& Ligand Effects | Very Important Paper| [U III {N(SiMe 2 tBu) 2 } 3 ]: A Structurally Authenticated Trigonal Planar Actinide Complex Conrad A. P. Goodwin, [a] Floriana Tuna, [a] Eric J. L. McInnes, [a] Stephen T. Liddle, [b] Jonathan McMaster, [b] Inigo J. Vitorica-Yrezabal, [a] and David P. Mills* [a] Abstract: We report the synthesis and characterization of the uranium(III) triamide complex [U III (N**) 3 ][1, N** = N- (SiMe 2 tBu) 2 À ]. Surprisingly, complex 1 exhibits a trigonal planar geometry in the solid state, which is unprecedent- ed for three-coordinate actinide complexes that have ex- clusively adopted trigonal pyramidal geometries to date. The characterization data for [U III (N**) 3 ] were compared with the prototypical trigonal pyramidal uranium(III) tria- mide complex [U III (N“) 3 ] (N” = N(SiMe 3 ) 2 À ), and taken to- gether with theoretical calculations it was concluded that pyramidalization results in net stabilization for [U III (N“) 3 ], but this can be overcome with very sterically demanding ligands, such as N**. The planarity of 1 leads to favorable magnetic dynamics, which may be considered in the future design of U III single-molecule magnets. Investigations into low-coordinate metal complexes (defined herein as coordination number, CN < 4) are legion, because they can exhibit interesting properties, [1] including small-mole- cule activation chemistry [2] and single-molecule magnet (SMM) behavior. [3] Low CN complexes usually contain sterically de- manding ligands to prevent oligomerization, [1] in which bulky monodentate amides are frequently utilized. [4] The bulky silyla- mide {N(SiMe 3 ) 2 } À (N“) has provided landmark low CN com- plexes; for example, three-coordinate [M III (N”) 3 ] com- plexes of Group 13 (M = Al, Ga, In, Tl) [5] and first row d-block (M = Ti–Co) [6] metals are trigonal planar (D 3h ) in the solid state, but Group 3, [6a, 7] lanthanide (Ln), [7] and actinide (An) [8] [M III (N“) 3 ] complexes exhibit trigo- nal pyramidal (C 3v ) solid-state geometries, although they have zero dipole moment in solution, inferring that they may become planar in this phase. [9] Pyramidal geo- metries persist for [Ln III (N”) 3 ] (Ln = Ce, Pr) in the gas phase, [10] but [Sc III (N“) 3 ] vapors are D 3h , with crystalline/gas-phase dis- crepancies for this complex attributed to crystal-packing ef- fects. [11] It is noteworthy that complexes, such as [Ln II (N”)(m- N“) 2 Na] (Ln = Eu, Yb) and [Sm II (N”)(m-N“) 2 M] (M = Na, K), have trigonal planar Ln coordination spheres, [12] but this geometry has not been previously observed in An complexes. f-Block metal centers favor high CNs, because Ln and An cat- ions have relatively large ionic radii and bonding regimes that are dominated by electrostatic contributions. [13] Low CN U III chemistry is burgeoning, driven by interesting small molecule activation reactions [14] and intrinsic SMM behavior. [15] Structural- ly characterized three-coordinate An complexes to date adopt exclusively trigonal pyramidal geometries rather than trigonal planar or T shaped (C 2v ), [16] although matrix isolation experi- ments [17] and calculations [18] have shown that monomeric UO 3 is T shaped. Both covalent [19] and electrostatic [10] arguments ac- count for the trigonal pyramidal geometry of [U III (N“) 3 ], [8, 20] hence, the most influential factor of these two for causing pyr- amidalization has never been established. Herein, we report the structurally characterized An complex, [U III (N**) 3 ](1, N** = N(SiMe 2 tBu) 2 À ), which adopts an unprecedented trigonal planar geometry for an actinide triamide complex. Complex 1 is close- ly related to [U III (N”) 3 ], allowing the contributions to pyramidali- zation to be assessed, together with the impact of geometry on magnetic (including dynamic) and electronic properties of U III complexes, for the future rational design of useful An materials. Complex 1 was prepared by a modification of the revised synthesis of [U III (N“) 3 ]. [8c] Compound [U III (I) 3 (THF) 4 ] [8c] was react- ed with 1.5 equivalents of [K{N(SiMe 2 tBu) 2 }] 2 in THF, followed by work-up and recrystallization from hexane to give 1 as dark purple needles in 62 % yield (Scheme 1). [21] Absorbances in the FTIR spectrum of 1 at u ˜ = 950, 825, and 761 cm À1 are attributed to the UNSi 2 stretching modes of the silylamide ligand. The asymmetric stretch (950 cm À1 ) is 40 cm À1 lower than that ob- served for [U III (N”) 3 ] (990 cm À1 ), [8a] which is of a similar magni- Scheme 1. Synthesis of 1. [a] C. A. P. Goodwin, Dr. F. Tuna, Prof. E. J. L. McInnes, Dr. I. J. Vitorica-Yrezabal, Dr. D. P. Mills School of Chemistry, The University of Manchester Oxford Road, Manchester, M13 9PL (UK) E-mail : david.mills@manchester.ac.uk [b] Prof. S. T. Liddle, Dr. J. McMaster School of Chemistry, The University of Nottingham University Park, Nottingham, NG7 2RD (UK) Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201404864.  2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons At- tribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Chem. Eur. J. 2014, 20, 14579 – 14583  2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 14579 Communication DOI: 10.1002/chem.201404864