Malcolm H. Chisholm

Lactide polymerization by well-defined calcium coordination complexes: comparisons with related magnesium and zinc chemistry.

Amide and alkoxide coordination complexes of calcium supported by beta-diiminato and bulky trispyrazolylborate complexes are reported together with their activity in lactide ring-opening polymerization; some are amongst the most active systems discovered to date.

New generation polymers: the role of metal alkoxides as catalysts in the production of polyoxygenates

Polyoxygenates such as polyesters and polycarbonates derived from readily renewable resources, e.g. lactide from corn and carbon dioxide, are attractive as environmentally friendly (green) alternatives to polymers derived from fossil fuels, and will play an increasing role in the market place as we move from petrochemical feedstocks. Metal–oxygen bonds to alkoxide ligands will play an increasingly important role as catalysts for ring-opening polymerizations, ring-opening copolymerizations and transesterification. This article briefly reviews current efforts in the development of single-site catalysts of the form LnMOR for the production of polyesters and polycarbonates.

Metal-Metal Bonds and Metal Carbon Bonds in the Chemistry of Molybdenum and Tungsten Alkoxides.

Abstract : Metal-metal bonds are found in alkoxides of molybdenum and tungsten when the metal atoms are in oxidation states 2 through 5. Metal-metal bonds may be locailized and multiple or single in order, or may be delocalized in cluster molecular orbitals. The structures of these metal alkoxides are quite different from those seen previously. Metal-metal bonds provide a reservoir of electrons for redox reactions: the reservoir may be tapped in oxidative- addition reactions and filled in reductive-eliminatin reactions. Alkoxide ligands may act as four or two electron donor ligands and may readily change between terminal and bridging sites. This allows for the facile interconnversion of saturated and unsaturated metal centers. As strong Pi-donor ligands, they can enhance backbonding to pi-acid ligands on the same metal. By pi-donating to vacant metal d orbitals, they may suppress metal-hydride abstraction from coordinated alkyl, alkylidene and alkylidyne ligands and they stabilize metals in high oxidation states. A variety of steric control can be engineered by choice of alkyl (RO0 groups and this may greatly influence structure, M-M bonding and reactivity of coordinated ligands.

The influence of the metal (Al, Cr, and Co) and the substituents of the porphyrin in controlling the reactions involved in the copolymerization of propylene oxide and carbon dioxide by porphyrin metal(III) complexes. 1. Aluminum chemistry.

The reactivities of aluminum(III) complexes LAlX, where L = 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TFPP), and 2,3,7,8,12,13,17,18-octaethylporphyirn (OEP) and X = Cl or OEt, have been studied with respect to their ability to homopolymerize propylene oxide (PO) and copolymerize PO and CO(2) to yield polypropylene oxide (PPO) and polypropylene carbonate (PPC), respectively, with and without the presence of a cocatalyst, namely, 4-dimethylaminopyridine (DMAP) or a PPN(+) salt where the anion is Cl(-) or N(3)(-). In the presence of a cocatalyst (0.5 equiv), the TFPP complex is the most active in copolymerization to yield PPC, with the latter being effective even at 10 bar CO(2). An increase in the PPN(+)X(-)/[Al] ratio decreases the rate of PPC formation and favors the formation of propylene carbonate, (PC). Studies of the polymers formed in reactions involving Al-alkoxide initiators and PPN(+) salts by mass spectrometry indicate that one chain is grown per Al center. These results are compared with earlier studies where the reactions display first order kinetics in the metal complex.

The tungsten-tungsten triple bond. 8. Dinuclear alkoxides of tungsten(III) and structural characterization of hexaisopropoxybis(pyridine)ditungsten, the first compound with four-coordinated tungsten atoms united by a triple bond

Abstract : The great majority of compounds containing metal-metal triple bonds of the x3mmx3 type where X is a monodentate ligand (including some where the ligand atmos are not all the same) have perfectly, or almost perfectly, staggered ethane-like rotational conformations. The great majority of compounds containing metal-metal quadruple bonds are of the type X4MMX4 ( again including those where more than one type of ligand is present) and have essentially eclipsed rotational conformations. The relatively few exceptions to each of these generalizations are of interest because they provide further insight into the nature of metal-metal multiple bonds.

Synthesis and X-ray crystal structures of the one-dimensional ribbon chains [MOBut•ButOH]∞ and the cubane species [MOBut]4 (M = K and Rb)

Abstract The series [MOBut·ButOH]∞ (M = K, Rb and Cs) have been prepared by the reaction of M with ButOH in THF. Single-crystal X-ray diffraction studies revealed for M = K or Rb that [MOBut·ButOH]∞ crystallizes from tetrahydrofuran-n-pentane at −20°C in the triclinic space group P 1 , with unit cell dimensions [K, Rb]: a = 9.862(3), 9.886(2) A; b = 9.929(4), 9.914(2) A; c = 6.330(2), 6.640(1) A; β = 90.66(2), 90.46(1)° and Z = 2, as a one-dimensional chain linked by hydrogen-bonding. The strong hydrogen-bonding within the chain O(2)O(7) (2.46, 2.44 A) also results in a near linear O(7)HO(2) angle (167.4, 172.61°) with the hydrogen atom closer to O(7) (1.22, 1.18 A), than O(2) (1.24, 1.27 A). These alcohol adducts may be readily converted into the compounds [MOBut]4 (M = K, Rb and Cs) by sublimation. Single-crystal X-ray diffraction studies reveal for M = K or Rb, that [MOBut]4 crystallizes from toluene at −20°C in the cubic space group P 4 3M; with unit cell dimensions [K, Rb]; a = 8.372(1), 8.514(1) A, and Z = 4, as cubane structures. Within the cubane-like structures the OMO angles are found to deviate only slightly from 90°, with OMO (90.18, 89.11°) and MOM (89.82, 90.89°) for M = K and Rb, respectively. The alcohol adducts undergo an alkoxide ligand exchange process that is rapid on the 1H NMR time scale at room temperature.

Conformational effects in β-diiminate ligated magnesium and zinc amides. Solution dynamics and lactide polymerization

Abstract The preparation of the compounds LMg(N i Pr 2 )(THF) ( 1 ); and LZnN i Pr 2 ( 2 ), are reported for L=the bulky β-diiminate ligand, CH(CMeN-2- t BuC 6 H 4 ) 2 . In solution compound 2 is shown to exist as a mixture of syn - and anti -rotamers that do not interconvert significantly. Compound 1 readily and reversibly dissociates THF in benzene-d 6 or toluene-d 8 from a site where THF is syn to the t Bu group of L. Both 1 and 2 are catalyst precursors for the ring-opening polymerization of lactides and it is shown that the syn -conformer of 2 reacts much faster than the anti . Polymerization of rac -lactide employing 2 in benzene or CH 2 Cl 2 or 1 in THF yield approximately 90% heterotactic PLA ( isi + sis ). These results are compared with related work by Coates [J. Am. Chem. Soc. 123 (2001) 3229]; and us [J. Chem. Soc., Dalton Trans. (2001) 222] and us employing the symmetric β-diiminate ligand CH(CMeN-2,6- i Pr 2 C 6 H 3 ) 2 .

Concerning the ring-opening polymerization of lactide and cyclic esters by coordination metal catalysts

A summary of the reactions involved in the ring-opening polymerization (ROP) of lactide (LA) to give polylactides (PLAs) is presented along with competing reactions. Particular attention is given to the stereoselective polymerization of rac-LA to give heterotactic PLA and meso-LA to give syndiotactic PLA by aluminum Schiff-based catalysts and to the development of highly active group 2 metal single-site catalysts. Melt or solvent-free polymerization is also described along with reactions that lead exclusively to cyclic-polylactides.

Electron delocalization in the S1 and T1 metal-to-ligand charge transfer states of trans-substituted metal quadruply bonded complexes

The singlet S1 and triplet T1 photoexcited states of the compounds containing MM quadruple bonds trans-M2(TiPB)2(O2CC6H4-4-CN)2, where TiPB = 2,4,6-triisopropylbenzoate and M = Mo (I) or M = W (I′), and trans-M2(O2CMe)2((N[i Pr ])2CC ≡ CC6H5)2, where M = Mo (II) and M = W (II′), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I′ but localized for II and II′ involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMoδδ* for both I and II but delocalized 3MLCT for I′ and localized 3MLCT for II′. These differences arise from consideration of the relative orbital energies of the M2δ or M2δ* and the ligand π∗ as well as the magnitudes of orbital overlap.

Chemistry of magnesium alkyls supported by 1,5,9-trimesityldipyrromethene and 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene. A comparative study

The compounds LMgBun(THF) and L′MgBun(THF) where L = 1,5,9-trimesityldipyrromethene and L′ = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene have been prepared from reactions between MgBun2 and the protonated ligands LH and L′H, respectively. Single crystal X-ray crystallographic studies reveal that in each compound the Mg2+ ion is in a distorted tetrahedral environment and further that the ligand L is more sterically demanding with respect to access to the Mg–Bun group. Related alkyl complexes (R = Me, Et, Prn, Pri, n-hexyl and CH2CH2Ph) were prepared from reactions involving LLi(THF) or L′Li(THF) and the appropriate RMgX (X = Cl or Br) and characterized by 1H NMR spectroscopy. Those where R = CH2CH2X (X = alkyl or phenyl) have characteristic α-CH2 proton resonances which are the part of an AA′XX′ pattern. Reactions with alcohols ROH (1 equiv.) give the kinetic products LMg(OR)(THF) and L′Mg(OR)(THF) which are isolable and kinetically persistent when R = a bulky group such as But and the structure of LMg(OBut)(THF) is reported and compared with that of the known compound L′Mg(OBut)(THF). With less bulky groups the compounds are labile toward ligand scrambling and the compound L′MgBun(THF) reacts with alcohols (2 equiv.) to give L′H and Mg(OR)2. Reactions with amines and carbon dioxide are described which indicate the greater reactivity of the Mg–Bun group relative to both the L′ and L ligand. With PhCHO, Ph2CO and cyclohexanone both LMgBun(THF) and L′MgBun(THF) react via β-hydrogen atom transfer to generate the appropriate alkoxide with the elimination of 1-butene. Similarly L-lactide reacts by β-H transfer to give poly-L-lactide (P-L-LA) and 1-butene while rac-lactide yields atactic polylactide in toluene–dichloromethane solutions but in the presence of ≥10 equiv. of THF, heterotactic PLA is formed. The ring-opening polymerization of e-caprolactone also is initiated by β-H transfer and is about ten times faster than for lactide; kp(LA) = 10.7 M−1 s−1vs. kp(CL) = 110 M−1 s−1. Interestingly, the presence of lactide completely suppresses the polymerization of e-caprolactone.