Redox Noninnocence of Nitrosoarene Ligands in Transition Metal Complexes

Tomson, Neil C.; Labios, Liezel A.; Weyhermüller, Thomas; Figueroa, Joshua S.; Wieghardt, Karl

doi:10.1021/ic2005979

Cited by 61 publications

(115 citation statements)

References 73 publications

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“…49 X-ray crystallography, X-ray absorption spectroscopy (XAS), and computational studies indicated that, in the solid state, complex 1a possessed a divalent palladium center and two monoanionic nitroxide radicals. 52 In addition, magnetic susceptibility measurements on crystalline samples of 1a showed that the nitroxide radicals exhibit weak antiferromagnetic coupling mediated by superexchange across the Pd center resulting in a ground-state, open-shell singlet (i.e., singlet diradical) and a triplet excited state that is thermally accessible at ca. 290 K (singlet−triplet gap = 0.4 kcal/mol).…”

Section: ■ Introductionmentioning

confidence: 99%

“…While the solid-state structure of Pd(κ 1 -N-PhNO) 2 -(CNAr Dipp2 ) 2 (1a) is established by X-ray crystallography and combustion analysis, as noted above, preliminary observations of its spectroscopic signatures in solution were inconsistent with its established magnetic properties. 52 Dissolution of forest green crystals of Pd(κ 1 -N-PhNO) 2 (CNAr Dipp2 ) 2 (1a) in benzene-d 6 or deuterated tetrahydrofuran (THF-d 8 ) produces a deep red solution. Analysis by 1 H and 13 C{ 1 H} NMR spectroscopy reveals the prominence of diamagnetic species in solution, with only one Ar Dipp2 environment apparent.…”

Section: ■ Introductionmentioning

confidence: 99%

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Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals

et al. 2015

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The redox-noninnocence of metal-coordinated C-organo nitrosoarenes has been established on the basis of solid-state characterization techniques, but the solution-phase properties of this class of metal-coordinated radicals have been relatively underexplored. In this report, the solution-phase properties and dynamics of the bis-nitrosobenzene diradical complex trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2 are presented. This complex, which is best described as containing singly reduced phenylnitroxide radical ligands, is shown to undergo facile nitrosobenzene dissociation in solution to form the metalloxaziridine Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2 and thus is not a persistent species in solution. An equilibrium between trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2, Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2, and free nitrosobenzene is established in solution, with the metalloxaziridine being predominantly favored. Efforts to perturb this equilibrium by the addition of excess nitrosobenzene reveal that the formation of trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2 is in competition with insertion-type chemistry of Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2 and is therefore not a viable strategy for the production of a kinetically persistent bis-nitroxide radical complex. Electronic modification of the nitrosoarene framework was explored as a means to generate a persistent trans-Pd(κ(1)-N-ArNO)2(CNAr(Dipp2))2 complex. While most substitution schemes failed to significantly perturb the kinetic lability of the nitrosoarene ligands in the corresponding trans-Pd(κ(1)-N-ArNO)2(CNAr(Dipp2))2 complexes, utilization of para-formyl or para-cyano nitrosobenzene produced bis-nitroxide diradical complexes that display kinetic persistence in solution. The origin of this persistence is rationalized by the ability of para-formyl- and para-cyano-aryl groups to both attenuate the trans effect of the corresponding nitrosoarene and, more importantly, delocalize spin density away from the aryl-nitroxide NO unit. The results presented here highlight the inherent instability of metal-coordinated nitroxide radicals and suggest a general synthetic strategy for kinetically stabilizing these species in solution.

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Section: ■ Introductionmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals

et al. 2015

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“…[22] Thus, innocence or non-innocence is not the permanent attribute of a ligand, but molecules can behave [8] as innocent or non-innocent ligands towards redox-active metals under specific circumstances, in a certain situation. While the potential for non-innocent ligand behavior is obvious for the very easily reducible and correspondingly employed TCNQ acceptor, [21] or for nitrosoarenes, [23] there are ligands with less eye-catching non-innocence. For instance, the substitutional modification [24] of the "NacNac" ligand [24] in [MeNC(Me)C(H)C(Me)NMe] -has made it potentially redox-active, i.e.…”

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confidence: 99%

The Shrinking World of Innocent Ligands: Conventionaland Non‐Conventional Redox‐Active Ligands

2012

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This essay for EurJIC's cluster issue on cooperative and redox non‐innocent ligands introduces the reader to redox‐active ligands, which range from the small archetypical NO+/•/– and O20/•–/2–systems via the classical 1,4‐dihetero‐1,3‐diene chelates (e.g. α‐diimine, dithiolene, or o‐quinone redox series) to π‐conjugated macrocycles. The increased attention paid recently to the redox activity of ligands in coordination chemistry has now prompted wider successful searches, resulting in the establishing of less‐conventional examples such as cyanide, carbon monoxide, thioethers, or acetylacetonate derivatives as non‐innocently behaving ligands. By considering situations with significantly covalent metal–ligand bonding, the cases of metal–oxo, metal–hydrido, and organometallic compounds will also be addressed, with a perspective on how pervasive non‐innocent ligand behavior is. The materials and reactivity potential of redox‐active ligands will be pointed out.

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“…Additionally,t he nitroxide aryl groups in complexes 8-10 adopt an in-plane orientation relative to the Mn center ( Figure 2B). This feature contrasts distinctly with structurally characterized h 2 -N,O nitrosoarene complexes (such as metalloxaziridines), [36] in which the aryl groups adopt aperpendicular orientation relative to the metal center on account of inplane M!p*(NO) backbonding.Accordingly,the solid-state structures of complexes 8-10 strongly suggest that the h 2 -ArNO ligands do not bind according to the standard DewarChatt-Duncanson model of M!L p backbonding.R ather, the structural properties of 8-10 indicate that the h 2 -ArNO ligands function as L 2 -type,n ot X 2 -type,d onors, [37] with an unpaired electron localized in aN Op*orbital oriented perpendicular to the plane of the Mn center. Ther oomtemperature EPR spectra of complexes 8-10 in toluene solution support this description and give rise to g iso values of 2.004, 2.008, and 2.005, respectively,i ndicative of organic radicals (see Figure 2C for the EPR spectrum of 8 and Figures S2.3 and S2.4 for the spectra of 9 and 10,r espectively).…”

Section: Angewandte Chemiementioning

confidence: 62%

“…Ther oomtemperature EPR spectra of complexes 8-10 in toluene solution support this description and give rise to g iso values of 2.004, 2.008, and 2.005, respectively,i ndicative of organic radicals (see Figure 2C for the EPR spectrum of 8 and Figures S2.3 and S2.4 for the spectra of 9 and 10,r espectively). Notably,w hile persistent nitroxide radical complexes are uncommon, structurally characterized examples exhibiting k 1 -N, [36,38,39] k 1 -O, [40] and bridging m-(1k 1 -N:2k 1 -O) [41] coordination modes have been reported. However,c omplexes 8-10 are unique examples of side-on bound h 2 -N,O nitroxide radicals.…”

Section: Angewandte Chemiementioning

confidence: 99%

Kinetic Destabilization of Metal–Metal Single Bonds: Isolation of a Pentacoordinate Manganese(0) Monoradical

et al. 2015

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For decades,t ransiently generated 17e À monoradicals have occupied ac entral role in the development of atom-transfer and radical-mediated processes in organometallic chemistry. [1][2][3] Ap rototypical example of this class is the zerovalent manganese pentacarbonyl radical [Mn(CO) 5 ]which has been extensively investigated for its role in radical-type olefin hydrogenation, [4,5] [7][8][9][10] or at higher temperatures by using either ultrafast spectroscopic [11] or nitrosoarene spin-trapping techniques. [12][13][14] Importantly,efforts to prepare stabilized variants of [Mn(CO) 5 ]bysubstitution of one or more CO ligands with more encumbering ligands such as phosphines (PR 3 )h ave been shown to extend the lifetime of [MnL 5 ]-type radicals to the order of hours in solution. [15][16][17][18] However,such derivatized [Mn(CO) 5Àn L n ]c omplexes have remained susceptible to facile dimerization/redistributionp rocesses when concentrated and isolable examples remain unknown. Additionally, investigations into the one-electron activation chemistry of [Mn(CO) 5Àn L n ]s pecies with small molecule substrates are limited. [16,17] Given the challenges faced in exploring the properties of [Mn (CO) ) 2 ]( Ar Dipp2 = 2,6-(2,6-(iPr) 2 C 6 H 3 ) 2 C 6 H 3 )f eaturing two m-terphenyl isocyanide ligands. [22,23] Thesterically encumbering nature of these isocyanidel igands effectively prevents the formation of MnÀMn single bonds,thus rendering this 17eÀ radical both isolable in the solid state and persistent in solution. Additionally,asaresult of the isolobal analogy between carbon monoxide (CO) and organoisocyanides (C NR), [22,24] ) 2 ]delivers 1 most cleanly and in the highest yield after isolation (92 %; Figure 1A). Upon isolation, 1 is stable for days in solution at elevated temperatures (C 6 D 6 ,808 8C) and shows no sign of dimerization and/or ligand redistribution when concentrated to asolid.In both solution and the solid state,c omplex 1 possesses several spectroscopic and structural features consistent with ad 7 monoradical formulation. The 1 HNMR spectrum of 1 in C 6 D 6 features broad, shifted resonance signals characteristic of ap aramagnetic species,w hereas determination of the

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Redox Noninnocence of Nitrosoarene Ligands in Transition Metal Complexes

Cited by 61 publications

References 73 publications

Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals

Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals

The Shrinking World of Innocent Ligands: Conventionaland Non‐Conventional Redox‐Active Ligands

Kinetic Destabilization of Metal–Metal Single Bonds: Isolation of a Pentacoordinate Manganese(0) Monoradical

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