Coordinating properties of some solvents towards the fragment CpFe(dppe)+

~

Poh'hedron Vol. 15, N o 5 6, pp. 997 1001, 1996 Copyright I 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 538796 $15 00 ~-0.00

Pergamon 0277-5387(95)00301-0

C O O R D I N A T I N G P R O P E R T I E S OF S O M E S O L V E N T S T O W A R D S THE F R A G M E N T CpFe(dppe) +

C. D[AZ* and N. YUTRONIC Departamento de Quimica, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Santiago, Chile

(Received 14 March 1995; accepted 13 June 1995) Abstract The coordination effects of 21 solvent ligands on the fragment CpFe(dppe) + [dppe = Ph2P(CH2)2PPh2, Cp = qS-CsHs] was investigated by UV vis spectroscopic methods. The shift of the lowest d-d absorption band depends on the a ~ d o n o > a c c e p t o r properties of the solvent ligand rather than typical properties of the solvent such as dielectric constant or donor and acceptor numbers. With chlorinated solvents some = interaction appears to exist.

Compounds containing the electron-rich fragments r/5-CsHsM(P1)(P2) + [M = Fe, Ru, (P1), (P2) = phosphine ligands] exhibit interesting features. ~6 They present catalytic properties in some C - - C bond forming and C - - C bond activating reactions, 7g as well as stabilizing alkylidene, 9 vinylidene 4 and formyl groups) ° Although several [(r/s-CsHs)Fe(P--P)L]PF6 complexes ( P - - P = diphosphine, L = neutral donor ligand) have been reported,' 3.6 very little work has been reported with those where L is a solvent molecule.ll 16 In contrast with other organometallic systems, some [CpFe(P--P)(solvent)]X (Cp = qS-CsH5) compounds appear to be somewhat stable. L2'15Dichloromethane solutions of the fragment CpFe(dppe) + [dppe = Ph2P(CH2)2PPh2] are stable and solids with variable amounts of solvent have been isolated. ,1,16 Thus, the complexes [CpFe(dppe)L]PF6 (L = T H F , acetone, R C N and Py) have been prepared and c h a r a c t e r i z e d1. 6"1-3'1 5 The T H F complex can be used as a useful precursor for the preparation of some cationic complexes. 6 Recently, we have studied the solvent effect on the electronic absorption spectra of the complexes CpFe(dppe)X (X = CI, I and CN). 17 In polar solvents, the complexes with X = Cl and I undergo ionization of the F e - - X bond to give the cationic

* Author to whom correspondence should be addressed.

solvated species [CpFe(dppe) (solvent)] +. In this paper we report a systematic solvent study on the fragment CpFe(dppe) + using UV vis spectroscopic methods.

R E S U L T S AND D I S C U S S I O N The species [CpFe(dppe)(solvent)] + were generated in solution by stirring the precursor complex CpFe(dppe)113 with T1PF6 in the respective solvent. The solvents were chosen considering two conditions:solubility of the [CpFe(dppe)(solvent)]PF6 complex formed and insolubility of the thallium iodide product. The selected solvents, together with some of their physical and physicochemistry properties, are shown in Table 1. Precipitation of TI! was observed in all cases. Visual inspection of the final colours of the solutions containing the complexes reflects their absorption properties : the amines were red-brown, the nitriles yellow orange and the chlorinated (CHCI3, CH2CI2) ethers, ketones, alcohols and P - - O solvents pale yellow. Spectral absorption data for the investigated systems are shown in Table 1. For some solvents no maximum was observed and so an approximate value was estimated from the shoulders. The transitions observed in the systems [CpFe(dppe)L] + have been assigned previously. 17 The lowest absorption band has been assigned to a d d transition. A simplified MO scheme lg for the 997

998

C. DfAZ and N. YUTRONIC Table 1. Energy maxima absorption data for [CpFe(dppe)(solvent)]PF6 complexes

Solvent Dioxane (Dxy Chloroform Piperidine Tetrahydrofuran (THF) Dichloromethane Pyridine Acetone (Ac) Dietilacetamide n-Butironitrile i-Butironitrile Trimethylphosphate (TMP) Benzonitrile (BN) Hexamethylphosphorictriamide (HMPA) Methanol Nitromethane (NM) Dimethylformamide (DMF) Acetonitrile (An) Ethylene sulphite Sulpholane (TMS) Dimethylsulphoxide (DMSO) Propanediol- 1,2-carbonate (PDC)

e"

DN ~'

2.2 4.8 5.8 7.6 9.1 12.3 20.7

19 < 10 51 20 < 10 33.1 17 32.2 16.6 15.4 23.0 11.9 38.8 19.0 2.7 26.6 14.1 15.3 14.8 29.8 15.1

20.3 20.2 20.6 25.2 30.0 33.6 35.9 36.5 38.0 41.0 42.0 48.9 69.0

2 (max) (nm) 430 ~450 ~ 500 441 440 ~ 540 420 438 453 449 ~419 450 438 413 < 450 456 <450 ~430 < 450 < 450

"Dielectric constant values from ref. 34. hDonor number values from ref. 22. ' Abbreviations given in parentheses.

CpFe(L2)L + system is shown in Fig. 1. Currently, the solvent effect on the metal coordination compounds is interpreted by the formation of substratesolvent outer-sphere complexes producing a specific metal-solvent interaction. 19,20However, in the complexes [CpFe(dppe)(solvent)]PF6 the solvent could act in two ways : a second coordination sphere interaction or a direct solvent-metal interaction. As expected for d d transitions, the lowest absorption

.--'°

.

"'"" .° iT

. . . . . . . "" """ -

L = n acceptor~ 4 p"'*l

/

..jJ _

_ A---L,'" ,'" k=adador .-~..L,"" L = rr d o d o r

@+

~,,,.Fe_ L--/ • P

P

Fig. 1. Simplified molecular orbital diagrams for d ~ CpML2L + complexes.

band for [CpFe(dppe)(solvent)] + complexes could not be affected by the solvent acting on a second coordination sphere. 21 Hence changes observed in the absorption spectra of the studied complexes could be due to a specific metallic fragment-solvent interactions. The bulk properties of the solvent such as dielectric constant or most specific interaction as the donor number D N 22 do not exhibit a clear relation with the observed 2. . . . as shown in Fig. 2. The acceptor number AN also gave an unacceptable relation with )...... The absorption properties appear to be determined by the coordination bonding properties of the solvent rather than by a bulk property of the solvent. Changes on varying the ligating properties of the ligand L can affect the H O M O and L U M O of the fragment CpFe(dppe)+, as shown in Fig. 1. Theoretical and experimental studies 18 have demonstrated that only the dn levels are significantly altered on varying the z - d o n o r acceptor, a-donor properties of the ligand. Amines absorb around 500 nm. The lowest energy of this transition arises from their high adonor properties, which destabilize the n levels, decreasing the transition energy, as shown in Fig. 2. On the other hand, typical n-acceptor ligand

Coordinating properties of solvents towards CpFe(dppe) *

999

60

S0

cI

40

S

0u

R-C-NR2 z

o >. u E

[~]

30

~

Amines

Fig. 3. HOMO [3a' orbital of CpFe(PH3)2 ~]-LUMO [(C--CI)* orbital of R--C] interaction of fragment CpFeL2 + with R--C1.

--~P:O 20

~S:O ~-~

~ elones,Alcohols,E lher s ]

Nilriles

~0

[~] ~ CHnC[4-n 0

I

4O0

RN02

I

I

500

600

~'mox

Fig. 2. ), .... for the complexes [CpFe(dppe)(solvent)] + versus the donor number DN for the solvent.

solvents such as nitriles 23 or sulphoxides 2° stabilize the ~ levels, increasing the d - d transition, as shown in Fig. 2. Similar spectral behaviour was observed for amines, nitriles and sulphoxides in M(CO)sL systems (M = Cr, Mo, W; L = neutral donor ligand).2° Dichloromethane and chloroform as solvent ligands cause unexpected absorptions around 450 nm. Halide ligands are g-donors 26and consequently absorptions could be observed at lower energy.t However, in halo-alkane-metal complexes, the M - - X bonds have been described as a-donation through a halogen orbital of highp-character to the metal. 26 Although in other halocarbon organometallic complexes 26 it has been suggested that the backbonding is negligible, in the complexes CpFe(dppe)(C1--R) + ( R = CICH2 and CHC12) some (M)d a(C--CI)* backbonding appears to exist. Symmetry arguments are favourable for such interaction, as shown in Fig. 3. These rr interactions could explain the high stability exhibited by dichloromethane solutions of [CpFe (dppe)] BF4. II.L6Thus, a yellow-orange crystalline solid with possible formula [CpFe(dppe)(CHzC12),]BF4 has been isolated. 16 Recently, stable dichloromethane complexes have been reported. 26'272~ In CHCI3 we were able to detect the absorption

tThe lowest absorption d - d band for CpFe(dppe) (X = CI, I) was observed at 600 nm. ~7

bands of CpFe(dppe)CP 7 in addition to the band at 448 nm, probably corresponding to the cationic complex CpFe(dppe)Cl3CH +. The formation of the chloride derivative can be explained by reaction of HCI, arising from CHC13, 29 with the iodide complex : CpFe(dppe)l + HCI

, CpFe(dppe)Cl + HI.

The fragment CpFe(dppe)+ with solvent ligands containing the C z O group (acetamide, formamide, PDC and acetone) absorb in the range 400450 nm, which suggests some ~-acceptor behaviour of the carbonyl group. In aldehyde and ketone transitions metal complexes, t/~ and ~/2 coordination of the ligands has been observed (see Fig. 4). 3°'3~ In the first case the ~ O groups coordinate via the oxygen atom (q~ coordination). In the second case, the coordination takes place through the CO bond (q2 coordination). For d 6 CpML2 fragments the respective CpML2(RtR2C~------O)+ complexes exist experimentally in both ~/~ and r/2 forms depending on the metal and on the R~ and R2 substituents. 3° For L = ketone, coordination is via the oxygen atom (t/~ coordination), while for L = PR3 ~/2coordination is favoured? ° Then is not rare that acetone coordinates by a q2 mode toward the CpFe(dppe) + fragment, as shown in Fig. 5. MO arguments are also consistent with this coordination mode. In fact, interaction of the H O M O

o

o=c~

[M],

[ M] -

/\ ql

[I C

i~2

Fig. 4. ~/~ and ~/2 coordination modes of ketone ligands towards [M] fragment.

1000

C. D[AZ and N. YUTRONIC General preparation o f [CpFe(dppe)(solvent)]PF6

0

/S/ Q_f'

Fe ~

II

C /"

CpFe(dppe)I (0.01 g, 0.015 mmol) and 0.01 g (0.015 mmol) of T1PF6 in 5 cm 3 of the respective solvent were stirred for 16 h at room temperature. After centrifugation of the solution to precipitate solid TII, the supernatant containing the complex was removed and placed in a 1 cm optical path cuvette. Acknowledgement--This work was financially supported

Fig. 5. Possible q2 mode of coordination of ketones towards the [CpFe(dppe)] + fragment.

by Fondecyt (Grants 1940588 and 1950305).

REFERENCES of the CpFe(P0(P2) fragment with the L U M O of a ketone 7r*(C--O) orbital is favourable considering symmetry arguments. On the other hand, for aldehydes with d 6 CpML2 fragments the r/2 coordination is preferred, and for D M F some (M)--(L)Tr backbonding is probable. The similar absorption behavior of N,N'-diethylacetamide suggests coordination to the metal through the ~ O group rather than the amide groups. Coordination of CH3NO2 to the fragment CpFe(dppe) + produces an absorption at higher energy than other oxygen donor solvent ligands, indicating some rc-acceptor ability of nitromethanc. Complexes with this solvent ligand have been prepared previously, 32the coordination probably being through the oxygen atom. Symmetry arguments indicate a favourable rc interaction between CpFe(dppe) + ~8and NO2R fragments. 33 The effect of coordination of alcohols and ethers correspond to that of a a-donor-rr-donor ligand, producing absorption similar to ketone and halocarbon ligands. Finally, as expected, a rough relation exists between 2m~x and the 10Dq parameter of the respective solvents acting as ligands in the [CpFe(dppe)L] + complexes. In effect, in spite of the fact that insufficient 10Dq values are available for iron(lI) complexes, by using 10Dq values for the CrL63+, C o L 6 2+ and NiL62+ series of complexes, 3s the following order can be established : NH3 (amines) > CH3CN > M e O H > acetamide (amides) > butanona (ketones) > D M S O > (R2N)3P--O , (RO)3P---O.

EXPERIMENTAL (CpFe(dppe)I was prepared as reported previously. ~3The solvents were all purified by standard methods. UV-vis absorption spectra were recorded using a Varian DMS-90 spectrophotometer.

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Coordinating properties of solvents towards CpFe(dppe) +

20. 21. 22. 23. 24. 25. 26.

27. 28.

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