Multiple Hydrogen Bond Stabilization of a Sandwich Complex of Sulfate between Two Macrocyclic Tetraamides

[Au(en)Cl2]Cl·2H2O, where en = ethylenediamine (1,2-diaminoethane), has been synthesized, and its structure has been solved for the first time by the single-crystal X-ray diffraction method. The complex has square-planar geometry about AuIII, and the anionic Cl- is located in the apical position and at a distance of 3.3033(10) Å compared to 2.2811(9) and 2.2836(11) Å for the coordinated Cl-. [Au(en)Cl2]Cl·2H2O belongs to the space group Pbca with a = 11.5610(15) Å, b = 12.6399(17) Å, c = 13.2156(17) Å, α = β = γ = 90°, and Z = 8. Bond lengths of Au−N are 2.03 Å. [Au(en)Cl2]Cl·2H2O is less thermally stable than [Au(en)2]Cl3 because of the replacement of two Cl ligands by a second en ligand in the latter. Cyclic voltammetry shows that the formal potential of AuIII/Au0 becomes more negative in the series [AuCl4]-, [Au(en)Cl2]+, and [Au(en)2]3+. 1H, 13C, and 31P NMR reveal that in an aqueous solution [Au(en)Cl2]+ bonds to guanosine 5‘-monophosphate, 5‘-GMP (1:1 mole ratio), via N(7), although the stability is not very high. NMR data also indicate that N(7)−O(6) or N(7)−phosphate 5‘-GMP chelation, as found in some gold(III) nucleotide complexes, is not present. The gold(III) complex undergoes hydrolysis at pH

2.5−3.0 and, therefore, N1 coordination to 5‘-GMP is not observed. No direct coordination between 5‘-GMP and [Au(en)2]Cl3 is observed.

NO is well-known for its trans effect. NO binding to ferrous hemes of the form (por)Fe(L) (L = neutral N-based ligand) to give the {FeNO}7 (por)Fe(NO)(L) product results in a lengthening of the axial trans Fe–L bond. In contrast, NO binding to the ferric center in [(por)Fe(L)]+ to give the {FeNO}6 [(por)Fe(NO)(L)]+ product results in a shortening of the trans Fe–L bond. NO binding to both ferrous and ferric centers involves the lowering of their spin states. Density functional theory (DFT) calculations were used to probe the experimentally observed trans-bond shortening in some NO adducts of ferric porphyrins. We show that the strong σ antibonding interaction of dz2 and the axial (L) ligand p orbitals present in the Fe(II) systems is absent in the Fe(III) systems, as it is now in an unoccupied orbital. This feature, combined with a lowering of spin state upon NO binding, provides a rationale for the observed net trans-bond shortening in the {FeNO}6 but not the {FeNO}7 derivatives.

Convergent preparative routes to new urea−pyrazolate dinucleating ligands are described. Metal complexes of these ligands have hydrogen bond donors that are proximal to the metal centers that interact with other coordinated species. This is exemplified by CoII dimers with CoII−μ-Cl−CoII motifs, in which the chloro ligand is involved in four intramolecular hydrogen bonds. These noncovalent interactions appear to influence the CoII−Cl bonds, which are unusually long, having lengths greater than 2.5 Å.

Metal–carbon bonds are known to form during the metal-catalyzed transformations of various organic compounds such as phenylhydrazines by the heme-containing proteins cytochrome P450, hemoglobin, and myoglobin. The preparation and characterization of synthetic organometallic porphyrins of the group 8 metals are thus of interest, and their properties help enlighten the general discussion of metalloporphyrin–carbon bond chemistry. We have prepared a representative set of (por)Ru(NO)R compounds (por = T(p-OMe)PP, T(p-CF3)PP; R = Me, Et) containing Ru–alkyl bonds trans to NO. We have determined the X-ray crystal structure of (T(p-OMe)PP)Ru(NO)Et, which represents the first crystal structure reported for any organometallic nitrosyl porphyrin with an alkyl ligand trans to NO; the structure reveals a significantly bent RuNO moiety at 153° in this {RuNO}6 compound. We have characterized the redox behavior of the (T(p-OMe)PP)Ru(NO)-containing compounds by cyclic voltammetry and infrared spectroelectrochemistry, and we have determined that the first oxidations are porphyrin-centered.

Metal ion function depends on the regulation of properties within the primary and second coordination spheres. An approach toward studying the structure−function relationships within the secondary coordination sphere is to construct a series of synthetic complexes having constant primary spheres but structurally tunable secondary spheres. This was accomplished through the development of hybrid urea−carboxamide ligands that provide varying intramolecular hydrogen bond (H-bond) networks proximal to a metal center. Convergent syntheses prepared ligands [(N‘-tert-butylureayl)-N-ethyl]-bis(N‘ ‘-R-carbamoylmethyl)amine (H41R) and bis[(N‘-tert-butylureayl)-N-ethyl]-(N‘ ‘-R-carbamoylmethyl)amine (H52R), where R = isopropyl, cyclopentyl, and (S)-(−)-α-methylbenzyl. The ligands with isopropyl groups H41iPr and H52iPr were combined with tris[(N‘-tert-butylureayl)-N-ethyl]amine (H6buea) and bis(N-isopropylcarbamoylmethyl)amine (H30iPr) to prepare a series of Co(II) complexes with varying H-bond donors. [CoIIH22iPr]- (two H-bond donors), [CoIIH1iPr]- (one H-bond donor), and [CoII0iPr]- (no H-bond donors) have trigonal monopyramidal primary coordination spheres as determined by X-ray diffraction methods. In addition, these complexes have nearly identical optical and EPR properties that are consistent with S = 3/2 ground states. Electrochemical studies show a linear spread of 0.23 V in anodic potentials (Epa) with [CoIIH22iPr]- being the most negative at −0.385 V vs [Cp2Fe]+/[Cp2Fe]. The properties of [CoIIH3buea]- (H3buea, tris[(N‘-tert-butylureaylato)-N-ethyl]aminato that has three H-bond donors) appears to be similar to that of the other complexes based on spectroscopic data. [CoIIH3buea]- and [CoIIH22iPr]- react with 0.5 equiv of dioxygen to afford [CoIIIH3buea(OH)]- and [CoIIIH22iPr(OH)]-. Isotopic labeling studies confirm that dioxygen is the source of the oxygen atom in the hydroxo ligands:  [CoIIIH3buea(16OH)]- has a ν(O−H) band at 3589 cm-1 that shifts to 3579 cm-1 in [CoIIIH3buea(18OH)]-; [CoIIIH22iPr(OH)]- has ν(16O−H) = 3661 and ν(18O−H) = 3650 cm-1. [CoIIH1iPr]- does not react with 0.5 equiv of O2; however, treating [CoIIH1iPr]- with excess dioxygen initially produces a species with an X-band EPR signal at g = 2.0 that is assigned to a Co−O2 adduct, which is not stable and converts to a species having properties similar to those of the CoIII−OH complexes. Isolation of this hydroxo complex in pure form was complicated by its instability in solution (kint = 2.5 × 10-7 M min-1). Moreover, the stability of the CoIII−OH complexes is correlated with the number of H-bond donors within the secondary coordination sphere; [CoIIIH3buea(OH)]- is stable in solution for days, whereas [CoIIIH22iPr(OH)]- decays with a kint = 5.9 × 10-8 M min-1. The system without any intramolecular H-bond donors [CoII0iPr]- does not react with dioxygen, even when O2 is in excess. These findings indicate a correlation between dioxygen binding/activation and the number of H-bond donors within the secondary coordination sphere of the cobalt complexes. Moreover, the properties of the secondary coordination sphere affect the stability of the CoIII−OH complexes with [CoIIIH3buea(OH)]- being the most stable. We suggest that the greater number of intramolecular H-bonds involving the hydroxo ligand reduces the nucleophilicity of the CoIII−OH unit and reinforces the cavity structure, producing a more constrained microenvironment around the cobalt ion.

Multiple deuterium exchange between DMSO-d6 and amide hydrogens in two hexaamido cryptand fluoride receptors has been verified by 19F and 2H NMR and FAB mass spectral studies. Structural results for one of the complexes indicate a tricapped trigonal prism hydrogen bond coordination geometry around an encapsulated fluoride, with hydrogen bonds from fluoride to six amide and three phenyl hydrogens.

C−N bond activation of the N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene occurs with Ru(PPh3)3(CO)HCl to give the C-2 bound 1-isopropyl-4,5-dimethylimidazol-2-ylidene complex Ru(C−IiPrHMe2)(PPh3)2(CO)HCl via loss of propene. In the presence of free carbene, this undergoes tautomerism to the corresponding imidazole compound Ru(N−IiPrHMe2)(PPh3)2(CO)HCl.

Crystal engineering principles were used to design three new co-crystals of paracetamol. A variety of potential co-crystal formers were initially identified from a search of the Cambridge Structural Database for molecules with complementary hydrogen-bond forming functionalities. Subsequent screening by powder X-ray diffraction of the products of the reaction of this library of molecules with paracetamol led to the discovery of new binary crystalline phases of paracetamol with trans-1,4-diaminocyclohexane (1); trans-1,4-di(4-pyridyl)ethylene (2); and 1,2-bis(4-pyridyl)ethane (3). The co-crystals were characterized by IR spectroscopy, differential scanning calorimetry, and 1H NMR spectroscopy. Single crystal X-ray structure analysis reveals that in all three co-crystals the co-crystal formers (CCF) are hydrogen bonded to the paracetamol molecules through O–H···N interactions. In co-crystals (1) and (2) the CCFs are interleaved between the chains of paracetamol molecules, while in co-crystal (3) there is an additional N–H···N hydrogen bond between the two components. A hierarchy of hydrogen bond formation is observed in which the best donor in the system, the phenolic O–H group of paracetamol, is preferentially hydrogen bonded to the best acceptor, the basic nitrogen atom of the co-crystal former. The geometric aspects of the hydrogen bonds in co-crystals 1–3 are discussed in terms of their electrostatic and charge-transfer components.

The conversion of inorganic NOx species to organo-N compounds is an important component of the global N-cycle. Reaction of a C-based nucleophile, namely the phenyl anion, with the ferric heme nitrosyl [(OEP)Fe(NO)(5-MeIm)]+ generates a mixture of the C-nitroso derivative (OEP)Fe(PhNO)(5-MeIm) and (OEP)Fe(Ph). The related reaction with [(OEP)Ru(NO)(5-MeIm)]+ generates the (OEP)Ru(PhNO)(5-MeIm) product. Reactions with the N-based nucleophile diethylamide results in the formation of free diethylnitrosamine, whereas the reaction with azide results in N2O formation; these products derive from attack of the nucleophiles on the bound NO groups. These results provide the first demonstrations of C–N and N–N bond formation from attack of C-based and N-based nucleophiles on synthetic ferric-NO hemes.