Dichromastibacumulenium-Ionen [η 6-aren(CO) 2CrSbCr(CO) 2aren-η 6] +: Stabilität in Abhängigkeit von η 6-gebundenen Arenliganden am Chromatom

Abstract

Abstraction of chloride by Lewis-acid reagents from the stibinidene complexes [{n 6-arene(CO) 2Cr} 2SbCl] (arene = Me 6C 6: 1a; 1,3,5-Me 3C 6H 3: 2a; C 6H 6: 3a) produced 4– 6, the allene type dichromastibacumulenium ions [arene(CO) 2CrSbCr(CO) 2arene] +, which were detected spectroscopically in solution. Their stability increases with an increasing number of methyl substituents at the arene ligand: salts with cation 6 (arene = C 6H 6) could not be isolated as such, while salts with 5 (arene = mes) or 4 (arene = hmb) could. However, 6 can be trapped with chelate bases L 2 (L 2 = 2,2′-bipyridine = bpy, 1,10-phenanthroline) yielding [{C 6H 6(CO) 2Cr} 2CrSb(L 2)Y ( 7– 9), while 5 and bpy gave unclear results and 4 is effectively inert towards bpy (IR, variable temperature UV/vis spectroscopy). 4– 6, in reverse of their formation, react instantaneously with anionic nucleophiles X − to give stibinidene complexes [{arene(CO) 2Cr} 2SbX] identified spectroscopically in solution. Investigation of the crystal structure of [{C 6H 6(CO) 2Cr} 2Sb(bpy)]PF 6 · 1 2 bpy, 7b · 1 2 bpy, indicated relatively long SbN bonds suggesting that the base is only weakly bound to the Sb + centre. These findings are in line with firstly the ease of decomposition of 7b in solution, spectroscopy suggesting that [{C 6H 6(CO) 2Cr} 2SbF] is formed, and secondly the ready displacement by anionic nucleophiles X − of the chelate base, where stibinidene complexes, 3, are formed. 7, 8 are easily accessible by means of a salt metathesis from [{C 6H 6(CO) 2Cr} 2SbX′] (X′  Cl, Br) and NaBPh 4 or KPF 6 in the presence of L 2. In this manner 4, 5 · BPh 4 were synthesized as well and isolated as analytically pure microcrystalline salts. The enhanced stability of 4 and 5 compared with 6 becomes apparent in their formation from 1a and 2a, respectively, and SbCl 3- 1a yields 4 · “[Sb 2Cl 7]” in an equilibrium reaction with two equivalents of SbCl 3 (upon cooling the equilibrium is slightly shifted back towards the educts); generating 5 from 2a requires 100–150 equivalents of SbCl 3, and 3a does not react with SbCl 3 under formation of 6. Thus, for cations [L n M SbML n ] + the following order of stability can be set up: L n M  Me 6C 6(CO) 2Cr > 1,3,5-Me 3C 6H 3(CO) 2Cr >. C 6H 6(CO) 2Cr ≈ Cp★(CO) 2Mn > Cp′(CO) 2Mn, while the stability of the stibinidene complexes [{L n M} 2SbX] increases from left to right. Steric factors are at least contributory to the low reactivity of [Me 6C 6(CO) 2CrSbCr(CO) 2C 6Me 6] + Complex fragments {η 6-arene(CO) 2Cr} proved to the suitable for the stabilization of strongly π-acidic, low coordinated ligands and may in special cases be markedly superior even to {Cp★(CO) 2Mn}.