The Structural Chemistry of N ‐Monolithium Borazines

Trimethylborazine (Me 3 B 3 N 3 H 3 , 1 ) reacts with RLi (R = n Bu, t Bu) in hexane solution by alkyl group exchange to give a mixture of borazines Me 3– n R n B 3 N 3 H 3 . In contrast, deprotonation of 1 to Me 3 B 3 N 3 H 2 Li ( 2 ) occurs upon treatment with MeLi in diethyl ether solution or RLi (R = n Bu, t Bu) complexed with tetramethylethylenediamine (TMEDA), pentamethyldiethylentriamine (PMDTA), or trimethylhexahydrotriazine (TMTA) in hexane solution, to give solvates of 2 . [(Me 3 B 3 N 3 H 2 )Li(OEt 2 )] ( 2a ), [(Me 3 B 3 N 3 H 2 )Li(tmta)] ( 2c ), and [(Me 3 B 3 N 3 H 2 Li) 4 (tmeda) 3 ] ( 2b ) contain dimeric units of 2 . In the case of 2b two of these dimers are bridged by one tmeda molecule, while the other two are coordinated to a lithium atom of one of its dimeric units, respectively. This molecule therefore contains tri‐ and tetracoordinated Li atoms. In contrast, [(Me 3 B 3 N 3 H 2 )Li(pmdta)] ( 2c ) is monomeric. The N ‐lithiation leads to a distortion of the planar B 3 N 3 ring system: the B–N bonds at the metallated N atoms are short, the next opposite B–N pair is the longest, followed by a shorter B–N bond involving the B atom in the para position. However, all B–N bonds are longer than in the parent compound 1 . Reaction of t Bu 3 B 3 N 3 H 3 ( 3 ) with t BuLi in the presence of triisopropylhexahydrotriazine (TIPTA) in hexane leads to the salt [(tipta) 2 Li][ t Bu 4 B 3 N 3 H 3 ] ( 4 ), while deprotonation occurs with t BuLi(tmta) to give monomeric [( t Bu 3 B 3 N 3 H 2 )Li(tmta)] ( 5 ) after refluxing the reaction mixture. Similarly, (Me 2 N) 3 B 3 N 3 H 3 ( 6 ) can be deprotonated with RLi in the presence of TMTA to give monomeric [{(Me 2 N) 3 B 3 N 3 H 2 }Li(tmta)] ( 7b ). The N ‐lithiation has almost no influence on the shielding of the boron nucleus of the borazine nuecleus. Attempts to obtain di‐ and trilithiated 1 as pure compounds failed: alkyl group exchange and borate formation as well as destruction of the borazine ring were detected by NMR spectroscopy. However, treatment of these reaction mixtures with MeBr gave Me 3 B 3 N 3 NH 3– n Me n ( n = 0–3). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)