Metallaborane reaction chemistry. A predicted and found tailored facile and reversible capture of SO2 by a B-frame-supported bimetallic: Structures of [(PMe2Ph)2PtPd(phen)B10H 10] and [(PMe2Ph)2Pt(SO2)Pd(phen) B10H10]

Article abstract of DOI:10.1039/b800984h

The formally closo twelve-vertex {ortho-M₂B₁₀} dimetallaborane system has been predictively tailored for reversible uptake of SO₂ across the metal-metal bond, as exemplified by the formation of [(PMe₂Ph)₂Pt(SO₂)Pd(phen)B₁₀H ₁₀] from [(PMe₂Ph)₂PtPd(phen)B ₁₀H₁₀]. The Royal Society of Chemistry.

Full text of DOI:10.1039/b800984h

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Metallaborane reaction chemistry. A predicted and found tailored facile
and reversible capture of SO2 by a B-frame-supported bimetallic:
structures of [(PMe2Ph)2PtPd(phen)B10H10] and
[(PMe2Ph)2Pt(SO2)Pd(phen)B10H10]w
Jonathan Bould and John D. Kennedy*
Received (in Cambridge, UK) 18th January 2008, Accepted 28th February 2008
First published as an Advance Article on the web 27th March 2008
DOI: 10.1039/b800984h
The formally closo twelve-vertex {ortho-M2B10} dimetallabor-
ane system has been predictively tailored for reversible uptake of
SO2 across the metal–metal bond, as exemplified by the forma-
tion of [(PMe2Ph)2Pt(SO2)Pd(phen)B10H10] from [(PMe2Ph)2-
PtPd(phen)B10H10].
binding strength were the shortness of metal–oxygen,
metal–carbon, and metal–sulfur distances, respectively, as well
as lengthening of the respective O–O, C–O and S–O distances.
Generally it was found that change in ligand from PMe2Ph to
PMe3 increased the binding strength, and from PMe2Ph to
PPh3 decreased it. A change from Pt2 via PtPd to Pd2
weakened the small-molecule coordination, and a change from
a phosphine to a nitrogen ligand strengthened it. However,
these general effects are not linearly additive, but can act either
synergically or antagonistically, as we have also found that
although a particular combination of ligands and metals may
sequester one type of molecule, say O2, more strongly than
another combination, it may well sequester another type of
molecule, say SO2, more weakly. A complex matrix of beha-
viour needs to be established in order to delineate thoroughly
the factors. Meanwhile, however, the indications were that a
mixed amine phosphine ligand system, with a combination of
Pd and Pt centres, would bind SO2 much more weakly than
[(PMe2Ph)4Pt2B10H10], suggesting that a reversibility of SO2
addition could thereby be engineered. Thus, the B3LYP/
LANL2DZ-calculated Pt–S and S–O distances of 2.5314 A
and 1.5005 A, respectively, in [(PMe2Ph)4(SO2)Pt2B10H10]
change to 2.654 A and 1.485 A in [(PMe2Ph)2Pt(SO2)Pd-
(phen)B10H10] (where phen = phenanthroline, N2C12H8).
We have thence confirmed this prediction experimentally by
investigation of the behaviour of SO2 with [(PMe2Ph)2PtPd-
(phen)B10H10].
We previously demonstrated that the closo-structured twelve-
vertex cluster compound [(PMe2Ph)4Pt2B10H10] shows a re-
markable haem mimicry, in that it exhibits a readily reversible
sequestration of molecular dioxygen across the intact
metal–metal linkage to give [(PMe2Ph)4(O2)Pt2B10H10].1–3
The other small unsaturated molecules SO2 and CO are,
by contrast, irreversibly sequestered to give [(PMe2Ph)4-
(SO2)Pt2B10H10] and [(PMe2Ph)4(CO)Pt2B10H10], respec-
tively.2,3 By alteration of the metal atoms, and/or their exo-
polyhedral ligands and/or the boron-cluster substituents, this
{M2B10} system is in principle uniquely tailorable for a variety
of small-molecule sequestration, delivery, exchange and acti-
vation.2–4 We now confirm this tailorability, with this preli-
minary report, of the engineering of the system by variation of
two of these factors—the metallic elements used and the
exopolyhedral metal ligands—to engender, now, reversibility
of SO2 sequestration. The direction of synthetic investigation
to efficiently achieve this goal was determined by predictive
DFT calculations.4 Although reversible addition of SO2, for
example to a single platinum atom, has long been known, and
also the instance of non-dissociable SO2 bound across a Pt–Pt
vector similarly long recognised,5 the reversibility of the latter
type of binding is novel. The directed engineering of such
reversibility, either to a single metal atom or to a metal–metal
vector, is as far as we are aware previously unprecedented.
DFT calculations were carried out on the formally closo
{M2B10H10} system, based initially on [(PMe2Ph)4Pt2B10H10],
by variation of metal centre between platinum and palladium,
and by variation of the ligands on the metal centres. The
species O2, CO and SO2 were used as model small molecules.
Structures were initially energy-minimised at the STO-3G*/
LANL2DZ level and subsequently refined at the B3LYP/
LANL2DZ level using the Gaussian 03 package.6 Criteria of
A sample of [(phen)PdB10H12] was made from B10H14 via
[(PPh3)2PdB10H12] essentially according to the procedure of
Siedle and Todd.7 Subsequently, a solution of K[HBEt3] (1.0
M in thf; 0.56 ml, corresponding to 560 mmol) was added to a
stirred toluene suspension of [(phen)PdB10H12] (113 mg, 280
mmol) at ꢀ70 1C. The mixture was then allowed to warm to ca.
ꢀ50 1C, whereupon [PtCl2(PMe2Ph)2] (151 mg, 280 mmol) was
added via tipper tube. The mixture was allowed to warm to
room temperature, with stirring, overnight. Filtration gave a
dark brown-purple microcrystalline solid, [(PMe2Ph)2PtPd-
(phen)B10H10] 1 (130 mg, containing a small amount of KCl:
ca. 150 mmol; ca. 54% of compound 1). Pure 1 was thence
recrystallisable from a solution in 1,2-Cl2C2H4 by overlayering
with C6H14, to give crystals suitable for a single-crystal X-ray
diffraction analysis (Fig. 1, upper).zy Compound 1 is robust
compared to [(PMe2Ph)4Pt2B10H10];2 for example, a solution
in acetone in air overnight showed no sign of decomposition.
The School of Chemistry of the University of Leeds, UK LS2 9JT.
E-mail: j.d.kennedy@leeds.ac.uk. E-mail: jbould@gmail.com
w CCDC 672233 and 672234. For crystallographic data in CIF or
other electronic format, see DOI: 10.1039/b800984h
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Fig. 2 Space-filling representations of the crystallographically deter-
mined molecular structures of (top) [(PMe2Ph)2PtPd(phen)B10H10] 1,
(bottom) [(PMe2Ph)2Pt(SO2)Pd(phen)B10H10
]
2, and (centre)
[(PMe2Ph)2Pt(SO2)Pd(phen)B10H10] with the elements of SO2 omitted
to show the underlying dimetal unit. It is seen that the P-organyl
groups of the {(PMe2Ph)2} assembly on the Pt atom of 1 (top) twist
out of the way (centre) to accommodate the SO2 molecule on the
{PtPd} unit (bottom). The opening up of access to the metal–metal
vector by the relative ‘flattening’ of the phenanthroline ligand sphere
versus the twisting phosphine ligand sphere is also noteworthy. The
overall substantial twisting and flattening required to accommodate
the SO2 molecule contributes entropically to the relatively high
reversibility of addition.
Fig.
1
ORTEP-type8 representations of the crystallographically
determinedz molecular structures of (top) purple [(PMe2Ph)2PtPd-
(phen)B10H10] 1 and (bottom) orange-yellow [(PMe2Ph)2Pt(SO2)Pd-
(phen)B10H10] 2, drawn with 50%-probability ellipsoids for the non-
hydrogen atoms. Methyl hydrogen atoms are omitted for clarity.
Selected interatomic distances for
1 are: from Pt(2) to Pd(1)
2.8744(3), to P(1) 2.3343(7), to P(2) 2.3399(7), to B(3) 2.336(3), B(6)
2.335(3), to B(7) 2.245(2), to B(11) 2.240(3) A; from Pd(1) to B(3)
2.261(3), B(6) 2.278(3), B(4) 2.195(3), B(5) 2.197(3), N(1) 2.176(2), and
to N(13) 2.199(2) A; the PPtP angle is 97.63(2)1. For 2, selected
interatomic distances are: from Pt(2) to Pd(1) 2.7880(5), to P(1)
2.3886(1), to P(2) 2.3664(1), to B(3) 2.343(4), B(6) 2.351(4), to B(7)
2.265(4), to B(11) 2.286(4), and to S(1) 2.4369(1) A; from Pd(1) to B(3)
2.245(4), B(6) 2.275(4), B(4) 2.236(4), B(5) 2.257(4), N(1) 2.222(3),
N(13) 2.180(3) and to S(1) 2.4161(1) A; S(1)–O(1) is 1.474(3) and
S(1)–O(2) is 1.474(3) A; the PPtP angle is 97.03(3)1, the PdSPt angle is
70.13(3) and the OSO angle is 113.7(2)1. The dihedral angles between
the Pt(2)P(1)P(2) and Pd(1)N(1)N(2) planes are approximately 65.1
and 59.01 for compounds 1 and 2, respectively.
2.355(2) A in [(PMe2Ph)4(SO2)Pt2B10H10]2 compare well to the
trend predicted in the DFT calculations, and the considerable
increase in the Pt–S distance in 2 marries well with the
observed occurrence of reversible SO2 uptake in this system,
compared to the irreversible formation of [(PMe2Ph)4-
(SO2)Pt2B10H10] from [(PMe2Ph)4Pt2B10H10].2 Any changes
in the S–O distances are too small to be significant compared
to the standard uncertainties in the experimental measure-
ments. Finally, it may be noted that a striking colour change in
the SO2-saturated solution of 1 in CH2Cl2, between yellow at
ca. ꢀ35 1C and yellow-purple at ambient temperature, indi-
cates the presence of an equilibrium condition for the addition
of SO2 to 1 to give 2 over this temperature range. As with the
reversibility of the addition of O2 to [(PMe2Ph)4Pt2B10H10],1,2
it is apparent that a significant component of the high entropy
factor implicit in the ready reversibility derives from the
required extensive repositioning of the ligand sphere about
the dimetal site (Fig. 2). In this regard, a referee has suggested
that the phenomenon that is often called ‘p-stacking’, as
manifested here in the parallel packing of the aromatic rings
of the phosphine ligands (Fig. 2, bottom right), may contri-
bute to the conformation of compound 2. However, it may be
Passage of SO2 through a solution of 1 in CH2Cl2 resulted in a
colour change from dark purple to lighter purple-yellow.
Reduced pressure, the passing of N2 gas through the solution,
or evaporation, promoted reversal of the process to regenerate
compound 1. NMR investigationy of the yellow solution
showed a compound of the same symmetry as compound 1,
and a single-crystal X-ray diffraction analysisz, on orange-
yellow crystals obtained from the yellow solution by over-
layering with C6H14 at room temperature, showed that SO2
addition had occurred across the metal–metal vector to give
orange-yellow [(PMe2Ph)2Pt(SO2)Pd(phen)B10H10] 2 (Fig. 1,
lower). The measured Pt–S distances of 2.4369(1) A in 2 and
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noted that there is also a related alignment of a phenyl ring
with one of the rings of the phenanthroline moiety in the
starting compound 1 (Fig. 2, top pair). In any event, the
thermodynamics of the equilibrium should be eminently ad-
justable by variations in substituents on the cluster and the
ligands and these will also be predictable by DFT calculation.
The DFT-predicted reversibility of SO2 addition to this type
of system is therefore confirmed, and the unique tailorability
of this system for general small-molecule reversible sequestra-
tion, delivery and potential reactivity activation is therefore
demonstrated. A particular advantage presented by the use of
the phenanthroline ligand is that it readily opens up the
system, using bidentate amine ligands and the well-established
synthetic techniques of organic chemistry, to a vast range of
property modification, such as tailoring for water solubility,
for attachment to polymer backbones, for electro-chemical
and chemical disposition on surfaces, for antenna attachment
for possible photoactivation, and for chiral and polar control
of the approach of molecules to the active dimetal site.
We thank Colin Kilner for guidance and technical assistance
with the collection of single-crystal X-ray diffraction data and
Simon Barrett for assistance with NMR spectroscopy.
y Selected NMR and mass spectrometric data for compounds 1 and 2
(CD2Cl2) [d in ppm, 31P rel. H3PO4, 11B rel. BF3(OEt2) and 1H rel.
SiMe4]. Compound 1: d(31P) (300 K): +2.61 [1J(195Pt–31P) 2764 Hz];
d(11B) [d(1H)] (300 K): 2BH +29.4 [+5.50], 2BH +26.7 [+4.21], 2BH
+21.2 [+4.27, +3.93, 1J(195Pt–1H) ca. 60 Hz], 2BH +19.7 [+4.15],
1
1BH ꢀ24.3 [+1.30, J(195Pt–1H) ca. 42 Hz]; d(1H)(PMe2Ph) (263 K):
+1.54 (6H) [N(31P–1H) ca. 8 Hz, 3J(195Pt–1H) ca. 26 Hz], +1.45(6H)
[N(31P–1H) ca. 10 Hz, 3J(195Pt–1H) ca. 23 Hz] and (sharp multiplets)
around +6.74 to 7.29 (phenyl) and +7.81 to +9.29 (phenanthroline).
HRMS TOF, ES+ obtained on an LCT Micromass instrument: found
877.2274, calc. for C28H40B10N2P2Pd1Pt1 877.2280. Compound 2:
d(31P) (300 K): ꢀ8.23 [1J(195Pt–31P) 2790 Hz]; d(11B) [d(1H)] (300 K):
1BH +25.1 [+5.09], 1BH +23.8 [+2.28], 2BH +22.2 [+4.37], 2BH
+21.7 [+4.37], 2BH +8.7 [+3.81] and 2BH ꢀ10.5 [+2.57,
1J(195Pt–1H) ca. 54 Hz]; d(1H)(PMe2Ph) (300 K) +1.86 (6H)
[N(31P–1H) ca. 11 Hz, 3J(195Pt–1H) ca. 25 Hz], +1.54 (6H)
[N(31P–1H) ca. 11 Hz, 3J(195Pt–1H) ca. 20 Hz] and (sharp multiplets)
around +7.36 to 7.49 (phenyl) and +7.86 to +8.87 (phenanthroline).
The compound was not amenable to mass spectrometric characterisa-
tion via a molecular ion, due to its ready loss of SO2; the spectra from
.
+
the crystalline sample of 2 showed only SO2
1 J. Bould, Y. M. McInnes, M. J. Carr and J. D. Kennedy, Chem.
Commun., 2004, 2380.
2 J. Bould, C. A. Kilner and J. D. Kennedy, Dalton Trans., 2005, 1574.
3 J. Bould, M. Bown, R. J. Coldicott, E. J. Ditzel, N. N. Greenwood,
I. Macpherson, P. MacKinnon, M. Thornton-Pett and J. D Kenne-
dy, (EUROBORON 3 Special Edition), J. Organomet. Chem., 2005,
690, 2701.
4 J. Bould and J. D. Kennedy, Abstracts Fourth European Meeting on
Boron Chemistry (EUROBORON 4), Bremen, Germany, 2–6
September, 2007, abstract no. O39, p. 56.
Notes and references
z X-Ray data for compound 1: crystal data for squeeze:
C28H40B10N2P2PdPt, M = 876.15, 0.48 ꢂ 0.15 ꢂ 0.10 mm3, triclinic,
5 See for example: D. C. Moody and R. R. Ryan, Inorg. Chem., 1977,
16, 1052, and references cited therein.
ꢀ
space group P1 (no. 2), a = 12.6626(12), b = 12.9085(12), c =
6 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N.
Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V.
Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V.
G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas,
D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J.
V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B.
Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, M. Challacombe, P. M. W. Gill, B. G. Johnson, W.
Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision B.05), Gaussian, Inc., Wallingford, CT, 2004.
14.5518(14) A, a = 71.088(5), b = 74.801(5), g = 81.734(6)1, V =
2167.0(4) A3, Z = 2, Dc = 1.343 g cmꢀ3, F000 = 852, Mo-Ka
radiation, l = 0.71073 A, T = 150(2) K, 2ymax = 57.71, 52 168
reflections collected, 11 253 unique (Rint = 0.0354). Final GooF =
1.040, R1 = 0.0225, wR2 = 0.0586, R indices based on 10 237
reflections with I 4 2s(I) (refinement on F2), 419 parameters, 6
restraints. Lp and absorption corrections applied, m = 3.732 mmꢀ1
.
Disordered 1,2-Cl2C2H4 solvent molecules were incorporated in the
model using PLATON SQUEEZE;9 CCDC 672233.
Crystal data for compound 2, [(PMe2Ph)2Pt(SO2)Pd(phen)B10H10
]
(CH2Cl2 trisolvate): C31H46B10Cl6N2O2P2PdPtS, 1194.99,
M
=
0.23 ꢂ 0.14 ꢂ 0.05 mm3, monoclinic, space group P21/n (no. 14),
a = 14.689(2), b = 17.188(2), c = 18.742(3) A, b = 98.046(8)1, V =
4685.2(12) A3, Z = 4, Dc = 1.694 g cmꢀ3, F000 = 2336, Mo-Ka
radiation, l = 0.71073 A, T = 150(2) K, 2ymax = 58.91, 109 904
reflections collected, 12 289 unique (Rint = 0.0633). Final GooF =
1.044, R1 = 0.0311, wR2 = 0.0729, R indices based on 10 072
reflections with I 4 2s(I) (refinement on F2), 509 parameters, 0
7 A. R. Siedle and L. J. Todd, Inorg. Chem., 1976, 15, 2838.
8 J. Farrugia, ORTEP-3, J. Appl. Crystallogr., 1997, 30, 565.
9 P. van der Sluis and A. L. Spek, Acta Crystallogr., Sect. A, 1990, 46, 194.
restraints. Lp and absorption corrections applied, m = 3.853 mmꢀ1
;
CCDC 672234.
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Chem. Commun., 2008, 2447–2449 | 2449