Recent Advances in Metallaaromatic Chemistry

Abstract

Metallaaromatics can be broadly defined as aromatic compounds in which one of the ring atoms is a transition metal. The metallabenzenes are one important class of these compounds that has undergone extensive study recently. Closely related species such as fused-ring metallabenzenes, heterometallabenzenes, π-coordinated metallabenzenes and metallabenzynes have also attracted considerable attention. Although many metallaaromatics can be considered as metalla-analogues of classic organic aromatic compounds, this is not always the case. Recent seminal studies have shown that metallapentalenes and metallapentalynes, which are metalla-analogues of the anti-aromatic compounds pentalene and pentalyne, are in fact aromatic and highly stable. Very unusual spiro-metallaaromatic compounds have also recently been isolated. In this concepts article, key features of all these intriguing metallaaromatic compounds are discussed with reference to the structural, spectroscopic, reactivity and theoretical studies that have been undertaken. These compounds continue to generate much interest, not only because of the contributions they make to fundamental chemical understanding, but also because of the promise of possible practical applications.

Rhodapentalenes: Pincer Complexes with Internal Aromaticity

“…Their NMR spectra are similar; in the 1 H NMR spectrum of 2a , C 1 H , C 3 H, and C 7 H were observed at 11.76, 7.91, and 12.83 ppm and in the spectrum of 2b, at 11.50, 8.07, and 13.08 ppm, respectively. These proton chemical shifts are consistent with those of the reported osmapentalenes (Zhu et al., 2014) and osmapentalynes (Zhu et al., 2013, Zhu and Xia, 2018, Zhuo et al., 2017) and are located in the metalla-aromatic region (Bleeke, 2001, Cao et al., 2014, Chen and Jia, 2013; Fernández et al., 2015, Frogley and Wright, 2018, Landorf and Haley, 2006, Saito, 2012, Wei et al., 2018, Wright, 2017). In the 13 C NMR spectra, the resonances of the metal-bound carbon atoms appeared at 239.66 (C1), 188.66 (C4), and 265.29 ppm (C7) for 2a and at 239.86 (C1), 187.62 (C4), and 261.20 ppm (C7) for 2b .…”

Dioxygen Activation by Internally Aromatic Metallacycle: Crystallographic Structure and Mechanistic Investigations

“…As depicted in Figure 3 A the ASE values of 2-1 (28.7 and 25.8 kcal mol −1 ) are comparable to the corresponding ASE values of previously reported metalla-aromatics ( Chen and Jia, 2013 ; Landorf and Haley, 2006 ; Fernández et al., 2015 ), indicating the aromaticity of 2 . The calculated NICS(1) zz values for the two metallacyclic rings of 2-2 are −10.3 (ring a) and −12.7 (ring b) ppm ( Figure 3 B), which are similar to those reported for other metalla-aromatics ( Wei et al., 2018 ; Frogley and Wright, 2018 ). The aromaticity of 2 is further supported by the ACID ( Herges and Geuenich, 2001 ; Geuenich et al., 2005 ), which can simulate the density and direction of the induced ring current in a molecular system under an external magnetic field.…”

The aromatic dianion metalloles

“…Significant progress has been made during the past few decades in metalla-aromatics, and these new aromatic systems have challenged and extended the concept of aromaticity remarkably. 3 …”