Cis-[PtBr2(PPh2(4-catechol))2]: Synthesis, crystal structure, and computational modelling of its binding to nanocrystalline TiO2

Article abstract of DOI:10.1039/b513354h

The complex cis-[PtBr₂(PPh₂(4-catechol))₂] 1 has been synthesized by cleavage of the four methyl groups from cis-[PtCl₂(PPh₂(4-veratrole))₂] using BBr ₃, followed by work-up in t

Full text of DOI:10.1039/b513354h

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cis-[PtBr2{PPh2(4-catechol)}2]: synthesis, crystal structure, and
computational modelling of its binding to nanocrystalline TiO2
Nigel T. Lucas,*† Andrew M. McDonagh,‡ Ian G. Dance, Stephen B. Colbran and Donald C. Craig
Received 20th September 2005, Accepted 12th December 2005
First published as an Advance Article on the web 6th January 2006
DOI: 10.1039/b513354h
The complex cis-[PtBr2{PPh2(4-catechol)} ] 1 has been synthesized by cleavage of the four methyl
groups from cis-[PtCl2{PPh2(4-veratrole)}22] using BBr3, followed by work-up in the presence of excess
bromide. An X-ray crystal structure of 1·(ethanol)2 confirms that the two catechol rings are adjacent to
each other and approximately parallel, and therefore well structured to act as double bidentate ligands
for adjacent metal atoms on the surface of a nanocrystal. The crystal packing of 1·(ethanol)2 involves
intermolecular hydrogen-bonding interactions and a parallel fourfold phenyl embrace between PPh2
moieties. Density functional calculations have demonstrated that conformational variability of the aryl
rings in cis-[PtBr2{PPh2(4-catechol)}2] is energetically feasible, and two conformations of
cis-[PtBr2{PPh2(4-catechol)}2] as a complex ligand for Ti atoms on the various surfaces of the anatase
and rutile structures of TiO2 have been assessed for geometrical commensurability. Three structural
models for adsorbates of cis-[PtBr2{PPh2(4-catechol)}2] on TiO2 are developed for anatase (110),
anatase (101), and rutile (001).
Introduction
Immobilization of metal complexes on metal oxide supports is
attracting continuing interest with applications in areas such as
photovoltaics, electronic display and catalysis.1 The immobiliza-
tion of such functional complexes is usually achieved though the
use of bi-functional ligands that bind the metal at one site and
the oxide surface at another site. The catechol-phosphine ligand
L1 (Scheme 1), first described by Raymond and co-workers,2
has the potential to bind to soft metals through the phosphine
atom, and to hard metals through the chelating catecholate group.
We have previously described the synthesis of metal complexes
[PdBr2(L1)2] and [W(CO)5L1] and the strong adsorption of these
complexes onto nanocrystalline TiO2.3 [PdBr2(L1)2] on TiO2 was
shown to catalyse the Sonogashira coupling of phenylacetylene
Scheme 1
and 4-iodonitrobenzene.
We present here the synthesis and crystal structure of the related
platinum complex [PtBr2(L1)2] (1) together with a molecular
modelling analysis of its adsorption on surfaces of the anatase
and rutile forms of TiO2.
spirit were used without drying but were purged with nitrogen
prior to use. Boron tribromide (Aldrich) was used as received.
Literature procedures were used to prepare [PtCl2(NCPh)2]4 and 4-
(diphenylphosphino)veratrole.2 1H and 31P NMR solution spectra
were recorded in CDCl3 (Cambridge Isotope Laboratories or
Aldrich) or d6-acetone (Aldrich) using a Bruker Avance 300
spectrometer (at 300 MHz for 1H, 121 MHz for 31P). The 1H NMR
spectra were referenced internally to residual non-perdeuterated
solvent and 31P spectra to 85% H3PO4.
Experimental
General
Reactions were carried out under an atmosphere of nitrogen using
standard Schlenk techniques. All solvents were analytical grade
(AR). Dichloromethane was dried and distilled over CaH2 under
a dinitrogen atmosphere. Acetone, diethyl ether, and petroleum
Synthesis of [PtCl2{PPh2(4-veratrole)}2] (2). A solution of
[PtCl2(NCPh)2] (604 mg, 1.28 mmol) in dichloromethane (10 mL)
was added to a solution of 4-(diphenylphosphino)veratrole (L2)
(870 mg, 2.70 mmol) in dichloromethane (15 mL) at room
temperature with no colour change observed. The resulting clear
pale-yellow solution was stirred for 2 h, the volume reduced to
ca. 5 mL and methanol added dropwise to give a pale-yellow
precipitate. The solution was cooled in ice and the precipitate
School of Chemical Sciences, The University of New South Wales, Sydney,
NSW, 2052, Australia. E-mail: n.lucas@chem.usyd.edu.au
† Present address: School of Chemistry, The University of Sydney, NSW
2006, Australia.
‡ Present address: Department of Chemistry, Materials and Forensics,
University of Technology Sydney, Sydney NSW 2007, Australia.
680 | Dalton Trans., 2006, 680–685
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collected on a sintered glass funnel, washed with methanol and
dried at the pump. Slow diffusion of diethyl ether into a
dichloromethane solution (ca. 4 mL) of the crude material yielded
white to pale-yellow crystals of [PtCl2{PPh2(4-veratrole)}2] 2
(524 mg, 45%) from which the solution was decanted and residue
dried in vacuo. 1H NMR (CDCl3): d 7.61–7.51 (m, 8H, Ph), 7.37–
7.29 (m, 4H, Ph), 7.24–7.16 (m, 8H, Ph), 6.98 (dd, 12 Hz, 2 Hz,
2H, C6H3), 6.91–6.83 (m, 2H, C6H3), 6.58 (dd, 8 Hz, 2 Hz, 2H,
C6H3), 3.84 (s, 6H, Me), 3.52 (s, 6H, Me). 31P NMR (CDCl3): 16.3
(s, 1JP,Pt = 3660 Hz).
Computer modelling
Alternative conformations of 1 were investigated using density
functional methods: program DMol3,6 with functional blyp,
numerical basis set dnp, and scalar relativistic corrections. The
modelling of these conformations on the surfaces of anatase and
rutile was done by least squares fitting of positions of the set of
three or four O atoms of the complex to sets of O atoms on the
surface, using the program InsightII.7 Models for surfaces of TiO2
were generated using CrystalMaker,8 which was used also for the
creation of published figures.
Results and discussion
Synthesis of [PtBr2{PPh2(4-catechol)}2] (1). A solution of
[PtCl2{PPh2(4-veratrole)}2] (330 g, 0.36 mmol) in dichloro-
methane (20 mL) was cooled to −80 ◦C and BBr3 (0.18 mL,
1.9 mmol) added dropwise and the stirred mixture was allowed
to warm to room temperature over 3 h. The volatile materials
were removed in vacuo, ca. 2 mL of methanol added with stirring
to quench any remaining BBr3, and the mixture again taken to dry-
ness in vacuo. The crude material was dissolved in acetone (20 mL)
and KBr (1.98 g, 16.6 mmol) was added and the suspension stirred
vigorously for 20 h. The mixture was transferred to a separating
funnel, dichloromethane (50 mL), water (100 mL) and two drops
of conc. HBr were added. The organic layer was extracted, dried
over Na2SO4, filtered and the solvent removed in vacuo to give an
oily white solid. The crude material was triturated with petroleum
spirit–acetone (9 : 1) and on standing (ca. 1 week) gave pale yellow
crystals of [PtBr2{PPh2(4-catechol)}2] 1 (205 mg, 60%). 1H NMR
(d6-acetone): d 8.32 (s, 4H, OH), 7.44–7.10 (m, 20H, Ph), 7.03 (br
t, 2H, C6H3), 6.72 (br d, 2H, C6H3), 6.09 (br s, 2H, C6H3). 31P
NMR (d6-acetone): 14.9 (s, 1JP,Pt = 3630 Hz).
Synthesis
[PtCl2{PPh2(4-veratrole}2]
2 was synthesized by the reac-
tion of [PtCl2(NCPh)2] with 4-(diphenylphosphino)veratrole3 in
1
dichloromethane at room temperature. The H NMR spectrum
shows the expected phenyl, veratrole ring, and methoxy reso-
nances. The 31P NMR spectrum contains a singlet flanked by
195Pt satellites at 16.4 ppm (1JP,Pt = 3660 Hz) and is consis-
tent with the cis geometry.9 Treatment of 2 with boron tri-
bromide cleaves the methyl groups from the veratrole units to
give a crude (diphenylphosphino)catechol platinum complex 1,
which was purified by recrystallisation from a dichloromethane–
diethyl ether mixture. This reaction is in contrast to that
of [PdCl2{PPh2(4-veratrole}2] where treatment with boron tri-
bromide cleaves the veratrole methyl groups but also decom-
plexes one of the phosphine ligands from the metal to afford
the dimeric species [Pd2Br4{(diphenylphosphino)catechol}2] plus
the (diphenylphosphino)catechol·BBr3 adduct.3 [PdBr2{PPh2(4-
catechol)}2] can be accessed via an alternative route that employs
L1·HBr.3 During the reaction of 2 with boron bromide, some
bromide for chloride exchange occurs. For this reason, the crude
reaction mixture was stirred with excess potassium bromide to
form 1 with a negligibly small amount of the chloride complex
present. The 1H NMR spectrum of 1 shows the expected phenyl,
catechol ring and hydroxy resonances while the 31P NMR spectrum
contains a singlet with satellites at 14.9 ppm (1JP,Pt = 3630 Hz),
indicative of cis geometry.
Crystallography
Crystals of 1·(ethanol)2 suitable for single-crystal X-ray analysis
were produced by slow diffusion of ethanol into a solution of 1
in dichloromethane at ambient temperature. Reflection data were
measured with an Enraf-Nonius CAD-4 diffractometer and were
not corrected for absorption. The positions of all atoms in the
asymmetric unit were determined by direct phasing (SIR92).5
The OH and methyl hydrogen atoms were first approximately
located from a difference Fourier map, then recalculated during
Crystal structure of 1
˚
The crystal structure of 1 is the first structure of a compound of
the type cis-[MX2{PPh2(4-catechol)}2] (X = halide). Complex 1
crystallizes from a dichloromethane/ethanol mixture in the space
the refinement such that O–H/C–H = 1.00 A; all other hydrogen
˚
atoms were included in calculated positions with C–H = 1.00 A.
¯
group P1 with one molecule of the complex and two molecules
of ethanol in the asymmetric unit. The molecular structure has
a slightly distorted cis square planar coordination (details in
Table 1), with the phosphine ligand substituents conformed such
that the two catechol rings are approximately parallel.
Crystal data. C36H30Br2O4P2Pt·2(C2H6O), M = 1035.6, T =
¯
294 K, triclinic, space group P1 (no. 2), a = 11.032(4), b =
˚
11.686(4), c = 15.342(5) A, a = 83.44(2), b = 87.94(2), c =
◦
3
−3
˚
87.97(2) , V = 1963(1) A , Z = 2, Dc = 1.75 g cm , l(Mo-Ka) =
5.751 mm−1, F(000) = 1016.0, scan mode h/2h, hmax = 23◦, 5426 in-
tensity measurements, 4624 independent observed reflections [I >
2r(I)], 4624 data and 237 parameters in the final refinement, R =
0.035, wR = 0.043 [I > 2r(I)], w = 1/[r2(F) + 0.0004F2], GOF =
◦
˚
Table 1 Coordination distances (A) and angles ( ) for 1
Pt1–Br1
Pt1–Br2
2.475(1)
2.482(1)
Pt1–P1
Pt1–P2
2.262(2)
2.275(2)
−3
˚
1.32, max./min. peak in final diff. map = 1.18/−1.74 e A .
Br1–Pt1–Br2
Br1–Pt1–P1
Br1–Pt1–P2
85.53(3)
91.91(5)
170.90(5)
Br2–Pt1–P1
Br2–Pt1–P2
P1–Pt1–P2
174.92(5)
86.30(5)
96.53(7)
CCDC reference number 280146.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b513354h
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Fig. 2 The hydroxyl domain of the crystal packing of 1·(ethanol)2.
There are two chains of four hydrogen bonds (red and white stripes,
O–H · · · O–H · · · O–H · · · O–H · · · O) linking two molecules of 1 and four
ethanol molecules (C dark green). Of the four OH groups per molecule
of 1, two are involved in these hydroxyl hydrogen bonded chains, and
the other two form O–H · · · Br hydrogen bonds, marked as orange/white
stripes.
¯
Fig. 1 The crystal packing of 1·(ethanol)2, space group P1: Pt black,
Br orange, P magenta, C green, O red, H cyan. The red enclosure draws
attention to the segregation of hydroxyl groups from the catechol groups
and the ethanol molecules (C atoms dark green). The black enclosure
outlines an intermolecular (OFF)(EF)2 embrace (P4PE) motif (see text),
and the region enclosed in blue contains sequences of Ph2P moieties,
between which there are no EF (edge-to-face) or OFF (offset-face-to-face)
motifs.
namely the centrosymmetric parallel fourfold phenyl embrace or
(OFF)(EF)2 combination (OFF = offset-face-to-face, EF = edge-
to-face) that is common in complexes of this type.11
The structure of 1 bound to TiO2 nanoparticles
Fig. 1 shows the conformation of the complex, its packing
in the crystal, and the main intermolecular domains. The two
catechol rings in each molecule are approximately parallel with
Examination of the crystal structure of 1 reveals that the two
catechol rings are adjacent to each other and approximately
parallel, and therefore well structured to act as double bidentate
ligands for adjacent metal atoms on the surface of a nanocrystal.
Thus, the structure of 1 allows us to evaluate how it could bind to
faces of TiO2 nanocrystals. Commonly available TiO2 nanocrystals
(e.g. Degussa P25) often contain a mixture of anatase and rutile
phases12 and so both TiO2 structures have been considered. The
extensive research on the structures of TiO2 surfaces has been
comprehensively reviewed.13
˚
an average separation of 3.30 A, and rotated so that the catechol
functions are staggered. This arrangement of rings is analogous
to the offset-face-to-face (OFF) arrangement of phenyl rings
that commonly occurs within and between molecules. Two other
related complexes, cis-[PtCl2(PPh3)2]10 and cis-[PtCl2{PPh2(C6H4-
3-CO2H)}2],10 have similar intra-complex inter-ligand OFF inter-
actions between phenyl groups, with inter-ring separations of 3.36
˚
and 3.30 A respectively.
The hydrophilic hydroxyl domain of 1·(ethanol)2 (red enclosure
in Fig. 1) is shown in detail in Fig. 2. Chains of hydrogen bonds O–
H · · · O–H · · · O–H · · · O–H · · · O link catechol groups and ethanol
OH groups, without intramolecular hydrogen bonds between
catechol groups. There are also catechol O–H · · · Br hydrogen
bonds. Geometrical details are in Table 2.
In our previous experiments3 a small excess of the base NEt3 was
used and so the two catechol groups will be significantly depro-
tonated when coordinated to Ti. The four donor O atoms of the
complex are not coplanar, which is a structural restriction because
the O atoms that they would replace on TiO2 crystallographic
faces are coplanar. Therefore we first investigated whether the
conformation of 1 that occurs in the crystal could change on
surface binding such that the four catecholate O atoms become
coplanar. Alternative conformations were generated by rotation
around the Pt–P and P–C bonds, and then optimised by density
functional calculations, as was the molecular structure in the
crystal. The most favourable alternative conformation 1b is shown
in Fig. 3, in comparison with the conformation 1a in the crystal.
Conformer 1b, which is 8 kcal mol−1 less stable, has the four
The crystal packing of 1·(ethanol)2 contains one inter-complex
embrace motif between PPh2 moieties (black enclosure, Fig. 1),
Table 2 Selected hydrogen bonding distances (H · · · A) for 1·(ethanol)2
a
˚
˚
(A) (O–H bond lengths normalized to 1.00 A)
O3–H1O3 · · · O1Et2
O1Et2–H1O1Et2 · · · O1i
O1–H1O1i · · · O1Et1
1.70
1.99
1.62
O1Et1–H1O1Et1 · · · O2
O2–H1O2ii · · · Br2
1.87
2.38
2.46
O4–H1O4ii · · · Br2
˚
catechol O atoms within 0.2 A of their mean plane. Both 1a and
1b were considered in modelling the binding to TiO2 surfaces,
a Symmetry codes: i = 1 − x, 1 − y, 1 − z; ii = x − 1, y, z.
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Fig. 3 An alternative conformation 1b of cis-[PtBr2{PPh2(4-catechol)}2],
in comparison with the crystal conformation 1a (both optimised by density
functional methods). Both are viewed normal to the parallel catechol
rings. In 1b some phenyl ring conformations are different, to maintain
appropriate intramolecular non-bonded interactions. The separations of
˚
˚
the parallel catechol rings are 3.56 A (1a), 3.54 A (1b).
which was done by comparing the geometry of their arrays of four
O atoms with the geometries of all sets of four O atoms on the
various surfaces of anatase and rutile. This analytical procedure
permits assessment of the commensurability of 1a and 1b with
chelation of Ti at any surface of TiO2. We also recognised the
possibility that one or two of the catechol OH groups might form
a hydrogen bond with O or OH within the TiO2 surface, instead
of chelating Ti.
Fig. 4 Representations of 1a bound to the anatase (110) surface. (a) Detail
of the bidentate plus monodentate chelation of two adjacent Ti atoms. (b)
Space-filling representation of three bound molecules of 1a, viewed normal
to the face.
For anatase (tetragonal, space group I42/amd) the (101) and
(100)(010) surfaces are observed, with some (001): the (101) face is
the most thermodynamically stable. For rutile (tetragonal, space
group P42/mmm) the observed faces are (110) (most stable),
(101)(011) and (001). Each of these surfaces was investigated,
together with anatase (110), with 1a and 1b.
Most combinations showed geometrical incompatibility be-
tween the O atoms of the complex and O atom positions
completing the coordination of surface Ti. Two good fits for three
of the O atoms of conformer 1a occur, with anatase (110) and
rutile (001). In these the fourth O of the ligand is not bound to
the surface. Fig. 4(a) shows detail for 1a bound to a section of
the anatase (110) surface, which, when neutral, contains four-
coordinate Ti atoms. One catechol chelates one Ti, while the
other catechol completes five-coordination of a nearby Ti. An
arrangement of three close molecules of 1a bound to the anatase
(110) surface is illustrated in Fig. 4(b).
Titanium atoms on the rutile (001) surface also have two vacant
coordination positions, and are arranged such that the complex
1a can chelate as a bidentate plus monodentate ligand. Details of
the coordination are shown in Fig. 5(a), and an arrangement of
four molecules of 1a on the surface is shown in Fig. 5(b).
Since anatase is the predominant component of the TiO2
¯
¯
material used in our experiments, and the (101)(011)(101)(011)
surfaces of anatase are most stable and prevalent in single
crystals,13 this surface type needs to be carefully considered as
the domain for binding of cis-[PtBr2{PPh2(4-catechol)}2]. The
neutral form of this surface has a sawtooth structure with two-
coordinate O atoms on the ridges and five-coordinate Ti atoms
on the slopes (see Fig. 6(a)), with surface reconstruction involving
Fig. 5 (a) Detail of 1a bound to the (001) surface of rutile. (b) Space-filling
representation of one of the possible arrangements of four molecules of 1a
bound to rutile (001), viewed normal to the face.
to bind to it. Two possibilities were identified. One has only one
deprotonated O of each catechol ligand coordinated to Ti, with
the other OH group hydrogen bonded to O atoms on the ridges
of the surface: this structure is disfavoured by the stereochemistry
shifts of ≤0.2 A.13 The array of positions for O atoms ligating this
˚
surface is essentially rectangular, which means that 1b is better able
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or rutile (001) surfaces. The third involves the alternative ligand
conformation 1b functioning as a double bidentate chelator of Ti
atoms at the anatase (101) surface. Other commonly occurring
surfaces of anatase and rutile are incommensurate with the
ligand. Because the TiO2 particles used in our experiments on
this system have dimensions of order 25 nm, edge-binding is
expected to be minor relative to face-binding, and has not been
evaluated.
Neutral balance of surface charge occurs in the model for 1 dou-
bly chelating anatase (101) (Fig. 6(b)), because each catecholate2−
replaces one surface O2−. The two other models involving 1
add 3− per ligand, to be compensated by protonation of three
surface O2−.14 Indication of the surface coverage is provided in
Figs. 4(b) and 5(b): the packing of molecules of cis-[PtBr2{PPh2(4-
catechol)}2] on the surface depends on the surface structure,
but each molecule covers at least nine surface Ti atoms. It is
not evident from the modelling that phenyl–phenyl interactions
between adjacent cis-[PtBr2{PPh2(4-catechol)}2] complexes, as
occur in the crystal, are likely to influence the adsorption of cis-
[PtBr2{PPh2(4-catechol)}2] on TiO2 surfaces. In all three models
the PtBr2 points away from the surface and should be accessible
for further reaction (e.g. catalysis) to take place.
Chelating ligands similar to but more flexible than cis-
[PtBr2{PPh2(4-catechol)}2] bound to TiO2 surfaces have been
investigated experimentally and theoretically, including phospho-
nic acid (HP(OH)2) on anatase (101),15 bi-isonicotinic acid (2,2ꢀ-
bipyridine-4,4ꢀ-dicarboxylic acid) on rutile (110),16 and a number
of carboxylic acids.13 STM images of benzoic acid on rutile (110)
Fig. 6 (a) The sawtooth structure of anatase (101), with five-coordinate
Ti on the slopes. The separation of Ti atoms in the [010] direction is
˚
3.79 A, which matches the separation of catechol rings in 1a and 1b. (b)
The favourable double-bidentate chelation of Ti in anatase (101) by 1b.
provide evidence for pair-association of the phenyl rings separated
17
at the ligating catecholate O atom, which is too close to linear.
The other possibility has 1b functioning as a double-bidentate
ligand, with one catecholate O atom of each catechol occupying
the sixth coordination position of Ti, and the other catecholate O
substituting for a ridge O atom of the surface. This coordination
mode is illustrated in Fig. 6(b).
˚
˚
by ca. 5.9 A, larger than the 3.3–3.7 A separation of the catechol
rings in cis-[PtBr2{PPh2(4-catechol)}2].
Acknowledgements
This research was supported by the Australian Research Council,
the Australian Partnership for Advanced Computing, and The
University of New South Wales.
Discussion
Density functional calculations have demonstrated the con-
formational variability of the aryl rings in cis-[PtBr2{PPh2(4-
catechol)}2], such that the separation of the catechol rings can
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˚
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©