The Doubly Base-Stabilized Diborane(4) [HB(I-hpp)]2 (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate): Synthesis by Catalytic Dehydrogenation and Reactions with S8 and Disulfides

Article abstract of DOI:10.1002/ejic.201000637

In this work we report on new experiments on the catalytic dehydrogenation of [H₂B(I-hpp)]₂ leading to the doubly base-stabilized diborane(4) [HB(I-hpp)]₂ featuring two hpp bridges (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) under mild conditions. Several dehydrogenation (pre)catalysts were tested. The best one turned out to be Ru₃(CO)₁₂, allowing quantitative dehydrogenation already at 60 °C. Subsequently we subjected [HB(I-hpp)]₂ to reactions with S₈ and disulfides (Ph₂S₂ and Bn₂S₂, Bn = benzyl). Reaction with S₈ leads to oxidative insertion of one sulfur atom into the B-B bond and formation of [HB(I-hpp)]₂(I-S). In the case of disulfides, substitution reactions leading to the doubly base-stabilized diborane(4) species [RSB(I-hpp)]₂ and HB(I-hpp) ₂BSR (R = Ph or Bn) compete with sulfuration again leading to [HB(I-hpp)]₂(I-S). Catalytic dehydrogenation of [H ₂B(I-hpp)]₂ leads to the doubly base-stabilized diborane(4) [HB(I-hpp)]₂. Oxidative sulfuration of the B-B bond in [HB(I-hpp)]₂ gives [HB(I-hpp)]₂(I-S) . A mixture ofsulfuration and substitution products is formed for reactions with disulfides. Copyright

Full text of DOI:10.1002/ejic.201000637

FULL PAPER
DOI: 10.1002/ejic.201000637
The Doubly Base-Stabilized Diborane(4) [HB(µ-hpp)]2 (hpp = 1,3,4,6,7,8-
hexahydro-2H-pyrimido[1,2-a]pyrimidinate): Synthesis by Catalytic
Dehydrogenation and Reactions with S8 and Disulfides
Nikola Schulenberg,[a] Oxana Ciobanu,[a] Elisabeth Kaifer,[a] Hubert Wadepohl,[a] and
Hans-Jörg Himmel*[a]
Keywords: Boron / Hydrides / Sulfur / Guanidines
In this work we report on new experiments on the catalytic
dehydrogenation of [H2B(µ-hpp)]2 leading to the doubly
base-stabilized diborane(4) [HB(µ-hpp)]2 featuring two hpp
with S8 and disulfides (Ph2S2 and Bn2S2, Bn = benzyl). Reac-
tion with S8 leads to oxidative insertion of one sulfur atom
into the B–B bond and formation of [HB(µ-hpp)]2(µ-S). In the
bridges (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyr- case of disulfides, substitution reactions leading to the dou-
imidinate) under mild conditions. Several dehydrogenation
(pre)catalysts were tested. The best one turned out to be
Ru3(CO)12, allowing quantitative dehydrogenation already at
60 °C. Subsequently we subjected [HB(µ-hpp)]2 to reactions
bly base-stabilized diborane(4) species [RSB(µ-hpp)]2 and
HB(µ-hpp)2BSR (R = Ph or Bn) compete with sulfuration
again leading to [HB(µ-hpp)]2(µ-S).
some relevance to this work the heterolytic cleavage of di-
sulfides by FLPs was recently studied.[13]
Introduction
Boron chemistry has regained considerable interest in re-
cent years due to a series of exciting new discoveries. Hence
a first neutral compound with a (base-stabilized) B=B bond
was published by Robinson et al.[1,2] Segawa, Yamashita
and Nozaki reported on the synthesis and characterization
of a first boron compound in which the boron center is
nucleophilic (e. g., allowing nucleophilic attack on benzal-
dehyde).[3,4] Braunschweig et al. stabilized a “π-boryl
Recently we reported the synthesis of new doubly base-
stabilized diborane(4) hydride species featuring bridging bi-
cyclic guanidinate ligands.[14–16] Hence [HB(µ-hpp)]2 can be
synthesized by thermal (catalytic) dehydrogenation of the
adduct hppH·BH3. So far we used for this purpose the com-
plex [Rh(1,5-cod)(µ-Cl)]2 as pre-catalyst and 110 °C.[16] In
an alternative route the diborane(4) B2Cl2(NMe2)2 is
brought to reaction either with the neutral bicyclic guani-
anion” with the aid of an N-heterocyclic carbene.[5,6]
A
2+ [17,18]
dine, e. g. hppH to give [(Me2N(H))B(µ-hpp)]2
,
or
number of complexes with borane, boryl, borylene and also
boride ligands were synthesized, showing exciting electronic
properties and a great potential for several applications.[7]
For example, boryl ligands were intensively applied in
metal-catalysed borylation reactions.[8] Recently even a first
oxoboryl complex has been synthesized.[9] Moreover, mo-
lecular boron compounds are of interest for hydrogen stor-
age and as hydrogen transfer reagents. Hence catalytic dehy-
drogenation of amine borane (H3N·BH3), featuring a high
hydrogen storage capacity (theoretically as high as 19.6% if
all six hydrogen atoms are summed up), is currently studied
intensively.[10] Frustrated Lewis pairs (FLPs) containing bo-
ron as Lewis acidic site were demonstrated to activate small
molecules and notably react reversibly with H2.[11,12] With
with a Li salt such as hppLi to give the extremely unstable
neutral [(Me2N)B(µ-hpp)]2, which is highly amenable to
protonation at the NMe2 groups.[19] These doubly base-sta-
bilized diborane(4) species exhibit a rich chemistry which
we only just started to explore. Our work is motivated by a
simple structure–reactivity concept. Hence the reactivity
can be effectively controlled by the ring sizes of the guanid-
inate (see below),[20] and also of course by the group 13 ele-
ment used.[21,22]
Results and Discussion
In the following we first discuss some new experiments
on the catalytic dehydrogenation leading to the doubly
base-stabilized diborane(4) [HB(µ-hpp)]2 and quantum
chemical calculations on the thermodynamics for hydrogen-
ation of this and related systems. Then reactions of [HB(µ-
hpp)]2 with elemental sulfur and two disulfides (diphenyl
disulfide, Ph2S2, and dibenzyl disulfide, Bn2S2) will be ana-
lysed.
[a] Anorganisch-Chemisches Institut,
Ruprecht-Karls-Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: +49-6221-545707
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000637.
View this journal online at
wileyonlinelibrary.com
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FULL PAPER
Dehydrogenation Reactions
HiPr2N·BH3 (at temperatures in the range 25 °C to
45 °C).[27] Dehydrogenation starts after an induction
period, during which a black suspension is formed.
In Figure 1 the gas-phase ∆G0 value calculated for hydro-
genation of several doubly base-stabilized diborane(4) spe-
cies of the general formula [HB(µ-guanidinate)]2 to give
[H2B(µ-guanidinate)]2 is plotted as a function of the bite
angle N–C–N of the bicyclic guanidinates in the product.
All bicyclic guanidinates considered in this plot were al-
ready synthesized (see ref.[23] for tbo and tbn, and ref.[24] for
tbd and tbu). Larger bite angles, which can simply be real-
ized by choice of smaller ring sizes of the guanidinate
bicycles, lead to a decrease of the ∆G0 value for hydro-
genation (the result of a linear fit is included in Figure 1).
Thus it should be possible to tune the system to achieve
optimal thermodynamic properties for reversibility (∆G ≈
0 kJmol–1). Reversible oxidative addition reactions are then
possible if the reaction barrier can be controlled by the aid
of a catalyst.[25] We showed in the past that the reverse reac-
tion, dehydrogenation of [H2B(µ-hpp)]2 to give [HB(µ-
hpp)]2, can be achieved quantitatively in the presence of
[Rh(1,5-cod)(µ-Cl)]2 as pre-catalyst in toluene solutions at
110 °C. This reaction is mildly exergonic at standard condi-
tions (∆G0 = –30 kJmol–1). The same complex was shown
to be a pre-catalyst [26] for dehydrogenation of the amine
boranes H3N·BH3, H(1,4-C4H8)N·BH2, HMe(PhCH2)N·
BH3, H2MeN·BH3, HMe2N·BH3, H2PhN·BH3, and
In the hope to find catalysts which allow dehydrogena-
tion at lower temperatures, we tested some other potential
catalysts, the results are summarized in Table 1. Only tem-
peratures of 80 °C or lower were considered. Most of the
complexes included in Table 1 were already applied for de-
hydrogenation of amine boranes. Thus already in 1989,
Ru3(CO)12 was claimed to catalyse dehydrogenation of
H3N·BH3 and Me3N·BH3 at 60 °C.[28] In our experiments
this carbonyl turned out to be a good pre-catalyst for dehy-
drogenation of [H2B(µ-hpp)]2. Directly upon addition of
Ru3(CO)12 to a toluene solution of the hydride the reaction
mixture turned to a brownish colour, indicating decomposi-
tion of the pre-catalyst and heterogeneous catalytic condi-
tions. Clean dehydrogenation to give [HB(µ-hpp)]2 already
occurs at 60 °C, and is completed in not more than 12 h
[for addition of 10% of the Ru3(CO)12 complex].[29] The
time needed for quantitative dehydrogenation can be fur-
ther reduced to 7 h by increasing the temperature to 80 °C.
Figure 2 shows the NMR spectra and the dehydrogenation
kinetics at this temperature. The spectra indicate that dehy-
drogenation proceeds cleanly to give [HB(µ-hpp)]2 as the
single product. To determine the concentration of starting
material and product, the integrals of the marked NMR
signals were calculated. Based on the assumption that the
sum of both integrals is constant, the rate of product forma-
tion appears to be exponential, in agreement with first-or-
der kinetics. From a linear fit, as provided in Figure 2, kobs
and t1/2 values of 0.0033 min–1 and 210 min, respectively,
were deduced.
The dicarbene complex formed in situ from Ni(1,5-
cod)2 and two equivalents of Ender’s carbene (1,3,4-tri-
phenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene) was shown
to dehydrogenate H3N·BH3 at 60 °C in 3 h (for addition
of 9% of the Ni catalyst),[30,31] releasing 2.5 equiv. H2 and
yielding polyborazylene (and maybe other products in ad-
dition). Interestingly, this complex turned out to be inactive
in the case of [H2B(µ-hpp)]2. No catalytic activity up to
80 °C was also found for Pd/C, Ni(PPh3)4, Co2(CO)8 and
[(C2H4)Pt(PPh3)2]. In the case of Cp2Ti, catalytic activity
was previously found at 110 °C. However, at 80 °C no dehy-
drogenation product of [H2B(µ-hpp)]2 was observed.
Figure 1. ∆G0 for hydrogenation of [HB(µ-guanidinate)]2 species as
a function of the N–C–N bite angle of the bicyclic guanidinate in
the hydrogenation product [H2B(µ-guanidinate)]2.
Table 1. Results of the catalytic dehydrogenation of [H2B(µ-hpp)]2.[a]
Catalyst (mol-%)
Conditions for quantitative reaction[a]
Product
[Rh(1,5-cod)(µ-Cl)]2 (2%)
Ru3(CO)12 (10%)
Ru3(CO)12 (10%)
Cp2Ti (2%)
Co2(CO)8 (2%)
43 h, 115 °C, toluene
12 h, 60 °C, toluene
7 h, 80 °C, toluene
[HB(µ-hpp)]2
[HB(µ-hpp)]2
[HB(µ-hpp)]2
no reaction
–//–
–//–
–//–
–//–
–//–
–//–
8 h, 80 °C, toluene
6 h, 80 °C, acetonitrile
72 h, 80 °C, toluene
65 h, 80 °C, acetonitrile
96 h, 110 °C, toluene
65 h, 80 °C, acetonitrile
72 h, 80 °C, toluene
Ni(PPh3)4 (2%)
Ni(1,5-cod)2, 2 Enders’ NHC (5%)
Ni(1,5-cod)2, 2 Enders’ NHC (5%)
(C2H4)Pt[P(C6H5)3]2 (10%)
Pd/C (2%)
[a] NMR experiments using 4 mg (0.013 mmol) of [H2B(µ-hpp)]2.
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Doubly Base-Stabilized Diborane(4)
rectly indicating the presence of guanidinate ligands in the
product. Thus the spectroscopic data point to oxidative in-
sertion of sulfur into the B–B bond. Moreover the mass
spectrometric data (EI) show an M+ peak at m/z = 332.2,
consistent with an overall product formula C14H26B2N6S
and insertion of one sulfur atom into the B–B bond. The
molecular structure of this product, [HB(µ-hpp)]2(µ-S), was
derived from X-ray diffraction experiments (see Figure 3).
As expected, the sulfur atom bridges almost symmetrically
the two B atoms [with B–S bond lengths of 188.6(2) and
189.4(2) pm]. The B···B separation amounts to 258.0(1) pm,
and is thus longer than that in [{HB(µ-hpp)}2(µ-H)]+
[222.9(4) pm].[16] As anticipated, the B–S–B bond angle of
86.07(9)° is close to 90° (92.3° in H2S). Interestingly, in
other experiments the analogous compound [HB(µ-hpp)]2-
(µ-O) was crystallized in low yield, being the product of the
reaction of [H2B(µ-hpp)]2 with traces of H2O. In this case
we were unable to isolate more than a few crystals suitable
for X-ray diffraction, and therefore we can present the mo-
lecular structure of this species (see Figure 4), but no other
analytical data. With 108.63(12)° the B–O–B angle is signif-
icantly larger than the B–S–B angle, being close to the H–
O–H bond angle of the H2O molecule in the gas phase
(104.5°).
Figure 2. 1H NMR spectra for the dehydrogenation of [H2B(µ-
hpp)]2 to give the doubly base-stabilized diborane(4) [HB(µ-
hpp)]2, catalysed by Ru3(CO)12 at 80 °C in toluene solutions. Dia-
grams of the kinetic studies. The dashed and solid lines result from
linear fits.
Reaction with Elemental Sulfur
Next we discuss the results of the reaction between
[HB(µ-hpp)]2 and S8. Addition of toluene to a solid mixture
of [HB(µ-hpp)]2 and S8 at room temp. resulted in an inten-
Figure 3. Molecular structure of [HB(µ-hpp)]2(µ-S). Ellipsoids
drawn at the 50% probability level. Hydrogen atoms attached to
sively coloured solution. At the beginning it was purple, carbon atoms were omitted for sake of clarity. Selected structural
parameters (bond lengths in pm, bond angles in deg.): S–B1
188.6(2), S–B2 189.4(2), N1–B1 156.5(2), N2–B2 157.0(2), N4–B1
157.7(2), N5–B2 155.5(2), N1–C1 134.6(2), N2–C1 134.4(2), N3–
C1 135.8(2), N4–C8 134.2(2), N5–C8 133.8(2), N6–C8 136.4(2),
then slowly turned brown at room temp. (see the photo-
graphs of the solution at various stages of the reaction pro-
vided in the Supporting Information), finally the colour
changed to turquoise upon heating of the reaction mixture
to 90 °C. The product precipitated upon cooling and was
re-crystallized from toluene to give colourless crystals. The
11B NMR spectrum of the product (in CD3CN) displayed
a sharp doublet centered at –1.98 ppm indicating the pres-
ence of B–H groups (1JB–H = 114.1 Hz) and tetra-coordi-
nated boron atoms. For comparison, in the case of [H2B(µ-
hpp)]2 (dissolved in toluene), [H2B(µ-tbo)]2 and [H2B(µ-
tbn)]2 (dissolved in benzene), triplets centered at –2.30,
–10.81 and –5.73 (isomer in which each boron atom is at-
tached to a five- and a six-membered ring) as well as –4.32/
–7.41 ppm (isomer in which one boron is attached to two
five-membered rings and the other to two six-membered
rings), respectively, were found in the 11B NMR spec-
tra.[32,20] In the case of [{HB(µ-hpp)}2(µ-H)]+ featuring a
bridging hydride, the 11B NMR shift measures –2.2 ppm.[16]
A strong band in the IR spectrum at 2359 cm–1 can be as-
signed to stretching modes ν(B–H). In addition a strong
B1–S–B2 86.07(9), N1–B1–N4 110.79(14), N2–B2–N5 109.76(15),
N1–C1–N2 118.74(15), N4–C8–N5 119.87(15).
Figure 4. Molecular structure of [HB(µ-hpp)]2(µ-O). Ellipsoids
drawn at the 50% probability level. Hydrogen atoms attached to
carbon atoms were omitted for sake of clarity. Selected structural
parameters (bond lengths in pm, bond angles in deg.): O–B1
142.1(2), O–B2 141.7(2), N1–B1 158.6(2), N2–B2 158.2(2), N4–B1
157.9(2), N5–B2 158.3(2), N1–C1 134.05(19), N2–C1 134.14(19),
N3–C1 136.00(18), N4–C8 133.99(17), N5–C8 134.07(18), N6–C8
136.35(19), B1–O–B2 108.63(12), N1–B1–N4 109.14(11), N2–B2–
band at 1559 cm–1 can be assigned to ν(C=N) modes, di- N5 108.54(12), N1–C1–N2 118.57(12), N4–C8–N5 118.14(13).
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Heterocycles containing boron and sulfur atoms in the for this mixture are shown in Figure 5 (a). The spectrum
ring were studied intensively in the past. These studies recorded after 15 min contains two relatively broad absorp-
mainly concentrated on tri-coordinated boron which can es- tions centered at 504 and 618 nm. After 23 min, an intense
tablish π-interactions.[33] Four representative examples with blue colour was obtained and the band at 618 nm gained
the structural motif B–S–B, for which XRD data are avail- in intensity (see Figure 5, a). The extinction coefficient
able, are provided in Scheme 1. The B–S bond lengths in amounts to 5814 Lmol–1 cm–1 at 50 °C based on the con-
these molecules measure 181.0(10)/178.9(14) pm in 1,[34] centration of S8. For longer reaction times the 618 nm band
179.5(9)/180.6(9) pm in 2,[35] 182.4(2) pm in 3,[36] and decreased again. In another set of experiments we used
186.1(2) pm in 4.[37] As anticipated, these distances are equimolar ratios of [HB(µ-hpp)]2 and S8. The UV/Vis spec-
shorter than the 188.6(2)/189.4(2) pm determined for tra recorded in these experiments are shown in Figure 5 (b).
[HB(µ-hpp)]2(µ-S), for which boron–sulfur π-bonding is not In this case the band at 504 nm is more intense than that
significant (four-coordinate B atoms).
at 618 nm. The intensity of the 504 nm band reaches a
maximum after 20 min and then decreases again. A brown
colour results and the extinction coefficient at 504 nm is
1357 Lmol–1 cm–1 at 50 °C based on the concentration of
S8. The spectra give useful information about the polysul-
fide ions present in the reaction mixture. There can be no
2–
doubt that S8 is responsible for the band at 504 nm, a
species that has been identified also as the first product of
electrochemical reduction of S8 in aprotic solvents.[38,39] Ad-
2–
dition of two more electrons to S8 generates the unstable
2–
S84–, which dissociates to S42–. The two dianions S4 and
2–
S8 are in equilibrium with S62–, which itself dissociates to
–
–
give S3 . S3 is well known to exhibit a characteristic ab-
sorption at 618 nm and is responsible for the blue colour of
electrochemically reduced sulfur in solution.[39,40] It also is
responsible for the blue colour of “Lapis lazuli”. Thus the
two intermediates S82– and S3– are involved in the reduction
Scheme 1. Some known heterocycles containing the B–S–B struc-
tural motif.
Quantum chemical (DFT) calculations were carried out
to obtain thermodynamic information about the reaction
between [HB(µ-hpp)]2 and S8 (see Scheme 2). As antici-
pated, the reaction leading to BII oxidation to BIII and re-
duction of elemental sulfur to S(-II) is significantly exer-
gonic. The ∆E, ∆H0 and ∆G0 values were calculated to be
–138, –132 and –121 kJmol–1, respectively.
Scheme 2. Sulfuration of the B–B bond in [HB(µ-hpp)]2 by elemen-
tal sulfur.
The mechanism of this slow reaction involves several in-
termediates with relatively long life times, resulting from
step-wise degradation of the S8 ring as judged from the dif-
ferent colours observed at various stages of the reaction (see
Supporting Information). In further experiments mixtures
of [HB(µ-hpp)]2 and S8 were dissolved in toluene and the
reaction followed by UV/Vis spectroscopy at 50 °C. In the
first set of experiments one equivalent of [HB(µ-hpp)]2 was
Figure 5. UV/Vis spectra recorded for the reaction between [HB(µ-
hpp)]2 and a) 1/8 equiv. of S8, and b) 1 equiv. of S8 in toluene solu-
treated with 1/8 equiv. of S8. The UV/Vis spectra recorded tions (see experimental details for more information).
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Doubly Base-Stabilized Diborane(4)
of S8 by [HB(µ-hpp)]2. This might imply the presence of the the exact product composition in the solution. Nevertheless,
2+
dication [HB(µ-hpp)]2 as an intermediate, an extremely from the inspection of the NMR spectroscopic data it can
interesting species.
be concluded that ca. 50% sulfuration and ca. 50% substi-
The reductive desulfuration of [HB(µ-hpp)]2(µ-S) was at- tution take place.
tempted by reaction with elemental zinc powder in boiling
The molecular structures of the four new product com-
toluene solutions. However, no reaction was observed. Even pounds are displayed in Figures 7, 8, 9 and 10. With
1
after 58 h in boiling toluene, the H and 11B NMR spectra 172.2(7)/173.2(6), 173.2(3), 173.0(4) and 173.6(4) pm,
gave no sign of decomposition or reaction of boron hydride respectively, the B–B bond lengths in [PhSB(µ-hpp)]2,
[HB(µ-hpp)]2(µ-S), underlining its remarkable stability.
[BnSB(µ-hpp)]2, HB(µ-hpp)2BSPh and HB(µ-hpp)2BSBn
are similar and close to the 177.2(3) pm measured for
[HB(µ-hpp)]2. The B–S bond lengths in the disubstituted
compounds [PhSB(µ-hpp)]2 and [BnSB(µ-hpp)]2 [193.9(5)/
194.6(5) and 194.0(2)/191.5(2) pm] are in average slightly
shorter than in the monosubstituted species HB(µ-hpp)2-
Reactions with Disulfides
Reactions between [HB(µ-hpp)]2 and Ph2S2 or Bn2S2
gave a mixture of three different products, namely the
sulfuration product [HB(µ-hpp)]2(µ-S) and the products of
substitution of one or both hydrides by thiolate (PhS or
BnS). By variation of the conditions for crystallization, all
these products can be isolated. We also were able to obtain
larger quantities of clean product by crystal picking. In this
way, it was possible to collect almost all standard analytic
data for the four species [PhSB(µ-hpp)]2, [BnSB(µ-hpp)]2,
HB(µ-hpp)2BSPh and HB(µ-hpp)2BSBn. However, because
of the presence of a product mixture, the yields cannot be
provided. Moreover, the product composition is very sensi-
tive to small variations of the reaction conditions.
Figure 6 shows a representative 11B{1H} NMR spectrum
of the reaction mixture obtained for reaction between
[HB(µ-hpp)]2 and Bn2S2. For comparison, the 11B{1H}
NMR spectra for clean solutions of [HB(µ-hpp)]2, its sul-
furation product [HB(µ-hpp)]2(µ-S), and the two substitu-
tion products [BnSB(µ-hpp)]2 and HB(µ-hpp)BSBn are also
included. The signals are relatively broad for the BII species,
and sharp for the BIII compound [HB(µ-hpp)]2(µ-S). It can
be seen that not a single product, but a mixture of the three
Figure 7. Molecular structure of [BnSB(µ-hpp)]2. Ellipsoids drawn
at the 50% probability level. Hydrogen atoms attached to carbon
atoms were omitted for sake of clarity. Selected structural param-
eters (bond lengths in pm, bond angles in deg.): B1–B2 173.2(3),
N1–B1 155.8(2), N2–B2 155.1(2), N4–B1 154.5(2), N5–B2 156.9(2),
S1–B1 194.0(2), S2–B2 191.5(2), N1–C1 133.6(2), N2–C1 145.6(2),
N3–C1 134.8(3), N4–C8 133.4(5), N5–C8 145.6(2), N6–C8
135.2(4), S1–C15 182.3(2), S2–C22 182.1(2), C15-C16 150.5(3),
products [HB(µ-hpp)]2(µ-S), [BnSB(µ-hpp)]2 and HB(µ- C22-C23 150.6(3), N1–B1–N4 111.56(14), N2–B2–N5 114.06(14),
B1–B2–S2 127.35(13), B2–B1–S1 126.83(12), N1–C1–N2 115.63(2),
N4–C8–N5 115.73(2), B1–S1–C15 103.36(9), B2–S2–C22
101.93(9), S1–B1–B2–S2 6.80(3).
hpp)BSBn has been formed. Due to the broad signals and
their overlapping, it turned out to be impossible to estimate
Figure 8. Molecular structure of [HB(µ-hpp)2BSBn]. Ellipsoids
drawn at the 50% probability level. Hydrogen atoms attached to
carbon atoms were omitted for sake of clarity. Selected structural
parameters (bond lengths in pm, bond angles in deg.): B1–B2
173.6(4), S–B2 194.1(3), S–C15 182.8(3), N1–B1 156.3(4), N2–B2
155.9(4), N4–B1 156.6(3), N5–B2 155.6(4), N1–C1 133.2(3), N2–
C1 133.9(3), N3–C1 136.2(3), N4–C8 133.7(3), N5–C8 133.4(3),
N6–C8 135.8(3), B1–B2–S 125.02(18), N1–B1–N4 112.9(2), N2–
B2–N5 112.6(2), N1–C1–N2 115.6(2), N4–C8–N5 115.7(2), B2–S–
C15 100.93(12).
Figure 6. 11B{1H} NMR spectra recorded for the reaction between
[HB(µ-hpp)]2 and Bn2S2 after 3,5 h at 90 °C. The 11B{1H} NMR
spectra recorded for b) [HB(µ-hpp)]2, c) [HB(µ-hpp)]2(µ-S), d)
[HB(µ-hpp)2BSBn], and e) [BnSB(µ-hpp)]2 are also shown for com-
parison. All spectra were recorded at 64 MHz in CD2Cl2 except d)
(in [D8]THF) and e) (in C6D6).
Eur. J. Inorg. Chem. 2010, 5201–5210
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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N. Schulenberg, O. Ciobanu, E. Kaifer, H. Wadepohl, H.-J. Himmel
FULL PAPER
out at 0 °C. However, 11B NMR spectra (C7D8, 64 MHz)
taken for reaction between [HB(µ-hpp)]2 and 1 equiv. Bn2S2
at 40 °C in an ultrasonic bath gave some indication for an
intermediate which might indeed be [(BnS)(H)B(µ-hpp)]2
(see the NMR spectra in the Supporting Information).
Hence in these experiments a doublet centered at δ =
4.97 ppm (1JBH = 112.5 Hz) appeared which first grew in
intensity, but decreased again in intensity with time, finally,
for 52 h at 40 °C it almost completely disappeared. [(RS)-
(H)B(µ-hpp)]2 might be responsible for this doublet. In ad-
dition, the broad signal at –1.19 ppm belonging to the start-
ing material disappeared, and a doublet at –0.81 ppm (1JBH
= 114.5 Hz) due to [HB(µ-hpp)]2(µ-S) increased in intensity.
Quantum chemical calculations were carried out to shed
light on the thermodynamic properties for the pathways
leading to the products HB(µ-hpp)2BSPh, [(PhS)B(µ-hpp)]2
and [HB(µ-hpp)]2(µ-S) via [(PhS)(H)B(µ-hpp)]2 as common
intermediate (see Figure 11). The formation of [(PhS)(H)-
B(µ-hpp)]2 from [HB(µ-hpp)]2 and Ph2S2 (∆G0
=
–50 kJmol–1), as well as the following elimination processes
leading to the observed products are all exergonic reactions.
The elimination of Ph2S to give [HB(µ-hpp)]2(µ–S) is the
most exergonic process, in line with the preferred formation
of this product in the experiments. Interestingly, dehydroge-
nation of [(PhS)(H)B(µ-hpp)]2 (∆G0 = –65 kJmol–1) is sig-
nificantly more exergonic than dehydrogenation of [H2B(µ-
hpp)]2 (∆G0 = –30 kJmol–1). The fairly large number of
atoms in the involved species causes long computational
times if transition states should be calculated. In prelimi-
nary calculations using H2S2 in place for Ph2S2, we ob-
tained a ∆G0 value of –78 kJmol–1 for oxidative addition to
give the intermediate species [(HS)(H)B(µ-hpp)]2 (see Sup-
porting Information). The reason for the significant dis-
crepancy between this value and the value obtained for
Ph2S2 (–50 kJmol–1) arises (at least partially) from the pres-
ence of S–H···S hydrogen bonding in [(HS)(H)B(µ-hpp)]2
(see Supporting Information). Reduction of the computa-
tional time for transition state calculations simply by re-
placement of the substituents by hydrogen atoms is there-
fore difficult. We have therefore set aside further calcula-
Figure 9. Molecular structure of [PhSB(µ-hpp)]2. Ellipsoids drawn
at the 50% probability level. Hydrogen atoms attached to carbon
atoms were omitted for sake of clarity. Selected structural param-
eters (bond lengths in pm, bond angles in deg.): B1–B2 172.2(7),
B3–B4 173.2(6), S1–B1 193.9(5), S2–B2 194.6(5), S3–B3 194.9(5),
S4–B4 196.2(4), N1–B1 155.1(6), N2–B2 156.1(6), N4–B1 154.7(5),
N5–B2 154.4(5), N1–C1 133.3(5), N2–C1 134.3(5), N3–C1
135.1(5), N4–C8 134.2(5), N5–C8 134.6(5), N6–C8 134.8(5), S1–
C20 177.5(2), S2–C26 179.7(2), N1–B1–N4 113.9(3), N2–B2–N5
112.3(3), B2–B1–S1 127.6(3), B1–B2–S2 126.8(3), N1–C1–N2
116.0(4), N4–C8–N5 115.0(3), S1–B1–B2–S2 8.141(23).
Figure 10. Molecular structure of [HB(µ-hpp)2BSPh]. Ellipsoids
drawn at the 50% probability level. Hydrogen atoms attached to
carbon atoms omitted for sake of clarity. Selected structural param-
eters (bond lengths in pm, bond angles in deg.): B1–B2 173.0(4),
S–B2 195.9(3), N1–B1 157.6(3), N2–B2 154.8(3), N4–B1 156.2(3),
N5–B2 156.1(3), N1–C1 133.9(3), N2–C1 134.6(3), N3–C1
135.3(3), N4–C8 133.8(3), N5–C8 135.1(3), N6–C8 136.0(3), S–C15
176.8(2), B1–B2–S 128.97(16), N1–B1–N4 111.28(18), N2–B2–N5
112.86(18), N1–C1–N2 115.09(19), N4–C8–N5 115.67(18), B2–S–
C15 103.30(11).
BSPh and HB(µ-hpp)2SBn [195.9(3) and 194.1(3) pm]. The
B–B–S bond angles are very similar, all within the region
126–129°.
The reaction of [HB(µ-hpp)]2 with R2S2 (R = Ph or Bn)
presumably leads first to the product of oxidative addition,
[(RS)(H)B(µ-hpp)]2. Unfortunately it was not possible to
Figure 11. ∆G0 values for the reaction between [HB(µ-hpp)]2 and
Ph2S2, leading to the three observed products [HB(µ-hpp)]2(µ-S),
[HB(µ-hpp)BSPh] and [PhSB(µ-hpp)]2 via [(PhS)(H)B(µ-hpp)]2 as
isolate this intermediate, even when the reaction was carried common intermediate.
5206
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 5201–5210

Doubly Base-Stabilized Diborane(4)
tions. The experiments indicate that the barriers for all three
elimination reactions starting from the intermediate are in
the same order.
Experimental Details
General Procedures: All reactions were carried out under a dry ar-
gon atmosphere using standard Schlenk techniques. Solvents were
dried with the appropriate drying agents, distilled and preserved
over molecular sieves (4 Å) prior to use. The bicyclic guanidine
hppH, dibenzyl disulfide and diphenyl disulfide were purchased
from Aldrich. Sulfur was purchased from Gruessing. Infrared spec-
tra were recorded using a BIORAD Excalibur FTS 3000. NMR
spectra were recorded on a Bruker Avance II 400 or on a Bruker
Avance DPX AC200. Elemental analyses were carried out at the
Microanalytical Laboratory of the University of Heidelberg. EI
mass spectra were obtained on a Finnigan MAT 8230 or on a
JEOL JMS-700 instrument.
Conclusions
The chemistry of the doubly base-stabilized diborane(4)
species [HB(µ-hpp)]2 (hpp = 1,3,4,6,7,8-hexahydro-2H-pyri-
mido[1,2-a]pyrimidinate) is described. This boron hydride
can be synthesized by catalytic dehydrogenation of the bi-
nuclear BIII hydride [H2B(µ-hpp)]2. The conditions and the
catalyst for this reaction were optimized. Quantum chemi-
cal calculations analyse the thermodynamics for hydrogena-
tion of [HB(µ-hpp)]2 and analogous species with other
bridging, bicyclic guanidinate substituents. These calcula-
tions support the assumption of an almost linear relation-
ship between the standard Gibbs free energy for hydrogena-
tion and the N–C–N bite angle of the guanidinate in the
hydrogenated product. The main aim of future work will be
[HB(µ-hpp)]2:
A Schlenk flask, charged with hppH (2.51 g,
18.03 mmol), trimethylamine borane (1.32 g, 18.03 mmol), [Rh(1,5-
cod)(µ-Cl)]2 (18 mg, 0.03 mmol) and toluene (75 mL) was heated
to reflux for 65 h. The brown solution was filtered, concentrated
and stored for several days at 3 °C to afford colourless crystals
of the clean product in 61% yield (1.65 g, 5.51 mmol). 1H NMR
(200 MHz, C6D6): δ = 3.57–3.32 (m, 8 H, CH2-N), 2.51–1.28 (m,
the optimization of the conditions for (catalytic) hydrogena- 8 H, CH2-N), 1.56–1.18 (m, 8 H, CH2) ppm. 11B NMR (128 MHz,
tion of [HB(µ-guanidinate)]2. They will concentrate on the
6–6 and 5–6 bicyclic guanidinates hpp and tbn as bridging
substituents.
C6D6): δ = –1.14 (br. s, BH) ppm. See ref. 16 for more analytical
data.
[HB(µ-hpp)]2(µ-S): A Schlenk flask was charged with [HB(µ-
hpp)]2 (150 mg, 0.50 mmol) and sulfur (16 mg, 0.06 mmol). After
addition of toluene (16 mL), the colour of the solution turned from
purple to dark brown. By heating the solution to 90 °C, the colour
of the solution turned into very dark turquoise, which lightens after
a while. After heating for 16 h, the hot turquoise solution was fil-
tered. By cooling to room temp., the product precipitated as a
colourless solid. It was filtered and washed with n-hexane
(1ϫ2 mL). After drying in vacuo the product was achieved in 82%
yield (136 mg, 0.41 mmol). Crystals suitable for x-ray diffraction
were grown by recrystallisation from toluene solutions.
C14H26B2N6S (332.08): calcd. C 50.63, H 7.89, B 6.51, N 25.31, S
9.66; found C 50.24, H 7.94, N 25.03. 1H NMR (600 MHz,
CD3CN): δ = 3.32–3.09 (m, 16 H, 2-H, 4-H), 1.82–1.77 (m, 8 H,
3-H) ppm. 13C NMR (150 MHz, CD3CN): δ = 152.8 (2 C, C-1),
49.0 (4 C, C-4/2), 48.1 (4 C, C-2/4), 23.7 (4 C, C-3) ppm. 11B NMR
The oxidative sulfuration of [HB(µ-hpp)]2 with S8 lead-
ing to [HB(µ-hpp)]2(µ-S) is an example for oxidative inser-
tion into the B–B bond. The reaction with the disulfides
Ph2S2 and Bn2S2 leads to [HB(µ-hpp)]2(µ-S) as well as the
substitution products [HB(µ-hpp)2SR] and [RS(µ-hpp)]2 (R
= Ph or Bn). All these products were structurally charac-
terized. The experimental results together with quantum
chemical calculations indicate that oxidative addition of the
disulfide first leads to [(PhS)(H)B(µ-hpp)]2, which acts as
common intermediate on the way to all three products. We
will extent our work to other disulfides, one of the aims
being the synthesis of species of the general formula
[RSB(µ-hpp)]2 by avoiding the sulfuration pathway.
These species possibly find applications as oxidation-
labile, chelating sulfur ligands. Scheme 3 summarizes the so
far studied reactions of the doubly base-stabilized dibo-
rane(4) species [HB(µ-hpp)]2.
1
(128 MHz, CD3CN): δ = –1.98 [d, JBH = 114.1 Hz, 2 B, B(H)S]
ppm. MS (EI): m/z = 332.2 [C H B N S]+. IR (KBr): ν = 2953
˜
14 26
2
6
(m) (C–H val.), 2938 (m) (C–H val.), 2855 (s) (C–H val.), 2359 (s)
(B–H val.), 1559 (s) (C=N val.), 1450 (m), 1397 (w), 1368 (w),
1312 (s), 1218 (s), 1173 (m), 1092 (s), 1048 (w) cm–1. Crystal
data for [HB(µ-hpp)]2(µ-S): C14H26B2N6S, Mr
=
332.09,
0.25ϫ0.20ϫ0.20 mm3, monoclinic, space group P21/n,
a
=
13.183(3), b = 8.076(2), c = 16.637(3) Å, β = 112.97(3)°. V =
1630.80(6) Å3, Z = 4, dcalc = 1.353 Mgm–3, Mo-Kα radiation
(graphite-monochromated, λ = 0.71073 Å), T = 100 K, θrange 1.69
to 30.02°, reflections measd. 9345, indep. 4753, Rint = 0.0206, final
R indices [IϾ2σ(I)]: R1 = 0.0540, wR2 = 0.1518.
[PhSB(µ-hpp)]2: A Schlenk flask containing [HB(µ-hpp)]2 (150 mg,
0.5 mmol), diphenyl disulfide (109 mg, 0.5 mmol) and toluene
(10 mL) was heated to reflux for 3 h. The solvent was removed
and the yellow residue dissolved in dichloromethane (7 mL). The
solution was overlayered with n-hexane and kept at 3 °C for 15 h.
After this, the solvents were removed in vacuo again and the light-
yellow residue was washed with tetrahydrofuran (2ϫ7 mL). The
white solid was dissolved in boiling acetonitrile and filtered hot.
By cooling the filtrate to room temp. colourless crystals of
[PhSB(µ-hpp)]2 appeared. Unfortunately it was not possible to ob-
tain enough crystalline material for an EA analysis. 1H NMR
Scheme 3. Reactivity of [HB(µ-hpp)]2 (hpp is symbolized by a
curved line connecting the two B atoms).
Eur. J. Inorg. Chem. 2010, 5201–5210
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5207

N. Schulenberg, O. Ciobanu, E. Kaifer, H. Wadepohl, H.-J. Himmel
(400 MHz, CD3CN): δ = 7.33 (dd, JHH = 8.3, JHH = 1.1 Hz, 4 H), 4.57 (s, 4 H, 15-H), 3.53–3.47 (m, 4 H, CH2-N), 2.99–2.93 (m,
FULL PAPER
3
4
3
4
H, 16-H), 6.98 (dt, JHH = 8.0, JHH = 1.6 Hz, 4 H, 17-H), 6.85
4 H, CH2-N), 2.30–2.17 (m, 8 H, CH2-N), 1.25–1.13 (m, 8 H, 3-
H) ppm. 13C NMR (150 MHz,C6D6): δ = 155.8 (2 C, C-1), 146.4
(2 C, C-16), 130.0 (4 C, C-17), 128.3 (4 C, C-18), 125.1 (2 C, C-
3
4
(tt, JHH = 7.4, JHH = 1.2 Hz, 2 H, 18-H), 3.41–2.96 (m, 16 H, 2-
H, 4-H), 1.76–1.46 (m, 8 H, 3-H) ppm. 13C NMR (100 MHz,
CD3CN): δ = 179.4 (2 C, C-1), 143.8 (2 C, C-15), 132.1 (4 C, 19), 47.0 (4 C, CH2-N), 40.2 (4 C, CH2-N), 32.1 (2 C, C-15), 22.2
C-16), 128.0 (4 C, C-17), 122.7 (2 C, C-18), 47.9 (4 C, C-4/2),
40.9 (4 C, C-2/4), 22.9 (4 C, C-3) ppm. 11B NMR (128 MHz,
CD3CN): δ = 2.38 (br. s, 2 B, BS) ppm. MS (EI): m/z = 516.3
[C26H34B2N6S2]+, 407.3 [C20H29B2N6S]+. HR-MS (EI): m/z =
516.2471 (27), 515.2485 (14), 407.2346 (100), 406.2383 (48). Crystal
(4 C, C-3) ppm. 11B NMR (128 MHz, C6D6): δ = 1.94 (br. s, 2 B,
B–S) ppm. MS (EI): m/z
=
544.2 [C28H38B2N6S2]+, 453.2
[C21H31B2N6S2]+. HR-MS (EI): m/z (%) = 544.2766 (27), 543.2814
(12), 453.2233 (100), 452.2260 (51). IR (KBr): ν = 3055 (w) (C–
˜
Harom val.), 3023 (w) (C–Harom val.), 2934 (m) (C–Haliph val.), 2847
data for [PhSB(µ-hpp)]2: C26H34B2N6S2, Mr
0.30ϫ0.30ϫ0.20 mm3, monoclinic, space group P21/c,
15.867(3), b = 21.383(4), c = 15.766(3) Å, β = 103.86(3)°. V =
=
516.34,
(m) (C–Haliph val.), 1568 (s) (C=N val.), 1488 (w), 1456 (m), 1395
(w), 1366 (w), 1317 (m), 1275 (m), 1219 (m), 1178 (w), 1096 (w),
1041 (m) cm–1. Crystal data for [BnSB(µ-hpp)]2: C28H38B2N6S2,
a
=
5193.4 Å3, Z = 8, dcalc = 1.321 Mgm–3, Mo-Kα radiation (graphite- Mr = 544.39, 0.16ϫ0.15ϫ0.10 mm3, monoclinic, space group P21/
monochromated, λ = 0.71073 Å), T = 100 K, θrange 1.36 to 27.93°,
reflections measd. 40500, indep. 12198, Rint = 0.1158, final R in-
dices [IϾ2σ(I)]: R1 = 0.0784, wR2 = 0.1573.
c, a = 17.676(9), b = 9.724(5), c = 16.854(10) Å, β = 106.437(8)°.
V = 2778(3) Å3, Z = 4, dcalc = 1.301 Mgm–3, Mo-Kα radiation
(graphite-monochromated, λ = 0.71073 Å), T = 100 K, θrange 2.40
to 31.00°, reflections measd. 69297, indep. 8848, Rint = 0.0730, final
R indices [IϾ2σ(I)]: R1 = 0.0540, wR2 = 0.1275.
HB(µ-hpp)2BSPh:
A Schlenk flask containing [HB(µ-hpp)]2
(300 mg, 1.00 mmol), diphenyl disulfide (218 mg, 1.00 mmol) and
toluene (20 mL) was heated to reflux for 3 h. By cooling to room
temp., a white solid precipitated. The solution was filtered and the
solid material was dissolved in boiling acetonitrile. Undissolved
particles were filtered and by cooling the filtrate to room temp.,
colourless crystals of [HB(µ-hpp)2BSPh] were obtained. The prod-
uct is stable in dichloromethane only for a few hours. The yield
was not determined. C20H30B2N6S (408.18): calcd. C 58.85, H 7.41,
HB(µ-hpp)2BSBn: A Schlenk flask charged with [HB(µ-hpp)]2
(105 mg, 0.35 mmol), dibenzyl disulfide (86 mg, 0.35 mmol) and
toluene (10 mL) was heated to 90 °C for 3 h. After cooling to room
temp., solid materials were filtered and the filtered solution was
reduced to approx. a half of its volume. By addition of n-hexane
(10 mL), a white solid precipitated. It was filtered again and crys-
tals as colourless plates could be grown from the filtered solution
by keeping it at room temp. for several days. The yield was not
determined. C21H32B2N6S (422.21): calcd. C 59.74, H 7.64, B 5.12,
N 19.91, S 7.59; found C 59.71, H 7.53, N 19.79. 1H NMR
(400 MHz, [D8]THF): δ = 7.18 (d, 3JHH = 7.5 Hz, 2 H, 17-H), 7.04
1
B 5.30, N 20.59, S 7.86; found C 59.07, H 7.41, N 19.97. H NMR
(400 MHz, CD2Cl2): δ = 7.18 (dd, 3JHH = 8.1, 4JHH = 1.0 Hz, 2 H,
3
3
16-H), 7.00 (t, JHH = 7.8 Hz, 2 H, 17-H), 6.84 (t, JHH = 7.4 Hz,
1 H, 18-H), 3.45–2.96 (m, 16 H, CH2-N), 1.95–1.59 (m, 8 H,C-
CH2-C) ppm. 13C NMR (100 MHz,CD2Cl2): δ = 156.3 (2 C, C-1),
143.6 (1 C, C-15), 130.1 (2 C, C-16), 126.9 (2 C, C-17), 121.1 (1 C,
C-18), 47.0 (2 C, CH2-N), 46.8 (2 C, CH2-N), 44.4 (2 C, CH2-N),
40.1 (2 C, CH2-N), 22.3 (4 C, C-3, C-6, C-10, C-13) ppm. 11B NMR
(128 MHz, CD2Cl2): δ = 2.38 (br. s, 1 B, B–S), –2.38 (br. s, 1B, B-
3
3
(t, JHH = 7.4 Hz, 2 H, 18-H), 6.93 (t, JHH = 6.9 Hz, 1 H, 19-H),
3.52 (s, 2 H, 15-H), 3.37–2.83 (m, 16 H, CH2-N), 1.95–1.37 (m, 8
H, 3-H, 6-H, 10-H, 13-H) ppm. 13C NMR (100 MHz, [D8]THF):
δ = 156.9 (2 C, C-1), 147.8 (1 C, C-16), 130.1 (2 C, C-17), 128.0 (2
C, C-18), 125.1 (1 C, C-19), 48.2 (2 C, CH2-N), 48.0 (2 C, CH2-
N), 46.0 (2 C, CH2-N), 41.5 (2 C, CH2-N), 35.5 (1 C, C-15), 23.9
(2 C, C-CH2-C), 23.5 (2 C, C-CH2-C) ppm. 11B NMR (128 MHz,
[D8]THF): δ = 3.63 (br. s, 1 B, B–S), –3.07 (br. s, 1B, B-H) ppm.
H) ppm. MS (EI): m/z = 407.3 [C H B N S]+. IR (KBr): ν =
˜
20 29
2
6
3044 (w) (C–Harom val.), 2956 (m) (C–Haliph val.), 2926 (m) (C–
Haliph val.), 2845 (s) (C–Haliph val.) 2283 (s) (B–H val.), 1578 (s)
(C=N val.), 1461 (m), 1397 (w), 1368 (w), 1313 (s), 1266 (s), 1261
(m), 1178 (w), 1092 (s), 1025 (s) cm–1. Crystal data for [HB(µ-
hpp)2BSPh]: C20H30B2N6S, Mr = 408.18, 0.35ϫ0.30ϫ0.30 mm3,
monoclinic, space group P2(1), a = 8.133(2), b = 15.218(3), c =
MS (EI): m/z = 422.3 [C H B N S]+. IR (KBr): ν = 3050 (w) (C–
˜
21 32
2
6
Harom val.), 3027 (w) (C–Harom val.), 2943 (m) (C–Haliph val.), 2838
(s) (C–Haliph val.), 2747 (w) (C–Haliph val.), 2679 (w) (C–Haliph val.),
2266 (m) (B–H val.), 1572 (s) (C=N val.), 1498 (w), 1452 (m), 1396
(w), 1363 (w), 1307 (s), 1274 (m), 1219 (s), 1178 (m), 1094 (m),
1045 (s) cm–1. Crystal data for [HB(µ-hpp)2BSBn]: C21H32B2N6S,
Mr = 422.21, 0.30ϫ0.25ϫ0.18 mm3, monoclinic, space group
P2(1), a = 16.024(3), b = 9.6990(19), c = 15.467(3) Å, β =
115.40(3)°. V = 2171.5(7) Å3, Z = 4, dcalc = 1.291 Mgm–3, Mo-Kα
radiation (graphite-monochromated, λ = 0.71073 Å), T = 100 K,
8.668(2) Å, β = 106.38(3)°. V = 1029.30 Å3, Z = 2, dcalc
=
1.317 Mgm–3, Mo-Kα radiation (graphite-monochromated, λ =
0.71073 Å), T = 100 K, θrange 2.45 to 27.46°, reflections measd.
4736, indep. 4717, Rint = 0.0000, final R indices [IϾ2σ(I)]: R1
0.0447, wR2 = 0.0837.
=
[BnSB(µ-hpp)]2: A Schlenk flask containing [HB(µ-hpp)]2 (105 mg,
0.35 mmol), dibenzyl disulfide (173 mg, 0.70 mmol) and toluene
(10 mL) was heated to reflux for 3.5 h. The solvent was removed,
the oily residue co-evaporated with toluene (2ϫ2 mL) and dried in
vacuo for several hours to remove benzylthiole. The yellow oil was
redissolved in toluene (6 mL) and overlayered with n-hexane (4 mL)
to precipitate the by-products. After storage for 17 h at 3 °C a
colourless non-crystalline solid appeared which was filtered. The
filtrate was again overlayered with n-hexane. This procedure was
repeated one more time. By storage at room temp. large yellow
crystals of [BnSB(µ-hpp)]2, suitable for x-ray diffraction were ob-
tained. However, only small amounts can be isolated (see text).
C28H38B2N6S2 (544.39): calcd. C 61.78, H 7.04, B 3.97, N 15.44, S
11.78; found C 62.11, H 6.86, N 15.85. 1H NMR (400 MHz, C6D6):
θrange 1.41 to 27.48°, reflections measd. 15796, indep. 4933, Rint
0.0741, final R indices [IϾ2σ(I)]: R1 = 0.0601, wR2 = 0.1435.
=
UV/Vis Studies: Solutions of sulfur (0.18 mg, 0.75 µmol or 1.54 mg,
6.00 µmol) and [HB(µ-hpp)]2 (1.80 mg, 6.00 µmol) in toluene
(2.5 mL) were prepared separately and mixed immediately before
starting the measurement. Then 3 mL of the solution were filled in
a 1 cm quarz cell. The time measurements was started upon reach-
ing the target temperature of 50 °C (ca 3 min after mixing). Mea-
surements were carried out on a Varian Cary 5000 UV/Vis/NIR
spectrophotometer equipped with a Specac 4000 High Stability
Temperature Controller. The spectra were automatically baseline-
corrected with the aid of the spectrum recorded for pure toluene.
3
4
3
δ = 7.84 (dd, JHH = 8.2, JHH = 1.3 Hz, 4 H, 17-H), 7.15 (t, JHH
X-ray Crystallographic Study: Suitable crystals were taken directly
out of the mother liquor, immersed in perfluorinated polyether oil,
= 7.5 Hz, 4 H, 18-H), 7.04 (tt, 3JHH = 7.3, JHH = 1.4 Hz, 2 H, 19-
4
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Doubly Base-Stabilized Diborane(4)
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Received: May 28, 2010
Published Online: August 24, 2010
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Eur. J. Inorg. Chem. 2010, 5201–5210