Synthesis and characterization of rare examples of stable potassium and arylcalcium triethylboranate complexes

Article abstract of DOI:10.1016/j.inoche.2010.08.018

Stabilization of potassium triethylboranate can be achieved in solution as well as in the solid state with tridentate 1,3,5-trimethyl-1,3,5-triazinane (tmta) as dimeric [(tmta)K(μ-H)BEt₃]₂ (1). The metathesis reaction of the post-Gri

Full text of DOI:10.1016/j.inoche.2010.08.018

Inorganic Chemistry Communications 13 (2010) 1466–1469
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and characterization of rare examples of stable potassium and arylcalcium
triethylboranate complexes
⁎
Sven Krieck, Helmar Görls, Matthias Westerhausen
Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena, August-Bebel-Str. 2, D-07743 Jena, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Stabilization of potassium triethylboranate can be achieved in solution as well as in the solid state with
tridentate 1,3,5-trimethyl-1,3,5-triazinane (tmta) as dimeric [(tmta)K(μ-H)BEt3]2 (1). The metathesis
reaction of the post-Grignard reagent [2,6-(tol)2C6H3-Ca(thf)3I] (2) with potassium triethylboranate yields
an unusual hydrogen bridged organocalcium contact ion pair as hydrocarbon soluble [(thf)(dme)Ca(C6H3-
2,6-tol2)HBEt3] (3) which shows no tendency to dismutate to the homoleptic derivatives.
© 2010 Elsevier B.V. All rights reserved.
Received 16 June 2010
Accepted 10 August 2010
Available online 18 August 2010
Keywords:
Arylcalcium complexes
Metathesis reactions
Hydride
Potassium
Boranate
Hydrides of main group metals as well as transition metals provide a
wide range of applications in synthetic chemistry as versatile catalysts
[1] and reducing reagents [2] as well as targets for hydrogen storage [3].
The binary hydride of calcium CaH2 is an insoluble colorless solid which
can be prepared from the elements at 400 °C and crystallizes under
standard conditions with a PbCl2 type structure [4]. This calcium(II)
hydride is rather inert and reacts only with acidic compounds such as
water or ammonia (yielding corresponding calcium(II) hydroxide or
amide). Due to the fact that solid CaH2 exhibits a very low reactivity
numerous attempts were undertaken to prepare soluble calcium
derivatives containing a reactive Ca–H bond. Harder and co-workers
demonstrated that R–Ca–H is a valuable synthon [5] for the hydroge-
nation of alkenes [6], an intermediate during hydrosilylation of ketones
[7], and reacts also as a strong reducing reagent [8]. Phenylcalcium
hydride (R=Ph) was prepared via a cocondensation reaction of calcium
with benzene yielding extremely reactive PhCaH [9,10]. Stability of an
organylcalcium hydride can be enhanced if R represents an extremely
bulky β-diketiminate ligand which is able to prevent the precipitation of
insoluble CaH2 via a dismutation reaction [11]. These heteroleptic
calcium hydride complexes crystallize as dimers containing the H atoms
in bridging positions [12]. Monomerization can be achieved via the
formation of borane adducts with Ca–H–B bridges [8,12,13]. Generation
of organoboranate adducts strongly enhances solubility in common
organic solvents [14]. As a consequence of a large lattice energy CaH2 is
nearly insoluble in organic solvents whereas the solubility of Ca(BH4)2
in ethereal solvents via formation of donor adducts [L2Ca(BH4)2]
[L=tetrahydrofuran (thf), 1,2-dimethoxyethane (dme), and diglyme]
is much larger [15–18]. Furthermore, there exist some structure–
property relationships between the heavier organoalkaline earth metal
chemistry and the organolanthanide(II) chemistry [19]. Baudry and co-
workers studied the steric and electronic control of the stability of
organolanthanide alkylborohydrides [20,21] and furthermore, some
complexes were structurally characterized such as e.g. [(dipp-nacnac)
Sm{(μ-H)BEt3}NH-C6H2-2,4,6-t Bu3] [22], [(C5Me5)2La{(μ-H)(μ-Et2)
BEt}] and [(C5Me5)2La(thf){(μ-H)(μ-Et)BEt2}] [23].
A broadportfolio of alkali metal organoborohydridesiscommercially
available as ethereal solutions which are used as hydride sources [24]
and synthons in preparative chemistry but due to their high reactivity
only very few of these compounds are structurally characterized. In the
case of potassium triethylboranate a ligand exchange of the mono-
dendate THF by sterically demanding tridendate 1,3,5-trimethyl-1,3,5-
triazinane (tmta) led to the formation of [(tmta)K(μ-H)BEt3]2 (1)
(Eq. (1)) and isolation succeeded without decomposition [25].
B
N
N
N
H
toluene
tmta
KBEt3H
ð1Þ
K
K
N
H
B
N
N
1
The molecular structure of the dimeric contact ion pair [(tmta)K(μ-H)
BEt3]2 (1) is represented in Fig. 1a [26]. The boron-bound hydrogen
⁎
Corresponding author. Fax: +49 3641 948102.
E-mail address: m.we@uni-jena.de (M. Westerhausen).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.08.018

S. Krieck et al. / Inorganic Chemistry Communications 13 (2010) 1466–1469
1467
a
b
Fig. 1. a: Molecular structure and numbering scheme of [(tmta)K-μ(H)BEt3]2 (1). The ellipsoids represent a probability of 40%, H atoms (with the exception of the bridging B–H) and
intercalated toluene molecules are neglected for clarity reasons. Selected bond lengths (pm): K1A–H1BA 256(3), K1A–H1B1 261(3), K1AA–H1BA 255(4), K1AA–H1B1 259(4), B1A–
H1B1 124(4); angles (°): K1A–H1B1–K1AA 117.4(8), H1B1–K1A–H1B2 62.6(8). b: Schematic representation of the chain-like structure of the dimeric units of [(tmta)K-μ(H)BEt3]2
(1) (η3-K-tmta interactions are represented as hollow sticks to the centroids of the nitrogen atoms of the tmta ligands for clarity reasons.).
atoms (av. B–H 125 pm) adopt bridging positions between two
potassium centers (av. K–H 258.6 pm) resulting in a rhombus-like
geometry with average K1–H–K1A and H1–K–H1B angles of 115.7° and
64.4°, respectively, and a non-bonding potassium contact K⋯K′ of 431.15
(14) pm. A similar structural fragment with angles of 106.5° and 73.5°,
respectively, was found in [{η6-C6H3-1,3,5-Me3)Na(μ-H)BEt3}2{Na(μ-H)
BEt3}2] [31] whereas tetrameric [(Et2O){Na(μ-H)BMe3}4] contains a
distorted heterocubane-type arrangement of the metal atoms and the
four-coordinate hydrides [32]. The coordination sphere of the potassium
atoms in (1) is completed by the tridentate tmta ligand (av. K–N
294.4 pm) resulting in a penta-coordination environment.
In the crystalline state intermolecular long-range agostic interac-
tions between the potassium centers and the ethyl groups of the
boranate functionalities (K⋯H–C) [K–C 330.3(4)–341.9(4) pm] addi-
tionally stabilize the complex via formation of an oligomeric chain-
like structure of the dimers (1), represented in Fig. 1b. This
aggregation leads to a tetrahedral arrangement of the ligands, due
to the tridentate tmta a 5+1 coordination number at K results.
Thus far bulky groups are employed in order to kinetically protect
Ca–H bonds. Based on our earlier findings that para-phenyl sub-
stituents destabilize arylcalcium halides [33] we investigated steric
protection by m-terphenyl groups. Furthermore, substitution of the
halides by bulkier anions such as amides and phosphanides
additionally stabilize the organocalcium derivatives [34]. The met-
athetical reaction of the heavier Grignard reagent [2,6-(tol)2C6H3-Ca
(thf)3I] (2) [35] with an equimolar amount of potassium triethyl-
borohydride yielded [(thf)(dme)Ca(C6H3-2,6-tol2)HBEt3] (3) which
was recrystallized from a solvent mixture of thf and dme (Eq. (2))
[36].
O
thf
KBEt3H (1 eq.)
THF / dme
thf
I
thf
Ca
Ca
O
-KI
thf
H
B
2
3
ð2Þ
The arylcalcium triethylboronate (3) is stable as solid and in
solution and shows no tendency to dismutate to the homoleptic
derivatives. In accordance to the findings of Hanusa and co-workers
[14e] regarding [(thf)2Ca(HBEt3){1,2,4-C5(SiMe3)3H2}] an abstraction
of the triethylborane moiety failed both by refluxing in hydrocarbons
(toluene, n-heptane) and drying in vacuo, respectively. Refluxing of a
THF solution led to fast decomposition of (3) by well-known ether
cleavage reactions [34b,37].

1468
S. Krieck et al. / Inorganic Chemistry Communications 13 (2010) 1466–1469
Appendix A. Supplementary material
CCDC 780762 and 773949 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
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As crystallographically shown for the crystalline state also spectro-
scopic investigations of (1) and (3) confirm the presence of the boranate
moiety and of metal–hydrogen–boron interactions in solution. In IR
spectra strong B–H stretching modes for (μ-H)-B fragments were
observed for (1) and (3) (1918 and 1926 cm−1) in comparison to
1935 cm−1 for [(thf)2Ca(HBEt3){1,2,4-C5(SiMe3)3H2}] [14e], 1924 and
1906 cm−1 for [(thf)(dipp-nacnac)Ca(μ-H)BsecBu3)] [12], as well as
1870–1950 cm−1 for M[HBEt3] (M=Li, Na, K) [39]. The 11B NMR
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at −15.9 (1) and −15.5 ppm (3) and are similar to −13.3 ppm for [(thf)
(dipp-nacnac)Ca(μ-H)BsecBu3] [12], and to values of Na[HBEt3] [39].
Stabilization of the hydrido complexes has been achieved by
coordination of Lewis acidic BEt3 to the hydride base whereas the
Lewis bases tmta or dme bind to the metal cation thus saturating the
coordination sphere and preventing oligomerization, dismutation and
precipitation of the otherwise insoluble corresponding metal hydrides.
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[25] Synthesis of [(tmta)K-μ(H)BEt3]2 (1): tmta (1.5 mL) was added dropwise to a
solution of commercially available KBEt3H in THF (5 mL, 5.0 mmol) at −25 °C.
After stirring for 4 h at −15 °C all volatiles were removed in vacuo and the residue
was extracted with toluene (6 mL). Storage at −20 °C led to crystallization of
colourless prisms of 1∙(PhMe) (1.05 g, 1.67 mmol, 67%). Physical data of 1∙
(PhMe): Dec. above 33 °C. Anal. Calc. for C31H72B2K2N6 (628.76 g mol−1): C,
59.22; H, 11.54; N, 13.37. Found: C, 59.30; H, 11.72; N, 13.16. 1H NMR
(400.25 MHz, 25 °C, [D6]benzene): δ −0.77 (2H, s(br), B–H), 0.54 (18H, s(br),
CH3, Et), 1.28 (12H, m(br), CH2, Et), 2.10 (18H, s, CH3, tmta), 3.09 (12H, s(br),
CH2N). 13C{1H} NMR (100.65 MHz, 25 °C, [D6]benzene): δ 11.2 (6C, CH2, Et), 12.0
(6C, CH3, Et), 39.7 (6C, CH2N, tmta), 77.8 (6C, CH3, triaz). 11B NMR (128.42 MHz,
25 °C, [D6]benzene): δ −15.9 (2H, m(br), B–H). MS (DEI, m/z [%]): 129 [100]
(tmta), 98 [62] (BEt3). IR (Nujol, KBr, cm−1) ν: 2025, m; 1918, m; 1682, s; 1607, s;
1455, vs; 1376, vs; 1262, s; 1234, m; 1157, m; 1116, vs; 1003, m; 914, s; 860, m;
728, s; 695, s; 622, m; 588, m.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG, Bonn-Bad Godesberg, Germany). We also acknowledge gener-
ous financial support by the Fonds der Chemischen Industrie (VCI,
Frankfurt/Main, Germany).
[26] Crystal Structure Determination: The intensity data for the compounds were
collected on a Nonius KappaCCD diffractometer using graphite-monochromated
Mo–Kα radiation. Data were corrected for Lorentz and polarization effects but not for
absorption effects [27,28]. The structures were solved by direct methods

S. Krieck et al. / Inorganic Chemistry Communications 13 (2010) 1466–1469
1469
(SHELXS [29]) and refined by full-matrix least squares techniques against Fo2
(SHELXL-97 [29]). The hydrogen atoms bound to the B atoms were located by
differenceFourier synthesis and refined isotropically. All other hydrogen atoms were
included at calculated positions with fixed thermal parameters. All non-hydrogen
atoms were refined anisotropically[29]. The crystal of 1 was a non-merohedral twin.
The twin law was determined by PLATON [30] to (−1, 0, 0/0, −1, 0/1.003, 0.919, 1).
The contribution of the main component was refined to 0.584(5). XP (SIEMENS
Analytical X-ray Instruments, Inc.) was used for structure representations. Crystal
KBEt3H (1.1 mmol, in THF) was added dropwise at −40 °C. After stirring for 12 h
at −25 °C the mixture was filtered and all volatiles were removed from the
mother liquor. The residue was extracted with a mixture of toluene and DME
(5 mL, 0.1 mL) and storage at −20 °C led to crystallization of colourless needles of
3 (0.41 g, 0.73 mmol, 70%). Physical data of 3: Melting point: 72 °C. Anal. Calc. for
C
34H51BCaO3 (558.66 g mol−1): Ca, 7.17. Found: Ca, 7.30. 1H NMR (400.25 MHz,
25 °C, [D6]benzene): δ −0.54 (1H, s(br), B–H), 0.62 (9H, m(br), CH2, Et), 1.34 (6H,
m(br), CH3, Et), 1.54 (4H, m, CH2, thf), 2.25 (6H, s, CH3), 2.88 (6H, CH3, dme), 2.99
3
Data for 1:
C
24H62B2K2N6, C7H8, Mr=626.75 g mol−1
,
colourless prism, size
(4H, CH2O, dme) 3.53 (4H, m, CH2O, thf), 7.07 (4H, d, JH–H =8.00 Hz, m′-CH),
0.04×0.04×0.04 mm3, triclinic, space group P ī, a=13.1010(5), b=13.6471(6),
c=14.0552(7) Å, α=113.540(2), β=115.009(3), γ=95.764(3)° V=1975.11(15)
Å3, T= −140 °C, Z=2, ρcalcd.=1.054 g cm−3, μ (Mo-Kα)=2.66 cm−1, F(000)=
692, 5865 reflections in h(−15/13), k(−16/14), l(0/16), measured in the range
2.27°≤Θ≤25.52° completeness Θmax =84.1%, 5865 independent reflections, 4808
7.29 (1H, t, 3JH–H =7.8 Hz, p-CH), 7.46 (4H, dd, 3JH–H =8.1 Hz, o′-CH), 7.60 (2H, d,
m-CH). 13C{1H} NMR (100.65 MHz, 25 °C, [D6]benzene): δ 12.8 (3C, CH2, Et), 15.4
(3C, CH3, Et), 20.9 (2C, CH3), 25.5 (2C, CH2, thf), 59.8 (2C, CH2, dme), 68.1 (2C,
CH2O, thf), 70.9 (2C, CH2O, dme), 126.0 (2C, m-CH), 127.5 (4C, o′-CH), 129.5 (1C,
p-CH), 129.8 (4C, m′-CH), 137.1 (2C, o-C), 138.9 (2C, p′-C), 142.2 (2C, i′-C), 151.3
(1C, i-C). 11B NMR (128.42 MHz, 25 °C, [D6]benzene): δ −15.7 (1H, m(br), B–H).
MS (DEI, m/z [%]): 341 [64] (C6H4-1,3-tol2), 169 [100] (C13H13), 98 [54] (BEt3). IR
(Nujol, KBr, cm−1) ν: 2923, vs; 2031, m; 1926, m; 1913, m; 1588, w; 1566, w;
1515, m; 1455, s; 1377, s; 1308, m; 1250, m; 1192, m; 1113, m; 1077, m; 1008, m;
981, m; 889, m; 856, m; 822, m; 785, m; 728, w; 695, s. For the NMR assignments,
atoms in the central ring are unprimed and atoms in the ortho phenyl groups are
primed.
reflections with
wR2obs=0.1486, R1all=0.0651, wR2all=0.1794, GOOF=0.747, largest difference
peak and hole: 0.318/−0.350 e Å−3
Fo N4σ(Fo), 392 parameters, 0 restraints, R1obs =0.0468,
.
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pp. 307–326.
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[38] Crystal Data for (3): C34H51BCaO3, Mr=558.64 g mol−1, colourless prism, size
0.04×0.04×0.04 mm3, triclinic, space group P ī, a=7.9825(4), b=10.7208(4),
c=19.9796(9) Å, α=79.128(3), β=84.611(2), γ=74.230(3)° V=1614.29(12)
Å3, T= −140 °C, Z=2, ρcalcd. =1.149 g cm−3, μ(Mo-Kα)=2.25 cm−1, F(000)=
608, 10175 reflections in h(−9/10), k(−11/13), l(−25/25), measured in the
range 2.89°≤Θ≤27.41° completeness Θmax =97.8%, 6861 independent reflec-
tions, Rint =0.0333, 4643 reflections with Fo N4σ(Fo), 354 parameters, 0 restraints,
R1obs =0.0750, wR2obs =0.1831, R1all =0.1168, wR2all =0.2107, GOOF=0.997,
[34] a) M. Gärtner, H. Görls, M. Westerhausen, Organometallics 26 (2007)
1077–1083;
b) M. Gärtner, H. Görls, M. Westerhausen, J. Organomet. Chem. 693 (2008)
221–227.
largest difference peak and hole: 1.414/−0.454 e Å−3
.
[39] P. Binger, G. Benedikt, G.W. Rotermund, R. Köster, Liebigs Ann. Chem. 717 (1968)
[35] S. Krieck, H. Görls, M. Westerhausen, J. Am. Chem. Soc. (2010), doi:10.1021/
ja105534w.
21–40.
[36] Synthesis of [(thf)(dme)Ca(C6H3-2,6-tol2)HBEt3] (3):
A solution of [2,6-
(tol)2C6H3-Ca(thf)3I] (2) (35 mL, 1.05 mmol) was prepared in THF [35] and