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 (η5-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 [(η5-C5H5)Re(NO)(PPh3)(PPh3)]+X− (2a-TsO−, 2b−TsO−, 2c-TfO−, 2d-TfO−; 87-96%). Reactions of 2a-d-X− with t-BuO−K+ give phosphido complexes (η5-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 [η5-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 (η5-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-Hückel MO calculations on the model compound (η5-C5H5)Re(NO)(PH3)(PH2). © 1988, American Chemical Society. All rights reserved.
author list (cited authors)
Buhro, W. E., Zwick, B. D., Georgiou, S., Hutchinson, J. P., & Gladysz, J. A.