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

Abstract

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.

Coordination Studies of a New Nonsymmetric Ditertiary Phosphane Bearing a Single Phosphaadamantane Cage

“…Yield 0.104 g, 66 %. 1 A similar procedure was also used for the preparation of the rhodium(III) and iridium(III) complexes 11 (78 %) and 12 (35 %) respec- X-ray Crystallography: Measurements were made with either a Bruker AXS SMART 1000 CCD area-detector diffractometer [45] for 3·CHCl 3 , 4, 5, 7·CHCl 3 , 10·CH 2 Cl 2 ·0.5C 4 H 10 O and 13·2CH 2 Cl 2 using sealed-tube graphite-monochromated Mo-K α radiation and narrow frame exposures (0.3°) in ω or a Bruker-Nonius 95 mm CCD kappa diffractometer [46,47] equipped with a rotating anode generator for 4. Cell parameters were refined from the observed (ω) angles of all strong reflections in each data set.…”

“…As part of continuing studies [1][2][3][4][5][6] in our group we are interested in developing new tertiary and symmetric ditertiary phosphane ligands using simple condensation reactions. We, and others, have found that ditertiary phosphanes such as those of general formula (R 2 PCH 2 ) 2 N(R) have interesting coordination properties [7,8] including their use in selfassembly reactions through covalent bonds or H-bonding contacts.…”

A novel fluorene-containing κ4-P2N2-tetradentate platinum(II) complex

“…2b,c,3, 4 We recently described a readily synthesised PNN¢N¢¢-tetradentate ligand and its diverse bonding capabilities to a range of late transition-metals. 5 Fluorene, diazofluorene or fluorenone groups have been used in organic synthesis 6 and for the preparation of metal complexes 7 including organometallic acetylides, 8 oligomers 9 and polymers. 10 However examples of monodentate or bidentate phosphine ligands bearing one or two fluorene groups are sparsely documented in the literature.…”

Late Transition‐Metal Complexes of 2‐Ph2PC6H4NHC(O)CH2NHCO2Bz: Observation of Three Distinct Ligation Modes (κ1‐P,κ2‐P/N and κ3‐P/N/N′)

“…This flexibility in ligand geometry mirrors our previous studies with a P/N/NЈ/NЈЈ-tetradentate system which likewise can adopt numerous coordination motifs. [25] The amide functional groups adopt various secondary hydrogen-bonding interactions and in solution, we believe some of these compounds exist as conformers due to hindered amide-bond rotation. Further studies are in progress and will be reported in due course.…”

“…This finding contrasts with the X-ray crystal structure of 2-{Ph 2 PC 6 H 4 C(H)=N}C 6 H 4 {NHC(O)CH 2 NHCO 2 -Bz}, recently reported by us, in which the -PPh 2 group points away from the N 3 core. [25] The hydrogen H(1) forms an intramolecular hydrogen bond to N (2) …”