Squaric acid metal complexes

X-Ray crystal structure analyses of both squaric acid158 (3,4-dihydroxy-3-cyclobutene-l,2-dione) and the dianion159 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 DAh symmetry... 

The effect of this composition has been subsequently improved to regenerate the oxidized metal in its reduced form by formation of a complex with squaric acid [151, 152] (Scheme 44). 

Complexes of Heavier Transition Metals Electrochemical Studies Luminescence Studies IV. Hydrogen Bonding and Other Weak Interactions in Complexes of Squaric Acid and Its Monosubstituted Derivatives References... 

Characteristic IR Stretching Frequencies (cm ) for Squaric Acid AND Metal Squarate Complexes... 

Infrared and UV/vis data have been used by several authors to identify the C=C, C=0, and M—O stretches in the complexes synthesized 15, 18-21, 37, 38, 41, 44, 50, 54, 56, 58, 59, 64-66, 69, 74, 78, 80, 82, 103). Except in the initial research on first-row transition metal complexes of squaric acid, where these data were used in proposing structures, IR and UV/vis analysis have been used as supporting evidence for the particular coordination mode of the ligand 19,21,22, 45, 52, 59, 65). Infrared spectroscopy has also been utilized in the study of mixed oxalate/squarate complexes 118), although not to the same extent as in complexes of the oxalate ion. For example, Scott et al. studied the IR properties of Co(III) oxalate complexes with the hgand in a variety of chelating/bridging situations 119). 

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... 

Some of the first metal complexes of squaric acid were synthesized by West and Niu(12). These complexes contained divalent transition metal ions such as Mn(II), Fe(II), Co(II), Ni(II), and trivalent metals such as Al(III), Fe(III), an Cr(III). The divalent metal complexes were initially assumed to have structure , but X-ray structural studies show that these metals form a three dimensional polymer network 5 with squaric acid(13). The trivalent complexes, however, are belived to have a dimeric structure 6, with uncoordinated oxygens (hereafter referred to as the diketosquarate derivatives) (12, 14, 15). In both the divalent and trivalent metal complexes, the geometry about the metal is octahedral (or approximately octahedral), which is the preferred geometry for these metal ions. 

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... 

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. 

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