Infrared, Raman and pre-resonance Raman spectroscopy of Rh2(O2CCH3)4(S(CH2Ph)2)2

Journal of Molecular Structure, 197 ( 1989 ) 105-112

105

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

I N F R A R E D , RAMAN A N D P R E - R E S O N A N C E RAMAN S P E C T R O S C O P Y OF Rh2(O2CCHa)4(S(CH2Ph)2) 2

ROBIN J.H. CLARK and ANDREW J. HEMPLEMAN

Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WCIH OAJ (Gt. Britain) (Received 26 August 1988)

ABSTRACT The infrared (3500-40 cm-1), Raman and pre-resonance Raman spectra (3150-20 cm -1 ) of the complex Rh2 (02CCH3) 4( S (CH2Ph) 2) 2 have been studied in detail at ca. 80 K and compared with those of other dirhodium tetraacetate complexes. Assignments for the key skeletal modes, vl[ v (RhRh) ] and v2[ v (RhO) ], are given. Although resonance conditions could not be reached, owing to the short wavelength (290 nm) of the first allowed electronic transition of the complex, pre-resonance effects are evident from the substantial enhancement of both Yl and (especially) v2 with 363.8-nm excitation. The vl value is in line with that expected on the basis of the previously established reciprocal relationship between the wavenumber of the rhodium-rhodium stretching fundamental and the rhodium-rhodium bond length.

INTRODUCTION

Earlier studies in this series have clarified the long-standing controversy regarding the assignment of the rhodium-rhodium stretching wavenumber, v(RhRh), in axially disubstituted complexes of the type Rh2(O2CCH~)4L2, where L = PPh3, AsPh3 or SbPh3 [ 1,2 ]; the key to the resolution of the controversy lay in the concomitant study of both 1sO and CD3 isotopomers by which procedure shifts occur to bands attributable to v (RhO) but not to ~ (RhRh). A further complex of this general type is Rh2 (02CCH3) 4(S (CH2Ph) 2) 2, which has now been studied in detail using infrared, Raman and pre-resonance Raman spectroscopy. The results add to the known data relating to the connection between v (RhRh) and the R h - R h bond distance, the hatter having been established recently by X-ray crystallography for the complex in question [3 ].

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106 [ Rha(OaCCH3)4(S(CH= Ph )a)a ] Xo= 5 i 4 . 5 nm

3100

2900

1600

KCI disc (a) double (b) triple

1200

-80K

BOO

400

0

Wavcnumbcr / ¢m ~

Fig. 1. R a m a n s p e c t r u m (3150-2850 a n d 1650-20 c m -1) o f R h 2 ( O 2 C C H ~ ) 4 ( S ( C H 2 P h ) 2 ) 2 as a KC1 disc at ca. 80 K w i t h 514.5 n m excitation. R e s o l u t i o n ca. 3 c m 1.

[Rh2(O2CCH3)~(S(CHzPh)z) 2] ko=363'8 nm

KC[ disc

i

ca.80 K

i' : , ! ! ,ii

T

,r

:: !i i

t"

I! I

i:,j :'

(3

r"

i

i;

F ';

;ii[ /

1500

1200

B00

400

Wovenumber,/cm - I

Fig. 2. P r e - r e s o n a n c e R a m a n s p e c t r u m ( 1650-20 c m -] ) o f Rh2 (02CCH3)4 (S ( C H 2 P h ) 2 )2 as a KC1 disc at ca. 80 K w i t h 363.8 n m excitation. R e s o l u t i o n ca. 5 c m 1. EXPERIMENTAL

Preparation of the complex Tetrakis-p-acetato-bis (dibenzylsulphide)dirhodium (II) was prepared as burgundy-coloured crystals and analysed as described previously [3 ].

107 TABLE 1 Wavenumber of bands observed in the Raman spectruma of Rh2 ( 02CCHa ) 4(S (CH2Ph) 2) 2at ca. 8O K P (cm -1) 25w,sh 37vw,sh 45w 56vw 72m,sh 87s 99m,sh ll3w,sh 123m 137w,sh 165s 172m 191w 213vs 218m 242vw 267m 297w,sh 303m 315vs 318s,sh 328m 344m 430vw 471w 481vw 538vw

Assignmentb

P

Assignment

(cm -I)

x X-sens vl, v (Rh-Rh) v(Rh-O) v(Rh-O) v2, v (Rh-O) t X-sens

561vw 619vw 679m 690vw 712vw 771vw 778vw 805vw 864vw 948w,sh 953m 1002m 1030vw 1158w 1207w 1246w,sh 1251m 1297w 1494vw 1582vw 1602m 2927m 2941w 3042w 3061w 3069w,sh

y X-sens s a(C-C-C) ~(C-S) v ¢(C-C) fy(C-H) }

r

X-sens

p~(CH2) }

v(C-C) p-ring bfl(C-H) cfl(C-H) q X-sens

}

pw(CH2) m v(C-C) l v(C-C) k v(C-C) v~(CH2) v(C-H) acetate

} u (C - H ) aromatic

a At 514.5 nm excitation, b The nomenclature is that ofD.H. Whiffen, J. Chem. Soc., (1956) 1350.

Instrumentation R a m a n s p e c t r a were r e c o r d e d on a S p e x 14018 ( R 6 ) s p e c t r o m e t e r in b o t h t h e double- a n d t r i p l e - m o n o c h r o m a t o r m o d e s (1800 line m m - 1 h o l o g r a p h i c gratings, 500 n m b l a z e ) , in c o n j u n c t i o n w i t h a C o h e r e n t C R 1 2 laser; t h e l a s e r p o w e r was h e l d at b e t w e e n 2 a n d 30 m W . R a m a n s a m p l e s were held as p r e s s e d KC1 discs a t ca. 80 K u s i n g a liquid n i t r o g e n cooled cell ( 3 1 5 0 - 2 0 c m - 1 ) . I n f r a r e d s p e c t r a w e r e r e c o r d e d at ca. 80 K ( B e c k m a n n R I I C c r y o s t a t ) as a KC1 disc ( 3 5 0 0 - 5 0 0 c m - 1 ) a n d as a p r e s s e d w a x disc ( 6 6 0 - 4 0 c m - ' ) a t a s p e c t r a l

108 TABLE 2 Wavenumber of bands observed in Rh2(O2CCH3)4(S (CH2Ph)2)2 at ca. 80 K P (cm -1 )

Assignment

165vw 267vw 299w x X-sens 314vs vl, ~ (Rh-Rh) 328w v(Rh-O) 344vs v2, v(Rh-O) 381vw 576vw y X-sens 656w ~, + v2 679w ~(C-S) 725w 5(0C0) 770vw r X-sens 777vw 806vw ~ 812vw ) pr(CH2) 865vw 885vw 952vw ~(C-C)

P (cm-')

the

pre-resonance

Raman

spectruma

of

Assignment

990vw j 7(C-H) 1002w p-ring 1018w p(CH3) 1034w b fl(C-H) 1049vw p(CH3) 1 0 7 7 v w dfl(C-H) l172vw 1183vw J aft(C-H) 1207vw q X-sens 1244vw "~ 1252vw j pw(CH2) 1292 vw 1307vw 1355vw 5s(CH3) 1489vw m ~(C-C) 1591w 1601w k v(C-C)

aAt 363.8 nm excitation. resolution of I c m - 1 with a B r u k e r 113V i n t e r f e r o m e t e r . Details of t h e procedures used in t h e sample p r e p a r a t i o n were similar to t h o s e used elsewhere [ 1 ]. RESULTS T h e R a m a n s p e c t r a o b t a i n e d for Rh2 (O2CCH3) 4 (S ( C H 2 P h ) 2 ) 2 w i t h b o t h 514.5 a n d 363.8 n m e x c i t a t i o n are s h o w n in Figs. 1 a n d 2, respectively, a n d t h e b a n d m a x i m a t o g e t h e r with t h e b a n d a s s i g n m e n t s are listed in T a b l e s 1 a n d 2, respectively. M o s t of t h e b a n d s above 400 c m - 1 are weak, b u t t h e r e are several s t r o n g b a n d s below this w a v e n u m b e r . T h e key a s s i g n m e n t to m a k e is t h a t of v ( R h R h ) . E a r l i e r work has clearly e s t a b l i s h e d a reciprocal r e l a t i o n s h i p bet w e e n v ( R h R h ) a n d the R h - R h b o n d l e n g t h in d i r h o d i u m t e t r a a c e t a t e complexes [2], viz. v ( R h R h ) occurs at 289, 297, 307 c m -1 for Rh2(O2CCH~)4L2, L = PPh3, AsPh3 or SbPh3, for w h i c h r ( R h R h ) is 2.450 [ 4 ], 2.427 [ 3 ] a n d 2.421 ~,, respectively [ 3 ]. Since r ( R h R h ) is 2.406 ~, for Rh2 (O2CCH3) 4 (S ( C H 2 P h ) 2 ) 2 [3] it follows t h a t v ( R h R h ) would be e x p e c t e d to lie above 307 c m - 1 for this complex. T h u s the v e r y s t r o n g b a n d at 315 cm -1 is assigned to p ( R h R h ) ; it is the m o s t i n t e n s e b a n d in t h e s p e c t r u m with 514.5 n m excitation, a c h a r a c t e r istic f e a t u r e of b a n d s a t t r i b u t e d to v ( R h R h ) .

109

[Rh~O£[H3]~{S(CH,zPh ]L~2] wax disc

600

ca.80 K

400

200

W o v e n u m b e r / c m -I

Fig. 3. FTIR spectrum (660-140 cm-1) of Rh2(O2CCHa)4(S(CH2Ph)2)2 as a wax disc at ca. 80 K. The medium R a m a n band at 344 c m - 1 is assigned to the totally symmetric u ( R h - O ) fundamental u2, since it occurs very close in wavenumber to the band so assigned (on the basis of 180 substitution) at 340 + 2 cm-1 in the R a m a n spectra of Rh2 (02CCH3) 2L2, L = PPh3 [ 1 ], AsPh3 [ 2 ] or SbPh3 [ 2 ]. This band wavenumber would be expected to be almost insensitive to axial substitution owing to the near identity of the Rh2 (O2CCH3) 4 cage in all four complexes (all R h - O bond distances lie in the range 2.045-2.040 A; all / R h R h O (av) in the range 87.5+0.4 ° [3,4]. Both ul, u ( R h R h ) , and u2, u(RhO), but particularly the latter show resonance e n h a n c e m e n t on changing the excitation wavelength from 514.5 to 363.8 nm. A weak combination band (at 656 cm-1) observed in the 363.8-nm spectrum has been assigned to ul + u2. Thus this spectrum displays pre-resonance effects; however, because the lowest allowed electronic band of the complex (at 290 n m ) is well removed from any available excitation line (even Ar 2+ or Kr 2+ ), it was not possible to reach resonance conditions. Five infrared bands (Fig. 3, Table 3 ) occurring at 389,384, 382,331 and 328 c m - 1 are assigned to u (RhO), whereas the M2 (02CR) 4 model predicts only two bands (a2u+eu in D4h symmetry) [5 ]. Both low-symmetry effects (to do with the methyl groups and the axial ligands) as well as site and factor group effects (there are two molecules per unit cell) [ 3 ] must be responsible for these splittings.

110 TABLE 3 Wavenumber of bands observed in the infrared spectrum of Rh2 (O2CCHs)4 (S (CH2Ph)2)2 at ca. 8O K P (cm -1) 3104vw 3087vw 3062w

Assignment

t

3047vw 3043vw 3036vw,sh 3028w 3025w 3016vw,sh 3011w 2993vw 2985vw 2974vw 2967vw 2940vw 2929w 2918vw,sh 2847vw 2838vw 2744vw 2679vw 2448vw 2428vw 2387vw 2380vw 2362vw 2352vw 2276vw 1982vw,sh ~ 1980vw J 1972vw 1960vw } 1955vw 1912vw,sh "~ 1906vw J 1900vw } 1885vw 1838vw 1830vw,sh } 1828vw 1817vw,sh 1814vw 1809vw 1778vw

v(C-H) aromatic

v(C-H) acetate ~ (C-H) acetate v(C-H) acetate va~(CH2

1341 + 1405 2 × 1341

2j h+j j+i h+i

h+g

g+ i

P (cm -1) 1666vw 1658vw 1645vw,sh 1600s,sh 1596vs 1584m,sh 1548vw 1533vw 1519vw 1497m 1494w,sh 1473vw 1456m 1454m 1448m,sh 1444m 1431m,sh 1427m 1412m 1405s 1387w,sh 1349m 1341w 1327vw 1324vw,sh 1297vw 1257w,sh 1252w 1247w 1245w 1240w,sh 1207vw,sh 1204vw 1200vw 1184vw ll80vw 1174vw 1172vw,sh ll60vw,sh 1158vw 1151vw 1146vw 111 lvw 1074w,sh

Assignment

k v(C-C) va~(CO0) 1 ~ (C-C)

J }

m lp(C-C)

n v(C-C) c~ (CH3) vs(COO)

} ~(CH2) and c~as(CH3)

f

5~(CH3)

} o ~ (C-C) } p,~(CH2) } q X-sens } aft(C-H) } cft(C-H)

d ft(C-H )

111 TAB LE 3 (continued ) P (cm-') 1769vw 1762vw 1701vw 1696vw 1691vw 1687vw 1029vw 1026vw 1020vw 1003vw 1001vw 994vw 991vw 986vw,sh 978vw 974vw 969vw 923w,sh 921w 917vw 889vw 885vw 864vw 853vw 849vw 846vw 842vw,sh 822vw 810vw,sh 808vw 805vw,sh 779w 775m 772m 770m,sh 732vw 711w,sh 702vs 695m 679w 664w,sh

Assignment }g+i } 2g bfl(C-H) } p(CH3) ~ p-ring J ~ } j y(C-H) J } h,(C-H) ~ ~ i 7(C-H) -

} g y(C-H) ~ } p,(CH2) ) -~ ~ r X-sens J ~ f y(C-H) ~ ~(OCO) and v 0(C-C) } v(C-S)

P (cm-') 1071w 1048vw 1045vw,sh 1038w 1035w,sh 1031vw 648vw 644vw 630w 622w 621w,sh 594w 591vw 584vw 575vw 563vw,sh 560w 484vw 477w 474vw,sh 470vw 418vw 389w 384w 382w,sh 346vw,sh 344vw 331vw,sh 328w 306vw 299vw 268vw 264vw 258vw 246vw 215vw 208vw 196vw 183vw

Assignment } dfl{C-H) } p(CH3) b fl(C-H) } out-of-plane pw(COO) s a(C-C-C) ~ in-plane ) p~(COO) } y X-sens

~ t X -sens )

~ v(Rh-O) }J u X-sens ~ v(Rh-O) ) x X-sens

112 T h e a s s i g n m e n t s of t h e a c e t a t e m o d e s are b a s e d u p o n detailed b a n d assignm e n t s p r e s e n t e d for Rh2 ( 0 2 C C H ~ ) 4 a n d Rh2 (O2CCH3) 4 ( H 2 0 ) 2 [ 6 ]. M a n y b a n d a s s i g n m e n t s , p a r t i c u l a r l y in the 1460-1400 a n d 720-690 c m - 1 regions, r e m a i n t e n t a t i v e owing to overlap of b a n d s a t t r i b u t a b l e to t h e a c e t a t e groups a n d to t h e axial ligands.

REFERENCES 1 R.J.H. Clark and A. J. Hempleman, Inorg. Chem., 27 (1988) 2225. 2 R.J.H. Clark and A. J. Hempleman, Inorg. Chem., 28 (1989) 92,746. 3 R.J.H. Clark, A. J. Hempleman, H. M. Dawes, M. B. Hursthouse and C. D. Flint, J. Chem. Soc. Dalton, (1985) 1775. 4 G.G. Christoph, J. Halpern, G. P. Khare, Y. B. Koh and C. Romanowski, Inorg. Chem., 20 (1981) 3029. 5 R.J.H. Clark, A. J. Hempleman and M. Kurmoo, J. Chem. Soc. Dalton, (1988) 973. 6 R.J.H. Clark and A. J. Hempleman, Croat Chem. Acta, 61 (1988) 313.