237422-63-8Relevant academic research and scientific papers
Remarkable effects of axial π* coordination on the Cr-Cr quadruple bond in dichromium paddlewheel complexes
Cotton, F. Albert,Daniels, Lee M.,Murillo, Carlos A.,Pascual, Isabel,Zhou, Hong-Cai
, p. 6856 - 6861 (1999)
It is well-known that donation of electron density into the σ* orbital of a Cr-Cr quadruple bond causes major lengthening of the Cr-Cr distance, and there is some prior evidence that a similar lengthening is caused by dative interaction with the π* orbitals. Some molecules have now been made that allow a definitive assessment of this axial π* effect. A molecule has been designed to ensure that there is axial donation into the π* orbitals but not onto the σ* orbital; ligands have been used in which the donor atoms are tethered to the bridging ligands in such a way that they can reach only the π* orbitals but not the σ* orbital. The ligands used for this purpose are the anions of 2,6-di(phenylimino)piperidine (DPhIP) and 2,2′-dipyridylamine (dpa). In the compound Cr2(DPhIP)4 four imino nitrogen lone pairs are suitably positioned to donate to the π* orbitals and the Cr-Cr bond length is 2.265(1) A. For direct comparison, the compound Cr2(PhIP)4 (PhIP is the anion of 2-(phenylimino)piperidine) was made and found to have a Cr-Cr distance of 1.858(1) A. In this case the ligand is very similar to DPhIP except that it has no donor nitrogen atoms available for axial π* donation. Thus, the cumulative effect of donation from four nitrogen atoms is very large, namely, 0.4 A in the Cr-Cr distance. The Cr2(dpa)4 molecule occurs in three different crystalline compounds, in all of which there are slightly different conformations, but the same Cr-Cr distance, 1.94 ± 0.01 A; these may be compared to that in the compound Cr2(mpa)4 (1.87 A) in which the bridging is quite similar but there are no tethered additional donor atoms.
