Dehydrocoupling reactions of borane-secondary and -primary amine adducts catalyzed by group-6 carbonyl complexes: Formation of aminoboranes and borazines

Article abstract of DOI:10.1021/ja904918u

Photoirradiation of a solution of BH₃·NHR₂ (1a: R = Me, 1b: R = 1/2C₄H₈, 1c: R = 1/2C ₅H₁₀, 1f: R = Et) containing a catalytic amount of a group-6 metal carbonyl complex, [M(CO)₆] (M = Cr, Mo, W), led to dehydrogenative B-N covalent bond formation to produce aminoborane dimers, [BH₂NR₂]₂ (2a-c, f), in high yield. During these reactions a borane σ complex, [M(CO)₅(η¹- BH₃·NHR₂)] (3), was detected by NMR spectroscopy. Similar catalytic dehydrogenation of bulkier amineboranes, BH ₃·NHⁱPr₂ (1d) and BH₃· NHCy₂ (1e, Cy = cyclo-C₆H₁₁), afforded monomeric products BH₂=NR₂ (4d, e). The reaction mechanism of the dehydrocoupling was investigated by DFT calculations. On the basis of the computational study, we propose that the catalytic dehydrogenation reactions proceed via an intramolecular pathway and that the active catalyst is [Cr(CO)₄]. The reaction follows a stepwise mechanism involving NH and BH activation. Dehydrocoupling of borane-primary amine adducts BH ₃·NH₂R (1g: R = Me, 1h: R = Et, 1i: R = ^tBu) gave borazine derivatives [BHNR]₃ (5g-i).

Full text of DOI:10.1021/ja904918u

Published on Web 09/22/2009
Dehydrocoupling Reactions of Borane-Secondary and
-Primary Amine Adducts Catalyzed by Group-6 Carbonyl
Complexes: Formation of Aminoboranes and Borazines
Yasuro Kawano,*,† Mikio Uruichi,‡,⊥ Mamoru Shimoi,*,† Seitaro Taki,†
Takayuki Kawaguchi,† Taeko Kakizawa,†,¶ and Hiroshi Ogino‡,§
Department of Basic Science, Graduate School of Arts and Sciences, UniVersity of Tokyo,
Meguro-ku, Tokyo 153-8902, Japan, Department of Chemistry, Graduate School of Science,
Tohoku UniVersity, Sendai 980-8578, Japan, and The Open UniVersity of Japan, Wakaba 2-11,
Mihama-ku, Chiba 261-8586, Japan
Received June 16, 2009; E-mail: ckawano1@mail.ecc.u-tokyo.ac.jp; cshimoi@mail.ecc.u-tokyo.ac.jp
Abstract: Photoirradiation of a solution of BH3 ·NHR2 (1a: R ) Me, 1b: R ) 1/2C4H8, 1c: R ) 1/2C5H10,
1f: R ) Et) containing a catalytic amount of a group-6 metal carbonyl complex, [M(CO)6] (M ) Cr, Mo, W),
led to dehydrogenative B-N covalent bond formation to produce aminoborane dimers, [BH2NR2]2 (2a-c,
f), in high yield. During these reactions a borane σ complex, [M(CO)5(η1-BH3 ·NHR2)] (3), was detected by
NMR spectroscopy. Similar catalytic dehydrogenation of bulkier amineboranes, BH3 ·NHiPr2 (1d) and
BH3 ·NHCy2 (1e, Cy ) cyclo-C6H11), afforded monomeric products BH2dNR2 (4d, e). The reaction
mechanism of the dehydrocoupling was investigated by DFT calculations. On the basis of the computational
study, we propose that the catalytic dehydrogenation reactions proceed via an intramolecular pathway
and that the active catalyst is [Cr(CO)4]. The reaction follows a stepwise mechanism involving NH and BH
activation. Dehydrocoupling of borane-primary amine adducts BH3 ·NH2R (1g: R ) Me, 1h: R ) Et, 1i: R
t
) Bu) gave borazine derivatives [BHNR]3 (5g-i).
Introduction
of amineboranes and phosphineboranes to produce aminobo-
rane and phosphinoborane oligomers and polymers.4,5
Transition metal-catalyzed dehydrocoupling reactions of
p-block element hydrides attract much attention for both
academic and practical reasons, and provide efficient synthetic
routes to new inorganic oligomers and polymers.1 This
methodology was first applied to the synthesis of polysilane
polymers,2 and to date, has been extended to oligomers and
polymers containing various p-block elements in the back-
bone.3 In the past few years, Manners and co-workers actively
studied Rh-, Ru-, and Ti-catalyzed dehydrocoupling reactions
Furthermore, this research area is currently expanding toward
the utilization of amineboranes as hydrogen storage materials.6-12
Since simple amineboranes (in particular BH3 ·NH3) have a large
hydrogen content and release H2 through the dehydrocoupling
reaction, they are promising candidates for hydrogen storage
materials. Thus, investigation of catalytic dehydrocoupling
(4) Jaska, C. A.; Manners, I. In Inorganic Chemistry in Focus II; Meyer,
G., Naumann, D., Wesemann, L., Eds.; Wiley-VCH: Weinheim, 2005;
pp 53-64.
(5) Staubitz, A.; Soto, A. P.; Manners, I. Angew. Chem., Int. Ed. 2008,
47, 6212.
(6) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg,
K. I. J. Am. Chem. Soc. 2006, 128, 12048.
† University of Tokyo.
‡ Tohoku University.
(7) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007,
129, 1844.
§ The Open University of Japan.
⊥ Present address: Institute of Molecular Science, 38 Nishigo-Naka,
Myodaiji, Okazaki 444-8585, Japan.
(8) Clark, T. J.; Whittell, G. R.; Manners, I. Inorg. Chem. 2007, 46, 7522.
(9) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, S. I.; Black, D. A.; Fagnou,
K. J. Am. Chem. Soc. 2008, 130, 14034.
¶ Present address: Laboratory of Proteomic Sciences, 21st Century COE
Program, Kyoto Pharmaceutical University, Yamashina-ku, Kyoto 607-8412,
Japan.
(10) Ka¨ss, M.; Friedrich, A.; Drees, M.; Schneider, S. Angew. Chem., Int.
Ed. 2009, 48, 905.
(1) (a) Gauvin, F.; Harrod, J. F.; Woo, H. G. AdV. Organomet. Chem.
1998, 42, 363. (b) Clark, T. J.; Lee, K.; Manners, I. Chem.sEur. J.
2006, 12, 8634. (c) Hamilton, C. W.; Baker, R. T.; Staubitzc, A.;
Manners, I. Chem. Soc. ReV. 2009, 38, 279.
(11) Reviews of chemical hydrogen storage: (a) Grochala, W.; Peter, P.;
Edwards, P. P. Chem. ReV 2004, 104, 1283. (b) Marder, T. B. Angew.
Chem., Int. Ed. 2007, 46, 8116.
(12) Thermolytic dehydrogenation of BH3 ·NH3 in ionic liquids: (a) Bluhm,
M. E.; Bradley, M. G.; Butterrick, R., III.; Kusari, U.; Sneddon, L. G.
J. Am. Chem. Soc. 2006, 128, 7748. Hydrogen release from BH3 ·NH3
through the action of acids: (b) Stephens, F. H.; Baker, R. T.; Matus,
M. H.; Grant, D. J.; Dixon, D. A. Angew. Chem., Int. Ed. 2007, 46,
746. Dehydrogenation of BH3 ·NH3 on mesoporous materials: (c)
Gutowska, A.; Li, L.; Shin, Y.; Wang, C. M.; Li, X. S.; Linehan,
J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw, W.; Gutowski, M.;
Autrey, T. Angew. Chem., Int. Ed. 2005, 44, 3578.
(2) Corey, J. Y. AdV. Organomet. Chem. 2004, 51, 1.
(3) For example: (a) Aitken, C.; Harrod, J. F.; Malek, A.; Samuel, E. J.
Organomet. Chem. 1988, 349, 285. (b) Choi, N.; Tanaka, M. J.
Organomet. Chem. 1998, 564, 81. (c) Neale, N. R.; Tilley, T. D. J. Am.
Chem. Soc. 2005, 127, 14745. (d) Fermin, M. C.; Stephan, D. W.
J. Am. Chem. Soc. 1995, 117, 12645. (e) Bo¨hm, V. P. W.; Brookhart,
M. Angew. Chem., Int. Ed. 2001, 40, 4694. (f) Waterman, R.; Tilley,
T. D. Angew. Chem., Int. Ed. 2006, 45, 2926.
9
14946 J. AM. CHEM. SOC. 2009, 131, 14946–14957
10.1021/ja904918u CCC: $40.75  2009 American Chemical Society

Catalytic Dehydrocoupling of Borane Adducts
A R T I C L E S
Chart 1
bidentate complex, but resulted in dehydrocoupling (formation
of a B-N covalent bond) of the amineborane to produce a cyclic
aminoborane dimer, [BH2NMe2]2 (2a). During the reaction, a
borane σ complex [Cr(CO)5(η1-BH3 ·NHMe2)] (3a) was de-
tected. We then found that the dehydrocoupling proceeded even
with use of a catalytic amount of [M(CO)6] and applied this
reaction to various borane-amine adducts.
In this paper, we report details of the dehydrocoupling
reactions of borane-amine adducts, including a mechanistic
study conducted by DFT calculations. This catalytic reaction is
important as a synthetic route to aminoboranes and borazines,
which can be utilized as precursors of BN ceramics,18 because
forcing conditions are not required here. A part of this work
has been published already.19 Formation of 3a and its decom-
position leading to 2a have also been reported by Heinekey and
co-workers.20 Other amineborane derivatives are mentioned in
their report.
reactions has influenced the development of new energy systems
such as fuel cells.
We previously reported the synthesis and properties of a series
of borane σ complexes, [M(CO)5(η1-BH3 ·L)] (M ) Cr, Mo,
W; L ) NMe3, PMe3, PPh3).13 In these compounds, the borane
ligand coordinates to the central metal through a B-H-M three-
center two-electron bond and is quite labile. The syntheses of
these compounds were accomplished by the photolysis of
[M(CO)6] in the presence of neutral borane adducts BH3 · L,
where L is a tertiary amine or phosphine (eq 1).
Results and Discussion
Dehydrocoupling of Borane-Secondary Amine Adducts
Catalyzed by [M(CO)6]. A THF solution of BH3 ·NHMe2 (1a)
and a catalytic amount (0.6 mol %) of [Cr(CO)6] was photolyzed
using a 450 W medium pressure Hg lamp for 1 h, after which
time the solution was stirred in the dark for 1 day. During the
photolysis, a gas was vigorously evolved, and a pale yellow
solution was produced. The gas evolution continued even after
the irradiation was terminated. However, it ceased after a day
of the reaction, and the yellow color disappeared to leave a pale
green suspension. From the resulting mixture, a cyclic ami-
noborane dimer [BH2NMe2]2 (2a)21 was isolated by sublimation
in 69% yield (eq 2). Because of the formation of the
boron-nitrogen covalent bond, this reaction is described as a
chromium-catalyzed dehydrogenative B-N coupling of 1a.
Interestingly, prolonged photoirradiation did not lead to
further dissociation of CO followed by the formation of bidentate
derivatives, [M(CO)4(η2-BH3 · L)]. This result was somewhat
surprising because the closely related borohydrides, [M(CO)4(η2-
BH4)]-, included a bidentate BH4 ligand.14-16 It is also in a
-
contrast to the behavior of monodentate diborane(4) complexes,
[M(CO)5(η1-B2H4 ·2PMe3)], which are converted to the bidentate
derivatives [M(CO)4(η2-B2H4 ·2PMe3)] through dissociation of
a carbonyl ligand induced by photoirradiation or heating.17 For
such a difference between the coordination modes of BH3 ·L
-
and BH4 (or B2H4 ·2PMe3), there are two possible rationales:
(1) Owing to the lability of the borane-metal linkage, further
irradiation of the complex predominantly causes borane dis-
sociation rather than CO expulsion.
(2) Steric interaction between the base on boron and a
carbonyl ligand prevents the formation of the bidentate species.
Indeed, in the hypothetical bidentate complex [Cr(CO)4(η2-
BH3 ·NMe3)], nonbonding distances between the carbonyl
oxygen and methyl carbon atoms were calculated to be shorter
than the sum of the van der Waals radii of oxygen and a methyl
group (Chart 1).
If the latter rationale is correct, the use of sterically less
demanding boranes might produce the bidentate species.
On the basis of such a hypothesis, we attempted the
photoreaction of [Cr(CO)6] with borane-dimethylamine adduct,
BH3 ·NHMe2 (1a). However, this trial did not afford the
Table 1 summarizes the results of photolysis of various
borane-secondary amine adducts in the presence of catalytic
amounts of [M(CO)6]. Pyrrolidineborane (1b) and piperidinebo-
rane (1c) underwent dehydrogenation like 1a to give
[BH2N(CH2)4]2 (2b) and [BH2N(CH2)5]2 (2c),22 respectively, in
high yields. At the early stage of these reactions, borane σ
complexes [Cr(CO)5(η1-BH3 ·NHR2)] (3a: R ) Me, 3b: R )
1/2C4H8, 3c: R ) 1/2C5H10) were detected by 1H and 11B NMR
spectroscopy. Details of compound 3 will be described in a later
(18) For example, Toury, B.; Miele, P.; Cornu, D.; Vincent, H.; Bouix, J.
AdV. Funct. Mater 2002, 12, 228.
(13) Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.; Katoh, K.; Uruichi,
M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704.
(14) Kirtley, S. W.; Andrews, M. A.; Bau, R.; Grynkewich, G. W.; Marks,
T. J.; Tipton, D. L.; Whittlesey, B. R. J. Am. Chem. Soc. 1977, 99,
7154.
(19) Shimoi, M.; Katoh, K.; Uruichi, M.; Nagai, S.; Ogino, H. Current
Topics in the Chemistry of Boron; Kabalka, G. W., Ed.; The Royal
Society of Chemistry: London; 1994, pp 293-296.
(20) Pons, V.; Denney, M. C.; Goldberg, K. I.; Heinekey, D. M. Prepr.
Symp. Am. Chem. Soc. DiV. Fuel Chem. 2005, 50, 51.
(15) Darensbourg, M. Y.; Bau, R.; Marks, M. W.; Burch, R. R., Jr.; Deaton,
J. C.; Slater, S. J. Am. Chem. Soc. 1982, 104, 6961.
(16) Wei, C.-Y.; Marks, M. W.; Bau, R.; Kirtley, S. W.; Bisson, D. E.;
Henderson, M. E.; Koetzle, T. F. Inorg. Chem. 1982, 21, 2556.
(17) (a) Katoh, K.; Shimoi, M.; Ogino, H. Inorg. Chem. 1992, 31, 670. (b)
Shimoi, M.; Katoh, K.; Ogino, H. J. Chem. Soc., Chem. Commun.
1990, 811.
(21) (a) Burg, A. B.; Randolph, C. L., Jr. J. Am. Chem. Soc. 1949, 71,
3451. (b) Haubold, W.; Schaeffer, R. Chem. Ber. 1971, 104, 513. (c)
No¨th, H.; Vahrenkamp, H. Chem. Ber. 1967, 100, 3353.
(22) (a) Storr, A.; Thomas, B. S.; Penland, A. D. J. Chem. Soc., Dalton
Trans. 1972, 326. (b) No¨th, H.; Wrackmeyer, B. Chem. Ber. 1974,
107, 3070. (c) Pasumansky, L.; Haddenham, D.; Clary, J. W.; Fisher,
G. B.; Goralski, C. T.; Singaram, B. J. Org. Chem. 2008, 73, 1898.
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 14947

A R T I C L E S
Kawano et al.
Table 1. Group-6 Metal-Catalyzed Dehydrogenation Reaction of Borane-Secondary Amine Adducts
yield of productsb/%
borane
catalyst(5 mol %)
conditiona
conversion/ % [BH2NR2]2(2) BH2dNR2(4) [BH2NR2]3 (6) BH(NR2)2 (7) B2H5(µ-NR2)(8)
BH3 ·NHMe2 (1a)
[Cr(CO)6]
[Cr(CO)6]
[Mo(CO)6]
[W(CO)6]
hν 1 h
95
100
90
84
97
97
98
100
71
100
24
90
95
94
95
94
93
97
93
88
98
-
-
-
53
5
57
0.2
0.2
0.7
2
0.1
0.2
-
2.5
8
0.3
2
0.3
2
0.6
2
2
-
2
-
-
-
-
-
-
-
-
2
1
1
0.4
0.1
0.1
0.8
2
2
1
6
11
12
11
17
14
6
3
2
1
3
3
-
-
-
-
-
-
10
-
-
9
hν 5 min + RT 1 dc
hν 1 h
hν 1 h
[Cr(CO)5(thf)]
RT 90 mind
[Cr(CO)5(η1-BH3 ·NMe3)] RT 60 mind
BH3 ·NH(CH2)4 (1b) [Cr(CO)6]
BH3 ·NH(CH2)5 (1c) [Cr(CO)6]
[W(CO)6]
hν 5 min + RT 1 dc
hν 1 h
hν 1 h
[W(CO)6]
hν 1 h + RT 1 dc
hν 1 h
BH3 ·NHiPr2 (1d)
[Cr(CO)6]
[W(CO)6]
[Cr(CO)6]
[Cr(CO)6]
[W(CO)6]
[W(CO)6]
71
hν 1 h
12
61
87
68
79
78
17
72
14
BH3 ·NHCy2 (1e)
BH3 ·NHEt2 (1f)
hν 8 h
hν 1 h
hν 1 h
hν 1 h + RT 1 dc
100
a Photolyses were carried out at 8 °C using a 450 W medium pressure Hg lamp. b The yield to the conversion, judged by NMR. c After the
photolysis, the sample was allowed to stand in the dark at RT. d In the dark.
section. Some minor products [BH2NR2]3 (6),23 BH(NR2)2 (7),24
and B2H5(µ-NR2) (8)25 were also detected after the reaction.
Dehydrogenation of 1a and 1c took place even when [W(CO)6]
was employed as a catalyst; however, the tungsten-catalyzed
reaction was slower than the chromium-mediated cases. Mo-
lybdenum carbonyl also catalyzed the dehydrocoupling of 1a.
It should be noted that the dehydrocoupling proceeds through
short-period photochemical initiation and subsequent thermal
reaction in the dark. In addition, the dehydrocoupling of 1a was
catalyzed even by [Cr(CO)5(thf)] and [Cr(CO)5(η1-BH3 ·NMe3)].
These observations indicate that [Cr(CO)5] acts as the active
catalyst or its precursor and that the dehydrocoupling proceeds
thermally after the active species is generated in the reaction
system.
was observed after 1 h of photolysis as the main component.
However, when the reaction mixture was allowed to stand for
1 day, 4f decreased dramatically and the dimer 2f alternatively
became the major product. Thus, in this case, the initial
formation of the monomeric product and its subsequent dimer-
ization were apparently observed.
Metal-Catalyzed Dehydrocoupling of Borane-Primary Amine
Adducts. Group-6 metal-catalyzed dehydrocoupling can also be
applied to borane-primary amine adducts, BH3 ·NH2Me (1g)
and BH3 ·NH2Et (1h). In these cases, however, further dehy-
drogenation proceeded to yield borazine derivatives 5g and 5h
in good yields (eq 4).23c,d,28 When the reaction of 1g was
monitored in benzene-d6, it was apparent that the precursor was
consumed within 1 h of photolysis as evidenced by 11B NMR.
At this stage, broad resonances appeared at 0 to -10 ppm in
the 11B NMR spectrum of the reaction mixture, and a consider-
able amount of a white viscous material precipitated. This is
assignable to intermediates in the borazine formation process,
a mixture of aminoborane oligomers [BH2NHMe]n. Further
irradiation and subsequent dark reaction at RT led to the
disappearance of the precipitate and formation of borazine 5g
in good yield. The ethyl derivative 1h dehydrocoupled similarly
to yield 5h. Unlike the methyl derivative, the intermediate
[BH2NHEt]n was considerably soluble in benzene-d6, and
showed intense signals at -5 to -10 ppm in the 11B NMR
spectrum. Thus, 1g and 1h undergo catalytic dehydrogenation
to produce oligomeric aminoboranes, which are then converted
to borazines 5. Recently, Ir-catalyzed formation of polymeric
aminoboranes, [BH2NHR]n (R ) H, Me), was reported by the
Manners and Heinekey groups.5,29
Bulky amineboranes, BH3 ·NHiPr2 (1d) and BH3 ·NHCy2 (1e),
showed different reactivity. Their dehydrogenation was much
slower than that of a-c. In the chromium-catalyzed reaction,
the conversion of 1d was 24% after 1 h of photolysis. More
importantly, the major products were monomeric aminoboranes
22c
i
BH2dNR2 (4d: R ) Pr, 4e: R ) Cy), and no formation of
the cyclic dimer was observed (eq 3). These results are parallel
to Manners’ Rh and Ru-catalyzed systems.26,27
The behavior of BH3 ·NHEt2 (1f) is of interest. In the
tungsten-catalyzed reaction, monomeric BH2dNEt2 (4f)21c,22c
(23) (a) Narula, C. K.; Janik, J. F.; Duesler, E. N.; Paine, R. T.; Schaeffer,
R. Inorg. Chem. 1986, 25, 3346. (b) Campbell, G. W.; Johnson, L.
J. Am. Chem. Soc. 1959, 81, 3800. (c) Bissot, T. C.; Parry, R. W.
J. Am. Chem. Soc. 1955, 77, 3481. (d) No¨th, H.; Wrackmeyer, B.
Chem. Ber. 1981, 114, 1150.
(24) (a) Burg, A. B.; Randolph, C. L., Jr. J. Am. Chem. Soc. 1951, 73,
953. (b) Ashby, E. C.; Kovar, R. A.; Culbertson, R. Inorg. Chem.
1971, 10, 900.
Table 2 summarizes the results of dehydrogenation of various
primary amine adducts. The formation of 5g was also catalyzed
(25) Phillips, W. D.; Miller, H. C.; Muetterties, E. L. J. Am. Chem. Soc.
1959, 81, 4496.
(26) (a) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem. Commun.
2001, 962. (b) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I.
J. Am. Chem. Soc. 2003, 125, 9424.
(28) (a) Hough, W. V.; Schaeffer, G. W.; Dzurus, M.; Stewart, A. C. J. Am.
Chem. Soc. 1955, 77, 864. (b) Ashby, E. C.; Kovar, R. A. Inorg. Chem.
1971, 10, 1524.
(27) Sloan, M. E.; Clark, T. J.; Manners, I. Inorg. Chem. 2009, 48, 2429.
9
14948 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

Catalytic Dehydrocoupling of Borane Adducts
A R T I C L E S
Table 2. Metal-Catalyzed Dehydrogenation Reaction of Borane-Primary Amine Adducts
yield of productsb/%
borane
catalyst (5 mol %)
conditiona
conversion/ %
[BHNR]3(5)
[BH2NHR]n
BH2dNHR (4)
BH(NHR)2 (7)
B2H5(µ-NHR) (8)
BH3 ·NH2Me(1g)
[Cr(CO)6]
hν 1 h
98
99
50
99
54
98
80
100
38
99
40
<30
80
<10
70
trace
35
22
75
2
>30
10
>40
10
>40
35
55
-
-
-
-
-
-
0.2
-
5
trace
trace
trace
trace
-
trace
trace
-
12
3
20
5
5
9
15
2
68
55
68
hν 8 h + RT 1 dc
hν 1 h
[Mo(CO)6]
[W(CO)6]
[Cr(CO)6]
hν 8 h + RT 1 dc
hν 1 h
hν 8 h + RT 1 dc
hν 1 h
BH3 ·NH2Et (1h)
hν 8 h + RT 1 dc
hν 8 h + RT 1 dc
hν 8 h + RT 1 dc
hν 8 h + RT 1 dc
20
[W(CO)6]
[Cr(CO)6]
[W(CO)6]
20
10
18
t
BH3 ·NH2 Bu (1i)
19
-
5
a Photolyses were carried out at 8 °C using a 450 W medium pressure Hg lamp. b The yield to the conversion, judged by NMR. c After the
photolysis, the sample was allowed to stand in the dark at RT.
by [Mo(CO)6] and [W(CO)6]. Bulky [BHNtBu]3 (5i)28b was
afforded in low yield (typically 19%), and significant amounts
of BH(NHtBu)2 (7i) and B2H5(µ-NHtBu) (8i, 55%)30 were
produced in the chromium-catalyzed dehydrogenation of 1i. The
tungsten-catalyzed reaction of 1i was very slow. When an
equimolar amount of the carbonyl complex was used, intense
signals assignable to
BH3 ·NH2 Bu)], appeared during the photoreaction, indicating
rather high stability of this species.
a
borane complex, [W(CO)5(η1-
t
Stoichiometric Reaction between 1a and [Cr(CO)6]. To know
what occurs during the dehydrogenation process, a stoichio-
metric reaction of 1a with [Cr(CO)6] was carried out and was
carefully monitored by NMR spectroscopy. The 11B NMR
spectral change is displayed in Figure 1. After 1 min of
photolysis, a significant amount of 1a was consumed, and
compound 2a (δ 11B ) 5.0 ppm) formed. Monomeric
BH2dNMe2 (4a, 37.5 ppm)21c and a small amount of trimeric
[BH2NMe2]3 (6a, 1.8 ppm)23 were also observed. At this time,
1
the H NMR spectrum (Figure 1, inset) exhibited a new broad
hump at -3.3 ppm. This is assigned to the BH protons of the
borane σ complex, [Cr(CO)5(η1-BH3 ·NHMe2)] (3a), in which
dimethylamineborane is coordinated to the chromium atom
through a B-H-Cr single bridge. The hump is characteristic
for the BH hydrogen atoms of borane complexes [M(CO)5(η1-
BH3 ·L)], in which the coordinated and terminal BH hydrogen
atoms rapidly exchange.13,31 The chemical shift value, -3.3
ppm, corresponds to the weighted average of those of the BH
protons. In 11B NMR, 3a resonates at higher field by 3.6 ppm
(-16.8 ppm) relative to free 1a. Such high field shifts of the
11B NMR signal have also been encountered in the formation
of other borane complexes, [M(CO)5(η1-BH3 ·L)],13 [CpMn-
(CO)2(η1-BH3 · L)],32 [Mn(CO)4(PR3)(η1-BH3 · PMe3)]+,33 and
[CpRu(PMe3)2(η1-BH3 · L)]+ (L ) tertiary phosphine and
amine).34 Compound 3a decomposed so rapidly, liberating the
aminoborane and H2, that it could not be isolated. However, its
structure was computationally deduced by geometry optimiza-
tion based on density functional theory (DFT, see later).
After 5 min of photolysis, the signal of 4a decreased in
Figure 1. 11B NMR (inset: 1H NMR) spectral change during the
dehydrocoupling reaction of 1a catalyzed by [Cr(CO)6]. 1a: BH3 ·NHMe2,
2a: [BH2NMe2]2, 3a: [Cr(CO)5(η1-BH3 ·NHMe2)], 4a: BH2dNMe2, 6a:
[BH2NMe2]3, 7a: BH(NMe2)2, 8a: B2H5(µ-NMe2), 9a: [Cr(CO)5(η1-
BH2NMe2BH2NMe2)].
new boron-coupled broad quartet appeared at -4.79 ppm. This
is assignable to the BH signal of [Cr(CO)5(η1-
BH2NMe2BH2NMe2)] (9a), which should be formed through
complexation between [Cr(CO)5] and 2a generated in the
reaction system. Formation of 9a was confirmed by a separate
experiment: when [Cr(CO)6] was photolyzed in the presence
1
intensity while that of 2a grew. In the H NMR spectrum, a
of 2a in C6D6, 9a was observed by NMR as expected. The 11
B
(29) Dietrich, B. L.; Goldberg, K. I.; Heinekey, D. M.; Autrey, T.; Linehan,
J. C. Inorg. Chem. 2008, 47, 8583.
NMR spectrum of this solution exhibits two triplet resonances
at 4.7 ppm, close to free 2a (5.0 ppm), and at -2.4 ppm, higher
field relative to 2a. When a toluene-d8 solution of 9a was cooled
(30) Schwartz, L. D.; Keller, P. C. J. Am. Chem. Soc. 1972, 94, 3015.
(31) Kawano, Y.; Kakizawa, T.; Yamaguchi, K.; Shimoi, M. Chem. Lett.
2006, 35, 568.
1
(32) Kakizawa, T.; Kawano, Y.; Shimoi, M. Organometallics 2001, 20,
3211.
to -80 °C, the high-field H NMR resonance collapsed into
the baseline. Further cooling to -90 °C revealed a new broad
resonance, assigned to the chromium-coordinated hydrogen
atom, at -12.0 ppm with 1H intensity. This indicates mono-
(33) Yasue, T.; Kawano, Y.; Shimoi, M. Angew. Chem., Int. Ed. 2003, 42,
1727.
(34) Kawano, Y.; Hashiva, M.; Shimoi, M. Organometallics 2006, 25, 4420.
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 14949

A R T I C L E S
Kawano et al.
Scheme 1
monomeric 4 and H2. The monomer then dimerizes to provide
2 if the substituents are small.
Noncatalytic dehydrogenation of adduct 1a at elevated
temperature39 was originally reported to occur through an
intramolecular mechanism.40 However, a bimolecular process
was later suggested for this reaction on the basis of results
obtained from deuterium-labeling experiments conducted by
Ryschkewitsch and Wiggins.41 A vacuum pyrolysis study by
Manners also supported the intermolecular pathway.26 Sepa-
rately, for hydrogen elimination from ammoniaborane, quantum
chemical calculations have shown an intramolecular pathway,
whichcanbecatalyzedbyBH3 generatedbyB-Ndissociation.42,43
Intramolecular H2 abstraction was suggested by Autrey and
co-workers for [RhCl(cod)]2-catalyzed dehydrocoupling of 1a
(cod ) 1,5-cyclooctadiene).44 They inferred that the active
species were soluble Rh4-6 clusters based on solution XAFS
and 11B NMR study. In contrast, for the same reaction, Manners
and co-workers proposed a mechanism involving intermolecular
B-N coupling,26 which occurs on the surface of in situ
generated colloidal metal.45,46 Autrey later suggested that with
the progress of the dehydrocoupling, the rhodium clusters
assembled with aminoborane-derived bridging ligands to gener-
ate insoluble polynuclear Rh complexes, which were oxidized
to give metallic rhodium.47 For the titanocene-catalyzed dehy-
drocoupling of 1a, originally reported by Manners,48 an in-
tramolecular reaction mechanism was proposed by Ohno based
on DFT calculations (Vide infra).49 Likewise, Chirik and co-
workers suggested an intramolecular pathway for their ti-
tanocene- and zirconocene-catalyzed reactions.50
dentate coordination of the [BH2NMe2]2 ligand and rapid
exchange between the geminal BH hydrogen atoms (eq 5). The
terminal BH signal coincided with that of free 2a and could
not be clearly observed. For this fluxional process, the free
energy of activation was evaluated to be 7.4 kcal mol-1 at 193
K. The interboron exchange was not observed up to 25 °C. Such
behavior of 9a is closely related to that of diborane(4)
derivatives, [M(CO)5(η1-B2H4 ·2PMe3)] (M ) Cr, W). In these
compounds, the site exchange between the Vicinal BH hydrogen
atoms is much slower than that between the geminal BH
protons.17,35 Unfortunately, isolation of 9a was frustrated
because of its lability.36
In the current group-6 metal-catalyzed system, the aminobo-
rane products appear to be generated through the intramolecular
process because of the formation of monomeric products in the
dehydrogenation of bulky boranes, 1d and 1e. This is rational-
ized by the tungsten-catalyzed reaction of 1f, which showed
formation of BH2dNEt2 (4f) and its slow dimerization giving
2f. Detection of 4a at an earlier stage of the stoichiometric
reaction of 1a with [Cr(CO)6] also supports the initial formation
of the monomer and its subsequent dimerization.
After the irradiation was terminated, the resonances of 9a
disappeared, while that of free 2a further increased due to
dissociation of the [BH2NMe2]2 ligand from the metal. In the
latter stage of the reaction, resonances of minor products,
BH(NMe2)2 (7a, 28.5 ppm) and B2H5(µ-NMe2) (8a, -18.0 ppm)
appeared. These species are known to be generated by dispro-
portionation of 4a.37,38
Reaction Mechanism of the Dehydrocoupling: Intramolecular
or Intermolecular. To discuss the reaction mechanism of the
dehydrocoupling, it is necessary to consider two possible
pathways, intermolecular and intramolecular reactions (Scheme
1). In the former mechanism, one of the hydridic BH atoms of
1 interacts in the coordination sphere with the acidic NH proton
of another molecule to release H2, forming a B-N covalent
bond. On the other hand, in the latter process, BH and NH
hydrogen atoms are eliminated from the same molecule to afford
Computational Study. To obtain further insights into the
reaction mechanism, we conducted theoretical calculations.
(39) Niedenzu, K.; Dawson, J. W. Boron-Nitrogen Compounds; Academic
Press Inc.: New York, 1965.
(40) Beachley, O. T., Jr. Inorg. Chem. 1967, 6, 870.
(41) Ryschkewitsch, G. E.; Wiggins, J. W. Inorg. Chem. 1970, 9, 314.
(42) (a) Nguyen, M. T.; Nguyen, V. S.; Matus, M. H.; Gopakumar, G.;
Dixon, D. A. J. Phys. Chem. A 2007, 111, 679. (b) Li, Q. S.; Zhang,
J.; Zhang, S. Chem. Phys. Lett. 2005, 404, 100.
(43) It has been proposed that the solid state thermolysis of ammoniaborane
proceeds via an intermolecular pathway. This reaction provides H2
and polymeric aminoborane [BH2NH2]x. (a) Smith, R. S.; Kay, B. D.;
Schmid, B.; Li, L.; Hess, N.; Gutowski, M.; Autrey, T. Prep. Pap.-
Am. Chem. Soc., DiV. Fuel Chem. 2005, 50, 112. (b) Stowe, A. C.;
Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T. Phys. Chem. Chem.
Phys. 2007, 9, 1831.
(44) Chen, Y.; Fulton, J. L.; Linehan, J. C.; Autrey, T. J. Am. Chem. Soc.
2005, 127, 3254.
(45) In a recent review, they mentioned the possibility of an intramolecular
mechanism. Clark, T. J.; Lee, K.; Manners, I. Chem. Eur. J. 2006,
12, 8634.
(35) Shimoi, M.; Katoh, K.; Kodama, G.; Kawano, Y.; Ogino, H. J.
Organomet. Chem. 2002, 659, 102.
(46) (a) Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 1334. (b)
Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 9776. (c) Jaska,
C. A.; Clark, T. J.; Clendenning, S. B.; Grozea, D.; Turak, A.; Lu,
Z.-H.; Manners, I. J. Am. Chem. Soc. 2005, 127, 5116.
(47) Fulton, J. F.; Linehan, J. C.; Autrey, T.; Balasubramanian, M.; Chen,
Y.; Szymczak, N. K. J. Am. Chem. Soc. 2007, 129, 11936.
(48) Clark, T. J.; Russell, C. A.; Manners, I. J. Am. Chem. Soc. 2006, 128,
9582.
(36) Quite recently, a rhodium complex of [BH2NMe2]2 was isolated and
structurally characterized. The cyclic aminoborane is ligated to the
rhodium atom in a bidentate fashion in this compound. Douglas, T. M.;
Chaplin, A. B.; Weller, A. S. J. Am. Chem. Soc. 2008, 130, 14432.
This report also described the formation of
a rhodium-
dimethylamineborane complex and its intermediacy in the Rh-catalyzed
borane dehydrocoupling reaction.
(37) No¨th, H.; Vahrenkamp, H. Chem. Ber. 1966, 99, 1049.
(38) Hall, R. E.; Schram, E. P. Inorg. Chem. 1971, 10, 192.
(49) Luo, Y.; Ohno, K. Organometallics 2007, 26, 3597.
(50) Pun, D.; Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2007, 3297.
9
14950 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

Catalytic Dehydrocoupling of Borane Adducts
A R T I C L E S
interaction with a filled chromium d orbital.53 The B-Hcoord
,
Scheme 2. Free-Energy Profile for Borane Dehydrocoupling via 3a
(Path I)a
Cr-H, and NH · · ·Cr interatomic distances are 1.268, 1.772, and
3.202 Å, respectively. In the transition state (TS1), the NH
proton is drawn toward the metal center, and the coordinated
BH bond is significantly stretched (B · · ·H(Cr) 2.460 Å). The
NH proton further approaches the chromium atom leaving an
aminoborane molecule to produce a dihydrogen complex
[Cr(CO)5(H2)].54 Importantly, this process has a large free energy
of activation, 35.2 kcal mol-1 and, thus, is not suitable for the
borane dehydrocoupling, which progresses smoothly at ambient
temperature. Despite repeated trials, we could not find a
plausible reaction mechanism that goes through 3a.
DFT calculations suggest that the active species of this
reaction was [Cr(CO)4]. Scheme 3 illustrates the reaction
mechanism of the [Cr(CO)4]-catalyzed dehydrocoupling (path
II). The optimized structures and geometrical parameters of the
concerned species are presented in Figure 3 and Table 3,
respectively. This reaction proceeds via a stepwise mechanism
involving both NH and BH bond activation. Borane 1a
coordinates to [Cr(CO)4] through the BH and NH hydrogen
atoms to generate a chelate complex 10a. The formation of 10a
is 2.6 kcal mol-1 endergonic, and 9.9 kcal mol-1 favorable in
electronic energy. Complex 10a undergoes NH insertion to
generate 11a through TS2 having an N-H-Cr three-center
interaction. 11a contains an amido-borane ligand which coor-
dinates to the chromium atom through the nitrogen lone electron
pair and a BH bond.55 The energy barrier for the NH activation
is 9.1 kcal mol-1, so this process occurs very easily. Subse-
quently, BH activation takes place to produce 12a. After this
step Cr-mediated H2 elimination of 1a has been attained, and
an aminoborane moiety is formed on the metal center. During
the transformation from 11a to 12a, the BH2NMe2 group rotates
90° around the Cr-N axis, leaving a hydride which was hitherto
bound to boron. In the key species 12a, the two hydrides, which
were originally from the BH and NH protons, are mutually
equivalent. This BH activation is the rate-determining step in
the whole catalytic reaction, and its energy barrier is 13.3 kcal
mol-1. This value is significantly reduced in comparison to those
for the noncatalytic and BH3-catalyzed intramolecular dehy-
drogenation of 1a, 40.2 and 26.9 kcal mol-1, respectively
(Scheme 4).
a The solvation free energies (kcal mol-1, 298 K, 1 atm) were evaluated
at the PBE0/SDD (Cr), aug-cc-pVDZ (other atoms) level using the CPCM
model (solvent ) benzene).
Figure 2. DFT-optimized structures of 3a, TS1, and [Cr(CO)5(H2)]· · ·4a.
Free energies relative to the separated reactants, 1a + [Cr(CO)5], are given
in kcal mol-1. Zero-point-corrected electronic energies are also given in
parentheses. Throughout this paper, indigo ) Cr, brown ) C, red ) O,
green ) B, blue gray ) N, pink ) H.
Geometry optimizations and vibration analyses were performed
by the DFT method with use of the PBE0 functional.51 The
single-point energies in solution were then calculated with larger
basis sets (see Experimental Section) applying the CPCM
model52 to the optimized geometries. First, we describe the
intramolecular process, which was suggested in the former
section.
Intramolecular Reaction. As mentioned already, a borane σ
complex 3a is observed during the Cr-catalyzed dehydrocou-
pling reaction of 1a. Scheme 2 shows the free energy profile
(∆Gsol) of an intramolecular reaction pathway containing this
complex as an intermediate (path I). The structures of each
species are exhibited in Figure 2. This mechanism involves
concerted hydrogen elimination. In complex 3a, the borane
ligand coordinates to chromium through a B-H-Cr single
bridge, and the NH proton also has a hydrogen-bond-like
As found in Table 3, in the chelate complex 10a the Cr-H(N)
bond distance (1.917 Å) is substantially longer than that of the
Cr-H(B) bond (1.766 Å). This reflects the weak coordination
of the protonic NH hydrogen atom. During the NH activation
step, the NH bond is greatly elongated (10a 1.055, TS2 1.480,
11a 2.458 Å), while the BH interatomic distance does not
change very much. However, the latter is increased dramatically
in the subsequent BH activation step. In 11a, TS3, and 12a,
the BH separations are 1.378, 2.305, and 2.825 Å, respectively.
(54) (a) Matthews, S. L.; Pons, V.; Heinekey, D. M. J. Am. Chem. Soc.
2005, 127, 850. (b) Sweany, R. L. J. Am. Chem. Soc. 1985, 107, 2374.
(c) Upmacis, R. K.; Gadd, G. E.; Poliakoff, M.; Simpson, M. B.;
Turner, J. J.; Whyman, R.; Simpson, A. F. J. Chem. Soc., Chem.
Commun. 1985, 27. (d) Church, S. P.; Grevels, F.-W.; Hermann, H.;
Schaffner, K. J. Chem. Soc., Chem. Commun. 1985, 30.
(51) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865.
(55) Quite recently, a related zirconocene-amidoborane complex was
isolated and structurally characterized. Forster, T. D.; Tuononen, H. M.;
Parvez, M.; Roesler, R. J. Am. Chem. Soc. 2009, 131, 6689. Harder’s
amidoborane calcium compound is also relevant. Spielmann, J.; Harder,
S. J. Am. Chem. Soc. 2009, 131, 5064. In addition, a lutetium imine-
stabilized borane complex, which was reported by Roesky and co-
workers very recently, has structural relevance. Meyer, N.; Jenter, J.;
Roesky, P. W.; Eickerling, G.; Scherer, W. Chem. Commun. 2009,
4693.
(52) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (b) Cossi,
M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24,
669.
(53) Selected papers for M · · ·HX hydrogen bonds: (a) Brammer, L.;
McCann, M. C.; Bullock, R. M.; McMullan, R. K.; Sherwood, P.
Organometallics 1992, 11, 2339. (b) Roe, D. M.; Bailey, P. M.;
Moseley, K.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1972,
1273.
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 14951

A R T I C L E S
Kawano et al.
Scheme 3. Free-Energy Profile for the Borane Dehydrocoupling Catalyzed by [Cr(CO)4] (path II)a
a Cr denotes [Cr(CO)4].
Figure 3. DFT-optimized structures of 10a, TS2, 11a, TS3, and 12a. Free energies relative to the separated reactants, 1a + [Cr(CO)4], are given in kcal
mol-1 (zero-point-corrected electronic energies are given in parentheses).
Table 3. Calculated Interatomic Distances (Å) for 10a, TS2, 11a,
TS3, and 12a
Scheme 5. Free-Energy Profile for the Isomerization of 13 to 14
10a
TS2
11a
TS3
12a
B-Hcood
N-H
1.275
1.055
2.428
2.442
1.766
1.917
1.653
1.334
1.480
2.337
2.142
1.728
1.573
1.570
1.378
2.458
2.380
2.099
1.703
1.619
1.518
2.305
2.659
2.516
2.229
1.634
1.649
1.448
2.825
2.625
2.416
2.175
1.650
1.650
1.457
Cr-B
Cr-N
Cr-H(B)
Cr-H(N)
B-N
Scheme 4. Non-Catalytic and BH3-Catalyzed Intramolecular
Dehydrogenation Reactions of 1a. Relative Solvation Free
Energies (zero-point-corrected electronic energies) are given in
kcal mol-1
distance is 2.416 Å, suggesting the contribution of metal back-
donation into the boron p orbital.
Aminoborane dissociation from 12a is a barrierless process,
and almost neutral in free energy. The resulting dihydride 13
cannot release H2 itself because of the trans configuration of
the two hydride ligands. However, it can isomerize with
deformation of the ligand arrangement. The resultant dihydrogen
adduct 14 releases H2 rapidly to regenerate the active species
[Cr(CO)4]. Scheme 5 and Figure 4 depict the isomerization of
13 to 14. It proceeds in two steps, where the highest reaction
barrier is 11.1 kcal mol-1 for the second step from 13′ to 14.
Coordinatively unsaturated [Cr(CO)4] is known to be gener-
ated photochemically.56 We found that it was also generated
thermally in the reaction system (Scheme 6 and Figure 5). Two
molecules of [Cr(CO)5], formed from dissociation of borane
In 12a, the B-N bond length, 1.457 Å, is an intermediate value
between the corresponding bond lengths of 1a (1.610 Å) and
4a (1.388 Å). In addition, the boron and nitrogen atoms are
slightly pyramidalized. The aminoborane ligand coordinates to
the chromium atom through the nitrogen electron pair with a
Cr-N bond distance of 2.175 Å. At the same time, the Cr · · ·B
(56) Fletcher, T. R.; Rosenfeld, R. N. J. Am. Chem. Soc. 1985, 107, 2203.
9
14952 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

Catalytic Dehydrocoupling of Borane Adducts
A R T I C L E S
Figure 4. DFT-optimized structures of the isomers of [Cr(CO)4H2] and transition states connecting them. Relative free energies are given in kcal mol-1
.
Zero-point-corrected electronic energies are given in parentheses.
Figure 5. DFT-optimized structures of 15, TS6, and 16. Relative free energies are given in kcal mol-1. Zero-point-corrected electronic energies are given
in parentheses.
Scheme 6. Free-Energy Profile for the Isomerization of the
Dihydride 13 to the Dihydrogen Complex 14
Recently, Luo and Ohno reported a theoretical treatment of
the titanocene-catalyzed dehydrocoupling of 1a.49 In this
reaction, the borane first coordinates to a TiCp2 fragment to
generate an adduct, [Cp2Ti(η1-BH3 ·NHMe2)]. Similarly to the
current Cr-catalyzed reaction, the dehydrogenation of this
complex proceeds via a stepwise mechanism including NH
cleavage followed by BH cleavage. In contrast, however, a
concerted H2 elimination mechanism has been shown for the
H2 elimination of ammoniaborane by an iridium pincer cata-
lyst.59
Intermolecular Reaction. We also computed the intermo-
lecular reaction pathway (path III). The acidity of the NH proton
may be enhanced in complex 3a because of the borane
polarization induced by the metal coordination. The dehydro-
genation could be initiated through the electrostatic interaction
of the acidic NH proton with a hydridic BH of another molecule
of 1a in the reaction system. Indeed, a related intermolecular
from 3a or photolysis of [Cr(CO)6], associate to give a loosely
bound dimer 15. In 15, [Cr(CO)5] fragments donate electron
density to each other from a carbonyl oxygen atom, where the
Cr· · ·Cr and Cr · · ·O separations are 3.812 and 2.395 Å,
respectively. One of the bridging carbonyl ligands then turns
reaction mechanism has been proposed by Denis for dehydro-
coupling of phosphineboranes catalyzed by B(C6F5)3.60
To form such an intermolecular NH · · ·HB dipole-dipole
interaction, the metal-coordinated dimethylamineborane needs
to point the NH proton toward the incoming molecule of 1a.
Such a geometry is realized in an isomer of the borane complex,
3a′ (Scheme 7). Association of 3a′ with free 1a occurs through
“dihydrogen bonds”61,62 to produce adduct 3a′· · ·1a. The adduct
formation is only 2.8 kcal mol-1 endergonic. In 3a′· · ·1a, the
BH · · · HN interatomic distances are 1.937-2.065 Å. Similar
association through BH· · ·HN interactions has been demon-
strated in the crystal structures of BH3 ·NR2BH2 ·NHR2 (R )
and migrates between the two chromium atoms to afford
[Cr(CO)6]· · ·[Cr(CO)4] (16).57 The transition state connecting
15 and 16, TS6, is only 3.4 kcal mol-1 higher than 15 and 14.1
kcal mol-1 higher than the separated reactants (two molecules
of [Cr(CO)5]) in free energy, so that this process can readily
occur under ambient conditions to release the active catalyst
into the solution. The thermal generation of [Cr(CO)4] readily
explains that the dehydrocoupling of 1a is catalyzed by
[Cr(CO)5(thf)] or [Cr(CO)5(η1-BH3 ·NMe3)] and progresses
smoothly in the dark.
The role of the σ complex 3a in the catalytic cycle should
also be considered. This complex acts as a reservoir of [Cr(CO)5]
in the reaction mixture. Complex 3a readily releases the borane
(59) Paul, A.; Musgrave, C. B. Angew. Chem., Int. Ed. 2007, 46, 8153.
(60) Denis, J.-M.; Fointos, H.; Szelke, H.; Toupet, L.; Pham, T.-N.; Madec,
P.-J.; Gaumont, A.-C. Chem. Commun. 2003, 54.
58
ligand with a reaction free energy of 6.1 kcal mol-1
.
Hence,
[Cr(CO)5], the source of the active catalyst, is supplied even
after the irradiation is switched off.
(61) Bakhmutov, V. I. Dihydrogen Bond, Principles, Experiments, and
Applications; Wiley-VCH: New York, 2008.
(62) Selected papers for dihydrogen bond: (a) Crabtree, R. H.; Siegbahn,
P. E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle, T. F. Acc. Chem.
Res. 1996, 29, 348. (b) Crabtree, R. H. Science 1998, 282, 2000. (c)
Bosque, R.; Maseras, F.; Eisenstein, O.; Patel, B. P.; Yao, W.; Crabtree,
R. H. Inorg. Chem. 1997, 36, 5505. (d) Lee, J. C., Jr.; Peris, E.;
Rheingold, A. L.; Crabtree, R. H. J. Am. Chem. Soc. 1994, 116, 11014.
(e) Xu, W.; Lough, A. J.; Morris, R. H. Inorg. Chem. 1996, 35, 1549.
(57) The existence of [Cr2(CO)10] after photolysis of [Cr(CO)6] has been
suggested. Fletcher, T. R.; Rosenfeld, R. N. J. Am. Chem. Soc. 1985,
107, 2203.
(58) Labilenatureofboraneligandshasbeenconfirmedexperimentally.Kawano,
Y.; Yamaguchi, K.; Miyake, S.; Kakizawa, T.; Shimoi, M.
Chem.sEur. J. 2007, 13, 6920.
9
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A R T I C L E S
Kawano et al.
Scheme 8. Non-catalytic, intermolecular dehydrocoupling of 1aa
Scheme 7. Intermolecular Dehydrogenation of 1a on Chromium
(Path III)a
a Solvation free energies and zero-point-corrected electronic energies (in
parentheses) relative to the separated reactants (two molecules of 1a) are
given in kcal mol-1
.
Figure 7. DFT (PBE0)-optimized structure of 2d.
BH3(η2-H2), which has been observed in cryogenic matrices.67,68
H2 release via TS7 results in the formation of a B-N covalent
bond to afford the coupling product 17a. This step is 3.9 kcal
mol-1 exergonic, and the activation barrier is slightly reduced
again compared with the noncatalytic reaction (Scheme 8).
Nonetheless, its value 52.8 kcal mol-1 is still very large, and
therefore, this type of intermolecular reaction is unlikely to
contribute to the borane dehydrocoupling.69
a Relative solvation free energies are given in kcal mol-1
.
As noted above, the [Cr(CO)4]-catalyzed intramolecular
reaction mechanism, path II, is strongly suggested for the
catalytic dehydrocoupling of 1a. Neither path I, which goes via
3a, nor path III, the intermolecular reaction pathway, are
favorable. Other computed possibilities for the reaction mech-
anism are presented in Supporting Information.
Dehydrogenation Reactions of Bulky Boranes. Finally, we
discuss the reactions of boranes containing bulky substituents.
In the NH activation step of path II, the amine moiety necessarily
approaches the metal center. As this occurs, bulky substituents
on nitrogen can give rise to steric repulsion between the amine
group and metal fragment to destabilize the transition state. This
explains the slow reaction of the bulky boranes.
Dimerization of methyl-substituted aminoborane 4a is 3.6 kcal
mol-1 favorable in free energy. In contrast, that of the isopropyl
derivative 4d is endergonic by 17.2 kcal mol-1. Indeed, its dimer
2d has a highly congested structure involving cross-ring steric
interaction between the isopropyl groups, as shown by the DFT-
based geometry optimization (Figure 7).
Figure 6. DFT-optimized structures of species regarding path III. Solvation
free energies and zero-point-corrected electronic energies (in parentheses)
relative to the separated reactants (3a′ + 1a) are given in kcal mol-1
.
Me, 1/2(CH2)4).26,63,64 Some phosphineboranes and hybrid
amineborane-phosphineborane species also show analogous
structural motifs.65,66
It is likely that the intermolecular reaction proceeds through
adduct 3a′· · ·1a. This complex successively releases two
equivalents of H2. In the first step, the acidic NH proton of the
ligated amineborane approaches one of the BH hydrogen atoms
of associated 1a to form an H-H bond (TS7, see Scheme 7
and Figure 6). In TS7, the H2 moiety interacts with a boron
atom in an η2-fashion. This is reminiscent of the structure of
Conclusion
In this paper, we described the dehydrocoupling reactions of
borane-secondary amine adducts, 1a-f, catalyzed by [M(CO)6]
(63) No¨th, H.; Thomas, S. Eur. J. Inorg. Chem. 1999, 1373.
(64) In crystal, free 1a adopts a chain-like structure, in which amineborane
molecules are linked by BH · · ·HN interactions. Molecules of primary
adduct 1g are linked in ribbons. Aldridge, S.; Downs, A. J.; Tang,
C. Y.; Parsons, S.; Clarke, M. C.; Johnstone, R. D. L.; Robertson,
H. E.; Rankin, D. W. H.; Wann, D. A. J. Am. Chem. Soc. 2009, 131,
2231.
(68) It has been reported that the alcoholysis of BH4- takes place through
a transition state involving a closely related H3B(η2-H2)· · ·OR-
structure. Filippov, O. A.; Filin, A. M.; Tsupreva, V. N.; Belkova,
N. V.; Lledo´s, A.; Ujaque, G.; Epstein, L. M.; Shubina, E. S. Inorg.
Chem. 2006, 45, 3086.
(69) In an additional note, the second H2 elimination from 17a does not
lead to ring closure giving 9a, but causes the formation of two
equivalents of 4a through the central B-N bond cleavage of the
BH3 ·NMe2BH2 ·NHMe2 ligand (see Supporting Information). Hence,
detection of monomeric aminoboranes 4 during the dehydrocoupling
is not incompatible with the possibility of intermolecular reaction,
although it has been excluded on the basis of the high activation barrier.
The formation of two molecules of 4a has also been pointed out by
Ohno in the thermolysis of BH3 ·NMe2BH2 ·NHMe2. Luo, Y.; Ohno,
K. Organometallics 2007, 26, 3597.
(65) Dorn, H.; Singh, R. A.; Massey, J. A.; Nelson, J. M.; Jaska, C. A.;
Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2000, 122, 6669. This
paper also reported Rh-catalyzed dehydrocoupling reactions of phos-
phine boranes.
(66) Jaska, C. A.; Lough, A. J.; Manners, I. Inorg. Chem. 2004, 43, 1090.
(67) (a) Tague, T. J., Jr.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 4970.
(b) Watts, J. D.; Bartlett, R. J. J. Am. Chem. Soc. 1995, 117, 825. (c)
Schreiner, P. R.; Schaefer, H. F.; Schleyer, P. v. R. J. Chem. Phys.
1994, 101, 7625.
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14954 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

Catalytic Dehydrocoupling of Borane Adducts
A R T I C L E S
Figure 8. Catalytic cycle of the dehydrocoupling reaction of 1a. Cr represents [Cr(CO)4].
THF according to the literature.70 [Cr(CO)6], [Mo(CO)6] (Strem),
and [W(CO)6], NaBH4 (Aldrich) were purchased and used as
received. 1H and 11B NMR spectra were recorded on a JEOL FX-
90Q or an R-500 spectrometer. IR spectra were recorded on a
JASCO FTIR-350 or a Bruker IFS66v spectrometer.
under photolytic conditions. This reaction primarily produces
aminoboranes 4a-f. When the amine substituents are small,
they dimerize to give cyclic boranes, [BH2NR2]2 (2a-c, R )
Me, 1/2C4H8, 1/2C5H10). In contrast, the dehydrogenation of
bulky amineboranes afforded monomers, BH2dNR2 (4d, e; R
Catalytic Preparation of [BH2NMe2]2 (2a). Samples of
BH3 ·NHMe2 (1a) (1090 mg, 18.50 mmol) and [Cr(CO)6] (24 mg,
0.11 mmol) were dissolved in THF (7 mL). This solution was
loaded in a Pyrex glass tube (20 mm o.d.) equipped with a high
vacuum stopcock and irradiated using a 450 W medium pressure
Hg lamp at 6 °C under vacuum. This led to vigorous gas evolution
and yielded a yellow solution. After the irradiation for 1 h, the
solution was stirred at room temperature for 24 h, and the resulting
pale green suspension was then evaporated to dryness. From the
solid residue, 2a (726 mg, 6.38 mmol, 69%) was isolated by
sublimation as a colorless crystalline solid (40 °C/1 × 10-4 torr).
The 1H and 11B NMR spectral data accorded with literature values.21
Dehydrogenation of Borane-Secondary Amine Adducts.
Typically, BH3 ·NH(CH2)4 (1b) (16 mg, 0.19 mmol) and [Cr(CO)6]
(2.0 mg, 9.2 × 10-3 mmol) were dissolved in C6D6 (ca. 0.5 mL)
and irradiated at 8 °C in a vacuum-sealed Pyrex NMR tube (5 mm
i
) Pr, Cy), as the main product.
The mechanism of this interesting reaction was investigated
by DFT calculation. The borane dehydrocoupling reaction is
catalyzed by [Cr(CO)4] and follows a stepwise intramolecular
pathway (path II). The whole catalytic cycle is illustrated in
Figure 8. It includes (i) generation of the active catalyst
[Cr(CO)4], (ii) borane coordination and dehydrogenation on
the chromium atom, and (iii) isomerization of the resulting
dihydride complex (regeneration of the active species). All three
of these processes take place with a reasonably low barrier. The
borane σ complex 3a, observed in the reaction mixture, acts as
a reservoir of [Cr(CO)5], which is a source of the active species.
We also reported here the dehydrocoupling of primary
adducts, BH3 ·NH2R (1g-i). These compounds underwent
multiple dehydrogenations to produce borazine derivatives 5.
In the course of the borazine formation, aminoborane oligomers,
[BH2NHR]n, are observed in the reaction mixture. Monomethy-
laminoborane is known to be the most stable in trimer,
[BH2NHMe]3.21a It is plausible that the cyclic trimer is in
equilibrium with the oligomers and that it undergoes metal
coordination followed by hydrogen elimination to yield borazines.
Group-6 metal carbonyl complexes are commercially avail-
able and easy to handle. Furthermore, in this system, the reaction
is not initiated only by mixing the chemicals, but is also
triggered photochemically. Thus, the use of transition metal
carbonyls may offer a more controllable catalytic system in
borane dehydrocoupling reactions. Attempts to construct other
such systems are currently underway.
1
o.d.). The progress of the dehydrogenation was monitored by H
and 11B NMR spectroscopy. Dehydrocoupling of other borane
adducts was carried out in similar fashion. The products were
1
characterized by comparing the H and 11B NMR data with the
literature.21c,22-25
During the reaction, borane
σ
complexes [M(CO)5(η1-
BH3 ·NHR2)] were detected unless the molybdenum catalyst was
employed. The tungsten-catalyzed dehydrogenation reaction of
BH3 ·NHCy2 (1e) was so slow that the formation of 4e was not
confirmed, but the borane complex [W(CO)5(η1-BH3 ·NHCy2)] was
observed. In the reaction of 1a-c, the complex of dimeric
aminoborane, [M(CO)5(η1-BH2NR2BH2NR2)], was also detected.
1
The H and 11B NMR data for these complexes are presented in
Table 4.
Dehydrogenation of Borane-Primary Amine Adducts.
BH3 ·NH2Me (1g) (8.5 mg, 0.19 mmol) and [Cr(CO)6] (2.0 mg,
9.2 × 10-3 mmol) were dissolved in benzene-d6 (ca. 0.5 mL) and
irradiated at 8 °C using a 450 W medium pressure Hg lamp. The
Experimental Section
All manipulations were carried out under high vacuum or dry
nitrogen atmosphere. Reagent-grade hexane, toluene, and THF
were distilled under nitrogen atmosphere from sodium-ben-
zophenone ketyl immediately before use. Benzene-d6 and
toluene-d8 were dried over potassium mirrors and vacuum
transferred into NMR tubes directly before use. BH3 · NHMe2
(1a), BH3 · NH(CH2)4 (1b), BH3 · NH(CH2)5 (1c), BH3 · NHiPr2
(1d), BH3 · NHCy2 (1e), BH3 · NHEt2 (1f), BH3 · NH2Me (1g),
1
reaction was followed by H and 11B NMR spectroscopy. After
1 h of photolysis, a white viscous material precipitated and the
reaction mixture exhibited a broad resonance at 0 to -10 ppm in
the 11B NMR spectrum. However, this precipitate disappeared by
further irradiation and subsequent dark reaction. After 8 h of
(70) (a) Bonham, J.; Drago, R. S. Inorg. Synth. 1967, 9, 8. (b) No¨th, H.;
Beyer, H. Chem. Ber. 1960, 93, 928. (c) Kelly, H. C.; Marchelli, F. R.;
Giusto, M. B. Inorg. Chem. 1964, 3, 431.
t
BH3 · NH2Et (1h), and BH3 · NH2 Bu (1i) were prepared by
treatment of NaBH4 with the corresponding ammonium salt in
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 14955

A R T I C L E S
Kawano et al.
Table 4. 1H and 11B NMR Spectral Data for Borane Complexes Observed in the Dehydrocoupling Reactions
cmpd
1H NMRa
11B NMRa
3
[Cr(CO)5(η1-BH3 ·NHMe2)] (3a)
3.30 (br, 1H, NH), 1.39 (d, JHH ) 6.0 Hz, 6H, CH3),
-16.8 (br)
-20.6 (br)
-3.3 (br, 3H, BH3)
[W(CO)5(η1-BH3 ·NHMe2)]
1.67 (d, JHH ) 5.5 Hz, 6H, CH3), -1.46 (br q, JBH
) 102 Hz, 3H, BH3)b
3
1
[Cr(CO)5(η1-BH3 ·NH(CH2)4)] (3b)
[Cr(CO)5(η1-BH3 ·NH(CH2)5)] (3c)
-3.06 (br q, JBH ) 102 Hz, 3H, BH3)c
-20.3 (br)
-19.3 (br)
1
3.0 (br, 1H, NH), 2.21, 1.63 (m, 2H × 2, NCH2),
1
0.68, 0.1-0.3 (m, 3H × 2, CH2), -3.36 (q, JBH
)
90 Hz, 3H, BH3)
[W(CO)5(η1-BH3 ·NH(CH2)5)]
[Cr(CO)5(η1-BH3 ·NHiPr2)]
2.28, 1.66 (m, 2H × 2, NCH2), 0.72, 0.30 (m, 3H ×
-23.8 (br)
-25.2 (br)
1
2, CH2), -1.54 (q, JBH ) 70 Hz, 3H, BH3)
3
3.55 (m, 2H, CHMe2), 0.83 (d, JHH ) 6.4 Hz, 6H),
3
0.77 (d, JHH ) 5.8 Hz, 6H) (diastereotopic CHMe2),
-3.04 (br, JBH ) 110 Hz, 3H, BH)d
1
3
[W(CO)5(η1-BH3 ·NHiPr2)]
3.15 (br, 1H, NH), 0.74 (d, JHH ) 5.0 Hz, 6H), 0.64
-29.1 (br)
3
(d, JHH ) 7.0 Hz, 6H) (diastereotopic CHMe2),
-1.35 (q, br, 3H, BH)c
[Cr(CO)5(η1-BH3 ·NHCy2)]
[W(CO)5(η1-BH3 ·NHCy2)]
[Cr(CO)5(η1-BH3 ·NHEt2)]
-3.0 (br, 3H, BH3)b e
-23.9 (br)
-28.4 (br)
-20.9 (br)
,
-1.2 (br, 3H, BH3)b e
,
3
1.90, 1.75 (m, 2H × 2, CH2), 0.38 (t, JHH ) 7.0 Hz,
1
6H, CH3), -3.29 (q, 3H, JBH ) 100 Hz, BH3)
[W(CO)5(η1-BH3 ·NHEt2)]
1.89, 1.73 (m, 2H × 2, CH2), 0.36 (t, JHH ) 7.0 Hz,
-25.5 (q, JBH ) 80 Hz)
3
1
1
6H, CH3), -1.58 (q, 3H, JBH ) 80 Hz, BH3)
[Cr(CO)5(η1-BH2NMe2BH2NMe2)] (9a)
1.84 (s, 12H, CH3), -4.79 (q, JBH ) 95 Hz, 2H,
4.7 (t, JBH ) 118 Hz, noncoordinated
BH2), -2.4 (t,1JBH ) 95 Hz,
Cr-BH2)
1
1
Cr-BH2)f
[W(CO)5(η1-BH2NMe2BH2NMe2)]
1.81 (s, 12H, CH3), -2.41 (q, JBH ) 93 Hz, 2H,
4.6 (t, JBH ) 114.7 Hz, noncoordinated
1
1
W-BH2)f
BH2), -6.6 (t, JBH ) 93.1 Hz,
1
W-BH2)
[Cr(CO)5(η1-BH2N(CH2)4BH2N(CH2)4)]
[Cr(CO)5(η1-BH2N(CH2)5BH2N(CH2)5)]
2.44, 2.38 (m, 4H × 2, NCH2), 1.30, 1.24 (m, 4H ×
4.7 (t, JBH ) 99 Hz, noncoordinated
1
2, CH2), -4.40 (br q, JBH ) 100 Hz, 2H, Cr-BH2)f
BH2), -5.2 (t, JBH ) 99 Hz,
1
1
Cr-BH2)
1
2.46, 2.18 (m, 4H × 2, NCH2), 1.00, 0.69 (m, 6H ×
-0.3(t, JBH ) 106 Hz, noncoordinated
2, CH2), -4.63 (q, JBH ) 95 Hz, 2H, Cr-BH2)f
BH2), -3.2 (t, JBH ) 95 Hz,
1
1
Cr-BH2)
[W(CO)5(η1-BH2N(CH2)5BH2(CH2)5)]
[Cr(CO)5(η1-BH3 ·NH2Me)]
2.46, 2.15 (m, 4H × 2, NCH2), 1.01, 0.69 (m, 6H ×
-0.2 (br, noncoordinated BH2), -7.2
2, CH2), -2.32 (q, JBH ) 110 Hz, 2H, Cr-BH2)f
(t, JBH ) 110 Hz, W-BH2)
1
1
1.47 (s, 3H, Me), -3.11 (br, 3H, BH)g
-17.7 (q, JBH ) 98 Hz)
1
t
[Cr(CO)5(η1-BH3 ·NH2 Bu)]
2.26 (br, 2H, NH2), 0.49 (s, 9H, CH3), -3.19 (q, br,
-27.4 (br)
-32.2 (br)
1
3H, JBH ) 110 Hz, BH3)
t
[W(CO)5(η1-BH3 ·NH2 Bu)]
2.20 (s br, 2H, NH2), 0.47 (s, 9H, CH3), -1.44 (q,
1
3H, JBH ) 120 Hz, BH3)
a NMR spectra were recorded in benzene-d6 at 500 MHz (1H) or 160.4 MHz (11B). b The NH resonance was too broad to be observed. c The other
signals were overlapped with resonances of coupling products. d The methyne signal could not be detected because of the overlap with the methyne and
BH resonances of free 1d. e The cyclohexyl protons were not resolved because they overlapped with signals of other compounds in the reaction system.
f The resonance of the terminal BH protons could not be observed because it was completely covered with the BH signals of free 2. g The NH resonance
was overlapped with that of the dehydrocoupling product, aminoborane oligomers.
photolysis and subsequent standing for 24 h at room temperature,
the main product was [BHNMe]3 (5g, 80%) as determined by NMR
spectroscopy. In a similar manner, the dehydrocoupling reactions
of various primary adducts were monitored in the presence of
[M(CO)6] (5 mol %).23c,d,28,30
with the effective core potentials (ECP) of Hay and Wadt with a
double-ꢀ valence basis set (LANL2DZ)71 augmented with f-
polarization functions (R ) 1.941).72 A double-ꢀ plus polarization
valence basis set augmented with diffuse functions, 6-31++G(d,p)
was employed for B, N, and BH and NH hydrogen atoms, which
directly participated in the dehydrocoupling. For all other atoms, a
standard 6-31G(d) basis set was applied. Geometry optimizations
were also conducted using several types of functionals for some
species. The structures given by the B3PW91,73,74 mPW1PW91,75
MPW1K,76 and PBEPBE51 functionals were essentially the same
as that of PBE0 except for very slight differences in bond distances
and angles. However, the B3LYP73,77 structure of 3a differed
substantially from the PBE0 geometry.78 Vibration analyses were
then performed at the same level of theory (PBE0) to characterize
t
The photoreaction of BH3 ·NH2 Bu (1i) with equimolar [W(CO)6]
provided a yellow solution, which showed NMR resonances
assigned to [W(CO)5(η1-BH3 ·NH2 Bu)]. Complexation of 1g with
t
chromium carbonyl was also confirmed. However, that of 1g with
tungsten carbonyl and coordination of 1h to chromium or tungsten
were not observed. The 1H and 11B NMR data of the borane
complexes observed are given in Table 4.
Photoreaction of 1a with [Cr(CO)6] in 1:1 Ratio. A C6D6 (0.5
mL) solution of 1a (4.1 mg, 6.9 × 10-2 mmol) and [Cr(CO)6] (15.0
mg, 6.9 × 10-2 mmol) was prepared and sealed in a Pyrex NMR
tube under high vacuum. The solution was photolyzed using a 450
W medium pressure Hg lamp. The 1H and 11B NMR spectra were
recorded after 1, 5, and 20 min of photolysis. After that, the mixture
was allowed to stand in the dark for 24 h at room temperature, and
NMR spectra were recorded.
(71) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt,
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Chem. Phys. Lett. 1993, 208, 111.
Computational Details. Stationary points were located by
density functional theory (DFT) using a hybrid functional PBE0,
which is also referred to as PBE1PBE.51 Chromium was described
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14956 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

Catalytic Dehydrocoupling of Borane Adducts
A R T I C L E S
the stationary points and to determine thermochemical parameters
including entropies. The connectivity of important transition states
was confirmed by performing intrinsic reaction coordinate (IRC)
calculations.79,80
The single point energies were then calculated with the DFT/
PBE0 level of theory. At this stage, Stuttgart-Dresden ECP and
valence triple-ꢀ plus f-polarization basis set (SDD) were used for
chromium,81 and Dunning’s double-ꢀ class basis set augmented
with polarization and diffuse functions, aug-cc-pVDZ, was em-
ployed to describe the other atoms.82 In addition, the CPCM
model52 was applied to estimate the energies in solution. To the
solvation free energies thus obtained, the Gibbs free energy
contributions, which were given in the vibration analyses, were
added to consider the entropic terms. The values of the solvation
free energies are given as those at 298 K and 1 atm in benzene.
BSSE corrections were not performed. All calculations were
performed with the Gaussian 03 package of programs.83
Acknowledgment. This work was supported by Grant-in-Aid
for Scientific Research on Priority Areas (No. 14078101, “Reaction
Control of Dynamic Complexes”) from Ministry of Education,
Culture, Sports, Science and Technology, Japan. We are grateful
to Professor Sakaki of Kyoto University for his kind suggestion in
the computational study. We thank Professor Koe of International
Christian University for the help of preparation of the manuscript.
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(77) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(78) In the B3LYP-optimized structure of 3a, the NH proton of the
BH3 ·NHMe2 ligand interacted with a carbonyl oxygen atom instead
of the chromium center.
Supporting Information Available: The Cartesian coordinates,
electronic energies, and Gibbs free energies of all the optimized
structures, the 11B spectral change during the Cr-catalyzed
dehydrocoupling of 1g, and complete ref 83. This material is
available free of charge via the Internet at http://pubs.acs.org.
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