Reactions of tetraamidinate zinc complexes
Reactions of tetraamidinate zinc complexes
{C[C(NiPr)2ZnMe]4} was also shown to be a suitable starting reagent for other multinary amidinate complexes. Reactions with aluminum trihalides AlX3 (X = Cl, Br, I) occurred with Me/X exchange and subsequent formation of the corresponding zinc halide complexes.[1]
Scheme 8: Methyl/halide exchange reaction; N-bonded i-Pr substituents are omitted for clarity.
Figure 6: Molecular structures of multinary zinc halide complexes.
These complexes react with two additional equivalents of MCl3 (M = Al, Ga) with formation of the corresponding Lewis acid-base adducts {C[C(NiPr)2ZnCl]4[MCl3]2} (M = Al, Ga),[2] while salt elimination reactions with MeLi, n-BuLi, EtMgBr, NaC≡CPh and CaH2 gave the correspondiing tetranuclear amidinate complexes of the general type {C[C(NiPr)2ZnR]4} (R = Me, Et, n-Bu, C≡CPh, H).[1]
Scheme 9: Salt metathesis reaction using organolithium or Grignard reagents; N-bonded i-Pr substituents are omitted for clarity.
The zinc hydride complex {C[C(NiPr)2ZnH]4} is very reactive and was used for the synthesis of acetylene complexes {C[C(NiPr)2Zn-C≡CH]4} and {C[C(NiPr)2Zn-C≡CH]4} by reaction with HC≡CR (R = Ph, H).[3]
Scheme 10: Hydrogen elimination reaction by the zinc hydride with acetylenes ; N-bonded i-Pr substituents are omitted for clarity.
Figure 7: Molecular structures of multinary zinc hydride and zinc acetylene complexes.
In an attempt to synthesize the protonated tetraamidine, we reacted {C[C(NR)2ZnMe]4} (R = i-Pr; P) with water.[4] Monitoring the reaction of {C[C(Ni-Pr)2ZnMe]4} by temperature dependent in situ 1H-NMR spectroscopy in C6D6 showed that this complex is almost stable against hydrolysis at ambient temperature, while heating to 55 °C exclusively yielded (i-PrN)2C and methanetrisamidine HC[C(NHi-Pr) Ni-Pr]3.
Figure 8: Temperature-dependent 1H NMR monitoring showing the hydrolysis of {C[C(Ni-Pr)2ZnMe]4} with water in C6D6 ({C[C(Ni-Pr)2ZnMe]4} ■, 2a ▲, C(Ni-Pr)2 ●).
In contrast, hydrolysis of {C[C(NPh)2ZnMe]4} yielded both tautomers C[C(NPh)N(Ph)H]2[C(HNPh)2] and HC[C(NHPh)NPh]3, which are the first structurally characterized CH-NH tautomers of an acyclic amidine.
Scheme 10: Hydrolysis reaction of {C[C(NR)2ZnMe]4} (R = Ph).
Figure 8: Molecular structures of the CH and NH tautomeric forms.
The trisamidines are reactive starting reagents in reactions with organometallic complexes and were shown to produce both homoleptic metal complexes in reactions with AlR3, GaR3, R2AlH and ZnR2.[4,5] For instance, reaction of HC[C(NHPh)NPh]3 with AlR3 occurred with elimination of alkane and formation of HC[C(Ni-Pr)2AlR2]3. Moreover, the N-H groups can be stepwise addressed in metalation reactions since the N-H acidity differs. As a consequence, heterobimetallic complexes were successfully synthesized.[5]
Figure 9: Stepwise metalation reaction and molecular structure of HC[C(NPh)2AlMe2]3 and {C[C(NPh)2]3[AlMe2]2[ZnMe]2}.
In addition, methanetrisamidines {HC[C(NR)NHR]3} (R = i-Pr; Ph) are valuable strating reagents in coordination chemistry.[6] Reaction with NiCl2(H2O)6 occurred with protonation and formation of {[C(C(NHi-Pr)2)3]2+[NiCl4]2−}, while the reaction with CuCl gave [C(C(N(i-Pr)CuCl)NHi-Pr)2(C(NHi-Pr)2)]. Both complexes show the N-H tautomeric form, resulting from a H-migration from carbon to nitrogen. In contrast, reactions of {HC[C(NPh)NHPh]3} with [M(NCMe)3(CO)3] (M = Cr, Mo, W) yielded octahedral complexes fac-[M(CO)3CH(C(NHPh)NPh)3], in which the C-H tautomeric form is preserved.
Figure 10: Molecular structures of a CuCl and Cr(CO)3 complex.
References
[1] B. Gutschank, S. Schulz, U. Westphal, D. Bläser, R. Boese, Organometallics 2010, 29, 2093.
[2] S. Schmidt, B. Gutschank, S. Schulz, D. Bläser, R. Boese, C. Wölper, Eur. J. Inorg. Chem. 2011, 28, 4464.
[3] B. Gutschank, S. Schulz, D. Bläser, R. Boese, C. Wölper, Organometallics 2010, 29, 6133.
[4] B. Gutschank, S. Schulz, M. Marcinkowski, G. Jansen, H. Bandmann, D. Bläser, C. Wölper, Angew. Chem. Int. Ed. 2012, 51, 10893.
[5] B. Gutschank, M. Bayram, S. Schulz, D. Bläser, C. Wölper, Eur. J. Inorg. Chem. 2013, 31, 5495.
[6] B. Gutschank, S. Schulz, D. Bläser, C. Wölper, J. Chem. Soc., Dalton Trans. 2014, 43, 2907.