Spectroscopic Evidence for Redox Isomerism in the 1,4-Diethynylbenzene-Bridged Heterobimetallic Cation [{Fe(dppe)Cp*}(μ-C≡CC₆H₄C≡C){Mo(dppe)(η-C₇H₇)}]PF₆

Abstract

International audienceReaction of [FeCl(dppe)Cp*] with [Mo(C≡CC6H4-4-C≡CH)(dppe)(η-C7H7)], 1, and NaBPh4 in methanol gives the alkynylvinylidene complex [{Fe(dppe)Cp*}{μ-C≡CC6H4(H)C═C}{Mo(dppe)(η-C7H7)}]BPh4, [2A]BPh4, which is deprotonated to form the heterobimetallic, 1,4-diethynylbenzene-bridged complex [{Fe(dppe)Cp*}(μ-C≡CC6H4C≡C){Mo(dppe)(η-C7H7)}], 3. The alkynylvinylidene compound [2A]BPh4 exists as the major component of an equilibrium mixture with [Fe(dppe)Cp*}{μ-C═C(H)C6H4C≡C}{Mo(dppe)(η-C7H7)}]BPh4, [2B]BPh4, and is formed as a consequence of proton migration between the Cβ carbons of the vinylidene and alkynyl ends of the bridging ligand. Cyclic voltammetric investigations reveal that 3 undergoes two reversible one-electron oxidations to cationic [3]+ and [3]2+, which have been isolated as the [PF6]− salts after chemical oxidation. Computational (DFT) studies on [3]n+ indicate that while the HOMO of neutral 3 is rather heavily localized on the Mo center, in [3]+ the frontier orbitals are more evenly distributed over both metals, with the concentration of spin density being sensitive to the relative disposition of the metal end-cap fragments about the diethynylbenzene ligand. Experimental investigations on [3]PF6 by IR and EPR spectroscopy provide evidence for the coexistence of redox isomers [3A]+ and [3B]+, in which spin density is localized at the molybdenum or iron center, respectively. The solution IR spectrum of [3]PF6 exhibits an unusual four-band pattern in the ν(C≡C) region, consistent with the observation of two isomeric forms of [3]PF6, which are "valence trapped" on the short time scale of IR spectroscopy. In the frozen solution EPR spectrum, at 120 K, the spectroscopic signatures of both paramagnetic end-caps Mo(dppe)(η-C7H7) and Fe(dppe)Cp* are observed. The properties of [3]n+ are discussed with reference to the parent homobimetallics [{MLx}2(μ-C≡CC6H4C≡C)]n+ [MLx = Fe(dppe)Cp*, [4]n+, and Mo(dppe)(η-C7H7), [5]n+, n = 0, 1, 2]

1,2-Diethynylbenzene-Bridged [Cp*(dppe)Fe]n+ Units: Effect of Steric Hindrance on the Chemical and Physical Properties

“…Higher stretching frequencies were computed for the CC triple bonds for the parent molecule m-1 (2051 and 2054 cm −1 ) for instance. 33 As experimentally observed (see Section 5), two significantly different ν CC vibration modes of different intensity are calculated for the mixed-valence complex o-1 + , namely, 1925 cm −1 (68 km/mol) and 2062 cm −1 (5652 km/mol); this value of high energy corresponds to the very short CC bond computed for o-1 + . For comparison, values of 1957 and 2024 cm −1 were computed for one asymmetrical rotamer of m-1 + .…”

“…The latter show in turn a SOMO that is 39% metal/46% carbon bridge in character for o-1 + , whereas that of p-1 + for instance is 58% metal and 31% carbon in character. 32,33 It has been observed for some complexes of the [Cp*-(dppe)Fe−CC−] + series that the tensor component g 1 is very sensitive to the conformation of the radical, whereas g 2 and g 3 components are almost not affected by the relative orientation of the redox centers and the bridging ligand. 46 The peak at g 1 ′ = 2.445 can be ascribed to a particular conformer trapped in the rigid glass, but considering the weakness of the corresponding signal, the abundance of the corresponding conformer in the solution is very weak.…”

“…33,34 A slightly more electron-rich character of o-1 with respect to p-1 reflects in the energy of the (Table 3). Two vibrational frequencies very close in energy at 2009 cm −1 (185 km/mol) (symmetrical) and 2014 cm −1 (431 km/mol) (antisymmetrical) are calculated for the neutral system o-1.…”

“…Much smaller asymmetry (if any) was computed for the related species p-1 + for instance. 33 As experimentally observed (see Section 3), the consequence of the steric hindrance between the spatially bulky metal fragments is a significant deviation from the linearity of the computed angles C(37)−C(38)−C(39) (165°, 165/163°, and 166°for n = 0, 1, and 2, respectively) and to a lesser extent Fe(1)−C(37)−C (38). This deviation from linearity leads to some charge accumulation on C (38).…”

Electronic Properties of Poly‐Yne Carbon Chains and Derivatives with Transition Metal End‐Groups

“…As the bridge and mixed‐valence states can be thermally populated, the electronic coupling between the metal termini results in an admixture of metal‐to‐ligand charge transfer (MLCT) electronic transitions via the bridge‐localized state and of inter‐valence charge transfer (IVCT) electronic transitions through the mixed‐valence states (see Figure ‐c) . A similar behavior was also characterized for the hetero‐metallic complex {[Fe(dppe)Cp*](µ‐C≡CC 6 H 4 C≡C)[Mo(dppe)(C 7 H 7 )]} + . DFT calculations on the latter have shown that the spin density distribution can be shifted from one metal to the other one in response to the relative orientation of the arene π‐system and the metal fragments.…”