Chemistry of Highly Electrophilic Binuclear Cations. 5. Addition and C−C and C−P Coupling Processes in the Reactions of 1-Alkynes with the Dimolybdenum Radicals [Mo₂(η⁵-C₅H₅)₂(μ-CO)₂(CO)₂(μ-L₂)][B{3,5-C₆H₃(CF₃)₂}₄] (L₂ = Ph₂PCH₂PPh₂ or Me₂PCH₂PMe₂)

Abstract

The reactions of radicals [Mo 2 Cp 2 (µ-CO) 2 (CO) 2 (µ-L 2 )](BAr′ 4 ) [Cp ) η 5 -C 5 H 5 ; L 2 ) Ph 2 PCH 2 -PPh 2 (dppm), Me 2 PCH 2 PMe 2 (dmpm)] with an excess of the 1-alkynes HCCR (R ) p-tol, t Bu, CO 2 Me) give two main type of products, the paramagnetic alkyne-bridged complexes [Mo 2 Cp 2 (µ-η 2 :η 2 -HC 2 R)(CO) 2 (µ-L 2 )](BAr′ 4 ) and the diamagnetic dicarbonyls [Mo 2 Cp 2 {µ-η 2 :η 3 -HCC(R)C(OH)}(CO) 2 (µ-L 2 )](BAr′ 4 ), which result from coupling of the incoming alkyne and a hydroxycarbyne ligand. The reaction of [Mo 2 Cp 2 (µ-CO) 2 (CO) 2 (µ-dmpm)](BAr′ 4 ) with HCC(CO 2 Me) gave additionally the compound [Mo 2 Cp 2 {µ-η 2 :η 2 ,κ-C(CO 2 Me)CPMe 2 CH 2 PMe 2 }-(CO) 3 ](BAr′ 4 ), resulting from coupling of the diphosphine and the alkyne. The relative amounts of the products in the above reactions were found to be strongly dependent on the alkyne used. The compounds [Mo 2 Cp 2 {µ-η 2 :η 3 -HCC(R)C(OH)}(CO) 2 (µ-L 2 )](BAr′ 4 ) (R ) t Bu, CO 2 Me) experience overall deprotonation by reaction with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), to give the corresponding neutral species [Mo 2 Cp 2 {µ-η 2 :η 2 -HCC(R)C(O)}(CO) 2 (µ-L 2 )] through an unexpected reduction/dehydrogenation pathway. This overall deprotonation is reversible, and reaction of the neutral complexes with HBF 4 ‚OEt 2 occurs specifically at the oxygen atom of the acyl group to give the starting cations as BF 4salts. Unexpectedly, the reduction of the alkyne-bridged radicals [Mo 2 Cp 2 (µ-η 2 :η 2 -HC 2 R)(CO) 2 (µ-L 2 )](BAr′ 4 ) with Na amalgam gave unstable paramagnetic derivatives that could not be characterized. The complexes [Mo 2 Cp 2 {µ-η 2 :η 3 -HCC( t Bu)C(OH)}(CO) 2 (µ-dmpm)](BAr′ 4 ), [Mo 2 Cp 2 {µ-η 2 :η 2 ,κ-C(CO 2 -Me)CPMe 2 CH 2 PMe 2 }(CO) 3 ](BAr′ 4 ), and [Mo 2 Cp 2 {µ-η 2 :η 2 -HCC(CO 2 Me)C(O)}(CO) 2 (µ-dppm)] were characterized through single-crystal X-ray diffraction studies. The solution structures of all new compounds are analyzed in the light of IR and NMR spectra, and plausible reaction pathways are proposed in order to explain the formation of the products isolated.

M−P versus MM Bonds as Protonation Sites in the Organophosphide-Bridged Complexes [M2Cp2(μ-PR2)(μ-PR‘2)(CO)2], (M = Mo, W; R, R‘ = Ph, Et, Cy)

“…Further interest in these species stems from the fact that coordination of the P−H bond of a phosphine ligand to a metal atom weakens that bond and is expected to result in specific reactivity. It also represents an intermediate step in the oxidative addition of P−H bonds of primary and secondary phosphines to unsaturated dimetal centers. 2d,, In this context, the results here reported provide the first evidence that M−P bonds compete efficiently with M−M bonds as protonation sites in organometallic complexes. Moreover, our results provide the first examples of stable dimolybdenum complexes displaying agostic P−H bonded bridging phosphines (μ-κ 2 -HPR 2 ), which in turn exhibit some novel features: (a) dynamic behavior in solution involving the intramolecular exchange of the hydrogen atom between μ-κ 2 -HPR 2 and μ-PR 2 ligands and (b) coexistence in solution with the corresponding hydride tautomers.…”

“…Organometallic compounds having multiple metal−metal bonds are species of interest due to their high reactivity toward a great variety of molecules under mild conditions, as exemplified by the wide chemistry developed around the unsaturated binuclear complexes [M 2 L 2 (CO) x ] (L = cyclopentadienyl or related ligand) . The presence of positive charges in this type of molecules greatly increases the electrophilic character of the dimetal center, which leads to enhanced reactivity and potential Lewis-acid catalysis, as shown by our recent studies on the unsaturated cations of type [M 2 Cp 2 (CO) x (μ-L‘ 2 )] n + (M = Mo, W; x = 2−4; L‘ 2 = diphosphine ligand; n = 1, 2), prepared through chemical oxidation of the corresponding electron-precise substrates [M 2 Cp 2 (CO) 4 (μ-L‘ 2 )] . An alternative synthetic approach to related unsaturated cations is the protonation of suitable precursors having multiple metal−metal bonds.…”

Alkyne to carbyne coupling reactions of the unsaturated methoxycarbyne-bridged complex [Mo2(η5-C5H5)2(μ-COMe)(μ-PCy2)(CO)2]

“…The intermetallic distance in 3a [2.8646(8) Å] is some ca. 0.1 Å shorter than that found in the related and isoelectronic cation [Mo 2 Cp 2 {l-j 2 :g 3 -CHC t BuC(OH)}(l-Me 2 PCH 2 PMe 2 )(CO) 2 ] + (2.963(1) Å) [10], a shortening effect expected from the replacement of a diphosphine by a phosphide ligand, and it is nearly identical to that measured in the also isoelectronic formimidoyl complex [Mo 2 Cp 2 (l-g,j:g,j-HCN t Bu)(l-PCy 2 )(CO) 2 ] (2.883(2) Å) [11]. In any case all these values are in agreement with the single MoÀMo bond that should be proposed for these 34-electron complexes on the basis of the EAN rule.…”

Reactions of the Carbyne-Bridged Radical Complex [Mo2(η5-C5H5)2(μ-CPh)(μ-PCy2)(μ-CO)]+ with Bidentate Ligands Having E–H Bonds (E = O, S, N)

“…Poor donor molecules with weak H–E bonds, such as silanes and borane adducts, are instead involved in H transfer to the O atom, then yielding the hydroxycarbyne complex [Mo 2 Cp 2 (μ-COH)­(μ-CPh)­(μ-PCy 2 )] + (Scheme ) . Since the chemistry of paramagnetic organometallic complexes with metal–metal bonds of orders higher than 1 has been little explored, − when compared to the extensively studied mononuclear organometallic radicals, , it was worth examining in depth the reactivity of 2 , which also would allow us to establish the full synthetic potential of the redox-induced reactions of the benzylidyne complex 1 . Moreover, it should be noted that the behavior of binuclear radicals also is of interest to better understand the transformations taking place at the active centers of several biomolecules such as the [FeFe] and [NiFe] hydrogenases .…”