CHEMISTRY

V. V. ZELENTSOV

STUDY OF THE IR SPECTRA OF INNER-COMPLEX COMPOUNDS OF URANYL WITH SCHIFF BASES AND DETERMINATION OF THE U=O BOND LENGTH

(Presented by Academician V. I. Spitsyn on 7 IV 1962)

The bands of the symmetric $\nu_1$ and asymmetric valence vibrations $\nu_3$ of the U=O bond in uranyl compounds lie in the ranges 830–880 cm$^{-1}$ and 900–940 cm$^{-1}$, respectively. If the $\nu_1$ band depends only slightly on the strength of the bond of the uranyl grouping with the ligands, then $\nu_3$ is rather sensitive to changes of this kind. The force constant of the U—O bond is related in a definite way to the shift of the band of asymmetric valence vibrations, which in turn is determined by the strength of the uranium—ligand bond. Thus, for example, in the region of 1000 cm$^{-1}$, a shift of the $\nu_3$ band of about 20 cm$^{-1}$ corresponds to a change in the force constant by approximately 0.2 mdyn/Å and in the U=O bond length by 0.01 Å. The IR spectra of inner-complex uranyl compounds with $\beta$-diketones (acetylacetone, benzoylacetone, and dibenzoylmethane) have been studied in works ($^{1,2,10,11}$). The influence on the position of the $\nu_3$ band of $\mathrm{UO}_2^{2+}$ in these inner-complex compounds by secondary ligands (water, pyridine, ammonia) was also studied.

In the present work we investigated the IR spectra (paste in Vaseline oil) in the region 700–1000 cm$^{-1}$ of solid inner-complex uranyl compounds with various azomethine derivatives. The influence on the shift of the $\nu_3$ band of such secondary ligands as water, methyl, ethyl, and butyl alcohols was investigated.

In all the complexes studied by us, an intense absorption band was found in the region 900–930 cm$^{-1}$, assigned to the asymmetric valence vibrations $\nu_3$ of the uranyl grouping, and a less intense $\nu_1$ band in the region 872–886 cm$^{-1}$.

The force constant $K_{\mathrm{U-O}}$ of the uranium—uranyl oxygen bond was calculated by the formula

$$ K_{\mathrm{U-O}} = 0.029447\, m_0 \left( \nu_1^2 + \frac{m_{\mathrm{U}}}{m_{\mathrm{O}} + m_{\mathrm{U}}}\nu_3^2 \right), $$

where $\nu_1$ and $\nu_3$ are the wave numbers (cm$^{-1}$), and $m_{\mathrm{O}}$ and $m_{\mathrm{U}}$ are the masses of oxygen and uranium in atomic units.

To calculate the U—O bond length, the well-known Badger equation was used ($^5$)

$$ R_{ij} = \beta K_{ij}^{-1/3} + d, $$

which, in the case of uranyl compounds, takes the following form ($^4$):

$$ R_{\mathrm{U-O}} = 1.08K_{\mathrm{U-O}}^{-1/3} + 1.17, $$

where $R_{\mathrm{U-O}}$ is the U—O bond length (Å), and $K_{\mathrm{U-O}}$ is the force constant in mdyn/Å.

Table 1 gives the vibrational frequencies of the uranyl grouping, the force constants, and the U—O bond lengths.

The $\nu_3$ band lies in the interval 901–930 cm$^{-1}$, which indicates approximately identical values of the force constants and U—O bond lengths.

The position of the \(\nu_3\) band for uranyl inner-complex compounds with one and the same Schiff base depends to some extent on the number and nature of the secondary ligands. Thus, for example, \(\nu_3\) and \(\nu_1\) of the uranyl complex with salicylal-o-oxyaniline, containing an alcohol molecule (ethanol and n-butanol), lie at 930 cm\(^{-1}\) and 872 cm\(^{-1}\), respectively, whereas when they are replaced by four molecules of water (I), the frequency \(\nu_3\) is 905 cm\(^{-1}\) at an unchanged value of \(\nu_1\). The water molecules are apparently bound to the uranyl ion or, in any case, change the character of the crystal field, which leads to a decrease in the force constant, i.e., to a certain increase in the length of the U—O bond.

In compound II, \(\nu_1\) and \(\nu_3\) are equal to 884 and 924 cm\(^{-1}\). This may also be explained by a change in the ligand field as a result of the transformation of crystalline I into X-ray-amorphous II upon drying. In V, obtained by drying crystalline VI, which contains a methanol molecule, an increase in the frequency \(\nu_1\) is also observed. If \(\nu_1\) for the latter is 872 cm\(^{-1}\), then for the inner-complex compound dried at \(160^\circ\), it increases to 886 cm\(^{-1}\). At the same time \(\nu_3\) remains practically unchanged. The force constant of the U—O bond in VIII is 7.15 mdyn/Å, which indicates approximately the same strength as in V and VI.

The strongest bond of the Schiff base with uranyl is formed in the case of VII. This is confirmed by the position of the \(\nu_3 = 901\) cm\(^{-1}\) band and is explained by the fact that 2-(6-bromo-2-oxy-1-naphthalamino)-pyridine, like salicylal-o-oxyanilinate, is a tridentate ligand. It is quite understandable that in VIII the length of the U—O bond (\(R_{\mathrm{U-O}} = 1.73\) Å) proves to be the greatest among all the uranyl inner-complex compounds we investigated, and the bond order of U—O the smallest. According to Zachariasen \((^6)\), the length of a single U—O bond is 2.12 Å, of a double bond 1.92 Å, and of a triple bond 1.84 Å.

The U—O bond lengths calculated by us compel the assumption that the order of this bond must be greater than two.

Using Gordy’s formula \((^7)\),

\[ K_{ij} = aN \left[(x_i x_j)/R_{ij}\right]^{3/4} + b, \]

which relates the force constant, length, and bond order to the electronegativities (\(x_i\) and \(x_j\)) of the bonded atoms, we calculated the bond order of U—O in these compounds (see Table 1). It turns out that the bond order

Table 1

Force constants, bond order, and lengths of the U—O bond

of the U—O bond is 2.2–2.3. This confirms the recently expressed assumption \((^8)\) that the presence of lone electron pairs (\(sp\)) of oxygen, on the one hand, and free electron orbitals (\(fd\)) of uranium—

on the other, allows for the possibility of the formation of additional bonds. In the limiting case, the bond order of the $\mathrm{U—O}$ bond may be equal to 4. There are indications [9] that uranium and oxygen in the uranyl ion are connected by one $\sigma$ bond and two $\pi$ bonds, which accounts for the high stability of the uranyl ion.

I consider it a pleasant duty to express my deep gratitude to Academician Vikt. I. Spitsyn for his interest in the work and to A. I. Grigor'ev for assistance in recording the IR spectra.

Moscow Institute of Physics and Technology

Received
20 III 1962

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