Synthetic Modeling of the Interactions of Nitrogen ... - ACS Publications

7 Synthetic Modeling of the Interactions of Nitrogen Oxides with Copper Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 17, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1995-0246.ch007

Proteins Copper Nitrosyl Complexes Relevant to Putative Denitrification Intermediates William B. Tolman University of Minnesota, 207 Pleasant Street S.E., Minneapolis, MN 55455

This chapter focuses on the chemistry of biomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed; the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented; and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NO - of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 2

-

THE CRITICAL ROLE OFMETALLOPROTEINS

in mediating interconversions of nitrogen oxides ( N O ) within the global nitrogen cycle is now wellestablished (1-3). In particular, copper-containing proteins from anaerobic bacteria function as catalysts for the dissimilatory reduction of nitrite ( N 0 ~ ) and nitrous oxide ( N 0 ) to gaseous nitric oxide ( N O , N 0 , or N ) . Dissimilatory nitrogen oxide reduction (denitrification) is a respiratory process with intriguing analogies to the dioxygen consumption performed by most organisms, but which differs from the assimilatory x

2

y

2

2

0065-2393/95/0246-0195/$08.54/0 © 1995 American Chemical Society

In Mechanistic Bioinorganic Chemistry; Thorp, H., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

2

196

MECHANISTIC BIOINORGANIC CHEMISTRY

reduction process b y w h i c h oxidized nitrogen compounds are r e d u c e d to a m m o n i a that is s u b s e q u e n t l y u s e d as a b u i l d i n g b l o c k f o r t h e s y n t h e s i s of nitrogen-containing biomolecules. Impetus for the study o f denitrification is p r o v i d e d b y t h e n u m e r o u s e n v i r o n m e n t a l c o n s e q u e n c e s (4) of the uptake or production o f the simple inorganic nitrogen compounds involved i n the process: N 0 " and N 0 ~ , important components o f fert i l i z e r s a n d agents r e s p o n s i b l e f o r l a k e e u t r o p h i c a t i o n ; N 0 , a g r e e n h o u s e gas t h a t h a s b e e n i m p l i c a t e d i n o z o n e d e p l e t i o n ; a n d N O , a n important pollutant and biological effector molecule. D e v e l o p i n g an u n derstanding o f the mechanisms b y w h i c h the copper-containing enzymes i n v o l v e d i n d e n i t r i f i c a t i o n r e d u c e n i t r o g e n o x i d e s is t h u s a n i m p o r t a n t research objective. 3

2

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 17, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1995-0246.ch007

2

Significant information o n the chemistry o f nitrogen oxide c o p p e r protein adducts can b e u n c o v e r e d b y examining i n detail the structures and r e a c t i v i t y o f a p p r o p r i a t e active site m o d e l c o m p l e x e s . T h i s c h a p ter w i l l focus o n t h e chemistry o f b i o m i m e t i c c o p p e r nitrosyl c o m plexes relevant to t h e N O - c o p p e r interactions i n proteins that are central players i n dissimilatory nitrogen oxide reduction. A d d i t i o n a l reasons for e m p h a s i z i n g c o p p e r nitrosyls i n c l u d e t h e i r p r e s u m e d form a t i o n i n e x p e r i m e n t s u s i n g N O as a n 0 a n a l o g u e a n d p r o b e o f d i o x y g e n - a c t i v a t i n g c o p p e r p r o t e i n s (5-13) a n d t h e r e c e n t i d e n t i f i c a t i o n o f N O as a p e r v a s i v e b i o m o l e c u l e t h a t h a s m e t a l s i t e s w i t h i n p r o t e i n s as a m a j o r t a r g e t (14-16). A f t e r t h e c u r r e n t s t a t e o f k n o w l e d g e o f N O - c o p p e r interactions i n nitrite reductase (NiR), a central denitrif y i n g e n z y m e , is b r i e f l y s u r v e y e d , t h e s y n t h e s e s , s t r u c t u r e s , a n d r e a c tivity o f c o p p e r n i t r o s y l m o d e l c o m p l e x e s p r e p a r e d to date w i l l b e presented, a n d t h e insight these m o d e l studies p r o v i d e into t h e mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates w i l l b e discussed. 2

The Copper Nitrite Reductase Nitrosyl Intermediate Several copper-containing N i R s have b e e n identified, b u t the most extensive structural a n d mechanistic studies have focused o n t h e e n z y m e f r o m Achromobacter cycloclastes (17-25). A 2 . 3 - À r e s o l u t i o n X - r a y c r y s tal s t r u c t u r e f o r t h i s N i R i n its o x i d i z e d f o r m at p H 5 . 2 has b e e n r e p o r t e d (17), a n d a r e p r e s e n t a t i o n o f t h e a c t i v e site is s h o w n i n F i g u r e 1. E a c h m o n o m e r i n the t r i m e r i c protein contains two c o p p e r ions, one o f w h i c h ( C u - 1 ) is l i g a t e d t o a c y s t e i n e , a m e t h i o n i n e , a n d t w o h i s t i d i n e r e s i d u e s i n a g e o m e t r y similar to that o f " t y p e 1 " c o p p e r centers i n proteins s u c h as p l a s t o c y a n i n (26). T h e s e c o n d " t y p e 2" c o p p e r i o n i n N i R ( C u 2) is o n l y 1 2 . 5 - À d i s t a n t f r o m t h e first a n d is b o u n d t o t h r e e h i s t i d i n e i m i d a z o l e s (two f r o m o n e m o n o m e r , t h e t h i r d f r o m a n a s s o c i a t e d s u b u n i t ) and a fourth small ligand i n an unusual tetrahedral arrangement. T h e


7.

197

Nitrogen Oxides and Copper Proteins

TOLMAN

Met 150


I

Me

His95

Ν His306 Η (from another Cu-II subunit)

Cu-I

12.5 À Figure 1. Schematic representation of the copper sites in nitrite reductase from Achromobacter cycloclastes (11).

c o p p e r ions are c o n n e c t e d to each other b y a d i p e p t i d e b r i d g e , C y s 1 3 6 - H i s - 1 3 5 . T h e f o u r t h s m a l l l i g a n d o n C u - 2 is p r e s u m a b l y a w a t e r m o l e c u l e , m a k i n g C u - 2 a g o o d c a n d i d a t e f o r t h e site o f s u b s t r a t e b i n d i n g and reduction. P r e l i m i n a r y o b s e r v a t i o n o f a d d i t i o n a l e l e c t r o n d e n s i t y at t h i s f o u r t h c o o r d i n a t i o n p o s i t i o n o f C u - 2 u p o n s o a k i n g crystals w i t h N 0 ~ is c o n sistent w i t h t h i s i d e a . T h u s , f r o m t h e s t r u c t u r a l d a t a i t w o u l d a p p e a r that C u - 1 is a t y p e 1 c e n t e r that functions t o transfer electrons t o t h e c a t a l y t i c C u - 2 i o n (See N o t e A d d e d i n P r o o f ) . I t h a s b e e n s u g g e s t e d , l a r g e l y o n t h e basis o f e l e c t r o n i c s t r u c t u r a l c o n s i d e r a t i o n s ( 2 7 , 28), t h a t the C y s - 1 3 6 - H i s - 1 3 5 l i n k b e t w e e n C u - 1 a n d C u - 2 is a possible c o n d u i t f o r e l e c t r o n t r a n s f e r b e t w e e n t h e t w o sites. A n a n a l o g o u s d i p e p t i d e bridge between the type 1 center and the catalytic tricopper cluster i n a s c o r b a t e o x i d a s e ( 2 9 , 30) m a y f u n c t i o n s i m i l a r l y . I n d e e d , o t h e r c l o s e similarities b e t w e e n p r o t e i n domains i n ascorbate oxidase a n d N i R have b e e n n o t e d (17). 2

T h e i d e a t h a t N 0 ~ b i n d i n g a n d r e d u c t i o n o c c u r s at C u - 2 i n N i R f r o m A . cycloclastes is also s u p p o r t e d b y r e c e n t m e c h a n i s t i c e x p e r i m e n t s 2

(24). I n e x a m i n a t i o n s o f e n z y m e p r e p a r a t i o n s h a v i n g m a x i m u m C u - 1 c o n t e n t b u t d e p l e t e d C u - 2 sites, s p e c i f i c a c t i v i t y ( p r o d u c t i o n o f N O from N 0 ~ ) was found to b e directly proportional to the C u - 2 content. T h i s r e s u l t a r g u e s against a p r e v i o u s s u g g e s t i o n (25) t h a t C u - 1 is t h e site o f c a t a l y s i s i n t h e e n z y m e a n d i m p l i e s t h a t t h e l o w s p e c i f i c a c t i v i t i e s of o t h e r c o p p e r N i R s that are r e p o r t e d to c o n t a i n o n l y t y p e 1 centers (31, 32) m a y b e d u e t o t h e fact t h a t t y p e 2 c e n t e r s a r e r e q u i r e d i n t h e s e 2


198


s y s t e m s , t o o (33). T h u s , it has b e e n p r o p o s e d t h a t s m a l l a m o u n t s o f t y p e 2 sites m a y a c t u a l l y he r e s p o n s i b l e f o r t h e o b s e r v e d a c t i v i t y i n t h e p u r p o r t e d type 1-only N i R s . F u r t h e r indications o f the critical substrate b i n d i n g r o l e o f t h e C u - 2 site i n c l u d e s p e c t r o s c o p i c d a t a a c q u i r e d f o r t h e N i R f r o m Alcaligenes xylosoxidans (33, 34), r e p o r t s o f p r e l i m i n a r y e l e c t r o n paramagnetic resonance ( E P R ) spectroscopy data that show c h a n g e s o n l y i n t h e A . cycloclastes N i R C u - 2 s i g n a l u p o n t r e a t m e n t o f s o l u t i o n s o f t h e e n z y m e w i t h excess N 0 ~ (24), a n d e l e c t r o c h e m i c a l experiments that c o m p a r e the apo, C u - 2 - d e p l e t e d , a n d fully m e t a l l a t e d e n z y m e s (35).


2

Several pieces o f evidence, albeit mostly indirect, implicate a nitrosyl a d d u c t t o C u - 2 as a k e y r e a c t i o n i n t e r m e d i a t e i n t h e g e n e r a t i o n o f N O or N 0 from N 0 " ( 2 i - 2 5 ) . T h u s , all proposed mechanisms o f nitrite r e d u c t i o n have i n c o m m o n an initial, reversible dehydration o f a coord i n a t e d N 0 ~ t o y i e l d a c o p p e r n i t r o s y l , w r i t t e n as C u - N O t o e m phasize the p r e s u m e d electrophilic character o f the b o u n d N O ligand a n d t h e n e e d f o r a n e l e c t r o n f r o m c o p p e r d u r i n g c a t a l y s i s (21-25) ( S c h e m e 1). T h e f o r m u l a t i o n o f t h e c o p p e r n i t r o s y l is also b a s e d o n a n a l ogies d r a w n to m o r e fully c h a r a c t e r i z e d nitrosyls o f i r o n - c o n t a i n i n g e n z y m e s a n d m o d e l c o m p l e x e s (36, 37). C e r t a i n l y , f o r m a t i o n o f the p r i n c i p a l e n z y m e r e a c t i o n p r o d u c t , f r e e N O , d u r i n g c a t a l y s i s is r e a d i l y e x p l a i n e d b y i n v o k i n g l i g a n d loss a n d C u ( I I ) g e n e r a t i o n f r o m s u c h a c o p p e r nitrosyl. Initial evidence for this species was p r o v i d e d b y N labeled nitrite- a n d azide-trapping experiments performed with cellf r e e e x t r a c t s f r o m A . cycloclastes, w h i c h w e r e i n t e r p r e t e d t o i n d i c a t e p a r t i t i o n i n g b e t w e e n pathways o f attack b y t h e l a b e l e d substrates o n t h e e l e c t r o p h i l i c C u - N O u n i t (21). I n c o r p o r a t i o n o f O f r o m H O i n t o p r o d u c t N 0 w h e n N O w a s i n t r o d u c e d as s u b s t r a t e i n w h o l e c e l l s has also b e e n c i t e d as s u p p o r t i n g e v i d e n c e f o r t h e n i t r o s y l i n t e r m e d i a t e (22), b e c a u s e it is k n o w n that s u c h e l e c t r o p h i l i c M - N O s p e c i e s u n d e r g o 2

2

+

2

+

1

+

+

l

s

5

2

l

s

2

+

2H

E-Cu

+

N0 " 2

E-Cu -IN0 ' +

2

H0 2

E-Cu -NO +

+

E-Cu + N 0

N0 '

+

2

E-Cu

+

2

E-Cu^ + NO +

E-Cu + N 0 +

2+

Scheme 1


2

7.

TOLMAN

199


r a p i d and reversible h y d r a t i o n - d e h y d r a t i o n processes that result i n ex c h a n g e o f o x y g e n i s o t o p e s (38-40). I n a d d i t i o n , a C u - N O s p e c i e s h a s b e e n i n v o k e d to explain t h e observed i n c o r p o r a t i o n o f Ν atoms f r o m b o t h N O a n d N 0 " i n t o p r o d u c t N 0 (23) ( S c h e m e 1). T h e r e v e r s i b l e bleaching o f the type 1 copper E P R and U V - v i s signatures i n N i R u p o n a d d i t i o n o f N O h a s also b e e n c i t e d as e v i d e n c e f o r t h e g e n e r a t i o n o f a C u - N O i n t e r m e d i a t e d u r i n g c a t a l y s i s , a l t h o u g h i t is p r o b a b l e t h a t t h e e n z y m e u s e d i n these e x p e r i m e n t s l a c k e d t h e C u - 2 site s u b s e q u e n t l y s h o w n t o b e l i n k e d t o h i g h s p e c i f i c a c t i v i t y (25). I n s u m , c i r c u m s t a n t i a l e v i d e n c e f o r t h e C u - N O d e n i t r i f i c a t i o n i n t e r m e d i a t e exists. H o w e v e r , it has n e v e r b e e n o b s e r v e d d i r e c t l y , a n d n o s p e c t r o s c o p i c e v i d e n c e h a s b e e n r e p o r t e d to e i t h e r c o n f i r m o r d i s p u t e its e l e c t r o n i c s t r u c t u r a l formulation. +

2


+

+

2

+

+

+

Studies of N O b i n d i n g to other copper proteins have been equally inconclusive, and the geometric and electronic structures o f the p u tative adducts that have b e e n r e p o r t e d r e m a i n obscure. I n attempts to p r o d u c e p r o t e i n - C u N O c o m p l e x e s , examples f r o m each o f the m a j o r classes o f c o p p e r - c o n t a i n i n g e n z y m e s h a v e b e e n t r e a t e d w i t h N O , i n c l u d i n g t y p e 1 [ e . g . , a z u r i n (5)], t y p e 2 [ e . g . , s u p e r o x i d e d i s m u t a s e (5)], t y p e 3 [e.g., h e m o c y a n i n ( 6 - 9 ) ] , a n d o t h e r s [e.g., g a l a c t o s e o x i d a s e (41, 42), c y t o c h r o m e c o x i d a s e (10-12), l a c c a s e (13), a n d a s c o r b a t e o x i d a s e (5)]. E v i d e n c e c i t e d i n t h e s e s t u d i e s i n s u p p o r t o f s u c c e s s f u l generation of copper nitrosyls have included the observation of bleaching o f E P R and U V - v i s spectroscopic features u p o n treatment of o x i d i z e d type 1 centers w i t h N O a n d restoration o f these signals u p o n p u r g i n g o f p r o t e i n s o l u t i o n s w i t h N o r u p o n p h o t o l y s i s ( 5 , 25), a n d t h e o b s e r v a t i o n o f a n u n u s u a l E P R s i g n a l w i t h a f e a t u r e at g < 2 with hyperfine coupling to the nitrosyl nitrogen upon treatment of r e d u c e d l a c c a s e w i t h N O (13). I n s o m e i n s t a n c e s , N O b i n d s b u t n o t t o t h e c o p p e r c e n t e r (41, 42). M o r e c o m m o n l y , c o m p l e x r e d o x c h e m istry ensues w h e n c o p p e r p r o t e i n s are e x p o s e d to N O , a n d m u c h w o r k is s t i l l n e e d e d t o d i f f e r e n t i a t e b e t w e e n N O b i n d i n g , c h a r g e - t r a n s f e r c o m p l e x formation, and electron-transfer processes i n these reactions and to d e t e r m i n e t h e mechanisms o f formation o f other n i t r o g e n ox i d e s ( N 0 " , N 0 , e t c . ) t h a t h a v e b e e n o b s e r v e d (5-13). 2

2

2

Copper Nitrosyl Complexes T h e need for structural, spectroscopic, and reactivity benchmarks w i t h w h i c h to compare and help interpret data acquired for all the copper p r o t e i n systems, N i R i n p a r t i c u l a r , has l e d to an interest i n p r e p a r i n g c o p p e r nitrosyl m o d e l c o m p l e x e s that m a y b e m o r e r e a d i l y c h a r a c t e r ized i n detail. Such complexes are rare and, until recently, none h a d been structurally characterized b y X - r a y crystallography. Solids having


200


the e m p i r i c a l formula C u X N O (X = C l or Br) have b e e n isolated from d e e p p u r p l e solutions f o r m e d u p o n exposure of c u p r i c halides to N O i n a n h y d r o u s a l c o h o l o r a c e t o n i t r i l e (43, 44). T h e p r i m a r y e v i d e n c e for the b i n d i n g of N O to c o p p e r i n these species was p r o v i d e d b y manometric N O - u p t a k e experiments and the observation of bands between 1830 and 1870 c m i n the infrared spectra of the solids. N o a d d i t i o n a l s t r u c t u r a l i n f o r m a t i o n has b e e n r e p o r t e d . T h e s e c o m p o u n d s h a v e b e e n u s e d as r e a g e n t s f o r n i t r o s a t i o n o f s e v e r a l c l a s s e s o f o r g a n i c m o l e c u l e s , s u g g e s t i n g t h a t t h e n i t r o s y l g r o u p is e l e c t r o p h i l i c (45). I n terestingly, the complexes react w i t h N ~ to f o r m N , N 0 , a n d the c u p r o u s h a l i d e , a r e a c t i o n r e l e v a n t t o t h a t w h i c h is p o s t u l a t e d t o o c c u r w h e n a z i d e is u s e d t o t r a p t h e p u t a t i v e e l e c t r o p h i l i c C u - N O NiR i n t e r m e d i a t e (21). 2


- 1

3

2

2

+

+

W e a k a n d r e v e r s i b l e b i n d i n g of N O to the t e t r a g o n a l Cu(II) c o m plexes C u L (L = dithiocarbamato, dithiophosphato, or 8-mercapt o q u i n o l i n a t o ) has also b e e n r e p o r t e d o n t h e basis o f o b s e r v e d b r o a d e n i n g o f E P R s i g n a l s u p o n e x p o s u r e o f t h e c o m p o u n d s t o N O (46). R e g e n e r a t i o n of the o r i g i n a l sharp spectra was o b s e r v e d u p o n subs e q u e n t p u r g i n g w i t h N o r A r . It w a s s u g g e s t e d t h a t t h e E P R s i g n a l broadening resulted from r a p i d exchange of b o u n d with u n b o u n d N O but, again, no other structural information (including infrared spectra i n this instance) w e r e r e p o r t e d that substantiated the hypothesis of N O adduct formation. 2

2

T h e first c o p p e r n i t r o s y l c o m p l e x t o b e 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 , 2 , w a s p r e p a r e d b y t r e a t m e n t o f a d i c o p p e r ( I ) c o m p o u n d , 1, w i t h N O + (47) ( S c h e m e 2). It w a s also s y n t h e s i z e d f r o m t h e d i c o p p e r ( I ) p r e c u r s o r , n - B u N ( N 0 ) , a n d t w o equivalents of H P F * E t 0 i n a r e a c t i o n that m o d e l s t h e p r o p o s e d i n i t i a l n i t r i t e d e h y d r a t i o n s t e p ( S c h e m e 1) c a r r i e d o u t b y c o p p e r N i R (48). A n X - r a y c r y s t a l l o g r a p h i c s t u d y r e v e a l e d a symmetrically b o u n d n i t r o s y l l i g a n d i n 2 w i t h b o n d distances C u - N = 2 . 0 3 6 (10) À a n d N - O = 1 . 1 7 6 (1) A . ( T h e n u m b e r s i n p a r e n t h e 4

2

6

2


7.

TOLMAN


201

ses r e p r e s e n t s t a n d a r d d e v i a t i o n s . ) T h e r e p o r t e d J>(NO) b a n d at 1 5 3 6 cm"

1

is c o n s i s t e n t w i t h t h e b r i d g e d n i t r o s y l s t r u c t u r e . F o r m u l a t i o n o f 2

as a C u ( I I ) - ( N O " ) - C u ( I I ) c o m p l e x , r e s u l t i n g f r o m t r a n s f e r o f t w o e l e c trons f r o m the dicopper(I) p r e c u r s o r to N O , was b a s e d o n the obser +

vation of square p y r a m i d a l c o p p e r geometries t y p i c a l for Cu(II) ions a n d a f e a t u r e i n t h e e l e c t r o n i c a b s o r p t i o n s p e c t r u m at 7 3 0 n m (ε ~ 5 0 0 ) a t t r i b u t a b l e t o a C u ( I I ) d d t r a n s i t i o n . T h e c o m p l e x has μ

Λ

= 0.59 M /CU B


a n d is E P R - s i l e n t , p r e s u m a b l y b e c a u s e t h e C u ( I I ) sites a r e s t r o n g l y a n tiferromagnetically coupled. D i n u c l e a r 2 a n d its d i c o p p e r ( I ) p r e c u r s o r h a v e b e e n s h o w n t o m e diate the formation of N ment of 2 with N 0

2

b r i d g e d dicopper(II)

2

0 from N 0

" o r N O (48). F o r e x a m p l e , t r e a t

2

" p r o d u c e d 1 m o l N 0 , 0.5 m o l 0 , and the oxo2

complex 3 (Scheme

2

3). Formation of N

2

0 and

c o p r o d u c t 3 w a s also i n d u c e d b y t r e a t i n g d i c o p p e r ( I ) c o m p l e x 1 w i t h N O

(Scheme

3). In similar reactions using related copper(I) starting

m a t e r i a l s , u n s t a b l e c o l o r e d i n t e r m e d i a t e s w e r e o b s e r v e d at l o w t e m p e r a t u r e , w h i c h w e r e f o r m u l a t e d as 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 d i copper(II) complexes containing two b r i d g i n g nitrosyl ( N O " ) ligands on the basis o f m a n o m e t r i c N O - u p t a k e m e a s u r e m e n t s , an o b s e r v e d

low

J>(NO) i n t h e I R s p e c t r u m ( 1 4 6 0 c m " ) , a n d t h e l a c k o f a n E P R s i g n a l . 1

Scheme 3


202


It w a s s u g g e s t e d t h a t a d i b r i d g e d d i n i t r o s y l s t r u c t u r e f o r t h e u n s t a b l e intermediates might facilitate t h e N - N c o u p l i n g reaction, an hypothesis analogous to t h e postulated necessity for a cis disposition o f two nitrosyls i n m o n o n u c l e a r transition m e t a l c o m p l e x e s that d e c o m p o s e to N 0 o r N (49). B o t h c o p p e r - p r o m o t e d N 0 g e n e r a t i o n r e a c t i o n s (2 p l u s N 0 " and 1 a n d analogues plus N O ) m o d e l p r o p o s e d N i R N 0 p r o d u c t i o n p a t h w a y s i n s o f a r as N - N b o n d f o r m a t i o n is i n d u c e d v i a t h e i n t e r m e d i a c y of a c o p p e r nitrosyl c o m p o u n d . A l t h o u g h a type 1 center 12.5-Â distant f r o m t h e c a t a l y t i c site i n N i R p r o b a b l y s u p p l i e s t h e n e e d e d e l e c t r o n s for n i t r i t e r e d u c t i o n , h o w e v e r , t h e s e c o n d m u c h closer c o p p e r i o n i n the synthetic system that participates i n n i t r o s y l c o m p l e x f o r m a t i o n a n d m e d i a t e s t h e o x y g e n a t o m t r a n s f e r r e a c t i o n s is a b s e n t i n t h e e n z y m e . T h u s , t h e relevance o f this m o d e l i n g c h e m i s t r y to N i R structure a n d f u n c t i o n is s o m e w h a t l i m i t e d . 2

2

2

2


2

M o t i v a t e d b y the lack o f well-characterized mononuclear models f o r c o p p e r N i R a c t i v e site c o m p l e x e s , w e h a v e b e g u n t o e x p l o r e t h e interactions o f simple nitrogen oxides w i t h molecules containing a single c o p p e r i o n (50-53)(See N o t e A d d e d i n P r o o f ) . T a k i n g o u r c u e f r o m p r e vious successful preparations o f reactive bioinorganic m o d e l complexes (i.e., see r e f e r e n c e s 5 4 a n d 5 5 ) , w e h a v e u s e d s t e r i c a l l y h i n d e r e d 3 , 5 s u b s t i t u t e d t r i s ( p y r a z o l y l ) h y d r o b o r a t e s (Tp**'; R = H , R ' = t-Bu; R = R ' = P h ; R = R ' = M e ) as s u p p o r t i n g l i g a n d s (56). T h e a r r a y o f p y r a z o l y l N - d o n o r s i n these ligands closely approximates t h e histidine i m i d a z o l y l g r o u p s c o o r d i n a t e d t o C u ( I I ) i n N i R f r o m A. cycloclastes. I n h i b i t i o n o f undesired polynuclear complex formation and increased kinetic stability of the targeted C u N O unit w e r e anticipated to result from the presence of the b u l k y substituents attached to the tris(pyrazolyl) frame. O u r initial synthetic strategy has i n v o l v e d t r e a t i n g suitable Cu(I) precursors w i t h N O , because w e reasoned that a m o n o n u c l e a r C u ( I I ) - ( N O " ) species w o u l d r e s u l t v i a a r e d o x p r o c e s s a n a l o g o u s t o , b u t i n v o l v i n g o n e less electron than, that i n v o l v e d i n t h e k n o w n reaction o f N O w i t h t h e dicopper(I) c o m p o u n d 1 to generate the C u ( I I ) - ( N O ~ ) - C u ( I I ) unit i n 2 (47). +

A n a r r a y o f n e w (57) a n d p r e v i o u s l y p r e p a r e d (58) Τ ρ ^ Ό ι φ s t a r t i n g materials were synthesized b y m i x i n g M T p ' ( M = K or T l ) w i t h C u C l in organic solvent (Scheme 4). X - r a y crystal structure determinations r e v e a l e d that a l l o f t h e c o m p l e x e s , w i t h t h e e x c e p t i o n o f 7, a r e d i m e r s R R

+

+

b r i d g e d b y t w o T p * * ' l i g a n d s i n t h e s o l i d state (57, 58) ( F i g u r e s 2 - 4 ) . F o r t h e d i m e r s w i t h R = R ' = M e o r P h (4 a n d 5 , r e s p e c t i v e l y ) , t w o p y r a z o l y l rings o f each ligand are b o n d e d to one Cu(I) i o n and t h e t h i r d is c o o r d i n a t e d t o t h e o t h e r m e t a l c e n t e r , r e s u l t i n g i n p l a n a r 3 - c o o r d i n a t e Cu(I) geometries i n the complexes (Figure 2). I n solution, b o t h molecules e x h i b i t d e c e p t i v e l y s i m p l e a n d sharp H N M R spectra that d o n o t b r o a d e n a p p r e c i a b l y u p o n c o o l i n g , w h i c h is c o n s i s t e n t w i t h t h e e x i s t e n c e 1


7.

TOLMAN


203

[Tp Cu] RR

2

4 Tp (ref. 58) 5 Tp 6 Tp'" Me2

Ph2


Bu

Tp Cu(3,5-Ph pz) RR

R' R = R* = Me; T p R = R' = Ph; T p R = H, R" = i-Bu; Tp

2

7 Tp Ph2 1

Me2

Ph2

3,5-Ph pz 2

f_Bu

Scheme 4

of r a p i d p y r a z o l y l exchange processes. I n contrast, t h e d i m e r f o r m e d w h e n R = H a n d R ' = f - B u (6) c o n t a i n s l i n e a r t w o - c o o r d i n a t e C u ( I ) i o n s and one uncoordinated pyrazolyl group o n each b r i d g i n g ligand (Figure 3). F l u x i o n a l p r o c e s s e s a r e s l o w e d i n t h i s c o m p l e x d u e t o t h e i n c r e a s e d steric b u l k o f the p y r a z o l y l 3-substituent, a l l o w i n g calculation o f acti vation parameters for apparent intramolecular pyrazole exchange v i a l i n e s h a p e a n a l y s i s o f v a r i a b l e t e m p e r a t u r e H N M R s p e c t r a [Δίί* = 1 1 . 7 (5) k c a l m o l " a n d AS* = - 9 (2) e u ] . F i n a l l y , m i x i n g M T p and 3,5diphenylpyrazole (3,5-Ph pz) with C u C l yielded the distorted tetra h e d r a l m o n o m e r 7 ( F i g u r e 4 ) . T h i s m o l e c u l e is also h i g h l y fluxional i n solution, w i t h exchange o f b o u n d w i t h added u n b o u n d pyrazole being p a r t i c u l a r l y f a c i l e b y Yi N M R s p e c t r o s c o p y . l

1

p h 2

2

l

D i m e r 6 a n d m o n o m e r 7 ( d i m e r 5 as w e l l , b u t m o r e s l o w l y ) d i s solved i n organic solvent r a p i d l y reacted w i t h N O to form deep r e d or orange solutions, respectively (52, 53). T h e solutions e x h i b i t e d s t r o n g I R a b s o r p t i o n s at 1 7 1 2 c m " ( T p ) and1720c m " (Tp ) t h a t w e r e a s s i g n e d as *>(NO) b a n d s o n t h e b a s i s o f t h e i r a p p r o p r i a t e shifts to l o w e r e n e r g y u p o n i s o t o p i c s u b s t i t u t i o n ( N O ) . F r o m these data alone it was evident that c o p p e r nitrosyl adducts h a d f o r m e d . T h e s e results also i n d i c a t e d that t h e T p l i g a n d causes t h e c o p p e r ion to b e more electron-deficient than that b o u n d to T p , a conclu sion corroborated b y t h e carbonyl-stretching frequencies o f t h e corresponding complexes T p C u ( C O ) K C O ) for T p = 2069 c m " (57); / / ( C O ) f o r T p = 2 0 8 6 c m " (59)]. 1

t B u

1

p h 2

1 5

p h 2

i B u

R B /

p h 2

i B u

1

1

T h e respective r e d and orange colors o f the N O - s a t u r a t e d solutions o f t h e C u ( I ) c o m p l e x e s r e s u l t f r o m e l e c t r o n i c a b s o r p t i o n f e a t u r e s at 4 9 8 n m ( T p ) a n d 4 7 8 n m ( T p ) , respectively (Figure 5). Placing the f B u

P h 2



ph2

2

ph2

ph2

2

Me2

2

Figure 2. Left: ORTEP drawing of[Tp Cu] , 5 (hydrogen atoms omitted for clarity). Right: ORTEP drawing of core of[Tp Cu] , 5, with phenyl groups removed and atom labels shown for all noncarbon atoms on one of the two symmetry equivalent Tp ligands (57). The structure is analogous to that of[Tp Cu] , 4 (58).



tBu

2

tBu

tBu

2>

Figure 3. Left: ORTEP drawing of[Tp Cu] 6 (hydrogen atoms omitted for clarity) (57). Right: ORTEP drawing of core of[Tp Cu] , 6, with tert-butyl groups removed and atom labels shown for the noncarbon atoms on one of the two inequivalent Tp ligands.



206


Figure 4. ORTEP drawing ofTp Cu(3,5-Ph pz), moved for clarity) (57). ph2

7 (hydrogen atoms re-

2

solutions u n d e r a v a c u u m or p u r g i n g w i t h N or A r caused these abs o r p t i o n b a n d s to b l e a c h , a n d H N M R a n a l y s i s o f t h e r e s u l t i n g c o l o r l e s s solutions i n d i c a t e d almost quantitative regeneration of the Cu(I) p r e c u r s o r s . T h u s , t h e b i n d i n g o f N O is r e v e r s i b l e . I n d e e d , m a n o m e t r i c measurements for the T p case i n d i c a t e d t h a t o n l y ~ 4 0 % o f t h e a v a i l a b l e c o p p e r i o n s a r e c o o r d i n a t e d t o N O i n s o l u t i o n at 2 3 ° C , w i t h i n c r e a s i n g a m o u n t s o f N O b o u n d as t h e t e m p e r a t u r e is l o w e r e d (53). T h e s e d a t a suggest that t h e C u ( I ) p r e c u r s o r s a n d N O are i n d y n a m i c e q u i l i b r i u m w i t h the r e s p e c t i v e n i t r o s y l adducts (Scheme 5), w i t h an a p p r o x i m a t e K = 1 4 0 M " . U s i n g the results of the m a n o m e t r y experiments, ext i n c t i o n coefficients for the e l e c t r o n i c absorption features of the n i t r o s y l c o m p o u n d s w e r e c a l c u l a t e d t o b e ~ 1 4 0 0 M c m , w h i c h suggests t h a t t h e a b s o r b a n c e s s h o u l d b e a t t r i b u t e d to c h a r g e - t r a n s f e r ( C T ) t r a n s i t i o n s . M o r e o v e r , a shift o f t h e a b s o r p t i o n to h i g h e r e n e r g y i n t h e c o m p o u n d w i t h the m o r e electron-deficient c o p p e r i o n ( T p ) favors a m e t a l - t o ligand charge transfer ( M L C T ) rather than a ligand-to-metal charge transfer ( L M C T ) assignment, an observation of some i m p o r t a n c e i n defining t h e e l e c t r o n i c s t r u c t u r e o f t h e c o p p e r n i t r o s y l u n i t (see l a t e r ) . 2

X

i B u

e q

1

_ 1

_ 1

p h 2

B y cooling an N O - s a t u r a t e d solution of 6 i n aromatic solvent, X - r a y quality crystals of T p C u ( N O ) , 8, w e r e o b t a i n e d a n d t h e r e s u l t i n g t B u



7.

TOLMAN

1

1

1

400

1

.

500 Wavelength

Figure 5. (52,

207


I

1

600

700

(nm)

UV-vis spectra of CH Cl solutions of the Tp 'Cu(NO) complexes 2

m

2

s t r u c t u r e ( F i g u r e 6) p r o v i d e d d e f i n i t i v e p r o o f t h a t t h e C u ( I ) d i m e r h a d b e e n c l e a v e d t o f o r m t h e first e x a m p l e o f a m o n o n u c l e a r c o p p e r n i t r o s y l c o m p l e x . B o t h t h e s h o r t C u - N O a n d N - O b o n d l e n g t h s ( 1 . 7 6 a n d 1.11 Â, respectively) i n 8 are t y p i c a l for transition m e t a l n i t r o s y l c o m p o u n d s (60, 61). T h e C u - N - O b o n d a n g l e is 1 6 3 ° , i n t e r m e d i a t e b e t w e e n t h e linear (180°) and bent (120°) metal nitrosyl extremes. In addition, the c o p p e r i o n i n 8 has a n a p p r o x i m a t e l y t e t r a h e d r a l g e o m e t r y ( c l o s e to l o c a l C symmetry) similar to that f o u n d i n other four-coordinate Tp or T p c o m p l e x e s o f b o t h C u ( I ) a n d C u ( I I ) (57, 6 2 , 63). 3v

f B u

i P r 2

It is d i f f i c u l t t o j u d g e t h e e x t e n t t o w h i c h e l e c t r o n t r a n s f e r f r o m Cu(I) to N O to f o r m a C u ( I I ) - ( N O ~ ) species h a d o c c u r r e d u p o n a d d u c t f o r m a t i o n b y c o n s i d e r i n g j u s t t h e I R [J>(NO) = 1 7 2 0 c m " , i n t h e r e g i o n o f o v e r l a p b e t w e e n l i n e a r a n d b e n t n i t r o s y l c o m p l e x e s (60, 61)] a n d X ray crystal structure results (tetrahedral geometry a n d C u - N - O angle = 1 6 3 ° ) . I n o r d e r to u n d e r s t a n d the e l e c t r o n i c s t r u c t u r e o f the C u N O u n i t i n 8 a n d its T p c o n g e n e r , a series o f E P R , m a g n e t i c c i r c u l a r d i c h r o i s m ( M C D ) , a n d near-IR absorbance spectroscopic studies w e r e u n 1

p h 2

y2[Tp'- Cu] B u

2

+ N O ^ Tp'-

B u

Cu(NO)

Scheme 5


M E C H A N I S T I C BIOINORGANIC CHEMISTRY


208

Figure 6. Left: ORTEP drawing of Tp Cu(NO), 8 (hydrogen atoms re moved for clarity) (52, 53). Right: ORTEP drawing of core ofTp Cu(NO), 8, with important metrical parameters noted. tBu

tBu

d e r t a k e n (52, 53). T h e c o m b i n e d r e s u l t s o f t h e s e i n v e s t i g a t i o n s a r e b e s t rationalized b y an electronic structural description i n w h i c h the copper i o n r e t a i n s i t s f u l l set o f d e l e c t r o n s a n d t h e u n p a i r e d e l e c t r o n is s u b stantially l o c a l i z e d o n the N O ligand. U s i n g the valence b o n d formalism, w h i c h does not take into account the relatively h i g h degree o f covalency c o m m o n l y a t t r i b u t e d t o t r a n s i t i o n m e t a l n i t r o s y l s ( 6 0 , 61, 64), a C u ( I ) N O * ( S D d e s c r i p t i o n is f a v o r e d . A q u a l i t a t i v e m o l e c u l a r o r b i t a l p i c t u r e that takes into account t h e c o v a l e n c y i n t r i n s i c to t h e C u - N O b o n d i n g c a n also b e c o n s t r u c t e d a n d u s e d t o e x p l a i n t h e s p e c t r o s c o p i c d a t a (61, 64) ( F i g u r e 7 ) ; t h i s v i e w h a s b e e n s u p p o r t e d b y d e t a i l e d a b i n i t i o c a l c u l a t i o n s p e r f o r m e d o n t h e m o d e l ( N H ) C u N O (53). A c c o r d i n g t o t h e 3

3

q u a l i t a t i v e d e s c r i p t i o n t h e u n p a i r e d e l e c t r o n r e s i d e s i n a π* o r b i t a l p r i marily localized on N O but having an antibonding interaction w i t h the d ,d z x

y z

set o n c o p p e r ( 4 e i n F i g u r e 7, w i t h d e g e n e r a c y s p l i t b y a J a h n -

Teller distortion involving bending of the nitrosyl). T h e

filled

levels

b e l o w have largely d orbital character, w i t h contributions from either σ Ν Ο o r 7 Γ * Ν Ο as s h o w n . E a c h v i e w o f the b o n d i n g o f N O to c o p p e r predicts that d d features in the visible a n d near-IR regions of the absorption a n d M C D spectra o f the n i t r o s y l c o m p l e x e s w i l l b e absent, an hypothesis that was s u p p o r t e d b y experiment. T h e M L C T assignment for the — 5 0 0 n m optical ab s o r p t i o n b a n d is also c o n s i s t e n t w i t h t h e b o n d i n g p i c t u r e s , b e c a u s e t h e filled,

e s s e n t i a l l y d o r b i t a l set p r o h i b i t s a n a l t e r n a t i v e L M C T a t t r i b u t i o n .

A metal d

π*ΝΟ transition to y i e l d an essentially C u ( I I ) - ( N O )

state s e e m s r e a s o n a b l e — a n d

_

is p r e d i c t e d b y a b i n i t i o t h e o r y


excited (53)—

7.

TOLMAN

4e (π NO-xz,yz)

0 Ν J Cu... Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 17, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1995-0246.ch007

209


Ν

Ν ^3v

|

• 3E

4 (ζ -σΝΟ)-||— 3 ι

2

0

!

3e(xy,xV)=ft=!J=

Ζ

2e (xz.yz + π NO)

|[ 1l

Figure 7. Qualitative molecular orbital diagram for the Tp 'Cu(NO) complexes (based on analogous diagrams for {MNO} systems described in references 61 and 64.) (Reproduced from reference 53. Copyright 1993 American Chemical Society.) m

10

because of the presence of l o w - l y i n g , only partially occupied, 7τ*ΝΟ o r b i t a l s . T h e g e n e r a t i o n o f s u c h a n e l e c t r o n i c e x c i t e d s u p p o r t e d b y o b s e r v a t i o n o f a n i n t e n s e f e a t u r e at — 5 0 0 n m i n s p e c t r u m , s p i n - o r b i t c o u p l i n g i n t h e e x c i t e d state C u ( I I ) i o n i n a s t r o n g p s e u d o A - t e r m (52, 5 3 ) .

primarily state w a s the M C D resulting

T h e substantial localization of the u n p a i r e d electron i n the g r o u n d state o f the c o p p e r n i t r o s y l s i n t h e N O - b a s e d o r b i t a l s also r e s u l t s i n u n u s u a l E P R s p e c t r o s c o p i c p r o p e r t i e s ( F i g u r e 8). S i m u l a t i o n s o f X and S-band E P R data for N O and N O complexes y i e l d e d the c o m m o n p a r a m e t e r s g„ = 1 . 8 3 , g = 2 . 0 0 , = 116 G , A = 66 G , and A = 3 0 G (52, 53). I n t e r e s t i n g l y , t h e E P R s i g n a l is not o b s e r v a b l e at t e m p e r a t u r e s a b o v e 4 0 K , i n d i c a t i n g t h e p r e s e n c e o f a facile r e l a x a t i o n process w h i c h is c u r r e n t l y u n d e r i n v e s t i g a t i o n ( H . Koteiche, W . E. Antholine, C .E. Ruggiero, and W . B. Tolman, un p u b l i s h e d results). Observation o f g values

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