Chemical and Electrochemical Oxidation of Diphenylphosphide-Bridged Hydrides [M₂(η⁵-C₅H₅)₂(μ-H)(μ-PPh₂)(CO)₄] and Anions [M₂(η⁵-C₅H₅)₂(μ-PPh₂)(CO)₄]^-(M = Mo, W)

Abstract

Reaction of the hydride complexes [M 2 Cp 2 (µ-H)(µ-PPh 2 )(CO) 4 ] (M ) Mo, W; Cp ) η 5 -C 5 H 5 ) with [FeCp 2 ]BF 4 proceeds in a 1:2 ratio to give the unsaturated tetracarbonyl salts [M 2 Cp 2 -(µ-PPh 2 )(CO) 4 ]BF 4 , which spontaneously (M ) Mo) or in the presence of CO (M ) W) transform into the corresponding electron-precise pentacarbonyls [M 2 Cp 2 (µ-PPh 2 )(µ-CO)-(CO) 4 ]BF 4 . In contrast, reaction of the above hydride complexes with [FeCp 2 ](BAr′ 4 ) (Ar′ ) 3,5-C 6 H 3 (CF 3 ) 2 ) proceeds in a 1:1 ratio to give the paramagnetic salts [M 2 Cp 2 (µ-H)(µ-PPh 2 )-(CO) 4 ](BAr′ 4 ). No further reaction takes place with [FeCp 2 ](BAr′ 4 ) unless a proton acceptor such as water or tetrahydrofuran is present, in which case the corresponding salts [M 2 Cp 2 -(µ-PPh 2 )(CO) 4 ](BAr′ 4 ) or [M 2 Cp 2 (µ-PPh 2 )(µ-CO)(CO) 4 ](BAr′ 4 ) are rapidly formed. Reactions of the anions [M 2 Cp 2 (µ-PPh 2 )(CO) 4 ] -(as (H-DBU) + salts, DBU ) 1,8-diazabicyclo[5.4.0]undec-7-ene) with [FeCp 2 ]BF 4 proceed stepwise to give first the 33-electron radicals [M 2 Cp 2 (µ-PPh 2 )(CO) 4 ], which can be isolated as very air-sensitive green solids, and then the unsaturated salts [M 2 Cp 2 (µ-PPh 2 )(CO) 4 ]BF 4 , which are identical to those formed from the neutral hydrides. The structure and dynamic behavior of the new compounds are analyzed on the basis of solution IR, 1 H, 31 P, and 13 C NMR or ESR spectroscopic data. In addition, the redox and other chemical transformations connecting all the new compounds are discussed on the basis of the synthetic and structural data, as well as cyclic voltammetry measurements carried out in dichloromethane on the neutral hydrides [M 2 Cp 2 (µ-H)(µ-PPh 2 )-(CO) 4 ] and the anions [M 2 Cp 2 (µ-PPh 2 )(CO) 4 ] -(as [N(PPh 3 ) 2 ] + salts).

Synthesis and Decarbonylation Reactions of Diiron Cyclopentadienyl Complexes with Bent-Phosphinidene Bridges

“…Only when this process is coupled to a proton transfer reaction due to the presence of a base, as noted above, would the unfavorable thermodynamics to reach compound 2d be overcome. A similar effect has been previously found in the chemical and electrochemical studies of the hydride derivatives [M 2 Cp 2 (μ-H)(μ-PPh 2 )(CO) 4 ] (M = Mo, W), where the second oxidation process to give [M 2 Cp 2 (μ-H)(μ-PPh 2 )(CO) 4 ] 2+ occurred irreversibly at a potential higher than that of the ferrocenium cation (0.1−0.25 V), but only in the presence of a weak base, yielding the monopositive cations [M 2 Cp 2 (μ-PPh 2 )(CO) 4 ] + . Unfortunately, in the case of 1d , the return sweep to negative potentials after the second oxidation shows a greatly diminished reduction wave corresponding to the first oxidation process, this being indicative of hampered diffusion of the analyte to the surface of the working electrode, most probably due to adsorption of the decomposition products formed after the second and irreversible oxidation of 1d , thus precluding the extraction of any further information from this experiment.…”

“…That the cis structure B would be more stable than the trans isomer A in the paramagnetic cation would not be unusual. For instance, we have shown previously that the oxidation of the phosphide−hydride complexes trans -[Fe 2 Cp 2 (μ-H)(μ-PPh 2 )(CO) 2 ] and trans -[M 2 Cp 2 (μ-H)(μ-PPh 2 )(CO) 4 ] (M = Mo, W) yields the paramagnetic cations [M 2 Cp 2 (μ-H)(μ-PR 2 )(CO) x ] + ( x = 2, 4) specifically with a cis geometry. , The radical intermediate B would then undergo a fast dehydrogenation process to give the diamagnetic intermediate C with a terminal phosphide ligand, this finally rearranging into a bridging position to give compounds 2 , specifically with a cis positioning of the terminal ligands. We note that a related dehydrogenation process most likely takes place in the reactions of the dinuclear radical species [Mo 2 Cp 2 (μ-PR 2 )(CO) 4 ] with secondary phosphines HPR′ 2 to give the mixed phosphide derivatives [Mo 2 Cp 2 (μ-PR 2 )(μ-PR′ 2 )(CO) 2 ] .…”

Transition-Metal Hydride Radical Cations

“…The ν­(C–O) pattern of [M 2 Cp 2 (μ-H)­(μ-PPh 2 ) (CO) 4 ] •+ (M = Mo, W) indicates a cis geometry in solution, which is very similar to that found for the neutral compound [Mo 2 {μ-(η 5 -C 5 H 4 ) 2 SiMe 2 }­(μ-H)­(μ-PMe 2 ) (CO) 4 ] [1959 (vs), 1925 (s), 1874 (vs), 1860 (sh) cm –1 ] with a cis Mo­(CO) 2 moiety due to the presence of the linked cyclopentadienyl ligands. An additional weak band at 2033 cm –1 for [Mo 2 Cp 2 (μ-H)­(μ-PPh 2 ) (CO) 4 ] •+ or 2023 cm –1 for the tungsten analogue with constant relative intensities were assigned to trans isomers …”

“…This is consistent with deprotonation of the radical cation followed by further oxidation of the resulting neutral radical in the same potential range . Similar ECE processes were also observed in the Cp 2 Fe + oxidations of H 3 Ir­(PMe 2 Ph) 3 , [FeH­(NCMe) (dppe) 2 ] + , CpRu­(PPh 3 ) 2 H, Cp*Ru­(dppf)­H, Cp*Ru­(PPh 3 )­(H) 3 , M 2 Cp 2 (μ-H)­(μ-PPh 2 ) (CO) 4 (M = Mo, W), and in other cases. In these systems, the addition of external base changes the one-electron process to a two-electron process.…”