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for the undisturbed, highly symmetric [Al(ORF)4]À
anion. By contrast, either no signal or a very broad
signal should be expected if the ethylzinc-cation
were coordinated to the oxygen atoms of the alumi-
nate.
Table 1. Bond distances, bond angles, and selected bond valences in different Et-Zn-
moieties.[a]
ZnÀC []
1.974
Zn-C-C [8]
Et-Zn+ calc. MP2/def2-QZVPP
113.1
112.0
113.7(9)
114.6(6)
115.9(4)/114.8
116.8
+
Et-Zn(OH2)2 calc. MP2/def2-QZVPP
1.903
The NMR spectra in o-difluorobenzene indicated
anion coordination, but conductivity measurements
also suggest some ion dissociation (see the Support-
ing Information). The vibrational spectra showed
splitting of several bands with respect to the non-
coordinated S4-symmetric anion, which indicated that
the anion was coordinated, similar to the anion coor-
dinated Li[Al(ORF)4] salt.[23] The spectrum of the mois-
ture-sensitive compound simplifies on measurement
in air and the coordinated Et-Zn+ cation decompos-
es, whereas the anion appears to be stable against
hydrolysis and becomes noncoordinated.[24] In one re-
action, we obtained a structurally characterized hy-
drolysis product, which contains a central Zn4O6 frag-
ment (see Figure S2 in the Supporting Information).
Et-Zn+ in EtZn[(FRO)3Al-F-Al(ORF)3]
Et-Zn+ in EtZn[Al(ORF)4][b]
1.938(8)
1.943(6)
1.948(5)/1.934
1.964(3)
1.964(6)
Et-Zn-Et[25] (exp./calc. MP2/def2-QZVPP)
[Et-Zn(OEt2)3]+[B(C6F5)4]À[7]
[Et-Zn(h3-C6H6)]+[CHB11Cl11]À·(C6H6)[3]
112.4(4)
ZnÀO1 []/
ZnÀO2 []/
ZnÀO3 []/
bond valence bond valence bond valence
[Et-Zn(OEt2)3]+[B(C6F5)4]À[7]
2.072(2)/0.326 2.031(2)/0.365 2.110(2)/0.294
Et-Zn+ in EtZn[(FRO)3Al-F-Al(ORF)3] 2.094(3)/0.307 2.101(3)/0.302
–
–
Et-Zn+ in EtZn[Al(ORF)4][b]
2.112(2)/0.293 2.058(2)/0.339
[a] Grouped according to increasing ZnÀC distance. Compounds prepared in this
work are shown in bold typeface; RF =C(CF3)3. [b] Arithmetic mean of disordered EtZn
groups.
becomes closer to the experimental distance in 4. Population
analyses suggest that the NPA charge of the zinc atom (Zn:
+0.97/+1.33) increases and the charge of the carbon (C:
Crystal structures of 2a and 4
The Et-Zn moiety in 2a and 4 is coordinated by two oxygen
atoms. Compared with the other two known ethylzinc cations
with coordinated solvent molecule(s), the ZnÀC bond in 2a
and 4 is shorter by 2.4 to 2.6 pm (Table 1). It is also shorter
than the ZnÀC bond in solid Et2Zn (Table 1).[25] The Zn-C-C
angle is between the two other ethylzinc cations and smaller
than the angle in diethyl zinc (Table 1). Only the Zn1ÀF11 con-
tact (2.71 ) is slightly shorter than the sum of the van der
Waals Radii of F and Zn. This is in agreement with our sugges-
tion that the coordinated Et-Zn moiety in those two structures
should be addressed, in analogy to the similar situation with si-
lylium ions, as ion-like; that is, displaying several of the proper-
ties of the truly free cation.[11] To get an idea of how strong the
ZnÀO interactions of 2a and 4 are, the bond valences of the
ZnÀO bonds of 2a, 4 and, for comparison, [Et-Zn(OEt2)3]+
[B(C6F5)4]À were calculated (Table 1).[26] The sum of the ZnÀO
bond valences for the ether adduct is 0.985, and that of 2a/4
only 0.609/0.632. Furthermore, the individual ZnÀO bond va-
lences in 2a/4 are at the lower end or smaller than the individ-
ual ZnÀO bond valences in [Et-Zn(OEt2)3]+. Of course, we com-
pare a trigonal with a tetrahedrally coordinated zinc atom, but
the overall trend is clear: the interaction between [Et-Zn]+ and
the aluminates is weaker than the interaction between EtZn+
and three ether molecules.
À0.51/À0.89) atom decreases; so, the ZnÀC bond becomes
more polarized and thus the Coulomb interaction becomes
greater and induces the unexpected shortening upon increas-
ing the coordination number from 1 to 3.
On the reactivity of the Et-Zn+ moiety
Comparison of the Kohn–Sham frontier orbitals from [Et-Zn]+
with the isoelectronic [Me3P-Au]+ orbitals show similarities
with respect to both shape and energies (Figure 3).[27] Given
that [Me3PAu]+ was used for hydroamination reactions of 1,3-
dienes, it was suspected that the ion-like ethylzinc-cation can
do the same.[28]
Hydroamination of alkynes
Recently, some of us reported an intermolecular hydroamina-
tion reaction catalyzed by [(h5-Cp*)2Zn2], [Cp*2Zn], and ZnEt2,
In free EtZn+, the MP2/def2-QZVPP calculated bond length
appears to be longer by 0.031 than that in Et-Zn[Al(ORF)4].
Given that MP2/def2-QZVPP is a reliable method close to the
basis set limit (cf. the calculated dZnÀC in Et2Zn in Table 1), we
were rather surprised to find dZnÀC of free EtZn+ to be longer
than that in 4. To understand this unusual phenomenon, fur-
ther calculations were performed: To mirror a similar environ-
ment to that in 4 around the gaseous EtZn+ cation, two water
molecules were added in an MP2/def2-QZVPP calculation.
Upon water coordination, dZnÀC is shortened by 0.071 and
Figure 3. Kohn–Sham frontier orbitals of the structure optimized [Me3PAu]+
cation (right, À11.43 eV) and the Et-Zn+ cation (left, À12.65 eV) at the BP86/
def2-SV(P) level (cut-off at 0.05 a.u.).
Chem. Eur. J. 2015, 21, 13696 – 13702
13698
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