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Molecular and Electronic Structures of Homoleptic Six-Coordinate Cobalt(I) Complexes of 2,2′:6′,2″-Terpyridine, 2,2′-Bipyridine, and 1,10-Phenanthroline. An Experimental and Computational Study Jason England,†,‡ Eckhard Bill,† Thomas Weyhermüller,† Frank Neese,† Mihail Atanasov,*,†,§ and Karl Wieghardt*,† †

Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34−36, D-45470 Mülheim an der Ruhr, Germany Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

§

S Supporting Information *

ABSTRACT: The crystal structures of nine homoleptic, pseudooctahedral cobalt complexes, 1−9, containing either 2,2′:6′,2″-terpyridine (tpy), 4,4′-di-tert-butyl-2,2′bipyridine (tbpy), or 1,10-phenanthroline (phen) ligands have been determined in three oxidation levels, namely, cobalt(III), cobalt(II), and, for the first time, the corresponding presumed cobalt(I) species. The intraligand bond distances in the complexes [CoI(tpy0)2]+, [CoI(tbpy0)3]+, and [CoI(phen0)3]+ are identical, within experimental error, not only with those in the corresponding trications and dications but also with the uncoordinated neutral ligands tpy0, bpy0, and phen0. On this basis, a cobalt(I) oxidation state assignment can be inferred for the monocationic complexes. The trications are clearly low-spin CoIII (S = 0) species, and the dicationic species [CoII(tpy0)2]2+, [CoII(tbpy0)3]2+, and [CoII(phen0)3]2+ contain high-spin (S = 3/2) CoII. Notably, the cobalt(I) complexes do not display any structural indication of significant metal-to-ligand (t2g → π*) π-back-donation effects. Consistent with this proposal, the temperature-dependent molar magnetic susceptibilities of the three cobalt(I) species have been recorded (3−300 K) and a common S = 1 ground state confirmed. In contrast to the corresponding electronic spectra of isoelectronic (and isostructural) [NiII(tpy0)2]2+, [NiII(bpy0)3]2+, and [NiII(phen0)3]2+, which display d → d bands with very small molar extinction coefficients (ε < 60 M−1 cm−1), the spectra of the cobalt(I) species exhibit intense bands (ε > 103 M−1 cm−1) in the visible and near-IR regions. Density functional theory (DFT) calculations using the B3LYP functional have validated the experimentally derived electronic structure assignments of the monocations as cobalt(I) complexes with minimal cobalt-to-ligand π-back-bonding. Similar calculations for the six-coordinate neutral complexes [CoII(tpy•)2]0 and [CoII(bpy•)2(bpy0)]0 point to a common S = 3/2 ground state, each possessing a central high-spin CoII ion and two π-radical anion ligands. In addition, the excited-states and ground state magnetic properties of [CoI(tpy0)2][CoI−(CO)4] have been explored by variable-temperature variable-magnetic-field magnetic circular dichroism (MCD) spectroscopy. A series of strong signals associated with the paramagnetic monocation exhibit pronounced C-term behavior indicative of the presence of metal-to-ligand charge-transfer bands [in contrast to d−d transitions of the nickel(II) analogue]. Time-dependent DFT calculations have allowed assignment of these transitions as Co(3d) → π*(tpy) excitations. Metal-to-ligand charge-transfer states intermixing with the Co(d8) multiplets explain the remarkably large (and negative) zerofield-splitting parameter D obtained from SQUID and MCD measurements. Ground-state electron- and spin-density distributions of [CoI(tpy0)2]+ have been investigated by multireference electronic structure methods: complete active-space selfconsistent field (CASSCF) and N-electron perturbation theory to second order (NEVPT2). Both correlated CASSCF/NEVPT2 and spin-unrestricted B3LYP-based DFT calculations show a significant delocalization of the spin density from the CoI dxz,yz orbitals toward the empty π* orbitals located on the two central pyridine fragments in the trans position. This spin density is of an alternating α,β-spin polarization type (McConnel mechanism I) and is definitely not due to magnetic metal-to-radical coupling. A comparison of these results with those for [NiII(tpy0)2]2+ (S = 1) is presented.

■

INTRODUCTION

observation of room temperature effective magnetic moments of 2.9−3.4 μB, values that are indicative of the presence of two unpaired electrons. However, it is now widely recognized that these ligands can exist as either diamagnetic neutral species (L0), paramagnetic (S = 1/2) π-radical anions (L•)−, or even the corresponding dianions (L2−)2− (L = tpy, bpy, and phen).3−6

+

The six-coordinate monocationic complexes [Co(tpy)2] , [Co(bpy)3]+, and [Co(phen)3]+, where tpy, bpy, and phen correspond to the generic abbreviations for 2,2′:6′,2″terpyridine, 2,2′-bipyridine, and 1,10-phenanthroline, respectively, have long been known and presumed to contain highspin CoI (d8; S = 1 ground state).1,2a−c These oxidation state assignments are largely based on the assumption that the aforementioned organic ligands bind as neutral entities and the © XXXX American Chemical Society

Received: October 19, 2015

A

DOI: 10.1021/acs.inorgchem.5b02415 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry This lends some uncertainty to the aforementioned electronic structure assignments. For instance, an alternative description is that these complexes possess one (L•)− ligand and a central high-spin CoII ion (d7, SCo = 3/2), which antiferromagnetically couple to one another to yield the reported triplet ground states. Each of the redox states of bpy, tpy, and phen possesses distinct metrical parameters, with the Cpy−Cpy and intrachelate C−N (C−Nchel) distances significantly shortening and lengthening, respectively, upon reduction.3−6 Furthermore, recent experimental results reinforce the notion that the neutral tpy0, bpy0, and phen0 ligands are very poor π acceptors,4,5 at least when coordinated to first-row transition-metal ions. As a consequence, the Cpy−Cpy and C−Nchel distances of these ligands in coordination compounds do not vary significantly, even within excellent X-ray crystallographic resolution (σ ≤ 0.005 Å), as a function of the dN electron configuration of the central metal ion. Therefore, high-quality, high-resolution X-ray structures of [Co(tpy)2]+, [Co(bpy)3]+, and [Co(phen)3]+ without static disorder problems would provide a means to discern the oxidation states of these species. Thus far, with the exception of a low-quality crystal structure of [Co(bpy)3]Cl·H2O,7 the required crystal structures have yet to be reported. Interestingly, in the aforementioned structure of [Co(bpy)3]Cl·H2O, the average Cpy−Cpy and C−Nchel bond lengths were reported to be 1.42 and 1.37 Å, respectively,7 which differ significantly from the values associated with bpy0 (1.47−1.49 and ∼1.35 Å, respectively) and closely resemble those of (bpy•)− (1.42−1.43 and ∼1.38 Å, respectively). Hence, if real, these bond lengths would indicate the presence of (bpy•)− radicals. However, they have very large estimated standard deviations σ = 0.02 Å, which precludes the safe assignment of the ligand oxidation level. Regardless, the original authors speculated that there may be significant π delocalization of d electron density into the lowset unoccupied molecular orbital (LUMO; π* orbital) of the neutral bpy ligands either by the formation of π-radical anion, as in [CoII(bpy•)(bpy0)2]+, or via a metal-to-ligand π-back-donation mechanism. As previously noted, there is a distinct difference between the electronic spectra of the CoI species8 [Co(tpy)2]1+, [Co(bpy)3]1+, and [Co(phen)3]1+ and their isoelectronic (and we show here isostructural) NiII counterparts,9 [NiII(tpy0)2]2+, [NiII(bpy0)3]2+, and [NiII(phen0)3]2+. The former display a number of bands in the visible and near-infrared regions with molar extinction coefficients (ε) of the order of 103 and 104 M−1 cm−1, whereas the latter have intensities not larger than 60 M−1 cm−1 and have been assigned throughout the literature as d → d transitions (3A2 → 3T1(F); 3A2 → 3T2(F)). Although in the case of [Co(bpy)3]1+ the observed bands have been interpreted as a series of metal-to-ligand charge transfer transitions,10 the spectra of the cobalt(I) species have yet to be unambiguously assigned and, in many ways, more closely resemble the spectra of ligand-radical monoanions5,6 than d → d transitions. Herein, high-resolution crystal structures of the monocations [Co(tpy)2]+, [Co(tbpy)3]+ (tbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), and [Co(phen)3]+ are reported for the first time, alongside those of the corresponding dications and a trication (complexes 1−9; Chart 1). In addition, we have recorded the temperature dependence (4−300 K) of their magnetic susceptibilities and measured the electronic spectra of all three monocations in the range 33300−6250 cm−1. Density functional theory (DFT-B3LYP) calculations for the tri-, di-,

Chart 1. Complexes Characterized by X-ray Crystallography [100(2) K] in This Work

and monocationic complexes have reliably reproduced the experimental structures and allowed unambiguous electronic structure assignments to be made. In addition, we have used DFT-B3LYP to investigate the molecular and electronic structures of the neutral complexes [Co(tpy)2]0 and [Co(bpy)3]0, the latter of which has been reported to possess either an S = 1/2 or 3/2 ground state,11 whereas the former has not yet been isolated and its ground state is unknown. Finally, we have recorded and analyzed the magnetic circular dichroism (MCD) spectrum of [Co(tpy)2]+ (S = 1) and report its ground-state electron- and spin-density distributions by multireference electronic structure methods, namely, the complete active space self-consistent field (CASSCF) and N-electron perturbation theory to second order (NEVPT2), and time-dependent density functional theory (TDDFT).

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RESULTS AND DISCUSSION Synthesis of Complexes. The earliest reports of the preparation of salts containing the monocations [Co(bpy)3]+ and [Co(phen)3]+, both with perchlorate as a counterion, involved reduction of the corresponding dications by sodium amalgam and sodium borohydride, respectively.2b,c Therefore, our initial efforts to access the monocationic species focused on the reduction of cobalt(II) salts using sodium amalgam. Although, via such means, we were able to grow crystals of black [Co(tpy)2](PF6) (4), dark blue [Co(tbpy)3](BF4) (7), and dark brown [Co(phen)3](BF4) (9) suitable for highresolution X-ray crystallography, analytically pure material for SQUID measurements could only be obtained in the latter case B


Article

Inorganic Chemistry Table 1. Crystallographic Data of Complexes chemical formula fw space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å Z T, K ρcalcd, g cm−3 reflns collected/2θmax unique reflns/I > 2σ(I) no. of param/restraints λ, Å/μ(Kα), cm−1 R1a/GOFb wR2c [I > 2σ (I)] residual density, e Å−3 chemical formula fw space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å Z T, K ρcalcd, g cm−3 reflns collected/2θmax unique reflns/I > 2σ(I) no. of param/restraints λ, Å/μ(Kα), cm−1 R1a/GOFb wR2c [I > 2σ (I)] residual density, e Å−3

1

2

3

4

5

C50H46CoF18N6O2P3 1256.77 C2/c (No. 15) 15.487(2) 16.240(2) 20.746(3) 90 101.593(3) 90 5111.4(12) 4 100(2) 1.633 68350/61.26 7841/6144 366/0 0.71073/5.44 0.0546/1.038 0.1396 +1.35/−0.97 6

C66H78Co3N6O9 1276.13 P21/c (No. 14) 18.432(2) 16.797(2) 20.428(2) 90 90.994(2) 90 6323.6(12) 4 100(2) 1.340 178786/61.98 20050/16466 791/7 0.71073/8.38 0.0329/1.058 0.0829 +0.79/−0.64

C34H22Co2N6O4 696.44 P1̅ (No. 2) 9.0566(6) 11.5661(6) 14.6130(8) 80.265(4) 88.642(5) 89.549(5) 1508.23(15) 2 100(2) 1.534 36456/66.20 11409/9215 415/7 0.71073/11.50 0.0373/1.063 0.0860 +0.48/−0.94

C38H38CoF6N6O2P 814.64 I4̅ (No. 82) 8.8263(5) 8.8263(5) 27.4049(11) 90 90 90 2134.9(2) 2 100(2) 1.267 8149/60.00 3057/2464 146/5 0.71073/5.03 0.0756/1.120 0.1843 +0.68/−0.37 8

C57H78CoF12N6O2P2 1212.12 C2/c (No. 15) 20.3973(12) 26.0964(10) 11.3912(9) 90 101.956(9) 90 5932.0(6) 4 100(2) 1.357 57408/66.22 11259/8593 378/7 0.71073/4.25 0.0516/1.091 0.1104 +0.75/−0.53 9

C70H88Co3N6O10 1350.25 P-1, No. 2 14.8166(13) 16.194(3) 16.527(3) 98.428(15) 95.691(12) 116.393(8) 3452.8(10) 2 100(2) 1.299 93264/66.24 26214/20266 840/138 0.71073/7.72 0.0446/1.024 0.1049 +0.77/−0.90

7 C54H72BCoF4N6 950.92 Pnna, No. 52 24.305(3) 16.826(2) 13.773(2) 90 90 90 5632.5(13) 4 100(2) 1.121 109128/52.00 5534/4698 305/0 0.71073/3.56 0.0600/1.088 0.1398 +0.79/−0.38

C38H27B2CoF8N7 814.21 P21/n, No. 14 18.0500(14) 9.3514(5) 22.009(2) 90 107.337(7) 90 3546.2(5) 4 100(2) 1.525 78821/59.98 10342/8914 538/20 0.71073/5.67 0.0508/1.180 0.1217 +0.52/−0.84

C43H34.5BCoF4N9.5 830.04 R-3, No. 148 14.2287(11) 14.2287(11) 34.0196(13) 90 90 120 5964.7(9) 6 100(2) 1.386 27336/66.18 5031/4611 183/14 0.71073/4.95 0.0364/1.083 0.0971 +0.60/−0.39

Observation criterion: I > 2σ(I). R1 = ∑||Fo| − |Fc||/∑|Fo|. bGOF = [∑[w(Fo2 − Fc2)2]/(n − p)]1/2. cwR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2, where w = 1/σ2(Fo2) + (aP)2 + bP, with P = (Fo2 + 2Fc2)/3.

a

In contrast, performing a similar reaction between the cobalt(0) starting material [Co2(CO)8]0 and bpy or phen is known to yield the cobalt(II)-containing salts [CoII(L)3][CoI−(CO)4]2 (L = bpy or phen).2d This proved to be true for 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine (ttpy) and tbpy (Chart 1) and allowed crystals of [CoII(ttpy)2][CoI−(CO)4]2 (2) and [CoII(tbpy)3][CoI−(CO)4]2 (6) suitable for X-ray crystallography to be obtained. Interestingly, it was found that the reaction of unsubstituted tpy with [Co2(CO)8]0 did not yield the cobalt(II) compound [CoII(tpy)2][CoI−(CO)4]2 but, instead, gave [Co(tpy)2][Co(CO)4] (3) as an analytically pure compound. The reason for this divergent outcome is presently unclear. To provide a point of comparison for analysis of the intraligand C−C and C−N bond distances, we have also grown single crystals of the cobalt(II) starting materials [CoII(tbpy)3]-

and required multiple recrystallizations, each followed by the mechanical separation of impurities. Instead, we sought to exploit the route of Behrens and Aquila, reported in 1967,2a via which the cobalt complexes [Co(tpy)2][Co(CO)4], [Co(bpy)3][Co(CO)4], and [Co(phen)3][Co(CO)4] were obtained in excellent yields by reaction of the requisite equivalents of ligand with [(CHD)2Co2(CO)4]0 (CHD = 1,3-cyclohexadiene) in a benzene solution under strictly anaerobic conditions at 80 °C. This reaction involves disproportionation of the cobalt(0) starting material to yield the desired monocation and a diamagnetic [CoI−(CO)4]− counteranion. Using similar reaction conditions, a combination of [(NBD)2Co2(CO)4]0 (NBD = 2,5-norbornadiene) and tbpy was found to yield analytically pure [Co(tbpy)3][Co(CO)4], for which SQUID measurements were performed. C


Article

Inorganic Chemistry (PF6)2 (5) and [CoII(phen)3](BF4)2 (8) and obtained their high-resolution X-ray structures. In addition, one diamagnetic cobalt(III) species [CoIII(p-tol-tpy)2](PF6)3 (1), where p-toltpy corresponds to 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine, has been prepared using standard procedures1 and its structure determined. X-ray Structure Determinations. The X-ray crystal structures of complexes 1−9 (see Chart 1 for notations) have been determined at high resolution using data collected at cryogenic temperatures (100 ± 2 K). The quality of these structures is, in general, very good (Table 1), with Co−N bond lengths possessing errors (3σ) of only ∼0.005 Å and those for the Cpy−Cpy and intrachelate C−N (C−Nchel) bonds being only ∼0.01 Å. This is sufficient to allow accurate determination of the ligand oxidation level. The structures of the three members of the electron-transfer series [Co(tpy)2]n+ (n = 3, 2, 1) are displayed in the following figures: the trication in crystals of 1 and the dication in 2 appear in Figure S1A (top and middle, respectively), and the monocation in 3 is shown in Figure 1. The structure of the

The most sensitive structural indicators of the oxidation level of bpy, tpy, and phen ligands are the Cpy−Cpy and C−Nchel bond lengths.3−6 Similarly, the oxidation and spin states of a central Co ion are often clearly identifiable from the observed Co−N distances. When these two sets of data are combined, accurate electronic structure assignments can be made. For example, complex 1 exhibits the shortest Co−N bonds of the series (Table 2) and a long average Cpy−Cpy distance of 1.472(3) Å and a short average C−Nchel bond length of 1.346(3) Å, which are very similar to those in uncoordinated neutral tpy0.12,13 Thus, the trication in 1 clearly possesses the electronic structure [CoIII(p-tol-tpy0)2]3+ (d6; S = 0).14 Similarly, the dication in complex 2 exhibits average Co− Ncentral and Co−Nterminal distances that are long and typical of octahedral high-spin CoII (d7; S = 3/2). The average Cpy−Cpy and C−Nchel bond lengths of 1.485(2) and 1.341(2) Å, respectively, are within the 3σ limits of the corresponding distances the same as those in 1, and are once again indicative of the presence of two neutral tpy0 ligands. Hence, as expected, the electronic structure of the dicationic component of 2 is best described as [CoII(tpy0)2]2+. The structures of the two monocations in 3 and 4, which are accompanied by the anions [Co I− (CO) 4 ]− and PF 6 − , respectively, are effectively identical. This shows that the nature of the anion and crystal-packing forces do not influence the structural parameters of the monocation. On the basis of the respective average Cpy−Cpy and C−Nchel distances in 3, it is clear that they contain only neutral tpy0 ligands. Furthermore, the π-acceptor properties of tpy0 are clearly very weak because if any cobalt-to-ligand π-back-donation is, indeed, present it fails to have a significant impact on the intraligand bond distances. Hence, it can be concluded that the monocation contains CoI (d8; S = 1) and its electronic structure is [CoI(tpy0)2]+. Similarly, the geometrical features of the dications in complexes 5 and 6 are identical within 3σ limits; they are also independent of the counteranion and crystal-packing forces. The structures of the two dications are shown in Figure S1B, and the salient bond distances are summarized in Table 3. The respective average Co−N bond lengths in 5 and 6 are long and characteristic of high-spin CoII. The experimental Cpy−Cpy bond lengths of all of the tbpy ligands are long (1.487−1.491 Å), and the C−Nchel bond lengths are observed in the narrow range 1.350−1.356 Å. This is, as anticipated, characteristic of

Figure 1. Structure of the monocation [Co(tpy)2]+ in 3.

monocation in 4 is effectively identical with that of 3 but is of poorer quality. As a consequence, it has been relegated to the Supporting Information (Figure S1A, bottom).

Table 2. Experimental and DFT-Calculated Bond Lengths (Å) in the Series [Co(tpy)2]n+ (n = 3, 2, 1, 0)

n = 3 (S = 0)

a

a

bond

exptl

1 2 3 4 5 6 7

1.948(2) 1.861(2) 1.950(2) 1.346(3) 1.346(3) 1.469(3) 1.474(3)

n = 2 (S = 3/2) b

calcd

exptl

1.994 1.894 1.997 1.344 1.344 1.471 1.471

2.147(1) 2.062(1) 2.138(1) 1.341(2) 1.340(2) 1.486(2) 1.484(2)

n = 2 (S = 1/2) c

calcd

exptl

2.219 2.096 2.206 1.340 1.340 1.485 1.483

2.083(4) 1.912(5) 2.083(4) 1.350(5) 1.350(5) 1.480(7) 1.480(7)

n = 1 (S = 1) d

calcd

exptl

2.114 1.935 2.141 1.348 1.347 1.478 1.477

2.131(1) 2.003(1) 2.129(1) 1.359(2) 1.355(2) 1.472(2) 1.475(2)

n = 0 (calcd)

calcd

S = 3/2

S = 1/2

2.200 2.059 2.225 1.352 1.350 1.476 1.479

2.189 2.053 2.246 1.372 1.366 1.454 1.464

2.241 2.055 2.209 1.365 1.371 1.465 1.456

This work, complex 1. bThis work, complex 2. cData from refs 14 and 20a. dThis work, complex 3. D


Article

Inorganic Chemistry Table 3. Selected Experimental Average Bond Distances (Å) in Complexes 5−9

a

complex

Cpy−Cpy

C−Nchel

Co−N

[CoIII(bpy0)3]3+ b 5 6 7 [CoIII(phen0)3]3+ c 8 9

1.466(6) 1.488(2) 1.490(2) 1.475(4) 1.415(4) 1.438(3) 1.436(1)

1.360(4) 1.350(2) 1.354(2) 1.353(3) 1.365(3) 1.362(3) 1.366(2)

1.933(2) 2.114(1) 2.121(1) 2.094(2) 1.940(2) 2.130(2) 2.111(1)

Sa 0 /2 3 /2 1 0 3 /2 1 3

Ground state. bData taken from ref 15. cData taken from ref 15.

the neutral ligands and electronic structures for the dications in 5 and 6 of [CoII(tbpy0)3]2+ (S = 3/2). From the structure of the monocation in 7 (Figure 2 and Table 3), which exhibits a long Cpy−Cpy distance of 1.475(4) Å

Figure 3. Structure of the monocation [CoI(phen0)3]+ in crystals of 9.

Magnetic Properties and Electronic Spectra. Consistent with it containing a low-spin CoIII ion, which is in line with published data for [CoIII(bpy0)3]3+ and [CoIII(phen0)3]3+,15 complex 1 was found to be diamagnetic. Temperaturedependent measurements (4−300 K) of the molar magnetic susceptibility for solid samples of the high-spin cobalt(II) complexes 5, 6, and 8 have been performed using a SQUID magnetometer in a magnetic field of 1 T. All three show temperature-independent effective magnetic moments, μeff, of ∼4.5 μB (the spin-only μeff for the S = 3/2 spin state is 3.87 μB) in the range 150−300 K (Figure S2), with g values of 2.40−2.50 (Table 4), which indicates S = 3/2 ground states (high-spin d7 Table 4. Parameters Used To Fit the Magnetic Susceptibility Data for the Cobalt(I) and Cobalt(II) Complexesa

Figure 2. Structure of the monocation [CoI(tbpy0)3]+ in crystals of 7.

complex

and a short average C−Nchel bond length of 1.353(3) Å, it is clear that three neutral tbpy0 ligands are present. Once again, the average Co−N bond length in the monocation of 2.094(2) Å is marginally shorter than those in the corresponding dication but consistent with the presence of a central CoI ion. Hence, from an X-ray structural standpoint, the monocation is best described as having a [CoI(tbpy0)3]+ (S = 1) electronic structure. Note, in contrast to [Co(bpy)3]Cl,7 no disorder is observed in the structural determination of 7. It is also noteworthy that the geometrical features of the N,N′coordinated ligands are effectively identical irrespective of the dn configuration of the central Co ion. This is consistent with minimal cobalt-to-ligand π-back-bonding and results from the poor π-acceptor properties of bpy0. Analysis of the structure of the monocation in 9, depicted in Figure 3, results in conclusions analogous to those outlined above for the corresponding tpy and tbpy complexes. The three phen ligands in 8 are, within experimental error (3σ), identical with those of the dication in 9 and a typical neutral phen0.16 Additionally, the Co−N distances in 8 are characteristic of sixcoordinate high-spin CoII, and those in 9 are marginally shorter and consistent with the presence of CoI. Therefore, we can assign the electronic structures 8 and 9 as [CoII(phen0)3]2+ (S = 3 /2) and [CoI(phen0)3]+ (S = 1), respectively.

parameter

3

7

9

Sb gc |D| (cm−1)d TIP (×10−6 emu)e θWeiss (K)

1 2.05 6.6 150 0

1 2.3 6.0 750 −3.6

1 2.3 4.0 120 −3.0

5 3

/2 2.4 41 500 0

6 3

/2 2.5 70 0 0

8 3

/2 2.5 80 0 −0.5

a Data fits have been performed with the constraint E/D = 0, for the sake of simplicity, and assumption of 0% paramagnetic impurities. b Ground spin state. cAverage g value. dAxial zero-field-splitting parameter. Sign not known. Positive and negative D values yielded equivalently good fits. eTemperature-independent paramagnetism.

configuration) for these cobalt(II) complexes. (For the sake of simplicity, fits of the experimental data were performed with the rhombicity parameter, E/D, set to zero.) Figure 4 displays the temperature dependence of the effective magnetic moments of solid samples of the cobalt(I) complexes 3, 7, and 9, and the parameters used to fit the data are summarized in Table 4 (once again, simulation of the experimental data was performed using E/D = 0). In the range 50−300 K, all three complexes display temperatureindependent effective magnetic moments, with values of 2.8 μB E


Article

Inorganic Chemistry Table 5. Solution-State UV−Vis−Near-IR Spectra of Nickel(II) and Corresponding Cobalt(I) Complexes complex II

0

2+

[Ni (tpy )2] [CoI(tpy0)2]+

[NiII(bpy0)3]2+ [CoI(tbpy0)3]+ [NiII(phen0)3]2+ [CoI(phen0)3]+

Figure 4. Plots of μeff versus temperature (T) for solid samples of cobalt(I) complexes 3 (red circles), 7 (black circles), and 9 (green circles). The solid lines are fits produced using the parameters listed in Table 4.

ν̃max, ×103 cm−1 (εmax, M−1 cm−1)

ref

12.5 (42), 18.2 (2), 19.6 (29), 22.5 (120), 29.0 7.7 (14 × 103), 10 (sh), 18.0 (7.2 × 103), 23.4 (9.8 × 103), 30.7 (33.0 × 103), 31.5 (sh), 36.1 (41.6 × 103), 42.6 (63.4 × 103) 11.6 (7), 12.7 (8), 19.2 (12), 21.0 6.9 (9.7 × 103), 8.6 (sh), 12.2 (3.1 × 103), 16.9 (6.3 × 103), 24.0 (sh), 26.0 (5.5 × 103), 34.7 (36.9 × 103) 11.6 (sh, 6), 12.8 (7), 19.3 (11) 7.0 (6.7 × 103), 7.2 (6.7 × 103), 8.9 (sh), 16.8 (0.7 × 103), 19.0 (1.0 × 103), 23.4 (2.7 × 103), 28.8 (1.5 × 103)

9 this work 9 this work 9 this work

would seemingly call into question our X-ray-structure-derived CoI oxidation state assignment. MCD Study of [CoI(tpy0)2][CoI−(CO)4]. In response, the magnetic properties of complex 3 have been further explored by using low-temperature MCD spectroscopy (5000−35000 cm−1). Samples of the compound dissolved in frozen butyronitrile showed strong MCD signals with pronounced C-term behavior (Figure 6). This is illustrated by the series of

for 3 and ∼3.2 μB for 7 and 8. Fitting the data yielded g values in the range 2.05−2.32 and zero-field-splitting parameters, |D|, of 4−7 cm−1. These data are in excellent agreement with those reported for isoelectronic nickel(II) complexes17 and support the notion that 3, 7, and 8 contain central high-spin CoI ions (d8). In an effort to obtain a more accurate measurement of the zero-field-splitting parameters for 3, variable-temperature variable-field (VTVH) magnetization measurements were performed (Figure S3). Fitting the resulting data (a g value of 2.05 was retained) yielded a |D| value of 5.7 cm−1. Unfortunately, unambiguous determination of the sign of D was not possible from these data. Figure 5 displays the electronic spectra (33300−6250 cm−1) of the cobalt(I) complexes 4, 7, and 9, which were recorded in

Figure 5. Electronic spectra of the cobalt(I) complexes 4 (black), 7 (blue), and 9 (red) recorded in a butyronitrile solution at 20 °C.

butyronitrile solution at 20 °C. The spectra are broadly similar regardless of the nature of the N-heterocyclic ligand coordinated (Table 5). More specifically, they exhibit a band in the near-IR region centered in the range 7800−6800 cm−1, which has a shoulder between 8600 and 10000 cm−1, and bands in the visible region at ∼18000 and ∼23000 cm−1. These bands are all quite intense, with molar extinction coefficients, ε, >103 M−1 cm−1. This is in stark contrast to bands in the corresponding isoelectronic nickel(II) complexes,9 which are restricted to the visible region and are of low intensity (ε < 50 M−1 cm−1). In the case of the nickel(II) complexes, assignment of the bands to d → d transitions is straightforward. In contrast, in many ways, the unusual spectra of complexes 4, 7, and 9 bear a greater resemblance to those seen for complexes of radical anions5,6 than to the spectra of the nickel(II) complexes, which

Figure 6. Electronic spectrum of [CoI(tpy0)2]+ in a butyronitrile solution and its deconvolution (top) and its MCD spectrum at 5 K and 7 T (bottom).

spectra recorded over the temperature range 1.8−78 K with B = 5 T applied field shown in Figure S4. The signals can be readily assigned to the [CoI(tpy0)2]+ cation because the [CoI−(CO)4]− anion is known to be diamagnetic and, hence, virtually MCDsilent. Furthermore, the large number of resolved transitions in the MCD spectrum of [CoI(tpy0)2]+ rules out an assignment of the intense bands as d−d transitions. In addition to recording full MCD spectra, we carried out VTVH measurements at selected peak positions by which the intensities of the most prominent MCD bands have been F


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Inorganic Chemistry

Figure 7. Experimental VTVH MCD data for four selected MCD bands of [CoI(tpy0)2]+ (S = 1) recorded at 1.6, 4.0, 6.0, 8.1, and 16 K and global spin-Hamiltonian simulations for spin S = 1 with g = 2, D = −11.9 cm−1, and E/D = 0.15 (solid lines). The polarization factors obtained for all monitored spectral positions are given in Table 6.

recorded at five different temperatures as a function of the applied field (isofield curves acquired in the range 0−8 T). Figure 7 shows a selection of the VTVH data together with the results of a global spin-Hamiltonian simulation that was performed according to the formalism laid out by Neese and Solomon.18 Such global simulations of the VTVH curves, in principle, provide an independent evaluation of the zero-field splitting of the ground-state manifold of the MCD active species. The best solution of the corresponding spin-Hamiltonian simulations for 3 has been obtained with D = −11.9(5) cm−1 and E/D = 0.15. The result is shown as solid lines in Figure 7, whereas the corresponding polarization factors are summarized in Table 6. However, a systematic inspection of the parameter space available for the simulations revealed that an equally good fit of the data can be achieved with a positive D value [D = +7.8(5) cm−1 and E/D = 0.05]. This ambiguity can be seen already from a one-dimensional scan of the error sums for the simulations shown as a function of D, in which E/D was set to zero and the g value to 2 (Figure S5), but the polarization factors for all transitions have been independently optimized for every D value. The resulting impossibility to determine the sign of D from the experimental MCD intensity data, as well as the same ambiguity found above for the magnetic susceptibility measurements, results directly from the low multiplicity of the triplet state of 3. The low number of ms energy gaps, i.e., only two are available, which both depend similarly on D and E/D, as well as the mixed nature of the ms wave functions found at finite values of E/D, in conjunction with the usual error bars of the experimental data does not offer sufficient characteristic

Table 6. Optical Transitions, Band Widths, and Polarization Factors Mij for 3 Obtained from the Global Fitting of Absorption and MCD Spectra Shown in Figure 6 band

energy, cm−1

fwhm,a cm−1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

29980 28991 28010 26200 24349 22232 20186 18800 16980 15491 15491 10728 8247 7604 6980

645 710 500 600 1650 1176 550 600 684 1164 1934 900 643 549 1083

Myz

Mxz

Mxy

wavelength,b nm

0.76

0.76

2.33

452

1.62 0.99 1.19

1.62 0.99 1.19

1.75 1.25 0.33

556 592 645

0.75

0.75

2.42

1080

0.56

0.56

3.30

1448

a

Full width at half-maximum of the MCD bands; absorption bands have been independently optimized. bPosition of VTVH measurement.

features in VTVH MCD or SQUID measurements on powder samples to clearly discriminate the sign of D. DFT Calculations. In order to confirm the electronic structures inferred from analysis of the X-ray crystallographic data, the members of the electron-transfer series, [Co(tpy)2]n, [Co(bpy)3]n, [Co(phen)3]n, where n = 3+, 2+, 1+, and 0, were investigated using DFT calculations with the B3LYP functional. The results are expressed using the broken-symmetry (BS) G


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Inorganic Chemistry formalism,19 and the accuracy of the calculations were gauged by comparison of the structural parameters of the geometryoptimized structures with those obtained experimentally. Other than a slight overestimation of the metal-to-ligand bond lengths on the average by up to 0.05 Å, as is typically observed with the B3LYP functional, the agreement is excellent. Table 7 lists the

distortions (Tables S2−S4). The magnitude of these distortions is much greater than that experimentally observed,20,21 which may derive from the gas-phase nature of the calculations. Again, in all cases, the calculated Cpy−Cpy and C−Nchel bonds are clearly indicative of the ligands being neutral. This conclusion is reaffirmed by the corresponding spin-density plots (Figures S6−S8) and qualitative FMO diagrams (Figures S10, S11, S13, S14, S18, and S19), which show Co II and minimal delocalization of the spin density onto the supporting ligands. These results for high- and low-spin [CoII(bpy0)3]2+ are in good agreement not only with the experiment but also with the DFT calculations previously reported by Hauser et al.21a The Monocations. In excellent agreement with the experimental structures, geometry optimizations for the monocations [Co(tpy)2]+, [Co(bpy)3]+, and [Co(phen)3]+ yielded ligands that appear to be coordinated in their neutral forms, thereby rendering the central metal ion CoI (d8, S = 1). More specifically, the Cpy−Cpy and C−Nchel bond distances within each complex are, within experimental error limits (3σ ∼ 0.01 Å), identical and agree well with those reported for the uncoordinated neutral ligands tpy0,12 bpy0,22 and phen0.16 Consistent with this statement, the occupied FMOs (Figures 11 and S15 and S20) have predominant 3d orbital character. However, the overlap integral between the α- and β-spin orbitals comprising the highest-energy doubly occupied molecular orbital is significantly less than unity; it ranges from 0.83 to 0.87 to 0.90 in [CoI(phen)3]+, [CoI(bpy)3]+, and [CoI(tpy)2]+, respectively. This is due to the incorporation of the ligand character into the β-spin orbital, which is reflected in a diminution of the cobalt character versus its α-spin partner (from ∼90% to ∼60%) . As can be seen from their qualitative FMO diagrams (Figures 11 and S16 and S21), this originates from limited π-back-donation from the CoI ion to π* orbitals of the ligands. A similar effect is not seen in the partner α-spin orbitals because of significant energetic stabilization deriving from exchange interactions. It should be highlighted that, although the calculations suggest that some metal-to-ligand πback-donation is present, it is insufficient to have a detectable impact upon either the experimental magnetic or the calculated structural parameters of the ligands. Figure 8 displays the Mulliken spin-density plots with spin population analyses for the geometry-optimized monocations [CoI(bpy)3]+ and [CoI(phen)3]+ (that for [Co(tpy)2]+ is shown in Figure 12 and will be additionally discussed in relation to the spectral behavior of the isoelectronic [NiII(tpy)2]2+ below). In [CoI(tpy0)2]+ (Figure 12 and Table S23), there are 2.30 unpaired electrons at the central metal ion and small, nonzero spins of −0.17 and −0.16 on the tpy ligands, which in both cases are largely localized on the C atoms at the 2, 4, and 6 positions of the central pyridine rings. Similar spin-density distributions are observed for [CoI(bpy0)3]+ and [CoI(phen0)3]+ but with two of the bidentate ligands bearing −0.11 to −0.14 spins and no spin on the third ligand. There is a direct correspondence between the location of the ligandcentered β spin in the spin-density plots (Figure 8) and that in the singly occupied molecular orbital (SOMO) involved in πback-bonding from the CoI ion to the π* orbitals of the ligands, which indicates that the former originates from the latter bonding interaction. In summary, the DFT calculations indicate that the monocations [CoI(tpy0)2]+, [CoI(bpy0)3]+, and [CoI(phen0)3]+ are best described as cobalt(I) complexes incorporating a minor amount of cobalt(II) character (∼26%).

Table 7. Selected Average Bond Distances (Å) from the DFT Geometry-Optimized Structures of the Six-Coordinate Cobalt Complexes III

complex

Cpy−Cpy

C−Nchel

Co−N

Ms

0

1.471 1.478 1.484 1.477 1.445 1.469 1.480 1.485 1.478 1.452 1.418 1.433 1.438 1.436

1.344 1.347 1.340 1.351 1.355 1.357 1.351 1.349 1.351 1.367 1.361 1.358 1.356 1.358

1.962 2.071 2.174 2.161 2.096 1.982 2.095 2.183 2.183 2.173 1.989 2.107 2.192 2.191

0 1 /2 3 /2 1 3 /2 0 1 /2 3 /2 1 3 /2 0 1 /2 3 /2 1

3+

[Co (tpy )2] [CoII(tpy0)2]2+

[CoI(tpy0)2]+ [CoII(tpy•)2]0 [CoIII(bpy0)3]3+ [CoII(bpy0)3]2+ [CoI(bpy0)3]+ [CoII(bpy•)2(bpy0)]0 [CoIII(phen0)3]3+ [CoII(phen0)3]2+ [CoI(phen0)3]+

calculated average Co−N, Cpy−Cpy, and C−Nchel bond lengths for all of the ground-state geometry-optimized structures. Selected bond lengths (Tables S2−S4), qualitative frontier molecular orbital (FMO) diagrams, additional Mulliken spindensity plots, and atomic coordinates for each of the geometryoptimized structures are provided in the Supporting Information. The Trications. The geometry-optimized structures of the three diamagnetic trications [Co(tpy)2]3+, [Co(bpy)3]3+, and [Co(phen)3]3+, obtained from restricted Kohn−Sham calculations, clearly display the characteristic structural features of octahedral low-spin cobalt(III) species. More specifically, they have the shortest Co−N distances of the series, and, most significantly, the Cpy−Cpy and C−Nchel distances closely resemble those of the uncoordinated neutral ligands. Thus, the oxidation state of the central Co ion is III+ (d6, low-spin), and the oxidation level of the ligands are those of the neutral molecules. This conclusion is borne out by the corresponding qualitative FMO diagrams (Figures S9, S12, and S17), which all contain three doubly occupied and two vacant molecular orbitals of majority 3d orbital character. The Dications. Geometry optimizations for both the highspin (S = 3/2) and low-spin (S = 1/2) electronic configurations of the dications [Co(tpy)2]2+, [Co(bpy)3]2+, and [Co(phen)3]2+ have been performed using the spin-unrestricted Kohn−Sham approach. For all three complexes, the high-spin configuration was found to be energetically the most stable (by 2.7, 6.2, and 5.7 kcal mol−1, respectively), which is consistent with our experimental observations. As is seen experimentally, the average Co−N bond lengths of the S = 3/2 solutions are approximately 0.1 Å longer than those of the S = 1/2 solutions and ∼0.2 Å longer than those of the corresponding trications, which is typical for six-coordinate high- and low-spin CoII (t2g6eg1 and t2g5eg2 in an octahedral ligand field). In addition, the low-spin complexes show significant tetragonal elongation, resulting from Jahn−Teller H


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Inorganic Chemistry

BS(4,1) solutions, respectively, which are 2.33 and 1.65 kcal mol−1 lower in energy than the Ms = 5/2 solution. From this, a BS(4,1) ground state can be inferred. Of the three ligands therein, one possesses Cpy−Cpy and C−Nchel bond lengths of 1.413 and 1.380 Å, respectively, which is suggestive of formulation as a (bpy•)−. The corresponding distances in the other two are very similar at about 1.45 and 1.35−1.36 Å, respectively. This is approximately the arithmetic mean of that expected for a neutral ligand and a radical monoanion, which is suggestive of a single electron delocalized over the two ligands [i.e., {(bpy)2}−].5b,6 On this basis, the electronic structure [CoII(bpy•)2(bpy0)]0 can be forwarded. Very similar results have been obtained for [Co(tpy)2]0, with the Ms = 1/2 and 3/2 states converging to BS(3,2) and BS(4,1) solutions that are 5.56 and 4.08 kcal mol−1 lower in energy than the Ms = 5/2 solution. In the Ms = 3/2 ground state, the average Cpy−Cpy and C−Nchel bond lengths in the tpy ligands of 1.446 and 1.355 Å, respectively, are the same in both spin states and suggestive of a [CoII(tpy•)2]0 valence formulation for the complex. The rather pronounced energy stabilization of the Ms = 3/2 state against the Ms = 1/2 structure is in agreement with the reported effective magnetic moments,11 which suggest an S = 3/2 ground state. The FMO compositions for Ms = 3/2 [Co(tpy)2]0 and [Co(bpy)3]0 (Tables S5 and S6) are very similar, inasmuch as they both display two “doubly occupied” molecular orbitals and three α-spin orbitals of largely 3d character (i.e., CoII), plus two ligand-centered SOMOs, one of which strongly antiferromagnetically couples with the lowest-energy metal-centered SOMO (overlap integral ∼0.5). The difference between the Ms = 3/2 and 1/2 solutions derives from whether the remaining ligandcentered unpaired electron aligns parallel or antiparallel to the metal-centered unpaired electrons. The two remaining available metal-centered unpaired electrons are in orbitals (eg* in octahedral symmetry) that are essentially orthogonal to the π-type ligand SOMOs and, therefore, result in a ferromagnetic interaction. This is reflected in the BS(3,2) solution by a nearzero overlap integral for this interaction (Tables S5 and S6). The resulting picture is most consistent with these neutral complexes possessing S = 3 / 2 [Co I I (tpy • ) 2 ] 0 and

Figure 8. Spin-density plots for the Ms = 1 ground states of [CoI(bpy)3]+ (top) and [CoI(phen)3]+ (bottom) from DFT-B3LYP calculations using optimized geometries. Contour surfaces of the spin density pertain to positive (spin majority, red) and negative (spin minority, yellow) contour values of the spin density of 0.003 spin bohr−3.

The Neutral Complexes. In order to probe their electronic structures, geometry optimizations have been performed for the six-coordinate complexes [Co(tpy)2]0 and [Co(bpy)3]0 in the Ms = 5/2, 3/2, and 1/2 spin states (the latter two were accessed using the flip-spin keyword), and the calculated Co−N, Cpy− Cpy, and C−Nchel bond lengths are summarized in Tables S2 and S3. Whereas the latter complex has been isolated as a black solid, for which the authors first claimed an S = 1/2 ground state and later an S = 3/2 ground state,11 the former has only been generated electrochemically,23 and its ground spin state has yet to be experimentally determined. In the case of [Co(bpy)3]0, geometry optimizations for the Ms = 1/2 and 3/2 states were found to converge to BS(3,2) and

Figure 9. Spin-density plots, relative energies, and intrachelate Cpy−Cpy distances (Å) for the Ms = 5/2, 3/2 and 1/2 ground states of [Co(tpy)2]0 (left, middle, and right, respectively) from DFT-B3LYP calculations using optimized geometries. Contour surfaces of the spin density pertain to positive (spin majority, red) and negative (spin minority, yellow) contour values of the spin density of 0.003 spin bohr−3. I


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Inorganic Chemistry

Figure 10. Spin-density plots, relative energies, and intrachelate Cpy−Cpy distances (Å) for the Ms = 5/2, 3/2, and 1/2 ground states of [Co(bpy)3]0 (left, middle, and right, respectively) from DFT-B3LYP calculations using optimized geometries. Contour surfaces of the spin density pertain to positive (spin majority, red) and negative (spin minority, yellow) contour values of the spin density of 0.003 spin bohr−3.

[CoII(bpy•)2(bpy0)]0 electronic structures. The corresponding spin-density plots for both the S = 1/2 and 3/2 solutions (Figures 9 and 10) reaffirm this conclusion. More specifically, the central Co ions carry ∼2.6 unpaired electrons, and the ligands in [Co(tpy)2]0 carry approximately 1 unpaired electron each, which equates to two (tpy•)− π-radical anions, and in [CoII(bpy•)2(bpy0)]0, approximately two unpaired spins are delocalized over the three bpy ligands. Ab Initio CASSCF/NEVPT2 Calculations of [CoI(tpy0)2][CoI−(CO)4] and Angular Overlap Model (AOM) Analysis. Octahedrally coordinated transition-metal ions with a d8 electron configuration give rise to a 3A(t2g6eg2) ground state and three 3T1,2(t2g5eg3) and 3T1(T2g4eg4) excited states. Transitions from the ground state usually manifest in three absorption bands of low intensity in the d−d absorption range. The complex [Ni(tpy)2]2+ (S = 1) is known to give rise to these transitions9a (see Table 5). In contrast, spectroscopically (cf. Figure 5) the cobalt(I) analogue [CoI(tpy0)2]+ obviously does not belong to this category. Nevertheless, it is useful to first determine the electron states of a hypothetical d8 configuration for [CoI(tpy0)2]+ (S = 1) and analyze in a following step the effect of metal-to-ligand charge transfer on its electronic spectrum and the magnetic properties (zero-field splitting). Calculations of the triplet electronic states of [CoI(tpy0)2]+ from nonrelativistic CASSCF and NEVPT2 calculations show the usual octahedral energy-level diagram (see Table S21) superimposed by splitting due to axial (D2d) and lower orthorhombic symmetry (Figure S22). An ab initio ligandfield analysis confirms the orbital energy order dz2 > dx2−y2 ≫ dxy > dxz, dyz, which is consistent with the tetragonal compression imposed by two rigid tpy0 ligands (see Figure S23 visualizing Co-ligand orbital interactions and AOM orbital energy expressions). Analysis of the CASSCF wave function in conjuction with the CASSCF and NEVPT2 state energies of the formally d8 complex [CAS(8,5) active space] affords characterization of the Co(3d)−N antibonding interactions as strong σ-donor character but only modest net π-donor character. AOM analysis allows one to quantify the Co−N interactions in terms of two parameters eσ and pyridine out-ofplane eπs for the four equatorial [eσ(eq), eπs(eq)] and two axial [eσ(ax), eπs(ax)] N-pyridine donors (Figure S23). Each N donor is involved in strong σ-bonding with two C atoms and, consequently, there is no N orbital for π overlap within each

pyridine plane; the corresponding energy parameter (eπc) is zero. Accounting for a deviation of the Ncentral−Co−Ncentral trans angle from 180° (experiment: 153.2°) and approximating eλ(eq)/eλ(ax) ratios in terms of a [R(ax)/R(eq)]6 distance dependence (R(ax) and R(eq) are the axial and equatorial Co− N bond distances; see Table S22), we find that a best fit of the 5 × 5 AOM ligand-field matrix to the ab initio results yields the Co−N AOM parameters given in Table 8. These parameters Table 8. Metal−Ligand Parameters Quantifying Energies of σ and π Out-of-Plane Antibonding, eσ and eπs, and Racah Parameters B and C (cm−1) from Ligand-Field Analysis of CASSCF and NEVPT2 Energies and in Connection with CASSCF Wave Functions ab initio method

eσ(ax)/ eσ(eq)

eπs(ax)/ eπs(eq)

B

C

standard deviation σAOM/σAILFT (cm−1)

CASSCF CASSCF + NEVPT2

5418/3738 6513/4494

914/631 1025/710

1060 1098

3993 2893

226/297 293/784

characterize the tpy0 ligands as strong σ donors but relatively weak net π donors. This is in contrast to the notion that the tpy0 ligands coordinated to a CoI ion are net π-acceptor ligands [in which case eπ(eq) and eπ(ax) are expected to be negative]. Thus, these ab initio calculations demonstrate the absence of a dominant π-back-donation in [CoI(tpy0)2]+ (S = 1). Metal-to-Ligand Charge Transfer in the Ground State of [CoI(tpy0)2]+ (S = 1). A relativistic spin−orbit-coupling calculation [CAS(8,5) and NEVPT2] of the zero-field parameter D in [CoI(tpy0)2]+ (S = 1) affords a value of −2.68 cm−1, which is 4 times smaller than the one deduced experimentally from VTVH MCD spectroscopy (D = −11.9 cm−1). Therefore, we carried out DFT calculations using the B3LYP functional, which is known to correctly predict the geometry, spin distribution, and energies of multiple-spin complexes. For [CoI(tpy0)2]+, ground states with S = 0, 1, and 2 are possible, whereas for S = 0, both closed- and open-shell singlet states are conceivable. Bond distances and N−Co−N angles of trans-positioned central N-pyridine donors of the two tpy0 ligands for each Ms state (0, 1, and 2) are listed in Table S22, as well as the relative energies and spin populations in Table S23. The Ms = 0 (BS) and Ms = 1 values are computed to J


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Inorganic Chemistry

Figure 11. Corresponding orbitals from spin-unrestricted DFT-B3LYP calculations of [CoI(tpy)2]+ with a Ms = 1 optimized geometry.

Figure 12. Spin-density plots for the Ms = 1 ground states of [CoI(tpy)2]+ (left) and [NiII(tpy)2]2+ (right) from DFT-B3LYP calculations using optimized geometries. Contour surfaces of the spin density pertain to positive (spin majority, red) and negative (spin minority, yellow) contour values of the spin density of 0.003 spin bohr−3.

be lowest in energy, with the Ms = 1 geometry slightly favored. This calculated geometry is in good agreement with the experiment. Spin populations for the Ms = 1 structure show an extension of 0.57 of one spin on Ncentral (0.133) and C (−0.441) on the o- and p-C atoms of the central pyridine (see

Table S23). The same extension is also reflected by the corresponding magnetic orbitals (Figure 11) and spin density (Figure 12). The spin density on the two N donors in the trans position of the central pyridines is of the same (positive) sign as that of the majority spin on Co (i.e., of the spin delocalization K


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Inorganic Chemistry type) and also of the m-C atoms of the central pyridines. As pointed out above, a pronounced negative spin density resides on the o- and p-C atoms. This density is of the spin polarization type (McConnel mechanism I) and indicates a pure spincorrelation phenomenon. It is definitely not of the metal-radical coupling type. Thus, in [CoI(tpy0)2]+ (S = 1), there is no indication for the presence of π-radical anions [(tpy•)−]. We propose the same phenomenon as that from the above DFT results to be operative in [CoI(bpy0)3]+ (S = 1) and [CoI(phen0)3]+ (compare Figure 8). We note here that spin-unrestricted wave functions with well-defined Ms values are not eigenfunctions of the S2 operator. The deviations depend on the value of Ms. As shown in Table S23, this deviation is negligibly small for the Ms = 2 structure, quite significant for Ms = 1, and largest for Ms = 0. It follows that the energy differences between the Ms = 1 and 0 structures (both are of the open-shell type) might be underestimated by the DFT-B3LYP functional. This is in agreement with the magnetic SQUID measurements, which show that there is no thermally accessible (up to 300 K) S = 0 state above the S = 1 ground state. Starting from optimized DFT-B3LYP geometries, we carried out CASSCF/NEVPT2 calculations. In these calculations, the active space has been extended with two σ-bonding ligand-based orbitals (the ones complementary to the 3d−dz2 and dx2−y2 molecular orbitals) and with two empty corresponding π*-ligand orbitals. The results from these calculations, relative energies, and spin populations are listed in Table S23. The results show that the energy of the S = 1 state (now described by a spin eigenfunction) is far below the alternative S = 0 and 2 states with spin populations that are compatible with the DFT-B3LYP results. In a quantitative sense, the rather ionic CASSCF wave function leading to an underestimation of the spin-delocalization part on N [spop(N)] is now close to zero but slightly negative rather than positive), and that of C [spop(C)] is negative but by 0.123 larger in absolute sign than the DFT-B3LYP results (see Table S23). Natural orbital occupation numbers reflect the multireference character of the S = 1 wave function of the ground state, and molecular orbital plots nicely illustrate the chargetransfer ground state of [Co(tpy)2]+ (Figure S24). We can conclude from these results that both DFT-B3LYP and CASSCF/NEVPT2 lead to a consistent description of the ground state of [Co(tpy)2]+. It is not of a pure d8 character but consists of a significant amount (26%) of d7π* and d6π*2 configurations. This is of importance for both the spectroscopy and magnetic anisotropy, i.e., for the origin of D. Metal-to-Ligand Charge-Transfer Effects on the Magnetic Anisotropy and Absorption Spectroscopy of [CoI(tpy0)2]+: The Origin of the Large Negative D. For a tetragonally distorted octahedral d8 complex, second-order perturbation theory yields eq 1, which relates D with the effective cobalt spin−orbit coupling constant ζeff and the energies of the electronic transitions from the 3B2 ground state into the sublevels 3E and 3B1 (symmetry notations for D2d) of the parent octahedral 3T2 term: ⎡ Δ(3B ) − Δ(3E) ⎤ 1 ⎥ D = ζeff 2⎢ ⎣ Δ(3E)Δ(3B1) ⎦

ΔE(3B1) − Δ(3E) 3 = − [eσ (ax) − eσ (eq)] − 4eπs(eq) + eπs(ax) 2

(2)

For an axially compressed geometry [eσ(ax) > eσ(eq) and with 4eπs(eq) > eπs(ax)], this expression predicts a negative D, which is expected to increase with the extent of the distortion. Spin− orbit coupling calculations employing NEVPT2 energy eigenvalues show leading contributions to the value of D = −2.68 cm−1 from the 3T2 excited state (−3.85 cm−1) followed by a significant effect of the 1T2 excited state of opposite sign (1.28 cm−1, not accounted for in eq 1). Taken together, this yields the correct sign of D but largely underestimates its magnitude (D ≈ −12 cm−1). How do charge-transfer states affect D? In order to systematically tackle charge-transfer state effects, specific calculations for each and every state that mixes with the ground state are necessary, a task that is practically not possible. To see roughly the effect of charge-transfer states on D, we have carried out state-averaged CASSCF calculations extending the active space of the five d orbitals with the two corresponding π*-ligand orbitals. Such an averaging over states that differ significantly in their nature yields an unrealistic ground-state spin distribution, which is now biased by the high weight of the metal-to-ligand charge-transfer configurations. In the present case, starting with the 10 S = 1 states of the nominal d8 configuration, a twice larger number of triplets from the singly excited 3d7π* configurations result, afford a computed D = −23.71 cm−1 from the various excited states, and identify, based on the electron configuration of each state, the matrix elements ⟨di|L̂ j|dk⟩ that contributed to the anisotropy. First of all, we note that the ground state is not dominated by d8 but is now solely of the 3d7π*1 charge-transfer type with three electrons on 3dxz,yz and one spin on the π*(e) ligand orbital pair of the same symmetry. The largest contribution to D stems from an excited state 5841 cm−1 above the ground state that differs by the spin distribution within the 3dxz,yz pair; the matrix ⟨dyz|L̂ z|dxz⟩ leads to the large and negative term in Di = −19.15 cm−1. The value of the total D is now 2 times as large as the experimental one, i.e., biased in an opposite direction compared with the one resulting from a CASSCF/NEVPT2 [CAS(8,5)] d-only model. Notwithstanding this numerical discrepancy, we propose that these calculations support the notion that the sign of D is negative. Effect of Metal-to-Ligand Charge Transfer on the Absorption Spectroscopy of [CoI(tpy0)2]+. The significant charge transfer in the ground state of [CoI(tpy)2]+ opens the question of how electronic excitations governing the absorption and MCD spectra are influenced by that effect? It is known that, at present, state-of-art-correlated methods cannot correctly describe charge-transfer excitations, nor is a single determinant DFT calculation able to be applied to the same purpose. We therefore decided to employ TDDFT using the appropriate B3LYP functional. Encouraged by the excellent agreement between the computed and experimental band profiles (Figure 13), we computed and assigned the rich absorption band maxima as deduced by a global fit of the absorption and MCD spectral profiles (Table 9). All excitations are assigned metal-toligand 3d8 → 3d7π*1 charge-transfer transitions. It is impressive that the TDDFT calculations correctly reproduce the order of magnitude of the intensity of the observed transitions. The lowest-energy excitations at 6890, 7604, and 8247 cm−1 have been assigned to e(3d) → π*(e) metal-to-ligand charge-transfer

(1)

The AOM expression for the energy difference Δ(3B1) − Δ(3E) reads L


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Inorganic Chemistry

Figure 13. Experimental (top) and TDDFT-B3LYP-computed (hwhm = 2000 cm−1) absorption band profiles of [CoI(tpy)2]+.

Figure 14. Theoretical TDDFT spectra of [CoI(tpy)2]+ and [NiII(tpy)2]2+. The inset shows the region of d−d absorptions for [NiII(tpy)2]2+ covered by metal-to-ligand charge-transfer transitions in the case of [CoI(tpy)2]+.

transitions. In D2d symmetry, these transitions give rise to four closely spaced nondegenerate levels that can mix electronically (via the lower symmetry) or vibronically, leading to the intensities of the simulated Gaussian curves. Likewise, closely spaced electronic levels observed at 28010, 28991, and 29980 cm−1 may gain their intensity from such mixings. Finally, taking such effects into account, we find a clear correlation between experimentally determined and theoretically computed oscillator strengths. Electronic Structure of [CoI(tpy0)2]+ in the Context of Classical Ligand-Field Theory: Comparison with the Isoelectronic [NiII(tpy)2]2+. It is illuminating to compare the electronic structures and spectra of [CoI(tpy)2]+ with [NiII(tpy)2]2+. Spin-density plots and theoretical TDDFT spectra for the two complexes are depicted in Figures 12 and 14, respectively, while spin populations on Co and Ni, the Ndonor atoms, and the residue reflecting spin delocalization over the C/H atoms are included in Table 10. On the basis of the values of the σ- and π-antibonding analysis, we can deduce that from the perspective of classical ligand-field theory the two complexes are very similar. However, on the basis of the spin-

Table 10. Spin Populations n for [CoI(tpy)2]+ and [NiII(tpy)2]2+ from Ms = 1 Ground-State DFT-B3LYP Calculations Using Optimized Geometries n(M) n(N) n(residue)

[CoI(tpy)2]+

[Ni(tpy)2]2+

2.26 0.06 −0.32

1.62 0.35 0.03

density plot and Table 10, we can conclude that there are important differences between the two complexes. While [Ni(tpy)2]2+ shows the typical pattern of a Werner-type complex with spin delocalization on the N donors,6d we find considerable spin density of alternating sign on the C atoms of the pyridine ligands trans to each other in [CoI(tpy0)2]+. This unusual electronic structure leads to dramatic differences in the spectroscopic behavior. While the nickel complex shows the typical pattern of d−d transitions of very low intensity in the

Table 9. Electronic Transitions As Revealed by a Global Fit of the MCD and Absorption Spectra of [Co(tpy)2]+ and Their Assignment Based on TDDFT-B3LYP Calculations Using a DFT-B3LYP-Optimized Geometry ΔE(MCD+ABS) exp. 6980 7604 8247 9250 10728

15491 16980 18800 20186 22232 24349 26200 28010 28991 29980

fexpεσ2π, M−1 cm−2 12.798 4.191 18.537 11.115

8.506 5.385 15.792 5.001 17.157 27.711 14.889 9.255 42.198

ΔE(TDDFT)

f TDDFT

6476 6784 8052 10500 10657 12791 13100 16256 17497 18008 20314 23351 24264 24426 28080 28614 29161

2 × 10−6 0.002 0.070 0.021 0.019 0.009 0.006 0.012 0.007 0.016 0.011 0.011 0.038 0.028 0.028 0.042 0.051

M

assignment B2 → 3B1,2; 3A1,2[e(3d)→e(π*)]β

3

3

B2 B2 3 B2 3 B2 3 B2 3 B2 3 B2 3 B2 3 B2 3 B2 3 B2 3 B2 3 B2 3 B2 3

→ → → → → → → → → → → → → →

3

A2[e(3d)→a2(π*)]β E[e(3d)→b1,a2(π*)]β 3 E[e(3d)→b1(π*)]β 3 E[e(3d)→a2(π*)]β 3 A1[b1(3d)→a2(π*)]β 3 E[e(3d)→b1(π*)]β 3 E[e(3d)→a2(π*)]β 3 E[e(3d)→a2(π*)]β 3 A1[b1(3d)→a2(π*)]β 3 E[b1(3d)→e(π*)]β 3 B1,2; 3A1,2[e(3d)→e(π*)]β 3 B1,2; 3A1,2[e(3d)→e(π*)]α 3 E[b1(3d)→e(π*)]α 3 B1,2; 3A1,2[e(3d)→e(π*)]α 3


Article

Inorganic Chemistry

of their high intensities, Kobayashi et al. have tentatively assigned the electronic transitions of [CoI(bpy0)3]+ as metal-toligand charge-transfer bands.10 We have shown here that Kobayashi’s proposal10 is essentially correct and, most importantly, the unusual features of the cobalt(I) spectra are not simply due to ground-state π-back-bonding from the metal to π* LUMOs of the ligands. It is noteworthy that in an elegant paper from 1970 Fitzgerald et al. compared the 1H NMR spectra of isoelectronic [NiII(bpy0)3]2+ and [CoI(bpy0)3]+ and concluded that “both σand π-delocalization mechanisms contribute to the observed contact shifts in these CoI complexes and that the dominant πdelocalization mechanism involves a direct overlap of the metal 2e orbitals with the highest filled π-symmetry ligand orbitals”.28 Experimental data for the neutral six-coordinate complexes [Co(tpy)2]0 and [Co(bpy)3]0 are scarce and are largely limited to syntheses and electrochemical identification in solution.11,23 In contrast, along with the corresponding monocationic and dicationic complexes [Co(pdi)2]+ (S = 1) and [Co(pdi)2]2+ (S = 1/2),29 where pdi represents generic N-aryl- or N-alkylsubstituted tridentate bis(imino)pyridine ligands (Chart 2), two

visible region followed by metal-to-ligand charge-transfer transitions above 30000 cm−1, the latter transitions are shifted down into the visible and near-IR regions in the spectrum of [CoI(tpy0)2]+, thus covering the region of d−d absorptions. The given changes correlate with the expected shift of the electronic states due to the d configuration to higher energy upon decreasing charge from 2+ to 1+ on going from [NiII(tpy)2]2+ to [CoI(tpy)2]+.

■

DISCUSSION The high-resolution crystal structure determinations for complexes 3, 4, 7, and 9 presented herein containing the monocations [CoI(tpy0)2]+, [CoI(tbpy0)3]+, and [CoI(phen0)3]+ exhibit Cpy−Cpy and C−Nchel bond lengths identical, within error limits (3σ) of only ±0.01 Å, with those reported for the uncoordinated molecules tpy0,12,13 bpy0,22 and phen0.16 This indicates that the redox level of the coordinated ligands is neutral and, by extension, the Co ions possess a 1+ oxidation state. DFT calculations support this electronic structure assignment. It is also informative to compare the structural data for the cobalt(I) complexes with those of the isoelectronic nickel(II) complexes [Ni I I (tpy 0 ) 2 ] 2 + , [NiII(bpy0)3]2+, and [NiII(phen0)3]2+ (Table S24).23−26 As a search of the Cambridge Crystallographic Database reveals, these dications have been structurally characterized many times in the past, and structures for the aforementioned purpose have been selected based on the following criteria: (1) no static disorder problems are present; (2) data collection was preferably performed at cryogenic temperatures (100−150 K); (3) the estimated standard deviations (σ) for the C−C and C−N bond lengths should not exceed 0.005 Å; (4) the R factor should be less than 5%. The experimentally observed average Ni−N, Cpy−Cpy, and C−Nchel bond lengths in the selected nickel(II) structures (Table S24) match exactly, within an error limit of ±0.01 Å, those of the cobalt(I) complexes and those reported for the uncoordinated ligands.12,16,22 Thus, the CoI monocations and corresponding NiII dications are isostructural and isoelectronic. An important observation in this study pertains to the inescapable fact that the neutral ligands in both sets of complexes do not display (within an error limit of ±0.01 Å) a significant structural perturbation because of metal-to-ligand πback-donation, a conclusion that contrasts with the discussion of the disordered structure of [Co(bpy)3]+ in ref 7. The same holds true for the corresponding CoII dications in both their high- and low-spin forms (S = 3/2 and 1/2, respectively).20 Gratifyingly, all aspects of the experimentally observed structures of the cobalt(I) and cobalt(II) complexes are almost perfectly reproduced in the DFT-calculated geometry-optimized structures. The above discussion renders the striking differences between the solution-state electronic spectra of the three monocationic cobalt(I) complexes and the isoelectronic dicationic nickel(II) complexes9 (Table 5) particularly intriguing. The most salient of these differences are the enormously enhanced intensities of the bands in the visible and near-IR regions in the cobalt(I) complexes, which are absent in the spectra of the nickel(II) complexes. As pointed out above, the electronic spectra of the nickel(II) species have throughout the literature been assigned, quite rightly, as Laporte-forbidden d → d transitions of an S = 1 d8 electronic configuration in a pseudooctahedral NiN6 coordination environment. Clearly, this cannot be true for the corresponding cobalt(I) species. Because

Chart 2. pdi Ligands

neutral [Co(pdi)2]0 (S = 3/2) complexes have been fully structurally and spectroscopically characterized.30 These neutral six-coordinate complexes, [Co(L1)2]0 and [Co(L2)2]0, possess S = 3/2 ground states that are fully populated in the temperature range 4−300 K. The ligand bond distances in the crystal structures of both complexes are indicative of the presence of two π-radical anions, (L1•)− and (L2•)−, thereby rendering the oxidation state of the Co ion as II+. On the basis of the Co−N bond distances, it can be concluded that the CoII ions therein are high-spin. BS DFT calculations for the [Co(pdi)2]0 species were consistent with the experiment and yielded results analogous to those detailed here for [Co(tpy)2]0 and [Co(bpy)3]0. More specifically, the unpaired spin of one of the two π-radical ligands antiferromagnetically couples to the “t2g” SOMO (dyz) of the high-spin CoII ion, and because of their orthogonality, the second ligand-centered unpaired electron couples in a ferromagnetic fashion to the remaining metal SOMOs (dz2 and dx2−y2), thereby affording the observed quartet ground state. However, it was suggested that this description is a “gross oversimplification” of the true ground state, which results from the failure of the BS wave function to accurately describe the multireference character of such systems, and that a more appropriate model involves a superexchange interaction of the two ligand-centered unpaired spins mediated by a doubly occupied orbital of the Co center.30 Last, it is interesting to note that the electronic structures of the two octahedral neutral iron(II) complexes [FeII(pdi•)2]0 and [FeII(tpy•)2]0, which are isoelectronic with the corresponding Co monocations, are also very similar to both, possessing S = 1 ground states attained by strong intramolecular antiferromagnetic coupling between the N


Article

Inorganic Chemistry

powdered solid samples were recorded in a 1 T magnetic field using a SQUID magnetometer (MPMS quantum design). MCD Spectroscopy. MCD experiments at cryogenic temperatures were carried out in a frozen butyronitrile solution (0.2 mM [CoI(tpy0)2][Co(CO)4]) with an Olis DSM17 CD spectrapolarimeter, while the sample was placed in an Oxford Spectromag SM4000 cryostat. Temperatures were varied in the range from 2 to 60 K and spectral energies from 5000 cm−1 (2000 nm) to 35000 cm−1 (285 nm). Selected spectra were subsequently simulated by using Gaussian line shapes in order to determine the band positions, widths, and intensities in comparison with the corresponding parameters for the absorption spectrum, recorded in a fluid butyronitrile solution. At selected MCD band positions, we carried out VTVH MCD experiments in which the MCD intensity is recorded as a function of variable temperature and variable magnetic field. Isotemperature curves are presented as a plot of the MCD intensity versus βB/kT, where β is the Bohr magneton, k is Boltzmann’s constant, B is the strength of the applied magnetic field, and T is the absolute temperature. The MCD intensities show strong C-term behavior because of the paramagnetism of the ground state of [CoI(tpy0)2][Co(CO)4] (S = 1), which were simulated by using the spinHamiltonian expressions worked out by Neese and Solomon.18

spins of two ligand π-radical anions and two “t2g” SOMOs of the central high-spin FeII(d6) ions.31,32

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CONCLUSIONS First, high-resolution X-ray crystal structures of salts of the monocations [CoI(tpy0)2]+, [CoI(bpy0)3]+, and [CoI(phen0)3]+ clearly show that these species contain neutral ligands tpy0, bpy0, and phen0 and, by extension, a central CoI ion (d8, S = 1). Although DFT calculations indicate a degree of metal-to-ligand π-back-bonding in these monocations, it is clearly minor because none of the ligands exhibit structural perturbations characteristic of its presence. Indeed, the observed structural parameters closely resemble, in all aspects, those of their isoelectronic dicationic counterparts [Ni II (tpy 0 ) 2 ] 2+ , [NiII(bpy0)3]2+, and [NiII(phen0)3]2+. Second, DFT calculations indicate that the neutral sixcoordinate species [CoII(tpy•)2]0 and [CoII(bpy•)3]0 possess S = 3/2 ground states, which are attained via intramolecular antiferromagnetic coupling of one ligand-centered unpaired spin with the “t2g” SOMO of the high-spin CoII ion and parallel alignment of the remaining ligand-centered unpaired spin with those in the Co “eg” orbitals. This outcome is fully consistent with the Goodenough−Kanamori rules and is similar to that experimentally observed for the [Co(pdi)2]0 complexes, although in the latter case, it was suggested to arise from a superexchange interaction between the two ligand-centered unpaired spins mediated by a doubly occupied t2g orbital. In summary, we have demonstrated that all redox processes in the electron-transfer series [Co(tpy)2]n+, [Co(bpy)3]n+, and [Co(phen)3]n+ (n = 3, 2, 1) are metal-centered. In other words, the trications, dications, and monocations contain neutral ligands and CoIII (low-spin d6), CoII (high- and low-spin d7), and CoI (high-spin d8), respectively. Only upon reduction to neutral species, which contain two π-radical anions and a central CoII (high-spin d7) ion, is a ligand-centered reduction observed. While [CoI(tpy)2]+ and [NiII(tpy0)2]2+ both possess a d8 ground state, there are large differences between their lowest excites states: metal-to-ligand charge transfer in the former case and classical Werner-type d−d transitions in the latter. This difference is due to the shift of the electronic multiplets split out from the d8 configuration to higher energy upon decreasing charge from 2+ for [NiII(tpy)2]2+ to 1+ for [CoI(tpy)2]+. These changes lower the energies of the metal-to-ligand [CoII(tpy•) (tpy0)]+ charge-transfer states and lead to an increase of the negative zero-field-splitting parameter D beyond the limits seen for Werner-type complexes.

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γ Δε = E 4πS

π

2π

∫0 ∫0 ∑ Ni(lx⟨Sx̂ ⟩Myzeff + ly⟨Sŷ ⟩Mzxeff + lz⟨Sẑ ⟩Mxyeff ) i

sin θ dθ Here, Δε/E is the MCD intensity, γ is a collection of constants, S is the total spin of the ground state, Ni is the Boltzmann population of the ith magnetic sublevel of the electronic ground state, lx,y,z are the directional cosine values of the angles between the magnetic field and molecular coordinate system, and ⟨Ŝx,y,z⟩ are the expectation values of the x, y, and z components of the spin operator Ŝ over the ith magnetic eigenstate, respectively. The Meff vw (v, w = x, y, z) values are effective transition dipole moment products, summarizing the effects of spin− orbit coupling that lead to finite MCD transition probabilities. X-ray Crystallographic Data Collection and Refinement of the Structures. Single crystals of complexes 1−9 were coated with perfluoropolyether, picked up with nylon loops, and mounted in the nitrogen cold stream of the diffractometer. Graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) from a molybdenum-target rotatinganode X-ray source was used throughout. Final cell constants were obtained from least-squares fits of several thousands of strong reflections. Intensity data were corrected for absorption using intensities of redundant reflections with the program SADABS.36 The structures were readily solved by Patterson methods and difference Fourier techniques. The Siemens ShelXTL37 software package was used for solution and rendering of the structures. ShelXL9738 was used for refinement. All non-H atoms were anisotropically refined, and H atoms were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. The checkCIF files for compounds 5 and 6 report B-alerts concerning some missing reflections at very low angles. These reflections were removed for the refinements because they were shaded by the beam stopper. Calculations. All DFT calculations were performed using version 3.0 of the ORCA software package.39 The geometries of all complexes were optimized, in redundant internal coordinates without imposing geometry constraints, and all subsequent single-point calculations were performed at the B3LYP functional.40 In all calculations, the def2TZVP basis set with f-polarization functions was applied to the Co center and the N atoms were coordinated to it, whereas the C and H atoms were described using the slightly smaller def2-SV(P) basis set.41 Auxiliary basis sets, used to expand the electron density in the calculations, were chosen to match the orbital basis sets.42 The RIJCOSX approximation was used to accelerate the calculations.43 The authenticity of each converged structure was confirmed by the absence of imaginary vibrational frequencies. Throughout this study, our computational results are described using the BS approach.19 As explained elsewhere,44 the nature of the

EXPERIMENTAL SECTION

Synthesis of Compounds. Unless stated otherwise, all syntheses were carried out in the absence of water and dioxygen, under an argon atmosphere, using standard Schlenk techniques or in a glovebox. All ligands, the starting materials [Co(H2O)6](BF4)2 and Co2(CO)8, and the reductant 10% sodium amalgam were all purchased from commercial vendors and used without purification. The starting materials [CoII(tpy0)2](PF6)2 and [Co2(nbd)2(CO)4] (nbd = 2,5norbornadiene) were prepared using published procedures.33,34 Complexes 1 and 5 were prepared using direct adaptations of published procedures.35 Details of the procedures for the syntheses of all other complexes are provided in the Supporting Information. Physical Measurements. Electronic spectra were recorded using a PerkinElmer Lambda 19 double-beam spectrophotometer (33300− 6250 cm−1). Variable-temperature (4−300 K) magnetization data of O


Article

Inorganic Chemistry solution is investigated from the corresponding orbital transformation, which from the corresponding orbital overlaps displays whether the system should be described as a spin-coupled or a closed-shell solution. Nonrelativistic energy levels and wave functions for [CoI(tpy0)2]+ were computed using the CASSCF method,45 averaging over the electron densities of all considered states and taking an active space with eight electrons distributed over five 3d molecular orbitals [CAS(8,5)]. To account for dynamical correlation, N-electron valence perturbation theory to second order (NEVPT2) was used as implemented in the program ORCA. Linear expressions for the ligand-field Hamiltonian were derived using the program AOMX.46

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(4) Irwin, M.; Doyle, L. R.; Krämer, T.; Herchel, R.; McGrady, J. E.; Goicoechea, J. M. Inorg. Chem. 2012, 51, 12301−12312. (5) (a) Scarborough, C. C.; Wieghardt, K. Inorg. Chem. 2011, 50, 9773−9793. (b) Scarborough, C. C.; Sproules, S.; Weyhermüller, T.; DeBeer, S.; Wieghardt, K. Inorg. Chem. 2011, 50, 12446−12462. (c) Bowman, A. C.; Sproules, S.; Wieghardt, K. Inorg. Chem. 2012, 51, 3707−3717. (6) (a) Wang, M.; Weyhermüller, T.; England, J.; Wieghardt, K. Inorg. Chem. 2013, 52, 12763−12776. (b) Bowman, A. C.; England, J.; Sproules, S.; Weyhermüller, T.; Wieghardt, K. Inorg. Chem. 2013, 52, 2242−2256. (c) Wang, M.; England, J.; Weyhermüller, T.; Wieghardt, K. Inorg. Chem. 2014, 53, 2276−2287. (d) Wang, M.; England, J.; Weyhermüller, T.; Wieghardt, K. Eur. J. Inorg. Chem. 2015, 2015, 1511−1523. (7) Szalda, D. J.; Creutz, C.; Mahajan, D.; Sutin, N. Inorg. Chem. 1983, 22, 2372−2379. (8) The electronic spectrum of [Co(bpy)3]+ has been reported in refs 2b (incomplete) and 5 (complete) and in: Schwarz, H. A.; Creutz, C.; Sutin, N. Inorg. Chem. 1985, 24, 433−439. (9) Electronic spectra of [Ni(tpy)2]2+: (a) Henke, W.; Reinen, D. Z. Anorg. Allg. Chem. 1977, 436, 187−200. (b) Baker, A. T.; Craig, D. C.; Rae, A. D. Aust. J. Chem. 1995, 48, 1373−1378. (c) Prasad, R.; Scaife, D. B. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 373−386. [Ni(bpy)3]2+: (d) Hancock, R. D.; McDougall, G. J. J. Chem. Soc., Dalton Trans. 1977, 67−70. (e) Palmer, R. A.; Piper, T. S. Inorg. Chem. 1966, 5, 864−878. (f) Vander Griend, D. A.; Bediako, D. K.; DeVries, M. J.; DeJong, N. A.; Heeringa, L. P. Inorg. Chem. 2008, 47, 656−662. (g) Robinson, M. A.; Curry, J. D.; Busch, D. H. Inorg. Chem. 1963, 2, 1178−1182. [Ni(phen)3]2+: (h) Basolo, F.; Hayes, J. C.; Neumann, H. M. J. Am. Chem. Soc. 1953, 75, 5102−5106. (i) Jørgensen, C. K.; et al. Acta Chem. Scand. 1955, 9, 1362−1377. (10) Kaizu, Y.; Torii, Y.; Kobayashi, H. Bull. Chem. Soc. Jpn. 1970, 43, 3296−3297. (11) (a) Herzog, S.; Klausch, R.; Lantos, J. Z. Chem. 1964, 4, 150. (b) Wulf, E.; Herzog, S. Z. Anorg. Allg. Chem. 1972, 387, 81−90. (12) Bowes, K. F.; Clark, I. P.; Cole, J. M.; Gourlay, M.; Griffin, A. M. E.; Mahon, M. F.; Ooi, L.; Parker, A. W.; Raithby, P. R.; Sparkes, H. A.; Towrie, M. CrystEngComm 2005, 7, 269−275. (13) Uncoordinated (p-tolyl-tpy0) data are taken from: (a) Liu, H.G.; Qiu, Y.-C.; Wu, J.-Z. Acta Cryst. Sect. E-Struct. Rep. Online 2007, 63, o4816-1−o4876-10. (b) Beves, J. E.; Chwalisz, P.; Constable, E. C.; Housecroft, C. E.; Neuburger, M.; Schaffner, S.; Zampese, J. A. Inorg. Chem. Commun. 2008, 11, 1009−1011. (c) Messina, M. T.; Metrangolo, P.; Resnati, G.; Quici, S.; Manfredi, A.; Pilati, T. Supramol. Chem. 2001, 12, 405−410. (14) Indumathy, R.; Radhika, S.; Kanthimathi, M.; Weyhermüller, T.; Nair, B. U. J. Inorg. Biochem. 2007, 101, 434−443. (15) (a) Jun, Q.; Zhang, C. Acta Crystallogr., Sect. E: Struct. Rep. Online 2010, 66, m12. (b) Harrowfield, J. M.; Koutsantonis, G. A.; Skelton, B. W.; Strong, A. J.; White, A. H. Z. Z. Anorg. Allg. Chem. 2010, 636, 808−817. (16) (a) Frohn, H.-J.; Hirschberg, M. E.; Westphal, U.; Flörke, U.; Böse, R.; Bläser, D. Z. Anorg. Allg. Chem. 2009, 635, 2249−2257. (b) Mahapatra, A. K.; Sahoo, P.; Hazra, G.; Goswami, S.; Fun, H.-K. J. Lumin. 2010, 130, 1475−1480. (17) [Ni(tpy)2][TCNQ]2 has D = +3.34 cm−1 and g = 2.156: Alonso, C.; Ballester, L.; Gutierrez, A.; Perpinan, M. F.; Sanchez, A. E.; Azcondo, M. T. Eur. J. Inorg. Chem. 2005, 2005, 486−495. (18) Neese, F.; Solomon, E. I. Inorg. Chem. 1999, 38, 1847−1863. (19) (a) Noodleman, L. J. Chem. Phys. 1981, 74, 5737−5743. (b) Noodleman, L.; Norman, J. G.; Osborne, J. H.; Aizman, A.; Case, D. J. Am. Chem. Soc. 1985, 107, 3418−3426. (c) Noodleman, L.; Davidson, E. R. Chem. Phys. 1986, 109, 131−136. (d) Noodleman, L.; Case, D. A.; Aizman, A. J. J. Am. Chem. Soc. 1988, 110, 1001−1005. (e) Noodleman, L.; Peng, C. Y.; Case, D. A.; Mouesca, J. M. Coord. Chem. Rev. 1995, 144, 199−244. (20) Spin transition in [CoII(tpy0)2]2+ followed by X-ray crystallography: (a) Kilner, C. A.; Halcrow, M. A. Dalton Trans. 2010, 39, 9008−9012. (b) Figgis, B. N.; Kucharski, E. S.; White, A. H. Aust. J.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02415. Summary of metrical parameters for [Co(bpy)3]Cl taken from ref 5, syntheses of complexes, temperature dependence of the magnetic moments for 5, 6, and 8, further information regarding the DFT calculations, including tables of atomic coordinates, bond distances, and energies, additional Mulliken spin-density plots, and qualitative FMO diagrams (PDF) Crystallographic information in CIF format for complexes 1−9 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address

‡ Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.E. is thankful to the Max Planck Society for a postdoctoral fellowship and to Andreas Göbels for technical assistance. DEDICATION Dedicated to Prof. Dr. Dirk Reinen, Philipps-Universität Marburg, Marbuerg, Germany, on the occasion of his 85th birthday.

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REFERENCES

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