Synthesis and reactivity of TADDOL-based chiral Fe(ii) PNP pincer complexes-solution equilibria between κ2P,N- and κ3P,N,P-bound PNP pincer ligands

Article abstract of DOI:10.1039/c5dt00832h

Treatment of anhydrous FeX₂ (X = Cl, Br) with 1 equiv. of the asymmetric chiral PNP pincer ligands PNP-R,TAD (R = iPr, tBu) with an R,R-TADDOL (TAD) moiety afforded complexes of the general formula [Fe(PNP)X₂]. In the solid state the

Full text of DOI:10.1039/c5dt00832h

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Holzhacker, B.
Stöger, M. D. D. Carvalho, L. Ferreira, E. Pittenauer, G. Allmaier, L. F. Veiros, S. Realista, A. Gil, M. J.
Calhorda, D. Müller and K. Kirchner, Dalton Trans., 2015, DOI: 10.1039/C5DT00832H.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/dalton

Page 1 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
1
Synthesis and Reactivity of Taddol-Based Chiral Fe(II) PNP Pincer Complexes -
Solution Equilibria between κ2P,N- and κ3P,N,P-Bound PNP Pincer Ligands
Christian Holzhacker,a Berthold Stöger,b Maria Deus Carvalho,c Liliana P. Ferreira,d,e Ernst
Pittenauer,b Günter Allmaier,b Luis F. Veiros,f Sara Realista,c Adrià Gil,c Maria José Calhorda,c Danny
Müller,a and Karl Kirchner*, a
Abstract
Treatment of anhydrous FeX2 (X = Cl, Br) with 1 equiv of the asymmetric chiral PNP pincer
ligands PNPꢀR,TAD (R = iPr, tBu) with an R,RꢀTADDOL (TAD) moiety afforded complexes of the
general formula [Fe(PNP)X2]. In the solid state these complexes adopt a tetrahedral geometry
with the PNP ligand coordinated in κ2P,Nꢀfashion, as shown by Xꢀray crystallography and
Mössbauer spectroscopy. Magnetization studies led to a magnetic moment very close to 4.9 ꢀB
reflecting the expected four unpaired dꢀelectrons (quintet ground state). In solution there are
equilibria between [Fe(κ3P,N,PꢀPNPꢀR,TAD)X2] and [Fe(κ2P,NꢀPNPꢀR,TAD)X2] complexes, i.e.,
the PNPꢀR,TAD ligand is hemilabile. At ꢀ50oC these equilibria are slow and signals of the nonꢀ
coordinated PꢀTAD arm of the κ2P,NꢀPNPꢀR,TAD ligand can be detected by 31P{1H} NMR
spectroscopy. Addition of BH3 to a solution of [Fe(PNPꢀiPr,TAD)Cl2] leads to selective boronation
of the pendant PꢀTAD arm shifting the equilibrium towards the fourꢀcoordinate complex
[Fe(κ2P,NꢀPNPꢀiPr,TADBH )Cl2]. DFT calculations corroborate the existence of equilibria between
3
fourꢀ and fiveꢀcoordinated complexes. Addition of CO to [Fe(PNPꢀiPr,TAD)X2] in solution yields
the diamagnetic octahedral complexes transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)X2], which react
further with Ag+ salts in the presence of CO to give the cationic complexes transꢀ[Fe(κ3P,N,Pꢀ
PNPꢀiPr,TAD)(CO)2X]+. CO addition most likely takes place at the five coordinate complex
[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)X2]. The mechanism for the CO addition was also investigated by DFT
and the most favorable path obtained corresponds to the rearrangement of the pincer ligand first
from a κ2P,Nꢀ to a κ3P,N,Pꢀcoordination mode followed by CO coordination to [Fe(κ3P,N,PꢀPNPꢀ
iPr,TAD)X2]. Complexes bearing tBu substituents do not react with CO. Moreover, in the solid
state none of the tetrahedral complexes are able to bind CO.
aInstitute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, Aꢀ1060 Vienna,
AUSTRIA
bInstitute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, Aꢀ1060
Vienna, AUSTRIA
cCentro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749ꢀ016 Lisboa,
PORTUGAL
dBiosystems and Integrative Sciences Institute, Faculdade de Ciências, Universidade de Lisboa, 1749ꢀ016
Lisboa, PORTUGAL
eDepartment of Physics, University of Coimbra, 3004ꢀ516 Coimbra, PORTUGAL
fCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049ꢀ
001 Lisboa.

Dalton Transactions
Page 2 of 29
View Article Online
DOI: 10.1039/C5DT00832H
2
Introduction
Neutral pyridineꢀbased PNP pincer ligands are widely utilized in transition metal chemistry due to their
combination of stability, activity and variability.1 They typically enforce a meridional κ3P,N,P
coordination mode, provided that three coordination sites are accessible at the metal center. As iron
PNP complexes are concerned, an important class of compounds are coordinatively unsaturated 16e
highꢀspin complexes of the type [Fe(κ3P,N,PꢀPNP)X2] (X = Cl, Br) obtained from the reaction of Fe(II)
halides with stoichiometric amounts of PNP ligands.2,3,4
We are currently focusing on the synthesis and reactivity of iron complexes containing PNP
pincer ligands based on the 2,6ꢀdiaminopyridine scaffold.5 In these ligands the aromatic pyridine ring
and the phosphine moieties are linked via NH, Nꢀalkyl, or Nꢀaryl linkers. In the course of our studies
we discovered that Fe(II) halides react with bulky PNP ligands such as N,N'ꢀbis(diꢀisoꢀ
propylphosphino)ꢀ2,6ꢀdiaminopyridine (PNPꢀiPr) and N,N'ꢀbis(diꢀtertꢀbutylphosphino)ꢀ2,6ꢀdiaminoꢀ
pyridine (PNPꢀtBu) to give the expected monoꢀchelated highꢀspin complexes [Fe(κ3P,N,PꢀPNPꢀR)X2]
(A). In contrast, with smaller PNP ligands such as N,N'ꢀbis(diphenylphosphino)ꢀ2,6ꢀdiaminopyridine
(PNPꢀPh) or N,N'ꢀbis(diethylphosphino)ꢀ2,6ꢀdiaminopyridine (PNPꢀEt), bisꢀchelated octahedral Fe(II)
complexes [Fe(κ3P,N,PꢀR)(κ2P,NꢀPNPꢀR)X]+ (B)6 were formed where the PNP pincer ligands are
coordinated in κ3P,N,Pꢀ and κ2P,Nꢀfashion (Scheme 1).
Scheme 1 Reactions of FeX2 with PNP ligands yielding three types of complexes with κ3P,N,Pꢀ and/or
κ2P,Nꢀbound PNP ligands.
In continuation of our studies on iron PNP complexes, we report here on the synthesis and reactivity of
chiral Fe(II) PNP pincer complexes based on R,RꢀTADDOL in combination with bulky iPr and tBu
substituents to prevent the formation of B.6 In the solid state these reactions afford complexes of the
type [Fe(κ2P,NꢀPNPꢀR,TAD)X2] (R = iPr, tBu) (C) where the PNP ligand is coordinated in κ2P,Nꢀ
fashion (Scheme 1). In solution, complexes with κ3P,N,Pꢀ and κ2P,Nꢀbound PNP ligands are in
equilibrium with one another undergoing reversible dechelation and rechelation of the aminophosphine
arm bearing the TAD moiety.

Page 3 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
3
Results and Discussion
Ligand and Complex Synthesis. The synthesis of the new chiral PNP ligands PNPꢀiPr,TAD
(2a) and PNPꢀtBu,TAD (2b) was achieved via a twoꢀstep procedure. First, addition of 1 equiv of PR2Cl
(R = iPr, tBu) to 2,6ꢀdiaminopyridine in the presence of base (NEt3 or nBuLi) afforded the monoꢀ
phosphinated compounds 1a and 1b, respectively. Subsequent addition of 1 equiv of TADꢀPCl in the
presence of NEt3 afforded then the desired PNP ligands 2a and 2b in 55 and 42% isolated overall
yield (Scheme 2). The optically pure ligands were isolated as air stable solids and were characterized
by 1H, 13C{1H}, and 31P{1H} NMR spectroscopy and HRMS.
Scheme 2 Synthesis of chiral TADDOLꢀbased PNP ligands
Treatment of anhydrous FeCl2 with 1 equiv of the PNP ligands PNPꢀiPr,TAD (2a) and PNPꢀ
tBu,TAD (2b) in THF at room temperature afforded coordinatively unsaturated complexes of the
general formula [Fe(PNPꢀiPr,TAD)Cl2] (3a) and [Fe(PNPꢀtBu,TAD)Cl2] (4a) in 79% isolated yields
(Scheme 3). The analogous bromide complexes 3b and 4b were obtained in similar fashion by
Scheme 3 Synthesis of [Fe(PNPꢀR,TAD)X2] (3, 4) – solution vs solidꢀstate behavior

Dalton Transactions
Page 4 of 29
View Article Online
DOI: 10.1039/C5DT00832H
4
straightforward complexation of the respective free PNP ligands with anhydrous ferrous dibromide (89
and 89% yield). All complexes are thermally robust pale yellow solids that are air sensitive in the solid
state and particularly in solution.
Solid State Studies. In the solid state, they were characterized by elemental analysis, SQUID
and Mössbauer spectroscopy. In addition, the molecular structures of 3b and 4a were determined by
Xꢀray crystallography. Structural views of 3b and 4a are depicted in Figures 1 and 2 with selected
bond distances and angles given in the captions. Surprisingly, these complexes adopt a distorted
tetrahedral geometry with the PNP ligands coordinated in κ2P,Nꢀfashion rather than in the typical
meridional κ3P,N,Pꢀcoordination mode. Such a coordination mode is unexpected and unusual for
pyridineꢀbased PNP pincer ligands in particular when taking into account that these complexes are
coordinatively unsaturated 14e species.
Figures 1 and 2
The magnetic properties of 3a, 3b, 4a, and 4b were investigated by means of SQUID
magnetometry and Mössbauer spectroscopy. Magnetic moments of ꢀeff = 4.95(1), 4.98(1), 4.97(1),
and 5.00(1)ꢀB for 3a, 3b, 4a, and 4b, respectively, were derived from the temperature dependence of
the inverse molar magnetic susceptibility, which was well described by a Curie law above 10 K (four
unpaired electrons). These values are in good agreement with the effective magnetic moment of HS
Fe(II) in the spin only approximation (4.9ꢀB). The thermal variation of the inverse molar magnetic
susceptibility and of χmT for all complexes is shown in Figure 3. Mössbauer spectroscopy of these
complexes at 78 K showed isomer shifts (IS) of 0.76ꢀ0.79 mm/s and large quadrupole splittings (QS)
in the range 2.93ꢀ3.22 mm/s (Table 1, Figure 4). All spectra confirm the exclusive presence of HS
Fe(II), with higher isomer shifts and lower quadrupole splittings for the Cl complexes in comparison
with the Br ones as was also previously observed and discussed for related Fe(II) complexes.7 As can
be seen from Table 1, the isomer shift and quadrupole splitting values are different from those
observed for highꢀspin Fe(II) complexes with similar ligands but different geometries. When compared
to compounds with square pyramidal coordination, it can be noticed that all tetrahedral complexes
exhibit lower IS and higher QS values.5c
Figures 3 and 4
Solution Studies. While the solid state structures of complexes 3a, 3b, 4a, and 4b could be
unequivocally established (3b and 4a by Xꢀray crystallography), the situation in solution turned out to
be more difficult due to the paramagnetic and complex nature of these compounds. It is not obvious
whether these complexes are fourꢀ or five coordinate containing κ2P,Nꢀ or κ3P,N,Pꢀbound PNP
ligands, respectively, or whether both species are present and, if this is the case, whether these are in
equilibrium with one another. Accordingly, several experiments were performed to provide information
on the solution structure and behavior of 3a, 3b, 4a, and 4b.

Page 5 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
5
ESIꢀMS studies (in the positive ion mode) of complexes 3a, 3b, 4a, and 4b in CH3CN/CH3OH
(9:1) solutions revealed that these complexes remain largely intact and fragments of the sodiated
complexes of [Fe(PNPꢀR,TAD)X2] ([M+Na]+) were observed at m/z 868.2, 886.1, 896.2, and 984.1,
respectively. Further abundant fragments are [MꢀX]+ (for 3a, 4a, 4b), [MꢀX+CH3OH]+ (for 3a, 4a), and
[MꢀXꢀHX+CH3OH]+ (for 4a) revealing the high affinity of CH3OH towards these Fe(II) complexes. A
representative positive ion ESI full scan mass spectra of 3b is depicted in Figure 5. In the inset, the
isotopic pattern of the [M+Na]+ ion is compared with the theoretical pattern, which turned out to
correlate quite well.
Figure 5
1
All complexes display large paramagnetic shifted H and 13C{1H} NMR solution spectra with
broad and featureless signals which, due to the complexity of the PNP ligands, were not assignable
and thus not informative. Interestingly, while at room temperature no 31P{1H} NMR signals could be
detected, at ꢀ50oC complexes 3 and 4 exhibit a broad signal at about 126 ppm, assignable to the nonꢀ
coordinated TAD arm of the κ2P,Nꢀbound PNPꢀR,TAD ligands based on the fact that the free ligands
2a and 2b give rise to signals at 134.6 and 132.4 ppm. This observation clearly points to an
equilibrium between κ2P,Nꢀ and κ3P,N,Pꢀbound species, i.e., the PNPꢀR,TAD ligand is hemilabile,
which is also supported by DFT calculations (vide infra). Related solution equilibria were proposed for
the pincer complex [Fe(PONOPꢀtBu)Cl2] bearing the bulky bisꢀphosphinte PONOPꢀtBu ligand.8
Moreover, addition of BH3—THF to a solution of 3a in CD2Cl2 leads to boronation of the pendant TAD
arm shifting the equilibrium towards the fourꢀcoordinate complex [Fe(κ2P,NꢀPNPꢀiPr,TADBH )Cl2] (5)
3
(Scheme 4). 31P{1H} NMR spectra of these studies are shown in Figure 6. An ESIꢀMS study of 5 in
CH3CN/CH2Cl2 gave rise to largely intact fragments of the sodiated species [5+Na]+ and [5ꢀCl]+
observed at m/z 882.1 and 824.1.
O
O
O
Ph
O
Ph
Ph
Ph
Ph
O
Ph
Ph
O
Ph
O
Ph
O
O
O
O
H3B
Cl
Ph
Cl
P
P
H
O
Ph
N
P
Cl
NH
NH
Ph
H3B THF
N
Fe
N
Fe
N
Fe
Cl
CD2Cl2
CD2Cl2
Cl
HN PiPr2
3a' (S = 2)
Cl
HN PiPr2
3a (S = 2)
HN PiPr2
5 (S = 2)
= detected by 31P{1H} NMR
Scheme 4 Solution equilibria and boronation of [Fe(κ2P,NꢀPNPꢀiPr,TAD)Cl2] (3a) and [Fe(κ3P,N,Pꢀ
PNPꢀiPr,TAD)Cl2] (3a’) with BH3—THF in CD2Cl2.

Dalton Transactions
Page 6 of 29
View Article Online
DOI: 10.1039/C5DT00832H
6
Figure 6
Reactivity towards CO. When CO was bubbled into acetone solutions of 3a and 3b for a few
minutes, an immediate color change from pale yellow to violet and blue, respectively, was observed
due the formation of transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr/TAD)(CO)Cl2] (6a) and transꢀ[Fe(κ3P,N,PꢀPNPꢀ
iPr/TAD)(CO)Br2] (6b). (Scheme 5). Pure crystalline materials had to be obtained by diffusion of nꢀ
pentane into a THF solution of 6a and 6b, respectively, since evaporation of the solvent resulted in
rapid CO loss and reformation of 3a and 3b. In the case of the bulkier tBu complexes 4a and 4b, no
CO addition was observed. The same behavior was found for [Fe(κ3P,N,PꢀPNPꢀtBu)X2]. Noteworthy,
in the solid state none of the tetrahedral complexes underwent CO addition, in contrast to the
reactions of the five coordinate complexes [Fe(κ3P,N,PꢀPNPꢀiPr)X2] reported previously.9 Complexes
6a and 6b were fully characterized by 1H, 13C{1H}, and 31P{1H} NMR and IR spectroscopy. The 31P{1H}
NMR spectra give rise to two doublets centered at 154.0/125.0 and 161.6/125.2 ppm, respectively,
with large JPP coupling constants of 278 and 261 Hz which are consistent with a transꢀP,P
configuration (Figure 6, spectrum d). In the 13C{1H} NMR spectra the CO ligands exhibit a single lowꢀ
intensity doublet of doublet resonance at 220.3 and 222.5 ppm. Complexes 6a and 6b give rise to one
band at 1986 and 1980 cmꢀ1 in the IR spectrum (cf 2143 cmꢀ1 in free CO). For comparison, in transꢀ
[Fe(κ3P,N,PꢀPNPꢀiPr)(CO)Cl2] and transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr)(CO)Br2] the CO stretching frequency is
observed at 1956 cmꢀ1, indicating that PNPꢀiPr,TAD is a weaker donor than PNPꢀiPr.5c
The solid state structure of 6b could be determined by singleꢀcrystal Xꢀray diffraction. Crystals
were grown under a CO atmosphere from a THF solution by diffusion of pentane. An ORTEP diagram
is depicted in Figure 7 with selected bond distances given in the captions. This complex adopts a
distorted octahedral geometry around the metal center with the bromide ligands being in trans position
to one another. The PNP ligand is now coordinated to the iron center in the typical tridentate
meridional κ3P,N,P mode.
Figure 7
Heating solid samples of 6a and 6b leads to regeneration of complexes [Fe(κ2P,NꢀPNPꢀ
iPr,TAD)Cl2] (3a) and [Fe(κ2P,NꢀPNPꢀiPr,TAD)Br2] (3b). However, loss of CO is slow with about 80%
conversion after 2h. The Mössbauer spectra of the complex 6b before and after being partially
decarbonylated (by heating at 100oC for 1h and 2h under vacuum) are shown in Figure 8. The
spectrum of 6b is well fitted to a quadrupole doublet with IS = 0.23(1) mm/s and QS = 1.19(1) mm/s
assignable to a typical low spin Fe(II) complex in a symmetric octahedral environment. The isomer
shift decrease observed from 3b to 6b is explained by the HSꢀLS change induced by the reaction with
CO. Accordingly, the decrease in the quadrupole splitting values is explained by the much less
asymmetric environment of Fe(II) in six coordination (6b) than in four coordination (3b).
Figure 8

Page 7 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
7
Scheme 5 Reaction of [Fe(PNPꢀiPr,TAD)X2] (3a,b) with CO in solution forming transꢀ[Fe(κ3P,N,Pꢀ
PNPꢀiPr,TAD)(CO)X2] (6a,b).
When compared with the hyperfine parameters of 6b spectrum (Table 1), it can be deduced that
60% and 21% of this contribution remains unreacted in the 1h and 2h annealed samples, respectively.
A second doublet appears in these partially decarbonylated samples, displaying hyperfine parameters
of IS = 0.78(1) mm/s, QS = 3.07(2) mm/s and IS = 0.79(1) mm/s, QS = 3.15(1) mm/s, for the 1h and
2h annealed samples, respectively. As can be seen from Table 1, the isomer shift and quadrupole
splitting values of [Fe(κ2P,NꢀPNPꢀiPr,TAD)X2] complexes are different from those observed for highꢀ
spin Fe(II) complexes with similar ligands but different geometries. In particular, when compared to
compounds with square pyramidal geometry, it can be noticed that all tetrahedral complexes exhibit
lower IS and higher QS values.5c Therefore, the second doublet detected in the decarbonylated
samples can be assigned to [Fe(κ2P,NꢀPNPꢀiPr,TAD)Br2] (3b), indicating that at higher temperatures
the CO release in the solid state does not induce the formation of the pentaꢀcoordinated [Fe(κ3P,Nꢀ
PNPꢀiPr,TAD)Br2] (3b’) complex.
The isomer shift and quadrupole splitting of complexes 3b, 3b’and 6b were calculated using a
DFT approach, as 0.873 (3b), 0.972 (3b’), and 0.478 (6b) mm/s and 2.694 (3b), 2.652 (3b’), and
0.675 (6b) mm/s, respectively. Despite the small deviation, the trends are well reproduced. The isomer
shift of 3b is much higher than that of 6b, as observed experimentally. The pentacoordinate
complexes often display isomer shifts ~0.1 higher than the tetrahedral ones (see examples in Table 1),
and our calculations also predict a similar difference (0.873 and 0.972 mm/s for 3b and 3b’,

Dalton Transactions
Page 8 of 29
View Article Online
DOI: 10.1039/C5DT00832H
8
respectively). These values suggest that the signal of a pentacoordinate species should have been
detected if such a species were present in the solid state, supporting the experimental results
described above.
Treatment of 6a and 6b with 1 equiv of AgSbF6 in THF in the presence of CO at room
temperature afforded selectively the cationic complexes transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)2X]+
(7a,b) in 80 and 81% isolated yields (Scheme 6). These complexes are thermally robust red solids that
are air stable both in the solid state and in solution for several days. In the 13C{1H} NMR spectrum the
two CO ligands are chemically and magnetically not equivalent thus giving rise to two lowꢀintensity
Scheme 6 Reaction of [Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)X2] (6a,b) with CO in the presence of Ag+
forming transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)2X]+ (7a,b) and attempted reaction with Na[HBEt3].
doublet of doublet resonances centered at about 207 ppm. In the IR spectrum the CO ligands exhibit
only one band at 2026 and 2021 cmꢀ1 for the mutually trans CO ligands which can be assigned to the
asymmetric CO stretching frequency. The symmetric CO stretching band is IR inactive and thus not
observed.
Finally, since the hydride complex [Fe(κ3P,N,PꢀPNPꢀiPr)(CO)(H)Br] turned out to be an
efficient hydrogenation catalyst for both ketones and aldehydes,10 we have attempted to prepare chiral
hydride complexes of the type [Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)(H)X] following an established
procedure. However, treatment of 6a or 6b with Na[HBEt3] lead to decomposition and the only
tractable products which could be isolated were the free PNP ligands.
Computational Studies. The change in coordination of the PNP ligand from a κ2P,N to a
κ3P,N,P mode was investigated by means of DFT calculations11 (see Computational Details) using a
model of complex 3b (R = iPr, X = Br) with a simplified TADDOL moiety (TAD’). In the model the
phenyl groups were replaced by Hꢀatoms for computational expediency. The energy profile for the
rearrangement between the fourꢀcoordinated complex with a κ2P,NꢀPNP ligand (A) and the pentaꢀ
coordinated species with a κ3P,N,PꢀPNP ligand (C) is shown in Scheme 7.
The mechanism for the coordination of the second Pꢀatom in [Fe(PNPꢀiPr,TAD’)Br2]
comprises two steps. The first is reꢀorientation of the pendant arm of the PNP ligand in the reactant,
with rotation along the CpyꢀNNH bond. In the second step, the Pꢀatom coordinates the metal. The
overall process has an energy barrier of 9.8 kcal/mol (corresponding to TSBC) and indicates a facile
process that should occur easily at room temperature. This is in agreement with 31P NMR studies

Page 9 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
9
(Figure 6). Since the fiveꢀcoordinated isomer C is only 3.6 kcal/mol less stable than the initial complex
A, the profile in Scheme 7 suggests that an equilibrium between the two forms may occur in solution.
Importantly, the performance of the model used in the calculations was tested through the comparison
of the stability difference obtained for the two isomers calculated with the real species, that is, with full
Scheme 7. Energy profile (kcal/mol) for the rearrangement between κ2P,N and κ3P,N,P in [Fe(PNPꢀ
iPr,TAD’)Br2] and comparison with [Fe(PNPꢀiPr)Br2].
TADDOL fragments in the PNP ligands. The results indicate that the fourꢀcoordinated species is more
stable than its fiveꢀcoordinated counterpart by 2.8 kcal/mol, a value that is only 0.8 kcal/mol smaller
than the one obtained with the simplified model and validates the use of that model in the calculations.
For comparison, the equilibrium between [Fe(κ3P,N,PꢀPNPꢀiPr)Br2] and [Fe(κ2P,NꢀPNPꢀiPr)Br2] was
also investigated (Scheme 7). This process proceeds via a single step but with a considerably higher
barrier of 17.2 kcal/mol and agrees with the fact experimentally no [Fe(κ2P,NꢀPNPꢀiPr)Br2] complex
was detected as the was no BH3 adduct formed. These results may also suggest that the hemilability
of the PNPꢀR,TAD ligand is not due to steric but electronic reasons.
Scheme 8. Energy profile (kcal/mol) for the reaction of CO addition to [Fe(κ3P,N,PꢀPNPꢀiPr,TAD’)Br2].
The energy values are referred to A, and the FeꢀCCO distances (Å) are indicated. The full curve

Dalton Transactions
Page 10 of 29
View Article Online
DOI: 10.1039/C5DT00832H
10
corresponds to the S = 2 potential energy surface (PES), and the dashed curve corresponds to the S
= 0 PES.
The energy profile calculated for CO addition to the fiveꢀcoordinated complex [Fe(κ3P,N,PꢀPNPꢀ
iPr,TAD’)Br2] is represented in Scheme 8. The first step of the mechanism occurs in the spin quintet (S
= 2) potential energy surface (PES). Starting point is the pair of reactants D, i.e., the fiveꢀcoordinated
complex and one CO molecule. From D CO addition occurs in a single step going through a rather
early transition state (TSDE) with a long FeꢀCCO bond distance (3.01 Å) indicating that CO coordination
is only starting once that transition state is reached. In the resulting intermediate E CO coordination is
established with a FeꢀCCO distance of 2.16 Å. This step is thermodynamically favorable (ꢁE = ꢀ3.1
kcal/mol) and has a negligible barrier of 0.7 kcal/mol. Since the product of the reaction is the low spin
(S = 0) CO adduct F, there must be a spin change from the quintet to the singlet spin PES in the last
step of the mechanism. In other words, this is a “spin forbidden” or “nonꢀadiabatic” reaction and, thus,
the change between PES occurs through a minimumꢀenergy crossing point (MECP). In the MECP
both the geometry and the energy of the molecules are the same in the two PES. Once that point is
reached along the reaction coordinate there is a given probability for the system to change spin state
and hop from one spin state to the other, completing the “spinꢀforbidden” reaction. Accordingly, in the
last step of the mechanism depicted in Scheme 7, the high spin CO complex, E (S = 2), changes to its
low spin isomer, F (S = 0), through a crossing point (CP) that has an intermediate geometry between
the two CO species. The moderate energy barrier associated with the spin exchange (6.7 kcal/mol)
reflects the small geometry modifications associated with this step. The final product F is 2.4 kcal/mol
more stable than the initial reagent, the fourꢀcoordinated species [Fe(κ2P,NꢀPNPꢀiPr,TAD’)Br2] (A).
From the two profiles it becomes clear that the most difficult process along the path is
coordination of the second Pꢀatom of the PNP ligand, rather than CO addition or spin change. In fact,
the highest barrier along the entire mechanism is associated with TSBC, i.e., the step corresponding to
the coordination of the dangling Pꢀatom, from B to C (see Scheme 6).
An alternative mechanism is CO addition to the fourꢀcoordinated complex [Fe(κ2P,NꢀPNPꢀ
iPr,TAD’)Br2] followed by a subsequent change in the coordination mode of the PNP ligand from κ2P,N
to κ3P,N,P with coordination of the free Pꢀatom. The energy profile for this process is depicted in
Scheme 9.

Page 11 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
11
Scheme 9. Energy profile (kcal/mol) for the reaction of CO addition to [Fe(κ2P,NꢀPNPꢀiPr,TAD’)Br2]
followed by exchange of the coordination of the pincer ligand from κ2P,N to κ3P,N,P. The energy
values are referred to A, and the FeꢀCCO distances (Å) are indicated.
The mechanism starts with a pair of nonꢀinteracting reactants G, the CO molecule and the
[Fe(κ2P,NꢀPNPꢀiPr,TAD’)Br2] complex. In the first step there is CO addition and formation of a fiveꢀ
coordinated CO complex H that maintains the PNP dangling arm existing in the initial complex. A
rather long FeꢀCCO distance of 2.80 Å in the corresponding transition state (TSGH) indicates that CO
coordination is just beginning when TSGH is reached along the reaction coordinate. This step occurs
with a very small barrier of 0.8 kcal/mol and the intermediate H formed is only 0.3 kcal/mol more stable
than the initial reagents. In the second step of the mechanism, from H to E, there is coordination of the
Pꢀatom of the PNP ligand, originating the high spin sixꢀcoordinated CO complex. Coordination of the
Pꢀatom in the CO species occurs in a single step, from H to E, through the transition state TSHE. In
TSHE, formation of the new Fe–P bond is still far from complete, as indicated by a distance of 3.64 Å,
which is 1.08 Å longer than the coordination distance in E. This step is practically thermoneutral (ꢁE =
0.6 kcal/mol) and the corresponding barrier (12.9 kcal/mol) is the highest of that path.
In the first mechanism, coordination of the loose Pꢀatom of the pincer ligand, with the
corresponding change from κ2P,N to κ3P,N,P occurs first and it is followed by CO addition. This path
corresponds to the profiles represented in Schemes 7 and 8, and its highest barrier is 9.8 kcal/mol. In
the second path, the order of the processes is reversed. First there is addition of CO to the fourꢀ
coordinated complex with κ2P,NꢀPNP, and only then occurs coordination of the free Pꢀatom. The
highest barrier obtained for this case, associated with the Pꢀcoordination step (TSHE in Scheme 9), is
12.9 kcal/mol and slightly higher by 3.1 kcal/mol than the barrier associated with the first mechanism.
Therefore, the first mechanism should be slightly favored.
This trend is supported by the fact that, despite their relatively low formal electron count, the
related four coordinate complexes [Fe(κ2P,NꢀPNꢀiPr)X2] (PNꢀiPr = Nꢀdiisopropylphosphinoꢀ2ꢀ
aminopyridine) did not react with CO at ambient temperature,12 while pentacoordinate complexes
[Fe(κ3P,N,PꢀPNPꢀiPr)X2] readily added CO to give octahedral complexes of the type [Fe(κ3P,N,Pꢀ
PNPꢀiPr)(CO)X2]. In general, the outcome of a CO addition to high spin iron(II) complexes depends on
the geometry and coordination number, the key factor being the overall ligand field of the ensemble of
ligands in the reactant. Accordingly, the lower the coordination number the less likely CO addition will
take place and reactions will be disfavored.13
Conclusions
We have prepared several Fe(II) complexes of the general formula [Fe(PNPꢀR,TAD)X2] (R = iPr, tBu;
X = Cl, Br) bearing asymmetric chiral pincer ligands based on R,RꢀTADDOL (TAD). In the solid state,
as shown by Xꢀray crystallography, Mössbauer and IR spectroscopy, these complexes adopt a
tetrahedral geometry with the PNP ligand coordinated in κ2P,Nꢀfashion with the TAD arm being not
coordinated. All complexes are paramagnetic with a quintet ground state. In solution there are
equilibria between [Fe(κ3P,N,PꢀPNPꢀR,TAD)X2] and [Fe(κ2P,NꢀPNPꢀR,TAD)X2] complexes and the

Dalton Transactions
Page 12 of 29
View Article Online
DOI: 10.1039/C5DT00832H
12
PNPꢀR,TAD ligand is hemilabile. DFT calculations indicate that the rearrangement of the pincer
ligands is an accessible process and, thus, equilibria between the fourꢀcoordinated species and their
fiveꢀcoordinated counterparts is expected to occur. Addition of CO to [Fe(PNPꢀiPr,TAD)X2] in solution
yields the diamagnetic octahedral complexes transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)X2], which react
further with Ag+ salts in the presence of CO to give the cationic complexes transꢀ[Fe(κ3P,N,PꢀPNPꢀ
iPr,TAD)(CO)2X]+. Complexes bearing tBu substituents do not react with CO. Moreover, in the solid
state none of the tetrahedral complexes are able to bind CO. Addition of CO most likely takes place at
the five coordinate complex [Fe(κ3P,N,PꢀPNPꢀiPr,TAD)X2]. The mechanism for this process was
investigated by DFT and the most favorable path obtained corresponds to the rearrangement of the
pincer ligand first from a κ2P,Nꢀ to a κ3P,N,Pꢀcoordination mode followed by CO coordination to
[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)X2]. The alternative path with a reversed order of the two processes is
competitive and the possible occurrence of both at the same time may not be discarded. Heating solid
samples of transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)2X]+ at 100oC under vacuum leads to regeneration of
complex [Fe(κ2P,NꢀPNPꢀiPr,TAD)X2]. Loss of CO is slow with about 80% conversion after 2h.
Mössbauer measurements show no evidence for the formation of [Fe(κ3P,N,PꢀPNPꢀiPr,TAD)X2]
suggesting that at higher temperatures the PNP ligands are flexible enough to change their
coordination modes also in the solid state.
Experimental Section
General. All manipulations were performed under an inert atmosphere of argon by using
Schlenk techniques or in a MBraun inertꢀgas glovebox. The solvents were purified according to
standard procedures.14 The deuterated solvents were purchased from Aldrich and dried over 4 Å
molecular sieves. TADꢀPCl was prepared according to the literature.15 1H, 13C{1H}, and 31P{1H} NMR
spectra were recorded on Bruker AVANCEꢀ250, AVANCEꢀ300 DPX, and AVANCEꢀ400
1
spectrometers. H and 13C{1H} NMR spectra were referenced internally to residual protioꢀsolvent, and
solvent resonances, respectively, and are reported relative to tetramethylsilane (δ = 0 ppm). 31P{1H}
NMR spectra were referenced externally to H3PO4 (85%) (δ = 0 ppm). 11B NMR spectra were
referenced externally to BF3—Et2O.
Magnetization measurements as a function of temperature were performed on powder samples
using a SQUID magnetometer (Quantum Design MPMS). The curves were obtained under 0.1 T, for
temperatures ranging from 10 to 300 K. The molar susceptibility values (
diamagnetism of the constituent atoms using Pascal constants.
χm) were corrected for
The 57Fe Mössbauer spectra were recorded in transmission mode at 78 K using a
conventional constantꢀacceleration spectrometer and a 50 mCi 57Co source in a Rh matrix. The
measurements were performed using a liquid nitrogen flow cryostat with a temperature stability of ±0.5
K. The velocity scale was calibrated using an αꢀFe foil. The spectra were fitted to Lorentzian lines
using the WinNormos software program, and the isomer shifts reported are relative to metallic αꢀFe at
room temperature.

Page 13 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
13
Mass spectrometric measurements were performed on an Esquire 3000plus 3Dꢀquadrupole ion
trap mass spectrometer (Bruker Daltonics, Bremen, Germany) in positiveꢀion mode by means of
electrospray ionization (ESI). Mass calibration was done with a commercial mixture of perfluorinated
trialkyltriazines (ES Tuning Mix, Agilent Technologies, Santa Clara, CA, USA). All analytes were
dissolved in a mixture of CH3CN and CH3OH “hypergrade for LCꢀMS Lichrosolv” quality (Merck,
Darmstadt, Germany) to form a concentration of roughly 1 mg/mL. Direct infusion experiments were
carried out using a Cole Parmer model 74900 syringe pump (Cole Parmer Instruments, Vernon Hills,
IL, USA) at a flow rate of 2 ꢀL/min. Full scans were measured in the range m/z 100ꢀ1100 with the
target mass set to m/z 1000. All mass calculations are based on the lowest mass (i.e. most abundant)
iron isotope (56Feꢀisotope). Mass spectra were averaged during data acquisition time of 1 to 2 min and
one analytical scan consisted of five successive micro scans resulting in 50 and 100 analytical scans,
respectively, for the final full scan mass spectrum.
Synthesis. [PN/NH2-iPr] (1a). 2,6ꢀDiaminopyridine (6.87 g, 62.95 mmol) was suspended with
NEt3 (8.7 mL, 62.95 mmol) in 1:1 mixture of THF/toluol (150 mL) and cooled to 0°C. PiPr2Cl (4.80 g,
31.47 mmol) was added slowly via syringe. The resulting mixture was allowed to reach room
temperature and was stirred overnight at 80°C. The solution was extracted at room temperature with
50 mL of a saturated NaHCO3 solution, followed by extraction with 50 mL of brine. The combined
organic phases were dried over Na2SO4, filtered and the solvent was removed under vacuum. The
product was purified via flash chromatography (ethyl acetate/petrol ether 2:1 → ethyl acetate) and
1
3
isolated as white crystals. Yield: 4.32 g (61%). H NMR (δ, CDCl3, 20°C): 7.25 (t, JHH = 7.9 Hz, 1H,
py4), 6.46 (d, 3JHH = 7.9 Hz, 1H, py3), 5.90 (d, 3JHH = 7.7 Hz, 1H, py5), 4.40 (d, 2JPH = 10.7 Hz, 1H, NH),
4.18 (s, 2H, NH2), 1.77 (m, 2H, CH(CH3)2), 1.07 (m, 12H, CH(CH3)2). 13C{1H} NMR (δ, CDCl3, 20°C):
2
3
159.86 (d, JCP = 20.3 Hz, py2), 157.28 (py6), 139.42 (d, 4JCP = 2.1 Hz, py4), 97.99 (d, JCP = 18.3 Hz,
1
2
py3), 97.99 (py5), 26.37 (d, JCP = 11.2 Hz, CH(CH3)2), 18.66 (d, JCP = 19.7 Hz, CH(CH3)2), 17.05 (d,
2JCP = 7.8 Hz, CH(CH3)2). 31P{1H} NMR (δ, CDCl3, 20°C): 47.4. MS (ESIꢀHRMS, CH3CN) (m/z): [M+H]+
(C11H21N3P) calcd 226.1468. Found 226.1453.
[PN/NH2-tBu] (1b). 2,6ꢀDiaminopyridine (5.17 g, 47.37 mmol) was dissolved in THF (120 mL)
and cooled to ꢀ10°C. nBuLi (9.6 mL, 24.16 mmol, 2.5 M) was added slowly via syringe, whereupon the
solution turned dark red and a precipitate was formed. The mixture was allowed to warm up to room
temperature and was stirred for 3 h. After cooling to ꢀ10°C PtBu2Cl (4.28 g, 23.69 mmol) was added
and the solution was then warmed to 80°C and stirred overnight. Workup was performed in the same
1
way as for compound 1a. The product was isolated as a pale yellow powder. Yield: 2.70 g (45%). H
NMR (δ, CDCl3, 20°C): 7.25 (t, 3JHH = 7.9 Hz, 1H, py4), 6.52 (dd, 3JHH = 8.0 Hz, 4JPH = 2.4 Hz, 1H, py3),
3
2
3
5.89 (d, JHH = 7.7 Hz, 1H, py5), 4.67 (d, JPH = 11.0 Hz, 1H, NH), 4.19 (s, 2H, NH2), 1.15 (d, JPH
=
12.0 Hz, 18H, C(CH3)3). 13C{1H} NMR (δ, CDCl3, 20°C): 160.25 (d, 2JCP = 21.4 Hz, py2), 157.29 (d, 4JCP
= 1.5 Hz, py6), 139.39 (d, 4JCP = 2.1 Hz, py4), 98.36 (d, 3JCP = 18.8 Hz, py3), 97.96 (py5), 33.98 (d, 1JCP
= 19.2 Hz, C(CH3)3), 28.08 (d, 2JCP = 15.2 Hz, C(CH3)3). 31P{1H} NMR (δ, CDCl3, 20°C): 58.2. MS (ESIꢀ
HRMS, CH3CN) (m/z): [M+H]+ (C13H25N3P) calcd 254.1786. Found 254.1788.
[PNP-iPr,TAD] (2a). To a solution of 1a (2.00 g, 8.84 mmol) in toluene (100 mL) NEt3 (1.9 mL,
13.26 mmol) was added. After cooling to 0°C, a solution of TADꢀPCl (5.16 g, 9.72 mmol) in toluene

Dalton Transactions
Page 14 of 29
View Article Online
DOI: 10.1039/C5DT00832H
14
(30mL) was added and the reaction mixture was stirred overnight at 80°C. The suspension was
filtered over a small pad of Celite® and the solvent was removed under reduced pressure. The product
was obtained as white powder in sufficient purity for subsequent reactions. If necessary, it can be
1
purified via flash chromatography (ethyl acetate/petrol ether/CH2Cl2 1:1:1). Yield: 5.75 g (90%). H
NMR (δ, CDCl3, 20°C): 7.74 – 7.63 (m, 4H, ph), 7.47 – 7.19 (m, 17H, ph, py4), 6.61 (dd, 3JHH = 8.1 Hz,
3
2
4JPH = 2.2 Hz, 1H, py3), 6.06 (d, JHH = 7.7 Hz, 1H, py5), 5.67 (d, JPH = 4.3 Hz, 1H, NHTAD), 5.32 (dd,
4
3
2
3JHH = 8.5 Hz, JPH = 2.8 Hz, 1H, CHTAD), 4.94 (d, JHH = 8.5 Hz, 1H, CHTAD), 4.43 (d, JPH = 11.2 Hz,
1H, NHiPr), 1.80 (m, 2H, CH(CH3)2iPr), 1.39 (s, 3H, CH3TAD), 1.12 – 1.01 (m, 12H, CH(CH3)2iPr), 0.34 (s,
3H, CH3TAD). 13C{1H} NMR (δ, CDCl3, 20°C): 159.96 (d, 2JCP = 20.4 Hz, py2), 154.19 (d, 2JCP = 16.2 Hz,
py6), 146.42 (Ph1), 145.58 (Ph1), 141.62 (Ph1), 140.95 (Ph1), 139.12 (py4), 128.96 (Ph), 128.52 (Ph),
128.49 (Ph), 128.15 (Ph), 127.71 (Ph), 127.67 (Ph), 127.61 (Ph), 127.37 (Ph), 127.28 (Ph), 127.16
(Ph), 112.12 (C(CH3)2TAD), 99.85 (d, 3JCP = 18.8 Hz, py3), 99.03 (d, 3JCP = 9.3 Hz, py5), 82.87 (CHTAD),
1
82.68 (CPh2), 82.46 (CHTAD, CPh2), 27.22 (CH3TAD), 26.37 (d, JCP = 10.8 Hz, CH(CH3)2iPr), 26.27 (d,
1JCP = 9.9 Hz, CH(CH3)2TAD), 24.89 (CH3TAD), 18.53 (d, 2JCP = 3.6 Hz, CH(CH3)2iPr), 18.33 (d, 2JCP = 3.8
2
2
Hz, CH(CH3)2iPr), 16.99 (d, JCP = 8.3 Hz, CH(CH3)2iPr), 16.90 (d, JCP = 7.7 Hz, CH(CH3)2iPr). 31P{1H}
NMR (δ, CDCl3, 20°C): 134.6 (TAD), 47.9 (iPr). MS (ESIꢀHRMS, CH3CN/CH3OH) (m/z): [M+H]+
(C42H48N3O4P2) calcd 720.3115. Found 720.3113.
[PNP-tBu,TAD] (2b). This compound was prepared analogously to 2a with 1b (0.86.g, 8.84
mmol), NEt3 (0.7 mL, 5.08 mmol), and TADꢀPCl (1.80 g, 3.38 mmol) as starting materials. Yield: 2.39 g
1
(94%). H NMR (δ, CDCl3, 20°C): 7.73 (m, 2H, Ph), 7.64 (m, 2H, Ph), 7.50 – 7.16 (m, 17H, Ph, py4),
6.63 (dd, 3JHH = 8.0 Hz, 4JPH = 2.2 Hz, 1H, py3), 6.10 (d, 3JHH = 7.8 Hz, 1H, py5), 5.55 (d, 2JPH = 2.4 Hz,
3
4
3
1H, NHTAD), 5.32 (dd, JHH = 8.4 Hz, JPH = 2.4 Hz, 1H, CHTAD), 4.99 (d, JHH = 8.4 Hz, 1H, CHTAD),
2
3
4.67 (d, JPH = 11.2 Hz, 1H, NHiPr), 1.22 (s, 3H, CH3TAD), 1.17 (d, JPH = 4.7 Hz, 9H, C(CH3)3tBu), 1.12
(d, 3JPH = 4.7 Hz, 9H, C(CH3)3tBu), 0.35 (s, 3H, CH3TAD). 13C{1H} NMR (δ, CDCl3, 20°C): 160.19 (d, 2JCP
2
= 21.8 Hz, py2), 153.90 (d, JCP = 19.0 Hz, py6), 145.99 (Ph1), 145.52 (Ph1), 141.74 (Ph1), 140.95 (s,
Ph1), 139.26 (py4), 128.93 (Ph), 128.54 (Ph), 128.19 (Ph), 127.71 (Ph), 127.47 (Ph), 127.36 (Ph),
127.15 (Ph), 127.06 (Ph), 126.65 (Ph), 112.36 (C(CH3)2TAD), 100.36 (d, 3JCP = 19.2 Hz, py3), 99.07 (d,
3
3
3JCP = 15.2 Hz, py5), 83.34 (CPh2), 82.66 (d, JCP = 7.6 Hz, CHTAD), 82.27 (d, JCP = 7.8 Hz, CHTAD),
1
82.12 (CPh2, 33.96 (d, JCP = 19.1 Hz, C(CH3)3tBu), 28.19 (C(CH3)3tBu), 27.95 (C(CH3)3tBu), 27.33
(CH3TAD), 25.39 (CH3TAD). 31P{1H} NMR (δ, CDCl3, 20°C): 132.4 (TAD), 58.6 (tBu). MS (ESIꢀHRMS,
CH3CN/CH3OH) (m/z): [M+H]+ (C44H52N3O4P2) calcd 748.3428. Found 748.3427.
[Fe(κ2P,NꢀPNP-iPr,TAD)Cl2] (3a). A suspension of anhydrous FeCl2 (86 mg, 0.68 mmol) and
2a (500 mg, 0.69 mmol) was stirred in THF (15 mL) at room temperature for 12 h. The solvent was
then removed under vacuum and the remaining solid redissolved in CH2Cl2 (15mL). Insoluble
materials were removed by filtration. The volume of the solution was reduced to about 1 mL and the
product was precipitated by addition of nꢀpentane (40 mL). After filtration the yellow product was
washed twice with nꢀpentane (10 mL) and dried under vacuum. Yield: 458 mg (79%). Anal. Calcd. for
C42H47Cl2N3O4P2Fe (846.55): C, 59.59; H, 5.60; N, 4.96%. Found: C, 58.99; H, 5.49; N, 5.10%. ESIꢀ
MS (m/z, CH3CN/CH3OH) positive ion: 868.2 [M+Na]+, 842.3 [M−Cl+CH3OH]+, 810.2 [M−Cl]+. ꢀeff
=
4.95(1)ꢀB.

Page 15 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
15
[Fe(κ2P,NꢀPNP-iPr,TAD)Br2] (3b). This complex was prepared analogously to 3a with
anhydrous FeBr2 (176 mg, 0.82 mmol) and 2a (600 mg, 0.83 mmol) as starting materials. Yield: 678
mg (89%). Anal. Calcd. for C42H47Br2N3O4P2Fe (935.46): C, 53.93; H, 5.06; N, 4.49%. Found: C,
54.09; H, 5.19; N, 4.52%. ESIꢀMS (m/z, CH3CN/CH3OH) positive ion: 956.1 [M+Na]+, 886.2 [MꢀBrꢀ
CH3OH]+, 854.2 [MꢀBr]+, 806.3 [M−BrꢀHBr+CH3OH]+. ꢀeff = 4.97(1)ꢀB.
[Fe(κ2P,NꢀPNP-tBu,TAD)Cl2] (4a). This complex was prepared analogously to 3a with
anhydrous FeCl2 (48 mg, 0.38 mmol) and 2b (300 mg, 0.40 mmol) as starting materials. Yield: 263 mg
(79%). Anal. Calcd. for C44H51Cl2N3O4P2Fe (874.60): C, 60.43; H, 5.88; N, 4.80%. Found: C, 60.22; H,
5.92; N, 4.95%. ESIꢀMS (m/z, CH3CN/CH3OH) positive ion: 896.2 [M+Na]+. ꢀeff = 4.98(1)ꢀB.
[Fe(κ2P,NꢀPNP-tBu,TAD)Br2] (4b). This complex was prepared analogously to 3a with
anhydrous FeBr2 (82 mg, 0.38 mmol) and 2b (300 mg, 0.40 mmol) as starting materials. Yield: 305 mg
(83%). Anal. Calcd. for C44H51Br2N3O4P2Fe (963.51): C, 54.85; H, 5.34; N, 4.36%. Found: C, 54.92; H,
5.39; N, 4.22%. ESIꢀMS (m/z, CH3CN/CH3OH) positive ion: 984.1 [M+Na]+, 882.2 [M−Br]+. ꢀeff
=
5.00(1)ꢀB.
Boronation of [Fe(κ2P,NꢀPNP-iPr,TAD)Cl2] (3a) in CD2Cl2. BH3—THF (26.0 ꢁL, 26.0 ꢁmol, 1.0
M in THF) was transferred to a NMRꢀtube containing 3a (20 mg, 23.6 ꢁmol) in CD2Cl2 (0.5 mL). A
31P{1H} and 11B NMR spectra were immediately measured giving rise to signals at 60.0 and ꢀ46.8 ppm,
respectively.
trans-[Fe(κ3P,N,P-PNP-iPr,TAD)(CO)Cl2] (6a). A solution of 3a (200 mg, 0.24 mmol) in THF (5
mL) was purged with CO for 3 min, whereupon the colour changed immediately from yellow to violet.
The product was crystallized by slow diffusion of nꢀpentane into a THF solution of 6a. The solvent was
decanted and the remaining solid was dried under vacuum for 30 min. Yield: 152 mg (74%). Anal.
Calcd. for C43H47Cl2N3O5P2Fe—C4H8O (946.66): C, 59.63; H, 5.86; N, 4.44%. Found: C, 59.52; H, 5.99;
N, 4.65%. 1H NMR (δ, acetoneꢀd6, 20°C): 8.09 (d, 2JPH = 8.1 Hz, 1H, NHiPr), 7.97 (m, 2H, Ph), 7.82 (m,
2
2H, Ph), 7.74 (d, JPH = 3.1 Hz, 1H, NHTAD), 7.56ꢀ7.47 (m, 4H, Ph), 7.42ꢀ7.35 (m, 4H, Ph), 7.29ꢀ7.19
3
3
3
(m, 9H, py4, Ph), 6.56 (d, JHH = 7.9 Hz, 1H, py3), 5.91 (d, JHH = 8.0 Hz, 1H, CHTAD), 5.87 (d, JHH
=
3
7.8 Hz, 1H, py5), 5.71 (d, JHH = 8.0 Hz, 1H, CHTAD), 3.14 (m, 1H, CH(CH3)2iPr), 2.98 (m, 1H,
3
CH(CH3)2iPr), 1.55 (dd, 3JHH = 7.0 Hz, 3JPH = 13.7 Hz, 3H, CH(CH3)2iPr), 1.54 (dd, JHH = 7.0 Hz, 3JPH
=
=
13.9 Hz, 3H, CH(CH3)2iPr), 1.45 (dd, JHH = 7.3 Hz, JPH = 16.0 Hz, 3H, CH(CH3)2iPr), 1.36 (dd, JHH
3
3
3
3
7.3 Hz, JPH = 15.9 Hz, 3H, CH(CH3)2iPr), 0.75 (s, 3H, CH3TAD), 0.66 (s, 3H, CH3TAD). 13C{1H} NMR (δ,
2
2
2
4
acetoneꢀd6, 20°C): 220.32 (dd, JCP = 22.3 Hz, JCP = 35.1 Hz, CO), 162.58 (dd, JCP = 8.0 Hz, JCP
=
2
4
12.9 Hz, py2), 157.81 (dd, JCP = 3.3 Hz, JCP = 23.5 Hz, py6), 144.07 (Ph1), 143.35 (Ph1), 141.75 (d,
3
3JCP = 5.5 Hz, Ph1), 140.29 (d, JCP = 8.2 Hz, Ph1), 139.46 (py4), 129.02 (Ph), 128.84 (Ph), 128.43
(Ph), 128.36 (Ph), 127.98 (Ph), 127.68 (Ph), 127.61 (Ph), 127.34 (Ph), 127.13 (Ph), 127.05 (Ph),
3
3
126.79 (Ph), 126.66 (Ph), 114.62 (C(CH3)2), 99.97 (d, JCP = 7.3 Hz, py3), 98.65 (d, JCP = 10.4 Hz,
2
2
py5), 88.91 (d, JCP = 8.3 Hz, CPh2), 88.56 (d, JCP = 19.9 Hz, CPh2), 80.97 (d, 3JCP = 2.5 Hz, CHTAD),
79.98 (d, 3JCP = 2.9 Hz, CHTAD), 26.44 (CH3TAD), 26.01 (CH3TAD), 25.86 (d, 1JCP = 22.0 Hz, CH(CH3)2iPr),
1
2
2
24.94 (d, JCP = 25.5 Hz, CH(CH3)2iPr), 18.69 (d, JCP = 4.3 Hz, CH(CH3)2iPr), 18.61 (d, JCP = 3.9 Hz,
2
CH(CH3)2iPr), 17.74 (CH(CH3)2iPr), 17.28 (CH(CH3)2iPr). 31P{1H} NMR (δ, CDCl3, 20°C): 154.0 (d, JPP
=

Dalton Transactions
Page 16 of 29
View Article Online
DOI: 10.1039/C5DT00832H
16
2
278.8 Hz, TAD), 125.1 (d, JPP = 278.9 Hz, iPr). IR (ATR, cmꢀ1): 1976 (νC=O). IR (CH2Cl2, cmꢀ1): 1986
(νC=O).
trans-[Fe(κ3P,N,P-PNP-iPr,TAD)(CO)Br2] (6b). This compound was prepared analogously to
6a with 3b (200 mg, 0.21 mmol) as starting material. Yield: 164 mg (80%). Anal. Calcd. for
C43H47Br2N3O5P2Fe—2C4H8O (1107.68): C, 55.30; H, 5.73; N, 3.79%. Found: C, 55.52; H, 5.81; N,
1
2
3.44%. HꢀNMR (δ, acetoneꢀd6, 20°C): 8.07 (d, JPH = 8.1 Hz, 1H, NHiPr), 8.02 (m, 2H, ph), 7.89 (m,
2
4
2H, ph), 7.76 (dd, JPH = 6.5 Hz, JPH = 3.0 Hz, 1H, NHTAD), 7.54 (m, 2H, Ph), 7.48 (m, 2H, Ph), 7.39
(m, 4H, Ph), 7.34 (t, 3JHH = 7.9 Hz, 1H, py4), 7.29 – 7.18 (m, 8H, Ph), 6.56 (d, 3JHH = 7.9 Hz, 1H, py3),
5.88 (d, 3JHH = 8.0 Hz, 1H, CHTAD), 5.82 (d, 3JHH = 7.8 Hz, 1H, py5), 5.72 (d, 3JHH = 8.0 Hz, 1H, CHTAD),
3
3
3.37 (m, 1H, CH(CH3)2iPr), 3.19 (m, 1H, CH(CH3)2iPr), 1.59 (dd, JHH = 7.2 Hz, JPH = 13.6 Hz, 3H,
3
CH(CH3)2iPr), 1.53 (dd, 3JHH = 7.2 Hz, 3JPH = 13.1 Hz, 3H, CH(CH3)2iPr), 1.47 (dd, JHH = 7.4 Hz, 3JPH
=
3
3
16.0 Hz, 3H, CH(CH3)2iPr), 1.37 (dd, JHH = 7.3 Hz, JPH = 16.2 Hz, 3H, CH(CH3)2iPr), 0.72 (s, 3H,
2
2
CH3TAD), 0.65 (s, 3H, CH3TAD). 13C{1H} NMR (δ, acetoneꢀd6, 20°C): 222.53 (dd, JCP = 22.2 Hz, JCP
=
2
4
2
4
34.9 Hz, CO), 162.58 (dd, JCP = 12.5 Hz, JCP = 7.3 Hz, py2), 157.81 (dd, JCP = 22.9 Hz, JCP = 3.3
3
3
Hz, py6), 144.04 (Ph1), 143.27 (Ph1), 141.84 (d, JCP = 5.5 Hz, Ph1), 140.10 (d, JCP = 8.2 Hz, Ph1),
139.34 (py4), 129.15 (Ph), 129.06 (Ph), 128.70 (Ph), 128.40 (Ph), 127.94 (Ph), 127.72 (Ph), 127.66
(Ph), 127.53 (Ph), 127.16 (Ph), 127.09 (Ph), 126.74 (Ph), 126.65 (Ph), 114.63 (C(CH3)2), 100.01 (d,
3JCP = 7.5 Hz, py3), 98.92 (d, 3JCP = 10.3 Hz, py5), 89.38 (d, 2JCP = 8.9 Hz, CPh2), 89.27 (d, 2JCP = 12.6
3
3
1
Hz, CPh2), 80.36 (d, JCP = 2.5 Hz, CHTAD), 79.74 (d, JCP = 2.9 Hz, CHTAD), 28.26 (d, JCP = 21.7 Hz,
1
2
CH(CH3)2iPr), 26.89 (d, JCP = 26.8 Hz, CH(CH3)2iPr), 26.42 (CH3TAD), 26.07 (CH3TAD), 18.84 (d, JCP
=
3.9 Hz, CH(CH3)2iPr), 18.50 (d, 2JCP = 4.3 Hz, CH(CH3)2iPr), 18.06 (CH(CH3)2iPr), 17.42 (s, CH(CH3)2iPr).
2
2
31P{1H} NMR (δ, CDCl3, 20°C): 161.61 (d, JPP = 261.4 Hz, TAD), 125.20 (d, JPP = 261.4 Hz, iPr). IR
(ATR, cmꢀ1): 1967 (νC=O). IR (CH2Cl2, cmꢀ1): 1980 (νC=O).
trans-[Fe(κ3P,N,PꢀPNP-iPr,TAD)(CO)2Cl]SbF6 (7a). CO was bubbled through a solution of 3a
(200 mg, 0.24 mmol) and AgSbF6 (82 mg, 0.24 mmol) in THF (15 mL) for 5 min. The solvent was
removed under vacuum and, the pink precipitate was dissolved in 15 mL of CH2Cl2. Insoluble
materials were removed by filtration and the volume of the solution was then reduced to about one
mL. The product was precipitated upon addition of nꢀhexane (40 mL) and dried under vacuum for 2 h.
Yield: 210 mg (81%). Anal. Calcd. for C44H47ClF6N3O6P2FeSb (1102.87): C, 47.92; H, 4.30; N, 3.81%.
1
Found: C, 47.82; H, 4.22; N, 3.89%. H NMR (δ, CD2Cl2, 20°C): 7.58 – 7.56 (m, 2H, Ph), 7.50 – 7.48
(m, 2H, Ph), 7.38 – 7.25 (m, 16H, Ph), 7.18 (bs, 1H, py4), 6.37 (m, 2H, NHTAD, py3), 5.67 (d, 2JPH = 8.1
Hz, 1H, NHiPr), 5.60 (d, 3JHH = 7.2 Hz, 1H, py5), 5.52 (d, 3JHH = 7.8 Hz, 1H, CHTAD), 5.48 (d, 3JHH = 7.8
Hz, 1H, CHTAD), 3.05 – 2.83 (m, 2H, CH(CH3)2iPr), 1.47 – 1.30 (m, 12H, CH(CH3)2iPr), 0.78 (s, 3H,
CH3TAD), 0.37 (s, 3H, CH3TAD). 13C{1H} NMR (δ, CD2Cl2, 20°C): 207.44 (dd, 2JCP = 23.8 Hz, 2JCP = 36.6
Hz, CO), 207.08 (dd, 2JCP = 24.4 Hz, 2JCP = 38.0 Hz, CO), 161.78 (m, py2), 156.10 (dd, 2JCP = 18.2 Hz,
3
3
4JCP = 2.7 Hz, py6), 142.73 (py4), 141.95 ( Ph1), 141.72 (d, JCP = 2.3 Hz, Ph1), 139.19 (d, JCP = 4.5
3
Hz, Ph1), 138.90 (d, JCP = 8.7 Hz, Ph1), 129.52 (Ph), 129.28 (Ph),129.07 (Ph), 129.05 (Ph), 128.57
(Ph), 128.54 (Ph), 128.42 (Ph), 128.31 (Ph), 128.20 (Ph), 127.71 (Ph), 127.67 (Ph), 126.94 (Ph),
115.79 (s, C(CH3)2), 102.54 (d, 3JCP = 11.2 Hz, 5JCP = 6.8 Hz, py3), 100.24 (d, 3JCP = 9.0 Hz, 5JCP = 3.3
2
2
3
Hz, py5), 93.68 (d, JCP = 13.0 Hz, CPh2), 93.12 (d, JCP = 19.5 Hz, CPh2), 79.47 (d, JCP = 3.6 Hz,

Page 17 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
17
1
3
CHTAD), 78.22 (d, 3JCP = 3.2 Hz, CHTAD), 32.16 (dd, JCP = 23.4 Hz, JCP = 1.9 Hz, CH(CH3)2iPr), 31.84
1
2
(d, JCP = 21.8 Hz, CH(CH3)2iPr), 26.82 (CH3TAD), 25.80 (CH3TAD), 17.68 – 17.48 (m, JCP = 1.1 Hz,
CH(CH3)2iPr). 31P{1H} NMR (δ, CD2Cl2, 20°C): 144.66 (d, 2JPP = 175.9 Hz, TAD), 111.87 (d, 2JPP = 175.9
Hz, iPr). IR (ATR, cmꢀ1): 2026 (νC=O).
trans-[Fe(κ3P,N,P-PNP-iPr,TAD)(CO)2Br]SbF6 (7b). This complex was prepared analogously
to 7a with 3b (200 mg, 0.21 mmol) as starting material. Yield: 180 mg (73%). Anal. Calcd. for Anal.
Calcd. for C44H47BrF6N3O6P2FeSb (1147.32): C, 46.06; H, 4.13; N, 3.66%. Found: C, 46.00; H, 4.20;
N, 3.70%. 1H NMR (δ, CD2Cl2, 20°C): 7.57 ꢀ 7.55 (m, 2H, Ph), 7.51 – 7.48 (m, 2H, Ph), 7.41 – 7.25 (m,
3
2
3
16H, Ph), 7.21 (d, JHH = 8.1 Hz, 1H, py4), 6.44 (d, JPH = 2.2 Hz, 1H, NHTAD), 6.40 (d, JHH = 8.1 Hz,
1H, py3), 5.66 (d, 2JPH = 8.5 Hz, 1H, NHiPr), 5.62 (d, 3JHH = 8.0 Hz, 1H, py5), 5.50 (d, 3JHH = 7.8 Hz, 1H,
3
3
CHTAD), 5.46 (d, JHH = 7.8 Hz, 1H, CHTAD), 3.00 – 2.84 (m, 2H, CH(CH3)2iPr), 1.47 (dd, JHH = 7.1 Hz,
3JPH = 3.0 Hz, 3H, CH(CH3)2iPr), 1.43 (dd, 3JHH = 7.1 Hz, 3JPH = 4.2 Hz, 3H, CH(CH3)2iPr), 1.39 (dd, 3JHH
3
3
3
= 7.2 Hz, JPH = 15.4 Hz, 3H, CH(CH3)2iPr), 1.35 (dd, JHH = 7.1 Hz, JPH = 16.3 Hz, 3H, CH(CH3)2iPr),
0.80 (s, 3H, CH3TAD), 0.37 (s, 3H, CH3TAD). 13C{1H} NMR (δ, CD2Cl2, 20°C): 207.76 (dd, 2JCP = 21.2 Hz,
2JCP = 37.9 Hz, CO), 207.38 (dd, 2JCP = 23.8 Hz, 2JCP = 42.3 Hz, CO), 161.75 (dd, 2JCP = 8.1 Hz, 4JCP
6.2 Hz, py2), 156.14 (dd, 2JCP = 18.5 Hz, 4JCP = 2.9 Hz, py6), 142.77 (Ph1), 141.90 (py4), 141.87 (Ph1),
=
3
3
139.23 (d, JCP = 4.3 Hz, Ph1), 138.91 (d, JCP = 8.8 Hz, Ph1), 129.51 (Ph), 129.28 (Ph), 129.06 (Ph),
129.00 (Ph), 128.61 (Ph), 128.54 (Ph), 128.43 (Ph), 128.35 (Ph), 128.21 (Ph), 127.69 (Ph), 126.91
3
3
2
(Ph), 115.77 (C(CH3)2), 102.50 (d, JCP = 6.3 Hz, py3), 100.18 (d, JCP = 9.2 Hz, py5), 94.01 (d, JCP
=
2
3
3
13.8 Hz, CPh2), 93.02 (d, JCP = 19.6 Hz, CPh2), 79.52 (d, JCP = 3.7 Hz, CHTAD), 78.25 (d, JCP = 3.3
1
3
1
3
Hz, CHTAD), 32.44 (dd, JCP = 23.7 Hz, JCP = 3.2Hz, CH(CH3)2iPr), 32.12 (dd, JCP = 22.4 Hz, JCP
=
2
3.2Hz, CH(CH3)2iPr), 26.85 (CH3TAD), 25.77 (CH3TAD), 18.01 (d, JCP = 1.1 Hz, CH(CH3)2iPr), 17.92 (bs,
CH(CH3)2iPr), 17.86 (CH(CH3)2iPr), 17.84 ( CH(CH3)2iPr). 31P{1H} NMR (δ, CD2Cl2, 20°C): 144.66 (d, 2JPP
= 169.8 Hz, TAD), 113.36 (d, 2JPP = 169.9 Hz, iPr). IR (ATR, cmꢀ1): 2021 (νC=O).
X-ray Structure Determination. Xꢀray diffraction data of 3b—THF, 4a—THF and 6b—2THF were
collected at T = 100 K in a dry stream of nitrogen on Bruker Kappa APEX II diffractometer systems
using graphiteꢀmonochromatized MoꢀKα radiation (λ = 0.71073 Å) and fine sliced φꢀ and ω-scans.
Data were reduced to intensity values with SAINT and an absorption correction was applied with the
multiꢀscan approach implemented in SADABS.16 The structures were solved by charge flipping using
SUPERFLIP17 and refined against F with JANA2006.18 Nonꢀhydrogen 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. H atoms connected to N located in difference Fourier maps. In
3b—THF and 4a—THF they were refined as riding on the parent N atom whereas in 6b—2THF the
positions were refined with the N–H distance restrained to 0.870(1) Å. Molecular graphics were
generated with the program MERCURY.19 Crystal data and experimental details are given in Table S1.
Computational Details Calculations were performed using the GAUSSIAN 09 software
package,20 and the B3LYP functional21 without symmetry constraints. This functional was shown to
perform well in mechanistic studies of spin forbidden reactions in closely related Fe system.9 The
optimized geometries were obtained with the Stuttgart/Dresden ECP (SDD) basis set22 to describe the
electrons of the iron atom. For all other atoms a standard 6ꢀ31G** basis set was employed.23

Dalton Transactions
Page 18 of 29
View Article Online
DOI: 10.1039/C5DT00832H
18
Transition state optimizations were performed with the Synchronous TransitꢀGuided QuasiꢀNewton
Method (STQN) developed by Schlegel et al,24 following a thorough search of the Potential Energy
Surfaces (PES). Frequency calculations were performed to confirm the nature of the stationary points,
yielding one imaginary frequency for the transition states and none for the minima. Each transition
state was further confirmed by following its vibrational mode downhill on both sides, and obtaining the
minima presented on the energy profiles.
The Minimum Energy Crossing Points (MECP) between PES of two different spin states were
determined using a code developed by Harvey et al.25 This code consists of a set of shell scripts and
Fortran programs that use the Gaussian results of energies and gradients of both spin states to
produce an effective gradient pointing towards the MECP. The energy values presented in the energy
profiles correspond to electronic energy, since MECP are not stationary points and, hence, a standard
frequency analysis is not applicable.
Mössbauer parameters (quadrupole splittings and isomer shifts) were evaluated by performing
singleꢀpoint calculations at the B3LYP21 level of theory with the ORCA software (Version 2.9.0),26 on
the optimized geometries. The value for quadrupole splittings is directly given by the program, while
the isomer shift was evaluated from the electron density at the Fe nucleus using the approach of
Neese.27 Fe was described by the triply polarized core properties basis set CP(PPP)28 and the other
atoms by the SV(P) basis set29 with the inner sꢀfunctions uncontracted; the auxiliary basis set SV/J30
was also used for these calculations. For the Fe atom, an enhanced integration grid was used, and the
overall integration accuracy was increased to 7.
Acknowledgements. Financial support by the Austrian Science Fund (FWF) is gratefully
acknowledged (Projects No. P24583ꢀN28 and P24202ꢀN17). The Xꢀray center of the Vienna
University of Technology is acknowledged for financial support and for providing access to the singleꢀ
crystal diffractometer. MDC, LPF and LFV acknowledge Fundação para a Ciência e Tecnologia,
Projecto Estratégico FCT (PEstꢀOE/QUI/UI0612/2013, PEstꢀOE/FIS/UI0261/2011 and PEstꢀ
OE/QUI/UI0100/2013) for financial support. The support and sponsorship of COST action CM1205
“Catalytic Routines for Small Molecule Activation (CARISMA)” is also acknowledged.
†Electronic supplementary information (ESI) available. Complete crystallographic data and technical
details in CIF format for 3b—THF, 4a—THF, and 6b—2THF (CCDC entries 1032898ꢀ1032900). 1H,
13C{1H}, and 31P{1H} NMR spectra of all complexes. Atomic coordinates for all DFT optimized
structures. For ESI and crystallographic data in CIF or other electronic format see DOI: xxxxxxx

Page 19 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
19
Figures
Figure 1. Structural view of [Fe(κ2P,NꢀPNPꢀiPr,TAD)Br2]—THF (3b—THF) showing 50% thermal
ellipsoids (most H atoms and solvent molecules omitted for clarity). Only one of the two
crystallographically independent complexes is shown. Selected bond lengths (Å) and bond angles (°):
Fe1ꢀBr1 2.3571(5), Fe1ꢀBr2 2.3785(5), Fe1ꢀP1 2.3776(8), Fe1ꢀN1 2.125(2), N1ꢀFe1ꢀP1 81.37(6), Br1ꢀ
Fe1ꢀBr2 117.78(2).
Figure 2. Structural view of [Fe(κ2P,NꢀPNPꢀtBu,TAD)Cl2]—THF (4a—THF) showing 50% thermal
ellipsoids (most H atoms and solvent molecules omitted for clarity). Selected bond lengths (Å) and
bond angles (°): Fe1ꢀCl1 2.2389(8), Fe1ꢀCl2 2.2453(8), Fe1ꢀP1 2.4150(8), Fe1ꢀN1 2.120(2), N1ꢀFe1ꢀ
P1 80.43(5), Cl1ꢀFe1ꢀCl2 115.70(3).

Dalton Transactions
Page 20 of 29
View Article Online
DOI: 10.1039/C5DT00832H
20
100
80
60
40
20
0
5.0
4.9
4.8
4.7
(a)
(b)
(c)
(d)
0
50
100
150
200
250
300
Temperature (K)
Figure 3. Temperature dependence of the inverse molar susceptibility of (a) [Fe(κ2P,NꢀPNPꢀ
iPr,TAD)Cl2] (3a), (b) [Fe(κ2P,NꢀPNPꢀiPr,TAD)Br2] (3b), (c) [Fe(κ2P,NꢀPNPꢀtBu,TAD)Cl2] (4a), and (d)
[Fe(κ2P,NꢀPNPꢀtBu,TAD)Br2] (4b), and of the effective magnetic moment of 4a. The straight lines
correspond to Curie law fittings to the experimental data.
Figure 4. 57Fe Mössbauer spectra of (a) [Fe(κ2P,NꢀPNPꢀiPr,TAD)Cl2] (3a), (b) [Fe(κ2P,NꢀPNPꢀ
iPr,TAD)Br2] (3b), (c) [Fe(κ2P,NꢀPNPꢀtBu,TAD)Cl2] (4a), and (d) [Fe(κ2P,NꢀPNPꢀtBu,TAD)Br2] (4b)
collected at 78 K.

Page 21 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
21
Figure 5. Positiveꢀion ESI full scan mass spectrum of [Fe(PNPꢀiPr,TAD)Br2] (3b) in CH3OH/CH3CN
(9/1). Inset shows the calculated and measured isotopic pattern of the sodiated complex [Fe(PNPꢀ
iPr,TAD)Br2] ([M+Na]+). Only signals containing the Fe isotopes of highest abundance (56Fe) are
annotated.

Dalton Transactions
Page 22 of 29
View Article Online
DOI: 10.1039/C5DT00832H
22
Figure 6 31P{1H} NMR spectra of (a) PNPꢀiPr,TAD (2a), (b) [Fe(κ2P,NꢀPNPꢀiPr,TAD)Cl2] (3a), (c)
[Fe(κ2P,NꢀPNPBH3ꢀiPr,TAD)Cl2] (5), and (d) transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)Cl2] (6a) in CD2Cl2
(spectra of 3a and 5 are referenced internally to PPh3 set to ꢀ5.6 ppm).

Page 23 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
23
Figure 7. Structural view of transꢀ[Fe(κ3P,N,PꢀPNPꢀiPr,TAD)(CO)Br2]—2THF (6b—2THF) showing 50%
thermal ellipsoids (H atoms and solvent molecules omitted for clarity). Selected bond lengths (Å) and
bond angles (°): Fe1ꢀBr1 2.4508(9), Fe1ꢀBr2 2.4482(8), Fe1ꢀC43 1.766(4), Fe1ꢀP1 2.258(1), Fe1ꢀP2
2.201(1), Fe1ꢀN1 1.996(3), P1ꢀFe1ꢀP2 163.91(4), N1ꢀFe1ꢀC43 179.3(1), Br1ꢀFe1ꢀBr2 177.33(2).

Dalton Transactions
Page 24 of 29
View Article Online
DOI: 10.1039/C5DT00832H
24
Figure 8. 57Fe Mössbauer spectra collected at 78 K of (a) transꢀ[Fe(κ3P,NꢀPNPꢀiPr,TAD)(CO)Br2] (6b),
(b) and (c) transꢀ[Fe(κ3P,NꢀPNPꢀiPr,TAD)(CO)Br2] (6b) after heating for 1h and 2h at 100oC under
vacuum, respectively, and (d) [Fe(κ2P,NꢀPNPꢀiPr,TAD)Br2] (3b).

Page 25 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
25
Table 1. Estimated hyperfine parameters from the 57Fe Mössbauer spectra of 3a, 3b, 4a, 4b and 6b
collected at 78 K and related Fe(II) complexes. IS: isomer shift; QS: quadrupole splitting.
T (K)
78
Fe(II)
HS
geometry
tetrahedral
tetrahedral
tetrahedral
tetrahedral
complex
IS (mm/s)
0.78(1)
0.76(1)
0.79(1)
0.77(1)
QS (mm/s)
2.93(1)
[Fe(κ2P,NꢀPNPꢀ
[Fe(κ2P,NꢀPNPꢀ
[Fe(κ2P,NꢀPNPꢀ
[Fe(κ2P,NꢀPNPꢀ
i
i
Pr,TAD)Cl2] (3a)
78
3.14(1)
HS
Pr,TAD)Br2] (3b
)
78
3.06(1)
HS
t
Bu,TAD)Cl2] (4a
Bu,TAD)Br2] (4b
)
78
3.22(1)
HS
t
)
transꢀ[Fe(κ3P,N,PꢀPNPꢀ
[Fe(κ3P,N,PꢀPNPꢀ
[Fe(κ2P,NꢀPNꢀ Pr)Cl2]b
i
Pr,TAD)(CO)Br2] (6b
)
78
0.23(1)
0.80(1)
0.680(2)
0.89
1.19(1)
2.56(1)
2.871(4)
2.40
LS
octahedral
298
HS
square pyramidal
i
Pr)Cl2]a
298
80
HS
HS
HS
HS
HS
LS
LS
HS
HS
tetrahedral
square pyramidal
square pyramidal
square pyramidal
square pyramidal
octahedral
i
[Fe((κ3N,N,NꢀiPrPDI)Cl2]c
[Fe(κ3N,N,NꢀHN{CH2CH2(P
i
Pr2)})Cl2]d
0.86
2.89
80
[Fe(κ3N,N,NꢀHN{CH2CH2(PCy2)})Cl2]d
[Fe(κ3N,N,NꢀHN{CH2CH2(P Bu2)})Cl2]d
transꢀ[Fe(κ3P,N,PꢀPNPꢀ Pr)(CO)Cl2]a
cisꢀ[Fe(κ3P,N,PꢀPNPꢀ Pr)(CO)Cl2]a
0.86
2.98
80
80
t
0.99
2.69
298
298
298
298
0.15(1)
1.56(1)
i
0.13(1)
0.85(1)
0.88(1)
1.09(1)
3.04(1)
3.14(1)
octahedral
i
transꢀ[Fe(κ2P,NꢀPNMeꢀPh)2Cl2]e
transꢀ[Fe(κ2P,NꢀPNMeꢀPh)2Br2]e
octahedral
octahedral
aRef. 5c, PNPꢀiPr = N,N'ꢀbis(diꢀisoꢀpropylphosphino)ꢀ2,6ꢀdiaminopyridine. bRef. 13, PNꢀiPr = Nꢀdiꢀisoꢀ
propylphosphinoꢀ2ꢀaminopyridine. cRef. 31, iPrPDI = 2,6ꢀ(2,6ꢀiPr2C6H3N=CCH3)2C5H3N. dRef 32.
eRef. 33, PNMeꢀPh = NꢀdiphenylphosphinoꢀNꢀmethylꢀ2ꢀaminopyridine.

Dalton Transactions
Page 26 of 29
View Article Online
DOI: 10.1039/C5DT00832H
26
References
1
For reviews on pincer complexes, see: (a) M. Albrecht and G. van Koten, Angew. Chem.,
Int. Ed. 2001, 40, 3750. (b) M. E. van der Boom and D. Milstein, Chem. Rev. 2003, 103
,
1759. (c) J. T. Singleton, Tetrahedron 2003, 59, 1837. (d) P. Bhattacharya and H. Guan,
Comment Inorg. Chem. 2011, 32, 88. (e) S. Schneider, J. Meiners and B. Askevold, Eur. J.
Inorg. Chem. 2012, 412. (f) D. MoralesꢀMorales and C. M. Jensen, Eds. The Chemistry of
Pincer Compounds; Elsevier: Amsterdam, 2007. (g) D. BenitoꢀGaragorri and K. Kirchner,
Acc. Chem. Res. 2008, 41, 201.
2
3
(a) W. V. Dahlhoff and S. M. Nelson, J. Chem. Soc. (A), 1971, 2184. (b) P. Giannoccaro, G.
Vasapollo, C. F. Nobile and A. Sacco, Inorg. Chim. Acta. 1982, 61, 69. (c) G. Müller, M. Klinga,
M. Leskelä and B. Z. Rieger, Anorg. Allg. Chem. 2002, 628, 2839.
(a) J. Zhang, M. Gandelman, D. Herrman, G. Leitus, L. J. W. Shimon, Y. BenꢀDavid and D.
Milstein, Inorg. Chim. Acta 2006, 359, 1955. (b) R. J. Trovitch, E. Lobkovsky and P. J. Chirik,
Inorg. Chem. 2006, 45, 7252.
4
5
E. M. Pelczar, T. J. Emge, K. KroghꢀJespersen and A. S. Goldman, Organometallics 2008, 27,
5759.
(a) D. BenitoꢀGaragorri, J. Wiedermann, M. Pollak, K. Mereiter and K. Kirchner, Organometallics
2007, 26, 217. (b) D. BenitoꢀGaragorri, M. Puchberger, K. Mereiter and K. Kirchner, Angew.
Chem., Int. Ed., 2008, 47, 9142. (c) D. BenitoꢀGaragorri, L. G. Alves, M. Puchberger, K.
Mereiter, L. F. Veiros, M. J. Calhorda, M. D. Carvalho, L. P. Ferreira, M. Godinho and K.
Kirchner, Organometallics 2009, 28, 6902.
6
7
(a) B. Bichler, M. Glatz, B. Stöger, K. Mereiter, L. F. Veiros and K. Kirchner, Dalton Trans 2014,
43, 14517. (b) M. Glatz, B. Bichler, M. Mastalir, B. Stöger, K. Mereiter, M. Weil, E. Pittenauer, G.
Allmaier, L. F. Veiros and K. Kirchner, Dalton Trans 2015, 44, 281.
C. Holzhacker, M. J. Calhorda, A. Gil, M. D. Carvalho, L. P. Ferreira, K. Mereiter, B. Stöger, E.
Pittenauer, G. Allmaier and K. Kirchner, Polyhedron 2014, 81, 45.
8
9
W.ꢀS. DeRieux, A. Wong and Y. Schrodi, J. Organomet. Chem. 2014, 772-773, 60.
D. BenitoꢀGaragorri, L. G. Alves, L. F. Veiros, C. M. StandfestꢀHauser, S. Tanaka, K. Mereiter
and K. Kirchner, Organometallics 2010, 29, 4932.
10
11
12
N. Gorgas, B. Stöger, E. Pittenauer, G. Allmaier, L. F. Veiros and K. Kirchner, Organometallics
2014, 33, 6905.
R. G. Parr and W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University
Press, New York, 1989.
C. Holzhacker, C. M. StandfestꢀHauser, M. Puchberger, K. Mereiter, L. F. Veiros, M. J.
Calhorda, M. D. Carvalho, L. P. Ferreira, M. Godinho, F. Hartl and K. A. Kirchner,
Organometallics 2011, 30, 6587.
13
14
D. BenitoꢀGaragorri, I. Lagoja, L. F. Veiros and K. A. Kirchner, Dalton Trans. 2011, 40, 4778.
D. D. Perrin and W. L. F. Armarego, Purification of Laboratory Chemicals, 3rd ed.; Pergamon:
New York, 1988.

Page 27 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
27
15
16
R. Kranich, K. Eis, O. Geis, S. Mühle, J. W. Bats and H.ꢀG. Schmalz, Chem. Eur. J. 2000, 6, 15,
2874.
Bruker computer programs: APEX2, SAINT and SADABS (Bruker AXS Inc., Madison, WI,
2012).
17
18
L. Palatinus and G. Chapuis J. Appl. Cryst. 2007, 40, 786.
Petříček, V.; Dušek, M.; Palatinus, L. JANA2006, the crystallographic computing system.
(Institute of Physics, Praha, Czeck Republic, 2006)
19
20
C. F. Macrae, P. R. Edgington, P. McCabe, E. Pidcock, G. P. Shields, R. Taylor, M. Towler and
J. van de Streek, J. Appl. Cryst. 2006, 39 453.
Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji,
M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J.
E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O.
Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G.
Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J.
B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
(a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648. (b) B. Miehlich, A. Savin, H. Stoll and H.
Preuss, Chem. Phys. Lett 1989, 157, 200. (c) C. Lee, W. Yang and G. Parr, Phys. Rev. B 1988,
37, 785.
21
22
23
(a) U. Haeusermann, M. Dolg, H. Stoll and H. Preuss, Mol. Phys. 1993, 78, 1211; (b) W.
Kuechle, M. Dolg, H. Stoll and H. Preuss, J. Chem. Phys. 1994, 100, 7535; (c) T. Leininger, A.
Nicklass, H. Stoll, M. Dolg and P. Schwerdtfeger, J. Chem. Phys. 1996, 105, 1052.
(a) A. D. McLean and G. S. Chandler, J. Chem. Phys. 1980, 72, 5639. (b) R. Krishnan, J. S.
Binkley, R. Seeger and J. A. Pople, J. Chem. Phys. 1980, 72, 650. (c) P. J. Hay, J. Chem. Phys.
1977, 66, 4377. (d) K. Raghavachari and G. W. Trucks, J. Chem. Phys. 1989, 91, 1062. (e) R.
C. Binning and L. A. Curtiss, J. Comput. Chem. 1995, 103, 6104. (f) M. P. McGrath and L.
Radom, J. Chem. Phys. 1991, 94, 511.
24
(a) C. Peng, P. Y. Ayala, H. B. Schlegel and M. J. Frisch, J. Comp. Chem., 1996, 17, 49. (b) C.
Peng and H. B. Schlegel, Israel J. Chem. 1994, 33, 449.
25
26
J. N. Harvey, M. Aschi, H. Schwarz and W. Koch, Theor. Chem. Acc. 1998, 99, 95.
Neese, F. ORCA – an ab initio, Density Functional and Semiꢀempirical Program Package,
Version 2.9.0, January 2012, MaxꢀPlanckꢀInstitut für Bioanorganische Chemie, Mülheim/Ruhr,
2012.
27
(a) F. Neese, Inorg. Chim. Acta 2002, 337, 181. (b) F. Neese, Coord. Chem. Rev. 2009, 252,
526.

Dalton Transactions
Page 28 of 29
View Article Online
DOI: 10.1039/C5DT00832H
28
28
(a) The ORCA basis set ‘CoreProp’ was used. This basis is based on the TurboMole DZ bases
developed by Ahlrichs and coworkers and obtained brom the basis set library under
ftp.chemie.uniꢀkarlsruhe.de/pub/basen. (b) The Ahlrichs (2d2fg,3p2df) polarization functions
were obtained from the Turbomole basis set library under ftp.chemie.uniꢀ
karlsruhe.de/pub/basen ScꢀZn: 2p functions from J. Wachters, Chem. Phys. 1970, 52, 1033
plus one fꢀfunction from the TurboMole library.
29
30
A. Schäfer, H. Horn and R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571.
The Ahlrichs auxiliary basis sets were obtained from the TurboMole basis set library under
ftp.chemie.uniꢀkarlsruhe.de/pub/jbasen HꢀBa; HfꢀAt: (a) K. Eichkorn, O. Treutler, H. Ohm, M.
Haser and R. Ahlrichs, Chem. Phys. Lett. 1995, 240, 283. (b) K. Eichkorn, F. Weigend, O.
Treutler and R. Ahlrichs, Theor. Chem. Acc. 1997, 97, 119.
31
32
33
S. C. Bart, K. Chlopek, E. Bill, M. W. Bouwkamp, E. Lobkovsky, F. Neese, K. Wieghardt and P.
J. Chirik, J. Am. Chem. Soc. 2006, 128, 13901.
K. L. Fillman, E. A. Bielinski, T. J. Schmeier, J. C. Nesvet, T. M. Woodruff, C. J. Pan, M. K.
Takase, N. Hazari and M. L. Neidig, Inorg. Chem. 2014, 53, 6066.
C. Holzhacker, M. J. Calhorda, A. Gil, M. D. Carvalho, L. P. Ferreira, B. Stöger, K. Mereiter, M.
Weil, E. Pittenauer, G. Allmaier, D. Müller, P. Weinberger and K. Kirchner, Dalton Trans. 2014,
43, 11152.

Page 29 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT00832H
For Table of contents use only
Synthesis and Reactivity of Taddol-Based Chiral Fe(II) PNP Pincer Complexes –
Solution Equilibria between κ2P,N- and κ3P,N,P-Bound PNP Pincer Ligands
Several Fe(II) complexes of the general formula [Fe(PNP-R,TAD)X2] (R = iPr, tBu; X = Cl, Br) bearing
chiral pincer ligands based on R,R-TADDOL (TAD) are described. In solution the PNP-R,TAD ligand is
hemilabile and complexes with κ2P,N- and κ3P,N,P-bound PNP ligands are present.