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

In 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) PRh(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.