Paula M. B. Piccoli

Synthesis and properties of a fifteen-coordinate complex: The thorium aminodiboranate [Th(H3BNMe2BH3)4]

The concept of coordination number is extremely useful and is widely employed to describe the local chemical environments of atoms. Originally defined by Alfred Werner in 1893, the coordination number is closely related to many other important properties such as atomic radius, molecular and electronic structure, and chemical reactivity. An important modification of Werner s original concept was the recognition that, for certain ligands such as ethylene, two linked atoms jointly occupy a single coordination site. This modified definition is widely used to describe both transitionmetal (d-block) and inner-transition-metal (f-block) complexes. An interesting question is: what is the largest possible coordination number? This question has recently been considered theoretically, and the 15-coordinate ion PbHe15 2+ has been predicted to be a bound species. The complexes tetrakis(cyclopentadienyl)uranium [UCp4] and its thorium analogue [ThCp4] are each connected to 20 atoms, [10] but the Werner coordination number of 12 (counting p bonds as occupying one site) is widely acknowledged to be more appropriate to describe the metal–ligand bonding in these compounds. Very high Werner coordination numbers are seen for metal complexes of the borohydride anion BH4 , which can coordinate to a single metal through as many as three hydrogen atoms. From an electronic perspective, each B-H-M interaction involves a separate electron pair, 14] and each BH-M interaction can be considered as a separate bond. Accordingly, [Zr(BH4)4], [15–17] [Hf(BH4)4], [15,16, 18] [Np(BH4)4], [19] and [Pu(BH4)4], [19] all have coordination numbers of 12, and [Th(BH4)4], [15, 16] [Pa(BH4)4], [19] and [U(BH4)4], [20] all of which are polymers in the solid state, have coordination numbers of 14. Some derivatives of these compounds also have high coordination numbers, such as the 14-coordinate tetrahydrofuran complex [U(BH4)4(thf)2]. [21] No complex of any kind, however, has been definitively shown to adopt a Werner coordination number higher than 14. Herein, we report the synthesis, single-crystal X-ray and neutron diffraction studies, and DFT investigations of the first 15-coordinate complex. DFT calculations suggest that this complex may adopt a 16-coordinate structure in the gas phase. This compound extends our recent studies of a new class of chelating borohydride ligands, that is, the aminodiboranates, some of which form highly volatile complexes that are useful as precursors for the chemical vapor deposition of thin films. Reaction of ThCl4 with four equivalents of sodium N,Ndimethylaminodiboranate, Na(H3BNMe2BH3), in tetrahydrofuran produced [Th(H3BNMe2BH3)4] (1), which could be isolated as colorless prisms by crystallization from diethyl ether. The IR spectrum of 1 contains strong bands at 2420 cm 1 that arise from terminal B–H stretches, and at 2264 and 2208 cm 1 that arise from bridging B-H···Th stretches. For comparison, [Th(BH4)4] contains a strong terminal B–H band at 2530 cm 1 and bridging B-H-M bands at 2270, 2200, and 2100 cm . The H NMR spectrum of 1 (C6D6 at 20 8C) contains peaks at d = 2.11 ppm (s, NMe2) and d = 4.23 ppm (br 1:1:1:1 q, JBH = 90 Hz, BH3); the terminal and bridging B–H units thus exchange rapidly on the NMR time scale. The B NMR spectrum consists of a binomial quartet at d = 2.75 ppm, which arises from coupling of the B nuclei with the three rapidly exchanging H nuclei (JHB = 90 Hz). For comparison, the B spectrum of [Th(BH4)4] consists of a quintet at d = 8.0 ppm (JBH = 86.5 Hz). Single-crystal X-ray and neutron diffraction studies of 1 reveal that it is monomeric with four chelating aminodiboranate ligands. The eight boron atoms describe a distorted D2d dodecahedral structure, in which boron atoms B1, B2, B2A, and B1A describe one planar trapezoidal array, and atoms B3, B4, B5, and B6 describe the other (Figure 1). The B2-Th1B2A and B4-Th1-B6 angles between wingtip boron atoms are almost linear at 172.61(12)8 and 171.85(13)8, respectively. Interestingly, seven of the eight Th···B distances (those for boron atoms B1–B5) range from 2.882(3) to 2.949(3) , but [*] S. R. Daly, Prof. G. S. Girolami School of Chemical Sciences University of Illinois at Urbana-Champaign 600 South Matthews Avenue, Urbana, IL 61801 (USA) Fax: (+ 1)217-244-3186 E-mail: girolami@scs.illinois.edu

Development and Loss of Ferromagnetism Controlled by the Interplay of Ge Concentration and Mn Vacancies in Structurally Modulated Y4Mn1―xGa12―yGey

The cubic intermetallic phase Y(4)Mn(1-x)Ga(12-y)Ge(y) (x = 0-0.26, y = 0-4.0) has been isolated from a molten gallium flux reaction. It presents a rare example of a system where ferromagnetism can be induced by controlling the vacancies of the magnetic centers. The Y(4)PdGa(12) type crystal structure is made up of a corner-sharing octahedral network of Ga and Ge atoms with Mn atoms at the centers of half the octahedra and Y atoms in the voids. At the highest Ge concentration, y = 4.0, the Mn site is nearly fully occupied, x = 0.05, and the samples are paramagnetic. At a lower Ge concentration, y = 1.0, Mn deficiency develops with x = 0.10. Surprisingly, strong ferromagnetism is observed with T(c) = 223 K. When Ge is excluded, y = 0, Mn is substantially deficient at x = 0.26 and ferromagnetism is maintained with a T(c) of approximately 160 K. In addition, a 6-fold modulated superstructure appears owing to an ordered slab-like segregation of Mn atoms and vacancies. Corresponding bond distortions propagate throughout the octahedral Ga network. Structure-property relationships are examined with X-ray and neutron diffraction, magnetic susceptibility, and electrical resistivity measurements.

SINGLE CRYSTAL NEUTRON DIFFRACTION FOR THE INORGANIC CHEMIST – A PRACTICAL GUIDE

Advances and upgrades in neutron sources and instrumentation are poised to make neutron diffraction more accessible to inorganic chemists than ever before. These improvements will pave the way for single crystal investigations that currently may be difficult, for example due to small crystal size or large unit cell volume. This article aims to highlight what can presently be achieved in neutron diffraction and looks forward toward future applications of neutron scattering in inorganic chemistry.

Structural, theoretical and spectroscopic studies of the dichloride hexahydrate cube [Cl2(H2O)6]2-.

The structure of the dichloride hexahydrate cube, [Cl(2)(H(2)O)(6)](2-), as a salt with the tris(diisopropylamino)cyclopropenium cation, [C(3)(N(i)Pr(2))(3)](+), has been determined by low-temperature X-ray and neutron-diffraction studies. H atoms not involved in O-H···Cl bonding are disordered over two 0.5 occupancy sites around the O(6) ring. Calculations of the dianionic cube in the gas phase show remarkably good agreement with the solid-state structures with the exception of short O-H bond distances around the O(6) ring that suggests the involvement of a dynamic process. The cluster was also characterised by single-crystal infrared spectroscopy, and vibrational wavenumbers were found to be in good agreement with hydrogen bonding distances. Dibromide and difluoride hexahydrates were also studied theoretically, and O···O distances were found to decrease in the order difluoride > dichloride > dibromide > (H(2)O)(6) and as O···O···O angles increased towards an almost planar ring in (H(2)O)(6). NMR spectra of a chloroform solution of the hydrated salt at -25 °C is consistent with cluster formation.

Neutron and X-ray investigations of the Jahn–Teller switch in partially deuterated ammonium copper Tutton salt, (NH4)2[Cu(H2O)6](SO4)2

The structural phase transition accompanied by a Jahn-Teller switch has been studied over a range of H/D ratios in (NH4)2[Cu(H2O)6](SO4)2 (ACTS). In particular, single-crystal neutron diffraction investigations of crystals with deuteration in the range 50 to 82% are shown to be consistent with previous electron paramagnetic resonance (EPR) experiments exhibiting a phase boundary at 50% deuteration under ambient pressure. Polycrystalline samples show that the two phases can co-exist. In addition, single-crystal neutron and polycrystalline X-ray diffraction pressure experiments show a shift to lower pressure at 60% deuteration versus previous measurements at 100% deuteration. (Less)