Anything One Can Do, Two Can Do, Too- And It's More Interesting

2 Anything One C a n D o , Two C a n D o , Too— And It's M o r e Interesting MALCOLM H. CHISHOLM

Downloaded by IOWA STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch002

Department of Chemistry, Indiana University, Bloomington, IN 47405

I should like to propose that all the types of reactions which have been established

for mononuclear transition metal

complexes will also occur for dinuclear transition metal complexes,

and furthermore, that the latter will show additional modes

of reactivity which are uniquely associated with the metal-metal bond. In this article, I shall support this proposal by illustrations taken from the reactions of dinuclear compounds of molybdenum and tungsten, two elements which enter into extensive dinuclear relationships (1). Coordination Numbers and Geometries For any transition element in a given oxidation state and n

d

configuration,

there

is

generally

a fairly well

defined

coordination chemistry. Ligand field stabilization energies are often

important, i f not dominant, and readily account for the

fact that mononuclear complexes of Cr(3+), Co(3+) and Pt(4+) are almost

invariably octahedral, while those of Cr(2+) and high

spin Co(2+) may be either 4- or 6-coordinate. The effect of the charge on the metal and attainment of an 18 valence shell of electrons are also two strong forces in determining preferred coordination numbers. High coordination numbers (7 and 8) are common for the early transition elements in their high oxidation states

(4+

and 5+),

while the latter

transition elements in

their low oxidation states often have low coordination numbers 0097-6156/81/0155-0017$05.75/0 ©

1981 American Chemical Society

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18

REACTIVITY

(2,

3 and 4 ) .

Similar

factors

control

O F

M E T A L - M E T A L

BONDS

the c o o r d i n a t i o n p r e f e r

ences of the d i n u c l e a r compounds of molybdenum and tungsten.

(M=M)^

Complexes

+

(2).

In

scores

of d i n u c l e a r molybdenum

compounds, and t o a l e s s e r extent ditungsten compounds, a c e n 4+ tral M u n i t has a M-M quadruple bond of c o n f i g u r a t i o n σ π*»δ . The metal atomic o r b i t a l s involved i n the MiM bond are d ( ° h 2

2

9

2


d

,d (π) and d xz yz xy

available

for

use

(δ). The remaining metal atomic o r b i t a l s are in

metal

ligand

bonding

giving

types of compounds shown schematically i n I and II Μ

=

Μ

=

— Μ

I

a plane

overall result the

atomic

geometry

about

the

of the formation atoms

These

majority II

is

distances the

of

s,

ρ , ρ , d χ y χ ~y 2

dimetal

center

16 valence

two a d d i t i o n a l the p

z

atomic

compounds

are of type

weak

i n the s o l i d

2

is

are formed

o r b i t a l s . The eclipsed

In these

shell

bonds

way, the ΕΑΝ r u l e i s

generally

bonds

of

with

orbitals

of each metal

satisfied.

At t h i s time,

I:

axial

c o o r d i n a t i o n to

An immediate analogy i s

The

absence

coordinated metal

pounds

with

leaves

the

ΜΞΜ bonds, d

configuration

x

2 .

y

2

ligand

seen with square

5 coordination

16 and 18 valence s h e l l e l e c t r o n i c c o n f i g u r a

respectively.

trigonally

In

are formed along the

c o o r d i n a t i o n number 4, but sometimes

observed g i v i n g

tions,

a

compounds,

as evidenced by long Mo-axial

state.

as

electrons.

c o o r d i n a t i o n chemistry of Pt(2+) which i s most often

planar is

a

utilize

atom and, i n t h i s

give

below. Μ —

ligand

of the M-M δ bond.

attain

of type II,

axis.

the

four metal

metal

compounds M-M

of type I,

using

metal

to two

II

In compounds in

rise

and

would

can

of

atoms, be

a

e.g.

M=M as

bond

i s common f o r com

understood:

orbitals

be paramagnetic

between two

degenerate

a

trigonal

field

and so a σ π δ

with the two unpaired

trons r e s i d i n g i n the δ o r b i t a l s .


2

ι >

2

elec

2.

Dinuclear

CHISHOLM

(ΜΞΜ) ä

ing

Transition

Compounds (3). M^

central

6 +

unit

molybdenum and tungsten the

formation

bonds tion of

a

(d-d , xz xz

d -d ) yz yz

rise

atoms to

Ilia,

ligands. atoms

to about

the

giving

ligands

below)

for

In

atoms

z 2

)

been

ΜΞΜ bond a r i s e s

these compounds,

is

atom

lie

which

the c o o r d i n a -

a plane,

either

giving

staggered,

atom

number 4, lie

in

to

staggered

a

the

four

plane,

the

ligand

and

the

on the requirements of

IVa,

eclipsed

IVb

(shown

situations.

Illb

1/

\

Μ = =

l

Μ

/

1/

Μ ΞΞΞΞΞ Μ

/\\

/\ IVa

IVb

coordination

the

structures

and

VI

below, in

from

depending upon the requirements of

metal

rise

in

are

Ilia

formed

for

commonly 3 or 4, but examples

the M-M bond depends

or intermediate

For

synthesized

and a degenerate p a i r of π

coordination

each

conformation

(4).

-d

structures

Illb,

Similarly,

bonded

z 2

each metal

state

or e c l i p s e d ,

recently

known. For c o o r d i n a t i o n number 3, the three

bonded to

ground

19

A large number of compounds c o n t a i n have

σ bond ( d

5 and 6 are also

Complexes

in which a c e n t r a l

number of the metal

ligand Downloaded by IOWA STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch002

of

Metal

a

of

numbers

5 and 6,

W (0 CNEt ) Me

reveal

2

2

2

that

pentagonal

4

2

which

less

and W ( 0 C N M e ) , 2

f i v e equivalent

plane

are

(using

metal

2

2

6

common,

shown

in V

bonds can r e a d i l y be s,

ρ ,

ρ ,


d

and


20

REACTIVITY

(M=M) at

Compounds

present,

(6).

not common.

i n both M o ( 0 P r )

with

respect t o each metal

2

based pyramidal

VII 0 = OPr

number 5 i s , however,

(7) and M o ( 0 B u ) ( C 0 )

( 8 ) , which have,

t

8

2

6

atom, t r i g o n a l

bipyramidal

and square

s t r u c t u r e s , r e s p e c t i v e l y . See VII and VIII below.

VIII 1

BONDS

Compounds containing M=M bonds are,

The c o o r d i n a t i o n

seen

1

O FM E T A L - M E T A L

0 = OBu

1


2.

10+

(M-M) and

of

Transition

Compounds.

tungsten

nature

Single

bridging

ligands

21

Complexes

M-M bonds

the +5 o x i d a t i o n

in

the

Metal

state (9L

formed by molybdenum are dependent

For example,

on the

MO^CI-JQ

is

paramagnetic

(JO) and does not show s t r u c t u r a l evidence (Y\) f °

a

Oxygen

M-M bond.

bond of

ligands,

however,

Mo^XîOPr )^

compounds

1

(]3)

with

number 6 i s tions

of

metal

which

Mo-to-Mo

seen

metal

inbetween

i n IX. d

cases

have

from the i n t e r a c -

their

An octahedral

the sake

the a v a i l a b l e

known

examples

Cp Mo (C0) 2

atoms

brevity,

I

I

in

dimers),

6

d

of

(M-M)

n

d

electrons

the l a t t e r

state

X = Br or C I

have

r e s t r i c t e d my a t t e n t i o n

electrons

n

are used t o form M-M

(2.448(1)

which

M-M bonds

i n which not

to M-M bonding.

both

contain

+1 (formally

Well

(ΜΞΜ) (J_5) and

are Cp^MoîCO)^

they

molybdenum are d^-d^

of the ΕΑΝ r u l e and the observed

and 3 . 2 3 5 ( 1 )

t o have M-M t r i p l e and s i n g l e

having

compounds

contribute

number

but by c o n s i d e r a t i o n s

distances

bond.

order and have considered only the

( J 6 ^ compounds

oxidation

M-M s i n g l e

^X

( J 4 ) and d i n u c l e a r

order

directed

geometry f o r each 1

There a r e , however, d i n u c l e a r compounds

fractional

2

of

L'-0^

of i n t e g r a l

lobes

OR

R

where a l l the a v a i l a b l e

bonds.

M-M

the s t r u c t u r e shown i n IX

i s thus well suited f o r t h i s type of d - d

For

the M-M

characterizations

1

here to M-M bonds

r

of 2 . 7 3 Ä . The c o o r d i n a t i o n

which

ligands.

X*^

all

to favor

The M-M bond a r i s e s

orbitals

the bridging

have

distances

OR

of

appear

(]2) as i s seen i n the recent s t r u c t u r a l

below Downloaded by IOWA STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch002

Dinuclear

CHISHOLM

8)

are

commonly considered

bonds, r e s p e c t i v e l y .


22

REACTIVITY

Ligand S u b s t i t u t i o n Mononuclear cally

labile

Cr(3+),

Co(2+) are ty

labile.


substitution

"inert" Group ples

are

classified

important

the

size of

as

kineti-

and undergo

in

of

metal

latter

ligands,

rates

the

electrons

ΕΑΝ

substitution

6 transition

determining

the

the

e.g.

and high

spin

differences in kinetic l a b i l i

valence

satisfy

BONDS

ligand s u b s t i t u t i o n r e a c t i o n s :

are i n e r t , while Cr(2+)

number

which

of the

be broadly

These dramatic

are

and the

complexes

can

r a t i o n a l i z e d by ligand f i e l d c o n s i d e r a t i o n s .

which

charge


(17)

i n e r t toward and Pt(4+)

are e a s i l y

factors

compounds

or

Co(3+)

Reactions

O F

of

formal

ligand positive

on the metal.

Many

are

substitutionally

by an i n i t i a l

l o s s of a l i g a n d .

carbonyl

rule

Other

compounds

provide

good exam

phenomenon. On the other hand, the c o o r d i n a -

t i v e l y unsaturated square planar complexes of the group 8 t r a n s i tion

elements

labilizing which is

allow

All

of

these

of

the

planar

control

in

considerations

position

(S 2).

and

to the group phenomenon,

octahedral

isolation

The

N

trans-effect

the

2

4

+

with

4

excess

of

carry

and tungsten.

(M=M)^

Kinetically,

addition

square

molybdenum

Thus, Mo R (PMe^) PMe^

for

kinetic

around the

(6).

in the trans

reactions

of

complexes isomers

of

and square planar compounds.

chemistry

ed

association

substitution,

documented

can

ligand

a group

undergoing

octahedral

tions

by

e f f e c t of

is

well

and

react

and

they

(ΜΞΜ)^

are

compounds PMe^

to

over Ligand

than

(ΜΞΜ) w i l l a

the d i n u c l e a r

substitution

reac

moieties are well document

+

slower

give

into

the

nmr

time-scale.

not exchange coordinated

single

a d i f f e r e n t phosphine w i l l

PMe^

resonance,

lead to r a p i d

but

scrambling

on the s y n t h e t i c t i m e - s c a l e . In

the

reagents

LiR

r e t e n t i o n of formed

which

reaction (2

between

equiv),

configuration then

anti-W Cl (NEt ) 2

substitution 08).

isomerizes

to

2

2

of

Kinetically a mixture

4

and

alkyllithium

Cl-by-R

occurs

with

anti-W R (NEt ) 2

of

anti

2

and

rotamers, with the l a t t e r being the favored rotamer. This t u t i o n r e a c t i o n can be viewed as an example of an S 2 f

2

4

gauche substi

process


is

in

2.

Dinuclear

CHISHOLM

Transition

which the new bond i s square the

plane

cogging

rotation Ligand

formed as

of

the

substitution

isomerization. substitution

(CH SiMe ) 3

readily

and

2

(CH SiMe ) 2

3

HNMe

Addition

yields

4

1,1-

whereas with C 0

of

give

Bu 0H

and

formed

to

1,12

while

the

not

react.

these

2

2

Clearly,

dinuclear

a

control in

3

rich

compounds

4

substitution and

1,2-Mo (NMe ) 2

do

not

to

2

2

isomerize

1,2-Mo (NMe ) 2

3

2

yields

1,2-isomer

be

2

respec

4

1,1-isomer

chemistry

remains

in the

(ΜΞΜ) r e a c t

4

2

l,T-Mo (NMe )(0 CNMe )(CH SiMe ) , 2

anti=—^

1,2-Mo (0Bu ) (CH SiMe ) , 2

to

(20).

seen

3

and

the

2

)

t

25°C),

2

is

and

atmos,

2

(1

mol than

2

1,1-

once

t

2

because

barrier

kinetic

center

2

which

of

Kcal

1 ,2-Mo Br (CH SiMe )

to

2

24

faster

example

of

arises

an energy

= ca.

t

dimolybdenum

respectively,

4 >

(21_).

tively,

a

produces

ft

broken w i t h i n a

rotamer

kinetically

Another

at

Hexane s o l u t i o n s

LiNMe

(E

thus

23

the o l d bond i s

groups

9

bond

is

ligand

2

NR

M=M

gauche

with

Complexes

Formation of the anti

effect

about

following.


(^9).

Metal

does

surrounds

explored

and

exploited.

Stereochemical Three

types

(1)

rotations

at

each

change

Lability of

stereochemical

about M-M bonds,

metal

center

and

(2) c i s ^ .

(3)

have been observed:

>trans

isomerizations

bridge^^terminal

ligand

ex

processes.

Rotations time-scale) bond.

In

have

the

pounds

ΜΞΜ bond

the

compounds c o n t a i n i n g σ ^ by

the and

rotation

of

is

than

ca.

steric

neutral

rotation

properties 2

15 Kcal

9 Kcal

2

the 3

2

and

4

to

be

ligands. show

E

which only

The com for

A c t

M-M

2

conformers

3

have

4

not been frozen

(22).

compounds M o ( 0 R ) L , nitrogen donor

2

moiety,

m o l " ^ . For M o M e ( C H S i M e ) , the b a r r i e r

mol'^

2

2

+

appears

of

(on the nmr

would rupture the δ (ΜΞΜ)^

1,2-Mo (NMe ) (CH SiMe )

out on the nmr t i m e - s c a l e The

are not observed

the c e n t r a l

configuration,

2

1,1-

less

about

and are not expected since t h i s

restricted

a

lability

6

2

where R = a l k y l

ligand,

or SiMe

contain three oxygen

3

and L =

atoms


and

24

R E A C T I V I T Y

one

nitrogen

25).

Thus,

which

is

the OR

atom coordinated

trans

each molybdenum

atom

B O N D S

(23,24,-

to the N atom. At low temperatures in toluene-dg,

nmr spectra are c o n s i s t e n t with the expected 2:1 r a t i o of

OR

described

ligands

MoÔPr )g(NCNMe ), an asymmetric

ca.

there i s +35°C,

ratio

is

exchange thus,

the

the

temperature,

all

time-scale.

For

nmr are

different

Mo^t p-NCNMe^) moiety (26). reveals

four

types

because

Here the low

of

OR ligands

r a t i o 1:1:2:2. Upon r a i s i n g the tempera-

a collapse

consistent

of

a pair

three OPr with

the

of these

s i g n a l s to give

resonances

1

view

that

in

cis

the

at

integral

^ ^ trans

OPr

1

r a p i d at one molybdenum, but not at the other. These

processes

parallel

nate

atoms

and 220 MHz,

3:2:1,

exchange

molybdenum

spectrum

in the expected i n t e g r a l ture,

on

central

limiting

Upon r a i s i n g

equivalent

the

2

temperature

above.

become

1



there are a p a i r of mutually trans OR ligands and one

ligands

the

of

to

O F

have

been

shown

the common l a b i l i t y

mononuclear

complexes

(27).

to

be

intramolecular

associated For

the

and

with f i v e c o o r d i -

dinuclear

compounds,

however, the f i f t h c o o r d i n a t i o n s i t e i s the other metal atom. The (M=M)

compounds (8),

MoÔPr ^

which

contain

b r i d g e ^ = ^ = terminal larly,

the

change

between

group

compounds

(7)

and

bridging

OR

ligands,

2

bridging (j>).

Organometal1ic

Reactions

MoÔBuÛ-CO) show

rapid

exchange on the nmr t i m e - s c a l e .

W Me (0 CNEt )

nmr t i m e - s c a l e

Tolman (28)

(M=M)

1

2

and

2

2

4

terminal

and W ( 0 C N M e ) 2

2

2

carbamato

6

Simi-

show

ligands

has suggested that a l l commonly o c c u r r i n g

on

exthe

organ-

ometal l i e r e a c t i o n s , i n c l u d i n g those that are important in c a t a l ysis,

can

be c l a s s i f i e d

microscopic tion,

(2)

reverse:

Lewis

(1)

by

f i v e named r e a c t i o n s ,

Lewis

acid a s s o c i a t i o n

base

and d i s s o c i a t i o n ,

and d e i n s e r t i o n (ligand migration r e a c t i o n s ) , tion

and

reductive

reductive

elimination,

decoupling.

With

the

each with

association

and

(5)

(4)

of

dissocia-

(3)

insertion

oxidative addi-

oxidative

exception

its

and

the

coupling simple


and Lewis

2.

Dinuclear

C H I S H O L M

acid

Transition

Metal

association-dissociation

Complexes

reactions,

25

we have

studied

exam-

p l e s of a l l of the aforementioned r e a c t i o n s .

Lewis

Base

Association

Dissociation.

alkoxides

MîOR)^ r e v e r s i b l y add donor

compounds

(23,24):

the

bulkiness

of

the

c.f.

(23,24)

position

R

addition reactions, Downloaded by IOWA STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch002

and

and

L.

of In

these

reversible

Mo = 2.242(1) Ä i n

Mo (OSiMe ) (HNMe ) ,

do

18

not

attain

However,

for

an

compounds

atoms

have

(CO)^

compounds

three

one,

reversible

the metal

atoms

configuration.

i n which the metal

the formation

the

Π6)

of

Mo-to-Mo

found

Reactions.

M (0R)^

compounds

2

compounds

(29).

An

catalysis

by t r a c e s

and

C0

the

Cp M ~ 2

2

two new

for

distances Cp Mo (C0) 2

2

of and

4

of

example

of

a

facile

is

in

the

reac

seen

which

2

These r e a c t i o n s

by a d i r e c t i n s e r t i o n mechanism ing

bonds

with Mo-to-

6

of e l e c t r o n s , e.g.

insertion-deinsertion reaction

between 2

since

2

ΜΞΜ

c.f.

3.235(1 ) Ä

Insertion-Deinsertion

2

3

respectively.

2

(0 C0R)

base

occur only with a reduction in M-M bond

to

and

2

electronic

shell

(M = Mo and W),

Π5)

Cp Mo (C0)^,

tions

shell

containing

bonds w i l l

from

2.448(1) Ä 2

2

a completed valence

metal-ligand order,

6

valence

Lewis

are e s s e n t i a l l y unchanged, 2

3

MîOR)^

dependent on

= 2.222(1) Ä i n Mo (0CH CMe ) 2

dinuclear

to give

equilibrium is

the M-M distances

Mo-to-Mo

ligands

The

were

give

shown

M (0R) ~ 2

4

to proceed

( i . e . not by a mechanism i n v o l v

alcohols)

with energies of

activation

of not greater than 20 Kcal m o l " . 1

Oxidative-Addition studying of

oxidative

ΡΓ 00ΡΓ ί

the

leads

ί

halogens

Mo (0Pr ) X n

2

6

additions, two

to

4

additions to

Cl , 2

(M-M)

Br

is

MoÔPr ^ and

I

change

achieved.

Kirkpatrick

to M o ^ O P r ) ^ (M=M)

1

2

Chuck 1

(7).

proceeds

2

where X = CI,

a stepwise

one,

Reactions.

in

Br M-M

A large

or

has

(ΜΞΜ) (30).

been

Addition

A d d i t i o n of each of

to

give

I.

In

these

bond order, number of

the

compounds oxidative-

from three

related

to

additions

have been noted, but await d e t a i l e d s t r u c t u r a l c h a r a c t e r i z a t i o n s .


26

R E A C T I V I T Y

The

formation

of

W^tNMe^)^ and PÔH addition: view

of

W^(p - H ) ( 0 P r ) ^

(excess)

WÔPr )^ the

the

reaction

i[W^(μ -Η) (OPr )^] skeleton

molecule

is

with

and long

(3.407(1) ft) W-to-W distances

is


Curtis

and coworkers

of

between

alternating

short

their

The

(2.446(1) Ä)

c o n s i s t e n t with the pres-

have also

in

below.

respectively.

documented a number of

studies

of the r e a c t i v i t y

Cp Mo (C0) . 2

2

4

Reductive ries

(32^)

reactions

2

shown

ence of W-to-W double and non-bonding d i s t a n c e s ,

oxidative-addition

B O N D S

(31_). An ORTEP

1

W^y-HÔ-^

centrosymmetric


can also be viewed as an o x i d a t i v e -

PrVrl -

central

in

n

2

1

O F

of

El imi n a t i o n s .

1,2-M R (NMe ) 2

2

2

between 1 , 2 - M C l ( N M e ) 2

equiv),

2

2

compounds

4

pathways

affords of

particularly

alkyl

the

4

(33,34)

i - P r , n-Bu,

opportunity of

groups

interesting

clear σ-alkyl

complexes

coordinated comparisons

of

the

Figure 1

structure (34).

This

of

an extensive

from

the

the view

to can

2

emphasizes

the

dimetal be

made

(2 and

3

decomposition centers. with

elements,

Some

mononu

which have

(35).

Mo Et (NMe ) 2

LiR

2

studying

se-

reaction

sec-Bu, t - B u , CH CMe

transition

been the subject of much i n v e s t i g a t i o n The

of

(ΜΞΜ) and a l k y l l i t h i u m reagents,

where R = CH^, E t ,

Ch^SiMe^,

The synthesis

2

4

the

molecule

is

virtual

C

?


shown

in

axis

of

2.

Dinuclear

C H I S H O L M

Transition

symmetry which

exists

the

were

hydrogens

Metal

f o r gauche located

of

the

C-H

bonds.

Two

27

1^-M^X^tNMe^J^

and

stereoview of the molecule i s

Complexes

refined,

and

is


are

limits shown

which, C-C

is

the

important

points

can

bond,

from

indicate (36^).

along group

relatively

is

short

e-H-to-Mo

the

M-M

absence

axis

CH—Mo

hydrogen

atoms

to

The

contained

distances

longer

molybdenum

be

are

molecules

on the d i s t a l

"what of

the

about Mo-N, to

2.4 Ä,

the

ethyl

groups

this

distances

central Mo-C

as

directly

bonded.

provide

β and γ

in

and

is

hydrogens

3.25(5) Ä,

6-H—Mo

planar,

atom

from

This

are each

introduces

molybdenum groups,

atoms

but

and

only

a

the

little

proximal methyl hydrogens and

rigid,

CH---Mo

groups the

distances

readily

answered:

but

allowing

rotations

atom of

in

is

distances of

between the methyl

molybdenum

to

ethyl

shown i n Figure 3. The

question

unit

The methyl

are

plane.

natural

Figure 4, the

the

are

as

atoms

the ΜΟΞΜΟ bond. This, leads one

closest

Mo^N^C^

Thus,

significant

and C-C bonds produces C H — M o

shown

ligand

this

methyl

This

any

which

as

within

conformation about the

hydrogen

the

across

type?"

a

(1) the

is,

which

one

involving

are the

to

units,

within

between

are the distances

wonder,

and

distances

other molybdenum atoms

keeping

seen:

two 3 -hydrogen

atom

of

(2) The MoNC^

the

shortest

the

the

together with the staggered

interaction

methyl

Newman p r o j e c t i o n below,

σ-bonded.

taken

aligned

Figure 2

of experimental e r r o r , p e r f e c t l y staggered. in

equidistant

ligand

in

All

shown which shows the o r i e n t a t i o n s

conformation about the C-C bonds of the ethyl the

compounds.

the

to

which

ethyl

2.3

protons it

is

ligands

two molybdenum atoms

Η — M o distances across the ΜΟΞΜΟ bond are the s h o r t e s t .


of not

thus

and the


REACTIVITY

O F


BONDS

Figure 1. An ORTEP view of the Mo Et (NMe ) molecule emphasizing the vir tual C axis of symmetry. Pertinent structural parameters are Mo Mo (M=M) = 2.206(1) A, Mo—N = 1.96 A (av), Mo—C = 2.18 A (av), Mo—Mo—N angle = 103° (av) and Mo—Mo—C angle = 101° (av). 2

2

2 h

2

Figure 2. Stereoview of the Μο Εί (ΝΜβ )ι, molecule looking down the bond. This stick view of the molecule emphasizes the positions of all the atoms. 2

2

2

Mo—Mo hydrogen


CHISHOLM

Dinuclear

Transition

Metal

Complexes


H(^0)

Mo(l)

Mo(2)

Figure 3. A line drawing of one Mo NC> fragment showing the Mo HC distances that arise to the two hydrogen atoms that are confined in the plane of the Mo—NC, unit. (Mo(2) H(44) 2.77 A; Mo(2) H(40) 3.18 A; Mo(l) H(40)2.96 A) 2

Figure 4. A line drawing of one Mo,-ethyl fragment showing the short CH Mo distance that arises from rotations about Mo—C and C—C bonds (Mo(l) Η 2.36 A;Mo(2) Η 2.97 A)


30

REACTIVITY

Although

the

below +100°C, reductive and

Bu.

(37)

is

type

fairly

of

compounds

C0

-

2

containing

2

2

products.

formed

and

cross-over

3

2

x

give

the

and Mo (PhN Ph)

4

2

3

2

has

been

experiments that

shown

this

by

of two

alkyl e.g. The

bonded

as the molybdenum 2

and CD CH D

the

appropriate

2

of

elimination

Pr

butane.

CH =CD

use

cause

quadruply

(38)

only

3

will

chemistry,

M-M

4

stable

when R = E t ,

1-butene +

When R = CH CD ,

it

substrates

alkenes,

+

BONDS

thermally

dialkylmononuclear

PhNNNHPh

Mo (0 CNMe ) 2

and

[(PPh ) Pt]

and

2

are

a number of

alkanes

common i n

2

compounds

4


reductive disproportionation

n

3

2

of

of

(PPh ) Pt(Bu )

are

2

elimination

addition


2

the a d d i t i o n of

This

ligands

Mo R (NMe )

O F

3

process i s

2

intramo

lecular. The r e a c t i o n between M o ( C H C H ) ( N M e ) 2

dg

has

been

build-up be

of

seen.

followed an

The

at

intermediate, spectrum

molecule

has

the

mid-point

of

the

2

4

2

2

and C0

4

nmr

in t o l u e n e -

2

spectroscopy.

namely M o E t ( N M e ) ( 0 C N M e ) 2

C

Mo-Mo

axis

2

2

to

of

bond,

2

this

2

2

2

is

found

following

esting

reaction

possibilities.

involved

in

the

The

slow

2

2

2

2

"Mo (NMe ) (0 CNMe ) 2

2

2

2

2

C0 -

Alternatively,

into

the Mo-NMe

2

2

Mo Et (0 CNMe ) , 2

pound

2

2

2

4

2

2

2

2

step

2

the

in

c.f.

(5)

2

4

reactive

slow of

2

the of

through for

poses two i n t e r

ethane

could

and

be

ethylene

which would then generate a species

2

highly

n

bonds

W Me (0 CNEt ) 2

2

reductive-elimination

from M o E t ( N M e ) ( 0 C N M e ) 2

2

that

(29)

2

2

is

the forma

t i o n of the intermediate M o E t ( N M e ) ( 0 C N M e ) 2

can

2

with the view

as

pathway

The

intermediate

symmetry passing

i.e.

The r e a c t i o n

2

2

e n t i r e l y consistent

a virtual

Μο (0Βυ^) (0 00Βυ^) ·

3

by

corresponding

shown i n F i g u r e 5 and i s the

2

-30°C

step

to

f u r t h e r r e a c t i o n with

could

involve

Mo Et (NMe ) (0 CNMe ) 2

2

the

which

2

2

2

2

structurally

then

rapidly

C0 2

2

to

insertion give

say,

c h a r a c t e r i z e d com

eliminates

ethane

and

ethylene.

Mo (NMe ) 2

2

6

+ ArNNNHAr

(excess)

-

Mo (NMe ) (ArN Ar) 2

2

4

3

2

(ΜΞΜ) + Me NH 2


(1)

2.

Dinuclear

CHISHOLM

Mo R (NMe ) 2

2

2

Transition

+ ArNNNHAr

4

Metal

(excess)

-

Mo R (NMe ) (ArN Ar) 2

Mo R (NMe ) 2

2

2

+ ArNNNHAr

4

Mo (ArN Ar) 2

We

have


pathways,

based

triazenes (3)

occurs. 3

has

relating

-

a

to

inclined

distinguish

toward

with

on

the

the

nature

When

structure

R =

of

where Ar = p - t o l y l ,

2

reactions

of

R.

Et

and

the

is

crystallographically

suppose

sonable

to

the M o R ( N M e ) ( A r N A r ) 2

that

2

2

of

unsaturated

molecules

alkane

reaction

former

observed

2

3

2

and alkene 2

time, nothing

CH

C

2

2

axis

generating

2

2

occurs

by

close

bond i s

symmmetry

i s not unrea initial

3

which

2

3

4

forma undergo

coordinatively react

rapidly

(M=M).

quantitative

it

is

and i r r e v e r s i b l e - no scrambling

of

a

rate

determining

9

CH—Mo

distance

formed, e l i m i n a t i o n of the

be

a

W R (NMe L ?

2

i s observed.

by the

ingly,

3

d e f i n i t i v e can be said concerning the

c y c l o m e t a l l a t i o n r e a c t i o n across

then

or

3

2

of

It

the

A p l a u s i b l e i n t e r p r e t a t i o n of these r e s u l t s tion

(2)

Mo Me (NMe ) ~

intimate mechanism of e l i m i n a t i o n beyond noting the f a c t s :

labels

with

reaction

n

compounds, which then

Mo (NMe ) (ArN Ar) 2

R =

Bu ,

3 proceeds v i a

2

intramolecular,

these

shown in Figure 6. The mole

with the excess t r i a z i n e to give M o ( A r N A r ) At t h i s

the

When

compound

imposed

tion

elimination

(3)

between

favoring

the two 4-coordinate molybdenum atoms.

of

(2)

2

2

occurs.

The molecular

(ArN Ar) , cule

2

(ΜΞΜ) + Me NH

2

2 and 3 above. The d i f f e r e n c e between

solely

reaction

3

+ alkane + 1-alkene + HNMe

analogy

1,

2

unable

are

on the

rests

3

(M=M)

been

we

in

2

(excess)

4

yet

shown

CH CMe , 2

as

though

pathway

and

3

2

31

Complexes

9

elimination slower

of

and e i t h e r

C0

shown i n Figure 4.

or

is

akin

to

a

suggested

Once the Mo-H

alkane i s r a p i d . Rather i n t e r e s t

step: 9

which

the M-M bond. This i s

alkene

final

step

i s that e l i m i n a

from the d i n u c l e a r center may analogous

reactions

triazines yield

involving

1 mole of


alkane


32

R E A C T I V I T Y

O F


B O N D S

Figure 5. H NMR spectrum recorded at —30°C, 100 MHz, during the reaction between Mo2Et (NMe ) and C0 , showing the formation of the intermediate Mo Et2(NMe )2(O CNMe ) , along with the products C H C H , and Mo (0 CNMe ) . The solvent is toluene-d and the protio impurities are indicated by (*). The signal noted by (**) corresponds to MoÔ^CNMe^^ which, because of its very low solubility, is mostly precipitated as it is formed. l

2

J

2

z k

li

2 h

2

g 2

2

h

2

6

2

2

8

Figure 6. An ORTEP view of the Mo>Me (NMe ) (C H N C H ) molecule (C,H = p-tolyl) viewed down the Mo=Mo bond emphasizing the C axis of molecular symmetry which relates the two 4-coordinate molybdenum atoms. The Mo Mo distance is 2.175(1) A and Mo—C = 2.193(4) A. 2

S

2 2

7

8

3

7

H 2

2


2.

Dinuclear

CHISHOLM

and as

yet

Transition

Metal

Complexes

uncharacterized tungsten

33

compounds

which

r e t a i n the

elements of the alkene. It tive

is

clear

from t h i s

brief

elimination

sequences

have

from the

presence of

summary that d i n u c l e a r reduc pathways

the dimetal

center,

a c c e s s i b l e to mononuclear complexes


Oxi dati ve-Coupli ng oxidative-coupling dimetal

center

and

and

are

which in

addition

Reducti ve-Decoupli ng.

in

uniquely to

those

(39).

reductive-decoupling

seen

arise

the work

of

of

Stone,

Examples

ligands

of

about

a

Knox

and t h e i r

or

functional

coworkers (40) and are not discussed here.

D i r e c t Attack on the M-M M u l t i p l e Bond A

number

groups

have

Cp Mo (C0) 2

2

enes In

small

been to

4

(41), all

of

found

give

allenes

of

4-electron

to

add

molecules

across

the

M-M

triple

bond

in

adducts C p M o ( C 0 ) ( u n ) , where un = a c e t y l 2

(42),

these donors

unsaturated

4

cyanamides

adducts, to

2

the

(43)

and thioketones

unsaturated

the dimetal

center,

fragments

(44).

act

as

thus reducing the M-M

bond order from three to one. The react

same group of

with

Mo (0Pr )g, n

2

characterized

by

a

observed

(45)

for

however,

with

a

unsaturated molecules have been found to but

full

as yet none of the adducts X-ray

study.

MoÔPr ) (NCNMe ) 1

6

mode

of

2

addition

The are

spectroscopic

entirely

directly

has been data

consistent,

analogous

to

that

observed above. Carbon monoxide has been shown the

ΜΞΜ bond of

VIII

before.

carbene-like class

of

Similar

We

2

have

addition

reactions

additions

reported

Mo (0Bu ) t

of

6

to

(8) to r e v e r s i b l y add across

give M o ^ O B u * ) ^ -CO)

previously

speculated

to

a M=M

bond

could

that

are

common

CO across

to

be

(46) one

dinuclear

Pd-Pd and Pt-Pt

(M=M). See that of

this

general

compounds.

bonds have been

(47).


34

REACTIVITY

O F


BONDS

A d d i t i o n of n i t r i c oxide to MîOR)^ compounds gives [M(0R) ~ 3

N0]

compounds. The l a t t e r do not c o n t a i n M-M bonds:

2

t u r e of

[Mo(0Pr ) N0] 3

shown i n VII, a formal

M-M bond i n


the

as

3

compounds

2

evidence

bonds

above)

by the

that

(2

equiv)

that

ligands

the such

WiOBu^)^-

also

and

2

have found that compounds c o n t a i n

cleaved

Mo (0Bu ) 2

6

the

addition

isocyanides.

(M=M)

t

by

reacts

are

eliminated azides to

2

from

to

give

both

2

the

certain

with each of molecular

and

t r i p l e bond i s

of

Very r e c e n t l y we have

a r y l azides ( >4 equiv) t Mo(NAr) (0Bu ) , r e s p e c t i v e l y . In

metal-metal

Mo(0) ?

reactions,

cleaved and the elements of 2Bu^0

metal

center.

Though

the

addition

of

low valent mononuclear t r a n s i t i o n metal complexes

is

known to g i v e

of

the ΜΞΜ bond seen in these r e a c t i o n s , once again,

the

In

addition

fact

has been s t r u c t u r a l l y confirmed f o r

are

(51)

the

aryl

a l s o supported ( i n

are r e a d i l y cleaved by donor

unsaturated molecules such as

oxygen t (OBu )

1

(49).

M=M

found

is

cited

Walton and coworkers (50) ing

MoÔPr )^

(ΜΞΝ->0). The absence of any d i r e c t

t r i p l e bonds

This

struc-

has been replaced by

1

[M(0R) N0]

pyridine.

r e l a t e d to that of

the

a Mo-to-Mo d i s t a n c e of 3.325(1) Ä (48).

bridged dimers

(NOMpy)

closely

the ΜΞΜ bond i n M o ^ O P r ) ^

structural

alkoxy

2

but has

sense,

two m e t a l - l i g a n d

to

is

1

a r y l i m i d o compounds

(52h

the ease of

cleavage

emphasizes

l a b i l i t y of the d i n u c l e a r center toward a d d i t i o n r e a c t i o n s .

Conclusions The

time

reactivity tial

for

catalytic

of

is

ripe

for

dinuclear

cyclic

has

of

by M u e t t e r t i e s ,

tively

hydrogenated to alkenes

rate

acetylene

et

determining adducts

metal

reactions,

step

al.

(53),

compounds. as

is

?

that

involves d

9

9

CO

the

The poten

It

for

has been

can be s e l e c by C p ^ o ^ C O ) ^ :

dissociation

(2) We

in

required

(1)

alkynes

(cj_s 2H-addition)

Cp Mo (C0) (R C ). ?

developments

already been r e a l i z e d .

shown,

the

exciting

transition

sequences

reactions,

truly

have

from

found


the that

2.

Dinuclear

CHISHOLM

μ-Η) (0Ρτ )

will

Ί

2

1-butene

to

speculate metal of

1 4

about

intimate

centers

will

f o r example

surely

mechanisms more

it

selectively

(54). While

impact fair of

35

Complexes

olefins:

ultimate is

prove

t h e i r analogues


the it

Metal

isomerize

cis-2-butene,

chemistry,

the

Transition

of

reactions

challenging

one can but

dinuclear

to say that

takes

transition

the e l u c i d a t i o n

at

dinuclear

and f a s c i n a t i n g

metal

than

did

at mononuclear c e n t e r s .

Acknowledgements I

thank

administered Naval

Research

by the American

Research

financial

Camille awards

and which

of

grateful Henry have

various

Society,

Science

aspects

of t h i s

Dreyfus allowed

my research

long

collaborations

Teacher-Scholar me a d d i t i o n a l

in this

area.

Fund

the O f f i c e

Foundation

to the A l f r e d P. Sloan

promoting time

the Petroleum Research

Chemical

and the National

support

particularly

Corporation,

for

work.

I

am also

Foundation and the Grant

degrees

Finally,

I

Program of

A&M U n i v e r s i t y ,

as well

as my own group

tions

are c i t e d i n the r e f e r e n c e s .

for

freedom i n

acknowledge my

with F. A l b e r t Cotton and coworkers

Texas

of

their

whose

at

contribu

Abstract All of the reaction chemistry surrounding mononuclear tran sition metal compounds could just as easily be carried out at a bimetallic center. reactions

Indeed, there should be additional types of

uniquely associated

with the M-M bond. This general

premise is discussed in light of the recent reactivity found for dimolybdenum and ditungsten compounds. For a given M X+ center, 2

the preferred coordination geometries,

ligand substitution beha

viors and dynamical properties (fluxional behavior) are noted. The ability of these compounds to undergo Lewis base association and

dissociation

reactions,

migrations, oxidative

reversible

insertions

or ligand

addition and reductive elimination reac


36

tions,

REACTIVITY OF METAL-METAL BONDS

as well

as direct the

additions across the M-M bond are

discussed.

Finally,

possible mechanisms for one of these

reactions,

reductive elimination of alkane and alkene from a


dimolybdenum center, is considered in detail.

Literature Cited 1. Cotton, F.A.; Wilkinson, G. "Advanced Inorganic Chemistry", 4th Edition, 1980, Wiley-Interscience. 2. For recent reviews, see (a) Cotton, F.A., Chem. Soc. Rev., 1975, 4, 27; (b) Cotton, F.A. Acc. Chem., Res., 1978, 11, 225; (c) Templeton, J.A. Prog. Inorg. Chem., 1979, 26, 211. 3. For recent reviews, see (a) Chisholm, M.H.; Cotton, F.A. Acc. Chem. Res., 1978, 11, 356 and (b) Chisholm, M.H. Faraday Society Symposium, 1980, 14, xxx. 4. For detailed calculations, see (a) ref. 2b; (b) Cotton, F.A.; Stanley, G.G.; Kalbacher, B.; Green, J.C.; Seddon, E.; Chisholm, M. H. Proc. Natl. Acad. Sci., U.S.A., 1977, 74, 3109; (c) Hall, M.B. J. Am. Chem. Soc., 1980, 102, 2104; (d) Bursten, B.E.; Cotton, F.A.; Green, J.C.; Seddon, E.A.; Stanley, G.G. J. Am. Chem. Soc., 1980, 102, 4579. 5. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Stults, B.R. Inorg. Chem., 1977, 16, 603. 6. Chisholm, M.H. Transition Metal Chemistry, 1978, 3, 321. 7. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Reichert, W.W. Inorg. Chem., 1978, 17, 2944. 8. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Kelly, R.L. J. Am. Chem. Soc., 1979, 101, 7645. 9. Cotton, F.A. Acc. Chem. Res., 1969, 2, 240. 10. Klemm, W.; Steinberg, H. Z. Anorg. Allgem. Chem., 1936, 227, 193. 11. Sands, D.E.; Zalkin, A. Acta Cryst., 1956, 12, 723. 12. See references to other studies cited in ref. 9. 13. Chisholm, M.H.; Kirkpatrick, C.C.; Huffman, J.C. J. Am. Chem. Soc., submitted.


2. CHISHOLM Dinuclear Transition Metal Complexes 37

14. 15. 16.


17.

18.

19.

20. 21. 22. 23. 24. 25.

Cotton, F.A.; Frenz, B.A.; Webb, T.R. J. Am. Chem. Soc., 1973, 95, 4431. Klinger, R.J.; Butler, W.; Curtis, M.D. J. Am. Chem. Soc., 1975, 97, 3535; idem, ibid, 1978, 100, 5034. Adams, R.D.; Collins, D.M.; Cotton, F.A. Inorg. Chem., 1974, 13, 1086. For general discussion, see (a) Basolo, F.; Pearson, R.G. "Inorganic Reaction Mechanisms", 2nd Ed., 1968, John Wiley Publishers; (b) Tobe, M.L. "Inorganic Reaction Mechanisms", 1972, T. Nelson Publishers; (c) Wilkins, R.G. "The Study of Kinetics and Mechanism of Reactions of Transition Metal Complexes", Allyn and Bacon Publishers, 1974. Chisholm, M.H.; Extine, M.W. J. Am. Chem. Soc., 1976, 98, 6393; Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Millar, M.; Stults, B.R. Inorg. Chem., 1977, 16, 320. See, Basolo, F.; Pearson, R.G. in "The Mechanisms of Inorganic Reactions", 2nd Ed., 1968, John Wiley and Sons Publishers, page 126. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Millar, M.; Stults, B.R. Inorg. Chem., 1976, 15, 2244. Chisholm, M.H.; Rothwell, I.P. J.C.S. Chem. Commun., 1980, xxx. Chisholm, M.H.; Rothwell, I.P. J. Am. Chem. Soc., 1980, 102, 5950. Chisholm, M.H.; Cotton, F.A.; Murillo, C.A.; Reichert, W.W. Inorg. Chem., 1977, 16, 1801. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Reichert, W.W. J. Am. Chem. Soc., 1978, 100, 153. See also the solid state structure and dynamical solution behavior of W (OPr ) (py) . Akiyama, M.; Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Haitko, D.A.; Little, D.; Fanwick, P.E. Inorg. Chem., 1979, 18, 2266. Chisholm, M.H.; Kelly, R.L. Inorg. Chem., 1979, 18, 2321. Muetterties, E.L. Acc. Chem. Res., 1970, 3, 266. i

2

26. 27.

6

2


REACTIVITY OF METAL-METAL BONDS

38

28. 29. 30.


31.

32. 33. 34. 35.

36.

37. 38. 39.

40.

41. 42.

Tolman, C.A. Chem. Soc. Rev., 1972, 1, 337. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Reichert, W.W. J. Am. Chem. Soc., 1978, 100, 1727. Chisholm, M.H.; Kirkpatrick, C.C.; Huffman, J.C. Inorg. Chem., in press. Akiyama, M.; Little, D.; Chisholm, M.H.; Haitko, D.A.; Cotton, F.A.; Extine, M.W. J. Am. Chem. Soc., 1979, 101, 2504. See Chapter 12 in this volume. Chisholm, M.H.; Haitko, D.A. J. Am. Chem. Soc., 1979, 101, 6784. Chisholm, M.H.; Haitko, D.A.; Huffman, J.C. J. Am. Chem. Soc., submitted. Kochi, J.K. "Organometallic Mechanisms and Catalysis", Academic Press Publishers, 1978, Ch. 12 and references therein. This contrasts with other significant and short CH---Mo interactions: Cotton, F.A.; Day, V.W. J.C.S. Chem. Commun., 1974, 415; Cotton, F.A.; LaCour, T.; Stanislowski, A.G. J. Am. Chem. Soc., 1974, 96, 754. Whitesides, G.M.; Gaasch, J.F.; Stedronsky, E.R. J. Am. Chem. Soc., 1972, 94, 5258. Cotton, F.A.; Rice, G.W.; Sekutowski, J.C. Inorg. Chem., 1979, 18, 1143. Reductive elimination: L M(H)R -> L M + R-H is generally much faster than by C-C bond formation. See, Norton, J.R. Acc. Chem. Res., 1979, 12, 139. Knox, S.A.R.; Stansfield, R.F.D.; Stone, F.G.A.; Winter, M.J.; Woodward, P. J.C.S. Chem. Commun., 1978, 221. See also, Chapter 13 in this volume. Bailey, W.I.; Chisholm, M.H.; Cotton, F.A.; Rankel, L.A. J. Am. Chem. Soc., 1978, 100, 5764. Bailey, W.I.; Chisholm, M.H.; Cotton, F.A.; Murillo, C.A.; Rankel, L.A. J. Am. Chem. Soc., 1978, 100, 802. n

n


2. CHISHOLM Dinuclear Transition Metal Complexes

43. 44.


45. 46. 47. 48. 49. 50. 51. 52. 53. 54.

39

Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Rankel, L.A. J. Am. Chem. Soc., 1978, 100, 807. Alper, H.; Silavwe, N.D.; Birnbaum, G.I.; Ahmed, F.R. J. Am. Chem. Soc., 1979, 101, 6582. Chisholm, M.H.; Kelly, R.L. Inorg. Chem., 1979, 18, 2321. Chisholm, M.H. Advances in Chemistry Series, 1979, 173, 396. See Chapters 8 and 9 in this volume and references therein. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Kelly, R.L. J. Am. Chem. Soc., 1978, 100, 3354. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Kelly, R.L. Inorg. Chem., 1979, 18, 116. Walton, R.A. Chapter 11 this volume. Chisholm, M.H.; Kirkpatrick, C.C.; Raterman, A, results to be published. Hillhouse, G.L. Ph.D. Thesis, Indiana University, 1980. Muetterties, E.L.; Slater, S. Inorg. Chem., in press. Akiyama, M.; Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Haitko, D.A.; Leonelli, J . ; Little, D. J. Am. Chem. Soc., in press.

RECEIVED November 21,

1980.


Anything One Can Do, Two Can Do, Too- And It's More Interesting

Recommend Documents