Copper(I) thiocyanate-amine networks: Synthesis, structure, and luminescence behavior

Article abstract of DOI:10.1021/ic200821f

A series of metal-organic networks of CuSCN were prepared by direct reactions with substituted pyridine and aliphatic amine ligands, L. Thiocyanate bridging is seen in all but 1 of 11 new X-ray structures. Structures are reported for (CuSCN)L sheets (L = 3-chloro- and 3-bromopyridine, N-methylmorpholine), ladders (L = 2-ethylpyridine, N-methylpiperidine), and chains (L = 2,4,6-collidine). X-ray structures of (CuSCN)L₂ are chains (L = 4-ethyl- and 4-t-butylpyridine, piperidine, and morpholine). A unique N-thiocyanato monomer structure, (CuSCN)(3-ethylpyridine)₃, is also reported. In most cases, amine ligands are thermally released at temperatures <100 °C. Strong yellow-to-green luminescence at ambient temperature is observed for the substituted pyridine complexes. High solid state quantum efficiencies are seen for many of the CuSCN-L complexes. Microsecond phosphorescence lifetimes seen for CuSCN-L are in direct contrast to the nanosecond-lifetime emission of CuSCN. MLCT associated with pyridine π* orbitals is proposed as the excitation mechanism.

Full text of DOI:10.1021/ic200821f

ARTICLE
pubs.acs.org/IC
Copper(I) Thiocyanate-Amine Networks: Synthesis, Structure, and
Luminescence Behavior.
Kayla M. Miller, Shannon M. McCullough, Elena A. Lepekhina, Isabelle J. Thibau, and Robert D. Pike*
Department of Chemistry, College of William and Mary, Williamsburg, Virginia 23187-8795, United States
Xiaobo Li, James P. Killarney, and Howard H. Patterson
Department of Chemistry, University of Maine, Orono, Maine 04469-5706, United States
S Supporting Information
b
ABSTRACT: A series of metalꢀorganic networks of CuSCN
were prepared by direct reactions with substituted pyridine
and aliphatic amine ligands, L. Thiocyanate bridging is seen
in all but 1 of 11 new X-ray structures. Structures are reported
for (CuSCN)L sheets (L = 3-chloro- and 3-bromopyridine,
N-methylmorpholine), ladders (L = 2-ethylpyridine, N-methyl-
piperidine), and chains (L = 2,4,6-collidine). X-ray structures of
(CuSCN)L2 are chains (L = 4-ethyl- and 4-t-butylpyridine,
piperidine, and morpholine). A unique N-thiocyanato mono-
mer structure, (CuSCN)(3-ethylpyridine)3, is also reported. In
most cases, amine ligands are thermally released at temperatures <100 °C. Strong yellow-to-green luminescence at ambient
temperature is observed for the substituted pyridine complexes. High solid state quantum efficiencies are seen for many of the
CuSCN-L complexes. Microsecond phosphorescence lifetimes seen for CuSCN-L are in direct contrast to the nanosecond-lifetime
emission of CuSCN. MLCT associated with pyridine π* orbitals is proposed as the excitation mechanism.
’ INTRODUCTION
As suggested in Scheme 1B, addition of a single L per Cu would
limit sulfur bridging, resulting in the formation of a 2D network,
or perhaps a 1D ladder structure. Addition of two L per Cu would
presumably lead to formation of simple decorated chains, like
those seen for CuCNLn.
In the course of our recent studies on the metalꢀorganic net-
work chemistry of copper(I) cyanide, we have noted that Cu(I)
centers in CuCN chains are readily and reversibly decorated by
amine and sulfide nucleophiles.1,2 Coordination of nucleophiles
shifts CuCN photoluminescence emission from the near UV
(392 nm) into the visible region. As a result, the reversible lumi-
nescence-inducing reaction of amines with CuCN has potential
use in sensing devices. In some cases, minor differences in
the amine (e.g., between piperidine, N-methylpiperidine, and
N-ethylpiperidine) result in remarkably different emission colors.2
X-ray analysis of CuCNLn complexes confirmed the addition of
amine or sulfide ligands to copper centers along the polymeric
chains, increasing the Cu coordination number from 2- to 3- or
4-coordinate (Scheme 1A, L = amine or sulfide ligand, “0” =
vacant coordination site).
Here we report the amine adduct chemistry of CuSCN. Both
CuCN and CuSCN are air-stable Cu(I) species; however, the
thiocyanate anion also stabilizes the Cu(II) oxidation state.3
Copper(I) thiocyanate has attracted attention as a p-type semi-
conductor for solar applications.4 Copper(I) thiocyanate differs
structurally from CuCN because of the extensive bridging capabil-
ities of sulfur. Three polymorphs of CuSCN are known,5 each of
which constitutes a 3D network containing 4-coordinate Cu and
S (Scheme 1B). Coordination of amines or other nucleophiles
would be expected to cause disruption of the CuSCN network.
To investigate the structural chemistry posited in Scheme 1B,
we set out to identify the various structural types of CuSCN-
amine compounds using the ligands shown in Chart 1, and also to
examine product luminescence behavior. There have been many
reports of the reaction of CuSCN with bidentate amines.6 Chelating
diamines form chains, and bridging diamines usually form sheets.
Imidazolidine-2-thione and a thiazolidine-2-thione thiocarbonyl
complexes of CuSCN have been reported.7,8 Several complexes
of CuSCN with simple amines were first noted in 1968.9 How-
ever, no structural data were reported. Aside from a few reports of
diamines behaving in monodentate fashion,6f,g,10 only a single
structural study of simple monodentate amine coordination to
CuSCN has appeared.11 In that paper the structures of 1:1
complexes (CuSCN)L (L = 2MePy and 26Lut), and 1:2 com-
plexes (CuSCN)L2 (L = 2MePy, 3MePy, 4MePy,12 24Lut, and
Quin) were reported. No mention has yet been made of photo-
luminescence behavior by the CuSCN-monoamine complexes.
Received: April 20, 2011
Published: July 05, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Scheme 1
single emission monochromator with a 75 W xenon lamp. Low tem-
perature steady-state photoluminescence measurements were achieved
by using a Janis St-100 optical cryostat equipped with a Honeywell tem-
perature controller. Liquid nitrogen was used as coolant. Lifetime
measurements were conducted using an Opolette (HE) 355 II UV
tunable laser with a range of 210ꢀ355 nm. The laser has a Nd:YAG
flashlamp pumped with a pulse repetition rate of 20 Hz and an average
output power 0.3 mW. The detection system is composed of a mono-
chromator and photomultiplier from a Jobin Yvon Ramanor 2000 M
Raman spectrometer. Data were collected by a Le Croy 9310C dual
400 MHz oscilloscope. The decays were averaged over 1000 sweeps and
fitted using a curve fitting method in Igor Pro 6.0. Solid state quantum
yields were measured on a Perkin-Elmer LS 55 spectrofluorimeter using
a modified version of Mann’s method,13 in which sample comparison is
made to a “perfect scatterer”, Fluorilon FW-99 (Avian Technologies,
Sunapee, NH). IR measurements were made on KBr pellets using a
Digilab FTS 7000 FTIR spectrophotometer. Thermogravimetric ana-
lyses (TGA) were conducted using a TA Instruments Q500 in the
dynamic (variable temp.) mode with a maximum heating rate of
50 °C/min to 300 °C under 60 mL/min. N2 flow. The theoretical struc-
ture and energy of electronic states of aromatic amines were determined
with Gaussian ’03 software (Gaussian Inc.).14 Density functional theory
(DFT) optimization calculations were performed with the B3LYP func-
tional15 and 6-31+g* basis set as implemented in the software. These
calculations were performed on the University of Maine supercomputer.
Syntheses. CuSCN(2MePy)2, 1a. While stirring, 5 mL of 2MePy
were added to solid CuSCN (0.182 g, 1.50 mmol), and the resulting
suspension was sealed in a vial under Ar and stirred at ambient tem-
perature for 4 d. The suspended solid was collected by means of filtration,
washed with diethyl ether, and then air-dried. An off-white powder was
isolated (0.409 g, 1.33 mmol, 88.6%). IR (KBr pellet, cmꢀ1) 2112, 801,
761, 727. Ligand loss was too rapid to allow for CHN analysis. TGA
Calcd for CuSCN(2MePy): 69.8. Found: 68.6 (25ꢀ45 °C). Calcd for
CuSCN: 39.5. Found: 37.7 (60ꢀ95 °C).
Chart 1
CuSCN(2MePy), 1b. Product 1a was placed under vacuum overnight,
producing an off-white powder (0.220 g, 1.02 mmol, 68.3%). IR (KBr
pellet, cmꢀ1) 2112, 800, 761, 727 Anal. Calcd for C7H7N2CuS: C, 39.15;
H, 3.29; N, 13.04. Found: C, 41.12; H, 3.06; N, 12.78. TGA Calcd for
CuSCN: 56.6. Found: 58.7 (55ꢀ90 °C).
CuSCN(3MePy)2, 2. The procedure was identical to that used for 1a
using 3MePy. A yellow powder was isolated (0.376 g, 1.22 mmol,
81.4%). IR (KBr pellet, cmꢀ1) 2197, 796, 763, 703. Anal. Calcd for
C13H14N3CuS: C, 50.72; H, 4.58; N, 13.65. Found: C, 51.41; H, 4.67; N,
13.67. TGA Calcd for CuSCN(3MePy): 69.8. Found: 67.1 (50ꢀ75 °C).
Calcd for CuSCN: 39.5. Found: 41.9 (75ꢀ105 °C).
CuSCN(4MePy)2, 3. The procedure was identical to that used for 1a
using 4MePy. A tan powder was isolated (87.4%). IR (KBr pellet, cmꢀ1
)
2122, 2096, 801, 768, 721. Anal. Calcd for C13H14N3CuS: C, 50.72; H,
4.58; N, 13.65. Found: C, 52.86; H, 4.56; N, 13.48. TGA Calcd for
CuSCN(4MePy): 69.8. Found: 69.2 (60ꢀ85 °C). Calcd for CuSCN:
39.5. Found: 40.4 (85ꢀ110 °C).
We considered it to be of interest to look for correlations in
photophysical behavior between structurally related groups.
CuSCN(2EtPy), 4. The procedure was identical to that used for 1a
using 2EtPy. A beige powder was isolated (92.7%). IR (KBr pellet, cmꢀ1
)
2163, 2104, 800, 752. Anal. Calcd for C8H9N2CuS: C, 42.00; H, 3.97; N,
12.24. Found: C, 42.00; H, 3.89; N, 12.12. TGA Calcd for CuSCN: 53.2.
Found: 54.0 (65ꢀ85 °C).
’ EXPERIMENTAL SECTION
Materials and Methods. All reagents were purchased from Aldrich
or Acros and used without purification. Commercial CuSCN (Aldrich)
was shown by FTIR to consist solely of the R-phase.8 Analyses for C, H,
and N were carried out by Atlantic Microlabs, Norcross, GA, or using a
Thermo Scientific Flash 2000 Organic Elemental Analyzer with a
Mettler Toledo XP6Microbalance. Steady-state photoluminescence
spectra were recorded with a Model QuantaMaster-1046 photolumi-
nescence spectrophotometer from Photon Technology International.
The instrument is equipped with two excitation monochromators and a
CuSCN(3EtPy)3, 5. The procedure was identical to that used for
1a using 3EtPy. A pale green powder was isolated (73.9%). IR (KBr
pellet, cmꢀ1) 2114, 2082, 809, 748, 708. Anal. Calcd for C22H27N4CuS:
C, 59.64; H, 6.14; N, 12.64. Found: C, 58.96; H, 5.49; N, 12.36. TGA
Calcd for CuSCN: 27.4. Found: 25.3 (40ꢀ160 °C).
CuSCN(4EtPy)2, 6. The procedure was similar to that used for 1a using
4EtPy. A dark colored solution resulted. Diethyl ether was layered onto the
solution under Ar, resulting in precipitation of a yellow powder overnight
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Inorganic Chemistry
ARTICLE
(88.1%). IR (KBr pellet, cmꢀ1) 2099, 2083, 826, 820, 783, 761. Anal.
Calcd for C15H18N3CuS: C, 53.63; H, 5.40; N, 12.51. Found: C,
53.63; H, 5.34; N, 12.43. TGA Calcd for CuSCN: 36.2. Found: 37.1
(65ꢀ85 °C).
CuSCN(MePyrrd), 17. The procedure was identical to that used for
1a using MePyrrd. A white powder was isolated (89.3%). IR (KBr
pellet, cmꢀ1) 2122, 870, 756. Anal. Calcd for C6H11N2CuS: C, 34.85; H,
5.36; N, 13.55. Found: C, 34.45; H, 5.36; N, 13.43. TGA Calcd for
CuSCN: 58.8. Found: 60.5 (55ꢀ75 °C).
CuSCN(Quin)2, 7. The procedure was identical to that used for 1a using
Quin. A yellow powder was isolated (67.7%). IR (KBr pellet, cmꢀ1) 2096,
805, 781. Anal. Calcd for C19H14N3CuS: C, 60.06; H, 3.71; N, 11.06.
Found: C, 59.24; H, 3.58; N, 11.02. TGA Calcd for CuSCN(Quin):
66.0. Found: 65.8 (90ꢀ115 °C). Calcd for CuSCN: 32.0. Found: 32.6
(115ꢀ145 °C).
CuSCN(NHEt2), 18. The procedure was identical to that used for
1a using NHEt2. A pale green powder was isolated (83.8%). IR
(KBr pellet, cmꢀ1) 2173, 2111, 2097, 820, 791, 766. Anal. Calcd for
C5H11N2CuS: C, 30.84; H, 5.69; N, 14.38. Found: C, 26.77; H, 5.00;
N, 14.50. TGA Calcd for CuSCN: 62.4. Found: 64.4 (35ꢀ60 °C).
(CuSCN)3(NMe2Cy)2, 19. The procedure was identical to that used
for 1a using NMe2Cy. A beige powder was isolated (97.5%). IR
(KBr pellet, cmꢀ1) 2114, 894, 858, 835, 777, 750. Anal. Calcd for
C19H34N5Cu3S3: C, 36.85; H, 5.53; N, 11.31. Found: C, 37.60; H, 5.40;
N, 10.82. TGA Calcd for CuSCN: 58.9. Found: 57.3 (60ꢀ80 °C).
X-ray Analysis. Single crystal determinations were carried out using
a Bruker SMART Apex II diffractometer using graphite-monochro-
mated Cu KR radiation.16 The data were corrected for Lorentz and
polarization17 effects and absorption using SADABS.18 The structures
were solved by use of direct methods or Patterson map. Least squares
refinement on F2 was used for all reflections. Structure solution, refinement,
and the calculation of derived results were performed using the SHELXTL19
package of software. The non-hydrogen atoms were refined anisotropi-
cally. In all cases, hydrogen atoms were located, then placed in theoretical
positions.
CuSCN(26Lut), 8. The procedure was identical to that used for 1a using
26Lut. A white powder was isolated (62.1%). IR (KBr pellet, cmꢀ1) 2120,
779, 768. Anal. Calcd for C8H9N2CuS: C, 42.00; H, 3.97; N, 12.24.
Found: C, 42.43; H, 4.03; N, 12.43. TGA Calcd for CuSCN: 53.2.
Found: 54.1 (70ꢀ105 °C).
CuSCN(246Coll), 9. The procedure was identical to that used for 1a
using246Coll. A tan powder was isolated (93.2%). IR (KBr pellet, cmꢀ1
)
2116, 851, 800. Anal. Calcd for C9H11N2CuS: C, 44.52; H, 4.57; N,
11.54. Found: C, 42.80; H, 4.52; N, 11.39. TGA Calcd for CuSCN: 50.1.
Found: 51.0 (75ꢀ110 °C).
CuSCN(4tBuPy)2, 10. The procedure was identical to that used for 1a
using 4tBuPy. A pale green powder was isolated (86.9%). IR (KBr
pellet, cmꢀ1) 2087, 829, 797. Anal. Calcd for C19H26N3CuS: C, 58.21;
H, 6.68; N, 10.72. Found: C, 56.61; H, 6.38; N, 10.47. TGA Calcd for
CuSCN: 31.0. Found: 29.8 (70ꢀ230 °C).
Powder diffraction analysis was carried out on the instrument described
above. Samples were ground and prepared as mulls using Paratone N oil.
Four 180 s frames were collected, covering 8ꢀ100° 2θ. Frames were
merged using the SMART Apex II software16 and were further processed
using DIFFRAC-Plus and EVA software.20 Simulated powder patterns
from single crystal determinations were generated using the Crystal-
lographica program.21 Experimental and calculated powder diffraction
results are provided in the Supporting Information.
CuSCN(3ClPy)2, 11a. The procedure was identical to that used for 1a
using 3ClPy. A yellow powder was isolated (91.8%). IR (KBr pellet, cmꢀ1
)
2118, 2100, 804, 766, 738, 694. Anal. Calcd for C11H8N3Cl2CuS: C,
37.89; H, 2.31; N, 12.05. Found: C, 37.40; H, 2.26; N, 11.99. TGA Calcd
for (CuSCN)3(3ClPy)2: 56.5. Found: 59.8 (50ꢀ70 °C). Calcd for
CuSCN: 34.9. Found: 35.5 (70ꢀ90 °C).
CuSCN(3BrPy)2, 12a. The procedure was identical to that used for 1a
using 3BrPy. A yellow powder was isolated (60.6%). IR (KBr pellet, cmꢀ1
)
2116, 2099, 802, 799, 763, 710, 693. Anal. Calcd for C11H8N3Br2CuS: C,
30.19; H, 1.84; N, 9.60. Found: C, 31.21; H, 1.93; N, 9.87. TGA Calcd
for (CuSCN)3(3BrPy)2: 63.9. Found: 63.8 (60ꢀ80 °C). Calcd for
CuSCN: 27.8. Found: 28.3 (80ꢀ110 °C).
’ RESULTS AND DISCUSSION
Synthesis. Amine (L) adduct powders were easily prepared by
stirring the solid CuSCN suspended in neat liquid amine. In the
absence of heating, a period of several days was required for
complete conversion. Alternatively, when the neat mixture was
heated to 70 °C in a sealed tube, the reactions proceeded more
rapidly (usually being completed in one night) and often produced
X-ray quality crystals. (The CuCN-L products reported in our
previous contributions2 can also be made via the ambient tem-
perature technique, as well as by the previously reported heated
tube syntheses.) Only in the case of 4EtPy did the CuSCN dis-
solve in the amine at ambient temperature. (Interestingly, identical
behavior was noted for CuCN in 4EtPy.) In this case the product
(6) was coaxed from solution by layering the mixture with diethyl
ether under inert atmosphere. The use of L = EtPipd, NEt3,
2ClPy and 2BrPy returned unreacted CuSCN at both tempera-
tures. All products except 5, 1a and 13a (as described below)
showed indefinite stability. Except as noted, the discussions
below center around compounds formed at ambient temperature
since complete reactions and homogeneous products were the
rule under these conditions.
CuSCN(Pipd)2, 13a. The procedure was identical to that used for 1a
using Pipd. A tan powder was isolated (79.6%). IR (KBr pellet, cmꢀ1
)
2167, 2111, 869, 847, 771, 757. Ligand loss was too rapid to allow for
CHN analysis. TGA Calcd for CuSCN(Pipd): 76.4. Found: 74.9
(35ꢀ50 °C). Calcd for CuSCN: 44.9. Found: 44.2 (50ꢀ105 °C).
CuSCN(Pipd), 13b. Product 13a was placed under vacuum line
overnight, producing a white powder. IR (KBr pellet, cmꢀ1) 2113,
870, 847, 770. Anal. Calcd for C6H11N2CuS: C, 34.85; H, 5.36; N, 13.55.
Found: C, 35.81; H, 5.43; N, 15.60. TGA Calcd for CuSCN: 58.8.
Found: 58.5 (60ꢀ105 °C).
CuSCN(MePipd), 14. The procedure was identical to that used for
1a using MePipd. A yellow powder was isolated (83.6%). IR (KBr
pellet, cmꢀ1) 2116, 2108, 860, 774, 753. Anal. Calcd for C7H13N2CuS:
C, 38.08; H, 5.93; N, 12.69. Found: C, 37.72; H, 5.92; N, 12.57. TGA
Calcd for CuSCN: 55.1. Found: 56.1 (55ꢀ85 °C).
CuSCN(Morph)2, 15. The procedure was identical to that used for 1a
using Morph. A white powder was isolated (92.8%). IR (KBr pellet, cmꢀ1
)
2089, 863, 763, 623. Anal. Calcd for C9H18N3CuO2S: C, 36.54; H, 6.13;
N, 14.20. Found: C, 36.49; H, 6.32; N, 14.13. CuSCN(Morph): 70.6.
Found: 69.4 (35ꢀ95 °C). TGA Calcd for CuSCN: 41.1. Found: 41.4
(95ꢀ130 °C).
The stoichiometry of the CuSCN-L products was readily
determined via thermogravimetric analysis (TGA), as previously
noted for CuCN-L products.2 Analytical TGA data are listed in
the Experimental Section. In all cases, except for that of the
molecular 3EtPy complex (5), heating of solid CuSCN-L sample
to 300 °C afforded clean recovery of CuSCN, allowing for ready
identification of CuSCN:L ratio. Stoichiometries were further
CuSCN(MeMorph), 16. The procedure was identical to that used for
1a using MeMorph. A white powder was isolated (86.8%). IR (KBr
pellet, cmꢀ1) 2174, 2118, 895, 864, 785, 758. Anal. Calcd for C6H11-
N2CuOS: C, 32.35; H, 4.98; N, 12.57. Found: C, 32.40; H, 4.62; N,
12.11. TGA Calcd for CuSCN: 54.6. Found: 56.1 (60ꢀ95 °C).
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Table 1. Synthetic and Structural Summary
liganda
productb
CuSCN/L
structure type
SCN bridging mode
Cu coordination no.
2MePyc
1a
1b
2
1:2
1:1
1:2
1:2
1:1
1:3
1:2
1:2
1:1
1:1
1:2
1:2
1:1
1:2
1:1
1:2
1:1
1:1
1:2
1:1
1:1
1:1
3:2
A, helical chaind
B, ladderd
A, planar zigzag chaind
A, planar zigzag chaind
B, ladder
S,N-bridged
S,S,N-bridged
S,N-bridged
S,N-bridged
S,S,N-bridged
N-bound
4-coordinate
4-coordinate
4-coordinate
4-coordinate
4-coordinate
4-coordinate
4-coordinate
4-coordinate
3-coordinate
3-coordinate
4-coordinate
3MePy
4MePy
2EtPy
3EtPy
4EtPy
Quin
3
4
5
D, monomer
6
A, planar zigzag chain
A, helical chaind
A0, helical chaind
A0, planar zigzag chain
A, helical chain
S,N-bridged
S,N-bridged
S,N-bridged
S,N-bridged
S,N-bridged
7
26Lut
246Coll
4tBuPy
3ClPye
8
9
10
11a
11b
12a
12b
13a
13b
14
15
16
17
18
19
C, rippled sheet
S,S,N-bridged
4-coordinate
3BrPye
Pipdc
C, rippled sheet
S,S,N-bridged
S,N-bridged
4-coordinate
4-coordinate
A, planar zigzag chain
MePipd
Morph
B, ladder
S,S,N-bridged
S,N-bridged
S,S,N-bridged
4-coordinate
4-coordinate
4-coordinate
A, planar zigzag chain
C, rippled sheet
MeMorph
MePyrrd
NHEt2
NMe2Cy
a No reaction with EtPipd, NEt3, 2ClPy, 2BrPy. b Products prepared at 25 °C, except as noted. c 1:2 Product formed initially, converting to 1:1 product
under vacuum. d Reference 11. e 1:2 Product formed at 25 °C and 1:1 product formed at 70 °C.
confirmed by elemental analysis. The CuSCN/L ratios found
were 1:1, 3:2, 1:2, and 1:3, see Table 1. Single phases were
formed in most cases and product identities were further con-
firmed by comparison of powder X-ray diffraction (PXRD) data
to PXRD patterns calculated from authentic crystal structures
(see Supporting Information).
failed to react with CuSCN under heating. For L = 26Lut, 246Coll,
and MePipd, PXRD and TGA showed evidence of unreacted
CuSCN in heated reactions. Crystals isolated from these 70 °C
reactions produced structures in agreement with ambient tem-
perature PXRD results. On the other hand, the 3ClPy and 3BrPy
heated reactions gave products showing 1:1 stoichiometry by
TGA and yielded 1:1 X-ray structures. These results were in
sharp contrast to the 1:2 ambient temperature products obtained
with 3ClPy and 3BrPy.
X-ray Structures. Eleven new X-ray structures emerged from
the current study. Refinement details for all structures are summar-
ized in Table 2, and selected bond lengths and angles are given in
Table 3. Among the CuSCN-L structures now known,6,7,10,11
several structural themes can be identified (see Chart 2). As was
predicted in Scheme 1, coordination of two L per Cu invariably
produces a polymer (A in Chart 2). Coordination of a single L
per Cu can lead to any of three outcomes: polymer A0 (0 = vacant
coordination site), ladder (double chain) B, or sheet network C.
While polymers A and A0 feature simple S,N-bridged thiocya-
nate, in networks B and C thiocyanate bridging is expanded to S,
S,N-type. New to this study is molecular structure D, which
features terminal N-bonded thiocyanate. As shown in Table 3,
the CuꢀSꢀC, SꢀCꢀN, and CꢀNꢀCu bond angles were
95.15(7)ꢀ110.76(7)°, 177.7(4)ꢀ179.39(19)°, and 156.3(3)ꢀ
175.62(14)°, suggesting that the SꢀCtN thiocyanate resonance
form dominates over SdCdN. Copperꢀamine bond lengths
appear to be slightly shorter for A0 (1.9479(19), 1.993(3) Å) than
for A (2.043(5)ꢀ2.1376(11) Å), B (2.000(3)ꢀ2.0919(18) Å),
C (2.012(2)ꢀ2.1279(12) Å), or D (2.0286(18), 2.0322(17),
2.0420(16) Å).
Among the amines investigated, three gave evidence of mixed
stoichiometry products: 2MePy, Pipd, and MeMorph. The initial
CuSCN-2MePy and CuSCN-Pipd products were identified using
TGA as being (CuSCN)L2 (1:2) compounds, 1a and 13a. How-
ever, L loss began immediately, making CHN analysis impos-
sible. Ligand loss ultimately resulted in 1:1 products 1b and 13b,
as confirmed by CHN analysis. X-ray structures for both 1:2 and
1:1 CuSCN complexes of 2MePy are known.11 PXRD of the 1:2
2MePy product 1a produced here did not match the calculated
pattern from reported compound CuSCN(2MePy)2, but 1b did
match the reported CuSCN(2MePy) pattern (Figures S1 and S2,
Supporting Information). The PXRD of 13a matched the cal-
culated pattern for the CuSCN(Pipd)2 (Figure S14, Supporting
Information). For the CuSCN-MeMorph product, both TGA
and CHN indicated pure CuSCN(MeMorph), 16. However,
PXRD indicated the presence of a minor component (Figure
S17, Supporting Information). Given the good quality analytical
data, this impurity is probably a polymorph. In all other cases
single products were found. The rapid progression of 2MePy loss
during the conversion: 1a (yellow emission) f 1b (blue emis-
sion) is shown as a series of photos under 365 nm excitation in
Figure S56 (Supporting Information).
The heated tube reaction products were usually found to be
identical to those made at ambient temperature, although NMe2Cy
7242
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Inorganic Chemistry
ARTICLE
Table 2. Crystal and Structure Refinement Data
4
5
6
9
10
11b
CCDC deposit no.
color and habit
size, mm
formula
808077
808078
808082
808079
808084
808083
colorless prism
0.22 ꢁ 0.10 ꢁ 0.09
C8H9CuN2S
228.77
yellow prism
0.44 ꢁ 0.10 ꢁ 0.09
C22H27CuN4S
443.08
yellow block
0.37 ꢁ 0.23 ꢁ 0.20
C15H18CuN3S
335.92
colorless prism
0.24 ꢁ 0.06 ꢁ 0.05
C9H11CuN2S
242.80
yellow plate
0.41 ꢁ 0.39 ꢁ 0.10
C19H26CuN3S
392.03
yellow plate
0.38 ꢁ 0.36 ꢁ 0.08
C6H4ClCuN2S
235.16
formula weight
space group
a, Å
P21/c (#14)
5.8113(2)
15.4887(6)
9.7322(4)
91.121(2)
875.82(6)
4
P21 (#4)
8.68570(10)
14.6171(3)
9.4291(2)
111.9520(10)
1110.32(4)
2
P21/n (#14)
15.3390(2)
5.85820(10)
18.4560(2)
112.7990(3)
1528.86(2)
4
P21/c (#14)
5.6098(3)
13.2503(8)
14.2590(9)
91.884(4)
1059.32(11)
4
P21/c (#14)
12.6385(2)
10.6785(2)
15.1085(2)
102.9720(10)
1987.01(6)
4
P21/n (#14)
3.78140(10)
24.2829(8)
8.3855(3)
91.0050(10)
769.87(4)
4
b, Å
c, Å
β, deg
volume, Å3
Z
F
calc, g cmꢀ3
1.735
1.325
1.459
1.522
1.310
2.029
F000
464
464
696
496
824
464
μ(Cu KR), mmꢀ1
temperature, K
residuals:a R; Rw
goodness of fit
5.257
2.365
3.223
4.381
2.553
9.128
100
100
100
100
100
100
0.0516; 0.1423
1.218
0.0210; 0.0522
1.025
0.0246; 0.0656
1.057
0.0243; 0.0629
1.067
0.0228; 0.0655
1.002
0.0282; 0.0843
1.040
12b
13a
14
15
16
CCDC deposit no.
color and habit
size, mm
formula
808080
808085
808081
816484
808086
colorless plate
0.18 ꢁ 0.12 ꢁ 0.03
C6H4BrCuN2S
279.62
colorless plate
0.33 ꢁ 0.19 ꢁ 0.02
C11H22CuN3S
291.92
colorless blade
0.26 ꢁ 0.11 ꢁ 0.05
C14H26Cu2N4S2
441.59
colorless blade
0.36 ꢁ 0.09 ꢁ 0.03
C9H18CuN3O2S
295.86
colorless plate
0.21 ꢁ 0.19 ꢁ 0.07
C6H11CuN2OS
222.77
formula weight
space group
a, Å
P21/n (#14)
3.8166(3)
24.2881(16)
8.4924(6)
91.714(4)
786.88(10)
4
Pnma (#62)
10.9672(4)
21.4419(8)
5.8806(2)
90
P21/c (#14)
11.7388(3)
13.0673(3)
12.6949(3)
112.1240(10)
1803.95(8)
4
Pnma (#62)
11.5188(10)
17.8699(6)
5.6997(2)
90
Pbca (#61)
6.81330(10)
10.6817(2)
23.2105(4)
90
b, Å
c, Å
β, deg
volume, Å3
1382.87(9)
4
1173.23(7)
4
1689.21(5)
8
Z
F
calc, g cmꢀ3
2.360
1.402
1.626
1.675
1.752
F000
536
616
9112
616
912
temperature, K
residuals:a R; Rw
goodness of fit
100
100
100
100
100
0.0287; 0.0745
0.0358; 0.0984
0.0250; 0.0623
0.0200; 0.0562
0.0180; 0.0521
1.005
1.091
1.082
1.034
1.075
2 2 1/2 for all data.
a R = R1 = ∑||Fo| ꢀ |Fc||/∑|Fo| for observed data only. Rw = wR2 = {∑w(Fo ꢀ Fc2)2/∑w(Fo ) }
2
The thiocyanate-S,N-bridged chain-type structure, A/A0, ap-
pears to be the most commonly encountered. Among these prod-
ucts two distinctions can be made: (1) (CuSCN)L2 (1:2, A) vs
(CuSCN)L (1:1, A0) and (2) zigzag vs helical chain type. In
addition to the known A/A0 complexes 1a (A, helical), 2(A, zigzag),
3 (A, zigzag), 7 (A, helical), 8 (A0, helical), and (CuSCN)(24Lut)2
(A, zigzag),11 new A/A0 compounds include 6 (A, zigzag), 9 (A0,
helical), 10 (A, helical), 13a (A, zigzag), and 15 (A, zigzag). Type
A/A0 also occurs for monodentate L = 2-cyanopyrazine and 4-hy-
droxypyrazine (both A, zigzag).6f Chain drawings of the new com-
plexes are shown in Figure 1. It will be noted that the Pipd ligand
in 13a is bound to Cu in equatorial fashion, while Morph in 15 is
bound axially, as are N-substituted amines MePipd and MeM-
orph in complexes 14 and 16 (see below). In contrast to the
type A cases, bond angles around Cu were within (10° of
tetrahedral.
Formation of the 1:1 type A0 complexes was observed only for
26Lut and 246Coll, (and not for 24Lut11). Substitution of the
crowded 2,6-aromatic positions seems to be required to exclude a
second L from the chain. The two examples of type A0 (8 and 9)
show a remarkable structural distinction. While 26Lut complex 8
features Cu bond angles within (3° of trigonal,11 the Namine
ꢀ
CuꢀNCS angle in 246Coll complex, 9, measures 150.51(8)°.
This angle makes the ring centroidꢀNCS distance roughly 3.4 Å,
suggesting a πꢀπ interaction.
Ladder structure B has previously been noted for 1b and
(CuSCN)L (monodentate L = 1-ethyl-2-methylpyrazine and
1,7-phenanthroline).6g,10a,11 New complexes 4 and 14 share this
arrangement (see Figure 2). As is typical of the rhomboid Cu2S2
core, CuꢀSꢀCu angles are quite acute (<80°) in all five cases.
behavior in CuCN(MeMorph),2 no Cu
O interactions
3 3 3
were seen for either 15 or 16 in the present study. In all 1:2
7243
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Inorganic Chemistry
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Table 3. Selected Bond Lengths and Angles for All Complexesa
Complex 4, (CuSCN)(2EtPy)b,c
CuꢀS
2.3380(15), 2.4865(14)
1.668(5)
CuꢀSꢀC
98.22(17), 106.10(18)
179.0(5)
SꢀC
SꢀCꢀN
CꢀN
1.151(7)
CꢀNꢀCu
SꢀCuꢀNCS
NamꢀCuꢀNCS
SꢀCuꢀNam
SꢀCuꢀS
159.6(4)
CuꢀNCS
CuꢀNam
1.991(5)
101.28(13), 103.17(14)
109.5(2)
2.026(5)
Cu Cu
2.7701(15)
108.56(14), 122.23(14)
109.99(5)
3 3 3
CuꢀSꢀCu
70.01(5)
b,d
Complex 5, (CuSCN)(3EtPy)3
SꢀC
1.642(2)
SꢀCꢀN
179.39(19)
CꢀN
2.049(7)-2.144(4)
2.028(2)
CꢀNꢀCu
163.12(18)
CuꢀNCS
CuꢀNam
NamꢀCuꢀNCS
NamꢀCuꢀNam
SꢀCuꢀNam
102.05(8), 105.20(7), 105.36(7)
111.54(7), 114.69(6), 116.37(7)
108.56(14), 122.23(14)
2.0286(18), 2.0322(17), 2.0420(16)
b,e
Complex 6, (CuSCN)(4EtPy)2
CuꢀS
2.123(4)
CuꢀSꢀC
103.77(5)
SꢀC
1.6558(16)
SꢀCꢀN
179.22(17)
CꢀN
CuꢀNCS
CuꢀNam
1.160(2)
CꢀNꢀCu
173.29(14)
1.9440(14)
SꢀCuꢀNCS
NamꢀCuꢀNCS
NamꢀCuꢀNam
SꢀCuꢀNam
111.10(4)
2.0833(13), 2.0873(13)
111.65(6), 113.33(6)
99.19(5)
110.25 (4), 110.77(4)
Complex 9, (CuSCN)(Coll)e,f
CuꢀS
2.3949(7)
1.649(2)
1.155(3)
1.868(2)
1.9479(19)
CuꢀSꢀC
97.13(8)
SꢀC
SꢀCꢀN
179.0(2)
CꢀN
CuꢀNCS
CuꢀNam
CꢀNꢀCu
SꢀCuꢀNCS
NamꢀCuꢀNCS
SꢀCuꢀNam
170.23(19)
105.50(6)
150.51(8)
103.99(6)
Complex 10, (CuSCN)(4tBuPy)2
b,e
CuꢀS
2.3131(4)
CuꢀSꢀC
98.76(5)
SꢀC
1.6580(14)
SꢀCꢀN
178.59(12)
CꢀN
CuꢀNCS
CuꢀNam
1.1571(19)
CꢀNꢀCu
176.72(12)
1.9390(12)
SꢀCuꢀNCS
NamꢀCuꢀNCS
NamꢀCuꢀNam
SꢀCuꢀNam
117.30(4)
2.0613(11), 2.1083(11)
110.00(5), 110.55(5)
101.24(4)
104.21(3), 112.20(3)
Complex 11b, (CuSCN)(3ClPy)b,c
CuꢀSꢀC
CuꢀS
2.3310(7), 2.3553(6)
1.670(3)
102.29(8), 103.58(9)
178.7(2)
SꢀC
SꢀCꢀN
CꢀN
CuꢀNCS
CuꢀNam
1.153(3)
CꢀNꢀCu
SꢀCuꢀNCS
NamꢀCuꢀNCS
SꢀCuꢀS
CuꢀSꢀCu
SꢀCuꢀNam
158.19(19)
1.974(2)
116.82(7), 117.18(7)
102.92(8)
2.064(2)
107.59(3)
107.59(3)
102.02(6), 108.88(6)
Complex 12b, (CuSCN)(3BrPy)b,c
CuꢀSꢀC
CuꢀS
2.3376(10), 2.3585(9)
1.677(4)
102.34(12), 104.84(12)
178.8(3)
SꢀC
SꢀCꢀN
CꢀN
CuꢀNCS
CuꢀNam
1.152(5)
CꢀNꢀCu
SꢀCuꢀNCS
NamꢀCuꢀNCS
156.3(3)
1.974(3)
116.21(9), 117.51(9)
102.10(12)
2.065(3)
7244
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Inorganic Chemistry
ARTICLE
Table 3. Continued
SꢀCuꢀS
108.73(4)
CuꢀSꢀCu
SꢀCuꢀNam
b,e
108.72(4)
102.05(8), 108.68(8)
Complex 13a, (CuSCN)(Pipd)2
CuꢀS
2.3066(11)
1.657(4)
1.155(5)
1.941(4)
2.127(2)
CuꢀSꢀC
103.69(14)
177.7(4)
SꢀC
SꢀCꢀN
CꢀN
CuꢀNCS
CuꢀNam
CꢀNꢀCu
172.8(3)
SꢀCuꢀNCS
NamꢀCuꢀNCS
NamꢀCuꢀNam
SꢀCuꢀNam
113.15(11)
108.71(7)
100.66(12)
108.71(7)
Complex 14, (CuSCN)(MePipd)b,c
CuꢀS
2.3232(6), 2.4074(6), 2.4048(6), 2.5934(6)
CuꢀSꢀC
96.52(7), 97.28(7), 104.98(7), 110.76(7)
178.60(19), 179.2(2)
SꢀC
1.659(2), 1.663(2)
SꢀCꢀN
CꢀN
1.162(3), 1.163(3)
CꢀNꢀCu
SꢀCuꢀNCS
NamꢀCuꢀNCS
SꢀCuꢀNam
SꢀCuꢀS
159.52(17), 164.53(17)
CuꢀNCS
CuꢀNam
1.9467(18), 1.9719(18)
2.0867(17), 2.0919(18)
2.7009(6), ca. 3.08
97.29(5), 100.79(5), 102.34(5), 110.25(5)
116.22(7), 123.26(7)
Cu Cu
102.68(5), 107.02(5), 116.03(5), 117.58(5)
102.603(19), 111.717(17)
3 3 3
CuꢀSꢀCu
68.284(17), 77.397(19)
b,e
Complex 15, (CuSCN)(Morph)2
CuꢀS
2.4410(6)
1.657(2)
CuꢀSꢀC
95.15(7)
SꢀC
SꢀCꢀN
179.22(18)
168.84(17)
107.10(5)
121.13(4)
100.14(6)
102.21(3)
CꢀN
CuꢀNCS
CuꢀNam
1.163(3)
CꢀNꢀCu
1.9399(18)
2.1376(11)
SꢀCuꢀNCS
NamꢀCuꢀNCS
NamꢀCuꢀNam
SꢀCuꢀNam
Complex 16, (CuSCN)(MeMorph)b,c
CuꢀSꢀC
CuꢀS
2.3677(4), 2.3720(4)
1.6601(16)
101.38(6), 109.68(6)
177.85(15)
SꢀC
SꢀCꢀN
CꢀN
CuꢀNCS
CuꢀNam
1.161(2)
CꢀNꢀCu
SꢀCuꢀNCS
NamꢀCuꢀNCS
SꢀCuꢀNam
SꢀCuꢀS
175.62(14)
1.9445(14)
99.04(4), 110.22(4)
121.08(5)
2.1279(12)
105.70(4), 114.14(4)
105.796(12)
CuꢀSꢀCu
121.321(17)
a Nam and NCS indicate amine and thiocyanate N atoms, respectively. b 4-Coordinate Cu atom. S,S,N bridging SCN. d Terminal N-bound SCN.
c
e S,N bridging SCN. f 3-Coordinate Cu atom.
This brings the copper atoms into relatively close proximity with
both CuSCN and CuS chains form tightly coiled figure-8 helices.
As with 11b and 12b and CuSCN(Pdz), the 16 sheet is decorated
with L on both faces. Oxygen atoms in adjacent sheets point toward
one another; there is a 2.5058(4) Å hydrogen-bond-like interaction
between O1 and a hydrogen atom (H4A) on an adjacent L.
Interestingly, each of the three EtPy isomers resulted in a dif-
ferent CuSCN ratio and structural type. Thus, the 2EtPy and 4EtPy
complexes (4 and 6) were of types B (1:1) and A (1:2), as de-
scribed above. The 3EtPy complex (5) revealed a remarkable 1:3
stoichiometry, which was confirmed by the determination of the
unique type D monomer structure. A simple tetrahedron, 5 reveals
an N-coordinated thiocyanate ligand and three L ligands with roughly
tetrahedral bond angles about Cu (102.05(8)ꢀ116.38(7)°). The
one another; in some cases the Cu Cu is slightly less than the
3 3 3
sum of their van der Waals radii (ca. 2.8 Å).22 Such is the case for
the single Cu Cu in 4 and for one of two Cu Cu in 14.
3 3 3
3 3 3
Putative opening of the Cu2S2 rings in ladder structure B pro-
duces an infinite sheet structure C similar to that reported for
CuSCN complexes of monodentate 5-bromopyrimidine (5BrPym)
and pyridazine (Pdz).6g,10a Isomorphic 3ClPy and 3BrPy complexes
11b and 12b, along with MeMorph complex 16, are C sheets (see
Figure 3). In 11b and 12b zigzag CuSCN chains are cross-linked
by zigzag CuS linkages. The rippled sheet produces alternating
inversion symmetry at the copper atoms, resulting in L decora-
tion on both faces of the sheet. Complexes 11b and 12b are iso-
structural with CuSCN(Pdz). In contrast, all CuꢀL units in
CuSCN(5BrPym) point in the same direction. Complex 16 is
structurally quite different that the other four C examples. In 16
shortest potential chain-forming Cu S distance measures
3 3 3
about 4.68 Å, confirming that this is a true monomer. Despite
the relative softness of Cu(I), N-coordination of thiocyanate
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Inorganic Chemistry
ARTICLE
Chart 2
Figure 3. X-ray structures of 11b (views along a- and b-axes) and 16
(views along a- and c-axes).
Figure 4. X-ray structure of 5.
(including multiple S-bridging), while higher charge density on
the metal and ligand π-acceptance favor N-attachment.3 Based
on this reasoning, the presence of three substituted pyridine
ligands in 5 would lead one to expect an N-coordinated SCN, as
is actually seen. The bridging SCN species in the present
structural study lent no particular support to the above claims,
which predict S,S,N-bridging for the σ-donor aliphatic amines
and S,N-bridging for the π-acceptor pyridine ligands. For
CuSCN-L structures, S,S,N-bridging is seen in 1:1 complexes
incorporating both aromatic ligand complexes 1b, 4, 11b, 12b and
aliphatic ligand complexes 14 and 16. Thiocyanate S,N-bridging
is also observed for both types of amines: 1a, 2, 3, 6ꢀ10,
(aromatic ligands) and 13a and 15 (aliphatic ligands). All of
these complexes (except 1:1 8 and 9) are of 1:2 stoichiometry. Pre-
sumably, 8 and 9 are prevented from producing either 1:2 chains
or 1:1 networks because of the demanding cone angles of 26Lut
and 246Coll. Thus, the number of amine ligands coordinated
appears to be a more significant factor than the donor/acceptor
nature of the ligands in determining the mode of SCN bridging.
Thermal Analysis. Thermogravimetric analysis (TGA) of
CuSCN-L networks revealed smooth loss of L, as seen previously
for CuCN-L complexes.1,2 Mass losses, temperature ranges, and
interpretations for the bulk powder samples are provided in the
Experimental Section; the TGA traces are included in the Sup-
porting Information. For (CuSCN)L>1 complexes stage-wise loss of
ligand, for example, (CuSCN)L2 f (CuSCN)L f CuSCN, was
usually obvious. Decomposition temperatures were very modest,
Figure 1. X-ray structures of 6, 9, 10, 13a, and 15. Key to Figures 1ꢀ4:
Copper and thiocyanate atoms shown as spheres. Amine ligands are
shown as wireframe. Color scheme for all X-ray figures: orange = Cu,
gray = C, blue = N, yellow = S, green = Cl, red = O. Hydrogen atoms
omitted.
Figure 2. X-ray structures of 4 and 14.
(as is seen for 5) is in fact more common than S-coordination in
related crystal structures.3
It has been claimed that reduced charge density on Cu and en-
hanced ligand σ-donation tend to favor S-coordinated thiocyanate
7246
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Table 4. Luminescence Results
complexa
excitation λmax, nm
emission λmax, nm (color)
stokes shift, cmꢀ1 b
lifetime, μs
quantum yield
1:2 3MePy, 2
1:2 4MePy, 3
1:1 2EtPy, 4
1:3 3EtPy, 5
1:2 4EtPy, 6
1:2 Quin, 7
341
387
342
367
386
358
337
341
367
360
367
493 (yellow-green)
480 (blue-green)
451 (blue)
9042
5006
7067
7755
5331
10840
6633
5573
8054
8694
8344
9.1
0.31
0.47
<0.03
0.60
0.04
<0.03
0.34
<0.03
0.34
0.66
0.48
9.1
0.19
6.7
513 (yellow-green)
486 (yellow-green)
585 (orange)
1.9
0.59
4.9
1:1 26Lut, 8
1:1 246Coll, 9
1:2 4tBuPy, 10
1:2 3ClPy, 11a
1:2 3BrPy, 12a
434 (blue)
421 (blue)
6.7
521 (yellow-green)
524 (yellow)
10.2
8.8
529 (yellow)
8.3
a Other isolated complexes showed no luminescence, except for 1b, which shows yellow-green emission, but was not further studied. b Calculated
between longest excitation λmax and shortest emission λmax
.
usually starting well below 100 °C. For L = substituted pyridine
ligands, CuSCN-L complexes initiated decomposition at tem-
peratures consistently 20ꢀ30 °C lower than those of the cor-
responding CuCN-L complexes. Thus, it appears that binding of
pyridine ligands to CuCN is slightly stronger than that to CuSCN.
No consistent trend was apparent between the initial decomposition
temperatures of CuSCN-L versus those of CuCN-L for aliphatic
amines.
lifetime (16 ns) for CuSCN indicates that this transition is spin
allowed. As a ligand for Cu(I), thiocyanate is a poorer acceptor for
MLCT compared to cyanide.
With the addition of substituted pyridines, the solid state
photophysical properties of the CuSCN-L compounds change
dramatically, producing long lifetimes (μs) and usually intense
emission. Luminescence results for the photoactive compounds
are collected in Table 4. Additional spectra are included in the
Supporting Information. The 2MePy complexes were not stu-
died because of their tendency to lose ligand. Each of the lumines-
cent CuSCN-L complexes showed a very broad excitation band
occurring between 341 and 387 nm. Some fine structure is evident
in these bands. The longer excitation wavelength for CuSCN-L
compared to that of CuSCN suggests that less disruption of the
network occurs during excitation, presumably since ligand addi-
tion has already broken up the 3D CuSCN lattice. The CuSCN-L
complexes generally show emission bands in the green-to-yellow
region. However, the 26Lut and 246Coll complexes 8 and 9 show
exceptional behavior with relatively short excitation (ca. 340 nm)
and emission (ca. 425 nm) wavelengths. Similar observations
were also noted for the ladder-structure complex 4. Species 4, 8,
and 9 are the only complexes reported herein that show blue
emission. Emission lifetime and quantum yield data were mea-
sured. In all cases lifetime curves were easily fit to single exponential
decays with values in the microsecond range. This suggests phos-
phorescent behavior, consistent with previous studies on copper-
(I) complexes.6h,25 Solid state quantum yields are quite high
(0.31ꢀ0.66) for complexes 2, 3, 5, 8, 10, 11a, and 12a.
Infrared Spectroscopy. Solid state IR spectroscopy of Cu-
SCN-L samples revealed the bands expected for metal thiocya-
nates in two regions: 2082ꢀ2197 (CtN) and 693ꢀ895
(SꢀC) cmꢀ1. Terminal metal N-thiocyanate and S-thiocyanate
bands are typically found near 2050 and 2100 cmꢀ1, respectively.8
Bridging SCN bands are found above 2100 cmꢀ1. In the current
study, all complexes except 5 are either confirmed or strongly
suspected to contain bridging SCN. Accordingly, all these com-
plexes showed CtN bands no lower in frequency than 2082 cmꢀ1
.
However, complex 5, which has been confirmed to contain an
N-bonded terminal thiocyanate, produced bands at 2114 and
2082 cmꢀ1, which would be more consistent with S-bonding or
bridging behavior. Although the lower frequency band can be
used to interpret the bonding mode of thiocyanate,8,23 complica-
tions are introduced by the presence of aromatic ligand modes in
the same region.
Luminescence Spectroscopy. Unlike solid CuCN, CuSCN
itself shows only very weak luminescence. The peak excitation for
CuSCN is found at short wavelength in the vicinity of 250 nm
with a broad emission band centered at 400 nm (see Figure 5).
This large Stokes shift of 16,300 cmꢀ1 is suggestive of significant
electronic configuration change, while the weak emission inten-
sity is due to a nonradiative deactivation pathway. We have pre-
viously reported that luminescence behavior in CuCN is the
result of excitation between π orbitals, specifically between
the dCu/πCN highest occupied molecular orbital (HOMO) and
the pCu/π*CN lowest unoccupied molecular orbital (LUMO).24
However, this is not the case for CuSCN. The reflectance results
for pure CuSCN powder show broad absorption from 240 to
330 nm, which is due to the presence of copper metal centered
transition (MC) and metal-to-ligand-charge-transfer (MLCT).
According to the excitation spectrum, the higher energy MC
transition, for example, d10fd9s1 or d10fd9p1, is responsible for
the weak emission at 400 nm. In the excited state, the copper
metal center becomes d9, and the electronic configuration change
leads to the observed large Stokes shift. Also the short luminescence
For most Cu(I) compounds, the long-lived emission usually
originates from MLCT transitions.26 We have found that no Cu-
SCN-L complex of aliphatic amines shows luminescent behavior.
The apparent requirement of ligand aromaticity for visible emission
indicates an MLCT transition, in which the empty π* orbitals of
amines accept electrons from copper. The excitation peak shapes
of all pyridine-based complexes are similar (Supporting Infor-
mation). However, the 26Lut and 246Coll complexes 8 and 9,
and the 2EtPy complex 4 exhibit short excitation wavelength, as
shown in Figure 5. Also interesting is the fact that 4, 8, and 9 have
the highest energy emission peaks (451, 434, and 421 nm, see
Table 4) and Quin complex 7 has the lowest (585 nm). The
pyridine-based complexes show intermediate emission energies
(ranging from 480 to 530 nm). Our density functional theory
(DFT) calculations show that the energy order of the LUMO
for these organic ligands is 26Lut (ꢀ94.75 kJ/mol) > pyridine
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Inorganic Chemistry
ARTICLE
temperature. The only exception is CuSCN(3ClPy)2, complex
11a. This is due to a decrease in C Cl separation where the
3 3 3
electronegative Cl raises the energy of the LUMO and decreases
the electron coupling, leading to an increase of the energy gap,
resulting in a blue shift in the emission peak at 77 K.
’ CONCLUSIONS
We have reported the facile reaction of various neat liquid amines
with solid copper(I) thiocyanate to produce amine-decorated
CuSCN networks. In several cases multiple CuSCN:L stoichio-
metries were noted. Compounds having CuSCN/L ratios of 1:2
are zigzag or helical chains decorated with amines. When the ratio
is 1:1 the products usually have ladder or sheet structures as a
result of S,S,N-thiocyanate bridging. However, when the amine is
a 2,6-disubstituted pyridine, (CuSCN)L chains are produced,
with 3-coordinate copper. 3-Ethylpyridine produces an unusual
monomeric (CuSCN)L3 product with terminal N-bound SCN.
In all cases, the amines are readily removed with heating. The
(CuSCN)L2 complexes with L = substituted pyridine are photo-
luminescent, showing excitation in the near UV and a broad emis-
sion near the yellow-green region. Microsecond lifetimes suggest
phosphorescence behavior. Absence of photoemission in all
species with aliphatic amine ligands suggests the importance of
π*-acceptor orbitals.
Figure 5. Luminescence spectra at 298 K and luminescence photo-
graphs under 365 nm light of (A) (CuSCN)(Quin)2, 7, (B) (CuS-
CN)(3MePy)2, 2, (C) (CuSCN)(26Lut), 8. (D) Luminescence spectra
of pure CuSCN powder at 298 K.
(ꢀ119.49 kJ/mol) > Quin (ꢀ180.71 kJ/mol). (The full table of
DFT results may be found in Table S1 in the Supporting Infor-
mation.) The extra conjugation provides further stabilization
while the methyl groups weaken the aromaticity. This result can
be used to understand the MLCT transition, in which aromatic
amines provide empty π* orbitals as the LUMO. The Quin
complex 7 has the smallest HOMOꢀLUMO gap and gives an
orange color; the 26Lut and 246Coll complexes 8 and 9 have the
large band gaps and exhibit blue emission. For pyridine-based
complexes, the energy differences between HOMO and LUMO
are intermediate and produce yellow to yellow-green color, depend-
ing on the groups on the aromatic ring. Presumably, the higher
energy emission seen in ladder complex 4 is associated with a MC
transition. It is noteworthy that this species shows the shortest
lifetime of any CuSCN-L.
’ ASSOCIATED CONTENT
S
Supporting Information. (1) X-ray powder patterns
b
(experimental and calculated) for all structurally characterized com-
pounds. (2) TGA traces for all compounds. (3) Additional lumi-
nescence spectra for all luminescent compounds. (4) Lumines-
cence photos for all emissive compounds and for the 1a f 1b
conversion. (5) DFT calculated orbital energy table and iso-
density maps. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Lowering of the LUMO symmetry through bending of the
polymer chain at copper appears to be responsible for bathochromic
emission shifts upon ligand addition. The situation is probably
similar to that of CuCN, which also shows unsaturation along the
chain. However, there are two very significant behavioral differ-
ences between CuCN-L and CuSCN-L: (1) aliphatic amines
produce luminescent complexes of CuCN, but not CuSCN, and
(2) the CuSCN-L complexes reported herein lack both the ad-
ditional low energy band and the thermochromic behavior noted
for many CuCN-L species. This may be because CN is a better
acceptor for MLCT than is SCN. There are two emissive path-
ways for CuCN-L compounds (Cu to CN and Cu to L) while
only one radiative deactivation pathway for CuSCN-L compounds
(Cu to L).
Corresponding Author
*Phone: 757-2212555. Fax: 757-2212715. E-mail: rdpike@
wm.edu.
’ ACKNOWLEDGMENT
Grateful acknowledgement is made to National Science
Foundation (CHE-0848109 and CHE-0315877) for support of
this research. We also acknowledge a Howard Hughes Medical
Institute grant through the Undergraduate Biological Sciences
Education Program to the College of William and Mary. We are
indebted to NSF (CHE-0443345) and the College of William
and Mary for the purchase of the X-ray equipment.
Most of the CuSCN-L complexes show modest emission
temperature dependence, which has been seen in many low-
dimensional materials of closed-shell transition metals.27 The emis-
sion red shift as the temperature decreases from room temperature to
77 K is due to the stacked metalꢀmetal distance contraction. The
long lifetime emission peaks are due to a 3MLCT, in which the
HOMO derives from the interaction between copper dz2, and the
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