The Structural Chemistry of N-Monolithium Borazines

Trimethylborazine (Me3B3N3H3, 1) reacts with RLi (R = nBu, tBu) in hexane solution by alkyl group exchange to give a mixture of borazines Me3nRnB3N3H3. In contrast, deprotonation of 1 to Me3B3N3H2Li (2) occurs upon treatment with MeLi in diethyl ether solution or RLi (R = nBu, tBu) complexed with tetramethylethylenediamine (TMEDA), pentamethyldiethylentriamine (PMDTA), or trimethylhexahydrotriazine (TMTA) in hexane solution, to give solvates of 2. [(Me3B3N3H2)Li(OEt2)] (2a), [(Me3B3N3H2)Li(tmta)] (2c), and [(Me3B3N3H2Li)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, [(Me3B3N3H2)Li(pmdta)] (2c) is monomeric. The N-lithiation leads to a distortion of the planar B3N3 ring system: the BN bonds at the metallated N atoms are short, the next opposite BN pair is the longest, followed by a shorter BN bond involving the B atom in the para position. However, all BN bonds are longer than in the parent compound 1. Reaction of tBu3B3N3H3 (3) with tBuLi in the presence of triisopropylhexahydrotriazine (TIPTA) in hexane leads to the salt [(tipta)2Li][tBu4B3N3H3] (4), while deprotonation occurs with tBuLi(tmta) to give monomeric [(tBu3B3N3H2)Li(tmta)] (5) after refluxing the reaction mixture. Similarly, (Me2N)3B3N3H3 (6) can be deprotonated with RLi in the presence of TMTA to give monomeric [{(Me2N)3B3N3H2}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 Me3B3N3NH3nMen (n = 03).