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Solid state and solution structures of rhodium and iridium poly(pyrazolyl)borate diene complexes

Adams, Christopher J., Anderson, Kirsty M., Charmant, Jonathan P. H., Connelly, Neil G., Field, Bevis A., Hallett, Andrew Jon and Horne, Matthew 2008. Solid state and solution structures of rhodium and iridium poly(pyrazolyl)borate diene complexes. Dalton Transactions (20) , pp. 2680-2692. 10.1039/b719223a

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The structures adopted by a range of poly(pyrazolyl)borate complexes [ML2Tpx] [M= Rh, Ir; L2 = diene; Tpx = Bp {dihydrobis(3,5-dimethylpyrazolyl)borate}, Tp {hydrotris(3,5- dimethylpyrazolyl)borate}, Tp {hydrotris(pyrazolyl)borate}, B(pz)4 {tetrakis(pyrazolyl)borate}] have been investigated. Low steric hindrance between ligands in [Rh(g-nbd)Tp] (nbd = norbornadiene), [Rh(g-cod)Tp] (cod = cycloocta-1,5-diene) and [Rh(g-nbd)Tp�] results in j3 coordination of the pyrazolylborate but [M(g-cod)Tp] (M= Rh, Ir) are j2 coordinated with the free pyrazolyl ring positioned above and approximately parallel to the square plane about the metal. All but the most sterically hindered Tpx complexes undergo fast exchange of the coordinated and uncoordinated pyrazolyl rings on the NMR spectroscopic timescale. For [Rh(g-cod){B(pz)4}], [Rh(g-dmbd)Tp] (dmbd = 2,3-dimethylbuta-1,3-diene) and [Rh(g-cod)TpPh] {TpPh = hydrotris(3- phenylpyrazolyl)borate} the fluxional process is slowed at low temperatures so that inequivalent pyrazolyl rings are observed. The bonding modes of the Tp� ligand (but not of other pyrazolylborate ligands) can be determined by 11B NMR and IR spectroscopy. The 11B chemical shifts (for a series of Tp� complexes) show the general pattern, j3 < −7.5 ppm < j2 and the m(BH) stretch j3 > 2500 cm−1 > j2. The electrochemical behaviour of the pyrazolylborate complexes is related to the degree of structural change which occurs on electron transfer. One-electron oxidation of complexes with Tp�, Tp and B(pz)4 ligands is generally reversible although that of [Ir(g-cod)Tp] is only reversible at higher scan rates and that of [Ir(g-cod){B(pz)4}] is irreversible. Of the complexes with the more sterically hindered TpPh ligand, only [Rh(g-nbd)TpPh] shows any degree of reversible oxidation. The ESR spectra of a range of Rh(II) complexes show coupling to both 14N and 103Rh nuclei in most cases but what appears to be coupling to rhodium and one hydrogen atom, possibly a hydride ligand, for the oxidation product of [Rh(g-nbd)TpPh].

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Publisher: RSC
ISSN: 1477-9226
Last Modified: 19 Mar 2016 22:08

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