Tris(borane) adducts of diphosphanylmethanides: The [H3BCH(PPh2BH3)2]- anion and its alkali metal complexes

Article abstract of DOI:10.1002/ejic.201402845

The reactivity of lithium complexes that contain the borane-modified diphoshanylmethanide ligand [CH(PPh₂BH₃)₂]^- towards different Lewis base adducts of BH₃ was studied to gain further insight into the mechanism of the isomerization of this derivative, which formally proceeds through a shift of one BH₃ group from the phosphorus atom to the carbon atom. Whereas the use of BH₃·THF in THF only resulted in the thf adduct of the starting material, [Li{CH(PPh₂BH₃)₂}(thf)₂] (1), the application of BH₃·SMe₂ in toluene resulted in the formation of the novel compound [(Li{H₃BCH(PPh₂BH₃)₂})_∞] (2). The subsequent addition of ethereal ligands led to the isolation of [Li{H₃BCH(PPh₂BH₃)₂}(Me₄thf)] (3) and [Li{H₃BCH(PPh₂BH₃)₂}(thf)₃] (4). Treatment of these complexes with stronger Lewis bases such as N,N,N',N'-tetramethylethane-1,2-diamine (tmeda) results in the removal of one phosphorus-bound BH₃ molecule and the formation of the [Ph₂PCH(BH₃)PPh₂BH₃]^- anion. These results indicate that the isomerization of [CH(PPh₂BH₃)₂]^- requires an additional BH₃ source and a rather strong Lewis base. Complexes 1-4 and the related derivatives [Li{CH(PPh₂BH₃)₂}(Me₄THF)] (5) and [K{H₃BCH(PPh₂BH₃)₂}(dme)₂] (6; dme = 1,2-dimethoxyethane) were characterized by multinuclear NMR spectroscopy and by single-crystal X-ray diffraction analysis.

Full text of DOI:10.1002/ejic.201402845

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DOI:10.1002/ejic.201402845
Tris(borane) Adducts of Diphosphanylmethanides:
The [H3BCH(PPh2BH3)2]– Anion and Its Alkali Metal
Complexes
Jens Langer,*[a] Robert Geitner,[a] and Helmar Görls[a]
Keywords: Lithium / Potassium / Phosphanes / Boranes / X-ray diffraction
The reactivity of lithium complexes that contain the borane-
(PPh2BH3)2}(thf)3] (4). Treatment of these complexes with
modified diphoshanylmethanide ligand [CH(PPh2BH3)2]– stronger Lewis bases such as N,N,NЈ,NЈ-tetramethylethane-
towards different Lewis base adducts of BH3 was studied to
gain further insight into the mechanism of the isomerization
of this derivative, which formally proceeds through a shift of
one BH3 group from the phosphorus atom to the carbon atom.
Whereas the use of BH3·THF in THF only resulted in the thf
adduct of the starting material, [Li{CH(PPh2BH3)2}(thf)2] (1),
1,2-diamine (tmeda) results in the removal of one phos-
phorus-bound BH3 molecule and the formation of the
[Ph2PCH(BH3)PPh2BH3]– anion. These results indicate that
the isomerization of [CH(PPh2BH3)2]– requires an additional
BH3 source and a rather strong Lewis base. Complexes 1–4
and the related derivatives [Li{CH(PPh2BH3)2}(Me4THF)] (5)
the application of BH3·SMe2 in toluene resulted in the forma- and [K{H3BCH(PPh2BH3)2}(dme)2] (6; dme = 1,2-dimethoxy-
tion of the novel compound [(Li{H3BCH(PPh2BH3)2})ϱ] (2). ethane) were characterized by multinuclear NMR spec-
The subsequent addition of ethereal ligands led to the isola- troscopy and by single-crystal X-ray diffraction analysis.
tion of [Li{H3BCH(PPh2BH3)2}(Me4thf)] (3) and [Li{H3BCH-
carbon atom. The preference of one potential donor over
Introduction
the other is closely related to the chosen metal and the co-
The chemistry of phosphane–borane-stabilized carban- ligands that are present. In the case of the complexes of the
ions and their metal complexes dates back to the pioneering s-block metals, there is a slight preference for the hydridic
work of Schmidbaur and co-workers[1] and has recently re- hydrogen atoms at the boron center over the carbanion,
ceived renewed interest on account of the importance of which is nevertheless often found in the coordination sphere
the alkali metal derivatives in particular in the synthesis of of the metal ions.[3,4] In these complexes, the phosphane–
various phosphane derivatives.[2] Izod and co-workers[3] as borane moiety shows remarkable stability in the presence
well as other groups[4] investigated the coordination chemis- of the adjacent nucleophilic carbanion, which allows the
try of these ligands towards a variety of main-group metals. broad use of those complexes in organic syntheses.[2] How-
Within these derivatives, these ambidentate ligands show ever, in a recent investigation on barium complexes, an
different coordination modes that often involve the phos- isomerization of such a ligand, namely, [CH(PPh2BH3)2]–,
phorus-bound BH3 groups in addition to the carbanionic by means of a formal shift of one of the BH3 groups from
Scheme 1. Proposed mechanism of the isomerization of the [CH(PPh2BH3)2]– anion.
[a] Institute of Inorganic and Analytical Chemistry,
the phosphorus atom to the carbon atom was observed.[5]
Friedrich Schiller University Jena,
Although the mechanism of this rearrangement is not fully
understood, this reaction offers a potential strategy for the
synthesis of novel anionic phosphane derivatives. Owing to
the high importance of phosphanes as ligands in an over-
Humboldtstrasse 8, 07743 Jena, Germany
E-mail: j.langer@uni-jena.de
http://www.lsac1.uni-jena.de/Mitarbeiter/Dr_+J_+Langer.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402845.
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whelming number of catalytic transformations,[6,7] the Lewis base was expected to show lower affinity toward the
aforementioned isomerization deserves further investiga- hard Lewis acid BH3 and therefore would make the meth-
tion.
anide carbon atom more competitive as a BH3 acceptor.
Additionally, related phosphorus-stabilized methanides The colorless crystalline material obtained was identified by
bearing BH3 substituents were observed as products or pro- X-ray diffraction analysis as the desired compound
posed as intermediates of B–H bond-activation reactions [(Li{H3BCH(PPh2BH3)2})ϱ] (2), although the rather low
between Li/Cl carbenoids and different BH3 adducts.[8]
quality of the obtained crystals only led to a structural mo-
Herein, we evaluate the affinity of alkali metal complexes tif of this complex (see Figure 1), thereby preventing further
of [CH(PPh2BH3)2]– towards different BH3 transfer agents discussion of bond lengths and angles.
to test the plausibility of one of the proposed mechanisms
(see Scheme 1) for the isomerization of this anion.
Results and Discussion
Lithium complexes that contain anionic [CH(PPh2BH3)2]–
are easily accessible by deprotonation of parent
[9]
CH2(PPh2BH3)2
with commercially available organo-
lithium compounds such as n-butyllithium and were there-
fore chosen as the starting point of this investigation. If this
reaction is performed in a solution of diethyl ether, the
ether-ligated derivative [Li{CH(PPh2BH3)2}(Et2O)2] is ac-
cessible in crystalline form.[4b] Although the methanide car-
bon atom does not show a bonding interaction with the
lithium cation in this complex, such interactions were found
in the related derivative [Li2{CH(PPh2BH3)2}2(Et2O)],
Figure 1. Structural motif of the polymeric chain of
[(Li{H3BCH(PPh2BH3)2})ϱ] (2). Hydrogen atoms are omitted for
clarity.
The complex was insoluble in non-coordinating solvents
which can be prepared under modified conditions.[4d] This such as benzene and toluene, which hampered its investiga-
finding underlines the notion that the methanide carbon tion in solution in the absence of additional Lewis bases.
atom, which carries a significant part of the negative charge To test the stability of the newly formed anion
of the anion,[4c] is sufficiently nucleophilic to form bonds [H3BCH(PPh2BH3)2]– of the lithium complex 2 in the pres-
with rather hard Lewis acids such as Li+ and therefore ence of different ethers, a number of these donor ligands
might be suitable to take up an additional BH3 group under were used as additives during recrystallization after success-
certain conditions. This uptake should result in the forma- ful formation of 2. Initially, 2,2,5,5-tetramethyltetra-
tion of the hitherto unknown anion [H3BCH(PPh2BH3)2]–, hydrofuran (Me4THF) was applied. Owing to steric crowd-
an envisioned intermediate[5b] in the isomerization depicted ing around its oxygen atom, this ether should be only
in Scheme 1.
weakly nucleophilic and was therefore in our opinion the
However, the addition of 1 equiv: of BH3·THF (in THF) most suitable candidate to prevent possible removal of la-
to a freshly prepared solution of [Li{CH(PPh2BH3)2}- bile BH3 groups. As the result of this experiment, the mono-
(Et2O)2] in diethyl ether did not lead to the isolation of the nuclear complex [Li{H3BCH(PPh2BH3)2}(Me4thf)] (3) was
desired product but to partial or complete exchange of the isolated in crystalline form. The structure of the compound,
neutral diethyl ether ligand and to the isolation of crystal- determined by X-ray diffraction experiments, is shown in
line samples of [Li{CH(PPh2BH3)2}(Et2O)2–n(thf)n] (n ≈ 1) Figure 2.
and [Li{CH(PPh2BH3)2}(thf)2] (1) upon cooling, depending
The uptake of the additional BH3 group by the formerly
on the conditions applied. An uptake of BH3 by the anionic sp2-hybridized methanide carbon atom led to sp3 hybridiza-
ligand was not detected in the isolated crystalline crop. tion of this atom. The observed distance between the
Compound 1 is easily accessible in good yields by carbon atom and the attached boron atom of 1.687(4) Å
recrystallization of the known diethyl ether ligated deriva- is slightly longer than the related single bond of
tive in THF. The molecular structure of 1 is depicted in 1.638(11) Å observed for isomer B (see Scheme 1) in
Figure S1 (see the Supporting Information).
[Ba4O{CH(PPh2BH3)2}2{Ph2PCH(BH3)(PPh2BH3)}][5] and
The isolation of 1 underlines the notion that the pro- the value of 1.66(3) Å reported for the carbon–boron single
posed uptake of an additional BH3 group requires more bond of the closely related carbodiphosphorane borane ad-
forceful conditions. The presence of competing Lewis bases duct [{(μ-H)H4B2}{C(PPh3)2}][B2H7].[10] The phosphorus–
such as diethyl ether and THF, especially when used in large carbon bond lengths of the diphosphanylmethanide moiety
excess as solvent, seems counterproductive for this en- in 3 show values of 1.814(2) Å (P1–C1) and 1.817(3) Å (P2–
deavor. Therefore, the original procedure was modified. Tol- C1), also typical of single bonds. Owing to the change in
uene or benzene were used as solvents during deprotonation hybridization, a smaller P1–C1–P2 angle of 116.91(13)° rel-
of the precursor CH2(PPh2BH3)2, and BH3·SMe2 was cho- ative to compound 1 [131.59(15)°] was observed. The lith-
sen as the BH3 source since dimethylsulfane as a rather soft ium cation interacts with the hydridic hydrogen atoms of
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The data obtained in [D8]THF (see the Experimental
Section and the Supporting Information) suggest that the
negative charge in these complexes is localized at the car-
bon-bound BH3 group. The hydridic hydrogen atoms of this
group resonated at rather high field strength and gave a
broad signal centered at δH = 0.30 ppm. In contrast, the
hydrogen atoms of the phosphorus-bound BH3 groups and
of the central methanide subunit are significantly de-
shielded relative to compound 1, and chemical shifts of δH
= 1.22 (P–BH3; 1: δH = 0.84 ppm) and 3.07 ppm (P2CH; 1:
δH = 0.68 ppm), respectively, were observed. In the 13C and
31P NMR spectra, the PCHP subunit gave rise to signals at
δC = 15.6 ppm and δP = 22.1 ppm, whereas two signals with
relative integrals of 2:1 were observed in the 11B NMR spec-
trum at δB = –37.4 (PBH3) and –30.8 ppm (CBH3).
To relate the solution data of the [D8]THF-ligated com-
pound formed during the NMR spectroscopic investigation
with additional data and to see how the use of a stronger
donor solvent such as THF might affect the interaction
between anion and cation, the thf-ligated complex
[Li{H3BCH(PPh2BH3)2}(thf)3] (4) was prepared in crystal-
line form. As shown in Figure 3, the complex contains
three-coordinated THF molecules per lithium center, which
leaves one coordination site of the preferably tetracoordi-
nated lithium cation for the interaction with the anionic
ligand. This interaction takes place through the carbon-
Figure 2. Molecular structure and numbering scheme of
[Li{H3BCH(PPh2BH3)2}(Me4thf)] (3). The ellipsoids represent a
probability of 40%. Hydrogen atoms except those of the BH3
groups and at C1 as well as a disorder of the Me4THF ligand are
omitted for clarity. Selected bond lengths [Å] and angles [°]: Li1–
H1(B1) 2.21(4), Li1–H1(B2) 2.12(3), Li1–H2(B2) 2.12(4), Li1–
H1(B3) 1.97(4), Li1–O1 1.841(7), C1–B3 1.687(4), C1–P1 1.814(2),
C1–P2 1.817(3), P1–B1 1.924(3), P2–B2 1.945(3); O1–Li1–H1(B1)
103.7(10), O1–Li1–H1(B2) 132.9(10), O1–Li1–H2(B2) 116.6(11),
O1–Li1–H1(B3) 104.1(11), B3–C1–P1 109.18(17), B3–C1–P2
108.16(19), P1–C1–P2 116.91(13), C1–P1–B1 116.55(14), C1–P2–
B2 117.09(13).
all three BH3 moieties of the anionic ligand. Whereas the
coordination mode of the BH3 group at P2 is best described
as κ2(H,H) with lithium–hydrogen distances of 2.12(3) and
2.12(4) Å, the other two BH3 groups show both κ1(H) coor-
dination with a shorter contact between the lithium ion and
the carbon-bound BH3 group [Li1–H1(B3) 1.97(4) Å] and
a rather long one to the second phosphorus-bound BH3
moiety [Li1–H1(B1) 2.21(4) Å]. Together with the oxygen
atom of the neutral ether ligand Me4THF, the three borane
moieties form a pseudotetrahedral coordination sphere
around the lithium center. The interaction between the cat-
ion and Me4THF seems to be variable. A positional disor-
der leads to two deviating locations of the ligand relative to
the lithium cation. Rather different distances between the
metal ion and the oxygen atom of 1.841(7) Å (Li–O1) and
2.045(8) Å (Li–O1A) were observed. Compound 3 is a rare
example of a metal complex that contains this coordinated
ether ligand[11] and to the best of our knowledge[12] is the
first structurally characterized derivative.
Figure 3. Molecular structure and numbering scheme of
[Li{H3BCH(PPh2BH3)2}(thf)3]·toluene (4). The ellipsoids represent
Dissolution of 2 and 3 in THF did not lead to reversion a probability of 40%. Hydrogen atoms except those of the BH3
groups and at C1 as well as the co-crystallized toluene are omitted
of the formation of the anion by liberation of BH3 as
underlined by the NMR spectroscopic data obtained for
these complexes in [D8]THF. Despite the enhanced donor
1.973(3), Li1–O3 1.954(2), C1–B3 1.6691(18), C1–P1 1.8329(12),
for clarity. Selected bond lengths [Å] and angles [°]: Li1–H1(B3)
1.994(18), Li1–H2(B3) 2.141(17), Li1–O1 1.965(3), Li1–O2
capacity of this ether, no formation of complex 1 was de-
tected in solution. Instead, the spectra obtained suggest a
breakup of the polymeric structure in the case of 2 as well
as rapid replacement of the neutral ligand in 3 by [D8]THF,
which leads to essentially identical spectra of these com-
pounds despite the signals of the liberated ligand.
C1–P2 1.8207(12), P1–B1 1.9257(15), P2–B2 1.9354(15); O1–Li1–
O2 100.05(11), O1–Li1–O3 108.14(12), O1–Li1–H1(B3) 95.5(5),
O1–Li1–H2(B3) 145.8(5), O2–Li1–O3 98.96(11), O2–Li1–H1(B3)
107.4(5), O2–Li1–H2(B3) 100.9(5), O3–Li1–H1(B3), 140.7(5), O3–
Li1–H2(B3) 94.9(5), B3–C1–P1 111.41(8), B3–C1–P2 112.87(9),
B3–C1–H1 106.1(10), P1–C1–P2 119.93(6), P1–C1–H1 100.0(9),
P2–C1–H1 104.5(10), C1–P1–B1 107.36(7), C1–P2–B2 120.06(6).
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Scheme 2. Reactivity of [CH(PPh2BH3)2]– towards neutral and anionic phosphane–borane adducts.
bound BH3 group, which shows an unsymmetrical κ2-coor- spectroscopic investigation of the product mixture (see the
dination mode [Li1–H1(B3) 1.994(18) Å, Li1–H2(B3) Supporting Information). The reaction is not limited to
2.141(17) Å]. The two other BH3 groups show no close con- lithium complexes but also works in the case of derivatives
tacts with the lithium ion and are oriented away from each that contain the softer Lewis acid potassium. Consequently,
other to minimize electrostatic repulsion.
a crystalline mixture of [K{H3BCH(PPh2BH3)2}(dme)2] (6;
The significance of complexes such as 2–4 for the isomer- dme = 1,2-dimethoxyethane) and Ph2PCH2PPh2BH3 was
ization of [CH(PPh2BH3)2]– to [H3BCH(PPh2BH3)(PPh2)]– observed from the reaction of [(K{CH(PPh2BH3)2})ϱ][5]
was further investigated, since in the original procedure for and CH2(PPh2BH3)2 after recrystallization from a mixture
the synthesis of [Ba4O{CH(PPh2BH3)2}2{Ph2PCH(BH3)-
(PPh2BH3)}],[5] the addition of an external BH3 source was
not mentioned. It was envisioned that the anionic ligand
[CH(PPh2BH3)2]– is not only a suitable BH3 group acceptor
as proven by the synthesis of 2–4, but also a potential do-
nor. This would lead to a redistribution of BH3 groups be-
tween two bis(borane) adducts [CH(PPh2BH3)2]– to form a
monoborane adduct [Ph2PCHPPh2BH3]– in addition to the
[H3BCH(PPh2BH3)2]– anion already observed in complexes
2–4 (see Scheme 2, left). However, heating of a freshly pre-
pared sample of [(Li{CH(PPh2BH3)2})n] in toluene without
additional donors to 80 °C for 12 h did not result in any
noticeable redistribution of BH3 groups, as judged by NMR
spectroscopy. The addition of 2 equiv. of Me4THF as a po-
tential BH3 “shuttle” does not change the outcome of this
experiment with respect to the bonding situation of the BH3
groups but led to the isolation of [Li{CH(PPh2BH3)2}-
(Me4thf)] (5) (see Figure S2 in the Supporting Information).
Figure 4. Molecular structure and numbering scheme of
[K{H3BCH(PPh2BH3)2}(dme)2] (6). The ellipsoids represent a
The isolation of 5 underlines the notion that the phos-
phorus–boron bond of the [CH(PPh2BH3)2]– anion is too
probability of 40%. Hydrogen atoms except those of the BH3
groups and at C1 are omitted for clarity. Selected bond lengths [Å]
and angles [°]: K1–H1(B1) 2.74(3), K1–H2(B1) 2.86(3), K1–H1(B2)
2.98(3), K1–H1(B3) 2.91(3), K1–H2(B3) 2.64(3), K1–O1 2.723(2),
K1–O2 2.7714(18), K1–O3 2.816(2), K1–O4 2.765(2), C1–B3
1.694(3), C1–P1 1.825(2), C1–P2 1.816(2), P1–B1 1.950(3), P2–B2
strong to be broken under the applied conditions. Nonethe-
less, the initial report[5] holds a clue about another BH3
source. It was mentioned that during transfer of
[CH(PPh2BH3)2]– from potassium to barium substantial
amounts of parent CH2(PPh2BH3)2 were formed. Since this 1.916(2); O1–K1–O2 62.33(6), O1–K1–O3 79.82(7), O1–K1–O4
83.20(7), O1–K1–H1(B1) 78.8(6), O1–K1–H2(B1) 91.2(6), O1–K1–
substance is a neutral molecule, it should be a more feasible
H1(B2) 140.3(5), O1–K1–H1(B3) 118.3(5), O1–K1–H2(B3)
BH3 donor than its anionic counterpart owing to a dimin-
156.6(6), O2–K1–O3 79.04(7), O2–K1–O4 130.82(7), O2–K1–
ished electrostatic attraction between the leaving BH3 group
H1(B1) 99.7(6), O2–K1–H2(B1) 72.3(6), O2–K1–H1(B2) 80.0(5),
and the remaining Lewis base. Heating a sample of
[(Li{CH(PPh2BH3)2})n] in toluene in the presence of sub-
stoichiometric amounts of CH2(PPh2BH3)2 indeed led to
O2–K1–H1(B3) 149.8(5), O2–K1–H2(B3) 139.1(6), O3–K1–O4
60.15(8), O3–K1–H1(B1) 156.4(6), O3–K1–H2(B1) 150.9(6), O3–
K1–H1(B2) 106.0(5), O3–K1–H1(B3) 131.0(5), O3–K1–H2(B3)
110.0(6), O4–K1–H1(B1) 107.1(6), O4–K1–H2(B1) 146.7(6), O4–
K1–H1(B2) 134.4(5), O4–K1–H1(B3) 76.4(5), O4–K1–H2(B3)
83.7(6), B3–C1–P1 108.77(14), B3–C1–P2 111.30(14), P1–C1–P2
consumption of CH2(PPh2BH3)2, and the formation of
[5b,13]
compound 2 and Ph2PCH2PPh2BH3
(major products)
in addition to further byproducts as judged by 31P NMR 115.40(11), C1–P1–B1 120.96(11), C1–P2–B2 118.15(11).
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Scheme 3. Reactivity of the [H3BCH(PPh2BH3)2]– anion towards tmeda.
1
Figure 5. Part of the H NMR spectrum of the reaction mixture of 2 and tmeda in [D8]THF. Left: P2CHB moiety of 2. Center: CH2
group of Me2NCH2CH2N(BH3)Me2. Right: P2CHB moiety of Li[Ph2PCH(BH3)(PPh2BH3)].
of toluene and dme. Attempts to separate 6 from this mix- Although this compound was only characterized in solu-
ture failed. Nevertheless, the molecular structure of 6 was tion, the NMR spectroscopic data indicates that this
determined by X-ray diffraction analysis (see Figure 4). As lithium complex contains the same [Ph2PCH(BH3)-
expected, the larger potassium cation shows a higher coor- (PPh2BH3)]– anion as observed in the barium complex
dination number than the lithium cation. In 6 the potas- [Ba4O{CH(PPh2BH3)2}2{Ph2PCH(BH3)(PPh2BH3)}].[5]
sium center is surrounded by the four oxygen atoms of the This result demonstrates that the complexes under investi-
dme ligands and the three BH3 groups of the anionic ligand gation are indeed potential intermediates in the isomeriza-
in an irregular arrangement. The average potassium–oxygen tion of [CH(PPh2BH3)2]–.
bond in 6 has a length of 2.769(2) Å, whereas the five potas-
sium–hydrogen bonds are 2.83(3) Å long on average. The
anionic ligand itself shows interatomic distances and angles
comparable to the values observed in 3 and 4.
Conclusion
As described above, alkali metal complexes that contain
A straightforward strategy for the synthesis of lithium
the novel anionic ligand [H3BCH(PPh2BH3)2]– are rather complexes that contain the novel tris(borane) adduct of di-
stable in the presence of ethereal Lewis bases like THF. phosphanylmethanide was developed by using BH3·SMe2
Against stronger bases such as N,N,NЈ,NЈ-tetramethyleth- as a borane source. During their synthesis, the use of tolu-
ane-1,2-diamine (tmeda), partial loss of a phosphorus- ene proved superior to ethereal solvents. Nevertheless, the
bound BH3 group was observed in solution (see Scheme 3). product of the reaction under these conditions,
The 31P NMR spectrum of the reaction mixture of 2 and [(Li{H3BCH(PPh2BH3)2})ϱ] (2), is, once formed, stable in
tmeda in [D8]THF shows the presence of a new species re- the presence of ethers. Whereas the derivative
lated to signals at δP = 22.8 and –8.8 ppm. The latter signal [Li{H3BCH(PPh2BH3)2}(Me4thf)] (3) contained one ether
1
stems from an unprotected phosphane moiety. In the H molecule per lithium center, the utilization of THF resulted
NMR spectrum, the methanide subunit of this species in the derivative [Li{H3BCH(PPh2BH3)2}(thf)3] (4), which
shows a broadened multiplet at δH = 2.73 ppm (see Fig- showed reduced interactions between the lithium cation and
ure 5), which is indicative of the still attached BH3 group. the anionic ligand owing to the uptake of three neutral
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diffraction analysis were obtained directly from the reaction mix-
ture.
THF molecules. The isolated compounds can act as borane
sources themselves in the presence of stronger Lewis bases
such as tmeda. In the presence of this substance, a phos-
phorus-bound BH3 group is partially removed and the an-
ionic phosphane ligand [Ph2PCH(BH3)(PPh2BH3)]– is
Synthesis of [(Li{H3BCH(PPh2BH3)2})ϱ] (2): BH3·SMe2 (0.1 mL,
1.69 mmol) was added with a syringe at ambient temperature to a
stirred solution of [(Li{CH(PPh2BH3)2})n], freshly prepared from
formed. This result indicates that the tris(borane) adducts CH2(PPh2BH3)2 (0.281 g, 0.68 mmol) and n-butyllithium (0.43 mL,
0.69 mmol, 1.6 m in hexane) in benzene (5 mL). Afterwards, the
reaction mixture was stirred for an additional 30 min. The white
precipitate that formed was collected with a Schlenk frit and dried
under vacuum. Yield: 0.245 g (0.57 mmol, 83.2%). C25H30B3LiP2
(431.835): calcd. C 69.53, H 7.00; found C 69.21, H 7.63. 1H NMR
(400 MHz, [D8]THF, 25 °C): δ = 0.30 (br., 3 H, CBH3), 1.22 (br.,
6 H, PBH3), 3.07 (m, 1 H, P2CHB), 7.05 (m, 8 H, m-CH Ph), 7.10
of bis(diphenylphosphanyl)methanide are potential inter-
mediates in the isomerization of [CH(PPh2BH3)2]– to
[Ph2PCH(BH3)(PPh2BH3)]–. However, this process requires
an external borane source as well as a sufficiently strong
Lewis base. Whether this simple strategy can be adapted to
the synthesis of other α-BH3-modified phosphanes is under
current investigation.
7
(m, 4 H, p-CH Ph), 7.78 (m, 8 H, o-CH Ph) ppm. Li{1H} NMR
(233 MHz, [D8]THF, 25 °C): δ = –1.1 (s) ppm. 11B{1H} NMR
(128 MHz, [D8]THF, 25 °C): δ = –37.4 (br., 2 B, PBH3), –30.8 (br.,
In addition, two unique metal complexes that contain the
sterically crowded ether ligand Me4THF, namely, [Li-
{H3BCH(PPh2BH3)2}(Me4thf)](3)and[Li{CH(PPh2BH3)2}- 1 B, CBH3) ppm. 13C{1H} NMR (100.6 MHz, [D8]THF, 25 °C): δ
= 15.6 (br., 1 C, P2CHB), 127.5 (d, 3JC,P = 9.5 Hz, 4 C, m-CH Ph),
(Me4thf)] (5), were synthesized and structurally charac-
terized.
3
4
127.8 (d, JC,P = 9.5 Hz, 4 C, m-CH Ph), 129.4 (d, JC,P = 1.0 Hz,
4
2 C, p-CH Ph), 129.6 (d, JC,P = 1.0 Hz, 2 C, p-CH Ph), 134.1 (d,
2JC,P = 8.4 Hz, 4 C, o-CH Ph), 134.2 (d, JC,P = 50.8 Hz, 2 C, i-C
1
2
1
Ph), 134.4 (d, JC,P = 8.4 Hz, 4 C, o-CH Ph), 135.0 (dd, JC,P
=
Experimental Section
54.4, JC,P = 6.5 Hz, 2 C, i-C Ph) ppm. 31P{1H} NMR (162 MHz,
[D8]THF, 25 °C): δ = 22.1 (br.) ppm. Crystals of 2 for X-ray diffrac-
tion experiments could be obtained directly from the reaction mix-
ture when BH3·SMe2 was added without stirring at ambient tem-
perature, and the resulting mixture was left undisturbed for 18 h.
3
General Methods: All manipulations were carried out under argon
by using standard Schlenk techniques. THF, 2,2,5,5-tetrameth-
yltetrahydrofuran, diethyl ether, toluene, and benzene were dried
with KOH and distilled from sodium/benzophenone under argon;
deuterated THF was dried with sodium, degassed, and saturated
with argon. The yields given are not optimized. 1H, 7Li{1H},
11B{1H}, 13C{1H}, and 31P{1H} NMR spectra were recorded with
Bruker AC 400 or AC 600 spectrometers. Chemical shifts are re-
ported in parts per million relative to Me4Si as an external stan-
dard. The residual signals of [D8]THF were used as internal stan-
dard in the 1H and 13C NMR spectra. 7Li, 11B, and 31P NMR
spectroscopic shifts are referenced to LiCl in D2O, 15% BF3·OEt2
in CDCl3, and 85% H3PO4 as external standard, respectively. Cou-
pling constants are given in Hertz. BH3·THF and BH3·SMe2 were
purchased from Aldrich. CH2(PPh2BH3)2 was synthesized accord-
ing to a known procedure.[9]
Synthesis of [Li{H3BCH(PPh2BH3)2}(Me4thf)] (3): Solid
CH2(PPh2BH3)2 (0.314 g, 0.76 mmol) was suspended in toluene
(5 mL), and a solution of n-butyllithium (0.48 mL of 1.6 m hexane
solution, 0.77 mmol) was added with a syringe to the stirred sus-
pension at ambient temperature. After 5 min of stirring, BH3·SMe2
(0.1 mL, 1.69 mmol) was added by syringe to the clear solution.
The resulting white solid dissolved upon addition of 2,2,5,5-
tetramethyltetrahydrofuran (1 mL). The clear solution was stored
at –20 °C overnight to give a white precipitate. The product was
collected with a Schlenk frit and dried under vacuum. Yield:
0.358 g (0.64 mmol, 83.9%). 1H NMR (400 MHz, [D8]THF, 25 °C):
δ = 0.30 (br., 3 H, CBH3), 1.17 (s, 12 H, CH3 Me4THF), 1.21 (br.,
6 H, PBH3), 1.81 (s, 4 H, CH2 Me4THF), 3.07 (m, 1 H, P2CHB),
7.05 (m, 8 H, m-CH Ph), 7.10 (m, 4 H, p-CH Ph), 7.78 (m, 8 H,
o-CH Ph) ppm. 7Li{1H} NMR (155 MHz, [D8]THF, 25 °C): δ =
–1.1 (s) ppm. 11B{1H} NMR (128 MHz, [D8]THF, 25 °C): δ =
–37.4 (br., 2 B, PBH3), –30.8 (br., 1 B, CBH3) ppm. 13C{1H} NMR
(100.6 MHz, [D8]THF, 25 °C): δ = 15.6 (br., 1 C, P2CHB), 30.1 (s,
4 C, CH3 Me4THF), 39.4 (s, 2 C, CH2 Me4THF), 81.0 (s, 2 C, OC
Me4THF), 127.5 (d, 3JC,P = 9.5 Hz, 4 C, m-CH Ph), 127.7 (d, 3JC,P
= 9.5 Hz, 4 C, m-CH Ph), 129.4 (d, 4JC,P = 2.3 Hz, 2 C, p-CH Ph),
Synthesis of [Li{CH(PPh2BH3)2}(thf)2] (1): Solid CH2(PPh2BH3)2
(0.666 g, 1.62 mmol) was suspended in diethyl ether (20 mL), and
a solution of n-butyllithium (1.0 mL of a 1.6 m hexane solution,
1.6 mmol) was added with a syringe to the stirred suspension at
ambient temperature. After the formation of a clear solution, THF
(1.5 mL) was added, and the resulting solution was stored at 0 °C.
After 24 h, the resulting white precipitate was collected with a
Schlenk frit and dried under vacuum. Yield: 0.813 g (1.45 mmol,
89.5%, crude product). The compound partially lost coordinated
THF upon prolonged drying. In some cases, the product partially
contained diethyl ether beside coordinated THF. In these cases, the
crude product was recrystallized from THF (ambient temperature
to –20 °C) to obtain pure 1. 1H NMR (400 MHz, [D8]THF, 25 °C):
δ = 0.68 (s, 1 H, P2CH), 0.84 (br., 6 H, PBH3), 1.76 (m, 8 H, CH2
THF), 3.62 (m, 8 H, OCH2 THF), 7.10 (m, 12 H, m-CH, p-CH
Ph), 7.71 (m, 8 H, o-CH Ph) ppm. 7Li{1H} NMR (155 MHz,
[D8]THF, 25 °C): δ = –0.6 (s) ppm. 11B{1H} NMR (128 MHz,
4
2
129.6 (d, JC,P = 1.9 Hz, 2 C, p-CH Ph), 134.1 (d, JC,P = 8.6 Hz,
4 C, o-CH Ph), 134.2 (d, 1JC,P ≈ 53 Hz, 2 C, i-C Ph), 134.4 (d, 2JC,P
1
3
= 8.5 Hz, 4 C, o-CH Ph), 135.0 (dd, JC,P = 54.4, JC,P = 6.5 Hz,
2 C, i-C Ph) ppm. 31P{1H} NMR (162 MHz, [D8]THF, 25 °C): δ =
22.0 (br.) ppm. Suitable crystals of 3 for X-ray diffraction experi-
ments were obtained directly from the reaction mixture at –40 °C.
Synthesis of [Li{H3BCH(PPh2BH3)2}(thf)3]·1toluene (4): Solid
CH2(PPh2BH3)2 (0.290 g, 0.70 mmol) was suspended in toluene
[D8]THF, 25 °C): δ = 5.1 (t, JC,P = 77.4 Hz, 1 C, P2CH), 26.3 (s, (5 mL), and a solution of n-butyllithium (0.38 mL of 1.6 m hexane
solution, 0.70 mmol) was added with a syringe to the stirred sus-
8 C, m-CH Ph), 127.8 (d, JC,P = 1.6 Hz, 4 C, p-CH Ph), 133.2 (d, pension at ambient temperature. After 5 min of stirring, BH3·SMe2
[D8]THF, 25 °C): δ = –33.6 (br.) ppm. 13C{1H} NMR (100.6 MHz,
1
2 C, CH2 THF), 68.2 (s, 2 C, OCH2 THF), 127.1 (d, 3JC,P = 8.8 Hz,
4
1
3
2JC,P = 9.1 Hz, 8 C, o-CH Ph), 142.9 (dd, JC,P = 55.4, JC,P
=
(0.1 mL, 1.69 mmol) was added to the clear solution. The resulting
1.4 Hz, 4 C, i-C Ph) ppm. 31P{1H} NMR (162 MHz, [D8]- white solid dissolved upon addition of THF (1 mL). The solution
THF, 25 °C): δ = 14.4 (br.) ppm. Suitable crystals of 1 for X-ray
was concentrated under vacuum until the remaining oil started to
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2014, 5940–5947
5945

www.eurjic.org
FULL PAPER
crystallize. The product was suspended in toluene (5 mL), removed
by filtration, and dried under vacuum. The compound partially lost
coordinated THF upon drying. Yield: 0.256 g (0.35 mmol, 49.1%).
CH Ph), 7.92 (m, 4 H, o-CH Ph) ppm. 11B{1H} NMR (128 MHz,
[D8]THF, 25 °C): δ = –37.8 (br., 2 B, PBH3), –28.6 (br., 1 B, CBH3)
ppm. 13C{1H} NMR (100.6 MHz, [D8]THF, 25 °C): δ = 13.0 (br.,
1H NMR (400 MHz, [D8]THF, 25 °C): δ = 0.30 (br., 3 H, CBH3), 1 C, P2CHB), 58.8 (s, 4 C, CH3 dme), 72.6 (s, 4 C, OCH2 dme),
1.22 (br., 6 H, PBH3), 1.77 (m, CH2 THF), 2.31 (s, 3 H, CH3 tolu-
127.7 (m, 8 C, m-CH Ph), 129.4 (m, 4 C, p-CH Ph), 133.6 (m, 8 C,
1
1
ene), 3.07 (m, 1 H, P2CHB), 3.62 (m, OCH2 THF), 7.05 (m, 8 H, o-CH Ph), 135.6 (d, JC,P = 45.8 Hz, 2 C, i-C Ph), 135.9 (d, JC,P
m-CH Ph), 7.10 (m, 4 H, p-CH Ph), 7.13 (m, 3 H, o-CH, p-CH = 72.3 Hz, 2 C, i-C Ph) ppm. 31P{1H} NMR (162 MHz, [D8]THF,
toluene), 7.19 (m, 2 H, m-CH toluene), 7.79 (m, 8 H, o-CH Ph)
ppm. 7Li{1H} NMR (155 MHz, [D8]THF, 25 °C): δ = –1.1 (s) ppm.
25 °C): δ = 21.2 (br.) ppm. Suitable crystals of 6 for X-ray diffrac-
tion experiments were obtained directly from the reaction mixture
11B{1H} NMR (128 MHz, [D8]THF, 25 °C): δ = –37.4 (br., 2 B, at –20 °C.
PBH3), –30.8 (br., 1 B, CBH3) ppm. 13C{1H} NMR (100 MHz,
[D8]THF, 25 °C): δ = 15.7 (br., 1 C, P2CHB), 21.4 (s, 1 C, CH3
Reaction of [Li{H3BCH(PPh2BH3)2}([D8]thf)3] with tmeda: A solu-
tion of compound 4 was generated in situ from 2 (19.5 mg,
0.045 mmol) and [D8]THF (0.4 mL), which was transferred to a
valved NMR spectroscopy tube. Afterwards, an excess amount of
tmeda (0.15 mL, 0.99 mmol) was added. The resulting mixture was
shaken for a few seconds and allowed to stand at ambient tempera-
ture for an additional 24 h. Thereafter, the mixture was
characterized by multinuclear NMR spectroscopy. In addition to
residual starting materials, Li{Ph2PCH(BH3)(PPh2BH3)} and
Me2NCH2CH2N(BH3)Me2 are the predominantly formed prod-
ucts.
toluene), 26.4 (s, CH2 THF), 68.3 (s, OCH2 THF), 126.1 (s, 1 C,
3
p-CH toluene), 127.6 (d, JC,P = 9.6 Hz, 4 C, m-CH Ph), 127.9 (d,
3JC,P = 9.6 Hz, 4 C, m-CH Ph), 128.9 (s, 2 C, m-CH toluene), 129.6
4
(d, JC,P = 1.8 Hz, 2 C, p-CH Ph), 129.7 (s, 2 C, o-CH toluene),
4
2
129.7 (d, JC,P = 2.0 Hz, 2 C, p-CH Ph), 134.3 (d, JC,P = 8.0 Hz,
2
4 C, o-CH Ph), 134.5 (d, JC,P = 8.0 Hz, 4 C, o-CH Ph), 134.3 (d,
1JC,P ≈ 50 Hz, 2 C, i-C Ph), 135.1 (dd, 1JC,P = 54.3, 3JC,P = 6.6 Hz,
2 C, i-C Ph), 138.4 (s, 1 C, i-C toluene) ppm. 31P{1H} NMR
(162 MHz, [D8]THF, 25 °C): δ = 22.1 (br.) ppm. Suitable crystals
of 4 for X-ray diffraction experiments were obtained by layering a
saturated solution of 4 in a mixture of toluene and THF with n-
heptane at –10 °C.
Analytical Data of Li{Ph2PCH(BH3)(PPh2BH3)}: 1H NMR
(400 MHz, [D8]THF, 25 °C): δ = 0.26 (br. m, 3 H, BH3), 0.70–1.65
(m, 3 H, PBH3), 2.73 (br. m, 1 H, P2CHB), 6.73 (m, 2 H, CH Ph),
6.78 (m, 1 H, CH Ph), 6.93 (m, 2 H, CH Ph), 6.99 (m, 1 H, CH
Ph), 7.02–7.18 (m, 7 H, CH Ph), 7.57 (m, 2 H, CH Ph), 7.65 (m, 2
H, CH Ph), 7.70–7.85 (m, 3 H, CH Ph) ppm. 11B{1H} NMR
(128 MHz, [D8]THF, 25 °C): δ = –40.8 (br., 1 B, PBH3), –31.4 (br.,
1 B, CBH3) ppm. 13C{1H} NMR (100.6 MHz, [D8]THF, 25 °C): δ
Synthesis of [Li{CH(PPh2BH3)2}(Me4thf)] (5): Solid CH2-
(PPh2BH3)2 (0.349 g, 0.85 mmol) was suspended in toluene (8 mL),
and a solution of n-butyllithium (0.55 mL of 1.6 m hexane solution,
0.88 mmol) was added to the stirred suspension at ambient tem-
perature. Hexane and butane were removed by distillation under
reduced pressure. The resulting white precipitate dissolved upon
addition of Me4THF (0.224 g, 1.75 mmol). The pale yellow solu-
tion was heated to 80 °C for 18 h. After cooling to ambient tem-
perature, the resulting white precipitate was collected with a
Schlenk frit and dried under vacuum. Yield: 0.333 g (0.61 mmol,
72.0%). 1H NMR (600 MHz, [D8]THF, 25 °C): δ = 0.64 (s, 1 H,
P2CH), 0.84 (br., 6 H, PBH3), 1.17 (m, 12 H, CH3 Me4THF), 1.81
(m, 4 H, CH2 Me4THF), 7.09 (m, 12 H, m,p-CH), 7.69 (m, 8 H,
3
= 14.8 (br., 1 C, P2CHB), 127.2 (s, 1 C, p-CH Ph), 127.45 (d, JC,P
= 7.2 Hz, 2 C, m-CH Ph), 127.5–127.8 (m, 7 C, 6 m-CH, p-CH Ph),
129.3 (br., 2 C, p-CH Ph), 133.7–134.0 (m, 4 C, o-CH Ph), 134.4
2
2
(d, JC,P = 21.3 Hz, 2 C, o-CH Ph), 134.9 (d, JC,P = 20.8 Hz, 2 C,
1
1
o-CH Ph), 135.9 (d, JC,P = 49.2 Hz, 1 C, i-C Ph), 136.4 (dd, JC,P
3
1
= 55.3, JC,P = 6.3 Hz,1 C, i-C Ph), 143.5 (d, JC,P = 22.1 Hz, 1 C,
i-C Ph), 143.9 (pseudo-t, 1JC,P ≈ 3JC,P ≈ 14.2 Hz, 1 C, i-C Ph) ppm.
31P{1H} NMR (162 MHz, [D8]THF, 25 °C): δ = –8.8 (d, JP,P
=
2
7
o-CH) ppm. Li{1H} NMR (233 MHz, [D8]THF, 25 °C): δ = –0.6
65.7 Hz, 1 P, PPh2), 22.7 (br., 1 P, PPh2BH3) ppm.
(s) ppm. 11B{1H} NMR (193 MHz, [D8]THF, 25 °C): δ = –33.9
(br.) ppm. 13C{1H} NMR (150 MHz, [D8]THF, 25 °C): δ = 5.3 (t,
Analytical Data of Me2NCH2CH2N(BH3)Me2: 1H NMR
(400 MHz, [D8]THF, 25 °C): δ = 2.19 (s, 6 H, NMe2), 2.55 (s, 6 H,
1JC,P = 77.6 Hz, 1 C, P2CH), 30.2 (s, 4 C, CH3 Me4THF), 39.5 (s,
3
3
3
BNMe2), 2.61 (t, JH,H = 6.8 Hz, 2 H, CH2), 2.81 (t, JH,H
=
2 C, CH2 Me4THF), 81.1 (s, 2 C, OC Me4THF), 127.3 (d, JC,P
=
=
2
6.8 Hz, 2 H, CH2) ppm; the signal of the BH3 group overlaps with
the signals of the BH3 groups of Li{Ph2PCH(BH3)(PPh2BH3)}.
11B{1H} NMR (128 MHz, [D8]THF, 25 °C): δ = –10.8 (s, 1 B,
NBH3) ppm. 13C{1H} NMR (100.6 MHz, [D8]THF, 25 °C): δ =
46.0 (s, 2 C, NMe2), 52.0 (s, 2 C, BNMe2), 55.4 (s, 1 C, CH2) 62.3
(s, 1 C, CH2) ppm.
9.9 Hz, 8 C, m-CH Ph), 127.9 (s, 4 C, p-CH Ph), 133.2 (d, JC,P
8.8 Hz, 8 C, o-CH Ph), 143.1 (d, 1JC,P = 57.4 Hz, 4 C, i-C Ph) ppm.
31P{1H} NMR (162 MHz, [D8]THF, 25 °C): δ = 14.3 (br.) ppm.
Suitable crystals of 7 for X-ray diffraction experiments were ob-
tained from the mother liquor of the reaction at –20 °C.
Formation of [K{H3BCH(PPh2BH3)2}(dme)2] (6): Solid [(K{CH-
(PPh2BH3)2})ϱ] (105 mg, 0.23 mmol) was suspended in toluene
(5 mL). After the addition of solid CH2(PPh2BH3)2 (96 mg,
0.24 mmol), the resulting suspension was heated to 80 °C (bath
temperature) for 30 h. Afterwards, the reaction mixture was al-
lowed to reach ambient temperature. Thereafter, dme (0.5 mL) was
added, and reaction mixture was stirred for 30 min. The remaining
solids were removed by filtration, and the clear solution was stored
at –20 °C for 2 d. The resulting crystalline mixture was isolated
by decantation and consisted predominantly of 6 in addition to
Ph2PCH2PPh2BH3 and minor amounts of the starting material.
Attempts to further purify 6 failed. Yield: 74 mg (crude product).
Structure Determinations: The intensity data for the compounds
were collected with a Nonius KappaCCD diffractometer by using
graphite-monochromated Mo-Kα radiation. Data were corrected
for Lorentz and polarization effects but not for absorption ef-
fects.[14,15] The structures were solved by direct methods
(SHELXS[16]) and refined by full-matrix least-squares techniques
2
against Fo (SHELXL-97[16]). The hydrogen atoms of compounds
5 and 6 (without the hydrogen atoms attached to C26) as well as
the hydrogen atoms of the borane groups of 1, 3, and 4 were located
by difference Fourier synthesis and refined isotropically. All other
hydrogen atoms were included at calculated positions with fixed
Analytical data of 6: 1H NMR (400 MHz, [D8]THF, 25 °C): δ = thermal parameters. The non-disordered, non-hydrogen atoms were
0.2–1.7 (br., 9 H, BH3), 3.03 (m, 1 H, P2CHB), 3.28 (s, 12 H, OCH3
dme), 3.44 (s, 8 H, OCH2 dme), 6.78 (m, 4 H, m-CH Ph), 6.90 (m, solution and refinement details are summarized in Table S1 (see the
refined anisotropically.[16] Crystallographic data as well as structure
2 H, p-CH Ph), 7.23 (m, 6 H, m-CH, p-CH Ph), 7.35 (m, 4 H, o-
Supporting Information). XP (SIEMENS Analytical X-ray Instru-
Eur. J. Inorg. Chem. 2014, 5940–5947
5946
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
Chem. 2013, 725, 11–14; k) K. Izod, C. M. Dixon, E. McMee-
kin, L. Rodgers, R. W. Harrington, U. Baisch, Organometallics
2014, 33, 378–386.
ments, Inc.) was used for structural representations. The crystals
of 2 were extremely thin and of low quality, which resulted in a
substandard data set. However, the structure is sufficient to show
connectivity and geometry. We will only publish the conformation
of the molecule and the crystallographic data. We will not deposit
the data of 2 in the Cambridge Crystallographic Data Centre data-
base. CCDC-1010309 (1), -1010310 (3), -1010312 (4), -1010313 (5),
and -1010314 (6) contain the supplementary crystallographic 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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Acknowledgments
The infrastructure of the Institute of Inorganic and Analytical
Chemistry was provided by the EU (European Fonds for Regional
Development, EFRE) and the Friedrich Schiller University, Jena.
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Received: September 9, 2014
Published Online: November 6, 2014
Eur. J. Inorg. Chem. 2014, 5940–5947
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