On the Electronic Structure of Ni^II Complexes That Feature Chelating Bisguanidine Ligands

Abstract

In this work we report on the syntheses and properties of several new Ni complexes featuring the chelating bisguanidines bis(tetramethylguanidino)benzene (btmgb), bis(tetramethylguanidino)naphthalene (btmgn), and bis(tetramethylguanidino)biphenyl (btmgbp) as ligands. All complexes were structurally characterized by single-crystal X-ray diffraction and quantum chemical calculations. A detailed inspection of the magnetic susceptibility of [(btmgb)NiX(2)] and [(btmgbp)NiX(2)] (X=Cl, Br) revealed a linear temperature dependence of chi(-1)(T) above 50 K, which was in agreement with a Curie-Weiss-type behavior and a triplet ground state. Below approximately 25 K, however, magnetic susceptibility studies of the paramagnetic d(8) Ni complexes revealed the presence of a significant zero-field splitting (ZFS) that results from spin-orbit mixing of excited states into the triplet ground state. The electronic consequences that might arise from the mixing of states as well as from a possible non-innocent behavior of the ligand have been explored by an experimental charge density study of [(btmgb)NiCl(2)] at low temperatures (7 K). Here, the presence of ZFS was identified as one potential reason for the flat angle-spherical Cl-Ni-Cl deformation potential and the distinct differences between the angle-spherical X-Ni-X valence angles observed by experiment and predicted by DFT. An analysis of the topology of the experimentally and theoretically derived electron-density distributions of [(btmgb)NiCl(2)] confirmed the strong donor character of the bisguanidine ligand but clearly ruled out any significant non-innocent ligand (NIL) behavior. Hence, [(btmgb)NiCl(2)] provides an experimental reference system to study the mixing of certain excited states into the ground state unbiased from any competing NIL behavior.

The Flexible Coordination Modes of Guanidine Ligands in Zn Alkyl and Halide Complexes: Chances for Catalysis

“…Preliminary calculations on olefin polymerization reactions with Ni complexes featuring chelating bis(guanidine) ligands indeed point to such an effect. [23] In future work we want to explore the catalytic properties of guanidine complexes making use of the stabilizing effect of the amino groups in …”

Redox Chemistry and Group 10 Metal Complexes of Aromatic Compounds with Bulky Bicyclic Guanidino Groups

“…138 pm) were significantly elongated with respect to free 1 [129.13 (14) pm], indicative of the presence of π-bonding. [16] The UV/Vis spectrum ( Figure 9) featured a broad band at around 460 nm, responsible for the orange color of the complex. As a result of coordination, the bands of the free ligand at 225, 267, and 306 nm are redshifted to 229, 286, and 331 nm.…”

“…[16,17] If further guanidino groups are introduced in positions four and five of the benzene ring, the redox-active, relatively strong electron donors 1,2,4,5-tetrakis(tetramethylguanidino)benzene (ttmgb) and 1,2,4,5-tetrakis(N,NЈ-dimethylethyleneguanidino)benzene (tdmegb) are obtained (Scheme 2), which can coordinate two metal centers. [18][19][20][21][22][23][24] Although oxidation, for example of ttmgb, generally leads directly to the dications (presumably due to ion pairing), the monocationic radical [ttmgb] .+ has been stabilized in a dinuclear Cu II complex.…”

Donor‐Acceptor Couples and Late Transition Metal Complexes of Oxidation‐Labile 1,4,5,8‐Tetrakis(guanidino)naphthalene Superbases

“…[16] Attracted mainly by their high basicity, guanidines and guanidinates were used intensively in coordination chemistry, [17] and were found to establish strong σ(NǞmetal) and also π(NǞmetal) bonds. [18] In the last years, we synthesized a number of late transition metal complexes of the GFA-2 ligands btmgb and btmgn, (see Scheme 1). [18][19][20] Interestingly, Alder's proton sponge 1,8-bis(dimethylamino)naphthalene is barely suitable as ligand due to steric encumbrance at the amino N atoms.…”

“…[18] In the last years, we synthesized a number of late transition metal complexes of the GFA-2 ligands btmgb and btmgn, (see Scheme 1). [18][19][20] Interestingly, Alder's proton sponge 1,8-bis(dimethylamino)naphthalene is barely suitable as ligand due to steric encumbrance at the amino N atoms. Up to date only a single example for a transition metal complex of this molecule is known.…”