16 Hydrogen-Deuterium Exchange in Hydrido Transition Metal Cluster Complexes Mass and Raman Spectrometric Characterization MARK A. ANDREWS, STEPHEN W. KIRTLEY, and HERBERT D. KAESZ Department of Chemistry, University of California, Los Angeles, CA 90024
The exchange of D O with a variety of hydrido-metal com 2
plexes is discussed. While the exchange of HRe(CO) with 5
D O is rapid, that of a variety of hydrido-metal cluster com 2
plexes is not observed. The exchange of D O with H Re (CO) 2
or H Os (CO) 2
3
10
2
2
8
on the other hand is found to be catalyzed by
Florisil chromatographic absorbent. Mass and Raman spec trometric means of characterization are discussed.
H
y d r i d o - m e t a l c a r b o n y l complexes are k n o w n to be t h e r m o d y n a m i c a l l y acidic ( I , 2, 3,4).
A broad range of a c i d i t y is observed w i t h H C o ( C O ) 4 ,
H V ( C O ) , a n d H F e ( C O ) n , reported to be strong acids (pK ~ 1); w i t h 6
2
a
3
H F e ( C O ) a n d H M n ( C O ) , reported to be weak acids (ρΚ 2
4
αι
5
~ 4.4 a n d pK ~ a
7.1, respectively); a n d w i t h H R e ( C O ) s , reported as only a " v e r y weak a c i d " ( J , 2 , 3 , 4).
T h e acidity of H R e ( C O ) i has been estimated by the slow equilibration 3
3
2
of the corresponding anions, H R e ( C O ) i 3
2
2 _
a n d H R e ( C O ) i ~ , w i t h a variety 2
3
2
of acids; for H R e ( C O ) i , Ρ & 1 is between 3 and 4 a n d pK 3
18 (3).
3
2
a2
α
is between 10 a n d
Q u i t e a contrast, however, is observed i n the rates of proton exchange
for mononuclear complexes compared with most polynuclear (cluster) complexes. T h e former w i t h t e r m i n a l l y bonded hydrogen atoms exchange protons f a i r l y rapidly; for the manipulation of D R e ( C O ) for instance, Beck, Hieber, and Braun 5
(4) advised the use of an absolutely dry apparatus.
In our experience (5) we found
it impossible to bake out all components of a conventional chemical high vacuum line and therefore found it necessary to condition its inner surfaces with D 0 prior 2
to evacuation and exposure to D R e ( C O ) s .
F a i l u r e to do so led to significant loss
of d e u t e r i u m content d u r i n g v a c u u m transfer of the deuterio complex. B y contrast, the deuterio-metal cluster complexes D R e ( C O ) i 3
3
2
(6) a n d
D R u ( C O ) i (7) proved to be stable t o w a r d proton exchange under c h r o m a t o 4
4
2
0-8412-0390-3/78/33-167-215/$05.00/0 © American Chemical Society
216
TRANSITION M E T A L HYDRIDES
graphic p u r i f i c a t i o n a n d other n o r m a l h a n d l i n g procedures. In these metal cluster complexes, the hydrogen atoms are i n a b r i d g i n g position rather than i n t e r m i n a l positions (8, 9, 10), a n d the coordinated c a r b o n y l groups on adjacent metal atoms are situated at close to van der W a a l s contacts. T h e structure of H R e ( C O ) i has thus far eluded a l l attempts by x-ray crystallography since no suitable single crystals have been found. T h e structure of the manganese analog, H M n ( C O ) i , has recently been completed (8). A b r i d g i n g position for h y drogen was found using F o u r i e r difference methods. O t h e r spectroscopic a n d structural evidence for b r i d g i n g hydrogen i n a variety of h y d r i d o - m e t a l cluster complexes is given i n a recent review (9). Access of external reagents to the b r i d g i n g hydrogen atoms is thus i m p e d e d (for further discussion, vide infra). It is understandable, therefore, that the rate of proton exchange i n these complexes w o u l d be rather slow despite their thermodynamic acidity (3). This is, however, not necessarily the case i n a l l cluster complexes, and our awareness of this developed over the series of observations described below. 3
3
3
2
3
2
Observations Leading to the Discovery ofH/D Metal Cluster Complexes
Exchange in
Hydrido-
T h e deuterio-metal cluster complexes mentioned above were synthesized f o l l o w i n g the routes indicated i n Reactions 1 and 2 ( T H F = tetrahydrofuran). T h e isotopic p u r i t y of the products was qualitatively checked by spectrophotom e t r i c means. W e have found that R a m a n more frequently than i r spectra display characteristic features sufficiently discernible to distinguish between the protonated and deuterated derivatives (9). T h e R a m a n absorptions are usually broad but are observed to shift a n d to become more narrow upon deuteration (9), similar to what has been observed for other compounds containing hydrogen or d e u t e r i u m i n a b r i d g i n g situation ( I I , 12). E x a m p l e s of this can be seen i n the spectra for H R e ( C O ) i a n d D R e ( C O ) i ( F i g u r e 1) or for H R u ( C O ) i and D R u ( C O ) i (Figure 2). T h e absorptions assigned to hydrogen or deuterium are shaded i n the corresponding spectra. In H R e ( C O ) i , the broad shaded absorption is centered at 1100 c m . In the corresponding modes of the deuter i u m complex, the center is at 785 c m , g i v i n g a ratio of ^ M H / ^ M D 1-40, i n good agreement w i t h that expected for the isotope shift (1.414). F u r t h e r discussions of the R a m a n spectral features are given i n the latter portion of this chapter. 3
4
4
3
2
3
3
2
4
4
2
2
3
3
2
- 1
- 1
=
reflux
R e ( C O ) i + N a B D / T H F — > » intermediate anions 2
0
4
D3PO4
—>·
D Re (CO) 3
3
1 2
(1)
cyclohexane extract
R u ( C O ) i + D ( l atm) 3
2
90°C 1 hr
2
D Ru (CO)i 4
octane sol'n
4
2
(2)
Figure
1.
Raman spectra,
Cary 81, He/Ne laser exciting
1
line 15 803 cm' ;
microcrystalline
samples
(25) t—'
to
md*
2000
1
4
Figure 2.
2ΚΧ)
X-
I
I
I
JL
Raman spectra,
1500
MRiiirm
4
D Rig[cq s
Solid SOTnpte
\
1000
l
900
800
700
line 15 803 cm~ ; microcrystalline
WOO 1300 1200 1100 Reman Shift fcrrf')
!
i
i
1
Cary 81, He/Ne laser exciting
1900 1700 1600
1
1
i
1
•
I
il
samples (9)
Chemical Reviews
500 4 0 0
Αν"
600
A-
1
ϋ
u ν
16.
Hydrogen-Deuterium
ANDREWS E T AL.
L_
219
Exchange
CALCULATED OBSERVED
Ru H (C0) 4
4
I
1—
—I—
732
736
740
744
il
736
740
744
748
I. I. 1— 756
752
748
L2
Ru D (C0) 4
752
4
L2
756
Journal of the American Chemical Society
Figure 3.
Calculated
and average observed peaks in the parent ion ΑΕΙ MS9 Spectrometer (7)
multiplets;
W e also have used mass spectrometry to characterize these derivatives. Analysis by this method, however, is complicated because of partial hydrogen loss on ionization, w h i c h leads to o v e r l a p p i n g isotopic multiplets for the parent ions. A n example of this can be seen i n the mismatch of calculated and observed peaks i n the parent ion multiplet shown for H R u 4 ( C O ) i 2 (top part of F i g u r e 3). T o deal w i t h this, a computer p r o g r a m , M A S P A N , was w r i t t e n to deconvolute the spectra i n terms of the various hydrogen-loss species, i.e. H R u 4 ( C O ) i 2 (n = 4, 3, 2, 1, and 0), whose lines contribute to the parent ion multiplet. F o r details of the program a n d its use see Refs. 7, 13, a n d 14. T h e effect of this deconvolution is an i m p r o v e d fit between calculated a n d observed spectra expressed by an R factor: 4
n
760
220
TRANSITION M E T A L HYDRIDES
Thus an R factor of 17.1% is obtained for the m a t c h between observed a n d calculated multiplets prior to correction for hydrogen loss, as shown at the top of F i g u r e 3. D e c o n v o l u t i o n of the observed spectrum, however, i m p r o v e d the fit to R = 6.5% for the composition H R u ( C O ) i ; η = 4, 6 5 % ; η = 3, 19%; η = 2 and η = 1, 0%; η = 0, 16% (7,14). D R u ( C O ) i , on the other hand, undergoes m u c h less fragmentation on ionization, as indicated by the m a t c h between ob served and calculated multiplets shown i n the lower scan of F i g u r e 3. F o r this, n
4
4
2
4
2
an R factor of 5.3% is obtained without deconvolution, reflecting undoubtedly the lower zero-point energy i n the b o n d i n g of deuterium as compared w i t h that of the hydrogen w i t h the metal atoms of the cluster. S i m i l a r l y , only slight fragmentation of hydrogen occurs i n the o s m i u m hydrides H O s ( C O ) i o a n d H O s ( C O ) i , paralleling greater stability i n the bonding of hydrogen to third-row transition metal atoms (9). A better fit of observed-to-calculated parent ion multiplets is obtained for these two hydrides, as shown i n F i g u r e 4; the R factor for these two spectra are 4.6% a n d 5.2%, respectively (7,14), without deconvo lution. 2
4
4
3
2
CALCULATED OBSERVED
0 s H ( C 0 ) ,Ί0 3
—V—
-V— 850
845
2
855
860
0s H (C0),'12 4
1090
•• •• υ 1092
I! I! I! li li 1094
1096
1098
4
Η l! II υ •• •• 1100
1102
1104
1106
1108
I IK)
Journal of the American Chemical Society
Figure 4. Calculated and average observed peaks in the parent ion multiplets; ΑΕΙ MS9 Spectrometer (7)
16.
Hydrogen-Deuterium
ANDREWS E T AL.
221
Exchange
D Os (CO),o 2
3
CALCULATED OBSERVED
847
850
Figure 5.
853
Calculated
856
859
862
and average observed peaks in the parent ion ΑΕΙ MS9 Spectrometer (7)
multiplet;
W e were thus taken by surprise when the mass spectrum of a sample believed to be D 2 0 s 3 ( C O ) i o ( F i g u r e 5) showed w i d e variation between the observed a n d calculated intensities (7).
T h e sample had been prepared from D and O s ( C O ) i 2 2
3
a n d p u r i f i e d by c h r o m a t o g r a p h y ; deconvolution analysis (R = 2.9%) i n d i c a t e d the sample consisted of a m i x t u r e of 9% D O s 3 ( C O ) i 2 , 42% H D O s ( C O ) i o , a n d 2
49% H O s ( C O ) i o . 2
3
3
W e subsequently learned that Keister a n d Shapley (15) had
been able to isolate pure D 2 0 s ( C O ) i o f r o m the same reaction of D2 a n d 3
O s ( C O ) i 2 ; however, their material had been p u r i f i e d by crystallization.
H/D
3
exchange i n our sample was thus indicated to occur d u r i n g the chromatographic w o r k - u p rather than d u r i n g the synthesis, as we h a d earlier suggested (7). T w o further instances of H / D exchange, these for D R e ( C O ) 8 came to 2
2
m i n d ; both were traceable to a chromatographic purification step.
O n e of these
occurred i n the photochemical reaction of D2 w i t h R e 2 ( C O ) i o (IS).
Three
products were isolated by us i n that reaction; n a m e l y , D R e ( C O ) s (distilled out of the reaction m i x t u r e w i t h the solvent) a n d D R e ( C O ) i a n d H R e 2 ( C O ) s 3
(isolated by c o l u m n chromatography).
4
2
A parallel and independent study of the
photochemical reaction of H2 with Re2(CO)io, giving the same products together with some H R e ( C O ) i 2 , was reported by Byers and B r o w n (16,17). 3
3
T h e isotopic
content of the products isolated i n our work was determined by ir and mass spectra (13).
T h e loss of d e u t e r i u m i n the product H R e 2 ( C O ) s , w h i c h was at first 2
p u z z l i n g , was soon connected w i t h a p r i o r instance of H / D exchange i n this de rivative that was observed d u r i n g a collaboration w i t h W . A . G . G r a h a m a n d co-workers on the R a m a n spectrophotometric characterization of the derivatives
TRANSITION M E T A L HYDRIDES
8
8
8
?
&
o
g
8
?
8
°
0 2200
20
40
60
80
0
20
40
60
80
1
A
7.
1900
Figure
2100 2000
U _-
1
11
U
1
i
1550
1250
Ronron SNft fcnV*)
050
m
1150
*——.
K>50
OOKJ οαπm *
Cary 81, He/Ne laser exciting
1450
Raman spectra,
1800
—
I LOMTRB non
1
(13)
550
JLA-J
I i 1
750 650
!
1
Inorganic Chemistry
850
line 15 803 cm'
950
1
450
\
i
350
224
TRANSITION M E T A L H Y D R I D E S
R e 2 ( C O ) H S i P h 2 a n d H R e ( C O ) (18). 8
2
2
2
8
T h e latter is f o r m e d d u r i n g c h r o m a
tography of the silyl derivative by loss of a diphenylsilylene group on the column. T h e R a m a n spectra of R e 2 ( C O ) 8 H S i P h and its di-deuterio derivative are shown 2
2
i n F i g u r e 6; a h i g h isotopic p u r i t y i n the latter is i n d i c a t e d b y appearance of the band at 1 2 8 0 c m at 1 7 9 0 c m
i n its spectrum (lower trace F i g u r e 6) replacing the absorption
- 1
i n the d i h y d r i d o species (upper scan).
- 1
by the ratio I ' M H / M D
=
isotopic substitution.
T h e two bands are related
1 3 9 8 , close to the value of 1.414 expected for such an T h e spectra of the d i r h e n i u m o c t a c a r b o n y l derivatives
obtained f r o m the silyl complexes are shown i n F i g u r e 7.
The D R e ( C O ) 2
2
8
ob
tained f r o m R e ( C O ) D S i P h (lower scan F i g u r e 7) shows the presence of an 2
8
2
2
appreciable amount of H R e ( C O ) . 2
2
8
T h u s , experiments i n d i c a t e d i n the title
of the next section were undertaken further to explore these exchanges.
Hydrogen-Deuterium
Exchange of H OsCO)\o 2
Florisil Chromatographic
and H ReCO)
on
2
Absorbent
W e first attempted exchange of the above mentioned hydrido-metal clusters by contact of their hydrocarbon solutions w i t h D 0 ; no exchange was observed. 2
S i m i l a r l y , contact of the h y d r o c a r b o n solutions of the hydrides w i t h D 3 P O 4 p r o d u c e d the same results.
T h u s , we proceeded to conditions a p p r o x i m a t i n g
those of chromatographic separation by contacting solutions of the hydrido-metal clusters w i t h deuterated F l o r i s i l (Florisil exchanged w i t h D 0 prior to its use). 2
Significant incorporation of d e u t e r i u m was observed under these conditions and the results are s u m m a r i z e d i n T a b l e I (13). T a b l e I. (A)
Analysis of Deuterated H R e ( C O ) 2
H Re (CO) 2
2
8
2
2
8
2
2
2
8
8
2
8
2
8
(B)H Os (CO) 2
3
5.5(10) 37.0(0) 55.5(17) 1.3(3) 0.8(4)
Species D Os (CO) HDOs(CO) H Os (CO) HOs (CO) Os (CO) 3
2
3
3
3
1 0
1 0
1 0
1 0
1 0
7.3(52) 0.0(0) 92.6(67) 0.0(0) 0.0(0)
and
H Os (CO)io 2
3
e
Run 2 Stat. %
b
6 38 56
— —
Obs. %
Stat. %
50.4(72) 35.4(47) 12.2(44) 1.4(26) 0.6(6)
46 43 11
b
— —
Run 2
Run 1
10
Obs.% 2
8
Run 1 Obs.%
Species D Re (CO) HDRe (CO) H Re (CO) HRe (CO) Re (CO)
2
Stat. %
b
1 13 86
— —
Obs. %
Stat. %
65.3(58) 32.5(29) 2.3(38) 0.0(0) 0.0(0)
69 28 3
b
— —
For details see Ref. 73; standard deviation of least significant digits given in parentheses. T h e total fractional deuterium content ( T D C ) is equal to the fraction of D 2 species plus one half of the fraction H D species. Random statistical distribution corresponds to ( T D C ) ^ for the D 2 species, 2 ( T D C ) (1 - T D C ) for the H D species and (1 - T D C ) for the H species. α
b
2
2
16.
Hydrogen-Deuterium
ANDREWS E T AL. T h e complex
H R e ( C O ) s exchanges 2
225
Exchange
about three times faster than
2
H Os3(CO)io, i n agreement w i t h the visual observation of greater adsorption of 2
the f o r m e r on the F l o r i s i l .
B o t h exchanges follow r a n d o m statistics w i t h i n the
experimental error of the analyses, as i n d i c a t e d i n T a b l e I.
T h i s implies that
adsorption a n d desorption on the support is c o m p e t i t i v e w i t h H / D exchange. O n e important characteristic of the two cluster hydrides w h i c h are observed to undergo H / D exchange on c h r o m a t o g r a p h i c support is their electronic u n saturation (9, JO). Because of this they are both expected to be more susceptible than the usual cluster complexes to nucleophilic attack; indeed the facile addition of L to H O s 3 ( C O ) i o , g i v i n g complexes of the type H O s ( C O ) i o L , recently have 2
2
been reported ( 2 i , 22).
3
This latter complex is believed to have one b r i d g i n g a n d
one t e r m i n a l h y d r o g e n , a n d the presence of a t e r m i n a l h y d r o g e n i n such an a d duct c o u l d confer u p o n it a higher rate of H / D exchange rate, as observed for H R e ( C O ) 5 (discussed above).
T h u s , the enhanced exchange rates for these h y -
drides m a y be attributed to reactions of the type ( N = n u c l e o p h i l i c site on the support) i n w h i c h the species w i t h the terminal M - H groups were those i n w h i c h H / D exchange w i t h D 0 took place. 2
A l t e r n a t i v e l y , the higher rate of exchange of b r i d g i n g h y d r o g e n i n the u n saturated clusters c o u l d result f r o m their greater accessibility to external bases. T h i s c o u l d be caused by the greater interligand separation, as can be seen by comparison of the structures for H R e ( C O ) i 2
also structure of H R e ( C O ) e ) (20). 2
2
3
2
_
(23) and H W ( C O ) 2
2
8
2
" (24), (see
Since oxygen-oxygen van der W a a l contacts
are about 3 A and even a small base such as water has a v a n der W a a l diameter of 2.8-3.0 A, it is clear that deprotonation i n a complex of type H R e ( C O ) i 2
3
2
requires a t u n n e l i n g process (cf. the several hour h a l f - l i f e for deprotonation of H Re3(CO)i 3
2
b y amines) (3).
In the type of complexes
represented by
H W ( C O ) s 2 , direct contact between a base a n d the cluster-bonded h y d r o g e n 2
2
is possible, w h i c h w o u l d greatly facilitate exchange.
226
TRANSITION M E T A L HYDRIDES
Regardless of the details of the exchange mechanism, it is clear that careful control experiments are necessary when w o r k i n g w i t h deuterated metal carbonyls, be they mononuclear or polynuclear.
Also, D 0 exchange catalyzed by a 2
c h r o m a t o g r a p h i c support m a y be a convenient m e t h o d for synthesizing certain metal deuterio complexes.
Vibrational Analysis of the Bridging Hydrogen
Absorptions
T h e v i b r a t i o n a l data for h y d r i d o - m e t a l cluster complexes, although useful i n characterization, present some difficulties i n analysis.
T h e absorptions for
h y d r o g e n b r i d g i n g between metals usually is observed i n the region 1600-800 cm
- 1
(9, 25) shifted a n d i n some cases considerably broadened c o m p a r e d w i t h
the t e r m i n a l m e t a l - h y d r o g e n stretching modes w h i c h usually are observed i n the region 2 2 0 0 - 1 6 0 0 c m
- 1
(9).
In some p o l y n u c l e a r h y d r i d e complexes, ab-
sorptions are seen above the b r i d g i n g region, and it is possible that these contain a pronounced a s y m m e t r y i n the position of the b r i d g i n g hydrogen.
T h i s is dis-
cussed at greater length below. T h e absorptions for b r i d g i n g h y d r o g e n also can d i s p l a y m u l t i p l i c i t y , as i n the pair of bands seen for H4Ru4(CO)i2 or H2Re2(CO)s (Figures 2 and 7, above). A discussion of these also is presented below.
F o r the b r i d g i n g absorptions:
very
broad bands can display a more complex pattern as e v i d e n c e d by a series of m i n i m a i n the spectrum of H R e 3 ( C O ) i 2 ( F i g u r e 1, above), or a pronounced 3
c o m p l e x i t y as evidenced i n the spectra of H R e s ( C O ) i 4 ( F i g u r e 3, Ref. 9) or H R e 2 ( C O ) s C l ( F i g u r e 24, Ref. 25).
Such patterns are too complex a n d spread
over too broad a spectrum of energy to be accountable as fundamental vibrations of b r i d g i n g h y d r o g e n alone.
C l a y d o n a n d Sheppard (26) have discussed the
effect of coincidences or near coincidences of overtones of l o w e r - l y i n g vibrations on the shape of the broad bands observed for h y d r o g e n - b r i d g i n g between n o n metal atoms; possibly the same methodology can be a p p l i e d to account at least in part for m a x i m a (or minima) observed i n the broader of the bridging hydrogen absorptions. F o r H R e 2 ( C O ) s (D h symmetry) (20), three M - H - M vibrational bands are 2
2
expected i n both the i r a n d R a m a n spectra, w i t h no coincidences between the two because of the center of s y m m e t r y (shown on page 227). T h e 2A
g
stretching and B
2u
energy (
