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Issue 42, 2007
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Cationic rhodium mono-phosphine fragments partnered with carborane monoanions [closo-CB11H6X6] (X = H, Br). Synthesis, structures and reactivity with alkenes

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Abstract

Addition of the new phosphonium carborane salts [HPR3][closo-CB11H6X6] (R = iPr, Cy, Cyp; X = H 1a–c, X = Br 2a–c; Cy = C6H11, Cyp = C5H9) to [Rh(nbd)(μ-OMe)]2 under a H2 atmosphere gives the complexes Rh(PR3)H2(closo-CB11H12) 3 (R = iPr 3a, Cy 3b, Cyp 3c) and Rh(PR3)H2(closo-CB11H6Br6) 4 (R = iPr 4a, Cy 4b, Cyp 4c). These complexes have been characterised spectroscopically, and for 4b by single crystal X-ray crystallography. These data show that the {Rh(PR3)H2}+ fragment is interacting with the lower hemisphere of the [closo-CB11H6X6] anion on the NMR timescale, through three Rh–H–B or Rh–Br interactions for complexes 3 and 4 respectively. The metal fragment is fluxional over the lower surface of the cage anion, and mechanisms for this process are discussed. Complexes 3a–c are only stable under an atmosphere of H2. Removing this, or placing under a vacuum, results in H2 loss and the formation of the dimer species Rh2(PR3)2(closo-CB11H12)25a (R = iPr), 5b (R = Cy), 5c (R = Cyp). These dimers have been characterised spectroscopically and for 5b by X-ray diffraction. The solid state structure shows a dimer with two closely associated carborane monoanions surrounding a [Rh2(PCy3)2]2+ core. One carborane interacts with the metal core through three Rh–H–B bonds, while the other interacts through two Rh–H–B bonds and a direct Rh–B link. The electronic structure of this molecule is best described as having a dative Rh(I) → Rh(III), d8 → d6, interaction and a formal electron count of 16 and 18 electrons for the two rhodium centres respectively. Addition of H2 to complexes 5a–c regenerate 3a–c. Addition of alkene (ethene or 1-hexene) to 5a–c or 3a–c results in dehydrogenative borylation, with 1, 2, and 3-B-vinyl substituted cages observed by ESI-MS: [closo-(RHC[double bond, length as m-dash]CH)xCB11H12−x]x = 1–3, R = H, C4H9. Addition of H2 to this mixture converts the B-vinyl groups to B-ethyl; while sequential addition of 4 cycles of ethene (excess) and H2 to CH2Cl2 solutions of 5a–c results in multiple substitution of the cage (as measured by ESI-MS), with an approximately Gaussian distribution between 3 and 9 substitutions. Compositionally pure material was not obtained. Complexes 4a–c do not lose H2. Addition of tert-butylethene (tbe) to 4a gives the new complex Rh(PiPr3)(η2-H2C[double bond, length as m-dash]CHtBu)(closo-CB11H6Br6) 6, characterised spectroscopically and by X-ray diffraction, which show coordination of the alkene ligand and bidentate coordination of the [closo-CB11H6Br6] anion. By contrast, addition of tbe to 4b or 4c results in transfer dehydrogenation to give the rhodium complexes Rh{PCy22-C6H9)}(closo-CB11H6Br6) 7 and Rh{PCyp22-C5H7)}(closo-CB11H6Br6) 9, which contain phosphine–alkene ligands. Complex 9 has been characterised crystallographically.

Graphical abstract: Cationic rhodium mono-phosphine fragments partnered with carborane monoanions [closo-CB11H6X6]− (X = H, Br). Synthesis, structures and reactivity with alkenes

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Supplementary files

Article information


Submitted
27 Jul 2007
Accepted
12 Sep 2007
First published
01 Oct 2007

Dalton Trans., 2007, 4829-4844
Article type
Paper

Cationic rhodium mono-phosphine fragments partnered with carborane monoanions [closo-CB11H6X6] (X = H, Br). Synthesis, structures and reactivity with alkenes

E. Molinos, S. K. Brayshaw, G. Kociok-Köhn and A. S. Weller, Dalton Trans., 2007, 4829
DOI: 10.1039/B711468K

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