Synthesis, Stereochemical Analysis, and Ligand-Transformation Reactions of New Classes of Bicapped Tricobalt Clusters, 5-C5H5-xMex)3(μ3-Y)>n (x=0,1,5), Containing Mixed ?-Acceptor X and ?-donor Y Capping Ligands (X = CO, NO; Y = NSiMe3, NC(O)NH2, NH): A Rational Pr...

Article abstract of DOI:10.1021/ja00279a043

The initial objective of this work was to prepare triangular metal clusters with capping NSiMe3 ligands and to use these clusters in subsequent ligand transformation reactions involving N-Si bond cleavage in order to produce an unprecedented bare pyramidal nitrido (μ3-N) cluster as well as to generate a high-yield pathway to nitrene (μ3-NH) clusters.The reactions of trimethylsilyl azide with Co(η⁵-C₅H_5-xMe_x)(CO)2 (x = 0, 1, 5) furnished the desired OC, Me3SiN-bicapped Co3(η⁵-C₅H_5-xMe_x)3(μ3-CO)(μ3-NSiMe3) clusters (x = 0, Ia; x = 1, Ib) and unexpectedly gave the OC, formamidonitrene-bicapped Co3(η⁵-C5H4Me)3(μ³-CO)(μ3-NC(O)NH2) cluster (IV) and OC, HN-bicapped Co3(η⁵-C5Me5)3(μ3-CO)(μ3-NH) cluster (IIc); the isolation of the nitrene capped IIc rather than the Me3SiN-capped Co3(η⁵-C5Me5)3(μ3-CO)(μ3-NSiMe3) cluster was attributed to intramolecular steric effects of the bulky pentamethylcyclopentadienyl ligands.The nitrene-capped Co3(η5-C₅H_5-xMe_x)3(μ3-CO)(μ3-NH) clusters (x = 0, IIa; x = 1, IIb) were subsquently obtained in >90percent yields by cleavage of the N-Si bond with fluoride ion.Attempted oxidations of Ia and Ib with NOBF4 instead gave rise in quantitative yields to the ON, HN-bicapped 5-C₅H_5-xMe_x)3(μ3-NO)(μ3-NO)(μ3-NH)>+ monocations (x = 0, IIIa; x = 1, IIIb); the formation of IIIa and IIIb involves both a N-Si bond cleavage of the μ3-NSiMe3 ligand by the tetrafluoroborate anion to form the μ3-NH ligand and a sample substitution of an isoelectronic μ3-NO+ ligand for the μ3-CO ligand.This unprecedented exchange (presumably via an association mechanism) of a NO+ capping ligand for a CO capping ligand was also shown to occur quantitatively when the OC, HN-bicapped clusters, IIa and IIb, were reacted with NOBF4.However, the HN-capped ligands in IIa and IIb were resistant to attempted hydrogen atom extraction and deprotonation transformations.Characterization of these 48-electron Co3(η⁵-C₅H_5-xMe_x)3(μ3-X)(μ3-Y) clusters with mixed ?-acceptor X and ?-donor (nonhybridized) Y ligands was achieved by IR, 1H NMR, mass spectral, and electrochemical measurements.X-ray diffraction studies were carried out on Co3(η⁵-C5Me5)3(μ3-CO)(μ3-NH) (IIc), the 5-C5H4Me)3(μ3-NO)(μ3-NH)>+ monocation (IIIb) as the tetraphenylborate salt, and the novel formamidonitrene-capped Co3(η⁵-C5H4Me)3(μ3-CO)(μ3-NC(O)NH2) (IV).A comparative structural-bonding analysis of these three clusters allowed a detailed assessment of the relative steric-bonding effects of the terminal C₅H_5-xMe_x ligands (x = 0, 1, 5) and the trimetal-capping ligands on the central Co(η3-X)(μ3-Y) cores.Co3(η⁵-C5Me5)3(μ3-CO)(μ3-NH) (IIc); molecular weight 625.6; monoclinic; P2₁/m; a = 10.516 (5) Angstroem, b = 17.859 (9) Angstroem, c = 8.443 (4) Angstroem, β = 111.76 (5) deg, V = 1473 (1) Angstroem³ at 22 deg C,; d(calcd) = 1.39 g/cm³ for Z = 2.Anisotropic least-squares refinement (with RAELS) of IIc, which possesses crystallographic C_s-m site symmetry, converged at R1(F) ...