Boryl-based pincer systems: New avenues in boron chemistry

Article abstract of DOI:10.1002/anie.200904795

"Chemical Equation Presented" To B or not to B? Incorporation of BH into tridentate ligands has recently allowed the isolation of the first borylbased pincer complexes. Functionalized diaminoborane and m-carbaborane (see picture) form Ir and Pd complexes,

Full text of DOI:10.1002/anie.200904795

Highlights
DOI: 10.1002/anie.200904795
Tridentate Boryl Ligands
Boryl-Based Pincer Systems: New Avenues in Boron
Chemistry**
Jarl Ivar van der Vlugt*
À
B H activation · boron · carboranes · aminoboranes ·
tridentate ligands
B
oron chemistry has long been dominated by main-group
One distinct role for boron in organometallic and
coordination chemistry has been as the central atom in
facially coordinating tridentate ligands, so-called scorpio-
nates.[5] However, boron has no interaction with the metal
center but functions purely as bridgehead atom in these ligand
scaffolds. The situation is markedly different in the recently
developed metallaboratranes, in which boron acts as a neutral
zero-electron donor/two-electron acceptor, also referred to as
Z-type ligand.[6] However, boryl-based chelating ligands have
been underdeveloped[7] and the development of boryl-cen-
tered tridentate compounds as ligands for transition metals,
let alone subsequent catalysis with metal-containing species,
has not been reported to date.
There has been a recent surge in the fundamental
coordination chemistry of various types of low-coordinate
boron-based scaffolds.[8] Notwithstanding these efforts, many
boron fragments utilized in coordination chemistry and in, for
example, borylation catalysis carry oxygen-linked substitu-
ents. Such compounds include pinacol derivatives (Bpin)2,
catecholate analogues B(cat), and boronic acids and esters
utilized in cross-coupling chemistry. This situation might have
limited the construction of suitable polydentate ligand
structures derived from boryl units, as the divalency of
oxygen severely limits side-arm functionalization. Moreover,
boryl species flanked by nitrogen-based groups are stronger
s donors than their oxygen analogues.[9]
inorganic chemistry, focusing on polyhedral boranes, borohy-
drides, and inorganic boranes.[1] The advent of carbaboranes
(or carboranes) has significantly altered this situation, and
boron has since been adopted by organic and inorganic
chemists alike with much success. Carbaboranes are versatile
frameworks that allow a variety of structural manipulations,
mainly directed toward the carbon fragments in the skel-
eton.[2] These clusters exhibit a high electron-delocalizing
ability, leading to very stable conjugate bases. Much recent
attention was drawn to their use as very weakly coordinating
anions[2c] and as building blocks for a variety of ligand
structures.[2d–g] Furthermore, application of boron-based li-
gands has recently received much attention, with a primary
focus on the various bonding modes of low-coordinate boron
species.[3] For synthetic purposes, catalytic aromatic boryla-
tion and hydroboration (formally the addition of a hydrogen
=
atom and a boryl group across a C C bond) are widely
applied in organic chemistry.
Boryl is the name for an anionic sp2-hybridized BH2
species and substituted derivatives thereof. Historically, metal
À
boryl (M BE2; E = OR, NR2, alkyl, etc.) chemistry developed
much later than metal alkyl chemistry. A primary reason is the
limited number of synthetic routes to boryl derivatives. For
À
instance, stable sources of BE2 hardly exist,[4] whereas
organolithium and Grignard reagents are commercially
available. The instability of BE2 species is not surprising,
given that boryl anions are coordinatively unsaturated.
Furthermore, the B H bond is inversely polarized compared
to a typical C H bond, that is, B
therefore boryl and borane species carry hydridic H atoms.
À
Boryl species BR2 are very strong s donors (and allow
significant tuning by side-group modification) as a result of
the electropositive nature of boron (also with respect to
carbon), and therefore they exhibit a strong trans effect as
well. This effect was studied both experimentally[10a] and
theoretically[10b,c] on Pt complexes, among others. Similar to
carbene ligands, boryl species have singlet ground states with
an empty p orbital (Scheme 1). However, p back-donation is
considered to be rather weak in most metal boryl com-
À
d+
dÀ
vs. CdÀ d+, and
H
À
À
H
À
[*] Dr. J. I. van der Vlugt
Supramolecular & Homogeneous Catalysis Group
van‘t Hoff Institute for Molecular Sciences
University of Amsterdam
Nieuwe Achtergracht 166, 1018 WV Amsterdam (The Netherlands)
Fax: (+31)20-5255604
plexes.[11] The bonding mode of a hydroborane ligand to a
2
À
metal ion can vary from s-bonded h -(B H) to hydridoboryl
E-mail: j.i.vandervlugt@uva.nl
Homepage: http://www.science.uva.nl/research/imc/HomKat/
[**] Financial support from the University of Amsterdam and the
Netherlands Organization for Scientific Research-Chemical Scien-
ces (NWO-CW) through a VENI Research Innovation Grant is
gratefully acknowledged. Prof. Joost Reek is thanked for continuous
support.
Scheme 1. Two limiting structures for the interaction of a hydroborane
with a metal center and the two modes of boryl binding.
252
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À
through oxidative addition of the B H bond. The differences
in reactivity for these compounds are pronounced, and recent
mechanistic investigations have yielded surprising kinetic
behavior for borane eliminations and activations as compared
to coordination and activation of other small molecules.
Pincer ligands have attracted long-lasting interest from a
large number of research groups over the past few decades[12]
À
À
and have been extensively studied for fundamental C H, C
À
C, and C N s-bond-activation processes, transition-metal
coordination chemistry, metal-mediated reactivity, and ho-
mogeneous catalysis. Some of the characteristic ligand
features include rigid, tridentate binding pockets; compati-
bility with a large number of transition metals; high tunability
at a number of key positions, intimately influencing the inner
and/or outer coordination sphere of the metal ion; and
incorporation of a variety of elements at the coordinating
bridgehead position. Besides the archetypical carbon-based
frameworks,[12,13] other novel backbones have amido,[14]
silyl,[15] or phosphido[16] units to provide heterodonor func-
tionality. Also, several neutral pincer ligands have been
intensely studied in the last years, including non-innocent
systems that undergo a reversible charge switch from neutral
to monoanionic.[17] Strikingly absent from this overview are
fragments derived from boron as the central atom.
Scheme 2. Preparation of a) SeBSe and b) SBS pincer ligands 4 and 6
based on a m-carbaborane scaffold and corresponding PdII complexes
5 and 7. Reproduced from Ref. [18] (Copyright: American Chemical
Society).
The established chemistry of N-heterocyclic carbene
frameworks incorporating low-valent main-group elements
that are isoelectronic to the boryl anion (e.g. C, Si, and Ge)
encouraged the application of chelating diamino substituents
to stabilize the boryl anion, containing a low-valent boron(I)
atom.[4] Two recent important contributions pave the way for
an exploration of the chemistry of pincer ligand systems based
on monoanionic boryl units as the central fragments and
transition-metal homogeneous catalysis therewith.
Scheme 3. Synthetic strategy to arrive at PBP ligand 9.
Mirkin and co-workers initiated a new chapter in carba-
borane chemistry by providing the first two ligand structures
the phosphine groups, with a signal at d = 26.4 ppm in the
preorganized for tridentate metal coordination with a direct
11B NMR spectrum.
[18]
I
À
À
M B bond. The bis(selenophenyl)carbaborane (SeBSe) 4
Oxidative addition of the B H bond to an Ir complex
and its bis(thiophenylether) analogue (SBS) 6 were synthe-
sized by two separate protocols (Scheme 2). Complexation to
PdII proceeded smoothly, albeit at reflux conditions to enable
proved facile, generating the first boryl-based pincer scaffold,
coordinated to an IrIII species, after stirring both precursors
for only 10 min at room temperature (complex 10). 11B NMR
spectroscopy showed a chemical shift difference of Dd = +
8.2 ppm upon coordination to iridium. The hydride appeared
as a triplet in the 1H NMR spectrum. Coupling with the
11B nucleus was absent for both hydride and phosphorus
atoms owing to quadrupolar relaxation phenomena. The
À
B H bond cleavage, which is also typical for aryl-based Pd
1
complexes with PCP ligands. H-coupled 11B NMR spectros-
copy proved very informative to establish selective rupture of
À
only one B H bond to generate the tridentate ligand frag-
ment. Pd complexes 5 and 7 were fully characterized,
including by X-ray crystallography. DFT calculations provid-
ed evidence that the electronic character of Pd in these
compounds is substantially different from well-known aryl-
based PCP-derivatives.
À
former indicates negligible, if any, B H interaction originat-
ing from s borane bond character. The nIrÀ band could be
H
detected at 2281 cmÀ1. X-ray crystallographic analysis con-
firmed the sp2 hybridization and established the stronger
s-bond donation of the PBP pincer versus related PCP
species. The related ethylene complex 11 was made straight-
forwardly by chloride abstraction (Scheme 4). This species
can be regarded as a model for one of the proposed
intermediates during alkene borylation.
Nozaki and co-workers, after their successful synthesis of
stable boryllithium and related isolable boryl precursors,[4]
now report on the elegant synthesis of a bis(phosphine)-
functionalized hydro(diamino)borane species 9.[19] o-Phenyl-
enediamine is converted into the bis(tert-butylphosphino)-
methyl analogue 8 in good yield by condensation with
formaldehyde and secondary phosphine (Scheme 3). Subse-
quent ring closure of the diamine groups using excess base-
stabilized BH3·SMe2 proceeded effectively, furnishing the
desired hydro(diamino)borane 9 in 78% after deprotection of
The development of tridentate scaffolds incorporating
boron as a pivotal, ligating element is noteworthy from a
number of perspectives. In contrast to the class of metal-
À
laboratranes, which feature an M(donor) B(acceptor) bond,
these pincer ligands contain a stable, strongly s-donating
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Highlights
E. Hey-Hawkins, Inorg. Chem. 2009, 48, 6072 – 6082; e) E. V.
Balagurova, D. N. Cheredilin, G. D. Kolomnikova, O. L. Tok,
F. M. Dolgushin, A. I. Yanovsky, I. T. Chizhevsky, J. Am. Chem.
Soc. 2007, 129, 3745 – 3753; f) M. Corsini, F. Fabrizi de Biani, P.
Zanello, Coord. Chem. Rev. 2006, 250, 1351 – 1372; g) Z. Xie,
Coord. Chem. Rev. 2006, 250, 259 – 272.
[3] G. J. Irvine, M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R.
Rice, E. G. Robins, W. R. Roper, G. R. Whittell, L. J. Wright,
Chem. Rev. 1998, 98, 2685 – 2722.
Scheme 4. Coordination of PBP ligand 9 in Ir complexes 10 and 11.
[4] a) Y. Segawa, Y. Suzuki, M. Yamashita, K. Nozaki, J. Am. Chem.
Soc. 2008, 130, 16069 – 16079; b) M. Yamashita, Y. Suzuki, Y.
Segawa, K. Nozaki, J. Am. Chem. Soc. 2007, 130, 16069 – 16079;
c) Y. Segawa, M. Yamashita, K. Nozaki, Science 2006, 314, 113 –
115.
[5] a) S. Trofimenko, Chem. Rev. 1993, 93, 943 – 980; b) S. Trofi-
menko, Scorpionates: The Coordination Chemistry of Polypyr-
azolylborate Ligands, University College Press, London, 1999.
[6] a) A. F. Hill, G. R. Owen, A. J. P. White, D. J. Williams, Angew.
Chem. 1999, 111, 2920 – 2923; Angew. Chem. Int. Ed. 1999, 38,
2759 – 2761; b) S. Bontemps, H. Gornitzka, G. Bouhadir, K.
Miqueu, D. Bourissou, Angew. Chem. 2006, 118, 1641 – 1644;
Angew. Chem. Int. Ed. 2006, 45, 1611 – 1614, and references
therein; for a debate on the bonding in these compounds, see
c) A. F. Hill, Organometallics 2006, 25, 4741 – 4743; d) G. Parkin,
Organometallics 2006, 25, 4744 – 4747.
[7] a) H. Braunschweig, M. Lutz, K. Radacki, Angew. Chem. 2005,
117, 5792 – 5796; Angew. Chem. Int. Ed. 2005, 44, 5647 – 5651;
b) C. E. F. Rickard, W. R. Roper, A. Williamson, L. J. Wright,
Organometallics 2002, 21, 1714 – 1718; c) C. E. F. Rickard, W. R.
Roper, A. Williamson, L. J. Wright, Organometallics 2002, 21,
4862 – 4872.
[8] a) Review: H. Braunschweig, C. Kollann, D. Rais, Angew. Chem.
2006, 118, 5380 – 5400; Angew. Chem. Int. Ed. 2006, 45, 5254 –
5274; boride: b) H. Braunschweig, R. D. Dewhurst, K. Kraft, K.
Radacki, Angew. Chem. 2009, 121, 5951 – 5954; Angew. Chem.
Int. Ed. 2009, 48, 5837 – 5840; metalloborylene: c) H. Braunsch-
weig, M. Burzler, R. D. Dewhurst, K. Radacki, Angew. Chem.
2008, 120, 5732 – 5735; Angew. Chem. Int. Ed. 2008, 47, 5650 –
5653.
boryl fragment. Side-arm functionalization can allow careful
tuning of desired ligand properties. The rich carbaborane
chemistry has now been expanded to include possible
applications as ligands in transition-metal catalysis. These
results open up new avenues in the coordination chemistry of
boron as well, with possibly exciting metal-mediated reac-
tivity.
À
B H bond activation has recently been postulated in
specific nitrogen-based scorpionate-like borate ligands to
facilitate metallaboratrane formation concomitant with the
formation of a metal hydride bond.[20] Species featuring
À
reversible B H bond activation could potentially serve as
hydride shuttling systems for metal-mediated transforma-
tions. It should be interesting to investigate the possible non-
innocent character of the novel tridentate boryl fragments
described, both with respect to reversible protonation of the
boron ion as well as potential redox behavior of some of these
ligand scaffolds and their metal complexes.
It remains to be seen whether alternative boryl-based
tridentate ligands are accessible through straightforward
synthetic methodologies. With respect to complexation, the
À
implied B H activation might not always occur readily, hence
hampering the clean preparation of metal species. Further-
more, pincer chemistry with carbaborane frameworks could
find broader application if more tunable (mixed) side-arm
groups could be introduced. It would be interesting to
investigate the precise nature of and control over the
electronic properties of boryl-based ligands. In this respect,
the suggestion that Pd might be formally zero-valent in
complexes 5 and 7 could have significant implications for
(carba)boryl-based coordination chemistry and the resulting
reactivity expected with such species.
[9] G. R. Clark, G. J. Irvine, W. R. Roper, L. J. Wright, J. Organo-
met. Chem. 2003, 680, 81 – 88.
[10] a) H. Braunschweig, K. Radacki, K. Uttinger, Chem. Eur. J.
2008, 14, 7858 – 7866, and references therein; b) J. Zhu, Z. Lin,
T. B. Marder, Inorg. Chem. 2005, 44, 9384 – 9390; c) K. C. Lam,
W. H. Lam, Z. Lin, T. B. Marder, N. C. Norman, Inorg. Chem.
2004, 43, 2541 – 2547.
[11] L. Dang, Z. Lin, T. B. Marder, Chem. Commun. 2009, 3987 –
3995.
[12] a) M. van der Boom, D. Milstein, Chem. Rev. 2003, 103, 1759 –
1792; b) D. Morales-Morales, C. M. Jensen, The Chemistry of
Pincer Compounds, Elsevier, Amsterdam, 2007.
[13] a) A. S. Goldman, A. H. Roy, Z. Huang, R. Ahuja, W. Schinski,
M. Brookhart, Science 2006, 312, 257 – 261; b) E. M. Schuster, M.
Botoshansky, M. Gandelman, Angew. Chem. 2008, 120, 4631 –
4634; Angew. Chem. Int. Ed. 2008, 47, 4555 – 4558; c) E.
Poverenov, I. Efremenko, A. I. Frenkel, Y. Ben-David, L. J. W.
Shimon, G. Leitus, L. Konstantinovski, J. M. L. Martin, D.
Milstein, Nature 2008, 455, 1093 – 1096.
The strongest selling point in favor of the introduction of
boron as a new member in the family of heterodonor-
functionalized (tridentate) ligands is fundamentally new or
highly altered (e.g. more reactive) properties of the resulting
metal complexes in catalytic transformations. So far these
claims have not been substantiated, but the combined specific
features of these EBE pincer complexes will undoubtedly
inspire many new developments in the area of boron-based
chemistry and catalysis.
Received: August 27, 2009
Published online: December 8, 2009
[14] a) M. D. Fryzuk, Can. J. Chem. 1992, 70, 2839 – 2845; b) L. Fan,
B. M. Foxman, O. V. Ozerov, Organometallics 2004, 23, 326 –
328; c) L. C. Liang, Coord. Chem. Rev. 2006, 250, 1152 – 1177.
[15] a) P. Sangtrirutnugul, T. D. Tilley, Organometallics 2007, 26,
5557 – 5568; b) M. C. MacInnis, D. F. MacLean, R. J. Lundgren,
R. McDonald, L. Turculet, Organometallics 2007, 26, 6522 –
6525; c) E. E. Korshin, G. Leitus, L. J. W. Shimon, L. Konstanti-
novski, D. Milstein, Inorg. Chem. 2008, 47, 7177 – 7189.
[1] T. B. Marder, Z. Lin, Contemporary Boron Chemistry I, Spring-
er, Berlin, 2008.
[2] a) V. I. Bregadze, Chem. Rev. 1992, 92, 209 – 223; b) J. Casanova,
The Borane, Carborane, Carbocation Continuum, Wiley, New
York, 1998; c) C. A. Reed, Chem. Commun. 2005, 1669 – 1677;
d) S. Stadlbauer, R. Frank, I. Maulana, P. Lꢀnnecke, B. Kirchner,
254
www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 252 – 255

Angewandte
Chemie
[16] N. P. Mankad, E. Rivard, S. B. Harkins, J. C. Peters, J. Am. Chem.
Soc. 2005, 127, 16032 – 16033.
[19] Y. Segawa, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2009,
131, 9201 – 9203.
[17] For a recent overview, see: J. I. van der Vlugt, J. N. H. Reek,
Angew. Chem. 2009, 121, 8990 – 9004; Angew. Chem. Int. Ed.
2009, 48, 8832 – 8846.
[20] a) I. R. Crossley, A. F. Hill, Dalton Trans. 2008, 201 – 203; b) N.
Tsoureas, M. F. Haddow, A. Hamilton, G. R. Owen, Chem.
Commun. 2009, 2538 – 2540.
[18] A. M. Spokoyny, M. G. Reuter, C. L. Stern, M. A. Ratner, T.
Seideman, C. A. Mirkin, J. Am. Chem. Soc. 2009, 131, 9482 –
9483.
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