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Amidinate ligands are utilized following the deprotonation of the amidine counterparts. Amidines are synthesized using anilines and various other compounds, including from imidoyl chlorides[3] and ortho esters.[2] To generate an organometallic complex, salt metathesis or protonolysis can be used. Typically, salt metathesis reactions are used from alkali salts (M = lithium, sodium, potassium) to implement larger metals.
Amidines can also be synthesized by using nucleophilic reagents with carbodiimides. Roesky & Kaufmann synthesized an ethynyl ferrocene substituted amidine by deprotonating the terminal hydrogen of the alkyne with n-butyllithium, then reacting the nucleophilic compound with the electrophilic carbodiimide (dipp = 2,6-diisopropylphenyl).[4]
Kays et. al. synthesized a series of magnesium amidinate complexes using alkyl magnesium reagents. Bisamidinate complexes were verified via single-crystal x-ray diffraction (XRD). According to the crystallographic data, the magnesium atoms were composed of four or five-coordinate monomers with tetrahedral or square pyramidal geometry. The four-coordinate complexes were favored with increasing steric bulk of the pendant aryl groups (dipp vs 3,5-dimethylphenyl (DMP)).[5]Beryllium-oxo Amidinate Complex[6]
Schulz & coworkers synthesized beryllium amidinate complexes using alkylberyllium reagents. The reaction with a carbodiimide yielded a homoleptic bisamidinate complex, whereas the reaction with a sterically hindered amidine yielded a monoamidinate. In the presence of oxygen, this complex reacts to form dimeric complex with ethoxide bridges, as determined via XRD.[6]Methyl-bridged Dimagnesium Amidinate Complex[7]
Rukiza et. al. synthesized a homoleptic η1 η1' magnesium amidinate complex through the addition of an alkyl magnesium species with a carbodiimide. Different complexes were then achieved by the addition of Grignard reagents to a similar carbodiimide, where methylmagesium iodide displayed a bridged methyl group between a K2 dimagnesium amidinate complex, and methylmagnesium chloride displayed bridged chlorides between two η2 magnesium complexes. The iodide complex showed promising results in the catalytic polymerization of ε-Caprolactone in the presence of benzyl alcohol. In the presence of THF, this same complex allowed for a mononuclear homoleptic magnesium complex.[7]
Forter et. al. synthesized a series of mononuclear iron (II) complexes, a three-coordinate iron mesitylene, a three-coordinate iron(dbabh) , and another a four-coordinate iron(dbabh)(Hdbabh). Upon photolyzing the four-coordinate system, the two ligands underwent an intramolecular C-C coupling. This complex displays unexpected C-H activation not seen in its guanidinate counterpart.[8]Ruthenium (II) P-cymene Complex[2]
Kaim et al. synthesized a mononuclear ruthenium (II) η6p-cymene complex. This complex was then oxidized using ferrocenium hexafluorophosphate, generating a stabilized nitrogen radical and proving the non-innocence of the ligand in redox processes. The absorption in the near-infrared region was studied to determine its chemical behavior, showing a λmax of 1230 nm.[2] Kaim et. al. also synthesized similar ruthenium (II) amidinate complexes using other ancillary ligands, including pyridine and 2,2'-bipyridine.[9]Oxygen ActivationIrridium Amidinate Complex[1]
Trifnov et. al. synthesized and functionalized phosphine-oxide and phosphine-imide amidine ligands with group 3 transition metal scandium. The metal bonded to the pendant group (oxygen or nitrogen), stabilized by further chelation. In the presence of borate and alkyl aluminum species, these complexes were used in the polymerization of heptene and cis 1,4 isoprene.[10]Scandium Phosphineoxide and Phosphineimide Complexes[10]Tetrazinc-oxo Amidinate Complex[11]
Gibson & coworkers synthesized a series of zinc amidinate complexes. Using an acetamidine and alkyl-zinc starting materials, a bisamidinate homoleptic zinc complex was generated and characterized via XRD. However, in some cases, a tetramethyl tetrazinc with a K2 amidinate complex was observed with a bridged oxide anion. With a more sterically bulkytert-butyl amidine, a reaction of the deprotonated ligand with zinc chloride yielded a dimeric bridged chloride species. Further salt metathesis reactions were carried out with potassium bis(trimethylsilyl)amide.[11]
Wheatley et. al. synthesized a mononuclear dimethylaluminum aminidate complex, which when treated with tert-butyllithium formed an aggregate compound. This compound consisted of two separate clusters, one representing the chemical formula {Li[Me2Al(Me)tBu]2}-, and the other [Li4(PhNC(Ph)NPh)6]+. The first component is a lithium bis(aluminate), and the second is a lithium amidinate
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Uhl and coworkers synthesized K2 digallium (II) amidinate complexes, featuring both homoleptic and heteroleptic ligands. These ligands were synthesized using varying stoichiometries of the amidinate material and showed a direct gallium-gallium bond, which was preserved from the starting material (Ga(CH(SiMe3)2)2.[13]Gallium Amidinate Complexes[13]Asymmetric Aluminum Amidinate Complex[3]
Ma & coworkers synthesized a variety of asymmetric (in respect to the aryl groups) dimethyl aluminum amidinate complexes. Specifically, these complexes were synthesized to compare the effects of the aromatic substituents on the catalytic ring opening polymerization of rac-lactide. A variety of electronic (fluorine, chlorine, methoxy etc.) and steric (tert-butyl, methyl etc.) substitution at the ortho and para positions were tested for selectivity of the products and kinetic factors, which show the smaller, electron-withdrawing substituents display faster turnover.[3]Bimetallic Dimethylaluminum Diamidinate Complex[14]
Otero et. al. synthesized an aryl bridged amidinate species functionalized with a bimetallic triethyl and trimethyl aluminum moiety. These complexes were compared to their monometallic counterparts in the ability to polymerize styrene, in which the bimetallic species showed faster conversion of starting materials.[14]
Lanthanide (II) Amidinate Complexes[15]Trifonov et. al. synthesized heteroleptic Ln(II) amidinates (Ln = samarium, yttrium). These lanthanide complexes vary from most literature examples with the 2+ oxidation state, and were synthesized using their respective MHMDS salts with the amidine starting material. These complexes show unique bonding of the lanthanide to the delocalized π-system of the aromatic ring. These complexes were used in the hydrophopshination of styrene, where secondary phosphines were preferably synthesized over their tertiary counterparts. They were also used in the hydrophosphination of tolane and showed little selectivity between E and Z isomers.[15]Lanthanum (II) Benzyl Amidinate Complex[16]
Hessen et. al. synthesized a trivalent lanthanide amidinate complex with pendant benzyl groups. This complex shows an η3binding mode to one of these pendant benzyl groups, and an η2 binding mode to the other benzyl group. The difference in coordination is obvious based on the bond angles, where the η3 has a bond angle of 87.0 and the η2 has a bond angle of 83.3.[16] The η3 is seen in other lanthanide complex examples.[17]Lutetium (III) Cp' (C5Me4(SiMe3)) Amidinate Complex[18]
Trifnov et. al. synthesized tris-tetramethylaluminate lanthanide (Ln = lanthanum, neodymium) amidinate complexes. One equivalent of methane and one equivalent of trimethyl aluminum were generated, preserving the tetramethylaluminate moities. The metals displayed different methyl bridging, where the lanthanum was bonded to 2 bridging methyl groups per aluminum, and the neodymium displayed 2 and 3 bridging methyl groups for the respective aluminates. These complexes were used for the polymerization of isoprene in the presence of a borate and an alkyl aluminum species.[19]Neodymium (III) Amidinate Complex[19]
