Crystal Structures of the Phosphorus-Boron Adducts n-Pr
3
P ´ BBr
3
and I
3
P ´ BBr
3
Christoph Aubauer, Katherine Davidge
1)
, Thomas M. Klapo
È
tke*, Peter Mayer, Holger Piotrowski,
and Axel Schulz
Munich, Ludwig-Maximilians-UniversitaÈt, Department of Chemistry
Received June 7
th
2000.
Abstract. The structures of the phosphorus-boron adducts n-
Pr
3
P ´ BBr
3
(trigonal, space group P 4 c1, Z =4,
a = 11.5423(6), b = 11.5423(6) and c = 13.8066(7) A
Ê
) and
I
3
P ´ BBr
3
(orthorhombic, space group Pnma, Z =4,
a = 12.761(2), b = 11.427(1), c = 7.3728(7) A
Ê
) were deter-
mined by X-ray crystallography. The P±B distance of
2.01(1) A
Ê
in I
3
P ´ BBr
3
is significantly longer than the P±B
bond in n-Pr
3
P ´ BBr
3
(1.95(1) A
Ê
). The different Lewis basi-
city of phosphorus halides, PX
3
(X = Cl, Br, I), and alkylpho-
sphines is discussed. The charge transfer and the bond situa-
tion in these donor-acceptor complexes is studied on the
basis of NBO analysis. Selected frequencies of n-Pr
3
P ´ BBr
3
obtained by Raman and infrared spectroscopy are assigned
and compared with the normal modes of I
3
P ´ BBr
3
.
Keywords: Crystal structure; NBO analysis; Lewis basicity;
Phosphorus boron halide complexes; Vibrational spectro-
scopy
Kristallstrukturen der Phosphor-Bor Addukte n-Pr
3
P ´ BBr
3
und I
3
P ´ BBr
3
Inhaltsu
È
bersicht. Die Strukturen der Phosphor-Bor-Addukte
n-Pr
3
P ´ BBr
3
(trigonal, Raumgruppe P 3 c1, Z =4,
a = 11.5423(6), b = 11.5423(6) and c = 13.8066(7) A
Ê
) und
I
3
P ´ BBr
3
(orthorhombisch, Raumgruppe Pnma, Z =4,
a = 12.761(2), b = 11.427(1), c = 7.3728(7) A
Ê
) wurden durch
RoÈntgenstrukturanalyse aufgeklaÈrt. Der P±B-Abstand in
I
3
P ´ BBr
3
ist mit 2.01(1) A
Ê
bedeutend laÈnger als in
n-Pr
3
P ´ BBr
3
(1.95(1) A
Ê
). Die unterschiedliche Lewis-Basizi-
taÈt von Phosphorhalogeniden, PX
3
(X = Cl, Br, I), und Al-
kylphosphinen wird diskutiert. Der Ladungstransfer und die
chemische Bindung in diesen Donor-Akzeptor-Komplexen
werden auf der Basis von NBO-Analysen erklaÈrt. Raman-
und Infrarot-Spektren von n-Pr
3
P ´ BBr
3
werden mit den
Normalschwingungen von I
3
P ´ BBr
3
verglichen.
Introduction
Boron trihalides are widely used in chemical synthesis
as Lewis acids and alkyl phosphines as Lewis bases.
Both species are of great importance in modern chem-
istry [1]. The central boron atom has only six valence
electrons and therefore, it represents an electron defi-
cient species which shows a strong affinity for electron
density especially from lone pairs of a donor molecule.
PX
3
(X = Cl, Br, I) molecules represent only weak do-
nor molecules whereas alkyl phosphines form stronger
Lewis acid-base complexes. Although Lewis acid-base
complexes of phospines with boron halides have been
known for over four decades, only a few X-ray data of
these species are available [2±5].
Recently, we have been interested in the calculation
of Raman and IR spectra of phosphorus halide com-
pounds and phosphorus boron halide adducts. Vibra-
tional assignments of the normal modes for these Le-
wis acid-base adducts have been made on the basis of
comparison between theoretically obtained and ex-
perimentally observed Raman and IR data. Moreover
we have investigated the bonding and electron trans-
fer within these species [6±8].
Previous work has shown that the donor behavior
of phosphorus trihalides depends on the electronega-
tivity of the halogen substituents [9, 10]. For the phos-
phorus trihalides, the order of stability for the 1 : 1 do-
nor-acceptor complexes with boron trihalides is
PCl
3
< PBr
3
< PI
3
[11].
In this paper we want to report on the structural
and electronic properties of n-Pr
3
P ´ BBr
3
and
I
3
P ´ Br
3
.
Results and Discussion
The Lewis acid-base adduct n-Pr
3
P ´ BBr
3
(1) was pre-
pared from the reaction of tri-n-propylphosphine with
boron tribromide in pentane in a 1 : 1 molar ratio
under nitrogen and recrystallized from toluene.
I
3
P ´ BBr
3
(2) was prepared according to the literature
[2]. Crystals of 2 were obtained by recrystallization
from CS
2
.
Z. Anorg. Allg. Chem. 2000, 626, 2373±2378 Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0044±2313/00/6262373±2378 $ 17.50+.50/0 2373
1)
present address: Department of Chemistry, University of
Glasgow, Glasgow G12 8QQ, Scotland (U.K.)
* Prof. Dr. Thomas M. KlapoÈtke,
Department of Chemistry, Ludwig-Maximilians-UniversitaÈt,
Butenandtstr. 5±13 (Haus D), D-81377 Munich, Germany
e-mail:tmk@cup.uni-muenchen.de
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