Douglas, Thomas M., Chaplin, Adrian B., Weller, Andrew S., Yang, Xinzheng and Hall, Michael B. (2009) Monomeric and oligomeric amine−borane σ-complexes of rhodium. Intermediates in the catalytic dehydrogenation of amine−boranes. Journal of the American Chemical Society, Vol.131 (No.42). pp. 15440-15456. doi:10.1021/ja906070r ISSN 0002-7863.

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Abstract

A combined experimental/quantum chemical investigation of the transition metal-mediated dehydrocoupling reaction of H(3)B center dot NMe(2)H to ultimately give the cyclic dimer [H(2)BNMe(2)](2) is reported. Intermediates and model complexes have been isolated, including examples of amine-borane sigma-complexes of Rh(I) and Rh(III). These come from addition of a suitable amine-borane to the crystallographically characterized precursor [Rh(eta(6)-1,2-F(2)C(6)H(4))(P(i)Bu(3))(2)][BAr(4)(F)] [Ar(F)= 3,5-(CF(3))(2)C(6)H(3)]. The complexes [Rh(eta(2)-H(3)B center dot NMe(3))(P(i)Bu(3))(2)][BAr(F)4] and [Rh(H)(2)(eta(2)-H(3)B center dot NHMe(2))(P(i)Bu(3))(2)][BAr(4)(F)] have also been crystallographically characterized. Other intermediates that stem from either H(2) loss or gain have been characterized in solution by NMR spectroscopy and ESI-MS. These complexes are competent in the catalytic dehydrocoupling (5 mol %) of H(3)B center dot NMe(2)H. During catalysis the linear dimer amine-borane H(3)B center dot NMe(2)BH(2)center dot NHMe(2) is observed which follows a characteristic intermediate time/concentration profile. The corresponding amine-borane sigma-complex, [Rh(P(i)Bu(3))(2)(eta(2)-H(3)B center dot NMe(2)BH(2)center dot NHMe(2))][BAr(4)(F)], has been isolated and crystallographically characterized. A Rh(I) complex of the final product, [Rh(P(i)Bu(3))(2){eta(2)-(H(2)BNMe(2))(2)}][BAr(4)(F)], is also reported, although this complex lies outside the proposed catalytic cycle. DFT calculations show that the first proposed dehydrogenation step, to give H(2)B=NMe(2), proceeds via two possible routes of essentially the same energy barrier: BH or NH activation followed by NH or BH activation, respectively. Subsequent to this, two possible low energy routes that invoke either H(2)/H(2)B=NMe(2) loss or H(2)B=NMe(2)/H(2) loss are suggested. For the second dehydrogenation step, which ultimately affords [H(2)BNMe(2)](2), a number of experimental observations suggest that a simple intramolecular route is not operating: (i) the isolated complex [Rh(P(i)Bu(3))(2)(eta(2)- H(3)B center dot NMe(2)BH(2)center dot NHMe(2))][BAr(4)(F)] is stable in the absence of amine-boranes; (ii) addition of H(3)B center dot NMe(2)BH(2)center dot NHMe(2) to [Rh(P(i)Bu(3))(2)(eta(2)-H(3)B center dot NMe(2)BH(2 center dot)NHMe(2))][BAr(4)(F)] initiates dehydrocoupling; and (iii) H(2)B=NMe(2) is also observed during this process.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Divisions:	Faculty of Science, Engineering and Medicine > Science > Chemistry
Library of Congress Subject Headings (LCSH):	Monomers, Oligomers, Amines, Boranes, Rhodium, Transition metal catalysts, Dehydrogenation
Journal or Publication Title:	Journal of the American Chemical Society
Publisher:	American Chemical Society
ISSN:	0002-7863
Official Date:	28 October 2009
Dates:	Date Event 28 October 2009 Published
Volume:	Vol.131
Number:	No.42
Page Range:	pp. 15440-15456
DOI:	10.1021/ja906070r
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Funder:	National Science Foundation (U.S.) (NSF), Robert A. Welch Foundation, Engineering and Physical Sciences Research Council (EPSRC), University of Oxford
Grant number:	CHE-0518074 (NSF), CHE-0541587 (NSF), DMS-0216275 (NSF), A0648 (RAWF), EP/E050743/1 (EPSRC)

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