Herein we report the synthesis of Cr imido complexes in bis(alkoxide) ligand environments and their nitrene transfer reactivity with isocyanides. The reaction of Cr 2 (OR) 4 (OR = OC t Bu 2 Ph) with bulky aryl or alkyl azide results in the formation of the trigonal-planar Cr(IV) mono(imido) complexes Cr(OR) 2 (NR 1 ), whereas less bulky aryl azides form the Cr(VI) bis(imido) complexes Cr(OR) 2 (NR 1 ) 2 . Cr(IV) mono-(imido) complexes undergo facile reaction with 1 equiv of 2,6-dimethylphenyl isocyanide (CNR 2 ) to form the corresponding carbodiimides R 1 NCNR 2 . In contrast, no reaction of Cr(OR) 2 (NR 1 ) 2 complexes with CNR 2 is observed. The reaction of Cr(OR) 2 (NR 1 ) with excess isocyanide leads to the isolation of the Cr(II) complex Cr(OR) 2 (CNR 2 ) 4 , along with the observation of the anticipated carbodimide product. Cr(OR) 2 (CNR 2 ) 4 , which can also be obtained by treating Cr 2 (OR) 4 with 4 equiv of isocyanide, reacts with azides N 3 R 1 (R 1 = adamantyl, mesityl) to produce the respective carbodiimides. Catalytic formation of carbodiimides R 1 NCNR 2 is observed from the mixtures of azides R 1 N 3 (R 1 = mesityl, 2,6-diethylphenyl, 2-isopropylphenyl, adamantyl) and several different aryl isocyanides CNR 2 using 2.5 mol % of Cr 2 (OR) 4 . ■ INTRODUCTIONCarbodiimides are widely used organic compounds that hydrolyze readily and thus serve as excellent dehydration agents. Large-scale applications of carbodiimides include as stabilizers and antihydrolysis agents in polymers production, as coupling agents in peptide synthesis, and in various organic transformations. 1−6 Carbodiimides are synthesized by a plethora of transition-metal-catalyzed routes, 7 including oxidative coupling of amines with isocyanides 8 and coupling of two isocyanates R′NCO with loss of 1 equiv of CO 2 , 9 as well as by transition-metal-free stoichiometric routes such as the reaction of isocyanates with phosphinimines (aza-Wittig) 10 and by tinmediated addition of silylamines to isocyanates. 11 Coupling of an [NR] (nitrene) group (originating in organoazides) with isocyanide is among the most attractive methods to synthesize carbodiimides, as (1) a large variety of azides are easily accessible using simple synthetic procedures, (2) both symmetrical and asymmetrical carbodiimides can be synthesized, and (3) the only side product in this reaction is the environmentally friendly dinitrogen. Therefore, numerous efforts have been made to make carbodiimides via coupling of isocyanides and organoazides. Most of these coupling reactions take place at low-coordinate late-transition-metal catalysts stabilized by nitrogen, carbon, or phosphorus ligands, where the MNR functionality is suggested to serve as the reactive intermediate. 12−21 The combination of the soft, electron-rich late transition metals and the hard π-donating imido functionality make the imido functionality reactive and thus susceptible to transfer. In contrast, imido compounds of earlier transition metals are significantly more stable, because the higher oxidation stat...

The iron bis(alkoxide) complex Fe(OR) 2 (THF) 2 (R = C t Bu 2 Ph), 1, was found to have strikingly different reactivity with various aryl azides, ArN 3 . 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) 2 Fe(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.

The reaction of HOR' (OR' = di-t-butyl-(3,5-diphenylphenyl)methoxide) with an iron(II) amide precursor forms the iron(II) bis(alkoxide) complex Fe(OR')(THF) (2). 2 (5-10 mol %) serves as a catalyst for the conversion of aryl azides into the corresponding azoarenes. The highest yields are observed for aryl azides featuring two ortho substituents; other substitution patterns in the aryl azide precursor lead to moderate or low yields. The reaction of 2 with stoichiometric amounts (2 equiv) of the corresponding aryl azide shows the formation of azoarenes as the only organic products for the bulkier aryl azides (Ar = mesityl, 2,6-diethylphenyl). In contrast, formation of tetrazene complexes Fe(OR')(ArNNNNAr) (3-6) is observed for the less bulky aryl azides (Ar = phenyl, 4-methylphenyl, 4-methoxyphenyl, 3,5-dimethylphenyl). The electronic structure of selected tetrazene complexes was probed by spectroscopy (field-dependent Fe Mössbauer and high-frequency EPR) and density functional theory calculations. These studies revealed that Fe(OR')(ArNNNNAr) complexes contain high-spin ( S = 5/2) iron(III) centers exchange-coupled to tetrazene radical anions. Tetrazene complexes Fe(OR')(ArNNNNAr) produce the corresponding azoarenes (ArNNAr) upon heating. Treatment of a tetrazene complex Fe(OR')(ArNNNNAr) with a different azide (NAr') produces all three possible products ArNNAr, ArNNAr', and Ar'NNAr'. These experiments and quantum mechanics/molecular mechanics calculations exploring the reaction mechanism suggest that the tetrazene functionality serves as a masked form of the reactive iron mono(imido) species.

Treatment of NiCl2(dme) and NiBr2(dme) (dme = dimethoxyethane) with 2 equiv of LiOR (OR = OC(t)Bu2Ph) 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)°.

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.