Iron(II) Polyamine Chemistry:  Variation of Spin State and Coordination Number in Solid State and Solution with Iron(II) Tris(2-pyridylmethyl)amine Complexes

Abstract

The synthetic system of Fe(SO 3 CF 3 ) 2 and one or two TPA ligands (TPA ) tris(2-pyridylmethyl)amine) affords a series of complexes that demonstrate the complexities of the solid-state and solution coordination chemistry of labile iron(II) even with a multidentate ligand. The low-spin [Fe(TPA)(CH 3 CN) 2 ](SO 3 CF 3 ) 2 (1-OTf) complex forms in acetonitrile, but the high-spin complex Fe(TPA)(SO 3 CF 3 ) 2 (2) forms in chloroform. The methanolbound complex [Fe(TPA)(CH 3 OH) 2 ](BPh 4 ) 2 (3) forms in the presence of the noncoordinating anion, BPh 4 -, and six-coordinate [Fe(TPA) 2 ](SO 3 CF 3 ) 2 (4-OTf) and eight-coordinate [Fe(TPA) 2 ](BPh 4 ) 2 (4-BPh 4 ) form in the presence of excess ligand. Their behavior in solution is explored by studying their magnetic properties and NMR spectra, which indicate the presence of spin and coordination equilibria. The crystal structures of these complexes are reported. Crystallographic parameters are as follows. 1-OTf‚CH 3 CN: C 26 H 27 F 6 FeN 7 O 6 S 2 , monoclinic, P2 1 /n, a ) 12.418(2) Å, b ) 16.192(4) Å, c ) 15.855(2) Å, β ) 92.09(2)°, Z ) 4. 2: C 20 H 18 F 6 FeN 4 O 6 S 2 , monoclinic, P2 1 /c, a ) 17.636(2) Å, b ) 9.659(1) Å, c ) 16.004(2) Å, β ) 113.29 (1)°, Z ) 4. 3‚CH 3 OH: C 69 H 70 B 2 -FeN 4 O 3 , monoclinic, P2 1 /n, a ) 17.525(1) Å, b ) 19.150(2) Å, c ) 17.703(1) Å, β ) 100.36(1)°, Z ) 4. 4-OTf: C 38 H 36 F 6 FeN 8 O 6 S 2 , monoclinic, Pc, a ) 10.236(1) Å, b ) 10.129(1) Å, c ) 19.251(1) Å, β ) 92.27(1)°, Z ) 2. 4-BPh 4 : C 84 H 76 B 2 FeN 8 , monoclinic, P2 1 /n, a ) 12.489(1) Å, b ) 14.189(1) Å, c ) 19.843(1) Å, β ) 102.84-(1)°, Z ) 2.

Mononuclear, Dinuclear, and Pentanuclear [{N,S(thiolate)}Iron(II)] Complexes: Nuclearity Control, Incorporation of Hydroxide Bridging Ligands, and Magnetic Behavior

“…(3) ) than in 1 and 2, with the short end occupied by the alkylthiolate (R = cyclohexyl) complex. [16] The tertiary amine to iron(ii) distances of 2.3875(13) and 2.3521 (16) [37] and [(TPA)Fe(Cl) 2 ], [33] which exhibit Fe II À N amine distances of 2.175(6)-2.220 (7) . A more analogous comparison to 1 and 2 would come from a series of five-coordinate TPA À iron(ii) complexes, however there is only one example of a five-coordinate iron(ii) species in which TPA binds in the typical tripodal mode, [(TPA)Fe(SC 6 H 2 -2,4,6-Me 3 )]ClO 4 , [34] and this complex has a normal Fe II ÀN amine bond length of 2.250(3) .…”

The Coordination Chemistry of FeCl3 and FeCl2 to Bis[2‐(2,3‐dihydroxyphenyl)‐6‐pyridylmethyl](2‐pyridylmethyl)amine: Access to a Diiron(III) Compound with an Unusual Pentagonal‐Bipyramidal/Square‐Pyramidal Environment

“…[11] TPA ligands are easily accessible, and their iron complexes have been extensively used as tools to understand the chemistry at the active site of some iron proteins, thus making this series of compounds one of the most widely investigated classes of complexes for inorganic chemists involved in the field of biomimetics. [1,[12][13][14] IronA C H T U N G T R E N N U N G (iii) derivatives are particularly stable, easy to handle, and, among other properties, their ability to coordinate catechol derivatives Abstract: Coordination of FeCl 3 to the title ligand yields a mononuclear iron-A C H T U N G T R E N N U N G (iii) complex 1, which was characterized by spectroscopic techniques and X-ray diffraction. The ligand is (k 3 -N) tridentate and the metal, which lies in a pseudo-octahedral environment, is bound to a phenolate group from the catechol substituent.…”

“…This coordination mode is now well established in the chemistry of substituted TPA ligands and obviously reflects a gain in stability with respect to a more sterically hindered environment. [12,24,30] The 1 H NMR spectra of the FeCl 3 complexes generally display broad and unresolved signals in the paramagnetic region. [24,25] Complex 1, with an almost silent trace, is no exception to this rule.…”

A Two‐in‐one Pincer Ligand and its Diiron(II) Complex Showing Spin State Switching in Solution through Reversible Ligand Exchange

“…In addition a shoulder at 409 nm and a weak band at 529 nm (e = 374 m À1 cm À1 ) arises upon cooling, likely attributable to d-d transitions. [10,20] 19 F NMR signal is observed at À22.8 ppm, indicating a rapid exchange of the bridging and terminal triflates on the NMR time scale. Upon cooling, the signal broadens further with a concomitant shift to higher fields (see the upper ten spectra from 323-238 K in Figure 6).…”

“…[6,7] In few cases, spin state variations have been associated with reversible solvent binding or with reversible substitution of solvent ligands by weakly coordinating anions. [9,10] Spin state switching in solution has thus been discussed for solution-based chemosensing or MRI contrast applications.[7] As a prominent example, Herges and Tuczek et al recently developed a porphyrin-based nickel(II) system that shows magnetic bistability in solution, induced by the light-driven reversible coordination of a tethered ligand to the nickel ion.[11]Here we report a diiron(II) complex that undergoes a reversible and essentially complete high-spin/low-spin transition in solution through a multistep temperaturedependent ligand exchange reaction. The diiron(II) complex is based on a novel type of binucleating ligand scaffold that is composed of two {PNN} pincer-type subunits, and that can be viewed as a dinucleating version of the bipyridine-based {PNN} pincer ligand I (Scheme 1).…”