Serendipitous CO- and Rh super(I)-Induced -Coupling of Bidentate Phosphaalkenes with Expulsion of Bis(trimethylsilyl)ketene Leading to Rh super(III)-Coordinated Dianionic Tetradentate Ligands

In the RhCl chelate complexes of C-bis(trimethylsilyl)-P-alkyl(dip henylphosphanyl)aminophosphaalkenes 1 [(Me sub(3)Si) sub(2)C=PN(R)PPh sub(2); 1a: R = 1-Ada; 1b: R = tBu], the P=C double bonds are preferentially eta super(2)-(P,C)-coordinated to Rh super(I), as shown by the structure determination of trans-[RhCl{(Me sub(3)Si) sub(2)C=PN(R)PPh sub(2)}] sub(2) (5a) and by the comparison of the super(31)P NMR spectra of cis- and trans-[RhCl{(Me sub(3)Si) sub(2)C=PN(R)PPh sub(2)}] sub(2) (5a,b), [RhCl(PPh sub(3)){(Me sub(3)Si) sub(2)C=PN(R)PPh sub(2)}] (6a,b), [RhCl(CO){(Me sub(3)Si) sub(2)C=P N(R)PPh sub(2)}] (7a,b) and [(cod)Rh( mu -Cl) sub(2)Rh{(Me sub(3)Si) sub(2)C=PN(R)PPh sub(2)}] (8a,b). With excess of [RhCl(cod)] sub(2), the ligand (Me sub(3)Si) sub(2)C=PN(CH sub(2)Ph)PPh sub(2) (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 arrow right Rh(Cl)cod coordination and the other one by bridging Rh( mu -Cl) sub(2)Rhcod. The reactions of the complex [RhCl(CO)(PPh sub(3)) sub(2)] with ligands 1a or 1b unexpectedly led, with elimination of bis(trimethylsilyl)ketene, to racemic Rh super(III)-chloro complexes [RhCl({Ph sub(2)PN(R)P} sub(2)C(S iMe sub(3)) sub(2))] (11a,b) containing the dianionic tetradentate chelating "bis-PNP" ligands (Me sub(3)Si) sub(2)C{P super((-))N(R)PPh sub(2)} sub(2) (12a,b) eta super(4)-coordinated to Rh super(III) (CN 5). The CO-induced formation of Rh super(III) complexes 11a and 11b also occurred when 7a and 7b were treated with another equivalent of ligand 1a or 1b, when [RhCl(CO) sub(2)] sub(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. The reaction of [RhCl(CO)(PPh sub(3)) sub(2)] with bidentate phosphaalkene ligands 1 [(Me sub(3)Si) sub(2)C=PN(R)PPh sub(2); 1a: R = 1-Ada; 1b: R = tBu] leads, with elimination of bis(trimethylsilyl)ketene, to racemic Rh super(III)-chloro complexes [RhCl({Ph sub(2)PN(R)P} sub(2)C(S iMe sub(3)) sub(2))] (11a,b) each containing a dianionic eta super(4)-coordinated ligand. CO is essential for the unusual ligand coupling reaction.