Synthesis and characterization of cationic dicarbonyl Fe(II) PNP pincer complexes

Abstract In the present work, we have prepared a series of octahedral Fe(II) complexes of the type trans-[Fe(PNP)(CO)2Cl]+—PNP are tridentate pincer-type ligands based on 2,6-diaminopyridine. These complexes are formed irrespective of the size of the substituents at the phosphorus sites and whether cis-[Fe(PNP)(Cl2)(CO)] or trans-[Fe(PNP)(Cl2)(CO)] are reacted with CO in the presence of 1 equiv of silver salts. X-ray structures of representative complexes are presented. Based on simple bonding considerations the selective formation of trans-dicarbonyl Fe(II) complexes is unexpected. In fact, DFT calculations confirm that trans-dicarbonyl complexes are indeed thermodynamically disfavored over the respective cis-dicarbonyl compounds, but are favored for kinetic reasons. Graphical abstract


Introduction
As part of our ongoing research on the synthesis and reactivity of iron(II) PNP pincer complexes [1][2][3], we recently prepared the cationic dicarbonyl complex trans-[Fe(PNP-iPr)(CO) 2 Cl] ? (PNP-iPr = N,N 0 -bis(diisopropyl)-2,6-diaminopyridine) (trans-2a) as shown in Scheme 1 [4]. The formation of this complex was somewhat unexpected as it features two CO ligands in a mutual trans position. In fact, simple bonding considerations suggest that the unobserved cis isomers are the more stable one. This was indeed also supported by DFT calculations. This complex is interesting, since the trans CO arrangement makes one of the CO ligands comparatively labile which can be replaced by other potential ligands. Accordingly, trans-[Fe(PNP-iPr)(CO) 2 Cl]X with X = BF 4 turned out to be an efficient precatalyst for the coupling of aromatic aldehydes with ethyl diazoacetate to selectively give 3-hydroxyacrylates rather than b-keto esters [5].
In continuation of our studies on iron PNP complexes, we herein report on the synthesis and reactivity of a series octahedral Fe(II) carbonyl complexes bearing both sterically little demanding as well as bulky PNP ligands in order to probe whether sterics influences the preference for a trans-over a cis-dicarbonyl arrangement. Moreover, we investigate the impact of the NR linker on the outcome of these reactions.

Results and discussion
Treatment of complexes cis-1b and trans-1c-1g (1f and 1g are mixtures of cis and trans isomers) with 1 equiv of Ag ? salts (with SbF 6 -, BF 4 -, or CF 3 SO 3 as counterions) in THF or acetone in the presence of CO at room temperature selectively afforded the cationic complexes trans-[Fe(j 3-P,N,P-PNP)(CO) 2 X] ? (trans-2b-2g) in 78-98 % isolated yields (Scheme 1). The respective cis-dicarbonyl complexes were not observed and, hence, sterics and also the amine linker (NR) apparently do not influence the preference for a trans-dicarbonyl geometry. This is also supported by DFT calculations (vide infra). These complexes are thermally robust red solids that are air stable both in the solid state and in solution for several days. Characterization was accomplished by elemental analysis and 1 H, 13 C{ 1 H}, 31 P{ 1 H} NMR and IR spectroscopy. In addition, the solid state structures of trans-2b, trans-2d, trans-2f, and trans-2g were determined by singlecrystal X-ray diffraction.
In the IR spectrum, as expected, the CO ligands exhibit only one band between 1979 and 2031 cm -1 for the mutually trans CO ligands which are assigned to the asymmetric CO stretching frequency. The symmetric CO stretching band is IR inactive and not observed. The 31 P{ 1 H} NMR spectrum of complexes trans-2b-2g show singlet resonances at 85.0, 92.3, 100.7, 96.7, 130.6, and 132.8 ppm, respectively. In the 13 C{ 1 H} NMR spectrum the two CO ligands exhibit a single low-intensity triplet resonance in the range of 207.2-211.8 ppm, thus clearly revealing that the two CO ligands are trans to one another.
Structural views of trans-2b, trans-2d, trans-2f, and trans-2g are depicted in Figs According to the calculations both cationic pentacoordinated intermediates A and B adopt a square pyramidal geometry where the Cl and the CO ligands, respectively, are in the apical position. The singlet ground state 1 B is the energetically favored species by 22.6 and 50.7 kJ mol -1 , respectively, over the singlet and triplet states of A ( 1 A, 3 A) (Fig. 5). In the case of B, no stable triplet state was found. A and B were found to interconvert readily via two pathways. 1 A is able to isomerize along the spin singlet surface (S = 0) to give 1 B with a small energy barrier of 11.3 kJ mol -1 . This reaction proceeds via transition state 1 TS AB . In the second pathway, 1 A undergoes two Scheme 1 consecutive spin state changes (spin crossover) from S = 0 to S = 1 and back to S = 0. The minimum energy crossing point 1 between the potential energy surfaces of the two spin states S = 0 to S = 1 (CP2) is easily accessible lying merely 1.3 kJ mol -1 above 1 A. The second spin state change from S = 1 to S = 0 proceeds via CP1 with a barrier of 19.3 kJ mol -1 .
Finally, the experimentally isolated trans-2c (which is actually is less stable than cis-2c by 17.2 kJ mol -1 ) is formed by an essentially barrierless addition of CO to 1 B which is the most stable and predominant species lying 50.7 kJ mol -1 lower in energy than 1 A. In general, CO addition at singlet intermediates is generally more favorable than at triplet intermediates as can be seen by examining the frontier orbitals of the relevant species. The LUMO of the pentacoordinated intermediates with a singlet spin state ( 1 A and 1 B) are formed mainly by z 2 -type orbitals centered at the Fe-atom and pointing towards the empty coordination position (Fig. 5). Therefore, these orbitals are ready to receive a pair of electrons from a ligand that occupies the sixth coordination site (CO in this 1 In the MECP both the energy as well as the geometry of the molecule are the same in the two spin states surfaces. Once that point (MECP) is reached, following the reaction coordinate, there is a given probability for the system to change spin state and hop from one PES to the other, giving rise to the ''spin-forbidden'' reaction. For more information about MECP and the kinetics of spin-forbidden reactions see for example Ref. [6]. case) and establish the corresponding r-bond. In the case of spin triplet intermediate ( 3 A), this orbital is occupied being, in fact, the highest single occupied molecular orbital (SOMO) of this species (Fig. 5). This is easily available to receive the electron pair from an incoming CO rendering addition of this ligand a difficult process. In fact, the first empty orbital (LUMO) in the case of the triplet intermediate corresponds to an x 2 -y 2 -type orbital which is centered on the metal and is antibonding (r * ) with respect to the four ligands in the equatorial plane.
Trans-[(chloro)[N 2 ,N 6 -bis(diphenylphosphanyl)pyridine-2,6-diamine](dicarbonyl)iron(II)] tetrafluoroborate (trans-[Fe(j 3 P,N,P-PNP-Ph)(CO) 2 Cl]BF 4 ) (trans-2b, C 31 H 25 BClF 4 FeN 3 O 2 P 2 ) Complex cis-1b (200 mg, 0.316 mmol) was dissolved in 10 cm 3 THF, CO gas was bubbled through the solution and 62 mg AgBF 4 (0.316 mmol) was added. After 4 h the red solution was filtered over Celite and the solvent was evaporated. The red powder was washed with 20 cm 3   X-ray structure determination X-ray diffraction data of trans-2a, trans-2c, trans-2e, and trans-2f (CCDC entries 1015363 (trans-2a), 1469956 (trans-2c), 1469957 (trans-2e), 1469958 (trans-2f),) were collected at T = 100 K in a dry stream of nitrogen on Bruker Kappa APEX II diffractometer systems using graphite-monochromatized Mo-Ka radiation (k = 0.71073 Å ) and fine sliced uand x-scans. Data were reduced to intensity values with SAINT and an absorption correction was applied with the multi-scan approach implemented in SADABS [9]. The structures of trans-2c, trans-2e, and trans-2f were solved by charge flipping using SUPERFLIP [10] and refined against with JANA2006 [11]. The structure of trans-2a was solved with direct methods and refined against F2 with the SHELX software package [12]. Nonhydrogen atoms were refined anisotropically. The H atoms connected to C atoms were placed in calculated positions and thereafter refined as riding on the parent atoms. The H atoms of the amine functionalities were located in difference Fourier maps and freely refined. Molecular graphics were generated with the program MERCURY [13].

Computational details
Calculations were performed using the GAUSSIAN 09 software package, and the OPBE functional without symmetry constraints as already described previously [14].