It is postulated that the copper(I) nitrite complex is a key reaction intermediate of copper containing nitrite reductases (Cu-NiRs), which catalyze the reduction of nitrite to nitric oxide (NO) gas in bacterial denitrification. To investigate the structure-function relationship of Cu-NiR, we prepared five new copper(I) nitrite complexes with sterically hindered tris(4-imidazolyl)carbinols [Et-TIC = tris(1-methyl-2-ethyl-4-imidazolyl)carbinol and iPr-TIC = tris(1-methyl-2-isopropyl-4-imidazolyl)carbinol] or tris(1-pyrazolyl)methanes [Me-TPM = tris(3,5-dimethyl-1-pyrazolyl)methane; Et-TPM = tris(3,5-diethyl-1-pyrazolyl)methane; and iPr-TPM = tris(3,5-diisopropyl-1-pyrazolyl)methane]. The X-ray crystal structures of all of these copper(I) nitrite complexes were mononuclear eta(1)-N-bound nitrite complexes with a distorted tetrahedral geometry. The electronic structures of the complexes were investigated by absorption, magnetic circular dichroism (MCD), NMR, and vibrational spectroscopy. All of these complexes are good functional models of Cu-NiR that form NO and copper(II) acetate complexes well from reactions with acetic acid under anaerobic conditions. A comparison of the reactivity of these complexes, including previously reported (iPr-TACN)Cu(NO2) [iPr-TACN = 1,4,7-triisopropyl-1,4,7-triazacyclononane], clearly shows the drastic effects of the tridentate ligand on Cu-NiR activity. The copper(I) nitrite complex with the Et-TIC ligand, which is similar to the highly conserved three-histidine ((His)3) ligand environment in the catalytic site of Cu-NiR, had the highest Cu-NiR activity. This result suggests that the (His)3 ligand environment is essential for acceleration of the Cu-NiR reaction. The highest Cu-NiR activity for the Et-TIC complex can be explained by the structural and spectroscopic characterizations and the molecular orbital calculations presented in this paper. Based on these results, the functional role of the (His)3 ligand environment in Cu-NiR is discussed.

A nitrous acid complex was identified by spectroscopic and kinetic analyses under stopped‐flow conditions as an intermediate in the reaction of copper(I) nitrite complex 1 with trifluoroacetic acid (see scheme). A reaction mechanism was proposed in which two protons are transferred in a stepwise manner to the nitrite anion. Intramolecular electron transfer from copper to the nitrite ligand occurs in the second protonation step.

63Cu NMR spectroscopic studies of copper(I) complexes with various N-donor tridentate ligands are reported. As has been previously reported for most copper(I) complexes, 63Cu NMR signals, when acetonitrile is coordinated to copper(I) complexes of these tridentate ligands, are broad or undetectable. However, when CO is bound to tridentate copper(I) complexes, the 63Cu NMR signals become much sharper and show a large downfield shift compared to those for the corresponding acetonitrile complexes. Temperature dependence of 63Cu NMR signals for these copper(I) complexes show that a quadrupole relaxation process is much more significant to their 63Cu NMR line widths than a ligand exchange process. Therefore, an electronic effect of the copper bound CO makes the 63Cu NMR signal sharp and easily detected. The large downfield shift for the copper(I) carbonyl complex can be explained by a paramagnetic shielding effect induced by the copper bound CO, which amplifies small structural and electronic changes that occur around the copper ion to be easily detected in their 63Cu NMR shifts. This is evidenced by the correlation between the 63Cu NMR shifts for the copper(I) carbonyl complexes and their nu(C[triple bond]O) values. Furthermore, the 63Cu NMR shifts for copper(I) carbonyl complexes with imino-type tridentate ligands show a different correlation line with those for amino-type tridentate ligands. On the other hand, 13C NMR shifts for the copper bound 13CO for these copper(I) carbonyl complexes do not correlate with the nu(C[triple bond]O) values. The X-ray crystal structures of these copper(I) carbonyl complexes do not show any evidence of a significant structural change around the Cu-CO moiety. The findings herein indicate that CO complexation makes 63Cu NMR spectroscopy much more useful for Cu(I) chemistry.

Dehydrative condensation of the hydroxopalladium complex (Tp(iPr2))(py)Pd-OH (1) with hydroperoxides (XOOH: X = H, t-Bu) produces the corresponding (hydroperoxo)-, (Tp(iPr2))(py)Pd-OOH (2a), and (tert-butylperoxo)palladium complexes, (Tp(iPr2))(py)Pd-OOBu(t) (3). Treatment of 2a with PPh(3) results in concomitant ligand displacement giving (Tp(i)(Pr2))(Ph(3)P)Pd-OOH (2b) and oxygenation of PPh(3) giving O=PPh(3). Further condensation between 1 and 2a gives the mu-kappa(1):kappa(1)-peroxo complex (Tp(iPr2))(py)Pd-OO-Pd(Tp(iPr2))(py) (4), while condensation between the bis(mu-hydroxo)dipalladium complex (PdTp(iPr2))(2)(mu-OH)(2) (7) with 2a affords the unsymmetrical mu-kappa(1):kappa(2)-peroxo complex (Tp(iPr2))(py)Pd-OO-PdTp(iPr2) (5). These peroxopalladium complexes 2-5 have been fully characterized by a combination of spectroscopic and crystallographic analyses, which lead to description of the O-O moieties in these complexes as peroxide (O(2)(2-)) with sp(3)-hybridized oxygen atoms. The OOH moiety in 2b is found to interact with the noncoordinated nitrogen atom of the pendant pyrazolyl group through hydrogen bond. The O(2) moieties in 2-5 turn out to be so nucleophilic (basic) as to add across carbon-heteroatom multiple bonds in acetonitrile and acetaldehyde to give the peroxometallacycle Tp(iPr2)Pd[OOC(Me)=N)]Pd(iPr2)(py)(8) (from 2, 4, and 5) and the acetato complex (Tp(iPr2))(py)Pd-OC(=O)CH(3) (9) (from 2-4), respectively. Methyl vinyl ether and styrene, CH(2)=CHY (Y = OMe, Ph), are converted to the corresponding methyl ketones, CH(3)C(=O)Y, upon treatment with 2-4.

Reaction of a hydroxopalladium complex bearing the Tp iPr 2 ligand, (Tp iPr 2 )(py)Pd-OH (1 iPr 2 ), with active methylene compounds, X-CH 2 -Y 2 [dicyanomethane (2a), methyl cyanoacetate (2b), benzoylacetonitrile (2c)], resulted in dehydrative condensation to afford the N/O-bound enolates, (Tp iPr 2 )(py)Pd-X-CH-Y 3 iPr 2 a-c. When the hydroxo complex 1 Me 2 with the less bulky Tp Me 2 ligand was allowed to react with 2 at 0 °C, similar N/O-bound enolato complexes, (Tp Me 2 )(py)Pd-X-CH-Y (3 Me 2 a-c), were obtained as kinetic products, which were gradually converted to the more stable C-bound enolato complexes (Tp Me 2 )(py)Pd-CHXY (4 Me 2 a-c) upon warming to 50 °C. X-ray crystallography of the N/O-and C-bound enolates reveals that (1) the Pd center adopts the square-planar or square-pyramidal geometry, (2) the structure of the C-bound isomer is consistent with the canonical structure with the localized bonding scheme, and (3) in the N/O-bound isomers the negative charge is delocalized over the X-CH-Y linkage to form the zwitterionic structure. The N-bound enolato complexes 3a,b obtained from cyano compounds further reacted with the cyano compounds to give the 1:2 condensates: i.e., the 2-cyanoethenylamido complexes (Tp R )(py)Pd-NH-C(CH 2 Y)dCCN-(Y) (6), whereas the C-bound enolates 4 Me 2 a,b showed no indication of the dimerization. Thus, the present study reveals that the reactivity of transition-metal enolates is dependent on their structures.

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A series of dendritic ligands with a 2,2'-bipyridine core was synthesized through the coupling of 4,4'-dihydroxy-2,2'-bipyridine with poly(aryl ether) dendrons. The corresponding dendritic Cu(OTf)2 catalysts were used for Diels-Alder and three-component condensation reactions. The dendritic Cu(OTf)2-catalyzed Diels-Alder reaction proceeded smoothly, and these dendritic catalysts could be recycled without deactivation by reprecipitation. Three-component condensation reactions such as Mannich-type reactions also proceeded not only in dichloromethane but also in water. Furthermore, a positive dendritic effect on chemical yields was observed in both Diels-Alder reactions and aqueous-media three-component condensation reactions.

Treatment of hydroperoxopalladium complexes, (Tp R )(py)Pd-OOH, with hydroxonickel complexes, [(µ-OH)NiTp RЈ ] 2 , when either Tp R or Tp RЈ is Tp iPr2 , results in dehydrogenation of an isopropyl group of the Tp iPr2 ligand to give heterobimetallic di-µ-hydroxo complexes bearing the 3-isopropenyl-substituted Tp ligand [HB(pz iPr2 ) 2 (pz 3-isopropenyl-5-iPr ). Similar dehydrogenation is observed for the reaction with the hydroxocobalt complex bearing the Tp iPr2 ligand. The dehydrogenated products are characterized by spectroscopic and crystallographic methods and a mechanism involving a heterobimetallic µ-peroxo intermediate formed via dehydrative condensation has been proposed for the oxidative dehydrogenation.