Derivative chemistry of [UCl2{B(pz)4}2]:: stability of complexes containing the fragments [U{B(pz)4}2] and [U{HB(pz)3}2]

Uranium tetrachloride reacts with two equivalents of K[B(pz)(4)] in THF affording [UCl2{B(pz)(4))(2)] (1) in 75% yield. Complex 1 is monomeric and crystallizes in the monoclinic space group C2/c with cell parameters a = 13.700(6), b = 12.759(2), c = 17.513(8) Angstrom, beta = 101.37(2)degrees, V = 3001(2) Angstrom(3), Z = 4. Derivatives [UCl(OR){B(pz)(4)}(2)] (R=C2H5 (2), Bu-t (3), C6H4-o-OMe (4) and C6H2-2,4,6-Me-3 (5)), [U((OBu)-Bu-t)(2){B(pz)(4)}(2)] (6), [U((SPr)-Pr-i)(2){B(pz)(4))(2)] (7) and [UCl(Me){B(pz)(4))(2)] (8) were obtained by reacting 1 with sodium alkoxides, with (NaSPr)-Pr-i or with LiMe. X-ray crystallographic analysis of 5 and 7 shows that uranium is eight-coordinate by the two eta(3)-[B(pz)(4)] ligands and by two monodentate coligands (5. crystallizes in the monoclinic space group C2/c with cell parameters a = 30.575(3). b = 9.929(1), c = 24.884(3) Angstrom, beta = 90.59(1)degrees, V = 7554(1) Angstrom(3), Z = 8; 7 crystallizes in the monoclinic space group C2/c with cell parameters a = 24.286(7), b = 9.471(2), c = 16.076(3) Angstrom, beta = 96.44(3)degrees, V = 3674(2) Angstrom(3), Z = 4). Extended Huckel molecular orbital (EHMO) calculations were used to get a better insight into the electronic properties of the ligand [B(pz)(4)] and to get some explanation on the relative stability of complexes containing the fragments '[U(B(pz)(4)}(2)] and '[U(HB(pz)(3)}(2)]'. (C) 1999 Elsevier Science S.A. All rights reserved.