A synthetic route to magnesium guanidinate compounds that avoids ligand redistribution is reported; selected derivatives are active pre-catalysts in the dimerization of aldehydes.

Spectroscopic, crystallographic, and computational studies of the substituent distribution about the "NCN" unit in a series of phospha(III)- and phospha(V)-guanidines, R(2)PC{NR'}{NHR'} and R(2)P(E)C{NR'}{NHR'} (R = Ph, Cy; R' = (i)Pr, Cy; E = S, Se), are reported. In the phosphorus(III) systems, the P-diphenyl substituted compounds are observed as only one isomer, shown by NMR spectroscopy to be the E(syn)-(alpha) configuration. In contrast, the corresponding P-dicyclohexyl derivatives exist as a mixture of E(syn)-(alpha) and Z(anti) in solution. Spectroscopic techniques are unable to determine whether the latter isomer exists as the alpha- or beta-conformer relative to rotation about the P-C(amidine)() bond; however, DFT calculations indicate a low-energy structure for the N,N'-dimethyl model complex in the beta-conformation. In their oxidized sulfo and seleno forms, the P-diphenyl compounds are present as an interconverting equilibrium mixture of the E(syn)-(beta) and Z(syn)-(beta) isomers in solution ( approximately 3:2 ratio), whereas for the P-dicyclohexyl analogues, the latter configuration (in which the nitrogen substituents are in a more sterically unfavorably cisoid arrangement about the imine double bond) is the dominant form. Intramolecular E...HN (E = S, Se) interactions are observed in solution for the Z(syn)-(beta) configuration of both P-substituted species, characterized by J(SeH) coupling in the NMR spectrum for the P(V)-seleno compounds and a bathochromic shift of the NH absorption in the infrared spectrum. An X-ray crystallographic analysis of representative Ph(2)P(E)- and Cy(2)P(E)-substituted species shows exclusively the E(syn)-(beta) configuration for the P-diphenyl substituted compounds and the Z(syn)-(beta) form for the P-dicyclohexyl derivatives, independent of the chalcogen and the nitrogen substituents. Results from a DFT analysis of model compounds fail to identify a compelling electronic argument for the observed preferences in substituent orientation, suggesting that steric factors play an important role in determining the subtle energetic differences at work in these systems.

Diisopropylcarbodiimide, (i)PrN[double bond, length as m-dash]C[double bond, length as m-dash]N(i)Pr, inserts into the lithium-phosphorus bond of in situ prepared "Ph(2)PLi(THF)(n)" to afford the lithium salt, [Li(Ph(2)PC{N(i)Pr}(2))(THF)(n)](x)(2a); alternatively, this compound can be made by deprotonation of the neutral phosphaguanidine, Ph(2)PC{N(i)Pr}{NH(i)Pr}(1a) with (n)BuLi. Displacement of the THF solvate in 2a is readily achieved with TMEDA to afford Li(Ph(2)PC{N(i)Pr}(2))(TMEDA)(3a). X-Ray crystallographic analyses show that 2a exists as a dimer in the solid state with a folded ladder structure and an N,N' chelating phosphaguanidinate, while 3a is monomeric with N,P-coordination of the ligand to lithium. Compound 2a reacts via a transmetallation pathway with AlMe(2)Cl to afford the dimethylaluminium complex, Al(Ph(2)PC{N(i)Pr}(2))Me(2)(4a), which can also be prepared by protonation of a methyl group of AlMe(3) using 1a. The formation of a series of dialkylaluminium compounds has been investigated employing this latter pathway using both 1a and the N,N'-dicyclohexyl analogue, Ph(2)PC{NCy}{NHCy}(1b), affording Al(Ph(2)PC{NR}(2))Et(2)(5a,b), Al(Ph(2)PC{NR}(2))(i)Bu(2)(6a,b) and the diphenylaluminium compound Al(Ph(2)PC{N(i)Pr}(2))Ph(2)(7a). The oily nature of most of the dialkyl compounds and high sensitivity to oxygen and moisture lead to difficulty in manipulation and characterization; however, NMR spectroscopy indicated highly pure products (>95%) upon removal of the solvent. The molecular structures of the crystalline examples 4a and 7a are reported, showing monomeric aluminium species with symmetrically chelating phosphaguanidinate ligands. The series of aluminium compounds AlLCl(2){L=[EC{NiPr}(2)](-): A, E=Me; B, E=Me(2)N; C, E=(Me(3)Si)(2)N and D, E=Ph(2)P} were investigated using density functional theory. In the more simple cases A and B, the delocalized electron density of the metallacycle was represented by a combination of the HOMO and an orbital of lower energy (A, HOMO-5; B, HOMO-6). The HOMO-1 in B was pi-bonded across the Me(2)N-C bond suggesting delocalization of electron density into the metallacycle. In the more complex systems C and D, delocalization within the metallacycle was less extensive due to the (Me(3)Si)(2)N- and Ph(2)P-moieties. A number of occupied orbitals in D, however, display phosphorus 'lone-pair' characteristics, indicating that these species have the potential to behave as Lewis bases in the formation of poly(metallic) systems.

The synthesis and catalytic properties of a series of magnesium compounds consisting of monoanionic, N,N'-chelating ligands (N∩N = amidinates, guanidinates, phosphaguanidinates) is reported. The compounds were synthesized by (i) insertion of a carbodiimide into an existing Mg-C or Mg-N bond, or (ii) protonolysis of an organomagnesium compound by a neutral pre-ligand. Structural analyses of mono- or bis-(chelate) compounds with general formula Mg(N∩N)X(L)n and Mg(N∩N)2(L)n (X = halide, aryloxide, amide; L = Et2O, THF; n = 0, 1 or 2) have been performed and the influence that the ligand substituent patterns have on the solid-state structures has been probed. Selected examples of the compounds were tested as (pre)catalysts for the polymerization of lactide, the dimerization of aldehydes and the hydroacetylenation of carbodiimides.

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