| 17955135 |
Molinos E, Brayshaw SK, Kociok-Kohn G, Weller AS: Cationic rhodium mono-phosphine fragments partnered with carborane monoanions [closo-CB11H6X6]- (X = H, Br). Dalton Trans. 2007 Nov 14;(42):4829-44. Epub 2007 Oct 1.
Synthesis, structures and reactivity with alkenes.. Addition of the new phosphonium carborane salts [HPR (3)][closo-CB (11) H (6) X (6)] (R = (i) Pr, Cy, Cyp; X = H 1a-c, X = Br 2a-c; Cy = C (6) H (11), Cyp = C (5) H (9)) to [Rh (nbd)(mu-OMe)](2) under a H (2) atmosphere gives the complexes Rh (PR (3)) H (2)(closo-CB (11) H (12)) 3 (R = (i) Pr 3a, Cy 3b, Cyp 3c) and Rh (PR (3)) H (2)(closo-CB (11) H (6) Br (6)) 4 (R = (i) Pr 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 (PR (3)) H (2)}(+) fragment is interacting with the lower hemisphere of the [closo-CB (11) H (6) X (6)](-) 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 H (2). Removing this, or placing under a vacuum, results in H (2) loss and the formation of the dimer species Rh (2)(PR (3))(2)(closo-CB (11) H (12))(2) 5a (R = (i) Pr), 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 [Rh (2)(PCy (3))(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), d (8)--> d (6), interaction and a formal electron count of 16 and 18 electrons for the two rhodium centres respectively. Addition of H (2) 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)(x) CB (11) H (12-x)](-) x = 1-3, R = H, C (4) H (9). Addition of H (2) to this mixture converts the B-vinyl groups to B-ethyl; while sequential addition of 4 cycles of ethene (excess) and H (2) to CH (2) Cl (2) 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 H (2). Addition of tert-butylethene (tbe) to 4a gives the new complex Rh (P (i) Pr (3))(eta (2)-H (2) C=CH (t) Bu)(closo-CB (11) H (6) Br (6)) 6, characterised spectroscopically and by X-ray diffraction, which show coordination of the alkene ligand and bidentate coordination of the [closo-CB (11) H (6) Br (6)](-) anion. By contrast, addition of tbe to 4b or 4c results in transfer dehydrogenation to give the rhodium complexes Rh{PCy (2)(eta (2)-C (6) H (9))}(closo-CB (11) H (6) Br (6)) 7 and Rh{PCyp (2)(eta (2)-C (5) H (7))}(closo-CB (11) H (6) Br (6)) 9, which contain phosphine-alkene ligands. Complex has been characterised crystallographically. |
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