## 1 Introduction  
Organophosphorus compounds containing imido groups capable of forming neutral chelate complexes with certain metals are widely used as catalytic systems [1; 2; 3], biologically active [4], and luminescent metal complexes [5], and in metal extraction processes [6; 7; 8; 9; 10; 11]. Synthetic approaches to various derivatives of imidophosphoric acids are widely represented [12; 13; 14], and the effectiveness of these compounds as rare-earth elements (REE) extractants is well established [14; 15; 16; 17]. Several esters of imidophosphoric acids (EIPAs) with general formula [OP(OR)\({}_{2}\)]\({}_{2}\)NH, where R = Me, Et, _n_-Pr, _i_-Pr, _n_-Bu, _n_-Hex, _n_-Oct, -C\({}_{2}\)H\({}_{4}\)OC\({}_{2}\)H\({}_{5}\), -C\({}_{2}\)H\({}_{4}\)F, -C\({}_{2}\)H\({}_{4}\)Cl, -CH(CH\({}_{2}\)Cl)\({}_{2}\), -CH=CHCF\({}_{2}\)CF\({}_{3}\), Ph, _o_-, _m_- and _p_-methylphenoxy groups, or containing mixed substituents [OP(OR)\({}_{2}\)]NH[P(OR\({}^{\prime\prime}\))\({}_{2}\)O], where R\({}^{\prime}\) and R\({}^{\prime\prime}\) correspond to Et and _n_-Bu, Me and Ph, Et and Ph, Ph and p-methylphenyl, were prepared and described [18]. However, most of these compounds are not even characterized by routine NMR spectroscopy or mass spectrometry. The first short-chain EIPAs were prepared by reaction between diphenoxyphosphamide (PhO)\({}_{2}\)P(O)NH\({}_{2}\) and phosphorus pentachloride with further successive treating of the resulting products with formic acid and alkali metal alkoxides or phenolates (Scheme 1) [19]. The simplest synthetic way to EIPAs is alcohols of imidodiphosphoric acid tetrachlorides (OPCI\({}_{2}\))2 NH with NaOR (R = alkyl, aryl) leading to the formation of the corresponding symmetrical esters [7; 18]. Short-chain EIPAs can be prepared by direct reaction of trichlorophosphazodichlorophosphonyl (TCDP or dichlorophosphinylphosphorimidic trichloride) Cl\({}_{3}\)P=N-POCl\({}_{2}\) with aliphatic alcohols using amine as an acceptor of HCl similarly as it usually used in aminophosphazene derivatives synthesis [20] or by treatment of the corresponding \"pentaesters\" with hydrochloric acid (Scheme 2) [21]. EIPAs with medium chain length or longer can be synthesized by the reaction of TCDP higher oligomer homologs with aliphatic alcohols or their alkoxides. The short-chain EIPAs form metal complexes containing deprotonated {OP(OR)\({}_{2}\)}\({}_{2}\)N\({}^{-}\) ligands. The spatial structure of a complex depends on the chemical properties of a metal. Phosphonyl oxygen atoms coordination -P=O-M-O=P- takes place in the complexes of {OP(OPh)\({}_{2}\)}\({}_{2}\)NH with Na, Ba, Ca, Cu, Fe (Figure 1a) [22; 23]. EIPAs also form lanthanide complexes similar to those with other metals (Figure 1b) [6; 24]. However, reaction of {OP(OPh)\({}_{2}\)}\({}_{2}\)NH with phenylmercury hydroxide PhHgOH leads to formation of covalent N-Hg-Ph bonds along with coordination of one phosphonyl oxygen atom with a mercury atom of a neighboring molecule (in this case, a dimeric association in the crystal structure is observed, Figure 1c) [25]. EIPAs represent a large and diverse class of organophosphorus compounds, which are widely used as REE extractants due to their ability to form metal complexes. Di-(2-ethylhexyl) phosphoric acid (D2 EHPA) and 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester (EHEHPA) are the most thoroughly studied among them [26; 27; 28; 29]. Since REE are poorly stripped from D2 EHPA complexes, EHEHPA is considered to be a perspective alternative with greater selectivity to REE and the possibility of stripping at lower acidities. However, EHEHPA has lower extraction efficiency in comparison with D2 EHPA. The use of extractant blends of D2 EHPA with various agents also demonstrates some improvements in extraction and separation processes [30; 31; 32] but does not eliminate all drawbacks of the base extractant. Hence, an active search for new highly efficient extractants and accurate study of their extraction properties are still of utmost actuality.