Linear Free Energy Relationships in Dinuclear Compounds. 2. Inductive Redox Tuning via Remote Substituents in Quadruply Bonded Dimolybdenum Compounds

Abstract

Syntheses and characterizations are reported for dimolybdenum(II) compounds supported by the diarylformamidinate (ArNC(H)NAr(-)) ligand, where Ar is XC(6)H(4)(-), with X as p-OMe (1), H (2), m-OMe (3), p-Cl (4), m-Cl (5), m-CF(3) (6), p-COMe (7), p-CF(3) (8), or Ar is 3,4-Cl(2)C(6)H(3)(-) (9) or 3,5-Cl(2)C(6)H(3)(-) (10). The (quasi)reversible oxidation potentials measured for the Mo(2)(5+)/Mo(2)(4+) couple were found to correlate with the Hammett constant (sigma(X)) of the aryl substituents according to the following equation: DeltaE(1/2) = E(1/2)(X) - E(1/2)(H) = 87(8sigma(X)) mV. Molecular structure determinations of compounds 1, 2, 5, and 10 revealed an invariant core geometry around the Mo(2) center, with statistically identical Mo-Mo quadruple bond lengths of 2.0964(5), 2.0949[8], 2.0958(6), and 2.0965(5) Å, respectively. Magnetic anisotropies for compounds 1-10 estimated on the basis of (1)H NMR data were similar and unrelated to sigma(X). Similarity in UV-vis spectra was also found within the series, which, in conjunction with the features of both molecular structures and (1)H NMR spectra, was interpreted as the existence of a constant upper valence structure across the series. Results of Fenske-Hall calculations performed for several model compounds paralleled the experimental observations.

Crystallographic and Spectroscopic Characterization of Tetrakis(μ-N,N′-diarylformamidinato)dichlorodirhenium(III,III) Compounds

“…The E 1/2 1 values for the Re 2 Cl 2 (µ-form) 4 series varied over 160 mV and E 1/2 2 values over 150 mV for 1؊4 (Table 2) and reflect the ability to use remote substituents to fine tune the electrochemical properties at a dinuclear core. [15,43,52,62] A similar trend is observed for Re 2 Cl 2 (µ-O 2 CR) 4 complexes with alkyl carboxylates. Plots of E 1/2 values versus the Taft σ* parameter, an indicator of the electron donating or withdrawing ability of the carboxylates, are linear.…”

“…mium(II,II), [52] dimolybdenum(II,II), [14] [15] and dinickel-(II,II) [43] complexes. In the previously studied dinickel sys-Synthesis tems, metalϪmetal bonding is absent with a long NiϪNi separation of 2.462(2) Å observed for Ni 2 [(3,5-Synthesis of compounds 1 and 3؊10 with the general Cl 2 C 6 H 3 )NCHN(3,5-Cl 2 C 6 H 3 )] 4 .…”

“…[63] For the series Ni 2 (µ-form) 4 [ρ ϭ 114 mV], [43] versus the E 1/2 1 value for Re 2 Cl 2 (µ-form) 4 series is 0.990 Cr 2 (µ-form) 4 [ρ ϭ 52.8 mV], [52] Mo 2 (µ-form) 4 [ρ ϭ 87.2 with ρ equal to 61 mV. When the average of the quasireversmV], [15] and Re 2 Cl 2 (µ-form) 4 [56 mV (E 1/2 1 ), 55 mV (E 1/2 2 )], ible reduction potentials observed at all scan rates is used, the magnitude of the value of ρ correlates to the ligand as shown in Figure 6, the correlation coefficient of the least character of the orbital where the loss of the electron (oxisquares fit to 8σ versus E 1/2 1 decreases [0.982] with a slight dation) or gain of the electron (reduction) occurs. For the decrease in ρ, 56 mV.…”

“…[44] A smaller reactivity conwhere X is the substituent on the formamidinate ligand and H is a stant is observed for the oxidation of the Mo 2 (µ-form) 4 hydrogen atom series [15] with a 2b 2g HOMO orbital with 87% Mo and 4% N character. [18] The reactivity constants calculated for the Re 2 Cl 2 (µ-form) 4 complexes 1؊4 are lower than either the dinickel or dimolybdenum series and similar for both reductions.…”

Tuning the electronic structure of Mo–Mo quadruple bonds by N for O for S substitution

“…29,30 The substitution Despite the often dramatic effect that changing the bridging atoms of the bridging ligand has on the electronic structure and properties of quadruply bonded paddlewheel compounds, there have been no systematic studies as to the origins of these effects.…”