Synthesis and reactivity of cobalt boryl complexes

Article abstract of DOI:10.1039/b516594f

The synthesis and reactivity of phosphine-cobalt boryls, were investigated. The reactivity studies have shown that alkyne diboration takes place and borylation of aryl bromides is possible with low selectivity. Boryl transfer was investigated initially by

Full text of DOI:10.1039/b516594f

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Synthesis and reactivity of cobalt boryl complexes
a
a
b
a
Christopher J. Adams, R. Angharad Baber, Andrei S. Batsanov, George Bramham,
a
a
b
c
Jonathan P. H. Charmant, Mairi F. Haddow, Judith A. K. Howard, Wai Han Lam,
Zhenyang Lin,* Todd B. Marder,* Nicholas C. Norman* and A. Guy Orpen
c
b
a
a
Received 22nd November 2005, Accepted 16th December 2005
First published as an Advance Article on the web 11th January 2006
DOI: 10.1039/b516594f
˚
The reaction between [Co(PMe
3
)
4
] and B
) or between [Co(PMe
) affords the paramagnetic
{B(4-Mecat)} ] and
] respectively, both of which have been
2
(4-Mecat)
2
(4-
(2.185 A). Herein we describe some preliminary results on the
further chemistry and reactivity of phosphine–cobalt boryls.
Mecat = 1,2-O
2
-4-MeC
(cat = 1,2-O
6
H
3
2
Ph)
4
]
and B (cat)
2
2
2
C
6
H
4
The reaction between [Co(PMe
3
)
4
] and B
or between [Co(PMe
afforded the complexes [Co(PMe
{B(4-Mecat)}
and [Co(PMe Ph)
{B(cat)}
lated yields and together with other unidentified products; both
and 3 were characterised by X-ray crystallography.‡ A view
of the molecular structure of 3 is shown in Fig. 1. Compounds
2
(4-Mecat)
2
12
(4-
and
13b
Co(II) bisboryl complexes [Co(PMe
Co(PMe Ph)
3
)
3
2
Mecat = 1,2-O
2
-4-MeC
6
H
3
)
2
Ph) ]
4
B
2
(cat)
2
3
)
3
2
] (2)
[
2
3
{B(cat)}
2
2
3
2
] (3) respectively,† albeit in low iso-
structurally characterised. ESR data and preliminary dibora-
tion and boryl transfer reactivity studies are also presented.
2
The reaction between [CoMe(PMe
Co(I) monoboryl complex [Co(PMe
3
)
4
] and B
2
(cat) affords the
2
)
3 4
{B(cat)}].
11
1,
2 (two crystallographically independent molecules) and 3
1
adopt square-based pyramidal structures with a phosphine in the
apical site and cis, basal boryl groups. All have similar Co–B
distances [1, 1.945(11), 1.970(11); 2, 1.947(6), 1.966(5), 1.954(6),
Transition metal boryl compounds continue to attract attention
not least because of their involvement in various metal catalysed
14
2
diboration and other borylation reactions. Boryl complexes have
now been characterised for many metals, one of the more unusual
of which is cobalt since many are reported to be paramagnetic.
N o¨ th and Schmid were the first to study cobalt boryl complexes
and in early work reported the 19-electron Co(II) species trans-
˚
1
1.960(6); 3, 1.948(3), 1.955(3) A], B–Co–B angles [1, 67.9(4); 2,
◦
6
8.8(2), 68.1(2); 3, 71.19(12) ] and B–B distances [1, 2.185; 2,
˚
2.211, 2.192; 3, 2.271 A] indicating that the acute B–Co–B angle
and short B–B distance in particular are fundamental features
for this type of Co(II) bisboryl complex. A molecular orbital
[
(
Co(dppe)
PMe
cis-[Co(dppe)
netic Co(I) complexes [Co(CO)
2
(BR
2
)
2
] (R = Ph, Br, I and derivatives with 1,2-
15
3
analysis, on the basis of DFT calculations, for the model complex
[Co(PH )} ] does reveal the presence of a three-centre
{B(O
CoB interaction (Fig. 2) consistent with some degree of weak B–
2
)
2
C
6
H
4
and 9-borafluorenyl), cis-[Co(dppe)
2
(BPh )X] and
2
4
3
)
3
2
C
2
H
2
2
2
(BX
2
)X] (X = halide) together with the diamag-
2
4
(BR
2
)] (R = Cl, Ph, NMe
2
and
5
B bonding involving one of the Co d orbitals and the in-phase
related phosphine substituted derivatives) and the dimethylgly-
oxime compounds [Co(dmg-BR
2
)
2
(BX
2
)(L)] (R = Ph, Bu; X
6
=
Ph, Cl, NMe
2
; L = PPh
3
, AsPh
3
, pyridine); much of this
7
work they subsequently reviewed. It was also reported that the
complex trans-[Co(dppe)
to the 20-electron diamagnetic anion trans-[Co(dppe)
2
(BPh ) ] could be reduced with sodium
2
2
−
3,7
2
(BPh ) ] .
2
2
Unfortunately, there were very little spectroscopic data reported
for any of these species and none were structurally characterised so
their true nature remains ambiguous. In later work, the complexes
8
1
9
[Co(CO)
4
{BH
2
(thf)}] and [Co(CO)
2
(g -dppm)(l-dppm)(BH )]
2
were described. Of particular interest were the reports that the
purported paramagnetic Co(II) bisboryls apparently acted as
efficient and versatile boryl transfer reagents in which the boryl
group could be transferred to a range of other transition metal
(
e.g. Cu, Ag, Au, Rh, Fe, Ni, Mn) or main group metal (e.g. Sn,
3,7,10
Pb) centres by reaction with the corresponding metal halide.
Despite this interest, the first structurally characterised cobalt
boryl containing an unsupported Co–B bond was the Co(II)
1
1
complex [Co(PMe
3
)
3
{B(cat)}
2
] (1) (cat = 1,2-O
2
C
6
H ), obtained
4
12
by us in 1996 from the reaction between [Co(PMe
3
)
4
]
and
13
B
2
(cat)
2
, which also exhibited an unusually short B–B interaction
a
University of Bristol, School of Chemistry, Bristol, UK BS8 1TS
b
Fig. 1 A view of the molecular structure of 3 showing the atom numbering
University of Durham, Department of Chemistry, Science Laboratories,
South Road, Durham, UK DH1 3LE
◦
scheme. Selected bond length ( A˚ ) and angle data ( ) include: Co(1)–B(1)
c
1.948(3), Co(1)–B(2) 1.955(3), Co(1)–P(1) 2.2237(7), Co(1)–P(2) 2.2510(7),
Co(1)–P(3) 2.2067(7); B(1)–Co(1)–B(2) 71.19(12).
The Hong Kong University of Science and Technology, Department of
Chemistry, Clear Water Bay, Kowloon, Hong Kong
1
370 | Dalton Trans., 2006, 1370–1373
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the reaction shown in eqn (1) was carried out which resulted in the
,
17
production and isolation of the Co(I) species 4 and MeB(cat).¶
The signal observed for 4 (52.5 ppm) was identical to that seen in
reaction solutions from which 1 was obtained, and the presence of
MeB(cat) (d B 34.8) was confirmed by comparison with previous
1
1
16
data.
[
CoMe(PMe
3
)
4
] + B
2
(cat)
2
→ [Co(PMe
3
)
4
{B(cat)}]
+
MeB(cat)
(1)
The structure of 4 was established by X-ray crystallography,
the results of which are shown in Fig. 4.* The 18-electron
14,16
Co(I) compound 4 adopts the expected
trigonal bipyramidal
geometry with the high trans-influence boryl group in an axial
site. Also by analogy with the rhodium system, the synthesis of
the Co(III) trisboryl complex 5 was attempted according to eqn
(
2). Prolonged reaction in toluene revealed the presence of a new
B NMR signal at 50.1 ppm assumed to be due to compound
, although the reaction did not proceed to completion and the
1
1
5
Fig. 2 A spatial plot of the three-centre, two-electron bonding molecular
orbital showing the weak interaction between Co and the two boron
centres.
trisboryl species was not isolated.
Co(PMe
{B(cat)}] + B
Co(PMe ]+PMe
{B(cat)}
[
3
)
4
2
(cat)
2
→
3
[
3
)
3
3
(2)
combination of the two ‘empty’ boron p orbitals from the
two B(O
2
C
2
2
H ) ligands which is consistent with these observed
structural features.
Compounds 1–3 are paramagnetic and exhibit ESR spectra
which are broad at room temperature and well-defined at low
temperature. Preliminary analysis of the spectrum of 2 (Fig. 3)§
presented here (g
the presence of a single unpaired electron in a dz2 orbital in
x
and g
y
> g
e
, g
z
≈ g
e
) is consistent with
accord with both the observed structure and the results of the
14
DFT calculations. The high field component shows significant
5
9
coupling to cobalt ( Co, 100%, I = −7/2), whilst the lower field
features show coupling to two equivalent phosphorus nuclei. No
1
1
appreciable coupling was observed to B.
Fig. 4 A view of the molecular structure of 4 showing the atom
◦
numbering scheme. Selected bond length ( A˚ ) and angle data ( ) in-
clude: Co(1)–B(1) 1.949(2), Co(1)–P(1) 2.1527(5), Co(1)–P(2) 2.1686(5),
Co(1)–P(3) 2.1687(5), Co(1)–P(4) 2.1859(5); B(1)–Co(1)–P(4) 174.04(6).
Preliminary reactivity studies involving both diboration catal-
ysis and boryl transfer were also carried out. With regard to
diboration, the reaction between B
2
(cat)
2
and the alkyne 4-
CF
3
C
6
H
4
C≡CC
6
H
4
-4-CF
3
in THF in the presence of 5 mol%
Fig. 3 The ESR spectrum of 2 recorded at 120 K. Selected data include:
◦
[
Co(PMe
3
)
4
] at 80 C for 24 h afforded good yields of the
A
1
(2P) ≈ 60 G, A
2
(2P) ≈ 75 G, A
3
(Co) = 80 G, g
1
= 2.239, g = 2.110,
2
diboration product (4-CF
CF ).†† Analysis of the reaction mixture by GC-MS (after
3
C
6
H
4
){B(cat)}C=C{B(cat)}(C
6
4
H -4-
g
3
= 1.985, giso = 2.116.
3
Since the Co(II) bisboryls are paramagnetic, it was not expected
treatment with pinacol to make the bis-B(pin) derivative which
is more easily analysed by GC-MS) showed not only the expected
cis isomer 6 but also a small amount of a compound with
1
1
31
that they would exhibit observable B or P NMR signals.
1
1
18
However, routine monitoring of reaction solutions by B NMR
consistently showed the presence of a broad signal around 50 ppm.
It was considered likely that such a signal was due to the presence
of either a diamagnetic Co(I) or Co(III) boryl species, most likely
the same mass and similar fragmentation pattern which we
assign to the trans isomer 7, as it also gave a singlet in the
1
9
1
F NMR spectrum and in the B(pin) region of the H NMR
either [Co(PMe
by analogy with the known corresponding rhodium species.
Accordingly, and guided by the analogous rhodium chemistry,
3
)
4
{B(cat)}] (4) or fac-[Co(PMe
3
)
3
{B(cat)}
3
] (5)
spectrum. Trans-diboration of alkynes has not been observed
1
6
previously (although we note an example of metal catalysed
19
trans-hydroboration ) and its appearance in this system is most
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Dalton Trans., 2006, 1370–1373 | 1371

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likely associated with the paramagnetic nature of the catalyst
although the detailed mechanism and reaction pathway remain
to be elucidated. The structure of the cis isomer, 6, was confirmed
by X-ray crystallography as a mixed B(pin)/B(cat) species.‡‡
C
36
H
41
B
2
CoO
4
P
3
, M = 711.15, monoclinic, a = 13.2266(6), b = 16.0716(7),
◦
3
c = 33.7307(15) A˚ , b = 96.30(1) , V = 7126.9(6) A˚ , T = 120(2) K, space
−
1
group C2/c, Z = 8, l = 0.654 mm , Rint = 0.0523 (for 33 424 measured
reflections), R1 = 0.0480 [for 6062 unique reflections with I > 2r(I)],
wR2 = 0.0962 (for all 7749 unique reflections).
§
ESR spectra were recorded by Dr Damien Murphy of the University of
Cardiff as toluene glasses at 120 K in a 4119HS cavity on a Bruker EMX
X-band spectrometer operating at 100 kHz field modulation.
¶
Preparation of 4: A sample of B
2 2
(cat) (0.016 g, 0.067 mmol) was added
17
to an orange-red solution of [CoMe(PMe
3
)
4
]
(0.027 g, 0.071 mmol)
3
in hexane (0.6 cm ) resulting in a light yellow solution and a small
amount of an unidentified green precipitate. The solution was filtered
and concentrated in vacuo resulting in the appearance of a small quantity
of yellow crystals of 4 after a few days, one of which was suitable for X-
ray crystallography. NMR (hexane/THF): B d 52.5, 4; 34.8, MeB(cat).
Satisfactory analytical data were not obtained due to persistent traces of
unidentified side products.
Boryl transfer was investigated initially by carrying out re-
actions between equimolar amounts of B
and a 4-substituted aryl bromide (4-BrC
temperature and analysing the reaction solution after 24 h by
GC-MS. For the arenes examined where X = H, CH and
CF
, the corresponding borylation products 4-{B(cat)}C
were observed in all cases. In addition, the bisborylated product
was observed in the reaction with ortho-
2
(cat)
2
, [Co(PMe
3
)
4
]
11
6
H
4
X) in THF at room
*
1
2
Crystal data for 4: C18
H
40BCoO
2
P
4
, M = 482.12, monoclinic, a =
3
◦
6.6032(12), b = 9.7012(7), c = 16.1590(12) A˚ , b = 104.3940(10) , V =
3
6
H X
4
3
−1
521.0(3) A˚ , T = 173(2) K, space group P2
/c, Z = 4, l = 0.945 mm
,
1
R
int = 0.0343 (for 15 951 measured reflections), R1 = 0.0294 [for 4639 uni-
1
,2-{B(cat)}
2
C
6
H
4
que reflections with I>2r(I)], wR2 = 0.0737 (for all 5793 unique reflections).
3
†
† To a stirred solution of B
2
(cat)
C≡CC
] (0.004 g, 0.011 mmol). The reaction mixture was
2
(0.061 g, 0.27 mmol) in THF (2.5 cm ),
dibromobenzene. Interestingly, the reaction between bromoben-
a sample of 4-CF
followed by [Co(PMe
then stirred at 80 C and samples were taken after 24 and 48 h for GC-MS
analysis. The samples were treated with excess pinacol before analysing by
GC-MS.
3
C
6
H
4
6 4 3
H -4-CF (0.080 g, 0.25 mmol) was added
zene and isolated 1 under similar conditions afforded PhB(cat),
3
)
4
◦
◦
whereas reaction with 4-iodotoluene at 80 C gave 4-tolylB(cat),
confirmed by single crystal X-ray diffraction.
In conclusion, these studies have shown that paramagnetic
Co(II) bisboryls can be formed from Co(0) precursors and
diborane(4) compounds, if somewhat unselectively and in low
isolated yields, but that Co(I), and possibly Co(III), species are
also formed. Preliminary reactivity studies have also demonstrated
that alkyne diboration takes place and that borylation of aryl
bromides, including dibromobenzene, is also possible albeit with
low selectivity and in modest yields. Further studies are in progress
to examine much of this chemistry in more detail.
‡
1
‡ Crystal data for 6: C28
H
24
B
2
F
6
O
4
, M = 560.09, monoclinic, a =
◦
˚
1.588(1), b = 13.554(1), c = 26.571(3) A, b = 101.58(1) , V = 4088.4(7)
A˚ , T = 120(2) K, space group P2/c, Z = 6, l = 0.116 mm , R
3
−1
=
int
0
.0467 (for 45 186 measured reflections), R1 = 0.0874 [for 6654 unique
reflections with I > 2r(I)], wR2 = 0.2090 (for all 9403 unique reflections).
The structure of 6 contains one molecule that has ordered B(pin) and
B(cat) groups and a second molecule which sits on a 2-fold axis and is thus
disordered with exact 1 : 1 disorder in the positions of the B(cat)/B(pin)
substituents. Both molecules also show disorder in their CF groups.
3
CCDC reference numbers 290418–290421. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b516594f
1
S. Aldridge and D. L. Coombs, Coord. Chem. Rev., 2004, 248, 535;
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Acknowledgements
We thank the EPSRC and the Hong Kong Research Grants
Council (HKUST6087/02P) for financial support and Frontier
Scientific, Inc., for generous gifts of diborane(4) compounds.
2
685; H. Wadepohl, Angew. Chem., Int. Ed. Engl., 1997, 36, 2441; M. R.
Smith III, Prog. Inorg. Chem., 1999, 48, 505.
2
T. B. Marder and N. C. Norman, Top. Catal., 1998, 5, 63; T. Ishiyama
and N. Miyaura, J. Organomet. Chem., 2000, 611, 392; T. Ishiyama and
N. Miyaura, Chem. Rec., 2004, 3, 271; T. Ishiyama and N. Miyaura,
J. Organomet. Chem., 2003, 680, 3; D. N. Coventry, A. S. Batsanov,
A. E. Goeta, J. A. K. Howard, T. B. Marder and R. N. Perutz, Chem.
Commun., 2005, 2172.
Notes and references
1
3b
†
Preparation of 2: A sample of B
2
(4-Mecat)
2
(0.070 g, 0.26 mmol) was
3
12
3 4
added to a solution of [Co(PMe ) ] (0.100 g, 0.28 mmol) in toluene (4 cm )
and the resulting mixture was stirred overnight at room temperature. After
removal of all volatiles in vacuo, the solid residues were extracted with
hexane and filtered. Slow evaporation of the solvent from the filtrate
afforded yellow crystals of 2 (0.03 g, 21%) one of which was suitable
3 G. Schmid and H. N o¨ th, Z. Naturforsch., Teil B, 1965, 20, 1008; G.
Schmid and H. N o¨ th, Chem. Ber., 1967, 100, 2899; G. Schmid, Chem.
Ber., 1969, 102, 191.
4 G. Schmid, W. Petz, W. Arloth and H. N o¨ th, Angew. Chem., Int. Ed.
Engl., 1967, 6, 696.
5 H. N o¨ th and G. Schmid, Allg. Prakt. Chem., 1966, 17, 610.
6 G. Schmid, P. Powell and H. N o¨ th, Chem. Ber., 1968, 101, 1205.
7 G. Schmid, Angew. Chem., Int. Ed. Engl., 1970, 9, 819.
8 J. D. Basil, A. A. Aradi, N. K. Bhattacharyya, N. P. Rath, C. Eigenbrot
and T. P. Fehlner, Inorg. Chem., 1990, 29, 1260.
9 D. J. Elliot, C. J. Levy, R. J. Puddephatt, D. G. Holah, A. N. Hughes,
V. R. Magnuson and I. M. Moser, Inorg. Chem., 1990, 29, 5015.
10 H. N o¨ th, H. Sch a¨ fer and G. Schmid, Angew. Chem., Int. Ed. Engl.,
1969, 8, 515; H. N o¨ th, H. Sch a¨ fer and G. Schmid, Z. Naturforsch., Teil
B, 1971, 26, 497.
11 C. Dai, G. Stringer, J. F. Corrigan, N. J. Taylor, T. B. Marder and N. C.
Norman, J. Organomet. Chem., 1996, 513, 273.
12 H. F. Klein, Angew. Chem., Int. Ed. Engl., 1971, 10, 343; H. F. Klein
and H. H. Karsch, Chem. Ber., 1975, 108, 944.
13 (a) R. J. Brotherton and W. G. Woods, US Pat., 3 009 941, 1961; (b) F. J.
Lawlor, N. C. Norman, N. L. Pickett, E. G. Robins, P. Nguyen, G.
Lesley, T. B. Marder, J. A. Ashmore and J. C. Green, Inorg. Chem.,
for X-ray crystallography. Preparation of 3: A sample of B
2
(cat)
2
(0.043 g,
1
2
0
0
.18 mmol) was added to a deep red solution of [Co(PMe
.18 mmol) in THF (3 cm ) and the resulting mixture was stirred at 60 C
2
Ph)
4
]
(0.110 g,
3
◦
for 1 h. After standing overnight at room temperature, a small quantity of
blue crystals precipitated which have not been identified. The remaining
red supernatant liquid was separated, all volatiles were removed in vacuo
and the solid residues extracted with hot hexane which afforded a small
quantity of orange crystals of 3 on cooling, one of which was suitable for
X-ray crystallography. Satisfactory analytical data were not obtained due
to the persistent presence of traces of unidentified side products in both
cases.
‡
1
9
Crystal data for 2: C23
H
39
B
2
CoO
4
P
3
, M = 553.00, triclinic, a =
2.3126(12), b = 16.2340(16), c = 16.3156(16) A˚ , a = 110.985(2), b =
◦
3
1.761(2), c = 108.279(2) , V = 2853.4(5) A˚ , T = 173(2) K, space
¯
−1
group P1, Z = 4, l = 0.795 mm , Rint = 0.0560 (for 30 455 measured
reflections), R1 = 0.0487 [for 6776 unique reflections with I > 2r(I)],
wR2 = 0.1206 (for all 12 934 unique reflections). Compound 2 contains two
crystallographically distinct molecules one of which shows disorder in the
position of the methyl substituent on the B(cat) ligand. Crystal data for 3:
1
372 | Dalton Trans., 2006, 1370–1373
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1
998, 37, 5282; (c) M. J. G. Lesley, N. C. Norman and C. R. Rice,
16 C. Dai, G. Stringer, T. B. Marder, A. J. Scott, W. Clegg and N. C.
Norman, Inorg. Chem., 1997, 36, 272.
17 H. F. Klein and H. H. Karsch, Chem. Ber., 1975, 108, 956.
18 R. Ll. Thomas, F. E. S. Souza and T. B. Marder, J. Chem. Soc., Dalton
Trans., 2001, 1650.
19 T. Ohmura, Y. Yamamoto and N. Miyaura, J. Am. Chem. Soc., 2000,
122, 4990.
Inorg. Synth., 2004, 34, 1; (d) C. N. Welch and S. G. Shore, Inorg.
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4 K. C. Lam, W. H. Lam, Z. Lin, T. B. Marder and N. C. Norman, Inorg.
Chem., 2004, 43, 2541; J. Zhu, Z. Lin and T. B. Marder, Inorg. Chem.,
2
1
1
005, 44, 9384.
5 The computational details are same as those described in ref. 14.
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1370–1373 | 1373