Organometallic crystal engineering with multidentate building blocks and template guest size effect. Supra-anionic organic frameworks obtained from cyclobutane-1,2,3,4-tetracarboxylic and trans-acotinic acids †

Abstract
The organic acids cyclobutane-1,2,3,4-tetracarboxylic C4H4(CO2H)4, and trans-acotinic acid C3H3(CO2H)3, have been treated in thf with the organometallic hydroxides [Co(η5-C5H5)2]+[OH], [Co(η5-C5Me5)2]+[OH], and [Cr(η6-C6H6)2]+[OH] prepared in situ from the oxidation of the corresponding neutral complexes, to yield the novel organic–organometallic crystalline materials [Co(η5-C5H5)2]+[C4H4(CO2H)3COO] 1, [Cr(η6-C6H6)2]+[C4H4(CO2H)3CO2]·H2O 2, and [Co(η5-C5Me5)2]+[C3H3(CO2H)2CO2(H)·C3H3(CO2H)2CO2]·H2O 3. Self-assembly of the monodeprotonated organic acid C4H4(CO2H)4 generates supra-anionic framework structures held together by O–H · · · O and O–H · · · O hydrogen bonds which accommodate the diamagnetic [Co(η5-C5H5)2]+ and paramagnetic [Cr(η6-C6H6)2]+ cations, respectively. Crystalline 1 does not form single crystals with defined shapes but rather an “enamel” like material which grows parallel to the crystallographic bc plane. The same reaction between trans-acotinic acid and [Co(η5-C5Me5)2]+[OH]generates a large honeycomb-type structure in [Co(η5-C5Me5)2]+[C3H3(CO2H)2CO2(H)·C3H3(CO2H)2CO2]·H2O 3. The effect of the size of the templating units [Co(η5-C5H5)2]+ and [Co(η5-C5Me5)2]+ is discussed. The previously unknown structure of the starting material [Co(η5-C5Me5)2]+[PF6] 4, used for the preparation of [Co(η5-C5Me5)2]+[OH], has also been determined.

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