Long-Range-¹³C-H-Kopplungen in einigen Cyclobutadien-eisentricarbonylen. Ein Beitrag zur Kenntnis der Bindungsverhältnisse im komplexgebundenen Cyclobutadien

Abstract

All observable long-range 13C-H-coupling constants have been measured for cyclobutadiene-iron-tricarbonyl, 1.2-dimethyl-cyclobutadiene-irontricarbonyl, trimethyl-cyclobutadiene-irontricarbonyl and tetramethyl-cyclobutadiene-irontricarbonyl. The couplings are assigned and discussed with respect to bonding problems in the four-membered ring. A quadratic structure of the cyclobutadiene-system with uniform C—C-bond order is postulated on the basis of these measurements.

(Cyclobutadiene)iron TricarbonylA Case of Theory before Experiment

“…Brune and co-workers investigated the bonding in (substituted-cyclobutadiene)iron tricarbonyls by measuring the long-range 13 C− 1 H coupling constants in their proton NMR spectra. , The main conclusion agreed with the results of the other structural and spectroscopic studies: the ligand has a square geometry with all C−C bonds of equal length. The results of a similar NMR study of ((para-substituted phenyl)cyclobutadiene)iron tricarbonyl complexes, (Z−C 6 H 4 C 4 H 3 )Fe(CO) 3 ; Z = H, CH 3 , F, Cl, Br) ( 13 C− 1 H and 1 H− 1 H coupling constants and ν(CO) in the IR spectra) suggested there is no conjugative π interaction between the phenyl and cyclobutadiene rings …”

“…However, A is still used.) Once it was known that (cyclobutadiene)iron tricarbonyl was stable and that it could be prepared by halogen elimination from vicinal dihalocyclobutenes with Fe 2 (CO) 9 , others used this procedure to prepare diverse alkyl-substituted cyclobutadiene−iron carbonyl complexes: 1,2-dimethyl-3,4-diethyl-, trimethylethyl-, 1-ethyl-2,3-dimethyl-, isopropyl-2,3-dimethyl-, 1,2-dimethyl-3,4-diisopropyl-, trimethylisopropyl-, 1,2-dimethyl-, tetramethyl-, trimethyl-, and 1,3-dimethylcyclobutadiene−iron tricarbonyl, as well as 23 and 24 6 Rowland Pettit (reproduced by permission of Prof. J.…”

Synthesis and Redox Behavior of Azulene-Substituted Benzene Derivatives and (η5-Cyclopentadienyl)[tetra- and di(6-azulenyl)cyclobutadiene]cobalt Complexes

“…A slice of the 2D spectrum in the region unequivocally exhibited the signals ( 11a , 1 J CH = 188 Hz; 11b , 1 J CH = 189 Hz) with a negligibly small coupling constant (<1 Hz) between the cyclobutadiene protons (part a of Figure ). These results clearly show the presence of the 1,2-disubstitution pattern in the cyclobutadiene ring in 11a and 11b 2 (a) Slice of 2D HMQC spectrum in the cyclobutadiene proton region and (b) 13 C satellite signals of the protons measured by 1 H NMR spectrum of 11b .

3 2D HMQC spectrum of 11b measured under nondecoupling conditions.

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Brom-cyclobutadien-eisentricarbonyl / Bromo-cyclobutadiene-irontricarbonyl

“…1. c. [5][6][7][8] ). In Übereinstimmung mit dem Spektrum ist nur eine solche Struktur, in der die beiden von C(l) ausgehenden Ringbindungen …”