Structural importance of secondary interactions in molecules: Origin of unconventional conformations of phosphine-borane adducts

Article abstract of DOI:10.1002/chem.200700649

The series of phosphine-borane adducts, Ph₂(H ₃C-C≡C)P-B(C₆F₅)₃ (8c), Ph(H₃C-C≡C)₂P-B(C₆F₅) ₃ (8b) and (H₃C-C≡C)₃P-B(C

Full text of DOI:10.1002/chem.200700649

FULL PAPER
DOI: 10.1002/chem.200700649
Structural Importance of Secondary Interactions in Molecules: Origin of
Unconventional Conformations of Phosphine–Borane Adducts
Patrick Spies, Roland Frçhlich, Gerald Kehr, Gerhard Erker,* and Stefan Grimme*[a]
Dedicated to Professor Hans J. Schäfer on the occasion of his 70th birthday
À ꢀ
À
Abstract: The series of phosphine–borane adducts, Ph
À ꢀ À ꢀ
G
ACHTREUNG
À
À
(8c), Ph
U
A
ACHTREUNG
pared. The X-ray crystal structure analyses revealed close to eclipsed conforma-
tions for all members of this series with average dihedral angles q(C-P-B-C) of
Keywords: boranes · donor–accept-
or systems · phosphines · quantum
AHCTREUNG
8.18 (8c), 12.38 (8b) and 20.38 (8a). Quantum chemical analysis of these com-
pounds revealed the importance of a subtle interplay between competing attractive
and repulsive secondary interactions, causing the surprising eclipsed conformation-
al preference for systems of this degree of complexity. Some cyclic phosphine–
borane adducts were studied for comparison.
chemical calculations
elucidation
· structure
Introduction
barrier (and hence the origin of the A/B energy difference)
of the parent hydrocarbon ethane has been (and probably
still is) a matter of controversial discussion.[2] For the higher
alkanes or systems with spatially (sterically) more demand-
ing substituents it seems commonly accepted that four-elec-
Most saturated hydrocarbons favour staggered conforma-
tions (A) substantially over their eclipsed ones (B).[1] The
latter are usually not even local minimum structures, but
rather they represent the transition-state geometries that are
passed upon the transition from one stable conformational
minimum structure to another. The nature of the energy
À
tron Pauli repulsion between in-plane C C s-bonds make
the eclipsed conformer B unfavourable. However, recently
there has been lively scientific discussion about the funda-
mental physical and chemical aspects of this interaction and
bonding concepts in general, as exemplified for the case of
[3]
À
the C C bond rotation in biphenyl.
One must realise that the overall situation of predicting
and understanding the preferred (conformational) structures
of molecules often becomes complicated by the interplay of
various types of secondary interactions between adjacent
subunits. In such situations energetic compensation effects
may lead to the observation of unusual or unexpected struc-
tural global minima, the appearance of which cannot be ra-
tionalised by any simplified general analysis.
The hetero-analogues of the alkanes are prominent exam-
ples of such a situation. Examples of both conformers A
and B were experimentally observed for differently substi-
[4,5]
À
À
tuted borane-phosphine adducts R3P BR’3.
The Ph3P B-
[a] P. Spies, Dr. R. Frçhlich, Dr. G. Kehr, Prof. G. Erker,
Prof. S. Grimme
(C6F5)3 system (2)[5] is a typical example of an eclipsed struc-
Organisch-Chemisches Institut, Universität Münster
Corrensstr. 40, 48149 Münster (Germany)
Fax : (+49)251-83-36503
À
ꢀ À
ture in the solid state, whereas the related Ph3P B(C C
A
CMe3)3 adduct (1)[4] exhibits a staggered conformation in
À
the crystal. We synthesised a series of P B addition prod-
E-mail: erker@uni-muenster.de
gimmes@uni-muenster.de
ucts that conceptually may be thought to be located be-
333
Chem. Eur. J. 2008, 14, 333 – 343
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

tween the extremes 1 and 2, namely the B
ꢀ À ꢀ À
A
2.064(2) (5a), 2.060(2) Š (5b)). Both compounds exhibit
distorted envelope-shaped conformations of the boraphos-
phacyclopentane core, but they are distinctly different from
each other. Compound 5a shows a conventional envelope
conformation with the tBu2P group adjacent to phosphorus
atom, representing the tip, and the C1-C2-C3-B1 moiety the
almost coplanar base. This leads to an arrangement of the
bulky substituents at P and B that is as close as possible to a
sterically favourable staggered orientation of the five-mem-
bered heterocyclic framework (dihedral angles q: C8-P1-B1-
C21 28.6(1), C8-P1-B1-C31 À96.5(1), C4-P1-B1-C31 47.7(1),
C4-P1-B1-C21 172.7(1)8) (Figure 1).
P
N
G
(C C CH3). The
preferred conformational structures of these heteroana-
logues of A and B in the crystal were determined by X-ray
diffraction. The observed structures were then analysed by
quantum chemical calculations to reveal the major secon-
dary interaction effects that were responsible for the occur-
rence of such favoured structural types. This revealed some
general features that need to be taken into account to un-
derstand or predict the correct structures of more complex
“alkane-like” molecular systems. We will begin our discus-
sion with a description of the structural features of a small
series of selected cyclic phosphine–borane adducts.
Results and Discussion
Syntheses and structural features of cyclic phosphine–
borane adducts: Allylbis(tert-butyl)phosphine (4a) was syn-
thesised by treatment of chlorobis(tert-butyl)phosphine (3a)
with allylmagnesium bromide.[6a] Subsequent reaction of 4a
with “Piersꢁ borane” [HB
(C6F5)2][6b] in toluene at reflux tem-
G
peratures resulted in a clean hydroboration reaction to yield
5a (28%) (Scheme 1). Compound 5a features a single
Figure 1. Views of the different envelope type structures of 5a (left) and
5b (right).
Compound 5b also features a distorted envelope confor-
mation in the crystal, but it is distinctly different from 5a
À
À
(see Figure 1). In 5b the central CH2 group forms the tip
of the envelope and C1-P1-B1-C2forms the base. This leads
to an eclipsed conformational arrangement of the bulky aryl
substituents at the adjacent phosphorus and boron centres
(dihedral angles q: C10-P1-B1-C21 À6.8(1), C4-P1-B1-C31
Scheme 1.
1H NMR tert-butyl resonance at d=1.25 ppm (18H), a
broad 31P NMR signal at d=+42ppm ( n1/2 =140 Hz) and a
typical 11B NMR resonance for a four-coordinated boron
atom at d=À10 ppm. A set of three 19F NMR signals was
observed at d=À128.5 (4F, o-), À159.5 (2F, p-) and
À165.0 ppm (4F, m-C6F5). The reaction of chlorodiphenyl-
phosphine (3b) with the allyl Grignard reagent followed by
hydroboration with HB
the five-membered P,B-compound 5b [51% isolated,
13C NMR of the P-(CH2)3-B unit at d=27.2 (1J
(P,C)=
42.6 Hz), 25.6 (2J
(P,C)=14.8 Hz), 23.7 ppm (br)]. Com-
À4.0(2),
C10-P1-B1-C31
À131.9(1),
C4-P1-B1-C21
121.1(1)8). It may be that this unconventional conformation
has become slightly favoured by some p-stacking interaction
of the C6H5 ring at phosphorus with the C6F5 ring at boron.
Such p–p arene interactions can be energetically quite sub-
stantial, especially when strong acceptors such as the penta-
fluorophenyl moiety are involved.[7] We note that the
C10–C15 phenyl group at P1 and the C21–C26 pentafluoro-
phenyl group at B1 are oriented close to parallel with the
framework (angle between the C10–C15 and C21–C26
planes: À24.88, C21···C10 separation: 3.093 Š).
(C6F5)2 proceeded similarly to give
AHCTREUNG
ACHTREUNG
pound 5b featured very typical NMR signals at d=22.6
(31P); À9 (11B) and À129.5 (o), À158.5 (p) and À164.2ppm
(m) (19F).
À
The corresponding cyclic six-membered P B adduct 6 was
prepared by treatment of Ph2PCl (3c) with butenylmagnesi-
um bromide to yield 4c (59%) followed by the hydrobora-
Single crystals were obtained for both 5a and 5b (from di-
ethyl ether). Both compounds in the crystal feature five-
membered heterocyclic structures, as expected, created by
internal adduct formation between the phosphine moieties
À
tion reaction with HB(C6F5)2 (see Scheme 1). Compound 6
C
was isolated in 66% yield as a white solid (31P NMR: d=
0.5, 11B NMR: d=À12ppm). It features a set of four
and the strong Lewis acid boron centres (d
(P1 B1)=
13C NMR methylene signals (C1–C4) at d=23.5 (1J
(P,C)=
A
334
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 333 – 343

Phosphine–Borane Adducts
FULL PAPER
32.2 Hz), 25.5 (2J
N
N
19.6 ppm (br). Compound 6 exhibits dynamic NMR spectra.
At 300 K in [D2]dichloromethane a sharp 19F NMR signal at
d=À163.0 ppm (m-C6F5), a slightly broadened p-C6F5 reso-
nance at d=À157.8 ppm, and a very broad o-C6F5 signal at
d=À128.4 ppm were observed. Upon lowering the monitor-
ing temperature (see Figure 2) the p-C6F5 resonance rapidly
Figure 3. Views of the conformational arrangements of the experimental-
ly determined global minimum chair structure (left) and a DFT calculat-
ed local minimum twist structure of 6 (right).
ised by dihedral angles C11-P1-B1-C41 À60.9(2), C11-P1-
B1-C31 57.9(2), C21-P1-B1-C41 61.6(2) and C1-P1-B1-C4
49.8(2)8.
A DFT calculation also finds a respective conventional
chair conformation as the global minimum; only the C6H5
and C6F5 groups are slightly rotated in the gas-phase calcula-
tion. However, the DFT calculation located a second confor-
mer as a local minimum, energetically only 1.7 kcalmolÀ1
less favourable than the global minimum chair conformer.
This isomer exhibits a distorted twist-conformation. It is
characterised by an eclipsed geometric arrangement of the
substituents along the P–B vector. The negative influence by
steric hindrance in this unconventional arrangement is appa-
rently more than compensated by some positive arene p–p
interaction between the coplanar C6H5/C6F5 pair at the P–B
unit (see Figure 3, right).
So far this study has shown that the favoured structures of
bulky phosphine–borane adducts may be influenced by a va-
riety of stabilizing and destabilizing forces, which may make
them difficult to predict. We therefore studied a related
series of acyclic phosphine–borane adducts in some detail,
all of which have favoured unconventional conformational
structures.
Figure 2. Temperature dependent 19F DNMR spectra of
([D2]dichloromethane, 564 MHz).
6
splits into a 1:1 pair of signals, whereas the o-C6F5 signals
decoalesces into a sharp 1:1 pair and a broad signal, each of
relative intensity 2. The m-C6F5 resonances of the system
are isochronous and do not change much with temperature.
The characteristic appearance of the temperature-depen-
dent 19F NMR signals of 6 indicates “freezing” of the inver-
sion process of the conventional cyclohexane chair frame-
work (see below) with decreasing temperature, which leads
À
to a differentiation between axial and equatorial C6F5 sub-
stituents, in addition to an increasingly hindered rotation
[8]
À
around the B C6F5 vectors. From the coalescence behav-
iour we estimated
a
Gibbs activation energy of
DG°inv
process and DG°rot
A
Syntheses and spectroscopic characterisation of new phos-
À
tion barrier of the first B C6F5 unit (the second value could
not be obtained from this experiment, see Figure 2).
phine–B(C6F5)3 adducts: The phosphines used for this study
G
were prepared analogously to published procedures
(Scheme 2). Thus, treatment of Ph2PCl with one molar
Single crystals of 6 suitable for X-ray crystal structure
analysis (see Figure 3) were obtained from diethyl ether.
The crystal features four-coordinate pseudotetrahedral
[9]
ꢀ À
equivalent of propynyllithium gave Ph2P(C C CH3) (7c).
A
The compound features a typical 31P NMR resonance at d=
(C C) IR band at 2183 cm . Simi-
larly, treatment of the related starting material PhPCl2 with
À1
À
ꢀ
boron and phosphorus centres (d
A
À32.0 ppm and a sharp n˜
A
Compound 6 exhibits a close to ideal chair conformation
with a nearly perfect staggering of the bulky aryl substitu-
ents along the P–B vector (see the respective projection in
Figure 3, left). The corresponding dihedral angle of the anti-
periplanar substituents amounts to C21-P1-B1-C31:
À179.6(2)8, the respective gauche orientations are character-
À ꢀ À
two molar equivalents of the Li C C CH3 reagent yielded
[10]
(31P NMR d=À43.0 ppm, IR: n˜ =
ꢀ À
PhP(C C CH3)2 (7b)
A
2182 cmÀ1), and three equivalents propynyllithium were
treated with PCl3 to yield P
(C C CH3)3 (7a) (31P NMR
A
[11]
ꢀ À
d=À86.3 ppm, IR: n˜ =2189 cmÀ1].
Chem. Eur. J. 2008, 14, 333 – 343
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
335

G. Erker, S. Grimme et al.
(Figure 4) shows that a new phosphorus–boron bond was
À
À
formed (P B: 2.157(3) Š). Although this P B linkage is
rather long, bond formation between the group 13 and
Scheme 2.
The reaction of 7c with B(C6F5)3 was carried out at ambi-
G
ent temperature in toluene. The usual workup (for details
see the Experimental Section) gave the adduct (8c) as a
ꢀ À
white solid in 78% yield. The PhP
N
N
adduct (8b) was prepared from the phosphine 7b and
tris(pentafluorophenyl)borane (54% isolated). Compound
8c features a 31P NMR resonance at d=6.4 ppm. The
11B NMR signal of 8c is observed at d=À8 ppm (cf. free
À ꢀ
À
B
(C6F5)3: d=60 ppm[12]). The Ph
E
G
The coordinated B(C6F5)3 unit shows NMR data that is
G
quite different from the free tris(pentafluorophenyl)borane
Lewis acid. It features a set of three 19F NMR multiplets at
d=À127.3 (o), À157.2( p) and À165.3 ppm (m). Free
B(C6F5)3 characteristically shows a much wider separation of
A
Figure 4. Two views of the molecular structure of 8c.
the respective p-F and m-F resonances (free B(C6F5)3: d=
A
À129.1 (o), À142.4 (p), À160.3 ppm (m)).[12] The 11B NMR
resonance of compound 8b is found at d=À7 ppm. The
adduct 8b features a sharp alkynyl IR band at 2204 cmÀ1.
Tris(pentafluorophenyl)borane was treated with tripropynyl-
phosphine (7a) in toluene at ambient temperature to give
the 1:1 adduct (8a) in 63% yield. It shows the typical NMR
group 15 element results in a severe distortion of the re-
maining bond angles at both the main group element cen-
tres upon going from the three- to four-coordinate states.
With respect to the free phosphine all distal C-P-C angles at
phosphorus have opened up to approximately 1048 upon
adduct formation (C1-P-C21 104.1(2), C1-P-C11 104.6(1),
C11-P-C21 104.2(2)8). The bond angles at phosphorus which
are proximal to the boron centre are much larger at 110.3(2)
(C1-P-B), 114.4(1) (C11-P-B), and 118.0(1)8 (C21-P-B). The
features of both
a tetra-coordinated phosphorus atom
(31P NMR: d=À38.0 ppm) and a tetra-coordinated boron
(11B NMR: d=À9 ppm). The 19F NMR spectrum shows a
typical set of corresponding C6F5 resonances at d=À126.0
(o), À155.2( p) and À163.2ppm ( m-C6F5). The 13C NMR sig-
ꢀ À
À
P C1 bond (1.718(4) Š) is much shorter than the adjacent
À
À
nals of the P
C
P–arene s-bonds (P C11 1.823(3), P C21 1.823(3) Š). The
À
ꢀ
A
C1 C2bond is in the typical range of a C C triple bond
(1.204(4) Š)[13] and the adjacent (sp)C2
N
3
À
À
amounts to a typical 1.461(5) Š. The three B C carbon s-
X-ray crystal structure analyses of the adducts: The struc-
bonds to the C6F5 rings were found in a very close range be-
À À
tures of all three B
C
tween 1.628(5) and 1.636(5) Š. The distal C B C angles are
found to be 111.0(3) (C31-B-C41), 114.0(3) (C41-B-C51)
and 115.6(3)8 (C31-B-C51). All three are larger than the
proximal angles (C31-B-P 102.7(2), C51-B-P 103.5(2), C41-
336
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 333 – 343

Phosphine–Borane Adducts
FULL PAPER
B-P 108.9(2)8). The conformation of the six substituents at
adduct 8b (110.8(2) (C11-B1-C21), 114.7(2) (C11-B1-C31),
116.3(2)8 (C21-B1-C31)).
Single crystals of the adduct 8a were obtained from
À
the P B unit in compound 8c is eclipsed. Typical dihedral
angles q that characterise this situation are, for example,
C11-P-B-C51: 10.2(2), C21-P-B-C31: 7.8(3) and C1-P-B-C41:
6.2(3)8.
diethyl
A
ether. As expected for a compound derived from the
À
sterically least encumbered phosphine in this series the P B
bond in 8a (2.062(3) Š) is the shortest of the three adducts.
In the crystal the molecule is C3-symmetric along the P–B
vector. Again the ipso-C(C6F5)-P-B angles (C11-B-P1
105.3(1)8) are markedly smaller than the acetylide-C-P-B
angles (C1-P1-B 113.6(1)8). Consequently, the “distal”
angles at boron (C11-B-C11* 113.3(1)8) increase and the re-
spective C1-P1-C1’ angles are decreased (105.1(1)8). The
P1–acetylide moieties are close to linear (P angles P1-C1-C2
Compound 8b shows similar structural features in the
À
crystal (Figure 5). As compared to 8c the P B bond in 8b
seems to be marginally stronger, as judged from its slightly
À
176.9(2), C1-C2-C3 178.0(2)8; bond lengths P1 C1: 1.737(2),
À
À
À
C1 C21.190()2, C2
C3: 1.463(3) Š). The B C6F5 linkage
À
amounts to B1 C11: 1.633(2) Š.
À ꢀ
À
The (H3C C C)3P B
(C6F5)3 adduct 8a again exhibits an
eclipsed conformation of the substituents at boron and phos-
phorous along the P–B vector (see Figure 6), although this is
the example in this series of adducts that features the largest
deviation from the ideal geometric situation. The corre-
sponding dihedral angle q (C1-P1-B1-C11) was found to be
20.38(1).
Quantum chemical calculations: To get more insight into
the origin of structural preferences for eclipsed or staggered
Figure 5. Two views of the molecular structure of 8b.
À
shorter length (8b; P1 B1: 2.125(2) Š). The overall confor-
mation in 8b is again eclipsed with pertinent dihedral angles
q being for example, À13.9(1) (C4-P1-B1-C21), À12.8(2)
(C1-P1-B1-C11) and À10.1(1)8 (C41-P1-B1-C31). The P–
À
acetylide bond lengths again are short (P1 C1 1.741(2),
À
P1 C4 1.740(2) Š) as compared to the remaining P–arene
À
linkage (P1 C41 1.813(2) Š). The bond angles at P1 fall
into two groups: being within a narrow range between
104.2(1) and 105.6(1)8 on the “distal” side, and amounting
to larger values of 109.3(1) (C4-P1-B1), 115.6(1) (C41-P1-
B1) and 116.3(1)8 (C1-P1-B1) in the “proximal” sector. Con-
versely, the “proximal” bond angles at B1 are smaller
(102.3(1) (C21-B1-P1), 102.6(1) (C31-B1-P1), 108.7(1)8
(C11-B1-P1)) than the “distal” angles at the borane end of
Figure 6. Two views of the molecular structure of 8a.
Chem. Eur. J. 2008, 14, 333 – 343
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
337

G. Erker, S. Grimme et al.
conformations depending on the substituents, we performed
a series of quantum chemical calculations first on two model
DFT-D method. Also the rotational barriers agree to within
0.1 kcalmolÀ1. For 10, the barrier is about the same as for
ethane (2.9 kcalmolÀ1), but only slightly larger than for 9
À
À
compounds PH3 BH3 (9) and PMe3 BMe3 (10) and second-
ly on the real systems 2 and
(R=H). For 9 the experimental value of the rotational bar-
[21]
8
A
rier is 2.47Æ0.05 kcalmolÀ1
.
This indicates that (when
ble crystal-packing effects. For
10 we also computed the poten-
electronic effects are neglected, for a recent discussion see
reference [22]) already the a-carbon atoms are located not
far away from regions of attractive vdW interactions. We
furthermore determined the torsional barrier for a larger
À
tial-energy curve for the P B
bond dissociation. The depth of
À
this potential and the associated force constant are strongly
(fixed) B P distance of 2.22 Š and found a decrease from
related to the ability for torsion around the B P bond as,
2.9 to 1.5 kcalmolÀ1. This already, at least in part, explains
why some compounds may favour the eclipsed conforma-
À
for example, steric interactions between the substituents
À
À
may be compensated by B P bond stretching. Before discus-
sing these systems in detail, a closer look at the appropriate
quantum chemical methods is necessary.
tion: for B P distances found experimentally (2.05–2.20 Š),
the inherent barrier is already so small that attractive vdW
interactions between the large substituents can overcompen-
sate.
The so-called steric interactions between the substituents
can be rationalised as the sum of repulsive Pauli and attrac-
tive electrostatic and van der Waals (vdW) interactions. The
latter are usually not accounted for by standard density
functional theory (DFT) methods that are normally used for
systems of this size (up to 68 atoms and about 1200 AO
basis functions). In our case we have many interatomic dis-
tances not far away from those of typical vdW minima (3.5–
4.5 Š) and thus, inclusion of vdW effects (the other terms
are accounted for quite accurately by DFT) is mandatory.
We do this with the recently developed dispersion correction
to DFT termed DFT-D,[17a] which has proven very successful
in many different chemical applications.[17,18] This approach
is used here together with the PBE density functional[19] and
its excellent performance for predicting structures and rota-
tional barriers in the P–B system is demonstrated for the
two model compounds 9 and 10. The comparison with refer-
À
The potential-energy curves for B P bond stretching in 10
(all other degrees of freedom fully relaxed at the DFT-D-
(2df,2p) level) are shown in Figure 7. The agree-
ment between DFT-D-PBE and SCS-MP2is almost perfect,
PBE/TZV
A
[20]
ence data obtained at very reliable MP2and SCS-MP2
levels is shown in Table 1.
À
For the P B bond lengths, the two methods agree to
within 0.01–0.02Š, while the bond angles are within 0.3 8.
Note that also the trend when going from the eclipsed to
Figure 7. Computed potential-energy curves (TZV(2df,2p) AO basis) for
A
À
À
À
B P bond stretching of Me3P BMe3 (10).
the staggered form (increase of R
A
the larger effect for R=Me is reproduced accurately by the
and the importance of the dispersion correction for DFT is
also evident from comparison with the plain PBE curve, in
which it is seen that it clearly misses some attraction in the
minimum region. The dissociation energy is found to be
only about 19 kcalmolÀ1, and around the minimum, bond
lengths changes of, for example, 0.3 Š increase the energy
Table 1. Comparison of structural data [Š, 8] and rotational barriers
[kcalmolÀ1] for the two model compounds R3P BR3 9 (R=H) and 10
À
(R=Me) at DFT-D-PBE/TZV
(2df,2p) and (SCS-)MP2/TZV
G
levels, respectively.
dihedral angle bond angle
R-B-P
À
Method
Conf.
d
A
barrier
by less than 2kcalmol À1. Thus, this P B interaction should
À
10: R=Me
MP2
DFTD-PBE
MP2
DFTD-PBE
9: R=H
MP2
DFTD-PBE
MP2
be considered as quite weak, which explains the wide varia-
A
A
B
B
1.96260.6
1.978
2.004
105.9
À
tions of the P B distances found experimentally.
60.5
0.0
0.0
105.8
106.3
106.2
2.8[a]
2.9[b]
The comparison between experimental and computed
geometrical parameters that are characteristic for the inves-
tigated question are given in Table 2. We do not show all
computed geometries here as these are visually not distin-
guishable from the X-ray data already shown. Perusing this
table, one finds that our calculations in all cases predict the
right conformer, that is, eclipsed in cases 2 and 8a–c. The
averaged dihedral angles and their trend towards conformer
2.026
A
A
B
B
1.934
1.922
1.963
1.947
60.2103.5
60.0
0.0
103.7
103.7
104.0
2.3[a]
2.3
DFTD-PBE
0.0
À1
[a] SCS-MP2level. [b] The barrier decreases to 1.5 kcalmol
at a
À
R(P B) of 2.22 Š.
G
338
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 333 – 343

Phosphine–Borane Adducts
FULL PAPER
Table 2. Comparison of calculated (DFT-D-PBE/TZVP’) and experimen-
tal structural data for 2 and 8a–c.[a]
2
8c
8b
8a
exptl calcd exptl calcd exptl calcd exptl calcd
À
d
A
2.18
105
2.22
105
0
2.16
114
8.1
2.22
104
1.8
2.13
105
12.3
2.22
103
12.6
2.06
105
20.3
2.18
103
14.7
C-B-P-C[c] 3.5
[a] Bond length in Š, bond angles and dihedral angles in degrees.
[b] Average of the three bond angles. [c] Average of the three dihedral
angles.
A when phenyl is replaced by the smaller CCMe group is
nicely reproduced by our calculations.
Figure 8. Space-filling model of 8a in the minimum conformation (left)
and a hypothetically staggered form (right).
For 8a, we also started the geometry optimisation in con-
formation A, but finally obtained the minimum B. This indi-
cates that for these systems only the eclipsed conformation
corresponds to a real minimum. We also performed a cross-
check and optimised 1 that experimentally prefers confor-
mation A, and found also in this case theoretically the right
structure. Although crystal packing effects cannot be ruled
out, these findings not only support our theoretical treat-
ment, but furthermore indicate that the general phenomen-
on probably represents an inherent property of the mole-
butyl groups on the boron side move in between two phenyl
groups on the phosphorous side.
Conclusion
A systematic series of Lewis acid/Lewis base adducts
A
N
À
ꢀ À
ꢀ À
cules. However, it is also noted that the B P distances are
G
G
ACHTREUNG
systematically overestimated by our calculations. Consider-
ing the good performance of DFT-D-PBE for the model sys-
tems discussed above, this is quite unexpected. Tentatively,
these differences between 0.04 and 0.12Š can be attributed
to some “compression” of the structures in the crystal.
But what about the intramolecular vdW interactions men-
tioned above? These are certainly of importance in the pres-
ent system but not decisive for the preferred conformation.
A first hint comes from computations with the pure PBE
functional for which the dispersion correction was switched
off. For example, for 8a we still obtained B as a minimum,
that is, no qualitative change of the structure was observed.
Although the separation of the vdW interaction between the
PBE functional and the dispersion correction is only qualita-
tive and one should not take this result too seriously, it
points to a different reasoning for the preference of an
eclipsed conformation in our systems.
À À À
their averaged characteristic dihedral angles q (C P B C)
ranging from 8.1 (8c) to 12.3 (8b) and 20.38 (8a). In a com-
parison with the preference of typical alkanes for staggered
conformations (A, q=608) the structural behaviour of the
adducts 8 and some similar systems towards favouring the
converse eclipsed conformations (B, ideal q=08) is striking.
From this structural behaviour, which is systematically dif-
ferent from that of the simple alkanes, it is evident that ad-
ditional attractive and/or repulsive components of interac-
tion between essential subunits of the molecular entities of
the adducts 8, other than the ubiquitous four-electron repul-
sion between localised bond orbitals, must be held responsi-
ble for this unusual behaviour. Our detailed theoretical
analysis has revealed the significance of the interplay of ad-
ditional attractive dispersion forces and specific components
of steric hindrance for the qualitative and quantitative de-
scription of the specific structural situations encountered in
these phosphine–borane addition products. Thus, energetic
compensation effects can become quite important for the
determination of the favoured structures, especially in sys-
tems such as 8, for which a shallow and soft potential of the
A closer look at the structures in a space-filling model
with atomic surfaces drawn at their vdW radius reveals a
strong interference of the substituents at phosphorus with
the B
Figure 8, in which we compare the minimum structure (left)
with hypothetically staggered conformation (dihedral
(C6F6)3 group. This is shown for example for 8a in
a
angles adjusted manually to 608 while keeping all other pa-
rameters fixed). In Figure 8 (right side), it is seen clearly
that this causes clashes of the fluorine atoms with the
carbon atoms of the triple bonds. Because the B(C6F6)3
group has a strong preference for a propeller-shape due to
otherwise too close F···F contacts, the energetically most
favoured pathway is thus the torsion around the B P bond
into conformation B. A similar explanation holds for the
preferred staggered conformation of 1, where the bulky tert-
P B bonds favour a rather pronounced response of the
overall system to even subtle changes in the periphery.
Structural determination by various such secondary effects
is probably not limited to such specific systems as the ones
used in this combined experimental/theoretical study, but is
likely to be encountered more frequently in other systems
of a similar topological complexity. It is good to see that re-
cently developed theoretical tools can adequately deal with
G
À
Chem. Eur. J. 2008, 14, 333 – 343
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
339

G. Erker, S. Grimme et al.
(22 mL, 10.8 mmol) yielded 1.53 g (6.4 mmol, 59%) of a colourless oil
(b.p. 1308C, oil pump vacuum). 1H NMR (300 MHz, [D6]benzene, 258C,
TMS): d=7.39 (m, 4H; Ph), 7.06 (m, 6H; Ph), 5.76 (m, 1H; =CH), 4.92
(m, 2H; =CH2), 2.13, 2.00 ppm (2m, 2”2H; -CH2CH2-); 13C{1H} NMR
the description (and consequently the prediction) of the
structural features of such complex molecular systems.[23]
(76 MHz, [D6]benzene, 258C, TMS): d=139.5 (d, J
ACHTREUNG
2
ACHTREUNG
(d, J
(d, 3J
17.1 Hz; PCH2), 27.8 ppm (d, 2J
A
Experimental Section
C
ACHTREUNG
AHCTREUNG
Materials: All reactions involving air- or moisture-sensitive compounds
were carried out under an inert gas atmosphere (Argon) by using
Schlenk-type glassware or in a glovebox. Solvents were dried and dis-
tilled prior to use. Unless otherwise noted, all starting materials were
commercially available and were used without further purification. Pro-
pynyllithium[24] and tris(pentafluorophenyl)borane[12] were synthesised ac-
cording to literature procedure.
(121 MHz, [D6]benzene, 258C, H3PO4): d=À15.0 ppm (n1/2 =5 Hz).
General procedure for the synthesis of cyclic phosphine–borane adducts
(5a, 5b, 6): Bis(pentafluorophenyl)borane was dissolved in toluene
(100 mL) and the enylphosphine (1 equiv) was added. Then the reaction
mixture was heated for two hours under reflux. After cooling to room
temperature the solvent was removed in oil-pump vacuum. The residue
was eluted in pentane (20 mL), isolated by filtration and dried in oil-
pump vacuum.
Techniques: The following instruments were used for physical characteri-
sation of the compounds: melting points: DSC 2010 TA-instruments; ele-
mental analyses: Foss–Heraeus CHNO-Rapid; NMR: Bruker AC 200 P
(11B: 64.2MHz; 31P: 81 MHz), AMX 300/AV 300 (1H: 300 MHz; 13C:
75 MHz, 31P: 121.5 MHz; 19F: 282.4 MHz), Varian UNITY plus NMR
spectrometer (1H: 599.9 MHz; 13C: 150.8 MHz; 31P: 242.8 MHz; 19F:
564.2MHz).
Hydroboration of 4a—formation of 5a: Reaction of bis(pentafluorophe-
nyl)borane (500 mg, 1.45 mmol) and 4a (271 mg, 1.45 mmol) yielded
217 mg
[D2]dichloromethane, 258C, TMS): d=2.10 (m, 4H; CH2-CH2-P), 1.69
(m, 2H; CH2-B), 1.25 ppm (d, 3J
(P,H)=12.5 Hz, 18H; tBu);
13C{1H} NMR (151 MHz, [D2]dichloromethane, 258C, TMS): d=148.1
(dm, 1J(F,C)=244 Hz; o-C6F5), 139.7 (dm, 1J
(F,C)=240 Hz; p-C6F5),
137.6 (dm, J
15.8 Hz; tBu), 29.9 (s; tBu), 22.3 (br; BCH2), 21.8 (d, 1J
PCH2), 21.1 ppm (d, 2J(P,C)=10.9 Hz; CH2); 19F NMR (564 MHz,
[D2]dichloromethane, 258C, CFCl3): d=À128.5 (br; o-C6F5), À159.5
(p-C6F5), (m-C6F5); (122 MHz,
31P{1H} NMR
(28%)
of
a
white
solid.
1H NMR
(600 MHz,
AHCTREUNG
X-ray crystal structure determinations: Data sets were collected with a
Nonius KappaCCD diffractometer equipped with a rotating anode gener-
ator. Programs used: data collection COLLECT (Nonius B.V., 1998),
data reduction Denzo-SMN,[28], absorption correction SORTAV[29] and
Denzo,[30] structure solution SHELXS-97,[31] structure refinement
SHELXL-97,[32] graphics SCHAKAL.[33]
A
ACHTREUNG
1
1
ACHTREUNG
A
(P,C)=
AHCTREUNG
AHCTREUNG
A
À165.0 ppm
CCDC-644001–644006 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[D2]dichloromethane, 258C, H3PO4): d=42ppm ( n1/2 =140 Hz);
11B{1H} NMR (160 MHz, [D2]dichloromethane, 258C, BF3OEt2): d=
À10 ppm (d, 1J
(P,B) ꢁ50 Hz, n1/2 =70 Hz); elemental analysis calcd (%)
A
for C23H24BF10P: C 51.91, H 4.55; found: C 51.82, H 4.25.
Allyldi-tert-butylphosphine (4a): Allylmagnesium bromide solution in
THF (1m, 33 mL, 33.0 mmol) was added to a mixture of di-tert-butyl-
chlorophosphine (5.00 g, 27.6 mmol) in THF (75 mL). The solution was
heated for 1 h under reflux. The reaction mixture was concentrated in
vacuo until approximately 15 mL remained, then pentane was added
(30 mL). After filtration and removal of the solvent in vacuo the residue
was distilled under reduced pressure (b.p. 428C, 0.25 mbar). A colourless
oil was obtained (2.5 g, 49%). 1H NMR (300 MHz, [D2]dichloromethane,
258C, TMS): d=5.9 (m, 1H; CH=), 5.01 (m, 2H; =CH2), 2.35 (m, 2H;
X-ray crystal structure analysis of 5a: formula C23H24BF10P, Mr =532.20,
colourless crystal 0.40”0.30”0.10 mm, a=9.5439(2), b=10.2216(2), c=
13.4103(3) Š,
a=70.592(1).
b=71.347(2),
g=72.095(1)8,
V=
1139.07(4) Š3, 1calcd =1.552gcm À3, m=0.213 mmÀ1, empirical absorption
correction (min/max transmission: 0.920/0.979), Z=2, triclinic, space
¯
group P1 (No. 2), l=0.71073 Š, T=198 K, w and f scans, 12463 reflec-
tions collected (Æh, Æk, Æl), [(sinq)/l]=0.67 ŠÀ1, 5470 independent
(Rint =0.037) and 4691 observed reflections [Iꢂ2s(I)], 322 refined param-
eters, R=0.037, wR2 =0.104, max/min residual electron density 0.34/
À0.34 eŠÀ3, hydrogen atoms calculated and refined as riding atoms.
CH2), 1.2ppm (d, 3J
[D2]dichloromethane, 258C, TMS): d=139.6 (d, 2J
115.2(d, 3J(P,C)=10.5 Hz; =CH2), 32.3 (d, 1J
(P,C)=22.5 Hz; tBu), 30.2
(d, 2J
(P,C)=21.4 Hz; CH2);
ACHTREUNG
AHCTREUNG
Hydroboration of 4b—formation of 5b: Bis(pentafluorophenyl)borane
(1.25 g, 3.6 mmol) and 4b (0.82g, 3.6 mmol) yielded 1.05 g (1.8 mmol,
A
ACHTREUNG
A
ACHTREUNG
51%) of
a
white solid, m.p. 1408C (DSC). 1H NMR (600 MHz,
31P{1H} NMR (122 MHz, [D2]dichloromethane, 258C, H3PO4): d=
60.9 ppm (n1/2 =95.2Hz).
[D6]benzene, 258C, TMS): d=6.98 (m, 4H, o-Ph), 6.90 (m, 2H, p-Ph),
6.79 (m, 4H, m-Ph), 2.05 (m, 4H, CH2-CH2-P), 1.86 ppm (m, 2H; CH2-
B); 13C{1H} NMR (151 MHz, [D6]benzene, 258C, TMS): d=148.3 (dm,
General procedure for the synthesis of alkenyldiphenylphosphines (4b,
4c): Chlorodiphenylphosphine (2mL, 2.4 g, 10.8 mmol) was dissolved in
diethyl ether (100 mL) and cooled down to 08C. A solution of the respec-
tive enylmagnesium bromide solution (1 equiv) in THF was added at
08C. The suspension was stirred for one hour at room temperature. The
solid was removed by filtration. The solvent was removed and the prod-
uct was isolated using vacuum distillation.
1J
(dm, 1J
(d, 4J(P,C)=2.8 Hz; p-Ph), 128.7 (d, 3J
1J(P,C)=50.3 Hz; i-Ph), 119.6 (br; i-C6F5), 27.2 (d, 1J
PCH2), 25.6 (d, 2J(P,C)=14.8 Hz; CH2), 23.7 ppm (br; CH2-B); 19F NMR
(564 MHz, [D6]benzene, 258C, CFCl3): d=À129.5 (o-C6F5), À158.5
A
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
Allyldiphenylphosphine
(4b):
Chlorodiphenylphosphine
(2mL,
A
10.8 mmol) and 1m allylmagnesium bromide/THF solution (11 mL,
10.8 mmol) yielded 1.28 g (5.6 mmol, 52%) of a colourless oil (b.p.
1078C, oil pump vacuum). 1H NMR (300 MHz, [D6]benzene, 258C,
TMS): d=7.38 (m, 4H; Ph), 7.06 (m, 6H; Ph), 5.77 (m, 1H, =CH), 4.91,
258C, H3PO4): d=22.6 ppm (n1/2 =80 Hz); 11B{1H} NMR (96 MHz,
[D6]benzene, 258C, BF3OEt2): d=À9 ppm (n1/2 =180 Hz); elemental
analysis calcd (%) for C27H16BF10P: C 56.68, H 2.82; found: C 56.86, H
2.99.
4.89 (2m, 2”H; =CH2), 2.71 ppm (d, 3J
ACHTREUNG
AHCTREUNG
X-ray crystal structure analysis of 5b: formula C27H16BF10P, Mr =572.18,
colourless crystal 0.40”0.25”0.15 mm, a=8.928(1), b=11.558(1), c=
13.358(1) Š, a=66.15(1). b=84.05(1), g=68.74(1)8, V=1173.4(2) Š3,
AHCTREUNG
G
ACHTREUNG
(P,C)=10.7 Hz; =CH2), 34.0 ppm (d, 1J
ACHTREUNG
1calcd=1.619 gcmÀ3
,
m=0.213 mmÀ1
,
empirical absorption correction
¯
31P{1H} NMR (121 MHz, [D6]benzene, 258C, H3PO4): d=À14.8 ppm
(n1/2=4 Hz).
(min/max transmission: 0.920/0.969), Z=2, triclinic, space group P1 (No.
2), l=0.71073 Š, T=198 K, w and f scans, 10859 reflections collected
(Æh, Æk, Æl), [(sinq)/l]=0.67 ŠÀ1, 5590 independent (Rint =0.032) and
4227 observed reflections [Iꢂ2s(I)], 352refined parameters, R=0.042,
But-3-enyldiphenylphosphine (4c): Chlorodiphenylphosphine (2mL,
10.8 mmol) and 0.5m but-3-enylmagnesium bromide/THF solution
340
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 333 – 343

Phosphine–Borane Adducts
FULL PAPER
wR2 =0.105, max/min residual electron density 0.38/À0.36 eŠÀ3, hydro-
(1.32g, 58%) as a white solid. The product was purified by sublimation
gen atoms calculated and refined as riding atoms.
(1008C, oil pump vacuum). 1H NMR (300 MHz, [D6]benzene, 258C,
TMS): d=1.40 ppm (d, 4J(P,H)=2.3 Hz, 9H; CH3); 13C{1H} NMR
A
Hydroboration of 4c—formation of 6: Bis(pentafluorophenyl)borane
(1.44 g, 4.1 mmol) and 4c (1.00 g, 4.1 mmol) yielded 1.58 g (2.7 mmol,
(101 MHz, [D6]benzene, 258C, TMS): d=103.3 (d, 2J
(P,C)=10.8 Hz;
N
ꢀ
CCH3), 71.5 (d, 1J
A
a
white solid, m.p. 1438C (DSC). 1H NMR (600 MHz,
ꢀ
66%) of
[D2]dichloromethane, 258C, TMS): d=7.49, 7.38 (2m, 10H; Ph), 2.64 (dt,
3J(H,H)=6.7 Hz, 2J
(P,H)=11.5 Hz, 2H; PCH2), 1.98 (br, 2H; CH2),
1.57 ppm (br, 4H; -CH2-CH2B);
13C{1H} NMR(101 MHz,
[D2]dichloromethane, 258C, TMS): d=148.2(dm, (F,C)=239 Hz;
1J
o-C6F5), 139.5 (dm, 1J(F,C)=234 Hz; p-C6F5), 137.4 (dm, 1J
(F,C)=
256 Hz; m-C6F5), 133.0 (d, 3J(P,C)=8.2Hz; o-Ph), 131.7 (d, 4J
(P,C)=
(P,C)=
(121.5 MHz, [D6]benzene, 258C, H3PO4): d=À86.3 ppm (n1/2 =10 Hz); IR
(KBr): n˜ =2189 cmÀ1 (vs, C C).
ꢀ
A
ACHTREUNG
Diphenylpropynylphosphine–tris(pentafluorophenyl)borane adduct (8c):
Tris(pentafluorophenyl)borane (0.67 g, 1.3 mmol) and diphenylpropynyl-
phosphine (0.29 g, 1.3 mmol) were dissolved in toluene (30 mL). The re-
action mixture was stirred for 2h at ambient temperature. The solvent
was removed until 5 mL remained in the flask. Pentane (20 mL) was
added and the product started to precipitate as a white solid. It was col-
lected by filtration, washed with pentane (3 mL) and dried in vacuum to
AHCTREUNG
A
ACHTREUNG
A
ACHTREUNG
2.5 Hz; p-Ph), 130.0 (d, 1J
C
ACHTREUNG
PCH2
CH2),
24.6 (d,
3J
A
(P,C)=32.2 Hz; PCH2), 19.6 ppm
2
1
yield 8c (0.75 g, 78%). M.p. 1408C (DSC, decomp); H NMR (600 MHz,
(br; BCH2); 19F NMR (564 MHz, [D2]dichloromethane, 258C, CFCl3):
d=À128.4 (br; o-C6F5), À157.8 (p-C6F5), À163.0 ppm (m-C6F5);
31P{1H} NMR(121 MHz, [D2]dichloromethane, 258C, H3PO4): d=0.5 ppm
(n1/2 =120 Hz); 11B{1H} NMR (64 MHz, [D2]dichloromethane, 258C,
BF3OEt2): d=À12ppm
[D2]dichloromethane, 258C, TMS): d=7.50 (m, 6H; Ph), 7.35 (m, 4H;
Ph), 2.00 ppm (d, 4J(P,H)=4.0 Hz, 3H; CH3); 13C{1H} NMR (151 MHz,
N
[D2]dichloromethane, 258C, TMS): d=148.8 (dm, 1J
o-C6F5), 140.6 (dm, 1J(F,C)=251 Hz; p-C6F5), 137.3 (dm, 1J
255 Hz; m-C6F5), 133.2(d, 3J(P,C)=9.6 Hz; o-Ph), 132.3 (d, 4J
ACHTREUNG
A
ACHTREUNG
G
ACHTREUNG
2.8 Hz; p-Ph), 129.0 (d, 2J
G
ACHTREUNG
(n1/2=164 Hz); elemental analysis calcd (%) for C28H18BF10P: C 57.37, H
3.09; found: C 57.60, H 3.32. For the determination of the Gibbs activa-
58.0 Hz; i-Ph), 115.6 (br; i-C6F5), 113.2(d, 2J
(P,C)=15.4 Hz; C ), 67.1
(P,C)=2.9 Hz; CH3);
tion energies equation DG°
A
G
N
(d, 1J
(P,C)=119.3 Hz; PC ), 5.4 ppm (d, 4J
ACHTREUNG
ꢀ
with the following parameters : ring inversion process: DG°inv (243 K,
511 Hz (188 K))=10.7Æ0.5 kcalmolÀ1; rotation barrier of the first B-C6F5
unit: DG°rot (275 K, 4320 Hz (188 K))=11.0Æ0.5 kcalmolÀ1).
31P{1H} NMR (121 MHz, [D2]dichloromethane, 258C, H3PO4): d=
6.4 ppm (n1/2 =85 Hz); 11B{1H} NMR (64 MHz, [D2]dichloromethane,
258C, BF3OEt2): d=À8 ppm (n1/2 =220 Hz); 19F NMR (282 MHz,
[D2]dichloromethane, 258C, CFCl3): d=À125.1 (o-C6F5), À155.1 (p-
C6F5), À163.1 ppm
X-ray crystal structure analysis of 6: formula C28H18BF10P, Mr =586.20,
colourless crystal 0.15”0.10”0.03 mm, a=9.451(1), b=10.780(1), c=
13.504(1) Š, a=83.21(1). b=76.56(1), g=68.09(1)8, V=1240.7(2) Š3,
1calcd =1.569 gcmÀ3, m=0.204 mmÀ1, empirical absorption correction (min/
(m-C6F5); IR (KBr): n˜ =2211 cmÀ1 (vs, C C); elemental analysis calcd
ꢀ
¯
max transmission: 0.970/0.994), Z=2, triclinic, space group P1 (No. 2),
X-ray crystal structure analysis of 8c: Single crystals were obtained from
diethyl ether, formula C33H13BF15P, Mr =736.21, yellow crystal 0.20”
0.15”0.15 mm, a=10.251(1), b=11.515(1), c=13.523(1) Š, a=85.89(1),
l=0.71073 Š, T=223 K, w and f scans, 11642reflections collected ( Æh,
Æk, Æl), [(sinq)/l]=0.67 ŠÀ1, 5897 independent (Rint =0.046) and 3723
observed reflections [Iꢂ2s(I)], 361 refined parameters, R=0.057, wR2 =
b=75.16(1), g=73.11(1)8, V=1476.5(2) Š3, 1calcd =1.656 gcmÀ3
,
m=
À3
0.214 mmÀ1
, empirical absorption correction (min/max transmission:
0.124, max/min residual electron density 0.28/À0.38 eŠ
, hydrogen
atoms calculated and refined as riding atoms.
¯
0.959/0.969), Z=2, triclinic, space group P1 (No. 2), l=0.71073 Š, T=
198 K, w and f scans, 18013 reflections collected (Æh, Æk, Æl), [(sinq)/
l]=0.62Š À1, 5981 independent (Rint =0.052) and 3327 observed reflec-
tions [Iꢂ2s(I)], 452refined parameters, R=0.056, wR2 =0.137, max/min
residual electron density 0.61/À0.34 eŠÀ3, hydrogen atoms calculated and
refined as riding atoms.
General procedure for the synthesis of alkynylphosphines (7a–c):[7–11]
A
suspension of propynyl lithium (1 equiv) in THF (100 mL) was cooled to
À788C. The molar equivalent of the respective chlorophosphine was
added and the resulting brown solution was stirred for one hour without
a cooling bath. For workup, the solvent was evaporated and the product
was extracted (under argon) with dry diethyl ether (3”30 mL) by using a
filter canula. The ether layer was evaporated to dryness and the product
was isolated by distillation or sublimation.
Phenyldipropynylphosphine–tris(pentafluorophenyl)borane adduct (8b):
Tris(pentafluorophenyl)borane (0.67 g, 1.30 mmol) was dissolved in tolu-
ene (30 mL). Phenyldipropynylphosphine (0.24 g, 1.30 mmol) was added
and the reaction mixture stirred for 2h at ambient temperature. The sol-
vent was removed until 5 mL remained in the flask. Pentane (20 mL) was
added and the product started to precipitate as a white solid. It was col-
lected by filtration, washed with pentane (3 mL) and dried in vacuum to
yield 8b (0.30 g, 54%) as a white solid. M.p. 1478C (DSC, decomp);
1H NMR (600 MHz, [D2]dichloromethane, 258C, TMS): d=7.75 (m, 2H;
Diphenylpropynylphosphine (7c):[9] Reaction of propynyl lithium (1.20 g,
26.1 mmol) with chlorodiphenylphosphine (4.5 mL, 24.4 mmol) yielded
7c (2.85 g, 52%) as a colourless oil, which becomes a solid after a few
hours. The product was purified by distillation (b.p. 1508C, oil pump
vacuum). 1H NMR (400 MHz, [D6]benzene, 258C, TMS): d=7.72(m,
4H; Ph), 7.07, 7.02(2m, 6H; Ph), 1.53 ppm (d,
CH3); 13C{1H} NMR (101 MHz, [D6]benzene, 258C, TMS): d=137.7 (d,
1J(P,C)=7.6 Hz; i-Ph), 132.9 (d, 2J
(P,C)=21.0 Hz; o-Ph), 129.0 (p-Ph),
128.8 (d, J
(d, 1J(P,C)=3.3 Hz; PC ), 4.9 ppm (d, 3J
(P,C)=1.1 Hz, CH3);
4J
(P,H)=1.5 Hz, 3H;
G
o-Ph), 7.51 (m, 1H; p-Ph), 7.41 (m, 2H; o-Ph), 1.98 ppm (d, 4J
4.1 Hz, 6H; CH3); (151 MHz, 600 MHz,
13C{1H} NMR
[D2]dichloromethane, 258C, TMS): d=149.1 (dm, 1J
(F,C)=240 Hz; o-
C6F5), 140.6 (dm, 1J(F,C)=251 Hz; p-C6F5), 137.3 (dm, 1J
(F,C)=249 Hz;
m-C6F5), 132.7 (d, 2J
(P,C)=12.1 Hz; m-Ph), 124.1 (d, 1J
A
A
ACHTREUNG
3
2
ACHTREUNG
ꢀ
G
AHCTREUNG
ꢀ
E
G
A
ACHTREUNG
31P{1H} NMR (121.5 MHz, [D6]benzene, 258C, TMS): d=À32.0 ppm
AHCTREUNG
(n1/2=3 Hz); IR (KBr): n˜ =2183 cmÀ1 (vs, C C).
ꢀ
A
N
2
1
ACHTREUNG
Phenyldipropynylphosphine (7b):[10] Reaction of propynyl lithium (1.17 g,
25.4 mmol) with dichlorophenylphosphine (1.35 mL, 10.3 mmol) yielded
7b (2.85 g, 66%) as a colourless oil. The product was purified by distilla-
tion (b.p. 1358C, oil pump vacuum). 1H NMR (300 MHz, [D6]benzene,
258C, TMS): d=7.98 (m, 2H; Ph), 7.23, 7.15 (2m, 3H; Ph), 1.56 ppm (d,
(br; i-C6F5), 110.5 (d, J
G
PC ), 5.3 ppm (d, 4J
ACHTREUNG
ꢀ
[D2]dichloromethane, 258C, H3PO4): d=À15 ppm (n1/2 =130 Hz);
11B{1H} NMR (64 MHz, [D2]dichloromethane, 258C, BF3OEt2): d=
À7 ppm (n1/2 =310 Hz); 19F NMR (564 MHz, [D2]dichloromethane, 258C,
CFCl3): d=À127.3 (o-C6F5), À157.2(br, p-C6F5), À165.3 ppm (m-C6F5);
ꢀ
4J(P,H)=1.9 Hz, 6H; CH3); 13C{1H} NMR (75 MHz, [D6]benzene, 258C,
U
IR
(KBr): n˜ =2204 cmÀ1 (vs, C C); elemental analysis calcd (%) for
C
TMS): d=135.3 (i-Ph), 132.4 (d, 2J
(P,C)=22.3 Hz; o-Ph), 129.3 (p-Ph),
G
128.8 (d, 3J
(P,C)=7.9 Hz; m-Ph) 104.2(d, 2J
(P,C)=6.6 Hz; CCH3), 74.1
A
ꢀ
C30H11BF15P: C 51.61, H 1.59; found: C 51.23, H 1.62.
(PC ), 4.8 ppm (CH3); 31P{1H} NMR (121 MHz, [D6]benzene, 258C,
ꢀ
X-ray crystal structure analysis of 8b: Single crystals were obtained from
diethyl ether, formula C30H11BF15P, Mr =698.17, colourless crystal 0.30”
0.30”0.15 mm, a=10.707(1), b=11.438(1), c=11.566(1) Š, a=91.71(1),
H3PO4): d=À43.0 ppm (n1/2 =20 Hz). IR (KBr): n˜ =2182 cmÀ1 (vs, C C).
ꢀ
Tripropynylphosphine (7a):[11,9c] Reaction of propynyl lithium (2.12 g,
46.1 mmol) with phosphorustrichloride (1.3 mL, 15.4 mmol) yielded 7a
b=101.31(1), g=93.91(1)8, V=1384.4(2) Š3, 1calcd =1.675 gcmÀ3
,
m=
Chem. Eur. J. 2008, 14, 333 – 343
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
341

G. Erker, S. Grimme et al.
0.223 mmÀ1
,
empirical absorption correction (min/max transmission
¯
[1] E. L. Eliel, S. H. Wilen, M. P. Doyle, Basic Organic Stereochemistry,
Wiley, New York, 2001.
0.936/0.967), Z=2, triclinic, space group P1 (No. 2), l=0.71073 Š, T=
198 K, w and f scans, 15421 reflections collected (Æh, Æk, Æl), [(sinq)/
l]=0.67 ŠÀ1, 6709 independent (Rint =0.039) and 5084 observed reflec-
tions [Iꢂ2s(I)], 426 refined parameters, R=0.045, wR2 =0.119, max/min
residual electron density 0.34/À0.31 eŠÀ3, hydrogen atoms calculated and
refined as riding atoms.
[2] a) V. Pophristic, L. Goodman, Nature 2001, 411, 565–568; b) F. M.
Bickelhaupt, E. J. Baerends, Angew. Chem. 2003, 115, 4315–4320;
Angew. Chem. Int. Ed. 2003, 42, 4183–4188; c) F. Weinhold, Angew.
Chem. 2003, 115, 4320–4326; Angew. Chem. Int. Ed. 2003, 42, 4188–
4194.
Tripropynylphosphine–tris(pentafluorophenyl)borane-adduct
(8a):
[3] a) C. F. Matta, J. Hernꢂndez-Trujillo, T. H. Tang, R. F. W. Bader,
Chem. Eur. J. 2003, 9, 1940–1951; b) J. Poater, M. Solꢃ, F. M. Bickel-
haupt, Chem. Eur. J. 2006, 12, 2889–2895; c) R. F. W. Bader, Chem.
Eur. J. 2006, 12, 2896–2901; d) J. Poater, M. Solꢃ, F. M. Bickelhaupt,
Chem. Eur. J. 2006, 12, 2902–2905.
[4] M. J. Bayer, H. Pritzkow, W. Siebert, Eur. J. Inorg. Chem. 2002,
2069–2072.
[5] H. Jacobsen, H. Berke, S. Dçring, G. Kehr, G. Erker, R. Frçhlich, O.
Meyer, Organometallics 1999, 18, 1724–1735.
Tris(pentafluorophenyl)borane (0.67 g, 1.30 mmol) and tripropynylphos-
phine (0.19 g, 1.30 mmol) were dissolved in toluene (30 mL). The reac-
tion mixture was stirred for 2h at ambient temperature. The solvent was
removed until 5 mL remained in the flask. Pentane (20 mL) was added
and the product started to precipitate as a white solid. It was collected by
filtration, washed with pentane (3 mL) and dried in vacuum to yield 8a
(0.54 g, 63%) as a white solid. M.p. 1608C (DSC, decomp); 1H NMR
(600 MHz, [D2]dichloromethane, 258C, TMS): d=1.92ppm (d, 4J
4.2Hz; CH 3); 13C{1H} NMR (151 MHz, [D2]dichloromethane, 258C,
TMS): d=149.2(dm, 1J(F,C)=240 Hz; o-C6F5), 140.6 (dm, 1J
(F,C)=
248 Hz; p-C6F5), 137.4 (dm, 1J
(F,C)=250 Hz; m-C6F5), 115.0 (br, i-C6F5),
ꢀ À
A
[6] a) S. Hietkamp, D. J. Stufkens, K. Vrieze, J. Organomet. Chem. 1976,
112, 419–428; b) W. E. Piers, T. Chivers, Chem. Soc. Rev. 1997, 26,
345–354.
A
ACHTREUNG
AHCTREUNG
108.9 (d, 2J
5.2ppm
(P,C)=25.6 Hz; C ), 64.1 (d, 1J
(d, (P,C)=3.5 Hz; CH3);
3J 31P{1H} NMR
(P,C)=156.8 Hz; PC ),
(121 MHz,
ꢀ
[7] a) J. H. Williams, Acc. Chem. Res. 1993, 26, 593–598; b) S. L. Cock-
roft, C. A. Hunter, K. R. Lawson, J. Perkins, C. J. Urch, J. Am.
Chem. Soc. 2005, 127, 8594–8595; c) Y. El-azizi, A. Schmitzer, S. K.
Collins, Angew. Chem. 2006, 118, 982–987; Angew. Chem. Int. Ed.
2006, 45, 968–973; and references cited in these articles.
[8] a) W. Ahlers, B. Temme, G. Erker, R. Frçhlich, T. Fox, J. Organo-
met. Chem. 1997, 527, 191–201; b) D. Vagedes, G. Erker, G. Kehr,
K. Bergander, O. Kataeva, R. Frçhlich, S. Grimme, C. Mück-Lich-
tenfeld, Dalton Trans. 2003, 1337–1344; c) S. Guidotti, I. Camurati,
F. Focante, L. Angellini, G. Moscardi, L. Resconi, R. Leardini, D.
Nanni, P. Mercandelli, A. Sironi, T. Beringhelli, D. Maggioni, J. Org.
Chem. 2003, 68, 5445–5465; d) F. Focante, R. Leardini, A. Mazzanti,
P. Mercandelli, D. Nanni, Organometallics 2006, 25, 2166–2172.
[9] a) C. Charrier, W. Chodkiewicz, P. Cadiot, Bull. Soc. Chim. France
1966, 167, 1002–1011; b) M. Simalty, M. H. Mebazaa, Bull. Soc.
Chim. France 1972, 554, 3532–3536; c) R.-M. Lequan, M.-J. Pouet,
M.-P Simonnin, Org. Magn. Reson. 1975, 392–400; d) C. Charrier,
M.-P Simonnin, W. Chodkiewicz, P. Cadiot, Compt. Rend. Acad. Sci.
Paris 1964, 258, 1537–2540.
ACHTREUNG
[D2]dichloromethane, 258C, H3PO4): d=À38.0 ppm (n1/2 =150 Hz);
11B{1H} NMR (64 MHz, [D2]dichloromethane, 258C, BF3OEt2): d=
À9 ppm (n1/2 =200 Hz); 19F NMR (282 MHz, [D2]dichloromethane, 258C,
CFCl3): d=À126.0 (o-C6F5), À155.2( p-C6F5), À163.2ppm ( m-C6F5); IR-
ꢀ
A
C27H9BF15P: C 49.13, H 1.37; found: C 48.95, H 1.68.
X-ray crystal structure analysis of 8a: Single crystals were obtained from
diethyl ether, formula C27H9BF15P, Mr =660.12, colourless crystal 0.30”
0.25”0.20 mm, a=17.629(1), b=17.629(1), c=15.180(1) Š, a=90.00, b=
90.00, g=120.008, V=4085.6(4) Š3, 1calcd =1.610 gcmÀ3, m=0.221 mmÀ1
,
empirical absorption correction (min/max transmission0.937/0.957), Z=6,
¯
hexagonal, space group R3 (No. 148), l=0.71073 Š, T=198 K, w and f
scans, 4893 reflections collected (Æh, Æk, Æl), [(sinq)/l]=0.66 ŠÀ1, 2140
independent (Rint =0.027) and 1542 observed reflections [Iꢂ2s(I)], 134
refined parameters, R=0.041, wR2 =0.114, max/min residual electron
density 0.17/À0.25 eŠÀ3, hydrogen atoms calculated and refined as riding
atoms.
Theoretical methods used: The quantum chemical calculations were per-
formed with slightly modified versions of the TURBOMOLE suite of
programs.[25] As AO basis, triple-zeta (TZV) sets of Ahlrichs et al.[26]
were employed. In the calculations of the model compounds, (2df,2p) po-
larisation functions were added. For the real systems (2df) polarisation
functions were used only for the phosphorus and boron atoms, while for
all other atoms a smaller (d,p) set was used. This basis was denoted as
TZVP’. In the (SCS-)MP2treatments for the correlation energy (frozen-
core approximation) and for the Coulomb operator in the DFT treat-
ments, the RI-approximation and the m4 numerical quadrature grid were
used.[27] For a detailed description of the dispersion correction used to-
gether with the PBE[19] density functional (termed DFT-D-PBE), see ref-
erence [17a]). The geometries of 2 and 5a–c were fully optimised at the
corresponding theoretical level without any symmetry restrictions. In the
model calculations and for the potential energy curves, the C3v or C3 sym-
metry, respectively, was used. As starting geometries, in most cases the
experimental X-ray structures were employed, which ensured that the
right minima with respect to rotations of the substituents were computed.
In selected cases, cross-checks were made by optimisations started from
[10] a) W. E. Davidsohn, M. C. Henry, Chem. Rev. 1967, 67, 73–106;
b) G. Märkl, W. Weber, W. Weiss, Chem. Ber. 1985, 118, 2365–2395.
[11] a) W. R. Cullen, D. C. Frost, W. R. Leeder, J. Fluorine Chem. 1971,
1, 227–233; b) R. E. Sacher, B. C. Pant, F. A. Miller, F. R. Brown,
Spectrochim. Acta, Part A 1972, 28, 1361–1373.
[12] a) A. G. Massey, A. J. Park, F. G. A. Stone, Proc. Chem. Soc. 1963,
212; b) A. G. Massey, A. J. Park, J. Organomet. Chem. 1964, 2, 245–
250; c) J. L. Pohlmann, F. E. Brinkmann, Z. Naturforsch. B 1965, 20,
1–4; d) J. L. Pohlmann, F. E. Brinkmann, Z. Naturforsch. B 1965,
20, 5–11; e) A. G. Massey, A. J. Park, J. Organomet. Chem. 1966, 5,
218–225.
[13] For other examples of B(C6F5)3-phosphine adducts and related com-
G
pounds see e.g.: a) D. C. Bradely, M. B. Hursthouse, M. Motevalli,
Z. Dao-Hong, J. Chem. Soc. Chem. Commun. 1991, 7–8; b) D. C.
Bradely, I. S. Harding, A. D. Keefe, M. Motevalli, D. H. Zheng, J.
Chem. Soc. Dalton Trans. 1996, 3931–3936; c) B. E. Carpenter,
W. E. Piers, R. McDonald, Can. J. Chem. 2001, 79, 291–295; d) B. E.
Carpenter, W. E. Piers, M. Parvez, G. P. A. Yap, S. J. Rettig, Can. J.
Chem. 2001, 79, 857–867; e) J. M. Denis, H. Forintos, H. Szelke, L.
Toupet, T.-N. Pham, P.-J. Madec, A.-C. Gaument, Chem. Commun.
2003, 54–55; see also reference [5], and references therein.
À
conformers with rotations around the B P bond.
À
[14] Structures of other related examples of R3P BR3 adducts: a) M. S.
Lube, R. L. Wells, Inorg. Chem. 1996, 35, 5007–5014; b) R. B.
Coapes, F. E. S. Souza, M. A. Fox, A. S. Batsanov, A. E. Goeta, D. S.
Yufit, M. A. Leech, J. A. K. Howard, A. J. Scott, W. Clegg, T. B.
Marder, J. Chem. Soc. Dalton Trans. 2001, 1201–1209; c) T. J. Male-
fetse, G. J. Swiegers, N. J. Coville, M. A. Fernandes, Organometallics
2002, 21, 2898–2904; d) N. Oohara, T. Imamoto, Bull. Chem. Soc.
Jpn. 2002, 75, 1359–1365; e) U. Monkowius, S. Nogai, H. Schmid-
baur, Dalton Trans. 2003, 987–991.
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie is gratefully acknowledged. We thank
the BASF AG for the generous donation of solvents.
342
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 333 – 343

Phosphine–Borane Adducts
FULL PAPER
[15] For the structures of other representative examples of Lewis base-B-
[17] a) S. Grimme, J. Comput. Chem. 2004, 25, 1463–1476; b) M. Piacen-
za, S. Grimme, ChemPhysChem 2005, 6, 1554–1558; c) M. Piacenza,
S. Grimme, J. Am. Chem. Soc. 2005, 127, 14841–14848; d) M. Parac,
M. Etinski, M. Peric, S. Grimme, J. Chem. Theory Comput. 2005, 1,
1110–1118.
[18] B. Bo Zou, K. Dreger, C. Mück-Lichtenfeld, S. Grimme, H. J. Schä-
fer, H. Fuchs, L. Chi, Langmuir 2005, 21, 1364–1370.
[19] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77,
3865–3868.
[20] S. Grimme, J. Chem. Phys. 2003, 118, 9095–9102.
[21] J. R. Durig, Y. S. Li, L. A. Carreira, J. D. Odom, J. Am. Chem. Soc.
1973, 95, 2491–2496.
[22] S. S. Mallajosyula, A. Datta, S. K. Pati, J. Phys. Chem. A 2006, 110,
5156–5163.
A
Kotila, J. Organomet. Chem. 1996, 518, 17–19; b) J. R. Galsworthy,
M. L. H. Green, M. Müller, K. Prout, J. Chem. Soc. Dalton Trans
1997, 1309–1313; c) M. J. G. Lesly, A. Woodward, N. J. Taylor, T. B.
Marder, Chem. Mater. 1998, 10, 1355–1365; d) S. Dçring, G. Erker,
R. Frçhlich, O. Meyer, K. Bergander, Organometallics 1998, 17,
2183–2187; e) D. J. Parks, W. E. Piers, M. Parvez, R. Atencio, M.
Zaworotko, Organometallics 1998, 17, 1369–1377; f) D. Vagedes, R.
Frçhlich, G. Erker, Angew. Chem. 1999, 111, 3561–3565; Angew.
Chem. Int. Ed. 1999, 38, 3362–3365; g) L. H. Doerrer, M. L. H.
Green, J. Chem. Soc. Dalton Trans. 1999, 4325–4329; h) R. E. La-
Pointe, G. R. Roof, K. A. Abboud, J. Klosin, J. Am. Chem. Soc.
2000, 122, 9560–9561; i) C. Bergquist, B. M. Bridgewater, C. J.
Harlan, J. R. Norton, R. A. Friesner, G. Parkin, J. Am. Chem. Soc.
2000, 122, 10581–10590; j) R. Duchateau, R. A. Van Santen, P. A.
Yap, Organometallics 2000, 19, 809–816; k) G. Kehr, R. Roesmann,
R. Frçhlich, C. Holst, G. Erker, Eur. J. Inorg. Chem. 2001, 535–538;
l) J. Zhou, S. J. Lancaster, D. A. Walker, S. Beck, M. Thornton-Pett,
M. Bochmann, J. Am. Chem. Soc. 2001, 123, 223–237; m) S. J. Lan-
caster, A. Rodriguez, A. Lara-Sanchez, M. D. Hannant, D. A.
Walker, D. H. Hughes, M. Bochmann, Organometallics 2002, 21,
451–453; n) F. Schaper, H.-H. Brintzinger, Acta Crystallogr. Sect. E
2002, 58, 77–78; o) D. Vagedes, G. Erker, R. Frçhlich, J. Organomet.
Chem. 2002, 641, 148–155; 2002, 651, 157; p) D. Vagedes, G. Kehr,
D. Kçnig, K. Wedeking, R. Frçhlich, G. Erker, C. Mück-Lichtenfeld,
S. Grimme, Eur. J. Inorg. Chem. 2002, 2015–2021; q) J. M. Black-
well, W. E. Piers, M. Parvez, R. McDonald, Organometallics 2002,
21, 1400–1407; r) D. Vagedes, G. Erker, G. Kehr, K. Bergander, O.
Kataeva, R. Frçhlich, S. Grimme, C. Mück-Lichtenfeld, Dalton
Trans. 2003, 1337–1344; s) I. C. Vei, S. I. Pascu, M. L. H. Green,
J. C. Green, R. E. Schilling, G. D. W. Anderson, L. H. Rees, Dalton
Trans. 2003, 2550–2557; t) S. Guidotti, I. Camurati, F. Focante, L.
Angellini, G. Moscardi, L. Resconi, R. Leardini, D. Nanni, P. Mer-
candelli, A. Sironi, T. Beringhelli, D. Maggioni, J. Org. Chem. 2003,
68, 5445–5465; u) A. J. Mountford, W. Clegg, R. W. Harrington,
S. M. Humphrey, S. J. Lancaster, Chem. Commun. 2005, 2044–2046;
v) J. Sanchez-Nieves, L.-M. Frutos, P. Royo, O. Castano, E. Herdt-
weck, Organometallics 2005, 24, 2004–2007; w) A. J. Mountford,
S. J. Lancaster, S. J. Coles, P. N. Horton, D. L. Hughes, M. B. Hurst-
house, E. L. Mark, Inorg. Chem. 2005, 44, 5921–5933; x) F. Froc-
tane, I. Camurati, L. Resconi, S. Guidotti, T. Beringhelli, G. DꢁAl-
fonso, D. Donghi, D. Maggioni, P. Mercandelli, A. Sironi, Inorg.
Chem. 2006, 45, 1683–1692; y) F. Froctane, R. Leardini, A. Mazzan-
ti, P. Mercandelli, D. Nanni, Organometallics 2006, 25, 2166–2172;
z) Review: G. Erker, Dalton Trans. 2005, 1883–1890.
[23] A good understanding of the bonding and conformational features
À
of R3P BR’3 adducts seems essential for the further development of
the recently discovered H2 activation features of some of those sys-
tems: a) G. C. Welch, R. R. S. Juan, J. D. Masuda, D. W. Stephan,
Science 2006, 314, 1124–1126; b) G. C. Welch, D. W. Stephan, J. Am.
Chem. Soc. 2007, 129, 1880–1881.
[24] a) V. Jäger, H. G. Viehe, Methoden Org. Chem. (Houben-Weyl), 4th
edition 1977, V/2a, 123; b) K. B. Starowieyski, A. Chojnowski, Kus-
mierek, Z. 1980, 192, 147–154; c) S. L. Bender, M. R. Detty, N. F.
Haley, Tetrahedron Lett. 1982, 23, 1531–1534.
[25] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kçlmel Chem. Phys.
Lett. 1989, 162, 165–169; TURBOMOLE (Vers. 5.6): R. Ahlrichs et
al, Universität Karlsruhe 2003. See also: http://www.turbomole.com.
[26] A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100, 5829–
5835; the basis sets are available from the TURBOMOLE home-
page via the FTP Server Button (in the subdirectories basen, jbasen,
and cbasen). See http://www.turbomole.com.
[27] a) K. Eichkorn, O. Treutler, H. ꢄhm, M. Häser, R. Ahlrichs, Chem.
Phys. Lett. 1995, 240, 283–290; b) K. Eichkorn, F. Weigend, O. Treu-
tler, R. Ahlrichs, Theor. Chem. Acc. 1997, 97, 119–124; c) F. Wei-
gend, M. Häser, Theor. Chem. Acc. 1997, 97, 331–340; d) F. Wei-
gend, A. Kçhn, C. Hättig, J. Chem. Phys. 2002, 116, 3175–3183.
[28] Z. Otwinowski, W. Minor, Methods Enzymology, 1997, 276, 307–
326.
[29] R. H. Blessing, Acta Crystallogr. Sect. A 1995, 51, 33–37; R. H.
Blessing, J. Appl. Crystallogr. 1997, 30, 421–426.
[30] Z. Otwinowski, D. Borek, W. Majewski, W. Minor, Acta Crystallogr.
Sect. A. 2003, 59, 228–234.
[31] G. M. Sheldrick, Acta Crystallogr. Sect. A 1990, 46, 467–473.
[32] SHELXL-97, program for crystal structure refinement, G. M. Shel-
drick, Universität Gçttingen, Gçttingen (Germany), 1997.
[33] E. Keller, Universität Freiburg, Freiburg (Germany), 1997.
[16] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, G. Orpen, R.
Taylor, J. Chem. Soc. Perkin Trans. 1987, S1–S19.
Received: April 27, 2007
Published online: September 25, 2007
Chem. Eur. J. 2008, 14, 333 – 343
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
343