Bonding, Luminescence, Metallophilicity in Linear Au3 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 Au3 or AgAu2 cores, and dinuclear Au2 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 Au3(μ3-PCNHCP,κP,κCNHC,κP)23 (OTf = CF3SO3; Au3L23; Chem. Commun. 2014, 50, 103-105) now also obtained by transmetalation from Ag3(μ3-PCNHCP,κP,κCNHC,κP)23 (Ag3L23), or in the mixed-metal tricationic Au2Ag(μ3-PCNHCP,κP,κCNHC,κP)23 (Au2AgL23). The latter was obtained stepwise by the addition of AgOTf to the digold(I) complex Au2(μ2-PCNHCP,κP,κCNHC)22 (Au2L22). The latter contains two dangling P donors and displays fluxional behavior in solution, and the Au···Au separation of 2.8320(6) Å in the solid state is consistent with metallophilic interactions. In the solvento complex Au3Cl2(tht)(μ3-PCNHCP,κP,κCNHC,κP)·MeCN (Au3Cl2(tht)L·MeCN), which contains only one L and one tht ligand (tht = tetrahydrothiophene), the metal chain is bent (148.94(2)°), and the longer Au···Au separation (2.9710(4) Å) is in line with relaxation of the rigidity due to a more "open" structure. Similar features were observed in Au3Cl2(SMe2)L·2MeCN. A detailed study of the emission properties of Au3L23, Au3Cl2(tht)L·MeCN, Au2L22, and Au2AgL23 was performed by means of steady state and time-resolved photophysical techniques. The complex Au3L23 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 Au3L23 complex demonstrated its usefulness as a deep-blue emitter in solution-processed OLEDs. Electrochemical and Raman spectroscopic studies were also performed on Au3L23. 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 and Au2L2 and pointed to Au···Au 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 and Au2L2 was studied by DFT, revealing contraction of the Au···Au distances and coordination geometry changes by association of the dangling P donor, respectively.