Multiple Bonds Between Metal Atoms / 15-Extended Metal Atom Chains

	Compound	Space	Ni···Ni, Å	µeff, µB	ref.
		Group
	[Ni3(dpa)4(N3)]PF6·3CH2Cl2	P21/n	2.389(2), 2.385(2)	3.2	53
	Ni3(dpa)4[Ag(CN)2]2·Me2CO	C2/c	2.4030(7)	NR	55
	[Ni3(dpa)4F2][Ni3(dpa)4(H2O)2](BF4)2·2MeOH	–
		P1	2.3888(7), 2.3917(7),	NR	56
			2.3924(7), 2.3896(7)
	[Ni3(dpa)4(C4O4Me)]BF4·Et2O	C2/c	2.400(1), 2.403(1)	NR	53
	Ni3(dpa)4(4-PyCO2)2	P21/n	2.4176(4), 2.4297(5)	NR	48
	Ni3(dpa)4(3-PyCO2)2	P21/n	2.4214(5), 2.4136(5)	NR	48
	[Ni3(dpa)4(4-PyCO2)2][ZnTPP]2	–
		P1	2.4212(6), 2.4067(6)	NR	48
	{[Ni3(dpa)4(4-PyCO2)2][MnTPP]}n(ClO4)n	C2/c	2.4088(4)	4.97	48
	{[Ni3(dpa)4(3-PyCO2)2][MnTPP]}n(ClO4)n	P21/c	2.4156(5), 2.4206(5)	5.28	48
	[Ni3(dpa)4(NCMe)2](PF6)2·3.14CH3CN	–
		P1	2.376(2), 2.371(2)	2.37	48
	Ni3(dpa)4(CN)2·CH2Cl2	Pnn2	2.4523(3)	2.68	54
	Ni3(dpa)4(NCS)2·CH2Cl2	Fddd	2.4285(9)	NR	67
	Ni3(dpa)4(NCNCN)2·2.5CH2Cl2	–
		P1	2.4044(8), 2.4082(8)	2.53	54
	Ni3(dpa)4(CCPh)2·0.3CH3OH	C2	2.477(1), 2.474(1), 2.4861(7), 2.4467(8)b	2.83	54
	Ni3(depa)4Cl2·0.5hexane	–
		P4n2	2.4325(3)	2.68	50
	[Ni3(depa)4(NCMe)2](PF6)2·0.33H2O	–
		Pn3n	2.415(1)	2.53	50
	[Ni3(PhPyF)4(NCMe)2](BF4)2	I41/a	2.469(5)	NR	51
	[Ni3(PhPyF)4Cl]Cl	P4/ncc	2.443(3), 2.454(3)	3.08	51
	Ni3(PhPyF)4Cl2	P43212	2.508(1), 2.503(1)	NR	51
	(NBu4)2[Ni3(BPAP)4]·2THF	C2/c	2.368(1)	diamagnetic	21

Berry

Chains Atom Metal Extended

679

Ni37+ Compounds

	Compound		Space	Ni–Ni, Å	µeff, µB	ref.
			Group
	[Ni3(dpa)4(PF6)2]PF6·5CH2Cl2		P2/n	2.2851(6), 2.2885(7)	2.0	15,44
	[Ni3(depa)4(PF6)2]PF6·3CH2Cl2		P21/n	2.296(1), 2.289(1)	1.79	50
				Cu36+ Compounds
	Compound	T,K	Space	Cu···Cu, Å	µeff, µB	ref.
			Group
	Cu3(dpa)4Cl2		Pnn2	2.4712(4)	NR	46
	Cu3(dpa)4Cl2·H2O		Pnn2	2.471(1)	2.4	45
	Cu3(dpa)4Cl2·CH2Cl2	298	Pnn2	2.492(2)	2.02	49
		160	Pnn2	2.4688(9)	NR	49
	Cu3(dpa)4Cl2·toluene		Pca21	2.4710(9), 2.4688(9)	NR	49
	Cu3(dpa)4Cl2·Et2O		P21/c	2.4672(8), 2.4735(8)	NR	49
	Cu3(dpa)4(BF4)2		P21/c	2.4035(8), 2.4029(8)	2.1	49
				Cu37+ Compounds
	Compound		Space	Cu···Cu, Å	µeff, µB	ref.
			Group
	[Cu3(dpa)4Cl2]SbCl6·2.86C2H4Cl2·0.792C6H12		I4/m	2.510(1), 2.516(1)	2.83	15
	[Cu3(dpa)4Cl2]SbCl6·2.44Me2CO		P21/c	2.506(1), 2.505(1)	NR	15

	680
15 Chapter		Bonds Multiple
		Atoms Metal Between

e f g h

Ru36+ Compounds

	Compound	Space	Ru–Ru, Å	µeff, µB	ref.
		Group
	Ru3(dpa)4Cl2·CH2Cl2	Pnn2	2.596(2)	diamagnetic	5
		Rh36+ Compounds
	Compound	Space	Rh–Rh, Å	µeff, µB	ref.
		Group
	Rh3(dpa)4Cl2·CH2Cl2	Pnn2	2.586(1)	1.9	5
a	S = symmetrical, U = unsymmetrical, sl. U = slightly unsymmetrical.
b Asymmetric unit contains two molecules.
c	These structures were reported (ref.11) to contain symmetrical molecules, but have been
	reinvestigated (ref. 26) and found to contain unsymmetrical molecules.
d The symmetrical arrangement in this structure is believed to be an artifact due to

pseudomerohedral twinning of the crystals (see ref. 26). Disorder in the positions of the Cr atoms leads to high esd’s.

Recently this has been shown to have the formula Cr3(dpa)– 4(CCPh)1.8Cl0.2; see ref. 25. This structure has been reinterpreted in space group I4, see ref. 31.

Asymmetric unit contains one and a half molecules.

Berry

Chains Atom Metal Extended

681

682Multiple Bonds Between Metal Atoms Chapter 15

15.9

A different model is based on results of DFT calculations.14 In this, the β and / orbital interactions are both considered to be negligible, leading to nine degenerate orbitals, three localized on each Cr atom (see 15.9b). By filling these orbitals and maximizing the spin multiplicity, the result is ten unpaired electrons. Since nine of these are localized on the individual Cr atoms in sets of three, they couple antiferromagnetically with each other. They also couple ferromagnetically (because of orthogonality) with the remaining electron in the μ nonbonding orbital to yield an S = 2 ground state and a net 3c3e sigma bond. The S = 5 excited state is calculated to lie 30.8 kcal mol-1 (> 10,000 cm-1) above the ground state, precluding any evidence of its population in the variable temperature magnetic susceptibility data.

Although no potential energy minimum for an unsymmetrical Cr3 chain is found, further calculations on Cr3(dpa)4Cl2 showed that a very unsymmetrical geometry exists in a quintet excited state at +10 kcal mol-1 vs that of the ground state.28 Though this observation does not justify an extremely unsymmetrical Cr3(dpa)4Cl2, it is proposed that exchanging the axial Cl ligands with an unsymmetrical set of ligands (e.g., Cl and PF6) could stabilize the unsymmetrical excited state and cause it to be favored.28

The Cr compounds of higher nuclearity, Crn2n+ and Crn(2n+1)+ with n 4, are listed in Table 15.2. These are synthesized similarly to the trinuclear complexes,12 but Cr5(tpda)4Cl2 and Cr7(teptra)4Cl2 have also been synthesized from CrCl2, H2tpda or H3teptra, and KOtBu in molten naphthalene.17 In the tetranuclear ion [Cr4(DPyF)4Cl2]2+, the Cr atoms pair up to form two isolated Cr24+ quadruply bonded units (av. Cr–Cr = 2.01 Å) with a distance of 2.73 Å separating them (see Fig. 15.3).22 The pentachromium complexes of the tpda2− ligand are the most studied, but the results are still controversial.12,29,30 Both localized and delocalized models for the structure of Cr5(tpda)4Cl2 have been proposed as shown schematically in 15.10. Crystallographic disorder in the positions of the metal atoms is an important issue in deciding whether the model of 15.10a (delocalized) or 15.10b (with alternating Cr–Cr quadruple bonds) is more applicable.30 The compounds Cr5(tpda)4(NCS)229 and heptanuclear Cr7(teptra)4Cl217 (shown in Fig. 15.4) have been reported as being consistent with model 15.10a, though the elongated thermal ellipsoids for the Cr atoms in the crystal structures suggest that 15.10b is probably a better description.30

Extended Metal Atom Chains

683

Berry

Cl

Cr

Cr

Cr

Cr

Cr

Cl

a

Cl

Cr

Cr

Cr

Cr

Cr

Cl

b

15.10

Fig. 15.3. Structure of the dication [Cr4(DPyF)4Cl2]2+.

Fig. 15.4. Structure of Cr7(teptra)4Cl2.

In the pentachromium complexes, interpretation of the magnetic data has also been de-

bated. For Cr5(tpda)4Cl2, the observed µeff of 4.0-4.2 µB has been interpreted as indicative of either two29 or four12,30 unpaired electrons, though it is not very close to either of the spin-only

values (2.83 and 4.90 µB, respectively) expected for these situations. A thorough and conclusive magnetic study of these pentachromium complexes has not been reported.

For the oxidized Cr5(tpda)43+ compounds [Cr5(tpda)4F2]BF4 and [Cr5(tpda)4F(OTf)]OTf,29 the structural results clearly indicate that model 15.10b is applicable, with the isolated Cr atom being the one oxidized to Cr(III) and responsible for the magnetic moment of 4.0 µB corresponding to three unpaired electrons.

Table 15.2. Structural data for EMACs having more than three metal atoms

Crn2n+ Compounds

	Compound	Cr Cr, Å	Cr···Cr, Å	µeff , µBb	ref.
	[Cr4(DPyF)4Cl2]Cl2·5Me2CO	1.9832(8)	2.709(1)	diamagnetic	22
	[Cr4(DPyF)4Cl2]Cl2·4MeOH	2.013(2), 2.001(2)	2.726(2)	diamagnetic	22
	Cr5(tpda)4Cl2·CH2Cl2	1.901(6), 2.031(6)	2.578(7), 2.587(6)	4.2	12
	Cr5(tpda)4Cl2·2Et2O·4CHCl3a	1.872(2), 1.963(3)	2.598(3), 2.609(2)	NR	30
	Cr5(tpda)4Cl2·Et2O	1.862(3), 1.931(3)	2.661(3), 2.644(3)	NR	30
		Outer Cr–Cr	Inner Cr–Cr
	Cr5(tpda)4Cl2·2Et2O·4CHCl3a	2.284(1), 2.284(1)	2.2405(8), 2.2405(8)	4.0	29
	Cr5(tpda)4(NCS)2	2.285(2), 2.285(2)	2.246(1), 2.246(1)	NR	29
	Cr7(teptra)4Cl2·6THF	2.291(2), 2.280(2)	2.243(2), 2.211(2),	NR	17
			2.215(2), 2.243(2)
		Crn(2n+1)+ Compounds
	Compound	Cr Cr	Cr···Cr	µeff , µBb	ref.
	[Cr5(tpda)4F2]BF4·1.5CH3CN·2H2O·THF	1.969(2), 2.138(2)	2.487(2), 2.419(2)	4.0	29
	[Cr5(tpda)4F(OTf)]OTf·2CHCl3	1.846(1), 1.922(1)	2.610(1), 2.596(1)	NR	29
		Co510+ Compounds
	Compound	Outer Co–Co	Inner Co–Co	µeff , µBb	ref.
	Co5(tpda)4(NCS)2·CH2Cl2·0.5Et2O·0.5H2O	2.276(2), 2.271(2)	2.232(2), 2.232(2)	1.90	7,43
	Co5(tpda)4Cl2·2CHCl3·Et2O	2.282(1)	2.235(1)	NR	43
	Co5(tpda)4(N3)2·2CH2Cl2·1/3H2O	2.258(1), 2.264(1)	2.223(1), 2.221(1)	NR	43
	Co5(tpda)4(CN)2·3CH2Cl2·Et2O	2.279(1), 2.286(1)	2.227(1), 2.231(1)	NR	43
	Co5(tpda)4(OTf)2·2CH2Cl2	2.253(1)	2.225(1)	NR	43

	684
15 Chapter		Bonds Multiple
		Atoms Metal Between

Co511+ Compounds

Compound	Outer Co–Co	Inner Co–Co	µeff, µBb	ref.
[Co5(tpda)4(NCS)2]ClO4	2.292(1), 2.276(1)	2.238(1), 2.243(1)	2.93	43
[Co5(tpda)4Cl2]ClO4·3CH2Cl2	2.300(2), 2.285(2)	2.246(2), 2.244(2)	3.18	43
[Co5(tpda)4(OTf)2]OTf	2.282(1), 2.290(1)	2.253(1), 2.241(1)	2.86	43
	Nin2n+ Compounds
Compound	Outer Ni···Ni	Inner Ni···Ni	µeff, µBb	ref.
Ni4(phdpda)4·C5H12	2.3269(6), 2.3280(6)	2.3010(6)	diamagnetic	8
Ni5(tpda)4Cl2·4CH2Cl2	2.385(2)	2.305(1)	4.0	7,68
Ni5(tpda)4(CN)2·CH2Cl2	2.400(3)	2.296(2)	3.7	68
Ni5(tpda)4(N3)2	2.379(2)	2.298(2)	3.85	68
Ni5(tpda)4(NCS)2·4CH2Cl2	2.367(2), 2.371(2)	2.298(2), 2.294(2)	3.94	68
[Ni5(tpda)4(NCMe)2](PF6)2·4MeCN	2.346(3)	2.291(2)	3.81	68
Ni5(tpda)4(NCFe(dppe)Cp)2	2.384(1)	2.306(1)	NR	38
Ni5(etpda)4Cl2·6CHCl3	2.389(2), 2.383(2)	2.304(2), 2.304(2)	4.3	65
Ni7(teptra)4Cl2·3CHCl3	2.383(1), 2.374(2)	2.310(1), 2.225(2), 2.215(2), 2.304(1)	4.0	8,69
Ni7(teptra)4(NCS)2·4CHCl3	2.375(2), 2.354(2)	2.300(2), 2.194(2), 2.206(2), 2.303(2)	NR	69
Ni9(peptea)4Cl2·10C2H4Cl2	2.391, 2.380, 2.375c	2.297, 2.253, 2.237, 2.243, 2.255, 2.286,	4.0	9
		2.296, 2.263, 2.247c
	Nin(2n+1)+ Compounds
Compound	Outer Ni–Ni	Inner Ni–Ni	µeff, µBb	ref.
[Ni5(tpda)4(H2O)(BF4)](BF4)2·4CH2Cl2	2.337(1), 2.300(1)	2.261(1), 2.245(1)	2.25	66
[Ni5(tpda)4(OTf)2]OTf·CH2Cl2·3.5H2O	2.358(2), 2.304(1)	2.276(2), 2.245(2)	2.75	66
[Ni5(etpda)4](PF6)3·4Me2CO	2.289(2), 2.292(2)	2.233(2), 2.235(2)	2.0	65

a This structure was determined twice and refined using two different models. b NR = Not reported.

c No values are listed in ref. 9. Ni···Ni distances are reported from the Cambridge database without esd’s.

Berry		Chains Atom Metal Extended
	685

686Multiple Bonds Between Metal Atoms Chapter 15

The model 15.10a has been used to calculate the band structure of a hypothetical Cr chain.17 The results indicate that Crn2n+ wires of the type 15.10a should be one-dimensional metallic conductors.

15.3 EMACs of Cobalt

Because the first synthesis of the parent tricobalt complex Co3(dpa)4Cl2 reported very low yields,4 its chemistry was not studied in detail until a straightforward, high yield synthesis was devised. Reaction of anhydrous CoCl2 with Lidpa in refluxing THF gives black microcrystalline Co3(dpa)4Cl2 in > 40% yield.31

3CoCl2 + 4Lidpa Α Co3(dpa)4Cl2 + 4LiCl

The chemistry of this complex is summarized in 15.11, and involves substitutions and oxidations. The chloride ions are easily exchanged for tetrafluoroborate or hexafluorophosphate anions by metathesis with the corresponding silver reagents.32,33 Replacement of Cl by Br, however, is slow, and complete reaction takes ~5 days using a 50 fold excess of NBu4Br.34 Co3(dpa)4(BF4)2 was found to react more quickly than the chloride precursor with pseudohalogens to form Co3(dpa)4X2 compounds with X = CN, NCS, and NCNCN.35

[Co3(dpa)4(NCMe)2](PF6)2

						Co3(dpa)4Br2
		2AgPF6
	Co3(dpa)4ClBF4	MeCN
						xs. NBu4Br
							[Co3(dpa)4Cl2]BF4
	AgBF4
		Co	(dpa)		Cl	2	NOBF4
		3	4
		2AgBF4
		Co3(dpa)4(BF4)2
	2NaCN						2KSCN
				2NaNCNCN
	Co3(dpa)4(CN)2						Co3(dpa)4(NCS)2
		Co3(dpa)4(NCNCN)2

15.11

Tricobalt complexes with axial Cl and CN ligands have also been reported for the ethyl substituted depa ligand (15.12).36

N N N

H

Hdepa

15.12

Extended Metal Atom Chains 687

Berry

The cyclic voltammogram of Co3(dpa)4Cl2 shows two reversible one-electron oxidation waves at E1/2 of 0.32 V and 1.24 V vs ferrocene. The oxidant NOBF4 was used to convert Co3(dpa)4Cl2 to the corresponding one-electron oxidized cation.37 Oxidation of Co3(depa)4Cl2 and Co3(depa)4(CN)2 are more easily achieved, due to the increased basicity of the depa ligand.36 Tricobalt chains with Fe, Ru or Cr complexes attached through axial cyanide linkages have been reported and electrochemical studies have shown that the axially coordinated metal ions are oxidized at essentially the same potential.38

Like the trichromium compounds, tricobalt compounds can exist with either equivalent or very different Co–Co distances.4,31,32,39 The solvates of Co3(dpa)4Cl2 with symmetrical and unsymmetrical structures have been viewed as examples of bond-stretch isomerism,31,40 although this claim has been debated.13,41 Because of this unusual situation, a wealth of crystallographic information has been obtained for this compound alone. As shown in Table 15.1, the structure has been determined with various interstitial solvent molecules, and at various temperatures.10,40

The dichloromethane solvates are unique in that the orthorhombic form (Co3(dpa)4Cl2·CH2Cl2,

–

Pnn2) and the tetragonal form (Co3(dpa)4Cl2·2CH2Cl2, I4) have symmetrical (D4) and unsymmetrical (C4) molecular structures, respectively, though they crystallize simultaneously from the same solution.10,39 The crystal habits of these solvates are sufficiently differentiable that they can be separated by hand under a microscope (see Fig. 15.5).

Fig. 15.5. STM images (above) and face-indexed drawings (below) of

Co3(dpa)4Cl2·CH2Cl2 (left) and Co3(dpa)4Cl2·2CH2Cl2 (right).

The molecular structure of Co3(dpa)4Cl2 in these crystals is temperature dependent, as shown in Table 15.1. Between 168 K and 109 K, the symmetric structure undergoes a phase transition from the orthorhombic Pnn2 form to a monoclinic Pn form. This breaks the crystallographically imposed equivalence of the Co–Co distances, though the compound remains symmetrical within experimental error (Co–Co distances of 2.3224(8) and 2.3214(8) Å at 109 K). For the unsymmetri-

688Multiple Bonds Between Metal Atoms Chapter 15

–

cal tetragonal I4 form, the Co–Co distances become more symmetrical as the temperature is lowered, reaching 2.3035(7) and 2.3847(8) Å at 20 K (as compared to 2.299(1) and 2.47(1) Å at room temperature).10

The other solvates of Co3(dpa)4Cl2 also have temperature dependent structures.40 Though most of these contain symmetrical molecules for which d(Co–Co) increases with temperature, there are a few that do not. For example, Co3(dpa)4Cl2·cyclohexane, like Co3(dpa)4Cl2·CH2Cl2, undergoes a phase transition between 213 K and 120 K, causing loss of the crystallographic equivalence of the Co–Co distances. The molecule, however, remains symmetrical at 120 K with Co–Co distances of 2.3127(5) and 2.3253(5) Å. Co3(dpa)4Cl2·THF crystallizes in the orthorhombic Pccn space group, and contains a slightly unsymmetrical molecule at 295 K, which becomes less symmetrical at lower temperature, contrary to the behaviour of tetragonal Co3(dpa)4Cl2·2CH2Cl2.

The most complex crystal form is Co3(dpa)4Cl2·1.75toluene·0.5hexane. Though this compound

–

crystallizes in the triclinic space group P1, the asymmetric unit contains two independent molecules which are both unsymmetrical at room temperature. The crystal structures determined at lower temperatures show increasing similarity of the Co–Co distances. At 90 K, one molecule is completely symmetrical (Co–Co = 2.3139(6) and 2.3196(6) Å) while the other is still slightly unsymmetrical (Co–Co = 2.3098(6) and 2.3660(6) Å).

Similar results were found for Co3(dpa)4Br2,34 but only one dichloromethane solvate was observed with equal Co–Co distances of 2.3234(6) Å at 240 K. The crystals of Co3(dpa)4Br2· 1.75toluene·0.5hexane show complex behavior similar to that of the chloride analog.

The molecular structure of Co3(dpa)4Cl2 was also characterized in solution by 1H and 13C NMR spectroscopy, despite the paramagnetism of the compound.42 In the 1H NMR spectrum, only four signals are detected and assigned to the pyridyl hydrogen atoms. The five signals of the 13C NMR spectrum are due to the pyridyl carbon atoms. The assignments are consistent with D4 molecular symmetry in solution. This could either be because the molecule actually is symmetrical in solution, or it could be that the molecule is unsymmetrical and that the central Co atom shifts positions more quickly than the timescale of the NMR experiment. Nevertheless, it should be noted that solutions made by dissolving crystals of either symmetrical or unsymmetrical Co3(dpa)4Cl2 result in the same NMR spectrum.

The only other unsymmetrical tricobalt compound known is Co3(dpa)4Cl(BF4), which has two different axial ligands.32 The tricobalt complexes with axial cyanide,35 dicyanamide,35 thiocyanate,35 and acetonitrile33 are all symmetrical with Co–Co distances ranging from 2.30 to 2.34 Å. Co3(dpa)4(BF4)2 (shown in Fig. 15.6) has the shortest Co–Co distances of the known symmetrical molecules (2.25 Å).32

Fig. 15.6. Structure of Co3(dpa)4(BF4)2.