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Bis ( cyclooctatetraenyl) neptunium( IV) and Bis ( cyclooctatetraenyl) plutonium (IV ) D. G. K~rraker,~’ J. A. Stone,aaE. R. Jones, Jr.,3b and N. EdelsteiP Contribution f r o m the Savannah River Laboratory, E. I. du Pont de Nemours and Company, Aiken, South Carolina 29801, and Lawrence Radiation Laboratory, University of California, Berkeley, California 94720. Received March 3, 1970 Abstract: Np(COT)* and PU(COT)~ (COT = C8Ha2-)were prepared by the reaction in THF solution of KzCOT with NpC14 and [(CzHS)4N]2P~C16, respectively. Infrared spectra of both were consistent with D8h molecular symmetry (“sandwich” compounds), and X-ray diffraction patterns showed both to be isomorphous with U(COT)*. The isomer shift of the Mossbauer spectrum of Np(COT)* was unusually positive for a Np4+ compound, indicating an electron contribution from the ligand to the bonding. The magnetic moments measured between 4.2 and 44’K were 2.43 BM for U(COT)z and 1.81 BM for Np(C0T)z; Pu(C0T)z was diamagnetic. A crystal-field model with an orbital reduction factor, k, of 0.8 is in reasonable agreement with the experimental results. benzene was more effective in removal of oxidizing impurities from THF than direct addition of solid LiA1H4. Preparation of Compounds. The preparation of Np(C0T)z followed the general procedure for the preparation of U(COT)2 as outlined by Streitwieser and Muller-Westerhoff.7 A THF solution of cyclooctatetraene was reduced with potassium metal at -20 to -40” in an inert atmosphere, and a stoichiometric quantity of NpClr in THF solution was added. This mixture was stirred for ca. 16 hr at room temperature, then the crude NP(COT)~was precipitated by the addition of deaerated water. The pure Np(C0T)z was recovered by extraction of the solid material with toluene and by vacuum evaporation of the toluene until the solid Np(C0T)z precipitated. Anal. Calcd for Np(C0T)t: Np, 53.3. Found: Np, 53.3. In the preparation of PU(COT)~, solid [(C2Hs)4N11PuCla was added directly to the THF solution of K2COT; other operations were identical with those used in the preparation and purifications of Np(C0T)z. Attempts to synthesize Pu(COT)2 with cs2Puclf, or Py2Puc16 (Py = CsH6N+)failed, although the preparation of U(COT)z with Py2UCle was successful. Anal. Calcd for Pu(COT)*: Pu, 53.5. Found: Pu, 51. Properties. The chemical properties of U(COT)z, Np(COT)z, and PU(COT)~ are nearly identical: all are stable toward water or dilute base, react rapidly with air to form oxides, and are sparingly soluble (ca. 10-3 M ) in benzene, toluene, cc14, CHCla, etc. Toluene solutions of NP(COT)~are yellow viewed in 1- to 2-cm layers, and blood red in >4-cm depths. PU(COT)~ is cherry red in toluene solutions. Mossbauer Spectra. Velocity spectra were measured at 4.2”K with a constant-acceleration Mossbauer spectrometer, using an alloy source of 241Am (3 wt %) in thorium metal. The specReagents. The preparation of UCL, NpC14, and [(CZH~)~N]Y trometerl0 and cryogenics systemll have been described previously. Measurements from 4.2 to 80°K were made with a variable-temPuCl6 has been previously r e p ~ r t e d . ~Cyclooctatetraene, benzene, perature spectrometer, using a t4IAm (5 wt %)- thorium metal and toluene were of reagent grade, and were further dried by passource held at 4.3’K. Preparation and properties of americiumsage through 4-A molecular sieve. Dissolved oxygen was removed thorium M6ssbauer sources have been described.12 The sources by repeatedly evacuating to about 0.3 atm. Tetrahydrofuran used in this work13 both emit a sharp single line with a very weak (THF) was purified by distillation from LiAlH4 in a nitrogen atsecondary line removed -2.0 cm/sec from the strong line. Abmosphere. The quality of THF was found to be a major factor in sorbers of Np(COT)2contained about 50 mg/cma of Np. the success of preparations; addition of LiAlH4 as a solution in Magnetic Measurements. Magnetic susceptibilities were measured with a Foner-type vibrating-sample magnet~meter’~ manu(1) The information in this article was developed under Contract No factured by Princeton Applied Research Corp., and operated in the AT(07-2)-1 with the U. S . Atomic Energy Commission. field of a 12-in. electromagnet. The magnetometer was calibrated (2) A preliminary account of this work was presented at the 180th with a nickel standard and with H ~ C O ( C N S ) ~ The . ’ ~ measurements National Meeting of the American Chemical Society, New York, N. Y . , reported here were made in uniform magnetic fields of 1-10 kOe. Sept 1969. (3) (a) Savannah River Laboratory, E. I. du Pont de Nemours & A variable-temperature, liquid helium dewar flask provided con-
The
reduction of cyclooctatetraene in tetrahydrofuran (THF) solution produces the planar CsHs2anion (COT),4s5 which can react with the anhydrous metal chlorides of transition metals to form M(COT), M2(COT)3, or M(COT)2 complexes.6 The preparation of U(COT)2 extended these complexes to actinide(IV) Crystallographic studiess show that the U(COT)2 molecule has a “sandwich” structure, with planar eight-member COT rings above and below the U4f ion in D8h molecular symmetry. An electronic configuration has been proposed’ for U(COT)2 that involves participation of the 5f orbitals in the bonding of the complex, with the orbitals of the two 5f electrons of the U4+ ion mixing with the degenerate Esu orbitals of the ligands in essentially a nonbonding combination. NP(COT)~and PU(COT)~were prepared to allow a more thorough investigation of the nature of the actinide(1V)-COT compounds. Mossbauer measurements on Np(COT)* provide information on the shielding of s orbitals by outer electrons, and thus on the participation of ligand electrons in the bonding. Magnetic susceptibility measurements on U(COT)z, Np(COT)2, and Pu(COT)2 (U4+, 5f2; Np4+, 5f3; Pu4+, 5f4) narrow the choice of applicable molecular models. Experimental Section
Co. Aiken, S . C. 29801. (b) ORAU Research Participant, Summer 1969. (c) Lawrence Radiation Laboratory, University of California, Berkeley, Calif. 94720. (4) H. P. Fritz and H. Keller, 2.Nuturforsch. E, 16, 231 (1961). (5) H. P. Fritz, Adcun. Orgonometal. Chem., 1, 239 (1963). (6) H. Brei1 and G. Wilke, Angew. Chem., Int. Ed. Engl., 5,898 (1966); German Patent 1191375, Studiengesellshaft Kohle, MulheimiRuhr; British Patent 1128128; Chem Absfr., 70,96963 (1969). (7) A. Streitwieser, Jr., and U. Muller-Westerhoff, J . Amer. Chem. Soc., 90,7364 (1968). (8) A. Zalkin and I> V.,">> Vd', and V , = V2". The energy inatrix involves matrix elenients of the type iJ,J,1 V."~J,J,) = [3J,' - J ( J l)]B," which are tabulated in the literature. Considering the J = 4 ions, U 4 + (3H,) and Pu4f (514),the lowest state will be either the doublet J , = =t4 or the singlet JL = 0, depending upon whether the crystal-field parameter B2" is positive or negative. A similar calculation for Np4+ (410,1) shows that the lowest state is = I/?. either J z = f or ,I2, The crystal-field parameter BZOis defined as B2" = ~ r ~ A ~ ~where ( r ~ )cr, is the second-degree operator equivalent factor and is a function of L and S, AZO is a parameter that depends upon the lattices and charges of the ions, and (Y') is the expectation value for the magnetic (50 electrons. The parameter Azo may be calculated exactly in the limit of a point-charge model, and is approximately constant for the three isoniorphous actinide(1V)-COT compounds. (r2:)is necessarily positive, so the algebraic sign of B2(' depends on the signs of cyJ and A,?". a J is negative for U 4 + and Np4+ and positive for Pu4+. Assuming A?"is positive, J , = f 4 is the lowest state for U4+,J,= i9 i : ! for Np4+,and J , = 0 for Pu4+. Experimentally, U(COT), and Np(COT)2 are paramagnetic, therefore, the assumption of A?" > 0 must be correct. Calculating the magnetic nioments from perf = glJ,jB, pL,ff=: 3.20 BM for U(COT)? and 3.27 BM for Np(COT)??and PU(COT)~ is diamagnetic. These calculated values for paramagnetic susceptibilities are larger than the experimental values, but may be brought to agree by correcting for the reduction in orbital size caused by covalent contributions to the metal-ligand bonding. Introducing the orbital reduction factor k , and replacing the conventional Zeeman operator
+
+
+
? j t 2 G
*
L +
j-i
with k L 4- 2% the Land6 g,,becomes g J - (1 - k)(2 - gJ).2782hA value of k = 0.8 agrees with experimental results. 1\25) I
