BINUCLEAR RHODIUM ALKYL AND ACYL A-FRAME COMPLEXES - SYNTHESIS, STRUCTURE AND REACTIVITY

The electrophilic alkylation of Rh-2(CO)(3)(dppm)(2) (2) with CH3SO3CF3 yields the mu-acyl A-frame complex Rh-2(mu-CH3CO)(CO)(2)(dppm)(2)(+) (3). Complex 3 is unstalbe and at 70 degrees C forms the A-frame species Rh-2(CH3)(mu-CO)(CO)(dppm)(2)(+) (4) via CO loss. Between 40 and 70 degrees C, both complexes 3 and 4 are observed in solution. Complex 3 reacts with CO to yield the terminal acyl complex Rh-2(CH3CO)(CO)(3)(dppm)(2)(+) generated in situ. The characterization of complexes 3, 4 and Rh-2(CH3CO)(CO)(3)(dppm)(2)(+) is based on NMR and IR spectroscopies, and single crystal X-ray diffraction methods for complexes 3 and 4. The crystals of 3 are monoclinic (space group P2(1)/c) having a unit cell of dimensions a=11.953(3), b=24.127(7), c=22.490(12) Angstrom, beta=103.00(3)degrees, V= 6310.1 Angstrom(3) and Z=4, with one molecule per asymmetric unit along with acetone and diethyl ether as solvents of crystallization. Complex 3 possesses an A-frame structure in which the Rh centers are bridged by two dppm ligands, and an acyl group occupies the bridgehead position with the carbonyl carbon bonded to Rh1 and the oxygen bonded to Rh2. The Rh-Rh separation of 2.952(3) Angstrom suggests a weak Rh-Rh interaction and the absence of a formal Rh-Rh single bond. The crystals of 4 are monoclinic (space group P2(1)/n) having a unit cell of dimensions a=13.919(4), b=15.682(3), c=23.758(9) Angstrom, beta=96.76(3)degrees, V=5149.8 Angstrom(3) and Z=4, with one molecule per asymmetric unit. Complex 4 possesses an unsymmetrical A-frame structure with the two Rh centers bridged by two dppm ligands and CO located in the bridgehead position. The Rh-Rh bond distance is 2.811(2) Angstrom. The reactivity of these complexes with H-2, phenyl acetylene and CO is also examined.