Ionic-type reactivity of 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyls: Regio- and stereoselective addition of hydracids

Article abstract of DOI:10.1021/ja903746p

Hydrogen chloride and trifluoromethane sulfonic acid readily add to the symmetrically substituted 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyl (1) and unsymmetrically substituted 1,3-dibora-2,4-diphosphoniobicyclo[1.1.0]butane (3) under mild conditions

Full text of DOI:10.1021/ja903746p

Published on Web 09/04/2009
Ionic-Type Reactivity of
1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls: Regio- and
Stereoselective Addition of Hydracids
Gad Fuks,†,‡ Nathalie Saffon,§ Laurent Maron,*,| Guy Bertrand,*,† and
Didier Bourissou*,‡
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry,
UniVersity of California RiVerside, RiVerside, California 92521-0403, Laboratoire He´te´rochimie
Fondamentale Applique´e, UniVersite´ de Toulouse, UPS, 118 route de Narbonne, F-31062
Toulouse, France, CNRS, LHFA UMR 5069, F-31062 Toulouse, France, Structure Fe´de´ratiVe
Toulousaine en Chimie Mole´culaire, UniVersite´ de Toulouse, UPS, FR2599, 118 Route de
Narbonne, F-31062 Toulouse, France, and UniVersite´ de Toulouse, INSA, UPS, LPCNO, 135
aVenue de Rangueil, F-31077 Toulouse, France, CNRS, LPCNO UMR 5215, F-31077 Toulouse,
France.
Received May 8, 2009; E-mail: guy.bertrand@mail.ucr.edu; dbouriss@chimie.ups-tlse.fr
Abstract: Hydrogen chloride and trifluoromethane sulfonic acid readily add to the symmetrically substituted
1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyl (1) and unsymmetrically substituted 1,3-dibora-2,4-
diphosphoniobicyclo[1.1.0]butane (3) under mild conditions, substantiating some formal analogies between
the PBPB diradicaloids D and alkenes. X-ray diffraction analyses carried out on all the resulting 1,3-diborata-
2,4-diphosphoniocyclobutanes (2, 4-6) revealed that the reactions proceed with complete stereo- and
regio-selectivity, and that an unusual t-Bu f i-Bu isomerization can occur at boron. DFT calculations shed
more light on the factors controlling the regioselectivity of the additions, its concerted versus stepwise
character, and support an original two-step mechanism for the t-Bu f i-Bu isomerization, involving a
carbocationic σ-borane adduct as a key intermediate.
Chart 1
Introduction
Due to their peculiar electronic structure, 1,3-diradicals have
attracted increasing interest over the last 20 years.1,2 The
archetypal systems, namely cyclobutane-1,3-diyls A and cy-
clopentane-1,3-diyls B have been characterized spectroscopi-
cally, both in the triplet3 and singlet4 ground states, and are
short-living species (Chart 1). In contrast, thanks to the unique
† University of California Riverside.
‡ CNRS, LHFA UMR 5069.
§ Universite´ de Toulouse.
| CNRS, LPCNO UMR 5215.
(1) For general reviews on diradicals, see:(a) Dougherty, D. A. Acc. Chem.
Res. 1991, 24, 88–94. (b) Berson, J. A. Acc. Chem. Res. 1997, 30,
238–244. (c) Berson, J. A. In ReactiVe Intermediate Chemistry; Moss,
R. A., Platz, M. S., Jones, M., Jr., Eds.; Wiley-Interscience: Hoboken,
NJ, 2004; pp 165-204. (d) Borden, W. T. In Encyclopedia of
Computational Chemistry; Schleyer, P. v. R., Ed.; Wiley: New-York,
1998; pp 708-722.
properties of heteroelements, singlet 1,3-diradicaloids (C-F)5-11
are stable at room temperature, and have recently been isolated
as crystalline compounds.
The electronic structure of compounds A-D has been
thoroughly investigated by computational methods.12,13 The
catenation of all these diradicals via covalent linkers has been
achieved, providing first insights into the factors influencing
the “communication” between the diradical sites.5m,6d,g,14 Ex-
amples of intramolecular rearrangements of A-D,5b,c,6c,e,f,15
especially ring closure, 1,2-shift reactions, and valence isomer-
izations that include bond-stretching isomerizations have been
reported. Moreover, although the instability of all-carbon
diradicals A and B almost totally precludes intermolecular
reactions,4b,c,e an extremely rich intermolecular reactivity has
been observed from 2,4-diphosphacyclobutane-1,3-diyls C,
(2) For recent reviews on heteroatom-containing diradicals, see:(a) Breher,
F.; Gru¨tzmacher, H. Angew. Chem., Int. Ed. 2002, 41, 4006–4011.
(b) Power, P. P. Chem. ReV. 2003, 103, 789–810. (c) Breher, F. Coord.
Chem. ReV. 2007, 251, 1007–1043.
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T.; Heubes, M.; Hrovat, D. A.; Kita, F.; Lewis, S. B.; Scheutzow, D.;
Wirz, J. J. Am. Chem. Soc. 1998, 120, 593–594. (b) Abe, M.; Adam,
W.; Nau, W. M. J. Am. Chem. Soc. 1998, 120, 11304–11310. (c) Abe,
M.; Adam, W.; Heidenfelder, T.; Nau, W. M.; Zhang, X. J. Am. Chem.
Soc. 2000, 122, 2019–2026. (d) Abe, M.; Adam, W.; Hara, M.; Hattori,
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9
10.1021/ja903746p CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 13681–13689 13681

A R T I C L E S
Fuks et al.
Chart 2
Chart 3
giving access to a variety of unusual phosphorus-containing
heterocycles.5d,e,g-i,k,l So far, the intermolecular reactivity of
1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyls D has been
poorly explored. Oxidation reactions have been reported with
CDCl3/Cl2 and Se/PhSeSePh, leading to the cis/trans-dichloro
and selenium adducts G and H, respectively. In addition, the
spontaneous formation of the 1,3-diborata-2,4-diphosphonio-
cyclobutanes I and J upon reaction with trimethyltin hydride
and bromotrichloromethane provided convincing evidence for
radical-type behavior of D (Chart 2).6b
Interestingly, in contrast to 2,4-diphosphacyclobutane-1,3-
diyls C, but similarly to cycloalkane-1,3-diyls A and B, the
frontier orbitals of 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-
diyls D are associated with in-phase (HOMO) and out-of-phase
(LUMO) combinations of 2p(B) orbitals, in line with some
through-space transannular π-bonding (Chart 3).6a,13 This sug-
gests that some analogy could be formally drawn between PBPB
systems D and alkenes, although in contrast with the latter no
BB σ-bond is present. This prompted us to investigate the
chemical behavior of diradicaloids D toward hydracids, and we
report here the first experimental and computational evidence
of ionic-type reactivity for these PBPB systems.
Results and Discussion
Reaction of PBPB Systems 1 and 3 with HCl. The sym-
metrically substituted 1,3-dibora-2,4-diphosphoniocyclobutane-
1,3-diyl 16a was treated with 1 equiv of hydrogen chloride (2
M solution in diethyl ether) in toluene at -78 °C. Within 5
min, the solution turned from intense yellow to colorless,
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1
indicating the complete consumption of 1. According to H
NMR spectroscopy, a new compound 2 was formed in >80%
yield (Scheme 1). The mass spectrum [EI, m/z ) 406 (M+)]
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1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls
A R T I C L E S
Scheme 1
Scheme 2
was consistent with the expected 1:1 addition product, and the
two signals observed at δ 5.6 and -15.0 ppm in the 11B NMR
spectrum indicate the presence of two inequivalent anionic
tetracoordinate boron centers. Besides the typical signals as-
sociated with the i-Pr groups at phosphorus and the t-Bu groups
at boron, the 1H NMR spectrum exhibits a broad signal
attributable to a BH proton at δ 3.06 ppm. Colorless crystals
(54% yield, mp ) 96 °C) were obtained from a saturated toluene
solution at low temperature, and the molecular structure of 2
was definitely confirmed by X-ray crystallography (Figure 1,
Table 2 in Experimental Section). Upon addition of HCl, the
BPBP four-membered ring is retained, but is noticeably distorted
from planarity (the interflap angle between the two PBB units
decreases from 180° in 1 to 154.5° in 2). The cancellation of
both through-space and through-bond BB interactions is ac-
companied by a substantial elongation of the PB bonds (from
1.89 Å in 1 to 1.99-2.05 Å in 2) and BB distance (from 2.57
Å in 1 to 2.82-2.83 Å in 2). The hydrogen and chlorine atoms
at the boron centers are disordered (the positions for the
hydrogen atom at boron were located and refined without
constraint), but the assignment of the cis relationship between
the two incoming atoms can be done without doubt. The
formation of 2 corresponds to the 1,3-addition of HCl across
the formal transannular π(BB) bond of the 1,3-dibora-2,4-
diphosphoniocyclobutane-1,3-diyl 1. The complete stereoselec-
tivity in favor of the cis addition strongly suggests a concerted
process.
cal reactivity has been observed for PBPB compounds adopting
cyclobutane-1,3-diyl and bicyclo[1.1.0]butane ground-state struc-
tures, including for the radical addition of HSnMe3.17
Upon addition of hydrogen chloride in pentane, the deep red
color characteristic of the unsymmetrically substituted 1,3-
dibora-2,4-diphosphoniobicyclo[1.1.0]butane 36g disappeared
within a few minutes, and a new compound 4 was obtained as
a single isomer in >90% yield, according to 1H NMR spectros-
copy (Scheme 2). The chemical formula of 4 was supported by
mass spectrometry [ESI, m/z ) 449 (M + Na+)], and diagnostic
signals were observed at δ 6.7 and -18.4 ppm in the 11B NMR,
1
and δ 3.39 ppm in the H NMR spectrum (BH proton). X-ray
quality crystals were obtained from a saturated pentane solution
at low temperature (37% yield, mp ) 141 °C) (Figure 2). The
hydrogen atom at boron was located and refined without
constraint. Here also, the addition of HCl induces some folding
of the PBPB four-membered ring (the interflap angle between
the two PBB units is 150.3°), and the two incoming atoms are
in cis relationship. The chlorine atom is bonded to the t-Bu-
substituted boron, while the hydrogen atom is linked to the Ph-
substituted boron center. Thus, the addition of HCl to the
unsymmetrically
substituted
1,3-dibora-2,4-
diphosphoniobicyclo[1.1.0]butane 3 proceeds with complete
regioselectivity and cis-stereoselectivity. Surprisingly, the chlo-
rine atom adds to the more sterically shielded boron center rather
than to the Ph-stabilized one, leading selectively to product 4.
Theoretical Study of the Addition of HCl to 3. To gain more
insight into the factors controlling the stereoselectivity and
regioselectivity of the addition of HCl to 3, density functional
theory (DFT) calculations were carried out. The actual PBPB
We then envisioned to probe the regioselectivity of the
addition reaction, and thus looked for a unsymmetrically
substituted PBPB substrate. Since, to date, the cyclobutane-1,3-
diyl ground-state structure has only been found for sym-
metrically substituted PBPB compounds,6a,c,e the study was
carriedoutontheunsymmetricallysubstitutedbicyclo[1.1.0]butane
3. Although this compound has a bicyclic ground-state structure,
it rapidly inverts in solution at room temperature via the
corresponding cyclobutane-1,3-diyl.16 In addition, similar chemi-
Figure 1. Molecular view of compound 2 in the solid state, with hydrogen
atoms omitted except at B2. Thermal ellipsoids projected at the 50%
probability level. For clarity, only one of the two independent molecules
present in the asymmetric unit is shown. Selected bonds distances (Å) and
angles (deg): P1-B1, 2.050(5); P1-B2, 1.994(5); P2-B1, 2.030(5); P2-B2,
1.990(5); P1-B1-P2, 86.8(2); B1-P2-B2, 89.1(2); P2-B2-P1, 89.5(2);
B2-P1-B1, 88.4(2).
Figure 2. Molecular view of compound 4 in the solid state, with hydrogen
atoms omitted except at B2. Thermal ellipsoids projected at the 50%
probability level. For clarity, only one independent molecule of the unit
cell is shown. Selected bonds distances (Å) and angles (deg): P1-B1,
2.036(3); P1-B2, 1.978(2); P2-B1, 2.048(2); P2-B2, 1.979(2); B1-Cl1,
1.901(3); P1-B1-P2, 86.4(1); B1-P2-B2, 87.7(1); P2-B2-P1, 89.9(1);
B2-P1-B1, 88.0(1).
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13683

A R T I C L E S
Fuks et al.
fairly small to claim with certainty that the reaction proceeds
under charge control.
The potential energy surface for the addition of HCl to 3
was then scrutinized (Figure 4). The formation of the 1,3-
diborata-2,4-diphosphoniocyclobutane 4 was predicted to be
highly exothermic (∆G -48.4 kcal/mol), and a transition state
TS3f4, connecting both compounds, was located 8.7 kcal/mol
higher in energy than 3. This profile is in good agreement with
the fast addition observed at low temperature, and the structure
of TS3f4 supports a concerted but strongly asynchronous
pathway for the addition of HCl to 3. The HCl molecule
approaches almost perpendicularly to the PBPB core of 3, with
a close B(Ph)/H contact (the corresponding distance is 1.55 Å
in TS3f4, as compared with 1.21 Å in the addition product 4),
substantial elongation of the H-Cl bond (from 1.29 Å in the
free form to 1.59 Å in TS3f4), but virtually no B(t-Bu)/Cl
interaction (the corresponding distance is 4.72 Å in TS3f4, as
compared with 3.6 Å for the sum of van der Waals radii19).
The regioisomeric adduct 4′ [with the chlorine atom bonded to
B(t-Bu) and the hydrogen atom linked to B(Ph)] was found 2.4
kcal/mol higher in energy than 4, while the related transition
Figure 3. Optimized geometries and Kohn-Sham representations of the
HOMO orbitals for the 1,3-dibora-2,4-diphosphoniobicyclo[1.1.0]butane 3
and the corresponding 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyl
structure 3′.
state TS3f4′ was found 0.6 kcal/mol lower in energy than TS3f4
.
This suggests that the regioselective formation of 4 is probably
thermodynamically rather than kinetically controlled, but the
small energetic difference predicted between the two pathways
prevents definitive conclusion.
Table 1. Mulliken Charges Computed for the
1,3-Dibora-2,4-diphosphoniobicyclo[1.1.0]butane 3 and the
Corresponding 1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyl
3′18
Mulliken charges
Reaction of PBPB Systems 1 and 3 with HOTf. To investigate
the influence of steric factors on the stereo- and regio-
selectivities observed in the reactions with HCl, compound 3
was then reacted with trifluoromethane sulfonic acid. One
equivalent of HOTf was added at -78 °C to a pentane solution
of the 1,3-dibora-2,4-diphosphoniobicyclo[1.1.0]butane 3. The
reaction proceeded more slowly than with HCl, as apparent from
the progressive discoloration of the reaction mixture over 2 h
at room temperature. Here again, a single product 5 was formed
(Scheme 3). The solubility of 5 in pentane suggested a covalent
rather than ionic structure. This contrasts with the formation of
the diphosphino-carbocation K observed by Niecke et al. upon
reaction of HOTf with a 2,4-diphosphacyclobutane-1,3-diyl C
[Mes* ) 2,4,6-(t-Bu)3C6H2].5i Compound 5 exhibits two 11B
NMR signals in the range typical for tetracoordinated anionic
B(t-Bu)
0.30
0.18
B(Ph)
0.18
-0.01
3
3′
system was considered in order to reliably take into account
substituent effects. Accordingly, the geometric parameters of
the 1,3-dibora-2,4-diphosphoniobicyclo[1.1.0]butane 3 computed
at the B3PW91/SDD(P,Cl),6-31G**(B,C,H) level of theory (BB
1.836 Å, PB 1.91-1.94 Å, fold angle 117.4°) nicely reproduced
those measured experimentally for the related system featuring
a ꢀ-naphthyl in place of phenyl group at boron (BB 1.884 Å,
PB 1.85-1.91 Å, fold angle 121.5°).6g The corresponding
cyclobutane-1,3-diyl structure 3′ (BB 2.569 Å, PB 1.91-1.92
Å, fold angle 165.8°) was found to be the transition state for
the inversion of the bicyclo[1.1.0]butane 3, but only 9.7 kcal/
mol higher in energy.
1
boron centers (δ 4.9 and -18.3 ppm) and a broad H NMR
signal at δ 3.85 ppm attributable to a BH proton. In addition,
the resonance observed at δ -78.1 ppm in the 19F NMR
spectrum is consistent with the presence of a trifluoromethane
sulfonate moiety. The formulation of 5 as a 1:1 adduct between
HOTf and 3 was further supported by mass spectrometry [EI,
m/z ) 540 (M+)], and definitely confirmed by X-ray diffraction
analysis (20% isolated yield, mp ) 155 °C, Figure 5). The
hydrogen atom (located and refined without constraint) and
trifluoromethane sulfonate group are bonded to the two boron
centers, in a cis relationship relative to the PBPB core. The BO
bond length (1.54 Å) is in the same range as those reported for
the few structurally authenticated tetracoordinated boron deriva-
tives featuring a covalently bonded trifluoromethane sulfonate
group (1.50-1.63 Å).20 Surprisingly, the hydrogen atom is
linked to the B(t-Bu) center and the trifluoromethane sulfonate
moiety is bonded to the B(Ph) boron center in 5. Thus, a
complete inversion of regioselectivity is observed upon addition
The HOMO orbitals of 3 and 3′ correspond formally to π-type
interaction between the two boron centers, involving pure 2p(B)
orbitals in 3′ and slightly hybridized orbitals in 3 (Figure 3).
Despite the dissymmetric substitution pattern, the HOMOs of
3 and 3′ are both almost equally distributed over the two boron
centers. It seems thus unlikely that orbital factors are responsible
for the regioselectivity upon the addition of HCl to 3. We then
considered electrostatic effects and noticed a small dissymmetry
in the atomic charges of the two boron centers. In both forms
3 and 3′, a slightly more positive atomic charge is predicted
for the B(t-Bu) center compared to the B(Ph) one (Table 1), in
agreement with the selective addition of Cl at B(t-Bu) and H at
B(Ph). However, the difference in the atomic charges remains
(16) All 1,3-dibora-2,4-diphosphoniobicyclo[1.1.0]butanes have been shown
by NMR to invert readily in solution at room temperature (the axial
and equatorial phosphorus substituents are magnetically equivalent).
(17) Scheschkewitz, D.; Amii, H.; Vranicar, L.; Bourissou, D.; Bertrand,
G. Unpublished results.
(18) Similarly, NBO calculations predicted atomic charges at B(t-Bu) higher
by about 0.1 unit than those at B(Ph) for both 3 and 3′ (see Supporting
Information).
(19) Batsanov, S. S. Inorg. Mater. 2001, 37, 871–885.
9
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1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls
A R T I C L E S
Figure 4. Energy profiles computed at the B3PW91/SDD(P,Cl),6-31G**(B,C,H) level of theory (free energies G at 25 °C including ZPE correction in
kcal/mol) for the addition of HCl to the 1,3-dibora-2,4-diphosphoniobicyclo[1.1.0]butane 3.
Scheme 3
Scheme 4
of HCl and HOTf to 3, suggesting that subtle steric and
electronic effects dramatically influence the outcome of the
reaction. In this respect, at this stage, the selective formation
of 5 was tentatively attributed to the extreme steric protection
of the B(t-Bu) boron center that may prevent the addition of
the trifluoromethane sulfonate group.
To further estimate the influence of the steric effects, the
addition of HOTf to the PBPB diradicaloid 1, featuring t-Bu
groups at both boron centers, was studied. The reaction was
complete after 1 h at -78 °C in toluene, as apparent from the
disappearance of the characteristic yellow color of the 1,3-
dibora-2,4-diphosphoniocyclobutane-1,3-diyl 1. The NMR (11B,
δ 6.2 and -17.9 ppm; 1H, δ 2.57 ppm; and 19F, δ -76.7 ppm)
and mass spectrometry [EI, m/z ) 520 (M+)] data for the
resulting product 6 were consistent with the 1,3-addition of
HOTf (Scheme 4). To unambiguously establish the structure
of compound 6, crystals were grown from a saturated pentane
solution at low temperature (45% yield, mp 105 °C) and an
X-ray diffraction study was carried out (Figure 6). Accordingly,
addition of HOTf had indeed occurred on the formal transan-
nular π(BB) bond of 1. But remarkably, the hydrogen atom
(located and refined without constraint) and trifluoromethane
sulfonate group are in a trans relationship relative to the PBPB
core. This result suggests that, in this case, the addition does
not proceed via a concerted but rather a stepwise mechanism.
Moreover, in contrast to 1, the substituents at the two boron
centers are no longer identical in 6; the TfO-substituted boron
(20) (a) Mu¨ller, M.; Eversheim, E.; Englert, U.; Boese, R.; Paetzold, P.
Chem. Ber. 1995, 128, 99–103. (b) Imamoto, T.; Asakura, K.; Tsuruta,
H.; Kishikawa, K.; Yamaguchi, K. Tetrahedron Lett. 1996, 37, 503–
504. (c) Gates, D. P.; Edwards, M.; Liable-Sands, L. M.; Rheingold,
A. L.; Manners, I.; McWilliams, A. R.; Guzei, I. A.; Yap, G. P. A.
Chem.sEur. J. 1998, 4, 1489–1503. (d) Carpenter, B. E.; Piers, W. E.;
Parvez, M.; Yap, G. P. A.; Rettig, S. J. Can. J. Chem. 2001, 79, 857–
867. (e) Clemente, D. A. Tetrahedron 2003, 59, 8445–8455. (f) Forster,
T. D.; Krahulic, K. E.; Tuononen, H. M.; McDonald, R.; Parvez, M.;
Roesler, R. Angew. Chem., Int. Ed. 2006, 45, 6356–6359. (g) Vidovic,
D.; Findlater, M.; Cowley, A. H. J. Am. Chem. Soc. 2007, 129, 8436–
8437. (h) Hudnall, T. W.; Gabba¨ı, F. P. Chem. Commun. 2008, 4596–
4597.
Figure 5. Molecular view of compound 5 in the solid state, with solvate
molecules and hydrogen atoms omitted except at B1. Thermal ellipsoids
projected at the 50% probability level. Selected bonds distances (Å) and
angles (deg): P1-B1, 2.000(6); P1-B2, 2.041(3); P2-B1, 1.995(4); P2-B2,
2.031(5); B2-O1, 1.541(5); P1-B1-P2, 90.4(2); B1-P2-B2, 89.7(2);
P2-B2-P1, 88.2(2); B2-P1-B1, 89.2(2).
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13685

A R T I C L E S
Fuks et al.
(Figure 7) suggests some secondary interaction between the t-Bu
substituent and the tricoordinate boron center: short H · · ·B
contact of 1.818 Å, slight elongation of the corresponding C-H
bond (1.126 vs 1.090 Å), and acute BCC bond angle (79.2°).
This interaction probably compensates to some extent the high
electron deficiency of the bis-onio borane moiety of I. This is
reminiscent to the contribution of hyperconjugation or “agostic”
bonding in B(t-Bu)3, as discussed recently by Downs, Parsons
et al. on the basis of X-ray diffraction and computational data.24
The t-Bu f i-Bu isomerization is predicted to be favored
thermodynamically, cation II being more stable than I by 10.7
kcal/mol.
The potential energy surface (PES) was then scrutinized in
order to find a low energy pathway between I and II. We first
located the transition state TSI associated with the transfer of a
hydrogen atom from one methyl group of the t-Bu substituent
to the tricoordinate boron center. TSI was found 12.4 kcal/mol
higher in energy than I (Figure 8). According to the optimized
geometry (Figure 9), it corresponds to an isobutylene π complex.
One boron center is formally pentacoordinate.25 As expected,
the boron center is more tightly bonded to the terminal carbon
atom (with B-CH2 and B-CMe2 distances of 1.841 and 1.915
Å, respectively). The isobutylene fragment retains some double
bond character with a CC bond length of 1.389 Å, and a
marginal pyramidalization of the two carbon centers (the sum
of the bond angles ΣR equals 354.3° for CCMe2 and 357.3° for
CCH2). Such π olefin/borane complexes have been recognized
as key intermediates in hydroboration reactions, but to the best
of our knowledge, have not been isolated nor characterized to
date.26-28 TSI leads to compound III, which is located 7.0 kcal/
mol higher in energy than I, by slippage of the boron center to
the terminal carbon atom. The B-CH2 bond shortens to 1.819
Å and the CCH2 center noticeably pyramidalizes (ΣR ) 347°).
The CCMe2 atom remains trigonal planar (359.6°) and no longer
interacts with the boron center (the BC distance reaches 2.503
Å). Compound III is thus best described as a 1,3-dihydro-1,3-
diborata-2,4-diphosphoniocyclobutane featuring a pendant car-
bocation. Upon rotation around the B-CH2 bond, another energy
minimum III′ could be located on the PES. It is significantly
Figure 6. Molecular view of compound 6 in the solid state, with hydrogen
atoms omitted except at B2. Thermal ellipsoids projected at the 50%
probability level. Selected bonds distances (Å) and angles (deg): P1-B1,
2.032(3); P1-B2, 1.986(2); P2-B1, 2.037(2); P2-B2, 1.974(3); B1-O1,
1.550(4); P1-B1-P2, 87.3(1); B1-P2-B2, 88.0(1); P2-B2-P1, 90.3(1);
B2-P1-B1, 87.8(1).
atom features an i-butyl instead of a t-butyl group. The
rearrangement of t-Bu into i-Bu substituent at boron is rare but
not unprecedented. It has been observed during the preparation
or distillation of sterically hindered t-butyl boranes.21 The
selective t-Bu f i-Bu isomerization observed upon addition of
HOTf to 1 parallels, to some extent, the reverse regioselectivity
noticed upon additions of HCl and HOTf to 3, and thereby
further supports the hypothesis that a t-Bu group at boron
prevents sterically the addition of the trifluoromethane sulfonate
group (that would have led to 6′).
As mentioned above, the trans arrangement of the hydrogen
atom and trifluoromethane sulfonate group in 6 indicates that
HOTf adds stepwise to the 1,3-dibora-2,4-diphosphoniocyclobu-
tane-1,3-diyl 1. Most likely, the reaction starts by protonation
of one of the electron-rich boron center of 1 leading to the
cationic intermediate I (Scheme 4). t-Bu f i-Bu isomerization
at the tricoordinate boron atom would then give compound II.
Lastly, addition of the trifluoromethane sulfonate to the sterically
accessible and planar B(i-Bu) center would finally afford the
trans product 6 that probably suffers from less steric congestion
than its cis isomer.22
(24) Cowley, A. R.; Downs, A. J.; Marchant, S.; Macrae, V. A.; Taylor,
R. A.; Parsons, S. Organometallics 2005, 24, 5702–5709.
Theoretical Study of the t-Bu f i-Bu Isomerization upon
Addition of HOTf to 1. To shed some light on the key
isomerization step I f II, DFT calculations were carried out.23
The two isomeric structures were located as energy minima on
the potential energy surface. In both cases, the incoming proton
is linked to a single boron center, any conceivable H-bridging
structure being disfavored by the rather large distance between
the two boron atoms. The optimized geometry for compound I
(25) For discussions about the existence and structure of BH5, see:(a) Tague,
T. J., Jr.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 4970–4976. (b)
Schreiner, P. R.; Schaefer, H. F.; Schleyer, P. v. R. J. Chem. Phys.
1994, 101, 7625–7632. (c) Watts, J. D.; Bartlett, R. J. J. Am. Chem.
Soc. 1995, 117, 825–826. (d) Jursic, B. S. J. Mol. Struct. 1999, 492,
97–103. (e) Kim, Y.; Kim, J.; Kim, K. H. J. Phys. Chem. A 2003,
107, 301–305. (f) Schuurman, M. S.; Allen, W. D.; Schleyer, P. v.
R.; Schaefer, H. F., III. J. Chem. Phys. 2005, 122, 104302/1–104302/
12.
(26) For computational studies of hydroboration, see:(a) Sundberg, K. R.;
Graham, G. D.; Lipscomb, W. N. J. Am. Chem. Soc. 1979, 101, 2863–
2869. (b) Nagase, S.; Ray, N. K.; Morokuma, K. J. Am. Chem. Soc.
1980, 102, 4536–4537. (c) Wang, X.; Li, Y.; Wu, Y.-D.; Paddon-
Row, M. N.; Rondan, N. G.; Houk, K. N. J. Org. Chem. 1990, 55,
2601–2609. (d) Long, L.; Lu, X.; Tian, F.; Zhang, Q. J. Org. Chem.
2003, 68, 4495–4498. (dd) Oyola, Y.; Singleton, D. A. J. Am. Chem.
Soc. 2009, 131, 3130–3131.
(21) For early reports on the rearrangement of t-Bu boranes, see:(a)
Hennion, G. F.; McCusker, P. A.; Marra, J. V. J. Am. Chem. Soc.
1957, 79, 5190–5191. (b) Hennion, G. F.; McCusker, P. A.; Marra,
J. V. J. Am. Chem. Soc. 1958, 80, 617–619. (c) Hennion, G. F.;
McCusker, P. A.; Marra, J. V. J. Am. Chem. Soc. 1958, 80, 3481–
3482. (d) Hennion, G. F.; McCusker, P. A.; Marra, J. V. J. Am. Chem.
Soc. 1959, 81, 1768. (e) McCusker, P. A.; Marra, J. V.; Hennion,
G. F. J. Am. Chem. Soc. 1961, 83, 1924–1928. (f) Rossi, F. M.;
McCusker, P. A.; Hennion, G. F. J. Org. Chem. 1967, 32, 450–452.
(g) No¨th, H.; Taeger, T. J. Organomet. Chem. 1977, 142, 281–288.
(22) The ionic pathway 1 f I f II f 6 is somewhat reminiscent of the
radical mechanism proposed to account for the reaction of 1 with
(27) For recent studies suggesting the transient formation of olefin
π-complex as key intermediates, see:(a) Scheideman, M.; Shapland,
P.; Vedejs, E. J. Am. Chem. Soc. 2003, 125, 10502–10503. (b) Clay,
J. M.; Vedejs, E. J. Am. Chem. Soc. 2005, 127, 5766–5767. (c)
Scheideman, M.; Wang, G.; Vedejs, E. J. Am. Chem. Soc. 2008, 130,
8669–8676. (d) Wang, G.; Vedejs, E. Org. Lett. 2009, 11, 1059–1061.
(28) For Van der Waals complexes between alkenes and BF3, see:
Herrebout, W. A.; van der Veken, B. J. J. Am. Chem. Soc. 1997, 119,
10446–10454. Attractive interactions have been recently predicted
computationally between 6π-electron rings and BH3: Kawahara, S.-
i.; Tsuzuki, S.; Uchimaru, T. Chem.sEur. J. 2005, 11, 4458–4464.
bromotrichloromethane.6
b
(23) As noted by one reviewer, ion pair contacts may affect the geometric
and energetic features of the reaction profile for the t-Bu f i-Bu
isomerization, but this effect could not be taken into consideration
here because of prohibitive computational times.
9
13686 J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009

1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls
A R T I C L E S
Figure 7. Optimized geometries for I and II.
Figure 8. Energy profile computed at the B3PW91/SDD(P,Cl),6-31G**(B,C,H) level of theory (free energies G at 25 °C including ZPE correction in
kcal/mol) for the t-Bu f i-Bu isomerization from I to II (i-Pr substituents at phosphorus and atomic charges omitted for clarity).
Figure 9. Optimized geometries for TSI, III,III′, and TSII.
more stable than III, and even 2.3 kcal/mol lower in energy
than I. In contrast to III, compound III′ retains some
B· · ·C(Me2) interaction [the corresponding distance is 2.107 Å,
and the BCC(Me2) bond angle is rather acute at 83.6°]. A short
contact is also observed between the hydrogen atom at boron
and the carbocationic center CC(Me2) (2.142 Å). The relation-
Figure 10. η2 and η1 forms for the [Cp2Zr(alkyl)(alkene)]+ complexes.
ship between the π complex TSI and the σ adducts III/III′ is
strongly reminiscent to that recently pointed out between the
η2 and η1 forms of [Cp2Zr(alkyl)(alkene)]+ complexes (Figure
symmetrically bridging hydrogen atom between C(H2) and
C(Me2) (the corresponding CH distances are 1.293 and 1.330
Å, respectively), while the other migrating hydrogen remains
exclusively bonded to the boron center (the cleaving BH and
forming CH bond lengths are 1.217 and 2.173 Å, respectively).
TSII is located 17.3 kcal/mol higher in energy than I.
The activation barrier for the overall process can thus be
estimated at about 20 kcal/mol (in the gas phase), in agreement
with a rather fast reaction at room temperature in the liquid
state. The isomerization of I into II is predicted to proceed in
10).29
Quite surprisingly, no transition state could be located for
the back transfer of a hydrogen atom from the boron center
directly to the C(Me2) atom. The formation of II rather results
from a concerted double 1,2-shift of hydrogen from B to C(H2)
and from C(H2) to C(Me2). Indeed, transition state TSII was
found to connect directly III′ and II. According to the optimized
geometry of TSII, the hydrogen shift from C(H2) to C(Me2) is
much more advanced than that from B to C(H2), with a quasi-
9
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A R T I C L E S
Fuks et al.
CH(CH3)2), 1.22 (pseudo-q,3JP,H ) 3JH,H ) 6.9 Hz, 6H, CH(CH3)2),
1.16 (s, 9H, C(CH3)3), 1.13 (s, 9H, C(CH3)3), 1.09 (pseudo-q,3JP,H
two steps: (i) transfer of a hydrogen atom from t-Bu to boron
(formal dehydroboration reaction), and (ii) an original double,
concerted, 1,2-shift of hydrogen from B to C(H2) and from C(H2)
to C(Me2). The high electron deficiency of the tricoordinate
boron atom in I and II probably explains why the transition
states TSI and TSII are accessible. In this respect, it is striking
to note that the carbocationic structure III′ is intermediate in
stability between the two cationic borane forms I and II.
) 3JH,H ) 6.9 Hz, 6H, CH(CH3)2), 1.05 (pseudo-q,3JP,H ) 3JH,H
)
6.9 Hz, 6H, CH(CH3)2). 13C{1H}-NMR (75.5 MHz, C6D6): δppm
3
35.2 (pseudo-t, C(CH3)3 JC,P ) 6.1 Hz), 31.7 (pseudo-t, C(CH3)3
1
3JC,P ) 4.1 Hz), 26.7 (pseudo-t, PCH, JC,P ) 23.5 Hz), 22.1 (s,
2
PCH), 22.0 (pseudo-t, CH(CH3)2, JC,P ) 6.9 Hz), 21.8 (s,
2
CH(CH3)2), 21.7 (pseudo-t, CH(CH3)2, JC,P ) 1.6 Hz), 21.0
2
(pseudo-t, CH(CH3)2, JC,P ) 2.0 Hz), B-C not observed. MS (EI
70 eV) m/z: 406 (M)+, 349 (M - tBu)+, 315 (M - tBu - Cl)+.
Synthesis of 1,3-Diborata-2,4-diphosphoniocyclobutane (4).
A 115 µL (0.231 mmol) portion of HCl (2 M in Et2O) was added
at -78 °C to a solution of compound 3 (90 mg, 0.231 mmol) in
pentane (2 mL). After the solution was stirred for 10 min at -78
Conclusion
HCl and HOTf were found to add readily to the PBPB core
of 1,3-dibora-2,4-diphosphonicyclobutane-1,3-diyls D under
mild conditions. The reactions proceed concertedly or stepwise,
but always with complete stereo- and regio-selectivity. X-ray
diffraction analyses carried out on all the resulting 1,3-diborata-
2,4-diphosphoniocyclobutanes (2, 4-6) revealed that the con-
certed versus stepwise character of the addition is controlled
by subtle steric and electronic effects and that the reaction may
be accompanied by an unusual t-Bu f i-Bu isomerization at
boron. DFT calculations support an original two-step mechanism
for the t-Bu f i-Bu isomerization involving a carbocationic
σ-borane adduct as key intermediate.
These results further illustrate that PBPB diradicaloids D
combine high thermal stability and versatile reactivity. The
addition of hydracids provides the first evidence of ionic-type
behavior and substantiates some formal analogy with alkenes,
in agreement with the symmetry of the frontier molecular
orbitals.
1
°C, the discoloration was complete. H NMR spectroscopy of the
crude mixture allowed to observe the formation of a major
compound (>92%). The solution was concentrated and kept in a
freezer to yield colorless crystals suitable for X-ray analysis. Yield
37 mg (37%); mp 141 °C. 31P NMR (201 MHz, CDCl3): δppm 6.3
(broad). 11B-NMR (160 MHz, CDCl3): δppm 6.7 (s, BCl), -18.4 (s,
1
3
BH). H NMR (500 MHz, CDCl3): δppm 7.44 (d, JH,H ) 7.8 Hz,
2H, Ph-o-CH), 7.19 (pseudo-t,3JH,H ) 7.8 Hz, 2H, Ph-m-CH), 7.13
(t,3JH,H ) 7.1 Hz, 1H, Ph-p-CH), 3.39 (broad, 1H, BH), 2.71 (m,
4H, PCH), 1.49 (pseudo-q,3JP,H ) 3JH,H ) 6.9 Hz, 6H, CH(CH3)2),
1.47 (pseudo-q,3JP,H
)
3JH,H ) 6.9 Hz, 6H, CH(CH3)2), 1.40
(pseudo-q,3JP,H ) JH,H ) 6.9 Hz, 6H, CH(CH3)2), 1.24 (pseudo-
3
q,3JP,H ) JH,H ) 6.9 Hz, 6H, CH(CH3)2), 1.14 (s, 9H, C(CH3)3).
3
13C{1H}-NMR (125 MHz, CDCl3): δppm 135.6 (s, Ph-o-CH), 127.0
(s, Ph-m-CH), 125.0 (s, Ph-p-CH), 30.7 (s, C(CH3)3), 27.2 (pseudo-
1
1
t, PCH, JC,P ) 22.7 Hz), 23.3 (pseudo-t, PCH, JC,P ) 7.2 Hz),
22.2 (s, CH(CH3)2), 21.7 (s, CH(CH3)2), 21.4 (s, CH(CH3)2), 20.5
(s, CH(CH3)2), BC not observed. MS (ESI+) m/z: 449 (M + Na)+,
391 (M - tBu)+, 315 (M - tBu - H + Na)+.
Experimental Section
Synthesis of 1,3-Diborata-2,4-diphosphoniocyclobutane (5).
A 20 µL (0.226 mmol) portion of HOTf was added at -78 °C to
a solution of compound 3 (90 mg, 0.231 mmol) in pentane (2 mL).
After the solution was stirred for 2 h at -78 °C, the discoloration
was complete. 1H NMR spectroscopy of the crude mixture allowed
observation of the formation of a major compound (>90%). The
solution was filtered and then concentrated and kept in a freezer to
yield colorless crystals suitable for X-ray analysis. Yield 25 mg
(20%); mp 155 °C. 31P NMR (121 MHz, C6D6): δppm 3.9 (broad).
11B-NMR (96 MHz, C6D6): δppm 4.9 (s, BOTf), -18.3 (s, BH). 19F-
NMR (300 MHz, C6D6): -78.1. 1H NMR (300 MHz, C6D6): δppm
7.49 (d, 3JH,H ) 7.5 Hz, 2H, Ph-o-CH), 7.25 (pseudo-t,3JH,H ) 7.5
Hz, 2H, Ph-m-CH), 7.18 (t, 1H, Ph-p-CH), 3.85 (broad, 1H, BH),
2.76 (m, 4H, CH(CH3)2), 2.37 (m, 4H, PCH), 1.43 (pseudo-q,3JP,H
All reactions were performed using standard Schlenk techniques
under an argon atmosphere. 31P, 1H, 11B, 19F and 13C spectra were
recorded on Bruker Avance 300 or 400 and AMX500 spectrometers.
31P, 1H, 11B, 19F and 13C chemical shifts are expressed with a positive
sign, in parts per million, relative to external 85% H3PO4, Me4Si,
FCCl3, and BF3 ·OEt2. Toluene was dried under sodium and distilled
prior to use, pentane was dried under CaH2 and distilled prior to
use. All organic reagents were obtained from commercial sources
and used as received. F3CSO3H and HCl (2 M solution in diethyl
ether) were purchased from Aldrich Chemicals. The 1,3-dibora-
2,4-diphosphoniocyclobutane-1,3-diyl 16a (t-Bu/t-Bu) and 1,3-
dibora-2,4-diphosphoniobicyclo[1.1.0]butane 36g (t-Bu/Ph) were
prepared according to literature procedures.
Synthesis of 1,3-Diborata-2,4-diphosphoniocyclobutane (2).
A 70 µL (0.140 mmol) portion of HCl (2 M in Et2O) was added at
-78 °C to a solution of compound 1 (50 mg, 0.140 mmol) in
toluene (2 mL). After the solution was stirred for 5 min at -78
3
) JH,H ) 7.5 Hz, 6H, CH(CH3)2), 1.33 (s, 9H, C(CH3)3), 1.26
3
(pseudo-q,3JP,H ) JH,H ) 7.5 Hz, 6H, CH(CH3)2), 1.24 (pseudo-
3
q,3JP,H ) JH,H ) 7.5 Hz, 6H, CH(CH3)2), 1.01 (pseudo-q,3JP,H
)
3JH,H ) 7.5 Hz, 6H, CH(CH3)2). 13C{1H}-NMR (75.5 MHz, C6D6):
ppm 136.2 (s, Ph-o-CH), 127.5 (s, Ph-m-CH), 125.7 (s, Ph-p-CH),
119.2 (q, 1JC,F ) 317.5 Hz, CF3), 31.6 (s, C(CH3)3), 28.8 (pseudo-
t, PCH, JC,P ) 23.2 Hz), 23.7 (pseudo-t, PCH, JC,P ) 7.0 Hz),
22.7 (s, CH(CH3)2), 22.3 (s, CH(CH3)2), 21.7 (s, CH(CH3)2), 21.5
(s, CH(CH3)2), BC not observed. MS (EI 70 eV) m/z: 540 (M)+,
470 (M - H - CF3)+, 391 (M - OTf)+.
1
°C, the discoloration was complete. H NMR spectroscopy of the
δ
crude mixture allowed the observation of the formation of a major
compound (>80%). The solution was concentrated and kept in a
freezer to yield colorless crystals suitable for X-ray analysis. Yield
29 mg (54%); mp 96 °C. 31P NMR (121 MHz, C6D6): δppm 12.2
(broad). 11B-NMR (96 MHz, C6D6): δppm 5.6 (s, BCl), -15.0 (s,
BH). 1H NMR (300 MHz, C6D6): δppm 3.06 (broad, 1H, HB), 2.19
1
1
(m, 4H, PCH), 1.47 (pseudo-q,3JP,H
)
3JH,H ) 6.9 Hz, 6H,
Synthesis of 1,3-Diborata-2,4-diphosphoniocyclobutane (6).
A 42 µL portion of (0.475 mmol) of HOTf was added at -78 °C
to a solution of compound 1 (175 mg, 0.473 mmol) in toluene (4
mL). After the solution was stirred for 1 h at -78 °C, the
(29) The key alkyl alkene intermediates [Cp2Zr(Me)(alkene)]+ in olefin
polymerization have been recently characterized spectroscopically. The
13C NMR data combined with DFT calculations support a strongly
dissymmetric coordination of the alkene with major contribution of
the η1 carbocationic structure:(a) Vatamanu, M.; Stojcevic, G.; Baird,
M. C. J. Am. Chem. Soc. 2008, 130, 454–456. (b) Sauriol, F.; Wong,
E.; Leung, A. M. H.; Donaghue, I. E.; Baird, M. C.; Wondimagegn,
Ziegler, T. Angew. Chem., Int. Ed. 2009, 48, 3342–3345. For related
[Cp2Zr(Ot-Bu)(alkene)]+ complexes, see: (c) Stoebenau, E. J., III;
Jordan, R. F. J. Am. Chem. Soc. 2003, 125, 3222–3223. (d) Stoebenau,
E. J., III; Jordan, R. F. J. Am. Chem. Soc. 2006, 128, 8162–8175.
1
discoloration was complete. H NMR spectroscopy of the crude
mixture allowed to observe the formation of a major compound
(>90%). The toluene was evaporated, pentane was added (4 mL),
and the solution was filtered. The solution was concentrated and
kept in a freezer to yield colorless crystals suitable for X-ray
analysis. Yield 112 mg (45%); mp 105 °C. 31P NMR (201 MHz,
C6D6): δppm 3.2 (broad). 11B-NMR (160 MHz, C6D6): δppm 6.2 (s,
BOTf), -17.9 (s, BH). 19F-NMR (200 MHz, C6D6): -76.7. 1H NMR
9
13688 J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009

1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls
A R T I C L E S
Table 2. Crystallographic Data for Compounds 2, 4, 5, and 6
2
4
5
6
empirical formula
formula weight
crystal system
space group
a, Å
C20H47B2ClP2
406.59
C44H86B4Cl2P4
853.15
C23H43B2F3O3P2S.1/2pentane
576.27
C21H47B2F3O3P2S
520.21
monoclinic
Cc
11.5692(7)
16.160(1)
15.866(1)
90
108.836(1)
90
2807.4(3)
4
triclinic
triclinic
monoclinic
C2/c
32.322(1)
11.0928(4)
19.4956(6)
90
114.759(2)
90
6347.5(4)
8
j
j
P1
P1
9.7254(9)
11.386(1)
22.736(2)
92.369(2)
96.916(2)
97.929(2)
2471.1(4)
4
13.177(1)
14.246(2)
14.621(1)
67.428(1)
88.668(1)
87.489(1)
2532.0(3)
2
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
densitycalcd, Mg/m3
abs coeff, mm-1
reflns collected
independent reflns
R1 (I > 2σ(I))
wR2
1.093
0.287
11077
6983
0.0589
0.1283
0.806 and -0.322
1.119
0.283
17763
8463
0.0370
0.0864
0.426 and -0.238
1.206
0.245
27869
4657
0.0483
0.1060
0.370 and -0.272
1.231
0.269
8203
4081
0.0313
0.0777
0.344 and -0.200
(∆/r)max (e Å-3
)
(500 MHz, C6D6): δppm 2.80 (m, 2H, PCH), 2.57 (broad, 1H, BH),
and hydrogen atoms have been described with a 6-31G(d,p)
double-ꢁ basis set.36 Calculations were carried out at the DFT level
of theory using the hybrid functional B3PW91.37,38 Geometry
optimizations were carried out without any symmetry restrictions,
the nature of the extrema (minimum) was verified with analytical
frequency calculations. The intrinsic reaction coordinate has been
followed using the IRC technique for all located transition states.
All these computations have been performed with the Gaussian 0339
suite of programs.
2.25 (m, 3H, PCH + CH(CH3)2), 1.38 (m, 8H, CH(CH3)2+ CH2),
3
1.32 (d, JH,H ) 6.7 Hz, 6H, CH(CH3)2), 1.26 (pseudo-q,3JP,H
)
3JH,H ) 7.1 Hz, 6H, CH(CH3)2), 1.24 (pseudo-q,3JP,H ) 3JH,H ) 7.2
Hz, 6H, CH(CH3)2), 1.22 (s, 9H, C(CH3)3), 1.21 (pseudo-q, 6H,
CH(CH3)2). 13C{1H}-NMR (125 MHz, C6D6): δppm 119.2 (q, JC,F
1
) 317.5 Hz, CF3), 34.8 (s, C(CH3)3), 30.0 (s, CH2(i-Bu)), 26.6
1
(pseudo-t, PCH, JC,P ) 7.0 Hz), 25.7 (s, CH(CH3)2(i-Bu)), 23.9
(pseudo-t, PCH, 1JC,P ) 24.0 Hz), 20.9 (m, PCH + CH(CH3)2(i-Pr)),
20.0 (s, 2 CH(CH3)2(i-Pr)), 19.7 (s, CH(CH3)2(i-Pr)), B-C not observed.
MS (EI 70 eV) m/z: 520 (M)+, 463 (M - tBu)+.
Acknowledgment. We are grateful to the NSF (CHE 0808825),
the French Ministry “Education Nationale, Enseignement Supe´rieur,
Recherche” for a Ph.D. Fellowship (G.F.), the Institut Universitaire
de France (L.M.), and CalMip (CNRS, Toulouse, France) for
calculation facilities. H. Gornitzka (LCC, Toulouse) is acknowl-
edged for his assistance in the X-ray diffraction analysis of
compounds 2 and 6.
Crystal Structure Determination of Compounds 2, 4, 5,
and 6 (Table 2). Data for all structures were collected at 193(2) K
using a oil-coated shock-cooled crystal on a Bruker-AXS CCD 1000
diffractometer with Mo KR radiation (λ ) 0.7103 Å). Semiempirical
absorption corrections were employed for compounds 2, 4, and 6.30
The structures were solved by direct methods (SHELXS-97)31 and
refined using the least-squares method on F2.32
Computational Details. Phosphorus and chlorine were treated
with a Stuttgart-Dresden pseudopotential in combination with their
adapted basis set.33 In all cases, the basis set has been augmented
by a set of polarization function (d for P and Cl).34,35 Carbon, boron,
Supporting Information Available: Complete ref 39 citation;
Cartesian coordinates for the optimized structures, H NMR
spectra and crystallographic data for compounds 2,4,5 and 6
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
1
(30) SADABS, Program for Data Correction, version 2.10; Bruker-AXS:
Madison, WI, 2003.
JA903746P
(31) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(32) Sheldrick G. M. SHELXL-97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(33) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys.
1993, 80, 1431.
(36) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(37) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(38) Burke, K.; Perdew, J. P.; Yang, W. Electronic Density Functional
Theory: Recent Progress and New Directions; Dobson, J. F., Vignale,
G., Das, M. P., Eds.; Plenum: New York, 1998.
(39) Frisch, M. J.; et al. Gaussian 03, D-02 ed.; Gaussian, Inc.: Pittsburgh,
PA, 2003.
(34) Ehlers, A. W.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth, A.;
Jonas, V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.
Chem. Phys. Lett. 1993, 208, 111.
(35) Maron, L; Teichteil, C. Chem. Phys. 1998, 237, 105.
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13689