Reactivity of Nitrido Complexes of Ruthenium(VI), Osmium(VI), and Manganese(V) Bearing Schiff Base and Simple Anionic Ligands

Abstract

Nitrido complexes (M≡N) may be key intermediates in chemical and biological nitrogen fixation and serve as useful reagents for nitrogenation of organic compounds. Osmium(VI) nitrido complexes bearing 2,2':6',2″-terpyridine (terpy), 2,2'-bipyridine (bpy), or hydrotris(1-pyrazolyl)borate anion (Tp) ligands are highly electrophilic: they can react with a variety of nucleophiles to generate novel osmium(IV)/(V) complexes. This Account describes our recent results studying the reactivity of nitridocomplexes of ruthenium(VI), osmium(VI), and manganese(V) that bear Schiff bases and other simple anionic ligands. We demonstrate that these nitrido complexes exhibit rich chemical reactivity. They react with various nucleophiles, activate C-H bonds, undergo N···N coupling, catalyze the oxidation of organic compounds, and show anticancer activities. Ruthenium(VI) nitrido complexes bearing Schiff base ligands, such as [Ru(VI)(N)(salchda)(CH3OH)](+) (salchda = N,N'-bis(salicylidene)o-cyclohexyldiamine dianion), are highly electrophilic. This complex reacts readily at ambient conditions with a variety of nucleophiles at rates that are much faster than similar reactions using Os(VI)≡N. This complex also carries out unique reactions, including the direct aziridination of alkenes, C-H bond activation of alkanes and C-N bond cleavage of anilines. The addition of ligands such as pyridine can enhance the reactivity of [Ru(VI)(N)(salchda)(CH3OH)](+). Therefore researchers can tune the reactivity of Ru≡N by adding a ligand L trans to nitride: L-Ru≡N. Moreover, the addition of various nucleophiles (Nu) to Ru(VI)≡N initially generate the ruthenium(IV) imido species Ru(IV)-N(Nu), a new class of hydrogen-atom transfer (HAT) reagents. Nucleophiles also readily add to coordinated Schiff base ligands in Os(VI)≡N and Ru(VI)≡N complexes. These additions are often stereospecific, suggesting that the nitrido ligand has a directing effect on the incoming nucleophile. M≡N is also a potential platform for the design of new oxidation catalysts. For example, [Os(VI)(N)Cl4](-) catalyzes the oxidation of alkanes by a variety of oxidants, and the addition of Lewis acids greatly accelerates these reactions. [Mn(V)(N)(CN)4]2(-) is another highly efficient oxidation catalyst, which facilitates the epoxidation of alkenes and the oxidation of alcohols to carbonyl compounds using H2O2. Finally, M≡N can potentially bind to and exert various effects on biomolecules. For example, a number of Os(VI)≡N complexes exhibit novel anticancer properties, which may be related to their ability to bind to DNA or other biomolecules.

Photocatalytic Oxygenation Reactions Using Water and Dioxygen

“…[47] Ethylbenzene was also hydroxylated to yield 1-phenylethanol using [47] The oxygen source in the oxygenatedp roducts was confirmed by using 18 Oi sotopic labeling to be H 2 O. [58] The photocatalytic oxygenation is started by photoinduced ET from [Ru(bpy) 3 [58] The quantum yield for the formationo fp-styrene oxide sulfonate in thep hotocatalytic oxygenation of p-styrene sulfonate with [(TMPS)Mn III (OH)] increased with increasing [Co III (NH 3 ) 5 Cl] 2 + concentration to reach ac onstantv alue close to 50 %, which corresponds to the maximum value for the one-photon one-electronprocess in Scheme 2. [35][36][37][38][39][40][41][42][43][44][45][46][47] [Ru(bpy) 3 ] 3 + was used as ao neelectron oxidant to produce high-valent metal-oxo complexes, but it can be replaced by much milder oxidants such as [Co III (NH 3 ) 5 Cl] 2 + under photoirradiation.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] Amongo xidants used in oxidation reactions, dioxygen (O 2 )i s the greenest, since the product of O 2 reduction is water or hydrogen peroxide. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Amongo xidants used in oxidation reactions, dioxygen (O 2 )i s the greenest, since the product of O 2 reduction is water or hydrogen peroxide.…”

“…[47] Such preference of cyclohexene over styrene in the catalytic oxygenation has also been observedu nder photocatalytic oxygenation of other alkenes by porphyrins. [35][36][37][38][39][40][41][42][43][44][45][46][47] [Ru(bpy) 3 ] 3 + was used as ao neelectron oxidant to produce high-valent metal-oxo complexes, but it can be replaced by much milder oxidants such as [Co III (NH 3 ) 5 Cl] 2 + under photoirradiation. [47] Ethylbenzene was also hydroxylated to yield 1-phenylethanol using [47] The oxygen source in the oxygenatedp roducts was confirmed by using 18 Oi sotopic labeling to be H 2 O.…”

“…[47] When the TMP ligand was replaced by TDCPP, [(TDCPP)Mn III ] + acts as a more reactive catalyst for the oxygenation of cyclohexene and styrene than [(TMP)Mn III (H 2 O) 2 ](PF 6 ), because of the smaller steric effect of the TDCPP ligand as comparedw ith that of TMP ligand and the high redox potential. [48][49][50][51][52][53][54][55][56][57][58] Scheme2 shows the catalytic cycle of the photocatalytic oxygenation of watersoluble substrates (S) with [Co III (NH 3 ) 5 Cl] 2 + as an oxidanta nd [Ru(bpy) 3 ] 2 + as ap hotocatalyst and am anganese(III)-hydroxo porphyrin ([(Por)Mn(OH)]) as an oxygenation catalyst. [47] Av ariety of high-valent metal-oxo complexes were produced by ET oxidation of metal complexes by one-electron oxidants, such as [Ru(bpy) 3 ] 3 + and cerium(IV) ammonium nitrate (CAN), with H 2 O, catalyzing oxygenation of substrates by using H 2 Oa sa no xygen source.…”

Sulfonated Schiff base Sn(IV) complexes as potential anticancer agents

“…Though a large number of Schiff bases of various types [76][77][78][79][80][81][82][83][84] has been reported, sulfonated Schiff bases [70][71][72][73][74][75] are rare and only a few metal complexes have been investigated structurally, and a restricted number were applied in catalysis, mainly in alkane [70,74] or alcohol oxidation [75] or in the nitro-aldol reaction [71]. Moreover, sulfonated Schiff bases display versatile coordination abilities, forming monomers, dimers, tetramers and polymers (both 1D and 2D) including solvatomorphs [70][71][72][73][74][75].…”