Unexpected pincer-type coordination (κ3-SBS) within a zerovalent platinum metallaboratrane complex

Article abstract of DOI:10.1039/b917733g

The first structurally characterised zerovalent platinum complex to contain a tridentate pincer-type coordination mode (κ³-SBS) is presented, raising further questions concerning the geometries and trans influence of Z-type ligands. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b917733g

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Unexpected pincer-type coordination (j3-SBS) within a zerovalent platinum
metallaboratrane complex†
Gareth R. Owen,*‡ P. Hugh Gould, Alex Hamilton and Nikolaos Tsoureas
Received 1st July 2009, Accepted 8th September 2009
First published as an Advance Article on the web 16th September 2009
DOI: 10.1039/b917733g
The first structurally characterised zerovalent platinum com-
plex to contain a tridentate pincer-type coordination mode
(j3-SBS) is presented, raising further questions concerning
the geometries and trans influence of Z-type ligands.
regarding the metal-borane interaction, we also set out to develop
a range of new hybrid ligands based on 2-mercaptopyridine (mp)
and 1-methyl-imidazoyl-2-thione (mt) units.13
A range of complexes containing the hybrid ligand, sodium
hydro(2-pyridylthione)bis(methimizole)borate, mpBm have been
prepared. During the course of our investigations, we synthesised
a zerovalent platinum complex expecting to obtain a trigonal
bipyramidal complex similar to that previously reported by
Crossely and Hill.14 Herein, we wish to report an unexpected
pincer-type coordination of the resulting mixed sulfur/borane
ligand, providing a square planar geometry at the zerovalent
platinum centre. The implication of trans influence of the borane
functionality is also discussed.
The coordination chemistry of Z-type ligand systems
(s-acceptors) is currently of considerable interest.1 In particular,
the existence of transition metal-borane compounds has been
widely debated ever since Shriver reported the first such compound
in 1963.2 This compound however, along with some further
examples,3 was later disproved.4 It was not until 1999 when
Hill provided conclusive proof and structural characterisation of
the first ‘metallaboratrane’ complex (1, Fig. 1)5 establishing an
important synthetic route to transition metal-borane complexes.
This report sparked considerable development in the field and a
number of subsequent synthetic routes have since been realised.1c–e
A wide range of metallaboratrane compounds have been prepared
based upon sulfur,6 phosphorus,7 nitrogen8 donors in addition to
other supporting units.9
The synthesis and structural characterisation of the diva-
4
lent octahedral complex [PtH(PPh3){k -SSBS-B(mt)3}]Cl (2) has
been previously reported.14,15 In the presence of DBU (1,8-
diaza[5.4.0]bicycloundec-7-ene), complex 2 underwent reductive
elimination, resulting in loss of HCl, to form the new complex
[Pt(PPh3){B(mt)3}] (3) (Scheme 1). In the absence of structural
characterisation, the coordination mode of the multidentate ligand
4
1
1
was assigned as k -SSBS on the basis on the H and 13C{ H}
NMR spectra which indicated only one chemical environment for
the ‘mt’ rings.
4
Fig.
1
Metallaboratrane complex [Ru{k -SSBS-B(mt)3(CO)(PPh3)]
(N–S = mt) (1).
We became interested in the borane functional group for
its potential application as a ‘hydride store’ within transition
metal mediated transformations. Indeed, some evidence has been
provided for reversible hydride migration between transition metal
and boron centres within metallaboratrane complexes.10,11 In rela-
tion to this, a number of investigations exploring the propensity for
hydride migration by varying the substituents at boron have been
carried out.8b Additionally, we set out to explore the nature of the
metal-boron interaction in more detail and for this purpose have
recently reported a new family of flexible scorpionate ligands based
on 2-mercaptopyridine.12 In an attempt to reduce the symmetry
of the resulting complexes and to provide systematic information
Scheme 1 Synthesis of divalent and zerovalent platinum complexes 3
and 4.
The hybrid ligand, Na[mpBm] (4)13 (Fig. 2) was utilised to
prepare the analogous divalent and zerovalent platinum complexes
[PtH(PPh3){B(mt)2(mp)}]Cl (5) and [Pt(PPh3){B(mt)2(mp)}] (6)
respectively.
The School of Chemistry, University of Bristol, Bristol, UK BS8 1TS.
E-mail: gareth.owen@bristol.ac.uk; Fax: 0117 925 1295; Tel: 0117 928 7652
† Electronic supplementary information (ESI) available: Full experimental
details for the preparation of 5 and 6. CCDC reference number 738770 for
6. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b917733g
Fig. 2 Hybrid scorpionate, Na[mpBm] (4).
Complex 5 was characterised by spectroscopic and analytical
methods (see ESI†) and was found to be analogous to complex
2. The signal corresponding to the phosphorus ligand was
‡ Royal Society Dorothy Hodgkin Research Fellow.
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1
particularly broad. The 31P{ H} NMR spectrum of 5 revealed
a single broad signal at 4.0 ppm (CD2Cl2, h.h.w. = 285 Hz) with
1
platinum satellites (1JPPt = 1025 Hz). The H NMR spectrum of
5 was consistent with the spectroscopic assignment and structural
characterisation for complex 2. The spectrum revealed a single
peak, with an integration for one proton, at -13.9 ppm with plat-
inum satellites (1JPtH = 1002 Hz) corresponding to the platinum-
hydride. This was also confirmed by infrared spectroscopy by the
presence of a band at 2182 cm-1. The spectrum further revealed
four doublets in the lowfield region of the spectrum at d 9.21,
d 7.92, d 7.16 and d 6.87, each integrating to one proton and
=
corresponding to the CH CH protons on the mt rings. This
observation indicates that the mt rings are in different chemical
environments. Two signals (each integrating for three protons)
were observed at 3.46 ppm and 3.47 ppm corresponding to the
two independent NCH3 groups. The two distinct chemical shifts
of mt ring protons show that they are cis to each other in the
complex and therefore one mt ring is trans to the hydride and
the other ring is trans to the mp ring. All other data agree with
the spectroscopic and structural assignment of complex 2 and
4
accordingly complex 5 has been assigned as [PtH(PPh3){k -SSBS-
B(mt)2(mp)}]Cl (Fig. 3).
Fig. 4 Crystal structure of complex 6,† hydrogens atoms [with the
exception of H(11)] have been omitted for clarity (thermal ellipsoids drawn
at the 50% level).
centre while the second mt ring is not coordinated and points
directly away from the metal centre. The conformation of this
ring places one of the CH units of the mt ring towards an
axial site on the platinum centre, the Pt(1)-H(11) distance of
Fig. 3 Highlighting the positions of the mt and mp rings within 5
(N–S = mt).
˚
2.728 A suggesting a weak interaction with the metal centre.
Complex 6, [Pt(PPh3){B(mt)2(mp)}], was obtained as a bright
orange solid in good yield. The formation of 6 was also confirmed
A previous study had shown a difference in the metal-sulfur
distances and boron-nitrogen bond distances between the mt and
mp heterocycles within complexes containing mpBm.13 Accordingly,
the boron-nitrogen distances, for the coordinated rings, are
by the spectroscopic and analytical data. The peak corresponding
1
to the phosphine ligand was found at 28.7 ppm (CD2Cl2, JPtP
=
1
˚
˚
1410 Hz) [c.f. 26.5 (CDCl3, JPtP = 1370 Hz) for 3]. The spectro-
scopic data were fully consistent with those reported for complex
3. The 1H NMR spectrum revealed only one chemical environment
for the two mt rings (two doublet signal, each integrating for two
protons, at 6.72 ppm and 8.50 ppm). Four signals integrating for
one proton each, corresponding to the mp ring protons were also
observed in the lowfield region of the spectrum. The equivalence
of the mt ring protons was therefore consistent with the trigonal
bipyramidal geometry that had been assigned to 3. In order to
confirm this assignment a crystallographic study was carried out
on 6. Single crystals were obtained by layering a dichloromethane
solution of 6 with hexane. Unexpectedly, a crystallographic study
B(1)-N(1) 1.559(3) A (mt) and B(1)-N(5) 1.579(3) A (mp). In
this case however, the corresponding platinum-sulfur distances
˚
are coincidentally the same [2.2998(5) A].
˚
The Pt(1)-B(1) bond distance in 6 is 2.129(2) A and the
˚
Pt(1)-P(1) distance is 2.3797(5) A. The former distance is typical
of metallaboratrane complexes.1d The Pt(1)-P(1) distance is also
typical of a zerovalent platinum-phosphine bond suggesting a only
a moderate trans influence for the borane functional group [c.f. the
15
˚
Pt-P distance in the divalent complex 2, 2.4626(9) A]. The trans
influence of s-acceptor ligand has invoked some discussion since
the labilisation of the ligand trans to it appears to be related to
the stabilisation of atypical geometries rather than direct orbital
competition.16 For example, ligands in the site trans to boron
within octahedral complexes have particularly large metal–ligand
distances. There are also a number of complexes which either
have no ligand (i.e. square based pyramidal complexes) or exhibit
reversible coordination at this coordination site.8a On the other
hand, trigonal bipyramidal complexes show only moderate trans
influences.
There are very few structurally characterised examples of d10
metallaboratrane complexes and only one based on platinum
(Fig. 5). Bourissou and Ozerov reported the tetracoordinate
platinum complex (7) which contains the ligand B(C6H4PiPr2)3
3
revealed an unexpected pincer-type coordination mode (k -SBS)
of the borane based ligand where one of the ligand arms (mt)
remains uncoordinated (Fig. 4).§ This was surprising since the
spectroscopic data suggested chemically equivalent environments
for the two mt rings.
The zerovalent platinum centre adopts a distorted square planar
geometry with cis-inter ligand angles in the range 81.04(6)–
98.434(11)◦. The S(1)-Pt(1)-S(2) angle is 163.15(2)◦ and the B(1)-
Pt(1)-P(1) angle is 172.32(6)◦. The boron atom is approximately
tetrahedral with angles in the range 106.54(15)–112.52(16)◦. One
each of the mt and mp rings are coordinated to the metal
50 | Dalton Trans., 2010, 39, 49–52
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suggesting that this was not the mechanism involved in the rapid
exchange of the mt rings in 6. We are currently investigating the
mechanism involved in the exchange of the mt rings in 6.
The authors would like to thank the Royal Society (G.R.O.),
the Leverhulme Trust (N.T.) and EPSRC (A.H.) for funding and
Johnson Matthey for the loan of platinum salts.
Fig. 5 Square planar, trigonal based pyramidal and trigonal bipyramidal
d10 complexes 7–9.
Notes and references
§ Crystal data for 6: C31H29BN5PPtS3, Mw = 804.64, monoclinic, space
˚
group P21/c, a = 15.9926(2) b = 10.12870(10), c = 20.0351(2) A,
4
which binds to the metal centre with a (k -PPBP) coordination
b = 106.5320(10)◦, Z = 4, number of reflections measured = 93339,
mode.7f,g In this case the trigonal based pyramidal geometry at
the metal centre is enforced by the ligand and there is no ligand
trans to the boron atom. There are also two related structures
unique = 9540 (Rint = 0.0343), The final wR(F2) was◦0.0246 for all data,
˚
T = 100 K. Selected bond lengths (A) and angles ( ) for 6: Pt(1)-B(1)
2.129(2), Pt(1)-S(1) 2.2998(5), Pt(1)-S(2) 2.2998(5), Pt(1)-P(1) 2.3797(5),
B(1)-N(1) 1.559(3), B(1)-N(3) 1.572(3), B(1)-N(5) 1.579(3), B(1)-Pt(1)-S(1)
81.04(6), B(1)-Pt(1)-S(2) 86.50(6), S(1)-Pt(1)-S(2) 163.15(2), B(1)-Pt(1)-
P(1) 172.32(6), N(1)-B(1)-N(3) 106.54(15), N(1)-B(1)-N(5) 112.52(16),
N(3)-B(1)-N(5) 109.01(15).
3
involving monovalent gold centres. The first of these, [Au{k -PBP-
PhB(C6H4PiPr2)2}Cl] (8), shows a pincer-type coordination mode
similar to that observed for 6. A chloride ion is coordinated in
the fourth coordination site.16a The second gold structure, [Au{k -
4
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PPBP-B(C6H4PiPr2)3}Cl] (9) shows that, in the presence of an
additional donor, a trigonal bipyramidal structure is obtained.7g
While complexes 7 and 8 are constrained to the geometries
of the respective ligands, it appears possible that complex 9
3
could interconvert between square planar (k -PBP) and trigonal
bipyramidal (k -PPBP) forms. Finally, Parkin has reported the
4
only zerovalent complexes metallaboratrane complexes based on
sulfur donors, [Pd{k -SSBS-B(mtBu)3}(PMe3)] (10) and [Pd{m-k -
4
1
S,k -SBS-B(mtBu)3]2 (11).6n The geometries at the palladium centre
3
are trigonal bipyramidal for 10 while complex 11 contains a non-
idealised geometry involving a bridging coordination mode of the
sulfur atoms.
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We were therefore intrigued by the coordination mode found
in complex 6. The square planar structure indicated by the solid
state structure did not correspond with the spectroscopic evidence
therefore implying a fluxional process where the coordinated
and uncoordinated mt rings rapidly exchange places. In con-
sideration to the complexes 7–11, we postulated two possible
mechanisms by which this rapid exchange could occur. Firstly,
this could involve simple association and re-association of the
uncoordinated ligand ‘arm’ and rearrangement between trigonal
bipyramidal (k -SSBS/PPh3) and square planar (k -SBS/PPh3)
geometries. A second possibility would involve dissociation of the
triphenylphosphine ligand to form a ‘T-shaped’ intermediate (k -
SBS). This would be followed by association of the third sulfur
4
3
3
4
arm (k -SSBS) forming a structure similar to 7. A ‘T-shaped’
coordination motif has previously been observed by Braunschweig
within a platinum(II) boryl complex.17 In order to explore this
further, we performed low temperature NMR experiments on
complex 6. The temperature of a CD2Cl2 solution containing 6
was reduced and the NMR spectra were recorded. No signific◦ant
changes were observed in the 1H NMR spectrum down to -80 C,
the spectrum still indicating one chemical environment for the
mt rings. The signal in the 11B{ H} NMR spectrum did broaden
1
significantly at this temperature, however remained centred at the
same chemical shift indicating no change in geometry at the boron
centre. In order to assess whether the phosphorus ligand was
rapidly dissociating and re-coordinating to the platinum centre,
tricyclohexylphosphine was added to a solution of 6. Although the
more basic phosphine did replace the triphenylphosphine ligand,
the reaction was particularly slow and did not go to completion
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52 | Dalton Trans., 2010, 39, 49–52
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©