The reaction of a dinucleating bis(iminopyridine) ligand L bearing a xanthene linker (L = N,N′-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine)) with Ni2(COD)2(DPA) (COD = cyclooctadiene, DPA = diphenylacetylene) leads to the formation of a new dinuclear complex Ni2(L)(DPA). Ni2(L)(DPA) can also be obtained in a one-pot reaction involving Ni(COD)2, DPA and L. The X-ray structure of Ni2(L)(DPA) reveals two square-planar Ni centers bridged by a DPA ligand. DFT calculations suggest that this species features NiI centers antiferromagnetically coupled to each other and their iminopyridine ligand radicals. Treatment of Ni2(L)(DPA) with one equivalent of ethyl propiolate (HCCCO2Et) forms the Ni2(L)(HCCCO2Et) complex. Addition of the second equivalent of ethyl propiolate leads to the observation of cyclotrimerised products by 1H NMR spectroscopy. Carrying out the reaction under catalytic conditions (1 mol% of Ni2(L)(DPA), 24 h, room temperature) transforms 89% of the substrate, forming primarily benzene products (triethyl benzene-1,2,4-tricarboxylate and triethyl benzene-1,3,5-tricarboxylate) in 68% yield, in a ca. 5 : 1 relative ratio. Increasing catalyst loading to 5 mol% leads to the full conversion of ethyl propiolate to benzene products; no cyclotetramerisation products were observed. In contrast, the reaction is significantly more sluggish with methyl propargyl ether. Using 1 mol% of the catalyst, only 25% conversion of methyl propargyl ether was observed within 24 h at room temperature. Furthermore, methyl propargyl ether demonstrates the formation of cyclooctatetraenes in significant amounts at a low catalyst concentration, whereas a higher catalyst concentration (5 mol%) leads to benzene products exclusively. Density functional theory was used to provide insight into the reaction mechanism, including structures of putative dinuclear metallocyclopentadiene and metallocycloheptatriene intermediates.

A new, potentially dinucleating xanthene-bridged bis(iminophenolate) ligand L (L = 6,6′-((1E,1′E)-((2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(azanylylidene))bis(methanylylidene))bis(2,4-di-tert-butylphenol)) has been synthesized and its coordination chemistry with zinc precursors featuring alkoxide, chloride, and ethyl leaving groups has been investigated. The reaction of a zinc precursor bearing two bulky alkoxides, Zn(Cl)(μ2-OR)2Li(THF) (OR = di-tert-butyl-phenylmethoxide), formed a mononuclear complex Zn(L) that was isolated as an H-bond adduct with HOR, Zn(L)·HOR. In contrast, the reaction of L (or its lithium salt) with diethylzinc (or zinc chloride) led to the formation of the corresponding dinuclear complexes Zn2(L)(Et)2 and Zn2(L)(μ2-Cl)4Li2(OEt2)2. X-ray crystallography revealed syn-parallel geometry for Zn2(L)(Et)2 (Zn⋯Zn distance of 4.5 Å) and anti-parallel geometry for Zn2(L)(μ2-Cl)4Li2(OEt2)2 (Zn⋯Zn distance of 6.7 Å). Zn2(L)(Et)2 was found to be somewhat unstable, demonstrating decomposition into Zn(L) and ZnEt2; this decomposition can be reversed by the addition of excess ZnEt2. Treatment of Zn2(L)(Et)2 with benzyl alcohol (BnOH) in deuterated benzene, toluene, or dichloromethane resulted in the formation of Zn2(L)(OBn)2, which was characterized by 1H and 13C NMR spectroscopy. Zn2(L)(OBn)2 was found to be active in the ring-opening polymerization of rac-lactide to afford heterotactically inclined PLA.

A series of nickel complexes with potentially redox active bis(aldimino)pyridine ligands [NNN] ([NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-arylmethanimine), where aryl = 2,6-diisopropylphenyl, mesityl, 4-methoxyphenyl, 4-trifluoromethylphenyl, and 3,5-bis(trifluoromethyl)phenyl) were synthesized, and their properties and reactivities were investigated as a function of the overall oxidation state of the system. (Ni[NNN])2+ complexes of ligands featuring bulky electron-rich substituents (1a-Br2 and 1b-Br2, [NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-(2,6-diisopropylphenyl)methanimine) and 1,1′-(pyridine-2,6-diyl)bis(N-mesitylmethanimine), respectively) demonstrated five electrochemical reduction events, the first three of which were quasi-reversible. In contrast, only two quasi-reversible reductions were observed for the less bulky and electron-deficient N-aryl substituents 4-(trifluoromethyl)phenyl and 3,5-bis(trifluoromethyl)phenyl. Chemical reduction of 1a-Br2 and 1b-Br2 with 1 equiv of KC8 or CoCp*2 forms (Ni[NNN])+ complexes of the general formula Ni[NNN]Br (2a-Br and 2b-Br). Structural, spectroscopic, and theoretical studies reveal that these complexes feature significant unpaired spin density on the metal, consistent with “nickel(I)” character. This behavior is in contrast with previously reported bis(ketimino)pyridine systems, in which at the (Ni[NNN])+ state the unpaired electron resided exclusively in the ligand. Further reduction forms a series of (Ni[NNN])0 complexes, in which all of the potentially tridentate [NNN] ligands bind via only one iminopyridine unit; the second arm is left unbound in most complexes. Variable temperature NMR spectroscopy demonstrates that bound and unbound arms exchange via a postulated tridentate intermediate. Electrochemical reduction, via three sequential one-electron reductions, of 1a-Br2 and 1b-Br2 in the presence of CO2/H+ forms an active catalyst for H2 evolution at a glassy-carbon electrode surface, again emphasizing the unique redox chemistry of the bulky bis(aldimino)pyridine nickel complexes.

The formally CoIV carbene Co(OR)2(═CPh2) is formed upon the reaction of diphenyldiazomethane with the cobalt bis(alkoxide) precursor Co(OR)2(THF)2. Structural, spectroscopic, and theoretical studies demonstrate that Co(OR)2(═CPh2) has significant high-valent CoIV═CPh2 character with non-negligible spin density on the carbene moiety.

In this paper, we report the synthesis and reactivity of a rare mononuclear chromium(II) bis(alkoxide) complex, Cr(OR′)2(THF)2, that is supported by a new bulky alkoxide ligand (OR′ = di-t-butyl-(3,5-diphenylphenyl)methoxide). The complex is prepared by protonolysis of square-planar Cr(N(SiMe3)2)2(THF)2 with HOR′. X-ray structure determination disclosed that Cr(OR′)2(THF)2 features a distorted seesaw geometry, in contrast to nearly all other tetra-coordinate Cr(II) complexes, which are square-planar. The reactivity of Cr(OR′)2(THF)2 with aldehydes, ketones, and carbon dioxide was investigated. Treatment of Cr(OR′)2(THF)2 with two equivalents of aromatic aldehydes ArCHO (ArCHO = benzaldehyde, 4-anisaldehyde, 4-trifluorbenzaldehyde, and 2,4,6-trimethylbenzaldehyde) leads cleanly to the formation of Cr(IV) diolate complexes Cr(OR′)2(O2C2H2Ar2) that were characterized by UV-vis and IR spectroscopies and elemental analysis; the representative complex Cr(OR′)2(O2C2H2Ph2) was characterized by X-ray crystallography. In contrast, no reductive coupling was observed for ketones: treatment of Cr(OR′)2(THF)2 with one or two equivalents of benzophenone forms invariably a single ketone adduct Cr(OR′)2(OCPh2) which does not react further. QM/MM calculations suggest the steric demands prevent ketone coupling, and demonstrate that a mononuclear Cr(III) bis-aldehyde complex with partially reduced aldehydes is sufficient for C–C bond formation. The reaction of Cr(OR′)2(THF)2 with CO2 leads to the insertion of CO2 into a Cr–OR′ bond, followed by complex rearrangement to form a diamagnetic dinuclear paddlewheel complex Cr2(O2COR′)4(THF)2, that was characterized by NMR, UV-vis, and IR spectroscopy, and X-ray crystallography.

Reduction of bis(aldimino)pyridine nickel(II) dihalide forms a non-planar bis(aldimino)pyridine nickel(I) halide. Unlike the previously reported square-planar bis(ketimino)pyridine nickel chloride, for which the spin density was localized on the ligand, the present species demonstrate Npy–Ni–X angles of 156°–162° and significant spin density at the metal inferred from EPR measurements and DFT calculations.

Treatment of NiCl2(dme) and NiBr2(dme) (dme = dimethoxyethane) with 2 equiv of LiOR (OR = OCtBu2Ph) forms the distorted trigonal planar complexes [NiLiX(OR)2(THF)2] (THF = tetrahydrofuran) 5 (X = Cl) and 6 (X = Br). The reaction of CuX2 (X = Cl, Br) with 2 equiv of LiOR affords the Cu(I) product Cu4(OR)4 (7). The same product can be obtained using the Cu(I) starting material CuCl. NMR studies indicated that the reduction of Cu(II) to Cu(I) is accompanied by the oxidation of the alkoxide RO– to form the alkoxy radical RO•, which subsequently forms tert-butyl phenyl ketone by β-scission. Treatment of compounds 1–4 ([M2Li2Cl2(OR)4], M = Cr–Co) with thallium hexafluorophosphate allowed the isolation of the distorted tetrahedral complexes of the form M(OR)2(THF)2 for M = Mn (8), Fe (9), and Co (10). Cyclic voltammetry performed on compounds 8–10 demonstrated irreversible oxidations for all complexes, with the iron complex 9 being the most reducing. Complex 9 shows a reactivity toward PhIO and Ph3SbS to form the corresponding dinuclear iron(III) complexes Fe2(O)(OR)4(THF)2 (11) and Fe2(S)(OR)4(THF)2 (12), respectively. X-ray structural studies were performed, showing that the Fe–O–Fe angle for complex 11 is 176.4(1)° and that the Fe–S–Fe angle for complex 12 is 164.83(3)°.

Herein we report the synthesis of Cr imido complexes in bis(alkoxide) ligand environments and their nitrene transfer reactivity with isocyanides. The reaction of Cr2(OR)4 (OR = OCtBu2Ph) with bulky aryl or alkyl azide results in the formation of the trigonal-planar Cr(IV) mono(imido) complexes Cr(OR)2(NR1), whereas less bulky aryl azides form the Cr(VI) bis(imido) complexes Cr(OR)2(NR1)2. Cr(IV) mono(imido) complexes undergo facile reaction with 1 equiv of 2,6-dimethylphenyl isocyanide (CNR2) to form the corresponding carbodiimides R1NCNR2. In contrast, no reaction of Cr(OR)2(NR1)2 complexes with CNR2 is observed. The reaction of Cr(OR)2(NR1) with excess isocyanide leads to the isolation of the Cr(II) complex Cr(OR)2(CNR2)4, along with the observation of the anticipated carbodimide product. Cr(OR)2(CNR2)4, which can also be obtained by treating Cr2(OR)4 with 4 equiv of isocyanide, reacts with azides N3R1 (R1 = adamantyl, mesityl) to produce the respective carbodiimides. Catalytic formation of carbodiimides R1NCNR2 is observed from the mixtures of azides R1N3 (R1 = mesityl, 2,6-diethylphenyl, 2-isopropylphenyl, adamantyl) and several different aryl isocyanides CNR2 using 2.5 mol % of Cr2(OR)4.

The iron bis(alkoxide) complex Fe(OR)2(THF)2 (R = CtBu2Ph), 1, was found to have strikingly different reactivity with various aryl azides, ArN3. Azides with methyl or ethyl groups in the ortho positions of the phenyl ring react catalytically via nitrene coupling to give azoarenes, ArNNAr. Catalyst loading as low as 1 mol % yields clean, quantitative conversion of aryl azides to azoarenes at room temperature in as little as 4 h. A combination of two different aryl azides leads to the catalytic formation of all three possible azoarenes, including the asymmetric one. In contrast, reactions with aryl azides lacking ortho substituents yield stable dimeric iron imido complexes of the form (RO)(THF)Fe(μ-NAr)2Fe(THF)(OR) (Ar = 4-(trifluoromethyl)phenyl, 5; Ar = phenyl, 6; Ar = 3,5-dimethylphenyl, 7), which do not undergo catalytic nitrene coupling. The isocyanide adduct Fe(OR)2(CNR)2 (4, R = 2,6-dimethylphenyl) was obtained from the reaction of Fe(OR)2(THF)2 with two equivalents of isocyanide. No C–N bond formation was observed in the reaction of compound 4 with azides or in the reaction of compounds 5–7 with isocyanides.

Herein we describe bimetallic di-nickel and di-copper complexes [Ni2(L)Br4] (1) and [Cu2(L)Br4(NCMe)2] (2) (L = (1E,1′E)-N,N′-(1,4-phenylenebis(methylene))bis(1-(6-(2,4,6-triisopropylphenyl)pyridin-2-yl)methanimine)) that bind oxalate intramolecularly to form [Ni2(L)Br2(C2O4)(NCMe)] (3) and [Cu2(L)Br2(C2O4)] (4). For the di-nickel complex 1, oxalate incorporation is accompanied by a significant colour change, from red-pink (1) to deep green (3). Mass spectrometric experiments demonstrate that the compound 1 is selective for oxalate versus related mono- and di-carboxylates tested. Oxalate can be released by the addition of slight excess of calcium bromide that forms insoluble calcium oxalate and restores the original Ni2(L)Br4 species. The product of the oxalate release was crystallized as [Ni2(L)Br4]·CaBr2(THF)4 species.

The iron(III) hexazene complex (RO)2Fe(μ-κ2:κ2-AdN6Ad)Fe(OR)2 (3) was synthesized via reductive coupling of 1-azidoadamantane at the iron(II) bis(alkoxide) complex Fe(OR)2(THF)2 (2). The X-ray crystal structure depicts electron delocalization within the hexazene moiety. Density functional theory studies propose the formation of an iron azide dimer on the route to hexazene, in which each azide is monoreduced and the iron centers are oxidized to the 3+ oxidation state.

The dinuclear complex Ni2L1(η2-CS2)2 (2), featuring iminopyridine ligation, is prepared by COD substitution from Ni2L1(COD)2 (1). Spectroscopic, structural, and theoretical data reveals significant activation of the metal-bound C–S bonds, as well as the different oxidation states of the iminopyridine in 1 (1−) and 2 (0).

The reactivity of dinucleating bis(iminopyridine) ligands bearing H (L1, (N,N′)-1,1′-(1,4-phenylene)bis(N-(pyridin-2-ylmethylene)methanamine)) or Me substituents (L2, (N,N′)-1,1′-(1,4-phenylene)bis(N-(1-(pyridin-2-yl)ethylidene)methanamine)) on the imine carbon atom with Ni(COD)2 (COD = 1,5-cyclooctadiene) has been investigated. Treatment of L1 with 2 equiv of Ni(COD)2 forms dinuclear Ni2(L1)(COD)2, whereas the reaction of L2 with 2 equiv of Ni(COD)2 leads to Ni2(L2)2, along with 1 equiv of Ni(COD)2. The compounds were characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis; the structure of Ni2(L2)2 was determined by XRD. Ni2(L2)2 exists as syn and anti stereoisomers in the solid state and in solution. DFT calculations suggest Ni(I) for both Ni2(L1)(COD)2 and Ni2(L2)2, with the radical anion localized on one iminopyridine fragment in Ni2(L1)(COD)2 and delocalized over two iminopyridine fragments in Ni2(L2)2. Both Ni2(L1)(COD)2 and Ni2(L2)2 undergo a reaction with excess diphenylacetylene, forming diphenylacetylene complexes. However, whereas Ni2(L1)(diphenylacetylene)2 decomposes upon removal of the excess diphenylacetylene, Ni2(L2)2 demonstrates a reversible disassembly/reassembly sequence upon the addition/removal of diphenylacetylene.

The electrochemical properties of two Ni(NNN)X2 pincer complexes are reported where X = Cl or Br and NNN is N,N′-(2,6-diisopropylphenyl)bis-aldiminopyridine. Cyclic voltammetry under 1 atm of CO2 suggests electrocatalytic CO2 reduction activity, however, bulk electrolysis shows a poor Faradaic efficiency for CO evolution with a high Faradaic yield for H2 evolution.

The dinuclear complex Ni2L1(η2-CS2)2 (2), featuring iminopyridine ligation, is prepared by COD substitution from Ni2L1(COD)2 (1). Spectroscopic, structural, and theoretical data reveals significant activation of the metal-bound C–S bonds, as well as the different oxidation states of the iminopyridine in 1 (1−) and 2 (0).

In this paper, we report the synthesis and reactivity of a rare mononuclear chromium(II) bis(alkoxide) complex, Cr(OR′)2(THF)2, that is supported by a new bulky alkoxide ligand (OR′ = di-t-butyl-(3,5-diphenylphenyl)methoxide). The complex is prepared by protonolysis of square-planar Cr(N(SiMe3)2)2(THF)2 with HOR′. X-ray structure determination disclosed that Cr(OR′)2(THF)2 features a distorted seesaw geometry, in contrast to nearly all other tetra-coordinate Cr(II) complexes, which are square-planar. The reactivity of Cr(OR′)2(THF)2 with aldehydes, ketones, and carbon dioxide was investigated. Treatment of Cr(OR′)2(THF)2 with two equivalents of aromatic aldehydes ArCHO (ArCHO = benzaldehyde, 4-anisaldehyde, 4-trifluorbenzaldehyde, and 2,4,6-trimethylbenzaldehyde) leads cleanly to the formation of Cr(IV) diolate complexes Cr(OR′)2(O2C2H2Ar2) that were characterized by UV-vis and IR spectroscopies and elemental analysis; the representative complex Cr(OR′)2(O2C2H2Ph2) was characterized by X-ray crystallography. In contrast, no reductive coupling was observed for ketones: treatment of Cr(OR′)2(THF)2 with one or two equivalents of benzophenone forms invariably a single ketone adduct Cr(OR′)2(OCPh2) which does not react further. QM/MM calculations suggest the steric demands prevent ketone coupling, and demonstrate that a mononuclear Cr(III) bis-aldehyde complex with partially reduced aldehydes is sufficient for C–C bond formation. The reaction of Cr(OR′)2(THF)2 with CO2 leads to the insertion of CO2 into a Cr–OR′ bond, followed by complex rearrangement to form a diamagnetic dinuclear paddlewheel complex Cr2(O2COR′)4(THF)2, that was characterized by NMR, UV-vis, and IR spectroscopy, and X-ray crystallography.

Reduction of bis(aldimino)pyridine nickel(II) dihalide forms a non-planar bis(aldimino)pyridine nickel(I) halide. Unlike the previously reported square-planar bis(ketimino)pyridine nickel chloride, for which the spin density was localized on the ligand, the present species demonstrate Npy–Ni–X angles of 156°–162° and significant spin density at the metal inferred from EPR measurements and DFT calculations.

Herein we describe bimetallic di-nickel and di-copper complexes [Ni2(L)Br4] (1) and [Cu2(L)Br4(NCMe)2] (2) (L = (1E,1′E)-N,N′-(1,4-phenylenebis(methylene))bis(1-(6-(2,4,6-triisopropylphenyl)pyridin-2-yl)methanimine)) that bind oxalate intramolecularly to form [Ni2(L)Br2(C2O4)(NCMe)] (3) and [Cu2(L)Br2(C2O4)] (4). For the di-nickel complex 1, oxalate incorporation is accompanied by a significant colour change, from red-pink (1) to deep green (3). Mass spectrometric experiments demonstrate that the compound 1 is selective for oxalate versus related mono- and di-carboxylates tested. Oxalate can be released by the addition of slight excess of calcium bromide that forms insoluble calcium oxalate and restores the original Ni2(L)Br4 species. The product of the oxalate release was crystallized as [Ni2(L)Br4]·CaBr2(THF)4 species.

The reaction of a dinucleating bis(iminopyridine) ligand L bearing a xanthene linker (L = N,N′-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine)) with Ni2(COD)2(DPA) (COD = cyclooctadiene, DPA = diphenylacetylene) leads to the formation of a new dinuclear complex Ni2(L)(DPA). Ni2(L)(DPA) can also be obtained in a one-pot reaction involving Ni(COD)2, DPA and L. The X-ray structure of Ni2(L)(DPA) reveals two square-planar Ni centers bridged by a DPA ligand. DFT calculations suggest that this species features NiI centers antiferromagnetically coupled to each other and their iminopyridine ligand radicals. Treatment of Ni2(L)(DPA) with one equivalent of ethyl propiolate (HCCCO2Et) forms the Ni2(L)(HCCCO2Et) complex. Addition of the second equivalent of ethyl propiolate leads to the observation of cyclotrimerised products by 1H NMR spectroscopy. Carrying out the reaction under catalytic conditions (1 mol% of Ni2(L)(DPA), 24 h, room temperature) transforms 89% of the substrate, forming primarily benzene products (triethyl benzene-1,2,4-tricarboxylate and triethyl benzene-1,3,5-tricarboxylate) in 68% yield, in a ca. 5:1 relative ratio. Increasing catalyst loading to 5 mol% leads to the full conversion of ethyl propiolate to benzene products; no cyclotetramerisation products were observed. In contrast, the reaction is significantly more sluggish with methyl propargyl ether. Using 1 mol% of the catalyst, only 25% conversion of methyl propargyl ether was observed within 24 h at room temperature. Furthermore, methyl propargyl ether demonstrates the formation of cyclooctatetraenes in significant amounts at a low catalyst concentration, whereas a higher catalyst concentration (5 mol%) leads to benzene products exclusively. Density functional theory was used to provide insight into the reaction mechanism, including structures of putative dinuclear metallocyclopentadiene and metallocycloheptatriene intermediates.