Boryl anion attacks transition-metal chlorides to form boryl complexes: Syntheses, spectroscopic, and structural studies on group 11 borylmetal complexes

Article abstract of DOI:10.1002/anie.200702369

(Chemical Equation Presented) Nucleophilic borylation: Transmetalation from boryllithium compounds to Group 11 transition-metal chlorides gives the corresponding boryl complexes (see scheme). Borylsilver and borylgold complexes that have three-center-two-

Full text of DOI:10.1002/anie.200702369

Communications
DOI: 10.1002/anie.200702369
Boryl Complexes
Boryl Anion Attacks Transition-Metal Chlorides To Form Boryl
Complexes: Syntheses, Spectroscopic, and Structural Studies on
Group 11 Borylmetal Complexes**
Yasutomo Segawa, Makoto Yamashita,* and Kyoko Nozaki*
Transition-metal boryl complexes[1] have been proposed as
intermediates in many catalytic transformations of boron-
containing substrates.[2] Since some isolated boryl complexes
show unique catalytic acitivity,[2,3] syntheses of boryl com-
plexes would provide the possibility for new catalytic
reactions. However,methods for the synthesis of boryl
complexes are still limited. In fact,the following three types
of reactions are the major methodologies to introduce a boryl
ligand:[1] 1) salt elimination through the reaction of anionic
metal carbonyl complexes with haloboranes;[4] 2) oxidative
addition of a boron–heteroatom bond to low-valent transition
metals;[5] and 3) s-bond metathesis reactions between alkyl
metal complexes and hydroboranes in the presence of light[6]
or oxygen-substituted metal complexes and diborane.[7,8]
Hence,generally obtainable boryl complexes should be
classified into 1) multicarbonyl complexes,2) metal com-
plexes of which the precursors are active for oxidative
addition,and 3) metal complexes that have appropriate
alkylmetal or alkoxymetal precursors. Thus,a new and
general methodology for the synthesis of boryl complexes
would enable access to new types of boryl complexes.
Recently,we reported the reduction of bromoborane 1a to
the corresponding boryllithium compound 2a [Eq. (1)].[9] The
nucleophilic character of 2a prompted us to develop the
introduction of boryl ligands via nucleophilic attack on
transition-metal halides.[10]
electron-rich metal center and to accelerate the Ru-catalyzed
olefin metathesis reaction.[13] Therefore,we decided to
À
synthesize the C C-saturated boryllithium compound 2b to
compare its electronic properties with 2a.
Herein,we report the synthesis of boryllithium species 2b
À
with a saturated C C backbone and reactions of 2a and 2b
with Group 11 metal chlorides to form the corresponding
borylmetal complexes by nucleophilic borylation.[14] Spectro-
scopic and structural studies on the boryl complexes are also
described.
À
The C C-saturated boryllithium species 2b was synthe-
sized by
a method similar to that reported for 2a
(Scheme 1).[9] Reduction of bromoborane precursor 1b with
N-heterocyclic carbene (NHC) ligands,which are isoelec-
tronic to boryllithium species 2a,are among the strongest
electron-donor ligands known.[11] Steric and electronic prop-
erties of NHCs can be easily tuned by modification of their
substituents and skeleton.[12] For example,the saturation of
Scheme 1. Generation of boryllithium compound 2b with a saturated
backbone.
an excess of lithium powder and a catalytic amount of
naphthalene in THF caused the appearance of a broad singlet
in the 11B NMR spectrum at dB = 51.9 ppm (h1/2 = 773 Hz)
attributable to 2b. The downfield shift of the 11B signal of 2b
relative to that of 2a (dB = 45.4 ppm)[9] obeyed the trend
observed for other diazaborolidine derivatives.[15] The THF
solution of 2b was quenched with Et3NHCl to give the
corresponding hydroborane 3b,which could be characterized
by X-ray crystallography (see the Supporting Information) in
88% yield of isolated product from 1b. Compound 2b was
À
the C C backbone of an NHC was reported to lead to a more
[*] Y. Segawa, Dr. M. Yamashita, Prof. Dr. K. Nozaki
Department of Chemistry and Biotechnology
Graduate School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo (Japan)
Fax: (+81)3-5841-7263
E-mail: makotoy@chembio.t.u-tokyo.ac.jp
nozaki@chembio.t.u-tokyo.ac.jp
1
also characterized by H and 13C NMR spectroscopy of the
[**] The authors thank Prof. Todd B. Marder for helpful discussions. This
work was supported by a Grant-in-Aid for Scientific Research on
Priority Areas (No. 17065005, “Advanced Molecular Transforma-
tions of Carbon Resources”) and for Young Scientists (B)
(18750027) from MEXT, Japan and by Kurata Memorial Hitachi
Science and Technology Foundation.
reaction mixture in [D8]THF.
IMes borylmetal complexes 4a,b–6a,b (IMes = N,N’-
bis(2,4,6-trimethylphenyl)-imidazole-2-ylidene;
unsaturated backbone, b denotes saturated backbone) and
PPh3–borylgold(I) complexes 7a,b were synthesized through
addition of boryllithium compounds 2a,b to equal amounts of
IMes- or PPh3-ligated Group 11 metal chlorides in THF
a
denotes
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Observed chemical shifts (NMR spectroscopy) of the 11B,
(Scheme 2). All complexes were characterized by spectro-
scopic,elemental,and X-ray analyses and were shown to have
a linear,two-coordinate structure (see Figures 1 and 2 for
structures of 5a and 7a; see Table 1 and the Supporting
Information for structures of the other complexes).[16] Com-
plexes 5a,b–7a,b are the first examples of fully characterized
borylsilver and borylgold complexes.
13C(carbene carbon atom) and 31P nuclei and selected bond lengths
obtained from crystallographic data for 4a,b–7a,b and reference
compounds 8–14.
À
À
Complex
dB
dC
dP
M B [Š]
M C/P [Š]
4a
4b
5a
5b
6a
6b
7a
7b
38.9
44.7
40.7
46.5
45.1
49.9
45.4
49.5
185.3
185.2
194.6
194.8
217.0
216.3
1.980(2)
1.983(3)
2.118(2)
2.122(4)
2.074(4)
2.069(3)
2.076(6)
2.086(5)
1.918(2)
1.915(3)
2.1207(18)
2.124(4)
2.078(4)
2.070(3)
2.3469(13)
2.3574(11)
57.7
57.8
free IMes (8)
219.7
178.7
184.0
173.4
IMesCuCl (9)
IMesAgCl (10)
IMesAuCl (11)
PPh3 AuCl (12)
PPh3 AuPh (13)
PPh3 AuMe (14)
2.056(7)
1.998(5)
2.235(3)
2.296(2)
2.279(8)
Scheme 2. Syntheses of Group 11 borylmetal complexes with boryl-
lithium compounds 2a,b (yields of isolated product).
33.5
44.0
47.7
to packing forces. Borylsilver complexes 5a,b possess two-
À
center-two-electron (2c-2e) Ag B bonds that are shorter than
the silver–boron interactions (2.35–2.76 Š)[17] in the previ-
ously reported hydroborane silver complexes,which have 3c-
2e bonds consisting of boron,hydrogen,and silver atoms.
Similarly,borylgold complexes 6a,b–7a,b also have 2c-2e
À
Au B bonds that are shorter than the gold–boron interactions
(2.14–2.68 Š)[18] in cage compounds containing gold and
boron atoms in which the boron atom forms one or more
multicenter–multielectron bonds.
The stronger trans influence of a boryl ligand[19,20] than
that of a chloride ligand is demonstrated by the smaller silver–
carbon coupling constants 1JC-Ag in 5a (81 and 88 Hz to 107Ag
and 109Ag,respectively) and 5b (83,95 Hz) than those in
[IMesAgCl] (10; 234,270 Hz). [21] It is noteworthy that strong
interaction between the central silver atom and boryl ligand
causes the signal of the olefinic protons in the five-membered
ring of 5a to be a doublet,owing to coupling with silver
(4JAg-H = 2 Hz,couplings to 107Ag and to 109Ag not resolved).[22]
From the chemical shifts,the following three features were
found (Table 1): 1) all boryl complexes 4a,b–7a,b showed
characteristic downfield 11B signals in the range of 38–50 ppm
as observed for common dialkoxyboryl or diaminoboryl
transition-metal complexes;[1] 2) in the 13C NMR spectra of
4a,b–6a,b,resonances for the carbene carbon atom were
shifted downfield compared to those of reference compounds
9–11.[21,23,24] Notably, 13C chemical shifts of the carbene carbon
atom in borylgold(I) carbene complexes 6a,b were close to
those of the free carbene IMes (8);[23] and 3) the differences in
the 13C or 31P chemical shifts between unsaturated system a
and saturated system b are generally small.
Figure 1. Crystal structure of 5a. Thermal ellipsoids are set at the 50%
probability level; hydrogen atoms are omitted for clarity.
Figure 2. Crystal structure of 7a. Thermal ellipsoids are set at the 50%
probability level; hydrogen atoms are omitted for clarity.
À
À
In the borylcopper complexes 4a,b,the Cu B and Cu C
bond lengths are shorter than those in the recently isolated
borylcopper complex [IMesCuB(pinacolate)] (15; 2.002(3)
and 1.937(2) Š).[8] The B-Cu-C angles in 4a,b are almost
linear (179.43(9) and 179.41(15)8),which satisfies the steric
repulsion between the bulky diaminoboryl and diaminocar-
bene ligands. This structure is in contrast to that of 15,which
has a bent B-Cu-C angle (168.07(16)8)[8] that is probably due
X-ray crystallographic analyses of 4a,b–7a,b further
support the large trans influence of boryl ligands (Table 1):
À
1) the M Ccarbene bonds in 5a,b and 6a,b are longer than those
of reference [IMesMCl] complexes 10 and 11 (M = Ag,[21]
Au[24]); and 2) the Au P bonds in 7a,b are longer than those
À
in a series of PPh3–AuI complexes 12–14[25] that have an
À
additional anionic ligand (ClÀ,Ph À,or Me ). The difference
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in trans influence between unsaturated and saturated systems
À
generally seems to be negligible,although the Au P bond in
À
7b is slightly longer than that in 7a. The order of M B and
À
M C bond lengths among 4a,b–6a,b is Cu < Au < Ag,which
reflects the ionic radii (Cu < Ag < Au) and the relativistic
effect[26] that leads the gold atom to be smaller than silver
atom. Borylgold complexes 6a,b and 7a,b showed no
significant intermolecular aurophilic interaction,[27] which is
probably due to the bulky boryl ligands.
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In conclusion,we have demonstrated the nucleophilic
substitution by boryllithium compounds 2a,b on Group 11
metal chloride complexes to form boryl complexes 4a,b–7a,b.
Complexes 5a,b–7a,b are the first examples of borylsilver and
À
borylgold complexes that have 2c-2e M B bonds. This
methodology may be applicable to the synthesis of other
boryl complexes. The NMR spectra and solid-state structures
of the resulting boryl complexes revealed that the boryl ligand
is one of the strongest known s donors.[20] Saturation of the
boryl ligand skeleton had little influence on its donor ability.
The further reactivity of these boryl complexes is under
investigation.
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Experimental Section
2b: In a glovebox, 1b (50 mg,106 mmol) and naphthalene (2.7 mg,
21 mmol) were dissolved in [D8]THF (1 mL). Lithium powder (7.4 mg,
1.07 mmol) was added to the solution at À458C,and the resulting
suspension was stirred for 35 h at À458C to afford a dark red
suspension. An aliquot of this suspension was transferred into a
screw-capped NMR tube to record the NMR spectra. 1H NMR
([D8]THF,500 MHz): d = 1.21 (d, J = 7 Hz,12H),1.22 (d, J = 7 Hz,
12H),3.44 (s,4H),3.84 (sep, J = 7 Hz,4H),6.97 (dd, J = 9 Hz,6 Hz,
2H),7.02 ppm (d, J = 7 Hz,4H); 13C NMR ([D8]THF,125 MHz): d =
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25.46 (CH3),25.55 (CH ),28.9 (CH),55.1 (CH 2),123.5 (CH),124.7
3
(CH),149.3 (quaternary C),149.7 ppm (quaternary C); 11B NMR
([D8]THF,160 MHz): d = 51.9 ppm (br s).
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4a,b–7a,b: In a glovebox, 1a or 1b (1 equiv) and naphthalene
(0.40 equiv) were dissolved in THF (10 mL per mmol of 1). Lithium
powder (10 equiv) was added to the solution at À458C,and the
resulting suspension was stirred for 20 h at À458C to afford a dark red
suspension. The suspension was filtered through a celite pad to
remove excess lithium and lithium naphthalenide. The filtrate was
added at À458C to a THF solution (see the Supporting Information
for each condition) of metal chloride complex (1.0 equiv),and the
resulting suspension was stirred for 1 h at room temperature. After
solvents were evaporated under reduced pressure,hexane was added
to the residue. The resulting suspension was filtered through a celite
pad,and the residue was washed with hexane. Volatiles were removed
from the filtrate,and recrystallization from toluene gave colorless
crystals of the desired product. An analytically pure sample was
obtained by recrystallization from hexane or toluene. Details of
spectroscopic and analytical data for each compound are described in
the Supporting Information.
Received: May 31,2007
Published online: July 30,2007
[14] Recently,Braunschweig indicated the possibility of a significant
development in the area of metal boryl complexes using boryl-
lithium,see: H. Braunschweig, Angew. Chem. 2007, 119,1990 –
1992; Angew. Chem. Int. Ed. 2007, 46,1946 – 1948; a related
synthesis of gallyl complexes using gallyl anions was reported,
see: S. P. Green,C. Jones,D. P. Mills,A. Stasch, Organometallics
2007, 26,3424-3430.
Keywords: B ligands · boron · boryl anion · Group 11 elements ·
.
structure elucidation
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Chemie
[16] Details of the crystal data and a summary of the intensity data
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(see the Supporting Information). ORTEP drawings of these
complexes are illustrated in Figures S1–S9 in the Supporting
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cooled mineral oil to the glass fiber and transferred to the
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(G. M. Sheldrick,SHELXL-97,Program for the Refinement of
Crystal Structures; University of Göttingen,Göttingen,Ger-
many, 1997) The intensities were corrected for Lorentz and
polarization effects. The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were refined isotropically in
the difference Fourier maps or placed using AFIX instructions.
CCDC-648270 (3b),CCDC-648271 ( 4a),CCDC-648272 ( 4b),
CCDC-648273 (5a),CCDC-648274 ( 5b),CCDC-648275 ( 6a),
CCDC-648276 (6b),CCDC-648277 ( 7a),and CCDC-648278
(7b) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
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À
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À
À
containing a B Ag bond. In all of these complexes,B Ag bonds
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À
bridging hydrogen atoms. The shortest example of a B Ag
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À
À
containing B Au bond. In all of these complexes,the B Au
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