We report the synthesis and structural characterization of the new monomeric borane complex H3B·hppH (hppH = 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidine), which represents the first example of a structurally characterized 1:1 complex between hppH and a group 13 element hydride. Significant intramolecular and, in the crystalline phase, intermolecular H···H contacts are established in this complex. It is shown that the complex can be used as a precursor to new dinuclear boron(II) compounds featuring a B–B single bond. Thus H2 elimination followed by dimerization of H3B·hppH leads to [(hpp)BH]2 with two bridging hpp units. The structural details derived from X‐ray diffraction measurements are reported.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

In this work we report on new experiments on the catalytic dehydrogenation of [H 2 B(µ-hpp)] 2 leading to the doubly base-stabilized diborane(4) [HB(µ-hpp)] 2 featuring two hpp bridges (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) under mild conditions. Several dehydrogenation (pre)catalysts were tested. The best one turned out to be Ru 3 (CO) 12 , allowing quantitative dehydrogenation already at 60°C. Subsequently we subjected [HB(µ-hpp)] 2 to reactions

A number of boronium ions with the general formula [R 2 BL 2 ] + (where L is a donor, such as an amine), some borenium [R 2 BL] + , and even borinium [R 2 B] + ions have been synthesized and structurally characterized. [1] In all these compounds, boron has a formal oxidation state of + III. In addition to the academic interest in the bonding properties in these species, some of them have found application as catalysts in polymerization [2] or Diels-Alder reactions.[3]Boronium cations are also efficient initiators for the dehydrogenation of ammonia-borane. [4] The boron atom of the boronium ions is more or less tetrahedrally coordinated by the two substituents R (for example, amido groups) and two donor ligands L (such as pyridine). In contrast, uncoordinated borinium species, such as the (dimethylamido)(2,2,6,6-tetramethylpiperidino)boron cation, [5] feature an almost linear NÀ BÀN unit.Herein we report the synthesis of the dication2+ (1; hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidate), the first representative of a new class of boron dications with the general formula2+ (R being an amido group) having two boron atoms in the formal oxidation state + II. The surprisingly simple synthesis of 1 involves treating diborane(4) B 2 Cl 2 (NMe 2 ) 2 , prepared from B 2 (NMe 2 ) 4 , [6] with two equivalents of the free base hppH [see Eq. (1)]. Presumably, the diborane(4) species [{Me 2 NB(hpp)} 2 ] forms initially, which then reacts with the released HCl to form the salt [{Me 2 (H)NB(hpp)} 2 ](Cl) 2 . The hpp ligands stabilize the dinuclear species and protect it from oxidation or disproportionation.The chloride salt of 1 can be crystallized as a dichloromethane solvate from a mixture of dichloromethane/hexane. The structure of 1 as determined by X-ray diffraction measurements is given in Figure 1. The B À B bond (174.6 pm) lies well within the range of typical BÀB single bonds. For example, gas-phase electron diffraction measurements of B 2 (NMe 2 ) 4 and B 2 (OMe) 4 gave B À B bond lengths of 176.2(1.1) and 172.0(6) pm, respectively. [7] Recently we reported the synthesis of [{HB(hpp)} 2 ][8] containing a slightly longer BÀB single bond (177.2(3) pm). The BÀN bonds to the hpp ligands in 1 are 153.9-155.2 pm long, and the bonds to the two NMe 2 H ligands have lengths of 160.1 and 160.6 pm. For comparison, the B À N bonds to the hpp ligands in [{HB-(hpp)} 2 ] fall within the range 156.3(3)-158.2(3) pm.[8] The B À NHMe 2 bond lengths are similar to those reported for amine adducts of BH 3 . In H 3 BNH 3 , [9] H 3 BNMe 3 [10] and H 3 B(quinuclidine), [11] BÀN bonds of 156.4, 161.6, and 160.8 pm, respectively, were measured in the solid state. The

No abstract

No abstract