Coordinated water/anion hydrogen bonds and Pd-H bond acidity in cationic palladium(II) aquo hydrides and the x-ray crystal and molecular structures of trans-[(Cy3P)2Pd(H)(H2O)]BF4 (Cy = cyclohexyl)

Abstract

Palladium(II) bis(tricyclohexylphosphine) cationic hydrides have been obtained by oxidative addition of the strong acids [H3O+][BF3OH-] or [H3O+][BF4-] to (PCy3)2Pd, 1; by this route, the compounds trans-[(Cy3P)2Pd(H)(H2O)]X (X = BF3OH, 2; X = BF4,4) have been isolated and characterized. A strong dependence of the stability of trans-[(Cy3P)2Pd(H)(L)]X complexes upon the L-X combinations was observed. When L = H2O, stable cationic hydrides are obtainable only with fluorine-containing anions (BF3OH-, BF4-, or PF6-), while immediate decomposition to 1 was observed when the metathesis of X- with BPh4- or B(n-Bu)4- was attempted. Stable complexes with X = BPh4 could be isolated when L = MeCn. No reaction was observed when 1 was reacted with [Et2OH+][BF4-]. These findings can be explained in principle by two hypotheses: (i) when the strength of the M-L bond is not sufficient to stabilize the [(R3P)2Pd(H)(L)]X complexes, further thermodynamic assistance is furnished by hydrogen-bond formation between the metal-bonded ligand L and the X counteranion, and/or (ii) depending upon the nature of L, the cationic hydrides are sufficiently acidic to decompose reactive anions such as BPh4- or B(n-Bu)4-. IR and crystallographic data on trans-[(Cy3P)2Pd(H)(H2O)]BF4 show the existence, in the solid state, of strong hydrogen bonds between the Pd-bound water molecule and the BF4- anion. A conductivity study on 4 and on the related hydride trans-[(Cy3P)2Pd(H)(MeCN)]BF4, 5, shows the existence of large concentrations of ions both in 4 and 5 solutions, though the values of LAMBDA-M (10-20% lower for 4 as compared to 5) could account for the presence in 4 of appreciable amounts of the hydrogen-bonded species. The aquohydride 4 has evidenced its acidic behavior by protonating strong and weak bases (Ph3C-, OH-, Et3N, and PCy3), and further information on its acid-base behavior has been obtained through the study of exchange reactions with D2O. The aquodeuteride trans-[(Cy3P)2Pd(D)(H2O)]BF4. 9, can in fact be obtained by reacting for a few minutes a solution of complex 4 with D2O while the d3 derivative trans-[(Cy3P)2Pd(D)(D2O)]BF4, 10, can only be obtained after prolonged reaction times. Complex 2 behaves similarly, and trans-[(Cy3P)2Pd(D)(H2O)]BF3OD, 11, or trans-[(Cy3P)2Pd(D)(D2O)]BF3OD, 12, can be prepared by controlling the reaction times with D2O. These data suggest that the acidity of the hydridic hydrogen is higher than the acidity of the hydrogens of the PdOH2 moiety. The last hypothesis is further confirmed by the reaction of trans-[(Cy3P)2Pd(D)(H2O)]BF4 with Ph3CLi, which yields a 70/30 Ph3C-d/Ph3C-h mixture. Crystals of 4 are monoclinic, C2/c, a = 30.587 (6) angstrom, b = 13.408 (4) angstrom, c = 19.121 (5) angstrom, beta = 99.23 (3)-degrees, V = 7740.2 angstrom 3, Z = 8, R = 0.040 for 3077 reflections with F(o) > 4-sigma-(F(o)

HP‐NMR Study of the Pd‐Catalyzed Methoxycarbonylation of Styrene Using Monodentate and Bidentate Phosphane‐Modified Systems

“…Leoni 8 ]THF with J P-H = 475 Hz, which is very similar to that observed for phosphetane 1-H + . [16] The values reported by McFarlane et al for a series of protonated organophosphorus compounds are also in agreement with the coupling value measured for 1-H + .…”

Synthesis, characterization and catalytic activity in the carbonylation of ethene of cis-[Pd(H2O)2(PPh3)2]X2·nH2O (X=p-CH3C6H4SO3, n=2; X=CH3SO3, n=0). X-ray structure of cis-Pd(H2O)2(PPh3)2](p-CH3C6H4SO3)2·2H2O and of cis-[Pd(H2O)2(PPh3)2](CH3SO3)2·2CH2Cl2

“…The IR spectrum of both Ia and IIa shows two broad absorptions at 3508, 3445 cm À1 and 3534, 3489 cm À1 , respectively, assigned to m as (O-H) and m s (O-H) of the water molecules [13,[22][23][24][25] …”

Active Pd(II) Complexes as Either Lewis Acid Catalysts or Transition Metal Catalysts

“…However, the generation of palladium hydride (H-Pd) from adventitious water and syn addition of H-Pd to the alkyne portion is highly likely to initiate the six-membered ring carbocyclization. Thus, cyclizations in the presence of an excess amount (600 mol%) of deuterated water (D 2 O) were examined [51,52]. Indeed, the products D-2a and D-3a, which were regiospecifically deuterated at the exo olefin in the a position to the ester functionality, were obtained with ~75% deuterium content (Scheme 8).…”

“…Taking advantage of hydride exchange between D 2 O and H-Pd to generate D-Pd species [69,70], the reactions under excess D 2 O (600 mol%) conditions were examined (Scheme 16). Consequently, the deuterated product at the vinylic position was obtained without diminution of catalytic activity under either polar or less polar conditions.…”