1,3,5‐Triazapentadiene Nickel(II) Complexes Derived from a Ketoxime‐Mediated Single‐Pot Transformation of Nitriles

Abstract

A series of cationic (2+) [Ni{HN=C(R)NHC(R)=NH}2](X)2 {R = 4‐(Cl)C6H4 (1), 3‐(NC)C6H4 (3), 4‐(NC)C6H4 (4) and Me (7); X = Cl– (1, 3, 4) or MeCOO–·H2O (7)} and neutral [Ni{HN=C(R)NC(R)=NH}2](solvate) {R = 3‐(Cl)‐4‐py (2), 3‐py (5) and 4‐py (6); solvate = MeOH and/or H2O; py = pyridyl} N,N‐chelating bis(1,3,5‐triazapentadiene/ato)nickel(II) [Ni(tap)2]2+/0 complexes has been easily generated by a ketoxime‐mediated single‐pot reaction of a nickel(II) salt [NiCl2·2H2O or Ni(MeCOO)2·4H2O] with 4‐chlorobenzonitrile, isophthalonitrile, terephthalonitrile, acetonitrile, 2‐chloro‐4‐cyanopyridine, 3‐cyanopyridine or 4‐cyanopyridine, respectively. The obtained compounds have been characterized by IR, 1H and 13C{1H} NMR spectroscopy, FAB‐MS(+) or ESI‐MS(+), elemental analyses and single‐crystal X‐ray diffraction [for 7 and solvated mono‐ {1a·(Me2CO)0.33·(MeOH)0.67} and bis‐deprotonated (2b·2Me2CO, 4b·CHCl3, 5b·Me2CO and 6b·MeOH) products, formed upon recrystallization of 1, 2, 4, 5 and 6, respectively]. The crystal structures of all compounds bear similar monomeric Ni(tap)2 units with a nearly square‐planar geometry. In addition, the structure of 7 features the formation of infinite 1D zig‐zag water–acetate chains {[(H2O)2(MeCOO)2]2–}n, which multiply interact with the [Ni(tap)2]2+ cations to generate a 2D hydrogen‐bonded supramolecular assembly.

Highly Reactive NiII‐Bound Nitrile–Oxime Coupling Intermediates Stabilized by Substituting Conventional Nitriles with a Dialkylcyanamide

“…The reactions of the Me 2 NCN/HON=C(R′)Me/NiCl 2 (R′ = Me, Ph) mixtures were conducted at 4:6:1 (or 4:4:1 for the preparation of [ 1 ]Cl; Scheme ) molar ratios of the reactants in non‐dried acetone at 50–70 °C in closed vials, conditions similar to those previously applied8a for the Ni II ‐mediated reactions between conventional nitriles and ketoximes. After 1 d of heating, we isolated crystals of [NiCl{H N =C(NMe 2 )O N =C(R′)Me} 2 ]Cl (R′ = Me, [ 1 ]Cl, 40 %; R′ = Ph, [ 3 ]Cl, 82 %; Figure 1).…”

“…Thus, if R is a group with a strong + M effect – that is, NMe 2 – the cyanamide‐derived part of [ 1 ] + , [ 2 ] 2+ , and [ 3 ] + forms the chelate fragment {R 2 N–C(–OR″)=NH(Ni II )}, which exhibits pronounced stability due to the positive charge delocalization (see theoretical calculations later) and is responsible for the stability of the complex at room temperature. The ligands with less strongly donating moieties and with no pronounced + M effect (R = Alk, Ar) are unstable in solution and were not isolated 8a…”

“…It is noteworthy that metal‐mediated reactions between ketoximes and nitriles also occur at Co II[9] and Zn II[10] metal centers, but these lead to amidines,9 carboxamides,10d and 1,2,4‐oxadiazoles,10a,10c,11 with no iminoacylated oximes being detected. However, iminoacylated oximes were postulated as plausible reaction intermediates 8a,10d,11…”

“…When nickel(II) salts were employed for the nitrile–oxime coupling, Kopylovich, Pombeiro, et al8a,8c,8d observed an unusual transformation (routes I and II, Scheme ) of RCNs into the appropriate (TAP)Ni II complexes (1,3,5‐triazapentadienes are commonly abbreviated as TAP; for a review on TAPs, see ref 128a,8c,8d. This route opens up easy access to TAPs, which otherwise are conventionally obtained by the hazardous two‐step Pinner synthesis.…”

“…When phthalonitrile is used as an RCN source, the formation of nickel phthalocyanines instead of (TAP)Ni II has been achieved in the “Ni II /ketoxime” systems (route III) 13. In all these works, the monofunctional nickel(II)‐mediated nitrile–ketoxime coupling was only postulated as one of the key steps of the cascade reaction leading to TAPs,8a,8c,8d but it was not confirmed experimentally, and therefore these intermediates were treated as elusive. Furthermore, when phthalonitriles were treated with ketoximes,13 the identified intermediates were formed, through the unusual double addition of oximes to the bis(nitriles), which is also different from the nitrile–oxime integration.…”