ABSTRACT
The bis-guanidyl compound H(2)C{hpp}(2) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) coordinates to palladium(ii) as a neutral, chelating N,N'-bidentate ligand. Structural analysis of PdMeCl(H(2)C{hpp}(2)) shows a non-planar metallacycle that is twisted relative to the square plane of the metal and an "anagostic" interaction with a C-H from the bridging methylene group. Generation of the cationic palladium complex, [PdCl(H(2)C{hpp}(2))][OTf] was achieved by halide abstraction from the dichloride PdCl(2)(H(2)C{hpp}(2)) using NaOTf. The product was identified as a mixture of different species in solution; in the solid-state, the molecular structure is dimeric, consisting of the mu,mu'-dichlorobridged dication. The new bis-guanidyl compound H(2)C{tbo}(2) (Htbo = 1,4,6-triaza-bicyclo[3.3.0]oct-4-ene) was synthesized, and structurally characterized. Coordination of this compound at palladium dichloride was accompanied by ionization of the Pd-Cl bond, and formation of the dication [Pd(H(2)C{tbo}(2))(2)][Cl](2). Structural analysis shows a significant reduction in the twisting of this ligand.
ABSTRACT
The bis-guanidino compound H(2)C{hpp}(2) (I; hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) has been converted to the monocation [I-H](+) and isolated as the chloride and tetraphenylborate salts. Solution-state spectroscopic data do not differentiate the protonated guanidinium from the neutral guanidino group but suggest intramolecular "-N-H...N=" hydrogen bonding to form an eight-membered C(3)N(4)H heterocycle. Solid-state CPMAS (15)N NMR spectroscopy confirms protonation at one of the imine nitrogens, although line broadening is consistent with solid-state proton transfer between guanidine functionalities. X-ray diffraction data have been recorded over the temperature range 50-273 K. Examination of the carbon-nitrogen bond lengths suggests a degree of "partial protonation" of the neutral guanidino group at higher temperatures, with greater localization of the proton at one nitrogen position as the temperature is lowered. Difference electron density maps generated from high-resolution X-ray diffraction studies at 110 K give the first direct experimental evidence for proton transfer in a poly(guanidino) system. Computational analysis of I and its conjugate acid [I-H](+) indicate strong cationic resonance stabilization of the guanidinium group, with the nonprotonated group also stabilized, albeit to a lesser extent. The maximum barrier to proton transfer calculated using the Boese-Martin for kinetics method was 2.8 kcal mol(-1), with hydrogen-bond compression evident in the transition state; addition of zero-point vibrational energy values leads to the conclusion that the proton transfer is barrierless, implying that the proton shuttles freely between the two nitrogen atoms. Calculations determining the gas-phase proton affinity and the pK(a) in acetonitrile both indicate that compound I should behave as a superbase. This has been confirmed by spectrophotometric titrations in MeCN using polyphosphazene references, which give an average pK(a) of 28.98 +/- 0.05. Triadic analysis indicates that the dominant term causing the high basicity is the relaxation energy.
ABSTRACT
The methylene-linked bis{guanidine}, H(2)C{hpp}(2) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), displays nucleophilic activity towards organic halides, including the activation of dichloromethane under ambient conditions affording the cyclic dication, [H(2)C{hpp}(2)CH(2)](2+)[Cl](2).
ABSTRACT
Protonation of the linked bis(guanidine), H(2)C{hpp}(2), affords isolated guanidinium salts that have been used to prepare a coordinatively saturated aluminium cation.
ABSTRACT
The methylene-linked bicyclic guanidine based on the 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) framework provides a versatile platform able to support trigonal-planar, tetrahedral, and square-planar metal centers.
Subject(s)
Guanidines/chemistry , Guanidines/chemical synthesis , Models, Molecular , Organometallic Compounds/chemistry , Organometallic Compounds/chemical synthesis , Bridged Bicyclo Compounds , Crystallography, X-Ray , Molecular Conformation , Molecular StructureABSTRACT
The synthesis and structure of copper(I) complexes supported by N-substituted bicyclic guanidines is described. The N-methyl-substituted bicyclic guanidine 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine (hppMe) reacted with copper(I) chloride to afford the ion-pair [Cu(hppMe)(2)][CuCl(2)] (1), a rare example of a compound containing an unsupported Cu...Cu interaction. The analogous reaction with CuI, however, afforded the molecular mu,mu-dihalo-bridged dimer [CuI(hppMe)](2) (2). Inclusion of a trimethylsilyl substituent at nitrogen provided a sufficiently sterically encumbered environment to support a two coordinate copper center in CuCl(hppSiMe(3)) (3). Compounds 1-3 have been fully characterized, including single-crystal X-ray diffraction studies.
Subject(s)
Copper/chemistry , Guanidine/chemistry , Organometallic Compounds/chemistry , Organometallic Compounds/chemical synthesis , Crystallography, X-Ray , Hydrogen Bonding , Ligands , Magnetic Resonance Spectroscopy , Molecular Conformation , Molecular Structure , Silanes/chemical synthesis , Silanes/chemistryABSTRACT
A series of structurally characterised, monovalent metal-halide complexes incorporating neutral amidine and guanidine ligands is reported. N,N'-diphenylbenzamidine reacted with copper(I) chloride to afford the bis-ligand complex [CuCl(PhC[NPh][NHPh])2]2 (1), that exists as a chlorine bridged dimer in the solid state, with a non-symmetrical distribution of NH...Cl interactions within the 'Cu2Cl2' metallacycle. In contrast, only one equivalent of the guanidine, Me2NC[NiPr][NHiPr] (2), is coordinated in the copper(I) iodide complex [CuI(Me2NC[NiPr][NHiPr])]2 (3), which was also isolated as the dimer with bridging halide atoms. The molecular structure of the bicyclic guanidine, 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-alpha]pyrimidine (hppH), is reported, revealing a hydrogen bridged dimer with extensive delocalisation throughout the ligand framework. Coordination of hppH to lithium chloride afforded the dimeric bis-ligand complex [LiCl(hppH)2]2 (4) in which each hppH molecule interacts with a different chlorine atom of the central 'Li2Cl2' core of the molecule via NH...Cl hydrogen bonding. In contrast the 2:1 ligand to metal complex is formed with silver(I) chloride to afford AgCl(hppH)2 (5), a unique example of a monomeric, three-coordinate silver chloride supported by nitrogen-based ligands. The series of mixed ligand complexes [CuX(hppH)(PPh3)]n (6, X = Cl, n= 1; 7, X = Br, n= 2; 8 X = I, n= 2) have also been synthesised and structurally characterised, allowing comparisons of the relative coordinating behaviour of hppH and PPh3 as neutral donors at copper(I) centres to be made.
Subject(s)
Amidines/chemistry , Copper/chemistry , Guanidines/chemistry , Organometallic Compounds/chemistry , Bromine Compounds/chemistry , Chlorine Compounds/chemistry , Crystallography, X-Ray , Hydrogen Bonding , Iodine Compounds/chemistry , Ligands , Models, Molecular , Molecular StructureABSTRACT
The series of poly(guanidyl)silanes RnSi[hpp](4-n) [hpp = 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidinate; R = Me, n= 1, 2; R = Ph, n= 1] have been synthesised and their coordination behaviour at copper(I) halides investigated.
ABSTRACT
The neutral, bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) coordinates to Cu(I) halides via the imine nitrogen, and the complexes are stabilized by intramolecular NH...X interactions (X = Cl, Br) or are partially dissociated (X = I) in the solid state; in all cases, fluxional behavior is observed in solution. Selected complexes have been tested as catalysts for the polymerization of methyl methacrylate.
