Solid-State NMR Study of Paramagnetic Bis(alaninato‑κ 2 N,O)copper(II) and Bis(1-amino(cyclo)alkane-1- carboxylato‑κ 2 N,O)copper(II) Complexes: Reflection of Stereoisomerism and Molecular Mobility in 13 C and 2 H Fast Magic Angle Spinning Spectra Ga ́ bor Szalontai,* ,† Ró bert Csonka, † Ga ́ bor Speier, † Jó zsef Kaizer, † and Jasmina Sabolovic ́ ‡ † Department of Chemistry, Faculty of Engineering, University of Pannonia, Egyetem utca 10, H-8201 Veszpre ́ m, Hungary ‡ Institute for Medical Research and Occupational Health, Ksaverska cesta 2, P.O. Box 291, HR-10001 Zagreb, Croatia * S Supporting Information ABSTRACT: Solid-state stereochemistry and mobility of paramagnetic copper(II) complexes formed by aliphatic amino acids (L-alanine, D,L-alanine, 1-amino-2-methyl-alanine) and 1-amino(cyclo)alkane-1-carboxylic acids (alkane = pro- pane, butane, pentane, hexane) as bidentate ligands has been studied by 13 C and 2 H solid-state fast magic angle spinning (MAS) NMR spectroscopy. We examined the prospective method to characterize solid-state paramagnetic compounds in a routine way. Both 13 C and 2 H MAS spectra can distinguish D,L and L,L diastereomers of natural and polydeuterated bis([D n ]alaninato)copper(II) (n = 0, 2, 8) complexes with axial and/or equatorial methyl positions (conformations) primarily due to different Fermi-contact (FC) contributions. The three-bond hyperfine couplings clearly show Karplus-like dependence on the torsional angles which turned out to be a useful assignment aid. Density functional theory calculations of the FC term and crystal structures were also used to aid the final assignments. The correlations obtained for bis(alaninato-κ 2 N,O)copper(II) complexes were successfully used to characterize other complexes. The usefulness of the 2 H MAS spectra of the deuterated complexes was underlined. Even the spectra of the easily exchangeable amine protons contained essential stereochemical information. In the case of a dimer structure of bis(1-aminohexane-1-carboxylato-κ 2 N,O)copper(II) both the 13 C and 2 H resolutions were good enough to confirm the presence of the cis and trans forms in the asymmetric unit. With regard to the internal solid-state motions in the crystal lattice, the obtained quadrupolar tensor parameters were similar for the D,L- and L,L- alaninato isomers and also for the cis-trans forms suggesting similar crystal packing effects, static amine deuterons involved in hydrogen bonding, and fast rotating methyl groups. ■ INTRODUCTION Stereochemistry of paramagnetic organometallic complexes formed by bidentate ligands is of general interest both in liquid and solid states. In crystalline solids single-crystal diffraction is the standard method for providing molecular structure. However, NMR has much to offer the chemist because it can both supplement and complement the diffraction results. 1a-c Unfortunately, NMR spectroscopy of paramagnetic compounds is often elusive as positions of resonances may change unpredictably and signals often broaden beyond detectio- n. 2a-d,3 Thus, information on spectra may remain unexploited. However, a recently proposed solid-state NMR methodology, coined by Ishii and co-workers as very fast magic angle spinning (VFMAS) method, 4-9 promised a breakthrough in such applications, at least for paramagnetic centers possessing a small magnetic susceptibility (electron spin S = 1 / 2 ). 2b For recording of NMR spectra of systems containing metal ions with larger susceptibilities and with S > 1 / 2 these methods have recently been combined with new pulse schemes utilizing short high-powered adiabatic pulses. 2c,d Kumara Swarmy et al. have pointed out very recently that nearly complete assignment of a paramagnetic metalorganic system (Cu-Cyclam) can be achieved by dipolar 1 H- 1 H and 1 H- 13 C correlations. 10 We too have recently demonstrated that the combination of 13 C and 2 H fast-MAS spectra of the crystals of anhydrous trans and aqua cis bis(L-valinato)copper(II) complexes is indeed a very effective tool for studying conformational disorder and molecular mobility in the solid state. 11 While the easily recordable 13 C fast-MAS spectra are informative on the local environment of the nuclei in the crystalline phase (e.g., the spectra provide the number of crystallographically inequivalent nuclei, which can be used in crystallographic space group determination too 1a,b ), the 2 H MAS spectroscopy can detect Received: December 14, 2014 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/ic502987e Inorg. Chem. XXXX, XXX, XXX-XXX