Borylene transfer under thermal conditions for the synthesis of rhodium and iridium borylene complexes

Article abstract of DOI:10.1002/anie.200801756

(Chemical Equation Presented) Borylene transfer reactions to [(η⁵-C₅R₅)M(CO)₂] (M = Rh, R = H; M = Ir, R = Me) using [(OC)₅Mo=BN(SiMe₃)₂] as a borylene source proceed at room temperature and generate terminal rhodium and iridium borylene complexes (see scheme). Iridium complex 1 was characterized by X-ray crystallography.

Full text of DOI:10.1002/anie.200801756

Angewandte
Chemie
DOI: 10.1002/anie.200801756
Borylene Complexes
Borylene Transfer under Thermal Conditions for the Synthesis of
Rhodium and Iridium Borylene Complexes**
Holger Braunschweig,* Melanie Forster, Thomas Kupfer, and Fabian Seeler
The recent development of the photochemical transfer of
borylene ligands from chromium and tungsten borylene
complexes to inorganic,[1] and more recently, organic[2] sub-
strates has shown the “DBX” fragment to be a useful synthon.
As is the case for many reactive species, the feisty nature of
the difficult-to-generate free borylenes[3] is tempered by
coordination to a transition metal, so that their reactivity,
which is reminiscent of the neighboring carbene species,[4] can
be harnessed in conventional syntheses.
terminal borylene species.[8] The H NMR spectrum shows a
singlet for the trimethylsilyl group at d = 0.21 ppm, which is
deshielded with respect to that of the molybdenum borylene
precursor (d = 0.15 ppm).[6] A single conspicuous band at n˜ =
1955 cmÀ1 in the solution IR spectrum of 4 suggests that a lone
CO ligand at rhodium was terminally coordinated. Unfortu-
nately, owing to its oily consistency, it was not possible to
obtain single crystals of 4 suitable for X-ray diffraction.
Borylene 4 is unstable in solution, which was revealed by
11B{1H} NMR spectroscopy. The 11B{1H} NMR spectrum
indicated the formation of a new boron-containing product
even under mild conditions (hexane at À358C). Conversion
into the new product was complete after about 15 days, at
which point red crystals of [{(h5-C5H5)(OC)Rh}2{m-BN-
(SiMe3)2}] (5) were isolated [Eq. (2)]. The rather moderate
yield of 37% for 5 is in agreement with its formation in a
nonstoichiometric reaction reminiscent of its cobalt conge-
ner.[9]
1
In particular, the above-mentioned transfer reaction has
proven a boon for the synthesis of borylenes with other metal
centers, and has uncovered a surprising synthesis of borirenes
from terminal or internal alkynes. However, with the
exception of one example,[5] this reaction requires activation
by UV light, and is thus limited to photochemically inert
substrates. Herein we describe the use of a terminal molyb-
denum borylene complex[6] which performs this task without
irradiation and at ambient temperatures. This new reactivity
is borne out in the synthesis of the first terminal borylene
complexes of the Group 9 elements rhodium and iridium.
These two metals have been the subject of intense inves-
tigation regarding their application in catalytic borylation
reactions. A variety of boryl complexes have therefore been
prepared,[7] however their complexes with lower-coordinate
boron have been overlooked owing to the lack of reliable
synthetic routes. The complexes presented herein go some
way in addressing this deficiency.
The 1H NMR spectrum of 5 has one singlet for the
trimethylsilyl group at d = 0.36 ppm, which is deshielded with
regard to that of the terminal precursor 4 (d = 0.21 ppm). A
When the rhodium dicarbonyl complex 2 was added to an
equimolar amount of 1 in C6D6 at ambient temperature,
multinuclear NMR spectroscopy revealed gradual consump-
tion of the starting materials within 16 h and formation of
what appeared to be the terminal borylene species [(h5-
=
C5H5)(OC)Rh BN(SiMe3)2] (4) with concomitant generation
of [Mo(CO)6], as indicated by a resonance at d = 201.49 ppm
in the 13C NMR spectrum for the latter [Eq. (1)]. After
workup, 4 was isolated as an analytically pure dark orange oil
in 67% yield.
The 11B{1H} NMR spectrum of 4 features a broad signal at
d = 75 ppm (w1/2 = 309 Hz) which is shifted upfield relative to
the signal for 1 (d = 91 ppm),[6] suggesting the formation of a
downfield-shifted broad singlet at d = 90 ppm (w1/2
=
1577 Hz) in the 11B{1H} NMR spectrum indicates the for-
mation of a new bridged borylene compound.[8]
[*] Prof. Dr. H. Braunschweig, M. Forster, Dr. T. Kupfer, Dr. F. Seeler
Institut für Anorganische Chemie
The atom connectivity of 5 was conclusively determined
by performing
a single-crystal X-ray diffraction study
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg(Germany)
Fax: (+49)931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
Braunschweig/index.html
(Figure 1). The complex crystallizes in the monoclinic space
group C2/c.[10] Two {(h5-C5H5)Rh(CO)} units are linked
through a metal–metal bond and a bridged borylene ligand.
The borylene complex 5 has an anti orientation of the (h5-
C5H5) and CO groups with respect to the Rh2B plane. These
À
three atoms form an isosceles triangle in which the B Rh
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
À
distances are 2.054(2) Š and the Rh Rh distance is
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Figure 2. Molecular structure of 7 in the solid state. Bond lengths [Š]
and angles [8]: B1–Ir1 1.892(3), B1–N1 1.365(4); Ir1-B1-N1 175.9(3).
Thermal ellipsoids are set at 50% probability. Hydrogen atoms are
omitted for clarity.
Figure 1. Molecular structure of 5 in the solid state. Bond lengths [Š]
and angles [8]: B1–Rh1 2.054(2), B1–Rh1A 2.054(2), B1–N1 1.399(3),
Rh1–Rh1A 2.668(3); Rh1-B1-N1-Si1 41.72(2), N1-B1-Rh1 139.51(5),
N1-B1-Rh1A 139.51(5), B1-Rh1-Rh1A 49.51(6), B1-Rh1A-Rh1 49.51(6).
Thermal ellipsoids are set at 50% probability, and hydrogen atoms are
omitted for clarity.
the trimethylsilyl group at d = 0.26 ppm, which is very slightly
deshielded realtive to that of the terminal borylene 1 (d =
0.15 ppm).[6]
The proposed constitution of 7 was confirmed by single-
crystal X-ray diffraction (Figure 2). Crystals of 7 were
obtained by cooling a concentrated hexane solution to
À708C; the complex crystallizes in the monoclinic space
group P21/n.[10]
À
2.668(3) Š. The B Rh bond length is similar to analogous
distances found in the bridged bisborylene complex [Rh4{m-
BN(SiMe3)2}2(m-Cl)4(m-CO)(CO)4], which features two three-
coordinate boron atoms linked to two rhodium centers
(2.004(3) Š, 2.076(3) Š, 2.051(3) Š).[5] The molecular geom-
etry also closely resembles that of the methylene-bridged
rhodium complex [(m-CH2){(h5-C5H5)Rh(CO)}2].[11] Herein
À
The Ir1 B1 bond of 7 (1.892(3) Š) is significantly shorter
À
the remarkably short Rh Rh distance of 2.665(1) Š also
than bonds of known iridium boryl complexes (1.991(6)–
indicates the presence of a metal–metal bond, which is
consistent with the description of this class of compounds as
dimetallacyclopropanes. The presence of the bulky trimethyl-
silyl groups imposes a twist of the Si1-B1-Si1A moiety with
respect to the Rh1-B1-Rh1A unit (41.72(2)8), as was pre-
viously observed in the bridged borylene complex [{(h5-
C5H4Me)Fe(CO)}2(m-CO){m-BN(SiMe3)2}] (6; 53(1)8). Thus,
2.093(7) Š)[14] and is consistent with a borylene complex. The
À
Ir1 B1 bond (1.892(3) Š) is very similar to the distance of the
=
Ir C double bond in the related half-sandwich iridum carbene
complex [(h -C5H5)Ir( CPh2)(PiPr3)] (1.904(5) Š),[15] but sig-
5
=
nificantly longer than that of terminal borylene complex [(h5-
[16]
=
C5H5)Mn( BCMe3)(CO)2] (1.809(9) Š). The central Ir-B-
N axis is slightly bent, having an angle of 175.9(3)8. The short
À
À
in connection with the elongated B N distance of 1.412(4) Š,
less effective backbonding from the nitrogen to the boron
B N bond length (1.365(4) Š), together with the trigonal-
planar geometry of the nitrogen atom, shows the presence of a
atom can be assumed in 6.[12] However, the B1 N1 bond
(1.399(6) Š) of 5 is only slightly longer than that observed in
the bridged manganese borylene complex [(m-BNMe2){(h5-
C5H5)Mn(CO)2}2] (1.390(1) Š),[13] indicating significant
B N double bond. The overall geometry of the M-B-N
À
=
moiety in 7 resembles that of the only other structurally
characterized neutral terminal half-sandwich amino-
borylene complex [(h -C5H5)(OC)3V BN(SiMe3)2]
5
[1b]
=
=
(V B
À
double-bond character of the B N linkage.
1.959(6) Š) and corresponds, together with the NMR spec-
troscopic data, to the description of a metal–boron bonding
As 4 can be regarded as an unstable terminal borylene
complex, we focused our attention on the transfer of the
borylene unit to a sterically more demanding late transition
metal system to protect the borylene fragment from scaveng-
ing a second metal centre. We chose [(h5-C5Me5)Ir(CO)2] (3)
because of the electron-rich and sterically demanding nature
of the iridium fragment owing to its bulky (h5-C5Me5)
substituent [Eq. (1)].
À
picture consisting of a strong B M s donation and somewhat
À
weaker M B p back donation.
Herein we have reported on the unprecedented borylene
transfer based on molybdenum borylene complex 1 at
ambient temperature, a discovery that could allow borylene
transfer to thermally and/or photochemically unstable pre-
cursors. The result is the formation of the first terminal
borylene complexes of rhodium and iridium, and indeed the
first borylene complex of iridium.
Monitoring the reaction of equimolar amounts of 1 and 3
in C6D6 at ambient temperature by multinuclear NMR
spectroscopy showed consumption of the starting materials
within 20 h and formation of a new species, which we assigned
5
=
as [(h -C5Me5)(OC)Ir BN(SiMe3)2] (7), with concomitant
Experimental Section
formation of [Mo(CO)6], as indicated by 13C NMR spectros-
All manipulations were conducted under an atmosphere of dry argon,
either inside a glove box or employing standard Schlenk techniques.
4: An orange-colored solution of 1 (0.21 g, 0.50 mmol) and 2
(0.13 g, 0.50 mmol) in benzene (3 mL) was stirred at ambient
temperature for 16 h. The solvent of the reaction mixture was then
removed in vacuo. The resulting dark orange oily residue was
dissolved in toluene (3 mL) and stored at À358C for 17 h to separate
[Mo(CO)6]. After filtration, all volatiles were removed in vacuo. The
copy.
The 11B{1H} NMR spectrum of 7 has a broad singlet at d =
67 ppm (w1/2 = 145 Hz), which is shifted upfield with regard to
the signal of the starting material 1 (d = 91 ppm),[6] suggesting,
as in the case of 4, the formation of a terminal borylene
complex.[8] The 1H NMR spectrum shows one new singlet for
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Angewandte
Chemie
resulting dark orange oil was dissolved in hexane (2 mL) with
subsequent filtration. The solvent was removed in vacuo and 4 was
isolated as an analytically pure dark orange oil (0.12 g, 67%).
Aldridge, Angew. Chem. 2005, 117, 7623 – 7626; Angew. Chem.
Int. Ed. 2005, 44, 7457 – 7460; c) D. L. Kays (nØe Coombs), J. K.
Day, S. Aldridge, R. W. Harrington, W. Clegg, Angew. Chem.
2006, 118, 3593 – 3596; Angew. Chem. Int. Ed. 2006, 45, 3513 –
3516.
2
1H NMR (500 MHz, C6D6, 258C, TMS): d = 5.34 (d, JRh,H = 0.5 Hz,
5H, C5H5), 0.21 ppm (s, 18H, SiMe3); 13C{1H} NMR (126 MHz, C6D6,
1
1
258C): d = 194.90 (d, JRh,C = 88.0 Hz, CO), 87.25 (d, JRh,C = 13.0 Hz,
[5] H. Braunschweig, M. Forster, K. Radacki, Angew. Chem. 2006,
118, 2187 – 2189; Angew. Chem. Int. Ed. 2006, 45, 2132 – 2134.
[6] B. Blank, M. Colling-Hendelkens, C. Kollann, K. Radacki, D.
Rais, K. Uttinger, G. R. Whittell, H. Braunschweig, Chem. Eur.
J. 2007, 13, 4770 – 4781.
C5H5), 3.50 ppm (s, SiMe3); 11B{1H} NMR (64 MHz, C6D6, 258C): d =
75 ppm (br s, w1/2 = 309 Hz). IR (toluene): n˜ = 1955 cmÀ1 (C O).
=
C,H,N analysis (%) calcd for C11H23NBRhOSi2: C 37.20, H 6.53,
N 3.94; found: C 37.37, H 6.45, N 3.31.
5: Compound 4 (0.05 g, 0.14 mmol) was dissolved in hexane
(1 mL) and stored at À358C for 15 d to yield a dark red crystalline
solid. The solid was dissolved in hexane (2 mL), filtered, and the dark
red solution was cooled to À358C. 5 was isolated as red crystals
[7] a) H. Braunschweig, C. Kollann, D. Rais, Angew. Chem. 2006,
118, 5380 – 5400; Angew. Chem. Int. Ed. 2006, 45, 5254 – 5274;
b) G. J. Irvine, M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R.
Rice, E. G. Robins, W. R. Roper, G. R. Whittell, L. J. Wright,
Chem. Rev. 1998, 98, 2685 – 2722; c) S. Aldridge, D. L. Coombs,
Coord. Chem. Rev. 2004, 248, 535 – 559.
[8] H. Braunschweig, M. Colling, Coord. Chem. Rev. 2001, 223, 1 –
51.
[9] Two equivalents of 4 form the dinuclear species 5 with formal
loss of one equivalent of the borylene [BN(SiMe3)2]. Data from
NMR spectroscopy did not provide conclusive evidence as to the
fate of this fragment.
1
(0.03 g, 37%). H NMR (500 MHz, C6D6, 258C, TMS): d = 5.28 (d,
5H, C5H5), 0.36 (s, 18H, SiMe3); 13C{1H} NMR (126 MHz, C6D6,
258C): d = 191.59 (d, 1JRh,C = 126.3 Hz, CO), 90.66 (s, C5H5), 5.89 ppm
(s, SiMe3); 11B{1H} NMR (64 MHz, C6D6, 258C): d = 90 ppm (br s, w1/
2 = 1577 Hz). IR (toluene): n˜ = 1975 cmÀ1, (C O). C,H,N analysis (%)
=
calcd for C18H28NBRh2O2Si2: C 38.39, H 5.01, N 2.49; found: C 37.91,
H 4.90, N 2.34.
7: A dark brown solution of 1 (0.21 g, 0.50 mmol) and 3 (0.20 g,
0.50 mmol) in benzene (3 mL) was stirred at ambient temperature for
20 h. The solvent was then removed in vacuo. All impurities were
removed by sublimation, and the resulting yellow solid was dissolved
in hexane (2 mL). After filtration, the solution was cooled to À708C,
[10] The crystal data for 5 and 7 were collected on a Bruker APEX
diffractometer with a CCD area detector and multilayer mirror
monochromated MoKa radiation. The structure was solved by
using direct methods, refined with the Shelx software package
(G. Sheldrick, Acta Crystallogr. Sect. A 2007, 64, 112 – 122) and
expanded by using Fourier techniques. All nonhydrogen atoms
were refined anisotropically. Hydrogen atoms were assigned
idealized positions and were included in structure factor
calculations. Crystal data for 5: C18H28BNO2Si2Rh2 Mr =
563.22, red blocks, 0.29 ” 0.22 ” 0.18 mm3, monoclinic, space
group P21/n, a = 14.9400(12), b = 9.5316(7), c = 15.6173(12) Š,
1
leading to formation of orange crystals of 7 (0.13 g, 48%). H NMR
(500 MHz, C6D6, 258C, TMS): d = 2.12 (s, 15H, C5Me5), 0.26 ppm (s,
18H, SiMe3); 13C{1H} NMR (126 MHz, C6D6, 258C): d = 181.94 (s,
CO), 96.23 (s, C5Me5), 11.78 (s, C5Me5), 3.29 ppm (s, SiMe3);
11B{1H} NMR (64 MHz, C6D6, 258C, TMS): d = 67 ppm (br s, w1/2
=
145 Hz). IR (toluene): n˜ = 1945 cmÀ1 (C O). C,H,N analysis (%)
calcd for C17H33NBIrOSi2: C 38.77, H 6.32, N 2.66; found: C 38.50,
H 6.15, N 2.65.
=
b = 99.8600(10)8, V= 2191.1(3) Š3, Z = 4, 1calcd = 1.707 mgmÀ3
,
m = 1.628 mmÀ1, F(000) = 1128, T= 173(2) K, R1 = 0.0182, wR2 =
0.0447, 2171 independent reflections (2F = 52.148) and 119
parameters. Crystal data for 7: C17H33BIrNOSi2, Mr = 526.63, red
block, 0.25 ” 0.12 ” 0.08 mm3, monoclinic, space group P21, a =
8.6070(2), b = 11.9484(3), c = 10.9962(2) Š, b = 99.9350(10)8,
Received: April 15, 2008
Published online: July 4, 2008
Keywords: boron · borylene complexes · iridium · rhodium
V= 1113.89(4) Š3, Z = 2, 1calcd = 1.570 gcmÀ3, m = 6.104 mmÀ1
,
.
F(000) = 520, T= 101(2) K, R1 = 0.0137, wR2 = 0.0330, Flack
parameter= 0.0633(41), 4215 independent reflections (2F =
52.028) and 219 parameters. CCDC-684930 (5) and CCDC-
684931 (7) 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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Angew. Chem. Int. Ed. 2008, 47, 5981 –5983
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