Control of the Charge Distribution and Modulation of the Class II–III Transition in Weakly Coupled Mo₂–Mo₂Systems

Abstract

Three novel dimolybdenum dimers [Mo2(DAniF)3]2(μ-OSCC6H4CSO), [Mo2(DAniF)3]2(μ-O2CC6H4CS2), and [Mo2(DAniF)3]2(μ-S2CC6H4CS2) (DAniF = N,N'-di(p-anisyl)formamidinate) have been synthesized and characterized by single-crystal X-ray diffractions. Together with the terephthalate analogue, the four compounds, denoted as [O2-O2], [OS-OS], [S2-S2], and [O2-S2], have similar molecular skeletons and Mo2···Mo2 separations (∼12 Å), but varying sulfur contents or symmetry. The singly oxidized complexes [O2-O2](+), [OS-OS](+), [S2-S2](+), and [O2-S2](+) display characteristic intervalence transition absorption bands in the near- and mid-IR regions, with differing band energy, intensity, and shape. Applying the geometrical length of the bridging group "-CC6H4C-" (5.8 Å) as the effective electron transfer distance, calculations from the Mulliken-Hush equation yield electronic coupling matrix elements (H(ab)) in the range 600-900 cm(-1). Significantly, this series presents a transition from electron localization to "almost-delocalization" as the carboxylate groups of the bridging ligand are successively thiolated. In terms of Robin-Day's scheme, [S2-S2](+) is best described as an intermediate between Class II and III, while [O2-O2](+) and [OS-OS](+) belong to Class II. It is unusual that the Class II-III transition occurs in such a weakly coupled system (H(ab) < 1000 cm(-1)). This is attributed to the d(δ)-p(π) conjugation between the Mo2 center and bridging ligand. By electrochemical and spectroscopic methods, the internal energy difference for [O2-S2](+) is determined to be 2250 ± 80 cm(-1), which controls the charge distribution of the cation radical. The experimental results and theoretical analyses illustrate that the unsymmetrical geometry leads to unbalanced electronic configurations and asymmetrical redox and optical behaviors.

Tuning the Electronic Coupling and Electron Transfer in Mo2 Donor–Acceptor Systems by Variation of the Bridge Conformation

“…The Mo–Mo bond lengths increase as the O chelating atoms of the bridge are replaced by S atoms, for example, 2.0859(11) Å for [ O 2 –(1,4 ‐ naph)–O 2 ] and 2.1082(6) Å for [ S 2 –(1,4 ‐ naph)–S 2 ]. In addition, the thiolated species show slightly longer Mo 2 ⋅⋅⋅ Mo 2 distances, as seen in the phenylene series (Table ) . Importantly, the three compounds differ in the coplanarity of the naphthyl group with the adjacent dimetal chelate rings, as defined by the torsion angles ϕ (Scheme ).…”

“…Thus, assembling two building blocks [Mo 2 (DAniF) 3 ] + with bridging ligands of type − E 2 C‐(1,4‐naph)‐CE 2 − (E=O or S), namely, 1,4‐naphthalenedicarboxylate, 1,4‐naphthalenedithiocarboxylate, and 1,4‐naphthalenetetrathiodicarboxylate, produced dimolybdenum dimers [{Mo 2 (DAniF) 3 } 2 (μ‐O 2 CC 10 H 6 CO 2 )] ([ O 2 –(1,4‐naph)–O 2 ]), [{Mo 2 (DAniF) 3 } 2 (μ‐OSCC 10 H 6 COS)] ([ OS–(1,4‐naph)–OS ]), and [{Mo 2 (DAniF) 3 } 2 (μ‐S 2 CC 10 H 6 CS 2 )] ([ S 2 –(1,4‐naph)–S 2 ]), respectively. In these Mo 2 assemblies, the three different [Mo 2 ] units [Mo 2 (DAniF) 3 (μ‐O 2 C)], [Mo 2 (DAniF) 3 (μ‐OSC)], and [Mo 2 (DAniF) 3 (μ‐S 2 C)] serve as D or A units in the MV compounds that differ in electron‐donating (electron‐accepting) ability, and the actual bridge (B) is commonly a naphthalene group, which tunes the electron transporting ability by twisting out of the CT platform to varying extents.…”

“…When the spacer is a phenylene group, this steric hindrance is released; for example, smaller torsion angles of about 9 and 23°, were found for [ O 2 –ph–O 2 ] and [ S 2 –ph–S 2 ], respectively . Interestingly, for the two thiolated species, the C4−C5 bonds that connect the [Mo 2 ] units and the naphthalene group are appreciably longer than those for the phenylene‐spaced analogues . The longer C−C single bonds reflect the poorer π conjugation of the CT platform.…”

Electronic Coupling and Electron Transfer between Two Dimolybdenum Units Spaced by a Biphenylene Group

“…The EPR peaks center at g 1. 28 Notably, for the two series, the g values increase as the number of sulfur atoms in the [Mo 2 ] units increases. This variation tendency is consistent with the results in electrochemical cyclic voltammograms and electronic and vibronic spectra, implying that, somehow, the g value is correlated with the extent of electron delocalization.…”

“…The MV complex series present a transition from weak (Class II) to moderately strong (Class II−III) coupling interactions. 28 Given the same auxiliary ligand and similar Mo 2 ···Mo 2 separations, the adiabatic ET rate is dependent on the chelating group (EE′C) which controls the potential energies of the donor and acceptor.…”