Low-valent ditantalum complex Ta2(μ-BH3)(μ-dmpm)3(η2-BH4)2: First dinuclear compound containing a bridging BH3 group with direct Ta-B bonds

Article abstract of DOI:10.1021/ja9817242

The reduction of TaCl₅ with LiBH₄ in the presence of the bidentate ligand bis(dimethylphosphino)-methane, dmpm, produces the novel BH₃-bridged ditantalum complex Ta₂(μ-BH₃)(μ-dmpm)₃(η₂-BH₄)₂ (1) and the acid- base adduct dmpm (BH₃)₂ (2). Two crystalline forms of 1 were obtained, and their structures have been determined by X-ray crystallography. The unique feature of 1 is that the bridging borane is bound only to the ditantalum metal center, forming a simple metallaborane cluster, Ta₂B, with averaged Ta-Ta and Ta-B bond distances of 2.7792[5] and 2.307[9]A?, respectively; the averaged Ta-B separation for the axially coordinated BH₄^- groups, which are bound to the Ta atoms through hydrogen atoms, is ~0.2 A? longer than that of the bound BH₃ group. The μ-BH₃ group has one exo-hydrogen atom with a B- h(exo) distance of 1.10-[8] A?. The distance between the endo-hydrogen atom and the boron atom is 1.23[8] A?. Each of the two endo-hydrogen atoms is also bound to a Ta atom with the Ta-H distance of 1.98[8] A?. Compound 1 represents the first example of a structurally characterized metallaborane complex containing a μ-BH₃ group bringing two metal centers. The structure of the air-stable compound 2 has also been determined; it shows a strong P to B dative bond with the P-B distance of 1.903(4) A?.

Full text of DOI:10.1021/ja9817242

9594
J. Am. Chem. Soc. 1998, 120, 9594-9599
Low-Valent Ditantalum Complex Ta2(µ-BH3)(µ-dmpm)3(η2-BH4)2:
First Dinuclear Compound Containing a Bridging BH3 Group with
Direct Ta-B Bonds
F. Albert Cotton,*,† Carlos A. Murillo,*,†,‡ and Xiaoping Wang†
Contribution from the Laboratory for Molecular Structure and Bonding, Department of Chemistry,
Texas A&M UniVersity, College Station, Texas, and Department of Chemistry,
UniVersity of Costa Rica, Ciudad UniVersitaria, Costa Rica
ReceiVed May 18, 1998
Abstract: The reduction of TaCl5 with LiBH4 in the presence of the bidentate ligand bis(dimethylphosphino)-
methane, dmpm, produces the novel BH3-bridged ditantalum complex Ta2(µ-BH3)(µ-dmpm)3(η2-BH4)2 (1)
and the acid-base adduct dmpm(BH3)2 (2). Two crystalline forms of 1 were obtained, and their structures
have been determined by X-ray crystallography. The unique feature of 1 is that the bridging borane is bound
only to the ditantalum metal center, forming a simple metallaborane cluster, Ta2B, with averaged Ta-Ta and
Ta-B bond distances of 2.7792[5] and 2.307[9] Å, respectively; the averaged Ta-B separation for the axially
-
coordinated BH4 groups, which are bound to the Ta atoms through hydrogen atoms, is ∼0.2 Å longer than
that of the bound BH3 group. The µ-BH3 group has one exo-hydrogen atom with a B-Hexo distance of 1.10-
[8] Å. The distance between the endo-hydrogen atom and the boron atom is 1.23[8] Å. Each of the two
endo-hydrogen atoms is also bound to a Ta atom with the Ta-H distance of 1.97[8] Å. Compound 1 represents
the first example of a structurally characterized metallaborane complex containing a µ-BH3 group bridging
two metal centers. The structure of the air-stable compound 2 has also been determined; it shows a strong P
to B dative bond with the P-B distance of 1.903(4) Å.
Introduction
carbonyl anions [H3BrM(CO)nL]-, where M ) Mn, Re, Co;
L ) CO, PPh3; and n ) 3, 4.5 No X-ray structure of such a
complex has been reported.
The simplest monoborane, BH3, forms donor-acceptor
adducts of general formula BH3‚L, where L ) Lewis base;1
the LfBH3 dative bond strength depends on the nature of the
donor. For example, the dative bond in BH3‚THF is weak, and
this adduct is often used as a borane source. When reacted
with stronger bases such as phosphines, BH3 forms more-stable
complexes of the type BH3‚P, where P ) phosphine ligand.
The phosphine adducts of BH3 have been the subject of several
spectroscopic2 and X-ray crystallographic studies.3 Conceiv-
ably, the Lewis base could be an electron-rich transition metal
unit; i.e., one in which the metal atom is in a formally low
oxidation state and thus one that contains d electrons available
for bonding to a Lewis acid.4 However, very few transition
metal complexes containing BH3 as an electron-accepting ligand
are known. Examples are the group 7 and group 8 metal
To our knowledge, direct coordination6 of BH3 to polynuclear
metal-containing units is known only for trinuclear clusters of
the type M3(CO)6(phosphine)3(BH3)(µ-H)2,7 and M3(CO)9(BH3)-
(µ-H)2,8 where M ) Fe, Ru, and Os. The ruthenium cluster
appears to exist in solution as two isomers, Ia and Ib. The
former has a boron atom capping the Ru3 triangle; it is directly
bound to one Ru atom but indirectly, via hydrogen atoms
bridging, to the other two Ru atoms. The second isomer (Ib)
arises from the rearrangement of the bridging hydrogen atoms
on the Ru3B core. Consequently, the BH3 group in Ia changes
* To whom correspondence should be addressed, at the Department of
Chemistry, Texas A&M University, P.O. Box 30012, College Station,
TX 77842-3012. Tel.: 409-845-4432. Fax: 409-845-9351. E-mail:
cotton@tamu.edu; murillo@tamu.edu.
† Texas A&M University.
‡ University of Costa Rica.
-
to a borohydride BH4 ion in Ib which caps the Ru3 triangle
through hydrogen atom bridges.
(1) Schmid, G. Angew. Chem., Int. Ed. Engl. 1970, 9, 819-830.
(2) (a) Power, W. P. J. Am. Chem. Soc. 1995, 117, 1800-1806. (b)
Cowley A. H.; Damasco, M. C. J. Am. Chem. Soc. 1971, 93, 6815-6821.
(c) Roudolph, R. W.; Schultz, C. W. J. Am. Chem. Soc. 1971, 93, 6821-
6822.
(3) See, for example: (a) Donaghy, K. J.; Carroll, P. J.; Sneddon, L. G.
Inorg. Chem. 1997, 36, 547-553. (b) Schmidbaur, H.; Stu¨tzer, A.; Bissinger,
P.; Schier, A. Z. Anorg. Allg. Chem. 1993, 619, 1519-1525. (c) Huffman,
J. C.; Skupinski, W. A.; Caulton, K. G. Cryst. Struct. Commun. 1982, 11,
1435-440.
(4) (a) Parry, R. W. Phosphorus Sulfur Silicon 1994, 87, 177-191. (b)
Housecroft, C. E. Polyhedron 1987, 6, 1935-1958.
(5) Parshall, G. W. J. Am. Chem. Soc. 1964, 86, 361-364.
(6) We define “direct coordination” as the formation of a bond between
a metal and a B atom not mediated by an H atom.
(7) Housecroft, C. E.; Matthews, D. M.; Edwards, A. J.; Rhengold, A.
L. J. Chem. Soc., Dalton Trans. 1993, 2727-2734.
(8) (a) Vites, J. C.; Eigenbrot, C.; Fehlner, T. P. J. Am. Chem. Soc. 1984,
106, 4633-4635. (b) Chipperfield. A. K.; Housecroft, C. E. J. Organomet.
Chem. 1988, 349, C17-C21. (c) Chung, J.-H.; Boyd, E. P.; Liu, J.; Shore,
S. G. Inorg. Chem. 1997, 36, 4778-4781.
S0002-7863(98)01724-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/02/1998

Ditantalum Complex with Direct Ta-B Bonds
J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9595
NMR spectra were recorded at 80.1 MHz, with an external 85% H3-
PO4 solution as standard (0.00 ppm). 11B NMR spectra were recorded
at 64.2 MHz, and chemical shifts were referenced to BF3‚diethyl ether
(0.00 ppm).
Formally, the BH3 group can also be considered a 2-electron
donor, in which a B-H σ bond is used to coordinate to a metal
center forming the so-called “σ complexes.” A few substituted
monoborane σ complexes, namely, Cp2Ti(PMe3)(HBcat) and
Cp2Ti(HBcat)2, where HBcat ) catecholborane, have been
recently reported.9 In Cp2Ti(PMe3)(HBcat) the Ti, H, and B
atoms form a triangle with a Ti-B bond length of 2.267(6) Å
and a 3c-2e bond between the Ti-H-B atoms (II); the Ti-H
bond distance is 1.61(5) Å. For Cp2Ti(HBcat)2, the Ti-B bond
distance is 2.335(5) Å.
Preparation of Ta2(µ-BH3)(µ-dmpm)3(η2-BH4)2 (1) and 1‚toluene.
A mixture of TaCl5 (1.08 g, 3.0 mmol) and LiBH4 (0.40 g, 18.4 mmol)
was refluxed in toluene for 20 h to give a black suspension. After this
suspension was cooled to ambient temperature, a black solid precipi-
tated. The colorless supernatant liquid was removed with a syringe,
and the solid was dried under vacuum. To the solid was added cold
(-70 °C) THF (30 mL) and the mixture was stirred at that temperature
for 30 min. Then a solution of dmpm (0.76 mL, 4.8 mmol) in THF
(10 mL) was introduced into the mixture. The cooling bath was re-
moved after 4 h and stirring was continued for 10 h at room temperature
to give a purple solution. The THF solvent was then removed under
vacuum and the remaining solid was first extracted with toluene (40
mL). Slow diffusion of hexanes (20 mL) into the toluene filtrate gave
purple crystals of 1‚toluene. Yield 0.41 g, 30%. A second extraction
with diethyl ether (40 mL) also produced a purple solution. After
filtration, the volume of the ether solution was reduced to 20 mL, then
layered with hexanes. Purple crystals of 1 were produced in 2 weeks,
(yield 0.024 g, 2%). 1H NMR δ 2.18 [t, 2H, PCH2P, J(31P-1H) ) 8.7
Hz], 2.09 (br m, 2H, PCH2P), 1.73 (m, 2H, PCH2P), 1.59, 1.36, 1.27
(36H, CH3), -0.26 [br q, 8H, BH4-, J(11B-1H) ) 93.0 Hz]. 31P{1H}
NMR δ 4.52 (m), -7.32, -7.55 (s). 11B{1H} NMR δ -28.5 (br, s,
BH4-), -33.5 (unresolved m, BH3). IR (KBr pellet) 3449 (br, m), 2962
(m), 2924 (w), 2902 (m), 2376 (s), 2341 (sh, s), 2203 (w), 2122 (w),
2090 (w), 2052 (w), 1412 (m), 1345 (m), 1296 (m), 1262 (s), 1107 (s),
1066 (m), 930 (vs, br), 869 (m), 804 (m, br), 766 (w), 746 (w), 729
As a part of our interest in extending the chemistry of metal-
metal-bonded compounds to low-valent tantalum, we used
LiBH4 as reducing agent to react with TaCl5. The products
isolated and the final oxidation states of the tantalum atoms
from the reduction of TaCl5 with LiBH4 have turned out to be
ligand-dependent and sensitive to subtle changes in reaction
conditions. Previously we reported that µ4,η2-B2H6 dianion
2-
bridged ditantalum(III) complexes10 resulted from the reaction
of TaCl5 with LiBH4 in the presence of arylformamidinate
ligands, [ArNC(H)NAr]-, where Ar ) phenyl or tolyl. We also
found that the reaction of potassium with crude Ta2(µ2-η4-B2H6)-
(µ-DTolF)4 produces [(B2H5)N(Tol)C(H)N(Tol)]Ta(µ2-NTol)2(µ2-
DTolF)2Ta[(Tol)NC(H)N(Tol)B2H5], in which a new chelating
ligand is formed by fusion of a B2H5 or a BH3 unit to a DTolF-
anion.11 Cleavage of formamidinate ligands can produce the
imido group NTol2- and several tantalum complexes containing
cleaved formamidine ligands have been characterized.12 In this
paper we report the synthesis and structural characterization of
a novel BH3-bridged ditantalum compound, namely, Ta2(µ-
BH3)(µ-dmpm)3(η2-BH4)2 (1), where dmpm ) bis(dimeth-
ylphosphino)methane. This compound was prepared by the
reduction of TaCl5 with LiBH4 in the presence of dmpm, and
isolated along with dmpm(BH3)2 (2). Compound 1 provides
the first example of a bridging BH3 complex to be crystallo-
graphically characterized in a dinuclear complex.
(w), 709 (m), 698 (m) 635 (m), 518 (br, w), 468 (w) cm-1
.
Preparation of dmpm(BH3)2 (2). A 1.0 M THF solution of BH3
(1.0 mL, 1.0 mmol) was added to a solution of dmpm (80 µL, 0.50
mmol) in toluene (4 mL) at 0 °C. After having been stirred for 1 h,
the resulting solution was layered with hexanes (6.0 mL) and placed
at -30 °C. Slow diffusion of hexanes into the solution gave crystals
of 2 (yield 0.075 g, 91%), mp 152-154 °C. 1H NMR δ 1.18 [dq, 6H,
BH3, J(11B-1H) ) 95.4 Hz, J(31P-1H) ) 10.8 Hz], 0.989 [t, 2H,
PCH2P, J(31P-1H) ) 11.4 Hz], 0.871 [d, 12H, P(CH3)2, J(31P-1H) )
10.2 Hz]. 13C NMR δ 25.40 [t, PCH2P J(13C-31P) ) 24.5 Hz], 13.65
[d, P(CH3)2, J(13C-31P) ) 39.9 Hz]. 31P{1H} NMR δ 5.28 [q, J(31P-
11B) ) 58.1 Hz]. 11B{1H} NMR δ -33.12 [d, J(11B-31P) ) 58.1 Hz].
IR (KBr pellet) 3074 (vw), 2997 (w), 2980 (w), 2923 (m), 2913 (w),
2878 (w), 2587 (w), 2377 (vs), 2239 (s), 2276 (w), 2253 (m), 1426
(sh,w), 1418 (m), 1366 (w), 1304 (sh, w), 1296 (m), 1193 (m), 1148
(m), 1132 (m), 1078 (sh, w), 1067 (s), 941 (br, vs), 899 (m), 870 (s),
820 (m), 793 (m), 765 (m), 709 (m), 668 (m), 576 (m), 554 (w) cm-1
.
Experimental Section
X-ray Structure Determinations. Geometric and intensity data for
1‚toluene and 2 were collected at -100 and -150 °C, respectively,
with a Nonius CAD4-S diffractometer. Detailed procedures have
previously been described.13 Data for compound 1 were gathered at
-60 °C with a Nonius FAST area detector system, utilizing the software
program MADNES.14
Unit cell refinement for 1‚toluene and 2 utilized 25 high-angle
reflections. In each case, the cell dimensions and Laue group were
confirmed by axial images. All data were corrected for Lorentz and
polarization effects. Three strong reflections measured periodically
throughout the data collection for compounds 1‚toluene and 2 showed
no significant decay. An empirical absorption correction based on six
æ-scans was applied for 1‚toluene and 2. For compound 1, we used
the program SORTAV15 to correct for absorption.
Materials. All manipulations were performed under an atmosphere
of argon by using standard Schlenk techniques. Solvents were purified
by conventional methods and were freshly distilled under nitrogen prior
to use. Tantalum(V) chloride and dmpm were purchased from Strem
Chemicals, Inc. Lithium borohydride and BH3‚THF (1.0 M in THF)
were purchased from Aldrich Chemical Co. Tantalum(V) chloride was
resublimed before use, but other chemicals were used as received.
Physical Measurements. IR spectra were recorded on a Perkin-
Elmer 16PC Fourier transform IR spectrophotometer as KBr pellets.
1H and 13C NMR spectral measurements were performed with a Varian
Unity 300 spectrometer. The 1H NMR spectra were recorded at 299.96
MHz; chemical shifts are referenced to trimethylsilane (0.00 ppm). 13
C
NMR spectra were recorded at 75.43 MHz; chemical shifts were
referenced to CDCl3 (77.0 ppm). 31P and 11B NMR spectral measure-
In all structures, the positions of heavy atoms were found in direct
methods E-maps by using the software solution program in SHELX-
ments were performed with a Varian-200 broad band spectrometer. 31
P
(9) (a) Muhoro, C. N.; Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1997,
36, 1510-1512. (b) Hartwig, J. F.; Muhoro, C. N.; He, X. J. Am. Chem.
Soc. 1996, 118, 10936-10937.
(13) (a) Bryan, J. C.; Cotton, F. A.; Daniels, L. M.; Haefner, S. C.;
Sattelberger, A. P., Inorg. Chem. 1995, 34, 1875-1883. (b) Scheidt, W.
R.; Turowska-Tyrk, I. Inorg. Chem. 1994, 33, 1314-1318.
(14) Pflugrath, J.; Messerschmidt, A., MADNES, Munich Area Detector
(New EEC) System, Version EEC 11/1/89, with enhancements by Enraf-
Nonius Corp., Delft, The Netherlands. A description of MADNES appears
in: Messerschmidt, A.; Pflugrath, J. J. Appl. Crystallogr. 1987, 20, 306-
315.
(10) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. J. Am.
Chem. Soc. 1996, 118, 4830-4833.
(11) Cotton, F. A.; Daniels, L. M.; Matonic, J. H.; Murillo, C. A.; Wang,
X. Polyhedron 1997, 16, 1177-1191.
(12) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. Inorg. Chem.
1997, 36, 896-901.
(15) Blessing, R. H. Acta Crystallogr. 1995, A51, 31-38.

9596 J. Am. Chem. Soc., Vol. 120, No. 37, 1998
Cotton et al.
Table 1. Crystal Data and Structure Refinement for 1, 1‚toluene,
and 2
compound
1
1‚toluene
2
empirical formula
formula weight
C15H53B3P6Ta2 C22H61B3P6Ta2 C5H20B2P2
813.72
905.86
163.77
crystal system
space group
a, Å
b, Å
c, Å
monoclinic
P21/c
orthorhombic hexagonal
Pnma
P6122
6.279(1)
6.279(1)
47.46(1)
90
17.076(2)
11.663(2)
15.8577(8)
99.257(8)
3117.0(7)
4
16.308(1)
16.086(1)
13.8755(7)
90
â, °
V, Å3
Z
3640.0(4)
4
1.653
1620.5(5)
6
1.007
D
calc, g/cm3
1.734
temperature, K
reflections collected
data/restraints/
parameters
213(2)
12726
4075/0/445
173(2)
2700
2438/6/262
123(2)
1842
708/0/79
final R indices [I >
2σ(I)], R1,a wR2b
R indices (all data),
R1, wR2
0.032, 0.080
0.035, 0.082
0.026, 0.067
0.038, 0.072
1.051
0.033, 0.079
0.048, 0.083
Figure 1. A perspective view of the molecular structure of 1.
goodness-of-fitc on F2 1.087
1.195
Nonhydrogen atoms are represented by their 40% probability ellipsoids.
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo )2]1/2
;
2
w ) 1/σ2(Fo ) + (a×p)2 + b×p, p ) [Max(Fo2 or 0) + 2(Fc2)]/3. a )
2
containing 1; according to the NMR spectra, these attempts were
all unsuccessful. The only method of separation that worked
in our hands, and it is far from being completely satisfactory,
was manual selection of the colorless crystals of 2 and separation
from those of compound 1, a painstaking process that must be
done in a drybox. However, a small amount of 2 could always
be detected by solution NMR (see below).
0.035 and b ) 9.34 for 1; a ) 0.025, b ) 26.93 for 1‚toluene; a )
0.033 and b ) 0.60 for 2. c Quality-of-fit ) w(|F| - |F|)2/(Nobs
-
1/2
Nparameters
) .
TL.16 Subsequent cycles of least-squares refinement followed by
difference Fourier synthesis produced the positions of the remaining
nonhydrogen atoms; they were refined anisotropically. Positions of
hydrogen atoms were found and located from the final difference
Fourier map. These hydrogen atoms, which are essential in defining
the structures, were included and refined isotropically. Compound 2
crystallized in the chiral space group P6122, which was confirmed by
the program XPREP of the SHELXTL package. However, the absolute
configuration of 2 could not be established from the Flack17 parameter
because of possible twinning of the crystal, as indicated by the structural
refinement. Crystal data are given in Table 1.
The structures of both crystalline forms of 1 have been
determined by X-ray crystallography. For compound 1 there
is one complete molecule in the asymmetric unit, and it is
depicted in Figure 1. Selected bond distances and angles for 1
are listed in Table 2. Compound 1 consists of two Ta atoms
joined by one BH3 group and three dmpm ligands; each of the
-
Ta atoms is also coordinated by an η2-BH4 anion located in
Results and Discussions
the axial position. A drawing of the molecule of 1 in 1‚toluene
is shown in Figure 2. Selected dimensions for 1‚toluene are
presented in Table 3. It has crystallographically imposed Cs
symmetry with a mirror plane passing through the methylene
carbon atoms of the three bridging dmpm ligands and the boron
atom of the bridging BH3 group. The trans-dmpm ligands adopt
a boat conformation with essentially identical Ta-P distances
in the range of 2.567(2) to 2.580(2) Å. The bridging borane is
trans to the third dmpm ligand at the equatorial position. Each
The reaction of TaCl5 with LiBH4 at reflux temperature in
toluene gave a black solid, which was then reacted with dmpm
in THF at low temperature to give Ta2(µ-BH3)(µ-dmpm)3(η2-
BH4)2 (1) (eq 1). After removal of THF, the product was
2TaCl5 + 10LiBH4 + 3dmpm THF8
Ta2(µ-BH3)(µ-dmpm)3(η2-BH4)2 + 4H2 +
10LiCl + 7BH3‚THF (1)
-
axial BH4 anion coordinates to a Ta atom through two
hydrogen atoms, H(11) and H(12), with Ta-H distances of 2.16-
(7) and 2.17(7) Å, respectively. The B(1)-Ta separation of
2.578(8) Å is comparable with the distances reported for
complexes with (3c,2e) Ta-H-B linkages.9,10 The bridging
boron atom B(2) is symmetrically coordinated to both tantalum
atoms with a B(2)-Ta distance of 2.306(8) Å, ∼0.26 Å shorter
than the distance between the axial B(1) atom and the Ta atom
for the borohydride units in which a Ta-H-B bond exists, and
only ∼0.04 Å longer than those reported for the catecholborane
complexes of titanium8 mentioned earlier. The distance between
the bridging boron and tantalum atoms in 1 is just slightly longer
than those observed in complexes containing direct Ta-B bonds
without hydrogen atom bridges. For example, the tantalaborane
complex Cp*TaCl2(B4H8) has a Ta-B bond distance of 2.249-
(12) Å;18 and for both isomers of the η1-boryl compound Cp2-
extracted with toluene. Crystals of 1‚toluene were obtained by
slow diffusion of hexanes into its toluene solution at 0 °C.
The presence of BH3 in the reaction mixture also led to the
formation of a dmpm-borane adduct, namely, dmpm(BH3)2 (2).
This was crystallized as a byproduct along with crystals of 1
from the toluene solution. Compound 1 is only slightly soluble
in ether, from which we also obtained a small crop of
nonsolvated crystals.
Compound 1 is sensitive to air and moisture; it burns im-
mediately when exposed to air. In dichloromethane, it decom-
poses to form insoluble dark solids. The similarity in the
solubility of 1 and 2 makes their separation difficult. Innumer-
able attempts to separate these two compounds by solvent ex-
traction were also hampered by the great instability of solutions
(16) SHELXTL (version 5.03); Siemens Industrial Automation: Madison,
WI.
(18) Aldridge, S.; Hasimoto, H.; Sang, M.; Fehlner, T. P. Chem. Commun.
1998, 207-208.
(17) Flack, H. D. Acta Cryst. 1983, A39, 876-881.

Ditantalum Complex with Direct Ta-B Bonds
J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9597
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1
Table 3. Selected Bond Distances (Å) and Angles (deg) for
1‚toluene.
Ta(1)-Ta(2)
2.7708(5) Ta(1)-B(1)
2.539(10)
2.585(10)
1.82(7)
2.02(7)
2.12(7)
2.568(2)
2.545(2)
2.575(2)
1.20(10)
1.19(7)
1.17(7)
1.12(8)
1.09(8)
1.12(10)
76.2(2)
Ta-Taa
2.7875(5) Ta-H(11)
2.578(8) Ta-H(12)
2.306(8) Ta-H(21)
2.580(2) B(1)-H(11)
2.569(2) B(1)-H(12)
2.567(2) B(1)-H(13)
1.852(5) B(1)-H(14)
1.843(5) B(2)-H(21)
1.839(5) B(2)-H(22)
1.821(7) P(2)-C(21)
1.839(8) P(2)-C(22)
1.850(7) P(3)-C(32)
2.16(7)
2.17(7)
2.02(6)
1.09(7)
1.08(7)
1.23(6)
1.24(6)
1.29(6)
1.11(8)
1.835(7)
1.833(8)
1.833(7)
Ta(1)-B(3)
2.304(9)
2.312(9)
2.25(8)
Ta(2)-B(2)
Ta(2)-B(3)
Ta(1)-H(13)
Ta-B(1)
Ta(1)-H(14)
Ta(1)-H(32)
Ta-B(2)
Ta(2)-H(23)
2.04(8)
Ta(2)-H(24)
Ta-P(1)
Ta(2)-H(33)
1.86(10)
2.583(2)
2.545(2)
2.572(2)
1.13(13)
0.99(8)
1.03(12)
1.16(7)
1.28(7)
Ta(1)-P(1)
Ta-P(2)
Ta(1)-P(3)
Ta(1)-P(6)
Ta-P(3)
Ta(2)-P(2)
Ta(2)-P(4)
P(1)-C(1)
Ta(2)-P(5)
B(1)-H(11)
P(2)-C(2)
B(1)-H(12)
B(1)-H(13)
C(3)-P(3)
B(1)-H(14)
B(2)-H(21)
P(1)-C(11)
P(1)-C(12)
P(3)-C(31)
B(2)-Ta-B(1)
Ta-B(2)-Taa
P(3)-Ta-P(2)
B(2)-H(21)-Ta
P(2)-Ta-B(1)
B(2)-Ta-P(1)
P(2)-Ta-P(1)
P(3)-Ta-Taa
B(1)-Ta-Taa
B(2)-H(22)
B(2)-H(23)
B(2)-H(24)
B(3)-H(31)
B(3)-H(32)
B(3)-H(33)
116.8(2)
B(2)-Ta-P(3)
B(2)-Ta-P(2)
P(3)-Ta-B(1)
P(3)-Ta-P(1)
B(1)-Ta-P(1)
B(2)-Ta-Taa
P(2)-Ta-Taa
P(1)-Ta-Taa
Ta-B(2)-Taa
143.3(2)
B(3)-Ta(1)-B(1)
B(3)-Ta(1)-P(6)
B(1)-Ta(1)-P(1)
B(1)-Ta(1)-P(3)
P(1)-Ta(1)-P(3)
H(32)-Ta(1)-Ta(2)
B(3)-Ta(1)-Ta(2)
P(6)-Ta(1)-Ta(2)
P(3)-Ta(1)-Ta(2)
H(33)-Ta(2)-B(3)
B(3)-Ta(2)-P(2)
B(3)-Ta(2)-P(5)
H(33)-Ta(2)-B(2)
P(2)-Ta(2)-B(2)
P(4)-Ta(2)-B(2)
117.8(4)
B(3)-Ta(1)-P(1)
P(6)-Ta(1)-P(1)
B(3)-Ta(1)-P(3)
P(6)-Ta(1)-P(3)
P(1)-Ta(1)-Ta(2)
B(1)-Ta(1)-Ta(2)
P(2)-Ta(2)-P(5)
P(2)-Ta(2)-P(4)
B(3)-Ta(2)-P(4)
P(5)-Ta(2)-P(4)
B(3)-Ta(2)-B(2)
P(5)-Ta(2)-B(2)
H(33)-Ta(2)-Ta(1)
P(2)-Ta(2)-Ta(1)
P(4)-Ta(2)-Ta(1)
74.4(3)
94.51(5)
85(3)
86.4(2)
98.2(3)
163.75(5)
94.05(3)
168.6(2)
75.3(3)
97.2(2)
99.23(5)
83.4(2)
52.8(2)
94.40(3)
93.25(3)
74.4(3)
144.8(2)
87.5(3)
84.3(3)
164.96(7)
81(2)
53.2(2)
94.67(5)
92.28(5)
29(3)
76.0(2)
142.9(2)
94(3)
85.4(2)
82.9(3)
94.16(8)
96.5(2)
99.11(7)
93.66(5)
170.1(3)
94.14(7)
163.03(7)
98.6(2)
99.42(7)
117.1(3)
97.1(2)
H(12)-B(1)-H(11) 108(7)
H(11)-B(1)-H(13) 110(5)
H(12)-B(1)-H(13) 115(6)
H(12)-B(1)-H(14) 105(7)
H(13)-B(1)-H(14) 119(6)
B(1)-H(12)-Ta
P(1)-C(1)-P(1)a
P(2)a-C(2)-P(2)
P(3)-C(3)-P(3)a
B(2)-H(21)-Ta
H(11)-B(1)-H(14)
B(1)-H(11)-Ta
98(6)
100(4)
100(4)
75(3)
95.04(5)
94.27(5)
H(22)-B(2)-H(21) 108(4)
112.5(5)
119.3(5)
117.9(5)
85(3)
H(21)-B(2)-H(21)a 108(5)
H(22)-B(2)-Ta
H(21)a-B(2)-Ta
133(2)
119(3)
H(23)-Ta(2)-Ta(1) 146(2)
B(3)-Ta(2)-Ta(1)
B(2)-Ta(2)-Ta(1)
H(31)-B(3)-H(33) 114(6)
H(33)-B(3)-H(32) 111(6)
H(33)-B(3)-Ta(1) 112(5)
53.0(2)
169.3(2)
a Symmetry transformations used to generate equivalent atoms: x,
-y + 1/2, z.
H(31)-B(3)-Ta(2) 133(4)
H(32)-B(3)-Ta(2) 121(3)
H(31)-B(3)-H(32) 106(5)
H(31)-B(3)-Ta(1) 134(4)
B(3)-H(32)-Ta(1)
B(3)-H(33)-Ta(2)
H(33)-B(3)-Ta(2)
Ta(1)-B(3)-Ta(2)
85(4)
99(6)
53(5)
73.8(3)
Table 4. Core Dimensions (Å or deg) Averaged for Cs Symmetry
H(32)-B(3)-Ta(1)
61(3)
compound
1
1‚toluene
average
Ta-Ta′
Ta-B1
2.7708(5)
2.304(9)
2.312(9)
2.02(7)
1.86(10)
1.28(7)
1.12(10)
1.09(8)
73.8(3)
85(4)
2.7875(5)
2.306(8)
2.7792[5]
2.307[9]
Ta-H1
B1-H1
2.02(6)
1.29(6)
1.97[8]
1.23[8]
B1-H2
Ta-B1-Ta′
Ta-H1-B1
1.11(8)
74.4(3)
85(3)
1.10[8]
74.1[3]
89[4]
99(6)
H1-B1-H2
H1-B1-H1a
114(6)
106(5)
111(6)
108(4)
108(5)
109[6]
110[6]
Figure 2. A perspective view of the molecular structure of 1 in 1‚
toluene. Nonhydrogen atoms are represented by their 40% probability
ellipsoids.
Figure 3. A view of the Ta2B core in 1 and 1‚toluene with a labeling
scheme that allows the comparison of the two independent molecules,
as given in Table 4.
TaH2(BO2C6H4), the Ta-B bond distances are 2.263(6) and
2.295(11) Å.19
Both independently determined molecular structures of 1 are
essentially the same and conform closely to Cs symmetry. Table
4 has the important inner dimensions of the molecules, defined
with reference to Figure 3. The averaged values for both mole-
cules are also listed in the last column of Table 4. The unique
feature in 1 is that the boron atom from the borane group in the
molecule is not bound to other elements except the ditantalum
metal center, thus forming a simple metallaborane cluster with
a Ta2B core. As shown in Figure 3, the µ-BH3 group has one
exo hydrogen atom with an averaged B(1)-H(2) distance of
1.10[8] Å; the distance between the endo hydrogen atom and
the boron atom is 1.23[8] Å. Each of the two endo hydrogen
atoms is also bound to a Ta atom with the Ta-H(1) distance
(19) Lantero, D. R.; Motry, D. H.; Ward, D. L.; Smith, III, M. R.; J.
Am. Chem. Soc. 1994, 116, 10811-10812.

9598 J. Am. Chem. Soc., Vol. 120, No. 37, 1998
Cotton et al.
Scheme 1
of 1.97[8] Å. Compound 1 represents the first example of a
structurally characterized compound with a µ-BH3 group
bridging two metal centers.
Figure 4. A drawing of the structure of 2, showing the atom numbering
scheme. Nonhydrogen atoms are represented by their 40% probability
ellipsoids.
This type of symmetrically bridging µ-BH3 group in a dimetal
unit has not been observed before. As shown in Scheme 1, the
bonding between the bridging BH3 group and the tantalum metal
centers in 1 may be described formally in one of three ways:
as a ditantalum(I,I) compound containing a 4-electron-donor
BH3 ligand (IIIa); as a Ta2(II,II) compound coordinated by a
6-electron-donor BH32- dianion (IIIb); or as a Ta2(I,I) complex
(IIIc), in which there is donation of electrons from the low-
valent Ta(I) atoms to the bridging boron atom. To be a
4-electron donor, the electron-deficient BH3 group in (IIIa) must
employ two B-H bonds to form two σ bonds to the Ta atoms.
Table 5. Selected Bond Distances (Å) and Angles (deg) for 2
P(1)-B
1.903(4) P(1)-C(2)
1.815(3) P(1)-C(3)
1.797(3)
1.796(4)
P(1)-C(1)
P(1)-C(1)-P(1A) 121.2(3)
C(3)-P(1)-C(1) 107.5(1)
C(2)-P(1)-C(3) 104.9(2)
C(3)-P(1)-B 111.9(2)
C(2)-P(1)-C(1)
C(1)-P(1)-B
C(2)-P(1)-B
101.6(2)
116.2(1)
113.7(2)
) 93.0 Hz). These are consistent with a fluxional behavior of
the coordinated BH4- ions in solution. The exchange between
the bridging and terminal hydrogen atoms in the coordinated
2-
In IIIb there is formally a BH3 anion, which is capable of
-
existing only under very strong reducing conditions.20 Thus
neither IIIa nor IIIb is likely to be the best description of the
structure of 1. The bonding scheme in IIIc, which has been
described as hyperconjugative interaction,2b includes donation
of d electrons from the low-valent Ta(I) metal centers to the
electron-deficient bridging BH3 ligand. We believe this model
provides the best qualitative explanation for the relatively short
Ta-B bond lengths and the increase in the endo B-H bond
distances coordinated to the Ta atoms, which are characteristic
for metallaborane complexes.21
BH4 ligands makes all the hydride signals equivalent on the
NMR time scale. The 1H NMR signals for the methylene
protons of the bridging dmpm ligands appear as three multiplets
centered at 2.18, 2.09, and 1.73 ppm. The signals for the methyl
groups are found at 1.59, 1.36, and 1.27 ppm.
Compound 2 was first isolated as a byproduct along with 1.
It is a simple borane phosphine adduct of the type BH3‚L, where
L is a Lewis base. Compound 2 can also be independently
prepared by adding two equivalents of BH3‚THF in THF to a
dmpm toluene solution (eq 2). It is noteworthy that 2 is air
In terms of the isolobal analogy, the Ta2(BH3) core has 12
electrons available for cluster bonding: 8 from 2Ta(I), 2 from
1BHexo, and 2 from 2Hendo atoms. Six pairs of electrons are
consistent with a trigonal bipyramidal parent deltahedron. Thus,
the 3 atom cluster core (2Ta + 1B) can be formulated as an
arachno-metalloborane, which is isoelectronic with a known
borane [B3H8]-,22 or as the hypothetical B3H9 molecule, for
which Wade’s rules predict a similar geometry. The difference
between compound 1 and [B3H8]- or B3H9 is that the electrons
in 1 can be used for metal-metal multiple bonding. In this
case, a TadTa double bond may exist, which is consistent with
the Ta-Ta distance of ∼2.78 Å observed in the solid-state
structure and the diamagnetism of compound 1 in solution.
The 31P{1H} NMR spectrum of 1 at room temperature shows
broad multiplets at 4.52 ppm and singlets at -7.32 and -7.55
ppm for the three bridging dmpm ligands. The 11B{1H} NMR
indicates the presence of two kinds of boron atoms with
chemical shifts of -28.5 and -33.5 ppm. The latter overlaps
with the signals of 2. The 1H NMR spectrum of 1 has a broad
quartet appearing as almost flat humps centered at -0.26 ppm,
(2)
stable and melts at 152 °C; dmpm itself is air-sensitive and has
a low melting point of -56 °C.23
A molecular drawing of 2 is shown in Figure 4. Selected
bond distances and angles are listed in Table 5. Compound 2
crystallizes in the hexagonal space group P6122. The molecule
has crystallographically imposed C2 symmetry with the two fold
axis passing through the methylene carbon atom C(1). The
P(1)-C(1) distance is 1.815(3) Å, and the distances between
the methyl carbon atoms C(2) and C(3), and atom P(1), are
1.797(3) and 1.796(4) Å, respectively. The P(1)-B distance
of 1.903(4) Å is comparable with those found in other
compounds containing a P to B dative bond.3
-
which can be assigned to the borohydride BH4 signals (JB-H
The NMR data for 2 are consistent with the solid-state
structure. The 31P{1H} NMR shows a broad quartet centered
at 5.28 ppm with J(31P-11B) ) 58.1 Hz. The P-B coupling
(20) Godfroid, R. A.; Hill, T. G.; Onak, T. P.; Shore, S. G. J. Am. Chem.
Soc. 1994, 116, 12107-12108.
(21) Housecroft, C. E. Boranes and Metalloboranes, Ellis Horwood:
Chichester, U.K. 1990.
(22) Mitchell, G. F.; Welch, A. J. J. Chem. Soc., Dalton Trans. 1987,
1017-1025.
(23) Karsch, H. H.; Schmidbaur, H.Z. Naturforsch. B Anorg. Chem. Org.
Chem. 1977, 32, 762-767.

Ditantalum Complex with Direct Ta-B Bonds
J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9599
constant is the same as that observed in the 11B NMR, in which
metallaborane complex. The Ta2BH3 core in 1 can be formu-
lated as an arachno-tantalaborane with a TadTa double-bond.
1
the 11B signal appears as a doublet at -33.12 ppm. The H
NMR spectrum of 2 shows the methyl protons as a doublet at
0.87 ppm and the methylene protons as a triplet at 0.99 ppm
[2J(31P-1H) ) 11.4 Hz]. The resonance of the coordinated
borane appears as broad humps of doublets of quartet centered
Acknowledgment. We are grateful to the Robert A. Welch
Foundation for financial support (grant no. A494), and the
Department of Chemistry at the University of Costa Rica. We
also thank L. M. Daniels for helpful crystallographic advice.
2
at 1.18 ppm [J(11B-1H) ) 95.4 Hz, J(31P-1H) ) 10.8 Hz].
Supporting Information Available: Tables of crystal data,
atomic coordinates, bond lengths and angles, anisotropic dis-
placement parameters, and H coordinates (21 pages, print/PDF).
CIF files are available online. See any current masthead page
for ordering information and Web access instructions.
Concluding Remarks
By using the robust dmpm ligand, the reduction of TaCl5
with LiBH4 produces the metallaborane compound 1, in which
the tantalum atom has a formal oxidation state of +1. The
unique feature of this compound is the µ-BH3 group bridging
the ditantalum metal centers symmetrically to give a simple
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