Total de bacterias identificadas en el análisis

(3 . 1 a ) R-H

In th e p re s e n t work th e fu n c t io n a lis e d phosphine lig a n d

( 3 . 2 ) was s y n th e s is e d in an a ttem p t t o o b ta in s i x membered c h e la t e -

s t a b i l i s e d a l c o h o l - o r a lk o x o - p la t i n u m ( I I ) com plex es.

Ph2P

OH

3 . 1 . 1 N onoden tate Oxygen Donor Com plexes o f Platin um

Hard lig a n d s c o n ta in in g th e e le c t r o n e g a t iv e ox ygen donor

atom would be ex p e c te d t o b in d o n ly v e r y weakly t o th e s o f t Lewis

a c i d , p l a t i n u m ( I I ) . 290 Oxygen don or com plexes o f th e Group V I I I

( 8 - 1 0 ) t r a n s i t i o n m eta ls e i t h e r ( 1 ) have s e v e r a l hard don ors

c o o rd in a te d t o th e m eta l, a m a n ife s ta tio n o f th e s y m b io tic

e f f e c t , 291 o r ( i i ) th e oxygen don or i s tra n s t o a lig a n d o f high

t r a n a in flu e n c e such as h y d rid e , a l k y l o r a r y l lig a n d s where the 292

a n t ls y m b io t ic e f f e c t promotes c o o r d in a t io n .

W ell c h a r a c te r is e d a lk o x o -p la tin u m com plexes a r e r a r e and

g e n e r a l l y u n s t a b le .293-295 Those com plexes which have been Is o la t e d

a r e o f t e n e x tre m e ly m o is tu re , oxygen and tem peratu re s e n s i t i v e . In

1986 th e f i r s t w e ll c h a r a c t e r is e d b u t h ig h ly r e a c t i v e b is - a lk o x o

p la tin u m com plex ( 3 . 4 ) was p rep ared by the m eta th e s is r e a c t io n o f

P^2p Q P h 3 S N. ✓ .

L x Ptv

4

2 N

q

OCH3 —

f

" ~l ^ K s 0 C H 3

Pt

_{2 No ci}

Ph/

( 3 . 3 )

Complex ( 3 . 4 ) when tr e a t e d w i t h c a rb o n m on o xid e , ge n e ra te d th e s t a b le b is -m e th o x y c a rb o n y l p r o d u c t 297 ( 3 . 5 ) (e q . 3 . 2 ) .

P^ 3p x / J C Hj 2 C O P^ P \ , C 0 , C H 3

C

R \

P ^ p / P , X 0 C H 3 p ^ p ' C OjC Hj

( 3 . 4 ) ( 3 . 5 )

The h i g h l y re a c t iv e m on o -alko x o p la tin u m com plex ( 3 . 6 ) un de rgoe s c a rb o n monoxide i n s e r t i o n t o form the m ono-methoxy c a rb o n y l p ro d u c t ( 3 . 7 ) (e q . 3 . 3 ) . eq. C O ? P "p, - C Hj Ph'! p X V 0 C H , ( 3 . 6 ) ( 3 . 7 )

o b s e rv e d alth ou gh th e in s e r t io n mechanism i s an alogou s t o those

p r e v io u s ly e s t a b lis h e d f o r i n s e r t io n in t o M-C b o n d s .298

Bryndza e t a l . 299 have shown th a t th e p r e v io u s ly unknown

a lk e n e in s e r t io n in t o a M-0 bond can o ccu r (e q . 3 . 4 ) . Again , no

in s e r t i o n in t o the P t-C bond was ob served and th e r e a c t io n p roceeds

CH3

OCH

₃ c h

3

c f

2

c f

2

o c h e q . 3 .4 3 (3 .8 )

by th e same mechanism as in s e r t io n s in t o M-C and M—H bonds in which

a s s o c ia t io n o f the alk en e to g i v e a f i v e c o o rd in a te com plex i s

f o llo w e d by in s e r t io n in t o th e P t-O bond. I t i s i n t e r e s t i n g t o n o te

t h a t in th e above two in s e r t io n r e a c t io n s , th e hard b a s e / s o ft a c id

com plex ( 3 . 6 ) does n o t depend on d is s o c ia t io n o f m ethoxide fo r

r e a c t i v i t y .

The d i f f i c u l t y in i s o l a t i n g alk o x o com plexes o f the Group

V I I I (8 -1 0 ) t r a n s it io n m etals has been a t t r ib u t e d t o th e commonly

h e ld p e rc e p tio n th a t such s p e c ie s have weak M-0 bonds due to the

weak i n t e r a c t io n o f the hard base w ith the r e l a t i v e l y s o f t Group

V I I I (8 -1 0 ) m etal c e n t r e s . 290 The dominant d eco m p o sitio n pathway o f

m eta l a lk o x id e s i s 6-hydrogen e lim in a t io n and th e e a s e w ith which

t o be due t o th e weak M-0 bonds lo w e r in g th e i n t r i n s i c b a r r i e r to

B-hydrogen e lim in a t io n r e a c t io n s by r a i s i n g th e ground s t a t e f r e e

p l a t i n u m ( I I ) a lk o x id e s i s a lo w e n ergy p r o c e s s but where both e th y l

and methoxo lig a n d s a re p r e s e n t as in compound ( 3 . 9 ) , d eco m p o sition

from e t h y l lig a n d B - e lim in a tio n p red om in a tes.

Thermodynamic s tu d ie s on [(d p pe)P t(O M e> 2 ] and [(d p pe)

P t ( CH^CH^)2 ) in d ic a t e th a t th e p resen ce o f n o n - a s s o c ia t in g p o la r bonds such as m ethoxide can a c c e le r a t e B -hydrogen e lim in a t io n from

b o th a lk y l and a lk o x o U g a n d a . These r e s u l t s a r e in c o n a is te n t w ith

th e assumption th a t the m eta l-o x yg e n bonds a re i n t r i n s i c a l l y

th erm odyn am ica lly weak. A r e c e n t study by Bryndza and Bercaw300 has

shown th a t m etal oxygen bonds f o r th e l a t e t r a n s i t i o n m etals a re

n o t weak th erm odyn am ica lly. In f a c t LnM-OH bonds a re found t o be

g e n e r a l l y s tr o n g e r than l^M -Cfsp* ) , Ln»-H o r I*nM-N bonds alth ou gh

n it r o g e n lig a n d s such as a c e t o n i t r i l e and p y r id in e a re found to

b in d much more r e a d i l y to th e platin um grou p m e ta ls . I t would

ap p ear th a t the l a b i l i t y o f many platin um ox ygen donor com plexes is

k i n e t i c in o r i g i n , as a r e s u l t o f th e e x is t e n c e o f f a c i l e 097

e n e r g ie s o f a lk o x id e s r e l a t i v e t o a l k y l s . • 297

Bryndza e t a l . have shown th a t d e co m p o sitio n o f

d e c o m p o s itio n pathways such as B-hydrogen e lim in a t io n , r a t h e r than

therm odynam ic in o r i g i n .

The e n ergy b a r r ie r s t o B-hydrogen e lim in a t io n from a lk y l and

a lk o x o lig a n d s may be c o n t r o l l e d by d e sig n o f com plexes w ith

a p p r o p r ia t e lig a n d s . F or exam ple, in c o r p o r a t io n o f M-0 bonds in t o

c h e l a t e r in g s le a d s t o i n h i b i t i o n o f th e B -h ydrogen e lim in s t io n

d e c o m p o s itio n pathway and c o n se q u en tly such compounds have g r e a t e r

c h e m ic a l s t a b i l i t y . 286,287

3 . 1 . 2 C h e la te S t a b i l i s e d Oxygen Donor Com plexes o f P la t in is i

R e ce n tly a number o f p latin u m a lk o x o com plex es have been

i s o l a t e d where th e P t - 0 lin k a g e s have been in c o rp o r a te d in t o

c h e l a t e r in g s . For example W i l l i s e t a l . 301 have p rep a red s t a b le

f l u o r o a lk o x id e s o f p la t i n u m ( I I ) u s in g th e b id e n t a t e p e r f lu o r o -

p in a c o l d ia n io n ( 3 . 1 0 ) w ith a ran ge o f phosph ine c o - lig a n d s .

The c h e la t in g platin u m a lk o x o com plexes have g r e a t e r

thermodynamic s t a b i l i t y than th e m onodentate p latin u m a lk o x id e s .

The c h e l a t i n g g l y c o l a t e platin um com plex (3 .1 1 ) on h e a t in g a t

th e monodentate a lk o x o com plex ( 3 . 4 ) which i s u n sta b le even a t room (3 .1 0 )

297 te m p e ra tu re .'

P r in g le e t a l . ^ »287 ^a v e s ^own th a t th e lig a n d s

Ph^PCH^CMe^OH (3 .1 2 ) and Ph^PCH^CH^OH (3 .1 3 ) form O-donor complexes

w ith p la t ln u m d l) which a re a i r , m o istu re and th erm a lly s t a b le .

They do n o t c o n ta in abnorm ally lo n g o r weak P t- 0 bonds.

r \

Ph2P

OH

Ph7P

|OH

R ea ctio n o f the b is-p h o sp h in e com plex (3 .1 4 ) w ith AgC104

ga v e th e d ic a t io n ic s p e c ie s (3 .1 5 ) (e q . 3 . 5 ).

HO

Cl

AgCIO*

Pt

---►

HO

N Cl

♦ 2 AgCl

( 3 . 1 4 ) in CDC13 i s a broad s i n g l e t w ith 195P t s a t e l l i t e s which,

when c o o le d t o -4 0 ° C sharpens t o an AX p a t t e r n . The c h e la t e

com plex (3 .1 6 ) in which th e a lc o h o l don or i s bonded t o th e m etal

was a s s ig n e d to t h is s p e c ie s . The f l u x i o n a l i t y in s o lu t io n i s due

t o in t e r c o n v e r s io n o f th e tau tom ers (3 .1 6 a ) and (3 .1 6 b ) by r a p id

in t r a m o le c u la r -OH exchange as shown in Scheme 3 .1 .

Scheme 3 .1 . F lu x io n a l b eh a viou r o f [ P t C l2 (Ph2PCH2CMe20H )2] i n

Even in the absence o f th e s i l v e r s a l t th e su pposedly weak

oxygen donor has d is p la c e d th e c h lo r o lig a n d .

T (3 .1 4 ) (3 .1 6 a )

" V

Cl OH

c r

(3 .1 6 b ) methanol..

D ep ro to n a tio n o f th e b is -p h o s p h in e com plex (3 .1 4 ) w ith NaOH in e t h a n o l, [L iN (S iM e 3 >2 ] in THF o r NEt3 in CDC13 gave th e b i s - a lk o x o c h e la t e complex (3 .1 7 ) ( e q . 3 . 6 ) . e q . 3.6

HO

p ^ 2

Cl

- 2 H *

_Ph»Ps

_{^ 0}

PT

--- -

_pt

h o

x ci

* 0

(3 .1 4 ) (3 .1 7 )

From th e c r y s t a l s t r u c t u r e o f (3 .1 7 ) i t was deduced th a t th e

P t-O bonds a r e the ex p ected le n g t h f o r a s i n g l e bond and show no

anomalous weakenin g. The b is - a lk o x o c h e la t e com plex (3 .1 7 ) i s a i r

s t a b l e and can be r e flu x e d in a l c o h o l i c KOH f o r 16 h w ith o u t any

s ig n o f d eco m p o sitio n . T h is i s in c o n t r a s t t o th e mononuclear b i s -

a lk o x o p la t in u m (II ) com plex [ Pt(OM e>2 (P h 2PCH2CH2PPh2 ) ] ( 3 . 4 ) which

decomposes a t 25 "*C in s o lu t io n and i s s e n s i t i v e to w a ter.

In document Evaluación Microbiológica de las aguas termales del balneario las Peñas, cantón Baños, provincia Tungurahua (página 88-100)