Serendipitous CO‐ and RhI‐Induced ­Coupling of Bidentate Phosphaalkenes with Expulsion of Bis(trimethylsilyl)ketene Leading to RhIII‐Coordinated Dianionic Tetradentate Ligands

Abstract

AbstractIn the RhCl chelate complexes of C‐bis(trimethylsilyl)‐P‐alkyl(diphenylphosphanyl)aminophosphaalkenes 1 [(Me3Si)2C=PN(R)PPh2; 1a: R = 1‐Ada; 1b: R = tBu], the P=C double bonds are preferentially η2‐(P,C)‐coordinated to RhI, as shown by the structure determination of trans‐[RhCl{(Me3Si)2C=PN(R)PPh2}]2 (5a) and by the comparison of the 31P NMR spectra of cis‐ and trans‐[RhCl{(Me3Si)2C=PN(R)PPh2}]2 (5a,b), [RhCl(PPh3){(Me3Si)2C=PN(R)PPh2}] (6a,b), [RhCl(CO){(Me3Si)2C=PN(R)PPh2}] (7a,b) and [(cod)Rh(μ‐Cl)2Rh{(Me3Si)2C=PN(R)PPh2}] (8a,b). With excess of [RhCl(cod)]2, the ligand (Me3Si)2C=PN(CH2Ph)PPh2 (1c) forms solid trinuclear complex 10 containing two Rh(Cl)cod moieties attached to the central chelate unit [Rh(Cl)(1c)]: one Rh atom is attached by (terminal) P→Rh(Cl)cod coordination and the other one by bridging Rh(μ‐Cl)2Rhcod. The reactions of the complex [RhCl(CO)(PPh3)2] with ligands 1a or 1b unexpectedly led, with elimination of bis(trimethylsilyl)ketene, to racemic RhIII–chloro complexes [RhCl({Ph2PN(R)P}2C(SiMe3)2)] (11a,b) containing the dianionic tetradentate chelating “bis‐PNP” ligands (Me3Si)2C{P(–)N(R)PPh2}2 (12a,b) η4‐coordinated to RhIII (CN 5). The CO‐induced formation of RhIII complexes 11a and 11b also occurred when 7a and 7b were treated with another equivalent of ligand 1a or 1b, when [RhCl(CO)2]2 was mixed with more than 2 equiv. of 1a or 1b, and even bubbling gaseous CO into a solution containing 6a provided 11a and bis(trimethylsilyl)ketene.