3,3‘-Bis(triphenylsilyl)biphenoxide as a Sterically Hindered Ligand on Fe(II), Fe(III), and Cr(II)

The structural coordination chemistry of the sterically hindered 3,3‘-bis(triphenylsilyl)-1,1‘-bi-2-phenoxide ligand ([TPSLO2]2-), an isosteric homologue of a chiral binaphthoxide ligand used in asymmetric induction, has been investigated on Fe(II), Fe(III), and Cr(II). The ligand diol (TPSL(OH)2, 2) can be prepared on a multigram scale from 2,2‘-dimethoxybiphenyl via a convenient, two-step ortho-metalation/silylation/deprotection sequence; the sodium salt of the ligand dianion (Na2[TPSLO2], 3) can be obtained by NaH deprotonation and was crystallographically characterized with two THF ligands bound to each cation. Protonolysis of Fe[N(SiMe3)2]2 or Cr[N(SiMe3)2]2(THF)2 with biphenol 2 in arene solvent gives [Fe(μ-TPSLO2)]2 (4) or (TPSLO2)Cr(THF)2 (9), respectively, while anion metathesis of FeCl2 or FeCl3/bipy with biphenoxide salt 3 in THF yields (TPSLO2)Fe(THF)2 (8) or ferric monomer (TPSLO2)FeCl(bipy) (10), respectively. Dimer 4 reacts with exogenous ligands to form monomeric ligand adducts of Fe(II):  with py, (TPSLO2)Fe(py)2 (5); with bipy, (TPSLO2)Fe(bipy) (6); with XyNC (Xy = 2,6-xylyl), (TPSLO2)Fe(CNXy)4 (7); and with THF, 8. Complexes were characterized in solution by 1H NMR and in the solid state by single-crystal X-ray diffraction. The 4-coordinate complexes (5, 6, 8, 9) adopt skew-distorted tetrahedral (for Fe(II)) or square planar (for Cr(II)) geometries; the 5- and 6-coordinate complexes (10 and 7) assume more typical distorted square pyramidal/trigonal bipyramidal and cis-octahedral stereochemistries. Dimer 4 possesses an unusual structure where each Fe(II) center is pseudo-4-coordinate and each biphenoxide ligand provides one terminal phenoxide donor, one bridging phenoxide, and a weak aromatic π-interaction from one of the phenyl groups of a SiPh3 substituent. The steric influence of the hindered biphenoxide ligand within the coordination sphere is revealed structurally through distortion of coordination polyhedra in the 4-coordinate species and through conformational and deformational changes within the biphenoxide ligand itself

3,3‘-Bis(triphenylsilyl)biphenoxide as a Sterically Hindered Ligand on Fe(II), Fe(III), and Cr(II)

The structural coordination chemistry of the sterically hindered 3,3‘-bis(triphenylsilyl)-1,1‘-bi-2-phenoxide ligand ([TPSLO2]2-), an isosteric homologue of a chiral binaphthoxide ligand used in asymmetric induction, has been investigated on Fe(II), Fe(III), and Cr(II). The ligand diol (TPSL(OH)2, 2) can be prepared on a multigram scale from 2,2‘-dimethoxybiphenyl via a convenient, two-step ortho-metalation/silylation/deprotection sequence; the sodium salt of the ligand dianion (Na2[TPSLO2], 3) can be obtained by NaH deprotonation and was crystallographically characterized with two THF ligands bound to each cation. Protonolysis of Fe[N(SiMe3)2]2 or Cr[N(SiMe3)2]2(THF)2 with biphenol 2 in arene solvent gives [Fe(μ-TPSLO2)]2 (4) or (TPSLO2)Cr(THF)2 (9), respectively, while anion metathesis of FeCl2 or FeCl3/bipy with biphenoxide salt 3 in THF yields (TPSLO2)Fe(THF)2 (8) or ferric monomer (TPSLO2)FeCl(bipy) (10), respectively. Dimer 4 reacts with exogenous ligands to form monomeric ligand adducts of Fe(II):  with py, (TPSLO2)Fe(py)2 (5); with bipy, (TPSLO2)Fe(bipy) (6); with XyNC (Xy = 2,6-xylyl), (TPSLO2)Fe(CNXy)4 (7); and with THF, 8. Complexes were characterized in solution by 1H NMR and in the solid state by single-crystal X-ray diffraction. The 4-coordinate complexes (5, 6, 8, 9) adopt skew-distorted tetrahedral (for Fe(II)) or square planar (for Cr(II)) geometries; the 5- and 6-coordinate complexes (10 and 7) assume more typical distorted square pyramidal/trigonal bipyramidal and cis-octahedral stereochemistries. Dimer 4 possesses an unusual structure where each Fe(II) center is pseudo-4-coordinate and each biphenoxide ligand provides one terminal phenoxide donor, one bridging phenoxide, and a weak aromatic π-interaction from one of the phenyl groups of a SiPh3 substituent. The steric influence of the hindered biphenoxide ligand within the coordination sphere is revealed structurally through distortion of coordination polyhedra in the 4-coordinate species and through conformational and deformational changes within the biphenoxide ligand itself

Reductive Cleavage of the N−N Bond: Synthesis of Imidoiron(III) Cubanes

Reductive Cleavage of the N−N Bond:  Synthesis of Imidoiron(III) Cubane

<i>Ortho</i>-Linked Polyaryloxide Ligands and Their Titanium Complexes

Ortho-linked polyphenols, quaterphenol L(OH)4, and moderately hindered terphenol PhL(OH)3 are developed as new multidentate polyaryloxide ligands for transition-metal chemistry. The polyphenols are synthesized using ortho-metalation and metal-catalyzed cross-coupling methodologies; the synthetic routes allow for facile electronic and steric modification of the basic ligand design. The Ti(IV) coordination chemistry of these ligands reveals a diverse collection of bridged structures:  dimeric [Ti(μ-PhLO3)(OiPr)]2 (P21/n, a = 12.2699(5) Å, b = 11.7957(5) Å, c = 21.238(1) Å, β = 94.551(1)°, Z = 2, T = 170(2) K), dimeric [Ti2(μ-PhLO3)2(μ-Cl)(Cl)(THF)] (P , a = 11.212(1) Å, b = 14.165(1) Å, c = 22.447(2) Å, α = 90.440(4)°, β = 93.345(4)°, γ = 111.164(4)°, Z = 2, T = 170(2) K), and trimeric [Ti3(μ,μ‘-LO4)(μ-OiPr)2(OiPr)6] (P21/n, a = 11.1022(5) Å, b = 18.7015(9) Å, c = 24.409(1) Å, β = 95.369(2)°, Z = 4, T = 170(2) K). The reaction of TiCl3(THF)3 with [PhLO3]3- results in oxidation of Ti(III) to Ti(IV) and formation of the oxo dimer [Ti(PhLO3)(THF)]2(μ-O) (P , a = 10.8649(6) Å, b = 12.1882(7) Å, c = 14.3349(9) Å, α = 65.602(3)°, β = 84.390(3)°, γ = 86.582(3)°, Z = 1, T = 200(2) K); the oxo group presumably originates from the THF solvent. The titanium centers in these environments are either 5- or 6-coordinate, with distorted square pyramidal/trigonal bipyramidal and distorted octahedral geometries, respectively; the polyphenoxide chelate ligands are capable of bridging multiple oxophilic titanium sites

[Fe₄S₄]^<i>q</i> Cubane Clusters (<i>q</i> = 4+, 3+, 2+) with Terminal Amide Ligands

Bis(trimethylsilyl)amide-ligated iron−sulfur cubane clusters [Fe4(μ3-S)4(N{SiMe3}2)4]z (z = 0, 1−, 2−) are accessible by the reaction of FeCl(N{SiMe3}2)2(THF) (1) with 1 equiv of NaSH (z = 0), followed by reduction with either 0.25 (z = 1−) or 1 equiv (z = 2−) of Na2S as needed. The anionic clusters are obtained as the sodium salts [Na(THF)2][Fe4S4(N{SiMe3}2)4] and [Na(THF)2]2[Fe4S4(N{SiMe3}2)4]; in the solid state, these two clusters both possess a unique contact ion pair motif in which individual sodium ions each coordinate to a cluster core sulfide, an adjacent amide nitrogen, and two THF donors. The monoanionic cluster can also be prepared as the lithium salt [Li(THF)4][Fe4S4(N{SiMe3}2)4] by the reaction of 1 with 1:0.5 LiCl/Li2S. The characterization of the three-membered redox series allows an analysis of redox trends, as well as a study of the effects of the amide donor environment on the [Fe4S4] core. Bis(trimethylsilyl)amide terminal ligation significantly stabilizes oxidized cluster redox states, permitting isolation of the uncommon [Fe4S4]3+ and unprecedented [Fe4S4]4+ weak-field cores

Reactivity of a Sterically Hindered Fe(II) Thiolate Dimer with Amines and Hydrazines

The sterically hindered Fe(II) thiolate dimer Fe2(μ-STriph)2(STriph)2 (1; [STriph]− = 2,4,6-triphenylbenzenethiolate) reacts with primary amines (tBuNH2, aniline) and N2H4 to form the structurally characterized addition complexes Fe(STriph)2(NH2tBu)2, Fe2(μ-STriph)2(STriph)2(NH2Ph)2, and Fe2(μ-η1:η1-N2H4)2(N2H4)4(STriph)4 in high yield. Chemical and NMR spectroscopic evidence indicate that the binding of these nitrogen donors is labile in solution and multispecies equilibria are likely. With arylhydrazines, 1 catalytically disproportionates 1,2-diphenylhydrazine to aniline and azobenzene, and it rearranges 1-methyl-1,2-diarylhydrazines to give, after treatment with alumina, mononuclear, trigonal bipyramidal Fe(III) complexes of composition Fe(ISQ)2(STriph), where [ISQ]− denotes an appropriately substituted bidentate o-diiminobenzosemiquinonate ligand. Complex 1 shows no reaction with hindered 1,2-dialkylhydrazines (isopropyl or tert-butyl) or tetrasubstituted 1,2-dimethyl-1,2-diphenylhydrazine

<i>Ortho</i>-Linked Polyaryloxide Ligands and Their Titanium Complexes

Ortho-linked polyphenols, quaterphenol L(OH)4, and moderately hindered terphenol PhL(OH)3 are developed as new multidentate polyaryloxide ligands for transition-metal chemistry. The polyphenols are synthesized using ortho-metalation and metal-catalyzed cross-coupling methodologies; the synthetic routes allow for facile electronic and steric modification of the basic ligand design. The Ti(IV) coordination chemistry of these ligands reveals a diverse collection of bridged structures:  dimeric [Ti(μ-PhLO3)(OiPr)]2 (P21/n, a = 12.2699(5) Å, b = 11.7957(5) Å, c = 21.238(1) Å, β = 94.551(1)°, Z = 2, T = 170(2) K), dimeric [Ti2(μ-PhLO3)2(μ-Cl)(Cl)(THF)] (P , a = 11.212(1) Å, b = 14.165(1) Å, c = 22.447(2) Å, α = 90.440(4)°, β = 93.345(4)°, γ = 111.164(4)°, Z = 2, T = 170(2) K), and trimeric [Ti3(μ,μ‘-LO4)(μ-OiPr)2(OiPr)6] (P21/n, a = 11.1022(5) Å, b = 18.7015(9) Å, c = 24.409(1) Å, β = 95.369(2)°, Z = 4, T = 170(2) K). The reaction of TiCl3(THF)3 with [PhLO3]3- results in oxidation of Ti(III) to Ti(IV) and formation of the oxo dimer [Ti(PhLO3)(THF)]2(μ-O) (P , a = 10.8649(6) Å, b = 12.1882(7) Å, c = 14.3349(9) Å, α = 65.602(3)°, β = 84.390(3)°, γ = 86.582(3)°, Z = 1, T = 200(2) K); the oxo group presumably originates from the THF solvent. The titanium centers in these environments are either 5- or 6-coordinate, with distorted square pyramidal/trigonal bipyramidal and distorted octahedral geometries, respectively; the polyphenoxide chelate ligands are capable of bridging multiple oxophilic titanium sites

Iron-Mediated Hydrazine Reduction and the Formation of Iron-Arylimide Heterocubanes

The reaction of Fe(N{SiMe3}2)2 (1) with 1 equiv of arylthiol (ArSH) results in material of notional composition Fe(SAr)(N{SiMe3}2) (2), from which crystalline Fe2(μ-SAr)2(N{SiMe3}2)2(THF)2 (Ar = Mes) can be isolated from tetrahydrofuran (THF) solvent. Treatment of 2 with 0.5 equiv of 1,2-diarylhydrazine (Ar′NH−NHAr′, Ar′ = Ph, p-Tol) yields ferric-imide-thiolate cubanes Fe4(μ3-NAr′)4(SAr)4 (3). The site-differentiated, 1-electron reduced iron-imide cubane derivative [Fe(THF)6][Fe4(μ3-N-p-Tol)4(SDMP)3(N{SiMe3}2)]2 ([Fe(THF)6][4]2; DMP = 2,6-dimethylphenyl) can be isolated by adjusting the reaction stoichiometry of 1/ArSH/Ar′NHNHAr′ to 9:6:5. The isolated compounds were characterized by a combination of structural (X-ray diffraction), spectroscopic (NMR, UV−vis, Mössbauer, EPR), and magnetochemical methods. Reactions with a range of hydrazines reveal complex chemical behavior that includes not only N−N bond reduction for 1,2-di- and trisubstituted arylhydrazines, but also catalytic disproportionation for 1,2-diarylhydrazines, N−C bond cleavage for 1,2-diisopropylhydrazine, and no reaction for hindered and tetrasubstituted hydrazines

Iron-Mediated Hydrazine Reduction and the Formation of Iron-Arylimide Heterocubanes

The reaction of Fe(N{SiMe3}2)2 (1) with 1 equiv of arylthiol (ArSH) results in material of notional composition Fe(SAr)(N{SiMe3}2) (2), from which crystalline Fe2(μ-SAr)2(N{SiMe3}2)2(THF)2 (Ar = Mes) can be isolated from tetrahydrofuran (THF) solvent. Treatment of 2 with 0.5 equiv of 1,2-diarylhydrazine (Ar′NH−NHAr′, Ar′ = Ph, p-Tol) yields ferric-imide-thiolate cubanes Fe4(μ3-NAr′)4(SAr)4 (3). The site-differentiated, 1-electron reduced iron-imide cubane derivative [Fe(THF)6][Fe4(μ3-N-p-Tol)4(SDMP)3(N{SiMe3}2)]2 ([Fe(THF)6][4]2; DMP = 2,6-dimethylphenyl) can be isolated by adjusting the reaction stoichiometry of 1/ArSH/Ar′NHNHAr′ to 9:6:5. The isolated compounds were characterized by a combination of structural (X-ray diffraction), spectroscopic (NMR, UV−vis, Mössbauer, EPR), and magnetochemical methods. Reactions with a range of hydrazines reveal complex chemical behavior that includes not only N−N bond reduction for 1,2-di- and trisubstituted arylhydrazines, but also catalytic disproportionation for 1,2-diarylhydrazines, N−C bond cleavage for 1,2-diisopropylhydrazine, and no reaction for hindered and tetrasubstituted hydrazines

Reactivity of a Sterically Hindered Fe(II) Thiolate Dimer with Amines and Hydrazines

The sterically hindered Fe(II) thiolate dimer Fe2(μ-STriph)2(STriph)2 (1; [STriph]− = 2,4,6-triphenylbenzenethiolate) reacts with primary amines (tBuNH2, aniline) and N2H4 to form the structurally characterized addition complexes Fe(STriph)2(NH2tBu)2, Fe2(μ-STriph)2(STriph)2(NH2Ph)2, and Fe2(μ-η1:η1-N2H4)2(N2H4)4(STriph)4 in high yield. Chemical and NMR spectroscopic evidence indicate that the binding of these nitrogen donors is labile in solution and multispecies equilibria are likely. With arylhydrazines, 1 catalytically disproportionates 1,2-diphenylhydrazine to aniline and azobenzene, and it rearranges 1-methyl-1,2-diarylhydrazines to give, after treatment with alumina, mononuclear, trigonal bipyramidal Fe(III) complexes of composition Fe(ISQ)2(STriph), where [ISQ]− denotes an appropriately substituted bidentate o-diiminobenzosemiquinonate ligand. Complex 1 shows no reaction with hindered 1,2-dialkylhydrazines (isopropyl or tert-butyl) or tetrasubstituted 1,2-dimethyl-1,2-diphenylhydrazine