Electronic structure and spectroscopy of luminescent heterobimetallic platinum(II)-rhodium(I), gold(I)-Rh(I), and silver(I)-Rh(I) complexes

(1)

3402 Inorg. Chem. 1993, 32, 3402-3407

Electronic Structure and Spectroscopy of Luminescent Heterobimetallic Pt(I1)-Rh(I),

Au(

I)

-Rh( I), and

Ag

(I)

-Rh( I) Complexes

Hon-Kay Yip,+ Hsiu-Mei Lin,* Yu Wang,* and Chi-Ming Che'Jv*

Departments of Chemistry, The University of

Hong

Kong, Pokfulam Road,

Hong

Kong, and

National Taiwan University, Taipei, Taiwan Received October 28, 1992

The spectroscopic and luminescent properties of the heterobimetallic

[PtRh(dppm)2(CN)2(CN-r-Bu)2JC104,

[AuRh-

(dppm)2(CN-t-Bu)2](C104)2,

and

[AgRh(dppm)2(CN-t-B~)2](C104)2

complexes were investigated. All the three

complexes display an intense '(do*

-

pu) transition at 473 nm (e,,,,, = 1.34 X lo4 M- cm-I), 455 nm (e,,, = 2.40

X lo4 M-1 cm-I), and 425 nm (e,,,,, = 1.68 X lo4 M-1 cm-l), respectively. A molecular orbital calculation on the

model complex

[PtRh(dmpm)2(CN)2(CN-t-Bu)2]+

(dmpm = bis(dimethy1phosphino)methane) suggests that the

du* orbital is mainly derived from the 4d,2 of Rh and the pu is made up of T* of isocyanide and 5pz of Rh. In each

case, the l(du*

-

pu) transition has substantial Rh+ ?r*(isocyanide, phosphine) charge-transfer character. Excitation

of solid samples of the complexes leads to phosphorescence derived from the 3(du*pu) excited state.

Introduction

The spectroscopy and photochemistry of heterobimetallic complexes have recieved much attention recently. Systems studied include various combinations of different metal ions such as dlo- main group ions (Au-Pb, Au-T1Ia), ds-main group ions (Pt-Tl,Ib Ir-T1,Ic Ir-PbId), and dI0-dS systems (Au-Pt,lc Au-Irlf,g). Our

recent work on the dI0-ds [A~IPt~~(dppm)2(CN)2]+ l e (dppm =

bis(dipheny1phosphino)methane) demonstrated that apart from the well-established homodinuclear ds-d8 complexes, heterobimetallic complexes are also very promising systems for photochemical studies. To further our study in metal-metal interaction and photophysics of heterobimetallic complexes, we examine the spectroscopic and photophysical properties of luminescent

dinuclear complexes

[PtI1Rh1(dppm)2(CN)2(CN-t-Bu)2]

Clod and

[MRh(dppm)2(CN-t-Bu)2](C104)2

(M = Au+, Ag+). The complexes are of special interest since they bears similar structure of the well-established d 8 4 dimers such as [Pt2(P205H2)4I6

and [Rhz( 1,3-diisocyanopropane)4]2+ 3 and dlo-dIo dimers such

as [ A u 2 ( d p p m ) ~ ] ~ + . ~ Since the completion of our work, Balch

and co-workers reported the spectroscopic properties and X-ray

crystal structure of

[Pt11Rh1(dppm)2(CN)2(CN-t-Bu)2]+.5

Experimental Section

Materials. [PtRh(dppm)2(CN)2(CO)C1],sa [MRh(dppm)2(CN-r-

Bu)2](C104)2 (M = Au+, Ag+),5b and frans-[Rh(PPh3)2(CO)CI]sC were prepared by the literature methods.

t The University of Hong Kong.

t National Taiwan Universitv.

(a) Wang, S.; Garz'on, G:; King, C.; Wang, J.-C.; Fackler, J. P., Jr. Inorg. Chem. 1989,28,4623. (b) Nagle, J. K.; Balch, A. L.; Olmstead, M. M. J . Am. Chem. SOC. 1988,110, 319. (c) Balch, A. L.; Neve, F.; Olmstead, M. M. J. Am. Chem. SOC. 1991, 113, 2995. (d) Balch, A. L.; Catalano, V. J.; Chatfield, M. A.; Nagle, J. K.; Olmstead, M. M.; Reedy, P. E. J. Am. Chem. SOC. 1991,113, 1252. (e) Yip, H.-K.; Che, C.-M.; Peng, S.-M. J. Chem. Soc., Chem. Commun. 1991, 1626. (f) Balch, A. L.; Catalano, V. J.; Olmstead, M. M. Inorg. Chem. 1990,29, 585. (9) Balch, A. L.; Catalano, V. J.; Noll, B. C.; Olmstead, M. M. J. Am. Chem. SOC. 1990, 112, 7558.

Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989, 22, 5 5 .

Mann, K. R.; Thich, J. A.; Bell, R. A.; Coyle, C. L.; Gray, H. B. Inorg.

Chem. 1980, 19, 2462.

(a) King, C.; Wang, J.-C.; Khan, N. I. Md.; Fackler, J. P. Inorg. Chem. 1989,28,2145. (b) Che, C.-M.; Kwong, H.-L.; Poon, C.-K.; Yam, V. W.-W. J . Chem. Soc., Dalton Trans. 1990, 3215.

Balch, A. L.; Catalano, V. J. Inorg. Chem. 1992, 31, 3934. (a) Hassan, F. S. M.; Markham, D. P.; Pringle, P. G.; Shaw, B. L. J.

Chem. Soc., Dalton Trans. 1985, 279. (b) Langrick, C. R.; Shaw, B. L.J. Chem. Soc.,Da/ton Trans. 1985,5 1 1. (c) Osborn, J. A,; Wilkinson, G. Inorg. Synth. 1990, 28, 77.

0020-166919311332-3402%04.00/0

Synthesis of [PtRh(dppm)~(CN)z(CN-t-Bu)z~lO4. tert-Butyl isocy-

anide (CN-t-Bu) (0.028 g, 0.34mmol) was added toa methanol suspension of [Pt11Rhl(dppm)2(CN)2(CO)(Cl)] (0.2g,0.17 mmol), and themixture was stirred a t room temperature. Efflorescence was observed upon addition, and the solution was stirred until a clear orange solution was obtained. The solution was then filtered, and an orange precipitate was obtained after adding LiC104 to the filtrate. The orange solid was recrystallized by diffusing diethyl ether into acetonitrile. Yield = 89%. Needlelike crystals obtained by this method were further used for X-ray crystallographic study. IR: 21 30 cm-I (v(C=N) of cyanide); 2145 cm-1 (v(C=N) of isocyanide). IH N M R : b (ppm) = 0.769 (s, 18H, methyl protons of the isocyanides), 4.32 (m, 4H, methylene protons of dppm), and 7.37, 7.80 (m, 40H, phenyl protons of dppm). 31P N M R data: b

(ppm) = 19.11 (d, P-Rh-P, IJ(Rh-P) = 105 Hz), 2.47 (pseudotriplet, P-Pt-P, IJ(Pt-P) = 2460 Hz). The structure of the complex was established by X-ray crystal analysis.'

Synthesis of ~rans[Rh(PPhs)z(CN-t-Bu)~]CIO4. To a methanol

suspension of trans-[Rh(PPh3)2(CO)CI] (0.2 g, 0.29 mmol) was added tert-butyl isocyanide (0.048 g, 0.58 mmol). A clear solution immediately resulted. The solution was filtered, and a yellow solid was obtained after adding LiClO4 to the filtrate. The product was recrystallized by slow diffusion of diethyl ether into an acetonitrile solution. Yield = 75%. IR: 2105 cm-l (s, sh) (v(C=N). IH NMR: b (ppm) = 0.61 (s, 18H, methyl protons of tert-butyl isocyanide), 7.2-7.8 (m, 40H, phenyl protons of PPh3). Anal. Calcd for RhC46H48N2P2C104: C, 61.8; H, 5.4; N , 3.1. Found: C, 61.4; H, 5.2; N, 3.0.

Instrumentation. Electronic absorption spectra were recorded on a

Milton Roy Spectronic 3000 array spectrometer. Room-temperature and 77 K emission spectra were recorded on a Spex 1681 spectrofluo- rometer. Emission lifetimes were measured by using a Spectra-Physics DCR-3 Nd:YAG pulsed laser with signal acquired by a Tektronix 2430 digitial analyzer. 31P and IH N M R spectra were recorded on a 270- M H z Jeol JMN-GSX 270 spectrometer. The chemical shifts were reported relative to H3P04.

Molecular Orbital Calculations. E H M O calculations were carried

out using the geometric parameters from the X-ray diffraction data.' The basis functions of Pt and Rh were taken from the literature.* Molecular orbital calculations were made with the I C O N program.9 For the wave function plots, the MOPLOT program and a locally developed contour plot routine were used.I0

(7) Che, C.-M.; Wang, Y. Unpublished result.

(8) (a) Schilling, B. E. R.; Hoffmann, R. J . Am. Chem. Soc. 1979, 101, 3456. (b) Wheeler, R. A.; Piela, L.; Hoffmann, R. J. Am. Chem. Soc. 1988, 110, 7302.

(9) Both ICON and MOPLOT programs are from QCPE, Department of Chemistry, Indiana University, Bloomington, IN 47405.

(10) Tsai, C. J. Masters Thesis, National Taiwan University, 1982.

0 1993 American Chemical Society

Downloaded by NATIONAL TAIWAN UNIV on August 20, 2009

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Luminescent Heterobimetallic Complexes Inorganic Chemistry, Vol. 32, No. 16, 1993 3403

wavelength (nm)

Figure 1. Absorption spectrum of an acetonitrile solution of [PtIIRhI- (dppm)2(CN)2(CN-?-Bu)2] c104 measured at room temperature.

Results

and Discussion

Heterobimetallic d8-d8Complex. The complex [PtRh(dppm)2- (CN)2(CN+Bu)2]C104 has been characterized by X-ray crys-

t a l l ~ g r a p h y . ~ It possesses essentially the same structure as the

one reported by Balch,s being composed of two face-to-face square planar P-Rh(CN-t-Bu)2-P and P-Pt(CN)2-P units. The in-

tramolecular Pt-Rh distance of 3.043(2)

A

of the perchlorate

complex7 is slightly shorter than 3.079(1)

A

found in the

hexafluorophosphate complex.s

Figure 1 shows the absorption spectrum of an acetonitrile

solution of [

Pt1IRh1(dppm)2(CN)z(CN-t-Bu)2]

C104 measured at

room temperature. The spectrum is featured by an intense

and a moderately intense absorption at 348 nm (E,,, = 5.09 X

103 M-l cm-l). The 473-nm band has its shape and intensity

similar to that of the '(da*

-

pa) transition commonly found in

dinuclear d 8 4 8 complexes such as [Pt2(P20sH2)IC or [Rhz- (1,3-diisocyanopropane)4]2+ and is assigned to it accordingly.

Interestingly, this band is situated in between the l(da*

-

pa)

transition energies of [Ptz(dppm)z(CN)4] (324 nm)I1 and [Rhr (dppm)2(CN-t-Bu)4]2+ (523 nm).l2 In order to understand the

influence of the Pt(I1) moiety on the l(d9

-

p,, ?r*) transition

of trans-[Rh(PR3)2(RNC)2]+, the absorption spectrum of truns- [Rh1(PPh3)2(CN-t-Bu)2]+ is examined. Figure 2 shows the

absorption spectrum of trans- [Rh1(PPh3)2(CN-t-Bu)2] + measured

in acetonitrile solution a t room temperature. According to previous spectroscopic assignment of [Rh(CNR)d],+" the two

intense absorption bands at 397 nm (c,,, = 5.50 X lo3 M-l cm-I)

and 317 nm (E,,, = 1.81 X 104M-1 cm-1) are assigned to the l(d,r

-

p,, T * ) and l(dxryr

-

p,, T * ) transitions, respectively. The

energy difference between the 473-nm band of [PtIlRhI(dppm)2-

(CN)2(CN-r-B~)2]+ and the l(d,r

-

p,, T * ) transition of trans-

[Rh1(CN-t-Bu)2(PPh3)2]+is4047

cm-'. Thisvalue is small when it is compared with the related difference in energy between the

l(da*

-

pa) transition of homodinuclear d * 4 8 complexes and

the I(d,i

-

pz, T * ) transition of their monomeric counterparts

(seeTable I). From table I, it is clear that intramolecular metal- metal distance is one of the factors which determine the amount of energy difference between the transitions.l4 However, it is also obvious that, among all the six complexes listed in Table I,

**[Pt1*Rh1(dppm)2(CN)2(CN-t-B~)2]**

+ has the shortest metal-metal (11) Che, C.-M.; Yam, V. W.-W.; Wong, W.-T.; Lai, T.-F. Inorg. Chem. (12) Balch, A. L. J . Am. Chem. SOC. 1976, 98, 8049.

(13) Isci, H.; Mason, W. R. Inorg. Chem. 1975, 14, 913.

absorption band centered at 473 nm (E,,, = 1.34 X lo4 M-1 cm-1 )

1989, 28, 2908. 0.62

i

\ I \ 0. 1 1 I I I l I I I l I l I I , I I I I I I I I I , I I I I I I l I 1 , I I l 2 0 Q 250 300 350 400 450 500 Wrwlmg&(nm)

Figure 2. Absorption spectrum of trons-[Rh1(PPh3)2(CN-t-Bu)2]+

measured in acetonitrile solution at room temperature. Table I. Energy Differences of '(du*

-

pu) Transitions of Binuclear d 8 4 * Complexes and '(d.2

-

pz) Transitions of Their Mononuclear Analogs

energy diff metal-metal

between separation

'(do*

-

pa) dists of and '(dz

-

pz) binuclear [Rh2(dpam)2(Co)2(C1)2la 6500 lo 3.396( l ) b [Rh2(TMB)4I2+ * 7824 3 3.26 3 [Ir2(TMBhI2+ 764lC 3.16(4)c [Ptz(dPPm)2(CN)41 5594 3.301(1) [ Rh2(dppm)2(CN-r-Bu)4l2+ 6068 lo [PtRh(dppm)2(CN)2(CN-t-B~)21+ 4047 3.043(2)

a dpam = bis(dipheny1arsino)methane. TMB = 2,5-dimethyl-2,5-

diisocyanohexane. Mague, J. T. Inorg. Chem. 1969,9, 1975. e Simth,

D. C.; Miskowski, V. M.; Mason, W. R.; Gray, H. B. J . A m . Chem. Soc.

1990,112,3759.

complex transitions (cm-1) complexes

(A)

distance, but theenergy differenceof the transitions is the smallest. We attribute this to the difference in energy of the interacting dzz orbitals of Pt and Rh.

Figure 3a displays a molecular orbital diagram for trans-[Rh- (PMe3)2(CN-t-Bu)2]+ based on an E H M O calculation (in our

calculation, the complex trans- [ R ~ ( P M ~ ~ ) z ( C N - ~ - B ~ ) ~ ] + rather

than

trans-[Rh(PPh3)z(CN-?-Bu)2]+

was used). The order of

the d-orbitals of Rh(1) in a square planar geometry should be

d9-9

>

d,i

>

(dxz, dyz)

>

d,, but in this complex the order is d3-9

>

d,i

>

d,,

>

d,

>

d,,, where the x-direction is along Rh-P and

the y-direction is along Rh-CN. The HOMO is a d t orbital of

Rh, and the LUMO is a combination of p, on Rh, T* of CN, and

(p,, dx,) on P. The two intensive absorption bands (397,317 nm)

shown in Figure 2 are assigned to transitions to this LUMO from d i ( R h ) and d,,(Rh), respectively. From an E H M O calculation,

the calculated energy differences for these two transitions are

3.565 and 3.629 eV, respectively. Figure 3b shows a molecular

orbital diagram for

**[Pt1IRh1(dmpm)2(CN)2(CN-r-Bu)*]+**

(the

dppm is replaced by (CH3)2PCH2P(CH3)2 in the calculation) based on the EHMO calculation. The orbital energies of the d-orbitals of Rh in this complex are roughly the same as that in

trans-[Rh(PMe3)2(CN-?-Bu)2]+.

Weak interaction between the (14) (a) Yersin, H.; Gliemann, G. Ann. N . Y. Acad. Sci. 1978,313,539. (b)

Yersin, H.; Gliemann, G.; Rossler, U. Solid Srote Commun. 1977, 21, 915.

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3404 Inorganic Chemistry, Vol. 32, No. 16, 1993

Rh [ R h ( P P h j )Z(CN-t-Bu 121'

- 4

.s

3! 5 P 1 5 %

Figure 3. Molecular orbital diagrams of (a, top) tram- [RhI(PPh&(CN-

t-Bu)*]+ and (b, bottom) [Pt11Rh1(dmpm)2(CN)2(CN-r-Bu)2]t.

4dz(Rh) and Sd,z(Pt) orbitals gives rise to d a and da* orbitals. The d a orbital is mainly from the Sdt(Pt), and da* is from 4dz2- (Rh). The calculation showed that the coefficients of the 4d,z

of Rh and 5d9 of Pt in the da* (da) orbitals are 0.82 (0.38) and

0.45 (0.60), respectively. The destabilization of da* is about

0.27 eV from the 4d,z orbital of Rh monomer complex (Figure

3a). A contour diagram showing the wave function of the da*

orbital is given in Figure 4a. The pa orbital in Figure 4b mainly

comes from Spy(Rh), A* orbitals of isocyanides, and (pz, dxz)

orbitals of phosphorus. The wave function of this orbital shown in Figure 4b is similar to that of the LUMO of trans-[Rh(PMe3)*- (CN-t-Bu)z]+. However, the stabilization of this orbital from the LUMO of the Rh monomer is quite substantial, i.e. 0.38 eV (Figure 3a,b). With these results, it is apparent that the 473-nm

band of

[PtRh(dppm)z(CN)2(CN-t-B~)2]+,

which is assigned to

the '(do* -pa) transition ('AI + 'AI, asuming a C2, symmetry

Yip et al.

, '. .

Figure 4. Contour plots of the wave functions of (a, top) du* and (b,

middle; c, bottom) paorbitals and ?r* of CN-of [PPRhI(dmpm)2(CN)2-

(CN-~-BU)~]+.

for the complex), has a significant extent of Rh

-

A* (isocyanide,

phosphine) charge-transfer character.

The transition at 317 nm of

rrans-[Rh(PPh3)2(CN-t-Bu)z]t

is red-shifted to 348 nm in

**[Pt*1Rh1(dppm)~(CN)2(CN-t-Bu)2]+.**

The latter absorption band is assigned to the l(dyz

-

pa)

(IBz

-

'AI) transition. The energy difference between the two transitions

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Luminescent Heterobimetallic Complexes Inorganic Chemistry, Vol. 32, No. 16, 1993 3405

wP.del@ 1 u'

Figures. 77 Kemissionspectrumof [Pt1IRh1(dppm)2(CN)z(CN-t-Bu)z]-

Clod measured in an n-butyronitrile glass solution.

is only about 28 10 cm-1 (cf. the calculated difference of 0.30 eV (2420 cm-I)). This value is smaller than the energy difference

between the I(d,z

-

p,, n*) and l(du*

-

pu) transitions

(calculated, 0.65 eV (5243 cm-l); experimental, 4047 cm-I). Such a finding is not unexpected since the interaction between the dyr orbitals is smaller than that between the d,z orbitals of the two metal ions.

Photoexcitation of a solid sample of [Pt11Rh1(dppm)2(CN)2- (CN-t-Bu)21C104 at 470 nm results in two emissions centered at 520 and 638 nm. The room-temperature lifetimes of the 520-

and 610-nm emissions are <20 ns and 0.1 ps, respectively. On

the basis of the lifetimes and Stokes shift of the emission from the lowest energy allowed absorption band, the 520-nm emission

is assigned to fluorescence l(du*lpd)

-

'(du*2pd') ('AI

-

'AI)

while the 610-nm one is assigned to phosphorescence 3(du*lpd)

-

I(du*2pdJ) (3Al

-

AI). These two emissions become

intensified and reduced in width a t low temperature. Figure 5

shows the 77 K emission spectrum measured in an n-butyronitrile

glass solution. The maxima of the two emission bands change

to 521 and 637 nm a t 77 K. The singlet (IAl) and triplet (3Al)

splitting, measured from the fluorescence and phosphorescence maxima, is 3495 cm-1. This value is very close to the 3500-cm-I splitting found in a number of Rh(1) monomers with chelating diphosphine ligands.l5

The excitation spectrum of the emission shows two bands centered at 351 and 483 nm, which correspond to the l(dy,-pu)

and '(do*

-

pu) transitions, respectively. A shoulder at 367 nm

is observed in the 351-nm excitation band.

The mononuclear trans- [Rh1(PPh3)2(CN-t-Bu)2]+ displays

emission a t low temperature. Figure 6 shows its 77 K emission

spectrum measured in an n-butyronitrile glass solution. The 580-

nm emission is assigned to the 3(p2, K*

-

d,z) phosphorescence.

The excitation spectrum of this emission consists of bands at 3 19,

347, 398, and 468 nm. The last two excitation bands are I(d,z

+ p,, K * ) and 3(d9

-

p,, n*) transitions, respectively. It is

found that the difference in phosphorescent energy between trans-

[Rh1(PPh3)z(CN-t-Bu),1+ and

[PtRh(dppm)z(CN)~(CN-t-B~)2]+

(1540 cm-I) is smaller than the corresponding difference in I(dZl

(15) Geoffroy, G. L.; Wrighton, M. S.; Hammond, G. S.; Gray, H. B. J . Am.

Chem. Soc. 1974, 96, 3105.

5 0 0 6d0 700

wavelength i run

Figure 6. 77 K emission spectrum of rrans-[Rh~(PPh~)z(CN-t-Bu)2]-

Clod measured in an n-butyronitrile glass solution.

-

p,, n*) and *(do*

-

pu) absorptions (4047 cm-').

In

other

words, the Stoke shift of emission of [ P t R h ( d ~ p m ) ~ ( C N ) 2 ( c N -

t-Bu)z]+ is smaller than that of trans- [Rh(PPh3)2(CN+Bu)z]+. This would mean a smaller excited-state distortion of the dinuclear complex than the Rh monomer.

Heterobimetallic d10-d8 Complexes. There are only few

examples of luminescent heterobimetallic dlo-da complexes, including [IrAu(dppm)z(CO)Cl]+ studied by Balch et a/." and

[MPt(dppm)2(CN)2]+ Ig and [MPt(dppm)z(C=CPh)~l+ ( M =

Au+, Ag+) studied by Che and co-workers.ICJ6 In order to further our understanding of the interaction between d* and dlo metal ions, we have investigated the spectroscopic properties of [MRh- (dppm)2(CN-t-Bu)2]2+ ( M = A d , Ag+), which were first synthesized by Shaw and c o - w o r k e r ~ . ~ ~

Analogous to the heterobimetallic complexes [PtRh(dppm)z-

(CN),(CN-~-BU)~]+ and [AuPt(dppm)~(CN)2]+, for [MRh-

(dppm)z(CN-t-Bu)2]2+, the antibonding interaction between the

d,z orbitals of Rh and M would give rise to du*, while the bonding interaction of the pz orbitals leads to pa. Figure 7 gives a qualitative molecular orbital diagram of [MRh(dppm)z(CN-t-

The absorption spectra of [AuRh(dppm)*(CN-r-Bu)~] (C104)2

and

[AgRh(dppm)2(CN-t-Bu)2](C104)~

measured at room temperature are shown in Figure 8. Both spectra exhibit two intense

absorption bands in the visible region with A,,, (tmax/M-l cm-1)

at 342 (9.44 X lo3) and 455 (2.4 X lo4) for [AuRh(dppm)2-

(CN-t-Bu)~](C10~)2and330nm

(1.19 X 104)and425 nm (1.68

X 104) for

[AgRh(dppm)2(CN-t-B~)~j(ClO~)~.

The455-nm band

of the former and 420-nm band of the latter are both assigned

to the '(do*

-

pu) transition as in the d 8 4 systems (see Figure

6). The 342- and 327-nm absorption bands are tentatively

assigned to the l(d,,(Rh)

-

pa) transitions of the complexes,

analogous to the 348-nm band of [PtRh(dppm)2(CN)~(CN-t-

B u ) ~ ] + . The spectrum of

[A~Rh(dppm)~(CN-r-Bu)21(ClO~)~

also shows a weak absorption at 525 nm (t = 6 X lo2 M-I cm-I), whichisassigned to theS(da*-pu) transition. TableIIcompares

the I(du* -pa) transitions of

[PtRh(dppm)z(CN)2(CN-r-Bu)2]+,

[A~Rh(dppm)~(CN-t-B~)2]~+, and [AgRh(dppm)2(CN-t-Bu)2]z+

with the I(d,z

-

p,, T*) transition of trans-[Rh(PPh&(CN-t-

Bu)~]+. The energy differences of the transitions between the dinuclear complexes and Rh(1) monomer are in the order Pt-

B U ) ~ ]

'+.

(16) Yip, H.-K.;Lin,H.-M.; Wang,Y.;Che,C.-M. Submittedforpublication.

(5)

3406 Inorganic Chemistry, Vol. 32, No. 16, 1993 Yip et al. p, of Rh. X* of isocyarudes and phosphines

--._

---.__ 0

do'

"'*, d,Z ut' Ag( 1 )

Figure 7. Molecular orbital diagram for [MRh(dppm)2(CN-r-Bu)zI2+

(M = Au+, Ag+). *, $ do __._-.---' __-

?I

200 250 300 350 4 0 0 4 5 0 500 5 5 0 600 Wavelength l n m l

Figure8. Room-temperature absorption spectra of acetonitrile solutions

of [AuRh(dppm)2(CN-r-Bu)2] (C104)2 (-) and [AgRh(dppm)2(CN-r-

B~)21(C104)2 (-

-

-).

Table 11. Energy Differences between l(du*

-

pu) Transitions

of the Dinuclear Complexes and the l(d12

-

p,) Transition

(25 188 cm-I) of rr~ns-[Rh(PPh~)~(CN-r-Bu)~]+

complex energy diff (cm-I)

[PtRh(dppm)2(CN)2(CN-f-B~)2]+ 4047

[AuRh(dppm)z(CN-z-B~)212+ 3210

[AgRh(dppm)2(CN-r-Bu)ll2+ 1659

(11)-Rh(1)

>

Au(1)-Rh(1)

>

Ag(1)-Rh(1). This suggests that

the extent of metal-metal interaction of thecomplexes may follow a similar order. The extent of orbital interaction is dependent

on the orbital overlap and energy difference between the

interacting orbitals. The spectroscopic result is, therefore, in accordance with the fact that the d-orbital energies of the metal

ions are in the order Rh(1)

>

Pt(I1)

>

Au(1)

>

Ag(1).

The two heterobimetallic d1O-d8 complexes also show intriguing luminescent properties. Figure 9 shows the emission spectra of [AuRh(dppm)z(CN-t-Bu)z] (C104)2 (excitation wavelength = 450 nm) measured in the solid form at room temperature and in a

77 K glass solution. While the solid exhibits a structureless

emission at 610 nm (lifetime = 0.5 rs), two emission bands

centered at 500 and 610 nm are found in the glassy solution. The two emissions are tentatively assigned to the fluorescence and phosphorescence derived from the l(da*pa) and 3(da*pa) states,

respectively. The excitation spectrum of the 77 K n-butyronitrile

glass emission is composed of four bands centered a t 339, 373, 466, and 546 nm. The last two excitation bands are assigned to

the l(da*

-

pa) and 3(da* -pa) transitions, respectively. Both

I

/ /

'\

\ 1

-500 600 700 sbo

500 660 700 SbO

Wavelcngh(nm)

Figure 9. Room-temperature (solid state, -) and 77 K (n-butyronitrile

glass (-

-

-) emission spectra of [AuRh(dppm)z(CN-t-Bu)z](ClO,)z.

I(da*

-

pu) and 3(da*

-

pa) transitions recorded in the 77 K

excitation spectrum are red-shifted from that recorded in the room-temperature absorption spectrum (see Figure 8), and this is not unreasonable since both transitions are expected to be accompanied by an increase of Au-Rh bond order.

The separation between the singlet and triplet of (da*

-

pa)

transitions, measured from the 77 K excitation spectrum, is about

3 140 cm-I. The value of splitting is comparable to the analogous singlet-triplet separation found in the MLCT transition of the

monomeric

trans-[Rh(PPh3)z(CN-r-B~)2]+

(3760 cm-l), sug-

gesting that in

[AuRh(dppm)z(CN)~(CN-t-Bu)2l2+

the l(da*

-

pa) transition can be viewed to be mainly derived from the Rh

-

A* (isocyanide, phosphine) transition.

[AgRh(dppm)2(CN-r-B~)2](C104)2

is found to be a weakly emissive solid a t room temperature. The solid-state emission a t

575 nm (lifetime

-

0.05 FS at room temperature) is slightly

red-shifted to 583 nm in a 77 K n-butyronitrile glass solution. It

is noted that the l(da*

-

pa) transition and 3(da*pa) emission

of

[AgRh(dppm)z(CN-t-Bu)2]2+

are both very close to the '(dz

-

A*, pz) transition and '(dtp,) emission of truns-[Rh(PPh&

(CN-t-Bu)2]. Thus,

[AgRh(dppm)z(CN-t-B~)2]~+

can be

con-

sidered to be an extreme case of dinuclear complex, in which the

perturbation of one metal on the other is small. The excitation

spectrum exhibits bands at 331,361, and 439 nm. The 439-nm

excitation band is due to the l(du*

-

pa) transition.

Balch and co-workers have reported the absorption and emission

spectra of a d10-d8 complex, [AuIr(dppm)~(CO)Cl]+. The l(da*

-

pa) transition of the complex is located at 440 nm, while the

1(5d9-6pZ, A * ) transitionof themononuclear trunr-[Ir(PPh,)2-

(CO)Cl] has , A, located at 387 nm. The energy difference of

3 1 12 cm-1 between the two transitions is quite significant, suggesting the orbitals of the Ir and Au ions are not well matched

and the da* orbital is predominantly contributed from the 5d9-

(Ir) orbital. From the spectroscopic study on [ P t R h ( d ~ p m ) ~ -

(CN)2(CN-t-Bu)2]+ and [ MRh(dppm)2(CN-r-Bu)~]+ ( M = Au+,

Ag+), it is reasonable to expect the l(da* + pa) transition of

[AuIr(dppm)z(CO)Cl]+ may contain to a certain extent the charge-transfer character of the Ir moiety.

Conclusion

In this work, the spectroscopic properties of several new luminescent heterobimetallic complexes are described. Although the metal-metal interaction of homcdinuclear d* metal complexes is well understood, the interaction between two metal ions with

(6)

Luminescent Heterobimetallic Complexes

different electronegativities still remains to be explored. The present study suggests that electronic spectroscopy is a useful tool to study this kind of metal-metal interaction. It is reasonable to expect that by judicious choice of auxiliary ligand, heterobimetallic Pt(I1)-Rh(1) complexes with long-lived electronic excited states could be synthesized and may be expected to display intriguing photochemical reactions.

Inorganic Chemistry, Vol. 32, NO. 16, 1993 3407

Acknowledgment. We acknowledge support from the Croucher

Foundation, the Research Grants Council of Hong Kong, and

the National Science Council (NSC) of Taiwan. C.-M.C. is

thankful for a visiting professorship administered by the National

Taiwan University. H.-K.Y. is thankful for a Croucher Stu-

dentship, administered by the Croucher Foundation of Hong Kong.