Angewandte
Chemie
Figure 4. Molecular structure of [IPr·ZnH(DMAP)2]OTf (5) with ther-
Figure 3. Molecular structure of IPr·ZnH(OTf)·THF (4) with thermal
ellipsoids at the 30% probability level. All carbon-bound hydrogen
atoms have been omitted for clarity. Selected bond lengths [ꢀ] and
angles [8] with values belonging to a second molecule in the
asymmetric unit listed in square brackets: Zn(1)–O(1) 2.068(3)
[2.078(3)], Zn(1)–O(4) 2.156(3) [2.124(3)], Zn(1)–H(1) 1.38(3)
[1.36(3)], Zn(1)–C(1) 2.031(3) [2.041(3)]; C(1)-Zn(1)-O(1) 103.43(13)
[101.78(12)], C(1)-Zn(1)-O(4) 106.69(12) [107.93(13)], O(1)-Zn(1)-
O(4) 94.31(12) [93.93(12)].
mal ellipsoids at the 30% probabilty level; all carbon-bound hydrogen
atoms and the OTfÀ anion have been omitted for clarity. Selected bond
lengths [ꢀ] and angles [8] with values due to a second molecule of 5 in
the asymmetric unit in square brackets: Zn(1A)–H(1A) 1.92(3)
[1.93(3)], Zn(1A)–C(1A) 2.071(4) [2.074(4)], Zn(1A)–N(3A) 2.074(5)
[2.135(4)], Zn(1A)–N(5A) 2.097(3) [2.059(4)]; C(1A)-Zn(1A)-N(3A)
104.92(14) [112.18(15)], C(1A)-Zn(1A)-N(5A) 112.84(14) [110.86(14)],
C(1A)-Zn(1A)-H(1A) 122.0(12) [117.2(13)].
(Figure S3).[11] A lack of dimerization of 4 might be attributed
to mutual repulsion between two [ZnH]+ units (overall charge
for each [ZnH]+ unit in 4 is + 0.545 by NBO analysis).[11]
Steric effects could also be responsible for promoting the
monomeric nature of 4 in the solid state as this species now
contains added bulk (relative to 1 and 3) in the form of
a bound OTfÀ group; however, 19F NMR spectroscopy
indicates that the OTfÀ unit in 4 is only weakly bound in
with the strong donor 4-dimethylaminopyridine (DMAP). In
the presence of two equivalents of DMAP, we were able to
displace both THF and OTfÀ from the coordination sphere of
zinc and generate the formal [ZnH]+ complex [IPr·ZnH-
(DMAP)2]OTf (5) (Scheme 1; Figure 4).[11] Compound 5
contains a tetrahedrally coordinated ZnII center with a slightly
À
elongated CIPr Zn bond [2.073(5) ꢁ avg.] in relation to the
triflato precursor 4 [2.036(4) ꢁ avg.]. The zinc-bound hydride
could be located in the electron difference map but again,
restraints were applied in the refinement of this residue. The
Zn···OTf distances in 5 are > 8.0 ꢁ and thus lie well outside
the sum of the van der Waals radii for Zn and O. 19F NMR
spectroscopy also supported the presence of an outer-sphere
OTfÀ ion in solution as a shielded resonance for this anion at
À78.7 ppm was noted in [D8]-THF.
À
[D8]-THF (d = À78.5 ppm). A terminal Zn H stretching band
was located in the IR spectrum of 4 at 1766 cmÀ1 which is
shifted to higher frequency in relation to the terminal ZnII
hydride vibrations in the ZnH2 adducts 1 and 2 (1625 and
1635 cmÀ1, respectively). The computed IR spectrum for 4
À1
À
also contains a prominent Zn H stretching band at 1765 cm
which supports our initial structural assignment (Fig-
ure S7).[11]
There has been recent literature precedence for the use of
either isolable or transiently generated zinc hydrides to
catalyze the hydrosilylation of activated unsaturated com-
pounds.[2b,5,22] Following the notion that the dual electrophilic
and hydridic character within the [ZnH]+ unit in 4 along with
the facile access to an open coordination site would encourage
substrate binding and hydride transfer from Zn, we decided to
investigate the catalytic hydrosilylation of benzophenone.
This portion of the study was buoyed by our computational
studies which clearly show the retention of hydridic character
at the zinc-bound H atom in 4 (NBO charge À0.534;
Figure 5).[11]
X-ray crystallography confirmed the presence of a mono-
meric zinc hydride adduct IPr·ZnH(OTf)·THF (4) with
À
À
elongated Zn O bonds involving the THF and OTf units
[2.140(4) and 2.073(4) ꢁ, respectively; average of the two
molecules in the asymmetric unit]. For comparison, IMes·Zn-
(OTf)2·THF[20] displays significantly shorter Zn O(Tf) dis-
À
tances of 1.966(2) and 1.973(2) ꢁ. The zinc-bound hydride
unit could be located in the electron difference map, however
restraints were applied during the refinement thus preventing
a detailed discussion of its metrical parameters. The adjacent
À
Zn CIPr bond length in 4 [2.036(4) ꢁ avg.] is similar to the
corresponding distance in the four-coordinate Zn adduct,
When benzophenone was combined with a stoichiometric
equivalent of MePhSiH2 in the presence of 0.1 mol% of 4 in
THF, the rapid and quantitative formation of the reduced
product Ph2C(H)OSi(H)MePh, Equation (3), was noted
IPr·ZnI2·THF [2.0419(19) ꢁ].[7]
We also attempted the one-electron reduction of
IPr·ZnH(OTf)·THF (4) with Na, KC8 and sodium naphtha-
lenide in ethereal solvents, and in each case obtained free
carbene IPr and Zn metal in place of the target ZnI complex
IPr·Zn(H)-Zn(H)·IPr.[21]
In order to further probe the electrophilicity of the zinc
center in IPr·ZnH(OTf)·THF (4), this species was treated
Angew. Chem. Int. Ed. 2014, 53, 9347 –9351
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