¹³C nuclear magnetic resonance study of five- and six-coordinated carbon in nonclassical organometallic compounds: Dimeric trialkyl-, tricyclopropyl-, and triarylaluminums and some nido and closo carboranes

Abstract

A 13C nuclear magnetic resonance spectroscopic study of dimeric trimethyl-, triethyl-, tricyclopropyl-, and triarylaluminums is reported. The five-coordinated bridging carbons are found consistently more shielded than the terminal carbons, in accordance with the increased p-character of the former. The nature of bridging two-electron three-centered Al-C-Al bonds is discussed. 13C nuclear magnetic resonance shifts of several nido and closo carboranes containing five and six coordinated carbons and their 13C-IH spin-spin coupling constants were also obtained. The relationship between the carbon chemical shifts and coordination number of the carbon atom is discussed. There is approximately a 20-to 40-ppm

Advances in Structural Studies on Alkylaluminum Species in the Solid State via Challenging 27Al–13C NMR Spectroscopy and X-ray Diffraction

“…The 1 H → 13 C CP-MAS spectrum, shown in Figure and Figure S2a, exhibits a broad signal spanning from −2 to −10 ppm, a chemical shift range typical of direct aluminum–carbon bonds. This range of 13 C chemical shifts is consistent with those measured for 1 in solution. , The complexity of the 13 C signal results from (i) the existence of several crystallographically inequivalent 13 C sites, (ii) the one-bond 27 Al– 13 C scalar-coupling, , and (iii) the second-order cross-terms between quadrupolar and dipolar interactions. , The 1 J Al–C coupling constants have not been determined for solid-state [25%- 13 C]- 2 , but they were found equal to 71 Hz for 1 dissolved in 1,2-dimethoxyethane at room temperature . In [25%- 13 C]- 2 , 27 Al– 13 C distances are of about 200 pm, which corresponds to a dipolar coupling constant of b Al–C /(2π) = −985 Hz, and the second-order shift of the outermost lines of a 13 C sextuplet is thus ca.…”

“…This range of 13 C chemical shifts is consistent with those measured for 1 in solution. 17,18 The complexity of the 13 C signal results from (i) the existence of several crystallographically inequivalent 13 C sites, (ii) the one-bond 27 Al− 13 C scalar-coupling, 16,17 and (iii) the second-order cross-terms between quadrupolar and dipolar interactions. 39,50 The 1 J Al−C coupling constants have not been determined for solid-state [25%- 13 C]-2, but they were found equal to 71 Hz for 1 dissolved in 1,2-dimethoxyethane at room temperature.…”

“…In this context, X-ray diffraction (XRD) and solid-state NMR constitute two complementary methods to probe respectively the long- and short-range orders in solid organoaluminum materials. − Single-crystal XRD provides detailed information on the internal lattice including cell dimensions, bond-lengths, and bond angles, but it is not an element specific method. Moreover, the preparation of single crystals may sometimes prove to be very troublesome.…”