Thermodynamic control of oxidative addition and reductive elimination processes in cis-bis(dimethoxyboryl)-bis(tricyclohexylphosphine)platinum(ii)

Article abstract of DOI:10.1039/c3cc41444b

cis-[(Cy₃P)₂Pt{B(OMe)₂}₂] (1) was obtained in quantitative yield and fully characterised. The thermal stability of 1 was investigated in solution using multinuclear NMR spectroscopy revealing a hitherto unknown temperature-dependent equilibrium between 1 and the corresponding starting materials [Pt(PCy₃)₂] and free B₂(OMe)₄.

Full text of DOI:10.1039/c3cc41444b

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Thermodynamic control of oxidative addition and
reductive elimination processes in cis-bis(dimethoxyboryl)-
bis(tricyclohexylphosphine)platinum(II)†
Cite this: Chem. Commun., 2013,
4
9, 5216
Received 25th February 2013,
Accepted 8th April 2013
Holger Braunschweig* and Alexander Damme
DOI: 10.1039/c3cc41444b
www.rsc.org/chemcomm
cis-[(Cy
3
P)
2
Pt{B(OMe)
2
}
2
] (1) was obtained in quantitative yield
and fully characterised. The thermal stability of 1 was investigated
in solution using multinuclear NMR spectroscopy revealing a
hitherto unknown temperature-dependent equilibrium between
1
B
and the corresponding starting materials [Pt(PCy
3 2
) ] and free
Fig. 1 Square-planar platinum(II) complexes cis-bis(boryl) complex (A), trans-
mono(boryl) complex (B), and trans-diboran(4)yl complex (C).
2
(OMe)
4
.
Oxidative addition and reductive elimination of E–E bonds at
metal centres are elementary steps in most late transition metal ligands in mutual trans-disposition has hitherto been observed.
catalysed transformations of organic substrates. Transition It is well known that reductive elimination processes are
metal boryl complexes are of immense importance in organic significantly facilitated when the respective ligands are situated
chemistry due to their intermediacy in metal-catalysed processes cis to each other, and the liberation of diboranes(4) from four-
1
2
3
such as hydroboration, C–H activation, and diboration. The coordinate Pt(II) bis(boryls) has been thoroughly studied due to
latter process, according to the Suzuki–Miyaura protocol, proceeds its relevance to the Suzuki–Miyaura process.
with the formation of a square-planar cis-bis(boryl) species (A) via
Thus, it was previously observed that the reaction of cis-
0
oxidative addition of the B–B bond of a diborane(4) to a low valent [(Ph
3
P)
2
Pt(BCat )
2
] (2) with B
2
Cat
2
leads to a mixture of 2 and
4
0
platinum complex (Fig. 1). However, 1,2-dihalodiboranes(4) were cis-[(Ph P) Pt(BCat) ] (3) and free diborane(4) B Cat (Cat =
3
2
2
2
2
0
8
recently shown to react with Pt(0) precursors selectively by catecholato; Cat = Cat-4-tert-butyl). Likewise, reaction of cis-
oxidative addition of B–X bonds (X = halide) – akin to the [(Ph P) Pt(BPin) ] (Pin = pinacolato) with various substrates
from such as CO, PPh , Me Sn–SnMe or B Cat leads to liberation
halomonoboranes – leading to the formation of diboran(4)yl of B Pin and formation of the corresponding Pt complexes cis-
P) PtL
3
2
2
0
4f,5
well-established synthesis of trans-[X(R
3
P)
2
Pt(BR
2
)] (B)
3
3
3
2
2
2
2
0
0
6
9
complexes trans-[X(R
3
P)
2
Pt(BR BR
2
)] (C) (Fig. 1). The latter [(Ph
3
2
2 3 3
] (L = CO, PPh , SnMe , BCat). All of these and
Pt(II) species B and C have been the subject of extensive studies further results indicate that facile reductive elimination of a
0
elucidating the very strong trans influence of R
2
B groups and diborane(4) from cis-bis(boryl) platinum complexes requires
related boron-centred ligands, which may easily surpass that of the presence of a further substrate – i.e. spontaneous reductive
hydrido or silyl ligands, i.e. ligands also commonly ascribed to elimination, which constitutes the reversal of the formation
5
a,7
exert a strong trans-influence.
of A, and thus would impede the Pt-mediated diboration
This trans-influence of boryl ligands, which increases with according to the mechanism – does not seem to occur. This
decreasing electronegativity of the boron-bound substituents, assumption received further support from recent mechanistic
is responsible for the exclusive formation of cis-bis(boryl) com- studies concerning the reductive elimination of alkylboranes
plexes A – to the best of our knowledge, no complex with boryl from trans-[Me(Cy
3 2
P) Pt–BCat] and the Pt-mediated dehydro-
4
d
coupling of CatBH to give B
2
Cat
2
.
All experimentally and
computationally derived results strongly suggested that reductive
elimination processes in such square-planar Pt(II) complexes,
Institut f u¨ r Anorganische Chemie, Julius-Maximilians-Universit a¨ t W u¨ rzburg,
Am Hubland, D-97074 W u¨ rzburg, Germany. E-mail: h.braunschweig@uni-wuerzburg.de;
Fax: +49 931 31 84623; Tel: +49 931 31 85260
including that of B
mediates and hence require the presence of a further substrate.
We turned our attention to the reactivity of tetramethoxy-
3 2
diborane (4) towards a low-valent platinum precursor [Pt(PCy ) ]
2 2
Cat , proceed only via six-coordinate Pt inter-
†
Electronic supplementary information (ESI) available: Experimental details
4d
including synthesis, X-ray crystallographic data (CIF files) of 1, tables with
NMR and crystallographic data of 1–6, as well as the depiction of the relevant
orbitals (HOMO ꢀ 1 and LUMO + 1). CCDC 913240. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc41444b
and herein we present the synthesis, characterisation and
5
216 Chem. Commun., 2013, 49, 5216--5218
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3 2 2 2
molecular structure of cis-[(Cy P) Pt{B(OMe) } ] (1) (Cy = cyclo-
hexyl). The title compound constitutes a unique system, as
reductive elimination of the precursor diborane from this
square-planar platinum(II) complex occurs without support of
a further promoting reagent.
Reaction of an excess B
2 4 3 2
(OMe) with [Pt(PCy ) ] in pentane
at low temperature (ꢀ25 1C) results in the oxidative addition
0
product of the B–B bond to the Pt centre, cis-[(Cy
3 2
P) -
Pt{B(OMe) ] (1). This can be isolated as colourless crystals
2 2
}
in almost quantitative yield, and characterised using multi-
nuclear NMR spectroscopy at low temperature (Fig. 3).
The identity of 1 was also unambiguously ascertained using
1
0
elemental analysis and X-ray diffraction.
1
The H NMR spectrum of 1 in solution (toluene-d
8
) at ꢀ40 1C
features two separate signals (d 4.26 and 3.76 ppm) for the
methyl groups shifted to a lower field with respect to the free
diborane(4) B
2
(OMe)
4
(d 3.54 ppm). Also, two broad signals
31
1
Fig. 3 Part of the P{ H} NMR spectra of a sample of 1 at different tempera-
in the C{ H} NMR spectrum for methyl groups (d 53.6 and tures. At ꢀ20 1C the oxidative addition process appears quantitative, while at
0.0 ppm) were observed. No signals for the platinum-bound 80 1C the mixture reverts completely to the reductive elimination products.
1
3
1
5
1
1
1
boron atoms could be detected in the B{ H} NMR spectrum at
low temperature. A P{ H} NMR signal at d 32.8 ppm, which is
3
1
1
shifted to a higher field by about 30 ppm with respect to the catechol-substituted bis(boryl) platinum complexes 2–6
1
0
3 2
starting material [Pt(PCy ) ] (d 62.0 ppm), was ascribed to (cf. ESI,† Table S1), which might be ascribed to the superior
complex 1. This signal shows a phosphorus–platinum coupling electron releasing ability of methyl relative to aryl substituents.
constant of 1460 Hz, lying in the typical range for cis- However, the P1–Pt1–P2 and B1–Pt1–B2 angles in these com-
1
0
11
bis(phosphine) platinum(II) complexes (cf. ESI,† Table S2).
plexes vary little (cf. ESI,† Table S1). The Pt–P separations in 1
Complex 1 crystallizes at ꢀ25 1C in the monoclinic space (2.372(1) Å) are only slightly smaller than those in corre-
group P2 /n as colourless blocks (Fig. 2). The molecular sponding complexes derived from catecholatodiboranes, i.e.
1
0
structure exhibits a square-planar arrangement around the cis-[(Cy
Pt centre ( Pt 361.081) and Pt–B distances (2.093(5) and (5) (2.393 Å), while PPh
3
P)
2
Pt(BCat)
2
] (4) (2.388 Å) and cis-[(Cy
3
P)
2
Pt(BCat )
2
]
P
II
3
(3: 2.354(2), 2.346(2) Å; 2: 2.344(3),
3
ligands (6: 2.340, 2.347 Å) show even
2.108(5) Å) that are about 5 pm longer than in corresponding 2.353(3) Å) and P(CH
2
Cy)
4
c,d,8
shorter Pt–P bond distances.
The B–B separation in 1
(
(
2.451 Å) is greater than that in 4 (2.356 Å), 5 (2.356 Å), and 6
2.309 Å), thus excluding a bonding interaction between the
boron atoms.
Most interestingly, the title compound was found to be
stable only at low temperature, while gentle warming in
solution induced reductive elimination of the two boryl ligands
with the formation of the starting materials B
2
(OMe)
4
and
[
(
Pt(PCy ) ]. Moreover, variable-temperature NMR spectroscopy
H, C{ H}, P{ H}) revealed a temperature-dependent equi-
3 2
1
13
1
31
1
librium between starting materials and the product (Fig. 3).
Thus, below ꢀ20 1C, only the cis-bis(boryl) complex 1 is present
in solution, while at 80 1C exclusively free diborane and
[
Pt(PCy
elimination of MeBCat from trans-[(Cy
which requires four weeks at 80 1C, the elimination of
3 2
) ] can be observed (Fig. 3). Compared to the reductive
0
0
3
2
P) Pt(Me)(BCat )],
4c
B
2
(OMe)
4
from 1 occurs within minutes.
The aforementioned process constitutes, to the best of our
knowledge, the first example of the reductive elimination of a
Fig. 2 Molecular structure of 1 in the solid state. Thermal ellipsoids displayed at
diborane (4) from a square-planar Pt(II) centre in the absence of
the 50% probability level. For clarity, only one of two molecules in the asymmetric
unit is displayed. Three molecules of co-crystallized pentane in the asymmetric unit any supporting substrate (i.e. from a truly four-coordinate
and hydrogen atoms were omitted, and the cyclohexyl groups were simplified.
Pt atom). In order to understand the process better, DFT
calculations with geometry optimizations were conducted at
the PBE level of theory on the bis(boryl) complexes cis-
Selected bond lengths and interatomic distances [Å], and bond angles [1]:
Pt1–B1 2.093(5), Pt1–B2 2.108(5), Pt1–P1 2.372(1), Pt1–P2 2.372(1), B1ꢁꢁꢁB2 2.451,
B1–O1 1.384(5), B1–O2 1.383(5), B2–O3 1.369(5), B2–O4 1.376(6), B1–Pt1–B2
[
3 2 2 2 3 2 2
(Cy P) Pt{B(OMe) } ] (1CAL) and cis-[(Cy P) Pt(BCat) ] (4CAL)
7
1.3(2), B1–Pt1–P1 89.7(1), P1–Pt1–P2 109.58(4), P2–Pt1–B1 159.5(1), P1–Pt1–B2
59.0(1), P2–Pt1–B2 90.3(1), O1–B1–O2 113.0(4), O3–B2–O4 113.9(4).
1
(Fig. 4). Computations revealed that the oxidative addition of
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HOMO ꢀ 1 is located strictly at the platinum and the LUMO + 1 can
10
be clearly assigned to the diborane(4) (ESI,† Fig. S4). The WBIs give
no indication of a Pt–B bond (0.005, 0.0018), instead the values for
the Pt–P bond increase (0.6289, 0.6278). The evaluation and com-
parison of the frontier orbitals of the diboranes(4) B (OMe)4
2
2 2
and B Cat in the starting materials, and the diborane(4) parts
in the transition states, reveal no electronic differences between
the compounds. The similar activation energies 1ADD - 1TS
ꢀ1
ꢀ1
(
18.60 kJ mol ) and 4ADD - 4TS (16.54 kJ mol ) suggest no
steric advantage for any reaction.
The synthesis of the cis-bis(boryl) platinum(II) complexes cis-
[
(Cy P) Pt{B(OMe) } ] (1) by oxidative addition of the B–B bond
3 2 2 2
of B (OMe) at low temperatures to the platinum(0) precursor
2
4
[Pt(PCy
3
)
2
] is reported herein. Full characterisation of the title
compound in solution and using X-ray diffraction revealed no
unusual structural features, but nevertheless 1 constitutes a
unique system as liberation of B (OMe) occurs under mild
2 4
conditions from the isolated complex. Thus, the square-planar,
four-coordinate complex 1 undergoes reductive elimination of a
diborane(4) in the absence of any supporting substrate – a
process that in the case of Pt(II) is believed to proceed exclusively
ꢀ
1
Fig. 4 Calculated parameters (DGTotal in kJ mol ) for the formation of the cis- via six-coordinate intermediates. Preliminary DFT studies revealed
bis(boryl) platinum complexes 1CAL and 4CAL and the transition states 1TS and 4TS
only a very small thermodynamic driving force for the oxidative
and the pre-configured adducts 1ADD and 4ADD
.
addition of B
tion for the observation of this unusual equilibrium facilitated by
] with the formation of the corre- oxidative addition and reductive elimination processes.
sponding cis-configured complex is an exergonic process (Fig. 4)
2 4 3 2
(OMe) to [Pt(PCy ) ], thus suggesting an explana-
3 2
the B–B bond to [Pt(PCy )
ꢀ1
Notes and references
for both B Cat (by ꢀ85.96 kJ mol ; all energies are corrected
2
2
1
0
ꢀ1
Gibbs free energies) and for B
2
(OMe)
4
(by ꢀ8.90 kJ mol ).
1
(a) I. Beletskaya and A. Pelter, Tetrahedron, 1997, 53, 4957;
(b) J. H. Brewster and E. Negishi, Science, 1980, 207, 44.
However, formation of the catecholato derivative 4CAL is signifi-
ꢀ1
2 (a) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J. M. Murphy and
J. F. Hartwig, Chem. Rev., 2009, 110, 890; (b) J. F. Hartwig, Chem. Soc.
Rev., 2011, 40, 1992.
cantly (by 77.06 kJ mol ) more favourable than that of the
methoxy derivative 1CAL. We were also able to locate the transi-
tion states 1TS and 4TS of both reaction paths. The two transition
states are markedly different. While 1TS is more comparable to
the product structure, the geometry of 4TS is more similar to that
of the starting materials. These findings are in accordance with
3 T. B. Marder and N. C. Norman, Top. Catal., 1998, 5, 63.
4
(a) A. Kerr, N. C. Norman, A. G. Orpen, M. J. Quayle, C. R. Rice,
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J. Starbuck, J. Chem. Soc., Dalton Trans., 1999, 1687; (c) H. Braunschweig,
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1
1
the Hammond-Postulate for early and late transition states.
This is also reflected by the geometry of 1TS, in which the
diborane(4) ligand is in a parallel orientation to the P–Pt–P-axis,
whereas in 4TS the diborane(4) ligand is in a perpendicular
orientation to this axis.
The reverse reactions of 1CAL and 4CAL through reductive
elimination of the diborane(4) ligand need to overcome quite
different activation barriers: 58.58 kJ mol for 4CAL and just
1.85 kJ mol for 1CAL, which further explains the equilibrium
of 1. In both reaction paths we found pre-configured adduct
formation of the platinum precursor and the diborane(4), which
(e) N. Lu, N. C. Norman, A. G. Orpen, M. J. Quayle, P. L. Timms and
G. R. Whittell, J. Chem. Soc., Dalton Trans., 2000, 4032; ( f ) G. J. Irvine,
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(
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5
ꢀ
1
ꢀ
1
1
A. Schneider, Chem. Rev., 2010, 110, 3924; (c) H. Braunschweig,
P. Brenner, A. M u¨ ller, K. Radacki, D. Rais and K. Uttinger,
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6
7
(a) H. Braunschweig, A. Damme and T. Kupfer, Angew. Chem., Int.
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ꢀ
1
was preferred over the separated precursors by ꢀ15.65 kJ mol for
ꢀ
1
1ADD and ꢀ43.92 kJ mol
for 4ADD. NBO calculations were
0
0
performed for the compounds 1 ADD and 4 ADD, which are derived
from the optimized adducts by exchange of PCy with PMe ligands,
3
3
0
using fixed geometry. In 4
we observe a bonding interaction
ADD
between the platinum centre and the unoccupied bonding p-orbital
of B Cat
in the LUMO + 1, while the HOMO ꢀ 1 represents the B–B
bonding interaction. This is reflected by the Wiberg bond indices
WBI) for the Pt1–B1 (0.3171) and Pt1–B2 (0.3486) bonds compared
8
9
G. Lesley, P. Nguyen, N. J. Taylor, T. B. Marder, A. J. Scott, W. Clegg
and N. C. Norman, Organometallics, 1996, 15, 5137.
C. N. Iverson and M. R. Smith, Organometallics, 1996, 15, 5155.
2
2
1
1
0 Experimental and theoretical details are provided in the ESI†.
1 (a) G. S. J. Hammond, J. Am. Chem. Soc., 1955, 77, 334;
(b) J. E. Leffler, Science, 1953, 117, 340.
(
0
to the platinum–phosphorus bonds (0.4938, 0.5167). For 1 ADD the
5
218 Chem. Commun., 2013, 49, 5216--5218
This journal is c The Royal Society of Chemistry 2013