Dehydrocoupling reactions of secondary and primary amine-borane adducts catalyzed by half-sandwich carbonyl complexes, [CpMn(CO)3], [(η6-C6H6)Cr(CO)3], and [CpV(CO)4]

Article abstract of DOI:10.1246/cl.2011.171

Dehydrocoupling reactions of amineborane adducts catalyzed by half-sandwich carbonyl complexes are described. Secondary amine-borane adducts released H₂ with catalytic action of [CpMn(CO)₃] (Cp: η⁵-C₅H₅

Full text of DOI:10.1246/cl.2011.171

doi:10.1246/cl.2011.171
Published on the web January 15, 2011
171
Dehydrocoupling Reactions of Secondary and Primary Amine­Borane Adducts Catalyzed
6
by Half-sandwich Carbonyl Complexes, [CpMn(CO) ], [(© -C H )Cr(CO) ], and [CpV(CO) ]
3
6
6
3
4
³
Taeko Kakizawa, Yasuro Kawano,* Kohsuke Naganeyama, and Mamoru Shimoi
Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo 153-8902
(
Received November 15, 2010; CL-100953; E-mail: ckawano1@mail.ecc.u-tokyo.ac.jp)
Dehydrocoupling reactions of amine­borane adducts cata-
lyzed by half-sandwich carbonyl complexes are described.
Secondary amine­borane adducts released H2 with catalytic
R R
N
hν / –H2
H
H
H
1/2
B
B
5
6
3
BH ·NHR2
action of [CpMn(CO) ] (Cp: © -C H ), [(© -C H )Cr(CO) ],
H
3
5
5
6
6
3
cat.
N
and [CpV(CO)4] under photoirradiation to produce dimeric or
monomeric aminoboranes. These results were parallel to the
1a: R = Me
b: R = Et
c: R = 1/2 C H
d: R = Cy
R R
1
1
1
[M(CO)6]-catalyzed system (M = Cr, Mo, and W); however,
2a: R = Me
2b: R = Et
c: R = 1/2 C H
5
10
the reactions were considerably slower. Dehydrocoupling of
BH3¢NH2Me afforded an aminoborane polymer, [BH2NHMe]n.
2
5
10
R
H
B
N
Intense attention is currently focused on transition-metal-
catalyzed dehydrocoupling reactions of amine­borane adducts.
This reaction provides aminoboranes and borazines, which are
potential precursors of BN ceramics. Moreover, this type of
reaction has been developed toward utilization of amine­boranes
1
H
R
3
d: R = Cy
Scheme 1. Half-sandwich carbonyl-catalyzed dehydrocoupling reac-
tions of secondary amine­boranes. cat. = [CpMn(CO)3], [(© -C6H6)-
6
(
in particular ammonia­borane) as chemical hydrogen storage
Cr(CO)3], and [CpV(CO)4].
materials because of the large hydrogen content and its effective
2
release. To date, it has been reported that borane dehydrocou-
3
4
4b
5
6,7
7­9
pling can be catalyzed by Sc, Y, Ti, Zr, Re, Ru, Rh,
to this solution resulted in gentle gas evolution to produce a red-
1
0,11
12
13
1
11
Ir,
and Ni complexes.
Recently, we reported dehydrocoupling of secondary and
primary amine­borane adducts catalyzed by group 6 metal
orange solution. After 1 h of photolysis, the H and B NMR
spectra of the resulting mixture indicated 32% conversion of 1a.
The major product was dimethylaminoborane dimer, [BH2-
1
4
11
17
carbonyls, [M(CO) ] (M = Cr, Mo, and W). Non-bulky
NMe ] (2a, ¤ B = 5.0), and a small amount of the monomer
2 2
6
11
17c
secondary amine adducts, BH3¢NHR2 (R = Me, Et, 1/2 C4H8,
and 1/2 C5H10), undergo the catalytic dehydrogenation under
photoirradiation to afford dimeric aminoboranes [BH NR ] ,
BH2=NMe2 (3a, ¤ B = 37.7) was also detected. Additional
standing for 24 h at room temperature was required for the
completion of the reaction. The final yield of 2a reached 93%,
while 3a almost disappeared at the end of the reaction (Scheme 1
and Table 1, Entries 1 and 2). Thus, the manganese complex has
a catalytic ability on the dehydrocoupling of 1a; however, the
2
2 2
while the reaction of bulky derivatives (R = i-Pr and Cy; Cy:
cyclo-C6H11) provides monomeric BH2=NR2. Furthermore,
primary amine adducts, BH3¢NH2R (R = Me and Et), release
two equivalents of H through the action of the metal carbonyl
reaction was considerably slower than in the [M(CO) ]-catalyzed
2
6
catalyst to give borazine derivatives [BHNR]3.
DFT calculations predicted a crucial role of an intermediate
involving a three-center two-electron interaction between the
metal atom and BH3 moiety in the catalytic cycle. This
suggests that metal fragments capable of interacting with a BH3
moiety can catalyze borane dehydrocoupling reactions. Based
on such an idea, we examined dehydrogenation of secondary
system (M = Cr, Mo, and W). The hexacarbonyl catalysts
complete the hydrogen elimination of 1a within 1 h of photolysis
1
4
or 24 h of standing after 5 min irradiation.
After the reaction, minor products (<1%), BH(NMe2)2 and
1
4
11
(®-Me2N)B2H5 were detected along with 2a by B NMR
spectroscopy (28.5 and ¹18.0 ppm, respectively). These prod-
11
ucts were identified by comparison of their B NMR chemical
1
8,19
and primary amine­boranes using half-sandwich carbonyls
shift values with the literature.
In addition, a weak resonance
6
11
[
CpMn(CO) ] and [(© -C H )Cr(CO) ], which are known to
appeared at ¹22.2 ppm in the B NMR spectrum after the
reaction. We tentatively assigned this signal to a borane ·
complex, [CpMn(CO)2(© -BH3¢NHMe2)] (4), because it ap-
3
6
6
3
react with tertiary amine­boranes under photoirradiation to
1
6
1
yield · complexes, [CpMn(CO)2(© -BH3¢L)] and [(© -C6H6)-
1
15,16
Cr(CO)3(© -BH3¢L)] (L = NMe3 and N(CH2CH2)3CH).
also employed a structurally relevant vanadium complex,
CpV(CO)4] as a precatalyst. In fact, these trials resulted in
We
peared at 9 ppm higher field than free 1a. Such a high-field-
11
shifted B NMR signal has been observed for a structurally
1
[
characterized manganese­borane complex, [CpMn(CO)2(© -
1
5
occurrence of catalytic borane dehydrocoupling reactions. In this
paper, we describe the results of this study. We also discuss the
reaction mechanism on the basis of DFT calculations.
BH3¢NMe3)], and other amine­borane and phosphine­borane
2
0
complexes.
6
A benzene chromium complex [(© -C6H6)Cr(CO)3] also
catalyzed the dehydrogenation of 1a to afford 2a. After 1 h of
photolysis, the conversion of 1a was 73%. The reaction was thus
rather faster than the Mn-catalyzed case, but still slower than the
A benzene-d6 solution of BH3¢NHMe2 (1a) containing a
catalytic amount of [CpMn(CO) ] (5 mol %) was prepared and
3
loaded in a flame-sealed Pyrex NMR tube. Near-UV irradiation
Chem. Lett. 2011, 40, 171­173
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1
72
Table 1. Manganese-, chromium-, and vanadium-catalyzed dehydrocoupling reactions of scondary amine­borane adducts
b
Conversion
Product distribution /%
[BH NR ] (2) BH =NR (3)
Entry
Borane
Catalyst (5 mol %)
Conditiona
/%
2
2 2
2
2
1
2
3
4
5
6
7
8
9
0
BH3¢NHMe2 (1a)
[CpMn(CO)3]
h¯ 1 h
h¯ 1 h + RT 1 d
h¯ 1 h
32
100
73
94
96
0
73
79
81
87
0
100
95
91
79
92
75
5
87
93
75
96
92
®
73
92
90
96
®
68
69
58
93
35
69
®
4
trace
1
trace
trace
®
6
[(© -C6H6)Cr(CO)3]
h¯ 1 h + RT 1 d
c
FL 1 d
RT 3 h (dark)
h¯ 1 h
[CpV(CO)4]
3
h¯ 1 h + RT 1 d
trace
trace
trace
®
6
5
5
trace
5
trace
63
c
FL 3 h
c
1
FL 1 d
1
1
RT 3 h (dark)
12
13
14
15
16
17
18
BH3¢NHEt2 (1b)
BH3¢NH(CH2)5 (1c)
BH3¢NHCy2 (1d)
[CpMn(CO)3]
h¯ 1 h + RT 2 d
h¯ 5 m + RT 2 d
6
[(© -C6H6)Cr(CO)3]
c
[CpV(CO)4]
FL 3 h
[CpMn(CO)3]
h¯ 1 h + RT 2 d
h¯ 1 h
6
[(© -C H )Cr(CO) ]
6
6
3
c
[CpV(CO)4]
FL 3 h
6
[(© -C6H6)Cr(CO)3]
h¯ 1 h + RT 1 d
a
b
c
Photolyses were carried out at 8 °C using a 450 W medium pressure Hg lamp. The yields of the products were judged by NMR. Under
fluorescent light.
[
M(CO) ]-catalyzed reactions. After the irradiation, the solution
hν / –H2
cat.=
H
B
H
H
N
6
was allowed to stand for 24 h at room temperature in the dark to
complete the dehydrogenation. Interestingly, this Cr-catalyzed
reaction proceeds even under fluorescent light. As shown in
Table 1 (Entry 5), product 2a was obtained almost quantitatively
by fluorescent light-irradiation for 24 h. Note that no reaction
occurred if the sample was kept in the dark throughout,
indicating that the photoirradiation was required to generate
the active catalyst.
BH ·NH Me
1/n
3
2
n
1e
[CpMn(CO)3]
(C H )Cr(CO) ]
Me
[
6
6
3
5e
H
Me
/3
B
Me
H
hν / –2H2
N
N
B
1
cat.= [M(CO)6]
M = Cr, Mo, W
B
H
N
Furthermore, the dehydrocoupling of 1a was also catalyzed
by [CpV(CO)4]. Near-UV photolysis of a benzene-d6 solution of
Me
1a and 5 mol % of the vanadium complex led to H2 evolution
with concomitant color change from orange to deep green. The
conversion was 73% after 1 h of photolysis, and was 79% after
additional standing for 1 d at room temperature. Fluorescent
light-induced dehydrocoupling of 1a took place again even in
Scheme 2. Transition-metal carbonyl complex-catalyzed dehydro-
coupling reactions of methylamine­borane 1e.
under photolytic conditions. Notably, the product of this reaction
was an aminoborane polymer, [BH NHMe] (5e, Scheme 2).
this V-catalyzed system. Under fluorescent light, H vigorously
2
2
n
evolved, and the conversion reached 81% in 3 h. However, the
hydrogen release terminated with the conversion of 87% even
though the reaction time was prolonged. This is probably
because a coordinatively unsaturated vanadium species, a
possible active catalyst, dimerizes to produce a deep green
When [CpMn(CO)3] was employed as a catalyst, the solution
color changed from yellow to red-brown with gentle gas
evolution with progress of the reaction. After 12 h of photolysis,
polymer 5e precipitated out as a white viscous material.
6
Likewise, [(© -C6H6)Cr(CO)3] also catalyzed hydrogen elimi-
21
dinuclear complex, [Cp V (CO) ], losing the catalytic activity.
nation of 1e to provide a similar precipitate almost quantitatively
after 3 h of photolysis. Product 5e was insoluble in benzene, but
2
2
5
Table 1 summarizes the results of dehydrocoupling reac-
tions of several secondary amine­borane adducts. By the
catalytic action of the half-sandwich carbonyl complexes,
1
11
moderately soluble in chloroform. The H and B NMR spectral
data of the product in chloroform-d well accorded with the
11
10
diethylamine­borane 1b and piperidine­borane 1c release H
literature values reported by Manners (¤ B = 0­10, broad).
2
17c,22a
similarly to 1a to afford [BH2NR2]2 (2b: R = Et,
2c:
Polymer 5e formed a highly viscous gel-like material to frustrate
complete removal of residual solvent.
2
2
R = 1/2 C5H10) in moderate to good yields. The dehydrogen-
ation of bulky BH ¢NHCy (1d) was very slow, and monomeric
Formation of the polymeric product contrasts the [M(CO) ]-
3
2
6
2
2a
6
BH2=NCy2 (3d)
was produced in low yield only if [(© -
catalyzed dehydrocoupling reaction of 1e, in which the polymer-
ic borane is first generated, but it undergoes further dehydrogen-
C6H6)Cr(CO)3] was employed as a catalyst.
1
4
A primary amine­borane adduct BH ¢NH Me (1e) also
ation to ultimately yield trimethylborazine [BHNMe]3. The
lower activity of the half-sandwich carbonyls should prevent the
3
2
liberated H2 catalyzed by half-sandwich carbonyl complexes
Chem. Lett. 2011, 40, 171­173
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1
73
[
Mn(CO)Cp] + 1a
References and Notes
³
Present address: Department of Medicinal Chemistry, Graduate School
of Medical Science, Kyoto Prefectural University of Medicine, Kita-ku,
Kyoto 603-8334, Japan.
0
.0 (0.0)
1
a) T. J. Clark, K. Lee, I. Manners, Chem.®Eur. J. 2006, 12, 8634.
b) C. A. Jaska, I. Manners, in Inorganic Chemistry in Focus II, ed. by
G. Meyer, D. Naumann, L. Wesemann, Wiley-VCH, Weinheim, 2005,
pp. 53­64. c) C. W. Hamilton, R. T. Baker, A. Staubitz, I. Manners,
Chem. Soc. Rev. 2009, 38, 279.
Mn
Mn
Mn
H
N
H
OC
H
N
OC
OC
Me
Me
H
H
N
Me
Me
H
Me
Me
2
3
4
5
6
a) W. Grochala, P. P. Edwards, Chem. Rev. 2004, 104, 1283. b) F. H.
Stephens, V. Pons, R. T. Baker, Dalton Trans. 2007, 2613.
M. S. Hill, G. Kociok-Köhn, T. P. Robinson, Chem. Commun. 2010, 46,
B
B
B
H
H
H
H
H
H
7
587.
a) T. J. Clark, C. A. Russell, I. Manners, J. Am. Chem. Soc. 2006, 128,
582. b) D. Pun, E. Lobkovsky, P. J. Chirik, Chem. Commun. 2007, 3297.
Y. Jiang, O. Blacque, T. Fox, C. M. Frech, H. Berke, Organometallics
009, 28, 5493.
a) M. Käß, A. Friedrich, M. Drees, S. Schneider, Angew. Chem., Int. Ed.
6
TS[6–7]
3.8 (–10.2)
7
0
.5 (–12.7)
–15.2 (–29.2)
9
2
2
009, 48, 905. b) A. Friedrich, M. Drees, S. Schneider, Chem.®Eur. J.
Mn
H
H
Mn
H
Mn
H
Mn
2009, 15, 10339. c) N. Blaquiere, S. Diallo-Garcia, S. I. Gorelsky, D. A.
Black, K. Fagnou, J. Am. Chem. Soc. 2008, 130, 14034.
a) C. A. Jaska, K. Temple, A. J. Lough, I. Manners, J. Am. Chem. Soc.
2003, 125, 9424. b) C. A. Jaska, K. Temple, A. J. Lough, I. Manners,
Chem. Commun. 2001, 962.
M. E. Sloan, T. J. Clark, I. Manners, Inorg. Chem. 2009, 48, 2429.
a) Y. Chen, J. L. Fulton, J. C. Linehan, T. Autrey, J. Am. Chem. Soc.
2005, 127, 3254. b) J. L. Fulton, J. C. Linehan, T. Autrey, M.
Balasubramanian, Y. Chen, N. K. Szymczak, J. Am. Chem. Soc. 2007,
129, 11936.
OC
OC
OC
H
OC
H
H
H
9
7
8
TS[8–9]
H
H
B
Me
Me
N
+
3a
+ 3a
+ 3a
–10.6 (–11.1)
8
9
–
4.1 (–4.6)
7.9 (7.4)
TS[7–8]
.0 (–3.9)
8
[
Mn(CO)Cp]+ H + 3a
2
–
2.5 (4.2)
10 A. Staubitz, A. P. Soto, I. Manners, Angew. Chem., Int. Ed. 2008, 47,
212.
6
1
1
M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey, K. I. Goldberg,
J. Am. Chem. Soc. 2006, 128, 12048.
R. J. Keaton, J. M. Blacquiere, R. T. Baker, J. Am. Chem. Soc. 2007, 129,
Scheme 3. The reaction pathway of the Mn-catalyzed dehydrocou-
pling of 1a predicted by DFT calculations (MPW1K). Relative free
energies are given in kcal mol . Zero point-corrected electronic
energies are also given in parentheses.
1
1
2
3
¹1
1844.
It has been documented that Co nanoparticles can be an active catalyst on
the ammonia­borane dehydrocoupling. T. J. Clark, G. R. Whittell, I.
Manners, Inorg. Chem. 2007, 46, 7522. The Pd/C-catalyzed dehydro-
coupling of ammonia­borane has also been reported. S.-K. Kim, W.-S.
Han, T.-J. Kim, T.-Y. Kim, S. W. Nam, M. Mitoraj, L. Piekoś, A.
Michalak, S.-J. Hwang, S. O. Kang, J. Am. Chem. Soc. 2010, 132, 9954.
Y. Kawano, M. Uruichi, M. Shimoi, S. Taki, T. Kawaguchi, T. Kakizawa,
H. Ogino, J. Am. Chem. Soc. 2009, 131, 14946.
borazine formation. This makes possible selective preparation of
either a borazine derivative or an aminoborane polymer by the
proper choice of a catalyst ([M(CO)6] or half-sandwich carbon-
yls).
1
4
5
The reaction mechanism of the Mn-catalyzed dehydrocou-
1
T. Kakizawa, Y. Kawano, M. Shimoi, Organometallics 2001, 20, 3211.
pling of 1a was studied by DFT calculations with use of the
6
MPW1K functional (Scheme 3).2
3,24
16 Photolysis of [(© -C6H6)Cr(CO)3] in the presence of BH3¢L (L = NMe3,
N(C2H4)3CH, and PMe3) produces the corresponding borane complex,
Presumably, the active
catalyst of this reaction is a 14 electron species, [Mn(CO)Cp].
Borane 1a coordinates to the manganese center through the BH
and NH hydrogen atoms to form a chelate intermediate 6. This
complex then undergoes NH activation via a transition state
TS[6­7], generating an amido(borane) species 7. This step is
followed by BH activation (through TS[7­8]) to release 3a. The
6
1
[
(© -C6H6)Cr(CO)2(© -BH3¢L)]. T. Kakizawa, Y. Kawano, M. Shimoi,
unpublished results.
1
7
a) A. B. Burg, C. L. Randolph, Jr., J. Am. Chem. Soc. 1949, 71, 3451.
b) W. Haubold, R. Schaeffer, Chem. Ber. 1971, 104, 513. c) H. Nöth,
H. Vahrenkamp, Chem. Ber. 1967, 100, 3353.
18 a) A. B. Burg, C. L. Randolph, Jr., J. Am. Chem. Soc. 1951, 73, 953.
b) E. C. Ashby, R. A. Kovar, R. Culbertson, Inorg. Chem. 1971, 10, 900.
1
9
W. D. Phillips, H. C. Miller, E. L. Muetterties, J. Am. Chem. Soc. 1959,
1, 4496.
resulting dihydride 8 liberates H through a dihydrogen adduct 9,
2
8
regenerating the active catalyst, while liberated aminoborane
dimerizes to produce 2a. The BH activation step has the highest
2
0
a) M. Shimoi, S. Nagai, M. Ichikawa, Y. Kawano, K. Katoh, M. Uruichi,
H. Ogino, J. Am. Chem. Soc. 1999, 121, 11704. b) Y. Kawano, K.
Yamaguchi, S. Miyake, T. Kakizawa, M. Shimoi, Chem.®Eur. J. 2007,
¹
1
barrier, 23.2 kcal mol (in free energy), in the reaction sequence.
This value is significantly larger than the activation energy for
13, 6920. c) T. Yasue, Y. Kawano, M. Shimoi, Angew. Chem., Int. Ed.
2003, 42, 1727. d) Y. Kawano, M. Hashiva, M. Shimoi, Organometallics
2006, 25, 4420.
‡
the [Cr(CO)6]-catalyzed dehydrocoupling of 1a (¦G = 13.3
¹1 14
kcal mol ). This well accounts for the lower activity of the
manganese catalyst. The active species [Mn(CO)Cp] can be
generated via disproportionation of [Mn(CO)2Cp], which is
formed by the photolysis of [CpMn(CO)3] or borane dissociation
from the · complex 4 (See Supporting Information).25 Another
reaction pathway, concerted H2 elimination that occurs on 16
electron [Mn(CO)2Cp], was shown to have an even higher
activation barrier.
21 W. A. Herrmann, J. Plank, Chem. Ber. 1979, 112, 392.
2
2
a) L. Pasumansky, D. Haddenham, J. W. Clary, G. B. Fisher, C. T.
Goralski, B. Singaram, J. Org. Chem. 2008, 73, 1898. b) A. Storr, B. S.
Thomas, A. D. Penland, J. Chem. Soc., Dalton Trans. 1972, 326. c) H.
Nöth, B. Wrackmeyer, Chem. Ber. 1974, 107, 3070.
23 a) B. J. Lynch, P. L. Fast, M. Harris, D. G. Truhlar, J. Phys. Chem. A
2000, 104, 4811. b) B. J. Lynch, D. G. Truhlar, J. Phys. Chem. A 2001,
105, 2936.
2
4
5
Basis sets. Geometry optimization and frequency analysis: SDD (Mn), 6-
1++G(d,p) (B, N, BH and NH hydrogen atoms), 6-31G(d) (C, H, O);
3
Single point energy (SCRF calculation, CPCM model (in benzene)):
SDD (Mn), aug-cc-pVDZ (other atoms).
Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
This work was financially supported by Grant-in-Aid for
Scientific Research (No. 21550056) from Ministry of Education,
Culture, Sports, Science and Technology, Japan.
2
Chem. Lett. 2011, 40, 171­173
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/