Phosphine and phosphine oxide ligand substitution and redistribution on tetrahedral cobalt(II) complexes in benzene

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PHOSPHINE AND PHOSPHINE OXIDE IJGAND SUBSTITUTION AND REDISTRIBUTION ON TETRAHEDRAL COBALT(II) COMPLEXES IN BENZENE JI-AN-C'I.AUDE PIERRARI). JEAN RIMBAUI.F ;rod RENI2~ P HL(iEI. I.abor:m,ire de Chimie Minerule. Univer,;il,2, de Reims. BP 347.51(162 Reims. France I Rote)red 20 .%lay 19'761

Abstract--The .,ub.,lituliun equilibria in benTene (at 25:('1: ('oXAPPh,b - ()PPh~ ~ ('oX~I()PPhO(PPh,) - PPh, ;md

('oX?(OPPhdlPPh ,I 4 ()PPh, ~ CoX21()PPh,), - I>Ph, ha'.e been follov, ed ,,pectrophotometricalb and equilibrium constant,, calculated. When X ('1. K , - 136 and K., 1.0: when X =. Hr. K, = 22 and K : - I h: when X I. K, 37 and K : = 1.4. rhe mixed complexe,, CoX,(OPPh,)IPPh,) ha;'e been isolated and their electronic *,pectra calculated. They are subject to redistribution equilibria in henzene 2('nX:(Ol~Ph ,)1PPh ,) ~ ('oX~(OPPh,l.. - ('oX?l t)Ph, h

'.*.ith K 11014 for X ('1:0.071 for X = Br anti (I.I)3q for X discu,,scd in term,, of electronic and steric effect,,.

INTRODUCTION I.itlle i-, known of the solution behavior of the c o m p l e x e s (ol
(11

I. l h e unusual order of the constants K. and K i,,

im,oluble (~)(l> But. the ,,oluhility can be increased b,. addition of an excess of triphen,vlphosphine. "Io elucidate thb, point, the foll'a'.Mng experiment '.,.as nlade. Different amounts (" of Co('l~(PPh,b were added to 25 pill amounts ,af benzene and stirred for IV,o davs. "l-he residue in each fll,sk is remo,..ed b.', tiltration and the spectra of the filtrate taken. Comparison of these speclra v,a,~ made with the spectrum of a •,olution of (o('l~lPPh,)? ~,ith a ten fold execs,, of PPh, (here ~,olubility wa, complete). The spectra are similar in aspect: lhe concentration of the complex in sohltion can then be calculated 1"Fable I). The effect of dilution on the equilihrium sugge,,ted h} Sacco[6]

(_oX,(PPhd(()PPh:) * ()PPh: ~ C o X : ( O P P h J : * PPh~. (2) This kind of ,,ub'qilUliOn should also exist for X = el. Br. The influence of the nature of the halogen on these equilihria will he the main ',ubject of this reporL EXPERIMENTAl, All experimcnl,, ;',ere carried out under a dr3 nitrogen il[rn,),,phere, nlt)MI} in it glo\e-b~,\ l'he preparat0on,,, the anal?,hc;d and the physical data on the compound,, I IR spectra, magneli,, susceptihilit,,,, powder pattern et..~ are gn',en el,ev, here[4] Soh~hilitv in btnzem,. (oX.at)Pl:)h,), complexes ha,.e a Io,.t solubilit,, in ben/ene {about [ It)().l mMI. l b e pbosphine compound,,, e\cepl for X - ('1. are more ,,oluble. l h e solubiliU. sequence in b o t h c a s e , , is [ ' B I . - e l . Spectral measurements ,,boy, that the,', are not dis~,ociated (Berr's la'..~, is obeyed} and do nt,{ form pentacoordinated specie., in presence of an execs., of OPPh, or PPh, for ('oX?lOPPh,J. or (7oX~(PPhO> re,,pectivel~ Ino change of the original .,pectra). The solutionh are ',er~ ,,ensilit e to ox!..ge~l, lhe pho,,phine being oxidized into phc.,.phine oxide[5], l'his ,,u,ceptihility lo'.a.ard oxidation in benzene ,,olutior., or e'..en m the solid Mate. increase,, fr,am ('1 It) I goluhilitr o[ ('o('l:tPPh,), .\re,,ta, Rt:.ssi anti Sacco[6] inditale 1hat 1hi', complex i,. practical1!, in,.cfluble in benzene and ,,uege,,t the fc~rnmtitm of a i'~ol!,lller v,.herea~ Sestilli. Furkmi and Fe,lucda[I] ',u,,pcct. in dich[oromelhane, the precipitation of

n(oCl.,q PPh,b ~ [(o('l:l PPh ,I, ],, - hi2 -. x )PPh,

,,hould increase the ratio r: in fact the contrar!, i,, ob,,er,.ed I~ addition, the residue does not show the presence of PPh,. Figure I shows the ',ariation of A ('r(solubilized complex) with ('. I h e curve A is compowd of two parts. Part (1) is concerned ,a ith an equilibrium: Ihe amonnl of precipitate increase,, con~,iderablv whereas the concentration of CoCI,(PPho., remain., 1o9.. In part 121. the dis,.oluti,an .ff the complex is a straight parallel to line (g! corresponding to fatal ,olubility Ir II. At the ,,ame time th,: a n l o u n [ o f residue tends to become constant. rhe ,,lope )if part I I ) o f cup, e A amt the positi,m M lhe break

1625

lahle I

Initial cobalt content × 10'

(oCl:t PPh,l: in solution , 10'

Ratio A

(('l

1~)

r ~:

0.58 ] 0.807 I. 140 1.522 1.834 1.925

(I.()76 II. 154 11.291 ql S:'l 0 8~,11 O935

0.131 (I. 191 11.2S5 0.t75 0.463 0.486

('obah in reskluc (_'(I--rl"

().5()'~ 0.(',5 ~ O849 11.951 0.985 I).989

10"

(. -r " 10 '

4 43 4 23 447 4 1t6 ~ 96 ~ 96

J.-C. PIERRARD et al.

1626 I

theless, the dibromo complex is still more sensitive than the diiodo complex.

!

B./"

NOTATION C analytical concentration of cobalt R ratio of concentration of OPPh, to cobalt a,/3 ratio of cobalt combined, respectively, in CoX2(PPh~)(OPPh0 and in CoX:(OPPh,), B concentration of PPh, added (in excess) molar extinction coefficient of the solution referred to the analytical concentration of cobalt e,~,~,,, ~,, extinction coefficients of CoX.,(PPh0~. CoX.,(OPPhJ (PPh~) and CoX.,(OPPh,)2 respectively D. 1 absorbance and optical path.

/ /"

/

/" /'

./././'

/

/./'/

/

///"

Spectrophotometric study A Beckman DK 2A instrument, with a thermostated cell compartment (T = 250_+ 0.2°C) is used. During the titration the cobalt concentration of the solutions are maintained constant whereas the concentration of OPPh., is increased. First equilibrium:

..°.-" I

I

1

2

--CoBr2(PPh02 --CoCI2(PPh02

C.10 3 Fig. I. Solubility of CoCI2(PPh~)2 in C,H,~.

C = 8.42.10 "M C = 1.23.10 ~M

and ratio (PPh, excess)/C = 6.0.

depends on the efficiency of dehydratation of the solvent. We propose, therefore, hydration of the complex by a reaction of the type: CoCI2(PPh)): + OHz ~ CoCI=, OH;,,, + 2PPh~.

Second equilibrium: The starting solution has an initial concentration of OPPh, so to have less than 1% of cobalt present in form of CoX2(PPh,)2.

(3)

--CoBrz(PPhO: --CoCIz(PPh02

C = 5.82.10-' M C = 1.29.10-'M

The equilibrium constant Ks can be written as and ratio (PPh~ excess)/C = 5.06.

Ks

4(1 - rJZC r(W - ( 1 - r)C)

(4)

Redistribution equilibrium: The spectra are taken with equimolar solution 0.1 mM in CoBr~(PPh~)2 and CoBr2(OPPh~I2.

where W is the concentration of water in the benzene. Relation (4) becomes: (K, -4)Cr =4 C - K,W+C(K, -8).

RESULTS

(5)

r

First substitution equilibrium The change in the spectra during titration of CoX2(PPh3)2 with OPPh3 is given in Fig. 2(a) X = Br and (b) X = C1. The presence of isobestic points as well as the graphical method of Coleman, Varga and Mastin[7] indicate clearly the presence in solution of only two absorbing species in equilibrium. The spectra are typical for pseudo-tetrahedral cobalt(ll) compounds, there is no formation of a pentacoordinated addition compound but only of a mixed complex CoX,(PPh3J(OPPh~) through equilibrium (1). We made similar observations with X =

From Table 1, CIr is assumed to stay constant as a first approximation in part (1) of curve A. This means that the solubility of the complex Cr is a linear function of C which is in agreement with the proposed equilibrium (3). From the slope of curve A part (1) one obtains Ks = 10.6, and the total water content is W = 1.7 x 10-3 M (30 mg/I). Let us take W = 2 mM and C = 1 mM, then to have less than 0.1% of the complex destroyed by water we must add an excess of PPh~ of around 5 times the concentration C. Under these conditions CoCI2(PPh3)~ dissolves. With X = I and Br, the constant Ks is probably lower, never-

Table 2. Nr of solution

0

1

2

3

4

5

6

7

8

9

10

X = Br

R

0

0.102

0.203

0.305

0.406

0.508

0.610

0.711

0.813

0.914

1.016

X = CI

R

0

0.082

0.165

0.247

0.330

0.412

0.495

0.577

0.660

0.742

0.825

Fable 3. Nr of solution

I

2

3

4

5

6

7

X = Br

R

0.89

1.78

2.66

3.55

4.44

5.33

6.22

X = CI

R

1.42

2.13

2.85

3.56

4.98

7.83

9.97

Pho~,phine and phosphinc oxide ligand substitution and redistribution of tetrahedral cobalt(lh complexes m benzene ,2

. ,

'

Second sub,,titution The titration spectra are given on Fig. 3 lal and [hi. The equilibrium constant K~ is given by:

. . . . .

"

I

/

K: --

:, ,

,

.

'...

..... :.

.

s'~•'~~. ...' ~', ... ,. ~--.x~l, ,,.',."~.,.,~. i Li' 9 \

.,7,

,,l'./~t.J /'.ii /.

7//,:/7

t9)

i-/7)

and '

K_,: ,

•

f o r X - Br

/3(1 l-~) ~t-fl)(R

,~, ,,i."//., /' i liZ~ / __~.. <~ . . . .

I,')2"~

i

fl(l+/3*B/c) I-/Ti(R-- I -/31

forX.

CI.

IlOi

,

Relation (8)is now rewritten as (11) 5' i.

4:Z.

4:'"

e :(:

65r5

vOC

zb:

~OO

E = t,,,(I - ~ ) +

t,,~

(11~

,~VE LENGTH prn

e,, is an experimental data. t,,, has been calculated before from the value of K,. Relations (9) and (11) or I I0) and (11) allow the estimation of K,., which was fourld, for X = Br, K~ = 1.6_+0.1 tat 3 wavelengths) and for X -- ('1. K: = 1.9--(I.1 (at 6 wavelengths).

Itl

Redistribution of the ligand,~ When the mixed complex is introduced into benzene.

/ i

it is subject to a redistribution equilibrium: 2CoX.4PPh a(OPPh~t ~ ( ' o X : ( P P h ~ l .

- CoX:(()PPh,L..

I l_")

The redistributien constant K is related to the preceeding ones bx "

_~., "OC

. qq

700

~,F:

600

,,a,%,¢! , : : I N G ~ .

Tb?

800

'.am;

II'l

Fig. 2. First equilibrium. Spectrophotometric titration of CoX:(PPhO.. solutions with t)PPh,. (a) X- Br. (b) X = ('1. Optical path: [a~ I = 1 ¢m from 350 to 450 nm. I = 5 cm from 550 to gtlllnm, lbl I - 5 d i n . 1131. The equilibrium constant K, is expressed as:

K'=(R-

Or

2

~)(I

a)

forX=Br

(61

for X = ('1.

~71

and K, =

a ( B I c - ~i (R-a)(I--~t)

A s the ligands PPh, and ()PPh, do not absorb at the wavelengths used. we can ,.,,'rite:

I)

It ",1

K = K~_/K,

"~'

e = T(-~ = t , , l l

- t~) + E,,,a

and the calculated values are given in Table 4. Nevertheless, it was possible to make a direct experimental determination of K for X = Br by measuring the al~sorbance at several wavelengths of an equimolar solution of CoBr:IPPh02 and CoBr,IOPPhO?. The value of K. at 70(J, 687.5 and 600 nm respectively u, as found to be; 0.114-,. 0.10 and 0.09. This value agrees with the calculated one ( l a h l e 41. When X = ('1. the low solubility of the complexes and the necessity of the presence of an excess of triphenylphosphine in the solution do not permit the ,,arne experiment. When these solutions ( X = Br. 1) ;,re heated, the equilibrium (12)is shifted from left to right: this mean', that the formation of the mixed complex through a redistribution of the ligands between the two complexe,, CoX.,(PPha: and CoX.qOPPhsL~ is exothermic. We ohserved also that dilution of these solutions has no effect on the redistribution equilibria. Table 4 summarizes the results of this stud~. In Fig. -I we give also the solution spectra in C~H,, of the c o m plexes CoX:(PPh3L, (experimentalL C o X , ( O P P h a , l e \ Table 4.

(81

G is taken from the spectra of the pure complex in solution. At a given wavelength, the two relations giving K, and t contain three unknowns K,. ot and t,. A least square-method already described[3] was used to get the best values of K,. The results were for X : Br, K, = 21.6-+0.2 (at 6 wavelengths) and for X = CI. K, = 136--- 2 (at 5 wavelengthsl.

X

('1

K, K., K

136 1.9 0.014

Ratio of mixed complex in solution

O.~1

Br

22

ll~;I

3-

1.6 0.0"~1

14 0 lilt;

11.6¢'

O "~'

1628

J.-C. PIERRARD

et al.

1.0

1

|

I

1.0

0.8

0.8 # t

0.6

0.6 t

f-

Pb

; ;

1\

/ 2,',, ,'", zu

Idd

¢J Z

/ 3 , ,' \ 14','k¢'~'

:I~ ,A /jr ] b , ~ , r " ~

~o.4 F ,:~I'I _!

~0.4 t/) oo
t

I

:1i

:I

\7."'"

I .-..-21

1\\'~',5--t

| 1111// t t ..

,

IJ

'

I

./

0.2

1 I 6OO 650 WAVELENGTH

700 Into ]

(a}

O IJ 5bo

I I 600 650 WAVELENGTH I" n m 3

I 700

J

(b)

Fig, 3. Second equilibrium. (a) X = Br, (b) X = CI. Optical path: I = 5 cm.

perimental) and CoX:(PPh3)(OPPhs) (calculated) for (a) X = CI, (b) X = Br, (c) X = I. DISCUSSION If we consider the values of K,. we see that the ease of replacement of one phosphine by a phosphine oxide on CoX.,(PPh3)2 follows the order: CI>>I>Br. This sequence of halogens is very unusual and has to be explained. The stability of the mixed complexes toward redistribution follows the same order and the K data indicate also that the stability is higher, in all cases, compared to a statistical redistribution. We could easily consider such a statistical redistribution considering the fact that the number of Co-P and Co--O bonds is the same on both sides of eqn (12). In that case we would have only 50% of mixed complex left over and K would be 0.25. The only system described of redistribution of ligands in such complexes is the one given by Schmidt and Yoke[5]. They find for the equilibrium CoCI,(PEh)., + CoCI,(OPEt0,,~2CoCI,(PEIO(OPEt3), in benzene, a value of the constant around 10, with a proportion of mixed complex around 60%. This is closer to a statistical redistribution than our lowest value found for X = Br. At least two factors have to be taken into account to explain these facts. First, we may look at the electronic structure of the starting complexes. The strength of the

Co-P bond depends partially on its [1-back bonding character. The phosphorus atom becomes a better I1acceptor for the metal electrons if the other groups attached to the metal are less electronegative; thus the strength of the Co-P bond follows the order I > Br > CI. The least strongly attached phosphines are those in CoCI,,(PPhO,, they are easily' substituted by a phosphine oxide. One can reach the same conclusion using the HSAB principle [8]. We can consider the entity CoX2 as an acid which is bound to soft (PPhO or hard (OPPh3) bases. The hardness of the acids decreases in the order CoCl2> CoBr2 > CoI2 and we find that the order of substitution of a phosphine by a phosphine oxide should follow the order CI > Br > I. If we now look at steric effects in these complexes we must remember that triphenylphosphine is a rather bulky ligand[9]. One can consider Col2(PPh3)., as a sterically hindered complex and upon substitution by replacing one phosphine by a phosphine oxide some of the steric interactions between the initial ligands are relaxed. Considering only this steric effect the substitution should follow the order I > Br > CI. This must be the reason for the unusual order found: the steric effect explains the position of the iodo complex in the sequence. The second substitution constant K2 is much lower than K~ and the sequence CI> B r > l is normal and is what would be expected from the electronic structure of the complexes. Nevertheless this low value of K2, which can be explained if we consider the usual Bjerrum

Pho',phme

and p h o , , p h i n c o x i d e ligand suhstitutic, n and re,Jistribulion ENEFI(; ' t,j(~ ,:) i

of t e t r a h e d r a l

( , , ' 4 ~(;:;

i

i

A

?50 .'~A'~ E k f N(~t FI

[, .,,]

I-ig. 4cal E N E RG',, .

.

.

.

.

.

,:oNdt(Ih

[ ....

]

I 6(;00 t - - r

~4()O0 i

c,.;

t' 5 ::

7~)0

'v', A \ , (_ L E N C i T H F i g 41b~

/50 [

,.]

,:tmlplexe,,

m

bell~erie

1~,2',~

1630

J.-C. PIERRARD et al. ENERGY

[cm"] 13000

15.000 I

!

12

8

O

O

600

650

700

750

WAVELENGTH

800

[nm]

Fig. 4(c)

Fig. 4. Spectra of the complexes. A. CoX2(PPh,)2; B, CoX2(OPPh3)2; O, CoX2(OPPh3)(PPhO calculated. (a) X = Br, (b) X =Cl, (c) X = 1. statistical factor, gives some stability to the mixed complex. This higher stability is also refleced by the values of the redistribution constant K and by the fact that the mixed complexes have been isolated as crystalline compounds[4]. The comparison of the spectra (Fig. 4) of the series of complexes CoX2L2 with the same ligand shows a shift of the 4A2~4T~(P) band toward higher energies when X goes from I to Br and to CI, in agreement with the spectrochemical series. For a given X, the spectrum of the mixed complex lies between the other two CoX2(PPh~)2 and CoX2(OPPh3)2. A practical hint when preparing and checking the purity of the complexes CoX2(PPh3h is to look at the spectra. For example in the case of X = I, an eventual oxidation is indicated by the presence of a shoulder or even a band at 640 nm and by the low value of the absorbance at 725 and 800 nm.

REFERENCES

1. L. Sestilli, C. Furlani and G. Festuccia, Inorg. Chim. Acta 4, 542 (1970). 2. C. A. McAuliffe. Transition Metal Complexes of Phosphorus, Arsenic and Antimony Ligands, p. 97. MacMillan Press, London (1973). 3. J. Rimbault and R. Hugel, lnorg. Nucl. Chem. Lett. 9, 1 (1973). 4. J. C. Pierrard, J. Rimbault and R. Hugel, Bull. Soc. Chim. (France) 1705 (1976). 5. D. D. Schmidt and J. T. Yoke, J. Am. Chem. Soc. 93, 637 (1971). 6. M. Aresta, M. Rossi and A. Sacco, lnorg. Chim. Acta 3, 227 (1%9). 7. J. Coleman, L. Varga and S. Mastin, Inorg. Chem. 9, 1015 (1970). 8. R. G. Pearson, Chem. Brit. 3, 103 (1%7). 9. C. A. Tolman, J. Am. Chem. Soc. 92, 2956 (1970).