Hypo-electronic triple-decker sandwich complexes: synthesis and structural characterization of [(Cp*Mo)₂{μ–η⁶:η⁶-B₄H₄E-Ru(CO)₃}] (E = S, Se, Te or Ru(CO)₃ and Cp* = η⁵-C₅Me₅)

Abstract

The syntheses and structural characterization of hypo-electronic di-molybdenum triple-decker sandwich clusters are reported. Thermolysis of [Ru₃(CO)₁₂] with an in situ generated intermediate obtained from the reaction of [Cp*MoCl₄] with [LiBH₄·THF] yielded an electron deficient triple-decker sandwich complex, [(Cp*Mo)₂{μ–η⁶:η⁶-B₄H₄Ru₂(CO)₆}], 1. In an effort to generate analogous triple-deckers containing group-16 elements, we isolated [(Cp*Mo)₂{μ– η⁶:η⁶-B₄H₄ERu(CO)₃}] (3: E = Te; 6: E = S; 7: E = Se). These clusters show a high metal coordination number and cross cluster Mo–Mo bond. The formal cluster electron count of these compounds is four or three skeletal electron pairs less than required for a canonical closo-structure of the same nuclearity. Therefore, these compounds represent a novel class of triple-decker sandwich complex with 22 or 24 valence-electrons (VE), wherein the “chair” like hexagonal middle ring is composed of B, Ru and chalcogen. One of the key differences among the synthesized triple-decker molecules is the puckering nature of the middle ring [B4RuE], which increases in the order S < Se < Ru(CO)₃ < Te. In addition, Fenske–Hall and quantum-chemical calculations with DFT methods at the BP86 level of theory have been used to analyze the bonding of these novel complexes. The studies not only explain the electron unsaturation of the molecules, but also reveal the reason for the significant puckering of the middle deck. All the compounds have been characterized by IR, ¹H, ¹¹B and ¹³C NMR spectroscopy in solution and the solid state structures were established by crystallographic analysis.