DE LA INFRACCIÓN A LA LEY DE PROTECCIÓN AL

A

CD

T

2,9, The im p o rta n t o r b it a l in te r a c tio n s between CH CoCdmg)^ (Cx) and [F e (C N )^ ]^ " (NP^~) o r H^O.

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F ig u re 29 d e t a ils th e p r in c ip a l o rb it a l in te r a c tio n s w it h in th e 1:1 complex o f n itro p r u s s id e and m e th ylco b a lo xim e . For th e a x ia l Isom er th e re i s a v e ry s tro n g

cf

cf' (FeCN) o r b it a l in th e n itro p r u s s id e (w hich app ro xim a te s to a n itro g e n lo n e p a ir ) and th e c o b a lt o r b it a l, coupled w ith a v e ry s m a ll "TT in t e r a c t io n in v o lv in g th e a x ia l -mr(CN) o r b it a ls o f th e n itro p r u s s id e and th e c o b a lt d^^ and d^^ o r b it a ls , a l l o f w hich are o cc u p ie d . N e ith e r in t h is 1:1 complex n or in th e v e ry sim p le m odel, t r a n s - [ (Q N )F e(II)H j^C N C o(III)H ^ 1^"*. i s th e re any in t e r a c tio n between

*

th e o ccupied c o b a lt d^^ and d^^ and th e unoccupied TT (CN) o r b it a ls o f th e a x ia l lig a n d . S ince c y a n o fe rrâ te s r e a d ily b in d th ro u g h

n itro g e n to sim p le e le c tr o p h ile s such as and

*

such ~rr b ack-bo n din g fro m th e e le c tr o p h ile to th e lig a n d "Tf o r b it a ls i s n o t necessary f o r th e s t a b ilis a t io n o f cya n o -b rid g e d complexes o f th e typ e 14-C-N-E. S tr u c tu r a l evidence in th e two lin k a g e iso m e rs o f [ (NH^)^Co-CN-Co(CN)j_3^^'^’'^^ ^ in d ic a te s th a t even when th e a p p ro p ria te o r b it a ls a re p re s e n t, th e -t t b a c k -b in d in g in t e r a c t io n appears to be n e g lig ib le in th e Co-CN bond as was c a lc u la te d h e re , a lth o u g h

“VI- back-bo n din g i s s ig n if ic a n t in th e Co-CN bonds.

T h e -M e th v lc o b a lo x im e fH e x a c v a n o fe rra te (II) Complex

The 1:1 complex o f h e x a c y a n o fe r ra te (II) and m e th ylco b a lo xim e i s v e ry s im ila r to th e cyano bound iso m e rs o f th e n itro p r u s s id e complex w ith m e th ylco b a lo xim e . The minimum energy o ccu rs a t a N-Co d is ta n c e

o f 1 «83A, where th e in t e r a c t io n energy (N-Co bond energy) is 1?4kJ m ol’~ \ le s s th a n th e in t e r a c t io n energy in th e [Fe(C N )^N O ]^" complex

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by some 60kJ m o l~ \ In com paring th e a x ia l N-Co o v e rla p p o p u la tio n s , th e h e x a c y a n o fe rra te (II) com plex has a v a lu e o f 0.297 as compared w ith 0.309 f o r th e n itr o p r u s s id e complex ( a x ia l is o m e r). For th e b in d in g o f n itro p r u s s id e v ia th e n it r o s y l lig a n d , th e c o rre sp o n d in g 0-Co o v e rla p p o p u la tio n i s o n ly 0.257» These c a lc u la tio n s a re e n t ir e ly

13

c o n s is te n t w ith th e p re v io u s ly d iscu sse d C NMR s tu d ie s , p o in tin g to a ra th e r weaker b in d in g o f h e x a c y a n o fe rra te (II) compared w ith n it r o - p ru s s id e . In c o n c lu s io n f o r th e b in d in g o f [Fe(CN)^NO] and o f [Fe(CN)^]^™ to m e th ylco b a lo xim e , th e Co~NC d is ta n c e s co rre sp o n d in g to th e energy m inim a a re s ig n if ic a n t ly lo n g e r th a n th e is o m e ric M-CN d is ta n c e s . These o b s e rv a tio n s and th e absence o f any nrf oonding between c o b a lt and th e b r id g in g cyano lig a n d a re in agreem ent w ith e x p e rim e n ta l r e s u lts on th e is o m e ric y * -cyano complexes [(NH^Co-

(CN)Co(CN) ] .

T h e-C o ba lt G o rrln iN itr o p ru s s ld e Complex

The in t e r a c t io n between n itr o p r u s s id e and th e c o b a lt c o r r in model (S p e cie s 3) was s tu d ie d as a fu n c tio n o f th e a x ia l N-Co d is ta n c e , s u b je c t to e s s e n tia lly th e same c o n s tr a in ts as m entioned i n th e m e th ylco b a lo xim e system . Only th e a x ia l isom er o f n itro p r u s s id e was

s tu d ie d f o r th e sake o f c o m p u ta tio n a l economy. The energy minimum f o r th e 1:1 complex was lo c a te d a t a Co-N d is ta n c e o f 1.75A ; t h i s i s

r a th e r s h o rte r th a n in th e cobaloxim e complex ( I . 80Â ). TTiis Co-N

O 1

d is ta n c e o f 1.751 co rre sp on d s to an in t e r a c t io n energy o f 288kJ mol , s u b s ta n tia lly h ig h e r th a n f o r th e cobaloxim e complex (235kJ mol ^) . In th e c o b a lt c o r r in :n itr o p r u s s id e complex th e o r b it a l in te r a c tio n s in v o lv e d a re v e ry s im ila r to th o se in th e s im p le r cobaloxim e model ;

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n e v e rth e le s s , th e N-Co o v e rla p p o p u la tio n i s 0.347 in th e c o r r in system compared w ith o n ly 0.309 i n th e cobaloxim e com plex. Thus, th e in te r a c tio n o f n itro p r u s s id e i s c a lc u la te d to be much s tro n g e r w ith th e c o b a lt c o r r in system (S p e cie s 3, Ï = NH^) th a n w ith a coba lo xim e . T h is re p ro d u ce s th e NMR e x p e rim e n ta l fin d in g s t h a t n itro p r u s s id e fo rm s f a r more s ta b le com plexes w ith cobalam ins th a n w ith [CHgCo(dmg)2 ] . A ltho u gh a l l o f these c o b a lt sp e cie s c o n ta in C o ( I I I ) , in th e

[CH^Co(dmg)2 ] th e c o b a lt i s bound to two u n in e g a tiv e (dmg)~ lig a n d s and to a n e g a tiv e CH^ a x ia l lig a n d , whereas in b oth aquocobalam in and S pecies 3 (% = NH^) th e a x ia l lig a n d i s n e u tr a l, e ith e r a b e n zim i­ d a zo le o r NH^ r e s p e c tiv e ly , and th e m a c ro c y c lic lig a n d i s o n ly u n in e g a tiv e . T h e re fo re on the se grounds a lo n e th e c o b a lt in [CH^Co- (dm g)^] i s expected to be r a th e r le s s e le c t r o p h ilic th a n in S pecies 3,

2—

o r in a n a tu ra l co b a la m in . N o n eth eless, th e b in d in g o f [Fe(CN)^NO] to [CHgCo(dmg)2 ] d em on stra te s t h a t such complex fo rm a tio n i s n o t a unique p ro p e rty o f cobalam ins (V ita m in

F u rth e r-C o m p a ris o n s -b e tw ee n -E xpe rim en ta l NMR S tu d ie s and EHMO C a lc u la tio n s

In 2:1 Co:Fe com plexes observed in N^iR e xp e rim e n ts, b o th s t e r ic and e le c t r o s t a t ic c o n s id e ra tio n s re q u ir e th e two c o b a lts to be bound to a p a ir o f tra n s lig a n d s in a c y a n o fe rra te . A lth o u g h th e r e s u lt s o f a 1:1 com plex (n itro p ru s s id e ;m e th y lc o b a lo x im e ) in d ic a te a s m a ll energy advantage f o r th e a x ia l bound cyano, th e a d d itio n a l b in d in g energy r e s u ltin g fro m c o m p le x a tio n o f th e c y a n o fe rra te to a second c o b a lt sp e cie s overcomes t h is sm a ll p re fe re n c e f o r a x ia l b in d in g . Thus, the 2 :1 com plexes o f n itr o p r u s s id e in v o lv e a tra n s p a ir o f e q u a to ria l

13

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In th e case o f th e [F e (C N ),]^ " a l l th e lig a n d s it e s y ie ld com plexes o f id e n t ic a l e n e rg y. C onsequently, th e re i s no therm o­ dynam ic im pedim ent to m ig ra tio n o f th e c ob a lt sp e cie s from one cyano lig a n d to a n o th e r in these com plexes. Such f lu x io n a l b e h a v io u r in th e case o f CFe(CN)g]^*" c o u ld e x p la in why th e s o lu tio n s ta te NMR

observed f o r th e [F e (C N )g 3^“ : cobalam in system co n ta in e d broad u n re s o lv e d resonances from w hich no c o u p lin g d a ta co u ld be d e riv e d , whereas, in th e analogous CFe(CN)[_NO]^ system v e ry sharp, w e ll re s o lv e d f i r s t o rd e r s p e c tra were alw ays observed.

In c o n c lu s io n , combined NFÎR s tu d ie s and EHMD c a lc u la tio n s have shown th a t th e n itro p r u s s id e a n io n b in ds to aquocobalam in to form b oth 1:1 and 1:2 n itro p ru s s id e :H 2 0 C b com plexes. The n it r o s y l lig a n d i s n o t c r u c ia l to t h is co m p lexing , s in c e h e x a c y a n o fe rra te (II) a ls o b in d s t o aquocobalam in, a lth o u g h in s o lu tio n i t appears to form a f lu x io n a l

system . In each case b rid g e s o f th e type Fe-C-N-Co a re proposed s in ce th e Co-N-C d is ta n c e s co rre sp o n d in g t o th e energy minim a a re s ig n if ­ ic a n t ly la r g e r tha n th e is o m e ric M-CN d is ta n c e s . Such b in d in g o f c y a n o fe rra te s a ls o o ccurs w ith th e sim p le aquom ethylcobaloxim e ([CH^Co(dmg)2H20] ) , b u t here th e b in d in g i s v e ry much weaker.

The fo rm a tio n o f th e 1:1 and 1:2 com plexes between n itro p r u s s id e and aquocobalam in re p o rte d h e re has been observed a t c o n c e n tra tio n s o f n itro p r u s s id e th a t a re somewhat h ig h e r tha n tho se commonly used in c li n i c a l p r a c tic e , the se a re t y p ic a lly 10“‘^mol dm"^ o r below . There­ fo r e , in th e succeeding c h a p te r (C hapter 4) th e h y p o te n s iv e i n y jjm b e h a vio u r o f these complexes i s s tu d ie d .

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3 .4 REFERENCES

1. ( a )A . I . S c o tt, G. B u rto n , and P.E. Eagerness; J . CHEM. SOC. CHEM. COMHJN., 1979, 199.

( b ) A * I, S c o tt, C.A, Townsend, K, Okada, M. % jiw a r a , R .J . Cushley, and P .J . Whitman; J . AM. CHEM. SOC., 1974, 1 6 , 8069.

2 . B .A . Narayanan and P .T . Manoharan; J . INORG. NOCL. C3-IEM., 1978, 40, 1993.

3 . A.D. James, R .8. M urray, S .F . L in c o ln , and E.H, W illia m s ; IN0R3