Bonding, Luminescence, Metallophilicity in Linear Au-3 and Au2Ag Chains Stabilized by Rigid Diphosphanyl NHC Ligands

The heterofunctional and rigid ligand N,N'-diphosphanyl-imidazol-2-ylidene (PCNHCP; P = P(t-Bu)(2)), through its phosphorus and two N-heterocyclic carbene (NHC) donors, stabilizes trinuclear chain complexes, with either Au-3 or AgAu2 cores, and dinuclear Au-2 complexes. The two oppositely situated PCNHCP (L) ligands that "sandwich" the metal chain can support linear and rigid structures, as found in the known tricationic Au(I) complex [Au-3(mu(3)-PCNHCP,kappa P,kappa C-NHC,kappa P)(2)](OTf)(3) (OTf = CF3SO3; [Au3L2](OTf)(3); Chem. Commun. 2014, 50, 103-105) now also obtained by transmetalation from [Ag3(mu 3-PCNHCP,?P,?C-NHC,?P)2](OTf)3 ([Ag3L2](OTf)3), or in the mixed-metal tricationic [Au2Ag(mu(3)-PCNHCP,kappa P,kappa C-NHC,kappa P)(2)](OTf)(3) ([Au2AgL2](OTf)(3)). The latter was obtained stepwise by the addition of AgOTf to the digold(I) complex [Au-2(mu(2)-PCNHCP,kappa P,kappa C-NHC)2](OTf)(2) ([Au2L2](OTf)(2)). The latter contains two dangling P donors and displays fluxional behavior in solution, and the AuAu separation of 2.8320(6) angstrom in the solid state is consistent with metallophilic interactions. In the solvento complex [Au3Cl2(tht)(mu(3)-PCNHCP,kappa P,kappa C-NHC,kappa P)](OTf)MeCN ([Au3Cl2(tht)L](OTf)MeCN), which contains only one L and one tht ligand (tht = tetrahydrothiophene), the metal chain is bent (148.94(2)degrees), and the longer AuAu separation (2.9710(4) angstrom) is in line with relaxation of the rigidity due to a more "open" structure. Similar features were observed in [Au3Cl2(SMe2)L](OTf)2MeCN. A detailed study of the emission properties of [Au3L2](OTf)(3), [Au3Cl2(tht)L](OTf)MeCN, [Au2L2](OTf)(2), and [Au2AgL2](OTf)(3) was performed by means of steady state and time-resolved photophysical techniques. The complex [Au3L2](OTf)(3) displays a bright (photoluminescence quantum yield = 80%) and narrow emission band centered at 446 nm with a relatively small Stokes' shift and long-lived excited-state lifetime on the microsecond timescale, both in solution and in the solid state. In line with the very narrow emission profile centered in the violet-blue region, fabrication of organic light-emitting devices (OLEDs) comprising the [Au3L2](OTf)(3) complex demonstrated its usefulness as a deep-blue emitter in solution-processed OLEDs. Electrochemical and Raman spectroscopic studies were also performed on [Au3L2](OTf)(3). Experimental results were rationalized by means of Wave-Function Theory (WFT) and Density Functional Theory (DFT). MP2 calculations gave a satisfactory description of the structures of the cationic complexes [Au3L2](3+) and [Au2L2](2+) and pointed to AuAu interactions having an electrostatic component owing to the dissimilar charge distribution in the chain caused by the heterofunctional ligand. The nature of the emitting states and their geometric distortions relative to the ground states in [Au3L2](3+) and [Au2L2](2+) was studied by DFT, revealing contraction of the AuAu distances and coordination geometry changes by association of the dangling P donor, respectively.