The protolytic stability of (dfepe)PtMe2 (dfepe = (C2F2)2PCH2CH2P(C2F5)2) and cis-(dfmp)2PtMe2 (dfmp = (C2F5)2PMe) and NMR characterization of their corresponding products in SbF5–HF superacid ...solvent mixtures are reported. Dissolution of (dfepe)Pt(Me)2 in 10 mol % of SbF5–HF at −60 °C resulted in the clean protonolysis of a single Pt–Me bond to form the cationic methyl complex (dfepe)Pt(Me)+; further conversion of (dfepe)Pt(Me)+ to (dfepe)Pt2+ occurred upon warming to −20 °C and followed pseudo-first-order kinetics (k = 1.4(2) × 10–2 min–1). In contrast, dissolution of the nonchelating analogue cis-(dfmp)2PtMe2 in 10 mol % of SbF5–HF at 20 °C evolved methane and cleanly produced the stable monomethyl complex trans-(dfmp)2Pt(Me)+. trans-(dfmp)2Pt(Me)+ is the most protolytically stable organometallic known: 33% conversion to the cis dicationic product cis-(dfmp)2Pt2+ requires 2 weeks in 10 mol % of SbF5–HF at 20 °C, whereas >90% conversion was observed in 30 h in 50 mol % of SbF5–HF. Dissolution of cis-(dfmp)2Pt(CD3)2 cleanly generated trans-(dfmp)2Pt(CD3)+, which subsequently underwent complete proton incorporation to produce trans-(dfmp)2Pt(CH3)+ within 1 h at 25 °C. This labeling study supports the reversible formation of the methane complex intermediate trans-(dfmp)2Pt(CH4)2+ under these conditions. Treatment of trans-(dfmp)2Pt(Me)+ in 10 mol % of SbF5–HF at −100 °C with 200 psi of H2 resulted in the clean formation of the dihydrogen complex trans-(dfmp)2Pt(Me)(η2-H2)+, which upon warming to −20 °C underwent methane loss and generated the hydride product trans-(dfmp)2Pt(H)+. The dihydrogen complex trans-(dfmp)2Pt(H)(η2-H2)+ has not been directly observed but has been implicated in exchange bradening behavior observed for trans-(dfmp)2Pt(H)+ under H2. Treatment of trans-(dfmp)2Pt(CD3)+ in 10 mol % of SbF5–HF at −40 °C with 200 psi of H2 cleanly produced trans-(dfmp)2Pt(CD3)(η2-H2)+ No significant H/D exchange into the Pt–CD3 group prior to trans-(dfmp)2Pt(H)+ formation was observed.
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Protonolyses of (dfepe)Pt(Me)X (dfepe = (C2F5)2PCH2CH2P(C2F5)2; X = O2CCF3, OSO2H, OSO2CF3, OSO2F) complexes in their respective neat acid solutions cleanly yield (dfepe)Pt(X)2 products with rates ...dependent on relative acid strengths. No (dfepe)Pt(Me)(X)2(H)+ intermediates were observed by variable-temperature NMR in dichloromethane. The (perfluoroaryl)phosphine analogue (dfppe)Pt(Me)2 (dfppe = (C6F5)2PCH2CH2P(C6F5)2) is much less resistant to protonolysis and rapidly converts to (dfppe)Pt(OTf)2 in HOTf at 20 °C. Kinetic studies for protonolysis in CF3CO2H(D) and CF3SO3H(D) solvents were carried out. Examination of ionic strength and chloride anion effects in trifluoroacetic acid indicate that prior association of anion to (dfepe)Pt(Me)X systems is not kinetically important. k H/k D values were obtained from competitive protonolysis studies (CF3CO2H, 9 ± 2 (20 °C); H2SO4, 7 ± 2 (100 °C); CF3SO3H, 2.7 ± 0.7 (100 °C)). In the case of CF3CO2H, separate kinetic runs in protio and deuterio acids gave a lower k H/k D value of 3.6(4). The data obtained in these studies do not differentiate between limiting SE2 and SE(oxidative) protonolysis mechanisms.
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The reaction of hydrogen with iridium and rhodium μ-S A-frame complexes has been investigated using parahydrogen-induced polarization (PHIP) NMR spectroscopy. The reaction of Ir2(μ-S)(CO)2(dppm)2 ...where dppm = bis(diphenylphosphino)methane with hydrogen proceeds by two addition pathways corresponding to oxidative addition inside (endo) and outside (exo) the pocket of the A-frame complex. The exo addition pathway yields a kinetically significant dihydride that has not been previously reported. This species is a key intermediate in the formation of the thermodynamic product of the reaction system. Also present are polarized resonances for the addition of hydrogen to the mixed metal A-frame, IrRh(μ-S)(CO)2(dppm)2, formed as a contaminant from rhodium present in the iridium trichloride starting material. To confirm this observation, IrRh(μ-S)(CO)2(dppm)2 was prepared independently and its reaction with parahydrogen was investigated, leading to results supporting endo addition H2 as the major pathway and exo addition as a minor reaction channel. The complex Rh2(μ-S)(CO)2(dppm)2, which had not previously been found to react with dihydrogen, generates polarized hydride signals under para-enriched hydrogen, showing that an equilibrium is established between the complex and its H2 adduct. In all of these cases, PHIP is used to detect species not readily seen by normal NMR spectroscopy and to investigate the mechanism of these reactions.
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An extension of prior protonolysis studies of platinum−carbon bonds to Pt−aryl bonds is reported. The protolytic stability of (dfepe)Pt(Ph)(O2CCF3) (dfepe = (C2F5)2PCH2CH2P(C2F5)2) in trifluoroacetic ...acid is found to be much less than (dfepe)Pt(Me)(O2CCF3), indicating that a concerted SE2 protonolysis mechanism is most likely operative in these electron-poor platinum systems. VT NMR experiments show that benzene coordination to the (dfepe)Pt2+ center in neat fluorosulfonic acid to form (dfepe)Pt(η6-C6H6)2+ at −80 °C is competitive with benzene dissociation in this weakly coordinating medium.
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Addition of excess HOSO2CF3 (HOTf) to CpRu(L)H (1) (L = dfepe = PR2CH2CH2PR2, R = C2F5) in CD2Cl2 under N2 produces a mixture of CpRu(L)(H)2+ (2a), CpRu(L)(H2)+ (2b), and CpRu(L)(OTf) (3) in a ratio ...of 1:5:2. Salts of the acid HOEt2+ are not strong enough to protonate 1. Complexes 2 slowly eliminate H2 to give 3; this reaction is slowed by adding excess HOTf. Of all such complexes CpRu(PR2CH2CH2PR2)(H2)+, R = alkyl and aryl, the dihydrogen complex 2b has the greatest acidity (similar to that of HOTf) and the related HD complex has the greatest J HD (29.1 Hz) because of the electron-withdrawing substituents R = C2F5. The reaction of 3 with 1 atm H2(g) proceeds much faster in the presence than in the absence of 1 equiv of HOTf to produce 1 and HOTf. This is a rare example of the production of a strong acid from H2(g) where the intermediate dihydrogen complex has been characterized. Reaction of Cp*Ru(L)Cl (4), Cp* = C5Me5, in dry CH2Cl2 at −78 °C with AgX salts under H2(g) (1 atm) gives mixtures of Cp*Ru(L)H (5) and Cp*Ru(L)(H)2+ (6), which have been identified by 1H NMR. 6 is deprotonated by traces of water or by PPh3 to give Cp*Ru(L)H (5). The addition of excess HOTf to mixtures of 5 and 6 under H2(g) produces 6. Complexes 3, 4, and 5 have been characterized by single-crystal X-ray diffraction. Complexes CpRu(L)Cl, 1, 4, and 5 have very positive redox potentials that indicate that the dfepe ligand has the electron-withdrawing power close to that of two carbonyl ligands.
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