The reactivity of Pd(PtBu3)2 towards the oxonium ion. Crystal structure of trans-tBu3P)2Pd(H)(CH3CN)>BPh4

Article abstract of DOI:10.1016/0022-328X(91)86352-Q

The reaction of Pd(P^tBu3)2 with the strong acids H3O⁺X^-, (X = BF3OH, BF4), gives the thermally unstable hydrides trans-<(^tBu3P)2Pd(H)(H2O)>X.The thermally stable hydrides trans-<(^tBu3P)2Pd(H)(CH3CN)>X were obtained by substitution of the water molecule by CH3CN.The reaction of the aquo-hydrides with CO, yielding tBu3)>2 and the crystal structure of trans-<(^tBu3P)2Pd(H)(CH3CN)>BPh4 are also reported.

Full text of DOI:10.1016/0022-328X(91)86352-Q

120
cationic hydride trans-[(tBu3P)2Pd(H)(CH3CN)]BPh4 has also been determined.
The role played by cationic palladium hydrides in some homogeneously-catalysed
processes has been recently reviewed by Zudin et al. [8].
Results and discussion
Treatment of toluene solutions of Pd(ptBu3)2 (1) at - 7 8 ° C with the strong
acids n 3 0 + X - (X = BF3OH, BF4) resulted in precipitation of white powdery solids,
characterization of which was prevented by their extreme thermal instability; they
could be filtered off at low temperature and sealed in ampoules under nitrogen, but
they rapidly darkened and decomposed above 0°C. Any attempt to prepare
solutions of the compounds failed, and rapid decomposition with the formation of a
black precipitate was observed when the powders were dissolved at low tempera-
tures in CDC13, thf-d8, CD3OD, or (CD3)2CO.
A reasonable suggestion for the structure of these products can be made on the
basis of an analogy with the outcome of the reaction of Pd(PCY3)2 with H 3 0 + X -
which gave the products of oxidative addition of H3O+ [4] i.e. trans[(CY3P)2Pd(H)
(H20)]X (3a, X = BF3OH; 2b, X = BF4), as white crystalline solids. The solid state
IR spectra of the aquo-hydrides 2a,b showed signals attributable to the Pd-bound
water molecule (vas(OH) 3520s, vs(OH) 3440s, 8(HOH) 1630s) and to the Pd-hy-
dride moiety (v(PdH) 2115w, 8(PdH) 720m); solution ~H NMR spectra in acetone-d6
showed the hydride resonance as a triplet (2j(PH) 4.3 Hz) at 8 - 1 8 . 3 [4].
If the analogy with the behaviour of the PCy3 derivative is correct, the expected
structure for the white powders obtained from 1 is trans-[(ptBu3)2Pd(H)(H20)]X
(3a, X = BF3OH; 3b, X = BF4). However, we were able to observe only the signals
attributable to the metal-bound water molecule in the solid state IR spectrum
(vas(OH) 3520s, v~(OH) 3440s, 8(HOH) 1630m), and no signals attributable to the
Pd-H moiety could be detected in the IR or in the ~H NMR spectra.
An indirect confirmation of the structure of 3a,b came from exchange experi-
ments with CH3CN. The solids 3a,b were filtered off at low temperature and rapidly
dissolved in cold CH3CN, and the pale yellow solutions were evaporated to dryness
to give the thermally stable compounds trans-[(ptBu3)2Pd(H)(CH3CN)]X (4a,b),
which were fully characterized: signals attributable to the nitrile ligand are observa-
ble in the IR spectrum (2320vw, 2280w [v(CN)]), and the 1H NMR spectrum
(CD3CN) (3 1.96 [s, 3HI); other signals in the ~H NMR spectrum arise from the
trans-tBu3 P ligands (1.50 [t, ~/(PH)+sJ(PH)=13.0 Hz, 54 H]) and from the
Pd-bound hydride (16.3 It, 2j(PH) = 9.0 Hz, 1H]).
Complexes 4a,b can also be prepared by treatment of Pd(P tBu3)2 with H30 + X -
in a 1/1 CH3CN/toluene mixture. The substitution of the metal-bound water
molecule by CH3CN was previously observed for the PCy3 derivatives 2a,b and
trans-[(CY3P)ePd(H)(CH3CN)]X (5a,b) were isolated. Both the aquo-hydrides 2a,b
and the nitrile derivatives 5a,b were shown to be thermally stable, though 2a,b were
much less stable than 5a,b towards anion exchange [3,4].
In the case of ptBu 3 derivatives prepared in this work the replacement of the
water molecule by CH3CN considerably increases the thermal stability of the Pd u
hydride. The greater affinity of the metal toward nitrile ligands than towards water
is well documented in the case of cationic platinum hydrides [9,10].