FORMATION AND MOLECULAR-STRUCTURE OF (ETA-5-CYCLOPENTADIENYL)(ETA-4-1,3-DIMESITYL-2,4-DIPHENYLCYCLOBUTADIENE)COBALT - COMPOUND WITH RESTRICTED ROTATION ABOUT AN ARYL-CYCLOBUTADIENE BOND

The reaction of η5-C5H5)Co(CO)2 (1) and mesitylphenylacetylene (3) has been found to result in the exclusive formation of a single cyclobutadiene-cobalt complex (η5-cyclopentadienyl)(η41, 3-dimesityl-2, 4-diphenylcyclobutadiene)cobalt (6) and a single cyclopentadienone-cobalt complex (η5-cyclopentadienyl)(η4-2, 4-dimesityl-3, 5-diphenylcyclopentadienone)cobalt (8), both of which exhibit a high degree of intramolecular steric crowding. The structures of 6 and 8 have been assigned on the basis of spectral considerations and unequivocally confirmed by means of a single-crystal X-ray crystallographic investigation of the former complex. The products 6 and 8 are those predicted on the basis of steric considerations of proposed cobaltacyclic intermediates in these types of reactions. Both 1H NMR and 13C NMR studies of 6 in solution indicate restricted rotation about the mesityl-cyclobutadiene bonds, and activation parameters for this process have been determined by dynamic ‘ H NMR studies. For a quantitative measure of the degree of crowding in 6, its crystal and molecular structures were determined by single-crystal X-ray diffraction methods. For comparison, those of 5, (η5-C5H5)Co(C4Ph4), were determined as well. Pertinent crystallographic data for 5 are as follows: space group P21/c, a = 10.293 (1) Å, b = 19.690 (3), Å, c = 12.904 (1) Å, β = 111.42 (1)°, volume = 2434 Å3, d(calcd) = 1.32 g cm-3, d(measd) = 1.30 g cm-3, Z = 4. For 6, the space group is P21 a = 8.573 (4) Å,b = 8.951 (7) Å,c = 20.324 (7) Å, β = 97.80 (3)°, volume = 1482.9 (2.5) A3, rf(calcd) = 1.265 g cm-3, (i(measd) = 1.25 g cm-3, and Z = 2. Both structures were refined by standard methods to final values of R(F) and Rw(F) of 0.029 and 0.027 (5) and 0.041 and 0.047 (6). The structure of the latter is highly hindered by interactions of the o-methyl groups of the mesityl substituents, while in 5 no serious barriers to torsional motion are found. These results are in agreement with the solution NMR behavior of the compounds. Furthermore, we find that the steric crowding present in 6 leads to distortions of the cyclobutadiene ring which are not found in the less hindered compound 5. Details of these steric phenomena are given in the text. © 1979, American Chemical Society. All rights reserved.