (red arrow) and diatropic (blue arrow) ring currents are shown). c, Partial key occupied π HOMOs and their energies (first row) together with their contributions to NICS(0) and NICS(1) zz (second row, in ppm) for model complex 3a′. d, The ACID plot of 3a′ contributed by the σ system (left) or the π system (right) is displayed with an isosurface value of 0.030 a.u. e, Isodesmic reactions for 3a′. ΔE indicates the energy gap between the products and reactants. All energies are given in kcal mol −1 .
Article https://doi.org/10.1038/s44160-022-00194-2 results of theoretical calculations confirm that the driving force of aromatization plays a vital role in the reaction. The π-aromaticity-driven ring opening of an antiaromatic metallacyclobutadiene followed by σ-aromaticity-driven ring reclosing resulted in the expansion of global aromaticity. Versatile aromaticity switches in these metallacycles have been observed, that is, from π-anti-/non-aromaticity to π-aromaticity and further to π-and σ-aromaticity. These findings offer a valuable supplement to ring contraction in small metallacycles and provide new insight into aromaticity-driven relay strategies in synthetic transformations.

General methods
Compounds S-1, S-2 and L1 were synthesized according to the literature 15 .

Synthesis of complex 1
Under a nitrogen atmosphere, a mixture of L1 (256 mg, 1.14 mmol), OsCl 2 (PPh 3 ) 3 (1.00 g, 0.95 mmol) and n Bu 4 NCl (1.00 g, 3.60 mmol) was stirred in CH 2 Cl 2 (25 ml) at r.t. for 15 min to give a brown solution. The solution was evaporated under vacuum to a volume of approximately 2 ml. Then, the solution was purified by column chromatography (alumina gel; eluent, CH 2 Cl 2 ) to give 1 (425 mg, 45%) as a yellow solid.

Data availability
All characterization data and experimental protocols are included in this Article and/or the Supplementary Information. Details of the synthesis and characterization of compounds S-1, S-2, L1, 2a-2c, 3a-3c, 4A, 5a, 5b and 6a can be found in the Supplementary Information. For general information, synthesis and characterization, see Supplementary Table 11. For kinetic study, see Supplementary Fig. 11. For the mechanism for the formation of compound 1,