Spin Changes Accompany Ultrafast Structural Interconversion in the Ground State of a Cobalt Nitrosyl Complex

Article abstract of DOI:10.1002/anie.201707508

Ultrafast, reversible intersystem crossing (ISC) is reported under ambient conditions for the electronic ground state of the pentacoordinate cobalt nitrosyl complexes, [CoX₂(NO)(PMePh₂)₂] (X=Cl, Br), in solution. ISCs on such short timescales are more typically observed in electronically excited states reached by absorption of ultraviolet or visible light. Singlet and triplet electron spin states of the complex, corresponding to two different isomers, are populated at room temperature, and the two isomers exchange on a timescale of a few picoseconds. Ultrafast two-dimensional infrared spectroscopy observes the change in wavenumber of the NO ligand band accompanying the isomerization and associated ISC on the (spin) adiabatic ground potential energy surface. Comparison of the dynamics of the chloro- and bromo-complexes shows that inertial effects of the ligand motion have a greater effect than spin–orbit coupling on determining the forward and reverse isomerization and ISC rates.

Full text of DOI:10.1002/anie.201707508

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Communications
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International Edition: DOI: 10.1002/anie.201707508
German Edition: DOI: 10.1002/ange.201707508
Ultrafast Dynamics
Spin Changes Accompany Ultrafast Structural Interconversion in the
Ground State of a Cobalt Nitrosyl Complex
Hugo J. B. Marroux, Basile F. E. Curchod, Charly A. Faradji, Timothy A. Shuttleworth,
Hazel A. Sparkes, Paul G. Pringle, and Andrew J. Orr-Ewing*
Abstract: Ultrafast, reversible intersystem crossing (ISC) is
reported under ambient conditions for the electronic ground
state of the pentacoordinate cobalt nitrosyl complexes, [CoX2-
(NO)(PMePh2)2] (X = Cl, Br), in solution. ISCs on such short
timescales are more typically observed in electronically excited
states reached by absorption of ultraviolet or visible light.
Singlet and triplet electron spin states of the complex,
corresponding to two different isomers, are populated at
room temperature, and the two isomers exchange on a timescale
of a few picoseconds. Ultrafast two-dimensional infrared
spectroscopy observes the change in wavenumber of the NO
ligand band accompanying the isomerization and associated
ISC on the (spin) adiabatic ground potential energy surface.
Comparison of the dynamics of the chloro- and bromo-
complexes shows that inertial effects of the ligand motion have
a greater effect than spin–orbit coupling on determining the
forward and reverse isomerization and ISC rates.
(PMePh2)2]. The isomerization of the chlorido complex is
shown in Figure 1. In the approximately square pyramidal
(SP) structure, the NO is formally a 1-electron donor with an
sp2 hybridized nitrogen atom bearing a lone pair of electrons
I
ntersystem crossing (ISC) between singlet and triplet
electron spin states is a common phenomenon in molecules
prepared photochemically in electronically excited states by
absorption of ultraviolet or visible radiation.[1] In contrast,
interchange between states of a molecule with different
electronic spin under ambient thermal conditions is unusual,
because it requires an adiabatic electronic ground state with
regions of different spin character connected by a structural
change. Here, we investigate the dynamics of spin change in
the electronic ground states of cobalt complexes containing
a nitrosyl ligand. We use two-dimensional infrared (2DIR)
spectroscopy to observe the rapid (few picosecond) exchange
between two isomers which differ in their spin multiplicities.
Similar spin dynamics are known for biological structures
such as hemeproteins.[2] However, the protein architecture
restricts these dynamics to the nanosecond to second time-
scales. The related phenomenon of spin-crossover is widely
studied in first-row transition metal complexes, but requires
an external stimulus to induce the ISC.[3]
Figure 1. Schematic structures of two isomers of the pentacoordinate
[CoCl2(NO)(PMePh2)2] complex and diabatic potential energy curves
(red and blue lines) along the isomer exchange coordinate. The
dashed black line shows the (spin) adiabatic ground state potential
energy surface.
to give a bent Co-N-O linkage. In the approximately trigonal
bipyramidal (TBP) isomeric structure, the NO is formally a 3-
electron donor with an sp hybridized nitrogen resulting in
a linear Co-N-O arrangement.[4] The oxidation states of the
Co are nominally + I in the TBP and + III in the SP
complexes. Structural isomerization is therefore accompanied
by a change in the formal oxidation state of the Co atom, and
the isomers are valence tautomers.[5]
Figure 2 shows molecular orbital (MO) diagrams for 5-
coordinate cobalt nitrosyl complexes with four identical
ligands.[4] The TBP and SP geometries are associated with
singlet and triplet states, respectively. In the SP complex, the
Two complexes with different halido ligands have been
*
studied,
[CoCl2(NO)(PMePh2)2]
and
[CoBr2(NO)-
{dyz; py NO } orbital is stabilized through interactions with
the other ligands, bringing it close enough in energy to the
{dxz; sp2 N } MO that a triplet spin-state becomes the ground
electronic configuration.[6] The existence of a paramagnetic
compound is supported by magnetic susceptibility and 31P-
{1H} NMR signal broadening measurements described in
Supporting Information (SI). The different electronic char-
acters of the two structures require the exchange dynamics to
be described by an adiabatic ground-state potential energy
surface (PES) constructed from diabatic PESs for the TBP
and SP isomers (Figure 1).
[*] Dr. H. J. B. Marroux, Dr. B. F. E. Curchod, C. A. Faradji,
Dr. T. A. Shuttleworth, Dr. H. A. Sparkes, Prof. P. G. Pringle,
Prof. A. J. Orr-Ewing
School of Chemistry, University of Bristol
Bristol, BS8 1TS (UK)
E-mail: a.orr-ewing@bristol.ac.uk
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201707508.
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(PMePh2)2]. The peaks at 1655 and 1765 cmÀ1 are, respec-
tively assigned to the triplet and singlet forms.[9b] In time-
dependent 2DIR spectra reported below, isomeric exchange
gives rise to characteristic off-diagonal (cross) peaks from
which the kinetics of exchange can be deduced.[11]
The computationally optimized geometries for the singlet
and triplet states of [CoCl2(NO)(PMePh2)2] are shown in
Figure 3. Computational methods are described in the SI. In
the singlet (distorted TBP) state, the Co-N-O angle is 1808
and the Cl-Co-Cl angle is 1038. The corresponding optimized
geometry for the triplet complex contains a bent nitrosyl
coordination with a Co-N-O angle of 1448 and a Cl-Co-Cl
angle of 1148. The computed intensities for the two isomer N-
O bands are similar, being 506 and 533 kmmolÀ1 for the
singlet and triplet, respectively. Hence IR intensities in
Figure 3 can be directly related to relative abundances.
Single crystal X-ray diffraction structures of both the [CoCl2-
(NO)(PMePh2)2] and [CoBr2(NO)(PMePh2)2] complexes sup-
port the co-existence of the two isomers (see SI).
Figure 2. Molecular orbital diagram of a pentacoordinated cobalt nitro-
syl complex adopting either a TBP (left) or SP (right) structure. The
figure is adapted from Ref. [4].
Relaxed scans of the Co-N-O angle were conducted from
the two electronic state minima while optimizing all the other
molecular degrees of freedom and maintaining the overall
spin character of the complex. The obtained potential energy
profiles are plotted in Figure 4. The triplet state minimum is
Most cobalt nitrosyl complexes adopt either the bent Co-
N-O (triplet)[6,7] or linear Co-N-O (singlet) structures at
thermal equilibrium,[8] but in selected cases the two geo-
metries are close enough in energy that both are accessible at
ambient temperature.[6,9] The chosen complexes are such
À
examples. Steady state IR absorption spectroscopy in the N
O stretching region differentiates between the two forms of
the complex, with the NO band of the triplet complex shifted
to lower wavenumber because greater localization of the
p* electron density on the nitrosyl ligand gives double-
bonded NOÀ character.[10] Figure 3 shows an FTIR spectrum
of a dichloromethane (DCM) solution of [CoCl2(NO)-
Figure 4. Relaxed energy scans of the Co-N-O angle on the triplet (red)
and singlet (blue) electronic states.
computed to lie 6.0 kJmolÀ1 lower in electronic energy than
the singlet minimum, corresponding to a Boltzmann popula-
tion of 92% in the triplet and 8% in the singlet state. The
calculations were performed for a gas-phase environment, but
are supported by the FTIR spectrum of [CoCl2(NO)-
(PMePh2)2] in cyclohexane, a weakly interacting solvent,
which indicates a similar population ratio (see SI, Figure S2).
2DIR spectra of [CoCl2(NO)(PMePh2)2] and [CoBr2-
(NO)(PMePh2)2] in DCM are shown in Figure 5 for early
and later waiting times (see SI for methods). The former
spectrum contains additional diagonal features at 1690,
1711 cmÀ1, and as a shoulder on the peak at 1793 cmÀ1
which originate from air reaction products. They are partic-
ularly evident in the Tw = 300 fs spectrum because this was
one of the last spectral time-points acquired. The effect on
Figure 3. FTIR spectra of DCM solutions of a) [CoCl2(NO)(PMePh2)2]
and b) [CoBr2(NO)(PMePh2)2] in the NO stretching region. Band
assignments to the two isomeric forms are shown by the computed
structures.
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coefficients and the transition dipole moments of the two
isomers, were adjusted. The obtained fits are shown in
Figure 5 as solid lines, and Table 1 reports the derived time
constants. A similar analysis was undertaken for the [CoBr2-
(NO)(PMePh2)2] complexes, for which the air degradation
product features appear on the diagonal at 1700, 1720 and
1800 cmÀ1.
Equilibrium constants, calculated from the ratio of the
exchange rate coefficients, are Ksinglet/triplet = 0.75 Æ 0.30 for
[CoCl2(NO)(PMePh2)2] and Ksinglet/triplet = 2.4 Æ 0.9 for [CoBr2-
(NO)(PMePh2)2]. The corresponding estimates from band
intensities in the FTIR spectra are 0.77 and 4.2. These latter
estimates assume equal transition dipole moments for the two
isomers, consistent with the outcomes of our electronic
structure calculations. Using the detailed balance relation
for the equilibrium, the changes in Gibbs free energy between
the triplet and singlet species in CH2Cl2 solution are evaluated
as 0.7 Æ 0.3 and À2.1 Æ 0.8 kJmolÀ1 for [CoCl2(NO)-
(PMePh2)2] and [CoBr2(NO)(PMePh2)2], respectively. The
triplet minimum is lower in energy in [CoCl2(NO)(PMePh2)2]
and the singlet minimum is lower in [CoBr2(NO)(PMePh2)2].
The rates of interconversion of the two isomers could be
controlled by either the nuclear dynamics, or the coupling
between singlet and triplet spin states. The heavier bromine
atoms should slow down the exchange dynamics through
inertial effects, but the greater spin–orbit coupling of the
heavier halogen atoms[13] is expected to enhance the mixing of
singlet and triplet states in regions of near degeneracy. The
larger time constant obtained for ISC from the higher to
lower energy isomer in [CoBr2(NO)(PMePh2)2] than in
[CoCl2(NO)(PMePh2)2] indicates that the inertial effect of
the heavier halogen ligands is the more important factor.
The observed changes in spin multiplicity in a pentacoor-
dinate cobalt nitrosyl complex are driven by environmental
fluctuations, and occur reversibly on timescales of a few
picoseconds. The precise evidence derived from our 2DIR
measurements should provide useful insights for the behav-
iour of cobalt nitrosyl complexes in biological systems, and for
spin-crossover dynamics.
Figure 5. 2DIR spectra of [CoCl2(NO)(PMePh2)2] (left column) and
[CoBr2(NO)(PMePh2)2] (right column) in DCM at early waiting times
(Tw =300 fs, top row) and averaged over intermediate waiting times
(middle row; Tw =2800 to 4500 fs for [CoCl2(NO)(PMePh2)2] and
Tw =3600 to 8500 fs for [CoBr2(NO)(PMePh2)2]) to highlight weak
cross peaks. Negative going features are shown in blue, and positive
features in red. The black dotted lines indicate the diagonals. The
bottom row shows time dependences of bleach spectral features
extracted from the 2DIR spectra by integration of band intensities at
each waiting time (filled circles) and fits to a kinetic model (solid
lines). Shaded regions correspond to 20% confidence intervals. The
regions integrated are shown by the coloured boxes in the spectra. The
cross-peak intensities have been multiplied by a factor of 12.
extracted exchange kinetics from growth of these impurities
was minimized by randomizing the waiting time delays during
data collection.
The off-diagonal peaks grow with increasing waiting time,
indicating exchange between cobalt nitrosyl isomers. Vibra-
tional relaxation and orientational diffusion cause a slower
decay. The kinetic data for [CoCl2(NO)(PMePh2)2] were
simultaneously fitted with a model which incorporated these
factors (see SI).[12] Four fitting parameters, the exchange
Acknowledgements
This work was supported by ERC Advanced Grant 290966
CAPRI. B.F.E.C. is supported by an EU Horizon 2020 Marie
Skłodowska-Curie Fellowship NAMDIA (710355). H.J.B.M.
acknowledges award of an EPSRC Doctoral Training Grant
Table 1: Exchange, vibrational relaxation and orientational decay time constants for [CoX2(NO)(PMePh2)2] (X=Cl, Br) complexes in dichloromethane.
[CoCl2(NO)(PMePh2)2] [CoBr2(NO)(PMePh2)2]
Singlet
Triplet
Singlet
Triplet
Vibrational lifetime
Orientational time constant
50.2Æ6.9 ps
239Æ21 ps
17.6Æ0.4 ps
507Æ20 ps
37.3Æ1.1 ps
117Æ12 ps
18.0Æ0.5 ps
103Æ14 ps
Exchange time constant
Singlet ! Triplet
Singlet ! Triplet
8.8Æ1.0 ps
31.0Æ6.2 ps
!
!
Singlet
Triplet
Singlet
Triplet
11.6Æ2.3 ps
12.9Æ2.6 ps
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Accepted manuscript online: September 4, 2017
Version of record online: September 22, 2017
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