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Lloret J, Estevan F, Lahuerta P, Hirva P, Perez-Prieto J, Sanau M: Dirhodium (II) compounds with bridging thienylphosphines: studies on reversible P,C/P,S coordination. Chemistry. 2009 Aug 3;15(31):7706-16.
Monocyclometalated compound [Rh (2){(C (8) H (4) S) P (C (8) H (5) S)(2)}(CH (3) CO (2) H)(2)(O (2) CCH (3))(3)] (1 a) and bis-cyclometalated compound [Rh (2){(C (8) H (4) S) P (C (8) H (5) S)(2)}(2)(CH (3) CO (2) H)(2)(O (2) CCH (3))(2)] (2 a) have been isolated from the reaction of dirhodium tetraacetate and tris (2-benzo [b] thienyl) phosphine (2 BTP) using low acidic solutions. By contrast, in pure acetic acid the reaction of Rh (2)(O (2) CCH (3))(4) with 2 BTP and tris (2-thienyl) phosphine (2 TP), followed by replacement of the axial acetate ligands by chlorides, led to [Rh (2){(2-C (8) H (5) S) P (2-C (8) H (5) S)(2)}(2) Cl (2)(O (2) CCH (3))(2)] (3 b) and [Rh (2){(2-C (4) H (3) S) P (C (4) H (3) S)(2)}(2) Cl (2)(O (2) CCH (3))(2)] (5 b), respectively. These new dirhodium (II) compounds possess equatorial bridging ligands in a phosphorous-sulfur (P,S) coordination mode. The reversible switching between the P,C and P,S bonding mode of the phosphine has been studied in the monocyclometalated [Rh (2){(C (4) H (2) S) P (C (4) H (3) S)(2)}(CH (3) CO (2) H)(2)(O (2) CCH (3))(3)] (6 a), which was selectively transformed into compound [Rh (2){(2-C (4) H (3) S) P (C (4) H (3) S)(2)}(CF (3) SO (3))(CH (3) CO (2) H)(O (2) CCH (3))( 3)] (7 c) in triflic acid media. Remarkably, compound 7 c reverts to the starting compound 6 a upon treatment with sodium acetate. Theoretical DFT calculations for both the P,C/P,S rearrangement and the base-promoted reversion have been performed to explain the experimental findings. Data suggest the P,C/P,S rearrangement occurs by means of a "concerted protonation-demetalation mechanism" followed by eta (2) coordination of the thienyl ring and subsequent isomerization to the S-eta (1)-coordination mode. In the reversion reaction, the base coordinated at the axial position would promote a concerted metalation-deprotonation mechanism. |
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