Complexes of Squaric Acid and Its

Complexes of Squaric Acid and Its Monosubstituted Derivatives Lincoln A. Hall and David J. Williams... 

Most of the complexes of squaric acid and its derivatives contain hydrogen bonds that appear to have a significant influence on the tertiary structure 19, 21, 22, 28, 30, 41, 56, 62, 65, 69, 76, 82, 90, 111, 112, 121, 123, 129, 131-137). The prevalence of hydrogen bonding in these complexes is not surprising, since the metals in most instances have... 

A variety of hydrophobic and hydrophilic squaraine rotaxane probes and labels such as 21a-21e c Rp and 22a-22e c Rp, containing reactive carboxylic functionalities and hydrophilic sulfo groups, are disclosed in a recent patent application [60]. It was shown that not only aniline-based squaraines 21a-21e but also heterocyclic squaraines 22a-22e can form stable pseudorotaxane complexes. The indo-lenine-based squaraine 22a forms rotaxane 22a C Rp. Importantly, also the sulfonated squaraine 22b could be successfully encapsulated in a Leigh-type, phenylene-based, tetralactam macrocycle to yield the water-soluble rotaxane 22b C Rp. Quatemized, indolenme-based squaraines do not form pseudorotaxanes probably because of sterical hindrance caused by /V-alkyl and 3,3 -dimethyl groups. On the other hand, quatemized benzothiazole (22c) and benzoselenazole (22d) squaraines could be embedded in a Leigh-type macrocycle to yield rotaxanes 22c C Rp and 22d C Rp, respectively. The hydrophilic, mono-reactive rotaxane 22e-NHS C Rp based on asymmetric squaraine, synthesized by a cross-reaction of squaric acid with the two different indolenines, was also obtained. 

Recent structural studies of squaric acid have shown it to form 1,3-bis(metal) complexes as well as 1,2-bidentate chelates with Ce(III) or with Cu(II). Stopped flow methods have been used to study the reaction between squaric acid and Fe(III). In solutions with [H ] <0.02 M, biphasic absorbance changes are observed due to operation of a number of parallel and series pathways. The initial reactions are complexations to form a 1 1 compound, [Fe(III)(H20)5(squarate)] , and a 2 1 compound, [Fe(III)2(0H)2(H20)6(squarate)] these are followed by a slower reaction leading to the reduction of Fe(III) to Fe(II) by the ligand. The reaction scheme, shown in Eq. (19)-(21), involves monomeric and dimeric squarate... 

X-Ray crystal structure analyses of both squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione) and the dianion are the usual benchmarks used in consideration of structural data of metal complexes. The relevant dimensions are shown in Table 9, from which it can be seen that there is substantial conjugation and C2 symmetry in the acid molecule while the dianion has symmetry... 

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