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

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)(4)NH2(CH2)(5)NH3 (1) and B5O6(OH)(4)NH2(CH2)(6)NH3 (2) and the adduct salt [H3N(CH2)(8)NH3] [B5O6(OH)(4)NH2 (CH2)(8)NH2B5O6(OH)(4)]center dot 2B(OH)(3)center dot 2H(2)O (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 angstrom 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) angstrom, alpha = 71.046(2), beta = 75.347(2), gamma = 77.236(2)degrees, V = 716.6(3) angstrom(3), Z = 2; 2 triclinic, P-1, a = 7.2376(5), b = 9.5921(6), c = 11.9329(7) angstrom, alpha = 71.694(2), beta = 72.749(2), gamma = 73.691(3)degrees, V = 734.86(8) angstrom(3), Z = 2; and 3, triclinic, P-1, a = 8.9871(5), b = 10.1041(6), c = 13.6052(8) angstrom, a = 84.6160(10), beta = 72.3950(10), gamma = 65.3050(10)degrees, V = 1069.13(11) angstrom(3), and Z = 1.