2,4-ETHANOTETRABORANE DERIVATIVES .2. SYNTHESIS, CHARACTERIZATION, AND GAS-PHASE STRUCTURES OF 2,4-(MECHCH(2))B4H8, 2,4-(TRANS-MECHCHME)B4H8, AND 2-PR-2,4-(MECHCH(2))B4H7 AND 4-PR-2,4-(MECHCH(2))B4H7
Pt. Brain et al., 2,4-ETHANOTETRABORANE DERIVATIVES .2. SYNTHESIS, CHARACTERIZATION, AND GAS-PHASE STRUCTURES OF 2,4-(MECHCH(2))B4H8, 2,4-(TRANS-MECHCHME)B4H8, AND 2-PR-2,4-(MECHCH(2))B4H7 AND 4-PR-2,4-(MECHCH(2))B4H7, Inorganic chemistry, 34(11), 1995, pp. 2841-2849
The compounds 2,4-(methylethano)tetraborane(10), (MeCHCH(2))B4H8 (1),
and 2,4-(trans-dimethylethano)tetraborane(10), (MeCHCHMe)B4H8 (2), syn
thesized from B4H10 and MeCH=CH2 or trans-MeCH=CHMe, respectively, hav
e been characterized and their molecular structures determined by gas-
phase electron diffraction and ab initio computations at the MP2/6-31G
level. The equilibrium structures of 2- and 4-n-propyl-2,4-(methylet
hano)tetraborane, 2-Pr-2,4-(MeCHCH(2))B4H7 (3) and 4-Pr-2,4-(MeCHCH(2)
)B4H7 (4), Obtained as side products in the synthesis of 1, have also
been characterized and their structures optimized using ab initio comp
utations. 3 and 4 represent the first examples of trisubstituted deriv
atives of tetraborane(10). The diffraction patterns of 1 and 2 are con
sistent with heavy-atom, C2B4, cages that are only slightly distorted
away from C-2v symmetry with twist angles of 0.5 and 0.8 degrees, resp
ectively, for the C(5)-C(6) bonds about the pseudo-C-2 axis. Other str
uctural parameters (r(a)) of the experimental geometries for 2,4-(MeCH
CH(2))B4H8 and 2,4-(MeCHCHMe)B4H8 respectively, include r[B(1)-B(2] (h
inge-wing) = 189.1(2) and 189.3(3), r[B(1)-B(3)] (hinge-hinge) 171.6(8
) and 171.2(9), r(B-C) = 161.2(9) and 161.5(11), and r(C-C) (skeleton)
= 156.2(9) and 156.7(11) pm; B(1)B(2)B(3) = 54.0(2) and 53.7(3)degrees
, and the dihedral (''butterfly'') angles between the planes B(1)B(2)B
(3) and B(1)B(4)B(3) are 100.4(2) and 100.4(3)degrees. These values ag
ree well with the ab initio (MP2/6-31G level) optimized molecular geo
metries and are supported by comparison of the calculated (IGLO) B-11
MMR chemical shifts, using both the MP2/6-31G and GED geometries, wit
h the experimental NMR data. The theoretical structures of 3 and 4 are
also supported by B-11 NMR chemical-shift calculations.