N-(2,4,6-Trimethylborazinyl)-substituted Boron, Aluminum and Titanium Compounds

Abstract

The N-lithioborazine LiH2N3B3Me3, 1, reacts with organoboron halides not only to the respective borazinyl organylboranes but also by Me/halogen exchange. (Me2N)2B-H2N3B3Me3 was obtained from 1 and (Me2N)2BCl. A new ten-membered B6N4 ring system, 5, results on treatment of Cl(Me2N)B-B(NMe2)Cl with 1. The B-N-borazinyl borazines 6 - 8 can be prepared from 1 and B-monohalo borazines. The synthesis of 2,4,6-trimethylborazinyl-aluminum and -titanium compounds is achieved only with mononuclear monohalides of Al(III) and Ti(IV). The 2,4,6-trimethylborazinyl- bis(piperidino)alane 9 and the tris(2,6-diisopropylphenoxo)-2,4,6-trimethylborazinyltitanium 10 were characterized by X-ray structure analysis. The distortion of the borazine ring by B and N substitution is discussed. In case of the N-substituted borazines YH2N3B3Me3 the B-N bonds of the YNB2 units are elongated, e. g. for Y = PBr2 or (RO)3Ti, while N lithiation leads to a shortening of these B-N bond. These changes of bond lengths are also reflected by changes in the B1-N2 and B3-N3 bond lengths which become shorter in the presence of electron-withdrawing groups, but longer in case of Li substitution. Also, the bond angles B1-N2-B2 and B2-N3-B3 are affected by an increase of up to 128°.

Structural Chemistry of Borazines

“…[2] The N atoms of borazines show Lewis basicity as demonstrated, for instance, by the ease of formation of 1:1 adducts with halides of aluminium, gallium or tin [63] whereas the B atoms show Lewis acidic character as revealed by the formation pyridine adducts [56,64] or the formation of borazinium cations such as (Me 3 B 3 N 3 H 4 ) + . [63,65,66] More recently, it has been shown that the BN bonds of borazines can become quite unequal by binding one N atom to a Li, Al, Si, P, As or Sb atom. [21][22][23][24] Also, borazinium cations, e.g.…”

“…[63,65,66] More recently, it has been shown that the BN bonds of borazines can become quite unequal by binding one N atom to a Li, Al, Si, P, As or Sb atom. [21][22][23][24] Also, borazinium cations, e.g. (Me 3 B 3 N 3 H 4 ) + , show three different BN bond lengths.…”

“…(Me 3 B 3 N 3 H 4 ) + , show three different BN bond lengths. [63][64][65][66] The ring planarity can be lost particularly with the introduction of bulky substituents. At the moment, the most striking examples are compounds 4, Me 3 B 3 N 3 H 2 Ti-(OtBu) 3 [24] and [(EtB=NR) 3 Cr(CO) 3 ].…”

Discovery of low energy pathways to metal-mediated BN bond reduction guided by computation and experiment

“… 53 – 55 In contrast, the B N bonds of borazine require polar reagents for reduction, and thus, the two properties of borazines most relevant to ionic hydrogenation, p K a (Table S-4 † ) and hydricity ( Scheme 3 ), were evaluated. Based on the absence of reactivity with hydride sources and many relatively strong acids, 27 , 56 the free borazines are predicted to be inert to hydride addition and protonation. In contrast, coordination to a metal fragment induces dramatic changes in both of these values.…”

N‐(2,4,6‐Trimethylborazinyl)‐Substituted Phosphanes, Arsanes, and Stibanes

“…[3,4] These proved to be versatile reagents for the preparation of hitherto unknown borazine derivatives of, for example, B, Al, Ge, or Ti. [5,6] Here, we describe borazinyl-substituted phosphanes, arsanes, and stibanes. Known P-containing borazines are derivatives of phosphorus esters in which either (RO) 2 P(O) 2 groups are bonded to the B atoms [7] or an (RO)PO 3 group connects two borazine units through a B-O-B bond.…”