Hydrides by Protonation Reactions - American Chemical Society

12 Reactivity Patterns in the Formation of Platinum (II) Hydrides by Protonation Reactions

Downloaded by CORNELL UNIV on October 31, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch012

D. MAX ROUNDHILL Department of Chemistry, Washington State University, Pullman, WA 99164

Protonation reactions of Pt(PPh ) with acids HX give a wide product range. When the conjugate base X- only poorly coordinates to Pt(II), compounds of type [PtH(PPh ) ]X are formed. The second-order high field H NMR spectrum and crystal structure of [PtH(PPh ) ](CF CO ) H show the com pound to be a planar Pt(II) complex. When the conjugate base strongly coordinates to Pt(II), the products are of the type trans-PtΗΧ(ΡΡh ) . With 1-ethynylcyclohexanol the product is the Pt(IV) dihydride PtH (C≡CC H (OH) )(PPh ) . Compounds HSCH CH SMe and HSCH CH SCH CH CH SMe react with Pt(PPh) to give hydrides PtH(SCH CH SMe)PPh and PtH(SCH CH SCH CH CH SMe)PPh , respectively. In the latter compounds, the terminal thioether is uncoordinated. Complexes PtH(OPPh )(HOPPh )L (L = ΡΡh , PMePh ) result from the oxidative addition of diphenyl phosphine oxide to PtL . Data are presented on the H and P NMR spectra of PtH(OPPh )(HOPPh )PMePh and PtH(OPPh )(F BOPh )PMePh . 3 3

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H

ydrides of Pt(II) are the most numerous of any transition metal hydride group. In addition to the presence of the hydride ligand, the complexes invariably have a coordinated phosphine, and synthetic routes to these compounds using both hydridic and protonic reagents have been reported (I ). The pure complexes are usually both air stable and kinetically inert. The purpose of this chapter is to show the diversity of hydrides that can be obtained from protonation reactions on zero-valent and di-valent triphenylphosphine platinum compounds, and to rationalize the type and nature of the product formed from the character of the acid HX. 0-8412-0390-3/78/33-167-160/$05.00/0 © American Chemical Society Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

12.

Formation of Pt(II) Hydrides by Protonation Reactions

ROUNDHILL

T h e zero-valent compound P t ( P P h ) undergoes a w i d e range of chemistry w i t h protonic acids. T h e initial product i n the protonation w i t h acids H X is the ionic c o m p o u n d [ P t H ( P P h ) ] X . U n d e r suitable conditions, this complex is the favored product a n d can be isolated i n h i g h y i e l d . Such a situation exists w h e n the a c i d H X is strong a n d the conjugate base X ~ is a sufficiently poor l i g a n d for Pt(II); it w i l l not undergo subsequent substitution reactions w i t h triphenylphosphine. E x a m p l e s of such conjugate bases are trifluoroacetate (2), sulfate a n d fluoroborate (3), a n d picrate (4). H o w e v e r , w h e n an a c i d is used that is weak and has a conjugate base that only poorly coordinates to Pt(II), such as acetic acid, the e q u i l i b r i u m i n (Reaction 1) can be shifted only to the h y d r i d e product i n the presence of an excess of acid, and the hydride product is not isolable from solution. F o r H C l , both an ionic, [ P t H ( P P h ) ] C l , a n d a covalent, * r a n s - P t H C l ( P P h ) h y d r i d e have been isolated. T h e former type is obtained i n donor solvents of h i g h dielectric constant, and the latter f o r m e d i n solvents such as benzene (3). 3

3

Pt(PPh ) + H X ^ 3

3

3

3

3


161

3

3

[PtH(PPh ) ] 3

-PPhs

3

frans-PtHX(PPh ) 3

(1)

2

T h e * H N M R spectra of complexes [ P t H ( P P h ) ] X have been interesting (2,5) because of the apparent difference i n the s p i n - s p i n c o u p l i n g patterns be tween the center line a n d the P t - s a t e l l i t e portion of the h i g h f i e l d resonance. T h i s spectrum shows the h y d r i d e resonance centered 5.83 p p m upfield of M e S i w i t h / ( H - P ( t r a n s ) ) of 164 H z and coupled to two apparently nonequivalent cistriphenylphosphines w i t h 7 ( H - P ( c i s ) ) of 13 a n d 17 H z . T h e P t satellite portion of the spectrum appears as a doublet of triplets w i t h / ( H - P ( t r a n s ) ) of 160 H z a n d / ( H - P ( c i s ) ) f 13 H z ( F i g u r e 1). A t that t i m e , we considered that this additional multiplicity was caused by second-order effects involving ^ ( P - P t ) and J ( P - P ) a n d not b y strong ion p a i r i n g between the cation a n d anion i n the complex leading to c h e m i c a l l y inequivalent phosphorus n u c l e i i n p y r a m i d a l geometry (2). T o verify this premise, we have d e t e r m i n e d the structure of the 3

3

195

4

2

2

1 9 5

2

2

0

2

Figure

1.

Center multiplet and upfield satellite of the H NMR of [ Ρ ί Η ( Ρ Ρ Λ ) ] ( Ο Ρ 0 0 ) ί / 1

3

3

3

2

spectrum

2

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

2

162

TRANSITION M E T A L HYDRIDES

c o m p o u n d [ P t H ( P P h ) 3 ] ( C F C 0 2 ) 2 H i n the solid state. 3

3

T h e molecule is strictly

planar a r o u n d p l a t i n u m , a n d the anion is too far away f r o m the metal center to be i n v o l v e d i n coordination (6).

T h e bond angles for P - P t - P are 99.6(2)° a n d

100.6(2)°, w i t h distances of 2.315(7) and 2.309(7)A for the P t - P distances of the m u t u a l l y trans-triphenylphosphines ( F i g u r e 2).

T h e P t - P distance of the t r i -

phenylphosphine trans to the h y d r i d e is significantly longer at 2.363(7)Â, reflecting the h i g h trans influence of the h y d r i d e ligand. T h i s structure confirms earlier ideas that the m u l t i p l i c i t y of the h i g h - f i e l d * H N M R pattern is a conseeffects (2, 7).

T h e planar geometry

for the ion


q u e n c e of second-order

[ P t H ( P P h ) ] also shows that the species is designated correctly as a complex of Pt(II) since, if a f o r m a l two-electron transfer f r o m Pt(0) to the proton h a d not occurred, the geometry of a protonated Pt(0) complex should be a distorted tetrahedron. 3

3

+

T h e second step i n Reaction 1 occurs w h e n X ~ is a strongly coordinating ligand for Pt(II). Consequently, complexes of type f r a n s - P t H X ( P P h ) are formed 3

2

w h e n the protonation of P t ( P P h ) is c a r r i e d out w i t h H C N ( 3 ) , imides (8), or 3

thioacids (9).

3

A n alternate route to these complexes J r a n s - P t H X ( P P h ) , h o w 3

2

ever, is to treat the c o m p o u n d P t ( P P h ) ( C H ) w i t h the appropriate a c i d H X . 3

2

2

4


12.


ROUNDHILL

T h i s latter procedure is favored for hydrides where X only poorly coordinates to Pt(II). T h e h y d r i d e complexes f r o m [ P t H ( P P h ) ] X have been investigated further with respect to both the addition of a second molecule of H X to give Pt(IV) d i h y d r i d e a n d the use of l i g a n d X that w i l l undergo further phosphine substitution. T h e P t ( I V ) d i h y d r i d e complex P t H C l ( P E t ) has been prepared by treating f r a n s - P t H C l ( P E t ) w i t h H C l (10), but it appears that the similar c o m p o u n d P t H C l ( P P h ) does not arise f r o m H C l a d d i t i o n to transP t H C l ( P P h ) ( I I ) . A n unusual Pt(IV) d i h y d r i d e nevertheless does result f r o m 1-ethynylcyclohexanol addition to P t ( P P h ) (Reaction 2). This complex initially was characterized spectroscopically (12) but more recently has been confirmed by crystal structure determination ( F i g u r e 3) (13). T h e h y d r i d e resonance is 3


163

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3

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2

2

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3

Figure

3.

3

Structure of (OH)) (PPhs) 2

PtH (C=CC H 2

6

w

2


164


12.9 p p m u p f i e l d to M e S i , a n d it is inferred f r o m the crystal structure that the 4

h y d r i d e ligands are m u t u a l l y trans.

Complexes such as these, f o r m e d b y o x i -

dative a d d i t i o n of a l k y n e C - H bonds to low-valent m e t a l centers, resemble proposed intermediates i n a l k y n e p o l y m e r i z a t i o n reactions. G e n e r a l l y , it is expected that H X a d d i t i o n to P t ( P P h ) w i l l occur readily 3

3

if H X is strong, or if the conjugate base X ~ is a sufficiently good ligand for Pt(II), that a reaction between

X ~ a n d the small e q u i l i b r i u m concentration

of

[ P t H ( P P h ) ] X w o u l d give P t H X ( P P h ) . 3

3

3

2

E x c e p t for 1-ethynylcyclohexanol, it appears that the a d d i t i o n of protonic a c i d to triphenylphosphine p l a t i n u m hydrides is unfavorable.

Nevertheless,

the existence of such complexes w i t h triethylphosphine ligands is proved suffiDownloaded by CORNELL UNIV on October 31, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch012

ciently since, i n addition to the isolation of complexes w i t h hydrochloric acid (JO, 14), good evidence is presented for the intermediacy of triethylphosphine Pt(IV) hydrides w i t h silanes a n d phosphines

(15,16).

A somewhat different type of product is obtained w h e n the conjugate base is a multidentate l i g a n d incorporating an anion that w i l l strongly coordinate to Pt(II).

A n example of such a ligand is 1,4-dithiapentane M e S C H C H S H . 2

When

2

the compound Pt(PPli3) is treated with this mixed thioether-thiol, the compound 3

P t H ( S C H C H S M e ) P P h is f o r m e d where both the thiolate and thioether are 2

2

3

coordinated (Reaction 3).

This reaction likely proceeds through initial formation Me (3)

of

an intermediate P t H ( S C H C H S M e ) ( P P h ) h a v i n g an 2

thioether group.

2

3

uncoordinated

2

Such a c o m p o u n d then must undergo r a p i d substitution of one

of the triphenylphosphines b y the thioether to f o r m the chelate complex

(17).

Interestingly, the presence of the chelating thioether group renders the compound isolable.

T h e stronger a c i d H S leads to a complex P t H ( S H ) ( P P h ) f r o m 2

3

2

P t ( P P h ) (18), but we have been unable to isolate characterizable products f r o m 3

3

the more weakly acidic thiols. Solutions containing excess thiol exhibit high-field resonances characteristic of platinum hydride complexes, but attempted isolation i n the absence of excess thiol causes reversion back to P t ( P P h ) . 3

3

W e have tried to extend this concept of using the thermodynamic advantage of the chelate effect (19) w i t h the c o m p o u n d 1,4,8-trithianone, M e S C H C H C H S C H C H S H . T h i s new l i g a n d can be prepared by first prep a r i n g the intermediate aldehyde M e S C H C H C H O f r o m the c u p r i c ion-catal y z e d a d d i t i o n of methanethiol to acrolein. T h i s intermediate c o m p o u n d is condensed w i t h 1,2-ethanedithiol i n the presence of boron trifluoride etherate to g i v e , after a d d i t i o n of base, the c y c l i c trithia compound M e S C H C H C H ( S C H ) , that cleaves w i t h c a l c i u m i n a m m o n i a to f o r m 1,4,8-trithanonane (20) (Reaction 4). T h i s c o m p o u n d reacts w i t h P t ( P P h ) to 2

2

2

2

2

2

2

2

2

2

2

3


3

12.

ROUNDHILL

Formation of Pt(II) Hydrides by Protonation

165

Reactions


(id) K O H

P t f P P r ^ + HSCH CH SCH CH CH SMe 2

2

2

2

—

21

Pt

J

+

2PPh

3

PhoP

( 5 )

give the h y d r i d e complex P t H ( S C H C H 2 S C H 2 C H C H 2 S M e ) ( P P h 3 ) (Reaction 5). T h e Ή N M R spectrum shows a high-field doublet ( 7(PH) = 18 H z ) centered 10.6 p p m upfield of M e S i . T h e remainder of the spectrum shows singlets 1.90 p p m downfield of M e S i for the methylenes of the ethane moiety, and resonances at 2.83,1.42, a n d 2.56 p p m downfield of M e S i for the methylenes of the propane group. Interestingly, this latter triplet ( 7 ( H H ) = 7 H z ) is unshifted f r o m the free l i g a n d , as is the methylene group at position 7 along the chain. Since the t e r m i n a l m e t h y l group at 2.10 p p m d o w n f i e l d of M e S i also is unshifted f r o m the free ligand position, the structure must be four-coordinate, w i t h the terminal thioether group uncoordinated. 2

2

2

4

4

4

2

4

P l a t i n u m hydrides h a v i n g a single, unsubstituted triphenylphosphine also have been prepared by the a d d i t i o n of diphenylphosphine oxide to P t ( P P h ) . D i p h e n y l p h o s p h i n e oxide is a weak protonic a c i d that exists p r e d o m i n a n t l y as this tautomer i n e q u i l i b r i u m w i t h diphenylphosphinous acid (Reaction 6). T h e c o m p o u n d probably is not m o n o m e r i c but likely is aggregated by hydrogen b o n d i n g (21 ). T h e reaction w i t h P t ( P P h ) readily occurs to give P t H ( O P P h ) ( H O P P h ) P P h . T h e c o m p o u n d shows an unusually intense band at 2000 c m " for the hydride. T h e h i g h - f i e l d line of the more soluble m e t h y l d i p h e n y l p h o s p h i n e c o m p o u n d is centered 3.9 p p m u p f i e l d of M e S i , the deshielded position 3

3

2

3

3

2

1

3

4

R P(H)Q ^ 2

R POH

'(6)

2

Pt(PPh ) + 2Ph P(H)Q — PtH(OPPh )(HOPPh )PPh + 2PPh 3

3

2

2

2

3


3

(7)


166


Figure

4.

H NMR spectrum (HOPPh )PMePh

l

2

of

PtH(OPPh )2

2

b e i n g a consequence of the h i g h trans influence of the phosphinito l i g a n d (22). This spectrum is shown i n F i g u r e 4 and the high-field line shows coupling to one trans ( 7 ( P H ) = 166 H z ) a n d two cis ( / ( P H ) = 24 H z , 10 H z ) phosphines. T h e spectrum also shows a peak 13.4 p p m d o w n f i e l d of M e S i w h i c h is the phosphinous a c i d hydrogen. T h e c o m p o u n d contains a h y d r i d i c hydrogen bonded to Pt(II) that can be removed by a d d i t i o n of strong m i n e r a l a c i d and a protonic h y d r o x y lie hydrogen that can be removed by titration w i t h strong base (23). W h e n the compound is treated with a solution of boron trif luoride i n diethyl ether, the a c i d i c hydrogen is replaced b y a B F group (Reaction 8). F i g u r e 5 shows the P N M R spectra of P t H ( O P P h ) ( H O P P h ) P M e P h a n d P t H ( O P P h ) ( F B O P P h ) P M e P h . T h e chemical shift and coupling data obtained from these spectra are given i n T a b l e I. T h e broadening effect of the B quadrupole on 2

2

4

2

3 1

2

2

2

2

2

2

2

n

R2

^ P T

R2

H + BF

3

—

^ P t ^

Rs

BF + HF 2

R2

( R = P h ; L = P P h PMePh ) 3

2


(8)

12.

T a b l e I.

3 1

P N M R D a t a for P t H ( O P P h ) ( H O P P h ) P M e P h 2

2

Pi

δ = 93.2 J ( P i P t ) = 2302 H z , J ( P i P ) = 28 H z , W i P a ) = 17 H z .

P

δ = 80.2 ^ ( P z P t ) = 2907 H z , J ( P P ) = 371 H z .

P

δ = 5.8 J ( P P t ) = 2224 H z .

2

l

1

3

2

a

2

2

2

167


ROUNDHILL

2

3

3

° δ is given in ppm downfield from 85% Η 3 Ρ Ο 4 .

Pi is trans to hydride and P2 is trans to P M e P h 2 -

a

3 1

3 1

P nucleus of P M e P h that is four bonds away.

Ρ nucleus two bonds away is apparent, but there is little broadening on the 2


A l t h o u g h these complexes f o r m a l l y contain a d i p h e n y l p h o s p h i n i t o and a diphenylphosphinous a c i d l i g a n d w i t h respective penta-valent and tri-valent phosphorus centers, i n reality it is more correct to consider the molecules as being conjugated, w i t h the proton hydrogen bonded between the pair of oxygens.

Such

a situation was suggested i n i t i a l l y by D i x o n (24), based on the position of *>(OH) i n the i r spectrum.

W e have v e r i f i e d this idea by observing that the Ρ N M R 3 1

spectrum of both P t ( O P P h ) ( H O P P h ) and P t [ O P ( O M e ) ] [ H O P ( O M e ) ] show 2

2

2

1

2

2

2

2

i l FB

Pt

2

JL

11 Figure

2

5.

3 1

P N M R spectrum of PtH(OPPh )(HOPPh )PMePh PtH(OPPh )(F BOPPh )PMePh 2

2

2

2

2

2

2


and

168

TRANSITION METAL HYDRIDES

only a single chemical shift. The respective values for these lines are 72.5 and 90.7 ppm downfield of 85% H3PO4. These hydrides form from diphenylphosphine oxide because of the strong coordination of the phosphorus-bonded diphenylphosphino ligands to the plat inum metal group of elements. Substitution of the additional tertiary phosphine from either Pt(PPh ) or Pt(PMePh ) is related to the chelate effect noted earlier with the thiolate ligands, except that now the additional ligand is bonded to the first one by a hydrogen bond rather than by an alkyl chain. Since diphenyl phosphine oxide exists as a hydrogen-bonded aggregate, it is possible that it is this adduct that reacts as a dimer with the phosphinous acid molecule already attached; however, until kinetic measurements are made on these systems, such questions will remain speculative. Platinum hydrides are significant in that they show the range of product types that are expected from protonation reactions and also the correlation of reactivity with acid HX and the ligands around platinum. This knowledge then can be used to suggest guidelines for designing homogeneous catalytic systems that facilitate the addition of protonic acids to unsaturated hydrocarbons. Of particular importance is the activation of Si-Η, N - H , P-H, O - H , and maybe C - H bonds to the addition of alkenes and alkynes. Currently, considerable work has been published on the hydrosilylation of alkenes and alkynes, but much less has been published on the synthesis and use of highly basic d and d complexes for the catalyzed addition of the these other functional groups. The major applied work currently in progress is the formation of adiponitrile from butadiene and H C N by using complexes Ni[P(OR) ] as catalysts, but it is likely that the range and scope of such catalytic processes will continue to expand, especially for such transformations as the conversion of alkenes into amines and ethers from ammonia and alcohols, respectively.


3

3

2

3

s

3

10

4

Acknowledgment This research was funded by the National Science Foundation and Cities Service Oil Company, Cranbury, New Jersey.

Literature Cited 1. Roundhill, D. M., "Advances in Organometallic Chemistry," Vol. 13, p. 273, Aca demic, New York, 1975. 2. Thomas, K., Dumler, J. T., Renoe, B. W., Nyman, C. J., Roundhill, D. M., Inorg. Chem. (1972) 11, 1795. 3. Cariati, F., Ugo, R., Bonati, F., Inorg. Chem. (1966) 5, 1128. 4. Tripathy, P. B., Roundhill, D. M , J. Organomet. Chem. (1970) 24, 247. 5. Bird, P., Harrod, J. F., Than, Κ. Α., J. Am. Chem. Soc. (1974) 96, 1222. 6. Caputo, R. E., Mak, D. K., Willett, R. D., Roundhill, S. G. N., Roundhill, D. M., Acta Crystallogr. (1977) B33, 215. 7. Dingle, T. W., Dixon, K. R., Inorg. Chem. (1974) 13, 846. 8. Roundhill, D. M., Inorg. Chem. (1970) 9, 254.


12. ROUNDHILL 9. 10. 11. 12. 13. 14. 15. 16.


17. 18. 19. 20. 21. 22. 23. 24.

Formation of Pt(II) Hydrides by Protonation Reactions 169

Roundhill, D.M.,Tripathy, P. B., Renoe, B. W., Inorg. Chem. (1971) 10, 727. Chatt, J., Shaw, B. L., J. Chem. Soc. (1962) 5075. Dumler, J. T., Roundhill, D. M., J. Organomet. Chem. (1971) 30, C35. Roundhill, D. M., Jonassen, H. B., J. Chem. Soc., Chem. Commun. (1968) 1233. Rasmussen, S. E., Mariezcurrena, R. Α., Acta Chem. Scand. (1973) 27, 2678. Anderson, D. W. W., Ebsworth, Ε. Α. V., Rankin, D. W. H., J. Chem. Soc., Dalton Trans. (1973) 854. Ebsworth, Ε. Α. V., Edward, J. M., Rankin, D. W. H., J. Chem. Soc., Dalton Trans. (1976) 1667. Ebsworth, Ε. Α. V., Edward, J. M., Rankin, D. W. H., J. Chem. Soc., Dalton Trans. (1976) 1673. Rauchfuss, T. B., Roundhill, D. M., J. Am. Chem.Soc.(1975) 97, 3386. Morelli, D., Segre, Α., Ugo, R., LaMonica, G., Cariati, S., Conti, F., Bonati, F., J. Chem. Soc., Chem. Commun. (1967) 524. Murro, D., Chem. Br. (1977) 13, 100. Rauchfuss, T. B., Shu, J. S., Roundhill, D. M., Inorg. Chem. (1976) 15, 2096. Kosolapoff, G. M., Maier, L., "Organic Phosphorus Compounds," Vol. 4, p. 491, Wiley, New York, 1972. Chatt, J., Heaton, B. T., J. Chem. Soc. A. (1968) 2745. Sperline, R. P., Roundhill, D. M., Inorg. Chem. (1977) 16, 2612. Dixon, K. R., Rattray, A. D., Can.J.Chem. (1971) 49, 3996.

RECEIVED July 19, 1977.


Hydrides by Protonation Reactions - American Chemical Society

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