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)}(CN

Abstract

From the reactions of the unsubstituted tantalacyclobutane complex [Ta{(CH 2 ) 3 -1,3}(CNN)-(O-t-Bu) 2 ] (CNN ) [C 6 H 4 (CH 2 N(Me)CH 2 CH 2 NMe 2 )-2] -) (1) with propene and styrene have been isolated in good yields the substituted tantalacyclobutane complexes [Ta{CH 2 CH(Me)-CH 2 -1,3}(CNN)(O-t-Bu) 2 ] (2) and [Ta{CH 2 CH(Ph)CH 2 -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 σ-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 Et 2 O with ethene. The reaction of 1 with carbon monoxide yields the oxytantalacyclopropane complex [Ta{C(O)((CH 2 ) 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 CdO function are ligated in the meridional plane with further η 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(dN-t-Bu)-CH 2 CH(R)CH 2 -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. 1 H and 13 C 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 η 2 -C,Nbonding of the CNN ligand through the aryl C ipso 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 R-carbon atom of the η 2 -bonded iminoacyl fragment, while the larger five-membered ring is a metallacylopentane system comprising the metal center and four carbon atoms.

Tantallaaziridines: from synthesis to catalytic applications

“…Furthermore, such compounds have been extensively characterized in the solid state. For instance, the CvN bond distance is frequently between 1.25 and 1.30 Å, 16,18,19,[25][26][27] which is typical for CvN bond distances in linear and cyclic imines 28 and is much shorter than the average C-N distance found for a tantallaaziridine (vide infra) (Table 1).…”

“…However, in this case the ligand is formally monoanionic, the carbon atom has only one R substituent, and there is a formal double bond between the carbon and nitrogen in the ring. Complexes of this type are most commonly synthesized by inserting isonitriles into existing Ta-E bonds (Scheme 2), where E can be C, [15][16][17][18] N, 19,20 or Si. 21 An alternative synthetic route involves the reduction of a tantalum(III) dimer with Zn powder in the presence of biarylamidinate ligands.…”

“…44 Based on frontier molecular orbital arguments, tantalum backdonation into the π* CvN orbital of the heterocycle substrate Table 1 Bond lengths and angles for some representative examples of iminoacyl-ligated complexes. 16,19,25 Metrical data for representative tantallaaziridines, 12,29,30 free isonitrile 31 and free imine 28 Scheme 4 Insertion of isonitrile into a Ta-C bond to form an iminoacyl species which reacts with another alkyl ligand to form a tantallaaziridine. 37 When R = tBu, the iminoacyl species can be isolated.…”

η2‐Iminoacyl and η2‐Acyl Monocyclopentadienyl Tantalum Complexes Bearing Oxo and Oxo‐Borane Ligands

“…δ = 240 ppm is the most apparent spectroscopic feature and confirms the presence of the η 2 -iminoacyl ligand in complexes 2. We previously reported [10] 3 , which afforded 2d in high yield. The formation of 2d was confirmed by 13 C and 19 F NMR spectroscopy.…”

“…[1] For a given metal atom, the stability and further evolution of these compounds are determined by the nature of the ancillary ligands. Many reaction pathways may follow to give a broad variety of products: [1] (a) migratory insertion of a second alkyl or aryl group to give η 2 -imine or η 2 -ketene complexes; [2][3][4][5] (b) intra-or intermolecular coupling of iminoacyl or acyl units affording diaza-or dioxobutene complexes; [6,7] (c) transfer of the NR or O moieties to the metal centre; [8][9][10][11] (d) insertion of a second CNR [12][13][14][15][16] or CO [17] molecule into the new M-C bond formed after the first insertion process and (e) hydrogen migration. [18,19] We reported the results of our studies on the insertion reactions of CNR into the Ta-C(methyl) bond of monocyclopentadienyl complexes of the type [TaCp*Cl x Me 4-x ] [8][9][10] for which processes (a) and (c) were observed.…”