Oxana Ciobanu

Synthesis of a stable B2H5(+) analogue by protonation of a double base-stabilized diborane(4).

On the basis of an analysis of the products formed in the course of reaction between B2H6 and the deuterated magic acid (FSO3D·SbF5), Olah et al. postulated the B2H5 + cation in 1988 as a short-lived intermediate formed by protonation of B2H6 and subsequent H2 elimination. [1] The cation had previously been observed in the gas phase after photoionization of B2H6. [2] However, a salt of this cationic boron hydride has not been synthesized on a preparative scale to date. Because of the scarceness of experimental information, the cation was the subject of several quantum-chemical calculations. These calculations found a global energy minimum structure with three bridging hydrogen atoms (Scheme 1). The distance between the two boron atoms is 149.5 pm according to HF/6-31G*, and 151.8 pm according to more recent QCISD(T)/6-311G** calculations. These values might argue for B B bonding, although it has been shown in many cases that a short distance does not automatically imply the presence of a significant chemical bond. To obtain more information, the cation was subjected to an NBO charge and Wiberg bond analysis. The NBO analysis suggested a charge of 0.20e on each boron atom, and the Wiberg bond analysis returned an index of 1.0 for the B B bond, which is clearly different to the situation in, for example, B2H6. The detailed description of the bonding situation in species such as this with multicenter bonds is still the subject of debate. Herein we report the synthesis of the first cationic binuclear borohydride [B2H3L2] , where L is the anionic guanidinate ligand 1,3,4,6,7,8-hexahydro-2H-pyrimido-[1,2a]pyrimidinate (hpp)). The geometry is similar to that of B2H5 , with two bridging hydrogen atoms being replaced by two hpp units. The synthesis commences with the complex hppH·BH3 (1), which can be dehydrogenated, via [BH2(hpp)]2 (2), to give the doubly base-stabilized diborane(4) [BH(hpp)]2 (3). Although we have previously reported the structure of 3, we have only now found a route to 3 in high yield and purity. The initial thermal dehydrogenation of 1 proved not to be the ideal route to 3 ; however, we recently showed that 1 can be catalytically dehydrogenated with [{Rh(1,5-cod)Cl}2] in toluene at 80 8C to give 2 (with a B···B separation of 306.5 pm). This species, dissolved in toluene, can be further dehydrogenated at 114 8C in the presence of catalytic amounts of [{Rh(1,5-cod)Cl}2] to yield 3 [Eq. (1)], with a B B bond

First dinuclear B(II) monocations with bridging guanidinate ligands: synthesis and properties.

Reaction between the diborane (4) B2Cl2(NMe2)2 and Li(hpp) (hpp-=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) leads to [(Me2N)B(mu-hpp)] 2. This species can be protonated by HCl.OEt2 to give [(Me2HN)B(mu-hpp)]2Cl2 featuring two B(II) cations with direct B-B bonding. The unsymmetrical monocation [(Me2N)B2(mu-hpp)2(NHMe2)]+ is also obtained. [(Me2HN)B(mu-hpp)]2Cl2 eliminates NHMe2 in a slow reaction leading to [ClB2(mu-hpp)2(NHMe2)]Cl and ultimately, presumably, to [ClB(mu-hpp)]2. We report the crystal structures of the two monocations [ClB2(mu-hpp)2(NHMe2)]Cl and [(Me2N)B2(mu-hpp)2(NHMe2)]Cl. The experimental results are accompanied by some quantum chemical density-functional theory calculations.