Phenylselenenylalkanes,their adducts with the dirhodium complex Rh2(MTPA)4 and ligand exchange mechanisms in solution as studied by 1H, 13C and 77Se NMR spectroscopy

Twelve secondary phenylselenenylalkanes and ‐cycloalkanes were studied by ¹H, ¹³C and ⁷⁷Se NMR spectroscopy in the presence of the chiral dirhodium complex Rh₂[(R)‐MTPA]₄ [Rh–Rh; MTPA‐H = (R)‐(+)‐methoxytrifluoromethylphenylacetic acid, Mosher's acid]. The 1 : 1 and 2 : 1 adducts were identified in solution at low temperatures. Two different mechanisms of ligand exchange, ‘switch’ and ‘replacement,’ were characterized and their energy barriers estimated and steric congestion during the exchange transitions is discussed. Coordination‐induced shifts Δδ(⁷⁷Se) are generally negative (shielding). For menthone bis(phenylselenoacetal) (7), these values indicate that a selection of the two selenium atoms occurs showing that 7 prefers complexation at the equatorial selenium atom whereas the axial selenium atom is hardly involved. Copyright © 2003 John Wiley & Sons, Ltd.     

Adamantanes as spherical nanosondes in adducts with a chiral dirhodium complex-discriminating enantiomers and probing spatial proximities

Three different kinds of substituted chiral adamantane molecules—adamantanones, dioxolanoadamantanes and dithiolano—adamantanes—were studied in the dirhodium experiment (NMR measurement with 1:1 molar mixtures with Rh^(II)₂[(R)‐(+)‐MTPA]₄ in CDCl₃). Their different behavior in adduct formation is described, and the possibility of determining enantiomeric purities and absolute configurations is explored. Detailed inspection of one‐ and two‐dimensional NMR experiments allowed for an interpretation of steric and electronic intra‐adduct interaction showing that the phenyl groups of Rh* tend to enwrap the bound adamantane ligand so that through‐space effects over a range of 6–7 Å away from the binding rhodium atom can be observed. Even slight differences in the relative orientation of phenyl groups can be monitored when comparing diastereomeric adducts via NMR signal dispersion. Copyright © 2011 John Wiley & Sons, Ltd.     

A DFT Study on the Mechanism of Rh2II,II‐Catalyzed Intramolecular Amidation of Carbamates

The potential‐energy surfaces of the reactions of dirhodium tetracarboxylate (Rh₂^II,II) catalyzed nitrene (NR) insertion into C H bonds were examined by a DFT computational study. A pure Becke exchange functional (B88) rather than a hybrid exchange functional (B3, BHandH) was found to be appropriate for the calculation of the energy difference between the singlet and triplet Rh₂^II,II–NH nitrene species. Rh₂^II,II–NR¹ (R¹=(S)‐2‐methyl‐1‐butylformyl) is thermodynamically more favorable with a free energy lower than that of Rh₂^II,II–N(PhI)R¹. The singlet and triplet states of Rh₂^II,II–NR¹ have similar stability. Singlet Rh₂^II,II–NR¹ undergoes a concerted NR insertion into the C H bond with simultaneous formation of the N H and N C bonds during C H bond cleavage; triplet Rh₂^II,II–NR¹ undergoes H atom abstraction to produce a diradical, followed by subsequent bond formation by diradical recombination. The singlet pathway is favored over the triplet in the context of the free energy of activation and leads to the retention of the chirality of the C atom in the NR insertion product. The reactivities of the C H bonds toward the nitrene‐insertion reaction follow the order tertiary>secondary>primary. Relative reaction rates were calculated for the six reaction pathways examined in this work.     

A novel heterobidentate chiral phosphine and its coordination chemistry in transition metal clusters

The optically active ligand R,R-PHAZAN (1,3-bis[(1R)-1-Phenylethyl]-2-(2-thienyl)-1,3,2-diazaphospholane) has been prepared and the products resulting from the reactions with Rh₆(CO)₁₅NCMe, H₃RhOs₃(CO)₁₂, and H₄Ru₄(CO)₁₂ have been investigated by X-ray crystallography and a variety of multinuclear NMR methods. X-ray studies show that PHAZAN can behave as a bidentate ligand in Rh₆(CO)₁₄(μ₂,κ²-R,R-PHAZAN) (with coordination through P and S) or a monodentate ligand (through P coordination) in H₄Ru₄(CO)₁₁(κ¹-R,R-PHAZAN) and NMR studies show that these structures are retained in solution. In Rh₆(CO)₁₄(μ₂,κ²-R,R-PHAZAN), edge-bridging coordination of PHAZAN results in the formation of an additional two novel chiral centres and these are observed in solution. Reaction of PHAZAN with H₃RhOs₃(CO)₁₂ results in cleavage of the thienyl group and formation of the phosphido cluster, H₂RhOs₃(CO)₁₁(μ₂-PNN), (PNN = 1,3-bis-(1-phenylethyl)-[1,3,2]diazaphospholidine-2-yl). A variety of NMR measurements show that the hydride site-occupancies in the solid state are retained in solution and there is evidence for interaction of an ortho-phenyl hydrogen and a hydride through “dihydrogen” bonding.     

Complexation of selenium to (R)‐Rh2(MTPA)4: thermodynamics and stoichiometry

Variable‐temperature ¹H and ⁷⁷Se NMR data for 3‐phenylselenenyl‐1‐phenyl‐1‐propene (1) in the presence of Rh₂(MTPA)₄ (Rh*) prove that the equilibria are strongly shifted towards the adduct Rh*···1; free selenide molecules cannot be detected as long as uncomplexed rhodium atoms are available. In the case of excess Rh*, both 1 : 2 and 1 : 1 adducts (Rh* vs 1) are formed, and the latter is slightly favoured. With excess selenide, the system strongly favours the complexation of two selenide molecules (1 : 2 adduct), i.e. one at each rhodium atom. In this situation, intermolecular selenide exchange can be monitored by variable‐temperature ¹H NMR spectroscopy and the energy barrier is estimated to be 54–55 kJ mol^?1. Copyright © 2001 John Wiley & Sons, Ltd.     

3‐Rhoda‐1,2‐diazacyclopentanes: A Series of Novel Metallacycle Complexes Derived From CN Functionalization of Ethylene

Rh‐containing metallacycles, [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NR)₂‐]Cl; TPA=N,N,N,N‐tris(2‐pyridylmethyl)amine have been accessed through treatment of the Rh^I ethylene complex, [(TPA)Rh(η²‐CH₂CH₂)]Cl ([ 1 ]Cl) with substituted diazenes. We show this methodology to be tolerant of electron‐deficient azo compounds including azo diesters (RCO₂N?NCO₂R; R=Et [ 3 ]Cl, R=iPr [ 4 ]Cl, R=tBu [ 5 ]Cl, and R=Bn [ 6 ]Cl) and a cyclic azo diamide: 4‐phenyl‐1,2,4‐triazole‐3,5‐dione (PTAD), [ 7 ]Cl. The latter complex features two ortho‐fused ring systems and constitutes the first 3‐rhoda‐1,2‐diazabicyclo[3.3.0]octane. Preliminary evidence suggests that these complexes result from N–N coordination followed by insertion of ethylene into a [Rh]?N bond. In terms of reactivity, [ 3 ]Cl and [ 4 ]Cl successfully undergo ring‐opening using p‐toluenesulfonic acid, affording the Rh chlorides, [(TPA)Rh^III(Cl)(κ¹‐(C)‐CH₂CH₂(NCO₂R)(NHCO₂R)]OTs; [ 13 ]OTs and [ 14 ]OTs. Deprotection of [ 5 ]Cl using trifluoroacetic acid was also found to give an ethyl substituted, end‐on coordinated diazene [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NH)₂‐]⁺ [ 16 ]Cl, a hitherto unreported motif. Treatment of [ 16 ]Cl with acetyl chloride resulted in the bisacetylated adduct [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NAc)₂‐]⁺, [ 17 ]Cl. Treatment of [ 1 ]Cl with AcN?NAc did not give the Rh?N insertion product, but instead the N,O‐chelated complex [(TPA)Rh^I ( κ²‐(O,N)‐CH₃(CO)(NH)(N?C(CH₃)(OCH?CH₂))]Cl [ 23 ]Cl, presumably through insertion of ethylene into a [Rh]?O bond.     

Design and Synthesis of Novel Chiral Dirhodium(II) Carboxylate Complexes for Asymmetric Cyclopropanation Reactions

A novel approach to the design of dirhodium(II) tetracarboxylates derived from (S)‐amino acid ligands is reported. The approach is founded on tailoring the steric influences of the overall catalyst structure by reducing the local symmetry of the ligand's N‐heterocyclic tether. The application of the new approach has led to the uncovering of [Rh₂(S‐^tertPTTL)₄] as a new member of the dirhodium(II) family with extraordinary selectivity in cyclopropanation reactions. The stereoselectivity of [Rh₂(S‐^tertPTTL)₄] was found to be comparable to that of [Rh₂(S‐PTAD)₄] (up to >99 % ee), with the extra benefit of being more synthetically accessible. Correlations based on X‐ray structures to justify the observed enantioinduction are also discussed.     

4‐(3,5‐Dimethyl‐1H‐pyrazol‐4‐ylmethyl)‐3,5‐dimethyl‐1H‐pyrazol‐2‐ium dihydrogen phosphate: a combined X‐ray and DFT study

The molecular structure of the title salt, C₁₁H₁₇N₄⁺·H₂PO₄⁻, has been determined from single‐crystal X‐ray analysis and compared with the structure calculated by density functional theory (DFT) at the BLYP level. The crystal packing in the title compound is stabilized primarily by intermolecular N—H...O, O—H...N and O—H...O hydrogen bonds and π–π stacking interactions, and thus a three‐dimensional supramolecular honeycomb network consisting of R₄²(10), R₄⁴(14) and R₄⁴(24) ring motifs is established. The HOMO–LUMO energy gap (1.338 eV; HOMO is the highest occupied molecular orbital and LUMO is the lowest unoccupied molecular orbital) indicates a high chemical reactivity for the title compound.     

Chiral Atropisomeric 2-Iodo-4,4′,6,6′-tetramethyl-2′-(diphenylphosphoryl)-1,1′-biphenyl-enantiodifferentiation by Rh2[MTPA]4-Adduct Formation and Conformational Analysis

Enantiodifferentiation of the chiral 2-iodo-4,4′,6,6′-tetramethyl-2′-(diphenylphosphoryl)-1,1′-biphenyl (2) can be accomplished easily by adding one mole equivalent of the enantiopure dirhodium complex Rh ^(II) ₂ [(R)-(+)-MTPA]₄ (Rh*). The internal competition of the two bindings sites in 2, the Ph ₂ P═O group, and the iodine atom was identified by variable-temperature ³¹ P NMR. Energy optimization (PM3) and ROESY spectroscopy of 2 in the absence and presence of Rh* reveal that 2 prefers a conformation in the adducts, which is the least stable one in the free molecule, i.e., adduct formation is accompanied by a rotation of the Ph ₂ P═O group about the C-2′─P bond.     

Cationic (Azulene)(diene)rhodium(I) Complexes: Spectroscopic,Dynamic, and Catalytic Properties

New [Rh^I(η⁵‐azulene)(η⁴‐diene)][BF₄] complex salts 3 – 5 (diene=8,9,10‐trinorborna‐2,5‐diene (nbd) and (1Z,5Z)‐cycloocta‐1,5‐diene (cod)) were synthesized according to a known procedure (Scheme 1). All of these complexes show dynamic behavior of the diene ligand at room temperature. In the case of the [Rh^I(η⁵‐azulene)(cod)]⁺ complex salts 3 and [Rh^I(η⁵‐guaiazulene)(nbd)]⁺ complex salt 4a (guaiazulene=7‐isopropyl‐1,4‐dimethylazulene), the coalescence temperature of the ¹H‐NMR signals of the olefinic H‐atoms was determined. The free energy of activation (ΔG; Table 1) for the intramolecular movement of the diene ligands exhibits a distinct dependency on the HOMO/LUMO properties of the coordinated azulene ligand. The DFT (density‐functional theory) calculated ΔG values for the internal diene rotation are in good to excellent agreement with the observed ones in CD₂Cl₂ as solvent (Table 2). Moreover, the ΔG values can also be estimated in good approximation from the position of the longest‐wavelength, azulene‐centered UV/VIS absorption band of the complex salts (Table 2). These cationic Rh^I complexes are stable and air‐resistant and can be used, e.g., as precursor complexes in situ in the presence of (M)‐6,7‐bis[(diphenylphosphino)methyl]‐8,12‐diphenylbenzo[a]heptalene for asymmetric hydrogenation of (Z)‐α‐(acetamido)cinnamic acid with ee values of up to 68% (Table 4).     

Amide‐Functionalized Naphthyridines on a RhII–RhII Platform: Effect of Steric Crowding,Hemilability, and Hydrogen‐Bonding Interactions on the Structural Diversity and Catalytic Activity of Dirhodium(II) Complexes

Ferrocene‐amide‐functionalized 1,8‐naphthyridine (NP) based ligands {[(5,7‐dimethyl‐1,8‐naphthyridin‐2‐yl)amino]carbonyl}ferrocene (L¹H) and {[(3‐phenyl‐1,8‐naphthyridin‐2‐yl)amino]carbonyl}ferrocene (L²H) have been synthesized. Room‐temperature treatment of both the ligands with Rh₂(CH₃COO)₄ produced [Rh₂(CH₃COO)₃(L¹)] ( 1 ) and [Rh₂(CH₃COO)₃(L²)] ( 2 ) as neutral complexes in which the ligands were deprotonated and bound in a tridentate fashion. The steric effect of the ortho‐methyl group in L¹H and the inertness of the bridging carboxylate groups prevented the incorporation of the second ligand on the {Rh^II–Rh^II} unit. The use of the more labile Rh₂(CF₃COO)₄ salt with L¹H produced a cis bis‐adduct [Rh₂(CF₃COO)₄(L¹H)²] ( 3 ), whereas L²H resulted in a trans bis‐adduct [Rh₂(CF₃COO)₃(L²)(L²H)] ( 4 ). Ligand L¹H exhibits chelate binding in 3 and L²H forms a bridge‐chelate mode in 4 . Hydrogen‐bonding interactions between the amide hydrogen and carboxylate oxygen atoms play an important role in the formation of these complexes. In the absence of this hydrogen‐bonding interaction, both ligands bind axially as evident from the X‐ray structure of [Rh₂(CH₃COO)₂(CH₃CN)₄(L²H)₂](BF₄)₂ ( 6 ). However, the axial ligands reorganize at reflux into a bridge‐chelate coordination mode and produce [Rh₂(CH₃COO)₂(CH₃CN)₂(L¹H)](BF₄)₂ ( 5 ) and [Rh₂(CH₃COO)₂(L²H)₂](BF₄)₂ ( 7 ). Judicious selection of the dirhodium(II) precursors, choice of ligand, and adaptation of the correct reaction conditions affords 7 , which features hemilabile amide side arms that occupy sites trans to the Rh–Rh bond. Consequently, this compound exhibits higher catalytic activity for carbene insertion to the C?H bond of substituted indoles by using appropriate diazo compounds, whereas other compounds are far less reactive. Thus, this work demonstrates the utility of steric crowding, hemilability, and hydrogen‐bonding functionalities to govern the structure and catalytic efficacyof dirhodium(II,II) compounds.     

A new method proposed for the determination of absolute configurations of α‐amino acids

Enantiopure α‐amino acids were converted to 4‐substituted 2‐aryl‐ and 2‐alkyl‐5(4H)‐oxazolones under partial racemization. These nonracemic mixtures were dissolved in CDCl₃, an equimolar amount of the chiral dirhodium complex [(R)? (+)? MTPA]₄ (MTPA‐H = Mosher's acid) was added, and the ¹H NMR spectra of the resulting samples were recorded (dirhodium method). The relative intensities of ¹H signals dispersed by the formation of diastereomeric adducts allow to determine the absolute configuration (AC) of the starting α‐amino acids. Binding atoms in the adducts were identified by comparing the ¹H and ¹³C chemical shifts of the oxazolones in the absence and presence of [(R)? (+)? MTPA]₄. Thereby, information about the scope and limits of this method can be extracted. A protocol how to use this method is presented. Copyright © 2008 John Wiley & Sons, Ltd.     

1H, 13C, 15N and 195Pt NMR studies of Au(III) and Pt(II) chloride organometallics with 2‐phenylpyridine

¹H, ¹³C, ¹⁵N and ¹⁹⁵Pt NMR studies of gold(III) and platinum(II) chloride organometallics with N(1),C(2′)‐chelated, deprotonated 2‐phenylpyridine (2ppy*) of the formulae [Au(2ppy*)Cl₂], trans(N,N)‐[Pt(2ppy*)(2ppy)Cl] and trans(S,N)‐[Pt(2ppy*)(DMSO‐d₆)Cl] (formed in situ upon dissolving [Pt(2ppy*)(µ‐Cl)]₂ in DMSO‐d₆) were performed. All signals were unambiguously assigned by HMBC/HSQC methods and the respective ¹H, ¹³C and ¹⁵N coordination shifts (i.e. differences between chemical shifts of the same atom in the complex and ligand molecules: Δ^1H_coord = δ^1H_complex ? δ^1H_ligand, Δ^13C_coord = δ^13C_complex ? δ^13C_ligand, Δ^15N_coord = δ^15N_complex ? δ^15N_ligand), as well as ¹⁹⁵Pt chemical shifts and ¹H‐¹⁹⁵Pt coupling constants discussed in relation to the known molecular structures. Characteristic deshielding of nitrogen‐adjacent H(6) protons and metallated C(2′) atoms as well as significant shielding of coordinated N(1) nitrogens is discussed in respect to a large set of literature NMR data available for related cyclometallated compounds. Copyright © 2009 John Wiley & Sons, Ltd.     

Syntheses and structural characterizations of the octahedral boride clusters [Rh2Ru4H(CO)16−2x (nbd) x B] (x=1,2) and gold(I) phosphine derivatives (nbd=norbornadiene)

The reaction of less than one equivalent of [Rh₂Cl₂(nbd)₂] with [Ru₄H(CO)₁₂BH]⁻, which contains a semi-interstitial boron atom, yields the heterometallic boride clustercis-[Rh₂Ru₄H(CO)₁₂(nbd)₂B] which has been characterized by spectroscopic and X-ray diffraction methods. The cluster has an octahedral core, consistent with an 86 electron count. Deprotonation yields the conjugate basecis-[Rh₂Ru₄(CO)₁₂(nbd)₂B]⁻ which has been isolated and fully characterized as the [(Ph₃P)₂N]⁺ salt. There is little structural perturbation upon going fromcis-[Rh₂Ru₄H(CO)₁₂(nbd)₂B] tocis-[Rh₂Ru₄(CO)₁₂(nbd)₂B]⁻ and neither cluster shows a tendency for the formation of thetrans skeletal isomer in contrast to the analogous carbonyl clustercis-[Rh₂Ru₄(CO)₁₆B]⁻. If the reaction of [Rh₂Cl₂(nbd)₂] with [Ru₄H(CO)₁₂BH]⁻ is allowed to proceed for 30 min and [R ₃PAuCl] (R=Ph, C₆H₁₁, 2-MeC₆H₄) is then added, the clusterscis-[Rh₂Ru₄(CO)₁₂(nbd)₂B(AuPR₃)] andcis-[Rh₂Ru₄(CO)₁₄(nbd)B(AuPR₃)] are formed in yields that are dependent upon the initial reaction period. The single crystal structures ofcis-[Rh₂Ru₄(CO)₁₂(nbd)₂B(AuPPh₃)] andcis-[Rh₂Ru₄(CO)₁₄(nbd)B(AuPPh₃)] are reported. In contrast to their all-carbonyl analoguescis-[Rh₂Ru₄(CO)₁₆B(AuPR ₃)] (R=Ph or C₆H₁₁), the nbd derivatives do not undergocis →trans skeletal isomerism.     

A New Model for Prediction of One Electron Reduction Potential of Nitroaryl Compounds

This paper introduces a simple model for prediction of one electron reduction potential [E(RNO₂/R ^? NO₂^–)] of various nitroaryl compounds. The new method uses energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in gas phase at the B3LYP/6‐311++G** level (ΔE_HOMO‐LUMO) and some structural parameters. It was used for 35 nitroaryl compounds including nitrobenzenes, nitrofurans, 2‐nitroimidazoles, 4‐nitroimidazoles, 5‐ninuintidazoles, nitroazaindoles, nitroacridines, and miscellaneous nitroaryl compounds. The root mean square (rms) percent deviation and the average absolute error of predictions of E(RNO₂/R ^? NO₂^–) relative to experiment were decreased from 12.4 % and 0.42 V to 3.5 % and 0.11 V, respectively, upon consideration of several structural parameters. Increment of the value of ΔE_HOMO‐LUMO and inclusion of specific polar groups can increase thermodynamic stability of these compounds.     

Rh2[(R)-(+)-MTPA]4 as an NMR auxiliary for the enantiodifferentiation of chiral secondary and tertiary phosphine–borane complexes

The direct chiral recognition of secondary and tertiary phosphine–borane complexes is made possible by applying the dirhodium method (NMR in the presence of Rh₂[(R)-(+)-MTPA]₄, Rh¹). Due to the acid lability of the phosphine–borane complexes, it is advisable to use deuterated benzene as solvent rather than deuterated chloroform. The decomposition of the phosphine–borane complexes and the resulting Rh¹–phosphine adducts are also studied.     

Dichloro­[(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)methane]zinc(II) and di‐μ‐chloro‐bis­{chloro­[(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)methane]cadmium(II)}

The Zn atom in dichloro­[(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)­methane]zinc(II), [ZnCl₂(C₁₁H₁₆N₄)], (I), is tetra­hedrally coordinated by two N atoms from one bis­(3,5‐dimethyl­pyrazol­yl)methane ligand and two terminal Cl atoms. The mol­ecule has no crystallographic symmetry. One H atom of the CH₂ group of the bis­(3,5‐dimethyl­pyrazol­yl)methane ligand inter­acts with a Cl atom of an adjacent mol­ecule to yield inter­molecular C—H⋯Cl contacts, thereby forming a one‐dimensional zigzag chain extending along the b axis. On the other hand, in di‐μ‐chloro‐bis­{chloro­[(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)methane]cadmium(II)}, [Cd₂Cl₄(C₁₁H₁₆N₄)₂], (II), each of the two crystallographically equivalent Cd atoms is penta­coordinated by two N atoms from one bis­(3,5‐dimethyl­pyrazol­yl)methane ligand, and by one terminal and two bridging Cl⁻ anions. The mol­ecule has a crystallographic centre of symmetry located at the mid‐point of the Cd⋯Cd line. One H atom of the CH₂ group of the bis­(3,5‐dimethyl­pyrazolyl)­methane ligand inter­acts with a Cl atom of an adjacent mol­ecule to produce pairwise inter­molecular C—H⋯Cl contacts, thereby affording chains of mol­ecules running along the c axis.     

NMR study of localization and mobility of 1‐phenylethanol enantiomers in homochiral metal‐organic sorbent Zn2(bdc)(S‐lac)(dmf)

Structural features of localization of chiral isomers of 1‐phenylethanol (R‐PhEtOH and S‐PhEtOH) and their mobility activation in homochiral metal‐organic [Zn₂(bdc)(S‐lac)(dmf)] sorbent were studied with ¹H and ¹³C NMR methods. ¹³C NMR chemical shifts do not show obvious advantage of selective interaction of molecule guests. But activation molecular mobility of S‐PhEtOH and R‐PhEtOH clearly indicates that stabilization of [Zn₂(bdc)(S‐lac)(dmf)]·S‐PhEtOH structure is more preferable than stabilization of [Zn₂(bdc)(S‐lac)(dmf)]·R‐PhEtOH structure. Copyright © 2015 John Wiley & Sons, Ltd.     

Two single‐enantiomer amidophosphoesters: a database study on the chirality of (O)2P(O)(N)‐based structures

The crystal structures of two single‐enantiomer amidophosphoesters with an (O)₂P(O)(N) skeleton, i.e. diphenyl [(R)‐(+)‐α‐methylbenzylamido]phosphate, (I), and diphenyl [(S)‐(?)‐α‐methylbenzylamido]phosphate, (II), both C₂₀H₂₀NO₃P, are reported. In both structures, chiral one‐dimensional hydrogen‐bonded architectures, along [010], are mediated by N—H…OP interactions. The statistically identical assemblies include the noncentrosymmetric graph‐set motif C(4) and the compounds crystallize in the chiral space group P2₁. As a result of synergistic co‐operation from C—H…O interactions, a two‐dimensional superstructure is built including a noncentrosymmetric R₄⁴(22) hydrogen‐bonded motif. A Cambridge Structural Database survey was performed on (O)₂P(O)(N)‐based structures in order to review the frequency of space groups observed in this family of compounds; the hydrogen‐bond motifs in structures with chiral space groups and the types of groups inducing chirality are discussed. The ^2,3J_X–P (X = H or C) coupling constants from the NMR spectra of (I) and (II) have been studied. In each compound, the two diastereotopic C₆H₅O groups are different, which is reflected in the different chemical shifts and some coupling constants.     

Understanding Electrogenerated Chemiluminescence Efficiency in Blue‐Shifted Iridium(III)‐Complexes: An Experimental and Theoretical Study

Compared to tris(2‐phenylpyridine)iridium(III) ([Ir(ppy)₃]), iridium(III) complexes containing difluorophenylpyridine (df‐ppy) and/or an ancillary triazolylpyridine ligand [3‐phenyl‐1,2,4‐triazol‐5‐ylpyridinato (ptp) or 1‐benzyl‐1,2,3‐triazol‐4‐ylpyridine (ptb)] exhibit considerable hypsochromic shifts (ca. 25–60 nm), due to the significant stabilising effect of these ligands on the HOMO energy, whilst having relatively little effect on the LUMO. Despite their lower photoluminescence quantum yields compared with [Ir(ppy)₃] and [Ir(df‐ppy)₃], the iridium(III) complexes containing triazolylpyridine ligands gave greater electrogenerated chemiluminescence (ECL) intensities (using tri‐n‐propylamine (TPA) as a co‐reactant), which can in part be ascribed to the more energetically favourable reactions of the oxidised complex (M⁺) with both TPA and its neutral radical oxidation product. The calculated iridium(III) complex LUMO energies were shown to be a good predictor of the corresponding M⁺ LUMO energies, and both HOMO and LUMO levels are related to ECL efficiency. The theoretical and experimental data together show that the best strategy for the design of efficient new blue‐shifted electrochemiluminophores is to aim to stabilise the HOMO, while only moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. Of the iridium(III) complexes examined, [Ir(df‐ppy)₂(ptb)]⁺ was most attractive as a blue‐emitter for ECL detection, featuring a large hypsochromic shift (λ_max=454 and 484 nm), superior co‐reactant ECL intensity than the archetypal homoleptic green and blue emitters: [Ir(ppy)₃] and [Ir(df‐ppy)₃] (by over 16‐fold and threefold, respectively), and greater solubility in polar solvents.