The Stabilization of Unusual Valence States and Coordination

This enabled us to isolate a discrete three-coordinated Cr(III) compound Cr(NPr 2 * ) 3 which, since it is a coordinatively unsaturated molecule, was ...
0 downloads 0 Views 720KB Size
Download PDF
22 The Stabilization of Unusual Valence States and Coordination Numbers by Bulky Ligands

Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch022

D. C. BRADLEY Queen Mary College, Mile End Rd., London El 4NS, Great Britain

The use of bulky organic ligands containing O- or N-donor atoms in stabilizing unusually low coordination numbers and oxidation states by metals is described. The work mainly concerns the effect of tertiary alkoxo groups and bulky dialkylamido groups such as NPr i and N(SiMe ) on transition metals and lanthanides. Supplementary ligands such as NO, THF, and P(C H ) are sometimes involved. The systems characterized include four-coordinated Nb(IV), Ta(V), Cr(II), and Cr(IV); three-coordinated M(III) (M = Al, Ga, Sc, Ti, V, Cr, Fe, and lanthanides), Μ(II) (Μ = Mn, Co, and Ni), and M(I) (M = Co, Ni, and Cu); and two­ -coordinated M(II) (M = Mn, Co, Ni, Zn, Cd, and Hg) and M(I) (M = Au). 2

6

3

2

5 3

" Q y t h e use of o r g a n i c l i g a n d s c o n t a i n i n g s u i t a b l e d o n o r a t o m s — v i z . : - °

XR

X

and χ =

w i t h X — 0 or S a n d χ =

1: X =

Ν or Ρ a n d χ — 2; a n d X — C

3 — a n d b y s u i t a b l e c h o i c e of t h e a l k y l or a r y l R groups, i t is

p o s s i b l e to v a r y the b u l k i n e s s of the l i g a n d a n d t h e r e b y c o n t r o l t h e c o o r d i n a t i o n n u m b e r of the m e t a l to w h i c h these l i g a n d s are

bonded.

I n g e n e r a l , this steric c o n t r o l leads to the i m p o s i t i o n of a n u n u s u a l l y l o w c o o r d i n a t i o n n u m b e r that often p r o d u c e s a discrete m o n o m e r i c

compound

t h a t is a p p r e c i a b l y v o l a t i l e a n d s o l u b l e i n o r g a n i c solvents.

A n early

e x a m p l e of a d e l i b e r a t e a t t e m p t to r e s t r i c t the c o o r d i n a t i o n i n m e t a l a l k o x i d e s , w h i c h are n o r m a l l y o l i g o m e r i c [ M ( O R ) ] n as a result of the a ?

presence of a l k o x o b r i d g e s , w a s t h e synthesis of t h e v o l a t i l e m o n o m e r i c z i r c o n i u m tetra-terf-butoxide Z r ( O C M e ) 3

m e t a l alkoxides, see

Refs. 2, 3, 4, a n d 5.)

4

(J).

( F o r reviews on

the

I n c e r t a i n cases, the conse-

266 King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

22.

Bulky

BRADLEY

267

Ligands

quences of l i g a n d steric h i n d r a n c e are severe e n o u g h to cause either a b r e a k d o w n of the l i g a n d or a n e l e c t r o n i c r e a r r a n g e m e n t i n v o l v i n g the c e n t r a l m e t a l t h a t leads to the s t a b i l i z a t i o n of a n u n u s u a l v a l e n c y state. T h e steric effect of the d i a l k y l a m i d o g r o u p ( X = greater t h a n t h a t of the alkoxo g r o u p ( X =

Ν; χ — 2 ) is m u c h

0; χ =

1) c o n t a i n i n g t h e

same a l k y l g r o u p , a n d recent w o r k has c o n c e n t r a t e d i n the field of t h e m e t a l d i a l k y l a m i d e s (4,

5).

B e s i d e s b i n a r y c o m p o u n d s ( M L * ) c o n t a i n i n g the u n i n e g a t i v e l i g a n d L =

X R , . , a n u m b e r of t e r n a r y ( M L ^ L ' y ) a n d q u a t e r n a r y ( M L J L ' y L " * )

c o m p l e x e s w e r e also i s o l a t e d b y use of s u p p l e m e n t a r y l i g a n d s L ' a n d L " , a n d there is scope for f u r t h e r e x p l o i t a t i o n of the

field.

I n this a c c o u n t ,

Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch022

w e r e s t r i c t the d i s c u s s i o n to m e t a l alkoxides a n d m e t a l d i a l k y l a m i d e s since the m e t a l a l k y l s are t h e subject of papers b y L a p p e r t et ah

{6,7).

F o r c o n v e n i e n c e of p r e s e n t a t i o n , the a v a i l a b l e m a t e r i a l , w h i c h relates p r e d o m i n a n t l y to the

first-row

t r a n s i t i o n metals, is o r g a n i z e d a c c o r d i n g

to the p a r t i c u l a r u n i n e g a t i v e l i g a n d t y p e .

Tertiary Alkoxo

Ligands

M o n o m e r i c species M(OR-tert)

x

h a v e b e e n c h a r a c t e r i z e d for t i t a ­

n i u m , v a n a d i u m , c h r o m i u m , z i r c o n i u m , a n d h a f n i u m (x = n i o b i u m a n d t a n t a l u m (x =

5).

4)

a n d for

W i t h c h r o m i u m i t was f o u n d t h a t l i m i t ­

i n g C r ( I I I ) to c o o r d i n a t i o n n u m b e r 4 i n the d i m e r i c C r ^ O B u ' ^

caused

instability and a remarkable facility toward valency disproportionation or o x i d a t i o n to the stable q u a d r i c o v a l e n t C r ( O B u ) i

4

(8, 9 ) .

m o l y b d e n u m f o r m e d a stable d i m e r i c t r i - t e r f - b u t o x i d e (OBu')

I n contrast,

(BuO) Mo==Mo3

w h i c h is d i a m a g n e t i c a n d p r e s u m a b l y b o u n d b y a m e t a l - m e t a l

3

t r i p l e b o n d (10, 11).

Y e t another i n t e r e s t i n g feature of c h r o m i u m is the

synthesis of a stable d i a m a g n e t i c n i t r o s y l C r ( N O ) ( O B u * )

3

i n w h i c h the

n i t r i c o x i d e is b e l i e v e d to act as a three-electron d o n o r w i t h f o r m a t i o n of a four-coordinated low spin c h r o m i u m ( I I ) b i l i t y of C r ( O B u ' ) 2

Cr(OBu ) f

6

compound

(12).

T h e insta­

a n d the s t a b i l i t y of b o t h C r ( N O ) ( O B u ' )

3

and

m u s t result f r o m the steric effects of t h e t e r t i a r y b u t o x o groups

4

since t h e less b u l k y n o r m a l a l k o x o groups f o r m v e r y stable p o l y m e r i c [ C r ( O R ) ] . c o m p o u n d s i n w h i c h t h e C r ( I I I ) has its u s u a l c o o r d i n a t i o n 3

a

n u m b e r of 6 ( o c t a h e d r a l ) .

Dialkylamido

Ligands

W i t h the s t e r i c a l l y less d e m a n d i n g d i m e t h y l a m i d o g r o u p , a r a n g e of binary

compounds

MNMe , 2

M ( N M e ) , and M ( N M e ) 2

5

2

6

M(NMe ) , 2

2

M(NMe ) , 2

has b e e n c h a r a c t e r i z e d (4,

3

5).

M(NMe ) , 2

4

However,

King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

268

INORGANIC

w i t h the higher homologues N R

2

COMPOUNDS WITH

(R =

UNUSUAL

PROPERTIES

E t , P r , B u , etc.), complications w

w

arise. W i t h N b ( V ) a n d T a ( V ) t h e p r e d o m i n a n t p r o d u c t s are N b ( N R ) 4 2

and T a ( N R ) ( N R ) 2

3

r e s p e c t i v e l y (13,

14).

T h i s i m p l i e s a greater sta­

b i l i t y of c o v a l e n t N b ( I V ) c o m p a r e d w i t h T a ( I V ) a n d a greater t e n d e n c y of t h e 5d e l e m e n t to f o r m a stable i m i d o d o u b l e b o n d T a = N R . C r ( I I I ) , the u n s t a b l e f o u r - c o o r d i n a t e d

dimer C r ( N R ) 2

2

c o n v e r t e d to the stable C r ( I V ) c o m p o u n d C r ( N R ) pounds C r R 18),

terf-butoxo

ligands.

( 8, 9, 15)

thus com­

4

w i t h b u l k y a l k y l groups w e r e r e c e n t l y p r e p a r e d (16,

4

17,

a n d t h e r e is n o d o u b t t h a t f o u r - c o o r d i n a t e d C r ( I V ) is q u i t e stable

i n non-aqueous

systems.

M o l y b d e n u m a n d tungsten behave differently

f r o m c h r o m i u m i n t h a t stable t r i s d i a l k y l a m i d e d i m e r s

Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch022

With

was readily

Stable C r ( I V )

2

paralleling the reaction w i t h

6

(NR ) 2

(R N) M=E=M3

2

are the p r e d o m i n a n t p r o d u c t s s t a r t i n g f r o m M o ( V ) a n d W ( V I )

3

c h l o r i d e s (10, M o and W

M o r e o v e r there are significant differences

11).

between

w i t h t h e 5d e l e m e n t p r e f e r r i n g to f o r m i m i d o c o m p o u n d s

W(NR) (NR ) . 2

2

2

W i t h the 3d elements M n , F e , C o , a n d N i , reactions of M C 1 or M C 1 2

w i t h L i N E t o c a u s e d d r a s t i c m o d i f i c a t i o n of the N E t

3

ligands w i t h re­

2

s u l t a n t f o r m a t i o n of o r g a n o - n i t r o g e n l i g a n d s t y p i f i e d b y t h e c o b a l t b i s chelated

complex

i n v o l v i n g N N ' - d i e t h y l b u t a n e - l ^ - d i i m i n e (19).

The

m e c h a n i s m s of these u n u s u a l reactions are n o w u n k n o w n , b u t i t seems l i k e l y that steric h i n d r a n c e p l a y s a s u b s i d i a r y r o l e w i t h e l e c t r o n i c factors p r e d o m i n a t i n g since t h e N E t 7Γ-Ν

2

g r o u p m a y f u n c t i o n as a σ-Ν

donor and a

d o n o r l i g a n d w h i c h leads to a b u i l d u p of n e g a t i v e c h a r g e o n t h e

central metal. T h e d i i s o p r o p y l a m i d o g r o u p N P r * is e x t r e m e l y b u l k y . T h i s e n a b l e d 2

us to isolate a discrete t h r e e - c o o r d i n a t e d C r ( I I I ) c o m p o u n d

Cr(NPr *) 2

3

w h i c h , since i t is a c o o r d i n a t i v e l y u n s a t u r a t e d m o l e c u l e , w a s e x c e e d i n g l y r e a c t i v e c h e m i c a l l y b u t stable t h e r m o d y n a m i c a l l y w i t h no t e n d e n c y d i s p r o p o r t i o n a t e to C r ( I V ) a n d C r ( I I ) (20, 21, 22).

to

W i t h nitric oxide, a

stable d i a m a g n e t i c m o n o n i t r o s y l C r ( N O ) ( N P r * ) f o r m e d r e a d i l y w h i c h 2

t o l e r a t e d e x c h a n g e of N P r * groups b y 2

N O ( R e a c t i o n s 1 a n d 2 ) (12). Cr(NO) (NPr

3

i

3

terf-butoxo

groups w i t h o u t loss of

It was d e m o n s t r a t e d b y v a r i a b l e t e m p e r a -

) + 2 B u O H -> C r ( N O ) ( O B u < ) ( N P r * ) + 3

2

2

2ΡΓ*ΝΗ 2

(1) C r ( N O ) (ΝΡΓ *) + 2

ture Ή

and

1 3

3

3BuOH

Cr(NO) (OBu'h +

SPr^NH

(2)

C N M R studies t h a t the i n t e r e s t i n g q u a t e r n a r y c o m p o u n d

Cr(NO)(OBu ) (NPr