ARTICLE DOI: 10.1002/zaac.201000420 N-Methyl-anilinoborane: Monomer, Dimers, Molecular Structure, and Rearrangement into μ-(N-Methyl-anilino)diborane(6). A Convenient Route to μ-(Amino)diboranes(6) Bernd Wrackmeyer,* [a] Elias Molla, [b] Peter Thoma, [a] Elena V. Klimkina, [a] Oleg L. Tok, [a] Tobias Bauer, [a] and Rhett Kempe [a] Keywords: Aminoboranes; μ-(Amino)diboranes(6); NMR spectroscopy; X-ray diffraction; Boron Abstract. N-Methyl-anilinoborane exists as a dimer in the solid state, shown by X-ray structural analysis, whereas in solution, a monomer– dimer equilibrium is present ( 1 H, 11 B and 13 C NMR spectroscopy). The phenyl groups are in trans positions in the crystalline dimer. In solu- tion, both possible isomers of the dimer exist in a trans/cis ratio of 3:2, in agreement with the energies for calculated optimized geometries Introduction Aminoboranes of the general formula R 1 R 2 N–BH 2 (mono- mers or dimers) are well known and readily available by vari- ous standard procedures. [1–3] It was shown that they can also be obtained by transition metal catalyzed dehydrocoupling starting from the corresponding amine borane adducts R 1 R 2 NH–BH 3 . [4,5] Potential applications of aminoboranes fo- cus on the presence of B–H bonds, which invite to studies of aminoboranes as precursors of BN ceramic materials. [6] Re- cently, some aspects of the synthetic potential of aminoboranes have been investigated, [7,8] of which the Pd-catalyzed C–B bond formation with aryl halides is particularly attractive. [7] The present knowledge on thermally induced reactions of secondary amines R 1 R 2 NH with Et 3 N–BH 3 is summarized in Scheme 1. In principle, this method appears to be the most convenient route towards monomeric aminoboranes R 1 R 2 N– BH 2 in bulk quantities (Scheme 1a), if the groups R 1 and R 2 are secondary alkyl groups. [2] Sterically less demanding sub- stituents lead to dimers (Scheme 1b). Problems are encoun- tered if the rate of the reaction of R 1 R 2 N–BH 2 with Et 3 N–BH 3 is comparable to that of the starting amine R 1 R 2 NH with Et 3 N– BH 3 , typical if one or both groups R 1 , R 2 are aryl groups (Scheme 1c). [2] Although appropriate changes in the experi- mental procedure (e.g. a large excess of Et 3 N–BH 3 ) help to suppress the competitive formation of (R 1 R 2 N) 2 BH, the desired * Prof. Dr. B. Wrackmeyer Fax: +49-921-552157 E-Mail: b.wrack@uni-bayreuth.de [a] Anorganische Chemie II Universität Bayreuth 95440 Bayreuth, Germany [b] Department of Chemistry Jahangirnagar University Savar, Dhaka-1342, Bangladesh Z. Anorg. Allg. Chem. 2011, 637, 401–405 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 401 [B3LYP/6-311+G(d,p) level of theory]. In solution, the monomer is rather reactive and disproportionates completely after several days to give a 1:1 mixture of bis(N-methyl-anilino)borane and μ-(N-methyl- anilino)diborane(6). This observation led to a convenient high yield synthesis of μ-(amino)diborane(6) derivatives, starting from bis(amino)boranes and borane/THF. products R 1 R 2 N–BH 2 (e.g. R 1 or R 2 = Ph), monomers or dimers (Scheme 1b), are difficult to obtain in pure state, [2] and there- fore, their properties in solution and in the solid state are not well studied. [2,3a] However, such aminoboranes deserve inter- est, since dimers, if present, can readily dissociate because of the reduced basicity of the nitrogen atoms. The monomers thus formed may be more reactive than dialkylaminoboranes owing to reduced B–N(pp)π interactions, considering the aryl group as a π acceptor competing successfully with the three-coordi- nate boron atom for the lone pair of electrons at the nitrogen atom. In this work, we report the molecular structure of N- methyl-anilinoborane dimer (1 2 ) and its behavior in solution. The latter part of this study prompted us to develop a new route to μ-(amino)diborane(6) derivatives. Scheme 1. Thermally induced reaction of secondary amines with Et 3 N–BH 3 to give aminoboranes, which may be monomers or dimers. In the case of R 1 = Me, R 2 = Ph, the reaction may proceed to give the bis(N-methyl-anilino)borane. Results and Discussion N-Methyl-anilinoborane, Structure and Properties N-Methyl-anilinoborane (1 2 ) was obtained as described, [2] and colorless crystals, grown from hexane, were studied by