posted on 2021-01-15, 19:33authored byBun Chan, Ying Luo, Masanari Kimura
In
the present study, we have examined hydride affinities relevant
to a range of group 13 and group 14 reductants. We use the high-level
W1X-G0, G4(MP2)-XK, and DSD-PBEP86 methods to obtain the RHA42 set
of accurate reductant hydride affinities. Assessment of DFT methods
with the RHA42 set shows that all functionals that we have examined
are fairly accurate. Overall, we find ωB97X-V to be the most
accurate. The MN12-SX screened-exchange functional and the nonhybrid
B97-D3BJ method also perform well, and they may provide a lower-cost
means for obtaining hydride affinities. The trend in the hydride affinities
suggests an increased reducing power when one moves down the periodic
table, e.g., with TlH3 being a stronger reductant than
BH3. We also find that group 13 hydrides are stronger reductants
than the group 13 analogues. In general, substitution of a hydrogen,
e.g., BH2+ → BHMe+, and the
formation of dimer, e.g., BH2+ → B2H5+, also lead to stronger reductants.
A notable observation is the small hydride affinities for silyl cations,
which are indicative of the potential of silanes as strong reducing
agents. In particular, poly(methylhydrosiloxane) (PMHS) cations are
associated with especially small hydride affinities owing to the presence
of intramolecular oxygen atoms that can stabilize the cation center.
We have further found the germanium analogues of the silanes to be
more reactive, and they may further widen the scope of main-group
hydride reducing agents.