Synthesis and structure of the first heterodinuclear bis(borylene) complexes

Article abstract of DOI:10.1039/c3cc43877e

We report a facile synthesis of the first heterodinuclear bis(borylene) complex by reaction of an iron bis(borylene) complex with [Pt(PCy ₃)₂], and its fully-reversible CO uptake and liberation reaction. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc43877e

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Synthesis and structure of the first heterodinuclear
bis(borylene) complexes†
Cite this: Chem. Commun., 2013,
49, 7593
Holger Braunschweig,* Qing Ye, Alexander Damme and Krzysztof Radacki
Received 23rd May 2013,
Accepted 1st July 2013
DOI: 10.1039/c3cc43877e
www.rsc.org/chemcomm
We report a facile synthesis of the first heterodinuclear bis(borylene) coordination sphere of transition metals remains a major chal-
complex by reaction of an iron bis(borylene) complex with [Pt(PCy3)2], lenge. To date, only three fully characterised examples have been
and its fully-reversible CO uptake and liberation reaction.
reported (1–3 in Scheme 2), which were exclusively obtained by
transferring the :BN(SiMe3)2 moiety from reliable borylene sources
Hypovalent free borylenes ‘‘:BR’’, unlike their isoelectronic counter- [(OC)5MQBN(SiMe3)2] (M = Cr, Mo, W) to the corresponding
part CO, are highly reactive and can only be generated as transient transition-metal substrates.5k,h,10 While severely limited in num-
species under drastic conditions1 or stabilised to a certain extent bers, bis(borylenes) have disclosed unique reactivity patterns far
upon coordination by carbenes.2 However, numerous computa- beyond the scope of their well-established mono(boryl) congeners,
tional studies have revealed a close relationship of borylenes with including tandem borylene transfer to yield unprecedented boron
the ubiquitous CO ligand in terms of coordination modes, and heterocycles11 and most notably borylene coupling10,12 and catena-
moreover, higher thermodynamic stability of the MQB linkage due tion.10 In particular, the latter, which was achieved by incorporation
to better s-donor and p-acceptor properties of borylene ligands in of four borylene units into the coordination sphere of one iron
comparison to CO.3 Since the synthesis of the first bridged borylene nucleus, represents a novel and potentially useful protocol for the
complexes,4 the common coordination modes of CO (i.e. terminal,5 generation of electron-precise B–B bonds (5 in Scheme 2).10,13
bridging,4,5i,6 semi-bridging,7 and triply bridging8) have thus been
Herein, we present a facile synthesis of the first hetero-
mimicked by borylene ligands (Scheme 1). In addition to the dinuclear bis(borylene) complexes and their intriguing CO
fundamental aspects of metal borylene bonding, significant interest absorption and desorption properties.
in this class of compounds stems from an increasing number of
applications in organometallic and organic synthesis, such as solution of [(OC)3Fe(BDur){BN(SiMe3)2}] (3) led to formation of the
borylene transfer and borylene-based metathesis.9
heterodinuclear bis(borylene) complex [(OC)3Fe{m-BN(SiMe3)2}-
Nonetheless, whilst homoleptic carbonyl complexes are well (m-BDur)Pt(PCy3)] (7) almost quantitatively (Scheme 3). The 11B
Addition of an equimolar amount of [Pt(PCy3)2] (6) to a C6D6
known, incorporation of more than one borylene moiety into the NMR spectrum of 7 displayed two broad resonances at dB
=
122 and 98 and the 31P{1H} NMR spectrum revealed a signal at
1
dP = 74.4 (Pt–PCy3, JPt–P = 4123.4). The concomitant formation of
free PCy3 was indicated by a 31P NMR resonance at dP = 9.8.
Scheme 1 Coordination modes of borylenes.
¨
¨
¨
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg,
¨
Am Hubland, D-97074, Wurzburg, Germany.
E-mail: h.braunschweig@mail.uni-wuerzburg.de; Fax: +49 931 31 84623;
Tel: +49 931 31 85620
† Electronic supplementary information (ESI) available: Experimental details
including synthesis and X-ray crystallographic data of 7 and 8. CCDC 940839
and 940840. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3cc43877e
Scheme 2 Published bis(borylene) and tetra(borylene) complexes.
Chem. Commun., 2013, 49, 7593--7595 7593
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borylene transfer process could be accomplished by removing the
iron fragment under a CO atmosphere in the form of [Fe(CO)5].
Hence, a solution of 7 in C6D6 was exposed to CO (1.2 bar) in a
sealed Young NMR tube, which resulted in a dramatic colour
change from dark brown to orange. Multinuclear NMR spectroscopy
revealed quantitative formation of a new boron- and phosphorus-
containing species, displaying 11B NMR resonances at dB = 134 and
98, which lie in the expected range for borylene complexes. Never-
theless, there was no sign for the formation of [Fe(CO)5] according
to the 13C NMR spectrum. Moreover, the 31P NMR spectrum showed
a new, broad signal at dP = 31.4. Interestingly, when all volatiles were
removed under vacuum, a colour change from orange back to dark
brown was observed, thus suggesting a reversible reaction. Indeed,
multinuclear NMR spectroscopy of the residue dissolved in C6D6
confirmed the regeneration of compound 7. These findings suggest
that CO might ligate to the Pt center. In fact, facile loss of CO is
known for dicarbonyl platinum complexes.14
Scheme 3 Preparation of compound 7.
In order to clarify the atom connectivity of the unknown
orange species, single crystals suitable for X-ray diffraction
analysis were obtained after exposing a suspension of 7 in
hexane to CO with stirring. The reaction mixture became
homogenous within several minutes. The obtained orange
hexane solution was kept under a CO atmosphere overnight
at ambient temperature, yielding orange crystals (Fig. 2).
The molecular structure of 8 confirmed our proposal of a
bis(borylene)-based platinum carbonyl complex (Scheme 4).
Bearing a carbonyl ligand, the Pt centre features a distorted
Fig. 1 Molecular structure of [(OC)3Fe{m-BN(SiMe3)2}(m-BDur)Pt(PCy3)] (7) in the
solid state. Thermal ellipsoids are depicted at the 50% probability level. For clarity,
ellipsoids of the cyclohexyl groups and hydrogen atoms have been removed.
Selected bond lengths (Å) and angles (1): Fe–B1 1.934(4), Fe–B2 1.945(3),
Pt–B1 2.004(3), Pt–B2 2.063(4), B2–N 1.393(4); B1–Fe–B2 96.88(15), Fe–B2–N
153.7(3), Fe–B1–C 155.0(2), Fe–B2–Pt 80.06(13), N–B2–Pt 126.2(2), Fe–B1–C
155.0(2), Pt–B1–C 123.2(2), Fe–B1–Pt 81.80(14).
Complex 7 was isolated as red crystals upon storage of the tetragonal planar geometry with an angular sum of 363.11. The
concentrated reaction solution at À35 1C overnight, in the increased steric congestion caused by introduction of an extra
monoclinic space group P21/c. The atom connectivity was ligand on platinum led to slight displacement of the boron-
clarified by X-ray diffraction analysis (Fig. 1). In comparison to bound substituents towards iron. As a result, the [BDur] moiety
the previously reported heterodinuclear monoborylene species adopts a symmetric bridging coordination mode with almost
[(OC)2(Me3P)Fe(m-CO)(m-BDur)Pt(PCy3)] (9),5n in which the borylene identical M–B1–C angles (M = Fe, 140.5(4)1; Pt, 139.9(4)1), while
moiety [:BDur] adopts a symmetric bridging coordination mode the BN(SiMe3)2 ligand retains its semi-bridging character as
with almost identical M–B bond distances (M = Fe, 1.970(4) Å; Pt, indicated by the bond angles of M–B2–N (M = Fe, 148.3(4)1; Pt,
1.976(4) Å), the borylenes in 7 both adopt a semi-bridging 133.7(4)1). In addition, the Pt–B separations are correspond-
coordination mode. The Pt–B separations are 7–12 pm longer ingly elongated by ca. 10 pm, while the Fe–B distances are
than the corresponding Fe–B bonds. The angles of Fe–B1–C slightly elongated by 1–5 pm. Remarkably, the central B–Fe–B–
(155.0(2)1) and Fe–B2–N (153.7(3)1) are similar to the corre- Pt ring becomes planar (Fig. 2, right), as indicated by the sum
sponding angles of the semi-bridged [:BN(SiMe3)2] moiety in
[(OC)4M(m-CO){m-BN(SiMe3)2}Pd(PCy3)] (M = Cr, Mo, W).7b,c
Furthermore, the overall geometry of the central Fe–B1–Pt–B2
fragment adopts a slightly bent ring structure (Fig. 1, right). The
almost orthogonal orientation of the boron-bound duryl and
bis(trimethylsilyl)amino substituents with respect to the BR3
plane is presumably due to steric reasons. Although the ortho-
gonal disposition of the amino substituent might preclude
effective p donation from N to the formally sp2-hybridised boron
atom, the B–N separation of 1.393(4) Å is comparable with
that (1.400(19) Å) of the boracumulene complex [CpRh(PCy3)-
{(Z2-B,C)–(SiMe3)2(NQBQCQCH2)}],15 thus indicating significant
Fig. 2 Molecular structure of [(OC)3Fe{m-BN(SiMe3)2}(m-BDur)Pt(PCy3)(CO)] (8) in
the solid state. Thermal ellipsoids are at the 50% probability level. For clarity,
ellipsoids of the phosphine ligand and hydrogen atoms have been removed.
Selected bond lengths (Å) and angles (1): Fe–B1 1.973(6), Fe–B2 1.944(6),
Pt–B1 2.096(6), Pt–B2 2.144(6), Pt–C1 1.920(6), C1–O1 1.148(6), B2–N 1.415(7);
B1–Fe–B2 107.3(2), Fe–B2–N 148.3(4), Fe–B1–C 140.5(4), Fe–B2–Pt 78.0(2),
N–B2–Pt 133.7(4), Pt–B1–C 139.9(4), Fe–B1–Pt 78.5(2), B2–Pt–C1 78.9(2),
C1–Pt–P 90.67(15), P–Pt1–B1 97.41(17), B2–Pt–B1 96.1(2).
B–N double bond character. In fact, this intriguing structural
feature is consistent with our previously reported semi-bridging
aminoborylene complexes.7b,c
As previous studies have proved that heterodinuclear bridging
borylene complexes act as key intermediates for intermetal borylene
transfer,5i,10 we addressed the question of whether the double
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Scheme 4 Reversible CO incorporation in, and loss from,
a bis(borylene)
platinum center.
(359.91) of angles within the four-membered ring. Although the
heterodinuclear borylene complexes 7–9 feature almost identical
Fe–Pt distances (ca. 2.57 Å) that are indeed less than the sum of
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The NMR spectroscopic features of complex 8 are in full
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diagnostic colour change.
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Chem. Commun., 2013, 49, 7593--7595 7595