The Polyhedral Metallaboranes Part I. Metallaborane Clusters with Seven Vertices and Fewer

Fine Tuning of Metallaborane Geometries: Chemistry of Metallaboranes of Early Transition Metals Derived from Metal Halides and Monoborane Reagents

“…Suitable crystals of 1 were obtained from a hexane solution at 7 8C. Compound 1 is described as a metallaborane built from two edge-fused V 2 B 2 tetrahedra, or as a bis-[B 2 H 6 ] 2À complex of [CpV] 2 4 + , giving each V center a formal oxidation state of + 3. The V À V distance is 2.787(2) , and the V À H distance for the V-H-B bridges averages 1.7785 .…”

“…The 11 B NMR spectrum indicates the presence of a single boron environment, a peak at d = À4.6 ppm is consistent with hydrogen-bridged rather than direct Ta À B linkages. From the MS analysis together with the 11 B NMR spectrum, 3 is formulated as [(Cp*Ta) 2…”

“…[1] The development of metallaborane chemistry has followed a similar pathway, with new species often obtained from conditions that favor the thermodynamic product, either through thermolysis or by simple metathesis reactions between a preformed polyborane anion and transition-metal halides. [2][3][4][5] Detailed investigations of the Co, [6] Rh, [7] Ir, [8,9] Fe, [10] Ru, [11] Re, [12,13] Cr, [14,15] Mo, [10,[16][17][18] W, [10,19,20] and Ta [21][22][23] systems reported to date reveal that metal identity affects products. For example, the earlier transition metals facilitate dehydrogenation leading to highly condensed clusters, such as closo-A C H T U N G -T R E N N U N G [(Cp*M) 2 B n H n + m ], (M = Re: [13] n = 7-10, m = 0; M = W: [12] n = 7, m = 2), whereas late transition elements form stable nido-or arachno-metallaboranes, such as nido-[(Cp*Ru) 2 B 10 H 16 ] [24] or arachno-1-[(Cp*IrH)B 4 H 9 ].…”

“…For example, the earlier transition metals facilitate dehydrogenation leading to highly condensed clusters, such as closo-A C H T U N G -T R E N N U N G [(Cp*M) 2 B n H n + m ], (M = Re: [13] n = 7-10, m = 0; M = W: [12] n = 7, m = 2), whereas late transition elements form stable nido-or arachno-metallaboranes, such as nido-[(Cp*Ru) 2 B 10 H 16 ] [24] or arachno-1-[(Cp*IrH)B 4 H 9 ]. [9] Metallaboranes that incorporate early transition metals are a sparsely explored area of cluster chemistry [2][3][4]25] and the challenge is to find synthetic routes. Traditionally, the focus has been on two broad but complementary approaches, namely a) insertion or fragmentation reactions of ready-assembled borane frameworks and b) condensation reactions of monoboron precursors [LiBH 4 ·thf] or [BH 3 ·thf].…”

“…Before this work, [27][28][29][30][31][32][33] except for coordination complexes containing the BH 4 À , B 2 H 6 2À , or B 3 H 8 À ligands, structurally authenticated compounds of the group 5 metals were confined to the electronically unsaturated tantalaborane [22] Metallaboranes, for example, [(Cp + Nb) 2 A C H T U N G T R E N N U N G (B 2 H 6 ) 2 ] [34] (Cp + = h 5 -C 5 Me 4 Et), have often been found as trace products in complex reactions, but their existence created synthetic goals. Thus, almost immediately, methods for the production of tantalaborane [(Cp*Ta) 2 A C H T U N G T R E N N U N G (B 2 H 6 ) 2 ] by Messerle, and of niobaborane [(Cp + Nb) 2 A C H T U N G T R E N N U N G (B 2 H 6 ) 2 ] by Wachter, were developed with good yields. [21,31] By contrast, examples of vanadaboranes are rare and, except for coordination complexes containing the BH 4 À and B 3 H 8 À ligands, [30,33] no structurally characterized vanadaboranes are known to date.…”