Gas-Phase Reaction of Tetraborane(10) and Ethyne: Molecular Structure of nido-1,2-C(2)B(3)H(7) in the Gas Phase.
The molecular structure of nido-1,2-C(2)B(3)H(7), 1, the principal volatile carborane generated in the quenched gas-phase reaction of B(4)H(10) and ethyne at 70 degrees C, has been determined by a combined analysis of gas-phase electron-diffraction data and rotation constants restrained by ab initio computations at the CCSD(T)/TZP' level. The structure is consistent with a geometry having C(s)() symmetry, similar to that of pentaborane(9). The apical position is occupied by a carbon atom, displaced toward B(4) from a position directly above the B(5).B(3) vector, and hydrogen atoms asymmetrically bridge the B-B bonds. The basal atoms are almost coplanar, C(2) lying ca. 2 degrees below the B(3)-B(4)-B(5) plane. Important experimental structural parameters (r(alpha) degrees /pm, angle(alpha)/ degrees ) are r[C(1)-C(2)] = 162.6(6); r[C(1)-B(3)] = 161.4(3); r[C(2)-B(3)] = 154.3(2); r[C(1)-B(4)] = 157.4(5); r[B(3)-B(4)] = 185.7(3); <B(3)-B(4)-B(5) = 80.9(1). In addition to this and the other previously reported carboranes, 2,3-C(2)B(4)H(8), 2-Me-2,3,4-C(3)B(3)H(6), and 4-Me-2-CB(5)H(8), several new derivatives have been identified among the volatile products. These include the dicarbahexaboranes 2,4-Me(2)-2,3-C(2)B(4)H(6) and 5-Et-2,3-C(2)B(4)H(7) (the major volatile products obtained when the reaction is allowed to go to completion, previously reported as tricarbahexaboranes) and the derivatives 2,5-Me(2)-2,3-C(2)B(4)H(6), 4-Et-2,3-C(2)B(4)H(7), 1-Me-2,3,4-C(3)B(3)H(6), 2-Me-2-CB(5)H(8), 3-Me-2-CB(5)H(8), and 2,3,4,5-C(4)B(2)H(6). The complex mechanism of the reaction is discussed in light of these new results.
Fox
Mark A.
MA
Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-0482.
Greatrex
Robert
R
Nikrahi
Alireza
A
Brain
Paul T.
PT
Picton
Matthew J.
MJ
Rankin
David W. H.
DW
Robertson
Heather E.
HE
Bühl
Michael
M
Li
Linda
L
Beaudet
Robert A.
RA