"Catch-Store-Release" Strategy for the Stabilization of 4,4'-Methylene Diphenyl Diisocyanate (4,4'-MDI).

4,4'-Methylene diphenyl diisocyanate (MDI) is an industrially crucial compound, being one of the most utilized linkers in the polyurethane industry. However, its long-term stability is limited due to dimerization to form insoluble uretdione. Herein we demonstrate an organometallic "catch-store-release" concept aiming at improving the long-term chemical stability of MDI. Treatment of MDI with two equivalents of selected N-heterocyclic carbenes (NHC) forms stable MDI-NHC adducts. Treatment of the adducts with CuCl forms metastable di-CuI complexes that undergo decomposition to re-form MDI (up to 85 %), along with Cu-NHC complexes. The yield of re-formed MDI can be improved (up to 95 %) by the release of the NHC ligands in the form of thiourea; this prevents subsequent MDI dimerization/polymerization by the carbenes. Furthermore, the need to separate MDI from the reaction mixture can be eliminated by the direct reaction of MDI-NHC complexes with alcohols (as models for diols), that form dicarbamate (as a model for polyurethane) quantitatively.

Synthesis, electronic structures, and reactivity of mononuclear and dinuclear low-valent molybdenum complexes in iminopyridine and bis(imino)pyridine ligand environments.

Molybdenum in redox non-innocent ligand environments features prominently in biological inorganic systems. While Holm and coworkers, along with many other researchers, have thoroughly investigated formally high-oxidation-state molybdenum (Mo(IV)-Mo(VI)) ligated by dithiolenes, less is known about molybdenum in other formal oxidation states and/or different redox-active ligand environments. This work focuses on the investigation of low-valent molybdenum in four different redox non-innocent nitrogen ligand type environments (mononucleating and dinucleating iminopyridine, mononucleating and dinucleating bis(imino)pyridine). The reaction of iminopyridine N-(2,6-diisopropylphenyl)-1-(pyridin-2-yl)methanimine (L1) with Mo(CO)3(NCMe)3 produced Mo(L1)(CO)3(NCMe). Mo(L1)(CO)3(NCMe) undergoes transformation to Mo(L1)(CO)4 upon treatment with CS2 or prolonged stirring in dichloromethane. The reaction of the open-chain dinucleating bis(iminopyridine) ligand N,N'-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine) (L2) similarly produced an hexacarbonyl complex Mo2(L2)(CO)6(NCMe)2 which also underwent transformation to the octacarbonyl Mo2(L2)(CO)8. Both complexes featured anti-parallel geometry of the chelating units. The oxidation of Mo(L1)(CO)3(NCMe) with I2 resulted in Mo(L1)(CO)3I2. The reaction of mononucleating potentially tridentate bis(imino)pyridine ligand (L3) (N-mesityl-1-(6-((E)-(mesitylimino)methyl)pyridin-2-yl)methanimine) with both Mo(CO)3(NCMe)3 and Mo(CO)4(NCMe)2 produced complexes Mo(L3)(CO)3(NCMe) and Mo(L3)(CO)4 in which L3 was coordinated in a bidentate fashion, with one imino sidearm unbound. The reaction of dinucleating macrocyclic di(bis(imino)pyridine) analogue (L4) led to the similar chemistry of Mo2(L4)(CO)6(NCMe)2 and Mo2(L4)(CO)8 complexes. Treatment of Mo(L3)(CO)3(NCMe) with I2 formed a mono(carbonyl) complex Mo(L3)(CO)I2 in which molybdenum was formally oxidized and L3 underwent coordination mode change to tridentate. The electronic structures of formally Mo(0) complexes in iminopyridine and bis(imino)pyridine ligand environments were investigated by density functional theory calculations.

Synthesis and Cu(I)/Mo(VI) Reactivity of a Bifunctional Heterodinucleating Ligand on a Xanthene Platform.

In an effort to probe the feasibility of a model of Mo-Cu CODH (CODH = carbon monoxide dehydrogenase) lacking a bridging sulfido group, the new heterodinucleating ligand LH2 was designed and its Cu(I)/Mo(VI) reactivity was investigated. LH2 ((E)-3-(((5-(bis(pyridin-2-ylmethyl)amino)-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthen-4-yl)imino)methyl)benzene-1,2-diol) features two different chelating positions bridged by a xanthene linker: bis(pyridyl)amine for Cu(I) and catecholate for Mo(VI). LH2 was synthesized via the initial protection of one of the amine positions, followed by two consecutive alkylations of the second position, deprotection, and condensation to attach the catechol functionality. LH2 was found to exhibit dynamic cooperativity between two reactive sites mediated by H-bonding of the catechol protons. In the free ligand, catechol protons exhibit H-bonding with imine (intramolecular) and with pyridine (intermolecular in the solid state). The reaction of LH2 with [Cu(NCMe)4]+ led to the tetradentate coordination of Cu(I) via all nitrogen donors of the ligand, including the imine. Cu(I) complexes were characterized by multinuclear NMR spectroscopy, high-resolution mass spectrometry (HRMS), X-ray crystallography, and DFT calculations. Cu(I) coordination to the imine disrupted H-bonding and caused rotation away from the catechol arm. The reaction of the Cu(I) complex [Cu(LH2)]+ with a variety of monodentate ligands X (PPh3, Cl-, SCN-, CN-) released the metal from coordination to the imine, thereby restoring imine H-bonding with the catechol proton. The second catechol proton engages in H-bonding with Cu-X (X = Cl, CN, SCN), which can be intermolecular (XRD) or intramolecular (DFT). The reaction of LH2 with molybdate [MoO4]2- led to incorporation of [MoVIO3] at the catecholate position, producing [MoO3(L)]2-. Similarly, the reaction of [Cu(LH2)]+ with [MoO4]2- formed the heterodinuclear complex [CuMoO3(L)]-. Both complexes were characterized by multinuclear NMR, UV-vis, and HRMS. HRMS in both cases confirmed the constitution of the complexes, containing molecular ions with the expected isotopic distribution.

Synthesis of a mononuclear magnesium bis(alkoxide) complex and its reactivity in the ring-opening copolymerization of cyclic anhydrides with epoxides.

Synthesis of a new mononuclear magnesium complex with a bulky bis(alkoxide) ligand environment and its reactivity in ring-opening polymerization (ROP) and ring-opening copolymerization (ROCOP) are reported. Reaction of n-butyl-sec-butylmagnesium with two equivalents of HOR (HOR = di-tert-butylphenylmethanol, HOCtBu2Ph) formed Mg(OR)2(THF)2. The reaction proceeded via the Mg(OR)(sec-Bu)(THF)2 intermediate that was independently synthesized by treating n-butyl-sec-butylmagnesium with one equivalent of HOR. Mg(OR)2(THF)2 led to active albeit not well-controlled ROP of rac-lactide. In contrast, well-controlled ROCOP of epoxides with cyclic anhydrides was observed, including efficient and alternating copolymerization of phthalic anhydride with cyclohexene oxide as well as rare copolymerization of phthalic anhydride with limonene oxide and terpolymerization of phthalic anhydride with both cyclohexene oxide and limonene oxide. In addition, novel copolymerization of dihydrocoumarin with limonene oxide is described.

Cooperative bimetallic reactivity of a heterodinuclear molybdenum-copper model of Mo-Cu CODH.

The synthesis of a heterodinucleating ligand LH2 (LH2 = (E)-3-(((2,7-di-tert-butyl-9,9-dimethyl-5-((pyridin-2-ylmethylene)amino)-9H-xanthen-4-yl)amino)methyl)benzene-1,2-diol) was undertaken toward a functional model of the bimetallic active site found in Mo-Cu carbon monoxide dehydrogenase (Mo-Cu CODH), and to understand the origins of heterobimetallic cooperativity exhibited by the enzyme. LH2 features a hard potentially dianionic catechol chelate for binding Mo(vi) and a soft iminopyridine chelate for binding Cu(i). Treatment of LH2 with either Cu(i) or M(vi) (M = Mo, W) sources leads to the anticipated site-selective incorporation of the respective metals. While both [CuI(LH2)]+ and [MVIO3(L)]2- complexes are stable in the solid state, [MVIO3(L)]2- complexes disproportionate in solution to give [MVIO2(L)2](NEt4)2 complexes, with [MVIO4]2- as the by-product. The incorporation of BOTH Mo(vi) and Cu(i) into L forms a highly reactive heterobimetallic complex [MoVIO3CuI(L)](NEt4)2, whose formation and reactivity was interrogated via1H NMR/UV-vis spectroscopy and DFT calculations. These studies reveal that the combination of the two metals triggers oxidation reactivity, in which a nucleophilic Mo(vi) trioxo attacks Cu(i)-bound imine. The major product of the reaction is a crystallographically characterized molybdenum(vi) complex [Mo(L')O2](NEt4) coordinated by a modified ligand L' that contains a new C-O bond in place of the imine functionality. This observed hydroxylation reactivity is consistent with the postulated first step of Mo-Cu CODH (nucleophilic attack of the Mo(vi)-oxo on the Cu(i)-bound electrophilic CO) and xanthine oxidoreductase (nucleophilic attack of Mo(vi)-oxo on the electrophilic xanthine carbon).