Synthesis, characterization, and electronic structure of new type of heterometallic boride clusters

The reaction of [Cp*TaCl₄], 1 (Cp* = η⁵-C₅Me₅), with [LiBH₄·THF] at −78 °C, followed by thermolysis in the presence of excess [BH₃·THF], results in the formation of the oxatantalaborane cluster [(Cp*Ta)₂B₄H₁₀O], 2 in moderate yield. Compound 2 is a notable example of an oxatantalaborane cluster where oxygen is contiguously bound to both the metal and boron. Upon availability of 2, a room temperature reaction was performed with [Fe₂(CO)₉], which led to the isolation of [(Cp*Ta)₂B₂H₄O{H₂Fe₂(CO)₆BH}], 3. Compound 3 is an unusual heterometallic boride cluster in which the [Ta₂Fe₂] atoms define a butterfly framework with one boron atom lying in a semi-interstitial position. Likewise, the diselenamolybdaborane, [(Cp*Mo)₂B₄H₄Se₂], 4 was treated with an excess of [Fe₂(CO)₉] to afford the heterometallic boride cluster [(Cp*MoSe)₂Fe₆(CO)₁₃B₂(BH)₂], 5. The cluster core of 5 consists of a cubane [Mo₂Se₂Fe₂B₂] and a tricapped trigonal prism [Fe6B3] fused together with four atoms held in common between the two subclusters. In the tricapped trigonal prism subunit, one of the boron atoms is completely encapsulated and bonded to six iron and two boron atoms. Compounds 2, 3, and 5 have been characterized by mass spectrometry, IR, ¹H, ¹¹B, ¹³C NMR spectroscopy, and the geometric structures were unequivocally established by crystallographic analysis. The density functional theory calculations yielded geometries that are in close agreement with the observed structures. Furthermore, the calculated ¹¹B NMR chemical shifts also support the structural characterization of the compounds. Natural bond order analysis and Wiberg bond indices are used to gain insight into the bonding patterns of the observed geometries of 2, 3, and 5