Coordination variability of Cu^I in multidonor heterocyclic thioamides: synthesis, crystal structures, luminescent properties and ESI-mass studies of complexes

Abstract

The chemistry of copper(I) with scarcely investigated heterocyclic thioamides, particularly 2,4,6-trimercaptotriazine, purine-6-thione, 2,4-dithiouracil, 2-thiouracil and pyrimidine-2-thione is described. The interaction of 2,4,6-trimercaptotriazine (tmtH₃) with [Cu(CH₃COO)(PPh₃)₂] resulted in a pair of bond isomers: [Cu(κ¹N-tmtH₂)(PPh₃)₂] (6a), [Cu(κ¹N,κ¹S-tmtH₂)(PPh₃)₂] (6b); with copper(I) bromide and PPh₃, it formed a trinuclear complex, [Cu₃Br₂(κ¹N,κ¹S, μ-S-tmtH₂)(PPh₃)₆] (7) with anionic tmtH₂⁻. The 2,4-dithiouracil with copper(I) halides (CuCl, CuBr) and PPh₃ yielded dinuclear complexes: [Cu₂(μ-Cl)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (4) and [Cu₂(μ-Br)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (5) with unusual eight-membered metallacyclic rings. Pyrimidine-2-thione (pymSH) is an N,S-chelated anion in a mononuclear complex, [Cu(κ¹N,κ¹S-pymS)(PPh₃)₂] (1), whereas 2-thiouracil (tucH₂) with copper(I) chloride and PPh₃ yielded a tetrahedral complex, [CuCl(κ¹S-tucH₂)(PPh₃)₂] (3). Purine-6-thione (purSH₂) coordinated to Cu^I in two different modes yielding mono- and di-nuclear complexes, [Cu(κ¹N,κ¹S-purS)(PPh₃)₂]·CH₃OH (2a) and [Cu₂(κ¹N,μ-S-purS)₂(PPh₃)₂] (2b). The existence of bond isomers (6a and 6b), synthesis of novel dinuclear (4 and 5) and rare trinuclear (7) complexes with unusual bonding patterns are the novel features of the present study. Complexes show intense emission bands in the 380–530 nm region with λ_em at 490 to 495 nm.

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Introduction

The coordination chemistry of heterocyclic thioamides with transition/main-group metals has been the focus of numerous studies, particularly with pyridine-2-thiones, imidazolidine-2-thiones, thiazolidine-2-thiones and a few other allied bases. These studies deal with the synthesis and structures of a variety of compounds, as well as their biochemical and other applications.^1–29 Among applications, copper complexes have shown antitumor, anti-inflammatory and antimicrobial properties.^2,16–18 There is keen interest in the chemistry of Cu(I)/Cu(II) with analogous N-based ligands because their complexes with 6-(2-chlorobenzylamino)purine,³⁰ 2,2′-biquinolines,³¹ substituted guanidines,³² phenanthroline³³ and quinolin-2(1H)-one-derived Schiff bases³⁴ have shown anticancer, antitumor, antifungal, antimicrobial and antibacterial properties.

Considering the importance of copper complexes with N/S donor ligands, it was decided to pursue copper chemistry with multifunctional heterocyclic thioamides, as shown in Scheme 1, having the propensity to bind to more than one metal ion. Coordination chemistry of copper(I) with pyrimidine-2-thione,^8–13,35 purine-6-thione,^36–38 2-thiouracil,³⁹ 2,4-dithiouracil^40,41 and 2,4,6-trimercaptotriazine is limited.^35,42,43 Herein, it is planned to react copper(I) salts with multifunctional heterocyclic thioamides (Scheme 1) with the objective of obtaining mono-, di- and polynuclear copper complexes by changing the number of donor atoms in organic thiobases. In the preparation of complexes, the reactions of pyrimidine-2-thione (pymSH, I), purine-6-thione (purSH₂, II), 2,4,6-trimercaptotriazine (tmtH₃, III), 2-thiouracil (tucH₂, IV) and 2,4-dithiouracil (dtucH₂, V) (Scheme 1) were carried out with copper(I) halides/Cu(CH₃COO)(PPh₃)₂. The resulting isolated mono-, di- and trinuclear complexes isolated were characterized using analytical data, spectroscopy, ESI-MS spectrometry and X-ray crystallography.

Scheme 1 Molecular structures of thiol ligands under study.

Results and discussion

Synthesis and IR spectroscopy

Scheme 2 shows a bonding pattern of the synthesized complexes. In the synthesis of complexes 1, 2 and 6, no refluxing was carried out because high temperature led to the formation of a solid which could not be crystallized, and thus the reaction mixture was kept undisturbed at room temperature. For the synthesis of complexes 3, 4, 5 and 7, a copper(I) halide was first reacted with a thio-ligand under magnetic stirring at room temperature, followed by the addition of PPh₃. Slow evaporation of the resulting solution gave pale yellow (3), light orange (4, 5) or yellow (7) crystals. The reaction of pyrimidine-2-thione (pymSH) with [Cu(CH₃COO)(PPh₃)₂] involved the removal of the N–H proton and the anionic pymS⁻ chelated to the {Cu(PPh₃)₂} moiety yielding the complex, [Cu(κ¹N,κ¹S-pymS)(PPh₃)₂] (1). Similarly, in the reaction of purine-6-thione (purSH₂) with [Cu(CH₃COO)(PPh₃)₂], the acetate removed one proton from an –NH– group.

Scheme 2 A view of bonding pattern of complexes synthesized.

The anionic purSH⁻ coordinated to Cu^I in two different modes yielding mono- and di-nuclear complexes, [Cu(κ¹N,κ¹S-purS)(PPh₃)₂]·CH₃OH (2a) and [Cu₂(κ¹N,μ-S-purS)₂(PPh₃)₂] (2b) from the same reaction. The crystals of 2a and 2b were manually separated. The interaction of tmtH₃ with [Cu(CH₃COO)(PPh₃)₂] resulted in a pair of bond isomers: [Cu(κ¹N-tmtH₂)(PPh₃)₂] (6a) and [Cu(κ¹N,κ¹S-tmtH₂)(PPh₃)₂] (6b) in the same unit cell (X-ray crystallography, vide infra). The reaction of tmtH₃ with copper(I) bromide in the presence of two moles of PPh₃ involved the removal of one –NH– hydrogen and uninegative tmtH₂⁻, yielding a trinuclear complex, [Cu₃Br₂(κ¹N,κ¹S, μ-S-tmtH₂)(PPh₃)₆] (7). The reaction of 2-thiouracil with copper(I) chloride in the presence of two moles of PPh₃ yielded light yellow crystals of a mononuclear complex, [CuCl(κ¹S-tucH₂)(PPh₃)₂] (3). The reaction of 2,4-dithiouracil with copper halides (CuCl, CuBr) in the presence of two moles of PPh₃ also removed one –NH– hydrogen and reacted with two CuX units yielding yellow complexes, [Cu₂(μ-Cl)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (4) and [Cu₂(μ-Br)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (5) with unusual bonding patterns. All the complexes except 2b were soluble in organic solvents such as chloroform and dichloromethane.

The IR spectral data of complexes is given in the Experimental section and that of the ligands is provided in the ESI.† The ν(N–H) band of the free pyrimidine-2-thione ligand at 3078 cm⁻¹ disappeared in complex 1. Similarly, the diagnostic ν(C–S) band at 1187 cm⁻¹ shifted to 1026 cm⁻¹ in the complex, which revealed that the thio-ligand is coordinating as anion through its N and S donors. The ν(N–H) bands in complexes 2–7 occur in the region 3082–3250 cm⁻¹, which are at different positions from those of the thio-ligands. In complex 3, the ν(O–H) band is assigned to 3447 cm⁻¹. In complexes 2–7, the ν(C–S) bands are assigned in the region 844–1184 cm⁻¹, which are generally at low-energy regions relative to the respective free thio-ligand. The ν(P–C_Ph) bands in the region of 1094 to 1091 cm⁻¹ indicated the presence of PPh₃ in the complexes.

Molecular structures

The crystal data of complexes are given in Table 1, and their important bond parameters are given in Table 2. Complexes 1, 2b, 3–5 and 7 crystallized in the triclinic crystal system with the space group P1̄, whereas other complexes (2a, 6) crystallized in the monoclinic crystal system with the space groups P2₁/n (2a) and P2₁(6).

Table 1 Crystal data for complexes 1–7

	1	2a	2b	3
Empirical formula	C40H33N2CuP2S	C42H37N4OCuP2S	C46H36N8Cu2P2S2	C40H34ClCuN2OP2S
M	699.24	771.30	953.97	751.68
T/K	296(2)	173(2)	295(2)	170(2)
Crystal system	Triclinic	Monoclinic	Triclinic	Triclinic
Space group	P1̄	P21/n	P1̄	P1̄
a(Å)	9.349(3)	9.0783(2)	12.6132(6)	10.8164(9)
b(Å)	11.140(4)	25.2036(6)	12.6674(6)	13.1393(10)
c(Å)	18.860(6)	16.9815(5)	17.0373(7)	14.7805(10)
α (°)	93.050(13)	—	88.319(3)	76.189(6)
β (°)	92.208(12)	102.171(3)	69.754(4)	81.704(6)
γ (°)	113.907(11)	—	65.581(4)	67.452(7)
V(Å^3)	1789.2(10)	3798.13(17)	2304.8(2)	1880.7(2)
Z	2	4	2	2
Dcalcd (g cm^−3)	1.298	1.349	1.375	1.327
μ (mm^−1)	0.788	0.753	2.966	0.826
F(000)	724	1600	976	776
Reflections collected	19105	39221	17070	19880
Unique reflections	7179 (Rint,0.0292)	9794 (Rint,0.0222)	9009 (Rint,0.0388)	9706 (Rint,0.0205)
Data/restraints/parameters	7179/0/416	9794/2/466	9009/0/544	9706/2/439
Reflens. With [I > 2σ(I)]	5988	8534	6944	7531
R Indices [I > 2σ(I)]
R1	0.0340	0.0395	0.0556	0.0433
WR2	0.0837	0.1027	0.1583	0.0960
R Indices (all data)
R1	0.0470	0.0481	0.0715	0.0660
WR2	0.1031	0.1095	0.1757	0.1122
Largest diff. Peak and hole	0.453 and −0.412 e Å^−3	0.360 and −0.219 e Å^−3	0.763 and −0.355 e Å^−3	0.546 and −0.381 e Å^−3

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	4	5	6	7
Empirical formula	C76H63ClCu2N2P4S2	C76H63BrCu2N2P4S2	C39H33N3O0.50CuP2S3	C111H92Br2Cu3N3P6S3
M	1354.81	1399.27	773.34	2100.32
T/K	173(2)	123(2)	100 (2)	173(2)
Crystal system	Triclinic	Triclinic	Monoclinic	Triclinic
Space group	P1̄	P1̄	P21	P1̄
a(Å)	17.6065(5)	17.5439(3)	12.864(4)	12.8825(5)
b(Å)	18.9904(6)	18.7998(3)	16.167(5)	13.2054(5)
c(Å)	22.8262(7)	22.7125(4)	17.506(5)	33.4043(1)
α (°)	66.994(3)	67.538(2)	—	93.2233(3)
β (°)	69.512(3)	69.421(2)	95.271(4)	96.511(3)
γ (°)	81.169(3)	80.7420(10)	—	119.084(4)
V(Å^3)	6579.4(3)	6477.95(19)	3625.4(19)	4893.4(3)
Z	4	4	4	2
Dcalcd (g cm^−3)	1.368	1.435	1.417	1.425
μ (mm^−1)	0.893	1.483	0.898	1.675
F(000)	2800	2872	1596	2148
Reflections collected	77752	134549	42507	50419
Unique reflections	31317 (Rint,0.0475)	65025 (Rint,0.0641)	14793 (Rint,0.0473)	23304 (Rint,0.0341)
Data/restraints/parameters	31317/2/1573	65025/0/1573	14793/1/875	23304/4/1124
Reflens. With [I > 2σ(I)]	19640	37449	12435	17703
R Indices [I > 2σ(I)]
R1	0.0542	0.0683	0.0495	0.0679
WR2	0.1041	0.1416	0.1121	0.1345
R Indices (all data)
R1	0.1037	0.1342	0.0675	0.0932
WR2	0.1234	0.1722	0.1215	0.1475
Largest diff. Peak and hole	0.849 and −0.497 e Å^−3	1.480 and −2.228 e Å^−3	1.012 and −0.904 e Å^−3	1.529 and −1.227 e Å^−3

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Table 2 Important bond lengths(Å) and bond angles(°)^b

Mono-nuclear complexes (1, 2a, 3, 6a, 6b)
	1	2a	3	6a^a	6b^a
a A case of bond isomerism (6a and 6b) in the crystal lattice.b Each of the complexes, 4 and 5, has independent molecules in the asymmetric unit.
Cu–N	2.144(2)	2.130(2)	—	2.015(4)	2.008(4)
Cu–S	2.419(1)	2.433(1)	2.381(1)	3.144(2) (Cu2–S22)	2.879(2) (Cu1–S12)
Cu–P	2.262(1), 2.251(1)	2.262(1), 2.277(1)	2.281(1), 2.256(1)	2.249(1), 2.273(1)	2.220(1), 2.254(1)
S–C	1.711(3)	1.715(2)	1.666(2)	1.680(5), 1.680(5), 1.652(5)	1.681(5), 1.664(5), 1.663(5)
N–Cu–S	69.10(6)	88.24(5)	—	56.14(12)	61.59(12)
P–Cu–P	126.25(3)	126.08(2)	124.53(3)	125.88(5)	120.03(5)
Cu–S–C	78.98(9)	93.60(7)	106.18(8)	—	69.40(17)

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Dinuclear complexes (2b, 4 and 5)
2b	Molecule 1	Molecule 2		Molecule 1	Molecule 2
Cu–N	2.036(3)	2.058(3)	N–Cu–S	85.85(8), 101.43(8)	87.66(9), 105.87(11)
Cu–S	2.362(1), 2.608(1)	2.345(1), 2.535(1)	Cu–S–Cu, Cu–S–C	70.60(3), 89.85(13)	67.52(4), 90.88(12)
Cu–P	2.223(1)	2.215(1)	S–Cu–S	109.40(3)	112.48(4)
Cu–Cu	2.880(1)	2.717(1)	N–Cu–P	127.62(8)	117.64(11)
S–C	1.710(3)	1.739(3)	P–Cu–S	103.35(3), 122.19(4)	110.98(5), 118.24(4)

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4	Molecule 1	Molecule 2	5	Molecule 1	Molecule 2
Cu–S	2.330(1), 2.344(1)	2.316(1), 2.362(1)	Cu–S	2.356(1), 2.311(1)	2.321(1), 2.337(1)
Cu–P	2.287(1), 2.283(1)	2.289(1), 2.290(1)	Cu–P	2.281(1), 2.283(1)	2.265(1), 2.291(1)
Cu–Cl	2.439(1)	2.462(1)	Cu–Br	2.553(1)	2.554(1)
S–C	1.693(3), 1.701(3)	1.695(3), 1.711(3)	S–C	1.717(3)	1.708(3)
Cu–S–C	112.14(11), 109.33(11)	110.58(11) 106.47(12)	Cu–S–C	107.42(9), 110.56(9)	112.94(9), 109.76(9)
Cu–Cl–Cu	146.92(4), (Cl)	145.94(4), (Cl)	Cu–Br–Cu	139.89(2), (Br)	140.94(2), (Br)
P–Cu–S	113.20(4), 106.13(3), 114.06(3), 105.66(3)	113.66(4), 105.52(4), 114.57(3), 104.43(3)	P–Cu–S	115.45(3), 104.25(3)	114.96(3) 105.86(2)
P–Cu–P	122.44(3), 121.14(3)	122.66(3), 122.36(3)	P–Cu–P	122.76(3), 122.77(2)	122.35(2), 121.31(3)
S–Cu–Cl	109.23(3), 110.38(3)	106.98(3), 108.82(3)	S–Cu–Br	109.73(2)	111.21(2)

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Trinuclear complex (7)
Cu–N	2.080(3)	S–C	1.693(4), 1.672(4), 1.631(4)	N–Cu–S	62.93(9)
Cu–S	2.395(1), 2.769(1) (Cu2–S1), 2.423(1)	Cu–Br	2.478(), 2.477(1)	P–Cu–P	121.19(4), 124.61(5)
Cu–P	2.270(1), 2.293(1),	S–Cu–Br	104.32(3), 109.13(3)	Cu–S–C	72.03(14), 109.84(14), 114.30(14)

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Mononuclear complexes

The pyrimidine-2-thiolate chelates to the copper atom in complex 1 through its N¹, S– donor atoms with the N–Cu–S bite angle of 69.10(6)° being the shortest, whereas the P–Cu–P angle {126.25(3)°} is the largest among the various angles around the Cu atom (Fig. 1). Purine-6-thiolate also chelates to the copper atom in 2a through its N⁷, S– donor atoms with the N–Cu–S bite angle of 88.24(5)° being the shortest, whereas the P–Cu–P angle {126.081(19)°} is the largest among angles around the Cu atom (Fig. 2). The geometry is severely distorted from the tetrahedron in two cases; however, the Cu–N, Cu–S and Cu–P bond distances are similar. Methanol was hydrogen-bonded to N4 nitrogen with an N–O distance of 2.801 Å, which is less than the sum of the van der Waals radius of N and O, viz. 3.05 Å, indicating weak O–H⋯N interaction.⁴⁴ The thiouracil did not chelate to Cu^I in complex 3, rather it bonded to the metal atom as a neutral ligand through its S-donor atom, whereas other donor atoms remained unbound (Fig. 3). As before, the P–Cu–P angle is the largest {124.53(3)°}, whereas other angles around the metal center fall in a narrow range, 103–109°. Complexes 6a and 6b exist as bond isomers in the same unit cell (Fig. 4). In 6a, the tmtH₂⁻ as mono anion is bonded to Cu^I through its one N donor atom only forming a three coordinate complex, and in 6b it is bonded to one N donor and to one S donor atom. Whereas, the Cu–N bond distances in 6a and 6b are nearly equal, although shorter than in 1 and 2a, the Cu–S distance of 2.879(2) Å in 6b is significantly longer than that in other mononuclear complexes (1, 2a, 3). In three-coordinate 6a, the Cu–S distance of 3.144(2) Å is still longer than 6b. The expected Cu–S bond distance is 2.61 Å, based on the ionic radii of Cu⁺ (0.91 Å) and S²⁻ (1.70 Å), and the Cu–S distances found in 6a and 6b complex species are significantly longer and reveal a weak Cu–S bond.⁴⁴ The Cu–S bonds in other complexes are similar to the literature trends.^1–15

Fig. 1 Molecular structure of [Cu(κ¹N,κ ¹S-pymS)(PPh₃)₂] 1.

Fig. 2 Molecular structure of [Cu(κ¹N,κ¹S-purSH)(PPh₃)₂]·CH₃OH 2a.

Fig. 3 Molecular structure of [CuCl(κ¹S-tucH₂)(PPh₃)₂] (3).

Fig. 4 Molecular structures of Cu(κ¹N-tmtH₂)(PPh₃)₂] 6a, [Cu(κ¹N,κ¹S-tmtH₂)(PPh₃)₂]·H₂O 6b (bond isomers).

Di- and trinuclear complexes

Complex 2a appears to lose one PPh₃ forming species, {Cu(purSH)(PPh₃)}, which dimerized via sulfur to yield the dinuclear complex 2b (Fig. 5). It exists as two independent entities: molecule 1 and molecule 2 in the asymmetric unit. Two molecules differ significantly in various bond lengths and bond angles, particularly pertaining to the bridging sulfur donor atoms. The N–Cu–S bite angles decrease to 85.85(8)° and 87.66(9)° vis-à-vis that 88.24(5)° of 2a. The central cores Cu₂S₂ that have unequal Cu–S distances form parallelograms and have Cu–S–Cu and S–Cu–S bond angles of 70.60(3)/67.52(4)° and 109.40(3)/112.48(4)° of molecules 1 and 2, respectively, and are typical of such cores observed in different dinuclear complexes.⁴⁷ Compound 4, [Cu₂(μ-Cl)(κ¹S,κ¹S-dtucH)(PPh₃)₄], exists as two independent molecules in the asymmetric unit with small differences in various bond parameters. In this hetero-bridged dinuclear complex, each copper is bonded to one S donor atom of dtucH anion, two P donor atoms of two PPh₃ molecules, and one bridging μ-Cl (Fig. 6a and b). The binding pattern of complex [Cu₂(μ-Br)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (5) is similar to that of complex 4 (Fig. 7a and b). Each metal center acquires tetracoordination in a distorted tetrahedral geometry. The trinuclear complex, [Cu₃Br₂(κ¹N,κ¹S,μ-S-tmtH₂)(PPh₃)₆] 7 has three types of Cu metal ions differently bonded (Fig. 8 and b). Here, Cu1 is bonded to one Br, two P and one bridging S donor atoms; Cu3 is bonded to one Br, two P and one S donor atoms; and finally, Cu2 is bonded to two P, one N and one bridging S donor atoms. The Cu2–S1 distance is the longest among various Cu–S distances found in complexes 1, 2, 3–5, except 6b, which is similarly bonded but has a longer distance than that observed in 7.

Fig. 5 Molecular structure of [Cu₂(κ¹N,κ²S-purSH)₂(PPh₃)₂] 2b.

Fig. 6 (a) Molecular structure of dinuclear [Cu₂(κ²Cl)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (4). (b) Skeletal structure of 4 (phenyl rings omitted).

Fig. 7 (a) Molecular structure of dinuclear [Cu₂(κ²Br)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (5). (b) Skeletal structure of 5 (phenyl rings omitted).

Fig. 8 (a) Molecular structure of [Cu₃Br₂(κ¹N,κ¹S,κ²S-tmtH₂)(PPh₃)₆] (7). (b) Skeletal view of 7(phenyl rings omitted).

Variations in C–S, Cu⋯Cu bond lengths and Cu–S–C bond angles

It is known that the C=S double-bond length is 1.62 Å, whereas the C–S single-bond length is 1.81 Å.^44,45 A comparison of C–S bond distances observed in different complexes (Table 2) reveal a wide variation from 1.631(4) to 1.739(3) Å. In complex 3, the thio-ligand tucH₂ binds to Cu^I as a neutral ligand, and the C–S distance of 1.666(2) Å exhibits a weakening of this bond on coordination but there is considerable double-bond character. Complexes 1, 2, 4 and 5 with pymS⁻(1), purSH⁻(2) and dtucH⁻(4, 5) uninegative anionic thio-ligands have C–S distances in the range of 1.693(3) (4, molecule 1)-1.739(3) (2b, molecule 2), suggesting significantly larger weakening of C–S bonds in the complexes. The C–S distances in three-coordinated 6a and tetracoordinated 6b are highly similar and have considerable double-bond character. Finally, in trinuclear complex 7, all three C–S distances are distinctly separate: 1.693(4) Å (bridging S), 1.672(4) Å (terminal S) and 1.631(4) Å (uncoordinated S). Regarding the Cu⋯Cu separation, the shortest distances are found in molecule 1{2.880(1) Å} and molecule 2 {2.717(1) Å}, which are comparable with 2.80 Å, which is twice the sum of the van der Waals radius of Cu.^46–48 In other cases, this metal-metal separation was well above 3.0 Å and is not cited in this paper.

The Cu–S–C angle of 78.98(9) in 1 changes to 93.60(7)° in 2a, the cases in which thio-ligands are N,S-chelating. When the thio-ligand is N,S-chelating –cum-S bridging as in 2b, the Cu–S–C angles slightly contract to 89.85(13) and 90.88(12)°. The Cu–S–C angle further changes to 69.40(17)° in 6b in which there is N,S-chelation. This angle lies in the range of 106–113° in the complexes 3, 4 and 5 where a thione sulfur is terminally (unidentate) bonded. Finally, the Cu–S–C angles in trinuclear cluster 7 are 72.03, 109.84(14) and 114.30(14)° depending on whether copper metal is chelated (Cu2, N,S-chelation), bonded to bridging S (Cu1) or nonbridging S(Cu3).

A comparison with literature reports

A comparison with literature reports reveals that pyrimidine-2-thione(pymSH) bonds to Cu^I as a neutral thio-ligand (S-bonded or N,S-bridged) in its complexes with copper(I) halides.^9–13,35 Complex 1, obtained from Cu(OAc)(PPh₃)₂ and pymSH in methanol-dichloroform, represents the first example in which the pymSH is chelating to Cu^I as the anion. Copper(II) acetate with pymSH and PPh3 in a 1 : 2 : 2 molar ratio in ethanol also formed the same product.³⁵ Here, it is inferred that pymSH did not rupture the Cu-halogen bonds; however, with acetate as the counter anion, it deprotonated pymSH and the thio-ligand acted as the anion. This behavior is unlike that shown by purine-6-thione (purSH₂), which ruptured both the Cu-halogen bonds (Cl, Br) in [Cu(κ²-N,S-purSH)(PPh₃)₂]·CH₃OH, or bonded to the metal center as the neutral ligand in its complexes with copper(I) halides.^36–38 With acetate as the counter anion, purSH₂ formed mononuclear Cu(κ¹N,κ¹S-purS)(PPh₃)₂]·CH₃OH 2a and dinuclear [Cu₂(κ¹N,μ-S-purS)₂(PPh₃)₂] 2b complexes. Thiouracil did not rupture Cu-halogen bonds and formed the complex 3, a behavior similar to the one noticed earlier in its only other trigonal planar complex, [CuCl(κ¹-S-tucH₂)₂].³⁹ Dithiouracil with copper(I) halides shows dual behaviour forming mononuclear [CuBr(κ²-P,P-dppbz)(κ¹-S-dtucH₂)] {dppbz = 1,2-bis(diphenylphosphanyl)benzene} and polynuclear complexes [Cu(μ-S,S-dtucH₂)(PPh₃)X]_n (X = Cl, Br, I) in which there is no Cu-halogen rupture,^40,41 as well as dinuclear complexes 4 and 5 in which there is a rupture of Cu-halogen bonds {one Cu–X bond per two CuX units is ruptured}. Complexes 4 and 5 display unusual eight-membered metallacyclic rings. The reaction of Cu(OAc)(PPh₃)₄ with a sodium salt of tmtH₃ (tmtH₂⁻Na⁺) only gave [Cu(κ¹N-tmtH₂)(PPh₃)₂] (6a), as reported earlier.⁴² This multidentate ligand has shown the formation of unusual bond isomers, [Cu(κ¹N-tmtH₂)(PPh₃)₂] (6a) and [Cu(κ¹N,κ¹S-tmtH₂)(PPh₃)₂] (6b) in which the thio-ligand is differentially bonded, Cu–N (6a) and N,S-chelated (6b). A few other tmtH₃ reported complexes are hexanuclear [(Cu₆(PPh₃)₆(μ₆-tmt)₂]⁴³ and polynuclear, [Cu(μ₃-S,S,S-tmtH₃)X]_n (X = Cl, Br, I).³⁵ Finally, tmtH₃ with copper(I) bromide in the presence of PPh₃ ligand loses one N–H proton and thio-ligand as anion tmtH₂⁻ binds to Cu^I ions in three different bonding modes, resulting in a novel trinuclear cluster [Cu₃Br₂(κ¹N,κ¹S,μ-S-tmtH₂)(PPh₃)₆] (7). The only other trinuclear cluster of copper(I) with heterocyclic thioamides, namely [Cu₃I₃L¹₃L² ] (L¹ = Ph₂P–CH₂–CH₂-PPh₂; L² = pyridine-2-thione) was reported from this laboratory.¹³

NMR (¹H and ³¹P) spectroscopy, electronic absorption and fluorescence spectroscopy

The ¹H NMR spectra of complexes 1–7 reveal proton signals because of the thio-ring protons and phenyl protons of PPh₃ (Experimental and ESI†). The PPh₃ protons appeared as complex multiplets, and in only a few cases, signals for o-H, m-H and p-H protons of phenyl rings could be separated. The complex 1 did not show a signal for N¹H proton, which supported that the thio-ligand coordinates to Cu^I as the anion. The H⁴ and H⁵ protons appear at 7.93 and 6.44 ppm, respectively, and are at high fields relative to the free pymSH ligand, which is attributed to the aromatization of the pyrimidine ring in the complex. The N⁹H, H² and H⁸ protons of complex 2a moved at a low field relative to the free purSH₂ ligand. Similarly, N^1,3H protons of tucH₂ in complex 3 appeared as a combined signal, which is at low field, and H⁵ and H⁶ signals are marginally affected relative to a free ligand. In dtucH₂ complexes 4 and 5, the expected N³H proton signals could not be identified because it was observed to be involved in H⋯X (X = Cl, Br) hydrogen bonding (X-ray crystallography, vide supra). The H⁵ and H⁶ protons move to a low field and appeared as a combined signal at 7.58 and 7.56 ppm in 4 and 5, respectively. Regarding H₃tmt complexes 6 and 7, while complex 6 exhibited one signal at 9.90 ppm assigned to N^3,5H protons, no NH signal could be detected in 7 probably due to broadening by quadrupolar nitrogen.

The ³¹P NMR spectrum of complex 1 showed one signal with a coordination shift of 2.84 ppm, suggesting that both phosphorus atoms are under identical chemical environments. Complex 2a with similar stoichiometry showed two ³¹P NMR signals, suggesting two types of PPh₃ groups. It shows the formation of 2b in solution. The coordination shifts of 1.60 and 3.34 ppm are assigned to complexes 2a and 2b, respectively. Thiouracil and dithiouracil complexes 3–5 showed one signal each with Δδ values of 1.29, 1.45 and 1.07 ppm, respectively. Complex 6 whose X-ray crystallography indicated the existence of bond isomers 6a and 6b revealed equivalent chemical environments for PPh₃ ligands in a solution state (Δδ, 6.10 ppm). Finally, complex 7 also showed a signal with Δδ = 2.13 ppm.

Complexes 1–7 show electronic absorption in the region of 259–450 nm (Fig. 9a and b). The absorption bands in the region 250–291 nm are because of π → π* transitions, whereas those in the region 334–370 nm are attributed to the metal to thio-ligand/n → π* transitions. To investigate if these copper(I) complexes display any emission properties, the complexes were excited in the region 270 to 291 nm. The complexes exhibited intense emission bands in the 380–530 nm region with λ_max at 490 to 495 nm (Fig. 10).

Fig. 9 (a) Electronic absorption spectra of complexes 1–3. (b) Electronic absorption spectra of complexes 4–7.

Fig. 10 Fluorescence spectra of complexes 1–7. (It may be noted that emission shown in the region of 540–586 nm is shown by DMSO in the absence of the sample and appears at twice the wavelength of excitation at 276 nm-spectrum of DMSO placed in ESI†).

ESI-mass spectral studies

The ESI-mass spectral data of most of the complexes (1–3, 5–7) have been obtained. The purpose of carrying out this study was manifold: to determine the molecular ion ([M]⁺), to understand the fragmentation pattern and the formation of new species, and to investigate the possibility of rupture of the Cu–P, Cu–S or C–S bonds. The C–S rupture is significant, particularly because of the presence of PPh₃ in the complexes, which might assist in such cleavage. The significant species identified are listed in Table 3 and are discussed. The most significant feature of this study is the formation of the species [Cu(PPh₃)₂]⁺, which is most abundant in complexes 3–7 and is reasonably intense in complexes 1 and 2. This shows that Cu–P bond is the most stable, and the rupture predominantly occurs at Cu–S/N bonds. Species that could be attributed only to [Cu(thio-ligand)_n]⁺were not observed. Nevertheless, there is a tendency to form mononuclear, dinuclear and trinuclear species. In complexes 3, 5, species containing both the thio-ligand and PPh₃ components were not observed; rather, only PPh₃ and halogens bonded to Cu^I were found to be present. In addition, there is no evidence to support the rupture of C–S bonds, and this could be attributed to the high stability of [Cu(PPh₃)₂]⁺ moiety, which favored the de-ligation of thio-ligand moiety. Mixed-ligand species containing both the thio-ligand and PPh₃ components in complexes 1, 2 and 6 support that thio-ligands remain intact in the reaction system without any degradation and recombine with Cu⁺ in different manners, generating unusual species, as given in Table 3. Only complex 6 showed the presence of a molecular ion, [M]⁺ (m/z obsd. 765.0; calcd. 763.1). Interestingly, ESI-mass has very clearly supported our contention that 2b appears to have been formed from 2a via the formation of the most intense species, [Cu(purSH)(PPh₃) +2H]⁺, which is strongly implicated in its spectrum (m/z, obsd. 477.0; calcd. 478.0). Finally, the spectrum of complex 1 has shown the formation of dinuclear and trinuclear complex species. Fig. 11 shows ESI-mass peak due to [Cu(PPh₃)₂]⁺ with an isotopic pattern found in complex 6, and Fig. 12 shows its molecular ion peak.

Table 3 ESI-mass data indicating assignment

Complex	% Intensity	m/z: obsd	m/z: calcd.	Species identified
1	20	587.1	587.1	[Cu(PPh3)2]^+
	100	761.0	761.0	[Cu2(pymS)(PPh3)2]^+
	15	935.0	935.0	[Cu3(pymS)2(PPh3)2]^+
	70	1023.1	1023.1	[Cu2(pymS)(PPh3)3]^+
	25	1197.1	1197.1	[Cu3(pymS)2(PPh3)3]^+
2	100	477.0	478.0	[Cu(purS)(PPh3) + 2H]^+
	40	803.0	803.0	[Cu2(purS)(PPh3)2 + 2H]^+
	20	587.1	587.1	[Cu(PPh3)2]^+
	10	1065.1	1065.2	[Cu2(purS)(PPh3)3 + 2H]^+
3	100	587.1	587.1	[Cu(PPh3)2]^+
5	100	587.1	587.1	[Cu(PPh3)2]^+
	10	729.0	729.0	[Cu2Br(PPh3)2]^+
	15	991.0	991.0	[Cu2Br(PPh3)3]^+
6	100	587.1	587.5	[Cu(PPh3)2]^+
	85	676.9	676.9	[Cu(tmtH2)2(PPh3)]^+
	25	765.0	763.1	[Cu(tmtH2)(PPh3)2]^+
	80	939.0	939.0	[Cu(tmtH2)2(PPh3)2]^+
7	100	587.1	587.1	[Cu(PPh3)2]^+

---

Fig. 11 ESI-mass peak due to [Cu(PPh₃)₂]⁺ with isotopic pattern (complex 6).

Fig. 12 ESI-mass peak due to molecular ion with isotopic pattern (complex 6).

Conclusion

The present investigation significantly contributes to copper-heterocyclic thioamide chemistry with scarcely investigated multifunctional thio-ligands, particularly purine-6-thione, thiouracils and 2,4,6-trimercaptotriazine. The formation of mono- and di-nuclear complexes (2a, 2b), display of bond isomerism by complex 6, formation of dinuclear complexes 4 and 5 with unusual bonding patterns, and the synthesis of rare trinuclear complex 7 are novel features of this study. The only other trinuclear cluster of copper(I) with heterocyclic thioamides, particularly [Cu₃I₃L¹₃L² ] (L¹ = Ph₂P–CH₂–CH₂–PPh₂; L² = pyridine-2-thione), was reported from this laboratory.¹³

Experimental

Materials and techniques

The precursor, particularly Cu(PPh₃)₂(CH₃COO), was prepared by the reduction of Cu(CH₃COO)₂ using a four-fold excess of PPh₃ in CH₃OH at a reflux temperature for 3 h.⁴⁹ Copper(I) halides were prepared by the reduction of CuSO₄·5H₂O using SO₂ in the presence of stoichiometric amounts of NaCl, NaBr, and NaI in water.⁵⁰ Organic ligands, namely 2,4,6-trimercaptotriazine, purine-6-thione, pyrimidine-2-thione, 2,4-dithiouracil and 2-thiouracil were procured from Sigma-Aldrich Ltd. Melting point was determined with a Gallenkamp electrically heated apparatus. IR spectra were recorded using KBr pellets on Varian 660 FT IR and Perkin Elmer FT IR Spectrometer in 4000–200 cm⁻¹ range. ¹H NMR spectra were recorded in CDCl₃ using a Bruker Avance II 400 NMR spectrometer at 400 MHz and JEOL AL300 FT ¹H NMR at 300 MHz using TMS as an internal reference. ³¹P NMR spectra were recorded in CDCl₃ with Bruker Avance II 400 NMR at 161.97 MHz spectrometers with o-phosphoric acid as the external reference (δ = 29.15 ppm). The ESI-MS mass spectra was recorded on a microTOF-QII 10356 in positive mode.

Synthesis of complexes

[Cu(κ¹N, κ¹S-pymS)(PPh₃)₂] (1). To a methanolic (5 mL) solution of pymSH (0.004 g, 0.036 mmol), a solution of [Cu(PPh₃)₂(CH₃COO)] (0.025 g, 0.039 mmol) in dichloromethane (5 mL) was slowly, and the clear solution was kept undisturbed at room temperature for a period of three to four days. The resulting pale yellow solution was allowed to evaporate slowly at room temperature. The pale yellow crystals of 1 were formed over a period of three days. Yield: 0.018 g, 72%; m.p 166–168 °C. Anal. calc. for C₄₀H₃₃N₂CuP₂S (699.24): C, 68.65; H, 4.72; N, 4.01. Found: C, 68.87; H, 5.02; N, 3.01%. Main IR peaks (KBr, cm⁻¹): ν(C–H), 3048m, 3013w, 3001w, 2925w; ν(C–C) + ν(C–N) + δ(C–H), 1566s, 1531s, 1479s, 1434s; 1370s, 1309w, 1238m, 1215w, 1183s, 1157w; ν(P–C), 1093s; 1070w, ν(C–S), 1026m; 997m, 850w, 796w, 768w, 747s, 695s, 651w, 619w, 516s, 505m, 467w. ¹H NMR (δ, ppm, J, Hz, CDCl₃): 7.93 (d, 1H, J, 4.2, Hz, H⁴), 7.33 (m, 31H, H⁶, o-H, m-H and p-H, PPh₃), 6.44 (t, 1H, J, 4.5, Hz, H⁵). ³¹P NMR (CDCl₃, δ ppm): −2.63 ppm, Δδ(δ_complex − δ_PPh3) = 2.84 ppm. UV-vis. data, DMSO, λ_max/nm,:[10⁻⁵ M] 250–400 nm. Fluorescence data (λ^em_max = 494 nm, λ^ex_max = 274 nm).

[Cu(κ¹N,κ¹S-purSH)(PPh₃)₂]·CH₃OH (2a). To a methanolic (5 mL) solution of purSH (0.006 g, 0.039 mmol), a solution of [Cu(PPh₃)₂(CH₃COO)] (0.025 g, 0.039 mmol) in dichloromethane (5 mL)was slowly added, and the clear solution was kept undisturbed at room temperature for a period of three to four days. The resulting pale yellow solution was allowed to evaporate slowly at room temperature. The pale yellow (2a) and yellow (2b) crystals were formed simultaneously over a period of three to four days, and the crystals were manually separated. [Cu(κ²-N,S-purS)(PPh₃)₂]·CH₃OH (2a) Yield: 0.016 g, 64%; m.p above 300 °C. Anal. calc. for C₄₂H₃₇N₄OCuP₂S (771.30): C, 65.34; H, 4.80; N, 7.26. Found: C, 65.56; H, 4.91; N, 7.43%. Main IR peaks (KBr, cm⁻¹): ν(N–H), 3080sh; ν(C–H), 3049m, 2960w, 2936w, 2816m; ν(C–C) + ν(C–N) + δ(C–H), 1592s, 1564s, 1479s, 1434s; 1374s, 1320s, 1233s, 1183w, 1129w; ν(P–C), 1094s; 1038m; ν(C–S), 942m; 853s, 793w, 748s, 696s, 646m, 603w, 516s, 431w. ¹H NMR (δ, ppm, J, Hz, CDCl₃): 14.45 (sb, N⁹H), 8.50 (s, 1H, H⁸), 8.22 (sb, 1H, –OH, CH₃OH), 7.68 (m, 1H, H²), 7.47 (m, 12H, o-H, PPh₃), 7.22 (m, 18H, m-H and p-H, PPh₃), 3.49 (s, 3H, CH₃OH). ³¹P NMR (CDCl₃, δ ppm): −2.13, −3.87 ppm, Δδ(δ_complex − δ_PPh3) = 3.34, 1.6 ppm. UV-vis. data, DMSO, λ_max/nm, ε/L mol⁻¹ cm⁻¹: [10⁻⁵ M] 334 (1.38 × 10⁴). Fluorescence data (λ^em_max = 490 nm, λ^ex_max = 266 nm).

[Cu₂(κ¹N,μ-S-purS)₂(PPh₃)₂] (2b). Yield: 0.004 g, 13%; m.p above 300 °C. Anal. calc. for C₄₆H₃₆N₈Cu₂P₂S₂ (953.97): C, 57.86; H, 3.77; N, 11.74. Found: C, 57.80; H, 3.19; N, 11.20%. Main IR peaks (KBr, cm⁻¹): ν(N–H), 3130sh; ν(C–H), 3051m, 2926w, 2870w, 2766w; ν(C–C) + ν(C–N) + δ(C–H), 1579s, 1477m, 1434s; 1385s, 1324m, 1230w, 1180m, ν(P–C), 1094m; 1025m, 938w; ν(C–S), 844m; 742s, 693s, 667w, 612m, 521s.

[CuCl(κ¹S-tucH₂)(PPh₃)₂] (3). To a solution of copper(I) chloride (0.025 g, 0.25 mmol) in acetonitrile, a solution of 2-thiouracil (0.032 g, 0.25 mmol) in methanol was added, followed by stirring for 1 h at room temperature. Ph₃P (0.132 g, 0.50 mmol) was added to the obtained precipitates and stirred until a clear solution was obtained. The slow evaporation of the solution at room temperature formed pale yellow crystals of 3. Yield: 0.12 g, 63%, M.p. 180–182 °C. Anal. calcd for C₄₀H₃₄ClCuN₂OP₂S (751.68): C, 63.86; H, 4.52; N, 3.72; Found: C, 63.85; H, 4.34; N, 3.56%. Main IR peaks (KBr, cm⁻¹): ν(O–H), 3447w; ν(N–H), 3130w; ν(C–H), 3048m, 3010w, 2960w; ν(C–N) + δ(C–H) 1584m, 1480s; 1432s, 1309m, ν(C–S) 1184m, 1154m; ν(P–C), 1091(s); 1027m, 998m, 923w, 851m, 743s, 694s, 517s. ¹H NMR (δ, ppm, J, Hz, CDCl₃): 13.58 (sb, 2H, N^1,3H), 5.82 (d, 1H, J = 7.8 Hz, H⁵), 7.02 (d, 1H, J = 7.8 Hz, H⁶), 7.19–7.42 (m, 15H, Ph₃P). ³¹P NMR (CDCl₃, δ ppm): −4.18 ppm, Δδ(δ_complex − δ_PPh3) = 1.29 ppm. UV-vis. data, DMSO, λ_max/nm, ε/L mol⁻¹ cm⁻¹: [10⁻⁵ M] 250–400 nm. Fluorescence data (λ^em_max= 493 nm, λ^ex_max= 270 nm).

[Cu₂(μ-Cl)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (4). To a solution of copper(I) chloride (0.025 g, 0.25 mmol) in acetonitrile (10 mL), a solution of 2,4-dithiouracil (0.036 g, 0.25 mmol) in methanol (5 mL) was added, followed by stirring for 24 h at low temperature (20 °C). Solid Ph₃P (0.132 g, 0.50 mmol) was added to the obtained orange precipitates. The contents were stirred until a clear solution was obtained. Slow evaporation of solution formed light orange crystals of [Cu₂(μ-Cl)(μ-S,S-dtucH)(PPh₃)₄] 4. Yield: 0.032 g, 19%, M.p. 178–180 °C. Anal. calcd for C₇₆H₆₃ClCu₂N₂P₄S₂ (1354.81): C, 67.32; H, 4.65; N, 2.07; Found: C, 67.02; H, 4.56; N, 2.18%. Main IR peaks (KBr, cm⁻¹): ν(N–H), 3250sh; ν(C–H), 3049w, 2995w, 2927w; ν(C–N) + δ(C–H), 1543s, 1507m, 1478m; 1433m, 1384w, 1271m, 1205w; ν(C–S) 1128m; ν(P–C), 1094m; 833m, 743s, 693s, 616w, 516s. ¹H NMR (δ, ppm, J, Hz, CDCl₃): 7.58 (m, 2H, H^5,6), 7.26 (m, 30H, o-H, m-H and p-H, PPh₃). ³¹P NMR (CDCl₃, δ ppm): −4.02 ppm, Δδ(δ_complex − δ_PPh3) = 1.45 ppm. UV-vis. data, DMSO, λ_max/nm, ε/L mol⁻¹ cm⁻¹: [10⁻⁵ M] 367 (7.54 × 10³), 262 (4.76 × 10⁴). Fluorescence data (λ^em_max = 495 nm, λ^ex_max = 276 nm).

[Cu₂(μ-Br)(κ¹S,κ¹S-dtucH)(PPh₃)₄] (5). This was prepared by the method used for the preparation of complex 4. Slow evaporation of the solution at room temperature formed light orange crystals of [Cu₂(μ-Br)(μ-S,S-dtucH)(PPh₃)₄] 5 (Yield: 0.024 g, 20%, M.p. 181–183 °C). Anal. calcd for C₇₆H₆₃BrCu₂N₂P₄S₂ (1399.27): C, 65.18; H, 4.50; N, 2.00; Found: C, 65.15; H, 4.21; N, 2.14%. Main IR peaks (KBr, cm⁻¹): ν(N–H), 3090w; ν(C–H), 3048m, 2970w, 2925m, 2854w; ν(C–N) + δ(C–H), 1541s, 1505m, 1479s; 1433s, 1384w, 1348w, 1309w, 1270m, 1204w, 1180w, 1156w; ν(C–S) 1126m; ν(P–C), 1093m; 1026w, 996w, 976w, 920w, 832m, 742s, 693s, 616w, 516s. ¹H NMR (δ, ppm, J, Hz, CDCl₃): 7.56 (m, 2H, H^5,6), 7.37 (m, 12H, o-H, PPh₃), 7.27 (m, 6H, p-H, PPh₃), 7.16 (m, 12H, m-H, PPh₃). ³¹P NMR (CDCl₃, δ ppm): −4.4 ppm, Δδ(δ_complex − δ_PPh3) = 1.07 ppm. UV-vis. data, DMSO, λ_max/nm, ε/L mol⁻¹ cm⁻¹: [10⁻⁵ M] 370 (8.21 × 10³), 262 (5.30 × 10⁴). Fluorescence data (λ^em_max = 493 nm, λ^ex_max = 273 nm).

[Cu(tmtH₂)(PPh₃)₂]·0.5H₂O (6) (bond isomers 6a,6b). To a methanolic (5 mL) solution of H₃tmt (0.007 g, 0.039 mmol), a solution of [Cu(CH₃COO)(PPh₃)₂] (0.025 g, 0.039 mmol) in dichloromethane (5 mL) was slowly added, and the clear solution was kept undisturbed at room temperature for a period of three to four days. The reaction mixture became pale yellow and was allowed to evaporate slowly at room temperature. The pale yellow-colored crystals were formed over a period of two to three days. Yield: 0.021 g, 70%; m.p above 300 °C. Anal. calc. for C₃₉H₃₃N₃O_0.50CuP₂S₃ (773.34): C, 60.52; H, 4.27; N, 5.43. Found: C, 59.88; H, 4.66; N, 5.49%. Main IR peaks (KBr, cm⁻¹): ν(N–H), 3110sh; ν(C–H), 3051m, 2959w, 2910m, 2831w; ν(C–C) + ν(C–N) + δ(C–H), 1532m, 1477s; 1434s, 1396w, 1353s, 1233s, 1171m, 1154w, 1138s; ν(P–C), 1095m; 1070w, 1026w, ν(C–S), 1014m; 998w, 917w, 893m, 882m, 849w, 743s, 693s, 618w, 517s, 492w, 454s. ¹H NMR (δ, ppm, J, Hz, CDCl₃): 9.90 (sb, 3H, 2NH+0.5H₂O), 7.36 (m, 18H, o-H and p-H, PPh₃), 7.24 (m, 12H, m-H, PPh₃). ³¹P NMR (CDCl₃, δ ppm): 0.63 ppm, Δδ(δ_complex − δ_PPh3) = 6.10 ppm. UV-vis. data, DMSO, λ_max/nm, ε/L mol⁻¹ cm⁻¹: [10⁻⁵ M] 346 (1.41 × 10⁴), 291 (2.66 × 10⁴). Fluorescence data (λ^em_max = 494 nm, λ^ex_max = 291 nm).

[Cu₃Br₂(κ¹N,κ¹S,μ-S-tmtH₂)(PPh₃)₆] (7). To a solution of copper(I) bromide (0.025 g, 0.25 mmol) in acetonitrile (10 mL), a solution of 2,4,6-trimercaptotriazine (0.031 g, 0.25 mmol) in methanol (5 mL) was added, followed by stirring for 24 h at room temperature. Solid Ph₃P (0.091 g, 0.50 mmol) was added to the obtained orange precipitates. The contents were stirred until a clear solution was obtained, and the slow evaporation of the solution at room temperature formed yellow crystals of 7, which are stable in mother liquor. Yield: 0.084 g, 68%, M.p. 200–203 °C. Anal. calcd for C₁₁₁H₉₂Br₂Cu₃N₃P₆S₃ (2100.32): C, 63.42; H, 4.38; N, 2.00; Found: C, 63.47; H, 4.51; N, 1.27%. Main IR peaks (KBr, cm⁻¹): ν(N–H), 3140w; ν(C–H), 3049m, 3010w, 2920w, 2855m; ν(C–N) + δ(C–H), 1524s, 1460s; 1433s, 1394m, 1362s, 1274w; ν(C–S), 1177s, 1126s; ν(P–C), 1093s; 1026w, 997w, 868w, 743w, 694s, 618w, 518s, 456w. ¹H NMR (δ, ppm, J, Hz, CDCl₃): 7.33 (m, 30H, o-H, m-H and p-H, PPh₃). ³¹P NMR (CDCl₃, δ ppm): -3.34 ppm, Δδ(δ_complex − δ_PPh3) = 2.13 ppm. UV-vis. data, DMSO, λ_max/nm, ε/L mol⁻¹ cm⁻¹: [10⁻⁵ M] 363 (1.41 × 10⁴), 262 (8.11 × 10⁴). Fluorescence data (λ^em_max = 495 nm, λ^ex_max = 273 nm).

X-ray crystallography

Single crystals of compounds were mounted on glass fibers, and data were collected on a Bruker CCD SMART1000 (1), Xcalibur, Eos, Gemini (2a, 4, 3, 7), Xcalibur, Ruby, Gemini (2b, 5) and Bruker APEX-II CCD (1) diffractometer equipped with a graphite monochromator and Mo–Kα radiation (λ = 0.71073 Å; 6, 2a, 1, 4, 5, 3, 7) and Cu-Kα radiation (λ = 1.54184 Å; 2b). The unit-cell dimensions and intensity data were measured at 100(2) K for 6, 173(2) for 2a, 4 and 7, 295(2) for 2b, 296(2) K for 1, 123(2) for 5 and at 170(2) for 3. The data were processed with SMART (data collection, cell refinement), SAINT (data reduction) (1), CrysAlisPro (data collection, cell refinement) (2a, 2b, 4, 5, 3, 7), CrysAlisPro RED (data reduction) (2a, 4, 3), CrysAlisPro (data reduction) (2b, 5 and 7),⁵¹ Bruker APEX2 (data collection), and Bruker SAINT (cell refinement, data reduction) (1).⁵² The structures were solved by direct methods using the program SHELXS-97 (ref. 53) (6, 2a, 2b, 4, 5, 3, 7), SIR-92 (1)⁵⁴ and refined by full-matrix least-squares techniques against F^{^2} using SHELXL-97.⁵⁵ The structure of 6 was refined as a racemic twin with components 0.54(1), 0.46(1).⁵⁶

Acknowledgements

Financial assistance from the University Grant Commission (BSR, UGC); the Council of Scientific and Industrial Research (CSIR), New Delhi; Emeritus Grant [21(0904)/12-EMR-II] to TSL; and the Department of Science and Technology (DST) ST for the X-ray diffractometer grant to the department are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: CCDC reference numbers 976978 (1), 976979 (2a), 976980 (2b), 976981 (3), 976982 (4), 976983 (5), 976984 (6) and 976985 (7). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra02748e

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