Dissertation

|3.3 NN-NHC-Ligand bbip: Toward Second Generation Catalysts|
by the findings, that all equivalent bond angles and distances in ip and bbip show insignificant
differences. As a result of that they will be discussed together.
The X-ray data indicated that the imidazole rings in ip/bbip and bip have identical C5-N3 and
C6-N4 distances in the range between 1.387(2) Å and 1.400(8) Å as well as C5-C6 in the range
between 1.365(4) Å and 1.386(9) Å. A significant asymmetry with shortening of the C13-N4 bond
and elongation of the C13-N3 bond is present in bip (d bip
bip
(C13-N4)
= 1.315(3) Å < d(C13-N3) = 1.357(2) Å)
which can be explained by the partial double bond character between C13 and N4 in this 1Himidazole
derivative. This effect is even visible when compared to ip and bbip (d bip
(C13-N4) = 1.315(3) Å
< d ip
bbip
bip
ip
(C13-N4)
= 1.331(4) Å ≈ d(C13-N4) = 1.322(8) Å, and d(C13-N3) = 1.357(2) Å < d(C13-N3) = 1.330(4) Å ≈
d bbip
(C13-N3)
= 1.330(9) Å), which are symmetric and therefore exhibit equidistant C13-N3 and C13-N4
bond lengths.
The higher symmetry of ip and bbip is further supported by the similar angles at N3 and N4
which vary between 107.7(6)° and 108.1(6)°, and as well at C5 and C6 atoms which vary between
105.8(6)° and 107.5(6)°. In bip, these opposing angles differ in contrary (∠ bip
(C13,N3,C5) = 106.9(16)°
> ∠ bip
(C13,N4,C6)
= 104.28(16)° and even more: ∠bip
(N3,C5,C6)
= 105.47(15)° < ∠bip
(N4,C6,C5)
= 110.56(15)°) and
as a result of that, they significantly differ from the corresponding angles in ip/bbip. Prominent
is the decrease of the angle at C13 in the imidazolium salts ip and bbip in comparison to the
uncharged bip which can be attributed to the mentioned changes in π-character of the C13 and
decreased delocalization in the imidazolium salts (∠ bip
(N3,C13,N4)
= 113.60(16)° > ∠ip
(N3,C13,N4) = 110.1(2)°
≈ ∠ bbip
(N3,C13,N4) = 110.9(6)°).
The various twist angles of the benzyl groups in bip and bbip and transoid orientation thereof
in the latter indicate that mainly crystal packing forces are responsible for the observed
conformations in the structures. In addition to the conformational changes of the benzyl
functionalities, a number of packing effects were observed in the crystal lattice of ip, bip and
bbip (for an example see figure 84).
In the crystal lattice of ip, hydrogen bonds between the protons at the N3 atom and the O1
of the carboxy functionality of trifuoroacetic acid and further between the O2 of the carboxy
functionality and the N4-proton of the next ip molecule (d ip
ip
(N3-O1)
= 2.671 Å and d(N4-O2) = 2.659 Å)
form helical chains with an alternating order of ipH + and CF 3 COO − molecules, here both
molecules act as bridges (see figure 83 (left) on page 118). Furthermore, π-π-interactions
(d ip
(π-π)
= 3.297 Å) between the opposingly directed planar aromatic phenanthroline systems cause
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