Crystal structure of dichlorido(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane)iron(III) hexafluoridophosphate

In contrast to other similar compounds, [FeCl2(C14H30N4)]PF6 is a monomer. Comparison with the mononuclear Fe2+ complex of the same ligand shows that the smaller Fe3+ ion is more fully engulfed by the cavity of the bicyclic ligand. Comparison with the μ-oxo dinuclear complex of an unsubstituted ligand of the same size demonstrates that the methyl groups of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane prevent dimerization upon oxidation.


Chemical context
The tendency for iron complexes to form oxido-bridged iron(III) species and ultimately hydrated iron oxides limits their utility, especially in aqueous media, as functional catalysts based on common ligands (Ortiz de Montellano, 1986). Even so, iron is one of the predominant metal ions found in biological catalytic systems (Jang et al., 1991;Wallar & Lipscomp, 1996;Boyington et al., 1993). A major feature of numerous synthetic catalysts having nitrogen donors and vacant coordination sites is their propensity to form dimers in which higher valent metal ions are present. One of us has produced iron(II) (Hubin et al., 2000) and iron(III) (Hubin et al., 2001) complexes of ethylene cross-bridged tetraazamacrocyclic ligands that are remarkably resistant to oxidative hydrolysis while still having available sites for binding of the metal ion to either a terminal oxidant or substrate. The ability of the complex to remain mononuclear, and thus catalytically useful, appears to hinge on the substitution pattern of the nonbridgehead nitrogen atoms of the bicyclic ligands (Hubin et al., 2001). Methyl or benzyl substitution results only in mononuclear complexes, even in the M 3+ (Hubin et al., 2001 or M 4+ (Yin et al., 2006) oxidation state, while oxidation of the unsubstituted-ligand complexes results in -oxido iron(III) dimers .
Recently, the iron(II) triflate complex of this same ligand, 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane, has been investigated as a catalyst for olefin oxidation by H 2 O 2 and was found to be an active catalyst with reactivity properties similar to [Fe(TPA)(OTf) 2 ] [TPA is tris(2-pyridyl- ISSN 2056-9890 methyl)amine; OTf is trifluoromethanesulfonate; Feng et al., 2011]. A key result of this study was that the location of two available cis binding sites on the metal ion is crucial for maximum catalytic activity. Very recently an Fe IV analogue has been reported, but no crystal structure data were given (England et al., 2015).

Structural commentary
The asymmetric unit of the title compound contains one complete Fe 3+ mononuclear cross-bridged cyclam complex and a single PF 6 À anion. The metal is hexacoordinate in the socalled cis-V geometry common for macrocycles of this type. It is coordinated by four nitrogen atoms of the macrocycle and two cis chloride ions, as shown in Fig. 1. The Fe-Cl bond lengths are similar to those of other comparable Fe 3+ complexes. The relatively long Fe-N bonds strongly suggest the Fe 3+ present is in a high-spin configuration.

Comparison with related structures
Structural characterization of an Fe 3+ mononuclear crossbridged cyclam complex has not been achieved prior to the present study. In the present case, even upon oxidation of the iron from Fe(II) to Fe(III), the methyl-substituted ligand does not allow dimerization to occur. We will now compare the observed structure with that of the lower valent analogue and to the iron(III) -oxido dimer of the unsubstituted analogue.
From a comparison of the Fe 3+ 4,11-dimethyl-1,4,8,11tetraazabicyclo[6.6.2]hexaadecane dichloride complex hexafluoridophosphate with the Fe 2+ analogue, the reduction in ionic radius of the iron ion upon oxidation is clear (Table 1). N ax -Fe 3+ -N ax is 166.8 (3) in the present structure, while N ax -Fe 2+ -N ax is 161.88 (5) in the reduced complex (Hubin et al., 2000). The smaller Fe 3+ ion is pulled further into the ligand cavity as the favored octahedral geometry is approached, as can be seen by viewing each complex down the N ax -Fe n+ -N ax axis (Fig. 2). Fe-N bond lengths are also affected, going from a mean of 2.255 Å in the Fe 2+ complex, to 2.209 Å in the Fe 3+ complex. Comparison of the present Fe 3+ monomer with the -oxido dimer complex is also informative. The crystal structure of the dimeric Fe 3+ complex (Hubin, 2003) is represented in Fig. 3. The Fe 3+ ion of this complex is also found in a pseudo-octahedral, six-coordinate geometry. Usually, these dimers contain five-coordinate metal cations, although dimers with six-and seven-coordinate cations are known (Murray, 1974). However, one monodentate chlorido ligand is maintained in this structure. Since the macrocyclic ligand is uncharged, the attractive Coulombic forces between the halide and the Fe 3+ ion may be enough to keep it bound. Also, the folded ligand conformation helps separate the ligands from each other, easing the steric interactions that might favor lower coordination numbers with more nearly planar ligands. The secondary amine/Fe 3+ bond lengths in the dimer are somewhat shorter than the tertiary amine/Fe 3+ bond lengths: the Fe-N(secondary) mean length is 2.153 Å while the Fe-N(tertiary) mean length is 2.239 Å . In the monomer, with all tertiary amines, the mean Fe-N bond length is 2.209 Å , shorter but not quite matching that of the shortest, ORTEP representation of the asymmetric unit with atoms drawn as 50% probability ellipsoids. secondary amine bonds in the dimer. The N ax -Fe-N ax mean bond angle is 161.6 in the dimer, while this value is 166.85 (19) in the monomer. Clearly, dimerization, and its associated steric consequences, pulls the Fe 3+ ion further out of the ligand cavity than it is in the Fe 3+ monomer. In fact, the dimer N ax -Fe-N ax bond angle is much closer to that of the Fe 2+ monomer at 161.88 (5) than that of the Fe 3+ monomer at 166.85 (19) . This steric consequence is consistent with the observation that the more sterically demanding methylsubstituted ligand prevents dimerization altogether.

Supramolecular features
There are no classical hydrogen bonds within the structure, but many C-HÁ Á ÁCl and C-HÁ Á ÁF intermolecular interactions exist ( Table 2). Pairs of complexes form dimers sustained by C-HÁ Á ÁCl interactions (HÁ Á ÁCl distances lie in the range 2.76 to 2.97 Å ) and further C-HÁ Á ÁCl interactions link these into tapes that run parallel to the b-axis. These tapes are stacked along the a and c axes. Between them lie PF 6 À anions, forming C-HÁ Á ÁF interactions to generate a threedimensional array of intermolecular contacts.
of Br 2 were added and the reaction was stirred for 15 minutes. Care must be taken when adding the bromine drops, as its vapor pressure and density tend to cause it to spurt out of the pipette. Practicing dispensing drops back into the bromine bottle (in the hood) can allow for successful dispensing. A bright yellow-orange precipitate formed immediately. The nitrogen gas was then allowed to bubble through the solution for 15 minutes to remove excess Br 2 . The flask was then stoppered and placed in a freezer for 30 minutes to complete the precipitation. The yellow-orange solid product was collected by vacuum filtration on a glass frit and washed with methanol and then ether. The product (0.428 g, 80% yield) was analytically pure as calculated with one-half molar equivalent of water of crystallization. Crystals suitable for X-ray diffraction were grown from ether diffusion into a dichloromethane solution.

Refinement
Standard data collection and refinement procedures were adopted. Crystal data, data collection and structure refinement details are summarized in Table 3.
Hydrogen atoms were placed using a riding model with fixed bond lengths and angles. For methylene groups U iso (H) was set at 1.2U iso (C) and for methyl groups U iso (H) was set at 1.5U iso (C).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refined as a two-component inversion twin.