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Communication

Structural Aspects of Cu(I)(κ2-X1,X2)(Y3) and Cu(I)(η2-X1,X2)(Y3) Complexes

by
Milan Melník
1,†,
Natalia Miklášová
2,
Veronika Mikušová
3 and
Peter Mikuš
1,4,*
1
Department of Pharmaceutical Analysis and Nuclear Pharmacy, Faculty of Pharmacy, Comenius University Bratislava, Odbojárov 10, SK-820 18 Bratislava, Slovakia
2
Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University Bratislava, Ilkovičova 6, Mlynská Dolina, SK-842 15 Bratislava, Slovakia
3
Department of Galenic Pharmacy, Faculty of Pharmacy, Comenius University Bratislava, Odbojárov 10, SK-820 18 Bratislava, Slovakia
4
Toxicological and Antidoping Centre, Faculty of Pharmacy, Comenius University Bratislava, Odbojárov 10, SK-820 18 Bratislava, Slovakia
*
Author to whom correspondence should be addressed.
†
Deceased author.
Inorganics 2026, 14(5), 142; https://doi.org/10.3390/inorganics14050142
Submission received: 14 July 2025 / Revised: 18 May 2026 / Accepted: 19 May 2026 / Published: 21 May 2026
(This article belongs to the Special Issue Applications and Future Trends for Novel Copper Complexes)
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Abstract

Structural parameters for over seventy complexes of the composition Cu(η2-X1×2)(Y3) or Cu(κ2-X1X2)(Y3) were analyzed in this work, being the third of a series of structural studies on three coordinated copper(I) complexes. Bidentate (X1X2) with monodentate (Y3) donor ligands build up distorted trigonal planar coordination spheres around copper(I) atoms. The bidentate ligands (X1X2) create three-, four-, and five-membered metallocyclic rings. The three-membered are: -C1-C2-Cu-C3; -B1=B2-Cu-Cl3; -P≡C2-Cu-C3, -B1-B2-Cu-X3, and B1-C2-Cu-C3. The X1-Cu-X2 angles indicate a total mean value of 44.2°. The four-membered complexes are -H1-B(H2)-H2-Cu-C3; -H1-B(Ph2)-H2-Cu-C3; -O1AlO2-Cu-N3; -O1CeO2-Cu-N3; -S1CP2-Cu-C3; -N1PN2-Cu-C3; -N1PS2-Cu-P3; -N1SiO1-Cu-Cl3; --N1CS2-Cu-C3; -Si1-NSi2-Cu-C3, and O1CO2-Cu-C3, and show a total mean value of the L-Cu-L angles of 71.0°. The five-membered are: -N1-C=C-N2-Cu-Y3 (more common) and N=C-C=N-Cu-C3. In this group, there are also copper(I) complexes in which the central Ns of five-membered metallocycle are “interlocked” in macrocycles. The X1-Cu-X2 angles exhibit an average value of 82.9°. There is a wide variety of monodentate (Y3) ligands in the studied complexes. The mean value of Cu-Y3 elongates with covalent radius (Å) of coordinate atoms in the sequence: 1.846(13) Å (N3, 0.75) < 1.884(21) Å (O3, 0.73) < 1.928(18) Å (C3, 0.77) < 2.126(18) Å (Cl3, 0.99) < 2.140(5) Å (S3, 1.02) < 2.194(4) Å (P3, 1.06) < 2.246(12) Å (Br3, 1.14) < 2.2445(18) Å (I3, 1.33). The data show that angular distortion from regular trigonal geometry grows in the following order: five-, four-, and three-membered.

Graphical Abstract

1. Introduction

Copper(I) complexes exhibit remarkable structural diversity and functional versatility, enabling their use in various applications [1,2,3,4,5,6]. Their ability to stabilize unusual oxidation states, form strong π-complexes, and engage in redox and photophysical processes makes them attractive in catalysis, materials science, and molecular sensing. For example, Cu(I) complexes with N-heterocyclic carbenes act as efficient catalysts in hydrophosphination and hydrosilylation reactions [7,8]. Their photoluminescent properties are exploited in OLED and sensing applications, particularly in three-coordinate or macrocyclic systems [9,10,11]. Additionally, Cu(I) forms reactive intermediates for selective C–H bond hydroxylation, mimicking metalloenzyme function [12,13]. The coordination of Cu(I) with soft ligands enables the stabilization of reactive species, such as NN’O adducts or low-coordinate halides, providing insight into fundamental bonding and reactivity [14,15]. Furthermore, their roles in supramolecular chemistry and as building blocks for functional frameworks highlight their structural tunability and electron-transfer capabilities [16,17,18,19].
Based on a variable combination of donor atoms in three-coordinate copper(I) compounds, these compounds were divided into two groups, namely (i) homo, Cu(XXX) (X = NL; CL; SL; PL; Cl; Br; I) and (ii) hetero [5]. The hetero were subdivided into two types, one creating, only by unidentate donor ligands, a three coordination inner sphere of the following types: Cu(NNY) (Y = O, C, Cl, S, Br, I); Cu(CCY) (Y = O, Cl, Br, I); Cu(SSY) (Y = N, Cl, I) Cu(SeSeY) (Y = Cl, Br, I); Cu(PPY) (Y = O, C, Cl, Br, I), Cu(ClClY) (Y = N, P); Cu(BrBrY) (Y = N, P); Cu(IIP); Cu(SClP); Cu(SBrP); and Cu(SIP) derivates [6].
Building upon our recent studies of three-coordinate copper(I) complexes [5,6], the present work provides a comprehensive structural analysis of compounds of the general formula Cu(η2-X1X2)(Y3) or Cu(κ2-X1X2)(Y3). As the third contribution in this systematic (three-coordinated) series, it not only expands the structural dataset but also refines the emerging understanding of coordination preferences and bonding characteristics of three-coordinate copper(I) systems. The factors that could lead to a better understanding of stereochemical interactions with the coordination of monomeric two- and three-coordinated copper(I) complexes are discussed. The investigated systems have been sorted by metallocycles and then subdivided according to the number of atoms involved in the metallocyclic rings.

2. Results and Discussion

New copper complexes, including those that are three-coordinated, are synthesized for their potential properties, such as catalytic properties; copper complexes [7] are highly active and selective precatalysts for the hydrophosphination of isocyanates, and catalytic carbonyl hydrosilylation activity in aqueous media [8]. Complexes with photophysical properties [9,10] have potential in light-emitting diode devices [16]. Some of them can play a role as starting material for the preparation of potentially valuable biologically active compounds [20,21]. A five-membered ring copper complex [22] has also been successfully used as an active catalyst in the preparation of arylated octafluoropentyl bromide building blocks with potential application in molecular electronics. On the other hand, a vast majority of the studied complexes, including those that are three-coordinated, were just synthesized and analyzed concerning their detailed chemical structures; however, their application potential has not been reported so far. Hence, this structural study, comparing distortion of derivatives within specific structural groups, could be helpful when assuming a relationship between, e.g., distortion and activity.
Analyzed structural data of Cu(η2-X1X2)(Y3) shows various metallocyclic rings. In general, two methods of preparation are reported for these derivatives. The first included the reaction of copper(II) salts in the presence of a ligand. These are mostly S and P donor molecules, serving as both ligand and reducing agents. The second, more common method includes direct interaction of the ligand with copper(I) salts in a nonaqueous solvent such as acetonitrile under an inert atmosphere (usually dry nitrogen).
The Cu(η2-X1X2)(Y3) and Cu(κ2-X1X2)(Y3) complexes involved in this study were divided into the groups characterized by: (i) three-membered metallocycles, (ii) four-membered metallocycles, and (iii) five-membered metallocycles. The complexes with five-membered metallocycles were subdivided into two subgroups, namely (iiia) metallocycles formed by unsaturated N1N2 donor ligands and (iiib) metallocycles “interlocked” in macrocycles. Other older relevant complexes of the structural formulas Cu(κ2-X1X2)(Y3) and Cu(η2-X1X2)(Y3) can be found in our previous review [19]. For the structures in this study, both the “hapto”-nomenclature and the κ-notation have been used [23].

2.1. Three-Membered Metallocycles in Cu(η2-X1X2)(Y3) and Cu(κ2-X1X2)(Y3) Derivatives

Three distinct complexes have been reported: monoclinic [Cu(η2-C6H6-C1C2)(C27H37N2-κ1-C3)]SbF6 (at 100 K) [24], orthorhombic [Cu(η2-C34H36B2-B1=B2)(κ1-Cl3)](C4H8O) (at 296 K) [25], and monoclinic [Cu(η2-P1≡C2O)(C22H35NP-κ1-C3)] (at 100 K) [26], in which bidentate ligands create a three-membered metallocyclic ring and monodentate ligand completed a distorted trigonal geometry about each copper(I) atom. The structure of [Cu(η2-C6H6-C1C2)(C27H37N2-κ1-C3)] is shown in Figure 1 [24] as an example.
Each bidentate ligand builds up a three-membered metallocyclic ring of the type, with the following structural data:
-
C1-C2-Cu-C3 [24], Cu-L(Å): 2.217 (C1), 2.129 (C2), 1.886 Å (C3); 37.4°(C1-Cu-C2), 156.3°(C1-Cu-C3), 164.3°(C2-Cu-C3);
-
B1=B2-Cu-Cl3 [25], Cu-L(Å): 2.143 (B1), 2.133 (B2), 2.176 Å (Cl3), 44.6°(B1-Cu-B2) 145.6° (B1-Cu-Cl3), 164.9°(B2-Cu-Cl3);
-
P1≡C2-Cu-C3 [26], Cu-L(Å): 2.224 Å (P1), 1.915 Å (C2), 1.911 Å (C3), 39.9°(P1-Cu-C2), 173.4°(P1-Cu-C3), 173.4°(C2-Cu-C3).
Noticeably, the sums of Cu-L (η2) grow in the following order: 4.13 Å [26] < 4.27 Å [25] < 4.35 Å [24], and at the same time the L-Cu-L (L = η2) bond angles diminish in the following order: 39.9° (P1-Cu-C2) [26] > 44.6° (B1-Cu-B2 [25] > 37.2° (C1-Cu-C2) [24]. The data correlate well with the type of donor atoms, which are for Cu-L distances in order (C1-C2) [24] < (B1=B2) [25] < (P1≡C2) [25].
In addition to the above-mentioned three-membered copper(I) metallocycles, a series of monoclinic three-membered copper(I) metallocycles Cu(κ2-B1B2)(X3) containing neutral σB−B diborane ligands has also been reported [27] (Figure 2). These complexes contain an X−Cu moiety (X=Cl, OTf, C6F5) that is nearly perpendicular to the B−B bond, and allows an almost equal interaction with boron atoms. The B-Cu bonds in these complexes are significantly shorter than in diborane (B=B) copper complexes [25].
In κ2-B-B bonded copper complexes containing a N-heterocyclic carbene ligand (NHC) [28], the elongation of the B-B bond is also revealed. In the case of copper diboriranide complexes [29], the σ-only coordination of diboriranide to the transition metal complexes does not affect the π-system.
The structural data for the three-membered metallocyclic ring of the type Cu(κ2-B1B2)(κ1-X3), Cu(η2-B1=B2)(κ1-X3), and Cu(κ2-B1C2)(κ1-X3) complexes are as follows:
-
B1-B2-Cu-X3 (X = OTf) [27], Cu-L(Å): 2.086 (B1), 2.068 (B2), 1.920 (O3), 51.2° (B1-Cu-B2), 140.1° (B1-Cu-O3), 163.3° (B2-Cu-O3).
-
B1-B2-Cu-C3 [28], Cu-L(Å): 2.177 (B1), 2.177 (B2), 1.949 (C3), 47.7° (B1-Cu-B2), 156.1° (B1-Cu-C3), 156.1° (B2-Cu-C3).
-
B1-B2-Cu-C3 [30], Cu-L(Å): 2.171 (B1), 2.135 (B2), 1.896 (C3), 47.2° (B1-Cu-B2), 154.5° (B1-Cu-C3), 157.8° (B2-Cu-C3).
-
B1=B2-Cu-Cl3 (Cl3-(C4H8O)3Li) [29], Cu-L(Å): 2.093 (B1), 2.071 (B2), 2.140 (Cl3), 49.0° (B1-Cu-B2), 158.7° (B1-Cu-Cl3), 152.0° (B2-Cu-Cl3).
-
B1-C2-Cu-C3 [31], Cu-L(Å): 2.121 (B1), 2.411 (C2), 1.943 (C3), 37.1°(B1-Cu-C2), 156.3° (B1-Cu-C3), 163.4° (C1-Cu-C3).
The sums of Cu–L interactions increase in the following order: 4.164 Å (B1=B2) [29] < 4.271 Å (range 4.154 Å–4.354 Å) (B1-B2) [27,28,30] < 4.532 Å (B1-C2) [31]. Conversely, the L–Cu–L bond angles increase across the series: 37.1° (B1-Cu-C2) [31] < 48.7° (B1-Cu-B2) [8,27,30] < 49.0° (B1-Cu-B2) [29]. The mean values of the corresponding L–Cu–L bond angles are also summarized in Table 1 in the Section 4.

2.2. Four-Membered Metallocycles in Cu(κ2-X1×2)(Y3) Derivates

In several other copper(I) complexes, a bidentate ligand builds up a distorted trigonal geometry about copper(I) atoms with a monodentate ligand; however, each bidentate ligand creates a variety of four-membered metallocyclic rings. Such complexes are: monoclinic (0.74) [Cu(κ2-H1-B(H2)-H2)(C24H37N-κ1-C3)] (0.26) (C24H37N-κ1-C4)(Cl)] (at 100 K) [32], triclinic [Cu(κ2-H1-B(Ph2)-C2C6H5)(κ1-C27H27N2-C3)] (at 185 K) [8], orthorhombic [Cu(C16F36AlO4-κ2-O1O2)(N2O-κ1-N3)] (at 133 K) [14], orthorhombic [Cu(C36H81Ce2O9-κ2-O1O2)(C7H10N2-κ1-N3)](C7H8)2 (at 113 K) [33], monoclinic [Cu(κ2-S1C(=S)P2(Ph2)(C28H40N2-κ1-N3)]C6H6 (at 150 k) [7], and triclinic [Cu(κ2-S1C(=NPh)P2(Ph2)(C28H39N2-κ1-C3)]C6H6 (at 150 K) [7]. The structure of [Cu(C16F36AlO4-κ2-O1O2)(N2O-κ1-N3)] [14] is shown in Figure 3 as an example.
Each bidentate (X1×2) ligand builds up a four-membered metallocyclic ring of the type with structural data:
-
H1-B(H2)-H2,C3 [32]: Cu-L (Å): 1.693 (H1), 1.696 (H2), 1.891 (C3), 69.0° (H1-Cu-H2); 139.2° (H1-Cu-C3); 151.3° (H2-Cu-C3);
-
H1-B(Ph2)-C2,C3 [8]: Cu-L (Å): 1.584 (H1), 2.188 (C2), 2.165 (C3); 72.0° (H1-Cu-C2), 136.0° (H1-Cu-C3), 143.0° (C2-Cu-C3);
-
O1-Al-O2, N3 [14]: Cu-L (Å): 2.099 (O1), 2.099 (O2), 1.885 (N3); 73.6° (O1-Cu-O2), 143.6° (O1-Cu-N3), 143.6° (O2-Cu-N3);
-
O1-Ce-O2, N3 [33]: Cu-L (Å): 2.090 (O1), 2.095 (O2), 1.927 (N3); 76.0° (O1-Cu-O2), 141.5° (O1-Cu-N3), 142.4° (O2-Cu-N3);
-
O1-Ce-O2, N3 [33]: Cu-L (Å): 2.025 (O1), 2.210 (O2), 1.919 (N3); 75.4° (O1-Cu-O2), 136.9° (O1-Cu-N3), 147.3° (O2-Cu-N3);
-
S1-C-P2, C3 [7]: Cu-L (Å): 2.369 (S1), 2.312 (P2), 1.927 (C3); 74.0° (S1-Cu-P2), 141.2° (S1-Cu-C3), 144.7° (P2-Cu-C3);
-
S1-C-P2, C3 [7]: Cu-L (Å): 2.311 (S1), 2.329 (P2), 1.929 (C3); 74.9° (S1-Cu-P2), 135.0° (S1-Cu-C3), 150.3° (P2-Cu-C3);
-
N1-P-N2, C3 [34]: Cu-L (Å): 2.002 (N1), 2.002 (N2), 1.824 (C3); 77.7° (N1-Cu-N2), 139.7° (N1-Cu-C3), 142.4° (N2-Cu-C3);
-
N1-P-N2, C3 [35]: Cu-L (Å): 1.971 (N1), 1.966 (N2), 1.832 (C3); 78.4° (N1-Cu-N2), 136.9° (N1-Cu-C3), 144.5° (N2-Cu-C3);
-
N1-N-N2, C3 [36]: Cu-L (Å): 2.562 and 2.701 (N1), 1.899 and 1.895 (N2), 1.834 and 1.833 (C3); 54.8° and 52.3° (N1-Cu-N2), 123.9° and 123.6° (N1-Cu-C3), 176.0° and 173.7° (N2-Cu-C3);
-
N1-C-N2, C3 [37]: Cu-L (Å): 1.891 (N1), 2.672 (N2), 1.876 (C3); 56.8° (N1-Cu-N2), 171.5° (N1-Cu-C3), 120.9° (N2-Cu-C3);
-
S1-C-S2, C3 [37]: Cu-L (Å): 2.310 (S1), 2.316 (S2), 1.878 (C3); 76.5° (S1-Cu-S2), 140.3° (S1-Cu-C3), 139.1° (S2-Cu-C3);
-
N1-P-S2, P3 [38]: Cu-L (Å): 1.960 (N1), 2.390 (S1), 2.154 (P3); 80.6° (N1-Cu-S1), 149.2°(N1-Cu-P3), 129.6° (S1-Cu-P3);
-
H1-B-H2, C3 [39]: Cu-L (Å): 1.716 (H1), 1.678 (H2), 1.887 (C3); 69.3° (H1-Cu-H2), 137.2° (H1-Cu-C3), 153.2° (H2-Cu-C3);
-
N1-Si-O1, Cl3 [40]: Cu-L (Å): 1.909 (N1), 2.596 (O2), 2.160 (Cl3); 69.0° (N1-Cu-O2), 173.1° (N1-Cu-Cl3), 112.5° (O2-Cu-Cl3);
-
Si1-N-Si2, C3 [41]: Cu-L (Å): 2.335 (Si1), 2.280 (Si2), 1.961 (C3); 69.0° (Si1-Cu-Si2), 139.6° (Si1-Cu-C3), 151.2° (Si2-Cu-C3);
-
Si1-N-Si2, I3 [41]: Cu-L (Å): 2.288 (Si1), 2.288 (Si2), 2.474 (I3); 70.5° (Si1-Cu-Si2), 144.7° (Si1-Cu-I3), 144.7° (Si2-Cu-I3);
-
N1-C-S2, C3 [42]: Cu-L (Å): 2.745 (N1), 2.146 (S2), 1.915 (C3); 63.9° (N1-Cu-S1), 122.1° (N1-Cu-C3), 173.7° (S1-Cu-C3);
-
O1-C-O2, C3 [43]: Cu-L (Å): 1.849 (O1), 2.811 (O2), 1.851 (C3); 86.6° (O1-Cu-O2), 133.0° (O1-Cu-C3), 174.6° (O2-Cu-C3).
The total mean values of Cu-L distance elongate in the following sequence: 1.673 (1.584–1.716) Å (H) < 2.167 (1.891–2.745) Å (N) < 2.188 Å (C) < 2.298 (1.849–2.811) Å (O) < 2.307 (2.146–2.390) Å (S) < 2.320 (2.312, 2.329) Å (P). In the three-membered ring complexes, the values are: 2.126 (2.071–2.177) Å (B) < 2.168 (1.915–2.411) Å (C) < 2.224 Å (P).
The total values of L-Cu-L angles for complexes of three- vs. four-membered rings are:
-
49.1 (37.1–79.2)° vs. 71.0 (range 52.3–86.6)° (X1-Cu-X2);
-
155.1 (140.1–173.4)° vs. 140.4 (range 122.1–173.1)° (X1-Cu-Y3);
-
161.9 (152.0–173.4)° vs. 147.9 (range 112.5–174.6)° (X2-Cu-Y3).
The data show that angular distortion from regular trigonal geometry occurs in both groups. The complexes with three-membered rings are somewhat more distorted.

2.3. Five-Membered Metallocycles in Cu(κ2-X1×2)(Y3) and Cu(η2-X1×2)(Y3) Derivatives

Interestingly, N-donor molecules prevail by far, although N-donor is hard. N-donors coordinate to the “soft” copper(I). The bidentate N-donor includes both saturated σ-donors, such as ethylenediamine and its derivatives, and unsaturated π-acceptors, such as phenanthroline and its derivatives, which exhibit different steric demands.

2.3.1. Five-Membered Metallocycles Formed by Unsaturated (κ2-N1N2) Donor Ligands

Mononuclear five-membered metallocycles where copper coordinates a hybrid guanidine amine ligand and a halide ion in a distorted trigonal planar mode have been disclosed [44]. Five-membered metallocycles, in which the bidentate ligands coordinate to copper exclusively through nitrogen atoms, have also been reported [45,46,47]. Each bidentate ligand builds up a five-membered metallocyclic ring with structural data:
-
N1-C2H4-N2, Cl3 [44]: Cu-L (Å): 1.906 (N1), 2.639 (N2), 2.129 (Cl3); 79.1° (N1-Cu-N2), 164.9° (N1-Cu-Cl3), 115.8° (N2-Cu-Cl3);
-
N1-C2H4-N2, X3 (X3 = I, Br, Cl) [44]: Cu-L (Å): 1.937 (N1, Cl3), 1.950 (N1, Br3), 1.982 (N1, I3); 2.311 (N2, Cl3), 2.267 (N2, Br3), 2.233 (N2, I3); 2.145 (Cl3), 2.275 (Br3), 2.459 (I3); 84.5° (N1-Cu-N2, Cl3), 85.2° (N1-Cu-N2, Br3), 85.6° (N1-Cu-N2, I3); 158.9° (N1-Cu-Cl3), 155.8° (N1-Cu-Br3), 143.0° (N1-Cu-I3); 116.3° (N2-Cu-Cl3), 118.7° (N2-Cu-Br3), 130.8° (N2-Cu-I3);
-
N1-C2H4-N2, Cl3 [45]: Cu-L (Å): 1.971 (N1), 2.159 (N2), 2.150 (Cl3); 85.0° (N1-Cu-N2), 150.2° (N1-Cu-Cl3), 123.9° (N2-Cu-Cl3);
-
N1-C2H4-N2, I3 [46]: Cu-L (Å): 1.969 (N1), 2.171 (N2), 2.435 (I3); 86.4° (N1-Cu-N2), 159.1° (N1-Cu-I3), 114.4° (N2-Cu-I3);
-
N1-C2H4-N2, N3 [47]: Cu-L (Å): 1.987 (N1), 2.033 (N2), 1.858 (N3); 83.3° (N1-Cu-N2), 142.6° (N1-Cu-N3), 133.5° (N2-Cu-N3).
In several copper(I) complexes, an unsaturated N-donor molecule forms five-membered metallocyclic rings. Such complexes are: monoclinic [Cu(η2-C38H26N9-N1,N2)(κ1-N3≡CCH3)]PF6 (at 293 K) [48], monoclinic [Cu(η2-C12H8N2-N1,N2)(C4BrF8-κ1-C3)] (at 145 K) [22], monoclinic [Cu(η2-C12H8N2-N1,N2)(C8H5F4O-κ1-C3)] (at 123 K) [20], monoclinic [Cu(η2-C16H14N2-N1,N2)(κ1-Cl3)] (at 153 K) [15], triclinic [Cu(η2-C12H8N2-N1,N2)(ButO)3SiS-κ1-S3] (at 153 K) [49], monoclinic [Cu(η2-C12H8N2-N1,N2)(C2F5-κ1-C3)] (at 123 K) [21], orthorhombic [Cu(η2-C12H8N3-N1,N2)(C15H28N2-κ1-C3)] (at 100 K) [9], orthorhombic [Cu(η2-C26H38N2O-N1,N2)(κ1-N3≡CCH3)]PF6.C4H10O (at 110 K) [12], monoclinic [Cu(η2-C16H22N2-N1,N2)(κ1-N3≡CCH3)]PF6 (at 110 K) [12], monoclinic [Cu(η2-C13H12N2-N1,N2)(κ1-N3≡CCH3)]PF6 (at 110 K) [13], triclinic [Cu(η2-C9H6N2-N1,N2)(κ1-N3≡CCH3)]PF6 (at 120 K) [47], monoclinic [Cu(η2-C14H10Br2N2-N1,N2)(κ2-N3≡CCH3)]BF4 (at 130 K) [45], triclinic [Cu(η2-C28H32N2-N1,N2)(C8H5O3-κ1-O3)] (at 173 K) [50], triclinic [Cu(η2-C25H32N2-N1,N2)(C8H5O3-κ1-O3)]CH2Cl2 (at 173 K) [51], triclinic [Cu(η2-C26H30N2O-N1,N2)(C8H5O3-κ1-O3)] (at 173 K) [51], triclinic [Cu(η2-C10H8N2-N1,N2)(C9H5O9-κ1-O3)] (at 298 K) [16], monoclinic [Cu(κ2-C12H28N2-N1,N2)(C8H4NO5-κ2-O3)] (at 173 K) [52], triclinic [Cu(κ2-C12H28N2-N1,N2)(C8H5O3-κ1-O3)] (at 173 K) [52], monoclinic [Cu(η2-C12H8N2-N1,N2)(P(o-tol)3-κ1-P3)]BF4 (at 123 K) [10], triclinic [Cu(η2-C12H8N2-N1,N2)(P(But)2(Ph2)-κ1-P3]BF4 (at 123 K) [10], triclinic [Cu(η2-C14H17N5O2-N1,N2)(κ1-Br3)] (at 100 K) [53], triclinic [Cu(η2-C14H17BrN4-N1,N2)(κ1-Br3)] (at 100 K) [53], monoclinic [Cu(η2-C11H18N4-N1,N2)(κ1-I3)] (at 120 K) [54], and monoclinic [Cu(η2-C16H28N6-N1,N2)(κ1-I3)] [55]. The structure of [Cu(η2-C12H8N2-N1, N2)(η1-C4BrF8-κ1-C3)] [22] is shown in Figure 4 as an example.
The more common unsaturated five-membered metallocycles are of the N-C=C-N type. The total mean value of Cu-N1,2 bond distance is 2.240 Å, and the N1-Cu-N2 bond angle is 83.0 °. There is a wide variety of monodentate donor ligands. The Cu-Y3 bond distance elongates in the following order: 1.846(13) Å (Y3=N3) < 1.884(21) Å (O3) < 1.928(30) Å (C3) < 2.126(18) Å (Cl3) < 2.140(16) Å (S3) < 2.194(22) Å (P3) < 2.246(12) Å (Br3) < 2.445(14) Å (I3). The total mean values of N1-Cu-Y3 and N2-Cu-Y3 bond angles are 151.7° and 121.9°, respectively. The data show that angular distortion from regular trigonal geometry occurs.

2.3.2. Five-Membered Metallocycles “Interlocked” in Macrocycles

There are copper(I) complexes in which N1, N2 donor molecules are also unsaturated, but the “central” five-membered metallocycles (N1-C=C-N2) are involved in macrocycles. Such complexes are monoclinic [Cu(η2-C30H30N2O4-N1,N2)(κ1-N3≡CCH3)]PF6∙0.5(CH2Cl2)∙0.5 (CHCl3) (at 293 K) [17], monoclinic [Cu(η2-C38H26O3N2-N1, N2)(κ1-I3)]CH2Cl2 (at 150 K) [56], monoclinic [Cu(η2-C42H42O4N2-N1, N2)(κ1-I3)]BF4 CH2Cl2 (at 90 K) [11], triclinic [Cu(η2-C46H34O4N2-N1, N2)(κ1-I3)](CHCl3)2 (at 110 K) [11], triclinic [Cu(η2-C46H34O4N2-N1,N2)(κ1-I3)]CH2Cl2 (at 110 K) [11], and monoclinic [Cu(η2-C27H31O3N3-N1,N2)(C45H58N2O3-κ1-C3)]PF6.CH2Cl2.CH3OH (at 100 K) [57]. The structure of [Cu(η2-C38H26O3N2-N1,N2)(κ1-I3)] [56] is shown in Figure 5 as an example. The total mean values of Cu-L bond distances are 2.055(22) Å (L=N1,2), 2.430(12) Å (I3), and 1.935 Å (C3). The mean values of L-Cu-L bond angles are 81.7(1.6)°(N1-Cu-N2), 136.8(2.4)°(N1-Cu-I3), and 142.6(2.6)°(N2-Cu-I3).
The interlocked macrocycles are of the following types: (N1COC6OC4OC6OCN2) [17], (N1C5OC5OC5OC5N2) [56], (N1C5OC6OC3OC6OC5N2) (N1C5OC6OC3OC6OC5N2) [11], and (N1C5OC6OC5O3OC6OC5N2) [11].
The structure of [Cu(η2-C27H31N3O3-N1N2)(C45H58N2O3-κ1-C3)]PF6 (at 100 K) [57] is unusual. The central five-membered metallocycle ring is built up by a bidentate (N1C6OC2OC2OC6NC2N2) ligand, which is parallel-overlapped by a monodentate (C14C6NC3NC5C14) ligand.
The monoclinic [Cu(η2-C11H24O2-O1, O2)(C30H32N2-κ1-C3)](C7H8) (at 100 K) [18] is the only example in which unsaturated O1, O2 donor ligands build up a five-membered metallocyclic ring of the type O1=C-C-O2. The C3 donor from the monodentate ligand occupied three coordination sites around the Cu(I) atom (O1O2C3). The values of Cu-L bond distances are 2.016 Å (L≡O1), 2.032 Å (O2), and 1.861 Å (C3). The values of L-Cu-L bond angles are 80.3° (O1-Cu-O2), 137.9° (O1-Cu-C3), and 141.8° (O2-Cu-C3).

3. Materials and Methods

In the structural study devoted to Cu(I) complexes, the Cambridge Crystallographic Data Center CSD version 5.45 (CCDC, Cambridge, UK) was used for the analysis of the structures and calculation of distortion values. The program Diamond (Diamond Version 3.2k, serial No: 1.3.2.20007208.2426; Crystal Impact, Bonn, Germany) was used to create the visualization of chemical structures. The complexes were selected for this analytical study according to well-defined structural group characteristics, i.e., Cu(κ2-X1×2)(Y3) or Cu(η2-X1×2)(Y3) composition formula possessing three- to five-membered metallocycles.

4. Conclusions

Structural data for over seventy copper(I) complexes of the composition Cu(η2-X1×2)(Y3) or Cu(κ2-X1×2)(Y3) were analyzed and classified. These complexes crystallized in three crystal classes: orthorhombic, triclinic, and monoclinic. The bidentate ligands X1×2 create a variety of metallocyclic rings, which are three-, four-, and five-membered. The monodentate ligands (Y3) completed the inner coordination sphere around the Cu(I) atoms. The metallocyclic rings are built up by a variety of atoms:
Three-membered: -C1-C2- [24],-B1=B2- [25], -P1≡C2- [26];
Four-membered: -H1-B(H2)-H2- [32],-H1-B(Ph2)-C2- [8], -O1-Al-O2- [14], -O1-Ce-O2- [33], and S1-C-P2- [7];
Five-membered: -N1C2N2- [9,10,11,12,13,15,16,17,18,20,21,22,48,49,51,52,53,55,56].
The total mean values of L-Cu-L angles for (A, three-), (B, four-), and (C, five-)membered metallocycles are summarized in Table 1.
Table 1. The total mean values of L-Cu-L angles for (A, three-), (B, four-), and (C, five-) membered metallocycles.
Table 1. The total mean values of L-Cu-L angles for (A, three-), (B, four-), and (C, five-) membered metallocycles.
A44.2 (37.1–51.2)° (X1-Cu-X2), 155.1 (140.1–173.4)° (X1-Cu-Y3), 161.9 (152–173.4)° (X2-Cu-Y3)
B71.0 (52.3–86.6)° (X1-Cu-X2), 140.4 (122.1–173.1)° (X1-Cu-Y3), 147.9 (112.5–174.6)° (X2-Cu-Y3)
C82.9 (79.1–86.4)° (X1-Cu-X2), 148.0 (140.0–164.9)° (X1-Cu-Y3), 121.9(114.4–139.2)° (X2-Cu-Y3)
The data show that angular distortion from regular trigonal geometry grows in the following order: five-membered < four-membered < three-membered metallocycles.

Author Contributions

Conceptualization, M.M., N.M. and P.M.; methodology, M.M., N.M. and P.M.; writing—original draft preparation, M.M. and P.M.; data curation, M.M. and N.M.; writing—review and editing, N.M., V.M. and P.M.; supervision, M.M. and P.M.; funding acquisition, P.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under the project No. 09I03-03-V04-00157 and the projects VEGA 1/0146/23 and KEGA 041UK-4/2024.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Acknowledgments

This work was supported by the Faculty of Pharmacy, Comenius University Bratislava. Structural data used in this study for discussion and calculations were obtained from the Cambridge Crystallographic Data Center (CCDC) with an institutional license of the Slovak University of Technology in Bratislava.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

C8H10N2(4-dimethyllamino)pyridine
C8H4NO5(4-nitrophenyl)oxoacetate
C8H5O3benzoylformate
C9H5O62,5-dicarboxybenzenecarboxylate
C10H8N24,4′-bipyridine
C12H8N21,10-phenanthroline
C15H28N2bis(di-isopropylamino)cyclophenylidene
C22H35NO(1-(2,6-diisopropylphenyl)-3,3-diethyl-5,5-dimethyl-pyrrolidin-2-ylidene)
C22H35NPcyclic (alkyl)(amino)carbene
C24H37N(2-(2,6-di-isopropylphenyl)-1,5-dimethyl-4-isopropyl-2-azabicyclo [2.2.2]octan-3-ylidene)
C27H36N2(1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene)
C30H32N2(1,3-bis[2,6-bis(propan-2-yl)phenyl]imidazol-2-ylidene)
C6F5 pentafluorophenyl substituent
P(But)2(Ph2)(1,1′-biphenyl(-2-yl)(di-t-butyl)phosphine)
P(o-tol)3[tris(2-methylphenyl)phosphine]
η2-C6H6Benzene
η2-C11H18N4(1,1,3,3-tetramethyl-2-((pyridine-2-yl)methyl)quanidine)
η2-C11H24O2(2,5-di-t-butyl-1-oxido-6-oxocyclohexa-2,4-diene-1-yl)
κ2-C12H28N2(N,N′-di-t-butyl-N,N′-dimethylethane-1,2-diamine)
η2-C12H8N3(2-(2-pyridyl)benzimidazolate)
η2-C13H12N2(1-phenyl-N-((pyridine-2-yl)methyl)methenimine)
η2-C14H10Br2N22,9-bis(bromomethyl)-1,10-phenanthroline
η2-C14H17BrN4N″-(6-bromoquinolin-8-yl)(N,N,N′,N′-tetramethylquanidine)
η2-C14H17N5O2(N,N,N′,N′-tetramethyl-N″-(6-nitroquinolin-8-yl)quinidine)
κ2-C16AlF36O4bis(1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-olato)-nitrosyliminoaluminium
η2-C16H14N2(2,7-dimethyl-1,10-phenanthrolidine)
η2-C16H22N2(1,7,7-trimethyl-N-((pyridine-2-yl)methyl)bicyclo [2.2.1]heptan-2-imine)
η2-C25H32N2(N-(1-(6-phenyl-2-pyridyl)ethylidene)-N-(2,6-diisopropylphenyl)amini)
η2-C26H30N2O(N-(1-(6-(3-methoxyphenyl)-2-pyridyl)ethylilidene)-N-(2,6-diisopropylphenyl)amine)
η2-C26H38N2O(17-((4-methylpyridin-2yl)methyl)iminoandrostane-3-ol)
η2C26H38N2O(1H-(((4-methylpyridin-2-yl)methyl)imino)androstan-3-ol)
η2-C27H31O3N3(1,3-bis(3′,5′-di-t-butyl [1,1′-biphenyl])-4-yl,imidazole-2-ylidene)
η2-C28H32N2(N-(1-(6-2,6-dimethylphenyl)-2-pyridyl)ethylidene)-N-(2,6-diisopropylphenyl)amine
η2-C30H30N2O4(4,15,22,27-tetraoxa-35,36-diazapentacyclo [26.2.2.210,6.21.15.9110,14]hexatriaconta-1(30)6.8.10(35),11.13.18.20.28.31.33-dodecaene)
η2-C34H36B21,2-bis(anthracen-9-yl)diborene
κ2-C36H81Ce2O9tris(μ-t-butoxo-hexakis(t-butoxy)di-cerium)
η2-C38H26O3N2[6,12,18-trioxa-32,35-diazaoctacylo [21.8.4.22,5,28,11,213,16,219,22,O26,34,O29,33]] tritetraconta-1(32),2,4.8,10,13,15,19,21,23(35)24,26,28,30,33,36,38,40,42–nonadecaene
η2-C42H42O4N2(6,13,19,26-tetraoxa-40.43—diazaheptacyclo [29.8.4.22,5.217,30114,18,O34,42O37,41] octatetraconta-1(39),2,4,14(46),15,17,27,29,31,33,35,37,40,42,44,47-hexadecaene)
κ1-C45H58N2O3(3,11-dimethyl-18,21,24-trioxa-3,11,33-triazatetra-cyclo [24.2.2.213,16,15,9]tritriaconta-1(28),5(33),6,8,13,15,26,29,31-nonaene)
κ2-HB(H2)Htetrahydridoborate
κ2-B(ph)2C6H6hydrido(triphenyl)borate
η2-P≡C-OPhosphaethynolate C6F5 pentafluorophenyl substituent
κ2-S1C(=S)P2(Ph2) (diphenylphosphanyl)carbonodithioate

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Inorganics 14 00142 g001
Figure 1. Structure of [Cu(η2-C6H6-C1C2)(C27H37N2-κ1-C3)] [24].
Figure 1. Structure of [Cu(η2-C6H6-C1C2)(C27H37N2-κ1-C3)] [24].
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Inorganics 14 00142 g002
Figure 2. Structure of [Cu(C49H44N2-κ2-B1B2)(OTf-κ1O)], where X = OTf (91% yield) [27].
Figure 2. Structure of [Cu(C49H44N2-κ2-B1B2)(OTf-κ1O)], where X = OTf (91% yield) [27].
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Inorganics 14 00142 g003
Figure 3. Structure of [Cu(C16F36AlO4-κ2-O1O2)(N2O-κ1-N3)] [14].
Figure 3. Structure of [Cu(C16F36AlO4-κ2-O1O2)(N2O-κ1-N3)] [14].
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Inorganics 14 00142 g004
Figure 4. Structure of [Cu(η2-C12H8N2-N1,N2)(C4BrF8-κ1-C3)] [22].
Figure 4. Structure of [Cu(η2-C12H8N2-N1,N2)(C4BrF8-κ1-C3)] [22].
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Inorganics 14 00142 g005
Figure 5. Structure of [Cu(η2-C38H26O3N2-N1,N2)(κ1-I3)] [56].
Figure 5. Structure of [Cu(η2-C38H26O3N2-N1,N2)(κ1-I3)] [56].
Inorganics 14 00142 g005
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Melník, M.; Miklášová, N.; Mikušová, V.; Mikuš, P. Structural Aspects of Cu(I)(κ2-X1,X2)(Y3) and Cu(I)(η2-X1,X2)(Y3) Complexes. Inorganics 2026, 14, 142. https://doi.org/10.3390/inorganics14050142

AMA Style

Melník M, Miklášová N, Mikušová V, Mikuš P. Structural Aspects of Cu(I)(κ2-X1,X2)(Y3) and Cu(I)(η2-X1,X2)(Y3) Complexes. Inorganics. 2026; 14(5):142. https://doi.org/10.3390/inorganics14050142

Chicago/Turabian Style

Melník, Milan, Natalia Miklášová, Veronika Mikušová, and Peter Mikuš. 2026. "Structural Aspects of Cu(I)(κ2-X1,X2)(Y3) and Cu(I)(η2-X1,X2)(Y3) Complexes" Inorganics 14, no. 5: 142. https://doi.org/10.3390/inorganics14050142

APA Style

Melník, M., Miklášová, N., Mikušová, V., & Mikuš, P. (2026). Structural Aspects of Cu(I)(κ2-X1,X2)(Y3) and Cu(I)(η2-X1,X2)(Y3) Complexes. Inorganics, 14(5), 142. https://doi.org/10.3390/inorganics14050142

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