Esr Study of Bivalent Rhodium Complexes Formed in Zeolites

14 Esr Study of Bivalent Rhodium Complexes Formed in

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Zeolites C L A U D E NACCACHE and Y O U N È S BEN TAARIT Institut de Recherches sur la Catalyse, 79 boulevard du 11 Novembre, 1918 - 69626 - Villeurbanne, France MICHEL BOUDART Department of Chemical Engineering, Stanford University, Stanford, Calif. 94305 ABSTRACT Rhodium (III) exchanged z e o l i t e s were prepared s t a r t i n g from NaY and [ R h ( N H ) C l ] aqueous s o l u t i o n [Rh(II)(NH ) ] was shown to form at the e a r l y stages of the thermal a c t i v a t i o n of the z e o l i t e around 480 K. At higher temperatures, 770-600°K, Rh(II) species bound d i r e c t l y to the lattice were formed. Part of these species a l s o r e s u l t e d from the thermal treatment and appeared to form μ-peroxo-species upon oxygen a d d i t i o n ; the symmetry of these v a r i o u s paramagnetic species i s d i s c u s s e d . 2+

3

5

2+

3

5

Introduction Recent s t u d i e s of t r a n s i t i o n metal complexes formed i n z e o l i t e s (J_, _2, 2» 4) suggest that the z e o l i t e framework could serve as a " s o l i d s o l v e n t " . Furthermore t r a n s i t i o n metal-ion-exchanged z e o l i ­ tes have shown c a t a l y t i c p r o p e r t i e s very s i m i l a r to those of s o l u ­ b l e t r a n s i t i o n metal complexes Ç5, 6 ) . More r e c e n t l y i t has been shown that ethylene was s e l e c t i v e l y dimerized to n-butenes over rhodium exchanged Y z e o l i t e and the authors suggested that zerov a l e n t w e l l dispersed rhodium atoms w i t h i n the z e o l i t e cages const i t u e d the source of a c t i v e s i t e s ( 5 ) . The object of t h i s paper i s to r e p o r t on the e s r measurements of rhodium exchanged Y z e o l i t e s to determine the o x i d a t i o n s t a t e and the environment of rhodium ions i n z e o l i t e a c t i v a t e d i n v a r i o u s c o n d i t i o n s . Experimental The rhodium Y form was obtained by s t i r r i n g NaY z e o l i t e (SK 40) i n an aqueous s o l u t i o n of rhodium pentaammine c h l o r i d e : |jlh(NH3)5Cl 3 Cl2 f ° 6 h. The pentaammine complex was prepared by r e f l u x i n g RhCl3 χ H2O i n concentrated ammonia at 80°C. Chemical a n a l y s i s f o r both sodium and rhodium showed that 5 rhodium comple­ xes were introduced per u n i t c e l l . The z e o l i t e samples were heated i n flowing oxygen while the temperature was slowly r a i s e d up to 480 Κ or 770 K. The samples were then outgassed at e i t h e r 480 Κ or 770 Κ and then t r a n s f e r r e d i n vacuo i n t o esr quartz tubes. 156 2+

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For a d s o r p t i o n experiments pure oxygen and carbon monoxide we­ re dehydrated over 5 A molecular s i e v e . 0 enriched 0^ (58 % i n 0 ) and C enriched CO (90 % i n C ) were obtained from the French Atomic Energy Agency and used without f u r t h e r p u r i f i c a t i o n . The esr measurements were performed on a V a r i a n E. l i n e spec­ trometer equipped with a dual c a v i t y and operating i n the X band mode, g-values were measured using D.P.P.H. as a standard (g = 2.0036). A l l esr spectra were recorded at 77 Κ unless otherwise stated. 1 7

l 7

1 3

13

Results The esr s p e c t r a of NaY form dehydrated up to 770 Κ and RhY heated up to 420 Κ revealed only a weak s i g n a l around g = 4 which i s a t t r i b u t e d to F e ^ i m p u r i t i e s . In c o n t r a s t paramagnetic species were formed when RhY z e o l i t e was heated at 480 K. The e s r spectrum of t h i s species (species A) i s shown i n f i g u r e l a . This spectrum c h a r a c t e r i z e d by gi = 2.09, g2 = 2.06, g3 = 1.97 was recorded a l s o at 295 Κ without a p p r e c i a b l e l i n e broadening. Hence i t appeared that s p i n - l a t t i c e r e l a x a t i o n time ( T j ) d i d not c o n t r o l the l i n e width. The spectrum was r e v e r s i b l y broadened by oxygen. RhY samples a c t i v a t e d at 770 Κ (02 - vacuo) revealed an a x i a l esr spectrum w i t h g, = 2.68 and g = 2.006, which could be obser­ ved at 77 Κ and 295 K. The g ^ component of t h i s species Β was p r o ­ g r e s s i v e l y s h i f t e d to = 2.60 by a d d i t i o n of small amounts of wa­ t e r . Furthermore no d i p o l a r l i n e broadening was observed i n the presence of oxygen. The temperature range over which ammonia evolved from decom­ p o s i t i o n of pentaammino-chlororhodium complexes was determined by t e s t i n g the emerging oxygen stream w i t h moist paper l i t m u s . Up to 480 Κ almost no ammonia was detected i n the e x i t stream which sug­ gests that no a p p r e c i a b l e r e l e a s e of the NH3 l i g a n d s has o c c u r r e d . Above 480 Κ ammonia was detected i n the emerging oxygen stream up to 620 K. Moreover the disappearance of the i n f r a r e d bands due to NH3 ligands when the RhY z e o l i t e was c a l c i n e d at 770 Κ confirmed t h a t | R h ( I I I ) ( N H 3 > 5 C 1 3 ^+ complex was completely decomposed. Carbon monoxide a d s o r p t i o n . The a d d i t i o n of ^co at 295 Κ to RhY a c t i v a t e d at 770 Κ r e s u l t e d i n the formation of a new paramagne­ t i c species (species C) ; i t s e s r spectrum shown i n f i g u r e lb i s t y p i c a l of a species of a x i a l symmetry whose g components are gj^ = 2.191, g = 1.991. The g^ component was f u r t h e r s p l i t i n t o two h y p e r f i n e l i n e s due to the i n t e r a c t i o n of the unpaired e l e c t r o n w i t h Rh nucleus (I = 1/2). This s i g n a l was not a l t e r e d by an evacua­ t i o n at 77 Κ but disappeared f o l l o w i n g d e s o r p t i o n at 295 Κ f o r 5 minutes. When l^CO was used the r e s u l t i n g esr spectrum ( f i g u r e l c ) showed that the p e r p e n d i c u l a r component was s p l i t i n t o a doublet while the p a r a l l e l component was s p l i t i n t o a doublet of doublets while the same f e a t u r e s observed when 12co was used a l s o appeared due to u n l a b e l l e d s p e c i e s . Hence a monocarbonyl species appears to be formed. The 13C0 a n i s o t r o p i c h y p e r f i n e s p l i t t i n g constants were A « = 105 Oe and A M = 115 Oe. T h i s spectrum was not observed a t +

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room temperature. ^ Oxygen adsorption. Upon O2 a d s o r p t i o n e i t h e r at 77 Κ or at 295 Κ on RhY a c t i v a t e d at 770 Κ the esr s i g n a l shown i n f i g u r e 2a with gj| = 2.015, gj_ 1.931 appeared (species D). This s i g n a l was unobservable at room temperature and r e v e r s i b l y broadened by i n c r e a ­ s i n g the oxygen p r e s s u r e . Furthermore experiments c a r r i e d out with 17o enriched molecular oxygen at a pressure low enough to avoid d i p o l a r l i n e broadening gave the esr spectrum shown i n f i g u r e 2b. Hyperfine l i n e s r e s u l t i n g from the i n t e r a c t i o n of the unpaired el e c t r o n with ^ 0 n u c l e i (I = 5/2) were not r e s o l v e d ; however the 17θ2 " adduct esr spectrum was considerably broader compared with that of the u n l a b e l l e d oxygen - adduct. As f o r CO - adduct the oxygen adsorption was r e v e r s i b l e at room temperature with the sub­ sequent disappearance of the paramagnetic oxygen - adduct. E f f e c t of water a d s o r p t i o n . The oxygen experiments were p e r ­ formed i n the absence of water t r a c e s . However a new esr spectrum at g] = 2.11, g • 2.016, g = 1.98 ( f i g u r e 2c) developed when RhY sample a c t i v a t e d at 770 Κ was exposed to the atmosphere. Subse­ quently a s e r i e s of RhY samples a c t i v a t e d at 770 Κ were exposed to water vapor alone or to water plus oxygen at room temperature. Wa­ t e r adsorption produced no e f f e c t while the simultaneous a d s o r p t i o n of water and oxygen r e s u l t e d i n the appearance of the esr s i g n a l shown i n f i g u r e 2c (species E ) . Furthermore the a d s o r p t i o n of wa­ t e r on a sample which has p r e v i o u s l y been contacted w i t h O2 to form the oxygen - adduct species D leads to the formation of spe­ c i e s Ε with the subsequent disappearance of species D. Species Ε was s t a b l e and observable at room temperature but was destroyed by outgassing the sample at about 673 K. F i n a l l y , a d s o r p t i o n of CO at 295 Κ followed by a d s o r p t i o n of O2 at 77 Κ on the same RhY sample p r e t r e a t e d at 770 Κ r e s u l t e d i n both species C and D thus suggesting that carbon monoxide and oxygen adducts i n v o l v e d d i f ­ ferent s i t e s . =

2

3

I n t e r p r e t a t i o n of the esr spectra A l l known s t a b l e Rh(III) species are diamagnetic w i t h low s p i n d^ c o n f i g u r a t i o n s . However thermal treatment of Rh(III) exchanged z e o l i t e s produced paramagnetic species as evidenced by the obser­ ved esr s i g n a l s . The l a r g e departure of the g values from g i s a c l e a r i n d i c a t i o n that the odd e l e c t r o n should be a s s o c i a t e d w i t h a rhodium species since rhodium has a l a r g e s p i n - o r b i t coupling which may account f o r the observed l a r g e g s h i f t s from g « ç Among the v a r i o u s o x i d a t i o n s t a t e s of rhodium only Rh(0) 4 d , Rh(II) 4 d7, Rh(IV) 4 d are paramagnetic w i t h s = 1/2. e

e

5

Species A : The powder esr spectrum of species A showed three p r i n c i p a l g-values which i n d i c a t e d that i t i s experiencing a rhombic c r y s t a l f i e l d . By i g n o r i n g the small d e v i a t i o n from a x i a l symmetry^one

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Figure 1. Esr spectra of (a) rhodium zeolite activated at 480 K; (b) C0 adsorbed on rhodium zeolite acti­ vated at 770 Κ; (c) the CO-adduct 12

13

,2.015

Figure 2. Esr spectra of (a) oxygen adsorbed on RhY activated at 770 K: the 0 -adduct; (b) the 0adduct; (c) the 0 -adduct con16

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might consider g. • 1.97 and gj^ • 1/2 (gi + g2> • 2.07, then g^* gespectrum of n e u t r a l rhodium i n (RhCl6)^~ has been reported (7) and i n t e r p r e t e d i n terms of Rh(0) under t e t r a g o n a l compression with g^ > gj^ τ» g , i n c o n t r a s t with the present r e ­ s u l t s . Furthermore s i n c e the formation of Rh(0) atoms w i t h i n the z e o l i t e cages would r e q u i r e a t h r e e - e l e c t r o n r e d u c t i o n per R h ( I I I ) , production of n e u t r a l rhodium i n the o x i d i z i n g atmosphere used du­ r i n g thermal a c t i v a t i o n i n oxygen i s h i g h l y u n l i k e l y . On the other hand assignement of s i g n a l A to Rh(IV) can be e q u a l l y r u l e d out on the f o l l o w i n g b a s i s : the 4 d5 low-spin c o n f i g u r a t i o n ground s t a t e would be and regarded as a s i n g l e hole i n the &2 s h e l l . In t h i s c o n f i g u r a t i o n the g-values would be very a n i s o t r o p i c with gj| much l e s s than g ( 8 ) . The s p i n - l a t t i c e r e l a x a t i o n would be very short and the corresponding esr spectra observed only at very low temperature. Indeed Rh(IV) i n MgO gave esr spectrum only at 20 Κ with g = 1.667 ( 9 ) . With the above c o n s i d e r a t i o n s obviously species A should be assigned to Rh(II) complex. For a d? system w i t h g^> g the odd e l e c t r o n i s l o c a l i z e d i n the metal cl\(dz ) o r b i t a l . Assu­ ming a rhodium (II) complex i n a square pyramidal c o n f i g u r a t i o n , the a.j o r b i t a l w i l l be d e s t a b i l i z e d w i t h respect to the 62(dxy) and (dxz,dyz) o r b i t a l s . The e l e c t r o n i c c o n f i g u r a t i o n f o r the metal com­ plex i s t h e r e f o r e Z^këa^ w i t h the unpaired e l e c t r o n i n the a\ o r b i t a l . T i l t i n g the f i f t h l i g a n d away from the t e t r a g o n a l a x i s r e s u l t s i n a s p l i t t i n g of the degenerate e o r b i t a l p a i r and the three p r i n c i p a l g-values given by the r e l a t i o n (10) : g = 2 N , g =2N +6Naj g = 2N + 6 Na2 where aj = \fe - E 2), a = X / ( E - E 2). Ν i s a normalized c o e f f i c i e n t and λ the reduced s p i n - o r b i t coupling constant allowing covalency (λ = 0.6 λ ) . S e t t i n g λ = 1235 cm" , the c a l c u l a t e d E - E 2 and E - E 2 energy d i f f e r e n c e s are r e s p e c t i v e l y 33,000 and 43,000 cm~i. Hence species A i s assigned to Rh(II)L5 complex i n a square pyramidal arrangement. E

s

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Β :

For the same reason s t a t e d above assignment to Rh(0) or Rh(IV) was r u l e d out. Since species Β showed esr spectrum with g^> g ^ g the unpaired e l e c t r o n i s i n a d z o r b i t a l . The g-value expressions given f o r species A are a l s o v a l i d i n t h i s case w i t h aj «• a2« Then E(dxz, dyz) - E ( d z ) = 11,000 cm" (RhClfc) " centres CM) gave esr spectrum at g ^ = 2.48, gjj =2.00, very c l o s e to those found f o r species B. The r e s u l t s are then c o n s i s t e n t with Rh(II) ions i n a t e t r a g o n a l l y d i s t o r t e d octahedral c r y s t a l f i e l d . u

e

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C :

CO r a d i c a l may be considered as a p o s s i b l e r e s u l t of the r e a c ­ t i o n of CO with RhY z e o l i t e . However t h i s suggestion was r u l e d out s i n c e CO r a d i c a l showed g-values c l o s e to g and very l a r g e hyper­ f i n e constants when ^CO was used (19). Moreover i t i s u n l i k e l y that CO was f o r m a l l y reduced or o x i d i z e d i n i t s adduct. We f e e l the ade

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duct i s best d e s c r i b e d as a CO-Rh(II) complex with the unpaired e l e c t r o n s p i n being mainly i n a rhodium o r b i t a l . Indeed the doublet with a s p l i t t i n g of about 18 Oe appearing at the gj| component i n d i ­ cates that the odd e l e c t r o n i s a s s o c i a t e d with rhodium d - o r b i t a l . The esr spectrum can be analyzed i n terms of an a x i a l l y symmetric g-tensor with g^ > g^ and i s c o n s i s t e n t with a (dxy, dyz, d x z ) ( d z ) ground s t a t e . Again from the t h e o r i t i c a l g^ expression given above a value of 24,000 cm" f o r the dxz, dyz dz transi­ t i o n was obtained. The gj and g^ components of the C 0 adduct were each s p l i t i n t o two l i n e s i n d i c a t i n g that t h i s s i g n a l i s due to the Rh(II) complex i n which a rhodium i o n i s coordinated to one carbon atom. The A j ^ and Aj| a n i s o t r o p i c C coupling constants pro­ v i d e , using the r e l a t i o n a = 1/3 (A j| + 2A^), a ^ ( C ) = 108.3 Oe then b ^ i p = 3.3 Oe. The molecular o r b i t a l c o e f f i c i e n t s C^ and Cj7p which c h a r a c t e r i z e the s p i n d e n s i t y on the r e s p e c t i v e 2s and 2p carbon o r b i t a l s can then be estimated, as C^s = a £ / l 1 1 0 . 8 , zp dip/ -4P d e n s i t i e s on 2s and 2p C o r b i t a l s are then 0.098 and 0.101 r e s p e c t i v e l y . The carbon donor o r b i t a l that mixes with the rhodium d z o r b i t a l has a carbon 2p/2s r a t i o c l o s e to 1 i n good agreement with the expected sp h y b r i d donor o r b i t a l f o r carbon monoxide. The t o t a l s p i n d e n s i t y d e l o c a l i s e d on the CO molecule i s about 0.20 hence the s p i n d e n s i t y on the metal o r b i t a l i s 0.80 which confirms our former suggestion that the odd e l e c t r o n i s mainly l o c a l i z e d on the rhodium metal o r b i t a l . From a molecular o r b i t a l standpoint the wave f u n c t i o n of the unpaired e l e c t r o n may be described as a l i n e a r combination of d z metal o r b i t a l and 3 σ sp carbon o r b i t a l . b

2

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1 3

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i s o

S Q

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Species D : The a n i s o t r o p i c esr s i g n a l due to the oxygen-adduct showed 8H Se % · The l a r g e departure of g ^ f r o m g ( A g = 0.07)and the absence of any r e s o l v e d h y p e r f i n e s p l i t t i n g when " 0 enriched mo­ l e c u l a r oxygen was used suggest that the unpaired e l e c t r o n o r b i t a l i s e s s e n t i a l l y metal d o r b i t a l i n c h a r a c t e r . However the l i n e broa­ dening observed when ^ 0 2 was used, r e s u l t e d probably from u n r e s o l ­ ved h y p e r f i n e s p l i t t i n g due to ^ 0 n u c l e i and i n d i c a t e s that a small e l e c t r o n s p i n d e n s i t y i s present on the oxygen n u c l e i i n the rho­ dium-oxygen complexes. The experimental order gy> g >g^ c o r r e s ­ ponds to the d? c o n f i g u r a t i o n with the unpaired e l e c t r o n i n the 4 d x - v o r b i t a l , the g values c a l c u l a t e d from the l i g a n d f i e l d theory being (12) : g „ - 2 + 8 λ/(Ε 2 _ 2 -Ε ), = 2-2λ/(Ε 2 2 - Ε ) From experimental g ^ value and *λ • 0.6 λο one ends'up w i t r r E - Εχ2 - 2 = 21,000 cm" . The e f f e c t i v e metal o x i d a t i o n s t a t e i n rhodium-oxygen complex i s 2, r e s u l t i n g probably from e l e c t r o n t r a n s f e r from metal i o n to oxygen through the Π* antibonding oxy­ gen o r b i t a l . >

>

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χ

%

χ

χ

1

x y

y

Species Ε : Since t h i s species was

formed from the r e a c t i o n of H2O

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Rh(II) - O2 species one could suggest that no change i n the formal o x i d a t i o n s t a t e of Rh(II) has occurred, but r a t h e r a change i n the c r y s t a l f i e l d symmetry. We f i n d that t h i s complex has g (gj) > 8yy (g2) > δχχ· These r e s u l t s are t y p i c a l of square p l a n a r or bipyramidal t r i g o n a l d? low-spin systems where the unpaired e l e c ­ t r o n s i t s mainly i n dxy o r b i t a l (Π_, \2. 20). Furthermore the o r thorhombic g-tensor obtained f o r species Ε implies that a d e v i a t i o n from pure t e t r a g o n a l or t r i g o n a l c r y s t a l f i e l d has occurred. Spe­ c i e s Ε may be a s c r i b e d to Rh(II) coordinated to oxygen and to 1^0. The esr s i g n a l of rhodium (II) tetraphenyl-porphyrin complex with gl = 2.089, g2 - 2.029, g = 1.99, v e r y c l o s e to the g-values ob­ tained f o r Ε s p e c i e s , has been reported (13) and t e n t a t i v e l y a s s i ­ gned to b i v a l e n t f o u r coordinate rhodium. z z

9

3

Discussion The experimental esr r e s u l t s and t h e i r i n t e r p r e t a t i o n s have shown that s e v e r a l b i v a l e n t rhodium species might be generated w i ­ t h i n the z e o l i t e cages s t a r t i n g from [Rh(III) ( N ^ ^ C l J complex. These r e s u l t s have proved the a b i l i t y of z e o l i t e s to s t a b i l i z e t r a n s i t i o n metal-ions i n unusual o x i d a t i o n s t a t e s . The thermal treatment at 473 Κ produced a r e d u c t i o n of Rh(III) i n t o Rh(II) i o n s . Two p o s s i b l e ways of Rh(II) ions formation may be suggested : e i ­ ther c h l o r i n e atom i s removed from the complex w i t h the concomitant r e d u c t i o n of Rh(III) l e a d i n g to [ R h ( N H ) 5 ] ^ or a r e d u c t i v e d i s s o ­ c i a t i o n of NH3, the r e s u l t i n g complex being [Rh(NH3)4Cl| . In both cases the rhodium complex would have a square pyramidal symmetry and should be l o c a t e d i n the surpercages. Because of s t e r i c r e s ­ t r i c t i o n s imposed by the ammine groups t h i s species appeared unreac t i v e to other ligands such as CO, 02· The e f f e c t of the thermal treatment at 500°C was to completely decompose £Rh(II) (^3)5} com­ p l e x . The bare Rh(II) ions hence migrate w i t h i n the z e o l i t e cages and coordinate w i t h l a t t i c e oxygen ions i n S j , S T . ' , S J J or S u » . I t i s obvious from our r e s u l t s t h a t , at l e a s t , p a r t of the t o t a l amount of Rh(II) i s l o c a l i z e d i n s i d e the hexagonal prisms. However q u a n t i t a t i v e measurements showed that only 10 % of the t o t a l rho­ dium content was found i n the form of Rh(II) ions s i t e d on S j . Hence one could suggest that e i t h e r a l a r g e amount of Rh(II) e s c a ­ pes esr d e t e c t i o n or that a d i s p r o p o r t i o n a t i o n r e a c t i o n 2Rh(II) Rh(I) + Rh(III) has occurred. P a r t i a l o x i d a t i o n of Rh(II) i n t o Rh(III) by O2 at 770 Κ could a l s o be achieved. The carbon monoxide and oxygen r e s u l t s prove that both suggestions are p l a u s i b l e . There are few mononuclear Rh(II) complexes. Magnetic p r o p e r t i e s of rhodium(II) acetate compounds i n d i c a t e d a diamagnetic dimer s t r u c t u r e R1I2(0AC)A w i t h a strong metal-metal i n t e r a c t i o n (14). Formation of rhodium ( I I - aquocomplex has been reported (15) the compound was found to be a b i n u c l e a r (Rti2(H20)io) complex. Considering the complex of (Rh(NH3)5) which a r i s e s from the c h l o r i n e atom removal, i t i s obvious that only one pentaammine complex i s present per supercage owing to the l a r g e s i z e of the rhodium complex. However as NH3 ligands were removed from the rhodium c o o r d i n a t i o n sphere, 2 +

+

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+

2+

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

14.

NACCACHE

E T AL.

Bivalent

Rhodium

Complexes

in

163

Zeolites

the Rh(II) ions tend to bind with l a t t i c e oxygen ions e i t h e r at the hexagonal windows or p o s s i b l y i n s i d e the hexagonal prisms. When two Rh(II) ions were present e i t h e r i n the supercage or i n the sod a l i t e c a v i t y , f o r example, i n Sx», and S j j t , or i n S J J , then met a l - m e t a l bonding would produce diamagnetic b i n u c l e a r Rh(II) complex t y p i c a l of rhodium (II) compounds (15. 16). 0n3 could r e p r e sent t h i s species as £(0 ~)3Rh - Rh(0 ~)33~^ with a C 3 or D symmetry. To allow a b e t t e r metal-metal binding i t i s probable that the rhodium ions move away from the t r i g o n a l axis thus lowering the symmetry of the c r y s t a l f i e l d they are e x p e r i e n c i n g . In some phosphine complexes metal-metal i n t e r a c t i o n over long d i s t a n c e s (4 Â ) has been suggested (17) ; such i n t e r a c t i o n would be r a t h e r small and would allow the i n s e r t i o n of a strong donor l i g a n d such as CO. Hence the b i n u c l e a r b i v a l e n t rhodium complex may p i c k up carbon monoxide with the subsequent formation of mononuclear b i v a l e n t rhodium-carbon monoxide adduct showing paramagnetic p r o p e r t i e s . As CO could not enter the s o d a l i t e cage i t appears that the b i n u c l e a r rhodium species r e a c t i n g w i t h CO was p r e v i o u s l y i n the supercage. The e f f e c t of CO was mainly to remove the metal-metal bond without changing the formal o x i d a t i o n s t a t e of R h ( I I ) . We have shown that oxygen was probably adsorbed on d i f f e r e n t s i t e s as those f o r CO.Rh(II) aquocomplex e x h i b i t e d r e a c t i o n w i t h oxygen s i m i l a r to Co(II) ammine complex forming the c l a s s i c a l μ-peroxo compound J ^ ^ O ^ Q Rh · However Rh(I) species might 2

2

+

V

3ll

a l s o be o x i d i z e d i n t o paramagnetic Rh(II) with the simultaneous formation of the superoxide i o n O2. In p a r t i c u l a r , i t was claimed that the Rh(I) cyclooctene complex was o x i d i z e d i n t o Rh(II) w i t h the formation of Oj (18). By contrast i t was reported that Rh(II) compounds could be reduced by hydrogen and r e o x i d i z e d by oxygen apparently without formation of O2 (13) . In the present study, we could see no evidence suggesting the formation of the superoxide ion as Rh(I) species were o x i d i z e d and therefore we favour the scheme where a s i n g l e O2 molecule r e a c t s w i t h two c l o s e enough Rh(I) ions g i v i n g r i s e to the μ-peroxo species ^ ^ ° ^ 0 ^ R h ( I I ) ^ the t r a n s f e r of two e l e c t r o n s i n t o the antibonding IT molecular o r ­ b i t a l of oxygen r e s u l t i n g i n the diamagnetic 0^~ i o n b r i d g i n g two Rh(II) i o n s . As the C0-adduct, the oxygen-adduct i s l o c a l i z e d i n the supercage. The o x i d a t i o n of rhodium from 1 to 2 o x i d a t i o n s t a ­ te upon oxygen a d s o r p t i o n which was r e v e r s i b l e i n the absence of water i s i r r e v e r s i b l e i n the presence of H2O. The probable r e a c ­ t i o n which occurs might be the c o o r d i n a t i o n of one H2O molecule per Rh atom to R h ( I I ) - OJ ~ Rh(II) i n a trans p o s i t i o n , hence the H2O molecule being l o c a l i z e d i n the s o d a l i t e cage. This geometry would show a t r i g o n a l bipyramidal arrangement of the ligands around the Rh i o n as proposed to i n t e r p r e t the esr r e s u l t s , the rhodium i o n being i n the plane of three l a t t i c e oxygen ions and i n trans p o s i t i o n one H2O and one oxygen atom s l i g h t l y away from the t r i g o ­ n a l a x i s . As a square planar Rh(II) complex could account f o r the esr r e s u l t s one could a l s o suggest that the a d s o r p t i o n of 0~ and u e

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

t 0

164

MOLECULAR SIEVES—II on two Rh(I) ions likely forms mononuclear Rh(II) species from

2 Rh(I) + 1/2 0

2

+ H0 2

2

Rh(II) OK).

In conclusion this work has provided information not only con­ cerning the zeolite as a unique host matrix for unusual transition metal oxidation states but also showed the solvent behaviour of the zeolite framework. Concerning the chemistry of rhodium, apart from the characte­ rization of paramagnetic rhodium species, the work emphasizes the a b i l i t y of rhodium complexes solvated in the zeolite framework to behave in similar fashion as in solution and particularly to a c t i ­ vate two chief reagents in organometallic chemistry, CO and oxygen. Acknowledgments C. NACCACHE wishes to thank the C.N.R.S. and the Stanford University Chemical Engineering department and National Science Foundation for trainships awards. The authors are also indebted to H. URBAIN for the chemical analysis and G. WICKER for the esr mea­ surements . Literature cited 1. Jermyn, J.W., Jonhson, T.J., Vansant, E . F . and Lunsford, J.H., J . Phys. Chem., (1973), 77, 2964. 2. Naccache, C., and Ben Taarit, Y . , Chem. Phys. Letters, (1971), 11, 11. 3. Laing, K . R . , Lubner, R.L. and Lunsford, J.H., Inorg. Chem., (1975), 14, 1400. 4. Vansant, E.F., and Lunsford, J.H., Third Internat. Conf. Mole­ cular Sieves, Adv. Chem. Series 121, Amer. Chem. Soc., (1973), 441. 5. Yashima, T . , Ushida, Y . , Ebisawa, M. and Hara, Ν . , J. Catalysis, (1975), 36, 320. 6. Pichat, P . , Vedrine, J.C., Gallezot, P. and Imelik, B . , J. Ca­ t a l y s i s , (1974), 32, 190. 7. Wilkens, J., De Graag, D.P. and H e l l , J . N . , Phys. L e t t . , (1965), 19, 178. 8. G r i f f i t h s , J . H . E . , Owen, J. and Ward, I . M . , Proc. Roy. Soc., (1963), Ser. A 219, 526. 9. Raizman, Α . , Suss, J . T . and Szapiro, S., Phys. L e t t . , (1970), 32 A, 30. 10. Krigas, T . , and Rogers, M . T . , J. Chem. Phys., (1971), 55, 3035. 11. Shock, J . R . , and Rogers, M . T . , J. Chem. Phys., (1975), 62, 2640. 12. Fujiwara, S., Watanabe, T. and Tadano, H . , J. Coord. Chem. (1971), 1, 195. 13. James, B.R. and Stynes, D . V . , J. Amer. Chem. Soc., (1972), 94, 6225. 14. Kitchens, J. and Bear, J.L., J. Inorg. Nucl. Chem., (1970), 32

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Bivalent Rhodium Complexes in Zeolites

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49. 15. Ziolkowski, J.J., and Taube, H . , B u l l . Acad. Science Poland, (1973), 21, 113. 16. Ziolkowski, J.J., B u l l . Acad. Science Poland, (1973), 21, 119. 17. Master, C. and Shaw, B . L . , J. Chem. Soc., (1971), A, 3679. 18. James, B . R . , Ng, F.T.T., and Ochiai, Ei., Canda., J. Chem., (1972), 50, 590. 19. Vedrine, J.C., and Naccache, C . , Chem. Phys. Letters, (1973), 18, 190. 20. Kuska, H.Α., and Rogers, M . T . , Martell Α . Ε . , Coordination Che­ mistry A . C . S . , Monograph Van Nostrand Reinhold C o . , New York, (1971), 1, 186.

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.