Thomas Onak

Correlation of closo-carborane 11B-H spin-coupling constants with structural features including cage “umbrella” angle

The magnitude of 1J(11B1H) values for the closo-carboranes are correlated to structural characteristics. Among the various parameters considered, both the number of adjacent cage carbon atoms and cage “umbrella” angle appear to contribute significantly to changes in the observed spin-coupling constants with smaller cage umbrella angles contributing to higher J(11B-H) values. A derived empirical method enables the prediction, or confirmation, of certain NMR assignments in those instances where some uncertainty previously existed.

Low temperature and paramagnetic ion effects on the proton and boron-11 nuclear magnetic resonance spectra of the triborohydride in (B3H8–)

The disappearance of 11B–H coupling is observed at low temperatures in the n.m.r. spectrum of the triborohydride ion. This phenomenon is attributed to electric quadrupole relaxation effects. Partial 11B–H decoupling is also effected by the addition of the paramagnetic manganese(II) ion. Only one type of proton in the 1H n.m.r. is observed down to –135° which allows for an upper limit of ca. 8 kcal./mole for ΔG‡ of the hydrogen tautomerization process.

Preparation and characterization of dichlorocyclopentadienylborane and attempted preparation of 1-chloro-2,3,4,5,6-pentacarba-nido-hexaborane cation

Abstract : Dichlorocyclopentadienylborane was obtained from the reaction of sodium cyclopentadienide with boron trichloride. Placement of the double bonds is probably 1,3 from available nuclear magnetic resonance data. Butyl lithium removes a proton from the cyclopentadienyl ring to give the aromatic Cl2B-C5H4(-) anion. Attempts to remove chloride ion from dichlorocyclopentadienylborane to form a B-chloro derivative of the nido C5BH6(+) resulted, instead, in a polymeric decomposition. (Author)

Correlation of proton charge assignments with aromatic-solvent-induced nuclear magnetic resonance shifts for the closo-carborane series C2BnHn+2 (n = 3 to 10)

Abstract The aromatic-solvent-induced PMR shifts (ASIS effect), observed for a series of closo -carboranes C 2 B n H n+2 ( n = 3 to 10), are correlated to PRDDO (an approximate molecular orbital method)-derived hydrogen charges, Q. Linear correlations between Δτ (ASIS) and Q (PRDDO) markedly improve when nearest-neighbor cage hydrogen effects are considered. From these relationships a procedure is developed to obtain the hydrogen charges from Δτ (ASIS) only.

Reactions of Me3P, Me3N and R2NH with the small closo-carboranes C2BxHx+2) (x = 3, 4, 5)

Abstract The reactivity of the small carboranes with Me3L (L = P or N) follows the order 1,5-C2B3H5 > 1,6-C2B4H6 > 2,4-C2B5H7. The smallest cage forms unstable adducts whereas 1,6-C2B4H6 gives the dipolar Me3LC2B4H6-. The largest cage does not react with Me3L, but Me2NH readily cleaves C2B5H7 to give Me2NH - BH3, a nonvolatile polymer which probably has the basic structure A, Me2NBHMe, and other volatile products that can formally be accounted for in terms of a disproportionation of the latter compound.

Reaction of bis(triethylphosphine)platinum and pentakis(t-butyl isocyanide)ruthenium with dicarba-nido-hexaboranes(8): molecular structures of [nido-µ4,5-{trans-(Et3P)2Pt(H)}-µ5,6-H-2,3-C2B4H6], [closo-1,1-(Et3P)2-2,3-Me2-1,2,3-PtC2B4H4], and [RuH(ButNC)5]+[nido-2,3-Me2-2,3-C2B4H5]–

Reaction of [Pt(PEt3)2] with nido-2,3-C2B4H8 or nido-2,3-Me2-2,3-C2B4H6 affords, respectively, [nido-µ4,5-{trans-(Et3P)2Pt(H)}-µ5,6-H-2,3-C2B4H6] and [nido-µ4,5-{trans-(Et3P)2Pt(H)}-µ5,6-H-2,3-Me2-2,3-C2B4H4], which on pyrolysis yield closo-platinacarbaboranes with nonadjacent and adjacent carbon atoms respectively; in contrast [Ru(ButNC)5] reacts with nido-2,3-Me2-2,3-C2B4H6 to give[RuH(ButNC)5]+[nido-2,3-Me2-2,3-C2B4H5]–.

NMR coupling, hybrid orbital character, and bond distances in boron hydrides

Abstract Boron-boron coupling constants for B(1)-B(3) of B 4 H 10 , B(1)-B(4) of CB 5 H 9 , and a number of other compounds have been determined. From both J ( 11 B-H) and J ( 11 B- 11 B) values for B,.H6, 11,11,0, and B 5 H 9 a set of s B -orbital populations have been derived, and found to compare favorably to those obtained from PRDDO wavefunctions. Also, J ( 11 B-H) is shown to be a function of bond distance and from this a more reasonable bond length of 1.30 A is suggested for the unique bridge hydrogen, bonded to the apex boron of B 5 H 11 ; and predictions of J = 14 (±10%) Hz for B(2)-H μ2,3 and J = 41 (±10%) Hz for B(3)-H μ2,3 of B 5 H 11 , as well as 24 (±10%) Hz for B(5)-H μ of 2,3-C 2 B 4 H 8 , are made.

Chemical and structural studies on the 2,4- and 2,3-isomers of the dicarba-nido-hexaborate(1–) ion, C2B4H7–, and some dipolar derivatives

The closo-carborane 1,6-C2B4H6 reacts with trimethylamine to form 5-Me3N+-nido-2,4-C2B4H6–. The latter compound rearranges in chloroform, or thermally, to the 3-Me3N+ isomer. The parent 2,4-C2B4H7– ion is obtained both from the action of NaH on either 3- or 5-Me3N+-2,4-C2B4H6– and from the slow reaction of closo-1,6-C2B4H6 with either NaH or LiH. The structures of 2,4-C2B4H7– and 2·3-C2B4H7– and the Me3N+-2,4-C2B4H6– isomers are correlated with 11B and 1H n.m.r. data.

Reactions of closo-1,5-C2B3H5 with Cl2 and with BMe3

Reaction of closo-1,5-C2B3H5 with Cl2 under reduced temperatures in an inert solvent gives 2-Cl-1,5-C2B3H4. Using a hot/cold reactor a mixture of BMe3 and 1,5-C2B3H5 is converted to a combination of B-mono-, di-, and tri-methyl derivatives of this smallest closo carborane. In addition, B-mono-, di-, tri-, and tetramethyl derivatives of 2,2-C2B3H4C2B3H4, as well as the parent dimer, are produced.

Antipodal hydrogen-hydrogen coupling in small cage carbon-boron compounds

Antipodal coupling (H, H) in the small closo carboranes is found to be surprisingly large and may indicate that there may be significant s-orbital bonding through the center of each of the cages. Typical JH,H values range from 6–14 Hz and were determined directly from white noise boron-decoupled proton spectra: H(1)…H(5) of 1,5-C2B3H5, 11.0 Hz; H(1)…H(6) of 1,2-C2B4H6, 11.1 Hz; H(3)…H(5) of I,2-C2B4H6, 6.9 Hz; H(1)…H(6) of 1,6-C2B4H6, 14.0 Hz; H(2)…H(4) of 1,6-C2B4H6, 7.1 Hz; H(1)…H(6) of CB5H7, 12.4 Hz; and H(1)…H(7) of 2,4-C2B5H7, 9.7 Hz. In contrast, nearest neighbor H…H coupling (e.g., H(2)… H(3) of C2B3H5) ranges from 1–7 Hz with an average value of around 2–3 Hz. A comparison of known antipodal cage distances with corresponding JH,H reveals an inverse correlation with shorter antipodal skeletal atom distances corresponding to larger coupling values. This relationship appears to be valid regardless of the cage atoms (C or B) attached to the hydrogen.