UNEQUIVOCAL PROOF OF SLOWED CHROMIUM TRICARBONYL ROTATION IN A STERICALLY CROWDED ARENE COMPLEX - AN X-RAY CRYSTALLOGRAPHIC AND VARIABLE-TEMPERATURE HIGH-FIELD NMR-STUDY OF (C6ET5COCH3)CR(CO)3

The low-temperature13C NMR spectrum of (C6Et5COCH3)Cr(CO)3is explicable only in terms of a single stereoisomer, i.e., the l-proximal-acetyl-2,4,6-distal-3,5-proximal conformer. The Cr(CO)3resonance is split 2:1 both in solution at -100 °C and in the CPMAS solid-state spectrum at 30 °C; these data demonstrate that Cr(CO)3rotation has stopped on the NMR time scale. Crystals of (C6Et5COCH3)Cr(CO)3are triclinic, of space group PĪ with a = 8.876 (2) Å, b = 9.458 (3) Å, c = 13.383 (4) Å, α= 95.26 (2)°, β = 98.89 (2)°, γ = 111.54 (2)°, V= 1018.9 (5) Å3, Dc= 1.29 g cm-3, Dm= 1.28 g cm-3for Z = 2, and R1= 0.0659 (R2= 0.0753) for 2673 unique reflections (R1= 0.0501, R2= 0.0547 for 2198 reflections with/> 2.5σ (l)). The alternating proximal-distal arrangement of substituents is in accord with the NMR data. These results are discussed in the context of previous reports on (C6Et6)Cr(CsO)2L complexes. © 1990, American Chemical Society. All rights reserved.