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vol.46 número3THERMAL STUDIES OF POLY(ESTERS) CONTAINING SILICON OR GERMANIUMUNA SINTESIS EFICIENTE DEL N,N',N''-TRIS- (2,3 DIHIDROXIBENZOIL)-1,1,1-TRIS-(AMINOMETIL)ANO, UN ANALOGO DE LA ENTEROBACTINA índice de autoresíndice de assuntospesquisa de artigos
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Boletín de la Sociedad Chilena de Química

versão impressa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.3 Concepción set. 2001

http://dx.doi.org/10.4067/S0366-16442001000300009 

EXPONENTIAL DECAY OF A CHARGE TRANSFER INDUCED BY
COORDINATION IN ORGANOMETALLIC COMPLEXES

CARLOS DIAZ* AND FERNANDO MENDIZABAL

Departamento de Química, Facultad de Ciencias, Universidad de Chile,
Casilla 653 Santiago, Chile.

*To whom correspondence should be addressed.
Fax: 56-2-2713888. E-mail: cdiaz@abello.dic.uchile.cl

(Received: March 6, 2001 - Accepted: June 15, 2001)

ABSTRACT

In the series of organometallic complexes N3P2(X)5P-SPACER-MLn X= Cl and OR, MLn= organometallic fragment , the D31 P-NMR coordination chemical shift parameter correlates with the length of the spacer, decaying exponentially with the length of it. The bridging spacers behave like a "molecular wire" showing dependence of the charge transfer with the distance, similar to electron transfer in donor-acceptor systems. Extended Hückel molecular orbital calculations on the model series N3P2(X)2 P-SPACER-MLn X=Cl- and OH- molecules shows that the changes of the Mulliken charges on the phosphorus and metal atoms upon coordination exhibits also an exponential dependence with the length of the spacer.

KEYWORDS: Charge transfer, molecular wire, organometallic, MO,

RESUMEN

En la serie de complejos N3P2(X)5P-ESPACIADOR-MLn X= Cl y OR, MLn= fragmento organometálico, el parámetro de corrimiento químico de coordinación D31P-NMR, correlaciona con la longitud del espaciador, decayendo exponencialmente con la longitud de éste. El puente espaciador se comporta similarmente a un "hilo molecular" mostrando una dependencia de la transferencia de carga con la distancia, similar a la transferencia de carga en sistemas dador-aceptor. Cálculos de orbitales moleculares del tipo Hückel extendido en la serie de moléculas N3P2(X)2 P-ESPACIADOR-MLn X=Cl- y OH- muestran que al ocurrir la coordinación, los cambios de las cargas de Mulliken sobre los átomos de fósforo y del metal exhiben también una dependencia con la longitud del espaciador.

PALABRAS CLAVES: Transferencia de carga, hilo molecular, organometálico,OM

INTRODUCTION

In chemistry there are several events where the electron transfer is involved. Electrical conduction through solid material, ionic conduction through a solution and electrochemical processes in the interface solution-electrode are some examples in which a transfer of electrons (electron transfer of one unit of charge ) occur [1].However when bonds are formed or broken to give a new compound, a charge transfer processes is also present. During the study of the organometallic derivatives of ciclophosphazenes [2] we have found that the phosphorus ring linked to the spacer transfers charge to an organometallic fragment, in a similar way to a "molecular wire" [3] . In previous works the mixed valence complexes CpFe(dppe)-X-MLn +z X= CN [4a] ,S-C5H4N [4b] , and TCNX [4c] ,X= E and Q, the metal-metal intramolecular electron transfer was studied by optical methods.In the systems here reported the charge transfer was evidenced by 31P-NMR data instead of optical methods and corroborated by theoretical calculations.

EXPERIMENTAL SECTION

Materials.
All solvents and chemical were of reagent grade, purchased commercially, and used without further purification except as noted below. CH2Cl2 was dried and distilled from CaH2. Reactions were carried out under a nitrogen atmosphere and using vacuum procedures.

Preparative
The synthesis of the complexes ( N3P3(OC6H5 )5OC6H4 CH 2CN M(P-P)Cp)PF6 M=Fe,Ru, P-P= dppe and PPh3 has been described elsewhere [2,4d] . The complexes; N3P3Cl4(C6 H5)P(C6H5)2 · MLn MLn= Cr(CO)5, Fe(CO)4[5,6] ; Mn(CO)2(bipy)2P(OPh)3 [6] , N3P3(OC6H 5)5OC6H5Cr(CO) 3[7] , N3P3(OCH2CH2 C6H5)6[Cr(CO3 )]6[7] and [N3P3(OC6H5 )5OC6H4CNFe(dppe)Cp)] PF6 [8] were prepared using methods previously described.

Measurements.
.31P-NMR spectra were recorded on a Bruker AMX 300 spectrometer in CD2Cl2 . Chemical shifts are reported in ppm relative to standard H3PO4 80% .

MO Calculations
MO calculations for the model complexes 11-14 (see MO results section) were performed through the extended Hückel method [9] , using the weighed Hij formula . The atomic parameters involved in our calculation are given in Table I. Geometrical parameters were fixed at average values from the molecular structure for 11 and 12. For the geometry of the fragments in complexes 13 and 14, the structural parameters from CpFe(dppe)NCCH3+ [10] and N3P3(OR)6 [11] were employed.

Table I. Atomic Parameters Used in the Calculations.


Atom

Orbital

Hii(ev)

x1ª

x2ª


Fe

4s

-9.17

1.9

4p

-5.32

1.9

3d

-12.6

5.35(0.5503)

2.00(0.6260)

Cr

4s

-8.66

1.70

4p

-5.24

1.70

3d

-11.22

4.950(0.5058)

1.80(0.6747)

C
2s
-21.4
1.625

 

2p
-11.4
1.625

 

O
2s
-21.4
1.625

 

2p
-14.8
2.275

 

N
2s
-26.00
1.950

 

2p
-13.40
1.950

 

P
3s
-18.6
1.60

 

3p
-14.0
1.60

 

H
1s
-13.6
1.3

 


ax is the Slater exponent whose coefficient of the double x expansion is given in parentheses.

RESULTS AND DISCUSSION

Figure 1 shows a chart with the studied complexes . 31P-NMR data for the series of complexes are displayed in Table II. The coordination shift, defined as D(coord)= dcomplex- dfree ligand, is a measure of the charge transfer upon coordination [12].


Fig.1. Structural Formula of the studied compounds.

Table II. 31P NMR Chemical Shift dataa, and Phosphorus-Metal Distanceb for the Cyclophosphazene Ring in Organometallic Complexes


Complex

d(31P)

Dcoord.

D ( P-M) A


1

33,1

5,8

4.62

2

32,5

5,4

4.47

3

31,9

6.0

4.60

4

9,7

1.4

6.0

5

17,5

0.1

12.19

6

8,9

1.1

8.6

7

8,7

1.6

8.6

8

5,6

3.4

8.8

9

11,7

0.18

13.19

10

11,5

0.38

13.36


a. Values in ppm. respect to 80% H3 PO4 as reference.
b. Values from crystallographic measurements as estimates from standard values (ref. 6,7,10 and 11).
c.D coord. = [d complex - d ligand] . d 31P values for free ligands are: complex 1,2,3: 37.9 ppm.;
0 complex 6,8.9: 10 ppm ;complex 4: 8,3 ppm ; complex 5: 17,5 ppm ;complex 7: 10.3 ppm; complex 10 .

Thus for the organometallic complexes series with cyclophosphazene as ligands an exponential dependence of the D (coord) on the length of the spacer was found (Fig. 2 ). This relationship has the form:

D31P=A·exp(-Br) (1)

where r is the distance from the cyclophosphazene phosphorus to the metal and A and B are constants. This equation is similar to the common expression for the distance dependence of the coupling matrix element for an electron transfer between a donor-acceptor pair linked by a bridge [13] eqn 2

HAB=H0 exp(-br)

(2)

where H0 is the coupling matrix-element for a donor-acceptor pair at the Van der Waals separation; r is the length of the variable bridge and b is a constant scaling the distance dependence. This analogy can be explained because D31P is a measure of the degree of charge transfer from the phosphazene ring to the metal upon coordination, and HAB is the interaction energy between the initial/final state in an electron transfer process from a donor to an acceptor linked by a bridge. A pictorial representation of the electron transfer in our system is shown in Figure 3.

Fig.2. Plots of D 31 P (coord.) vs the spacer distance

Fig.3. Pictorial representation of the initial and final state of the charge transfer between the phosphorus atom and the metal.

Because the charge transfer occurs between the ground-states Yi and Yf , the electron transfer may be described by an extension of Mulliken's Theory of donor-acceptor interactions [14]. From a plot of ln(D31P) vs the distance of the spacer, the constant B (slope of the plot) can be obtained. Because (D31P) is a measure a degree of the charge transfer between the phosphorus atom an the metallic atom, and comparing the equations (1) and (2), we can conclude that B=b=0,38 . Here b reflects the degree of charge migration from the phosphorus atom to the metal upon coordination. Considering that the right side of equation (2) is also related to the electron transfer rate between two centers [13,15] ,equation (3)

ket = k exp[-b(r-r0)]

(3)

it can be concluded that the calculated b value for the systems N3P2(X)2 P-SPACER-MLn is smaller than those determined from the relationship between ln ket and the donor - acceptor distance for randomly oriented donor and acceptors in rigid matrices 0,8 Å £b£1,2Å[16]. It is also smaller than those observed for the rate of decay of coupling with distance in metalloprotein systems (b@1Å ) [13a,17]. However, comparable b values (0.3-0.5Å) were obtained in photoconduction experiments performed in multilayered structure dyes and fatty acids [13a]. The analogy arising from comparison of equations (1) (2) and (3) can be viewed as the electron transfer in the ground state involving some charge separation, which indeed involves some electron transfer [18].

Most interesting is the comparison with electron transfer across the self-assembled monolayer on the electrodes, which follows the equation [19].

i = i0 exp[-br]

(4)

where i0 is the observed current, r is the thickness of the electrode film and b is the electron tunneling constant. In order to corroborate the charge transfer deduced from the 31P chemical shift, we have performed Hückel MO calculations on the model molecules shown in Figure 4 . From Figure 5 and Table III we can see that the net charge transfer, both on the phosphorus atom and on the metal atoms, also decays exponentially with the spacer length. It is interesting to note that the variation in the net charge on the metal atom, which could not be observed experimentally, follows similar behavior with the length of the spacer. We call attention to the fact that notwithstanding the different metal and the different nature of the bridge, the charge transfer through the spacer follows a regular behavior. Although the observed behavior occur in solution and in the ground state, the spacer behave analogue to a "molecular wire" [3]. This is an interesting feature in relation to the field of "molecular electronic"[20] specifically to the electronic devices with electronic shift register memory based on electron transfer reactions [21], as well as in molecular devices with information based on electron transfer [22]. These results are also in agreement with recent theoretical studies [23] of the distance dependence of electronic coupling H, through an alkyl chain; nearly exponential decay of H with the distance between the donor D and the acceptor A in A-(CH2 )n-D systems was found. Most interesting, recent theoretical studies [22] have shown that the changes in electrostatic potential between molecules as a form of molecular information coding also follows an exponential decay with the distance between the molecules. Surprisingly in spite of the different nature of the spacer, involving s and p bonds, the electron transfer exhibits a regular behavior depending only on the length of the spacer. This can be explained assuming some tunnel contribution to the electron transfer mechanism.

Table III. Calculated Changes of net Charges(e) on Phosphorus and Metal Atoms Upon Coordination according to Hückel Methods.


Model Complex

D(P,Charge)

D(M,Charge)

Hückel

Hückel


N3P3(Cl)5PH2Cr(CO)5

0.21

0.481

N3P3(OH)5C6H4Cr(CO)3

0.007

0.248

N3P3(OH)5OC6H4CN[Fe]

~0

0.049

N3P3(OH)5OC6H4CH2CN[Fe]

~0

0.07


Fig.4. Structural formula of the models used in the theoretical calculations


Fig.5. Plots of calculated net charge transfer on the phosphorus (A) and on the metal atom (B) vs the length of the spacer.

It can be concluded that the charge transfer on coordination depends mainly on the length of the wire. Most detailed theoretical studies for understanding the above described experimental results are in progress

ACKNOWLEDGEMENT

Financial support from FONDECYT (Projects 1990038 and 1000672) is gratefully acknowledged.

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