SYNTHESIS AND ELECTROPHILE-INDUCED DISPROPORTIONATION OF THE NEUTRAL FORMYL (ETA-C5H5)RE(NO)(PPH3)(CHO)
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The crystalline, thermally stable neutral formyl (-C5H5)Re(NO)(PPh3)(CHO) (3) is synthesized by reaction of cation [(-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 [(-C5H5)Re(NO)(CO)2]+BF4 (1) with C6H5I+O/CH3CN (oxidative removal of CO) and PPh3. At 50105 C in appropriate solvents, 3 decomposes (in variable yields) to rhenium hydrides. 3 is reduced by BH3THF to (-C5H5)Re(NO)(PPh3)(CH3) (4). When 3 is reacted with CH3SO3F or CF3CO2H, facile formyl ligand disproportionation occurs: 4 and [(-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 [(-C5H5)Re(NO)(PPh3)(CH2)]+X (5), which can be isolated as a stable solid. 5 is further characterized by preparing [(-C5H5)Re(NO)(PPh3)(CH2L+)] adducts where L = pyridine (6a), 2,6-dimethylpyridine (6b), PPh3 (7a), and P(-C4H9)3 (7b). Reaction of 5 or 6a with excess CH3O yields (-C5H5)Re(NO)(PPh3)(CH2OCH3) (8). Addition of 0.5 equiv of CH3SO3F to 8 gives 4, (CH3)2O, and [(-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 39a854. Reaction of 3 with CF3CO2H is suggested to occur similarly; initial formation of unstable hydroxymethylidene intermediate [(-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 (-C5H5)Re(NO)(PPh3)(CH2OH) (11) are detailed. Syntheses of (-C5H3)Re(NO)(PPh3)(COOH) (2a + NaOH) and (-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.