Solid State Nuclear Magnetic Resonance Study of Heavy Metal Hydrides

18 Solid State Nuclear Magnetic Resonance Study of Heavy Metal Hydrides A. T. NICOL and R. W. VAUGHAN

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Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125

A group of heavy metal hydrides, Th H , H Os (CO) , 4

H Os (CO) , and H Ru (CO) 4

4

12

4

4

12

15

2

3

10

have been examined with

both conventional-pulsed and high-resolution solid state nuclear magnetic resonance techniques. Results of lineshape analysis, relaxation time measurements, and line shifts are summarized for the solid state binary hydride, Th H . 4

15

Ex-

perimental lineshapes for the carbonyl hydrides are compared with theoretical calculations for small spin systems, and solid state chemical shift tensors are also reported.

T

his

chapter

presents

results

of

N M R studies

of

several

heavy

metal hydrides, i n c l u d i n g both a s u m m a r y of completed work b y L a u et al.

(J) o n a b i n a r y h y d r i d e , T h H 4

1 5

, a n d p r e l i m i n a r y results on several c a r b o n y l

hydrides, H O s ( C O ) i o , H O s ( C O ) , and H R u ( C O ) i . T h e binary hydride, 2

3

4

4

1 2

4

4

2

T h H i 5 , has attracted interest recently with the discovery of its super-conducting 4

properties (2) a n d the c a r b o n y l hydrides are metal cluster hydrides (3) w h i c h are of interest as models i n the study of catalysis (4). Efforts to understand the state of hydrogen i n metals a n d metal hydrides have i n v o l v e d the use of N M R for m a n y years.

T h i s study combines the con-

ventional solid state N M R techniques w i t h more recently developed high-resolution, solid state N M R techniques (5,6).

Conventional N M R techniques furnish

i n f o r m a t i o n o n d i p o l a r interactions a n d thus can furnish static geometrical i n formation o n hydrogen positions a n d information o n proton motion w i t h i n such solids.

T h e newer m u l t i p l e pulse techniques suppress p r o t o n - p r o t o n dipolar

interaction a n d allow i n f o r m a t i o n o n other, smaller interactions to be obtained. This chapter reports what the authors believe is the first observation of the powder pattern of the chemical shift tensor of a proton that is directly bonded to a heavy metal. These materials are a l l particularly w e l l suited to a N M R investigation since they are simple i n structure w i t h no more than one or two c h e m i c a l l y different 0-8412-0390-3/78/33-167-248/$05.00/0 © American Chemical Society

Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

18.

NICOL A N D V A U G H A N

protons present.

Th Hi 4

stantial extent (7-14).

5

NMR Study of Heavy Metal

Hydrides

249

is stoichiometric a n d has been characterized to a sub

It contains two structurally different kinds of protons,

w h i l e the c a r b o n y l cluster hydrides are thought to contain only a single k i n d of structural proton (3).

Experimental

Details

Proton N M R measurements were made at 56.4 M H z on a spectrometer that was described previously (15). T\ was measured w i t h a 180°--1~-90° pulse se quence, a n d lineshapes were d e t e r m i n e d f r o m free-induction decay signals f o l l o w i n g a 1-2 Msec 90° pulse. T was measured w i t h a 90° x-pulse followed by an attenuated t/-pulse whose length was v a r i e d f r o m 10 Msec to 40 msec. F o r the m u l t i p l e pulse studies, a n eight-pulse cycle that has been discussed in detail previously (5,6,16,17,18) was used. C y c l e time, t , the time required for a single eight-pulse cycle, was 4 2 - 4 8 Msec for measurements reported here. T h e temperature range of 4 0 ° - 4 6 0 ° Κ has been achieved w i t h two probes of different constructions. T h e probe w i t h a temperature range of 100° - 4 6 0 ° Κ used nitrogen gas as the coolant. T h e low temperature probe, connected through a l i q u i d h e l i u m transfer line to a l i q u i d h e l i u m D e w a r flask, used h e l i u m as coolant. For line shift measurements w i t h the eight-pulse cycle between 180° Κ and r o o m temperature, the reference was acetyl chloride. Its frequency was m e a sured relative to a spherical tetramethylsilane (TMS) sample at room temperature, and a l l results are reported relative to this T M S on the τ scale, (τ = σ + 10 p p m , w h e r e σ is the signed c h e m i c a l shift used i n solid state N M R . ) A t lower t e m peratures, the reference was a single crystal of C a ( O H ) , oriented i n the magnetic f i e l d such that the major axis of its proton c h e m i c a l shift tensor was parallel to the external f i e l d (19). Thus, it is assumed that the proton c h e m i c a l snift of the C a ( O H ) r e m a i n e d unchanged as the temperature was varied. The T h H samples were prepared b y C a m e r o n Satterthwaite a n d co workers (1,2) a n d consisted of two polycrystalline samples that were determined to be w i t h i n 1% of the stoichiometric composition, T n H ± o . i 5 - T h e samples differed p r i m a r i l y i n the pressure a n d temperature used i n their synthesis. T h e sample h y d r i d e d under lower pressure ana temperature conditions (1 a t m of H and a temperature cycle i n i t i a t i n g at 800° Κ a n d d r o p p i n g to 450° Κ before re m o v i n g the H ) is labeled the L P sample, a n d the one h y d r i d e d under higher pressure a n d temperature (1100°K a n d ~ 1 0 , 0 0 0 psi of H ) is labeled H P . T h e sample preparation a n d characterization of this stoichiometric c o m p o u n d is apparently critical since the results of the present study differ significantly f r o m the previous N M R studies (12,13,14), and some difference was detected between the L P a n d H P samples themselves. T h e c a r b o n y l h y d r i d e samples were f u r nished k i n d l y b y John R. Shapley (20).


l p

c

2

2

4

1 5

4

15

2

2

2

Results and Discussion TI14H15. Th Hi5: 4

T h r e e kinds of i n f o r m a t i o n are obtained f r o m the samples of

i n f o r m a t i o n on r i g i d lattice structure f r o m free i n d u c t i o n decays, on

proton m o t i o n f r o m relaxation t i m e measurements, a n d on internal fields f r o m peak locations that were found using the m u l t i p l e pulse techniques.


Figure 1

TRANSITION M E T A L HYDRIDES


250

illustrates the free-induction decay of T h H i 5 , a n d a beat structure is clearly 4

present.

T h i s decay was temperature independent below room temperature,

a n d no difference was noted between the two samples.

It is possible to extract

two structural parameters from such a rigid lattice line shape since both the second a n d f o u r t h m o m e n t of the line were obtained by fitting the experimental free i n d u c t i o n decay (21, 22).

Results of early x-ray studies furnished a basis upon

w h i c h one c o u l d hypothesize the proton locations w i t h i n the structure (9), and more recent neutron d i f f r a c t i o n results (JO) have c o n f i r m e d the predicted


18.



structure.

Hydrides

251

F u r t h e r confirmation was obtained b y c o m p a r i n g theoretical second

and fourth moments (21, 22) that were calculated f r o m this structure w i t h these experimental values.

T h e experimental and calculated values are, respectively:

19.5 G and 20.1 G for the second moment and 844 G and 895 G for the fourth 2

2

moment.

4

4

A second way to compare the theoretical values w i t h the experimental

line shape is to use a n e m p i r i c a l function of the f o r m , exp (—a t /2) sin (bt)/bt 2

2

(23), a n d to calculate values of a a n d b f r o m the theoretical second a n d fourth moments.

T h e curve i n F i g u r e 1 was generated i n this fashion using the theo

retical second a n d fourth moments, a n d it fits w e l l w i t h the experimental re sults. T o extract information on the proton motion f r o m N M R measurements, Downloaded by CORNELL UNIV on October 26, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch018

one can use the following approximate equations appropriate for the various N M R relaxation times measured i n this work (24, 25, 26). — ~7 M r T 2

for

2

(1)

τΜ «1 ι/2

2

2

± « i ί ι 3

7

2

M

2

_ L _ ωο τ

f

o

r

ω

ο

» ι

Τ

() 2

ζ

1 + 4ωι τ

Tip

ζ

ζ

where ω = L a r m o r frequency corresponding to external field, H ; o>i = L a r m o r frequency corresponding to rf locking field, Η υ τ = correlation time for proton motion a n d M = second m o m e n t of proton resonance. These equations are d e r i v e d f r o m a single correlation t i m e model for dipolar relaxation of protons, and if one assumes further that τ obeys an Arrhenius relation, the corresponding activation energies, Δ Ε , c a n be obtained f r o m the temperature dependence of the relaxation parameters. 0

0

2

! = ! r T β

Δ

Ε

/ *

(4)

τ

0

Thus, τ can be estimated b y several different measurements, and Δ £ is obtained by fitting the slope of a plot of In ( 1 / T ) , In ( 1 / T i ) , or In (l/T ) as a function of 1 / T . 2

lp

T h e relaxation results (I ) for the L P sample can be summarized. Line-shape measurements indicated that motional n a r r o w i n g started at around 60° C , a n d by 110°C, the lines appeared completely L o r e n t z i a n . T h e activation energy obtained f r o m T above 120°C was 16.3 ± 1.2 k c a l / m o l . 2

T h e observed T\ can have two contributions: 1 / T i = 1 / T i + 1/T\d. T\ results f r o m relaxation effects caused b y conduction electrons, a n d Τ id results f r o m dipolar relaxation effects caused b y the motion of nuclear spins. A t t e m peratures below 110°C, the conduction electron effect appears to dominate T\ since a plot of Τ ι vs. 1 / T can be fit with a straight line passing through the origin. At higher temperatures, the relaxation caused b y the proton motion becomes e


e


252

dominant and by subtracting out the conduction electron-contribution, one obtains an activation energy for proton motion of 18.0 ± 3.0 k c a l / m o l , w h i c h agrees w e l l w i t h the activation energy f r o m the T data. 2

T w o sets of T\ data were obtained at substantially different holding fields, H i = 4.7 G and H i = 20.6 G . T h e measured T\ values are proportional to H i at a fixed t e m p e r a t u r e over the temperature range studied. T h i s is what the m o d e l for proton m o t i o n discussed above (26) w o u l d predict a n d is strong e v i dence that T i p is d o m i n a t e d b y lattice motion. A c t i v a t i o n energies for proton m o t i o n of 10.9 ± 0.7 k c a l / m o l are obtained f r o m the slopes of the lines fitted through both sets of T\ data. T h e difference between the activation energies for proton m o t i o n (16.3 ± 1.2 a n d 18.0 ± 3.0 k c a l / m o l above 120°C a n d 10.9 ± 0.7 k c a l / m o l below 80° C ) obtained from the N M R relaxation rates is w e l l outside the limits of experimental error. T h i s suggests a more complex m e c h a n i s m for proton motion than a simple, single correlation time-activated process a n d could i m p l y more than a single m e c h a n i s m for proton motion. p

2

p


p

In addition to activation energies, Equations 1, 2, and 3 can be used to obtain estimates of the correlation time, r , for the proton motion. These results are i l lustrated i n F i g u r e 2. However, the estimation of correlation times depends upon the n u m e r i c a l values for constants relating the relaxation times (T\, T , T\ ) to the correlation t i m e , r , i n Equations 1, 2, a n d 3, a n d these constants are defined only w i t h i n factors of two or three. As shown i n F i g u r e 2, the τ values obtained f r o m 72 data by using E q u a t i o n 1 are approximately 2.6 times the τ values f r o m the Τ γ data using E q u a t i o n 2. U s i n g E q u a t i o n 3, the τ values d e r i v e d f r o m T data at different H i h o l d i n g fields agree w i t h one another. F u r t h e r m o r e , both r curves (i.e., above 120°C a n d below 8 0 ° C ) extrapolate to the same region i n the intermediate temperature range, i n d i c a t i n g that the available data give a rather consistent picture of the proton motion i n the T l ^ H i s sample. 2

p

lp

A d d i t i o n a l evidence for the a p p l i c a b i l i t y of the above data interpretation is obtained b y using E q u a t i o n 3 to predict values of l / ( T i ) m i n . These are 2.8 Χ 1 0 s e c and 6.4 Χ 1 0 s e c for H = 4.7 G and 20.6 G , respectively, and are consistent w i t h the T\ and T data. T h e motional properties reported here differ significantly f r o m previous measurements: the activation energy obtained f r o m a temperature above 120°C is two to three times the values obtained i n Ref. 13, a n d motional n a r r o w i n g of the lineshape occurs at a significantly higher t e m perature than that i n Ref. 12. p

4

- 1

3

p

- 1

x

2

L i t t l e difference was found between the N M R results on the L P a n d H P samples at either room temperature or as the temperature was raised, although the Τ ι for the H P sample was 10% shorter than that for the L P sample. A f t e r maintaining the samples at temperatures near 200°C for 1 hr., a major difference was noted u p o n cooling, w i t h the H P sample e x h i b i t i n g a m a r k e d t i m e - t e m perature hysteresis of a l l of the measured N M R properties w h i l e the L P sample exhibited no t i m e - t e m p e r a t u r e hysteresis i n any of the N M R parameters m e a sured. A more dramatic manifestation of this is that after the sample was cooled to room temperature, the proton motion took weeks to return to the original state


18.

253

NMR Study of Heavy Metal Hydrides


(°C)


200

ΙΟ

8

'

100

'—·—->

1

2D

I

·

I

0

»

2.5

I

ι

I

ι

3.0

l/T

ι

I

L_J

l_J

3.5

(ΙΟ'^Κ' ) 1

Figure 2. Estimated correlation times, τ, for proton motion as a function of inverse temperature in Th Hi : ( • ) τ values estimated from T data; (O)from T\ data; (A) from T i with H i = 20.6 g; and (•) from T with H i = 4.7 g below 100° C. 4

5

2

p

ip

as i n d i c a t e d b y the T , Γχ, a n d T data (see F i g u r e 3). A f t e r the phenomena were observed the first time, the experiment was repeated a m o n t h later, a n d 2

i p

the same effect was observed i n T\ T , a n d Γ ι y

2

ρ

measurements.

Γι is dominated at room temperature by conduction electron effects w h i l e Τ2 a n d T i are controlled b y motional properties of the protons. T h e large time-temperature hysteresis observed i n all of these parameters indicates strongly a phase change i n the m a t e r i a l on heating. T h e hysteresis is large enough that it is d i f f i c u l t to understand without m o v i n g the t h o r i u m atoms to new locations p


254


since the m o b i l i t y of the protons is such that they w o u l d relocate i n times short compared with these. T h e fact that samples of the same composition and physical characteristics can behave so d i f f e r e n t l y under m i l d heating is i n d i c a t i v e that there is still m u c h to be learned about this complex material.

D e t a i l e d x-ray

studies as a function of sample preparation and temperature are being conducted by C . B. Satterthwaite's research group at this time. M u l t i p l e - p u l s e measurements were p e r f o r m e d on both the L P a n d H P samples at 20° a n d — 80° C , a n d w h e n no differences were noted, lower t e m perature measurements were performed only on the L P sample. M u l t i p l e - p u l s e spectra for the L P sample are illustrated i n F i g u r e 4 together with the eight-pulse spectrum of the reference used for the low-temperature measurements, Ca(OH)2-


T h e lineshapes observed are q u i t e broad, a n d the line center is a f u n c t i o n of temperature.

T h e line w i d t h was separated into three contributions b y per

f o r m i n g three related m u l t i p l e - p u l s e measurements ( i ).

These i n d i c a t e d that

the m a i n contributions to the l i n e w i d t h came f r o m both relaxation a n d secondorder dipolar effects.

T h e m a x i m u m possible field inhomogeneity H a m i l t o n i a n

is estimated to be less than 16 p p m by this means, w h i c h indicates that the corn-

>700h

Ό

10

10' J Ο

1

1

1

10

20

30

/

20

30

T I M E (hour)

40

L, /J/

40

>700

/—I

-40

• I 0

,

Ι

40

kHz

F i g u r e 3. Log of the inverse of the spin lattice relax ation time ( T i ) , the spin-lattice relaxation time in the rotating frame ( T i p ) , and the line shape as a function of time for the HP sample after the sample was heated to 460°K and brought back to room temperature. - 1

-1


18.

NMR

NICOL A N D VAUGHAN

Study of Heavy

\

T h

Hydrides

255

4 .5 H

\

298°K

\v


Metal

I25°K

Ca(OH) A 2

/

1

0

\

1

1

1 2 KHz

1

1

3

4

46°K

Figure 4. The eight-pulse line shape and the peak locations of the Γ / ι Η (LP) powder sample as a function of temperature using a Ca(OH) single crystal as reference. The reference is oriented such that the major principal axis of the proton chemical shift tensor is parallel to the external magnetic field. A shift to the left signifies an increase in the value of σ, i.e., the in ternal magnetic field at the proton site is larger in 7 7 ι Η than in Ca(OH) . 4

1 5

2

4

bination of H

0

1 5

2

inhomogeneity, field distortions (caused by susceptibility effects),

a n d c h e m i c a l a n d K n i g h t shift anisotropics do not a d d u p to more than this value. A l t h o u g h the f i e l d inhomogeneity H a m i l t o n i a n is small, the line center moves by a p p r o x i m a t e l y 50 p p m as the temperature is lowered f r o m room temperature to 46° K .

T h i s is illustrated i n F i g u r e 4.

T h e observed shift was

corrected (reduced by u p to 20%) for temperature-dependent bulk susceptibility (demagnetization effect) (27) of the sample before constructing F i g u r e 4, a n d consequently, the experimentally observed shift was larger than that indicated i n this figure. T h e absolute c h e m i c a l shifts of protons i n d i a m a g n e t i c solids are t y p i c a l l y near 30 p p m (28), a n d the K n i g h t shift caused by conduction electrons through contact interaction is estimated to be —31.2 p p m , using the experimental Γι Τ value of 180 K - s e c a n d the K o r r i n g a relation (29):


256


h

le

1

B o t h of these contributions are temperature independent a n d are smaller than the temperature-dependent

shift observed

experimentally.

Temperature-

dependent shifts have been reported i n rare earth intermetallic compounds (30, 31) a n d attributed to h y p e r f i n e couplings w i t h the rare earth electron spin. A d d i t i o n a l l y , the m a g n i t u d e of temperature-dependent shifts found i n several c o m p o u n d s w i t h the A - 1 5 structure (i.e.,

5 1

V and

6 9

7 1

G a i n V G a (32,33) have 3

been correlated w i t h the super-conducting transition temperature of these m a terials, a n d an explanation has been suggested (34) based on the necessity of a


sharp peak i n the electron density of states at the F e r m i level.

The tempera-

ture-dependent shift reported here for protons i n T h H i is large when compared 4

5

with the expected size of the proton chemical or temperature-independent K n i g h t shift a n d is m u c h smaller than the temperature-dependent shifts seen on other n u c l e i (32, 33). Carbonyl Hydrides.

T h i s section presents p r e l i m i n a r y results obtained

f r o m the a p p l i c a t i o n of both conventional a n d multiple-pulse N M R techniques to a series of metal cluster compounds, H Os (CO)i2. 4

4

H O s ( C O ) i o , H R u ( C O ) i , and 2

3

4

4

2

A review by H . D . Kaesz (3), together w i t h more recent articles

(35-43), summarizes the present knowledge of the structure of these a n d closely related molecules.

Some information on proton positions has been obtained from

N M R studies of similar molecules dissolved i n nematic l i q u i d crystals (39, 40) but i n general, the proton positions i n such s y m m e t r i c molecules have been i n ferred f r o m the x-ray-determined geometry of the heavier atoms.

X - r a y studies

in most cases were performed on derivatives containing a large ligand to facilitate crystallization rather than on the parent globular molecule.

Thus, efforts to

obtain i n f o r m a t i o n on the geometry a n d motional properties of protons w i t h i n these structures directly w i t h N M R techniques appear w o r t h w h i l e .

Solid state

N M R is suited particularly for examining the highly symmetric parent molecules and would, i n contrast, be more difficult to apply to the more structurally complex derivatives.

These materials present a relatively tractable situation for proton

N M R studies since available data indicate that a l l protons i n the structures are equivalent, a n d the carbonyl groups isolate the protons somewhat w i t h i n a single molecule f r o m interactions w i t h protons on neighboring molecules.

T h u s , we

are faced w i t h either two- or four-particle problems that must be solved i n first order to understand the N M R lineshapes.

F o r solids containing a limited number

of spin V particles, the d i p o l a r lineshape can be calculated i n detail a n d thus is 2

more informative than i n the many-particle case (as the T h H i discussed above). 4

5

G . E . P a k e first calculated the two-spin V particle lineshape (23, 44), a n d later 2

the tetrahedral four-spin lineshape was calculated by Bersohn and Gutowsky (45). A single crystal spectra calculation for the rectangular four-spin p r o b l e m was reported by J. Itoh et a l . (46), a n d more recently, E i c h h o f f a n d Z a c h m a n (47) discussed powder patterns for four protons on a rectangle.

T h e situation of the


18.



Hydrides

257

molecules to be considered here is, i n a d d i t i o n , c o m p l i c a t e d b y the presence of a fraction of the metal nuclei w i t h a nonzero dipole moment. has a spin of 3 / 2 a n d a large quadrupole moment (~2 X 1 0 ~

In particular, 2 4

1 8 9

Os

c m ) a n d is found 2

i n a natural abundance of 16.1% w h i l e r u t h e n i u m has t w o isotopes w i t h spin 5 / 2 — " R u (12.8% natural abundance) a n d H OS3(CO)io. 2

1 0 1

R u (17% natural abundance).

F i g u r e 5 presents a proton N M R absorption spectrum for

H2ÛS3(CO)io that was obtained by F o u r i e r transformation of the free induction


decay.

T h e s p e c t r u m was taken at r o o m temperature, but no difference was

H 0 s ( C 0 ) Ίο , 2

3

300°K

Figure 5. The proton free-induction decay line shape for H Os$(CO)i . The points in the center of the spectrum where no line was drawn are caused partly by probe impurities. 2

0

noted as the temperature was lowered to 140° K, thus i m p l y i n g an absence of any change i n molecular motions fast enough to average the dipolar interactions. This spectrum is q u a l i t a t i v e l y similar to that expected f r o m a pair of r i g i d isolated protons (23, 44); however, it does deviate significantly.

F o r example, the sepa-

ration between the two inner peaks should be one-half the separation of the two outer peaks (shoulders), but the inner peak separation is somewhat greater than this.

Possibly more important, the intensity of the shoulder peaks is greater than

w o u l d be expected f r o m a single pair interaction. It does not appear possible


258


to account for such deviations w i t h interactions f r o m protons on n e i g h b o r i n g molecules, a n d we presently are investigating the possibility of m u l t i p l e proton positions (i.e., a double potential well) and that the anomalous effects.

Since

1 8 9

1 8 9

O s might be p r o d u c i n g the

O s has a large quadrupole moment, and the osmium

site s y m m e t r y is lower than c u b i c , it is l i k e l y that the o s m i u m spin is not i n a Z e e m a n state but p r i m a r i l y is under the influence of a very large q u a d r u p o l e interaction, w i t h the Zeeman interaction serving only as a perturbation.

The

ability of non-Zeeman state spins to produce anomalous effects i n N M R spectra is w e l l documented (48, 49), a n d we presently are investigating whether or not the effects observed i n F i g u r e 5 can be simulated analytically on this basis.

The

results are important i n a t t e m p t i n g to extract p r o t o n - p r o t o n distances f r o m a


spectrum such as that i n F i g u r e 5.

F o r example, a second m o m e n t calculated

f r o m the spectrum i n F i g u r e 5 is 1.6 G , and if we assume 0.1 G for the combined 2

2

effects of intermolecular proton interactions a n d p r o t o n - o s m i u m interactions

Figure

6. Multiple-pulse proton line shape H20s(CO)ioat room temperature

for

(proper for the case where the o s m i u m is i n a Zeeman state), we obtain a p r o t o n - p r o t o n distance of 2.5 A .

If one accepts that the m a i n - p e a k splittings i n

F i g u r e 5 were p r o d u c e d by a proton pair alone, one obtains values near 2.55 A; if one uses the shoulder spacings, one obtains values greater than 2.6 A. T h u s , such distortions must be understood to extract accurate p r o t o n - p r o t o n dis tances. F i g u r e 6 is a plot of the proton N M R spectrum obtained f r o m H O s ( C O ) i o 2

w h e n using an eight-pulse cycle (5, 6,16,17,18)

3

to suppress the effects of pro

t o n - p r o t o n d i p o l a r interactions. T h e curve results f r o m a computer fit that as sumes the lineshape is caused b y the c h e m i c a l shift tensor.

T h e center of the

s p e c t r u m is near τ = 19 p p m , a n d thus it agrees reasonably w i t h that expected f r o m the solution N M R results ( r = 21.7 p p m (37)).

T h e three p r i n c i p a l values

of the tensor, a c c o r d i n g to this fit, are at τ values 5.6 p p m , 19.9 p p m , a n d 31.6 ppm.

Since a p p r o x i m a t e l y o n e - t h i r d of the proton pairs interact w i t h a near


18.


H 0s (C0) 4

4

H 0s (C0)

| 2

4

~IOO°K


259


4

l 2

~300°K

Figure 7. Free-induction decay line shapes for H Os (CO)1 near 100°Κ and room temperature. The lack of complete symmetry in the line shape may be caused by a probe impurity signal located slightly to the left of the spectra s center. 4

H Ru (C0) 4

4

4

2

H Ru (C0)

1 2

4

~I20°K

.

4

~300°K

Figure 8. Free-induction decay line shapes for H Ru (CO)\ near 120°Κ and room temperature. The sharp peak located slightly left of the center in the room-temperature spectrum (and the asymmetry in the lower temperature spectrum) is caused partially by probe impurities. 4

4

2


| 2

260


neighbor osmium with a spin, these values can furnish an upper limit to the proton c h e m i c a l shift anisotropy (~26

p p m ) i n this environment.

T h i s is, to our

knowledge, the first direct observation of a c h e m i c a l shift tensor for a proton believed to be bonded d i r e c t l y to a transition metal. T h e size of the anisotropy of the c h e m i c a l shift tensor is similar to that f o u n d i n several proton tensors (5, 6) a n d i n particular, is similar to the O-H—Ο hydrogen-bonded proton tensor (5, 6,28).

Thus, the phenomena responsible for p r o d u c i n g the large r c h e m i c a l

shifts that are unique to the direct bonding of the proton to a transition metal ion does not necessarily produce a large c h e m i c a l shift tensor anisotropy, as has been speculated for the case of t e r m i n a l l y bonded protons (50). H O s ( C O ) i A N D H R u ( C O ) i . Figures 7 a n d 8 illustrate the w i d e - l i n e 4

4

2

4

4

2


spectra of these two materials. As indicated above, these cluster hydrides consist of nearly tetrahedral units of four metal atoms a n d are thought to have b r i d g i n g hydrogens on four of the six m e t a l - m e t a l bonds (3, 35, 36, 37).

T h e two

H O s ( C O ) i 2 spectra are similar a n d indicate that the protons i n this molecule 4

4

are not involved i n a motional process sufficient to average the dipolar interaction d i f f e r e n t l y at 100° a n d at 300°K.

T h e proton spectrum taken of H O s ( C O ) i 4

4

2

w i t h the eight-pulse cycle (5, 6, i 6 , 17, 18) was centered at r = 30 p p m but is otherwise similar to F i g u r e 6 w i t h a p p r o x i m a t e l y the same c h e m i c a l shift a n isotropy.

Thus, the same remarks can be made about the c h e m i c a l shift spectra

of H O s ( C O ) i 2 as were m a d e for H O s ( C O ) i o . 4

4

2

3

In p a r t i c u l a r , there are no i n

dications that molecular motions are rapid enough to average either the chemical shift or dipolar spectra.

O n e cannot eliminate the possibility of a highly restricted

motion w i t h a small activation energy (51,52) that could produce similar motional averaging at 100° a n d 300°K.

T h e h i g h molecular s y m m e t r y of H O s ( C O ) i 4

4

2

makes it difficult to envision such a motional process that w o u l d not average the c h e m i c a l shift or dipolar spectra, except possibly for a double potential w e l l i n the o s m i u m - h y d r o g e n o s m i u m b o n d . T h e two H R u ( C O ) i 2 spectra i n F i g u r e 8 present a different picture. 4

4

The

r o o m temperature spectrum is similar to, although slightly narrower than, the two H O s ( C O ) 4

4

1 2

spectra i n F i g u r e 7.

however, quite different i n shape.

T h e 120°K H R u ( C O ) i spectrum is, 4

4

2

A change i n shape of a w i d e l i n e N M R spec

t r u m as a function of temperature n o r m a l l y indicates that t h e r m a l l y activated nuclear motions can average partially the dipolar interaction at the higher temperature; one suspects that the protons are moving sufficiently rapidly at room temperature to narrow partially the dipolar line width. F i g u r e 9, w h i c h illustrates the eight-pulse spectrum of H R u ( C O ) i 2 taken at two different temperatures 4

4

(—45° a n d 2 2 ° C ) , confirms the presence of molecular m o t i o n as the spectrum changes significantly oVer this temperature range.

Also note that the center of

this m o t i o n a l l y averaged tensor is at τ = 25 p p m c o m p a r e d w i t h 28 p p m i n so lution (32,48).

Since the c h e m i c a l shift tensor can be averaged w i t h a m o t i o n

slower than that r e q u i r e d to affect the p r o t o n - p r o t o n d i p o l a r interaction, a m a r k e d effect occurs i n the c h e m i c a l shift pattern.

T h u s , we have c o n f i r m e d

qualitatively the results obtained b y J . R. Shapley (54) w h i c h i n d i c a t e d that


18.



Hydrides

261

m o t i o n a l n a r r o w i n g occurs i n H R u ( C O ) i 2 at r o o m temperature, a n d w e f i n d 4

4

no evidence for similar m o t i o n a l processes i n H O s ( C O ) i 2 i n the same temper 4

4

ature range. As i n d i c a t e d i n the i n t r o d u c t i o n to this section, these materials are p a r t i c u larly interesting since the n u m b e r of interacting spins is l i m i t e d by the separation between molecules that is created b y the carbonyl groups; thus, one can compare results

with

detailed

lineshape

calculations.

F o r these

t w o materials,

h O s ( C O ) i 2 a n d H R u ( C O ) i 2 , one must deal w i t h a four-proton p r o b l e m 4

4

4

4

possibly m o d i f i e d b y the presence of q u a d r u p o l a r n u c l e i w i t h small d i p o l e m o ments.

T h e four-spin V p r o b l e m has been the subject of a n u m b e r of papers, 2

but there is only one, the work of E i c h h o f f and Z a c h m a n n (47), dealing w i t h the spin V particles arranged on a rectangle (the case here i f one assumes the sug-


2

H Ru (C0) 4

22°C

°

4

| 2

\ °

0

-45°C

lOppm (

PP

Ο

Ο

_

ο cP

P

Figure 9. Multiple-pulse proton spectra for r Y R u ( C O ) i 2 at 22° and —45°C. The increased width of the lower temperature spec trum indicates less motional averaging of the chemical-shift powder pattern as the temperature is lowered. 4

4

gested edge-bridging structures), and it is not i n a f o r m that can be used without more detailed calculations. Thus, at this early point i n our study we cannot make the same comparisons w i t h detailed lineshapes as w e d i d for H O s ( C O ) i o 2

3

H o w e v e r , such calculations are i n progress a n d w e w i l l hopefully be able to make such detailed comparisons i n the future. Acknowledgment T h i s work was supported b y the E n e r g y Research a n d D e v e l o p m e n t A d ministration, a n d laboratory equipment purchased w i t h funds f r o m the N a t i o n a l Science F o u n d a t i o n was used.

A . T . N i c o l expresses appreciation for partial

support f r o m a n I . B . M . predoctoral fellowship.


262

TRANSITION METAL HYDRIDES

Literature Cited


1. 2. 3. 4. 5.

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1977.


Solid State Nuclear Magnetic Resonance Study of Heavy Metal Hydrides

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