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).

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)

LEONI, PIERO;PASQUALI, MARCO;
1991-01-01

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).
1991
Leoni, Piero; Sommovigo, M; Pasquali, Marco; Midollini, S; Braga, D; Sabatino, P.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/17211
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