Rhodium(I) complexes of a PBP ambiphilic ligand: Evidence for a metal→borane interaction

Article abstract of DOI:10.1002/anie.200503649

To and fro: Diphosphanylborane derivative 1 behaves as a tridentate, ambiphilic ligand towards rhodium(I) fragments (see picture). The presence of metal-→borane interactions in the resulting square-pyramidal complexes is highlighted by structural analyses

Full text of DOI:10.1002/anie.200503649

Angewandte
Chemie
1999 that Hill reported the first solid-state structure of a
borane complex.[5a] The synthesis of the ruthenaboratrane B
clearly represents a major breakthrough, the borane moiety
ꢀ
being generated in the coordination sphere of the metal by B
H activation of a tris(azolyl)borate. Following the same
strategy, a fewrelated metallaboratranes (Os, Pt, Rh, Ir, Co)
have recently been prepared.[5b–h] Notably, the contribution of
M!B interactions[6] has also been pointed out in the bonding
description of the tantalocene–borataalkene h2-complexes
C[7,8] and the boryl-bridged complexes D–F (cat = 1,2-
O2C6H4).[9]
Ambiphilic Ligands
DOI: 10.1002/anie.200503649
Rhodium(i) Complexes of a PBP Ambiphilic
Ligand: Evidence for a Metal!Borane
Interaction**
SØbastien Bontemps, Heinz Gornitzka,
Ghenwa Bouhadir, Karinne Miqueu, and
Didier Bourissou*
Besides the fundamental aspects associated with the
bonding description of such transition metal!borane inter-
actions, newinsights might also be expected for their use in
organometallic catalysis. Indeed, ligands featuring pendant
borane moieties can be reasonably assumed to interact with
the coordination sphere of transition metals not only by
Transition-metal complexes of Group 13 elements,[1] and
especially of boron,[2] have attracted considerable interest
over the last decade. A wide variety of coordination modes
have been characterized structurally and investigated theo-
retically.[3] In particular, the possibility for Lewis acids ER3
(E = B, Al…) to behave as s-acceptor ligands has been
demonstrated, although examples of such M!ER3 interac-
tions remain very rare. Indeed, despite the structural charac-
terization of the transition metal!alane complex Et4N+-
[CpFe(CO)2(AlPh3)]ꢀ (A) as early as 1979,[4] it was not until
[10,11]
activating an M X bond
or anchoring a substrate,[12] but
ꢀ
also by coordinating as s-acceptor ligands. With this in mind,
we have recently initiated a research program aimed at
exploring the use of so-called ambiphilic ligands. Here we
report the synthesis of a diphosphanylborane derivative and
its tridentate coordination to RhI fragments. The presence of
a Rh!B interaction has been demonstrated by both struc-
tural analyses and DFT calculations.
To disfavor an intramolecular phosphane!borane inter-
action, a phenyl ring was chosen as a rigid two-carbon linker
for the target PBP ambiphilic ligand.[13] Starting from readily
available (o-bromophenyl)phosphane 1,[14] the desired ligand
2 was obtained in 70% yield by bromine–lithium exchange
followed by electrophilic trapping with dichlorophenylborane
(Scheme 1). The monomeric structure of 2 was suggested by
mass spectrometry and confirmed by multinuclear NMR
[*] S. Bontemps, Dr. H. Gornitzka, Dr. G. Bouhadir, Dr. D. Bourissou
Laboratoire HØtØrochimie Fondamentale et AppliquØe du CNRS
(UMR 5069)
UniversitØ Paul Sabatier
118, route de Narbonne, 31062 Toulouse Cedex 09 (France)
Fax: (+33)5-6155-8204
E-mail: dbouriss@chimie.ups-tlse.fr
Dr. K. Miqueu
Laboratoire de Chimie ThØorique et Physico-Chimie MolØculaire
(UMR 5624)
Avenue de l’UniversitØ, BP 1155, 64013 Pau Cedex (France)
[**] We are grateful to the CNRS, UPS, and the French Ministry of
Research and New Technologies (ACI JC4091) for financial support
of this work. IDRIS (CNRS, Orsay, France) is acknowledged for
calculation facilities.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Synthesis of the diphosphanylborane 2.
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spectroscopy. Indeed, the 31P NMR chemical shift for 2 (d =
11 ppm) is very similar to that of 1 (d = 9 ppm), and a broad
signal is observed at d = 43 ppm in the 11B NMR spectrum, as
expected for a triaryl borane.
The presence of an occupied, nonbonding dz2 orbital
makes square-planar d8-ML4 fragments ideal candidates for
the study of metal!borane interactions.[15] The propensity of
2 for tridentate coordination through P!M and M!B
interactions was thus investigated by treating it with half an
equivalent of [{Rh(m-Cl)(nbd)}2] (nbd = 2,5-norbornadiene)
in dichloromethane (Scheme 2). The yellowprecipitate that
Scheme 2. Synthesis of the rhodium(i) complexes 3 and 4.
spontaneously formed at room temperature was collected by
filtration and analyzed spectroscopically. The 31P NMR spec-
trum at room temperature in solution is only poorly resolved,
with several broad signals being observed, probably due to
conformational restrictions. At ꢀ608C, however, one of the
conformers becomes largely predominant. It exhibits two
magnetically differentiated phosphorus atoms (d = 74.7 and
64.2 ppm (JP, P = 24.3 Hz)) coupled to rhodium (1JP, Rh = 151.9
and 160.0 Hz, respectively), thereby unambiguously establish-
ing the coordination of both phosphorus atoms to the
rhodium center. Moreover, the mass spectrum is consistent
with a dimeric structure of general formula [{RhCl(2)}2]. All
these features were confirmed by an X-ray diffraction study
(Figure 1a).[16] Complex 3 adopts a centrosymmetric and
planar[17] chloro-bridged structure in the solid state. The
rhodium center is surrounded by two phosphorus and two
chlorine atoms organized in a perfectly planar arrangement
(ꢀRha = 359.88). The bite angle of the diphosphane skeleton
(P-Rh-P = 98.58) is at the lower limit of those observed for
related POP diphosphanes featuring donor diphenylether
backbones (102–1238).[18] The two phenyl linkers adopt
slightly different conformations (Rh-P-Cipso-Cortho torsion
angles of 17.88 and 24.18), in agreement with the inequiva-
lence of the two phosphorus atoms in the 31P NMR spectrum
in solution at ꢀ608C and in the solid state at room temper-
Figure 1. Molecular views of 3 (a) and 4 (b) in the solid state.
Hydrogen atoms have been omitted for clarity. The thermal ellipsoids
are shown at 50% probability for (a) and 50% probability for (b).
ꢀ
ꢀ
Selected bond lengths [Š] and angles [8]: 3: Rh1 P1 2.2507(13), Rh1
ꢀ
ꢀ
ꢀ
P2 2.2710(13), Rh1 B1 2.306(3), Rh1 Cl2 2.4381(13), Rh1 Cl1
2.4492(13); P1-Rh1-P2 98.52(4), C13-B1-C19 116.3(3), C13-B1-C12
113.3(3), C19-B1-C12 109.2(3), P1-Rh1-B1 81.18(10), P2-Rh1-B1
ꢀ
80.41(10), B1-Rh1-Cl2 103.81(9), B1-Rh1-Cl1 106.09(10); 4: Rh1 P2
ꢀ
ꢀ
ꢀ
2.2560(12), Rh1 P1 2.2856(12), Rh1 B1 2.295(5), Rh1 N1 2.153(4);
P2-Rh1-P1 97.59(4), C20-B1-C19 117.6(4), C20-B1-C26 114.1(4), C19-
B1-C26 108.5(4), N1-Rh1-B1 110.75(16), P2-Rh1-B1 83.55(13), P1-Rh1-
B1 80.06(13), N1-Rh1-Cl1 83.63(10).
As a first evaluation of the strength of the Rh!B
interaction, complex 3 was then treated with N,N-dimethyl-
aminopyridine (DMAP) in dichloromethane (Scheme 2).
Although the 1H and 13C NMR spectra revealed the presence
of a coordinated DMAP in the ensuing complex 4, no
significant modification was observed by 11B and 31P NMR
spectroscopy. An X-ray diffraction analysis (Figure 1b)[16]
revealed that cleavage of the chloro bridge had taken place
due to the coordination of the Lewis basic pyridine to the
metal center rather than to the boron atom. It is noteworthy
that the Rh!B interaction is not significantly affected, as
clearly indicated by the similarity in the geometric parameters
of both complexes. Indeed, the rhodium center in the
monomeric complex 4 also adopts an overall square-pyrami-
ature (d = 76.9 (1JP, Rh = 166.6 Hz) and 66.5 ppm (1JP, Rh
=
162.8 Hz)). Interestingly, due to the folding of this rather
rigid ligand, the boron atom comes close to the metal center,
almost perpendicularly to the square coordination plane (P-
ꢀ
ꢀ
Rh-B = 81.18 and 80.38). The Rh B distance (2.306 Š) is
dal geometry (P-Rh-P bite angle of 97.68), the Rh B distance
noticeably longer than the metal–boron distances observed in
metallaboratranes (2.09–2.21 Š). Although these values
cannot be rigorously compared to each other as the position
trans to the borane is occupied by a donor ligand in
metallaboratranes but not in complex 3, the presence of a
Rh!B interaction in 3 is further supported by the pyramid-
alization of the boron environment (ꢀBa = 338.88) and by the
significant high-field shift of the 11B NMR resonance signal
(Dd = ꢀ23 ppm compared to that of the free ligand 2).
(2.295 Š) is only marginally shorter than in 3, and the boron
environment is still significantly pyramidalized (340.38).
To gain further insight into the nature of the Rh–B
interaction, ab initio calculations[19] were performed for the
mononuclear complex 4* featuring model diphosphanylbor-
ane and DMAP ligands (methyl groups at phosphorus,
hydrogen atoms at nitrogen). The key features of complex 4
ꢀ
ꢀ
(Rh B and Rh P bond lengths and boron pyramidalization)
could be very well reproduced, especially at the BP86/[CEP-
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Angewandte
Chemie
31G(Rh),6-31G*(C,P,B,N,Cl,P,H)] level of theory (Table 1).
Notably, the two-center, two-electron bonding interaction
between the metal center and borane moiety is apparent from
the frontier molecular orbitals of 4* (major contribution from
to the s-acceptor ligand is accompanied by some hybrid-
ization of the formerly vacant 2p(B) orbital (the correspond-
ing natural atomic orbital (NAO) decomposition of the
NLMO features 12.7% of s character).[19] Although the
precise value for this donor–acceptor interaction is mean-
ingless (in the range 30–58 kcalmolꢀ1, depending on the level
of calculations), its magnitude is remarkable and in good
agreement with the observation of the cleavage of the chloro
bridge and retention of the Rh!B interaction in the reaction
of complex 3 with DMAP.
In conclusion, evidence for Rh!B interactions has been
provided by both structural analyses and DFT calculations for
the 16-electron square-pyramidal RhI complexes 3 and 4
derived from the ambiphilic PBP ligand 2. This highlights that
these very rare transition metal!borane interactions 1) are
readily accessible by coordination of preformed borane-
containing ligands, 2) exist even in complexes not featuring
donor ligands in the position trans to the Lewis acid, and
3) are resistant to the action of a Lewis base such as DMAP.
The influence of such M!B interactions on the reactivity of
the resulting complexes is currently under investigation, as
are variations of the metal fragment and of the stereoelec-
tronic properties of the ambiphilic ligand.
Table 1: Selected bond lengths [Š] and angles [8] for complexes 4 and 4*.
ꢀ
Rh P
ꢀ
Rh B
P-Rh-P
P-Rh-B ꢀRha ꢀBa
X-ray
2.286(1) 2.295(5) 97.59(4) 80.1(1) 360.2 340.2
2.256(1)
2.296
2.265
2.282
2.249
2.281
2.251
83.6(1)
78.13
81.38
78.61
82.04
78.51
81.97
78.63
82.10
B3LYP/
2.458
2.392
2.384
2.381
97.52
97.39
97.38
97.41
359.3 344.6
359.2 342.8
359.2 339.2
359.2 342.2
LanL2DZ(Rh)[a]
BP86/
LanL2DZ(Rh)[a]
BP86/CEP-
31G(Rh)[a]
BP86/SDD(Rh)[a] 2.274
2.241
[a] The 6-31G*basis set was used for C,P,B,Cl,N,P,H.
the dz2(Rh) orbital to the HOMO (Figure 2a) with little
distortion due to its interaction with a vacant boron orbital,
and antibonding combination of the dz2(Rh) and 2p(B)
orbitals in the LUMO (Figure 2b)). This qualitative descrip-
Experimental Section
All reactions and manipulations were carried out under an atmos-
phere of dry argon, using standard Schlenk techniques. Unless
otherwise stated, the NMR spectra were recorded at 293K.
2: nBuLi (2.5m in hexane, 5.2 mL, 12.9 mmol) and dichlorophe-
nylborane (0.84 mL, 6.5 mmol) were successively added at ꢀ408C
and ꢀ788C, respectively, to a solution of 1 in toluene (20 mL). After
warming to room temperature, the volatiles were removed under
vacuum, the residue was dissolved in diethyl ether (20 mL), and the
salts were removed by filtration. Diphosphanylborane 2 (2.04 g, 69%)
was obtained as a white solid upon removal of the solvent. M.p. 95–
978C; 11B NMR (128.4 MHz, C6D6): d = 43.1 ppm; 31P{1H} NMR
(162.0 MHz, C6D6): d = 11.0 ppm; MS (EI, 70 eV): m/z: 474 [M]+.
3: A solution of 2 (1.00 g, 2.1 mmol) in CH2Cl2 (6 mL) was added
at ꢀ788C to a solution of [{Rh(m-Cl)(nbd)}2] (486 mg, 1.1 mmol) in
CH2Cl2 (10 mL). After warming the mixture to room temperature
and stirring for 1 h, complex 3 (921 mg, 72%) was collected by
filtration. Crystals suitable for X-ray crystallography were obtained
from a saturated toluene solution at ꢀ48C. M.p. 2058C; 11B NMR
(160.5 MHz, CDCl3): d = 20.0 ppm; 31P{1H} NMR (161.8 MHz, solid
1
1
state): d = 76.9 (d, JP, Rh = 166.6 Hz), 66.5 ppm (d, JP, Rh = 162.8 Hz);
103Rh NMR (15.8 MHz, CDCl3, 213K) d = ꢀ7365 ppm; MS (DCI/
NH3): m/z: 1242 [M+NH4]+; C,H analysis (%) calcd. for
C30H41BClP2Rh: C 58.80, H 6.74; found: C 58.99, H 7.11.
Figure 2. Molekel plots (cutoff: 0.04) of the HOMO (a), LUMO (b),
ꢀ
and NLMO (c) accounting for the Rh B interaction in the model
4: A solution of DMAP (40 mg, 0.33 mmol) in CH2Cl2 (2 mL) was
added at ꢀ788C to a solution of 3 (200 mg, 0.16 mmol) in CH2Cl2
(10 mL). After warming to room temperature, complex 4 (203 mg,
83%) was collected by filtration. Crystals suitable for X-ray
crystallography were obtained from a saturated THF/pentane solu-
complex 4* at the BP86/[CEP-31G(Rh),6-31G*(C,P,B,N,Cl,P,H)] level of
theory.
tion was further confirmed by a natural bond orbital (NBO)
analysis, second-order perturbation theory being particularly
suitable for the description of donor–acceptor interactions. A
natural localized molecular orbital (NLMO) was found to
tion at ꢀ208C. M.p. 1378C; 11B NMR (160.5 MHz, CDCl3): d =
1
19.4 ppm; 31P{1H} NMR (202.5 MHz, CDCl3): d = 66.8 (dd, JP, Rh
=
169.8, 2JP, P = 31.0 Hz), 65.3 ppm (dd, 1JP, Rh = 144.5, 2JP, P = 31.0 Hz);
103Rh NMR (15.8 MHz, CDCl3): d = ꢀ7552 ppm.
ꢀ
account for the Rh B bonding interaction (Figure 2c) with
Received: October 14, 2005
about 79.3% contribution from the rhodium lone-pair (dz2
)
and major “delocalization tails” (17.1%) from a vacant boron
orbital. In agreement with the pyramidalization of the boron
environment, this transfer of electron density from the metal
Keywords: s-acceptor ligands · ab initio calculations · boranes ·
phosphanes · rhodium
.
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[12] Boron analogs of 2,3-O-isopropylidene-2-3-dihydroxy-1,4-bis-
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space group P42212, a = b = 20.711(11), c = 15.503(9) Š, V=
6650(7) Š3, Z = 8, m(MoKa) = 0.713 mmꢀ1, crystal size 0.2 ” 0.3 ”
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=
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for the bent structure of metallocenylboranes, and the delocal-
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Scheibitz, M. Bolte, J. W. Bats, H.-W. Lerner, I. Nowik, R. H.
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0.1009), 453 parameters, R1(I>2s(I)) = 0.0312, wR2(all data) =
0.0711, largest diff. peak and hole: 0.750 and ꢀ0.337 eŠꢀ3. 4:
¯
C41H59BClN2OP2Rh, M = 807.01, triclinic, space group P1, a =
10.0198(8), b = 10.2336(8), c = 21.9239(18) Š, a = 100.279(2),
b = 91.578(2), g = 115.263(1)8, V= 1986.8(3) Š3, Z = 2, m-
(MoKa) = 0.612 mmꢀ1
,
crystal size 0.1 ” 0.3 ” 0.4 mm3, 8716
reflections collected (5588 independent,
Rint = 0.0822), 452
parameters, R1(I>2s(I)) = 0.0434, wR2(all data) = 0.0966, lar-
gest diff. peak and hole: 1.018 and ꢀ0.578 eŠꢀ3. Data for all
structures were collected at 133(2) K for an oil-coated, shock-
cooled crystal on a Bruker-AXS CCD 1000 diffractometer (l =
0.71073 Š). The structures were solved by direct methods
(SHELXS-97),[20] and refined using the least-squares method
on F2.[21] CCDC-286287 (3) and -286288 (4) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[17] G. Aullón, G. Ujaque, A. Lledós, S. Alvarez, P. Alemany, Inorg.
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[19] See Supporting Information for details.
[20] G. M. Sheldrick, Acta Crystallogr. Sect. A 1990, 46, 467 – 473.
[21] SHELXL-97, Program for Crystal Structure Refinement, G. M.
Sheldrick, University of Göttingen, 1997.
[7] a) K. S. Cook, W. E. Piers, R. McDonald, Organometallics 1999,
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[10] For metallocene complexes featuring pendant boryl groups, see:
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[11] The activation of a nickel–methyl bond by a phosphanylalane
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1614
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Angew. Chem. Int. Ed. 2006, 45, 1611 –1614