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

Reaction of [Pt(XylNC)4](PF6)2 with NaBH4 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-D3 helical cage, [Pt2Na(μ-dpnapy)3(XylNC)2](PF6) (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 Pt0(XylNC) fragments cap each end of
the double-tripodal {(dpnapy)3Na}+ 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, [Pd2Na(μ-dpnapy)3(XylNC)2](PF6) (4b), was prepared by reaction
of [Pd3(XylNC)6] with dpnapy and NaPF6. The terminal isocyanide ligands of complexes 4a,b
were readily replaced by CO (1 atm) to give [M2Na(μ-dpnapy)3(CO)2](PF6) (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 [Pt2Na(H)(μ-dpnapy)3(XylNC)2](PF6)2 (5) and [Pt2Na(H)2(μ-dpnapy)3(XylNC)2](PF6)3 (6). Complexes 5 and 6 were isolated from alternative reactions of [Pt3(XylNC)6] with
dpnapy and NaPF6 in the presence of regulated amounts of HPF6. 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)3Na} 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 Å and causes slight but appreciable distortion around the central
Na ion with an average Na−N distance of 2.42 Å for the Pt0 side and 2.54 Å 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 Epa1 = −0.14 V (vs Ag/Ag+)
and 0.15 V, corresponding to Pt02 → Pt0PtI → PtI2 with some concomitant structural changes,
and the hydride encapsulation made the Pt center redox inactive in 5 and 6, as Pt0(PtH) →
PtI(PtH) (5) and (PtH)2 were redox silent (6).