Oxocarbon dianions

A recent theoretical investigation of oxocarbon dianions suggested that the dianion of the cyclic carbon monoxide trimer, (CO)32-, is aromatic.79... 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

As an illustrative example taken from the current literature, consider the variation of C-C and C-O bond lengths in the deltate species CsO2- held within a dinuclear organometallic uranium(IV) complex. In a remarkable synthesis, this cyclic aromatic oxocarbon dianion is generated by the metal-mediated reductive cyclotrimerization of carbon monoxide, as indicated in the reaction scheme... 

The rhodizonate dianion C60g (Fig. 20.4.19) is a member of a series of planar monocyclic oxocarbon dianions C 0 (n = 3, deltate n = 4, squarate n = 5, croconate n = 6 rhodizonate) which have been recognized as nonbenzenoid aromatic compounds. However, this six-membered ring species... 

The self-assembly methodology can be extended to linkers containing oxygen. Oxalate 62 and oxocarbon dianions 63 and 64 are rigid molecules that react immediately with tecton 31 in water/acetone to form neutral metallomacro-cycles 65, 66, and 67, respectively, in 90-98% yields <2005IC7130>. 

Stabilization of Cyclic Oxocarbon Dianions by Hydrogen Bonding with Urea/Thiourea... 

Trapping Cyclic Oxocarbon Dianions in the Crystalline State... 

Charge-localized and -delocalized Valence Tautomeric Forms of Cyclic Oxocarbon Dianions... 

For over four decades, interest in the family of cyclic oxocarbon dianions has focused on their aromaticity and molecular structure. Theoretical studies have indicated that aromaticity decreases with increasing ring size, and CeOg possesses only a small degree of aromatic character [47, 51]. Furthermore, while it is firmly established that the... 

A theoretical study in 2000 concluded that while the oxocarbon dianions C 0 (n = 3-5) all favor ) /, symmetry, the QOg" ground state has C2 symmetry, and all isomeric structures are very close in energy, e.g. the energy difference is 0.34 kcal mor between the and nonplanar C2 forms [47]. Notably the measured dimensions of the relatively unstable CeOg" species in 24 and 25 conform to idealized Dch and C2v molecular symmetry, corresponding to respective valence tautomeric structures that manifest nonbenzenoid aromatic and enediolate character (Figure 8.53). 

In continuing and most elegant studies, Mak and coworkers have exploited crystal engineering principles to capture and thereby stabilize hitherto rare or even uncharacterized species within a crystalline manifold.For example, monocyclic oxocarbon dianions C 0 for n = 3, 4, 5, and 6 have been characterized in this manner. As an illustrative example, the squarate dianion, 404 (Figure 26a), has been trapped in a crystal structure with formulation PiUN]2[C404 ]- 4(NH2)2C=S 2H20. The anions are located in a three-dimensional host lattice whereby zigzag... 

Of the known cyclic oxocarbon acids, the systems based on squaric (68) and croconic (69) acids have been most widely studied. The loss of two protons from these acids gives rise to aromatic dianions as shown in equations (18) and (19), and these can coordinate to metal anions in a variety of ways. Unidentate coordination (70,77) is known for both systems but is not common. Simple bidentate chelate coordination (78) is also relatively uncommon but is observed in a number of croconate complexes. The squarate anion adopts this mode only with larger cations, such as the group 2 and lanthanide metals, and then only in association with additional bridging interactions. Bridging coordination modes dominate the chemistry of these anions, some of which are shown here (71-76), (79-81). The various modes of coordination can usually be distinguished by IR spectroscopy, and the use of NMR spectroscopy has also been investigated. 

The U(III) complex 345a reacts with CO at atmospheric pressure to give the U(IV) complex 346, which is the first reported deltato complex . The same authors have reported the surprising formation of the squarato complex 347 from CO and 345b . In both cases, CO is the only source of the dianionic oxocarbon ligands. 

Celtic acid (70) is an air-stable, easily prepared (Section II.F) solid which is readily bis-methylated". Its derived dianion (359) represents the first member of the potentially aromatic cyclic oxocarbons described by the formula Calculations suggest that... 

This is the first synthesis of an oxocarbon from a CO2 carbon source and its synthesis may be considered as the product of successive 2e" reductions of CO2. The first reduction gives carbonate plus CO, the second then reduces this liberated CO to the squarate dianion (Eq. 2). However, it must be noted that... 

This study provides the first reasonably precise molecular dimensions of the rhodi-zonate dianion, which lends support to the aromaticity of this nonbenzenoid cyclic oxocarbon. Notably, the measured C-C bond lengths [1.421(5) — 1.458(5) A] of the rhodizonate in 21, which exhibits approximate D(,h molecular symmetry, are significantly shorter than the corresponding values (1.488 and 1.501 A) in Rb2C60e [46] and the calculated values (1.500 and 1.501 A) for the C2 structure of this dianion [47]. Compound 21 provides yet another example of the use of urea and its derivatives for stabilizing elusive molecular anions such as allophanate [5h] and dihydrogen borate [5c] in a hydrogen-bonded host lattice. 

Figure 5.2 Electronic spectra of oxocarbons (a) squarate 2, (b) croconate 3, and (c) rhodi-zonate 4 dianions in aqueous solutions.

Patton and West studied the electrochemistry of these species. The radical anions of squarate 2, croconate 3, and rhodizonate 4 were characterized in dichloromethane using the electron paramagnetic resonance (EPR) technique [17]. Likewise, Carr, Fabre, and collaborators obtained the UV/visible and EPR spectra of these radical dianions, produced electrochemically in dimethylfor-mamide [18]. The oxidation potential of the oxocarbonic acids was determined in perchloric acid solution using platinum electrodes. The oxidative process was proposed to proceed in two stages, beginning with the transfer of charge from substrates at the electrode. Subsequently, the oxidation product is desorbed from the electrode and hydrated [19]. 

The most studied pseudo-oxocarbons derived from the croconate dianion 3 are croconate violet [3,5bis(dicyanomethylene)cyclepentane-l,2,4-trionate] 17 and croconate blue [2,4,5-tris(dicyanomethylene)cyclepentane-l,3-dionate] 18 [16b, 17-19]. They can be obtained from the reaction between croconate 3 and malononitrile (H2C(CN)2), as can be seen in Scheme 5.3. These derivatives are of interest because of their reversible electrochemical character [20, 21], their strong absorption maxima in the visible region, which determine their intense colors (molar absorptivity in the range of 10 lmol cm ), their delocalized Jt-system,... 

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