Insertion of Carbon Monoxide into Metal-Nitrogen Bonds. Synthesis, Chemistry, Structures, and Structural Dynamics of Bis( pentamethylcyclopentadienyl) Organoactinide Dialkylamides and µ2-Carbamoyls

This paper reports the synthesis and characterization of chlorobis(pentamethylcyclopentadienyl)thorium and -uranium dialkylamides, M[µ-(CH3)sC5]2(NR2)C1 (M = Th, U; R = CH3, C2H5), and bis(dialkylamides), M[µ-(CH3)5C5]2(NR2)2 (M = Th, U, R = CH3; µ= U, R = C2H5). NMR studies indicate restricted rotation about the M-NR2 bonds. The amide compounds undergo facile migratory insertion of carbon monoxide to produce the corresponding j 2-carbamoyl complexes M[ij-(CH3)5C5]2[7;2-CONR2]Cl, M[µHCH3)5C5]2[772-CONR2]NR2, and M[µ-(CH3)5C5]2[i,2-CONR2]2 which were characterized by a variety of chemical and physicochemical methods. The infrared spectra of these compounds exhibit unusually low C-0 stretching frequencies (1490-1560 cm-1) for carbamoyl complexes, indicative of strong metal-oxygen bonding. The molecular structures of Th[7j-(CH3)5C5]2[7;2-CON(C2H5)2] Cl and U[i;-(CH3)5C5] 2[tj2-CON(CH3)2] 2 have been determined by single-crystal X-ray diffraction techniques. The former complex crystallizes in the monoclinic space group P2fn (an alternate setting of Pife-C2#1) with four molecules in a cell of dimensions: a = 9.097 (2) A,b = 21.291 (4) A, c = 14.031 (2) Á, and ß = 95.78 (2)°; the latter complex crystallizes in the monoclinic space group P2c-C\h with four molecules in a unit cell of dimensions: a = 9.277 (4) A, b = 16.T43 (5) A,c= 18.977 (6) Á, and β = 130.55 (3)°. For the chloro complex, least-squares refinement led to a value for the conventional R index (on F) of 0.080 for 3156 independent reflections having 20MoKa < 55° and I > 3 (7), while for the bis(carbamoyl), least-squares refinement led to a conventional R (on F) of 0.036 for 3689 independent reflections having 20MoKa < 55° and I > 3 (7). The M[(CH3)5C5]2 fragments of both molecules are of the “bent sandwich” M(C5H5)2X2 configuration, with the carbamoyl ligands bound in a 2 fashion. The chloro complex is disordered, with two isomers (one with the C-0 vector pointing toward the Cl atom, one with it pointing away) having cocrystallized in nearly equal proportions. The Th-O distances are 2.383 (31) and 2.460 (16) Á; the average Th-C distance is 2.418 (20) A; the Th-C-O angles accompanying the above distances are 70.1 (15) and 74.5 (1)°, respectively. In solution, the two isomers of the analogous uranium complex are in equilibrium ( = 1.2 ± 0.1 kcal/mol; AS = 8 ± 1 eu) with interconversion rapid on the NMR time scale at room temperature (at -80 eC, AG* = 8.9 ± 0.5 kcal/mol). In the bis(carbamoyl), the U-O distances are 2.370 (5) and 2.342 (7) A, the corresponding U-C distances 2.405 (8) and 2.402 (9) A, and the corresponding U-C-0 angles 73.0 (4) and 71.8 (5)°. The structural and spectral data evidence dative nitrogen lone-pair donation to the “carbenoid” carbamoyl carbon atom. The complex Th[jj-(CH3)5C5]2[N(CH3)2]CH3 was also prepared; carbonylation occurs preferentially at the metal-carbon bond to yield Th[jj-(CH3)5C5]2[N(CH3)2](j;2-COCH3).