Silver−Phosphine Complexes of the Highly Methylated Carborane Monoanion [closo-1-H-CB11Me11]-

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Abstract
The synthesis of the silver(I) salt of the highly methylated carborane anion [closo-1-H-CB11Me11]- is described, Ag[closo-1-H-CB11Me11] 1, which in the solid state shows close intermolecular Ag···H3C contacts. Addition of various monodentate phosphines to 1 results in the formation of the complexes (R3P)Ag[closo-1-H-CB11Me11] [R = Ph, 2; cyclohexyl (C6H11), 3; (3,5-Me2-C6H3), 4]. All these complexes show close intermolecular Ag···H3C contacts in the solid state that are considerably shorter than the sum of the van der Waals radius of methyl (2.00 Å) and the ionic radius of silver(I) (1.29 Å). For 2 and 3 there are other close intermolecular Ag···H3C contacts in the solid state, arising from proximate carborane anions in the crystal lattice. Addition of methyl groups to the periphery of the phosphine ligand (complex 4) switches off the majority of these interactions, leaving essentially a single cage interacting with the cationic silver−phosphine fragment through three CH3 groups. In solution (CD2Cl2) Ag···H3C contacts remain, as evidenced by both the downfield chemical shift change and the significant line-broadening observed for the cage methyl signals. These studies also show that the metal fragment is fluxional over the surface of the cage. The Ag···H3C interactions in solution may be switched off by addition of a stronger Lewis base than [closo-1-H-CB11Me11]-. Thus, addition of [NBu4][closo-1-H-CB11H5Br6] to 2 affords (Ph3P)Ag[closo-1-H-CB11H5Br6], while adding Et2O or PPh3 affords the well-separated ion-pairs [(Ph3P)(L)Ag][closo-1-H-CB11Me11] (L = OEt25, PPh36,) both of which have been crystallographically characterized. DFT calculations on 2 (at the B3LYP/DZVP level) show small energy differences between the possible coordination isomers of this compound, with the favored geometry being one in which the {(Ph3P)Ag}+ fragment interacts with three of the {BCH3} vertices on the lower surface of the cage, similar to the experimentally observed structure of 4.