Synthesis and electrophile-induced disproportionation of the neutral formyl triphenylphosphinenitrosyl-.eta.-cyclopentadienylrhenium formyl ((.eta.-C5H5)Re(NO)(PPh3)(CHO))
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The crystalline, thermally stable neutral formyl (η-C5H5)Re(NO)(PPh3)(CHO) (3) is synthesized by reaction of cation [(η-C5H5)Re(NO)(PPh3)(CO)]+BF4− (2a) with either Li(C2H5)3BH in THF or NaBH4 in THF/H2O. Precursor 2a is in turn prepared by the sequential treatment of [(η-C5H5)Re(NO)(CO)2]+BF4− (1) with C6H5I+O−/CH3CN (oxidative removal of CO) and PPh3. At 50–105 °C in appropriate solvents, 3 decomposes (in variable yields) to rhenium hydrides. 3 is reduced by BH3·THF to (η-C5H5)Re(NO)(PPh3)(CH3) (4). When 3 is reacted with CH3SO3F or CF3CO2H, facile formyl ligand disproportionation occurs: 4 and [(η-C5H5)Re(NO)(PPh3)(CO)]+ salts form. Potential intermediates in these disproportionations are independently synthesized. Reaction of 4 with Ph3C+X− (X = PF6, BF4) at −78 °C affords the cationic methylidene complex [(η-C5H5)Re(NO)(PPh3)(CH2)]+X− (5), which can be isolated as a stable solid. 5 is further characterized by preparing [(η-C5H5)Re(NO)(PPh3)(CH2L+)] adducts where L = pyridine (6a), 2,6-dimethylpyridine (6b), PPh3 (7a), and P(η-C4H9)3 (7b). Reaction of 5 or 6a with excess CH3O− yields (η-C5H5)Re(NO)(PPh3)(CH2OCH3) (8). Addition of 0.5 equiv of CH3SO3F to 8 gives 4, (CH3)2O, and [(η-C5H5)Re(NO)(PPh3)(CHOCH3)]+SO3F− (9a) in a 1.0:1.0:1.1 ratio. On the basis of hydride transfer reactions observed between 3 and 5, 3 and 9a, and 8 and 5, and low temperature 1H NMR monitoring, the CH3SO3F-induced disproportionation of 3 is proposed to involve the sequence of intermediates 3→9a→8→5→4. Reaction of 3 with CF3CO2H is suggested to occur similarly; initial formation of unstable hydroxymethylidene intermediate [(η-C5H5)Re(NO)(PPh3)(CHOH)]+X− (X = CF3CO2, 10a) can be observed by 1H and 13C NMR spectroscopy. When X = CF3S03, this salt can be isolated. Attempts to prepare the proposed hydroxymethylidene intermediate (η-C5H5)Re(NO)(PPh3)(CH2OH) (11) are detailed. Syntheses of (η-C5H3)Re(NO)(PPh3)(COOH) (2a + NaOH) and (η-C5H5)Re(NO)(PPh3)(H) (2a + (CH3)3N+O−/LiAlH4) are also described, and the relevance of the above reactions to catalytic CO reduction is discussed. © 1982, American Chemical Society. All rights reserved.
author list (cited authors)
Tam, W., Lin, G. Y., Wong, W. K., Kiel, W. A., Wong, V. K., & Gladysz, J. A.