Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study

Abstract

The three- and four-membered redox series [Cr(PDP)] (z = 1-, 2-, 3-) and [Mo(PDP)] (z = 0, 1-, 2-, 3-) were synthesized to study the redox properties of the pincer ligand PDP (HPDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine). The monoanionic complexes were characterized by X-ray crystallography, UV/vis/NIR spectroscopy, and magnetic susceptibility measurements. Experimental and density functional theory (DFT) studies are consistent with closed-shell PDP ligands and +III oxidation states (d, S = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M(PDP)] (M = Cr, Mo), which were further investigated via chemical oxidation and reduction. For molybdenum, one-electron oxidation yielded Mo(PDP) which was characterized by X-ray crystallography, UV/vis/NIR, and magnetic susceptibility measurements. The experimental and computational data indicate metal-centered oxidation to a Mo complex (d, S = 1) with two PDP ligands. In contrast, one- and two-electron reductions were found to be ligand centered resulting in the formation of PDP radicals, in which the unpaired electron is predominantly located on the central pyridine ring of the ligand. The presence of ligand radicals was established experimentally by observation of ligand-to-ligand intervalence charge transfer (LLIVCT) bands in the UV/vis/NIR spectra of the dianionic and trianionic complexes and further supported by broken-symmetry DFT calculations. X-ray crystallographic analyses of the one-electron-reduced species [M(PDP)] (S = 1, M = Cr, Mo) established structural indicators for pincer reduction and showed localization of the radical on one of the two pincer ligands. The two-electron-reduced, trianionic complexes (S = 1/2) were characterized by UV/vis/NIR spectroscopy, magnetic susceptibility measurements, and EPR spectroscopy. The electronic structures of the reduced complexes are best described as containing +III metal ions (d) antiferromagnetically coupled to one and two radical ligands for the dianionic and trianionic species, respectively.

Luminescence and Light‐Driven Energy and Electron Transfer from an Exceptionally Long‐Lived Excited State of a Non‐Innocent Chromium(III) Complex

“…Spin polarization (s pathway) imposes some b spin density to all six nitrogen donor atoms ( Figure S7 a; À0.08) and increases the a spin density at chromium above three.T his is in accordance with a( t 2g ) 3 electron configuration of the chromium(III) ion and the magnetic data ( Figure S5). Furthermore,the spin density is consistent with that obtained for [Cr(tpy) 2 ] 3+ ,[Cr(bpy) 3 ] 3+ and [Cr(MePDP) 2 ] 1À with 3.21, 3.27 and 3.21 a electrons at the chromium centers,respectively, [1,2] at the B3LYP/def2-TZVP level of theory.…”

“…[Cr-(ddpd) 2 ] 3+ is reduced to the corresponding labile d 4 chromium(II) complex at E1 = 2 = À1.11 Va nd irreversibly at the ddpd ligand at À1.94 Vv s. FcH/FcH + . [3,6] On the other hand, [Cr(tpy) 2 ] 3+ (E1 = 2 = À0.53, À0.95, À1.45, À2.37 V) [2] and [Cr( t bpy) 3 ] 3+ (E1 = 2 = À0.63, À1.15, À1.72, À2.34, À2.67, À2.90 V) [1a] exhibit several reversible one-electron reduction steps,w hich have all been assigned to ligand centered reductions yielding the corresponding radical anions and dianions coordinated to Cr 3+ . [ Cr( t bpy) 3 ] n+ and [Cr(tpy) 2 ] n+ (n = 2, 1) exhibit characteristic intense absorption bands in the red to near-infrared spectral region.…”

“…As trongly growing interest in chromium(III) complexes, especially with polypyridyl ligands,a rises from two perspectives,n amely from the ambiguity of the ground state electronic structures of their reduced congeners (redox noninnocence) [1,2] and their-for first row transition metal complexes-outstanding luminescent properties with long-lived spin-flip emission from doublet states. [3][4][5] Thet ype of polypyridine ligand determines both, redox and photophysical properties of chromium(III) complexes.The classical electron transfer series [Cr( t bpy) 3 ] n+ and [Cr(tpy) 2 ] n+ (n = 3, 2, 1, 0) exclusively comprise ligand-centered redox couples and the chromium center retains its oxidation state + III throughout ( t bpy = 4,4'-di-tert-butyl-2,2'-bipyridine,t py = 2,2':6',2''-terpyridine;Scheme 1). [1] Analogous results have been obtained for [Cr( Me PDP) 2 ] nÀ (H 2 Me PDI = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine) complexes.…”

“…[1] Analogous results have been obtained for [Cr( Me PDP) 2 ] nÀ (H 2 Me PDI = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine) complexes. [2] On the other hand, the [Cr(ddpd) 2 ] 3+/2+ redox couple featuring the electron-rich polypyridine ligand ddpd involves ap urely metal centered process giving chromium(II) (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine;S cheme 1). [3,6] Bis(terpyridine)chromium(III) [Cr(tpy) 2 ] 3+ and other classical pyridine complexes are weakly emissive (Table 1).…”

“…We surmised that the tpe ligands should exert as trong ligand field in [Cr(tpe) 2 ] 3+ .T his should place the lowest energy doublet states below the lowest energy quartet excited states in ah omoleptic chromium(III) complex leading to phosphorescence.I nc ontrast to D 2 -symmetric mer-[Cr-(ddpd) 2 ] 3+ and D 3 -symmetric [Cr(N\N) 3 ] 3+ metal complexes, [M(tpe) 2 ] 3+ complexes feature an inversion center.According to Laportesr ule for dd transitions in centrosymmetric complexes,t he inversion center should affect the absorption and emission properties. [49] Furthermore,t pe could be susceptible to ligand-based redox chemistry (ligand non-innocence) similar to t bpy,tpy or Me PDP 2À , [1,2] contrasting ddpd as aredox-innocent spectator ligand. [6] In this study,wee xploit the complex [Cr(tpe) 2 ] 3+ with an Earth-abundant metal ion as ap otential substitute for the classical, precious metal containing chromophore [Ru-(bpy) 3 ] 2+ in luminescence,a sw ell as in photoinduced energy and electron transfer reactions.S ingle crystal X-ray diffraction, [50][51][52][53][54] NIR luminescence quantum yields [55] and lifetimes, variable temperature luminescence and step-scan FT-IR spectroscopy, [56][57][58] electrochemistry and spectroelectrochemistry,S tern-Volmer analyses as well as quantum chemical calculations [59][60][61][62][63][64][65][66][67][68][69] confirm the proposed design guidelines.…”