MetalMetal Bond Making and Breaking in Binuclear Complexes with

8 Metal-Metal Bond Making and Breaking in Binuclear Complexes with Phosphine Bridging

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch008

Ligands ALAN L. BALCH Department of Chemistry, University of California—Davis, Davis, CA 95616 Polyfunctional phosphine ligands make useful bridging ligands for constructing binuclear metal complexes. This article is concerned with the use of two ligands - bis(diphenylphosphino)methane (dpm) and 2-diphenylphosphinopyridine (Ph Ppy)-in promoting metal-metal bond forming and breaking reactions. Three specific topics will be covered. These are the ability of dpm to modify the stability of Rh-Rh bonds in interrelated compounds of Rh(I), Rh(II) and Rh(III), the utility of dpm bridging ligands in allowing novel metal-metal bond opening and closing in low valent rhodium and palladium complexes, and the usefulness of PhPpy in the stepwise construction of binuclear complexes. 2

2

Modification of Metal-Metal Bonding in Rhodium Complexes by a Bridging Diphosphine. The yellow, planar complexes, (RNC) Rh , undergo novel self-association reactions in concentrated solution to form the blue or violet dimers, (RNC) Rh2 , via reaction (1) (1,2). The equilibrium constants for this reaction are strongly +

4

2+

8

L (1)

+

2L—Rh—L L

L \

L / Rh---

/

\

l_

2+

/

L = RNC influenced by solvent and by the size of the substituent on the isocyanide ligand. Not unexpectedly (t-BuNC) Rh with its bulky substituents is most reluctant to dimerize. Substitution of the bidentate dpm for half of the isocyanide ligands produces the blue, dimeric cation 1. (3) These dimers, which can be prepared with various isocyanide substituents including t-butyl, show no tendency to dissociate into monomers. Similar stabilization of the dimeric structure can be achieved by the use of bifunctional isocyanides like 1,3-diisocyanopropane. (4,5) These form qua+

4

0097-6156/81/0155-0167$05.00/0 © 1981 American Chemical Society In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

REACTIVITY

168

OF

METAL-METAL

BONDS

2 +

P—Ph

Ph-P 'CH ' Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch008

2

druply bridged cations 2. The c o l o r and s t a b i l i t y o f the Rh-Rh bonds in ( R N C ) R h , 1 and 2 may be understood by reference to 2 +

8

2

2 +

2 the molecular o r b i t a l diagram i n Figure 1. This diagram shows the i n t e r a c t i o n o f the o u t - o f - p l a n e o r b i t a l s , which are the o r b i t a l s of the complex most s t r o n g l y a f f e c t e d by s e l f - a s s o c i a t i o n . The occupied o r b i t a l s of the dimer are s t a b i l i z e d s i n c e the upper and lower sets of dimer o r b i t a l s , which are derived from the mono mer aig and a u o r b i t a l s r e s p e c t i v e l y , have the same symmetries. Consequently, the net Rh-Rh i n t e r a c t i o n i s bonding; although the f o r m a l l y antibonding Rh-Rh σ o r b i t a l i s doubly occupied. The blue c o l o r r e s u l t s from a lowering o f the HOMO-LUMO gap upon d i merization. The spectra of a r e l a t e d p a i r of complexes, monomer i c ( M e N C ) R h ( P P h ) and dimeric ( M e N C K R h ( d p m ) , are compared i n Figure 2. Thg lowest allowed t r a n s i t i o n in the monomer r e s u l t s from the d + Lnr (ag + l b ) e x c i t a t i o n . In the dimers, the t r a n sition l b + 2ag i s i n v o l v e d , and the energy gap i s smaller. Not only does the i n c o r p o r a t i o n of the dpm l i g a n d i n t o 1_ i n h i b i t d i s s o c i a t i o n , i t a l s o p r o h i b i t s f u r t h e r Rh-Rh bond forma tion. The c a t i o n s (RNCKRh can f u r t h e r a s s o c i a t e to f o r m t r i m e r s v i a r e a c t i o n (2). A d d i t i o n a l l y the isocyanide bridge complex 2 also s e l f a s s o c i a t e s v i a r e a c t i o n (3) to form a tetrarhodium species. However the dpm-bridged dimers, 1_, show no evidence f o r s e l f a s s o c i a t i o n even at the highest c o n c e n t r a t i o n s . Undoubtedly 2

+

2

Z 2

3

2+

2

2

2

l u

l u

+

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


8.

BALCH

Binuclear

Complexes

with

Phosphine

Bridging

Ligands

3biu

2b

2 b,,

1u 3a

(

2b,,, ib,. 2a

ibi

u

a , d 2-

^

R,P\

/CNR

1a

q

R,P\

/CNR

R

3

P ^

Rh

Rh RNC

•d 2 7

7

PR

3

RNC R

3

ι P

X

^CNR Rh

^PR

3

RNC

PR

3

j /CNR Rh

RNC^

Figure 1.

Partial molecular

X

P R

3

orbital diagram for the interaction complexes

of two planar Rh(I)


169


170

REACTIVITY OF M E T A L - M E T A L

BONDS

WAVELENGTH (nm)

Figure 2. Electronic NC),,Rh (dpm) ][PF ], 2

2

6

spectrum of (A) a 0.20-mL acetonitrile solution of [(CH and (B) a 0.40-mL acetonitrile solution of [(CH NC) Rh(PPh ) [PF ] . 3

s

3 2

2

e 2

Each solution is contained in a 1.0-mm pathlength cell. The markers show the position of the low-energy maxima for the cases where the isocyanide substituent is changed to η-butyl, cyclohexyl, and X-butyl. Increasing the bulk of the isocyanide substituent in creases the Rh - - - Rh separation and increases the energy of the b -» 2a transition. 1

lu


g

Binuclear

BALCH

8.

Complexes

L

L

1

(2)

\

+

L-Rh-L

+

L

L

X

L

/

Rh-

with

L

I

Rh

\

Phosphine

/

L

Bridging

L

2 +

I /

L

L

L

I /

L

3

+

Rh----Rh

/' L L

L L

L

I /

Rh

171

Ligands

/· L L

/' L L


the bulky phenyl groups at each end o f these dimers p r o h i b i t s the approach of a u n i t along the Rh-Rh v e c t o r .

(3)

2

The same pattern of dimer s t a b i l i z a t i o n at the expense of monomeric or more h i g h l y polymerized forms i s a l s o e v i d e n t i n the behavior of these rhodium complexes toward o x i d a t i o n . Treatment of the dimer 1_ with i o d i n e , bromine, or t r i f l u o r o m e t h y l d i s u l f i d e r e s u l t s i n t r a n s - a n n u l a r , o x i d a t i v e - a d d i t i o n to form the brown dimers, 3_, v i a r e a c t i o n (4) (3,6). As i n s p e c t i o n o f Figure 1 shows Ph

Ph

Ph—Ρ

P—Ph

I

L (4) Ph—Ρ

X = I, 2

+ X

Ρ—Ph

Pf/' ^ C H Br , 2

2

^

Ph—P

^ Ph

(SCF ) 3

Ph

2

X

I/

L

X

~~Rfr

XH x

2

1/

X—Rh-

Rh—X

Ph—Ρ Ph^ ^ C H

P—Ph

.Ρ—Ph 2

^

Ph

5

such two-electron o x i d a t i o n should strengthen and shorten the Rh-Rh bond by removing e l e c t r o n s from the p r i m a r i l y anti bonding l b i orbital. These dimers, once formed, are s t a b l e to the p r e s ence of a d d i t i o n a l q u a n t i t i e s o f oxidant. That i s , they cannot be r e a d i l y converted i n t o Rh(III) complexes. In c o n t r a s t o x i d a t i o n of (RNCKRh gives a v a r i e t y of products whose formation can be c o n t r o l l e d by l i m i t i n g the amount of oxidant used. (7-10) The r e a c t i o n s involved are shown i n equations 5-7. The d i n u c l e a r and t r i nuclear rhodium c a t i o n s have been s t r u c t u r a l l y c h a r a c t e r i z e d by X-ray c r y s t a l l o g r a p h y . (8,10) The s t r u c t u r e of the t r i nuclear c a t i o n i s shown i n Figure 3. These metal-metal bonded products u


172

REACTIVITY

L

L

I

L

I


L

L

1/

I

V/ I'

L

L L 1/ X—Rh

L-Rh-L

/ !

L

L

2 +

Rh—

L/ ' L

L

L

1/

/ C

(14)

Ph

2

Ph—Ρ ρ

BONDS

P—Ph

u

^CH£

X

Ph

3.3542(9)A f o r X= Cl @)

of b i n u c l e a r complexes. It should be noted that there i s a rather poor r e l a t i o n between the i n f r a r e d absorption due to the bridging carbonyl group and the metal-metal separation. Some data are given in Table 1. Of p a r t i c u l a r i n t e r e s t i s the 67 c n r d i f f e r ence in v(C0) f o r the i s o e l e c t r o n i c and i s o s t r u c t u r a l complexes Pd (dam) (y-CO)Cl and P t ( d p m ) ( y - C O ) C l . (23) 1

2

2

2

2

2

2

2-Diphenylphosphinopyridine, s t r u c t i o n o f Binuclear Complexes

a Ligand f o r the Stepwise Con

2-Diphenylphosphinopyridine, These i n c l u d e : t r a n s - R h ( C O ) C l ( P h P p y ) , P d ( P h P p y ) C l , Ru(C072~Cl ( P h P p y ) , and R u ( C 0 ) ( P h P p y ) . The a b i l i t y o f these species to bind a second metal i s under a c t i v e i n v e s t i g a t i o n . 2

2

2

2

3

9

2

2

2

2

2

3


2


2

2

2

2

2

2

2

2

2

2

2

2

Rh (Ph Ppy) (u-CO)Cl

2

2

Ir (dpm) (p-C0)(u-S)(C0)

2

3

2

2

Rh (dpm) (p-C0)(y-C {C0 CH } )C1

2

Rh (dpm) (y-CO)Br

2

Rh (dpm) (y-C0)(y-Cl)(C0)

2

Pt (dpm) (u-CO)Cl

2

2

2

2

Pd (dam) (u-CO)Cl

2

2

2

Pd (dpm) (p-C0)Cl

Compound

I

2

15, 26 28 29 34

1638 1865 1745 * 1700 1760 1797

2.838(1) 2.7566(8) 3.3542(9) 2.843(2) 2.612(1)

33

19, 22 23

1723

3.162(4)

Reference

3.274(8)

1

19

v(C0) cm" 1705

Μ · · · Μ d i s t a n c e (A)

P r o p e r t i e s o f Complexes with B r i d g i n g Carbonyl Ligands

Table



178

REACTIVITY OF

CH -PPh

2

CH -PPh

2

2

2

METAL-METAL

BONDS

The r e a c t i o n s of Rh(C0)Cl(Ph Ppy) o f f e r an idea of what can be expected of complexes o f t h i s type. (32,33) Some p e r t i n e n t r e a c t i o n chemistry i s summarized i n Chart 1. The r e a c t i o n o f Rh(C0)Cl(Ph Ppy) with R h ( y - C l ) ( C 0 K y i e l d s Rh (Ph Ppy) (y-CO)C l , with v(C0) at 1797 c m " . The s t r u c t u r e o f t h i s b i n u c l e a r complex as determined by an X-ray c r y s t a l l o g r a p h i c study i s shown i n Figure 5. The geometry i s that of a d i s t o r t e d molecular Aframe with a nearly planar R h ( y - C 0 ) C l unit. Each rhodium cent e r possesses 16-valence e l e c t r o n s and consequently t h i s complex i s a rare example o f an e l e c t r o n d e f i c i e n t complex containing a b r i d g i n g carbonyl group and a metal-metal bond. This complex d i f f e r s from the expectations of r e a c t i o n 15 i n t h a t the Ph Ppy ligands have become r e o r i e n t e d so that they have a h e a d - t o - t a i l o r i e n t a t i o n A r a t h e r than the head-to-head o r i e n t a t i o n B. 2

2

2

2

2

2

2

2

2

1

2

2

2

2

Reaction of Rh(C0)Cl(Ph Ppy) with ( 1 , 5 - c y c l o o c t a d i e n e ) P d C l y i e l d s RhPd(Ph Ppy) (CO)Cl with a terminal carbonyl l i g a n d i n d i cated by i n f r a r e d absorption at 2040 c m ' . Extension o f the concept o f r e a c t i o n (15) to t h i s system would p r e d i c t s t r u c t u r e 8 f o r t h i s complex. The demonstrated tendency f o r the trans-Ph Ppy ligands to adopt the h e a d - t o - t a i l arrangement might lead to the expectation o f s t r u c t u r e 9_. However n e i t h e r of these s t r u c t u r e s would p r e d i c t a carbon absorption on the Rh(I) center to be great2

2

2

2

2

3

1

2


Binuclear

Complexes

with

Phosphine

Bridging

Ligands


B A L C H


R E A C T I V I T Y


180

O F

M E T A L - M E T A L

B O N D S

Figure 5. Perspective drawing of Rh (Ph Ppy)4^-CO)Cl . Some bond lengths are: Rh—Rh, 2.612(1); Rh(l)—Cl(l), 2.355(1); Rh(2)—Cl(2) 2.355(1); Rh(l)—P(ll 2.206(1); Rh(2)—P(2), 2.215(1); Rh(l)—N(l), 2.116(5); Rh(2)—N

MetalMetal Bond Making and Breaking in Binuclear Complexes with

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