Quantifying factors that influence metal ion release in photocaged complexes using ZinCast derivatives

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PAPER

Quantifying factors that inﬂuence metal ion release in photocaged complexes

using ZinCast derivatives†

Celina Gwizdala,^aCharlene V. Singh,^aTracey R. Friss,^aJohn C. MacDonald^band Shawn C. Burdette*^b

Received 19th January 2012, Accepted 12th March 2012

DOI: 10.1039/c2dt30135k

Two generations of nitrobenzhydrol-based photocages for Zn²⁺have been prepared and characterized. The

ﬁrst series includes the tridentate ZinCast-1 utilizes a bis-pyridin-2-ylmethyl-aniline ligand that forms a

5,5-chelate ring upon metal binding. The related photocages ZinCast-2 with a N-[2-(pyridine-2-yl)ethyl]-

N-(pyridine-2-ylmethyl)aniline (5,6-chelate ring) and ZinCast-3 with a N,N-bis[2-(pyridine-2-yl)ethyl]

aniline (6,6-chelate ring) were synthesized for comparative studies. The complexes formed by the ions

Cu²⁺, Zn²⁺and Cd²⁺with three ZinCast and their photoproducts (ZinUnc) were interrogated by UV-Vis

spectroscopy. The studies indicate that ZinCast-1 forms complexes of the highest stability and ZinCast-3

exhibits the most signiﬁcant changes in metal afﬁnity upon uncaging. These results suggest that the

changes in nitrogen atom donor ability as well as the initial complex stability must be considered to

design a photocage with the desired properties. The composite results were used to design ZinCast-4 and

ZinCast-5, the second generation photocages that incorporate an additional adjacent ether ligand into the

Zn²⁺chelator.

with release mechanisms analogous to those exploited in Ca²⁺

Introduction

systems.^10–12ZinCast-1 integrates a N,N-dipicolylaniline (DPA)

chelating moiety with a nitrobenzhydrol caging group.¹¹Pho-

Molecular photocages undergo irreversible photoreactions to

trigger the release of a bioactive product.^1,2The photocaging

substituent renders the analyte inactive prior to photolysis. Upon

uncaging, concentration ﬂuxes of the released photoproduct

perturb biochemical processes. The response elicited can be

monitored and quantiﬁed with an orthogonal analytical tech-

nique. Uncaging can be focused in space or modulated in time

the so concentration of analyte and location of its release can

be controlled. A variety of bioactive molecules have been suc-

cessfully incorporated into photocages facilitating the study of

numerous intra- and extracellular processes.^2,3In addition to

photocaged molecules, divalent metal ion photocaged complexes

tolysis of [Zn(ZinCast-1)(H₂O)₂]²⁺is initiated by the light-

induced abstraction of the benzylic hydrogen atom onto the

excited nitro group yielding aci-nitro intermediate (Fig. 1). The

intermediate decomposes to a nitrosobenzophenone photopro-

duct [Zn(ZinUnc-1)]²⁺.¹¹After photolysis, the [Zn–DPA]²⁺

complex stability is decreased due to delocalization of the

anilino lone pair onto the carbonyl oxygen of

have had

a signiﬁcant impact on the study of signal

transduction.^4–6Photocaged complexes helped provide a deeper

understanding of Ca²⁺-signaling in neurons as well as other

Ca²⁺-induced processes in non-neuronal cells. While physiologi-

cal signaling roles of Ca²⁺are well-established, similar functions

for Zn²⁺have been proposed, but not proven unequivocally.^7–9

To facilitate the study of Zn²⁺signaling in biology, we are

developing both Cast and Cleav photocaged Zn²⁺complexes

^aUniversity of Connecticut, 55 North Eagleville Road, Storrs, CT

06269-3060, USA

^bWorcester Polytechnic Institute, 100 Institute Road, Worcester, MA

01609-2280, USA. E-mail: scburdette@wpi.edu

†Electronic supplementary information (ESI) available: Additional spec-

troscopic data, XLﬁt spectral ﬁtting protocols, complete X-ray tables and

NMR spectra for new compounds. CCDC reference numbers 864178.

For ESI and crystallographic data in CIF or other electronic format see

DOI: 10.1039/c2dt30135k

Fig. 1 Uncaging action of ZinCast-1. Upon irradiation, the nitrobenz-

hydrol (red) is converted into a benzophenone. The decrease in electron

density on the aniline nitrogen atom due to the new resonance interaction

between the aniline lone pair and the carbonyl oxygen reduces the

binding afﬁnity of the uncaged ligand.

8162 | Dalton Trans., 2012, 41, 8162–8174

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nitrosobenzophenone. Photolysis of [Zn(ZinCast-1)]²⁺in

aqueous solution (50 mM HEPES, 100 mM KCl, 20% DMSO,

pH 7) leads to ∼400-fold decrease in Zn²⁺afﬁnity.

upon uncaging, ZinCast-1 exhibited more dramatic changes in

Zn²⁺afﬁnity (ΔK_d= 400, ΔK = 2.1). To compare photocages,

the change in Zn²⁺afﬁnity due to photolysis of ZinCast photoc-

age is expressed as ratio of photoproduct’s stability constant

(ZinUnc K_d) to the photocage’s stability constant (ZinCast K_d),

ΔK_d= (ZinUnc K_d)/(ZinCast K_d). All changes can also been

expressed as ΔK, where ΔK = (ZinCast K)/(ZinUnc K) and K =

−log[K_d].

In subsequent studies, the metal binding properties of

ZinCast-1 and its photoproduct ZinUnc-1 were evaluated in

EtOH and CH₃CN; in addition, the aqueous data (50 mM

HEPES, 100 mM KCl, pH 7.0; “buffer”) was re-acquired using

EtOH as a co-solvent to avoid the spectral overlap between

ZinCast-1 and DMSO (Table 1). The metal binding afﬁnity of

ZinCast-1 and ZinUnc-1 are inﬂuenced by the solvent. While

Zn²⁺afﬁnity is the highest in CH₃CN (K_d= 9.7 μM), the ΔK_dis

the largest in aqueous solution. Replacement of DMSO with

EtOH reduces ΔK_dby a factor of ∼4. The reduction in ΔK_dis

caused by weakened stability constant of [Zn(ZinCast-1)]²⁺in

the presence of EtOH. Changes in ligand solvation by ethanol

and subsequent competition for Zn²⁺ions by solvent molecules

are hypothesized to decrease complex stability.¹⁸The reduced

basicity of anilino nitrogen in ZinUnc-1 moderates the solvation

differences and results in nearly identical binding constants in

both aqueous media.

Given the solvent-based afﬁnity variability, as well as the goal

of tuning photocaged complex properties for biological appli-

cations, new ZinCast derivatives were sought to systematically

study trends in metal ion binding afﬁnity. Since ligand structure

also inﬂuences metal ion afﬁnity, studying variations of the

ZinCast-1 framework provides a simple means to investigate the

metal binding properties of photocaged complexes. Two comp-

lementary ZinCast derivatives where envisioned where the alkyl

linker between the aniline nitrogen atom and the pyridine moiety

is extended from a methylene to an ethylene spacer. This strategy

provides three related photocages where the magnitude of the

ΔK_dshould be independent of ancillary donor groups (pyridine)

and changes in the anilino nitrogen atom basicity (Fig. 2).

Zn²⁺possesses a great degree of conformational ﬂexibility

because of its d¹⁰conﬁguration. Zn²⁺frequently forms

The magnitude of the decrease in metal binding constant in

ZinCast-1 after photolysis is an order of magnitude greater than

that measured for nitrobenzhydrol-based Ca²⁺photocages.¹³The

efﬁcacy of photocaged complexes for probing biological signal-

ing is directly related to the change in metal ion afﬁnity upon

uncaging. Rationally predicting the binding afﬁnity changes

at the design stage would greatly expedite the development of

photocaged complexes with the desired properties. There are

well-established qualitative rules for predicting relative metal ion

afﬁnity in multidentate ligands based on chelate ring size, so

study of related ZinCast derivatives could provide insight into

rules for predicting photocaging properties.

Results and discussion

Design and synthesis of ZinCast photocages

The uncaging mechanism in nitrobenzhydrol-based photocages

involves light-initiated formation of an aci-nitro intermediate that

subsequently decomposes into a nitrosobenzophenone photopro-

duct.^15,16In Cast¹⁷and Nitr⁴metal photocaged complexes,

photoproduct formation triggers partial delocalization of a metal-

bound aniline lone pair onto the carbonyl oxygen, which reduces

the chelator’s metal ion afﬁnity. The change in metal binding

equilibrium after uncaging increases the concentration of free

metal ion. Practical photocages must release sufﬁcient quantities

of the metal ion analyte to trigger the biological process under

investigation, so the photocage must possess a relatively high

afﬁnity before uncaging that is drastically reduced following

photolysis. Initially, the binding afﬁnity of ZinCast-1 was deter-

mined spectrophotometrically by titrating 25 μM ligand sol-

utions with Cu²⁺, Zn²⁺or Cd²⁺and monitoring the erosion of

the absorption band at λ ∼ 260 nm that corresponds to the metal-

binding aniline group. In aqueous solution (50 mM HEPES,

100 mM KCl, pH 7.0, 20% DMSO), ZinCast-1 exhibited a K_d

of 14 μM (log K = 4.9) and a photolysis quantum yield of 0.7%.

While the ΔK_dfor the Ca²⁺photocage Nitr-5 decreases ∼40-fold

Table 1 Metal binding properties of ZinCast-1^a

K_d

M²⁺

Solvent

[M(ZinCast-1)]²⁺

[M(ZinUnc-1)]²⁺

ΔK_d

ΔK

Zn²⁺

CH₃CN

EtOH

20% EtOH–buffer

20% DMSO–buffer

(9.7 0.4) × 10⁻⁶

(1.3 0.2) × 10⁻⁵

(6.1 0.1) × 10⁻⁵

(1.4 0.1) × 10⁻⁵

(9.7 0.2) × 10⁻⁶

(3.5 0.1) × 10⁻⁵

(5.6 0.1) × 10⁻³

1

3

92

1.0

1.1

1.8

2.1

400

Cu²⁺

Cd²⁺

CH₃CN

EtOH

20% EtOH–buffer

NA^b

1

2

NA^b

1

1.1

(7.2 0.1) × 10⁻⁶

(8.5 0.1) × 10⁻⁶

(7.2 0.3) × 10⁻⁶

(1.5 0.1) × 10⁻⁵

CH₃CN

EtOH

20% EtOH–buffer

(7.9 0.3) × 10⁻⁶

(1.7 0.1) × 10⁻⁵

(3.7 0.1) × 10⁻⁴

(3.1 0.1) × 10⁻⁵

(8.2 0.2) × 10⁻⁴

(1.8 0.1) × 10⁻²

4

48

49

1.1

1.5

2.0

^aThe 1 : 1 M–L binding constants were calculated with XLﬁt.^{14 b}Cu²⁺oxidizes aniline compounds in CH₃CN, so no binding constants could be

measured.

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complexes of both octahedral and tetrahedral geometry. Zn²⁺

complexes of the tridentate ligands bis(2-pyridylmethyl)amine

(BPA) and bis(2-pyridyl-2-ethyl)amine (BPEA) have been

characterized crystallographically.¹⁹The complexes adopt dis-

torted mononuclear octahedral and dinuclear tetrahedral struc-

tures respectively, where the open coordination sites are

occupied by water molecules and/or perchlorate anions. The dis-

tortion of the geometry in the BPA ligand structure is the result

of a small bite angle (N_py–Zn–N_amineangles of 79.99 and

79.69°) that leads to formation of a long N_amine–Zn bond

(N_amine–Zn 2.142 Å vs. N_py–Zn 2.080 Å). The N_amine–Zn bond

is shorter in the BPEA complex (N_amine–Zn 2.022 Å vs. N_py–Zn

2.012 Å) because of the tendency of six-membered chelate rings

to adopt a cyclohexane-like conformation (N_py–Zn–N_aniline

angles of 101.4 and 100.4°). While the Zn²⁺complex of the

hybrid N-[2-(pyridine-2-yl)ethyl]-N-(pyridine-2-ylmethyl)amine

has not been characterized structurally, the N_amine–M bond

should have an intermediate length between that of the BPA and

BPEA complexes. A systematic change in N_amine–Zn interaction

should provide a context for the ΔK values for different ZinCast

ligands, which utilize aniline analogs of these chelators. Metal

complexes with six-membered rings should experience a more

profound decrease in afﬁnity following photolysis due to greater

proximity of anilino nitrogen and the metal. Conformational

ﬂexibility of Zn²⁺as well as Cu²⁺allows for investigation of

different binding motifs in ZinCast photocages. The BPA

complex with Cu²⁺adopts distorted mononuclear square-pyrami-

dal geometry with all three nitrogen atoms coordinating and

chloride ions at the remaining two sites.²⁰High proximity of the

coordinating N_pywith an average distance of N_py–Cu of 2.006 Å

should induce high stability for Cu²⁺complexes as well as small

changes in afﬁnity upon photolysis.

The 5,5-ligand N-phenyldi(2-picolyl)aniline (10) and 6,6-

ligand N,N-bis[2-(pyridine-2-yl)ethyl]aniline (8) were prepared

by alkylation of aniline with picolylchloride hydrochloride and

vinyl pyridine, respectively. The asymmetric 5,6-ligand was pre-

pared by controlled addition of vinyl pyridine (6) to give N-[2-

(pyridine-2-yl)ethyl]aniline (9) followed by reductive amination

with 2-pyridinecarbaldehyde. ZinCast-1 can be prepared by the

classical TMSOTf-promoted (trimethylsilyl triﬂuoromethanesul-

fonate) electrophilic aromatic substitution reaction in 58.1%

yield after isolation and puriﬁcation (Scheme 1).¹¹The TMSOTf

methodology was used to prepare ZinCast-2 and ZinCast-3 in

79.6 and 70.9% yield, respectively. The lutidine base and

TMSOTf were added incrementally to maximize yields. All

three compounds were isolated as yellow solids after puriﬁcation

by ﬂash chromatography. ZinCast-1 also can be prepared by

either a benzophenone reduction strategy or a Pd-catalyzed

cross-coupling reaction described previously (Scheme 2).¹¹The

Pd-based methods are particularly convenient for making nitro-

benzhydrol derivatives that cannot be prepared with TMSOTf

methodology.

Fig. 2 Structure of ZinCast-2 and ZinCast-3. In ZinCast-1, the two

methylene linkers between the nitrogen atom and the pyridine ring lead

to the formation of 5,5-chelates with the guest metal ion. ZinCast-2 and

ZinCast-3 systematically replace the methylene spaces with ethylene

units that lead to 5,6- and 6,6-chelates upon metal ion binding.

Scheme 1 Synthesis of ZinCast photocages.

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8164 | Dalton Trans., 2012, 41, 8162–8174

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Scheme 2 Alternative synthetic pathways to ZinCast-1.

Table 2 pK_aof DMACast and DMAUnc^a

pK_a

Solvent

[H(DMACast)]⁺

[H(DMAUnc)]⁺

ΔpK_a

CH₃CN

EtOH

20% EtOH–buffer

4.8 0.1

4.0 0.1

2.5 0.2

∼1.5

1.8 0.2

1.9

2.7

2.2

^aValues obtained by UV-vis spectroscopy by monitoring changes at

∼265 nm for DMACast and 350 nm for DMAUnc.

Scheme 3 Synthesis of DMACast and DMAUnc.

Aniline basicity

Metal-binding properties of ZinCast photocages

The introduction of an electron withdrawing carbonyl group

upon uncaging modulates the electron density on the nitrogen

atom. The electron density correlates with the nitrogen atom’s

basicity, and insight into the variation would facilitate designing

photocages with improved properties. To quantify the changes,

the nitrobenzhydrol adduct of N,N-dimethylaniline (DMACast)

and the corresponding photoproduct (DMAUnc) were prepared.

DMACast was prepared using TMSOTf in 89.6% yield

(Scheme 3), and photolysis of DMACast with 1000 W xenon

lamp provided DMAUnc in 59.8% yield. DMACast allows

quantiﬁcation of the aniline basicity independent of the pyridine

ligands, which are susceptible to protonation. In aqueous solu-

tion dimethylanilinium and pyridinium ions exhibit pK_avalues

of 5.1 and 5.2, respectively.^21,22In protic solvents, the apparent

pK_aof the DMACast aniline nitrogen is 4.0, which decreases to

1.8 for DMAUnc as measured by monitoring changes in the

absorption spectrum at ∼265 nm (Table 2). The trend in pK_a

changes with DMACast is consistent with the metal ion unca-

ging behaviour of ZinCast-1, but the magnitude of change in

ΔpK_ais larger than that for Zn²⁺with the photocage (ΔK = 1.8)

under the same conditions. Differences are even more pro-

nounced in CH₃CN where ΔpK_afor DMACast is 1.9, but there is

no measurable change in log K for ZinCast. The results suggest

an important correlation between ligand geometric constraints,

metal–nitrogen bonding interactions and ΔK.

Metal binding between select divalent cations and the ZinCast

photocages were investigated spectrophotometrically in CH₃CN,

EtOH and aqueous 20% EtOH (50 mM HEPES, 100 mM KCl,

pH 7.0). Metal ion insolubility/hydrolysis combined with the

weak afﬁnity of the new photocages prevented acquisition of a

complete data set in aqueous solution and ethanol (Table 3). An

aniline-derived absorption band for [Zn(ZinCast-1)]²⁺was

observed at λ_max∼ 260 nm while the same absorption band for

[Zn(ZinCast-3)]²⁺was shifted to λ_max∼ 275 nm (Fig. 3). In

addition to bathochromic shifting of the aniline-derived absorp-

tion band, a new absorption band of low extinction coefﬁcient

(ε = 6600) at λ ∼ 355 nm developed. The erosion of these bands

upon the addition of metal ions allowed binding afﬁnity to be

quantiﬁed. Binding studies of ZinUnc-1 and ZinUnc-2 in

CH₃CN were carried out by monitoring of formation of the

absorption band at ∼290 nm and erosion of the benzophenone

charge transfer band at ∼345 nm. [Zn(ZinUnc-3)]²⁺exhibited a

bathochromic shift of λ_minto ∼360 nm and formation of two

absorption bands at ∼265 and ∼323 nm (Fig. 4).

Metal ions binding in ZinCast-1, ZinCast-2 and ZinCast-3

photocages and the corresponding photoproducts, lead to for-

mation of metal complexes with 5,5-, 5,6- and 6,6-chelate rings,

respectively. Large and medium sized metal ions typically favor

ﬁve-membered chelate rings with a small N–M–N angle (∼70°)

and long N–M bonds (∼2.5 Å), whereas small metal ions prefer

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Table 3 Metal binding properties of ZinCast-2 and ZinCast-3^a

K_d

M²⁺

Solvent

[M(ZinCast-2)]²⁺

[M(ZinUnc-2)]²⁺

ΔK_d

ΔK

[M(ZinCast-3)]²⁺

[M(ZinUnc-3)]²⁺

ΔK_d

ΔK

Zn²⁺

CH₃CN

EtOH

(8.6 0.3) × 10⁻⁶

(2.5 0.1) × 10⁻⁴

(4.8 0.1) × 10⁻⁵

NA^b

6

1.2

(3.3 0.1) × 10⁻⁵

(3.8 0.1) × 10⁻³

NA^b

115

1.9

NA^b

∼3.1 × 10⁻³

NA^b

Cu^{2+ c}

Cd²⁺

EtOH

(7.4 0.2) × 10⁻⁶

(1.1 0.1) × 10⁻⁵

1.5

1.0

(2.9 0.2) × 10⁻⁵

NA^b

CH₃CN

EtOH

(1.3 0.1) × 10⁻⁵

(6.5 0.1) × 10⁻⁴

(2.5 0.2) × 10⁻⁵

NA^b

1.9

1.1

(1.8 0.1) × 10⁻⁴

NA^b

^aThe 1 : 1 M–L binding constants were calculated with XLﬁt.^{14 b}M²⁺afﬁnity was too weak to be accurately assessed. Metal ion insolubility/

hydrolysis at higher concentrations precluded accurate determination of binding afﬁnity in aqueous conditions. ^cCu²⁺oxidizes aniline compounds in

CH₃CN, so no binding constants could be measured.

Fig. 4 Titration of 25 μM ZinUnc-1 (top) and ZinUnc-3 (bottom) with

Fig. 3 Titration of 25 μM ZinCast-1 (top) and ZinCast-3 (bottom) with

Zn(ClO₄)₂in CH₃CN. The absorbance was ﬁt to a 1 : 1 binding isotherm

Zn(ClO₄)₂in CH₃CN. The absorbance was ﬁtted to a 1 : 1 binding iso-

(inset). The error bars represent the variance in the measurements over

therm (inset). The error bars represent the variance in the measurements

three trials.

over three trials.

Comparison of bond distances in Zn²⁺complexes with BPA

and BPEA shows a more pronounced elongation of the N_aniline

–

six-membered chelate rings with larger N–M–N angles (∼100°)

and a short N–M bond (∼1.6 Å).²³While these conformations

reduce strain energy, they have a signiﬁcant effect on the stability

of the metal complexes. A medium size ion such as Zn²⁺will

prefer to coordinate a tridentate ligand with 5,5-chelate rings

such as DPA. As a consequence complex stabilities increase sys-

tematically 5,5 > 5,6 > 6,6 as predicted. Increasing the length of

the link between anilino nitrogen and pyridine has an overall

destabilizing effect on metal complexes of ZinCast photocages.

This effect is even more pronounced for the metal complexes of

ZinUnc which results in steadily increasing change in metal

afﬁnity (ΔK_d) upon uncaging.

Zn bond (Δ = 0.120 Å) than in N_py–Zn (Δ = 0.066 Å),¹⁹which

is consistent with similar [Zn(DPA)]²⁺structures.^24–26These

results suggest that the N_aniline–Zn bond in [Zn(ZinCast-1)]²⁺

might be longer than the N_aniline–Zn bond in [Zn(ZinCast-3)]²⁺

complex. Since stability of the studied photocaged complexes

decreases in opposing direction to the shortening of the N_aniline

–

Zn bond, we hypothesize the N_py–Zn interactions are more sig-

niﬁcant to complex stability in ZinCast-1. This behavior is also

consistent with the trends in ΔK_d. A longer N_aniline–Zn bond in

ZinUnc-1 would signiﬁcantly reduce the effect of decreased

electron availability after uncaging which leads to a small ΔK_d

8166 | Dalton Trans., 2012, 41, 8162–8174

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Table 4 Metal binding properties of ZinCast-4 and ZinCast-5^a

K_d

M²⁺

Solvent

[M(ZinCast-4)]²⁺

[M(ZinUnc-4)]²⁺

ΔK_d

ΔK

[M(ZinCast-5)]²⁺

[M(ZinUnc-5)]²⁺

ΔK_d

ΔK

Zn²⁺

EtOH

EtOH–buffer

(1.2 0.2) × 10⁻⁵

(1.1 0.3) × 10⁻⁵

(1.1 0.1) × 10⁻⁵

(5.6 0.1) × 10⁻⁵

1

4

1.0

1.2

(1.5 0.2) × 10⁻⁵

(1.4 0.3) × 10⁻⁵

NA^b

Cu²⁺

Cd²⁺

EtOH

(7.5 0.1) × 10⁻⁶

(8.2 0.1) × 10⁻⁶

1

1.0

(1.4 0.1) × 10⁻⁵

NA^b

EtOH

EtOH–buffer

(7.0 0.2) × 10⁻⁶

(9.8 0.1) × 10⁻⁶

(1.1 0.1) × 10⁻⁵

(4.9 0.1) × 10⁻⁵

2

4

1.0

1.2

(9.8 0.2) × 10⁻⁶

(1.3 0.1) × 10⁻⁵

NA^b

^aThe 1 : 1 M–L binding constants were calculated with XLﬁt.^{14 b}The ZinCast-5 photoproduct could not be isolated for analysis of its metal binding

properties.

and relatively high afﬁnity for Cu²⁺and Zn²⁺. In contrast, for

[Zn(ZinUnc-3)]²⁺where the N_aniline–M is predicted to be shorter,

the availability of the lone pair of electrons is enhanced. Sub-

sequent photolysis and delocalization of the lone pair on the car-

bonyl oxygen leads to more efﬁcient release of the coordinated

metal and large ΔK_d. Ligands with 5,5-chelates effectively coor-

dinate metal ion with all three nitrogen atoms. Inclusion of an

additional carbon in the backbone of the ligand enhances the

contribution of the N_aniline–Zn bond to complex stability.

Stability constants for complexes of ZinCast and ZinUnc

follow the hard soft acid base (HSAB) principle and conform to

the Irving–Williams series. All stability constants decrease

according to the following pattern: Cu²⁺> Zn²⁺> Cd²⁺,

however change in afﬁnity ΔK_dhas an opposing trend: Cu²⁺

< Zn²⁺< Cd²⁺. This pattern implies that as the afﬁnity for the

metal increases, coordination by the pyridyl nitrogen atoms

becomes more signiﬁcant. While the pK_aof the anilino nitrogen

is affected by photolysis of the all the ZinCast photocages, the

reduced nucleophilicity will only diminish cation afﬁnity if

the aniline nitrogen atom and the metal ion guest are in close

proximity (Table 4).

primarily on resonance interactions between the aniline lone pair

and the carbonyl oxygen formed upon photolysis, an analogous

resonance interaction between the aniline lone pair and the aci-

nitro intermediate generated upon exposure to light appears to

reduce the quantum yield of uncaging.²⁹The competing con-

siderations of high metal ion afﬁnity and maximizing the unca-

ging efﬁciency make the design of photocages that use this

release mechanism particularly challenging. Since inductive

effects might also contribute to changes in metal ion afﬁnity,

ZinCast-5, where the positions of the DPA and methoxy group

were exchanged, also was prepared for analysis (Scheme 4).

With the less electron-donating methoxy group in the para pos-

ition, we hypothesized that the quantum yield might be improved

without signiﬁcantly compromising the ΔK_d.

The binding afﬁnities of ZinCast-4 and ZinCast-5 solutions

were measured for the same metal ions and under the same con-

ditions as the original ZinCast photocages. Since ZinCast-1

exhibited no measurable ΔK_din CH₃CN, no analogous measure-

ments were performed for ZinCast-4 and ZinCast-5. While the

photoproduct of ZinCast-4 photocage, ZinUnc-4, could be syn-

thesized on a preparative scale to allow for direct determination

of metal ion afﬁnities, decomposition of ZinUnc-5 made iso-

lation of the photoproduct impossible. Unlike the ﬁrst-generation

ZinCast photocages, which exhibit much lower metal ion

afﬁnities in aqueous solution, the stability of ZinCast-4 and

ZinCast-5 complexes are insensitive to increasing solvent

polarity. This suggests that ZinCast-4 and ZinCast-5 are less sol-

vated than the original photocages, which leads to decreased

competition for binding.

Design of second-generation ZinCast photocages

While changing ZinCast-1 structure led to improvements in ΔK_d,

increasing the Zn²⁺afﬁnity while preserving a relatively high

ΔK_dis necessary to apply ZinCast photocages to problems in

biology. Cells typically maintain [Zn²⁺] concentration as sub-nM

levels, and an effective photocage cannot perturb this homeosta-

sis prior to uncaging. We reasoned that increasing the denticity

of ZinCast-1 by incorporating a weakly coordinating methyl

ether would lead to increased complex stability without compro-

mising metal ion release upon uncaging. The high afﬁnity

observed in structurally related ﬂuorescent sensors for Zn²⁺with

multiple additional chelating ligands suggest that additional

ligands appended to the ether oxygen might impair metal ion

release.^27,28The second-generation photocage ZinCast-4 that

incorporates an aryl ether ligand with the DPA ligand was pre-

pared by Pd-catalyzed cross-coupling between the boronic ester

of the ligand and the corresponding aldehyde in 43% yield

(Scheme 4).

While [Zn(ZinCast-4)]²⁺possess a six-fold increase in Zn²⁺

afﬁnity in aqueous solution compared to [Zn(ZinCast-1)]²⁺only

small changes in afﬁnity occur following photolysis. X-Ray crys-

tallography of an analogous model ligand suggests that any Zn–

O_etherinteraction in solution, if any, is probably weak since no

coordination is observed in the solid state (Fig. 5). Analogous

Cu²⁺complexes also exhibit no ether coordination.³⁰Instead of

coordination, the methoxy group may increase the electron

density of the aryl ring and therefore modulate the anilino nitro-

gen–carbonyl resonance interaction. While ΔK_dfor ZinCast-5

could not be determined, without a resonance interaction with

the aniline lone pair, it is unlikely the ΔK_dwould be large.

In contrast to the Zn²⁺complexes, Cd²⁺appears to beneﬁt from

coordination to the methoxy oxygen. The [Cd(ZinCast-4)]²⁺and

[Cd(ZinCast-5)]²⁺complexes possess stability constants that are

In all the Cast and Cast-like photocaged complexes described

to date, the aniline nitrogen occupies the position para to the

pendent caging group. Although metal ion release relies

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Scheme 4 Synthesis of ZinCast-4 and ZinCast-5.

Fig. 6 Photolysis of ZinCast-1 in CH₃CN. Irradiation at 350 nm leads

to the erosion of the absorption band a 270 nm associated with ZinCast-

1 and the concurrent formation of a band at 350 nm characteristic of the

benzophenone photoproduct ZinUnc-1.

Fig. 5 ORTEP diagram of ZinCast-4 model complex [Zn(23)]Cl₂

showing 50% thermal ellipsoids and selected atom labels. Hydrogen

atoms are omitted for clarity.

∼40 fold higher than the K_dfor [Cd(ZinCast-1)]²⁺in aqueous

solution. The major disadvantage of the increased stability for

these complexes is lack of signiﬁcant reduction in Cd²⁺afﬁnity

in the corresponding photoproducts. The pattern of enhanced

stability of metal-cage complex and consequential decrease in

ΔK_dseen for the discussed Cd²⁺complexes supports the hypoth-

esis that the addition of strongly coordinating pending groups to

ZinCast photocages would signiﬁcantly compromise Zn²⁺ion

release efﬁciency.

The uncaging process can be quantiﬁed by monitoring changes

in absorption of ZinCast and the corresponding photoproduct

ZinUnc (Fig. 6). Quantum yields (Φ) were determined by irra-

diating 25 μM solutions of free ZinCast photocages in three

solvent systems with a 150 W source (λ = 350 nm). Measure-

ment of changes at λ_max∼ 350 nm due to formation of ZinUnc

as well as calibration of the light intensity by iron oxalate actino-

metry allowed for calculation of Φ for the conversions. All three

ZinUnc derivatives were prepared in bulk by photolysis of the

ZinCast photocages in CH₃CN followed by column chromato-

graphy (Scheme 1). Working with ZinUnc on a preparative scale

allows for accurate determination of extinction coefﬁcients as

well as metal binding constants.

Uncaging of ZinCast photocages

The efﬁciency of the light-induced conversion of nitrobenzhydro

l based photocages of ZinCast-1, -2, -3 into the corresponding

nitrosobenzophenones can be evaluated spectrophotometrically.

The Φ obtained for all three apo ZinCast derivatives were

<1% in CH₃CN and buffered aqueous solution, but slightly

higher in EtOH (Table 5). In the presence of Zn²⁺, the Φ of

8168 | Dalton Trans., 2012, 41, 8162–8174

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Table 5 Quantum yields of ZinCast and DMACast compounds.

Solvents not shown when values could not be determined because of

Zn²⁺ion insolubility/hydrolysis

Φ_photolysisε_{λ=350 nm}

(%)

/

Efﬁciency,

Compound

ZinCast-1

Solvent

cm⁻¹M⁻¹Φε

CH₃CN

EtOH

0.7 0.3 5300

1.6 0.7 5000

EtOH–buffer 0.7 0.1 5600

37

80

39

ZinCast-2

ZinCast-3

DMACast

CH₃CN

EtOH

0.5 0.1 5400

1.5 0.2 5200

27

78

20

Scheme 5 Synthesis of m-OMeCast and m-DMACast.

EtOH–buffer 0.4 0.1 5100

of the methoxy group. m-DMACast with an electron donating

nitrogen in meta position experiences the lowest Φ (0.3 0.1);

however, like ZinCast-5, m-DMACast undergoes a secondary

reaction process following photolysis that makes direct compari-

sons of Φ difﬁcult. Substitution of dimethylamine for a methoxy

group leads to increase in Φ (0.8 0.2), which correlates with

previously observed trend in Φ change as a function of the sub-

stituent’s electron donating ability. UV-Vis spectra of ﬁrst and

second generation photocages share the same spectral features

with absorption features at ∼255, 300 and 345 nm. A stronger

absorption of light at 255 nm may account for the larger Φ

measured for ZinCast-5 and m-OMeCast, while increased

absorption at λ ∼ 300 nm seems to be associated with the

unstable photoproducts of ZinCast-5 and m-DMACast that

undergo rapid secondary reactions. The two photocages with

aniline groups in the meta position initially appear to form nitro-

sobenzophenone based photoproducts initially, but instability of

these compounds leads to the formation of unidentiﬁed ﬂuor-

escent compounds.

CH₃CN

EtOH

0.6 0.1 5600

1.5 0.1 5200

34

78

11

EtOH–buffer 0.2 0.1 5700

CH₃CN

EtOH

0.6 0.1 5500

1.5 0.1 5200

33

78

36

EtOH–buffer 0.6 0.1 6000

[Zn(ZinCast-1)]²⁺CH₃CN

EtOH

0.3 0.1 5400

2.2 0.2 5100

16

112

28

EtOH–buffer 0.5 0.3 5700

[Zn(ZinCast-2)]²⁺CH₃CN

0.4 0.2 5600

1.8 0.2 5300

22

95

EtOH

[Zn(ZinCast-3)]²⁺CH₃CN

1.2 0.2 6300

76

[H(DMACast)]⁺CH₃CN

EtOH

5.1 0.6 6600

1.9 0.1 5300

337

101

ZinCast-3 doubled in CH₃CN. The low efﬁciency of photolysis

in nitrobenzhydroyl photocaged complexes has been attributed

to a resonance interaction between the anilino nitrogen and aci-

nitro intermediate, which perturbs the uncaging process.^12,29The

predicted short N_aniline–Zn bond in [Zn(ZinCast-3)(H₂O)₂]²⁺

would make the aniline lone pair less likely to engage in reson-

ance interactions. A decrease in resonance interaction is consist-

ent with the higher Φ measured for the complex. The same

reasoning provides insight into the Φ obtained for DMACast and

[H(DMACast)]⁺, where the addition of HCl increases photolysis

efﬁciency 7-fold. Like metal ion binding, protonation of the

anilino nitrogen would engage the aniline lone pair in a bonding

interaction inhibiting its engagement in resonance interactions.

While formation of photoproducts was followed with UV-Vis

spectroscopy for the ﬁrst-generation compounds initially, quanti-

tative analysis of photolysis Φ was repeated by HPLC. The

average Φ for ZinCast-1, -2 and -3 is the same as the Φ obtained

for ZinCast-4 (0.6 0.2), suggesting that addition of methoxy

group to ZinCast-1 has minimal inﬂuence on photolysis

efﬁciency. Changing the position of the methoxy group from

meta to para appears to reduce resonance with the aci-nitro inter-

mediate and increases the quantum yield. ZinCast-5 possesses

the highest quantum yield of all the photocages screened

(Φ = 1.0 0.1).

Conclusion

We have synthesized three ZinCast photocages based on triden-

tate ligands of bis(pyridin-2-ylmethyl)aniline ligand (5,5 chelate

ring), N-[2-(pyridine-2-yl)ethyl]-N-(pyridine-2-ylmethyl)aniline

(5,6 chelate ring) and N,N-bis[2-(pyridine-2-yl)ethyl]aniline to

study how size of the chelate rings effect metal ion binding

afﬁnity. While the 5,5 chelate rings impart complex stability, the

tighter binding also compromises Zn²⁺release upon uncaging.

ZinCast-3, which utilizes the 6,6-metal chelate, exhibits the

biggest changes in afﬁnity upon photolysis. We hypothesize that

a shorter N_amine–M bond in ZinCast-3 induces greater depend-

ability of the complex on the lone pair of electrons of the anilino

nitrogen. Delocalization of this pair of electrons onto carbonyl

oxygen in the photoproduct signiﬁcantly reduces stability of the

metal complex and leads to larger ΔK_d.

Addition of methoxy ligands to the ZinCast-1 affords ZinCast-

4 and ZinCast-5; however, no signiﬁcant perturbation in the

metal binding afﬁnity or the efﬁciency of photolysis was

observed. The latter conclusion is supported by investigation into

the photochemistry of the model complexes m-OMeCast and m-

DMACast and DMACast that all uncage with similar quantum

yields. While the low quantum yield and weak afﬁnity compro-

mise the ability of these ZinCast ligands to buffer Zn²⁺under

physiologically relevant conditions, these studies provide an

important roadmap for designing photocaged complexes with

To further understand the contribution of the resonance

interaction between the aci-nitro intermediate and differently

substituted aryl groups to quantum yields, m-OMeCast and

m-DMACast were prepared by Pd cross-coupling reaction from

the corresponding boronic acids (Scheme 5). Changes in Φ for

all four photocages vary slightly depending on relative position

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predictable properties. The application of these ﬁndings to the

construction of photocages for biological applications is ongoing.

120.9 112.3, 110.4, 108.4, 71.5, 57.1, 56.6, 52.1, 36.0 ppm. IR

(KBr) 3100, 3008, 2934, 1612, 1591, 1571, 1516, 1470, 1435,

1392, 1329, 1270, 1206, 1179, 1150, 1063, 987, 815, 796,

748 cm⁻¹. HRMS (+ESI): Calc. for MH⁺, 501.2138. Found,

501.2107.

Experimental

(4-{Bis[2-(pyridine-2-yl)ethyl]amino}phenyl)(4,5-dimethoxy-2-nitro-

phenyl)methanol (4, ZinCast-3). Synthesis of ZinCast-3 followed

the same procedure as ZinCast-1 using 8 (0.188 g, 0.620 mmol),

11 (0.170 g, 0.805 mmol), 2,6-lutidine (0.11 mL, 0.95 mmol),

TMSOTf (0.15 mL, 0.83 mmol), 1 M TBAF in THF (1.86 mL,

1.86 mmol). The product was puriﬁed on basic alumina

(1 : 4 hexanes–EtOAc) affording orange solid (0.226 g, 70.9%).

Synthesis

General experimental. All materials were in the highest purity

commercially available from Acros Organic or TCI America.

Solvents were sparged with argon and dried in Seca Solvent

Puriﬁcation System. N-Phenyldi(2-picolyl)aniline (10),¹¹

N-[2-(pyridine-2-yl)ethyl]aniline (7),³¹N,N-bis[2-(pyridine-2-

yl)ethyl]aniline (8),³²and [4-(dimethylamino)phenyl](4,5-

dimethoxy-2-nitrophenyl)methanol (20, DMACast)²⁹were

prepared according to known procedures. All chromatography

and TLC were performed on silica from Silicycle or activated

1

Mp 55–57 °C. H NMR (CDCl₃, 400 MHz) δ 8.50 (2 H, d, J =

4 Hz), 7.59 (1 H, s), 7.51 (2 H, t, J = 8 Hz), 7.46 (1 H, s), 7.16

(2 H, d), 7.07 (4 H, m), 6.71 (2 H, d, J = 8 Hz), 6.49 (1 H, s),

3.97 (3 H, s), 3.92 (3 H, s), 3.61 (4 H, t, J = 12 Hz), 3.03 (1 H,

s), 2.95 (4 H, t, J = 12 Hz). ¹³C NMR (CDCl₃, 100 MHz) δ

159.9, 153.7, 149.8, 148.0, 147.4, 140.3, 136.7, 135.5, 129.9,

128.8, 123.8, 121.7, 112.2, 110.5, 108.5, 71.6, 56.8, 56.7, 51.5,

36.2 ppm. IR (KBr) 3105, 2933, 1611, 1517, 1436, 1358, 1329,

1270, 1207, 1177, 1063 cm⁻¹. HRMS (+ESI): Calc. for MH⁺,

515.2294. Found, 515.2288.

basic alumina from Acros Organics. H and ¹³C NMR spectra

1

were recorded with Bruker 400 MHz spectrometer and refer-

enced to CDCl₃. IR spectra were recorded on a Nicolet 205

FT-IR instrument and samples were analyzed as KBr pellets.

High-resolution mass spectra were obtained on micromass

Q-Tof-2^TMmass spectrometer operating in positive ion mode.

The instrument was calibrated with Glu-ﬁbrinopeptide B 10

pmol μL⁻¹by using a 50 : 50 solution of CH₃CN–H₂O with

0.1% acetic acid.

N-[2-(Pyridine-2-yl)ethyl]-N-(pyridine-2-ylmethyl)aniline (9). 2-

Pyridinecarbaldehyde (0.55 mL, 4.88 mmol) was added to a stir-

ring solution of 7 (0.645 g, 3.25 mmol) in 30 mL of dichlor-

oethane. After 30 min, NaBH(OAc)₃(1.03 g, 4.88 mmol) and

an additional 30 mL of dichloroethane was added. The reaction

mixture was stirred for 12 h at room temperature, the solvent was

removed under reduced pressure, 60 mL of water was added and

the pH of the aqueous mixture was adjusted to between 8–9 with

5 M NaOH. The product was extracted into CH₂Cl₂(3 × 60 mL)

and the combined organic fractions were dried over anhydrous

Na₂SO₄, ﬁltered and the solvent was removed. Flash chromato-

graphy on silica with a solvent gradient (EtOAc to 50 : 1

EtOAc–CH₃OH) yielded the product as a clear yellow oil

[4-(Bis-pyridin-2-ylmethylamino)phenyl](4,5-dimethoxy-2-nitrophenyl)

methanol (1, ZinCast-1). Trimethylsilyl triﬂuoromethanesulfonate

(TMSOTf) (1.25 mL, 6.88 mmol) was added dropwise to a

mixture of 4 (0.373 g, 1.36 mmol), 6-nitroveratraldehyde (11,

0.372 g, 1.76 mmol) and 2,6-lutidine (0.94 mL, 8.13 mmol) in

degassed CH₂Cl₂. The resulting solution was stirred for 12 h.

Tetrabutylammonium ﬂuoride (TBAF) in THF (1 M, 4.30 mL,

15.4 mmol) was added to the reaction mixture. After 30 min the

solvent was removed under reduced pressure and product was

puriﬁed on basic alumina with EtOAc affording a yellow solid

(0.383 g, 58.1%). Mp 113–114 °C. ¹H NMR (CDCl₃, 400 MHz)

δ 8.55 (2 H, d, J = 4.0 Hz), 7.60 (3 H, m), 7.41 (1 H, s), 7.21

(2 H, d, J = 8.0 Hz), 7.15 (2 H, t, J = 8.0 Hz), 7.09 (2 H, d, J =

12.0 Hz), 6.61 (2 H, d, J = 8.0 Hz), 6.47 (1 H, s), 4.76 (4 H, s),

3.95 (3 H, s), 3.92 (3 H, s), 2.93 (1 H, s). ¹³C NMR (CDCl₃,

100 MHz) δ 158.6, 153.4, 149.7, 147.9, 147.7, 139.9, 136.9,

134.8, 130.8, 128.4, 122.1, 120.8, 112.4, 110.1, 108.1, 77.3,

71.3, 57.3, 56.4, 56.3 ppm. IR (KBr) 3285, 2939, 2848,

1610–1570, 1517, 1473–1463, 1439, 1380, 1353, 1326, 1266,

1219, 1181, 1154, 1061 cm⁻¹. HRMS (+ESI): Calc. for MH⁺,

487.1981. Found, 487.1946.

(4,5-Dimethoxy-2-nitrophenyl)(4-{[2-(pyridine-2-yl)ethyl](pyridine-

2-ylmethyl)amino}phenyl)methanol (3, ZinCast-2). Synthesis of

ZinCast-2 followed the same procedure as ZinCast-1 using 9

(0.58 g, 2.0 mmol), 11 (0.55 g, 2.6 mmol), lutidine (0.35 mL,

3.0 mmol), TMSOTf (0.47 mL, 2.59 mmol), 1 M TBAF in THF

(6.00 mL, 6.00 mmol). The product was puriﬁed on basic

alumina with neat EtOAc affording a yellow solid (0.801 g,

79.6%). Mp 60–62 °C. ¹H NMR (CDCl₃, 400 MHz) δ 8.49

(2 H, d, J = 4 Hz), 7.52 (4 H, m), 7.08 (6 H, m), 6.62 (2 H, d,

J = 8 Hz), 6.46 (1 H, s), 4.49 (2 H, s), 3.95 (3 H, s), 3.91

(3 H, s), 3.84 (2H, t, J = 8 Hz), 3.08 (3 H, m). ¹³C NMR

(CDCl₃, 100 MHz) δ 159.5, 159.3, 153.6, 149.7, 147.9, 147.7,

140.2, 136.9, 136.7, 135.2, 130.4, 128.6, 123.8, 122.1, 121.7,

1

(0.801 g, 85.2%). H NMR (CDCl₃, 400 MHz) δ 8.54 (2 H, d,

J = 4 Hz), 7.52 (2 H, m, J = 8 Hz), 7.18 (2 H, t, J = 8 Hz), 7.11

(4 H, m), 6.70 (3 H, m), 4.56 (2 H, s), 3.90 (2 H, t, J = 8 Hz),

3.15 (2 H, t, J = 8 Hz). ¹³C NMR (CDCl₃, 100 MHz) δ 159.7,

159.6, 149.8, 149.7, 147.9, 136.9, 136.6, 129.5, 123.8, 122.0,

121.6, 120.8, 116.8, 112.4, 57.2, 52.0, 36.1 ppm. IR (KBr)

3061, 3008, 2926, 1590, 1570, 1506, 1472, 1433, 1390, 1353,

1273, 1244, 1198, 1162, 1147, 1049, 989, 943, 747, 694 cm⁻¹

HRMS (+ESI): Calc. for MH⁺, 312.1477. Found, 312.1463.

.

[4-(Bis-pyridin-2-ylmethylamino)phenyl](3,4-dimethoxyphenyl)

methanone (15). A 10 g potion of 84% polyphosphoric acid

(PPA) was added to a suspension of 2,3-dimethoxybenzoic acid

(14, 1.09 g, 5.99 mmol) in 5 mL of CH₂Cl₂, and the resulting

mixture was heated to 80 °C. Compound 3 (1.65 g, 5.99 mmol)

was added to the red reaction mixture, which was stirred for an

additional 2 h at 80 °C. The dark red reaction mixture was

cooled to 0 °C and the PPA was neutralized by adding 6 M

NaOH dropwise until the solution reached a pH of 9–10. The

product was extracted into CH₂Cl₂(3 × 50 mL) and the com-

bined organic extracts were dried over Mg₂SO₄. Flash chromato-

graphy on basic alumina with diethyl ether afforded 4 as a white

solid (2.24 g, 84.9%). Mp 135–136 °C. ¹H NMR (CDCl₃,

400 MHz) δ 8.60 (2 H, d, J = 4.0), 7.70 (2 H, d, J = 8.0 Hz),

7.64 (2 H, t, J = 8.0 Hz), 7.37 (1 H, s), 7.32 (1 H, d, J = 8.0

8170 | Dalton Trans., 2012, 41, 8162–8174

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Hz), 7.24 (2 H, d, J = 8.0 Hz), 7.20 (2 H, t, J = 8.0 Hz), 6.85

(1 H, d, J = 8.0 Hz), 6.75 (2 H, d, J = 8.0 Hz, ArH), 4.90

(4 H, s, N–CH₂–Py), 3.92 (3 H, s, OCH₃), 3.90 (3 H, s, OCH₃).

¹³C NMR (CDCl₃, 100 MHz) δ 194.1, 157.9, 152.3, 151.8,

150.1, 149.0, 137.1, 132.7, 131.6, 127.0, 124.5, 122.5, 120.9,

112.5, 111.7, 109.9, 57.4, 56.2 ppm. IR (KBr) 3076, 3007,

2963, 2834, 1625, 1461, 1437, 1414, 1390, 1365, 1340, 1321,

115.1, 78.6, 57.6 ppm. IR (KBr) 3048, 3006, 2927, 1862, 1585,

1494, 1469, 1435, 1383, 1357, 1278, 1254, 1235, 1195, 1177,

1151, 1145, 1090, 1049, 1043, 665, 545, 458 cm⁻¹. HRMS

(+ESI): Calc. for MH⁺, 402.0467. Found, 402.0458.

Bis-pyridin-2-ylmethyl[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-

2-yl)phenyl]amine (18). Compound 5 (0.307 g, 0.766 mmol),

KOAc (0.226 g, 2.30 mmol), PdCl₂(4.1 mg, 3 mol%), PPh₃

(12.1 mg, 6 mol%), bis(pinacolato)diboron (B(Pin)₂, 0.214 g,

0.843 mmol) and DMSO (10 mL) were combined in a Schlenk

tube. After performing three freeze–pump–thaw cycles, the tube

was ﬁlled with N₂, sealed and heated at 80 5 °C for 12 h. The

mixture was cooled, diluted with H₂O (25 mL) and extracted

with EtOAc (3 × 25 mL) and dried with Na₂SO₄. Flash chrom-

atography on alumina (1 : 1 petroleum ether–EtOAc) yielded 6 as

a white solid (0.162 g, 52.8%). Mp 201–202 °C. ¹H NMR

(CDCl₃, 400 MHz) δ 8.59 (2 H, d, J = 4.0 Hz), 7.63–7.58 (4 H,

m), 7.20 (2 H, d, J = 8.0 Hz), 7.17 (2 H, t, J = 8.0 Hz), 6.69 (2 H,

d, J = 8 Hz), 4.85 (4 H, s), 1.29 (12 H, s). ¹³C NMR (CDCl₃,

100 MHz) δ 158.6, 150.7, 150.0, 137.1, 136.6, 122.3, 120.9,

111.9, 83.5, 77.6, 57.2 ppm. IR (KBr) 616, 652, 750, 810, 964,

1038, 992, 958, 941, 877, 841, 818, 702, 627, 510, 406 cm⁻¹

HRMS (+ESI): Calc. for MH⁺, 440.1974. Found, 440.2010.

.

[4-(Bis-pyridin-2-ylmethylamino)phenyl](4,5-dimethoxy-2-nitro-

phenyl)methanone (16). Cu(NO₃)₂·2.5H₂O (1.27 g, 5.46 mmol)

was added to a solution of 15 (1.60 g, 3.64 mmol) in acetic

anhydride (10 mL) at 0 °C and the reaction mixture was stirred

for 12 h. Ice and 25 mL of 0.4 M EDTA were added to the sol-

ution and the pH was adjusted to 9 with 6 M NaOH. The

product was extracted into CH₂Cl₂(3 × 50 mL) and the com-

bined organic extracts were dried over Mg₂SO₄. Flash chromato-

graphy on alumina (17 : 3 CH₂Cl₂–EtOAc) followed by

recrystallization from toluene yielded 16 as a white solid (1.00 g,

1

56.7%). Mp 200–201 °C. H NMR (CDCl₃, 400 MHz) δ 8.59

(2 H, d, J = 4.0 Hz), 7.69 (1 H, s), 7.64 (2 H, t, J = 8.0 Hz),

7.58 (2 H, d, J = 12.0 Hz), 7.19 (4 H, m), 6.78 (1 H, s), 6.69

(2 H, d, J = 12.0 Hz), 4.87 (4 H, s), 3.99 (3 H, s), 3.92 (3 H, s).

¹³C NMR (CDCl₃, 100 MHz) δ 191.7, 157.5, 154.0, 152.8,

150.2, 149.4, 137.2, 131.9, 131.6, 125.5, 122.6, 120.9, 112.0,

110.2, 107.1, 57.3, 56.8, 56.7 ppm. IR (KBr) 3076, 3007, 2963,

2834, 1647, 1592–1570, 1548, 1522, 1453–1440, 1408–1390,

1340, 1284, 1265, 1221, 1196, 1190, 1182, 1145, 1065, 933,

871, 824, 753, 614 cm⁻¹. HRMS (+ESI): Calc. for MH⁺,

485.1825. Found, 485.1780.

[4-(Bis-pyridin-2-ylmethylamino)phenyl](4,5-dimethoxy-2-nitrophenyl)

methanol (ZinCast-1, 1). Compound 5 (0.875 g, 1.99 mmol) was

dissolved in CH₃CN, chilled to 0 °C and NaBH₄(753 mg,

1.99 mmol) was added slowly over a period of 1–2 min. After

the addition was complete, the reaction mixture was slowly

heated to 70 °C over a period of 2 h and stirred for an additional

12 h. The reaction mixture was diluted into 10 mL of ice water

and 1 mL of conc. HCl, and the pH was adjusted to 9 with 6 M

NaOH. The product was extracted into CH₂Cl₂(3 × 30 mL) and

the combined organic extracts were dried over Mg₂SO₄. Flash

chromatography on alumina (4 : 6 CH₂Cl₂–EtOAc) followed by

recrystallization from toluene–hexanes yielded ZinCast-1 as ﬁne

yellow crystals (0.324 g, 33.5%).

(4-Iodophenyl)bis-pyridin-2-ylmethylamine (17). Compound 10

(0.507 g, 1.84 mmol) was dissolved in CH₃CN (10 mL) and

chilled to 0 °C. [K(18-crown-6)]ICl₂(0.970 g, 1.94 mmol) was

added and the reaction mixture was stirred at room temperature

for 30 min. After removing the solvent, the reaction residue was

diluted with 25 mL of water and the pH was adjusted to 9 with 6

M NaOH. The product was extracted into EtOAc (3 × 25 mL),

washed with saturated NaHSO₃(25 mL) and brine (3 × 25 mL)

the combined organic extracts were dried over MgSO₄. Flash

chromatography on alumina (1 : 1 hexanes–EtOAc) followed by

recrystallization from hexanes yielded 5 as pale yellow needles

(0.687 g, 93.0%). Mp 104–105 °C. ¹H NMR (CDCl₃, 400 MHz)

δ 8.59 (2 H, d, J = 8.0 Hz), 7.62 (2 H, t, J = 8.0 Hz), 7.39 (2 H,

d, J = 8.0 Hz), 7.22 (2 H, d, J = 4.0 Hz), 7.18 (2 H, t, J = 8.0

Hz), 6.49 (2 H, d, J = 8.0 Hz), 4.79 (4 H, s). ¹³C NMR (CDCl₃,

100 MHz) δ 158.5, 150.1, 148.0, 138.1, 137.1, 122.4, 121.0,

991, 1141, 1372, 1470, 1606, 2928, 2973, 3046, 3082 cm⁻¹

HRMS (+ESI): Calc. for MH⁺, 402.2353. Found, 402.2337.

.

[4-(Bis-pyridin-2-ylmethylamino)phenyl](4,5-dimethoxy-2-nitrophenyl)

methanol (1, ZinCast-1). Compound 6 (0.150 g, 0.374 mmol), 11

(79.0 mg, 0.374 mmol), tris-1-naphthylphosphine (P(Nap)₃),

15.4 mg, 10 mol%), PdCl₂(6.6 mg, 10 mol%), K₂CO₃(0.155 g,

1.12 mmol) and THF (10 mL) were combined in a Schlenk tube.

The tube was sealed and mixture was stirred at 60 5 °C for

60 h. Solvent was removed under reduced pressure and to the

resulting dark brown residue was added 50 mL of water, the

mixture was extracted with CH₂Cl₂and dried with MgSO₄.

Flash chromatography on alumina with EtOAc provided

ZinCast-1 as ﬁne yellow crystals (95.2 mg, 52.3%).

2-Methoxy-N,N-bis(pyridine-2-ylmethyl)aniline (23). Synthesis

of 23 followed the same procedure as 9 using anisidine (22,

2.23 g, 18.2 mmol), pyridinecarboxyl aldehyde (6.14 mL,

54.6 mmol), NaBH(OAc)₃(11.5 g, 54.6 mmol). The product

was puriﬁed on basic alumina (3 : 7 EtOAc–hexanes) affording a

1

white solid (4.65 g, 83.8%). Mp 76–77 °C. H NMR (CDCl₃,

400 MHz) δ 8.46 (2 H, d, J = 4 Hz), 7.53 (2 H, t, J = 8 Hz),

7.47 (2 H, d, J = 8 Hz), 7.05 (2 H, t, J = 4 Hz), 6.86 (3 H, m),

6.72 (1 H, m), 4.50 (4 H, s), 3.82 (3 H, s). ¹³C NMR (CDCl₃,

100 MHz) δ 159.9, 152.8, 149.0, 139.3, 136.4, 122.5, 122.4,

121.8, 120.9, 120.8, 111.8, 58.8, 55.6 ppm. IR (KBr) 2840,

1590, 1569, 1502, 1471, 1462, 1436, 1432, 1427, 1352, 1240,

1179, 1154, 1029 cm⁻¹. HRMS (+ESI): Calc. for MH⁺,

306.1606. Found, 306.1618.

4-Bromo-2-methoxy-N,N-bis(pyridine-2-ylmethyl)aniline (24).

Compound 23 (0.52 g, 1.7 mmol) was dissolved in CH₃CN

(10 mL) and [K(18-crown-6)]Br₃(1.02 g, 1.9 mmol) was added

and the reaction mixture was stirred at room temperature for 2 h.

After removing the solvent, the reaction residue was diluted with

25 mL of water and the pH was adjusted to 9 with 6 M NaOH.

The product was extracted into EtOAc (3 × 25 mL), washed with

saturated NaHSO₃(25 mL) and brine (25 mL) the combined

organic extracts were dried over MgSO₄. Flash chromatography

on alumina (3 : 7 EtOAc–hexanes) yielded 24 as white solid

(0.638 g, 97.1%). Mp 130–131 °C. ¹H NMR (CDCl₃, 400 MHz)

δ 7.47 (2 H, d, J = 4 Hz), 7.54 (2 H, d, J = 8 Hz), 7.41 (2 H, d,

Dalton Trans., 2012, 41, 8162–8174 | 8171

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J = 8 Hz), 7.07 (2 H, t, J = 8 Hz), 6.94 (1 H, s), 6.82 (1 H, d, J =

8 Hz), 6.70 (1 H, d, J = 8 Hz), 4.46 (4 H, s), 3.81 (3 H, s). ¹³C

NMR (CDCl₃, 100 MHz) δ 159.3, 153.5, 149.1, 138.5, 136.5,

123.6, 122.3, 122.1, 122.0, 115.2, 114.6, 58.7, 55.9 ppm. IR

(KBr) 2840, 1589, 1568, 1496, 1476, 1457, 1444, 1398, 1375,

58.5, 55.7 ppm. IR 1589, 1569, 1494, 1462, 1433, 1402, 1375,

1296, 1239, 1178, 1148, 1094, 1047, 1024, 994, 943, 798,

761 cm⁻¹. HRMS (+ESI): Calc. for MH⁺, 432.0573. Found,

432.0572.

2-Methoxy-N,N-bis(pyridine-2-ylmethyl)-5-(tetramethyl-1,3,2-

dioxoborolan-2-yl)aniline (29). Synthesis of 29 followed the

same procedure as compound 18 using 28 (0.20 g, 0.46 mmol),

KOAc (0.14 g, 1.38 mmol), PdCl₂(2.5 mg, 3 mol%), PPh₃

(7.3 mg, 6 mol%), B(Pin)₂(0.13 g, 0.51 mmol). The product

was puriﬁed on basic alumina (6 : 13 EtOAc–hexanes) affording

1351, 1322, 1273, 1237, 1191, 1176, 1153, 1121, 1025 cm⁻¹

.

HRMS (+ESI): Calc. for MH⁺, 384.0711. Found, 384.0736.

2-Methoxy-N,N-bis(pyridine-2-ylmethyl)-4-(tetramethyl-1,3,2-

dioxaborolan-2-yl)aniline (25). Compound 24 (0.51 g,

1.3 mmol), KOAc (0.0.39 g, 3.9 mmol), Pd₂dba₃(15 mg, 2 mol

%), DavePhos (21 mg, 4 mol%), B(Pin)₂(1.01 g, 3.9 mmol) and

dioxane (15 mL) were combined in a Schlenk tube. After per-

forming three freeze–pump–thaw cycles, the tube was ﬁlled with

N₂, sealed and heated at 110 5 °C for 48 h. The mixture was

cooled, washed with H₂O (2 × 15 mL) and dried with Na₂SO₄.

Flash chromatography on alumina (2 : 3 EtOAc–hexanes)

1

yellow solid (0.45 g, 51%). Mp 151–152 °C. H NMR (CDCl₃,

400 MHz) δ 8.45 (2 H, d, J = 8 Hz), 7.52 (4 H, m), 7.40 (2 H,

m), 7.04 (2 H, m), 6.85 (1 H, d, J = 8 Hz), 4.49 (4 H, s), 3.83

(3 H, s), 1.25 (12 H, s). ¹³C NMR (CDCl₃, 100 MHz) δ 160.1,

156.0, 148.9, 139.1, 136.4, 130.4, 127.7, 122.5, 121.7, 111.1,

83.6, 58.9, 55.5, 25.0 ppm. IR (KBr) 3060, 2975, 2931, 2837,

1571, 1510, 1472, 1381, 1354, 1316, 1296, 1270, 1238, 1198,

1175, 1141, 1125, 1101, 1054, 1043, 1023, 996, 963, 949, 887,

851, 813, 773, 759. HRMS (+ESI): Calc. for MH⁺, 432.2458.

Found, 432.2434.

1

yielded 25 as white solid (0.549 g, 96.5%). Mp 98–99 °C. H

NMR (CDCl₃, 400 MHz) δ 7.47 (2 H, d, J = 8 Hz), 7.53 (2 H, t,

8 Hz), 7.41 (2 H, d, J = 12 Hz), 2.23 (2 H, m), 7.06 (2 H, t, J =

8 Hz), 6.80 (1 H, d, J = 8 Hz), 4.56 (4 H, s), 3.84 (3 H, s), 1.28

(12 H, s). ¹³C NMR (CDCl₃, 100 MHz) δ 159.6, 151.6, 149.0,

142.2, 136.5, 128.3, 122.2, 121.9, 119.4, 117.4, 100.1, 83.7,

58.5, 55.7, 25.0 ppm. IR (KBr) 2975, 1602, 1591, 1568, 1513,

1472, 1432, 1408, 1382, 1372, 1348, 1273, 1231, 1175, 1157,

1142, 1103, 1033, 965, 854, 769, 687 cm⁻¹. HRMS (+ESI):

Calc. for MH⁺, 432.2458. Found, 432.2448.

{3-[Bis( pyridine-2-ylmethyl)amino]-4-methoxyphenyl}(4,5-

dimethoxy-2-nitrophenyl)methanol (30, ZinCast-5). Compound 29

(0.35 g, 0.82 mmol), 11 (0.17 g, 0.82 mmol), P(Nap)₃(34 mg,

10 mol%), PdCl₂(15 mg, 10 mol%), K₂CO₃(0.40 g,

2.46 mmol). The product was puriﬁed on basic alumina

(1 : 9 hexanes–EtOAc) affording yellow solid (0.12 g, 30%). Mp

1

{4-[Bis(lyridin-2-ylmethyl)amino]-3-methoxyphenyl}(4,5-dimethoxy-

2-nitrophenyl)methanol (26, ZinCast-4). Synthesis of ZinCast-4

followed the same procedure as ZinCast-1. Compound 26

(1.00 g, 2.32 mmol), 11 (0.49 g, 2.32 mmol), P(Nap)₃(96 mg,

10 mol%), PdCl₂(41 mg, 10 mol%), K₂CO₃(0.96 g,

6.96 mmol). The product was puriﬁed on silica (1 : 19 CH₃OH–

EtOAc) affording yellow solid (0.509 g, 43.0%). Mp

135–136 °C. H NMR (CDCl₃, 400 MHz) δ 8.39 (2 H, d, J =

8 Hz), 7.50 (3 H, t, J = 8 Hz), 7.37 (2 H, d, J = 8 Hz), 7.12

(1 H, s), 7.03 (2 H, t, J = 2 Hz), 6.91 (1 H, d, J = 8 Hz), 6.79

(1 H, d, J = 8 Hz), 6.70 (1 H, s), 6.30 (1 H, d, J = 4 Hz), 5.28

(2 H, s), 4.42 (4 H, t, J = 16 Hz), 3.92 (3 H, s), 3.86 (3 H, s),

3.79 (3 H, s), 2.80 (1 H, s). ¹³C NMR (CDCl₃, 100 MHz) δ

159.6, 153.4, 152.3, 148.9, 147.8, 140.0, 139.3, 136.4, 134.6,

134.4, 122.3, 121.8, 121.2, 119.5, 111.4, 110.3, 108.1, 71.41,

58.68, 56.54, 55.71 ppm. Ir (KBr) 3107, 2939, 1596, 1569,

1521, 1507, 1471, 1433, 1360, 1335, 1276, 1248, 1235, 1211,

1186, 1159, 1145, 1115, 1095, 1066, 1047, 1028, 1005, 986,

972, 949, 889, 817, 797, 772, 753, 747, 734 cm⁻¹. HRMS

(+ESI): Calc. for MH⁺, 517.2087. Found, 517.2103.

1

180–182 °C. H NMR (CDCl₃, 400 MHz) δ 8.44 (2 H, d, J = 4

Hz), 7.54 (3 H, m), 7.42 (2 H, d, J = 8 Hz), 7.19 (1 H, s), 7.06

(2 H, t, J = 8 Hz), 6.92 (1 H, s), 6.75 (1 H, d, J = 8 Hz), 6.58

(1 H, d, J = 8 Hz), 6.42 (1 H, s), 4.45 (4 H, s), 3.91 (3 H, s),

3.86 (3 H, s), 3.78 (3 H, s), 3.28 (1 H, s). ¹³C NMR (CDCl₃,

100 MHz) δ 159.7, 153.5, 152.5, 149.0, 148.0, 140.5, 139.0,

136.5, 136.1, 134.5, 122.4, 121.9, 120.3, 119.1, 111.1, 110.7,

108.2, 71.4, 58.6, 56.6, 56.5, 55.7 ppm. IR (KBr) 3269, 3009,

2936, 2835, 1591, 1571, 1519, 1466, 1439, 1431, 1325, 1273,

1256, 1218, 1159, 1067, 1034, 797, 765, 745 cm⁻¹. HRMS

(+ESI): Calc. for MH⁺, 517.2087. Found, 517.2070.

(4,5-Dimethoxy-2-nitrophenyl)(2-methoxyphenyl)methanol

(m-

OMeCast, 33). 3-Methoxyphenylboronic acid (31, 0.73 g,

0.4.68 mmol), 11 (0.54 g, 2.34 mmol), P(Nap)₃(48 mg, 5 mol

%), PdCl₂(21 mg, 5 mol%), K₂CO₃(0.155 g, 1.12 mmol) and

THF (10 mL) were combined in a Schlenk tube. The tube was

sealed and mixture was stirred at 65 5 °C for 24 h. The reac-

tion mixture was cooled, ﬁltered and solvent was removed. Flash

chromatography on silica (2 : 3 EtOAc–hexanes) yielded a

5-Iodo-2-methoxy-N,N-bis(pyridine-2-ylmethyl)aniline (28). To

a solution of 5-iodo-2-methoxyaniline (27, 1.00 g, 4.02 mmol)

in CH₃CN (40 mL) was added 2-(bromomethyl)pyridine hydro-

bromide (4.05 g, 16.1 mmol), potassium hydrogen phosphate

(K₂HPO₄) (2.80 g, 16.1 mmol) and KI (0.33 g, 2.01 mmol).

After the reaction mixture was reﬂuxed overnight, the solvent

was removed, 20 mL of water was added and pH was adjusted to

∼9. The crude product was extracted with EtOAc (3 × 20 mL)

and dried over MgSO₄. Flash chromatography on basic alumina

(2 : 3 EtOAc–hexanes) yielded product as yellow oil (1.09 g,

1

yellow solid (0.435 g, 58.2%). Mp 83–84 °C. H NMR (CDCl₃,

400 MHz) δ 7.61 (1 H, s), 7.25 (2 H, m), 6.90 (2 H, m), 6.82

(1 H, d, J = 8 Hz), 6.50 (1 H, s), 3.94 (6 H, d, J = 8 Hz), 3.79

(3 H, s), 2.88 (1 H, s). ¹³C NMR (CDCl₃, 100 MHz) δ 159.9,

153.6, 148.2, 143.6, 140.6, 134.0, 129.7, 119.4, 113.4, 112.9,

110.9, 108.3, 71.7, 56.6, 55.4 ppm. IR (KBr) 3543, 1605, 1580,

1513, 1503, 1488, 1463, 1447, 1436, 1399, 1329, 1264, 1207,

1182, 1161, 1146, 1059, 1048, 1017, 987, 891, 880, 869, 788,

780 cm⁻¹. HRMS (+ESI): Calc. for MH⁺, 302.1028. Found,

302.1028.

1

63.0%). H NMR (CDCl₃, 400 MHz) δ 8.48 (2 H, d, J = 4 Hz),

7.56 (2 H, t, J = 8 Hz), 7.41 (2 H, d, J = 8 Hz), 7.15 (2 H, m),

7.08 (2 H, t, J = 8 Hz), 6.57 (1 H, d, J = 8 Hz), 4.47 (4 H, s),

3.77 (3 H, s). ¹³C NMR (CDCl₃, 100 MHz) δ 159.2, 152.8,

149.2, 141.2, 136.5, 131.2, 129.5, 122.3, 122.0, 113.7, 83.3,

(4,5-Dimethoxy-2-nitrophenyl)([2-(dimethylamino)phenyl]methanol

(m-DMACast, 34). The synthesis of 16 followed the same

8172 | Dalton Trans., 2012, 41, 8162–8174

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4-[(4,5-Dimethoxy-2-nitrosophenyl)carbonyl]-2-methoxy-N,N-bis

(pyridine-2-ylmethyl)aniline (35, ZinUnc-4). Photolysis of

ZinCast-4 (53 mg, 0.10 mmol) followed by chromatography

yielded yellow solid (21 mg, 41%). Decomp. 78–80 °C. ¹H

NMR (CDCl₃, 400 MHz) δ 8.48 (2 H, d, J = 4 Hz), 7.66 (1 H, s),

7.59 (2 H, J = 8 Hz), 7.36 (2 H, d, J = 8 Hz), 7.09 (3 H, m), 6.95

(1 H, d, J = 8 Hz), 6.66 (1 H, d, J = 8 Hz), 6.33 (1 H, s), 4.68 (4

H, s), 4.00 (3 H, s), 3.88 (3 H, s), 3.78 (3 H, s). ¹³C NMR

(CDCl₃, 100 MHz) δ 194.6, 160.0, 158.9, 155.9, 151.0, 150.3,

149.4, 145.2, 139.3, 136.8, 131.3, 126.8, 122.2, 121.8, 117.2,

111.3, 109.8, 92.3, 58.3, 57.0, 56.4, 55.9 ppm. IR (KBr) 2361,

1645, 1594, 1508, 1471, 1442, 1256, 1223, 1148, 1079, 857,

750. HRMS (+ESI): Calc. for MH⁺, 499.1981. Found, 499.2020.

procedure as compound 34. 3-(N,N-Dimethylamino)boronic acid

(32, 0.32 g, 1.94 mmol), 11 (0.21 g, 0.97 mmol), P(Nap)₃

(20 mg, 5 mol%), PdCl₂(8.6 mg, 5 mol%), K₂CO₃(0.40 g,

2.91 mmol). The product was puriﬁed on basic alumina

1

(1 : 1EtOAc–hexanes) affording orange oil (0.754 g, 79.6%). H

NMR (CDCl₃, 400 MHz) δ 7.58 (1 H, s), 7.15 (1 H, t, J =

8 Hz), 6.76 (1 H, s), 6.63 (1 H, d, J = 8 Hz), 6.58 (1 H, d, J =

8 Hz), 6.45 (1 H, d, J = 4 Hz), 3.91 (6 H, d, J = 4 Hz), 2.91

(6 H, s), 2.77 (1 H, d, J = 4 Hz). ¹³C NMR (CDCl₃, 100 MHz) δ

153.5, 150.9, 148.1, 142.9, 140.6, 134.3, 129.4, 114.9, 112.3,

111.5, 110.9, 108.2, 72.1, 56.5, 40.7 ppm. IR (KBr) 3514, 2935,

1601, 1578, 1514, 1460, 1436, 1328, 1266, 1212, 1151, 1059,

1030, 993, 964, 866, 795, 764, 732, 696. HRMS (+ESI): Calc.

for MH⁺, 333.1450. Found, 333.1436.

Spectroscopy. Absorption spectra were recorded on a Cary 50

UV-visible spectrophotometer run under Varian’s Cary WinUV

software. Analytical photolysis reactions were performed on a

Hitachi F-4500 spectrophotometer run under FL Solutions 2.0

software and equipped with 150 W Xe lamp and 700 V photo-

multiplier tube. All spectra were acquired at 25 °C in quartz cuv-

ettes with path length of 1 cm and cell volume of 3.0 mL. Stock

solutions of ZinCast photocages was prepared at mM concen-

trations in DMSO and diluted to prepare 25 μM solutions in the

solvent of choice. All metal binding constants and quantum

yields for ﬁrst-generation ZinCast photocages were determined

by previously described procedures.^11,17,33,34

Binding constants. Stock solutions of ZinCast-1, -2, -3, -4 or

-5, ZinUnc-1, -2, -4 or -3 and the metal perchlorate salts of

Cu²⁺, Zn²⁺and Cd²⁺were prepared in mM concentration in

spectrophotometric grade CH₃CN. A 25 μM solution of the

ligand was prepared in 3000 μL of the solvent and titrated in tri-

plicate with each of the metal salt stock solutions. Absorbance

spectra were corrected for dilution and the conditional dis-

sociation constant (K_d) was calculated using XLﬁt.¹⁴

Quantum yields. Photochemistry of the second generation

photocages was evaluated by irradiating 0.25 mM solutions pre-

pared in HPLC grade CH₃CN with a 150 W source at 350 nm.

The change in concentration of ZinCast-1, -2, -3 DMACast was

evaluated by UV-vis spectroscopy by monitoring the absorption

increase at ∼350 nm. For ZinCast-4, -5, m-OMeCast and m-

DMACast the photolysis was followed with an Agilent 1100

HPLC equipped with a Grace Apollo 5 μm silica 4.6 × 250 mm

column as previously described.²⁹m-OMeCast and m-DMACast

were eluted with 5 to 7% iPrOH–hexanes gradient elution, while

ZinCast-4 and ZinCast-5 were eluted with 5% iPrOH–EtOAc

and 15% iPrOH–EtOAc, respectively. Quantum yields were cal-

culated with the following formula:

General procedure for synthesis of ZinUnc compounds. A

solution of ZinCast in CH₃CN (0.03 M) was placed in a quartz

cuvette and irradiated for ∼12 h with a 1000 W Xe lamp. The

resulting solution was evaporated to dryness and puriﬁed by

ﬂash chromatography on basic alumina (3 : 10 hexanes–EtOAc).

4-[(4,5-Dimethoxy-2-nitrosophenyl)carbonyl]-N-[2-(pyridine-2-

yl)ethyl]-N-(pyridine-2-ylmethyl)aniline (12, ZinUnc-2). Photoly-

sis of 3 (50 mg, 0.10 mmol) followed by chromatography lead

to isolation of ZinUnc-2 as a green solid (23 mg, 48%). Mp

1

137–139 °C. H NMR (CDCl₃, 400 MHz) δ 8.52 (2 H, d, J = 4

Hz), 7.70 (2 H, d, J = 8 Hz), 7.55 (2 H, q, J = 8 Hz), 7.10 (4 H,

m), 7.01 (1 H, d, J = 8 Hz), 6.65 (2 H, d, J = 8 Hz), 6.29 (1 H,

s), 4.59 (2 H, s), 3.98 (5 H, m), 3.88 (3 H, s), 3.13 (2 H, t, J = 8

Hz). ¹³C NMR (CDCl₃, 100 MHz) δ 194.0, 160.1, 158.8, 157.8,

156.0, 152.2, 150.2, 150.0, 149.9, 140.3, 137.1, 136.8, 132.9,

127.6, 123.8, 122.4, 121.9, 120.6, 111.3, 109.8, 91.8, 57.0, 56.9,

56.4, 51.9, 35.9 ppm. IR (KBr) 1651, 1602, 1591, 1566, 1556,

1523, 1509, 1469, 1444, 1432, 1386, 1358, 1326, 1299, 1260,

1224, 1196, 1146, 1083 cm⁻¹. HRMS (+ESI): Calc. for MH⁺,

483.2027. Found, 483.2006.

4-[(4,5-Dimethoxy-2-nitrosophenyl)carbonyl]-N,N-bis[2-(pyridine-

2-yl)ethyl]aniline (13, ZinUnc-3). Photolysis of

4 (50 mg,

0.097 mmol) followed by chromatography lead to isolation of

ZinUnc-3 as a green solid (22 mg, 46%). Decomp. 99–101 °C.

¹H NMR (CDCl₃, 400 MHz) δ 8.52 (2 H, d, J = 4 Hz), 7.74

(2 H, d, J = 8 Hz), 7.54 (2 H, t, J = 8 Hz), 7.07 (5 H, m), 6.70

(2 H, d, J = 8 Hz), 6.30 (1 H, s), 4.02 (3 H, s), 3.89 (3 H, s),

3.70 (4 H, t, J = 8 Hz); 3.98 (4 H, t, J = 8 Hz). ¹³C NMR

(CDCl₃, 100 MHz) δ 193.9, 160.1, 158.9, 156.0, 151.8, 150.1,

149.8, 140.5, 136.7, 133.0, 126.9, 123.7, 121.8, 110.9, 109.9,

91.7, 57.0, 56.4, 51.2, 35.9 ppm. IR (KBr) 2362, 1645, 1595,

1509, 1471, 1443, 1257, 1224, 1148, 1079 cm⁻¹. HRMS

(+ESI): Calc. for MH⁺, 497, 2183. Found, 497.2170.

ðΔ½cageꢀ=ΔtÞ

Φ ¼

N_A

4-[(4,5-Dimethoxy-2-nitrophenyl)carbonyl]-N,N-dimethylaniline

(21, DMAUnc). Photolysis of DMACast (20, 6.4 mg,

0.020 mmol) followed by chromatography yielded yellow solid

I

where I is source intensity measured by iron oxalate actinometry

and N_Ais Avogadro’s number.

1

(3.0 mg, 49%). Mp 166–168 °C. H NMR (CDCl₃, 400 MHz) δ

7.73 (2 H, d, J = 8 Hz), 7.12 (1 H, s), 6.58 (2 H, d, J = 8 Hz),

6.30 (1 H, s), 4.01 (3 H, s), 3.89 (3 H, s), 3.03 (6 H, s). ¹³C

NMR (CDCl₃, 100 MHz) δ 194.0, 160.0, 155.9, 154.0, 150.1,

140.3, 132.6, 126.8, 110.7, 109.9, 91.7, 56.9, 56.4, 40.2 ppm.

IR (KBr) 1587, 1508, 1452, 1433, 1319, 1296, 1255, 1226,

1192, 1183, 1142, 1079, 895, 817, 750 cm⁻¹. HRMS (+ESI):

Calc. for MH⁺, 315.1345. Found, 315.1369.

Collection and reduction of X-ray data

Structural analysis was carried out in the X-Ray Crystallographic

Facility at Worcester Polytechnic Institute. A colorless plate of

[Zn(23)]Cl₂having approximate dimensions of 0.02 × 0.10 ×

0.20 mm was covered in paratone oil on 100 μ MiTeGen polyi-

mide micromounts was mounted on a Bruker-AXS APEX CCD

Dalton Trans., 2012, 41, 8162–8174 | 8173

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Table 6 Crystallographic parameters for [Zn(23)]Cl₂

Notes and references

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Formula

M_r

ZnCl₂N₃OC₁₉H₁₉

441.64

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Space group

a/Å

b/Å

c/Å

P2₁/c

7.7523(3)

25.2982(11)

10.7609(4)

106.849(2)

2019.82

4

1.452

1.493

100(2)

β/°

V/Å³

Z

D_c/g cm⁻³

μ/cm⁻¹

T/K

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14 XLﬁt, 5.1.0.0, ID Business Solutions Limited, Guildford, UK,

2009.

Total no. data

No. unique data

Obs. data^a

No. parameters

R^b

17 291

3997

2910

236

0.0608

0.0756

0.569, −0.264

c

wR₂

Max., min. peaks/e Å⁻³

^aObservation criterion: I > 2σ(I). ^bR = ∑||F_o| − |F_c||/∑|F_o|. ^cwR₂

=

2 2

2 2 1/2

[∑(w(F_o²− F_c) )/∑w(F_o) ]

.

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Table 7 Selected bond lengths (Å) and angles (°) for [Zn(23)]Cl₂with

estimated standard deviations in parentheses

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4994.

Zn(1)–N(1)

Zn(1)–N(4)

Zn(1)–Cl(2)

2.098(2)

2.414(2)

2.2711(9)

Zn(1)–N(2)

Zn(1)–Cl(1)

2.105(2)

2.2944(7)

19 Y. Gultneh, A. R. Khan, D. Blaise, S. Chaudhry, B. Ahvazi,

B. B. Marvey and R. J. Butcher, J. Inorg. Biochem., 1999, 75, 7–18.

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N(1)–Zn(1)–N(2)

N(1)–Zn(1)–Cl(1)

N(2)–Zn(1)–N(4)

N(2)–Zn(1)–Cl(2)

N(4)–Zn(1)–Cl(2)

141.15(8)

N(1)–Zn(1)–N(4)

N(1)–Zn(1)–Cl(2)

N(2)–Zn(1)–Cl(1)

N(4)–Zn(1)–Cl(1)

Cl(1)–Zn(1)–Cl(2)

73.52(8)

99.86(6)

101.99(6)

96.93(5)

112.81(3)

102.04(6)

73.65(8)

98.36(6)

150.25(6)

diffractometer equipped with an LT-II low-temperature device.

Diffraction data were collected at 100(2) K using graphite mono-

chromated Mo-Kα radiation (λ = 0.71073 Å) using the omega

scan technique. Empirical absorption corrections were applied

using the SADABS program.³⁵The unit cells and space groups

were determined using the SAINT+ program.³⁵The structures

were solved by direct methods and reﬁned by full matrix least-

squares using the SHELXTL program.³⁵Reﬁnement was based

on F²using all reﬂections. All non-hydrogen atoms were reﬁned

anisotropically. Hydrogen atoms on carbon atoms were all

located in the difference maps and subsequently placed at ideal-

ized positions and given isotropic U values 1.2 times that of the

carbon atom to which they were bonded. Hydrogen atoms

bonded to oxygen atoms were located and reﬁned with isotropic

thermal parameters. Mercury 1.4.2 software was used to examine

the molecular structure.³⁶The crystallographic data and reﬁne-

ment parameters for [Zn(23)]Cl₂are given in Table 6 with

selected bond lengths and angles in Table 7. The 50% thermal

ellipsoid plot is shown in Fig. 5.

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Acknowledgements

This work was supported by the Worcester Polytechnic Institute

and by NSF grant CHE-0955361 and C. V. S. by NSF-REU

program (CHE-0754580).

36 CCDC Mercury 1.4.2, CCDC, Cambridge, UK, 2009.

8174 | Dalton Trans., 2012, 41, 8162–8174

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Quantifying factors that influence metal ion release in photocaged complexes using ZinCast derivatives

DOI: 10.1039/c2dt30135k

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