ABSTRACT
The synthesis and structural characterization of the first [1.1]chromarenophanes and the first [1.1]molybdarenophanes are described. A salt-metathesis reaction of [2-(Me 2NCH 2)C 6H 4]AlCl 2 with freshly prepared [Cr(LiC 6H 5) 2].TMEDA (TMEDA = N, N, N', N'-tetramethylethylenediamine) resulted in the dialumina[1.1]chromarenophane [{2-(Me 2NCH 2)C 6H 4}Al(eta (6)-C 6H 5) 2Cr] 2 ( 2a). The poor solubility of 2a in organic solvents prompted us to synthesize the new intramolecularly coordinated aluminum- and gallium dichlorides [5- tBu-2-(Me 2NCH 2)C 6H 3]ECl 2 [E = Al ( 3a), Ga ( 3b)] in which the phenyl group was equipped with a tert-butyl group. Salt-metathesis reactions of 3a and 3b, respectively, with freshly prepared [M(LiC 6H 5) 2].TMEDA (M = Cr, Mo) resulted in four new [1.1]metallarenophanes of the general type [{5- tBu-2-(Me 2NCH 2)C 6H 3}E(eta (6)-C 6H 5) 2M] 2 [E = Al, M = Cr ( 4a); E = Ga, M = Cr ( 4b); E = Al, M = Mo ( 5a); E = Ga, M = Mo ( 5b)]. 2a, 4a, b, and 5a, b have been structurally characterized by single-crystal analysis [ 2a.1/2C 6H 12: C 48H 56Al 2Cr 2N 2, monoclinic, P2 1/ c, a = 9.9117(9) A, b = 19.9361(16) A, c = 10.638(2) A, alpha = 90 degrees , beta = 112.322(5) degrees , gamma = 90 degrees , Z = 2; 4a.2C 6H 6: C 62H 72Al 2Cr 2N 2, monoclinic, P2 1/ c, a = 10.9626(9) A, b = 19.3350(18) A, c = 12.4626(9) A, alpha = 90 degrees , beta = 100.756(5) degrees , gamma = 90 degrees , Z = 2; 4b.2C 6H 6: C 62H 72Cr 2Ga 2N 2, monoclinic, P2 1/ c, a = 10.8428(2) A, b = 19.4844(4) A, c = 12.4958(2) A, alpha = 90 degrees , beta = 100.6187 degrees , gamma = 90 degrees , Z = 2; 5a.2C 6H 6: C 62H 72Al 2Mo 2N 2, triclinic, P1, a = 10.4377(4) A, b = 11.6510(4) A, c = 11.6514(4) A, alpha = 73.545(3) degrees , beta = 89.318(2) degrees , gamma = 76.120(2) degrees , Z = 1; 5b.2C 6H 6: C 62H 72Ga 2Mo 2N 2, triclinic, P1, a = 10.3451(5) A, b = 11.6752(6) A, c = 11.6900(5) A, alpha = 73.917(3) degrees , beta = 89.550(3) degrees , gamma = 76.774(2) degrees , Z = 1]. All five [1.1]metallarenophanes 2a, 4a, b, and 5a, b crystallize as anti isomers with both Me 2N donor groups in exo positions ( C i point group symmetry). The new [1.1]metallarenophanes show NMR spectra that can be interpreted as being caused by time-averaged C 2 h symmetrical species, which is consistent with the findings of their molecular structures in the solid state. Variable-temperature (1)H NMR measurements for 4a, b and 5a, b (500 MHz; -90 to 90 degrees C) revealed only peak broadening in the lower temperature range of -70 to -90 degrees C. (1)H NMR saturation transfer difference experiments did not show an expected anti-to-anti isomerization, rendering the new [1.1]metallacyclophanes rigid on the NMR time scale. Electrochemical measurements were performed for 4a, b and 5a, b. However, reproducible cyclic voltammograms could only be obtained for the two gallium species 4b and 5b, revealing the expected weak communication between the two transition-metal atoms in both compounds (class II).
ABSTRACT
The first [1]molybdarenophanes were synthesized and structurally characterized. The aluminum and gallium compounds [(Me2Ntsi)Al(eta6-C6H5)2Mo] (2a) and [(Me2Ntsi)Ga(eta6-C6H5)2Mo] (2b) [Me2Ntsi = C(SiMe3)2(SiMe2NMe2)] were obtained from [Mo(LiC6H5)2].TMEDA and (Me2Ntsi)ECl2 [E = Al, Ga] in analytical pure form with isolated yields of 74% (2a) and 52% (2b). The silicon-bridged species [Ph2Si(eta6-C6H5)2Mo] (2c) was synthesized from [Mo(LiC6H5)2].TMEDA and Ph2SiCl2. Compound 2c was isolated as a crystalline material in an approximately 90% overall purity, from which a single crystal was used for X-ray analysis. The molecular structures of all three [1]molybdarenophanes 2a-c were determined by single-crystal X-ray analysis. The ring-tilt angle alpha was found to be 18.28(17), 21.24(10), and 20.23(29) degrees for 2a, 2b, and 2c, respectively. Variable temperature NMR measurements of 2a and 2b (-80 to 80 degrees C; 500 MHz) showed a dynamic behavior of the gallium species 2b but not of compound 2a. The dynamic behavior of 2b was rationalized by assuming that the Ga-N donor bond breaks, inversion at the nitrogen atom occurs, and a rotation of the Me2Ntsi ligand takes place followed by a re-formation of the Ga-N bond on the other side of the gallium atom. The analysis of the signals of meta and ortho protons of 2b gave approximate values of DeltaG not equal of 59.6 and 59.1 kJ mol-1, respectively. Compound 2b reacted with [Pt(PEt3)3] to give the ring-open product [(eta6-C6H6)Mo{eta6-C6H5[GaPh(Me2Ntsi)]}] (3b). The molecular structure of 3b was deduced from a single-crystal X-ray determination. The formation of the unexpected platinum-free product 3b can be rationalized by assuming that benzene reacted with 2b in a 1:1 ratio. Through a series of 1H NMR experiments with 2b it was shown that small amounts of donor molecules (e.g., THF) in benzene are needed to form 3b; in the absence of a donor molecule, 2b is thermally stable.
