Dalton Dynamic Article Links Transactionspeople.wm.edu/~rdpike/Reprints/DaltonTrans201211663.pdf · suggest that the relatively high energy emission seen in dimers is due to cluster-centred

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Network formation and photoluminescence in copper(I) halide complexeswith substituted piperazine ligands†

Jason P. Safko,a Jacob E. Kuperstock,a Shannon M. McCullough,a Andrew M. Noviello,a Xiaobo Li,b

James P. Killarney,b Caitlin Murphy,b Howard H. Patterson,b Craig A. Baysec and Robert D. Pike*a

Received 8th June 2012, Accepted 18th July 2012DOI: 10.1039/c2dt31241g

The synthesis, X-ray structures and photophysics of ten complexes of CuX (X = I or Br) with bridgingN-substituted and N,N′-disubstituted piperazines (Pip) are presented. Depending on the steric demand ofthe Pip substituents, the complexes fall into four categories: (CuX)4(Pip)2, which are networks of linkedCu4X4 cubane units, (CuX)2(Pip), which are chains of linked Cu2X2 rhombs, and (CuX)2(Pip)2 or(CuX)4(Pip)4, which are simple rhomboid dimers and cubane tetramers. A combination of spectroscopicstudies and DFT calculations was used to investigate the luminescence of the products. The resultssuggest that the relatively high energy emission seen in dimers is due to cluster-centred (XMLT/metal-centred) excitations for the aliphatic amines and MLCT (d → π*) for aromatic amines, and low energyemission seen in the tetramers is the result of cluster-centred transitions. The (CuI)2(Pip) complexes act assensor materials, undergoing irreversible reaction with aliphatic and aromatic amines (Nu) in the vapourstate, irreversibly producing cubanes (CuI)4Nu4, with corresponding production of long wavelengthemission.

Introduction

We have recently reported the reaction of liquid and gaseousamines and sulphides with copper(I) salts CuCN and CuSCNunder ambient conditions.1 When nucleophile uptake isaccompanied by changes in luminescence emission behaviour,these reactions provide the foundation for potential chemicaldetection systems. The key to this behaviour is the presence ofcoordinative vacancies at 2- or 3-coordinate Cu(I) sites. So, forexample, the 2-coordinate metal sites in the infinite –Cu–CN–Cu–CN– chains readily coordinate one or two amine ligandswhen exposed to liquid or gaseous amine.

Copper(I) iodide represents another system that shows evi-dence of photophysical changes in the presence of nucleophiles.2

Copper(I) iodide is a p-type semiconductor having a bandgapof about 3.1 eV.3 The bandgap is associated with promotionof a 3d electron to the 4s or 4p subshell. This results in

photoluminescence emission centred near the UV/vis transition.CuI exists in the 3-D networked γ-form (which has the cubicZnS structure) at ambient temperatures.4 This 3-D network isbroken down when CuI is reacted with nucleophiles.

The tendency of CuI to form topological knots offers amarked contrast to the omnipresent chain-like structures formedby CuCN and CuSCN. A great many simple N-, S-, and P-donorcomplexes of CuI are known.5 The structures of these complexesare usually based on any of four motifs: the rhomboid Cu2I2dimer, the cubane Cu4I4 tetramer (or occasionally higher orderclusters, such as Cu6I6), or the infinite Cu∞I∞ zigzag or stairstep polymers, see Scheme 1(A–D). Cross-linking of CunInknots or chains by a bridging ligand (LL) opens up a widevariety of network structures, see Scheme 1(E–H). Examples ofeach type of network are known: (CuI)2(LL)2 type E,6

(CuI)4(LL)2 type F,6e–g,7,8 CuI(LL) type G,6a,b,9 and (CuI)2(LL)type H.6a,b,9a,b,10

The CuI–LL products E–H, while usually photoluminescent,are not expected to be responsive to incoming nucleophiles. Thisis because all the copper(I) centres are four-coordinate, and there-fore lack available reaction sites for incoming nucleophiles. If,on the other hand, these networks were constructed from three-coordinate Cu centres, the remaining coordinative vacancywould allow for potential reaction with added nucleophiles (Nu).Copper(I) materials that undergo photoemissive changes uponexposure to Nu would be candidates for sensors. In the currentstudy, we show that control of CuI cluster formation and coppercoordination number is enabled through steric manipulation ofbridging N,N′-disubstituted piperazine ligands, see Chart 1.

†Electronic supplementary information (ESI) available: X-ray powderdiffraction and TGA traces, luminescence spectra and cluster-basedorbital plots for Y. CCDC 877782–877791. For ESI and crystallographicdata in CIF or other electronic format see DOI: 10.1039/c2dt31241g

aDepartment of Chemistry, College of William and Mary, Williamsburg,VA 23187-8795, USA. E-mail: [email protected]; Fax: +757-221-2715;Tel: +757-221-2555bDepartment of Chemistry, University of Maine, Orono, ME04469-5706, USA. E-mail: [email protected];Fax: +207-581-1191; Tel: +201-587-1178cDepartment of Chemistry and Biochemistry, Old Dominion University,Norfolk, VA 23529, USA. E-mail: [email protected];Fax: +757-683-4628; Tel: +757-683-4097

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 11663–11674 | 11663

http://dx.doi.org/10.1039/c2dt31241g

http://dx.doi.org/10.1039/c2dt31241g

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Herein we report ten new CuI and CuBr complexes of types A,B, and F, as well as a new type of 3-coordinate Cu(I) network,(CuI)2(LL) (type I, Scheme 2) that shows significant lumines-cence emission changes during reaction with nucleophilevapours. Spectroscopic and computational studies of the variouscomplexes are reported.

Experimental

Materials and methods

All reagents were purchased from Aldrich or Acros and usedwithout purification, except for ligands 4, 5, 7, and 8, whichwere prepared using modified literature methods,11 and CuBr,which was recrystallized from HBr. (CuI)4(pyridine)4 was pre-pared according to the literature.12 Analyses for C, H, and Nwere carried out by Atlantic Microlabs, Norcross, GA or using aThermo Scientific Flash 2000 Organic Elemental Analyzer witha Mettler Toledo XP6 Microbalance. Thermogravimetric ana-lyses (TGA) were conducted using a TA Instruments Q500 inthe dynamic (variable temp.) mode with a maximum heating rateof 50 °C min−1 to 300 °C, under 60 mL min−1 N2 flow.

Bulk syntheses

(CuBr)4(N-Methylpiperazine)2, (CuBr)4(1)2. N-Methylpiper-azine (1, 0.501 g, 5.00 mmol) was dissolved in 25 mL of MeCN.CuBr (1.435 g, 10.0 mmol) was dissolved in a separate 50 mLportion of MeCN under Ar. The solution of 1 was added to theCuBr solution via syringe. A white precipitate formed withinseconds of addition. The suspension was refluxed for 3 h toensure complete reaction. The white precipitate was collected viafiltration, washed with MeCN and ethyl ether, and dried undervacuum (1.143 g, 1.477 mmol, 59.1%). Anal. calcd forC10H24N4Cu4Br4: C, 15.52; H, 3.12; N, 7.24. Found: C, 15.50;H, 3.16; N, 7.20.

(CuI)4(N-Methylpiperazine)2, (CuI)4(1)2. The compound wasprepared in an analogous fashion to (CuBr)4(1)2, using5.00 mmol of 1 and 10.0 mmol of CuI, yielding 2.057 g ofproduct (2.138 mmol, 75.8%). Anal. calcd for C10H24N4Cu4I4:C, 12.48; H, 2.51; N, 5.82. Found: C, 12.36; H, 2.39; N, 5.63.TGA calcd for CuI: 79.2. Found: 79.7 (125–180 °C).

(CuBr)4(N,N′-Dimethylpiperazine)2, (CuBr)4(2)2. The com-pound was prepared in analogous fashion to (CuBr)4(1)2, using5.00 mmol of 2 and 10.0 mmol of CuBr, yielding 1.561 g ofproduct (1.946 mmol, 77.9%). Anal. calcd for C12H28N4Cu4Br4:C, 17.97; H, 3.52; N, 6.98. Found: C, 18.23; H, 3.45; N, 6.98.

(CuI)4(N,N′-Dimethylpiperazine)2, (CuI)4(2)2. The compoundwas prepared in analogous fashion to (CuBr)4(1)2, using5.00 mmol of 2 and 10.0 mmol of CuI (2.171 g, 2.193 mmol,93.0%). Anal. calcd for C12H28N4Cu4I4: C, 14.58; H, 2.85; N,5.66. Found: C, 14.57; H, 2.77; N, 5.67. TGA calcd for CuI:76.9. Found: 77.1 (145–180 °C).

(CuI)2(N,N′-Diethylpiperazine), (CuI)2(3). The compoundwas prepared in analogous fashion to (CuBr)4(1)2, using5.00 mmol of 3 and 10.0 mmol of CuI, yielding 1.619 g ofproduct (3.095 mmol, 61.9%). Anal. calcd for C8H18N2Cu2I2:C, 18.37; H, 3.47; N, 5.35. Found: C, 18.34; H, 3.37; N, 5.32.TGA calcd for CuI: 72.8. Found: 73.4 (115–135 °C).

(CuI)2(N,N′-Dibenzylpiperazine), (CuI)2(4). The compoundwas prepared in analogous fashion to (CuBr)4(1)2, using5.00 mmol of 4 and 10.0 mmol of CuI, yielding 2.326 g ofproduct (3.593 mmol, 71.9%). Anal. calcd for C18H22N2Cu2I2:

Chart 1

Scheme 2

Scheme 1

11664 | Dalton Trans., 2012, 41, 11663–11674 This journal is © The Royal Society of Chemistry 2012

C, 33.40; H, 3.43; N, 4.33. Found: C, 33.33; H, 3.39; N, 4.40.TGA calcd for CuI: 58.8. Found: 59.6 (150–190 °C).

(CuI)2(N,N′-bis-Phenethylpiperazine), (CuI)2(5). The com-pound was prepared in analogous fashion to (CuBr)4(1)2, using3.81 mmol of 5 and 7.57 mmol of CuI, yielding 1.295 g ofproduct (1.918 mmol, 50.3%). Anal. calcd for C20H26N2Cu2I2:C, 35.57; H, 3.88; N, 4.15. Found: C, 35.61; H, 3.88; N, 4.14.TGA calcd for CuI: 56.4. Found: 57.1 (160–210 °C).

(CuI)4(N-Diphenylmethylpiperazine)4, (CuI)4(6)4. The com-pound was prepared in analogous fashion to (CuBr)4(1)2, using5.00 mmol of 6 and 5.00 mmol of CuI, yielding 0.984 g ofproduct (0.556 mmol, 44.5%). Anal. calcd for C68H80N8Cu4I4:C, 46.11; H, 4.55; N, 6.33. Found: C, 46.71; H, 4.54; N, 6.44.TGA calcd for (CuI)5(6)4: 88.6. Found 88.6 (185–205 °C).Calcd for CuI: 43.0. Found: 43.5 (205–245 °C).

(CuI)2(N,N′-bis(2-Pyridylmethyl)piperazine), (CuI)2(7). Thecompound was prepared in analogous fashion to (CuBr)4(1)2,using 2.09 mmol of 7 and 4.00 mmol of CuI, yielding 0.640 gof product (0.986 mmol, 49.3%). Anal. calcd for C16H20N4Cu2I2:C, 29.60; H, 3.10; N, 8.63. Found: C, 30.24; H, 3.17; N, 8.65.TGA calcd for CuI: 58.7. Found: 56.8 (160–210 °C).

(CuI)2(N-(2-Pyridylmethyl)-N′-diphenylmethylpiperazine)2,(CuI)2(8)2. The compound was prepared in analogous fashion to(CuBr)4(1)2, using 3.43 mmol of 8 and 10.0 mmol of CuI, yield-ing 0.998 g of product (0.935 mmol, 54.5%). Anal. calcd forC23H25N3Cu2I2: C, 51.74; H, 4.72; N, 7.87. Found: C, 51.79;H, 4.68; N, 7.86. TGA calcd for CuI: 52.6. Found: 55.8(260–290 °C).

Crystal growth

Ligand (1.00 mmol) and CuI or CuBr (2.00 mmol) were com-bined in 5 mL of MeCN. The mixture was purged with Ar andplaced in a thick-walled threaded tube (Ace Glass 8648-03). Themixture was heated at 100 °C for 72 h. After cooling, themixture was filtered and washed with ethyl ether. Single-crystalX-ray diffraction quality crystals were isolated from most reac-tions. This procedure was used to prepare crystals of (CuBr)4(1)2,(CuI)4(1)3, (CuBr)4(2)2, (CuI)4(2)2, (CuI)2(3), and (CuI)2(4).Crystals of (CuI)2(5), (CuI)4(6)4, (CuI)2(7), and (CuI)2(8)2 wereproduced by layering MeCN solutions of ligand (20 mM) overCuI (40 mM) in 5 mm i.d. tubes.

X-Ray structure determinations

Single crystal determinations were carried out using a BrukerSMART Apex II diffractometer using graphite-monochromatedCu Kα radiation.13 The data were corrected for Lorentz andpolarization14 effects and absorption using SADABS.15 Thestructures were solved by use of direct methods or Pattersonmap. Least squares refinement on F2 was used for all reflections.Structure solution, refinement and the calculation of derivedresults were performed using the SHELXTL16 package of soft-ware. The non-hydrogen atoms were refined anisotropically.In all cases, hydrogen atoms were located and then placed intheoretical positions.

Theoretical methods

Models of copper(I) halide clusters were optimized usingGaussian 0317 and the mPW1PW91 exchange-correlation (xc)functional. The copper atoms were represented by the Ermler–Christiansen relativistic effective core potential (RECP) basisset18 modified to include the 4p contractions of Couty and Hall19

and an additional d-type diffuse function (7s9p7d)/[4s4p5d]. TheWadt–Hay RECP basis sets for the iodine centers were augmen-ted with a set of diffuse and polarization functions.20 Basis setsfor the carbon and nitrogen atoms were the split-valencetriple-ζ plus polarization functions (TZVP) representations ofDunning.21 Hydrogen basis sets were double-ζ quality.22 Fromthe optimized structures, singlet excitations up to 250 nm werecalculated using time-dependent DFT (TD-DFT) using thehybrid mPW1PW91 functional. In contrast to our previous studyof CuCN23 where pure functionals were more effective in repro-ducing the excitation energies, the localization of the photo-luminescence of the CuI clusters is better suited to hybridfunctionals, as has been found for d6 transition metal complexes.24

Adiabatic singlet–triplet gaps were determined as the energydifference between the ground state singlet and the lowest excited-state triplets, corrected for zero-point vibrational energy.

Spectroscopy

Steady-state photoluminescence spectra of the complexes wererecorded with a Model QuantaMaster-1046 photoluminescencespectrophotometer from Photon Technology International. Theinstrument is equipped with two excitation monochromators anda single emission monochromator with a 75 W xenon lamp. Lowtemperature steady-state photoluminescence measurements wereachieved by using a Janis St-100 optical cryostat equipped witha Honeywell temperature controller. Liquid nitrogen was usedas coolant. Lifetime measurements were conducted using anOpolette™ (HE) 355 II UV tunable laser with a range of210–355 nm. The laser has a Nd:YAG flashlamp pumped with apulse repetition rate of 20 Hz and an average output power0.3 mW. The detection system is composed of a monochromatorand photomultiplier from a Jobin Yvon Ramanor 2000 M Ramanspectrometer. Data were collected by a Le Croy 9310C dual400 MHz oscilloscope. The decays were averaged over 1000sweeps and fitted using a curve fitting method in Igor Pro 6.0.

The spectra of nucleophile adducts for (CuI)2(3), and(CuI)2(4) were collected using a fiber optic luminescence spectro-photometer composed of a 365 nm LED source, 365 nmoptical band pass filter and Ocean Optics USB2000+ diode arraydetector. The fiber optic bundle consisted of six excitation fiberoptic lines around a single emission fiber, according to themethod of Mann et al.25 The system output was calibrated usingan Ocean Optics HL-2000-FHSA calibrated light source. Datawere processed using the Ocean Optics Spectral Suite software.

Results and discussion

Synthesis

Direct combination of the N,N′-disubstituted piperazines 1–8with CuI (or CuBr) as solutions in acetonitrile immediatelyproduced solid complexes. Suspensions were refluxed to help


ensure product uniformity. All products were white or yellowcrystalline powders. While the CuI products showed excellentstability in air, the CuBr compounds oxidized slowly, picking uplight green colour over the course of several weeks at roomtemperature. TGA and combustion elemental analyses were usedconfirmed the metal to ligand ratios. TGA resulted in smoothligand loss in the case of iodide compounds, yielding CuI (seeESI†); however TGA results for the bromides were not readilyinterpretable. Single crystals for all compounds were formedfrom unstirred solventothermal reaction mixtures in acetonitrileat 100 °C or layered CuX and ligand solutions at 25 °C. In allbut one case, that of CuBr-1, the products of both proceduresrevealed identical metal to ligand ratio.

X-Ray crystal structures

The intention of using N,N′-substituted piperazine ligands 1–8was to force the formation of three-coordinate copper(I) centres

via steric control. The anticipated result was polymeric networkI (Scheme 2). This arrangement is unusual, but has beenreported for the CuI complex of phenazine.26 Crystal structureswere solved for the CuBr compounds of ligands 1 and 2 and theCuI compounds of all eight ligands. Structure solution data arecompiled in Table 1 and selected bond length and angle infor-mation is provided in Table 2.

The desired structure I was produced for the CuI compoundsof three ligands: 3, 4, and 5. These ligands are each symmetric,having ethyl, benzyl, or phenethyl substituents on both nitrogenatoms. In the structures of (CuI)2(3), (CuI)2(4), and (CuI)2(5)Cu2I2 rhombs are linked into chains (Fig. 1). The acute anglesaround iodine produce sub van der Waals Cu⋯Cu interactions of2.4837(14), 2.4716(11), and 2.4826(14) Å for (CuI)2(3),(CuI)2(4), and (CuI)2(5), respectively. Copper attachment topiperazine is axial in all cases. The independent unit in (CuI)2(3)and (CuI)2(4) consists of CuI and half the ligand. These poly-meric chains form a zigzag pattern. The situation is different for

Table 1 Crystal and structure refinement data

(CuBr)4(1)2 (CuI)4(1)3 (CuBr)4(2)2 (CuI)4(2)2 (CuI)2(3) (CuI)2(4)

CCDC no. 877785 877787 877782 877784 877783 877786Color and habit Yellow prism Colourless blade Colourless prism Colourless plate Colourless plate Colourless prismSize, mm 0.27 × 0.17 × 0.12 0.29 × 0.10 × 0.06 0.22 × 0.11 × 0.05 0.39 × 0.24 × 0.08 0.27 × 0.15 × 0.06 0.42 × 0.14 × 0.10Formula C10H24Br4Cu4N4 C15H37Cu4I4N6 C12H28Br4Cu4N4 C12H28Cu4I4N4 C4H9CuIN C9H11CuINFormula weight 774.13 1063.27 802.18 990.14 261.56 323.63Space group Fdd2 (#43) P21/n (#14) C2/c (#15) C2/c (#15) P21/c (#14) P21/n (#14)a, Å 33.4094(14) 8.6108(3) 26.2013(6) 27.3307(15) 7.1527(2) 7.7387(3)b, Å 37.0836(16) 24.0135(7) 14.0499(3) 14.3197(8) 12.5997(4) 16.7925(5)c, Å 6.5516(3) 13.5384(4) 13.9931(3) 14.3175(8) 7.8483(3) 8.2424(3)α, ° 90 90 90 90 90 90β, ° 90 92.104(2) 121.9660(10) 121.052(2) 90.712(2) 110.6170(10)γ, ° 90 90 90 90 90 90Volume, Å3 8117.0(6) 2797.52(15) 4370.10(17) 1059.32(11) 707.25(4) 1002.52(6)Z 16 4 8 8 4 4ρcalc, g cm−3 2.534 2.525 2.438 2.740 2.456 2.144F000 5888 1988 3072 3648 488 616μ(Cu Kα), mm−1 13.933 38.190 12.973 44.408 37.728 26.796Temperature, K 100 100 100 100 100 100Residuals:a R; Rw 0.0247; 0.0573 0.0436; 0.1119 0.0198; 0.0453 0.0371; 0.0970 0.0287; 0.0732 0.0257; 0.0646Goodness of fit 1.089 1.112 1.185 1.124 1.177 1.120

(CuI)2(5) (CuI)4(6)4 (CuI)2(7) (CuI)2(8)2

CCDC no. 877791 877790 877788 877789Color and habit Colorless blade Colorless plate Yellow needle Yellow prismSize, mm 0.34 × 0.18 × 0.02 0.15 × 0.12 × 0.05 0.23 × 0.05 × 0.04 0.24 × 0.12 × 0.11Formula C20H26Cu2I2N2 C68H80Cu4I4N8 C16H20Cu2I2N4 C46H50Cu2I2N6Formula weight 675.31 1771.16 649.24 1067.80Space group P21/c (#14) P1̄ (#2) P1̄ (#2) P21/n (#14)a, Å 8.4848(3) 8.90150(10) 8.6487(2) 9.6835(3)b, Å 20.3374(7) 19.1000(3) 9.9098(2) 22.3183(6)c, Å 13.1364(4) 21.8081(3) 12.4157(3) 10.0190(3)α, ° 90 114.2040(10) 112.8310(10) 90β, ° 92.548(2) 92.7600(10) 91.449(2) 93.8350(10)γ, ° 90 90.6140(10) 95.4470(10) 90Volume, Å3 2264.56(13) 3376.02(8) 974.07(4) 2160.45(11)Z 4 2 2 2ρcalc, g cm−3 1.981 1.742 2.214 1.641F000 1296 1744 616 1064μ(Cu Kα), mm−1 23.758 16.127 27.613 12.733Temperature, K 100 100 100 100Residuals:a R; Rw 0.0419; 0.1221 0.0380; 0.0963 0.0306; 0.0762 0.0210; 0.0497Goodness of fit 1.022 1.011 1.066 1.097

a R = R1 = Σ||Fo| − |Fc||/ Σ|Fo| for observed data only. Rw = wR2 = {Σ[w(Fo2 − Fc

2)2]/Σ[w(Fo2)2]}1/2 for all data.


(CuI)2(5), which has two independent CuI units and two half-independent ligands. In this case the chain forms a square wavepattern, having two Cu2I2 rhomb orientations and two ligandorientations. (There appears to be a second C-centred monoclinicpolymorph of (CuI)2(5), which has the zigzag chain pattern seenfor (CuI)2(3) and (CuI)2(4), but a high quality structure solutionwas not obtained for this polymorph.) Ligand substituents onadjacent chains fit together zipper-style in all three structures.The Cu2I2 units on adjacent chains are located far apart from oneanother. The closest inter-chain Cu⋯I and Cu⋯Cu approach dis-tances are about 6.0 and 6.5 Å, 8.1 and 8.7 Å, and 5.9 and 7.5 Åfor (CuI)2(3), (CuI)2(4), and (CuI)2(5), respectively. (CuI)2(4)shows a remarkably close intra-chain interaction between anortho phenyl C–H and copper (C5⋯Cu1 = 3.016(4) Å,H5⋯Cu1 = 2.4684 Å), which is absent in the other twostructures.

The smaller methyl- and dimethylpiperazine ligands 1 and 2allowed the formation of cubane or related networks typified byμ3-I. The symmetrical 2 gives rise to isomorphous structures

(CuBr)4(2)2 and (CuI)4(2)2, which are type F networks (seeFig. 2). All bond lengths and angles were similar to knowncubane complexes and networks.5–8 As is typical, relativelyshort Cu⋯Cu distances (2.6234(5), 2.9919(12) Å) were present.All N,N′-dimethylpiperazine (2) ligands were bonded to copperatoms at axial positions. Supramolecular networks are formedfrom macrocyclic (Cu4X4)6(2)6 hexagonal rings. Identical honey-comb superstructures are visible viewing the networks downtheir b-axis, c-axis, and a/b-axis diagonal. Hexagonal layers arestacked directly above one another producing channels, albeitfairly narrow ones, of about 11.5 Å diameter, between piperazinecarbons, passing through the networks in each of the aforemen-tioned directions. In each viewing direction all of the Cu4X4

units are aligned, as are four of the six ligands, while the remain-ing pair of ligands show alternating positions. This 3-D arrange-ment does not match that of either known (CuI)2(piperazine)structure,6h,10 which are of types E and H. Neither is it analo-gous to any of the six known (CuI)4(DABCO)2 structures(DABCO = 1,4-diazabicyclo[2.2.2]octane), all of which are

Table 2 Selected bond lengths and angles for all complexes

(CuBr)4(1)2Cu–Br 2.4317(11)–2.7170(9) Cu–Br–Cua 64.26(3)–71.22(3)Cu–N 1.950(4)–2.105(4) Cu–Br–Cub 96.70(4)–142.20(4)Cu–Cu 2.7427(13)–2.9522(11) Br–Cu–Br 102.46(4)–119.21(5)

Br–Cu–N 99.56(13)–127.21(12)N–Cu–N 156.91(18)

(CuI)4(1)3Cu–I 2.6218(16)–2.7537(15) Cu–I–Cua 57.68(4)–62.06(4)Cu–N 2.062(8)–2.165(8) I–Cu–I 112.21(5)–115.02(6)Cu–Cu 2.5994(19)–2.768(2) I–Cu–N 100.5(2)–116.1(2)(CuBr)4(2)2Cu–Br 2.4956(5)–2.5975(5) Cu–Br–Cua 61.322(13)–67.480(14)Cu–N 2.091(2)–2.100(2) Br–Cu–Br 102.865(16)–116.219(17)Cu–Cu 2.6234(5)–2.8007(5) Br–Cu–N 98.37(6)–126.76(6)(CuI)4(2)2Cu–I 2.6732(9)–2.7195(9) Cu–I–Cua 58.93(3)–67.31(3)Cu–N 2.141(5)–2.148(5) I–Cu–I 100.48(3)–119.92(3)Cu–Cu 2.6686(12)– 2.9377(13) I–Cu–N 99.47(13)–124.41(15)(CuI)2(3)Cu–I 2.5431(7), 2.5859(8) Cu–I–Cua 57.92(3)Cu–N 2.042(4) I–Cu–I 122.08(3)Cu–Cu 2.4837(14) I–Cu–N 111.59(11), 126.33(11)(CuI)2(4)Cu–I 2.5507(6), 2.5945(6) Cu–I–Cua 57.41(2)Cu–N 2.045(3) I–Cu–I 122.59(2)Cu–Cu 2.4716(11) I–Cu–N 112.50(9), 124.85(9)(CuI)2(5)Cu–I 2.5245(10), 2.5363(10), 2.5853(10), 2.5711(10) Cu–I–Cua 58.12(3), 58.16(3)Cu–N 2.044(5), 2.048(6) I–Cu–I 121.33(4), 122.38(4)Cu–Cu 2.4826(14) I–Cu–N 111.22(15), 111.74(16), 125.26(16), 127.20(15)(CuI)4(6)4Cu–I 2.6090(7)–2.7832(8) Cu–I–Cua 56.23(2)–60.99(2)Cu–N 2.052(4)–2.078(4) I–Cu–I 109.64(2)–117.02(3)Cu–Cu 2.6145(10)–2.7045(9) I–Cu–N 92.40(12)–113.68(11)(CuI)2(7)Cu–I 2.5593(6), 2.5729(6), 2.6094(5), 2.6614(6) Cu–I–Cua 58.873(16), 59.386(17)Cu–N 2.061(3), 2.073(3), 2.214(3), 2.237(3) I–Cu–I 122.59(2)Cu–Cu 2.5673(7) I–Cu–N 104.46(8)–120.79(8)

N–Cu–N 80.25(12), 82.22(12)(CuI)2(8)2Cu–I 2.5428(3), 2.6627(4) Cu–I–Cua 61.102(13)Cu–N 2.065(2), 2.2302(18) I–Cu–I 118.898(13)Cu–Cu 2.6479(7) I–Cu–N 100.07(5), 104.50(5), 121.94(5), 123.67(5)

N–Cu–N 79.93(7)

aWithin cluster. bNot within cluster.


roughly of type F, with each exhibiting a different supramole-cular structure.8

The unsymmetrical ligand 1 produced very different structureswith CuBr and CuI. The product (CuBr)4(1)2 exhibited a uniquenetwork of type J (Fig. 3). As with the CuI complex of 1 (seebelow), this unusual arrangement probably results becauseN-alkyl-substituted nitrogen atoms coordinate metal atoms inaxial positions while N–H piperazines coordinate equatorial-ly.1a,6h,10,27 This creates a Cu–centroid–Cu angle of about 121°.In (CuBr)4(1)2 the Cu4Br4 cubane units have been partiallyopened, with one Cu atom swung out. This atom is 3-coordinate,

being coordinated by one cubane Br and two 1 ligands (N–Hend). Bonding of the “swung out” Cu–N is significantly shorter(1.950(4), 1.954(4) Å) than the cluster Cu–N bonds (2.097(4),2.105(4) Å). Two of the remaining Cu atoms have the normal4-coordinate cubane bonding arrangement (three Br bonds andone N bond). The final Cu is bonded to three Br atoms within itscube and one from the adjacent cubane, resulting in directlinkage of the cubanes into a chain. Cross-linking of thesechains by 1 produces tiled macrocycles consisting of two opencubanes, two “swung out” copper atoms and four 1 ligands. Thefour bromine atoms bridge 2, 2, 3, and 4 copper atoms. Onlytwo Cu⋯Cu interactions are present in the open cubane units(2.7427(13), 2.9522(11) Å). One structure that is somewhatrelated to (CuBr)4(1)2 has been reported.27 The network(CuBr)4(L)3·3H2O, L = (S)-1,4-diallyl-2-methylpiperazine formsa chain composed of closed Cu4Br4 cubane units linked througha single Cu via μ4-Br. The three Cu atoms in the cube are cross-linked in a trigonal pattern by the substituted piperazine ligands.In the case of (CuBr)4(1)2, the standard cubane network ratio isconserved, but the distribution of ligands is atypical.

A structurally uncharacterized (CuI)4(1)2 phase is producedthrough bulk reaction of CuI and 1 in acetonitrile. In contrast,the sealed tube reaction in acetonitrile produced a complexhaving the formula (CuI)4(1)3. The novel type K network(CuI)4(1)3 is constructed of Cu4I4 cubanes that are linked by asingle bridging 1 ligand (Fig. 4). The remaining two Cu atomsin the cube are capped by monodentate 1 units connected to themetal centres only by axial N–H units. The polymeric chainspropagate parallel to the c-axis. The usual six Cu⋯Cu inter-actions are apparent within the cubes (2.6200(19)–2.768(2) Å).

Fig. 1 X-Ray structures: A, (CuI)2(3), B, (CuI)2(4), C, (CuI)2(5).X-Ray structure drawing key: H atoms omitted for clarity, ligands shownas wireframe, Cu orange, I violet, N blue, C grey.

Fig. 2 The X-ray structure of (CuI)4(2)2. One layer of 3-D honeycombnetwork shown.

Fig. 3 The X-ray structure of (CuBr)4(1)2. (A) View of tiled layerdown c-axis. (B) View of single cross-linking chain down a/b-axisdiagonal.

Fig. 4 The X-ray structure of (CuI)4(1)3.


This is a rare example of a 1-D polymer of cubane units.7k,28

The current structure opens the door to an as-yet unknown classof polymers having the formula (CuX)4(μ-LL)L2, in which μ-LLis a bridging ligand that forms the chain, while monodentateligands (L) cap the cubane units, preventing growth in otherdirections.

The formation of a type I network for N,N′-dibenzylpiperazineligand (4) left open the possibility that the diphenylmethyl sub-stituent might also be used to further sterically restrict the spacearound Cu(I) centres. However, as is evident from the simplecubane (type A) structure of (CuI)4(6)4, the N-diphenylmethylsubstituent appears to be too large to allow coordination at thesubstituted nitrogen atom. The complex of CuI with 6 showsattachment of the ligand only through the N–H group, equato-rially, as expected. This compound is a fairly typical cubane(Fig. 5), with Cu⋯Cu distances of 2.6142(10)–2.7044(9) andinternal bond angles Cu–I–Cu and I–Cu–I of 56.22(2)–60.98(2)and 109.64(3)–117.02(3), respectively.

When the N-2-pyridylmethylpiperazines 7 and 8 are reactedwith CuI, the result is chelation (Fig. 6). The complex (CuI)2(7)is analogous to (CuI)2(4) insofar that it is a polymer of linkedCu2I2 rhombs. However, in the case of pyridyl-bearing ligand

4-coordinate copper centres are produced. As a result of the che-lation, the chains are relatively compact, lacking the closespacing of R-groups on adjacent chains seen in (CuI)2(4). Thepolymerization seen for (CuI)2(7) is lacking when ligand 8 isused due to the inability of the bulky diphenylmethyl-bearing Nto attach to Cu. The result is a simple type A rhomboid dimer(CuI)2(8)2, which is half crystallographically independent. Inboth (CuI)2(7) and (CuI)2(8)2, rhomboidal Cu⋯Cu interactionsare present (2.5673(7) and 2.6479(7) Å respectively).

X-Ray powder diffraction

Powder diffraction (PXRD) was carried out on all bulk reactionproducts and the results compared to calculated powder patternsgenerated from the single crystal X-ray results (see ESI†). Many,but by no means all, of the overlaid calculated and experimentaldiffractograms indicated a match between the bulk products andthe crystals. As expected, the PXRD traces of bulk (CuI)4(1)2and calculated (CuI)4(1)3 did not match. In addition, mismatchbetween experimental and calculated PXRD were seen for(CuBr)4(1)2, (CuI)2(5), and (CuI)2(7) products, even though thestoichiometries of the bulk and crystal products were identical.Polymorphism in these systems is not remarkable given, e.g., thefact that there are six known (CuI)4(DABCO)2 phases.

Spectroscopy

All of the complexes reported herein were examined at ambientand liquid nitrogen temperature. All complexes were found to bephotoluminescent at 77 K, and all but (CuI)2(4) photolumines-cent at room temperature. The data are shown in Table 3. Themost striking result concerns the difference between cubane andrhomboid dimer copper(I) species. The cubanes show relativelylow energy (LE) emission bands above 500 nm, while thedimers show high energy (HE) bands at wavelengths of less than500 nm.

The 4-coordinate cubane complexes showed excitation bandscentred in the region of 320–350 nm and very broad LE emis-sion bands centred in the region of 560–600 nm. This behaviourwas seen both for the simple cubane (CuI)4(6)4, and the net-worked cubanes (CuI)4(1)3, (CuBr)4(2)2, and (CuI)4(2)2, all ofwhich exhibited yellow to orange emission. Lifetime valuesdetermined for (CuI)4(6)4 and (CuI)4(2)2 were similar (20 and2.6 μs for the former and 7.6 μs for the latter) and relativelylong, indicating the phosphorescent nature of the transition. Thisphotophysical behaviour in (CuX)4L4 cubanes (X = I, Br, Cl) isassociated with the well-documented clustered centred (3CC)transitions of the Cu4X4 units.5j,29–32 These 3CC transitionsrepresent a combination of two phenomena: halide to metalcharge transfer (3XMCT) and metal-centred transfer (3MC).Both charge transfers are associated with largely Cu 4s-orbitalbased LUMOs. The 3XMCT HOMO is largely halide p-orbital-based, and 3MC HOMO largely Cu 3d-orbital-based. The sig-nificant cluster reorganization associated with transition to theexcited state gives rise to relatively large Stokes shifts, as wasnoted in the present species (12 000–14 000 cm−1). Anadditional HE band has been seen for cubanes bearing aromaticligands. This band is absent in all the cubane species reportedFig. 6 X-ray structures: A, (CuI)2(7) and B, (CuI)2(8)2.

Fig. 5 The X-ray structure of (CuI)4(6)4.


here since the piperazine ligands are aliphatic and lack the π*orbitals necessary for a halide to ligand charge transfer (3XLCT)process.

As noted above, the more sterically demanding piperazineligands resulted in 3-coordinate, type I complexes (CuI)2(3),(CuI)2(4), and (CuI)2(5). These species are of particular interestdue to their ability to react with gaseous nucleophiles with a cor-responding alteration in emission behaviour (see below). Thetype I networks show strikingly different emission behaviourfrom that of the other species in the present study. Their lumines-cence spectra showed peak excitation near 320 nm and sharpemission bands centred around 450 nm. In the case of (CuI)2(4),luminescence response was observed only at reduced temp-erature. The lifetime value for (CuI)2(3) of 6 μs is again consistentwith phosphorescence. Stokes shifts were in the range8500–11 000 cm−1. The significant degree of reorganizationimplied by these values and the computational results discussedbelow, suggests that the luminescence activity in these com-plexes also results from 3CC transitions. However, the smallerclusters offer fewer donor orbitals, so the 3CC is dominated by3d10 → 4s13d9 3MC. The absence of room temperature emissionin the case of (CuI)2(4) is intriguing, given its structural simi-larity to the other two type I complexes, (CuI)2(3) and (CuI)2(5).It is possible that the close phenyl⋯copper contact seen in(CuI)2(4) allows for non-radiative relaxation at ambienttemperature.

The polymeric complex (CuI)2(7) and the dimeric complex(CuI)2(8)2 both contain 4-coordinate Cu atoms in rhomboid clus-ters resulting from chelation by 2-pyridylmethyl piperazinegroup(s). Of the complexes studied herein, (CuI)2(7) and(CuI)2(8)2 show the lowest energy excitation features (358 nm inboth cases). Their emission bands are of moderate breadth andare found at less than 500 nm, yielding small Stokes shifts of6500–7800 cm−1. These are the only complexes amongst thosestudied herein that contain aromatic ligands. Experimental andcomputational studies of (CuX)2L4 have revealed a variety ofemission wavelengths that have been assigned to 3CC transitionsin the absence of pyridyl π*-derived orbitals, and largely to3MLCTwhen pyridyl ligands are present.32–34

Computational results

Since the luminescence of (CuI)4L4 cubane tetramers has beenextensively studied (L = NH3,

30 PH3,30 and pyridine31), we set

out to understand the luminescence response of the rhomboiddimers using TD-DFT studies. Photoluminescence of the poly-meric copper(I) iodide materials is assumed to be localized onindividual Cu2I2 clusters for the purposes of our DFT calcu-lations. The model clusters replace the linking piperazines withtrimethylamine, (CuI)2(NMe3)2 (Y) or N,N-dimethyl-2-pyridin-amine (DMP), (CuI2)(DMP)2, (Z) to cap the copper centers. Twoconformations of Z (Z1 (C2) and Z2 (Ci)) were calculated corre-sponding to (CuI)2(7) and (CuI)2(8)2, respectively.

The DFT(mPW1PW91) bond distances and angles for Y (Ci

symmetry, Table 4) agree well with the X-ray structures of(CuI)2(3), (CuI)2(4), and (CuI)2(5) (Table 2) and previous DFTand MP2 studies of a [Cu(NH3)I]2 dimer.35 The Cu2I2 clustercore is distorted such that there are two distinct Cu–I distances,as was observed in the crystal structures and previous compu-tational studies34a of CuX rhombic dimers. The Cu⋯Cu dis-tances in Y and Z are less than the sum of the copper van derWaals radii, demonstrating some metal–metal bonding contri-bution from these d10 centers.36 The Cu⋯Cu bond distances forthe isomers of Z1 and Z2 (2.538 Å) are slightly longer thanthose in Y due to the increase in coordination number, but stillless than the sum of the van der Waals radii. These distances are

Table 3 Luminescence results

Complex Temp., K Excitation λmax, nm Emission λmax, nm (colour) Stokes Shift, cm−1a Lifetime, μs

(CuI)4(1)3 298 336 593 (orange) 12 900 —77 347 580 11 600

(CuBr)4(2)2 298 334 589 (orange) 13 000 —77 331 619 14 100

(CuI)4(2)2 298 321 525 (yellow) 12 100 7.477 309 558 14 400

(CuI)2(3) 298 330 444 (blue) 7780 6.077 307 438 9740

(CuI)2(4) 298 [Non emissive]77 301 448 10 900

(CuI)2(5) 298 319 449 (blue) 9080 —77 306 447 10 300

(CuI)4(6)4 298 327 573 (orange) 13 100 20, 2.677 325 605 14 300

(CuI)2(7) 298 358 496 (yellow-green) 7770 7.0, 0.7(CuI)2(8)2 298 358 466 (blue) 6470 —

aCalculated between longest excitation λmax and shortest emission λmax.


shorter than those calculated at the B3LYP/SDD level for a [Cu-(PH3)2I]2 cluster (2.895 Å), although the Cu⋯Cu distances inthese Cu2X2 clusters are highly dependent upon X and L.35b

The short Cu⋯Cu distances observed in Cu2X2 clusters, inspite of the formal zero net bonding between the d10–d10 Cucenters, has been attributed to both the mixing of the s- andp-type Cu AOs with the d-type MOs to form bonding MOs37

and the transfer of electron density from the halides into thes- and p-type Cu AOs, which stabilizes the cluster and reinforcesCu⋯Cu bonding.35 The similar energies of the copper andiodide AOs lead to highly covalent cluster-type MOs (Fig. 7),which favor the tight rhomboidal structure with a short Cu⋯Cudistance.34a Additional factors that contribute to the covalency ofthe cluster include the similarity in electronegativity between Cu

and I relative to lighter halogens and the ability of the larger I pAOs to better span the distance between the metal centers.Nevertheless, the Cu⋯Cu interaction is weak, as suggested bythe Wiberg bond index (0.089) and the relative character of themetal–metal interaction in certain MOs (e.g., HOMO−4 andHOMO−6 of Y, see ESI†).

The calculated excitation and emission properties of Y, Z1 andZ2 are in good agreement with experimental spectroscopy of theCuI materials. For Y, two transitions with significant non-zerooscillator strengths were found in UV/vis region. The longestwavelength band, attributed to the HOMO–LUMO excitation(328.6 nm), is lower in intensity than HOMO−1 → LUMO(271.9 nm). The relative oscillator strengths of these two bandssuggest that the longer wavelength XMCT HOMO–LUMO exci-tation is hidden under the more intense CC HOMO−1 →LUMO transition. The HOMO may be described as having pre-dominately iodine p character as previously discussed.32 HOMO−1 is an unusual MO insofar that it involves three-centerbonding character between the iodine p fragments and the outerlobes of the Cu d AOs. The LUMO is σ bonding between sphybrid AOs on the Cu centers.

The emission wavelength of Y was estimated from the adia-batic S0 − T gap (424 nm) calculated from the optimized struc-ture of the triplets and is in relatively good agreement with theexperimental λmax for 3–5 (444–448 nm). For 3Y (Fig. 7), thepopulation of the Cu σ-bonded LUMO and the loss of bondingin HOMO−1 results in a decrease in the Cu⋯Cu bond distance(2.291 Å) and a breakdown in the rhomboidal structure of thecluster (Cu–IA = 2.643 Å; Cu⋯IB = 3.455 Å).

For the four-coordinate models Z1 and Z2, the calculated sig-nificant excitations originate from HOMO−2 (Fig. 8, analogousHOMO−1 in Y) to the LUMO, a π* MO of the pyridine ring.Note that previous computational studies that do not calculatethe probability of transition assign the excitation as HOMO →LUMO. A similar excitation is predicted from an examination ofthe LUMO of a related four-coordinate Cu2I2 cluster with a P^Nligand.38 The wavelengths of this transition (Z1, 389 nm; Z2,390 nm) are somewhat longer than the experimental values of358 nm, possibly due to solid state contributions not accountedfor in the gas-phase model. Assignment of the HOMO−2 →LUMO excitation as a 3MLCT transition is consistent with therelatively small Stokes shifts observed for (CuI)2(7) and (CuI)2(8)2.

Table 4 DFT(mPW1PW91) bond distances and angles for the (CuI)2(NMe3)2 (Y) and (CuI)2(DMP)2 (Z1 and Z2) model clusters

Y Z1 Z2

d(Cu–Cu), Å 2.487 2.538 2.538d(X–X), Å 4.607 4.653 4.659d(Cu–X), Å 2.598, 2.637 2.666, 2.672 2.263, 2.283d(Cu–N), Å 2.062 2.082,a 2.263b 2.080,a 2.261b

∠(Cu–X–Cu), ° 56.7 61.5 60.2∠(Cu–X–Cu), ° 123.3 122.7 122.8qCu 0.540 0.566 0.566qX −0.606 −0.619 −0.633WBICu–Cu 0.089 — 0.078HOMO–LUMO 329 (0.023) — —HOMO–LUMO−1 272 (0.116) — —HOMO–LUMO−2 — 389 (0.080) 390 (0.083)

a Pyridyl. bAmine.

Fig. 7 (A) DFT orbitals involved in the excitation spectrum of(CuI)2(NMe3)2, Y. (B) Calculated bond lengths and angles for singletand triplet (CuI)2(NMe3)2, Y.


Formation of tetramers from dimer chains

The 3-coordinate type I networks (CuI)2(3), (CuI)2(4), and(CuI)2(5) show irreversible changes in their luminescence emis-sion in response to gas phase amine and sulphide nucleophiles(Nu). This behaviour is not observed for the other complexesprepared herein, all of which contain 4-coordinated copper(I).The 3-coordinate nature of the copper centres suggests that thechanges seen in the type I complexes could be due to the simpleassociation of the Nu to the Cu centre, as has been demonstratedfor low-coordinate Cu(I) in CuCN and CuSCN polymers.1d,e,39

However, the irreversible nature of the Nu-induced emissionchanges in the present case is suggestive of a more significantchemical change.

Samples of (CuI)2(3) and (CuI)2(4) were exposed to a satu-rated atmosphere of various Nu species at room temperature in asealed vessel for ten minutes. The white product powders wereexamined for luminescence emission using a fibre optic spectro-photometer using LED excitation at 365 nm. The resulting data

are shown in Table 5 and Fig. 9. Importantly, the emission wave-lengths for each Nu adduct were virtually identical, irrespectiveof whether the (CuI)2(3) or (CuI)2(4) substrate was used. More-over, these emission wavelengths were in excellent agreementwith the values for (CuI)4(Nu)4, where the latter is known.29a

This finding strongly suggested that the addition of Nuto the dimer chains was producing (CuI)4(Nu)4 via reaction (1),Pip = 3 or 4.

2ðCuIÞ2ðPipÞ ðsÞ þ 4Nu ðgÞ ! ðCuIÞ4ðNuÞ4 ðsÞ þ 2Pip ðsÞ ð1Þ

Fig. 8 DFT orbitals involved in the excitation spectrum of the isomersof (CuI)2(DMP)2, Z1 and Z2.

Table 5 Luminescence emission data for nucleophile adducts of (CuI)2(4) and (CuI)2(3)

(CuI)2(4) Emissiona (CuI)2(3) Emissiona Literatureb

Nucleophile, Nu

Residual (CuI)2(4)Intensity (444 nm),×10−3 μW cm−2 nm−1

(CuI)4Nu4λmax, nm

(CuI)4Nu4 Intensity,×10−3 μW cm−2 nm−1

(CuI)4Nu4λmax, nm

(CuI)4Nu4 Intensity,×10−3 μW cm−2 nm−1

(CuI)4Nu4λmax, nm

Pyridine 1.00 580 7.54 581 4.80 580a

2-Methylpyridine — 482 2.83 470 9.77 —3-Methylpyridine 0.92 590 5.88 600 0.99 5882-Chloropyridine <0.2 — <0.2 — <0.2 —3-Chloropyridine <0.2 554 4.24 553 2.84 537c

Piperidine 1.57 600 1.88 580 3.40 590a

N-Methylpiperidine 2.10 568 0.32 565 0.46 —Morpholine 1.25 655 1.15 654 2.07 625a

N-Methylmorpholine 1.92 — <0.2 — <0.2 —Pyrrolidine 0.51 640 0.93 642 1.10 —N-Methylpyrrolidine <0.2 610 5.20 608 1.20 —Diethylamine <0.2 619 6.12 609 0.76 —Diethyl sulphide 1.60 559 0.47 552 0.94 —Tetrahydrothiophene 2.00 — <0.2 666 0.38 —

a Solid state, 298 K. bRef. 29a. c In toluene, 298 K.

Fig. 9 Emission spectra of (A) (CuI)2(3) and (B) (CuI)2(4) afterexposure to various Nu’s (excitation at 365 nm). The arrow indicates theresidual emission from (CuI)2(3). (C) Photos under 365 nm irradiationof Nu vapour-exposed (CuI)2(4) samples (right-to-left), top row: Nu =pyridine, 2-methylpyridine, 3-methylpyridine, 3-chloropyridine, piper-idine, bottom row: N-methylpiperidine, morpholine, N-methylmorpho-line, pyrrolidine, tetrahydrothiophene, and dimethyl sulphide.


In contrast to (CuI)2(4), (CuI)2(3) has a HE band that servedas a marker for the unreacted dimer chain during Nu vapourexposure (see arrow in Fig. 9, see also Table 5). In all cases thisband was diminished, and in some cases it was extinguished,during vapour exposure. Assuming that diminution of this bandrepresents the chemical conversion of (CuI)2(3), it is noteworthythat reaction (1) was occurring even in cases for whichno product emission was evident, such as for Nu = 2-chloro-pyridine. The apparent lack of emission in the vapour-exposedproduct was therefore not due to lack of reaction, but rather tothe formation of products that are not emissive at roomtemperature.

In order to confirm the conversion of CuI-piperazine polymersto CuI–Nu tetramers, (CuI)2(3) was suspended in a 5% pyridine/toluene solution and stirred for two hours at room temperature.X-ray powder diffraction of the filtered powder revealed com-plete conversion of (CuI)2(3) to (CuI)4(pyridine)4, as confirmedby comparison to an authentic sample of the tetramer (see ESI†).This result supported the conclusion that piperazine ligands suchas 3 are readily displaced by aromatic nucleophiles such aspyridine.

Conclusions

We have prepared a variety of substituted piperazine complexesof CuI and several of CuBr. The least sterically demandingmethyl piperazines enable the formation of cubane nodes, whichare linked to form 1-, 2-, or 3-D networks through piperazinebridging, see Fig. 2–5. Moderately large ethyl, benzyl, and phen-ethyl piperazines produce 3-coordinate Cu(I) Cu2I2 dimers,which link into 1-D chains with 3-coordinate metal centresforming the structures shown in Fig. 1. The very large diphenyl-methyl substituent obviates coordination, while the 2-pyridyl-methyl substituent chelates copper and thus restricts networkformation, yielding the structures shown in Fig. 6. All of thecomplexes studied photoemit at low temperature (and all but onedo so at ambient temperature). Excitation occurs in the near UVand has been determined to be the result of cluster centredtransitions for the dimers and the tetramers. The latter are foundat longer wavelengths due to their greater degree of delocaliza-tion. In the pyridyl-containing ligands, excitation is associatedwith MLCT to ligand π* orbitals. Based on spectroscopic data,the dimer chain complexes react with gaseous amines spon-taneously and irreversibly to form cubane tetramers throughreplacement of the piperazine ligands. This reaction enablesthese compounds to act as sensors for amine vapour.

Notes and references

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2 (a) H. D. Hardt and A. Pierre, Z. Anorg. Allg. Chem., 1973, 402, 107–112; (b) E. Cariati, X. Bu and P. C. Ford, Chem. Mater., 2000, 12, 3385–3391.

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