Bulky Guanidinato Nickel(I) Complexes: Synthesis, Characterization, Isomerization, and Reactivity Studies

Abstract

Reactions of lithium complexes of the bulky guanidinates [{(Dip)N}(2)CNR(2)](-) (Dip=C(6)H(3)iPr(2)-2,6; R=C(6)H(11) (Giso(-)) or iPr (Priso(-)), with NiBr(2) have afforded the nickel(II) complexes [{Ni(L)(μ-Br)}(2)] (L=Giso(-) or Priso(-)), the latter of which was crystallographically characterized. Reduction of [{Ni(Priso)(μ-Br)}(2)] with elemental potassium in benzene or toluene afforded the diamagnetic species [{Ni(Priso)}(2)(μ-C(6)H(5)R)] (R=H or Me), which were shown, by X-ray crystallographic studies, to possess nonplanar bridging arene ligands that are partially reduced. A similar reduction of [{Ni(Priso)(μ-Br)}(2)] in cyclohexane yielded a mixture of the isomeric complexes [{Ni(μ-κ(1)-N-,η(2)-Dip-Priso)}(2)] and [{Ni(μ-κ(2)-N,N'-Priso)}(2)], both of which were structurally characterized. These complexes were also formed through arene elimination processes if [{Ni(Priso)}(2)(μ-C(6)H(5)R)] (R=H or Me) were dissolved in hexane. In that solvent, diamagnetic [{Ni(μ-κ(1)-N-,η(2)-Dip-Priso)}(2)] was found to slowly convert to paramagnetic [{Ni(μ-κ(2)-N,N'-Priso)}(2)], suggesting that the latter is the thermodynamic isomer. Computational analysis of a model of [{Ni(μ-κ(2)-N,N'-Priso)}(2)] showed it to have a Ni-Ni bond that has a multiconfigurational electronic structure. An analogous copper(I) complex [{Cu(μ-κ(2)-N,N'-Giso)}(2)] was prepared, structurally authenticated, and found, by a theoretical study, to have a negligible Cu···Cu bonding interaction. The reactivity of [{Ni(Priso)}(2)(μ-C(6)H(5)Me)] and [{Ni(μ-κ(2)-N,N'-Priso)}(2)] towards a range of small molecules was examined and this gave rise to diamagnetic complexes [{Ni(Priso)(μ-CO)}(2)] and [{Ni(Priso)(μ-N(3))}(2)]. Taken as a whole, this study highlights similarities between bulky guanidinate ligands and the β-diketiminate ligand class, but shows the former to have greater coordinative flexibility.

The Ligand‐Based Quintuple Bond‐Shortening Concept and Some of Its Limitations

“…Dimers of Cr I , such as those observed herein, are difficult to obtain, because a variety of "side reactions" have to be avoided. [24] X-ray crystal structure analysis, magnetic data, electronic structural calculations (see below), IR data, and reaction with CCl 4 (no formation of CHCl 3 ) indicated that no bridging hydrides are present in 12. [11,21] Arene sandwich complexes can be formed, if aromatic solvents are used.…”

Binuclear complexes of Ni(i) from 4-terphenyldithiophenol

“…One can differentiate between structural motifs in which electronic coupling of the Ni(I) ions either occurs through bonding to (i) bridging conjugated π-systems in both syn-and antarafacial fashion, [1][2][3][4][5][6][7][8] (ii) bridging amido, 9 phosphido, 10 thiolato, 11,12 sulfido, 13 halogenido, [14][15][16][17] and hydrido 18 ligands in the form of Ni 2 (μ-X) 2 cores, (iii) bridging diphosphine, 19,20 and biphenyldiyl 15,21,22 ligands, or in the form of unsupported Ni-Ni bonds. One can differentiate between structural motifs in which electronic coupling of the Ni(I) ions either occurs through bonding to (i) bridging conjugated π-systems in both syn-and antarafacial fashion, [1][2][3][4][5][6][7][8] (ii) bridging amido, 9 phosphido, 10 thiolato, 11,12 sulfido, 13 halogenido, [14][15][16][17] and hydrido 18 ligands in the form of Ni 2 (μ-X) 2 cores, (iii) bridging diphosphine, 19,…”

“…[23][24][25] Antiferromagnetic coupling through a ligand and direct Ni-Ni bonding results in diamagnetic behaviour in most instances, albeit triplet ground states have been proposed for the Ni 2 (μ-Br) 2 core bound to the neutral form of a redox active 1,8-naphthyridine diimine ligand. 2,5,9 The reversible formation of reactive Ni(I) species appears to represent one of the two major modes of reactivity displayed by binuclear Ni(I) complexes. In the biradical state the Ni-Ni interaction is non-bonding and dissociation is a favourable and facile process.…”

A Mononuclear Iron(II) Bis(guanidinate) Complex: Synthesis, Structure, and Reactivity

“…Monoanionic guanidinate anions of the general formula [(RN) 2 C(NR′ 2 )] – are close analogues of amidinates. They are N ‐donor ligands which can form stable complexes with a variety of metal ions . These two ligand systems feature a similar Y‐shaped structure containing a NCN core.…”

“…Afterwards, a number of transition metal guanidinate complexes were reported in the literature . However, only a handful of guanidinate complexes of the late 3d metals (Fe→Ni) have been reported , . Richeson and co‐workers have studied the chemistry of several Fe II and Fe III guanidinate complexes derived from N , N′ , N" ‐trisubstituted [(RN) 2 C(NHR)] – (R = i Pr, Cy) ligands .…”