The Synthesis, Electrochemistry, and Reactivity of Binuclear Copper(I

6 The Synthesis, Electrochemistry, and Reactivity of Binuclear Copper(I) Complexes as Mimics of Protein Active Sites

Downloaded by UNIV LAVAL on May 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch006

R O B E R T R. G A G N É , R O B E R T P. K R E H ,

1

and J O H N D O D G E

California Institute of Technology, Division of Chemistry and Chemical Engineering, Pasadena, C A 91125

Several

binuclear

cuprous

complexes

were synthe-

sized as potential models for binuclear copper protein sites. Appropriate

modifications

of the

polydentate

ligand systems resulted in a wide range of reduction potentials. In this manner, utilizing only nitrogen and oxygen ligands, high reduction potentials comparable to those of the protein binuclear sites were achieved. The new binuclear copper(I) complexes appear to be only tricoordinated in solution but in some instances demonstrate significant copper(I)-copper(I) interactions in the solid state. The CO and O reactivities of these com2

pounds also were explored.

A n u n d e r s t a n d i n g o f the interactions b e t w e e n d i o x y g e n a n d transi t i o n m e t a l sites i n p r o t e i n s is v a l u a b l e f r o m a p e d a g o g i c a l s t a n d p o i n t as w e l l as f o r p o s s i b l e p r a c t i c a l a p p l i c a t i o n s t o e l e c t r o n transfer catalysis [e.g., t h e r e d u c t i o n o f d i o x y g e n to w a t e r i n f u e l c e l l s ( I ) ] . F o r c o p p e r p r o t e i n s , a b i n u c l e a r a c t i v e s i t e is u t i l i z e d for a d i r e c t interaction with molecular oxygen. These proteins include h e m o c y a n i n , w h i c h is r e s p o n s i b l e for r e v e r s i b l e o x y g e n a t i o n i n t h e b l u e b l o o d o f v a r i o u s m o l l u s k s a n d a r t h r o p o d s (2); t y r o s i n a s e , w h i c h c a t a l y z e s t h e o x i d a t i o n o f s u b s t r a t e s b y d i o x y g e n i n a v a r i e t y o f or g a n i s m s (3,4); a n d l a c c a s e , w h i c h c a t a l y z e s t h e r e d u c t i o n o f d i o x y g e n to w a t e r a n d i s f o u n d i n A s i a n l a c q u e r trees a n d w h i t e r o t f u n g i (5, 6 ) . 1

Current address: University of Georgia, Department of Chemistry, Athens, GA 30602 0065-2393/82/0201-0139$06.00/0 © 1982 A m e r i c a n C h e m i c a l Society Kadish; Electrochemical and Spectrochemical Studies of Biological Redox Components Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

140

BIOLOGICAL R E D O X COMPONENTS

S e v e r a l i n t e r e s t i n g features i n these c o p p e r proteins w e r e

noted

and investigated: 1. A l t h o u g h t h e e x a c t n u m b e r a n d g e o m e t r y o f t h e l i g a n d s b o u n d to t h e copper centers is u n k n o w n , nitrogenous l i g a n d s ( h i s t i d i n e , i n p a r t i c u l a r ) w e r e i m p l i c a t e d (7). A p e r o x i d e m o i e t y b r i d g i n g t h e t w o c u p r i c ions i n o x y h e m o c y a n i n a n d o x y t y r o s i n a s e w a s p o p u l a r i z e d (8-

11).


2 . I n t h e o x i d i z e d f o r m s , a l l t h r e e b i n u c l e a r c u p r i c sites a r e s t r o n g l y a n t i f e r r o m a g n e t i c a l l y c o u p l e d (12, 13). 3 . T h e t y r o s i n a s e a n d l a c c a s e b i n u c l e a r sites e x h i b i t t w o e l e c t r o n r e d u c t i o n s at p o t e n t i a l s t h a t a r e r a t h e r h i g h ( ^ 0 . 3 6 V v s . N H E ) for t h e p r o p o s e d a l l n i t r o g e n - o x y g e n c o p p e r c o o r d i n a t i o n (13-16). T h e s e p r o p e r t i e s a n d t h e d i o x y g e n b i n d i n g o f t h e s e c o p p e r sites h av e b e e n addressed b y d i r e c t study o f the proteins a n d investigations of appropriate

model compounds.

Some

success

w a s attained i n

m i m i c k i n g t h e b i n u c l e a r c o p p e r a c t i v e sites i n t e r m s o f l i g a n d e n v i ronments, redox properties, magnetic interactions, a n d d i o x y g e n b i n d i n g ( J 7 - 2 8 ) . T o h e l p e l u c i d a t e the relationships b e t w e e n l i g a n d e n v i ronments, redox potentials, a n d dioxygen reactivity w e investigated t h e c h e m i s t r y o f a series o f b i n u c l e a r c o p p e r ( I ) c o m p l e x e s . T o c l e a r l y define

a n d preserve

polydentate

the coordination sphere around

each

copper,

l i g a n d s w e r e e m p l o y e d (29-32). W e a p p r o x i m a t e d t h e

p r o p o s e d p r o t e i n l i g a n d e n v i r o n m e n t as c l o s e l y as p o s s i b l e , a n d t h u s , o n l y nitrogen a n d o x y g e n donors w e r e used. T h e first s e r i e s o f b i n u c l e a r c o p p e r c o m p o u n d s t o b e d i s c u s s e d were generated b y modifications on the m a c r o c y c l i c l i g a n d system i n C o m p o u n d 1.

1

Kadish; Electrochemical and Spectrochemical Studies of Biological Redox Components Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

6.

GAGNÉ ET AL.

The

second

Binuclear Copper(I) Complexes

system

employs

the

potentially

141

binucleating

Ν,Ν,Ν',Ν'-tetrakis(2-pyridylmethyl)ethylendiamine

ligand

(TPEN).


TPEN

T h r o u g h variation o f the polydentate l i g a n d s , u n u s u a l copper(I) coor dination environments

were found, a n d a w i d e range o f reduction

potentials were realized.

Binuclear Systems Derived from 2-Hydroxy-5-Methylisophthalaldehyde O u r i n i t i a l w o r k o n b i n u c l e a r s y s t e m s f o c u s e d o n C o m p o u n d 1, s y n t h e s i z e d p r e v i o u s l y (31-33). T h i s b i n u c l e a r c o m p l e x e x h i b i t s a t w o - s t e p r e d u c t i o n at - 0 . 5 2 a n d - 0 . 9 1 V v s . N H E . T h e s e v e r y n e g a t i v e p o t e n t i a l s i n d i c a t e the d e s t a b i l i z a t i o n o f copper(I) r e l a t i v e to c o p per(II), w h i c h most l i k e l y results from the square-planar arrangement o f " h a r d " o x y g e n a n d n i t r o g e n l i g a n d s . [ S t r u c t u r a l a n d e l e c t r o n i c fac tors i n f l u e n c i n g c o p p e r r e d u c t i o n p o t e n t i a l s w e r e d o c u m e n t e d (34).] T o s t a b i l i z e copper(I) a n d t h e r e b y raise the r e d u c t i o n potentials t o w a r d t h e h i g h v a l u e s f o u n d for t y r o s i n a s e a n d l a c c a s e , b i n u c l e a t i n g l i g a n d s that c o n t a i n s o m e w h a t "softer" aromatic nitrogen l i g a n d s i n a m o r e flexible f r a m e w o r k (to a l l o w a t e t r a h e d r a l g e o m e t r y a r o u n d e a c h c o p p e r ) w e r e d e s i g n e d . T h i s a p p r o a c h l e d to t h e s y n t h e s i s o f C o m p l e x e s 2-4, s h o w n i n S c h e m e I a l o n g w i t h v i s i b l e a b s o r p t i o n a n d m a g n e t i c d a t a (35). [ D u r i n g t h e c o u r s e o f t h i s w o r k , c o m p l e x e s s i m i l a r t o 3 a n d 4 w e r e r e p o r t e d b y o t h e r s (36)]. I n a l l t h r e e c o p p e r ( I I ) c o m p l e x e s , b o t h c o p p e r a t o m s are p r e s u m a b l y b o u n d t o a n a r o m a t i c n i t r o gen. T h i s b o n d i n g was d e m o n s t r a t e d b y a n x-ray structural d e t e r m i n a t i o n o f C o m p o u n d 4 (37). (See T a b l e I for c o m p o u n d n u m b e r s a n d s t r u c t u r e s . ) E a c h c o p p e r ( I I ) is a c t u a l l y p e n t a c o o r d i n a t e d ; o n e c o p p e r a t o m is b o u n d to t h e o x y g e n o f a w a t e r m o l e c u l e a n d t h e o t h e r c o p p e r is b o u n d t o t h e o x y g e n o f t h e h y d r o x y b r i d g e o f a n a d j a c e n t m o l e c u l e . T h e e l e c t r o c h e m i s t r y o f t h e s e n o n m a c r o c y c l i c c o m p l e x e s , 2-4, is i r r e v e r s i b l e as s h o w n i n F i g u r e 1 for C o m p o u n d 2, i n d i m e t h y l f o r m a m i d e ( D M F ) . T h i s irreversibility m a y b e a result o f the relatively


142

BIOLOGICAL REDOX COMPONENTS


R

λ (€)

Meff

Scheme I. Preparation of binuclear copper(II) -copper(II) (34). Spectra were recorded in methanol solution at 25°C. susceptibilities are given in B.M., measured at 25°C, and for diamagnetism, but not for TIP.

/ C u

complexes Magnetic corrected

l a b i l e h y d r o x y b r i d g e , w h i c h is a p o o r l i g a n d for c o p p e r ( I ) . T h e i n t r o duction of pyrazole d i d produce reversible electrochemical behavior, a l s o s h o w n i n F i g u r e 1. S i m i l a r r e s u l t s w e r e o b s e r v e d for C o m p o u n d 3, but C o m p o u n d 4 gave irreversible electrochemistry e v e n w i t h added pyrazole. C o n t r o l l e d p o t e n t i a l e l e c t r o l y s i s ( C P E ) at - 0 . 3 5 a n d - 0 . 7 0 V o f C o m p o u n d 2, p l u s an e q u i v a l e n t a m o u n t o f p y r a z o l e , i n d i c a t e d that e a c h w a v e c o r r e s p o n d s to a o n e - e l e c t r o n p r o c e s s . N o a t t e m p t w a s m a d e to i s o l a t e a m i x e d - v a l e n t s p e c i e s ; b u t t h e b i n u c l e a r c u p r o u s c o m p l e x , C o m p o u n d 5 , w a s s y n t h e s i z e d b y C P E at - 0 . 7 0 V (n = 2.0) of an acetonitrile solution containing C o m p o u n d 2 and an equivalent a m o u n t o f p y r a z o l e (35). C o m p o u n d 5 also was synthesized d i r e c t l y from cuprous starting materials. S i m i l a r reactions w i t h 2 - a m i n o m e t h y l p y r i d i n e or 2( 2 ' - a m i n o e t h y l ) p y r i d i n e as s i d e a r m s a n d w i t h p y r a z o l e a n d 3 , 5 d i m e t h y l p y r a z o l e as b r i d g i n g l i g a n d s g a v e C o m p l e x e s 6 - 8 . (see T a b l e



2-(pyridyl)methyl 2-(2'-pyridyl)ethyl 2-(4'-imidazoyl)ethyl 2-(pyridyl)methyl 2-(pyridyl)methyl 2-(2'-pyridyl)ethyl 2-(2'-pyridyl)ethyl phenylmethyl phenylmethyl 2-phenylethyl 2-phenylethyl 1-propyl 2-propyl f-butyl f-butyl phenyl p-(dimethylamino)-phenyl p-acetylphenyl

Sidearm (R)

+ e~Q

+ 2

(X)

Bridge

Cu(I)Cu(I)L

+ e~ â C u ( I I ) C u ( I ) L

+3

hydroxide hydroxide hydroxide pyrazolate 3,5-dimethyl-pyrazolate pyrazolate 3,5-dimethyl-pyrazolate pyrazolate 3,5-dimethyl-pyrazolate pyrazolate 3,5-dimethyl-pyrazolate pyrazolate pyrazolate pyrazolate 3,5-dimethyl-pyrazolate pyrazolate pyrazolate pyrazolate

Cu(II)Cu(I)L

Cu(II)Cu(II)L

+

d d d -0.452 -0.374 -0.344 -0.267 -0.081 0.005 -0.078 0.009 -0.076 0.001 0.053 0.080 -0.032 -0.048 0.008

-0.211 -0.190 -0.110 -0.113 0.146 0.206 0.128 0.205 0.146 0.193 0.240 0.239 0.144 0.146 0.152

-0.91

E|°

d d d

-0.52

Binuclear Copper Complexes and Reduction Potentials

d

c

6

a

Note: These values were measured by differential pulse polarography in D M F using a platinum indicating electrode. Potentials are given in V vs. N H E . Potentials were measured vs. ferrocene as an internal redox couple,then corrected to vs. a value of 0.40 V for ferrocene vs. N H E (24, 38, 39). These values for η were determined by CPE at a potential 200 mV more positive than E{. This value was determined by C P E at -1.16 V (31, 32). Irreversible electrochemistry.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Compound Number

Table I.



144


V O L T S (nhe) Figure 1. Cyclic voltammograms of DMF solutions of and Compound 2 containing one equivalent of pyrazole mograms were observed with a platinum electrode, using electrolyte at a scan rate of 200 mV/s. (Reproduced from 1979, American Chemical Society.)

Compound 2 ( ), (—). Both voltam0.1 M TBAP as the Ref. 35. Copyright

I a n d S c h e m e II.) T h e c o o r d i n a t i o n e n v i r o n m e n t a r o u n d e a c h c o p p e r i l ) o f these c o m p l e x e s was e x p e c t e d to b e a t e t r a h e d r a l arrange ment o f one oxygen a n d three nitrogen ligands. A crystallographic analysis o f C o m p o u n d 7, d i d not y i e l d the e x p e c t e d structure, h o w ever.

Crystallographic Analysis of Compound 7 T h e a c t u a l s t r u c t u r e o f 7 is d e p i c t e d i n F i g u r e 2 a l o n g w i t h p e r t i n e n t b o n d d i s t a n c e s (35). T h e p y r i d i n e n i t r o g e n a t o m s a r e n o t c o o r d i n a t e d a n d e a c h c o p p e r is b o u n d to t h r e e l i g a n d a t o m s i n w h a t is a l m o s t a T - g e o m e t r y , w i t h the largest angles b e i n g b e t w e e n the i m i n e a n d p y r a z o l a t e n i t r o g e n s . T h e c o p p e r - n i t r o g e n b o n d s are shorter t h a n m o s t d i s t a n c e s t h a t w e r e r e p o r t e d for c o p p e r ( I ) c o m p l e x e s w i t h n i t r o g e n l i g a n d s ( 2 2 , 23, 40-43). T h e a v e r a g e l e n g t h s are 1.88 À for t h e c o p p e r - p y r a z o l e n i t r o g e n b o n d s a n d 1.90 Â for t h e c o p p e r - i m i n e n i t r o g e n b o n d s . T h e c o p p e r - o x y g e n d i s t a n c e s are s u b s t a n t i a l l y l o n g e r , averaging 2.09 A . These b o n d lengths, a l o n g w i t h the v e r y large



6.

GAGNÉ E T A L .

Binuclear

Copper(I)

Complexes

145

Figure 2. Molecular structure of Compound 7, including labels for noncarbon atoms and selected bond lengths (35). Bond angles: NlCul-N3,167.6°; Ol-Cul-Nl, 91.7°; 01-Cul-N3, 97.Γ ; N2-Cu2-N4, 170.7°; 01-Cu2-N2, 91.5°; and 01-Cu2-N4, 96.8°.

N - C u - N b o n d a n g l e s (ca. 1 7 0 ° ) m a y s u g g e s t s o m e d e g r e e o f d i c o o r d i n a t e d c h a r a c t e r , w h i c h m a y b e e s p e c i a l l y f a v o r a b l e for c o p p e r ( I ) i n teractions w i t h these nitrogenous l i g a n d s . T h e i n t r a m o l e c u l a r c o p p e r - c o p p e r d i s t a n c e , 3 . 3 0 4 A , is l o n g e n o u g h to m a k e a n y d i r e c t i n t e r a c t i o n u n l i k e l y . H o w e v e r , a n i n t e r m o l e c u l a r interaction apparently does occur b e t w e e n copper atoms. T h e m o l e c u l e s l i n e u p i n an infinite array s u c h that r e l a t i v e l y close contact (2.968 À) exists b e t w e e n c o p p e r atoms o f adjacent m o l e c u l e s ( F i g u r e 3). T h e o v e r l a p p i n g l i g a n d atoms o f n e i g h b o r i n g m o l e c u l e s a l s o c o m e c l o s e to o n e a n o t h e r . T h e m e a n p l a n e c o n t a i n i n g t h e p h e n o x i d e o x y g e n , the i m i n e n i t r o g e n , a n d the three carbons c o n -



146


Figure 3. Crystal packing of Compound 7, showing a section of an infinite stack of molecules that forms in the solid state. The three molecules are corresponding ones from three different unit cells, translated along the b axis (which lies in the vertical direction in the orientation of this figure). All copper atoms lie in the same plane, parallel to the page. The pyrazolate groups are directed out of the page, toward the reader. The shortest copper-copper distance, 2.97 Â, occurs between atoms in neighboring molecules, connected by dashed lines in the figure. The closest intermolecular Cu-H contacts also are shown ( 35).

n e c t i n g t h e s e a t o m s is s i t u a t e d at a n a v e r a g e d i s t a n c e o f 3 . 2 0 Â a w a y from the c o r r e s p o n d i n g p l a n e d i r e c t l y b e l o w i n the n e i g h b o r i n g m o l e c u l e . T h i s d i s t a n c e is s o m e w h a t s h o r t e r t h a n t h e s e p a r a t i o n u s u a l l y o b s e r v e d b e t w e e n s t a c k e d 7 r - d e l o c a l i z e d m o l e c u l e s (44-46). T h u s , an i n t e r m o l e c u l a r c o p p e r - c o p p e r b o n d and/or an interac t i o n b e t w e e n t h e d e l o c a l i z e d 7r-systems o f a d j a c e n t m o l e c u l e s a p p e a r s


6.

GAGNÉ ET AL.

147


to b e f a v o r e d i n t h e s o l i d state. C o p p e r ( I ) - c o p p e r ( I ) i n t e r a c t i o n s w i t h m e t a l s e p a r a t i o n s as s h o r t as 2 . 4 5 A are k n o w n , b u t a l l p r e v i o u s l y characterized species w i t h proposed copper(I)-copper(I)

interactions

h a v e at l e a s t o n e b r i d g i n g l i g a n d b e t w e e n t h e t w o i n t e r a c t i n g c o p p e r a t o m s (47-49). T h e s t r u c t u r e s o f t h e a n a l o g o u s s p e c i e s , 5 - 8 , are p r o b a b l y s i m i l a r to t h i s s t r u c t u r e f o u n d for C o m p o u n d 7 .


Synthesis and Solution Structures of Compounds 9-19 T h e s t r u c t u r e o f C o m p o u n d 7 s u g g e s t e d t h e s y n t h e s i s o f a series o f c o m p o u n d s that h a d n o d o n o r atoms o n the s i d e a r m s (R), that is, C o m p l e x e s 9 - 1 9 , T a b l e I. T h e s e c o m p o u n d s w e r e s y n t h e s i z e d a n a l o g o u s l y to t h e s y n t h e s i s o f C o m p o u n d s 5 - 8 ( S c h e m e I I ) . A l l c u p r o u s c o m pounds were fully characterized b y elemental analysis, I R a n d N M R s p e c t r o s c o p y , a n d m a s s s p e c t r o s c o p y o n s e l e c t e d c o m p o u n d s (35). T h e s e c o m p o u n d s also p r o v e d to b e c r y s t a l l i n e solids, stable i n the absence o f d i o x y g e n . B e c a u s e the polydentate l i g a n d systems e m p l o y e d p r o v i d e o n l y t r i c o o r d i n a t i o n for e a c h c o p p e r , t h e s e c o m p o u n d s are p r e s u m e d to b e t r i c o o r d i n a t e d i n s o l u t i o n . A l l c o m p o u n d s that h a v e n o n b u l k y sidearms are b r o w n i n the s o l i d state, t h i s c o l o r b e i n g d u e , i n p a r t , to a 6 0 0 - n m b a n d o b s e r v e d i n t h e s o l i d state n u j o l m u l l s p e c t r u m ( C u r v e A i n F i g u r e 4 ) . T h i s b a n d is n o t p r e s e n t i n t h e s p e c t r u m o f t h e c o m p l e x w i t h t-butyl sidearms, C o m p o u n d 1 5 , w h i c h is r e d i n t h e s o l i d state as w e l l as i n s o l u t i o n ( C u r v e C , i n F i g u r e 4 ) . T h i s 6 0 0 - n m a b s o r p t i o n a l s o is a b s e n t i n t h e s p e c t r a o f C o m p o u n d s 14 a n d 1 6 . T h e 6 0 0 - n m b a n d m a y b e a t t r i b u t e d to a n i n t e r m o l e c u l a r i n t e r a c t i o n t h a t m a y b e i n h i b i t e d b y t h e p r e s e n c e o f l a r g e £ - b u t y l s i d e a r m s . T h i s i n t e r a c t i o n a p p e a r s to b e o n l y a s o l i d state p h e n o m e n o n , b e c a u s e t h e 6 0 0 - n m b a n d w a s n o t f o u n d i n s o l u t i o n s p e c t r a (e.g., C u r v e B , F i g u r e 4 ) , e v e n i n v e r y c o n c e n t r a t e d s o l u t i o n s . T h e s o l u t i o n s t r u c t u r e o f C o m p o u n d 7 a p p e a r s to b e s i m i l a r to t h a t f o u n d i n t h e s o l i d state w i t h t h e a b s e n c e o f t h e i n t e r m o l e c u l a r c o p p e r - c o p p e r i n t e r a c t i o n . T h e p y r i d i n e nitrogens d o not a p p e a r to b e b o u n d i n b e n z e n e s o l u t i o n , b e c a u s e t h e N M R s i g n a l for t h e p r o t o n o n t h e c a r b o n a d j a c e n t to t h e p y r i d i n e n i t r o g e n o f C o m p o u n d 7 o c c u r s at t h e s a m e p o s i t i o n , w i t h i n e x p e r i m e n t a l e r r o r , as t h e c o r r e s p o n d i n g p r o t o n f r o m 2 - ( 2 ' - a m i n o e t h y l ) p y r i d i n e ( i . e . , 8.44 ± 0 . 0 2 p p m δ f r o m T M S ) . I n fact, a l l c o m p o u n d s t h a t c o n t a i n p y r i d i n e o n t h e s i d e a r m s e x h i b i t t h i s r e s o n a n c e at t h e s a m e p o s i t i o n , w i t h i n e x p e r i m e n t a l e r r o r ( i . e . , 8.42 ± 0.04 p p m δ f r o m T M S ) . T h e r e s o n a n c e o f t h i s p r o t o n w o u l d b e e x p e c t e d to s h i f t d o w n f i e l d o n b i n d i n g to a c o p p e r ( I ) i o n . ( W e o b s e r v e d t h i s effect for t h e l i g a n d i V , ] V , i V ' , ] V ' - t e t r a k i s ( 2 - p y r i d y l m e t h y l ) e t h y l e n e d i a m i n e ( T P E N ) , w h e r e b i n d i n g o f c o p p e r ( I ) to t h e p y r i d i n e n i t r o g e n s c a u s e s a d o w n f i e l d s h i f t o f 0 . 1 3 p p m for t h e r i n g p r o t o n



148


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ι 500

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ι

600

700

I

WAVELENGTH ( n m ) Figure 4. Electronic absorption spectra. Key: A, Compound 11 in the solid state; B , Compound 11 in a hexane solution; and C , Compound 15 in the solid state. (Reproduced from Ref. 35. Copyright 1979 American Chemical Society.) n e a r e s t n i t r o g e n ( 5 0 ) . D o w n f i e l d shifts o f 0 . 4 - 0 . 7 p p m w e r e o b s e r v e d for t h e p r o t o n i n t h e 2 - p o s i t i o n o f a n i m i d a z o l e r i n g o n b i n d i n g c o p p e r ( I ) ( 5 1 ) . F u r t h e r m o r e , s h a r p r e s o n a n c e s w e r e o b s e r v e d for t h e p y r a z o l e p r o t o n s o f t h e s e c o m p o u n d s (e.g., a d o u b l e t at 7 . 8 7 p p m a n d a t r i p l e t at 6 . 4 3 p p m f o r C o m p o u n d 14). T h i s o b s e r v a t i o n i n d i c a t e s t h a t t h e r e is n o e q u i l i b r i u m b e t w e e n b o u n d a n d u n b o u n d p y r a z o l e ; hence, the p y r a z o l a t e b r i d g e m u s t b e totally b o u n d to the copper(I) i o n s o r t o t a l l y d i s s o c i a t e d i n b e n z e n e s o l u t i o n s ( u n l i k e l y , at b e s t ) .

Electrochemical Analysis for the Series 5-19 A l l b i n u c l e a r copper(I) c o m p o u n d s presented i n T a b l e I e x h i b i t e d c y c l i c v o l t a m m e t r y s i m i l a r t o t h e s o l i d l i n e i n F i g u r e 1, b u t a w i d e r a n g e o f r e d u c t i o n p o t e n t i a l s w e r e o b s e r v e d (35). S e v e r a l t r e n d s c a n be o b s e r v e d from T a b l e I. I n the f o l l o w i n g discussion, the formal p o t e n t i a l s for t h e s e c o n d r e d u c t i o n , ( £ £ ) , E q u a t i o n 1, w i l l b e u s e d s i n c e these processes w e r e m o r e r e v e r s i b l e a n d s l i g h t l y m o r e s y s t e m a t i c


6.


GAGNÉ E T AL.

Cu(II)Cu(I)L


t h a n t h e E{potentials. b o t h processes.

+

+ e~ â

Cu(I)Cu(I)L

149

(1)

T h e general trends, h o w e v e r , are the same w i t h

T h e first p o i n t o f i n t e r e s t is t h e effect o f p o s s i b l e b i n d i n g sites ( i . e . , p y r i d i n e ) i n t h e s i d e a r m s . T h e f o l l o w i n g series o f p o t e n t i a l s (E{) w a s observed: C o m p o u n d 11, ( - 0 . 0 7 8 V ) « C o m p o u n d 9, ( - 0 . 0 8 1 V ) > C o m p o u n d 7, ( - 0 . 3 4 4 V ) > C o m p o u n d 5, ( - 0 . 4 5 2 V ) . T h i s finding indicates that the c o m p l e x e s c o n t a i n i n g the p y r i d i n e rings p r o v i d e a m o r e f a v o r a b l e e n v i r o n m e n t for c o p p e r ( I I ) r e l a t i v e t o c o p p e r ( I ) w h e n c o m p a r e d to t h e i r b e n z e n e r i n g a n a l o g u e s . B e c a u s e t h e p y r i d i n e n i t r o g e n s d o n o t a p p e a r to b e b o u n d to t h e c o p p e r ( I ) i o n s ( i n s o l u t i o n or i n t h e s o l i d state), t h e effect o f t h e s e p y r i d i n e r i n g s m u s t b e l a r g e l y o n t h e o x i d i z e d c o p p e r ( I I ) sites. H e n c e , t h e p y r i d i n e s a p p a r e n t l y b i n d to t h e c o p p e r ions o n o x i d a t i o n , C o m p o u n d 2 0 , a n d dissociate f r o m copper(I) on reduction, C o m p o u n d 7, ( S c h e m e III). I n the s i m i l a r c u p r i c c o m p l e x , 4 , t h e i m i d a z o l e n i t r o g e n s w e r e s h o w n to b e c o o r d i n a t e d i n t h e s o l i d state x - r a y s t r u c t u r e ( 3 7 ) . M o s t l i k e l y , s i d e a r m p y r i d i n e n i t r o g e n s w o u l d a l s o c o o r d i n a t e to c o p p e r ( I I ) . T h e d e p e n d e n c e o f t h e c y c l i c v o l t a m m o g r a m o n s c a n rate (as a r e s u l t o f t h e p r o c e s s i n S c h e m e I I I ) was not investigated. C o m p o u n d s 5 - 1 9 also i n d i c a t e that the m e t h y l p y r i d i n e s i d e a r m p r o v i d e s a b e t t e r e n v i r o n m e n t for c o p p e r ( I I ) t h a n does the e t h y l p y r i d i n e sidearm. T h i s c o n d i t i o n m a y b e a result o f the g e o m e t r y o f t h e s i d e a r m s a n d t h e i r r e l a t i v e a b i l i t y t o b i n d to t h e c o p per(II) centers. T h e i n t r o d u c t i o n o f m e t h y l substituents on the pyrazolate bridges c a u s e s a n i n c r e a s e i n t h e r e d u c t i o n p o t e n t i a l (Ε{) as f o l l o w s : C o m p o u n d 6, ( - 0 . 3 7 4 V ) > C o m p o u n d 5, ( - 0 . 4 5 2 V ) a n d C o m p o u n d 10, ( + 0 . 0 0 5 V ) > C o m p o u n d 9 , ( - 0 . 0 8 1 V ) . A n i n d u c t i v e effect o f t h e m e t h y l g r o u p s w o u l d s t a b i l i z e c o p p e r ( I I ) r e l a t i v e to c o p p e r ( I ) , b u t t h e r e v e r s e t r e n d is a c t u a l l y o b s e r v e d . T h e s t e r i c b u l k o f t h e m e t h y l g r o u p s

Scheme III.


150


m a y b e r e s p o n s i b l e for t h e r e l a t i v e d e s t a b i l i z a t i o n o f c o p p e r ( I I ) . T h e s e m e t h y l groups m a y i n h i b i t the b i n d i n g o f b o t h p y r i d i n e (from the s i d e a r m s ) a n d d i m e t h y l f o r m a m i d e ( s o l v e n t ) to t h e o x i d i z e d c o p p e r ( I I ) i o n s b y p a r t i a l l y b l o c k i n g t h e f o u r t h , s q u a r e - p l a n a r b i n d i n g site a r o u n d each copper(II). Furthermore, s h i e l d i n g the copper centers f r o m t h e s o l v e n t m o l e c u l e s w i t h h y d r o p h o b i c g r o u p s ( s u c h as t h e s e m e t h y l s ) m a y n o t a l l o w t h e p o l a r D M F m o l e c u l e s to e f f i c i e n t l y s o l v a t e the c h a r g e d copper(II) species. R e l a t i v e stabilization o f copper(I) also r e s u l t s f r o m m o r e b u l k y s i d e a r m s , as r e f l e c t e d i n t h e f o l l o w i n g series (E ): C o m p o u n d 1 5 , ( + 0 . 0 5 3 V ) > C o m p o u n d 1 4 , ( + 0 . 0 0 1 V ) > C o m p o u n d 13, ( - 0 . 0 7 6 V ) . f


2

F i n a l l y , a n e l e c t r o n i c effect o n E{ is o b s e r v e d i n t h e series o f c o m p o u n d s t h a t h a v e p h e n y l r i n g s b o n d e d d i r e c t l y to t h e i m i n e n i t r o gens: C o m p o u n d 19, (+0.008 V ) > C o m p o u n d 17, ( - 0 . 0 3 2 V ) > C o m p o u n d 18, ( - 0 . 0 4 8 V ) . H e r e , the e l e c t r o n - w i t h d r a w i n g c a r b o n y l sub stituent results i n relative copper(I) stabilization, w h i l e the electrond o n a t i n g d i m e t h y l a m i n e substituent results i n relative C u ( I I ) stabiliza t i o n . S i m i l a r effects o f r e m o t e s u b s t i t u e n t s w e r e r e p o r t e d r e c e n t l y for a series o f m o n o n u c l e a r copper(II) c o m p l e x e s (24). T h e o x i d a t i o n / r e d u c t i o n o f these c o m p o u n d s i n t w o one-electron s t e p s w a s t h e e x p e c t e d b e h a v i o r for t w o i n t e r a c t i n g m e t a l c e n t e r s (31, 3 2 ) . T h i s s e q u e n t i a l b e h a v i o r w a s o b s e r v e d for t h e s e r i e s o f b i n u c l e a r c o m p l e x e s , 2 1 , recently investigated (52). S t e p w i s e oxidation/re d u c t i o n w a s a l s o f o u n d for m a n y r u t h e n i u m ( I I ) d i m e r s , i n w h i c h closer p r o x i m i t y a n d greater interactions b e t w e e n the r u t h e n i u m c e n ters c o r r e l a t e s w i t h a g r e a t e r s e p a r a t i o n o f t h e t w o r e d o x p r o c e s s e s ( a n d larger c o m p r o p o r t i o n a t i o n constants) ( 5 3 - 5 6 ) . [ A r e v e r s i b l e t w o e l e c t r o n r e d u c t i o n w a s o b s e r v e d for w e l l - s e p a r a t e d c u p r i c i o n s ( 2 7 ) ,

21


6.

GAGNÉ ET AL.


151

a n d a curious exception to the stepwise electrochemistry e x h i b i t e d b y s t r o n g l y i n t e r a c t i n g m e t a l s is r e p o r t e d f o r a series o f b i n u c l e a r t r i k e t o n a t o c o p p e r ( I I ) c o m p l e x e s (25).]


Binuclear Systems Derived from Ν, Ν,Ν',Ν '-Tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) T h e a p p a r e n t p r e f e r e n c e o f c u p r o u s i o n s for l o w e r c o o r d i n a t i o n n u m b e r s ( i . e . , t w o , t h r e e , a n d four) ( 3 5 , 4 7 - 4 9 , 5 7 ) s u g g e s t e d t h e p o s s i b i l i t y o f t h e h e x a d e n t a t e l i g a n d , T P E N , p e r f o r m i n g as a b r i d g i n g l i g a n d b e t w e e n t w o c o p p e r ( I ) i o n s . I n d e e d , a b i n u c l e a r c u p r o u s c o m p l e x , 22, e m p l o y i n g t h e T P E N l i g a n d w a s s y n t h e s i z e d as s h o w n i n S c h e m e I V (50). T h e s t r u c t u r e o f C o m p o u n d 22, w a s s o m e w h a t s u r p r i s i n g a n d i s s h o w n i n F i g u r e 5 along w i t h pertinent b o n d lengths a n d angles. Rather than the expected tricoordination or tetracoordination (in c l u d i n g B F as a l i g a n d ) , e a c h c o p p e r a p p e a r s t o b i n d o n l y t w o n i t r o g e n l i g a n d s at c l o s e d i s t a n c e s ( ~ 1 . 9 Â ) , w i t h a t h i r d , w e a k C u - N i n t e r a c t i o n (2.30 Â ) . F u r t h e r m o r e , t h e t w o s t r o n g l y b o u n d n i t r o g e n ligands, on a given copper, originate from p y r i d i n e rings o n opposite e n d s o f the e t h y l e n e d i a m i n e b r i d g e . A p p a r e n t l y , the m o l e c u l e adopts t h i s c o n f o r m a t i o n t o m a x i m i z e t h e c o p p e r - c o p p e r i n t e r a c t i o n (2.78 A ) . T h e s t r u c t u r e s p r e s e n t e d h e r e i n for C o m p o u n d s 7 ( F i g u r e 2 ) a n d 22 ( F i g u r e 5) i n d i c a t e a p r e f e r e n c e o f c o p p e r ( I ) for a s o m e w h a t l i n e a r arrangement o f t w o nitrogen ligands. [ C o o r d i n a t i o n o f this type was o b s e r v e d i n other b i n u c l e a r a n d m o n o n u c l e a r copper(I) c o m p l e x e s ( 5 8 - 6 0 ) . ] H o w e v e r , i n b o t h o f t h e p r e s e n t cases t h e t h i r d l i g a n d ( i . e . , p h e n o l a t e o x y g e n or t e r t i a r y n i t r o g e n ) i s r a t h e r " h a r d " a n d h e n c e u n f a v o r a b l e for c o p p e r ( I ) . T h e r e f o r e , t h e o b s e r v e d c o o r d i n a t i o n o f t h e s e c o m p o u n d s m a y l a r g e l y r e f l e c t p r e f e r e n c e for r e l a t i v e l y " s o f t " l i g a n d s . B o t h s y s t e m s (7 a n d 22) a p p a r e n t l y foster a d i r e c t c o p p e r - c o p p e r i n t e r a c t i o n . [ T h e r e i s a t h e o r e t i c a l b a s i s for a n a t t r a c t i o n b e t w e e n c o p p e r ( I ) i o n s (49)]. 4

C o m p o u n d 22, u n d e r g o e s a n e x t e n s i v e l i g a n d r e a r r a n g e m e n t i n t h e p r e s e n c e o f c a r b o n m o n o x i d e t o f o r m t h e d i c a r b o n y l , 23 ( S c h e m e I V ) (50). T h e c r y s t a l s t r u c t u r e o f t h i s c o m p l e x r e v e a l e d a p s e u d o t e t r a h e d r a l g e o m e t r y as s h o w n i n F i g u r e 6 . E a c h c o p p e r is b o u n d to t h r e e n i t r o g e n l i g a n d s , w i t h t w o short C u - N distances o f ca. 2.04 A ( b e t w e e n c o p p e r a n d the p y r i d i n e nitrogens) a n d o n e l o n g C u - N distance o f ca. 2.17 A (between copper a n d the tertiary a m i n e nitrogen). A l t h o u g h the s h o r t e r C u - N b o n d l e n g t h s a r e w i t h i n t h e r a n g e e x p e c t e d for C u ( I ) - N b o n d s (29, 30, 47-49, 61-63), t h e C u ( I ) - N ( t e r t i a r y a m i n e ) d i s t a n c e is l o n g , p o s s i b l y i n d i c a t i n g a l o w e r affinity b e t w e e n t h e c u p r o u s i o n a n d t h e " h a r d " t e r t i a r y a m i n e l i g a n d . T h e C u - C b o n d l e n g t h s a v e r a g e 1.80


152


1) C u ( C H C N ) B F , CH CN 3

4

4

3


2)

Cu (TPEN)(C0) (BF ) 2

2

4

2

(23)

CH3OH

Cu^CTPENKBF^

(22)

Scheme IV.

A , a n d t h e C - O b o n d l e n g t h s a v e r a g e 1.11 A w i t h a C u - C - O a n g l e o f 176°. T h e s e v a l u e s are i n a g r e e m e n t w i t h other p r e v i o u s l y r e p o r t e d tetracoordinated a n d pentacoordinated copper(I) carbonyls (30,

61-63).

Carbon Monoxide and Dioxygen Reactivity I n t h e T P E N s y s t e m ( i . e . , 22 a n d 23) t h e c o o r d i n a t i o n e n v i r o n m e n t about c o p p e r d e p e n d s o n the presence or absence o f a p o t e n t i a l f o u r t h l i g a n d ( C O ) . I n c o n t r a s t , t h e series o f C o m p l e x e s 5-19 s h o w e d n o t e n d e n c y t o b i n d C O o r p y r i d i n e as a f o u r t h l i g a n d . T h e r e a s o n for t h i s d i f f e r e n c e is n o t c l e a r b e c a u s e m o l e c u l a r m o d e l s i n d i c a t e t h a t C o m p l e x e s 5-19 c o u l d f o r m a p s e u d o t e t r a h e d r a l g e o m e t r y w i t h t h e addition of a fourth ligand. T h u s , a special stabilization apparently o c c u r s for c u p r o u s i o n s i n C o m p l e x e s 5-19 s u c h t h a t s i g n i f i c a n t e n e r g y w o u l d b e l o s t o n t h e b i n d i n g o f a f o u r t h l i g a n d (60). T o further c o m p l i c a t e the issue o f c a r b o n m o n o x i d e b i n d i n g to copper(I), a n u m b e r o f square-planar, tetracoordinated cuprous c o m p l e x e s ( i n c l u d i n g 1, η = 1) b i n d C O as a fifth l i g a n d (29-32, 40, 41). O b v i o u s l y , t h e g e o m e t r y a n d t y p e o f l i g a n d s a r o u n d c o p p e r ( I ) affect its C O b i n d i n g a b i l i t y . T h e n a t u r e o f t h e s e effects, h o w e v e r , r e m a i n s obscure. [ H e m o c y a n i n w i l l b i n d only one m o l e c u l e o f carbon m o n o x i d e p e r b i n u c l e a r c o p p e r ( I ) s i t e (64).]


6.

GAGNÉ ET AL.

Binuclear

Copper(I)

Complexes

153

A l l copper(I) c o m p o u n d s reported herein react i r r e v e r s i b l y w i t h o x y g e n i n s o l u t i o n . C o m p o u n d s 5 - 1 9 f o r m g r e e n or b r o w n p r o d u c t s from orange d i m e t h y l f o r m a m i d e solutions a n d C o m p o u n d s 21 a n d 22 form b l u e products from colorless acetonitrile solutions. These autoxidation p r o d u c t s w e r e not c h a r a c t e r i z e d .

Conclusions Stable

binuclear

copper(I)

complexes

were

prepared

using


p o l y d e n t a t e l i g a n d s . T h e s t r u c t u r e s r e p o r t e d i n t h i s c h a p t e r , as w e l l as

+

Figure 5. ORTEP diagrams of Cu (TPENf , including selected bond lengths and angles ( 50). The dication lies on a twofold rotation axis, perpendicular to the page in the upper drawing and parallel to the page in the lower one. 2




154

2

Figure 6. ORTEP drawing of Cu (TPENXCO) \ including selected bond lengths (50). This molecule, unlike Cu (TPENf , does not have exact twofold symmetry. 2

2

+

2

t h e s t r u c t u r e s f o u n d for o t h e r c u p r o u s s y s t e m s , d e m o n s t r a t e t h a t a w i d e variety o f coordination environments can be assumed by copper(I) ions. T h e n u m b e r a n d g e o m e t r y o f l i g a n d s p r e f e r r e d b y c o p p e r ( I ) a r e e x t r e m e l y s e n s i t i v e t o m i n o r l i g a n d a l t e r a t i o n s . T h e s e effects are n o t w e l l - d e f i n e d , a n d , as a r e s u l t , o n e m u s t e x e r c i s e c a u t i o n i n t h e p r e d i c t i o n o f s t r u c t u r e s for c u p r o u s c o m p o u n d s . W i t h the exception o f C o m p o u n d s 2 - 4 , 2 1 , a n d 22, w h i c h e x h i b i t e d i r r e v e r s i b l e e l e c t r o c h e m i s t r y , a l l b i n u c l e a r c o p p e r c o m p l e x e s ex h i b i t t w o o n e - e l e c t r o n r e d o x p r o c e s s e s at w e l l - d e f i n e d p o t e n t i a l s . B y s y s t e m a t i c v a r i a t i o n s i n t h e b r i d g i n g ( X ) a n d s i d e a r m ( R ) g r o u p s (see S c h e m e II a n d T a b l e I), a w i d e range o f r e d u c t i o n potentials was r e a l i z e d . T h e h i g h e s t r e d u c t i o n p o t e n t i a l s w e r e o b s e r v e d for C o m p l e x 16 i n w h i c h t h e c o p p e r ( I ) c e n t e r s a r e s o m e w h a t b u r i e d w i t h i n t h e h y d r o p h o b i c s u b s t i t u e n t s . W i t h r e d u c t i o n p o t e n t i a l s o f E{ = 0 . 2 3 9 V a n d Ε 2 = 0 . 0 8 0 V , t h e l i g a n d e n v i r o n m e n t i n 16 r e p r e s e n t s a s u b s t a n tial i m p r o v e m e n t o v e r the o r i g i n a l square-planar, tetracoordinated l i g a n d e n v i r o n m e n t p r o v i d e d b y t h e s t a r t i n g m o d e l , 1. T h e s e h i g h r e d u c t i o n potentials are c o m p a r a b l e to those o f the p r o t e i n b i n u c l e a r sites (13-16); h e n c e , i t is n o t u n r e a s o n a b l e to b e l i e v e t h a t t h e s e p r o teins u t i l i z e o n l y n i t r o g e n and/or o x y g e n l i g a n d s a r o u n d e a c h copper. I n d e e d , d i c o o r d i n a t i o n a n d / o r t r i c o o r d i n a t i o n c a n b e c o n s i d e r e d for t h e r e d u c e d f o r m o f t h e b i n u c l e a r sites.



6.

GAGNÉ ET AL.


155

The oxidation/reduction of these compounds in well-separated, one-electron steps contrasts with the available electrochemical infor mation on the binuclear protein sites. Although no electrochemical data are available on hemocyanin, for both laccase and tyrosinase a single potential was associated with the overall two-electron reduction of the binuclear copper site (13-16, 65). Correlations between the electrochemical behavior of the new compounds, 5-19, and of the proteins are difficult because of solvation effects, the different tech niques employed, possible nonequilibria in the proteins, and other reasons (65, 66). Nonetheless, the electrochemical behavior reported for the new compounds suggests that the two-electron reduction of laccase and of tyrosinase is not a simple reduction of two equivalent copper ions that strongly interact through bridging ligands.

Acknowledgments We greatly appreciate assistance given by Richard E . Marsh and Michael McCool, and financial support from the National Institutes of Health.

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The Synthesis, Electrochemistry, and Reactivity of Binuclear Copper(I

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