Correction: Mechanistic details of the cobalt-mediated dehydrogenative dimerization of aminoquinoline-directed benzamides.

[This corrects the article DOI: 10.1039/D0SC02066D.].

Crystal structure and vibrational spectra of bis(2-isobutyrylamidophenyl)amine: a redox noninnocent ligand.

The molecular structure of bis(2-isobutyrylamidophenyl)amine (H3LNNN) has been determined from single-crystal X-ray diffraction data. The crystal packing of H3LNNN is governed by the N-H···O and C-H···O hydrogen-bonding and C-H···π stacking interactions between the vicinal molecules. The intermolecular interactions in the crystal structure of H3LNNN have been also examined via Hirshfeld surface analysis and fingerprint plots. The Hirshfeld surface analysis showed that the important role of N-H···O and C-H···π interactions in the solid-state structure of H3LNNN. The molecular structure, vibrational frequencies, and infrared intensities of H3LNNN were computed by ab initio HF and DFT (B3LYP, B3PW91, and BLYP) methods using the 6-31G(d,p) basis set. The computed theoretical geometric parameters were compared with the corresponding single crystal structure of H3LNNN. The harmonic vibrations calculated for the title compound by the B3LYP method are in good agreement with the experimental IR spectral data. The theoretical vibrational spectrum of the H3LNNN compound was interpreted through potential energy distributions using the SQM Version 2.0 program. The performance of the used methods and the scaling factor values were calculated with PAVF Version 1.0 program.

Mechanistic details of the cobalt-mediated dehydrogenative dimerization of aminoquinoline-directed benzamides.

Key mechanistic features of the cobalt-mediated and aminoquinoline-directed dehydrogenative aryl-aryl coupling were investigated computationally and experimentally. A series of CoII and CoIII complexes relevant to the proposed reaction cycle have been synthesized and characterized. Stoichiometric reactions and electrochemical studies were used to probe the role of different additives in the reaction pathway. Computationally, three different mechanisms, such as charge neutral, anionic, and dimetallic were explored. It is shown that the mono-metallic anionic and charge neutral mechanisms are the most favorable ones, among which the former mechanism is slightly more encouraging and proceeds via the: (a) concerted-metalation-deprotonation (CMD) of the first benzamide C-H bond, (b) PivOH-to-PivO- rearrangement, (c) CMD of the second benzamide C-H bond, (d) C-C coupling, (e) product formation facilitated by the amide nitrogen re-protonation, and (f) catalyst regeneration. The rate-determining step of this multi-step process is the C-C coupling step. The computational studies suggest that the electronics of both the aryl-benzamide and pyridine fragments of the aminoquinoline-benzamide ligand control the efficiency of the reaction.

The Mechanism of Rhodium-Catalyzed Allylic C-H Amination.

Herein, the mechanism of catalytic allylic C-H amination reactions promoted by Cp*Rh complexes is reported. Reaction kinetics experiments, stoichiometric studies, and DFT calculations demonstrate that the allylic C-H activation to generate a Cp*Rh(π-allyl) complex is viable under mild reaction conditions. The role of external oxidants in the catalytic cycle is elucidated. Quantum mechanical calculations, stoichiometric reactions, and cyclic voltammetry experiments concomitantly support an oxidatively induced reductive elimination process of the allyl fragment with an acetate ligand proceeding through a Rh(IV) intermediate. Stoichiometric oxidation and bulk electrolysis of the proposed π-allyl intermediate are also reported to support these analyses. Lastly, evidence supporting the amination of an allylic acetate intermediate is presented. We show that Cp*Rh(III)2+ behaves as a Lewis acid catalyst to complete the allylic amination reaction.

Oxygen Activation by Co(II) and a Redox Non-Innocent Ligand: Spectroscopic Characterization of a Radical-Co(II)-Superoxide Complex with Divergent Catalytic Reactivity.

Bimetallic (Et4N)2[Co2(L)2], (Et4N)2[1] (where (L)(3-) = (N(o-PhNC(O)(i)Pr)2)(3-)) reacts with 2 equiv of O2 to form the monometallic species (Et4N)[Co(L)O2], (Et4N)[3]. A crystallographically characterized analog (Et4N)2[Co(L)CN], (Et4N)2[2], gives insight into the structure of [3](1-). Magnetic measurements indicate [2](2-) to be an unusual high-spin Co(II)-cyano species (S = 3/2), while IR, EXAFS, and EPR spectroscopies indicate [3](1-) to be an end-on superoxide complex with an S = 1/2 ground state. By X-ray spectroscopy and calculations, [3](1-) features a high-spin Co(II) center; the net S = 1/2 spin state arises after the Co electrons couple to both the O2(•-) and the aminyl radical on redox non-innocent (L(•))(2-). Dianion [1](2-) shows both nucleophilic and electrophilic catalytic reactivity upon activation of O2 due to the presence of both a high-energy, filled O2(-) π* orbital and an empty low-lying O2(-) π* orbital in [3](1-).

Lanthanide(III) di- and tetra-nuclear complexes supported by a chelating tripodal tris(amidate) ligand.

Syntheses, structural, and spectroscopic characterization of multinuclear tris(amidate) lanthanide complexes is described. Addition of K3[N(o-PhNC(O)(t)Bu)3] to LnX3 (LnX3 = LaBr3, CeI3, and NdCl3) in N,N-dimethylformamide (DMF) results in the generation of dinuclear complexes, [Ln(N(o-PhNC(O)(t)Bu)3)(DMF)]2(µ-DMF) (Ln = La (1), Ce (2), Nd(3)), in good yields. Syntheses of tetranuclear complexes, [Ln(N(o-PhNC(O)(t)Bu)3)]4 (Ln = Ce (4), Nd(5)), resulted from protonolysis of Ln[N(SiMe3)2]3 (Ln = Ce, Nd) with N(o-PhNCH(O)(t)Bu)3. In the solid-state, complexes 1-5 exhibit coordination modes of the tripodal tris(amidate) ligand that are unique to the 4f elements and have not been previously observed in transition metal systems.

Cobalt catalyzed sp3 C-H amination utilizing aryl azides.

A dinuclear Co(ii) complex supported by a modular, tunable redox-active ligand system is capable of selective C-H amination to form indolines from aryl azides in good yields at low (1 mol%) catalyst loading. The reaction is tolerant of medicinally relevant heterocycles, such as pyridine and indole, and can be used to form 5-, 6-, and 7-membered rings. The synthetic versatility obtained using low loadings of an earth abundant transition metal complex represents a significant advance in catalytic C-H amination technology.

Antitumor properties of five-coordinate gold(III) complexes bearing substituted polypyridyl ligands.

In an on-going effort to discover metallotherapeutic alternatives to the chemotherapy drug cisplatin, neutral distorted square pyramidal gold(III) coordination complexes possessing 2,9-disubstituted-1,10-phenanthroline ligands {[((R)phen)AuCl3]; R = n-butyl, sec-butyl} have been previously synthesized and characterized. A structurally analogous gold(III) complex bearing a 6,6'-di-methylbipyridine ligand ([((methyl)bipy)AuCl3]) has been synthesized and fully characterized to probe the effect of differing aromatic character of the ligand on solution stability and tumor cell cytotoxicity. The two compounds [((sec-butyl)phen)AuCl3] and [((methyl)bipy)AuCl3]) were subsequently assessed for their stability against the biological reductant glutathione, and it was found that the [((sec-butyl)phen)AuCl3] complex exhibits slightly enhanced stability compared to the [((methyl)bipy)AuCl3] complex and significantly higher stability than previously reported square planar gold(III) complex ions. Furthermore, these complexes were tested for cytotoxic effects against existing lung and head and neck cancer cell lines in vitro. The [((sec-butyl)phen)AuCl3] complex was found to be more cytotoxic than cisplatin against five different tumor cell lines, whereas [((methyl)bipy)AuCl3] had more limited in vitro antitumor activity. Given that [((sec-butyl)phen)AuCl3] had significantly higher antitumor activity, it was tested against an in vivo tumor model. It was found that this complex did not significantly reduce the growth of xenograft tumors in mice and initial model binding studies with bovine serum albumin indicate that interactions with serum albumin proteins may be the cause for the limited in vivo activity of this potential metallotherapeutic.

Erratum: N-tert-Butyl-2-methyl-propanamide. Corrigendum.

The name of one of the authors in the paper by Kluge et al. [Acta Cryst. (2011), E67, o2143] is corrected.[This corrects the article DOI: 10.1107/S1600536811028947.].

N-tert-Butyl-2-methyl-propanamide.

The title compound, C(8)H(17)NO, crystallizes with two independent mol-ecules in the asymmetric unit. In the crystal, inter-molecular N-H⋯O hydrogen bonding is observed between neighboring mol-ecules, forming continuous mol-ecular chains along the c-axis direction.

Oxygen activation and intramolecular C-H bond activation by an amidate-bridged diiron(II) complex.

A diiron(II) complex containing two µ-1,3-(κN:κO)-amidate linkages has been synthesized using the 2,2',2''-tris(isobutyrylamido)triphenylamine (H(3)L(iPr)) ligand. The resulting diiron complex, 1, reacts with dioxygen (or iodosylbenzene) to effect intramolecular C-H bond activation at the methine position of the ligand isopropyl group. The ligand-activated product, 2, has been isolated and characterized by a variety of methods including X-ray crystallography. Electrospray ionization mass spectroscopy of 2 prepared from(18)O(2) was used to confirm that the oxygen atom incorporated into the ligand framework is derived from molecular oxygen.

Catalytic dioxygen activation by Co(II) complexes employing a coordinatively versatile ligand scaffold.

The ligand bis(2-isobutyrylamidophenyl)amine has been prepared and used to stabilize both mononuclear and dinuclear cobalt(II) complexes. The nuclearity of the cobalt product is regulated by the deprotonation state of the ligand. Both complexes catalytically oxidize triphenylphosphine to triphenylphosphine oxide in the presence of O(2).

Transition metal complexes supported by a neutral tetraamine ligand containing N,N-dimethylaniline units.

First-row transition metal-halide complexes of tris(2-dimethylaminophenyl)amine, L(Me), have been synthesized and characterized. X-ray crystallographic studies on [Co(L(Me))Br]BPh(4), [Ni(L(Me))Cl]BPh(4), [Fe(L(Me))Cl]BPh(4), and [Cu(L(Me))Cl]BF(4) have been performed, and in all cases the ligand produces five-coordinate complexes with distorted trigonal bipyramidal coordination geometries. Where possible, comparisons have been made to the structures of related neutral tripodal ligands. Spectroscopic and magnetic studies of these complexes are also described. The Cu(I)-carbonyl complexes [Cu(L(Me))(CO)]PF(6) and [Cu(Me(6)tren)(CO)]PF(6) (Me(6)tren = tris(N,N-dimethylaminoethyl)amine) have also been prepared. Infrared spectroscopic investigations of these carbonyl complexes confirm that L(Me) is a less electron donating ligand than Me(6)tren and indicate that L(Me) can impart a different coordination number in the solid-state.

Chelating tris(amidate) ligands: versatile scaffolds for nickel(II).

The synthesis and characterization of nickel complexes supported by a family of open-chain, tetradentate, tris(amidate) ligands, [N(o-PhNC(O)R)(3)](3-) ([L(R)](3-) where R = (i)Pr, (t)Bu, and Ph) is described. The complexes [Ni(L(iPr))](-), [Ni(L(tBu))](-), and [Ni(L(Ph))(CH(3)CN)](-) have been characterized by solution-state spectroscopic methods and single crystal X-ray diffraction. Each ligand gives rise to a different primary coordination sphere about the nickel centre. These studies indicate that the ligands' acyl substituents can be used to regulate the coordination mode of the amidate donors to nickel and the coordination number of the nickel centres. In addition, the ability of these complexes to bind cyanide has been explored. These experiments demonstrate that only one of these complexes, [Ni(L(iPr))](-), is able to irreversibly bind cyanide and can be used to assemble [Et(4)N](3)[Ni(L(iPr))(mu(2)-CN)Co(L(iPr))], a cyanide bridged, heterobimetallic complex. The synthesis and characterization of the cyanide containing complexes, including magnetic susceptibility studies, are described.

Effect of methylene spacers on the spectral, electrochemical, and structural properties of bis(4,4'-disubstituted-2,2'-bipyridyl) ruthenium(II) dye analogues.

Two new ruthenium complexes, [Ru(L1OMe)2(NCS)2] and [Ru(L2OMe)2(NCS)2] (where L1OMe = 2,2'-bipyridine-4,4'-di(methyl ethanoate) and L2OMe = 2,2'-bipyridine-4,4'-di(methyl propionate)), have been synthesized and characterized using spectroscopic and electrochemical techniques. The [Ru(L2OMe)2(NCS)2] complex has also been characterized by X-ray diffraction studies and reveals a distorted octahedral coordination geometry about the ruthenium center. Cyclic voltammetry studies of both complexes exhibit quasi-reversible Ru(II/III) couples and a number of ligand reduction events. The complex with one methylene spacer between the bipyridyl ligand and the methyl ester functional group, [Ru(L1OMe)2(NCS)2], was particularly unstable under reducing conditions. The properties of these complexes are compared with the ethyl ester analogue of the N3 photosensitizer [Ru(L0OEt)2(NCS)2].

Synthesis and characterization of gold(III) complexes possessing 2,9-dialkylphenanthroline ligands: to bind or not to bind?

In an effort to discover potential alternatives to the anti-cancer drug cisplatin, the synthesis of gold(III) polypyridyl coordination complexes was pursued. Specifically, this report describes the synthesis and characterization of a series of 2,9-dialkyl-1,10-phenanthroline (Rphen) gold(III) coordination complexes (R = n-butyl, sec-butyl, and tert-butyl). Due to the steric hindrance imparted by the alkyl substituents, these ligands do not react with HAuCl4 to form square-planar gold(III) dichloride complex ions, as is the case with 1,10-phenanthroline, but instead form salts comprised of [AuCl(4)](-) anions and protonated 2,9-dialkylphenanthroline cations (compounds 1 and 2). In an effort to facilitate direct binding between the substituted phenanthroline and the gold(iii) metal center, reactions were carried out between the ligand and NaAuCl4 in the presence of a Ag(I) salt. The precipitation of one equivalent of AgCl afforded the formation of neutral, distorted square-pyramidal gold(iii) trichloride complexes (compounds 3 and 4). Primary or secondary substitutions at the alpha carbon of the alkyl substituent allow direct metal-ligand coordination, whereas a tertiary substituent inhibits chelation and results only in the formation of a salt comprised of a protonated phenanthroline cation and a [AuCl2]- anion (compound 5). Compounds 1-4 have been characterized by 1H NMR, UV/vis, IR spectroscopy, and X-ray crystallography.

Tripodal phenylamine-based ligands and their CoII complexes.

The syntheses of two phenylamine-based ligand systems, N(o-PhNH(2))(3) and N(o-PhNHC(O)(i)Pr)(3), are reported. These ligands readily coordinate to Co(II) to form monomeric complexes. X-ray diffraction studies establish that the [N(o-PhNC(O)(i)Pr)(3)](3-) ligand stabilizes the Co(II) ion in a trigonal-monopyramidal coordination environment. The axial coordination site in this complex is accessible and, upon cyanide coordination, generates an electrochemically active species.

The coordination chemistry of "[BP3]NiX" platforms: targeting low-valent nickel sources as promising candidates to L3Ni=E and L3Ni(triple bond)E linkages.

A series of divalent, monovalent, and zerovalent nickel complexes supported by the electron-releasing, monoanionic tris(phosphino)borate ligands [PhBP3] and [PhBPiPr3] ([PhBP3] = [PhB(CH2PPh2)3]-, [PhBPiPr3] = [PhB(CH2PiPr2)3]-) have been synthesized to explore fundamental aspects of their coordination chemistry. The pseudotetrahedral, divalent halide complexes [PhBP3]NiCl (1), [PhBP3]NiI (2), and [PhBPiPr3]NiCl (3) were prepared by the metalation of [PhBP3]Tl or [PhBPiPr3]Tl with (Ph3P)2NiCl2, NiI2, and (DME)NiCl2 (DME = 1,2-dimethoxyethane), respectively. Complex 1 is a versatile precursor to a series of complexes accessible via substitution reactions including [PhBP3]Ni(N3) (4), [PhBP3]Ni(OSiPh3) (5), [PhBP3]Ni(O-p-tBu-Ph) (6), and [PhBP3]Ni(S-p-tBu-Ph) (7). Complexes 2-5 and 7 have been characterized by X-ray diffraction (XRD) and are pseudotetrahedral monomers in the solid state. Complex 1 reacts readily with oxygen to form the four-electron-oxidation product, [[PhB(CH2POPh2)2(CH2PPh2)]NiCl] (8A or 8B), which features a solid-state structure that is dependent on its method of crystallization. Chemical reduction of 1 using Na/Hg or other potential 1-electron reductants generates a product that arises from partial ligand degradation, [PhBP3]Ni(eta2-CH2PPh2) (9). The more sterically hindered chloride 3 reacts with Li(dbabh) (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) to provide the three-coordinate complex [kappa2-PhBPiPr3]Ni(dbabh) (11), also characterized by XRD. Chemical reduction of complex 1 in the presence of L-type donors produces the tetrahedral Ni(I) complexes [PhBP3]Ni(PPh3) (12) and [PhBP3]Ni(CNtBu) (13). Reduction of 3 following the addition of PMe3 or tert-butyl isocyanide affords the Ni(I) complexes [PhBPiPr3]Ni(PMe3) (14) and [PhBPiPr3]Ni(CNtBu) (15), respectively. The reactivity of these [PhBP3]NiIL and [PhBPiPr3]NiIL complexes with respect to oxidative group transfer reactions from organic azides and diazoalkanes is discussed. The zerovalent nitrosyl complex [PhBP3]Ni(NO) (16) is prepared by the reaction of 1 with excess NO or by treating 12 with stoichiometric NO. The anionic Ni(0) complexes [[kappa2-PhBP3]Ni(CO)2][nBu4N] (17) and [[kappa2-PhBPiPr3]Ni(CO)2][ASN] (18) (ASN = 5-azoniaspiro[4.4]nonane) have been prepared by reacting [PhBP3]Tl or [PhBPiPr3]Tl with (Ph3P)2Ni(CO)2 in the presence of R4NBr. The photolysis of 17 appears to generate a new species consistent with a zerovalent monocarbonyl complex which we tentatively assign as [[PhBP3]Ni(CO)][nBu4N], although complete characterization of this complex has been difficult. Finally, theoretical DFT calculations are presented for the hypothetical low spin complexes [PhBP3]Ni(NtBu), [PhBPiPr3]Ni(NtBu), [PhBPiPr3]Ni(NMe), and [PhBPiPr3]Ni(N) to consider what role electronic structure factors might play with respect to the relative stability of these species.

Utilization of hydrogen bonds to stabilize M-O(H) units: synthesis and properties of monomeric iron and manganese complexes with terminal oxo and hydroxo ligands.

Non-heme iron and manganese species with terminal oxo ligands are proposed to be key intermediates in a variety of biological and synthetic systems; however, the stabilization of these types of complexes has proven difficult because of the tendency to form oxo-bridged complexes. Described herein are the design, isolation, and properties for a series of mononuclear Fe(III) and Mn(III) complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethyl]aminato ([H(3)1](3-)), which creates a protective hydrogen bond cavity around the M(III)-O(H) units (M(III) = Fe and Mn). The M(III)-O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the M(III)-O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [Fe(III)H(3)1(O)](2-) complex also can be made using sulfoxides. These findings support the proposal of a high valent M(IV)-oxo species as an intermediate during dioxygen cleavage. Isotopic labeling studies show that oxo ligands in the [M(III)H(3)1(O)](2-) complexes come directly from the cleavage of dioxygen: for [Fe(III)H(3)1(O)](2-) the nu(Fe-(16)O) = 671 cm(-1), which shifts 26 cm(-1) in [Fe(III)H(3)1((18)O)](2-) (nu(Fe-(18)O) = 645 cm(-1)); a nu(Mn-(16)O) = 700 cm(-1) was observed for [Mn(III)H(3)1((16)O)](2-), which shifts to 672 cm(-1) in the Mn-(18)O isotopomer. X-ray diffraction studies show that the Fe-O distance is 1.813(3) A in [Fe(III)H(3)1(O)](2-), while a longer bond is found in [Fe(III)H(3)1(OH)](-) (Fe-O at 1.926(2) A); a similar trend was found for the Mn(III)-O(H) complexes, where a Mn-O distance of 1.771(5) A is observed for [Mn(III)H(3)1(O)](2-) and 1.873(2) A for [Mn(III)H(3)1(OH)](-). Strong intramolecular hydrogen bonds between the urea NH groups of [H(3)1](3-) and the oxo and oxygen of the hydroxo ligand are observed in all the complexes. These findings, along with density functional theory calculations, indicate that a single sigma-bond exists between the M(III) centers and the oxo ligands, and additional interactions to the oxo ligands arise from intramolecular H-bonds, which illustrates that noncovalent interactions may replace pi-bonds in stabilizing oxometal complexes.

Isolation of monomeric Mn(III/II)-H and Mn(III)-complexes from water: evaluation of O-H bond dissociation energies.

The syntheses and properties of the monomeric [MnIII/IIH31(OH)]-/2- and [MnIIIH31(O)]2- complexes are reported, where [H31]3- is the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethyl)]aminato. Isotope-labeling studies with H218O confirmed that water is the source of the terminal oxo and oxygen in the hydroxo ligand. The molecular structures of the [MnIIH31(OH)]2- and [MnIIIH31(O)]2- complexes were determined by X-ray diffraction methods and show that each complex has trigonal bipyramidal coordination geometry. The MnIII-O distance in [MnIIIH31(O)]2- is 1.771(4) A, which is lengthened to 2.059(2) A in [MnIIH31(OH)]2-. Structural studies also show that [H31]3- provides a hydrogen-bond cavity that surrounds the MnIII-O(H) units. Using a thermodynamic approach, which requires pKa and redox potentials, bond dissociation energies of 77(4) and 110(4) kcal/mol were calculated for [MnIIH31(O-H)]2- and [MnIIIH31(O-H)]-, respectively. The calculated value of 77 kcal/mol for the [MnIIH31(O-H)]2- complex is supported by the ability of [MnIIIH31(O)]2- complex to cleave C-H bonds with bond energies of <80 kcal/mol.