A Redox Series of Aluminum Complexes: Characterization of Four Oxidation States Including a Ligand Biradical State Stabilized via Exchange Coupling

Myers, Thomas W.; Kazem, Nasrin; Stoll, Stefan; Britt, R. David; Shanmugam, Maheswaran; Berben, Louise A.

doi:10.1021/ja2015718

Cited by 96 publications

(92 citation statements)

References 38 publications

(35 reference statements)

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“…The separation of the two events is 300 mV in both 2 and 3, and it is most likely that the observed differences in the Ph I 2 P 2−/3− redox couple stems from electrostatic buildup on the complexes as more electrons are added. 16,17 Additionally, two overlapping and irreversible oxidation events were observed for complexes 2 (at 0.31 and 0.46 V) and 3 (at 0.34 and 0.48 V), respectively. We speculate that these events involve two electrons each and correspond to the Ph HI 2 P −/2− and Ph HI 2 P 0/− couples.…”

Section: T He Addition Of O−h Bonds Across the Metal−ligand Bondsmentioning

confidence: 90%

Aluminum–Amido-Mediated Heterolytic Addition of Water Affords an Alumoxane

Myers

Berben

2013

Organometallics

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Addition of the O−H bonds in water across the aluminum− nitrogen bond of a molecular aluminum−amido complex affords an alumoxane. The reaction of ( Ph I 2 P 2− )AlH (1) with water forms dimeric [( Ph HI 2 P − )AlH](μ-O) (2) under mild conditions. Upon reaction of 2 with excess water [( Ph HI 2 P − )Al(OH)](μ-O) (3) is formed with liberation of H 2 . T he addition of O−H bonds across the metal−ligand bonds of a metal complex, essentially an acid−base process, represents an alternative to oxidative addition of OH bonds at a single metal center. This elementary step has been shown to initiate the activation of alcohols toward reactions such as dehydrogenative coupling and amide formation. 1 The activation of water, via addition of the O−H bond across a metal− ligand bond, has also been reported by Milstein and co-workers, who showed formation of a ruthenium hydroxide complex. 2 Examples of heterolytic bond activation by group 13 metal complexes include activation of weak C−H bonds and of the N−H bonds in amines, 3,4 but we are not aware of examples of O−H cleavage.The bis(imino)pyridine (I 2 P) ligand system has been employed for its redox noninnocence in a variety of transition-metal complexes. 5 Reports of the chemical noninnocence of this ligand are rarer and include radical coupling to form new C−C bonds between ligands and the transfer of alkyl or hydride groups to various positions on the ligand backbone. 6 Budzelaar and co-workers showed that alkylation of a variety of I 2 P ligands by main-group-metal alkyls can occur at any site on the pyridine ring in addition to the imine carbons and that the selectivity is influenced by both the identity of the metal and the alkyl used. 7 In order to access the potential of metal−ligand cooperative pathways observed in other pincer ligand systems, it is necessary to design (I 2 P)M complexes that preferentially access two-electron acid−base reactions over radical or redox processes. 8 Due to their tendency of discrete complexes containing the Al−O−Al moiety (alumoxanes) to form oligomers, their formation is a synthetic challenge. 9 Most commonly, alumoxanes are formed through the interaction of water with Al−H, Al−R, or Al−X bonds (R = alkyl, X = halide), 10

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Section: T He Addition Of O−h Bonds Across the Metal−ligand Bondsmentioning

confidence: 90%

Aluminum–Amido-Mediated Heterolytic Addition of Water Affords an Alumoxane

Myers

Berben

2013

Organometallics

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“…[4,[25][26][27][28][29] Only recently, the monoanion LC À and the doubly reduced dianion L 2À were investigated as ligands in more detail. Recent reports showed a series of bis(a-iminopyridine)-metal complexes featuring aluminum [30] and the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) [31,32] with the monoanionic, and thus paramagnetic p-radical form of the ligand in coordination complexes. The a-iminopyridine N-2,6-diisopropylphenylimino-2-pyridine (IPy) has also been introduced in lanthanide chemistry.…”

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confidence: 99%

Sodium and Potassium Salts of Mono‐ and Dianionic α‐Iminopyridines

Nayek

Arleth

Trapp

et al. 2011

Chemistry A European J

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[a] a-Diimine (1,4-diaza-1,3-butadiene) ligands are well established in main-group, transition-metal, and lanthanide chemistry. [1][2][3][4][5][6][7] Beside their application as ligands for latetransition-metal complexes, which can function as active and selective catalysts for a-olefin polymerization, [8] a-diimine and related ligands are of special interest due to their ability to undergo redox chemistry.[9] They have frequently been described as "non-innocent" ligands. [10][11][12] For example, in lanthanide chemistry, it has been found that the formal oxidation state of Eu [13] and Yb [14] depends on the nature of the substituents on the a-diimine ligand and on the ligand on the metal center. Thus, in both cases, the a-diimine acts either as neutral ligand or as radical anion. Reduction of adiimines, using either alkali metals or electrochemical methods, leads in each case to the corresponding radical anions [15][16][17][18] and eventually to diamagnetic dianions, [17][18][19][20][21][22][23][24] albeit at much more negative potentials. In contrast to the well-established chemistry of a-diimines, the related a-iminopyridines are much less investigated. The three different redox states of a-iminopyridines are shown in Scheme 1. The neutral system L 0 has been extensively used as ligand for late transition metals. [4,[25][26][27][28][29] Only recently, the monoanion LC À and the doubly reduced dianion L 2À were investigated as ligands in more detail. Recent reports showed a series of bis(a-iminopyridine)-metal complexes featuring aluminum [30] and the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) [31,32] with the monoanionic, and thus paramagnetic p-radical form of the ligand in coordination complexes. The a-iminopyridine N-2,6-diisopropylphenylimino-2-pyridine (IPy) has also been introduced in lanthanide chemistry. [33,34] In this context, a potassium derivative of the monoanionic form of a-iminopyridine was prepared in situ, but not further characterized.[34] The doubly reduced dianion has only been observed in two magnesium complexes so far. [35] In contrast, the reaction of "GaI" with a-iminopyridine leads to reductive coupling of the coordinated ligand to give neutral diamido-digallium complexes. [36] Based on this information, we were interested in a comprehensive study of the properties of the alkali metal salts of the monoanionic p-radical and the doubly reduced dianion of a-iminopyridine in solution and in the solid state. We report here the synthesis, characterization and magnetic properties of sodium and potassium salts of the mono-and dianionic a-iminopyridines.N-2,6-Diisopropylphenylimino-2-pyridine (IPy) was prepared according to literature procedures.[37] As a first entry point, we examined the electrochemical properties of IPy using cyclic voltammetry at room temperature in THF. As can be seen from Figure 1, we detected a quasi-reversible one-electron reduction process centered at E 0 1=2 = À2.57 V (vs. the ferrocene/ferrocenium couple, Fc/Fc + ), with a peakto-peak separation of 15...

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“…In both solid state and solution, 2 is dark green, which is consistent with the semiquinonate form of ligands. [8c], The thf solution of 2 exhibits a strong absorbance at 374 nm (3418 mol –1 L cm –1 ) and a weak and broad absorbance at 700 nm (505 mol –1 L cm –1 ) in UV/Vis spectrum, which is ascribed to ligand‐based π–π* transitions because of the [Kr] electronic configuration of Y III , . Absorbance at 408 nm (1900 mol –1 L cm –1 ) and 590 nm (3983 mol –1 L cm –1 ) was observed for [( MeO NNN sq· )ZrCl 2 (thf)] and [( MeO NNN sq · )TaCl 3 ] containing the semiquinonate form of a similar ligand, respectively.…”

Section: Resultsmentioning

confidence: 99%