Photo-redox reactivity of titanium-oxo clusters: Mechanistic insight into a two-electron intramolecular process, and structural characterisation of mixed-valent Ti(iii)/Ti(iv) products

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ARTICLE

Photo-redox reactivity of titanium-oxo clusters: mechanistic

insight into a two-electron intramolecular process, and structural

characterisation of mixed-valent Ti(III)/Ti(IV) products.

Received 00th January 20xx,

Accepted 00th January 20xx

Tobias Krämer,^bFloriana Tuna ^cand Sebastian. D. Pike *^a

DOI: 10.1039/x0xx00000x

Small titanium-oxo-alkoxide clusters, [TiO(OR)(O₂PR’₂)]₄, synthesised by the stoichiometric reaction of Ti(OⁱPr)₄, phosphinic

acid and water, undergo a photo-redox transformation under long-wave UV light. The photo-reaction generates blue

coloured, mixed-valence Ti(III)/Ti(IV)-oxo clusters alongside acetone and isopropanol by-products. This reactivity indicates

the ability for photoactivated charge separation to occur in even the smallest of Ti-oxo clusters. EPR and NMR spectroscopic

studies support a photo-redox mechanism that occurs via an intramolecular, two-electron pathway, directly relating to

current doubling effects observed at TiO₂photoanodes in the presence of alcohols. The rate of photo-reaction is solvent

dependent, with donor solvents supporting the formation of low coordinate Ti(III) sites. The nature of the electronic

transition is identified by DFT and TDDFT calculations as an oxygen to titanium charge transfer and it is possible to finetune

the UV absorption onset observed by changing the phosphinate ligand. A two-electron photo-reduced cluster,

[Ti₄O₄(O₂PPh₂)₆], forms spontaneously from the photo-reaction and its structure is identified by X-ray crystallography with

supporting DFT calculations. These indicate that [Ti₄O₄(O₂PPh₂)₆] is high-spin and contains two ferromagnetically coupled

electrons delocalised over the Ti₄core. [Ti₄O₄(O₂PPh₂)₆] undergoes rapid oxidation in air in the solid-state and performs a

remarkable single-crystal to single-crystal transformation, to form a stable cluster-superoxide salt.

8, 21

clusters than that required for bulk TiO_2.

These enlarged

Introduction

TiO₂is an important semiconductor material, with applications

ranging from sun creams and pigments, to self-cleaning

absorption energies are consistent with quantum confinement

effects which are estimated to occur in TiO₂particles smaller

than ~2 nm.²²Considering this effect from a molecular

perspective, the frontier molecular orbitals in Ti-oxo clusters

comprise of a combination of atomic orbitals (O 2p orbitals in

the HOMO and Ti 3d orbitals in the LUMO). As clusters increase

in size towards nanoparticles a combination of a greater

number of atomic orbitals increases the spread of available

energy states and pushes the HOMO and LUMO closer together,

with the orbitals accumulating towards a band structure.

However, it should be noted that in molecular systems the

supporting ligands,²³anion or cation dopants,^{24, 25}and cluster

shape may all affect the charge transfer absorption onset in

titanium-oxo clusters, and may alter the lowest energy

electronic transition from an oxygen-to-metal charge transfer

(OMCT, analogous to the band-gap transition in TiO₂) into a

ligand-to-metal charge transfer (LMCT) in dye substituted

clusters.^13-16

If photo-excitation of titanium-oxo clusters is conducted in

the presence of a hole quencher (e.g. an alcohol or alkoxide

ligand) the excited hole can be transferred away from the

cluster, resulting in a formally reduced molecule with Ti(III)

sites, which is typically blue in colour (Fig. 1). Further

decomposition of an oxidised alcohol (e.g. a radical species) can

result in the production of a further electron and proton, which

we show can be passed back onto the metal-oxo cluster. Such a

two-electron photo-redox process is referred to as “current

windows,

antibacterial

surfaces,

photovoltaics

and

photocatalysis.^1-7Many applications arise from the ability to

absorb UV light across the band gap (3.03-3.2 eV), promoting

electrons from the predominantly oxygen-based valence band

to the titanium (3d)-based conduction band. Atomically precise

titanium-oxo clusters, with sizes of up to 52 Ti centres,^8-12act as

molecular relations to TiO₂and due to their precise molecular

structures offer great opportunities for detailed mechanistic

insight. In particular, these systems may act as useful models for

dye-sensitised solar cells.^13-16Other applications of this family

of clusters include as photocatalysts, fluorescent labels, vertices

within metal organic frameworks (MOFs), or nano-building

blocks for hybrid materials,^{17, 18}and due to their good solubility

they may act as single source precursors for thin films.^{19, 20}

Titanium-oxo clusters are also able to absorb UV light, which

promotes an electron into the titanium (3d) orbitals (Fig. 1(a)).

The minimum excitation energy is generally larger in small

^a.Department of Chemistry, University of Cambridge, Lensfield Road, CB2 1EW, UK

^b.Department of Chemistry, Maynooth University, Maynooth, Co. Kildare, Ireland

^c.School of Chemistry and Photon Science Institute, University of Manchester,

Oxford Road, Manchester, M13 9PL, UK

Electronic Supplementary Information (ESI) available: NMR, ESI-MS and IR spectra

for complexes

1-8, crystallography details, in-situ NMR, EPR and UV/vis

spectroscopy data. See DOI: 10.1039/x0xx00000x

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phosphinate supported titanium-oxo-alkoxide clusters are

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synthesised in this way and their reactivity with UV light

−

examined. The use of phosphinate ligands (R₂PO₂) to stabilise

the clusters is favourable for several reasons: oxygen donors

maintain the relationship with bulk metal oxides; variable R

groups allow solubility to be optimised; the ³¹P nucleus allows

for in-situ multinuclear NMR spectroscopy;³⁹and, unlike

carboxylates,^{19, 40}competing esterification reactions between

ligand and alkoxide are not observed – allowing excellent

stoichiometric control of added moisture. The small clusters

allow detailed mechanistic examination of the photo-reaction

using spectroscopic methods, revealing that two alkoxide

ligands upon the same cluster can be converted into a ketone

and a free alcohol by a photo-oxidation process. This process

leaves a doubly photo-reduced mixed-valent Ti-oxo cluster

which is observed by EPR spectroscopy, and can be structurally

characterised using X-ray crystallography and further

rationalised with DFT calculations.

Figure 1. Conceptual diagram showing a photo-excitation process in a titanium-oxo

cluster, (a) UV light promotes an electron from oxygen-based orbitals to titanium orbitals

(OMCT), (b) hole quencher donates an electron to fill the photogenerated hole and

becomes an unstable radical, (c) organic radical may decompose (current doubling

effect) releasing a proton and an electron which is passed onto the Ti (3d) orbitals of the

cluster. The final product is coloured due to electronic transitions (d) in the visible range

(e.g. d-d transitions or intervalence charge transfer).

doubling” in the analogous reaction at TiO₂photoanodes (Fig.

1).^{6, 26, 27}

There have been multiple reports of the photo-reduction of

titanium-oxo clusters, MOFs, and bulk or nanoscale TiO₂under

UV light, leading to blue or black coloured materials, with the

Ti(III) sites identified by electron paramagnetic resonance (EPR)

spectroscopy. For instance, the titanium based MOF COK-69,

(Ti₃(μ₃-O)(μ₂-O)₂(H₂O)(1,4-C₆H₄(CO₂)₂)₃), may be photo-reduced

in ethanol to give a blue solid containing one Ti(III) site per Ti₃

vertex.²⁸Isotope studies reveal that this process occurs via

proton-coupled electron transfer with ethanol as reductant,

with an oxo site gaining a proton during the process. Further

notable examples are the reversible yellow to dark-purple

colour change in dye-sensitised Ti₆O₆(OiPr)₆(9-AC)₆(9-AC = 9-

anthracenecarboxylate) as electrons are transferred from the

dye to Ti upon irradiation,¹³and the photo-reduction of cluster

Ti₆O₃(OⁱPr)₁₄(1,2-C₆H₄(CO₂)₂)₂in the solid-state, which resulted

in a darkening of the surface of the crystalline material.²⁹

Detailed studies have also investigated the photo-reduction of

TiO₂(or ZnO) nanoparticles in the presence of alcohols

describing the ability to store electrons within the conduction

band of these semiconductors, which may then undergo

onward reactivity via proton-coupled electron transfer.^30-33

These coloured photo-reduced titanium-oxo species relate to

partially reduced TiO₂materials which may be blue or black in

colour, and may be employed as photocatalysts which operate

using visible light.^34-36

Results and discussion

Synthesis and characterisation of titanium-oxo clusters

The reaction of Ti(OⁱPr)₄with one equivalent of

dicyclohexylphosphinic acid (Cy₂PO₂H) forms the dimeric

1

complex [Ti(OⁱPr)₃(Cy₂PO₂)]₂( ) and ⁱPrOH. An X-ray structure of

1

(Fig. 2) shows the phosphinate ligands bridge between the

two Ti centres, which are 5-coordinate and adopt a pseudo

trigonal-bipyramidal structure. This dimeric structure is

consistent with solution NMR spectra collected in d₈-toluene

(Figs. S1-S7).

The analogous reaction of Ti(OⁱPr)₄with one equivalent of

diphenylphosphinic acid (Ph₂PO₂H) in d⁸-toluene or CDCl₃

solvent generates a set of nine signals in its ³¹P NMR spectrum

(Figs. S8, S10); these nine signals are consistently present in an

exact 1:1:2:1:3:1:1:1:1 ratio which suggests a single compound

(

2

) with multiple ³¹P environments. The ¹H NMR spectrum

shows that half of the Ti(OⁱPr)₄remains unreacted (Fig. S9),

suggesting that

2 2 can be

has a formula [Ti(OⁱPr)₂(O₂PPh₂)₂]_n.

formed directly by reacting Ti(OⁱPr) with two equivalents of

1

Ph₂PO₂H, and isolated as a powder. The H NMR spectrum of

isolated 2, is consistent with the expected integrals for the

formula [Ti(OⁱPr)₂(O₂PPh₂)₂]_nwith multiple phosphinate and

alkoxide environments (Fig. S11), however, no crystalline

material of

2 was retrieved. It is expected that 2 is most likely

an asymmetric hexameric species, with n = 6 (since the sum of

³¹P integrals = 12). A minor impurity species was also identified

Despite the many reports which discuss photo-reduction,

there is a scarcity of structural characterisation of the highly

reactive coloured Ti(III) containing products, which typically

react rapidly with air, reducing O₂to superoxide radicals.^{24, 37}

There are also only isolated reports of chemically reduced

titanium-oxo clusters, such as the blue octahedral cluster

Cp₆Ti₆O₈with formally two Ti(III) centres, formed via reaction of

Cp₂Ti(CO)₂with H₂/CO.³⁸

Whilst much success in building Ti-oxo clusters has been

achieved by solvothermal synthetic methods, the hydrolysis of

Ti-alkoxide units with a stoichiometric quantity of water is an

attractive and controlled approach. In this report, small

in some of the ³¹P NMR spectra of

2, which has a 2:2:1 ratio of

³¹P signals (23.6, 20.5, 18.7 ppm) (Fig. S8, S12-S14); this species,

previously observed by Schmidt and co-workers,⁴¹could be

separated from

solid-state structure was collected characterising the complex

as Ti₃(OⁱPr)₇(Ph₂PO₂)₅(

) (Fig. S15).

The remaining Ti−OⁱPr bonds in

hydrolysis and condensation can be initiated with a controlled

2 due to its preferable solubility in hexane. A

3

1

and 2 are susceptible to

amount of water.

single compound (

1

4

reacts with 1 equivalent of water to form a

) after heating at 60°C overnight, with a

2 | J. Name., 2012, 00, 1-3

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Figure 2. Reaction of Ti(OⁱPr)₄with R₂PO₂H and subsequent hydrolysis, with solid-state structures of 1 and 4, hydrogen atoms omitted for clarity. Ellipsoids displayed at 50%

probability. Selected bond lengths (Å) and angles (°) 1: P1−O1, 1.5054(17); P1−O2, 1.5260(17); Ti1−O1, 2.0050(16); Ti1−O2, 1.9837(16); Ti1−OⁱPr (range), 1.793(2)-1.807(2);

O1−Ti1−O2, 83.08(7); O1−Ti1−O4, 169.50(8). 4: Ti1-O1, 1.937(2); Ti1-O1’, 2.121(2); Ti1-O1’’, 1.912(2); Ti1-O2, 1.790(2); Ti1-O3, 2.011(3); Ti1-O4, 2.021(3); P1-O3, 1.522(3); P2-O4,

1.518(3); Ti1···Ti1’, 2.9105(12); Ti1-O1-Ti1’, 98.25(10)-99.16(10); O3-P1-O3’, 114.2(2); O4-P2-O4’, 114.9(2)

single ³¹P NMR signal at 56.3 ppm (Figs. S16-S17).

from hexane at reduced temperatures as a tetrameric cluster, [Cy₂PO₂]⁻in

4

crystallises higher energies (3.74 ±0.2 eV) than the clusters. Introducing

or causes a red shift in the absorption onset (

, 3.59 ±0.1 eV) and this effect is extended when

1

4

1,

[TiO(OⁱPr)(Cy₂PO₂)]₄(Fig. 2). The solid-state structure identified 3.53 ±0.1 eV;

4

by single crystal X-ray diffraction is fully consistent with the the [Ph₂PO₂]⁻ligand is used in

2

and

5

(2

, 3.50 ±0.1 eV;

5

, 3.47

1

solution H and ³¹P NMR spectra (Figs. S18-S20). The structure ±0.1 eV).⁴³The electronic absorptions in Ti(OR)₄,

1

,

2

,

4

and

5

is very similar to the previously reported [Ph₂PO₂]⁻analogue, are considered to be OMCT in nature (see calculations below),

[TiO(OⁱPr)(Ph₂PO₂)]₄( ).^{42, 43}

may also be straightforwardly in contrast to LMCT previously reported in dye functionalised

titanium-oxo clusters and many Ti MOFs.^13-17π-π* transitions in

was the free Ph₂PO₂H ligand (and similar peaks in the spectra of

is and ) occur at high energies (>4.5 eV) (Fig. 3) and are not

and have an absorption

relate to other phosphinate supported onset similar to that of other carboxylate and alkoxide ligated

heterocubane clusters with a M₄(

₃-O₄) core (M = V, Mn, Mo, Ti-oxo clusters^{10, 21, 23}but greater than the bandgap in bulk (>2

In, Sn, W) that have been previously reported;^{42, 44-48}

nm) TiO₂. This is expected for small clusters which have discreet

The solid-state structures of and reveal that the frontier orbitals rather than the extended band structure found

phosphinate ligands are delocalised with all P−O bonds of in TiO₂(N.B. the Ti₄O₁₆cores of and span around 0.64 nm,

similar lengths. Each Ti centre adopts an approximately t ( , 3.53 ±0.1 eV; , 3.59 ±0.1 eV) and this effect is extended

octahedral geometry, each coordinated to three oxo ligands, when the [Ph₂PO₂]⁻ligand is used in

and , 3.50 ±0.1 eV;

two phosphinate O donor atoms and an isopropoxide. Each Ti 3.47 ±0.1 eV).⁴³The electronic absorptions in Ti(OR)₄,

and

forms two shorter bonds to ₃-oxo centres (1.91 - 1.94 Å) and are considered to be OMCT in nature (see calculations below),

one slightly longer bond to the third ₃-oxo (2.12 - 2.16 Å), such in contrast to LMCT previously reported in dye functionalised

that the structure may be viewed as two Ti₂O₂square units titanium-oxo clusters and many Ti MOFs.^13-17π-π* transitions in

bridged by four phosphinate ligands. The Ti−Ti distances within the free Ph₂PO₂H ligand (and similar peaks in the spectra of

the Ti₂O₂square unit are shorter than between the squares and ) occur at high energies (>4.5 eV) (Fig. 3) and are not

and have an absorption

5

produced by adding two equiv. of water to a 1:1 mixture of

2

and Ti(OⁱPr)₄and heating to 60°C overnight (Fig. 2). Whilst

5

2

i

previously reported as the DMSO or PrOH solvate, here

5

crystallised from toluene as

structures of and

5

·toluene (Figs. S21-25).^{42, 43}The expected to overlap with OMCT.

4

5

4

5



4

5

4

5

1

4

2

5

(2

5,

1, 2, 4



5



2

5

(2.91 Å vs 3.07-3.12 Å respectively), with similar magnitudes to expected to overlap with OMCT.

4

5

the Ti−Ti distances in Brookite (2.95 & 3.06 Å).⁴⁹

onset similar to that of other carboxylate and alkoxide ligated

Ti-oxo clusters^{10, 21, 23}but greater than the bandgap in bulk (>2

nm) TiO₂. This is expected for small clusters which have discreet

frontier orbitals rather than the extended band structure found

UV spectroscopy and electronic structure of Ti-oxo clusters

The UV spectra of precursor Ti(OⁱPr)₄(and related tetramer

[Ti(OEt)₄]₄), 1, 2, 4, 5, and free ligands R₂PO₂H (R = Ph or Cy) are

shown in Fig. 3. They reveal that all Ti complexes show similar

oxygen-to-metal charge transfer (OMCT) absorption onsets,

ranging between 3.74-3.47 eV, which may be calculated using

the Tauc plot method using spectra from relatively

in TiO₂(N.B. the Ti₄O₁₆cores of

4

and

5

span around 0.64 nm,

radius of

below the exciton Bohr

TiO₂).²²

Figure 3. UV spectra of complexes

and free R₂PO₂H ligands (see Fig S26-31 for spectra over

concentrations). Absorptivity of and determined in terms of concentration

1

,

2

,

4

and

5

in comparison to Ti(OR)₄complexes

a

range of

1,

2

,

4

5

concentrated solutions ([Ti] = 1.75 and 0.88 mM, Figs. S26-

of Ti (M[Ti]⁻¹cm⁻¹). Dashed lines show onset of absorptions to highlight the

50

S38).^21,

All spectra show expected Beer-Lambert

differences between the Ti species (collected with [Ti] = 1.75 mM), plotted vs

concentration profiles over the concentration range [Ti] = 1.75-

dashed scale bar to right hand side. Solvent = pentane (or CH₂Cl₂for

Ph₂PO₂H, N.B. identical spectra for in pentane and CH₂Cl₂).

2, 5 and

i

0.05 mM. The precursor alkoxides [Ti(OR)₄]_n(R = Pr or Et)

exhibit essentially identical spectra and begin to absorb at

4

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Figure 4. Dominant donor/acceptor NTO pairs (isovalue = 0.05 au) for the first electronic excitation of a)

4

and b)

5

(ZORA-B3LYP/def2-TZVP(-f)/def2-SV(P)),

including percentage contributions of each pair to the transition. The isosurfaces of the electron difference densities betwe en the excited and ground states are

contoured at 0.003 au. White regions correspond to the depletion of the electron density during the transition while blue regions correspond to the accumulation of

electrons.

below the exciton Bohr radius of TiO₂).²²

Density Functional Theory (DFT) calculations were carried electronic transitions in

out in order to elucidate the electronic structures of the above case (Fig. 4). Both clusters show a donor state comprised of a

titanium-oxo clusters. Optimised geometries of clusters and combination of μ₃-oxo and alkoxide oxygens which upon

dependent DFT (TD-DFT) calculations of the singlet low-energy

and confirm that this is indeed the

4

5

4

5

are in very good agreement with their X-ray crystallographic excitation donates into a combination of the Ti 3d orbitals, with

counterparts (Fig. S39 and Table S1). As seen from the a high degree of similarity in their transition characters. The TD-

calculated frontier orbitals (Figs. S39-41), the HOMOs of both

and are predominantly localised on the oxygen atoms (O 2p) are lower than the calculated HOMO-LUMO gap as expected,

with contributions from the oxo ( : 10.8%, : 23.6%), alkoxide and in reasonable agreement with experiment. Time-

: 36.9%, : 17.6%) and phosphinate ( : 15.7%, : 2.0%) oxygen dependent calculations generally describe the optical gap more

groups (Table S2). There is also a minor contribution (~7%) to accurately (Table S3), owing to the configuration interaction

4

DFT vertical excitation energies for 4 (3.97 eV) and 5 (3.93 eV)

5

4

5

(4

5

4

5

the HOMO in

substituents. In

4

5

from the ipso-carbons of the cyclohexyl expansion of the electronically excited state as a linear

there is a negligible contribution of the combination of one-electron transition configurations. Whilst

diphenylphosphinate oxo centres to the HOMO orbital, possibly the changes in the energy gap between

4 and 5 are relatively

as a result of the more electron withdrawing Ph substituents, small, it shows the opportunity to fine-tune the HOMO-LUMO

however, some admixture (~29%) from phenyl π orbitals is energy gap and the relative orbital energies by subtle changes

present. Both LUMOs are similarly comprised of Ti 3d orbitals in ligand backbone, without affecting the nature of the HOMO-

(82–85%), forming pairwise bonding interactions between LUMO transitions (i.e. OMCT); a useful strategy for designing

adjoining Ti centres. The LUMO in

energy by 0.06 eV compared to . As a result, one finds an

overall reduced HOMO-LUMO gap in (4.88 eV) relative to

5 is slightly stabilised in photocatalysts.

4

5

4

Photo-redox reactivity

Complexes

and precursor Ti(OⁱPr)₄were irradiated with

(4.94 eV), consistent with the experimental absorption onsets.

It appears that the greater electron withdrawing nature of the

1, 2, 4, 5

long wave (LW-UV, 4 W) or medium wave (MW-UV, 6 W) UV

lamps under inert atmosphere (N₂) to investigate photo-redox

reactivity (LW-UV: ‘365 nm’ output range ~3.8-3.1 eV, MW-UV:

‘302 nm’ output range ~4.4-3.3 eV). By absorbing a photon

greater than the HOMO-LUMO gap, OMCT can occur, with an

electron promoted to occupy a Ti based orbital. If the hole

which forms upon an oxygen-based orbital is quenched, then a

photo-reduced complex is formed in which Ti(III) sites are

found; these induce a colouration to the solution (typically

phenyl substituent in the [Ph₂PO₂]^–ligand of

cyclohexyl groups in

5

, relative to the

4

, subtly influences the nature of the

frontier orbitals. We note that the calculations overestimate the

energy gap relative to the experimental value, a phenomenon

that is well-known for hybrid functionals due to their tendency

to over-delocalise unoccupied states.^{51, 52}The above results

suggest that the first excited state in both

4 and 5 corresponds

to a charge transfer from oxygen to the metal core (OMCT). The

leading Natural Transition Orbitals (NTOs) generated from time-

blue).^{24, 28, 53}A small excess (~30 equiv.) of PrOH was added to

i

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d⁸-toluene solutions of the complexes in glass NMR tubes (with (Figs. 5c, S46 and S48) corresponding to a formally-forbidden

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an internal reference capillary of CH₂Cl₂and PPh₃in CDCl₃) and two-electron spin flip (ΔM_s= ±2), indicating the presence of

the samples irradiated with UV light (N.B. the glass tube will complexes with S = 1, e.g. two unpaired electrons in a single

begin filtering photons >3.7 eV). NMR spectra were collected molecule. Comparison of the spectra at various temperatures

after 1, 3 and 5 hours of irradiation. Both

1 and 2 slowly between 10 and 100 K indicate that the minor signal at 1700 G

decompose under irradiation (LW or MW-UV) to give is retained over the entire temperature range (Fig. S46). Upon

precipitates but give no blue colour (Fig. S42); TiOⁱPr₄is cooling, the spectra broaden and shift to higher fields, indicating

unreactive under LW-UV, but very slowly photo-reduces under a decrease in the g value.⁵⁵Simulation of the spectra using an S

MW-UV light to give a pale blue solution, with acetone = 1 model, gave a good agreement with the experimental

produced as the oxidised component (Fig. S42-43). Both

undergo photo-reduction under LW-UV light, highlighting = 1.868 at 10 K (or 1.926 and 1.938 respectively at 100 K) for

lower energy photo-excitation in these complexes relative to photo-reduced , and g_xy= 1.880 and g_z= 1.905 for photo-

TiOⁱPr₄. The photo-redox reaction of

and occurs with reduced (10 K). These spectra conclusively show the presence

4 and results (Figs. S49-50) and produced values of g_xy= 1.860 and g_z

5

4

5

conversion of some isopropoxide ligands into acetone (the of S = 1 complexes in the photo-reduced solutions, although we

oxidised product) and also isopropanol (maintaining proton cannot discount a contribution also from S = ½ species with

balance, vide infra). The solution of

blue/grey colour without precipitate, and analysis of the NMR

spectra reveal 88% consumption of after 5 h (Fig. 5a and S44, across the visible region (maxima: from

] = 9.4 μmol, [ⁱPrOH] = 320 μmol); however, prolonged nm, with smaller peaks at 406 nm (

irradiation (>5 h) causes formation of a dark solid (vide infra). observed, Fig. S51-52). These absorptions are indicative of d-d

undergoes slower photo-reduction with only 10% consumption transitions and/or intervalence Ti(III) Ti(IV) electron

after 4 h with LW-UV, but 88% if MW-UV is used ([

] = 9.7 μmol, transfer.^{29, 37, 53}The presence of S = 1 clusters are also supported

[HOiPr] = 320 μmol, Fig. S44) to give a blue solution (Fig. 5a). by the electronic absorption spectra predicted from TDDFT

5

changes colour to a dark similar g values.

The UV spectra after irradiation show broad absorptions

, 693 nm; from , ~700

) and 480 nm ( ) also

5

4

[

5

4

–

4

The presence of paramagnetic Ti(III) species is confirmed by calculations. The absorption maxima positions for the

UV and EPR spectroscopy of the blue solutions. X-band EPR photoreduced solutions are favourably reproduced (Fig. S53)

spectra of photo-irradiated solutions of

4 or 5 (frozen at 10 K) using doubly-reduced cluster models in their triplet ground

show resonance signals consistent with Ti(III) centres (Figs. 5c, states (S = 1). The d-d and intervalence transition character of

S45-S50; for previously reported examples g = 1.84-1.99).^{54, 55}these absorption bands is confirmed by difference density plots.

Both spectra display a small signal at approximately 1700 G The TDDFT spectra of singly-reduced cluster species (S = ½) were

Figure 5. a & b) Photographs of toluene solutions of 4 and 5 (with 30 equiv. ⁱPrOH) before and after irradiation (a, 13 h, MW-UV; b, 3 h, LW-UV). c) X-band EPR spectrum at 10 K of 4

after photo-reduction with MW-UV (7.5 h, [4] = 10 mM, 9:1 toluene:ⁱPrOH) with small signal at ~1700 G enhanced. d & e) Normalised NMR integrals of 4, acetone and ⁱPrOH during

MW-UV irradiation in toluene (d: 30 equiv ⁱPrOH; e: 30 equiv pyridine), Dotted lines show the expected amount of acetone that will be produced by a 1-electron redox process (0.5

equiv per cluster) or a 2-electron redox process (1 equiv per cluster), estimated from the consumption of ¹H integrals of 4. N.B. 4* and the ³¹P signal at 49 ppm remain uncharacterised.

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also calculated (Fig. S53), suggesting that these could also play [Ti₄O₄(O₂PR₂)₄(OⁱPr)₂]^●●+ acetone + ⁱPrOH, in_View_wh_Ari_tc_ich_{le O}t_nw_lino_e

a role as chromophores in solution with absorption bands electrons are transferred to the cluster un^Dit^Op^Ir^:o¹⁰d^.1u⁰c³i⁹n^/g^Ct⁹w^SCo⁰T¹²i(⁴I¹II^A)

spanning a wide spectral range.

sites that are likely stabilised by donor solvents. Such a process

The irradiation of

5

in toluene without additional ⁱPrOH was resembles current doubling effects observed at TiO₂

also followed by NMR spectroscopy - in this case negligible photoanodes in the presence of sacrificial electron donors (Fig.

26, 27

photo-reduction occurs over 5 h. Various additives (alcohols, 1).^6,

Current doubling refers to the generation of two

pyridine, THF and water) were tested, and all were found to conduction band electrons after photo-excitation by a single

t

increase the rate of photo-reduction (Fig. S54). As BuOH, THF photon, one electron being directly photo-excited, and the

and pyridine were all found to accelerate photo-reduction, the second electron injected into the conduction band after

additive is not required to undergo a redox transformation. decomposition of the photo-oxidised organic radical. It is

Therefore, it appears that the additive plays a role as a donor interesting that this effect is observed for a cluster molecule as

ligand which may stabilise low coordinate Ti(III) sites generated this indicates that the second electron is preferentially donated

through photo-reduction. This is supported by the colour of the to the originally photo-excited cluster – e.g. the entire process

photo-reduced solutions which is dependent upon the additive is likely intramolecular. Such findings are highly relevant for

or solvent.

After photo-reaction of

photocatalysis as two-electron processes are key for bond

or 5, acetone and PrOH are breaking/forming steps. It has been reported that photo-

i

4

observed by ¹H NMR spectroscopy. It is known that alcohols can reduction of only up to 16% of the Ti atoms can occur in 3 nm

quench photogenerated holes,^{24, 53}and in the photo-reaction amorphous TiO₂nanoparticles, therefore it is noteworthy that

some of the OⁱPr substituents on

4 or 5 are oxidised to acetone. 50% of the Ti atoms are reduced in the small Ti₄clusters;

To proceed to acetone the [OⁱPr]⁻unit must accept one hole perhaps the molecular nature and interaction with organic

(lose an electron) to form a radical [ⁱPrO]^●species followed by ligands makes a higher Ti(III) content accessible.^{32, 33}

decomposition of the radical to give acetone, plus one electron

The photo-redox reaction of 4 or 5 in the presence of 30

and one proton. The excess proton can then react with a further equiv. pyridine occurs with the generation of a vivid blue colour

i

[OⁱPr]⁻substituent to form PrOH. Solutions of

4 and 5 with (Fig. 6b). The intense blue colour suggests an additional

excess (~30 equiv.) of either ⁱPrOH or pyridine were monitored electronic interaction occurs in the photo-reduced complexes in

by NMR spectroscopy during photo-irradiation (Figs. 5d, 5e, 6d, the presence of pyridine. This is attributed to the introduction

S55-59). The spectra confirm that the production of acetone is of Ti(III) to pyridine(π*) charge transfer absorptions, which are

linked to the formation of the same quantity of ⁱPrOH, and both indeed implied by TDDFT calculations (Fig. S53). A deep blue

organic products grow in at the same rate as the consumption solution generated from irradiation of

of starting cluster. This suggests that an overall two-electron toluene with 30 equiv. pyridine was concentrated under

reaction scheme occurs: vacuum and deep blue crystals grew out of the blue solution.

[Ti₄O₄(O₂PR₂)₄(OⁱPr)₄]

5 (~10 h, LW-UV) in



Figure 6. a) UV/vis. spectra of a solution of 5 under LW-UV irradiation in the presence of pyridine ([Ti] = 7.3 mM). Solid lines: toluene solution with 30 equiv. pyridine; dashed lines:

neat pyridine solution. b) photograph of a toluene solution of 5 with 30 equiv. pyridine after 3h LW-UV photo-reduction. c) photograph of a pyridine solution of 5 after 3 h LW-UV

photo-reduction. d) Normalised NMR integrals of clusters, acetone and ⁱPrOH during irradiation with expected quantities of acetone produced relative to clusters consumed in the

cases of 1, 2 and 4-electron photo-reduction per cluster shown by dotted lines. N.B. Extra minor ³¹P NMR signals (with a 2:1 ratio) were observed during prolonged irradiation; these

were accompanied by Ti−OⁱPr signals in the ¹H NMR spectrum, suggesting a diamagnetic complex, and are termed ‘4*’ or ‘5*’. The signals of 4*/5* slowly convert back to 4/5 if left

without irradiation. e) Solid state structure of 6, hydrogen atoms omitted for clarity. Ellipsoids displayed at 50% probability. 6 co-crystallises with 5 (27% 6) with all atoms identically

placed except for pyridine group (replacing an [OⁱPr]⁻group in 5).

6 | J. Name., 2012, 00, 1-3

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The X-ray diffraction data revealed the expected unit cell for

and shows co-crystallisation of (colourless) starting material

5

identified by distinct ³¹P NMR signals (Figs. 7a,b and_VieS_w6_A3_rt;_icf_leo_Or_nt_lh_ine_e

DOI: 10.1039/C9SC01241A

similar exchange reaction with EtOH see Supporting note 2).

(73%) alongside a photo-reduced complex, most likely [Ti(IV)_(4-After heating

_x)Ti(III)_xO₄(OⁱPr)_(4-x)(O₂PPh₂)₄(pyridine)_x] ( ) (27%), in which at was formed as the major species (³¹P NMR: 31.9 ppm, Figs. S64-

5

with 100 equiv of ^tBuOH at 70°C for 7 days 5^Bu4

6

least one anionic [OⁱPr]⁻is replaced by a neutral pyridine. One 65). A minor doublet signal was also observed, due to a small

pyridine is clearly modelled in the minor crystalline component, proportion of 5^Bu3, with the compositions of both 5^Bu4and 5^Bu3

however, due to ligand disorder and the overlapping structure confirmed by electrospray ionisation mass spectrometry (ESI-

of

pyridine is present (Fig S61 and Supporting note 1). The confirming the expected geometry (Fig. S67). A solution of 5^Bu4

5

, it is difficult to determine if more than one coordinated MS, Fig. S66). A crystal structure of 5^Bu4was collected

t

i

structure with x = 1 is shown in Fig. 6e and was also modelled with excess BuOH + 4 equiv. PrOH was exposed to LW-UV

using DFT calculations. The calculated frontier orbitals imply irradiation, however, no photo-reduction of 5^Bu4occurred,

that in this S = ½ model the unpaired spin density is located indicating that Ti-OⁱPr units are essential for photo-reduction (as

predominantly upon the Ti atom coordinated to pyridine (55 %) ^tBuO⁻cannot be oxidised) which must process via an

(Fig. S61 and Table S4).

Whilst solutions of

intramolecular pathway. In contrast, mixtures of 5^Bu1

,

5^Bu2and

4

and pyridine generate the expected 1 5^Bu3all undergo photo-reduction, and interestingly 5^Bu3(as a

equiv. of acetone and ⁱPrOH per cluster consumed (Figs, 5e and mixture with 5^Bu4with 30 equiv. THF or pyridine) generates

S58), solutions of

5

and pyridine begin in the same manner, but exclusively acetone and ^tBuOH upon irradiation, with no

after 25 minutes irradiation the quantity of acetone/ⁱPrOH rises production of PrOH (and no reaction of 5^Bu4, Fig. S68-69). This

i

to ~2 equiv. per cluster consumed (Figs. 6d, S59). This indicates supports an intramolecular, two-electron process in which

a four-electron redox reaction in which all four Ti-OⁱPr units of

5

acetone and alcohol are generated from the same cluster unit.

are consumed, clearly highlighting the important role of the Conversely, if an [ⁱPrO]^●radical was ejected from 5^Bu3(following

donor solvent. Furthermore, if pyridine is used as the neat electron photo-reaction) subsequent intermolecular

solvent, photo-reduction of

yields a red/purple solution (Figs reaction with a different cluster of 5^Bu3is expected to produce

a

1

5

t

i

6a,c). In contrast, neat pyridine solutions of

blue (Fig. S62).

4 irradiate to deep a mixture of BuOH and PrOH. Compound 4 undergoes similar

alkoxide exchange reactions, albeit at a slower rate, and a

t

Clusters

4

and

5

undergo slow alkoxide exchange in the solution of 4^Bu3

/

4^Bu4similarly generates BuOH and acetone

i

presence of excess alcohols. The mixed alkoxide species under MW-UV irradiation, with no production of PrOH. Whilst

[Ti₄O₄(O₂PPh₂)₄(OⁱPr)_x(O^tBu)_y] (y = 1, 5^Bu1; y = 2, 5^Bu2(2 isomers); several mechanistic pathways are possible, a process is

y = 3, 5^Bu3; y = 4, 5^Bu4) can be prepared as mixtures during the suggested in Fig. 7c, in which the photogenerated [ⁱPrO]^●radical

slow alkoxide exchange of

5

with ^tBuOH, and all can be is retained within the coordination sphere of the cluster, and

Figure 7. a) Alkoxide exchange products from 5 and ^tBuOH and the organic products released on irradiation with UV light (red curves represent [O₂PPh₂]⁻). b) ³¹P {¹H} NMR spectra

of 5 + 300 equiv. ^tBuOH in toluene after heating to 70ᵒC for various time periods. c) Suggested intramolecular mechanism for formation of acetone and alcohol from one cluster unit,

square Ti₂O₂face shown for clarity.

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rapid H atom transfer to the adjacent alkoxide group occurs

across the Ti₂O₂square face, to directly release acetone and

alcohol.

View Article Online

DOI: 10.1039/C9SC01241A

Oxidation of the blue photo-reduced solutions by exposure

to air results in an almost instantaneous colour change to pale

yellow solutions. Various unidentified ³¹P NMR signals are

i

observed directly after oxidation with air. If excess PrOH is

present, upon standing in air the solution slowly regenerates

starting material (e.g. 37%

5 reformed over 1 week). The

regeneration of complex under air completes a catalytic cycle

5

for the oxidation of ⁱPrOH to acetone, with water also generated

(see supporting note 3). Understanding the effect of additives,

solvents, and ligands is essential for designing effective

photocatalysts, and the studies detailed here should be useful

when utilising metal-oxo clusters or metal oxide materials.⁵⁶

Structural characterisation of photo-reduced clusters and their re-

oxidation under air

Figure 8. a) Solid-state structure of 7 (hydrogen atoms omitted for clarity, ellipsoids

displayed at 50% probability) Selected bond lengths (Å) and angles (°) 7: Ti1-O1, 1.970(2);

Ti1-O2, 2.025(3); Ti1···Ti1’, 2.933(2); P1-O2, 1.522(4); Ti1-O1-Ti1’, 96.25(16), O2-P1-O2’

115.9(3). b) Calculated α-spin SOMOs (isovalue = 0.05 au); c) a spin density plot and

Loewdin atomic spin populations.

In the absence of strong donor solvents (e.g. pyridine), loss of

alkoxide during the photo-reaction leaves a vacant coordination

site at the Ti(III) centres. This can be compensated by ligand

exchange of one remaining monodentate alkoxide for a

bidentate phosphinate. After irradiation of a toluene solution of

i

5

with 30 equiv PrOH for >2.5 h, small dark coloured cubic

insoluble material that crystallises, whilst the residual soluble Ti

crystals spontaneously form in the reaction flask overnight. The

solid-state structure revealed these dark crystals to be

complexes remain uncharacterised. Irradiation of

presence of two extra equivalents of Ph₂PO₂H, generates

5

in the

as

7

comprised of high symmetry Ti₄O₄(Ph₂PO₂)₆(

−4 3 m) - a cluster that has retained the Ti₄O₄core from

7

, space group = I

but

an insoluble black powder which can be isolated and analysed

by I.R. spectroscopy. The I.R. spectrum recorded under inert

atmosphere shows a series of well-defined stretches consistent

with a high symmetry molecule (Fig. S74). Upon exposure to air,

5

which now has exclusively phosphinate ligands (Fig. 8a). The

cluster is a symmetrical cube (Ti−O−Ti, 96.3ᵒ) with an average

oxidation state of +3.5 per Ti centre, which is supported by

bond-valence sum calculations (Table S5). This indicates that

two electrons occupy the Ti 3d based molecular orbitals in the

solid

7 rapidly changes colour to bright yellow/orange and the

IR spectrum evolves to give broader, less-defined signals (Fig.

S75). We were only able to isolate mg scale quantities of very

cluster. The Ti−₃-O bond length in 7 (1.970(2) Å) and Ti···Ti

air sensitive 7 by the photo-reaction of 5 + 2 Ph₂PO₂H. Scale-up

distance (2.933(2) Å) are intermediary to the longer and shorter

of the photo-reaction proved challenging due to slow reaction

times, air sensitivity and the formation of mixtures of starting

material and product, nevertheless, it was possible to analyse

the product formed from oxidation of 7. Solid-state EPR

distances found in asymmetrical 5. 7 is isostructural with the

previously reported mixed valent complex Mn₄O₄(Ph₂PO₂)₆.⁴⁷

The experimental findings are further corroborated by a DFT

electronic structure analysis of 7, which predicts the electronic

spectroscopy of this oxidised product

(7-ox) at room

triplet state to lie energetically below the closed-shell singlet

(ΔE = +0.26 eV) and open-shell diradical singlet states (ΔE =

+0.06 eV), the latter modelled using the broken-symmetry

approach (Table S6, Figs. S71-73). The symmetrical geometry of

the Ti₄O₄cluster core in the triplet ground state is well

reproduced by the calculations, with average Ti–oxo and TiTi

distances of 1.95 Å and 2.90 Å, respectively (Fig. S70, Table S7).

The unpaired electrons in two singly occupied molecular

orbitals (SOMOs, Fig. 8b) are coupled ferromagnetically,

resulting in a S = 1 total spin per molecular cluster. The spin

density is evenly delocalised over four Ti centres with Loewdin

populations of approximately 0.5e^–per atom (Fig. 8c), fully

consistent with the assigned average oxidation state of +3.5 per

Ti centre and the EPR results.

temperature revealed a paramagnetic signal consistent with a

superoxide anion (Fig. S76, g = 2.017, 2.006, 2.001), implying the

formation of a Ti(IV)-superoxide complex upon oxidation.³⁷The

dication [Ti₄O₄(Ph₂PO₂)₆]²⁺could also be observed by ESI-mass

spectrometry, after 7-ox was dissolved in CH₂Cl₂(Fig. S77),

suggesting a formulation for 7-ox as [Ti₄O₄(Ph₂PO₂)₆][O₂]₂.

7-ox appears to be rather stable in the solid-state under

ambient conditions, with the superoxide signal observed by EPR

spectroscopy several days after oxidation. The yellow/orange

colour of the compound is similar to that observed in alkali

metal and titanium based superoxides.⁵⁷Diffuse reflectance

UV/visible spectroscopy showed a broad absorption with a

maximum value at ~412 nm (Fig. S78) consistent with similar

observations upon photo-reduced and re-oxidised TiO₂

nanoparticles.³³

The migration of two extra phosphinate ligands required in

the formation of

7 suggests the phosphinate ligands may be

Single crystals of

7 also react slowly with air to give

exchanged in solution.³⁹The formation of

7 generates an

yellow/orange 7-ox whilst maintaining single crystallinity.⁵⁸The

8 | J. Name., 2012, 00, 1-3

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View Article Online

DOI: 10.1039/C9SC01241A

Figure 9. a) Oxidation of single crystals of 7 under air. Crystals undergo reaction from

surface downwards. b) Packing diagram of 7 (with phenyl groups in stick notation and

hydrogen atoms omitted) showing symmetrical nature of the complex and possibility for

incorporation of small anions within the lattice.

transition progresses smoothly through a bi-colour crystalline

intermediate in which the edge of the crystal is oxidised whilst

the centre remains in its reduced form (Fig. 9a). For ~0.5

mm³crystals, the oxidation requires ~3 days under ambient

conditions. The final yellow/orange crystal diffracts weakly at

high angle, but the original unit cell is confirmed, and the data

Figure 10. Solid state structure of 8 (hydrogen atoms omitted for clarity, ellipsoids

displayed at 50% probability) and calculated α-spin SOMO (isovalue = 0.05 au) along

with a spin density plot and Loewdin atomic spin populations. Selected bond lengths (Å)

and angles (°) 8: Ti1-O1, 1.925(2); Ti1-O1’, 1.957(2); Ti1-O2, 2.056(2); Ti2-O1, 2.053(2);

Ti2-O2, 1.9319(19); Ti2-O2’, 1.953(2); Ti1-O8, 1.795(3); Ti-OPOCy₂range, 2.010(2)-

2.032(2); P-O range, 1.519(2)- 1.528(2); Ti1···Ti1’, 2.9470(12); Ti2···Ti2’, 2.9114(12),

Ti1···Ti2, 3.0080(8); Ti1···Ti2’; 3.0466(9), Ti1-O1-Ti1’, 98.76(9); Ti2-O2-Ti2’, 97.08(9); O-P-

O range, 114.48(14)-115.52(19).

collected fits the structural model of

7 with an R factor of 14%

(for , R = 4%). The data suggest that the Ti₄O₄(Ph₂PO₂)₆unit is

7

retained, although no other clear areas of electron density were

identified. Whilst only approximate bond lengths and angles are

available from this dataset (Fig. S79), it is noteworthy that the

valence sum calculations, supported by molecular orbital

Ti−phosphinate bonds in 7-ox appear shortened compared to

7

calculations, indicate that

8 contains two Ti(IV) sites (with

(from 2.03 to ~1.95 Å) and the Ti···Ti distances have slightly

increased (from 2.93 to ~3.01 Å); bond-valence sum analysis is

now consistent with all Ti centres in the +4 oxidation state

(Table S5). It appears that the cluster is oxidised to

[Ti₄O₄(Ph₂PO₂)₆]²⁺and, although the counterion has not yet

been verified by crystallography (likely due to partially occupied

identical sites in the high symmetry structure), the

incorporation of [O₂]⁻into the lattice via reaction with air is

consistent with a formula of [Ti₄O₄(Ph₂PO₂)₆][O₂]₂for 7-ox. The

coordinated alkoxide ligands) and two sites with average

valence of 3.5 (Table S5, S8, Figs S80-82), suggesting

delocalisation of one 3d electron across two Ti centres (e.g.

within one of the two Ti₂O₂square units). This assessment is

borne out in the contour surface of the SOMO and associated

atomic spin populations (Fig. 10b), both indicating notable

accumulation of electron spin (0.73e^–) on Ti2 and Ti2’ of the

lower Ti₂O₂square. It was very challenging to produce bulk

samples of

8

in the absence of starting material

4

, which

pseudo-spherical shape of

7 influences the crystal packing,

restricted analysis. Upon oxidation of mixtures of

8/4 under air,

causing a body centred cubic arrangement within the unit cell.

Small pockets between the clusters could possibly

accommodate a superoxide anion upon oxidation. Titanium (III)

complexes can be used as colorimetric oxygen indicators,⁵⁹

often useful in monitoring glovebox inert atmospheres, with an

indicative blue to yellow colour change in the presence of O₂;

the powder turns a bright yellow/orange colour and solid-state

EPR spectroscopy and diffuse-reflectance spectroscopy reveal

signals for Ti(IV)-superoxide (from modelled EPR spectrum, g =

2.0182; 2.007; 2.0023, Figs. S78 and S83), similar to that found

for 7-ox

,

such that 8-ox is best formulated as

[Ti₄O₄(OⁱPr)₂(Cy₂PO₂)₅][O₂]. The cation [Ti₄O₄(OⁱPr)₂(Cy₂PO₂)₅]⁺

compound

7 acts as a solid-state example of this process.

was also identified by ESI-MS after 8-ox was dissolved in CH₂Cl₂

Furthermore, the ability to trap and store superoxide anions has

interesting possibilities with respect to antibacterial surfaces or

onward chemical reactivity.^{5, 7, 57}

(Fig. S84). The structure of

8 does not follow the two-electron

photo-redox process supported by EPR and NMR experiments

and may suggest that competing one-electron pathways are

also possible; alternatively electron transfer may occur

between photo-reduced species in solution, which may account

4

is rather soluble in hydrocarbon solvents, far more so than

(and

) due to the [Cy₂PO₂]⁻ligand, however, blue crystals of

a photo-reduced cluster could be slowly grown directly from a

5

7

the formation of crystals of

exposure.

8

only after a prolonged UV

photo-irradiated solution of

30 equiv. PrOH) in hexane solvent. These blue crystals show a

4 after prolonged irradiation (with

i

solid-state structure with the formula Ti₄O₄(Cy₂PO₂)₅(OⁱPr)₂(

in which only one-electron reduction of the original cluster has

occurred (Fig. 10a). retains a structure comprised of two Ti₂O₂

8)

Conclusions

8

squares with a longer Ti−O bond between the squares. Bond-

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Small phosphinate supported Ti₄-oxo clusters have been nitrogen. d₈-toluene was dried by stirring over CaH₂, ‘extra-dry’

View Article Online

DOI: 10.1039/C9SC01241A

prepared in a straightforward stoichiometric reaction. The acetone was purchased from Acros Organics, these and all other

clusters absorb LW-UV light via an oxygen-to-metal charge dry solvents and reagents were degassed by bubbling with N₂

transfer process, with an absorption onset which is adjustable for 30 minutes or freeze pump thaw cycles and stored over

by ligand design. The absorption onset falls at higher energies molecular sieves under nitrogen.

than in bulk TiO₂consistent with other small Ti-oxo clusters and

1. [Ti(OⁱPr)₃(Cy₂PO₂)]₂: 75 mg (0.33 mmol) of Cy₂PO₂H was

in line with quantum confinement effects, but at lower energies placed in a Schlenk flask with a stirrer bar and dissolved in 3 mL

than simple Ti(OR)₄precursor molecules. The clusters undergo of Hexane. To this 93 mg (0.33 mmol) of TiOⁱPr₄was added and

productive photo-redox reactivity under UV irradiation, with the mixture stirred overnight. The solution was then evacuated

the oxidation of isopropoxide substituents to acetone (and to ~0.5 mL and cooled to −20°C overnight to yield colourless

isopropanol) and the formation of reduced Ti species containing crystals. The remaining solution was removed, and the crystals

Ti(III) centres. EPR and NMR spectroscopy analysis of the dried under vacuum. 49 mg isolated crystalline yield (33%). ³¹P

irradiation process, alongside reactivity of mixed alkoxide {¹H} NMR spectroscopy (d⁸-toluene, 162 MHz): δ 45.5

cluster 5^Bu3are consistent with an intramolecular two-electron ¹H NMR spectroscopy (d⁸-toluene, 400 MHz): δ 5.09 (6H, sept

H

photo-redox pathway in which one molecule of acetone and (J_HH= 6 Hz), Ti-OⁱC Me₂), 1.35 (36H, d (J_HH= 6 Hz), Ti-OⁱCHMe₂),

alcohol are generated from the same cluster after photo- 1.2-2.1 (44H, m, C₆H₁₁). Elemental Analysis (predicted): % C,

excitation. This leaves a two-electron reduced cluster, such as 55.30 (55.51); % H, 9.54 (9.54). N.B.

1

was not stable during ESI-

+

7. 7 ]

is a rare example of a structurally characterised photo- MS, instead hydrolysis products [Ti₄O₄(OⁱPr)_2/3(Cy₂PO₂)_5/4

reduced Ti-oxo cluster that exists in a triplet ground state. Such were observed.

two-electron reactivity is consistent with current doubling

. [Ti(OⁱPr)₂(Ph₂PO₂)₂]₆:76.8 mg (0.35 mmol) of Ph₂PO₂H was

2

effects observed at TiO₂photoanodes and is of interest with placed in a Schlenk and suspended in 2 ml CH₂Cl₂. To this 50 mg

respect to photocatalysis in which a two-electron bond (0.18 mmol) of TiOⁱPr₄was added. The solution was monitored

breaking/forming step is typically required. Reactivity of the directly by ³¹P NMR spectroscopy and some

3 is identified as a

clusters shows that both alkoxide and phosphinate ligands may by-product. Removal of solvent led to a colourless solid. 50 mg

undergo exchange in solution, and whilst competitive isolated (44 % yield). ³¹P {¹H} NMR spectroscopy (CDCl₃, 162

decomposition and/or one-electron photo-redox pathways MHz): δ 23.9 (1P), 23.8 (1P), 23.6 (2P), 21.5 (1P), 21.3 (3P), 19.6

remain possible, it is clear that a propensity for retention of the (1P), 19.5 (1P), 16.8 (1P), 16.6 (1P). ³¹P {¹H} NMR spectroscopy

stable Ti₄O₄core remains throughout these photo-reactions. (d⁸-toluene, 162 MHz): δ 24.3 (1P), 23.9 (1P), 23.8 (2P), 21.83

1

Donor solvents, such as pyridine, accelerate the photo-redox (1P), 21.79 (3P), 20.0 (1P), 19.9 (1P), 16.6 (1P), 16.4 (1P). H

process, supporting the formation of low-coordinate Ti(III) NMR spectroscopy (CDCl₃, 400 MHz): δ 6.7-8.0 (120H, m,

centres. Understanding the effects of additives, solvents and Ph₂PO₂), 5.02 (3H, septet, Ti-OⁱC

HMe₂), 4.92 (6H, septet, Ti-

ligands within metal-oxo clusters is important for developing OⁱC

their use in efficient photocatalytic processes. These studies OⁱC

H

Me₂), 4.85 (1H, septet, Ti-OⁱC

Me₂), 4.03 (1H, septet, Ti-OⁱC

Elemental

H

Me₂), 4.49 (1H, septet, Ti-

H

Me₂), 1.26-0.68 (72H, m, Ti-

should also give insight into photo-reactivity at the surfaces of OⁱCHMe₂).

Analysis

(predicted

for

metal oxides,^30-32for which detailed mechanistic study is often [Ti(OⁱPr)₂(Ph₂PO₂)₂]₆.(2.7 CH₂Cl₂), N.B. sample precipitated from

challenging, and are relevant in studies of partially reduced CH₂Cl₂and thoroughly dried): % C, 57.30 (57.27); % H, 5.60

(black/blue) TiO₂used as a photocatalytic material.^34-367 and

react with oxygen to form superoxide complexes, and single

crystals of complete this reaction within the single crystalline placed in Schlenk flask and suspended in 3 mL CH₂Cl₂. To this 50

8

(5.51)

. Ti₃(OⁱPr)₇(Ph₂PO₂)₅: 64 mg (0.29 mmol) of Ph₂PO₂H was

3

7

phase, allowing for the product of oxidation to be studied mg (0.23 mmol) of TiOⁱPr₄was added and the reaction mixture

directly by X-ray crystallography. These results show that stirred for 1 hour. The solvent was removed under vacuum and

productive photochemistry with charge separation can occur in 10 mL of Hexane added to give a suspension. The flask was

even the smallest of metal-oxo clusters and highlight these heated to 40ᵒC overnight whilst stirring and then the solution

molecules as useful systems for mechanistic studies.

was filtered (and any solid discarded). The solvent was removed

under vacuum to yield a white powdered solid. 25 mg isolated

powder (26%). ³¹P {¹H} NMR spectroscopy (d⁸-toluene, 162

MHz): δ 23.6 (2P), 20.5 (2P), 18.7 (P), ¹H NMR spectroscopy (d⁸-

toluene, 400 MHz): δ 8.08-7.76 (20H, Ph₂PO₂), 7.14-6.85 (30H,

Experimental

All manipulations were undertaken using a nitrogen filled

glovebox or using a Schlenk line, unless otherwise stated.

Ph₂PO₂H and Ti(OⁱPr)₄were used directly from suppliers,

Cy₂PO₂H was synthesised by an amended literature prep⁶⁰

(details included in the supporting information). As an air

sensitive liquid, all additions of Ti(OⁱPr)₄were transferred by

syringe within the glovebox (measured by negative weight of

donor flask). THF, hexane and toluene were dried by refluxing

over sodium (and benzophenone for THF), pentane and

dichloromethane were dried by refluxing over CaH₂, all under

Ph₂PO₂), 5.37 (3H, br, Ti-OⁱC

Ti-OⁱC

HMe₂), 5.04 (4H, sept (J_HH= 6 Hz),

H

Me₂), 1.54-1.40 (18H, br, Ti-OⁱCHMe₂), 1.26 (24H, d (J_HH

= 6 Hz), Ti-OⁱCHMe₂). Elemental Analysis (predicted): % C, 58.15

(59.21); % H, 6.28 (6.07).

4. [TiO(OⁱPr)(Cy₂PO₂)]₄: 810 mg (3.52 mmol) of Cy₂PO₂H was

placed in a Schlenk flask with a stirrer bar and dissolved in 20

mL of Hexane. To this 1 g (3.52 mmol) of TiOⁱPr₄was added and

the mixture stirred for 30 minutes. In a separate Schlenk, 63 L

of water was added to 1.5 mL of acetone, and this mixture

10 | J. Name., 2012, 00, 1-3

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Page 11 of 13

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added dropwise to the reaction solution whilst stirring. The the solid further washed with CH₂Cl₂. 4 mg (12% yield) of 7 was

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solution was then heated to 50°C for 48 hours whilst stirring, isolated as a dark powder which turns bri^Dg^Oht^I:y¹⁰e^.l¹l⁰o³w^9//^Co⁹r^Sa^Cn⁰g¹e²⁴o¹n^A

before concentrating under vacuum to ~1 mL and cooling to exposure to air. Elemental analysis after oxidation in air

−20°C overnight to yield a white microcrystalline powder. The (predicted for [Ti₄O₄(Ph₂PO₂)₆][O₂]₂): % C, 54.21 (53.30); % H,

remaining solution was removed, and the powder dried under 3.94 (3.73). ESI-MS after oxidation in air (CH₂Cl₂):

vacuum. 520 mg isolated crystalline yield (42%). ³¹P {¹H} NMR [Ti₄O₄(Ph₂PO₂)₆]₄²⁺m/z = 779.2, calc. 779.0

spectroscopy (CDCl₃, 162 MHz): δ 56.3. ³¹P {¹H} NMR

spectroscopy (d⁸-toluene, 162 MHz): 55.0. ¹H NMR irradiated with MW-UV light for several hours and the blue

spectroscopy (CDCl₃, 400 MHz): δ 5.04 (4H, sept (J_HH= 6 Hz), Ti- solution left to stand. A small quantity of thin blue crystals of

OⁱC

Me₂), 2.15 (8H, d, Cy), 1.96 (8H, d, Cy), 1.77 (24H, m, Cy), grow on the flask. ESI-MS after oxidation in air (CH₂Cl₂):

1.67 (8H, s, Cy), 1.49 (16H, m, Cy), 1.26 (24H, d (J_HH= 6 Hz), Ti- [Ti₄O₄(OⁱPr)₂(Cy₂PO₂)₅]⁺m/z 1519.5, calc. 1519.6

8 4 was

. [Ti₄O₄(Cy₂PO₂)₅(OⁱPr)₂]: A sealed hexane solution of

δ

8

H

OⁱCHMe₂), 1.19 (24H, m, Cy). ESI-MS (CH₂Cl₂): [M-H]⁺m/z =

1409.6, calc. 1409.5; [Ti₄O₄(OⁱPr)₃(Cy₂PO₂)₄]⁺m/z = 1349.6, calc.

1349.5. Elemental Analysis (predicted): % C, 51.08 (51.15); % H,

8.25 (8.30)

[CCDC 1902111–1902116 and 1912033–1912035 contain the

supplementary crystallographic data for this paper.]

Conflicts of interest

5

.toluene [TiO(OⁱPr)(Ph₂PO₂)]₄.C₆H₅Me: 768 mg (3.52 mmol)

There are no conflicts to declare.

of Ph₂PO₂H was placed in a Young’s tap flask with a stirrer bar

and suspended in ~30 mL of Toluene. To this 1 g (3.52 mmol) of

TiOⁱPr₄was added and the mixture stirred for 30 minutes. In a

Acknowledgements

separate Schlenk, 63 L of water was added to 1.5 mL of dry

acetone, and this mixture added dropwise to the reaction

solution whilst stirring. The solution was then heated to 60°C for

24 hours whilst stirring, before concentrating under vacuum to

~2 mL and cooling to −20°C overnight to yield a colourless

crystalline product. The remaining solution was removed and

the product dried under vacuum. 470 mg isolated crystalline

yield (36%). ³¹P {¹H} NMR spectroscopy (d⁸-toluene, 162 MHz):

We thank the Herchel Smith Fund (SDP) and Department of

Chemistry, Maynooth (TK) for financial support. SDP thanks

Prof. Dominic Wright, University of Cambridge, for supplying

laboratory space and use of a glovebox for experimental work

and Prof. Clare Grey for access to an FTIR spectrometer in a

glovebox. TK wishes to acknowledge the DJEI/DES/SFI/HEA Irish

Centre for High-End Computing (ICHEC) for the provision of

computational facilities and support. We acknowledge the

National EPSRC UK EPR Facility at Manchester and Adam

Brookfield for support with the EPR measurements. We are also

grateful for the helpful and constructive comments from

reviewers when revising this manuscript.

1

δ 32.5. H NMR spectroscopy (d⁸-toluene, 400 MHz): δ 8.01

(16H, m, Ph), 7.10 (8H, m, Ph), 7.02 (16H, m, Ph), 4.93 (4H, sept

(J_HH= 6 Hz), Ti-OⁱC Me₂), 1.17 (24H, d (J_HH= 6 Hz), Ti-OⁱCHMe₂).

H

ESI-MS (CH₂Cl₂): [M-H]⁺

m/z = 1361.0, calc. 1361.2;

[Ti₄O₄(OⁱPr)₃(Ph₂PO₂)₄]⁺m/z = 1301.0, calc. 1301.1. Elemental

Analysis (predicted for [TiO(OⁱPr)(Ph₂PO₂)]₄.(0.66 C₆H₅Me), note

partial loss of solvent of crystallisation during drying): % C, 54.62

(54.61); % H, 5.18 (5.20)

Notes and references

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5^Bu4. [TiO(O^tBu)(Ph₂PO₂)]₄.(C₆H₅Me)_0.5: 70 mg (0.05 mmol)

of

5

.toluene was placed in a Schlenk and dissolved in 5 mL of

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black microcrystalline product, the solution was removed and

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DOI: 10.1039/C9SC01241A

TOC text and graphic

The photo-reactivity of titanium-oxo clusters is investigated, revealing an intramolecular,

solvent assisted, two-electron redox process that generates blue-coloured Ti(III)/Ti(IV)

clusters.

Article Doi

Photo-redox reactivity of titanium-oxo clusters: Mechanistic insight into a two-electron intramolecular process, and structural characterisation of mixed-valent Ti(iii)/Ti(iv) products

DOI: 10.1039/c9sc01241a

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Article abstract of DOI:10.1039/c9sc01241a

Full text of DOI:10.1039/c9sc01241a

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