Synthesis of benzannulated N-heterocyclic carbene ligands by a template synthesis from 2-nitrophenyl isocyanide.

The reaction of 2-nitrophenyl isocyanide 2 with [M(CO)5(thf)] (M=Cr, Mo, W) yields the isocyanide complexes [M(CO)5(2)] (3: M=Cr; 4: M=Mo; 5: M=W). Complexes 3-5 react with elemental tin under reduction of the nitro function of the isocyanide ligand to give the complexes with the unstable 2-aminophenyl isocyanide ligand. The coordinated 2-aminophenyl isocyanide ligand in all three complexes reacts spontaneously under intramolecular nucleophilic attack of the primary amine at the isocyanide carbon atom to yield the complexes with the NH,NH-benzimidazol-2-ylidene ligand (6: M=Cr; 7: M=Mo; 8: M=W). An incomplete reduction of the nitro group in 3-5 is observed when hydrazine hydrate is used instead of tin. Here the formation of complexes with a coordinated 2-hydroxylamine-functionalized phenyl isocyanide [(CO)5M-CN-C6H(4-)-2-N(H)-OH] is postulated and this unstable ligand again undergoes intramolecular cyclization to give the NH,NOH-stabilized benzimidazol-2-ylidene complexes 9-11. The tungsten derivative 11 can be allylated stepwise by a deprotonation/alkylation sequence first at the OH and then at the NH position to yield the monoallylated and diallylated species 12 and 13. The molecular structures of 3-5 and 12-13 were established by X-ray crystallography.

An unsymmetrical tripodal ligand with NOS2-donor set: coordination chemistry with nickel(II) and zinc(II).

The synthesis of the novel tripodal ligand [N(CH2CH2CH2OH)(CH2CH2SH)2] H3-4 is reported. The aliphatic tetradentate ligand is equipped with an unsymmetrical NOS2 donor set. It reacts with Ni(OAc)2 x 4H2O or Zn(BF4)2 x xH2O to give the complexes [Ni(H-4)]2 5 and [Zn(H-4)]4 6, respectively. The molecular structures of 5 and 6 have been determined by X-ray diffraction. In both cases multinuclear, mu-thiolato-bridged complexes, wherein the ligand coordinates with only three (NS2) of the four donor groups, had formed. The dinuclear complex 5 adopts a butterfly geometry and contains nickel(II) ions in a square-planar NS3 coordination environment. Cyclic voltammetry experiments indicate that the nickel centers in 5 are electron-rich but not overly sensitive toward oxidation. Complex 6 is tetranuclear and the four thiolato-bridged metal centers form a ring. It shows a distorted tetrahedral coordination geometry for the zinc(II) ions in an NS3 coordination sphere. In both complexes the hydroxyl functionalized ligand arm of the tripodal ligand remains uncoordinated.

Template synthesis of benzannulated N-heterocyclic carbene ligands.

The reaction of 2-azidophenyl isocyanide (7) with [M(CO)(5)(thf)] (M=Cr, W) yields the isocyanide complexes [M(CO)(5)(7)] (M=Cr 8, M=W 9). Complexes 8 and 9 react with tertiary phosphines such as triphenylphosphane at the azido function of the isocyanide ligand to give the 2-triphenylphosphiniminophenyl isocyanide complexes 10 (M=Cr) and 11 (M=W). The polar triphenylphosphiniminophenyl function in complexes 10 and 11 can be hydrolyzed with H(2)O/HBr to afford triphenylphosphane oxide and the complexes containing the unstable 2-aminophenyl isocyanide ligand. This ligand spontaneously cyclizes by intramolecular nucleophilic attack of the primary amine at the isocyanide carbon atom to yield the 2,3-dihydro-1H-benzimidazol-2-ylidene complexes 12 (M=Cr) and 13 (M=W). Double deprotonation of the cyclic NH,NH-carbene ligands in 12 and 13 with KOtBu and reaction with two equivalents of allyl bromide yields the N,N'-dialkylated benzannulated N-heterocyclic carbene complexes 14 (M=Cr) and 15 (M=W). The molecular structures of complexes 9 and 11-15 were confirmed by X-ray diffraction studies.

Dinuclear complexes with bis(benzenedithiolate) ligands.

As a part of a broader study directed towards helical coordination compounds with benzenedithiolate donors, we have synthesized the bis(benzenedithiol) ligands 1,2-bis(2,3-dimercaptobenzamido)ethane (H(4)-1) and 1,2-bis(2,3-dimercaptophenyl)ethane (H(4)-2). Both ligands form dinuclear complexes with Ni(II), Ni(III) and, after air-oxidation, Co(III) ions under equilibrium conditions. Complexes (NEt(4))(4)[Ni(II)(2)(1)(2)] (11 b), (NEt(4))(2)[Ni(III)(2)(1)(2)] (13), and Na(4)[Ni(II)(2)(2)(2)] (14) were characterized by X-ray diffraction. In all complexes, two square-planar [Ni(S(2)C(6)H(3)R)(2)] units are linked in a double-stranded fashion by the carbon backbone and they assume a coplanar arrangement in a stair-like manner. Cyclic voltammetric investigations show a strong dependence of the redox potential on the type of the ligand. The substitution of 1(4-) for 2(4-) on nickel (-785 mV for 11 b versus -1130 mV for 14, relative to ferrocene) affects the redox potential to a similar degree as the substitution of nickel for cobalt (-1160 mV for [Co(2)(1)(2)](2-)/[Co(2)(1)(2)](4-), relative to ferrocene). The redox waves display a markedly less reversible behavior for complexes with the shorter bridged ligand 2(4-) compared to those of 1(4-).

Coordination Chemistry of N-Alkylbenzamide-2,3-dithiolates as an Approach to Poly(dithiolate) Ligands: 1,4-Bis[(2,3-dimercaptobenzamido)methyl]benzene and Its Chelate Complex with the (C(5)H(5))Ti Fragment.

The bidentate dithiolate ligands N,N-diethyl-2,3-dimercaptobenzamide (H(2)-1), bis(N,N-diethyl-2,3-dimercaptoterephthalamide (H(2)-2), and 1,4-bis(hydroxymethyl)-2,3-dimercaptobenzene (H(4)-3) were synthesized from 2,3-dimercaptobenzoic acid or 2,3-dimercaptoterephthalic acid. The air-sensitive ligands form metallocene complexes of the type [(eta(5)-C(5)H(5))(2)Ti(1)] (13), [(eta(5)-C(5)H(5))(2)Mo(1)] (15), [(eta(5)-C(5)H(5))(2)Ti(3)] (16), and [(eta(5)-C(5)H(5))(2)Ti(2)] (17). Complexes 15 and 16 were characterized by X-ray diffraction. Selected crystallographic details for 15 are as follows: formula C(21)H(23)MoNOS(2); M = 465.49; Pbca; a = 12.536(3), b = 14.313(3), c = 22.463(3) Å; V = 4031(2) Å(3); Z = 8; R = 3.56 and R(w) = 4.49 for 2111 structure factors (F(o)(2) >/= 3sigma(F(o)(2))) and 254 refined parameters. The molybdenum complex 15 shows an almost planar Mo(&mgr;-S)(2)C(2) chelate ring. Selected crystallographic details for 16 are as follows: formula C(18)H(18)O(2)S(2)Ti; M = 378.35; P&onemacr;; a = 10.1778(11), b = 11.5806(14), c = 22.967(3) Å; alpha = 96.42(1), beta = 101.74(1), gamma = 108.82(1) degrees; V = 2462.3(5) Å(3); Z = 6; R = 4.79 and R(w) = 13.27 for 5426 structure factors (I >/= 2sigma(I)) and 766 refined parameters. All titanocene derivatives assume the envelope conformation. The free activation energy for the flip around the S-S axis was determined for 13 to be 69 kJ/mol. The bis(dithiolate) ligand 1,4-bis[(2,3-dimercaptobenzamido)methyl]benzene (H(4)-4) was prepared from 2,3-dimercaptobenzoic acid and converted into the dinuclear air-stable titanocene complex [{(eta(5)-C(5)H(5))(2)Ti}(2)(4)] (19). Complex 19 reacts with HCl/CHCl(3) with liberation of free H(4)-4 while reaction with NMe(4)Cl results in an intramolecular dithiolate shift under formal liberation of [(C(5)H(5))(3)TiCl] and formation of the square-pyramidal chelate complex (NMe(4))[(eta(5)-C(5)H(5))Ti(4)], (NMe(4))[20]. Crystallographic details for (NMe(4))[20].CH(2)Cl(2) are as follows: formula C(32)H(35)Cl(2)N(3)O(2)S(4)Ti; M = 740.67; P&onemacr;; a = 11.579(4), b = 12.210(4), c = 14.016(4) Å; alpha = 112.28(2), beta = 94.46(3), gamma = 104.22(3) degrees; V = 1744.9(10) Å(3); Z = 2; R = 5.35 and R(w) = 12.84 for 2870 structure factors (I >/= 2sigma(I)) and 397 refined parameters. The chelate rings in [20](-) assume an endo/exo conformation.