Unsymmetrically Substituted Bis(phosphino)Ferrocenes Triggering Through-Space (31)(P, P ')-Nuclear Spin Couplings and Encapsulating Coinage Metal Cations

We describe unsymmetrically substituted di-tert-butylated 1,1'-bis(phosphino)ferrocenes, with phosphino substituents R = [5-methyl]-2-furyl = Fu, and R' = phenyl (4a), i-propyl (4b). A modular synthetic approach was applied from the di-tert-butylated ferrocene platform (1), which leads to the formation of new diphosphines by using 1,1'-bis(diiodo)-3,3'-bis(tert-butyl)ferrocene (2) as synthetic precursor. In contrast to the cousin non-alkylated unsymmetrically substituted diphosphino-ferrocenes which were reported up to now, these diphosphines showed strong (P-31, P')-nonbonded ("through-space") nuclear spin-spin coupling. The strength of such internuclear spin-spin coupling constant (SSCCs) J(PP') ranges between 17 and 35 Hz, and as such, they are not associated with traditional "through-bond" (4)J(PP). The characterizations of these ferrocene derivatives by X-ray diffraction in the solid state, and by multinuclear NMR in solution, evidence that the strong SSCCs are attributable to the conformational constraint which is imposed to the ferrocene backbone by the introduction of alkyl substituents on this platform. The molecular structures resolved in the solid state showed long P center dot center dot center dot P mutual separation up to 6.9168(8) angstrom, while in solution, a (P, P')-phosphorus lone-pair overlap clearly occurs. This testifies to a sufficient degree of rotational flexibility of the alkylated ferrocene backbone. We examined the coordination chemistry of these diphosphines toward coinage metals: Cu, Ag, and Au. The constrained unsymmetrically substituted diphosphines 4a and 4b allowed the selective formation of dinuclear two-coordinate linear gold(I) complexes with strong Au center dot center dot center dot Au aurophilic interactions. They also lead to rare tetrahedral homoleptic d(10) monocationic complexes, with all the coinage metals, by structuring the encapsulation of the cation in a highly constrained environment where the stacking interactions of the (hetero)aromatic substituents of the phosphino groups are decisive. The resulting AA'XX' P-31 NMR signature in solution of the complexes was simulated and analyzed, as well as the stacking interactions between the pairs of furyl rings that were illustrated by noncovalent interactions computation.