A New Class of Hydrido-Bridged Platinum ... - ACS Publications

8

Downloaded by UNIV OF TEXAS AT DALLAS on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch008

A New Class of Hydrido-Bridged Platinum Complex with Application in Catalysis MIGUEL CIRIANO, MICHAEL GREEN, JUDITH A. K. HOWARD, MARTIN MURRAY, JOHN L. SPENCER, F. GORDON A. STONE, and CONSTANTINOS A. TSIPIS Department of Inorganic Chemistry, The University, Bristol BS8 1TS, England

The Pt(O) monophosphine complex [Pt(C H ) {(C H ) P}] reacts with tri-organosilanes to give diplatinum compounds [PtSiR '(μ-H){(C H ) P})] [R 'Si = EtMe Si, Me PhSi, Et Si, Me PhCH Si, ClMe Si, Cl Si, and (EtO) Si]. An x-ray dif fraction study of [PtSiEt (μ-H){(C H ) P}] revealed a dihe dral angle of 21° between the planes defined by the two SiPtP units, but failed, expectedly, to locate the bridging hydride li gands. An unusual feature of the diplatinum complexes is the absence in their H NMR spectra of a high-field resonance above 12 τ. Detailed NMR studies on [PtSi(OEt) (μ-H)(Bu2 MeP)] showed that these compounds are dynamic, and the signal corresponding to the hydrido bridge occurs at about 7 τ. Evidence is presented for asymmetric Ρt(μ-Η) Ρt bridges lead ing to concomitant, multicenter Pt-Η···Si bonding. The complexes are efficient hydrosilylation catalysts. [PtSiMe Ph(μ-Η){(C Η ) Ρ}] decomposes thermally to give, in low yield, a bis(dimethylsilyl)diplatinum compound, [PtH(SiMe ){(C H ) P}] . 2

3

2

6

11

3

2

2

3

2

4 2

2

3

6

11 3

2

3

3

3

6

11

3

2

1

3

t

2

2

2

6

2

T

he

discovery

11

3

6

11

2

3

2

complexes

of P t ( O ) is p r o m p t i n g

r a p i d expansion of organo p l a t i n u m chemistry.

( J ) of pure-olefin

T h e olefin groups i n c o m

pounds such as [Pt(cod) ] (cod = cyclo-octa-l,5-diene) or [Pt(C2H ) ] are displaced 2

4

easily i n a variety of reactions (2,3,4,5,6).

3

Particularly important complexes that

are d e r i v e d f r o m these o l e f i n - p l a t i n u m species are the bis(ethylene)tertia r y p h o s p h i n e p l a t i n u m derivatives [ P t ( C H ) ( R P ) ] [ R P = M e P , M e P h P , 2

4

2

3

3

3

2

M e P h P , P h P , a n d ( c y c l o - C H n ) P ] (7), w h i c h effectively c a n be sources of the 2

3

6

3

R P P t fragment i n synthesis since they lose ethylene readily. 3

I n this chapter,

0-8412-0390-3/78/33-167-lll/$05.00/0 © American Chemical Society

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

112

TRANSITION M E T A L HYDRIDES

w e report reactions w i t h tri-organosilanes that give d i p l a t i n u m complexes [ P t S i R V H ) ( R P ) ] 2 (8). Most of the work so far has been carried out with the tricyclohexylphosphine 3

3

complex [ P t ( C H 4 ) 2 ( ( C H i i ) P ) ] , w h i c h on treatment w i t h the silanes i n toluene 2

6

3

or petroleum ether gives i n h i g h y i e l d (60-90%) yellow, air-stable crystalline


compounds [ P t S i R a V H W C ^ n f o P J k [ r V S i

=

EtMe Si, Me PhSi, 2

2

Et Si, 3

Journal of the Chemical Society, Chemical Communication

Figure 1. Molecular structure of [PtSiEt^-H)(CefinhTk. Pt(D-P(l), 2.282(9); Pt(2)-P(2\ 2.271(9); Pt(l)-Si(l) 2.33(1); Pt(2)-Si(2), 2.34(1) A; < Si(l)Pt(l)-Pt(2\ 124.9(3); Si(2)-Pt(2)-Pt(l) 123.5(3); P(l)-Pt(l)-Pt(2), 131.4(3); P(2)-Pt(2)-Pt(l\ 133.9(3)° f

y

M e P h C H S i , C l M e S i , C l S i , and (EtO) Si]. 2

2

2

3

P-31 ( H decoupled, chemical shift l

3

relative to H P 0 ) N M R studies on these complexes gave singlet signals that had 3

two sets of

1 9 5

4

P t satellites arising from molecules with one and two active platinum

n u c l e i , the latter b e i n g second order ( Α Α ' Χ Χ ' system), e.g. for [PtSi(OEt) (M3

H ) ( ( C H ) P } ] , δ = - 6 6 . 3 p p m [ J(?Pt) 6

1 1

3

3127 H z a n d 7 ( P P t ) 82 H z ] . T h e d i 2

l

2

nuclear nature of the complexes was c o n f i r m e d by a single crystal x-ray d i f fraction study on [ P t S i E t ( M - H ) ( ( C H 11 ) P| ] (Figure 1) (8). 3

6

3

2

T h e six atoms, S i ( l ) ,


8.

Hydrido-Bridged

CIRIANO E T AL.

Platinum

P t ( l ) , P ( l ) ; Si(2), Pt(2), P(2) c o m p r i s i n g the m a i n skeleton of the molecule, are significantly non-co-planar, with a dihedral angle of 21° between the S i ( l ) P t ( l ) P ( l ) a n d Si(2)Pt(2)P(2) planes. T h e P t - P t distance (2.692 Å) is not unusual (2). N o b r i d g i n g h y d r o g e n atoms were located. Attempts to detect the h y d r i d o ligands b y i r a n d N M R spectroscopy were i n i t i a l l y inconclusive. In the i r , broad bands were observed i n the range 1695-1545 c m " w h i c h could be attributed to P u > H ) P t groups. T h e U N M R spectra above 12 r revealed no resonance attributable to a p l a t i n u m h y d r i d e . Nevertheless, i n spite of what was considered at first to be the absence of a N M R signal caused b y a metal h y d r i d e , there was c h e m i c a l evidence for Pt(M-H)Pt bridges. Thus, treating [ P t S i M e P h ( M - H ) ( ( C H i ) P ) ] 2 w i t h the ligands B u W C , M e P , or M e P h A s gave the t e r m i n a l h y d r i d o complexes, c i s - [ P t H ( S i M e P h ) ( ( C H n ) P } L ] [ L = B u W C , M e P , or M e P h A s ] , w h i c h were evidently f o r m e d b y cleavage of the Pt(^-H)Pt* bridges. T h e spectroscopic properties of these products were as expected, and there was no d i f f i c u l t y i n observing either the ir bands or the H N M R resonances caused b y P t H , e.g. for ris-[PtH(SiMe Ph)( ( C e H n ) P K B u N C ) ] , * w ( P t H ) , 2100 c m " ; 13.95 τ [d, 7(PH) 20, 7(PtH) 960 H z . Moreover, the dinuclear complexes reacted w i t h ethylene to give [ P t ( C H ) ( ( C 6 H i i ) P } ] , w i t h triphenylphosphite to give [Pt((PhO) P| ], and w i t h carbon monoxide to give [Pt(M-COJKCeHntaPita; the silane M e P h S i H also was f o r m e d i n the reaction w i t h ( P h O ) P . 1


113

Complex

2

3

l

2

6

1

3

2

6

2

3

3

2

X

2

4

2

t

3

2

1

3

3

4

2

3

Deuteration studies also p r o v i d e d evidence for h y d r i d o bridges i n the d i p l a t i n u m compounds. It was expected that reaction of [ P t ( C H ) | ( C 6 H n ) P } ] w i t h a deuteride R ' S i D w o u l d give complexes [ P t S i R ' ( M - D ) l ( C H n ) P } ] . In practice, syntheses of the latter compounds were not clean; [Pt(M-H)Pt]-containing species were produced simultaneously because of D - H exchange. This exchange c o u l d occur b y reversing some of the steps i n the hydrosilylation mechanism, discussed later. A n initial experiment involved treating [ P t ( C H ) ( ( C 6 H n ) P } ] with E t S i D . The H-decoupled P N M R spectrum showed a resonance at —63.2 p p m [ 7 ( P P t ) , 3229; 7(PPt), 60 H z , as expected for a [Ρ·Ρΐ(μ-Χ)Ρΐ·Ρ] (X = Η or D ) system, but the signal was broad because of P - D coupling. In other experiments, [ P t ( C H ) ( ( C H ) P } ] reacted with M e P h S i D . T h e ir spectrum of the product showed a band at 1163 c m " with a m u c h weaker band at 1610 c m . Because of the relation between these bands, it is reasonable to assign the former to [Pt(M-D)Pt] a n d the latter to [Pt(M-H)Pt]. N o b a n d at 1163 c m " is observed i n the spectrum of [ P t S i M e P h ( M - H ) ( ( C 6 H ) P ) ] . Reaction of the partially deuterated product w i t h trimethylphosphine gave a mixture of the mononuclear deuterated a n d nondeuterated hydrides c i s - [ P t D ( S i M e P h ) ( ( C e H ) P K M e a P ) ] [ * w ( P t D ) , 1470 c m " ; D N M R : r , 12.9 d of d, J(PD) 23.8 a n d 3.2,7(PtD) 122 H z ] a n d c w - [ P t H ( S i M e P h ) ( ( C H ) P | ( M e P ) ] [ i w ( P t H ) , 2054 c m " ; H N M R r , 12.85 d of d , 7(PH) 162 a n d 22 H z ] . T h e relative i n tensities of the i r bands and N M R signals showed that the deuterated species p r e d o m i n a t e d i n this mixture of mononuclear hydrides. 2

3

2

1

4

3

4

2

3

6

2

3

2

3

3

3 1

2

3 1

2

4

2

6

11

2

3

2

1

- 1

1

2

11

3

2

2

11

1

3

2

2

1

6

n

3

3

l


114

TRANSITION M E T A L HYDRIDES

F r o m these results, it appears that the complexes [ P t S i R 3 ' ( M - H ) ( R P ) ] u n 3

2

dergo reactions i n w h i c h they give either the fragment P t H ( S i R ' ) ( R s P ) , subse 3

quently captured by a σ-donor molecule L , or they reform Pt(O) complexes, e.g. [Pt|(PhO) P) ] a n d R ' S i H . 3

4

Moreover, i n spite of the absence of a h i g h - f i e l d * H

3

N M R signal, the i r spectra p r o v i d e good evidence for the [Ρί(μ-Η)Ρί] group. T h e e n i g m a of the apparently absent P t H N M R signal i n the H N M R l

spectra of the d i p l a t i n u m compounds at first appeared to be compounded by the results of P u d d e p h a t t et a l . (9). These workers recently have characterized d i nuclear bis(diphenylphosphino)methane cationic complexes of p l a t i n u m con t a i n i n g both b r i d g i n g a n d t e r m i n a l h y d r i d o ligands. 2


I n the

compound

[ P t H 2 ( M - H ) ( d p p m ) ] C l - ( d p p m = P h P C H P P h ) , for example, the N M R +

2

2

2

2

resonances for t e r m i n a l P t H a n d b r i d g i n g P t ( M - H ) P t appear at 16.8 r ^ / ( P t H ) 1 1 6 2 , / ( P t H ) 116 H z ] and 15.8 τ [ / ( P t H ) 540 H z ] , respectively. 2

1

Thus, it appears

that i n a c o m p o u n d c o n t a i n i n g a s y m m e t r i c a l [ P t ( M - H ) P t ] group, a h i g h f i e l d N M R signal near 16 τ is expected.

T h e absence of a signal i n this region i n

complexes [ P t S i R 3 ' ( M - H ) ( R P ) ] indicates that either the resonance is not seen 3

2

because the molecules are u n d e r g o i n g d y n a m i c behavior i n solution a n d the measurement (at + 3 0 ° C ) is being m a d e near the coalescence temperature, or the P t ( M - H ) P t bridge b o n d i n g is of a nonclassical k i n d , g i v i n g rise to a h y d r i d e resonance w i t h an unusual c h e m i c a l shift.

In v i e w of these considerations de

tailed N M R experiments were c a r r i e d out on the c o m p o u n d [ P t S i ( O E t ) 3 ( M - H ) (BitfMeP)]* T h i s complex was prepared b y reaction of ( E t O ) S i H w i t h 3

[Ρ^0 Η ) (Βυ 2

4

2

2

ι

M e P ) ] , a n d was studied to s i m p l i f y the 5 - 1 0 τ region of the N M R spectrum. Complexes [Ρί8ίΒΥ(μ-Η)|(ΰ6Ηιι) Ρ!] , because of the cyclohexyl groups and alkyl 3

2

substituents on silicon, show N M R bands that tend to obscure signals below 10 r.

M o r e o v e r , the chosen complex is considerably more soluble than the cyclo-

hexylphosphine derivatives.

T h e resonances observed i n the * H spectrum of

[ P t S i ( O E t ) 3 ( M - H ) ( B u 2 M e P ) ] , recorded at + 7 5 ° , + 2 5 ° , a n d - 6 0 ° C , are listed l

2

in Table I. Resonances caused by the protons i n the B u * M e P and S i ( O E t ) groups 2

were assigned readily.

3

O f considerable significance was the observation of a

signal i n the region 6 . 6 - 7 r that h a d a c h e m i c a l shift w h i c h v a r i e d w i t h temper ature.

T h e nature of the platinum satellites of this resonance clearly showed that

it was caused by a proton bonded to p l a t i n u m w h i c h , i n the l i m i t i n g low t e m perature spectrum, was associated m o r e closely w i t h one m e t a l a t o m than w i t h the other.

It was undergoing d y n a m i c behavior at elevated temperatures so that

there was a time-averaged / ( P t H ) value of 681 H z at 75°C.

Moreover, ^ { ^ P t }

I N D O R measurements at 3 0 ° C gave a p l a t i n u m resonance at —44 p p m (mea sured relative to 21.4 M H z ( M e S i = 100 M H z ) ) w i t h / ( P t P ) = 3122 H z a n d av 4

erage / ( P t H ) = 678 H z .

This

1 9 5

P t - H c o u p l i n g value agrees w e l l w i t h that 1

measured f r o m the H spectrum at 7 5 ° C . l

T h e c o u p l i n g constants ^ ( P t H ) 894

H z a n d / ( P t H ) 456 H z obtained f r o m the spectrum of [ P t S K O E t ^ M - H K B u ^ 2

M e P ) ] at - 6 0 ° C bracket the value observed [ / ( P t H , 540 H z ] (9) for the group 2

[Pt( -H)Pt] in [ P t H ( d p p m ) ] + C l M

2

3

2


8.

Hydrido-Bridged

CIRIANO E T AL.

T a b l e I.

+75°C

2

6.0 7 7.05 681 52


2

PtH

3

-60° C r

A New Class of Hydrido-Bridged Platinum ... - ACS Publications

Recommend Documents