1,2‐, 1,7‐ and 1,12‐Dicarba‐closo‐dodecaborane(12) Derivatives Revisited by ¹³C NMR Spectroscopy and DFT Calculations. First Observation of Isotope‐Induced Chemical Shifts ¹Δ^10/11B(¹³C), and the Signs and Magnitudes of Coupling Constants ¹J(¹³C,¹³C) and ¹J(¹³C,¹¹B)

Abstract

The origin of broadening of 13C(carborane) NMR signals of 1,2‐, 1,7‐ and 1,12‐dicarba‐closo‐dodecaboranes(12) and several diphenylsilyl derivatives has been examined in detail and could be traced only partially to unresolved 13C–11B spin‐spin coupling. Other contributions to the line widths arise from 13C–1H dipole‐dipole interactions and, in particular, from isotope‐induced chemical shifts 1Δ10/11B(13C), observed here for carboranes for the first time. In the case of 1‐diphenylsilyl‐1,2‐dicarba‐closo‐dodecaborane(12), the coupling constant 1J(13C,13C) = 9.3 Hz was measured in natural abundance of 13C. The small value of this coupling constant and its negative sign is predicted by calculations based on optimised structures [B3LYP/6‐311+G(d,p) level of theory] of the parent carboranes and 1‐silyl‐1,2‐dicarba‐closo‐dodecaborane(12) as a model compound [calcd. 1J(13C,13C) = –10.5 Hz]. Calculated coupling constants 1J(13C,11B) are small (<7 Hz), in contrast to published assumptions, and of either sign, whereas 1J(11B,11B) are all positive and range up to 15 Hz.

Synthesis and Reactivity of 4,5‐[1,2‐Dicarba‐closo‐dodecaborano(12)]‐1,3‐diselenacyclopentane: Opening of the Icosahedron to Give a Zwitterionic Intermediate and Conversion into 7,8‐Dicarba‐nido‐undecaborate(1−)

“…However, in general the calculated data agree well with experimental data if these become available. This is verified here for ), [39] in agreement with the fairly sharp 13 C-A C H T U N G T R E N N U N G (1,2) NMR spectroscopic signal (Figure 1), whereas the calculated coupling constants…”

“…[37][38][39] Experimentally, these parameters are often difficult to determine on account of fast 11 B nuclear spin relaxation. However, in general the calculated data agree well with experimental data if these become available.…”

Reactions of 1,3,2‐Diselenaphospholanes with Lewis Acids: Borane and (Pentamethylcyclopentadienyl)rhodium and ‐iridium Dichloride

“… [a] Coupling constants n J ( 31 P, 13 C) are given in parentheses (±0.5 Hz); n J ( 77 Se, 13 C) are given in square brackets [±0.5 Hz]; n J ( 103 Rh, 13 C) are given in angled brackets <±0.5 Hz>; 1 Δ 10/11 B[ 13 C(carb)]12 are given in braces {±0.5 ppb}; isotope‐induced chemical shifts 1 Δ are given in ppb, and the negative sign denotes a shift of NMR spectroscopic signal of the heavy isotopomer to lower frequency; n.o. : not observed. …”

Applications of nuclear shielding

“…and range up to 15 Hz. 166 The preparation and crystal structures of four ortho-carboranyl-nitrogen compounds, PhCboN:N(C 6 H 4 Me-4) (1), PhCboNHNH(C 6 H 4 Me-4) (2), MeCboNHNHPh (3) and PhCboNHOH (4) (Cbo = 1,2-C 2 B 10 H 10 ; nitrogen groups at cage carbon C1, Ph or Me at C2), the last as a 1,4-dioxane solvate, were reported. The 11 B NMR chemical shifts of their antipodal boron atoms reflect the p-bonding characteristics of the nitrogen substituent.…”