Molybdenum tricarbonyl complexes of 1-substituted borepins

Article abstract of DOI:10.1021/om970017z

The reaction of 1-substituted borepins C₆H₆BX, where X = Me, Ph, and Cl, with Py₃Mo-(CO)₃ and BF₃·OEt₂ affords the corresponding borepin molybdenum tricarbonyl complexes. Similar reaction o

Full text of DOI:10.1021/om970017z

1884
Organometallics 1997, 16, 1884-1889
Molybd en u m Tr ica r bon yl Com p lexes of 1-Su bstitu ted
Bor ep in s
Arthur J . Ashe, III,* Samir M. Al-Taweel,1 Christian Drescher,
J eff W. Kampf, and Wolfram Klein
Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109-1055
Received J anuary 13, 1997X
The reaction of 1-substituted borepins C6H6BX, where X ) Me, Ph, and Cl, with Py3Mo-
(CO)3 and BF3‚OEt2 affords the corresponding borepin molybdenum tricarbonyl complexes.
Similar reaction of 1-methoxyborepin affords (C6H6BF)Mo(CO)3. The reaction of (C6H6BCl)-
Mo(CO)3 with appropriate nucleophiles affords the boron-substituted complexes (C6H6BX)-
Mo(CO)3, where X ) H, OCH3, OH, O1/2, N(iPr)2, N(CH2)5, N(c-C6H11)2, and NBnMe. All
1
complexes (C6H6BX)Mo(CO)3 have been compared using H NMR, 11B NMR, and 13C NMR
spectroscopies. The X-ray structure of (C6H6BN(iPr)2)Mo(CO)3 shows that Mo is η6-
coordinated to the ring carbon atoms but not to B. Rates of rotation about the B-N bonds
of (C6H6BN(iPr)2)Mo(CO)3 and (C6H6BNBnMe)Mo(CO)3 have been measured using variable-
temperature NMR spectroscopy.
In tr od u ction
The recent availability of the seven-membered ring
borepins (3)12 has allowed exploration of their ligand
properties. We have previously reported on Cr(CO)3
complexes of ring-fused borepins 913 and 1014 and have
made preliminary reports on Mo(CO)3 complexes of
1-substituted borepins 6a ,15 6c,15 and 6d .16 We now
report in detail on Mo(CO)3 complexes of a larger series
of 1-substituted borepins, 6a -l and on the structure
of 6i.
Fully unsaturated boron heterocycles (1, 2, 3 Chart
1) are versatile ligands toward transition metals.2 The
ligand properties of the five-membered ring borole (1)2,3
and the six-membered ring boratabenzene (2)2,4-8 have
been extensively investigated. Typical complexes are
illustrated by structures 4c9 and 5c.10 For both ring
systems, exocyclic π-donor substituents can interact
strongly with boron in a manner which can change the
properties of the ligand. For example, the X-ray struc-
ture of 4c9 shows that the phenylborole ring is η5-
coordination to the metal. However, when the borole
ring bears a strong π-donor as in complexes 7 and 8,
the ring is only η4-coordinated to the metal.11 In the
same manner, B-Mn bonding is much stronger in 5c10
than that in 5l.7a
Resu lts a n d Discu ssion
Syn th eses. 1-Substituted borepins 3b-d are easily
prepared by the exchange reaction of 1,1-dibutylstan-
nepin (11)17 with the appropriate boron halide.12 Al-
ternatively, the reaction of 1-chloroborepin (3d ) with
nucleophiles affords 1-substituted borepins 3a ,15 3e-
l.12 Thus, borepins with a large variety of substituents
are available for study.
X Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) Permanent address: Department of Chemistry, Mu’tah Univer-
sity, Mu’tah/Al-Karak, J ordan.
1-Alkyl- and 1-arylborepins are conveniently con-
verted to their Mo(CO)3 complexes by reaction with
tricarbonyltris(pyridine)molybdenum and boron trifluo-
ride etherate, Scheme 1. In this manner, we have
obtained (1-methylborepin)Mo(CO)3, 6b, as a red air-
sensitive oil and the phenyl derivative 6c as red crystals
which are only modestly air sensitive. In the case of
6c, it is interesting that the Mo(CO)3 group is coordi-
nated to the borepin rather than the phenyl ring.15
Similar metal coordination to the borepin as opposed
to the benzocyclic ring has been observed for 9.13 These
data suggest that the borepin moieties of 9 and 6c are
(2) (a) Herberich, G. E. In Comprehensive Organometallic Chemistry
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford,
U.K., 1995; Vol. 1, p 197. (b) Herberich, G. E. In Comprehensive
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W.,
Eds.; Pergamon: Oxford, U.K., 1982; Vol. 1, p 381.
(3) Herberich, G. E.; Wagner, T.; Marx, H.-W. J . Organomet. Chem.
1995, 502, 67. Herberich, G. E.; Carstensen, T.; Ko¨ffer, D. P. J .; Klaff,
N.; Boese, R.; Hyla-Kryspin, I.; Gleiter, R.; Stephen, M.; Meth, H.;
Zenneck, U. Organometallics 1994 13, 619 and the prior papers in this
series.
(4) (a) Herberich, G. E.; Greiss, G.; Heil, H. F. Angew. Chem., Int.
Ed. Engl. 1970, 9, 805. (b) Herberich, G. E.; Ohst, H. Adv. Organomet.
Chem. 1986, 25, 199.
(5) Ashe, A. J ., III; Meyers, E.; Shu, P.; Von Lehmann, T.; Bastide,
J . J . Am. Chem. Soc. 1975, 97, 6865.
(6) (a) Herberich, G. E.; Schmidt, B.; Englert, U.; Wagner, T.
Organometallics 1993, 12, 2891. (b) Herberich, G. E.; Schmidt, B.;
Englert, U. Organometallics 1995, 14, 471. (c) Herberich, G. E.;
Englert, U.; Schmidt, M. U.; Standt, R. Organometallics 1996, 15, 2707.
(7) (a) Ashe, A. J ., III; Kampf, J . W.; Mu¨ller, C.; Schneider, M.
Organometallics 1996, 15, 387. (b) Ashe, A. J ., III; Kampf, J . W.; Waas,
J . R. Organometallics 1997, 16, 163.
(12) Ashe, A. J ., III; Klein, W.; Rousseau, R. Organometallics 1993,
12, 3225.
(13) Ashe, A. J ., III; Drone, F. J . J . Am. Chem. Soc. 1987, 109, 1879;
1988, 110, 6599.
(8) (a) Bazan, G. C.; Rodriguez, G.; Ashe, A. J ., III; Al-Ahmad, S.;
Mu¨ller, C. J . Am. Chem. Soc. 1996, 118, 2291. (b) Bazan, G. C.;
Rodriguez, G.; Ashe, A. J ., III; Al-Ahmad, S.; Kampf, J . W. Organo-
metallics, submitted for publication.
(9) Herberich, G. E.; Boveleth, W.; Hessner, B.; Ko¨ffer, D. P. J .;
Negele, M.; Saive, R. J . Organomet. Chem. 1986, 308, 153.
(10) Huttner, G.; Gartzke, W. Chem. Ber. 1974, 107, 3786.
(11) Herberich, G. E.; Hessner, B.; Ohst, H.; Raap, I. A. J . Organ-
omet. Chem. 1988, 348, 305.
(14) Ashe, A. J ., III; Kampf, J . W.; Kausch, C. M.; Konishi, H.;
Kristen, M. O.; Kroker, J . Organometallics 1990, 9, 2944.
(15) Ashe, A. J ., III; Kampf, J . W.; Nakadaira, Y.; Pace, J . M. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1255.
(16) Ashe, A. J ., III; Kampf, J . W.; Klein, W.; Rousseau, R. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1065.
(17) (a) Nakadaira, Y.; Sato, R.; Sakurai, H. Chem. Lett. 1987, 1451.
(b) Nakadaira, Y.; Sato, R.; Sakurai, H. J . Organomet. Chem. 1992,
441, 411.
S0276-7333(97)00017-4 CCC: $14.00 © 1997 American Chemical Society

Molybdenum Tricarbonyl Complexes
Organometallics, Vol. 16, No. 9, 1997 1885
Ch a r t 1
Sch em e 1a
Sch em e 2a
a
Key: (i) Py3Mo(CO)3/BF3‚OEt2.
the better ligands toward the group 6 metal tricarbonyl
groups.
Borepins bearing Lewis acid sensitive substituents
cannot be converted directly to their Mo(CO)3 complexes
by using the BF3‚OEt2-mediated ligand transfer. The
reaction of 1-methoxyborepin (3h ) with Py3Mo(CO)3/
BF3‚OEt3 gives 6e rather than 6h . A similar reaction
of (1-diisopropylamino)borepin (3i) leads to largely
intractable products, although small quantities of 6e
a
Key: (ii) LiAlEt3H; (iii) 2HNR2; (iv) H2O; (v) HOCH3; (vi)
Al2O3; (vii) MgSO4.
1
complex cannot be prepared directly from the very labile
free 1-H-borepin 3a .
were detected using H NMR spectroscopy.
The reaction of 1-chloroborepin (3d ) with Py3Mo(CO)3/
BF3‚OEt2 afforded 64% of the corresponding Mo(CO)3
adduct 6d . 6d is quite moisture sensitive. Unless all
manipulations involving 6d are performed in glassware
which has been silylated with hexamethyldisilazane and
subsequently flame-dried in vacuum, the products are
contaminated with hydrolysis products 6f and 6g. The
chloride 6d is a useful precursor to a variety of 1-sub-
stituted borepin Mo(CO)3 complexes. The reaction of
6d with methanol in ether gives the methoxy complex
6h in 87% yield. Treating 6h with alumina affords the
crystalline hydroxy complex 6f, which on drying over
anhydrous MgSO4 was converted to the bis(oxide) 6g.
The above syntheses have provided us with a series
of borepin Mo(CO)3 complexes with a range of substit-
uents. It is now appropriate to compare their proper-
ties.
NMR Sp ectr a . The borepin-Mo(CO)3 complexes
display characteristic NMR spectra. The 1H NMR
spectra in C6D6 are summarized in Table 1.18 The
signals for the six ring protons form an [AA′BB′CC′]
pattern. For all derivatives, the signals for the â-pro-
tons occur at lowest field (δ 4.84-5.20) as a broad
multiplet, while the relative positions of the R-proton
doublet (δ 3.75-4.85) and γ-proton multiplet (δ 4.26-
4.85) are more variable. Analysis of the spectra by
The reaction of 6d with secondary amines gives good
yields of corresponding aminoborepin complexes. In this
manner we have obtained complexes as crystalline
solids 6i-l, Scheme 2. Finally, the reaction of 6d with
Li(AlEt3H) in THF affords 1-H-borepin(tricarbonyl)-
molybdenum (6a ) as a stable crystalline product. This
3
computer simulation (RACCOON) shows that the J H -H
R
â
3
(12.0-11.4 Hz) and J H -H (7.8-7.5) vary little in the
series. Comparison of the signals of 6 with those of 3
â
γ
(18) The 1H NMR spectra are subject to rather large solvent shifts.
See refs 12 and 14.

1886 Organometallics, Vol. 16, No. 9, 1997
Ta ble 1. 1H NMR P a r a m eter s of (C6H6BX)Mo(CO)3 (6)a
Ashe et al.
X (compd)
δ(HR, ∆)
δ(Hâ, ∆)
δ(Hγ, ∆)
3J H
3J H
δ(X)
R-Hâ
â-Hγ
H (6a )
CH3 (6b)
Ph (6c)
Cl (6d )
F (6e)
OH (6f)
O1/2 (6g)
OMe (6h )
N(iPr)2(6i)
4.62 (dd, 3.47)b
4.31 (d, 3.33)
4.86 (d, 3.24)
4.45 (d, 3.15)
4.14 (dd, 2.91)d
3.84 (d, 3.02)
4.37 (d, 2.73)
4.08 (d, 2.91)
3.99 (d, 2.78)
5.15 (br, 2.45)c
5.07 (br, 2.35)
5.33 (br, 2.29)
4.84 (br, 2.41)
4.96 (br, 2.36)
5.05 (br, 2.18)
5.16 (br, 2.17)
5.15 (br, 2.15)
5.14 (br, 1.92)
4.52 (m, 2.37)
4.52 (m, 2.27)
4.64 (m, 2.21)
4.26 (m, 2.31)
4.34 (m, 2.24)
4.48 (m, 2.02)
4.53 (m, 2.08)
4.53 (m, 2.03)
4.77 (m, 1.72)
11.4
7.8
n.o.c
1.08
11.5
11.6
11.4
11.4
11.5
11.5
11.7
12.0
7.8
7.8
7.8
7.7
7.7
7.5
7.8
7.8
7.32-7.46 (m), 7.84 (d)
3.07
3.42
3.72 (sept, J ) 7 Hz)
1.25, 1.23 (d, J ) 7 Hz)
3.27 (d), 2.71 (t), 1.6-1.2 (m)
2.55 (m), 1.8 (m), 1.7-1.5 (m),
1.2-1.0 (m)
7.3-7.1 (m), 4.10 (d, J ) 16.2 Hz)
3.89 (d, J ) 16.2 Hz), 2.44 (s)
N(CH2)5 (6j)
N(c-C6H11)2 (6k )
3.75 (d, 2.95)
4.20 (d)e
5.15 (m, 1.93)
5.20 (m)
4.77 (m, 1.70)
4.85 (m)
12.0
7.8
NBnMe (6l)
3.85 (m, 2.86)
5.16 (m, 1.94)
4.76 (m, 1.73)
a
b 3
All δ(C6D6) values in ppm; ∆ ) δ(C6H6BX) - δ(C6H6BXMo(CO)3). 3J H
and 3J H
in Hz.
J
) 6.0 Hz. c Br ) broad; n.o. )
RHâ
âHγ
CH-BH
not observed. J CH-BF ) 3.2 Hz. e Free ligand unknown.
d
3
Ta ble 2. 13C NMR Ch em ica l Sh ift Va lu es (δ) for (C6H6BX)Mo(CO)3 (6) a n d th e Ch a n ges on
Com p lexa tion (∆)a
X (compd)
δ(CR, ∆)
n.o.c
98.8 (br, 51.2)c 112.0 (34.3)
δ(Câ, ∆)
δ(Cγ, ∆)
δ(CO)
δ(X)
H (6a )b
112.8 (36.4)
97.4 (39)
214.0
215.5
214.3
213.9
214.0
214.0
215.0
214.0
218.4
218.3
218.5
CH3 (6b)b
Ph (6c)b
101 (33.7)
96.8 (38.6)
95.9 (39.5)
95.9 (38.1)
95.5 (37.1)
95.8 (37.5)
95.7 (37.0)
96.2 (34.7)
96.0 (35.5)
96.2
6.0 (br)
98.1 (br, 51.4)
92 (br, 58)
88 (br, 61)
n.o.
112.1 (36.1)
109.4 (38.9)
110.7 (d, 38.8)e
111.3 (35.0)
111.4 (35.6)
111.5 (34.7)
109.8 (28.9)
110.0 (31.2)
109.8
127.7(m), 129.0(p), 133.4(o), Cipso n.o.
Cl (6d )b
F (6e)b
OH (6f)b
O1/2 (6g)b
OMe (6h )b
N(iPr)2 (6i)d
N(CH2)5 (6j)d
n.o.
88 (br, 52)
84 (br, 54)
83 (br, 55)
53.4
46.6(NCH), 22.61, 22.95(Me)
48.1(NCH2), 27.7, 25.6 (CH2)
53.2(NCH), 33.4, 32.8, 26.5, 25.9, 25.4 (CH2)
55.4(NCH2), 36.7(Me)
N(c-C6H11)2 (6k )d 85 (br)f
NBnMe(6l)b
83 (br, 56)
111.53, 111.18 (30.6) 96.28, 96.16 (35.4) 218.2
a
b
d
All δ values in ppm; ∆ ) δ(C6H6BX) - δ((C6H6BX)Mo(CO)3). Solvent, C6D6. c n.o. ) not observed, br ) broad. Solvent, CDCl3.
CH-BF
e 3
J
) 17.0 Hz. f Free ligand unknown.
Ta ble 3. 11B NMR Ch em ica l Sh ift Va lu es (δ) of
show that the upfield shifts on complexation are in the
usual range.19 The complexation shifts are similar to
those between cycloheptatriene and (C7H8)Mo(CO)3.20
Com p lexes (C6H6BX)Mo(CO)3 w ith th e Ch a n ge on
Com p lexa tion (∆)a ,b
X (compd)
δ (∆)
X (compd)
δ (∆)
The 13C NMR chemical shifts of the borepin-Mo(CO)3
complexes 6a -l are collected in Table 2. In all cases,
the signals due to the R-carbon atoms are broad due to
11B coupling, while the other signals are sharp. The
â-carbon signals are invariably at lowest field and occur
over a very narrow range (δ 109-112). The range of
the γ-carbon signals is also small (δ 96-101), while the
R-carbon signals are most variable (δ 83-99). Relative
to the corresponding free borepins, the R-carbon signals
experience the largest complexation shift (52-61 ppm).
The complexation shifts of the â- and γ-carbons are in
the range of 39-29 ppm. In this respect also the
borepin-Mo(CO)3 complexes resemble cycloheptatriene-
Mo(CO)3.21
The 11B NMR chemical shift values of 6 and the shifts
on complexation are collected in Table 3. The complexes
6b-l display 11B chemical shift values over a very
narrow range (δ 27.0-31.7), while the parent borepin
complex 6a has a significantly higher field shift (δ 23.1).
This is the usual effect since the complexes of BH-
derivatives of boratabenzenes,22 1,2,5-thiadiborolenes,23
and boroles24 also show high-field 11B signals.
H (6a )
23.1 (d, J ) 94 Hz, 24.9) O1/2 (6g)
29.7 (10.6)
29.7 (10.3)
27.0 (7.0)
28.4 (5.6)
CH3 (6b)
Ph (6c)
Cl (6d )
F (6e)
31.7 (23.1)
28.1 (20.7)
30.5 (17.4)
30.5 (11.6)
29.9 (10.4)
OMe (6h )
N(iPr)2 (6i)
N(CH2)5 (6j)
N(c-C6H11) (6k ) 28.7c
OH (6f)
NBnMe (6l)
29.0 (6.5)
a
Solvent, C6D6. b ∆ ) δ(C6H6BX) - δ((C6H6BX)Mo(CO)3). c Free
ligand unknown.
complexation. However, the aminoborepins show only
modest (5.6-7.0 ppm) shifts on complexation, while the
oxygen- and fluorine-substituted borepins show inter-
mediate shifts on complexation (10.3-11.6 ppm). This
complexation shift suggests that borepins with π-donor
substituents are more weakly coordinated to the Mo-
(CO)3 moiety.25 However, the chemical shift values of
the complexes themselves do not seem to be sensitive
to the strength of complexation.
Str u ctu r es. The molecular structures of 6c15 and
6d 16 are illustrated in parts a and b of Figure 1,
respectively, while selected bond distances are listed in
Table 4. These structures have been discussed in
The borepins with poor π-donor substituents (H, Me,
Ph,Cl) show an upfield shift of about 20 ppm on
(22) Ashe, A. J ., III; Butler, W.; Sandford, H. F. J . Am. Chem. Soc.
1979, 101, 7066.
(23) Siebert, W.; Full, R.; Edwin, J .; Kinberger, K.; Kru¨ger, C. J .
Organomet. Chem. 1977, 131, 1.
(19) Elschenbroich, Ch.; Salzer, A. Organometallics; VCH Publish-
ers: New York, 1989; pp 307, 298.
(20) Gu¨nther, H.; Wenzl, R. Z. Naturforsch. 1967, 22B, 389.
(21) Olah, G. A.; Yu, S. H. J . Org. Chem. 1976, 41, 1694.
(24) Herberich, G. E.; Negele, M.; Ohst, H. Chem. Ber. 1991, 124,
25.
(25) Similiar trends have been found for the 1,2,5-triaborolene-Fe-
(CO)3 complexes. See ref 23.

Molybdenum Tricarbonyl Complexes
Organometallics, Vol. 16, No. 9, 1997 1887
F igu r e 1. Comparison of the molecular structures of (a) 6c, (b) 6d , and (c) 6i.
carbonyl (12).26 In both 6i and 12, the six-coordinated
Ta ble 4. Selected Bon d Dista n ces (Å) of
(C6H6BX)Mo(CO)3 for X ) Cl, P h , a n d N(iP r )2 (6d ,
carbon atoms are close to coplanar while the uncoordi-
nated ring atoms lie above this plane away from the
Mo(CO)3 unit (0.37 Å for 6i and 0.73 Å for 12). The
C-C bond distances of 6i show a clear distinction
between single and double bonds (1.35-1.46 Å), which
is typical of cycloheptatriene metal complexes. The
B-C bonds of 6i (1.55 Å) are longer than those of 6c
and 6d , indicating weak B-C π-bonding, while the
exocyclic B-N bond (1.39 Å) is quite short, indicating
strong B-N π-bonding.
6c, a n d 6i)
bond
6d (X ) Cl)
6c (X ) Ph)
6i (X ) N(iPr)2)
B-CR
1.514(3)
1.398(3)
1.421(3)
1.411(3)
1.802(2)
2.486(4)
2.417(2)
2.362(2)
2.325(2)
1.529(4)a
1.391(4)a
1.422(4)a
1.408(5)
1.578(4)
2.523(3)
2.408(8)a
2.363(34)a
2.337(11)a
1.549(5)
1.368(6)
1.432(6)
1.363(13)
1.394(7)
2.769(5)
2.424(4)
2.336(4)
2.320(4)
CR-Câ
Câ-Cγ
Cγ-Cγ′
B-X
B-Mo
CR-Mo
Câ-Mo
Cγ-Mo
These changes are readily explained. The π-donation
of the nitrogen lone pair of 6i saturates the boron,
attenuating its ability to π-bond to the ring carbon
atoms. Since these additional electrons would make the
B-Mo interaction antibonding, the B-Mo distance
lengthens. The net effect of changing the exocyclic
substituent is to change the ligand character of the
borepin. For poor π-donor substituents (Cl, Ph, and
presumably H and CH3), the borepin is an η7-aromatic
ligand. For good donors (NR2), the borepin becomes an
η6-trienyl ligand.
Computational studies, (HF/6-31G*)16,28 and MP2/6-
31G*),29 on the uncomplexed aminoborepin have indi-
cated that the molecule adopts a boat conformation
similar to that of cycloheptatriene.30 A boat conforma-
tion is also suggested from indirect experimental evi-
dence based on analysis of the 1H NMR spectrum of 3i.14
On complexation of 3i, as in cycloheptatriene,26b the
major change is a flattening of the bow of the boat,
presumably to effect better overlap with the metal
orbitals in 6i and 12. As in other metal-cyclohep-
tatriene complexes, the alternating single-double bond
pattern is altered little in complexation.26,27
a
Average value.
preliminary communications, however, it is necessary
to recall their salient features in order to compare them
with 6i. The structures of 6c and 6d , which are very
similar, show nearly planar borepin rings which are η7-
bound to the Mo(CO)3 unit. The B-Mo distances (2.523-
(3) Å for 6c and 2.486(4) Å for 6d ) are slightly longer
than the C-Mo distances (2.33-2.42 Å), and the boron
atom is slightly displaced above the ring plane (0.04-
0.05 Å). This conforms to the pattern shown by the
transition metal complexes of most boron heterocycles
and is consistent with the the larger size of boron. The
small range of C-C bond distances (1.39-1.42 Å) shows
that the π-bonds are highly delocalized, while the short
intraring B-C distances (1.529(4) Å for 6c and 1.514-
(3) Å for 6d ) indicate significant π-bonding. These data
clearly show that 1-phenyl and 1-chloroborepins behave
as aromatic ligands.
For comparison, it was of considerable interest to
obtain a crystal structure of a Mo(CO)3 complex of a
borepin bearing a strong donor substituent. A crystal
of 6i suitable for X-ray diffraction was obtained by
recrystallization from pentane. An ORTEP diagram of
the molecular structure of 6i, which shows the number-
ing scheme used in the refinement, is illustrated in
Figure 1c. In contrast to the structures of 6c and 6d ,
the Mo(CO)3 group of 6i is only η6-coordinated to the
six carbon atoms of the boracycloheptatriene ring. The
In contrast, X-ray structural data on 3d 16 and com-
putational data on 3a ,21 3b,21,22 and 3d 16 indicate that
these borepins are planar aromatic rings with a sub-
(26) (a) Dunitz, J . D.; Pauling, P. Helv. Chim. Acta 1960, 43, 2188.
(b) Hadley, F. J .; Gilbert, T. M.; Rogers, R. D. J . Organomet. Chem.
1993, 455, 107.
(27) For Mo(CO)3 complexes, see: Temmel, P. O.; Weidenhammer,
K.; Wienand, H.; Ziegler, M. L. Z. Naturforsch. 1975, 30B, 699.
Sommer, M.; Weidenhammer, K.; Weinand, H.; Ziegler, M. L. Z.
Naturforsch. 1978, 33B, 361.
(28) Schulman, J . M.; Disch, R. L. Organometallics 1989, 8, 733.
(29) Schulman, J . M; Disch, R. L. Mol. Phys. 1996, 88, 213.
(30) Traetteberg, M. J . Am. Chem. Soc. 1964, 86, 4265.
1
B-Mo distance of 2.769(5) Å of 6i is about /4 Å longer
than that of 6c and 6d and must be too long for
significant metal-boron bonding. There is a strong
resemblance of 6i to cycloheptatrienemolybdenum tri-

1888 Organometallics, Vol. 16, No. 9, 1997
Ashe et al.
stantial delocalization of the π-electrons. The major
structural charge on complexation is to reduce the C-C
bond alternation from (0.058 Å in 3d to (0.023 Å in
6d . Thus, delocalization of the π-bonding is enhanced
on complexation.
B-N Rota tion Ba r r ier s. The strong B-N π-bond-
ing in Mo(CO)3 complexes of aminoborepins is indicated
by the large rotational barriers about the B-N bonds.
The 13C NMR spectrum of 6i in C6D6 at 25 °C shows
signals at δ 22.61 and 22.95 for the pairs of diaste-
reotopic iPr methyl groups. The methyl groups are
nonequivalent due to the slow rotation about the B-N
bond. On heating 6i to 54 °C the signals coalesce,
indicating a barrier height of 16.5 ( 0.5 kcal/mol.
An analogous process involving a slow B-N rotation
can be detected from examining the NMR spectra of 6l.
1
At ambient temperature, the H NMR spectrum shows
two signals for the nonequivalent benzyl protons. The
13C NMR spectrum of C6D6 solutions of 6l shows two
signals each for Câ and Cγ, but the signals for CR are
too broad to detect the presumed nonequivalence. On
heating to 62 °C, the signals at δ 96.253 and 96.133 due
to the nonequivalent γ carbon atoms coalesce, indicating
a barrier height of 17.6 ( 0.5 kcal/mol.
The measured barriers to rotation about the B-N
bonds of 6i and 6l are nearly identical. Thus, there are
no significant energetic differences between the rotation
of the NBnMe and N(iPr)2 groups past the Mo(CO)3 unit.
A similar conclusion had been drawn previously in the
boratabenzene series. The barrier to B-N rotation in
6l is identical to that found for 3l (18.0 ( 0.5 kcal/mol).31
Thus, complexation has no effect on the barrier and
presumably the B-N π-bond strength. We argue that
the B-N groups are electronically isolated from the six
ring carbon atoms in both aminoborepins and their Mo-
(CO)3 complexes.
The B-N rotational barriers of analogous tricarbonyl
metal complexes 4i,32 5i,7a and 6i have been found to
be 17.9, 16.5, and 17.5 kcal/mol, respectively. Since the
barriers are similar, the B-N π-bond strengths must
be similar. In each case, the pendant nitrogen is
strongly π-bonded to boron while the π-bonding between
boron and the ring carbon atoms must be weak. Elec-
tronically these compounds are better represented by
structures 4i′, 5i′, and 6i′ rather than 4i, 5i, and 6i,
respectively.
THF was added to a suspension of tris(pyridine)molybdenum
tricarbonyl (0.5 g, 1.2 mmol) in 10 mL of THF. Boron
trifluoride etherate (0.5 mL, 4.1 mmol) was added, and the
reaction mixture was allowed to stir at 25 °C for 1 h. The
solvent was then removed in vacuo leaving a brown residue,
which was extracted with pentane. Removal of pentane left
30 mg (22%) of a red air-sensitive oil. High-resolution EI-
MS: calcd for C10H911B98MoO3, 285.9699; found, 285.9688. IR
(CDCl3, ν(CO)): 2003, 1947, 1919 cm-1
.
Tr ica r bon yl(1-flu or obor ep in )m olybd en u m (6e). A so-
lution of 1-methoxyborepin (400 mg, 3.25 mmol) in 2 mL of
methylene chloride was added slowly to a suspension of tris-
(pyridine)molybdenum tricarbonyl (1.36 g, 3.25 mmol) in 20
mL of ether. Then boron trifluoride etherate (1.9 mL, 15.6
mmol) was added dropwise with stirring. After 4 h at 25 °C,
the solvent was removed under vaccum and the residue was
extracted with pentane. The orange-red filtrate was cooled
to -78 °C, and BF3 gas was bubbled through the solution for
3 min. The reaction mixture was warmed to 25 °C and filtered.
The filtrate was evaporated to dryness under vacuum leaving
an orange-red oil (650 mg, 69%), which on cooling to -15 °C
formed long red-orange needles, mp 53 °C. This compound is
extremely moisture sensitive. 19F NMR (C6D6): δ -136.9. EI-
MS m/z (relative intensity): 290 (49, M+ for C9H611BF98MoO3,
206 (100, M+ - 3CO). High-resolution EI-MS: calcd for
C9H611BF98MoO3, 289.9448; found, 289.9460. IR (CDCl3, ν-
Exp er im en ta l Section
(CO)): 2010, 1948, 1924 cm-1
.
Tr ica r bon yl(1-h yd r oxybor ep in )m olybd en u m (6f) a n d
1,1′-Bis(tr ica r bon yl(bor ep in )m olybd en u m ) Oxid e (6g).
Tricarbonyl(1-methoxyborepin)molybdenum (250 mg, 1 mmol)
was dissolved in 20 mL of ether and added to a chromotogra-
phy column (Al2O3). Nonpolar impurities were eluted with
pentane, while the product was eluted with ether. Slow
evaporation of the solvent left the hydroxy compound as
orange-yellow clusters, mp 123-126°. EI-MS m/ z (relative
Gen er a l Rem a r k s. All reactions were carried out under
an atmosphere of nitrogen or argon. Solvents were dried using
standard procedures. The mass spectra were determined by
using a VG-70-S spectrometer, while the NMR spectra were
obtained by using either
a Bruker WH-360 or WH-300
spectrometer on solutions in CHCl3, C6D6, or CD2Cl2 as noted.
The 1H NMR and 13C NMR spectra were calibrated using
signals from the solvents referenced to Me4Si, while external
BF3‚OEt2 and CCl3F were used to calibrate the 11B NMR and
19F NMR spectra, respectively. The IR spectra were obtained
using a Nicolet 5DXB FT-IR instrument. Experimental
procedures have previously been reported for 6a ,15 6c,15 and
6d .16
intensity): 288 (21, M+ for C9H711B98MoO4), 204 (70, M+
3CO), 98 (100, Mo). IR (CDCl3, ν(CO)): 2007, 1946, 1917 cm-1
-
.
The crude hydroxy compound from above was dissolved in
20 mL of ether, and 2 g of anhydrous MgSO4 was added. After
the mixture was allowed to stand for 20 h, the solvent was
removed under vacuum and the product was extracted with
pentane. Crystallization at -78 °C afforded 200 mg (80%) of
yellow needles, mp 170-172°. EI-MS m/ z (relative inten-
sity): 554 (38, M+ for C18H1211B298Mo2O7). High-resolution
EI-MS: calcd for C18H1211B298Mo2O7, 553.8863; found, 553.8899.
Tr ica r b on yl(1-m et h ylb or ep in )m olyb d en u m (6b ).
A
solution of 1-methylborepin (50 mg, 0.48 mmol) in 10 mL of
(31) Ashe, A. J ., III; Klein, W.; Rousseau, R. J . Organomet. Chem.
1994, 468, 21.
(32) Herberich, G. E.; Ohst, H. Chem. Ber. 1985, 118, 4303.
IR (CDCl3, ν(CO)): 2006, 1944, 1917 cm-1
.

Molybdenum Tricarbonyl Complexes
Organometallics, Vol. 16, No. 9, 1997 1889
Ta ble 5. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for 6ia
Tr icar bon yl(1-m eth oxybor epin )m olybden u m (6h ). Tri-
carbonyl(1-chloroborepin)molybdenum (245 mg, 0.8 mmol) was
stirred for 15 h in 0.5 mL of methanol with 1 g of anhydrous
MgSO4. The liquid phase was decanted, and the solvent was
removed under vacuum. The residue was extracted with
pentane. On cooling to -78 °C, the solution gave 210 mg (87%)
of orange-red air- and moisture-sensitive crystals, mp 110-
115 °C. Prolonged manipulation of this compound led to
product contamination by the corresponding hydroxy borepin
complex. EI-MS m/ z (relative intensity): 302 (40, M+ for
empirical formula
fw
temperature
wavelength
cryst syst
C15H20BMoNO3
369.07
178(2) K
0.710 73 Å
orthorhombic
space group
unit cell dimensions
Pnma (No. 62)
a ) 19.360(6) Å
b ) 12.551(3) Å
c ) 6.723(2) Å
1633.6(8) Å3, 4
1.501 Mg/m3
C
10H911B98MoO4), 218 (100, M - 3CO). High-resolution EI-
MS: calcd for C10H911B98MoO4, 301.9648; found, 301.9665. IR
(CDCl3, ν(CO)): 2005, 1945, 1915 cm-1
volume, Z
density (calcd)
abs coeff
F(000)
cryst size
θ range for data collection
limiting indices
0.810 mm-1
.
752
Tr ica r b on yl(1-(d iisop r op yla m in o)b or e p in )m olyb -
d en u m (6i). Diisopropylamine (72 mg, 0.7 mmol) was added
dropwise with stirring to a solution of tricarbonyl(1-chlo-
roborepin)molybdenum (100 mg, 0.33 mmol) in 15 mL of
hexane at 25 °C. A white precipitate formed immediately.
After 10 min, the mixture was filtered and the clear filtrate
was concentrated under vacuum. Crystallization at -20 °C
afforded 70 mg (63%) of orange crystals, mp 152 °C (dec). EI-
0.50 × 0.24 × 0.14 mm
2.66-28.67 deg
-26 e h e 26, 0 e k e 17,
0 e l e 9
no. reflns collected
9277
no. independent reflns
abs corr
max and min transmission
refinement method
2183 (R(int) ) 0.0504)
XEMP
0.605 and 0.497
full-matrix least-squares on F2
2178/0/136
MS m/ z (relative intensity): 371 (35, M+ for C15H2011B98
-
data/restraints/parameters
goodness-of-fit on F2
final R indices (I > 2σ(I))
R indices (all data)
1.386
MoNO3), 285 (100, M+ - 3CO). High-resolution EI-MS: calcd
R1 ) 0.0430, wR2 ) 0.0951
R1 ) 0.0531, wR2 ) 0.1066
0.909 and -1.050 e‚Å-3
for C15H2011B98MoNO3, 371.0590; found, 371.0588. IR (hexane)
(ν(CO)): 1996, 1937, 1911 cm-1
°C): δ 22.61, 22.95 (iPr CH3)
.
13C NMR (90.5 MHz, Tc)54
largest diff. peak and hole
a
The molecule occupies a mirror plane in the crystal lattice.
The iPr group is disordered across this plane in two orientations,
each at 50% occupancy. Refinement in the lower symmetry space
group Pna2(1) results in ordered iPr groups; however, the remain-
der of the structure then exhibits problems typically associated
with refinement in an incorrect space group. Since the majority
of the X-ray scattering reveals a centrosymmetric structure, we
have opted for the disordered model.
Tr ica r bon yl(p ip er id in obor ep in )m olybd en u m (6j). Pi-
peridine (43 mg, 0.5 mmol) was added dropwise with stirring
to a solution of tricarbonyl(1-chloroborepin)molybdenum (60
mg, 0.2 mmol) in 15 mL of hexane. A precipitate formed
immediately. The reaction mixture was filtered after 10 min,
affording a clear solution, which was concentrated under
vacuum. Crystallization at -20 °C gave 30 mg (42%) of orange
crystals, mp 87 °C (dec). EI-MS m/ z (relative intensity): 355
(35, M+ for C14H1611B98MoNO3), 271 (80, M+ - 3CO), 84 (100).
High-resolution EI-MS: calcd for C14H1611B98MoNO3, 355.0277;
391.0277; found, 391.0284. 13C NMR (90.5 MHz, 25 °C, C6D6,
Tc ) 61 °C): δ 96.28, 96.16 (Cγ).
found, 355.0273. IR (pentane, ν(CO)): 1997, 1937, 1913 cm-1
.
X-r a y Str u ctu r e Deter m in a tion . Crystals of 6i were
obtained by recrystallization from pentane. Crystallographic
data are collected in Table 5. An ORTEP diagram of the
molecular structure of 6i showing the numbering scheme used
in refinement is illustrated in Figure 1c. Table 4 gives the
more important distances for non-hydrogen atoms. A list of
observed and calculated structural factors is available from
A.J .A. on request.
Tr ica r b on yl(1-(d icycloh e xyla m in o)b or e p in )m olyb -
d en u m (6k ). Dicyclohexylamine (127 mg, 0.72 mmol) was
added dropwise with stirring to a solution of tricarbonyl(1-
chloroborepin)molybdenum (110 mg, 0.36 mmol) in 15 mL of
pentane at 25 °C. A white precipitate formed after 5 min. After
1 h at 25 °C, the mixture was filtered and the filtrate was
concentrated under vacuum. Crystallization at -20 °C gave
60 mg (37%) of red crystals, mp 151 °C (dec). EI-MS m/ z
(relative intensity): 451 (4, M+ for C21H2811B98MoNO3), 363 (13,
M+ - 3CO), 138 (100). High-resolution EI-MS: calcd for
Ack n ow led gm en t. We thank the National Science
Foundation (Grant No. CHE-922467) for generous fi-
nancial support. C.D. and W.K. thank the Deutscher
Akademischer Austauschdienst for fellowships. Com-
pound 6b was prepared by Ms. J ennifer Pace in partial
fulfillment of the B.S. degree. Ms. Manuela Schneider
measured the variable-temperature 13C NMR spectrum
of 6i.
C
21H2811B98MoNO3, 451.1216; found, 451.1218. IR (C6D6, ν-
(CO)): 1988, 1925, 1892 cm-1
.
Tr ica r bon yl(1-(N-ben zyl-N-m eth yla m in o)bor ep in )m o-
lybd en u m (6l). A solution of N-benzyl-N-methylamine (95
mg, 0.79 mmol) in 0.5 mL of dry benzene was added dropwise
with stirring to a solution of tricarbonyl(1-chloroborepin)-
molybdenum (120 mg, 0.39 mmol) in 1.0 mL of dry benzene
at 25 °C. A white precipitate formed immediately, and the
color changed to red. After 1 h, 5 mL of pentane was added
and the resulting suspension was filtered, affording a clear
filtrate. On cooling to -20 °C, 90 mg (60%) of product was
isolated as red crystals, mp 152 °C (dec). EI-MS m/ z (relative
intensity): 391 (39, M+ for C17H1611B98MoNO3), 307 (100, M+
- 3CO). High-resolution EI-MS: calcd for C17H1611B98MoNO3,
Su p p or tin g In for m a tion Ava ila ble: Tables of bond
distances and angles, positional values, anisotropic thermal
parameters of non-hydrogen atoms, and positional parameters
of hydrogen atoms of 6i (3 pages). Ordering information is
given on any current masthead page.
OM970017Z