Experimental Organometallic Chemistry - American Chemical Society

Chapter 8

Spectroscopic Characterization of Inorganic and Organometallic Complexes by Metal and High-Pressure NMR

Downloaded by IMPERIAL COLLEGE LONDON on February 19, 2015 | http://pubs.acs.org Publication Date: November 24, 1987 | doi: 10.1021/bk-1987-0357.ch008

D. Christopher Roe Central Research and Development Department, Experimental Station, Ε. I. du Pont de Nemours & Co., Wilmington, DE 19898

Practical guidelines are offered for obtaining NMR spectra either of metals or of samples which are subjected to conditions of high pressure. Both techniques are becoming routinely available as a means for characterizing organometallic complexes. Applications of metal NMR are no longer limited to static characterization in terms of sample homogeneity or trends in chemical shift. Examples are drawn to emphasize structural characterization and dynamic studies. A novel sapphire NMR tube is described which permits routine high resolution operation up to 2,000 psi. The tubes can be used in any spectrometer, and have allowed the study of fundamental reactions of transition metal carbonyl complexes under conditions which would otherwise bring about sample decomposition. The f i r s t part of t h i s review introduces some of the p r a c t i c a l considerations which may be required to obtain an NMR spectrum of a t r a n s i t i o n (or other) metal of interest. An underlying theme i s that such studies are no longer limited to the determination of chemical s h i f t or sample homogeneity, and l i t e r a t u r e examples (through mid-1986) have been selected with t h i s more elaborate emphasis i n mind. The second part of the review deals with a recent development i n "high-pressure" NMR which f a c i l i t a t e s the study of systems under pressures up to a few thousand p s i . An advantage of the approach described i s that i t may be implemented r e l a t i v e l y e a s i l y and requires no hardware modification to the spectrometer. Metal NMR The p r i n c i p a l reason f o r the increasing importance of metal NMR as a means f o r sample characterization i s simply the more general a v a i l a b i l i t y of commercial broad-band NMR spectrometers, with a t y p i c a l range of RF components from 5 - 500 MHz. The range from low frequency metals, such as rhodium, on up to r e l a t i v e l y high frequency nuclei such as phosphorus can t y p i c a l l y be spanned using

0097-6156/87/0357-0204S06.00/0 © 1987 American Chemical Society

In Experimental Organometallic Chemistry; Wayda, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Characterization of Inorganic and Organometallic Complexes 205


just one or two probes, and the advent of wide-bore superconducting iiiagnets has permitted the advantage of large sample volumes (e.g., tubes 20 mm i n diameter holding 10-12 mL). Instrumental Considerations. Of necessity f o r multinuclear observation, one must be able to conveniently tune and match the broad-band probe to the desired frequency. I f software f a c i l i t i e s for accomplishing t h i s are not d i r e c t l y available on the spectrometer, the probe tuning curve can be displayed on an o s c i l l i s c o p e using a sweep generator (e.g., Wavetek Model 10621) and a tuning bridge (e.g., Wiltron Model 62BF50). Externally adjustable capacitors i n the probe are then adjusted to tune the probe response to the desired frequency, and to match the response to a 50-ohm load (1). The high-Q probes t y p i c a l l y employed i n high r e s o l u t i o n NMR are s u f f i c i e n t l y sensitive that a well-tuned probe at room temperature may become detuned by more than 1 MHz by changing the temperature to -80°C. Similar effects may be noted f o r samples having substantially d i f f e r e n t d i e l e c t r i c properties, and f a i l u r e to account f o r these tuning differences may lead to longer pulse lengths and markedly degraded S/N. Once an operating frequency has been chosen, the search f o r resonance i s f a c i l i t a t e d by using the widest available sweep width ( t y p i c a l l y around 100 KHz, depending on d i g i t i z e r speed) and quadrature detection. However, 90° pulse lengths t y p i c a l f o r high resolution spectrometers are not capable of providing uniform excitation across such spectral widths (2) and, i n f a c t , regions of n u l l excitation occur at o f f s e t frequencies related to 1/(pulse length). The chances of observing a signal which occurs i n the sweep window can therefore be improved by using smaller pulse widths (e.g.,10 /is) and r e l a t i v e l y rapid recycle times. Once the chemical s h i f t region of interest has been defined, i t may be desirable to operate the spectrometer frequency close to resonance and, f o r c e r t a i n more sophisticated experiments, to determine accurate 90° pulse lengths. The l a t t e r task i s greatly aided i f a readily-detected set-up sample can be found (preferably with d i e l e c t r i c properties s i m i l a r to that of the sample of i n t e r e s t ) . In general, 90° pulse lengths less than 50 /is are highly desirable. Superconducting magnets conventionally have vertically-mounted (Helmholtz c o i l ) probes which permit ready sample access, but which also have some undesirable features. The alternative i s a sideways-mounted solenoidal probe, which t y p i c a l l y gives approximately a factor of 2 improvement i n S/N and markedly better H^ pulse c h a r a c t e r i s t i c s . For high-loss samples (e.g., concentrated s a l t solutions i n water), H^ homogeneity may be so poor i n a conventional probe that i t i s impossible to achieve a 180° pulse, and i n such cases a solenoidal probe would be required f o r T^ or 2-dimensional experiments. Thus, f o r special cases involving low s o l u b i l i t y or s e n s i t i v i t y , or unacceptably long pulse lengths, a sideways-mounted probe may o f f e r s u f f i c i e n t advantage to merit the associated expense, the inconvenience of sample loading, and the d i f f i c u l t y of shimming an unconventional probe o r i e n t a t i o n . One of the p r i n c i p a l goals of probe and amplifier design i s to produce short pulse lengths both f o r good power d i s t r i b u t i o n c h a r a c t e r i s t i c s , and also f o r improved performance of the more demanding pulse sequences which w i l l f i n d increasing use i n



206

EXPERIMENTAL ORGANOMETALLIC CHEMISTRY

multinuclear NMR (3). Since these pulse sequences can involve 90° and 180° pulses on both the observe and the decoupler channels, a reasonably sophisticated pulse programmer i s required f o r t h e i r implementation. Elaborate pulse sequences are also required i n e f f o r t s to minimize the e f f e c t s of acoustic ringing, which can be a s u f f i c i e n t l y serious problem below about 20 MHz that i t can overwhelm the signal of i n t e r e s t . The sources of acoustic ringing have been described (2,4), and, of the solutions proposed, a p a r t i c u l a r l y e f f e c t i v e one given i s given by E l l i s ( E l l i s , P. D., discussion at the Nato Meeting Advanced Study I n s t i t u t e on Multinuclear NMR Spectroscopy, S t i r l i n g , 1981; cf (5)); a b r i e f discussion and further apparent improvements are given by Eckert and Yesinowski (6). A number of generally useful hints and techniques are discussed i n (7). Nuclear Properties A f f e c t i n g Detectability• Among the d i f f i c u l t i e s that can be experienced i n t r y i n g to observe a given metal i s the extremely large range of chemical s h i f t s which may occur; a notable example i s Pt, whose chemical s h i f t range spans about 13,000 ppm. While the l i t e r a t u r e may provide guidelines i n locating a metal chemical s h i f t f o r a p a r t i c u l a r oxidation state (8-11), i t may nevertheless be required to search a number of spectral windows because of pulse power and sweep width l i m i t a t i o n s . I t should also be noted that r e l i a b l e chemical s h i f t comparisons require temperature regulation since, i n some cases, s h i f t s can vary by as much as 1 ppm per degree. While spin-1/2, or dipolar, metals are often associated with narrow resonance l i n e s , t h i s useful feature i s sometimes obtained only at the expense of long T^ relaxation times. Some improvement i n d e t e c t a b i l i t y may r e s u l t from broad-band H decoupling, although the observed NOE depends on the extent to which dipolar interactions contribute to the relaxation mechanism, and i s u n l i k e l y to approach i t s maximum possible value ?n/2yy- A small NOE can actually lead to a dereased or n u l l signal f o r negative-y metals such as Sn, Cd, Rh, etc. In cases which involve resolved coupling to protons, an a t t r a c t i v e alternative involves p o l a r i z a t i o n transfer experiments such as DEPT (3,12). While t h i s approach requires c a l i b r a t i o n of decoupler pulse lengths f o r a given output l e v e l (2,13), the advantages include enhancements greater than those provided by NOE and r e p e t i t i o n rates related to the proton T^ (usually shorter than the metal T^) . The enhancement i s proportional to d is independent of the dipolar contribution to the relaxation mechanisms of the metal. In practice, p o l a r i z a t i o n techniques such as DEPT are not generally suitable f o r quadrupolar n u c l e i . Quadrupolar relaxation tends to be very e f f i c i e n t i n the nonsymmetric environments l i k e l y to be encountered i n cases of synthetic i n t e r e s t , and t h i s relaxation e f f e c t i v e l y decouples any nearby protons. The t y p i c a l r e s u l t of t h i s relaxation i s a broad unresolved l i n e whose S/N r a t i o can be b u i l t up only by rapid pulse r e p e t i t i o n rates. The factors involved i n quadrupolar and other types of relaxation are informatively discussed by Kidd (14). a n

Examples Involving Dipolar Metals. The dipolar metals are grouped i n Table I along with a rough indication of t h e i r ease of detection.


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Typical work with the " d i f f i c u l t " group of nuclei would require large sample volumes and high s o l u b i l i t y . A notable absentee from the Table i s Yb(II); although i t i s moderately receptive ( i n p r i n c i p l e ) , and resonates at a convenient frequency, I am unaware of any successful observation of t h i s nucleus i n solution (despite a number of known attempts). There i s an enormous l i t e r a t u r e concerning the dipolar metals, and e a r l i e r reviews (8,10,15) constitute an important source of reference information. Recent contributions include trends i n Pt (16) and Sn (17) chemical s h i f t s , and i n Sn-X coupling constants

Table I. Easy

1 1 9

Moderately

89

C l a s s i f i c a t i o n of Dipolar Metals Sn,

Y,

7 7

1 9 5

Pt,

Se,

1 2 5

2 0 5

3,

T1

Te,

1 1 3

Cd,

1 9 9

H , g

2 0 7

Pb

difficult

Hard

183

W,

1 0 3

Rh,

1 0 9

Ag,

5 7

Fe,

1 8 7

0s

a

The ordering i s thought to r e f l e c t a combination of ease of detection and degree of u t i l i t y . ^ F e i s extremely d i f f i c u l t to observe without enrichment, and Os i s even less receptive than i t s quadrupolar spin isotope.

(17-21). Studies of Pt-Sn complexes frequently combine NMR observation of both nuclei (22,23), and use of both these probes has been of evident value i n homogeneous c a t a l y s i s (24,25). Of perhaps greater i n t e r e s t i s the f a c t that t i n NMR has been used d i r e c t l y to study ligand dynamics. Two-dimensional exchange spectroscopy (NOESY) was used to elucidate i n a q u a l i t a t i v e manner the mechanism of isomerization of the d i t i n complex CH [(CgHg)Sn(SCHgCHg) NCH ] (26). À quantitative approach to t h i s sort of isomerization i s therefore possible, either by means of phase-sensitive 2D NOESY (27) or by a complete set of ID magnetization transfer experiments (28JT Tin magnetization transfer has i n f a c t been elegantly applied to a quantitative evaluation of cis-trans isomerization and Me S ligand exchange i n SnCl *2Me S (29). Variable temperature NMR lineshapes of metal nuclei can also provide information r e l a t i n g to the mechanism of dynamic processes. Fluxional behavior i n complexes I and I I can e a s i l y be followed 2

2

2

4

2

PPh, I (PPh-)-Pt-H I CpM(C0) 3

1 31 by H or Ρ NMR,

I M = Mo II M = W 3

but the observed lineshapes do not immediately


3

2



208

suggest what sort of process should be used to model the exchange. Such ambiguities are removed by the s i m p l i c i t y associated with the variable temperature Pt NMR spectra shown i n Figure 1. The f a c t that the outside l i n e s of the spectrum remain sharp throughout the VT range indicates that the process involves a mutual exchange of the phosphines: the cis-trans isomerization must proceed i n such a way that both the hydride and phosphine ligands remain attached to the metal. While the Pt lineshapes also provide a reasonable estimate of the exchange rates, more accurate rate information may be derived from the corresponding Ρ spectra i n which the i n t r i n s i c linewidths are narrower. A d i f f e r e n t sort of dynamic e f f e c t can be encountered when observing a spin-1/2 metal which i s scalar coupled to a quadrupolar nucleus. In general, the lineshape of such a spin-1/2 nucleus I depends (i) on the T« of the quadrupolar nucleus S and ( i i ) on the magnitude of the scalar coupling J j g , such that narrow l i n e s observed at low temperature become increasingly broad as the temperature i s increased ( i n contrast to the normally observed behavior). In the high temperature region, the long quadrupolar T^ corresponds to a slow relaxation rate, and i f t h i s rate i s slow r e l a t i v e to J , the spin-1/2 nucleus I sees 2S+1 equally populated s i t e s ; under these conditions, the spectrum of I consists of 2S+1 equally intense l i n e s . The vanadium-coupled tungsten spectra i l l u s t r a t e d i n Figure 2 represent an intermediate case i n which the quadrupolar vanadium nucleus i s only p a r t i a l l y "decoupled" from the tungsten at the highest accessible temperature (30). In addition to the v a r i a t i o n of the measured quadrupolar T^, the simulations i n Figure 2 include an additional small exchange rate contribution required to provide the optimal f i t to the data. Chemical exchange can a f f e c t these lineshapes i n a c h a r a c t e r i s t i c way, but i n practice the exchange e f f e c t s tend to be rather subtle i n comparison with the quadrupolar e f f e c t s . In systems containing a number of metal atoms, questions concerning connectivity can be answered i n p r i n c i p l e i f J coupling between the metals can be observed. The s t r u c t u r a l characterization of heteropolyanions, f o r example, has been enormously aided by modern NMR techniques which r e l y f o r t h e i r success on the presence of scalar coupling. For example, the s i m i l a r i t y of W-W coupling constants i n L i g [PVoW-. QO^Q] leads to ambiguity i n the normal 1-D yi spectrum, whereas the 2D-INADEQUATE r e s u l t s shown i n Figure 3 permit a unique assignment of the possible p o s i t i o n a l isomers (30). 89 lbd

Additional references of interest may be noted f o r °*Y (31), 7 7

S e (32,33),

2 0 7

1 2 5

Te

(34),

1 1 3

Cd

(35-38),

1 0 3

R h (39),

1 9 9

H g (40) and

P b (41).

Examples Involving Quadrupolar Metals. The quadrupolar metals are grouped i n Table I I , again with an i n d i c a t i o n of t h e i r ease of detection. The c l a s s i f i c a t i o n i s considerably more arbitrary than f o r the dipolar metals since the ease of detection w i l l depend markedly on the degree of symmetry around the metal and the magnitude of the quadrupole coupling constant. Where a choice e x i s t s , the most l i k e l y preferred isotope i s indicated.


8.

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Characterization of Inorganic and Organometallic Complexes


60 °C

ι 30.000 f

t^»J#«jl*#*^ l 30 °C

,|

1

M

5300··

1

20 °C

1400 e"

0°C

260 β"

-60 °C

Of

H

1

1

1

Figure 1. Observed and calculated Pt NMR spectra f o r I at indicated temperatures (A. H. Janowicz and D. C. Roe, unpublished data).


209


210


Figure 2 . 183

Observed and calculated lineshape of the - 8 1 . 1

W NMR resonance of L i g [ S Î V W ^ O ^ Q ] as a function of

ppm

temperature. and t

Simulations use the values ^J^_g_y = 1 1 . 5 Hz

= 4 0 s" . 1


8.

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Characterization of Inorganic and Organometallic Complexes

\

ο -ο

1

τ ο -ο


τ

7

H Ο -ο

H

ο -ο

Ko —I 400

Λ.

I » ι »

-70

1

ι

1

ι

-80

I

ι

ι

0

200

J ι ι

-90

I

I

ι

ι

1

I

J

I

-200

jU.Jt

J ι

-100

1

ι ι I ι ι ι ι—J

-110

183

t

-400

I

-120

I

I

I

I

Hz

J ι

-130

ΡΡΜ

51

Figure 3. 2D-INADEQUATE W NMR spectrum ( V-decoupled) of a - l , 2 - L i [ P V W 0 ] , 0.4M i n D 0 at 30°C. 5

2

1 0

4 0

2


211


212


As i s the case f o r the dipolar metals, the number of applications involving quadrupolar metals i s f a r too great to cover i n a short review such as t h i s . Dechter's coverage (10) of the l i t e r a t u r e from 1978 through September, 1983 i s p a r t i c u l a r l y timely i n updating e a r l i e r reviews, and selected references which are thought to be currently representative w i l l simply be mentioned here. Trends i n the Co chemical s h i f t s of a wide variety of organocobalt complexes have been usefully interpreted i n terms of s t r u c t u r e - a c t i v i t y relationships i n homogeneous c a t a l y s i s (42) and i n terms of the influence of various pi-ligands on structure and

Table I I .

Relatively

C l a s s i f i c a t i o n of Quadrupolar Metals

7

Li,

23

27

Moderately

9

Be,

difficult

99

Ru,

Very d i f f i c u l t

25

Mg, K,

73

Ge,

4 5

Na, A1,

Sc,

51

59

V,

Co,

93

3,

Nb

easy

Useless^

4 7

>

4 9

1 3 3

1 0 5

1 8 9

71

C r , Ga,

87

Rb,

9 1

95

Z r , Mo,

Cs

39

7 5

5 3

Ti,

4 3

As,

Pd,

1 1 5

0s,

1 9 3

55

C a , Mn, 8 7

Sr,

In,

1 7 5

Ir,

1 9 7

9 9

6 1

Tc,

Lu,

1 7 7

Au,

2 0 9

63

N i , Cu, 1 2 1

Sb,

Hf, Bi,

1 8 1

1 3 7

Zn,

Ba,

Ta,

2 3 5

6 7

1 8 7

1 3 9

La

Re,

U

a

The ordering can only be approximate. The nuclear properties of t h i s group make i t extremely unlikely that NMR spectra w i l l provide useful information f o r organometallic complexes.

chemical s t a b i l i t y (43). A l NMR has naturally figured i n the characterization of organoaluminum compounds and, f o r example, has permitted the d i s t i n c t i o n between octahedral and tetrahedral s i t e s i n a series of alkoxide and s i l o x i d e complexes (44). An apparently i n t e r e s t i n g dynamic equilibrium between four- and five-coordination i n diorgano[(2-pyridyl)methoxy]aluminum complexes (45) has been discounted i n that the observations leading to t h i s conclusion stem i n part from a spurious signal found to be present when the sample tube contained only solvent (46). This example focuses attention on a general caveat that must be heeded when observing such broad resonances from quadrupolar metal n u c l e i : the absence (or presence) of background signals a r i s i n g from the probe or sample tube alone must be confirmed p r i o r to signal i d e n t i f i c a t i o n and interpretation. In favorable cases, aspects normally associated with spin-1/2 systems can feature i n the study of quadrupolar metals. For example, variable temperature Sc NMR spectra provided evidence



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for a monomer-dimer equilibrium f o r Cp^Sc i n toluene (47). In addition, homonuclear J-correlation (CuSY) experiments have bgen successfully applied to determine vanadium connectivités v i a Jy_g_y i n heteropolyanions, even when the breadth of the resonances obscures the spin-spin coupling (30). In the case of Nag H [PV^Chrt]> the off-diagonal peaks c o r r e l a t i n g the resonances at -57/ and -593 ppm (Figure 4) indicate that these l a t t e r peaks derive from the main Keggin framework, while the peak at -526 ppm shows no such c o r r e l a t i o n and i s assigned to vanadium i n the capping group. Such detection of connectivity patterns through J coupling i n otherwise broad, featureless spectra may have implications f o r s t r u c t u r a l studies of other quadrupolar nuclei where i t i s possible to trade a problem of inherent resolution f o r a problem of sensitivity. Additional references are noted f o r Ru (48), Mo (49-51), eo

87

55

R b (52), M n (53), Ca,

K, and

9 9

T c (54),

Zn (57).

1 3 9

La

(55),

6 3

25

C u (56) and Mg,

In the l a t t e r case, use i s made of

i s o t o p i c a l l y enriched metals to overcome the low natural abundance of the NMR preferred isotopes f o r Mg, Ca and Zn, and i n t h i s way to study the i n t e r a c t i o n of these metals with the regulatory protein, calmodulin. I t i s hoped that the above examples provide an i n d i c a t i o n of the role metal NMR can play i n characterizing certain organometallic and inorganic complexes. In favorable cases, i t may be seen that metal NMR provides the most d i r e c t insight to the s t r u c t u r a l d e t a i l s of multinuclear systems, and to the dynamics and mechanism of exchange processes occurring around the metal center. High Pressure NMR The extent to which pressure has been neglected as an experimental variable i s due, i n large part, to the lack of ready a v a i l a b i l i t y of appropriate pressure c e l l s . This portion of the review describes a means by which high resolution NMR spectra can be obtained f o r samples subjected to pressures up to 2000 p s i with what i s considered to be a reasonable margin of safety. The key t o t h i s accomplishment i s to take advantage of the t e n s i l e strength and other properties of sapphire tubing. While the emphasis i s on these new sapphire NMR tubes, the relationship to other high pressure NMR techniques w i l l be described f i r s t . Conventional glass NMR tubes can often be used up t o 10 atmospheres (ca. 150 p s i ) , but they become increasingly unreliable at higher pressures and low temperatures. Repeated use which leads to the development of micro scratches on the surface of the glass can also lead to f a i l u r e under conditions which had previously been successfully achieved. Nevertheless, t h i s approach may be very convenient f o r work with gases such as H , CO and 0 , and heavy-walled glass tubes f i t t e d with a symmetrical Teflon valve are commercially available (e.g., from Wilmad or other sources). The valve can be mated to a standard Swagelok f i t t i n g on a 1/16" l i n e from a cylinder, and these devices provide perhaps the most convenient entry to a useful, a l b e i t limited, pressure region. 2

2



214


ι ι ι I

ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—Γ

4000

Figure 4. 60°C.

0 i

2000

0

-2000

-4000

0 1

2-D V - V COSY spectrum of Na

Q

Hz

EL [?Y, V ] at A

A0



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Very high pressures ( e . g . , i n excess of 2000 atm) can be achieved by means of high pressure probes, but these are r e l a t i v e l y demanding to construct (58) and tend to require a f u l l - t i m e commitment. The rewards that can be achieved by such efforts are exemplified by the elegant studies of the dynamic structure of l i q u i d s (59) or the conformational isomerization of cyclohexane (60), and by the estimation of volumes of a c t i v a t i o n for a number of chemical processes (58,61). Intermediate pressures of 10 - 100 atm are of i n t e r e s t i n that they are associated with the chemistry of reactant gases i n s o l u t i o n , where such pressures permit a corresponding increase i n the accessible concentrations of the gases compared to those a v a i l a b l e i n the glass tubes described above. Areas where t h i s sort of c a p a b i l i t y might be useful include k i n e t i c studies which require the systematic v a r i a t i o n of gas concentration, and studies i n v o l v i n g reactive intermediates which decompose by d i s s o c i a t i v e loss of ligands such as CO. Another sort of a p p l i c a t i o n might involve s u p e r c r i t i c a l f l u i d s which can be maintained at modest pressures. Construction and Operation of the Sapphire NMR Tube. Sapphire was chosen as the material for tube construction because i t s excellent t e n s i l e strength c h a r a c t e r i s t i c s can be uniformly retained i n a tube grown i n t a c t as a single c r y s t a l . Tubes 5 mm i n o . d . , with 0.8 mm w a l l and sealed at one end, were chosen for our prototype and were purchased from Saphikon, Inc. (51 Powers Street, M i l f o r d , NH 03055); at the time of our o r i g i n a l purchase, the cost of the tubes was approximately $300 each. The open end of the tube i s sealed by means of a nonmagnetic titanium a l l o y valve (Figure 5 ) . The numbered d e t a i l s are 1, valve body; 2, valve stem drive handle; 3, stem drive and packing gland; 4, nonrotating stem; 5, packing assembly; 6, gas i n l e t port for 1/16 i n . tubing; 7, assembly screw ( t o t a l of 4 ) ; 8, Viton O-ring s e a l ; 9, tube mounting flange; 10, epoxy sealant; 11, spinner turbine; and 12, sapphire tube. The base plate or flange for the valve i s epoxied to the tube (62) and the upper valve assembly i s attached by means of four threaded b o l t s . The o r i g i n a l valve design has been reduced to 25 mm o . d . i n order to accommodate the dimensions of conventional bore superconducting magnets, and the weight of the valve and tube assembly i s approximately 74 g. Samples can be syringed i n t o the tube through the opening i n the flange, and the valve bolted i n place; for a i r - s e n s i t i v e compounds, these operations are conveniently c a r r i e d out inside a drybox. The tube i s mounted on the spinner and housed inside a safety s h i e l d c o n s i s t i n g of a polymethylmethacrylate c y l i n d e r , baseplate and cap; the c y l i n d e r i s long enough to accommodate the tube, spinner and lower portion of the v a l v e . With the spinner supported by a p a i r of p l a s t i c b o l t s across the c y l i n d e r , the upper valve assembly i s accessible for pressurization from the appropriate gas c y l i n d e r . The gas i s introduced through 1/16" s t a i n l e s s s t e e l tubing f i t t e d with a standard Valco 1/16 f e r r u l e and male nut which screws i n t o the side of the v a l v e . Rotation of the valve stem handle effects the opening and c l o s i n g of the v a l v e . Once the sample i s sealed under pressure, i t may be detached from the l i n e and gently rocked on i t s side i n order to mix the gas with the solution. B



216


Figure 5. Schematic drawing of T i - a l l o y valve and sapphire tube assembly.


8. ROE



The pressurized sample can be introduced into the probe without d i r e c t exposure to the tube. The baseplate and cap of the safetys h i e l d are removed, and the remaining assembly i s made to rest on top of the magnet above the probe stack. The tube i s restrained by means of a removable loop of s t r i n g or lasso around the stem drive (part 3 i n Figure 5) while the p l a s t i c bolts supporting the spinner are removed. The tube can then be lowered to the top of the probe stack where i t can be supported by the eject a i r system and lowered into the probe. Performance of the Sapphire NMR Tube. The tube and valve are s u f f i c i e n t l y l i g h t weight and symmetrical that they spin and achieve a t y p i c a l resolution of about 1 Hz. In hydrostatic pressure t e s t i n g , the burst pressure of one tube was found to be approximately 14,500 p s i , and i t i s therefore believed that operations up to 2000 p s i may be c a r r i e d out with a reasonable margin of safety. Obviously however, a l l due caution must be exercised and operator exposure to a pressurized tube must be avoided. I t i s as yet unclear whether extended use w i l l eventually a f f e c t the performance of these tubes. In our hands, the tubes have been operated without incident, over a period of two years, from -140 to +150°C at nominal pressures up to 1200 p s i . Examples of High Pressure Studies. The sapphire tubes have been used to study CO exchange rates with Co (C0)g at pressures up to 1100 p s i (63). C NMR spectra obtained between 40 and 80°C reveal that free CO i n solution i s exchanging slowly with the CO*s coordinated to cobalt. Since lineshape information at these and higher temperatures i s complicated by broadening caused by the quadrupolar cobalt nucleus, magnetization transfer techniques were used to measure the exchange rates as a function of temperature and pressure. Carbon monoxide 99% C-enriched was used i n order to overcome the low S/N associated with the p o t e n t i a l l y long C T^'s of the coordinated CO. Consistent with a small value f o r the equilibrium d i s s o c i a t i o n constant (Equation 1), the observed exchange rates were found to be independent of pressure and 2

16

Co (C0) 2

g

300ms, we r e g a r d as

0097-6156/87/0357-0223S06.00/0 © 1987 American Chemical Society


224


unambiguously c l a s s i c a l , h a v i n g t e r m i n a l M-H bonds o n l y . The second, w i t h Ί± < 125ms a r e unambiguously n o n c l a s s i c a l . We p r e f e r the f o r m u l a t i o n s shown i n T a b l e I , i n which t h e r e a r e one o r two d i h y d r o g e n l i g a n d s per m e t a l because f a c i l e d i s p l a c e m e n t o n l y o f the H2 l i g a n d s o c c u r s w i t h N2 o r phosphine t o g i v e c l a s s i c a l s p e c i e s . The t h i r d group has i n t e r m e d i a t e v a l u e s o f The Ύ\ v a l u e a l s o depends on the r o t a t i o n o f the m o l e c u l e , as measured by T , the r o t a t i o n a l c o r r e l a t i o n time. This w i l l vary a c c o r d i n g t o the moment o f i n e r t i a o f the m o l e c u l e and the v i s c o s i t y o f the s o l v e n t . T h i s v a r i a b i l i t y , we t h o u g h t , might account f o r the i n t e r m e d i a t e v a l u e s o b s e r v e d i n T a b l e I. On c h a n g i n g the temper a t u r e the Ύ\ s h o u l d go t h r o u g h a minimum when T = 0.63/ω, where ω i s the NMR f r e q u e n c y . (4,6,9) The T\ v a l u e a t the minimum Τχ(πιίη) i s independent o f the f a c t o r s mentioned above and so i s a u s e f u l c r i t e r i o n f o r the s t r u c t u r e d e t e r m i n a t i o n . These v a l u e s a r e shown i n T a b l e I I f o r a range o f examples which have been s t u d i e d t o d a t e . The a m b i g u i t i e s apparent from the d a t a o f T a b l e I a r e removed and we a r e now a b l e t o a s s i g n the s t r u c t u r e s o f a l l the s p e c i e s s t u d i e d .


c

c

The T i ( n , m i n ) v a l u e s so o b t a i n e d r e f l e c t r e l a x a t i o n due t o d i p o l e c o u p l i n g s (a) w i t h i n the H2 l i g a n d and (b) between t h i s l i g a n d and o t h e r d i p o l a r n u c l e i i n the complex. We have t r i e d t o a l l o w f o r (b) i n eq. 2. Ti(n,min, c o r r d . )

- 1

= T^in^in)"

1

- Ti(c,min)

- 1

(2)

Once a g a i n , we assume T;[(c,min) i s 200ms. F o r the r e s u l t i n g T ^ ( n , min, c o r r d . ) we can o b t a i n an H-H d i s t a n c e from eq. 3. (4,6,9) This d i s t a n c e i s a l s o g i v e n i n T a b l e II. The numbers a r e o n l y s l i g h t l y

{T^DD)}" ^ O ^ V r ^ l l 1

+

cA

2 c

)

4x^(1 +4cA )} (3) 2

+

c

( γ = gyromagnetic ratio, h = Planck's constant, r = internuclear distance, x = rotational correlation time, ω = Larmor frequency) c

a f f e c t e d by our c h o i c e o f T ^ ( c , m i n ) , e.g., a change o f 10ms t y p i c a l l y changes r by 0.001A. The d i f f e r e n c e i n r e l a x a t i o n r a t e s between the c l a s s i c a l and n o n c l a s s i c a l group i s so l a r g e t h a t e r r o r s i n measurement o f T^ ( c a . 10%) a r e r e l a t i v e l y u n i m p o r t a n t . Paramagnetic samples would be u n s u i t a b l e , however, because a l l the r e l a x a t i o n times would be short. T h i s means t h a t , so as n o t t o be l e d a s t r a y by a paramagnetic sample, i t i s n e c e s s a r y t o v e r i f y t h a t the T\ s o f the l i g a n d p r o t o n s are normal (3) even i n a s u p p o s e d l y d i a m a g n e t i c c a s e . The p r e s e n c e o f a q u a d r u p o l a r n u c l e u s nearby ( e . g . , the m e t a l ) may a f f e c t the h y d r i d e r e s o n a n c e s , but i t s h o u l d not s i g n i f i c a n t l y a f f e c t the T\ as measured by i n v e r s i o n - r e c o v e r y . (The p r e s e n c e o f a q u a d r u p o l a r m e t a l n u c l e u s u s u a l l y l e a d s t o r a p i d r e l a x a t i o n o f the m e t a l s p i n s t a t e s which w i l l g i v e r i s e t o sharp M-_H r e s o n a n c e s . In u n u s u a l s i t u a t i o n s t h e r e may be slow r e l a x a t i o n and broad M-_H resonances may be seen, but the Τγ o f the M-H p r o t o n w i l l not be a f f e c t e d . ( 9 ) ) The p r e s e n c e o f o t h e r d i p o l a r n u c l e i ( e . g . , 31p) s h o u l d not s i g n i f i c a n t l y a f f e c t T i because o f the much g r e a t e r d i s t a n c e i n v o l v e d and i n some c a s e s the lower v a l u e s o f the gyromagnetic r a t i o . The r e s i d u a l 1

y


8.1

Table I.

Complex

5

3

2

c

+


2

2

2

2

2

2

3

4

3

2

5

3

4

2

2

+

2

2

6

2

2

3

3

3

3

2

2

s

3

s

18.5

2

2

3

f

4

2

2

s

25 25 33

f

+

5

2

12 20

290,618 540 485 44 540 148 166 3000

6

3

d

110

Cp*ReH ReH L MoH (PMePh )4 MoH (H )(PMePh )4 WH4(PMePh )4 [WH (PMePh )4] WH (PMe Ph) H Fe(CO) 5

37 30

e

2

II

CD C1 ,-•80°C CB Cl2>-•80°C CD2CI2,-•80°C t o l u e n e ,-70°C t o l u e n e ,-70°C t o l u e n e ,-70°C t o l u e n e ,-70°C CD C1 ,-•70°C t o l u e n e ,-70°C t o l u e n e ,,70°C t o l u e n e ,-70°C t o l u e n e ,-70°C CD C1 ,-•70°C t o l u e n e ,-70°C CD2CI2,--70°C t o l u e n e ,-70°C t o l u e n e ,-70°C 2

d

e

2

2

II

b

Tx(n)(ms) Conditions

c

+

2

3

820 48 ,73 30 ,390 24 38 820 78 79

2

2

5

The Τ χ v a l u e s o f some h y d r i d e s

T^Cms)

IrH (PCy ) [IrH (H )2(PCy3)2] [IrH(H )(bq)L ] FeH (H )(PEtPh )3 RuH (H )(PPh3) OsH (P{p-tol} )3 ReH (H )L ReH (H )dpe

225

Detection of Dihydrogen Complexes

CRABTREE ET AL.

3

3

Refs. 5,6 5,6 6,7 8 8 8 9a 9a 9a 9a 9a 9a 9a 9a 9a 9a 9a

L = PPI13, bq = 7 , 8 - b e n z o q u i n o l i n a t e , Cy = c y c l o h e x y l . by i n v e r s i o n - r e c o v e r y , ± 10%. The v a l u e s a r e somewhat s o l v e n t dependent and because o f t h e x term, temperature dependent, b c a l c u l a t e d as shown i n t h e t e x t . f o r the resonance a s s i g n e d t o t h e c l a s s i c a l hydrides. assigned t o the n o n c l a s s i c a l h y d r i d e s . u n p u b l i s h e d n e u t r o n d i f f r a c t i o n d a t a a r e s a i d (10) t o show d i s o r d e r e d but c l a s s i c a l s t r u c t u r e f o r t h e PPrPti2 complex. Perhaps d i f f e r e n c e i s r e a l and due t o t h e change i n p h o s p h i n e , o r t h e r e may be a change o f s t r u c t u r e on g o i n g t o t h e s o l i d s t a t e . ^ These r e l a x a t i o n times a r e i n t e r m e d i a t e between those which i n d i c a t e an unambiguously c l a s s i c a l s t r u c t u r e (>300ms), and one which would suggest a n o n c l a s s i c a l s t r u c t u r e (

Experimental Organometallic Chemistry - American Chemical Society

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