Synthesis and characterization of the silylated hexaphenyl carbodiphosphorane [Me3SiC(PPh3)2][CF 3SO3]

Article abstract of DOI:10.1002/zaac.201300545

The addition of trimethylsilyl trifluoromethanesulfonate TMS-OTf (CF ₃SO₃ = OTf, triflate) to hexaphenyl carbodiphosphorane PPh₃=C=PPh₃ (1) in toluene yields the silylated carbodiphosphorane [Me₃SiC(PPh₃)₂][OTf] (2). Compound 2 represents the first silylated carbodiphosphorane characterized in solution and in the solid state. 2 is an air-sensitive compound but stable in solution and in the solid state in an inert atmosphere as shown by heteronuclear NMR experiments and also by X-ray diffraction analysis. Compound 2 crystallizes in the monoclinic space group P2₁/n with the cell dimensions a = 1161.7(1), b = 1714.4(1), c = 1903.3(1) pm; β = 102.74(1)° and Z = 4. Structure, frontier orbitals, and dissociation energies for 2 were determined by density functional theory-based computations highlighting the character of 2 as a Lewis acid adduct of a carbon(0) compound. Copyright

Full text of DOI:10.1002/zaac.201300545

ARTICLE
DOI: 10.1002/zaac.201300545
Synthesis and Characterization of the Silylated Hexaphenyl
Carbodiphosphorane [Me3SiC(PPh3)2][CF3SO3]
Istemi Kuzu,*[a] Nis-Julian H. Kneusels,[a] Mona Bauer,[a] Bernhard Neumüller,*[a] and
Ralf Tonner*[a]
Keywords: Carbodiphosphoranes; NMR spectroscopy; Silicon; Density functional calculations
Abstract. The addition of trimethylsilyl trifluoromethanesulfonate
ments and also by X-ray diffraction analysis. Compound 2 crystallizes
TMS-OTf (CF3SO3 = OTf, triflate) to hexaphenyl carbodiphosphorane in the monoclinic space group P21/n with the cell dimensions a =
PPh3=C=PPh3 (1) in toluene yields the silylated carbodiphosphorane 1161.7(1), b = 1714.4(1), c = 1903.3(1) pm; β = 102.74(1)° and
[Me3SiC(PPh3)2][OTf] (2). Compound 2 represents the first silylated Z = 4. Structure, frontier orbitals, and dissociation energies for 2 were
carbodiphosphorane characterized in solution and in the solid state. 2
is an air-sensitive compound but stable in solution and in the solid
state in an inert atmosphere as shown by heteronuclear NMR experi-
determined by density functional theory-based computations high-
lighting the character of 2 as a Lewis acid adduct of a carbon(0) com-
pound.
Introduction
The presentation of the first carbodiphosphorane
Ph3P=C=PPh3 (1) in 1961 by Ramírez et al. opened the door
towards a new class of compounds with intriguing chemical
and physical properties.[1a] The unusual topology of 1 in the
solid state with a bent arrangement of the P–C–P unit[2] and
the strong nucleophilic character of the central carbon
atom[1b,3] were the driving forces for the investigation of the
donor abilities of carbodiphosphoranes.[4,9,10] First, the bond-
ing situation in 1 was described by resonance forms rendering
the bis ylide (B) and the heterocumulene (C) character of this
compound (see Scheme 1).[1] Additionally, Kaska et al. related
to 1 as an “unsaturated carbon complex with a formally zero-
valent central atom stabilized by phosphine groups” (which
refers to resonance form A in Scheme 1) and, in this context,
used the term “bis-phosphine carbon complex”.[4] The concept
of carbones was later introduced by Frenking and Tonner
based on chemical bonding analysis for carbodiphosphoranes
and carbodicarbenes[5a–5d] (see also the discussion in referen-
ces[5e,5f] regarding the ylide and carbone formulation) and ex-
tended further by Frenking et al.[5g] These findings were
Scheme 1. Resonance forms (A, B, and C), and bonding modes of
carbodiphosphorane 1 with one (D) or two Lewis acids (E).
* Dr. I. Kuzu
Fax: +49-6421-2825653
E-Mail: istemi.kuzu@chemie.uni-marburg.de
* Prof. Dr. B. Neumüller
E-Mail: neumuell@chemie.uni-marburg.de
* Dr. R. Tonner
E-Mail: tonner@chemie.uni-marburg.de
[a] Philipps-Universität Marburg
Fachbereich Chemie
substantiated by experimental work by Petz[6] and other
groups.[9,10]
Therefore, the hexaphenyl carbodiphosphorane 1 is best de-
scribed as a divalent carbon(0) atom stabilized by two tri-
phenylphosphine donor ligands. These strong donor-acceptor
interactions stabilize this type of CL2 compounds.[7] Addition-
ally, two lone pairs of electrons are centred at the carbon(0)
atom with σ and π symmetry and do not participate in the
donor-acceptor bonds.[5a,5b,5c,6] Generally, the two lone pairs
Hans-Meerwein-Straße
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Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300545 or from the au-
thor.
Z. Anorg. Allg. Chem. 2014, 640, (2), 417–422
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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I. Kuzu, B. Neumüller, R. Tonner et al.
ARTICLE
3
residing at the carbon(0) atom are capable of donating to one 92.9 Hz (N = |1JCP + JCP|). The remaining para carbon atoms
(D) or two electrophiles (E, see Scheme 1).[8]
show a singlet only.[17] Furthermore, the silicon atom in 2
Several examples of such carbodiphosphorane complexes gives rise to a triplet in the 29Si{1H} NMR spectrum at δSi
and adducts have been presented in the literature with main –1.0 ppm with a two bond coupling to the phosphorus atoms
group elements[9] and transition metal electrophiles.[10]
(2JSiP = 5.4 Hz).[18] The singlet resonance signal in the 19F
We are interested in the silylation of carbodiphosphoranes NMR spectrum at δF –77.9 ppm shows no signs of bonding
and their application as precursors in organometallic synthesis. interactions between the cationic unit and the triflate anion.
1
For example, silylated ylides turned out to be a powerful tool
The precise assignment of the H and 13C{1H} resonance
in generating salt free trialkylphosphonium alkylides.[11] signals were carried out on the basis of 2D NMR experiments
Therefore, we resumed work done on silicon in carbodiphos- (HMQC, HMBC, and NOESY: see Supporting Information).
phorane chemistry. The first compound of this type, In particular, the 3J long range coupling of the carbon(0) atom
[Me3SiC(PMe3)2]+, was described by Schmidbaur and Tronich to the protons of the TMS group yields the anticipated cross
in the course of their studies on substituted ylides.[11a] Subse- peak in the HMBC spectrum of 2+ (Figure S3, Supporting In-
quent publications of the Schmidbaur group on the reaction of formation). Additionally, unambiguous proof for the stability
[12]
[7a]
the carbodiphosphoranes C(PPh2Me)2
and C(PMe3)2
of cation 2+ in solution is given by the NOEs between the
with trimethylsilyl chloride (TMSCl) provided a dichloride protons of the TMS group and the aromatic protons in ortho
salt [(Me3Si)2C(PPh2Me)2]2+ and a transylidation product and meta position (Figure 1).
Me3P=C(H)–P(Me2)=C(H)–SiMe3, respectively. Uedelhoven
The positive ESI mass spectrum shows the molecular peak
et al. obtained [Ph3SiC(PMe3)2]+ from the reaction of a transi- (M)+ at m/z 609.2287, which is assigned to the cation 2+
tion metal substituted ylide with trimethylphosphane.[13] Later (calcd. 609.2296). The infrared and Raman spectra of 2 are
on, Bestmann et al. reported on a reaction of carbodiphosphor- rich in bands and dominated by the skeleton vibrations of the
ane 1 with TMSCl including 1H, 13C{1H}, and 31P{1H} phenyl groups (see Supporting Information). Therefore, the as-
NMR spectroscopic data for the obtained product signment of the bands to the key vibrations is not possible.[19]
[Me3SiC(PPh3)2]+.[14] Recently, Alcarazo et al. obtained But, in the Raman spectrum, a pair of new bands are present
[EtMe2SiC(PPh3)2][HB(C6F5)3] from an Si–H bond activation at 692 cm–1 and 641 cm–1. These can be assigned to the total
reaction of EtMe2SiH with the frustrated lewis pair (FLP) symmetric A1 and twofold degenerate E vibration of the Si–
1/B(C6F5)3.[15] Herein, we present our results regarding the CH3 bonds of the TMS group.[20] In the IR spectrum, the bands
synthesis and characteristics of silylated carbodiphosphorane of the typical vibrations of the free triflate anion[21] are super-
1.
imposed by the bands of the carbodiphosphorane vibrations.
However, two characteristic features are apparent in the IR
spectrum of 2: (i) a medium intense band at 894 cm–1, which
represents the vibration of the newly formed (CH3)3Si–C(0)
bond and (ii) the vibration of the now weaker P–C(0)–P bonds
resulting in a medium intense band at 994 cm–1. These experi-
mental results are in very good agreement with the computed
vibrational frequencies for compound 2 {corresponding com-
puted wavenumbers: ν[(CH3)3Si–C(0)] = 874 cm–1, ν[P–C(0)–
P] = 991 cm–1}.[22]
Results and Discussion
Carbodiphopsphorane 1 can be silylated quantitatively by
reacting it with trimethylsilyl triflate (TMS-OTf) in toluene.
The precipitated ionic compound [Me3SiC(PPh3)2][OTf] (2)
can be recrystallized from CH2Cl2/n-pentane to obtain single
crystals of 2. In solution, compound 2 is stable in an inert
atmosphere. The 31P{1H} NMR spectrum of 2 in CDCl3 shows
only one singlet at δP 26.2 ppm, which is comparable to the
value found by Bestmann et al. (δP 25.3 ppm).[14] Typically,
adducts of 1 and cationic units of the type [EC(PPh3)2]+ fall
Single crystals of 2 suitable for X-ray diffraction studies can
be obtained by layering a CH2Cl2 solution of 2 with n-pentane.
The molecular structure of 2 is shown in Figure 2.[23]
1
The molecular structure of 2 shows discrete cationic and
anionic units. The cation 2+ adopts a planar arrangement
around the central C1 atom (sum of angles at C1: 359.8°),
which points to a sp2 hybridization at C1. The TMS group
shows local C3v symmetry. The CTMS–Si–CTMS bond angles
are below the ideal value for tetrahedrons (103.6–107.7°) and
the C(0)–Si–CTMS bond angles lie above this value (112.1–
114.6°), which is due to steric repulsion between the TMS
group and the phenyl rings. The Si1–C1 bond length of
190.6 pm is slightly longer than for Si–C single bonds but fits
well with the value expected for single bonds of silicon atoms
with sp2 hybridized carbon atoms.[24] The P–C1 bond lengths
(172.9 pm and 173.2 pm) are about 10 pm longer than in 1 and
therefore clearly substantiate the decrease of the P–C1 bond
into this region. In the corresponding H NMR spectrum only
the resonance signals of the phenyl protons (δH 7.41–
7.57 ppm) and the TMS group (δH –0.24 ppm) are present.
However, the resonance signal of the TMS group appears at
even lower frequencies than denoted by Bestmann et al.
(δH +0.1 ppm). Significant differences to the findings of Best-
mann are apparent in the 13C{1H} NMR spectrum of 2: for the
carbon(0) atom the anticipated triplet can be found at even
lower frequency at δC –0.89 ppm with a coupling constant of
1JCP = 73.4 Hz[16] (cf. Bestmann: δC 2.2 ppm, JCP = 119 Hz).
1
The resonance signals of the phenyl groups are located in the
aromatic region with the characteristic virtual coupling pattern
of the corresponding carbon atoms to the two phosphorus
atoms (AAЈX spin system). The carbon atoms in ortho and
meta position appear as virtual triplets, and the carbon atoms order upon reaction of 1 with TMS-OTf. They are similar to
in ipso position give rise to an apparent doublet with N = P–C(0) bond lengths in other mono adducts of 1, e.g. the BeCl2
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Z. Anorg. Allg. Chem. 2014, 417–422

Silylated Hexaphenyl Carbodiphosphorane [Me3SiC(PPh3)2][CF3SO3]
1
Figure 1. Section of H NOESY NMR spectrum of 2 at 300 K in CDCl3; mixing time 300 ms.
adduct.[9a] The P1–C1–P2 bond angle of 119.9° lies in the
range of values observed for isostructural carbodiphosphorane
adducts.[25]
In order to obtain a better insight into the bonding situation
and stability of compound 2+, we performed quantum chemical
calculations and examined the frontier (Kohn-Sham) orbitals
and dissociation energies.
Figure 3 shows the optimized structure of 2+ and the corre-
sponding values for selected bond lengths and angles, which
are in good agreement with the experimental results.
Figure 2. Molecular structure of [Me3SiC(PPh3)2][OTf] (2) with 50%
probability for thermal ellipsoids. Hydrogen atoms are omitted for
clarity. Selected bond lengths /pm and angles /°: C1–P1 172.9(2), C1–
P2 173.2(2), C1–Si1 190.6(2), P1–C8 180.7(2), P1–C2 182.1(2), P1–
C14 182.4(2), P2–C26 181.4, P2–C20 181.8(2), P2–C32 182.3(2),
Si1–C38 187.6(2), Si1–C39 187.3(2), Si1–C40 187.9(2); P1–C1–P2
119.9(1), P1–C1–Si1 122.3(1), P2–C1–Si1 117.6(1), P1–C1–P2 119.9,
P1–C1–P2 119.9, C1–Si1–C38 112.11(9), C1–Si1–C39 111.72(9), C1– Figure 3. Computed (BP86/def2-SVP) structure of 2+ in comparison
Si1–C40 114.61(9), C38–Si1–C39 106.40(9), C38–Si1–C40 107.72(9),
C39–Si1–C40 103.63(9).
to results from X-ray structural analysis (in italics). Bond lengths in
Å, angles in degrees. Hydrogen atoms are omitted for clarity.
Z. Anorg. Allg. Chem. 2014, 417–422
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419

I. Kuzu, B. Neumüller, R. Tonner et al.
ARTICLE
As can be seen in Figure 4, the calculated HOMO of 2+
exhibits mainly lone pair character at the central carbon atom,
characteristic for carbon(0) compounds. This is underlined by
a strong negative partial charge (NBO) at the central carbon
atom (–1.54 e).
Scheme 2. Free energies of dissociation (BP86/def2-TZVPP//BP86/
def2-SVP) for compounds 2+ (a) and hypothetical 32+ (b).
in 4+ is present at δH 1.86 ppm (2JPH = 5.3 Hz). In the corre-
sponding 31P{1H} NMR spectrum a new signal appears at δP
21.6 ppm, which is typical for protonated carbodiphosphorane
[HC(PPh3)2]+.[28]
Regarding this decomposition reaction of 2 in aerobic condi-
tions, we envisaged an additional protonation of 2 in an inert
atmosphere with trifluoromethanesulfonic acid (HOTf). Ad-
dition of one equivalent of HOTf to a solution of 2 in CDCl3
gives rise to a new resonance signal in the 31P{1H} NMR spec-
trum at δP 20.0 ppm. However, the resonance signal of starting
material 2 is still present at δP 26.2 ppm. The corresponding
1H NMR spectrum shows resonance signals of unreacted 2 and
free TMS(OTf). Also, a new triplet is present at δH 5.51 ppm
Figure 4. HOMO (BP86/def2-SVP, –13.4 eV) for 2+. Hydrogen atoms
are omitted for clarity.
Based on these data compound 2+ can best be described with
the resonance form shown on the left in Figure 5, including a
single bond between silicon and the central carbon atom,
which has a remaining lone pair of electrons with p character.
The P–C(0) bonds are weakened and therefore elongated due
to the newly formed Si–C bond.
2
with J(1H, 31P) of 15.9 Hz, which corresponds to the twofold
protonated carbodiphosphorane 1.[29] This observation points
to the formation of an intermediate that is not stable in solution
and reacts immediately with another HOTf molecule to give
the final product [H2C(PPh3)2]2+ (52+) with elimination of the
(SiMe3)+ group (see Scheme 3). Finally, full conversion of 2+
to 52+ is achieved by adding another equivalent of HOTf to
the reaction mixture.
Figure 5. Resonance forms of 2+.
In a further step, we computationally investigated the
strength of the C–Si bond in 2+. As shown in Scheme 2, the
+
free energy of dissociation for the SiMe3 cation is
65.1 kcal·mol–1 and thereby at the upper range for main-group
adducts of 1.[5c,9] Since it is known that 1 can form adducts to
two Lewis acids, we performed calculations for the attachment
+
of a second SiMe3 cation. Although this resulted in a struc-
Scheme 3. Presumed reaction course of protonation of compound 2+
to final decompostion product 52+ via hypothetical intermediate (IM,
not observed); 31P{1H} and 1H NMR spectroscopic data in CDCl3,
OTf anions are omitted for clarity.
ture, which is a minimum on the potential energy surface, the
dication [(Me3Si)2C(PPh3)2]2+ (32+) exhibits a negative free
energy of dissociation for one TMS group (–64.3 kcal·mol–1)
and is therefore not regarded as a viable target for chemical
synthesis (Scheme 2).[26]
As stated above, in solution compound 2 is stable in an inert
atmosphere. But, if the inert atmosphere is no longer pre-
served, conversion of compound 2 to the main decomposition
Conclusions
We present herein the first molecular structure of the sil-
product [HC(PPh3)2]+ (4+)[27] is observed. This can also be ylated hexaphenyl carbodiphosphorane 2 alongside with de-
monitored by 1H and 31P{1H} NMR spectroscopy by exposing tailed 2D and heteronuclear NMR spectroscopic experiments.
a NMR sample of 2 (in CDCl3) to air. In the 1H NMR spectrum Furthermore the molecular structure of 2 is presented together
the typical triplet of the proton attached to the carbon(0) atom with electronic properties obtained by DFT methods. Experi-
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Silylated Hexaphenyl Carbodiphosphorane [Me3SiC(PPh3)2][CF3SO3]
C. P. Smith, B. Hansen, N. McKelvie, J. Am. Chem. Soc. 1967,
89, 6273–6276; c) R. Appel, F. Knoll, W. Michel, W. Morbach,
H.-D. Wihler, H. Veltmann, Chem. Ber. 1976, 109, 58–70; d) R.
Appel, F. Knoll, H. Schöler, H.-D. Wihler, Angew. Chem. Int. Ed.
Engl. 1976, 15, 702–703.
ments concerning further reactivity of 2 and of carbodiphos-
phorane 1 towards different silicon reagents are in progress
and will be presented in due course.
[2] A. T. Vincent, P. J. Wheatley, J. Chem. Soc. Dalton Trans. 1972,
617.
Experimental Section
Synthesis of [Me3SiC(PPh3)2][OTf] (2): To a stirred solution of 1
(70 mg, 0.13 mmol) in toluene (10 mL) TMS-OTf (1 equiv.,
0.13 mmol, 24 μL) was added whereupon a precipitate is formed. The
suspension is filtered after 2 h and the solid residue is dried in high
vacuum. Yield: quantitative. The crude product can be recrystallized
from DCM/n-pentane to obtain single crystals of 2. M.p.: 249 °C. Ele-
mental analysis for C41H39F3O3P2SSi (758.853): C 63.81 (calcd.
[3] a) J. S. Driscoll, D. W. Grisley Jr., J. V. Pustinger, J. E. Harris,
C. N. Matthews, J. Org. Chem. 1964, 29, 2427–2431; b) H. J.
Bestmann, W. Kloeters, Angew. Chem. 1977, 89, 55; c) H. J. Best-
mann, W. Kloeters, Tetrahedron Lett. 1977, 18, 79–80; d) H. J.
Bestmann, W. Kloeters, Tetrahedron Lett. 1978, 19, 3343–3344.
[4] W. C. Kaska, D. K. Mitchell, R. F. Reichelderfer, J. Organomet.
Chem. 1973, 47, 391–402.
[5] a) G. Frenking, R. Tonner, Pure Appl. Chem. 2009, 81, 597–614;
b) R. Tonner, G. Frenking, Chem. Eur. J. 2008, 14, 3260 –3272;
c) R. Tonner, G. Frenking, Chem. Eur. J. 2008, 14, 3273–3289;
d) R. Tonner, G. Frenking, Angew. Chem. Int. Ed. 2007, 46, 8695–
8698; e) H. Schmidbaur, Angew. Chem. Int. Ed. 2007, 46, 2984–
2985; f) G. Frenking, B. Neumüller, W. Petz, R. Tonner, F. Öxler,
Angew. Chem. Int. Ed. 2007, 46, 2986–2987; g) N. Takagi, T.
Shimizu, G. Frenking, Chem. Eur. J. 2009, 15, 8593–8604.
[6] R. Tonner, F. Öxler, B. Neumüller, W. Petz, G. Frenking, Angew.
Chem. Int. Ed. 2006, 45, 8038–8042.
[7] For other carbodiphosphoranes of the type C(L)(LЈ) see: L = LЈ
= PMe3: a) H. Schmidbaur, O. Gasser, M. S. Hussain, Chem. Ber.
1977, 110, 3501–3507; H. Schmidbaur, O. Gasser, Angew. Chem.
Int. Ed. Engl. 1976, 15, 502–503; L = L’ = PPh2Me: b) H.
Schmidbaur, G. Haßlberger, U. Deschler, U. Schubert, C. Kapp-
enstein, A. Frank, Angew. Chem. Int. Ed. Engl. 1979, 18, 408–
409; U. Schubert, C. Kappenstein, B. Milewski-Mahrla, H.
Schmidbaur, Chem. Ber. 1981, 114, 3070–3078; L = L’ = PPhMe2:
c) H. Schmidbaur, O. Gasser, M. S. Hussain, Chem. Ber. 1977,
110, 3501–3507; H. Schmidbaur, T. Costa, Chem. Ber. 1981, 114,
3063–3069; H. Schmidbaur, T. Costa, B. Milewski-Mahrla, U.
Schubert, Angew. Chem. Int. Ed. Engl. 1980, 19, 555–556; L =
PPh2–C(H2)n–PPh2 with n = 2–4: d) H. Schmidbaur, T. Costa,
Chem. Ber. 1981, 114, 3063–3069; L = PPh3, LЈ = PR3 with R =
Alkyl: e) R. Appel, G. Erbelding, Tetrahedron Lett. 1978, 19,
2689–2692; Additional recapitulation is given in: f) H. Schmid-
baur, R. Herr, C. E. Zybill, Chem. Ber. 1984, 117, 3374–3390; H.
Schmidbauer, A. Schier, Angew. Chem. Int. Ed. 2013, 52, 176–
186.
1
64.89), H 5.17 (5.18), S 4.12 (4.23)%. H NMR (CDCl3): δ = –0.24
(s, 9 H, –SiCH3), 7.41 (m, 12 H, H–Cmeta), 7.51 (m, 12 H, H–Cortho),
7.57 (m, 6 H, H–Cpara). 13C{1H} NMR (CDCl3): δ = –0.89 (t, JPC
=
1
73.4 Hz, PCP), 5.4 (–SiCH3), 127.5 (vd, AAЈX: N = 92.9 Hz, PCipso),
129.2 (vt, Cmeta), 133.3 (s, Cpara), 134.2 (vt, Cortho). 31P{1H} NMR
1
(CDCl3): δ = 26.2 (s, JPC = 75.2 Hz). 29Si{1H} NMR (CDCl3): δ =
1
–1.0 (t, JSiP = 5.4 Hz). 19F NMR (CDCl3): δ = –77.9. IR (ATR): ν˜ =
3066 vw, 2963 vw, 1589 vw, 1481 vw, 1436 w, 1266 m, 1222 w, 1186
vw, 1140 w, 1094 m, 1031 m, 1013 m, 994 m, 941 vw, 894 m, 839 w,
749 m, 694 m, 635 m, 571 vw, 560 sh, 517 m, 508 m, 438 w, 397 w,
370 w, 346 vw, 331 vw, 320 w, 295 vw, 286 vw, 272 w, 261 w, 236
w, 218 w, 206 w, 163 vw, 145 vw, 136 vw, 126 vw, 114 w, 94 w, 84
w, 75 w, 54 m, 38 vs cm–1. Raman (crystalline): ν˜ = 1193 vw, 1101
w, 1034 s, 1005 vs, 754 vw, 712 vw, 692 w, 641 vw, 618 w, 574 vw,
349 vw, 312 vw, 280 vw, 263 w, 244 w, 235 vw, 215 w, 199 vw, 155
vw, 113 m, 100 w cm–1. HR-ESI-MS (79 eV, 150 °C): m/z = 609.2287
(2)+ (calcd. 609.2296).
Computational Details: Geometry optimizations without symmetry
constraints were carried out using the Gaussian09 optimizer[30] with
Turbomole6[31] energies and gradients with the functional and basis
set combination BP86[32]/def2-SVP.[33] Stationary points were charac-
terized as minima and thermodynamic corrections derived by calculat-
ing the Hessian matrix analytically at this level of theory.[34] Improved
energies were derived with the basis set def2-TZVPP[33] based on these
structures. If not otherwise noted, all computed energies given are free
energies (ΔG, T = 298.15 K, p = 1 atm) on the BP86/def2-TZVPP//
BP86/def2-SVP level of approximation.
[8] It is also possible that all four electrons participate in σ- and
π-donation to the same lewis acid
E simultaneously, see
references[9b,10b]
.
Natural partial charges were calculated with the same method em-
ploying the Natural Population Analysis[35] as implemented in Tur-
bomole.
[9] Be: a) W. Petz, K. Dehnicke, N. Holzmann, G. Frenking, B. Ne-
umüller, Z. Anorg. Allg. Chem. 2011, 637, 1702–1710; W. Petz,
K. Dehnicke, B. Neumüller, Z. Anorg. Allg. Chem. 2011, 637,
1761–1768; BH3 and BR3: b) W. Petz, F. Öxler, B. Neumüller, R.
Tonner, G. Frenking, Eur. J. Inorg. Chem. 2009, 4507–4517; B.
Inés, M. Patil, J. Carreras, R. Goddard, W. Thiel, M. Alcarazo,
Angew. Chem. Int. Ed. 2011, 50, 8400–8403; J. S. Driscoll, D. W.
Grisley Jr., J. V. Pustinger, J. E. Harris, C. N. Matthews, J. Org.
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era, M. Heitbaum, G. Frenking, R. Tonner, B. Neumüller, Inorg.
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Supporting Information (see footnote on the first page of this article):
2D NMR spectra (HMQC, HMBC, NOESY), IR and Raman spectra,
crystallographic data, molecular structure, bond lengths and angles of
compound 2. Cartesian coordinates and SCF energy of computed 2+.
Acknowledgements
The authors thank the reviewers for valuable comments. I. K. would
like to thank Prof. S. Dehnen for generous support and Prof. W. Petz
for helpful discussions. R. T. thanks the HLR Stuttgart, the CSC Frank-
furt, the HHLR Darmstadt, and the Hochschulrechenzentrum Marburg
for providing computational resources and Prof. G. Frenking for fruit-
ful discussions.
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Received: October 28, 2013
Published Online: December 9, 2013
422
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