Covalency Of Neptunium(iv) Triscyclopentadienyl


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DP-MS-78-62

COVALENCY OF NEPTUNIUM(IV) TRISCYCLOPENTADIENYL COMPOUNDS FROM MOSSBAUER SPECTRA by D. G. Karraker and J. A. Stone Savannah River Laboratory E. I. du Pont de Nemours &Co. Aiken, SC 29801

r-------NOTICE------,
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Paper proposed for presentation at the Southeastern ACS Meeting Savannah, Georgia November 8-11, 1978

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DP-MS-78-62

COVALENCY OF NEPTUNIUM(IV) TRISCYCLOPENTADIENYL COMPOUNDS FROM MOSSBAUER SPECTRA* D. G. Karraker and J. A. Stone Savannah River Laboratory
E. I. du Pont de Nemours &to.
Aiken, SC 29801

ABSTRACT
Isomer shifts measured from the 237Np Mossbauer spectra of

NpCp30R (R = alkyl) NpCp3R and NpCpAr (Ar ~ aryl) are used as a

·measure of the covalency in Np(IV) ligand bonding. The isomer

.

n

.

shifts m NpCp3 Bu and NpCp3C6H4C 2Hs show a strong cr character

to the Np-n- Bu and Np-C 6H4C2H5 bonds. The cr character of Np-OR

bonding is definite, but less pronounced. The comparatively low

covalency in the bonding of NpCp4 is ascribed to longer Np-C bonds in NpCp 4 than in NpCp3'+ compounds. The 237Np isomer shift

in Np(MeCp)Cl3•2THF indicates that'the MeCp ligand is a-bonded

in this compound.

* The information contained in this article was developed during
the c.ourse of work under Cortt:ta.ct No. AT(07-:-2)-l with tht:: U.S. Department of Energy.
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-.
INTRODUCTION
The U(IV) compounds with cyclopentadiene (HCp), UCp 3Cl and UCp4 were among the first actinide organometallic compounds discovered. 1' 2 The analogous Np(IV) compounds, NpCp 3Cl and NpCp4 were synthesized later, and shown to have identical properties. 3' 4 A number of derivatives have been prepared from UCp 3Cl, SUGh as UCp30R (R =alkyl), UCp3R and UCp3Ar (Ar = aryl). 5 - 10 Substituted
cyclopentadienes, such as MeCp, indenyl (In) and C5H4CH 20 have
also been prepared for structural studies of the UCp 3+ compounds. 11 • 12 This study is concerned with the preparation of the Np(IV)
analogs of the UCp3+ compounds and their study by Mossbauer spec-
-+ -
troscopy. For several NpCp 3 compounds, intermediate relaxation effects did not produce interpretable Mossbauer spectra, so Cp3NpOR and Cp3NpAr compounds substituted on_either the Cp, R, or Ar ligands were prepared and studied. Previous investigations have shown that the isomer shift ofthe Mossbauer spectrum reflects covalent contributions to the bonding of Np+ 4 ion. 13 ' 14 These covalent effects are the major interest of this study.
EXPERIMENTAL General
All compounds were prepared in the dry, purified argon atmosphere of a glove box. Samples were sealed under an argon· atmosphere in plastic holders for Mossbauer measurements, as performed previously. 13 ' 14 Solvents (THF, 1,2-DME, toluene, petroleum ether
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. .
and ethyl· ether) were purified by standard methods in an argon atmosphere. Neptunium was analytically determined by destruction of a sample in dilute acid and alphaTcounting an aliquot of this solution. Chloride was determined by titration with standard silver nitrate to potentiametric end-point. Organic ligands were estimated by gas chromatography of the decomposition products, formed by reacting the compounds with water or ethanol.
Preparation of Compounds NpCp3Cl, Np(MeCp)3Cl, NpCp3Br•DME
NpCp3Cl was prepared by reacting NpCl~ and TlCp in 1,2dimethoxyethane (DME), as described by Marks, et al. 15 Substitution of TlMeCp or NpBr~ produced Np(MeCp) 3Cl or NpCp 3Br•DME, respectively.. The product of this reaction may be only 70-80% pure. To obtain pure material (>95%) the crude produce was dissolved in toluene, ~iltered, and vacuum evapprated. The solid product was heated at 100°C in vacuum for 8 to 16 hours to remove ex~.:t::ss solvent. In the single preparation of NpCp 3Br, ,solvent removal was incomplete. Calcd. for NpCp3Br•DME: Np 40.05%; Br, 13.6%. Found: Np, 40.6%; Br 14.5%.
Np In3Cl, Npln3•xTHF
Npin3Cl was prepared by reacting NpCl~ and Kin in tetrahydrof4ran (THF) solution, as described by Laubereau, et al. 11 The product of the reaction at room temperature contained only trace amounts of chloride; and its Mossbauer spectrum showed only a
-~-

Np(III) species, consistent with a Npln3•xTHF product. Mixing the reagents at -88°C yielded a mixture with a Cl/Np ratio of 0.15, assumed to be a mixture of Npln3Cl•xTHF and Npin 3•xTHF from its Mossb~uer spectrum.

These compounds were prepared by metathesis of NpCp3Cl (for example) with LiBH~, 5 or a potassium alkoxide. 16 Stoichiometric amounts were stirred in toluene for 24 to 72 hqurs, the solids (LiCl, KCl) filtered, and the filtrate vacuum-evaporated to recover the products. Compounds prepared in this inanner, and their analyses are tabulated below.

Compound
NpCp3BH~ Np(MeCp)3BH~
NpCp30iC3H7 Np(MeCp)3oicsH1 NpCp30CH(CF3)2 NpCp 3otr.,, He

Np%, Calculated
53.04 48.4 48.3 44.2 39.6 46.9

Np%, Found
48.9 46.9 45.0 42.3 40.2 44.4

NpCp

n
3

Bu

was

prepared

by

adding

an

equimolar

quantity

of

commercial butyl lithium in hexane to NpCp3Cl suspended in diethyl P.thP.r Ht -78°C, 8 The solution was stirred as it warmed to room

temperature, then the ether was removed by vacuum. The remaining

solids were dissolved in toluene, filtered and the product re-
covered by vacuum evaporation of the toluene. Calcd. for NpCp3 n R11:

48.5% Np. Found: 47.9% Np; Cl not detected.

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. ''
Butane produced by decomposition of a sample with ethanol was measured by gas chromatography as 84% of theory. Mossbauer spectraof several preparations always showed NpCp~ and NpCp3•3THF impurities.
These compounds were prepared only in an impure state, by reacting NpCp3Cl with an ether solution of the phenyl lithium 17 at -78°C, similar to the method for preparing NpCp 3nC~H 9 • After stirring a short time at room temperature, the ether was removed by vacuum, and the residue extracted with toluene. The toluene solution was evaporated by vacuum to recover the product. For
NpCp 30 the best preparation had a Cl/Np ratio of 0.52, indicating
52%of the sample was unreacted NpCp3Cl. Destruction of a sample with ethanol and benzene analysis by gas chromatography showed
40% NpCp 30; the Mossbauer spectrum showed a NpCp~ impurity. The
NpCp3(C 6 H~CzHs) and Np(MeCp)3(C6H~CzHs) compounds were further purified by extraction with petroleum ether (boiling range 20-40°C).
Np{MeCp)Cl3•2THF This compound was prepared by the reaction of stoichiometric quantities of TlMeCp and NpCl~ in THF solution, similar to the literature preparation of UCpC1 3 •2THF. 18 ' 1 ~ The reaction mixture was stirred for 24 hours at room temperature, filtered, and a crude product obtained by evaporation of the filtrate. The crude product was washed with toluene to remove a probable Np(MeCp) 3Cl impurity, and vacuum-dried. Calcd. for Np(MeCp)Cl 3•2THF: NP, 41.95%;
Cl~ 18.50%. Found: Np, 42.4%; Cl, 19.55%; Cl/Np = 3.05.
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t

. . .-

RESULTS

Mossbauer spectra were obtained for all 16 compounds pre-

pared· Representative spectra for the compounds studied are

shown in the five figures: Figure l, Np(MeCp) 3 BH~; Figure 2,

.

n

Np(MeCp)3Cl, NpCp3Br•DME, Npin3Cl•xTHF; Figure 3, NpCp3 Bu;

Figure 4, Np (MeCph OiC 3H 1; and Figure 5, Np (MeCp) Cl 3•2THF.

Except for NpMeCpC1 3, none of the spectra are well-resolved. The spectra of all the Cp 3Np+ compounds investigated had their

resolution decreased to varying degrees by relaxation broadening.

In extreme cases, the resolution was so poor that no useful in-

formation could be obtained from the spectra of Np(MeCp)3Cl,

Npin3Cl•xTHF, NpCp3Br•DME (Figure 2) and NpCp3BH~. and NpCp30 (not shown). The spectra of NpCpaOi C3H7, NpCp30t Bu, and.

NpCp3C6HsC2Hs were poorly resolved, but could be interpreted

within a rather generous error. Poor resolution occurs because the relaxation time of 237Np

nuclei is about the same as the 62 nsec-half-life of the 59.5 keV

level. At relaxation times much faster than 62 nsec, the spectra is either a single line, or quadruple-split pattern, like NpCp~. 1 ~

At much slower relaxation times, the spectrum is magneticallysplit, like Np(C 8H8 ) 2. 13 Intermediate relaxation effects in

Np(IV) compounds usually show some correlation with the distance

betweeri Np(IV) ions. Relaxation effects on Mossbauer spectra can

sometimes be relieved by measuring a similar compound with bulkier

ligands, such as [(C2Hs)~N]2NpCl6, which has a well-resolved

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spectrum, instead of Cs2NpCl 6 , which has an uninterpretable spectrum because of intermediate relaxation effects. This approach was used in this study with partial success. For example, NpCp3BH4 and NpCp30 have spectra that could not be interpreted, so measurements were made with Np(MeCp) 3.BH 4 and NpCp 3C6H5C 2H5 • Interpretation of the spectra requires the assumption that the isomer shifts are essentially the same for both the substituted and unsubstituted ligands.
The Mossbauer parameters for the compounds studied are listed in Table 1, with some previously reported data included for comparison. Isomer shifts are referred to NpAl2 = 0. ·
DISCUSSION
The isomer shift in t~e 237Np Mossbauer spectra depends upon the shielding of the 6s orbitals by electron density in the inner orbitals, principally the Sf orbitals. 20 ' 21 As the. Sf orbitals add electrons proceeding from Np+ 7 to Np+ 3 , the isomer shift be~ comes progressively more positive, from approximately -6.S em/sec to 3.5 em/sec. Similarly, as electron density is contributed to the Sf orbitals from the ligands bonded to Np(IV)', the isomer shift of the NpH ion is shifted from its normal position of -0.4 em/sec toward the normal value for Np+ 3, 3.S em/sec. The isomer shift of the Np+ 4 ion in Np(IV) organometallic compounds reflects the differences in the electron density contributed by the ligands, which are equivalent to differences in covalent contributions of the ligands bonding to the Np(IV) ion.
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Covalency Of Neptunium(iv) Triscyclopentadienyl