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

The electrophilic alkylation of Rh-2(CO)(3)(dppm)(2) (2) with CH3SO3CF 3 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 spec ies Rh-2(CH3)(mu-CO)(CO)(dppm)(2)(+) (4) via CO loss. Between 40 and 7 0 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 spectroscop ies, 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 uni t 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 molecul e 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 occ upies the bridgehead position with the carbonyl carbon bonded to Rh1 a nd 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 s ingle bond. The crystals of 4 are monoclinic (space group P2(1)/n) hav ing a unit cell of dimensions a=13.919(4), b=15.682(3), c=23.758(9) An gstrom, beta=96.76(3)degrees, V=5149.8 Angstrom(3) and Z=4, with one m olecule per asymmetric unit. Complex 4 possesses an unsymmetrical A-fr ame structure with the two Rh centers bridged by two dppm ligands and CO located in the bridgehead position. The Rh-Rh bond distance is 2.81 1(2) Angstrom. The reactivity of these complexes with H-2, phenyl acet ylene and CO is also examined.