Alexander S. Novikov

Difference in Energy between Two Distinct Types of Chalcogen Bonds Drives Regioisomerization of Binuclear (Diaminocarbene)PdII Complexes

The reaction of cis-[PdCl2(CNXyl)2] (Xyl = 2,6-Me2C6H3) with various 1,3-thiazol- and 1,3,4-thiadiazol-2-amines in chloroform gives a mixture of two regioisomeric binuclear diaminocarbene complexes. For 1,3-thiazol-2-amines the isomeric ratio depends on the reaction conditions and kinetically (KRs) or thermodynamically (TRs) controlled regioisomers were obtained at room temperature and on heating, respectively. In CHCl3 solutions, the isomers are subject to reversible isomerization accompanied by the cleavage of Pd-N and C-N bonds in the carbene fragment XylNCN(R)Xyl. Results of DFT calculations followed by the topological analysis of the electron density distribution within the formalism of Baders theory (AIM method) reveal that in CHCl3 solution the relative stability of the regioisomers (ΔGexp = 1.2 kcal/mol; ΔGcalcd = 3.2 kcal/mol) is determined by the energy difference between two types of the intramolecular chalcogen bonds, viz. S···Cl in KRs (2.8-3.0 kcal/mol) and S···N in TRs (4.6-5.3 kcal/mol). In the case of the 1,3,4-thiadiazol-2-amines, the regioisomers are formed in approximately equal amounts and, accordingly, the energy difference between these species is only 0.1 kcal/mol in terms of ΔGexp (ΔGcalcd = 2.1 kcal/mol). The regioisomers were characterized by elemental analyses (C, H, N), HRESI+-MS and FTIR, 1D (1H, 13C{1H}) and 2D (1H,1H-COSY, 1H,1H-NOESY, 1H,13C-HSQC, 1H,13C-HMBC) NMR spectroscopies, and structures of six complexes (three KRs and three TRs) were elucidated by single-crystal X-ray diffraction.

Fine-tuning halogen bonding properties of diiodine through halogen–halogen charge transfer – extended [Ru(2,2′-bipyridine)(CO)2X2]·I2 systems (X = Cl, Br, I)

The current paper introduces the use of carbonyl containing ruthenium complexes, [Ru(bpy)(CO)2X2] (X = Cl, Br, I), as halogen bond acceptors for a I2 halogen bond donor. In all structures, the metal coordinated halogenido ligand acts as the actual halogen bond acceptor. Diiodine, I2, molecules are connected to the metal complexes through both ends of the molecule forming bridges between the complexes. Due to the charge transfer from Ru–X to I2, formation of the first Ru–X⋯I2 contact tends to generate a negative charge on I2 and redistribute the electron density anisotropically. If the initial Ru–X⋯IA–IB interaction causes a notable change in the electron density of I2, the increased negative charge accumulates on the second iodine, IB. The increased negative charge on IB reduces its ability to act as a halogen bond donor. With the [Ru(bpy)(CO)2Cl2] complex, the electron density of the I2 molecule remains isotropic and it acts as a symmetrical halogen bond donor for two metal complexes. With [Ru(bpy)(CO)2Br2] and [Ru(bpy)(CO)2I2], the Ru–X⋯IA–IB⋯X–Ru bridges are unsymmetrical with a stronger and shorter Ru–X⋯IA contact and a weaker and longer Ru–X⋯IB contact. In these cases, the negative charge is accumulated on the more weakly bonded IB atoms. QTAIM calculations were used to analyze the strength of the interactions and charge distribution in the metal complex and I2 molecule in detail. In accordance with the experimental data, the QTAIM analyses show that the charge difference between the two ends of the I2 molecule is increased in the order Ru–Cl⋯I2 < Ru–Br⋯I2 < Ru–I⋯I2.

A family of heterotetrameric clusters of chloride species and halomethanes held by two halogen and two hydrogen bonds

Two previously reported 1,3,5,7,9-pentaazanona-1,3,6,8-tetraenate (PANT) chloride platinum(II) complexes [PtCl{HNC(R)NCN[C(Ph)C(Ph)]CNC(R)NH}] (R = tBu 1, Ph 2) form solvates with halomethanes 1·1¼CH2Cl2, 1·1⅖CH2Br2, and 2·CHCl3. All these species feature novel complex-solvent heterotetrameric clusters, where the structural units are linked simultaneously by two C–X⋯Cl–Pt (X = Cl, Br) halogen and two C–H⋯Cl–Pt hydrogen bonds. The geometric parameters of these weak interactions were determined using single-crystal XRD, and the natures of the XBs and HBs in the clusters were studied for the isolated model systems (1)2·(CH2Cl2)2, (1)2·(CH2Br2)2, and (2)2·(CHCl3)2 using DFT calculations and Baders AIM analysis. The evaluated energies of the weak interactions are in the range 0.9–3.0 kcal mol−1. The XBs and HBs in the reported clusters are cooperative. In the cases of (1)2·(CH2Cl2)2 and (1)2·(CH2Br2)2, the contribution of the HBs to the stabilization of the system is dominant, whereas for (2)2·(CHCl3)2 contributions of both types of the non-covalent interactions are almost the same. Crystal packing and other forces such as, e.g. dipole–dipole interactions, also affect the formation of the clusters.

Diiodomethane as a halogen bond donor toward metal-bound halides

A 1,3,5,7,9-pentaazanona-1,3,6,8-tetraenate (PANT) chloride platinum(II) complex (1) was obtained via the metal-mediated double coupling of 2,3-diphenylmaleimidine with both nitrile ligands in trans-[PtCl2(NCtBu)2]. Compound 1 was then co-crystallized with diiodomethane forming solvate 1·½CH2I2. The XRD experiment reveals that this solvate displays the halogen bonds H2C(I)–I⋯Cl–Pt and hydrogen bonds I2C(H)–H⋯Cl–Pt, which join two complex and one CH2I2 molecules in a heterotrimeric supramolecular cluster. Inspection of the CCDC database reveals only one example of the halogen bond H2C(I)–I⋯I–Pt between the CH2I2 molecule and metal-coordinated halide in the structure of VEMWOA. In VEMWOA, CH2I2 serves solely as a halogen bond donor with no hydrogen bond contribution. Results of the Hirshfeld surface analysis and DFT calculations (M06/DZP-DKH level of theory) followed by topological analysis of the electron density distribution within the formalism of Baders theory (QTAIM method) for both 1·½CH2I2 and VEMWOA confirmed the formation of these weak interactions. The evaluated energies of halogen bonds involving CH2I2 are in the 2.2–2.8 kcal mol−1 range.

Trinuclear (aminonitrone)ZnII complexes as key intermediates in zinc(ii)-mediated generation of 1,2,4-oxadiazoles from amidoximes and nitriles

Aliphatic and aromatic amidoximes RC(NH2)NOH (R = Et, tBu, Ph, o-ClC6H4) react with Zn(OAc)2·2H2O in Me2CO giving [Zn(OAc)2{RC(NH2)NOH}2] complexes bearing N-bound amidoximes, which are involved in a moderate strength (7.3–11.9 kcal mol−1 by the DFT calculations) intramolecular resonance-assisted hydrogen bonding between the oxime HO group and the oxo group of the acetate ligand. The complexes [Zn(OAc)2{RC(NH2)NOH}2] react with excess Zn(OTf)2 in acetone accomplishing trinuclear species [Zn3(μ2-OAc)2{μ2-RC(NH2)N(H)O}4(H2O)6](OTf)4 featuring both O-ligated amidoximes—stabilized in the aminonitrone tautomeric form—and bridging acetate ligands. The aminonitrone trinuclear species were also prepared directly via the reaction of the amidoximes with Zn(OTf)2 in EtOAc; ethyl acetate in this reaction plays the role of the acetate donor and OAc− is generated in situ via ZnII-mediated hydrolysis of EtOAc. Although [Zn(OAc)2{RC(NH2)NOH}2] are inactive toward dimethylcyanamide, the [Zn3(μ2-OAc)2{μ2-RC(NH2)N(H)O}4(H2O)6](OTf)4 complexes readily react with Me2NCN giving, as a result of ZnII-mediated amidoxime–cyanamide coupling, the O-carbamidine amidoxime complexes [Zn(OTf)2{RC(NH2)NOC(NMe2)NH}2]. All synthesized compounds were characterized by HRESI-MS, FTIR, 1H-, CP-MAS TOSS 13C{1H}-, and 13C{1H} NMR, and additionally by single-crystal X-ray diffraction for eight species. Different types of non-covalent interactions in the obtained solid-state structures were studied by DFT calculations (M06-2X/6-311+G(d,p) level of theory) and topological analysis of the electron density distribution within the formalism of Baders theory (QTAIM method).

Addition of N-nucleophiles to gold(iii)-bound isocyanides leading to short-lived gold(iii) acyclic diaminocarbene complexes

Reaction of [AuCl3(CNR1)] (R1 = Xyl, Cy, (S)-CHMePh) with amines unexpectedly proceeds via the redox pathway giving gold(I)–isocyanides and imines, while the addition of benzophenone hydrazone to the isocyanide ligand in [AuCl3(CNR1)] at RT leads to short-lived gold(III) acyclic diaminocarbene complexes [AuCl3{C(NHNCPh2)NHR1}].

Metal-mediated generation of triazapentadienate-terminated di- and trinuclear μ2-pyrazolate NiII species and control of their nuclearity

1,3,5-Triazapentadienate-terminated di- and trinuclear nickel(II) complexes featuring bridging azolate ligands, [Ni2(μ2-azolate)2(TAP)2] (TAP = HC(OMe)NC(OMe)H; azole = 3,5-Me2pyrazole 2, 3,5-Ph2pyrazole 3) and [Ni3(μ2-azolate)4(TAP)2] (azole = 3,5-Me2pyrazole 4, indazole 5), were obtained from systems Ni2+/NCNR2/azole systems in MeOH. The terminal TAP ligands in the [Ni2(μ2-azolate)2(TAP)2] and [Ni3(μ2-azolate)4(TAP)2] species originate from the previously unreported cascade NiII-mediated and chelation-driven reaction between cyanamides and methanol. The oligomeric species and also [Ni(TAP)2] (1) are subject to interconversions that depend on the reactants involved and the reaction conditions. The control of the nuclearity of the complexes can be achieved by changing the amount of azoles or by their protonation, alteration of the steric hindrance of the substituents in the heterocycles, and by changing the reaction temperature. Complexes 1–4 were characterized using elemental (C, H, N) analyses, 1H, 13C{1H} NMR, FTIR, HRESI-MS, TG-DTA, X-ray crystallography, and 5 was characterized using HRESI-MS and X-ray crystallography. Unconventional metallophilic contacts NiII⋯NiII were observed in dimer 3 in the solid state (the distance for Ni⋯Ni is 2.99 A, whereas the double Bondis vdW radius for Ni is 3.26 A) and the reality of these interactions was confirmed theoretically by the topological analysis of the electron density distribution (AIM method). The estimated energy for these non-covalent Ni⋯Ni interactions (ca. 4 kcal mol−1) fills the gap in the reported energies of the metal⋯metal interactions in a series comprising of NiII⋯NiII (this work), PdII⋯PdII (4.3–6.0 kcal mol−1), and PtII⋯PtII (3.9–11.7 kcal mol−1).

Halogen Contacts‐Induced Unusual Coloring in BiIII Bromide Complex: Anion‐to‐Cation Charge Transfer via Br⋅⋅⋅Br Interactions

A yellow bromobismuthate {(2-BrPy)2 H}[BiBr4 ] (1, 2-BrPy=2-bromopyridinium) transforms into the unusually deeply colored cherry-red (2-BrPyH)2 [BiBr5 ] (2). A combination of structural studies and theoretical calculations confirms that the appearance of short non-covalent Br⋅⋅⋅Br interactions (≈3.3 Å) in 2 is responsible for the anion-to-cation charge transfer (LP(Brligand )→σ*(Br-C)), yielding dramatic changes in optical behavior. This effect opens the way towards novel halogen bonding-templated halometalate-based hybrid materials with enhanced optical properties.

PdII-mediated integration of isocyanides and azide ions might proceed via formal 1,3-dipolar cycloaddition between RNC ligands and uncomplexed azide

Reaction between equimolar amounts of trans-[PdCl(PPh3)2(CNR)][BF4] (R = t-Bu 1, Xyl 2) and diisopropylammonium azide 3 gives the tetrazolate trans-[PdCl(PPh3)2(N4t-Bu)] (67%, 4) or trans-[PdCl(PPh3)2(N4Xyl)] (72%, 5) complexes. 4 and 5 were characterized by elemental analyses (C, H, N), HRESI+-MS, 1H and 13C{1H} NMR spectroscopies. In addition, the structure of 4 was elucidated by a single-crystal X-ray diffraction. DFT calculations showed that the mechanism for the formal cycloaddition (CA) of N3− to trans-[PdCl(PH3)2(CNMe)]+ is stepwise. The process is both kinetically and thermodynamically favorable and occurs via the formation of an acyclic NNNCN-intermediate. The second step of the formal CA, i.e. cyclization, is rate limiting. Despite the fact that the substitution of CNMe by the N3− ligand is slightly thermodynamically favorable, we were unable to find paths on the potential energy surface for hypothetical CA between uncomplexed isocyanide and palladium-bound azide. Thus, we believe that the experimentally observed palladium tetrazolate complexes are, in fact, generated from the negatively charged uncomplexed azide and the positively charged metal-bound isocyanide species, and this reaction path is favorable from the viewpoint of Coulomb attraction.

Oxidation of olefins with H2O2 catalysed by salts of group III metals (Ga, In, Sc, Y and La): epoxidation versus hydroperoxidation

The catalytic activity of aqua complexes of the group III metals [M(H2O)n]3+ (M = Ga, In, Sc, n = 6; M = Y, n = 8; M = La, n = 9) towards the oxidation of olefins with H2O2 was investigated in detail by theoretical (DFT) methods. It was predicted and then confirmed in a preliminary experiment that these complexes formed from simple soluble salts in aqueous medium are able to efficiently catalyse the olefin oxidation. The reaction occurs via two competitive reaction channels which are realized concurrently, i.e. (i) hydroperoxidation of the allylic C atom(s) via a radical Fenton-like mechanism involving HO˙ radicals and leading to alkyl hydroperoxides ROOH and (ii) epoxidation of the CC bond through a one-step mechanism involving oxygen transfer from the hydroperoxo ligand in an active catalytic form [M(H2O)n−k(OOH)]2+ (M = Ga, In, Y, La, k = 2; M = Sc, k = 1) to the olefin molecule and leading to epoxides and/or trans-diols. Other concerted and stepwise mechanisms of the epoxidation were also considered but found less favourable.