Abstract
The complexes (Et3NH)2[Mn(Cat)3] and K2[Mn(3,5-(f-Bu)2Cat)3]*6CH3CN (Cat = catecholate) have been synthesized, and the latter has been characterized by X-ray diffraction (trigonal system, space group J?3, a = 14.760 (9) Á, c = 50.752
(32) Á, Z = 6, final R = 6.9%, final R„ = 7.1%). The [Mn(3,5-(r-Bu)2Cat)3]2" anion has crystallographic threefold symmetry with short Mn-0 bond lengths (1.922 (3) and 1.891 (3) A) and no evidence of either dynamic or static Jahn-Teller distortion. The electron paramagnetic resonance spectrum of the [Mn(Cat)3]2" ion at 77 K is characteristic of a d3 system with large zero-field splitting. The magnetic behavior of K2[Mn(3,5-(r-Bu)2Cat)3]-6CH3CN is fully consistent with that expected for a simple d3 system. The present structural, magnetic susceptibility, and magnetic resonance results establish that these essentially identical chelates are tris(catecholate)manganese(IV) rather than (semiquinone)bis(catecholate)manganese(III) complexes. The present structural results also show that the “bite distance” of catechol (2.58 A) is too short to offer a regular octahedral environment to Mn(IV). This fact has a pervasive influence on the chemistry of the system. It causes trigonal compression of the Mn(IV) ion and thereby gives rise to the large zero-field splitting observed. In the Mn(III)-catechol system, it actually determines the solution chemistry. Because Mn(III) is even larger than Mn(IV) and has either two or four elongated bonds (by the Jahn-Teller theorem), three catechol ligands cannot span all six coordination sites in a chelating fashion. The short bite distance of catechol therefore effectively prevents formation of [Mnln(Cat)3]3". Moreover, the difficulty of forming a tris complex of Mn(III) with a catechol-like ligand may contribute significantly to stabilization of [Mn(3,5-(r-Bu)2Cat)3]2TM as tris(catecholate)manganese(IV) instead of (semiquinone)bis(catecholate)manganese(III).