Structural studies and matrix photochemistry of tetramethyloxorhenium(vi), (CH3)4ReO, and related compounds

Abstract

The crystal structure of tetramethyloxorhenium(VI), (CH₃)₄ReO, 1, at 150 K reveals molecules with a square-based pyramidal skeleton and dimensions very similar to those in the gas phase. The related binuclear compound [(CH₃)₃Re(O)]₂O, 2, has likewise been shown to feature rhenium centres with a similar coordination geometry linked by a linear Re–O–Re bridge and carrying terminal ReO bonds that are mutually trans. Salient average dimensions (distances in Å, angles in °) are as follows: 1 Re–C 2.110(8), ReO 1.684(4), ORe–C 112.6(2), C–Re–C 81.5(5) and 134.8(3); 2 Re–C 2.095–2.134(11), ReO 1.675(6), Re–O 1.851(1), ORe–C 103.0–113.5(4), C–Re–C 79.2(4) and 132.5–137.0(4). Density functional theory (DFT) calculations have been performed to predict ionization energies for (CH₃)₄ReO. In the light of these results, the photoelectron spectrum has been reassigned. IR spectroscopic measurements have been used to chart the reactions activated by irradiating solid argon matrices doped with 1 with broad-band UV-visible light (200 ≤ λ ≤ 800 nm). The spectra have been interpreted on the basis of additional experiments with (CD₃)₄ReO, 1-d₁₂, and by comparison either with the vibrational properties forecast by calculations or with those of known related molecules. Hence photoexcitation is shown to result in the elimination of methane and the formation of the novel methylidene–rhenium(VI) compound H₂CRe(O)(CH₃)₂, 3. The results are discussed in relation to the photochemistry of methyltrioxorhenium(VII) in similar circumstances.