[1]Molybdarenophanes: Strained Metallarenophanes with Aluminum, Gallium, and Silicon in Bridging Positions

The first [1]molybdarenophanes were synthesized and structurally characterized. The aluminum
and gallium compounds [(Me2Ntsi)Al(η6-C6H5)2Mo] (2a) and [(Me2Ntsi)Ga(η6-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(η6-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 α was found to be 18.28(17), 21.24(10), and 20.23(29)° for 2a,
2b, and 2c, respectively. Variable temperature NMR measurements of 2a and 2b (−80 to 80 °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 ΔG≠ of 59.6 and 59.1 kJ mol-1, respectively. Compound 2b reacted with [Pt(PEt3)3]
to give the ring-open product [(η6-C6H6)Mo{η6-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.