Caption: A ruthenium–carbon triple bond has been constrained in a five-membered ring to generate ruthenapentalynes. The metallacycle of ruthenapentalyne is highly strained due to the bent Ru≡C–C moiety, considerably deviating from its linear nature.
Transition metal carbyne complexes hold an important place in chemistry, and a wide variety of metal carbyne complexes have been prepared and exploited in organic and organometallic synthesis. However, the construction of cyclic metal carbyne complexes is challenging because of the linear geometry nature of the sp-hybridized carbyne carbon. Although the first metal carbyne complex was obtained in 1973, cyclic metal carbyne complexes remained elusive until 2001. In comparison with cyclic metal carbyne complexes with third-row transition metals, their first- and second-row counterparts are usually less stable due to the decreasing strength of transition metal–carbon bonds along with the ascending of a column in the periodic table. Therefore, it is not surprising that the cyclic metal carbyne complexes with non-third-row transition metals have not yet been accomplished to date.
Researchers from Xiamen University have synthesized unprecedented cyclic ruthenium carbyne complexes, that is, ruthenapentalynes. The carbyne carbon angle of the ruthenapentalyne is only 130.2(3)°, deviating nearly 50° from the ideal value (180°) of sp-hybridized carbon angle. The research findings have been published in Science Advances (2018, 4, eaat0336).
The researchers are dedicated to the synthesis and applications of polydentate carbon ligands in recent years. They have designed and synthesized a series multiyne chains (they term them carbolongs) which could readily chelate metal centers to yield all-carbon-ligated polydentate chelates. In this study, ruthenapentalynes were easily obtained by one-pot reactions of triyne chains with commercially available RuCl2(PPh3)3. “Generally, the coordinating atoms in polydentate chelates are primarily drawn from a variety of donor heteroatoms, including N, O, P and S. Our designed carbon chains can efficiently wrap osmium or Ruthenium centre like a scarf to generate chelates starting from commercially available substrates under ambient conditions on a multigram scale.” said Professor Haiping Xia, the lead of this project.
Although the bent Ru≡C–C moiety shows extremely strained carbyne carbon angle, these ruthenapentalynes exhibit good stability. The solid sample is persistent in air at 60 °C for at least 3 h. “DFT calculations revealed that the inherent aromaticity plays a key role in the stabilization of the ruthenapentalyne,” explains Associate Professor Hong Zhang, who conceived the theoretical work, “Aromaticity generally leads to stabilization, thus it is not surprising that most documented examples of cyclic metal carbynes exhibit aromaticity.”
Synergistic interplay of aromaticity and ring strain results in unique ambiphilic reactivity of ruthenapentalyne, which is highly reactive toward both nucleophiles and electrophiles. Specifically, the first [2+2] cycloaddition reaction of ruthenium carbyne complexes with alkynes was realized in this ruthenapentalyne system. “This study provides a valuable complement to the chemistry of metal carbyne, which would further boost the development of new reactions and new reagents.” says Qingde Zhuo, the first author of the study, who does his postdoctoral research in Collaborative Innovation Center of Chemistry for Energy Materials, China.
The full report can be found at: https://www.eurekalert.org/pub_releases/2018-06/xu-stm061518.php
More information, including a copy of the paper, can be found online at http://advances.sciencemag.org/content/4/6/eaat0336.
