electronic structure of binuclear platinum complexes

SPECIAL ISSUE ON PLATINUM GROUP CHEMISTRY, PART I

ELECTRONIC STRUCTURE OF BINUCLEAR PLATINUM COMPLEXES Axel Klein Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany CONTENTS Abstract 1. Introduction 2. Electrochemistry 3. ESR spectroscopy 4. Optical spectroscopy 5. UV/Vis/NIR spectroelectrochemistry 6. Conclusion 7. References

283 283 288 293 294 296 298 299

ABSTRACT Reports on investigations on the electronic structure of binuclear platinum complexes were reviewed over the past decade, focusing on compounds with organic bridging ligands that separate the metal centers but are potentially able to mediate metal-metal interaction.

1. INTRODUCTION Binuclear and oligonuclear complexes of platinum group elements have met enormous interest in this decade due to their numerous potential applications. Their well established photophysical and photochemical properties determine such molecules as potential antenna molecules for light harvesting, light induced catalysis or electrocatalysis.1"5 A quite efficient way to assemble polynuclear complexes is the use of defined bridging ligands.

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Vol. 20, No. 4, 2000

mainly

of

Electronic Structure of Binuclear Platinum

the

multinuclear

polyimine

complexes

type1'6"'Extensive

of

studies

are

reported

1 3 710 12

ruthenium,

osmium, " ·

Complexes

"

rhodium

on and

iridium. 1314 In contrast, much less work has been devoted to comparable platinum or palladium complexes. Interest in platinum complexes of that type is fed by their interesting properties found in cancer research15"21 or catalysis.22'23 The present review focuses on homobinuclear complexes of platinum and their electronic structure as determined by various techniques. Heterobimetallic compounds24"31 that have met some interest in the last few years are not discussed here. The interesting class of homobimetallic or oligometallic compounds showing direct Pt Pt interaction will be reviewed only briefly in this introduction. It is well known that compounds like [(bpy)Pt(X)2]

(X

thienyl)pyridine), 40

=

CI,

CN),32"39

Br,

[Pt(thpy)2]

(thpy

[Pt(terpy)X]+,41"43 or [Pt(quaterpy)]2+

44

=

2-(2-

form

inler-

molecular stacks in the solid state and also in solution at high concentrations, as can be seen from characteristical spectroscopic phenomena e.g. excimer emission.45"47 These w/ermolecular Pt Pt distances range approximately from 3 to 4.5 A. Molecules with short /Miramolecular Pt Pt contacts are formed using bridging anions like X", RO", RS" or RSe" (R = H, alky I or aryl),48-56 small nitrogen triazenido. bridging

containing 58,59

bridging

ligands

like pyrazolato, 57

amidinato

or

Also new complexes with alkinyl or phosphinoalkyne

ligands,60"62

bidentate

bis(diphenylphosphino)methane)

63 64

phosphines

like

dppm

(dppm

=

or the well studied phosphonates have

been reported.65"67 In addition, biorelevant molecules like arginine or nucleobases are able to bridge two or more platinum centers.68"74 The Pt Pt distances for these compounds range between 2.8 and 3.5 A. Another interesting molecule is formed by two para-benzoquinones

and 4,5-

diazafIuoren-9-one, all three of which bridge two platinum(O) centers forming a binuclear complex. The palladium derivative was structurally characterized and a Pd-Pd distance of 2.775 A was found. 75 A dative Pt-Pt bond in the solid state (d Pt-Pt = 2.723 Ä) but non-bonding interaction in fluid solution,

as seen

from NMR experiments,

is reported

diphenylphosphinocyclopentadienyl bridged binuclear complex.

76

for a Shorter

Pt Pt distances are found in compounds where a real Pt-Pt bond has to be discussed.77"79 In contrast to the compounds mentioned above, this paper will deal with homobinuclear platinum complexes, where a bridging ligand separates the platinum centres by at least 5 A, but is potentially able to mediate the


Axel Klein

Reviews in Inorganic Chemistry

interaction of the two platinum centers via its unsaturated π system. The ligands used were mainly nitrogen containing aromatic or iminic ligands. The design of the ligands allows not only to vary the Pt Pt distance and connectivity but also the Pt-ligand interaction (Scheme 1).

tdidien

bpip

Scheme 1: Schematic representation of the bridging ligands used in this work, bpym = 2,2'-bipyrimidine, bptz = 3,6-bis(2-pyridyl)-l,2,4,5-tetrazine tdidien = l,4-bis(l-(N,N'-dimethylaminoethyl)iminoethyl)benzene (Bis(N,N-dimethylaminoethyl)terephthaldiimide), bpip = 2,5-bis(l-phenyliminoethyl)pyrazine,.

Results will be presented from various electrochemical and spectroscopic methods that have been used to probe for interactions of the metal centers and the bridging ligand and also for the interaction between the metal centres. The synthesis and characterization of the platinum compounds shown in Scheme 2 were previously described. 80 " 84 For 5 the molecular structure could be determined from single crystal X-ray diffraction; the Pt-Pt distance is 5.751 A. 83


Vol. 20, No. 4, 2000


Φ

Complexes

V)

ω Έ

Φ 2

α.

/\

mK3 ν

σ>ΐ

/ α.

/ \ α>

V) 0) Έ

bpip > tdidien, parallel to decreasing nitrogen content and increasing electron density in the central phenyl-type core.

4. OPTICAL SPECTROSCOPY One of the typical properties of binuclear platinum complexes of that type is the appearance of low-energy optical transitions. If the bridging ligands possess low-lying π*-orbitals they are usually assigned to charge transfer transitions. Due to the coordination of two electrophilic platinum centres they are found at very low energies (often in the NIR region). Table 4 lists some absorption data.

Table 4 UV/Vis/NIR absorption data of parent complexes a compound

CT (3)

CT (2)

CT(1)

1 2 3 4 5 7 8 9

374 309 301 362, 375 372sh 370sh, 398sh

482sh 410 395 432 402 428sh, 511sh 348 690 465sh 557, 603, 654 510sh 471 411 334

588 535, 595 520, 576 627 623 600 461, 477sh 852 588sh, 641sh, 698 765 670sh, 734, 805sh 534 473 447

Μ 11 11 13 Μ 15 a

440 414 418 400 372 328 243

Absorption maxima (in nm) as measured in CH2C12.


Axel Klein

Reviews in Inorganic

Chemistry

The energy of the lowest transition decreases along the series tdidien > bpym > bpip > bptz. Hence the optical transition simply reflects the relative energy of the π* LUMO of the bridging ligand. Such a comparison for complexes with various Pt fragments is not so straightforward, since the transitions are not necessarily of the same character. For the platinum(II) complexes at least two d(Pt)-7i* (MLCT) transitions are observed. For the PtMe4 complexes the long-wavelength absorption has σ(Με-Ρί-Με)-π* (SBLCT) character, the compounds are therefore photolabile. The transitions at higher energies are of MLCT type. The character of the low-energy absorption in PtMe3X complexes is assigned to a ρπ(Χ)-π* (LLCT) • ·

94

transition. For the mixed complex 7 the absorption spectrum (Figure 1) is the exact superposition of the spectra of the two homoleptic complexes 4 and 5. So the chromophores seem to be isolated from each other.

300

400

500 600 λ [nm]

700

800

Fig. 1: Absorption spectra of 4 (upper), 7 (middle) and 5 (lower) in CH2C12


Vol. 20, No. 4, 2000


Complexes

A similar behaviour was reported by Eisenberg et al. for the dipyridocatecholate bridged complex 18 shown in Scheme 6.95 96

18 Scheme 6: Schematic representation of complex 18. Although the absorption spectrum appears to arise from overlapping, independent transitions of separate isolated chromophores, the authors assume some interaction between the platinum centres because the expected emission of the [(tdt)Pt(dbcat)] chromophore is quenched. For the binuclear complex [Ph2Pt(dpp)PtCl2] (dpp = 2,3-bis(2-pyridyl)pyrazine) also no emission was found, whereas both monomelic complexes are luminescent.6 Whether this is a hint for a platinum-platinum interaction, or whether the excited state structure and dynamics have simply changed from the mononuclear to the dinuclear complex, is still unclear. So far, we have not observed luminescence for any of the binuclear complexes presented here. The main reason for this failure might be that the spectroscopic range for such emissions would lie beyond 900 nm and such emissions are assumed to be quite weak.

5. UV/VIS/NIR SPECTROELECTROCHEMISTRY The UV/Vis/NIR spectra of the reduced forms of the dinuclear platinum complexes have been recorded using spectroelectrochemical techniques, that is the in situ electrolysis in an optical transparent ihin-/ayer electrochemical (OTTLE) cell.97 First of all, this technique provides an excellent tool to probe for long-term reversibility of electrochemical processes. It was shown for the present complexes that the first reduction is fully reversible for all the



Axel Klein

complexes but 13. Also many of the (ligand-based) second reductions were reversible under these conditions. The oxidized forms of the Pt mesityl complexes, however, are stable for some minutes at ambient temperature. Table 5 shows some selected data for some in situ prepared radical anionic complexes. Table 5 UV/Vis/NIR absorption data of radical anionic ligands and platinum complexes 3 compound b

IL (3)

IL (2)

IL(1)

369

517, 548

819, 930

{2}-

373

501

892

{4}" -

469

580

872

376 375

507

912

503

954, 1124

373

492, 525

959, 1128

280

460

700

{10}"

375

468, 596

735, 840

{11}' {12}' {bptz} °

313 398

540 475

630

240

353, 482sh

{9}'

290

553, 613sh d

{tdidien}

416

540

{bpym}

(5>{7}" {bpip}

c

1056

673

539 682 895 {UK 408 538 634 {15}" a Generated by in situ electrolysis in THF/0.1 Μ Bu 4 NPF 6 . MeCN. d Residual MLCT.

b

In DMF.

c

In

The data confirm that the reduction occurs mainly at the bridging ligands. Thus, the absorption spectra of the radicals anions are very similar to those of the free ligands. The long-wavelength absorptions are assigned to transitions from the Tt*(SOMO) to higher LUMOs. From MO calculations it should be anticipated that the energy of the lowest transition for both the platinum complexes and the free ligands should decrease in the series tdidien > bpip > bpym > bptz. 98 ' 99 This is exactly what is found for the first three systems. The bptz system, however, does not show the expected transition at very lowenergy that should be anticipated from the very close lying second LUMO of the same symmetry. 99 We can explain this speculatively by assuming that

297

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Vol. 20, No. 4, 2000


Complexes

either this transition lies in the IR region of the spectrum, or that the two MO levels have become degenerate in the platinum complex. Investigations on this subject are ongoing. At present we cannot correlate the observed transition energies with the influences from the platinum fragments. More detailed assignments based on theoretical calculations are warranted. A good probe for the metal-metal interaction in mixed-valent species is the observation of a intervalence charge transfer band (IVCT). The energy, line-width and intensity allows to classifiy mixed-valent species according to Marcus and Hush's theory.100"102 For the above binuclear complex 16 (Scheme 4) an IVCT band at 704 nm (ε = 700 Μ"1 cm"1) was found. 91 For the mixed-valent species {9}+ a IVCT band could not be observed due to the lability of this species but further efforts are ongoing.

6. CONCLUSION The systematic investigation of the electronic properties of binuclear platinum(II) or platinum(IV) complexes with a number of unsaturated nitrogen containing bridging ligands has revealed specific interactions between the bridging ligand and the platinum fragments, as well as metalmetal interactions mediated by the bridging ligands. Electrochemical data reflect

both

types

of

interactions

very

well.

Spectroelectrochemical

techniques (UV/Vis/NIR or ESR) have allowed to locate the redox sites for the individual processes, e.g ligand-based reductions and platinum-based oxidations. Also, they gave further evidence on the reversiblity of these reactions. Futhermore, ESR spectroscopy showed that, apart from the coupling constant of the platinum centers to the unpaired electron, the g anisotropy is also a means to determine the extent of mixing between metal and ligand orbitals and thus enables to probe for the ligand-metal interaction. All

binuclear

complexes

exhibit

several

long-wavelength

electronic

absorptions. They can be assigned to charge transfer transitions (CT), their character (MLCT, SBLCT, LLCT etc.) depends on the nature of the Pt fragment. The consequences of ligand-metal or metal-metal interaction on the optical properties (absorption or emission) are more difficult to understand and require additional detailed studies.

298


Axel Klein


ACKNOWLEDGEMENTS Prof. W. Kaim (University of Stuttgart) is thanked for financial support, Rainer F. Winter is acknowledged for helpful comments.

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