Triple bonds between tungsten atoms with ancillary dimesitylboroalkoxide ligands. Preparations, properties and structures of W 2(NMe 2) 4[OB(Mes) 2] 2 and W 2(OBu t) 4[OB(Mes) 2] 2

Abstract

From the reaction between W 2(NMe 2) 6 and dimesitylborinic acid, (Mes) 2BOH (2 equiv.), in toluene, the golden- yellow crystalline compound W 2(NMe 2) 4[OB(Mes) 2] 2 ( 1) has been isolated and characterized (elemental analysis, 1H, 13C{ 1H}, 11B NMR spectroscopy, IR spectroscopy and a single crystal X-ray diffraction study). At −160 °C, a=39.379(7), b=13.649(2), c=21.918(4) Å, β=123.12(1)°, Z=8 and space group C2/ c. The compound W 2(OBu t) 4[OB(Mes) 2] 2 ( 2) has been similarly characterized as a dark red crystalline compound obtained from the reaction between W 2(OBu t) 6 and (Mes) 2BOH (2 equiv.) in toluene. At −159 °C, a=17.164(2), b=19.773(2), c=18.490(2) Å, β=102.91(1)°, Z=4 and space group C2. In both compounds there are unsupported WW bonds of distance 2.3068(13) and 2.3521(15) Å for compound 1 and 2, respectively. In 1 there is a central ethane like W 2N 4O 2 core with the gauche conformation, WN=1.94(2) (av.) and WO=1.93(1) Å. The coordination geometry at nitrogen is trigonal planar and the NC 2 units are aligned along the WW axis as found for related compounds. In compound 2 the W 2O 4O′ 2 skeleton is eclipsed and most surprisingly the WO′ distances to the OB(Mes) 2 ligands are shorter 1.81(1) Å than the WO distances 1.94(1) and 1.90(1) Å to the alkoxide ligands. In both 1 and 2 the coordination geometry at boron is trigonal planar and the OB distances fall in the range 1.37(3)–1.41(3) Å, and are statistically equivalent to that of the free borinic acid. Compound 2 has crystallographically imposed C 2 symmetry and the OB(Mes) 2 groups are syn. These are the first structurally characterized boroalkoxides coordinated to the (WW) 6+ moiety and comparisons of WO π bonding are made with respect to related OR and OSiR 3 compounds.