Arturo Espinosa Ferao

Reactivity of terminal phosphinidene versus Li–Cl phosphinidenoid complexes in cycloaddition chemistry

Reaction of the thermally generated transient phosphinidene complex 2 and the Li-Cl phosphinidenoid complex 7 with N-methyl-thiophene-3-carbaldimine (3) was investigated. In the first case a mixture of azaphosphiridine complex 4 and bicyclic azaphospholene complex 5 was obtained; the reaction of complex 7 yielded selectively 5. DFT modelling of the reaction of 7 with 3 supports a nucleophilic formal [4+1] cycloaddition.

Electrochemical and Fluorescent Ferrocene-Imidazole-Based Dyads as Ion-Pair Receptors for Divalent Metal Cations and Oxoanions

In the tricyclic bis(heteroaryl)substituted ferrocenyl-imidazo-quinoxalines 7 and 8, the presence of redox and fluorescent units at the heteroaromatic core, which can act as a ditopic binding site, made these molecules potential candidates as electro-optical ion-pair recognition receptors. In this context, both molecules behave as ion-pair receptors for cations and anions, which individually had demonstrated their ability to form the corresponding cationic and anionic complexes. These receptors also show an important enhancement of anion binding by co-bound cations, whereas no affinity of the free receptors by the anion is observed. Similarly, receptors 7 and 8 display a dramatic increase in the cation binding by the action of their anionic complexes, while no affinity of the free receptors by the cations was detected. Interestingly, both receptors exhibit a remarkable enhancement of anions and cations binding, although no affinity of the free receptors by the ions is observed. In all cases, the ion-pair formation is detected by a perturbation of the redox potential of the ferrocene moiety and a remarkable enhancement in the emission band.

Coordination of CO to low-valent phosphorus centres and other related P–C bonding situations. A theoretical case study

The multi-faceted bonding of CO in molecular phosphorus compounds is described using calculated P–C bond strengths as a criterion. Full compliance matrices at coupled cluster level of HPCO (1a), singlet oxaphosphirane-3-ylidene HP(η2-CO)), the dimer (HPCO)2 as well as PCH, HPCH2 and H2P–CH3 were calculated to obtain quantifiable data and enable comparison. The quest for CO coordination and activation was examined for phosphaketenes 1a–f: the P–C compliance constants (in A mdyn−1) reveal a clear trend that shows a weakening of the P–CO bond strength from 1a to mono-ligation as in [(OC)5W{P(CO)Me}] (1c) (0.301), in H3BP(CO)Me (1b) (0.322), to bis-ligation as in [{(OC)5W}2P(CO)R] (1f) (0.488) to (H3BP)2(CO)Me (1d) (0.649). Availability of p-type electron density at phosphorus drastically strengthens the P–CO bond and weakens the C–O bond via π–back-donation, bis complexes are better described as weak CO (C→P) adducts to phosphorus. In complexes [(OC)5W{P(CO)R}] the CO activation by phosphorus equals that of CO activation through tungsten in pentacarbonyltungsten complexes. A comparative study of various CO bonding motifs in molecular compounds indicates that acyclic (2) or cyclic diphospha-urea derivatives (2–5) or isomers (6) display P–CO bond strengths (compliance constants range 0.502–0.640) well below that of the P–C bond of H2P–CH3 (0.364), thus providing insight into the bonding and the ease of CO extrusion, experimentally known for some cases. A highly unusual adduct of CO was obtained in silico through two-fold P-ligation in diphosphiren-3-ones 2a–d, the parent compound of which was found to be properly described as a side-on (PP)→(CO) complex, in contrast to its aza-analogue 2aN. A drastic weakening of the P–CO bond strength is observed from P2CO (2a) (0.502) to the C2-symmetrical (H3BP)2CO (2b) (0.913); the latter represents an extreme case of a weakly bound CO. Furthermore, calculated 31P NMR shifts and scalar 1J(P,E) couplings were correlated with P–CO and PC–O compliance constants as a tool for experimentalists.

Unprecedented Ring–Ring Interconversion of N,P,C‐Cage Ligands

The novel N,P,C-cage complexes 5 a-f and 6 a-f have been obtained by the reaction of the P-pentamethylcyclopentadienylphosphinidene complex 2, generated thermally from 2H-azaphosphirene complex 1, with N-methyl-C-arylcarbaldimines 3 a-f. Li/Cl phosphinidenoid complex 8 reacted with 3 a,b to give N,P,C-cage complexes 6 a,b, whereas with 3 c-f, complexes 6 c-f were obtained in negligible amounts only. Both types of ligand N,P,C-cage structures 5 and 6 were found to be in an unprecedented equilibrium, with 5 a,f as the predominant species. Transient electrophilic terminal phosphinidene complexes 10 a-f serve as intermediates in both ligand interconversions (5 a,f↔6 a,f), as evidenced through trapping reactions with phenylacetylene and N-methyl-C-phenylcarbaldimine, thus leading to the novel N,P,C-cage complexes 13 b and 15. DFT calculations predicted a small difference in the relative energies of the two types of N,P,C-cage ligands, and a remarkable stabilisation of the aminophosphinidene complex 10 as the common precursor, thereby providing an insight into this surprising 5-ring-3-ring interconversion. In depth analysis of intermediate 10 revealed the occurrence of both through-bond (conventional inductive/mesomeric effects) and through-space (non-covalent interactions) mechanisms, which amount to 67.8 and 14.4 kcal mol(-1), respectively, and account for the remarkable stabilisation of this intermediate.

Stimuli‐Responsive Frustrated Lewis‐Pair‐Type Reactivity of a Tungsten Iminoazaphosphiridine Complex

Reactions of 3-imino-azaphosphiridine complexes 1 a,b with carbodiimides 2 a,b, isocyanates 3 a,b, and carbon dioxide are described. Whereas exchange of the carbodiimide unit occurs in the first case, an overall ring expansion takes place with phenyl isocyanate (3 a) and carbon dioxide to yield complexes 4 and 5 bearing novel 1,3,5-oxazaphospholane ligands; the isopropyl derivative 3 b did not react under these conditions. DFT calculations provide insight into the pathway of the reaction with carbon dioxide with model complex 1 c, revealing effects of initial non-covalent interactions with the substrate onto the ring bonding, thus triggering an initially masked frustrated Lewis-pair-type behavior.

Cycloaddition of P-C Single Bonds: Stereoselective Formation of Benzo-1,3,6,2-trioxaphosphepine Complexes via a Ditopic van der Waals Complex.

While phosphaalkenes and phosphanes are known to participate in [4+n] cycloaddition reactions, P-C single bonds are inert in this respect. Herein, reactions of oxaphosphirane complexes with tetrachloro-ortho-benzoquinone are presented that reveal a stereoselective reaction of the endocyclic P-C bond to afford benzo-1,3,6,2-trioxaphosphepine complexes. High-level DFT calculations provide evidence that the final product is derived from a sequence of three consecutive steps involving a ditopic van der Waals complex.

Coordination of N2 and Other Small Molecules to the Phosphorus Centre of RPW(CO)5: A Theoretical Study on the Janus Facets of the Stabilization/Activation Problem

Bonding of neutral terminal phosphinidene tungsten(0) complexes stabilized by ligands bound to phosphorus (ligand-to-P) have been studied using a large testbed of predominantly N-ligands. Most complexes (1 a-r,v-z and 2 l) exhibit a pyramidal P centre with a formal single ligand-to-P bond order. Three ligand-to-P complexes (1 s-u) exhibit planar sp2 -hybridization of P and P,N bonds featuring double bond character. All derivatives with C-ligands (1 v,x-z) exhibit a ligand-to-P bond strength intermediate between the P-N single and double bonded species. Remarkably, dinuclear complexes containing CN- (1 p,w), N3- (1 r) and N2 (1-3 t) as bridging ligands between two phosphorus centres show intertwined terminal W(CO)5 fragments. In particular, the case of N2 bridging ligand (1-3 t) represents a noteworthy example of activation, displaying a weakened N,N bond and two sets of very strong P,N double bonds, the latter resembling the activation-picture of dinitrogen by transition metal complexes. Bond dissociation energies (BDEs) of ligand-to-P bonds do not show any correlation with a range of bond strength descriptors, but display some meaningful, roughly linear variation with the ligand softness. Based on Haalands definition of dative bonding, a reasonable linear correlation of BDE with dativity (d1 ) and the dative covalence energy (DCE1 ) was found. A moderate correlation was also obtained with the vertical ionization potential of the ligand (I(L)) as well as with the ligands HOMO energy.

C i -Symmetry, [2 × 2] grid, square copper complex with the N4,N5-bis(4-fluorophenyl)-1H-imidazole-4,5-dicarboxamide ligand: structure, catecholase activity, magnetic properties and DFT calculations

A new square tetranuclear copper complex of formula Cu4(LH)4·4DMF with the dinucleating amide ligand N4,N5-bis(4-fluorophenyl)-1H-imidazole-4,5-dicarboxamide (LH3) is reported herein. This ligand and the complex have been characterized by elemental analysis, FTIR, NMR, mass and UV-visible spectroscopy, as well as magnetic and electrochemical measurements. The single crystal X-ray diffraction study showed each Cu centre in a distorted square pyramidal environment, the square plane being formed by the extended coordination of two sets (N,N and N,O) of donor atoms from pairs of different ligands. The Cu4 unit has a Ci symmetry and crystallises in the P[1 with combining macron] space group in between DMF layers. The electronic spectrum of the complex exhibits a d–d transition at 676 nm. The complex also displays mild catecholase activity in DMF solution by using 3,5-di-tert-butylcatechol as substrate. Variable temperature magnetic measurements reveal an antiferromagnetic interaction between the copper centres with two different coupling constants of −143.4 and −169.0 cm−1. DFT calculations made on a single Cu4 unit from the crystal structure corroborated the antiferromagnetic coupling, the spin density of the lower-energy broken-symmetry state being consistent with an alternating +−+− singlet state operating at low temperature.

The structure of Cu(II) and Hg(II) complexes of bispyrenyl azine revisited

AbstractAzine ligands show preference for the antiperiplanar conformation, but their 2:2 ligand-metal complexes can exhibit a central N4M2 core in the most stable chair arrangement, as in the case of the model lithium complex, with the ligand displaying a synclinal conformation that is not stable in the free ligand. According to DFT calculations, complexation of Cu2+ with bis(1-pyrenyl)azine (1) affords a C2-symmetric [1·Cu2+]2 species with a planar central N4Cu2 core exhibiting a weak cuprophilic interaction. The pendant pyrenyl substituents are brought close and parallel to each other, therefore accounting for the π-stacking that is responsible for excimer fluorescence. In case of Hg2+, complexation by the same ligand affords a C2-symmetric complex with an essentially linear N–Hg–C coordination environment at every metal center and without intermetallic interaction. T-Stacking interactions between pyrenyl groups explain the overall rigidity required for enhancement of fluorescence. Graphical abstractThe structures of 2:2 complexes of Hg(II) and Cu(II) with bispyrenyl azine are reported