Nickel(II) complexes stabilized by bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine: Synthesis and characterization of complexes stabilized by a hydrogen bonding network

Abstract

Hydrogen bonds in metalloproteins are key in directing reactivity yet to be achieved in synthetic systems. We have been developing a synthetic system that uses hydrogen-bonding interactions to modulate the secondary coordination around a transition metal ion. This was accomplished with the ligand bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine (H₂pmb), which contains two carboxyamido units appended from pyridine rings. Several nickel complexes were prepared and structurally characterized. In particular, we found that the appended carboxyamido groups either provide intramolecular H-bond donors or can be converted to bind directly to a metal center. We established that the complex Ni^IIH₂pmb(Cl)₂ can be sequentially deprotonated with potassium tert-butoxide, causing coordination of the carboxyamido oxygen atoms and concomitant loss of the chloro ligands. The chloro ligands were also removed with silver(I) salts—in the presence of acetate ions, the complex Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) was isolated, in which an intramolecular H-bonding network occurs between the H₂pmb ligand and the coordinate acetato ligands.

1. Introduction

Hydrogen bonds (H-bonds) within the secondary coordination sphere are important in controlling the structure and function of metal complexes.ⁱ This is typified within the active sites of metalloproteins, wherein protein-derived H-bonding networks affect a variety of structural and physical properties that are ultimately correlated to activity.ⁱⁱ These findings have inspired the development of synthetic systems that attempt to create similar H-bonding networks within the secondary coordination sphere of transition metal complexes.ⁱⁱⁱ The most common designs utilize multidentate ligands that contain a metal ion-binding pocket and appended functional groups that can position H-bond donors/acceptors proximal to bound metal ions. In addition, most of these ligands form relatively rigid cavities in order to promote formation of intramolecular H-bonds. Several systems have featured the carboxyamidopyridyl moiety, which has been successful in forming intramolecular H-bonds to other coordinated species (Fig. 1A). First introduced by Masuda within a symmetrical tripodal ligand,^iv several derivatives now exist that are being used to probe H-bonding effects.^v,vi

Fig. 1.

The carboxyamido unit used to form an intramolecular H-bond (A), to form a bidentate ligand (B), and the H₂pmb ligand used in this study (C).

In the structure depicted in Fig. 1A, the pyridyl groups coordinate to the metal center and the carboxyamido portion serves as the H-bond donor to another ligand. However, another binding mode for the carboxyamidopyridyl moiety is also possible, in which the pyridyl nitrogen and the carbonyl oxygen atoms both bind to a metal ion (Fig. 1B).^5,6,vii This bidentate chelation could be a convenient means for controlling access to a metal ion and modulating the properties of metal complexes. Toward these goals we have been exploring the Ni(II) coordination chemistry with the ligand bis(N-pivalamido-2-pyridylmethyl)benzylamine (H₂pmb) that contains two carboxyamidopyridyl units (Fig. 1C). This report describes the syntheses, molecular structures, and physical properties for a series of Ni(II) complexes with this ligand that illustrate the utility of incorporating carboxyamidopyridyl units into the design of metal complexes.

2. Experimental

2.1. Preparative Methods and Syntheses

All reagents were purchased from commercial sources and used as received, unless otherwise noted. Solvents were purged with argon and dried over columns containing Q-5 (supported redox catalyst for dioxygen removal) and molecular sieves. Syntheses of air-sensitive metal complexes were conducted in a Vacuum Atmospheres, Co. (Hawthorne, CA) drybox under an argon atmosphere. The H₂pmb ligand was prepared following a literature procedure.^viii Elemental analyses for Ni^IIH₂pmb and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) consistently gave low values for the percentages of carbon and nitrogen. We thus report high-resolution mass spectral data for these complexes.

2.2. Preparation of Complexes

2.2.1. {(Bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine)(dichloro)}nickelate(II) (Ni^IIH₂pmbCl₂)

A solution of H₂pmb (50 mg, 0.103 mmol) in 3 mL of methanol was treated with NiCl₂·6H₂O (25 mg, 0.052 mmol). The solution turned dark green and was stirred for 10 minutes. Orange crystals were obtained through diffusion with diethyl ether into methanol (46 mg, 72%). Found: C, 55.44; H, 6.22; N, 11.24%. Ni^IIH₂pmbCl₂·1/2MeOH (C_29.5H₃₉N₅O_2.5NiCl₂) requires C, 55.95; H, 6.21: N, 11.06%. FT-IR/cm⁻¹ (Nujol): ν(NH) = 3297; λ_max (THF, nm (ε, M⁻¹cm⁻¹)) 20 °C: 461 (124), −100 °C: 429 (89), λ_max (Acetone, nm (ε, M⁻¹cm⁻¹)) 20 °C: 457 (101) and −90 °C: 417 (53), λ_max (MeCN, nm, (ε, M⁻¹cm⁻¹)) 20 °C: 452 (88) and −40 °C: 446 (79), λ_max (DCM, nm (ε, M⁻¹cm⁻¹)) 20 °C: 452 (100) and −80 °C: 442 (42), λ_max (MeOH, nm, (ε, M⁻¹cm⁻¹)) 20 °C: 597 (11) and −90 °C: 555 (32); μ_eff = 2.86.

2.2.2. {(Bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine)(diacetato)}nickelate(II) (Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc))

A solution of Ni^IIH₂pmbCl₂ (58 mg, 0.081 mmol) in 3 mL of 1:1 MeCN:THF was treated with Me₄NOAc (22 mg, 1.6 mmol). The orange solution was stirred for 15 minutes, then AgBF₄ was added and allowed to stir for an additional 10 min then filtered over Celite to remove AgCl. The volitles were removed and the resulting blue solid was redissolved in THF. Blue crystals were obtained via pentane diffusion into THF solution (45 mg, 84%). FT-IR/cm⁻¹ (Nujol): ν(NH) 3406, 3315; λ_max (THF, nm (ε, M⁻¹cm⁻¹)) 20 °C: 651 (16) and −100 °C: 639 (39), λ_max (MeCN, nm (ε, M⁻¹cm⁻¹)) 20 °C: 656 (20) and −40 °C: 645 (26), λ_max (Acetone, nm (ε, M⁻¹cm⁻¹)) 20 °C: 646 (19) and −80 °C: 645 (33); μ_eff = 2.84; HRMS (ES+): Exact mass calcd for C₃₁H₄₀N₅NiO₄ [M − OAc], 604.2433. Found 604.2408.

2.2.3. {(Bis[N-(6-pivalamido-2-pyridylmethyl)][O-carboxyamido]benzylamine])(chloro)} nickelate(II) (Ni^IIHpmbCl)

Under argon, a solution of Ni^IIH₂pmbCl₂ (50 mg, 0.081 mmol) in 3 mL of THF was treated with KOBu^t (9.1 mg, 0.081mmol) and was allowed to stir for 20 minutes. Green crystals were obtained via pentane diffusion into a THF solution (46 mg, 98%). FT-IR/cm⁻¹ (Nujol): ν(NH)3264, 3180; λ_max (THF, nm (ε, M⁻¹cm⁻¹)) 20 °C: 655 (35) and −100 °C: 639 (39). λ_max (DCM, nm (ε, M⁻¹cm⁻¹)) 20 °C: 635 (25) and −90 °C: 635 (44); μ_eff = 2.88; HRMS (APCI): Exact mass calcd for C₂₉H₃₆N₅NiO₂Cl [M + H], 580.1989. Found 580.1995.

2.2.4. {(Bis[N-(6-pivalamido-2-pyridylmethyl)-O-carboxyamido]benzylamine])}-nickelate(II) (Ni^IIpmb)

Under argon, a solution of Ni^IIH₂pmbCl₂ (25 mg, 0.041 mmol) in 3 mL of 1:1 THF:MeCN was treated with KOBu^t (9.1 mg, 0.081 mmol) and was allowed to stir for 20 min. Volatiles were removed under reduced pressure. The resulting green residue was dissolved in THF and crystals were obtained via pentane diffusion (13 mg, 59%). Found: C, 63.77; H, 7.58; N, 11.16%. Ni^IIpmb·THF (C₃₃H₄₃N₅O₃Ni) requires C, 64.30; H, 7.03: N, 11.36%. FT-IR/cm⁻¹ (Nujol): ν(NH) 3347; λ_max (THF, nm (ε, M⁻¹cm⁻¹)) 20 °C: 446 (29); μ_eff = 2.76; HRMS (ES+): Exact mass calcd for C₂₉H₃₅N₅NiO₂ [M + H], 544.222. Found 544.2215.

2.3. Physical Methods

NMR spectra were obtained on either Bruker DRX-400 MHz or Bruker Advance 500 MHz spectrometers. NMR solvents were used as received. Electronic spectra were recorded with an Agilent 8453 spectrophotometer. FT-IR spectra were collected on a Varian 800 Scimitar Series FT-IR spectrophotometer and are reported in wavenumbers. Mass spectra were recorded on a Waters LCT Premier mass spectrometer operated in EI or APCI mode. Magnetic moments were determined using Evan’s method in DMSO.^ix Elemental analyses were performed on a Perkin-Elmer 2400 Series II CHNS analyzer.

2.4.1. Crystallographic Structural Determination

Intensity data for the complexes were collected using a Bruker SMART APEX II diffractometer using graphite-monochromated Mo Kα radiation (λ=0.71073 Å). The SMART^x program package was used to determine the unit-cell parameters and for data collection (25 s/frame scan time) for a sphere of diffraction data. The raw frame data was processed using SAINT^xi and SADABS^xii to yield the reflection data files. Subsequent calculations were carried out using the SHELXTL^xiii program. The structure was solved by direct methods and refined on F² by full-matrix least-squares techniques. The analytical scattering factors^xiv for neutral atoms were used throughout the analysis.

2.4.1.1. Ni^IIH₂pmbCl₂

A green crystal of approximate dimensions 0.15 × 0.21 × 0.23 mm was used, giving diffraction symmetry of 2/m and systematic absences consistent with the monoclinic space group P2₁/c. The structure was solved by direct methods and refined on F² by full-matrix least-squares techniques. Hydrogen atoms associated with atoms C(20), C(21), and C(22) were included using a riding model.

2.4.1.2 Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc)·CH₃CN

A blue crystal of approximate dimensions 0.07 × 0.26 × 0.27 mm was employed, giving diffraction symmetry of 2/m and the systematic absences consistent with the monoclinic space groups P2/ and Pc. It was later determined that space group Pc was correct. Hydrogen atoms were either located from a difference-Fourier map and refined (x,y,z and Uiso) or were included using a riding model. There were two molecules of the formula-unit and two molecules of acetonitrile solvent present. Attempts to solve the structure in space group P2/c were unsuccessful. Analysis of the data using the PLATON^xv program package suggested the space group assignment was correct. The structure was refined using the TWIN instruction^xvi, BASF = 0.372.

2.4.1.3. Ni^IIHpmbCl

A gold-green crystal of approximate dimensions 0.15 × 0.30 × 0.35 mm gave diffraction symmetry of 2/m and the systematic absences consistent with the monoclinic space group P2₁/n that was later determined to be correct. Hydrogen atoms were included using a riding model. There were two molecules of the formula-unit present (Z = 8).

2.4.1.4. Ni^IIpmb(THF)

The crystal of this complex isolated contained one coordinated THF molecule. A colorless crystal of approximate dimensions 0.13 × 0.13 × 0.22 mm was mounted on a glass fiber and transferred to the diffractometer. There were no systematic absences or any diffraction symmetry other than the Friedel condition. The centrosymmetric triclinic space group P1̄ was assigned and later determined to be correct. Hydrogen atoms were included using a riding model. Carbon atoms C(9), C(10), C(11), C(30), C(31), C(32) and C(33) were disordered and included using multiple components, partial site-occupancy-factors and isotropic thermal parameters. At convergence, wR2 = 0.1032 and Goof = 1.028 for 381 variables refined against 6753 data (0.78Å), R1 = 0.0399 for those 5622 data with I > 2.0σ(I).

3. Results and discussion

3.1. Synthesis and Solution Studies

The ligand H₂pmb is similar to other dipyridylamine ligands used in coordination chemistry.^xvii The key difference is the appended carboxyamido groups that can affect both the primary and secondary coordination spheres. To explore this premise we have prepared a series of Ni(II) complexes, starting with the parent complex, Ni^IIH₂ pmb(Cl₂) (Scheme 1). This complex was readily prepared by treating a methanolic solution of H₂pmb with Ni^IICl₂. The complex was isolated via vapor diffusion of diethyl ether to afford orange crystals in a 72% yield.

Scheme 1.

Preparative route to Ni^IIH₂pmb(Cl)₂.

The appended carboxyamido groups in Ni^IIH₂pmb(Cl)₂ could be deprotonated under basic conditions, causing loss of the chloro ligands with concomitant amidate coordination to the nickel center (Scheme 2). For example, the monochloro complex, Ni^IIHpmb(Cl) was produced in nearly quantitative yield when Ni^IIH₂ pmb(Cl₂) was allowed to react with 1 equiv of KOBu^t in THF.

Scheme 2.

Preparative routes to Ni^IIHpmb(Cl), Ni^IIpmb(THF), and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc).

In addition, Ni^IIpmb was isolated (59% yield) when 2 equiv of KOBu^t were reacted with Ni^IIH₂pmb(Cl)₂. Note too that Ni^IIpmb was prepared from NiHpmbCl and 1 equiv of the base. Removal of the chloro ligands was also achieved using AgBF₄ in the presence of acetate. In this case, two acetate ions coordinate to the nickel center, affording the complex Ni^IIH₂pmb(κ²-OAc) (κ¹-OAc) in a yield of 84%.

All the Ni^II complexes are paramagnetic at room temperature with solution effective magnetic moments between 2.8–2.9 μ_BM. These values are near the spin-only value of 2.83 μ_BM expected for an S = 1 spin system.^xviii The chloro complexes and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) have temperature-dependent optical properties, in which a red-shift in the absorbance bands occurred with decreased temperature. For instance, the 20°C absorbance spectrum of Ni^IIH₂ pmb(Cl)₂ in acetone has a peak at λ_max = 452 nm that shifts to at λ_max = 442 nm at −80°C.

3.2. Molecular Structures

The molecular structures of the nickel complexes were examined in the solid-state using X-ray diffraction methods. Thermal ellipsoid diagrams of Ni^IIH₂pmb(Cl)₂ and Ni^IIHpmb(Cl) are found in Fig. 2, and Ni^IIpmb(THF) and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) are shown in Fig. 3. Crystal, data collection, and refinement parameters are found in Table 1. Selected angles and distances are given in Tables 2 and 3.

Fig. 2.

Thermal ellipsoid diagrams of Ni^IIH₂ pmb(Cl₂) (A) and Ni^IIHpmb(Cl) (B). The ellipsoids are drawn at the 50% probability level and all non-carboxyamido hydrogen atoms are removed for clarity.

Fig. 3.

Thermal ellipsoid diagrams Ni^IIpmb(THF) (A) and Ni^IIH₂ pmb(κ²-OAc)(κ¹-OAc) (B). The ellipsoids are drawn at the 50% probability level and all non-carboxyamido hydrogen atoms are removed for clarity.

Table 1.

Crystallographic data for Ni^IIH₂pmb(Cl)₂, Ni^IIHpmb(Cl), Ni^IIpmb(THF), and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc)·CH₃CN.

Complex	NiIIH2pmb(Cl)2	NiIIHpmb(Cl)	NiIIpmb(THF)	NiIIH2pmb(κ2-OAc)(κ1- OAc)2·CH3CN
molecular formula	C29H37NiN5O2Cl2	C29H36NiN5O2Cl	C33H43NiN5O3	C35H46NiN6O6
formula weight	617.25	580.79	616.43	705.49
T (K)	153(2)	148(2)	98(2)	153(2)
space group	P21/c	P21/n	P1̄	Pc
a (Å)	9.3786(5)	17.9869(18)	9.8123(5)	13.452(2)
b (Å)	18.1064(9)	11.6153(12)	11.8641(6)	14.834(3)
c (Å)	17.4684(9)	28.718(3)	13.4619(7)	17.896(3)
α (deg)	90	90	82.8894(7)	90
β (deg)	93.4637(6)	102.5698(14)	82.2732(7)	102.073(2)
γ (deg)	90	90	88.6757(7)	90
Z	4	8	2	4
V (Å3)	2960.9(3)	5856.0(10)	1540.95(14)	3492.4(10)
δcalc (Mg/m3)	1.385	1.318	1.329	1.342
Ra	0.0382	0.0627	0.0399	0.0363
Rwb	0.1028	0.1494	0.0969	0.0735
GOFc	1.053	1.098	1.028	1.004
Largest diff. peak (eÅ3)	1.803	1.450	1.011	0.447

^aR = [Σ|ΔF|/Σ|F_o|].

^bR_w = [Σω(ΔF)²/ΣωF_o²].

^cgoodness of fit on F².

Table 2.

Selected bond distances and angles for Ni^IIH₂pmb(Cl)₂ and Ni^IIHpmb(Cl).

Complex	NiIIH2 pmbCl2	NiIIHpmbCl
Distances (Å)
Ni1—N1	2.031(2)	2.081(3)
Ni—N2	2.125(2)	2.067(3)
Ni—N3	2.134(2)	2.006(3)
Ni—Cl1	2.291(1)	2.315(1)
Ni—Cl2	2.269(1)	–
Ni—O2	–	2.007(3)
Cl1···N4	3.211(6)	–
Cl1···N5	3.230(6)	–
O2···N4	–	2.978(1)
Angles (deg)
N1—Ni1—N2	81.64(7)	80.71(13)
N1—Ni1—N3	81.69(7)	82.89(13)
N2—Ni1—N3	163.24(7)	102.26(13)
N1—Ni1—Cl1	114.31(5)	94.24(9)
N1—Ni1—Cl2	101.50(5)	–
N2—Ni1—Cl1	92.81(5)	101.76(9)
N2—Ni1—Cl2	92.82(5)	–
N3—Ni1—Cl1	92.44(5)	155.00(10)
N3—Ni1—Cl2	92.20(5)	–
Cl1—Ni1—Cl2		–
N1—Ni1—O2	–	169.56(12)
N2—Ni1—O2	–	105.94(12)
N3—Ni1—O2	–	87.76(12)
Cl1—Ni1—O2	–	92.27(8)

Table 3.

Selected bond distances and angles for Ni^IIpmb(THF) and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc)·CH₃CN.

Complex	NiIIpmb(THF)	NiIIH2pmb (κ2 -OAc) (κ1-OAc))·CH3CN
Distances (Å)
Ni—N1	2.166(2)	2.079(2)
Ni—N2	2.022(2)	2.117(2)
Ni—N3	2.052(2)	2.120(2)
Ni—O1	2.001(2)	–
Ni—O2	2.013(2)	–
Ni—O3	–	2.110(2)
Ni—O4	–	2.144(2)
Ni—O5	–	1.999(2)
Ni—O3(THF)	2.175(2)	–
O3···N5	–	3.063(8)
O5···N4	–	3.000(8)
Angles (deg)
N1—Ni—N2	77.72(7)	80.49(9)
N1—Ni—N3	77.09(7)	82.59(9)
N2—Ni—N3	109.08(7)	163.07(8)
N1—Ni—O1	107.91(6)	–
N2—Ni—O1	81.83(7)	–
N3—Ni—O1	168.93(6)	–
O1—Ni—O2	89.72(6)	–
O2—Ni—N2	97.14(6)	–
O2—Ni—N3	87.08(6)	–
N1—Ni—O3	–	154.16(8)
N1—Ni—O4	–	92.09(8)
N1—Ni—O5	–	110.99(8)
N2—Ni—O3	–	98.27(8)
N2—Ni—O4	–	91.54(8)
N2—Ni—O5	–	89.70(8)
N3—Ni—O3	–	97.21(8)
N3—Ni—O4	–	89.67(8)
N3—Ni—O5	–	95.81(8)
O3—Ni—O4	–	62.08(7)
O3—Ni—O5	–	94.78(8)
O4—Ni—O5	–	156.75(8)

3.2.1. Molecular Structures of Ni^IIH₂pmb(Cl)₂ and Ni^IIHpmb(Cl)

The complexes Ni^IIH₂pmb(Cl)₂ and Ni^IIHpmb(Cl) crystallized in the monoclinic space groups P2₁/c and P2₁/n, respectively. The asymmetric unit for Ni^IIHpmb(Cl) contained two crystallographically independent, but chemically identical, molecules. Only the molecule containing Ni1 will be included in this discussion.

The complex Ni^IIH₂pmb(Cl)₂ is five-coordinate with a coordination geometry that is best described as distorted trigonal bipyramidal (Fig. 2A). The nickel center is positioned nearly within (0.007 Å) the trigonal plane defined by the chloro ligands and the apical nitrogen atom N1. The H₂pmb ligand binds in a meridional manner, with N1 and pyridyl nitrogen (N2, N3) atoms coordinated to the nickel center. The Ni–N1 bond distance of 2.031(2) Å is greater than those of the Ni–N_py bonds, which have an average bond distance of 2.129(2) Å. The binding of H₂pmb to Ni^IIpmb(Cl)₂ is comparable to that reported for the Ni^II complexes of the ligand, N,N-bis[(phenylpyridin-2-yl)methyl] benzylamine) (1).^xvii Ligand 1 is similar to H₂pmb, differing only in having appended phenyl groups instead of carboxyamido units. For instance, in the Ni^II(1)(CH₃CN)(H₂O) complex, 1 also binds meridionally and has similar Ni N bond lengths.

The chloro ligands occupied the remaining coordinate sites with Ni–Cl1 and Ni–Cl2 bond distances of 2.291(1) and 2.269(1) Å. The difference in the Ni–Cl bond length is attributed to the presence of an intramolecular H-bonding network that surrounds Cl1. Two intramolecular H-bonds are present involving Cl1 and NH groups of the appended carboxyamido units. The N4–H and N5–H bonds vectors are directed toward Cl1 producing N···Cl1 distances of 3.211(3) and 3.230(3) Å, which are within the heavy atom distances normally associated with H-bonds.^xix

Our structural analyses confirmed that treating Ni^IIH₂pmb(Cl)₂ with a base caused deprotonation of one of the carboxyamido groups to form Ni^IIHpmb(Cl). A comparison of the molecular structures of these two complexes showed that a large structural change occurs upon deprotonation. Ni^IIHpmb(Cl) has a molecular structure that is closer to square pyramidal (Fig. 2B). The N1, N2, and N3 nitrogen atoms of [Hpmb]⁻ ligand coordinates to the nickel center, similar to what was found in Ni^IIH₂pmb(Cl)₂, but in a facial manner. Both the N1–N2 and N1–N3 bond distances of 2.067(3) and 2.006(3) Å are significantly shorter in Ni^IIHpmb(Cl) compared to those in Ni^IIH₂pmb(Cl)₂. Moreover, the carboxyamidato oxygen atom O₂ in Ni^IIHpmb(Cl) coordinates to the nickel center in Ni^IIHpmb(Cl) with a Ni1–O2 bond distance equal to 2.007(3) Å. The final coordination site is filled by the lone chloro ligand that binds at a Ni1–Cl1 bond length of 2.315(1) Å. Unlike in Ni^IIH₂pmb(Cl)₂ the chloro ligand does not form a H-bond—rather, a intramolecular H-bond is formed between the appended carboxyamido groups (the N4···O2 distance is 2.978(3) Å).

3.2.2. Molecular Structures of and Ni^IIpmb(THF) and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc)·CH₃CN

The complexes Ni^IIpmb(THF) and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) crystallized in the triclinic space group P1̄ and the monoclinic space group Pc, respectively. For Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc)·CH₃CN, there were two independent, but chemically equivalent, molecules in the asymmetric unit. Only the molecule containing Ni1 will be discussed.

The molecular structure of Ni^IIpmb(THF) corroborated that deprotonation of both appended carboxyamido groups of H₂pmb caused complete loss of the chloro ligands and coordination of the carbonyl oxygen atoms to the nickel center (Fig. 3A). Facial coordination of the N1, N2, and N3 atoms was observed in Ni^IIpmb(THF), a binding of the [pmb]²⁻ ligand that is similar to the way [Hpmb]⁻ binds in Ni^IIHpmb(Cl). In Ni^IIpmb(THF) the carbonyl oxygen atoms O1 and O2 are also coordinated with Ni1–O1 and Ni1–O2 bond distances of 2.011(2) and 2.013(2) Å. Note that unlike in the molecular structures of the Ni^II–chloro complexes, Ni^IIpmb(THF) is six-coordinate, with a THF molecule occupying the final coordination site (Ni1–O3(THF) = 2.175(2) Å).

We have also found that the chloro ligands in Ni^IIH₂pmb(Cl)₂ are removed with Ag^I salts in the presence of acetate ions. The resulting complex, Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) is six-coordinate (Fig. 3B), in which the H₂pmb ligand is bonded to the nickel ion in a meridional manner similar to what was observed in Ni^IIH₂pmb(Cl)₂. The N1–N2 and N1–N3 bond lengths are statistically the same at 2.117(2) and 2.120(2) Å, respectively and the Ni1–N1 bond distance is 2.079(2) Å. The two acetate ions coordinate differently: one is bidentate, having Ni1–O3 and Ni1–O4 bond distances of 2.110(2) and 2.144(2) Å, whereas the other is monodentate, having a Ni1–O5 bond length of 1.999(2) Å. Intramolecular H-bonds were also found in Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) between the acetate ions and the H₂pmb ligand. The heavy atom N4···O5 and N5···O3 distances are approximately at 3.0 Å, which is within the range considered for H-bonds.^xx

4. Summary

Previous studies have shown the utility of the pyridylcarboxyamido unit in ligand design. We have further demonstrated its usefulness in controlling both the primary and secondary coordination spheres in transition metal complexes. Both Ni^IIH₂pmb(Cl)₂ and Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) have H-bonding networks involving the H₂pmb and bound chloro or acetato ligands. Sequential deprotonation of Ni^IIH₂pmb(Cl)₂ led to the binding of the carboxyamido oxygen atoms with concurrent loss of the chloro ligands. Alteration in the H-bonding networks was also observed upon carboxyamido binding and was completely absent in Ni^IIpmb(THF). The ability to affect structural changes within both coordination spheres could translate into a method for regulating functional properties, such as substrate binding, in a manner reminiscent of what is found in metalloproteins.