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Salts of Ferricinium Cation and its Homologues: NMR Investigation, II EPR and NMR Spectra of sym-Polymethyl Substituted Ferricinium Cations

R. B. Materikova*, V. N. Babin, S. P. Solodovnikov, I. R. Lyatifov, P. V. Petrovsky, and E. I. Fedin Institute of Organoelement Compounds, U.S.S.R. Academy of Sciences, Vavilova 28, Moscow, 117813, U.S.S.R.

Z. Naturforsch. 3 5 b , 1415-1419 (1980); received June 2, 1980

Ferricinium, sym-Polymethylferricinium, Spin Derealization, NMR Spectra, E R R Spectra

The EPR, iH and 13C N M R spectra of methylferricinium hexafluorophosphates, [(MenCp)2Fe]PFö, n — 0-5, have been recorded. HFS constants have been estimated for all the ligand atoms. The general pattern of the spin density distribution is shown to depend on the superposition of the various mechanisms of spin derealization. For the majority of the cations the determining factors has been found to be represented by spin polarization over the jr-skeleton.

The electronic structure and mechanisms of spin density derealization in a ferricinium cation and some of its mono- and disubstituted derivatives have been the subject of a number of radiospectro-scopic studies [1-5]. However, now there is no consensus of opinion among the different authors as to the mechanisms of spin derealization in these hard-to-study compounds. We have synthesized a set of methylferrocenes with identically substituted ligands, the number of the methyl groups being from 0 to 5 [6]; then we have obtained hexafluoro-phosphates of the corresponding methylferricinium cations [7] and studied their XH and 13C NMR and also EPR spectra.

In a previous paper we have discussed the mech-anism of spin derealization in ferricinium and 1,1'-dimethylferricinium cations [5]. In this work we make an attempt to find out how the cumulation of methyl substituents in cyclopentadienyl rings determines the splitting of the double-degenerate 2E2g level and how this affects the change in the spin density derealization in the corresponding ferricinium cations. As it was found by us earlier by X-ray analysis [8] the adjacent methyl groups are withdrawn from the cyclopentadienyl ligand plane due to steric hindrance. This phenomenon, in turn, changes the parameters of the XH and 13C spectra of the corresponding methylferrocenes [6]. It was of interest to elucidate if the steric interaction produces any effect on the HFS constants in methyl-ferricinium cations.

* Reprint requests to Dr. R. B. Materikova. 0340-5087/80/1100-1415/$ 01.00/0

Experimental The synthesis and properties of the methylfer-

ricinium hexafluorophosphates were reported ear-lier [7].

The EPR spectra of these salts (polycrystalline samples) were studied in quartz ampoules on an EPR-3 (X-range) spectrometer at a magnetic field modulation frequency of 20 kHz and a temperature of 15-20 K.

The XH NMR spectra of these salts in acetone-de were recorded on a Bruker WP-60 spectrometer operated at 15.1 MHz, 30 °C.

The 13C NMR spectra of the salts (acetone being used as the solvent and the internal standard) were recorded on a Bruker WH-180 spectrometer oper-ated at 45.28 MHz, 35 °C in the 13C NMR spectra Fourier transform mode without *H decoupling.

Results Figure 1 presents the EPR spectrum for deca-

methylferricinium and Table I - the g-factor com-ponents. As it is seen from Table I, the g-factor anisotropy decreases noticeably with the introduc-tion of the first methyl group, and then it increases again along the series. According to the existing ideas [2] this implies that the splitting value of the 2E2g degenerate state initially increases under the

ßM«,Cp),F«f 20 K

1416 R. B. Materikova et al. • Salts of Ferricinium Cation and its Homologues: NMR Investigation

Table I. Values of g-factor components and low-symmetric distortion parameters* <5 for methylferri-cinium cations.

Cation gll g l 6 [cm - 1]

[CP2Fe]+ 4.40 1.39 540 [(MeCp)2Fe]+ 3.90 1.76 750 [(l,3-Me2Cp)2Fe]+ 4.51 1.40** 502 [(l,2,4-Me3Cp)2Fe]+ 4.50 1.40** 506 [(Me4Cp)2Fe]+ 3.14 1.17 1360 [(Me5Cp)2Fe]+ 4.54 1.46 490

* The k value is assumed as 1, <5 values are calculated from g|j data.

** The signal is very wide, the value is approximate.

effect of lateral distortion and then it decreases again as the molecule approaches the more sym-metrical form. In fact, the g-factor anisotropy of ferricinium and decamethylferricinium cations is approximately the same (Table I). On the contrary, di- and octamethylferricinium cations for which the largest lateral distortion is probable, are charac-terized by a noticeable decrease in g||. Thus in this series of cations the EPR spectra show no significant increase of the parameter d with the number of methyl groups but this parameter rather remains within 500-1300 cm-1.

Table II lists the isotropic paramagnetic shifts for the iH and 13C NMR spectra of the cations under study. In a previous paper [5] we have cal-culated the pseudocontact and contact components of the XH and 13C NMR isotropic shifts in ferricinium and dimethylferricinium cations (1 and 2, respec-tively). It has been shown that the g-factor ani-sotropy method gives the results identical to those with the more rigorous method based on the mag-netic susceptibility components. Therefore the pres-

ent work uses the former method [9] to obtain the pseudocontact components of the isotropic NMR shifts. For the geometrical factors of the cations [(Me„Cp)2Fe]+, n — 0-5, we used the data of the X-ray analysis performed for ferrocene [10, 11], ferricinium [12, 13], l,r-dimethylferricinium [14], octamethylferroeene [8] and its cation [15], deca-methylferrocene [8, 16, 17] and its cation [18]. Consideration of the geometry of these compounds has shown that the introduction of methyl groups and oxidation produce little effect on the Fe-C lengths (an increase of 0.02-0.04 Ä for the cationic form). Therefore we used the same values of the geometrical factors [5] for the entire series of the cations.

Table III presents the obtained contact and pseudocontact components of the isotropic *H and

Fig. 2. Ai constant signs for the various nuclei of cyclopentadienyl ligands in cations 1 - 6 .

13C shifts and the HFS constants (Ai). Fig. 2 shows the signs of the Ai constants for each atom in the ligands. When discussing the HFS signs, the fol-lowing facts appear to be most noticeable:

(1) in the majority of the cations (except for 1,1'-di- and sym-octamethylferricinium) the carbon atoms of the cyclopentadienyl rings have negative Ai values;

(2) the positive Ai sign for the ring carbons appears in the key positions of the most asymmet-rical cations - dimethyl- (2) and octamethylferrici-nium (5), the distribution of the Ai signs and values having a C2-symmetry;

Table II . Isotropic paramagnetic shifts in the XH and 13C N M R spectra of methy lferricinium hexafluorophos-phates (in ppm).

Nucleus [(Cp2Fe)]+ [(MeCp)2Fe]+ [(1,3-Me2Cp)2Fe]+ [(l,2,4-Me3Cp)2Fe]+ [(Me4Cp)2Fe]+ [(Me5Cp)2Fe]+

C(Cp) — 246.0 — 1 9 7 . 0 — 1 9 0 . 2 —185 .1

— 165.1 — 156.6

H(Cp) — 25.0 — 29.0 — 32.0 — 30.5 — 25.0 C(Key) — 330.0 — 279.6 — 215.8

— 235.2 — 184.5 — 210.9

— 183.0

C(Me) 23.3 25.6 22.8 24.7

21.1 25.6

29.5

H(Me) 11.5 25.5 26.6 35.0

28.3 36.8

38.3

1417 R. B. Materikova et al. • Salts of Ferricinium Cation and its Homologues: NMR Investigation

Table III . Pseudocontact, (A H/H)P S , (I), Fermi-contact, {A H/H)F, (II), shifts for XH and 13C nuclei of ferricinium hexafluorophosphates (in ppm) and the corresponding HFS constants, AI • 104, (III) (in Tesla units).

C(Cp) H(Cp) C(Key) C(Me) H(Me)

[(CP2Fe)]+ I — 3 4 2 . 0 — 9.0 II 96.0 — 1 6 . 0 I I I — 0.791 0.187

[(MeCp)2Fe]+ I — 2 3 1 . 0 — 6.0 — 2 3 1 . 0 9.0 19.0 II 34.0 — 2 3 . 0 — 99.0 14.3 — 7.5 I I I — 0.108 0.292 0.319 — 0.046 0.095

[(1,3 -Me2Cp )2Fe]+

I — 3 5 7 . 0 — 9.5 — 3 5 7 . 0 14.0 29.6 I I 166.8 — 2 2 . 5 77.4 11.6 — 4.1

171.9 I I I — 0.641 0.340 — 0.305 — 0.040 0.076

— 0.668 [(l,2,4-Me3Cp)2Fe]+

I — 3 5 7 . 0 — 9.5 — 3 5 7 . 0 14.0 29.6 I I 191.9 — 2 1 . 0 141.2 8.8 — 3.0

121.8 10.7 5.4 I I I — 0.737 0.243 — 0.550 — 0.029 0.061

— 0.473 — 0.038 — 0.076 [(Me4Cp)2Fe]+

I — 1 6 6 . 0 — 4.4 — 1 6 6 . 0 6.5 13.7 H 9.4 — 2 0 . 6 — 17.9 14.6 14.6

— 42.7 19.1 23.1 I I I — 0.049 0.323 0.093 — 0.076 — 0.228

0.221 — 0.099 — 0.361 [(Me5Cp)2Fe]+

I — 3 5 0 . 0 13.5 29.0 H 167.0 16.0 9.3 I I I — 0.481 — 0.027 — 0.099

(3) all the C - H fragments of the cyclopentadienyl rings show alternating Ai signs when passing from carbon to hydrogen;

(4) the C-CH3 fragments have four possible var-iants of the Ai sings, i.e. "hyperconjugation" - in dimethylferricinium; the one typical of direct de-localization - in decamethylferricinium (6) and vicinal C-CH3 fragments of hexamethylferricinium (4). Moreover, there exist two unusual variants: in the C-CH3 groups of octamethylferricinium and isolated C-CH3 fragments of hexa- and tetramethyl-ferricinium (3).

Discussion The negative Ai sign of C(Cp) suggests that it is

the spin-polarization mechanism which is mainly responsible for the derealization of the unpaired electron in the entire series of the cations, except, may be, for the octamethylferricinium cation. It then follows from these results that the unpaired

electron density at the iron atom is close to unity, so the orbital reduction factor should be taken to be equal to unity instead of the usually employed 0.8 [19]. The fact that the unpaired electron density is mainly localized at the iron atom is presumably responsible for the great complexity of the total spin delocalization pattern. In fact, there is no unique chief reason determining the nature of spin derealization in these compounds. Therefore even slight disturbances are capable to introduce signifi-cant changes into spin derealization pattern. This sensitizes the pattern to the effect of various factors. That is why the only spin-delocalization mechanism is not essential and there exist some other spin de-localization mechanisms valid for these systems. It is believed that at least three derealization mechanisms are operative in the cations under study, viz.:

(1) the above mentioned spin-polarization mech-anism involving the jr-frame;

1418 R. B. Materikova et al. • Salts of Ferricinium Cation and its Homologues: NMR Investigation

(2) the spin-polarization mechanism involving the a-skeleton;

(3) the direct derealization mechanism involving the excited configuration.

In fact, the alternation of the signs in the C - H fragments suggests that spin polarization can lead to the appearance of an unpaired electron density on the or-skeleton orbitals of the cyclopentadienyl ligands. Its further extension to the hydrogen atoms occurs by the McConnell mechanism [20] and re-sults in the appearance of a positive spin density thereon.

As it has been stated above, the sets of the HFS constant signs in the adjacent and isolated C-CH3 fragments are different for the cations 2—4 and 6. In the vicinal C-CH3 fragments all the atoms have a negative Ai sign, and the general pattern corre-sponds to the domination of the direct spin density derealization mechanism over the cr-skeleton. Pre-sumably such a situation could be attributed to the steric interaction withdrawing the methyl group from the cyclopentadienyl ring plane. In fact, the X-ray analysis data demonstrate that in cation 5, octa- and decamethylferrocene the methyl groups are bent towards the external side of the molecule [8, 15-18]. The bent influences on the chemical shifts in the XH and 13C NMR spectra of deca- and other vicinal methylferrocenes [6]. For example, the carbon signals of the methyl groups in 13C NMR spectra can be divided by the number of the adja-cent methyls into the following three groups: iso-lated, having one adjacent methyl group and having two such groups. The protons of the methyl groups can be treated in the same manner. The same pat-tern is also observed in the similar series of cobalt-icinium salts [21]. It should be noted that the pop-ulation of different conformations in sterically hin-dered methyl groups becomes unequal as reported by Grant and coworkers [22, 23]. This fact could complicate the spin density derealization to the methyl group by the "usual" hyper conjugation mechanism. We may believe therefore that spin polarization over the cr-skeleton occurs in ferrici-nium and methylferricinium cations along with a spin-polarization mechanism involving ^-skeleton orbitals. Earlier such mechanisms were proposed by Rettig and Drago for the metallocenes of V, Cr, Co, Ni [24] and then by Anderson and Rai for fer-ricinium [4]. By this reason a very small fraction of

the unpaired electron expands over the cr-skeleton orbitals when the spin density distribution over the C-CH3 fragments by the hyperconjugation mech-anism is complicated (see Table III).

In a previous paper [5], when discussing the spin density distribution patterns in cations 1 and 2, we emphasized the appearance of a positive spin density in the key positions of the cyclopentadienyl rings of cation 2. The same feature is also typical of the key carbons of another maximum-distorted cation - octamethylferricinium (5). Spin derealiz-ation pattern in cations 2 and 5 can be understood if account is taken of the fact that the methyl groups remove degeneracy of the e levels in aro-matic systems. These phenomena have been studied in detail for xylene anion radicals [25, 26]. It has been shown that stabilization of either an antisym-metric (p-xylene) or symmetric (ra- and o-xylene) component of the e2-orbital occurs in these radicals depending on the mutual position of the methyl-groups.

Similarily it may be assumed that in cation 2 the electron-donor methyl group produces a stabilizing effect on the anti-symmetric components of the degenerate e-orbitals, particularly the eig-orbital. On the contrary, the C - H fragments in the rings of cation 5 can be regarded as electron-accepting "substituents" with respect of the C-CH3 fragments. This will lead to an additional stabilization of the symmetric components of the e-orbitals. Thus cations 2 and 5 have a different arrangement of the eig-orbital components participating in the spin-polarization distribution of spin density. The result is that a positive sign of spin density appears at the Ci atoms in cation 2 due to : 1) a node of the antisymmetric eig-orbital component existing in this point and 2) the direct derealization of the unpaired electron density from the neighbouring aig-orbital [5].

It is possible that the 2E2g and 2Aig configurations are also close to each other in cation 5 but the sym-metric eig-orbital component has the largest values of atomic coefficients at the Ci atoms. There-fore cation 5 at the Ci atoms has a negative spin density whereas at the remaining atoms of the ring the thermally populated 2 Aig state provides a posi-tive spin density. Sometimes they propose an as-sumption of the accidental degeneration of the 2Aig

and 2E2g states in such systems [27]. However, to

1419 R. B. Materikova et al. • Salts of Ferricinium Cation and its Homologues: NMR Investigation

provide support for this interpretation, an additional NMR study is required.

At present it is rather difficult to give an ex-haustive interpretation of the observed pattern of spin density distribution in the cations of the entire

series. Additional experimental evidence may be necessary to describe this series adequately.

The authors wish to thank A. V. Kulikov and R. S. Agabekyan for the help in measuring the EPR spectra.

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