Page 4 - Interaction of Multiple Bonded and Unsaturated Heavier Main Group Compounds with Hydrogen, Ammonia, Olefins, and Related Molecules

Reactivity of Multiple Bonded and Main Group Compounds Power

SCHEME 2. Activation of H 2 by a Digermyne

SCHEME 3. Reaction of H 2 with Various Distannynes (Ar -4-X = C 6 H 2 -
0
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2,6(C 6 H 3 -2,6-Pr 2 ) 2 -4-X Where X = H (i.e., Ar ), SiMe 3 , or F) 3,5-Pr 2 Ar* =
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C 6 H-2,6(C 6 H 2 -2,4,6-Pr 3 ) 2 -3,5-Pr 2 )

FIGURE 3. Calculated relative energies and selected geometric param-
eters for various isomer forms of (Ar SnH) 2 , (Ar*SnH) 2 , and
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[(3,5 Pr 2 Ar*)SnH] 2 at the B3W91. Numbers in parentheses refer to the
1 28
HOMOLUMO energy separation is less than 50 kcal mol . stability of the (Ar SnH) hydrides relative to Ar SnSnAr . 0
0
0
A possible reaction sequence is thus shown in Scheme 2 where
H 2 is activated synergistically via interaction of its σ and σ* (Figure 3) showed that as the bulk of the substituents increased
orbitals with the HOMO (π) and LUMO (n þ ) of the digermyne the bridged form (III) and the distannene form (II) became more
(cf. Figure 1). This scheme gives Ar GeGe(H) 2 Ar initially disfavored. Oddly, only the symmetric Ar Sn(μ-H) 2 SnAr and
0
0
0
0
which then rearranges to its symmetric isomer, Ar (H)GeGe- Ar*Sn(μ-H) 2 SnAr* are seen in the crystal phase even though
0
0
0
(H)Ar . The unsymmetric Ar GeGe(H) 2 Ar was stabilized as theyarecalculatedtobetheleaststable(albeitbyonlyca.7kcal
0
1
0 29
the adduct Ar (PMe 3 )GeGe(H) 2 Ar . Calculations 30 on the mol ) of the three structural isomers. The apparent inconsis-
0
distannyne/H 2 reaction indicate a similar initial activation tency is probably due to packing forces which can favor the
0
0
pathway in which the unsymmetric Ar SnSn(H) 2 Ar is gener- symmetricallybridgedisomers.Theexistenceofisomericforms
ated initially. However, neither a distannene nor a distan- of the hydrides is unique to the tin species and mirrors earlier
nane nor a monostannane product was seen. Instead, findings on related tin compounds with small organic substit-
for most terphenyl substituents, a symmetrically bridged uents, such as Me or Ph instead of hydrogen, which can also
Sn(II) hydride structure of the type ArSn(μ-H) 2 SnAr was iso- have unsymmetric structures. 33
lated. With very bulky terphenyl groups such as C 6 H- In parallel work, Stefan and Bertrand showed that hydro-
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2,6(C 6 H 3 -2,6-Pr 2 ) 2 -3,5-Pr 2 (Ar*-3,5-Pr 2 ), the unsymmetric gen could be activated under mild conditions by phosphine
i i 34 35
stannylstannane 3,5-Pr 2 Ar*SnSn(H) 2 Ar*-3,5-Pr 2 can be ob- boranes (reversibly) and also by carbenes which could
tained (Scheme 3). 31 also activate ammonia. The unifying theme of these results
More detailed investigations 32 showed that the tin hy- and ours was that the activation occurred by the use of
drides could also be synthesized by reduction of aryl tin donor and acceptor orbitals in a synergistic fashion at one or
halideprecursorswithavarietyofreducingagents.Calculations more main group elements (B, C, P, Ge, or Sn). In this sense,
630 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 627–637 ’ 2011 ’ Vol. 44, No. 8