Transition Metal Hydrides - American Chemical Society

14

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Photochemistry of Transition Metal Hydride Complexes GREGORY L. GEOFFROY, MARK G. BRADLEY, and RONALD PIERANTOZZI Department of Chemistry, The Pennsylvania State University, University Park, PA 16802

The photochemical studies of transition metal hydride com plexes that have appeared in the chemical literature are re viewed, with primary emphasis on studies of iridium and ru thenium that were conducted by our research group. The pho 5

tochemistry of the molybdenum hydride complexes [Μο(η -

C H ) M ] and [MoH (dppe) ] (dppe = 5

5

2

2

4

2

Ph PCH CH PPh ), 2

2

2

2

which eliminate H upon photolysis, is discussed in detail. 2

The photoinduced elimination of molecular hydrogen from di -and polyhydride complexes of the transition elements is pro posed to be a general reaction pathway.

T

ransition metal h y d r i d e complexes

have become a n important class

of compounds i n inorganic a n d organometallic chemistry, a n d the f i e l d has

expanded tremendously since the 1955 report (I) of the first t h e r m a l l y stable h y d r i d e complex, [ R e ^ - C s H s ^ M ] . M a n y reviews describing the properties of m e t a l hydrides were published (2-7), and i n a recent literature survey (8) of the three catalytically important metals, r u t h e n i u m , r h o d i u m , and i r i d i u m , over 2000 k n o w n h y d r i d e complexes were found.

Transition metal hydrides are es

sential i n m a n y homogeneous catalytic reactions, are useful synthetic i n t e r m e diates, are p r o m i s i n g as hydrogen a n d energy storage systems, a n d have been proposed as important intermediates for obtaining molecular hydrogen f r o m water (9). A l t h o u g h there are m a n y k n o w n h y d r i d e complexes that are important i n homogeneous catalysis, relatively f e w photochemical studies were conducted on these compounds before we began our investigations. Because transition metal h y d r i d e complexes are used i n homogeneous catalysis a n d m i g h t be used i n the photoassisted production of H f r o m water, we initiated a study of their photo c h e m i c a l properties. 2

T h e studies that were conducted b y other workers are reviewed, a n d our previously published work on ruthenium and i r i d i u m hydrides is presented. This 0-8412-0390-3/78/33-167-181/$05.00/0 © American Chemical Society Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

182

TRANSITION M E T A L HYDRIDES

is followed b y a discussion of the photochemical properties of the m o l y b d e n u m hydrides [ M o ^ - C s H s ) ^ ] a n d [ M o H ( d p p e ) ] (dppe = P h P C H C H P P h ) , 4

2

2

2

2

2

that show interesting photochemistry a n d lead to the photogeneration of very reactive complexes. O n e of the first reports concerning the photochemical properties of metal hydrides was a statement by Sacco a n d Aresta (10) that i n sunlight [ F e H ( N ) 2

( P E t P h ) ] undergoes reversible loss of hydrogen (see Reaction 1). 2

2

It was pro-

3

posed that irradiation induces formation of coordinatively unsaturated [ F e ( N ) 2

( P E t P h ) ] , a n d i n a subsequent step, iron inserts into an ortho C - H bond of P P h 2

3

(orthometallation).

3

In a separate report, K o e r n e r von Gustorf a n d co-workers

presented evidence that N , not H , is lost upon photolysis, but they gave few 2

details (11).

2

Darensbourg (12) reported that irradiation of [ F e H ( N ) ( P E t P h ) ] 2

i n the presence of excess carbon monoxide r r a n s - [ F e ( C O ) ( P E t P h ) ] (see Reaction 2). 3

2

2

2

3

yields [ F e ( C O ) ( P E t P h ) ] a n d 4

2

A l t h o u g h the photochemistry of

2

this complex is not resolved, the complex probably loses H

2

and N

2

upon i r r a -

d i a t i o n , a n d the nature of the f i n a l product probably depends upon reaction conditions. G r e e n a n d co-workers showed that uv i r r a d i a t i o n of [ W ( T 7 - C 5 H ) H ] so5

5

2

2

lutions results i n [ W ( 7 - C H ) H ( R ) ] or [ W ( r / - C H ) R ] formation, where R ?

5

5

5

5

2

is derived from the solvent (IS,14,15).

5

5

2

2

F o r example, irradiation of the dihydride

complex i n benzene produces [ W ( T 7 - C 5 H ) H ( C H 5 ) ] (14), 5

5

2

a n d i n methanol,

6

[ W ( 7 7 - C H ) H ( O M e ) ] a n d [ W ( î 7 - C H ) M e ( O M e ) ] are f o r m e d (15). 5

5

5

5

2

5

5

These

2

reactions presumably occur through photoinduced H

elimination and yield

2

reactive tungstenocene, w h i c h inserts into a C - H or O - H b o n d of a solvent molecule. In 1971 K r u c k a n d co-workers showed that uv i r r a d i a t i o n of the m o n o h y dride [ I r H ( P F ) ] produced H 3

4

2

a n d [ I r ( P F ) ] (16). 2

3

A l t h o u g h this reaction

8

apparently occurs w i t h a very low q u a n t u m y i e l d , [ I r ( P F ) ] reacts w i t h water 2

3

8

to regenerate [ I r H ( P F ) ] (16), w h i c h completes a cycle for the photochemical 3

4

generation of H from water. 2

These workers showed that irradiation of the cobalt

analog [ C o M ( P F ) ] produces the h y d r i d e - a n d phosphide-bridged 3

complex

4

shown i n Reaction 3 (17).

E l i m i n a t i o n of H f r o m [ C o H ( C h e l ) ( P R ) ] + ( C h e l 2

2

3

2

= 2,2 -bypyridine,l,10-phenanthroline) is light accelerated (18), and photolysis /

of [ F e ( 7 7 - C H ) H ( C O ) ] leads to [ F e ( 7 7 - C H ) ( C O ) ] f o r m a t i o n (19). 5

5

5

2

5

2

5

5

2

2

Ellis

2

and co-workers recently demonstrated that although replacement of H by C O 2

i n [ V H ( C O ) ( d i a r s ) ] (diars = o r i / i o - ( M e A s ) C H ) does not occur t h e r m a l l y , 3

3

2

2

6

4

i r r a d i a t i o n r e a d i l y yields [ V H ( C O ) ( d i a r s ) ] (see Reaction 4). 4

[FeH (N )(PEtPh ) ] 2

2

2

3

[FeH (N )(PEtPh ) ] + C O 2

2

2

3

H

2

+ [FeH(C H PEtPh)(N )(PEtPh ) ] 6

4

2

2

2

(1)

[Fe(CO) (PEtPh )] 4

2

+ *rans-[Fe(CO) (PEtPh ) ] 3

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

2

2

(2)

14.

Transition Metal Hydride

GEOFFROY ET AL

2[CoH(PF ) ] 3

Complexes

183

CoiPF )

(3)

(PF ) Co'

4

3

3

3

F

F

lvh (CO) (diars)] + C O 3

H

3

2

a

+

[VH(CO) (diars)]

(4)

4

In connection w i t h a study of the photochemical properties of several d i oxygen complexes of i r i d i u m , Geoffroy, G r a y , a n d H a m m o n d (21 ) observed that irradiation

of

argon-purged

solutions

of

[IrH (Ph PCH CH PPh ) ] , 2

2

2

2

2

+

2

[ I r H ( P h P C H C H P P h ) ] + , and [ I r C l H ( C O ) ( P P h ) ] eliminated H and formed 2

2

2

2

2

3

2

2

the stable Ir(I) complexes [ I r ( P h P C H C H P P h ) ] , [ I r ( P h P C H C H P P h ) ] + , 2

and [IrCl(CO)(PPh ) ]. 3

2

2

2

2

+

2

2

O f the d i h y d r i d e s studied, only [ I r C l H ( C O ) ( P P h ) ]

2

2

3

2

loses hydrogen thermally, and photolysis is the only k n o w n method for effecting H

2

elimination from the diphosphine complexes.

T h e nature of the active excited

state i n these complexes was not identified, and the mechanism of H elimination 2

was not unambiguously d e t e r m i n e d . Results and

Discussion

[IrClH2(PPh3)3] a n d mer- a n d / a c - f l r H ^ P P h a f o ] . O u r first objective was to test the generality of photoinduced e l i m i n a t i o n of molecular hydrogen f r o m stable d i - and p o l y h y d r i d e transition element complexes, and we first examined the well-characterized triphenylphosphine complexes, [ I r C l H ( P P h ) ] , mer2

[IrH (PPh ) ], and / a c - [ I r H ( P P h ) ] . 3

3

3

3

3

3

3

3

[ I r C l H ( P P h ) ] was first prepared by 2

3

3

V a s k a , w h o reported it to be an a i r stable, light-sensitive w h i t e solid (22) whose configuration was determined by N M R and ir analysis (see Structure 1) (23).

The

PPh,

PPh,

complex also was prepared by Bennett and M i l n e r (24) by the irreversible addition of H

2

to [ I r C l ( P P h ) ] . 3

3

C o m p o u n d s mer-[IrH (PPh ) ] 3

3 3

and

fac-[IrH (PPh ) ] 3

3 3

are easily prepared (25) by heating N a [ I r C l ] , P P h , and N a B H i n ethanol, and 2

have Structures 2 a n d 3 (26). of hydrogen.

6

3

4

A l l three complexes are resistant to t h e r m a l loss

F o r example, we showed that there is no H

2

loss w h e n solutions

are p u r g e d w i t h an inert gas or w h e n solid samples are heated to 150° C for 24 hr under v a c u u m .


184


mer-[ I r H ( PPh ,) ] :i

:

/ac-[IrH,( PPh,) J

3

W e observed (27) that [ I r C l H ( P P h ) 3 ] is quite photosensitive, a n d w h e n 2

3

solid samples or solutions of the complex are irradiated w i t h sunlight or fluorescent light, a r a p i d color change f r o m w h i t e to orange occurs.

Mass spectral analysis

of the gases above the irradiated solid samples shows a large amount of hydrogen. T h e electronic absorption spectral changes that occur d u r i n g 366-nm photolysis of a degassed 2.3 X 1 0 " M C H C 1 solution are shown i n F i g u r e 1. 2

2

2

T h e spectrum

of [ I r C l H ( P P h ) ] is featureless below 300 n m , a n d as i r r a d i a t i o n proceeds, a 2

3

3

new b a n d appears at 449 n m .

T h i s band is identical i n position and shape to that

displayed by a [ I r C l ( P P h ) ] sample prepared by the reaction of [ I r C l ( N ) ( P P h ) ] 3

w i t h P P h (28). 3

3

2

3

2

W h e n photolysis is followed i n the ir spectral region, solutions

show a steady decrease i n intensity of the m e t a l h y d r i d e vibrations at 2215 a n d 2110 c m

- 1

, a n d no new bands appear between 1800 a n d 2300 c m

- 1

.

T h e mass spectral analysis a n d the electronic a n d i r spectral changes d e m onstrate that h y d r o g e n is e l i m i n a t e d f r o m [ I r C l H ( P P h ) ] u p o n photolysis a n d 2

400

450

3

3

500

550

WAVELENGTHM Figure 1. Electronic absorption spectral changes accompanying 366-nm photolysis of a 2.3 X I 0 ~ M degassed CH Cl solution of [IrClH (PPh ) ] (27) 2

2

2

2

s s


14.

that [IrCl(PPh ) ] is the p r i m a r y photoproduct 3

185

Transition Metal Hydride Complexes

GEOFFROY ET AL.

If the photolysis is not prolonged

3

(50 cycles) through the photoinduced H

elimination-thermal H

2

reactions without any observable loss of complex. d u c e d b y i r r a d i a t i o n w i t h λ < 400 n m .

addition

2

E l i m i n a t i o n of H can be i n 2

T h e q u a n t u m y i e l d of e l i m i n a t i o n ,

measured at 254 n m b y m o n i t o r i n g the g r o w t h of the 4 4 9 - n m b a n d of [ I r C l ( P P h ) ] , is 0.56 ± 0.03. 3

3

Scheme 1 [IrClH (PPh ) ] £ ± 2

3

3

Η,

Η

[IrCl(PPh ) ] + H , 3

3

//

T h e r m a l e l i m i n a t i o n of H f r o m [ I r H ( C O ) ( P M e P h ) ] proceeds i n a concerted fashion, a n d t h e r m a l e l i m i n a t i o n of H f r o m other p o l y h y d r i d e complexes of i r i d i u m probably occurs b y a similar mechanism. W e f o u n d that photolysis (λ = 366 n m ; 15 m i n irradiation) of a degassed C H C 1 solution con taining equimolar amounts of [ I r C l H ( P P h ) ] and [ I r C l D ( P P h ) ] gave H and D , a n d no H D was detected by mass spectrometry (27). T h e absence of H D i n the gases above the irradiated solution of the m i x t u r e indicates that the pho toinduced elimination of H is also concerted since elimination of H ~ ( D ~ ) , H-(D-), or H ( D ) w o u l d lead to detectable amounts of H D . F u r t h e r evidence for the concerted pathway of photoinduced e l i m i n a t i o n comes f r o m previous studies (21 ) where irradiation of [ I r C l H ( C O ) ( P P h ) ] i n toluene d i d not yield bi-benzyl, an expected product if h y d r o g e n atoms were f o r m e d i n the photoprocess. 2

2

2

2

2

+

2

2

2

3

3

2

2

3

3

2

2

2

+

+

2

3

2

Preparation of [ I r H ( P P h ) ] according to literature procedures (25) gives a mixture of facial and meridional isomers that are separated by recrystallization from benzene-methanol. Irradiation of a degassed benzene solution of the synthetic m i x t u r e yields a r a p i d decrease i n intensity of the 1740 c m ν\ .γ\ of the meridional isomer, a slow decrease of the 2080 c m *>i _H of the facial isomer, 3

3

3

- 1

- 1

r


χ

186


a n d no new i / _ vibrations. F o r m a t i o n of H was v e r i f i e d by mass spectral analysis of the gases above the irradiated solutions. Irradiation of a degassed benzene solution of pure mer-[IrH (PPh ) ] shows electronic absorption spectral changes similar to those observed for photolysis of [ I r C l H ( P P h ) ] , w i t h new absorption shoulders at 375 a n d 430 n m . T h e expected p r i m a r y photoproduct f r o m both isomers, [ I r H ( P P h ) ] , should be more reactive toward ortho-metallation than [ I r C l ( P P h ) ] . E v a p o r a t i o n of solvent f r o m i r r a d i a t e d solutions of m e r - [ I r H ( P P h ) ] yields an orange solid (see Structure 4) that we characterized as an ortho-metallated derivative [ i r ( C H P P h ) ( P P h ) ] , f o r m e d through photoelimination of a second H molecule (see Scheme 2) (27). F u r t h e r support for the f o r m u l a t i o n of Structure 4 is that the photoproduct can be converted quantitatively into [ I r H ( P P h ) ] by stirring a benzene solution of the complex under a H atmosphere (27). E v i d e n c e for the initial photoproduction of [ I r H ( P P h ) ] is that i r r a d i a t i o n of mer- a n d / a c - [ I r H ( P P h ) ] under a C O atmosphere leads to formation of [ I r H ( C O ) ( P P h ) ] (27). T h i s reaction does not occur thermally; therefore, [ I r H ( C O ) ( P P h ) ] probably is formed by [ I r H ( P P h ) ] scavenging C O . I r

2

H

3

3 3

2

3

3

3

3

3

3

3

3

3

6

4

2

3

2

2

3

3

3

2

3

3

3

3

3

3

3

3

3

3

3

Scheme 2 [IrH (PPh,),]

[IrH(PPh,) ] + H ,

;{

H,

3

+

If photolysis of mer- o r / a c - [ I r H ( P P h ) ] is conducted under a H atmosphere, a different product is obtained (27). Prolonged photolysis of [ I r H ( P P h ) ] i n benzene under a H purge yields a white precipitate with a single i> _ at 1948 c m (KBr). This white solid is insoluble i n water, acetone, benzene, c h l o r o f o r m , a n d dichloromethane, and further characterization proved impossible. A complex of similar color a n d solubility, w i t h an identical ir spectrum, was prepared by C h a t t a n d co-workers (26). O r i g i n a l l y , it was formulated as [ I r H ( P P h ) ] but later was proposed (SO) to be [ I r H ( P P h ) ] . Irradiation under H suppresses the p r i n c i p a l photoreaction pathway ( H loss) and allows a second p a t h w a y ( P P h loses) to be observed. 3

3

Ir

3

3

3

3

2

2

- 1

H

3

3

2

5

2

3

2

2

3

[IrH (PPh ) ] -X 3

3

[IrH (PPh ) ] + P P h

3

H

5

3

2

3

2


(5)

14.

187


G E O F F R O Y E T AL.

ζ*

Z

2

x*-y X J

Ν S^

^

\

1 Ir -

" " "

. 1 1 ,

xz

yz

/ •

1 V

xy

xz

yz

\

•

>

s

/

\

*\

V\

\

Ν\ ΝΝ Ν \

\

^'

\

1 t

σ

L tfxy

H

2

ci$-(lrH L ] 2

L

Figure 2. Molecular orbital energy cis-dihydride complexes of iridium. complex in octahedral geometry with left, and the H molecular orbitals 2

4

level diagram for six-coordinate, The metal d orbitals for an IrL vacant cis sites are shown on the are indicated on the right (27). 4

T o determine the nature of the photoactive excited state i n these h y d r i d e complexes, we examined the electronic absorption spectra of a series of i r i d i u m h y d r i d e complexes. T h e spectra showed only a few shoulders on a rising ab sorption into the uv, and no definitive excited state assignments could be made. H o w e v e r , a molecular orbital d i a g r a m for an i r i d i u m d i h y d r i d e complex was d r a w n (see F i g u r e 2). T h e orbitals on the left are d orbitals i n proper or d e r i n g for an I r L complex that is distorted to f o r m an octahedron w i t h vacant cis sites. T h e σ a n d σ* orbitals of molecular hydrogen are shown on the right. In a c i s - [ I r H L ] complex, hydrogen bonding occurs through interactions of the Η - σ orbital with d and the Η - σ * orbital with d 2- 2 (bonding and antibonding combinations are formed i n each case). T h e exact position of the Η - σ and Η - σ * orbitals, relative to the metal d orbitals, influences the final molecular orbital 4

2

2

4

xy

2

x

y

2


2

188


order although the positioning i n F i g u r e 2 appears reasonable i n view of the low electronegativity of H . Similarly, if hydrogen is added as two distinct H atoms 2

rather than a H

molecule, an electronic transition that depopulates σ ΐ- 2 χ

2

populates a * 2 _ 2 should weaken the m e t a l - h y d r o g e n b o n d i n g . x

υ

y

χ

ν

i r i d i u m , a n d e l i m i n a t i o n of H

2

is predicted.

or

T h e transition

—• σ * 2 _ 2 leaves the complex w i t h zero net b o n d i n g between H

a 2- 2 x

y

2

and

A l t h o u g h the exact nature of the

photoactive excited state was not elucidated f r o m spectral measurements, w e v i e w the complex i n terms of the M O d i a g r a m shown i n F i g u r e 2 a n d propose that σ * 2 _ 2 is populated i n the photoactive state. χ

ν

A similar molecular orbital

d i a g r a m based on s e m i - e m p i r i c a l calculations was d r a w n for analogous 0 , S , 2

a n d S e adducts of i r i d i u m (SI ).

2

E l e c t r o c h e m i c a l studies of these complexes

2

suggest that d u r i n g the reduction of the adducts, the electron goes into an orbital of the

σ* 2- 2 χ

ν

type (32).

N o t e that i r r a d i a t i o n of these d i - o x y g e n (21 ) a n d d i -

sulfur (33) adducts leads to efficient e l i m i n a t i o n of 0

2

and S . 2

T h e [ I r C l H ( P P h ) 3 ] - [ I r C l ( P P h ) 3 ] system serves as a m o d e l storage system 2

3

3

for h y d r o g e n a n d energy (27).

[ I r C l ( P P h ) ] readily takes u p H to store it as 3

3

2

[ I r C l H ( P P h 3 ) ] , a n d then releases it on d e m a n d b y i r r a d i a t i o n w i t h uv light or 2

3

w i t h sunlight. W h e n H adds to [ I r C l ( P P h ) 3 ] , a p p r o x i m a t e l y 1 5 - 2 0 k c a l / m o l 2

3

of energy is released (S4): kcal/mol.

[IrCKPPhsfo] + H — [ I r C l H ( P P h ) ] + 1 5 - 2 0 2

2

3

3

Irradiation of [ I r C l H ( P P h ) ] w i t h 366 n m (78 k c a l / m o l ) gives H 2

3

3

a n d [ I r C l ( P P h ) 3 ] that c a n be stored separately. 3

2

W h e n the components react

to r e f o r m [ I r C l H ( P P h ) ] , a p p r o x i m a t e l y 1 5 - 2 0 k c a l / m o l of energy is released 2

3

3

a n d this amount of energy was stored.

Because of i r i d i u m ' s h i g h cost, a system

l i k e this can serve only as a m o d e l . [RuClH(COXPPh )3], [RuH^COXPPhak], and [RuClHiCO^PPhafe]. W e e x a m i n e d (35) the w e l l - c h a r a c t e r i z e d r u t h e n i u m compounds [ R u C l H ( C O ) ( P P h ) ] (see Structure 5), [ R u H ( C O ) ( P P h ) ] (see Structure 6), a n d [ R u C l H ( C O ) ( P P h ) ] (see Structure 7). Structures 5 a n d 6 are ideal for photo 3

3

3

2

2

3

3

3

2

chemistry comparison since 6 is d e r i v e d f r o m 5 b y h y d r i d e substitution of c h l o ride.

T h e three complexes are easily prepared a n d relatively air stable i n the

solid state.

Solutions, however, slowly decompose w h e n exposed to air. T h e

d i h y d r i d e , [ R u H ( C O ) ( P P h ) 3 ] , is resistant to H loss u n d e r t h e r m a l conditions, 2

3

2

a n d no reaction was observed when the complex was heated for prolonged periods under vacuum.


14.

189


GEOFFROY ET AL.

400

500

600

WAVELENGTH

M

Figure 3. Electronic absorption spectral changes resulting from 366-nm irradiation of a J 0 " M C H C / solution of [RuClH(CO)(PPhzkJ (35) 3

2

2

T h e complex [ R u C l H ( C O ) ( P P h ) ] is light sensitive, a n d i r r a d i a t i o n of a degassed 10~ Αί benzene solution of [ R u C l H ( C O ) ( P P h ) ] w i t h 366 n m results i n the electronic absorption spectral changes shown i n F i g u r e 3. As irradiation proceeds, the solution changes f r o m yellow to purple, and new absorption bands appear a n d increase i n intensity at 520 and 470 n m . T h e final spectrum (see F i g u r e 3) is identical to that of an authentic [ R u C l H ( P P h ) ] sample prepared b y the reaction of [ R u C l ( P P h ) ] w i t h E t N and H (36), w h i c h suggests that the photochemical reaction expressed i n Reaction 6 occurs. Similar spectral changes were obtained i n degassed toluene a n d dichloromethane solutions, but irradiation i n the presence of oxygen leads to r a p i d decomposition because of the air sensitivity of [ R u C l H ( P P h ) ] . 3

3

3

3

3

3

2

3

4

3

3

3

2

3

C o n f i r m a t i o n of [ R u C l H ( P P h ) ] photogeneration comes f r o m a catalysis experiment. [ R u C l H ( P P h ) ] was reported (37) to be one of the most efficient homogeneous catalysts for the hydrogénation of t e r m i n a l olefins. A deoxygenated benzene solution, 1 0 " M i n [ R u C l H ( C O ) ( P P h ) ] a n d 3 X 1 0 " M i n 1-hexene, under a H atmosphere showed no H uptake before irradiation, but r a p i d hydrogen uptake occurred u p o n photolysis w i t h 366 n m . 3

3

3

3

3

2

3

3

2


2

190


T h e ir changes that occur d u r i n g photolysis of [ R u C l H ( C O ) ( P P h ) ] indicate 3

3

that the overall reaction is not as simple as that expressed i n Reaction 5. a smooth decrease i n the *>OK) of [ R u C l H ( C O ) ( P P h ) ] at 1925 c m 3

new weak bands appear at 1965 a n d 2040 c m

- 1

.

3

- 1

Although occurs, two

These two bands are charac-

teristic (38) of [ R u C l H ( C O ) ( P P h ) ) , w h i c h probably is formed by the reaction 2

3

2

of [ R u C l H ( C O ) ( P P h ) ] with photoreleased C O (see Reaction 7). 3

3

complex is photosensitive but does not lose carbon monoxide.

T h e dicarbonyl

Instead it appears

to undergo photoisomerization (35) as do other [ R u X ( C O ) L ] complexes (39). 2

2

2

Consequently, it was not possible to achieve quantitative conversion of [ R u C l H ( C O ) ( P P h ) ] into the catalyst [ R u C l H ( P P h ) ] , and the best yield we obtained 3

3

3

3

was about 85% (35). T h e q u a n t u m y i e l d measured at 313 n m for C O e l i m i n a t i o n f r o m [ R u C l H ( C O ) ( P P h ) ] is 0.06 ± 0.02. 3

3

Because of the air sensitivity of [ R u C l H ( P P h ) ] , 3

the y i e l d was d e t e r m i n e d by i r r a d i a t i n g a C H C 1 2

2

3

solution of [ R u C l H ( C O ) -

( P P h ) ) i n a degassed a n d sealed uv cell. Since the reaction vessel was sealed, 3

3

reverse reaction w i t h C O was not prevented, a n d the measured q u a n t u m y i e l d should be considered a lower l i m i t . Irradiation of a degassed benzene solution of [ R u H ( C O ) ( P P h ) ] results 2

i n the electronic absorption spectral changes shown i n F i g u r e 4.

3

3

As irradiation

proceeds, the solutions change f r o m colorless to yellow, a n d a shoulder appears

400

500

600

WAVELENGTH(nm)

Figure 4. Electronic absorption spectral changes resulting from 366-nm irradiation of a I 0 ~ M benzene solution of [RuH CO(PPhzhJ (35) 3

2


14.

191


GEOFFROY E T AL.

[RuClH(CO)(PPh ) ] 3

C O + [RuClH(PPh ) ]

3

3

(6)

3

[RuClH(CO)(PPh ) ] + C O — [RuClH(CO) (PPh ) ] + P P h 3

3

2

[RuH (CO)(PPh ) ] 2

3

[RuH (CO)(PPh ) ] 2

3

3

co

2

(7)

3

[Ru(CO)(PPh ) ]

3

H

3

3

(8)

3

+ [Ru(CO) (PPh ) ] + P P h

2

3

3

2

(9)

3

a n d increases i n intensity at 400 n m . Mass spectral analysis of the gases above an irradiated, degassed benzene solution showed substantial amounts of molecular h y d r o g e n a n d no evidence of C O , w h i c h suggests that i r r a d i a t i o n of [ R u H ( C O ) ( P P h ) ] leads to photoinduced H elimination (see Reaction 8). T h e p r i m a r y photoproduct expected f r o m H loss, [ R u ( C O ) ( P P h ) ] , should be extremely reactive. T h e only material isolated f r o m irradiated, degassed benzene solutions was an amorphous yellow solid that showed ir bands suggestive of an ortho-metallated product (35). 2

3

3

2

2

3

3

A l t h o u g h we could not isolate a n d characterize a pure product f r o m photolysis of [ R u H ( C O ) ( P P h ) ] i n benzene, we could trap the proposed intermediate by irradiation under carbon monoxide. W h e n irradiation was conducted under C O , the ir and P N M R spectral changes showed that [ R u ( C O ) ( P P h ) ] was p r o d u c e d quantitatively (see Reaction 9) (35). T h e product could be isolated pure by solvent concentration. 2

3

3

3 1

3

3

2

T h r e e r u t h e n i u m h y d r i d e complexes were examined, a n d each showed a different photochemistry (35). Irradiation of [ R u C l H ( C O ) ( P P h ) ] leads to C O loss, [ R u H ( C O ) ( P P h ) ] undergoes photoinduced reductive e l i m i n a t i o n of molecular hydrogen, and [ R u C l H ( C O ) ( P P h ) ] undergoes photoisomerization. U n f o r t u n a t e l y , the electronic absorption spectra of the three complexes are not clearly resolved a n d therefore reveal little about the nature of the excited states that give the three different reactivities. It is significant, however, that the replacement of chloride b y h y d r i d e w h e n going f r o m [ R u C l H ( C O ) ( P P h ) ] to [ R u H ( C O ) ( P P h ) ] completely changed the photochemistry. W e believe that photoinduced e l i m i n a t i o n of H is such a favored reaction that w h e n a complex has two hydrogens i n cis positions it w i l l undergo H loss, regardless of other possible reaction pathways. 3

2

3

3

3

2

3

2

3

2

3

3

3

2

2

[MO(T7 -C5H5)2H2] a n d [MoH^dppefe]. O u r studies of the d i - and trihydride complexes of r u t h e n i u m and i r i d i u m , described above and published previously (27,35), a n d those of other workers (discussed at the b e g i n n i n g of this chapter), indicate that photoinduced e l i m i n a t i o n of molecular hydrogen is a c o m m o n reaction pathway for d i - a n d p o l y h y d r i d e complexes. T o demonstrate the photoreaction's generality a n d its utility for generating otherwise unattainable, extremely reactive metal complexes, we have begun to study the photochemistry of p o l y h y d r i d e complexes of the early transition metals. W e focused initially 5


192


on m o l y b d e n u m a n d e x a m i n e d [ M o ( 7 j - C 5 H ) H ] a n d [ M o H ( d p p e ) ] , 5

5

2

2

4

and

2

r e e x a m i n e d the previously studied [ W ( T 7 - C 5 H ) H ] ( 1 3 , 1 4 , 15). 5

5

2

2

T h e [ M O ( T 7 - C 5 H ) H ] a n d [ W ( i 7 - C 5 H ) H ] complexes are prepared by 5

5

5

2

2

5

2

2

the reaction of M0CI5 a n d W C 1 6 , respectively, w i t h N a B H are isolated as air-sensitive yellow solids (40).

and N a C s H , and

4

5

B o t h complexes are resistant to

t h e r m a l h y d r o g e n loss a n d can b e recovered unchanged f r o m s u b l i m a t i o n at 1 0 0 ° C under v a c u u m .

[ M O ( Î J - C 5 H ) H ] is photosensitive, a n d irradiation of 5

5

2

2

a degassed iso-octane solution of the complex w i t h 366-nm light gives a rapid color change f r o m y e l l o w to b r o w n . longed photolysis.

A r e d - b r o w n precipitate is obtained after p r o -

A c c o m p a n y i n g the irradiation is a smooth decrease i n intensity

of the characteristic I>M-H of [ M O ( T 7 - C H 5 ) H ] at 1847 c m 5

brations appear i n the J> -H region.

5

2

2

- 1

,

a n d no new v i -

T h e H N M R spectrum of the r e d - b r o w n l

M

precipitate a n d its color suggest its identity as the p o l y m e r i c [ M O ( T 7 - C 5 H ) ] 5

species previously described by T h o m a s (41, 42).

5

2

X

Mass spectral analysis of the

gases above the i r r a d i a t e d solutions shows a considerable amount of H .

The

2

apparent reaction sequence is o u t l i n e d i n Reaction 10.

In contrast to [ W ( r ; 5

C s H ) H ] , photolysis of [ Μ ο ( 7 7 - 0 Η ) Η ] i n aromatic solvents does not produce 5

2

5

2

5

5

2

2

any products arising f r o m C - H insertion, a n d only the p o l y m e r i c m a t e r i a l is observed. F u r t h e r evidence for i n i t i a l generation of molybdenocene (see 10) comes f r o m t r a p p i n g experiments.

Reaction

Irradiation under a carbon monoxide

or an acetylene purge, for example, leads to near quantitative f o r m a t i o n of the previously characterized [ M o ( 7 7 - C H ) C O ] (42) a n d [ M o ( 7 7 - C H ) ( C H ) ] 5

adducts (43).

5

5

5

2

5

5

2

2

2

These c a n be separated a n d p u r i f i e d by fractional s u b l i m a t i o n

a n d i d e n t i f i e d by their i r , N M R , a n d mass spectra.

Irradiation of [ M o ( r 7 - C 5 H ) H ] with excess P P h or P E t leads to formation 5

5

2

2

3

3

of the new tertiary phosphine adducts, [ M o ( 7 7 - C s H ) P R ] . E l e c t r o n i c ab 5

5

2

3

sorption spectral changes are obtained w h e n a 1.1 X 1 0 M hexane solution of 4

[ M O ( T 7 - C 5 H ) H ] is irradiated w i t h 366 n m with excess P P h (see F i g u r e 5), and 5

5

2

2

3

the isosbestic points at 285 a n d 270 n m (not shown) suggest a clean conversion. T h e phosphine adducts can be isolated f r o m the photolysis m i x t u r e by fractional sublimation. 80°C ( 1 0

3

[ M o ( 7 7 - C H ) P P h ] a n d [ M o ( 7 7 - C H ) P E t ] sublime at 90° and 5

5

5

2

5

3

5

5

2

3

m m H g ) , respectively, whereas unreacted [Mo(77 -C5H5) H ] sublimes 5

at 5 0 ° - 6 0 ° C .

2

2

T h e P P h adduct often contains small quantities of P P h i m p u r i t y , 3

3

but the P E t adduct can be isolated pure.

B o t h adducts were characterized b y

their N M R , i r , a n d mass spectra.

5

3

[ M o ( T 7 - C 5 H ) P E t ] , for example, shows a 5

2

3

doublet at 6.17 τ ( / P _ H = 5 H z ) i n its H N M R spectrum assigned to the r 7 - C H 5

l

protons, a n d a singlet at 34.9 p p m i n its nated P E t . 3

3 1

5

5

P N M R spectrum assigned to c o o r d i

[ M o ( î 7 - C 5 H ) P P h ] shows corresponding resonances at 6.18 τ ( / P _ H 5

5

2

3

= 4 H z ) a n d 18.9 p p m .


14.

193


GEOFFROY E T AL.

300

400

500

600

WAVELENGTH(nm) Figure 5. Electronic absorption spectral changes obtained during 366-nm irradiation of a 1.1 X 10~ M hexane solution of [Moi^-C H ) H ] and excess Ρ Ρ Λ 4

b

5 2

2

3

T h e phosphine complexes provide a thermal route to other molybdenocene adducts since the m o l y b d e n u m - p h o s p h o r u s b o n d appears to be labile.

When

solutions of [ M o ( 7 7 - C 5 H ) P E t ] react w i t h C O or diphenylacetylene, formation 5

5

2

3

of the corresponding adduct results (see Reaction 11).

[Mo^-CslWRs] + L ^

[Mo(7 5-C H )2L] + P R 7

5

5

(11)

3

T h e q u a n t u m y i e l d for H e l i m i n a t i o n f r o m [ M O ( T 7 - C 5 H ) H ] , measured 5

2

5

2

2

at 366 n m i n hexane solution i n a degassed a n d sealed spectrophotometer cell, is 0.10. H

2

T h i s value should be treated as a lower l i m i t since reverse reaction of

w i t h photogenerated molybdenocene was not prevented.

This compares with

a value of 0.01 that we obtained for H elimination from [ W ( T 7 - C H ) H ] under 5

2

5

5

2

2

similar photolysis conditions. W e conducted experiments to probe the mechanism of H [MO(T7 -C H5) H ]. 5

5

2

2

elimination f r o m

Mass spectral analysis of gases above irradiated toluene-ds

2

solutions showed p r e d o m i n a n t H

2

p r o d u c t i o n w i t h less than 10% H D .

Since

toluene is a n efficient h y d r o g e n atom scavenger, the absence of H D indicates that free h y d r o g e n atoms are not produced to a large extent (if at all) d u r i n g photolysis; therefore, an i n t r a m o l e c u l a r e l i m i n a t i o n process is suggested. d i a t i o n of [ M o ( 7 7 - C H ) D ] i n C D , C H , or C D 5

5

5

2

2

6

6

6

6

6

6

P P h gave D / H D mixtures i n an approximate 3:2 ratio. 3

2

Irra

solutions containing excess

Since the toluene so

lution experiments showed that free hydrogen atoms are apparently not produced, and since heterolytic cleavage of a M o - H b o n d is u n l i k e l y , substantial D duction should arise from concerted D elimination. 2

2

pro

T h e H D presumably comes


194


f r o m secondary t h e r m a l reactions of photogenerated [ M O ( T 7 - C 5 H ) ] w i t h u n 5

reacted [ M o ( 7 7 - C H 5 ) D ] . 5

2

5

c o n t a i n i n g η ,η -€ Η ι

5

5

5

2

These reactions c o u l d give d i m e r i c intermediates

2

rings (e.g., Structure 8), w h i c h c o u l d lose H D to give d i

5

m e r i c products s i m i l a r to those described by G r e e n a n d co-workers

(45).

8

T h e electronic absorption spectra of [ M O ( T 7 - C 5 H ) H ] and [ W ( T 7 - C 5 H ) H ] 5

5

2

reveal little about the nature of the active excited state.

5

2

5

2

2

T h e y each show an i n

tense peak at 270 n m (e ~ 5000) w h i c h m a y be attributed to a m e t a l - t o - C s H s charge transfer, and a shoulder near 3 1 5 - 3 3 0 n m (e « 800-2000) that cannot be assigned readily.

Neither complex shows the ligand field transitions at low energy

characteristic of [ M O ( T 7 - C 5 H ) C 1 ] (45), but this is expected because of the high 5

5

2

2

l i g a n d f i e l d strength of the h y d r i d e ligand.

T h e photochemistry is, however,

consistent w i t h a molecular orbital d i a g r a m calculated for [ Μ ο ( τ / - θ 5 Η ) Η ] 5

by D a h l and co-workers (46) (see F i g u r e 6).

5

2

2

Significantly, the highest occupied

molecular orbital, 8α i , is the principal bonding orbital between M o and H . 2

The

6fo orbital, w h i c h could easily be populated by excitation, is strongly antibonding 2

between M o a n d H . D e p o p u l a t i o n of 8α ι or population of 6fc should greatly 2

2

weaken the M o - H b o n d i n g , a n d H 2

2

loss w o u l d be expected f r o m such excited

states. The

complex

[MoH (dppe) ]

[MoCl (dppe)] with N a B H 4

4

4

is synthesized

2

easily

by

reaction

of

i n the presence of excess d p p e (47), a n d can be iso

lated as a pale yellow solid w h i c h is relatively stable to t h e r m a l loss of H . 2

De

composition does occur, however, on prolonged heating under N or v a c u u m , 2

but clean conversion to a single product does not occur.

A l t h o u g h the structure

of [ M o H ( d p p e ) ] has not been d e t e r m i n e d by x-ray d i f f r a c t i o n , the analogous 4

2

[ M o H ( P M e P h ) ] c o m p l e x has been shown to possess the M o H P 4

2

4

4

4

core below.

It is best described as two inter-penetrating tetrahedra w i t h the hydrogen atoms f o r m i n g an elongated tetrahedron and the phosphine ligands f o r m i n g a flattened tetrahedron. is similar (48).

Spectral evidence indicates that the structure of [ M o H ( d p p e ) ] 4

T h e * H and

3 1

2

P N M R spectral data suggest that [ M o H ( d p p e ) ] 4

is f l u x i o n a l at or near room temperature

(48).

φ

Mo

Ο Η Ο


2

14.

GEOFFROY ET AL

Transition Metal Hydride

Complexes


195

1%

TRANSITION M E T A L H Y D R I D E S

Irradiation of degassed benzene solutions of the complex w i t h 3 6 6 - n m light induces a r a p i d color change f r o m bright y e l l o w to orange.

T h i s color change

is a c c o m p a n i e d b y a steady decrease i n intensity of the m o l y b d e n u m - h y d r i d e stretch at 1745 c m "

1

a n d the h y d r i d e resonance at τ = 13.6 p p m .

singlet at —83.5 p p m i n the

3 1

The initial

P N M R spectrum of the complex i n benzene solution

decreases i n intensity as the photolysis proceeds.

Mass spectra, gas c h r o m a t o

g r a p h i c , a n d T o e p l e r - p u m p analysis of the gases above the i r r a d i a t e d solutions show H 2 f o r m a t i o n w i t h two moles released per mole of complex irradiated. Solvent evaporation from the irradiated solutions gave a very air-sensitive orange solid that showed no m e t a l - h y d r i d e vibrations i n the ir, and no hydride resonances i n its H N M R spectrum. l

T h e absence of evidence for a ligating h y d r o g e n suggests the f o r m u l a t i o n of the product as [Mo(dppe) ] or its d i m e r .

T h e relatively l o w solubility of the

2

complex has thus far prevented molecular weight measurements.

A n ortho-

metallated derivative, w h i c h m i g h t be expected, w o u l d be expected to show evidence for a m o l y b d e n u m - h y d r i d e bond.

T h e overall photochemical reaction

that is consistent w i t h this formulation and w i t h the quantity of hydrogen released is g i v e n i n Reaction 12. [MoH (dppe) ] - X 2 H 4

2

2

+ [Mo(dppe) ]

(12)

2

F u r t h e r evidence for the f o r m u l a t i o n g i v e n for the product comes f r o m its reactivity. Irradiation of [ M o H ( d p p e ) ] under a C O atmosphere (or a d d i t i o n of C O to a solution of the orange product) gave a m i x t u r e of cis- a n d trans[ M o ( C O ) ( d p p e ) ] , as shown b y their characteristic (49,50) i r spectra w i t h bands at 1853 a n d 1770 c m " (cis) a n d 1812 c m " (trans). Photolysis i n the presence of ethylene gave a n orange solid that displayed a H N M R spectrum i n d i c a t i v e of a n ethylene complex but not i d e n t i c a l to that reported (51) f o r [ M o ( C H ) (dppe) ]. F u r t h e r characterization of this complex is u n d e r way. 4

2

2

2

1

1

l

2

4

2

2

Irradiation of [ M o H ( d p p e ) ] under a n N atmosphere results i n v i r t u a l l y quantitative f o r m a t i o n of [ M o ( N ) ( d p p e ) ] . In a t y p i c a l experiment, 225 m g of [ M o H ( d p p e ) ] i n 100 m L of N - s a t u r a t e d benzene was i r r a d i a t e d w i t h 366 n m for 12 hr. C o n c e n t r a t i o n of the solution to 10 m L , f o l l o w e d b y a d d i t i o n of 75 m L of methanol, gave 221 m g of [ M o ( N ) ( d p p e ) ] (93% yield). T h e bisdinitrogen complex was characterized by its P N M R spectrum (δ = —64.9 ppm) a n d its i r spectrum (52) (J>N=N = 1972 c m " ) that showed no r e m a i n i n g [ M o H ( d p p e ) ] . This synthetic procedure represents a considerable improvement i n y i e l d a n d convenience over previously reported methods (52, 53) that gave yields f r o m 1 3 - 6 0 % , d e p e n d i n g o n starting m a t e r i a l a n d reaction conditions. L e n g t h y isolation a n d p u r i f i c a t i o n steps are not r e q u i r e d , a n d the o v e r a l l y i e l d f r o m [ M o C l ( d p p e ) ] through [ M o H ( d p p e ) ] is about 70%. 4

2

2

2

4

2

2

2

2

2

2

2

3 1

1

4

2

4

4

2

N o experiments have been conducted to determine the e l i m i n a t i o n m e c h a n i s m (although it is l i k e l y concerted), a n d poor spectral properties have p r e c l u d e d q u a n t u m y i e l d measurements. T h e reaction does appear efficient,


14.

197


GEOFFROY ET AL.

however, since 250 m g of [ M o H ( d p p e ) ] c a n be converted entirely to [ M o ( N ) 4

(dppe) ]

2

2

2

w i t h a few hours of irradiation.

2

Summary Although only relatively few hydride complexes have been examined, several aspects c o n c e r n i n g their photoreactivity are b e c o m i n g clear.

First, it appears

that i r r a d i a t i o n of complexes c o n t a i n i n g o n l y one h y d r i d e l i g a n d w i l l lead to t y p i c a l p h o t o c h e m i c a l reactions.

F o r example, photolysis of [ R u C l H ( C O ) -

( Ρ Ρ η ) ] leads to C O loss as do m a n y m e t a l carbonyls, a n d irradiation of [ R u 3

3

C l H ( C O ) ( P P h 3 ) ] gives photoisomerization analogous to that observed (39) for 2

2

a series of [ R u X ( C O ) L ] ( X = h a l i d e ; L = tertiary phosphine) complexes. 2

2

In

2

particular, we do not believe that photoelimination of H , Η·, or H " is important. +

E v e n Adamson's photochemical rules (54) predict that loss of H , Η·, or H ~ +

should not occur often since the weakest l i g a n d - f i e l d axis is rarely the h y d r i d e c o n t a i n i n g axis. W e p r o v i d e d strong evidence that a complex w i t h t w o or more hydrogens w i l l lose H w h e n irradiated regardless of h o w t h e r m a l l y resistant it is to H loss 2

(27, 35).

2

W e believe that this is a general reaction for d i - a n d p o l y h y d r i d e s of

a l l the transition elements, a n d it has been observed for V , M o , W , F e , R u , C o , and Ir. W e are investigating a series of polyhydride complexes to test this general concept a n d to explore the possibility of generating extremely reactive m e t a l complexes that cannot be obtained thermally but w h i c h might be derived through photoinduced H

2

elimination.

Experimental T h e complexes [ M O ( T 7 - C H ) H ] (40), [ M o ( r y - C H 5 ) D ] (40), [ W C H ) H ] (40), [ M o C l ( d p p e ) J (47), a n d [ M o H ( d p p e ) ] (47) were p r e p a r e d b y p u b l i s h e d procedures. T r i p h e n y l p h o s p h i n e was obtained f r o m A l d r i c h C h e m i c a l C o . and was rœiystallized f r o m benzene/ethyl alcohol before use. A l l other chemicals were reagent grade a n d were used witnout further purification. A l l solvents were d r i e d b y standard methods a n d rigorously degassed before use. Manipulations a n d reactions w i t h air-sensitive compounds were carried out under an argon atmosphere unless otherwise stated. General Irradiation Procedures. Irradiations were conducted at 366 n m using a 4 5 0 - W H a n o v i a , medium-pressure H g l a m p equipped w i t h C o r n i n g Glass 0 - 5 2 a n d 7-37 filters (/ 1 0 " e i n s t e i n / m i n ) , a 100 W B l a k - R a y B 1 0 0 A l a m p e q u i p p e d w i t h a 3 6 6 - n m narrow bandpass filter, or a 3 5 0 - n m Rayonet photoreactor. T h e c o m p l e x to be studied was placed i n a n evacuable q u a r t z uv c e l l or i n a Schlenk tube. A f t e r degassing, the appropriate solvent was distilled onto the sample. Solutions for i r studies were transferred i n a n inert atmosphere glovebox to 0.5-nm path length, N a C l solution ir cells. Solutions were irradiated w i t h the appropriate l a m p , a n d electronic a n d i r spectra were recorded p e r i o d ically. Samples for H a n d P N M R spectra were prepared s i m i l a r l y , a n d the N M R tubes sealed u n d e r v a c u u m . L a m p intensities were measured b y ferrioxalate actinometry. 5

5

5

2

2

5

4

5

2

5

2

4

5

2

2

2

7

L

3 1


5

198


P h o t o l y s i s of [ M o ^ - C s H s ) 2 H ] . Irradiation of degassed iso-octane, benzene, a n d hexane solutions of [ M O ( T 7 - C 5 H ) H ] w i t h uv light led to change i n color f r o m y e l l o w to b r o w n . Prolonged photolysis led to a r e d - b r o w n pre cipitate that showed i r bands of coordinated T ^ - C S H S . T h e H N M R spectrum of this precipitate showed a complex pattern i n the cyclopentadienyl region but no evidence for a M - H resonance. T h e q u a n t u m y i e l d of H loss f r o m [Μο(τ7 C H ) H ] was determined by 366-nm irradiation (J = 1.42 Χ 1 0 " einstein/min) of thoroughly degassed n-hexane solutions placed i n sealed uv cells. T h e photoreaction was m o n i t o r e d by measuring the decrease i n absorbance o f the [ M o ( 7 7 - C H ) H ] b a n d at 270 n m . T r a p p i n g E x p e r i m e n t s w i t h C O a n d H C ^ C H . Photolysis of deoxygenated iso-octane solutions of [ M o ( i 7 - C 5 H ) H ] under a C O purge gave a color change f r o m yellow to green. Solvent evaporation at — 1 0 ° C y i e l d e d a green solid that was p u r i f i e d by sublimation at 2 5 ° - 3 0 ° C (10~ m m H g ) . T h e green sublimed product showed a single i>c=o at 1910 c m i n its i r spectrum, a singlet at 5.70 r i n its H N M R spectrum, a n d a parent ion at 258 m je i n its mass spec t r u m . These data i m p l i e d that the product was i d e n t i c a l to the previously characterized [ M o ( 7 7 - C H ) C O ] (42). Photolysis of deoxygenated iso-octane solutions of [ M O ( T 7 - C H ) H ] i n the presence of an H C ^ C H purge gave a yellow-to-orange color change. Solvent evaporation y i e l d e d an orange-brown solid that was p u r i f i e d by sublimation at 2 5 ° - 3 5 ° C ( 1 0 " m m H g ) to give orange crystals. T h e Ή N M R spectrum of the product showed two singlets (at 2.32 and 5.70r) identical to those of the previously characterized [ M o ( 7 7 - C H ) ( C H ) ] (43). P r e p a r a t i o n of [ M o ^ - C s H s f e P E t a ] a n d [ M o ^ - C s H s f e P P h a ] . Irradiation of a solution of 100 m g of [ M O ( T 7 - C 5 H ) H ] a n d a tenfold excess of P P h i n 75 m L of degassed iso-octane gave a color change f r o m y e l l o w to red-orange. R e m o v a l of the solvent y i e l d e d a red-orange solid that sublimed at 8 0 ° - 9 0 ° C (10~ m m H g ) , and that was shown to be [ M o ( 7 7 - C H ) P P h ] by its spectral data. (The product is often contaminated w i t h co-sublimed P P h a n d [ M O ( T 7 - C 5 H ) H ].) Photolysis of a solution containing 100 m g of [ M O ( T / - C 5 H 5 ) H ] and a tenfold excess of P E t i n 7 5 m L of iso-octane gave a color change similar to that of the P P h reaction. Solvent removal followed by prolonged p u m p i n g to remove the excess P E t yielded a red-orange material that sublimed at 7 0 ° - 8 0 ° C . T h i s substance was characterized as [Mo(?7 -C5H5) PEt ). T h e product is collected more easily by solvent evaporation a n d r e m o v a l of unreacted [ M o ( r / - C 5 H ) H ) by subli m a t i o n at 50° C (10~ m m H g ) . T h i s method essentially gave quantitative conversion based on reacted d i h y d r i d e . M e c h a n i s t i c E x p e r i m e n t s . Sample solutions of [ M O ( T 7 - C 5 H ) H ] a n d [ M o ( 7 j - C 5 H ) D ] were prepared by distilling appropriate degassed solvent into Schlenk tubes that contained the complex. Trie sample solutions were subjected to short-term photolysis (

Transition Metal Hydrides - American Chemical Society

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