Zinc complexes of TtzR,Me with O and S donors reveal differences between Tp and Ttz ligands: Acid stability and binding to H or an additional metal (TtzR,Me = tris(3-R-5-methyl-1,2,4-triazolyl)borate; R = Ph, tBu)

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Zinc complexes of Ttz^R,Mewith O and S donors reveal differences between Tp

and Ttz ligands: acid stability and binding to H or an additional metal

(Ttz^R,Me= tris(3-R-5-methyl-1,2,4-triazolyl)borate; R = Ph, tBu)†

Mukesh Kumar,^aElizabeth T. Papish,*^aMatthias Zeller^band Allen D. Hunter^b

Received 14th March 2011, Accepted 26th April 2011

DOI: 10.1039/c1dt10429b

Alkylzinc complexes, (Ttz^R,Me)ZnR¢ (R = tBu, Ph; R¢ = Me, Et), show interesting reactivity with acids,

bases and water. With acids (e.g. ﬂuorinated alcohols, phenols, thiophenol, acetylacetone, acetic acid,

HCl and triﬂic acid) zinc complexes of the conjugate base (CB), (Ttz^R,Me)ZnCB, are generated. Thus the

B–N bonds in Ttz ligands are acid stable. (Ttz^R,Me)ZnCB complexes were characterized by ¹H,

¹³C-NMR, IR, MS, elemental analysis, and, in most cases, single crystal X-ray diffraction. The four

coordinate crystal structures included (Ttz^R,Me)Zn(CB) [where R = Ph, CB (conjugate base) = OCH₂CF₃

(2), OPh (6), SPh (8), p-OC₆H₄(NO₂) (10); R = tBu, CB = OCH(CF₃)₂(3), OPh (5), SPh (7)*,

p-OC₆H₄(NO₂) (9) (* indicates a rearranged Ttz ligand)]. The use of bidentate ligands resulted in

structures [(Ttz^Ph,Me)Zn(CB) (CB = acac (12), OAc (14))] in which the coordination geometries are ﬁve,

and intermediate between four and ﬁve, respectively. Interestingly, three forms of

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) (10) were analyzed crystallographically including a Zn coordinated water

molecule in 10_H₂_O, a coordination polymer in 10_CP, and a p-nitrophenol molecule hydrogen bonded to a

3

2

triazole ring in 10_Nit. Ttz ligands are ﬂexible since they are capable of providing k or k metal binding

and intermolecular interactions with either a metal center or H through the four position nitrogen (e.g.

in 10_CPand HTtz^tBu,Me·H₂O, respectively). Preliminary kinetic studies on the protonolysis of LZnEt (L =

Ttz^tBu,Me, Tp^tBu,Me) with p-nitrophenol in toluene at 95 ^◦C show that these reactions are zero order in acid

and ﬁrst order in the LZnEt.

of hydrogen bonding⁵or metal coordination at the additional

Introduction

nitrogen at the four position⁶(Chart 1). The resulting enhanced

solubility in protic solvents³makes Ttz complexes ideal for green

chemistry applications in water and alcoholic solvents. With these

concepts in mind, we were interested in exploring the chemistry

of TtzZn(alkyl) complexes towards protic reagents as an entry

point for creating biomimetic structures.

Scorpionate ligands, which include the well known

tris(pyrazolyl)borate (Tp) ligands,¹facially cap a transition

metal leaving an open site for the formation of biomimetic

structures or catalysis.²The under utilized tris(triazolyl)borate

(Ttz) ligands³can be thought of as analogous to Tp ligands in

terms of sterics and the ability to structurally approximate the

coordination of three histidines to a metal center in an active

site of an enzyme, but Ttz ligands are slightly weaker electron

donors than Tp ligands.⁴Ttz ligands also allow for the possibility

^aDepartment of Chemistry, Drexel University, 3141 Chestnut St., Philadel-

phia, PA, 19104, USA. E-mail: elizabeth.papish@drexel.edu

^bDepartment of Chemistry, Youngstown State University, 1 University Plaza,

Youngstown, OH, 44555

† Electronic supplementary information (ESI) available: Crystallographic

data for compounds 2, 3, 5, 6, 7, 8, 9, 10_Nit, 10_CP, 10_{H O}, 12, 14 and

18·H₂O·CH₂Cl₂in CIF format. Experimental details for t²he synthesis of

Tp^tBu,MeZn((p-OC₆H₄(NO₂)), Fig. SI-1 illustrating hydrogen bonding in the

Chart 1 Hydrogen bonding or metal coordination through the four

position nitrogen.

crystal structure of 10_H_O, UV-Vis spectra of Ttz^tBu,MeZn((p-OC₆H₄(NO₂))

2

and Tp^tBu,MeZn((p-OC₆H₄(NO₂)) (Fig. SI-2 and SI-3), kinetic details and

plots (Fig. SI-4 to SI-16), and mass spectra of the new compounds (Fig.

SI-17 to SI-30). CCDC reference numbers 764042–764053, 804900. For

ESI and crystallographic data in CIF or other electronic format see DOI:

10.1039/c1dt10429b

The wealth of Tp literature shows that in order to create

biomimetic structures with coordination numbers four and ﬁve

for late ﬁrst row transition metals it is necessary to employ bulky

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Tp ligands.⁷Similarly, bulky Ttz ligands are needed to form

observed with these ligands, often utilizing the fourth position ni-

trogen for hydrogen bonds or metal coordination, thus illustrating

signiﬁcant differences between Ttz and Tp complexes.

biomimetic structures. Prior to our work, only tris(triazolyl)borate

1

ligands with minimal steric bulk (Ttz^H,Hand Ttz^Me,Me

)

had

been synthesized and complexed to metal ions.^3,8,9This lack of

steric bulk often favors the formation of octahedral (Ttz^R,R¢)₂M

complexes, which are typically unreactive.^3,8,9We have contributed

two new ligands for bioinorganic modeling:¹Ttz^tBu,Meand Ttz^Ph,Me

which provide large and intermediate steric bulk, respectively.^4,5,10

Results and discussion

Reactivity of zinc alkyl complexes with neutral, acidic and basic

reagents

The t-butyl group in Ttz^tBu,Meacts as a tetrahedral enforcer

Our attempts to react bulky alkylzinc complexes of the type

(Ttz^tBu,Me)ZnR with water have presumably failed due to this

reaction being slow. Thermodynamically, the equilibrium between

(Ttz^tBu,Me)ZnR and (Ttz^tBu,Me)ZnOH should lie very far to the

right towards the zinc hydroxide species¹⁵because the pK_aof

water (15.74)¹⁶is much lower than the pK_aof ethane or methane

(methane pK_a= ~50).¹⁷Others have also observed sluggish

reactions between bulky alkylzinc complexes and water, alcohols¹⁸

or other electrophiles.¹⁹Similarly, we do not see any products

formed when (Ttz^tBu,Me)ZnR complexes are combined with simple

alcohols like methanol or ethanol which have pK_avalues similar

to water (15.5 and 15.9 respectively).¹⁶Upon prolonged heating

of (Ttz^tBu,Me)ZnEt with water in THF, the ligand does slowly

decompose to give triazole as the isolated product; presumably this

reaction goes through a putative (Ttz^tBu,Me)ZnOH intermediate.

In contrast, acidic reagents lead to cleavage of the Zn–Et bond

in bulky alkylzinc complexes at rates that are much faster yet still

comparatively slow, as reactions take days at elevated tempera-

tures. (Ttz^tBu,Me)ZnEt and (Ttz^Ph,Me)ZnEt react with various acids

including CF₃CH₂OH, phenol, (CF₃)₂CHOH, acetylacetone, p-

nitrophenol, thiophenol, and acetic acid to form simple complexes

of the conjugate base (CB) (Scheme 1). (Ttz^tBu,Me)ZnEt also reacted

4,5,11

(coordination number four)

except for when small bidentate

ligands like acetate or nitrate are present (allowing coordination

number ﬁve or between four and ﬁve).⁵The phenyl group in

Ttz^Ph,Meresults in a more varied range of coordination modes

with coordination numbers four, ﬁve and six all possible.^10,12

Thus, Ttz ligands are equally ﬂexible to Tp ligands in terms

of steric and electronic modiﬁcation, as our group has shown

that steric modiﬁcation is possible and recent reports from Dias

and coworkers have shown that electron withdrawing ﬂuoroalkyl

groups can be placed on the Ttz ligands.¹³

We recently communicated the synthesis and structures of

three new alkylzinc complexes of the Ttz ligands: (Ttz^tBu,Me)ZnEt,

(Ttz^tBu,Me)ZnMe, and (Ttz^Ph,Me)ZnEt.¹⁴Remarkably, (Ttz^tBu,Me)ZnEt

and (Ttz^tBu,Me)ZnMe are water stable under mild conditions and

even co-crystallize with water. In contrast, when less steric bulk is

employed in (Ttz^Ph,Me)ZnEt water sensitivity is observed and this

molecule reacts with water to form (Ttz^Ph,Me)₂Zn.^12,14We postulated

that with stronger acids than water, new modes of reactivity would

be observed. Indeed, (Ttz)Zn(alkyl) complexes do react with acids,

alcohols, acetylacetone, phenols and thiophenol to form structures

that are four or ﬁve coordinate. A rich variety of structures are

Scheme 1 Four and ﬁve coordinate products formed through the reaction of (Ttz^R,Me)ZnEt with protic reagents.

7518 | Dalton Trans., 2011, 40, 7517–7533

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Table 1 Acidity data and products formed through the reaction of alkylzinc complexes with protic reagents

20

Reagent

pK_a

Product from rxn with (Ttz^tBu,Me)ZnEt

Product from rxn with (Ttz^Ph,Me)ZnEt

CF₃CH₂OH

PhOH

(CF₃)₂CHOH

Acetylacetone

p-(NO₂)C₆H₄OH

PhSH

CH₃COOH

HBF₄

HCl

12.5

10.0

9.3

8.95

7.1

(Ttz^tBu,Me)Zn(OCH₂CF₃) (1)

(Ttz^tBu,Me)Zn(OPh) (5)

(Ttz^Ph,Me)Zn(OCH₂CF₃) (2)

(Ttz^Ph,Me)Zn(OPh) (6)

(Ttz^Ph,Me)Zn(OCH(CF₃)₂) (4)

(Ttz^Ph,Me)Zn(acac) (12)

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))^a(10)

(Ttz^Ph,Me)Zn(SPh) (8)

(Ttz^Ph,Me)Zn(OAc) (14)

NA^b

(Ttz^tBu,Me)Zn(OCH(CF₃)₂) (3)

(Ttz^tBu,Me)Zn(acac) (11)

(Ttz^tBu,Me)Zn(p-OC₆H₄(NO₂)) (9)

(Ttz^tBu,Me*)Zn(SPh) (7)

(Ttz^tBu,Me)Zn(OAc) (13, ref. 5)

Htz^tBu,Me

6.5

4.75

-0.4

-7

(Ttz^tBu,Me)ZnCl (15, ref. 11)

(Ttz^tBu,Me)Zn(O₃SCF₃) (16)

NA^b

CF₃SO₃H

-13

NA^b

^aThree different structures resulted: one is a coordination polymer with the formula shown (10_CP), one has a water solvent molecule coordinated (10_{H O}),

2

and one has a p-nitrophenol solvent molecule present (10_Nit). ^bThe water sensitivity of (Ttz^Ph,Me)ZnEt precluded us from using reagents that are not easily

dried. * Indicates rearranged ligand.

with HCl and triﬂic acid to form new zinc complexes, but the water

sensitivity of (Ttz^Ph,Me)ZnEt precluded us from using these acids

since they are difﬁcult to dry. The general structure in every case

is (Ttz^R,Me)Zn(CB), and in our experience all reagents with pK_a

less than or equal to 12.5 slowly gave conversion to products

in moderate yields (see pK_avalues in Table 1²⁰). Presumably,

this reaction involves protonation of the ethyl group to generate

ethane as a leaving group; further details of the mechanism are

discussed below. Two of these products [(Ttz^tBu,Me)ZnOAc (13)⁵

and (Ttz^tBu,Me)ZnCl (15)¹¹] are known compounds that our group

has reported previously and in this study both were characterized

(Ttz^tBu,Me)Zn(p-OC₆H₄(NO₂)) (9) (Scheme 1, Fig. 1) both strongly

absorb UV-visible light. Furthermore, (Ttz^tBu,Me)ZnEt is not water

sensitive, unlike (Ttz^Ph,Me)ZnEt (see Introduction). In order to

probe the effect of the third nitrogen in the triazole ring on the

mechanism of protonolysis, a comparison of the reaction kinetics

of (Tp^tBu,Me)ZnEt (17) and 9 with p-nitrophenol was done. We

recently reported the structure of complex 17,²¹which represents

an ideal point of comparison since it is sterically very similar to

(Ttz^tBu,Me)ZnEt and differs only by the change from triazole to

pyrazole.

The rate of protonolysis in toluene was measured by using a

large excess (pseudo ﬁrst order conditions) of LZnEt (L = Ttz^tBu,Me

,

1

by H-NMR and IR and additionally 15 was characterized by

MS. The fourteen new compounds have all been characterized

Tp^tBu,Me), and p-nitrophenol was chosen as the limiting reagent

since it absorbed strongly. While both p-nitrophenol and LZn(p-

OC₆H₄(NO₂)) absorbed at the wavelengths of interest [l = 337 nm

{e = 1851, 13851 M^-1cm^-1for p-nitrophenol and 9, respectively}

and l = 347 nm {e = 1378, 14823 M^-1cm^-1for p-nitrophenol and

(Tp^tBu,Me)Zn(p-OC₆H₄(NO₂)), respectively}], the concentration of

p-nitrophenol at each time can be calculated since the initial

concentration of p-nitrophenol was known and equals [LZn(p-

OC₆H₄(NO₂))] + [p-nitrophenol] at any given time (see Supporting

Information†). Thus we were able to monitor the disappearance

of p-nitrophenol; this reaction is slow typically requiring 22 h at

95 ^◦C to consume all of the p-nitrophenol (Fig. 2b), but is best

behaved for the ﬁrst 6–10 h (Fig. 2a). Surprisingly, this reaction is

zero order in p-nitrophenol with the rate approximately constant

as p-nitrophenol is completely consumed (Fig. 2b), and using the

method of initial rates gives more ideal behavior during the ﬁrst

6 h (see Fig. 2a and caption). Variation of the ratio between p-

nitrophenol and LZnEt from ~12 to 32 fold excess of the latter

allowed a determination of the reaction order of 1 with respect

to LZnEt (Fig. 3). Thus the rate law is –d[p-nitrophenol]/dt =

k[LZnEt]¹[p-nitrophenol]⁰and converting k_obsto k gives average

values of k = 7 ¥ 10^-7s^-1and k = 9 ¥ 10^-7s^-1for protonolysis of

(Ttz^tBu,Me)ZnEt and (Tp^tBu,Me)ZnEt, respectively.

1

spectroscopically by H, ¹³C-NMR (see Table 2), IR (see exper-

imental section), high resolution MS and/or elemental analysis,

and in twelve cases recrystallization produced crystals suitable for

structure determinations by X-ray diffraction (Fig. 1,5–15).

This rate law is consistent with a rate determining step that does

not depend on acid (p-nitrophenol) concentration. The rates for

hydrolysis of Tp and Ttz alkyl zinc complexes are qualitatively

similar, given the errors involved in such measurements, and

therefore we do not see an inﬂuence of the extra nitrogen in the

triazole ring on the rate of protonolysis. We propose that the

rate determining step involves a rearrangement of LZnEt, and

since the bulky t-butyl groups may prevent a close approach of

Fig. 1 Molecular diagram of (Ttz^tBu,Me)Zn(p-OC₆H₄(NO₂)) (9). Hydrogen

atoms are omitted for clarity. Ellipsoids are shown at 50% probability.

An investigation of the mechanism of acid protonolysis of

the zinc alkyl bond was undertaken with p-nitrophenol, which

serves as an ideal test case since the acid and the product

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Table 2 NMR spectroscopic data of 1–12, 14 and 16

Compound

d(¹H)/ppm^a

d(¹³C)/ppm^b

(Ttz^tBu,Me)Zn(OCH₂CF₃) (1)

1.57 (s, 27H, (CH₃)₃C), 2.02 (s, 9H, CH₃), 4.32 (q, 2H,

13.22 (CH₃), 30.09 C(CH₃)₃, 33.42 C(CH₃)₃, 62.35 (q,

CH₂CF₃, ²J_C-F= 35.28 Hz), 157.76 (5-tz), 174.39 (3-tz);

(CDCl₃): 13.64 (CH₃), 29.89 C(CH₃)₃, 33.00 C(CH₃)₃,

61.94 (q, CH₂CF₃, ²J_C-F= 35.28 Hz), 126.55 (q,

CH₂CF₃, ¹J_C-F= 314.4 Hz), 157.48 (5-tz), 173.94 (3-tz)

13.36 (CH₃), 65.96 (q, CH₂CF₃, ²J_C-F= 32.19 Hz),

128.48, 129.79, 131.39 (Ph), 159.16 (5-tz), 164.51

(3-tz); ^c(CDCl₃): 13.74 (CH₃), 65.47 (q, CH₂CF₃,

²J_C-F= 31.22 Hz), 127.69, 128.09, 129.42, 131.12 (Ph),

159.19 (5-tz), 164.08 (3-tz),

13.24 (CH₃), 30.20 (C(CH₃)₃), 33.38 (C(CH₃)₃), 75.44

(m, CH(CF₃)₂), 157.82 (5-tz), 174.64 (3-tz); (CDCl₃):

13.11 (CH₃), 29.27 (C(CH₃)₃), 32.41 (C(CH₃)₃), 74.08

(septet, CH(CF₃)₂, ²J_C-F= 31.0 Hz), 123.23 (unresolved

quartet, CF₃), 156.59 (5-tz), 173.59 (3-tz)

CH₂CF₃, ³J_H-F= 8.7 Hz)

(Ttz^Ph,Me)Zn(OCH₂CF₃) (2)^c

(Ttz^tBu,Me)Zn(OCH(CF₃)₂) (3)

2.73 (s, 9H, CH₃), 4.25 (q, 2H, CH₂),7.38 (m, 9H,

meta, para-Ph), 8.64 (d, 6H, ortho-Ph, ³J_C-H= 7.8 Hz)

1.52 (s, 27H, (CH₃)₃C)), 1.99 (s, 9H, CH₃), 5.22 (m,

1H,CH(CF₃)₂; (CDCl₃): d 1.39 (s, 27H, (CH₃)₃C)),

2.57 (s, 9H, CH₃), 4.88 (septet, 1H, CH(CF₃)₂)

(Ttz^Ph,Me)Zn(OCH(CF₃)₂) (4)^c

(Ttz^tBu,Me)Zn(OC₆H₅) (5)

2.12 (s, 9H, CH₃), 4.24 (m, 1H, CH(CF₃)₂), 7.14–7.29

13.29 (CH₃), 74.65 (m, (CH(CF₃)₂), 128.06, 128.33,

129.77, 131.42 (Ph), 159.28 (5-tz), 164.81 (3-tz)

(9H, meta and para-Ph), 8.37 (d, 6H, ortho-Ph, ³J_H-H

6.5 Hz)

=

1.56 (s, 27H, (CH₃)₃C)), 2.04 (s, 9H, CH₃), 6.81–7.39

(m, 5H, C₆H₅)

13.18 (CH₃), 30.29 C(CH₃)₃, 33.63 C(CH₃)₃, 116.31,

117.25, 120.38, 121.77, 130.28 (Ph), 157.95 (5-tz),

174.55 (3-tz)

13.38 (CH₃), 119.25, 119.50, 128.62, 128.86, 129.44,

131.26 (Ph), 159.21 (5-tz), 164.66 (3-tz)

(Ttz^Ph,Me)Zn(OC₆H₅) (6)

(Ttz^tBu,Me*)Zn(SPh) (7)

2.17 (s, 9H, CH₃), 6.70–7.11 (m, 14H, meta, para-Ph

and OPh), 8.61 (m, 6H, ortho-Ph)

1.31 (s, 9H, (CH₃)₃C)), 1.56 (s, 18H, (CH₃)₃C)), 2.21 (s, 13.37, 14.06 (CH₃), 30.36, 30.62 C(CH₃)₃, 33.19, 33.63

6H, CH₃), 2.49 (s, 3H, CH₃), 6.82 (m, 3H, meta,

para-SPh), 7.66 (d, 2H, ortho-SPh, J = 8.0 Hz);

(CDCl₃): 1.38 (s, 9H, (CH₃)₃C)), 1.58 (s, 18H,

(CH₃)₃C)), 2.28 (s, 6H, CH₃), 2.62 (s, 3H, CH₃),

6.95–7.07 (m, 3H, meta, para-SPh), 7.32 (d, 2H,

ortho-SPh, J = 8.0 Hz)

C(CH₃)₃, 123.50, 128.82, 132.37, 14043 (Ph), 157.92,

161.90 (5-tz), 168.89, 173.61 (3-tz)

(Ttz^Ph,Me)Zn(SPh) (8)

2.19 (s, 9H, CH₃), 6.49–7.12 (m, 14H, meta, para-Ph

and SPh), 8.49 (d, 6H, ortho-Ph, J = 7.5 Hz)

13.42 (CH₃), 122.84, 129.03, 129.15, 129.36, 129.39,

131.04, 132.11 (Ph), 159.04 (5-tz), 165.29 (3-tz)

(Ttz^tBu,Me)Zn(p-OC₆H₄(NO₂)) (9)

1.45 (s, 27H, (CH₃)₃C)), 2.08 (s, 9H, CH₃), 6.86 (d, 2H, 13.23 (CH₃), 30.20 C(CH₃)₃, 33.50 C(CH₃)₃, 116.27,

C₆H₄, ³J_H-H= 7.5 Hz), 8.32 (d, 2H, C₆H₄, ³J_H-H= 7.0

Hz)

121.05, 126.93 (C₆H₄), 158.31 (5-tz), 174.42 (3-tz)

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) (10_CP

(Ttz^tBu,Me)Zn(acac) (11)

)

2.16 (s, 9H, CH₃), 6.27 (br, 2H, C₆H₄), 6.91–7.02 (m,

9H, meta and para-Ph), 7.84 (d, 2H, C₆H₄, ³J_H-H= 7.5

Hz), 8.31 (d, 6H, ortho-Ph, ³J_H-H= 8.5 Hz)

1.57 (s, 27H, (CH₃)₃C)), 1.85 (s, 6H, CH₃-acac), 2.11

(s, 9H, CH₃), 5.45 (s, 1H, CH-acac)

13.49 (CH₃), 120.26, 126.49, 128.25, 128.57 (C₆H₄),

128.82, 128.89, 129.54, 131.53 (Ph), 159.73 (5-tz),

164.76 (3-tz)

13.48 (CH₃), 28.19 (CH₃acac), 30.03 C(CH₃)₃, 33.66

C(CH₃)₃, 101.61 (CH acac), 157.28 (5-tz), 173.03

(3-tz), 193.16 (CO, acac)

13.45 (CH₃), 27.47 (CH₃acac), 100.74 (CH acac),

128.78, 128.93, 129.74, 131.46 (Ph), 158.49 (5-tz),

163.98 (3-tz), 193.47 (CO acac)

(Ttz^Ph,Me)Zn(acac) (12)

1.34 (s, 6H, CH₃acac), 2.24 (s, 9H, CH₃), 5.11 (s, 1H,

CH, acac), 7.11–7.21 (m, 9H, meta, para-Ph), 8.27 (d,

6H, ortho-Ph, J = 9.0 Hz)

(Ttz^Ph,Me)Zn(OAc) (14)

1.89 (s, 3H, CO₂CH₃), 2.12 (s, 9H, CH₃), 7.10–7.16 (m, 13.36 (CH₃), 21.71 (CO₂CH₃), 127.78, 129.11, 130.35,

9H, meta, para-Ph), 8.35 (d, 6H, ortho-Ph, J = 8.1 Hz)

(CD₃OD): 1.44 (s, 27H, (CH₃)₃C)), 2.62 (s, 9H, CH₃)

130.79 (Ph), 158.94 (5-tz), 164.39 (3-tz), 181.37 (C O)

10.34 (CH₃), 28.58 C(CH₃)₃, 33.31 (CH), 121.82 (q,

CF₃, ¹J_C-F= 318.4 Hz), 154.00 (5-tz), 164.12 (3-tz)

(Ttz^tBu,Me)Zn(OSO₂CF₃) (16)

a

¹H-NMR are recorded in C₆D₆or noted; ^{b 13}C-NMR recorded in C₆D₆or noted; ^cthe expected quartets for CH₂CF₃and CH(CF₃) were obscured by

the phenyl ring resonances and could not be assigned.

1

acid, the logical possibility is a change in coordination mode of

H(Ttz^tBu,Me)·H₂O which has been characterized by H-NMR, IR,

FAB–MS and elemental analysis. It is surprising that we isolate

the H⁺salt of the Ttz ligand rather than the ammonium salt of

the ligand, especially since NH₃is more basic than 1,2,4-triazole

based on pK_avalues (9.2²³and 2.27,²⁴respectively) reported in

water. The other product is presumably Zn(OH)₂. This H⁺salt of

the Ttz ligand crystallized from methanol and dichloromethane

(1 : 1) as H(Ttz^tBu,Me)·H₂O·CH₂Cl₂(Fig. 4) and the crystal structure

shows extensive hydrogen bonding as discussed below. This pattern

of reactivity is similar to that observed with Ttz zinc halides;

(Ttz^tBu,Me)ZnCl has been treated with bases in an effort to isolate

a biomimetic zinc hydroxide complex, but most hydroxide sources

3

2

L from k to k . This could be followed by fast protonation of

2

the k -LZnEt molecule (Scheme 2). Of course we cannot rule out

other mechanistic possibilities, especially with other acids besides

p-nitrophenol and other solvents, more polar than toluene, that

could stabilize charged intermediates and provide greater amounts

of acid dissociation to give free protons. However, the relatively

slow rates of most protonolysis reactions described herein and

3

2

the presence of k to k isomerizations in the Tp literature²²

suggests that this is a reasonable mechanism for acid protonolysis

in toluene.

In contrast, to the reactivity with acids, the use of basic reagents

typically gave loss of ligand from Zn. With alkyl zinc reagents,

(Ttz^tBu,Me)ZnEt reacts with NH₄OH in water–methanol to yield

give [M][Ttz^tBu,Me] (M⁺¹= NMe₄, Na⁺, K⁺) as seen with the

+

reagents NaOH, KOH, and NMe₄OH.²⁵

7520 | Dalton Trans., 2011, 40, 7517–7533

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Scheme 2 Proposed mechanism of protonolysis of LZnEt (L = Ttz^tBu,Me, Tp^tBu,Me) with p-nitrophenol in toluene.

Fig. 3 Determination of reaction order with respect to LZnEt for

protonolysis with p-nitrophenol as the limiting reagent.

CH₂Cl₂molecules contribute to a relatively large R value for this

structure. Notably, this structure shows that Ttz complexes can

bind protons in ways that Tp ligands cannot.

The crystallographically characterized structures of the type

(Ttz^R,Me)Zn(CB) can be divided into three classes: four coordinate

structures (2, 3, 5, 6, 7, 8, 9, 10), ﬁve coordinate structures (12), and

those with a coordination geometry intermediate between these

extremes (14). Interestingly, three unique structures were obtained

for (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) (10); all were four coordinate

structures but two had an N₃O donor set (10_Nit, 10_CP) and

one contained an N₂O₂donor set (10_H₂_O) (Chart 2). The X-ray

crystallographic data and selected bond lengths and angles are

summarized in Tables 3, 4, and 5.

Fig. 2 a) Typical kinetics run: consumption of p-nitrophenol in the

presence of 19.3 fold excess of (Ttz^tBu,Me)ZnEt shows that reaction is zero

order in p-nitrophenol. b) Reaction is approximately ﬁrst order for entirety

of kinetics run, but is well behaved for just the ﬁrst 6–10 h. Given the long

reaction time (22 h) and high temperature (95 ^◦C), partial decomposition

is a possibility at long reaction times, and thus the ﬁrst six hours have been

used to calculate rate constants (method of initial rates).

The

four

coordinate

ﬂuorinated

alkoxides

[(Ttz^Ph,Me)Zn(OCH₂CF₃) (2) and (Ttz^tBu,Me)Zn(OCH(CF₃)₂)

(3)] (see Fig. 5 and 6) show unremarkable Zn–O and Zn–

Bond lengths, angles and coordination geometries

N_avgbond distances (Table 2) that are similar to those in

27

The zinc free structure H(Ttz^tBu,Me)·H₂O·CH₂Cl₂will be described

ﬁrst, followed by the four coordinate zinc structures, and lastly the

ﬁve coordinate zinc structures will be described.

H(Ttz^tBu,Me)·H₂O·CH₂Cl₂(18·H₂O·CH₂Cl₂) (Fig. 4a, Table 4)

has hydrogen bonding interactions connecting triazole rings with

neighboring water molecules. The four position nitrogen in each

triazole ring is hydrogen bonded to water, and the protonated

triazole nitrogen N9 shows the shortest hydrogen bond with an

(Tp^Cum,Me)Zn(OCH₂CF₃) (1.830(7) A and ~2.05 A, respectively),

˚

(Tp^tBu,Me)ZnOEt (1.825(2) A and ~2.06 A, respectively), and

28

˚

(Tp^Ph,Me)ZnOMe (1.843 A and ~2.07 A, respectively). The steric

inﬂuence of both the bulky t-butyl groups and the ﬂuorinated

alkoxide is evident in that the Zn–O–C angles increase from 126^◦

to 145^◦from 2 to 3. A similar trend was seen for (Tp^Ph,Me)Zn_◦OMe

and (Tp^tBu,Me)ZnOEt which showed Zn–O–C angles of 130.7 and

141.7^◦, respectively.^15,28

15

˚

N ◊ ◊ ◊ O distance of just 2.578(9) A. The other hydrogen bonding

Considering the four coordinate complexes formed from the

reaction of alkylzinc complexes with stronger acids, it appears

that the Zn–O distances increase slightly as the weaker conjugate

bases are employed, presumably due to weaker coordination. For

example, the two phenoxides [(Ttz^R,Me)Zn(OPh)] (5 and 6, see

Fig. 7 and 8) have Zn–O distances of 1.837(2) (R = tBu) and

distances are longer, but still indicate a relatively strong hydrogen

bond,²⁶with N ◊ ◊ ◊ O distances of 2.615(9) to 2.756(9) A. These

hydrogen bonds form a two dimensional network in the crystal

structure (Fig. 4b). The other bond lengths and angles are

unremarkable, with the triazole rings canted away from each other

as expected for an uncoordinated scorpionate. The disordered

˚

1.833(2) (R = Ph) but these distances increase up to 1.873(2) A

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Fig. 4 (a) Molecular diagram of H(Ttz^tBu,Me)·H₂O·CH₂Cl₂(18·H₂O·CH₂Cl₂). CH₂Cl₂is omitted for clarity. A water molecule is shown to illustrate

hydrogen bonding interactions. Ellipsoids are shown at 50% probability. (b) Formation of a two-dimensional structure via H-bonds in this structure.

[Color code: red-oxygen, blue-nitrogen, pink-boron].

˚

(9), 1.862(1) A (10_Nit) and 1.894(2) A (10_CP) (Fig. 1, 11 and 12) on

average for the different structures obtained with p-nitrophenolate

coordinated. The Zn–O distances for 9 and 10 are somewhat longer

The greatest difference between structures of similar coordina-

tion geometry is found for the Zn–chalcogen–C angle, which is

inﬂuenced by both steric and electronic factors. The complexes of

the Ttz^tBu,Meligand generally show larger Zn–O–C angles than

the corresponding complexes of the Ttz^Ph,Meligand (Table 6).

By expanding these angles beyond the ideal covalent value of

109.5^◦(presumably through bonds that have signiﬁcant ionic

than the value of 1.849 A for the comparable (Tp^tBu,Me)Zn(p-

˚

OC₆H₄(NO₂)).²⁹The Zn–N_avgdistances do not vary much from

structure to structure (see Table 2) and are consistent with other

values in the literature.^15,27,28,29

7522 | Dalton Trans., 2011, 40, 7517–7533

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Table 3 Summary of X-ray data of (Ttz^Ph,Me)Zn(OCH₂CF₃) (2), (Ttz^tBu,Me)Zn(OCH(CF₃)₂) (3), (Ttz^tBu,Me)Zn(OPh)·PhOH (5·PhOH), (Ttz^Ph,Me)Zn(OPh)

(6), (Ttz^tBu,Me*)Zn(SPh) (7), and (Ttz^Ph,Me)Zn(SPh) (8). All structures were measured at 100 K using graphite monochromated Mo-Ka radiation

Compound

2

3

5·PhOH

6

7

8

Moiety formula

Crystal color

Crystal habit

C₂₉H₂₇BF₃N₉OZn C₂₄H₃₈BF₆N₉OZn C₆₀H₉₀B₂N₁₈O₃Zn₂

C₃₃H₃₀BN₉O Zn

Colorless

Plate

C₂₇H₄₂BN₉SZn

Colorless

Block

C₃₃H₃₀BN₉SZn

Colorless

Plate

Colorless

Block

Colorless

Plate

Colorless

Block

Crystal size/mm

0.37 ¥ 0.34 ¥ 0.27 0.55 ¥ 0.30 ¥ 0.11

0.55 ¥ 0.30 ¥ 0.24

9.887(3)

20.091(6)

16.297(5)

90

90.627(7)

90

0.55 ¥ 0.25 ¥ 0.10

16.954(2)

11.5994(13)

17.447(2)

90

116.398(2)

90

0.44 ¥ 0.24 ¥ 0.13 0.42 ¥ 0.39 ¥ 0.15

˚

a/A

17.1768(9)

11.6552(7)

16.1882(9)

90

115.8620(10)

90

41.250(2)

44.237(3)

14.6555(9)

90

9.2539(11)

17.075(2)

18.952(2)

90

2994.7(6)

Orthorhombic

P2₁2₁2₁

4

11.842(3)

15.757(4)

17.293(5)

72.951(4)

89.883(4)

89.978(4)

3085.1(14)

Triclinic

P-1

˚

b/A

˚

c/A

a/^◦

b/^◦

g /^◦

3

˚

Volume/A

2916.3(3)

Monoclinic

P2₁/c

26743(3)

Orthorhombic

Fdd2

3236.9(17)

Monoclinic

P2₁/c

3073.3(6)

Monoclinic

P2₁/c

Crystal system

Space group

Z

4

32

2

4

Dc/g cm^-3

1.482

0.903

1.309

0.8

1.297

0.799

6247

7959

1.0062

0.0406, 0.1032

0.0563, 0.1114

1.394

0.843

5446

7588

1.057

0.0433, 0.1019

0.0754, 0.1207

1.333

0.923

1.423

0.904

m/mm^-1

Reﬂections (I > 2s(I)) 6090

10738

6826

7240

11797

15017

Independent reﬂections 7027

12986

1.001

Goodness-of-ﬁt on F²

R₁^a, wR2^b[I > 2s(I)]

R₁^a, wR2^b(all data)

1.035

1.091

0.0332, 0.0822

0.0373, 0.0853

1.043

0.0469, 0.0963

0.0697, 0.1075

0.0342, 0.0883

0.0411, 0.0931

0.0415, 0.0945

0.0534, 0.0992

ꢀ

^aR1 =

| | F_o| - | F_c| |/ | F_o|. ^bwR2 = { [w(F_o- F_c²)²]/ [w(F_o²)²]}

.

2

1/2

Table 4 Summary of X-ray data of (Ttz^tBu,Me)Zn(p-OC₆H₄(NO₂)) (9), (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·(p-HOC₆H₄(NO₂)) (10_Nit), (Ttz^Ph,Me)Zn(p-

OC₆H₄(NO₂)) (10_CP), (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·H₂O (10_{H O}), (Ttz^Ph,Me)Zn(acac) (12), (Ttz^Ph,Me)Zn(OAc) (14) and H(Ttz^tBu,Me)·H₂O·CH₂Cl₂(18_H

).

₂O CH₂Cl₂

2

All structures were measured at 100 K using graphite monochromated Mo-Ka radiation

Compound

9

10_Nit

10_CP

10_H2O

12

14

18_{H2O CH2Cl2}

Moiety formula C₂₇H₄₁BN₁₀O Zn C₃₃H₂₉BN₁₀O₃Zn C₇₃H₆₆B₂N₂₀O₆Zn₂C₄₂H₄₀BN₁₀O₄Zn C₃₂H₃₂BN₉O₂Zn C₂₉H₂₈BN₉O₂Zn C₂₂H₄BCl₂N₉O

Crystal color

Crystal habit

Colourless

Plate

Yellow

Plate

Yellow

Block

Colourless

Plate

Colourless

Plate

Colourless

Block

Colourless

Block

Crystal size/mm 0.45 ¥ 0.24 ¥ 0.08 0.44 ¥ 0.41 ¥ 0.19 0.42 ¥ 0.37 ¥ 0.27 0.55 ¥ 0.32 ¥ 0.12 0.55 ¥ 0.45 ¥ 0.04 0.38 ¥ 0.26 ¥ 0.10 0.55 ¥ 0.36 ¥ 0.22

˚

a/A

13.9148(16)

15.3546(16)

14.7509(15)

90

103.469(2)

90

13.5636(12)

14.5278(13)

36.290(3)

87.8045(14)

79.9926(14)

64.3664(13)

6343.5(10)

Triclinic

P-1

25.0768(17)

19.0144(13)

29.281(2)

90

13961.9(16)

Orthorhombic

Pbca

8

1.4

0.756

13729

16.6709(14)

14.6250(12)

18.4633(14)

90

115.455(2)

90

18.670(8)

16.257(7)

10.399(4)

90

97.551(9)

90

3129(2)

Monoclinic

P2₁/c

4

17.0859(14)

11.2068(8)

16.7846(12)

90

117.7980(10)

90

16.290(8)

18.323(9)

20.877(10)

90

93.436(11)

90

˚

b/A

˚

c/A

a/^◦

b/^◦

g /^◦

3

˚

Volume/A

3064.9(6)

Monoclinic

P2₁/n

4064.6(6)

Monoclinic

P2₁/n

2843.0(4)

Monoclinic

P2₁/c

6220(5)

Monoclinic

P2₁/c

Crystal system

Space group

Z

4

2

4

8

Dc/g cm^-3

1.365

0.848

1.398

0.644

25643

1.348

0.659

7989

1.382

0.83

3742

1.427

0.909

5845

1.133

0.238

7000

m/mm^-1

Reﬂections (I > 5589

2s(I))

Independent

reﬂections

7562

37424

17337

10059

5511

8411

10839

Goodness-of-ﬁt 1.021

1.018

1.042

1.041

1.115

1.008

1.090

on F²

R₁^a, wR2^b[I >

2s(I)]

0.0424, 0.0897

0.0524, 0.1122

0.0886, 0.1262

0.0369, 0.0907

0.0536, 0.1025

0.0424, 0.1156

0.0577, 0.1252

0.1014, 0.2569

0.1416, 0.2837

0.0488, 0.0915

0.0823, 0.1066

0.1047, 0.2499

0.1651, 0.2883

R₁^a, wR2^b(all

data)

0.0670, 0.1015

ꢀ

2

1/2

^aR1 =

| | F_o| - | F_c| |/ | F_o|. ^bwR2 = { [w(F_o- F_c²)²]/ [w(F_o²)²]}

.

character) these alkoxide and phenoxide groups can avoid the

bulky t-butyl groups. Expected covalent bond angles for sulfur

are closer to 90^◦(observed bond angles are 92^◦for H₂S and

99^◦for S(CH₃)₂)³⁰due the use of p orbitals for bonding with

very little s character.^30aThus, (Ttz^Ph,Me)ZnSPh (8, Fig. 10) has

an average Zn–S–C bond angle of 105.6(2)^◦and the phenyl

rings of the thiophenoxide and the Ttz^Ph,Meligand are able to

interdigitate. However, with the increased bulk of the Ttz^tBu,Me

ligand, (Ttz^tBu,Me)ZnSPh is not observed. The tight bond angle

required for S would cause a steric clash (see Chart 3) and provides

a reasonable explanation for why ligand isomerization occurs

to produce (Ttz^tBu,Me*)ZnSPh (7, where * indicates a rearranged

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◦

˚

Table 5 Selected bond lengths (A) and angles ( )

(Ttz^Ph,Me)ZnO(CH₂CF₃) (2)

N1–Zn1

N7–Zn1

O1–Zn1–N4

N4–Zn1–N1

N4–Zn1–N7

C28–O1–Zn1

2.041(1)

2.088(1)

126.8(1)

90.5(1)

92.3(1)

125.7(1)

N4–Zn1

O1–Zn1

O1–Zn1–N1

O1–Zn1–N7

N1–Zn1–N7

2.032(1)

1.845(1)

126.5(1)

117.1(1)

94.6(1)

(Ttz^tBu,Me)Zn(OCH(CF₃)₂) (3)

N1–Zn1

N7–Zn1

O1–Zn1–N1

O1–Zn1–N7

2.052(2)

2.067(2)

126.4(1)

123.0(1)

91.9(1)

N4–Zn1

O1–Zn1

O1–Zn1–N4

N1–Zn1–N4

N4–Zn1–N7

2.060(2)

1.855(2)

120.5(1)

96.6(1)

N1–Zn1–N7

C22–O1–Zn1

89.2(1)

45.0(2)

[(Ttz^tBu,Me)Zn(OC₆H₅)]·(C₆H₅OH) (5).

N3–Zn1

N9–Zn1

O1–Zn1–N9

N9–Zn1–N6

2.067(2)

2.033(2)

134.8(1)

92.8(1)

N6–Zn1

O1–Zn1

O1–Zn1–N6

O1–Zn1–N3

N6–Zn1–N3

2.065(2)

1.837(2)

115.6(1)

118.6(1)

92.3(1)

N9–Zn1–N3

92.8(1)

C22–O1–Zn1

141.7(2)

(Ttz^Ph,Me)Zn(OC₆H₅) (6)

N1–Zn1

N7–Zn1

O1–Zn1–N4

N4–Zn1–N1

N4–Zn1–N7

C28–O1–Zn1

2.044(2)

2.065(2)

129.4(1)

94.1(1)

93.7(1)

132.2(2)

N4–Zn1

O1–Zn1

O1–Zn1–N1

O1–Zn1–N7

N1–Zn1–N7

2.027(2)

1.833(2)

117.7(1)

120.9(1)

92.3(1)

(Ttz^tBu,Me*)Zn(SC₆H₅) (7)

N2–Zn1

N9–Zn1

N2–Zn1–N9

N9–Zn1–N5

N9–Zn1–S1

C22–S1–Zn1

2.045(2)

2.050(2)

87.3(1)

95.7(1)

115.6(1)

102.3(1)

N5–Zn1

S1–Zn1

N2–Zn1–N5

N2–Zn1–S1

N5–Zn1–S1

2.066(2)

2.223(1)

91.5(1)

129.8(1)

126.6(1)

(Ttz^Ph,Me)Zn(SC₆H₅) (8)

N1–Zn1

N7–Zn1

N1–Zn1–N4

N4 –Zn1–N7

N4–Zn1–S1

C28–S1–Zn1

2.053(3)

2.077(3)

96.9(1)

89.9(1)

119.2(1)

105.0(1)

N4–Zn1

S1–Zn1

N1–Zn1–N7

N1–Zn1–S1

N7–Zn1–S1

2.076(3)

2.228(1)

88.9(1)

123.3(1)

129.5(1)

(Ttz^tBu,Me)Zn(p-OC₆H₄(NO₂)) (9)

N3–Zn1

N9–Zn1

O1–Zn1–N6

N6–Zn1–N3

2.042(2)

2.071(2)

117.2(1)

90.0(1)

N6–Zn1

O1–Zn1

O1–Zn1–N3

O1–Zn1–N9

N3–Zn1–N9

2.035(2)

1.873(2)

131.9(1)

117.6(1)

97.3(1)

N6–Zn1–N9

94.2(1)

C22–O1–Zn1

134.4(2)

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·(p-HOC₆H₄(NO₂)) (10_Nit

)

N1–Zn1

2.043(2)

2.029(2)

125.7(1)

93.7(1)

94.8(1)

129.8(1)

N4–Zn1

O1–Zn1

O1–Zn1–N4

O1–Zn1–N1

N4–Zn1–N1

2.041(2)

1.857(2)

122.9(1)

118.8(1)

92.9(1)

N7–Zn1

O1–Zn1–N7

N7–Zn1–N4

N7–Zn1–N1

C28–O1–Zn1

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) (10_CP

N19–Zn1

O1–Zn1

N4–Zn1

N12–Zn2

Zn2–N3

)

2.021(2)

1.893(1)

2.032(2)

2.028(2)

2.054(2)

122.5(1)

N3–Zn2

O4–Zn2

N7–Zn1

N15–Zn2

2.054(2)

1.895(1)

2.013(2)

2.014(2)

O1–Zn1–N7

O1–Zn1–N19

104.5(1)

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Table 5 (Contd.)

N7–Zn1–N19

N7–Zn1–N4

O4–Zn2–N15

N15–Zn2–N12

N15–Zn2–N3

C28–O1–Zn1

108.3(1)

97.9(1)

115.6(1)

97.7(1)

106.0(1)

136.3(1)

O1–Zn1–N4

N19–Zn1–N4

O4–Zn2–N12

O4–Zn2–N3

N12–Zn2–N3

C61–O4–Zn2

110.2(1)

113.8(1)

126.1(1)

106.8(1)

102.6(1)

129.8(1)

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·H₂O (10_H

O1–Zn1

N1–Zn1

Zn1–O2–C28

O2–Zn10–N7

)

₂O

2.024(2)

2.027(2)

127.8(1)

128.6(1)

98.5(1)

O2–Zn1

N7–Zn1

O2–Zn1–O1

O2–Zn1–N1

O1–Zn1–N1

1.887(2)

2.020(2)

100.7(1)

125.7(1)

92.2(1)

O1–Zn1–N7

N7–Zn1–N1

100.6(1).

(Ttz^Ph,Me)Zn(acac) (12)

N3–Zn1

N9–Zn1

2.089(9)

2.154(8)

1.976(6)

125.0(6)

89.7(3)

103.2(3)

88.8(3)

91.3(3)

90.5(3)

N6–Zn1

O1–Zn1

2.103(7)

2.003(7)

O2–Zn1

C29–O1–Zn1

O2–Zn1–O1

O1–Zn1–N3

O1–Zn1–N6

O2–Zn1–N9

N3–Zn1–N9

C31–O2–Zn1

O2–Zn1–N3

O2–Zn1–N6

N3–Zn1–N6

O1–Zn1–N9

N6–Zn1–N9

125.6(6)

114.0(3)

148.8(3)

96.7(3)

164.6(3)

82.4(3)

(Ttz^Ph,Me)Zn(OAc) (14)

N1–Zn1

N7–Zn1

2.067(2)

2.097(2)

2.583(2)

105.0(2)

123.6(1)

98.0(1)

N4–Zn1

O1–Zn1

2.031(2)

1.918(2)

O2–Zn1

Zn1–O1–C28

O1–Zn1–N4

N4–Zn1–N1

N1–Zn1–N7

O1–Zn1–N1

O1–Zn1–N7

N4–Zn1–N7

127.7(1),

117.6(1)

88.2(1)

91.18(1)

Chart

2

Structure variation for Ttz^Ph,Mezinc complexes with the

p-nitrophenolate ligand.

ligand) as shown in Fig. 9. Such rearrangements are well known

in Tp chemistry especially with ligands of intermediate steric

bulk. In the rearranged ligand, one triazole ring contains a t-

butyl group in the ﬁve position allowing space for the SPh ligand

(see Fig. 9). This complex has a Zn–S–C bond angle of 102.3^◦

and thus we do not see signiﬁcant change in the bond angle

from complex 8 to 7 despite a change in ligand. In this case,

there is not a large steric difference for the SPh group due to

the ligand rearrangement in the latter molecule. Complexes in the

literature similarly show that Zn–O–C bond angles [e.g. 145.0^◦and

152.7^◦for (Tp^tBu,Me)Zn(p-OC₆H₄tBu);²⁹157.5^◦for (Tp^tBu,Me)Zn(p-

OC₆H₄NO₂);²⁹and 132.6(2)^◦for (Tp^Cum,Me)Zn(p-OC₆H₄(NO₂))²⁷]

Fig. 5 Molecular diagram of (Ttz^Ph,Me)ZnO(CH₂CF₃) (2). The disorder

in the ﬂuorine atoms is not shown and hydrogen atoms are omitted for

clarity. Ellipsoids are shown at 50% probability.

are much larger than Zn–S–C bond angles [e.g. 102.9^◦for

(Tp^Ph,Me)Zn(p-SC₆H₄Me)³¹and 105.7^◦for (Tp^Ph,Me)Zn(SPh)³²] and

the largest Zn–O–C bond angles are seen with bulky Tp^tBu,Me

ligands. Zn–S and Zn–N_avgdistances in 7 and 8 (Table 6) are

consistent with similar complexes in the literature.^31,32

Only one structure was observed for (Ttz^tBu,Me)Zn(p-

OC₆H₄(NO₂)) (9) (Fig. 1) despite efforts to introduce water as a

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Fig. 8 Molecular diagram of (Ttz^Ph,Me)Zn(OC₆H₅) (6). Hydrogen atoms

are omitted for clarity. Ellipsoids are shown at 50% probability.

Fig. 6 Molecular diagram of (Ttz^tBu,Me)Zn(OCH(CF₃)₂) (3). Hydrogen

atoms are omitted for clarity. Ellipsoids are shown at 50% probability.

Chart 3 Explanation of why steric factors preclude the formation of

(Ttz^tBu,Me)ZnSPh.

seen between the triazole rings of (Ttz^tBu,Me)ZnOC₆H₅(5) and

a co-crystallized phenol molecule, with an N1–O2 distance of

˚

2.749(3) A. These N–O distances are consistent with the standard

deﬁnition of a hydrogen bonding interaction.²⁶

Fig. 7 Molecular diagram of [(Ttz^tBu,Me)Zn(OC₆H₅)]·(C₆H₅OH) (5). El-

lipsoids are shown at 50% probability. Co-crystallized, disordered phenol

and hydrogen atoms are omitted for clarity.

When p-nitrophenol is limited to a 1 : 1 ratio with the

(Ttz^Ph,Me)ZnEt starting material and air and humidity are rig-

orously excluded, a coordination polymer of (Ttz^Ph,Me)Zn(p-

OC₆H₄(NO₂)) forms (10_CP) (See Chart 2 and Fig. 12). In this

structure, each ligand contains two triazole rings bound to zinc in

the normal fashion (through the two position N), but one triazole

ring is bound to a neighboring zinc via the four position nitrogen.

Thus it is clear that the four position nitrogen can bond to an

electropositive element, either H or a metal in the case of 10_Nit

and 10_CP, respectively. To our knowledge, this is the ﬁrst instance

of a triazole this bulky coordinating to a metal through the four

position nitrogen. Such behavior is quite common with less bulky

triazoles and has been used to form metal organic frameworks and

coordination polymers.⁶As a result of each ligand coordinating to

coligand, which is in contrast the p-nitrophenolate ligand coupled

with the decreased steric bulk of the Ttz^Ph,Meligand that has led

to three different structures. When excess p-nitrophenol is used

in the synthesis of 10, we obtain [(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·(p-

HOC₆H₄(NO₂))] (10_Nit). The coordination geometry at the zinc

in 10_Nit(Fig. 11) is similar to that observed in the other four

coordinate structures, but the presence of hydrogen bonding inter-

actions differentiates this structure. There are three independent

molecules in the asymmetric unit of 10_Nit; they all are composed

of (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) units but they differ slightly in

the bond lengths, angles and close contacts to neighboring

molecules. The ligands bound to Zn1, Zn2 and Zn3 each have

one triazole ring that has a hydrogen bonding interaction to the

OH of p-nitrophenol with N–O distances of 2.661(2), 2.748(3)

2

1

two metals (k and k ), O–Zn–N bond angles decrease and N–Zn–

3

N bond angles increase relative to those seen with k -Ttz ligands,

and this brings all the angles closer to the tetrahedral ideal value.

When the recrystallization of 10 is carried out with exposure to

ambient air, a third structure [(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·H₂O,

˚

and 2.688(3) A, respectively. A similar interaction had been

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◦

Table 6 Comparison of bond lengths (A) and angles ( ) for the four coordinate structures of the form Ttz^R,MeZnX

˚

Ttz^tBu,Meligand

Ttz^Ph,Meligand

X, compound

OCH(CF₃)₂, 3

OPh, 5

SPh, 7

ONit, 9

OCH₂CF₃, 2

OPh, 6

SPh, 8

ONit, 10_Nit

ONit, 10_CP

Zn–O (or Zn–S)

Zn–N_avg

Zn–O–C (or

Zn–S–C)

1.855(2)

2.060(2)

145.0(2)

1.837(2)

2.055(1)

141.7(2)

2.223(1)

2.054(2)

102.3(1)

1.873(2)

2.049(2)

134.4(2)

1.845(1)

2.053(1)

125.7(1)

1.833(2)

2.045(2)

132.2(2)

2.224(2)^a

2.068(2)^a

105.6(2)

1.862(1)^b

2.045(1)^b

130.7(1)^b

1.894(2)^a

2.027(1)^a

133.1(2)^a

^aAverage of values for two unique environments, ^bAverage of values for three unique environments.

Fig. 11 Molecular diagram of (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·(p-HOC₆H₄-

(NO₂)) (10_Nit). Ellipsoids are shown at 50% probability. Hydrogen atoms

not involved in H bonding and the other two crystallographically

independent molecules with similar coordination environments around

Zn2 and Zn3 are omitted for clarity.

Fig. 9 Molecular diagram of (Ttz^tBu,Me*)Zn(SC₆H₅) (7). Hydrogen atoms

are omitted for clarity. Ellipsoids are shown at 50% probability.

Fig. 10 Molecular diagram of (Ttz^Ph,Me)Zn(SC₆H₅) (8). Hydrogen atoms

and the second independent molecule with a similar coordination envi-

ronment around Zn2 are omitted for clarity. Ellipsoids are shown at 50%

probability.

Fig. 12 Molecular diagram of (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) (10_CP). Hy-

drogen atoms and phenyl rings are omitted for clarity. Ellipsoids are shown

at 50% probability.

10_H₂_O] forms which contains a water molecule bound to zinc in

addition to the p-nitrophenolate ligand (See Chart 2 and Fig. 13).

angles are in between tetrahedral and trigonal bipyramidal but

closer to the latter. For example, Zn1, N1, N7 and O2 are nearly

in a plane and the angles around the zinc with these atoms are

100.6(7)^◦, 125.7(6)^◦and 128.6(6)^◦. The O1 atom caps the trigonal

pyramid and the angles between O1, Zn, and O2, N7 or N1

are 100.7(6)^◦, 98.5(6)^◦and 92.2(6)^◦, respectively. Two hydrogen

2

The Ttz ligand is bound in a k fashion (Zn–N distances 2.027(2)

˚

and 2.020(2) A) and so the zinc is ligated by two oxygens and two

nitrogens in a roughly four coordinate environment. Although

˚

the Zn1–N4 distance (2.929(1) A) is too long to be considered a

bond, there appears to be a weak interaction because the bond

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Fig. 14 Molecular diagram of (Ttz^Ph,Me)Zn(acac) (12). Phenyl rings and

hydrogen atoms are omitted for clarity. Ellipsoids are shown at 50%

probability.

Fig. 13 Molecular diagram of (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·H₂O (10_{H O}).

2

All hydrogen atoms but those of the water molecule are omitted for clarity.

Ellipsoids are shown at 50% probability.

bonding interactions connect O1 to neighboring triazole rings

˚

with O–N distances of 2.764(2) and 2.677(2) A and these hydrogen

bonds link these molecules to form a two dimensional network

˚

(see Fig. SI-1†). The Zn–O1 distance (2.024(2) A) with the neutral

˚

water molecule is greater than the Zn–O2 distance (1.887(2) A)

with p-nitrophenolate, thus the trigonal pyramidal structure allows

for large bond angles between the strongest donor groups.

With bidentate ligands, (Ttz^Ph,Me)Zn(acac) (12) is clearly ﬁve

coordinate but (Ttz^Ph,Me)Zn(OAc) (14) has a zinc atom which is

between four and ﬁve coordinate. In complex 12 (Fig. 14), the

structure is closer to a square pyramid than a trigonal bipyramid

(t = 0.263 where 0 is a perfect square pyramid and 1 is a perfect

trigonal bipyramid).³³Atoms N6, N9, O1 and O2 form the base

of the square pyramid and N3 is at the apex. The chelate ring

containing the acetylacetonate ligand is planar, and all angles in

this ring are 124.9–126(4)^◦except the O–Zn–O angle which is

89.7(3)^◦. The Zn–N and Zn–O bond distances are very similar

to those in the analogous (Tp^Ph,Me)Zn(acac) and (Tp^Ph)Zn(acac)

complexes in the literature, but there are signiﬁcant differences

in the coordination geometry. While both of the literature acac

complexes are described as roughly trigonal bipyramidal, the

values of the t parameter are 0.767 for (Tp^Ph)Zn(acac)³⁴and 0.443

for (Tp^Ph,Me)Zn(acac).³⁵Thus changing the ligand from Tp^Phto

Tp^Ph,Meto Ttz^Ph,Mein zinc acac complexes produces a gradual shift

from trigonal bipyramidal towards square pyramidal geometry.

The structure of complex 14 (Fig. 15), features a semibidentate³⁶

acetate ligand, thus the structure is midway between coordination

numbers four and ﬁve. If the weak coordination of O2 is neglected,

a distorted tetrahedron results with bond angles (average angles

are O1–Zn–N = 123^◦and N–Zn–N = 92^◦) similar to those in

the four coordinate structures in Fig. 5–11. Inclusion of O2 in

the coordination sphere of zinc gives a ﬁve coordinate structure

that is approximately a trigonal bipyramid (t = 0.661) with

O2 and N7 axial (O2–Zn–N7 = 167.4(7)^◦). The Zn–O distance

Fig. 15 Molecular diagram of (Ttz^Ph,Me)Zn(OAc) (14). Hydrogen atoms

are omitted for clarity. Ellipsoids are shown at 50% probability.

38

39

(1.923 A), (Tp^Ms)Zn(OAc) (1.947 A) and (Ttz^tBu,Me)Zn(OAc)

˚

5

(1.978 A). While all of the mentioned LZn(OAc) structures have

a semibidentate acetate ligand, the Zn–O (weakly coordinated)

Ms 39

˚

distance is shorter (around 2.36 A) with bulkier ligands (L = Tp

,

Ttz^tBu,Me).⁵For comparison, Zn–O (weakly coordinated) distances

Py,Me 37

38

˚

are 2.5–2.6 A with less bulky ligands (L = Tp

,

Tp^Fur,Me

,

Ttz^Ph,Me). In contrast, (Tp^tBu,Me)Zn(OAc)⁴⁰and (Tp^tBu)Zn(OAc)⁴¹

both have clearly monodentate acetate ligands so an overarching

trend is not present. The Zn–N average distances are similar in all

˚

of the above structures, ranging from 2.05–2.09 A.

Reactivity involving the cleavage of B–N bonds

The stability of the B–N bonds in the Ttz ligands towards strong

acids is noteworthy, even in the presence of HCl and triﬂic acid

(pK_avalues of -7 and -13, respectively) the B–N bonds remain

intact and zinc complexes of the conjugate base are formed. Only

in the case of HBF₄(pK_a= 0.4, intermediate in strength of the

acids used, see Table 1) do we see B–N bond cleavage, presumably

due to the absence of a suitable anion to coordinate to the zinc.

Treatment of (Ttz^tBu,Me)ZnEt with HBF₄results in formation of

the known 3-t-butyl-5-methyl-1,2,4-triazole⁵as conﬁrmed by IR,

¹H-NMR and mass spectrometry. Similarly, Tp^R,R¢and related

complexes are known to decompose (usually in the presence of

˚

for the strongly coordinated O1 is 1.918(2) A and is similar

37

to that seen in (Tp^Py,Me)Zn(OAc) (1.919 A), (Tp^Fur,Me)Zn(OAc)

˚

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acids) yielding inadvertent pyrazole complexes via B–N bond

cleavage.^42–45However, cleavage of B–N bonds in Tp complexes

occurs under comparatively mild conditions, including in the

presence of water from imperfectly dried solvents.⁴⁶Overall, it

appears that Ttz complexes have greater B–N bond stability in

acids and water as compared with Tp complexes, and this may

offer an advantage for the Ttz ligand class.

The cleavage of B–N bonds with HBF₄is related to the

formation of (Ttz^tBu,Me*)ZnSPh in that the latter reaction involves a

1,2-borotropic shift in which B–N bonds are broken and reformed

to give an isomerized product. Both reversible B–N bond cleavage

to give 1,2-borotropically shifted products and irreversible B–N

bond cleavage to give pyrazole and pyrazolate products are well

known and recently summarized in the introduction of a paper by

Young et al.⁴⁷The signiﬁcant difference between our work and the

reactions in the literature is that most prior B–N bond cleavage

and isomerization involved less bulky ligands, typically Tp^iPr. In

the work reported herein, B–N bond cleavage to give triazole or

rearranged products is seen only with strong acids or to relieve

steric congestion in (Ttz^tBu,Me*)ZnSPh.

that the pK_aof zinc bound water is too high to be deprotonated

under neutral conditions when an anionic O donor is present.

Lastly, the (Ttz^R,Me)ZnSPh complexes (7, 8) feature N₃S donor sets

that bear resemblance to metalloenzymes that contain a mixture

of histidine residues and cysteines^2cor thiols including the thiol

inhibitor bound form of metallo-b-lactamase.^32,51

We have also seen that B–N bond cleavage can occur with strong

acids to decompose the Ttz ligand and produce free triazole. Re-

versible B–N bond cleavage is also involved in the 1,2-borotropic

shift that gave the rearranged product (Ttz^tBu,Me*)ZnSPh (7) which

relieves steric strain in order to accommodate a small Zn–S–C

angle. However, these B–N bond cleavage reactions are relatively

rare in the greater context of Ttz chemistry (as is similarly true in

Tp chemistry), and so this facet of the reactivity is not a major

detriment to the Ttz ligand class. In general, Ttz complexes are

quite acid stable as evidenced by the formation of (Ttz^tBu,Me)ZnCl

(15) and (Ttz^tBu,Me)ZnOTf (16) with intact B–N bonds in the

presence of strong acids. Our kinetic studies of protonolysis of

LZnEt (L = Ttz^tBu,Meand Tp^tBu,Me) suggest that the origin of this acid

stability is kinetic in nature, as the zero order dependence on acid

suggested that slow rearrangement of the zinc complex (probably

via a bidentate L) is the rate determining step. Thus once the

(Ttz)Zn(CB) (CB = conjugate base) complexes are formed, they are

most likely acid stable due to the inaccessibility of the B–N bonds.

However, Ttz zinc complexes are base sensitive, as evidenced

by disengagement from Zn and formation of H(Ttz^tBu,Me), which

further illustrates that the basicity of the extra nitrogen leads to

signiﬁcant differences (cf . Tp ligands). Thus the Ttz ligands are

versatile, robust, and allow for convenient entry to good structural

models for zinc metalloenzymes with coordination numbers four

and ﬁve and secondary interactions through the four position

nitrogen to either H or a metal.

Conclusions

The reactions and complexes described herein illustrate how

slight changes in the presence of hydrogen bonding interactions,

sterics and the electron donor ability of a ligand can create

different coordination geometries. The (Ttz^Ph,Me)Zn(acac) (12)

complex has a square pyramidal geometry, but the closely related

(Tp^Ph)Zn(acac) and (Tp^Ph,Me)Zn(acac) complexes^34,35are trigonal

bipyramidal in geometry with a slightly stronger electron donor.

Similarly, the Ttz^Ph,Meand p-nitrophenolate complexes of zinc can

form as either a coordination polymer with an N₃O donor set with

one N bound through the four position nitrogen (10_CP), a complex

with an N₃O donor set and p-nitrophenolate hydrogen bonded

(10_Nit), or a water complex with an N₂O₂donor set (10_H₂_O). The

Ttz ligand allows for changes in coordination number by changing

Experimental

2

denticity. For example, 10_Hhas a k -Ttz^Ph,Meligand, 10_CPhas

O

2

1

Ttz^Ph,Mebound k to one metal and k to another, and the remaining

General procedures

3

Ttz complexes are all bound to the zinc in a k fashion. Likewise,

the zinc acetate complexes with bulky Tp and Ttz ligands vary

between four and ﬁve coordinate structures with our most recent

contributions (Ttz^tBu,Me)Zn(OAc)⁵and (Ttz^Ph,Me)Zn(OAc) (14) both

intermediate between square pyramidal and trigonal bipyramidal

geometries but closer to the latter.

All experiments were performed under an atmosphere of dry N₂

using Schlenk techniques and an M. Braun UNILAB glove-box.

Ttz^tBu,MeZnEt, Ttz^tBu,MeZnMe, and Ttz^Ph,MeZnEt were prepared by

following the recently reported procedures.¹⁴C₆H₅OH, C₆H₅SH,

p-NO₂-C₆H₄OH, (CF₃)₂CHOH, CF₃CH₂OH, CF₃SO₃H, HBF₄

and HCl (2.0 M in ether) were purchased from Acros and Aldrich

and used without further puriﬁcation. Solvents were dried using

an M. Braun solvent puriﬁcation system with alumina columns or

were freshly distilled from drying agents using standard methods.

NMR spectra were recorded using either a 300 MHz or a 500

MHz Varian Unity Inova NMR spectrophotometer. All the NMR

data is presented in Tables 1 and 2. The required number of

carbon signals is observed in each compound which supports the

formulation of the newly synthesized molecules. IR spectra were

recorded on a Perkin–Elmer Spectrum One Fourier-transform

IR absorption spectrophotometer. High resolution (HR) mass

spectrometry was performed on VG70SE double focusing, triple

quadruple mass spectrometer equipped with FAB or CI ionization

capability. Elemental analyses were performed by Robertson

Microlit, Ledgewood, NJ.

This variability in structure mirrors how enzymes are able

to control coordination geometry and function through slight

changes in the hydrogen bonding interactions and the amino acids

in the secondary coordination sphere. Coordinative ﬂexibility is

necessary for zinc containing enzymes which must frequently

change coordination number with each step of a reaction. Further-

more, many of our complexes provide structural approximations

of the ﬁrst coordination sphere in zinc enzyme active sites. The

tetrahedral complexes that feature an N₃O donor set (crystallo-

graphically characterized in 2, 3, 5, 6, 9, 10_CP, and 10_Nit) are similar

in structure to the active site in carbonic anhydrase which has three

histidines and a hydroxide.⁴⁸Both N₃O and N₂O₂donor sets can be

seen in enzymes that contain the 2-His-1-carboxylate facial triad.⁴⁹

It is noteworthy that 10_Hforms an aqua complex rather than a

O

2

hydroxide ligand; these results and others in the literature⁵⁰suggest

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Single crystal X-ray diffraction studies

were added to the NMR tube and the mixture was agitated for

~10 min. H-NMR spectra were taken immediately afterwards,

1

Diffraction data for all compounds were collected using a Bruker

AXS SMART APEX CCD diffractometer using monochromatic

Mo-Ka radiation with the omega scan technique. Single crystals

of the compounds were mounted on Mitegen micromesh mounts

and data were collected at 100 K. Data were collected, unit

cells determined, and the data integrated and corrected for

absorption and other systematic errors using the Apex2 suite of

programs.⁵²The structures were solved by direct methods and

reﬁned by full matrix least squares against F²with all reﬂections

using SHELXTL 6.14.⁵³All non-hydrogen atoms were reﬁned

anisotropically. Carbon and boron bonded hydrogen atoms as well

as alcohol H atoms were placed in calculated positions and were

reﬁned with isotropic displacement parameters 1.2 (C–H/B–H) or

1.5 (CH₃/OH) times that of the adjacent carrier atom. Methyl and

hydroxyl H atoms were allowed to rotate around the C–X bond at

a ﬁxed angle to best ﬁt the experimental electron density. Water H

hours later, days later, and ﬁnally two weeks later. Spectra showed

the presence of (Ttz^tBu,Me)ZnEt with the Zn–Et group’s resonances

showing the correct integration relative to the resonances from the

Ttz^tBu,Meligand, indicating no detectable hydrolysis. However, after

prolonged exposure of (Ttz^tBu,Me)ZnEt to water, a slight change in

the ratio of starting material to internal standard was noticed.

In another typical experiment, complex (Ttz^tBu,Me)ZnEt (0.020

g) was mixed with 2 mL of water and 2 mL of methanol to form

a suspension which was stirred for 48 h at room temperature.

The ¹H-NMR spectrum of the resultant product showed no

evidence of hydrolysis of the Zn–Et group. Similarly, complex

(Ttz^tBu,Me)ZnEt (0.020 g) was mixed with 4 mL of water and 2 mL

of dichloromethane to form a clear mixture which was stirred for

24 h at room temperature. The ¹H-NMR spectrum of the resultant

product showed no evidence of hydrolysis of the Zn–Et group.

Similar experiments with complex (Ttz^tBu,Me)ZnMe indicated that

it too was water stable.

˚

atoms were reﬁned with an O–H distance of 0.84(2) A and with

isotropic displacement parameters 1.5 times that of the adjacent

oxygen atom. (For further details see supporting information†).

Formation of (Ttz^Ph,Me)₂Zn from (Ttz^Ph,Me)ZnEt

A solution of (Ttz^Ph,Me)ZnEt (0.050 g, 0.086 mmol) in 2 mL

of CH₂Cl₂was combined with 0.5 mL of water. The contents

were stirred for 24 h at room temperature. Then the solvent was

removed and ¹H-NMR and IR spectra showed the formation

of (Ttz^Ph,Me)₂Zn. This material was puriﬁed by recrystallization

from CH₂Cl₂and hexane (1 : 1). The yield of (Ttz^Ph,Me)₂Zn was

0.044 g, 49%. Single crystal X-ray diffraction conﬁrmed the

structure of (Ttz^Ph,Me)₂Zn, which was identical to (Ttz^Ph,Me)₂Zn

prepared previously in our laboratory by a different route and

fully characterized.¹²

Kinetic studies and method

All the kinetic experiments were performed on a Perkin Elmer

Lambda 35 UV-Vis spectrometer equipped with a Peltier constant

temperature bath. A kinetic method was developed and the

parameters for the kinetic time scan method are as follows: scan

range of 800–200 nm, cycle time of 15 min, resolution of 1 nm,

and a scan rate of 480 nm min^-1.

Representative procedure for kinetics study between LZnEt and

p-nitrophenol

Synthesis of (Ttz^tBu,Me)Zn(OCH₂CF₃) (1)

A 1.5 mL solution of Ttz^tBu,MeZnEt (2.888 mM) or Tp^tBu,MeZnEt

(2.520 mM) was transferred by syringe to a specially designed

air tight UV/Vis cell and then 1.5 mL of p-nitrophenol (0.092

mM) was added to this cell by a syringe and the cell was closed

immediately with a Teﬂon adaptor to avoid the longer exposure

to the air. The contents were then mixed thoroughly and placed

in a preheated (95 ^◦C) UV-Vis spectrophotometer chamber and

the data collection began with the mixing time as time zero of

the reaction. (The parameters of the method used are described

in the ﬁrst paragraph of this section). A total of 90 cycles (22.5

h) were recorded in each experiment. The appearance of a new

peak at 337 or 347 nm was followed to monitor the progress of

the reaction. The concentration of p-nitrophenol consumed and

product formation during the reaction was calculated by using eq.

2 and 3 (see supporting information†). To understand the effect

of the concentration of LZnEt on the rate of the reaction, these

reactions were repeated with varying concentration of LZnEt and

constant concentration of p-nitrophenol (the plots are given in

supporting information, see Fig. SI-4–SI-16†).

A solution of (Ttz^tBu,Me)ZnEt (0.020 g, 0.039 mmol) in 0.5 mL of

C₆D₆was combined with CF₃CH₂OH (0.008 g, 0.080 mmol) in

an NMR tube. The NMR tube was then sealed under an N₂

atmosphere and heated to 65–70 ^◦C for 7 days. The progress

of the reaction was monitored by ¹H-NMR spectra taken at

different time intervals. After completion of the reaction, a white

product was isolated by removing the solvent under vacuum. This

product was puriﬁed by recrystallization from hexane to produce

(Ttz^tBu,Me)Zn(OCH₂CF₃) (1) in 71% yield (0.016 g).

IR (cm^-1): 2543.42 (nB–H); HRCI–MS: m/z = 590.267179 [M

+ H]⁺(experimental), 590.269243 [M + H]⁺(calculated), all peaks

showed the expected isotopic pattern.

In a similar manner complex 2–12 were prepared using the

amounts of reagent, temperature and time noted:

(Ttz^Ph,Me)Zn(OCH₂CF₃)

(2). (Ttz^Ph,Me)ZnEt

(0.050

g,

0.086 mmol), CF₃CH₂OH (0.016 g, 0.16 mmol), 65–70 ^◦C, and 5

d. The product was puriﬁed by recrystallization from a mixture of

toluene and hexane (1 : 1) to produce (Ttz^Ph,Me)Zn(OCH₂CF₃) (2)

in 75% yield (0.042 g). IR (cm^-1): 2547.88 (nB–H); HRCI–MS:

m/z = 650.174900 [M + H]⁺(experimental), 650.1675 [M + H]⁺

(calculated), all peaks showed the expected isotopic pattern.

Attempts at hydrolysis of (Ttz^tBu,Me)ZnEt and (Ttz^tBu,Me)ZnMe

In a typical experiment, complex (Ttz^tBu,Me)ZnEt and hexamethyl-

cyclotrisiloxane (internal standard) was dissolved in 0.6 mL of

(Ttz^tBu,Me)Zn(OCH(CF₃)₂) (3). (Ttz^tBu,Me)ZnEt (0.020 g,

0.039 mmol), (CF₃)₂CHOH (0.013 g, 0.077 mmol), 65–70 ^◦C and

6 d. The product was puriﬁed by recrystallization from hexane to

1

C₆D₆in an NMR tube and an H-NMR spectrum was taken to

conﬁrm the presence of starting compound. Five drops of H₂O

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produce (Ttz^tBu,Me)Zn(OCH(CF₃)₂) (3) in 67% yield (0.017 g). IR

(cm^-1): 2543.42 (nB–H); HRCI–MS: m/z = 658.257970 [M + H]⁺

(experimental), 658.2487 [M + H]⁺(calculated), all peaks showed

the expected isotopic pattern.

mixture of toluene and hexane (1 : 1) under anhydrous conditions

to produce (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) as light yellow solid (10_CP

)

in 68% yield (0.021 g). IR (cm^-1): 2556.79 (nB–H); HRCI–MS:

m/z = 689.190412 [M + H]⁺(experimental), 689.18085 [M + H]⁺

(calculated), all peaks showed the expected isotopic pattern.

(Ttz^Ph,Me)Zn(OCH(CF₃)₂) (4). (Ttz^Ph,Me)ZnEt (0_◦.030 g,

0.052 mmol), (CF₃)₂CHOH (0.016 g, 0.095 mmol), 65 C and 4

d. The product was puriﬁed by recrystallization from a mixture

of toluene and hexane (1 : 1) to produce (Ttz^Ph,Me)Zn(OCH(CF₃)₂)

(4) in 75% yield (0.028 g). However, crystals suitable for X-ray

diffraction could not be obtained. IR (cm^-1): 2538.97 (nB–H);

HRCI–MS: m/z = 718.159698 [M + H]⁺(experimental), 718.1633

[M + H]⁺(calculated), all peaks showed the expected isotopic

pattern.

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·H₂O (10_H2O). The recrystalliza-

tion of 10_CP(0.021 g) from a 5 ml solution of benzene and hexane

(1 : 1) in air at room temperature for one week afforded light yellow

crystals of 10_H2O(Yield: 51%, 0.011 g). The formation of 10_H₂_Ofrom

10_CPprobably occurred in the presence of adventitious water from

solvent or atmosphere. The presence of adventitious water is also

discerned in the crystal structures of Ttz^tBu,MeZnR (R = Et, Me).¹⁴

IR (cm^-1): 2559.65 (nB–H), 3385 (br, nO–H (H₂O)). Elemental

analysis, found C 56.30, H 4.27, N 18.92, calcd for 10_H2OC 55.99,

H 4.41, N 19.79.

(Ttz^tBu,Me)Zn(OC₆H₅)

(5). (Ttz^tBu,Me)ZnEt

(0_◦.021

g,

0.040 mmol), C₆H₅OH (0.004 g, 0.043 mmol), 65–70 C and 10

d. This product was puriﬁed by recrystallization from a mixture

of toluene and hexane (1 : 1) to produce (Ttz^tBu,Me)Zn(OC₆H₅) (5)

in 72% yield (0.017 g). IR (cm^-1): 2544.18 (nB–H); HRCI–MS:

m/z = 584.296680 [M + H]⁺(experimental), 584.2896 [M + H]⁺

(calculated), all peaks showed the expected isotopic pattern.

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂))·(p-HOC₆H₄(NO₂))

(10_Nit).

(Ttz^Ph,Me)ZnEt (0.031 g, 0.054 mmol), p-HOC₆H₄(NO₂) (0.015

g, 0.108 mmol), 65–70 ^◦C and 24 h. This product was puriﬁed

by recrystallization from a mixture of toluene and hexane

(4 : 1) under anhydrous conditions to produce (Ttz^Ph,Me)Zn(p-

OC₆H₄(NO₂))·(p-HOC₆H₄(NO₂)) (10_Nit) as light yellow crystals in

(Ttz^Ph,Me)Zn(OC₆H₅)

(6). (Ttz^Ph,Me)ZnEt

(0.030

g,

1

45% yield (0.020 g). H-NMR (C₆D₆): d 2.16 (s, 9H, CH₃), 6.20

0.052 mmol), C₆H₅OH (0.005 g, 0.053, 65–70 ^◦C and 4 d.

This product was puriﬁed by recrystallization from a mixture of

toluene and hexane (1 : 1) to produce (Ttz^Ph,Me)Zn(OC₆H₅) (6) in

72% yield (0.024 g). IR (cm^-1): 2543.01 (nB–H); HRCI–MS: m/z =

643.198299 [M]⁺(experimental), 643.195783 [M]⁺(calculated), all

peaks showed the expected isotopic pattern.

(br, 4H, C₆H₄), 6.90–7.03 (m, 9H, meta and para-Ph), 7.80 (d,

4H, C₆H₄, ³J_H-H= 9.6 Hz), 8.29 (d, 6H, ortho-Ph, ³J_H-H= 7.8 Hz);

IR (cm^-1): 2550.29 (nB–H), 3305.12(br, nOH (p-nitrophenol));

HRCI–MS showed (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) as M: m/z =

689.185582 [M + H]⁺(experimental), 689.188686 [M + H]⁺

(calculated).

(Ttz^tBu,Me*)Zn(SPh) (7). (Ttz^tBu,Me)ZnEt (0.054 g, 0.104 mmol),

PhSH (0.023 g, 0.209 mmol), 65–70 ^◦C and 4 d. This product was

puriﬁed by recrystallization from a mixture of dichloromethane

and hexane (1 : 1) to produce (Ttz^tBu,Me*)Zn(SPh) (7) in 71%

yield (0.044 g). IR (cm^-1): 2581.98 (nB–H), HRCI–MS: m/z =

600.271873 [M + H]⁺(experimental), 600.274665 [M + H]⁺

(calculated), all peaks showed the expected isotopic pattern.

Elemental analysis, found C 53.71, H 7.11, N 20.73, calcd for

7 C 53.96, H 7.04, N 20.98.

(Ttz^tBu,Me)Zn(acac) (11). (Ttz^tBu,Me)ZnEt (0.035 g, 0.067 mmol),

acetylacetone (0.013 g, 0.129 mmol) 70 ^◦C and 4 d. This

product was puriﬁed by recrystallization from a mixture of

dichloromethane and hexane (1 : 1) to produce (Ttz^tBu,Me)Zn(acac)

(11) in 73% yield (0.029 g). IR (cm^-1): 2542.77 (nB–H), 1598.38

(nC O); HRCI–MS: m/z = 589.300319 [M]⁺(experimental),

589.300248 [M]⁺(calculated), all peaks showed the expected

isotopic pattern. Elemental analysis, found C 52.04, H 7.39, N

20.59, calcd for 11 C 52.85, H 7.51, N 21.33.

(Ttz^Ph,Me)Zn(SPh) (8). (Ttz^Ph,Me)ZnEt (0.026 g, 0.045 mmol),

PhSH (0.009 g, 0.082 mmol), 65–70 ^◦C and 18 h. This product

was puriﬁed by recrystallization from a mixture of toluene and

hexane (1 : 1) to produce (Ttz^Ph,Me)Zn(SPh) (8) in 71% yield (0.021

g). IR (cm^-1): 2548.21 (nB–H), 1603.23 (nC O); HRCI–MS:

m/z = 660.178038 [M + H]⁺(experimental), 660.172593 [M +

H]⁺(calculated), all peaks showed the expected isotopic pattern.

Elemental analysis, found C 59.10, H 4.60, N 18.39, calcd for 8 C

59.97, H 4.68, N 19.07.

(Ttz^Ph,Me)Zn(acac) (12). (Ttz^Ph,Me)ZnEt (0.024 g, 0.041 mmol),

acetylacetone (0.008 g, 0.079 mmol), 65–70 ^◦C and 2d. This

product was puriﬁed by recrystallization from a mixture of

dichloromethane and hexane (1 : 1) to produce (Ttz^Ph,Me)Zn(acac)

(12) in 71% yield (0.019 g). IR (cm^-1): 2542.82 (nB–H), 1593.04

(nC O); HRCI–MS: m/z = 649.208096 [M + H]⁺(experimental),

649.206347 [M + H]⁺(calculated), all peaks showed the expected

isotopic pattern. Elemental analysis, found C 59.11, H 4.91, N

19.42, calcd for 12 C 59.05, H 4.96, N 19.37.

(Ttz^tBu,Me)Zn(p-OC₆H₄(NO₂)) (9). (Ttz^tBu,Me)ZnEt (0.040 g,

0.077 mmol), p-NO₂-C₆H₄OH (0.011 g, 0.079 mmol), 65–70 ^◦C

and 8d. This was puriﬁed by recrystallization from a mixture of

toluene and hexane (1 : 1) to produce (Ttz^tBu,Me)Zn(p-OC₆H₄(NO₂))

(9) in 79% yield (0.038 g). IR (cm^-1): 2553.03 (nB–H); HRCI–MS:

m/z = 629.285148 [M + H]⁺(experimental), 629.27474 [M + H]⁺

(calculated), all peaks showed the expected isotopic pattern.

Synthesis of (Ttz^tBu,Me)Zn(OAc) (13).

A

solution of

(Ttz^tBu,Me)ZnEt (0.010 g, 0.019 mmol) in 2 mL of CH₂Cl₂

was combined with acetic acid (0.002 g, 0.033 mmol) in 1 mL of

water. The contents were stirred for 2 h at room temperature and

1

then the solvents were removed. The H-NMR and IR spectral

data showed the formation of (Ttz^tBu,Me)Zn(OAc) (13), which is a

known compound,⁵in 76% yield (0.008 g).

(Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) (10_CP). (Ttz^Ph,Me)ZnEt (0.026 g,

0.045 mmol), p-NO₂-C₆H₄OH (0.007 g, 0.050 mmol), 65–70 ^◦C

and 24 h. This product was puriﬁed by recrystallization from a

Synthesis of (Ttz^Ph,Me)Zn(OAc) (14).

A

solution of

(Ttz^Ph,Me)ZnEt (0.027 g, 0.047 mmol) in 5 mL of CH₂Cl₂

was combined with CH₃COOH (0.003 g, 0.050 mmol) under

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Dalton Trans., 2011, 40, 7517–7533 | 7531

©

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an N₂atmosphere. The contents were stirred for 2 h at room

temperature and then the solvent was removed and a white solid

was isolated. This solid was puriﬁed by recrystallization from a

mixture of toluene and hexane (1 : 1) to produce (Ttz^Ph,Me)Zn(OAc)

(14) in 85% yield (0.024 g). IR (cm^-1): 2545.65 (nB–H), 1603.23

(nC O); HRCI–MS: m/z = 610.180479 [M + H]⁺(experimental),

610.17503 [M + H]⁺(calculated), all peaks showed the expected

isotopic pattern. Elemental analysis, found C 56.75, H 4.45, N

20.36, calcd for 14 C 57.03, H 4.62, N 20.64.

Acknowledgements

We thank NSF CAREER and Drexel University for ﬁnancial

support, T. Wade for mass spectrometry experiments, and A. W.

Addison for helpful discussions. The diffractometer was funded

by NSF grant 0087210, by Ohio Board of Regents grant CAP-491,

and by YSU.

References

Synthesis of (Ttz^tBu,Me)ZnCl (15).

A

suspension of

1 Abbreviations: Tp^R,R¢= tris(3-R-5-R¢-pyrazolyl)borate, Ttz^R,R¢= tris(3-

R-5-R¢-1,2,4-triazolyl)borate.

2 (a) S. Troﬁmenko, Scorpionates: The Coordination Chemistry of

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699–767. See also references therein.

(Ttz^tBu,Me)ZnEt (0.028 g, 0.054 mmol) in 6.0 mL MeOH–

H₂O (5 mL/1 mL) was combined with HCl (26.0 mL, (2.0 M in

diethyl ether), 0.052 mmol) at room temperature. After stirring

for 10 min, the mixture became a clear solution and was stirred

for 2 h. The solvents were removed and the product was extracted

1

in 10 mL benzene, ﬁltered and dried. H-NMR, IR, and CI–MS

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spectral data showed the formation of (Ttz^tBu,Me)ZnCl (15), which

is a known compound,¹¹in 51% yield (0.014 g).

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Synthesis of (Ttz^tBu,Me)Zn(OSO₂CF₃) (16). A suspension of

(Ttz^tBu,Me)ZnEt (0.070 g, 0.135 mmol) in 3.0 mL MeOH–H₂O

(2 mL/1 mL) was combined with CF₃SO₃H (0.020 g, 0.133 mmol)

in 1 mL of water. The suspension became clear after 5 min of

stirring at room temperature and was stirred for 1 h. The solvents

were then removed and the residue obtained was washed with 5

ml of CH₂Cl₂and 5 mL of hexane. A white solid was isolated

and characterized as (Ttz^tBu,Me)Zn(OSO₂CF₃) (16). The yield of

16 was 0.039 g, 46%. IR (cm^-1): 2565.70 (nB–H); HRCI–MS:

m/z = 640.215094 [M + H]⁺(experimental), 640.20765 [M +

H]⁺(calculated), all peaks showed the expected isotopic pattern.

Elemental analysis, found C 41.09, H 6.32, N 20.44, calcd for 16

C 41.23, H 5.82, N 19.67.

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Reaction of (Ttz^tBu,Me)ZnEt with HBF₄to form triazole.

A

solution of (Ttz^tBu,Me)ZnEt (0.020 g, 0.039 mmol) in 2.0 mL MeOH

was combined with HBF₄(0.003 g, 0.034 mmol) in 1 mL of

water. The contents were stirred for 1 h at room temperature and

then the solvents were removed. The ¹H-NMR, IR and FAB–MS

spectral data showed the formation of Htz^tBu,Me, which is a known

compound reported previously.⁵

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2010, 39, 59–61.

Reaction of (Ttz^tBu,Me)ZnEt with NH₄OH to form

15 H. Brombacher and H. Vahrenkamp, Inorg. Chem., 2004, 43, 6042–

6049.

H(Ttz^tBu,Me)·H₂O (18·H₂O).

A

solution of (Ttz^tBu,Me)ZnEt

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Inorg. Chem., 1999, 38, 4593–4600; (c) Also, sluggish reactivity between

(Tp^tBu,Me)ZnH and methanol has been reported, see: C. Bergquist, L.

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(0.093 g, 0.18 mmol) in 5 mL of MeOH was combined with

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The contents were stirred for overnight at room temperature and

a clear solution obtained. The solvents were then removed and

the product was extracted in methanol (5 mL). A white solid

was obtained after removal of methanol and washing the solid

with hexane (2 mL). The product was puriﬁed by recrystallization

from methanol and dichloromethane (1 : 1) mixture at room

temperature to produce H(Ttz^tBu,Me)·H₂O (18·H₂O) in 89% yield

1

(0.071 g, 0.16 mmol). H-NMR (CD₃OD): 1.31 (s, 27H, tBu),

2.21 (s, 9H, CH₃); IR (cm^-1): 3359–3198 (br, n(O–H of water)),

2964 (n(C–H)), 2477 (n(B–H)), 1591 (n(C N)); FAB–MS: m/z =

428.5 [M + H]⁺(experimental), 428.35 ([M + H]⁺(calculated);

Elemental analysis, found C 56.64, H 8.78, N 28.09, calcd for

H(Ttz^tBu,Me)·H₂O C 56.63, H 9.05, N 28.30.

7532 | Dalton Trans., 2011, 40, 7517–7533

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The Royal Society of Chemistry 2011

Dalton Trans., 2011, 40, 7517–7533 | 7533

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Article Doi

Zinc complexes of TtzR,Me with O and S donors reveal differences between Tp and Ttz ligands: Acid stability and binding to H or an additional metal (TtzR,Me = tris(3-R-5-methyl-1,2,4-triazolyl)borate; R = Ph, tBu)

DOI: 10.1039/c1dt10429b

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Full text of DOI:10.1039/c1dt10429b

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