Diamine pentaborates with B-N bonds – zwitterions and tethered polyborates

Abstract

Reactions of 1,n-diaminoalkanes (n = 5, 6, 8) with excess boric acid in the absence of added solvent resulted in novel pentaborate derivatives containing N → B dative bonds. These diaminoalkane-pentaborate adducts include analogous zwitterions, B₅O₆(OH)₄NH₂(CH₂)₅NH₃ (1) and B₅O₆(OH)₄NH₂(CH₂)₆NH₃ (2) and the adduct salt [H₃N(CH₂)₈NH₃][B₅O₆(OH)₄NH₂(CH₂)₈NH₂B₅O₆(OH)₄]·2B(OH)₃·2H₂O (3). Compounds 1 and 2 form unusual zwitterion pairs, whereas 3 exhibits a large borate dianion comprised of two anionic pentaborate moieties tethered together by a doubly B-N bonded diaminooctane group. These compounds exhibit similar B-N bond lengths in the 1.622–1.638 Å range and nearly tetrahedral geometries around Lewis acceptor boron atoms. Compound 1 crystallized in the triclinic space group P-1 with a = 7.4366(17), b = 9.585(2), c = 11.116(3) Å, α = 71.046(2), β = 75.347(2), γ = 77.236(2)°, V = 716.6(3) ų, Z = 2; 2 triclinic, P-1, a = 7.2376(5), b = 9.5921(6), c = 11.9329(7) Å, α = 71.694(2), β = 72.749(2), γ = 73.691(3)°, V = 734.86(8) ų, Z = 2; and 3, triclinic, P-1, a = 8.9871(5), b = 10.1041(6), c = 13.6052(8) Å, a = 84.6160(10), β = 72.3950(10), γ = 65.3050(10)°, V = 1069.13(11) ų, and Z = 1.