Partial Insertion of the 9-BBN Unit into the nido-B10 Framework: Preparation and Structural Characterization of (9-BBN)B10H13 and [(9-BBN)B10H12]-

Abstract

Na[B10H13] reacts with 9-Br-BBN (9-BBN = 9-bora[3.3.1]bicyclononane) in CH2Cl2 to produce (9-BBN)B10H13, 1, in high yield. Single-crystal X-ray diffraction analysis shows that the boron atom of the 9-BBN unit in 1 is partially inserted into the nido-B10 structure through a highly asymmetric bridge between B(5) and B(6) on the B10 cluster with distances of B(BBN)−B(5) = 1.740(6) Å and B(BBN)−B(6) = 2.079(6) Å. There is also an agostic interaction of the terminal H(5b) on B(5) with B(BBN). NMR studies confirm that this interaction is maintained in solution. Compound 1 is unstable in coordinating solvents, resulting in some cases in the cleavage of the 9-BBN unit from the decaborane skeleton. When 1 is placed in Et2O, NMR spectra suggest that (Et2O)(9-BBN)B10H13, 2, is formed in which an Et2O molecule associates with the boron B(BBN) of the 9-BBN unit while the interactions of that boron with B(6) and with H(5b) are eliminated, leaving only a direct bond between B(BBN) and B(5). Reaction of 1 with “Proton Sponge”, 1,8-bis(dimethylamino)naphthalene, in CH2Cl2 gives [PSH][(9-BBN)B10H12], 3, (PSH = protonated “Proton Sponge”). Single-crystal X-ray diffraction analysis of 3 indicates a structure similar to that of nido-[B11H14]-, with the boron of the 9-BBN unit approximately occupying a vertex site of the pentagonal face of a nido-B11 cluster. The B(BBN) boron is inserted into a bridging site between B(5) and its crystallographically imposed mirror equivalent B(10‘) of the decaborane cluster with B(BBN)−B(5)/B(BBN)−B(10‘) distances of 1.884(4) Å and a B(5)−B(10‘) distance of 1.823(5) Å. On the other hand the distance of the B(BBN) boron to B(6) and to the mirror equivalent B(9‘) is an abnormally long 2.142(4) Å, which indicates that the B(BBN) boron is not fully accommodated in the formation of a nido-[B11H14]- analogue. The novel interaction of the 9-BBN fragment with the decaborane cluster in 1 and the partial insertion of the 9-BBN moiety into the nido-B10 unit observed in 3 are the first examples of such interactions observed between a monoboron unit and a decaborane cluster. Crystal data: 1, Pbca (No. 61), a = 11.924(3) Å, b = 13.205(6) Å, c = 19.655(5) Å, Z = 8; 3, Pnma (No. 62), a = 15.359(3) Å, b = 13.932(3) Å, c = 12.565(4) Å, Z = 4.