TUNING METAL-TO-METAL DISTANCES IN LINEAR TRICHROMIUM UNITS

A series of complexes composed of three chromium atoms linearly arrang ed and surrounded by four dipyridyl amido anions, dpa, of the type [Cr -3(dpa)(4)](n+), n = 2, 3, have been prepared. They show a remarkable variation of the Cr-Cr distances that vary from equal to highly unequa l. These complexes are made by reacting CrCl2 and Lidpa, which gives i ntermediate quadruply bonded Cr-2( dpa)(4) species (1.CH2Cl2, 2) and t hen the trinuclear complex [Cr-3(dpa)(4)Cl-2] (3 toluene). Compound 3 has chemically equivalent Cr-Cr distances (2.365(2) and 2.345(2) Angst rom). The linear, trinuclear complexes [Cr-3(dpa)(4)(CCPh)(2)] (4) and [Cr-3(dpa)(4)Cl(PF6)] (5.CH2Cl2), obtained from the reaction of 3 and LiCCPh and TIPF6, respectively, show a very different arrangement of the Cr-Cr distances. Whereas the acetylide complex 4 retains a symmetr ic pair of Cr-Cr-Cr bonds (2.4149(14) and 2.3223(14) Angstrom), 5 is v ery unsymmetrical, with Cr-Cr-Cr distances of 2.0085(13) and 7.6130(12 ) Angstrom. A one-electron oxidation of 3 by various oxidizing agents allows the isolation of [Cr-3(dpa)(4)Cl-2]Cl (6.CH2Cl2 . THF), [Cr-3(d pa)(4)Cl-2]PF6 (7.CH2Cl2 . C6H14) and [Cr-3(dpa)(4)Cl-2]FeCl4 (8.CH2Cl 2). Again there is great variability in the Cr-Cr-Cr unit; the distanc es vary from totally symmetric in 6 (2.3006(9) and 2.3006(9) Angstrom) to moderately different in 7 (2.199(3) and 2.369(3) Angstrom) to mark edly different in 8 (2.0089(12) and 2.5618(14) Angstrom). The latter i s best described as containing a quadruply bonded Cr-2(4+) unit and an isolated five-coordinate Cr3+ moiety. (C) 1998 Elsevier Science S.A.