Metal complexes of diiminodiphosphines. Structural and reactivity patterns

Kappa what? Insights into the coordination modes of N2P2 ligands

While first synthesized more than three decades ago, complexes supported by N2P2 ligands have seen renewed interest due to the synthesis of new ligands, expansion of their reactivity, and catalytic applications. Possessing both soft phosphines and hard nitrogen donors, N2P2 ligands can accommodate various metal geometries and coordination modes thanks to their capability to act as bidentate, tridentate or tetradentate ligands. This short review will explore how metals bind to these ligands and also highlight the complexes' reactivity and catalytic abilities.

Stabilization of bis(chlorogermyliumylidene)s within bifunctional PNNP ligand frameworks and their reactivity studies

The diiminodiphosphine (L_im) and diaminodiphosphines (l-NH and l-NMe) with a bifunctional PNNP ligand framework have been employed to host two [GeCl]⁺ units leading to the formation of bis(chlorogermyliumylidene) 1-3, respectively. The synthetic route involves a 1 : 2 stoichiometric reaction between the PNNP ligand and GeCl₂·dioxane and the subsequent addition of two equivalents of chloride abstracting agent. Compound 1 is unstable towards coordinating solvents and Lewis bases, resulting in the displacement of the GeCl unit and the formation of rearranged products 4 and 5. However, the diaminodiphosphine coordinated Ge(ii) bis(monocation)s 2 and 3 proved to be stable and revealed their electrophilic behaviour towards the Lewis bases studied.

Exploiting metal-ligand bifunctional reactions in the design of iron asymmetric hydrogenation catalysts

This is an Account of our development of iron-based catalysts for the asymmetric transfer hydrogenation (ATH) and asymmetric pressure hydrogenation (AH) of ketones and imines. These chemical processes provide enantiopure alcohols and amines for use in the pharmaceutical, agrochemical, fragrance, and other fine chemical industries. Fundamental principles of bifunctional reactivity obtained by studies of ruthenium catalysts by Noyori's group and our own with tetradentate ligands with tertiary phosphine and secondary amine donor groups were applied to improve the performance of these first iron(II) catalysts. In particular the correct positioning of a bifunctional H-Fe-NH unit in an iron hydride amine complex leads to exceptional catalyst activity because of the low energy barrier of dihydrogen transfer to the polar bond of the substrate. In addition the ligand structure with this NH group along with an asymmetric array of aryl groups orients the incoming substrate by hydrogen-bonding, and steric interactions provide the hydrogenated product in high enantioselectivity for several classes of substrates. Enantiomerically pure diamines or diphenylphosphino-amine compounds are used as the source of the asymmetry in the tetradentate ligands formed by the condensation of the amines with dialkyl- or diaryl-phosphinoaldehydes, a synthesis that is templated by Fe(II). The commercially available ortho-diphenylphosphinobenzaldehyde was used in the initial studies, but then diaryl-phosphinoacetaldehydes were found to produce much more effective ligands for iron(II). Once the mechanism of catalysis became clearer, the iron-templated synthesis of (S,S)-PAr2CH2CH2NHCHPhCHPhNH2 ligand precursors was developed to specifically introduce a secondary amine in the precatalyst structures. The reaction of a precatalyst with strong base yields a key iron-amido complex that reacts with isopropanol (in ATH) or dihydrogen (in AH) to generate an iron hydride with the Fe-H bond parallel to the secondary amine N-H. In the AH reactions, the correct acidity of the intermediate iron-dihydrogen complex and correct basicity of the amide are important factors for the heterolytic splitting of the dihydrogen to generate the H-Fe-N-H unit; the acidity of dihydrogen complexes including those found in hydrogenases can be estimated by a simple additive ligand acidity constant method. The placement of the hydridic-protonic Fe-H···HN interaction in the asymmetric catalyst structure influences the enantioinduction. The sense of enantioinduction is predictable from the structure of the H-Fe-N-H-containing catalyst interacting with the ketone in the same way as related H-Ru-N-H-containing catalysts. The modular construction of the catalysts permits large variations in order to produce alcohol or amine products with enantiomeric excess in the 90-100% range in several cases.

Intermolecular alkene and alkyne hydroacylation with beta-S-substituted aldehydes: mechanistic insight into the role of a hemilabile P-O-P ligand

A straightforward to assemble catalytic system for the intermolecular hydroacylation reaction of beta-S-substituted aldehydes with activated and unactivated alkenes and alkynes is reported. These catalysts promote the hydroacylation reaction between beta-S-substituted aldehydes and challenging substrates, such as internal alkynes and 1-octene. The catalysts are based upon [Rh(cod)(DPEphos)][ClO(4)] (DPEphos=bis(2-diphenylphosphinophenyl)ether, cod=cyclooctadiene) and were designed to make use of the hemilabile capabilities of the DPEphos ligand to stabilise key acyl-hydrido intermediates against reductive decarbonylation, which results in catalyst death. Studies on the stoichiometric addition of aldehyde (either ortho-HCOCH(2)CH(2)SMe or ortho-HCOC(6)H(4)SMe) and methylacrylate to precursor acetone complexes [Rh(acetone)(2)(DPEphos)][X] [X=closo-CB(11)H(6)Cl(6) or [BAr(F) (4)] (Ar(F)=3,5-(CF(3))(2)C(6)H(3))] reveal the role of the hemilabile DPEphos ligand. The crystal structure of [Rh(acetone)(2)(DPEphos)][X] shows a cis-coordinated diphosphine ligand with the oxygen atom of the DPEphos distal from the rhodium. Addition of aldehyde forms the acyl hydride complexes [Rh(DPEphos)(COCH(2)CH(2)SMe)H][X] or [Rh(DPEphos)(COC(6)H(4)SMe)H][X], which have a trans-spanning DPEphos ligand and a coordinated ether group. Compared to analogous complexes prepared with dppe (dppe=1,2-bis(diphenylphosphino)ethane), these DPEphos complexes show significantly increased resistance towards reductive decarbonylation. The crystal structure of the reductive decarbonylation product [Rh(CO)(DPEphos)(EtSMe)][closo-CB(11)H(6)I(6)] is reported. Addition of alkene (methylacrylate) to the acyl-hydrido complexes forms the final complexes [Rh(DPEphos)(eta(1)-MeSC(2)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X] and [Rh(DPEphos)(eta(1)-MeSC(6)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X], which have been identified spectroscopically and by ESIMS/MS. Intermediate species in this transformation have been observed and tentatively characterised as the alkyl-acyl complexes [Rh(CH(2)CH(2)CO(2)Me)(COC(2)H(4)SMe)(DPEphos)][X] and [Rh(CH(2)CH(2)CO(2)Me)(COC(6)H(4)SMe)(DPEphos)][X]. In these complexes, the DPEphos ligand is now cis chelating. A model for the (unobserved) transient alkene complex that would result from addition of alkene to the acyl-hydrido complexes comes from formation of the MeCN adducts [Rh(DPEphos)(MeSC(2)H(4)CO)H(MeCN)][X] and [Rh(DPEphos)(MeSC(6)H(4)CO)H(MeCN)][X]. Changing the ligand from DPEphos to one with a CH(2) linkage, [Ph(2)P(C(6)H(4))](2)CH(2), gave only decomposition on addition of aldehyde to the acetone precursor, which demonstrated the importance of the hemiabile ether group in DPEphos. With [Ph(2)P(C(6)H(4))](2)S, the sulfur atom has the opposite effect and binds too strongly to the metal centre to allow access to productive acetone intermediates.

Coordination Studies of a New Unsymmetrical κ4-PNN‘N‘ ‘-Tetradentate Ligand: Stepwise Formation and Structural Characterization

The two-step synthesis of a new unsymmetrical ligand 2-[Ph2PC6H4C(H)=N]C6H4[N(H)COCH2N(H)CO2Bz], 2.HH, via acid-catalyzed Schiff base condensation of 2-(H2N)C6H4[N(H)COCH2N(H)CO2Bz], 1, with 2-Ph2PC6H4(CHO) in refluxing EtOH is reported. The multidentate ligand 2.HH, isolated in ca. 60% yield, exhibits an array of ligation modes, as exemplified by coordination studies with NiII, PdII, PtII, and AuI mononuclear metal precursors. Hence, reaction of 2.HH with AuCl(tht) (1:1 molar ratio, tht = tetrahydrothiophene) affords AuCl(2.HH), 3, in which the ligand behaves as a classic, neutral two-electron phosphorus donor. In contrast, reaction with MCl2(cod) (M = Pt, Pd; cod = cycloocta-1,5-diene) affords the corresponding dichloro complexes MCl2(2.HH) (4a M = Pt; 4b M = Pd) in which kappa2-P/N-chelation through both P and imino N-donor atoms is observed. Likewise, treatment of Pd(CH3)Cl(cod) with 2.HH gave Pd(CH3)Cl(2.HH), 4c, in which the imino nitrogen is trans to the methyl ligand. Cycloocta-1,5-diene elimination from, and single methyl protonation of, Pt(CH3)2(cod) with 1 equiv of 2.HH in toluene at ambient temperature affords the neutral complex Pt(CH3)(2.H-), 5a, in which 2.H- functions effectively in a kappa3-PNN' coordination mode. The dichloro compounds 4a or 4b undergo smooth N(H) deprotonation with tBuOK to give 6a\6a' and 6b\6b' in which 22- acts as a dianionic kappa4-PNN'N' ' ligand. The corresponding square-planar, diamagnetic, nickel(II) complex 6c\6c' was prepared in excellent yield from NiCl2.6H2O, 2.HH, and tBuOK. Variable-temperature NMR experiments confirm 6a\6a' and 6b\6b' exist, in solution, as a pair of conformational (anti and syn) isomers due to restricted rotation about the N-CO2Bz group. This feature is also borne out by single-crystal X-ray studies of anti-6a.CHCl3, syn-6a'.H2O, anti-6b.CHCl3, and anti-6c.CH2Cl2. To the best of our knowledge, we believe these constitute the first examples of crystallographically characterized conformers of a tetradentate ligand incorporating a P-donor center. All new compounds reported have been fully characterized by a combination of spectroscopic (NMR, FT-IR, ES-MS) and analytical methods. Furthermore, single-crystal X-ray studies have also been undertaken on compounds 2.HH, 3, 4a, and 5a.Et2O.