Carbodiphosphorane C(PPh3)2 as a single and twofold lewis base with boranes: Synthesis, crystal structures and theoretical studies on [H3B{C(PPh3)2}] and [{(μ-H)H 4B2}{C(PPh

Article abstract of DOI:10.1002/ejic.200900691

The donor-acceptor complex [(H₃B)[C(PPh₃) ₂}] (2) has been synthesized by treating B₂H₆ with C(PPh₃J₂ and its geometry determined by X-ray structure analysis. Treatment of 2 with

Full text of DOI:10.1002/ejic.200900691

FULL PAPER
DOI: 10.1002/ejic.200900691
Carbodiphosphorane C(PPh3)2 as a Single and Twofold Lewis Base with
Boranes: Synthesis, Crystal Structures and Theoretical Studies on
[H3B{C(PPh3)2}] and [{(µ-H)H4B2}{C(PPh3)2}]+
Wolfgang Petz,*[a] Florian Öxler,[a] Bernhard Neumüller,*[a] Ralf Tonner,[a,b] and
Gernot Frenking*[a]
Keywords: Boranes / Density functional calculations / Donor-acceptor systems / Bond energy / Ab initio calculations
The donor-acceptor complex [(H3B){C(PPh3)2}] (2) has been
synthesized by treating B2H6 with C(PPh3)2 and its geometry
determined by X-ray structure analysis. Treatment of 2 with
DME yields the complex [{(µ-H)H4B2}{C(PPh3)2}][B2H7] (4),
which has also been isolated and structurally characterized.
Compound 4 is the first complex of a carbodiphosphorane
where the carbon donor atom binds with its two-electron
lone pairs to two main-group Lewis acids larger than protons.
This reaction is likely to occur via initial formation of [(H3B)2-
{C(PPh3)2}] (6), which subsequently reacts with B2H6, with
loss of a hydride, to yield 4. Quantum chemical calculations
of 2, 4+ and 6 show that the carbon–boron bonds in these
complexes are very strong, and analysis of the bonding situa-
tion using the EDA, NBO and AIM methods reveals typical
bonding patterns between the divalent carbon(0) moieties
and one or two Lewis acids. The carbon donor atom of the
carbodiphosphorane moiety remains strongly negatively
charged even in the cation 4+.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
bene complexes [M]ǟCR2 and in ylides, which are usually
written as R2C=PR3,[6] are further examples of formally
sp2-type carbon ligands.[7] The donor-acceptor complexes
EǟCR2=PR3 and EǟCH2=M(L)x, where E is a Lewis
acid, are the only known compounds with an sp3-carbon
donor atom.[8]
1. Introduction
Neutral donor ligands with a lone electron pair at carbon
play an important role in organometallic chemistry and can
be divided into several types, depending on the coordina-
tion at the carbon atom. Carbon monoxide and isonitriles,
both of which possess singly coordinated carbon, are well-
known ligands with an sp-type carbon atom that mainly
form complexes with low-valent transition metals of the
type [M]ǟCO and [M]ǟCNR.[1] Fischer-type carbyne
complexes can be similarly formulated with singly coordi-
nated carbon donor atoms ([M]–ǟCR+).[2] Complexes
which have a naked carbon atom as ligand ([M]ǟC) are a
recently synthesized novel type of carbon-donor com-
pound.[3] These carbon complexes were found to be isolobal
to CO.[4] In the series of sp2-type carbon ligands, the iso-
lable N-heterocyclic carbenes (NHC) have attracted much
interest in recent years as σ-donors in numerous complexes
which have found numerous applications in homogeneous
catalysis.[5] The carbene fragments in the Fischer-type car-
The carbodiphosphorane C(PPh3)2 (1),[9] which has a
strongly bent geometry, is an unusual carbon ligand.[10] In
contrast to NHC compounds, 1 has two lone electron pairs
at the divalent carbon(0) atom, which is stabilized by two
phosphane groups through strong donor-acceptor bonds of
the type LǞCǟL.[11] Carbodiphosphoranes are examples
of CL2 compounds where L is a strong σ donor.[12] The
latter compounds have been termed carbones[13] to distin-
guish them from carbenes, which have only one lone elec-
tron pair. The related carbone C(NHC)2, which contains
benzoannelated NHC ligands instead of phosphanes, was
recently synthesized and structurally characterized[14] after
the geometry and stability of the parent carbodicarbene
had been predicted by quantum chemical calculations.[15]
Since carbodiphosphoranes have two lone electron pairs
at carbon, they should be able to bind either one Lewis acid
E, to yield a complex of type A (Scheme 1), or two Lewis
acids to give a complex of type B (Scheme 1). Some com-
plexes of type A bound to Lewis acids containing group 13
atoms, such as the adducts of 1 with InMe3 and AlBr3,
which have been characterized by X-ray analyses, are
known.[16] Related adducts A with transition-metal[17] or
other main-group Lewis acids[18] are also known. Similarly,
[a] Fachbereich Chemie der Philipps-Universität,
Hans-Meerwein-Strasse, 35043 Marburg, Germany
E-mail: petz@staff.uni-marburg.de
neumuell@chemie.uni-marburg.de
frenking@staff.uni-marburg.de
[b] Centre for Theoretical Chemistry and Physics, New Zealand
Institute for Advanced Study, Massey University Albany,
Private Bag 102904, North Shore MSC, 0745 Auckland, New
Zealand
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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FULL PAPER
adducts of type B with two Lewis acids E are known in the reactions are typical for Lewis acid adducts of 1, which
cations (H2C{PPh3}2)2+, [Ag{HC(PPh3)2}2]3+ [11]
,
(HMeC- makes spectroscopic characterization somewhat problem-
atic and crystals for X-ray analyses difficult to obtain. Thus,
if the precipitate is dissolved in dry DMSO gas evolution is
observed and a main signal at δ = 21 ppm is found in the
31P NMR spectrum. In order to understand these solvent
reactions, we treated the precipitate with dry thf at room
temperature. After gas evolution ceased, a colorless insolu-
ble microcrystalline material had formed. The 31P NMR
spectrum of the solution showed three signals at δ = 39.4,
21.1, and 20.1 ppm in a 1:6:4 ratio. After allowing the mix-
ture to stand for a couple of days, small colorless crystals
were found to have grown along with larger yellow crystals.
The colorless crystals were identified as the salt
{PPh3}2)2+[19] and in [Cl2Au2{µ-C(PPh3)2}].[20]
(HC{PPh3}2)[B3H8] (3·THF), whereas the yellow crystals
were found to be 2·THF. Both compounds are only slightly
soluble in thf. The 31P NMR spectrum of the solute finally
showed peaks at δ = 39.1 and 21.1 ppm in a 1:2.7 ratio. The
crystals of 2·THF are slightly soluble in DMSO, without
gas evolution, and the 31P NMR spectrum exhibits a singlet
at δ = 21.00 ppm. To avoid reaction with a potential proton
donor such as thf we treated the initial precipitate with dry
dimethoxyethane (DME). A suspension was obtained, al-
though in this case with little or no gas evolution. Small
colorless crystals, which turned out to be the salt-like com-
pound [{(µ-H)H4B2}{C(PPh3)2}][B2H7] (4), as shown by X-
ray analysis, formed within several hours. The supernatant
DME solution showed two signals in the 31P NMR spec-
trum at δ = 39.4 and 21.2 ppm. Additionally to 4 a few
other crystals could be isolated from DME, which turned
out to be the salt (HC{PPh3}2)[BH4] (5). The proton in 5
probably comes from internal redox processes during the
formation of B–B bonds and H2.
The unusual behavior of the initial precipitate indicates
that 3 and 4 are not formed in the first reaction step but are
solvent-induced consecutive products. We therefore suppose
that the precipitate from 1 and B2H6 in toluene consists of
a mixture of 2 and the bis-adduct [(H3B)2{C(PPh3)2}] (6).
The latter product is considered to be highly reactive and
responsible for the gas evolution in protic solvents. Quan-
tum chemical calculations suggest that both the mono-ad-
duct 2 and the bis-adduct 6 are stable at ambient tempera-
ture as the BH3 group is small enough to allow two ligands
to be attached to the two lone electron pairs of 1 with for-
Scheme 1. Schematic representation of the bonding situation in car-
bodiphosphoranes and the change after complexation with (a) one
and (b) two Lewis acids.
To date, no examples of a complex of type B where 1
binds two main-group Lewis acids other than a proton or
proton/methyl are known. Somewhat surprisingly, no com-
plex of type A with a boron-containing Lewis acid BX3 has
been isolated and structurally characterised either. The
complex with the parent Lewis acid BH3 is of particular
interest as adducts of BH3 with carbon-based Lewis bases
are rare and crystal structures of carbon donors have only
been published for an N-heterocyclic carbene[21] and an
ylide,[22] although microwave structures have been reported
for H3BCO[23] and H3BCNMe.[23] The latter species have
also been studied by quantum chemical methods.[24] In
1981, for example, Schmidbaur and co-workers reported IR
and NMR spectroscopic data for a BH3 adduct with a cy-
clic carbodiphosphorane, although no structural data were
reported.[25] Herein we report the first synthesis of a com-
plex of 1 with the parent borane ligand BH3 to be charac-
terized by X-ray structure analysis. We also present experi-
mental evidence for the synthesis of a complex where 1
binds two boron Lewis acids in the unusual adduct [{(µ-H)-
H4B2}{C(PPh3)2}][B2H7] (4). The results of quantum chem-
ical calculations on these complexes and a bonding analysis
of the compounds are also given.
2. Results
When a toluene solution of 1 was brought into contact mation of a type B compound.[12b] The isolation of 4 may
with a bulb containing freshly prepared gaseous B2H6, col- be a hint that the predicted adduct 6, which subsequently
orless crystals formed at the surface of the solution after undergoes the observed reaction, is the main product in tol-
several days. These were subsequently identified as the ad- uene. This is supported by the fact that only the initial tolu-
dition compound [H3B{C(PPh3)2}] (2). For preparative ene precipitate gives gas evolution upon dissolution in
purposes, B2H6 was bubbled, with stirring, into a solution DMSO; the products 2, 3, and 4 obtained from thf or DME
of 1 in toluene to yield a yellowish precipitate; the reaction only dissolve. However, to date we have been unable to con-
occurs immediately, even at low temperature. However, the firm or to exclude the presence of 6. The salt-like adduct 4
result of this reaction is more complicated than a simple is the first compound of type B in which 1 coordinates to
addition to give 2 directly. The resulting precipitate is pre- main-group Lewis acids in a bridging manner. This adduct
sumably a mixture of compounds and is insoluble in tolu- can be considered to be derived from 6 by abstraction of a
ene and other non-polar solvents. Further reaction occurs hydride ion, as shown in Scheme 2. We should point out
in polar solvents such as CH2Cl2, acetonitrile, and thf or here that the fragment {(µ-H)H4B2}+ is known from phos-
related solvents, with gas evolution in most cases. Solvent phane complexes.[26]
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Carbodiphosphorane as a Lewis Base with Boranes
the Td symmetry of the [BH4]– anion is disrupted by the
H···H contact to the cation. Additionally, a medium-strong
band at 1078 cm–1 can be attributed to the BH2 defor-
mation vibration.[30]
Various 31P NMR studies allowed us to assign the signals
at δ = 39.4, 20.1, and 21.1 ppm from measurements in thf
and DME to compounds 4, 3, and 2, respectively.
Scheme 2. Suggested reaction course for the formation of 4 from
carbodiphosphorane and bis-borane.
The cation of 4 can also be seen as a formal derivative
of B2H6 in which a bridging hydride species is replaced by
the two-electron-pair donor 1 to yield a 14-electron B2H5C+
fragment. A similar bonding situation is present in the un-
stable compounds B2H5X (X = halogen)[27] and in related
fragments with X = P[26] or Fe.[28] The postulated bis-ad-
duct 6 may abstract a proton from thf to finally produce
the salt 3, and dismutation into 2 and 4, with hydride trans-
fer to B2H6, may occur in DME.
Crystal Structures
X-ray analyses of 2–5 were performed to gain more in-
sight into the structure of the complexes and their bonding
situation. Colorless crystals of the solvent-free addition
compound 2 were obtained by allowing a solution of 1 in
toluene to stand under an atmosphere of B2H6 for several
days. Yellow crystals of 2·THF and small colorless crystals
of 3·THF were obtained from thf as described above. Very
small crystals of 4 and needles of 5·DME formed upon ad-
dition of DME to the crude product obtained from 1 and
B2H6 in toluene. Details of the structure determination are
summarized in the Experimental Section; bond lengths and
angles are given in Tables 1, 2, 3, and 4. The molecular
structures of 2 and 4 are depicted in Figures 1 and 2, respec-
tively, whereas the structures of 3 and 5, which contain
known components, are not shown and will not be dis-
cussed further.
The IR spectrum of 2 in Nujol mull shows medium
strong absorptions at 2171, 2207, and 2257 cm–1 which are
indicative for the B–H vibration of an end-on BH3 group.
In principle two stretching vibrations, with E and A1 sym-
metry, are expected for νas(BH) and νs(BH), respectively.
The three bands observed may arise from a symmetry re-
duction which causes the E vibration to split into two
bands. Two strong bands at 1130 and 1143 cm–1 belong to
BH2 deformation vibrations.[29] The spectrum of 4 is more
complicated and shows medium to strong bands for BH
stretching vibrations between 2600 and 2210 cm–1; BH2 de-
formation vibrations appear at 1186 and 1175 cm–1 but can-
not be assigned with certainty. The IR spectra of 3 and 5
are governed by the skeleton vibrations of the (HC-
{PPh3}2)+ cation. The vibrational spectrum of 3 exhibits a
broad and unresolved intense band at 2388 cm–1 and a less
intense broad band at 2074 cm–1, which can be assigned to
the B–H stretching vibrations of the anion. Further me-
dium intensity bands at 1059 and 1030 cm–1, which do not
arise from vibrations of the cation, are due to BH2 defor-
mation modes. The IR spectrum of 5 shows three strong
and broad bands in the region of B–H stretching fre-
quencies at 2280, 2214, and 2137 cm–1, thus indicating that
Table 2. Selected bond lengths [Å] and angles [°] in 3·THF.
P(1)–C(1)
P(1)–C(8)
P(2)–C(1)
P(2)–C(26)
C(1)–H(1)
B(1)–B(2)
1.700(6)
1.797(6)
1.714(5)
1.797(6)
0.95
P(1)–C(2)
P(1)–C(14)
P(2)–C(20)
P(2)–C(32)
B(1)–B(3)
B(2)–B(3)
C(1)–P(1)–C(8)
C(1)–P(1)–C(14)
C(2)–P(1)–C(14)
P(1)–C(1)–H(1)
P(2)–C(1)–H(1)
B(3)–B(2)–B(1)
1.810(6)
1.810(6)
1.817(6)
1.803(6)
1.78(2)
1.76(2)
114.6(3)
113.8(3)
106.8(3)
115.4
1.79(2)
C(1)–P(1)–C(2) 108.9(3)
C(8)–P(1)–C(2) 103.3(2)
C(8)–P(1)–C(14) 108.6(3)
P(1)–C(1)–P(2)
129.2(3)
B(3)–B(1)–B(2) 59.1(6)
B(2)–B(3)–B(1) 60.7(7)
115.4(2)
60.2(7)
Table 1. Selected bond lengths [Å] and angles [°] in 2/2·THF. Theoretical values at the BP86/SVP level of theory are given in italics.
P(1)–C(1)
P(1)–C(8)
P(2)–C(1)
P(2)–C(26)
C(1)–B(1)
B(1)–H(2)
C(1)–P(1)–C(2)
C(1)–P(1)–C(14)
C(2)–P(1)–C(14)
C(1)–P(2)–C(20)
C(1)–P(2)–C(32)
P(1)–C(1)–P(2)
P(2)–C(1)–B(1)
C(1)–B(1)–H(2)
H(1)–B(1)–H(2)
H(2)–B(1)–H(3)
1.681(3)/1.687(5)
1.832(3)/1.821(4)
1.689(3)/1.696(5)
1.838(3)/1.811(4)
1.673(4)/1.695(8)
1.21(4)/1.23(5)
112.9(1)/113.1(2)
116.7(1)/115.3(2)
104.3(1)/102.3(2)
108.4(1)/113.4(2)
118.3(1)/117.8(2)
130.5(2)/125.1(3)
111.2(2)/114.1(4)
108(2)/115(2)
1.711
1.867
1.712
1.863
1.689
1.246
110.9
116.9
103.8
109.1
117.8
128.2
118.7
111.5
109.2
109.9
P(1)–C(2)
P(1)–C(14)
P(2)–C(20)
P(2)–C(32)
B(1)–H(1)
B(1)–H(3)
C(1)–P(1)–C(8)
C(2)–P(1)–C(8)
C(8)–P(1)–C(14)
C(1)–P(2)–C(26)
1.825(2)/1.827(5)
1.803(3)/1.820(5)
1.818(3)/1.794(6)
1.806(3)/1.821(5)
1.12(3)/1.18(5)
1.856
1.853
1.848
1.858
1.243
1.239
117.1
104.8
101.9
116.7
104.7
111.6
107.1
111.0
107.9
1.14(3)/1.10(7)
113.2(1)/117.0(2)
105.7(1)/102.1(2)
102.9(1)/105.2(2)
116.2(1)/112.5(2)
C(20)–P(2)–C(26) 107.3(1)/105.7(2)
P(1)–C(1)–B(1)
C(1)–B(1)–H(1)
C(1)–B(1)–H(3)
H(1)–B(1)–H(3)
117.3(2)/120.1(3)
113(1)/107(2)
114(3)/106(3)
107(2)/109(4)
102(2)/113(3)
112(2)/107(4)
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Table 3. Selected bond lengths [Å] and angles [°] in the cation of 4. Theoretical values for 4+ at the BP86/SVP level of theory are given
in italics.
B(1)–C(1)
B(1)–H(1)
B(1)–H(3)
B(2)–H(1)
B(2)–H(5)
C(1)–P(2)
P(1)–C(2)
P(2)–C(26)
1.67(2)
1.30(18)
1.19(16)
1.40(20)
1.30(29)
1.80(1)
1.829(3)
1.826(5)
1.842(8)
56(1)
49(9)
132(8)
134(8)
91(10)
56(1)
44(8)
119(10)
117(8)
116(10)
68(2)
112(1)
115(1)
114.3(6)
111.9(5)
111.9(5)
1.644
1.348
1.208
1.347
1.216
1.829
1.842
1.842
1.835
55.3
B(1)–B(2)
B(1)–H(2)
B(2)–C(1)
B(2)–H(4)
C(1)–P(1)
P(1)–C(8)
P(1)–C(14)
P(2)–C(20)
1.86(4)
1.24(18)
1.66(3)
0.80(20)
1.79(1)
1.829(1)
1.843(4)
1.830(1)
1.877
1.214
1.648
1.215
1.822
1.832
1.843
1.845
P(2)–C(32)
C(1)–B(1)–B(2)
B(2)–B(1)–H(1)
B(2)–B(1)–H(2)
C(1)–B(1)–H(3)
H(1)–B(1)–H(3)
C(1)–B(2)–B(1)
B(1)–B(2)–H(1)
B(1)–B(2)–H(4)
C(1)–B(2)–H(5)
H(1)–B(2)–H(5)
B(2)–C(1)–B(1)
B(1)–C(1)–P(1)
B(1)–C(1)–P(2)
C(1)–P(1)–C(8)
C(1)–P(1)–C(14)
C(1)–P(2)–C(20)
C(1)–B(1)–H(1)
C(1)–B(1)–H(2)
H(1)–B(1)–H(2)
B(2)–B(1)–H(3)
H(2)–B(1)–H(3)
C(1)–B(2)–H(1)
C(1)–B(2)–H(4)
H(1)–B(2)–H(4)
B(1)–B(2)–H(5)
H(4)–B(2)–H(5)
B(2)–C(1)–P(1)
B(2)–C(1)–P(2)
P(1)–C(1)–P(2)
C(1)–P(1)–C(2)
C(1)–P(2)–C(26)
C(1)–P(2)–C(32)
105(9)
111(8)
113(10)
121(9)
101(10)
100(8)
120(10)
103(10)
137(9)
101(10)
121(1)
101.0
118.7
102.7
120.8
115.2
100.8
117.3
101.8
114.8
116.7
114.8
110.1
124.3
113.4
113.0
110.6
45.8
118.9
114.3
101.0
55.1
45.9
123.8
114.4
101.8
69.5
107.0
118.6
112.4
112.6
115.4
108(1)
121.1(9)
111.6(5)
110.9(6)
113.2(5)
Table 4. Selected bond lengths [Å] and angles [°] in 5·DME.
C(1)–P(1)
C(1)–H(1)
P(1)–C(2)
P(2)–C(26)
P(2)–C(20)
B(1)–H(3)
1.697(3)
0.85(3)
1.822(2)
1.796(3)
1.816(2)
1.05(3)
C(1)–P(2)
P(1)–C(8)
P(1)–C(14)
P(2)–C(32)
B(1)–H(2)
B(1)–H(4)
1.707(3)
1.802(2)
1.817(2)
1.822(2)
0.94(3)
1.11(3)
B(1)–H(5)
1.02(4)
129.1(2)
111(2)
112.9(1)
110.6(1)
113.0(1)
P(1)–C(1)–P(2)
P(2)–C(1)–H(1)
C(1)–P(1)–C(8)
C(1)–P(2)–C(20)
C(1)–P(1)–C(32)
P(1)–C(1)–H(1) 120(2)
C(1)–P(1)–C(2) 109.9(1)
C(1)–P(1)–C(14) 114.8(1)
C(1)–P(1)–C(26) 113.4(1)
Figure 2. X-ray structure of the cation of 4+: [{(µ-H)H4B2}-
{C(PPh3)2}]+ showing bond lengths [Å] and angles [°]. Values cal-
culated at the BP86/SVP level of theory are given in italics.
Crystal Structure of 2
This compound crystallizes from toluene with no ad-
ditional solvent molecules. The molecular structure is de-
picted in Figure 1. There are no close contacts between the
individual molecules. The environment of the ylidic carbon
atom C(1) is not exactly planar but shows a slight pyrami-
dalization Ϫ the sum of the angles is 359.0(2)°. Theoretical
calculations predict an angle of 358.5°. Thus, the C atom is
located about 0.1 Å out of the plane containing the boron
and the two phosphorus atoms. The B–C distance is
1.673(4) Å, which is about 0.07 Å longer than in the related
Figure 1. X-ray structure of [H3B{C(PPh3)2}] (2) showing bond
lengths [Å] and angles [°]. Values calculated at the BP86/SVP level
of theory are given in italics.
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Carbodiphosphorane as a Lewis Base with Boranes
NHC adduct;[21] in both cases the boron atoms are bonded tion metal complex [µ-[CpFe(CO)2]B2H5].[28] The B–B dis-
to sp2-carbon atoms. The P–C bond lengths, which are very tances in these compounds are 2.00 and 1.77 Å, respec-
short in the starting compound 1 (1.61 Å) and correspond tively; that of 4 lies in between these values.
to a partial double bond, have a mean value of 1.685(3) Å,
Crystal Structure of 5·DME
which is in the range of other [X3E{C(PPh3)2}] com-
pounds.[16] The P–C–P angle of 130.5(2)° is close to that in
the (HC{PPh3}2)+ cations of 3 and 5.
The salt-like compound 5·DME can be considered as be-
ing derived from 2 upon hydrogenation and splitting of the
incoming H2 into H+ and H– to form weak H···H bridges
with a C(1)–B(1) distance of 4.082(4) Å. The bridge be-
tween the proton of the cation and a [BH4]– anion demon-
strates the weakly acidic character of this proton, which
agrees with the strongly basic character of 1. The molecular
structure of the cation in 5 is very similar to that of the
cation in 3 as no clear contacts exist to the solvent molecule.
The B–H bond lengths vary between 0.98(3) and 1.11(3) Å.
The other parameters of the (HC{PPh3}2)+ cation are nor-
mal and similar to those found for 3 and other salts with
this cation.[16]
Crystal Structure of 2·THF
The thf molecule in the unit cell of 2·THF is disordered
over two positions, both of which could be refined. It has
no close contacts to 2 and does not change the bonding
parameters of the adduct shown in Figure 1 markedly. The
slight pyramidalization at C(1) found in 2 is confirmed,
with the sum of the angles at C(1) being 359.3°; the carbon
atom lies 0.09 Å above the P2B plane. The C(1)–B(1) bond
length of 1.695(8) Å is even longer, whereas the P(1)–C(1)–
P(2) angle is more acute by about 5°. In light of the higher
measuring temperature (193 vs. 100 K for solvent-free 2),
the thermal ellipsoids of the atoms are larger.
3. Theoretical Studies
We calculated the mono- and bis-borane complexes
[H3B{C(PPh3)2}] (2) and [(H3B)2{C(PPh3)2}] (6) and the
cation of the hydrogen-bridged complex [{(µ-H)H4B2}-
{C(PPh3)2}]+ (4+) using DFT and ab initio methods. The
calculated structures of 2 and 4+ closely match the results
for the X-ray structure analysis (see Figures 1 and 2).
Tables 1 and 3 give experimental and calculated values for
other bond lengths and angles, all of which are in good
agreement. The optimized structure of the experimentally
unknown bis-borane complex 6 is shown in Figure 3.
Crystal Structure of 3·THF
The cation of the salt-like compound 3 exhibits no un-
usual bonding parameters.[16] The thf molecule forms an
O···H contact to the proton of the ylidic carbon atom with
a C(1)···O(1) distance of 3.38(1) Å. No further contacts ex-
ist between the cation and the [B3H8]– anion. Although nu-
merous compounds with the [B3H8]– anion have been de-
scribed,[31] only the crystal structure of the salt
[BzNMe3][B3H8] is known;[32] the B–B distances in both
compounds range between 1.76 and 1.80 Å.
Crystal Structure of 4
The unit cell of 4 contains two independent cations, the
parameters of which differ only slightly. The molecular
structure of one of the cations is depicted in Figure 2. The
B–C distances are similar to those of 2 and 2·THF. The P–
C distances increase by about 0.12 Å on going from 2 to 4
and approach a normal single bond between an sp3-carbon
atom and a phosphorus atom. The carbon atom C(1) is in
a distorted tetrahedral environment with a narrow B–C–B
angle of 68(2)°, while the P–C–P angle is 121.1(8)°, a value
which is similar to that in the dication (H2C{PPh3}2)2+[33]
or the trication [{(Ph3P)2CH}ǞAgǟ{HC(PPh3)2}]3+ [11]
,
but about 10° smaller than in the monocation (HC-
{PPh3}2)+ and most of the type-A addition compounds
such as 2 or related compounds.[16] When 1 bridges two
Au atoms, as in [Cl2Au2{µ-C(PPh3)2}], this angle is slightly
narrower than in 4.[20] The B(1)–B(2) distance of 1.86(4) Å
Figure 3. Optimized geometry of [(H3B)2{C(PPh3)2}] (6) at the
BP86/SVP level of theory showing calculated bond lengths [Å] and
angles [°].
The data in Figures 1 and 2 indicate that the C–B bonds
can be considered to be a nonbonding interaction; the car- in cation 4+ are much shorter (1.644 Å) than in the neutral
bon atom provides four electrons to stabilize the B2H5+ cat- bis-borane complex 6 (1.730 Å). This is reasonable because
ion. Similar compounds in which a hydride anion in B2H6 the positively charged B2H5+ ligand in 4+ should be a much
has been replaced by a four-electron donor are rare and stronger Lewis acid than neutral (BH3)2 in 6. It is interest-
include the anion (µ-{Ar(BH3)P}B2H5)–[26] and the transi- ing to compare the calculated bond strength of the borane
Eur. J. Inorg. Chem. 2009, 4507–4517
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4511

W. Petz, F. Öxler, B. Neumüller, R. Tonner, G. Frenking
Table 5. Calculated bond dissociation energies (BDEs) for the borane complexes 2, 4+, and 6 (in kcalmol–1).
FULL PAPER
Molecule
Ligand
BP86/TZVPP//BP86/SVP MP2/TZVPP//BP86/SVP
SCS-MP2/TZVPP//BP86/SVP
[H3B-C(PPh3)2] (2)
BH3
BH3
(BH3)2
B2H5
32.9
23.0
55.9
44.2
33.2
77.4
38.7
27.0
65.7
[(H3B)2-C(PPh3)2] (6)
[(H3B)2C(PPh3)2] (6)
+
[{(µ-H)H4B2}{C(PPh3)2}]+ (4+)
127.0
149.1
144.6
ligands in 2, 4, and 6. The theoretically predicted bond
dissociation energy (BDE) of the BH3 ligand in 2 at the
BP86/TZVPP//BP86/SVP level of theory is 32.9 kcalmol–1,
+
whereas the BDE of the B2H5 ligand in 4+ is
127.0 kcalmol–1 (Table 5). The calculated BDE of the sec-
ond BH3 ligand in 6 is still rather high at 23.0 kcalmol–1,
which gives a total bond energy for the borane ligands of
55.9 kcalmol–1. The MP2 method gives higher BDEs, al-
though they exhibit the same trend for all compounds inves-
tigated. The SCS correction, which is known to yield much
more accurate bond energies,[34] brings the MP2 values of
the BDEs closer to the BP86 results. All calculations indi-
+
cate that the B2H5 ligand is very strongly bonded to the
carbodiphosphorane.
Table 6 gives the atomic partial charges in 2, 4+, and 6 in
comparison with the parent carbodiphosphorane C(PPh3)2.
The very large negative partial charge at the central carbon
atom in C(PPh3)2 decreases slightly with increasing number
of bonded boron atoms. However, there is still a negative
charge of more than one electron at the central atom in 6
and even in the cation 4+. This observation can be under-
stood by considering the bonding situation of the divalent
carbon(0) compound in carbodiphosphorane and its com-
plexes.[12b] The charge flow from the divalent C0 atom to
the Lewis acid is partly compensated by an increased charge
flow from the phosphane ligands to carbon. The NBO
method gives an optimized Lewis structure with a 2e,3c
bond in the B1–H1–B2 moiety of 4+, which has an occupa-
Figure 4. Optimized geometry of [{(µ-H)H4B2}{C(PH3)2}]+ (4+[H])
(a) and [(H3B)2{C(PH3)2}] (6[H]) (b) at the BP86/SVP level of
theory showing calculated bond lengths [Å] and angles [°].
The EDA method gives an in-depth analysis of the do-
tion number of 1.98 e. There are also two single bonds be- nor–acceptor interactions in 4+(H) and 6(H). The numeri-
tween C1 and the boron atoms with occupation numbers cal results are shown in Table 7. The very strong interaction
of 1.87 e as well as two single bonds between C1 and the energy (∆Eint) between the borane ligand and the CDP is
phosphorus atoms with occupation numbers of 1.96 e, largely compensated in the cation 4+(H) by a high prepara-
which are normal values for a tetracoordinate carbon atom. tion energy that results from distortion of the B2H5+ ligand
For a more detailed discussion of the bonding situation from the triply bridged energy minimum structure. How-
it is useful to consider the hydrogen-substituted model com- ever, 4+(H) is much more strongly bonded than 6(H), as
pounds 4+(H) and 6(H). The most important geometrical can be seen above for the real systems. The donor–acceptor
parameters for these two compounds (see Figure 4) indicate interactions in 4+(H) and 6(H) come mainly from the or-
that they are valid models for 4+ and 6, in agreement with bital term ∆Eorb, which is stronger in both these com-
earlier studies on other carbone complexes.[12]
pounds than the electrostatic term ∆Eelstat. The breakdown
Table 6. Atomic partial charges (NBO) at the BP86/TZVPP//BP86/SVP level of theory for 2, 4+, and 6.
Molecule
q(B)
q(C)
q(P)
q[C(PPh3)2]
C(PPh3)2
–
–1.43
–1.30
–1.11
–1.12
1.52
1.61/1.63
1.66
–
[H3B-C(PPh3)2] (2)
[(H3B)2-C(PPh3)2] (6)
[{(µ-H)H4B2}{C(PPh3)2}]+ (4+)
–0.45
–0.43
–0.24/–0.26
0.48
0.89
0.15
1.66
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 4507–4517

Carbodiphosphorane as a Lewis Base with Boranes
of the orbital interactions into contributions which come
We also employed the Atoms-In-Molecules (AIM)
from orbitals with different symmetry reveals interesting in- method to elucidate the bonding situation in 4+(H) and
formation about the donation of the σ and π lone-pair or- 6(H). Figure 6 shows the contour-line diagrams of the La-
bitals of the CDP. The molecules have C2v symmetry, there- placian ٌ2ρ(r) for the two molecules in the BCB plane.
fore donation of the σ lone-pair is associated with the
∆Eσ(a1) term whereas donation of the π lone-pair is associ-
ated with the ∆EπЌ(b1) term. Figure 5 shows the relevant
orbitals in 4+(H), which show up as HOMO-1 and HOMO-
4. The rather small contributions from the ∆Eδ(a2) and
∆Eπʈ(b2) terms (Table 7) result from the donation of lower-
lying occupied orbitals of C(PH3)2.
Table 7. EDA (BP86/TZ2P) results for complexes 4+(H) and 6(H).
The interacting fragments are the closed-shell Lewis base C(PH3)2
+
and the Lewis acids (LA) B2H5 and (BH3)2, respectively. Both
compounds have been optimized with C2v symmetry constraints.
Figure 6. Contour-line diagrams [ٌ2ρ(r)] for complexes 4+(H) (left)
and 6(H) (right) in the BCB plane (BP86/TZVPP//BP86/SVP). So-
lid lines indicate areas of charge concentration [ٌ2ρ(r) Ͻ 0] whereas
dashed lines show areas of charge depletion [ٌ2ρ(r) Ͼ 0]. The thick
solid lines connecting the atomic nuclei are the bond paths. The
thick solid lines separating the atomic basins indicate the zero-flux
surfaces crossing the molecular plane.
Energy values are given in kcalmol–1. The P–C–P plane is defined
as in-plane (πЌ) while the B–C–B plane is defined as out-of-plane
(πʈ).
4+(H) Ǟ B2H5+ + C(PH3)2 6(H) Ǟ 2 BH3 + C(PH3)2
∆Eint
∆EPauli
∆Eelstat
–222.4
293.1
–209.8
–305.7
–192.5
–4.5
–94.8
–14.0
111.0
97.9
–121.7
284.5
–185.4
–220.9
–94.6
–8.1
–110.0
–8.3
46.4
[a]
(40.7%)
(59.3%)
(63.0%)
(1.5%)
(31.0%)
(4.6%)
(45.6%)
(54.4%)
(42.8%)
(3.7%)
(49.8%)
(3.7%)
[a]
∆Eorb
∆Eσ(a1)[b]
The plots for both molecules show a region of charge
concentration [ٌ2ρ(r) Ͻ 0, solid lines] at the carbon atom
which points towards the electron-deficient boron atoms.
This is in agreement with the assignment of two donor or-
bitals at carbon. There are bond critical points and associ-
ated bond paths between carbon and both boron atoms in
4+(H) and 6(H). In the former complex, bond paths are
found between the boron atoms and the bridging hydrogen
atom, although there is no B–B bond critical point. Instead,
a ring critical point is found for the CB2H moiety of 4+.
In summary, the theoretical results clearly show that 4+
is the first main-group adduct of a carbodiphosphorane
with two Lewis acids larger than protons where both lone-
pairs of the CDP donor are engaged in donor–acceptor
bonds. Furthermore, the bonding analysis shows that there
are two carbon–boron bonds rather than a 2e,3c bond in
4+.
∆Eδ(a2)[b]
∆EπЌ(b1)[b]
∆Eπʈ(b2)[b]
∆Eprep
∆Eprep (LA)
∆Eprep [C(PH3)2]
∆E (= –De)
38.1
8.2
–75.4
13.1
–111.4
[a] The percentage values in parentheses give the contribution to
the total attractive interactions ∆Eelstat + ∆Eorb. [b] The percentage
values in parentheses give the contribution to the total orbital inter-
actions ∆Eorb
.
4. Summary
Figure 5. Molecular orbitals of 4+(H) at the BP86/SVP level of
The results of this work can be summarized as follows.
The donor–acceptor complex [(H3B){C(PPh3)2}] (2) has
been synthesized by treating B2H6 with C(PPh3)2 in toluene
and its geometry determined by X-ray structure analysis.
theory showing the bonding interaction between C(PH3)2 and
+
B2H5
.
HOMO-1 (left, –0.422 eV) and HOMO-4 (right,
–0.502 eV).
The EDA results in Table 7 suggest that donation of the Treatment of the insoluble precipitate from this reaction
π lone-pair of the CDP moiety in 4+(H), which contributes with DME yields the complex [{(µ-H)H4B2}{C(PPh3)2}]-
31.0% to ∆Eint, is only half as strong as the donation of the [B2H7] (4), which was also isolated and structurally charac-
σ lone-pair, which contributes 63.0%. The π contribution terizeds. Compound 4 is the first complex of a carbodiphos-
becomes much more important in the neutral complex phorane where the carbon donor atom binds to two main-
6(H), however, where the ∆EπЌ(b1) term accounts for group Lewis acids larger than protons through its two-elec-
49.8% of ∆Eint whereas the ∆Eσ(a1) term accounts for only tron lone pairs. It is likely that the reaction takes place after
42.8%. This can be explained by taking into account the initial formation of [(H3B)2{C(PPh3)2}] (6), which sub-
geometry of the complexes Ϫ the H-bridged structure sequently reacts with B2H6, with loss of H–, give 4. Quan-
4+(H) has a very acute B–C–B angle of 70.8°, whereas the tum chemical calculations on 2, 4+, and 6 have shown that
B–C–B angle in 6(H) is 125.6°.
the carbon–boron bonds in these complexes are very strong.
Eur. J. Inorg. Chem. 2009, 4507–4517
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4513

W. Petz, F. Öxler, B. Neumüller, R. Tonner, G. Frenking
FULL PAPER
ture was stirred mechanically for several minutes and was then al-
lowed to stand at room temperature. The 31P NMR spectrum of
the supernatant solution showed three signals at δ = 39.4, 21.1,
and 20.1 ppm in a 1:6:4 ratio, which were attributed to 4+, 2, and
(HC{PPh3}2)+, respectively. Two sorts of crystals, which were sepa-
rated mechanically, grew upon allowing the suspension to stand for
several days. The majority of these crystals were colorless crystals
of the salt-like compound 3·THF and a few were orange crystals
of 2·THF. The crystals of 2·THF are only slightly soluble in DMSO
(no gas evolution was observed) and exhibit a singlet at δ =
Analysis of the bonding situation using the EDA, NBO,
and AIM methods has revealed a typical bonding pattern
between divalent carbon(0) compounds and one or two
Lewis acids. The carbon donor atom of the CDP moiety
remains strongly negatively charged even in the cation 4+.
5. Experimental Section
General: All operations were carried out under argon in dried and
degassed solvents using Schlenk techniques. The solvents were
thoroughly dried and freshly distilled prior to use. IR spectra were
recorded with a Nicolet 510 spectrometer and 31P NMR spectra
with a Bruker AC 200 spectrometer. Elemental analyses were per-
formed by the analytical service of the Fachbereich Chemie der
21.0 ppm in the 31P NMR spectrum. 2·THF: IR (Nujol mull): ν =
˜
2293 (w br), 2257 (m), 2207 (m), 2170 [s; all ν(BH)], 1481 (m), 1435
(s), 1142 (s), 1130 (s), 1103 (s), 1061 (m), 1030 (w), 891 (m), 820
(w), 750 (s), 710 (m), 694 (m), 532 (m), 525 (w), 519 (m), 511 (w),
500 (m) cm–1. 3·THF: IR (Nujol mull) ν = 2386 (s br), 2045 [m br;
˜
Universität Marburg. Compound
1
was prepared from
all ν(BH)], 1478 (m), 1437 (s), 1312 (w), 1232 (m), 1182 (w), 1161
(w), 1138 (w), 1098 (s), 1072 (m), 1059 (m), 1030 (s), 1013 (s), 991
(s), 908 (m), 804 (m), 766 (m), 750 (s), 743 (s), 720 (s), 712 (s), 700
(s), 694 (s), 689 (s), 559 (s), 540 (s), 517 (s) cm–1.
(ClC{PPh3}2)Cl and P(NMe2)3 according to a modified literature
procedure.[35] B2H6 was prepared as described previously.[36]
CCDC-736303 (for 2), -736304 (for 2·THF), -737305 (for 3·THF),
-736306 (for 4), and -736307 (for 5·DME) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif; see also Table 8.
DME: Addition of 2 mL of DME (freshly dried with potassium)
to about 120 mg of the precipitate caused only slight gas evolution
and a suspension was obtained. The supernatant solution showed
signals at δ = 39.6 and 21.5 ppm in a 1:6 ratio, which were assigned
to 4+ and 2, respectively; a very small signal at δ = 20.6 ppm is
probably due to the cation (HC{PPh3}2)+. Small crystals, which
turned out to be the salt-like complex 4, separated upon allowing
the suspension to stand for several days. A DMSO solution of the
crystals of 4 exhibits a singlet at δ = 37.5 ppm; two further signals
with very low intensities at δ = 21.4 and 20.9 ppm were assigned
to 2 and the cation (HC{PPh3}2)+, respectively. 4: IR (Nujol mull):
Reaction of 1 with B2H6: In a Schlenk tube, 0.29 g of 1 (0.53 mmol)
was layered with approx. 5 mL of toluene. Due to the low solubility
of 1 in cold toluene, only a dilute solution formed. Without stirring,
the Schlenk tube was connected to a flask filled with freshly pre-
pared B2H6. Slow diffusion of gaseous borane into the solution
resulted, after about one week, in the formation of colorless, nee-
dle-like crystals of 2 in the upper part of the Schlenk tube in low
yields. The crystals of 2 were separated mechanically from unre-
acted crystals of 1.
ν = 2507 (w), 2456 (m), 2417 (m), 2240 (s), 2211 [s; all ν(BH)], 1481
˜
(w), 1464 (s), 1435 (s), 1186 (m), 1175 (m), 1144 (m), 1099 (s), 1069
(w), 1040 (m), 1026 (m), 999 (w), 986 (w), 872 (w), 762 (w), 748
(m), 739 (w), 714 (s), 693 (s), 534 (s), 525 (s), 507 (s), 498 (m) cm–1.
Further large crystals found in the DME suspension were found to
In a similar procedure, a slow stream of gaseous B2H6 was passed
through a stirred solution of 1.56 g of 1 (2.92 mmol) in 10 mL of
toluene at room temperature for about half an hour. A colorless to
pale-yellow precipitate formed immediately, and the mixture was
stirred mechanically for another hour. The precipitate was then fil-
tered off and dried in vacuo to give a microcrystalline powder.
Analysis for 2. C37H33BP2 (550.38): calcd. C 80.74, H 6.04. For 6:
C37H36B2P2 (564.21): calcd. C 78.76, H 6.43; found C 76.73, H
be the salt 5·DME. IR (Nujol mull): ν = 2280 (s), 2213 (s), 2135
˜
[m, ν(BH4)], 1478 (m), 1435 (s), 1366 (w), 1236 (m), 1190 (m), 1101
(s), 1076 (m), 1032 (m), 1013 (m), 991 (s), 855 (m), 808 (m), 770
(m), 762 (m), 745 (s), 716 (s), 694 (s), 559 (s), 544 (s), 519 (m),
494 cm–1 (m). The bands at 1032, 1013, and 991 cm–1 are typical
for the cation (HC{PPh3}2)+; salts with this cation are scarcely sol-
uble in DME.
7.09. IR (Nujol mull): ν = 2683 (w), 2537 (w), 2461 (m), 2392 (s
˜
br), 2211 (m), 2041 (m br), 1588 (w), 1481 (m), 1435 (s), 1312 (w),
1265 (m), 1260 (m), 1186 (w), 1142 (m), 1103 (s), 1061 (m), 1032
(s), 999 (m), 955 (w), 947 (w), 800 (m), 748 (s), 720 (s), 692 (s), 625
(w), 607 (w), 544 (m), 543 (s), 523 (m), 502 (s) cm–1. The IR spec-
trum of the precipitate from toluene does not contain bands be-
longing to the cation (HC{PPh3}2)+, which forms in contact with
polar solvents such as thf or DME. This cation must therefore form
upon treatment of the precipitate with these solvents, as described
below.
DMSO: The toluene precipitate dissolved in DMSO with gas evol-
ution. The 31P NMR spectrum showed a single signal at δ =
20.8 ppm.
Dioxane: The precipitate is only slightly soluble in dioxane and the
supernatant solution exhibited singlets at δ = 39.6, 25.0, and
21.2 ppm in a 2:0.1:2.4 ratio. No crystals separated upon standing
for several days.
Acetone: The precipitate dissolved with gas evolution and the solu-
tion showed a singlet at δ = 19.7 ppm in the 31P NMR spectrum.
The precipitate of this reaction is insoluble in nonpolar hydro-
carbons such as benzene, toluene, or hexane. For further charac-
terization, attempts were made to dissolve the precipitate in polar
solvents such as DCM, thf, DME, DMSO, and others. In all cases
gas evolution was observed and the results upon contacting the
precipitate with various solvents are as follows:
Acetonitrile: The 31P NMR spectrum of the supernatant solution
showed seven signals (relative intensities in brackets) at δ = 19.37
(7.7), 19.87 (2.1), 20.71 (2.8), 21.41 (4.4), 22.25 (7.7), 23.60 (11.8),
24.40 (6.0), and 36.0 (1.0) ppm. After 18 h only the signals at δ =
19.37 (2.0), 19.87 (1.0), and 21.41 ppm (1.8) remainedd.
CH2Cl2: The product dissolved in DCM with gas evolution. The
31P NMR spectrum of the solution exhibits signals at δ = 38.4,
36.5, and 20.3 ppm in a 1:3:6 ratio; the latter signal can be attrib-
uted to the cation (HC{PPh3}2)+.
Theoretical Methods: Geometry optimizations without symmetry
constraints were carried out using the Gaussian03 optimizer[37] to-
gether with TurboMole5[38] energies and gradients at the BP86[39]
/
THF: Addition of 2 mL of dry thf to about 100 mg of the precipi-
def-SVP[40] level of theory. A minimal basis set (benzene BS) was
tate caused gas evolution and a suspension was obtained. The mix-
used for the phenyl rings of the PPh3 groups, except for the α-C
4514
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 4507–4517

Carbodiphosphorane as a Lewis Base with Boranes
Table 8. Crystal data and structure-refinement details.
2
2·THF
3·THF
4
5·DME
Formula
C37H33BP2
550.43
11.942(1)
16.756(2)
15.200(2)
90
101.56(1)
90
0.3ϫ0.22ϫ0.08
2979.8(6)
4
C41H41BOP2
622.54
16.205(2)
10.941(1)
18.669(2)
90
93.05(1)
90
0.49ϫ0.34ϫ0.32
3305.3(6)
4
C41H47B3OP2
650.20
C37H42B4P2
591.89
12.794(2)
34.484(9)
16.256(3)
90
104.65(2)
90
0.12ϫ0.12ϫ0.06
6939(2)
C41H45BO2P2
642.52
9.865(1)
19.727(1)
18.509(1)
90
99.94(1)
90
0.28ϫ0.19ϫ0.06
3547.9(4)
4
Mw [gmol–1]
a [Å]
12.251(2)
12.298(2)
13.797(2)
69.93(2)
69.44(2)
74.47(2)
0.22ϫ0.11ϫ0.06
1802.3(5)
2
b [Å]
c [Å]
α [°]
β [°]
γ [°]
Crystal size [mm]
Volume [pm3 ϫ106]
Z
8
dcalcd. [gcm–3]
Crystal system
Space group
Diffractometer
Radiation
Temperature [K]
µ [cm–1]
1.227
1.251
1.198
1.133
1.203
monoclinic
Cc (Nr. 9)
IPDS II (Stoe)
monoclinic
P21/c (Nr. 14)
IPDS I (Stoe)
triclinic
monoclinic
P21/c (Nr. 14)
IPDS I (Stoe)
monoclinic
P21/c (Nr. 14)
IPDS II (Stoe)
¯
P1 (Nr. 2)
IPDS I (Stoe)
Mo-Kα
193
1.5
52.40
100
1.7
52.08
193
1.64
52.96
193
1.133
52.46
193
1.57
51.84
2θmax [°]
Index range
–14ՅhՅ14
–20ՅkՅ20
–18ՅlՅ18
17180
–19 ՅhՅ19
–13ՅkՅ13
–22ՅlՅ22
31691
–15ՅhՅ15
–15ՅkՅ15
–17ՅlՅ16
17891
–15ՅhՅ15
–42ՅkՅ42
–20ՅlՅ20
59536
–12ՅhՅ11
–24ՅkՅ24
–22ՅlՅ22
25339
Reflections collected
Independent reflections (Rint
Observed reflections
[Fo Ͼ 4σ(Fo)]
)
5780 (0.078)
6313
6611 (0.1045)
13431 (0.8377)
6581 (0.0935)
4218
374
2499
384
2947
457
886
215
3903
438
Parameters
Absorption correction
Structure solution
numerical
direct methods
SIR-92[58]
SHELXL-97[59]
H2 to H9 of [B3H8]–
were refined free[a]
–
direct methods
SHELXS-97[57]
direct methods
SIR-92[58]
direct methods
SIR-92[58]
direct methods
SIR-92[58]
Refinement against F2
H atoms
H1, H2, H3
H1, H2, H3
–
H1 to H5
were refined free[a] were refined free[a]
were refined free[a]
Flack parameter
R1
wR2 (all data)
Max. residual electron density
[10–6 epm–3]
0.09(8)
0.0376
0.0767
–
–
–
0.0825
0.2224
0.0827
0.2424
0.0663
0.2562
0.0451
0.0951
0.18
0.57
1.03
0.165
0.262
[a] Calculated positions with common displacement parameter.
atom. Stationary points were characterized as minima by calculat-
ing the Hessian matrix analytically at this level of theory.[41] Kohn–
Sham orbitals were taken from these calculations. Single-point en-
ergies for the BP86/def-SVP (hereinafter SVP) optimized geome-
tries were calculated with the MP2 method[42] applying the frozen-
core approximation for non-valence shell electrons and with BP86
using the def2-TZVPP[43] basis set (hereinafter TZVPP). The reso-
lution-of-identity method was applied for the BP86 and MP2 calcu-
lations.[44] MP2 energies were also calculated including the spin-
component-scaled (SCS) correction proposed by Grimme,[45] using
the standard parameters. NBO[46] analyses were carried out with
the internal module of Gaussian03 at the BP86/TZVPP level of
theory. The electronic charge distribution was also analyzed using
the Atoms-In-Molecules (AIM) method,[47] which was performed
at the BP86/TZVPP level of theory with a locally modified version
of the AIMPAC program package.[48]
frozen-core approximation. This level of theory is denoted BP86/
TZ2P. An auxiliary set of s, p, d, f, and g STOs was used to fit the
molecular densities and to represent the Coulomb and exchange
potentials accurately in each SCF cycle.[51] Scalar relativistic effects
were incorporated by applying the zeroth-order regular approxi-
mation (ZORA) in all ADF calculations.[52]
The interatomic interactions were investigated by means of an en-
ergy decomposition analysis (EDA) developed independently by
Morokuma[53] and by Ziegler and Rauk.[54] This bonding analysis
focuses on the instantaneous interaction energy, ∆Eint, of a bond
A-B between two fragments A and B in the particular electronic
reference state and in the frozen geometry of AB. This interaction
energy is divided into three main components; see Equation (1).
∆Eint = ∆Eelstat + ∆EPauli + ∆Eorb
(1)
For the bonding analysis, some molecules were optimized with a
C2v symmetry constraint using the ADF2006.01 program pack-
age.[49] As above, BP86 was chosen applying uncontracted Slater-
type orbitals (STOs) as basis functions.[50] The latter basis sets for
all elements have triple-ζ quality augmented by two sets of polar-
ization functions (ADF-basis set TZ2P). Core electrons (i.e., 1s for
second- and [He]2s2p for third-period atoms) were treated by the
The term ∆Eelstat corresponds to the quasi-classical electrostatic in-
teraction between the unperturbed charge distributions of the pre-
pared atoms and is usually attractive. The Pauli repulsion, ∆EPauli
,
is the energy change associated with the transformation from the
superposition of the unperturbed electron densities of the isolated
fragments to the wave function, which properly obeys the Pauli
principle through explicit antisymmetrization and renormalization
Eur. J. Inorg. Chem. 2009, 4507–4517
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4515

W. Petz, F. Öxler, B. Neumüller, R. Tonner, G. Frenking
FULL PAPER
(N = constant) of the product wave function.[49a] ∆EPauli comprises
the destabilizing interactions between electrons of the same spin on
either fragment. The orbital interaction, ∆Eorb, accounts for charge
transfer and polarization effects.[55] The ∆Eorb term can be decom-
posed into contributions from each irreducible representation of
the point group of the interacting system. The molecules investi-
gated show C2v symmetry, which makes it possible to distinguish
between σ-contributions (a1) and π-contributions arising from in-
plane (b2) πʈ orbitals and out-of-plane (b1) πЌ orbitals. The energy
contributions from orbitals which possess δ symmetry (a2) are neg-
ligible for the investigated molecules; see Equation (2).
[12] a) R. Tonner, G. Frenking, Chem. Eur. J. 2008, 14, 3260; b) R.
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1999, 18, 619; b) J. Sundermeyer, K. Weber, K. Peters, H. G.
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∆Eorb (C2v) = ∆Eσ(a1) + ∆Eδ(a2) + ∆EπЌ(b1) + ∆Eπʈ(b2)
(2)
To obtain the bond dissociation energy (BDE; by definition with
the opposite sign to ∆E), the preparation energy, ∆Eprep, which
gives the relaxation of the fragments into their electronic and geo-
metric ground states, must be added to ∆Eint; see Equation (3).
∆E (= –BDE) = ∆Eint + ∆Eprep
(3)
[19] W. Petz, B. Neumüller, unpublished results.
Further details regarding the EDA method and its application to
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Received: July 1, 2009
Published Online: September 16, 2009
Eur. J. Inorg. Chem. 2009, 4507–4517
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4517