Calcium complexes containing oxalamidinate ligands as catalysts for ε-caprolactone polymerization

Abstract

A series of calcium complexes containing oxalamidinate ligands is described. Reactions of oxalamidinate ligand precursors, [PhN=C{NH(CH2)2OMe}-C{NH(CH2)2OMe}=NPh] (1), [PhN=C{NH(CH2)2NMe2}-C{NH(CH2)2NMe2}=NPh] (2), [Ph(TriMe)N=C{NH(CH2)2OMe}-C{NH(CH2)2OMe}=NPh(TriMe)] (3), [Ph(TriMe)N=C{NH(CH2)2SMe}-C{NH(CH2)2SMe}=NPh(TriMe)] (4), [Ph(TriMe)N=C{NHCH2Py}-C{NHCH2Py}=NPh(TriMe)] (5), with two molar equivalents of Ca[N(SiMe3)2]2(THF)2 gave calcium oxalamidinate complexes, [Ca{N(SiMe3)2}(THF)(PhN)C{N(CH2)2OMe}-]2 (6), [Ca{N(SiMe3)2}(THF)(PhN)C{N(CH2)2NMe2}-]2 (7), [Ca{N(SiMe3)2}(THF)(Ph(TriMe)N)C{N(CH2)2OMe}-]2 (8), [Ca{N(SiMe3)2}(THF)(Ph(TriMe)N)C{N(CH2)2SMe}-]2 (9), [Ca{N(SiMe3)2}(THF)(Ph(TriMe)N)C{NCH2Py}-]2 (10), respectively. The molecular structure of complex 7 was further characterized by the single crystal X-ray diffraction technique. The catalytic activities of complexes 6-10 toward the ring opening polymerization of ε-caprolactone were studied. Complex 8 exhibited excellent activity for the polymerization of ε-caprolactone with controlled molecular weights and polydispersities.

Metal–Ligand Interplay Governs the Coordination Flexibility and Catalytic Performance of Ca (II) , Mg (II) , and Zn (II) Complexes Supported by Benzamidinate‐Oxazolinate Type Ligands

“…Overall, these structural features confirm that while complexes 2 and 3 share the same octahedral coordination framework, the calcium center exhibits longer and more labile metal-nitrogen interactions than magnesium, highlighting a greater degree of coordination flexibility in the Ca 2+ system. The observed Ca-N and Mg-N distances fall within reported ranges for calcium-and magnesium-nitrogen complexes, confirming the validity of the coordination environments [65,[79][80][81][82]. In contrast to the six-coordinate geometries observed for complexes 2 and 3, complex 4 adopts a five-coordinate distorted trigonal bipyramidal geometry (τ₅ = 0.83) [83], making it the only five-coordinate species in this series.…”

“…In particular, monoanionic N,N′-chelating ligands such as β-diketiminates, amidinates, and related frameworks have been extensively explored because of their strong chelation ability, tunable steric, and electronic properties . These ligands can effectively stabilize reactive metal centers while allowing systematic modification of ligand substituents, thereby providing an important platform for investigating structure-activity relationships in ROP catalysis [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65]. Previous studies, including our own, have demonstrated that amidinatesupported zinc and magnesium complexes exhibit notable catalytic activity toward cyclic ester polymerization and that subtle variations in ligand architecture can result in significant differences in polymerization efficiency and controlled behavior (Figure 1, I and II) [66].…”

Metal–Ligand Interplay Governs the Coordination Flexibility and Catalytic Performance of Ca (II) , Mg (II) , and Zn (II) Complexes Supported by Benzamidinate‐Oxazolinate Type Ligands

“…Overall, these structural features confirm that while complexes 2 and 3 share the same octahedral coordination framework, the calcium center exhibits longer and more labile metal-nitrogen interactions than magnesium, highlighting a greater degree of coordination flexibility in the Ca 2+ system. The observed Ca-N and Mg-N distances fall within reported ranges for calcium-and magnesium-nitrogen complexes, confirming the validity of the coordination environments [65,[79][80][81][82]. In contrast to the six-coordinate geometries observed for complexes 2 and 3, complex 4 adopts a five-coordinate distorted trigonal bipyramidal geometry (τ₅ = 0.83) [83], making it the only five-coordinate species in this series.…”

“…In particular, monoanionic N,N′-chelating ligands such as β-diketiminates, amidinates, and related frameworks have been extensively explored because of their strong chelation ability, tunable steric, and electronic properties . These ligands can effectively stabilize reactive metal centers while allowing systematic modification of ligand substituents, thereby providing an important platform for investigating structure-activity relationships in ROP catalysis [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65]. Previous studies, including our own, have demonstrated that amidinatesupported zinc and magnesium complexes exhibit notable catalytic activity toward cyclic ester polymerization and that subtle variations in ligand architecture can result in significant differences in polymerization efficiency and controlled behavior (Figure 1, I and II) [66].…”

Ligands with Two Monoanionic N,N‐Binding Sites: Synthesis and Coordination Chemistry

“…In addition, mononuclear complexes of type U are also known and examples incorporating B, Mo, Ru, and Si have been isolated so far. Finally, quite a few dinuclear type W ′′ complexes have been reported in the literature, which are all based on oxalamidinates, and cover the elements Al, B, Ca, Co, Ga, In, Li, Mo, Ni, Pd, Rh, Ru, Ti, U, and Zn . These are either directly obtained from the bis(amidine) or from their alkali‐metal salts by salt metathesis .…”

“…[14i] Monometallic type T complexes incorporating Ta or Zr are also selectively synthesized in the same way, [15c] whereas with lanthanum and samarium dinuclear type Y-complexes or even higher aggregates such as at etranuclearl anthanum complex, are obtained. [14i] In addition, mononuclear complexes of type U are also known and examples incorporatingB , [40] Mo, [41] Ru, [42] and Si [43] have been isolated so far.F inally,q uite af ew dinuclear type W'' complexes have been reported in the literature, which are all based on oxalamidinates,a nd cover the elements Al, [44] B, [45] Ca, [46] Co, [47] Ga, [44] In, [44] Li, [48] Mo, [41a] Ni, [15d, 49] Pd, [49b, 50] Rh, [15i] Ru, [42] Ti, [51] U, [48] and Zn. [47, 49b, 52] These are either directly obtainedf rom the bis(amidine) [15d, 42, 44, 45, 49b,c, 50b, 52] or from their alkali-metal salts by salt metathesis.…”

Hybrid amidinates and guanidinates of main group metals

“…On the other hand the cerium complex is dinuclear withbridging Cl atoms and the coordination neighborhood of the metal being monocapped octahedron with two ligands, where only one is tridentate, and two chlorine atoms.Scheme 9: Synthesis of magnesium oxalamidinato complexesBisamidinato ligands (oxalamidinates) prepared by the procedures from Scheme 3 and 9 were used for preparation of aluminium and magnesium complexes[19] with pendant amino and ether groups which have been structurally investigated by crystallographic techniques metals is trigonal bipyramidal with the arrangement of central atoms outside of amidinato pocket. Modification of ligands by using thioether or 2-methypyridyl groups and variation steric properties of the aromatic ring together with previously used ligands were employed in synthesis of calcium compounds[31] which have similar structures as analogous Mg and Al complexes. The catalytic activities of selected Ca complexes toward the ring opening polymerization of ε-caprolactone were studied.…”

Metal–Ligand Interplay Governs the Coordination Flexibility and Catalytic Performance of Ca (II) , Mg (II) , and Zn (II) Complexes Supported by Benzamidinate‐Oxazolinate Type Ligands

“…Overall, these structural features confirm that while complexes 2 and 3 share the same octahedral coordination framework, the calcium center exhibits longer and more labile metal-nitrogen interactions than magnesium, highlighting a greater degree of coordination flexibility in the Ca 2+ system. The observed Ca-N and Mg-N distances fall within reported ranges for calcium-and magnesium-nitrogen complexes, confirming the validity of the coordination environments [65,[79][80][81][82]. In contrast to the six-coordinate geometries observed for complexes 2 and 3, complex 4 adopts a five-coordinate distorted trigonal bipyramidal geometry (τ₅ = 0.83) [83], making it the only five-coordinate species in this series.…”

“…In particular, monoanionic N,N′-chelating ligands such as β-diketiminates, amidinates, and related frameworks have been extensively explored because of their strong chelation ability, tunable steric, and electronic properties . These ligands can effectively stabilize reactive metal centers while allowing systematic modification of ligand substituents, thereby providing an important platform for investigating structure-activity relationships in ROP catalysis [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65]. Previous studies, including our own, have demonstrated that amidinatesupported zinc and magnesium complexes exhibit notable catalytic activity toward cyclic ester polymerization and that subtle variations in ligand architecture can result in significant differences in polymerization efficiency and controlled behavior (Figure 1, I and II) [66].…”

Ligands with Two Monoanionic N,N‐Binding Sites: Synthesis and Coordination Chemistry

“…In addition, mononuclear complexes of type U are also known and examples incorporating B, Mo, Ru, and Si have been isolated so far. Finally, quite a few dinuclear type W ′′ complexes have been reported in the literature, which are all based on oxalamidinates, and cover the elements Al, B, Ca, Co, Ga, In, Li, Mo, Ni, Pd, Rh, Ru, Ti, U, and Zn . These are either directly obtained from the bis(amidine) or from their alkali‐metal salts by salt metathesis .…”

“…[14i] Monometallic type T complexes incorporating Ta or Zr are also selectively synthesized in the same way, [15c] whereas with lanthanum and samarium dinuclear type Y-complexes or even higher aggregates such as at etranuclearl anthanum complex, are obtained. [14i] In addition, mononuclear complexes of type U are also known and examples incorporatingB , [40] Mo, [41] Ru, [42] and Si [43] have been isolated so far.F inally,q uite af ew dinuclear type W'' complexes have been reported in the literature, which are all based on oxalamidinates,a nd cover the elements Al, [44] B, [45] Ca, [46] Co, [47] Ga, [44] In, [44] Li, [48] Mo, [41a] Ni, [15d, 49] Pd, [49b, 50] Rh, [15i] Ru, [42] Ti, [51] U, [48] and Zn. [47, 49b, 52] These are either directly obtainedf rom the bis(amidine) [15d, 42, 44, 45, 49b,c, 50b, 52] or from their alkali-metal salts by salt metathesis.…”

Hybrid amidinates and guanidinates of main group metals

“…On the other hand the cerium complex is dinuclear withbridging Cl atoms and the coordination neighborhood of the metal being monocapped octahedron with two ligands, where only one is tridentate, and two chlorine atoms.Scheme 9: Synthesis of magnesium oxalamidinato complexesBisamidinato ligands (oxalamidinates) prepared by the procedures from Scheme 3 and 9 were used for preparation of aluminium and magnesium complexes[19] with pendant amino and ether groups which have been structurally investigated by crystallographic techniques metals is trigonal bipyramidal with the arrangement of central atoms outside of amidinato pocket. Modification of ligands by using thioether or 2-methypyridyl groups and variation steric properties of the aromatic ring together with previously used ligands were employed in synthesis of calcium compounds[31] which have similar structures as analogous Mg and Al complexes. The catalytic activities of selected Ca complexes toward the ring opening polymerization of ε-caprolactone were studied.…”

Metal–Ligand Interplay Governs the Coordination Flexibility and Catalytic Performance of Ca (II) , Mg (II) , and Zn (II) Complexes Supported by Benzamidinate‐Oxazolinate Type Ligands

“…Overall, these structural features confirm that while complexes 2 and 3 share the same octahedral coordination framework, the calcium center exhibits longer and more labile metal-nitrogen interactions than magnesium, highlighting a greater degree of coordination flexibility in the Ca 2+ system. The observed Ca-N and Mg-N distances fall within reported ranges for calcium-and magnesium-nitrogen complexes, confirming the validity of the coordination environments [65,[79][80][81][82]. In contrast to the six-coordinate geometries observed for complexes 2 and 3, complex 4 adopts a five-coordinate distorted trigonal bipyramidal geometry (τ₅ = 0.83) [83], making it the only five-coordinate species in this series.…”

“…In particular, monoanionic N,N′-chelating ligands such as β-diketiminates, amidinates, and related frameworks have been extensively explored because of their strong chelation ability, tunable steric, and electronic properties . These ligands can effectively stabilize reactive metal centers while allowing systematic modification of ligand substituents, thereby providing an important platform for investigating structure-activity relationships in ROP catalysis [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65]. Previous studies, including our own, have demonstrated that amidinatesupported zinc and magnesium complexes exhibit notable catalytic activity toward cyclic ester polymerization and that subtle variations in ligand architecture can result in significant differences in polymerization efficiency and controlled behavior (Figure 1, I and II) [66].…”

Ligands with Two Monoanionic N,N‐Binding Sites: Synthesis and Coordination Chemistry

“…In addition, mononuclear complexes of type U are also known and examples incorporating B, Mo, Ru, and Si have been isolated so far. Finally, quite a few dinuclear type W ′′ complexes have been reported in the literature, which are all based on oxalamidinates, and cover the elements Al, B, Ca, Co, Ga, In, Li, Mo, Ni, Pd, Rh, Ru, Ti, U, and Zn . These are either directly obtained from the bis(amidine) or from their alkali‐metal salts by salt metathesis .…”

“…[14i] Monometallic type T complexes incorporating Ta or Zr are also selectively synthesized in the same way, [15c] whereas with lanthanum and samarium dinuclear type Y-complexes or even higher aggregates such as at etranuclearl anthanum complex, are obtained. [14i] In addition, mononuclear complexes of type U are also known and examples incorporatingB , [40] Mo, [41] Ru, [42] and Si [43] have been isolated so far.F inally,q uite af ew dinuclear type W'' complexes have been reported in the literature, which are all based on oxalamidinates,a nd cover the elements Al, [44] B, [45] Ca, [46] Co, [47] Ga, [44] In, [44] Li, [48] Mo, [41a] Ni, [15d, 49] Pd, [49b, 50] Rh, [15i] Ru, [42] Ti, [51] U, [48] and Zn. [47, 49b, 52] These are either directly obtainedf rom the bis(amidine) [15d, 42, 44, 45, 49b,c, 50b, 52] or from their alkali-metal salts by salt metathesis.…”

Hybrid amidinates and guanidinates of main group metals

“…On the other hand the cerium complex is dinuclear withbridging Cl atoms and the coordination neighborhood of the metal being monocapped octahedron with two ligands, where only one is tridentate, and two chlorine atoms.Scheme 9: Synthesis of magnesium oxalamidinato complexesBisamidinato ligands (oxalamidinates) prepared by the procedures from Scheme 3 and 9 were used for preparation of aluminium and magnesium complexes[19] with pendant amino and ether groups which have been structurally investigated by crystallographic techniques metals is trigonal bipyramidal with the arrangement of central atoms outside of amidinato pocket. Modification of ligands by using thioether or 2-methypyridyl groups and variation steric properties of the aromatic ring together with previously used ligands were employed in synthesis of calcium compounds[31] which have similar structures as analogous Mg and Al complexes. The catalytic activities of selected Ca complexes toward the ring opening polymerization of ε-caprolactone were studied.…”