Orbital Symmetry Control of Electronic Coupling in a Symmetrical, All-Carbon-Bridged “Mixed Valence” Compound: Synthesis, Spectroscopy, and Electronic Structure of [{Mo(dppe)(η-C₇H₇)}₂(μ-C₄)]ⁿ⁺ (n = 0, 1, or 2)

Abstract

The cycloheptatrienyl molybdenum alkynyl complex [Mo(CCH)(dppe)(η-C7H7)], 1 (dppe = Ph2PCH2CH2PPh2), undergoes oxidative dimerization on reaction with [FeCp2]PF6 in thf at −78 °C to give the bis(vinylidene) [{Mo(dppe)(η-C7H7)}2(μ-CCH-CHC)][PF6]2, [2][PF6]2. Deprotonation of [2][PF6]2 with KOBut yields butadiyndiyl-bridged [{Mo(dppe)(η-C7H7)}2(μ-CC-CC)], 3, which undergoes in situ aerial oxidation to give [{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6], [3]PF6, as the major product. The cyclic voltammogram of [3]PF6 exhibits a series of four redox processes indicative of sequential formation of [{Mo(dppe)(η-C7H7)}2(μ-C4)] n+ (n = 0, 1, 2, 3, 4) with the comproportionation constant, K C, for [3]PF6 of 1.9 × 107. Spectroscopic investigations on [3]PF6 by IR, Raman, NIR, and EPR spectroscopy reveal properties characteristic of a d5/d6 mixed valence complex with a localized electronic structure and an estimated intramolecular electron transfer rate in the range 108–1010 s–1. The experimental NIR spectrum of [3]PF6 is consistent with the predicted spectral characteristics of a three-state model for bridge-mediated, electron transfer in a weakly coupled, symmetrical mixed valence system. The dication [3][PF6]2 was isolated by chemical oxidation and structurally characterized; magnetic susceptibility measurements on [3][PF6]2 in the temperature range 2–300 K reveal strong antiferromagnetic coupling with the exchange coupling constant J ab (defined according to the Hamiltonian Ĥspin = –J ab ·Ŝa ·Ŝb) determined as −406 (±3) cm–1.

Ligand Redox Non‐Innocence in Transition‐Metal σ‐Alkynyl and Related Complexes

“…of [FeCp 2 ]PF 6 at -78°C to yield the bimetallic bis(vinylidene), which is then readily deprotonated to form the bimetallic butadiynediyl complexes [{M(dppX)(L)} 2 (1,4-μ-CϵC-CϵC)] (Scheme 17). [112,[135][136][137][138] Scheme 17. Oxidative coupling and subsequent deprotonation to yield bimetallic μ-1,3-diyne-1,4-diyl complexes.…”

“…Nevertheless, sufficient spin density is associated with the alkynyl ligand in [M(CϵCR)(dppe)(η 7 -C 7 H 7 )] + systems to give rise to hyperfine couplings with the alkynyl substituent R in both alkynyl [19][20][21]201] and polyynyl [155] systems, as well as C β -C β coupling processes in the least sterically encumbered derivatives. [113,138,155] In the case of four-legged piano-stool structures such as trans-[Mo(CϵCR)(CO)(PMe 3 ) 2 CpЈ] and cis-[Mo(CϵCR)-(CO)(dppe)CpЈ] (CpЈ = Cp, Cp*; Figure 5), there are important differences in the nature of the frontier orbitals brought about by the position of the carbonyl ligand and the requirement of the system to maximise back-bonding and minimise the overall energy. These orbital characteristics are preserved on one-electron oxidation and account for the different degree to which the metal centre and alkynyl ligand support the unpaired electron.…”

Electron‐Transfer Studies of trans‐Platinum Bis(acetylide) Complexes

“…Furthermore, transition metal acetylides with two redox-active metal fragments as terminal groups can be considered as molecular wires, because -CϵC-linkers can act as electronic bridges. [29,32,38] Ferrocenyl units have often been used as redox-active end groups owing to their well-known reversible redox behavior and stability. [39] Platinum complexes are suitable for the study of electronic interactions through the metal center; [40] however, different results have been reported regarding the influence of an introduced platinum motif on electronic communication.…”

Refining the Interpretation of Near‐Infrared Band Shapes in a Polyynediyl Molecular Wire

“…[1,3,[5][6][7][8][9][10][11][12] The degree of delocalisation along the [{L n M}A C H T U N G T R E N N U N G (m-C CC C)A C H T U N G T R E N N U N G {ML n }] backbone, and hence the nature of the redox-derived products, is sensitive to the identity of the metal end-capping group. By varying the terminal cap, and thus the metal d-orbital/carbon p-system interactions, [{L n M}A C H T U N G T R E N N U N G (m-C CC C)A C H T U N G T R E N N U N G {ML n }] + complexes ranging from weakly (class II in the Robin-Day scheme; e.g., [5] to strongly coupled (class III; {ML n } = [FeA C H T U N G T R E N N U N G (dppe)Cp*]) systems can be obtained.…”

“…[17] However, the NIR spectra of MV complexes often feature complex absorption envelopes due to the partial resolution of vibrational fine structure, [18] or multiple IVCT processes due to low symmetry at the metal centre [5] and/or by spin-orbit coupling in the case of complexes of the heavier metals. [19] The compound […”