N-Atom transfer: Via thermal or photolytic activation of a Co-azido complex with a PNP pincer ligand

Article abstract of DOI:10.1039/c7dt01712j

Thermal or photolytic activation of well-defined mononuclear [Co(N₃)(PNP)] (PNP = 2,2′-bis(diisopropylphosphino)-4,4′-ditolylamido) results in the structurally characterized dinuclear species [Co(μ-N;κ³-P,N,N-PN^PN)]₂ (3), with two N-bridging phosphiniminato bridgeheads. Density Functional Theory (DFT) calculations indicate the intermediacy of a mononuclear cobalt-nitrido complex, followed by N-migratory insertion into a Co-P bond. Reaction of 3 with two equiv. HCl leads to rupture of the dimer with formation of mononuclear [CoCl(PN^PN^H)] (4) by protonation of the N-bridges.

Full text of DOI:10.1039/c7dt01712j

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N-Atom transfer via thermal or photolytic
activation of a Co-azido complex with a
Cite this: DOI: 10.1039/c7dt01712j
PNP pincer ligand†
Received 10th May 2017,
Accepted 12th May 2017
a
V. Vreeken,a L. Baij,a B. de Bruin, a M. A. Sieglerb and J. I. van der Vlugt
*
DOI: 10.1039/c7dt01712j
rsc.li/dalton
Thermal or photolytic activation of well-defined mononuclear temperatures, migratory insertion of the N-atom into a metal–
[Co(N3)(PNP)] (PNP = 2,2’-bis(diisopropylphosphino)-4,4’-ditolyl- carbene bond and subsequent H-atom abstraction (from a
amido) results in the structurally characterized dinuclear species sacrificial H-atom donor) resulted in a cobalt-bound imidazo-
[Co(μ-N;κ3-P,N,N-PNPN)]2 (3), with two N-bridging phosphinimi- lin-2-imine fragment (Scheme 1, A).10 Similar intramolecular
nato bridgeheads. Density Functional Theory (DFT) calculations reactivity was previously observed upon photolysis or thermo-
indicate the intermediacy of
a mononuclear cobalt–nitrido lysis of bis(imino)pyridine cobalt azide complexes, leading to
complex, followed by N-migratory insertion into a Co–P bond. N-insertion into a ligand C–H bond to form B, with a putative
Reaction of 3 with two equiv. HCl leads to rupture of the dimer transient Co-nitrido proposed as intermediate.11
with formation of mononuclear [CoCl(PNPNH)] (4) by protonation
of the N-bridges.
We recently described the photolysis of [Ni(N3)(PNP)]
(PNP = 2,2′-bis(diisopropylphosphino)-4,4′-ditolylamido) in a
benzene solution, resulting in clean transformation to
complex C, [Ni(Ph)(PNPNH)].12 DFT-studies resulted in the pro-
Transition metal complexes featuring multiple bonded
terminal nitrido (MuN) ligands are attracting interest, as
these species are proposed to play important roles in N2 split-
ting, N-atom transfer reactions and in biochemical processes.1
Terminal nitrido complexes with metals up to Group 8 are
well-known but bona fide MuN complexes with metals from
Group 9 onwards are still rare due to a mismatch between the
strongly π-donating nitrido (N3−) ligand and the electron-rich
metal centers, leading to increasingly filled antibonding
π-orbitals and overall destabilization of the M–N bond.2
posal of
a mechanism involving a transient terminal
Ni–nitrido complex, with N-migratory insertion into a Ni–P
bond and intermolecular C–H activation of benzene solvent to
provide the final product. Mindiola and coworkers reported
the thermolysis of [Fe(dbabh)(PNP)] (dbabh = 2,3:5,6-dibenzo-
7-azabicyclo[2.2.1]hepta-2,5-diene), generating anthracene and
Typically, azido complexes are used as precursors to gene-
rate the nitrido species via either thermolysis, chemical oxi-
dation or photolysis. For Group 9 metals, this has resulted in
isolable terminal Ir–nitrido compounds,3,4 but terminal
nitrido- or nitridyl5 complexes with Rh proved too reactive for
isolation and only species arising from follow-up reactivity
have been structurally characterized to date.6–9 A Co(IV)–nitrido
complex stabilized by a bis(NHC)(aryloxido)-amine ligand
scaffold was detected by EPR spectroscopy at 10 K. At higher
aHomogeneous, Bioinspired and Supramolecular Group, van ‘t Hoff Institute for
Molecular Sciences, University of Amsterdam, Science Park 904,
1098 XH Amsterdam, the Netherlands. E-mail: j.i.vandervlugt@uva.nl
bSmall Molecule X-ray Crystallography, John Hopkins University, Baltimore, MD,
USA
†Electronic supplementary information (ESI) available: Experimental, crystallo-
graphic and computational data. CCDC 1518623 (3) and 1527630 (4). For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c7dt01712j
Scheme 1 N-Insertion products from activation of first-row metal
nitrido precursors.
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dimer D after N-insertion into the Fe–P bond. The formation
We succeeded to obtain single crystals, suitable for X-ray
of a transient FeuN species was proposed as a possible inter- structure determination, from a cold pentane solution. The
mediate in this reaction.13 We herein report the formation of molecular structure of the centro-symmetric dimeric Co2II-
[Co(N3)(PNP)] complex 2 from Cl-precursor 1 and the fate of species is depicted in Fig. 1. The nitrogen atoms of two mono-
this azido-complex upon thermolysis and photolysis, seeking anionic phosphiniminato units act as bridging donors to two
to answer the following questions: (i) how do activation of 2 tetrahedral CoII centers, similar as in the aforementioned iron
and the resulting reactivity compare to the Ni analogue and analogue, providing [Co(μ-PNPN)]2 (3).13,20 Photolysis in
other Group 9 examples and (ii) are intermediates or products benzene led to selective formation of the same dinuclear
of the azide activation interesting targets for follow-up complex, with no sign of intermolecular C–H activation as
chemistry.
observed for the Ni-analogue.12 The phosphiniminato unit is
Following a modified literature procedure,14 dark blue proposed to originate from thermal activation of the inorganic
[CoCl(PNP)] complex 1 was prepared in high yield by the azide fragment and subsequent insertion of a transient nitrido
addition of CoCl2 to in situ prepared (PNP)Li (Scheme 2). N-atom into a Co–P bond, resulting in formation of the asym-
Subsequent efficient salt metathesis between 1 and NaN3 led metric dianionic PNPN ligand. Two of the backbone phenyl
to the desired dark green [Co(N3)(PNP)] species (2) in 95% rings are directed away from the diamonoid Co2N2 center. The
yield. Solution IR spectroscopy of 2 showed absorption bands other phenyl rings are nearly co-planar (centroid–centroid
(νN = 2052 and 2025 cm−1) and a shoulder (νN = 2043 cm−1
in the expected region for metal-azido species (Fig. S2†).15 The
)
distance 4.485 Å) and almost perpendicular to the Co2N2 center.
3
3
The 1H NMR spectrum of complex 3 ranges from 25 to
UV-vis spectrum of 2 (Fig. S3†) contains a strong absorption −5.5 ppm and shows relatively sharp signals that allow for
at 302 nm (ε = 12 100 M−1 cm−1) and a shoulder at 338 nm (ε = integration and also some degree of (nuclear spin) multiplicity
6800 M−1 cm−1). The IR spectrum of 2-15N shows the expected determination (see ESI†). All CHx groups of the ligand frame-
shifts of the azide bands (νN = 2043 and 2036 cm−1 and a work are inequivalent, signifying overall asymmetry in the
3
1
shoulder at 2010 cm−1). The paramagnetic H NMR spectrum binding motif of the PNPN skeleton. The 1H NMR chemical
ranges from 44 ppm to −25 ppm. At room temperature the µeff shifts for all signals are temperature dependent, showing
is 1.77µB (using Evans’ method),16 suggestive of an S = 1/2 spin uncomplicated Curie-like behavior indicative of a single contri-
ground state. X-band EPR spectroscopy reveals a slightly buting spin state in the temperature range of −60 °C to 80 °C
rhombic, almost axial spectrum (Fig. S4†), with the following (see ESI†). The effective magnetic moment of 3 in toluene at
g-values based on spectral simulation g11 = 3.000 (ACo 270 MHz), 20 °C of µeff = 2.75µB (Evans’ method), is consistent with an
g22 = 2.008 (ACo NR) and g33 = 1.950 (ACo NR). The combined S = 1 ground state of the dinuclear complex.
results confirm the formation of a low-spin square planar CoII
A plausible reaction mechanism for the azide activation
species. A preliminary X-ray crystallographic connectivity plot and N-insertion was investigated using DFT calculations. Both
for the molecular structure of this species supports this assign- the doublet and the quartet spin states were considered using
ment (see ESI†). No azide activation of 2 was observed under the BP86 DFT functional (Fig. 2).21 In accord with the experi-
electrochemical conditions, as
a reversible one-electron mental data, starting material 2 is clearly most stable in the
oxidation was found by cyclic voltammetry (Fig. S5†), with the S = 1/2 state.
redox-active ligand being the suggested locus for this
event.17,18 Using TGA-DSC, we established that 2 undergoes an
exothermic event around 169 °C, associated with a mass loss
of −3.1%, indicative of decomposition of the azido group.19
Large scale thermolysis led to formation of a dark red
pentane-soluble and air-sensitive complex 3 with no apparent
azide stretch in the IR spectrum.
Fig. 1 Displacement ellipsoid plots (50% probability level) of one of the
two crystallographically independent molecules of 3. Hydrogen atoms
omitted for clarity. Selected bond lengths (Å) and angles (°): Co1–Co2
2.5679(8); Co1–N1 1.965(3); Co1–N2 1.982(3); Co1–N4 1.927(3);
Co2–N2 1.938(3); Co2–N3 1.976(3); Co2–N4 1.989(3); Co1–P1
2.3148(11); Co2–P3 2.3313(11); P2–N2 1.580(3); P4–N4 1.582(3);
Co1–N2–Co2 48.34(9); N2–Co1–N4 97.97(12).
Scheme 2 Synthetic scheme for complexes 2–4, starting from
[CoCl(PNP)] 1.
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To the best of our knowledge, the potential of a metal-
bound phosphiniminato to act as proton-responsive ligand
has hardly been addressed to date.23 To assess if the sterically
congested Co–(μ-N) bond can interact with acidic H–E frag-
ments, 3 was reacted with an equimolar amount of HCl (2 M
in Et2O). This generates paramagnetic brown solid 4 in 56%
yield after work-up (as well as some blue CoCl2 precipitate),
which is characterized by an IR absorption band at ν =
3375 cm−1 (Fig. S8†) that is assigned to an PvN–H stretch.12
X-ray diffraction studies of single red crystals confirmed the
structure of 4 as mononuclear [CoCl(PNPNH)] (Fig. 4), with
cobalt in a distorted tetrahedral geometry (∠N1–Co1–Cl1
132.53(10)°); (N2–Co1–P1 122.18(10)°). The preparative route
to obtain the protonated iminophosphorane fragment and the
metal geometry are different from the Ni analogue recently
reported by us (involving C–H activation of benzene and a
square planar NiII).12 The dimeric nature of 3 does not impede
reactivity of the Co–(PvN) bond with HCl in a heterolytic
manner. Complex 3 can be viewed as containing two masked
mononuclear coordinatively unsaturated CoII species, with the
anionic phosphiniminato moiety potentially available as
internal base for metal–ligand bifunctional activation of polar
substrates.24
Fig. 2 Proposed pathway for formation of 3 from 2 based on DFT
calculations (BP86-D3, def2-TZVP). Energies in kcal mol−1 relative to
S = 1/2 spin state of 2. Dimerization of I-2 to form 3 is likely a low-
barrier step, but this step was not calculated.
Activation of the azide leading to N2 release proceeds via
transition state TS1, which is high in energy (30.0 kcal mol−1
)
but much lower than the activation barrier calculated for the
previously reported [Ni(N3)(PNP)] species (46.2 kcal mol−1 12
)
In summary, the activation of [Co(N3)(PNP)] (2) and sub-
sequent reactivity of the proposed transient cobalt nitrido is
subtly but significantly different than for the nickel congener.
Both thermal and photolytical pathways are accessible with 2.
Loss of N2 results in insertion of the α-Nazide atom into a Co–P
bond to create an anionic phosphiniminato fragment and
dimeric structure 3, featuring a diamonoid Co2N2 core with
bridging N-donors. No evidence for C–H activation of arene
solvent is observed for Co. DFT calculations support the ener-
getic accessibility of a cobalt nitrido intermediate (with sub-
stantial nitridyl character), with facile follow-up trapping by
the ligand.
and in line with an accessible thermal activation pathway.
Expulsion of N2 results in nitrido structure I-1, which also is
most stable in the doublet spin state. The spin density in I-1 is
distributed over the Co center (0.58) and the nitrido N atom
(0.39) (Fig. 3), giving the latter significant nitridyl character.
The Wiberg and Mayer bond orders for the cobalt–nitrido
fragment were calculated to be 1.777 and 1.949, respectively
(see ESI†).
Insertion of the nitrido N into the Co–P bond via transition
state TS2, with a relatively low barrier of +6.9 kcal mol−1 for S =
1/2, leads to species I-2. This T-shaped intermediate is equally
stable in the doublet and quartet spin state, with at least one
vacant coordination site to reach a square planar geometry.
Dimerization of two species I-2 to form final product 3 is there-
fore a likely process. Indeed, the energy of 3 with S = 1 was
calculated to be −38.0 kcal mol−1 22
The transition state from
.
I-2 to 3 was not calculated, but it is likely a low-barrier process
considering the coordinatively unsaturated nature of I-2.
Fig. 4 Displacement ellipsoid plot (50% probability level) of 4.
Hydrogen atoms omitted for clarity, except for H1 on N2. Selected bond
lengths (Å) and angles (°): Co1–Cl1 2.2549(11); Co1–N1 1.959(3); Co1–
N2 1.960(3); N2–P1 1.597(3); Co1–P2 2.3271(11); N1–Co1–P2 85.20(9);
N2–Co1–P2 122.18(10); N1–Co1–Cl1 132.53(10); N1–Co1–N2 95.65(13);
Co1–N2–P1 117.23(18).
Fig. 3 Spin density plot for I-1 (S = 1/2) (BP86-D3, def2-TZVP).
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This research is funded by an ERC Starting Grant (EuReCat, 14 A. R. Fout, F. Basuli, H. Fan, J. Tomaszewski,
Agreement 279097) to J. I. v. d. V. We thank Prof. Dr Joost
N. H. Reek for interest in our work, Ed Zuidinga for MS
analysis, Christophe Rebreyend for assistance with EPR
spectroscopy and Dr Linda S. Jongbloed for help with NMR
spectroscopy using Evans’ method.
J. C. Huffman, M. H. Baik and D. J. Mindiola, Angew.
Chem., Int. Ed., 2006, 45, 3291–3295. For a related Co(PNP)
system, see: M. J. Ingleson, M. Pink, H. Fan and
K. G. Caulton, J. Am. Chem. Soc., 2008, 130, 4262–4276. For
ligand and metal oxidation in a square planar Co(PNP)
platform, see: P. O. Lagaditis, B. Schluschaß, S. Demeshko,
C. Würtele and S. Schneider, Inorg. Chem., 2016, 55, 4529–
4536.
15 The two bands may stem from multiple accessible
conformations in solution: M. Käß, J. Hohenberger,
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24 The full reactivity scope for 3 is currently under investi-
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