Europium(II) and Ytterbium(II) Cyclic Organohydroborates: An Europium(II) Complex with an Agostic Interaction

Article abstract of DOI:10.1021/ic030249s

Lanthanide bis((cyclooctane-1,5-diyl)dihydroborate) complexes (THF) ₄Ln{(μ-H)₂BC₈H₁₄}₂ (Ln = Eu, 1; Yb, 2) were synthesized by a metathesis reaction between (THF) _xLnCl₂ and K[H₂BC₈H₁₄] in THF in a 1:2 molar ratio. Attempts to prepare the monosubstituted lanthanide cyclic organohydroborates (THF)_xLnCl{(μ-H)₂BC ₈H₁₄} were unsuccessful. On the basis of the molecular structure and IR spectrum of 1, there is an agostic interaction between Eu(II) and one of the α-C-H hydrogens from the {(μ-H)₂BC ₈H₁₄} unit. No such interaction was observed for 2. The coordinated THF in 1 and 2 can be removed under dynamic vacuum, but the solvent ligands remain bound to Yb when 2 is directly dissolved in Et₂O or toluene. In strong Lewis basic solvents, such as pyridine or CH₃CN, attack of the Yb-H-B bridge bonds results. Decomposition of 2 to the 9-BBN dimer in CD₂Cl₂ was observed by ¹¹B and ¹H NMR spectroscopies. Compound 2 was reacted with 2 equiv of the hydride ion abstracting reagent B(C₆F₅)₃ to afford the solvent-separated ion pair [Yb(THF)₆][HB(C ₆F₅)₃]₂ (3). Complexes 1, 2, and 3 were characterized by single-crystal X-ray diffraction analysis. Crystal data: 1 is orthorhombic, Pna2₁, a = 21.975(1) A, b = 9.310(1) A, c = 16.816(1) A, Z = 4; 2 is triclinic, P1, a = 9.862(1) A, b = 10.227(1) A, c = 10.476(1) A, α = 69.87(1)°, β = 76.63(1)°, γ = 66.12(1)°, γ = 1; 3.Et₂O is triclinic, P1, a = 13.708(1) A, b =14.946(1) A, c = 17.177(1) A, α = 81.01(1)°, β = 88.32(1)°, γ = 88.54(1)°, Z = 2.

Full text of DOI:10.1021/ic030249s

Inorg. Chem. 2004, 43, 692−698
Europium(II) and Ytterbium(II) Cyclic Organohydroborates: An
Europium(II) Complex with an Agostic Interaction
Xuenian Chen, Soyoung Lim, Christine E. Plecˇnik, Shengming Liu, Bin Du, Edward A. Meyers, and
Sheldon G. Shore*
Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210
Received August 7, 2003
Lanthanide bis((cyclooctane-1,5-diyl)dihydroborate) complexes (THF)4Ln{(µ-H)2BC H }2 (Ln ) Eu, 1; Yb, 2) were
8
14
synthesized by a metathesis reaction between (THF)xLnCl2 and K[H BC H ] in THF in a 1:2 molar ratio. Attempts
2
8 14
to prepare the monosubstituted lanthanide cyclic organohydroborates (THF)xLnCl{(µ-H)2BC H } were unsuccessful.
8
14
On the basis of the molecular structure and IR spectrum of 1, there is an agostic interaction between Eu(II) and
one of the R-C−H hydrogens from the {(µ-H)2BC H } unit. No such interaction was observed for 2. The coordinated
8
14
THF in 1 and 2 can be removed under dynamic vacuum, but the solvent ligands remain bound to Yb when 2 is
directly dissolved in Et2O or toluene. In strong Lewis basic solvents, such as pyridine or CH CN, attack of the
3
Yb−H−B bridge bonds results. Decomposition of 2 to the 9-BBN dimer in CD Cl2 was observed by 11B and 1H
2
NMR spectroscopies. Compound 2 was reacted with 2 equiv of the hydride ion abstracting reagent B(C F ) to
6
5 3
afford the solvent-separated ion pair [Yb(THF)6][HB(C F ) ] (3). Complexes 1, 2, and 3 were characterized by
6
5 3 2
single-crystal X-ray diffraction analysis. Crystal data: 1 is orthorhombic, Pna21, a ) 21.975(1) Å, b ) 9.310(1) Å,
c ) 16.816(1) Å, Z ) 4; 2 is triclinic, P1h, a ) 9.862(1) Å, b ) 10.227(1) Å, c ) 10.476(1) Å, R ) 69.87(1)°, â
) 76.63(1)°, γ ) 66.12(1)°, Z ) 1; 3‚Et2O is triclinic, P1h, a ) 13.708(1) Å, b ) 14.946(1) Å, c ) 17.177(1) Å,
R ) 81.01(1)°, â ) 88.32(1)°, γ ) 88.54(1)°, Z ) 2.
Introduction
been reported.5 However, there are few reports on divalent
lanthanide [BH4]- systems,6 and to the best of our knowl-
edge, no synthetic studies have been published on lanthanide-
(II) organohydroborates. It is worthy of note that Zanella et
al.7 characterized the first actinide organohydroborate Cp3U{(µ-
H)2BC8H14} that incorporates the [H2BC8H14]- ligand, the
organohydroborate anion of the 9-BBN dimer.8
Though all the lanthanides can exist in the divalent state
in crystalline, alkaline earth metal halide lattices, only Sm(II),
Eu(II), and Yb(II) are readily accessible under normal
reaction conditions.1,2 The first divalent lanthanide metal-
lacarborane complex, [Yb(C2B9H11)(DMF)4], was reported
by Hawthorne and co-workers.3 Shortly thereafter, our
laboratory prepared a ytterbium(II) decaborate compound,
(CH3CN)6Yb{(µ-H)2B10H12}‚2CH3CN, in which the [B10H14]2-
unit coordinates to Yb(II) through two Yb-H-B bridge
bonds.4 In addition to this work on polyhedral boron
hydrides, abundant studies on Ln(III) tetrahydroborates have
Recently, we synthesized a series of group 4 and 5
metallocene cyclic organohydroborate systems containing the
[H2BC4H8]-, [H2BC5H10]-, and [H2BC8H14]- ligands.9 Some
of these complexes exhibit unusual reactivity with B(C6F5)3
to afford cationic or neutral products according to the
solvent’s coordinating ability.10,11 In an effort to extend
divalent lanthanide borohydride chemistry, the organohy-
* To whom correspondence should be addressed. E-mail: shore@
chemistry.ohio-state.edu.
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692 Inorganic Chemistry, Vol. 43, No. 2, 2004
10.1021/ic030249s CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/13/2003

Eu(II) and Yb(II) Cyclic Organohydroborates
droborate complexes (THF)4Ln{(µ-H)2BC8H14}2 (Ln ) Eu,
1; Yb, 2) were prepared. A striking feature in the molecular
structure of 1 is the existence of an agostic interaction
between Eu(II) and the R-C-H bond of a {H2BC8H14} group.
Although a number of agostic interactions involving Ln(III)
have been reported,12 examples of Ln(II) interactions are less
abundant,13 and apparently, no Eu(II) cases appear to have
been established previously. Here we provide details on the
preparation, structural characterization, and reactivity of the
lanthanide(II) disubstituted cyclic organohydroborates, 1 and
2.
revealed the presence of 2. No other hydroborate complex
was detected. Products from the 1:1 reactions were isolated
and characterized by elemental analysis, single-crystal X-ray
diffraction, H NMR, and IR spectroscopy, all of which
substantiated the formation of 1 and 2.
The 1:1 complex (THF)xLnCl{(µ-H)2BC8H14) is probably
formed as a transient intermediate, but a reverse metathesis
reaction between two molecules of the monosubstituted
derivative could occur, thereby producing (THF)4Ln{(µ-
H)2BC8H14}2 and (THF)xLnCl2. Since (THF)xLnCl2 has
limited solubility in THF, precipitation of the dichloride
would drive the equilibrium on to the formation of the 1:2
complex (eq 2).
1
Results and Discussion
Preparation of (THF)4Ln{(µ-H)2BC8H14}2 (Ln ) Eu,
1; Yb, 2). Lanthanide bis((cyclooctane-1,5-diyl)dihydrobo-
rate) complexes 1 and 2 are synthesized by reacting (THF)x-
LnCl2 with K[H2BC8H14] in a 1:2 molar ratio at room
temperature in THF (eq 1).
2(THF)xLnCl{(µ-H)2BC8H14} h
(THF)4Ln{(µ-H)2BC8H14}2 + (THF)xLnCl2 (2)
Molecular Structure of 1. The molecular structure of 1
is shown in Figure 1. Crystal data are given in Table 1, and
selected bond distances and angles are listed in Table 2. The
coordination geometry about the Eu(II) center can be
described as a distorted octahedron formed by two 9-BBN
hydroborate ligands cis to each other and four THF ligands.
Each 9-BBN hydroborate ligand is attached to Eu(II) through
two Eu-H-B bridges. Of the Eu‚‚‚B distances, 2.794(6)
and 2.920(7) Å, the shorter distance is believed to be a
consequence of an agostic interaction between Eu and the
C(11)-H(11) bond that draws B(1) closer to Eu. The average
Eu-O bond length is 2.61 Å. The B(1)-Eu-B(2) angle,
103.3(2)°, is considerably distorted from the regular cis-
octahedral angle of 90°. The steric repulsion of the two
When the reagents are stirred overnight in Et2O, no
apparent reaction ensues since (THF)xLnCl2 is insoluble in
that solvent. Complexes 1 and 2 are stable in THF, Et2O,
and toluene solution. They are unstable in the solid state
when they are stored at room temperature under vacuum or
nitrogen. The solids lose the coordinating solvent, and their
bright yellow color gradually changes to a faint yellow. The
resulting materials are not soluble in Et2O or toluene.
Attempts to prepare the monosubstituted lanthanide cyclic
organohydroborates, (THF)xLnCl{(µ-H)2BC8H14}, were un-
successful. Slow dropwise addition of K[H2BC8H14] (1 equiv)
in THF to a solution of (THF)xLnCl2 afforded only the
disubstituted complexes 1 and 2. Boron-11 NMR spectra of
the 1:1 reaction mixture of (THF)xYbCl2 and K[H2BC8H14]
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Inorganic Chemistry, Vol. 43, No. 2, 2004 693

Chen et al.
Table 1. Crystallographic Data for (THF)4Eu{(µ-H)2BC8H14}2 (1), (THF)4{(µ-H)2BC8H14}2 (2), and [Yb(THF)6][HB(C6F5)3]2 (3)
1
2
3‚Et2O
a
empirical formula
fw
space group
a, Å
b, Å
C32H64B2EuO4
686.41
Pna21 (No. 33)
21.975(1)
9.310(1)
16.816(1)
90
C32H64B2YbO4
707.58
C64H61B2F30YbO7
1706.79
P1h (No. 2)
13.708(1)
14.946(1)
17.177(1)
81.01(1)
88.32(1)
88.54(1)
3473.5(4)
2
1.632
-73
1.477
0.0478
0.1400
P1h (No. 2)
9.862(1)
10.227(1)
10.476(1)
69.87(1)
76.63(1)
66.12(1)
901.8(2)
1
1.295
-3
2.622
0.0445
0.1101
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
90
90
3440.24(7)
4
1.325
-123
1.855
D
calcd, g‚cm-3
T, °C
F, mm-1
R1 [I > 2σ(I)]b
wR2 (all data)c
0.0314
0.0702
a Substitutional disorder results in the following formula: [Yb(THF)5.5(OEt2)0.5][HB(C6F5)3]2‚Et2O. b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) {∑w(Fo
-
2
Fc )2/∑w(Fo )2}1/2
.
2
2
tural difference. In the {(µ-H)2BC8H14} ligand that contains
B(1), the Eu-C(11) distance of 3.116(6) Å is more than 1
Å shorter than the distance of Eu to the other R-carbon
(C(15); 4.402 Å). Additionally, the Eu-B(1)-C(11) angle
of 85.5(3)° is much smaller than the corresponding Eu-
B(1)-C(15) angle of 168.2(4)°. Such distortion results from
an agostic interaction between the C(11)-H(11) bond and
the Eu atom. The Eu-H(11) distance 2.68(5) Å, is within 3
esd’s of the Eu-H-B(1) bridge bonds (Eu-H(1), 2.48(4)
Å; Eu-H(2), 2.57(4) Å). That this is an agostic interaction
is further supported by the presence of a band at 2760 cm-1
in the IR spectrum of 1. The distance of Eu to the other
R-hydrogen, H(15), is 4.800 Å well beyond consideration
for an agostic interaction.
On the other hand, the {(µ-H)2BC8H14} moiety involving
B(2) coordinates symmetrically to Eu. This symmetric
coordination constrains the distances, Eu‚‚‚C(21) (4.176 Å)
and Eu‚‚‚C(25) (4.016 Å), and angles, Eu-B(2)-C(21)
(131.2(4)°) and Eu-B(2)-C(25) (122.7(5)°), to be similar.
The distance of Eu to the R-hydrogens, H(21) and H(25)
(average 4.073 Å), is well outside the range anticipated for
an agostic interaction.14 The two Eu-H-B(2) bridge bonds
(Eu-H(3), 2.43(6) C; Eu-H(4), 2.30(6) Å) appear to be
shorter than those for Eu-H-B(1).
Molecular Structure of 2. The molecular structure of 2
is shown in Figure 2. Crystal data and selected bond distances
and angles are provided in Tables 1 and 3. The asymmetric
unit is comprised of Yb(1) on an inversion center, one {(µ-
H)2BC8H14} unit, and two THF solvent ligands. Inversion
through the Yb(II) center generates the remainder of the
molecule. The THF ligands are disordered, with each ligand
adopting two positions with ca. 50% occupancy. Only one
of these arrangements is shown in Figure 2. The full set of
THF orientations is depicted in the Supporting Information.
The coordination geometry around the Yb(II) atom consists
of a distorted octahedral arrangement of two {(µ-H)2BC8H14}
groups and four THF ligands. The {(µ-H)2BC8H14} units are
Figure 1. Molecular structure of (THF)4Eu{(µ-H)2BC8H14}2 (1) showing
25% probability thermal ellipsoids. Bridging and R-C-H hydrogens are
drawn with arbitrary thermal ellipsoids. All other hydrogen atoms are
omitted for clarity.
Table 2. Selected Bond Distances and Angles for
(THF)4Eu{(µ-H)2BC8H14}2 (1)
Bond Distances (Å)
Eu-O(1)
Eu-O(2)
Eu-O(3)
Eu-O(4)
Eu-H(1)
Eu-H(2)
Eu-H(3)
Eu-H(4)
Eu-C(11)
Eu-H(11)
Eu‚‚‚B(1)
2.591(4)
2.635(5)
2.597(3)
2.603(5)
2.48(4)
2.57(4)
2.43(6)
2.30(6)
3.116(6)
2.68(5)
2.794(6)
Eu‚‚‚B(2)
2.920(7)
1.17(4)
1.17(5)
1.25(5)
1.22(6)
4.402
4.800
4.176
4.190
4.016
B(1)-H(1)
B(1)-H(2)
B(2)-H(3)
B(2)-H(4)
Eu‚‚‚C(15)
Eu‚‚‚H(15)
Eu‚‚‚C(21)
Eu‚‚‚H(21)
Eu‚‚‚C(25)
Eu‚‚‚H(25)
3.956
Angles (deg)
O(1)-Eu-O(2)
O(1)-Eu-O(3)
O(1)-Eu-O(4)
O(2)-Eu-O(3)
O(2)-Eu-O(4)
O(3)-Eu-O(4)
B(1)-Eu-B(2)
B(1)-Eu-O(1)
B(1)-Eu-O(2)
85.1(2)
83.2(1)
155.9(2)
77.9(1)
79.2(1)
75.8(1)
103.3(2)
90.2(2)
96.2(2)
B(1)-Eu-O(3)
171.5(2)
109.5(2)
85.5(3)
168.2(4)
131.2(4)
122.7(5)
119(3)
B(1)-Eu-O(4)
Eu-B(1)-C(11)
Eu-B(1)-C(15)
Eu-B(2)-C(21)
Eu-B(2)-C(25)
B(1)-C(11)-H(11)
B(1)-C(15)-H(15)
115(3)
{(µ-H)2BC8H14} units forces the THF ligands toward each
other; the O(1)-Eu-O(4) angle between trans-THF ligands
is 155.9(2)° and one of the cis-THF angles, O(2)-Eu-O(3),
measures 77.9(1)°.
A comparison of the interactions between Eu and the two
cyclic organohydroborate ligands reveals a significant struc-
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Organomet. Chem. 1983, 250, 395.
694 Inorganic Chemistry, Vol. 43, No. 2, 2004

Eu(II) and Yb(II) Cyclic Organohydroborates
Figure 2. Molecular structure of (THF)4Yb{(µ-H)2BC8H14}2 (2) showing
15% probability thermal ellipsoids. Bridging and R-C-H hydrogens are
drawn with arbitrary thermal ellipsoids. All other hydrogen atoms are
omitted for clarity.
Table 3. Selected Bond Distances and Angles for
(THF)4Yb{(µ-H)2BC8H14}2 (2)a
Bond Distances (Å)
Yb(1)‚‚‚B(1)
Yb(1)-H(A)
Yb(1)-H(B)
Yb(1)-O(1)
Yb(1)-O(2)
2.876(7)
2.59(11)
2.59(13)
2.424(11)
2.462(6)
B(1)-H(A)
B(1)-H(B)
B(1)-C(1)
B(1)-C(5)
1.09(11)
1.15(12)
1.59(1)
1.59(1)
Figure 3. Solid-state (KBr) IR spectra of the complexes (THF)4Eu{(µ-
H)2BC8H14}2 (1) and (THF)4Yb{(µ-H)2BC8H14}2 (2) in the C-H stretching
region.
Angles (deg)
Comments on the Differences between the Structures
of 1 and 2. As reported above the solid-state structure of
complex 1 possesses a cis arrangement of [H2BC8H14]-
ligands while a trans arrangement occurs in complex 2.
Analysis of crystal packing gives no obvious reasons for the
difference between these two structures. On steric grounds
the trans arrangement is favored. Thus, the occurrence of
the cis arrangement most likely implies the influence of the
agostic bond. The question arises as to why the two
complexes are structurally different. As indicated from IR
spectra, below the agostic interaction occurs in solution as
well as in the solid state. In the simplest terms, on the basis
of charge density, size of the metal with respect to radius,
ytterbium would be expected to be more likely to interact
with the R-C-H hydrogen because of its higher positive
charge density. However, the larger size of the europium
favors a larger coordination number and this might be the
driving factor for the formation of the agostic bond.
H(A)-Yb(1)-H(B)
H(A)-B(1)-H(B)
O(1)-Yb(1)-O(2)
O(1)-Yb(1)-B(1)
O(2)-Yb(1)-B(1)
46(3)
128(9)
94.7(4)
87.7(3)
89.9(2)
O(1)-Yb(1)-H(A)
O(1)-Yb(1)-H(B)
O(2)-Yb(1)-H(A)
O(2)-Yb(1)-H(B)
104(2)
70(3)
74(2)
106(3)
a Symmetry transformation used to generate equivalent atoms: -x + 2,
-y - 2, -z.
trans to each other along the elongated axis, and each is
bound to Yb though two Yb-H-B bridges. Equatorial
oxygen atoms of the THF ligands are coplanar with Yb(II).
The THF rings are parallel to each other and are perpen-
dicular to the equatorial plane. This differs from the C5H5N
ring arrangement, “four-bladed right- and left-handed propel-
lers,” in (C5H5N)4Yb(BH4)2.6a
The Yb-O distances, 2.421(11) and 2.462(6) Å, compare
favorably with those in Cp2Yb(THF) (2.412(5) Å)15 and η5-
bis(Pr-indenyl)Yb(THF)2 (2.47(1), 2.77(7) Å).13a The Yb-
(1)-H(A) and Yb(1)-H(B) distances of the Yb(1)-H-B
bridges are 2.59(11) Å and 2.59(13) Å. The Yb‚‚‚B distance
of 2.876(7) Å is noticeably longer than the 2.666(6) and
2.692(5) Å lengths in (CH3CN)4Yb(BH4)2 and (C5H5N)4Yb-
(BH4)2.6a This dissimilarity mainly results from the two
distinct coordination modes of the ligands; [H2BC8H14]- is
bidentate while [BH4]- is tridentate.6a Variations in M‚‚‚B
distances between complexes with µ2- and µ3-BH4 coordina-
tion modes typically fall within the range 0.16-0.34 Å.5b In
contrast, compounds having bidentate borohydride ligands,
such as [H2BC8H14]-, [H2BC4H8]-, [H2BC5H10]-, and
[H2BH2]-, do not differ significantly in their M‚‚‚B dis-
tances.9 Besides affecting the Yb‚‚‚B distance, the coordina-
tion mode also influences the stability of the organohydrob-
orate and borohydride complexes. For instance, the doubly
bridged system in 2 is prone to attack by strong Lewis basic
solvents, whereas triply bridged lanthanide tetrahydroborates
are stable in those solvents.6a
IR and NMR Spectra. Complexes 1 and 2 generally
exhibit similar IR spectra in the solid-state. Bands assigned
to Ln-H-B stretches appear from 2012 to 2075 cm-1 for 1
and from 2035 to 2118 cm-1 for 2. A striking discrepancy
between the spectra is apparent in the C-H stretching region.
Besides the normal C-H stretches from 2979 to 2834 cm-1,
an additional absorption at a lower frequency, 2760 cm-1,
is present in 1’s spectrum (Figure 3). The appearance of this
extra stretch is consistent with an agostic interaction between
Eu(II) and an R-C-H bond of the organohydroborate ring.12a,i
An IR spectrum of 1 in benzene solution displays a weak
broad shoulder at 2759 cm-1. Not unexpectedly, the agostic
interaction is weaker in solution than in the solid state. It is
of interest to note that IR evidence for agostic interactions
between a C-H bond and a lanthanide metal are relatively
rare12a,f,g compared to such interactions between a C-H bond
and a transition metal,16 the first example of which was
1
suggested by Trofimenko16d based upon IR and H NMR
spectra and later established by Cotton and co-workers17
through single-crystal X-ray analysis.
(15) Tilley, T. D.; Andersen, R. A.; Spencer, B.; Ruben, H.; Zalkin, A.;
Templeton, D. H. Inorg. Chem. 1980, 19, 2999.
Inorganic Chemistry, Vol. 43, No. 2, 2004 695

Chen et al.
The paramagnetism of the Eu(II) atom (f7) in 1 prevents
the acquisition of NMR spectra that are easily interpreted.
However, Yb(II) (f 14) is diamagnetic, and NMR spectra for
2 were acquired in a variety of solvents (see Experimental
Section).
Table 4. Selected Bond Distances and Angles for
[Yb(THF)5.5(Et2O)0.5][HB(C6F5)3]2 (3)a
Bond Distances (Å)
Yb(1)-O(1)
Yb(1)-O(2)
Yb(1)-O(3)
Yb(2)-O(4)
2.376(4)
2.384(3)
2.389(4)
2.399(4)
Yb(2)-O(5)
Yb(2)-O(6)
B(1)-H(1)
B(2)-H(2)
2.387(4)
2.411(4)
1.09(5)
1.10(5)
The 11B NMR spectrum of 2 in THF consists of a triplet
at δ -11.2 (1JBH ) 69 Hz), which is downfield relative to
Angles (deg)
1
O(1)-Yb(1)-O(2)
O(1)-Yb(1)-O(3)
O(2)-Yb(1)-O(3)
90.0(1)
90.7(2)
90.8(1)
O(4)-Yb(2)-O(5)
O(4)-Yb(2)-O(6)
O(5)-Yb(2)-O(6)
91.0(2)
87.3(2)
88.6(1)
the signal of the precursor K[H2BC8H14] (δ -16.3; JBH
)
73 Hz).8b Triplets are also apparent in the 11B NMR spectra
of 2 in Et2O (δ -9.5; 1JBH ) 65 Hz) and toluene (δ -10.1;
1JBH ) 65 Hz).
a Symmetry transformations used to generate equivalent atoms: (A) -x,
-y + 1, -z; (B) -x + 1, -y, -z + 1.
1
The H NMR spectrum of 2 in d8-THF includes a broad
multiplet from δ 1.84 to 1.41 and a singlet at δ 0.84. The
former is designated as the â- and γ-hydrogens of the
organohydroborate ring and the latter is assigned to the
R-protons.9a The bridging hydrogen resonance, δ 1.63, was
Dissolution of 2 in CH2Cl2 produces a white precipitate
in a colorless solution. The 11B NMR spectrum consists of
one signal at δ 27.2, which is assigned to the 9-BBN dimer.9a
The 1H NMR spectrum of 2 in CD2Cl2 confirms the
formation of the 9-BBN dimer.9a Though clearly the
{HBC8H14} moieties in 2 dimerize, the resulting environment
around the Yb(II) center is uncertain.
Reactions with B(C6F5)3. Treatment of 2 with 2 equiv of
the hydride ion abstracting reagent B(C6F5)3 yields the
solvent-separated ion pair complex [Yb(THF)6][HB(C6F5)3]2
(3) and the 9-BBN dimer, (µ-H)2(BC8H14)2 (eq 3). On the
basis of an 11B NMR spectrum of the reaction solution, the
located by comparing the H and H{11B} NMR spectra.
Proton spectra of 2 in d10-Et2O and d8-toluene have similar
chemical shifts for the {(µ-H)2BC8H14} unit, but peaks
corresponding to the R-hydrogens of ligated THF (d10-Et2O,
δ 3.92; d8-Tol, δ 4.06) are also visible. The signal from the
â-hydrogens of the THF ring overlaps with the multiplet for
the organohydroborate â- and γ-protons.
1
1
The procedure of stirring 2 in Et2O, removing the solvent,
and redissolving the solid in Et2O promotes the complete
replacement of THF by Et2O and forms (Et2O)xYb{(µ-
H)2BC8H14}2. The 1H NMR spectrum of this solid dissolved
in either does not contain any THF resonances. In turn, the
solid can be redissolved in d8-THF, and the 1H NMR
spectrum illustrates that the coordinated Et2O molecules in
(Et2O)xYb{(µ-H)2BC8H14}2 are displaced by the stronger
nucleophile THF to regenerate 2.
(THF)4Yb{(µ-H)2BC8H14}2 + 2B(C6F5)3 THF8
[Yb(THF)6][HB(C6F5)3]2
+ (µ-H)2(BC8H14)2 (3)
3
anion HB(C6F5)3]- and the 9-BBN dimer (28.7 ppm) were
formed in less than 30 min. With increasing time, the 9-BBN
dimer changed to the trialkyl borane (δ 57.6), which is also
produced in the abstraction reaction between Cp2Ti{(µ-
H)2BC8H14} and B(C6F5)3.10b The 11B NMR spectrum of 3
in THF consists of a doublet (1JBH ) 90 Hz) at δ -24.5,
which is consistent with the [HB(C6F5)3]- anion.
Reactions occur when 2 is dissolved in the strong Lewis
basic solvents pyridine and CH3CN. Symmetrical cleavage
of the double hydrogen bridge transpires in pyridine to
generate the HBC8H14‚Pyr adduct. In the 11B NMR spectrum
of 2 in pyridine, two signals, a broad unresolved doublet at
δ -1.6 for HBC8H14‚Pyr and a triplet at δ -9.6 for 2, are
evident. (The chemical shift of the HBC8H14‚Pyr adduct was
verified by acquiring the 11B NMR spectrum of the 9-BBN
dimer in pyridine.) Since the intensity ratio of the two peaks
did not alter over time, the system may exist in equilibrium.
Besides HBC8H14‚Pyr, we were unable to identify the other
cleavage product. The reaction between 2 and CH3CN is less
obvious. After 30 min, the 11B NMR spectrum of 2 in
CH3CN contains the signal for 2 (δ -10.3) and a downfield
singlet at δ 48.5 for a trialkyl borane. After 1 day, the
intensity of the organoborane peak gradually increases, while
2’s resonance disappears.
Crystals of 3 were grown from a saturated Et2O solution
and contained 2 mol of Et2O per unit cell as solvent of
crystallization. The cations are on inversion centers. There
are two unique [HB(C6F5)3]- anions in the asymmetric unit.
The boron atoms have approximate tetrahedral geometry and
their structural features are not unusual,10 although some
disorder is observed in the C6F5 groups of one of the anions.
There are two unique cations in the unit cell, each of which
having Yb2+ surrounded by six oxygen atoms in an ap-
proximately octahedral configuration (average Yb-O, 2.39
Å; average O-Yb-O, 89.7°: Table 4). These structural
parameters closely resemble those of the similar cation in
[Yb(THF)6][Co(CO)4]2 (average Yb-O, 2.388 Å; average
O-Yb-O, 90.0°).18 Both cations in this structure are
disordered. One of these, Yb1(THF)62+, is shown in Figure
4 with the positionally disordered atoms C22′, C32′, and C33′
omitted for clarity. The second cation, [Yb2(THF)6]2+, in
addition to positional disorder in the equatorial THF ligands,
has both THF and Et2O present in the axial positions, each
(16) (a) McGrady, G. S.; Downs, A. J.; Haaland, A.; Scherer, W.; McKean,
D. C. Chem. Commun. 1997, 1547. (b) Jordan, R. F.; Bradley, P. K.;
Baenziger, N.; LaPointe, R. E. J. Am. Chem. Soc. 1990, 112, 1289.
(c) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203. (d) Trofimenko, S. Inorg. Chem. 1970, 11, 2493.
(e) Trofimenko, S. J. Am. Chem. Soc. 1968, 90, 4754.
(17) (a) Cotton, F. A.; Frenz, B. A.; Stanislowski, A. G. Inorg. Chim. Acta
1973, 7, 503. (b) Cotton, F. A.; Jeremic, M.; Shaver, A. Inorg. Chim.
Acta 1972, 6, 543.
(18) Plecˇnik, C. E.; Liu, S.; Liu, J.; Chen, X.; Meyers, E. A.; Shore, S. G.
Inorg. Chem. 2002, 41, 4936.
696 Inorganic Chemistry, Vol. 43, No. 2, 2004

Eu(II) and Yb(II) Cyclic Organohydroborates
The empirical absorption correction was applied with the SORTAV
program21 provided by MaXus software.22 The positions of the
heavy atoms, Yb and Eu, were revealed by the Patterson method.
The structures were refined using the SHELXTL-97 (difference
electron density calculations and full-matrix least-squares refine-
ments) structure solution package.23 Data merging was performed
using the data preparation program supplied by SHELXTL-97. After
all non-hydrogen atoms were located and refined anisotropically,
hydrogen atom positions were calculated assuming standard
geometries. Bridge hydrogens in 1 and 2, the R-hydrogens of the
organohydroborate ring in 1, and the terminal B-H hydrogens of
the anions in 3 were located and refined isotropically.
The Yb atom in 2 resides on a special position, (1, -1, 0).
Tetrahydrofuran ligands in 2 are disordered; two random orienta-
tions were resolved for each ligand. The atoms were split and the
occupancies were refined isotropically. Positions of the hydrogen
atoms on the disordered THF carbons were not calculated. The two
Figure 4. Molecular structure of the cation in [Yb(THF)6][HB(C6F5)3]2
(3) showing 25% probability thermal ellipsoids. Hydrogen atoms are omitted
for clarity.
1
with approximately 50% occupancy. Detailed representations
of the anions and cations are given in the Supporting
Information.
Yb atoms in 3 are located on special positions (Yb(1), (0, /2, 0);
1
Yb(2), (1/2, 0, /2)). Some of the THF carbon atoms and penta-
fluorphenyl carbon and fluorine atoms are disordered. Additionally,
substitutional disorder is observed in the cation with Yb(2); carbon
atoms bonded to O(4) are distributed over two sites, THF or Et2O
carbon positions, and each component contributes approximately
50%. Thus, the formula is [Yb(THF)5.5(OEt2)0.5][HB(C6F5)3]2‚Et2O
which includes a molecule of cocrystallized Et2O. The disorder in
3 was treated in the same manner as it was for 2.
Experimental Section
General Comments. All manipulations were carried out on a
standard high vacuum line or in a drybox under an atmosphere of
nitrogen. Tetrahydrofuran, diethyl ether, toluene, and pyridine were
dried over sodium/benzophenone and freshly distilled prior to use.
Acetonitrile and methylene chloride were stirred over P2O5 for 10
days before being distilled for use. Hexane was stirred over
concentrated sulfuric acid for 2 days and then decanted and washed
with water. Next the hexane was stirred over sodium/benzophenone
for 1 week, followed by distillation into a storage bulb containing
sodium/benzophenone. Celite was dried by heating at 150 °C under
dynamic vacuum for 5 h. Ammonia (Matheson) was distilled from
sodium immediately prior to use. Ammonium chloride (Fisher) was
recrystallized from anhydrous methanol and vacuum-dried at 120
°C prior to use. Ytterbium metal (40 mesh, Strem) was used as
received. Europium (ingot, Strem) was packaged under mineral oil,
washed with hexane, and vacuum-dried. The metal was cut into
small pieces before use. Tetrahydrofuran-free K[H2BC8H14] was
prepared according to the literature procedure.8a Potassium hydride
(Aldrich) was received as an oil slurry, washed with hexane,
vacuum-dried, and stored in the drybox. 9-Borabicyclo[3.3.1]nonane
dimer (Aldrich) was used as received. Solvated lanthanide di-
chlorides were synthesized according to the published method19
with the modifications listed below. Elemental analyses were
performed by Galbraith Laboratories, Inc., Knoxville, TN. NMR
spectra were recorded on a Bruker AM-250 spectrometer. 1H NMR
spectra were obtained at 250.1 MHz, and referenced to residual
solvent protons. 11B NMR spectra were obtained at 80.3 MHz, and
externally referenced to BF3‚OEt2 in C6D6 (δ ) 0.00 ppm). Infrared
spectra were recorded on a Mattson-Polaris FT-IR spectrometer
with 2 cm-1 resolution.
Preparation of (THF)4Eu{(µ-H)2BC8H14}2, 1. In a drybox, a
50 mL flask containing a Teflon-coated magnetic stir bar was
charged with 152 mg (1.00 mmol) of Eu metal and 107 mg (2.00
mmol) of NH4Cl. The flask was connected to a vacuum line and
evacuated. Ammonia (25 mL, liquid) was condensed into the flask
at -78 °C and stirred at that temperature. A reaction occurred
immediately with evolution of H2 and formation of a golden-yellow
suspension. After 40 min of stirring, the NH3 was removed in vacuo
and a light gray-colored solid, (NH3)xEuCl2, remained. (THF)xEuCl2
was prepared by stirring the NH3-solvated complex in THF,
pumping off the solvent, and repeating this procedure two more
times to ensure that all of the ammonia was replaced. The flask
was then charged with 324 mg (2.00 mmol) of K[H2BC8H14] in
the drybox and evacuated. Next, 25 mL of THF was condensed
into the flask at -78 °C, and the mixture was warmed to room
temperature and stirred overnight. During this time the color of
the solution changed to an orange-brown with formation of a
precipitate. Filtration of the reaction mixture yielded a yellow-brown
colored filtrate. The filtrate was cooled to 0 °C and slow removal
of the THF solvent under vacuum provided yellow X-ray quality
crystals of 1. Complete removal of the solvent at room temperature
gave a yellow solid, which became faint yellow after washing with
hexane and vacuum-drying. As determined by elemental analysis,
this procedure promoted the loss of three THF ligands ((THF)Eu-
{(µ-H)2BC8H14}2). Anal. Calcd for C20H40OEuB2 (loss of three THF
solvent ligands): C, 51.08; H, 8.57; B, 4.60. Found: C, 50.81; H,
X-ray Structure Determination. Single-crystal X-ray diffraction
data were collected on a Nonius KappaCCD diffraction system,
which employs graphite-monochromated Mo-KR radiation (λ )
0.71 073). Single crystals of 1, 2, and 3 were each mounted on the
tip of a glass fiber coated with Fomblin oil (a pentafluoropolyether).
Unit cell parameters were obtained by indexing the peaks in the
first 10 frames and refined by employing the whole data set. All
frames were integrated and corrected for Lorentz and polarization
effects using the DENZO-SMN package (Nonius BV, 1999).20
(20) Otwinowsky, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Macromolecular Crystallography,
Part A; Carter, C. W., Jr., Sweet, R. M., Eds.; Methods in Enzymology
276; Academic Press: New York, 1997; pp 307-326.
(21) (a) Blessing, R. H. Acta Crystallogr. 1995, 51A, 33. (b) Blessing, R.
H. J. Appl. Crystallogr. 1997, 30, 421.
(22) Mackay, S.; Gilmore, C. J.; Edwards, C.; Tremayne, M.; Stuart, N.;
Shankland, K. MaXus: A Computer Program for the Solution and
Refinement of Crystal Structures from Diffraction Data; University
of Glasgow: Glasgow, Scotland, Nonius BV: Delft, The Netherlands,
and MacScience Co. Ltd.: Yokohama, Japan, 1998.
(23) Sheldrick, G. M. SHELXTL-97: A Structure Solution and Refinement
Program; University of Go¨ttingen: Germany, 1998.
(19) Howell, J. K.; Pytlewski, L. L. J. Less Common Met. 1969, 18, 437.
Inorganic Chemistry, Vol. 43, No. 2, 2004 697

Chen et al.
8.20; B, 4.58. A yield of 406 mg of (THF)Eu{(µ-H)2BC8H14}2 (86%
based on K[H2BC8H14]) was obtained. Solid-state IR (KBr): 2979
(s), 2918 (vs), 2887 (vs), 2837 (vs), 2760 (s), 2688 (w), 2657 (w),
2075 (s), 2012 (m), 1651 (w), 1557 (w), 1539 (m), 1446 (m), 1402
(m), 1336 (w), 1313 (w), 1282 (w), 1232 (m), 1103 (w), 1079 (w),
1038 (m), 988 (w), 952 (w), 927 (w), 891 (w), 819 (m), 744 (w)
cm-1. Solution IR (benzene, νC-H): 2976 (s), 2917 (vs), 2892 (s),
at room temperature, the 11B NMR spectrum (80 MHz) of the
reaction solution contained two signals, δ 48.5 for an organoborane
and δ -10.3 for 2. The ratio of organoborane to 2 was 0.04. This
ratio changed over time: 2 h, 0.14; 5 h, 0.30; 10 h, 1.20. After 24
h, 2 completely reacted with CH3CN to form the organoborane.
C. CD2Cl2. When 2 was dissolved in CD2Cl2 in a NMR tube, a
white precipitate was immediately produced. The 11B NMR
spectrum (80 MHz) of the reaction solution showed one peak at δ
27.2, which was assigned to the 9-BBN dimer (the same chemical
shift was obtained when the 9-BBN dimer is dissolved in CD2-
2869 (vs), 2837 (vs), 2759 (br, sh) cm-1
.
Preparation of (THF)4Yb{(µ-H)2BC8H14}2, 2. In a procedure
similar to the one described above for 1, 173 mg (1.00 mmol) of
Yb metal and 107 mg (2.00 mmol) of NH4Cl were reacted in 25
mL of NH3. The light green solid (NH3)xYbCl2 was converted to
(THF)xYbCl2, which was combined with 324 mg (2.00 mmol) of
K[H2BC8H14] in 25 mL of THF. The mixture was stirred overnight,
during which time the color of the solution changed to orange-
brown with formation of a precipitate. Filtration of the reaction
mixture yielded a yellow-brown colored filtrate. The filtrate was
cooled to 0 °C and slow removal of the solvent afforded yellow
X-ray quality crystals of 2. Complete removal of the solvent at
room temperature gave a yellow solid, which became faint yellow
after being washed with hexane and vacuum-dried. As determined
by elemental analysis, this procedure promoted the loss of two THF
ligands ((THF)2Yb{(µ-H)2BC8H14}2). Anal. Calcd for C24H48O2-
YbB2 (loss of two THF solvent ligands): C, 51.17; H, 8.59; B,
3.84. Found: C, 50.97; H, 8.48; B, 3.74. A yield of 497 mg of
(THF)2Yb{(µ-H)2BC8H14}2 (88% based on K[H2BC8H14]) was
obtained. Solid-state IR (KBr): 2975 (s), 2913 (vs), 2863 (vs), 2834
(vs), 2684 (w), 2655 (w), 2118 (m), 2087 (m), 2035 (m), 1696
(m), 1649 (m), 1556 (m), 1450 (m), 1413 (m), 1335 (m), 1283
1
Cl2). A resonance corresponding to 2 was absent. The H NMR
spectrum (250 MHz) of the reaction solution further confirmed that
the 9-BBN dimer had been produced.
Reaction of 2 with B(C6F5)3. Formation of [Yb(THF)6][HB-
(C6F5)3]2, 3. A 50 mL flask was charged with 177 mg (0.250 mmol)
of 2 and 256 mg (0.500 mmol) of B(C6F5)3. Approximately 20 mL
of THF was condensed into the flask at -78 °C. The flask was
warmed to room temperature and stirred for 1 h. Initially, the 11B
NMR spectrum (80 MHz) of the reaction solution indicated the
1
presence of the [HB(C6F5)3]- anion (δ -24.2 (d, JBH ) 90 Hz))
and the 9-BBN dimer (δ 28.7). After some time, the 9-BBN dimer
changed to an organoborane (δ 57.6 (br s)). The solvent was
removed and the yellow residue was washed with 3 × 5 mL of
hexane. Crystals of 3 were grown from a saturated Et2O solution
1
at -40 °C. The single-crystal X-ray diffraction structure and H
NMR spectrum of 3 indicated that Et2O and THF were coordinated
to the Yb cation. Some solvent ligands were removed by vacuum-
drying solid 3. As determined by elemental analysis, one Et2O and
two THF ligands remained in the molecule ([Yb(OEt2)(THF)2][HB-
(C6F5)3]2). Anal. Calcd for C48H28O3F30YbB2 (one Et2O and two
THF remain): C, 40.67; H, 1.99%. Found: C, 40.18; H, 1.94%. A
yield of 189 mg of [Yb(OEt2)(THF)2][HB(C6F5)3]2 (53% based on
B(C6F5)3) was obtained. Solid-state IR (KBr): 2983 (s), 2912 (s),
2842 (m), 2375 (m), 2317 (m), 1645 (s), 1603 (w), 1553 (sh), 1513
(vs), 1468 (vs), 1401 (m), 1373 (m), 1331 (w), 1316 (w), 1274 (s),
1172 (w), 1109 (vs), 968 (vs), 913 (m), 892 (m), 860 (m), 787
(m), 1205 (m), 1158 (w), 1105 (w), 1037 (s), 881 (w), 822 (w),
1
798 (w) cm-1
.
11B NMR (80 MHz, THF): δ -11.2 (t, JBH ) 69
1
Hz). H NMR (250 MHz, d8-THF): δ 1.84-1.41 (m, â- and γ-H
of {(µ-H)2BC8H14}, â-H of THF), 0.84 (s, R-H of {(µ-H)2BC8H14}).
1H{11B} NMR (250 MHz, d8-THF): δ 1.63 (s, µ-H). 11B NMR
(80 MHz, Tol): δ -10.1 (t, JBH ) 65 Hz). H NMR (250 MHz,
d8-Tol): δ 4.06 (br s, R-H of THF), 2.23-1.46 (m, â- and γ-H of
{(µ-H)2BC8H14}, â-H of THF), 1.23 (s, R-H of {(µ-H)2BC8H14}).
1H{11B} NMR (250 MHz, d8-Tol): δ 1.82 (s, µ-H). 11B NMR (80
MHz, Et2O): δ -9.5 (t, JBH ) 65 Hz). H NMR (250 MHz, d10-
Et2O): δ 3.92 (br s, R-H of THF), 1.90-1.50 (m, â- and γ-H of
{(µ-H)2BC8H14}, â-H of THF), 0.89 (s, R-H of {(µ-H)2BC8H14}).
1H{11B}NMR (250 MHz, d10-Et2O): δ 1.68 (s, µ-H).
1
1
(w), 757 (m), 728 (w), 686 (w), 663 (m), 602 (m), 567 (m) cm-1
.
1
1
1
1
11B NMR (80 MHz, THF): δ -24.5 (d, JBH ) 90 Hz). H NMR
3
(250 MHz, d8-THF): δ 3.14 (q, CH2 of Et2O, JHH ) 7 Hz), 0.87
3
(t, CH3 of Et2O, JHH ) 7 Hz). 1H{11B} NMR (250 MHz,
d8-THF): δ 3.91 (s, HB).
Reaction of 2 with Pyridine, CH3CN, and CD2Cl2. A.
Pyridine. Complex 2 (12 mg, 0.02 mmol) was dissolved in 0.5
mL of pyridine in a NMR tube. The color of the solution was violet.
The tube was attached to the vacuum line, cooled to -78 °C,
evacuated, and flame-sealed. The 11B NMR spectrum (80 MHz)
of the reaction solution included two resonances, δ -1.6 for
HBC8H14‚Pyr and δ -9.6 for 2. The ratio (approximately 1:3) of
the two peaks did not change with time. The chemical shift of the
9-BBN adduct was verified by dissolving the 9-BBN dimer in
pyridine.
Acknowledgment. This work was supported by the
National Science Foundation through Grants CHE 99-01115
and CHE 02-13491.
Supporting Information Available: Three crystallographic files
in CIF format and figures showing molecular structures of 2 and 3
1
and relevant H NMR, 11B NMR, and IR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC030249S
B. CH3CN. In a NMR tube, 2 was dissolved in CH3CN, and the
color of the solution changed from yellow to orange. After 30 min
698 Inorganic Chemistry, Vol. 43, No. 2, 2004