Neutral copper-phosphido-borane complexes: Synthesis, characterization, and use as precatalysts in Csp-P bond formation

Article abstract of DOI:10.1039/c2cc30723e

Copper-phosphido-borane complexes were synthesized and isolated for the first time. Their structures were experimentally and computationally investigated. They were shown to display catalytic activity in C_sp-P bond formation.

Full text of DOI:10.1039/c2cc30723e

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COMMUNICATION
Neutral copper–phosphido-borane complexes: synthesis, characterization,
and use as precatalysts in Csp–P bond formationw
¸
¨
Ibrahim Abdellah,a Elise Bernoud,a Jean-Francois Lohier,a Carole Alayrac,a Loıc Toupet,b
Christine Lepetitcd and Annie-Claude Gaumont*a
Received 1st February 2012, Accepted 24th February 2012
DOI: 10.1039/c2cc30723e
Copper–phosphido-borane complexes were synthesized and
isolated for the first time. Their structures were experimentally
and computationally investigated. They were shown to display
catalytic activity in Csp–P bond formation.
Scheme 1 Synthesis of copper–phosphido-borane complex 2.
Metal phosphido-boranes M(R2PBH3), where M is generally
an alkali, are well known as nucleophilic or basic entities.1
They have been widely used for the synthesis of phosphine
ligands through P-reprotonation, alkylation, bromination, or
oxidation.2 Interestingly, these anions show in fact a more
complex reactivity (dual reactivity) in relation with their
ditopicity3 and a rich coordination chemistry.4 The development
of transition metal catalyzed synthesis of phosphine borane
derivatives, valuable surrogates of phosphines,5 has recently drawn
attention towards terminal transition metal–phosphido-borane
complexes, which are the key to rationalize the mechanism
and stereochemical outcome (when P-stereogenic derivatives
are the goal) of these reactions. For example, the mechanism
and enantiodiscriminating step of palladium-catalyzed cross
coupling reactions between phosphine-boranes and aryl or
alkenyl derivatives have been established through the isolation
of palladium–phosphido-borane complexes.6 In contrast, copper–
phosphido-borane derivatives have never been investigated
presumably because copper-catalyzed carbon–phosphorus bond
forming reactions have only emerged recently as a useful low
cost and low toxicity alternative to the analogous palladium-
catalyzed transformations.7 Our recent findings that secondary
phosphine boranes readily underwent copper-catalyzed reactions
with 1-bromoalkynes under very mild conditions8 (rt to 60 1C
in a few hours) stimulated our interest for the yet unknown
copper–phosphido-borane complexes. We believe that these
species should play an active role in copper-catalyzed
P-functionalization of phosphine-borane derivatives, by analogy
to the recent mechanistic studies dealing with copper-catalyzed
carbon–heteroatom bond forming reactions.9–11 Indeed in the
nitrogen, oxygen and sulfur series, copper(I) nucleophiles
(amidates, imidates, phenoxides, etc.) have been proposed as
the reactive intermediates. Similarly, a Cu(I) terminal phosphido
intermediate was recently reported to be the active species in a
copper catalyzed P-alkylation reaction.12 However, its high reac-
tivity precluded its isolation and full characterization. We report
here the first examples of well-defined neutral copper–phosphido-
borane complexes, and the investigation of their structure and
reactivity through experimental and theoretical studies.
Our initial attempts to isolate a copper–phosphido-borane
complex consisted in treating diphenylphosphine borane 1 with
a stoichiometric amount of BuLi, and adding CuI (1 equiv.) and
phenanthroline (1 equiv.), subsequently, to the resulting lithiated
anion at 0 1C (Scheme 1).8
The low temperature 31P NMR revealed the instant formation
of a new copper complex (dP = À22 ppm), which was assigned
to the copper phosphido species [Ph2P(BH3)Cuphen] (2) based
on the 1H and 13C NMR spectra. However, all attempts to
isolate this complex and to grow crystals failed. In order to
improve the stability of this complex, we decided to introduce an
additional ancillary electron donating ligand into the coordi-
nation sphere of copper and chose for this purpose, a phosphine
derivative. Indeed secondary phosphines are known to tightly
bond transition metals.13 The synthesis of the targeted complex
4a was achieved within two steps (Scheme 2): synthesis of a
copper(I) complex 3a, which is structurally related to the well-
defined complex [Ph3PCuIphen] reported by Venkataraman
et al.,14 followed by nucleophilic substitution of the iodide by
potassium diphenylphosphido borane.
a
´
Laboratoire de Chimie Moleculaire et Thio-organique,
UMR CNRS 6507, INC3M, FR 3038, ENSICAEN & Universite
´
de Caen, 6 boulevard du Marechal Juin, F-14050 Caen, France.
E-mail: annie-claude.gaumont@ensicaen.fr;
´
Fax: +33 2 31 45 28 77; Tel: +33 2 31 45 28 73
Institut de Physique de Rennes, UMR CNRS 6251, Universite de
b
´
´
´
Rennes 1, 263 avenue du General Leclerc, F-35042 Rennes, France
c CNRS, LCC (Laboratoire de Chimie de Coordination),
205 route de Narbonne, F-31077 Toulouse, France.
E-mail: christine.lepetit@lcc-toulouse.fr; Fax: +33 5 61 55 30 03;
Tel: +33 5 61 33 31 36
d Universite´ de Toulouse, UPS, INPT, LCC,
F-31077 Toulouse, France
w Electronic supplementary information (ESI) available: Experimental
details, characterization data including X-ray structures, NMR spectra
of new compounds and computational details. CCDC 857695–857697.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc30723e
c
4088 Chem. Commun., 2012, 48, 4088–4090
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Fig. 2 X-Ray structures of complexes 4a (left) and 4b (right).
Scheme 2 Synthesis of copper–phosphido-boranes 4.
Interestingly, the length of the newly formed Cu–P2(phosphido-
borane) bond is close to that of the Cu–P1(phosphine) bond,
suggesting that both Cu–P bonds are rather similar.
The nature of the copper phosphido-borane bond in complexes 4
was further investigated using electron localization function
(ELF)15 and atoms in molecules (AIM)16 theoretical studies
(see Section VI, ESIw). ELF topological analysis is indeed a
unique tool for chemical bonding studies, as it provides a
partition of the molecular space into core and valence basins,
the latter being in one-to-one correspondence with Lewis-type
bonds and lone pairs.15b The populations of ELF disynaptic
basins V(Cu, Pi), i = 1, 2, calculated at the B3PW91/DGDZVP
level using the experimental geometry of complex 4a, are very
close (2.27 e and 2.15 e respectively) and lie in the range of those
previously reported for dative metal–phosphine bonds (see
Fig. S1, ESIw).17 From this ELF picture, the Cu–phosphido-borane
(i = 2) and the Cu–phosphine bond (i = 1) appear therefore
very similar, the slightly lower population of the Cu–P2 bond
being in agreement with its longer length (Table 1). The small
positive AIM atomic charge of P2 (+0.8) as compared to the
one calculated for P1 (+1.5) is reminiscent of its phosphido
character exemplified in the (PPh2)À ligand (+0.6, see Table S1,
ESIw). The hydrogen atoms of the borane moiety exhibit a
strong hydride character as indicated by their negative AIM
charge (À0.6). This hydride character was already reported to
drive the affinity of Li cations to the borane moiety after
deprotonation of diphenylphosphine borane.3
Fig. 1 X-Ray structures of complexes 3a (left) and 3b (right).
Table 1 Selected bond lengths (A) and angles (1) in complexes 3a, 3b,
4a, and 4b
3a
3b
4a
4b
Cu–N(1)
Cu–N(2)
Cu–I
Cu–P(1)
Cu–P(2)
P(1)–H
P(2)–B
2.046(16)
2.082(14)
2.634(3)
2.175(7)
—
1.000
—
113.38(18)
—
2.0810(17)
2.0860(17)
2.6741(11)
2.1864(10)
—
1.29(3)
—
99.40(4)
—
2.1439(13)
2.0711(13)
—
2.2427(5)
2.2640(5)
1.363(19)
1.9480(19)
—
2.095(2)
2.098(2)
—
2.2505(8)
2.2609(6)
1.26(4)
1.941(3)
—
P–Cu–I
P–Cu–P
121.105(19)
115.95(3)
Equimolar amounts of CuI, 1,10-phenanthroline and diphenyl-
phosphine were reacted in dichloromethane (À78 1C to rt). After
filtration the expected new copper(I) complex 3a was obtained in
94% yield. Compound 3b having diethylphosphine as a neutral
ligand was also prepared to compare its reactivity with that of
complex 3a. These new complexes proved to be air-stable and were
fully characterized spectroscopically (see ESIw). Moreover their
structure was unambiguously confirmed by X-ray diffraction
analysis (Fig. 1, Table 1 and ESIw).
Next, complexes 3a and 3b were reacted with TMSOK
(1.1 equiv.) and diphenylphosphine borane (1 equiv.) in dichloro-
methane at À78 1C. Substitution of the iodide by the in situ
formed potassium diphenylphosphido-borane readily occurred
affording cleanly copper phosphido-boranes 4a and 4b as
red solids in 82% and 89% yields respectively (Scheme 2).
Both complexes were found air- and moisture sensitive. They
were stable at low temperature, but started decomposing
after a few hours at room temperature. Their structures were
unambiguously confirmed by X-ray diffraction analysis (Fig. 2,
Table 1 and ESIw).
Near-frontier molecular orbitals (MO) of various phosphorus
ligands are compared in Fig. S2 (see ESIw). Similarly to triphenyl-
or secondary diphenylphosphine, the HOMO of diphenyl-
phosphido-borane is strongly polarized on the lone pair of
phosphorus, but its higher energy is in agreement with a
stronger s-donating properties of this ligand as compared to
a standard phosphine. Regarding MOs of complex 4a (see Fig. S3,
ESIw), the HOMO is related to a nonbonding interaction of
a d orbital of the metal centre with the lone pair of P2.
The bonding MOs related to the Cu–P1 and Cu–P2 s-bonds
are deeper but closer in energy. Their relative energy order
therefore suggests a comparable strength of both s-bonds.
However the heterolytic bond dissociation energy of the
Cu–P2 bond in 4a is calculated to be larger than the one of
the Cu–P1 bond by more than 16 kcal molÀ1 (see Table S2,
ESIw). The negative Gibbs energy, calculated for the dissociation
of HPPh2, suggests that complex 4a can easily yield complex 2
(Scheme 1).
Both complexes 4a and 4b have a tetrahedral-like coordination
structure. In both cases, the replacement of the iodide in complex
3a or 3b by the phosphido-borane (complex 4a or 4b) leads to the
increase in the Cu–P(phosphine) bond length (Table 1).
The above theoretical studies therefore suggest that the
phosphido-borane ligand is of L type in the Green formalism18
just as triphenylphosphine (see Fig. S4 in ESIw), but is expected
c
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Chem. Commun., 2012, 48, 4088–4090 4089

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2 See for example: (a) T. Imamoto, Y. Saitoh, A. Koide, T. Ogura
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Scheme 3 Catalytic activity of complexes 4 in Csp–P bond formation.
van Leeuwen, H. Hiemstra, J. N. H. Reek and J. H. van Maarseveen,
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to display stronger s-donor properties according to the frontier
MO analysis.
Next the ability of complex 4a to act as a catalyst was
tested in the reaction between diphenylphosphine borane 1
and 1-bromohexyne 5 (Scheme 3). Gratifyingly, the use of
10 mol% of 4a allowed a full conversion to be reached at room
temperature, and afforded alkynylphosphine borane 6 as the
sole product in 80% isolated yield.
Similar results were obtained with 4b. This result suggests
that copper–phosphido-borane complex 4a may be an inter-
mediate in the phosphination of alkynyl bromides or may lead
to such an intermediate.
6 (a) D. S. Glueck, Chem.–Eur. J., 2008, 14, 7108; (b) D. S. Glueck,
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7 For phosphines as coupling partners, see: (a) D. Gelman, L. Jiang
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The relative chemical reactivity of the Cu–P1 and Cu–P2
bonds was further theoretically investigated using Fukui indices.
The Fukui function was introduced by Parr and Yang as the
response of the electron density of the molecular system to a
change in the global number of electrons.19 The sensitivity of the
copper complex 4a towards electrophilic attack, such as that of
an alkynyl halide, was investigated using Fukui fÀ functions
condensed on ELF or AIM basins.20 The largest value is
obtained for the Cu–phosphido-borane bond (fÀ
= 0.22)
ELF
that is therefore expected to be the most reactive with a
bromoalkyne (see Fig. S5, ESIw). A sizeable value (fÀELF = 0.14)
is also calculated for the metallic centre and for the P–B bond
8 E. Bernoud, C. Alayrac, O. Delacroix and A.-C. Gaumont, Chem.
Commun., 2011, 47, 3239.
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633; (b) G. Franc, Q. Cacciuttolo, G. Lefevre, C. Adamo,
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C. D. Incarvito and J. F. Hartwig, J. Am. Chem. Soc., 2008,
130, 9971.
(fÀ
= 0.12), while the Cu–P1 bond (fÀ
= 0.03) is
ELF
ELF
predicted to be insensitive to the electrophilic attack of the alkynyl
halide. This theoretical result is in good agreement with the
selective formation of 6 from the reaction of bromoalkyne 5 with
phosphine 1 catalyzed by complex 4b bearing a diethylphosphine
ligand (Scheme 3).
11 (a) G. O. Jones, P. Liu, K. N. Houk and S. L. Buchwald, J. Am.
Chem. Soc., 2010, 132, 6205; (b) E. R. Strieter, B. Bhayana and
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13 See, for example: (a) S. I. M. Paris, J. L. Petersen, E. Hey-Hawkins
and M. P. Jensen, Inorg. Chem., 2006, 45, 5561; (b) M. Itazaki,
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30, 828.
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92, 5379; (b) B. Silvi and A. Savin, Nature, 1994, 371, 683.
16 R. F. W. Bader, Atoms in Molecules, Clarendon Press, Oxford,
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17 (a) I. Abdellah, C. Lepetit, Y. Canac, C. Duhayon and R. Chauvin,
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2010, 30, 1.
In conclusion, we described the first examples of isolable
neutral copper phosphido-borane complexes, which clearly
take advantage of the stabilization provided by the phosphine
ligand. These unprecedented well-defined copper(I) complexes
were fully characterized spectroscopically and by X-ray analysis.
Additional structural information was provided by theoretical
calculations. Finally experimental studies demonstrated that
copper–phosphido-borane complexes 4 were able to catalyze
the P-alkynylation reaction of secondary phosphine-boranes.
Further studies will focus on the mechanism of this reaction.
This work was supported by CNRS and Crunch (interregional
organic chemistry network). The ‘‘Region Basse-Normandie’’ and
´
ERDF funding (ISCE-Chem & INTERREG IVa program) are
gratefully thanked for financial support. COST ACTION CM0802
‘‘PHOSCINET’’ is also acknowledged for support. The computa-
tional studies were performed using HPC resources from CALMIP
(Grant 2011 [0851]), and from GENCI-[CINES/IDRIS]
(Grant 2011 [085008]).
19 R. G. Parr and W. Yang, J. Am. Chem. Soc., 1984, 106, 4049.
20 (a) F. A. Bulat, E. Chamorro, P. Fuentealba and A. Toro-Labbe,
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Notes and references
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¨
c
4090 Chem. Commun., 2012, 48, 4088–4090
This journal is The Royal Society of Chemistry 2012