Synthesis and Electrophile-Induced Disproportionation of the Neutral Formyl (η-C₅H₅)Re(NO)(PPh₃)(CHO)

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)(CH₂L⁺)] 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.