Cyclolinear Oligo- and Poly(iminoborane)s: The Missing Link in Inorganic Main-Group Macromolecular Chemistry

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Title: Cyclolinear Oligo- and Poly(iminoborane)s: The Missing Link in

Inorganic Main Group Macromolecular Chemistry

Authors: Ozan Ayhan, Nicolas Alexander Riensch, Clemens

Glasmacher, and Holger Helten

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Cyclolinear Oligo- and Poly(iminoborane)s: The Missing Link in

Inorganic Main Group Macromolecular Chemistry

Ozan Ayhan, Nicolas A. Riensch, Clemens Glasmacher, and Holger Helten*

Dedicated to Prof. Dr. Jun Okuda on the occasion of his 60th birthday

Abstract: The reaction of n-C₈H₁₇B[N(Me)SiMe₃]₂(1) with n-

C₈H₁₇BCl₂(2a) yielded, instead of a linear poly(iminoborane), the

aminoborane n-C₈H₁₇B(Cl)N(Me)SiMe₃(4) and after cyclotrimeriza-

tion the borazine cyclo-(n-C₈H₁₇BNMe)₆(6). Side reactions that result

in borazine formation were effectively suppressed when 1,3-bis(trime-

thylsilyl)-1,3,2-diazaborolidines 7 were employed as the co-mono-

mers in combination with dichloro- or dibromoboranes 2, 8. Si/B ex-

change polycondensation led to oligo(iminoborane)s 11a,b,ac,d. Al-

ternative synthetic routes to such species involve Sn/B exchange of

1,3-bis(trimethylstannyl)-2-n-octyl-1,3,2-diazaborolidine (16) and n-

C₈H₁₇BBr₂(8a), and the initiated polycondensation of the dormant

imparts unique characteristics to these materials, thus enabling

their use, for example, for (opto)electronic, sensory, and imaging

applications.^[5,6,7]Surprisingly, well-characterized truly inorganic

boron-containing linear polymers became accessible not until

2008 through work by Manners and co-workers with their report

of the first poly(aminoborane)s (PABs, IV).^[8]These species may

be regarded as inorganic polyolefin analogues. Since then, poly-

(aminoborane)s have become a quickly evolving field of re-

search.^[9]PABs can be used as polymeric precursors to shaped

boron nitride,^[8,10]piezoelectric PAB derivatives can possibly be

prepared,^[11a]and, like boron-rich compounds in general, PABs

monomer 14 in the presence of a Brønsted acid (HCl, HOTf, or HNTf₂). are of potential interest for the use as boron delivery agents in

While the attempt to obtain an oligo/poly(iminoborane) with phenyl

side groups yielded only insoluble material, the incorporation of aryl

groups was proven for a derivative having both phenyl and n-octyl B-

substituents (11ac) as well as for a derivative with 4-n-butylphenyl

side groups (11d). The highest-molecular-weight sample was ob-

tained of 11ac. Featuring about 18 catenated BN units on average,

this is the closest approach to a poly(iminoborane) to date.

boron neutron capture therapy (BNCT) of cancer.^[11b]

H H

B

R

Si

R

R R

Si

R R

P

N

O

N

n

R H

n

I

II

III

IV

H

N

Mes

B

Tip

B

Introduction

N

H

n

N

V

H

Polymers with a backbone composed of inorganic main group ele-

ments are of considerable interest as they often exhibit useful

properties and functions that complement those of classical or-

ganic macromolecular materials.^[1]Probably the most prominent

examples of such “inorganic polymers” are the well-known poly-

siloxanes (silicones, I, Figure 1), which are noted for their excep-

tional low-temperature flexibility and thermo-oxidative stability.^[2]

The particular value of the related polyphosphazenes (II) has

been recognized as well.^[3]Thermal and mechanical properties of

these macromolecules are effectively tuned by variation of their

side groups. Polysilanes (III) have aroused significant research

interest owing to the discovery of σ conjugation along the polymer

backbone,^[4]and they have been used as polymeric precursors to

shaped SiC ceramics and as photoresists for microlithographic

applications.

B

Tip

N

VI

HB

N

BH

n

H

N

R

B

H

B

N

H

HN

N

R'

B

N

B

N

BH

NH

VIII

n

HB

HN

B

VII

NH

B

Figure 1. Polysiloxanes (I), polyphosphazenes (II), polysilanes (III), poly(amino-

borane)s (IV), poly(p-phenylene diimidoborane) (V; Mes = mesityl), and poly(p-

phenylene iminoborane) (VI; Tip = 2,4,6-triisopropylphenyl), poly(iminoborane)s

(VII; R, R’ = aryl, alkyl, or H), polyborazylene (VIII; approximate structure).

The element boron has proved to be a valuable component in π

conjugated inorganic–organic hybrid polymers. The boron atom

BN and CC units are isoelectronic and isosteric, and the incor-

poration of BN fragments at specific positions in organic com-

pounds has been successfully exploited for quite some time to

produce novel hybrid materials with structural similarities to their

all-carbon congeners but in some cases fundamentally altered

electronic features.^[5j,8,9,12]In terms of unsaturated B=N moieties,

the applicability of this concept to polymer chemistry has just been

demonstrated.^[5j,13-15]We recently reported the B=N-containing in-

organic–organic hybrid polymers V^[14]and VI.^[15]The latter can be

regarded as BN analogue of poly(p-phenylene vinylene) (PPV).

[*]

O. Ayhan, N. A. Riensch, C. Glasmacher, Dr. H. Helten

Institute of Inorganic Chemistry, RWTH Aachen University

Landoltweg 1, 52056 Aachen (Germany)

E-mail: holger.helten@ac.rwth-aachen.de

http://www.ac.rwth-aachen.de/extern/helten/index.html

Supporting information for this article is given via a link at the end of

the document.

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For the synthesis of these materials we developed a novel Si/B Results and Discussion

exchange polycondensation approach which proceeds via facile

spontaneous B–N bond formation with cleavage of volatile

Me₃SiCl or Me₃SiBr as the condensation by-product, respectively.

The parent unsaturated B=N polymer, poly(iminoborane) (PIB,

VII), however, has not been synthesized and characterized by

means of current state-of-the-art analysis methods so far, al-

though it has been the subject of several theoretical studies.^[16]

Formally, VII is isoelectronic to polyacetylene, however, theory

predicts rather insulating properties for this inorganic polymer.^[16b]

In several papers, in particular those which discuss the solid-state

pyrolysis of ammonia–borane (NH₃·BH₃, AB), the term “polyimi-

noborane” or the formula [BHNH]_nappears,^[17]though it is not al-

ways clear if a linear polymer is meant. During the dehydrogena-

tion process of AB an insoluble solid is formed as an intermediate

that has the approximate composition BNH₂. Recent solid-state

NMR studies,^[18]however, revealed that the constitution of this

material should not be described as a linear poly(iminoborane)

but rather as a polyborazylene (VIII), that is in this case a poorly

defined network of partially fused borazine rings.^[19,20]

In 1962, Burch, Gerrard, and Mooney reported on the thermol-

ysis of di(alkylamino)phenylboranes, (RNH)₂BPh, which gave, de-

pending on the substituent R on nitrogen, either borazine deriva-

tives, (NRBPh)₃, a mixture of borazine and non-cyclic products, or

the non-cyclic products only.^[21]The interpretation of the latter as

linear macromolecules was based on IR spectroscopy and visco-

simetry. In the 1980s, Paetzold and co-workers reported on the

generation of monomeric iminoboranes and subsequent trapping

thereof at low temperature.^[22,23]In some cases, besides the re-

spective iminoborane trimers (i.e., borazines), insoluble waxy ma-

terials were formed and isolated for which the constitution of linear

poly(iminoborane)s (VII, R = R’ = alkyl) was proposed. This

assignment was based on elemental analysis and mass spectro-

metry data and on the observation that one derivative thermally

transformed into the corresponding borazine.^[22a]

Overall, the intended synthesis of linear poly(iminoborane)s

from rational precursors often seems to be hampered by the facile

formation of the respective borazines, which are formally inorgan-

ic benzene analogues. Recently, we briefly communicated the

synthesis and characterization of a well-defined, processable

oligo(iminoborane) comprising a linear chain of 12–14 BN units

on average.^[24]With our synthetic approach, that is, linking the ad-

jacent nitrogen centers of the main chain pairwise via an ethylene

bridge, unwanted side reactions that result in the formation of a

borazine were effectively prevented. Herein, we report full details

of these studies, and we present novel methods for the synthesis

of oligo(iminoborane)s, one of which involves previous isolation of

a dormant form of the monomer which is activated for polymeri-

zation by a Brønsted acid. Furthermore, we prepared a series of

new oligo/poly(iminoborane) derivatives, by which it is demon-

strated that the physical properties of these materials can be ef-

fectively tuned by variation of their side chains. In this study, the

highest-molecular-weight derivative to date was isolated, which

features about 18 catenated BN units on average.

Attempted synthesis of a fully linear poly(iminoborane). In an

attempt to prepare an essentially linear poly(iminoborane) we per-

formed the reaction of diaminoborane 1 with dichloro-n-octylbo-

rane (2a) (Scheme 1). Based on our previous experience with N-

silylamines combined with chloro- or bromoboranes,^{[5j,13e,14,15]}we

anticipated that 1 and 2a should undergo spontaneous Si/B ex-

change in solution with elimination of Me₃SiCl and formation of a

new B–N bond to give the linear diborazene 3 in the first step. As

this species comprises a linear chain of four alternating B and N

atoms, unwanted follow-up reactions to yield small cyclic species

should be less likely. In particular, the formation of the six-mem-

bered ring compound borazine 6 should be impossible – as long

as redistribution processes are absent.^[25]We anticipated that 3

should then further oligomerize in a linear fashion to afford the

desired poly(iminoborane). The n-octyl groups (herein denoted as

Oct) at boron were chosen in order to impart good solubility to the

product in organic solvents. However, when we mixed 1 with 2a

in CH₂Cl₂, instead of the expected condensation, a ligand ex-

change reaction occurred.^[26]

Oct

B

Oct Oct

CH₂Cl₂

Me₃Si

SiMe₃

Me₃Si

B

+ OctBCl₂

N

Cl

– Me₃SiCl

Me Me

1

2a

3

CH₂Cl₂

Oct

Me

Oct

N

Oct

B

N

B

N

Me₃Si

B

2/3

2

N

Cl – 2 Me₃SiCl

Me

B

Me

4

Oct

6

CH₂Cl₂

SiMe₃

– 2 Me₃SiCl

Me₃Si

2

N

+ 2 OctBCl₂

Me

5

2a

Scheme 1. Reaction between diaminoborane 1 and dichloro-n-octylborane (2a)

with formation of 4 and subsequent cyclotrimerization of the latter to give 6 (Oct

= n-C₈H₁₇), and independent synthesis of 4.

The formation of the aminoborane^[27]4 as a mixture of two dia-

stereomers that differ from one another by the relative configura-

tion at the partial B=N double bond (i.e., E/Z isomers) was evi-

denced by ¹H and ¹¹B NMR spectroscopy from the reaction solu-

tion. The ¹¹B{¹H} NMR spectrum showed a signal at δ_B= 43.8 ppm

which is reasonably assigned to 4. For comparison, the related

aminoborane derivative Cl(tBu)B=N(iPr)SiMe₃resonates at δ_B=

1

44.7 ppm.^[28]The H NMR spectrum displayed two sets of reso-

nances for the N–CH₃and the N–Si(CH₃)₃protons corresponding

to the E/Z isomers of 4. To further unequivocally confirm the for-

mation of 4 in this reaction, we prepared it by an independent syn-

thesis via the condensation of 2a with heptamethyldisilazane (5)

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(a)

Me₃Si

(Scheme 1). This gave identical NMR data. Subsequently, 4 un-

derwent further spontaneous condensation with cyclotrimerization

to quantitatively yield the borazine 6^[29]within 16 h at room temper-

ature.

Oct

B

H

N

+

OctB(NMe₂)₂

13

toluene, reflux

12

Oct Oct

Polycondensation of cyclolinear building blocks via Si/B ex-

change. As the approach described above was not effective in

preventing the formation of the borazine 6, we chose a different

strategy to achieve linear BN catenation: We introduced a short

hydrocarbon bridge, that is, an ethylene group, between the nitro-

gen atoms of the monomer, i.e. 7a (Scheme 2). Upon polycon-

densation with 2a or 8a, the ethylene bridges would pairwise link

the adjacent N centers of the chain in the targeted cyclolinear

product 11a. This should preclude the formation of a borazine, or

small BRNR cycles in general, for steric reasons. At the same

time, redistribution processes that might occur in the first step of

the synthesis should be unfavored as a result of the chelate effect.

Related approaches have been followed previously by Neilson et

al., but, to the best of our knowledge, the identification of a poly-

or an extended oligo(iminoborane) has not been achieved in their

studies.^[30]

– Me₂NH

B

Me₃Si

N

NMe₂

1) LDA, Et₂O, r.t.

2) OctB(Cl)NMe₂

14

(b)

Me₃Si

15

)

Oct

B

(

H

– LiCl

N

12

Scheme 3. Synthesis of model compound 14.

The reaction of 1,3-bis(trimethylsilyl)-1,3,2-diazaborolidine^[31]

7a with either dichloro- (2a) or dibromo-n-octylborane (8a) was

performed in different chlorinated non-donor solvents (CH₂Cl₂,

CD₂Cl₂, and CDCl₃) and at various temperatures (Scheme 2 and

Table 1). In a typical experiment (e. g., entry 2 in Table 1), 7a and

2a were mixed in dichloromethane at ambient temperature to give

a 1 M starting concentration of the reactants. Monitoring of the

reaction in CD₂Cl₂by ¹¹B{¹H} and ¹H NMR spectroscopy revealed

that both 7a and 2a were immediately consumed with initial for-

mation of 9a^[24](see Figure S42 in the Supporting Information).

Even after short reaction times, the spectra showed that further

oligomerization had occurred. The ¹¹B resonance for the B–Cl end

groups of the growing chain (at around 44.5 ppm) continuously

decreased in intensity. One broad signal remained in the ¹¹B{¹H}

NMR spectrum for the bulk boron atoms of the oligomer 11a^Cl,

which in the course of the reaction was shifted slightly upfield to δ

= 31 ppm. In the ¹H NMR spectrum, a common signal appeared

for the protons of the ethylene bridge, centered at δ = 3.32 ppm.

Small peaks at δ = 3.00–3.25 and 3.45–3.65 ppm remained which

we assign to the ethylene protons of the rings at the chain ends.

The proton resonance for the SiMe₃end group was detected at δ

= 0.15 ppm, that is, at identical chemical shift as that of the SiMe₃

group of 14. Concomitant formation of the volatile condensation

by-product Me₃SiCl was evidenced by its ¹H resonance at δ = 0.45

ppm. Quantitative evaluation of the signals over time revealed that

conversion of the reactive groups leveled off at about 85 %. This

may be associated with a rate reduction due to a marked increase

in the viscosity of the solution. Estimation of the degree of poly-

merization after 14 days by using Carothers’ equation yielded DP_n

≈ 7. Then, Me₃SiNMe₂(6 mol%) was added to deactivate the B–

Cl end groups of 11a^Cl. The end-capped product 11a^NMe2was

purified by adding the mixture to an excess amount of anhydrous

acetonitrile, which resulted in separation of 11a^NMe2from solution.

This afforded oligo(iminoborane) 11a^NMe2as a highly viscous am-

ber fluid in 83 % yield.^[24]

Oct

Oct Oct

solvent

B

Me₃Si

SiMe₃

B

Me₃Si

N

X

+ OctBX₂

– Me₃SiX

7a

2a:

8a:

X = Cl

X = Br

9a: X = Cl

10a: X = Br

xn

–(n–1) Me₃SiX

Oct Oct

Me₃SiNMe₂

– Me₃SiX

B

Me₃Si

B

Me₃Si

N

NMe₂

N

X

n

11a^NMe2

11a^Cl: X = Cl

11a^Br: X = Br

Scheme 2. Polycondensation of 1,3-bis(trimethylsilyl)-2-n-octyl-1,3,2-

diazaborolidine (7a) and dichloro- (2a) or dibromo-n-octylborane (8a) by Si/B

exchange and subsequent end-capping (Oct = n-C₈H₁₇).

We successfully synthesized the oligo(iminoborane) 11a using

our Si/B exchange polycondensation approach (Scheme 2).^[24]

Additionally, we prepared compound 14 as a molecular model

system for 11a (i. e., 14 is equivalent to 11a^NMe2with n = 1). In our

previous communication we presented the synthesis of 14 by

transamination between 12 and 13 (Scheme 3a).^[24]We now

found that 14 can be obtained more conveniently by deprotona-

tion of 12 with LDA, followed by salt elimination with chlorodimeth-

ylamino-n-octylborane (15) (Scheme 3b). The latter is easily ac-

cessed via ligand redistribution between 2a and 13.

Similarly, reactions of 7a with the dibromoborane 8a gave 11a^Br

via 10a, besides Me₃SiBr as the condensation by-product, as evi-

denced by ¹H and ¹¹B{¹H} NMR spectroscopy. Subsequent end-

capping with Me₃SiNMe₂furnished 11a^NMe2as well. Irrespective

of the reaction time (cf. entries 1 and 2 in Table 1), temperature,

solvent, and the use of 2a vs. 8a, gel permeation chromatography

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(GPC) analysis of the products revealed comparable molecular

weight distributions with M_n= 1.51 – 1.81 kDa and M_W= 1.56 –

2.05 kDa (Figure 2a). This corresponds to chain lengths of about

DP_n= 5 – 6 repeat units on average.

Mass loss occurred in basically two steps. In a narrow tempera-

ture range at about 350ꢀ°C, 72.9 % of the initial mass was lost,

which matches well with the fraction of the n-octyl groups. In the

second, broader step starting from 450ꢀ°C, the sample lost further

9.0 % of its mass, corresponding to an expulsion of the ethylene

bridges. The overall mass loss at 1000ꢀ°C amounts to 81.9 %. The

remaining 18.1 % correspond well to the fraction of BN in 11a^NMe2

being 16.3 %.

Table 1. GPC results for 11a/11a’ prepared under various reaction

conditions and GPC results for 11ac and 11d.

Entry

Co-

mono-

mers

Solvent

T/

°C

Reac

-tion

time

M_n/

kDa

M_W/

kDa

DP_n

1

2

3

4

5

6

7

8

9

7a, 2a

7a, 8a

7a, 2a

7a, 8a

7a, 2a

7a, 8a

16, 8a

CH₂Cl₂

CDCl₃

CH₂Cl₂

20

40

61

20

14 d

24 h

1.72

1.78

1.54

1.51

1.57

1.51

1.81

1.77

1.75

2.03

1.94

1.62

1.56

1.61

1.78

2.05

2.00

2.10

6

5

6

Figure 2. GPC trace of 11a^NMe2(in THF, versus polystyrene standards) obtained

by (a) Si/B exchange polycondensation of 7a and 2a in CH₂Cl₂at 20 °C and (b)

by activation of 14 with 2 equiv. of HOTf.

Polycondensation by Sn/B exchange. We also explored

tin/boron exchange as an alternative polymerization strategy. For

the synthesis of organoborane polymers featuring B–C linkages

along the backbone, Sn/B exchange polycondensation, which in

that case proceeds via B–C coupling, is a well-established meth-

od.^[6b]Different from polymerization by Si/B exchange at car-

bon,^[6s]this type of reaction usually does not require a catalyst to

proceed. In our reaction, 1,3-bis(trimethylstannyl)-1,3,2-diazabo-

rolidine 16 and dibromoborane 8a, as expected, underwent

smooth polycondensation at ambient temperature to give 11’a^Br

with concomitant release of Me₃SnBr (Scheme 4). The proton res-

onance of the Me₃Sn end group was detected at δ = 0.29 ppm.

The reactive B–Br end group was deactivated by subsequent re-

action with Me₃SiNMe₂to give 11’a^NMe2. The GPC analysis re-

vealed a molecular weight distribution for 11’a^NMe2(Table 1, entry

8) comparable to that of 11a^NMe2obtained by the Si/B exchange

reactions described in the previous section.

14,

HCl

CH₂Cl₂/

Et₂O

10

14,

HOTf

CDCl₃

20

96 h

24 h

2.24

1.91

2.86

2.33

7

6

11^[a]

14,

HNTf₂

CD₂Cl₂

12^[b]

13

7b, 2b

7a, 2c

7d, 2d

CH₂Cl₂

20

96 h

24 h

2.43

2.50

2.16

2.48

3.77

2.81

13

9

14^[a]

6

[a] In this case, the GPC results from the product 11d^Clbefore end-

capping are given because some degradation had occurred during the

end-capping process (see the Supporting Information). [b] The GPC re-

vealed a multimodal distribution; the data given corresponds to the

higher molecular-weight fraction.

The oligo(iminoborane) 11a^NMe2was further analyzed by multi-

nuclear NMR spectroscopy (including H DOSY), UV–vis spec-

Oct

B

Oct

B

Oct

B

n

CH₂Cl_2,r.t.

1

Me₃Sn

SnMe₃

Me₃Sn

Br

N

– (2n–1) Me₃SnBr

troscopy, elemental analysis, dynamic light scattering (DLS,

Figure 3a), small-angle X-ray scattering (SAXS), differential scan-

ning calorimetry (DSC), and thermogravimetric analysis (TGA).

The NMR data of 11a^NMe2are consistent with those of its precur-

+

n OctBBr₂

n

11'a^Br

16

8a

Me₃SiNMe₂

– Me₃SiBr

1

sor 11a^Cldescribed beforehand. The H resonance of the NMe₂

Oct

B

end group of 11a^NMe2was detected at δ = 2.68 ppm, i. e., the

chemical shift being virtually identical to that of the NMe₂group in

the model compound 14. A detailed discussion of the analytical

data for 11a^NMe2has been provided in our previous communica-

tion.^[24]It is noteworthy that 11a^NMe2exhibits a very low glass tran-

sition temperature, T_g= −71ꢀ°C. The results from the TGA confirm

the stabilizing effect of the ethylene bridge in 11a^NMe2(Figure 3c).

B

Me₃Sn

NMe₂

N

n

11'a^NMe2

Scheme 4. Polycondensation of 16 with 8a by Sn/B exchange.

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2n HX

CH₂Cl₂(Et₂O)

Polycondensation of 14 through initiation with a Brønsted

acid. In the polymerizations described so far the true monomer

(e.g., 9a, 10a) generally had remained elusive. Although 9a was

observed by NMR spectroscopy in the reaction of 7a with 2a,

some follow-up condensation was already visible at an early stage.

Therefore, in our next approach we decided to employ a dormant

form of the monomer which is isolable but might be activated un-

der certain conditions to undergo polymerization. We anticipated

that compound 14 could serve this purpose. Previous examples

have shown that in some cases the amino group of an aminobo-

rane can be substituted by Cl upon reaction with anhydrous

HCl.^[32]For this, an excess of the acid (ca. 2 equiv.) is usually

needed to suppress the reverse reaction by irreversible formation

of the corresponding ammonium salt.

Oct

B

Oct Oct

Oct

B

n

B

Me₃Si

N

NMe₂

N

NMe₂/X

– n [H₂NMe₂]X

– n Me₃SiX

n

14

– (n–1)

11a

(2n–2) HX

Me₃SiNMe₂

– (n–1)

Me₃SiX

[H₂NMe₂]X

– Me₃SiX

Oct Oct

B

Me₃Si

(n–1)

N

X

B

Me₃Si

14

+

N

NMe₂

n

9a:

X = Cl

X = Cl, OTf, NTf₂

11a^NMe2

17:

18:

X = OTf

X = NTf₂

Scheme 5. Polymerization of the dormant monomer 14 initiated by Brønsted

acids.

Indeed, when a solution of 14 in CH₂Cl₂was treated with HCl in

Et₂O (2 equiv.), polycondensation occurred to give the oligo(imi-

noborane) 11a (Scheme 5). Monitoring of the reaction by ¹¹B{¹H}

NMR spectroscopy clearly showed the formation of B–Cl species.

At about δ = 44 ppm a broad signal appeared (see Figure S61 in

the Supporting Information) which we assign to the B–Cl group of

initially formed 9a, and in the further course of the reaction, to the

end group of 11a^Cl. This signal decreased over time, consistent

with elongation of the chains through condensation. Consequent-

ly, the formation of 11a is initiated by replacement of NMe₂with

Cl, thus, generating the reactive monomer 9a, which undergoes

facile subsequent polycondensation. Volatile Me₃SiCl is released,

and the salt [H₂NMe₂]Cl formed as the second by-product is easily

removed by filtration in the work-up procedure. According to the

formal stoichiometry of the reaction (cf. Scheme 5), exactly 2n −

2 equivalents of HCl are required to afford the NMe₂-end-capped

Variation of the side groups. We then aimed at tailoring the

properties of oligo/poly(iminoborane)s through variation of the

side groups. For the synthesis of the new derivatives we chose

the Si/B exchange polycondensation approach using appropriate

combinations of a 1,3-bis(trimethylsilyl)-1,3,2-diazaborolidine and

a dichloroborane (Scheme 6). Our first target was the derivative

11b having n-butyl side chains attached to the boron centers. The

formation of 11b^Clproceeded just as smoothly as that of 11a^Cl.

Subsequently, it was end-capped to give 11b^NMe2. Its identity was

unambiguously ascertained by multinuclear NMR spectroscopy.

Analysis of 11b^NMe2by GPC yielded a multimodal distribution,

which is presumably due to some decomposition during the mea-

surement. The higher molecular-weight fraction showed weight

averages of M_n= 2.43ꢀkDa and M_W= 2.48ꢀkDa, suggesting a de-

gree of polymerization of DP_n= 13. Analysis of 11b^NMe2by DLS

yielded a hydrodynamic radius of R_h= 2.0ꢀnm for 11b in n-pentane

(Figure 3b), which is well comparable to that of 11a (R_h= 2.2 nm,

Figure 3a).

1

form of 11a, i.e., 11a^NMe2. However, the result of a H NMR end

group analysis did not fully match that of the GPC analysis. It is

plausible that some B–Cl end groups had remained (see Figure

S63 in the Supporting Information). In order to fully transform

these into B–NMe₂groups, Me₃SiNMe₂was subsequently added

which afforded 11a^NMe2. The integral of the ¹H NMR signal for the

SiMe₃end group, on the other hand, was somewhat too low.

Hence, we assume that partial desilylation of the substrate had

occurred during polymerization in the presence of the acid. Over-

all, our investigations revealed that the best results are obtained

with approximately 2 equiv. of HCl. The use of a greater excess

turned out to be unfavorable, probably due to protonation of nitro-

gen atoms in the chain and chain scission and/or salt formation.

We also performed analogous reactions of 14 with the stronger

Brønsted acids HOTf and HNTf₂. Here, similar observations were

made. The salt by-product, [H₂NMe₂]OTf, obtained from the reac-

R

B

R'

B

R

B

n

CH₂Cl₂

Me₃Si

SiMe₃

N

Cl

n

N

– (2n–1) Me₃SiCl

+ n

R'BCl₂

11b^Cl

:

7a: R' = Oct

7b: R' = Bu

2a: R' = Oct

2b: R' = Bu

R = R' = Bu

R = R' = Ph

:

11c^Cl

:

7c:

R' = Ph

2c:

R' = Ph

11ac^Cl

11ca^Cl

11bc^Cl

R = Oct, R' = Ph

R = Ph, R' = Oct

R = Bu, R' = Ph

Me₃SiNMe₂

– Me₃SiCl

:

11cb^Cl: R = Ph, R' = Bu

R

B

R'

B

Me₃Si

NMe₂

N

1

tion to 17 was unambiguously identified by H and ¹⁹F{¹H} NMR

n

11b^NMe2

11ac^NMe2

:

R = R' = Bu

spectroscopy. During the reaction with HNTf₂,^{[33] 11}B{¹H} NMR

spectroscopic monitoring evidenced the formation of B–NTf₂spe-

cies (i.e., 18 and the respective growing chain) by a signal at

about δ = 39–40 ppm (see Figure S82 in the Supporting Informa-

tion). From each of these reactions the oligo(iminoborane) 11a

was isolated and subjected to GPC analysis (Table 1, entries 9–

11). The best result was obtained with 2 equiv. of HOTf in CDCl₃.

This yielded the highest molecular weight for 11a of all reactions

described so far, i.e., M_n= 2.24ꢀkDa and M_W= 2.86ꢀkDa, corres-

ponding to a number average degree of polymerization of DP_n= 7.

:

R = Oct, R' = Ph

Scheme 6. Polycondensation of 1,3-bis(trimethylsilyl)borolidines (7) and dichlo-

roboranes (2) by Si/B exchange and subsequent end-capping of the soluble

derivatives.

The appearance of 11b was that of a sticky fluid with a markedly

higher viscosity compared to 11a. Despite the shorter side chains,

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11b was readily soluble in THF, CH₂Cl₂, chloroform, and n-pen-

tane. Its DSC trace showed a glass transition at T_g= −20.1ꢀ°C.

The shape of the TGA trace of 11b (Figure 3d) was similar to that

of 11a (Figure 3c). Likewise, it showed one sharp and one broad

step, corresponding to expulsion of the alkyl side groups and the

ethylene bridges, respectively. Although the percentages of mass

loss for the individual steps, 67.7 and 7.9 %, do not fit as well as

in the case of 11a with the ratio of the two fragments expulsed,

the overall mass loss of 75.6 %, leaving 24.4 %, corresponds well

to the BN fraction of 11b being 23.6 %.

interchanging the substituents by performing the reaction be-

tween 7a and 2c, after end-capping and work-up, yielded the am-

ber colored solid product 11ac^NMe2which was readily soluble in

THF, CH₂Cl₂, chloroform, and n-pentane. The results from GPC

analysis suggested mass averages of M_n= 2.50ꢀkDa and M_W

=

3.77ꢀkDa for 11ac^NMe2(Table 1, entry 13, and Figure 4a). This cor-

responds to chains of on average 9 repeat units or 18 BN units,

respectively, which is the longest-chain iminoborane oligomer and

the closest approach to a poly(iminoborane) unambiguously char-

acterized by GPC so far.

Figure 3. Intensity-weighted size distribution of (a) 11a^NMe2and (b) 11b^NMe2in

n-pentane by DLS (additional peak # assigned to aggregates of 11b^NMe2), TGA

trace of (c) 11a^NMe2, and (d) 11b^NMe2

.

Our next target was to incorporate aromatic side groups, which

was expected to significantly alter the mechanical and thermal

properties of the new materials. We first performed the reaction of

the B-phenyl substituted co-monomers 7c and 2c under Si/B poly-

condensation conditions. This, however, yielded only insoluble

material. Shortly after 7c and 2c were mixed in dichloromethane

at room temperature, the formation of a precipitate was observed.

Figure 4. GPC trace of (a) 11ac^NMe2and (b) 11d^NMe2(in THF, versus polystyrene

standards), DSC trace of (c) 11ac^NMe2and (d) 11d^NMe2, and TGA trace of (e)

11ac^NMe2and (f) 11d^NMe2

.

1

An H NMR spectrum from the overlaying solution showed the

As the strategy to incorporate both alkyl and phenyl side chains

was generally working, we next attempted to combine both fea-

tures in one substituent. For this, we devised 4-n-butylphenyl as

the B-substituent, with which an aryl group can be attached to

each boron center while butyl groups are present to impart solu-

bility, though having minor influence on the steric and electronic

situation at the boron centers (Scheme 7). Indeed, the reaction

between 7d and 2d afforded 11d^Clas a solid yet readily soluble

product. GPC suggested mass averages of M_n= 2.16ꢀkDa and M_W

= 2.81ꢀkDa, corresponding to DP_n= 6 (Table 1, entry 14, and Fig-

ure 4b). After deactivation of the end groups with Me₃SiNMe₂, the

molecular weight of the resulting 11d^NMe2was somewhat de-

creased (M_n= 1.73ꢀkDa and M_W= 2.29ꢀkDa), presumably due to

slight degradation during the end-capping process.

formation of Me₃SiCl, thus, confirming that some condensation

had occurred. However, nearly the entire boron content of the

sample had precipitated, and the colorless solid obtained after fil-

tration did not dissolve in any of the common organic solvents

tested. Therefore, we decided in the next step to attempt the syn-

thesis of poly(iminoborane) derivatives with mixed substituents,

one of which being phenyl and the other an alkyl chain.

Combinations of a phenyl and an n-butyl substituent at the two

boron centers, i.e., reactions of 7b with 2c and 7c with 2b yielded

insoluble products, 11bc^Cland 11cb^Cl, as well. It should be noted

though, that precipitation only occurred at a later stage of the poly-

merization process. From the reaction of dichloro-n-octylborane

(2a) with the B-phenyl derivative 7c, a partially soluble product

was obtained. GPC analysis of the dissolved parts showed a

rather weak signal corresponding to a mass of approximately

1ꢀkDa while most of the product 11ca did not dissolve. Finally,

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bonded phenyl groups in 11c^NMe2are twisted with respect to the

respective BR₃planes by 39.2° on average (in the tetramer). The

adjacent phenyl rings along the chain are close to parallel oriented

with interplanar angles of on average 35.7°. The mean distance

between the ipso-carbon atoms is 3.146 Å.

n

CH₂Cl₂

B

Me₃Si

B

Me₃Si

SiMe₃

+ n

Cl

N

Cl

n

N

B

– (2n–1) Me₃SiCl

Cl

11d^Cl

2d

7d

Me₃SiNMe₂

– Me₃SiCl

B

Me₃Si

N

NMe₂

n

11d^NMe2

Figure 5. Optimized structure of 11c^NMe2(n = 4, H-atoms omitted for clarity;

B3LYP-D3(BJ)/def2-SV(P)).

Scheme 7. Polycondensation of 1,3-bis(trimethylsilyl)borolidine 7d and dichlo-

roborane 2d by Si/B exchange and subsequent end-capping.

The new oligo(iminoborane) derivatives 11ac^NMe2and 11d^NMe2

Conclusions

were further characterized by multinuclear NMR spectroscopy (in-

1

This study has demonstrated that skeletal backbone stabilization

is an effective approach to prevent the unwanted formation of

small cyclic (by)products such as borazines in the targeted syn-

thesis of poly(iminoborane)s. While an attempt to use acyclic co-

monomers 1 and 2a in a Si/B exchange reaction did not afford

linear BN catenation, but exclusive formation of borazine 6 in-

stead, the use of 1,3-bis(trimethylsilyl)-1,3,2-diazaborolidines 7

together with 2 or 8 cleanly yielded the oligo(iminoborane)s

11a,b,ac,d. Furthermore, two alternative synthetic routes to such

species were disclosed in this study: Sn/B exchange of 13 and 8a,

and initiated polycondensation of the dormant monomer 14 in the

presence of a Brønsted acid (HCl, HOTf, or HNTf₂). The latter

method utilizes in situ transformation of dormant B–NMe₂groups

into active B–X groups (X = Cl, OTf, or NTf₂), which are then prone

to undergo facile condensation with the N–SiMe₃termini.

Effective stabilization of the backbone in the obtained oligo(imi-

noborane)s through the ethylene bridges was further supported

by our TGA studies. With the synthesis of 11ac and 11d we

achieved the incorporation of aromatic side groups, and DSC

measurements showed that the glass transition temperatures

systematically increase with the aryl group content (in the order,

11a < 11b < 11ac < 11d). Derivative 11ac, with about 18 cate-

nated BN units on average, is the closest approach to a poly(imi-

noborane) to date. Currently, we are exploring further synthetic

routes that eventually should lead to high-molecular-weight poly-

(iminoborane)s, and we are probing the potential of these new

materials for applications in materials science.

cluding H DOSY), UV–vis spectroscopy, DLS, DSC, and TGA.

Dynamic light scattering revealed hydrodynamic radii of R_h= 2.2

(11ac^NMe2) and 1.5 nm (11d^NMe2) in CH₂Cl₂, respectively. Differen-

tial scanning calorimetry revealed glass transitions at 0.1 °C

(11ac^NMe2) and 15.8 °C (11d^NMe2) (Figure 3c,d). This trend con-

firms the expected increase in the crystallinity of the product with

increasing ratio of phenyl groups attached to boron. Both deriva-

tives 11ac^NMe2and 11d^NMe2readily dissolve in most of the com-

mon organic solvents. However, it is noteworthy that for 11d^NMe2

n-pentane is a notably poorer solvent. In the TGA traces of

11ac^NMe2and 11d^NMe2the two stages of mass loss are not re-

solved (Figure 3e,f). Both compounds show major mass loss in a

broad temperature range starting at about 150 °C. Up to 1000 °C,

11ac^NMe2lost 81.9 % of its weight. The residual mass amounts to

18.1 %, which fits to the theoretical fraction of boron and nitrogen

being 17.9 %. The residual 14.2 % of 11d^NMe2at 1000 °C fit like-

wise well to the theoretical fraction of BN in 11d^NMe2being 14.0 %.

Computational studies. In order to gain insight into structural

features of the new compounds, we carried out geometry optimi-

zations on DFT niveau including dispersion correction (B3LYP-

D3(BJ)/def2-SV(P)).^[34-38]In our preliminary communication^[24]we

reported calculations on oligomers of the B-methyl substituted de-

rivative 11e^NMe2, which serves as a truncated model system for

11a,b^NMe2and 11a,b^NMe2. We now additionally calculated the B-

phenyl derivative 11c^NMe2with chain lengths of n = 1 to 4 repeat

units (Figure 5). The backbone structure of 11c^NMe2is very similar

to that of 11e^NMe2. The boron and the nitrogen centers are trigonal-

planar coordinated, and in 11c^NMe2the mean angle sums amount

to 360.0° (at B) and 359.8° (at N), respectively. The N-B-N and

the B-N-B planes are not fully coplanar but have an average tor-

sion angle of 20.2° (in the tetramer; in 11e^NMe2, 19.5°). For both

derivatives this causes a helical structure with a periodicity of

about 5 structural units as the most stable conformation. The B-

Experimental Section

General procedures. All manipulations were performed under an atmos-

phere of dry argon using standard Schlenk techniques or in an MBraun

glovebox. Solvents (dichloromethane, n-pentane, and diethylether) were

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dried and degassed by means of an MBraun SPS-800 solvent purification

system. CDCl₃and CD₂Cl₂for NMR spectroscopy were dried and de-

gassed at reflux over CaH₂and freshly distilled prior to use. Boron trichlo-

ride solution (1ꢀM in hexanes), n-butyllithium solution (2.5ꢀM in hexanes),

boron trichloride, 1-butene, bromobenzene, 1-bromo-4-iodobenzene, 1-

bromobutane, chlorotrimethylstannane, HCl solution (2ꢀM in diethyl ether),

heptamethyldisilazane (5), and trifluoromethanesulfonic acid were com-

mercially purchased and used as received. Dimethylammonium chloride,

methylammonium chloride, and potassium hydroxide were dried in vacuo

prior to use. Dimethylamine and methylamine were generated from their

ammonium chlorides with potassium hydroxide and recondensed prior to

use. 1-Octene, triethylamine and ethylenediamine were dried and de-

gassed at reflux over Na and freshly distilled prior to use. Pentamethylsil-

azane was dried and degassed by inert-gas distillation from CaH₂. Tri-

methylsilyl chloride, trimethylsilane, and triethylsilane were purified by in-

ert-gas distillation. Dichloro-n-octylborane (2a),^[39]n-butyldichloroborane

(2b)^[40]and dibromo-n-octylborane (8a)^[41]were prepared according to

methods described in the literature for boranes with other alkyl substitu-

ents.^[42]1-bromo-4-trimethylsilylbenzene,^[43]dichlorophenylborane (2c),^[44]

lithium diisopropylamide,^[45]bistriflamidic acid,^[46]and trimethylsilylben-

zene^[47]were prepared according to procedures described in the literature.

NMR spectra were recorded at 25ꢀ°C on a Bruker Avance II-400 spectro-

meter or on a Bruker Avance III HD spectrometer operating at 400 MHz.

Chemical shifts were referenced to residual protic impurities in the solvent

(¹H) or the deuterio solvent itself (¹³C) and reported relative to external

SiMe₄(¹H, ¹³C, ²⁹Si), BF₃·OEt₂(¹¹B), or CFCl₃(¹⁹F) standards. Mass spec-

tra were obtained with the use of a Finnigan MAT95 spectrometer employ-

ing electron ionization (EI) using a 70ꢀeV electron impact ionization source.

UV/vis-spectra were obtained using a Jasco V-630 spectrophotometer.

DSC measurements were performed on a Netzsch DSC 204 F1 phoenix

device with a heating/cooling rate of 10ꢀK min^–1. TGA measurements were

performed on a Mettler-Toledo TGA/SDTA 851e device at a heating rate

of 5ꢀK min^–1under N₂. GPC chromatograms were recorded on an Agilent

1100 Series, using a flow rate of 1ꢀmL min^–1in THF at 25ꢀ°C, calibrated

against polystyrene standards. Dynamic light scattering (DLS) measure-

ments were performed in CH₂Cl₂(c = 2.5ꢀgꢀL^–1) at 20ꢀ°C with an ALV 5000

E autocorrelator equipped with a red laser (λ = 633ꢀnm). The time-resolved

signal of two Single Photon Counting Modules (SPCM-CD 2969; Perkin

Elmer) was cross-correlated. Hereby, the CONTIN analysis was per-

formed in an angular dependent way. For each measurement (sampling

time 90ꢀs), the intensity-weighted decay-time τ distributions (as obtained

from the field autocorrelation function obtained by use of the Siegert rela-

tion)^[48]were analyzed in respect to multimodality, where for each diffusive

mode its (intensity-) average decay rate Γ (1/τ) was extracted (if necessary,

the probability factor of the CONTIN algorithm was adjusted in order to

increase resolution). Then, the decay rates were plotted against the

squared length of the scattering vector q². The slope gave the diffusion

coefficient D and its value was transformed to the hydrodynamic radius R_h

by the Stokes-Einstein equation.

Synthesis of bis(trimethylsilyl(methyl)amino)-n-octylborane (1). To a

solution of lithium trimethylsilyl(methyl)amide (2.17ꢀg, 19.9ꢀmmol) in diethyl

ether (10ꢀmL) dichloro-n-octylborane (1.94ꢀg, 9.96ꢀmmol) was added drop-

wise at −78ꢀ°C. It was stirred for 1.5ꢀh, warmed to ambient temperature,

and stirred for another 2ꢀh. The volatiles were removed in vacuo and n-

pentane (30ꢀmL) was added to the residue. It was filtered off and washed

with n-pentane. Distillation at 15ꢀmbar and 112ꢀ°C yielded a colorless oil.

¹H NMR (400ꢀMHz, CDCl₃): δ = 2.57 (s, 6ꢀH, NCH₃), 1.28 (m, 12ꢀH), 0.90

(t, 3ꢀH), 0.79 (t, 2ꢀH, BCH₂), 0.13 (s, 18ꢀH, Si(CH₃)₃)ꢀppm; ¹¹B{¹H} NMR

(128ꢀMHz, CDCl₃): δ = 42.2 (s)ꢀppm.

Attempted synthesis of the diborazene 3 and successive formation

of chloro(trimethylsilyl(methyl)amino)-n-octylborane (4) and N,N’,N’’-

trimethyl-B,B’,B’’-tri-n-octylborazine (6).^[49]To a solution of bis(trimeth-

ylsilyl(methyl)amino)-n-octylborane (0.3409ꢀg, 1.0378ꢀmmol) in CDCl₃

(1ꢀmL) dichloro-n-octylborane (0.2007ꢀg, 1.0295ꢀmmol) was added. After

45ꢀmin it was evidenced by ¹H NMR that the aminoborane 4 had formed

quantitatively as two isomers. Isomer 1: ¹H NMR (400ꢀMHz, CDCl₃): δ =

2.84 (s, 3ꢀH, NCH₃), 1.46 (br, 2ꢀH, BCH₂), 1.28 (m, 12ꢀH), 0.89 (t, 3ꢀH), 0.26

(s, 9ꢀH, Si(CH₃)₃)ꢀppm; Isomer 2: ¹H NMR (400ꢀMHz, CDCl₃): δ = 2.75 (s,

3ꢀH, NCH₃), 1.46 (br, 2ꢀH, BCH₂), 1.28 (m, 12ꢀH), 0.89 (t, 3ꢀH), 0.28 (s, 9ꢀH,

Si(CH₃)₃)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 43.8 (s)ꢀppm. Addition-

ally, the formation of some borazine 6 was evidenced.

After 16ꢀh it was confirmed that the borazine 6^[49]had formed quantitatively:

¹H NMR (400ꢀMHz, CDCl₃): δ = 2.94 (s, 3ꢀH, NCH₃), 1.46–1.20 (m, 12ꢀH),

1.03 (br, 2ꢀH, BCH₂), 0.89 (t, 3ꢀH)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ

= 36.8 (s)ꢀppm.

Alternative synthesis of 4 and formation of 6. To a solution of dichloro-

n-octylborane (0.551ꢀg, 2.83ꢀmmol) in CH₂Cl₂(2ꢀmL) was added slowly and

dropwise heptamethyldisilazane (5) (0.489ꢀg, 2.78ꢀmmol) at −78ꢀ°C. After

10ꢀmin, the cooling bath was removed. ¹H and ¹¹B {¹H} NMR of the solution

immediately after the start of the reaction confirmed the formation of

aminoborane 4. After 1ꢀd, the NMR spectra revealed the formation of the

borazine 6.

Synthesis of chlorodimethylamino-n-octylborane (15). To dichloro-n-

octylborane (5.848ꢀg, 30.00ꢀmmol) bis(dimethylamino)-n-octylborane

(6.366ꢀg, 30.00ꢀmmol) was added and the mixture was stirred for 30ꢀmin.

1

Distillation at 25ꢀmbar and 115ꢀ°C afforded the product quantitatively. H

NMR (400ꢀMHz, CDCl₃): δ = 2.89 (s, 3ꢀH, N–CH₃), 2.82 (s, 3ꢀH, N–CH₃),

1.44 (t, 2ꢀH, CH₂), 1.29 (m, 10ꢀH, alkyl), 1.04 (t, 2ꢀH, CH₂), 0.89 (t, 3ꢀH,

CH₃); ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 39.1 (s)ꢀppm.

Synthesis of 3-(dimethylamino-n-octylboryl)-2-n-octyl-1-trimethylsil-

yl-1,3,2-diazaborolidine (14).^[24]To a solution of 2-n-octyl-1-trimethylsilyl-

1,3,2-diazaborolidine (12)^[24](0.1429ꢀg, 0.5619ꢀmmol) in toluene (0.7ꢀmL)

was added bis(dimethylamino)-n-octylborane (0.1509ꢀg, 0.7112ꢀmmol).

The mixture was heated at reflux for 5ꢀd. The volatile compounds were

removed in vacuo yielding crude 14 as a yellowish viscous liquid which

was evaporated at 160ꢀ°C and 4ꢀ·ꢀ10⁻²ꢀmbar and recondensed at −196ꢀ°C

to give 14 as a colorless liquid (yield 15ꢀ%). ¹HꢀNMR (400ꢀMHz, CDCl₃): δ

= 3.26–3.13 (m, 4ꢀH, N–C₂H₄–N), 2.68 (br, 6ꢀH, N(CH₃)₂), 1.28 (m, 24ꢀH,

CH₂), 0.89 (t, 6ꢀH, CH₃), 0.82 (t, 2ꢀH, B–CH₂), 0.72 (t, 2ꢀH, B–CH₂), 0.15 (s,

9ꢀH, Si(CH₃)₃)ꢀppm; ¹¹B{¹H}ꢀNMR (128ꢀMHz, CDCl₃): δ = 37.1 (s)ꢀppm;

¹³C{¹H}ꢀNMR (101ꢀMHz, CDCl₃): δ = 49.3 (s, B–N–CH₂–CH₂–N–Si), 47.7

(s, B–N–CH₂–CH₂–N–Si), 39.3 (br, N(CH₃)₂), 33.4 (s, CH₂), 33.2 (s, CH₂),

32.0 (s, CH₂), 29.7 (s, CH₂), 29.6 (s, CH₂), 29.4 (s, CH₂), 29.4 (s, CH₂),

26.6 (s, CH₂), 25.6 (s, CH₂), 22.7 (s, CH₂), 16.2 (br, B–CH₂), 14.9 (br, B–

CH₂), 14.1 (s, CH₃), 0.8 (s, Si(CH₃)₃)ꢀppm; ²⁹Si{¹H}ꢀNMR (79.5ꢀMHz,

CDCl₃): δ = 3.3 (s)ꢀppm; MS (EI, 70 eV): m/z (%) = 421.5 ([M⁺], 9), 308.4

([M⁺−C₈H₁₇], 100), 239.4 ([M⁺−C₁₁H₂₅BN], 53), 73.2 ([C₃H₉Si⁺], 16);

Synthesis of lithium trimethylsilyl(methyl)amide. MeNH₂(26.2ꢀg,

0.844ꢀmol) was recondensed onto a frozen (N_2(l)cooled) solution of chloro-

trimethylsilane (36.67ꢀg, 0.3376ꢀmol) in diethyl ether (340ꢀmL). Subse-

quently the mixture was allowed to warm to ambient temperature and was

stirred overnight. A colorless precipitate formed which was filtered off and

washed with n-pentane. The volatiles were removed from the filtrate and

the residue was distilled (41.47 mmol, 4.28 g, 12 %). The product was

dissolved in diethyl ether (40ꢀmL) and a solution of n-BuLi (2.5 M, 16.2ꢀmL,

40.5ꢀmmol) in hexanes was added dropwise at −78ꢀ°C. After 1ꢀh, the solu-

tion was warmed to ambient temperature and it was stirred for 2ꢀh. The

volatiles were removed in vacuo. The product was received quantitatively

as a colorless solid and was used without further purification for the next

step.

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12

HRMS: found m/z = 421.4195, theo. mass for

C

23

¹H₅₃¹⁴N₃¹¹B₂²⁸Si =

(79.5ꢀMHz, CDCl₃): δ = 3.3 (s)ꢀppm; MS (EI, 70ꢀeV): m/z (%) = 270.4 ([M⁺],

27), 255.4 ([M^+ꢀ−ꢀCH₃], 57), 213.3 ([M^+ꢀ−ꢀC₄H₉], 100), 199.3 ([M⁺− C₅H₁₀],

26), 155.3 ([M⁺− C₆H₁₅Si], 16), 73.3 ([C₃H₉Si⁺], 92).

421.4189; UV/vis (THF): λ_max= 248ꢀnm (ε = 1.3ꢀ·ꢀ10^3ꢀLꢀcm⁻¹ꢀmol⁻¹).

Alternative synthesis of 14. To a solution of 2-n-octyl-1-trimethylsilyl-

1,3,2-diazaborolidine (0,5086ꢀg, 2.000ꢀmmol) in diethyl ether (2ꢀmL) was

added a solution of lithium diisopropylamide (0.2678ꢀg, 2.500ꢀmmol) in

diethyl ether (2ꢀmL). The mixture was stirred for 30ꢀmin and

chlorodimethylamino-n-octylborane (0.5293ꢀg, 2.600ꢀmmol) was added.

After 30ꢀmin, the volatiles were removed in vacuo and n-pentane (4ꢀmL)

was added to the residue. It was filtered off and the volatiles and by-

products were removed at 5ꢀ·ꢀ10⁻²ꢀmbar and 70ꢀ°C yielding the product 14

as a yellowish liquid quantitatively.

Synthesis of bis(dimethylamino)phenylborane.^[52]Dimethylamine (ca.

175ꢀmmol) was slowly recondensed into a solution of dichloro(phenyl)bo-

rane (4.6ꢀg, 29ꢀmmol) in n-pentane (60ꢀmL) at −78ꢀ°C. The mixture was

allowed to reach ambient temperature while stirring overnight. The preci-

pitate was filtered off and washed with n-pentane (3 x 10ꢀmL). Removal of

the volatiles in vacuo yielded the product quantitatively as a colorless oil

which was used without further purification for the next step. ¹H NMR

(400ꢀMHz, CDCl₃): δ = 7.47–7.28 (m, 5ꢀH), 2.71 (s, 12ꢀH, N(CH₃)₂)ꢀppm;

¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 32.6 (s)ꢀppm.

Synthesis of 1,3-bis(trimethylstannyl)-2-n-octyl-1,3,2-diazaborolidine

(16). To

a

solution of 2-n-octyl-1,3,2-diazaborolidine (0.9275ꢀg,

Synthesis of 2-phenyl-1,3,2-diazaborolidine.^[53]To a solution of bis(di-

methylamino)phenylborane (5.1ꢀg, 29ꢀmmol) in toluene (15ꢀmL) at −78ꢀ°C

ethylenediamine (2.3ꢀmL, 35ꢀmmol) was added slowly and dropwise. The

mixture was allowed to reach ambient temperature while stirring overnight.

Removal of the volatiles in vacuo yielded a colorless solid. It was stirred in

n-pentane for 30ꢀmin and filtered off. The solid was dried in vacuo. ¹H NMR

(400ꢀMHz, C₆D₆): δ = 7.53–7.47 (m, 2ꢀH), 7.29–7.24 (d, 3ꢀH), 3.15 (s, 4ꢀH,

N–C₂H₄–N), 2.55 (br, 2ꢀH, NH)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, C₆D₆): δ =

32.8 (s)ꢀppm.

5.093ꢀmmol) in diethyl ether (15ꢀmL) at −78ꢀ°C a solution of lithium diiso-

propylamide (1.1875ꢀg, 11.098ꢀmmol) in diethyl ether (8ꢀmL) was added.

After 1.5ꢀh, the clear solution was warmed to ambient temperature. A solu-

tion of chlorotrimethylstannane (2.31ꢀg, 11.6ꢀmmol) in diethyl ether (9ꢀmL)

was added and the mixture was stirred overnight. The volatiles were re-

moved in vacuo and n-pentane (30ꢀmL) was subsequently added. It was

filtered off and washed with n-pentane (2 x 10ꢀmL). The volatiles of the

filtrate were removed in vacuo and the product used without further purifi-

cation. ¹H NMR (400ꢀMHz, CDCl₃): δ = 3.32 (s, 4ꢀH, N–C₂H₄–N), 1.27 (m,

12ꢀH), 0.89 (t, 3ꢀH), 0.57 (t, 2ꢀH, BCH₂), 0.26 (s, 18ꢀH, Sn(CH₃)₃) ppm;

¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 38.4 (s)ꢀppm.

Synthesis of 1,3-bis(trimethylsilyl)-2-phenyl-1,3,2-diazaborolidine

(7c). To

a suspension of 2-phenyl-1,3,2-diazaborolidine (1.46ꢀg,

10.0ꢀmmol) in THF (100ꢀmL) at 0ꢀ°C n-BuLi in hexanes (8.2ꢀmL, 2.5ꢀM,

20.5ꢀmmol) was added. After 2ꢀh the mixture had reached ambient temper-

ature and chlorotrimethylsilane (2.7ꢀmL, 21ꢀmmol) was added while the

mixture became clear. The solution was stirred overnight and the volatiles

were removed in vacuo. n-Pentane (20ꢀmL) was added to the residue and

the mixture was filtered off and washed with n-pentane (2 x 10ꢀmL). Re-

moval of the volatiles and distillation at 5ꢀ·ꢀ10⁻²mbar and 90ꢀ°C yielded the

product as a colorless oil (1.5ꢀg, 5.1ꢀmmol, 51 %). ¹H NMR (400ꢀMHz,

CDCl₃): δ = 7.31–7.22 (m, 5ꢀH), 3.41 (s, 4ꢀH, N–C₂H₄–N), −0.15 (s, 18ꢀH,

Si(CH₃)₃)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 36.4 (s)ꢀppm; ¹³C{¹H}

NMR (101ꢀMHz, CDCl₃): δ = 132.2 (s, ortho-C), 127.2 (s, para-C), 127.0

(s, meta-C), 48.2 (s, N–C₂H₄–N), 0.3 (s, Si(CH₃)₃)ꢀppm; ²⁹Si{¹H} NMR

(79.5ꢀMHz, CDCl₃): δ = 4.6 (s)ꢀppm.

Synthesis of n-butylbis(dimethylamino)borane.^[50]Trichloroborane

(10.76ꢀg, 91.84ꢀmmol) and 1-butene (5.72ꢀg, 102.1ꢀmmol) were condensed

into a flask and dissolved in n-pentane (100ꢀmL) at −78ꢀ°C. Triethylsilane

(14.4ꢀmL, 90.2ꢀmmol) was added slowly dropwise. After 1ꢀh, n-pentane

(100ꢀmL) was added and dimethylamine (24ꢀg, 0.54ꢀmol) was condensed

into the solution. The mixture was allowed to reach ambient temperature

while stirring overnight. The mixture was filtered off and the precipitate was

washed with n-pentane (2 x 40ꢀmL). Removal of the volatiles and distilla-

tion at 15ꢀmbar and 63ꢀ°C yielded the product as colorless oil. ¹HꢀNMR

(400ꢀMHz, CDCl₃): δ = 2.66 (s, 12ꢀH, N(CH₃)₂), 1.37–1.21 (m, 4ꢀH), 0.91 (t,

3ꢀH), 0.75 (t, 2ꢀH, BCH₂)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 34.1

(s)ꢀppm.

Synthesis of 2-n-butyl-1,3,2-diazaborolidine.^[51]To a solution of n-bu-

tylbis(dimethylamino)borane (9.24ꢀg, 60.2ꢀmmol) in toluene (60ꢀmL) at 0ꢀ°C

ethylenediamine (4.4ꢀmL, 66ꢀmmol) was added slowly and dropwise. The

mixture was allowed to reach ambient temperature while stirring overnight.

Removal of the volatiles in vacuo yielded a colorless oil in quantitative yield.

¹H NMR (400ꢀMHz, CDCl₃) δ = 3.30 (s, 4ꢀH, N–C₂H₄–N), 2.60 (br, 2ꢀH),

1.24–1.35 (m, 4ꢀH), 0.87 (t, 3ꢀH), 0.70 (s, 2ꢀH)ꢀppm. ¹¹B{¹H} NMR (128ꢀMHz,

CDCl₃) δ = 35.7 (s)ꢀppm.

Synthesis of 4-n-butyl-1-trimethylsilylbenzene.^[54]To a solution of 1-

bromo-4-trimethylsilylbenzene (11.59ꢀg, 50.61ꢀmmol) in THF (100ꢀmL) held

at ambient temperature with a water bath n-BuLi in hexanes (24ꢀmL, 2.5ꢀM,

60ꢀmmol) was added dropwise. After 30ꢀmin, 1-bromobutane (5.3ꢀmL,

50ꢀmmol) was added and the mixture was stirred overnight. Water (50ꢀmL)

and diethylether (100ꢀmL) were added and the phases were separated.

The aqueous phase was extracted with diethylether (3 x 20ꢀmL) and the

combined organic phases were washed with brine. After drying with

MgSO₄and filtration, the solvent was removed with a rotary evaporator.

Further drying in vacuo yielded the product as a colorless oil (10.12ꢀg,

49.13ꢀmmol, 97 %). ¹H NMR (400ꢀMHz, CDCl₃): δ = 7.45 (d, 2ꢀH, aryl), 7.19

(d, 2ꢀH, aryl), 2.61 (t, 2ꢀH, butyl-C1), 2.27 (m_c, 2ꢀH, butyl-C2), 1.38 (m_c, 2ꢀH,

butyl-C3), 0.94 (t, 3ꢀH, butyl-C4), 0.27 (s, 9ꢀH, Si(CH₃)₃)ꢀppm.

Synthesis of 2-n-butyl-1,3-bis(trimethylsilyl)-1,3,2-diazaborolidine

(7b). To a solution of 2-n-butyl-1,3,2-diazaborolidine (7.53ꢀg, 60.0ꢀmmol) in

n-pentane (110ꢀmL) and diethyl ether (55ꢀmL) at −78ꢀ°C n-BuLi in hexanes

(2.5ꢀM, 55.2ꢀmL, 138ꢀmmol) was added dropwise. The mixture was stirred

for 2ꢀh at −78ꢀ°C and another 1ꢀh at ambient temperature. It was cooled to

−78ꢀ°C again and chlorotrimethylsilane (18.1ꢀmL, 138ꢀmmol) was added.

The mixture was allowed to reach ambient temperature while stirring over-

night. The volatiles were removed in vacuo and n-pentane (50ꢀmL) was

added to the residue. It was filtered off and the volatiles removed in vacuo.

Distillation at 0.1ꢀmbar and 60ꢀ°C yielded 7b as a colorless oil (12.97ꢀg,

Synthesis of 4-n-butylphenyldichloroborane (2d). To a solution of BCl₃

(18.24ꢀg, 155.7ꢀmmol) in CH₂Cl₂(120ꢀmL) at −78ꢀ°C 4-n-butyl-1-trimethyl-

silylbenzene (21.593ꢀg, 104.62ꢀmmol) was added dropwise. The solution

changed over yellow to a dark orange over time. The mixture was allowed

to reach ambient temperature while stirring overnight. Removal of the vol-

atiles and distillation at 7ꢀ·ꢀ10⁻²mbar and 55ꢀ°C yielded the product as a

colorless oil in quantitative yield. ¹H NMR (400ꢀMHz, CDCl₃): δ = 8.07 (d,

2ꢀH, aryl), 7.30 (d, 2ꢀH, aryl), 2.70 (t, 2ꢀH, butyl-C1), 1.64 (m_c, 2ꢀH, butyl-

C2), 1.38 (m_c, 2ꢀH, butyl-C3), 0.95 (t, 3ꢀH, butyl-C4)ꢀppm; ¹¹B{¹H} NMR

(128ꢀMHz, CDCl₃): δ = 54.9 (s)ꢀppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃): δ =

1

48.0ꢀmmol, 80 %). H NMR (400ꢀMHz, CDCl₃) δ = 3.21 (s, 4ꢀH, N–C₂H₄–

N), 1.24–1.38 (m, 4ꢀH), 0.90 (t, 3ꢀH), 0.70–0.77 (m, 2ꢀH), 0.13 (s, 18ꢀH)ꢀppm.

¹¹B{¹H} NMR (128ꢀMHz, CDCl₃) δ = 37.7ꢀppm; ¹³C{¹H} NMR (101ꢀMHz,

CDCl₃): δ = 47.8 (s, N–C₂H₄–N), 28.8 (s, butyl-C2), 26.4 (s, butyl-C3), 13.9

(s, butyl-C4), 13.8 (br, butyl-C1), 0.6 (s, Si(CH₃)₃)ꢀppm; ²⁹Si{¹H} NMR

10.1002/chem.201705913

Chemistry - A European Journal

FULL PAPER

151.3 (s, para-C), 137.1 (s, ortho-C), 131.4 (br, ipso-C), 128.4 (s, meta-C),

36.0 (s, butyl-C1), 33.1 (s, butyl-C2), 22.3 (s, butyl-C3), 13.9 (s, butyl-

C4)ꢀppm.

NMR data for 11a^Cl: ¹H NMR (400ꢀMHz, CDCl₃): δ = 4.00–2.80 (m, 4 H; N-

C₂H₄-N), 1.80–0.60 (m, 34 H; alkyl), 0.19–0.13 (m, 0.16 H; Si-(CH₃)₃end

group) ppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 31.1 (br) ppm.

For the temperature-dependent experiments the reaction mixtures were

heated to 40ꢀ°C or 61ꢀ°C, respectively, over the duration of the reaction.

The experiment at 61ꢀ°C was conducted in CDCl₃.

Synthesis of 4-n-butylphenylbis(dimethylamino)borane. Dimethyl-

amine (ca. 360ꢀmmol) was slowly recondensed into a solution of (4-n-butyl-

phenyl)dichloroborane (13.0850ꢀg, 62.103ꢀmmol) in n-pentane (120ꢀmL) at

−78ꢀ°C. The mixture was allowed to reach ambient temperature while

stirring overnight. The precipitate was filtered off and washed with n-pen-

tane (3 x 20ꢀmL). Removal of the volatiles in vacuo yielded the product as

a colorless oil (13.55ꢀg, 58.37ꢀmmol, 94 %). ¹H NMR (400ꢀMHz, CDCl₃): δ

= 7.26 (d, 2ꢀH, aryl), 7.12 (d, 2ꢀH, aryl), 2.66 (s, 12ꢀH, N(CH₃)₂), 2.60 (t, 2ꢀH,

butyl-C1), 2.30 (m_c, 2ꢀH, butyl-C2), 1.38 (m_c, 2ꢀH, butyl-C3), 0.94 (t, 3ꢀH,

butyl-C4)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 32.8 (s)ꢀppm.

Synthesis of 11'a^NMe2. To a solution of 16 (507.7ꢀmg, 1.000ꢀmmol) in

CH₂Cl₂(0.5ꢀmL) a solution of 8a (283.8ꢀmg, 1.000ꢀmmol) in CH₂Cl₂

(0.5ꢀmL) was added at ambient temperature. After 24h, all volatiles were

removed in vacuo, yielding the intermediate product 11'a^Br. ¹H NMR

(400ꢀMHz, CDCl₃): δ = 3.66–2.87 (m, 4ꢀH, N–C₂H₄–N), 1.80–0.79 (m, 34ꢀH,

alkyl), 0.29 (s, 0.77ꢀH, Sn(CH₃)₃end group) ppm; ¹¹B{¹H} NMR (128ꢀMHz,

CDCl₃): δ = 37.3 (s) ppm; GPC (THF): M_n= 1.73ꢀkDa, M_W= 2.00ꢀkDa.

Synthesis of 2-(4-n-butylphenyl)-1,3,2-diazaborolidine. To a solution of

(4-n-butylphenyl)-bis(dimethylamino)borane (13.30ꢀg, 58.37ꢀmmol) in tolu-

ene (60ꢀmL) at −78ꢀ°C ethylenediamine (4.1ꢀmL, 61ꢀmmol) was added

slowly and dropwise. The mixture was allowed to reach ambient tempera-

ture while stirring overnight. Removal of the volatiles in vacuo yielded a

highly viscous liquid. The product was used without further purification. ¹H

NMR (400ꢀMHz, CDCl₃): δ = 7.47 (d, 2ꢀH, aryl), 7.19 (d, 2ꢀH, aryl), 3.51 (s,

4ꢀH, N–C₂H₄–N), 3.07 (br, 2ꢀH, NH), 2.64 (t, 2ꢀH, butyl-C1), 1.63 (m_c, 2ꢀH,

butyl-C2), 1.39 (m_c, 2ꢀH, butyl-C3), 0.95 (t, 3ꢀH, butyl-C4)ꢀppm; ¹¹B{¹H}

NMR (128ꢀMHz, CDCl₃): δ = 33.1 (s)ꢀppm.

11'a^Brwas dissolved in CH₂Cl₂(1mL) and treated with pentamethylsil-

azane (ca. 0.2ꢀmmol) at ambient temperature to deactivate the end groups.

After 30ꢀmin, all volatiles were removed in vacuo yielding 11'a^NMe2. ¹H NMR

(400ꢀMHz, CDCl₃): δ = 3.92–2.82 (m, 4ꢀH, N–C₂H₄–N), 2.82–2.54 (br,

0.09ꢀH, N(CH₃)₂end group), 1.75–0.60 (m, 34ꢀH, alkyl), 0.28 (s, 1.16ꢀH,

Sn(CH₃)₃end group)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 36.7 (br)

ppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃): δ = 48.9 (br, N–C₂H₄–N), 33.7 (s,

CH₂), 33.5 (s, CH₂), 32.0 (s, CH₂), 29.7 (s, CH₂), 29.5 (s, CH₂), 27.4 (br,

CH₂), 27.1 (br, CH₂), 25.9 (br, CH₂), 25.6 (br, CH₂), 22.7 (s, CH₂), 18.7 (br,

B–CH₂), 15.9 (br, B–CH₂), 14.1 (s, CH₃), -6.1 (s, Sn(CH₃)₃end group)ꢀppm;

GPC (THF): M_n= 1.77 kDa, M_W= 1.98ꢀkDa.

Synthesis of 2-(4-n-butylphenyl)-1,3-bis(trimethylsilyl)-1,3,2-diaza-

borolidine (7d). To a solution of 2-(4-n-butylphenyl)-1,3,2-diazaborolidine

(ca. 58ꢀmmol) in CH₂Cl₂(180ꢀmL) triethylamine (34ꢀmL, 248ꢀmmol) was

added. After 20ꢀmin chlorotrimethylsilane (17.3ꢀmL, 136ꢀmmol) was added

while a large amount of precipitate formed. After 60ꢀh the most part of the

precipitate had dissolved. The volatiles were removed in vacuo and n-pen-

tane (60ꢀmL) was added to the residue. The mixture was filtered off and

washed with n-pentane (2 x 60ꢀmL). Removal of the volatiles and distilla-

Synthesis of 11a^NMe2by activation of 14 with HCl. To a solution of 14

(0.1870ꢀg, 0.4437ꢀmmol) in CH₂Cl₂(0.35ꢀmL) a solution of HCl in diethyl

ether (0.44ꢀmL, 2ꢀM, 0.88ꢀmmol) was added at ambient temperature. It was

stirred for 1ꢀd and the volatiles were removed in vacuo. n-Pentane (1ꢀmL)

was added to the residue and it was filtered off and washed with n-pentane

(2 x 0.5ꢀmL). The volatiles were removed in vacuo, yielding the amber

colored product. ¹H NMR (400ꢀMHz, CDCl₃): δ = 3.87–2.90 (m, 4ꢀH, N–

C₂H₄–N), 2.70 (br, 0.50ꢀH, N(CH₃)₂end groups), 1.78–0.56 (m, 34ꢀH, alkyl),

0.17 (s, 0.11ꢀH, Si(CH₃)₃end groups)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz,

CDCl₃): δ = 38.9 (br), 34.1 (sh) ppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃): δ =

49.1 (s, N–C₂H₄–N), 33.7–32.5 (m, CH₂), 32.0 (s, CH₂), 30.2–29.2 (m,

CH₂), 26.6–23.8 (m, CH₂), 22.7 (s, CH₂), 16.7 (br, B–CH₂), 16.1 (br, B–

CH₂), 14.1 (s, CH₃), 1.0 (s, Si(CH₃)₃end group), 0.8 (s, Si(CH₃)₃end

group)ꢀppm; GPC (THF): M_n= 1.75ꢀkDa, M_W= 2.10ꢀkDa.

1

tion at 5 · 10⁻²mbar and 110ꢀ°C yielded 7d in quantitative yield. H NMR

(400ꢀMHz, CDCl₃): δ = 7.17 (d, 2ꢀH, aryl), 7.06 (d, 2ꢀH, aryl), 4.19 (s, 4ꢀH,

N–C₂H₄–N), 2.59 (t, 2ꢀH, butyl-C1), 1.60 (m_c, 2ꢀH, butyl-C2), 1.33 (m_c, 2ꢀH,

butyl-C3), 0.92 (t, 3ꢀH, butyl-C4), −0.14 (s, 18ꢀH, Si(CH₃)₃)ꢀppm; ¹¹B{¹H}

NMR (128ꢀMHz, CDCl₃): δ = 36.6 (s)ꢀppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃):

δ = 144.7 (s, para-C), 136.0 (br, ipso-C), 132.1 (s, ortho-C), 127.1 (s, meta-

C), 48.1 (s, N–C₂H₄–N), 35.6 (s, butyl-C1), 33.7 (s, butyl-C2), 22.2 (s, bu-

tyl-C3), 14.0 (s, butyl-C4), 0.4 (s, Si(CH₃)₃)ꢀppm; ²⁹Si{¹H} NMR (79.5ꢀMHz,

CDCl₃): δ = 4.5 (s)ꢀppm; MS (EI, 70ꢀeV): m/z (%) = 346.4 ([M⁺], 32), 331.4

([M^+ꢀ−ꢀCH₃], 100), 147.2 ([M^+ꢀ−ꢀC₈H₂₂BNSi₂], 35), 73.3 ([C₃H₉Si⁺], 24).

Synthesis of 11a^NMe2by activation of 14 with HOTf in CDCl₃. To a solu-

tion of 14 (42ꢀmg, 0.10ꢀmmol) in CDCl₃(0.25ꢀmL) in a Young tube a solution

of HOTf (15ꢀmg, 0.10ꢀmmol) in CDCl₃(0.25ꢀmL) was added. The mixture

was shaken and a precipitate formed. ¹H NMR indicated that a partial reac-

tion had taken place (see Figure S65). Another portion of HOTf (15ꢀmg,

0.10ꢀmmol) in CDCl₃(0.2ꢀmL) was added. After 4ꢀd the volatiles were re-

moved in vacuo and n-pentane (1ꢀmL) was added to the residue. It was

filtered off and washed with n-pentane (2 x 0.25ꢀmL). The ¹H and ¹⁹F{¹H}

NMR of the precipitate confirmed its constitution as [Me₂NH₂]OTf (¹H NMR

(400ꢀMHz, CD₂Cl₂): δ = 7.51 (br, 2ꢀH, NH₂), 2.79 (s, 6ꢀH, N(CH₃)₂)ꢀppm;

¹⁹F{¹H} NMR (400ꢀMHz, CD₂Cl₂) δ = −79 (s)ꢀppm).^[55]From the filtrate the

volatiles were removed in vacuo, yielding the amber colored intermediate

Synthesis of 11a^{NMe2 [24]}

To a solution of 7a (326.5ꢀmg, 1.000ꢀmmol) in

.

CH₂Cl₂(0.5ꢀmL) a solution of 2a (194.9ꢀmg, 1.000ꢀmmol) or 8a (283.8ꢀmg,

1.000ꢀmmol) in CH₂Cl₂(0.5ꢀmL) was added at ambient temperature. After

24ꢀh, pentamethylsilazane (ca. 0.2ꢀmmol) was added to deactivate the

reactive end groups. After another 30ꢀmin, the volatiles were removed in

vacuo. ¹H NMR (400ꢀMHz, CDCl₃): δ = 3.66–2.87 (m, 4ꢀH, N–C₂H₄–N),

2.80–2.55 (m, 0.09ꢀH, N(CH₃)₂end group), 1.80–0.60 (m, 34ꢀH, alkyl),

0.19–0.13 (m, 0.10ꢀH, Si(CH₃)₃end group)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz,

CDCl₃): δ = 31.1 (br)ꢀppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃): δ = 48.8 (s, N–

C₂H₄–N), 35.5 (s, N(CH₃)₂end group, from ¹H,¹³C HSQC NMR measure-

ment), 33.7 (s, CH₂), 33.5 (s, CH₂), 32.1 (s, CH₂), 32.0 (s, CH₂), 29.9 (s,

CH₂), 29.7 (s, CH₂), 29.6 (s, CH₂), 29.5 (s, CH₂), 27.4 (s, CH₂), 25.9 (s,

CH₂), 22.8 (s, CH₂), 22.7 (s, CH₂), 18.7 (br, B–CH₂), 15.9 (br, B–CH₂), 14.1

(s, CH₃), 0.8 (s, Si(CH₃)₃end group)ꢀppm; UV/vis (THF): λ_max= 269ꢀnm (ε

1

product. H NMR (400ꢀMHz, CDCl₃): δ = 3.78–2.94 (m, 4ꢀH, N–C₂H₄–N),

1.73–0.62 (m, 34ꢀH, alkyl), 0.17 (s, 0.07ꢀH, Si(CH₃)₃end groups)ꢀppm;

¹¹B{¹H} NMR (128ꢀMHz, CD₂Cl₂): δ = 37.2 (br), 32.4 (s) ppm; GPC (THF):

M_n= 2.20ꢀkDa, M_W= 2.67ꢀkDa.

The product was dissolved in CH₂Cl₂(1mL) and treated with pentamethyl-

silazane (ca. 0.2ꢀmmol) at ambient temperature to deactivate the end

groups. After 30ꢀmin, all volatiles were removed in vacuo. ¹H NMR

(400ꢀMHz, CDCl₃): δ = 4.10–2.95 (m, 4ꢀH, N–C₂H₄–N), 2.86–2.60 (m,

0.61ꢀH, N(CH₃)₂end group), 1.78–0.62 (m, 34ꢀH, alkyl), 0.23–0.13 (m,

=

0.86ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹); DLS (n-pentane): R_h

= 2.2ꢀnm, D =

4.1ꢀ·ꢀ10⁻¹⁰ꢀm²ꢀs⁻¹; DOSY NMR (CDCl₃): D = 4.4ꢀ·ꢀ10⁻¹⁰ꢀm²ꢀs⁻¹; GPC (THF):

M_n= 1.78ꢀkDa, M_W= 1.94ꢀkDa; DSC: T_g= −71ꢀ°C.

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0.08ꢀH, Si(CH₃)₃end group)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 37.3

(br), 32.6 (br) ppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃): δ = 48.9 (br, N–C₂H₄–

N), 33.5 (s, CH₂), 33.0 (s, CH₂), 32.0 (s, CH₂), 29.6 (s, CH₂), 29.4 (s, CH₂),

27.0 (br, CH₂), 25.2 (br, CH₂), 22.7 (s, CH₂), 18.4 (br, B–CH₂), 16.3 (br, B–

CH₂), 14.1 (s, CH₃), 1.0 (s, Si(CH₃)₃end group)ꢀppm; GPC (THF): M_n=

2.24 kDa, M_W= 2.86ꢀkDa.

1.00ꢀH, Si(CH₃)₃end group) ppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 34.0

(br) ppm; UV/vis (THF): λ_max= 234ꢀ(ε = 0.14ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹), 265ꢀ(ε =

0.11ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹), 305ꢀ(ε

=

0.052ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹), 340 (ε

=

0.043ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹)ꢀnm; DLS (n-pentane): R_h

4.4ꢀ·ꢀ10⁻¹⁰ꢀm²ꢀs⁻¹; DSC: T_g= −20.1ꢀ°C.

= 2.0ꢀnm, D

Synthesis of 11c^Cl. To a solution of 7c (290.4ꢀmg, 1.000ꢀmmol) in CH₂Cl₂

(0.5ꢀmL) a solution of 2c (158.8ꢀmg, 1.000ꢀmmol) in CH₂Cl₂(0.5ꢀmL) was

added at ambient temperature. Immediately after the mixing of the reac-

tants, an insoluble colorless precipitate formed. ¹H NMR of the solution

showed evidence for Me₃SiCl formation. H NMR (400ꢀMHz, CDCl₃): δ =

0.45 (s, 9ꢀH, ClSi(CH₃)₃).

Synthesis of 11a^NMe2by activation of 14 with HOTf in CD₂Cl₂. To a so-

lution of 14 (42ꢀmg, 0.10ꢀmmol) in CD₂Cl₂(0.25ꢀmL) in a Young tube a so-

lution of HOTf (15ꢀmg, 0.10ꢀmmol) in CD₂Cl₂(0.25ꢀmL) was added at am-

bient temperature. While the mixture was shaken a precipitate formed. ¹H

NMR indicated that a partial reaction had taken place. Another portion of

HOTf (15ꢀmg, 0.10ꢀmmol) in CDCl₃(0.2ꢀmL) was added. After 1ꢀd, the vol-

atiles were removed in vacuo and n-pentane (1ꢀmL) was added to the res-

idue. It was filtered off and washed with n-pentane (2 x 0.25ꢀmL). The vola-

tiles were removed from the filtrate in vacuo, yielding the amber colored

intermediate product. ¹H NMR (400ꢀMHz, CDCl₃): δ = 3.71–2.94 (m, 4ꢀH,

N–C₂H₄–N), 1.82–0.62 (m, 34ꢀH, alkyl), 0.17 (s, 0.02ꢀH, Si(CH₃)₃end

groups)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CD₂Cl₂): δ = 37.1 (br), 34.1 (br) ppm;

GPC (THF): M_n= 1.58ꢀkDa, M_W= 1.79ꢀkDa.

1

Synthesis of 11ac^NMe2. To a solution of 7a (326.5ꢀmg, 1.000ꢀmmol) in

CH₂Cl₂(0.5ꢀmL) a solution of 2c (158.8ꢀmg, 1.000ꢀmmol) in CH₂Cl₂

(0.5ꢀmL) was added at ambient temperature. After 24ꢀh, pentamethylsil-

azane (ca. 0.2ꢀmmol) was added to deactivate the reactive end groups.

After another 30ꢀmin, the volatiles were removed in vacuo. ¹H NMR

(400ꢀMHz, CDCl₃): δ = 7.83–6.70 (m, 5ꢀH, aryl), 4.00–2.16 (m, 4.30ꢀH, N–

C₂H₄–N & N(CH₃)₂end group), 1.92–0.51 (m, 17ꢀH, alkyl), 0.20–0.04 (m,

0.54ꢀH, Si(CH₃)₃end group) ppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 25.7

(br) ppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃): δ = 133.7 (br, phenyl), 128.4 (br,

phenyl), 127.0 (br, phenyl), 126.2 (br, phenyl), 50.0 (br, N–C₂H₄–N), 33.4

(br, CH₂), 32.0 (s, CH₂), 29.7 (s, CH₂), 29.4 (s, CH₂), 26.9 (br, CH₂), 25.5

(br, CH₂), 22.7 (s, CH₂), 18.5 (br, B–CH₂), 15.6 (br, B–CH₂), 14.1 (s, CH₃),

0.76–0.57 (m, Si(CH₃)₃end group)ꢀppm; ²⁹Si{¹H} NMR (79.5ꢀMHz, CDCl₃):

δ = 4.7 (s)ꢀppm; UV/vis (THF): λ_max= 265 (ε = 0.11ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹), 285

(ε = 0.12ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹), 339 (ε = 0.028ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹) ꢀnm; DLS

(CH₂Cl₂): R_h= 2.2ꢀnm, D = 2.2ꢀ·ꢀ10⁻¹⁰ꢀm²ꢀs⁻¹; DOSY NMR (CDCl₃): D =

2.2ꢀ·ꢀ10⁻¹⁰ꢀm²ꢀs⁻¹; GPC (THF): M_n= 2.50ꢀkDa, M_W= 3.77ꢀkDa; DSC: T_g=

0.1ꢀ°C.

The intermediate product was dissolved in CH₂Cl₂(1mL) and treated with

pentamethylsilazane (ca. 0.2ꢀmmol) at ambient temperature to deactivate

the end groups. After 30ꢀmin, all volatiles were removed in vacuo. ¹H NMR

(400ꢀMHz, CDCl₃): δ = 3.68–2.99 (m, 4ꢀH, N–C₂H₄–N), 2.91–2.46 (m,

3.16ꢀH, N(CH₃)₂end group), 1.73–0.60 (m, 34ꢀH, alkyl), 0.23–0.12 (m,

0.70ꢀH, Si(CH₃)₃end group)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 36.6

(br) ppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃): δ = 48.8 (br, N–C₂H₄–N), 33.5

(s, CH₂), 33.3 (s, CH₂), 32.0 (s, CH₂), 29.7 (s, CH₂), 29.4 (s, CH₂), 26.9 (br,

CH₂), 26.4 (br, CH₂), 25.7 (br, CH₂), 25.2 (br, CH₂), 22.7 (s, CH₂), 18.1 (br,

B–CH₂), 15.8 (br, B–CH₂), 14.1 (s, CH₃), 0.8 (s, Si(CH₃)₃end group)ꢀppm;

GPC (THF): M_n= 1.64 kDa, M_W= 1.85ꢀkDa.

Synthesis of 11ca^Cl. To a solution of 7c (290.4ꢀmg, 1.000ꢀmmol) in CH₂Cl₂

(0.5ꢀmL) a solution of 2a (194.9ꢀmg, 1.000ꢀmmol) in CH₂Cl₂(0.5ꢀmL) was

added at ambient temperature. Several hours after mixing of the reactants,

an insoluble colorless precipitate formed. ¹H NMR of the overlaying solu-

tion showed evidence for Me₃SiCl formation. ¹H NMR (400ꢀMHz, CDCl₃): δ

= 0.45 (s, 9ꢀH, ClSi(CH₃)₃).

Synthesis of 11a^NMe2by activation of 14 with 1 equivalent of HNTf₂in

CD₂Cl₂. To a solution of the 14 (42ꢀmg, 0.10ꢀmmol) in CD₂Cl₂(0.25ꢀmL) in

a Young tube a solution of HNTf₂(28ꢀmg, 0.10ꢀmmol) in CD₂Cl₂(0.25ꢀmL)

was added at ambient temperature and the mixture was shaken. After 1ꢀd

the volatiles were removed in vacuo and n-pentane (1ꢀmL) was added to

the residue. It was filtered off and washed with n-pentane (2 x 0.25ꢀmL).

The volatiles were removed from the filtrate in vacuo, yielding the amber

colored intermediate product. H NMR (400ꢀMHz, CDCl₃): δ = 3.65–3.01

(m, 4ꢀH, N–C₂H₄–N), 2.70 (br, 0.69ꢀH, N(CH₃)₂end groups), 1.65–0.58 (m,

34ꢀH, alkyl), 0.15 (s, 1.20ꢀH, Si(CH₃)₃end groups)ꢀppm; ¹¹B{¹H} NMR

Synthesis of 11bc^Cl. To a solution of 7b (270.4ꢀmg, 1.000ꢀmmol) in CH₂Cl₂

(0.5ꢀmL) a solution of 2c (158.8ꢀmg, 1.000ꢀmmol) in CH₂Cl₂(0.5ꢀmL) was

added at ambient temperature. Several hours after mixing of the reactants,

an insoluble colorless precipitate formed. ¹H NMR of the overlaying solu-

tion showed evidence for Me₃SiCl formation. ¹H NMR (400ꢀMHz, CDCl₃): δ

= 0.45 (s, 9ꢀH, ClSi(CH₃)₃).

1

(128ꢀMHz, CD₂Cl₂): δ = 35.8 (br), 31.4 (sh) ppm; GPC (THF): M_n

1.91ꢀkDa, M_W= 2.33ꢀkDa.

=

The intermediate product was dissolved in CH₂Cl₂(1mL) and treated with

pentamethylsilazane (ca. 0.2ꢀmmol) at ambient temperature to deactivate

the end groups. After 30ꢀmin, all volatiles were removed in vacuo. ¹H NMR

(400ꢀMHz, CDCl₃): δ = 3.68–3.02 (m, 4ꢀH, N–C₂H₄–N), 2.88–2.51 (m,

1.94ꢀH, N(CH₃)₂end group), 1.67–0.60 (m, 34ꢀH, alkyl), 0.19–0.16 (m,

0.10ꢀH, Si(CH₃)₃end group)ꢀppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 36.7

(br), 31.1 (sh)ꢀppm; ¹³C{¹H} NMR (101ꢀMHz, CDCl₃): δ = 48.9 (br, N–C₂H₄–

N), 33.5 (s, CH₂), 33.2 (s, CH₂), 32.0 (s, CH₂), 29.7 (s, CH₂), 29.4 (s, CH₂),

27.5 (br, CH₂), 27.1 (br, CH₂), 25.6 (br, CH₂), 25.2 (br, CH₂), 22.7 (s, CH₂),

18.0 (br, B–CH₂), 15.8 (br, B–CH₂), 14.1 (s, CH₃), 0.8 (s, Si(CH₃)₃end

group)ꢀppm; GPC (THF): M_n= 1.35ꢀkDa, M_W= 1.62ꢀkDa.

Synthesis of 11cb^Cl. To a solution of 7c (290.4ꢀmg, 1.000ꢀmmol) in CH₂Cl₂

(0.5ꢀmL) a solution of 2b (138.8ꢀmg, 1.000ꢀmmol) in CH₂Cl₂(0.5ꢀmL) was

added at ambient temperature. Shortly after mixing of the reactants, an

insoluble colorless precipitate formed. H NMR of the overlaying solution

showed evidence for Me₃SiCl formation. ¹H NMR (400ꢀMHz, CDCl₃): δ =

1

0.45 (s, 9ꢀH, ClSi(CH₃)₃).

Synthesis of 11d^NMe2. To a solution of 7d (346.5ꢀmg, 1.000ꢀmmol) in

CH₂Cl₂(0.5ꢀmL) a solution of 2d (214.9ꢀmg, 1.000ꢀmmol) in CH₂Cl₂

(0.5ꢀmL) was added at ambient temperature. After 24ꢀh, pentamethylsil-

azane (ca. 0.2ꢀmmol) was added to deactivate the reactive end groups.

After another 30ꢀmin the volatiles were removed in vacuo. The product

11d^NMe2was mostly insoluble in n-pentane, which was used to wash

11d^NMe2several times. ¹H NMR (400ꢀMHz, CDCl₃): δ = 7.68–6.13 (m, 8ꢀH,

aryl), 3.43–1.93 (m, 8ꢀH, N–C₂H₄–N & butyl-CH₂& N(CH₃)₂end group),

1.93–0.64 (m, 14ꢀH, alkyl), −0.19–(−0.29) (m, 0.71ꢀH, Si(CH₃)₃end group)

ppm; ¹¹B{¹H} NMR (128ꢀMHz, CDCl₃): δ = 32.4 (br) ppm; ¹³C{¹H} NMR

Synthesis of 11b^NMe2. To a solution of 7b (270.4ꢀmg, 1.000ꢀmmol) in

CH₂Cl₂(0.5ꢀmL) a solution of 2b (138.8ꢀmg, 1.000ꢀmmol) in CH₂Cl₂

(0.5ꢀmL) was added at ambient temperature. After 24ꢀh, pentamethylsil-

azane (5; ca. 0.2ꢀmmol) was added to deactivate the reactive end groups.

After another 30ꢀmin, the volatiles were removed in vacuo. ¹H NMR

(400ꢀMHz, CDCl₃): δ = 3.66–2.87 (m, 4ꢀH, N–C₂H₄–N), 2.83–2.53 (m,

0.04ꢀH, N(CH₃)₂end group), 1.80–0.60 (m,ꢀ18ꢀH, alkyl), 0.21–0.12 (m,

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(101ꢀMHz, CDCl₃): δ = 134.6 (br, aryl), 126.4 (br, aryl), 63.3 (br, N–C₂H₄–

N), 35.9 (s, butyl-C1), 33.5 (s, butyl-C2), 22.5 (s, butyl-C3), 14.0 (s, butyl-

C4); UV/vis (THF): λ_max= 231 (ε = 0.26ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹), 302ꢀ(ε =

Baumgartner, RSC Adv. 2013, 3, 11334–11350; h) A. Wakamiya, S.

Yamaguchi, Bull. Chem. Soc. Jpn. 2015, 88, 1357–1377; i) F. Jäkle, Top.

Organomet. Chem. 2015, 49, 297–325; j) H. Helten, Chem. Eur. J. 2016,

22, 12972–12982; k) L. Ji, S. Griesbeck, T. B. Marder, Chem. Sci. 2017,

8, 846–863; l) S.-Y. Li, Z.-B. Sun, C.-H. Zhao, Inorg. Chem. 2017, 56,

8705–8717; m) H. Helten, “Conjugated Inorganic–Organic Hybrid

Polymers”, in Encyclopedia of Inorganic and Bioinorganic Chemistry

(Ed.: R. A. Scott), John Wiley, Chichester, 2017, DOI:

10.1002/9781119951438.eibc2496.

0.013ꢀ·ꢀ10^5ꢀLꢀcm⁻¹ꢀmol⁻¹)ꢀnm; DLS (CH₂Cl₂): R_h

=

1.5ꢀnm,

D

=

3.4ꢀ·ꢀ10⁻¹⁰ꢀm²ꢀs⁻¹; DOSY NMR (CDCl₃): D = 4.0ꢀ·ꢀ10⁻¹⁰ꢀm²ꢀs⁻¹; GPC (THF):

M_n= 1.73ꢀkDa, M_W= 2.29ꢀkDa; DSC: T_g= 15.8ꢀ°C.

Computational methods. DFT calculations were carried out with the

TURBOMOLE V7.0.1 program package.^[34]Optimizations were performed

with Becke’s three parameter exchange-correlation hybrid functional

B3LYP^[35]in combination with the valence-double-ζ basis set def2-

SV(P).^[36]The empirical dispersion correction DFT-D3 by Grimme was

used including the three-body term and with Becke-Johnson (BJ) damp-

ing.^[37]The stationary points were characterized as minima by analytical

vibrational frequency calculations,^[38]which revealed the absence of imagi-

nary frequencies.

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Pammer, J. Mater. Chem. C 2017, 5, 1730–1741; r) D. Marinelli, F.

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Acknowledgements

Funding by the German Research Foundation (DFG) through the

Emmy Noether Programme and the Research Grant HE 6171/4-

1 and support by the COST action CM1302 (SIPs) is gratefully

acknowledged. We thank Dr. K. Beckerle for GPC and DSC mea-

surements, Prof. Dr. W. Richtering for DLS access, and Prof. Dr.

J. Okuda for generous support and helpful discussions.

Keywords: BN compounds • boron • hybrid materials • inorganic

polymers • main group polymers

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Entry for the Table of Contents

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O. Ayhan, N. A. Riensch, C.

Glasmacher, H. Helten*

Page No. – Page No.

Cyclolinear Oligo- and

Poly(iminoborane)s: The Missing Link

in Inorganic Main Group

Macromolecular Chemistry

Well-defined oligo(iminoborane)s with up to 18 catenated BN units on average were

prepared by Si/B or Sn/B exchange polycondensation as well as initiated

polymerization of a dormant monomer. The cyclolinear backbone imparts high

stability and precludes the unwanted formation of borazine by-products. The

properties of the new materials are effectively tuned by variation of their side

groups.

Article Doi

Cyclolinear Oligo- and Poly(iminoborane)s: The Missing Link in Inorganic Main-Group Macromolecular Chemistry

DOI: 10.1002/chem.201705913

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