Electron-Rich Diplatinum(0) Metallocryptate Promoting Novel Successive Encapsulation of Acidic Hydrides

Abstract

Reaction of [Pt(XylNC) 4 ](PF 6 ) 2 with NaBH 4 in the presence of 2,7-bis(diphenylphosphino)-1,8-naphthyridine (dpnapy) afforded a diplatinum(0) metallocryptate with a tightly encapsulated sodium ion in a pseudo-D 3 helical cage, [Pt 2 Na(μ-dpnapy) 3 (XylNC) 2 ](PF 6 ) (4a; XylNC = 2,6-xylyl isocyanide). The three linear dpnapy ligands in complex 4a are bundled by a sodium ion through six Na-N bonds, and two Pt°(XylNC) fragments cap each end of the double-tripodal {(dpnapy) 3 Na} + unit through three Pt-P bonds. Each electron-rich platinum(0) center adopts a tetrahedral geometry with 18 valence electrons. An analogous dipalladium(0) complex, [Pd 2 Na(μ-dpnapy) 3 (XylNC) 2 ](PF 6 ) (4b), was prepared by reaction of [Pd 3 (XylNC) 6 ] with dpnapy and NaPF 6 . The terminal isocyanide ligands of complexes 4a,b were readily replaced by CO (1 atm) to give [M 2 Na(μ-dpnapy) 3 (CO) 2 ](PF 6 ) (M = Pt (4c), Pd (4d)). In the presence of H + ions, the electron-rich diplatinum metallocryptate 4a interestingly demonstrated successive encapsulation of protons as platinum-bound hydrides, resulting in the formation of [Pt 2 Na(H)(μ-dpnapy) 3 (XylNC) 2 ](PF 6 ) 2 (5) and [Pt 2 Na(H) 2 (μ-dpnapy) 3 (XylNC) 2 ]-(PF 6 ) 3 (6). Complexes 5 and 6 were isolated from alternative reactions of [Pt 3 (XylNC) 6 ] with dpnapy and NaPF 6 in the presence of regulated amounts of HPF 6 . Whereas the structures of 5 and 6 are closely similar to that of 4a, one and two hydrides are trapped into the small room comprised of the {Pt(PCN) 3 Na} framework in complexes 5 and 6, respectively. The hydride attached to the Pt center at the opposite site of isocyanide with the Pt atom deformed slightly toward trigonal-bipyramidal geometry. The structure of 5 is asymmetric with the hydride fixed onto a platinum center, and any site-exchange behavior of the hydride was not observed even in solution. The hydride encapsulation in 5 drags the encountered platinum atom inside by ca. 0.17 A and causes slight but appreciable distortion around the central Na ion with an average Na-N distance of 2.42 A for the Pt° side and 2.54 A for the PtH side. The structural distortion caused by the hydride encapsulation might have influenced the other Pt center to be less reactive and stabilize the asymmetric structure of 5. Electrochemical measurements for 4a, 5, and 6 were performed to reveal that complex 4a underwent two irreversible one-electron oxidation processes at E p a 1 = -0.14 V (vs Ag/Ag + ) and 0.15 V, corresponding to Pt 0 2 → Pt 0 Pt I → Pt I 2 with some concomitant structural changes, and the hydride encapsulation made the Pt center redox inactive in 5 and 6, as Pt 0 (PtH) → Pt I (PtH) (5) and (PtH) 2 were redox silent (6).