& Silicon Chemistry Bis[N,N’-diisopropylbenzamidinato(À)]silicon(ii ): Lewis Acid/Base Reactions with Triorganylboranes Konstantin Junold, [a] Johannes A. Baus, [a] Christian Burschka, [a] CØlia Fonseca Guerra, [b] F. Matthias Bickelhaupt,* [b, c] and Reinhold Tacke* [a] Abstract: Reaction of the donor-stabilized silylene 1 (which is three-coordinate in the solid state and four-co- ordinate in solution) with BEt 3 and BPh 3 leads to the for- mation of the Lewis acid/base complexes 2 and 3, respec- tively, which are the first five-coordinate silicon com- pounds with an Si ÀB bond. These compounds were struc- turally characterized by crystal structure analyses and by multinuclear NMR spectroscopic studies in the solid state and in solution. Additionally, the bonding situation in 2 and 3 was analyzed by quantum chemical studies. Donor-stabilized silylenes with amidinato ligands have attract- ed much attention in recent years. [1, 2] Compound 1 is the only bis(amidinato)silicon(ii ) complex described so far. [2k] It exists as a three-coordinate silylene in the solid state, but is four-coordi- nate in solution (benzene) and formally reacts in oxidative ad- dition and nucleophilic substitution reactions as a four-coordi- nate silicon(ii ) species. [2k–n] To further characterize its reactivity profile, we have now studied Lewis acid/base reactions of 1 with triorganylboranes. Compared to other reaction types, such as oxidative addition and nucleophilic substitution reac- tions, relatively little is known about the reaction of silylenes with boranes. [3] To the best of our knowledge, silylene–borane adducts with triethylborane and triphenylborane have not yet been reported. In continuation of our ongoing systematic studies on higher-coordinate silicon(ii ) and silicon(iv) complexes (for recent publications, see ref. [2k–n] and [4]), we have succeeded in synthesizing the neutral silylene–borane adducts 2 and 3 (Scheme 1), the first examples of five-coordinate silicon com- pounds with an Si ÀB bond. Compounds 2 and 3 were synthe- sized by treatment of 1 with one molar equivalent of triethyl- borane (2, 75 % yield) and triphenylborane (3, 82 % yield), re- spectively. The identities of 2 and 3 were established by ele- mental analyses, crystal structure analyses, [5] and NMR spectro- scopic studies in the solid state and in solution (including VT NMR experiments). [6] The crystal structure of 3 was determined with the toluene solvate 3·C 6 H 5 CH 3 . The molecular structures of 2 and 3 are de- picted in Figures 1 and 2 (hydrogen atoms are omitted for clarity), with selected bond lengths and angles in the respec- tive figure legends. The silicon coordination polyhedra of 2 (two crystallographi- cally independent molecules; both with disordered ethyl groups) and 3 (disordered solvent molecules) are strongly dis- torted trigonal bipyramids (Berry distortion: 2, 31.7/32.2 %; 3, 25.1 %), [7] with the boron atom in an equatorial position. The N ax -Si-N ax angles are in the range 147.23(9)–150.22(8)8, and the sum of the equatorial bond angles is 359.9–360.08. The axial Si ÀN distances are in the range 1.9767(19)–2.048(2)  and are significantly longer than the equatorial ones (1.8039(18)– 1.828(2) ). The Si ÀB bond lengths amount to 2.076(4)/2.077(3) (2) and 2.067(3)  (3) and are very similar to those reported for silylene–borane adducts with three- or four-coordinate silicon (1.9624(5)–2.108(2) ). [3] The isotropic 11 B chemical shifts of 2 and 3 in the solid state and in solution are very similar, whereas larger differences were observed in the 29 Si NMR spectra (2, Dd 29 Si = 13.8/ 9.1 ppm ; 3, Dd 29 Si = 6.4 ppm; Table 1). These shift differences may be explained by a rapid exchange of the four amidinato- nitrogen sites in solution, affecting the electronic environment of the silicon atom. As can be seen from the different isotropic 29 Si chemical shifts of the two crystallographically independent molecules of 2 (Dd 29 Si = 4.7 ppm), even small differences in the Scheme 1. Syntheses of compounds 2 and 3. [a] K. Junold, J. A. Baus, Dr. C. Burschka, Prof. Dr. R. Tacke Universität Würzburg, Institut für Anorganische Chemie Am Hubland, 97074 Würzburg (Germany) Fax: (+ 49) 931-31-84609 E-mail : r.tacke@uni-wuerzburg.de [b] Dr. C. Fonseca Guerra, Prof. Dr. F. M. Bickelhaupt VU University Amsterdam, Department of Theoretical Chemistry Amsterdam Center for Multiscale Modeling (ACMM) De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands) E-mail : f.m.bickelhaupt@vu.nl [c] Prof. Dr. F. M. Bickelhaupt Radboud University Nijmegen, Institute for Molecules and Materials Heyendaalseweg 135, 6525 AJ Nijmegen (The Netherlands) Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201403995. Chem. Eur. J. 2014, 20, 12411 – 12415 # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 12411 Communication DOI: 10.1002/chem.201403995