Correlations between Transition-Metal NMR ... - ACS Publications

Chapter 17

Correlations between Transition-Metal NMR Chemical Shifts and Reactivities M . Bühl

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0732.ch017

Organisch-Chemisches Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

Modern methods based on density-functional theory (DFT) can describe relative activation barriers of organometallic reactions, i.e. relative reactivities, as well as the transition-metal N M R chemical shifts of the reactant complexes involved. It is thus possible to reproduce or rationalize observed correlations between these properties or to predict new ones. NMR/reactivity correlations that could be reproduced theoretically ("intrinsic correlations") are summarized. Newly predicted NMR/ reactivity correlations are discussed for the ethylene polymerization with V(=O···X)R or V(=Y)R catalysts. When X or Y are varied (X = AlH , Li+, SbF , H+; Y = N H , O , S, Se), both 3

3

3

5

51

the olefin insertion barrier and the V chemical shift are computed to change in a parallel way, albeit with a different sensitivity for both types of compounds. V N M R spectroscopy could thus be a valuable tool for a screening of potential polymerization catalysts. 51

Catalytic speedup of many reactions is indispensable for their practical application in synthesis. Homogeneous catalysis with transition-metal complexes continues to receive much attention in that respect (7), and many research efforts are devoted to the "tailoring" of ligands for special catalytic purposes. Rational design of such catalysts, however, usually requires detailed knowledge of the reaction mechanism(s) involved, which is often very difficult to elucidate experimentally. The modern tools of computational chemistry, on the other hand, may provide such important information concerning the structures and energetics that characterize the catalytic cycle. This is true in particular because the currently available variants of density-functional theory (DFT) perform quite well for the description of these properties (2) and allow the treatment of ever larger, increasingly realistic, systems. One of the reasons for the difficult experimental characterization of catalytic mechanisms is that the intermediates involved are usually so short-lived that the single

240

© 1999 American Chemical Society

Facelli and de Dios; Modeling NMR Chemical Shifts ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

241

most powerful analytical tool, N M R spectroscopy, cannot be applied. Fourteen years ago it was discovered that this technique may be useful for a more direct estimation (or prediction) of substituent effects on the rate of a catalytical reaction: in the pyridine synthesis catalyzed by [Co(C H X)(COD)] (COD = 1,5-cyclooctadien), the 5

4

5 9


catalytic activity and the C o chemical shifts were found to be correlated for various substituents X (J). Since then, several systems were identified empirically for which transition-metal chemical shifts can be correlated with rate constants of stochiometric reactions (for instance, equations 1 and 2) (4,5), and another such relationship was reported for the catalytic activity in C O 2 hydrogénation (equation 3) (6). Also, stability constants of transition-metal complexes sometimes parallel their ô(metal) values (7). Rh(C H X)(CO) (1) + P R 5

4

2

Fe(C H )(CO) R (2)+ P R 5

5

2

3

3

Rh(C H X)(PR )(CO) 5

4

3

Fe(C H )(PR )(CO)(COR) 5

5

3

(1) (2)

£2

R, C0 + H 2

2

HCOOH

(3)

There is certainly no general, universal connection between reactivity and chemical shift, but apparently substituents can affect either one in a similar, parallel way. Once such a correlation is established it can be of great practical use because reactivities or catalytic activities of newly prepared compounds can be estimated from their N M R spectra alone. With the advent of appropriate DFT-based methods, N M R properties of transition-metal compounds have now become amenable to theoretical computations (8). Suitable density functionals have been identified (9) which permit calculations of transition-metal chemical shifts with reasonable accuracy, typically within a few percent of the respective shift ranges. Thus, it is now possible to investigate possible NMR/reactivity correlations for transition-metal complexes from first principles; several such studies have already been undertaken (10,11,12). The first part of the present paper summarizes attempts to reproduce or rationalize NMR/reactivity correlations known empirically. The second part is devoted to the search for prospective candidates for new such correlations, with the emphasis on catalytic reactions. Reproducing NMR/Reactivity Correlations The first successful reproduction of an empirical NMR/reactivity correlation was achieved for the substitution reaction at a rhodium center, equation 1. For several substituents X at the cyclopentadienyl ring, the logarithm of the observed rate constant


242 1 0 3

had been found to correlate linearly with the corresponding R h chemical shift of the reactant complex (4a) (see Figure l a for a schematic representation). For a model reaction (involving PH3 instead of PPh ), the suggested associative, "ring-slippage" mechanism (equation 4) was confirmed at the BP86/ECP1 level (see Computational Details at the end) and formation of the ^-intermediate 4 was indicated to be the ratedetermining step (11). 3

X

+PR

x 3

\

Rh y


oc

x

o

co

c

(4)

/ y \

p

R

(4)

-CO

3

c ο

When the substituents X at the Cp ring are varied (from X = Η to CI, N M e 2 , and NO2), the barrier for formation of 4 is successively reduced. Both the trend and the magnitude of this variation are consistent with the experimental k ^ values. Except 0

s

1 0 3

for a minor detail, the observed, concomitant deshielding of the R h nucleus is qualitatively reproduced at the SOS-DFPT-IGLO/II level. Thus, the overall NMR/reactivity relation in this system is well reproduced with DFT-based methods (Figure 1). A E calc [kcal/mol] a

log k

obs

-1300

-1200

-1100

-1050

103

-950

-850

103

8( Rh) expt

6( Rh) calc

Figure 1. a) Empirical correlation between rate constants (log k ^ ) and 103

ô ( R h ) in Rh(C H X)(CO) 5

4

2

complexes (Adapted from ref. 4a). b) 103

Correlation between A E and ô ( R h ) values computed for the same a

compounds (From the data given in réf. 11).


243 The first theoretical rationalization of an NMR/reactivity correlation was offered for an insertion reaction involving iron complexes 2, equation 2. With increasing bulkiness of the alkyl ligands at iron, PPh -induced insertion of CO into the Fe3

57

C(alkyl) bond proceeds more readily. At the same time, the F e nucleus becomes more deshielded (4b). The relative ease of insertion was suspected to be related to variations in the Fe-C(alkyl) bond strength. Indeed, in the series 2a (R = Me), 2b (R = Bu), and 2e (R = f-Pr), a significant elongation of the Fe-C bond and a notable decrease of the corresponding bond dissociation eneergy (BDE) was computed at the BP86/AE1 level (10), entirely consistent with the observed increase of reactivity in this sequence. Despite serious shortcomings of the SOS-DFPT-IGLO method for a larger range of F e chemical shifts, the trend in the 5( Fe) values between 2a, 2b, and 2c was well reproduced (10). The same qualitative trend is computed at the GIAO-B3LYP level (Table I), even if the individual chemical shifts appear to be overestimated at that level. The latter functional is preferable because of its good performance over the whole F e chemical-shift range (9,13). It is probably safe to assume that the insertion barriers into the Fe-C(alkyl) bonds will decrease with the BDEs (even though the range of k fc values, which cover two orders of magnitude, suggest a narrower span of actual insertion barriers than obtained for the BDEs, which vary by 11 kcal/mol). Nevertheless, the data in Table I constitute a plausible rationalization of the observed NMR/reactivity correlation.


57

57

57

0

s

Table I. Observed and Computed Kinetic Parameters and 8( Fe) Values for F e ( C H ) ( C O ) R Complexes 57

5

a

BDE »

(UBP86)

Me Bu i-Pr

51.9 46.9 40.8

2

57

b

R

5

k bs

6( Fe)d

C

0

b

SOS-DFPT B3LYP 2

2.1-101.6101.6

1

a

286 345 401

e

Exp.

684 716 796

808 912 g 984

b

c

c

_1

[kcal/mol], AE1 basis. From referecne (10), basis II. [s ], from reference (4b). [ppm], relative to Fe(CO) . From reference (9). SThis work. d

e

5

For C 0 hydrogénation with rhodium-based catalysts 3 bearing chelating phosphine ligands (equation 3), the observed correlation between catalytic activities and R h chemical shifts (6a) appeared to be mediated by the steric requirements of the ligands, in particular by the bite-angle. First, this parameter itself (as measured in the solid state) was found to correlate with the ô ( R h ) values (6a). Second, an ab 2

1 0 3

103


244 initio study of the catalytic cycle has indicated that dissociation of the product from the rhodium-bis(phosphine) complex is the rate-determining step (14); the "ease" of this dissociation was related to the so-called "accessible surface" at the metal fragment (6b), which in turn depends on the steric demand of the ligands, and is quite sensitive to the bite angle. When the explicit dependence of 5( Rh) on the bite angle was computed for model compound Rh(acac)(PH ) (5), however, only minor variations were found, much smaller than the range observed experimentally (see Figure 2a) (15). What other structural features could be responsible for the observed trend in ô ( R h ) ? It had been noted earlier that not only the bite angles but also the Rh-P distances r in complexes 3 show substantial variations (up to nearly 0.05 Â), and in fact a roughly linear correlation of the two had been found (6b). Consequently, 8( Rh) should also correlate with r. This empirical correlation is depicted in Figure 2b, together with the DFT results for 5 as a function of r. Indeed, both data sets show a similar variation of 5(!0 Rh). 103

3

2

103


103

3

Complexes of type 3 with four-membered chelate rings (not included in the data of Figure 2) are a special case: the observed, substantial deshielding of the Rh nucleus (around ca. 1100 ppm) cannot be explained by the Rh-P bond lengths (15). According to the model calculations for 5, it is the tilt of the lone pairs at phosphorus away from the Rh-P axis that is responsible for the deshielding in this case. The results summarized in Figure 2 imply that the observed correlation between 8( Rh) and catalytic activities for complexes 3 may be indirect rather than intrinsic. The activity is attributed to steric effects related to the bite angle which in turn affects the chemical shifts only indirectly via concomitant changes in the Rh-P bond lengths. 1 0 3

103

Predicting NMR/Reactivity Correlations In the preceding chapter it has been shown that the DFT methods currently available can be used to reproduce relative trends in both reactivities and transition-metal N M R chemical shifts. Thus, NMR/reactivity correlations can be modeled theoretically, at least when relative reactivities are reflected in relative energies on the potential energy surfaces (activation barriers, BDEs). It should in principle also be possible to predict new such correlations. This is done in the following, with the emphasis on olefin polymerization with vanadium-based catalysts. ? i 0 II i^\*'*.. 3 R Lewis-acid complexes 6 are oxovanadium(V) species and are polymerization (16). System 6 R

R

2

6 X = Al(CH SiMe ) R , R , R = C H S i M e , OSiPh 2

1

2

3

3

3

2

3

3

7 X = A1H a R =R =R = Me b R =R = Me, R = OMe formed upon addition of the aluminium alkyl to mildly active catalysts for homogeneous ethylene appeared to be a promising candidate for an 3

]

2

1

2

3

3


245 ô(

103

Rh)


1400

Ρ

80

85

90

95

2.20

2.16

100 105

2.24

2.28

R h - P [A]

P-Rh-P[deg.]

103

Figure 2. a) Dependence of 5( Rh) in bis(phosphine complexes) on the P-Rh-P bite angle; experimental for chelates 3 (Adapted from ref. 15), computed for model compound 6 (GIAO-B3LYP). b) The same for the Rh-P distance r. A E calc. [kcal/mo a

26

1

-

"Τ Γ

24

-

y

\

20

1

1

= NH

Λ

OsΝX = none \

22

1

0

\

Se \ A I H

3

18 SbF \

ο

5

16

s

-

Li

+

14

a)

12 10 500

H

ο

+

-

\

\

\

I

1000

1500

1

2000

51

6( V) cale, [ppm]

L

2500 500

b) 1

1000

1

1

1

1500

2000

2500

51

5( V) calc. [ppm]

Figure 3. a) Predicted correlation between the ethylene-insertion barrier ΔΕ and

Correlations between Transition-Metal NMR ... - ACS Publications

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