SYNTHESIS, STRUCTURE, DYNAMIC BEHAVIOR, AND REACTIVITY OF RHENIUM PHOSPHIDO COMPLEXES (ETA-5-C5H5)RE(NO)(PPH3)(PR2) - THE GAUCHE EFFECT IN TRANSITION-METAL CHEMISTRY
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Reactions of (5-C5H5)Re(NO)(PPh3)(X) (X = OTs (OSO2-p-Tol), OTf (OSO, CF, )) with PR, H (R = Ph (a), p-Tol (b), Et (c), t-Bu (d)) give secondary phosphine complexes [(5-C5H5)Re(NO)(PPh3)(PPh3)]+X (2a-TsO, 2bTsO, 2c-TfO, 2d-TfO; 87-96%). Reactions of 2a-d-X with t-BuOK+ give phosphido complexes (5-C5H5)Re(NO)(PPh3)(PR2) (4a-d; 79-99%). Optically active, configurationally stable (+)-(S)-4b is analogously prepared. NMR experiments show 4a-d to have very low PR2 phosphorus inversion barriers (12.6-14.9 kcal/mol). The rapid alkylation of 4a by CH2C12 to give [5-G5H5)Re(NO)(PPh3)(PPh2CH2Cl)]+Cr (79%) shows the PR2 phosphorus to be highly nucleophilic, and 4a and 4d are easily oxidized (02, PhIO) to phosphine oxides (5-C5H5)Re(NO)(PPh3)(P(=O)R2) (34-60%). The X-ray crystal structures of 4a and 4d show that the PR2 phosphorus lone pairs make 59-60 torsion angles with the rhenium d orbital HOMO. It is proposed that avoided overlap between these orbitals (gauche effect) is an important Re-PR2 conformation-determining factor. This proposal is supported by extended-Hckel MO calculations on the model compound (5-C5H5)Re(NO)(PH3)(PH2). 1988, American Chemical Society. All rights reserved.
