Synthesis, crystal structure and reactivity of the dihydroborate [Li(Et2O)][B(C6F5)2(H)2]

Article abstract of DOI:10.1016/S0277-5387(00)00428-9

The new dihydroborate [Li(Et₂O)][B(C₆F₅)₂(H)₂] (1) has been prepared and characterised by single-crystal X-ray diffraction. In the solid state 1 exhibits a one-dimensional chain structure that is a result of intermolecular Li-F interactions. Reaction between 1 and one equivalent of Cp₂TiCl or 0.5 equivalents of Cp₂TiCl₂ gives the violet compound Cp₂Ti{(μ-H)₂B(C₆F₅)₂} (2), which has also been characterised by single-crystal X-ray diffraction. Addition of two equivalents of PMe₃ to 2 does not result in elimination of a borane:phosphine adduct, but yields the ionic compound [Cp₂Ti(PMe₃)₂][H₂B(C₆F₅)₂] (3). (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00428-9

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1579–1583
Synthesis, crystal structure and reactivity of the dihydroborate
[Li(Et2O)][B(C6F5)2(H)2]
Richard E. Douthwaite *
Inorganic Chemistry Laboratory, South Parks Road, Oxford, OXI 3QR, UK
Received 21 February 2000; accepted 10 May 2000
Abstract
The new dihydroborate [Li(Et2O)][B(C6F5)2(H)2] (1) has been prepared and characterised by single-crystal X-ray diffraction. In
the solid state 1 exhibits a one-dimensional chain structure that is a result of intermolecular LiꢀF interactions. Reaction between
1 and one equivalent of Cp2TiCl or 0.5 equivalents of Cp2TiCl2 gives the violet compound Cp2Ti{(m-H)2B(C6F5)2} (2), which has
also been characterised by single-crystal X-ray diffraction. Addition of two equivalents of PMe3 to 2 does not result in elimination
of a borane:phosphine adduct, but yields the ionic compound [Cp2Ti(PMe3)2][H2B(C6F5)2] (3). © 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: Hydroborates; Lithium; Titanium(IV); Bonding; Crystal structures
1. Introduction
matic solvents such as benzene. The 11B{1H} NMR
spectrum of 1 shows a single resonance at l= −29.9,
indicative of borate species and the H coupled spec-
trum a triplet (J=81 Hz) ascribed to two protons
Tetrahydroborate [BH4]− and its derivatives are used
extensively as reducing agents in organic chemistry [1].
In addition d- and f-block metal complexes of BH4−
have a rich chemistry and have been investigated as
pre-catalysts for olefin polymerisation, hydrogenation
and as precursors for transition metal hydrides and
borides [2]. Reported herein is the synthesis, characteri-
sation and some reactivity of the dihydroborate
[BH2(C6F5)2]−, a compound related to the C6F5 substi-
tuted boranes B(C6F5)3 and B(C6F5)2H, which have
been investigated as co-catalysts for Zeigler–Natta
polymerisation [3].
1
directly bonded to a boron atom. In addition, elemental
1
analysis and integration of the H NMR spectrum of
several independently prepared samples of 1 showed a
constant ratio of 1:1 between Et2O and [(H)2B(C6F5)2]−
. IR spectroscopy of 1 shows two absorptions at 2318
and 2380 cm−1 attributed to BꢀH vibrations. This was
determined from comparison with the IR spectrum
(H/D 1.37) of the deuterated isotopomer [(Et2O)Li][(m-
D)2B(C6F5)2] (D2]-1) prepared from B(C6F5)2Cl and
excess LiD. Attempts to synthesize the mixed isoto-
pomer [(Et2O)Li][(m-D)(m-H)B(C6F5)2] from BH(C6F5)2
and LiD and also reaction between B(C6F5)2D and
LiH gave mixtures of [(Et2O)Li][(m-D)2−x(m-
H)xB(C6F5)2] (where x=0–2), a result of H/D ex-
change similar to that reported between BH4− and
BD4− [4].
Determination of the single-crystal X-ray structure of
1 confirmed the proposed formulation (Fig. 1). The
asymmetric unit of 1 contains half of the molecule and
the remainder of the molecular structure is generated by
mirror symmetry. The geometry about each lithium
atom is a pseudo-trigonal bipyramid with a BꢀH hydro-
2. Results and discussion
Reaction at 25°C between B(C6F5)2Cl and excess LH
in Et2O gave [(Et2O)Li][(m-H)2B(C6F5)2] (1) in 90%
yield. Compound 1 decomposes slowly in air and is
very soluble in ethers and has good solubility in aro-
* Present address: School of Chemistry, University of Birmingham,
Edgbaston, Birmingham, B15 2TT, UK. Tel.: +44-121-414-7816;
fax: +44-121-414-4403.
E-mail address: r.e.douthwaite@bham.ac.uk (R.E. Douthwaite).
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00428-9

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R.E. Douthwaite / Polyhedron 19 (2000) 1579–1583
gen atom and Et2O oxygen atom occupying apical
positions. The equatorial sites are comprised of the
remaining BꢀH hydrogen and two ortho fluorine atoms
from an adjacent [(H)2B(C6F5)2]− moiety (Li1ꢀF1
,
1.972(7) A). This intermolecular bonding interaction
gives rise to a very unusual one-dimensional chain
structure shown in the packing diagram (Fig. 2). A few
examples of solid state LiꢀF interactions have been
structurally characterised, including fluorobenzenes co-
ordinated to lithium hexamethyldisilazide dimers, one
of which incorporating para-difluorobenzene, crys-
tallises in a one-dimensional polymer structure (LiꢀF
,
1.999(8) A) [5].
Fig. 3. Molecular structure of 2. All hydrogen atoms were calculated.
Cp hydrogen atoms have been removed for clarity. Selected bond
,
lengths (A) and angles (°): Ti(1)ꢀB(1) 2.440(3), Ti(1)ꢀH(1) 1.91(1),
B(1)ꢀH(1) 1.21(1), CpcentꢀTi(1) 2.03 H(1)ꢀB(1)ꢀH(2) 100.8(7),
H(1)ꢀTi(1)ꢀH(2) 58.1(2), CpcentꢀTi(1)ꢀCpcent 138.0.
Initial exploration of the chemistry of 1 shows it to
be effective as a reagent for the preparation of transi-
tion metal complexes of [(H)2B(C6F5)2]−. An example
has been previously described from reaction between
Cp2ZrH2 and two equivalents of the borane BH(C6F5)2
to give Cp2Zr{(m-H)2B(C6F5)2}2 [6]. However, 1 is a
reagent of more general utility for the preparation of
this class of compound. For example, reaction between
Cp2TiCl and 1 in C6H6 yields the violet Ti(III) complex
Cp2Ti{(m-H)2B(C6F5)2} (2). Compound 2 is stable as a
solid in air for hours and has good solubility in aro-
Fig. 1. Structure of 1. Et2O hydrogen atoms have been removed for
,
clarity. Selected bond lengths (A) and angles (°): Li(1)ꢀH(1) 2.139(9),
Li(1)ꢀH(2) 1.884(6), Li(1)ꢀF(1) 1.972(7), H(1)ꢀB(1)ꢀH(2) 111.6(5),
Li(1)ꢀO(1) 1.879(7), H(1)ꢀLi(1)ꢀH(2) 55.2(5), H(1)ꢀLi(1)ꢀO(1)
159.9(6), F(1)ꢀLi(1)ꢀO(1) 106.3(5), H(1)ꢀLi(1)ꢀF(1) 85.9(4).
1
matic and ether solvents. No signals in the H or 11B
spectrum of 2 could be observed; however, 19F NMR
spectroscopy did yield relatively sharp resonances at l
−158.9 and −166.2, facilitating assessment of product
purity. The IR spectrum of 2 displays bands at-
tributable to BꢀH vibrations at 2073, 2008 and 1367
cm−1 assigned from comparison with the deuterium
isotopomer Cp2Ti{(m-D)2B(C6F5)2} ([D2]-2) prepared
from [D2]-1 and Cp2TiCl. The IR spectrum of 2 is
indicative of bidentate coordination of the dihydrobo-
rate [2] and the molecular structure obtained from a
single-crystal X-ray diffraction study corroborates this
(Fig. 3). The m-H atoms were calculated; however, it is
clear from the disposition of the B(C6F5)2 fragment
relative to Cp2Ti that the hydrogen atoms occupy sites
in a plane containing Ti(1) and B(1).
Comparison between the IR and structural data of 2
and the known tetrahydroborate Cp2Ti{(m-H)2BH2}
(TiꢀB 2.440(3) A, IR w=1936 cm−1) [7] indicate
,
greater ionic character in the bonding between Cp2Ti
and [(H)2B(C6F5)2] [8]. Perhaps the clearest evidence of
this is reaction between 2 and strong s-donor ligands
Fig. 2. Packing diagram of 1 illustrating 1-D chain structure along a
axis. All hydrogen atoms have been removed for clarity.

R.E. Douthwaite / Polyhedron 19 (2000) 1579–1583
1581
such as PMe3. Under ambient conditions it has been
reported that no reaction occurs between Cp2Ti{(m-
H)2BH2} and PMe3 [7]. In contrast, on addition of two
equivalents of PMe3 to a toluene solution of 2, a blue
viscous oil immediately precipitates that can be crys-
tallised from THF/pentane to give a blue microcrys-
talline solid (3). Compound 3 is extremely sensitive to
air and is insoluble in non-polar organic solvents, spar-
ingly soluble in diethyl ether and very soluble in THF.
Spectroscopic data and elemental analysis are consis-
tent with the formulation [Cp2Ti(PMe3)2][H2B(C6F5)2].
Cp2TiCl2 were purchased from Aldrich and used as
received. Cp2TiCl [9] and (C6F5)BCl [3] were prepared
1
according to a literature method. H, 13C, 11B and 19F
NMR spectra were recorded at 500, 125.7, 160.6 and
470.4 MHz, respectively, at probe temperature on a
Varian Unity 500 spectrometer. Proton and carbon
spectra were referenced internally to the residual sol-
vent proton and 13C resonance, respectively, relative to
tetramethylsilane. Boron spectra were referenced exter-
nally to Et2O:BF3 and fluorine externally to CClF3.
Infrared spectra were recorded on a Mattson–Polaris
FT interferometer. Microanalyses were obtained at the
Inorganic Chemistry Laboratory, Oxford.
1
No H NMR resonance was observed for 3 that could
be ascribed to the [Cp2Ti(PMe3)2]+ fragment and the
1
31P NMR spectrum was featureless. The H NMR did
however contain a 1:1:1:1 signal at l 2.40 (1J(B, H)=
86.0 Hz) and the 11B NMR a triplet at l −26.0. In
addition, IR spectroscopy showed BꢀH stretches (confi-
rmed by reaction between [D2]-2 and two equivalents of
PMe3) at 2314 and 2361 cm−1. On steric grounds and
the similarity between the IR data of compounds 1 and
3 in comparison to 2, and the observation of NMR
signals attributable to [H2B(C6F5)2]−, an ionic formula-
tion for 3 seems likely. This is a surprising result as
reaction between transition metal tetrahydroborates
and s-donors (L) invariably result in formation of
borane adducts L:BH3. This reaction type has however
been observed in an alternative preparation of 2 that
gives the adduct Et2O:B(C6F6)H as a by-product. Addi-
tion of two equivalents of 1 to Cp2TiCl2 in benzene
gave 2 and Et2O:B(C6F5)2H with concomitant elimina-
tion of H2 (Eq. (1)), illustrating that in this system 1
acts as a reducing agent in a similar manner to LiBH4.
3.1. [(Et2O)Li ][(v-H)2B(C6F5)2] (1)
To a solid mixture of (C6F5)2BCl (20 g, 52.6 mmol)
and LiH (2 g, 251.8 mmol) was added Et2O (100 ml)
and the white mixture stirred at room temperature (r.t.)
for 12 h. The volatiles were removed under reduced
pressure and the resulting white solid extracted with
Et2O (2×50 ml) to give a colourless solution. Removal
of the volatiles and washing with pentane (2×20 ml)
gave 1 as a white powder (90% yield). Crystals were
obtained by crystallisation from benzene. Anal. Found:
C, 45.00; H, 2.87; B, 2.50; Li, 1.64. Calc. C, 44.90; H,
1
2.83; B, 2.53; Li, 1.62.%. H NMR (500 MHz, benzene-
3
d6): l=2.89 (q, J(H, H)=7 Hz, 4H, OCH2CH3), 2.00
1
1
(1:1:1:1, J(B, H)=81.0 Hz, 2H, BH2), 0.71 (t, J(H,
H)=7 Hz, 6H, OCH2CH3); 11B NMR (160.6 MHz,
benzene-d6): l= −29.9 (t, 1J(B, H)=81.0 Hz); 19F
NMR (470.4 MHz, benzene-d6): l= −135.5 (m, 2F),
−160.6 (m, 1F), −164.3 (m, 2F); 13C{1H} NMR
(125.8 MHz, benzene-d6): l=148.48 (d, 1J(C, F)=
2 (1)+Cp2TiCl2“(2)+Et2O:B(C6F5)2H+0.5H2
1
+2LiCl
(1)
231.0 Hz), 139.06 (d, J(C, F)=246.7 Hz), 137.5 (d,
1J(C, F)=250.5 Hz), 120.6 (br. s), 66.1, 13.9; IR (KBr
pellet): w(BH2)=2380 cm−1 (br m), 2314 (br m).
In conclusion, the high yield synthesis of 2 shows that
the new dihydroborate 1 is an effective reagent for the
simple preparation of transition metal derivatives of
[H2B(C6F5)2]−. It was further observed that unusual
reactivity is exhibited by compound 2 in comparison to
the [BH4]− analogue, presumably a consequence of the
electron withdrawing effect of the C6F5 groups. The
presence of two bulky electron withdrawing C6F5
groups will also attenuate the propensity of
[H2B(C6F5)2]− to act as a reducing agent. Compound 1
and transition metal derivatives of [H2B(C6F5)2]− may
therefore find use in the selective reduction of organic
substrates [1].
3.2. Cp2Ti{(v-H)2B(C6F5)2] (2): method 1
To solid mixture of [(Et2O)Li][(m-H)2B(C6F5)2] (500
mg, 1.17 mmol) and Cp2TiCl (290 mg, 1.17 mmol) was
added benzene (30 ml) and the green mixture stirred at
r.t. for 90 min to give a white solid and violet superna-
tant. The volatiles were removed under reduced pres-
sure and the resulting violet solid was extracted with
benzene (2×30 ml) or until the washings were colour-
less. Removal of the volatiles and washing with ice cold
Et2O (2×5 ml) gave 2 as a violet microcrystalline solid
(82% yield). Crystals were obtained by crystallisation
from hot toluene. Anal. Found: C, 50.37; H, 2.28; B,
2.04; Ti, 9.03. Calc.: C, 50.33; H, 2.30; B, 2.06; Ti,
9.12%. 19F NMR (470.4 MHz, benzene-d6): l= −
158.9 (br s, 1F), −166.2 (br s, 2F) (only two reso-
nances could be found); IR (KBr pellet): w(BH2)=2075
cm−1 (br m), 2008 (br m), 1373 (s).
3. Experimental
All manipulations were performed under nitrogen in
a dry box or using standard Schlenk techniques. All
solvents were dried over the appropriate drying agent
and distilled under nitrogen. LiH, LiD, PMe3, and

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R.E. Douthwaite / Polyhedron 19 (2000) 1579–1583
3
,
3.3. Method 2
99.151(8)°, V=912.1(2) A , T=294 K, space group
P21/m, Z=4, v(Mo Ka)=0.16 mm−1, 3512 measured
reflections, 1688 independent (Rint=0.046), 958 reflec-
tions with I\3|(I), R(F)=0.055, wR(F)=0.076, dis-
order is present in the ethyl groups of Et2O. The best
results were obtained by refining over two sites the
occupancies of each carbon atom adjacent to oxygen.
Hydroborate hydrogens were located and refined
isotropically. The remaining Et2O hydrogens were
placed in calculated positions and allowed to ride on
their corresponding carbon atoms with fixed thermal
parameters.
At r.t. benzene was added (30 ml) to a solid mixture
of [(Et2O)Li][(m-H)2B(C6F5)2] (500 mg, 1.17 mmol) and
Cp2TiCl2 (136 mg, 0.55 mmol). On stirring, the red
reaction mixture immediately became green with evolu-
tion of a gas. Over a further period of 1 h, gas
evolution ceased and a violet supernatant developed
with precipitation of a white solid. The volatiles were
removed under reduced pressure and the solid washed
with pentane (2×10 ml). Final work-up was as de-
scribed in method 1 (80% yield).
3.4. [Cp2Ti(PMe3)2][H2B(C6F5)2] (3)
4.2. Crystal data for 2
To a violet benzene (20 ml) solution of Cp2Ti{(m-
H)2B(C6F5)2} (200 mg, 0.30 mmol) was added a ben-
zene (10 ml) solution of PMe3 (46 mg, 0.70 mmol).
Immediately the reaction developed a dark blue colour
and after 5 min gave an oily blue solid precipitate and
colourless supernatant. The volatiles were removed un-
der reduced pressure and the blue powder was washed
with pentane (2×10 ml) and crystallised from 3:1
THF–pentane (3 ml) at −80°C to give 3 as a blue
crystalline solid (95% yield). Anal. Found: C, 50.85; H,
3.30; B 1.55; P, 8.97; Ti, 6.93. Calc.: C, 50.32; H, 3.15,
B, 1.62; P, 9.28; Ti, 7.17%. 1H NMR (500 MHz,
C22H12B1F10Ti1, M=525.03, triclinic, a=11.6410(9),
,
b=12.7490(12), c=15.7680(13) A, h=90.754(9), i=
3
,
109.448(11), k=110.35(1)°, V=2046.8(8) A , T=220
−1
(
K, space group P1, Z=2, v(Mo Ka)=0.51 mm
10197 measured reflections, 7713 independent (Rint
,
=
0.045), 5972 reflections with I\3|(I), 626 parameters,
R(F)=0.058, wR(F)=0.062. Hydroborate hydrogen
atoms were placed in calculated positions with fixed
thermal parameters and then refined with anisotropic
displacement parameters. The remaining Cp hydrogens
were placed in calculated positions and allowed to ride
on their corresponding carbon atoms with fixed thermal
parameters.
1
THF-d8): l=2.40 (1:1:1:1, J(B, H)=86.0 Hz, BH2);
11B NMR (160.6 MHz, THF-d6): l=26.0 (t, 3J(B,
H)=86.0 Hz); IR (KBr pellet): w(BH2)=2361 cm−1
(br m), 2314 (br m).
5. Supplementary material
Supplementary data are available from the Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK, on request, quoting the
deposition numbers CCDC 137188 (1) and 137187 (2).
4. Structural studies
Crystals of [(Et2O)Li][(m-H)2B(C6F5)2] (1) and
Cp2Ti{(m-H)2B(C6F5)2} (2) were isolated under dinitro-
gen. Crystals of 1 were flame sealed in glass capillaries
and crystals of 2 were covered with a polyfluoroether
and mounted on glass fibres. Data were collected on an
Enraf–Nonius DIP2000 image plate diffractometer
with graphite monochromated Mo Ka radiation (u=
Acknowledgements
I would like to thank St. John’s College, Oxford for
a Junior Research Fellowship and Professor M.L.H.
Green FRS for additional financial support.
,
0.71069 A). The images were processed with the DENZO
and SCALEPACK programs [10]. All solution, refinement
and graphical calculations were performed using the
CRYSTALS and CAMERON software packages [11,12].
Crystal structures were solved by direct methods using
the SIR92 program [13] and were refined by full-matrix
least-squares on F. All non-hydrogen atoms were
refined with anisotropic displacement parameters unless
otherwise stated below.
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