Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 17, Issue 10, Pages 3011-3020Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201002517
Keywords
aminoboranes; catalysis; dehydrocoupling; density functional calculations; iridium; reaction mechanisms
Categories
Funding
- University of Oxford
- CNRS
- ANR [ANR-09-BLAN-0184-01 HyBoCat]
- Universite de Toulouse (UPS, Aide Ponctuelle a la Cooperation)
- EPSRC [EP/E02971X/1]
- Heriot-Watt University
- Engineering and Physical Sciences Research Council [EP/E02971X/1] Funding Source: researchfish
- EPSRC [EP/E02971X/1] Funding Source: UKRI
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The Ir-III fragment {Ir(PCy3)(2)(H)(2)}(+) has been used to probe the role of the metal centre in the catalytic dehydrocoupling of H3B center dot NMe2H (A) to ultimately give dimeric aminoborane [H2BNMe2](2) (D). Addition of A to [Ir(PCy3)(2)(H)(2)(H-2)(2)][BAr4F] (1; Ar-F =(C6H3(CF3)(2)), gives the amineborane complex [Ir(PCy3)(2)(H)(2)(H3B center dot NMe2H)][BAr4F] (2a), which slowly dehydrogenates to afford the aminoborane complex [Ir(PCy3)(2)(H)(2)(H2B-NMe2)][BAr4F] (3). DFT calculations have been used to probe the mechanism of dehydrogenation and show a pathway featuring sequential BH activation/H-2 loss/NH activation. Addition of D to 1 results in retrodimerisation of D to afford 3. DFT calculations indicate that this involves metal trapping of the monomer-dimer equilibrium, 2H(2)BNMe(2) reversible arrow [H2BNMe2](2). Ruthenium and rhodium analogues also promote this reaction. Addition of MeCN to 3 affords [Ir(PCy3)(2)(H)(2)(NCMe)(2)][BAr4F] (6) liberating H2B-NMe2 (B), which then dimerises to give D. This is shown to be a second-order process. It also allows on-and off-metal coupling processes to be probed. Addition of MeCN to 3 followed by A gives D with no amine-borane intermediates observed. Addition of A to 3 results in the formation of significant amounts of oligomeric H3B center dot NMe2BH2 center dot NMe2H (C), which ultimately was converted to D. These results indicate that the metal is involved in both the dehydrogenation of A, to give B, and the oligomerisation reaction to afford C. A mechanism is suggested for this latter process. The reactivity of oligomer C with the Ir complexes is also reported. Addition of excess C to 1 promotes its transformation into D, with 3 observed as the final organometallic product, suggesting a B-N bond cleavage mechanism. Complex 6 does not react with C, but in combination with B oligomer C is consumed to eventually give D, suggesting an additional role for free aminoborane in the formation of D from C.
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