Remi Chauvin

NHC-derived bis(amidiniophosphine) ligands of Rh(I) complexes: versatile cis-trans chelation driven by an interplay of electrostatic and orbital effects.

The synthesis, structure, and electronic properties of a dicationic bis(amidiniophosphine) based on an o-phenylene bridge is described. In spite of the presence of two facing P-conjugated positive charges, this electron-poor diphosphine is shown to act as a versatile chelating ligand in a series of stable rhodium(I) complexes. IR analysis of a carbonyl complex showed that the sigma-donating versus pi-accepting character of the cationic ligand is comparable to that of a neutral trialkylphosphite. While the ligand is trans-chelating at a neutral Rh(I) center, it switches to cis-chelating at a cationic Rh(+) center. This and other unusual geometrical features revealed by X-ray diffraction analyses are interpreted by a subtle interplay between antisymbiotic trans preference, electrostatic repulsion, and relative Lewis acidity of the transition metal centers.

Diaminocarbene and Phosphonium Ylide Ligands: A Systematic Comparison of their Donor Character

The coordinating properties of the diaminocarbene (A) and phosphonium ylide (B) ligand types have been investigated systematically through a test family of C,C-chelating ligands containing two moieties of either kind. The overall character of o-C6H4A(a)B(b) ligands (a + b = 2) has been analyzed from the IR CO stretching frequencies of isostructural complexes [(eta(2)-C6H4A(a)B(b))Rh(CO)2][TfO]. The test moieties A = NC2H2N(+)(Me)C(-) and B = Ph2P(+)CH2(-) were first considered. While the ligands bearing at least one diaminocarbene end (AA, a = 2 and AB, a = 1) could be generated (and trapped by complexation), the bis-ylide case BB (a = 0) proved to be awkward: treatment of the dication C6H4(P(+)Ph2Me)2 with n-BuLi indeed lead to the Schmidbaurs carbodiphosphorane Ph3PCPPh2Me, through an unprecendented ylido-pentacoordinated phosphorane which could be fully characterized by NMR techniques. The bis-ylide ligand type C6H4B2 could however be generated by bridging the phosphonium methyl groups by a methylene link (B2 = (P(+)Ph2CH(-))2CH2), preventing the formation of the analogous highly strained carbodiphosphorane. The three complexes [(eta(2)-C6H4A(a)B(b))Rh(CO)2][TfO] were fully characterized, including by X-ray diffraction analysis and (103)Rh NMR spectroscopy. Comparison of their IR spectra indicated that the A2 type bis-NHC ligand is less donating than the hybrid AB type, which is itself less donating than the B2 type bis-ylide ligand. The excellent linear variation of the nu(CO) frequencies vs a (= 0, 1, 2) shows that the coordinating moieties act in a pseudoindependent way. This was confirmed by DFT calculations at the B3PW91/6-31G**/LANL2DZ*(Rh) level. It is therefore demonstrated that a phosphonium ylide ligand is a stronger donor than a diaminocarbene ligand.

Flexible diphosphine ligands with overall charges of 0, +1, and +2: critical role of the electrostatics in favoring trans over cis coordination.

The influence of the formal electrostatic interaction on the cis/trans coordination mode at a PdCl(2) center is investigated in a family of isostructural flexible diphosphine ligands Ph(2)P-X-C(6)H(4)-Y-PPh(2), where X and Y stand for neutral or cationic N,C-imidazolylene linkers. While the neutral and monocationic diphosphine spontaneously behave as classical cis-chelating ligands, only the dicationic diphosphine, where the electrostatic repulsion between the formal positive charges specifically takes place, is observed to behave as a trans-chelating ligand. The crucial role of electrostatics is analyzed on the basis of (31)P NMR data in solution and X-ray diffraction data in the crystal state. Comparative theoretical studies of the cis- and trans-chelated complexes, including EDA, static (31)P NMR, MESP, and AIM analyses, have been undertaken on the basis of DFT calculations in the gas phase or in the acetonitrile continuum. Whereas the cis-coordination mode is shown to be thermodynamically favored for the neutral ligand, the trans-coordination mode is found to be preferred for the dicationic homologue. The stereochemical preference is thus shown to be parallel to the expected effect of the formal electrostatic interaction. The results open perspectives for control of the cis- and trans-chelating behavior of flexible bidentate ligands by more or less reversible charge transfer at the periphery of the coordination sphere of a metallic center.

P(CH)P pincer rhodium(I) complexes: the key role of electron-poor imidazoliophosphine extremities.

The coordination chemistry of a potentially pincer-type dicationic meta-phenylene-bis(imidazoliophosphine) ligand 3 to neutral and cationic carbonylrhodium(I) centers has been investigated. Similarly to what was observed previously for its ortho-phenylene counterpart, 3 was found to bind to the RhCl(CO) fragment in a trans-chelating manner that makes possible a weak Rh-C(H) interaction, inferred from the nonbonding but relatively short Rh-C and Rh-H contacts observed in the solid state structure of the dicationic adduct (3)RhCl(CO) (5). Formation of the target PCP-type pincer complex could not be triggered despite multiple attempts to deprotonate the central C-H moiety in the initial dicationic adduct 5, or in the tricationic species [(3)Rh(CO)](+) (8) generated by abstraction of the chloride ion from 5. Complex 8 was identified on the basis of NMR and IR analyses as a Rh(I)-stabilized P(CH)P-intermediate en route to the anticipated classical PCP-type pincer complex. Analysis of the electronic structure of this intermediate computed at the density functional level of theory (DFT level) revealed a bonding overlap between a Rh d-orbital and π-orbitals of the m-phenylene ring. NBO analyses and calculated Wiberg indices confirm that this interaction comprises an η(1)-C-Rh bonding mode, with only secondary contributions from the geminal C and H atoms. Although the target PCP-type pincer complex could not be generated, treatment of the tricationic intermediate with methanol induced a P-CN(2) bond cleavage at both imidazoliophosphine moieties, resulting in the formation of a dicationic open pincer species, that is, a nonchelated bis((MeO)PPh(2))-stabilized aryl-Rhodium complex that is the κC-only analogue of the putative κP,κC,κP-PCP complex sought initially. Theoretical studies at the DFT level of experimental or putative species relevant to the final C-H activation process ruled out the oxidative addition pathway. Two alternative pathways are proposed to explain the formation of the open pincer complex, one based on an organometallic α-elimination step, the other based on an organic aromatization-driven β-elimination process.

The Missing Entry in the Agostic–Anagostic Series: Rh(I)–η1-C Interactions in P(CH)P Pincer Complexes

The missing entry, namely, the C-anagostic or η(1)-C interaction, closing the agostic-anagostic series of metal-CH(aryl) interactions is found in a bis(amidiniophosphine) P(CH)P pincer rhodium complex. The three entries, namely, agostic η(2)-(C,H), anagostic (related to hydrogen bonding, thus recoined here as H-anagostic), and C-anagostic interactions, are unambiguously characterized by electron localization function (ELF) topological analysis. Other theoretical tools such as noncovalent interaction (NCI) analysis and multicenter electron delocalization indices (MCIs) support the ELF characterization. A η(2)-(C,H) agostic interaction is evidenced by a disynaptic V(C,H) or trisynaptic V(M,C,H) ELF basin with a significant quantum topological atoms in molecules (QTAIM) atomic contribution of the metal M and a large covariance (in absolute value) with the metal core basin C(M). The C-anagostic η(1)-C interaction is characterized by a disynaptic V(M,C) basin, a weak covariance (in absolute value) of V(C,H) and C(M) populations, and a negligible QTAIM atomic contribution of M to V(C,H). The relevance of these ELF signatures is evidenced in a selected series of related rhodium and osmium complexes.

From Neutral to Anionic η1-Carbon Ligands: Experimental Synthesis and Theoretical Analysis of a Rhodium−Yldiide Complex

Deprotonation of a cationic rhodium complex of a chelating semistabilized phosphino-phosphonium sulfinyl-ylide ligand afforded the neutral complex of the corresponding yldiide ligand. Despite the limited stability of the yldiide complex its structure was ascertained by ESI MS and multinuclear (1)H, (31)P, (13)C, and (103)Rh NMR spectroscopy. DFT calculations were carried out at the B3PW91/6-31G*/LANL2DZ*(Rh) level to derive a reasonable gas-phase structure for the yldiide complex or model thereof and gain insight into their electronic structure. ELF and AIM topological analyses were used to investigate the metal-ligand bonding and estimate the electron transfer resulting from proton abstraction. ELF weighting of the resonance forms of the anionic free yldiide ligand (Ar(3)PC(-)-S(O)Ar) was compared to the corresponding weighting of previously reported neutral counterparts (XCY), namely, bis-alpha-zwitterionic bisylides of phosphonium, sulfonium, or iminosulfonium moieties (X, Y = PR(3), SR(2), S(NMe)R(2)). The results suggest that the phosphonium sulfinylyldiide can be regarded as a tris-alpha,beta-zwitteranionic bisylide (Ar(3)P(+)-C(2-)-S(+)(-O(-))p-Tol).

Chemical algebra. I: Fuzzy subgroups

Using the notion of fuzzy subset, the algebraic formulation of the constant of stereogenic pairing equilibria between skeletal analogs (previously disclosed) is connected to symmetry group theory. A distinction is introduced between geometrical (skeletal) symmetry and topographical (numerical parameters) symmetry. In order to describe “topographical symmetry”, a formal extended definition of a subgroup is proposed. Fuzzy subsets of the skeletal groupG are endowded with a structure which can be defined without referring to the geometrical representation of the abstract group isomorphic toG. The relevance of these propositions is evidenced by their “integer interpretation” meeting basic definitions of group theory, as well as by their role in expressing chemical pairing constants.

Tandem acylation-complexation of a chlorophosphine by carbonylferrates

Chlorodiphenylphosphine reacts with lithium acyltetracarbonylferrates to give the corresponding mononuclear acylphosphine iron complexes.

Chemical algebra. II: Discriminating pairing products

The algebra of stereogenic pairing equlibria is presented in a very general context. Starting from the notions of fuzzy subgroup and conjugacy link, chemical pairing constants between molecular speciesu andv having a skeletal symmetry groupG are formulated as “pairing products” on aG-Hilbert space. “Discriminating pairing products”K are defined by the conditions: “K ⩾ 1” and “K = 1 → the representative vectors of the paired species areG-equivalent”. WhenG has only two elements, the pairing product is always discriminating. For several skeletal symmetries, if the vectors are “enantiomorphic (v = σu, σ2 =e, σ ≠G), thenK is greater than 1 and reaches 1 only ifu is “achiral”: “chirality indexes” and general “permutational indexes” are then defined fromK(u σu). The general model is illustrated by some examples.

Direct Evaluation of Cyclic Contributions to the π Energy of Conjugated Hydrocarbons from Strongly Localized Zero-Order Pictures†

This paper presents a new procedure for identifying that part of the pi electronic energy of conjugated hydrocarbons which results from cyclic circulation of electrons around a ring. It first shows that one may calculate perturbatively the ground state energy of the Hückel Hamiltonian from a strongly localized Kekulé-type zero-order wave function. The contributions due to cyclic circulation of the electrons appear explicitly, in terms of the interatomic hopping integral t, at the second order in cyclobutadiene (where it is equal to -t (antiaromatic)) and at third order in benzene, where its value is 0.5t (aromatic). Conjugated isomers of benzene are also considered. The cyclic circulation contributions for an N-membered ring are shown to depend strongly on the molecular graph in which it is embedded. A general expression is found for the cyclic contribution to the pi energy of a ring, the Kekulé graph of which contains N double bonds alternating with N single bonds. It is the energy of the ring, plus the sum of the energies of the N subsystems that result from one double-bond removal, minus the sum of the energies of the N open systems that result from one single-bond cut. This new aromaticity index, ACE(MC), may be seen as the enthalpy of a hyperhomodesmotic chemical equation. In contrast to the index ACE(DC) previously defined from a double cut of the ring, the multiple-cut ACE(MC) exhibits the expected asymptotic disappearance of the cyclic energy as the ring size tends to infinity. In the multiple-cut approach, aromaticity persists in bond-alternating rings, but, in contrast to the total pi energy, the purely cyclic contribution tends to resist distortion. Extension of the approach to charged, branched and heterosubstituted rings are discussed, as well as its ab initio transcription.