Synthesis, Characterization, and Properties of Doubly Alkynyl Bridging Dinuclear Cyclometalated Iridium(III) Complexes

Bis­(cyclometalated) diiridium complexes
[Ir­(ppy)2(μ–κCα:η2-CCR)]2 (R =
Tol 1, C6H4OMe-4 2, 1-Np 3, SiMe3 4, tBu 5), stabilized by a double
alkynyl bridging system, have been synthesized by alkynylation of
[Ir­(ppy)2(μ-Cl)]2 with excess of the appropriate
LiCCR (1:6 or 1:10 molar ratio). Complexes 1–3 were alternatively generated by treatment of [Ir­(ppy)2(MeCN)2]­(OTf) with 2.5 equiv of LiCCR.
However, the related reaction with LiCCtBu evolves with the formation of mixtures from which the unexpected
dinuclear complex [Ir­(ppy)2(μ-CH2CN)]2 6 was crystallized, as confirmed by X-ray diffraction
studies. Complexes 1–5 have been
characterized by NMR, IR, absorption and emission spectroscopies,
cyclic voltammetry, and mass spectrometric methods. Characterization
indicates that 1–3 and 5 are obtained as a unique diastereoisomer (ΛΛ/ΔΔ),
whereas 4 gives the two diastereoisomers (ΔΛ
and ΛΛ/ΔΔ). Single crystals of [Ir­(ppy)2(μ-CCTol)]2 1 contains
the inversion related ΛΛ/ΔΔ isomers, and crystals
of 4 (chosen from a mixture of two different types of
crystals) consist of the meso-ΔΛ isomer.
Electrochemical studies showed the presence in 1–3 and 5 of two consecutive one-electron-oxidation
IrIII/IrIV processes, whereas 4 displayed only one irreversible oxidation peak. In degassed fluid
solutions, complexes 1–5 are emissive
in the 505–515 nm region with quantum yields higher (ϕ
= 0.007–0.024) than that of the dichloro bridged precursor
(0.0037). The influence of the alkynyl substituent in the emissive
state at room and at low temperatures has been investigated. For the
naphthylacetylide derivative 3, a clear switch from a
mixed charge transfer 3MLCT/3L′LCT excited
state at 298 K (515 nm) to a characteristic long-lived low lying naphthalene
emission at low temperature (77 K) is observed. DFT and TD-DFT calculations
were performed on complexes 1 and 3 in the
ground and triplet states to gain insight into the structural, electronic,
and photophysical properties.