Selective synthesis of unsymmetrical diboryl ptii and diaminoboryl CuI Complexes by B-B activation of unsymmetrical diboranes(4) {pinB-B[(NR)2C6H4]}

Article abstract of DOI:10.1002/ejic.201402234

Diaminoboryl ligands are currently intensively investigated because of their unique coordination chemical properties, for example, as part of pincer ligands. Owing to synthetic restrictions, however, access to diaminoboryl complexes is limited. Unsymmetrical diborane(4) derivatives comprising a dialkoxy- and a diaminoboron moiety provide efficient access to diaminoboryl complexes either by oxidative addition or by σ-bond metathesis reactions. Especially, the latter is complementary to existing methods and overcomes the existing limitations. This is illustrated by the modular synthesis of four unsymmetrical diborane(4) derivatives and their application in the selective preparation of unprecedented unsymmetrical diboryl Pt^II complexes as well as sterically little encumbered diaminoboryl Cu^I complexes.

Full text of DOI:10.1002/ejic.201402234

SHORT COMMUNICATION
DOI:10.1002/ejic.201402234
Selective Synthesis of Unsymmetrical Diboryl PtII and
Diaminoboryl CuI Complexes by B–B Activation of
Unsymmetrical Diboranes(4) {pinB–B[(NR)2C6H4]}
Corinna Borner[a] and Christian Kleeberg*[a]
Keywords: Boron / B–B activation / Boryl complexes / Copper / Platinum
Diaminoboryl ligands are currently intensively investigated
because of their unique coordination chemical properties, for
example, as part of pincer ligands. Owing to synthetic restric-
tions, however, access to diaminoboryl complexes is limited.
Unsymmetrical diborane(4) derivatives comprising a dialk-
oxy- and a diaminoboron moiety provide efficient access to di-
aminoboryl complexes either by oxidative addition or by σ-
bond metathesis reactions. Especially, the latter is comple-
mentary to existing methods and overcomes the existing
limitations. This is illustrated by the modular synthesis of four
unsymmetrical diborane(4) derivatives and their application
in the selective preparation of unprecedented unsymmetrical
diboryl PtII complexes as well as sterically little encumbered
diaminoboryl CuI complexes.
porated in pincer-type ligand frameworks.[2,3] To exploit the
potential of diaminoboryl ligands II in full, versatile, flexi-
ble, and modular synthetic access to their metal complexes
as well as to the necessary boron precursors is required.
We envisaged that unsymmetrical diborane(4) derivatives,
consisting of one moiety I and one moiety II, would be
particularly suitable, as they should be converted into di-
aminoboryl complexes by either oxidative addition or for-
mal σ-bond metathesis reactions (routes 1 and 3). Such un-
symmetrical diboranes(4) have only occasionally been re-
ported together with the respective symmetrical com-
pounds. Only recently did a selective synthesis of the un-
symmetrical diborane(4) pinB–Bdan (pin = pinacolato, dan
= 1,8-diaminonaphthalene) appear in the scientific litera-
ture.[7] However, this compound was only used as a reagent
in borylation reactions.[7a,7b]
Introduction
Boryl ligands have been intensively studied for their
unique coordination chemical properties and because of
their crucial role as active intermediates of transition-metal-
catalyzed borylation reactions.[1–6] Whereas dialkoxyboryl
and diaryloxyboryl ligands [(RO)2B (I)] are particularly well
explored, diaminoboryl ligands [(RRЈN)2B (II)] have only
recently become a topic of intense research with potential
applications, for example, in catalysis.[1–4] Boryl complexes
of ligands of types I and II are generally synthesized by
(1) reaction of metal nucleophiles/bases or metals in a low
oxidation state with suitable boryl ligand precursors (e.g.,
containing B–X, B–H, or B–E bonds) following substitu-
tion or oxidative addition pathways,[1,3–5] (2) the reaction of
a boryl anion with a metal precursor,[1,2] and (3) heterolytic
cleavage (e.g., by σ-bond metathesis) of symmetrical dibor-
anes by transition-metal complexes (only for type I).[1,6]
However, all these methods suffer from inherent restrictions
on the type of metal precursor, from limitations of the boryl
anion synthesis to sterically bulky diaminoboryl moieties
II, or are restricted to boryl systems of type I.[1,5b] This is
especially unsatisfying, because ligands of type II are of
prime interest as their steric and electronic properties are
expected to be tunable by appropriate substitution, analo-
gous to the well-established isoelectronic and isostructural
N-heterocyclic carbene (NHC) ligands. Moreover, these li-
gand types are particularly interesting, as they can be incor-
Herein, we report a protocol for the synthesis of unsym-
metrical diboranes(4) of the type pinB–B[(NR)2C6H4] (R
= Me, Bn, TMS). The versatile protocol is modular, as it
introduces the R substituent at a late stage by a simple sub-
stitution reaction and, hence, may be adapted to other sub-
stituents R.
Results and Discussion
On the basis of the procedure described for the prepara-
tion of pinB–Bdan, the unsymmetrical diborane(4) pinB–
Bdab (1, dab = 1,2-diaminobenzene) was obtained selec-
tively from pinacol, 1,2-diaminobenzene, and B2(NMe2)4 in
47% yield (Scheme 1), with only little contamination of the
symmetrical compounds B2pin2 and B2dab2 (100:9:3 GC–
MS area ratio).[8] By reaction of 1 with tBuLi and suitable
electrophiles, the bis-N,NЈ-substituted derivatives pinB–
[a] Institut für Anorganische und Analytische Chemie, Technische
Universität Carolo-Wilhelmina zu Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
E-mail: ch.kleeberg@tu-braunschweig.de
https://www.tu-braunschweig.de/iaac/personal/kleeberg
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402234.
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SHORT COMMUNICATION
Bdmab [2a, dmab = 1,2-di(methylamino)benzene], pinB–
Bdbab [2b, dbab = 1,2-di(benzylamino)benzene], and pinB–
Bdtab [2c, dtab = 1,2-di(trimethylsilylamino)benzene] were
obtained (Scheme 1).[8–10] In an attempt to prepare 2a di-
rectly from pinacol, 1,2-di(methylamino)benzene, and
B2(NMe2)4 under the conditions established for 1, unsym-
metrical product 2a was only obtained in minor amounts
along with B2pin2 and B2dab2 (10:58:6 GC–MS area ra-
tio).[8] This suggested that the presence of the primary
amine functionality was crucial for the selective formation
of unsymmetrical diborane(4) compounds such as 1.
Scheme 2. Synthesis of diaminoboryl complexes 3a/b and 4a/b.
O2]: B–Pt 2.06–2.08 Å, P–Pt 2.35 Å, B–Pt–B 73.4–75.6°}
showed that the pinB–Pt and P2–Pt1 distances were in a
comparable range, whereas the B–Pt–B angles were slightly
smaller.[5b,5c] A comparison with the diaminoboryl com-
plexes [(PPh3)2Pt(B{(NiPr)2C6H4})(Me3Sn)] (5) [B–Pt
2.085(6) Å, trans-P–Pt 2.376(1) Å] and [(PPh3)2Pt{B-
(NMe2)2}(Me3E)] [E = Sn (6a): B–Pt 2.136(4) Å, trans-P–
Pt 2.369(1) Å; Ge (6b): B–Pt 2.139(6) Å, trans-P–Pt
2.377(1) Å] was also informative.[4a] Whereas similar B–Pt
distances were observed for 3a, 3b, and 5, a significantly
larger B–Pt distance was found for 6a/b, which suggested a
significant influence of the backbone of the diaminoboryl
ligand on its coordination properties.
Scheme 1. Synthesis of diboranes(4) 1 and 2a–c.
To demonstrate the use of 2a–c as precursors for di-
aminoboryl metal complexes, the novel unsymmetrical di-
boranes were treated with [(PPh3)2Pt(C2H4)] and [(IDipp)-
CuOtBu], respectively, two metal complexes known to form
boryl complexes readily upon reaction with tetraalkoxydi-
boranes [IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene].[5,6] Unprecedented unsymmetrical diboryl plati-
num complexes 3a and 3b were indeed formed selectively
from 2a and 2b, whereas 2c was unreactive under the condi-
tions, possibly because of steric bulk (Scheme 2). Quantita-
tive conversion into 3a and 3b, respectively, was only ob-
tained if the formed ethene was removed under reduced
pressure. The presence of an equilibrium between diborane
2a and oxidative addition product 3a was further corrobo-
rated by the increased amount of 2a observed by NMR
spectroscopy upon the addition of 1-hexene or diphenylace-
tylene to 3a.[5g,8] Noteworthy, an equilibrium between 2a
and 3a (and 2b and 3b, respectively) was also observed in
the absence of an additional ligand in C6D6 solution ac-
cording to the 1H–1H NOESY/EXSY spectroscopic data
[28:1 (3a/2a) and 14:1 (3b/2b) ratio].[11] Thus, the oxidative
addition of 2a/b to [(PPh3)2Pt(C2H4)] as well as the re-
ductive elimination of 2a/b from 3a/b was kinetically and
thermodynamically feasible. A similar equilibrium oxidative
addition was recently studied in some detail for the reaction
of B2(OMe)4 with [{P(Cy)3}2Pt] (Cy = cyclohexyl).[5a]
Figure 1. Molecular structure of 3a (a) and 3b (b). Selected bond
lengths and angles for 3a: Pt1–B1 2.078(3) Å, Pt1–B2 2.067(3) Å,
Pt1–P1 2.340(1) Å, Pt1–P2 2.332(1) Å, B1–B2 2.448(5) Å, B1–Pt1–
P1 163.10(8)°, B2–Pt1–P2 160.85(9)°, B1–Pt1–B2 72.4(1)°, P1–Pt1–
P2 103.72(3)°; for 3b: Pt1–B1 2.092(2) Å, Pt1–B2 2.080(2) Å, Pt1–
P1 2.3599(7) Å, Pt1–P2 2.3394(6) Å, B1–B2 2.465(3) Å, B1–Pt1–P1
161.65(7)°, B2–Pt1–P2 167.72(7)°, B1–Pt1–B2 72.41(9)°, P1–Pt1–
P2 102.23(2)°. For clarity, hydrogen atoms are omitted and only
ipso carbon atoms of the phenyl groups are shown; thermal ellip-
soids are drawn at the 50% probability level.[8,10]
The differences in the coordination properties, specifi-
The molecular structures of 3a/b were determined by sin- cally the different trans influence, between a diaminoboryl
gle-crystal X-ray structure analysis.[8,10] For both PtII com- and a dialkoxyboryl ligand should be reflected in the trans-
plexes, a distorted square-planar coordination is observed Pt–P distances.[4b,12] However, the observed differences are
(Figure 1). A comparison of the geometrical data of 3a/b to small and NMR spectroscopy, namely, the Pt–P coupling
symmetrical diboryl complexes of the type [(PPh3)2Pt- constants, is better suited to characterize the ligand proper-
(B(OR)2)2] {(OR)2 = pin, [C2H3(Ph)O2], [C2H2(CO2Me)2- ties.
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SHORT COMMUNICATION
195Pt,31P coupling constants within [(PPh3)2PtIIL2] com-
plexes have been widely used to probe the trans influence
of the L ligand trans to the respective phosphine ligand.[13]
With the aid of H–1H NOESY and H–31P HMBC NMR
spectroscopic data, the coupling constants were assigned to
JP,Pt(trans-Bdmab) = 1715 Hz and JP,Pt(trans-Bpin) =
1591 Hz for 3a and JP,Pt(trans-Bdbab) = 1589 Hz and
JP,Pt(trans-Bpin) = 1530 Hz for 3b.[8] These data suggest a
stronger trans influence of the Bpin ligand than the diami-
noboryl ligand. This does not correlate well with the order
of the trans influences of different boryl ligands [(RRЈN)2]-
BϾ pinBϾ catB (cat = catecholato) established both com-
putationally and structurally within the series [(PR3)2Pt-
(X)(BR2)] of complexes.[12,4b] At the same time, the differ-
ences in the trans-Bpin P,Pt coupling constants for 3a, 3b,
and [(Bpin)2Pt(PPh3)2] (JP,Pt = 1504 Hz) indicate a signifi-
cant influence of the cis-boryl ligand.[5b] Corroborating the
latter, significantly different trans-boryl coupling constants
of JP,Pt = 1392, 1200, and 1560 Hz were reported for 6a/b
and 5. This series illustrates also the influence of the back-
bone of the diaminoboryl ligand on its coordination prop-
erties (see above).[4a]
1
1
Figure 2. Molecular structure of 4a (a) and 4b (b). Selected bond
lengths and angles for 4a: Cu1–B1 1.995(4) Å, Cu1–C9 1.930(4) Å,
B1–Cu1–C9 174.8(2)°, angles between the mean planes (Cu1, B1,
N1, N2) and (Cu1, C9, N3, N4) 88.3(8)°; for 4b (molecule A/B;
numbering for molecule A only): Cu1–B1 1.994(4)/1.986(4) Å,
Cu1–C21 1.932(3)/1.926(3) Å, B1–Cu1–C21 172.1(1)/178.9(1)°,
angles between the mean planes (Cu1, B1, N1, N2) and (Cu1, C21,
N3, N4) 34.6(1)/55.7(1)°. Hydrogen atoms are omitted for clarity
and only ipso carbon atoms of the Dipp groups are shown. Ther-
mal ellipsoids are drawn at the 30 and 50% probability level for 4a
and 4b, respectively.[8,10]
tures of 4a and 4b (molecule A and B) are comparable with
respect to the approximate linear coordination of the Cu
atoms and the similar B–Cu and Ccarbene–Cu distances (Fig-
ure 2). However, the angles between the mean planes of the
diaminoboryl and the NHC ligand vary significantly and
are clearly not only dependent on the steric demand of the
boryl ligands but are rather governed by the minimization
of repulsive interactions between the peripheral substituents
at both ligands. Bond lengths and angles as well as the
chemical shifts in the 11B NMR spectra fit well in the series
of reported NHC–CuI boryl complexes with both dialk-
oxyboryl and sterically demanding diaminoboryl moieties:
[(IDipp)CuBpin] [Cu–B 2.002(3) Å, Cu–C 1.937(2) Å, C–
Cu–B 168.1(2)°], [(IMes)Cu(B{C2H2(DippN)2})] [Cu–B
1.980(2) Å, Cu–C 1.918(2) Å, C–Cu–B 179.43(9)°], and
[(IMes)Cu(B{C2H4(DippN)2})] [Cu–B 1.983(3) Å, Cu–C
1.915(3) Å, C–Cu–B 179.4(2)°].[6a,2d]
A detailed analysis of the coordination chemical proper-
ties of the (unsymmetrical) cis-diboryl platinum complexes,
especially regarding the trans influence and hence the bond-
ing situation (e.g., contribution of σ- and π-bonding, a pos-
sible p–p B–B interaction, influence of the ligand back-
bone) of the boryl ligands has to await more detailed, espe-
cially computational, studies.[3g,5]
It should be emphasized that neither during the synthesis
of 2a–c nor during the study of 3a/b was there evidence for
scrambling reactions; hence, the formation of symmetrical
diborane(4) compounds or PtII complexes thereof was not
observed. Nevertheless, the [(PPh3)2Pt(OBpin)(OBdbab)]
complex was characterized crystallographically as one of
the decomposition products of 3b upon exposure to air.[8]
Reaction of 2a and 2b with [(IDipp)CuOtBu] yielded se-
lectively diaminoboryl complexes 4a and 4b (Figure 2),
respectively, whereas for 2c, again no reaction was observed
by in situ NMR spectroscopy.[8] Moreover, from the reac-
tion of 2a with CuOtBu in the presence of PPh3, a few
crystals of the [Cu5(PPh3)2(OtBu)(Bdmab)4] cluster were
obtained, which proved that B–B bond activation was not
specific to the (IDipp)Cu fragment.[8,10] The selectivity of
the B–B bond cleavage reaction in favor of the formation
of diaminoboryl complexes may be explained by the higher
Lewis acidity of the Bpin moiety, which favored the forma-
tion of pinB–OtBu and diaminoboryl complexes 4a/b. This
was also indicated by the formation of the Lewis acid/base
adduct [K(18-crwon-6)(pin(tBuO)B–Bdmab)] upon reac-
tion of 2a with [K(18-crown-6)OtBu].[8,10]
Conclusions
In conclusion, unsymmetrical diborane(4) compounds of
the type pinB–B[(NR)2C6H4] (2a–c; R = Me, Bn, SiMe3)
were efficiently prepared by functionalization of parent
compound 1 (R = H) with electrophiles and provide facile
synthetic access to diaminoboryl complexes either by oxi-
dative addition or by selective σ-bond metathesis reactions.
The findings presented may facilitate the exploration of this
unique class of complexes by providing access to substitu-
tion patterns previously inaccessible by the boryllithium
route and/or complexes of metals that do not readily un-
dergo oxidative addition of B–X bonds.
Compound 4a crystallizes with one molecule in the
asymmetric unit in the orthorhombic space group type
Fdd2. The Bdmab moiety is disordered over two equally
occupied positions compromising slightly the geometrical
data obtained.[8,10] Complex 4b crystallizes in the triclinic
space group type P1 with two independent molecules in the
Supporting Information (see footnote on the first page of this arti-
cle): All experimental and spectroscopic details (including in situ
NMR experiments) as well as crystallographic and additional
asymmetric unit (molecule A/B).[8,10] The molecular struc- structural data.
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SHORT COMMUNICATION
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Acknowledgments
C. K. and C. B. thank the Fonds der Chemischen Industrie for
generous support through a Liebig stipend and a PhD stipend. The
authors thank BASF SE for a gift of B2(NMe2)4 and the Deutsche
Forschungsgemeinschaft (DFG) for financial support.
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Received: March 28, 2014
Published Online: April 23, 2014
Eur. J. Inorg. Chem. 2014, 2486–2489
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