Synthesis of Novel Molybdaboranes from (η⁵-C₅R₅)MoCl_nPrecursors (R = H, Me;n= 1, 2, 4)

Abstract

Reaction of Cp*MoCl4 (1), or (Cp*MoCl2)2 (2), Cp* = η5-C5Me5, with BH3·THF ultimately generates the Mo(II) cluster (Cp*Mo)2B5H9 (7), together with the Mo(III) species (Cp*MoCl)2B4H10, 4. Prereduction of 2 before reaction with BH3·THF yields only 7. The structure of 4 consists of two Cp*Mo units bridged by two chlorides and a [B2H5(B2H5)]2- ligand in which the two diboron moieties are connected by a B−B−B three center bond. Closer inspection of the reaction by 11B and 1H NMR reveals the existence of three intermediate species (Cp*MoCl)2B2H6 (3), (Cp*MoCl)2B3H7 (5), and (Cp*Mo)2(B2H6)2 (6). Each of these species has been characterized spectroscopically, and crystal structures have been obtained for 3 and 5. Compound 3 features molybdenum centers bridged by two chlorides and an ethane-like [B2H6]2- ligand such that the B−B bond is perpendicular to the Mo−Mo bond. Replacing one terminal H by [B2H5] generates 4. The structure of 5 is based on a trigonal bipyramidal Mo2B3 core, and the molecule is electronically unsaturated although the Mo−Mo distance (3.096 Å) precludes the existence of multiple bonding between the metal centers. 5 exists as a relatively stable molecule despite having too few electrons and too few atoms to adopt a capped structure based on a polyhedron with fewer vertexes. Comparison of MO descriptions of the electronic structure of 5 with that of the later transition metal species (Cp*Co)2B3H7 (8) shows that this stabilization is derived from the appropriate energy match between Cp*Mo and borane based orbitals which elevates the energy of the Mo−B antibonding LUMO, a cluster orbital which would normally be filled, into the region of unoccupied orbitals. The concentration vs time behavior for the final products 4 and 7, for the intermediates 3, 5, and 6, for the monoboron species BH3·THF and BH2Cl, and selected non-boron containing species is used to define a pathway for the molybdaborane cluster condensation. With 1, use of LiBH4 as the monoboron source yields 6 as the primary product via 3 as an intermediate, whereas prereduction of 2 with [Et3BH]- results in the formation of 7. The varied cluster building abilities of BH3·THF vs LiBH4 originate in the differing reduction and coordination properties of the two monoboranes. Investigation of the analogous Cp = η5-C5H5 system reveals similar chemistry albeit simpler and on a shorter time scale.

Fine-Tuning of Metallaborane Geometries: Chemistry of Iridaboranes Derived from the Reaction of [(Cp*Ir)2HxCl4−x] (x=0-2; Cp*=η5-C5Me5) with LiBH4

“…Stepwise addition of a BH fragment: As a general rule, reaction of dimeric monocyclopentadienylmetal chlorides with boranes yields dimetallaboranes as the major products, but the number of BH vertices in the cluster depends on the metal elements and the boranes used, for example, M 2 B n : Rh, n 2; [6] Co, [4] Rh, [6] Ru, [6] n 3; Cr, [43] n 4, Mo, [9] W, [10] n 5; Re, [12] mination ( Figure 6) is fully consistent with the NMR data. It constitutes the second structurally characterized ªborallylº species with the general formula [{Cp*M(CO)}B 3 H 7 ] (M Co, Ir).…”

“…Indeed, we have demonstrated the reaction of binuclear [Cp*MCl n ] complexes (Cp* h 5 -C 5 Me 5 ) with LiBH 4 is a general route to metallaboranes. Detailed investigations of the Co, [4] Rh, [5,6] Ru, [6] Cr, [7] Mo, [8,9] and W [10,11] systems have been reported to date. Metal identity affects products.…”

Metallaboranes of the Early Transition Metals: Direct Synthesis and Characterization of [{(η5‐C5Me5)Ta}2BnHm] (n=4, m=10; n=5, m=11), [{(η5‐C5Me5)Ta}2B5H10(C6H4CH3)], and [{(η5‐C5Me5)TaCl}2B5H11]

“…[11] The development of metallaborane chemistry has followed a similar pathway, and new species are often isolated under conditions that favor the thermodynamic product, either through thermolysis or by simple metathesis reactions between a preformed polyborane anion and transition-metal halides. [3a-b, 12] Fehlner et al have studied reactions between cyclopentadienyl transition-metal halides [Cp*MCl n ] (Cp*= h 5 -C 5 Me 5 ) and monoborane reagents BH 3 ·thf, LiBH 4 ·thf, and BHCl 2 ·SMe 2 ,which often afford metallaboranes (M = Cr, [13] Mo, [14] W, [15] Re, [16] Ru, [17] Co, [18] Rh, [19] and Ir. [20] ) in modest to high yield under mild conditions.…”

Direct insertion of sulfur, selenium and tellurium atoms into metallaborane cages using chalcogen powders

“…proposed as an intermediate species leading to [(g 5 -C 5 Me 5 -Mo) 2 B 5 H 9 ][46]. Therefore, compounds 5-7 have been postulated to be generated by the insertion of two chalcogen atoms into the core geometry of [(g 5 -C 5 Me 5 Mo) 2 B 4 H 8 ] with the elimination two molecules of H 2 (Scheme 2).…”