Coordination compounds of monoborane - Lewis base adducts: Syntheses and structures of [M(CO)5(η1-BH3·L)] (M = Cr, Mo, W; L = NMe3, PMe3, PPh3)

Article abstract of DOI:10.1021/ja990828p

Photolysis of [M(CO)₆] (M = Cr, W) in the presence of BH₃·L (L = NMe₃, PMe₃, PPh₃) gave isolable borane complexes [M(CO)₅(η¹-BH₃·L)] (1a, M = Cr, L = PMe₃; 1b, M = Cr, L = PPh₃; 1c, M = Cr, L = NMe₃; 2a, M = W, L = PMe₃; 2b, M = W, L = PPh₃; 2c, M = W, L = NMe₃). In products 1 and 2, the monoborane - Lewis base adduct coordinates to the metal center through a B-H-M three-center two-electron bond, which was confirmed by X-ray structural analyses of 1a, 2a, and 2b at low temperature. The X-ray crystal structural analysis of 1c at ambient temperature also showed the same coordination mode, although the positions of hydrogen atoms on the boron were not determined. The ¹H NMR spectra of 1 and 2 exhibit only one BH signal at -2 to -3 ppm with an intensity of 3H in the temperature range of -80 °C to room temperature. This indicates that the coordinated BH and terminal BH's are rapidly exchanging in solution even at low temperature. When [Mo(CO)₆] was used as a precursor, the formation of the corresponding molybdenum - borane complexes, [Mo(CO)₅(η¹-BH₃·L)] (3a, L = PMe₃; 3b, L = PPh₃; 3c, L = NMe₃), was observed by NMR spectroscopy, but the complexes could not be isolated because of their thermal instability. Complexes of pyridineborane [M(CO)₅(η¹-BH₃·NC₅H ₅)] (1d, M = Cr; 2d, M = W) were also observable by NMR spectroscopy. Fenske - Hall MO calculations for the model compound [Cr(CO)₅(η¹-BH₃·PH₃)] (1e) demonstrated that the bonding between the borane and metal can be described as donation of the bonding electron pair of BH to the a₁ orbital of [Cr(CO)₅], and that π back-donation from the metal d orbital to the antibonding σ* orbital of BH is negligible. Compounds 1-3 can be regarded as model compounds of the methane complex [M(CO)₅(CH₄)], which is observed in the photolyses of [M(CO)₆] in methane matrixes. Structural and spectroscopic features of the ligated borane are discussed and compared with those of related compounds.

Full text of DOI:10.1021/ja990828p

11704
J. Am. Chem. Soc. 1999, 121, 11704-11712
Coordination Compounds of Monoborane-Lewis Base Adducts:
Syntheses and Structures of [M(CO)5(η1-BH3‚L)] (M ) Cr, Mo, W; L
) NMe3, PMe3, PPh3)
Mamoru Shimoi,*,† Shin-ichiro Nagai,† Madoka Ichikawa,† Yasuro Kawano,† Kinji Katoh,‡
Mikio Uruichi,‡ and Hiroshi Ogino‡
Contribution from the Department of Basic Science, Graduate School of Arts and Sciences, UniVersity of
Tokyo, Meguro-ku, Tokyo 153-8902, Japan, and Department of Chemistry, Graduate School of Science,
Tohoku UniVersity, Sendai 980-8578, Japan
ReceiVed March 15, 1999. ReVised Manuscript ReceiVed October 5, 1999
Abstract: Photolysis of [M(CO)6] (M ) Cr, W) in the presence of BH3‚L (L ) NMe3, PMe3, PPh3) gave
isolable borane complexes [M(CO)5(η1-BH3‚L)] (1a, M ) Cr, L ) PMe3; 1b, M ) Cr, L ) PPh3; 1c, M )
Cr, L ) NMe3; 2a, M ) W, L ) PMe3; 2b, M ) W, L ) PPh3; 2c, M ) W, L ) NMe3). In products 1 and
2, the monoborane-Lewis base adduct coordinates to the metal center through a B-H-M three-center two-
electron bond, which was confirmed by X-ray structural analyses of 1a, 2a, and 2b at low temperature. The
X-ray crystal structural analysis of 1c at ambient temperature also showed the same coordination mode, although
1
the positions of hydrogen atoms on the boron were not determined. The H NMR spectra of 1 and 2 exhibit
only one BH signal at -2 to -3 ppm with an intensity of 3H in the temperature range of -80 °C to room
temperature. This indicates that the coordinated BH and terminal BH’s are rapidly exchanging in solution
even at low temperature. When [Mo(CO)6] was used as a precursor, the formation of the corresponding
molybdenum-borane complexes, [Mo(CO)5(η1-BH3‚L)] (3a, L ) PMe3; 3b, L ) PPh3; 3c, L ) NMe3), was
observed by NMR spectroscopy, but the complexes could not be isolated because of their thermal instability.
Complexes of pyridineborane [M(CO)5(η1-BH3‚NC5H5)] (1d, M ) Cr; 2d, M ) W) were also observable by
NMR spectroscopy. Fenske-Hall MO calculations for the model compound [Cr(CO)5(η1-BH3‚PH3)] (1e)
demonstrated that the bonding between the borane and metal can be described as donation of the bonding
electron pair of BH to the a1 orbital of [Cr(CO)5], and that π back-donation from the metal d orbital to the
antibonding σ* orbital of BH is negligible. Compounds 1-3 can be regarded as model compounds of the
methane complex [M(CO)5(CH4)], which is observed in the photolyses of [M(CO)6] in methane matrixes.
Structural and spectroscopic features of the ligated borane are discussed and compared with those of related
compounds.
Introduction
CH bond. Zaric´ and Hall have proposed a C-H-M single
bridge for the bonding between methane and metal in [W(CO)5-
(CH4)], based on a theoretical study.4 However, owing to the
extreme instability of [M(CO)5(CH4)], little is known of its
detailed structure and chemistry. A closely related and more
stable molecular species could be helpful in understanding
coordination of alkanes.
The photolysis of M(CO)6 (where M ) Cr, Mo, W) in inert
media such as a liquid alkane (i.e., CH4 at low temperature) or
a liquid noble gas has been shown to give M(CO)5 and CO.
The resulting M(CO)5 is extremely reactive and reacts even with
CH4 or with a noble gas atom within 1 ps to produce [M(CO)5-
(solv)] (solv ) CH4 or noble gas atom). The coordinated alkane
or noble gas atom in such complexes is extremely labile and
can be very easily displaced. [M(CO)5(solv)] can be detected
in low-temperature matrixes by spectrocopic techniques.1,2 At
room temperature, its lifetime is reported to be of microsecond
order.3 The alkane-coordinated species is of considerable interest
because of its possible role in the catalytic activation of the
The BH4- ion is isoelectronic with the CH4 molecule and is
known to form relatively stable borohydride compounds con-
taining metal-hydrogen-boron three-center two-electron bonds.5
For group 6 metal carbonyl complexes of BH4-, however, only
the bidentate complexes [M(CO)4(η2-BH4)]- are known,6 so a
direct comparison with [M(CO)5(alkane)] is impossible. The first
(3) (a) Hermann, H.; Grevels, F.-W.; Henne, A.; Schaffner, K. J. Phys.
Chem. 1982, 86, 5151. (b) Hodges, P. M.; Jackson, S. A.; Jacke, J.;
Poliakoff, M.; Turner, J. J.; Grevels, F.-W. J. Am. Chem. Soc. 1990, 112,
1234.
(4) Zaric´, S.; Hall, M. B. J. Phys. Chem. A 1997, 101, 4646.
(5) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77, 263 and references
cited therein.
(6) (a) 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. (b) Darensbourg, M. Y.; Bau, R.; Marks, M. W.; Burch, R. R., Jr.;
Deaton, J. C.; Slater, S. J. Am. Chem. Soc. 1982, 104, 6961. (c) 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.
† University of Tokyo.
‡ Tohoku University.
(1) (a) Vogler, A. In Concepts of Inorganic Photochemistry; Adamson,
A. W., Freischauer, P. W., Eds.; John Wiley and Sons: New York, 1975;
pp 274-276. (b) Alway, D. G.; Barnett, K. W. Photochemical Process in
Cyclopentadienylmetal Carbonyl Complexes. In Inorganic and Organo-
metallic Photochemistry; Wrighton, M. S., Ed.; Advances in Chemistry
Series 168; American Chemical Society: Washington, DC, 1978.
(2) (a) Kelly, J. M.; Hermann, H.; Koerner von Gustorf, E. J. J. Chem.
Soc., Chem. Commun. 1973, 105. (b) Simon, J. D.; Xie, X. J. Phys. Chem.
1986, 90, 6751. (c) Joly, A. G.; Nelson, K. A. J. Phys. Chem. 1989, 93,
2876.
10.1021/ja990828p CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/04/1999

Coordination Compounds of Borane-Lewis Base Adducts
J. Am. Chem. Soc., Vol. 121, No. 50, 1999 11705
unidentate borohydride complex was reported by Bommer et
al.7 They prepared [Cu(η1-BH3OAc)(PMePh2)3], to which they
assigned a unidentate structure on the basis of spectroscopic
evidence. The compound was the first to contain a definite
single, unsupported metal-hydrogen-boron bond. They then
synthesized [Cu(η1-BH4)(PMePh2)3]; the structure of this mol-
ecule has been determined unequivocally by X-ray8,9 and
neutron diffraction9 methods.
Scheme 1
The reality of a metal-hydrogen-boron three-center covalent
bond stabilizing the borohydride complexes was subjected to
some questions because of the charges on the metal (1+) and
-
the BH4 (1-). Were these charges necessary to provide an
ionic contribution for the formation of the stable structures? A
neutral borane ligand was required in order to test this point.
The neutral compound, B2H4‚2PMe3, was prepared,10 and it was
demonstrated that it could serve as a bidentate ligand using
three-center metal-hydrogen-Boron bonds.11 Compounds pre-
pared and subjected to X-ray structural verification included
[ZnCl2(η2-B2H4‚2PMe3)],11 [Ni(CO)2(η2-B2H4‚2PMe3)],12 and
several others.13 While Ni(CO)4 reacts with B2H4‚2PMe3 at room
temperature under mild conditions, the reaction stops unless the
liberated CO is removed during the reaction. Other metal carbo-
nyls were even less reactive. Group 6 metal complexes M(CO)6
did not react with B2H4‚2PMe3 when they were combined.
With the additional step of photolyzing M(CO)6 (M ) Cr,
W) in the presence of B2H4‚2PMe3, we synthesized two types
of chromium and tungsten complexes of B2H4‚2PMe3.14 The
first complexes appear to derive from M(CO)5 and involve a
unidentate [M(CO)5(η1-B2H4‚2PMe3)]. These compounds are
heat-sensitive (as is the monodentate copper-borohydride
complex, [Cu(η1-BH4)(PMePh2)3]) and liberate another molecule
of CO to give [M(CO)4(η2-B2H4‚2PMe3)], in which the borane
ligand is bidentate. The process may be represented as in
Scheme 1. This reaction sequence appears to occur more readily
with molybdenum than it does with chromium or tungsten, since
no unidentate molybdenum complex was ever isolated; only the
bidentate molybdenum complex was isolated.
Results
Syntheses and Crystal Structures of Borane Complexes.
When chromium or tungsten hexacarbonyl was photolyzed in
the presence of BH3‚L (L ) PMe3, PPh3, NMe3) in toluene or
hexane using a medium-pressure Hg lamp, solid [M(CO)6]
gradually dissolved and a yellow color developed, accompanied
by evolution of CO. Removal of the solvent and subsequent
recrystallization of the residue afforded novel borane complexes
[M(CO)5(η1-BH3‚L)] (1a, M ) Cr, L ) PMe3; 1b, M ) Cr, L
) PPh3; 1c, M ) Cr, L ) NMe3; 2a, M ) W, L ) PMe3; 2b,
M ) W, L ) PPh3; 2c, M ) W, L ) NMe3) as yellow crystals
in good yields (eq 1).16 These are the first metal complexes of
In view of the foregoing facts, it seemed that, by using the
photolysis technique, it should be possible to synthesize group
6 metal carbonyl complexes containing BH3‚L as a ligand. Here
we report the synthesis and the molecular structure of borane
complexes of the type [M(CO)5(η1-BH3‚L)]. The Lewis bases
L are PMe3, PPh3, and NMe3. These complexes have great
importance as a model system of the methane complexes since
BH3‚L is isoelectronic with methane. The properties of the
B-H-M bond in solution and the results of MO calculations
are also discussed. A part of this work has been published
already.15
neutral monoborane-Lewis base adducts with an unsupported
M-H-B bond.17,18 They are also the second examples of fully
characterized σ complexes of binary [M(CO)5], following
[M(CO)5(η1-B2H4‚2PMe3)] (4). When [Mo(CO)6] was used as
a precursor, formation of the corresponding molybdenum
complexes [Mo(CO)5(η1-BH3‚L)] (3a, L ) PMe3; 3b, L ) PPh3;
(15) Shimoi, M.; Katoh, K.; Uruichi, M.; Nagai, S.; Ogino, H. In Current
Topics in the Chemistry of Boron; Kabalka, G. W., Ed.; The Royal Society
of Chemistry: London, 1994; pp 293-296.
(7) (a) Bommer, J. C.; Morse, K. W. J. Chem. Soc., Chem. Commun.
1977, 137. (b) Bommer, J. C.; Morse, K. W. Inorg. Chem. 1979, 18, 531.
(c) Bommer, J. C.; Morse, K. W. Inorg. Chem. 1980, 19, 587.
(8) (a) Atwood, J. L.; Rogers, R. D.; Kutal, C.; Grutsch, P. A. J. Chem.
Soc., Chem. Commun. 1977, 593. (b) Kutal, C.; Grutsch, P. A.; Atwood, J.
L.; Rogers, R. D. Inorg. Chem. 1978, 17, 3558. This study indicated a linear
metal-hydrogen-boron bond. Two subsequent X-ray studies and a neutron
diffraction study support a bent M-H-B bond.9
(9) Takusagawa, F.; Fumagalli, A.; Koetzle, T. F.; Shore, S. G.;
Scmitkons, T.; Fratini, A.; Morse, K. W.; Wei, C. W.; Bau, R. J. Am. Chem.
Soc. 1981, 103, 5165. (b) Liu, F.-C.; Liu, J.; Meyers, E. A.; Shore, S. G.
Inorg. Chem. 1999, 38, 2169.
(16) Representation of σ complexes is somewhat confusing. The
geometries of the M-H-B linkage in boron hydride complexes are
essentially “end-on”, and M‚‚‚B distances are sufficiently long, particularly
-
in unidentate complexes (bidentate BH4 complexes necessarily involve
close contact between metal and boron).5 Thus, η1-BH4 and η2-BH4 represent
-
unidentate and bidentate BH4 ligands, respectively, where one and two
BH σ bonds interact with the metal. On the other hand, silanes coordinate
to metals in “side-on” fashion through a single Si-H σ bond, which is
represented as “η2-silane”. Similarly, “η2-H2” means a side-on dihydrogen
ligand coordinating through a single σ bond. Throughout this paper, the
unidentate borane ligands in complex 1-3 are represented as “η1-BH3‚L”.
(10) Parry, R. W. Phosphorus, Sulfur Silicon Relat. Elem. 1994, 87, 177.
(11) Snow, S. A.; Shimoi, M.; Ostler, C. D.; Thompson, B. K.; Kodama,
G.; Parry, R. W. Inorg. Chem. 1984, 23, 511.
(12) Snow, S. A.; Kodama, G. Inorg. Chem. 1985, 24, 795.
(13) Shimoi, M.; Katoh, K.; Tobita, H.; Ogino, H. Inorg. Chem. 1990,
29, 814.
(14) (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.

11706 J. Am. Chem. Soc., Vol. 121, No. 50, 1999
Shimoi et al.
Figure 2. ORTEP diagram of [Cr(CO)5(BH3‚NMe3)] (1c) (thermal
elipsoids at 30% probability).
Figure 1. ORTEP diagram of [Cr(CO)5(BH3‚PMe3)] (1a) (thermal
elipsoids at 30% probability).
3c, L ) NMe3) was confirmed by NMR spectroscopy. However,
isolation of 3 was not successful due to rapid decomposition.
This is attributable to the particular lability of the molybdenum
complexes among group 6 metal compounds.19 Also, in the
B2H4‚2PMe3 system, the unidentate molybdenum complex [Mo-
(CO)5(η1-B2H4‚2PMe3)] was not isolated.14 Even in the Cr and
W systems, the complexes of pyridineborane, [M(CO)5(η1-BH3‚
NC5H5)] (1d, M ) Cr; 2d, M ) W), were not stable enough to
be isolated. We note that even prolonged photolysis did not
give bidentate complexes of formula [M(CO)4(η2-BH3‚L)]. This
contrasts with the group 6 metal-tetrahydroborate compounds,
for which only the bidentate complexes [M(CO)4(η2-BH4)]- are
obtained.6 Characterization of the new complexes was ac-
complished by NMR and IR spectroscopy, elemental analysis,
and single-crystal X-ray diffraction, by which the coordination
mode of the borane ligand in the complexes was unequivocally
established. We present the crystal structures of the compounds
first, and the spectroscopic features will be discussed later.
ORTEP diagrams of 1a, 1c, 2a, and 2b are provided in
Figures 1-4, respectively, with the atomic numbering schemes.
Tables 4-7 summarize key geometrical parameters for 1a, 1c,
2a, and 2b, respectively. As evident in the figures, the borane-
Lewis base adduct coordinates to the metal center through a
B-H-M single bridge in these compounds. Each metal atom
in the complexes has five carbonyl ligands, and the geometry
around the central atom is approximately octahedral when the
BH is regarded as one ligand. The positions of the hydrogen
atoms attached to boron were determined by difference Fourier
synthesis for 1a, 2a, and 2b, for which the data collection was
carried out at low temperature. The conformations around the
B-P axis are all staggered. For 1c, the reflection data were
collected at ambient temperature, and the coordinates of the
hydrogen atoms were calculated assuming a staggered confor-
mation around the B-N bond. As the structural features of 1a,
Figure 3. ORTEP diagram of [W(CO)5(BH3‚PMe3)] (2a) (thermal
elipsoids at 30% probability).
Figure 4. ORTEP diagram of molecule A of [W(CO)5(BH3‚PPh3)]
(2b) (thermal elipsoids at 30% probability). Hydrogen atoms on
aromatic rings are omitted for clarity.
1c, 2a, and 2b are almost identical except for the Lewis base
on the boron atom, the details of the structure of 1a, which has
smaller standard deviations, are primarily described below.
When the C(5)-Cr axis is extended beyond the Cr atom, it
crosses the midpoint of the B-H(1) bond. Although the Cr-
H(1)-B bridge is tilted with a bond angle of 130(8)°, the
coordination mode of BH3‚PMe3 is essentially “end-on”. This
clearly indicates that the bonding between the chromium and
the borane is a three-center two-electron bond. The bond angle
of Cr-H(1)-B is roughly comparable to the corresponding
M-H-B angles in the η1-tetrahydroborate complexes, [(Ph2-
MeP)3Cu(η1-BH4)] (121.7°)9 and [V(η1-BH4)2(dmpe)2] (140°)
(dmpe ) PMe2CH2CH2PMe2).20 An η1-borohydride complex
(17) Two complexes with a BH3-inner base adduct coordinating to the
metal center, [(Me3P)2(OC)Fe{CH(SMe)S‚BH3}] (6)17a and [Cp(CO)2Mo-
(P(BH3)Ph{N(SiMe3)2})] (7),17b have been reported. (a) Khasnis, D. V.;
Toupet, L.; Dixneuf, P. H. J. Chem. Soc., Chem. Commun. 1987, 230. (b)
McNamara, W. F.; Duesler, E. N.; Paine. R. T.; Ortiz, J. V.; Kolle, P.;
No¨th, H. Organometallics 1986, 5, 380.
(18) Quite recently, coordination of a BH3‚inner base adduct to Rh in a
bidentate fashion was reported: Mac`ıas, R.; Rath, N. P.; Barton, L. Angew.
Chem., Int. Ed. Engl. 1999, 38, 162.
(19) Elschenbroich, Ch.; Salzer, A. Organometallics; A Consice Intro-
duction, 2nd ed.; VCH: Weinheim, 1992; p 232.
(20) Jensen, J. A.; Girolami, G. S. J. Am. Chem. Soc. 1988, 110, 4450.

Coordination Compounds of Borane-Lewis Base Adducts
Table 1. NMR Spectral Data for Borane Complexesa
J. Am. Chem. Soc., Vol. 121, No. 50, 1999 11707
1H NMR (200 MHz)
compound [M(CO)5(BH3‚L)]
BH3‚L
BH3‚L
11B{1H} NMR (28.8 MHz)
[Cr(CO)5(BH3‚PMe3)] (1a)
[Cr(CO)5(BH3‚PPh3)] (1b)
[Cr(CO)5(BH3‚NMe3)] (1c)
[Cr(CO)5(BH3‚NC5H5)] (1d)
0.34 (d, J(PH) ) 11.1 Hz)
6.8-7.1, 7.2-7.4 (m)
1.60 (s)
7.64 (d, J ) 5.2 Hz, 2-CH)
6.5-6.7 (m, 4-CH)
6.1-6.3 (m, 3-CH)
0.30 (d, J(PH) ) 11.2 Hz)
6.8-7.1, 7.2-7.4 (m)
1.60 (s)
7.64 (d, J ) 5.4 Hz, 2-CH)
6.5-6.7 (m, 4-CH)
6.1-6.3 (m, 3-CH)
0.40 (d, J(PH) ) 11.1 Hz)
6.8-7.1, 7.2-7.5 (m)
1.60 (s)
-3.8 (br q, J(BH) ) 85 Hz)
-2.8 (br, J(BH) ) 80 Hz)
-3.3 (br q, J(BH) ) 86 Hz)
-2.0 (br q, J(BH) ) 84 Hz)
-42.8 (d, J(BP) ) 71 Hz)
-41.9 (br)
-13.4 (br)
-18.2 (br)
[W(CO)5(BH3‚PMe3)] (2a)
[W(CO)5(BH3‚PPh3)] (2b)
[W(CO)5(BH3‚NMe3)] (2c)
[W(CO)5(BH3‚NC5H5)] (2d)
-2.1 (br q, J(BH) ) 82 Hz)
-1.2 (br, J(BH) ) 80 Hz)
-1.5 (br q, J(BH) ) 81 Hz)
-0.1 (br q, J(BH) ) 86 Hz)
-47.0 (d, J(BP) ) 74 Hz)
-46.3 (br)
-17.7 (br)
[Mo(CO)5(BH3‚PMe3)] (3a)
[Mo(CO)5(BH3‚PPh3)] (3b)
[Mo(CO)5(BH3‚NMe3)] (3c)
-2.0 (br q, J(BH) ) 87 Hz)
-1.0 (br, J(BH) ) 85 Hz)
-1.4 (br q, J(BH) ) 86 Hz)
-15.4
a NMR spectra were recorded in C6D6.
Table 2. IR Spectral and Analytical Data for Isolated Complexes
IR (KBr)/cm-1
compd
ν(BH)
ν(CO)
anal. found (calcd)
1a
2455 (w), 2410 (w) 2065 (m), 1995 (sh), 1950 (sh), 1930 (vs), 1890 (s), 1870 (s),
1840 (sh)
C, 33.78; H, 4.30 (C, 34.08; H, 4.29)
1b
1c
2450 (w), 2405 (w) 2055 (m), 1992 (m), 1980 (m), 1950 (sh), 1920 (vs), 1890 (s)
2460 (w), 2430 (w) 2140 (w), 2060 (m), 1995 (s), 1950 (vs), 1930 (vs), 1895 (s),
1870 (s)
C, 58.88; H, 4.10 (C, 59.01; H, 3.88)
C, 35.15; H, 4.60; N, 5.07 (C, 36.26; H, 4.56;
N, 5.29)
2a
2b
2c
2450 (w), 2400 (w) 2075 (m), 1982 (sh), 1940 (sh), 1925 (vs), 1880 (s), 1860 (s)
2460 (w), 2420 (w) 2075 (w), 1990 (m), 1975 (m), 1940 (sh), 1918 (vs), 1885 (m) C, 45.78; H, 3.14 (C, 46.04; H, 3.02)
2460 (w), 2420 (w) 2070 (m), 1985 (s), 1940 (vs), 1920 (vs), 1880 (s), 1860 (s)
C, 22.98; H, 2.72 (C, 23.22; H, 2.92)
C, 24.26; H, 2.71; N, 3.21 (C, 24.21; H, 3.05; N,
3.53)
[FeH(dppe)2(η1-BH4)] (dppe ) PPh2CH2CH2PPh2), with a
significantly large M-H-B angle (162°), has also been
reported.21 The bond distance of B-H(1) (1.12(11) Å) is longer
by ca. 10% than those of B-H(2) and B-H(3) (0.97(17) and
0.94(7) Å, respectively). This is attributed to the decrease of
the bond order between B and H(1) due to coordination to the
chromium atom. Similar lengthening of the B-H bonds has
been observed in the foregoing η1-BH4 compounds and the
complexes of a borane-inner base adduct, [(Me3P)2(OC)Fe-
{CH(SMe)S‚BH3}] (6)17a and [Cp(CO)2Mo(P(BH3)Ph{N(Si-
Me3)2})] (7),17b although the M-H-B bond angles of 6 and 7
complexes (around 10%)23 as well as complexes with an agostic
CH bond (<10%).24 Thus, the borane complexes 1 and 2 may
be classified as unstretched σ complexes.25,26
The interatomic distance between Cr and C(5) (trans carbonyl
to the borane ligand) is substantially shorter (1.95 Å) than those
between Cr and the carbon atoms of the carbonyl ligands in
the cis positions (2.03-2.05 Å). This is explained by the
enhanced back-donation to the trans carbonyl ligand caused by
the poor π-acidity of the borane ligand (vide infra). Moreover,
the carbonyl ligands in the cis positions slightly tilt toward BH3‚
L. This “umbrella effect” also reflects the σ donor character of
the borane ligands in 1 and 2.27
Spectroscopic Features and Dynamic Behavior of 1-3.
Based on the crystal structures, it could be expected that
complexes 1-3 would reveal a signal for the coordinating BH
as well as one for the terminal BH atoms in a 1:2 intensity
(23) (a) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120. (b) Crabtree, R. H.
Acc. Chem. Res. 1990, 23, 95. (c) Morris, R. H.; Jessop, P. G. Coord. Chem.
ReV. 1992, 92, 155. (d) Heinkey, D. M.; Oldham, W. J., Jr. Chem. ReV.
1993, 93, 913.
(24) (a) Green, M. L. H. J. Organomet. Chem. 1983, 205, 395. (b) Schulz,
A. J. Science 1983, 220, 197. (c) Brookhart, M.; Green, M. L. H.; Wang,
L. L. Prog. Inorg. Chem. 1988, 36, 1.
(25) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789.
(26) Only a few examples are known for stretched η2-H2 complexes (e.g.,
[Re(η2-H2)H5{P(p-C6H4Me)3}2])26a and unstretched η2-silane complexes
(e.g., [(OC)4WdC(NMe2)SiHMes2],26b [(Cp2Ti)2(µ-SiH2Ph)2], and [(Cp2-
Ti)2(µ-H)(µ-SiH2Ph)]26c). (a) Brammer, L.; Howard, J. A. K.; Johnson, O.;
Koetzle, T. F.; Spencer, J. L.; Stringer, A. M. J. Chem. Soc., Chem.
Commun. 1991, 241. (b) Schubert, U.; Schwarz, M.; Mu¨ller, F. Organo-
metallics 1994, 13, 1554. (c) Aitken, C. T.; Harrod, J. F.; Samuel, E. J.
Am. Chem. Soc. 1986, 108, 4059.
(27) (a) Crabtree, R. H.; Lavin. M. Inorg. Chem. 1986, 25, 805. (b)
Simpson, C. Q., II; Hall, M. B. J. Am. Chem. Soc. 1992, 114, 1641. (c)
Fischer, R. A.; Miehr, A.; Priermeier, T. Chem. Ber. 1995, 128, 831. (d)
Weiss, J.; Stetzkamp, D.; Nuber, B.; Fischer, R. A.; Boehme, C.; Frenking,
G. Angew. Chem., Int. Ed. Engl. 1997, 36, 70.
are much narrower than those of 1 and 2. The degree of B-H
bond extension in 1 and 2 is less pronounced than that of the
Si-H bond in many of the η2-hydrosilane complexes (around
20% elongation in [CpMn(CO)2(η2-HSiR3)] and related sys-
tems)22 but is rather close to those of most η2-dihydrogen
(21) (a) Baker, M. V.; Field, L. D. J. Chem. Soc., Chem. Commun. 1984,
996. (b) Bau, R.; Yuan, H. S.; Baker, M. V.; Field, L. D. Inorg. Chim.
Acta 1986, 114, L27.
(22) (a) Schubert, U. AdV. Organomet. Chem. 1990, 30, 151. (b) Schubert,
U.; Ackermann, K.; Wo¨rle, B. J. Am. Chem. Soc. 1982, 104, 7378. (c)
Schubert, U.; Bahr, K.; Mu¨ller, J. J. Organomet. Chem. 1987, 327, 357.
(d) Schubert, U.; Mu¨ller, J.; Alt, H. G. Organometallics 1987, 6, 469. (e)
Colomer, K.; Corriu, R. J. P.; Manzin, C.; Viox, A. Inorg. Chem. 1982,
21, 368. (f) Schubert, U.; Scholz, G.; Mu¨ller, J.; Ackermann, K.; Wo¨rle,
B.; Stansfield, R. F. D. J. Organomet. Chem. 1986, 306, 303.

11708 J. Am. Chem. Soc., Vol. 121, No. 50, 1999
Shimoi et al.
Table 3. Crystal Data for 1a, 1c, 2a, and 2b
1a
1c
2a
2b
sample
formula
fw
color of crystal
temp/°C
crystal system
space group
system absence
a/Å
[Cr(CO)5(η1-BH3‚PMe3)]
C8H12BCrO5P
281.90
yellow
-60
monoclinic
[Cr(CO)5(η1-BH3‚NMe3)]
C8H12BCrO5N
265.00
yellow
20
[W(CO)5(η1-BH3‚PMe3)]
C8H12BO5PW
413.81
yellow
-50
monoclinic
[W(CO)5(η1-BH3‚PPh3)]
C23H18BO5PW
600.03
yellow
-90
triclinic
monoclinic
P21/n (variant of no. 14)
P1h (no. 2)
(h 0 l): h + l ) 2n + 1; (0 k 0): k ) 2n + 1
12.540(5)
10.065(2)
12.387(5)
90.0
12.596(6)
9.785(2)
12.014(5)
90.0
117.46(6)
90.0
1314.0(7)
4
1.34
12.629(3)
10.491(2)
24.302(7)
9.261(2)
90.33(2)
101.27(2)
94.43(2)
2308.0(9)
4
b/Å
c/Å
10.195(3)
12.363(4)
90.0
118.02(2)
90.0
1405.1(7)
4
1.96
R/deg
â/deg
118.62(3)
90.0
1372.1(9)
4
γ/deg
V/Å3
Z
d
calcd/g cm-3
1.36
1.73
µ (Mo KR)
crystal size/mm
radiation
9.84
9.10
88.2
54.0
0.43 × 0.20 × 0.13
0.20 × 0.15 × 0.10
0.20 × 0.13 × 0.10
0.30 × 0.20 × 0.10
Mo KR (λ ) 0.710 69 Å)
monochromator
reflcns measd
2θ range/deg
graphite
(h, +k, +l
3-60
2θ - ω
(h, +k, +l
3-60
(h, +k, +l
3-50
(h, (k, +l
3-50
scan mode
ω scan width/deg
ω-scan rate/deg min-1
no. of unique data
no. of data used with
|Fo| > 3σ(Fo)
no. of params refined
R
1.575 + 0.30 tan θ
2.0
4290
1541
1.1 + 0.30 tan θ
0.84 + 0.30 tan θ
1.2 + 0.30 tan θ
8.0
7143
5527
2.0 (3-40°), 1.0 (40-50°)
2.0
4300
2510
2465
1193
158
158
158
560
0.087
0.098
2.38
0.16
0.62
0.063
0.106
0.81
0.10
0.40
0.049
0.057
2.52
0.089
1.32
0.050
0.063
1.70
0.060
1.66
Rw
GOOF
largest shift/esd, final cycle
max resid electron dens/Å3
Table 4. Key Geometrical Parameters for [Cr(CO)5(BH3‚PMe3)]
(1a)
Table 6. Key Geometrical Parameters for [W(CO)5(BH3‚PMe3)]
(2a)
Distances (Å)
Distances (Å)
Cr-H(1)
B-H(1)
B-H(3)
Cr-C(1)
Cr-C(3)
Cr-C(5)
1.94(10)
1.12(11)
0.94(7)
1.90(1)
1.90(1)
1.84(1)
Cr‚‚‚B
2.79(1)
0.97(17)
1.85(1)
1.89(1)
1.89(2)
W-H(1)
B-H(1)
B-H(3)
W-C(1)
W-C(3)
W-C(5)
2.01(9)
1.14(10)
0.92(12)
2.05(1)
2.05(2)
1.97(1)
W‚‚‚B
2.86(2)
1.14(28)
1.85(1)
2.04(1)
2.05(2)
B-H(2)
B-P
B-H(2)
B-P
Cr-C(2)
Cr-C(4)
W-C(2)
W-C(4)
Angles (deg)
Angles (deg)
Cr-H(1)-B
H(1)-Cr-C(1)
130(8)
94(3)
P-B-H(1)
H(1)-Cr-C(2)
95(5)
82(3)
W-H(1)-B
H(1)-W-C(1)
128(7)
96(2)
P-B-H(1)
H(1)-W-C(2)
92(5)
80(2)
Table 5. Key Interatomic Distances (Å) for [Cr(CO)5(BH3‚NMe3)]
(1c)
region) of 1a is shown in Figure 5. The broad quartet of the
BH protons does not decoalesce, even at -80 °C, although it is
sharpened by quadrupole-induced thermal decoupling.28 The
activation energy ∆Gq for the hydrogen exchange process at
193 K is estimated to be less than 30 kJ mol-1, assuming
conventional values for the original chemical shifts for the
bridging BH and the terminal BH’s in 1-3. This value is much
lower than the energy expected for dissociation of the borane
ligand (see later section). Thus, the BH scrambling proceeds
through the concerted site exchange of the hydrogen atoms in
the coordination sphere rather than a dissociation-recombination
mechanism. At the transition state, the borane ligand should
interact in an η2-fashion with the metal center. Fluxional
behavior due to the exchange between the bridging and terminal
hydrogen atoms was also observed for the unidentate B2H4‚
Cr‚‚‚B
2.873(15)
1.91(1)
1.92(1)
1.82(1)
B-N
Cr-C(2)
Cr-C(4)
1.534(14)
1.89(1)
1.88(2)
Cr-C(1)
Cr-C(3)
Cr-C(5)
1
ratio in the H NMR spectra. In the actual spectra, however,
1-3 show only one BH signal at -1.0 to -3.8 ppm with integral
intensity of three protons (Table 1). The observed chemical shifts
of the BH signals are considered to be the weighted average of
typical values of chemical shifts for the terminal BH protons
and that for the metal-coordinating BH proton (in general,
hydrogen atoms terminally bound to boron resonate at 1-2 ppm,
while signals of BH atoms coordinated to transition metals are
found around -10 ppm). This clearly indicates that the bridging
hydrogen atom and the terminal hydrogen atoms in 1-3 are
exchanging their positions faster than the NMR time scale in
solution (Scheme 2). This process is so fast that it is not frozen
(28) (a) Lindner, H. H.; Onak, T. J. Am. Chem. Soc. 1966, 88, 1890. (b)
Bushweller, C. H.; Beall, H.; Grace, M.; Dewkett, W. J.; Bilofsky, H. S. J.
Am. Chem. Soc. 1971, 93, 2145. (c) Bommer, J. C.; Morse, K. W. Inorg.
Chem. 1981, 19, 589.
1
out, even at low temperature. The H NMR spectrum (the BH

Coordination Compounds of Borane-Lewis Base Adducts
Table 7. Key Geometrical Parameters for [W(CO)5(BH3‚PPh3)] (2b)
J. Am. Chem. Soc., Vol. 121, No. 50, 1999 11709
molecule A
molecule B
molecule A
molecule B
Distances (Å)
W-H(1)
B-H(1)
B-H(3)
W-C(1)
W-C(3)
W-C(5)
1.91
1.38
1.05
2.030(14)
2.052(12)
1.953(11)
1.75
1.19
0.97
W‚‚‚B
2.845(13)
0.87
1.900(15)
2.065(13)
2.080(13)
2.812(14)
1.10
1.936(14)
2.055(15)
2.034(11)
B-H(2)
B-P
W-C(2)
W-C(4)
2.054(13)
2.041(12)
1.984(12)
Angles (deg)
W-H(1)-B
H(1)-W-C(3)
119.0
79.9
146.3
101.0
P-B-H(1)
H(1)-W-C(4)
96.5
87.2
109.5
92.8
as an equilibrium mixture of the Nb(III) η2-borohydrido complex
and the Nb(V) boryl(dihydrido) complex.33 However, a similar
equilibrium (between borane complexes and boryl(hydrido)
complexes) is not observed in 1-3.
1
The apparent coupling constants J(BH) of 1-3 are 80-87
Hz, which are smaller by ca. 10 Hz than those of free BH3‚L.
1
1
Since the J(BHb) and J(BHt) values are averaged by the fast
scrambling of BH's, the real values of 1J(BHb) can be estimated
to be around 60 Hz on the assumption that the value of 1J(BHt)
is the same as that of free BH3‚L. This value corresponds to
ca. 65% of 1J(BH) of free BH3‚L (95.0 Hz for BH3‚PMe3, 92.5
Hz for BH3‚PPh3, 91.5 Hz for BH3‚NMe3) and falls into the
range of the reduction percentage of the J values of unstretched
σ complexes (e.g., J(HD) values in most η2-HD complexes are
1
Figure 5. BH region in the H NMR spectrum of 1a.
22-35 Hz, whereas that of free HD is 43 Hz: Jcoord/Jfree
)
50-80%).23 For [CpRe(CO)2(cyclo-C5H10)], the alkane complex
directly observed by NMR,34 the coupling constant of the
coordinating CH was observed to be 96 Hz, which is reduced
by 74% from that of free cyclopentane. In the 11B NMR spectra,
the coordinated boranes in 1-3 resonate at higher field than
free boranes.
Scheme 2
In the IR spectra of 1 and 2, two bands assignable to the
terminal BH stretching are observed at 2400-2460 cm-1. These
frequency values are slightly higher than those for free BH3‚L.
We were not able to identify the ν(BHbrid) bands, probably
because they are overlapped by the strong ν(CO) bands. The
average position of the ν(CO) bands (1950-1870 cm-1) is ca.
30 cm-1 lower in frequency than that for [M(CO)6]. This also
reflects the weak π-accepting ability of the BH3‚L ligand.
Coordination of BH3‚L to the metal in 1-3 is very weak,
and the boranes readily dissociate in solution. When solid
[M(CO)5(η1-BH3‚L)] (M ) Cr, W) was dissolved in C6D6 under
vacuum, an intractable brown precipitate gradually formed, and
the signals of free BH3‚L appeared in the NMR spectra.
Complexes 1a and 2a liberate the BH3‚PMe3 ligand under high
vacuum even in the solid state. This is attributable to the
volatility of BH3‚PMe3. Dissociation of borane ligands from
the complexes is more significant in the presence of other
ligands. When a solution of 1a or 2a was photolytically
generated in a sealed NMR tube and left at room temperature,
growth of the signal of free BH3‚PMe3 and decrease of that of
the complex were observed with concomitant precipitation of
[M(CO)6]. This demonstrates that BH3‚PMe3 in the complex is
replaced by CO in the closed system, which is the reverse
reaction of eq 1. Irradiation of the resulting mixture reproduces
complex 1a or 2a. In the Mo system, the borane complexes are
2PMe3 complex [M(CO)5(η1-B2H4‚2PMe3)] (4), in which the
exchange took place not only between geminal hydrogens but
also with vicinal hydrogens.14
Similar fluxionality has been reported for an η1-silylborohy-
dride complex [Cp2Ta(PMe3){η1-BH3(SiHt-Bu2)}] (∆Gq ) 34.7
kJ mol-1 at 215 K).29 Similarly, only a coalesced BH signal
was observed for the η1-tetrahydroborate complex, [(MePh2P)3-
Cu(η1-BH4)],7 and the iron complex of a borane-inner base
adduct, 6.17a On the other hand, in another borane-inner base
complex, 7, the bridging hydrogen and the terminal hydrogen
atoms are separately observed.17b Exchange between bridging
and terminal hydrogen atoms has also been found for bidentate
-
BH4 complexes, but the barriers to bridging-terminal inter-
change are generally larger than those of 1-3.5 For example,
the free energy of activation for the Ht-Hb interchange in [Mo-
(CO)4(η2-BH4)]- was reported to be 42 kJ mol-1 at 231 K.6
Kubas and co-workers recently reported an η2-SiH4 complex
[Mo(depe)2(CO)(η2-SiH4)] (depe ) PEt2CH2CH2PEt2), which
is in equilibrium with a silyl(hydrido) complex.16,30,31 Intercon-
version between an η2-H2 complex and a dihydrido complex is
also occasionally found in polyhydride compounds.23,32 Like-
wise, a niobocene derivative, Cp2Nb(H2Bcat), is known to exist
(29) Jiang, Q.; Carroll, P. J.; Berry, D. H. Organometallics 1993, 12,
177.
(30) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Bryan, J. C.; Unkefer, C. J.
J. Am. Chem. Soc. 1995, 117, 1159.
(31) Lin et al. suggested a possibility that Kubas’s complex [Mo(depe)2-
(CO)(η2-SiH4)] is in equilibrium not with the hydrido(silyl) complex but
with an isomer generated by the rotation of the silane ligand: Fan, M.-F.;
Jia, G.; Lin, Z. J. Am. Chem. Soc. 1996, 118, 9915.
(32) Kubas, G. J.; Unkefer, C. J.; Swanson, B. I.; Fukushima, E. J. Am.
Chem. Soc. 1986, 108, 7000.
(33) Hartwig, J. F.; De Gala, R. J. Am. Chem. Soc. 1994, 116, 3661.
(34) Recently, a rhenium alkane complex, CpRe(CO)2(cyclopentane), was
directly observed by NMR: Geftakis, S.; Ball, G. E. J. Am. Chem. Soc.
1998, 120, 9953.

11710 J. Am. Chem. Soc., Vol. 121, No. 50, 1999
Shimoi et al.
ligand in 1-3 as donation of the bonding electron pair of BH
to the vacant a1 orbital of [M(CO)5]. This is consistent with the
end-on coordination mode of the borane ligand in 1-3. It also
accounts well for the fact that the BH is not stretched so much
during the complexation, and the bond distance between Cr and
C(5) is significantly shorter than those between Cr and carbon
atoms of cis carbonyls in 1a, 1c, 2a, and 2b. Finally, the bonding
orbital generated by the a1 of [Cr(CO)5] and the next HOMOs
of BH3‚PH3 is strongly characterized by the latter. This can
explain the lability of the borane ligand and the fluxional
behavior found in the complexes.
Recently, the σ complexes of tricoordinate catecholborane
(HBcat), [Cp2Ti(HBcat)2] and [Cp2Ti(PMe3)(HBcat)], were
reported by Hartwig and co-workers.38-40 In these compounds,
HBcat ligands bind to Ti(II) in side-on fashion. The MO
calculation for [Cp2Ti{HB(OH)2}2] (a model compound of [Cp2-
Ti(HBcat)2]) showed that HBcat ligands function as an acceptor
with the empty p orbital of the boron atom.38,40 Our study
demonstrates that tricoordinate boranes and tetracoordinate
borane-Lewis base adducts are quite different in their manner
of coordination to a metal.
Complexes 1-3 as Model Compounds of Alkane Com-
plexes. Coordination of an alkane to an unsaturated metal center
has been one of the most interesting interactions in organome-
tallic chemistry.41-43 The first evidence for the existence of such
interactions came from studies on photochemically generated
[M(CO)5] (M ) Cr, Mo, W) in alkane matrixes. Turner and
co-workers demonstrated that [M(CO)5(alkane)] and [M(CO)5-
(noble gas)] complexes are produced after the photolyses of
Figure 6. Correlation diagram of [Cr(CO)5] with BH3‚PH3 obtained
by Fenske-Hall MO calculations.
extremely labile and the coordinated BH3‚L is rapidly replaced
by CO in the system, so 3 could not be isolated.
[M(CO)6] in alkane and noble gas matrixes, respectively.44
A
dihydrogen complex [M(CO)5(H2)] has also been reported as
the short-lived product of photolysis of [M(CO)6] in an inert
medium containing H2.3,45 These species are, however, ex-
tremely labile, so structural detailes are not clear. Concerning
the bonding between the alkane and metal in [M(CO)5(alkane)],
it is possible to suppose a C-H-M three-center two-electron
bond as found in intramolecular agostic interactions.24 Indeed,
a recent theoretical study indicates that the most stable geometry
of [W(CO)5(CH4)] has a bent C-H-M linkage.4 The borane
complexes 1-3 are closely related to the methane complexes
[M(CO)5(CH4)], since BH3‚L is isoelectronic with CH4.
For 1-3, the structural features (lengthening of the BH bond
on coordination is small and the M-H-B angle is wide) and
spectroscopic data (reduction of the 1JBH coupling constants is
also small) indicate that the borane complexes correspond to
an earlier stage on the reaction coordinate of oxidative addition
if we adopt the notion that the bonding in σ complexes is
“incomplete oxidative addition”. This is very similar to most
η2-H2 complexes and complexes with an agostic C-H-M
interaction. Moreover, as described in the previous section, π
Discussion
Molecular Orbitals of the Borane Complexes. To better
understand the bonding in the borane complexes 1-3, Fenske-
Hall MO calculations were carried out for a model compound
[Cr(CO)5(η1-BH3‚PH3)] (1e).35 Figure 6 illustrates the correlation
diagram of the molecular orbitals of 1e. The bonding orbital
between Cr and the borane ligand is generated by the overlap
of the empty a1 orbital of [Cr(CO)5] with one of the degenerate
next HOMOs, which has the character of a BH bonding orbital,
of BH3‚PH3 (the HOMO of BH3‚PH3 is essentially a bonding
orbital of BP). The almost triply degenerate HOMOs of 1e are
derived from the “t2g” set of [Cr(CO)5]. They are completely
nonbonding and scarcely include components of the atomic
orbitals of the bridging hydrogen and boron. In other words,
there is almost no back-donation from the filled d orbital of
chromium to the BHσ* orbital. One of the reasons should be
the fact that [Cr(CO)5] is predominantly a σ acceptor and only
a weak π donor.36 However, it is still in contrast to the orbital
interactions in [Cr(η2-H2)(CO)5] shown by Saillard and Hoff-
mann, where Cr back-donation into the H2 σ* orbital is small
but significant.37 The doubly degenerate LUMOs of BH3‚PH3
are essentially of PHσ* character, in which the coefficients of
the AOs of the BH’s and the boron atom are very small. The
antibonding σ* orbital of BH is energetically so high (93.0 eV,
not depicted in Figure 6) that it cannot interact with the dxz
orbital of Cr, even though it has allowed symmetry. Thus, we
can describe the bonding between the metal and the borane
(38) Hartwig, J. F.; Muhoro, C. F.; He, X.; Eisenstein, O.; Bosque, R.;
Maseras, F. J. Am. Chem. Soc. 1996, 118, 10936.
(39) Muhoro, C. F.; Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1997,
36, 1510.
(40) Muhoro, C. N.; He, X.; Hartwig, J. F. J. Am. Chem. Soc. 1999,
121, 5033.
(41) Hall, C.; Perutz, R. N. Chem. ReV. 1996, 96, 3125.
(42) Sun, X.-Z.; Grills, D. C.; Nikiforov, S. M.; Poliakoff, M.; George,
M. W. J. Am. Chem. Soc. 1997, 119, 7521.
(43) Coordination of heptane to an iron-porphirin in the solid state has
been suggested: Evans, D. R.; Drovetskaya, T.; Bau, R.; Reed, C. A.; Boyd,
P. D. W. J. Am. Chem. Soc. 1997, 119, 3633.
(44) Graham, M. A.; Poliakoff, M.; Turner, J. J. J. Chem. Soc. (A) 1971,
2939.
(45) (a) Graham, M. A.; Perutz, R. N.; Poliakoff, M.; Turner, J. J. J.
Organomet. Chem. 1972, 34, C34. (b) Poliakoff, M.; Turner, J. J. J. Chem.
Soc., Dalton Trans. 1974, 2276. (c) Perutz, R. N.; Turner, J. J. J. Am. Chem.
Soc. 1975, 97, 4791.
(35) (a) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972, 11, 768. (b)
Bursten, B. E.; Fenske, R. F. J. Chem. Phys. 1977, 67, 3138. (c) Bursten,
B. E.; Jensen, R. J.; Fenske, R. F. J. Chem. Phys. 1978, 68, 3320. (d) Fenske,
R. F. Pure Appl. Chem. 1988, 60, 1153.
(36) Zang, S.; Dobson, G. R.; Brown, T. L. J. Am. Chem. Soc. 1991,
113, 6908.
(37) Saillard, J.-Y.; Hoffmann, R. J. Am. Chem. Soc. 1984, 106, 2006.

Coordination Compounds of Borane-Lewis Base Adducts
J. Am. Chem. Soc., Vol. 121, No. 50, 1999 11711
vacuum. BH3‚PMe3, BH3‚PPh3, BH3‚NMe3, and BH3‚NC5H5 were
prepared by treatment of B2H6 with the corresponding bases under
vacuum.53 [Cr(CO)6] and [Mo(CO)6] (Strem) and [W(CO)6] (Aldrich)
Scheme 3
1
were used as purchased. H NMR spectra were recorded on Varian
XL-200 and JEOL GX-400 spectrometers. 11B and31P NMR spectra
were recorded on JEOL FX-90Q and JEOL R-500 spectrometers. IR
spectra were recorded on JASCO IR-810, JASCO FTIR-350, and
Bruker IFS66v spectrometers. NMR data of the new complexes obtained
in this work are listed in Table 1. IR and analytical data for the isolated
compounds are listed in Table 2.
back-donation from the metal d orbital to the BH σ* orbital of
the borane ligand is negligible in 1-3. This is in sharp contrast
to the η2-silane and η2-H2 complexes, in which these σ ligands
have been shown to be π-acceptors (it is particularly significant
for η2-silanes) theoretically37,46 and experimentally.47 Thus, the
π-acidities of the σ ligands are in the following order: silanes
> H2 > BH3‚L. It is presumed that the poor accepting ability
of BH3‚L is very similar to those of alkanes. A question arises
here: Why does BH3‚L form more stable complexes with
[M(CO)5] than H2 despite its poor π-acidity? A possible
explanation is that, because BH is more basic than H2 (and CH),
the BH bonding orbitals are higher than those of H2 and CH in
energy, and the BH bond is polarlized in a B(+)-H(-) fashion.
This idea is supported by the fact that complexes of less basic
pyridineborane, 1d and 2d, are not stable enough to be isolated.
For the [Cr(R3SiH)(CO)5] system, silanes possessing electron-
releasing groups bind more strongly to [Cr(CO)5].36 This trend
is similar to the case for the borane complexes and is opposed
to other metal-silane systems.
Synthesis of [Cr(CO)5(η1-BH3‚PMe3)] (1a). A Pyrex glass tube (22
mm o.d.) was charged with [Cr(CO)6] (67 mg, 0.30 mmol) and BH3‚
PMe3 (27 mg, 0.30 mmol). Toluene (8 mL) was transferred to the
sample tube by conventional trap-to-trap distillation. The resulting
suspension was photolyzed using a 450-W medium-pressure Hg lamp
with stirring at 0 °C under high vacuum. During the photolysis, a yellow
color developed, and evolution of CO was observed. After irradiation
of the suspension for 1 h, the evolved CO in the reaction vessel was
completely removed by freeze-pump-thaw cycles, and then the
mixture was irradiated again for 1 h. The resulting solution was
evaporated to dryness at -15 °C. Recrystallization of the yellow residue
from pentane at -20 °C afforded 1a (70 mg, 0.25 mmol, 88%) as air-
and moisture-sensitive yellow crystals.
Other borane complexes were prepared by a similar procedure. The
yields were as follows: [Cr(CO)5(η1-BH3‚PPh3)] (1b), 94%; [Cr(CO)5-
(η1-BH3‚NMe3)] (1c), 87%; [W(CO)5(η1-BH3‚PMe3)] (2a), 93%;
[W(CO)5(η1-BH3‚PPh3)] (2b), 88%; [W(CO)5(η1-BH3‚NMe3)] (2c),
24%. Mass spectra of these compounds gave poor data, probably due
to their easy fragmentation. Therefore, only the mass spectral data of
1c and 2c are given. 1c (EI, 25 eV): m/z 265 (0.7, M+ based on 184W),
237 (1.7, M+ - CO), 209 (5.7, M+ - 2CO), 153 (M+ - 4CO), 125
(26.4, M+ - 5CO), 192 (8.8, Cr(CO)5+), 164 (10.4, Cr(CO)4+), 136
(9.5, Cr(CO)3+), 108 (33.0, Cr(CO)2+), 80 (100, Cr(CO)+), 52 (85.2,
Cr+), 72 (32.0, BH2‚NMe3+). 2c (EI, 25 eV): m/z 369 (9.9, M+ - CO
based on 184W), 352 (100, W(CO)6+), 341 (3.2, M+ - 2CO), 324 (13.3,
W(CO)5+).
In the 1H NMR spectrum of the alkane complex [CpRe(CO)2-
(cyclo-C5H10)], the coordinating hydrogen atom is observed as
a coalesed signal with the geminal hydrogen atom at -100 °C.34
Theoretical work has also shown that the activation energy for
the scrambling of the hydrogen atoms in [W(CO)5(CH4)] is very
small.4 These properties of the alkane complexes are similar to
the fluxional character of the borane complexes 1-3. The
dynamic behavior of 1-3 is also closely related to the isotope
exchange in intermediate methane complexes, which proceeds
faster than elimination of CH3D (Scheme 3).48-50 Periana and
Bergman first proposed the role of a methane σ complex as an
intermediate for the rhodium system based on careful labeling
experiments.48 In all cases, HD site exchange was shown to be
faster than methane elimination. Similar hydrogen scrambling
Formation of [Mo(CO)5(η1-BH3‚PMe3)] (3a). A Pyrex NMR tube
equipped with a high-vacuum stopcock was charged with [Mo(CO)6]
(ca. 0.05 mmol) and BH3‚PMe3 (0.05 mmol) and evacuated. Benzene-
d6 (0.5 mL) was introduced into the NMR tube. The resulting suspension
was irradiated at ca. 5 °C using a 450-W medium-pressure Hg lamp,
with removal of the evolved CO every 1 h. After 3 h of photolysis, the
1
NMR tube was flame-sealed, and a H NMR spectrum was quickly
recorded. The signals assignable to 3a were observed at positions close
to those of 1a and 2a. The formation of [Mo(CO)5(η1-BH3‚PPh3)] (3b)
was confirmed by a similar method.
1
has been observed by H NMR for a methylosmium complex,
[Cp*Os(H)Me(dmpm)]+ (dmpm ) PMe2CH2PMe2).51 Although
the borane ligand in 1-3 readily dissociates as described above,
the interchange of the BH’s is much faster than its dissociation.
As mentioned above, the properties of 1-3 are very similar
to those of alkane complexes. The alkane in [M(CO)5(alkane)]
is occasionally termed a “token ligand”.52 This work has
visualized the “token ligand” by the use of isoelectronic borane-
Lewis base adducts.
Attempted Synthesis of [Mo(CO)5(η1-BH3‚NMe3)] (3c). Photolysis
of a hexane suspension of [Mo(CO)6] and BH3‚NMe3 was carried out
using a procedure similar to that for 1a. After photolysis (4 h
irradiation), the resulting yellow solution was filtered and cooled at
-20 °C to provide 3c (61 mg, 0.20 mmol, 46%) as a yellow solid.
However, the product rapidly decomposed to give a brown solid.
Observation of [M(CO)5(η1-BH3‚NC5H5)] (1d, M ) Cr; 2d, M
) W). The photoreaction of [M(CO)6] (M ) Cr or W) with BH3‚
1
NC5H5 was monitored by H NMR spectroscopy. Signals assignable
to 1d and 2d were observed in the NMR spectra. However, intensities
of the signals reached the upper limit where they were smaller than
those of free BH3‚NC5H5. Attempts to isolate the complexes were not
successful.
Experimental Section
All manipulations were carried out under high vacuum or a dry
nitrogen atmosphere. Reagent-grade pentane, hexane, toluene, and THF
were distilled under a nitrogen atmosphere from sodium-benzophenone
ketyl just before use. Benzene-d6 and toluene-d8 were dried over
potassium mirrors before use and transferred into NMR tubes under
X-ray Crystal Structure Determination. Crystals of 1a, 1c, 2a,
and 2b were obtained by cooling their pentane solutions at -20 °C.
The intensity data for the complexes were collected on a Rigaku AFC-5
(for 1a, 2a, and 2b) and Rigaku AFC-6A (for 1c) four-circle
diffractometers using graphite-monochromated Mo KR radiation (λ )
0.710 73 Å). Data collection for 1a, 2a and 2b was carried out at -60,
-50, and -90 °C, respectively. That for 1c was carried out at 20 °C.
The reflection data were corrected with Lorentz and polarization factors
but not for absorption and extinction. The space groups were determined
from the systematic absence. Crystallographic and experimental data
for these crystals are listed in Table 3.
(46) Schubert, U. J. Organomet. Chem. 1983, 205, 395.
(47) Butts, M. D.; Bryan, J. C.; Luo, X.-L.; Kubas, G. J. Inorg. Chem.
1997, 36, 3341.
(48) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 7332.
(49) Bullock, R. M.; Headford, C. E. L.; Kegley, S. E.; Norton, J. R. J.
Am. Chem. Soc. 1985, 107, 727.
(50) Gould, G. L.; Heinkey, D. M. J. Am. Chem. Soc. 1989, 111, 5502.
(51) Gross, C. L.; Girolami, G. S. J. Am. Chem. Soc. 1998, 120, 6605.
(52) Dobson, G. R.; Hodges, P. M.; Healy, M. A.; Poliakoff, M.; Turner,
J. J.; Firth, S.; Asali, K. J. J. Am. Chem. Soc. 1987, 109, 4218.
(53) Hewitt, F.; Holliday, A. K. J. Chem. Soc. 1953, 530.

11712 J. Am. Chem. Soc., Vol. 121, No. 50, 1999
Shimoi et al.
The structures of all complexes were solved by the heavy atom
method. All the non-hydrogen atoms were located and refined applying
anisotropic temperature factors. For 1a and 2a, the coordinates of the
hydrogen atoms bound to the boron atom were determined by difference
Fourier synthesis and refined isotropically. For 2b, the hydrogen atoms
bound to the boron atom were found and then fixed. For 1c, the
positions of hydrogen were calculated assuming B-H distances of 1.09
Å and a staggered conformation around the B-N axis and then fixed.
Atomic scattering factors for non-hydrogen atoms and hydrogen atoms
were taken from refs 54 and 55, respectively. Calculations were
performed using the program UNICS III.56
The Fenske-Hall Molecular Orbital Calculation. The molecular
orbitals were calculated for [Cr(CO)5(η1-BH3‚PH3)] (1e) by Fenske-
Hall methods.35 The geometry of 1e was idealized to the Cs symmetry.
Calculations for BH3‚PH3 were performed assuming the C3V symmetry.
For the required parameters of the atoms, the values given by R. F.
Fenske et al. were used as presented.
Supporting Information Available: Tables of positional
parameters, anisotropic temperature factors, bond distances, and
bond angles for 1a, 1c, 2a, and 2b; positional parameters of
the model compound 1e for the Fenske-Hall MO calculations
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(54) International Tables for X-ray crystallography; Kynoch Press:
Birmingham, 1974; vol. IV.
(55) Stewart, R. F.; Davidson, R. E.; Simpson, W. T. J. Chem. Phys.
1965, 42, 3175.
(56) Sakurai, T.; Kobayashi, M. Rikagaku Kenkyujo Hokoku 1979, 55,
69.
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