Comprehensive Synthesis\nand Structural Trends in Tetramethyl\nDiglycolamide (TMDGA) Nitrate Complexes with Lanthanides and Americium

Complexes of <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethyl diglycolamide\n(TMDGA),\na hydrophilic diglycolamide (DGA) proposed as an aqueous phase holdback\nreagent, have been crystallized for the majority of the lanthanide\nseries (excluding promethium), yttrium, and americium to deepen our\nstructural understanding of trivalent metal ion (<i>M</i><sup>3+</sup>) DGA coordination compounds in the presence of nitrate\ncounter-anions. The presented collection of 16 complexes with accompanying\nsingle-crystal structures, taking formulas [<i>M</i>(TMDGA)<sub>3</sub>]​[<i>M</i>(NO<sub>3</sub>)<sub>6</sub>]\n(<i>M</i> = La, Ce, Pr, Nd, Sm, Am), [<i>M</i>(TMDGA)<sub>3</sub>]​[<i>M</i>(NO<sub>3</sub>)<sub>5</sub>(H<sub>2</sub>O)]<sub>1–<i>x</i></sub>​[<i>M</i>(NO<sub>3</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub><i>x</i></sub>​(NO<sub>3</sub>)<sub>1+<i>x</i></sub> (<i>M</i> = Eu, Gd, Tb, Dy, Ho, Er, Tm,\nYb), [<i>M</i>(TMDGA)<sub>3</sub>]<sub>2</sub>​[<i>M</i>(NO<sub>3</sub>)<sub>4</sub>​(H<sub>2</sub>O)<sub>2</sub>]<sub>0.75</sub>​[<i>M</i>(NO<sub>3</sub>)<sub>5</sub>​(H<sub>2</sub>O)]<sub>1.25</sub>​(NO<sub>3</sub>)<sub>2.75</sub>·H<sub>2</sub>O (<i>M</i> =\nLu), and [<i>M</i>(TMDGA)<sub>3</sub>]​[<i>M</i>(NO<sub>3</sub>)<sub>5</sub>​(H<sub>2</sub>O)]​(NO<sub>3</sub>)·CH<sub>3</sub>OH (<i>M</i> = Y) were all\nsynthesized via solvent diffusion of reaction mixtures containing\nthe metal nitrate <i>M</i>(NO<sub>3</sub>)<sub>3</sub>·nH<sub>2</sub>O and TMDGA. Single-crystal X-ray diffraction analyses of\nthese new structures show that each TMDGA complex comprises of three\nTMDGA ligands coordinating the metal ion via carbonyl and etheric\noxygen atoms forming [<i>M</i>(TMDGA)<sub>3</sub>]<sup>3+</sup> cations. Spectroscopy measurements under high pressure displayed\nnotable differences in the <i>f</i> → <i>f</i> transition shifting between that of Nd(III) and Am(III). Shifting\nof transitions by 2.0 nm were observed in Nd up to 8.50 ± 0.09\nGPa, while Am saw shifting between 11.0 to 13.5 nm at pressures up\nto 20.06 ± 1.90 GPa. The local geometry in these complexes is\na distorted spherical capped square antiprism (CSAPR-9) except for\nof the yttrium complex, which exhibits a distorted spherical tricapped\ntrigonal prismatic (TCTPR-9) geometry. The anions that form concomitantly\nwith the TMDGA complexes are composed of hexanitrato species for the\nearly lanthanide ions (lanthanum to samarium); whereas the remaining\nsmaller lanthanides did not possess sufficiently large ionic radii\nto coordinate six bidentate nitrate anions, instead, one or two nitrate\nanions are situated in the outer sphere. The systematic progression\nof changes in the anionic environments of these complexes outlines\nthe changing coordination habits afforded by the lanthanide contraction.