Electronic structure of dioxolene adducts of rhodium(III) tetraazamacrocyclic complexes

Macrocyclic Chromium(III) Catecholate Complexes

The synthesis and structural, electrochemical, spectroscopic, and magnetic characterizations of Cr^III(HMC) catecholate and semiquinonate complexes are reported herein, where HMC is 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. cis-[Cr(HMC)(Cat)]⁺ complexes (Cat = catecholate, [1]⁺; tetrachlorocatecholate, [2]⁺; and 3,5-di-tert-butylcatecholate, [3]⁺) were prepared from the reaction between appropriate catechol and [Cr^III(HMC)Cl₂]Cl reduced in situ by zinc. Chemical oxidation of [3]⁺ by FcPF₆ resulted in cis-[Cr(HMC)(SQ)]²⁺ ([3]²⁺, SQ = 3,5-di-tert-butylsemiquinonate). Single crystal X-ray diffraction studies revealed the cis-chelation of the Cat/SQ ligand around the Cr metal center and confirmed the Cat/SQ nature of the ligands. Reversible oxidations of Cat to SQ were observed in the cyclic voltammograms of [1]⁺-[3]⁺, while the Cr^III center remains redox inactive. The absorption spectrum of the SQ complex [3]²⁺ exhibits an intense spin-forbidden transition in solution. Time-delayed phosphorescence spectra recorded at 77 K revealed that all catecholate complexes emit from the ²E state, while [2]⁺ also emits from the ²T₁ state. Temperature-dependent magnetic susceptibility measurements indicate the Cat complexes exist as S = 3/2 systems, while the SQ complex behaves as an S = 1 system, resulting from strong antiferromagnetic coupling of the S = 3/2 Cr center with the S = 1/2 SQ radical. Density functional theory (DFT) shows the similarities between the SOMOs of [1]⁺ and [2]⁺ and differences in their LUMOs in the ground state.

Bonding Properties of the 1,2-Semiquinone Radical-Anionic Ligand in the [M(CO)(4-n)(L)(n)(DBSQ)] Complexes (M = Re, Mn; DBSQ = 3,5-di-tert-butyl-1,2-benzosemiquinone; n = 0, 1, 2). A Comprehensive Spectroscopic (UV-Vis and IR Absorption, Resonance Raman, EPR) and Electrochemical Study

Rhenium and manganese complexes of the 3,5-di-tert-butyl-1,2-benzosemiquinone (DBSQ) ligand, [M(CO)(4)(DBSQ)], fac-[M(CO)(3)(L)(DBSQ)], and cis,trans-[M(CO)(2)(L)(2)(DBSQ)], with a widely varied nature of co-ligand(s) (L = THF, Me(2)CO, MeC(O)Ph, py, NEt(3), Ph(3)PO, SbPh(3), AsPh(3), PCy(3), P(OPh)(3), PPh(3), dppe-p, PPh(2)Et, P(OEt)(3), PEt(3)) were generated in solution and characterized as valence-localized molecules containing the radical-anionic DBSQ ligand bound to Re(I) or Mn(I) metal atoms. This is evidenced by the following. (i) Carbonyl stretching frequencies nu(C&tbd1;O) and average force constants k(av) are typical for Mn(I) or Re(I) carbonyls. (ii) Frequencies of the intra-dioxolene C=O bond stretching vibration, nu(C=O), lie within the 1400-1450 cm(-1) range which is diagnostic for coordinated semiquinones. (iii) EPR spectra indicate a very small spin density on the metal atom (0.2% < a(M)/A(iso) > 2.6%). (iv) Absorption spectra show Re(I) --> DBSQ MLCT electronic transitions characterized by a resonant enhancement of the Raman peaks due to the nu(C&tbd1;O) and intra-DBSQ nu(C=O) vibrations. (iv) Finally, the electrochemical pattern consists of DBSQ/DBQ and DBSQ/DBCat ligand-localized redox couples. All these properties are, in a limited range, dependent on the nature and, especially, the number of co-ligands L, indicating a small delocalization of the singly occupied MO of the DBSQ ligand over the metal atom. The extent of this delocalization may be finely tuned by changing the co-ligands, although in absolute terms, it remains rather limited, and the DBSQ ligand behaves toward Re(I) and Mn(I) as a very weak pi-acceptor only. The changes of the electronic properties of the metal center induced by the co-ligands are mostly compensated by more flexible M --> CO pi back-bonding as is manifested by large variations of the average C&tbd1;O stretching force constant.