A para-hydrogen investigation of palladium-catalyzed alkyne hydrogenation

The complexes [Pd(bcope)(OTf)(2)] (1a), where bcope is (C8H14)PCH2-CH2P(C8H14), and [Pd((t)bucope)(OTf)(2)] (1b), where (t)bucope is (C8H14)PC6H4CH2P(Bu-t)(2), catalyze the conversion of diphenylacetylene to cis- and trans-stilbene and 1,2-diphenylethane. When this reaction was studied with para-hydrogen, the characterization of [Pd(bcope)(CHPhCH2Ph)](OTf) (2a) and [Pd((t)bucope)(CHPhCH2Ph)](OTf) (2b) was achieved. Magnetization transfer from the alpha-H of the CHPhCH2Ph ligands in these species proceeds into trans-stilbene. This process has a rate constant of 0.53 s(-1) at 300 K in methanol-d(4) for 2a, where Delta H-double dagger = 42 +/- 9 kJ mol(-1) and Delta S-double dagger = -107 +/- 31 J mol(-1) K-1, but in CD2Cl2 the corresponding rate constant is 0.18 s(-1), with Delta H-double dagger = 79 +/- 7 kJ mol(-1) and Delta S = 5 +/- 24 J mol(-1) K-1. The analogous process for 2b was too fast to monitor in methanol, but in CD2Cl2 the rate constant for trans-stilbene formation is 1.04 s(-1) at 300 K, with Delta H = 94 +/- 6 kJ mol(-1) and Delta S = 69 +/- 22 J mol(-1) K-1. Magnetization transfer from one of the two inequivalent beta-H sites of the CHPhCH2Ph moiety proceeds into trans-stilbene, while the other site shows transfer into H-2 or, to a lesser extent, cis-stilbene in CD2Cl2, but in methanol it proceeds into the vinyl cations [Pd(bcope)(CPhCHPh)(MeOD)](OTf) (3a) and [Pd((t)bucope)(CPhCHPh)(MeOD)](OTf) (3b). When the same magnetization transfer processes are monitored for 1a in methanol-d(4) containing 5 mu L of pyridine, transfer into trans-stilbene is observed for two sites of the alkyl, but the third proton now becomes a hydride ligand in [Pd(bcope)(H)(pyridine)](OTf) (5a) or a vinyl proton in [Pd(bcope)(CPhCHPh)(pyridine)](OTf) (4a). For 1b, under the same conditions, two isomers of [Pd((t)bucope)(H)(pyridine)](OTf) (5b and 5b') and the neutral dihydride [Pd((t)bucope)(H)(2)] (7) are detected. The single vinylic CH proton in 3 and the hydride ligands in 4 and 5 appear as strong emission signals in the corresponding H-1 NMR spectra.