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)(2)-C= PN(R)PPh2; 1a: R = 1-Ada; 1b: R = tBu], the P=C double bonds are preferentially eta(2)-(P,C)-coordinated to Rh-I, as shown by the structure determination of trans-[RhCl{(Me3Si)(2)C= PN(R) PPh2}](2) (5a) and by the comparison of the P-31 NMR spectra of cis- and trans-[RhCl{(Me3Si)(2)C= PN(R)PPh2}](2) (5a,b), [RhCl(PPh3)-{( Me3Si)(2)C= PN(R) PPh2}] (6a,b), [RhCl(CO){(Me3Si)(2)C= PN(R)PPh2}] (7a, b) and [(cod) Rh(mu-Cl)(2)Rh{(Me3Si)(2)C= PN(R)PPh2}] (8a,b). With excess of [RhCl(cod)](2), the ligand (Me3Si)(2)C=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(mu-Cl)(2)Rhcod. The reactions of the complex [RhCl(CO)(PPh3)(2)] with ligands 1a or 1b unexpectedly led, with elimination of bis(trimethylsilyl) ketene, to racemic Rh-III-chloro complexes [RhCl({Ph2PN(R)P}(2)C(SiMe3)(2))] (11a,b) containing the dianionic tetradentate chelating "bis-PNP" ligands (Me3Si)(2)C{P(-)N(R) PPh2}(2) (12a,b) eta(4)-coordinated to Rh-III (CN 5). The CO-induced formation of Rh-III 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(tri-methylsilyl)ketene.