Tantalacyclobutane complexes containing the potentially C,N,N'-Coordinating ligand [C6H4(CH2N(Me)CH2CH2NMe2)-2]- (CNN) and their reactivity with carbon monoxide and tert-butyl isocyanide. X-ray molecular structures of [Ta{CH2CH(Me)CH2-1,3} (CNN)(O-t-Bu)2], [Ta{CH2CH(Ph)CH2-1,3} (CNN) (O-t-Bu)2], [Ta{C(O)((CH2)3-1,3)} (CNN) (Ot-Bu)2], and [Ta{C(=N-t-Bu)CH2CH(Ph)CH2-1,4} (CNN (O-t-Bu)2]

From the reactions of the unsubstituted tantalacyclobutane complex [Ta{(CH2)3-1,3}(CNN)(O-t-Bu)2] (CNN = [C6H4(CH2N(Me)CH2CH2NMe2)-2]-) (1) with propene and styrene have been isolated in good yields the substituted tantalacyclobutane complexes [Ta{CH2CH(Me)CH2-1,3}(CNN)(O-t-Bu)2] (2) and [Ta{CH2CH(Ph)CH2-1,3}(CNN)(O-t-Bu)2] (3), respectively. The X-ray molecular structures of 2 and 3 show them to be seven-coordinate pentagonal bipyramidal species in which the meridional ligation comprises C,N,N'-bonding of the aryldiamine ligand CNN and -bonding of two carbons of the metallacycle; the two alkoxide groups occupy the apical positions. The organyl substituent of the metallacycle is positioned on the -carbon. Complexes 2 and 3 can both be converted back into 1 by reacting them in Et2O with ethene. The reaction of 1 with carbon monoxide yields the oxytantalacyclopropane complex [Ta{C(O)((CH2)3-1,3)}(CNN)(O-t-Bu)2] (4), which has been isolated as an air-stable white solid in 59% yield. The X-ray molecular structure of 4 reveals that the carbon and oxygen atoms of the C=O function are ligated in the meridional plane with further 3-C,N,N'-ligation of CNN and the two alkoxides as found in 2 and 3. Reaction of the tantalacyclobutane complexes 2-4 with 1 equiv of t-BuNC affords the 1:1 insertion products [Ta{C(=N-t-Bu)CH2CH(R)CH2-1,4}(CNN)(O-t-Bu)2] (R = H (5), Me (6), and Ph (7), respectively). Complexes 5 and 7 have been isolated as yellow solids (yields: 5, 62%; 7, 50%), while 6 has been prepared and characterized in situ. 1H and 13C NMR spectroscopy of 6 and 7 indicate for each complex the presence of two isomers in solution. The X-ray molecular structure of 7 shows 2-C,N-bonding of the CNN ligand through the aryl Cipso atom and the N(Me) donor atom and, most importantly, the presence of two tantalacyclic rings of different sizes that have a common Ta-C bond. The smaller three-membered ring comprises the metal center with the nitrogen and the -carbon atom of the 2-bonded iminoacyl fragment, while the larger five-membered ring is a metallacylopentane system comprising the metal center and four carbon atoms