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

Extended Metal Atom Chains 689

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The temperature dependence of the Co–Co distances in these compounds is mirrored to a degree by the temperature dependence of their magnetic moments. Co3(dpa)4Cl2 has been shown to exist in an equilibrium between low spin (S = ½) and high spin (S = 3/2 or S = 5/2) states.4,10,40,42 At low temperatures, only the S = ½ state is populated, but as the temperature is increased, population of the high spin state occurs which leads to a temperature dependence of µeff as shown in Fig. 15.7. This spin crossover phenomenon has been studied in other tricobalt complexes also.33,34,35 For Co3(dpa)4Cl2, the magnetic data at high temperatures show incomplete population of the high spin state and therefore it is not possible to determine whether this state is S = 3/2 or S = 5/2. For the ethyl-substituted Co3(depa)4Cl2, however, population of the high spin state is complete at 400 K and this state clearly has S = 3/2.36

Fig. 15.7. Plot of µeff vs T for s-Co3(dpa)4Cl2.

A qualitative model of the symmetrical Co3(dpa)4Cl2 molecular orbitals accounts for the spin equilibrium.36 By filling the 21 cobalt based electrons in the MO scheme described earlier for chromium, the result is that all the / and β orbitals are filled, and a 3c3e μ bond exists in the compound as shown in 15.13a. The spin crossover process to achieve an S = 3/2 state therefore involves removing an electron from the /* orbitals and placing it in the μ* orbital as in 15.13b (resulting in longer

Co–Co distances in the high spin state). This accounts for the lengthening of the Co–Co bonds with increasing temperature because population of the high spin state implies population of the μ* orbital, and the reason that the Co–Co bond distances in Co3(depa)4Cl2 (2.3787(7) Å) are longer than those of Co3(dpa)4Cl2 (2.3369(4) Å) is because in the former, the high spin state is nearly 90% occupied at room temperature, whereas in the latter it is only ~50 % occupied.

Density functional calculations on Co3(dpa)4Cl2 are consistent with this scheme and support the view of a three-center three-electron bond in the symmetrical complex, very similar to the DFT results for Cr3(dpa)4Cl2.13,14 The calculations have shown that the potential energy surface of symmetrical Co3(dpa)4Cl2 (which is the only observed potential energy minimum) is very broad and that distortions to C4 symmetry cause changes in energy of only 1 to 4 kcal mol-1 vs the symmetrical ground state.13b No potential energy minimum could be found, however, for an unsymmetrical complex.13 Scheme 15.14 summarizes the DFT results in which the ground 2A2 state undergoes two different types of distortions. If the Co–Co distances lengthen in a symmetrical manner (from the middle towards the right in 15.14), spin crossover to the symmetrical 4B state occurs while as ¨d(Co–Co) becomes larger (i.e. the compound becomes more unsymmetrical), spin crossover to the 4A state can be achieved. The molecular geometry

690Multiple Bonds Between Metal Atoms Chapter 15

of Co3(dpa)4Cl2 in the 4A excited state was calculated and found to be very similar to that observed in the unsymmetrical compound. It is postulated that population of this state at low temperatures gives rise to the unsymmetrical “isomer” of Co3(dpa)4Cl2, despite the large energy difference of 18 kcal mol-1 vs the ground state. It should be mentioned that no transitions to spin sextet states were postulated in this study.

15.13

15.14

Extended Metal Atom Chains 691

Berry

The DFT calculations also explain the behavior of the one-electron oxidized [Co3(dpa)4Cl2]BF4. As shown in Table 15.1, the Co–Co distances (2.32 Å) in this cation are not only equivalent, but similar to those of the neutral species.37 The major structural difference between Co3(dpa)4Cl2 and [Co3(dpa)4Cl2]+ is that the Co–Cl bond lengths are 0.15 Å shorter in the latter.37 The lack of change in the Co–Co distances and the major change in the Co–Cl distances is consistent with the DFT calculation indicating that the SOMO of Co3(dpa)4Cl2 (containing the electron which is removed upon oxidation) has Co–Co nonbonding character and Co–Cl antibonding character.13 Moreover, the oxidized [Co3(dpa)4Cl2]+ cation undergoes two stepwise, thermal, spin crossover transitions (evidenced by magnetic susceptibility measurements in the solid state and in solution) from the S = 0 ground state to an intermediate S = 1 state, and then an S = 2 state.37 The partial MO diagram in 15.15 accounts for this.37

15.15

In addition, the polypyridylamido EMACs are helical and therefore chiral. They exist as Ρ and ¨ enantiomers as shown in 15.16.

15.16

As seen in Table 15.1, often these compounds crystallize in noncentrosymmetric space groups. The compound [Co3(dpa)4(NCMe)2](PF6)2 has been found in the centrosymmetric

–

group P1 and also in the noncentrosymmetric and chiral group P21.33 The monoclinic P21 crystals were examined and the absolute configuration of several of these were determined crystallographically. Crystals containing only the Ρ or ¨isomers were separated this way, and circular dichroism spectra were obtained for solutions of the Ρ and ¨ enantiomers.33 These spectra, shown in Fig. 15.8, are related by mirror symmetry, as expected for an enantiomeric pair. This experiment also shows that the pure enantiomers do not racemize in solution, because conversion from the Ρ to the ¨ isomer would involve the difficult process of interchanging the constrained pyridyl hydrogen atoms shown in 15.3.

692Multiple Bonds Between Metal Atoms Chapter 15

Fig. 15.8. CD spectra of ¨- (solid line) and Ρ- (dashed line) [Co3(dpa)4(NCCH3)2](PF6)2.

The complex structural behavior of the tricobalt complexes is not seen to any degree in the pentacobalt complexes with the tpda ligand. These are prepared in useful yield from CoCl2, H2tpda and KOBut in molten naphthalene in an Erlenmeyer flask.7,43 This chemistry is summarized in 15.17.

2KOtBu

+

H2N	N	NH2	N	Cl	thf	N	N	N	N	N
							H		H

CoCl2, KOtBu, nBuOH, naphthalene

	Co5(tpda)4(OTf)2						NaN3
				NaSCN
	TlOTf			Co5(tpda)4(NCS)2	AgClO4
		Co5(tpda)4Cl2				Co5(tpda)4(N3)2
					NaN3
	NaCN		elec.	elec.	AgOTf	[Co5(tpda)4(OTf)2]OTf
			NBu4ClO4
				NBu4ClO4

Co5(tpda)4(CN)2 [Co5(tpda)4Cl2]ClO4 [Co5(tpda)4(NCS)2]ClO4

15.17

Upon treatment of Co5(tpda)4Cl2 or Co5(tpda)4(NCS)2 with AgOTf, oxidation occurs yielding [Co5(tpda)4(OTf)2]OTf, while use of the non-oxidizing TlOTf yields Co5(tpda)4(OTf)2. The one-electron oxidized [Co5(tpda)4Cl2]+ and [Co5(tpda)4(NCS)2]+ cations are readily obtained by bulk electrolysis of a solution containing the neutral Co510+ compound and NBu4ClO4 at

Eappl. = +0.55 V vs Ag/AgCl.43 The reaction is monitered by UV-Vis spectroscopy, and the products are obtained by recrystallization when the reaction is complete.

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In contrast to the tricobalt complexes, all Co5(tpda)42+/3+ compounds have symmetrical chains (i.e. the inner two Co–Co distances are indestinguishable within experimental error, as are the outer two distances). The outer Co–Co distances are typically ~ 0.05 Å longer than the inner ones due to interactions of the outer Co atoms with the axial ligands. All of the Co–Co distances are short (2.22 to 2.30 Å); therefore five-center Co–Co bonds are proposed.

The Co5(tpda)42+ complexes are paramagnetic with one unpaired electron (µeff of 1.90 µB) and the Co5(tpda)43+ complexes contain two unpaired electrons (µeff = 2.86 - 3.18 µB). These observations were rationalized by the MO scheme presented by Peng43 shown in 15.18 filled with the 35 d electrons from the five cobalt atoms. Therefore, the unpaired electron of the Co5(tpda)42+ complexes is believed to occupy the μ3 nonbonding orbital. Since the oxidized species have triplet ground states, upon oxidation the electron therefore is said to be removed from the β*5 orbital.

15.18

694Multiple Bonds Between Metal Atoms Chapter 15

15.4 EMACs of Nickel and Copper

Polynickel(II) complexes have no nickel-nickel bonds, but are included in this book for two reasons: EMACs of nickel provide examples of the longest discrete metal chains,9 and more importantly, oxidation of Nin2n+ EMACs to Nin(2n+1)+ involves the formation of delocalized Ni–Ni bonds.44 Tricopper complexes also have no Cu–Cu bonds, but are included for the following reasons: (1) Dipyridylamido tricopper complexes were the first EMACs to be recognized as such and structurally characterized.45,46 (2) Oxidation of Cu3(dpa)4Cl2 provides a remarkable contrast to the oxidation of Ni3(dpa)4Cl2.15 (3) Tricopper complexes are known for ligands other than those already described in this chapter which may be useful in the future for synthesizing metal-metal bonded EMACs of other metals.

The earliest known polypyridylamido EMAC, Ni3(dpa)4Cl2, was synthesized in 1968 by high temperature reaction of NiCl2(Hdpa)2 (an octahedral, mononuclear Ni(II) complex with two cis chelating Hdpa ligands) with KOBun in molten naphthalene.2 The product was characterized by elemental analysis, room temperature magnetic susceptibility measurements, a molecular weight determination, IR and UV-Vis spectroscopy. Based on these measurements, a structure (15.19) with two square planar Ni atoms and one tetrahedral Ni atom was proposed.2

N	N	Cl	N	N
N	Ni	N Ni N	Ni	N
N	N	Cl	N	N

15.19

While at the time, this structure was reasonable, an X-ray crystallographic study showed over 20 years later that the compound possesses the linear structure shown in Fig. 15.1.3 Synthetic routes to this compound are various, and are summarized in the following equations:

3NiCl2(Hdpa)2 + 4KOBun BunOH Ni3(dpa)4Cl2 + 4KCl + 2Hdpa

naphthalene

3NiCl2 + 4Lidpa	thf		Ni3(dpa)4Cl2 + 4LiCl
3NiCl2(Hdpa)2 + 4MeLi			thf	Ni3(dpa)4Cl2	+ 4LiCl + 4CH4	+ 2Hdpa

ButOH

3NiCl2 + 4Hdpa + 4KOBut naphthalene Ni3(dpa)4Cl2 + 4KCl + 2ButOH

All four of these reactions give Ni3(dpa)4Cl2 in high yields,2,3,15,47,48 and the product is easily purified by recrystallization from dichloromethane and hexanes. Method 2 has been used to synthesize a Ni36+ chain with the ligand BPAP,21 and an analogous method starting with CuCl2 was used to obtain Cu3(dpa)4Cl2 in high yields.49 Method 4 claims the highest yield48 (95 %) and has been used to synthesize an ethyl-substituted analog, Ni3(depa)4Cl2.50 Trinickel complexes of the unsymmetrical formamidinate ligand PhPyF have also been synthesized, but only in low yields as minor reaction products.51 From Ni3(dpa)4Cl2, many new derivatives have been made by substitution of different axial ligands. These Ni3(dpa)4X2 compounds are known for X = NO3,52 N3,53 MeCN,50 C>N,54 NCNCN,54 C>CPh,54 Ag(CN)2,55 mixed F and H2O ligands,56 C4O4Me,53 and also carboxylates.48 The latter three sets of ligands have been used to

Extended Metal Atom Chains 695

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connect together trinickel units either by hydrogen bonding, direct connection, or through another metal-containing linker as shown in 15.20.

(BF4)2

H2 O

	Ni
		Ni
			Ni
			O	H	F Ni	Ni	Ni	F
			H
								n
		Me			BF4			Me
	O						O
	O	O	Ni	Ni	Ni	O		O
	O						O
								n
								ClO4
N	O Ni	Ni	Ni	O		N	Mn	N	O
	O				O				O
									n
				Mn		= MnTPP

15.20

Ni3(dpa)4Cl2 has been shown to react incompletely with X anions to give products with mixed Cl and X ligands, so two methods have been developed to improve this type of reaction. In one, Ni3(dpa)4Cl2 is first allowed to react with AgBF4 to remove the Cl anions and then the desired axial ligand is added.

Ni3(dpa)4Cl2	1. 2AgBF		4	Ni3(dpa)4X2
	2. X	-

In the other, [Ni3(dpa)4(NCMe)2](PF6)2 is first prepared from Ni3(dpa)4Cl2 by reaction with AgPF6 in MeCN.50 This complex with labile acetonitrile ligands then reacts quickly with X anions to give Ni3(dpa)4X2.54

Ni	(dpa)	Cl	2	+ 2AgPF	6	MeCN			[Ni3(dpa)4(NCMe)2](PF6)2
3	4
[Ni3(dpa)4(NCMe)2](PF6)2 + 2NaX								methanol		Ni3(dpa)4X2	+ 2NaPF6

696Multiple Bonds Between Metal Atoms Chapter 15

The bis-phenylacetylide complex is prepared in high yield from [Ni3(dpa)4(MeCN)2](PF6)2 by reaction with sodium hydroxide and phenylacetylene in methanol.54

methanol

[Ni3(dpa)4(NCMe)2](PF6)2 + 2NaOH + 2HCCPh

Ni3(dpa)4(CCPh)2 + 2NaPF6 + 2H2O

Oxidation of Ni36+ to Ni37+ is quite difficult because the potential for this process is high

(E1/2 = 0.908 V vs Ag/AgCl for Ni3(dpa)4Cl2). Reaction of Ni3(dpa)4Cl2 with excess AgPF6 leads to the formation of the oxidized Ni3(dpa)43+ cation.15,44 The blue crystalline compound

[Ni3(dpa)4](PF6)3 is unstable at room temperature. Solutions revert to Ni36+ within a day, and the solid decomposes in air in about a week. For the ethyl-substituted analog Ni3(depa)4Cl2, the potential for this process is 0.130 V lower (due to the increased bacisity of the depa ligand), and the blue compound resulting from oxidation (i.e. [Ni3(depa)4](PF6)3) is stable for several weeks, even in solution:50

Ni3(dpa)4Cl2 + 3AgPF6 Α [Ni3(dpa)4](PF6)3 + 2AgCl + Ag

The Ni···Ni distances in Ni36+ compounds range from 2.37 Å in Ni3(BPAP)42- to 2.51 Å in Ni3(PhPyF)4Cl2 (see Table 15.1). In all of these compounds, the Ni···Ni distances are similar enough to consider the Ni36+ core as having idealized D4 symmetry. No unsymmetrical Ni36+ compounds are known. All Ni3(dpa)42+ compounds have strongly bound axial ligands, which cause the terminal Ni2+ ions to be high spin. This is manifested in the magnetic properties (vide infra), and also structurally in the fact that the Ni–N distances for the terminal, 5-co- ordinate Ni atoms are typically ~0.2 Å longer than the Ni–N distances for the central Ni(II) species (which is square planar and thus diamagnetic). A compound with the PhPyF ligand is known with only one axial ligand, namely [Ni3(PhPyF)4Cl]Cl, which contains two diamagnetic square planar Ni atoms and only one high spin, five coordinate Ni(II) ion which is responsible for the observed µeff of 3.08 µB.51 The only Ni3 compound known without axial ligands is Ni3(BPAP)42- (shown in Fig. 15.9) which has three square planar Ni(II) units with equivalent Ni–N bond lengths, and is diamagnetic.21

Fig. 15. 9. Structure of the dianion [Ni3(BPAP)4]2-.

Oxidation of Ni3(dpa)4Cl2 to [Ni3(dpa)4](PF6)3 causes major structrural changes to the trinickel unit.15,44 Most notably, the Ni–Ni distances in the Ni37+ compounds are 0.07 Å shorter than the

Extended Metal Atom Chains 697

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shortest distances in any Ni36+ compound and 0.14 Å shorter than the Ni···Ni distances in the precursor Ni3(dpa)4Cl2 (2.43 Å). The axial PF6 anions are not strongly coordinated to the terminal nickel atoms (Ni···F distances are over 2.4 Å), and the Ni–N distances for all three nickel atoms are shorter than in the precursor. These structural results are explained by the formation of 3c Ni–Ni bonds in the Ni37+ species. A similar result is observed in the ethyl-substituted analog.

In contrast to this, the Cu···Cu distances in Cu3(dpa)4Cl2 are in the range of 2.47 to 2.49 Å,45,46,49 while the oxidized [Cu3(dpa)4Cl2]SbCl6 has Cu···Cu separations of 2.51 to 2.52 Å.15 The modest increase of ~0.05 Å in the Cu···Cu distances upon oxidation is the result of increased electrostatic repulsion between the more highly charged Cu atoms. These results show inter alia that neither the Cu36+ nor the Cu37+ compounds have Cu–Cu bonds. In the oxidized species, the central Cu atom with shorter Cu–N distances (1.89 Å, as compared to the outer Cu–N distances of 2.06 Å) is believed to be the Cu atom oxidized to CuIII.

If a delocalized MO scheme such as the one shown in 15.9 is considered for Ni36+ compounds, the 24 d electrons would fill all the bonding, nonbonding, and antibonding MOs leaving no net bond. Moreover, the compound is expected to be diamagnetic, since all the MOs are occupied by an electron pair. This cannot be the case, however, because Ni3(dpa)4Cl2 is paramagnetic at room temperature.3,47 The reason for this apparent discrepancy is that since there are no Ni–Ni bonds, each Ni atom behaves independently. The central Ni atom is square planar and diamagnetic while the outer ones are five coordinate and high spin with S = 1. The spins of the two outer Ni atoms couple antiferromagnetically so that µeff is a complex function of temperature as shown in Fig. 15.10. All known Ni36+ compounds show this type of magnetic behavior except for [Ni3(BPAP)4]2− which, as mentioned above, is diamagnetic since it has no axial ligands.21

Fig. 15.10. Plot of µeff vs T for Ni3(dpa)4Cl2.

Oxiation to Ni37+ changes the magnetic behavior of the trinickel chain. The magnetic moment of 2.0 µB for [Ni3(dpa)4](PF6)3 is constant over the entire temperature range signifying that there is only one unpaired electron delocalized over the Ni3 chain.44 More evidence for delocalization of this electron comes from EPR measurements. The X-band EPR spectra of Ni3(dpa)43+ and Ni3(depa)43+ are axial, and the g components are split into three lines, consistent with coupling of the unpaired electron with the two axially coordinated fluorine atoms of the molecule.15 Thus, the unpaired electron is believed to reside in the three-center μ* orbital, which has small but significant contributions from the axial ligands.

Exchange coupled multinuclear Cu(II) complexes are perhaps the most well studied systems in the field of magnetochemistry.57 As may be expected from the vast work done on dinuclear Cu24+ paddlewheel-type complexes,58 the tricopper complexes Cu3(dpa)4Cl2 and Cu3(dpa)4(BF4)2

698Multiple Bonds Between Metal Atoms Chapter 15

show antiferromagnetic coupling between the three nonbonded d9 Cu(II) ions.49 In the oxidized Cu37+ complex, only two unpaired electrons remain, and these couple antiferromagnetically.15 This is consistent with the view that the central, square planar Cu atom is the one oxidized to a d8 Cu(III) species. Further evidence is seen in the crystal structure (vide supra) and in the electronic spectrum. The band at 487 nm is assigned to the d8 square planar CuIII species, and a band at 1310 nm is believed to be an intervalence charge transfer band.

Several complexes with non-metal-metal bonded tricopper chains are known which employ bridging ligands other than dpa. For example, Cu(II) complexes of tetradentate bis-pyridyl or bis-pyrimidyl formamidinates are known for those ligands shown in 15.21.59,60

H

DPyF

N N N N

N H N

DPmF

N N N N

H

DMPyF

N N N N

15.21

Table 15.3 summarizes the structural information and magnetic data for these compounds and also for the known Cr36+, Fe36+, and Co36+ complexes which employ DPyF and related ligands. An unsymmetrical Cr36+ chain with the DPyF ligands has been characterized which, as mentioned in Section 15.2,22 can be described as having a short Cr–Cr quadruple bond and a longer Cr···Cr separation leaving an isolated high spin Cr2+ ion. The corresponding Co36+, Cu36+, and Fe36+ compounds are isomorphous to [Cr3(DPyF)4](PF6)2, but do not show any sign of metal-metal bonding.61 The M···M distances in this group of complexes (except for the Cr36+ compound) are all long, ranging from 2.64 Å in [Cu3(DPmF)4](OTf)260 to 2.78 Å in [Fe3(DPyF)4](PF6)2 and 2.87 Å in [Co3(DPyF)4](PF6)2.61 Rather than forming metal-metal bonds in these complexes, the outer two metal atoms are pulled away from the central, squareplanar one by the extra dangling pyridyl groups resulting in a distorted octahedral geometry for the former as in 15.22.

N	N	N	N
N	N	N	N
M	M		M
N	N	N	N
N	N	N	N

15.22