A Xanthene-Based Mono-Anionic PON Ligand: Exploiting a Bulky, Electronically Unsymmetrical Donor in Main Group Chemistry

The synthesis of a novel mono-anionic phosphino-amide ligand based on a xanthene backbone is reported, togetherr with the corresponding GaI complex, (PON)Ga (PON = 4-(di(2,4,6-trimethylphenyl)phosphino)-5-(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene). The solid-state structure of (PON)Ga (obtained from X-ray crystallography) reveals very weak O⋅⋅⋅Ga and P⋅⋅⋅Ga interactions, consistent with a R2 NGa fragment which closely resembles those found in one-coordinate amidogallium systems. Strong N-to-Ga π donation from the amido substituent is reflected in a very short N-Ga distance (1.961(2) Å), while the P⋅⋅⋅Ga contact (3.076(1) Å) is well outside the sum of the respective covalent radii. While the donor properties of the PON ligand towards GaI are highly unsymmetrical, oxidation to GaIII leads to much stronger coordination of the pendant phosphine as shown by P-Ga distances which are up to 20 % shorter. From a steric perspective, the PON ligand is shown to be significantly bulkier than related β-diketiminate systems, a finding consistent with reactions of (PON)Ga towards O-atom sources that proceed without oligomerization. Despite this, the enhanced P-donor properties brought about by oxidation at gallium are not sufficient to quench the reactivity of the highly polar Ga-O unit. Instead, intramolecular benzylic C-H activation is observed across the Ga-O bond of a transient gallanone intermediate.

Probing the non-innocent nature of an amino-functionalised β-diketiminate ligand in silylene/iminosilane systems

Electron-rich β-diketiminate ligands, featuring amino groups at the backbone β positions ("N-nacnac" ligands) have been employed in the synthesis of a range of silylene (Si^II) complexes of the type (N-nacnac)SiX (where X = H, Cl, N(SiMe₃)₂, P(SiMe₃)₂ and Si(SiMe₃)₃). A combination of experimental and quantum chemical approaches reveals (i) that in all cases rearrangement to give an aza-butadienyl Si^IV imide featuring a contracted five-membered heterocycle is thermodynamically favourable (and experimentally viable); (ii) that the kinetic lability of systems of the type (N-nacnac)SiX varies markedly as a function of X, such that compounds of this type can be isolated under ambient conditions for X = Cl and P(SiMe₃)₂, but not for X = H, N(SiMe₃)₂ and Si(SiMe₃)₃; and (iii) that the ring contraction process is most facile for systems bearing strongly electron-donating and sterically less encumbered X groups, since these allow most ready access to a transition state accessed via intramolecular nucleophilic attack by the Si^II centre at the β-carbon position of the N-nacnac ligand backbone.

Photoinduced One-Electron Reduction of Alkyl Halides by Dirhodium(II,II) Tetraformamidinates and a Related Complex with Visible Light

Various substituted dirhodium tetraformamidinate complexes, Rh(2)(R-form)(4) (R = p-CF(3), p-Cl, p-OCH(3), m-OCH(3); form = N,N'-diphenylformamidinate), and the new complex Rh(2)(tpgu)(4) (tpgu = 1,2,3-triphenylguanidinate) have been investigated as potential agents for the photoremediation of saturated halogenated aliphatic compounds, RX (R = alkyl group). The synthesis and characterization of the complexes is reported, and the crystal structure of Rh(2)(tpgu)(4) is presented. The lowest energy transition of the complexes is observed at approximately 870 nm and the complexes react with alkyl chlorides and alkyl bromides under low energy irradiation (lambda(irr) > or = 795 nm), but not when kept in the dark. The metal-containing product of the photochemical reaction with RX (X = Cl, Br) is the corresponding mixed-valent Rh(2)(II,III)X (X = Cl, Br) complex, and the crystal structure of Rh(2)(p-OCH(3)-form)(4)Cl generated photochemically from the reaction of the corresponding Rh(2)(II,II) complex in CHCl(3) is presented. In addition, the product resulting from the dimerization of the alkyl fragment, R(2), is also formed during the reaction of each dirhodium complex with RX. A comparison of the dependence of the relative reaction rates on the reduction potentials of the alkyl halides and their C-X bond dissociation energies are consistent with an outer-sphere mechanism. In addition, the relative reaction rates of the metal complexes with CCl(4) decrease with the oxidation potential of the dirhodium compounds. The mechanism of the observed reactivity is discussed and compared to related systems.

Dirhenium Paddlewheel Compounds Supported by N,N‘-Dialkylbenzamidinates: Synthesis, Structures, and Photophysical Properties

Dirhenium(III,III) compounds Re2(DMBA)4Cl2 (1, DMBA=N,N'-dimethylbenzamidinate) and Re2(DEBA)4Cl2 (2, DEBA=N,N'-diethylbenzamidinate) were synthesized via molten reactions between Re2(OAc)4Cl2 and the corresponding amidine. Re2(DMBA)4(NO3)2 (3) was obtained through reacting Re2(DMBA)4Cl2 with AgNO3. Single crystal X-ray diffraction studies revealed that the Re-Re distances in compounds 1-3 are 2.212(1), 2.217(1), and 2.173(1) A, respectively, which are consistent with the presence of a Re-Re quadruple bond. Voltammetric studies revealed that compound 2 exhibits two quasireversible couples, an oxidation and a reduction, and an irreversible reduction, while compound 1 displays irreversible couples at similar potentials. The three complexes exhibit 1deltadelta* absorption as a shoulder at approximately 440 nm (epsilon approximately 1500 M(-1) cm(-1)). Upon excitation of solid samples or CH2Cl2 solutions of 2 with visible light, emission is observed at 824 nm (77 K) and 833 nm (298 K), respectively. The luminescence is assigned as arising from the 3deltadelta* excited state.

New chemistry of the triply bonded divanadium (V2 4+) unit and reduction to an unprecedented V2 3+ core

We report here the synthesis and characterization of seven new divanadium compounds with the paddlewheel geometry bridged by nitrogen-donating ligands. Five of these contain the diamagnetic V(2)(4+) core with short V-V distances of less than 2.0 A, consistent with a formal triple bond. The V-V distances vary with the basicity of the bridging ligands; more basic ligands such as those of the guanidinate type have the shorter metal-metal separations, and those with the less basic formamidinate groups have longer separations. One compound, V(2)(DPhF)(4) (DPhF = the anion of N,N'-diphenylformamidine), has been reduced by one electron, and two structures, [K(THF)(3)]V(2)(DPhF)(4) and [K(18-crown-6)(THF)(2)]V(2)(DPhF)(4), have been obtained. These have an unprecedented V(2)(3+) core where the formal oxidation state of each vanadium atom is +1.5. The decrease in V-V bond distance and the multiline EPR spectrum in the reduced species provide evidence that these two molecules contain a bond of order 3.5.

Structural and catalytic properties of bis(guanidine)copper(I) halides

The neutral, bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) coordinates to Cu(I) halides via the imine nitrogen, and the complexes are stabilized by intramolecular NH...X interactions (X = Cl, Br) or are partially dissociated (X = I) in the solid state; in all cases, fluxional behavior is observed in solution. Selected complexes have been tested as catalysts for the polymerization of methyl methacrylate.

Synthesis and Characterization of HC{C(Me)N(C6H3-2,6-i-Pr2)}2MX2 (M = Al, X = Cl, I; M = Ga, In, X = Me, Cl, I): Sterically Encumbered β-Diketiminate Group 13 Metal Derivatives

A series of group 13 metal complexes featuring the beta-diketiminate ligand [[(C(6)H(3)-2,6-i-Pr(2))NC(Me)](2)CH](-) (i.e., [Dipp(2)nacnac](-), Dipp = C(6)H(3)-2,6-i-Pr(2)) have been prepared and spectroscopically and structurally characterized. The chloride derivatives Dipp(2)nacnacMCl(2) (M = Al (3), Ga (5), In (8)) were isolated in good yield by the reaction of 1 equiv of Dipp(2)nacnacLi.Et(2)O (2) and the respective metal halides. The iodide derivatives Dipp(2)nacnacMI(2) (M = Al (4), Ga (6), In (9)), which are useful for reduction to afford M(I) species, were made by a variety of routes. Thus, 4 was obtained by treatment of the previously reported Dipp(2)nacnacAlMe(2) with I(2), whereas the gallium analogue 6 was obtained as a product of the reaction of "GaI" with Dipp(2)nacnacLi.Et(2)O, and 9 was obtained by direct reaction of InI(3) and the lithium salt. The methyl derivatives Dipp(2)nacnacMMe(2) (M = Ga (7), In (10)), which are analogous to the previously reported Dipp(2)nacnacAlMe(2), were synthesized by the reaction of GaMe(3) with Dipp(2)nacnacH (1) or by reaction of the indium chloride derivative 8 with 2 equiv of MeMgBr in diethyl ether. The compounds 3-10 exist as colorless, air- and moisture-sensitive crystalline solids. Their X-ray crystal structures feature nearly planar C(3)N(2) arrays in the Dipp(2)nacnac ligand backbone with short C-C and C-N distances that are consistent with a delocalized structure. However, there are large dihedral angles between the C(3)N(2) plane and the N(2)M metal coordination plane which have been attributed mainly to steric effects. The relatively short M-N distances are consistent with the coordination numbers of the metals and the normal/dative character of the nitrogen ligands. The compounds were also characterized by (1)H and (13)C NMR spectroscopy. (1)H NMR data for 7 revealed equivalent methyl groups whereas the spectrum of 10 displayed two In-Me signals which indicated that ring wagging was slow on the (1)H NMR time scale.

Synthesis and Characterization of Functionalized N,N'-Diphenylformamidinate Silver(I) Dimers: Solid-State Structures and Solution Properties

Functionalized N,N'-diphenylformamidines and their deprotonated silver(I) complexes have been synthesized: silver(I) N,N'-di(4-alkyl)phenylformamidinate (alkyl = methyl, ethyl, n-butyl, and n-hexadecyl) 1-4; silver(I) N,N'-di(4-trifluoromethyl)phenylformamidinate 5, silver(I) N,N'-di(3-methoxy)phenylformamidinate 6, silver(I) N,N'-di(3-methylmercapto)phenylformamidinate 7, silver(I) N,N'-di(2-methoxy)phenylformamidinate 8, silver(I) N,N'-di(2-methylmercapto)phenylformamidinate 9. The effects of increasing the coordination number of the silver(I) centers by donor substituents on the phenyl groups have been investigated by solution and solid-state studies. Variable-temperature proton NMR (223-303 K) for 1-4 shows coupling between the proton attached to the amidinate carbon and the (107/109)Ag centers at room temperature which is unaffected by cooling (2). For the four-coordinate complexes, 8 and 9, such coupling is only observed on cooling. Molecular weight measurements recorded in solution by vapor pressure osmometry at 310 K show some aggregation to higher molecular weight species than simple dimers for 1-4 and 6, but 8 and 9 exist as discrete dimeric species. Measurement of thermal stability shows the expected increase in stability with increasing coordination number. Compounds 8 and 9 were structurally characterized by X-ray methods. Both show four-coordinate silver dimers bridged by two amidinate ligands with additional longer interactions with the ether oxygens or thioether sulfurs.

Preparation and Structure of Two Ditungsten Compounds Synthesized with the 3,5-Dichlorophenylformamidinate Ligand

The structure, preparation, and spectroscopy of a W(2)(II, II) paddlewheel complex with four formamidinate ligands is reported. Upon exposure to air and moisture in solution, the W(2)(II, II) core undergoes an oxidative addition reaction to form a W(2)(III, III) edge-sharing bioctahedral (ESBO) complex with two bridging hydroxides. The products, W(2)(&mgr;-DCPF)(4) (1) and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) (2) where DCPF is [(3,5-Cl(2)C(6)H(3))NC(H)N(3,5-Cl(2)C(6)H(3))(-)], were characterized by UV-vis and (1)H NMR spectroscopy. Structural data are presented for W(2)(&mgr;-DCPF)(4) with four bridging formamidinate ligands and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) with the formamidinate ligands in both chelating and bridging positions. Crystallographic data for W(2)(&mgr;-DCPF)(4) (1) and W(2)(&mgr;-OH)(2)(&mgr;-DCPF)(2)(eta(2)-DCPF)(2) (2) are as follows: (1), C(68)H(60)Cl(16)N(8)O(4)W(2), a = 12.6906(3) Å, b = 12.7818(4) Å, c = 13.0450(3) Å, alpha = 109.173(1) degrees, beta = 93.865(1) degrees, gamma = 103.746(1) degrees, triclinic, P&onemacr; (No. 2), and Z = 1 and (2), C(59)H(35)Cl(16)N(8)O(2)W(2), a = 17.1524(1) Å, b = 20.6568(3) Å, c = 19.3959(3) Å, beta = 102.791(1) degrees, monoclinic, C2/c (No. 15), and Z = 4.

Linear Free Energy Relationships in Dinuclear Compounds. 2. Inductive Redox Tuning via Remote Substituents in Quadruply Bonded Dimolybdenum Compounds

Syntheses and characterizations are reported for dimolybdenum(II) compounds supported by the diarylformamidinate (ArNC(H)NAr(-)) ligand, where Ar is XC(6)H(4)(-), with X as p-OMe (1), H (2), m-OMe (3), p-Cl (4), m-Cl (5), m-CF(3) (6), p-COMe (7), p-CF(3) (8), or Ar is 3,4-Cl(2)C(6)H(3)(-) (9) or 3,5-Cl(2)C(6)H(3)(-) (10). The (quasi)reversible oxidation potentials measured for the Mo(2)(5+)/Mo(2)(4+) couple were found to correlate with the Hammett constant (sigma(X)) of the aryl substituents according to the following equation: DeltaE(1/2) = E(1/2)(X) - E(1/2)(H) = 87(8sigma(X)) mV. Molecular structure determinations of compounds 1, 2, 5, and 10 revealed an invariant core geometry around the Mo(2) center, with statistically identical Mo-Mo quadruple bond lengths of 2.0964(5), 2.0949[8], 2.0958(6), and 2.0965(5) Å, respectively. Magnetic anisotropies for compounds 1-10 estimated on the basis of (1)H NMR data were similar and unrelated to sigma(X). Similarity in UV-vis spectra was also found within the series, which, in conjunction with the features of both molecular structures and (1)H NMR spectra, was interpreted as the existence of a constant upper valence structure across the series. Results of Fenske-Hall calculations performed for several model compounds paralleled the experimental observations.