Reactions of Bis(pentafluorophenyl)borane with Cp₂Ta(CH₂)CH₃:  Generation and Trapping of Tantalocene Borataalkene Complexes

Abstract

plus 1 equiv of H 3 CB(C 6 F 5 ) 2 . The pathway to 1 involves stepwise attack of borane first at the methylene group, followed by attack at the methyl group, which undergoes alkyl/hydride exchange with the second equivalent of HB(C 6 F 5 ) 2 . The product of HB(C 6 F 5 ) 2 addition to the methylene ligand, methyl hydride complex Cp 2 -Ta(CH 2 B(C 6 F 5 ) 2 )(µ-H)(CH 3 ), 2, can be intercepted by carrying out the reaction in hexane at low temperature, a medium in which it is nearly totally insoluble. This complex eliminates methane at higher temperatures in a first-order decomposition process (∆H q ) 20.4(5) kcal mol -1 and ∆S q ) -2.0(2) cal mol -1 K -1 ). The product, a borataalkene complex formulated as Cp 2 Ta[CH 2 B(C 6 F 5 ) 2 ], 3, is unstable and cannot be isolated. DFT calculations support its formulation and show that it is present as a singlet/triplet mixture, accounting for the observed paramagnetism of solutions containing 3. While 3 cannot be isolated or spectroscopically probed, it can be trapped if 2 is allowed to decompose in the presence of t BuNC or CO, giving the products Cp 2 Ta[η 2 -CH 2 B(C 6 F 5 ) 2 ](L) (L ) t BuNC, 4; CO, 5). Both of these compounds have been structurally characterized, and the structural and spectroscopic data for these compounds support an η 2 bonding description for the borataalkene ligand which is reminiscent of the commonly held Dewar-Chatt-Duncanson model for alkene bonding to transition metals. DFT calculations on 5 and the model complex Cp 2 Ta[η 2 -CH 2 BH 2 ](CO), 6, provide further support for this description. The facile conversion of this ligand from an η 2 to an η 1 bonding mode is proposed to account for some H/D exchange processes observed in both dihydride 1 and methyl hydride complex 2. Plausible mechanisms for both of these processes are proposed.

Crystal field arguments to explain the trans labilisation within transition metal–borane complexes

“…A number of examples of complexes containing metalborane coordination have now been reported [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]. Metal-borane complexes have been synthesised from nearly all of the late transition metals (groups [8][9][10][11].…”

“…Despite the classification of this mode of coordination for some time, they had received little attention. There were few examples of Z-type ligands claimed prior to the discovery of the metallaboratrane compounds and many of these examples were also later disproved [32][33][34][35][36][37][38][39][40][41][42]. This can be ascribed to the instability of the resulting compounds.…”

Structural Changes Accompanying the Stepwise Population of a BC π Bond

“…[1,[4][5][6] While the chemistry of borataalkenes has not been widely investigated, studies have shown that they can act as nucleophilic reagents [4][5][6] or as ligands in transition-metal complexes. [7][8][9] Electrogenerated diborataalkenes, such as [p-(Mes 2 B)C 6 H 4 (BMes 2 )] 2À , have also been detected using spectroelectrochemistry. [10][11][12] Interestingly, however, the structural changes accompanying the stepwise population/depopulation of a boron-carbon p bond have not been studied experimentally.…”

Synthesis and redox properties of a para-borylated phenylxanthenium cation\m{1}

“…As expected, the dimesitylboryl moiety is efficiently π-conjugated with the p-phenylene backbone as evidenced by the relatively small dihedral angle of 23.6 • formed between the planes defined by C(20)-B(1)-C(29) and C(3)-C(4)-C(5). On the other hand, the dihedral angle of 66.0 • formed by the C(8)-C(7)-C (19) and C(2)-C(1)-C(6) planes indicates that the xanthenium unit is not efficiently conjugated with the rest of the molecule.…”

“…Borataalkenes, which have been known for several decades, can be regarded as borylated carbanions stabilized by π-conjugation of the carbanion lone pair with the empty p-orbital of the boron atom [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Recently we have demonstrated that borataalkene derivatives can also be obtained by reduction of borylated carbocationic derivatives such as [C] + [20,21].…”