NMR Methods DOI: 10.1002/anie.201202116 7 Li Residual Quadrupolar Couplings as a Powerful Tool To Identify the Degree of Organolithium Aggregation** Ann-Christin Pçppler, Helena Keil, Dietmar Stalke,* and Michael John* Organolithium and lithium amide compounds rank among the most widely used reagents in synthesis, [1] and knowledge of their structure and/or aggregation state is of great importance. [2] Structural studies in solution (where the reactions are mostly carried out) have been dominated by NMR spectroscopy, traditionally using 13 C and 6/7 Li chemical shifts [3] or lithium–carbon couplings. [4] More recently, Williard et al. could show that diffusion-ordered spectroscopy (DOSY) [5] is an efficient tool to discriminate different aggregates of n-butyllithium (nBuLi; 2) by comparison with reference substances of defined molar masses. [6] Nevertheless, in many cases the differentiation is hampered by similar masses of various donor-ligated aggregates or rapid dissoci- ation of the ligands. We could recently characterize 2- thienyllithium (2-ThiLi; 3) with various donor bases in toluene by applying relatively time-consuming 1 H, 7 Li- HOESY experiments [7] and comparing the distance informa- tion from these data with the crystal structures. [8] The rather long measurement time and the additional interpretation and comparison of the data clearly underline the need for a more straightforward method to judge on aggregation. Herein, we present such a method based on 7 Li residual quadrupolar couplings (RQCs) that were obtained with text book lithium compounds (Scheme 1) in swollen polystyrene gels. The importance of anisotropic NMR spectroscopic parameters for structure elucidation of organic compounds has been underlined by various publications in recent years. Meanwhile, residual dipolar couplings (RDCs), [9] residual chemical shift anisotropies (RCSAs), [10] and 2 H residual quadrupolar couplings (RQCs) [11] are conveniently measured using liquid-crystalline phases, such as poly(g-benzyl-l-gluta- mate) (PBLG) [12] or stretched polymer gels (strain-induced alignment in a gel; SAG). [13] Nevertheless, applications of such alignment media to inorganic or organometallic com- pounds [14] or quadrupolar nuclei other than 2 H [15] are still rare. Quadrupolar couplings originate from the interaction between the nuclear electric quadrupole moment Q (nuclear spin I > 1 = 2 ) and the electric field gradient (efg, tensor V) produced by a non-symmetric electronic environment of the nucleus. The interaction averages to zero in isotropic solution, but gives rise to a splitting into 2I lines if the molecule is weakly aligned along the magnetic field axis (as described by the alignment tensor A). Thus, in the case of 7 Li (I = 3/2), a triplet is expected with line separations given by Equa- tion (1). [16] Dn Q ¼ eQ 2h X i;j¼x;y;z A ij V ij ð1Þ Where e is the elementary charge, h Plancks constant, A ij and V ij are the components of the alignment and efg tensors in the molecular axis system x,y ,z (second-order quadrupolar effects are neglected). The formula implies that in molecules which are spherically shaped (A ij = 0) or contain 7 Li in a highly symmetric environment (V ij = 0) no splitting is expected. [17] We chose to study 7 Li rather than 6 Li (I = 1) because of its higher sensitivity and larger quadrupole moment (4.01 vs. 0.081 fm 2 ) which should give rise to RQCs comparable to 2 H RQCs in typical C– 2 H groups. The first challenge was the design of an alignment medium that would withstand highly reactive organolithium compounds. We chose polystyrene (PS) cross-linked with divinylbenzene (DVB) which has no functional groups that could possibly be attacked. The polymer sticks were swollen directly with solutions of the respective lithium compound in [D 8 ]toluene. Toluene was chosen as solvent because it is chemically rather inert, closely resembles the polymer environment, and easily allows the deaggregation of the lithium compound by stoichiometric addition of more polar solvents or donor bases. [8] Additionally, we slightly modified the preparation reported by Luy et al. [18] in that we abstained from using 2,2’-azobis(2-methylpropionitrile) (AIBN) as the initiator of the polymerization. Instead, the reactants were thoroughly degassed and thermally polymerized at 115 8C for Scheme 1. The common organolithium and lithium amide reagents used. The degree of aggregation n depends on the solvent or stoichiometric donor base D (DME = dimethoxyethane, PMDETA = N,N,N’,N’’,N’’-pentamethyldiethylentriamine). Aggregation: n = 6 hexa- mer, n = 4 tetramer, n = 2 dimer, n = 1 monomer. If two numbers are given for n, an equilibrium is present. [*] A.-C. Pçppler, H. Keil, Prof.Dr. D. Stalke, Dr. M. John Institut für Anorganische Chemie der Universität Gçttingen Tammannstrasse 4, 37077 Gçttingen (Germany) E-mail: dstalke@chemie.uni-goettingen.de mjohn@gwdg.de [**] We kindly acknowledge funding from the DFG Priority Programme 1178, the DNRF funded Centre of Materials Crystallography, and the doctoral programme Catalysis for Sustainable Synthesis, provided by the Land Niedersachsen. We appreciate chemical donations from CHEMETALL. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201202116. A ngewandte Chemi e 7843 Angew. Chem. Int. Ed. 2012, 51, 7843 –7846  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim