Solid-state 115 In NMR study of indium coordination complexesw Fu Chen, a Guibin Ma, a Ronald G. Cavell, a Victor V. Terskikh b and Roderick E. Wasylishen* a Received (in Cambridge, UK) 18th August 2008, Accepted 22nd September 2008 First published as an Advance Article on the web 8th October 2008 DOI: 10.1039/b814326a The feasibility of solid-state 115 In NMR studies is demonstrated by an examination of four different coordination complexes: indium(III) acetylacetonate, indium(III) tris(tropolonato), indium(III) triiodide bis(tris(4-methoxyphenyl)phosphine oxide) and indium(III) trichloride tris(2,4,6-trimethoxyphenyl)- phosphine. The results provide information about the electric field gradients and magnetic shielding at the indium nuclei through the nuclear quadrupolar and chemical shift parameters, respectively. The C Q values in these four complexes range between 106.0  2.0 and 200.0  4.0 MHz, while the magnetic shielding anisotropies fall in the range from 85  15 to 550  60 ppm. Finally, this research demonstrates that solid-state 115 In NMR studies are facilitated by performing experiments at the highest possible magnetic-field strengths, and that NMR offers a promising tool for the characterization of indium compounds. Indium coordination complexes find many important applica- tions ranging from materials chemistry to nuclear medicine. 1–5 One well-known example, indium(III) acetylacetonate, In(acac) 3 , is a versatile precursor for preparing a variety of materials, including pure or doped In 2 O 3 nanocrystals used as transparent semiconductors. 1 In(acac) 3 is also a promising co- catalyst in advanced organic synthesis. 2 The radioactive nuclide, 111 In, incorporated in indium(III) tris(tropolonato), In(trop) 3 , is commonly used as a labeling agent in diagnostic nuclear medicine. 3 The adducts of indium trihalide or indium trialkyl compounds with phosphine ligands are environment- friendly, single-source, precursors for preparing a wide range of InP-based semiconductors. 4 Indium complexes are reason- ably well studied in the solution state and are usually char- acterized by coordination numbers ranging from two to eight. 5 However, structural information for these complexes in the solid state is still scarce. Here we demonstrate that 115 In NMR spectroscopy is an ideal technique to characterize indium compounds in the solid state. Indium-115 is a spin-9/2 nucleus with a quadrupolar moment, Q = 81.0 fm 2 , largest of all naturally occurring isotopes of the main group elements. 6 The large value of Q usually results in proportionally large nuclear quadrupolar coupling constants, C Q ( 115 In), for indium compounds, which lead to very broad NMR line shapes that are difficult to acquire experimentally. Previously, solid-state 115 In NMR studies have been mainly confined to the tetrahedral tetra- haloindate, InX 4  , and octahedral hexahaloindate, InX 6 3 , anions, for which symmetry ensures that the electric field gradient, EFG, at the In nucleus is small resulting in C Q ( 115 In) values that are typically less than 50 MHz. 7 In the past, C Q ( 115 In) values greater than 50 MHz have been deter- mined using either nuclear quadrupole resonance spectroscopy or microwave spectroscopy; for example, see ref. 8. In an earlier 31 P NMR study of solid Br 3 In–P(4-C 6 H 4 - (OCH 3 )) 3 , we found negligible residual 115 In– 31 P dipolar coupling in spectra acquired with magic angle spinning (MAS) and predicted that C Q ( 115 In) in this complex must be less than 32 MHz. 9 This observation suggested that 1 : 1 adducts of indium trihalide and triarylphosphine ligands should be amenable to investigation by solid-state 115 In NMR spectroscopy. For non-integer spin quadrupolar nuclei, the central NMR transition, m I = 1/2 2 m I = 1/2, is not perturbed by the first- order quadrupolar interaction. 10 In second-order, the breadth of central transition is inversely proportional to the magnetic field, B 0 ; 10 thus, one expects linewidths that scale as B 0 1 (in Hz) or B 0 2 (in ppm). The purpose of this contribution is to demonstrate the feasibility of acquiring 115 In NMR spectra of indium coordi- nation complexes in solids where the EFG at the In nucleus is significant. To acquire broad central transition powder patterns Scheme 1 Molecular structures. a Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2. E-mail: roderick.wasylishen@ualberta.ca; Fax: +1 (1)780 492 8231; Tel: +1 (1)780 492 4336 b Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ontario, Canada K1A 0R6 w Electronic supplementary information (ESI) available: Sample pre- paration, NMR experimental details, NMR spectra acquired at 11.75 T, ZORA DFT calculation details and single-crystal X-ray diffraction data for I 3 In[OP(p-Anis) 3 ] 2 CCDC 699098 and Cl 3 In(TMP) CCDC 699099. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b814326a This journal is  c The Royal Society of Chemistry 2008 Chem. Commun., 2008, 5933–5935 | 5933 COMMUNICATION www.rsc.org/chemcomm | ChemComm